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A technique for producing large single crystals of lead telluride Cannon, G. Harry (George Harry) 1954

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A TECHNIQUE FOR PRODUCING LARGE SINGLE CRYSTALS OF LEAD TELLURIDE by George Harry Cannon A Thesis Submitted in Partial Fulfilment of the Requirements for the Degree of MASTER OF SCIENCE in the Department of Physics We accept this thesis as oonforming to the standard required from candidates for the degree of MASTER OF SCIENCE Members of th® Department of Physics THE UNIVERSITY OF BRITISH COLUMBIA October, 1954 A b s t r a c t The s t e p s t o be t a k e n i n d e v e l o p i n g a t e c h n i q u e o f g r o w i n g s i n g l e c r y s t a l s a r e o u t l i n e d . Two t h e o r i e s on c r y s t a l g r o w t h a r e l i g h t l y t r e a t e d . The r e c o r d o f t h e e x p e r i m e n t a l w o r k done i n c l u d e s t h e f o l l o w i n g . i The d e v e l o p s i e n t o f a n a x i a l t e m p e r a t u r e g r a d i e n t i n a f u r n a c e t o o p e r a t e i n t h e t e m p e r a t u r e r a n g e s 6 0 0 ° C t o 1 4 0 0 ° C i s d i s c u s s e d . A c o n t r o l l e r t o c o n t r o l t h e t e m p e r a t u r e o f t h i s f u r n a c e t o ± 1 $ ? i s d e v e l o p e d . A r e c o r d i n g u n i t t o r e c o r d t h e t e m p e r a t u r e v a r i a -t i o n s w i t h t i m e ( o r w i t h p o s i t i o n i n t h e f u r n a c e ) i s i n c o r p o r a t e d . The c o n s i d e r a t i o n s f o r p r e p a r i n g a n d t e s t i n g p u r e PbTe a r e d i s c u s s e d . T h i s i n c l u d e a t a b l e o f t h e s p e c t r a l l i n e s o f t e l l u r i u m , c o m p i l e d f r o m t h e I - l . I . T . T a b l e o f W a v e l e n g t h s . A t e c h n i q u e f o r p r o d u c i n g a s i n g l e c r y s t a l i s o u t l i n The a t t e m p t s t o p r o d u c e a s i n g l e c r y s t a l o f PbTe a r e d e s c r i b e d . The d e g r e e s o f s u c c e s s a n d f a i l u r e a r e d i s c u s s e d . A c k n o w l e d g e m e n t s I w o u l d l i k e t o t e n d e r my t h a n k s t o t h e f o l l o w i n g , w i t h o u t whose h e l p s u c c e s s i n t h i s u n d e r t a k i n g w o u l d n o t h a v e b e e n p o s s i b l e . The D e f e n c e R e s e a r c h B o a r d o f C a n a d a w h i c h h a s f i n a n c e d t h e p r o j e c t a n d h a s e m p l o y e d t h e a u t h o r f o r t h e summer o f 1 9 5 3 . The D e p a r t m e n t o f P h y s i c s and t h e 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 f o r e n c o u r a g e m e n t i n s t a r t i n g t h e u n d e r t a k i n g and f o r e m p l o y i n g t h e a u t h o r for t h e summer o f 1 9 5 4 . D r . A . I I . C r o o k e r f o r h i s c o n t i n u a l e n c o u r a g e m e n t a n d w h o l e h e a r t e d c o o p e r a t i o n i n o v e r c o m i n g t h e many d i f f i c u l t i e s d u r i n g t h e w o r k . H r . J o h n L e e s f o r h i s u n s e l f i s h n e s s i n g i v i n g o f h i s own t i n e t o a i d t h e a u t h o r w i t h t h e g l a s s x v o r k . Table of Contents Chapter I Chapter II Chapter III Chapter IV Chapter V Introduction to the Problem Theory Experimental Considerations A. General B. Equipment ( i ) Furnace ( i i ) Control ( i i i ) Recording C. Materials ( i ) General ( i i ) Preparation ( i i i ) Testing D. Technique ( i ) General ( i i ) Production Runs Results Discussions A TECHNIQUE FOR PRODUCING LARGE SINGLE CRYSTALS OF LEAD TELLUR3DE Chapter I. Introduction Research in fields of physics such as spectroscopy, elec-tronics, etc. often needs large crystals free from flaws with special physical properties for making observations and measure-ments. In some cases these crystals may be obtained from nature but more often they must be prepared a r t i f i c i a l l y in the labora-tory either because of short supply of the former or because of special properties found only in the l a t t e r . In the case of large single crystals of lead telluride they must be prepared a r t i f i c i a l l y . Crystals of lead telluride are of interest because of their semi-conductor properties and their photo-electric response in the infra-red region. In order that observation and measurement of physical characteristics of lead telluride may be made without the complications caused by crystal boundaries and interfaces, and also be made on a macro-scopic scale, a large single crystal is needed. To develop a technique for the production of large single crystals was the problem undertaken in the work reported on in this thesis. Laboratory techniques for growing crystals are many and varied (1) depending on the substance from which the growth must -tic take place. These methods include growth^ of crystals from - 2 -solution, solid state, and from the melt, etc.. The method which is selected for making the lead telluride crystals i s the last one mentioned; i.e. from the melt. Again, the melt method of producing crystals has been per-formed with many techniques, each one successful for the material used. For producing lead telluride crystals i t was decided to use the Stockbarger method having two furnaces in cascade at different temperatures with a sharp temperature gradient separ-ating them. By lowering a boule through this static temperative program at the correct rate i t should be possible to produce a single crystal throughout the melt. The problem then resolves i t s e l f into three parts: (1) To produce; (i) a controlled furnace with the proper static;, temperature program along i t s axis, , ( i i ) a controller which w i l l maintain the temperature program with a tolerance set down later in this thesis, ( i i i ) a lowering mechanism and stand to permit lowering of the boule through the temperature program, (iv) a recording device to measure and record the temperature program in the furnace. - 3 -(2) To prepare; (i) the raw materials used in the process, ( i i ) a method of checking for purity and, i f necessary, a method of producing the materials in the laboratory. (3) To develop a technique for growing the crystals. The following pages w i l l report on what steps have been taken; what results have been achieved; and what is suggested for further investigation. - 4 -C h a p t e r I I T h e o r y Two t h e o r i e s o f c r y s t a l g r o w t h h a v e b e e n p r o p o u n d e d . O n e , on t h e r m o d y n a m i c a l g r o u n d s i s b y G i b b s , C u r i e , tfulff a n d o t h e r s . The o t h e r , an a t o n i c t h e o r y o f c r y s t a l g r o w t h , h a s b e e n d e v e l o p e d b y V o l m e r , K o s s e l , S t r a n s k i a n d o t h e r s . T h i s l a t t e r c o n s i d e r s t h e g r o w t h o f an i d e a l l y p e r f e c t c r y s t a l . I n g e n e r a l , a c r y s t a l m u s t h a v e i t s s t a r t f r o m a s m a l l b e g i n n i n g . I n c o n s i d e r a t i o n o f t h e e a r l y t h e o r y G i b b s made u s e o f t h e a n a l o g y o f l i q u i d d r o p s a n d a p p l i e d t h e c o n d i t i o n s g o v e r n -i n g t h e g r o w t h o f w a t e r d r o p l e t s i n a m i s t t o t h e g r o w t h o f a c r y s t a l . The f r e e e n e r g y w h i c h r e s i d e s on t h e s u r f a c e s e p a r a t i n g t h e two p h a s e s r e t a r d s t h e f o r m a t i o n o f t h e s e c o n d p h a s e i n s i d e t h e f i r s t . The c o n d i t i o n f o r t h e s t a b i l i t y o f a n i s o l a t e d d r o p o f a f l u i d i s t h a t i t s s u r f a c e f r e e e n e r g y , a n d h e n c e i t s a r e a , i s a m i n i m u m . I n a s i m i l a r way f o r a c r y s t a l i n e q u i l i b r i u m w i t h i t s s u r r o u n d i n g s a t c o n s t a n t t e m p e r a t u r e a n d p r e s s u r e , t h i s c o n d i t i o n i m p l i e s t h a t t h e G i b b s f r e e e n e r g y w i l l be a m i n i m u m f o r a g i v e n v o l u m e . T a k i n g t h e v o l u m e f r e e e n e r g y p e r u n i t v o l u m e a s a c o n s t a n t t h r o u g h o u t t h e c r y s t a l , t h e c o n d i t i o n b e c o m e s n \ Q i F i = m i n i m u m (1) 1 w h e r e ^ i s t h e s u r f a c e f r e e e n e r g y p e r u n i t a r e a o f t h e i t h f a c e o f a r e a F_. on a c r y s t a l b o u n d e d b y n f a c e s . T h u s t h o s e f a c e s w i l l d e v e l o p w h i c h l e a d t o a m i n i m u m t o t a l s u r f a c e f r e e e n e r g y f o r a g i v e n v o l u m e . W u l f f e x t e n d e d t h i s t h e o r y b y s h o w i n g t h e r e l a t i o n b e t w e e n t h e G i b b s e q u i l i b r i u m s h a p e o f a c r y s t a l a n d t h e r e l a t i v e s u r f a c e f r e e e n e r g i e s o f t h e f a c e s . C o n s i d e r i n g a p o i n t P w i t h i n t h e c r y s t a l , l e t p ^ be t h e p e r p e n d i c u l a r f r o m P on t h e i t h f a c e o f a r e a . T h e n t h e t o t a l v o l u m e V o f t h e c r y s t a l w i l l be e q u a l t o t h e v o l u m e s o f t h e p y r a m i d s w i t h P a s v e r t e x a n d t h e f a c e s a s b a s e s . n (2) T h e r e f o r e V = 1 r — p . p . a n d t h e t o t a l f r e e e n e r g y i s ( 3 ) E = > (J—. F . " T T 1 1 Now i f one s e t o f f a c e s g r o w s a t t h e e x p e n s e o f a n o t h e r t h e c h a n g e i n v o l u m e dV may be x v r i t t e n a s dV = £ ^ F . d P . (4) 1 a n d f r o m (2) n (5) dV = 1 ? ( F - d p . p . d F . ) 3" L— l * i * i i I f we k e e p t h e v o l u m e c o n s t a n t d f B 0 a n d we h a v e f r o m (4) a n d (5) p . d F . o 0 ( 6 ) t l l - 6 -F r o m (1) if-, t h e t o t a l f r e e e n e r g y i s c o n s t a n t we h a v e b y n e g l e c t i n g a n y v a r i a t i o n o f <J w i t h F n ( 7 ) T h u s f r o m e q u a t i o n s ( 6 ) and ( 7 ) t h e f a c e s f o r w h i c h i t i s p o s s i b l e t o v a r y F n- i n d e p e n d e n t l y m u s t be s o s i t u a t e d t h a t P i ^ G c r r ( 8 ) T h e r e f o r e , t h e c r y s t a l s h o u l d f o r m a p o l y h e d r o n s u c h t h a t t h e p e r p e n d i c u l a r d i s t a n c e s f r o m a p o i n t P , w i t h i n t h e c r y s t a l , a r e p r o p o r t i o n a l t o t h e s p e c i f i c f r e e e n e r g i e s o f t h e a p p r o p r i a t e f a c e s . T h i s p i c t u r e i m p l i e s t h a t on a c r y s t a l t h e v e l o c i t i e s o f g r o w t h o f d i f f e r e n t f a c e s i n t h e d i r e c t i o n s o f t h e n o r m a l s a r e p r o p o r t i o n a l t o t h e a p p r o p r i a t e s p e c i f i c s u r f a c e f r e e e n e r g i e s . T h i s i s n o t t r u e f o r a n y f i n i t e r a t e o f g r o w t h o f t h e c r y s t a l f o r t h e G i b b s c r i t e r i o n i s a p p l i c a b l e o n l y t o a c r y s t a l i n e q u i l i b r i u m w i t h i t s s u r r o u n d i n g s . I n c o n s i d e r i n g t h e l a t t e r t h e o r y a n i d e a l c r y s t a l w i l l o n l y g r o w i f t h e r e i s a s t e p a l o n g w h i c h g r o w t h c a n t a k e p l a c e . I f we h a v e a s o l i d , m e l t i n t e r f a c e we c a n c o n s i d e r t h a t m o l e c u l e s a r e l e a v i n g a n d e n t e r i n g t h e m e l t c o n t i n u a l l y due t o t h e r m a l a g i t a t i o n . I f t h e e n e r g y o f e v a p o r a t i o n i s g r e a t e r t h a n t h e b i n d i n g e n e r g y o f t h e s o l i d p l u s t h e e n e r g y o f r e - e n t r y , t h e c r y s t a l w i l l m e l t . B u t i f on t h e o t h e r h a n d , t h e e n e r g y o f e v a p o r a t i o n (due t o t e m p e r a t u r e ) i s l e s s t h a n t h e b i n d i n g e n e r g y p l u s t h e r e - e n t r y e n e r g y , t h e c r y s t a l w i l l g r o w . C o n s i d e r t h e - 7 -b i n d i n g energy to be a f u n c t i o n of area; then i f two or three f a c e s are being a t t r a c t e d by adjacent molecules, the b i n d i n g energy w i l l be g r e a t e r than with only one f a c e being a t t r a c t e d . So that i f the growth i s by the advancing of a step, the b i n d i n g e n e r g i e s along the step are g r e a t e r than on a c r y s t a l f a c e o n l y . Thus growth i s more l i k e l y along a s t e p than on a face o n l y . The energy of r e - e n t r y i s due to the degree of s u p e r c o o l i n g . Thus we see that f o r c r y s t a l growth there should be a s t e p formed on the face as a seed and a supercooled s o l u t i o n f o r the mother l i q u o r . We should next c o n s i d e r the p r o d u c t i o n of the seeds or more commonly, n u c l e a t i o n . Ey s u p e r c o o l i n g , a molecular l a y e r can be d e p o s i t e d upon which growth nay take p l a c e . The s i z e of t h i s m o lecular l a y e r depends on the degree of s u p e r c o o l i n g . I f t h i s m o lecular i s l a n d i s l a r g e i t w i l l grow f a s t e r than a s m a l l i s l a n d . I f the given volume i n which growth may take p l a c e i s s m a l l the l a r g e nucleus w i l l use up the a v a i l a b l e m a t e r i a l s t a r v i n g out the small n u c l e i . In t h i s manner, then, a l a r g e degree of super-c o o l i n g i n a s m a l l volume should produce a s i n g l e l a r g e nucleus that may grow. A l l other s m a l l n u c l e i ! w i l l disappear and the r e s u l t w i l l be a s i n g l e c r y s t a l . Thus, i f we can produce a l a r g e degree of s u p e r c o o l i n g i n a s n a i l volume of a melt, a s i n g l e c r y s t a l w i l l grow. I f , having s t a r t e d the c r y s t a l , we move the r e s t of the melt i n t o the r e g i o n f o r s u p e r c o o l i n g at a r a t e not f a s t e r than the growth r a t e of the c r y s t a l , the s i n g l e c r y s t a l w i l l grow throughout the whole melt. - 8 -This single nucleation then depends on the two conditions, f i r s t that the degree of supercooling reaches at least to some minimum value and second, that the volume i n which c r y s t a l growth i s taking place i s smaller than some maximum. Thus, i n theory i t w i l l be possible to grow a single c r y s t a l throughout the whole melt i f the nucleation requirements of a large supercooling i n a very small volume can be met. Chapter III 1 Experimental A. General j The Stockbarger (2) method of growing crystals has I been successfully used for growing many types of single crystals. In this method a static temperature program is set up on a vertical axis through two furnaces, one above the other. The upper section of the program (in the top furnace) i s set about 50°C above the melting point of the material being used. The lower section (in the bottom furnace) i s about 50°C below the melting point. The temperature gradient between the two sections should be about 100°C in one inch. The temperature in the lower section should be reasonably constant for several inches of length along the axis. The time variation of <r°c temperature should be better than (£3^ over a thirty minute period or longer. When a suitable program i s developed a crucible i s lowered through the freezing point after the contents: has been well melted in the upper furnace. To start a single crystal growing the amount of supercooling i s ad-justed by setting the temperature of the plateau in the lower furnace to the number of degrees below the freezing point to produce a single nucleus. The following are the steps taken in attempting to -10 -f u l f i l the requirements stated above. B. Equipment. (i) FURNACE The f i r s t furnace used was fourteen inches in diameter and fourteen inches high, outside dimensions. It consisted of an alundum core one and one half inches inside diameter and one and three quarters inches outside diameter around which was wound a platinum heating element. This was surrounded with a bulk insulation made by Johns-Manville called Sil-0-Cel C - 3 which was contained in a cylinder of sixteen guage galvanized iron. The bottom of the cylinder was a piece of quarter inch transite, fourteen inches outside dia-meter and two inches inside diameter. The top of the Sil-0-Cel was surmounted by a layer of fire-brick sawed to form a seg-ment disc and a disc of quarter inch transite with a five inch hole in the centre. A plug of porous f i r e brick closed the top of the alundum core and a shaped f i r e brick f i t t e d into the brick and transite superstructure. Each of these plugs was bored to permit passage of a nickel lowering wire. The heating element consisted of number twenty two Brown and Sharpe guage platinum wire wound two and one quarter turns per inch for the top seven inches and two turns per inch for the bottom seven inches at the point between the two windings a c h i l l ring was placed. This consisted of half inch thick Alpha brass twelve inches in diameter with a two and one half inch centre hole. The space between the c h i l l ring and 11 -and the furnace core i s f i l l e d with a c o i l of nichrome wire. Opposite the c h i l l ring and inside the furnace is a s i l i c a diaphragm one quarter inch thick, one and one half inches outside diameter, and five eighths inch inside, diameter which separates the two sections of the furnace. Directly below the s i l i c a diaphragm the controller thermo-couple (Pt - Pt 10% RL) i s inserted. The bottom of the furnace tube was blocked by a piece of fine brick. The temperature program was charted by lowering a thermocouple down through the furnace, measuring the T - C output on a Rubicon potentiometer, and converting this to temperature. The program shown on graph 1 with unbroken line was measured for the furnace described above. This program was not considered satisfactory; f i r s t , because the gradient was not steep enough; second, because the temperature in the upper chamber was not pro-perly distributed; and f i n a l l y , because the plateau in the lower furnace was not long enough. Coupled with the above three objections the time variation in temperature at any one pint was too large, as shown by the spread indicated on the curve. To reach a proper program i t was intended to attempt to correct the temperature traverse to follow the dotted line on graph 1. - 12 -F i r s t a s i l i c a tube was. inserted into the lower furnace to increase the heat content. In this way i t i s intended to reduce the time variations of tempera-ture to a minimum. Second, a stainless steel tube was i n -serted in the upper furnace. This was also to increase the heat content of the upper furnace for better temperature con-t r o l . But more than that, i t was intended to cause a re-generation in the furnace by conducting some of the heat downwards. Finally by blocking the bottom of the furnace to reduce updrafts, thermal currents were reduced. The result of these changes i s shown in graph 2. F i r s t , the plateau in the lower furnace has been exten-ded to an acceptable minimum length. Then the gradient has been steepened to 85°C in one inch and the maximum tempera-ture in the upper furnace has been reduced by 40°C, resulting in a better temperature distribution in the upper furnace. And f i n a l l y , the time variations of temperature have been reduced from at lOOOOC to at 1000°C. This new program was considered to be just acceptable. To improve on this program i t was considered necessary to rewind the heating c o i l which would require new platinum wire since the present material was baked on with alundum cement and could not be reclaimed as wire. It was decided at this stage to make an attempt - 13 -to produce a crystal. This attempt w i l l not be described further at this point but w i l l come under further sections of this chapter, such as "materials" and "technique". But, i t i s sufficient to say that the attempt to produce a cry-stal was delayed by the furnace element open circuiting. This was due to an expansion of the alundum core when the stainless steel insert corroded at the high temperatures to which i t was subjected. An oxide layer was deposited between the tube and the core which produced the pressure to break the alundum core. Thus, a second furnace was b u i l t . It was intended to use the experience of the f i r s t furnace to improve the program to something approximating the dotted curve on graph 1. To do this, no extreme changes in design were requied, except to change the distribution of the heating c o i l to introduce the power into the furnace in a more favorable fashion. This change was not calculated, but a judicious guess from inspecting graphs 1 and 2 suggested that more power was needed from the nine inch mark to the fourteen inch mark and from the four inch mark to the seven inch mark. To do this the new winding was distributed on the core such that at the bottom of the c o i l i t was two turns per inch reducing gradually to one and three quarter turns - 14 -per inch at the seven inch mark. Then the c o i l increased to two and three quarters turns per inch reducing gradually to two and one quarter turns per inch at the top. At the seven inch nark a space of three quarters of an inch was l e f t on the core without winding on i t . At this position the c h i l l ring was to be placed and the controller thermo-couple inserted. The platinum was not baked on with alun-dum cement this time but was tacked to the core at several places with Saureisen porcelain cement. The core used to support the winding was again made of RA 98 Norton Alundum but this time i t had an inside diameter of one and one half inches and an outside diameter of two inches with a length of fourteen inches. The furnace was now reassembled as before. In addition the lower furnace had inserted in i t , in place of the s i l i c a tube, f i r s t a piece of alundum tubing then a piece of monel tubing. This was a l l held up in place by a silicon plug that closed completely the bottom of the tube producing, essentially, a dead a i r space in the furnace. In the upper furnace the same inserts as in the lower furnace were made. These inserts were to both increase the heat content of the furnace and provide regen-eration in the furnace and thus a more even temperature distribution. The new temperature program is recorded now on - 16 -graph 3 . It is seen f i r s t that in the lower furnace there is a plateau at least three inches long, of a con-stant temperature. This w i l l be the annealing section of the furnace. The gradient i s approximately one hundred degrees in one inch. For about four inches in the upper furnace the variation of temperature i s approximately thirty degrees, or essentially constant for the melting section of the furnace. The large drop in temperature in the f i r s t three inches and the last two inches i s an effect which could have been compensated for to some ex-tent with a wiser choice of taper on the heater windings. The time variation of temperature has also been reduced with the increase in heat content of the furnace by adding the inserts and increasing the thickness of the alundum core. The measured differences in temperature at nine hundred degrees i s now t- 1.5°C