Papers by Author: Jun Guo Li

Abstract: In this paper, coated ZrSiO₄-ZrB₂ powder was prepared by sol-gel method. The ZrSiO₄ coating can improve the oxidation resistance of ZrB₂ at high temperature. The effect of N (n_ZrSiO4/n_ZrB2), sintering temperature, holding time and catalytic hydrolysis condition of tetraethoxysilane (TEOS) on coated ZrSiO₄-ZrB₂ powder was discussed. The modern testing techniques such as XRD, SEM and TG-DSC were used to analyze the crystal phase compositions, microstructures and the oxidation resistance property. The results of experiments showed that the oxidation resistance of coated ZrSiO₄-ZrB₂ powder was excellent when N is 0.7, and TEOS is alkaline-catalyzed hydrolyzed.

Abstract: The Zr₂Al₄C₅ ceramic was successfully fabricated by the spark plasma sintering at 1800 °C for 10 min under uniaxial 20 MPa pressure in vacuum using a mixed raw materials of Zr, Al, Si and graphite powders. The X-ray diffraction analysis results showed that the unexpected Zr₂Al₃C₅ phase rather than target compound Zr₂Al₄C₅ formed in the sintered samples. An initial Zr:Al:C molar ratio of 2:4.2:4.8 for raw powders, and even 55 mol.% excess Al, did not lead to a phase transformation from Zr₂Al₃C₅ to Zr₂Al₄C₅. When 4 wt.% Si was induced in the starting powders, the major phase became Zr₂Al₄C₅ and no obvious Zr₂Al₃C₅ was detected in the sintered samples with an initial Zr:Al:C molar ratio of 2:6.2:4.8 (55 mol.% excess Al). The introduction of Si could suppress and even remove additional ZrC, and Si atoms would exclusively occupy the site of Al to make Zr₂Al₄C₅ become a stable solid solution. The scanning electron microscopy observation showed that the as-synthesized Zr₂Al₄C₅ grains had elongated, rod-like and/or plate-like shapes. The mechanical properties of the sintered Zr₂Al₄C₅ ceramic were also investigated, and it showed a hardness of 11.06±0.34 GPa and a fracture toughness of 4.6 ± 0.4 MPa×m^1/2.

Abstract: Powder metallurgy (PM) method to fabricate 6.5% silicon steel attracts much attention due to the lower energy consumption and cost. In this paper, Fe-6.5%Si alloy was prepared by spark plasma sintering (SPS) from the mixture of Fe and FeSi powders with different size. The sintering process was investigated through the shrinkage behavior, phase change and microstructure. The results show that the shrinkage starts from 500°C, lower than conventional sintering methods. The sintering temperature and FeSi powder size influence the reaction between Fe and FeSi.

Abstract: In this work, ZrB2 powder coated with ZrO2 (ZrB2 @ ZrO2) were applied to promote the densification of ZrB2 prepared by pulsed electric current sintering (PECS). The coating process and sintering behavior were investigated. While the pH value was about 9, the ZrB2 particles were successfully coated with ZrO2 by co-precipitation methods. The thickness of ZrO2 layer was about 3-5nm. The coated powder was sintered at 1600°C ~1950°C by PECS in vacuum. For comparison, the pure ZrB2 powder was sintered at the same conditions. The relative density of the sample coated with ZrO2 was 97.8% at PECS 1950°C, which was higher than that of sample without coating. The densification process can be divided into three stages. The ZrO2 coating layer plays an important role on the densification of ZrB2.

Abstract: The purpose of this study was to investigate the oxidation of ZrB2/ZrO2 (ZZ) and ZrB2/ZrO2/SiC (ZZS) ceramics. The ceramics were fabricated by spark plasma sintering (SPS) at 1900°C and exposed to ten-minute oxidation cycles in stagnant air at 1200°C in a box furnace with molybdenum disilicide heating elements. Results of relative density, surface phase change and the rate of weight growth show that the addition of ZrO2 improved the sintering properties of ZrB2 ceramics. While the resistance to oxidation declined with the increase content of ZrO2. And the addition of SiC improved the resistance to oxidation of ZrB2/ZrO2 composites significantly.

Abstract: A series of zirconia porous ceramics with different density are fabricated with commercial zirconia powder and zirconia hollow balls by pressureless sintering technology. The microstructure and phase transformation are characterized respectively by SEM and XRD testing methods. The result indicates that the density and compressive strength depend greatly on zirconia powder content at the same sintering temperature, and elevating sintering temperature just has a little effect on the density and compressive strength for the samples of the same zirconia powder content. The XRD diffraction patterns analysis shows that elevating sintering temperature is helpful to eliminate monoclinic zirconia and the best sintering temperature should be beyond 1700°C.

Abstract: The reaction mechanism of silicon and iron composite powders was clarified during the fabrication of high silicon iron sheet with the Si-content of 6.5wt% by Direct Powder Rolling (DPR) technique. The changes of phase composition and structure evolvement were mainly studied. It is found that a local graded structure, Fe-Fe(Si)-Fe3Si(Si)-Si, forms when sintering at 950-1000oC, which plays an important role in the DPR process. Fe3Si(Si) phase keeps higher content of Si, and Fe(Si) phase remains the state with much lower Si-content, thus provides good mechanical proprieties of rolling and cutting. Then, the subsequent sintering at about 1200oC improves the density and makes the distribution of Si homogeneous in the final high silicon iron sheets.