Papers by Author: Hai Long Wang

Abstract: Different molar ratio of HfB₂ and ZrB₂ had been mixed, and 30 vol.% SiC was selected as sintering additives. The mixing powders were sintered by hot pressing at 1900 °C for 1 h under a pressure of 20 MPa in Ar atmosphere. X-ray diffraction, scanning microscopy and Archimedes’s method were used to characterize the phase, microstructure and density of the sintered composites. Meanwhile, the hardness, the fracture toughness and flexural strength of the obtained composites were considered too. It can be found that the (Zr,Hf)B₂ solid solutions were formed by HfB₂ and ZrB₂ during the sintering. The flexural strength of (Zr,Hf)B₂-SiC composites increased with the amount of HfB₂ increasing, which reached (332±40) MPa for the composites content of 70% HfB₂. Which fracture toughness was (2.22±0.25) MPa·m^1/2. The highest Vickers’ harness of was (24.8±3.4) GPa for the composites content of 50% HfB₂.

Abstract: ZTA/cordierite composites were prepared using ZrO₂, Micro-Al₂O₃/Nano-Al₂O₃ and cordierite as raw materials by pressureless sintering. The influence of Nano-Al₂O₃ particles content on phase composition, microstructure and thermal shock resistance of ZTA/cordierite were investigated. The results show that m-ZrO₂, t-ZrO₂ and Al₂O₃ existed in the matrix and t-ZrO₂ content increased with the increase of Nano-Al₂O₃ powders content. By adding Nano-Al₂O₃ powders and ultrasonic dispersion, which can refine grain and promote sintering, the grain size is uniform, the porosity is less and some grains were pulled out, the fracture mode changes from intergranular fracture to intergranular fracture and transgranular fracture. The thermal shock test was carried out at 300°C-1000°C, the critical thermal shock temperature of ZTA ceramic without adding cordierite and Nano-Al₂O₃ particles is 800°C and the residual strength retention rate of the material is only 32.4%. But by adding Nano-Al₂O₃ powers and ultrasonic dispersion 30 min,the residual strength retention rate, relative density, flexural strength and fracture toughness of ZTA/cordierite sintered at 1550°C for 4 h increased greatly and were up to 73.6%, 97.2%, 436 MPa and 4.6 MPa.m^1/2, respectively.

Abstract: Polycrystalline cubic boron nitride (PcBN) composites were sintered by high pressure and high temperature sintering (HPHT) at 1450 °C for 3 min under a pressure of 5.0 GPa. Aluminium,boron carbide and carbon in the starting mixture reacts with cubic boron nitride (cBN) to form Al₃BC₃ and AlN bonding among cBN grains during sintering. X-ray diffraction (XRD) and Scanning electron microscope (SEM) were used to analyze phases and micro-structure of the sintered samples. The dense structure of super hard cBN grains bonded together with Al₃BC₃ and AlN offers superior hardness and high strength. The Vickers hardness of PcBN composites was 45±5 GPa, and the strength of PcBN composites was 345±15 MPa.

Abstract: SiC-Al₃BC₃-cBN composites with different contents of cBN were fabricated by high pressure and high temperature sintering (HPHT) at 1450 °C for 3 min under a pressure of 5.0 GPa using SiC, Al, B₄C and C as additives. The effect of SiC content on the density and mechanical properties of SiC-Al₃BC₃-cBN composites was investigated. X-ray diffraction (XRD) and Scanning electron microscope (SEM) were used to analyze phases and micro-structure of the sintered samples. The hardness of SiC-Al₃BC₃-cBN composites decreased with the increasing of SiC content, However, the fracture strength of SiC-Al₃BC₃-cBN composites increased with the increasing of SiC content.

Abstract: Dense ZrB2-SiC composites were fabricated by high pressure and high temperature sintering(HPHT) at 1500 oC for 3 min under a pressure of 5 GPa. The vickers’ hardness of ZrB2-SiC composite is 25 GPa. The flexural strength of the ZrB2-SiC composite is 300 MPa with the amount of SiC increased to 30wt.%, which increased after the composites oxidized at high temperature. The generated glass phase like SiO2 sealed pores and cracks on the surface of the ZrB2-SiC composite to increase the mechanical properties of the ZrB2-SiC composite. After oxdized at 1500oC for 45 mins, the dense oxidized film formed on the surface of ZrB2-SiC composites to improve the resistance oxi dation of ZrB2 ceramics.

