Effect of carbide particles on the ablation properties of tungsten composites

The high temperature ablation behavior of tungsten composites containing carbides produced by vacuum hot pressing is studied as a function of reinforcement chemistry (ZrC and TiC) and content using a self-made oxyacetylene ablation equipment. A dynamic responding multiwavelength pyrometer was employed to measure the temperature of the ablation surface, and a thermocouple was employed to measure the temperature of the back surface during the time that a specimen was being ablated. The mass and linear ablation rates are lower in composites containing ZrC, decreasing with increasing particle content in both composites system. The values of the mass and linear ablation rates were in the order from high to low: W>30TiC/W>40TiC/W>30ZrC/W>40ZrC/W (30TiC/W stands for 30 vol.% TiC particle content in the W matrix, the same below). The important temperature curves of the ablation surfaces of specimens were successfully detected online. Ablated surfaces and vertical sections of the specimens were investigated using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Thermochemical oxidation of tungsten, TiC, and ZrC was the main ablation mechanism of ZrC/W and TiC/W composites. These ablation behaviors are discussed based on the thermophysical and chemical properties of both the composite systems.     

Microstructural and electrical properties of sintered tungsten trioxide

Tungsten trioxide sintered wafers were prepared from WO₃ powder obtained when ammonium paratungstate is decomposed in air at moderate temperature. Two wafer series of five samples were sintered under the same conditions in the temperature range 600–1000 °C. One of these wafers series was submitted to a subsequent annealing at 700 °C under a hydrogen atmosphere. All samples were characterized at room temperature by X-ray diffraction and electrical measurements. X-ray spectra show that WO₃ ceramic presents a mixture of the triclinic and monoclinic phases before the reduction process. After the reduction process, WO₂ and four hydrogen tungsten bronze phases are present in wafers. Capacitance measurements showed that the samples submitted only to the sintering process changed the dielectric constant with the frequency according to the Debye model. The reduced WO₃ shows a semiconductor behavior, as determined by electrical resistivity measurements.     

添加剂对Ag/陶瓷复合触点材料浸润性及电性能的影响

采用固相反应法制备了LaFe0.25Ni0.75O3陶瓷并加入少量添加物作为改性剂,对其与Ag的浸润性,材料的电阻率以及Ag/陶瓷复合电接触材料的电性能分别进行了研究,结果显示,改性剂使陶瓷材料与Ag之间的浸润性发生明显的变化,且作用效果与改性剂的加入方式有关,添加剂的加入使材料的电阻率增大,同时一定程度上改变了陶瓷在银基体中的分布状态,从而对电接触材料的使用性能产生较大的影响.     

Sr掺杂对WO3陶瓷电学性质及微观结构的影响

利用传统的陶瓷制备工艺制备了Sr掺杂的WO3陶瓷样品,测量了样品的显微结构和电学性质.结果表明,对于Sr掺杂来说,浓度为2 mol%是一个临界点,在这一点处电学性质和微观结构都将发生显著的变化.掺杂浓度为0.2 mol%的样品表现出较高的非线性系数8.7.实验中发现部分样品表现出电学的不稳定性,我们认为这种不稳定性与相共存以及由此导致的极化有关.     

Microstructure and tribological properties of alumina ceramic with laser-dispersed tungsten additions

The surface of slightly porous, commercially available alumina ceramic was laser-modified with tungsten and/or zirconia additions. The thickness of the resulting multiphase surface layers ranged from 300–800 m depending on the chemical composition and the parameters of the laser process used. Microstructure and worn surfaces were analysed by scanning electron microscopy and energy- dispersive X-ray spectroscopy. Mechanical properties were characterized by using Vickers hardness and nanoindentor testing. Tribological tests were carried out on the surface-modified ceramics using a ball-on-block tribometer. All tests were conducted in unlubricated oscillating sliding contact against balls of alumina in laboratory air at room temperature, relative humidities varying between 3% and 80%, and in distilled water. The multiphase surface layers showed a total volume fraction of second phases up to 40 vol% embedded in the alumina matrix, whereas the average size of the alumina grains was substantially reduced compared with the substrate ceramic. Tungsten dispersoids were distributed homogenously in the ceramic matrix and eutectic Al2O3–ZrO2 phase occurred along the boundaries of the alumina crystallites. Mechanical and tribological properties varied as a function of the microstructure of the laser-modified ceramics, i.e. type and volume fraction of the second phases, and both friction and wear were substantially reduced compared with the commercially available monolithic alumina ceramic used for reference. Friction coefficient and amount of linear wear of the ceramics decreased with increasing relative humidity of the surrounding air. © 1998 Kluwer Academic Publishers     

Effect of inter-electrode spacing on structural and electrical properties of RF sputtered AlN films

An attempt has been made to correlate the morphological and electrical properties of RF sputtered aluminum nitride (AlN), with target to substrate distance (D _ts) in sputter chamber. AlN films, having thickness around 3,000 Å, were deposited on silicon substrates with different D _ts values varying from 5 to 8 cm. XRD results indicated that the crystallinity of c-axis oriented films increase significantly with decrease in D _ts and the FTIR absorption band of the films became prominent at shorter D _ts. The surface roughness increased from 1.85 to 2.45 nm with that in D _ts. A smooth surface with smaller grains was found at shorter D _ts. The capacitance–voltage (C–V) measurements revealed that the insulator charge density (Q _in) increased from 3.3 × 10¹¹  to 7.3 × 10¹¹ cm^?2 and the interface state density (D _it) from 1.5 × 10¹¹  to 7.3 × 10¹¹ eV^?1cm^?2 with the increase in D _ts.     

