Papers by Author: Gui Wu Liu

Abstract: The Fe-C composites were fabricated by a combination of high-energy ball milling of Fe-C powder mixtures and plasma activated sintering process. An orthogonal experiment in four factors (including original powder composition, sintering temperature, applied pressure and holding time) and three levels was employed to investigate the effects of preparation technology on mechanical properties (bending strength and hardness) of the Fe-C composite. The experimental results show that the crystalline Fe₃C phase can be produced by the rapid sintering process, though it is cannot form theoretically due to the high Gibbs free energy, and more or less holes and composition segregation phenomenon coexist in the composite. The original powder composition plays the leading role in both the mechanical properties of the Fe-C composites. However, the effects of the other parameters on the bending strength and hardness of the composite are somewhat different. The optimal technology combinations for the bending strength and hardness are obtained as follows: 50Fe+50Fe₃C/1373 K/400 s/20 MPa and 50Fe+50Fe₃C/50 MPa/1273 K/400 s, respectively.

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: Mullite whiskers were prepared from SiC powders in molten Al₂(SO₄)₃-Na₂SO₄ mixture salts at different temperatures. The morphology and phase composition of resulting whiskers were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) techniques. Mullite tiny fiber clusters with diameter about 50 nanometers and lengths of over several microns were obtained in 900°C mixture molten salts system. A new oxidation-dissolution mechanism was proposed for explanation mullite whiskers growth.

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: A hierarchical porous carbon derived from phenolic resin (PF) was processed using ethylene glycol (EG) and starch as double porogens. The influences of composition of starting mixture, including the two porogens and PF, on morphological properties and microstructure of the porous carbon were investigated. It was demonstrated that the content of starch and the relative content of EG to PF played key roles in determining the number, size and formation of the hierarchical pores, which in turn led to changes in the properties and the microstructure of the porous carbon. In particular, the number of the first-level pores (diameter ~10–40 μm) increased with the content of starch increasing, and the high relative content of EG to PF contributed to the formation of the second-level pores (diameter ~0.5–2 μm), which were closely related to the formation mechanisms of the two-level pores, respectively. Under the present experimental conditions, sufficiently high starch content can result in the microstructural abnormalities, such as the incomplete decomposition of starch and the formation of the third-level pores which originated from the stack of discrete carbon particles.

Abstract: A route based on a technique of polymerization - induced phase separation and pyrolysis (PIPSP) has been developed to fabricate complex-shaped SiC parts. The capability of this process to produce complex component shapes has been demonstrated, and corresponding reactive mechanisms have been also discussed. Three types of porous carbon preforms, i.e. mesoporous carbon monoliths (MCMs), hierarchical porous carbon monoliths (HCMs) and porous carbon foam (PCFs) were obtained, which has different pore size distributions. The pore structures of the preforms can be controlled through changing starting mixture composition and polymerizing conditions. The apparent porosity of the preform was changed from 19.9 to 60%, which was a key parameter to obtain dense SiC parts. After reactive infiltration of the preform with Si, the SiC parts were obtained. Geometry of SiC parts were controlled by molds. The dimension shrinkage of SiC parts was less than 3% before/after siliconization and no distortion occurred. Compared with other molds assistance route, the wax mold assistance route was a most potential technique to fabricate SiC parts industrially because of its suitable forming precision, recycled mold materials and low-cost. The mechanism of the reactive infiltration of MCMs was different from that of the reactive infiltration of preforms with bigger pore size, i.e. the pore channels of MCMs were restructured at transitional stage of reaction.

Abstract: High purity alumina/stainless steel joints were produced via activated molybdenummanganese (Mo-Mn) route using 72Ag-28Cu solder. Microstructures of the metallized ceramic and joint sections were observed by scanning electron microscopy. Joint strength was tested by shear-loading method. Some process factors were characterized and analyzed, which include temperature, holding time and heating and cooling rate in ceramic metallization process. The effects of Ni plating and succedent annealing were also investigated. Experimental results show that, migration of glassy phases is the main mechanism of the ceramic metallization. Glass migration direction is from metallizing layer to ceramic side. In the ranges of temperature and holding time of metallization, joint strength firstly increases and then falls with temperature raising and time extending. More fully sintered metallizing layer can be obtained while the temperature increases from 1200oC to 1500oC, and the time prolongs from 30min to 60min. Over-sintering of the metallizing layer will take place with metallizing temperature of 1600 oC and overlong holding time of 70min, which reduces the joint strength. The slower heating and cooling rate, and the annealing after Ni plating both help enhance the seal strength, due to relieving or eliminating interlayer residual thermal stress. However, too slow heating and cooling rate, such as 5 oC /min, is equivalent to overlong holding time and finally also decline the strength. A thin Ni coating helps solder wet metallizing surface, and stops solder erode metallizing layer.