Dielectric properties of spark plasma sintered AlN/SiC composite ceramics

In this study, we have investigated how the dielectric loss tangent and permittivity of AlN ceramics are affected by factors such as powder mixing methods, milling time, sintering temperature, and the addition of a second conductive phase. All ceramic samples were prepared by spark plasma sintering (SPS) under a pressure of 30 MPa. AlN composite ceramics sintered with 30wt%–40wt% SiC at 1600℃ for 5 min exhibited the best dielectric loss tangent, which is greater than 0.3. In addition to AlN and β-SiC, the samples also contained 2H-SiC and Fe₅Si₃, as detected by X-ray difraction (XRD). The relative densities of the sintered ceramics were higher than 93%. Experimental results indicate that nano-SiC has a strong capability of absorbing electromagnetic waves. The dielectric constant and dielectric loss of AlN-SiC ceramics with the same content of SiC decreased as the frequency of electromagnetic waves increased from 1 kHz to 1 MHz.     

Optimum sintering conditions for optical properties of translucent aluminum nitride ceramics

High translucency is one of the excellent properties of AlN ceramics because of its wide optical band gap energy of 6.2 eV. We have achieved success in producing AlN ceramic tube of 98% total visible light transmittance at 0.6 mm thick tube wall by applying an improved sintering technique. This AlN ceramic was produced by sintering at 1880℃ using Ca3Al2O6 as a sintering additive and in reduction atmosphere to remove the sintering additive from the final sintered material. After the sintering, the annealing ...     

Effect of CuO addition on the sintering temperature and microwave dielectric properties of CaSiO3– Al2O3 ceramics

CuO-doped CaSiO3–1 wt% Al2O3 ceramics were synthesized via a traditional solid-state reaction method, and their sintering behavior,microstructure and microwave dielectric properties were investigated. The results showed that appropriate CuO addition could accelerate the sintering process and assist the densification of CaSiO3–1 wt% Al2O3 ceramics, which could effectively lower the densification temperature from1250 1C to 1050 1C. However, the addition of CuO undermined the microwave dielectric properties. The optimal amount of CuO addition was found to be 0.8 wt%, and the derived CaSiO3–Al2O3ceramic sintered at 1100 1C presented good microwave dielectric properties of εr?7.27,Q f?16,850 GHz and τf? 39.53 ppm/1C, which is much better than those of pure CaSiO3 ceramic sintered at 1340oC(Q f?13,109 GHz).The chemical compatibility of the above ceramic with 30 Pd/70 Ag during the cofiring process has also been investigated, and the result showed that there was no chemical reaction between palladium–silver alloys and ceramics.& 2014 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.     

Porous SiC ceramics fabricated by quick freeze casting and solid state sintering

Porous SiC ceramics with uniform microstructure were fabricated by quick freezing in liquid nitrogen and solid state sintering.Poly(vinyl alcohol)(PVA) was added as binder and pore morphology controller in this work.The microstructure and mechanical properties of porous SiC ceramics could be controlled by the composition of the aqueous slurries.Both solid content of the slurries and PVA content impacted on the pore structures and mechanical properties of the porous SiC ceramics.The solid content of slurries and PVA content varied from 60 to 67.5 wt%and 2-6 wt%,respectively.Besides,the grain morphology of ceramics was also tailored by changing the sintering temperature from 2050 to 2150 ℃.Porous SiC ceramics with an average porosity of 42.72%,flexural strength of 59.28 MPa were obtained at 2150 ℃ from 67.5 wt% slurries with 2 wt% PVA.     

Investigation of compaction and sintering behavior of SiC powder after ultra-fine treatment

Silicon carbide ceramics were prepared with SiC powder treated by the fluidized bed opposed jet mill as raw materials, and the effects of the ultra-fine treatment mechanism on the compaction and sintering behavior of SiC ceramics were investigated. The results showed that the compacts had higher density and microstructure homogeneity when the sintering temperature of the compact was decreased; and that the surface microstructure, densification and mechanical properties of the sintered body could be ameliorated obviously.     

Effect of heating rate on the densification of NdFeB alloys sintered by an electric field

This study introduces a novel method of electric field sintering for preparing NdFeB magnets. NdFeB alloy compacts were all sintered by electric fields for 8 min at 1000℃ with different preset heating rates. The characteristics of electric field sintering and the effects of heating rate on the sintering densification of NdFeB alloys were also studied. It is found that electric field sintering is a new non-pressure rapid sintering method for preparing NdFeB magnets with fine grains at a relatively lower sintering temperature and in a shorter sintering time. Using this method, the sintering temperature and process of the compacts can be controlled accurately. When the preset heating rate increasing from 5 to 2000℃/s the densification of NdFeB sintered compacts gradually improves. As the preset heating rate is 2000℃/s, Nd-rich phases are small, dispersed and uniformly distributed in the sintered compact, and the magnet has a better microstructure than that made by conventional vacuum sintering. Also, the maximum energy product of the sintered magnet reaches 95% of conventionally vacuum sintered magnets.     

