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UBC Theses and Dissertations

Electronic processes in anodic A1₂O₃ Cochran , John Francis 1951

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ELECTRONIC PROCESSES IN ANODIC AI2O3 by John F r a n c i s Cochran (iff A t h e s i s s u b m i t t e d i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r t h e degree o f MASTER OF APPLIED SCIENCE i n t h e Department o f PHYSICS We a c c e p t t h i s t h e s i s as conforming t o t h e s t a n d a r d r e q u i r e d f rom c a n d i d a t e s f o r t h e degree o f MASTER OF APPLIED SCIENCE Members o f t h e Department o f P h y s i c s THE UNIVERSITY.. OF BRITISH COLUMBIA August, 1951 ABSTRACT Some o f the p r o p e r t i e s o f the l u m i n e s c e n c e which accompanies the o x i d a t i o n o f aluminum i n weak a c i d s , such as b o r i c a c i d , have been i n v e s t i g a t e d . An a p p a r a t u s to measure e l e c t r o n i c c u r r e n t d i r e c t l y has been b u i l t and used t o c o r r o b o r a t e p e k k e r and U r q u h a r t ' s t h e o r y o f e l e c t r o n i c c o n d u c t i o n t h r o u g h a n o d i c o x i d e l a y e r s . Anderson's t h e o r y o f o x i d e growth has been sub-s t a n t i a t e d by o x i d i z i n g p l a t e s i n an O x a l i c a c i d s o l u t i o n and comparing t h e t h i c k n e s s e s of the o x i d e l a y e r s w i t h t h e i r c a l c u l a t e d t h i c k n e s s e s . The p o r o s i t y of l a y e r s formed i n O x a l i c a c i d has been o b t a i n e d as a f u n c t i o n o f c u r r e n t den-s i t y . A m o d i f i c a t i o n o f Anderson's t h e o r y o f the growth o f porous l a y e r s has been advanced. ACKNOWLEDGEMENTS The research described i n t h i s thesis was supported by grants from the National Research Council of Canada. I wish to thank the National Research Council of Canada for a bursary granted to me 1950-19.51. I sincerely appreciate the help given to me by Dr. A.J. Dekker who suggested and directed the problem. Thanks also to G.W. Williams and E.K. Darby v/ho gave me considerable assistance with the electronic c i r c u i t s used i n t h i s research. TABLE OF CONTENTS page I • INTRODUCTION . . 1 (1) A n o d i c B a r r i e r L a y e r 1 (2) Porous A l g O j L a y e r 3 (3) T h e o r i e s o f A l g O j growth 4 I I EXPERIMENTAL 7 P a r t (A) ' Luminescence I n A n o d i c B a r r i e r L a y e r s 7 (a) A p p a r a t u s 7 (b) R e s u l t s 11 P a r t (B) Porous A l 2 O j L a y e r s 16 (a) A p p a r a t u s 17 lb) R e s u l t s 18 DIAGRAMS -F i g u r e F a c i n g Page ( 1 ) V o l t a g e a c r o s s t h e A n o d i c b a r r i e r as a f u n c t i o n o f time f o r o x i d a t i o n a t c o n s t a n t c u r r e n t 2 ( 2 ) Schematic diagram o f a porous o x i d e l a y e r 3 ( 3 ) The Aluminum p l a t e s used i n t h e p r e s e n t e x p e r i m e n t s 7 (4) 600 v o l t power s u p p l y 8 ( 5 ) C u r r e n t R e g u l a t o r 8 (6) Response c u r v e o f the 931-A p h o t o m u l t i p l i e r 8 ( 7 ) C i r c u i t d i a g r a m t o i l l u s t r a t e how t h e 931-A. p h o t o m u l t i p l i e r i s connected t o the a m p l i f i e r and n e g a t i v e v o l t a g e s u p p l y 8 (8) D.C. A m p l i f i e r 9 ( 9 ) R e g u l a t e d 1 0 0 0 v o l t s u p p l y 9 ( 1 0 ) O x i d a t i o n c e l l 10 ( 1 1 ) E f f i c i e n c y C e l l 10 ( 1 2 ) R e c i p r o c a l c a p a c i t y v . s . V o l t a g e f o r an a n o d i c b a r r i e r l a y e r o x i d i z e d i n ammonium c i t r a t e s o l u t i o n 1 1 (13) V o l t a g e and i n t e n s i t y v . s . Time f o r an o d i c b a r r i e r l a y e r s o x i d i z e d i n ammonium c i t r a t e s o l u t i o n . C u r r e n t d e n s i t i e s f rom 1 t o 1 . 7 m.a ./cm2 1 2 (14) V o l t a g e and i n t e n s i t y v . s . time f o r a n o d i c b a r r i e r l a y e r s o x i d i z e d ' i n ammonium c i t r a t e s o l u t i o n . C u r r e n t d e n s i t i e s f r o m 2 t o 3 m.a ./cm2 12 ( 1 5 ) V o l t a g e and i n t e n s i t y v . s . time f o r an o d i c b a r r i e r l a y e r s o x i d i z e d i n ammonium c i t r a t e s o l u t i o n ; c u r r e n t d e n s i t y = 1 . 7 m.a./cm2 I n t e n s i t y w i t h an w i t h o u t a f i l t e r o v e r the p h o t o m u l t i p l i e r 13 V o l t a g e and i n t e n s i t y v . s . t i m e f o r a n o d i c b a r r i e r l a y e r s o x i d i z e d i n ammonium c i t r a t e s o l u t i o n ; c u r r e n t d e n s i t y = 2 .7 m.a./cm2 I n t e n s i t y w i t h and w i t h o u t a f i l t e r o v e r the p h o t o m u l t I p l i er T r a n s m i s s i o n o f the C o r n i n g f i l t e r fr33^1 V o l t a g e and E f f i c i e n c y v . s . time f o r an a n o d i c b a r r i e r l a y e r o x i d i z e d i n ammonium b o r a t e s o l u t i o n V o l t a g e as a f u n c t i o n of time f o r a sample o x i d i z e d i n H2SQ4 R e c i p r o c a l c a p a c i t y and i n c r e a s e i n w e i g h t as a f u n c t i o n o f time f o r a sample o x i d i z e d i n H2SO4 P e r o s i t y as a f u n c t i o n o f c u r r e n t d e n s i t y f o r p l a t e s o x i d i z e d i n O x a l i c a c i d " INTRODUCTION I f Aluminum i s made the anode i n "an e l e c t r o l y t i c o e l l f i l l e d w i t h the p r o p e r e l e c t r o l y t e i t becomes c o a t e d w i t h an o x i d e l a y e r . Two t y p e s o f l a y e r a r e p o s s i b l e de-pendi n g on t h e e l e c t r o l y t e " u s e d . (1) The A n o d i c B a r r i e r L a y e r I n weak a c i d s such as b o r i c a c i d i . , ammonium b o r a t e s o l u t i o n o r ammonium n i t r a t e s o l u t i o n the aluminum becomes co a t e d w i t h a h a r d , t h i n l a y e r o f o x i d e . That t h i s o x i d e c o a t i n g i s v e r y u n i f o r m has been shown by P.D. Lomer (1) from an i n v e s t i g a t i o n o f anode b a r r i e r l a y e r t h i c k n e s s e s u s i n g i n t e r f e r e n c e f r i n g e s . From e l e c t r o n d i f f r a c t i o n s t u d i e s made on the b a r r i e r l a y e r B a s s ' ( 2 ) c o n c l u d e d t h a t t h i s t y pe o f o x i d e has t h e amorphous s t r u c t u r e t y p i c a l o f su b s t a n c e s l i k e g l a s s , a l t h o u g h from X-ray d i f f r a c t i o n p i c -t u r e s Yerwey (3) c o n c l u d e d t h a t i t has a s e m i - c r y s t a l l i n e s t r u c t u r e denoted by y 1 - A l 2 0 j . The tf1 -A1 2 0 ^ s t r u c t u r e o f Verwey i s a f a c e c e n t r e d c u b i c oxygen l a t t i c e t h r o u g h -which t h e A l 5 + i o n s a r e d i s t r i b u t e d i n a s t a t i s t i c a l f a s h i o n — 7 0 % i n o c t a h e d r a l h o l e s , 30°/<> I n t e t r a h e d r a l h o l e s . F o r v o l t a g e s up t o lOOv a c r o s s the b a r r i e r l a y e r the o x i d a t i o n p r o c e s s i s almost 100% e f f i c i e n t , t h a t i s , most o f t h e oxygen formed a t the anode i s c o n v e r t e d i n t o i i Eig.U) - V o l t a g e a c r o s s the c r y s t a l l i n e l a y e r as a f u n c t i o n o f time f o r o x i d a t i o n a t c o n s t a n t c u r r e n t . 