over a period of six minutes for the cycle. The problem of improving this w i l l be discussed under controls in the next section. A Dexion rack was built to hold the furnace and ancillary equipment such as the dropping mechanism. The rack was mounted on rubber casters to reduce shock. The dropping mechanism was made from an electric clock. It was mounted on a flange which was attached to a move-able plate. This plate was mounted on another moveable plate that could be moved at right angles to the f i r s t and also locked. The whole was mounted on the top of the Dexion rack. (See photograph 1). Fi r s t , the whole mechanism could be moved to give a rough positional adjustment. Then a fine adjustment can be made with the moving of the two plates at right angles such that any position in the ^"Yplane may be selected. In this way the crucible can be positioned exactly in the furnace to drop axially down the furnace without having contact with the sides. This again i s to reduce mechanical shock to a minimum, ( i i ) CONTROL To control the furnace temperature two essen-t i a l s are necessary. F i r s t , for the static program the temperature must be able to be held constant. The time variation in temperature about the median should be less than -©35^ over a period of a half hour or longer. Second, the temperature must be able to be reduced at a controlled rate for annealing after the crystal i s made. To do both of these a Wheelco Potentiotrol, which was available, was used. The essential principle of this control i s electronic in nature. The output of a Pt - Pt 10$ Rh thermocouple which is inserted in the furnace is applied to a moving c o i l galvanometer. (See diagram 1). A metal - 11 -f l a g i s fastened to the c o i l and swings so that i t passes through the pickup coils, varying the inductance. This causes variations to an electronic tube (presumably a D.C. amplifier) which in turn operates a relay which i s cascaded with a micro switch, and this operates the on - off relay of the furnace. As the temperature increases, the output of the thermocouple increases, swinging the metal f l a g . When the preset temperature and the temperature of the thermo-couple coincide the flag varies the coupling so that the tube operates the relays cutting off the current. When the temperature of the thermocouple i s reduced due to radiation and conduction the flag swings out from the pickup coils varying the inductance, operating the electronic tube, actuating the relays and turning on the current in the furnace heaters. The sensitivity of this control i s sufficient for controlling the furnace to the specifications l a i d down but for the in e r t i a l effect. This is to say, the temperature of the furnace tends to swing past the limits set by the controller causing a larger variation in temper-ature than that at which the controller reacts. This i s due when heating to the temperature difference between the element and the furnace averaging out as a function of their heat capacities. When cooling this i s due to a f i n i t e heating time being necessary for the element and the surrounding volume. - 18 -This variation in temperature was evident on the f i r s t boule when i t resulted in rings being made on the outside of the boule (see photograph 1). To correct for this the following procedure was adopted. A rheostat was placed across the points of the current controlling relay. Then, with the furnace temperature at the operating point, the Variac which controls the current across the heating c o i l is reduced so that the heater w i l l not quite increase the tempera-ture of the furnace. The Variac then i s adjusted so that the current is about one half ampere above this point. Then the rheostat i s adjusted so that the current is one half ampere below this point with the relay open. This w i l l give a control of the temperature to within 1°C at 90C-OC. This variation follows the cycle of one and one half minutes at high current with thirty to forty minutes at low current, depending on the temperature of the room which varies throughout the night as the heat was turned off in the building. In the i n i t i a l heating of the furnace, due to the temperature coefficient of resistance of platinum, the voltage across the furnace must be kept low. But as the temperature of the furnace increases and thus the re-sistance increases i t i s necessary to raise the voltage so that the power input to the furnace is sufficient to keep the temperature increasing. This is accomplished by having - 19 -a Variac across the input, thus the voltage across the coils can be adjusted. The maximum current in the coils should not exceed ten amperes. Precautions must be taken in the event of the furnace being up to temperature and the power f a i l i n g , to ensure that the current does not exceed ten amperes i f the furnace has cooled when the power comes back on. To do this a circuit (see diagram 2) using two relays was made so that one relay held the other relay in while the current was on. But on a power failure the relays dropped out and must be reset by hand before the power w i l l come back on. If the power has been off for too long a period, the Variac can be readjusted to keep the current below 10 amperes. The disadvantage of this circuit was that, when drop in the line voltage for a few seconds took place, the furnace would be unnecessarily shut down. To avoid this d i f f i c u l t y a new circuit (see diagram 3) was devised using an amperite delay switch. The amperite thermal delay switch took approximately thirty seconds to close and about the same time to open. The circuit was devised so that the manual switch across the points of the thermal delay when placed at START would close a l l relays. When the thermal switch closed this wculd act as a holding device for the others. Now the manual switch should be placed at RUN. If the power failed and came back on within thirty seconds, the thermal delay would not have opened so that whole circuit xtfould s t i l l be operative. In this time the furnace would not have cooled sufficiently to cause a large increase in current. But i f the power fai l e d and stayed off for more than thirty seconds, the thermal delay would open, open circuiting the unit and closing down the furnace. Since several power failures were experienced during this work i t i s suggested that a further addition be made to this control unit. If the power f a i l s i t would be advantageous'to automatically change over to an auxiliary supply such as the exisiting D.C. supply in the building. The suggested circuit is shown on diagram 4. A start has been made on this change but has not been completed at this writing. The second essential for the control was to be able to reduce the temperature at some predetermined rate. Again, the Wheelco Potentiotrol w i l l do this. The setting of the temperature at which this unit., w i l l control is done by moving an arm through an arc of several inches. This arm can be locked in any desired position or can ride on a cam which is driven by a small synchronous motor. The radius of the cam at any point determines the tempera-ture at which the control operates for that setting. The motor is geared down so that the cam makes one revolution every twenty four hours. - 21 -The cans are made from sixteenth inch thick aluminum. A one inch hole i s made in the centre of the cam and a key slot i s f i l e d in the edge of the hole. The cooling program is scribed onto the plate and then the cam is cut out. The edge of the cam is then f i l e d smooth. To scribe a cooling program onto the cam two methods were used. F i r s t , a piece of graph paper was glued to the aluminum and the curve was laid out on the graph paper. Then the resulting curve was cut out on a band saw. Since the rate of cooling was continuous i t was found more advantageous to make a j i g out of brass to which the aluminum plate i s clamped. A post i s placed in the centre which has a circumference equal to the differences in rad i i of the start and finish of the cam. Around this post i s wound a piece of fine wire to which a stylus has been attached. The length of the wire is adjusted to have the stylus at the largest radius on the rotation around the post and the shortening of the wire due to being wound on the post des-cribes a spiral of constant reduction in radius. This gives the type of cooling curve used in the production of the crystals. No cooling curve faster than the normal rate with the power turned off is possible. This i s the one limit on choice of cooling programs. With any other cooling program i t is necessary to reduce the current in the heating c o i l (by lowering the Variac) as the temperature i s reduced so as to keep the duty cycle approximately 50%. This is also necessary to maintain the current through the c o i l below the ten ampere maximum, ( i i i ) RECORDING EQUIPMENT Measurement of the temperature programs in the furnace was made by dropping a thermocouple through the furnace in place of the crucible. The thermocouples used were either type S (Pt - Pt 10% Rh) or Type R (Pt - Pt 13% Rh). The measurement of the potentials generated was either by a Rubicon or Leeds Northrup potentiometer. The thermocouples were lowered one inch at a time and the po-tential measurement was taken. The f i r s t programs were measured in this way-and plotted on a graph. In order that the programs could be measured in a way which would more approximate the actual conditions a dynamic method was devised. A Brown recording potentiometer was con-verted so that the chart drive was four and one half inches per hour and the sensitivity was f u l l scale de-flection in 3 seconds. The scale on this potentiometer was 0 - 1 0 mv. This was used in conjunction with a type S thermocouple which would measure 0° - 1040° C - 23 -for 0 - 10 av range. The thermocouple was suspended from a wheel which was rotated by means of the dropping mechanism. The rate of dropping of the thermocouple was one inch per hour. A l l the latter programs were measured and recorded in this manner. Then the information was transferred to the graph paper. It was intended to replace the suspension wire for the crucible by a tube of nickel so that a thermocouple core could be inserted and the thermocouple dropped coincident with the crucible. In this way a more exact record of the program the crucible saw could be made. At the time of this writing this has not been done. C. Materials. (i) GENERAL One of the main problems in growing crystals is obtaining the proper materials from which the crystal must grow. The considerations i s , in most cases, one of purity of the material. If i t is intentional to add a slight trace of impurity i t is a problem to make the material homogeneous. In the case of lead telluride purity i s what we are concerned with. Prepared lead telluride was purchased along with tellurium metal and lead. As a preliminary check a spectro-gram was taken comparing the lead telluride against the lead and tellurium (see photograph2.). Prom this spectrogram i t is evident that the prepared lead telluride contains impurities. So i t was decided to produce the lead telluride in the laboratory. - 2 4 -( i i ) P R E P A R A T I O N To do t h i s s t o i c h i o m e t r i c p r o p o r t i o n s o f l e a d a n d t e l l u r i u m a r e m i x e d i n a c l e a n q u a r t z g l a s s t u b e . The t u b e a n d c o n t e n t s a r e e v a c u a t e d . A f l a m e i s p l a c e d a t t h e e n d o f t h e