Thermionic Emission Microscopy of Metals. Part I. General

Abstract

An electrostatic emission microscope operating with an accelerating voltage of 10–25 kv is briefly described. Factors determining image quality, particularly contrast and resolution, are discussed and the interpretation of images on the basis of the Schottky emission equation is considered. The instrument can be operated at magnifications of 250, 1000, 2500, and 4000× with a resolution consistently better than 1000 A. It has been found that the iron group metals can be activated easily using barium formate rather than evaporated Ba or Sr. The level of thermionic emission from such surfaces is determined primarily by the partial pressure of chemical reducing agent at the surface (such as carbon) to reduce BaO to free barium. The effect of various impurities and alloying metals is discussed from the standpoint of interpretation of emission images. The most important single agent for promoting emission from Ni, Cu, and Fe, etc., is carbon. Examples of emission images include nickel, stainless steel, and plain carbon steel. Complications such as surface roughness are demonstrated with micrographs and the limitations of the technique pointed out.