Liquid phase assisted densification of superhard B6O materials

B₆O-based materials are known as some of the hardest materials after diamond and cubic boron nitride with a hardness of 45 GPa measured on single crystals. Several attempts were made to produce B₆O materials by hot pressing, but without success. Based on thermodynamic considerations the possibility of the use of sintering additives was discussed and the developed concepts were validated by densification of the materials using FAST (field assisted sintering technique)/SPS methods and analysing the microstructure and properties of the resulting materials. Two groups of materials were found to be suitable for the densification: transition metals which form borides with B₆O, the elements of the first to fourth main groups of the periodic table and the rare earths (Sc, Y, and lanthanides) which are in equilibrium in the oxide form with B₆O and form a liquid phase during densification at 1700–1900 °C. Superhard dense B₆O materials were produced and their properties investigated.     

Densification and properties of lime with V2O5 additions

Sintering of lime was carried out in the presence of V₂O₅ by a single firing process. A pure limestone was crushed, mixed with 1, 2 and 4 wt.% V₂O₅, pelletised and fired between 1550 and 1650 °C. The sintered lime was evaluated by bulk density, apparent porosity, microstructure, hydration resistance and hot modulus of rupture (HMOR) at 1300 °C. Incorporation of V₂O₅ forms liquid phase with lime at elevated temperature and influences the densification process by liquid phase sintering. As a result bulk density of sinters improved and they become more hydration resistant due to the larger grain size of the lime phase. The hot strength increased up to a certain temperature followed by deterioration because of the pressure of higher amount of liquid phase.     

Low temperature sintering and microwave dielectric properties of Ba3Ti5Nb6O28 with ZnO–B2O3 glass additions for LTCC applications

The effects of ZnB₂O₄ glass additions on the sintering temperature and microwave dielectric properties of Ba₃Ti₅Nb₆O₂₈ have been investigated using dilatometer, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and a network analyzer. The pure Ba₃Ti₅Nb₆O₂₈ system showed a high sintering temperature (1250 °C) and had the good microwave dielectric properties: Q × f of 10,600 GHz, ɛ_r of 37.0, τ_f of −12 ppm/°C. It was found that the addition of ZnB₂O₄ glass to Ba₃Ti₅Nb₆O₂₈ lowered the sintering temperature from 1250 to 925 °C. The reduced sintering temperature was attributed to the formation of ZnB₂O₄ liquid phase and B₂O₃-rich liquid phases. Also the addition of ZnB₂O₄ glass enhanced the microwave dielectric properties: Q × f of 19,100 GHz, ɛ_r of 36.6, τ_f of 5 ppm/°C. From XPS and XRD studies, these phenomena were explained in terms of the reduction of oxygen vacancies and the formation of secondary phases having the good microwave dielectric properties.     

Fe2O3粉加入量对镁钙质中间包干式料性能的影响

以w(CaO)≈20%的镁钙砂(其粒度为5～3、3～1、1～0.2、≤0.2和≤0.074 mm)为主原料制成镁钙质中间包干式料,并研究了加入不同量Fe2O3粉(其质量分数分别为0、1%、2%、3%、4%)对干式料性能的影响。结果表明:1)加入Fe2O3粉可以增加镁钙质干式料的低温和中温强度,但加入质量分数超过3%后,干式料的低温强度明显下降,高温烧后显气孔率上升,体积密度下降,体积膨胀加剧;综合分析认为,镁钙质干式料中Fe2O3粉的加入质量分数在2%～3%较适当。2)加入Fe2O3粉的镁钙质干式料抗熔渣侵蚀性能较好,这是由于加入的Fe2O3与镁钙砂中的CaO反应生成了低熔点矿物铁酸二钙,促进了材料的烧结,从而提高了其抗熔渣侵蚀性。     

Bond characteristics and microwave dielectric properties of high-Q materials in li-doped Zn3B2O6 systems

