The Vaporization Coefficient of Silica
Authors: Ingersoll Nolan , Costa Gustavo , Jacobson Nathan S. 
Published: February 2020

Free surface or Langmuir evaporation of a solid oxide often leads to a vapor flux that 1-4 orders of magnitude lower than that calculated from equilibrium vapor pressures [1]. This factor is termed the “vaporization coefficient” and clearly has a large impact on determining vaporization rates.
The most common method of measuring vaporization coefficients is to measure the vapor flux from a free surface and compare it to the vapor flux calculated from equilibrium vapor pressures. In this study, we used a vacuum thermogravimetric analyzer (TGA) to measure the vapor flux from a silica coupon. Initially we had deposition on the coupon of W and Ta oxides from the furnace element and heat shields. This problem was eliminated with a graphite tube surrounding the coupon. The coupons were amorphous, however the temperatures of vaporization (1475 to 1675°C) induced surface crystallization to a surface layer of cristobalite. This was verified with x-ray diffraction (XRD) and scanning electron microscopy (SEM).
Thermodynamically, the vaporization of SiO2(cristobilite) leads SiO2(g), SiO(g), O2(g), O(g). The pressures were calculated from a free energy minimization code [2] and converted to vapor fluxes with the Hertz-Knudsen-Langmuir equation. These calculated fluxes were compared to the measured fluxes and the vaporization coefficient is simply the quotient of the two. These data are given in Table 1 below.
Table 1. Vaporization coefficients obtained by comparison of free surface vapor flux to calculated flux.

Temperature (°C)
J(calculated)
J(measured)

Vaporization Coefficient

mg/cm2-hr
mg/cm2-hr

1475
3.797459
0.0168

4.42E-03

1525
10.20388
0.0451

4.42E-03

1575
25.95741
0.1421

5.47E-03

1625
62.79001
0.3606

5.74E-03

1675
144.9916
0.7064

4.87E-03

Other investigators have determined the vaporization coefficient for silica with the Knudsen cell method and variation of the orifice geometry [3]. This method is discussed and results for silica [4-7]. The two methods will be discussed as well as differences and similarities in the results.
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