Key Engineering Materials Vols. 512-515

Abstract: CaAl₁₂O₁₉-Al₂O₃ composite ceramics were synthesized by in situ solid state reaction using CaCO₃ and industrial α-Al₂O₃ powders. The effects of sintering temperature and ratio of CA₆ (CaAl₁₂O₁₉) content on phases and microstructure of the final composite ceramics were studied by XRD and SEM. The result showed that sheet-like CA₆ appeared in the materials when sintering temperature was higher than 1400°C and grew up with the increased of the sintering temperature. The optimum sintering temperature was 1600°C, when the sample containing 20wt% CA₆ was sintered at 1600°C, the bulk density was 3.21g/cm³, the apparent porosity achieved 9.94% and the bending strength gained 149.88MPa. With the increasing of CA₆ content in the final composite ceramics, the bulk density and the bending strength increased firstly and then decreased. The lowest apparent porosity 1.42%, a density of 3.60g/cm³ and the highest bending strength of 265.10MPa were achieved for the sample containing 10wt% CA₆ sintered at 1600°C.

Abstract: In this study, Al2O3 fiber composite papers with a certain strength and flexibility were prepared by pressure filtration. Microstructure and pore size of the composite papers with different density were investigated and thermal conductivities were measured by plane table thermo-conductivity meter at 600°C to 1200°C. Then the relationship between density ρ and thermal conductivity λ was studied, and the optimum density of Al2O3 fiber composite papers at different temperature was determined.

Abstract: Silica fibers are widely used in superhigh velocity missile’s antenna housing and windows.The effects of heat treatment temperatures, heat treatment atmosphere，dampproof process and impurities contents on the dielectric properties of silica fiber reinforced silica composites were sdutied.Electric network apparatus was used to test the dieletric constants and dielectric loss of silica fiber reinforced silica composites treated at different conditions.The results indicate that silica fiber reinforced silica composites must have dampproof process.The dielectric properties of silica fiber reinforced silica composites was better with little carbon.The dielectric constant was about 3,and the dielectric loss was less than 5×10-3.The minimum transmission coefficient was 68.2%, the average transmission coefficient was 78.5% ~ 86.2%, which showed the radome had good wave penetrating performance.

Abstract: Quartz ceramic has a great potential use in engineering ceramic materials. However, due to the cristobalite transformation in sintering process, the mechanical properties are weakened. The influence of Al3+ additions on bending strength and open porosity of quartz ceramics was determined by adding Al2O3 (sample 1) and aluminum sulfate solution (sample 2). The content of Al3+ ranged from 1% to 7%. Both samples were sintered at 1200°C for 2h. The bulk density of samples was measured by Archimede’s displacement method and the flexural strength of the specimens was tested using a conventional three point flexural method. The addition of 3% Al3+ of aluminum sulfate leads to better properties (bending strength at 14Mpa and open porosity at 23%). Glass theory was applied to explain the mechanisms of inhibiting crystallization of Al3+ additions.

Abstract: Highly porous silica ceramics were produced by gel casting with egg white protein (EWP) as a foaming agent and evaluated by measuring the porosity, mechanical strength and microstructure. After drying, the green bodies were debindered and sintered at 1300°C for 4 h. The influences of the EWP content on the open porosity as well as the strength were investigated. The porosity of the obtained porous silica ceramics was within 70.2%-86.1%, the bending strength decreased from 6.81 MPa to 5.48 MPa. It was found that the strength dependence of the porosity was well described by a modified exponential relation of the form σ=σ0exp(-BP), where P is the porosity and B is a constant (B = 1.37 was obtained by fitting the experimental data). This finding is in agreement with other literature data and seems to indicate a common feature of all porous ceramics.

Abstract: Silica fibers are widely used in superhigh velocity radome and antenna windows. The methods of eliminating the agents on the surface silica fibers were sdudied in this paper. SEM was engaged in studying the effects of silica fibers disposed by different methods,and the tensile strength of disposed silica fbers was compared with the originals. The results show that the heat treatment in the high temperature was a useful means to eliminate the agent on the surface of silica fiber, but there was great thermal damage for silica fibers,and the tensile strenth decreased significantly. The combination of volvent treatment and heat tratment could eliminate the agent on tht surface of silica fibers completely,and the tensile strength retained more, which was a good means to eliminate the agents on the surface of silica fiber.

Abstract: Grafting methacryloxypropyltrimethoxysilane (MPS) onto the surface of silica nanoparticles is the most common approach in surface chemical modification. Traditionally, ethanol or toluene was used as solvent in the grafting process. However, using ethanol as solvent brought about poor grafting result and toluene being regulated was also not an ideal grafting solvent. In order to overcome these disadvantages, we widen the grafting solvents and a novel solvent of xylene was utilized to prepare MPS grafting silica cryogels in this study. The hydrolysis of MPS was in the mixture of methanol and water using diluted hydrochloric acid solution as a catalyst. Both original and MPS-grafted samples were characterized by fourier transform infrared spectrometer (FTIR), thermogravimetric analysis (TG) and other methods. The results showed that xylene is an effective solvent in the process of grafting and the MPS was chemically bonded onto the surfaces of nano-silica particles.

Abstract: Nonionic triblock copolymers (P123) templated SBA-15 silicas since 1998 have attracted great interests due to their larger pores, thicker pore walls, etc. Following that, extensive works were devoted to tailor the textural and morphological properties of SBA-15s, among which the SBA-15 with plugs formed in mesoschannels, called plugged hexagonal templated silica (PHTS), was one of famous examples. This work presents a simple but effective method to prepare PHTS-type SBA-15s based on the partitioned cooperative self-assembly process using P123 as template and cheap sodium silicate as silica precursor, which is in contrast with most other works using TEOS as silica precursor. Various techniques, including XRD, TEM and N2 sorption, have been employed to characterize the resultant PHTS-type SBA-15s. For this method, synthetic parameters, especially the interval time and aging temperatures, were found to have great influence on the plugged mesostructures.

Abstract: Promoting of mullite generation has been studied by replacing kaolinite with pyrophyllite because of mullite has excellent strength and thermal shock resistance. Effects of promoting of mullite generation and vitrification by replacing kaolinite with pyrophyllite on the mechanical and thermal properties were investigated. Addition of 45-55% pyrophyllite as a replacement of kaolinite (pyrophyllite (45-55%)-feldspar (30%)-clay (20%)) could vitrify samples (water absorption: 0.05%, bulk density: 2.66g/cm3) and improve the strength (122MPa) of samples fired at 1280°C. In ternary porcelain system, pyrophyllite-feldspar-clay, mullite generation of samples with 50% pyrophyllite reaches about 78.7% and thermal expansion coefficient is 5.4×10-6/K. Beyond 50% pyrophyllite addition, quartz and cristobalite phases increased. And thermal expansion coefficient of samples decreased with increasing of mullite amount.

Abstract: In this paper, porous mullite ceramics with an apparent porosity up to 81 % were fabricated by foam-gelcasting using mullite powder as raw material with solid loading of 40 vol.%. The monomers content and sitering temperature have obvious effect on the properties of porous mullite ceramics. The apparent porosity of the prepared samples was in the range of 75~82%, compressive strength, was in the range of 3.0~16.02 MPa, and thermal conductivity was between 0.14 and 0.47 W/(m•K). A complex porous microstructure was formed, where large spherical pores contained small cellular pores on their internal walls.