The bond characteristics, Raman spectroscopy, and microwave dielectric properties of Zn_3-xLi_2x(BO₃)₂ ceramics prepared by solid-state reaction method were investigated. According to the complex chemical bond theory, the bond ionicity and lattice energy of the B–O bond were proved to contributed more to the electric polarization and phase structure stability than that of A-site bond. Thus, the B–O bond had a dominant effect on the dielectric constant and Q × f values. The optimization of the τ_f value can be attributed to the bond valence. Moreover, the shift and full width at half maximum of the Raman peak were closely related to the dielectric constant and Q × f values, respectively. On the whole, Li⁺ substitution contributed greatly to improve the temperature stability and reducing the dielectric loss of Zn_3-xLi_2x(BO₃)₂ ceramics. Additionally, Zn_2.99Li_0.02(BO₃)₂ ceramics sintered at 850 °C exhibited satisfactory microwave dielectric properties of ε_r=6.59, Q × f=122,030 GHz, τ_f=−76.9 ppm/°C, and had good chemical compatibility with silver.     

Physicomechanical characteristics of superhard polycrystalling materials

Experimental investigations of the hardness, facture toughness, and elastic characteristics (longitudinal velocity of sound waves, Young's modulus, and Poisson coefficient) of almost poreless polycrystalline ceramic materials based on corundum, higher borides of aluminum and calcium, boron and silicon carbides, boron (cubic) and aluminum nitrides, tungsten-free TiB₂-based solid alloy, cordierite ceramics, ceramics from a mixture of copper titanate and copper aluminate, and metalloceramic materials of the B₄C-SiC-Al-Si system are described. It is established that a linear correlation between the hardness and the velocity of sound waves exists for ceramic and metalloceramic materials. It is shown that ceramic materials can be classified in accordance with their rigidity and capacity for absorbing the energy of ballistic impact, the value of the Poisson coefficient, and a combined criterion comprising hardness, fracture toughness, Young's modulus, and the longitudinal velocity of sound waves. The physicomechanical properties of cermets in the B₄C-SiC-Al-Si system can be controlled by changing the amount of the metallic phase and the annealing time. Translated from Ogneupory, No. 9, pp. 2 – 5, September, 1995.     

Microwave dielectric properties and low-temperature sintering of Ba3Ti4Nb4O21 ceramics with B2O3 and CuO additions

The low sintering temperature and the good dielectric properties such as high dielectric constant (ɛ_r), high quality factor (Q × f) and small temperature coefficient of resonant frequency (τ_f) are required for the application of chip passive components in the wireless communication technologies. In the present study, the sintering behaviors and dielectric properties of Ba₃Ti₄Nb₄O₂₁ ceramics were investigated as a function of B₂O₃–CuO content. Ba₃Ti₄Nb₄O₂₁ ceramics with B₂O₃ or CuO addition could be sintered above 1100 °C. However, the additions of both B₂O₃ and CuO successfully reduced the sintering temperature of Ba₃Ti₄Nb₄O₂₁ ceramics from 1350 to 900 °C without detriment to the microwave dielectric properties. From the X-ray diffraction (XRD) studies, the sintering behaviors and the microwave dielectric properties of low-fired Ba₃Ti₄Nb₄O₂₁ ceramics were examined and discussed in the formation of the secondary phases. The Ba₃Ti₄Nb₄O₂₁ sample with 1 wt% B₂O₃ and 3 wt% CuO addition, sintered at 900 °C for 2 h, had the good dielectric properties: ɛ_r = 65, Q × f = 16,000 GHz and τ_f = 101 ppm/°C.     

中国超硬材料新态势

文章分析了具有中国特色的六面顶压机大型化、群控、远程控、高精度控等硬件问题。同时指出要与组装设计、合成工艺等软件部分紧密结合起来,才能获得优质的制品。文章在研究中国超硬材料的水平和特色的基础上,还指出了我国超硬材料存在的主要差距及目标。     

Preparation and properties of B6O/TiB2-composites

B₆O/TiB₂ composites with varying compositions were produced by FAST/SPS at temperatures between 1850 and 1900 °C following a non-reactive or a reactive sintering route. The densification, phase and microstructure formation and the mechanical and thermal properties were investigated. The comparison to an also investigated pure B₆O material showed that the addition of TiB₂ in a non-reactive sintering route promotes the B₆O densification. Further improvement was obtained by sintering reactive B–TiO₂ mixtures which also results in materials with a finer grain size and thus in enhanced mechanical properties. The fracture toughness was significantly improved in all composites and is up to 4.0 MPa m^1/2 (SEVNB) and 2.6–5.0 MPa m^1/2 (IF method) while simultaneously a high hardness of up to 36 GPa (HV_0.4) and 28 GPa (HV₅) could be preserved. The high temperature properties at 1000 °C of hardness, thermal conductivity and CTE were up to 20 GPa, 18 W/mK and 6.63 × 10^?6/K, respectively.     

