Zhou Fu Wang

Effects of Rare Earth Oxides on Microstructures and Properties of Magnesia Refractories

The effects of Y2O3, La2O3 and Nd2O3 on the sintering, microstructure and mechanical properties of magnesia refractories were investigated. Addition of rare earth oxide (ReO) to magnesia refractories increases the bulk density, decreases the porosity and improves the mechanical strength of the refractories. The improved sinterability was attributable to the vacancies generation associated with the solid-solution reactions between MgO and ReO. In the samples with ReO, rare earth silicate phases form at magnesia grain boundaries, providing additional bonding between magnesia grains and between magnesia grains and matrix. Consequently, the samples with ReO showed much higher high temperature strengths than those without ReO.

Effect of atmosphere annealing on the magnetic and dielectric properties of YMn0.8Fe0.2O3 multiferroic ceramics

YMn0.8Fe0.2O3 ceramics were annealed in oxygen and nitrogen atmosphere, respectively, and the magnetic and dielectric properties were investigated. The structures of the annealed ceramics were maintained in hexagonal P63cm space group. The magnetic structures of the annealed ceramics were not uniform, and predominantly antiferromagnetic with weak ferromagnetic properties at low temperature was detected. Cluster glass state behavior was observed in the O2-annealed ceramics and absent in the N2-annealed ceramics, which indicated the weakened ferromagnetic interaction in the latter. This was related to the decreased oxygen content in the N2-annealed ceramics. Because of the varied valence distribution of Mn and Fe ions in the ceramics annealed in different atmosphere, the dielectric properties were obviously modified. The dielectric permittivity and dielectric loss were smaller in the O2-annealed ceramics, which should be related to the lower content of Mn2+ ions in the mixed-valence structure of Mn ions. These results indicated that the magnetic and dielectric properties of YMn0.8Fe0.2O3 ceramics were strongly dependent on the oxygen vacancy content and the mixed-valence structure of magnetic ions.

Preparation and Characterization of Al2O3 Coated Graphite Powders

Heterogeneous-nucleation formation of Al2O3 coatings on graphite powders via a controlled hydrolysis process of aluminum chloride was investigated, and surface morphology and properties of the coated graphite were examined. The formed Al2O3 coatings were uniform and essentially amorphous. They were bonded well with graphite via chemical interaction between their O atoms and C atoms on graphite surface. Graphite particle size increased from 7.61 µm (as-received graphite) to 9.53 µm (the coated graphite), revealing that average thickness of the coating was ~1µm. Compared with those of as-received graphite, specific surface area and fractal dimension of the coated graphite also increased, which was caused by the porosity present in the coating. The isolectric point (IEP) of the coated graphite (~9) was close to that of pure Al2O3, indicating that the electro-kinetic behavior of the coated graphite was similar to that of Al2O3.

Preparation and Morphology of Mullite Whiskers by Sol-Gel Method

Mullite whiskers were prepared by sol-gel method. The preparing conditions, the morphology and crystallite phase of the mullite whiskers were studied by Thermal analysis, XRD and SEM. The results demonstrate that the uniform mullite whiskers with high aspect ratio were obtained in properly preparing conditions and appropriate amount of fluorides.

Light Burning Condition of Preparing Dolomite Clinker Using Natural Dolomite

The influence of the light burning temperature on the sintering property of nature dolomite has been investigated by two-step sintering process in the temperature range 1500 °C to 1600 °C. The resulting bulk densities and apparent porosities of the sintered dolomite samples were examined, and analyzing the sintered dolomite by scanning electron microscopy and X-ray diffraction were performed. The results showed light burned at 850 °C for 3 h, the main phases of the dolomite with 3-5 grain size were MgO, CaO and little CaCO3, and then fired at 1600 °C,the density of sintering dolomite reached to 3.38 g/cm3, the apparent property was 1.2 %, the size of MgO grain up to 3.75 μm . However when dolomite light burned at 1050 °C for 3 h, the main phases were MgO and CaO, and then fired at 1600 °C,the density of sintering dolomite only was 3.30 g/cm3, the apparent property was 2.3 %, the size of MgO only was 3.05 μm .

