Low pressure synthesis of boron nitride with (C2H5)2O · BF3 and Li3N precursor

Cubic boron nitride(c-BN) was synthesized through benzene thermal method at a lower temperature of 300 °C by selecting liquid (C₂H₅)₂O · BF₃ and Li₃N as reactants. Hexagonal boron nitride(h-BN) and orthorhombic boron nitride(o-BN) were also obtained. The samples were characterized by X-ray powder diffractometry and Fourier transformation infrared spectroscopy. The results show that all the BF₃, BCl₃ and BBr₃ in the same family compounds can react with Li₃N to synthesize BN since the strongest bond of B-F can be broken. Compared with BBr₃, liquid (C₂H₅)₂O · BF₃ is cheaper, less toxic and more convenient to operate. Li₃N not only provides nitrogen source but also has catalytic effect on accelerating the formation of c-BN at low temperature and pressure. Foundation item: Projects(20273007,50372006) supported by the National Natural Science Foundation of China     

Structure and mechanical properties of ZrO2-mullite nano-ceramics in SiO2-Al2O3-ZrO2 system

ZrO2-mullite nano-ceramics were fabricated by in-situ controlled crystallizing from SiO2-Al2O3-ZrO2 amorphous bulk. The thermal transformation sequences of the SiO2-Al2O3-ZrO2 amorphous bulk were investigated by X-ray diffraction, infrared spectrum, scanning electron microscope and differential scanning calorimetric. And the mechanical properties of the nano-ceramics were studied. The results show that the bulks are still in amorphous state at 900 ℃ and the t-ZrO2 forms at about 950 ℃ with a faint spinel-like phase which changes into mullite on further heating. ZrO2 and mullite become major phases at 1 100 ℃ and an amount of m-ZrO2 occur at the same time. The sample heated at 950 ℃ for 2 h and then at 1 100 ℃ for 1 h shows very dense and homogenous microstructure with ball-like grains in size of 20-50 nm. With the increase of crystallization temperature up to 1 350 ℃, the grains grow quickly and some grow into lath-shaped grains with major diameter of 5 μm. After two-step treatment the highest micro-hardness, flexural strength and fracture toughness of the samples are 13.72 GPa, 520 MPa and 5.13 MPa·m1/2, respectively.     

Processing and performance of 2D fused-silica fiber reinforced porous Si3N4 matrix composites

Two-dimension （2D） fused-silica fiber reinforced porous silicon nitride matrix composites were fabricated using slurry impregnation and cyclic infiltration with colloidal silica sol. The microstructure and fracture surface were characterized by SEM, the mechanical behavior was investigated by three-point bending test, and the dielectric constant was also measured by impedance analysis. The microstructure showed that the fiber and the matrix had a physical bonding, forming a clearance interface. The mechanical behavior suggested that the porous matrix acted as crack deflection, and the fracture surface had a lot of fiber pull-out. However, the interlaminar shear strength was not so good. The dielectric constant of the composites at room temperature was about 2.8-3.1. The relatively low dielectric constant and non-catastrophic failure indicated the potential application in the radome materials field. 2008 University of Science and Technology Beijing. All rights reserved.     

Microstructure and mechanical properties of WC-20wt% Co/ZrO2（3Y）cermet composites

The cermet composites WC-20wt%Co/ZrO2（E6）with four different comtents of ZrO2（3Y）were prepared by normal vacuum sinter processing;the optical microscope and SEMwere used to characterize their microstructures.The hardness.bending strength and impact toughness of the specimens were determined.The experimental results show that ZrO2（3Y） particles in WC-20wt%Co matrix are sphcrical particles in different sizes which are distributed uniformly in Co phases and WC phases,the bending strength and impact toughness of the WC-20wt%Co cermet composites added ZrO2（3Y）improve remarkably.but the hardness values have little change.     

Preparation of BN／SiO2 ceramics by PIP method

The precursor infiltration and pyrolysis(PIP) method for preparation of BN/SiO2 composites was used to improve mechanical properties, dielectric properties and feasibility of high temperature dielectric parts with large dimensions and complex shapes. In the processing procedure, the porous BN ceramic matrix was first successfully prepared by compacting the mixed powders of B and BN and then sintering them at a certain temperature under normal pressure of N2. The polycarbosilane(PCS) solution was vacuum infiltrated into porous BN ceramics at the room temperature and then at 800℃ in the air to depolimerize out amorphous SiO2, and sintered further at 1 300℃ in N2 to get BN/SiO2 composites. The microstructure of materials was studied by means of X-ray diffraction and electron probe micro analysis. The thermo-decomposition mechanism of PCS was investigated by a TG-DTA and infrared (IR) spectrum analysis. The flexural strengths were measured by the three-point bending method. The dielectric constant and the loss tangent were measured by the wave-guide method. The results show BN/SiO2 composites were fabricated. The obtained composites posses a flexural strength of 61.96 - 93.31 MPa, the dielectric constant in the range of 3.50 - 3.78 and the order of magnitude of the loss tangent at 10^-3 , which are good for the high tempera turedielectric parts with large size and complex shapes.     

