Papers by Author: Guan Jun Qiao

Abstract: A porous SiC ceramic reinforced copper-matrix composite was fabricated by spontaneous infiltration of molten Cu-24at%Si alloy into the reinforcement. The influence of process parameters on the infiltration behavior and microstructure of the as-prepared SiC/Cu composite investigations showed that infiltration temperature had an important influence on the infiltration behavior, and higher infiltration temperature which decreased the viscosity of molten Cu-24Si was beneficial to the penetration. Besides, the degree of infiltration increased with the increase of dwelling time at 1600°C. SiC particles were bonded together by sintering additives to form the porous reinforcement, which can be maintained after spontaneous infiltration. The interfacial bond between SiC and Cu-24Si alloy was tight, and no obvious interfacial reaction layer was observed in the as-prepared composite.

Abstract: A three dimensional (3D) SiC/Cu-Si composite with bicontinuous structure was fabricated by spontaneous infiltration method, using porous recrystallized SiC ceramic with porosity of 37% as 3D network reinforcement and Cu alloy (Cu-18Si) as matrix. The phase composition, microstructure, and thermo-physical properties of the as-prepared 3D-SiC/Cu-Si composite were investigated. The experimental results showed that the Cu-18Si alloy could fully penetrate into the porous SiC ceramic at 1600 °C for 2 h spontaneously. SiC and Cu₁₅Si₄ phases were identified in the as-prepared composites. The interfacial bond between SiC and Cu-Si alloy was tightly and no severe interfacial reaction was observed. The thermal conductivity and coefficient of thermal expansion of the as-prepared 3D-SiC/Cu-Si composite were changed from 89.8 to 55 W·m^-1·K^-1 and 7.512 to 9.64×10^-6 °C^-1 with the temperature increased from room temperature to 500 °C, respectively.

Abstract: In this paper, the compact SiO₂-B₂O₃-Al₂O₃-MgO-F machinable ceramics were prepared through melt-cast and sintering method. The phase composition and microstructure of the ceramics were analyzed and observed using XRD and SEM, and the relationship between processing and microstructure were discussed. The results showed that, after the heat treatment, the main phase of the ceramics changed from fluorophlogopite (mica like) to fluoramphibole (rod like). When the crystallization temperature at 950°C, the nuclei growth rate of the mica glass-ceramics was higher. Meanwhile, a small quantity of forsterite (Mg₂SiO₄) was also crystallized out in the process. The best heat treatment process was nucleated at 630°C for 2h and crystallized at 950°C for 2h.

Abstract: The effects of Cr₂O₃ additions on the mechanical and electrical properties of alumina ceramics were investigated. The phase composition, organizational structure of ceramics were analyzed and observed using XRD and SEM. The results show that doping with small amount (<7wt.%) of Cr₂O₃ can improve the mechanical (15% for bending strength, 83% for Vickers hardness) and electrical properties (minimum resistivity 9.8 × 10¹²Ω·cm), but when doping amount above 7wt.% , these properties were decreased.

Abstract: Combustion reaction in laminated Ni and Al foils was ignited by plasma activated sintering (PAS) to synthesize metal-intermetallic laminated composites (MILCs). The microstructure evolution in the reaction and the post-heat treatment was investigated. The results showed that thermal explosion (TE) reaction were happened between Ni and Al foils at the melting point of Al. The reaction was incomplete due to the heat loss through the thick foils. The produced phases, Ni₂Al₃ and NiAl₃, were converted to a compositionally gradient series of intermetallic phases in the nal microstructures of the intermetallic layers after the post heat treatment.

Abstract: SiC/0~20% graphite (volume fraction) composites were successfully fabricated by pressureless sintering at 1700 °C for 2 h in nitrogen atmosphere. The impregnating and heat treatment process for the samples with silica sol/phenolic impregnant was carried out at 1450 °C for 2 h in nitrogen atmosphere. The Vickers hardness, oxidation resistance and thermal shock property of the composites before and after treatment were tested, and the microstructure and phase composition were characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). The results shown that the Vickers hardness of SiC/20% graphite was increased from 3.35 GPa to 4.90 GPa by heat treatment, and the oxidation resistance and thermal shock property of the heat treated samples were also improved significantly. The SEM and XRD results revealed that the formation of new SiC particles by the reaction of silica sol and phenolic was the main reason for the mechanical properties improvements.

