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Interface reactions between iron alloys and magnesium oxide single crystals Rose, Daneil Joseph 1962

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INTERFACE REACTIONS BETWEEN IRON ALLOYS AND MAGNESIUM OXIDE SINGLE CRYSTALS by DANIEL JOSEPH ROSE A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in the Department of METALLURGY We accept this thesis as conforming to the standard required from candidates for the degree of MASTER OF APPLIED SCIENCE. Members of the Department of Metallurgy. THE UNIVERSITY OF BRITISH COLUMBIA February 1962 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h - C o l u m b i a , I agree t h a t , t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by t h e Head o f my Department o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f Metallurgy The U n i v e r s i t y o f B r i t i s h Columbia, Vancouver 8, Canada. Date March 22, 1962 ABSTRACT The wettability of MgO by liquid iron was studied using the sessile drop technique in vacuo at 1550 ° C . Specimens of vacuum-cast iron were melted on single crystals of optical-grade magnesium oxide. Tests were limited to two minutes because of iron volatilization. Chemical reaction at the liquid-vapor interface caused the contact angle to decrease from 1 1 7 ° to 6 5 ° during the f irst minute. Reaction between the iron and oxygen provided through the dissociation of MgO resulted in formation of an FeO layer over the drop surface. The FeO was drawn to the base of the drop where i t accumulated as an annular Interfacial deposit. After solidification, the FeO decomposed to magnetite with separation of iron. Within the peripheral annulus the interface showed no sign of chemical attack. This effect was related to formation of a protective interfacial monolayer from electropositive impurities in i t ia l ly present in the iron. The major constituent of the monolayer appeared to be silicon although positive identification was not achieved. Chemical reaction occurred in each case where alloying additions to the iron of T i , V, Cr, Nb, Ta, and Zr were made. Variation of contact angle with concentration was studied. Both V and Cr improved the wettability. The effect of Zr, T i , Nb, and Ta on the wettability was not determined because oxide deposits around the basal periphery of the drops restricted expansion. Complete interfacial attack was observed with additions of Zr, T i , and V whereas additions of Nb, Ta, and Cr resulted in an annular interfacial deposit around the drop perimeter similar to the pure Fe-MgO specimens. This phenomena supported the existence of an interfacial monolayer. ACKNOWLEDGEMENT The author gratefully acknowledges Professor W. M.. Armstrong, Dr. A. C. D. Chaklader, and Mrs. A. M. Armstrong for their advice, guidance, and assistance during the investigation. The author is also indebted to Clayburn-Harbison Limited who • provided the necessary financial assistance for this work in the form of a Fellowship. i i i . TABLE OF CONTENTS Page I. INTRODUCTION 1 A. General Discussion of Metal-Ceramic Bonding 1 B. Metal-Oxide Binary Systems 2 C. The Sessile Drop Technique 3 D. The Effect of Solute Additions to the Liquid-Metal Drop . . 6 E. Factors influencing Selection of the Fe-MgO System and Alloying Components . . . . . 8 F. Previous Work 1 0 G. Specific Purpose of the Present Investigation 1 2 II. EXPERIMENTAL 1 3 A. Materials 1 3 1 . Magnesium Oxide 1 3 2 . Iron and Additive Metals 1 3 B. Apparatus 1 9 1 . Furnace 1 9 2 . Vacuum System 2 3 3 » Optical System 2 3 C. Specimen Preparation 2k 1 . Magnesium Oxide 2k 2 . Iron and Additive Metals . 25 D. Experimental Procedure 2 5 1 . Sessile Drop Experiments 2 5 2 . X-ray Diffraction . 27 3 . Metallographic Observation . . . . . . . . . . . . . . 2 7 i v . Table of Contents Continued Page I I I . RESULTS AND DISCUSSION . . . . . . . . . . 29 A. Pure Fe-MgO System 29 1. Resu l ts and C a l c u l a t i o n s 29 a) Molten Iron - - S o l i d MgO System 29 i . S e s s i l e drop experiments 29 i i . Thermodynamic c a l c u l a t i o n s . . . . . . . . 3k b) S o l i d Iron - - S o l i d MgO System . . 3^4-i , • Weight measurement 3^-i i . Meta l lographic observat ions 3^  i i i . X - ray r e s u l t s . . . 37 2. D iscuss ion 0^ B. Iron A l l o y - - MgO System kQ 1. Resul ts and C a l c u l a t i o n s k-8 a) Molten Drop - - S o l i d MgO System . . 14-8 b) S o l i d Drop - - S o l i d MgO System 56 i . I n t e r f a c i a l observat ions 56 a . Macroscopic 56 b. Microscop ic 56 i i . X - ray r e s u l t s 59 2. D iscuss ion . 60 IV. CONCLUSIONS 65 V. RECOMMENDATIONS FOR FURTHER WORK . 66 VI. APPENDICES -. 67 VII . BIBLIOGRAPHY . ' 95 . V . LIST OF FIGURES ^ Wo. Page 1. Sessile Drop Parameters . . . 5 2. Photographs of Apparatus . . . 20 3. Schematic Diagram of Apparatus 21 k. Susceptor Assembly and Sample Specimen 22 5. Specimen Mounted for Metallographic Observation . 28 6. Contact Angle Vs. Time for Pure Iron System . 30 7* Sessile Drop Photographs . 31 8. Interface Energy Vs. Time . 33 9. Interface Types as seen through MgO Plate . 35 10C o n t a c t Angle Vs. Basal Diameter 38 11. Photomicrographs of Interface 39 12. Lattice Parameter Vs. Atomic Percent Iron ^7 13. Contact Angle Vs. Time (Ta-alloyed Iron) 50 lk. Contact Angle Vs. Time (Nb-alloyed Iron) . . . 51 15. Contact Angle Vs. Time (Cr-allpyed Iron) . . . 52 16. Contact Angle Vs. Time (V-alloyed Iron) 53 17. Contact Angle Vs. Time (Ti-alloyed Iron) 5k 18. Contact Angle Vs. Time (High Carbon Steel) 55 19. Interfacial Deposits 57 20. Interface Structures . . . . . 58 21. Plot of x Vs. /3 75 z 22. Plot of/x] and/z] Vs. /3 . 76 Wjrf = 90° = 90° ' 23. Plot of xj_ Vs. / 3 77 b 2k.. Plot of zj_ V s . ^ 78 b 25. Histogram of Surface Tension Variation 82 26. Vapor Pressures of Iron 86 27. Phase Diagrams of Fe-0 and FeO-MgO Systems 90 v i . LIST OF TABLES No. Page I. Free Energies of Oxide Formation 9 II. Spectrochemical Analysis of MgO . . . . . . ik III. Properties of Materials Used 15 IV. Published Surface Energies of Magnesium Oxide l 6 V. Spectrographs Analyses of Me'tals Used . 17 VI. Summary of Liquid Iron Surface Tension Values . 18 VII. Interface Energies at Various Times 32 VIII. Interface Annulus Measurements 36 IX. Sessile Drop Data 68 X. Table of Liquid Iron Surface Tension Values 80 XI. X-ray Data 88 INTERFACE REACTIONS BETWEEN IRON ALLOYS AND MAGNESIUM OXIDE SINGLE CRYSTALS I. INTRODUCTION A. General Discussion of Metal-Ceramic Bonding The superior strength, corrosion, and thermal shock properties of such metal-ceramic combinations as enamelled metals, cermets, and dispersion-hardened alloys have stimulated interest in the nature of metal-ceramic inter-actions.. Since these materials are" heterogeneous (i .e. composites of distinct metal and ceramic phases) the interfacial bond strength must compare favorably with the. bulk strength of the weakest component i f the substance is to be used for industrial application. Investigators of the bond mechanism have employed both the crystallographic and chemical approach. The crystallographic approach attempts to explain interfacial properties through structural con-figurations: ;and interatomic bond relations, whereas the chemical approach endeavours to describe interfacial properties in terms of energetic quantities derived from thermodynamic relations. In addition, the effects of diffusion, mechanical stresses, and thermal stresses upon the interface have been> studied to some extent. The ultimate aim of bond investigations is the evolution of fundamental theories that wil l aid in predicting the behaviour of systems which have not been studied." In order to avoid the complexity of multi-component systems, most studies have treated binary systems. These studies have been complicated in many cases, however, by the presence of trace impurities which affect the interfacial bond. - 2 -B . M e t a l - O x i d e B i n a r y S y s t e m s I n c o m p a r i s o n t o m e t a l - c a r b i d e a n d m e t a l - n i t r i d e c e r m e t s , t h e d e v e l o p -m e n t o f m e t a l - o x i d e c e r m e t s h a s b e e n o f m i n o r i m p o r t a n c e . H o w e v e r t h e a b u n d a n c e o f o x i d e s - r e l a t i v e t o c a r b i d e s a n d n i t r i d e s w a r r a n t s f u r t h e r i n v e s t i g a t i o n i n t h i s f i e l d . F a c t o r s a f f e c t i n g m e t a l - o x i d e . i n t e r f a c e s h a v e b e e n d e t e r m i n e d l a r g e l y t h r o u g h w e t t a b i l i t y s t u d i e s o f s o l i d o x i d e s b y m o l t e n m e t a l s . T h e s i g n i f i c a n t p a r a m e t e r s i n s u c h a s t u d y a r e t h e s o l i d - l i q u i d i n t e r f a c e e n e r g y ( ^ S L ^ a n c ^ w o r k o f a d h e s i o n (W ) . H e r e a f t e r i n t h i s r e p o r t , " i n t e r f a c i a l " w i l l r e f e r s p e c i f i c a l l y t o t h e s o l i d - l i q u i d i n t e r f a c e u n l e s s o t h e r w i s e i n d i c a t e d . T h e i n t e r f a c e e n e r g y h a s b e e n d e f i n e d b y G l a s s t o n e 1 a s t h e w o r k r e q u i r e d t o e n l a r g e t h e s u r f a c e o f s e p a r a t i o n b y o n e u n i t a r e a . A l l e n a n d K i n g e r y d e f i n e t h e w o r k o f a d h e s i o n a s t h e d e c r e a s e i n e n e r g y i n b r i n g i n g t o g e t h e r a u n i t a r e a o f l i q u i d s u r f a c e a n d a u n i t a r e a o f s o l i d s u r f a c e t o f o r m a u n i t a r e a o f i n t e r f a c e . A l o w XQL o r h i g h b e t w e e n t h e l i q u i d m e t a l a n d s o l i d o x i d e u s u a l l y r e s u l t s i n a s t r o n g b o n d b e t w e e n t h e s o l i d m e t a l a n d s o l i d o x i d e . A v a r i e t y o f c o n f i g u r a t i o n s m a y f o r m i n t h e i n t e r f a c i a l r e g i o n : 1. n e w o x i d e s r e s u l t i n g f r o m d i r e c t c h e m i c a l r e a c t i o n b e t w e e n t h e l i q u i d m e t a l a n d s o l i d o x i d e , 2. o t h e r c o m p o u n d s ( c a r b i d e s , n i t r i d e s , g a s e s e t c ) f o r m e d t h r o u g h r e a c t i o n ; o f t h e o x i d e w i t h s o m e i m p u r i t y i n t h e l i q u i d m e t a l , 3 . l i q u i d m e t a l c o n t a i n i n g a n i m p u r i t y i n e x c e s s o f t h e b u l k c o n c e n t r a t i o n : : o f t h a t i m p u r i t y , k. s o l i d s o l u t i o n s f o r m e d t h r o u g h d i f f u s i o n o f t h e l i q u i d m e t a l ( o r i t s i m p u r i t i e s ) i n t o t h e o x i d e . - 3 -T h e i n t e r f a c i a l c o n f i g u r a t i o n p r e s e n t i n t h e m o l t e n l i q u i d m a y b e a l t e r e d u p o n s o l i d i f i c a t i o n a n d s u b s e q u e n t c o o l i n g d u e t o s e p a r a t i o n o f e u t e c t i c s , f o r m a t i o n o f i n t e r m e t a l l i c c o m p o u n d s , p h a s e c h a n g e s , e t c . S i n c e t h e s e p h e n o m e n a m a y c r e a t e l a r g e s t r e s s e s i n t h e i n t e r f a c i a l r e g i o n , a l o w i n t e r f a c e e n e r g y i n t h e l i q u i d - s o l i d s y s t e m d o e s n o t n e c e s s a r i l y r e s u l t i n a s t r o n g m e t a l - c e r a m i c b o n d . N o t o n l y m u s t t h e s t r e s s e s p r o d u c e d "by n e w p h a s e s b e . - c o n s i d e r e d , b u t a l s o t h e r m a l s t r e s s e s e s t a b l i s h e d d u r i n g r a p i d c o o l i n g o f t h e s y s t e m . C . T h e S e s s i l e D r o p T e c h n i q u e T h e s e s s i l e d r o p t e c h n i q u e i s v a l u a b l e i n d e t e r m i n i n g t h e w e t t a b i l i t y o f a n o x i d e s u r f a c e b e c a u s e : ' 1. i t a l l o w s t h e s i m u l t a n e o u s d e t e r m i n a t i o n o f l i q u i d s u r f a c e 3 t e n s i o n a n d i n t e r f a c e e n e r g y , 2 . t h e d r o p p r o v i d e s a l a r g e p l a n a r i n t e r f a c e w h i c h f a c i l i t a t e s t h e i d e n t i f i c a t i o n o f i n t e r f a c i a l c o n f i g u r a t i o n s , d i f f u s i o n , k s t r e s s e s , e t c . , 5 3» ' s e s s i l e d r o p t h e o r y i s t h e r m o d y n a m i c a l l y j u s t i f i e d , k. t h e m e t h o d i s a d a p t a b l e t o n u m e r o u s m e t a l - o x i d e s y s t e m s w i t h o u t a p p a r a t u s m o d i f i c a t i o n ( a s i d e f r o m p o s s i b l e t e m p e r a t u r e l i m i t a t i o n s ; ; 7 5. e x p e r i m e n t a l r e s u l t s a r e r e a s o n a b l y p r e c i s e . I f a l i q u i d d r o p i s r e s t i n g o n a s o l i d s u r f a c e a s i n F i g u r e 1. t h e s h a p e o f t h e d r o p i s d e t e r m i n e d b y a n e q u i l i b r i u m b e t w e e n g r a v i t a t i o n a l f o r c e s t e n d i n g t o f l a t t e n t h e d r o p a n d s u r f a c e - t e n s i o n a l f o r c e s t e n d i n g t o 2 . '• s p h e r o i d i z e t h e d r o p . T h e r e l a t i o n b e t w e e n t h e f o r c e s a c t i n g o n t h e p e r i -8 p h e r y o f t h e s o l i d - l i q u i d i n t e r f a c e w a s f i r s t i m p l i e d b y Y o u n g a n d l a t e r s t a t e d b y Dupre* i n t h e e q u a t i o n , ^ S L <*SV ^ L V c o s 9 w h i c h i s d e r i v e d f r o m a h o r i z o n t a l r e s o l u t i o n o f t h e e n e r g y v e c t o r s : - h -; JfgL = solid-liquid interface energy o'sy- =• solid-vapor interface energy •^^ Y = liquid-vapor interface energy where 9 = contact angle'' measured through the liquid Under the influence of the three interface energies the"drop may exist in equilibrium at any angle between 0° and l80° . The drop is said to "wet" the surface i f i t possesses a contact angle less than 9 0 ° ' o Direct measurementsof 9 becomes inaccurate at angles greater than 90 • 11 In order to avoid this problem,.Bashforth and Adams developed a method (Appendix II.) for calculating'9 and O'LV ^ r o m dimensions x, x', z, z' shown in Figure 1. 