2. AI2O3 ( 2 ) , (4). I f the current i s kept constant during the oxidation the voltage across the oxide layer varies -with time as indicated i n F i g . ( l ) . Curves I and I I are f o r e l e c t r o l y t e s having a low and a high r e s i s t i v i t y respectively. The terminal voltage i s c a l l e d the "spark p o t e n t i a l " because i n the neighbor hood of the time indicated by b F i g . ( l ) the oxide, i f observed i n a dark room, becomes studded with random flashes of l i g h t . Guhtherschulze and Betz (5) found that the spark pote n t i a l i s a function of the concentration of the el e c t r o l y t e used. For oxidation i n ammonium borate solution they found Vsp = -a In c + constant c-concentration of the ammonium borate solution. f o r any given current density. For solutions with a s p e c i f i c resistance of 500 item the spark potential Is approximately 500 v o l t s . A s p e c i f i c resistance of 100 .item corresponds to approx-imately. 400 v o l t s . When the spark potential i s reached oxide growth stops, although current i s . s t i l l flowing. F i n a l thick-nesses are of the order of l/Zyii • The formation of the oxide layer i s accompanied by luminescence: the i n t e n s i t y of the emitted l i g h t increases with time during oxidation at constant current and i s greater for larger current densities. E. Gumminski (6) found the l i g h t to have a continuous spectrum extending from wavelengths of 3700 A° to 5900 A° with a maximum near 4600 A°. The spect-r a l d i s t r i b u t i o n depends upon the type and amount of impurity i n the aluminum. I " i g . ( 2 ) - s c h e m a t i c d i agram of a porous l a y e r . " b 11 i s the t h i c k n e s s o f the base l a y e r . 3 . F i n a l l y , t h e system AI-AI2O35 - e l e c t r o l y t e e x h i b i t s r e c t i f y i n g p r o p e r t i e s ( 7 ) . (2) The Porous L a y e r Aluminum o x i d i z e d i n s t r o n g l y a c i d s o l u t i o n s such as d i l u t e H2SO4 o r o x a l i c a c i d becomes c o a t e d w i t h an o x i d e l a y e r o f u n i f o r m t h i c k n e s s , but t h e o x i d e i s p i e r c e d by c i r c u l a r h o l e s o r i e n t e d a t r i g h t a n g l e s t o the m e t a l s u r f a c e , F i g . ( 2 ) , and d i s t r i b u t e d i n a random f a s h i o n over t h e o x i d e s u r f a c e ( 8 ) . T h i s s t r u c t u r e i s c l e a r l y i l l u s t r a t e d i n photo m i c r o g r a p h s by Huber ( 8 ; . The d i a m e t e r o f t h e pores i s e s t i m a t e d t o be be-tween 1/10 and l/10Cy{ by Rummel ( ? ) , Dekker and van Ge e l (10), B u r w e l l , Smudski and May (11). Dekker and van G e e l (10) have shown t h a t the r a t i o o f pore volume t o t o t a l volume i s about 1/3 f o r o x i d a t i o n i n O x a l i c a c i d and t h a t the p o r o s i t y d e c r e a s e s w i t h i n c r e a s i n g c u r r e n t d e n s i t y . The pores do not extend t o the aluminum but end i n a t h i n , u n i f o r m base l a y e r o n l y a few hundred angstroms t h i c k . (12), (13), 14). D u r i n g o x i d a t i o n a t c o n s t a n t c u r r e n t t h e v o l t a g e i n c r e a s e s r a p i d l y a t f i r s t but becomes c o n s t a n t a f t e r a minute o r two. The t h i c k n e s s o f t h e porous l a y e r i s p r o p o r t i o n a l t o tim e ( 9 ) , a l t h o u g h t h e t h i c k n e s s o f t h e base l a y e r remains c o n s t a n t (12). The t h i c k n e s s o f the base l a y e r I n c r e a s e s w i t h c u r r e n t d e n s i t y and d e c r e a s e s w i t h i n c r e a s i n g a c i d i t y o f t h e e l e c t r o l y t e (12). By u s i n g an O x a l i c a c i d s o l u t i o n o f pH = 3 .3 and a c u r r e n t d e n s i t y o f 10 m.a./cm2 Guminski (14J produced 4 . porous layers with very thick base layers, the f ina l voltage across the layer being of the order of 1 .50-250 volts . These coatings with the thick base layers luminesced, the l ight having the same appearance as the l ight produced during the formation of Anodic Barrier layers. Edwards and Keller (4) found that for oxidation in oxalic acid or dilute sulphuric acid the weight of the oxide coating accounted for only 60-807<> of the aluminum lost from the anode, although the current efficiency measured from the loss in weight by the aluminum anode v/as almost 1 0 0 % . The crystal l ine structure of porous AI2O3 layers seems to depend on the electrolyte used for the oxidation. From electron diffraction patterns Yoshida (15) finds that the rings from porous AI2O3 formed in Oxalic acid agree neither with the rings from pure A l nor with those from ^ i - A ^ P ^ . He supposes that the oxide is composed of minute crystals of 2T-A1205. Layers formed in H2SO4 showed the patterns obtained from hot water treated anodic films i . e . they have a crystal structure, similar to oi -AJ.2O3-.-H2O. oi -AI2O3.