The use of a field-emission microscope for the formation and study of whiskers produced on tungsten as a result of chloromethane adsorption

Projections, believed to be whiskers of tungsten or its carbide, were formed on a tungsten emitter as a result of the adsorption of chloromethanes and the application of an electrostatic field. The projections were stable below 1200°K and in fields less than 240 MV cm-1. From the voltages required for constant emission current the whisker growth rate γ was measured. At a constant chloromethane pressure, γ was constant for the first four minutes of growth, but when the pressure P of the gas was varied, the growth rate followed the expression γ = k(P - Pc)1/n, where k and n are constants, and Pc is the critical pressure necessary for growth. This equation is interpreted on the basis of a mechanism previously proposed by Gundry & Tompkins. It is suggested that the chloromethane is first adsorbed in a precursor state and is then transformed into a final chemisorbed state. The latter state is pictured as a mobile gas-metal complex which decomposes at the whisker tip (this being heated by field-emitted electrons) with the consequent growth of whiskers. For such whiskers, the position at which the derived Fowler-Nordheim plot deviated from linearity enabled the corresponding current density j to be found. The diameter and length of the whiskers were estimated to be ca. 15 Å and ca. 1000 Å, respectively.