High pressure synthesis of rocksalt type of AlN.

Abstract

Aluminium nitride with the wurtzite structure, w-A1N, has attracted special attention as an alternative ceramic substrate of high power applications in microcircuits.1 The conspicuous advantages are high thermal conductivity greater than 100 W/mK2> and with a low coefficient of thermal expansion which, in the temperature range of interest (20-200°C), closely matches that of silicon. It also has a high electrical resistivity, a moderate dielectric loss, and good resistance to thermal shock and thermal decomposition. On the analogy of other III-V compounds with the wurtzite structure, A1N is expected to assume the cubic structure at high pressures. Van Vechten3~ predicted that, at 90 GPa and room temperature, A1N would transform to a metallic phase with the a-tin structure. Kieffer et a1.41 reported a new phase in the system AlN-ALO:, which they claimed to be cubic A1N. The new phase, however, was proved later to be AION, occurring in hot pressing experiments in the system. ~~ On the basis of the shock wave experiments on A1N, Kondo et al.°~ suggested a transformation to a denser phase at about 21 GPa. Despite of more general searches,7~-9 no definite evidence has been obtained for the existence of the new dense phase of A1N. It is the purpose of the present paper to report the successful synthesis of the new cubic phase of A1N at high pressures and temperatures. The fine powder of w-A1N (grain size, <1.0 µm; purity, >99%) kindly provided by Tokuyama Soda Co. Ltd. was used as the starting material. High pressure and high temperature experiments were carried out using a uniaxial split sphere apparatus driven by 1000 ton press (USSA-1000) at the Institute for Study of the Earth's Interior. The assembly of tungsten carbide cubes with 5 mm truncation was used with semisintered magnesia octahedron of 10 mm edge length as pressure medium. The A1N starting material was charged into a cylindrical tantalum heater embedded in the octahedron. The detailed sample assembly was similar to those described elsewhere.1o1 The sample temperature was monitored by the Pt/Pt13 % Rh thermocouple. The pressure values were calibrated at room temperature by the conventional fixed points of Bi III-V (7.7 GPa), ZnS metallic transition (15.6±0.6 GPa) and GaAs metallic transition (18.2±0.7 GPa). The experimental conditions were varied in the ranges of 11.5-16.5 GPa and

Pressure enhancement of the isostructural cubic decomposition in Ti1−xAlxN

“…This is in line with the experimental findings in Ref. 19 that the structural transition starts at 14-16.5 GPa and that the rock salt structure is metastable at ambient conditions. The crossing point of the enthalpies of the cubic and hexagonal phases shifts from x = 0.71 at 0 GPa ͑Ref.…”

Dynamic properties of ceramic materials

“…The phase transformation has been reported by Vollstadt, et al (1990) in static high pressure experiments, and by Kondo, et al (1982) in shock wave experiments. In the latter study, the discrete data, obtained with pin and electromagnetic gage techniques, clearly indicated the presence of the phase transformation.…”

Phase stability of ScN-based solid solutions for thermoelectric applications from first-principles calculations

“…The group-13 nitrides AlN, GaN, and InN crystallize in the hexagonal wurtzite structure as their ground state but can be transformed into the rock salt phase under high pressure. The B1 rock salt phase of AlN created at high pressure can be metastable also at ambient conditions 57 while B1 GaN has been stabilized as thin interlayers. 58 AlN is also known to be soluble as a metastable rock salt alloy in several rock salt transition metal nitrides, in particular, Ti 1Àx Al x N. 26 Thus, for our purpose, it is of interest to investigate their mixing tendency with ScN.…”