Solubility limits of selenium in copper
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D. L. V A N W A G O N E R ,
AND C. H. PITT
The solid solubility limit of selenium in copper was determined experimentally ranging from 20 ppma at 675~ to 92 parts per million atomic at 900~ These results are extrapolated to 3 ppma at 500~ and 220 ppma at the melting point of copper using thermodynamic relationships. The enthalpy and entropy of dilute solution, AHs and ASs are found to be 15.1 Kcal and 5.52 e.u. respectively. The meaning of the thermodynamic quantities AHs and ASs is discussed.
MANY
d i f f e r e n t t e c h n i q u e s have been used for d e t e r m i n i n g solid s o l u b i l i t y l i m i t s in m e t a l alloy s y s t e m s . In g e n e r a l , t h e s e a p p r o a c h e s have b e e n i n a p p l i c a b l e w h e r e the s o l u b i l i t y l i m i t o c c u r s in v e r y dilute concentrations. For many industrial applications, dilue i m p u r i t y c o n c e n t r a t i o n s can have s i g n i f i c a n t i m p a c t on the b e h a v i o r of c e r t a i n m e t a l s . F o r e x a m p l e , as little as 6 ppma of s e l e n i u m in copper can r a i s e the r e c r y s t a l l i z a t i o n t e m p e r a t u r e of p u r e copper by 40oc. * TO determine the solubility limit of selenium in copper, two concepts were used. First, high temperature solute distributions were obtained by annealing copper specimens at the desired temperature for an extended period of time. The specimens were then quick quenched to freeze in the high temperature solute distribution. This technique was first developed for copper systems by Smart and Smith. z Having frozen in the high temperature conditions, it is possible to investigate the solute distribution through low temperature resistivity studies. At liquid helium temperatures, the resistivity becomes almost exclusively a function of the impurity distribution. These two techniques were effectively combined to study the interaction of silver and selenium impurities in copper.S It was this study which indicated the potential of the approach to the study of solubility limits. Below solid solubility limits, low temperature resistivity is a l i n e a r f u n c t i o n of the m i n o r e l e m e n t conc e n t r a t i o n . Above the s o l u b i l i t y l i m i t , the alloy is composed of two p h a s e s ; the solid s o l u t i o n phase and s m a l l p r e c i p i t a t e s of a s e c o n d phase. The s e c o n d phase p r e c i p i t a t e s c o n t r i b u t e v e r y l i t t l e to r e s i s t i v i t y . T h e r e f o r e , once the solid s o l u b i l i t y l i m i t is r e a c h e d , a d r a m a t i c change in slope will o c c u r when the r e s i s tivity is plotted a g a i n s t the solute e l e m e n t c o n c e n t r a tion. The change in slope of the low t e m p e r a t u r e r e s i s t i v i t y c u r v e d e f i n e s the s o l u b i l i t y l i m i t . THE RMODYNAMIC S T r a d i t i o n a l l y the e q u i l i b r i u m s o l u b i l i t y of dilute s o lutions m a y be a p p r o x i m a t e d by the e x p r e s s i o n : X = exp I -
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