-H2O i s formed from ^ - A ^ O j by heating with water (4). ( 3 ) Theories of Oxide Growth Theories of aluminum oxide growth have been ad-vanced by Verwey (16), Mott ( 1 7 ) , and Anderson (18)'. Mott and Verwey picture oxide growth occurring due to the migration of A l ^ + ions from the metal to the oxide-electrolyte interface 3 under the i n f l u e n c e o f the i n t e n s e f i e l d p r e s e n t ( a p p r o x i m a t e l y 7 10' v o l t s / c m . ) . T h i s would, mean o x i d e growth o n l y a t t h e o x i d e -e l e c t r o l y t e i n t e r f a c e . - On t h e b a s i s of t h i s t h e o r y i t i s d i f f -i c u l t t o see how t h e porous l a y e r c o u l d keep on growing w i t h time a l t h o u g h e l e c t r i c a l l y s h o r t e d by t h e po r e s f i l l e d w i t h e l e c t r o l y t e . Anderson proposes t h a t o x i d e growth o c c u r s b o t h by -z + p— outward d i f f u s i o n o f A l ^ and t h e in w a r d m i g r a t i o n o f 0 from the e l e c t r o l y t e ; 1/3 the c u r r e n t t h r o u g h the o x i d e i s c a r r i e d by A l ^ + i o n s , the r e s t by 0 2~ i o n s ; t h e r e f o r e , 2/3 o f the o x i d e growth o c c u r s a t the A l - A l ^ O j i n t e r f a c e . AL/ i o n s r e a c h i n g the o x i d e - e l e c t r o l y t e i n t e r f a c e form o x i d e i n s o l u t i o n s l i k e b o r i c a c i d , but pass d i r e c t l y i n t o s o l u t i o n when s t r o n g a c i d s a r e used. A c c o r d i n g t o Anderson, d u r i n g t h e f o r m a t i o n o f the b a r r i e r l a y e r , a t c o n s t a n t v o l t a g e , t h e c u r r e n t w i l l d e c r e ase as the t h i c k n e s s i n c r e a s e s u n t i l a s t a t i o n a r y s t a t e i s reached i n w h i c h the r a t e o f growth i s e q u a l t o the r a t e o f a c i d a t t a c k . Once i n i t i a t e d a t a number o f p o i n t s t h e a c i d a t t a c k c r e a t e s a s i t u a t i o n s i m i l a r to t h a t o f charged p o i n t s o p p o s i t e a p l a n e c o n d u c t o r . T h i s would cause the o x i d e t o f o r m i n s p h e r i c a l caps around t h e s e p o i n t s , t h e r e b y l e a d i n g t o a porous s t r u c t u r e . Dekker and U r q u h a r t (12 J c a r r i e d out experiments on porous l a y e r s u s i n g , not c o n s t a n t v o l t a g e , but c o n s t a n t c u r r e n t . They p o i n t out t h a t on the b a s i s o f Anderson's p i c t u r e i t i s d i f f i c u l t t o see how e q u i l i b r i u m c o u l d be e s t a b -l i s h e d between r a t e of growth and a c i d a t t a c k i f the c u r r e n t , and t h e r e f o r e r a t e of growth, i s constant. These authors t h e r e f o r e propose t h a t the t o t a l c u r r e n t Is p a r t l y i o n i c and p a r t l y e l e c t r o n i c . The e l e c t r o n i c p o r t i o n i s normally very s m a l l , but at some c r i t i c a l t h i c k n e s s o f the b a r r i e r l a y e r the e l e c t r o n i c c u r r e n t i n c r e a s e s r a p i d l y , w i t h a corresponding decrease In the i o n i c c u r r e n t , u n t i l r a t e o f growth of oxide and a c i d a t t a c k are In e q u i l i b r i u m . The l a r g e Increase i n e l e c t r o n i c c u r r e n t i s ex p l a i n e d on the b a s i s o f a shower phenomenon In which e l e c t r o n s , i n t r o d u c e d from the e l e c t r o l y t e , are a c c e l e r a t e d by the l a r g e f i e l d In the oxide and r a i s e l a t t i c e e l e c t r o n s to the conduction band by means of c o l l i s i o n s . The f o l l o w i n g r e p o r t i s concerned both -with Anodic b a r r i e r l a y e r s and w i t h porous AlgO^ l a y e r s . In p a r t (A) evidence i s o f f e r e d r o r the e x i s t a n c e o f an e l e c t r o n i c c u r r e n t f l o w i n g through the Anodic b a r r i e r l a y e r and an attempt i s made to q u a l i t a t i v e l y r e l a t e the magnitude of the e l e c t r o n i c c u r r e n t to the i n t e n s i t y of the luminescence produced. P a r t (B) i s concerned w i t h porous l a y e r s ; an e x p l a n a t i o n i s o f f e r e d f o r the formation o f porous l a y e r s which i s a m o d i f i c a t i o n of Anderson's p i c t u r e . 1 —Glyptal F i g . ( 3 ) - The alurainum p l a t e s used i n the p r e s e n t experiments, I I EXPERIMENTAL P a r t A - Luminescence i n Anodic B a r r i e r Layers Aluminum p l a t e s were cleaned and o x i d i z e d at constant current and temperature i n ammonium c i t r a t e s o l u t i o n . The i n t e n s i t y of luminescence was measured using a 931-A p h o t o m u l t i p l i e r tube and curves of volt a g e and i n t e n s i t y were p l o t t e d as f u n c t i o n s of time. Thicknesses of the l a y e r s were obtained from t h e i r c a p a c i t i e s as measured i n mercury. (a) Apparatus ( l ) Samples The aluminum p l a t e s , Fig. ( 3 ) , were cut from sheet aluminum approximately 1/2 in.m. t h i c k . The edges of the p l a t e s were p o l i s h e d w i t h 0,00 and 000 emery paper and washed i n organic s o l v e n t s . G l y p t a l was baked on the stems by heating at 200°0 f o r s e v e r a l hours; t h i s g l y p t a l c o a t i n g confines the area of o x i d a t i o n to the p l a t e proper. The p l a t e s were given a f i n a l c l e a n i n g w i t h 10% KOH s o l u t i o n j u s t before o x i d a t i o n , then washed w i t h 2% HNOj, d i s t i l l e d water and l a s t l y w i t h the o x i d i z i n g s o l u t i o n . The aluminum was s u p p l i e d by the Aluminum Co. of Can. L t d . i n two a l l o t m e n t s . Nominally both allotments were Al c a n 23-0 f u l l y annealed aluminum, but spectroscopic a n a l y s i s revealed a d i f f e r e n c e In p u r i t y ; throughout the r e s t of t h i s 1/2 RK60 10 h tOh. Fig..K 5 ) - u.C. c u r r e n t r e g u l a t o r f o r c u r r e n t s between 20 and 200 ma. The o u t p u t o f t h e D.u. power s u p p l y i s c o n n e c t e d t o t h e 1 I I l I I r | • | 1000 v. O-regulated 10 meg W V W -I meg Photo-Cathode I meg Dytoodes I meg I meg Anode Amplifier F i g . ( 7 ) - C i r c u i t diagram t o i l l u s t r a t e how the 931-A photo-m u l t i p l i e r i s connected t o the a m p l i f i e r and n e g a t i v e v o l t a g e s u p p l y . 8. paper the two batches o f aluminum w i l l be r e f e r r e d t o as "pure" and "impure" aluminum r e s p e c t i v e l y . Pure Aluminum - Fe - t s % Ga?03 - .oc Z Cu Mn - 0 -OS-- <o\ II II «l - -OS " Zn - o 003 II " S i -<o-7 II Y- . 0 0 ? " Impure Aluminum - Fe Cu - o-G - o \ % It Caz03- .«9 % Mn - l-o II OS " S i - » * _ . 