Synthesis of Zirconium Titanate-Based Materials by Colloidal Filtration and Reaction Sintering

Zirconium titanate exhibits crystallographic anisotropy in thermal expansion, which makes it a suitable candidate for low thermal expansion materials. In this work, zirconium titanate has been synthesized by reaction sintering the green bodies, which have been obtained by colloidal filtration of concentrated suspensions of yttria-tetragonal zirconia polycrystals (Y–TZP) and titania. Powders were mixed in a 50/50 mol% ratio (ZT50) to obtain pure zirconium titanate. Rheological characterization of the suspensions has allowed the establishment of optimum green processing conditions. Sintering has been performed at 1400°C for 2 h, and the obtained materials have been characterized by X-ray diffraction, and scanning electron microscopy with energy-dispersive X-ray microanalysis. The ZT50 material has Zr₅Ti₇O₂₄ as the major phase, although Y₂ ((Zr_0.3Ti_0.7)₂O₇) and unreacted Y–TZP can still be detected.     

Direct Synthesis and Sintering of Silicon Nitridenitanium Nitride Composite

Electrical discharge machining is an attractive machining technique which requires electrically conductive ceramic materials. Silicon nitride based composites with 35–40% of TiN possess electrical conductivity high enough to use this machining technique. The composite powder was prepared by combustion synthesis and densified by pressure sintering (sintering aids 6 wt% of Y₂O₃ and 4 wt% of A1,0₃). High fracture toughness, K _c 10.5 MPa^.m^1/2, is typical for the composite. It can be expected that this material can compete with pure silicon nitride in applications requiring complex machining.     

原位反应法合成聚丙烯/蒙脱土复合材料及其表征

以未改性蒙脱土为原料,采用原位反应增容技术制备了聚丙烯(PP)／蒙脱土(MMT)复合材料,并对复合材料的结构进行了表征。结果表明：MMT的阻燃效果比Al(OH)3／Mg(OH)2复合阻燃体系好,且不降低PP的力学性能;在原位反应时,反应性单体的双键部分接枝到PP主链上,同时极性基团部分与MMT发生相互作用,形成插层结构,层间距离由1．17nm增加到1．32nm,MMT与PP间显示出良好的界面粘合形态,但MMT主要以200～300nm的初级粒子分散于PP中,只有少量是以纳米尺寸的插层结构存在。     

烧结气氛对合成镁铁铝系复合尖晶石的影响

以电熔镁砂、活性氧化铝、氧化铁和石墨作为原料,按质量比24:61:10:5进行配料,制得φ20 mm×10 mm的试样,在空气气氛、还原气氛(埋碳)、氮气气氛三种不同烧结气氛条件下,1550℃ ×3 h高温处理.采用XRD、SEM、EDS等手段对烧后试样进行了表征.分析结果表明:在三种不同烧结条件下,都合成了铁含量不同的镁铁铝复合尖晶石;在还原气氛烧结条件下,试样中镁铁铝复合尖晶石相中铁是+2价的,在空气气氛和N2气氛烧结条件下,试样中镁铁铝复合尖晶石相中铁是+3价的;试样的晶面间距随铁固溶量的增加而增大;还原气氛烧结条件相比其他两种烧结气氛,晶粒尺寸大且合成率高.     

成型方式对反应烧结碳化硼基复合材料性能的影响

本文针对干压成型结合冷等静压成型对反应烧结碳化硼基复合材料样品性能的影响进行研究.考察了成型压力、保压时间、加压方式对压坯密度和强度的影响.结果表明:采用100 MPa干压2 min,再经过150 MPa冷等静压保压1 min,压坯密度由干压压制时最大密度1.86 g/cm3升高到1.96 g/cm3,压坯强度可达到2.0 MPa.保压时间和加压方式对压坯的密度、强度的影响不大.干压结合冷等静压相对于干压成型,在相同的烧结温度下样品性能有明显的提高,烧结后样品密度为2.44 g/cm3,显微硬度为2394 kg/cm2,三点抗弯强度为241 MPa.     

NaB5C–B5/C Composite Ceramics Prepared by Reaction Sintering in Na Vapor

Reaction sintering of sodium carbaboride (NaB₅C) was performed by heating compacts of mixtures of amorphous boron (B) and carbon black (C) powders (B/C molar ratio, 5/1) at 1173 K in Na vapor. The ceramics obtained from the compacts of B₅/C powder mixed with a ball mill (compact density, 1.67 Mg/m³) were the composites of NaB₅C (74 mass%) and unreacted B₅/C (26 mass%). The bulk density and bending strength of the composite ceramics were 2.04 ± 0.03 Mg/m³ and 320.9 ± 10.4 MPa, respectively. Transmission electron microscope observation revealed that nanometer‐size amorphous grains of B or C were included in the matrix of NaB₅C.     

