High temperature oxidation of tungsten wires in O 2 -Ar mixtures
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EXPERIMENTAL The experimental procedure and apparatus used in the present investigation are essentially the same as that used in the previous study on the vaporization kinetics of tungsten in pure argon.* Briefly, 1.53x 10-2 cm diam, 5.08 cm long straight tungsten wires were mounted vertically in a copper filament holder and placed into a Varian ultrahigh vacuum system. After the system was evacuated in the 10-1~to 10-I* atm range, the filaments were degassed. The vacuum vessel was then back-filled with an O2-Armixture of analyzed oxygen contentcontaining less than 1 ppm HzO and COz. The tungsten wires were resistively heated using a constant voltage power supply. In all cases, the wires were so-called doped tungsten containing about 100 ppm of impurities mainly consisting of AI, Si, and K. The rate of volatilization for a given temperature and oxygen partial pressure was obtained by measuring the weight loss per unit area as a function of time. The weight loss was obtained by comparing the weight of a segment of the constant temperature length of the filament* with the weight of an unreacted filament of *Temperatureprofilesshowthat approximately70 pct of the resistivelyheated tungstenfilamentis at a constanttemperature,that is approximately-+5 to 10 K. the same length and initial diameter. Unfortunately, an in situ gravimetric method could not be used because it is virtually impossible to inductively heat a 1.53• -2 cm diam tungsten wire in 0.789 atm of argon to temperatures near 3000K. However, since the standard ( u n r e a c t e d ) f i l a m e n t s for any given r u n w e r e n o r m a l l y within 5 to 10 ~g of each other and weight loss r a n g e d b e t w e e n 250 and 500 ~g, the i n d i r e c t g r a v i m e t r i c method was v e r y s a t i s f a c t o r y . All weight m e a s u r e m e n t s w e r e made on a M e t t l e r pan m i c r o b a l a n c e . B r i g h t n e s s t e m p e r a t u r e was m e a s u r e d with a m i c r o optical p y r o m e t e r , which was f r e q u e n t l y checked with a National B u r e a u of S t a n d a r d ' s c a l i b r a t e d p y r o m e t e r . E m i s s i v i t y c o r r e c t i o n s w e r e made u s i n g the data of DeVos, 4 while window c o r r e c t i o n s were m e a s u r e d e x perimentally. RESULTS
F. J. HARVEY is Senior Scientist, WestinghouseResearch Laboratories, Pittsburgh, Pa. 15235. Manuscript submitted November 27, 1972. METALLURGICALTRANSACTIONS
A summary of the rates of volatilization of tungsten as a function of temperature and oxygen partial pressure are shown in Fig. 1. The results have been ohVOLUME 4, JUNE 1973-1513
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9 pB^ =1. 18 x z
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pB =7.74 x 10-5 atm
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9 P0B =3.71x10- atm
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o p~2 =1. 73 x 10-5 at m
3) The r a t e s of v o l a t i l i z a t i o n a r e 3 to 5 o r d e r s of m a g n i t u d e g r e a t e r than the r a t e of v a p o r i z a t i o n of t u n g s t e n in high p u r i t y a r g o n at 0.789 a t m . ~ The r e s u l t s of s o m e s u p p l e m e
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