Papers by Author: Hai Long Wang

Abstract: A novel cost-effective pressureless sintering method has been developed to prepare polycrystalline cubic boron nitride (PcBN) compacts. The effect of feldspar as sintering aids was analyzed in this paper. Various amounts of feldspar from 5 to 15 wt% were added to cBN powders, and the pressureless sintering was conducted at temperatures ranging from 900°C to 1200°C under an air atmosphere. The microstructure, phase, density and microhardness of the as-obtained PcBN compacts were measured and correlated to amounts of Si added and to sintering temperatures. The sample showed superior sintering behavior in comparison to those fabricated using hot pressed sintering. The results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that feldspar diffused homogeneously and tightly bonded with cBN. But hBN appeared when the sintering temperature even at 900°C, which dramatically affected the property of PcBN compacts. A PcBN compact with a relative density of 91% was obtained at 1100°C and its microhardness was as high as 1000HV.

Abstract: Nano-Fe particles coating Al2O3 composite powders were prepared by heterogeneous precipitation method with nanometer -Al2O3 and Fe(NO3)3•9H2O as raw materials. The composite powders were analyzed by DSC-TG, XRD,SEM and Zeta potential. Results showed that Fe coating Al2O3 nanometer composite powders were obtained in the condition of being sintered at 500°C for 30min and reduced at 700°C for 1h in H2. The coating Fe nanometer particles are in the shape of sphericity with diameter about 30nm and the dispersion of the powders is uniform. Al2O3/Fe composite ceramics were obtained by hot-pressing (30MPa). The mechanical properties of the composite were investigated after hot press at different temperatures. With the increasing of Fe content in composite ceramics, the hardness of the composite is decreased. Fracture toughness of 10mol%Fe content is 5.62MPa after sintered at 1400°C, which is increasing 57% high than that of monolithic Al2O3 ceramics.

Abstract: SiO2-SiC composite particles were prepared using a sol-gel process. BaTiO3 powders were synthesize through solid-state reaction. They were mixed as microwave absorbents with Fe3O4 powders to obtain the complex absorption. Epoxide resin (EP) was used as matrix and solidified with the mixtures. The techniques of DSC-TG, XRD were used to characterize the composite particles and the obtained compacts. A vector network analyzer was used to measure the reflectivity of the SiO2-SiC-based composites. The effects of the aborbents’ contents on the reflection of the microwave absorption materials were disscussed. It was found that SiO2-SiC composites could be prepared using sol-gel process and BaTiO3 powders could be synthesize through solid-state reaction. The results indicated that SiO2-SiC composite is contribute to absorb microwave, where SiO2-SiC: BaTiO3: Fe3O4 = 6:2:2 (vol %), the frequency region in which the maximum reflectivity is more than -10 dB is 5.4-7.6 GHz.

Abstract: The characteristic of Angang blast furnace slag was studied by X-ray fluorescence spectrometry, DSC, X-ray diffraction and SEM. SiO2-Al2O3-CaO system glass-ceramics have been obtained successfully from slag with other additives. The properties of slag-based glass-ceramics were analyzed in this paper. It has been found that nucleation temperature is in the range of 600~700 °C, and crystallization temperature is in the range of 850~950 °C. The crystals phase is 2 CaO⋅ Al2O3⋅ SiO2. The chemical and mechanical properties of slag-based glass-ceramics are superior to the properties of clay brick.

Abstract: SiC particulate-reinforced Fe composites were prepared by a powder metallurgy (PM) and conventional atmospheric sintering method. X-ray diffraction (XRD), scanning electron microscopy (SEM) techniques were used to characterize the obtained composites. The coating layer of Cu on SiC particles can suppress the reaction between SiC and Fe until 1250oC. The maximum microhardness of 283 Hv appears at near 1250oC. Substantial reaction occurs at above 1250oC, which leads to the deterioration in the microstructure and related properties. The inter-metallic compounds of FeSi or Fe2Si were detected which contributed to the enhancement of the interface between SiC and Fe.

Abstract: SiCw/3Y-ZrO2/A12O3 composites were prepared. The composite powders were prepared by a precursor method. Samples were conventionally sintered at 1500°C and 1580°C for 2h in air. The density, phases, microstructure, hardness of the SiCw/3Y-ZrO2/A12O3 composites were investigated. It was found that the relative density of the samples decreased as the increasing content of SiCw. The optimum mechanical properties may be obtained in the SiCw/3Y-ZrO2/A12O3 composites with 5 vol.% SiCw.

Abstract: SiC-based boundary layer capacitors were prepared by hot pressing. XRD, TEM and the high-resolution TEM techniques were used to characterize the sintered samples. It was found that the width of the grain boundary within the SiC-based boundary capacitors was about 200 nm. Extremely high dielectric constant of >2,400,000 appeared in a wide temperature range from 590oC to 730oC, with the maximum of >2,900,000. The critical temperature was about 500oC. Space charge polarization was detected as the temperature increased. Nano grains in the boundary phase were observed, which might enhance the space charge behavior.

Abstract: CaCu3Ti4O12 (CCTO) barrier layer ceramic capacitor was prepared by a two-step sintering process. The CCTO powders were pre-synthesized at 900oC by solid-state reaction and the effects of the amount of CuO on the formation of the CCTO powders were investigated. The CCTO ceramics were prepared by the second-step sintering. It was found that the abnormal grain growth and inhomogeneous microstructure are controlled by the amount of excessive CuO. The optimized CuO content in the composites is ~14 wt%. The maximum permittivity is 115,000 (1 kHz, 210oC).

Abstract: SiC/Cu composites were prepared by spark plasma sintering under different uniaxial pressure. X-ray diffraction (XRD), SEM techniques were used to characterize the sintered samples. It was found that higher pressure led to the transformation of Cu into Cu2O. The microhardness of the composites was improved by SiC reinforcements. The optimised pressure during the spark plasma sintering was about 50 MPa with the maximum hardness of 1.36 GPa.

Abstract: Copper coated silicon carbide clusters were used to fabricate nanocomposite. Compacts were isostatically pressed and heated in nitrogen atmosphere. Microstructure observations were carried out to show the spheroid growth of the coated clusters. Spheroid growth was found to proceed through coalescence of smaller spheroids of the coated Cu/SiC composite particles. The densification process contains steps of agglomeration – kernel-shell formation – slumping movement. Grain growth of the adherent Cu particles is suppressed due to the constraint of rigid SiC particles. This is the deterministic characteristic of the coated composite particles.