Advances in Science and Technology Vol. 88

Abstract: Highly porous Si₃N₄ ceramics were prepared by aqueous gelcasting using Si₃N₄ poly-hollow microspheres as pore-forming agent and the effect of calcining temperature of Si₃N₄ poly-hollow microspheres on their properties was investigated. With the increase of the calcining temperature, the surfaces of the Si₃N₄ poly-hollow microspheres become coarser and coarser due to more rod-like β-Si₃N₄ grains appearing on their surfaces. Only the β-Si₃N₄ phase is observed in the porous Si₃N₄ ceramics regardless of the calcining temperature. The Si₃N₄ poly-hollow microspheres distribute uniformly in the Si₃N₄ green samples and porous Si₃N₄ ceramics, and they contact with each other, which could restrict the shrinkage, warpage and cracking of the final material. With the increase of the calcining temperature, the porosity of the porous Si₃N₄ ceramics increases, while shrinkage, flexural strength and fracture toughness decrease.

Abstract: In this work a new method of nanoand ultrafine powder of tungsten oxide and carbide synthesis by means of combinating carbon carrier supported classic liquid-phase precipitation and low-temperature microwave treatment was offered. The full range of intermediate substances obtained during thermolysis, reduction and carbidization precursors to final products were presented. The thermolysis of tungstic acid with the formation of tungsten oxide and carbide ultrafine particles of different modifications were studied. It was shown, that cooling ammonium tungstate solution to 4 °C, and use of carbon carrier on the precipitation stage can increase specific surface area from 20 to 100 m²g^-1. With the use of SEM precursors particles size were examined (∼200 nm) and the morphology of initial, intermediate and final products was shown.

Abstract: Polymer Matrix Composites (PMC) are often used in lightweight applications due to their excellent mechanical properties combined with a low density. The manufacturing technologies are fully developed and raw materials are cheap. The limiting factor of these reinforced polymers is the maximum service temperature. Ceramic Matrix Composites (CMC) are suitable for service temperatures up to 1500 °C and more. These composites are composed of ceramic matrices combined with ceramic fibers based on alumina or silicon carbide. This class of composites is handicapped by the high cost of processing and raw materials and therefore only attractive for applications in astronautics and military aviation. Composite materials, bridging the gap between PMC and CMC, are manufactured by the use of polysiloxanes, carbon-and basalt fibers. Such competitive free formable Hybrid-composites are capable for service temperatures up to 800 °C in oxidative atmosphere. In order to make the material attractive also for series applications, manufacturing technologies like filament wet winding, Resin Transfer Moulding (RTM) or pressing techniques are employed. Beside the improved thermal resistivity in comparison to reinforced polymers and light metals, a major benefit of SiOC composites is investigated in the field of friction materials. The excellent properties in wear resistance and an adjustable coefficient of friction make it an interesting alternative for CFC and CMC.

Abstract: Microwave heating was discovered more than 60 years ago. And nowadays, it became popular for the use as domestic ovens. Microwave has also been utilized for the industrial processes, such as drying and roasting. On the other hand, there have been many applications of microwave, being investigated for materials' processing and environmental technologies. They are attempted to take advantage of some specific characteristics in microwave heating, which differs from that of the conventional one. Rapid heating, internal heating, selective heating are the features to be taken into consideration. Moreover, so-called "non-thermal effect" is the additional feature in which researchers are particularly interested. In this article, it is intended to describe fundamental aspects in microwave heating and introduce some selected topics of research projects performed in our research group. They include researches on fabrication of some functional materials and on handling industrial wastes etc. In this article, it is also intended to interpret the phenomena observed in these applications from the fundamental view points of electromagnetic wave interaction with materials.

