Ferroelectrics of Ultrafine Particle Size: III, Thin Barium Titanate Layers for Capacitors

The large change in the dielectric constant of barium titanate near 120°C was smoothed out by the use of very small particle sizes and additions of tantalum oxide. Unsupported thin sheets of the material showed the same electrical properties as thick pellets. Stacked capacitors made from these sheets, using platinum electrodes, showed a maximum capacitance change over the range 25° to 180°C of 19 to 27%, based on the room-temperature value.     

Ferroelectrics of Ultrafine Particle Size: II, Grain Growth Inhibition Studies

Inhibition of grain growth in pure, fine-particle-size barium titanate bodies by coating the surfaces of the barium titanate particles with tantalum oxide was so effective that doping with a sintering aid was necessary for densification of the bulk material at temperatures below the melting point. A correlation, which was independent of starting particle size, was established between inhibitor concentration and final grain size of the fired bodies. Barium titanate bodies of predetermined grain size not larger than 2000 A were obtained even with extended firing schedules. In the smallest grain sizes, the Curie point was shifted below the measured temperature range, and hysteresis losses were reduced markedly. The dielectric constant had a smooth variation from approximately 3000 at room temperature to 800 at 180°C.     

Synthesis of Ultrafine and Spherical Barium Titanate Powders Using a Titania Nano-Sol

A novel synthetic method for the preparation of spherical, homogeneous, and ultrafine barium titanate (BaTiO₃) powders is described. An aqueous titania nano-sol was prepared by peptizing coarse aggregate of hydrous titania with nitric acid. BaTiO₃ powders could be synthesized through a simple reflux method using the titania nano-sol and barium hydroxide. As decreasing the titanium concentration, the particle size of the resulting spherical BaTiO₃ powder was increased from 40 to 130 nm and the porosity also increased. It was revealed that the smaller as-prepared BaTiO₃ powder was less porous and became more tetragonal with less intragranular pores after annealing. With this method, a highly tetragonal BaTiO₃ powder ( c / a ∼1.008) with a particle size of 120.0 nm was successfully prepared and would be very suitable for the thinner dielectrics in higher capacitance multilayer ceramic capacitors.     

Ultrafine Barium Titanate Powders via Microemulsion Processing Routes

Three processing routes have been used to prepare barium titanate powders, namely conventional coprecipitation, single-microemulsion coprecipitation using diether oxalate as the precipitant, and double-microemulsion coprecipitation using oxalic acid as the precipitant. A single-phase perovskite barium titanate was obtained when the double-microemulsion-derived oxalate precursor was calcined for 2 h at a temperature of as low as 550°C, compared to 600°C required by the single-microemulsion-derived precursor. A calcination for 2 h at >700°C was required for the conventionally coprecipitated precursor in order to develop a predominant barium titanate phase. It was, however, impossible to eliminate the residual TiO₂ impurity phase by raising the calcination temperature, up to 1000°C. The microemulsion-derived barium titanate powders also demonstrated much better powder characteristics, such as more refined crystallite and particle sizes and a much lower degree of particle agglomeration, than those of the conventionally coprecipitated powder, although they contained ∼0.2 wt% BaCO₃ as the impurity phase.     

Hydrothermal Synthesis of Bismuth Titanate Powders

Bismuth titanate was synthesized under hydrothermal conditions from an amorphous bismuth–titanium precursor gel. The gel was formed by mixing a bismuth acetate complex with titanium butoxide and then adding the solution dropwise into 6 M NaOH. The resulting gel suspension was reacted under hydrothermal conditions at temperatures ranging from 160° to 200°C to form crystalline bismuth titanate. The gel crystallization kinetics increased with temperature, which resulted in 100% crystalline bismuth titanate in 5 h at 200°C. Wavelength-dispersive spectroscopy data indicated that sodium was incorporated into bismuth titanate during processing, and X-ray diffractometry suggested that the powder was composed of the Bi₅Ti₄O₁₅ phase. Transmission electron microscopy micrographs showed that the gel particles decomposed to 100–200 nm crystalline bismuth titanate particles during hydrothermal processing.     

Particle Size Distributions in Calcined Powders

The changes in particle size distributions in bayerite and magnesium hydroxide on calcination are reported. In general, size distributions in calcined powders very closely resemble those obtained before calcination. Theoretical changes in particle volume, and hence in volume equivalent diameter, can be calculated from weight-loss data and corresponding densities. Possible reasons for deviation from the theoretical size reduction ratio are also discussed.     