0 * 5 - . . Ni < OS •• (2) P.C. Power Supply D.C. power was s u p p l i e d by a 200 m.a., 600 v o l t v a r i a b l e output supply. F i g (4). The r i p p l e was l e s s than .05% at f u l l l o a d . ( 3 ) Current Regulator The current through the oxide l a y e r s could be held constant to 1/3 m.a. or b e t t e r w i t h the r e g u l a t o r of F i g ( 5 ) . A 1% change i n current causes a 50 v o l t change i n voltage across the 6AS7. (4) P h o t o m u l t i p l i e r The i n t e n s i t y measurements were made w i t h a 531-A p h o t o m u l t i p l i e r tube which has the s p e c t r a l s e n s i t i v i t y i l l u s t r a t e d i n F i g ( 6 ) . I t was connected to the D.C. a m p l i f i e r and a 1 0 0 0 v o l t supply as shown i n F i g ( 7 ) . The tube was sealed i n t o a glas s jacket and both the tube and i n -side of the jacket were coated, except f o r windows, w i t h black enamel. The windows l i m i t e d the angular subtend of the photo-cathode at the aluminum p l a t e to an area w e l l i n s i d e the B,C are zero adjustments for the meter. 6J7 6T — 1 2 meg 1500A. F i g . ( 8 ) -»k 4 . i I --. J F i g . ( 9 ) R e g u l a t e d 1000 v o l t power s u p p l y used i n c o n j u n c t i o n w i t h t h e 931-A p h o t o - m u l t i p i i e r . b o u n d a r i e s o f the p l a t e . T h i s arrangement made.the i n t e n s i t y measurements independent o f changes i n d i s t a n c e between t h e photo-cathode and o x i d e l a y e r . 1 ( 5 ) D.C A m p l i f i e r - F i g . ( 8 ) A 12 v o l t d r y c e l l w i t h 13 megohms i n s e r i e s p r o v i d e d a r e f e r e n c e v o l t a g e w i t h w h i c h t o check t h e s t a b i l i t y o f t h e a m p l i f i e r . The r e s p o n s e o f t h e p h o t o - m u l t i p l i e r and a m p l i f i e r was checked by t h e two s o u r c e method and found to be l i n e a r w i t h l i g h t i n t e n s i t y o v e r the r e q u i r e d range f o r v o l t a g e s between 9 0 0 and 1 2 0 0 v o l t s . No f a t i g u e e f f e c t s were d e t e c t e d over t h e range o f l i g h t i n t e n -s i t i e s used i n t h e s e e x p e r i m e n t s . ( 6 ) 1 0 0 0 V o l t N e g a t i v e Power Supply - F i g ( 9 ) P r o v i d e s r e g u l a t e d v o l t a g e f r o m 9 0 0 - 1 3 0 0 v o l t s w i t h a r e g u l -a t i o n f a c t o r o f 1 0 0 0 . (7) C a p a c i t y Measurements The c a p a c i t y of t h e aluminum o x i d e l a y e r was measured by u s i n g t h e A l p l a t e as one p l a n e e l e c t r o d e and mercury as t h e opposing e l e c t r o d e . The t h i c k n e s s o f the l a y e r can be c a l c u l a t e d f r o m i t s c a p a c i t y u s i n g the f o r m u l a f o r t h e c a p a c i t y o f a p l a n e p a r a l l e l p l a t e condenser. T h i c k n e s s e s o b t a i n e d from c a p a c i t y measurements and from the w e i g h t o f o x i d e formed on the sample agreed v e r y w e l l p r o v i d i n g the o x i d e l a y e r was h e a t e d t o 4 5 0°C f o r a few minutes t o d r i v e out absorbed w a t e r . F i g . ( 1 0 ) - O x i d a t i o n c e l l . (A) Aluminum c o o l i n g c o l l . (B) Tungsten cathode. (C) A i r d r i v e n s t i r . (D) 931-A p h o t o - m u l t i p l i e r t u o e . (E) Aluminum p l a t e . 2 mm. capillary -3 cm. tubing F i g . ( 1 1 ) - E f f i c i e n c y C e l l . 1 0 . Impedance measurements were made w i t h a G e n e r a l Radio c a p a c i t a n c e b r i d g e , type 716-B, w i t h n u l l d e t e c t o r t y p e 1 2 3 1 - A . ( 8 ) O x i d a t i o n C e l l - F i g . ( 1 0 ) The temperature o f the b a t h v/as k e p t c o n s t a n t t o 2/j?°C by means o f t h e a i r d r i v e n s t i r and A l c o o l i n g c o i l . Temperature t h r o u g h o u t t h e e l e c t r o -l y t e was u n i f o r m t o l/j?°C. S t r a y l i g h t i n t e n s i t y was m i n i m i z e d by e n c l o s i n g t h e c e l l i n a b l a c k box. (9) M e t e r s V o l t a g e a c r o s s the o x i d e was measured w i t h an E l e c t r o n i c s I n s t r u m e n t s m u l t i m e t e r , model 44, a c c u r a t e to. 1/3% f u l l s c a l e . C u r r e n t was measured w i t h a Weston m i l i -v o l t m e t e r shunted t o g i v e a 1 0 0 and a 2 0 0 m i l i a m p range, a c c u r a t e t o 2 / 3 % f u l l s c a l e . ( 1 0 ) E f f i c i e n c y C e l l The d e v i c e o f F i g . ( 1 1 ) was c o n s t r u c t e d and used to measure t h e e f f i c i e n c y o f t h e o x i d a t i o n p r o c e s s . A sample o f c o n v e n i e n t d i m e n s i o n s was i n s e r t e d i n t o one o f t h e t h r e e c e n t i m e t e r tubes and o x i d i z e d f o r a known l e n g t h o f t i m e ; any gas g i v e n o f f c o l l e c t e d i n t h e 2 m.m. c a p i l l a r y . A p l a t i n u m e l e c t r o d e was used to bubble O2 gas i n t o the second c a p i l l a r y t o f o r m a gas column the same l e n g t h as t h a t o b t a i n e d f rom o x i d i z i n g the aluminum; t h e charge r e -q u i r e d was c a l c u l a t e d from the time and t h e c u r r e n t p a s s i n g t h r o u g h t h e e l e c t r o d e . S i n c e b o t h gas columns were under i d e n t i c a l c o n d i t i o n s t h i s same amount o f charge must have been 400 CO o > 300 200 100 0 ( 1 2 ) - R e c i p r o c a l c a p a c i t y v s . v o l t a g e f o r an- aluminum p l a t e o x i d i z e d a t 20°C i n ammonium c i t r a t e s o l u t i o n , s p e c i f i c r e s i s t a n c e = 2 0 7 ohm-cm. C u r r e n t d e n s i t y = 1 . 6 7 ma/cm 2. 3 4 in ifieroforatfs responsible f o r the volume of gas c o l l e c t e d from the sample. Knowing the charge and the time i t took to pass through the oxide l a y e r the e l e c t r o n i c current may be computed. The e f f i c i e n c y of the o x i d a t i o n process i s defined as the r a t i o of ( t o t a l current - e l e c t r o n i c current) to the t o t a l current through the oxide. The main source of e r r o r i n t h i s experiment i s due to the abs o r p t i o n of oxygen by the e l e c t r o l y t e and can be minimized i n two ways: (1) P r e s a t u r a t i o n of the e l e c t r o l y t e by passing a stream of gas through each column p r i o r to performing the experiment. (2) A d j u s t i n g the current d e n s i t y through the platinum electrode so that the bubbles from i t and the sample are about the same s i z e . The time f o r the bubbles to tr a v e r s e the column of e l e c t r o l y t e w i l l then be the same i n each case. (b) Results Unless s t a t e d otherwise the f o l l o w i n g exper-iments were done using the "pure" aluminum. I t was found that the f i e l d s trength i n the aluminum oxide l a y e r s remains constant even a f t e r the spark p o t e n t i a l has been reached. This point i s I l l u s t r a t e d i n P i g . ( 1 2 ) where 1/c i s a measure of the thickness of the l a y e r . Since the f i e l d s t r e n g t h i s constant during o x i d a t i o n , the voltage across the l a y e r at any time Is p r o p o r t i o n a l to the t h i c k n e s s ; the pro-p o r t i o n a l i t y f a c t o r depending on the current d e n s i t y . 