加入Al粉和Si粉对低碳MgO-Al_2O_3-C材料性能的影响

以电熔镁砂、电熔镁铝尖晶石及鳞片石墨为原料,以酚醛树脂作结合剂,在MgO-A l2O3-C材料中加入A l粉和Si粉,混练后在180 MPa压力下成型,185℃12 h热处理,制成了加入4.5%质量分数A l粉、0~3.5%质量分数Si粉的AS系列试样和同时加入2%质量分数石墨的ASC系列试样,研究了试样的常温物理性能、高温抗折强度、抗热震性及其相组成和显微结构。结果表明:(1)AS系列中,仅加入4.5%A l粉试样的高温抗折强度从未加A l粉的3.0 MPa提高到22.3 MPa,热震后抗折强度保持率从60%提高到77%;再加入1.5%Si粉后,高温抗折强度提高到27.9 MPa,热震后抗折强度保持率提高到79%;(2)加入A l粉、Si粉和石墨的ASC系列试样的高温抗折强度略高于AS系列试样的,热震后抗折强度保持率在71%~75%之间;(3)这类复合材料具有良好的热态强度和抗热震性的原因为:A l粉、Si粉在埋炭加热过程中与C、N2原位生成针状A lN和SiC等非氧化物,填充、穿插在方镁石和尖晶石骨架中,起到增强、增韧的作用。     

《超硬材料工程》的定位和目标

超硬材料行业有过辉煌的过去,也会具有光明的未来,这是《超硬材料工程》兴旺发达的基础.文章叙述了超硬材料发展的现状,展望了超硬材料行业发展的方向,以此为基础定位行业期刊《超硬材料工程》的发展方向和目标,并组建编辑委员会.认为刊物应该成为业内各界人士交流和获取信息的窗口,为超硬材料行业的美好明天而努力.     

B6O materials with Al2O3/Y2O3 additives densified by FAST/SPS and HIP

Fully densified B₆O materials with Al₂O₃/Y₂O₃ sintering additives amounts systematically varied between 0 and 15 vol.% and Al₂O₃/(Al₂O₃ + Y₂O₃) molar ratios of 0.05–1 were prepared by FAST/SPS and HIP at sintering temperatures between 1725 °C and 1900 °C. Their densification and microstructure were correlated with measured mechanical properties. The addition of low additive amounts in the range of 2–3 vol.% was found to increase the fracture toughness and strength from 2.0 MPa m^1/2 (SEVNB) and 420 MPa for pure B₆O to about 3.0 MPa m^1/2 and 540 MPa, but it had no effect on the hardness, which remained at a high level of 30–36 GPa (HV_0.4). Higher additive contents did not yield a further improvement in the toughness but resulted in a reduction in hardness and strength.     

红柱石微粉加入量对红柱石材料结构与性能的影响

以红柱石和电熔白刚玉为骨料,以α-Al2O3微粉、SiO2微粉、红柱石微粉为基质料,外加4%PVA混练,压制成型后于110℃烘干,于1400℃保温6h烧成,研究了红柱石微粉添加质量分数分别为0、2.7%、5.4%、10.8%对红柱石材料结构与性能的影响。研究结果表明:在本试验条件下,随着红柱石微粉含量的增加,试样的常温强度、弹性模量下降,高温抗折强度升高;最适宜的红柱石微粉加入质量分数为5.4%。     

超硬材料及制品现状与展望

作者欲通过了解的基本情况,分析行业总体形势,掌握行业发展动态,为企业经营者的决策提供参考依据.文章介绍了超硬材料单晶、复合片、细料、微米级(含纳米级)料等设备及产品的行业最新情况,并提供了产量和质量的数据.还简述了金刚石工具已形成的两个方向,并列举了部分石材精加工用的金刚石工具及其发展.     