非氧化物陶瓷超细粉的制备

非氧化物陶瓷以优异的综合性能，在冶金、化工、机械、电子等领域有着广阔的应用前景．为了制备出适于烧结的非氧化物陶瓷超细粉体，人们进行了大量的研究工作，本综述了非氧化物陶瓷超细粉的研究进展，其中涉及机械粉碎法、金属元素反应法、金属氧化物碳热还原(氮化)法、聚合物热解法、气相化学反应法及溶剂热合成法等，并对金属氧化物碳热还原(氮化)法的进展进行了较为详细的描述，     

无团聚PbTiO3超细粉末的制备

本文设计了一种以乙二醇和丁醇混合溶液作为溶剂,以钛酸丁酯和醋酸铅作为原料制备钛酸铅超细粉末的溶胶—凝胶新工艺。利用红外光谱分析确定了复合醇盐(C_4H_9O)_3≡Ti-O-Pb-(CH_3CO_2)的合成温度在80℃以上。凝胶在100℃下干燥仅需要1小时,经干燥后的凝胶在1500℃下热处理2小时就可以得到结晶的、粒径小于0.1μm的无团聚PbTiO_3超细粉末。     

Hydrothermal Synthesis of Acicular Lead Titanate Fine Powders

A pure, acicular lead titanate (PbTiO₃) fine powder with a white color has been prepared by hydrothermal synthesis. It is a new phase of PbTiO₃ with I 4 symmetry, cell parameters of a = 12.358 Å and b = 14.541 Å, and a density of 6.80 g.cm⁻³. The influences of pH (12.5 to 14.4), Pb/Ti ratio (1.0 to 1.6) in the feedstock, reaction temperature (130° to 230°C), time (0.25 to 4 h), starting materials, and additives on the formation of acicular PbTiO₃ under hydrothermal conditions have been investigated. The acicular PbTiO₃ with I 4 symmetry, referred to as the PX phase, can be converted to the perovskite-type (PE phase) of PbTiO₃ at about 605°C while its acicular morphology is essentially unchanged. The preferable conditions for preparing pure acicular PX-phase PbTiO₃ are that the pH is 13.0 to 14.0, Pb/Ti ratio is >1.3, reaction temperature is 170° to 200°C, time is 0.5 to 1.0 h, titanium butoxide (Ti[O(CH₂)₃CH₃]₄) is the starting material, and poly(vinyl alcohol) is an additive. The acicular grain of the PX phase is usually less than 100 nm in diameter and more than 1000 nm in length.     

Solvothermal Synthesis and Characterization of Barium Titanate Powders

Cubic BaTiO₃ powders have been synthesized from gel powders after thermal treatment in an alcohol solution and characterized using X-ray diffractometry, transmission electron microscopy, and other techniques in detail. The borderline reaction conditions, such as the reaction temperature and time, for the synthesis of crystalline BaTiO₃ powder in different solvents are established. The single-phase BaTiO₃ powder has low agglomeration and seems to have a regular morphology. The formation of BaTiO₃ powders via solvothermal reaction is more difficult, in comparison to hydrothermal processing. The crystalline powders, which have a small particle size (∼20–60 nm) and narrow particle-size distribution, can be synthesized in methanol, ethanol, or n -propanol systems. Unlike the hydrothermal reaction, tetragonal-phase BaTiO₃ powders cannot be prepared via the solvothermal reaction, even with alkali catalysts.     

Particle Size Distribution of Coprecipitated Mn-Zn Ferrite Powders

The particle size distribution of 0.07- to 0.35-μm powders was measured by quantitative electron microscopy using an areal analysis. Measurements of at least 15 particles were required to characterize each size distribution. The specific surface area calculated from the size distributions satisfactorily agreed with that measured by the BET method. For the powders with surface area of 4.4 to 9.7 m²/g, the particle size distributions were generally narrow. In many cases the distributions were near log normal.     

Synthesis of Ultrafine Ceria Powders by Mechanochemical Processing

The synthesis of ultrafine cerium dioxide (CeO₂) powders via mechanochemical reaction and subsequent calcination was studied. Anhydrous CeCl₃ and NaOH powders, along with NaCl diluent, were mechanically milled. A solid-state displacement reaction—CeCl₃+ 3NaOH → Ce(OH)₃+ 3NaCl—was induced during milling in a steady-state manner. Calcination of the as-milled powder in air at 500°C resulted in the formation of CeO₂ nanoparticles in the NaCl matrix. A simple washing process to remove the NaCl yielded CeO₂ particles ∼10 nm in size. The particle size was controlled in the range of ∼10–500 nm by changing the calcination temperature.     

