Rj Richard Bruls

The temperature dependence of the Young's modulus of MgSiN2, AlN and Si3N4

The temperature dependence of the Youngs modulus of MgSiN2 and AlN was measured between 293 and 973 K using the impulse excitation method and compared with literature data reported for Si3N4. The data could be fitted with . The values of the fitting parameters E0 and T0 are related to the Debye temperature, and the parameter B to the harmonic character of the bond.

Ab initio band structure calculations of Mg3N2 and MgSiN2

Ab initio band structure calculations were performed for MgSiN2 and Mg3N2. Calculations show that both nitrides are semiconductors with direct energy gaps at . The valence bands are composed mainly of N 2p states hybridized with s and p characters of the metals. The bottom of the conduction band consists of the s characters of Mg and N for Mg3N2, as well as for MgSiN2, while the characters of Si are higher in energy. The optical diffuse spectra show an energy gap of about 2.8 eV for Mg3N2 and 4.8 eV for MgSiN2, in agreement with the calculated values.

Preparation and characterisation of MgSiN2 powders

The powder preparation of MgSiN2 was studied using several starting mixtures (Mg3N2/Si3N4, Mg/Si3N4 and Mg/Si) in the temperature range 800–1500°C in N2 or N2/H2 atmospheres. The phase formation was followed with TGA/DTA and powder X-ray diffraction (XRD). At 1250°C Mg/Si mixtures did not yield single phase MgSiN2 whereas for Mg/Si3N4 and Mg3N2/Si3N4 mixtures nearly single-phase powders were obtained. The Mg/Si3N4 mixtures yielded MgSiN2 at the lowest processing temperature but the Mg3N2/Si3N4 mixtures yielded the most pure MgSiN2 powder with respect to secondary phases. The main secondary phase detectable with XRD was MgO when starting from Mg3N2/Si3N4 or MgO and metallic Si when starting from Mg/Si3N4 mixtures. When the processing starting from Mg3N2/Si3N4 mixtures was optimised MgSiN2 powders containing only 0.1 wt % O could be prepared. Using XRD the solubility of oxygen in the MgSiN2 lattice was estimated to be at maximum 0.6 wt %. The MgSiN2 powder was oxidation resistant in air till 830°C. The morphology and particle size were studied with the scanning electron microscope (SEM) and the sedimentation method. Two different kinds of morphology were observed determined by the morphology of the Si3N4 starting material.

Anisotropic thermal expansion of MgSiN2 from 10 to 300 K as measured by neutron diffraction

The lattice parameters of orthorhombic MgSiN2 as a function of the temperature have been determined from time-of-flight neutron powder diffraction. The results indicate that MgSiN2, just like several other adamantine-type crystals, exhibits a relatively small thermal expansion coefficient at low temperatures. This is ascribed to a strongly bonded 3D, relatively open, crystal structure which is characteristic of highly covalent bonded materials. The anisotropic linear thermal expansion behaviour could be qualitatively related to the characteristics of the crystal structure. The least dense packed crystallographic direction showed the smallest anisotropic linear expansion coefficient.

Preparation, characterization and properties of MgSiN2 ceramics

MgSiN2 ceramics with and without sintering additives were prepared by hot uni-axial pressing. For the sintered samples the lattice parameters, secondary phases, density, oxygen and nitrogen content, microstructure, oxidation resistance, hardness, elastic constants, linear thermal expansion coefficient and thermal diffusivity were determined. By suitable processing, fully dense MgSiN2 ceramics with an oxygen content <1.0 wt % could be obtained. The size of the MgSiN2 grains increased with increasing hot-pressing temperature and time. Transmission electron microscopy (TEM) showed the absence of grain-boundary phases and that secondary phases are present as separate grains in the MgSiN2 matrix. Scanning thermal microscopy (SThM) thermal imaging revealed a thermal barrier at the grain boundaries. The influence of the microstructure as well as the oxygen content and defect chemistry on the thermal diffusivity is limited. Supported by some theoretical considerations it is concluded that the thermal conductivity of the MgSiN2 ceramic samples is determined by intrinsic phonon-phonon scattering and will not exceed 35 W m-1K-1 at 300 K in agreement with the maximum observed value of 21–25 W m-1 K-1.

On the Debye temperature in the Slack approximation for an estimation of the thermal conductivity of nonmetallic compounds

The value of the Debye temperature of the acoustic modes, as required in the application of Slack’s approximation [G. A. Slack, Solid State Physics, edited by F. Seitz, D. Turnbull, and H. Ehrenreich, (Academic, New York, 1979), Vol. 34, pp. 1–71] for the thermal conductivity, is not always available. It is shown that for these cases, the value at the minimum of the curve of the Debye temperature versus the temperature obtained from specific-heat data, gives a good approximation.