0 10 20 30 40 50 60 70 80 90 TIME in minute* FI@. ( 1 5 ) Velfeage -mwA tmmmMg? m. Item -Jfiwr oxidised a t 2 0 ° 0 i s asiaomirasa e i t ^ t © so lwt£oa r s^eel f i© , ^ s l s # a a e © * 3®$ ©tarsia* 32kp:eim©& nurabero J^Mafc® -m$$®B& v s . $ia@;9 J>EISI0& . marabera iiiiia©$© ^slss fc l^ InS^iist^ v s . 1&jm& (1) m& igeaslty and ant (2*} it 1.6? 5*. 5 0 0 4 0 0 co o > 3 0 0 2 0 0 1 0 0 F i g . ( 1 4 ) Voltage and i n t e n s i t y vs. time Tor ( 1 ) and ( 1 " ) p l a t e s o x i d i z e d at 2 0 ° 0 i n ammonium c i t r a t e s o l u t i o n , s p e c i f i c r e s i s t a n c e ( 2 ) and { 2 " ) = 553 ohm-cm. Unprimed numbers i n d i c a t e voltage vs. time, primed numbers ( 3 ) and ( 3 " ) i n d i c a t e r e l a t i v e I n t e n s i t y vs.time. L e t t e r s i n d i c a t e i n f l e c t i o n p o i n t s . current d e n s i t y = 1 . 9 8 ma/cm2 » it = 2 . 6 5 " n it = 2 . 9 2 » 12. F i g s . (13) and (14) are a t y p i c a l s e t of v o l t a g e , i n t e n s i t y v .s. time graphs, o b t a i n e d from a set or p l a t e s o x i d i z e d i n ammonium c i t r a t e s o l u t i o n at constant c u r r e n t . Two important f e a t u r e s of these graphs a r e : (1) The r e l a t i v e i n t e n s i t y f o r each c u r r e n t d e n s i t y does not become measurable u n t i l the v o l t a g e across the l a y e r i s about 100 v. S i m i l a r l y , at about 300 v. the slope of the i n t e n s i t y v . s . time graph suddenly i n -c r e a s e s . (2) To every change (denoted by an unprimed l e t t e r ) In the slope of a V-T curve there corresponds a d e f i n i t e change (denoted by a primed l e t t e r ) i n the c h a r a c t e r o f the c o r r e s p o n d i n g R.I. - T curve. Corresponding r e g i o n s on the V-T and R.I.-T curves are l a b e l l e d w i t h the same l e t t e r . P o i n t (2) i s i l l u s t r a t e d v ery c l e a r l y at the spark p o t e n t i a l where the r a t e of growth of the oxide l a y e r becomes very small (x,y,z F i g . ( 1 3 ) , r , s , t F i g . ( 1 4 ) ) . Luminescent i n t e n s i t i e s at d i f f e r e n t c u r r e n t d e n s i t i e s cannot be compared, because the s p e c t r a l d i s t r i b u t i o n of the emitted l i g h t depends upon f i e l d s t r e n g t h . The response curve of the p h o t o - m u l t i p l i e r Is v e r y peaked so t h a t the output s i g n a l f o r d i f f e r e n t f i e l d s i s not p r o p o r t i o n a l to the f l u x d e n s i t y i n c i d e n t on the photo-cathode. The change i n s p e c t r a l d i s t r i b u t i o n v/as detected i n the f o l l o w i n g way: two p l a t e s were o x i d i z e d , under i d e n t i c a l 20 25 30 TIME 35 in minutes F i g . ( 1 5 ) Voltage and i n t e n s i t y vs. time f o r p l a t e s o x i d i z e d i n ammonium c i t r a t e s o l u t i o n at 20°C, s p e c i f i c r e s i s t a n c e = 2 0 7 ohm-cm, current d e n s i t y = 1 . 6 7 ma/cm2. TJnprimed numbers i n d i c a t e voltage vs. time, primed numbers i n d i c a t e r e l a t i v e i n t e n s i t y vs. time ( 2 ) and ( 2 " ) i n t e n s i t y measurements made without the f i l t e r i n place. ( 1 ) and ( 1 " ) i n t e n s i t y recorded w i t h the corning f i l t e r f 3 2 8 7 i n place.. 0 5 10 15 20 25 TIME in minuttt i Fig.(16) Voltage and I n t e n s i t y v s . t . f o r p l a t e s o x i d i z e d at 20°C i n NH4 c i t r a t e s o l u t i o n , s p e c i f i c r e s i s t a n c e = 207 ohm-cm, i = 2.65 ma/cm2. TJnpriraed numbers i n d i c a t e v vs.t.,primed numbers R.I.vs.t. ( 3 ) and ( 3 " ) i n t e n s i t y without - the f i l t e r ^3387. (4) and 14") i n t e n s i t y w i t h the f i l t e r i n p l a c e . 4200 4400 4600 4800 5000 5200 WAVELENGTH in ongstroms 1 3 . c o n d i t i o n s , a t a c u r r e n t d e n s i t y o r 1 . 6 7 ma/cm2. The I n t e n s i t y v . s . time graph was measured f o r the f i r s t p l a t e I n the u s u a l way ( F i g . 1 5 - 2 and 2 " ) . F o r the second p l a t e a C o r n i n g f i l t e r $ 3 3 8 7 ( F i g . 1 7 ) was I n t e r p o s e d between t h e o x i d e l a y e r and the p h o t o - m u l t i p l i e r ( F i g.I5 - 1 and 1"). The experiment was r e -peated f o r a c u r r e n t d e n s i t y o f 2 . 6 5 ma/cm 2; the r e s u l t s a r e i l l u s t r a t e d i n F i g . ( I D ) . To reduce the d a t a p l o t t e d I n F i g . ( 1 5 ) i t i s n e c e s s a r y t o compare i n t e n s i t i e s f o r a g i v e n t h i c k n e s s of t h e aluminum o x i d e l a y e r ; t h e r e f o r e , c u r v e s ( l ) a n d ( l ' ) a r e s h i f t e d a l o n g the time a x i s u n t i l the v o l t a g e c u r v e s ( 1 ) and ( 2 ) c o i n c i d e . Now a t any time c o r r e s p o n d i n g p o i n t s on each of t h e f o u r graphs r e f e r t o t h e same t h i c k n e s s o f o x i d e . The r a t i o s o f t h e i n t e n -s i t y w i t h a f i l t e r t o i n t e n s i t y w i t h o u t a f i l t e r f o r v a r i o u s t i m e s a re l i s t e d i n T a b l e I . I n t h e same way- T a b l e I I was p r e -p a red from F i g . ( 1 6 ) by s h i f t i n g ' c u r v e s 4 and 4" w i t h r e s p e c t to 3 and 3 " . TABLE I I = 1 . 6 7 ma/cm 2 I]_ = r e l a t i v e i n t e n s i t y w i t h f i l t e r i n t e r p o s e d 12 = r e l a t i v e i n t e n s i t y w i t h no f i l t e r Time-minutes I i '/l2 14.5 .76" 17 .77 22.5 .76" 26.5 .78 28.5 .80 30.5 .75 34.5 .66 37.5 .58 40.5 .52 0 10 2 0 3 0 4 0 5 0 TIME in minutes F i g . ( 1 8 J - V o l t a g e and e f f i c i e n c y v s . time f o r an "impure" sample o x i d i z e d a t 20°C i n Ammonium b o r a t e s o l u t i o n , s p e c i f i c r e s i s t a n c e = 217 cm. C u r r e n t d e n s i t y -4.03 ma/cm 2. 14. Time-minutes H A 2 6 .80 8 .68 10 .63 12 . 6 5 14 . 6 5 16 . 6 6 18 .41 20 .40 TABLE II I = 2.65 ma/cm2 From this data i t may be concluded that: (1) The spectral distr ibution of the l ight remains essentially constant unt i l the ,spark potential is reached. (2) In the neighbourhood of the spark potential the blue wave-lengths in the spectrum become much more pronounced. (3) The ratio I 1 / I 2 i s much smaller for the high current den-sity so that the blue wavelengths in the emitted light become more pronounced the higher the f i e ld strength. Because no more "pure" aluminum v/as available the efficiency measurements had to be made using the impure alum-inum Fig.