Abstract: ZrB₂-Cu composite is a new electrical contact materials in the integration of high conductivity, high wear resistance and good mechanical strength. In this paper, ZrB₂-Cu composites were prepared by hot-pressing sintering at 800~900 °C under a pressure of 20 MPa.The densification of ZrB₂-Cu composites was improved by the addition of nickel using an electroless metal plating technique. X-ray diffraction and scan electron microscopy were used to analyze the phase and microstructure of ZrB₂-Cu composites. The results showed that ZrB₂-Cu composites with 60 vol % Cu which was sintered at 900 °C had a higher relative density, highest flexural strength of 381 MPa and higher hardness of 2.16 GPa(HV). ZrB₂-Cu composites with 50 vol % Cu which was sintered at 900 °C had higher flexural strength of 297 MPa and the highest hardness of 2.66 GPa.

Abstract: Porous silicon nitride (Si₃N₄) ceramics green were prepared by sol-gel and freeze drying processing. Al₂O₃ and MgO were selected as sintering additives. Porous Si₃N₄ ceramics were sintered at 1200~1300 °C. The porosity of porous Si₃N₄ ceramics reached 60~80%, the pore size of porous Si₃N₄ ceramics dried by freeze drying is less than 5μm. Two kinds of pores were formed, including open pores with pore size of 1~5μm and closed pores pore size with the nanometer level. The compression strength of porous Si₃N₄ ceramics was 15~25MPa. Thermal conductivity of porous Si₃N₄ ceramics was 0.08-0.1 W/m·K.

Abstract: The SiC/Cu composite is one of the "structural-functional" materials. It shows good mechanical properties and very high thermal, high electrical conductivity etc. But the co-dispersion, wetting and bonding between SiC and Cu interface are of practical importance in the preparation of SiC/Cu composites. In this work, surface treatment techniques such as high-temperature oxidation, acid dipping and alkaline wash were adopted separately on silicon carbide particles, in order to improve the wettability and physical and chemical compatibility between silicon carbide and copper, then we used the replacement reaction method and decomposition-reduction reaction method to generate Cu coating on the surface of silicon carbide. The results shown that, the surface of silicon carbide particle which treated by alkaline wash was cleaner and more rough than that only treated by high-temperture oxidation, moreover, the specific surface of the particle was increased, which resulted in a compact layer of Cu coating. for the same silicon carbide particles, the effect of the Cu coating prepared by decomposition reaction method was better than that by reduction reaction method.

Abstract: Coal gangue was used as the main material to fabricate floor tiles in this paper. The mixture of coal gangue, quartz, feldspar and bentonite, with the weight percentage of 55, 20, 20 and 5, was wetly milled, dried, and then pressed into green compacts. The obtained compacts were sintered at 1180-1240 oC and the floor tiles were obtained. The obtained tiles were characterized by XRD, SEM, linear shrinkage, water absorption, bulk density and flexural strength. The results indicate that the tiles were composed of glassy phases, quartz and mullite phases. The phase composition, microstructure, physico-mechanical properties of the samples change with the sintering temperature. The tile sintered at 1220 oC achieves the linear shrinkage, water absorption and bulk density values of 6.18%, 0.16%, 2.45 g/cm3, respectively. Its flexural strength reaches the maximum of 92.0 MPa.

Abstract: Using the density functional theory method, we have characterized the geometrical structures and adsorption energy of water clusters adsorption on graphite surface. When one water molecule inter- acts with graphite surface, one of the H-O bonds formed hydrogen-bond with carbon atom in graphite sheet; in the two water molecules structure, the linear dimmer nearly parallel to the graphite surface, and also formed the hydrogen-bond; when the number of water molecules increased to six, all the H-O bond that point to the graphite surface has formed Hydrogen-bond with it. The binding energy of the water clusters with a graphite surface depends only on the number of water molecules that form hydrogen bond.