Part VII - Surface Depletion During Effusion

SURFACE depletion can be a significant source of error in the use of the effusion method for determining the thermodynamic properties of alloys, particularly in the case of solid alloys.' However, as yet this problem has not been treated quantitatively by experimentalists in the field.1-3 The method necessitates diffusive transport of material out of the source alloy to obtain a relevant measurement. The general character of the system is shown in Fig. 1. The diffusive flux (nD) at the alloy surface is equal to the effusive flux (nE): where D is the diffusion coefficient of the diffusing substance, C is its concentration, X is the distance from the surface into the source alloy, p is the density of the alloy, and AS is the actual available surface of the alloy. The concentration of the diffusing substance at the surface is CS, and Here fE is the Clausing factor, AE is the area (sq cm) of the orifice, y is the activity coefficient of the substance in the alloy, PO is the vapor pressure (Torr) of the pure substance, M is the molecular weight of the effusing species, and T is the absolute temperature. The differential equation for diffusive transport in the source alloy is Fick's second law. Its solution is dependent on the geometry of the source alloy. Two boundary conditions must be established to obtain the proper solution, the first being Eq. [1], or its equivalent. The solutions for several important forms of the source alloy are considered below.