(18). The characteristics of the oxide formed from this aluminum are markedly different from those of the oxide formed from the pure samples: the luminescence has a pronounced orange color (compared to pale green from the pure samples) and is orders of magnitude more intense; the voltage-time curves are very different, e.g. compare F ig . (18). with Fig.( l3) curve (1). The important features of the efficiency curve are: 1 5 . ( 1 ) I n i t i a l l y the e f f i c i e n c y I s v e r y h i g h , but as the t h i c k n e s s i n c r e a s e s so does the e l e c t r o n i c c u r r e n t . A t about 1 0 0 v a c r o s s t h e l a y e r t h e e l e c t r o n i c c u r r e n t becomes almost c o n s t a n t w i t h t i m e . ( 2 ) A f t e r the s p a r k i n g v o l t a g e has been r e a c h e d th e t o t a l c u r r e n t t h r o u g h t h e o x i d e l a y e r i s e l e c t r o n i c c u r r e n t . These g e n e r a l f e a t u r e s w i l l a l s o be t r u e f o r t h e "pure" samples. The r e s u l t s o f t h e f o r e g o i n g experiments may be q u a l i t a t i v e l y u n d e r s t o o d on the b a s i s o f the p i c t u r e suggested by Dekker and U r q u h a r t ( 1 2 ) . I t i s supposed t h a t d u r i n g o x i d -a t i o n e l e c t r o n s a r e e j e c t e d f rom t h e n e g a t i v e 0 2 ~ i o n s i n s o l u t i o n i n t o t h e c o n d u c t i o n band o f t h e o x i d e where t h e y are a c c e l e r a t e d . by a f i e l d o f about 10^v/cm. These e l e c t r o n s pass i n t o t h e m e t a l o n l y o c c a s i o n a l l y g i v i n g up energy, e i t h e r i n i o n i z i n g im^ . p u r i t y atoms o r r e l e a s i n g t r a p p e d e l e c t r o n s . As the t h i c k n e s s o f the l a y e r approaches the mean f r e e p a t h of an e l e c t r o n i n the o x i d e t h e number o f s e c o n d a r y e l e c t r o n s r a i s e d to the con-d u c t i o n band r a p i d l y i n c r e a s e s , c o n s e q u e n t l y t h e l u m i n e s c e n t i n t e n s i t y a l s o i n c r e a s e s . T h i s i n c r e a s e o c c u r s when the v o l t a g e a c r o s s the o x i d e l a y e r i s about 1 0 0 v o l t s c o r r e s p o n d i n g t o a t h i c k n e s s o f 3 x 1 0 " ^ cm. As the t h i c k n e s s i n c r e a s e s more and more e l e c t r o n s a r e r a i s e d to the c o n d u c t i o n band by p r i m a r y , and p o s s i b l y by s e condary, e l e c t r o n s u n t i l a s t a t e o f e q u i l i b r i u m i s r e a c h e d i n w h i c h as many e l e c t r o n s recombine o r a r e r e t r a p p e d as a r e r e l e a s e d . A t t h i s p o i n t t h e l u m i n e s c e n t i n t e n s i t y i s 16. almost c o n s t a n t with. t i m e . A t a t h i c k n e s s of around 10"* cm, c o r r e s p o n d i n g t o J 0 0 v o l t s , t h e p r i m a r y e l e c t r o n s o b t a i n enough energy under the i n f l u e n c e o f t h e h i g h f i e l d t o r a i s e e l e c t r o n s f r o m the f u l l band t o the c o n d u c t i o n band. T h i s causes a l a r g e i n c r e a s e i n the b l u e wavelengths o f the l u m i n e s c e n c e and l e a d s to d i a l e c t r i c breakdown o r s p a r k i n g . The i n f l e c t i o n p o i n t s on t h e v - t c u r v e s denote a sudden i n c r e a s e i n e l e c t r o n i c c u r r e n t (dv p r o p o r t i o n a l t o d t i o n i c c u r r e n t ) . T h i s i n c r e a s e c o u l d be due t o a p o s i t i v e space charge i n the o x i d e , which would b r i n g the c o n d u c t i o n band e n e r g i e s c l o s e r t o the e l e c t r o n e n e r g i e s o f the i o n s i n s o l u t i o n . At s p a r k i n g t h e space charge must b r i n g the two s e t s o f l e v e l s almost i n t o c o i n c i d e n c e , s i n c e most o f the c u r r e n t i s t h e n e l e c t r o n i c . The dependence o f the spectrum on f i e l d s t r e n g t h may be due to a s h i f t i n energy l e v e l s caused by l a t t i c e d i s t o r t i o n a t the h i g h f i e l d s t r e n g t h s e n c o u n t e r e d i n t h e s e e x p e r i m e n t s . P a r t B - Porous Oxide L a y e r s C l e a n e d aluminum p l a t e s {"impure" samples) were o x i d i z e d , u s i n g c o n s t a n t c u r r e n t , b oth i n d i l u t e H2SO4 s o l u t i o n and i n O x a l i c a c i d s o l u t i o n . The c o n s t a n c y of the r a t e o f growth o f the o x i d e was checked by p l o t t i n g t h e i n c r e a s e i n w e i g h t o f t h e sample and the c a p a c i t y o f t h e o x i d e l a y e r i n mercury as f u n c t i o n s o f t i m e . The t h i c k n e s s o f an o x i d e l a y e r was measured by 17. mounting and p o l i s h i n g the sample as described i n a report to the Proc. Amer. Soc. Test. Mat., (19). The oxide coating i s v i s i b l e under high m a g n i f i c a t i o n and i t s thickness can be measured w i t h a c a l i b r a t e d eyepiece. The known thicknesses of s e v e r a l oxide l a y e r s are compared to the thicknesses com-puted from Faraday's law. Anderson's theory of oxide growth (18) i s s u b s t a n t i a t e d . (a) Apparatus The same equipment and samples as described i n part A were used f o r these experiments;- the samples were cut from the "impure" aluminum. The sample stems were not i n s u l a t e d w i t h g l y p t a l . Before each weighing or ca p a c i t y measurement the samples were heated to 450°C f o r JO minutes, and since g l y p t a l chars at t h i s temperature the stems were Ins u l a t e d by o x i d i z i n g them to 400 v o l t s i n ammonium borate s o l u t i o n . The c a p a c i t y of the oxide l a y e r as measured i n ammonium borate s o l u t i o n was obtained by measuring the imp-edance of an aluminum - AI2O3 - e l e c t r o l y t e - platinum c e l l , the aluminum and platinum being the el e c t r o d e s . Dekker and Urquhart (12) pointed out that the impedance measured between the aluminum and platinum electrodes w i l l be the impedance of the oxide l a y e r p r o v i d i n g t h a t the measuring frequency i s high enough, and that the contact between counter-electrode and e l e c t r o l y t e i s almost p e r f e c t . For the present measurements precautions were taken to insure t h a t these co n d i t i o n s were f u l f i l l e d . V o l t a g e a c r o s s t h e porous l a y e r as a f u n c t i o n o f t i m e . P l a t e s o x i d i z e d i n 2% H2SO4 s o l u t i o n a t 15°C. 1 1 1 ' f J I L 400 500 600 TIME in seconds (1) c u r r e n t d e n s i t y = 1 . 