This review article aims to provide an updated and comprehensive description of the development of the Electric Current Activated/assisted Sintering technique (ECAS) for the obtainment of dense materials including nanostructured ones. The use of ECAS for pure sintering purposes, when starting from already synthesized powders promoters, and to obtain the desired material by simultaneously performing synthesis and consolidation in one-step is reviewed. Specifically, more than a thousand papers published on this subject during the past decades are taken into account. The experimental procedures, formation mechanisms, characteristics, and functionality of a wide spectrum of dense materials fabricated by ECAS are presented. The influence of the most important operating parameters (i.e. current intensity, temperature, processing time, etc.) on product characteristics and process dynamics is reviewed for a large family of materials including ceramics, intermetallics, metal–ceramic and ceramic–ceramic composites. In this review, systems where synthesis and densification stages occur simultaneously, i.e. a fully dense product is formed immediately after reaction completion, as well as those ones for which a satisfactory densification degree is reached only by maintaining the application of the electric current once the full reaction conversion is obtained, are identified. In addition, emphasis is given to the obtainment of nanostructured dense materials due to their rapid progress and wide applications. Specifically, the effect of mechanical activation by ball milling of starting powders on ECAS process dynamics and product characteristics (i.e. density and microstructure) is analysed. The emerging theme from the large majority of the reviewed investigations is the comparison of ECAS over conventional methods including pressureless sintering, hot pressing, and others. Theoretical analysis pertaining to such technique is also proposed following the last results obtained on this topic.

Consolidation/synthesis of materials by electric current activated/assisted sintering

Grain Boundary Complexions

Advances in solid state white lighting technologies witness the explosive development of phosphor materials (down-conversion luminescent materials). A large amount of evidence has demonstrated the revolutionary role of the emerging nitride phosphors in producing superior white light-emitting diodes for lighting and display applications. The structural and compositional versatility together with the unique local coordination environments enable nitride materials to have compelling luminescent properties such as abundant emission colors, controllable photoluminescence spectra, high conversion efficiency, and small thermal quenching/degradation. Here, we summarize the state-of-art progress on this novel family of luminescent materials and discuss the topics of materials discovery, crystal chemistry, structure-related luminescence, temperature-dependent luminescence, and spectral tailoring. We also overview different types of nitride phosphors and their applications in solid state lighting, including general ...

Down-Conversion Nitride Materials for Solid State Lighting: Recent Advances and Perspectives.

Structural ceramic nanocomposites are reviewed with emphasis on the Al 2 O 3 SiC and Si 3 N 4 SiC systems. The incorporation of a nanosized second phase, such as SiC, into a ceramic matrix can lead to an improvement in mechanical properties. It is still unclear, however, whether those improvements can directly be related to an intrinsic ‘nanocompossite effect’ or to other factors. This review is divided into three parts. First, basic processing routes for nanocomposites, namely conventional powder processing, sol-gel processing and polymer pyrolysis are presented in detail. Second, the mechanical properties of different nanocomposites are compared. Finally, models which attempt to explain the improvements in these properties are explored. It will be shown that the strength increase can best be related to a reduction in processing defect size. For applications the most interesting properties of nanocomposites are their wear, creep and high temperature performance.

Structural ceramic nanocomposites

Ultra-precision grinding is primarily used to generate high quality and functional parts usually made from hard and difficult to machine materials. The objective of ultra-precision grinding is to generate parts with high surface finish, high form accuracy and surface integrity for the electronic and optical industries as well as for astronomical applications. This keynote paper introduces general aspects of ultra-precision grinding techniques and point out the essential features of ultra-precision grinding. In particular, the keynote paper reviews the state-of-the-art regarding applied grinding tools, ultra-precision machine tools and grinding processes. Finally, selected examples of advanced ultra-precision grinding processes are presented.

Ultra-precision grinding

Positive temperature coefficient of resistivity (PTCR) materials have become very important components, and among these materials barium titanate compounds make up the most important group. When properly processed these compounds show a high PTCR at the Curie temperature (the transition temperature from the ferroelectric tetragonal phase to the paraelectric cube phase). In the first half of this paper literature related to the resistivity-temperature behaviour is discussed. As explained by the well established Heywang model, the PTCR effect is caused by trapped electrons at the grain boundaries. From reviewing experimental results in the literature it is clear that the PTCR effect can not be explained by assuming only one kind of electron trap. It is concluded that as well as barium vacancies, adsorbed oxygen as 3d-elements can act as electron traps. In the second half of this paper, the influence of the processing parameters on the PTCR related properties is discussed. Special emphasis is placed on the phenomenon that the conductivity and grain size decrease abruptly with increasing donor concentration above ∼ 0.3 at%. Several models explaining this phenomenon are discussed and apparent discrepancies in experimental data are explained.

