Evaluation of D As4-Ar at 760 to 834 K
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D. C. LYNCH is Assistant Professor of Metallurgy, and D. SHOKRI is a Graduate Student, Department of Mining, Metallurgical and Ceramic Engineering, University of Washington, Seattle, WA 98195. Manuscript submitted August 6, 1979.
subjected to a v a c u u m and then warmed to remove any residual acetone. The gas in the bulb of the cell must be saturated with arsenic vapor. Jones and Philipp 4 reported t h a t arsenic is readily vaporized at temperatures above 473 K. However, to insure that the gas was saturated, two cells of different sizes were used. The dimensions of the cells are reported in Table I. Although metallic arsenic is known to vaporize as As, As2, A s 3 and As4, only the latter has a significant vapor pressure at the temperatures of interest in this study? The partial pressure of A s 4 is approximately six orders of magnitude greater than that of PAsz; As2 having the second greatest pressure. The general form of the equation of binary gaseous diffusion is given by: 6 Jl -
- PrDI_2 dXl RT dZ
+ X l ( J l q- J2)
[l]
where D ~-2 = the binary diffusion coefficient; J],J2 = flux of component 1 and 2; P r = total pressure; R = gas constant; T = absolute temperature; X] = mole fraction of component l; Z -- distance. Evaluation of Eq. [1] for the diffusion of As 4 through a stagnant Ar layer yields: W
- P r D As4_ArA R TI in (1 -
F = 299.7
X~,s,)
[2]
where F = total flow, mol As4/s; W = rate of weight loss of cell, g/s; A = cross sectional area of the diffusion path, cm2; [ = length of diffusion path, cm; X cAs 4 -- mole fraction of As4 vapor in the cell. The experimental conditions, the experimental results and the values of Dgs4_Ar calculated from Eq. [2] are listed in Table I. Once steady state conditions were achieved, the rate at which the cell lost weight was recorded. These values were found to be independent of the gas flow rate at gas velocities at and above l0 c m / s at the furnace temperature, and the calculated values of the diffusivity were found to be independent of the cell size at temperatures above 710 K. Thus, it is concluded
Table I. Summary of Experimental Results S m a l l Cell
Temperature deg K 764 787 800 805 809 814 821 832 Large Cell 760 787 810 810.5 822 834
l
= 3.2 c m
Rate g-mol As4/s
1.13 2.22 3.42 3.83 4.48 5.06 6.41 9.17
10-s 10 8 10-8 10-8 10 8 10-8 10 8 10 8
l = 3.3 cm 2.69. 10-8 6.31 ~ 10-8 1.29 ~ 10 -7 1.29. 10 -7 1,85
9 10 -7
2.79
9 10 -7
A
= 0.096 c m 2
P As4 atm. 0.057 0.110 0.157 0.180 0.200 0.230 0.275 0.360 A = 0.283 c m 0.051 0.110 0.209 0.210 0.279 0.390
D As4.Ar cm2/s 0.40 0.41 0.44 0.43 0.44 0.43 0.45 0.47 2
0.37 0.38 0.43 0.42 0.44 0.45
ISSN 0360-2141/80/0911-0531500.75/0 METALLURGICAL TRANSACTIONSB 9 1980 AMERICAN SOCIETY FOR METALS AND VOLUME 11B, SEPTEMBER 1980--531 THE METALLURGICAL SOCIETY OF AIME
that the powdered arsenic yielded a saturated gas phase which in turn served as the As 4 source. The Chapman-Enskog kinetic theory predicts that the diffusivity increases roughly as the 1.65 power of temperature at high temperatures. 7 Ac
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