Measurement of Atlanta Aerosol Size Distributions: Observations of Ultrafine Particle Events

As part of EPRI's Aerosol Research Inhalation Epidemiology Study (ARIES), measurements of aerosol size distributions in the 3 nm to 2     

Particle Size Control of Barium Titanate Prepared from Barium Titanyl Oxalate

Barium titanates formed from decomposition of barium titanyl oxalates (BT-oxalates) exhibit the same powder state as that of the Bt-oxalates, so that controlling the particle size and shape of the starting BT-oxalates is necessary. In the present study, BT-oxalates were precipitated from aqueous solution under various aging conditions: aging fluids, temperature, and time. The particle size of the spherical BT-oxalates was controlled at 0.4 μm by aging at 25°C for 3 h, and the diameter of the barium titanate formed was similar to that of the BT-oxalates.     

Dependence of the Crystal Structure on Particle Size in Barium Titanate

The effect of the sample particle size on the crystal structure and the Curie temperature of BaTiO₃ powder has been investigated in the particle size range 0.1 to 1.0 μm. The transformation from tetragonal to cubic symmetry occurs at a critical particle size of 0.12 μm at room temperature, and the Curie temperature drops below room temperature at the critical particle size.     

钢渣超细粉的粒度分布与均匀程度的研究

将钢渣助磨剂与钢渣进行混合后,利用行星式球磨机进行粉磨得钢渣超微粉并且对其进行物性分析.利用激光粒度分析仪测定钢渣超微粉的粒径分布d90、d50和d10,并且计算出钢渣超微粉的粒度分布宽度比系数Z和粒径分布宽度H.研究溶质浓度、分散剂加入量、超声功率和超声分散时间对钢渣超微粉粒径分布与均匀程度的影响.结果表明:钢渣超微粉的化学成分与物相组成复杂,存在极性物质与胶凝活性物质,易形成具有吸附性的多孔结构.钢渣超微粉的最佳分散条件:分散介质为无水乙醇、溶质(钢渣超微粉)浓度为3.0 g/L、分散剂(PVP K30)加入量为2.5％、超声功率为500 W、超声分散时间为45 min.以钢渣超微粉最佳分散条件对钢渣超微粉的粒径分布与均匀程度进行重复性测试,其测试结果具有良好的重复性.     

Effect of Particle Size on the Room-Temperature Crystal Structure of Barium Titanate

The room-temperature tetragonal-to-cubic transformation in BaTiO₃ powders with decreasing particle size has been carefully studied, using materials prepared mainly by hydrothermal methods. Hydrothermal BaTiO₃ powders exhibited a more uniform particle size distribution than oxalate-route powders, with X-ray diffraction and electron microscopy indicating that powders 0.19 μm in size were fully cubic while powders 0.27 μ were completely tetragonal (within a 5% detection limit for cubic material) at room temperature. The tetragonal-to-cubic transformation temperature was also found to lie in the range of 121°± 3°C for BaTiO₃ powders with room-temperature ( c/a ) values > 1.008. No transformation could be detected using differential scanning calorimetry for BaTiO₃ particles with a ( c/a ) > 1.008 at room temperature. BaTiO₃ powder with a particle size just too small (0.19 μm) to be tetragonal at room temperature remained cubic down to 80 K. Different models for the cubic-to-tetragonal room-temperature transformation are discussed. Hydroxyl ions do not appear to greatly affect the cubic-to-tetragonal transformation, which appears to be essentially dependent on particle size. It is concluded that a model based on surface free energy, as previously discussed for the monoclinic-to-tetragonal transformation at room temperature of fine ZrO₂ particles, is consistent with the experimental data.     

Carbon Dioxide laser Synthesis of Ultrafine Silicon Carbide Powders from Diethoxydimethylsilane

Ultrafine SiC powders have been synthesized from diethoxydimethylsilane by laser-induced vapor-phase reactions. The powders produced under different flow rates of the reactant vapor have been characterized by chemical analysis, transmission electron microscopy, infrared spectros-copy, and X-ray diffractometry. The results show that the powders are less than 30 nm in average particle diameter, uniformly distributed, spherical in shape, and amorphous. The oxygen and free carbon of the powders vary with the vapor flow rate of the reactant. Annealing of the powders at 1873 K in nitrogen for 1 h results in a complete transformation from amorphous SiC to β-SiC and a small amount of α-SiC.     

超细炸药粉体粒度测试条件的优化

采用Brookhaven 90 Plus型激光粒度分析仪,对超细PETN样品的粒度及粒度分布进行了测试,研究了测试样品制备过程中的影响因素.通过实验得出测试样品制备的最佳工艺条件为:以蒸馏水为悬浮载体,六偏磷酸钠为最佳分散剂,最优间隔振荡时间80 s,最适合的质量浓度是220～300 μg/mL.将最佳条件下的测试结果与样品的SEM照片进行对比,二者吻合较好.此方法可以推广到其他超细炸药的测定.