AlN／MAS微晶玻璃复合材料的热物理性能

通过X射线衍射、扫描电镜和热物理性能测试，研究AIN引入量和温度对在900-1000℃真空热压烧结制备的AIN／MAS玻璃陶瓷复合材料热物理性能的影响。研究结果表明：复合材料的热膨胀系数随着AIN引入量和测试温度的增加而增加；热导率随着AIN引入量的增加而增加；随着测试温度的升高，复合材料的热导率随着AIN引入量的变化呈现不同的特征；AIN引入量为20％（体积分数）时，样品的热导率随着测试温度的升高而升高，表现出明显的非晶态物质的导热特性；AIN引入量为50％时，样品的热导率呈现先升高后稍许降低的导热特性。     

Low-temperature sintering and microwave dielectric properties of Li2MgTi3O8 ceramics doped with BaCu（B2O5）

The influences of BaCu(B₂O₅) (BCB) addition on sintering, microstructure and microwave dielectric properties of Li₂MgTi₃O₈ ceramics were investigated using X-ray diffractometry, scanning electron microscopy and microwave dielectric measurements. The experimental results show that a small amount of BaCu(B₂O₅) addition can effectively reduce the sintering temperature to 900 °C, and induce only a limited degradation of the microwave dielectric properties. Typically, the best microwave dielectric properties of ɛ _r=24.5, Q×f =24 622 GHz, τ _f=4.2×10⁻⁶ °C⁻¹ are obtained for 1.0% BCB-doped Li₂MgTi₃O₈ ceramics sintered at 900 °C for 3 h. The BCB-doped Li₂MgTi₃O₈ ceramics can be compatible with Ag electrode, which may be a strong candidate for low temperature co-fired ceramics applications.     

Effect of sintering additives on properties of Si3N4-BN composites fabricated via die pressing and precursor infiltration and pyrolysis route

The Si3N4-BN composites have been prepared via die pressing and precursor infiltration and pyrolysis route using borazine as precursor, and the effect of sintering additives on properties of the composites has been investigated. After sintering additives are adopted, the α to β phase transition of Si3N4 and the mechanical properties of the composites at both room temperature and high temperature are all increased with small extent. When using Y2O3+Al2O3 as additives, the phase transition of Si3N4 and the mechanical properties of the composites have better results. The β-Si3N4 content is 17.47%. The fl exural strength, elastic modulus and fracture toughness of the composites are 188.74 MPa, 84.34 GPa and 2.96 MPa?m1/2, respectively. After exposed at 1 000 ℃ in the air for 15 min, the fl exural strength of the composites is 154.62 MPa with a residual ratio of 81.92%. The elongated β-Si3N4 grains appear in all composites with different sintering additives. Relatively more rod like β-Si3N4 grains can be observed in composites with Y2O3+Al2O3 as additives, making it to possess better mechanical properties.     

Effect of nucleating agents on microstructure and mechanical properties of SiO2-Al2O3-ZrO2 glass-ceramics

SiO2-Al2O3-ZrO2 glasses with different nucleating agents were crystallized under special processing schedule. The microstructure and mechanical properties of the glass-ceramics in SiO2-Al2O3-ZrO2 system were investigated by differential scanning calorimetry, scanning electron microscopy, X-ray diffraction and three-point bending method. The results show that ZrO2 is not an effective nucleating agent in SiO2-Al2O3-ZrO2 system, while TiO2 is effective for the separation of spinel, and P2O5 facilitates solubility of ZrO2 in glass and crystallization. The main crystalline phases of the glass-ceramics are spinel, anorthite and tetragonal zirconia. With the increase of ZrO2 content in the glass, glass-ceramics show higher bending strength （120 MPa） than others.     

