Papers by Author: Hui Qing Fan

Abstract: A SiC ceramic coating for high temperature and anti-oxidation was prepared by high temperature reaction and PVD methods inside industrial synthetic furnace of SiC. Its phase composition and microstructure were characterized by using XRD, SEM,and its high temperature anti-oxidation property were studied by isothermal oxidation tests.The results showed that the coating is composed of mainly α-SiC and β-SiC,the coating is uniform, compact, perfect grain growth, without crack, closely with the C/C materials,the thickness is about 600μm. After 10h oxidation in air at 1773 k the weight loss of the coating was only 0.3%,which implies that the coating has excellent anti-oxidation ability.

Abstract: The new electrostrictive ceramics have been produced from the (1-x) K0.5Na0.5NbO3-xSrTiO3 (KNN-ST, x=0.1-0.25) system by using conventional mixed-oxide methods. Sintering temperatures rise with increasing SrTiO3 content (x=0.1-0.25) and are in a very narrow range. The x-ray diffraction patterns indicate that with the increasing of SrTiO3 up to 0.25, KNN-ST ceramics with a single perovskite structure exhibit a transition from orthorhombic to cubic and no other impure phase appeared. The dielectric and relaxor ferroelectric properties of KNN-ST ceramics are investigated with the different SrTiO3 content. Also, the strain of these ceramics induced by applied electric fields have been studied. The electrostrictive response is similarly as in the classical PMN (lead magnesium niobate), but lower (order of the 10-5) than PMN (order of the 10-3). Furthermore, this system shows translucent, high dielectric constant, thus suggests possible applications in electric-optic devices, electromechanical transducer applications.

Abstract: Pb(Zr0.95Ti0.05)O3 powders were prepared by one-step pyrolysis process from non-aqueous Pechini method. Porous Pb(Zr0.95Ti0.05)O3 ceramics were obtained by sintering the chemically prepared powders and the grain growth in the porous ceramics was investigated. For the powders calcined at 450 °C for 10 h by one-step pyrolysis process, the grain growth exponent value was observed to be 3.3 while the apparent activation energy was 193 kJ/mol.

Abstract: Lanthanum-modified bismuth titanate, Bi3.25La0.75Ti3O12 (BLT), powders were prepared from polymerized precursors using citric acid as a chelating agent and ethylene glycol as an esterification agent. The effect of calcination temperature on the microstructure and morphology of powders were studied. It was shown that the pure and normally stoichiometric phase of BLT powders could be obtained at relative low temperature from 550o to 700oC even if the bismuth content is not excess in the starting precursors while the secondary phase could be detected at both lower and higher calcination temperatures. The shape of the BLT grains is also strongly influenced by the calcination temperature.

Abstract: Ba0.6Sr0.4TiO3 nanopowders (~ 80 nm in size) were prepared by low temperature hydrothermal process. The process and synthesis mechanism were discussed in detail. Furthermore, the powders were pressed into disc-shaped pellets and sintered at different temperatures. The frequency dependence of the dielectric constant and loss angle tangent of the sintered BST ceramics are measured from room temperature to 500°C and a diffuse dielectric anomaly by the dielectric relaxation was found.

Abstract: Relaxor-based ferroelectrics, 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 (PMN-PT) with a pyrochlore-free phase, was prepared by a modified polymerized complex process. The reactive columbite (MgNb2O6) phase was synthesized by the polymerized complex method at 1050oC for 4 h with 3wt% excess MgO. Lead acetate and tetrebutyl titanate were used to synthesize pyrochlore-free PMN-PT powder via the columbite route, from 800oC to 900oC, in air for 4 h. X-ray diffraction and scanning electron microscopy were used to detect the perovskite phase evolution and the presence of the pyrochlore phase in PMN-PT. The formation of perovskite PMN-PT is a function of the time and temperature conditions of the precursor calcinations, and an optimum condition for the thermal decomposition of the precursor was determined to avoid the formation of the pyrochlore phase.

Abstract: Lead magnesium niobate-lead titanate (0.8Pb(Mg1/3Nb2/3)O3-0.2PbTiO3, PMN-PT) thick films in the thickness range about 75 μm have been successfully fabricated on Au-coated Al2O3 substrates by electrophoretic deposition (EPD). Non-aqueous colloidal suspensions suitable for EPD were prepared by mixing ultrasonically PMN-PT particles in ethanol with pH=6.0. The effect of EPD process parameters such as deposition voltage, deposition time and the specific deposition mass of PMN-PT particles were investigated. The EPD parameters were optimized in order to obtain crack-free, high-quality uniform ceramic films. The deposited pyrochlore-free PMN-PT thick films were sintered at 1000oC for 30 min, and the phase evolvement and the microstructure of the film were characterized by X-ray diffraction and scanning electron microscope.

Abstract: To adjust the morphology and crystallinity of the chemically prepared Zr-riched` lead zirconate titanate (Pb(Zr0.95Ti0.05)O3) powders, polyethylene glycol (PEG 20000) was used in aqueous solution (modified Pechini method). Thermal analysis of the as-synthesized gel showed two distinct stages, which indicates the volatilization of water and the decomposition of polymers, respectively. When the gel were calcined at 400 oC, the powders consisted mainly of perovskite PZT accompanied with small amounts of secondary phases. The higher the calcination temperature applied, the stronger the intensity of XRD, which means the crystallinity were improved. Scanning electron microscopy of as-prepared powders suggested that the powders were soft agglomerate particles when calcined at 600 to 800 oC. Compared with the normal Pechini method, PEG could help the formation of crystalline phase and soft agglomerate. When the powders compacts were sintered at 975 to 1125 oC, high pure (>99.5%) of tetragonal phase ceramics were got having relative density of 85% to 94%.

Abstract: A relaxor ferroelectric material, Pb(Mg1/3Nb2/3)O3 (PMN) powder with a pyrochlore-free phase, was prepared by using a one-step calcinations method. The pyrochlore phase was prevented by coating Mg(OH)2 on Nb2O5 particles. Pure-perovskite PMN powder was synthesized by a single calcination of the coating powder mixed with appropriate amounts of PbO at 900 oC for 2 h. The pyrochlore-free perovskite PMN powders were sintered to 96.9% density at 1100 oC. The sintered PMN ceramics exhibited a dielectric constant maximum of ~11 700 at room temperature at a frequency of 1 kHz.

Abstract: Relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN) crystals were grown at low temperature by a single-step hydrothermal process, using the mixture of PbO, MgO and Nb2O5 as precursors. The precursors were just heated to 150 oC for 24 h. The effect of the KOH concentration on the morphology and microstructure of PMN crystals was systematically investigated. The optimum KOH concentration was found to be 11M, which led to the stabilization of the perovskite crystals against the pyrochlore phase, and to the growth of high-quality PMN crystals suitable for subsequent characterization. According to morphological evolution during the reaction, two possible growth mechanisms are proposed. When the KOH concentration is 11M, at the early stage of the reaction, the starting materials dissolve, and then promote the formation of spherical intermediate pyrochlore phase followed by the formation of primary cubic PMN crystals. At the same time, a platelet-shaped intermediate pyrochlore phase also forms and then grows into larger platelet-shaped PMN crystals.