Thermodynamic calculation of the binary systems M-Ga and investigation of ternary M-Ga-N phase equilibria (M=Ni, Co, Pd, Cr)

Abstract

A thermodynamic modeling of the binary systems Ni-Ga, Co-Ga, Pd-Ga, and Cr-Ga has been performed using the CALPHAD method. This modeling is focused on a simplified description of the solid-state equilibria. From published data on binary nitrogen systems, isothermal sections of the ternary M-Ga-N systems have been calculated by extrapolation. Ternary samples were prepared from M and GaN powders in various ratios, pressed to pellets, and annealed between 500 and 700 °C for up to 162 h. Phase analyses were carried out by x-ray diffraction, and the results compared to the calculated ternary phase equilibria; the comparisons indicate that M+GaN are not in equilibrium in any of these systems. Experimental results show that the M+GaN reactions are sluggish. In the ternary Ni-, Co-, and Pd-Ga-N systems, no ternary compounds were observed. The solid reaction products are essentially the metal-rich binary intermetallics (Ni3Ga, CoGa, Pd2Ga). Thermodynamic calculations show that, at elevated (local) pressure, reaction slows down. Pressure buildup must have occurred inside the tightly pressed pellet, since only small amounts of nitrogen gas were found to have escaped from the pellet. In the Cr-Ga-N system, two ternary compounds are known to exist along the Cr-GaN section. Therefore, the reaction of GaN with Cr to form the ternary phases could occur without gas liberation. Even this reaction is sluggish and was not completed in the samples investigated.