t u b e h e a t i n g t h e m i x t u r e t o s t a r t t h e r e a c t i o n . The r e -a c t i o n i s e x o t h e r m i c a n d u p o n b e i n g s t a r t e d t r a v e l s t h r o u g h t h e w h o l e m i x t u r e . P r e c a u t i o n mus t be t a k e n t o k e e p a l l c o n t a m i n -a t i o n o u t o f t h e p r o c e s s i f h i g h p u r i t y i s t o be g a i n e d . A l l g l a s s w a r e m u s t be w e l l c l e a n e d . The c l e a n i n g p r o c e s s w h i c h was f i n a l l y u s e d was s u c c e s s i v e i m m e r s i o n s i n n i t r i c a c i d , s o d i u m h y d r o x i d e , s u l p h u r i c a c i d a n d t h e n a b o i l i n g i n c h r o n i c a c i d i n a fume c l o s e t . E a c h s t e p was f o l l o w e d b y a w a s h i n g i n d i s t i l l e d w a t e r . The t a b l e on w h i c h t h e p r e p a r a t i o n was made was c o v e r e d w i t h a g l a s s t o p w h i c h a l s o was w e l l c l e a n e d . G l a s s t o n g s f o r h a n d l i n g t h e m a t e r i a l w e r e made b y t h e g l a s s b l o w e r . The f i r s t p r o d u c t o f t h e l a b o r a t o r y was n o t s u b j e c t t o a l l t h e p r e c a u t i o n s l i s t e d a b o v e . F o r i n s t a n c e , t h e g l a s s t o n g s h a d n o t b e e n made and t h e c l e a n i n g p r o c e s s o n l y c o n s i s t e d o f a b o i l i n g i n c h r o m i c a c i d . The r e s u l t o f t h e f i r s t a t t e m p t was s t i l l a l e a d t e l l u r i d e w h i c h c o n t a i n e d i m p u r i t i e s . A s e c o n d a t t e m p t a t p r o d u c i n g l e a d t e l l u r i d e was m a d e . T h i s t i m e a l l p r e c a u t i o n s w e r e t a k e n a s l i s t e d a b o v e . - 2 5 -Besides this, a flame was played on the mixture at the end of the evacuation process to heat i t gradually and thus try to drive any trapped air out. The second attempt at making the lead telluride was successful. According to the spectrogram taken, a high degree of purity had been gained, ( i i i ) TESTING The method of testing for purity was spectro-scopic. From the H.I.T. Tables of Wavelength of Spectral Lines two separate tables were compiled; Table 1, the Spectral Lines of Lead and Table 2, the Spectral Lines of Tellurium. These were used along with the Principal Spectral Lines of the elements from the K.I.T. Tables for checking the purity. The spectrograms were taken on a Hilger Spectro-graph with FII Spectroscopic plates. The material was excited in a cored carbon open arc. The current through the arc was adjusted to a constant to .give approximately the same excita-tion for each sample. For calibration the iron spectrum was photographed. The methods used for checking for impurities were by comparison. On the f i r s t plate Lead, Lead Telluride (purchased), and Tellurium spectrin were placed side by side. The lead telluride was checked for lines that did not appear in lead and tellurium separately. The wavelengths of these lines were checked against p r i n c i p a l lines of elements. In this way the impurities were identified. As a further check - 26 -t h e l i s t s o f s p e c t r a l l i n e s o f l e a d a n d t e l l u r i u m i n T a b l e s 1 a n d 2 e s t a b l i s h e d i f a l i n e d i d n o t b e l o n g t o l e a d o r t e l l u r i u m . P l a t e 2 i s a c a l i b r a t i o n o f i r o n a g a i n s t l e a d , t e l l u r i u m a n d l e a d t e l l u r i d e . P l a t e 3 c o m p a r e s l a b o r a t o r y p r e -p a r e d l e a d t e l l u r i d e a g a i n s t p u r c h a s e d l e a d t e l l u r i d e ; a l s o l a b o r a t o r y p r e p a r e d l e a d t e l l u r i d e a g a i n s t l e a d a n d t e l l u r i u m . P l a t e s 4 a n d 5 c o m p a r e two l a b o r a t o r y p r e p a r e d P b T e ' s a g a i n s t e a c h o t h e r a n d a g a i n s t t h e p u r c h a s e d P b T e . P h o t o g r a p h i c p r i n t s o f p l a t e s 1 - 5 a r e shown a s P l a t e I - p r i n t 2 ; p l a t e I I - p r i n t 3 ; p l a t e I I I - p r i n t 4 ; p l a t e IV - p r i n t 5 ; p l a t e V - p r i n t 6 . On i n s p e c t i o n o f t h e s e p l a t e s i t i s s e e n t h a t t h e s e c o n d g r o u p o f l a b o r a t o r y p r e p a r e d l e a d t e l l u r i d e i s e s s e n -t i a l l y f r e e f r o m i m p u r i t i e s . T h i s was t h e m a t e r i a l u s e d i n t h e c r y s t a l g r o w i n g p r o c e s s . D . C r y s t a l G r o w i n g T e c h n i q u e , ( i ) G E N E R A L H a v i n g p r o d u c e d a n a c c e p t a b l e f u r n a c e , a n d h a v i n g p r e p a r e d t h e l e a d t e l l u r i d e , i t now o n l y r e m a i n s t o d e v e l o p t h e t e c h n i q u e f o r g r o w i n g t h e c r y s t a l . To do t h i s i t was i n t e n d e d t o s e t up a m e t h o d t o r e d u c e t h e v a r i a b l e s t o a m i n i m u m . L e a d t e l l u r i d e i s v e r y r e a c t i v e w i t h o x y g e n a t h i g h t e m p e r a t u r e . I t i s a l s o h i g h l y t o x i c . B e c a u s e o f t h e s e two t h i n g s i t i s n e c e s s a r y t o c o n t a i n t h e m e l t i n a n e v a c u a t e d c a p s u l e . The m a t e r i a l f r o m w h i c h t h e c a p s u l e i s made m u s t be a b l e t o w i t h s t a n d t e m p e r a t u r e s t o 1 2 0 0 ° C o r b e t t e r . I t m u s t n o t r e a c t w i t h l e a d t e l l u r i d e . I t m u s t be a b l e t o be e v a c u a t e d . - 27 -a n d s e a l e d a f t e r h a v i n g b e e n f i l l e d w i t h t h e l e a d t e l l u r i d e . To m e e t t h e s e s p e c i f i c a t i o n s a m a t e r i a l c a l l e d V i t r e o s i l was u s e d . I t i s 99% SiC>2 a n d comes a s t u b i n g w i t h 1 cm d i a m e t e r b o r e a n d a b o u t 1 mm w a l l . The g l a s s b l o w e r f o r m e d t h i s i n t o c a p s u l e s h a v i n g a p o i n t e d t i p w i t h a n a n g l e o f a b o u t 7 5 ° t o t h e c o n e . The c a p s u l e h a d s t r a i g h t w a l l s f o r a b o u t 3 i n c h e s a n d t h e n w e r e d r a w n down t o a b o u t 1 mm d i a m e t e r b o r e t o p e r m i t s e a l i n g . The b o r e i n c r e a s e d a g a i n t o 1 cm d i a m e t e r f o r a n o t h e r i n c h o r s o . L e a d t e l l u r i d e was g r o u n d i n t o a p o w d e r w i t h a m o r t a r a n d p e s t l e . I t was t h e n p u t i n t o t h e c a p s u l e s o t h a t t h e c a p s u l e was a b o u t t w o t h i r d s f u l l . The c a p s u l e was j o i n e d on t o a v a c u u m s y s t e m a n d pumped d o w n . I n t h e c a s e o f t h e f i r s t t h r e e c a p s u l e s , t h e y w e r e p l a c e d i n an o v e n a n d b a k e d f o r s e v e r a l h o u r s a t 5 0 0 ° C . T h i s was t o d r i v e a l l o x y g e n o u t o f t h e c a p s u l e . I n t h e o t h e r c a s e s a f l a m e was p l a y e d on t h e c a p s u l e f o r s e v e r a l m i n u t e s t o a t t e m p t t o d r i v e a l l o x y g e n o u t . A f t e r b e i n g x- re l l e v a c u a t e d , t h e c a p s u l e i s s e a l e d o f f a n d a s m a l l r i n g o f q u a r t z i s a t t a c h e d t o t h e t o p t o a s s i s t i n l o w e r i n g . L o w e r i n g i s a c h i e v e d b y s u s p e n d i n g t h e c a p s u l e f r o m a n i c h r o m e w i r e e i g h t e e n i n c h e s l o n g . The w i r e i s s u s p e n d e d f r o m a c o r d w h i c h i s w o u l d a r o u n d a s m a l l d r u m . The d r u m i s r o t a t e d b y means o f a c l o c k m o t o r ; one r e v o l u t i o n i n t w e l v e h o u r s . I n t h i s way b y c h a n g i n g t h e d i a m e t e r o f t h e d r u m t h e r a t e o f d r o p p i n g c a n be c o n t r o l l e d . The r a t e s o f d r o p p i n g i n -v e s t i g a t e d s o f a r w e r e f r o m . 9 6 cm p e r h o u r t o . 4 8 cm p e r h o u r . - 28 -S e v e n r u n s h a v e b e e n m a d e . I n p r e p a r i n g f o r a r u n t h e f u r n a s e i s b r o u g h t up t o t e m p e r a t u r e a n d a l l o w e d t o s t a b i l i z e i t s e l f o v e r a p e r i o d o f t w e n t y - f o u r t o f o r t y - e i g h t h o u r s . T h e n t h e f u r n a c e i s c h a r g e d w i t h a c a p s u l e a n d t h e c a p s u l e i s p o s i t i o n e d i n t h e h o t t e s t s e c t i o n o f t h e f u r n a c e . A g a i n a p e r i o d o f f r o m t w e n t y - f o u r t o f o r t y - e i g h t h o u r s i s a l l o w e d f o r t h e m a t e r i a l t o m e l t c o m p l e t e l y a f t e r w h i c h t h e d r o p p i n g may b e g i n . B e f o r e t h e d r o p p i n g i s s t a r t e d , i t i s n e c e s s a r y t o p o s i t i o n t h e d r o p p i n g m e c h a n i s m s o t h a t no c o n t a c t i s made w i t h a n y p a r t o f t h e f u r n a c e t h r o u g h o u t - t h e p e r i o d o f t h e d r o p . T h i s i s t o r e d u c e m e c h a n i c a l s h o c k t o a m i n i m u m . F o r t h i s same r e a s o n t h e c o r d i s u s e d i n t h e s u s p e n s i o n t o a c t a s a s h o c k a b s o r b e r . To a d j u s t t h e c a p s u l e f o r v e r t i c a l p o s i t i o n a m e t a l s c a l e i s p l a c e d on t h e f u r n a c e r a c k . I n t h i s way t h e p o s i t i o n o f t h e c a p s u l e c a n be c h e c k e d a t a n y t i m e . T h i s i s e s s e n t i a l so t h a t t h e c a p s u l e c a n be p o s i t i o n e d i n t h e m e l t i n g s e c t i o n o f t h e f u r n a c e t o s t a r t w i t h a n d l a t e r s t o p p e d w h i l e s t i l l i n t h e a n n e a l i n g s e c t i o n o f t h e f u r n a c e . A f t e r t h e c a p s u l e h a s b e e n w e l l m e l t e d , d r o p p i n g i s s t a r t e d b y s t a r t i n g t h e c l o c k m o t o r . The t e m p e r a t u r e o f t h e f u r n a c e now s h o u l d be w a t c h e d s o - t h a t v a r i a t i o n s a r e k e p t t o a m i n i m u m . T h i s i s done b y a d j u s t i n g t h e " h i g h " a n d " l o w " c u r r e n t s o f t h e f u r n a c e . When t h e c a p s u l e h a s a l l r e a c h e d t h e a n n e a l i n g s e c t i o n o f t h e f u r n a c e , w h i c h may t a k e f r o m f o u r t e e n t o t w e n t y -. - 29 - . f o u r h o u r s * t h e m o t o r i s s t o p p e d . The c a p s u l e i s l e f t i n t h i s p o s i t i o n w h i l e t h e f u r n a c e i s c o o l e d s l o w l y . A p r e p a r e d cam on t h e N h e e l c o P o t e n t i o t r o l a d j u s t s t h e s e t t i n g s o t h a t t h e c o o l i n g r a t e i s c o n t r o l l e d . A s a m p l e c o o l i n g c u r v e m e a s u r e d on t h e B r o w n r e c o r d i n g P o t e n t i o m e t e r i s shown on g r a p h 8. A f t e r t h e f u r n a c e h a s "been c o o l e d t o r o o m t e m p e r a - , t u r e t h e c a p s u l e i s r e m o v e d . The q u a r t z i s c u t o f f w i t h a d i a m o n d w h e e l and t h e b o u l e i s r e m o v e d . I f s u c c e s s f u l , t h i s w i l l be a s i n g l e c r y s t a l . To r e c a p i t u l a t e t h e s t e p s t a k e n : ( 1 ) The c a p s u l e i s f i l l e d , e v a c u a t e d , a n d s e a l e d . ( 2 ) The f u r n a c e i s b r o u g h t up t o t e m p e r a t u r e a n d a l l o w e d t o s t a b i l i z e . (3) A r e c o r d o f t h e t e m p e r a t u r e p r o g r a m i n t h e f u r n a c e s h o u l d be t a k e n a t t h i s p o i n t by d r o p p i n g a t h e r m o c o u p l e t h r o u g h t h e f u r n a c e and r e c o r d i n g i t s o u t p u t on a B r o w n r e c o r d i n g p o t e n t i o m e t e r . (4) The f