水热体系合成NaA/X复合沸石

在无模板剂条件下,采用水热反应体系合成NaA/X复合沸石,研究了原料配比及凝胶化条件对NaA/X复合沸石合成的影响.结果表明,该反应体系钠的优化配比为n(SiO2)∶n(Al2O3)=2.8,反应的适宜碱度条件为:n(H2O)∶n(Na2O) =25～30、n(Na2O)∶n(SiO2)=1.6～3.0,碱度过高或过低均会导致X型沸石的消失.产物为复合沸石,粒径为150nm,分散度较好.产物的钙离子交换能力达到300 mg/g,镁离子交换能力优于纯4A、X及高铝P型沸石.     

Reaction Sintering of Submicrometer Silicon Powder

Submicrometer silicon powder synthesized by thermally induced, vapor-phase reaction in an aerosol reactor was densified by reaction sintering to form Si₃N₄. The densification behavior of these spherical, nonagglomerated particles in the 0.1 to 0.2 μm size range is described. Competition between densification by silicon sintering and reaction product infilling was observed at typical reaction sintering temperatures. In addition, pore growth during the early stages of silicon presintering adversely affected nitrogen permeability and nitriding rates. The factors influencing the nitridation of ultrafine silicon powders are discussed.     

羟基磷灰石-玻璃复合陶瓷的微波烧结

采用沉淀法合成羟基磷灰石粉体,将R2OAl2O3B2O3SiO3体系玻璃粉按一定的比例与HAP粉混合,采用等静压成型和干压法成型2种成型方法对羟基磷灰石玻璃复合粉体成型,分别在1150℃、1200℃、1250℃下微波烧结。利用XRD、IR和SEM等手段对烧结过程中的相变和陶瓷显微结构进行研究,结果表明随着烧结温度的升高,羟基磷灰石玻璃复合陶瓷的结构逐渐致密;烧结温度低于1200℃时主晶相没有发生明显变化,当烧结温度达到1250℃时等静压成型的样品中HAP发生了明显的分解;等静压成型的羟基磷灰石—玻璃复合陶瓷的致密度优于普通干压法成型的陶瓷。     

反应烧结法合成铁铝尖晶石中二氧化硅的赋存状态

以Fe2O3、Al2O3和Si O2为原料,按照Fe O和Al2O3质量比为40.8:59.2配料,并在合成体系中引入质量分数为1%的二氧化硅,在高纯氮气下于1 600℃保温4 h。用X射线衍射、扫描电子显微镜、能谱和透射电子显微镜等对试样进行了表征,利用Fe O-Al2O3-Si O2三元相图对二氧化硅的存在状态进行了分析。结果表明:引入的二氧化硅没有进入铁铝尖晶石晶格,而是以Fe O-Al2O3-Si O2三元组分的非晶态形式存在于铁铝尖晶石晶粒间。在二氧化硅质量分数为1%时,含Si O2组分的非晶相呈不连续状态分布,未改变铁铝尖晶石晶粒间的直接接触。     

SiC-YAG陶瓷复合材料的烧结致密性研究

本文基于机械混合法和液相烧结法制备SiC-YAG陶瓷复合材料,并对SiC-YAG陶瓷复合材料在烧结过程中的质量损耗、致密化等情况进行了研究,比较了铝钇摩尔比、烧结温度和保温时间对SiC-YAG陶瓷复合材料烧结致密性的影响.研究结果表明:在实验研究条件范围内,当烧结温度为1850℃时,复合材料的气孔率最低,相对密度最大;复合陶瓷材料的相对密度和收缩率在保温时间小于30 min时,变化较大,大于30 min时,变化很小;当铝钇摩尔比为1.5时,质量流失较少,复合材料的相对密度最高.     

Inclusion Size and Sintering of Composite Powders

Ceramic powders containing dispersed inert particulates often exhibit dramatically lower densification rates than the pure powder. This decrease results from the shrinkage incompatibilities of the matrix and the inclusion during densification. The analyses of this phenomenon all predict that the shrinkage rate is only dependent on the overall volume fraction of the inert second phase, and not on the particle size. Experiments on ZnO containing dispersed SiC particles of different size showed that the retarding effect of the second phase increases significantly with decreasing particle size, at constant SiC volume fraction.     

Microwave-Assisted Reaction Sintering of Bismuth Titanate-Based Ceramics

Bismuth titanate-based ceramics, pure and doped with 5 mol% WO₃, were prepared by reaction-sintering using a microwave oven. High densities, ≥96% of theoretical density were achieved with very short thermal treatments, with soaking time ranging from 5 to 15 min. Relatively small platelet-like grains were formed, and microstructure seemed not to be strongly affected by doping nor soaking time. Electrical measurements were performed and, as expected, a decrease of electrical conductivity was observed in WO₃-doped materials. The results of reaction-sintered samples, both conventional as well as microwave sintered, are compared with those of samples prepared with previously calcined powders and sintered using microwaves as well as a conventional electric furnace.     