Effect of Micro-Sized Alumina Powder on the Hydration Process of Calcium Aluminate Cement

Micro-sized alumina powder is widely used in low cement high-alumina refractory castables. The hydration of calcium aluminate cement can be affected by adding micro-sized alumina powder. This work addresses the hydration of a commercial cement at 25°C with 50 wt% micro-sized alumina powder added. The hydration heat was measured by isothermal micro-calorimetry. The phase composition and microstructure of the hydration products at the designated times were studied by XRD and SEM, respectively. The results showed that micro-sized alumina powder accelerated the dissolution of CAC. The induction period of the cement hydration reduced from 13h to 4.5h, and the hydration heat within 50hs was increased from 469J/g to 587J/g with the addition of micro-sized alumina powder. The morphology of the hydrates was flocculent amorphous at the beginning, and then transformed to short rods or cubic shape with micro-sized alumina powder added.

Mechanical and Thermal Properties of MgAl2O4-Y3Al5O12 Ceramic Composites

MgAl2O4-Y3Al5O12 ceramic composites were prepared using fused spinel and a Y2O3 micropowder as the raw materials. The microstructure and thermal properties of the composites were characterized by X-ray diffraction, scanning electron microscopy, laser flash diffusivity measurements. The mechanical properties were also determined. MgAl2O4-Y3Al5O12 ceramic composites are composed of spinel and garnet structures. The thermal expansion coefficients of MgAl2O4 and MgAl2O4-Y3Al5O12 ceramics are similar. The measured thermal diffusivity decreases gradually with increasing temperature. Thermal conductivity of the composites is in the range of 3.3-5.8 W∙m-1∙K-1 from 400°C to 900°C.

Effect of WO3 on the Performances of Cordierite Ceramics Synthesized by Using Kyanite as Raw Materials

In order to synthesize cordierite ceramics with low thermal expansion coefficient and good properties, in our work, the cordierite ceramics were prepared by using talc, natural containing zirconium kyanite, common kyanite and industrial Al2O3 as raw materials, introducing the right amount of WO3 (introducing tungsten acid) as catalyst. The effects of the introduced WO3 on the phase composition, sintering characters, microstructure and thermal expansion coefficient of the cordierite ceramics were investigated. The results show that the introduction of WO3 can eliminate the intermediate phase magnesia-alumina spinel and promote the formation of cordierite; the as-prepared cordierite ceramics synthesized by using natural containing zirconium kyanite as raw materials have high densification degree and low thermal expansion coefficient (1.53×10-6/°C, Rt~1000 °C).

The In Situ Preparation of MgAl2O4/YAG Eutectic Composites by Reaction Sintering Using Induction Heating

High density MgAl2O4/YAG (yttrium aluminum garnet) eutectic composites were successfully prepared by in-situ reaction sintering using induction heating (IH). The effects of IH time and the starting materials composition on the phase composition and microstructure were investigated. The results showed that the eutectic composites consisted of only MgAl2O4 and YAG phase could be prepared by IH in a short time in a range of Al2O3/MgO ratio between 1~1.54. Compared to conventional sintering (CS), the solubility of Al2O3 in spinel is remarkably enhanced under the synergistic effects of high temperature and induced electromagnetic field. The higher YAG content facilitates the formation of more eutectic liquid phase, which favors to obtain a more homogeneous and denser interpenetrating network structure.

Effects of Al 2 O 3 on the Structure and Properties of Calcium-Magnesium-Silicate Glass Fiber

Calcium-magnesium-silicate glass fiber is a kind of candidate materials for aluminosilicate ceramic fiber in high temperature resistant field. However, the large thermal shrinkage limits its rapid development and industrial application in high temperature insulation field. It has been known that the shrinkage under high temperatures is mainly affected by the structure and crystallization mechanisms of glass fibers. Thus, Al2O3 was chosen as additive in the chemical composition of glass fiber to investigate the glassy network structure, crystallization and dissolution properties of calcium-magnesium-silicate glass fiber by DTA, XRD and ICP-AES techniques. The results show that with the addition of Al2O3, the glassy network structure was strengthened and the precipitation of crystals was inhibited for heat-treated fibers. As for the dissolution properties in physiological fluids, though the weight losses, changes of pH values and leached ions concentration lowered slightly with the addition of Al2O3 for the intensified network structure, fibers still present high dissolution rates.