u r n a c e i s c h a r g e d w i t h t h e c a p s u l e . ( 5 ) The c a p s u l e i s p o s i t i o n e d a n d l e f t s o t h a t t h e b o u l e w i l l c o m p l e t e l y m e l t . (6) The c a p s u l e i s d r o p p e d t h r o u g h t h e t e m p e r a t u r e g r a d i e n t a t a c o n s t a n t r a t e i n t o t h e a n n e a l i n g p a r t o f t h e f u r n a c e a n d s t o p p e d t h e r e . ( 7 ) The f u r n a c e g o e s t h r o u g h a c o n t r o l l e d c o o l i n g p r o g r a m . ( 8 ) The b o u l e i s e x t r i c a t e d , ( i i ) PRODUCTION RUNS The f i r s t t h r e e c a p s u l e s were f i l l e d f r o m a common s o u r c e m a t e r i a l , e v a c u a t e d , a n d s e a l e d a t t h e same t i m e . W h i l e e v a c u a t i n g t h e y w e r e p l a c e d i n a n o v e n a t 5 0 0 ° C f o r s e v e r a l h o u r s t o d r i v e o u t a l l t r a p p e d o x y g e n . S i n c e t h e y w e r e s u b j e c t t o t h e same c o n d i t i o n s t h r o u g h o u t , i t c a n be a s s u m e d t h a t b e f o r e be ing -p u t i n t o t h e f u r n a c e , t h e t h r e e c a p s u l e s a r e i d e n t i c a l t h u s r e d u c i n g one v a r i a b l e . S i n c e t h e f u r n a c e mast be r e s e t f o r e a c h r u n , t h e t e m p e r a t u r e p r o g r a m may c h a n g e i n l e v e l f o r e a c h a t t e m p t . B u t a s t h e d i s t r i b u t i o n c a n n o t be c h a n g e d , t h i s b e c o m e s a n o t h e r f i x e d f a c t o r . W i t h r e s p e c t t o t h e m e l t i n g o f t h e b o u l e i n t h e u p p e r f u r n a c e , t h e o n l y r e s t r i c t i o n on t h i s i s t h a t i t m u s t be c o m p l e t e . To e n s u r e t h a t i t i s c o m p l e t e i n a l l c a s e s a n e x t r a l o n g m e l t i n g p e r i o d h a s b e e n u s e d e a c h t i m e . T h i s c o u l d be c u t down l a t e r t o t h e m i n i m u m i f s o d e s i r e d b u t i t s h o u l d h a v e no u n -d e s i r a b l e e f f e c t i n p r o d u c t i n n b y b e i n g l o n g . I n ^ t h e n e x t s t e p a n o t h e r v a r i a b l e i s e n c o u n t e r e d . I t i s p o s s i b l e t o r e p e a t t h e r a t e o f d r o p p i n g f o r s u b s e q u e n t c a p s u l e s h u t f i n d i n g t h e p r o p e r d r o p p i n g r a t e i s one o f t h e m a j o r p r o b l e m s e n c o u n t e r e d . F i n a l l y , t h e a n n e a l i n g p r o g r a m o n l y h a s one r e -s t r i c t i o n on i t . I t m u s t n o t he t o o f a s t . I n s e l e c t i n g a s e v e n t y two h o u r c o o l i n g p e r i o d w h i c h i s s l o w a t t h e b e g i n n i n g a n d i n -c r e a s i n g i n r a t e ( s e e g r a p h 8 ) i t was f e l t t h a t t h e a n n e a l i n g was s u f f i c i e n t l y l o n g t o be n e g l e c t e d a s a v a r i a b l e . T h u s w i t h t h e f i r s t c a p s u l e s t h e o n l y two v a r y i n g - 31 -c o n d i t i o n s w e r e t h e r a t e o f d r o p a n d t h e d e g r e e o f s u p e r c o o l i n g . I n d r o p p i n g t h e f i r s t c a p s u l e t h e t e m p e r a t u r e d i s -t r i b u t i o n on g r a p h 4 was p l o t t e d i n t h e p r o c e s s ( S t e p 3 ) . T h i s g r a p h mus t be c o r r e c t e d f o r r o o m t e m p e r a t u r e o f 2 2 ° C . T h i s i s a c o n s t a n t a n d s h o u l d be a d d e d t o a l l g r a p h r e a d i n g s . The m e l t i n g p o i n t o f l e a d t e l l u r i d e i s 9 0 5 ° C , s o f o r t h e f i r s t r u n t h e maximum d e g r e e . o f s u p e r c o o l i n g i s a p p r o x i -m a t e l y 3 5 ° C . The r a t e o f d r o p u s e d was . 7 6 cm p e r h o u r . S i n c e t h e d e g r e e o f s u p e r c o o l i n g a n d t h e r a t e o f d r o p , a r e n o t d e f i n e d i n a n y l i m i t s i t i s n e c e s s a r y t o f i n d t h e r i g h t c o m b i n a t i o n e m p i r i c a l l y . The r a t e o f d r o p m u s t be s u c h t h a t t h e i n t e r f a c e b e t w e e n t h e l i q u i d and s o l i d p h a s e s r e m a i n s a t t h e same l e v e l i n t h e f u r n a c e . T h i s i s t o s a y , t h e r a t e m u s t be s l o w e n o u g h s o t h a t t h e h e a t i n t h e b o u l e c a n be c a r r i e d away a n d t h e i n t e r f a c e r e m a i n s h o r i z o n t a l a t a l l t i m e s . The r a t e o f . 7 6 cm p e r h o u r was a j u d i c i o u s g u e s s b u t f r o m t h e r e s u l t s o f t h e f i r s t b o u l e i t m u s t be r e a s o n a b l y c l o s e t o c o r r e c t . A s e r i e s o f r i n g s w e r e f o u n d on t h e f i r s t b o u l e a b o u t . 5 mm a p a r t . T h i s was f o u n d t o be due t o t h e v a r i a t i o n s i n t e m p e r a t u r e a s t h e c o n t r o l l e r a t t e m p t e d t o k e e p i t c o n s t a n t . . B y a d d i n g t h e r e s i s t a n c e i n p a r a l l e l w i t h t h e c o n t r o l r e l a y p o i n t s and t h u s h a v i n g a " h i g h " a n d " l o w " c u r r e n t r a t h e r t h a n " o n " a n d " o f f " c u r r e n t i t was p o s s i b l e t o make t h i s e f f e c t n e g l i g i b l e . The s e c o n d r u n f o l l o w e d t h e same p a t t e r n a s t h e f i r s t . The m e l t i n g t i m e , d r o p p i n g t i m e a n d c o o l i n g r a t e w e r e t h e s a m e . The o n l y two c h a n g e s w e r e ; f i r s t , a b e t t e r s t a b i l i z e d - 32 -t e m p e r a t u r e a n d s e c o n d , a l a r g e r d e g r e e o f s u p e r c o o l i n g . The t e m p e r a t u r e was s t a b i l i z e d b y t h e a d d i t i o n o f t h e r h e o s t a t s u c h t h a t , when p r o p e r l y a d j u s t e d t h e t e m p e r a t u r e s w i n g was a t o t a l o f two c e n t i g r a d e d e g r e e s i n a f i f t y m i n u t e p e r i o d . The t e m p e r a t u r e o f t h e f u r n a c e was r e d u c e d b y f i f t e e n d e g r e e s a t t h e a n n e a l i n g l e v e l i n t h i s way p e r m i t t i n g a p o s s i b l e s u p e r c o o l i n g o f f i f t y c e n t i g r a d e d e g r e e s . A l a r g e v o l t a g e d r o p on t h e l i n e a t a b o u t 6 A . M . c a u s e d t h e E a i n s C o n t r o l C i r c u i t ( d i a g r a m 2j) t o o p e n c i r c u i t a n d t h e f u r n a c e was o f f f o r a b o u t t e n m i n u t e s . T h i s was a t a c r i t i c a l t i m e when t h e b o u l e was p a s s i n g t h r o u g h t h e g r a d i e n t . I t i s f e l t t h a t t h i s had a n u n d e s i r a b l e e f f e c t on t h e b o u l e r e s u l t i n g i n a d i s l o c a t i o n due t o a c h a n g e i n t h e d e g r e e o f n u c l e a t i o n . The r e s u l t i n g b o u l e u p o n e x t r a c t i o n f r o m t h e c a p s u l e b r o k e up i n t o l a r g e s i n g l e c r y s t a l s b u t n o t one e x t e n d i n g t h r o u g h -o u t t h e w h o l e m e l t . The t h i r d r u n was u n f o r t u n a t e i n t h a t a f t e r t h e d r o p h a d b e e n m a d e , b u t b e f o r e t h e f u r n a c e was c o o l e d , i t was f o u n d t h a t t h e c o n t r o l t h e r m o c o u p l e i n t e r m i t t e n t l y o p e n c i r c u i t e d c a u s i n g t h e "Wheelco P o t e n t i o t r o l t o k e e p t h e f u r n a c e s t a b l e a t a much h i g h e r t e m p e r a t u r e t h a n t h a t f o r w h i c h i t was s e t a n d r e c o r d e d . I n f a c t , t h e c a p s u l e h a d n o t p a s s e d t h r o u g h t h e f r e e z i n g p o i n t f o r l e a d t e l l u r i d e a t a l l . The t h e r m o c o u p l e was r e p a i r e d a n d r e p l a c e d w i t h o u t r e m o v i n g t h e c a p s u l e f r o m t h e f u r n a c e . I t was t h e n n e c e s s a r y t o f i n d t h e new s e t t i n g o n t h e c o n t r o l l e r t o a r r i v e a t t h e t e m p e r a t u r e n e e d e d . To do t h i s a - 33 -t h e r m o c o u p l e was i n s e r t e d i n t h e f u r n a c e a l o n g s i d e t h e c a p s u l e , a n d a d j u s t m e n t s w e r e m a d e . The t h e r m o c o u p l e w h i c h was i n s e r t e d t o u c h e d t h e c a p s u l e n e a r t h e t i p a n d p r e s u m a b l y b y some r e a c t i o n o p e n e d t h e c a p s u l e d i s c h a r g i n g t h e m e l t i n t o t h e f u r n a c e . The d e c o n t a m i n a t i o n o f t h e f u r n a c e was done b y r e m o v i n g t h e m o v e a b l e p a r t s a n d c l e a n i n g t h e m . T h e n b y h e a t i n g t h e f u r n a c e t o a t e m p e r a t u r e o f 1 2 0 0°C t h e r e s t o f t h e PbTe was e v a p o r a t e d o u t o f t h e c h a n b e r . T h r e e new c a p s u l e s w e r e made , f i l l e d , e v a c u a t e d a n d s e a l e d . The m a t e r i a l u s e d f o r t h e f i l l i n g was t h e f i r s t l a b o r a t o r y p r e p a r e d p r o d u c t . T h i s t i m e t h e h e a t i n g w h i l e e v a -c u a t i n g was done w i t h a f l a m e f o r a s h o r t e r p e r i o d . A d e p a r t u r e i n c a p s u l e d e s i g n was t r i e d i n t h a t one was b u i l t w i t h a s m a l l r o u n d b a l l on t h e t i p s e p a r a t e d f r o m t h e m a i n c a p s u l e b y a c o n s t r i c t i o n . Run 4 was s e t w i t h a t e m p e r a t u r e d i s t r i b u t i o n s i m i l a r t o g r a p h 4 , w h i c h g i v e a p o s s i b l e s u p e r c o o l i n g o f t h i r t y -f i v e c e n t i g r a d e d e g r e e s . The c h a n g e i n t h i s r u n was t o i n c r e a s e t h e r a t e o f d r o p p i n g t o . 9 6 cm p e r h o u r . W h i l e t h e a n n e a l i n g was b e i n g done a d r o p i n t h e p o w e r r e s u l t e d i n t h e e q u i p m e n t b e i n g s h u t down s o t h a t t h e f u r n a c e q u i c k l y c o o l e d f r o m 8 5 0°C t o 500°C. T h i s f a s t c o o l i n g may h a v e h a d a d e t r i m e n t a l e f f e c t on t h e b o u l e . R u n 5 h a d a t e m p e r a t u r e d i s t r i b u t i o n a s p l o t t e d i n t h e s o l i d l i n e on g r a p h 5 . I t i s i n t e r e s t i n g t o n o t e a l s o t h e d o t t e d l i n e on g r a p h 5 . T h i s i s a p l o t o f t h e " i d e a l " p r o g r a m a i m e d a t i n b u i l d i n g t h e f u r n a c e . The p r o d u c e d d i s t r i b u t i o n c o m p a r e s - 3 4 -f a v o r a b l y w i t h t h e " i d e a l " . F o r R u n 5 t h e c a p s u l e w i t h t h e b a l l on t h e t i p was u s e d . The a n n e a l i n g p l a t e a u was 55 c e n t i g r a d e d e g r e e s b e l o w t h e f r e e z i n g p o i n t . The d r o p p i n g r a t e was . 7 6 cm p e r h o u r . T h i s was e s s e n t i a l l y t h e same p r o g r a m a s f o r R u n 2 . A s a p r e c a u t i o n a g a i n s t t h e f u r n a c e t u r n i n g o f f due t o l a r g e f l u c t u a t i o n s i n t h i s l i n e v o l t a g e , a m a i n s s u p p l y c o n t r o l c i r c u i t w i t h a d e l a y ( d i a g r a m 3 ) h a s b e e n made a n d i n s t a l l e d . F o r R u n 6 a t e m p e r a t u r e p r o g r a m was a d j u s t e d a n d p l o t t e d a s on g r a p h 6 . T h i s g i v e s a n a n a e a l i n g p l a t e a u t w e n t y -f i v e c e n t i g r a d e d e g r e e s b e l o w t h e f r e e z i n g p o i n t . The d r o p p i n g r a t e was a d j u s t e d t o ,48 c m ' p e r h o u r . The c o o l i n g c u r v e was r e c o r d e d on t h e B r o w n R e c o r d i n g P o t e n t i o m e t e r . The c o o l i n g was i n t e r r u p t e d due t o a p o w e r f a i l u r e i n t h e a r e a s o t h a t t h e t e m -p e r a t u r e d r o p p e d 6 0 ° i n a f e w m i n u t e s . A n a u x i l i a r y D.C. s u p p l y was t e m p o r a r i l y h o o k e d up t o m i n i m i z e t h e d r o p t o 6 0 ° a n d b r i n g i t q u i c k l y b a c k t o n o r m a l . T h i s s u d d e n d r o p may h a v e a f f e c t e d t h e c r y s t a l g r o w t h . F o r R u n 7 t h e t e m p e r a t u r e p r o g r a m o f g r a p h 7 was s e t up i n t h e f u r n a c e . The d r o p p i n g r a t e o f . 