6 7 ma/cm 2 ( 3 ) « '» = 4 . 7 5 1 1 18. (b) R e s u l t s V o l t a g e time graphs f o r d i f f e r e n t c u r r e n t d e n s i t i e s are i l l u s t r a t e d In F i g . ( 1 9 ) ( t h i s experiment done w i t h "pure" aluminum). I t i s important to note that the sl o p e of the i n i t i a l s t r a i g h t l i n e p o r t i o n of the curve i s the same at the i n i t i a l dv f o r a "pure" p l a t e o x i d i z e d i n ammonium c i t r a t e or b o r i c a c i d dt a t the same c u r r e n t d e n s i t y . F i g .(20) i s an example of the graphs o b t a i n e d when the i n v e r s e c a p a c i t y as measured i n mercury and the i n c r e a s e i n weight o f the sample are p l o t t e d a g a i n s t time. For a l l p l a t e s the c a p a c i t y as measured i n ammonium borate s o l u t i o n remained constant throughout the o x i -d a t i o n , In agreement w i t h Dekker and Urquhart (12). The t h i c k n e s s of the oxide l a y e r may be computed from Faraday's laws. I f Anderson Is assumed to be c o r r e c t 2/3 of the c u r r e n t forms oxide a t the A l - A ^ O ^ i n t e r f a c e , t h e r e f o r e b = 7.07 I t 71 x 10~ 6 cm. fo -r > 2 I i n ma/cm ft, d e n s i t y o f AlgO*, y c u r r e n t e f f i c i e n c y (1 amp hour g i v e s 298.4 mg. of 02) Porous l a y e r s cannot grow at the AlgO^ e l e c t r o l y t e i n t e r f a c e because the h i g h r e s i s t a n c e oxide i s short c i r c u i t e d by the pores f i l l e d w i t h e l e c t r o l y t e . Most of the cur r e n t w i l l flow from the oxide i n t o the e l e c t r o l y t e through the base of the pores. The p o r o s i t y of the oxide, pi , i s defined as of = area of the pores as measured on a plane p a r a l l e l to the aluminum surface area of the aluminum p l a t e As a consequence of the columnar s t r u c t u r e of the pores oi = pore Volume.  T o t a l oxide volume oi i s obtained as f o l l o w s : the o r i g i n a l weight of the A l p l a t e i s &-0f<#, t - • A Q - area of p l a t e t - thickness of A l flA. - d e n s i t y of A l .= 2 . 7 0 gms/cm^ A f t e r o x i d a t i o n the weight of the r e s i d u a l A l plus the A1 2 0 ^ i s A 0 foA U-4/?b) + A s ( l - * ) b f c A s - Surface area of the p l a t e = 2 Ao f0 - d e n s i t y of AI2O3 Here we have used the r e s u l t obtained by Edwards and K e l l e r (4) that the r a t i o of the thickness of AI2O3 to the thickness of A l from which i t was formed equals 3 / 2 . The f a c t o r 4/3b = (2) (2/ j5)b i s used because the oxide 2 0 . forms on both sides of the aluminum sample. The increase i n weight of the sample a f t e r o x i d a t i o n i s w - A s ( 1 - * ) b£ - ( 2 / 3 ) ( 2 A 0 ) b f * e or as a consequence of 2A 0 = ,AS w = A s b f ( l - , ^ )f0 -2/3 fi&J gras. e>c • may be c a l c u l a t e d i f w and b are known In Tables I , I I , and I H are l i s t e d values of b ob-t a i n e d • experimentally and from the current, d e n s i t y . In the c a l c u l a t i o n s Q = 3.2 the. value given by Burgers, Claassen and Zernike (20). For each e l e c t r o l y t e the current e f f i c i e n c y was found to be independent of current d e n s i t y , w i t h i n experimental e r r o r . The- e f f i c i e n c y measurements v/ere accurate to 3% and r e p r o d u c i b l e to 1%. Thickness measurements were accurate to 3%. Table I P l a t e s o x i d i z e d at 20°C i n a s o l u t i o n of O x a l i c a c i d saturated at 20°C. E f f i c i e n c y , ^ , i s 8 7 % . i ma/cm2 Area - c m 2 Increase In wt .gmsxlO^ Time minutes << b measured A b c a l c u l a t e d 1 . 0 2 40 . 0 - 1 7 6 800 . 5 2 1 5 . 5 1 5 . 7 1.92 40 . 8 +120 420 . 3 8 1 6 . 4 1 3 . 5 3 . 8 8 40 . 8 +271 240 . 3 3 "16.8 1 7 . 9 7 . 9 2 2 0 . 0 +216 1 3 5 . 2 7 2 0 . 2 2 0 . 6 1 5 . 9 1 0 . 0 +130 71 .24 2 0 . 7 2 1 . 6 1 . 9 2 40 . 8 — — 431 — — 16 . 2 1 5 . 9 2 1 . Table I I P l a t e s o x i d i z e d at 20°C i n a s o l u t i o n of 4% H2SG4 by volume = 91% f 2 1 ma/cm Area cm 2 Increase i n wt.gmsxlO^ Time min. •(calculated -using b meas b measured /* b c a l c u l a t e d 0 . 9 9 1 . 9 1 3 . 8 7 8 . 0 0 1 5 . 9 , .._ 41 . 0 41 . 0 41 . 0 1 9 . 8 1 0 . 0 • - 1 7 9 0 - 1 0 8 1 50 294 9 6 0 4 3 5 240 1 2 0 71 . 9 2 1 .42 . 2 1 . 1 7 14 . 2 1 9 . 1 2 3 . 3 3 4 . 3 2 0 . 4 1 8 . 2 1 9 . 9 2 0 . 6 2 3 . 0 Table I I I P l a t e s o x i d i z e d at 20°G" i n a s o l u t i o n of 8% H 2 S 0 4 by volume. °J = 97% r 2 1 ma/cm^ A-cm2 Increase i n wt. gmsxlO 4 -Time min. < c a l c u l a t e d using b meas b measured b c a l c u l a t e d 0 . 9 9 40 . 7 -2043 9 9 0 mm —• 21.3 1 . 9 2 41.0 — 420 17 . 5 22.6 3 . 8 6 41.0 - 7 5 8 240 •93 11 . 8 19.9 8 . 2 5 19.2 +179 120 • 3 3 26.2 21.2 15.9 9 . 6 +236 70 .22 35.2 2 3 . 8 The p l a t e s o x i d i z e d i n Ox a l i c a c i d support Anderson's theory of oxide growth, but the r e s u l t s obtained from the p l a t e s o x i d i z e d i n H2SO4 are i n c o n c l u s i v e . The d i f f e r e n c e between b measured and b c a l c u l a t e d at higher current d e n s i t i e s i n the case of the H2SO4 e l e c t r o l y t e may be due to using = 3 . 2 i n the c a l c u l a t i o n s . Yoshida (15) repor t s that the oxide l a y e r s formed i n H2SO4 have the o( - A l 2 0 3»H20 s t r u c t u r e . Probably these l a y e r s D e k k e r a v a n G e e l 8 12 16 l t in ma./crrf F i g . ( 2 1 ) - jVorosity as a f u n c t i o n o.f current d e n s i t y f o r p l a t e s o x i d i z e d i n O x a l i c a c i d . 