Microwave dielectric properties of LiNb3O8 ceramics with TiO2 additions

The microwave dielectric properties of LiNb₃O₈ ceramics were investigated as a function of the sintering temperature and the amount of TiO₂ additive. LiNb₃O₈ ceramics, which were calcined at 750 °C and sintered at 1075 °C for 2 h, showed a dielectric constant (ɛ_r) of 34, a quality factor (Q × f₀) of 58,000 GHz and a temperature coefficient of resonance frequency (τ_f) of −96 ppm/°C, respectively. The density of the samples influenced the properties of these properties. As the TiO₂ content increased in the LiNb₃O₈–TiO₂ system, ɛ_r and τ_f of the material were increased due to the mixing effect of TiO₂ phase, which has higher dielectric constant and larger positive τ_f. The 0.65LiNb₃O₈–0.35TiO₂ ceramics showed a dielectric constant ɛ_r of 46.2, a quality factor (Q × f₀) of 5800 GHz and a temperature coefficient of resonance frequency τ_f of near to 0 ppm/°C.     

Densification and properties of magnesia-rich magnesium-aluminate spinel derived from natural and synthetic raw materials

Magnesia rich magnesium aluminate spinel (MgO: Al₂O₃=2:1) was developed by reaction sintering of Indian natural magnesite of Salem region as well as from synthetic caustic magnesia with calcined alumina. Dilatometric study of the green compacts was carried out to evaluate the spinelisation and sintering behaviour of both the samples. Green samples were heat treated between 1400 to 1600 °C and characterised in terms of densification behaviour, high temperature flexural strength, microstructure and phase development. Spinel and periclase are the major phases in both the samples, where as forsterite is found only in the sample developed from Indian magnesite due to presence of silica as impurities.     

Effect of the type and quality of antioxidant additions on properties of materials of the system Al2O3-SiC-C

Results are presented for a study of the apparent density and ultimate strength in compression of developed materials of the system Al₂O₃-SiC-C, modified by introduction of phosphate addition and complex antioxidant (Al + Si + phosphate addition). It is established that introduction of a phosphate addition together with a complex antioxidant (Al + Si) to the composition of corundum-graphite SiC-containing refractory based on an ethyl silicate binder leads to compaction of the structure and formation of materials of prescribed phase composition with increased oxidation resistance. Translated from Novye Ogneupory, No. 5, pp. 28–31, May 2008.     

Additive-free superhard B4C with ultrafine-grained dense microstructures

A unique combination of high-energy ball-milling, annealing, and spark-plasma sintering has been used to process superhard B₄C ceramics with ultrafine-grained, dense microstructures from commercially available powders, without sintering additives. It was found that the ultrafine powder prepared by high-energy ball-milling is hardly at all sinterable, but that B₂O₃ removal by gentle annealing in Ar provides the desired sinterability. A parametric study was also conducted to elucidate the role of the temperature (1600–1800 °C), time (1–9 min), and heating ramp (100 or 200 °C/min) in the densification and grain growth, and thus to identify optimal spark-plasma sintering conditions (i.e., 1700 °C for 3 min with 100 °C/min) to densify completely (>98.5%) the B₄C ceramics with retention of ultrafine grains (∼370 nm). Super-high hardness of ∼38 GPa without relevant loss of toughness (∼3 MPa m^1/2) was thus achieved, attributable to the smaller grain size and to the transgranular fracture mode of the B₄C ceramics.     

The synthesis and microwave dielectric properties of Mg3B2O6 and Mg2B2O5 ceramics

Single-phase Mg₃B₂O₆ and Mg₂B₂O₅ ceramics were synthesized and then structurally and dielectrically characterized. The highest Qxf value of 230,900 GHz was obtained for a Mg₃B₂O₆ ceramic with a density of 97% and 1000-μm grains. Considerably lower Qxf values (10,000–32,000 GHz) were determined for the Mg₂B₂O₅ ceramic. Mg₃B₂O₆ and Mg₂B₂O₅ exhibited permittivities (?) of 7.2 and 6.2–7.0, respectively. Both ceramics showed negative temperature coefficients of resonant frequency (τ_f) of ?18 to ?45 ppm/°C.