Abstract: A self-made sectional die made of high-performance graphite, SiC ceramic and Ni-based superalloy was firstly designed and developed. The TiO₂ ceramic, with original TiO₂ powders of average particle size ~25 nm and 80 wt.% anatase + 20 wt.% rutile, was fabricated by plasma activated sintering (PAS) at 500600 °C for 3 min under applied uniaxial pressure of 2001000 MPa using the sectional die. The influences of sintering temperature and applied pressure on the density, phase transformation and grain growth of the TiO₂ ceramic were investigated. The results showed that the sintering temperature and applied pressure played key roles in determining the relative density, phase composition and grain size of the TiO₂ ceramic. The relative density and grain size increased and the anatase phase transformed into the rutile phase slowly or quickly as the sintering temperature or the applied pressure increased. In particular, the increase of sintering temperature was very advantageous to the phase transformation, and the increase of applied pressure was quite effective to inhabit the grain growth. All the averaged grain sizes of TiO₂ ceramics were less than 100 nm in the present experimental conditions. Moreover, the relative density of the sintered ceramic were over 95% when the optimized sintering parameters were 600 °C × 500MPa or 500 °C × 1000MPa. The TiO₂ ceramics were composed of only the rutile phase when the applied pressure and the sintering temperature were not less than 300 MPa and 550 °C, respectively.

Abstract: In this research, the SiC/Al/h-BN composite ceramics with different SiC grain size were fabricated by the method of preparing the machinable pre-sintered body through Plasma Activated Sintering (PAS), which has the advantage of complex shape formation for precision parts. By hardening treatment, the SiC/Al/h-BN composite ceramics would be changed to SiC/AlN composites, and the relatively higher hardness and mechanical properties could be obtained accordingly. The phase transformation and microstructure were observed and the mechanical and other properties were also measured. The results showed that, for different matrix SiC particle size, change tendency of the bending strength was different with the heat treatment temperature change. And the bending strength of composites with larger SiC grain size was higher than that with smaller SiC grain size.

Abstract: The preparation of interconnected mesoporous carbon monoliths (MCMs) derived from phenolic resin/ethylene glycol mixtures based on polymerization-induced phase separation have been investigated for fabrication of complex-shape SiC ceramics. The effect of the ethylene glycol content, curing catalyst and the curing temperature on the pore structure and pore distribution of carbon monoliths has also been studied, with emphasis on controlling the apparent porosity and pore size distribution. Fractal dimensions (DF) was proposed to evaluate the morphologies of carbon monoliths by using the box counting method. The results show that interconnected mesoporous carbon monoliths with narrow pore size distribution were obtained by changing the curing temperature and the content of ethylene glycol, curing catalyst in the resin mixtures and its mechanism was discussed in this paper. In this paper, interconnected mesoporous structure was attributed to the mechanism of spinodal decomposition (SD), which was discussed in detail. Carbon monoliths inherit their porosity from cured resins where it was formed as a result of phase separation of resin-rich and glycol- rich phases.

Abstract: In this work, a facile approach combining polymerization-induced phase separation and starch-templating is presented to synthesize hierarchically macro/mesoporous carbons. The obtained porous carbons have bimodal macropores with pore diameters of 10~60 μm and 3~5 μm and 3D interconnected mesopores with pore diameters of 5~40 nm. The large macropores and the small macropores are obtained by the thermal decomposition and the closely stacking of starch particles, respectively. The 3D interconnected mesopores are developed through polymerization-induced phase separation between ethylene glycol and phenolic resin via spinodal decomposition mechanism. These as-prepared hierarchically macro/mesoporous carbons may have great potential for applications as electrodes materials for batteries, fuel cells, and supercapacitors due to their facile synthesis, unique hierarchical porous structure, and large BET surface areas (~ 610 m2/g).