22. when f i r s t formed a r e 6 -AI2O3, but l o c a l h e a t i n g and the c a t a l -y t i c a c t i o n o f H^SO^ cause the s t r u c t u r e t o p a r t i a l l y change t o ' -Al202 .HgO. The l a y e r would t h e n be a m i x t u r e o f o x i d e and mono-hydrate w i t h a d e n s i t y dependent on the degree of homo-g e n e i t y . Any d i s c r e p a n c y between b c a l c . and b.meas. due to a c i d a t t a c k a t the e l e c t r o l y t e - o x i d e i n t e r f a c e i s n e g l i g i b l e . Hass (2) found t h a t a t 20°C a 127<> s o l u t i o n o f H2SG4 d i s s o l v e d o n l y 2 x 10"° gms/cm 2/hr = 14 x 1 0 ~ 8 cm/hr o f y -A1 20^. The w e i g h t o f o x i d e d i s s o l v e d per hour was almost independent o f c o n c e n t r a t i o n be-tween 3 and 127° but was s t r o n g l y dependent on t e m p e r a t u r e . Dekker and van Gee1 (10) measured oL by o x i d i z i n g p l a t e s f i r s t i n o x a l i c a c i d , t h e n i n b o r i c a c i d , dv f o r the o x i d a t i o n d t i n b o r i c a c i d was much g r e a t e r t h a n d v f o r a bare p l a t e o x i d i z e d d t I n t h e same s o l u t i o n , t h e r e f o r e i t was supposed t h a t d u r i n g the o x i d a t i o n o f a porous l a y e r i n b o r i c a c i d the pores become f i l l e d w i t h o x i d e . These a u t h o r s assumed \ d t /b I d t / b - r a t e o f change o f v o l t a g e oi = — • d u r i n g t h e o x i d a t i o n o f a / dv* bare p l a t e i n b o r i c a c i d I d t Jo Vdt/p - r a t e o f change o f v o l t a g e d u r i n g t h e o x i d a t i o n o f a porous l a y e r i n b o r i c a c i d I n Table I V a r e l i s t e d v a l u e s o f oi o b t a i n e d from d a t a g i v e n by Dekker and van G e e l ( 1 0 ) , and i n F i g . ( 2 1 ) t h e i r oi a l o n g w i t h the p o r o s i t i e s l i s t e d i n Table. I a r e p l o t t e d a g a i n s t current d e n s i t y . Agreement between the two sets of p o r o s i t i e s i s good. Table 17 P o r o s i t y as a f u n c t i o n of current d e n s i t y from data obtained by Dekker and van Geel. The authors do not st a t e what concen t r a t i o n of o x a l i c a c i d they used f o r t h e i r experiments. ( d t ) b (dt)p rate of change of voltage f o r base i-ma/cm2 ( d t j j 3 - v/minj /dv \dt> 'by£dv* = ol Idt/p A l plate -15.5 v/min. 1.21 46.0 .34 2.22 54.0 .29 rate of change of 3.04 ' 57.8 . 2 7 voltage i n boric 6 . 0 5 63.0 .25 acid for the 9.10 64.2 .24 porous layer. 13.4 64.2 .24 I f oi = (dt)b/dv\ i t must be concluded that oxide / l d t / p growth takes place only at the o x i d e - e l e c t r o l y t e boundary, i n d i r e c t o p p o s i t i o n to Anderson's theory. The d i f f i c u l t y i s removed upon the c o n s i d e r a t i o n of experimental technique. Dekker and van Geel washed the porous l a y e r s and d r i e d them f o r 15 minutes at 450°C before o x i d i z i n g them i n b o r i c acid.y-AlgO^ forms <K - A l 2 0 j . H 20 when heated w i t h water ( 4 ) . Therefore, i n the experiments of Dekker and van Geel the pores were coated w i t h the mono-hydrate which allowed Al?+ to migrate to the e l e c t r o l y t e but which prevented the m i g r a t i o n of O2"* inwards. 24. P o r o s i t i e s c a l c u l a t e d from the data of Tables I I and I I I probably have l i t t l e meaning. I t can be s a i d , however, that at low current d e n s i t i e s porous l a y e r s formed i n HgSO^ . have a very l a r g e p o r o s i t y , probably about 80%. Thickness measurements on the l a y e r s formed at 1 and 2 ma/cm2 were u n s a t i s f a c t o r y because the oxide was so porous i t crumbled while being mounted f o r observation. The preceding experiments s u b s t a n t i a t e Anderson's theory of oxide growth although h i s p i c t u r e of the formation of porous l a y e r s i s not a p p l i c a b l e . The suggestion by Dekker and Urquhart (12) that the thickness of the base l a y e r i s l i m i t e d by a change from I o n i c to e l e c t r o n i c conduction i s discounted by the high e f f i c i e n c i e s obtained i n these exper-iments. The f o l l o w i n g p i c t u r e of the formation of porous l a y e r s f i t s a l l the a v a i l a b l e data. At f i r s t the oxide l a y e r grows at a "normal" r a t e I.e. a l l the current converted to oxide, and a c i d a t t a c k i s neg-l i g i b l e , s o : t h e voltage i s i n c r e a s i n g r a p i d l y w i t h time. A f t e r about a minute, a c i d a t t a c k i s i n i t i a t e d at a number of p o i n t s on the surface of t h i s b asic l a y e r and holes are r a p i d l y eaten i n t o the oxide. A c i d a t t a c k depends on current d e n s i t y , prob-ably due to l o c a l heating of the e l e c t r o l y t e . The v o l t a g e l e v e l s o f f , and f o r high current d e n s i t i e s drops, since the holes tend to short c i r c u i t the base l a y e r . At f i r s t the pores grow both wider and deeper, but a f t e r they have reached a c r i t ^ -i c a l depth most of the current w i l l flow through the pore bottoms causing them to grow deeper, but not.wider. A s t a t i o n a r y s t a t e i s reached when the current through the pores i s j u s t l a r g e enough-to cause r a t e of growth and a c i d a t t a c k to be i n e q u i l i b r i u m . The l a r g e r the t o t a l current the smaller must be the pores, and hence t o t a l pore area, so that I may be l a r g e enough to cause a c i d a t t a c k to balance oxide growth. ( 2 (3 ( 4 (5 ( 6 (7 ( 8 (9 ( 1 0 ( 1 1 BIBLIOGRAPHY ( 1 ) P.D. Lomer, P r o c Phys. Soc. Lond B, 6 £ , 8 1 8 , 1 9 5 0 G. Hass, J . Opt. Soc. Amer. ^9_, 5 3 2 , 1 9 4 9 I.J.W. Verwey, F. K r i s t a l l o g r (A) 21, 3 1 7 , 1 9 3 5 J.D. Edwards, F. K e l l e r , Trans. Electrochem Soc.22., 1 3 5 , 1 9 4 1 A. Gttntherschulze and H. Betz, Z. Physik, 1 0 0 , 5 3 9 , 1 9 3 6 K. Guminski, B u l l . Acad. Polon. Sc. L e t . , 5 - 4 , 145, 1 9 3 6 A.J. Dekker and H.M.A. TJrquhart, J . of App.Phys. 2 1 _ , 7 0 8 , 1 9 5 0 K . Huber, J . C o l l o i d . S c i . 1, 1 9 7 , 1 9 4 8 Th. Hummel,' Z. Physik 9_9_, 5 1 8 , 1 9 3 6 A.J. Dekker and Yf.Ch.van Geel, P h i l i p s Reo.Rep. 2 , 3 1 3 , 1 9 4 ? R.L. B u r w e l l , P.A.' Smudski, and T.P. May, J.Chem.Soc. 69_, 1 5 2 5 , 1 9 4 7 (12) A.J. Kekker, H.M.A. TJrquhart, Can.J.Res. B 28, 541, 1 9 5 0 ( 1 3 ) R.L. Bu r w e l l , T.P.May, J . Electrochem Soc., 9_4, 1 9 5 , 1 9 4 8 (14) K. Guminski, B u l l . I n t . Acad. Polon. Sc. L e t . , A, 7-10,133,1947 (15) S. Yoshida, J.Phys.Soc. Japan, 1 7 5 , 1 9 4 8 ( 1 6 ) S.J.W. Verwey, Physlca 2 , 1 0 5 9 , 1 9 3 5 ( 1 7 ) N.F. Mott',-Trans. Faraday Soc. £6, 472, 1 9 4 0 (18) S. Anderson, J.App. Phys. 1 £ , 477, 1944 ( 1 9 ) Report on Methods of Testing.Oxide Coatings on A l , Appendix I I , Report of Committee B17 Proc. Amer. Soc. Test. Mat., vol.37, I , 261, 1 9 3 7 (20) W.G. Burgers, A. Cloassen, J. Fer n i k e , Z. Physik 22, 5 9 3 , 1 9 3 2 

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