Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 183face of a crystal. Evaporation can be accomplished by resistance heating, r.f.induction heating, radiation heating, electron bombardment or by sputteringcaused by ion bombardment. The general techniques have been reviewed by anumber of author^^^-^^ and vapour pressure oersus temperature data36 give auseful guide to the conditions necessary for evaporating a particular material.The simplest method of evaporation is by direct heating of a wire or ribbon.If the vapour pressure at the melting point is too low for successful evaporation,the material can be wound or electroplated on a higher melting point material,usually tungsten. For materials not readily available in wire form, films can beevaporated from pieces lodged in a tungsten spiral or placed in an inert crucible.Low melting point metals such as sodium have been prepared as films by distillationfrom a side tube into a larger vessel. Electron bombardment heating of thematerial or the crucible can be used instead of direct heating. Sputtering of metalsand semiconductors by bombardment with inert gas ions" is also widely applicable(see Section 2.1.1e). A list of films prepared by the various techniques is givenin Table 1.One of the disadvantages of evaporated films is that their crystallinity is notusually well defined. The BET surface area is often much larger than the geometricalarea and may be proportional to the weight of evaporated material4oi4zs 50*51indicating a very porous structure. The physical and chemical properties may thusbe quite different from those of the bulk material. This problem can be overcometo some extent by sintering the film before use and it is preferable to heat the filmto a higher temperature than that to be used in carrying out the experiments.Low melting point metals and Group IB metals sinter above - 80" C. Metalsmelting between 1400 and 3000" C show an increasing surface area with meltingpoint because they become increasingly difficult to sinter.TABLE 1PREPARATION OF EVAPORATED METAL FILMS~Technique Material ReferenceDirect evaporation of a wireOverwinding on tungstenEvaporation from tungstenspiral or crucibleElectroplating on tungstenElectron bombardment heatingSputtering with inert gas ionsDistillationTi, V, Cr, Fe, Co, Ni, W, Mo, Ta, Nb, 33, 37, 38, 42Ir, Pd, Pt, RhCu, Ag. Au, Zn, Co 37, 38Ge, Si 40,43Mn 38.39C 41Ge, Si, Ga, Sb, Ti, V, Cr, Fe, Co, Ni,Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf,Ta, W, Re, Os, Ir, Pt, Au, Th, U, Al,BeNa, K, Ca, Ba, As, Sb, Bi43, 44,4546, 47, 48, 49References pp. 270-278