Dissoaciation of alkaline-earth triple carbonates inthe pressure range 10 -4-10 Torr

Abstract

Alkaline-earth carbonates are used in the manufacture of electron-emissive coatings on electrodes in low-pressure gas-discharge lamps. The dissociation temperature of the carbonates, as used in the coating process, has been determined as a function of C02 pressure, with the evolved gas detection technique. Measurements have been made on both coprecipitated composite Ca-Sr-Ba carbonate crystals, and on mixtures of the three components, in two pressure ranges around 10 -3 and S Torr 1 1 1 Torr = 1 mm Hg. . The composite crystals generally dissociate in three steps. The experimental pressure-temperature relations of the decompositions, obtained for rates of heating between I and 20 K min-1, are compared with the equilibrium p vs. T relations of the separate compounds. as taken from thermodynamic data in the literature. The experimental pressures found are about I to 2 orders of magnitude below the equilibrium pressure. In a few additional experiments it was found that the single compound BaC0 3 decomposed below 10 -3 Torr in two distinct steps; the lower the pressure the more separated the dissociation temperature.