Papers by Author: Yun Hua Xu

Abstract: This paper prepared Cu-ZrO₂ composite by a modified internal oxidation in order to improve the high temperature strength of resistance Spot-welding electrode. The results showed that with the increasing of the cold deformation, Hardness increased while electrical conductivity decreases. Hardness reached 100HV and electrical conductivity was up to 86%IACS after cold drawn in 56%. With the increasing of annealing temperature and annealing time, Hardness of the composite decreased more slowly compared with the conventional Cu-Cr-Zr alloy and was higher than that of the Cu-Cr-Zr alloy after annealed above 700°C for 2h and at 600°C for 5h , which was attributed to the dispersion strengthening effect of the Zirconia particles. Thus, the Cu-ZrO₂ composite has good high-temperature stability and can be used for the electrode

Abstract: Cu-coated Ti3SiC2 powder was fabricated by electroless copper plating method from CuSO4 using poisonless ascorbic acid (C6H6O4(OH)2) as reducing agent. Sensibilization and activation on Ti3SiC2 powder surface was performed by Sn-Ag catalyst system. Surfactants lauryl sodium sulfate (SDS) and Polysorbate 80 (Tween-80) were used as modifiers to control copper particle sizes and shapes. It is found that good pretreatment and activation can obtain fully-formed copper coating. The optimal copper deposition rate can reach to 2.053 g.dm-2.h-1 at 80°C . The best coating effect can be obtained in the samples modifying with 12g/L SDS+8ml/L Tween-80.

Abstract: In the paper, tungsten carbide (WC) particles can be in-situ synthesized by applying electromagnetic field to the system consisting of tungsten wires and gray cast iron melt at 1573 K. The microstructures and wear-resistant properties of composites reinforced by both WC particles and the residual tungsten wires were investigated by XRD, SEM, EDS, micro-hardness and pin-on-disc wear measurements. The results show that, with enhancing frequencies of electromagnetic field from 0 to 5 KHz, the amounts of in-situ WC particles increased and of the residual tungsten wires gradually decreased until tungsten wires completely reacted. Due to the higher hardness of in-situ WCp(2100-2231HV0.1) and the strong interfacial bonding, the composites displayed an excellent wear-resistant properties. When the frequency was 5 kHz, the wear loss for the composite fabricated was optimal and 2.69 times lower than that of reference samples.

Abstract: To investigate the possibility and reaction route of fabrication high purity Ti3SiC2 powders using coarse reactants by a relatively simpler method, Ti/Si/TiC mixtures (74μm in average size) added proper amount of aluminum were heated in vacuum sintering furnace with different sintering scheme. The effects of Al addition and temperature on the purity of resultant Ti3SiC2 powder were also examined. The results showed that over 99wt.% content Ti3SiC2 powders could be obtained at optimal temperature 1410°C-1420°C holding 90-120min with 0.10-0.20 Al addition (molar ratio). XRD, SEM and EDS results indicated that Ti5Si3 and TiCx were the main intermediate phases which redounded to synthesize Ti3SiC2 powder subsequently.

Abstract: The present research describes a simple low-temperature synthesis route of fabricating porous YVO4:Sm nanoplates via a chemical co-precipitation method using commercially available Y2O3, NH4VO3, Sm2O3 and ethylene glycol as the reacting precursors. The as-synthesized YVO4:Sm was thermally treated at 300°C and 600°C for 2 h which is much lower than that of the conventional preparation methods. The obtained samples were characterized by FTIR, XRD, TEM and PL. The photoluminescence measurement revealed that the luminescence intensity was significantly increased with increasing annealing temperature.

Abstract: Li2MSiO4(M=Mn, Co, Ni) is a potential high capacity cathode material because of its outstanding properties that exchange of two electrons per transition metal atom is possible and the theoretical capacity of Li2MSiO4 can reach as high as 330 mAhg-1. In this family, the cathode performance of Li2MnSiO4 synthesized by solution route has been published recently. However, it seems that the cycle life of Li2MnSiO4 fell short of our expectation. In this work, the Li2Mn0.7Fe0.3SiO4 cathode material was synthesized by traditional solid-state reaction method. The prepared powder was consisted of majority of Li2Mn0.7Fe0.3SiO4 and minor impurities which were examined by XRD. FESEM morphology showed that the products of Li2Mn0.7Fe0.3SiO4 and Li2MnSiO4 have similar particle size (about 50-300 nm). The electrochemical performance of Li2Mn0.7Fe0.3SiO4, especially for reversible capacity and cycle life, exhibited better than those of Li2MnSiO4.

Abstract: Under high impact energy, nano-structured surface layers of Hadfield steel and annealed AISI 1045 steel were investigated in the present paper. It has been observed that a so-called “black layer” for Hadfield steel and “white layer” for AISI 1045 steel has been formed, respectively. That definitely will give rise to a change of wear mechanism. The wear tests showed that the wear weight loss curve of Hadfield steel will be bent down after some critical impact numbers. The wear curve of the AISI 1045 steel, however, shows a step-like characteristic with increasing impact numbers. It can be found from microstructural examination that high density twin bands of subsurface for Hadfield steel were produced, which have good plastic deformation coordination with bulk material. Cracks are usually initiated in the “black layer” underneath 12 µm in depth, and the worn debris sizes were also observed in nano-scale. Nano scaled wear controls the whole wear process. For the annealed AISI 1045 steel, cracks are mainly initiated between the interface of the “white layer” and sub-surface deformation layer. Debris is in micron-scale and spalled in the flake-like style. The wear weight loss is, therefore, greater than that of Hadfield steel. The result showed from the wear tests of Hadfield steel and AISI 1045 steel that nanocrystalized process of subsurface becomes one of control factors to affect wear losses and wear mechanism under high impact energy.

Abstract: It has been well known that Hadfield steel behaviors excellent wear resistance under high impact energy. Up to now there exist many theories to explain the wear mechanism of Hadfield steel. In this research subsurface microstructure evolution process of Hadfield steel was investigated after high energy impact experiments. It was shown from high resolution electron microscope (HRTEM) examination of subsurface microstructure that nanocrystallized austenite grains have been formed in the procedure of the reaction and rearrangement of high density dislocations under the heavy plastic deformation, sub-grains as a transitional structure and, finally, the formation of nano austenite grains. On the other side, the interactions of twins and stack faults or dislocations and stack faults make austenite crystals transform to amorphous solid. With increasing impact cycles the sizes of nano-grains were decreased and the amorphous volumes were increased further. A large amount of nano-sized grains embedded in bulk amorphous matrix were fully developed, which will dominate the wear of the steel. In the subsurface no martensitic transformation was observed.