12 • Dorsey developed a simpler but less accurate method of deriving ^LY r r o m measurement of the two dimensions x and b (also shown in Figure 1 . ) . The error in Bashforth and Adams' method has been quoted within , 6, 13 • . .6 * 270 whereas the error in Dorsey's method may be ± yj° • I*1 both cases the principal error arises from the simultaneous existence of several different ih 7 9's around.the drop periphery . Baes1 attempts to minimize this error by measuring the drop dimensions from five different viewing directions. The accuracy of the sessile drop technique is slightly dependent up-on the volume of the drop, which can be described by the parameter x . From z a consideration of Baes' results, which support the use of large drops, and Mack's discussion1^, which favors small drops, the optimum x value/apparently z lies somewhere between 1.15 and 1 .70. For 9 between 0° and 90° , O'LV c a n n ° t e^ calculated. The contact angle can be obtained, however, byjassuming the drop to be a spherical segment1^. The contact angle is then given by 9 = 2 tan"1 h , x where h and x are the dimensions shown in Figure 1. - 5 -Acute Contact Angle Figure 1. S e s s i l e Drop Parameters - 6 -Ellefson and Taylor"^ state that this -assumption may be in error because 15 gravitational forces destroy the spherical relationship although Mack's results show that the maximum error introduced by gravity effects is 1°10' Advancing and receding peripheries of drops of a specific liquid , o!7 have formed equilibrium angles which differ by as much as 40 . Although this has been described as a wetting hysteresis effect (implying some 18 metastable configuration), Bartell and Wooley maintain that the angle formed in either case is a true equilibrium angle. For the advancing case, the final angle depends on the pretreatment of the unwetted surface. For the receding case, the final angle is independent of pretreatment because the liquid is receding over a previously wetted surface. Advancing peri-pheries are more desirable for sessile-drop experiments because they reflect the pretreated surface condition of the solid. D. The Effect of Solute Additions to the Liquid Metal Drop • Attempts have recently been made to improve the wettability of an oxide by a particular metal through solute additions to that metal. Solute atoms in general may: 1. form oxides through reaction with oxygen at either the solid-liquid or liquid-vapor interface, 2. adsorb at the metal-oxide interface without entering into chemical reaction, 3. be surface-active in the liquid, affecting V'LV-' h. have no effect. Solutes of type 2. usually decrease the interface:. . energy and improve the wettability without forming new compounds having undesirable thermal, mechanical, and physical properties. - 7 -F a c t o r s i n f l u e n c i n g the p r e f e r e n t i a l i n t e r f a c i a l a d s o r p t i o n 5 6 1Q 20 21 of c e r t a i n s o l u t e s have been s t u d i e d by s e v e r a l i n v e s t i g a t o r ' y ' ' . They have c a l c u l a t e d the excess i n t e r f a c i a l s o l u t e c o n c e n t r a t i o n u s i n g Gibb's Adsorption Theory, which, f o r s e s s i l e drop a p p l i c a t i o n , takes the form of the equation P = lk- d0 SL R T d l n a 2 where P = excess i n t e r f a c i a l c o n c e n t r a t i o n (mol/cm ) a = b u l k a c t i v i t y of the s o l u t e ^ S L = i n t e r f a c e energy T = absolute temperature R = gas constant 20 Gamma has been i n t e r p r e t e d more c l e a r l y by Guggenheim as the number of moles of component (per u n i t area of f r e e surface) i n excess of what would be present i f the b u l k composition remained uniform r i g h t up t o the i n t e r -f a c e . I f the s o l u t e obeys Henry's Law, the mole f r a c t i o n can be s u b s t i t u t e d f o r a c t i v i t y i n the equation. . ,-The degree of i n t e r f a c i a l s o l u t e a t t r a c t i o n i s c o n t r o l l e d not only by the p r o p e r t i e s of the s o l u t e atoms, but a l s o by the nature of the 22 oxide s u r f a c e . Murray has shown t h a t c a t i o n s may r e t r a c t below the oxide surface due to the d i r e c t i o n a l i n t e r a t o m i c bond d i s t r i b u t i o n at the s u r f a c e . The oxygen i o n domination then'creates an e l e c t r o n e g a t i v e s u r f a c e . When a l i q u i d drop i s placed on the s u r f a c e , s o l u t e atoms are a t t r a c t e d t o the i n t e r f a c e i n p r o p o r t i o n t o t h e i r e l e c t r o p o s i t i v i t y . To a f i r s t approximation, the adsorbed s o l u t e atoms form an i o n i c metal-oxygen bond^ w i t h the surface oxygen i o n s . P r e f e r e n t i a l a d s o r p t i o n of one s o l u t e over another may t h e r e -f o r e be approximately p r e d i c t e d through a c o n s i d e r a t i o n of the f r e e energy of formation (AF^. ) of the s o l u t e oxides at t h a t temperature. Hence s t a b l e oxide-formers such as T i , Zr, and Th would be expected t o adsorb p r e f e r e n t i a l l y t o Cr, Mn, and Fe which are l e s s s t a b l e oxide-formers. - o -E. Factors Influencing Selection of the Fe-MgO System and Alloying Components Experimental evidence in support of the preferential interfacial adsorption theory is meager. Results that would establish a definite cor-relation between A F ^ ..of solute oxides and their preferential interfacial adsorption could be obtained through a study of alloying effects on some metal-oxide system. The choice of system is limited primarily by the amount of thermodynamic data and surface energy information available. In this work/ iron was chosen for the metal component because the 23 activities of several alloying components in liquid iron have been measured . Furthermore, liquid surface tension values for pure iron are known and the effects of various solute elements on this surface tension have been ascert-ained. The moderate attraction between iron and oxygen makes possible the selection from a wide range of solute elements that would adsorb on an oxygen-dominated surface preferentially to iron. In liquid steels, these elements would be referred to as deoxidizing agents. Magnesium oxide was chosen as the ceramic because of its availa-b i l i ty in the form of large cleavable single crystals. There is reason to believe that freshly-cleaved surfaces are the most contaminant-free oxide IT surfaces obtainable . The solid surface energy (X sv) of the MgO (100) cleavage face is more accurately known than any other oxide Xgy, although determinations of this quantity, in general, are s t i l l in a rather primitive pk state . Since mechanical preparation of cleaved MgO surfaces is unnecessary, surface energy variations are minimized. The stability of the oxide precludes direct chemical reduction except by the strongest oxide-former solutes. The possibility of developing Fe-MgO combinations for specific industrial applications was considered in choosing these two components. The 25 use of Ni-MgO and Co-MgO cermets : has already been reported . In view of the similarity of Fe to Co and Ni, and the numerous uses of irons and steels, development of dispersion-hardened iron alloys or cermets.: may be industrially feasible. Solid surfaces tend to lower their surface energy by adsorption 22 of a gas . Since the decrease in solid surface energy of MgO associated with adsorption of various gases is not known, i t was decided that the experiments would be conducted in a moderate vacuum. The vapor pressure of liquid iron near its melting point prohibits the use of a high vacuum. The solute elements were chosen in i t ia l ly from a comparison of c standard free energies of formation ( .A F - 0 ) of the oxides at 1500°C. as 26 compiled by Tripp and King . This data is shown in Table I. Standard free energies do not allow accurate prediction of preferential interfacial adsorption of one solute over another. Free energies of solution in liquid iron, reduction of oxygen potential (associated with vacuum), interaction effects, etc. must be considered. Table I aids an approximate selection, however. Elements chosen were T i , Si , V, Ta, Nb, Cr, Mn, and C. TABLE I. -AF° of the Oxides 1550 Group 1, 2, 3 Group k Group 5 Group 6, 7, 8 Eeduce MgO C a 0 j BeO SrO A1 20 3 212 210-19k 175 Th02 219 Ce20o 211 Zr0 2 180 uo 2 182 - • MgO 173 TiO 166 V2°3 VO 123 MnO 119 w CD Ce02 165 117 Cr 20o Mn30^ 101 •H-P Ti 20o "163 Ta20e V02 ' 117 90 W -H a H T i 2 0 5 15U l lU Mn 20 3 73* •H -H >> Ti0 2 ll+5 NbO Ilk* 0 CH H CQ Si0 2 130 Nb02 108* r H O •3 O CO 129 Nb20c 102* PM P20H-V2°5 93* 76* FeO 73 Cu20 23 C02 62 P 2 0 5 63* Mo02 63 CO -P CuO 5 Sn02 58 As20o 58* WOo 6 l 0 0 PbO 35 Sb203 31* - WO2 59 s "t BiO 25* Fe 2 0 3 5^* « SbOp 23* CoO 52 * = extrapolated values NiO Ul - 1 0 -F. Previous Work , The Fe-MgO system has been studied to some extent using the sessile drop technique. Kingery and Humenik^  measured the surface tension (fay)' of Armco iron on MgO in helium at 1550°C. to be 1 2 ^ 0 ergs/cm^. They observed an interfacial discoloration of the MgO plaque implying that some chemical reaction had occurred. Rapid volatilization of the drop in vacuum discouraged 6 further study in this direction. Humenik and Kingery studied the Fe-MgO system in vacuo but their observations were again obscured by volatilization of the iron. Furthermore, the contact angle decreased with time. Since Dupre's equation applied ini t ia l ly , they extrapolated the contact angle to zero time at melt ( 1 2 3 ° ) and calculated the interfacial \^^: l---:'/'J :' :r'::--'i: :.'-' energy using this angle. The decreasing contact angle was attributed to some surface reaction which was not investigated in detail. A zero-time o contact angle of 1 3 0 was measured in helium indicating that adsorbed gas had affected the surface energy balance. In hydrogen .a non-metallic surface film formed on the liquid drop which completely restricted its flow. They surmised that the film was produced by a surface reaction or atmospheric conditions within the furnace. Sintered MgO plaques rather than single crystals were used throughout their experiments. Observations on the Fe-BeO system by the same authors revealed that surface porosity seems to have little,.effect on the equilibrium contact angle. A variation of 2 ° with zero to1 ten percent porosity was noted. The surface of a sintered MgO plaque would probably display other crystal faces than the ( 1 0 0 ) face as well as numerous edges and corners. The surface energy of the ( 1 0 0 ) MgO face has been established as 1 0 9 0 (± 20$) 2 2 ergs/cm^ at 0°K . Lennard-Jones and Taylor have shown that MgO has an (Oil) surface energy of 3 9 ^ 0 ergs/cm^ and a crystal edge energy of 2 7 X 1 0 ^  ergs/cm. The presence of either could thus cause a wide local variation in solid.surface energy on a sintered plaque. - 11 -P r e f e r e n t i a l i n t e r f a c i a l adsorpt ion has been evident i n the r e s u l t s of some authors . Kingery found i n the Fe-Al^O^ system that 0.06$ S i i n the i r o n was s u f f i c i e n t to form a s i l i c o n monolayer at the s o l i d - l i q u i d i n t e r f a c e . Humenlk and Kingery^ observed i n the same system that O.Ok'fo t i tan ium i n the i r o n g reat l y decreased the interface:...", energy without a f f e c t i n g the l i q u i d 2 surface t e n s i o n . A l l e n and Kingery d iscovered the same e f f e c t with carbon i n i r o n . Although the carbon d i d not a f f e c t the l i q u i d surface t e n s i o n , carbon add i t ions of approximately 4$ caused bubble evo lu t ion fromothe drop that compl icated contact angle measurements. They assumed the gas to be carbon monoxide. 