Synthesis of Calcium Hydroxyapatite-Tricalcium Phosphate (HA-TCP) Composite Bioceramic Powders and Their Sintering Behavior

Composite (biphasic) mixtures of two of the most important inorganic phases of synthetic bone applications-namely, calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂ (HA)) and tricalcium phosphate (Ca₃(PO₄)₂ (TCP))-were prepared as submicrometer-sized, chemically homogeneous, and high-purity ceramic powders by using a novel, one-step chemical precipitation technique. Starting materials of calcium nitrate tetrahydrate and diammonium hydrogen phosphate salts that were dissolved in appropriate amounts in distilled water were used during powder precipitation runs. The composite bioceramic powders were prepared with compositions of 20%-90% HA (the balance being the TCP phase) with increments of 10%. The pellets prepared from the composite powders were sintered to almost full density in a dry air atmosphere at a temperature of ~1200°C. Phase-evolution characteristics of the composite powders were studied via X-ray diffractometry as a function of temperature in the range of 1000°-1300°C. The sintering behavior of the composite bioceramics were observed by using scanning electron microscopy. Chemical analysis of the composite samples was performed by using the inductively coupled plasma-atomic emission spectroscopy technique.     

Zirconium Carbide–Tungsten Cermets Prepared by In Situ Reaction Sintering

Zirconium carbide–tungsten (ZrC–W) cermets were prepared by a novel in situ reaction sintering process. Compacted stoichiometric zirconium oxide (ZrO₂) and tungsten carbide (WC) powders were heated to 2100°C, which produced cermets with 35 vol% ZrC and 65 vol% W consisting of an interpenetrating-type microstructure with a relative density of ∼95%. The cermets had an elastic modulus of 274 GPa, a fracture toughness of 8.3 MPa·m^1/2, and a flexural strength of 402 MPa. The ZrC content could be increased by adding excess ZrC or ZrO₂ and carbon to the precursors, which increased the density to >98%. The solid-state reaction between WC and ZrO₂ and W–ZrC solid solution were also studied thermodynamically and experimentally.     

High‐Hardness Diamond Composite Consolidated by Spark Plasma Sintering

A high‐hardness diamond‐based composite was synthesized by spark plasma sintering (SPS) under 100 MPa, using SiC‐coated diamond powder prepared via chemical vapor deposition (CVD). SiC layers 20–40 nm were uniformly deposited on diamond powders by a rotary CVD technique. The SiC‐coated diamond powder was consolidated with SiO powder by SPS at the sintering temperature of 1873 K, resulting in the formation of fully compacted mosaic microstructure with the Vickers hardness of 36 GPa.     

Reaction Sintering and Properties of Silicon Oxynitride Densified by Hot Isostatic Pressing

Silicon oxynitride ceramics were reaction sintered and fully densified by hot isostatic pressing in the temperature range 1700°C to 1950°C from an equimolar mixture of silicon nitride and silica powders without additives. Conversion to Si₂N₂O increases steeply from a level around 5% of the crystalline phases at 1700°C to 80% at 1800°C, and increases a few percent further at higher temperatures. α -Si₃N₄ is the major residual crystalline phase below 1900°C. The hardness level for materials containing 85% Si₂N₂O is approximately 19 GPa, comparable with the hardness of Si₃N₄ hot isostatically pressed with 2.5 wt% Y₂O₃, while the fracture toughness level is around 3.1 MPa. m^1/2, being approximately 0.8 MPa.m^1/2 lower. The three-point bending strength increased with HIP temperature from approximately 300 to 500 MPa.     

Synthesis of Ceramic and Composite Materials Using a Combination of Self-Propagating High-Temperature Synthesis and Spark Plasma Sintering (Review)

Combustion, Explosion, and Shock Waves - This review deals with the potential of combining self-propagating high-temperature synthesis (SHS) and spark plasma sintering (SPS) for obtaining...     

High-Pressure Self-Combustion Sintering of Alumina-Titanium Carbide Ceramic Composite

Al₂O₃-TiC ceramic composites were fabricated by highpressure self-combustion sintering (HPCS). The samples had a unique microstructure which was quite different from that of commercial Al₂O₃-TiC materials made by conventional processing. A possible mechanism for combustion reaction is suggested on the basis of the microstructure of the samples and the thermodynamical data. The HPCS sample had a higher K _IC value than commercial Al₂O₃-TiC materials.