Abstract: We measured the microwave and millimeter-wave behaviors of ceramics and metal powders as a basis for developing microwave and millimeter-wave heating technology. In this study, nitride powders were subjected to microwave absorption measurements at elevated temperature. These measurements were performed using a system comprising a vector network analyzer, a circular wave-guide fixture, and a vacuum furnace. The sample’s microwave absorbency was evaluated by the change in reflection power from the sample in the circular wave-guide fixture under vacuum heating. Measurements were performed at approximately 24 GHz and at temperature up to approximately 1100°C. Boron nitride (BN) exhibited almost no absorption of microwaves up to 1100°C, similar to the results of a low-loss oxide powder such as alumina. Dichromium mononitride (Cr2N) powder exhibited a rather high absorption of microwaves at room temperature. During vacuum heating, absorbency of Cr2N started to decrease at 300°C, and became almost zero at temperature greater than 600°C, similar to the behavior of iron powder. This result indicated that roughly packed Cr2N powder absorbs microwave radiation; however, during vacuum heating, the compact body was sintered and the whole sample body became a reflector of microwaves. This change was irreversible, and the sample maintained its reflection behavior upon the cooling. When a mixed powder of Cr2N (20 mass%) and Al2O3 was heated under vacuum, the compact sample exhibited microwave absorption that increased with increasing temperature. This change was reversible upon cooling. These results indicated that the mixed powder did not become reflector of microwaves; Cr2N particles were not electrically connected each other because the Al2O3 particles kept the Cr2N particles separate during the vacuum heating process.

Abstract: Since FAST/SPS technology ensures an adequate and fast consolidation of different materials the demand for suitable pressing tool materials is growing. Graphite based materials are widely used and well known as FAST/SPS tool materials but they also have some unwanted or weak properties, for example, a possible reaction or reduction of sample material as well as low mechanical values compared to other tool materials. In this paper, an approach for the FAST/SPS consolidation of ZTA-Ti (C,N) and TiN-TiB₂ based cutting tool inserts in newly developed FAST/SPS tool materials which can be used for fully automated FAST2 devices is presented. Furthermore, a general strategy for the further development of FAST/SPS tool materials is focused.

Abstract: Microwave sintering has emerged in recent years as a new, fast, cheap and green technology for sintering a variety of materials. The main advantages of microwave heating can be summarized as follow: reduced processing times, energy costs and environmental benefits. Nevertheless, understanding how this specific heating drives to obtain ceramic materials with a combination of unique, structural and functional properties is the big challenge. The present work shows the different and improved properties achieved with β-eucryptite nanocomposite ceramic materials by microwave heating compared with the conventional method. Microcracking evolution in addition to the microstructure of the sintered materials along the several thermal cycles has been studied. Mechanical properties changes observed can be related to this process. Thus, the microwave technique is a promising tool for sintering new materials by controlling the composition of the phases, chemical reactivity and nanostructure, using up to 70% less energy in the whole sintering process than conventional heating. This technique becomes part of the new and innovative technologies "eco-green".

Abstract: Graphene is a promising component for next-generation high-performance structural and multifunctional composite materials. Graphene deposited onto nanofibers of high aspect ratio can serve as reinforcement agent for improving ceramic fracture toughness and electroconductivity. It was found that quality and quantity of graphene sheets on the fiber surface essentially depends on the pyrolysis of carbon source conditions such as gas flow, duration, temperature and the composition of the gas mixture. The alumina/graphene composites of 10 and 15 wt% of nanofibers covered by graphene were produced by spark plasma sintering (SPS) at 1380 °C. Both composites show improvement in mechanical and electrical properties as compared to the monolithic alumina. The main advantage of the graphene growth on the fibers surface is a lack of complicated step of constituents mixing. Graphene platelets are believed to act as toughening agents prevailing crack propagation under loading.

Abstract: Tantalum diboride was synthesized and sintered from metallic tantalum powder and amorphous boron powder in one technological process using Spark Plasma Sintering method. The precursors were: tantalum with grain size below 5μm and boron with particle size in the range of 1-2μm. Tantalum powder, before mixing with boron, was subjected to high-energy milling under argon atmosphere in order to reduce specific surface area. The process should be carried out without air, due to protection against the influence of oxygen. During reactive sintering SPS process oxidation participation should be limited because of high exothermic reactions. Morphologies of the powders before and after milling were studied using SEM. Reactive sintering processes were carried out at temperatures from 1800°C up to 2200°C at 48MPa. Sintering duration was in the range of 1-30min. Volume changes of samples and temperature increase during the synthesis were observed and determined. The result of X-Ray phase composition analysis and microstructure observations using SEM are presented. Relative density, Young's modulus, Vickers hardness and fracture toughness of the materials were determined. During the reactive sintering the material of only one phase TaB₂, with high level of densification, was obtained.