Papers by Author: Yi Wu

Abstract: β-Sialon-cubic boron nitride (cBN) composites were prepared by the high temperature (1700°C) and high pressure (5GPa) sintering. β-Sialon chemical general composition can be represented as Si_6-zAl_zO_zN_{8 –z} (Z is the number of Al atoms which are substituted by Si atoms, and the range of z value is usually between 0-4.2[1].).The effects of different formula with corresponding to z values on microstructure and mechanical properties of β-Sialon-cBN composites were studied. The relative test results were verified by computer simulation. The study result showed: with the z values of β-Sialon increasing, the aluminum and oxygen content increased. The densification and mechanical properties of β-Sialon-cBN composites decreased gradually.

Abstract: Polycrystalline perovskite-type manganite La_0.65Sr_0.2Na_0.15MnO₃ was prepared by sol-gel method. An X-ray diffraction measurement showed that the sample was a single phase. The Curie temperature of the sample was determined to be 350K. The maximum magnetic entropy change |ΔS_M| corresponding to a 1T magnetic field variation was found to be 1.08 J/kg K and about 40.6 J/kg of relative cooling power was obtained near the Curie temperature. The first-order or the second-order on the phase transition of the manganite was distinguished by Banerjee criteria.

Abstract: This work focus on the effects of C vacancy on wetting of Fe to TiC/Fe at the cermet interfaces. We do the whole work using the first-principles density functional theories. The ideal work of adhesion of the pure interface is not big enough, comparing with the expeimental value. Our calculations suggest that the C vacancy at the interface is a very important factor for interface banding of TiC/Fe cermet composite. An adequate quantities of C vacancies at the interface can improve the wetting of TiC/Fe interfaces.

Abstract: First-principles plane-wave pseudopotential calculations of the electron structure and energetics of the interfaces of clean and Ni-doped TiC/Fe are reported. We predicted the atomic structure, bonding, and the interface binding energy of TiC(100)/Fe(100) and TiC(100)/Fe(110). By comparing the interface bonding energy and the total charge density distribution, the interface have priority to combine in TiC(100)/Fe(100) and TiC(100)/Fe(110) ways, where the former’s interface binding energy is higher. So the structure of TiC(100)/Fe(100) is more stable. The doped Ni atoms have preferential access to Fe-based body and form FeNi alloy, and enhance the interface bonding energy, thus effectively reducing the system energy of TiC(100)/Fe(100) and TiC(100)/Fe(110) interfaces, increasing the bonding strength and stability of interfaces of the composite materials.

Abstract: Pyrite nano-powder was synthetized in a high-pressure solvothermal process in the ethanediol solvent, with Fe(NO₃)₃·9H₂O and NH₂CSNH₂ as the raw materials. The sample was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and the results show that the product has a pure phase in a typical cubic crystal. The effects of temperature, aging time and surfactant on the shape of the crystallites were investigated systematically. The nano-powder synthetized shows itself in various micro-shapes such as granule, globular and flake, with its diameter ranging from hundreds of nanometers to one micrometer.

Abstract: Steel-bonded cemented carbide GT35 was fabricated from natural ilmenite by in-situ carbothermic reduction and vacuum pressureless sintering. The effects of C content on the mechanical properties and microstructures of GT35 composites were investigated. As an excellent reducer, carbon provides an impetus for a series of reductive process. C content determines the melting point of the steel matrix, and directly affects the mechanism of dissolving and segregating of TiC particles, forming a netlike microstructure. Besides, the liquid steel whose viscosplasticity is determined by C content was coated on the surface of the composite and refrained the release of CO. The results showed that the good mechanical properties of steel-bonded cemented carbide GT35 composite were obtained with 0.9wt.% C at the same sintering conditions and the density reached 6.12g/cm3, the average bending strength was over 1229MPa after heat treatment, the mean hardness HRC was 69.4. It also showed that in-situ reduction of natural limonite was a feasible way to fabricate good performance GT35 composite with a relatively low cost.

Abstract: In the present study, steel-bonded cemented carbide GT35 has been fabricated from natural ilmenite by in-situ carbothermic reduction and vacuum pressureless sintering. The effect of different carbon source on the mechanical properties and microstructure of GT35 composite was studied. The quality of the composite synthesized from natural graphite is higher than those from other carbon source. Some good results were obtained from experiments: the density is about 6.2g/cm3, and the bending strength is 1390MPa, which higher than that of other samples. The structures and ingredient of different carbon source were analyzed to elucidate their different performance in the reactions.

Abstract: Based on the empirical electron theory (EET) of solids and molecules, the valence electron structures (VESs) of TiC-Mo(Ni)-Fe system were calculated by building proper structure model. The results indicate that additives of Mo and Ni improve the interface conjunction factors of the cermets in different ways. By adding Mo, the VESs of the ceramic phase are improved for the formation of the rim phase (Ti1-xMox)C, which leads to the enhancement of the interface conjunction, while the improvement of the VESs on metal phase by adding Ni is due to the formation of the Fe100-yNiy. Mo and Ni additives increase the interface electron density of cermets, that is, the adding of the Mo and Ni enhance the overlapping grade of the electron cloud on interface and increase the binding energy of the interface, which is propitious to the wettability. The best wettability was found at x=0.5 or y=30.

Abstract: Well-crystallized LiFePO4 cathode materials were synthesized by NaCl-KCl molten salt method. The effect of heating time on the structure, morphology and electrochemical properties of the materials were studied in detail. Single olivine-type LiFePO4 phase was obtained, and the morphology and particle size of the powders could be controlled by changing the synthesis conditions. It was shown that LiFePO4 cathode material synthesized at 680°C for 3h with the salt content N=2 (defining the molar ratio of NaCl-KCl/LiFePO4 as N) had a narrow particle-size distribution and spherical or quasi-spherical shape. Meanwhile, the tap density of the cathode material reached the maximum of 1.501gcm-3. The charge-discharge test indicated that the initial charge and discharge specific capacity reached 138mAhg-1 and 125mAhg-1 respectively at the current density of 0.3mAcm-2.

Abstract: Steel-bonded cemented carbide GT35 was fabricated from natural ilmenite by in-situ carbothermic reduction and vacuum pressureless sintering. The effects of Mo-doped on the mechanical properties and microstructures of GT35 composites were investigated. That the good mechanical properties of steel-bonded cemented carbide GT35 with 9wt% Mo were obtained at the same sintering condition that the density reached 6.223g/cm3 and the average bending strength reached about 1155.6MPa after heat treatment, but the hardness dropped the lowest value HRC 60.94 and the difference was not apparent with the change of Mo content. The additive of Mo is beneficial to be refinement of carbide particle size, sphericity of grains and formation of compact composite. In-situ reduction of natural ilmenite was a feasible way to fabricate good performance composite GT35 with a relative low cost.