7 6 cm p e r h o u r was u s e d . The c a p s u l e w h i c h was d r o p p e d was f i l l e d w i t h t h e r e m a i n i n g m a t e r i a l w h i c h h a d b e e n f i r s t p r o d u c e d i n t h e l a b o r a t o r y . I t was f i l l e d , e v a c u a t e d a n d s e a l e d i n t h e same way a s t h e s e c o n d g r o u p o f t h r e e c a p s u l e s . A t t h e t i m e o f t h i s w r i t i n g no f u r t h e r a t t e m p t s h a v e b e e n made t o p r o d u c e s i n g l e c r y s t a l s o f P b T e . The e q u i p m e n t h a s b e e n t a k e n o v e r t o make c r y s t a l s o f o t h e r s u b s t a n c e s n e e d e d i n r e s e a r c h i n t h e n u c l e a r r e s o n a n c e w o r k . - 35 -C h a p t e r I V R e s u l t s Of t h e s e v e n m e l t s a t t e m p t e d d u r i n g t h i s w o r k t h r e e h a d a r e a s o n a b l e amoun t o f s u c c e s s , one h a d some s u c c e s s , two had v e r y p o o r s u c c e s s , and one was n e v e r c o m p l e t e d . No m e l t r e s u l t e d i n a s i n g l e c r j ^ s t a l t h r o u g h o u t t h e w h o l e b o u l e . The one t h a t was n o t c o m p l e t e d was R u n 3. I n t h i s c a s e t h e e x p e r i e n c e g a i n e d s h o w e d t h e n e c e s s i t y f o r a m e a s u r e m e n t o f t h e t e m p e r a t u r e d i s t r i b u t i o n e a c h t i m e b e f o r e a n a t t e m p t t o g r o w a c r y s t a l i s m a d e . I t f u r t h e r s h o w e d t h a t t h e r e i s a r e a c t i o n b e t w e e n q u a r t z a n d a l u n d u m a b o v e 1100°C. The two r u n s w i t h v e r y p o o r s u c c e s s w e r e n u m b e r s 4 a n d 7. B o t h o f t h e s e m e l t s w e r e p e r f o r m e d w i t h a m a t e r i a l k n o w n t o c o n -t a i n i m p u r i t i e s . I t i s c o n s i d e r e d t h a t t h e i m p u r i t i e s c o u l d c a u s e n u c l e a t i o n a n d t h u s p r o d u c e a p o l y c r y s t a l l i n e s t r u c t u r e . I n b o t h o f t h e s e c a p s u l e s a h e a v y d i s c o l o r a t i o n t o o k p l a c e on t h e q u a r t z , p r e s u m a b l y c a u s e d b y a v o l a t i l e i m p u r i t y d e p o s i t i n g a s a s l u d g e on t h e c a p s u l e w a l l . The b o u l e i t s e l f s h o w e d a d i s c o l o r a -t i o n . The t o p o f t h e b o u l e i n e a c h o f t h e l a s t f o u r m e l t s was d i s c o l o r e d due t o i m p u r i t i e s . I n t h e f i r s t two r u n s , t h i s s l u d g e and d i s c o l o r a t i o n d i d n o t show u p , s i n c e t h e f i r s t P b T e u s e d was c o n s i d e r e d t o be w i t h o u t m e a s u r a b l e i m p u r i t y . T h u s t h e l a c k o f s u c c e s s i n p a r t f o r R u n s 4 a n d 7 i s b l a m e d on t h e i m p u r i t y o f t h e m a t e r i a l u s e d . I n t h e c a s e o f R u n 4 i t may a l s o be t h a t t h e - 36 -d r o p p i n g r a t e was t o o f a s t . R u n 7 s h o w s d i s c o l o r a t i o n o f t h e c r y s t a l f a c e s t h r o u g h o u t t h e b o u l e , s u g g e s t i n g t h a t t h e i m p u r i t y i s t h r o u g h o u t t h e Ttfhole m e l t . The one m e l t t h a t s h o w e d some s u c c e s s was R u n 5. T h i s was t h e a t t e m p t t o u s e a c a p s u l e w i t h a s m a l l p i l o t c a p s u l e j o i n e d t o t h e l a r g e one b y a c o n s t r i c t i o n . T h i s m e l t r e s u l t e d i n a s i n g l e c r y s t a l s h e l l a b o u t 1 mm t h i c k g r o w i n g a r o u n d a p o l y c r y s t a l l i n e c o r e . The c o r e s eems t o be l o n g p o l y g o n a l p i l l a r s g r o w i n g t h r o u g h t h e m e l t . On t h e t o p o f t h e b o u l e t h e p o l y g o n a l p a t t e r n o f d i s -c o l o r a t i o n i s p r e s e n t s h o w i n g i m p u r i t i e s a g a i n b u t p r e s u m a b l y a t t h e r a t e o f d r o p o f t h e c a p s u l e t h e i m p u r i t i e s w e r e p u s h e d a h e a d o f t h e i n t e r f a c e . The r e a s o n f o r a p o l y c r y s t a l l i n e s t r u c t u r e may r e f e r b a c k t o i m p u r i t i e s , b u t a l s o may be due t o s e v e r a l s e e d n u c l e i b e i n g p r o d u c e d i n t h e s m a l l s p h e r e a n d a l l g r o w i n g , n o t j u s t o n e . Of t h e t h r e e c a p s u l e s w i t h r e a s o n a b l e s u c c e s s i t i s i n t e r e s t i n g t o n o t e t h a t one o f t h e m , R u n 6, was made w i t h l e a d t e l l u r i d e w h i c h h a d a q u e s t i o n a b l e p u r i t y . B u t t h e d r o p p i n g r a t e o f t h i s m e l t was s l o w e r t h a n t h e o t h e r s . I t i s p r e s u m e d t h a t t h e i m p u r -i t i e s r o d e t h e i n t e r f a c e b e t w e e n t h e two p h a s e s , w h i c h p o s s i b l y c a n be done a t t h e s l o w e r d r o p p i n g r a t e . On c l e a v i n g , t h i s b o u l e r e s u l t e d i n s e v e r a l s e c t i o n s w i t h g o o d c l e a v a g e p l a n e s . On r o t a t i n g t h e s e p l a n e s t h r o u g h a n i n c i d e n t l i g h t t h e r e f l e c t i o n i s o n l y maximum a t one t i m e i n d i c a t i n g a s u r f a c e o f a t l e a s t p a r a l l e l f a c e s a n d , i n d e e d , i t i s c o n s i d e r e d t h e s e t o be s i n g l e p l a n e s a s f o r a s i n g l e c r y s t a l . I n f a c t , t h i s i s u s e d a s j u s t i f i c a t i o n f o r c o n s i d e r i n g a l a r g e s i n g l e c r y s t a l h a s b e a n p r o d u c e d . The o t h e r t w o r e a s o n a b l e s u c c e s s e s w e r e R u n 1 a n d R u n 2. T h e s e w e r e p r o d u c e d a t a f a s t e r d r o p p i n g r a t e a n d a s l i g h t l y h i g h e r d e g r e e o f s u p e r c o o l i n g t h a n f o r R u n 6. T h e s e s t i l l r e s u l t e d i n l a r g e s i n g l e c r y s t a l s b e c a u s e t h e r e was n o t t h e i m p u r i t y t o c a u s e a u x i l i a r y n u c l e a t i o n . I n - b o t h c a s e s t h e c o o l i n o f t h e f u r n a c e was done f a s t e r t h a n f o r Run 6. I t i s c o n s i d e r e d t h a t t h e s t r a i n s c s e t ' - u p ^ i b y i m p r o p e r a n n e a l i n g c a u s e d t h e s i n g l e c r y s t a l s t o s h a t t e r so t h a t t h e r e s u l t was n o t a s i n g l e c r y s t a l t h r o u g h t h e w h o l e m e l t b u t s e v e r a l l a r g e s i n g l e c r y s t a l s m a k i n g up t h e m e l t . I t i s i n t e r e s t i n g t o n o t e t h a t - . . b o t h o f t h e b o u l e s c o n t a i n i n g r e l a t i v e l y p u r e P b T e r e s u l t e d i n - l a r g e s i n g l e c r y s t a l s . The c a p s u l e s i n b o t h c a s e s d i d n o t c o n t a i n s l u d g e , b u t a t h i n l a y e r o f y e l l o w compound p r o b a b l y a t e l l u r a t e o r a s i l i c a t e s u g g e s t i n g t h a t t h e r e i s some o x y g e n i n t h e c a p s u l e a f t e r s e a l i n g . T h i s d i d n o t a f f e c t t h e g r o w t h o f t h e c r y s t a l i n a n y o b v i o u s m a n n e r . A f l a t c l e a v a g e p l a n e 1 cm l o n g b y .75 cm w i d e h a s b e e n c l e a v e d f r o m t h e b o u l e o f R u n S . T h i s means t h a t a s i z e a b l e s i n g l e c r y s t a l was p r o d u c e d . The b o u l e o f R u n 2 b r o k e up i n t o s e v e r a l c r y s t a l s ; o n e , a c e n t i m e t e r l o n g a n d f o u r m i l l i m e t r e s s q u a r e ; a n o t h e r , t h r e e q u a r t e r s o f a c e n t i m e t e r l o n g a n d 5 m i l l i m e t r e s s q u a r e . T h e s e r e s u l t s g i v e a n i n d i c a t i o n t h a t i f t h e c o r r e c t amount o f s u p e r c o o l i n g , t h e c o r r e c t d r o p p i n g r a t e a n d a p u r e compound w e r e u s e d w i t h t h e p r e s e n t f u r n a c e , e t c . , a s i n g l e c i ~ y s t a l t h r o u g h o u t t h e m e l t w o u l d r e s u l t . - 38 -C h a p t e r V D i s c u s s i o n P r o b a b l y t h e m o s t i m p o r t a n t c o n c l u s i o n t h a t c a n be d r a w n f r o m t h e w o r k done i s t h a t a f u r n a c e ( s e e d i a g r a m 5 ) a n d c o n -t r o l l e r h a v e b e e n c o n s t r u c t e d i n w h i c h l a r g e s i n g l e c r y s t a l s c a n be made o f m a t e r i a l s t h a t h a v e m e l t i n g p o i n t s u n d e r 1 4 0 0 ° C . I t h a s b e e n s e e n f r o m t h e w o r k t h a t f o u r i m p o r t a n t f a c t o r s h a v e t o be c o n s i d e r e d i n p r o d u c i n g l a r g e s i n g l e c r y s t a l s f r o m t h e m e l t . • The f i r s t i m p o r t a n t f a c t o r i s one o f p u r i t y o f t h e m a t e r i a l s u s e d . F i r s t , t h e s o u r c e m a t e r i a l s m u s t be p u r e . T h i s c o n c l u s i o n i s r e a c h e d b y c o n s i d e r i n g t h e r e c o r d o f R u n s 1, 2 v e r s u s R u n s 4 , 5 , 7 , made d u r i n g t h i s w o r k . I f t h e m a t e r i a l i s n o t made p u r e d u r i n g t h e c h e m i s t r y o f t h e p r o d u c t i o n i t c a n b e made p u r e b y a z o n e r e f i n i n g s u c h a s was p r e s u m a b l y e x p e r i e n c e d b y t h e b o u l e o f R u n 6 . I n t h i s m a n n e r t h e i m p u r i t i e s a r e moved a l o n g t o the e n d b y t h e i n t e r f a c e b e t w e e n t h e p h a s e s . T h i s i n d i c a t e s t h e n e e d o f a z o n e r e f i n i n g f u r n a c e t o be b u i l t i f f u r t h e r c r j / s t a l g r o w i n g i s t o be c a r r i e d o n . I n t h i s way b e t t e r c o n t r o l o f t h e s o u r c e m a t e r i a l w i l l be p o s s i b l e . A l o n g w i t h t h e p r o b l e m o f p u r i t y o f t h e s o u r c e m a t e r i a l i s l i n k e d c l e a n l i n e s s i n h a n d l i n g t h e m a t e r i a l d u r i n g p r o d u c t b n . C o n t a m i n a t i o n c a n t a k e p l a c e due t o i n s u f f i c i e n t c a r e i n h a n d l i n g t h e m a t e r i a l s . S t i l l c o n s i d e r i n g p u r i t y o f t h e m e l t , i t i s n e c e s s a r y t o t a k e p r e c a u t i o n s o f p a s s i b l e c h e m i c a l r e a c t i o n s d u r i n g t h e p r o c e s s . - 39 -L o o k i n g a t t h e c a p s u l e s o f R u n s 1 a n d 2 w h e r e p r e c a u t i o n s t o w a r d s c l e a n l i n e s s w e r e s t r i c t l y r e g a r d e d , a n d w h e r e p u r i t y o f t h e s o u r c e m a t e r i a l was c l o s e l y c h e c k e d , a r e a c t i o n t o o k p l a c e d u r i n g t h e r u n p r e s u m a b l y w i t h o x y g e n . I t i s p o s s i b l e t h e n t h a t t h e m i x t u r e b e f o r e p r o d u c i n g t h e PbTe s h o u l d be done w h i l e p a s s i n g a warm h y d r o g e n a t m o s p h e r e o v e r t h e m a t e r i a l s . R e a c t i o n w i t h t h e c a p s u l e m u s t a l s o be c h e c k e d f o r and a s u i t a b l e m a t e r i a l u s e d i n m a k i n g t h e c a p s u l e . A l l t h e s e p o i n t s o f p r e c a u t i o n a r e n e c e s s a r y s i n c e i m p u r i t i e s i n t h e m e l t c a n p r o d u c e a u x i l i a r y n u c l e a t i o n w h i c h p r o d u c e