Reactions of powders of zirconium and niobium carbides in their homogeneity ranges with some acids

Abstract

In the work described below a study was made of the reactions of zirconium and niobium carbides in their homogeneity ranges with some mineral acids. It was considered of great interest also to investigate the gaseous products of acid decomposition of the carbides. Experiments were carried out, using hydrochloric, sulfuric, and nitric acids as reactants, on powders of the zirconium carbides ZrC0.7~, ZrCo.80, ZrC0.88, and ZrC0.92 and of the niobium carbides NbC0.63 , NbC0.69 , NbC0.84 , and NbC0.94. The zirconium and niobium carbides were prepared by synthesis from the elements. Data on the chemical compositions of the carbides aregiven in Table 1. A study was made of the chemical stability and reaction kinetics of the above-mentioned zirconium and niobium carbides in 6 N, 12 N, and 18 N sulfuric, hydrochloric, and nitric acids. Chemical stability was investigated at a 100-fold excess of acid relative to the weight of the carbide sample, during 2-h heating at a constant acid concentration. As a measure of chemical stability the percentage of the insoluble residue was employed (Table 2). The insoluble residue was subjected to chemical analysis. From Table 2it will be seen that the chemical stability of the carbides grows with rise in their carbon content and that the reactions of the carbides with nonoxidizing acids produce no change in their chemical composition. In order to develop methods for the materials chemical analysis of the carbides, a study was made of the kinetics of their reactions with the acids. Comparable results for the reactions of powders of various specific surfaces were obtained by expressing experimental data in grams per square meter (amount of reacted carbide from unit surface of carbide powder). The reaction Mnetics of the carbides in the acids was determined as follows. A 1.0000-g carbide powder sample was placed in a conical flask with a ground reflux condenser, after which 100 ml of an acid of the required concentration was poured into the flask and then boiled for 2 and 5 h at the temperatures indicated in Tables 2and3. After 5, 10, 15, 30, 45, 60, 90, and 120 rain or 1, 2, 3, and 5h, 0.2, 1, or 2ml of the solution was withdrawn with a pipet, each withdrawal being followed by the addition to the flask of 0.2, 1, or 2 ml of the same acid in order to maintain the solvent concentration constant. The amount of zirconium or niobium which had passed into the solution was determined photometrically with xylenol orange [5, 6], trilonometricall y [7], or by the Cupferron gravimetric method [8]. The results obtained are depicted in Figs. 1-4. The greatest reactivity in the homogeneity ranges was found to be exhibited by the carbides of lowest carbon content, ZrC0.75 and NbC0.63. The greatest chemical stability was shown by carbides of compositions close to stoiehiometric. This was a general finding, applicable to the reactions with all the nonoxidizing acids, both boiling and cold. In cold 12 N sulfuric acid 0.017 g/m 2 of ZrC0.92 and 0.126 g/m 2 of ZrC0.75 decomposed during 24-h te~t.% This fact can be explained as follows. During the formation of steichiometric