Search

The membrane microstructure and phase composition of Ruddlesden–Popper-type La 2 NiO 4+ δ ceramics, which were prepared by field-assisted sintering technique/spark plasma sintering (FAST/SPS) process or by conventional pressing and pressureless sintering were investigated. As starting material, a La 2 NiO 4+ δ powder, with an average particle size of 0.2 μm was used. The grain-size distribution of the resulting membranes varied from 0.015 μm 2 for FAST/SPS sintered ceramic to 6.11 μm 2 for pressureless sintered membrane. The microstructure analysis of membranes was performed by transmission and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy. Phase transition from orthorhombic to tetragonal crystallographic structure in FAST/SPS material was investigated by temperature-dependent X-ray powder diffraction. The effect of sintering technique and grain size on the oxygen permeation performance of the membranes is discussed with respect to impurities in the material.

Hexagonal boron nitride is a material with a unique combination of mechanical, chemical and electrical properties and therefore of considerable technical and commercial interest. Nevertheless, there exists only very limited knowledge concerning the microstructure and electrical properties of such materials. In this work different materials produced by SPS from ‘turbostratic’ and well-crystallised powders are compared with commercial materials in terms of densification, microstructure and electrical properties. The turbostratic powders could be densified at temperatures as low as 1500–1600 °C, but recrystallisation of the grains took place at much higher temperatures (1800–1900 °C). The electrical resistivity of the investigated materials reached values of up to 1015 Ω cm and strongly depended on the microstructure. The main factor influencing the resistivity was the amount and nature of the grain boundary phase.

Binderless TiC–WC–Cr 3 C 2 hard materials with different compositions were prepared by FAST (field-assisted sintering technique) and by SPS from mixtures of TiC, WC and Cr 3 C 2 . The effect of the microstructure on the properties was evaluated in as-densified materials and materials heat treated at 1900 °C. The mechanical properties were determined and the microstructures were characterised using X-ray diffraction and scanning electron microscopy. A cubic (Ti, W, Cr)C phase was found to form during sintering. Hardness values of up to 23 GPa were observed. For the TiC-rich materials annealing did not have a significant effect on the hardness, but in the case of the WC-rich materials it resulted in a reduction in the hardness due to pore formation.

B 6 O/TiB 2 composites with varying compositions were produced by FAST/SPS at temperatures between 1850 and 1900 °C following a non-reactive or a reactive sintering route. The densification, phase and microstructure formation and the mechanical and thermal properties were investigated. The comparison to an also investigated pure B 6 O material showed that the addition of TiB 2 in a non-reactive sintering route promotes the B 6 O densification. Further improvement was obtained by sintering reactive B–TiO 2 mixtures which also results in materials with a finer grain size and thus in enhanced mechanical properties. The fracture toughness was significantly improved in all composites and is up to 4.0 MPa m 1/2 (SEVNB) and 2.6–5.0 MPa m 1/2 (IF method) while simultaneously a high hardness of up to 36 GPa (HV 0.4 ) and 28 GPa (HV 5 ) could be preserved. The high temperature properties at 1000 °C of hardness, thermal conductivity and CTE were up to 20 GPa, 18 W/mK and 6.63 × 10 −6 /K, respectively.

During Field Assisted Sintering Technology (FAST) the temperature differences at two different positions were investigated using two pyrometers, an internal and an external one. Two substances, an electrically conductive (tungsten carbide) and a non-conductive material (96 wt.% silicon nitride with 2 wt.% alumina and yttria) were used to monitor the temperature differences between both pyrometers during heating, sintering shrinkage and dwell time by varying die geometry and heating rate. It was shown that the temperature distribution is strongly influenced by the electrical conductivity of the material as well as by tool design and setup. The alpha–beta transformation of silicon nitride was analyzed to predict the radial temperature distribution within the sample. For comparison and for visualization a dynamical FE model including piston movement for simulating sintering shrinkage was introduced. With this, a complete time dependent FAST run could be simulated. The modeled differences in temperature distribution are in good agreement with real temperature measurements as well as phase analyses.