Fabrication and Mechanical Properties of TiC／TiAl Composites

TiC/ TiAl composites with different TiC content were fabricated by rapid heating technique of spark plasma sintering ( SPS ). The effect of TiC purticles on microstructure and mechanical properties of TiAl matrix was imestigated. The results indicate that grain sizes of TiAl matrix decrease and mechanical properties are improved because of the addition of TiC particles. The composites display a 26.8% increase in bending strength when 10wt% TiC is added and 43.8% improvement in fracture toughness when 5ut% TiC is added compared to values of TiC-free materials. Grain-refinement and dispersion-strengthening were the main strengthening mechanism. The improvement of fracture toughness was due to the deflexion of TiC particles to the crack.     

Effect of relative density on cyclic oxidation resistance properties of MoSi2

MoSi2 powders were fabricated respectively by mechanical alloying technique and sintering at different temperatures to prepare materials with different relative densities. The relative oxidation behavior of all MoSi2 materials at 1 473 K was investigated by TGA,SEM and XRD. The results show that the ＂pesting＂ is not found in all materials after being oxidized for 480 h. The density has no essential relation to the ＂pesting＂. The oxidation curve of specimens with lower density shows two-step oxidation kinetics. Both the first stage （0-1 h） and the second stage （1-480 h） nearly obey linear kinetics,but the oxidation rates are obviously different. The oxidation kinetics of MoSi2 with higher relative density nearly follows parabolic law. The mass gains of MoSi2 with the lowest relative density （78.6%） and the highest relative density （94.8%） are increased by 10.390 and 0.135 mg/cm^2,respectively. The oxide scale of materials with lower densities is non-protective and makes the oxygen diffusion easy. A dense scale in the material with higher density is formed,which acts as a diffusion barrier to the oxygen atoms to penetrate into the matrix,showing much better high temperature oxidation resistance. The phases distribution of oxidation scale from the outside to the inside is SiO2→Mo5Si3→MoSi2.     

Low-temperature oxidation behavior of MoSi2 powders

The oxidation behavior of molybdenum disilicide (MoSi2) powders at 400, 500, and 600℃ for 12 h in air were investigated by using X-ray diffraction (XRD) and transmission electron microscopic (TEM) techniques. Significant changes were observed in volume, mass, and color. Especially at 500℃, the volume expansion was found to be as high as 7-8 times, the color changed from black to yellow-white, and the mass gain was about 169.34% after 8 h, with SiO2 and MoO3 as main reaction products. The gains in volume and mass were less at 400 and 600℃ compared with those at 500℃, probably due to the less reaction rate at 400℃ and the formation of silica glass scale at 600℃, which would protect the matrix and restrain the diffusion of oxygen and molybdenum. Thus,the accelerated oxidation behavior of MoSi2 powder appeared at 500℃ and the volume expansion was the sign of accelerated oxidation.     

FHA bioceramic composite materials enhanced by ZrO2

Experiments for investigating the problem to improve the mechanical properties of FHA [Ca₁₀(PO₄)₆F(OH)] biomaterial have been presented in this paper. ZrO₂ micro-particle, as strengthening phase, is added into FHA matrix material to make a composite biomaterial. Various mechanical properties were tested. Distribution behaviour of ZrO₂ particle in sintered material and phase structure changes of material at elevated temperature were investigated by means of X-ray and SEM. Some biologic experiments were also carried out on animals to estimate the biological function of the composite material. Project supported by the National Natural Science Foundation of China Synopsis of the first author Ruan Jianmin, associate professor, born in 1957, specializing in powder metallurgy as well as bioceramic materials     

稀土Nd掺杂对Sr（Fe0.5Nb0.5）O3基介质陶瓷巨介电性能的影响

铁铌酸锶是一种重要的介质材料,具有复合钙钛矿结构,同其他介质材料相比,具有相对高的介电常数的优点,在高储能等领域有重要的应用潜力,是一种用途广泛的功能陶瓷材料．实验以Sr（Fe0.5Nb0.5）O3基陶瓷为研究对象,研究了稀土Nd掺杂对该系陶瓷巨介电性能的影响．结果表明：随着Nd抖掺杂量的增加,陶瓷样品的介电常数逐渐变大;随着温度的上升,陶瓷体系的介电常数也逐渐增大;Nd抖含量x≤0．10时,陶瓷样品有着良好的频率稳定性,x=0．10的陶瓷样品在100kHZ下有着良好的温度稳定性．     

玻璃纤维的含量对复合材料的力学性能影响及表征研究

为研究不同质量分数的玻璃纤维对增强聚丙烯复合材料力学性能的影响,选择直径为10μm的玻璃纤维制备复合材料小样.测试在不同质量分数时材料的拉伸强度、弯曲弹性模量等力学性能,并应用扫描电镜(SEM)对其微结构进行表征.结果表明:玻璃纤维质量分数对复合材料的取向分布有很大影响,随着玻璃纤维质量分数增加,拉伸强度增大,但弯曲弹性模量、弯曲强度变化不明显.此外,随着质量分数的增加,复合材料的脆性变大;SEM分析表明复合材料中玻璃纤维有一定的取向且分布相对均匀,玻璃纤维和复合材料基体结合良好.     

