Key Engineering Materials Vols. 512-515

Abstract: Porous mullite ceramics reinforced by mullite fiber have been prepared by vacuum filtering with MgO as sintering aid and starch as pore-forming agent. The influence of different sintering temperature and mullite fiber content on porosity and strength of porous mullite ceramics was investigated. The results showed that the compressive strength of the mullite ceramics increased with increasing sintering temperature. With the increase of mullite fiber content, the compressive strength of porous mullite ceramics appeared to increase at first and then decrease. And the peaking strength could reach 211MPa with 36.8% porosity when 7 vol% mullite fiber was added and sintering temperature of 1500 oC was adopted.

Abstract: Porous anorthite/mullite composite ceramics with different mullite content were fabricated by foam-gelcasting, using CaCO₃, SiO₂, α-Al₂O₃ as raw material for anorthite phase and mullite powder for mullite phase. Effects of mullite powder content on bulk density, porosity, compressive strength and thermal conductivity of the porous composite ceramics were researched. It has been shown that mullite powder content has great effect on microstructure and properties of the porous anorthite⁄mullite composite ceramics. The open porosity of the prepared porous anorthite⁄mullite composite ceramics is in the range of 58.7 %~77.5 %, the compressive strength is between 4.2 and 30.9 MPa, and the thermal conductivity is in the range of 0.18 ~1.47 W⁄(m·K).

Abstract: Porcelain insulator has a long history of being used in power transmission and distribution due to the good strength, chemical resistance capacity and mature manufacturing technology. However, there are some disadvantages of porcelain materials which could be the barrier for the development of porcelain insulators such as brittleness, which could lead to severe hurt to personnel and power station after a burst failure. In this paper, mullite fibers are used in order to improve the mechanical properties of porcelain material and further enhance the anti-burst behavior of porcelain insulators. The samples were prepared by traditional wet method for ceramic materials and several material characterization methods including XRD, SEM were employed to evaluate the reinforcement effect. The test result showed that with the addition of mullite fibers both the bending strength and the fracture toughness of porcelain materials have been largely improved. The investigation provided a meaningful pathway for further development not only for porcelain insulators but also for the power industry.

Abstract: In this paper, a fine pure Magnesium aluminate spinel powder was synesised by Gel solid reaction,using Al(NO₃)₃ 6H₂O and Mg(NO₃)₂6H₂O as raw materials under given condition. The precursor was formed by polymer gelling.The phase changed according to heating temperatures from 700°C to 900°C.The single phase of Magnesium aluminate spinel was obtained by calcining the precursor at temperatures higher than 900°C. The phase changing process and relationship among calcination parameters, phase composition and image of particle are studied to determine the optimal conditions for the synthesis of fine Magnesium aluminate spinel powder. The powder obtained was characterized by XRD,TEM etc. The results of analysis showed that the ultrafine powder is of spinel phase, high purity, good dispersion.This process for preparing fine Magnesium aluminate spinel powder was very simple and inexpensive.The results of analysis showed that the ultrafine powder was of mullite phase, high purity, good dispersion, and the particle were homogeneous.

Abstract: Boehmite nanoparticles with different shapes and lengths were used in the preparation of the boehmite films and coatings, and as the fillers for an organic polymer. Then, it was investigated how the shapes and lengths of the particles reflected the characteristics of the films, coatings and fillers. The films were prepared by casting an aqueous sol of the nanoparticles on a Teflon-lined plate, drying and peeling. The coatings were fabricated on porous alumina tubes or graphite sheets by dipping the substrates into a sol of the nanoparticles. The boehmite-polyvinyl alcohol (PVA) composite films, in which the nanoparticles are fillers, were prepared by casting a mixture of a boehmite nanoparticle sol and PVA. The films, coatings and composites were characterized by TEM, TG-DTA, gas permeability, pore size distribution and thermal expansion measurements. In the films, fibrous particles with aspect ratios over 50 aggregated in parallel to each other to form slit-like micropores, on the other hand, plate- and rod-like particles with aspect ratios below 10 randomly aggregated to form meso pores. Corresponding to this, for the coatings on porous alumina tubes and graphite sheets, the fibrous particles gave much low gas permeability and high thermostability, respectively, compared with the plate-like and rod-like particles. For the boehmite-PVA composite films, the fibrous particles gave lower thermal expansion than the plate-like and rod-like particles.