7 . Baes ; :sums up the e f f e c t of var ious a l l o y i n g elements on the surface tens ion of l i q u i d i r o n with the statement that s u r f a c e - a c t i v e impur i t i es are u s u a l l y elements possess ing l i m i t e d s o l u b i l i t y i n the l i q u i d metal ( i . e . the non-metals of group V, VI and VI I ) . Kurk j ian and 19 -Kingery add that s u r f a c e - a c t i v e impur i t i es o f ten have a la rge r atomic '13 s i ze and lower l i q u i d surface t e n s i o n than the so lvent . Halden and Kingery found that carbon and n i t rogen are not s u r f a c e - a c t i v e i n i r o n whereas oxygen and sulphur are' h igh l y s u r f a c e - a c t i v e and may form surface monolayers at concentrat ions below 0.1$. k Economos and Kingery observed i n t e r f a c i a l reac t ions between var ious metal -ceramic combinations i n c l u d i n g Si-MgO, Ti -MgO, Zr-MgO and Nb-MgO. In each of these systems they reported a new i n t e r f a c i a l phase which was not so lub le i n e i t h e r oxide or m e t a l . The p o s s i b i l i t y of Ta reducing MgO to some 2 8 extent was mentioned by Shepherd because of i t s e f f i c i e n c y as an oxygen-g e t t e r " . - 12 -The diffusion of iron into magnesium oxide has been investigated 29 by Turnbull using tracer techniques and more recently by Vasilos and 30 Wuensch using X-ray and electron probe methods. The diffusion coefficient r ° 1 0 of metallic iron in MgO determined by Turnbull between 1060 and 13^ 0 was D = 9 ,5 (10~9) - 25,900 e RT He then substituted this value into the solution of Fick-' s equation, viz: C = GQ (TT^Dt)* , 0 e I T 5 t and studied concentration gradients. Vasilos and Wuensch calculated the activation energy for iron diffusion in MgO to be 1.7 ev (compared to Turnbull's value of 1.12 ev); 31 Carter measured the rate of diffusion of FegO^ into MgO and claims the mechanism,to be counter-diffusion of Mg + + and F e + + + ions through a relatively rigid oxygen lattice. 32 The FeO^ MgO diffusion studies, of Rigby and Cutler show that vacancies diffuse into the MgO in approximate proportion to the number of F e + + + already present in the MgO. Furthermore, the area through which iron has diffused becomes visibly discolored. G. Specific Purpose of the Present Investigation The purpose of the present investigation was to make a comprehensive wettability study of the Fe-MgO system using the sessile drop technique in vacuo at 1550°C. The effects on wettability of various alloying elements in the liquid iron were to be determined and compared in the hope that the pre-.; ferential interfacial adsorption trend noted by Kingery et a l . could be verified for this system. Interfacial reaction products were to be identified where possible by X-ray analysis and their structural configurations invest-igated' by metaJUographic techniques. Mechanisms of the reactions were to be proposed on the basis of these results. Vacuum-cast high-purity iron and single crystals of optical-grade MgO were to be used throughout the experiment. - 13 -II. EXPERIMENTAL A. Materials '•.'- ;."••••'•' 1. Magnesium Oxide Originally, synthetically-produced ultra-high-purity single crystals of magnesium oxide (MgO), which-are now being prepared by General 33 Electric , were to be used but these proved to be.too small for sessile drop supports. Hence high purity single crystals of optical-grade MgO supplied by Norton Company were used i n a l l experiments. The average purity of these crystals was determined by spectrochemical analyses on eight samples as shown in Table II. A purity comparison to Baker and Adamson reagent grade MgO powder i s included. MgO has an ionically-bonded- rocksalt structure of two inter-penetrating face-centred-cubic" lattices.. Magnesium ions occupy one set of l a t t i c e sites while oxygen ions"occupy the other set. Significant properties of MgO and the various metals used in this investigation are l i s t e d i n Table m. The solid surface energy (Xgy) o r M S ° has been estimated by both experimental' and.theoretical methods.' Values published by various authors are i n disagreement, as shown in Table IV. • The value chosen for this work was 1090 ergs/cm^ at 0°K, which was derived from the most recent and thorough 22 investigation . 2. iIron and Additive Metals The high-purity iron wa.s supplied by Crucible Steel Company of America under the trade name of Ferrovac E cold-drawn 5/8; inch rod. A spectrographs analysis of this material together with analyses of solute metals used is shown in Table V. Samples of zonesrefined iron and high-carbon steel shot were also analysed. The li q u i d surface tension of pure iron has been measured by several investigators. A summary of this'work i s presented in Table VI. - 14 -TABLE II. Spectrochemical Analysis•of: MgO Samples Internal Standard - Mg 30Jk~ Al •" Element Wavelength 3082 •Mn " L"-2576 , . Fe 2600 Si ' 2516 Cu 321+7 Cr 4254 Ca 3969 Weight Per: -cent Baker and Adamson Reagent Grade MgO .006 .0005' .011 .006 • .001 .006 .014 Specimen 1, .03k .0017 .009 .020 .0013 .006 .006 2. .029 .0014 .013 .018 .0033 .003 .007 3v .022 ;.ooi6:> .012 .017 .0020 .005 .005 k. .034 .0016' .008 .018 .0007 .(X>'3 .004 5. .022: ".0041 .011 .011 .0005 .008 .008 6. .023 : .0041 .005 .007 .0005 .006 .006,-. . 7 . .010 . .0029 .006 .007 .0011 .007 .005 8. .010 .0018 .007 .006 .0006 .008 ... '005 Average Analysis .023 .0024. • 009 .013 .0013 .006 .006 Data supplied "by J . H. Kelly (Steel Company of Canada Limited). - 15 -TABLE III. Properties of Materials Used Density Xy at M.P. Vapor.->Frensure M.P. Electro- Metallic (gms/cc) (ergs/cm^) (mm), 1^50°c. negativity Radius (&), MgO 2 8 0 0 Mg + + 1 . 9 2 0 — 4 . 0 8 Fe 7 . 2 0 1835 ' 0 . 0 7 1535 I . 165 Ti 4 .50 i46o 0.01 1727 1 . 6 0 1 . 3 2 4 V 5 . 9 6 1697 < 0 . 0 0 1 1697 1 . 9 0 1 . 2 2 4 Ta 1 6 . 6 0 2 8 6 0 < 0 . 0 0 1 2996 1 . 9 0 ' 1 . 3 4 3 Nb 8.40 2 0 3 0 < 0 . 0 0 1 2 5 0 0 1 . 8 0 1 . 3 4 2 Cr 7 . 2 0 - 2 . 0 1900 2 . 2 0 •I.I76 Zr 6 . 4 4 - < 0 . 0 0 1 2127 1 . 6 0 1 . 5 9 7 Ref. 34 Ref. 35 Ref. 36 Ref. 36 Ref. 37 Ref. 37 T A B L E I V . P u b l i s h e d S u r f a c e E n e r g i e s o f MgO A u t h o r D a t e S u r f a c e E n e r g y ( e r g s / c m 2 ) M e t h o d F r i c k e J u r a a n d G a r l a n d L e n n a r d - J o n e s a n d T a y l o r K i n g e r y L i v e y a n d M u r r a y M u r r a y 19^3 1952 1953 1954 1956 i 9 6 0 1459 1040 1362 C a l c u l a t i o n C a l o r i m e t r i c C a l c u l a t i o n 1360 ± 20$ E s t i m a t e , , 1090 E s t i m a t e f r o m L a t t i c e E n e r g i e s 1090 ± 220 t o g e t h e r w i t h H e a t o f s o l . d a t a . N O T E : A l l v a l u e s f o r (100) c r y s t a l f a c e a t 0 ° K . TABLE Y. Spectrographic Analyses of Metals Used Metals C, o2'-. - W 2 - V /Fb :,Ta Mo Zr, Ti Fe"1:. Si:- P . : . s :. .Ni :Cr • Mg Al MgO . 0 0 9 . 0 1 3 - 0 0 6 - 0 2 3 Ferra'vac Fe - 0 1 3 . 0 0 8 3 . 0 0 0 3 . 0 0 5 ND ND . 0 0 6 <T . 0 1 " bal 0 . 0 1 T :•: - 0 0 2 .-T • . 0 1 0 . 0 0 2 •-T-V Zone-E Fe - 0 1 2 . 0 0 5 ND ND -005 T . 0 2 0 bal ND T - 0 0 3 ND - 0 1 0 . 0 0 2 T Cr ND . 0 0 6 ND -• T j . 0 0 1 • T .; • • 0 5 .ko . 2 0 ND . 0 9 - 0 1 - 0 0 3 .h V -05 . 0 9 . 0 5 bal Fb . 0 5 - 0 7 - 0 3 bal . 1 5 . 0 1 .01 . 0 1 Ta - 0 5 . 0 2 . 0 1 • 0 5 bal . 1 8 . 0 1 . 0 4 . 0 1 Ti - 0 3 . 0 1 . 0 5 bal .Oh - 0 3 Zr . 1 0 . 0 1 . 0 5 bal - 0 6 . 1 2 - 0 5 Hi C Steel . 9 0 - 0 0 3 ND ND . 0 0 7 T .. . 0 2 bal . 2 0 - 0 0 3 - 0 1 3 • 0 7 • 0 3 . 0 0 2 - 0 2 Ferrovac Fe . 0 0 8 (after.melt-ing on MgO) . 0 0 5 ND ND . 0 0 5 T . 0 1 bal ND T - 0 0 2 T . 0 1 - 0 0 1 ND TABLE VI. Summary of Liquid Iron Surface Tension Values Ref. No. Temp. °C. #LV Method Iron Type Chemical Analysis Al Cu FD Si C (wt. io) P 0 S N 5 1550 1450 Sess. Drop N.R.C. vac. cast 6 1550 1400 1450 Sess. Drop 11 11 Armco Electrolytic 2 1570 1700 11 11 N . R i C . vac. cast .002 .01 <.01 .04 .005 .002 .010 .005 .00089 13 1570 1720 II 1! Ferrovac E .003K.001 T .01 .0031 .0072 .005 .00051 38 1550 1380 II II Electrolytic 25 1550 1940 Estimate Pure 3 1550 1240 Sess. Drop Armco 39 1550 840 Pendant Drop 4o 1550 1835* 15 Sess. Drop Purified Carbonyl .001 .0003 <.001 <.001 <.0008 T <.0005 This Work 1550 1426 Sess. Drop FerrovacE 1* 2* .003 .01 .001 .01 .007 a? .oo4 T 0.01 -013 .002 .OO83 T .005 .0003 .003 1* Invoice Analysis 2* Spectrographic analysis performed by Coast Eldridge Chemists - 19 -B. Apparatus The sessile drop furnace described: .'in detail by previous workers 35..> 4l w a g m o di f ied slightly for this work. Modifications were such that: 1. a vacuum of 10"^ mm of mercury could be attained in ten minutes, 2. drop measurements could be made after melting at ten-second time intervals through the use of ultra.-, high-speed self-developing film. Figure 2. shows photographs of the equipment and Figure 3. represents a schematic diagram of the apparatus. 2 1 , 21 1. Furnace The molybdenum susceptor design used by Clarke et a l . was altered to reduce radiation heating of the vycor tube and prolong the l i fe of the susceptor itself . Dimensions were altered to: Length ( i a « ) Diameter (in.) Thickness (in.) SUSCEPTOR RADIATION SHIELD 1.5 3-5 k.O 5.0 0- 75 0.75 1- 75 1.50 0.020 0.005 0.005 0.005 Rose Clarke et al . Rose Clarke et al . The assembly is shown in Figure k. The insertion of two sintered alumina spacers near the ends of the radiation shield, prevented sagging and subsequent closure of the loop. Furnace power was supplied by a Lepel (model T-10-3) high-frequency induction generator,and temperatures were measured through a pyrex viewing window to ± 5°C with a Hartmann and Braun disappearing-filament optical pyrometer. Aside from the optical-flat correction of or 23°C. DeCleene showed that emissivity corrections with this apparatus were negligible. - 20 -Figure 2. Photographs of Apparatus OPTICAL SYSTEM FURNACE ASSEMBLY 1. Ground glass. 13- •Induction coil . 2. Vertical adjustment screw. lh. Heating element, radiation shield and specimen 3. Horizontal adjustment track. 15. Thermocouple gauge. h. Focussing screw. 16. Ionization gauge. •5. . Adjustable bellows. 17- Gas inlet control. 6. Ocular lens. 18. Viewing window. 7- Objective lens,shutter and ir i s diaphram.19- Brass fittings. 8. Vertical adjustment screw. 20. Optical pyrometer. 9- Water-cooled optical flat. 21. Light source interchangeable with pyrometer. 10. Water-cooled brass fitt ing. 22. Polaroid camera. 11. Magnetic shutter. 23. Pivoting mirror. 12. Vycor tube. Figure 3. Schematic Diagram of Apparatus ro h-1 - 22 -Figure h. Susceptor Assembly and Sample Specimen The high vapor pressure of iron at its melting point (0.1 mm Hg) created a problem in the furnace tube. Iron vapor condensed rapidly on the tube interior near the cooling jackets so that the tube itself became a susceptor shortly after melting. Experimental tests were thus limited to two minutes in order to avoid implosion of the vycor tube. During tests, heat dissipation from the tube was accelerated by blowing a continuous stream of cold air across the tube. 2. Vacuum System The two-stage o i l diffusion pump used by Clarke et a l . was - 5 inadequate for this work because i t required one hour to reach 10 mm Hg and frequently became contaminated with metal vapors and oxides. It was therefore replaced by a Speed!vac mercury-vapor pump (model 2M2A'0- This pump, operating in conjunction with a mechanical fore pump, produced a vacuum of 10 ^  mm Hg in ten minutes. 