Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview

A systematic study of various spark plasma sintering (SPS) parameters, namely temperature, holding time, heating rate, pressure, and pulse sequence, was conducted to investigate their effect on the densification, grain-growth kinetics, hardness, and fracture toughness of a commercially available submicrometer-sized Al 2 O 3 powder. The obtained experimental data clearly show that the SPS process enhances both densification and grain growth. Thus, Al 2 O 3 could be fully densified at a much lower temperature (1150°C), within a much shorter time (minutes), than in more conventional sintering processes. It is suggested that the densification is enhanced in the initial part of the sintering cycle by a local spark-discharge process in the vicinity of contacting particles, and that both grain-boundary diffusion and grain-boundary migration are enhanced by the electrical field originating from the pulsed direct current used for heating the sample. Both the diffusion and the migration that promote the grain growth were found to be strongly dependent on temperature, implying that it is possible to retain the original fine-grained structure in fully densified bodies by avoiding a too high sintering temperature. Hardness values in the range 21-22 GPa and fracture toughness values of 3.5 ± 0.5 MPa.m 1/2 were found for the compacts containing submicrometer-sized Al 2 O 3 grains.

Spark Plasma Sintering of Alumina

Recent advances in semiconductor thermoelectric physics and materials are reviewed. A key requirement to improve the energy conversion efficiency is to increase the Seebeck coefficient (S) and the electrical conductivity (σ) while reducing the electronic and lattice contributions to thermal conductivity (κe + κL). Some new physical concepts and nanostructures make it possible to modify the trade-offs between the bulk material properties through changes in the density of states, scattering rates, and interface effects on electron and phonon transport. We review recent experimental and theoretical results on nanostructured materials of various dimensions: superlattices, nanowires, nanodots, and solid-state thermionic power generation devices. Most of the recent success has been in the reduction of lattice thermal conductivity with the concurrent maintenance of good electrical conductivity. Several theoretical and experimental results to improve the thermoelectric power factor (S2σ) and to reduce the Lorenz ...

Recent Developments in Semiconductor Thermoelectric Physics and Materials

Nanocrystals of CsPbX3 perovskites are promising materials for light-emitting optoelectronics because of their colloidal stability, optically tunable bandgap, bright photoluminescence, and excellent photoluminescence quantum yield. Despite their promise, nanocrystal-only films of CsPbX3 perovskites have not yet been fabricated; instead, highly insulating polymers have been relied upon to compensate for nanocrystals’ unstable surfaces. We develop solution chemistry that enables single-step casting of perovskite nanocrystal films and overcomes problems in both perovskite quantum dot purification and film fabrication. Centrifugally cast films retain bright photoluminescence and achieve dense and homogeneous morphologies. The new materials offer a platform for optoelectronic applications of perovskite quantum dot solids.

Efficient Luminescence from Perovskite Quantum Dot Solids

Monolithic ceramics are not widely used as structural materials because of their brittleness. Ceramic matrix composites, in which whiskers 1-3 or fibres 4-7 of strong ceramics such as silicon carbide or silicon nitride are embedded in a ceramic matrix, offer improved toughness and strength because the energy of cracks may be dissipated at the whisker/matrik interface. Here we describe a strong ceramic composite with a different kind of microstructure, made by unidirectional solidification of an Al 2 O 3 / GdAlO 3 eutectic mixture. This composite has a microstructure in which continuous networks of single-crystal Al 2 O 3 and single-crystal GdAlO 3 interpenetrate without grain boundaries. Rather than brittle fracture, the material displays plastic deformation at 1,873K owing to dislocation motion, as in metals. The high strength and resistance to brittle failure of this material at such high temperatures augurs well for applications in mechanical engineering.

A ductile ceramic eutectic composite with high strength at 1,873 K

Silicon carbide ceramics which 5 mass% Al2O3 and 2 mass% Y2O3 were added to were prepared by the spark plasma sintering (SPS) method under the conditions of 30 MPa and 5 min. Mechanical properties at room temperature were eximined. The SPS brought dense silicon carbide ceramics at a sintering temperature of 1800°C, which was about 200°C lower than that of the hot-pressing process. The silicon carbide obtained by SPS had higher strength and fracture toughness than those obtained by hot-pressing. The results suggest that the inside temperature of the sintered bodies during spark plasma sintering was higher than the measured temperature.

Effect of Spark Plasma Sintering on Densification and Mechanical Properties of Silicon Carbide

Preparation and mechanical properties of Al2O3-15wt.%ZrO2 composites

Thermoelectric modules composed of eight pairs of p-type Ca 2.7 Bi o3 Co 4 O 9 (Co-349) and n-type CaMn 0.98 Mo 0.02 O 3 (Mn-113) bulks were constructed using Ag electrodes and paste including powder of the n-type oxide. The former bulks were prepared by hot pressing. On the other hand, the latter were densified using a cold isostatic pressing technique and sintered in atmospheric pressure. Dimensions of both oxide legs were 5 mm wide and thick and 4.5 mm high. An alumina plate was used as a substrate, and there was no alumina plate on the other side of the modules. When the substrate side was heated, the module could generate up to 1.0 V and 0.17W of open circuit voltage (V O ) and maximum power (P max , respectively, at a hot-side temperature of 1273 K (furnace temperature as a heat source) and a cold-side temperature of 298 K (circulated water temperature) in air. But internal resistance R I reached a value of 1.5 Ω, which is about six times higher than the calculated one from resistivity of both p- and n-type bulks. When the substrate side was cooled, V O and P max reached 0.7V and 0.34 W of V O and P max , respectively, at a furnace temperature of 1273 K.

https://ceramics.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1744-7402.2007.02173.x

Power Generation of a p‐Type Ca3Co4O9/n‐Type CaMnO3 Module

Centrifugal casting of Al2O3–15 wt.%ZrO2 ceramic composites

Zirconia (3mol% Y2O3) matrix composites reinforced with 20vol% α-alumina whiskers were prepared by spark plasma sintering (SPS) at 1200 to 1500°C under 30MPa for a short holding time of 5min. The room temperature mechanical properties were measured. The SPS brought about dense alumina whisker/zirconia composites at a sintering temperature lower than that in the hot-pressing process. The composites obtained by SPS had strengths, hardness and fracture toughness higher than those by hot-pressing under the same sintering temperatures. The zirconia matrix grains in the composites prepared by SPS grew larger than those by hot-pressing.

Takahiro Tanaka

Papers

Effect of Spark Plasma Sintering on Densification and Mechanical Properties of Silicon Carbide

Preparation and mechanical properties of Al2O3-15wt.%ZrO2 composites

Power Generation of a p‐Type Ca3Co4O9/n‐Type CaMnO3 Module

Centrifugal casting of Al2O3–15 wt.%ZrO2 ceramic composites

Usefulness of Spark Plasma Sintering on Densification and Mechanical Properties of Alumina Whisker/Zirconia Composites