Papers by Author: Ji Qiang Gao

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Authors: Rong Zhen Liu, Bo Wang, Ji Kuan Cheng, Jian Feng Yang, Ji Qiang Gao
Abstract: A novel method for preparing porous silicon carbide ceramics with high porosity had been developed by recrystallization of green bodies composed of α-SiC, β-SiC, remnant silicon and incompletely-reacted carbon. Fine microstructure and uniform pore structure of the resultant porous silicon carbide ceramics was obtained. The green bodies of porous ceramics were prepared by the precursor powder which contained α-SiC, carbon black and silicon powder. The precursor powder was sintered at 1600°C under Vacuum circumstance to obtain the green bodies; the sintering process is same with the reaction sintering silicon carbide. Then the green bodies were sintered to 2300°C for half an hour to recrystallization. The incompletely-reacted carbon was fully reacted with silicon. And the remnant silicon was excluded during the recrystallization process to create porous structures. The influence of composition of the precursor powder and the fabrication process (the moulding pressure) on the microstructure of sintering bodies was analyzed. X-ray diffractometry demonstrated the transformation of β-SiC to α-SiC during the recrystallization process. The density and the porosity of this material was 1.027g/cm3 and 67% respectively.
Authors: Guan Jun Qiao, Hong Jie Wang, Ji Qiang Gao, Zhi Hao Jin
Abstract: High purity alumina ceramic and Kovar alloy were brazed by Nickel and Titanium foils stacked as Ni/Ti/Ni layer structure. Airtight joints were achieved with shear strength more than 30MPa, after brazing at 995oC for 30min. A sandwich structure was observed in solder, which means an a-Ti solid solution belt at mid part and Ti2Ni intermetallics belts at both sides. The main reaction product at alumina/solder interface was Ni2Ti4O, a complex oxide with structure similar to Ti2Ni, which is the bonding agent and transition from the solder’s metallic crystal lattice to alumina crystal lattice. To simulate the actual serving conditions, a thermal cyclic experiment was undertaken at the 200oC-600oC temperature range. The results showed that shear strength of joint increased dramatically after thermal cycles. This interesting phenomenon is attributed to the annealing effects of thermal cycles, which released residual stress in brazed joints remarkably.
Authors: Jie Liu, Guang Wei Fan, Pei De Han, Jian Sheng Liu, Ji Qiang Gao, Jian Feng Yang
Abstract: Effects of nitrogen content on hot ductility of duplex stainless steels have been investigated. With the increase of nitrogen content in the duplex stainless steels, mechanical strength increased, while hot ductility and elongation decreased. With the same strain rates and deformation degree, the high nitrogen content led to the high optimum hot ductility temperature for the high nitrogen DSS alloy. These results indicated the importance of control over the shape and volume fraction of phases in duplex stainless steels to achieve the optimum hot ductility.
Authors: Jian Guang Bai, Bo Wang, Guang Liang Liu, Ji Qiang Gao, Jian Feng Yang, Jing Zhong Zhao, Zhen Li
Abstract: In this paper, cordierite-mullite multiphase ceramics material was prepared using cordierite powder, mullite particles, fused silica, magnesia and alumina as main starting material. Effects of addition of 2%~10% SiC on the thermal expansion, flexural strength and thermal shock resistance were studied, and the fracture surface morphology was observed with Scanning Electron Microscope (SEM). The results showed that the multiphase ceramics material’s thermal expansion coefficient and flexural strength had little change. The thermal shock resistance of cordierite-mullite multiphase ceramics material varied as increasing-decreasing with the increase of SiC content, when the content of SiC was as high as 4%, the highest conservation rate of the flexural strength after 1100°C~water(3 times) was 72.08%, and the thermal shock resistance of cordierite-mullite multiphase ceramics material was superior.
Authors: Xiao Chen Liu, Li Tong Guo, Ji Qiang Gao, Tian Wen Guo, Li Liu, Shi Bao Li, Xiao Na Li
Abstract: The aim of this investigation is to study the effect of SnO coatings on the bond strength between titanium and porcelain. The coatings were produced by sol-gel method and heat-treatment at 300°C. Once the coatings have been formed on the titanium substrates, self-made porcelain was fused. The specimens with pre-oxidation before porcelain fused were set as control group. It was shown SnO gel formed completely at 300°C and was composed by SnO and SnO2 in the form of a uniform film. The bond strength between titanium and porcelain of the specimens with SnO coatings was greater than those in control group statistically, which might prove that the coatings reduced the oxidation of titanium surface in the porcelain fusing progress. EDS result showed almost all fractures between titanium and porcelain occurred at the oxide layer. Si and Sn might attend the reaction at interface. It was concluded that the SnO coatings produced in this study can improved the titanium-porcelain bond strength.