Microstructure and microwave dielectric properties of CaO-B2O3-SiO2 glass ceramics with various B2O3 contents

The effects of B2O3 addition on both the sintering behavior and microwave dielectric properties of CaO-B2O3-SiO2 (CBS) glass ceramics were investigated by Fourier transform infrared spectroscopy (FTIR),X-ray diffractometry (XRD) and scanning electron microscopy (SEM).The results show that the increasing amount of B2O3 causes the increase of the contents of [BO3],[BO4] and [SiO4],which deduces the increase of CaB2O4 and α-SiO2 and the decrease of CaSiO3 correspondingly.No new phase is observed throughout the...     

Mechanical properties of 3D carbon/carbon composites by nanoindentation technique

Nanoindentation tests were conducted to investigate the near-surface mechanical properties of the individual components (fiber and matrix) for three-dimensional reinforced carbon/carbon composites (3D C/C). Optical microscope and polarizing light microscope were used to characterize the microstructure of 3D C/C. The microscopy results show that large number of pores and cracks exist at both bundle/matrix interface and pitch carbon matrix. These defects have important effect on the mechanical behavior of 3D C/C. The in situ properties for components of 3D C/C were acquired by nanoindentation technique. Relative to the matrix sample, the fiber samples have more larger values for modulus, stiffness and hardness. However, there is no significant difference of modulus and stiffness among fiber samples with different directions.     

Degradation of p-nitrotoluene by O3/H2O2 process and oxidation mechanism

The degradation of p-nitrotoluene by O3/H2O2 process in a bubble contact column was investigated. Effects of the molar ratio of hydrogen peroxide to ozone,pH value and t-butanol on the oxidation process were discussed. It was found that the proper H2O2/O3 molar ratio for the degradation of p-nitrotoluene was around 0.6, different pH values and the presence of t-butanol highly influenced the removal efficiency of p-nitrotoluene. 5-methyl-2-nitrophenol, 2-methyl-5-nitrophenol, (4-nitrophenyl) methanol, 5-(hydroxymethyl)-2-nitro phenol, acetic acid, 2-methylpropane diacid and 2-(hydroxylmethyl)propane diacid were identified as degradation intermediates and products through GC-MS. Radical reaction mechanism and degradation pathway were proposed based on the results of experiments. It is deduced that the benzene ring of p-nitrotoluene can be only destroyed by hydroxyl radicals through a polyhydroxy intermediate pathway. Then unstable polyhydroxy intermediates can be oxidized to different acids with low molecular weight rapidly.     

La3+掺杂BaAl2Si2O8陶瓷的结构及微波介电性能

采用传统固相反应合成Ba_1-xLa_xAl₂Si₂O₈(x=0, 0.005, 0.01, 0.015, 0.02),探讨不同La³⁺取代量对Ba_1-xLa_xAl₂Si₂O₈(BAS)陶瓷晶体结构和微波介电性能的影响,根据第一性原理计算讨论离子的取代位置,比较不同位置的形成能。结果表明：La³⁺最有可能取代Ba²⁺;La³⁺能有效地促进六方相向单斜相的转变,当x=0.02时,转化率达到98.71%,提高了品质因数(Q×f值)和谐振频率温度系数(τ_f);烧结温度1400 ℃、x =0.01时Ba_0.99La_0.01Al₂Si₂O₈陶瓷表现出最佳介电性能,Q×f为25674 GHz,密度ρ为3.30 g/cm³,介电常数ε_r为6.938,谐振频率温度系数τ_f 为−42.79×10^–6/ ℃。     

Low-cost preparation of boron nitride ceramic powders

The amorphous boron nitride ceramic powders were prepared at 750-950 ℃ by the low-cost urea route, and the effects of preparation temperatures, molar ratios of the raw materials and oxidation treatment on the composition, structure and surface morphology of the products were investigated through FT-IR, XRD and SEM. The results show that the products ceramize and crystallize gradually with the increase of the temperature. When the molar ratio and reaction temperature are 3:2 and 850 ℃, respectively, the products have high purity, compact structure and nice shape. The oxidation treatment at 450 ℃ will not impair the composition and structure of boron nitride but effectively remove the impurities.