Abstract: The thermal properties of the magnesium hydroxide crystalline powder samples, which were prepared by us, were investigated by first-principles method. The calculated results show that the forbidden band of phonon appears in both 14～19 THz and 33～102 THz. There are 3 acoustic lattice wave branches and 12 optical ones, where two horizontal acoustic branches are degeneracy and two horizontal optical ones are degeneracyhe. Moreover, The constant-volume specific heat quickly increases at low temperature; but it tends to be flat at high temperature. Besides, the Debye temperature quickly increases to 483K from temperature 0K to 10K; but from temperature 100K to 1000K, it is linear with temperature approximately, whose increasing rate is about 1.333; at temperature of 1000K, the Debye temperature is 1930K, reaching its maximum.

Abstract: Different compositions (aY₂O₃+bCeO₂)-0.4YCr_0.5Mn_0.5O₃ (a+b=0.6) were prepared via a conventional solid-state reaction at 1200°C, and sintered under air atmosphere at 1600°C. XRD patterns analysis has revealed that for 0<a<0.6, the major phases present in the calcined bodies are Y₂O₃, CeO₂ and orthorhombic perovskite YCr_0.5Mn_0.5O₃ phase, respectively. SEM and EDAX observations confirm the obtaining of three-phased composite ceramics. The brighter regions are the Y₂O₃ and CeO₂ phase, whereas the darker are perovskite phase. All the NTC thermistors prepared show a similar linear relationship between the ln of the resistance and the reciprocal of the absolute temperature, indicative of NTC characteristics. For 0≤a≤0.6, the plotting curves of resistance- temperature characteristic are none linear on the wide temperature range, exhibit two branches. The obtained B_25/150 and B_700/1000 constants of the thermistors are in the range 3600-4400K and 6700-12000K. The magnitude order of the resistivity at 25°C is of 10⁶-10⁷Ωcm and activation energies vary from 0.313 to 1.029 eV at low and high temperatures, respectively. These compounds, having good NTC characteristics in a wide range of temperatures, could be applied as potential candidates for NTC thermistors from ambient to 1100°C.

Abstract: The utilization of lightweight refractories plays an important role in reducing the energy consumption of industrial furnaces. In this paper, MgAl₂O₄-CaA1₁₂O₁₉ high temperature composite was synthesized via solid state reaction using magnesite, dolomite and industrial alumina as raw materials. The influences of raw materials and reaction temperature on phase compositions and microstructure of the composite were investigated by XRD and SEM，respectively. The parameters to prepare MgAl₂O₄-CaA1₁₂O₁₉ high temperature composite were optimized. The results show that the optimum reaction conditions for synthesizing MgAl₂O₄-CaA1₁₂O₁₉ composite is the CA₆/MA weight ratio of 2:3, and the reaction temperature of 1500°C for 4h. The CaA1₁₂O₁₉ crystals showed laminated or plate-like structure, and the MgAl₂O₄ showed spherical morphology. The reaction temperature had little effect on the phase compositions of MA-CA₆ composite in this experiment. The content of Al₂O₃ in the raw material affected the phase composition of MA-CA₆ composite.With the increase of the CaA1₁₂O₁₉ amount, the bending strength of the composite decreased.

Abstract: This paper aims to study preparation and mechanism of hollow ceramic microspheres. With Al+Cr₂O₃+SiO₂+sucrose+epoxy resin as reactive system, Al2O3-based hollow ceramic microspheres are synthesized and reaction mechanism of hollow ceramic microspheres is established. Results show that the quenching products consist of hollow spherical or irregularly spherical particles and irregularly powders. Spherical or irregularly spherical particles are hollow structure and constitutive of Al₂O₃-SiC diphase and a few Cr₃Si、Cr₇C₃. The preparation of hollow ceramic microspheres includes igniting by heating, forming ceramic thawy droplets and fast cooling. The great deal of air produced by burning epoxy resin and sucrose is the key reason why hollow ceramic microspheres are formed.

Abstract: A new type of double pore size activated alumina/activated carbon multiphase material was made by using waste aluminum slag and activated carbon as main raw materials. The microstructures and pore size distribution of the new material were characterized by SEM and BET techniques. Iodine adsorption value was used to evaluate the adsorption performance of the multiphase materials, and the benzene adsorption capability was also measured. The results show that the specific surface area of the activated alumina/activated carbon multiphase materials is 261.95m²/g, pore volume can reach 0.25m³/g, and the iodine adsorption value is 381.97mg/g. BET pore size distribution shows that micropores and mesopores both exist in the composite material. The micropores range from 0.6 to 1.4nm, and the mesopores range from 3.0 to 7.0nm. The benzene adsorption capability of this new material is excellent, with a saturated adsorption capacity of 241.00 mg/g, much better than pure activated alumina or activated carbon.