Method for the estimation of imperfection density by thermoelectric means
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D. P O L L O C K
A method is developed for the e s t i m a t i o n of the mol f r a c t i o n of i m p e r f e c t i o n s in c o l d - w o r k e d m e t a l ; this is b a s e d upon an analogy with the t h e r m o d y n a m i c s of r e v e r s i b l e c e l l s . T h i s method cannot d i f f e r e n t i a t e between types of l a t t i c e i m p e r f e c t i o n s or m e c h a n i s m s of r e c o v e r y . T h i s technique shows that four s t a g e s a r e involved in the r e c o v e r y of c o l d - w o r k e d p l a t i n u m above 100~ T h e s e s t a g e s c o r r e s p o n d c l o s e l y to the r e c o v e r y b e h a v i o r of p l a t i n u m noted by M i u r a et al. and c o n f i r m t h e i r findings.
ESTIMATES of imperfection densities can be made by means of techniques which include electrical resistivity, calorimetry, electron microscopy, X-ray transmission and reflection, decoration, and etching. Each of these methods has optimum areas of application as well as limitationsJ It is considered that methods which rely on the selection of more or less randomly selected cross sections for estimations of imperfection densities are not as representative as those utilizing bulk effects. Calorimetric techniques provide such estimates but they require special equipment and are difficult to perform. 2 Koehler3 suggested that thermoelectric measurements could provide additional data. Approximations based on this property are representative of the volume of the specimen and are much more readily made than are those based upon calorimetry. Considerable work has been done on the change in thermoelectric power as a measure of imperfection densities. 4-7 This has been based upon electron-imperfection interactions. Most of this work has been done at temperatures at or below room temperature. The present work employs an analogy to the thermodynamics of reversible cells to estimate the mol fraction of imperfections and to observe changes in recovery phenomenaat elevated temperatures.
1) BASIS FOR T H E R M O E L E C T R I C APPROXIMATION C o n s i d e r a t h e r m o c o u p l e c o m p o s e d of a n n e a l e d p i e c e s of a given pure m e t a l in the i d e n t i c a l m e t a l l u r g i c a l condition. No e l e c t r o m o t i v e force will be gene r a t e d when such a couple is s u b j e c t e d to a t e m p e r a t u r e d i f f e r e n c e between i t s m e a s u r i n g and r e f e r e n c e j u n c t i o n s . T h i s r e s u l t s f r o m the fact that the T h o m p son effects in the legs a r e equal and opposite in sign and cancel each other. If, however, a c o l d - w o r k e d piece of the s a m e pure m e t a l i s s u b s t i t u t e d for one of the a n n e a l e d legs of this couple, a s m a l l e l e c t r o m o t i v e force will be g e n e r a t e d . The T h o m p s o n effects a r e no longer equal. This s m a l l d i f f e r e n c e a r i s e s f r o m the change in the e l e c t r o n i c s p e c i f i c heat of the worked t h e r m o e l e m e n t . 8 Since DANIEL D. POLLOCKis
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