Electron-microscopic investigation of the reaction of water vapor with single crystals of graphite—I. Reaction with edge atoms

Abstract

An experimental investigation of the initial stages of the reaction of highly purified single crystals of graphite with water vapor in helium at high temperatures is described. Water vapor concentrations ranged from 250 vpm to 22,000 vpm and reaction temperatures ranged from 850°C to 1075°C. Rates of reaction were determined by measuring the rates of increase in diameters of etch pits produced from lattice vacancies in cleaved surfaces of the crystals; the etch pits were decorated with gold to make them visible in an electron microscope. This technique limited the temperature range over which the reaction could be studied at any given concentration, but, in general, an interval of more than one hundred degrees could be covered. The apparent activation energy of the reaction varied with water vapor concentration, increasing steadily from 68 kcal-mole −1 at 22,000 vpm to 109 kcal-mole −1 at 250 vpm. The apparent order of the reaction, with respect to water vapor concentration, was 0.89 at all temperatures studied and appeared to be concentration independent in the range 500 vpm to 22,000 vpm; however, there are indications that a higher order may prevail at lower concentration.