3. Optical System Molten iron drops magnified eight times could be viewed with ease on the ground glass plate shown in Figure 3' (# l ) « Tne light intensity at this magnification, however, was insufficient for photographic exposures. Clarke and co-workers found that a moderately fast.film (Ferrania Sh - ASA daylight 250) required a twenty-second time exposure at : f l l . to yield a sharp drop image. Since their tests often ran for 45 minutes, time exposures were satisfactory. It was discovered in preliminary investigations on the Fe-MgO system that the contact angle changed rapidly during the f irst two minutes. Time exposures were thus inadequate.. The optical system was then modified to include a Polaroid Land Camera employing ultra^high-speed self-developing film (Polaroid 3000 - ASA daylight 3200) as shown in Figure 3. - 24 -At room temperature a pivoting mirror (# 23) allowed focusing of the drop on a ground glass plate (# l) with the aid of a light source (#'21). At 1500°C. the mirror could be quickly t i lted up, permitting light to reach the Polaroid camera. Since exposures of 1 of a second at f64 were sufficient 100 to yield a sharp image, problems of vibration encountered by Clarke et a l . were eliminated. Furthermore, the developing time of Polaroid 3 0 0 0 is ten seconds so that photographs at fifteen-second time intervals during a-single experiment were possible. C. Specimen Preparation 1. Magnesium Oxide Materials to be used in surface tension measurements require scrupulous handling in order to avoid surface contamination. Both magnesium oxide and metallic specimens were prepared with care in this respect. Large single crystals of magnesium oxide were cleaved (with chisel and hammer) along -£l.00^ planes into small rectangular plates approximately 20 X 20 X 1 mm. The mechanical stresses introduced in the specimens during cleavage were relieved through annealing in vacuo at o 1100 C. for three hours. Thermal stresses were avoided by decreasing the temperature gradually following annealing. Adamson points out that cleaved surfaces have a wide local variation in surface energy due to surface irregularities such as edges, corners, cleavage steps, microscopic pits, etc. Experiments were conducted to determine a suitable etching procedure for removal of these irregularities. Specimen 42 immersion in phosphoric acid (at 100°C.) was mentioned by Evans as a s u i t r able method for disposal of cleavage steps. Microscopic observation revealed that no further removal of surface irregularities occurred beyond five minutes of immersion. A l l cleaved and annealed plates were thus etched in acid for five minutes, rinsed quickly in water, and dried instantly with a blast of warm air in order to prevent magnesium hydroxide formation on the surface. - 25 -2. Iron and Additive Metals Preliminary experiments showed that small iron cylinders (0.250" in diameter by 0.250" in length), when melted on magnesium oxide plates, in i t ia l ly possessed an advancing periphery. The iron supplied in the form of 5/8" rod was thus machined down to 0.250" on a lathe. Sections 0.250" in length were then cut from the rod with a jeweler's saw. The iron cylinders were rinsed in acetone, immersed in concentrated hydrochloric acid to remove oxide films, rinsed in alcohol, and dried. Specimens were placed immediately in the furnace tube and the tube evacuated to minimize further oxidation. A l l handling was performed with metal forceps.. Where metal additions were necessary to the iron specimen, a small hole was drilled in one end of the cylinder and a portion of the additive placed in the well as shown in Figure h.- Similar care was taken in the preparation and handling of the alloying metals. D. Experimental Procedure 1. Sessile Drop Experiments After preparation, the iron and magnesium oxide specimens were each weighed to the nearest 0.1 milligram. Alloying additions were made to the iron by weighing a portion of the additive metal to give the desired weight percent. A micrometer was used to measure the diameter of.each iron cylinder to the nearest 0.001 inches. This measurement was recorded for purposes of determining the photograph magnification. A magnesium oxide specimen was placed in the susceptor and upon i t an iron cylinder containing the appropriate additive was set as shown in Figure k. The furnace tube was then assembled and the system pumped down - 26 -to a vacuum of one micron. A lamp was used to project an image of the iron specimen on the ground glass plate (# 1 in Figure 3)' Provisions for focusing the image were made through a sliding bellows (#4). The cylinder image diameter on the glass plate was measured with microcalipers to allow calculation of the optical magnification. Purified hydrogen was introduced through a needle valve until the pressure had increased to 500 microns. Hydrogen reduction of the specimen was carried out at 500°C. After cooling the system was again pumped down to one micron and the mercury diffusion pump placed in operation. Within ten minutes the pressure decreased to -5 5 X 10 mm of Hg. The temperature was slowly increased to a point near 1000°C. where adsorbed hydrogen remaining in the furnace tube ionized, creating a bright blue flash. It was hoped that the ionized hydrogen would reduce a l l final traces of oxide on the metal surface. After a slight pause, the remaining hydrogen was removed by the diffusion pump and the vacuum returned to 5 X 10"^ mm, At this point the cold air stream was directed at the furnace tube and the temperature slowly increased to 1550°C. Prior to melting, the incandescent solid cylinder was viewed on the ground glass plate. Several preliminary experiments revealed that the time elapsed from in i t ia l to complete melting varied from three to five seconds. At the instant in i t i a l melting was observed a timer was started, the camera shutter was closed, and the mirror pivoted up to allow photography of the specimen. Self-developing photographs of the molten drop were taken at appropriate time intervals up to two minutes. Four seconds was subtracted from the observed time to give the true time after complete melting. At approximately 120 seconds, the iron vapor pressure reached 0.4 X 10 mm whereupon gas discharge occurred, the power was decreased, and the specimen cooled to room temperature. Dimensions for calculating contact angle and surface tension were measured on the photographs with microcalipers and dividers. 2 . X-ray Diffraction The iron drops were sheared from the'magnesium oxide plate. The discolored interfacial material adhering to the metal was scraped off with a sapphire rod and prepared for X-ray identification using the powder diffraction technique. Since each alloyed specimen required a different i target material, exposures varied from three to six hours. Diffraction lines on each photograph were indexed and attempts made to correlate the patterns to materials in the ASTM X-ray F i l e . 3. Metallographic Observation The iron drop (bonded to the MgO plate) was mounted in a lucite mould and half the specimen ground away as shown in Figure 5 . A cross-section of the interface was polished and examined under the microscope for visible interfacial structures. F i g u r e 5. Specimen Mounted f o r M e t a l l o g r a p h i c O b s e r v a t i o n - 29 -III. RESULTS AND DISCUSSION A. Pure Fe-MgO System 1, Results and Calculations a. Molten iron-solid MgO system 1. Sessile drop experiments The sessile drop dimensions (x, z, x', z') obtained from the photographs were recorded together with time after complete melting as in Appendix I. From this data the contact angle and liquid surface tension values were calculated using the method outlined in Appendix II. The time-dependence of the contact angle was then determined as in Figure 6. Specimens melted in a gas-fired furnace with an air atmospheres possessed a constant contact angle of 2 1 ° after ten minutes. Photographs of a typical specimen (No, 12) in Figure 7. shows the decreasing contact angle with time. A wide variation in calculated liquid surface tension values was noted. Since no trend was obvious, the possibility of a statistical variation was investigated, A statistical analysis of the OLV v a l u e s w a s made (Appendix III). The variation followed a normal distribution with a mean 2 2 value of 1^26 ergs/cm and a standard deviation of 139 ergs/cm . In Dupre's equation the contact angle can decrease only i f V s L and/or d . e c r e a s e s * Assuming the liquid surface tension to be approx-imately constant (1U26 ergs/cm ), the changing contact angle can be attributed entirely to a decrease i n ^ ^ . o^ a f irst approximation, Dupre's equation wil l then relate the contact angle to (Table VII) from which the time-dependence may be determined as in Figure 8. - 31 -Figure 1. Sessile Drop Photographs :• . - 32 -TABLE.VII. • " Pure Fe-MgO System Change of Interface Energy With Time Calculated'from .tfgL = 906 - 1426 cos 9 (9's obtained from curve of 9 Vs. time) Time 9" cos 9 -1426 cos 9 VsL (sec.) (degrees) ergs/cm2 0 117.0 - 0.4695 + 669.5 1575.5 5 109.0 - 0.3420 • +487.7 1393.7 10 101.0 - 0.2079 * + 296.4 1202.4 15 93.0 - 0.0698 + 99-5 1005.5 20 85.0 + 0.0872 - 124.3 781.7 25 80.0 + 0.1736 - 247.5 658.5 30 76.2 + 0.2385 «' 340.1 565.9 35 "73.4 + 0.2857 -.407.4 498.6 40 71.2 + 0.3223 - 459.6 446.4 45 69.5 0.3502 -4499.4 406.6 50 -67.8 + 0.3778 - 538.7 367.3 59' ...66.8 • + 0.3939 ' - 561.7 . 344.3 •60 66.0 + 0.4067 - 579-9 326.I 65 65.5 + 0.4147 - 591.3 314.7 70 65.1 + 0.4210 - 600>3 305.7 75 65.0 + 0.4226 - 602.6 303.4 120 65.0 + 0.4226 - 602.6 303.4 - 33 -i6oo Time (sec) After Complete Melting Figure 8. Interface Energy Vs. Time The i n i t i a l c o n t a c t angle ( a t zero t ime a f t e r m e l t i n g ) was d e t e r ' rained by two methods f o r comparison w i t h Humenik and K i n g e r y ' s va lue ( 1 2 3 ° ) . A v a l u e o f 1 1 7 ° * 1 ° was o b t a i n e d from F i g u r e 6 , , which i s based e n t i r e l y on photographic d a t a . T h i s v a l u e was v e r i f i e d by a second method t o be d e s c r i b e d l a t e r . i i . Thermodynamic c a l c u l a t i o n s The f r e e energy o f i r o n ox ide f o r m a t i o n a t 1550°C(correc ted f o r oxygen p o t e n t i a l ) was determined as i n Appendix I V . The e q u i l i b r i u m oxygen p r e s s u r e over magnesium o x i d e , molybdenum o x i d e , and f e r r o u s oxide a t 1550°C. 8 8 "8 was c a l c u l a t e d t o be 4 .23 X 10* a t m . , 1.58 X 10 a t m , , and 0.42 X 10* atm, o r e s p e c t i v e l y . The vapor p r e s s u r e of i r o n over l i q u i d i r o n a t 1550 C. i s ' 8 63^0 X 1 0 ' atm. (by i n t e r p o l a t i o n o f L o f t n e s s ' d a t a - see Appendix I V ) . b , S o l i d i r o n - - s o l i d • M g O system i« Weight measurements The t o t a l weight l o s s t o the Fe-MgO specimens a f t e r 120 seconds was 3*7 percent (average of n ine spec imens) . i i , M e t a l l o g r a p h i c o b s e r v a t i o n s The i n t e r f a c e of a l l specimens, when viewed through the MgO p l a t e , possessed a b l a c k annulus under the drop of u n i f o r m t h i c k n e s s around the c i rcumference as shown i n Type 1, F i g u r e 9« The annulus appeared t o be a t h i n d e p o s i t o f some new i n t e r f a c i a l phase. I n s i d e the annulus a s h i n y m e t a l l i c s u r f a c e was p l a i n l y v i s i b l e . The i n n e r boundary of the annulus appeared t o correspond w i t h the p o s i t i o n of the mol ten drop p e r i p h e r y a t z e r o t i m e . Attempts were made t o v e r i f y t h i s r e l a t i o n through a comparison of m i c r o s c o p i c measurements o f the annulus d iameter w i t h b a s a l drop d iameters measured i n the m o l t e n s t a t e (Table V I I I ) . Three specimens w i t h t r u l y c i r c u l a r p e r i p h e r i e s were examined (Specimen Nos, 13, 15 and 2 1 ) . Type 3 Figure 9 . I n t e r f a c e Types as seen through Mgo P l a t e - 36 TABLE VIII. Interface Annulus Measurements Specimen 13 Magn. = 9 .98 Time Measured (sec.) Basal Diameter ( 2 x ' for 9>90°) (2x for 9 < 90°) (inches)  True Basal Angle I.D. of Diameter (degrees) Black Annulus = M.B.D. inches (measured with micro-scope, inches) Magn. 13 29 50 2(1 .434) 2(1 .586) 2(1 .691) 0.287 0.318 0.339 99 77 65 0.258 Specimen 15 Magn. = 8.84 3 2(1 .305) 0.295 112 0.289 12 2(1 .493) 0.338 99 26 2(1 .775) 0.402 76 55 2(1 .896) 0.430 66 Specimen 21 Magn. = 8 . 