Rate‐Determining Process of Surface Reaction in the Phosphidation of Silver

A study was undertaken concerning the kinetics and mechanism of the phosphidation of silver at 450°C in phosphorus vapor at 44.0 to 80.0 kPa. X‐ray diffraction patterns and an electron probe microanalysis of the reaction products demonstrated that the films were composed of single layers of . A marker experiment indicated that silver was apparently the component that diffused across these films. In the early stage, the phosphidation obeyed a linear rate law and its rate‐determining process was regarded as the surface reaction; that is, the chemisorption of molecules generated by dissociation of a molecule. When the thickness of a phosphide layer reached a critical value, the reaction kinetics exhibited a gradual transition from the linear to the parabolic rate law as a result of the rate‐determining process changing from adsorption to solid‐state diffusion.