Characteristics of aerosol-synthesized AlN particles

Particulate characteristics were investigated for AlN synthesized by a thermochemical reaction of aluminium aerosol with ammonia gas. The product powders had a decreasing specific surface area between 15.7 and 36.7 m²g^–1, with increasing reaction temperature from 1100–1500 °C, and the powders with large surface area were strongly hygroscopic. Although the powders were severely aggregated and had a small amount of unreacted aluminium, light milling and post heat treatment made them ultrafine and completely converted. When sintered with 0.5% yttrium at 1900 °C, full densification and high thermal conductivity of about 130 Wm^–1 K^–1 were obtained.     

Preparation and properties of SiO2 matrix composites doped with AlN particles

SiO₂ matrix composites doped with AlN particles were prepared by hot-pressing process. Mechanical properties of SiO₂ matrix composites can be greatly improved by doping with AlN particles. Flexural strength and fracture toughness of 30 vol%AlN-SiO₂ composite sintered at 1400°C reached 200 MPa and 2.96 MPa·m^1/2. XRD analysis indicated that, up to 1400°C, no chemical reaction occurred between SiO₂ matrix and AlN particles suggesting an excellent chemical compatibility of SiO₂ matrix with AlN particles. The influences of hot-pressing temperature and the content of AlN particles on dielectric properties of SiO₂-AlN composites were studied. The temperature and frequency dependency of dielectric properties of SiO₂-AlN composites were also studied. Residual flexural strength of SiO₂-AlN composites decreased with increasing temperature difference. The critical temperature difference was estimated about 600°C.     

退火温度对ZnO陶瓷薄膜低压压敏特性的影响

应用新型溶胶-凝胶法制备了ZnO陶瓷薄膜,研究了退火温度对ZnO陶瓷薄膜低压压敏电阻电性能的影响.结果表明,采用溶胶掺杂在550℃退火条件下可形成Zn7Sb2O12及ZnCr2O4相,且在退火温度范围内(550～950℃)基本上没有焦绿石相形成.当退火温度达到750℃以后,Sb2O3已全部转变为稳定性好的尖晶石相,同时存在Bi2O3、ZnO的挥发.采用适当的退火温度,可得到具有优良电性能的ZnO陶瓷薄膜低压压敏电阻,其压敏电压低于5 V,非线性系数可达20,漏电流密度小于0.5μA/mm2.     

Influence of the seed layer on structural and electro-acoustic properties of sputter-deposited AlN resonators

The growth of high-quality AlN films has been studied by reactive sputtering onto Mo electrodes with Ni, Ti, and TiW seed layers and subsequent integration into thin film bulk acoustic wave resonators. The crystalline structure and morphology of the Mo and c-axis oriented AlN films were found to vary strongly with seed layer material and thickness. The smoothest Mo electrodes were obtained on thin Ti films. Reactive sputtering of AlN on top of these optimized electrodes resulted in a dense columnar grain structure with a well-aligned (002) crystal orientation and good electro-acoustic properties, including an effective coupling coefficient of 6.89% and quality factor above 1000.     

晶体结构对化合物电学性质的影响

化合物的导电性包括电子(空穴)导电及离子导电两大部分.新材料的发展已将化合物的电学性质提到了急需研究的位置.随着对化合物电学性质研究的不断深入,发现了许多新现象,新性质和新材料,如氧化物高温超导体,稀土半导体,稀土巨磁阻材料等.这些都是电子(空穴)导电的实际应用.而由于新能源探索需适应保护环境的需要,传统的氧离子和质子导体成了人们的首选研究对象,以开发它们在燃料电池、氧分离膜,催化等方面的实际应用.由于不等价置换,价态变化及氧离子具有大的双电荷与阳离子基体的强相互作用及高迁移率,使得大多数的研究工作集中于化合物的结构畸变分析及精化,试图发现新的结构相或者新的电学材料.     

Irreversible anomalies in the electrical properties of oxides with the tetragonal tungsten bronze structure

The anomalies in the electrical properties of (Pb,Sr,Ba)₄Li₂Nb₁₀O₃₀ solid solutions were studied by measuring dielectric permittivity and electrical conductivity as functions of temperature and time (during isothermal holding). The anomalies were shown to occur in defect-rich ferroelectrics. Both the dielectric permittivity and conductivity vary exponentially with time during isothermal holding. The effects of heating rate and UV irradiation time are analyzed. The results can be accounted for in the framework of a fluctuation-cluster model.     

