Heat transfer coefficient in aluminum alloy die casting
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Discussion of "Electrochemical Determination of the Free Energy of Formation of SnO2"*
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I 1200
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Tempecoture,K
S. S E E T H A R A M A N AND L . - I . S T A F F A N S S O N T h i s d i s c u s s i o n p e r t a i n s to t h e c o m p a r i s o n of t h e G i b b s e n e r g y of f o r m a t i o n of SnO2 r e p o r t e d b y B a m a n a r a y a n a n a n d B a r w i t h t h e r e s u l t s of a n u m b e r of s i m i lar measurements c a r r i e d o u t e a r l i e r 1-~ a n d w h i c h h a v e n o t b e e n r e f e r r e d to b y R a m a n a r a y a n a n a n d B a r , the latest among them being ours. 5 We used Ni-NiO as the reference electrode and there was no Pyrex seal separating the electrodes. O u r a t t e m p t s to u s e c h r o m e l w i r e s a s e l e c t r i c a l l e a d s in c o n t a c t w i t h m o l t e n t i n w e r e n o t s u c c e s s f u l a b o v e 1300 K d u e to s e r i o u s a t t a c k on t h e m a n d h e n c e we were forced to switch over to chromium cermet (Met a m i c 612, M o r g o n Co., U . K . ) e l e c t r o d e s w h i c h w e r e f o u n d to f u n c t i o n w e l l . T h e AG~n O v a l u e s r e p o r t e d b y R a m a n a r a y a n a n a n d B a r a g r e e we211 w i t h o u r r e s u l t s . B u t , a s t h e r e f e r e n c e e l e c t r o d e a s w e l l a s t h e s o u r c e of A G ~ f o r t h e r e f e r e n c e e l e c t r o d e a r e d i f f e r e n t f r o m o u r s , it w o u l d o n l y b e p r o p e r to ' c o n v e r t ' t h e i r e x p e r i m e n t a l d a t a to N i NiO reference electrode so that a meaningful comparison could be made. Hence their reported equation for the cell EMF: E (mY) = 624.4was recalculated the cell:
0 . 1 6 6 T (K) + 3.8
[11
u s i n g t h e e q u a t i o n f o r t h e E M F of
(-) Ni, NiO // ZrO~ - CaO//
0 . 0 7 0 4 6 T (K) + 0 . 1 9
[2]
v a l i d in t h e t e m p e r a t u r e r a n g e 924 to 1328 K. T h i s EMF expression has been recommended by Rapp and S h o r e s 7 a s t h e b e s t . T h e r e s u l t i n g e q u a t i o n of t h e E M F for the cell:
(-) Sn, SnO2//
ZrO2 - CaO//NiO,
Ni (+)
w i U then b e : E (mY) = 2 7 7 . 7 2 -
0 . 0 9 5 5 4 T (K) • 3 . 9 9 .
[3]
T h i s h a s b e e n i n c l u d e d in F i g , 1 t a k e n f r o m o u r p u b l i c a t i o n . 5 It c a n b e s e e n t h a t t h e a g r e e m e n t i s e x c e l l e n t . *T. A. RAMANARAYANAN and A. K. BAR: M e t . Trans. B, 1978, vol. 9B, pp. 485-86. S. SEETHARAMAN and L.-I. STAFFANSSON are Research Associate and Professor, respectively, Department of Theoretical Metallurgy, Royal Institute of Technology, S-100 44 Stockholm 70, Sweden. Discussion submitted December 281 1978. METALLURGICAL
TRANSACTIONS B
1. G. Petot-Ervas, R. Farhi, and C. Petot: J. Chem. Thermodyn., 1975, voI. 7, pp. 1131-36. 2. T. Palamutcu: Ph.D. Thesis, Imperial College of Science and Technology, London, 1970. 3. T. Oishi, T. Hiruma, and J. Moriyama: Nippon Ginzaku Gakkaishi, 1972, vol. 36, pp.
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