7 5 7 2(1 .265) .0.289 106 0.273 20 2(1 .415) 0.324 87 46 2(1 .532) 0.351 70 78 2(1.564) 0.358 65 Contact angle was plotted versus basal diameter (Figure 10). A linear relation was obtained. The angle where the inside diameter of the annulus f e l l upon the line was determined to be 117°, .116°, and 117° respectively for the three specimens mentioned above. The air-oxidized specimen possessed a completely discoloured interface similar to that of Type 3, Figure 9. When the specimens were mounted, sectioned, and observed microscopically, photographs of the interface were made. In the center of the drop, no visible evidence of either diffusion or new interfacial configurations was observed (Figure 11.). Near the circumference, however, the interfacial annulus appeared in cross-section as a new interfacial configuration (Figure 11.) which" increased in thickness from zero at the inner boundary to approximately ten microns at the drop periphery. No interfacial fractures were observed in either the vacuum-melted or air-oxidized specimens.' Three of the iron drops bonded to the MgO plates were submitted to a semi-quantitative spectrographic analysis. The average impurity concentration is given in Table V. for comparison with the pre-melted material. No appreciable impurity reduction is evident. i i i . X-ray results X-ray photographs of the interfacial material revealed the presence of magnetite (Appendix V). Examination of the Fe-0 phase diagram (Appendix VI) revealed that wustite converts to Fe^ O^ with separation of metallic iron. In support of this' observation, the photomicrograph of the interface (Figure 11,) shows arms of what appears to be pure iron penetrating the interfacial compound. Figure 1 0 . Contact Angle Vs. Basal Diameter - 3 9 -Sketch of Specimen Cross-Section showing Magnetite D i s t r i b u t i o n around Drop Periphery Annular C e n t r a l I n t e r f a c i a l Region I n t e r f a c i a l Region Annular I n t e r f a c i a l Region ( X 2 0 0 0 ) C e n t r a l I n t e r f a c i a l Region ( X 2 0 0 0 ] F i g u r e 11. Photomicrographs of I n t e r f a c e - ko -X-ray patterns of the air-oxidized specimen revealed that the entire drop had oxidized and solidified to Fe^O^. 2. Discussion The observed decreasing contact angle with time supports the observations of Humenik and Kingery^; Evaluation of the time-angle relation-ship deserves some discussion. The validity of applying Bashforth and Adams' procedure is rather questionable since their tables are based upon equil-ibrium drop shapes. Angle calculations were confounded by various sources of error. The spreading drop prevented the acquisition of sharp photographic images necessary for accurate dimension determination. Iron volatilization contributed to a lesser extent to the image obscurity. If the MgO plate was not placed precisely horizontal during furnace assembly, the unbalanced gravitational.force produced asymmetrical spreading subsequent to melting. Occasionally this force caused drop migration across the plate. Thus the combined effects of spreading, volatilization, migration, and unbalanced force distribution often created asymmetrical drops of indeterminate shape. For this reason, photographs were often discarded. Those that appeared symmetrical were retained for measurement but a considerable scatter of experimental points, s t i l l exists (Figure 6 . ) . Since the drop shape depends directly.on the liquid surface tension, asymmetry may account partially for the scatter in^y values (Figure 25.). However other causes are probably more important in this respect. The decreasing angle indicates a chemical reaction at the solid-liquid and/or liquid-vapor interfaced ^3. Thermodynamic calculations show that oxygen produced through the dissociation of MgO will oxidize the iron at 1550°C. (Appendix IV). The free energy change associated with the reaction is -8,620 cal/g-atom of oxygen. Ferrous oxide forming on the surface of the - kl -drop wil l be in the liquid state at this temperature. Kozakevitch^ has determined the liquid surface tension of FeO at 1M-20°C. to be 52'5 ergs/cm . A surface layer.of liquid FeO would therefore be expected to lower the 19 surface tension of the iron . The liquid surface tension determined by statistical analysis.in this work (lk26 ergs/cm^) is much lower than other reported values (Table VI), the highest being 19^0 ergs/cm^ Since the purity of the iron is also different (Table VI), surface-active impurities may be contributing to the decrease as well as iron oxide formation. Oxygen available for iro n' oxidation may be supplied from two sources: the atmosphere surrounding the drop surface, and the solid-liquid interface. The atmosphere is continually replenished by dissociating MgO. k5 Since the associated ;magnesium vapor is insoluble in liquid iron i t accumulates in the furnace tube. Apparently FeO forms on the drop surface. Evidence.of FeO formation at the solid-liquid interface exists only around the periphery. The central interfacial surface reveals no visible dis-coloration or iron oxide deposit as does the periphery (Figure 9«)« This phenomenon cannot be explained satisfactorily on the basis of available experimental results but two hypotheses are presented: 1. Oxygen liberated through the dissociation of MgO at the solid-liquid interface does not form a ferrous oxide layer. The formation may be prevented by some unfavorable interfacial energy change associated with the establishment of an F e ( i ) " Fe0(-]_)--Mg0£s \, structure. As surface oxidation proceeds the FeO layer probably thickens. The thickness is limited by the force of gravity which attracts the slag to the basal peri-phery. A thick layer of FeO would be expected to cause the ^ v °^ "^ e drop to approach the value of pure FeO (525 ergs/cm^).. The observed however, was reduced to approximately lk25 ergs/cm^. At the basal periphery, the accumulating FeO affects the balance of the three interfacial energies - 42 -related by Dupre's equation (^3L? &SV7 ^ ' L V ^ * I n o r ( l e r ' t o compensate for this effect, the contact angle must change. If the advancing periphery is not able to push the accumulated rim of liquid FeO before i t , i t may override the oxide. The inundated FeO would continue to dissolve MgO since they form a solid solution in both the liquid and solid state (Figure 27.). MgO raises the melting point of FeO so that partial solid-ification would be possible although no FeO solid solutions were detected during X-ray analysis. The remaining liquid FeO would convert to Fe^Oi^ with separation of iron upon solidification. 2. . Subsequent to melting, electropositive impurities in the iron (such as Al,. T i , Si etc.) are attracted to the solid-liquid interface and form a stable monolayer with the oxygen-dominated surface of the MgO. The electronegative surface is thus neutralized, which prevents further attraction of electropositive impurities. This configuration may be stable enough to prevent dissociation of the MgO. Meanwhile surface oxidation of the iron has. been proceeding since melting. The electropositive impurities within the bulk continue to be oxidized until their respective deoxidation equilibria are satisfied. The reaction products form a slag with the surface FeO layer but their concentrations are insufficiently large to be detected by X^ray analysis. The unlimited supply of oxygen at the slag surface continues to oxidize the iron and the liquid FeO, attracted by gravity, builds up around the basal periphery. Spreading then proceeds due to the energy unbalance as described previously. The periphery inundates the liquid oxide and advances until a ternary equilibrium is reached at the solid-liquid-vapor interface between the liquid iron, liquid FeO, and solid MgO. From Figure 6. this equilibrium angle appears to be 65°. If the tests had been continued beyond two minutes, however, perhaps this angle would have decreased s t i l l further since the air-oxidized specimen possessed an angle of 21°, It is more likely that this angle corresponds to the equilibrium angle between pure liquid FeO and solid MgO. The experimental results seem to support the second explanation. In view of the apparent presence of interfacial FeO around the periphery, the interfacial energy configuration of this structure seems stable. Formation of an FeO layer across the entire interface appears favorable,; therefore, unless some structure is preventing oxidation. The possibility of an interfacial monolayer forming from minor electropositive impurities in the iron is supported by the calculations in Appendix VII. For high-purity iron containing 0,01 wt. percent silicon, the total number of atoms present in a typical drop is 21,400 X lO 1 ^ atoms. Only 2 , 7 3 X 10^ atoms are required to form a monolayer, Hence in order to ensure the elimination of impurity effects upon monolayer formation, i t would be necessary to purify the iron below approximately 0.01 ppm of silicon. The interface energy (^gjO can be calculated from Dupre's equation only i f equilibrium exists between the three interfacial energies acting on the drop periphery (Figure 1.). The calculation of O'SL ^ n Table VII (assuming constant O'LV = 1426 ergs/em^) therefore is very approximate in the region of changing contact angle. Not only is the system far. from equilibrium, but XLV is probably decreasing because of the influence of the surface FeO layer. After seventy seconds the contact angle remains steady at 65° so that the final value obtained ( 3 0 3 ergs/cm^) is probably reasonably accurate. The unknown geometrical arrangement of phases at the interface periphery prevents, a clear interpretation of this value. It may be the interfacial energy between Fe^N -FeO^N, Fe(-]_\-Fe0(s), Fe^  \-Mg0^  g \ , or any combination of these three. - kk -The work of adhesion is given by the equation = TTe + tfLV(l + cos 9) where TT^  is the equilibrium surface pressure of a monolayer film of the liquid material on the solid surface^. Since /f|"^  is only zero when the liquid has a vanishing vapor pressure, the usual simplification to the equation, cannot be made for the Fe-MgO system. The value of,-^ r'e for this system has not been published although this quantity could be determined from a study of adsorption isotherms, which is beyond the scope of this work. Hence the work of adhesion was not calculated from the experimental results. The system under study is Ini t ia l ly governed by kinetic mechanisms. A study of.the reaction kinetics is difficult , however, because of the complex geometry of the system, which introduces transport problems and concentration gradients. Eventually local equilibria are probably established in various parts of the system but the number of components present (gases, vapors, impurities, etc.) preclude any calculations. Hence either a kinetic or thermo-dynamic study of the system is not feasible unless simplifications in the system geometry are made. The most important factor affecting the system appears to be the oxygen potential. An unlimited supply of oxygen for iron oxidation is made available through dissociation of MgO. Magnesium oxide may sublime directly to the molecular form and then dissociate, or dissociate at the surface and kl vaporize as elemental magnesium and oxygen. Kingery and Wygant claim the molecular vapor pressure of MgO to be., given by the equation l 0 S PMgO = -27.320 .+ 10.25 - 45 -Q Solving the equation at 1 5 5 0 ° C , "the MgO pressureeequals 1 7 7 0 X 1 0 " atm. (Appendix IV). The oxygen pressure due to the dissociation of MgO was 8 calculated to "be 4 . 2 3 X 1 0 atm. On the basis of these calculations the 48 molecular species appears to be predominant. However Altman and Searcy -8 state that molecular MgO vapor has a partial pressure less.than 3 X 1 0 atm. at 2030 ° K . In support of this statement, two other investigators^, 5 0 have shown the gaseous concentration present over solid MgO to be in the order: Mg, £>2> MgO. Hence the molecular species appears to be of minor importance. Speculation as to the relative rates of dissociation of MgO and transport of oxygen to the liquid iron surface is unfounded. The possibility of drop volatilization contributing to the decreasing angle is not excluded. It is unlikely that a volume decrease of three percent in two minutes would greatly affect the overall change (from 1 1 7 ° to 6 5 ° ) . The diffusion of iron into" the MgO was assessed through the application of Fick's Law (Appendix VIII). After two minutes the" maximum -5 42 iron concentration ratio achieved, C , is 5 X 1 0 . Evans found that ' c 0 the minimum impurity concentration of iron in MgO to cause a slight yellow discoloration was approximately 0 . 