Authors: Ji Hua Chen, Bin Zhang, Zhi Hao Jin, Ji Qiang Gao, San Jun Zhao
Abstract: This study was conducted to evaluate the influence of surface roughness on anchoring strength between ceramic and coating glass. Commercial Vita alpha glass-ceramic was selected as the coating glass, while the zirconia/alumina (Z/A) composite as the substrate. Anchoring strength between substrate and coating-glass was tested, and microstructure of the junction was also observed. From the experiments we can find the following results. There was significant difference in the mean strength of different surface roughness groups and HF acid-etching also had a favorable effect on matching properties. The highest amount of anchoring strength was occurred at the high scope of roughness after HF acid-etching and surface roughness will also affect the microstructure of the junction. Therefore, we can draw the conclusions that with the limit of this research, roughness will influence matching properties between ceramic substrate and coating-glass, while hydrofluoric acid etching was also suggested in the matching procedure. Testing matching properties between ceramic substrate and coating-glass should consider the influence of surface roughness.
Authors: Jian Feng Yang, Ji Qiang Gao, Guo Jun Zhang, Ichiro Hayashi, Tatsuki Ohji
Abstract: Porous Si3N4 ceramics with different pore morphology have been fabricated, utilizing either organic whiskers or starch as the fugitive agents, through slip-casting and die-pressing technique, respectively. The obtained porous ceramics have rod shaped or equiaxial pore morphology, originated from there two kinds of pore forming agents. The mechanical properties were investigated. The strength decreased considerably when small amount of whiskers were added, however, further increase in the whisker content only cause a moderate decrease of the strength. Gas permeability were measured for the samples with high whisker content of 60 vol% (corresponding to porosity of about 45% in the sintered bodies), and was compared with the counterpart contained the same porosity in which pores were equiaxial. The flexural strength of the samples with these two types of fugitive particles was almost the same, but the permeability of samples with rod-shaped pores were much higher than that with equiaxial pores, which can be understood in terms of a short pass model.
Authors: Yuan Lu, Jian Feng Yang, Shao Yun Shan, Ji Qiang Gao, Zhi Hao Jin
Abstract: In this paper, porous Si3N4 ceramics were fabricated by carbothermal reduction between silicon dioxide and carbon. The influences of four types of sintering additives on the microstructure and mechanical properties of the porous Si3N4 ceramics were investigated. XRD analysis proved complete formation of a single-phase β-Si3N4. SEM analysis showed that the resultant porous Si3N4 ceramics occupied fine microstructure and a uniform pore structure. The sintered sample with Lu2O3 as sintering additive showed finer, a higher aspect ratio β-Si3N4 grains. The addition of Eu2O3 accelerated the densification of porous Si3N4 ceramics, decreased the porosity and increased the flexural strength.
Authors: Bo Wang, Rui Guo, Ji Qiang Gao, Jian Feng Yang
Abstract: The barium aluminum silicate-silicon nitride (BAS-Si3N4) matrix-ceramic composite was fabricated using pressureless sintering, at temperatures ranging from 1720°C, which is below the melting point of BAS, to 1850°C. The effect of processing conditions on sinterability, crystalline structure, microstructure and mechanical properties was evaluated. It was demonstrated the BAS glass-ceramic served as an effective liquid-phase-sintering aid, to attain high densities and completed the α-Si3N4–β-Si3N4 phase transformation, and remained as a structural matrix that was reinforced by the rod-like β-Si3N4 grains. Si3N4 grains nucleated and grow in random directions in an almost completely crystallized matrix of hexacelsian BAS. High flexural strength (665±40 MPa) and fracture toughness (7.74 MPa•m1/2) could be obtained from 30wt%BAS-70wt%Si3N4 samples that have been sintered at 1800°C for 120 min with a fine-grained microstructure.
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