Zur Kenntnis des Calciumcarbides. II. Über die Darstellung von reinem Calciumcarbid und die Auffindung einer dritten kristallisierten Phase des Calciumcarbides

Abstract

1. Es wird ein nahezu reines (99,2%, iges) weißes Calciumcarbid, das außer Spuren von CaO und Kohlenstoff keine Verunreinigungen enthielt, erstmalig dargestellt durch Erhitzen von reinem Calciumcyanamid mit und ohne Kohlenstoff (Umkehrung der Azotierreaktion).

The polymorphism of calcium carbide

“…However, another unexpected observation was made: The diffraction patterns of the pulverized samples invariably now showed the lines of carbide II exclusively. 1 This was in conspicuous disagreement with the previous results (6,7), according to which carbide II was to be obtained in the presence of calcium cyanamide only. In these latest experiments, however, the nitrogen was carefully prevented from entering the carbide melt; accordingly, the analysis of the pulverized product showed that only 0.05-0.10 per cent of nitrogen was present.…”

“…Especially the spot corresponding to an interplanar spacing of 2.85Á., on the equator of the diagram,-by far the strongest reflection,-could in no way be interpreted as a reflection from phase II. Furthermore, it corresponded perfectly to the strongest line of the powder pattern of phase III (7), and the remaining reflections, which could not be ascribed to phase II, corresponded entirely satisfactorily to the lines of the powder x-ray pattern of phase III. The conclusion was to be drawn from these results that the carbide, made in the electric-arc furnace under the described precautions, originally had structure III, as expected, but had changed its crystal lattice, during the process of grinding in the mortar,-that is, by mechanical deformation,-into that of carbide II.…”

“…After grinding at lower temperatures, that is, at 200°and 350°C., neither carbide III nor carbide II was found to be present in more than minor quantities. Instead, carbide I was obtained which heretofore had been observed only in the presence of calcium sulfide or of an "excess of calcium" (7). The new observation showed that the tetragonal form I, too, was to be considered simply an enantiotropic crystal modification of the compound CaC2 and that the presence of sulfide or "excess of calcium" was not required for its formation.…”

“…The "pure" carbide, used in the experiments of this investigation, was made exclusively by the method described above, that is, in the electric-arc furnace. It was preferred to the methods described formerly (6,7), since it yielded far greater quantities of carbide, free from sulfur and nitrogen and containing at least 90 per cent CaC2. According to all previous experience, the slight content of calcium oxide and free carbon (graphite) had no influence upon the transformations, except as discussed on page 802, and therefore was neglected in the following considerations.…”

“…THE PREPARATION OF CALCIUM CARBIDES I, II, AND III IN THE ELECTRIC-ARC FURNACE Calcium carbide III, the phase occurring in the absence of impurities such as calcium sulfide, can be obtained by either the reaction CaO + 3C = CaC2 + CO or the reaction CaCN2 + C = CaC2 + Ns in a vacuum furnace, or from calcium metal and carbon in a closed steel tube (7). It has also been shown that this form, when prepared in one of these ways, is the most active one in the reaction with nitrogen.…”