Principles and Applications of the Iodide Process

The thermodynamic factors favoring deposition of elements from vapors of their iodides are the low heats of formation of those iodides and the ease with which diatomic iodine dissociates at moderately high temperatures. The vapor pressure of the element and the stability of its lower iodides determine the temperature and pressure range in which deposition can occur. The lower pressure limit for deposition at a given temperature is set by the vapor pressure of the deposited element and the upper limit by the stabilities of the lower iodides. At very low pressures, deposition is a first order heterogeneous reaction, every iodide molecule which strikes the surface adding to the deposit. As the pressure is increased, the deposition rate falls off and eventually becomes zero order, the rate being governed by that at which I atoms evaporate from the saturated surface. At higher pressures the deposition reaction is diffusion‐controlled. Factors influencing the character of the deposits, the removal of impurities, and the deposition of alloys are discussed.