Dramatic effect of multiwalled carbon nanotubes on the electrical properties of alumina based ceramic nanocomposites

We report recent work on electrical properties of multiwalled carbon nanotubes (MWNTs)/alumina composites. The composites with different contents of MWNTs were consolidated by spark plasma sintering and their temperature dependence dc conductivity was scrutinized in the temperature range from 5 to 300 K. The analysis of the temperature dependence of the conductivity suggests that for temperatures higher than 50 K, conduction can be ascribed to thermal fluctuation induced tunneling of the charge carriers through insulating barriers between MWNTs, while at temperatures below 50 K, the conduction can be attributed to three dimensional variable range hopping through MWNTs network in the alumina matrix. The frequency dependence of the conductivity was studied from 5 to 1.3 × 10⁷ Hz. The universality of the ac conduction in MWNT/alumina composites was examined by construction of master curve.     

Influence of laser annealing on the structural properties of sputtered AlN:Ag plasmonic nanocomposites

We propose a process of deposition of plasmonic nanocomposites comprising magnetron sputtering of AlN:Ag multilayers combined with intermediate steps of flash annealing. When the AlN matrix structure was amorphous, thermal annealing induced the break-up of silver layers and the formation of homogeneously distributed spherical nanoparticles. On the other hand, in the case of a nanocrystalline AlN matrix, the larger nanoparticles were observed to form only at an interfacial and a surface zone. Further treatment by laser annealing was employed in order to photo-modulate the localized surface plasmon resonances (LSPRs) by promoting ripening of the nanoparticles. Using high resolution transmission electron microscopy, it was observed that laser annealing caused nanoparticle enlargement with a concurrent improvement of their separation, while retaining their average spherical shape. Optical reflectance measurements showed that better LSPR was obtained when the AlN matrix was amorphous due to the restrained nanoparticle ripening inside nanocrystalline AlN. Roughening at the film/substrate interface and film degradation after laser annealing at the employed radiation wavelength where reduced compared to similar samples grown by pulsed laser deposition. Based on finite difference time domain simulations and X-ray reflectivity measurements, this was attributed to the improved quality of the AlN matrix.     

Influence of ZrO2 particles on the optical properties of pc-LEDs

Experimental and modeling progress and results aiming to increase the color uniformity of hemisphere-type pcW-LEDs are proposed. By adding micrometric zirconia particles, the light scattering is enhanced, that induces a decrease of phosphor necessary to obtain a specific CCT. The optical model is able to determine the optical properties (CCT, angular CCT distribution, chromaticity and packaging efficiency) in a hemisphere LED for various amount of YAG-phosphor and ZrO₂-zirconia particles. Based on previous process, the work is in the first step to fit the effective radius and refractive index that will be implemented in the optical model. In the second step, the optical model is compared with the experimental measurement to determine the absorption coefficient and conversion efficiency of a package of silicone resin containing phosphor and zirconia. Finally, the model, confirmed by selected experimental results, allows determining the optical properties of any king of package that lead to a CCT emission ranging from 4500 K to 6500 K. The analysis of these data is interpreted by comparing, for a specific CCT, the phosphor loss, the packaging efficiency and the angular distribution of CCT. The results show that if the effort is targeted on one of the previous parameters, there is always a counterpart on the other ones. Increasing the color uniformity will induce to decrease the packaging efficiency.     

Effects of Ag particles on sintering and electrical properties of ZnO-based varistor

ZnO-based varistors containing Ag particles (abbreviated as Z-Ag) were prepared using the conventional solid-state reaction method. The sintering and electrical properties of Z-Ag composites show that the composites can be achieved at a lower sintering temperature (920 °C) relative to that of a commercial ZnO-based varistor. The composites possess non-ohmic behavior analogous to that of the ZnO-based varistor, and the nonlinear voltage can be easily controlled by the content of Ag particles in the ceramic matrix. Meanwhile, the dielectric constant and dissipation factor indicate that the composites have enhanced dielectric properties at room temperature with increasing content of Ag particles, especially at frequencies of 0.5-30 kHz. The mechanisms involved are discussed.     

制备工艺对PAR电双稳薄膜电特性的影响

采用真空蒸发的方法将有机电双稳材料PAR制成Al/PAR/Al夹层结构.详细研究了薄膜厚度、电极面积、退火处理和自然放置等因素对薄膜电性能的影响.结果表明,采用适当的工艺,能够获得性质均匀、电特性良好的电双稳薄膜(转变电压的分散性小于10%,延迟时间基本小于5μs,转变时间在20ns左右).Al/PAR/Al结构可以简单等效为电阻与电容的并联,而退火与自然放置将使薄膜结构趋于稳定.