The positive temperature coefficient of resistivity in barium titanate

From thermal diffusivity measurements of sintered AIN at temperatures ranging from 100 to 1000 K, the phonon mean free path of AIN was calculated in order to investigate phonon scattering mechanisms. The calculated mean phonon scattering distance was increased with decreasing temperature. The mean phonon-defect scattering distances were respectively limited to about 50 nm at temperatures ranging from 100 to 270 K and about 30 nm at temperatures ranging from 100 to 700 K, for AIN specimens with a room-temperature thermal conductivity of 220 and 121 Wm−1 K−1 containing 0.1 and 1.4 wt % oxygen, respectively. These short phonon-defect scattering distances were considered to correspond to the separation of oxygen-related internal defects in AIN grains. Calculation of the mean phonon scattering frequencies indicated that the phonon scattering is dominated by phonon-defect scattering at temperatures below 270 K for an AIN specimen with an oxygen content of 0.1 wt %, and at temperatures below 350 K for an AIN specimen with an oxygen content of 1.4 wt %.

Phonon scattering and thermal conduction mechanisms of sintered aluminium nitride ceramics

Chemical reactions to increase thermal conductivity by decreasing oxygen contents during AlN sintering with an Y2O3 additive in a reducing nitrogen atmosphere with carbon were investigated. They were: Al2O3 + N2 + 3CO ⇋ 2AlN + 3CO2, Al2Y4O9 + N2 + 3CO ⇋ 2AlN + 2Y2O3 + 3CO2 and Y2O3 + N2 + 3CO ⇋ 2YN + 3CO2. Some of the CO2 gas reduced to CO gas in the presence of carbon by a chemical reaction: CO2 + C ⇋ 2CO. These reactions were confirmed by examining oxygen contents, the grain boundary phases of the sintered AlN, and the trapped CO and CO2 gases in the sintered bodies. These reducing reactions proceed with increasing sintering temperature and periods, and hence the thermal conductivity is increased.

Sintering chemical reactions to increase thermal conductivity of aluminium nitride

High-thermal-conductivity SiC ceramic with very small amount of BeO additive, having a value of 270 W m −1  K −1 corresponding to roughly 50% of the intrinsic SiC value of 490 W m −1  K −1 , is characterized by high-resolution transmission electron microscopy. Crack in BeO crystalline phase, strain contrast between BeO and SiC, and stacking disorders in SiC grains are observed, and they are considered as reducing factors on the thermal conductivity. Presence of secondary phase including Fe, Ti, Al and Ni elements and Be 2 SiO 4 phase reveals that liquid in the BeO–SiO 2 –SiC system forms partially, traps metal impurities to purify SiC grains and precipitates at triple-grain junctions. A distinctive feature of intergranular film between the SiC grains is also reported.

Microstructural characterization of high-thermal-conductivity SiC ceramics

Polycrystalline Si 3 N 4 samples with different grain-size distributions and a nearly constant volume content of grain-boundary phase (6.3 vol%) were fabricated by hot-pressing at 1800°C and subsequent HIP sintering at 2400°C. The HIP treatment of hot-pressed Si 3 N 4 resulted in the formation of a large amount of β-Si 3 N 4 grains ∼10 μm in diameter and -50 μm long, and the elimination of smaller matrix grains. The room-temperature thermal conductivities of the HIPed Si 3 N 4 materials were 80 and 102 W.m -1 .K -1 , respectively, in the directions parallel and perpendicular to the hot-pressing axis. These values are slightly higher than those obtained for hot-pressed samples (78 and 93 W.m -1 .K -1 ). The calculated phonon mean free path of sintered Si 3 N 4 was -20 nm at room temperature, which is very small as compared to the grain size. Experimental observations and theoretical calculations showed that the thermal conductivity of Si 3 N 4 at room temperature is independent of grain size, but is controlled by the internal defect structure of the grains such as point defects and dislocations.

Kozo Ishizaki

Papers

The positive temperature coefficient of resistivity in barium titanate

Phonon scattering and thermal conduction mechanisms of sintered aluminium nitride ceramics

Sintering chemical reactions to increase thermal conductivity of aluminium nitride

Microstructural characterization of high-thermal-conductivity SiC ceramics

Effect of Grain Size on the Thermal Conductivity of Si3N4