s d i s -l o c a t i o n s o r p o l y c r y s t a l l i n e s t r u c t u r e s i n t h e c r y s t a l g r o w t h . The n e x t two f a c t o r s t h a t h a v e t o be c o n s i d e r e d a r e v a r i a b l e s i n t h e t e c h n i q u e . O n e , t h e d e g r e e o f s u p e r c o o l i n g , a f f e c t s t h e r a t e o f n u c l e a t i o n . The r a t e o f n u c l e a t i o n i n c r e a s e s e x p o n e n t i a l l y w i t h t h e d e g r e e o f s u p e r c o o l i n g s o t h a t b y a d j u s t i n g t h e d e g r e e • o f s u p e r c o o l i n g t h e p r o d u c t i o n o f a l a r g e n u c l e u s f o r m i n g a s i n g l e c r y s t a l c a n be d o n e . B u t t h i s p r o d u c t i o n o f t h e l a r g e n u c l e u s i s a l s o d e p e n d e n t on t h e o t h e r f a c t o r , t h a t o f t h e r a t e o f d r o p p i n g . I n o r d e r t h a t t h e v o l u m e c r y s t a l l i z i n g a t a n y t i m e i s s m a l l , t h e r a t e o f d r o p m u s t be s m a l l . T h i s c r i t e r i o n t h a t t h e c r y s t a l l i z i n g v o l u m e be s m a l l i s one f o r t h e p r o d u c t i o n o f a l a r g e s i n g l e n u c l e u s w h i c h w i l l g r o w . T h a t i s t o s a y , i f t h e c r y s t a l l i z i n g v o l u m e i s s m a l l e n o u g h a n d t h e d e g r e e o f s u p e r -c o o l i n g l a r g e e n o u g h , a n u c l e i w i l l be p r o d u c e d w h i c h i s s o l a r g e t h a t o n l y one c a n e x i s t . A n d s o wc f i n d t h e d e g r e e o f s u p e r -c o o l i n g a n d t h e d r o p p i n g r a t e a r e i n t e r d e p e n d e n t . The v a l u e s o f t h e s e two f a c t o r s m u s t be a r r i v e d a t e m p i r i c a l l y . B u t , i t i s - 4 0 -considered from the experimental r e s u l t s that the rate of drop f o r PbTe with t h i s furnace should be l e s s than .5 cm per hour. The f i n a l f a c t o r which must be considered i s that of annealing. Since the s t r u c t u r e of the c r y s t a l depends on the i n t e r n a l f o r c e s i n the c r y s t a l , annealing i s important. The main point here i s that the c o o l i n g while i n the s o l i d phase can not be too slow. But on the other hand, i f i t i s too f a s t s t r a i n s are set up i n s i d e the c r y s t a l which may d i s t o r t the c r y s t a l , r e -ducing i t to a p o l y c r y s t a l l i n e from a s i n g l e c r y s t a l s t r u c t u r e . As an example of t h i s i n Runs 1 and 2 the annealing was too quick due to power t r o u b l e . Thus, s e v e r a l large c r y s t a l s were produced instead of one s i n g l e c r y s t a l . In respect to the equipment that has been made, s e v e r a l p o i n t s are worth n o t i n g . The temperature d i s t r i b u t i o n of the furnace now compares f a v o r a b l y to the proposed program. This can be seen i f we check the a c t u a l d i s t r i b u t i o n (the s o l i d l i n e on graph 5) w i t h the proposed program (dotted l i n e on graph 5 ) . An improve-ment could be made on a f u t u r e furnace (a) i f more power was introduced at the ends to reduce end e f f e c t and (b) i f the r a t i o of power introduced i n t o the upper furnace to the power introduced i n t o the lower furnace was l a r g e r to produce a steeper g r a d i e n t . The temperature c o n t r o l i s very s a t i s f a c t o r y , being « ^ /°C v a r i a t i o n over a period of f i f t y minutes when adjusted p r o p e r l y . To increase t h i s c o n t r o l w i t h the e x i s i t i n g equipment i t i s necessary to increase the r a t i o of the heat content of the furnace to the heat content of the heating element. I t i s considered t h a t v e r y l i t t l e i m p r o v e m e n t c a n be made w i t h o u t d r a s t i c c h a n g e s i n d e s i g n . F i n a l l y , i n r e s p e c t t o t h e a t t e m p t s t o make s i n g l e c r y s t a l s , l a r g e s i n g l e c r y s t a l s w e r e p r o d u c e d i n . t h r e e o f t h e s i x c o m p l e t e d a t t e m p t s . S u c c e s s i n p r o d u c i n g a s i n g l e c r y s t a l t h r o u g h o u t the w h o l e m e l t wa s n o t met due t o a c c i d e n t a l v a r i a t i o n s f r o m t h e p r o p e r t e c h n i q u e . I t i s c o n c l u d e d t h a t i f t h e p r o p e r t e c h n i q u e : , , a s d i s c u s s e d , i s f o l l o w e d a s i n g l e c r y s t a l t h r o u g h o u t t h e m e l t c a n be p r o d i i c e d w i t h t h e e q u i p m e n t d e s c r i b e d i n t h i s t h e s i s . - 42 -T a b l e I S P E C T R A L L I N E S OF L E A D C o m p i l e d f r o m M I T ' T a b l e o f W a v e l e n g t h I n t e n s i t y W a v e l e n g t h C l a s s I n t e n s i t y W a v e l e n g t h C l a s s C U T ) ( M I T ) • A r c S p a r k A r c S p a r k 50 7 2 2 9 . 1 1 I I 8 5 9 1 0 . 7 B a n d h e a d 1 0 0 7193 . 6 I I 2 0 h l 2 5 8 9 5 . 7 0 10 7 1 6 5 . 1 I I 20 5 8 5 7 . 6 7 I 2 7 1 1 4 . 7 I I 4 0 5 7 6 7 . 9 I I 1 0 7 0 9 9 . 7 8 10 5 7 1 3 . 8 I I 4 0 7 0 5 0 . 7 I I '20 5 6 9 2 . 2 6 50 7013 . 2 4 5 6 6 0 . 1 I I 2 6 8 7 2 . 0 I I 3 5 6 1 7 . 7 B a n d h e a d 10 6 7 9 0 . 8 I I 4 0 5 6 0 8 . 8 I I 15 6 7 8 5 . 9 8 5545 . 1 5 0 0 6 6 6 0 . 0 I I 4 0 5 5 4 4 . 6 I I 25 6 5 6 9 . 4 I I 25 5523 . 5 I 15 6 5 5 8 . 7 • I I 25 5 4 7 2 . 4 I I 25 . 6 5 2 7 . 8 I I l O h 5 3 7 2 . 5 35 6 5 1 8 . 2 I I 4 0 5 3 7 2 . 1 I I 3 6433 . 6 B a n d h e a d 4 0 5 3 6 7 . 3 I I 2 6 4 2 2 . 9 I I 3 5353 .8 B a n d h e a d 25 6 3 4 5 . 0 I I 2 0 5 3 0 6 . 8 I I 3 6 3 4 2 . 0 B a n d h e a d 10 2h 5 2 0 1 . 4 4 1 5 6 3 3 9 . 8 I I 2 5 1 9 1 . 4 I I 4 0 6 3 1 1 . 5 2 0 5 1 8 9 . 2 20 6 2 3 5 . 4 4 25 5 1 6 3 . 8 I I 50 6 2 2 9 . 7 8 5 1 6 2 . 3 B a n d h e a d 5d 6 2 0 3 . 7 I I 25 5 1 5 5 . 8 I I od 6 1 9 8 . 8 I I 4 h 5143 . 1 4 3 0 6 1 8 1 . 9 I I 3 5 1 3 8 . 2 B a n d h e a d l O h 6 1 6 9 . 4 6 5 1 0 9 . 6 I I 50 6 1 6 0 I I 6 5 0 8 1 . 2 I I 1 5 h l 6 1 1 0 . 7 8 4 0 5 0 7 4 . 6 I I 2d 6 0 9 4 . 0 I I . 4 0 5 0 7 0 . 7 I I 2 0 0 6 0 7 5 . 8 I I 10 5 0 6 3 . 1 I 3 5d 6 0 4 1 . 4 I I 35 5 0 4 9 . 3 I I 2 5 h l 5 0 1 1 . 9 8 20 5 0 3 2 . 2 I I 10 6007.5C I I 20 4 5 0 0 5 . 4 3 4 0 h l 3 h 6 0 0 1 . 8 8 6 4983 . 8 B a n d h e a d - 43 -I n t e n s i t y (Km A r c S p a r k W a v e l e n g t h C l a s s I n t e n s i t y ( J U T ) A r c S p a r k W a v e l e n g t h C l a s s 3 4 9 1 2 . 7 I I 3 4 1 9 5 . 5 I I 2 4 8 9 5 . 6 I I 20 1 0 4 1 6 8 . 0 4 5 6 4 8 3 6 . 3 I I 5 4 1 5 2 . 9 3 I I 5 4833 . 7 I I 1 0 4 1 4 1 . 4 2 4 8 1 6 . 9 B a n d h e a d 5 4 1 2 8 . 2 1 8 4 8 0 4 . 5 I I 4 4 1 1 3 . 2 7 I I 1 0 4 8 0 2 . 2 3 5 4 1 1 0 . 7 7 I I 20 4 7 9 8 . 5 2 5 4 0 9 4 . 6 8 5 4 7 9 8 . 4 I 2 4 0 7 7 . 6 1 6 4 7 8 8 . 1 I I 20 20 4 0 6 2 . 1 4 4 5 4 6 8 4 . 9 I I 2 0 0 0 r 3 0 0 r 4 0 5 7 . 8 2 0 I 5 4 6 6 5 . 5 I I 6 6 4 0 1 9 . 6 3 9 2 4 6 0 5 . 4 3 5 3 9 G 7 . 5 I I 10 4 5 8 2 . 3 4 I I 3 0 3 9 7 1 . 3 I I 4 4 5 8 1 . 3 I I 5 Oh 3 9 5 1 . 9 4 I 10 4 5 7 9 . 1 5 I I 5 3 9 4 3 . 8 0 7 4 5 7 1 . 7 2 I 2 3 9 2 7 . 7 9 2 4 5 5 7 . 2 I I 2 3 9 2 5 . 2 3 4553 . 7 B a n d h e a d 4 0 3 9 0 9 . 1 7 I I 4 4 5 4 4 . 8 I I 2 3 8 9 6 . 9 I I 5 4 5 3 4 . 6 9 5 3 8 9 4 . 6 I I 3 4 4 7 6 . 3 I I 2 3 8 7 4 . 6 5 2 4 4 2 8 . 7 I I 2 3 8 7 2 . 6 4 4 4 1 0 . 4 B a n d h e a d 1 0 0 3 8 5 4 . 0 5 3 I 2 4 3 8 6 . 0 I I 5h 3 3 4 1 . 9 1 10 4 3 5 2 . 7 60 3 8 4 1 . 6 2 3 4 3 5 1 . 5 I I 50 3 8 3 2 . 8 3 6 4 2 9 6 . 7 1 I I 2 3 8 2 9 . 2 I I 7 4 2 9 3 . 3 4 I I 20 3 8 2 7 . 2 I I 3 0 4 2 7 2 . 6 3 T l O h 3 7 8 6 . 2 4 3 I I 2 4 2 7 2 . 5 5 I I 1 0 3 7 8 4 . 0 I I 1 0 4 2 4 2 . 4 7 I I 1 5 0 6 0 h 3 7 3 9 . 9 4 7 2 4 2 4 2 . 2 0 I I 10 3 7 3 4 . 8 I I 2 4 2 3 2 . 4 3 I I 20 3 7 2 3 . 8 3 I 4 2 2 9 . 0 B a n d h e a d 2 3 7 2 3 . 8 7 I - 44 -I n t e n s i t y W a v e l e n g t h C l a s s I n t e n s i t y W a v e l e n g t h C l a s s ( M I T ) ( M I T ) A r c S p a r k A r c S p a r k 10 3 7 1 4 . 0 5 I I 2 3 4 3 1 . 3 5 10 3 6 9 9 . 2 I I 5 3 4 2 9 * 6 I I 4 0 1 3 6 8 9 . 3 0 9 I I 2 9 b h l 3 4 0 1 . 9 5 3 6 8 9 . 2 0 1 l O e a 3 3 8 9 . 4 I I 2 3 6 3 9 . 0 I I 1 5 s x 3 3 6 1 . 5 8 I 3 0 0 50 3 6 8 3 . 4 7 1 I 1 5 e a 3 3 0 9 . 2 I I 2 3 6 7 4 . 9 5 l O s x 3 2 7 9 . 3 3 50 7 3 6 7 1 . 5 0 3 5 h k l 3 2 7 6 . 4 4 70 3 6 7 1 . 3 9 6 0 s x 3 2 7 6 . 1 9 2 3 6 6 5 , 4 8 5 I I 2 0 h 3 2 6 2 . 3 5 3 2 3 6 6 5 . 0 5 I 2 s x 3 2 6 1 . 2 1 3 0 0 5Oh 3 6 3 9 . 5 8 0 I 5 0 e a 3 2 5 1 . 0 5 3 6 2 0 . 3 5 4 0 s x 3 2 4 2 . 8 6 20 3 6 0 1 . 8 I I 3 0 k l 3 2 3 2 . 3 5 3 3 0 3 5 9 3 . 1 2 l O s x 3 2 3 1 . 2 3 3 3 5 9 2 . 9 2 1 5 s x 3 2 2 7 . 0 8 4 0 3 S 8 9 . 9 2 I 5 0 h 3 2 2 0 . 5 3 8 20 3 5 8 6 . 4 4 1 5 e a 3 2 1 7 . 9 I I 2 0 0 20 3 5 7 2 . 7 3 4 2 s x 3 1 9 0 . 8 9 5 3 5 6 7 . 1 2 3 1 9 0 . 0 6 20 3 5 6 2 . 8 9 l O O k l 3 1 7 6 . 5 4 I 2 3 5 3 3 . 9 1 2 e a 3 1 7 3 . 5 I I 5 3 5 3 0 . 3 5 l O s x 3 1 4 5 . 6 0 I 5 3 5 0 5 . 1 5 l O O k l 3 1 3 7 . 8 3 I 10 3 5 0 1 . 9 I I 3 s x 3 1 2 9 . 6 3 I 3 0 b h 3 4 8 5 . 7 5h 3 1 1 8 . 9 2 3 0 3 4 8 3 . 3 9 2 3 1 1 8 . 1 9 2 3 4 7 6 . 2 5 l O O e a 3 1 1 7 . 7 50 3 4 6 3 . 6 I I 2 s x 3 1 1 1 . 3 8 2 3 4 5 3 . 0 I I 1 0 3 1 0 2 . 8 7 4 I 1 0 3 4 5 1 . 9 0 I 3 0 3 0 8 9 . 0 9 3 I 80 3 4 5 1 . 7 0 I I 20 3 0 8 7 . 0 3 9 1 0 3 4 5 1 . 6 1 9 I I 1 0 3 0 6 8 . 5 I I 10 3 4 5 0 . 0 I I l O s x 3 0 6 2 . 4 4 1 0 3 4 3 7 . 3 6 I 2 s x 3 0 6 1 . 1 3 - 4 5 I n t e n s i t y W a v e l e n g t h C l a s s (JUT) A r c S p a r k 1 0 0 3 0 4 3 . 9 0 I 2 0 s x 3 0 3 1 . 6 5 2 s x 3 0 2 5 . 6 1 1 0 - 3 0 1 7 . 4 6 2 5 e a 3 016*4 I I 2 s x 3 0 1 0 . 1 9 1 0 g s 3 0 0 2 . 7 4 I I 2 0 e a 2 9 8 6 . 9 I I 2 2 9 8 0 . 1 6 2 k l 2 9 7 3 . 0 0 . 125 2 9 4 8 . 7 1 9 I I 2 h k l 2 9 2 6 . 6 4 6 l O e a 2 9 1 4 . 5 I I 1 0 g s 2 8 8 7 . 1 9 I I l O O r 2 8 7 3 . 3 1 6 2 0 e a 2 8 7 3 . 0 5 2 8 6 8 . 1 4 8 60 2 8 6 4 . 2 5 7 2 s x 2 8 6 0 . 6 4 2 e a 2 S 4 5 . 2 I I 20 2 8 4 0 . 6 6 I I 5 0 0 r 2 8 3 3 . 0 6 9 I 1 5 0 r 2 8 2 3 . 1 8 9 I 2 5 0 r h 2 8 0 2 . 0 0 3 2 2 7 7 2 . 7 1 I I 2 0 e a 2 7 1 7 . 5 I I 2 h s x 2 7 1 7 . 3 7 15wh 2 6 9 7 . 5 2 7 5 2 6 8 4 . 7 6 I I 3-00wh 2 6 6 3 . 1 6 6 l O O k l 2 6 5 0 . 4 5 h s x 2 6 5 0 . 2 6 2 2 6 3 7 . 7 2 2 0 e a 2 6 3 4 . 3 I I 5 0 h z 2 6 2 8 . 2 6 3 I I I n t e n s i t y W a v e l e n g t h C l a s s O U T ) A r c S p a r k 2 0 0 r 2 6 1 4 . 1 7 8 5 0 r 2 6 1 3 . 6 5 3 5 e a 2 6 0 8 . 4 lOOwh 2 5 7 7 . 2 6 3 1 0 0 2 5 7 6 . 5 5 10 2 5 6 8 . 4 3 1 0 0 2 5 6 2 . 2 8 2 s x 2 5 5 0 . 2 9 2 s x 2 5 3 4 . 7 8 2 s x 2 5 2 7 . 0 3 l O O g s 2 5 2 6 . 6 2 5 s x 2 5 0 8 . 9 0 2 s x 2 5 0 0 . 5 0 2 s x 2 4 9 5 . 5 8 150wh 2 4 7 6 . 3 7 9 2 0 s x 2 4 6 1 . 5 1 15 Own 2 4 4 6 . 1 8 8 2 e a 2 4 4 5 . 1 lOOw 2 4 4 3 . 8 3 8 l O h k l 2 4 2 8 . 6 4 4 7 5 h z 2 4 1 1 . 7 3 5 50 2 4 0 1 . 9 4 6 35 2 3 9 9 . 5 8 3 2 5 0 0 23 9 3 . 7 9 4 4 0 2 3 8 8 . 7 7 4 3 2 3 5 9 . 5 8 60 2 3 3 2 . 4 2 6 3 2 2 9 6 . 8 5 2 h 2 2 8 0 . 8 5 4 2 2 5 7 . 5 0 4 0 2 2 5 3 . 9 5 3 Or 2 2 4 6 . 8 8 8 15 2 2 4 2 . 6 1 0 50w 2 2 3 7 . 4 2 6 50 2 2 1 8 . 0 8 - 46 -I n t e n s i t y W a v e l e n g t h C l a s s I n t e n s i t y W a v e l e n g t h C l a s s (KITT OUT) A r c S p a r k 5 0 0 0 r 2 2 0 3 . 5 0 5 I I 50 2 1 8 7 . 8 5 6 2 1 8 1 . 6 6 I I 40w 2 1 7 5 . 5 8 l O O O r 2 1 6 S . 9 9 4 I 15 2 1 5 9 . 5 2 3 Or 2 1 1 5 . 0 2 15 2 1 1 2 . 0 3 20w 2 1 1 1 . 7 4 3 Or 2 0 8 8 . 4 3 12 2 0 8 7 . 5 8 2 0 1 g 2 0 7 4 . 0 8 r 2 0 6 1 . 7 4 12 2053 . 3 2 I 5 2 0 2 2 . 