0 2 0 wt. percent. This corresponds to an approximate relative iron concentration of 2 5 X 1 0 ~ 5 . Hence no discoloration from diffusion as observed by Rigby and Cutler^ would be expected. In support of this conclusion, neither the front view (Figure 9 ' ) n or trieccross-section (Figure 1 1 . ) of the interface showed visible discoloration. The black magnetite annulus In cross-section (Figure 11.) appears to be a somewhat lamellar conglomeration of iron and Fe^O^. The layer displaces the MgO rather than deposits on the top of the solid. This might be expected since the liquid FeO dissolves MgO as the periphery advances. The increasing thickness is probably due to the augmented supply of solvent (FeO) available as oxidation proceeds. - k6 -The structure of this interfacial layer can be explained through a consideration of phase transformations during cooling. As the temperature is decreased, the iron drop solidifies before the FeO so that a liquid phase exists between two solid phases (Fe and MgO). Since iron separates from the FeO upon solidification, solid nucleation sites would influence this separation. The iron lamellae in Figure 11. appear' to be connected to the iron drop so that the solid iron probably provided the nucleation sites. The nuclei then grew into the Fe^ Ol). upon further cooling. Alternatively, the MgO may act as a nucleation site for Fe^O^. Since Fe^ Ol). and MgO are both face-centred-cubic, the lattice parameters can be compared for purposes of matching the oxygen atom distribution on the 51 (lOO) plane of the unit cells. Wells "observed that an approximate linear relationship occurs between the lattice parameter and percent iron in iron oxides (Figure 12 . ) for the transition ^FegO^ Fe^O^ FeO. If Rooksby's lattice parameter for Fe^O^ (published in ASTM X-ray File) is substituted for Well's value on the line as shown, the relationship is even more closely linear. Since the Fe^O^ lattice parameter is almost exactly twice the MgO lattice parameter, i t is conceivable that Fe^O^ nucleation would be promoted on the MgO surface. In any case, the resulting interlocking structure (Figure 11.) would be expected to produce a strong bond, aside from] possible thermal stresses. The interfacial slip lines shown in Figure 9*.(pure iron specimen) kl reveal plastic deformation of. the MgO. Hasselman7 who observed similar lines in Ni-MgO specimens, attributed the cause to thermal stresses resulting from non-uniform cooling rates across the MgO plate. Since fractures were not visible in any of the specimens, the ultimate strength of the MgO was not exceeded. F i g u r e 1 2 . L a t t i c e Parameter Vs. Atomic Percent I r o n - 48 -B. Iron Alloy — MgO Systems 1. Results and Calculations a. Molten drop — solid MgO system Photographs of the sessile drops were obtained at various time intervals and the dimensions recorded as in Appendix I. In a l l cases the alloyed drop images were more distinct than the pure iron images (compare Photograph 4 with 1, Figure •7--)« Furthermore, the basal region of the drops possessed an optical shadow not observed on pure iron drops. The height of this shadow decreased with time and occasionally the shadow disappeared within the two minute test. Experiments were conducted with alloy additions of zirconium (Zr), titanium (Ti), niobium (Nb), tantalum (Ta), vanadium (V), chromium (Cr), and drops of high carbon steel. Subsequent to melting, gas discharge ensued whenever additions of silicon or manganese were made. Humenik and Kingery^ also encountered the high vapor pressure problem of silicon in vacuum at 1550°C. Hence tests with this alloy were discontinued. Vanadium-alloyed specimens displayed an erratic behaviour after 75 seconds. The uniformly spreading drops suddenly contracted to angles near 65°• Photographs of the high carbon steel drops were often discarded because gas evolution followed by drop pulsation, contraction, and migration was common. Zirconium-alloyed drops seemed to explode violently after approx-imately twenty seconds. Because' the scattered material dissolved the susceptor and destroyed radiation shielding, only four tests were conducted. Consequently the results were insufficient for either time or concentration effects to be determined. - h9 -From the drop dimensions the contact angles and liquid surface tensions were calculated (as in Appendix II). The time-dependence of the angles at various alloy concentrations were then plotted as in Figures 13 to 18. Since no trend was observed in the liquid surface tension variation with alloy concentration;/ the values were included in the statistical analysis of Appendix III which led to the derivation of 1U26 ergs/cm for pure iron. In sessile drop experiments where no chemical reaction occurs, the contact angle usually remains constant with time but varies with alloy concentration in the drop. The normal procedure is then to plot interface energy (determined from contact angle by Dupre's equation) versus log concentration and determine interfacial alloy adsorption ( if any) by applying Gibb.'s Theory (refer to Introduction). Since chemical reaction was observed in a l l cases where alloy additions were made in this work, the interpretation of interface energy remains obscure and inaccurate for reasons discussed in the previous section of this report.-» 50 -- 51 -- 52 -Time (sec) After Complete Melting Figure 15. Contact Angle Vs. Time (Cr-Alloyed Iron) - 53 -120 0 20 kO 60 00 WO 120 Time (sec) After Complete Melting Figure l 6 . Contact Angle Vs. Time (V-Alloyed Iron) - <jk -- 56 tu Solid drop - - solid MgO system i . Interfacial observations (a) Macroscopic The interface of a l l alloyed specimens, when viewed through the MgO plate, appeared as one of the three types shown in Figure 9- Those similar to the pure iron interface (i .e. Type l) were the Nb-, Ta-, and Cr-alloyed specimens. Completely discolored interfaces (i^e. Type 3) were displayed by the T i - , V-, and Zr-alloyed samples. A completely shiny interface with no peripheral discoloration was observed under the steel drops«. Vertical fracture through the MgO plate around the drop periphery was observed in the high T i - and high Nb-alloyed specimens. In a l l other specimens, slip lines were observed at the interface (as shown in Figure 9«) (b) Microscopic In order to complete the microscopic study i t was necessary to examine the center and periphery of the interface at. both high and low alloy concentrations. In some cases (Ti - , Zr- , Kb-, and Ta-alloyed specimens) the interfacial deposit increased in thickness very rapidly near the periphery as shown in Specimen 32, Figure 19- In others the interfacial layer was either approximately constant in thickness across the entire interface (V-alloyed specimen 35, Figure 19 )> or non-existent (steel specimen 56, Figure 1 9 « ) ' No interfacial layer was observed in the center of the Nb-, Ta-, and Cr-alloyed specimens. The interfacial deposit in the low Ti-alloyed specimen (specimen 27, Figure 20) differs from that of the high Ti-alloyed specimen (specimen 22, Figure 20.). In specimen 27 the interfacial phase contains a precipitate (presumably iron) which eventually grows into two distinct phases (similar Specimen 35 Centre and Periphery (X2000) Specimen 56 Centre and Periphery (X2000) (in a l l photos, light region is iron, dark region is MgO) Figure 19. Interfacial Deposits Specimen 27 (X2000) Specimen 22 (X2000) Specimen 32 (X300) Specimen 37 (X2000) Figure 20. Interface Structures to specimen 22) near the periphery. The phase in contact with the iron is rust-brown whereas the phase adjacent to the MgO has an olive-green appearance. Only one type of interfacial "deposit was observed (specimen 37, Figure 20.) in the Zr-alloyed specimens at a l l concentrations. The overall layer thickness increased with concentration. Both the Nb- and Ta-alloyed specimens displayed rather peculiar interfacial structures. At high Nb contents the periphery appeared as in specimen 36, Figure 20. The peripheral bead is deposited on top of the MgO rather than at the expense of the MgO as in the T l case (specimen 27, Figure 20.) The ascicular structure of the bead, greatly magnified, reveals a lamellar region as in specimen 36a, Figure 20. The Ta-alloyed specimens did not exhibit an ascicular structure in the peripheral bead (specimen 32, Figure 19«) but similar lamella/twere visible at higher magnifications (specimen; 32:, Figure 20^). In both the Nb- and Ta-alloyed specimens, the height of the bead (H, Figure 20.) increased with concentration. The peripheral deposits in the Cr-alloyed samples were similar to the pure iron specimens except that the precipitate was horizontal as in the V-alloyed specimens (specimen 35, Figure 19«) rather than vertical (refer to Figure 11.). i i . X-ray Eesults Attempts were made to identify the interfacial deposits at both low and high alloy concentrations by powder pattern analysis. In some cases the quantity of material present was insufficient for identification (Gr-and V-alloyed samples). In others, distinct complex patterns were obtained (Nb-, Ta-, and low Ti-alloyed specimens) but these could not be identified with compounds in the ASTM X-ray Fi le . The presence of T i ?0o in both - 6 0 -the peripheral and. interfacial. region of the high Ti-alloyed samples was established. This compound was present only in the peripheral deposit of the low Ti-alloyed specimens. The interfacial deposit yielded a complex unidentifiable diffraction pattern.•• Since identification of reaction products was restricted, a thermodynamic assessment of the reactions occurring could not be made. Nor could a kinetic analysis of the systems be performed because of con-centration gradients, geometrical factors, chemical interactions, etc. 2. Discussion The decreasing contact angle observed with a l l alloy additions indicates a chemical reaction in the system. The reaction, in each case, presumably was the oxidation of the alloy component. Since the central interface remained shiny in the Nb-, Ta-, and Cr-alloyed samples, the. oxidation mechanism is probably similar to that postulated on page 42. The oxidation species that collect over the surface of the drop in this case are greatly enriched in the alloying component. They may temporarily form a sheath which depresses iron oxide formation and/or iron volatilization. This would account for the sharp images obtained. As the surface oxide accumulates, gravitational forces may force i t toward the basal periphery. The shadowed region observed in the photor graphs is probably due to the dissimilar emissivities of the surface oxide and iron (or iron oxide). As surface oxidation proceeds and the alloy concentration in the drop decreases, the rate of alloy oxidation may also decrease. Insufficient material is formed to replenish the shadowed region which is settling toward the base. Hence the height of the region decreases with time. The presumption that the oxide layer is being continually drawn toward the basal periphery is supported by the large rim deposits observed, for example, in specimen 32, in Figure 19. - 61 -Where Zr, T i , and V additions were made, the oxidation mechanism must be slightly different since discoloration of the entire interface was observed after solidification. Apparently the electropositive components that stabilized the solid-liquid interface from attack by Nb, Ta, and Cr are not able to prevent attack by Zr, T i , and V. Hence oxide deposits of these metals build up across the entire interface as well as on the drop surface. This new development does not prevent peripheral accumulation of reaction products, however, because gravitational forces s t i l l operate on the surface layer. Because of the difficulty in making a thermodynamic assessment of these systems i t is not feasible to speculate as to which electropositive components are preventing interfacial attack in some cases and not in others. In view of the in i t ia l impurity concentration of the pure iron and free energy considerations, silicon (which is present as 0.01 wt$J appears to be a major constituent of the monolayer. The erratic behaviour of the steel drops was probably due to the formation and evolution of carbon monoxide bubbles as observed.by Allen and Kingery^. Contraction of the V-alloyed drops to angles near 65° may be a result of the interfacial configuration. The entire interface is discolored so that presumably a vanadium oxide phase forms across the interface and the drop spreads toward some iron - vanadium oxide - MgO equilibrium angle. When the vanadium content of the drop becomes depleted, however, iron oxide, which is continually forming, dilutes the vanadium oxide layer. The: peri-phery reacts to this influence by returning to the iron-iron oxide-MgO equilibrium angle. After solidification, the interfacial phase resembles the magnetite structure observed in pure iron except that the precipitate lies parallel (specimen 35, Figure 19«) rather than perpendicular to the - 62 -interface. The precipitate appears to be metallic iron but positive identification was not possible. The interfacial layer is probably some complex vanadium-oxide-enriched magnetite. The interfacial distribution supports the hypothesis that pure FeO would extend over the entire inter-face in the pure iron-MgO specimens unless some monolayer was stabilizing the region as described. Figures 13.' to 18. which show the contact'angle - time - con-centration relation for various alloy additions illustrate three trends: 1. Contact angles which decrease ini t ia l ly at the same rate then stabilize at a level dependent on concentration (Ta, Nb). 