Papers by Author: Jin Yong Zhang

Abstract: Recently work shown that an imperfection interface can be deduced by a fast sintering, which termed as Self-propagating High-temperature Synthesis plus Quick Pressing (SHS/QP). In order to investigate the effect of imperfection interface on the transport properties of function ceramics, the SHS/QP was applied to fabricate permittivity BaTiO₃ in this article. It was found that this ultra-fast sintering method could not only lead to a broaden-amorphous grain boundary but also semi-conductive grain body simultaneously. It is interesting that the new rapid route can result in the giant permittivity of BaTiO₃.

Abstract: Reliability of indentation method in carbon nanotube reinforced ceramics had been proposed in some works, especially of indentation toughness. In this work the traditional indentation hardness of the Multi-wall carbon Nanotube reinforced alumina composites was studied. It is found that the Vickers hardness and nanoindentation hardness of MWNT reinforced alumina fabricated by SPS decreased from about 20GPa to 16GPa at a critical load force about 1kg. While pure alumina showed a Vickers hardness of 20.6GPa in the whole load force range. When the load force or displacement into surface increasing, continuous recorded hardness, contact stiffness and modulus of the MWNT reinforced alumina showed quite different behavior compared with the pure alumina, especially at middle period of indentation. It was believed that different properties in vertical and horizontal directions of the MWNT were the key. MWNT was compressed and deformed as a soften component in the whole indentation processing. And tensile stress caused by MWNT in composite became significant until an enough strain attained and decreased when a critical stress was over. The results mean the definition of hardness or similar ones seemed need deep description in this type of material.

Abstract: The ZrC oxidization mechanism in air was carefully investigated by the phase composition and grain morphology through DSC, X-ray diffraction and SEM respectively at different temperature. It was found that the oxidization of ZrC start at about 400°C by oxygen atom entering the interstitial sites to substitute part of Carbon. Zr₂O and ZrO₂ can be formed at a temperature of about 600°C and 1000°Crespectively. And ZrC will completely transfer to ZrO₂ at 1000°C for 5hrs. No obvious grain morphology change was found under 600°C.

Abstract: Self-propagating high-temperature synthesis with reduction process was used to fabricate LaB6 ceramic powder from La2O3-B2O3-Mg system. The combustion reaction carried out in inert atmosphere (Ar) and the combustion temperature was about 1600°C. The products were washed with dilute hydrochloric acid and distilled water, respectively, to remove LaBO3, Mg3B2O6, MgO and other impurities. The influence of compact pressure and addition of SHS diluent on the grain size was studied. The result indicates that as the compact pressure increased, the mean size of powder decreased gradually. The addition of diluents reduced the synthesis temperature, and as the amount of diluent increased, the grain size of LaB6 powder was found to reduce continuously. Microstructure analyses show that the mean grain size of LaB6 powder prepared by SHS with reduction process is less than 500 nm, which is finer compared with the grain size prepared by traditional process.

Abstract: Two different kinds of mullite precursors with composition 3Al2O3•2SiO2 (3:2) were prepared by conventional drying ethanol solution and spray-drying aqueous solution of aluminum nitrate nanohydrate and tetraethoxysilane, respectively. The results of scanning electron microscope (SEM) indicate that one powder consists of irregular particles with size of 1-10μm, the other powder is made of inhomogeneously sized hollow spherical particles with mean size of 0.5-5μm. The TG-DTA curves indicate the hollow spherical particles are unfavorable to eliminate the decomposed products. After the precursors were sintered by Spark Plasma Sintering at 1450°C for 10min, the microstructures of the former are made of fine equiaxed grains with sizes of around 0.5μm, the latter consist of elongated grains distributed in the matrix of fine grains with imhomogenous size of 0.5~10μm due to the liquid phase forming. The different microstructures lead to the former sintered body is transparent, while, the sample from spray-drying is opaque.

Abstract: The densification behavior and mechanical properties of B4C by SPS at 1900°C for 2 min with additions of Al2O3 up to 10 wt.% were investigated. When the additional of 10 wt.%, the relative density was 99% and the hardness went to 33GPa. Sinterability was greatly improved by the addition of a small amount of Al2O3, and the SPS technology with high hearing rate is particularly attractive for prepare of poorly sinterable materials. The improvement was attributed to the enhanced mobility of elements through the Al2O3 near the melting temperature. As a result of this improvement in the density, mechanical properties, such as hardness, fracture toughness, increased remarkably.

Abstract: The equiatomic multicomponent CoCrFeNiCuAl high-entropy alloy powder was synthesized by mechanical alloying. The effects of milling time and heat treatment on the structure and morphology of the ball milled alloy were investigated. Single BCC solid solution structure appears when the alloy is ball milled more than 30h. The 60h ball milled alloy powder shows a mean particle size of 3 μm, which is actually hard agglomerations of nanosized crystals with crystalline size less than 10nm. The 60h ball milled alloy exhibits good chemical homogeneity. The single BCC solid solution structure transforms to a BCC and a FCC phases when annealled at 600°C for 1h, which can be attributed to the supersaturatable solid solution formation during the mechanical alloying process.

Abstract: Flowerlike bicephalous titanium diboride (TiB2) nanowhisker clusters were fabricated by planetary ball-milling. The pure cleavage theory and crack growth mechanism were used to explain the formation process. The high-resolution transmission electron microscopy and nano electronic beam diffraction show that TiB2 superhard material always has a cleavage plane along the direction of [0001] zone axe and [10 0] zone axe because of its big interplane distance, weak bonding force between planes and low index directions. The flowerlike bicephalous TiB2 nanowhisker can be gained after repeated cleavage under high strains. The discovery could open a new path to fabricate C32 type superhard ceramic nanowhiskers.

Abstract: Carbon nanotubes reinforced alumina was fabricated by spark plasma sintering method. When adding 0.2wt% nanotubes, the fracture toughness of the composites prepared increases 19% compared with the pure alumina ceramics. The effect of sintering schedule on microstructure and mechanical properties is investigated systematically. Microstructure studies reveal that at high sintering temperature, the nanotubes tend to gather in the gaps surrounded by three or more grains in a flocculent state, which leads to poor mechanical properties. Raman spectrum indicates that long sintering duration may cause serious nanotubes destruction and lower the mechanical properties.

Abstract: ZrC fine powder has been prepared by self-propagating high-temperature synthesis (SHS) using exothermic reaction of ZrO2-C-Mg system. By theoretical calculating, the adiabatic temperature (Tad) for the system is about 2235K enough to react as SHS process. The Tad observed during experiment is 1850K. The results show that high pure ZrC powder is obtained with appropriate Mg contents. The scanning electron micrograph shows that the average size of ZrC particles is about 2μm.