0 7 - 47 -Intensity (MIT) Arc Spark 18 35 5 30 15 Table II SPECTRAL LINES OF TELLURIUM Compiled from KIT Table of Wavelength Wavelength Class 7280.9 7191.08 7144.61 7135.34 7103.43 I I Intensity (MIT) Arc Spark 15 5 30 100 15 Wavelength Class 6659.80 6658.07 6649.72 6648.52 6640.59 5 5 70 50 15 7097.57 7049.68 7038.95 7015.89' 6959.08 50 50 181 5 30 6637.00 6628.7 6613.4 6603.04 6596.32 70 i— o 30 30 50 6930.51 6924.66 6913.18 6885.04 6877.94 30 15 30 501 121 6584.93 6587.63 6574.50 6563.95 6553.5 5w 6870.56 I 5 6546.59 5 6867.53 30 6541.89 501 6854.7 I 100 6537.01 70 6843.94 I 5 6528.35 50w 6837.65 15 6491.81 30 6832.46 70 6487.09 5 6796.66 30 6474.28 5 6782.54 3L 6469.13 I 30 6761.34 7 6462.9 I 50 6751.40 5 6457.80 3 0 673 6.32 15w 6456.7 I 30 6708.34 5 6446.7 I 5 6688.49 1000 6437.06 5 6687.36 70 6422.96 7w 6686.7 6409.4 30 6686.03 18 6405.9 70 6676.01 50 6396.46 5 6672.70 15 6390.19 50 6670.6 70 6367.10 30 6661.06 5 6363.93 - 48 -Intensity Wavelength Class Intensity Wavelength Class (JUT) (MIT) Arc Spark Arc Spark 3 0 6345.51 5 633 0.04 5 6324.38 30 6298.64 3 6277.7 15 6106.55 " I 30 6082.45 I 50 6073.35 5L 6055.82 100 6047.44 18s 6273.41 I 30 6022.06 70 6266.21 . 7w 6020.8 30 6260.51 30 6019.51 30 6259.44 100 6014.49 12 6255.9 I 3L 6013.49 150 6245.61 •- / 1 30 6008.62 15 6243 .24 15 6007.97 300 6230.80 15 6001.34 50 - 6221.48 75 5993.94 30 6211.49 75 5993.12 15 6203.29 75 5985.64 70 6202.29 250 5974.70 5 6195.05 50 5972.64 5 6187.56 25 5954.96 50 6183.39 75 5936,21 30 6180:58 I 15 5 932.22 6181.94 8L 5925.25 5 6171.83 25 5896.65 15 6167.24 15 5891.43 100 6166.84 8 5888.89 3w 6162.4 I 3s 5874.6 12w 6160.2 I 15 5871.80 15 6155.16 15 5866.11 70 6153.89 8 5859.7 2 6148.1 I 50 5858.55 5 6146.57 75 5851.09 30 6144.32 8 5845.02 30 6142.24 25 5843.31 15 6140.28 15 5835.16 70 6136.31 I 25 5828.63 Intensity Wavelength Class O U T ) Arc Spark Intensity Wavelength Class O U T ) Arc Spark 50 5826.45 50 5824.19 50 5803.07 9 5789.22 8 5787.05 8 5785.23 15 5777.25 35 5770.92 6 5769.1 70 5765.25 25 5763.92 15 5762.60 250 5755.87 8 5746.31 8 5745.75 70 5741.66 8 5733.5 8 5692.96 25 5679.71 15 5672.24 15 5667.86 50 5666.26 25 5655.15 25 5652.06 25 5651.51 250 5649.30 25 5630.66 , 25 5618.47 15 5592.90 15 5582.77 8 5580.46 15 5569.38 8 5565.57 25 5536.73 50 5488.07 8 5480.83 50 5479.13 15 5465.17 75 544 9.82 8 5426.05 25 5410.41 25 5366.91 15 5366.28 8 5350.41 8 5321.02 25 5304.99 15 5303.72 25 5256.36 8 5244.21 15 5238.07 8 5219.46 15 5172.99 8 5164.00 8 5149.90 8 5148.7 8 5133.23 15 5130,99 8 5112.12 8 5065,74 8 5060.44 15 5037,97 8 5020.44 25 5000.87 15 4925.25 30 4919.12 15 4912.05 30 4901,14 150 4894.94 70 4893.58 15 4875.53 - 50 -Intensity Wavelength Class (JUT) Arc Spark 800 4866.22 50 4865.13 800 4864.10 50 4842.88 800 4831.29 50 4827.14 70 4796.10 70 4784.85 50 4771.56 30 4769.73 150 4766.03 15 4765.03 50 4738.67 70 4731.27 50 4729.83 15 4726.91 70 4711.16 70 4706.53 300 4686.95 15L 4681.06 15 4676.88 30 4670.11 800 4654.38 15 4645.22 70 4641.19 50 4630.57 15 4621.27 800 4602.37 15 4586.98 70 4569.71 15 4562.45 300 4557.84 30 4555.27 50 4552.10 15 4551.04 Intensity Wavelength Class (MIT) Arc Spark 15 4550.36 50 4546.64 50 4537.07 15 4535.83 30 4529.54 15 4522.22 30 4514.05 50 4498.59 30 4489.33 30 4485.77 800 4478.73 15 4455.28 70 4434.96 50 4411.78 50 4410.95 15 4405.49 100 4401.89 70 43 98.45 100 4396.00 70 4377.10 50 4373.00 30 4368.09 400 43 64.02 50 43 61.27 30 4349.29 15 4346.86 15 4332.54 30 4325.77 30 4323.79 30 4321.96 70 4294.25 70 4293.35 70 4285.84 15 4279.48 50 4276.68 - 51 Intensity Wavelength Class (MIT) Arc Spark 70 4273.40 30 4264.32 300 4261.08 50 4256.10 70 4251,15 15 4246.47 50 4238.46 100 4229.42 15 4211.32 15 4197.22 30 4183.99 50 4163.53 30 4127.34 15 4127.04 15 4113.60 50 4101.07 3 00 4073.57 70 4048.89 15 4047.18 15 4029.73 30 4011.68 100 4006.50 5 3988.33 10 3981.74 10 3 975.90 10" 3969.18 10 3 947.93 5 3936.21 5 3 931.43 5 3918.50 10 3644.46 25 3644.27 25 3617.55 5 3612.90 5 3611.77 Intensity Wavelength Class (MIT) Arc Spark 5 3589.78 350 3585.34 15 3552.15 15 3521.27 5 3480.28 10 3456.84 5 3449.57 10 3442.22 10 3438.73 15 3406.77 25 3374.10 15 3365.17 50 3362.83 10 3358.12 5 3356.89 35 3352.11 5 3350.68 35 3345,93 25 3340.07 15 3323.06 5 3301.48 5 3301.18 10 3282.67 10 3278.77 15 3277.50 35 3256.81 25 3229.52 5 3228.27 35 8211.20 5 3201.06 10 3194,41 10 3193.59 10 3193.12 5 3189.87 10 3188.37 - 52 -Intensity Wavelength Class tar) Arc Spark 5 3186.46 10L 3159.31 10L 3158.23 5. 3154.24 5 3152.81 15 3151.46 10 3145.18 5 3141.62 5 3136.15 10 3132.58 15 3128.74 10 3125.91 10 3120.36 25 3117.99 15 3112.00 5 3105.95 10 3104.41 5L 3101.45 5L 3097.33 10 3 087.52 5L 3085.40 5 3083.91 25 3 076.57 100 3073.53 10 3073.01 10 3063.18 5 3 052.44 10 3050.01 35 3049.36 350 3047.00 5 3040.46 10 3034.62 5 3033.35 5 3029.90 15 3028.45 Intensity Wavelength Class (MIT) Arc Spark 100 3023.29 350 3017.51 20 3015.09 5 3013.63 25 3012.05 5 3009.96 50 3006.35 10 3004.87 10 2998.89 50 2997.05 15 2995.67 5 2990.73 10 2988.97 15 2985.47 10 2983.24 15 2980.02 30 2 977-. 94 100 2975.91 50 2973.69 300 2976.21 25 2959.43 10 2955.55 10 2954.82 15 2951.48 100 2949.52 5 2 944.96 100L 2942.16 5 2 940; 95 15 2937.90 5 2936i77 5 2931.88 10 2930.17 15 2928.23 25 2925.37 15 2924.02 - 5 3 Intensity Wavelength Class (JUT) Arc Spark 5 2 921.97 50 2919.96 10 2911.35 5 2908.36 15 2906.98 10 2900.52 30 2893.27 5 2872.18 10 2869.72 100 2868.86 5 2867.67 2 2842.02 30 2841.18 10 2839.02 10 2831.48 15 2827.20 5 2821.57 5 2813.96 10 2809.90 300 2793.24 30 2791.95 10 2778.08 5 2772.64 10 2767.16 5 2759.51 10 2753.64 10 2752.21 10 2751.59 5L 2746.38 25 2745.57 10 2740.37 5L 2738.50 25 2737.90 5 2736;84 30 2726.36 Intensity Wavelength Class (JUT) Arc Spark 30 2724.25 50 2711.61 25 2703.54 5 2700.17 15 2697 .,67 50 2695.55 15 2694.55 10 2689.06 5 2686.48 30 2684.52 5 2683.76 10 2680.60 5 2677.16 25 2662.11 30 2661.13 10 2657.72 15 2649.80 10 2648.61 5 2644.84 10 2642.09 15 2640^35 5 2639.17 350 2635.55 10 2628.24 15 2625.12 15 2622.26 O 2616.91 5 2608.18 10 2605.89 5 2595.33 30 2591.35 15 2586.04 15 2581.05 50 2579.24 25 2577.57 - 5 4 -Intensity Wavelength Class Intensity Wavelength Class (MIT) (MIT) Arc Spark Arc Spark 15 2573.14 10 2412.06 15 2568il3 50 2411.38 150 2564.58 5 2410.88 10 2562.28 5 2407.54 25 2559.71 30 2403.66 5 2549.21 ".' 100 ^2403.00 5 2544.89 10 2401.81 5 2537.19 25 2389.79 10 2533.05 600 300 2385.76 30 2530.70 I 500 300 2383.25 10 2500.88 300 2499.75 10 2499.22 15 2494.67 15 2490.33 30 2374.94 10 2372.86 50 2369.92 10 2367.00 5 2361.64 100 2489.20 10 2485.24 10 2481.22 5 2479.85 300 2469.62 50 2467.72 5 2456.90 5 2455.54 10 2452.53 5 ' 2447.92 5 2359.53 10 2358.23 5 2356.60 5 2355.08 5 2354.90 5 2348.74 10 2343.83 15 2336.15 5 2334.91 15 2332.32 25 2438.80 25 2436.63 15 2434.73 10 2434.35 10 2433.82 50 2329.12 25 2327.45 5 2326.22 5 2322.22 10 2320.09 10 15 2431.78 I 5 2311.81 10 2431.45 10L 2304.29 100 2426.39 2288.90 10 2 2420.14 I 2287.84 50 2415.59 2286.71 - 55 -Intensity Wavelength Class Intensity Wavelength Class (MIT) (MIT) Arc Spark Arc Spark 15 2226.07 10 2216.61 5 2209.45 200 2 2208.83 50 2207.17 5 2196.07 5 2187.22 10 2178.13 100 2159.79 15 2148.04 150 2147.19 25 2143.54 600 2142.75 10 2121.75 10 2117.46 30 2109.83 100 2107.63 100 2107.20 10 2101.74 5 2081.88 400 .2081.03 15 2067.94 30 2066.49 5 2061.03 5 2044.22 3 00 2039.79 5 2024.20 50 5 2002.72 600 2001.59 30 2000.2 - 56 -T a b l e I I I RECORD OP RUNS A T T E M P T E D R u n D r o p p i n g R a t e S u p e r c o o l i n g S u c c e s s c m / h r 1 . 7 6 350 g o o d 2 . 7 6 5 0 ° g o o d 3 . 7 6 — i n c o m p l e t e 4 . 5 6 35° p o o r 5 . 7 6 55° f a i r 6 . 4 8 2 5 ° g o o d 7 . 7 6 45° p o o r B I B L I O G R A P H Y ( 1 ) B u c k l e y H . E. C r y s t a l G r o w t h . J o h n W i l e y a n d S o n s I n c . T h i s r e f e r e n c e i n c l u d e s a c o m p r e h e n s i v e c o v e r a g e o f t h e r e f e r e n c e s on c r y s t a l g r o w t h t o 1 9 5 1 ( 2 ) S t o c k b a r g e r D . P , A l s o r e f e r r e d t o w e r e : R . S . I . 7 , 1 3 3 , 1 9 3 6 ( 3 ) V e n a er A . J . C r y s t a l G r o w t h a n d D i s l o c a t i o n s , ( 4 ) D a v e y P. D . ( 5 ) L a w s o n W. D . ( 6 ) R . S m i t h H o p k i n s A S t u d y o f C r y s t a l S t r u c t u r e a n d i t s A p p l i c a t i o n s A I l e t h o d o f G r o w i n g S i n g l e C r y s t a l s o f PbTe a n d P b S e . J . A . P . 2 2 , 1 4 4 4 , 1 9 5 1 . O x y g e n F r e e S i n g l e C r y s t a l s o f P b T e , P b S e , a n d P b S . J . A . P . 2 3 , 4 9 5 , 1 9 5 2 C h a p t e r s i n t h e C h e m i s t r y o f L e s s F a m i l i a r E l e m e n t s . Ii bo 900 300 --- - -- ---- - -- - - -— - - - - -T - - - -- -1 | -! H _ *• / J? IB '}\ A / s * % \ 1 r I fi q 7 -e 7 1 ft J E ~ f T r ~t V > \ * 1. 1 - J I i 7 ? • 1 j *-L —4 I _j % 1 T 7 —• 1 J * - | — / F — \ 7 \ V • <, \ \ L s 1 \ r \ > 1 l J _ \ 1 • v. -\ 1 I" _ 1 1 1 T 1 t — f 7cc 600 5 op 8 to IZ. $ 0 4 1 i 1 1 j... . 1 ... - - \ ' i 1 i _ --• t —|— - -— ---- ... --4 . . -\ - - . . . - T . . . -- -1 - - --- -- - -i - j " i -.1 . . i - 1 i ** .i ; r i r T 1 f - - ! 1 1 l 1 ; | --• -4- I r i | i ! - 1 - - - i - - I Tl L . ' . I 4 ... i 'Si ...i i_ . . . / . . . v| 1 t ' s { / i S •> i _ V \ fa . . . i r { - ! r r f.. \ I — 1 K I '4 ( 1 i ... -— t - . . . r-- - \ - i • -i-4 -\ - -il t - • •-. . . ~'f -I • j . . . -«... -1 1 -- . . . - 1 . . . - . . . . - P T ! ' I . . .—H-1 p r r r 1 - 4 4 - — - v~ —U-I--. ^ — j - - - — f— — -U L . . . - - . -.1... - . . . - 4 1.1. . . . . . . - n ; j - ! . . . -- J r --U ! _ i j B •' i— -j •>.' i I j i - H — K J . . . - i ! —r - i IT i : 3" C £ t - fc n? t . _ i - _ . . . - B-| _ -- 'X- —f, .(IS -. . . J "t f t t i >-) R i i 4 * . L 0 ~| 'N -V • i S \ - r ] r s V . I" — r . . . / - - . . . , -\ k - _ - :- _. 4 - - 4— , r j -- . . . . 1 i ! i '• | : i A ! T -! - -. . . . . . . -1 ! i " -}--4-- • * yj >"A i ._ f i . . . . . . - r - -i t r' r _ ! -- I ! - * ! | i 1 i „ I i -- 1 ! '• L r-! j 1 i 4 _ . . . - - 1 I i - -• . . . - ...j -- . . . 1 ! '—1— T - 1 . . . . -- - - . . . - 1 p. i - - i . . . . . . i -- ! I 1 ! • ! [ j I - ;~T j ! }. —t— i I ' i i i I ( 1 - [ T | i — i — • 1 i s j _ L _ i .J. L, 7 o o boo 6 /o 100 r 1 ~V . . 4 . . . 1 \~ i — — ~m-- i - — - ' 1 -- ! H . . . . . . . . . —1—1—f— --4-- —t --- . . . — 1 i 1 i • l L_ , • M ---i ! i -. . . 4 . I -•--1 ! i -- - i _ . . . . ! I ; i ! i , — r - - • . . . . ! ' i i I • . .j j ( P r h -• WW i i ?~ Br - i 1 - 1 1 • 1 k . . . \ - s i 1 ft -ft M 1 < t J C ) \ t Si If* i x - - \ -- -. . . . •- - -•'+---j—1 - • J_ -| 1 - -- i—j-^ j i - -- - __ 1 - i - . . . - I | — -- - ---- i . . . - r p - - -i . . . - 1 . . . ! 1 1 -- 1 1 i \ - t . . . I _ . \ - - 1 t - -V -j s > . . . 1 ... -. . . -• . . . . ..p. _ - . . . . . . -- --- -[ 1 1 - I _ - 4- -i . i. 1 _ i... 1._ - f r 1 - - . . . . . . -- -> -'j 1 ft 7oo 5 0 a 506 f£r]p A 2 " : 4 6 / ^ - 9 '4* PlAGRAn t 3 COIL MOWBD &Y CURRENT SY THERMOCOUPLE '2 tfisLAY CO/L. OPEpATeD BY TUB£- ourfi>uT £HCL05E0 RELAY 7<E*iP£RA TUilE. SETTING /NDEX TAMPS TUPE POINTED f _ HOLD I M C, RfLrt Y j 10 V O L T S U P P L Y START -RuH SWITCH CONTKOLHISJ ^ r V A R I A C WHPF1.C.O C O I L D.a^ram 2, H A I N 5 C O N T R O L CIRCUIT RE.LW 110 V O L T S U P P L Y ATI PERIT£ T U E R n A i -SV/TCH C3 = 3 S T A R T - RUN S W I T C H RELAY . T O V A R I A C (v5£E D I A G R A H 3 ) DiasTam3. D E L A Y E D MAINS CONTROL CIRCUIT lio V O L T D . C S U P P L Y CLOSED rn_ R C U A V I C O I u V V V v 4- AUXILIARY D-C SUPPLY . CONTROL 14 ... i l l - D - Cf.L. CI b * w ™ 5 F U R N A C E * P H O L O G R A P H I C . p«if>rr 1 A PHOTOGRAPHIC PRINT PHOTOGRAPHIC PAIIMT 4-«nie.RoPn*rTO<3«ePH OF CRvsTfita r v \ i c « o Ptt&rcxsaapn op u)ftatf s«\<a*j» c ft V S T A is. 

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