Dariusz Zientara

Investigation of γ-Alon Structural Evolution during Sintering and Hot-Pressing

Dense polycrystalline aluminium oxynitride with spinel structure, -alon, is noted for its excellent thermal properties, high-temperature mechanical properties, low dielectric constant, thermal expansion coefficients and intrinsic transparency extending from ultraviolet to mid-infrared wavelengths. The dense materials were made by reactive pressureless sintering or hot-pressing of the SHS derived powders. Powders were synthesized from mixtures of aluminium and corundum powders of different proportions. The products of the SHS synthesis were composed mostly of -alon and in lesser extend of aluminium nitride. Ground powders were pressureless sintered at temperatures of 1800-2100°C for 2-6hs as well as hot-pressed at 1750-1950°C for 1 h under 25 MPa in nitrogen flow. The present work is focused on phase evolution of -alon materials during pressureless sintering and hot-pressing. The structural changes of -alon, a Al2O3 and AlN were also examined.

Mechanical and thermal properties of tungsten carbide – graphite nanoparticles nanocomposites

Abstract Previous studies concerning pure tungsten carbide polycrystalline materials revealed that nanolayers of graphite located between WC grains improve its thermal properties. What is more, pressure-induced orientation of graphene nano platelets (GNP) in hot pressed silicon nitride-graphene composites results in anisotropy of thermal conductivity. Aim of this study was to investigate if addition of GNP to WC will improve its thermal properties. For this purpose, tungsten carbide with 0.5–6 wt.% of GNP(12)-additive underwent hot pressing. The microstructure observations performed by SEM microscopy. The anisotropy was determined via ultrasonic measurements. The following mechanical properties were evaluated: Vickers hardness, bending strength, fracture toughness KIc. The influence of GNP(12) addition on oxidation resistance and thermal conductivity was examined. It was possible to manufacture hot-pressed WC-graphene composites with oriented GNP(12) particles, however, the addition of graphene decreased both thermal and mechanical properties of the material.

Dense γ-Alon Materials Derived from SHS Synthesized Powders

Dense polycrystalline aluminium oxynitride with spinel structure, γ-alon, is noted for its excellent thermal properties, high-temperature mechanical properties, low dielectric constant, thermal expansion coefficients and intrinsic transparency extending from ultraviolet to mid-infrared wavelengths. The conventional way for synthesis of γ-alon powder is high-temperature reaction of aluminium nitride and corundum in pure nitrogen or a vacuum. The dense materials are made by reactive pressureless sintering or hot-pressing of a powder compact. This work is focused on preparation of γ-alon materials derived from SHS synthesized powders. The powders for sintering were synthesized from mixtures of aluminium and corundum powders of different proportions. The products of the SHS synthesis were composed mostly of γ-alon and aluminium nitride with small amount of non-reacted substrates. Ground powders were hot-pressed at 1750, 1850 and 1950°C for 1 h under 25 MPa in nitrogen flow. Such procedure allowed dense material composed of pure γ-alon with good mechanical properties to obtain.

Synthesis of Bi6Fe2Ti3O18 Aurivillius Phase by Wet Chemical Methods

The Aurivillius phases in the Bi-Fe-Ti-O system showing multiferroic properties arouse an increasing interest due to their wide potential applications in electronics. These compounds were usually prepared by high temperature solid-state reaction method using respective oxide powders mixed in required stoichiometry. An excess of bismuth oxide was often added due to its evaporation during heat treatment. The mixture of the oxide powders were calcined in air between 700 and 900°C for several or even for several dozen hours. In the present paper the Bi6Fe2Ti3O18 Aurivillius phase was prepared by direct solid state reaction between respective oxides and by co-precipitation – calcination method. Mixture of the oxides and co-precipitated gels were calcined at different temperatures and X-ray diffraction analysis was used for identification of phase composition of the products.

Composites Produced by SHS Method – Current Development and Future Trends

Basic stages of progress in composite materials, prepared by SHS method, from a scientific approach to a promising and rapidly developing applications are discussed in this paper. The systematic review of different forms of composites prepared directly by SHS or by SHS-origin precursors is presented. Powders are usually the starting material for manufacturing of ceramic and a lot of attention has been paid to find new routes for synthesis powders in form of nano or micro particles. The present work is aimed at efficient and convenient powder processing by SHS as an important target for future composites technology. The use of SHS may bring a considerable development in ceramic technology, by enabling a manufacturing of sinterable, high-purity nano or micro powders. It can be demonstrated in different ceramic systems explored by the authors and coworkers using SHS e.g. (a) Si-C-N, (b) Al-O-N as well as (c) Ti-Si-C-N. Rapid combustion conditions were successfully used to manufacturing composite powders and nanopowders suitable for preparing multiphase composite materials having controlled properties.