2. Contact angles which decrease at a rate which increases with alloy concentration and do not stabilize at a particular level (Cr, V). 3. Contact angles which decrease at constant rate to a level independent of.concentration after a period of time which depends on alloy concentration (Ti). The behaviour of specimens in group 1. does not manifest the wettability properties of the system. It results from a peculiar structural i configuration due to the accumulation of reaction products around the drop periphery as illustrated in specimen 32, Figure 19. The deposit on top of the MgO acts as a barrier that prevents the periphery from advancing. Both the rate of barrier formation and final height are dependent on the in i t ia l alloy concentration within the drop. At high concentrations, a high barrier forms rapidly and restricts the drop expansion. At low concentration a low barrier forms slowly and allows partial '.expansion of the drop before restriction is imposed. -.63-Group 2. displays the wettability of the MgO by the alloyed iron because oxide deposits around the periphery do not form a barrier. Oxide accumulation may slightly retard the advancing periphery as evidenced in the slope change of the curves after one minute. This effect is more pronounced in the Cr-alloyed specimens because more oxide is available for accumulation around the periphery. In the V-alloyed specimens, the oxide formed is distributed uniformly across the interface whereas in the Cr-alloyed specimens, the oxide accumulation is confined to the periphery-. The accelerated rate of angle decrease with allpy concentration suggests that the equilibrium angle for the.iron - (V or Cr) oxide - MgO system is probably much smaller than the iron - iron oxide - MgO system. The reason for the behaviour of group 3' is less certain. Apparently the equilibrium angle for the iron - Ti oxide - MgO system at low concentrations is 105°. The precipitated microstructure in specimen 27, Figure 20. (which exists across the entire interface) resembles the pure iron counterpart. Near the periphery the accumulating oxide appears to dissolve the MgO rather than deposit on top of i t as in the' Ta- and Nb-alloyed s'amples. At high concentrations, however, the oxide probably deposits at the periphery too rapidly for complete dissolution of the MgO so that a barrier is formed which restricts the periphery. At intermediate concentrations (e.g. curve for 0.46$, Figure 17») a barrier forms ini t ia l ly because of the high rate of Ti oxide formation. This barrier begins to dissolve MgO and effectively decreases in height. Since the depleted drop cannot replenish the oxide supply and strengthen the barrier, the periphery begins to advance and eventually the barrier disappears at the expense of the MgO. The periphery o then advances to its equilibrium value of 105 . - 6h -The curve for the steel sample (Figure 18.) resembles the curve for the pure Fe-MgO system except that the final angle is much higher ( 8 5 ° ) . The carbon is probably retarding iron oxide formation and hence preventing the periphery from advancing to the iron - iron oxide - MgO equilibrium angle of 65° . Since thermodynamic and kinetic calculations were confined by lack of information and complexity, and the reaction products were unident-ifiable in almost a l l cases, the precise oxidation mechanism for a particular alloy addition was not determined. Furthermore, because phase diagrams of the binary systems of iron oxide and oxides of the alloying elements have not been published, not even prediction of reaction products was possible. IV. CONCLUSIONS Where chemical reaction occurs, the sessile drop technique does not necessarily manifest the wettability properties of the system. The drop behaviour may be governed by (a) interfacial energy relationships or (b) the nature of the reaction product deposit. Furthermore, the cal-culation of wettability properties and interfacial energies is inaccurate because of the kinetic character of the parameters involved. Kinetic analysis of the reaction mechanism is complicated by geometry factors, concentration gradients, transport problems, etc. The quantity of reaction products formed in sessile-drop experiments is insufficient for positive identification and subsequent prediction of possible mechanisms. A macro-scopic study of the interface would probably yield more definite information about the chemical reactions involved. Chemical reactions occurringthroughout this work prevented any quantitative study of the preferential interfacial adsorption of electro-positive' alloy components originally intended. The iron reacted with oxygen released through the dissociation of MgO and formed a.surface oxide layer which reduced the liquid surface tension and decreased the interfacial energy. The oxide accumulated around the basal periphery of the drop and solidified as wustite, which decomposed to magnetite and iron. Interfacial attraction and monolayer formation of electropositive impurities in the iron may have occurred since no evidence of reaction was observed within the annular oxide deposit. Results of alloying experiments revealed complete interfacial attack with alloy additions of T i , Zr, and V, and annular deposits similar to pure iron where additions of Ta, Nb, and Cr were made. A consideration of free energy data together with in i t i a l impurity concentration of the iron suggests that silicon is probably one of the major constituents stabilizing the interface. - 66 -Chemical reaction was evident, when the iron was alloyed with Ta, Nb, Cr, V, T i , and Zr. Presumably the reactions were oxidation of the alloying component. The wettability of the MgO by the iron improved with increasing concentration of Cr and V. In the case of Nb, Ta, T i , and Zr additions, however, no wettability conclusions could be drawn because the drop behaviour was complicated by peripheral oxide deposits that restricted drop expansion. 'r V. RECOMMENDATIONS FOR FURTHER WORK Further work on the Fe-MgO system could probably be carried on to greater advantage with iron of higher purity. A macroscopic system would provide sufficient quantities of reaction products for identification and kinetic mechanisms could be investigated without complication from geometrical factors, transport.problems, and concentration gradients. In the case of alloy additions to the iron, preliminary studies that would establish phase diagrams for the iron oxi'de--alloy oxide systems would be useful. The effect of various atmospheres (particularly oxygen) on the reaction kinetics could then be studied. Once this information is gained, a sessile drop study could then be made to determine the. reaction behaviour of single crystals of MgO. The nature of concentration gradients and diffusion around the interfacial area.could be studied using a micro-probe analyzer. Perhaps then the existence of a monolayer could be established. VI. APPENDICES APPENDIX I. Sessile Drop Data - 68 -TABLE IX. Sessile Drop Data Pure Fe-MgO Specimen Time After Mag.' of ' , , k ' Q No. Complete Melting.' Photo. x z x z °LV . AV ( s e c m) - inches - ^ erg/cm degrees 10 10 7-78 1.452 1.181 1.429 I..336 .1285 103 25 I.626 1.228 74 50 1.810 I.185 66 110 1.885 1.208 65 11 17 1.364 1.330 88 35 I.526 1.204 77 80 1.650 1.050 65 12 2 7.90 1.332 1.120 1.281 1.444 1308 113 9 7.90 1.387 1.215 1.360 1.417 1520 102 4o 1 . 6 0 9 1.155 71 105 1.763 1.118 65 13 13 9.98 1.4.34 1.267 1428 99 29 1.586 1.255 77 50 I.691 1.075 65 14 24 1.525 1.300 81 40 1.607 1.170 72 80 1.642 1.025 64 15 2 8.84 I.365 1.197 1.305 1-535 1545 112 12 8.84 I.513 1.256 1.493 I.370 1235 99 26 1.775 1.369 76 55 I.896 1.241 66 16 5 7.70 1.420 I.190 1.372 1.495 1535 110 24 1.644 1.307 77 50 1.788 1.175 67 120 1.830 1.167 65 17 16 1.428 1.400 89' 37 1.635 1.204 73 18 33 1.690 1.295 75 44 1.754 1.242 70 19 9 9.05 1.334 1.165 -1.311 1.383 1355 103 20 l 7.88 1.330 1.134 1.267 1.464 1465 11^ 21 7 8.75 1.300 1.133 1.265 1.362 1352 106 20 1.415 1.350 87 46 1.532 1.076 70 78 1.564 .991 65 10 Determinations Average 1403 ± 10$ Fe + Ti - MgO - 69 -Specimen f> Ti Time Mag. of erg/cm 9AV degrees No. in Fe After Complete Melting (sec.) Photo. X z inches X' z' 22 3.85 15 8 . 0 0 1.257 1.084 1.092 1.590 1385 133 4o 8 . 0 0 1.264 1.086 I .069 1.588 1350 133 2k 3.85 1 8.71 1.282 1.130 1.079 1.718' 1470 133 6 8 . 7 1 1.304 1.147 1.107 1.718 1485 133 90. 8.71 1.310 1.138 i . 0 8 7 1.710 1345 . 134 25 1.00 2 8.52 1.215 1.072 1.070 1.578 1415 132 7 8.52 " 1.215 1.072 1.070 1.578 .1415 132 21 8.52 1.215 1.072 1.070 1.578 1415 132 38 •8.52 1.246 1.108 1.137 1.524 1580 120 44 8.52 1.297 1.131 1.233 1.436 1455 112 57 8.52 1.327 1.144 1.296 1.378 1355. 105 26 0.46 l • 7.52 1.276 1.078 1.108 1.588 1375 132 •16 7.52 1.276 1.078 1.108 1.588 1375 132 30 7.52 1.329 1.134- 1.268 1.464 1610 . 114 36 7.52 1.382 I .160 I .352 1.380 1545 105 90 7 .52 1.382 1.160 1.352 I .380 1545 105 27 0.22 18 8.78 1.233 I .096 1.139 1.510 i 4 4 o 119 22 8.78 1.245 1.092 1.195 1.413 1325 112 28 0.22 2 7-55 1.270 1.090 1.084 I .626 1525 134 7 7-55 1.270 1.090 1.084 1.626 1525 134 29 0.22 30 8.97 1.391 1.221 1.368 1.462 1585 104 70 8.97 1.393 1.220 1.367 1.460 1555 105 30 0.11 90 7 .73 1.348 ' 1.138 1.314 1.387 1425 105 31 0.11 2 7 .73 1.246 -1.060 1.173. 1.377 1310 115 5 7.73 i . 2 9 8 1.132 1.234 1.434 1760 111 20 7-73 1 .373 1.130 1.346 1.300 1250 104 50 7-73 1.372 1.130 1.346 1.301 1255 104 Fe + Ta - MgO $ Ta 32 5 .00 20 8.97 1;472 1.267 1.424 1.589 1475^ 109.5 90 8*97 1.472 1.267 1.424 1.589 1475 109.5 39 0 . 0 8 l 7 .98 1.355 1.148 1.312 1.458 l 4 i o 112 10 .7.98 1.350 1.142 "1.313 1.450 1395 111 20 7 .98 1.415 1.154 1.387 1.340 1185 106.5 45 7 .98 1.317 1.092 1.297 1.292 114.5 104 60 7 . 9 8 1.315 1.097 1.295 1.290 1180 104 40 0.01 5 8.36 1.468 1.253 1.425 1.570 1605 UO . 5 15 8.36 1.479 1.252 1.434 1.486 1525 IO6 . 5 4 l 0 . 0 1 35 1.489 1.291 82 50 1.486 1.290 82 85 1.368 1.185 82 60 0 .03 20 8 . 0 1 • ' 1.427 1.192 1.410 1.369 1410 102 33 8 . 0 1 1.452 1.226 1.449 1.322 1570 95-5 75 8 . 0 1 1.457 1.229 1.451 1.313 1565 95 61 0.21 40 8.27 1.419 1.191 1.379 1,482 1375 109 80 8.27 1.422 1.190 1.369 1.483 1320 110 Fe + Nb - MgO - 7 0 -Specimen $Nb. Time Mag. of ^LV No. i n Fe A f t e r Photo. X z x' . z' 9AV Complete ergs/cm degrees M e l t i n g inches (sec.) 3 6 8 . 8 0 5 8 . 5 5 1 . 5 2 7 1 . 2 7 4 1 . 4 9 0 1 . 5 3 3 1 3 9 0 1 0 7 1 5 8 . 5 5 1 . 5 2 7 1 . 2 7 4 1 . 4 9 0 1 . 5 3 3 1 3 9 0 1 0 7 5 0 . 8 . 5 5 1 . 5 2 7 1 . 2 7 4 1 . 4 9 0 1 - 5 3 3 1 3 9 0 1 0 7 9 0 8 . 5 5 1 . 5 2 7 1 . 2 7 4 1 . 4 9 0 1 . 5 3 3 . 1 . 3 9 0 1 0 7 42 0 . 0 1 7 8 . 9 3 1 . 4 3 4 1 . 2 4 4 1 . 3 9 7 1 . 4 2 4 , 1 5 3 5 1 0 6 . 7 . 2 5 1 . 4 7 4 1 . 3 5 9 8 5 8 5 1 . 4 7 6 1 . 3 5 9 8 5 4 3 0 . 0 0 5 5 ' 8 . 8 7 1 . 3 8 4 1 . 2 2 1 : 1 . 3 3 3 1 . 4 8 6 1 6 8 0 1 0 7 . 5 1 2 8 . 8 7 I . 4 3 6 1 . 1 9 1 1 . 4 1 2 1 . 4 o 8 1 1 1 0 1 0 3 . 5 3 3 1 . 5 4 0 1 . 2 5 2 7 8 6 0 1 . 5 4 6 1 . 2 2 8 7 7 1 0 0 1 . 5 4 5 1 . 2 2 8 7 7 • 5 8 . 0.0k 1 0 8 . 9 0 I . 3 8 0 1 . 2 0 5 1 . 3 5 0 1 . 4 4 2 1 5 0 5 1 0 5 1 5 8 . 9 0 1 . 4 2 0 1 . 1 9 8 1 . 4 0 5 . 1 . 3 7 2 1 2 1 5 1 0 1 4 5 8 . 9 0 1 . 4 2 1 1 . 1 9 9 1 . 4 0 5 1 . 3 7 2 1 2 1 5 1 0 1 5 9 0 . 3 0 1 0 8 . 7 1 1 . 4 2 8 1 . 2 3 0 1 . 3 9 0 1 . 4 9 6 1 4 9 5 ' 1 0 7 40 8 . 7 1 1 . 4 2 8 1 . 2 3 0 1 . 3 9 0 I . 4 9 6 1 ^ 9 5 1 0 7 7 0 . 8 . 7 1 1 . 4 2 8 1 . 2 3 0 1 . 3 9 0 1 . 4 9 6 1 4 9 5 1 0 7 Fe + V - Mg 50 3 5 ' 6 . 0 0 5 . 1 . 5 1 0 1 . 3 2 0 8 2 2 0 1 . 7 5 0 P . 9 9 6 5 9 5 0 1 . 7 9 9 0 . 7 4 0 4 5 . 4 4 1 . 1 0 6 1 . 6 1 0 1 . 2 6 8 7 6 1 1 1 . 6 8 0 1 . 1 9 4 7 1 2 5 1 . 8 0 2 1 . 0 4 1 6 0 4 5 6 . 1 3 5 1 . 5 1 7 1 . 2 9 0 8 1 1 3 1 . 7 4 4 1 . 0 9 6 64 46 0 * 5 5 1 8 . 9 5 1 . 2 9 5 1 . 1 5 7 1 . 2 2 8 l . 4 l l 1 6 2 0 1 0 9 6 1 . 3 8 7 1 . 2 5 4 84 2 1 1 . 5 8 8 1 . 0 0 5 6 5 4 1 I . 6 5 8 0 . 8 7 2 5 6 4 7 0 . 0 4 1 0 1 . 4 9 1 1 . 3 1 5 8 3 2 2 1 . 5 3 8 1 . 1 5 2 7 4 40 1 . 5 9 1 1 . 0 1 2 65 5 0 1 . 6 6 0 0 . 9 9 5 6 2 48 8 . 3 2 6 1 . 5 4 8 1 . 2 1 0 7 6 1 5 1 . 6 4 2 0 . 9 5 6 6 0 4 3 • 1 . 7 3 9 0 . 7 4 0 46 4 9 0 . 6 5 0 8 . 8 9 1 . 2 3 8 1 . 1 1 4 1 . 1 7 1 1 . 4 2 4 1 6 5 5 1 1 1 . 5 5 1 . 3 9 8 1 . 3 4 5 • 8 8 2 0 1 . 6 2 8 1 . 0 3 4 6 5 5 0 0 . 4 2 4 1 . 4 1 6 1 . 3 1 2 8 6 : 2 3 1 . 5 8 3 I.O65 6 8 4 3 1 . 6 8 5 0 . 9 3 8 5 8 , 7 5 1 . 7 5 3 O . 7 6 6 4 7 Fe + Gr - MgO - 71 -Specimen No. f Cr in Fe Time After Complete Melting' (sec.) Mag. of Photo. X z x' inches z' erg/cm 9AV degrees 33 4.00 5 " 8.90 1.507 1.251 1.501 1.338 1225 95 15 1.802 1.087 62 30 I.916 0.852 48 90 I.968 0.735 4i 34 0.19 2 8.96 1.384 1.225 1.314 I.588 1685 112 5 • 8.96 1.439 1.244 1.401 1.502 1490 106 15 I.819 1.442 77 45 I.925 I.038. 57 8o 1.937 O.988 54 51 9-72 0 8.57 1.333 1.149 1*248 I.529 1355 117 6 1.554 1.308 80 25 1.702 D.756 48 4o 1.668 0.640 42 65 1.666 0.620 41 52 2.00 2 "8,93 1.269 1.141 1.180 1.433 1725 112 11 1-578 1.211 75 4o I.628 0.754 50 115 1.940 0.798 45 57 0.03 4 8,88 1,302 1,128 1.260 1.398 1250 :.io8 22 1.821 1.322 72 50 1.882 I.067 59 100 1.897 0.995 55 Steel - MgO 12 8.91 0.967 0.900 O.936 I.123 1540 107 20 8,91 0.963 0,900 0.928 0.100 1620 106 4o " 8.91 0.941 0.889 0.927 0.988 1825 99.5 1 8.37 0.965 0.872 0.892 1.208 1200 II7.5 7 8.37 0.988 0,906 0.943 1.204 1490 . .112 20 8.37 1.015 0,930 0.993 1.158. 1530. 105.5 115 1.004 O.965 . ,88 240 0.984 0.925 86.. 5 Average ^ L y 1551 Cast Fe (.6 det'ns) io Zr Fe + Zr. - MgO 37 - 3-73' 5 • 8.91 1.320 1.132 I.063 I.780. 1190 147.5 12 8.9I 1.348 I.I70 1.056 1.850 1360 147.5 38 .41 50 8.90 1.3l3 1.148 I.O96 I.698 1370 133.5 Zone Refined Iron - Mg . 6 4 10 9.03 I.518 1.308 I.465 1.629 1565 109 14 9.03 1.541 1.319 1.508 1.582 I5IO 106 40 1.573 1.455 84 60 1.732 1.297 74 90 I.831 1.232 68 APPENDIX II. Bashforth and Adams Method Bashforth and Adams Method - 73 -For purposes of error determination, specimen number 36 (Fe + 8.80$ Nb) was examined. Measurements on the photograph yielded: x = 1.527 ± .001 inches z = 1.274 ±-.003 . x' = 1.490 * .001 " z'= 1.533 ± .001 " x = I..1I99 ± .003 z From Figure 21. 1.1+40 ± .030 where fe = gdb2 From Figure 222. g = gravity., constant x = 0.852 * .002 z = 0.711 ± .004 d = density of liquid Fe b b b = radius of curvature . of drop apex It becomes apparent that the accuracy of z and z, is much less -than b that of x and x . Hence most accurate value of b b = I.527 '* .001 = 1.792 ± .006 inches 0.852 * .002 . b2= 3.211 ± .022 (inches) ^ = ; gdb 2 .-4o Most accurate value of density (d) for iron is 7«2 g/cc . Gravity. • constant^ g = 980.7 cm/sec2. One inch - 2.54 centimeters. Magnification = 8.55 * .03. Hence Y T T r = 980.7(7.2) (3-211 * .022)(2.54)2 ° L Y (8:55 *.03)r(i.44 .03) = 1390 ± 49 = 1390 * 3.-52^  To find contact angle ,0: x' = O.83I ± .004 z' = O.855 * .004 b Bashforth and Adams Method Continued - lk -From Figure 23 • Q = 107 ± 3 ° From Figure 2k. s o , o 9 = 106.5 * 1 Hence the more accurate value is obtained from the z' measurement. 7 The Baes error factor at x = 1.199 is 6.8. Since the uncertainty z in x = 0.25$, the uncertainty in y^y using Baes' method is 0.25(6.8) = 1.7$' z This method thus gives an approximate estimation of the error from direct analysis of x . z Since ^gyls known within ± 20$, and ^ y within * 25$, the calculated from Young and Dupre's equation (applied.to non-equilibrium conditions) is probably within * 50$ At angles close to 90°, the point of curvature becomes, difficult to determine and hence the accuracy in determining z rapidly decreases. Total accuracy therefore decreases. APPENDIX I I I . S t a t i s t i c a l A n a l y s i s of L i q u i d I r o n Surface Tension Values TABLE X. Liquid Iron Surface Tension Values Specimen \/ y , „ / y Np Specimen v y ^ \2 N o < «LV 0LV-1U26 CflLV-lU26) . N o . OLV OLV-1426 («LV-l426) 10 1285 -141 19,881 12 1308 -118 13,924 1520 + 94 8,836 13 1428 + 2 4 15 1545 +119 14,161 1235 - 91 8,281 16 1535 +109 11,881 19 1355 - 71 5 , 0 4 l 20 1465 + 39 1,521 21 1352 - 74 5,476 22 1385 - 4 l 1,681 1350 - 76 5,776 24 1470 + 44 1,936 1485 + 59 3,481 1345 - 81 6,561 25 1415 - l l 121 1415 - l l 121 1415 - l l 121 1580 +154 23,716 1455 + 29 841 1355 - 71 5,041 26 1375 - 51 2,601 1375 - 51 2,601 1610 +184 33,856 1545 +119 14,161 1545 +119 14,161 27 l44o + 14 196 1325 -101 10,201 9,801 28 1525 + 99 1525 + 99 9,801 29 1585 +159 25,281 1555 +129 16,641 30 1425 - l l 31 1310 -116 13,456 1760 +334. 111,556 1250 -176 30,976 1255 -171 29,241 32 1475 + 49 2,401 1475 + 49 2,401 39 1410 - 16 256 1395 - 31 96l 1185 - 2 4 l 58,081 1145 -281 78,961 1180 -246 60,516 40 1605 +179 32,o4l • 1525 + 99 9,801 60 1410 - 16 256 1570 +144 20,736 1565 +139 19,321 61 1375 - 51 2,601 1320 -106 11,236 1,296 36 1390 - 36 1390 -.36 • 1,296 1390 - 36 1,296 1390 - 36 1,296 42 ' 1^ 35 +104 10,816 43 1680 +254 64,516 1110 -316 99,856 58 1505 + 79 6 , 2 4 l 1215 -211 44,521 1215 -211 44,521 59 1495 + 69 . 4,761 1495 + 69 4,761 1495 + 69 4,761 46 1620 +194 37,636 49 1655 +229 52,441 33 1225 -201 4o,4oi 34 1685 +259 67,081 1490 + 64 4,096 5,04l 51 1355 - 71 52 1725 +299 89,401 57 . 1250 -176 30,976 37 1190 -236. 55,696 1360 - 66 4,356 38 1370 - 56 3,136 - 81 -Results of Statistical Analysis of  Liquid Iron Surface Tension Values Mean tfLV = £ X LV N 106988 = 1426 erg/cm2 .75 Standard Deviation, (5 ~ / %S ^LV-1426)' l , Ma, 201 75 N 138.7 erg/cm2 Portion of Surface Tension Values Within: 1426 * <T 1426 * 2()" 1426 ± 3 ^ Experimental Curve of Author 68.0$ 94.7$ 100.0$ Theoretical Normal Distribution Curve 68.26$ 95.46$ 99.7 $ tr1 •!p. H ) P? f3 CD 1-3 CD P CD H-o p CD 4 1019.9 U J _ 1158.6 1297.3 1426,0 1564.7 ro 1703.4 1842.1 1. ro 1 ro ro + ro + IVJl + O J ro Number of Measurements in Region 0 . 5 0 " Designated ro ON 00 O H ro H ..ON CD ro o *l ure ro vn • K H-cn 0 d-CD O d" CD 4 P O CD H j H' CO O d" P S» cn d-H-O cn H j d -H-f O H' H U cn CO d -o CD H -O H3 P CD P O cn H> X CD H H-s CD P c+ P 4 O P - S8 " APPENDIX IV. Thermodynamic Calculations - 84 -Thermodynamic C a l c u l a t i o n s Oxygen pressure over magnesium oxide: Magnesium oxide u s u a l l y d i s s o c i a t e s i n t o i t ' s gaseous elements i . e . M g O C s ) - * M g ( g 0 + i 0 2 ( g ) For t h i s r e a c t i o n , A F ° = 176,500 - U7.5 T ^ Assuming s t o i c h i o m e t r i c d i s s o c i a t i o n , pMg = 2p0 2 A F ° = -RT l o g (pMg)(pO ?) 2 = -RT l o g 2(p0 2) at T = 1823°K 2' 3/2 p0 2 = 4.23 X 10" 8 atm. 2 . Oxygen pressure over molybdenum oxide: M o 0 2 ( s ) — * M o ( s ) + ° 2 ( g ) A F° = 137 ,000 - 3 9 . 4 T 2 3 = -RT l o g p 0 2 at T = 1823°K p 0 2 = 1 .57 X 1 0 " 8 atm. 3 . Oxygen pressure over f e r r o u s oxide: F e 0 ( l ) — > • F e ( l ) + ^ ( g ) A F ° = 5 6 , 8 3 0 - I I . 9 4 T 2 3 = -RT l o g ( p 0 2 ) 2 at T = I.82 3°K P 0 2 = 0.42 X 10 a t m . Thermodynamic Calculations Continued h. Iron vapor pressure over l i q u i d i r o n : 36 The vapor pressure of metals was studied "by Loftness • , who reports the temperatures at which the i r o n vapor pressure equals 0.001, 0.01, 0.1, 1.0, and 10 mm- of mercury. These were plotted i n Figure 2 6 . o , and the pressure at 1550 C. interpolated from the graph to "be O . O M - 7 mm. of mercury. 5. Vapor pressure of molecular magnesium oxide: MgO log P, - 2 7 , 3 2 0 + 1 0 . 2 5 T 4 7 MgO at T =•1823°K pMgO - 8 = 1 7 7 0 X 1 0 atm. 6 . Reduction of MgO by i r o n : MgO (s) + Fe (1) FeO (1) + Mg (g) A F = A(RT log p 0 2 ) = U . 5 7 5 ( l 8 2 3 ) ( - 0 . 9 9 ) - 8 » 2 6 0 cal/gmole APPENDIX V. X-ray Data TABLE XI. -88 -X-ray Data Co Kc< (Fe f i l t e r ) 30 KV, 10 ma, 5 hours Experimental Values A.S.T.M. Published Values Line i / i o d MgO F e3°4 1 20 4.82 4.86 30 2 6o 2.95 • 2.97 60 3 100 2.52 2.53 100 4 20 2.427 2.431 10 2.425 10 5 90 . 2.097 2.106 100 2.097 50 6 . 30 1.713 1.714 4o 7 6o 1.614 1.615 60 8 90 1.486 1.489 52 1.484 70 9 10 1.325 1.326 10 10 20 1.278 1.279 30 l l 10 1.266 1.270 4 1.266 10 12 20 1.120 1.120 20 13 40 1.089 1.091 50 14 20 1.046 1.053 5 1.048 20 15. 10 .987 .988 10 16 30 .967 .966 2 .968 • 4o 17 40 • 937 .942 .938 30 Cu Tarf •set Co Tart set Lattice parameter of Peg^ determined from experimental results. 4, = 8.378 % APPENDIX VI. Phase Diagrams Atom % oxygen - 90 -3000 2600 2200 1800 \J 0 0? 0.4 22 26 Wt. % oxygen 1400 1000 F«0-M80 zooo 1600 1200 -1 1 r-Liquid / 0 FeO Magnesio-wustite _i i i i I i_ zo 10 60 80 100 MgO Figure 21. Phase Diagrams of Fe-0 and FeO-MgO Systems - 91 -APPENDIX VII. Calculation of Monolayer Formation from Impurities in the Iron - 92 C a l c u l a t i o n of Monolayer Formation from  Impur i t ies i n the Iron To f i n d i n t e r f a c e area f o r a t y p i c a l drop!: weight = 1 g x dimension = 1.365 inches M a g n i f i c a t i o n = 8 .84 Hence diameter = 2(l.365)(2."54) = 0.786; cm 8.84 Area = 1f (O.786)2 = 0.485 cm 2 Take s i l i c o n as a sample impur i t y . I n i t i a l concentrat ion i s 0.01 wt.$ (by spectrographic a n a l y s i s ) . I f a monolayer forms i n the SiO^ s t ructure then each oxygen atom on the s o l i d sur face requ i res \ s i l i c o n atom. To f i n d the number of surface oxygen atoms on the (100) face of the MgO: The MgO l a t t i c e parameter = 4.213 & (ASTM X - ray F i l e ) Since MgO i s f a c e - c e n t e r e d - c u b i c 2 oxygen atoms occupy (4.213 X 1 0 - 8 ) 2 = 17.7 X 1 0 - 1 ^ cm 2 -l6 2 1 oxygen atom therefore occupies 8.85 X 10 cm The number of oxygen atoms at i n t e r f a c e equals 0.485 c -+ l U 8T85T10- 1 6 ) 5 . 4 7 X 10 atoms 1 4 Number of s i l i c o n atoms requ i red f o r Si0 2 = 2.73 X 10 atoms-Number of atoms i n drop i n i t i a l l y = .0001 (6 .02 X 102^) = 14 2 8 , 1 21,400 X 10 /atoms Thus only 0.01$ of impuri ty atoms present i n i t i a l l y i s s u f f i c i e n t to form a monolayer. APPENDIX VIII. Diffusion Calculations Diffusion Calculations Calculation of diffusion distance of Fe into MgO (after Turnbull) • i - x 2 Fick'-s Law C = CQ ( D t ) 2 e , j is k Dt only valid i f 1. uniformly • infinite layer of Fe 2. infinitely thick diffusion medium. (Condition 1. would be upset by an impurity monolayer). Turnbull concludes elements with atomic radii. >^ 1.32 wil l not 37 diffuse. The atomic radius of Si = 1.32. Turnbull measured diffusion of solid iron into MgO in temperature range of 134-0° -*• 1060°C. For purposes of illustration, this calculation wil l involve extrapolation of his results to melting point even though experiments were carried out at 15° above the melting point. -25,900 p D = 9.5 (10 Y) ^ RT cm /sec. = 9.5 d o " 9 ) -fZ^2£ =9.5 d o " 9 ) - 7 ' 2 k e 1.98(1808) e = 9.5 (io~ 9 ) (.00072) -12 2 = 6.83 X 10 cm /sec. Let t = 120 sec. Fick's law now becomes C = 5.08(10"^) 3.28 X 10~ 9 u o Hence at x = 0 2 = 5.08 (10~5) Co ; VII..BIBLIOGRAPHY - 95 -1. Glasstone, S., "Textbook of Physical Chemistry", Van Nostrand, N.Y. (1946). 2. Allen, B. C , and Kingery, W. D., Trans. A.I.M.E. 215, 30 (1959). 3. Kingery,. W.D., and Humenik, M. J r . , J . Phys. Chem. 57, 359 (1953). 4. Economos, G. , and Kingery, W. D., J . Am. Cer. 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