CuO掺杂对10NiO-NiFe2O4陶瓷致密化烧结及其力学性能的影响

采用冷压成型和常压烧结工艺制备CuO掺杂10NiO-NiFe2O4陶瓷,研究CuO掺杂量对10NiO-NiFe2O4陶瓷的致密度、抗折强度及热震稳定性的影响。结果表明,在1 300℃烧结温度下,掺杂CuO能够有效提高10NiO-NiFe2O4陶瓷的致密度,其中添加6%CuO试样的相对密度（95.55%）最大;掺杂CuO可大幅提高10NiO-NiFe2O4陶瓷的抗折强度,其中添加8%CuO试样的抗折强度（139MPa）最高,且在800℃下经过1次和3次热循环后,该试样的抗折强度损失率仅分别为5.7%和8.6%,表现出良好的抗热震性能。     

Densification behavior of mechanically milled Cu–8 at% Cr alloy and its mechanical and electrical properties

Synthesis and consolidation behavior of Cu–8 at%Cr alloy powders made by mechanical alloying with elemental Cu and Cr powders,and subsequently,compressive and electrical properties of the consolidated alloys were studied.Solid solubility of Cr in Cu during milling,and subsequent phase transformations during sintering and heat treatment of sintered components were analyzed using X-ray diffraction,scanning electron microscopy and transmission electron microscopy.The milled powders were compacted applying three different pressures(200 MPa,400 MPa and 600 MPa)and sintered in H2atmosphere at 900 1C for 30 min and at 1000 1C for 1 h and 2 h.The maximum densification(92.8%)was achieved for the sample compacted at 600 MPa and sintered for 1000 1C for 2 h.Hardness and densification behavior further increased for the compacts sintered at 900 1C for 30 min after rolling and annealing process.TEM investigation of the sintered compacts revealed the bimodal distribution of Cu grains with nano-sized Cr and Cr2O3precipitation along the grain boundary as well as in grain interior.Pinning of grain boundaries by the precipitates stabilized the fine grain structure in bimodal distribution.     

Mechanical properties and friction behaviors of CNT/AlSi<Subscript>10</Subscript>Mg composites produced by spark plasma sintering

The mechanical properties and friction behaviors of CNT/AlSi₁₀Mg composites produced by spark plasma sintering (SPS) were investigated. The results showed that the densities of the sintered composites gradually increased with increasing sintering temperature and that the highest microhardness and compressive strength were achieved in the specimen sintered at 450℃. CNTs dispersed uniformly in the AlSi₁₀Mg matrix when the addition of CNTs was less than 1.5wt%. However, when the addition of CNTs exceeded 1.5wt%, the aggregation of CNTs was clearly observed. Moreover, the mechanical properties (including the densities, compressive strength, and microhardness) of the composites changed with CNT content and reached a maximum value when the CNT content was 1.5wt%. Meanwhile, the minimum average friction coefficient and wear rate of the CNT/AlSi₁₀Mg composites were obtained with 1.0wt% CNTs.     

稀土氧化物对液相烧结碳化硅的影响

采用AIN-稀土氧化物系统烧结助剂,通过液相烧结制备了碳化硅陶瓷．研究了不同稀土氧化物对烧结行为、烧结体显微结构和力学性能的影响．结果表明：Nd2O3-A1N,La2O3-A1N系统烧结失重较大,无法达到完全致密．Y203-A1N或AIN-Nd203-Y2O3,A1N-La203-Y20。系统可以在较低的温度下实现致密化;所有样品的晶粒细小,尺寸为1-2μm,无异常长大现象;在烧结体中除有高温烧结液相之外,还形成了稀土硅铝酸盐新相;致密的碳化硅陶瓷烧结体不但具有高硬度,而且断裂韧性高达6-8MPa·m^1/2     

High temperature tensile properties,fracture behaviors and nanoscale precipitate variation of an Al-Zn-Mg-Cu alloy

In order to assess Al-Zn-Mg-Cu alloys as potential high temperature structural materials,the hardness,tensile properties and fracture behaviors of 7085 Al alloy were investigated at various temperatures from room temperature to 175℃.High-resolution transmission electron microscopy was used to investigate the evolutions of precipitates at different temperatures,particularly on the relationship between microstructural evolution and tensile strength.The results reveal that both the microstructure and mechanical properties of the alloy are quite sensitive to the environmental temperature.As the temperature increases,the hardness and strength decrease while the elongation and reduction of area increase.As tensile testing temperature rises,the strain hardening exponent(n) decreases due to the thermal softening effect.The fracture mode of the alloy transforms from mixture of intergranular and transgranular fracture to completely transgranular dimples when tensile testing temperature reaches 150℃.The precipitate sequence during high temperature tests is coincident with that of aging.With the increase of tensile testing temperature,the mean precipitate radius grows larger,and the distribution of grain boundary precipitates transforms from continuous to discontinuous.     

机械合金化法制备的Cu/SiC纳米复合材料的组织、热性能和力学性能评估

Nano-sized silicon carbide (SiC: 0wt%, 1wt%, 2wt%, 4wt%, and 8wt%) reinforced copper (Cu) matrix nanocomposites were manufactured, pressed, and sintered at 775 and 875°C in an argon atmosphere. X-ray diffraction (XRD) and scanning electron microscopy were performed to characterize the microstructural evolution. The density, thermal expansion, mechanical, and electrical properties were studied. XRD analyses showed that with increasing SiC content, the microstrain and dislocation density increased, while the crystal size decreased. The coefficient of thermal expansion (CTE) of the nanocomposites was less than that of the Cu matrix. The improvement in the CTE with increasing sintering temperature may be because of densification of the microstructure. Moreover, the mechanical properties of these nanocomposites showed noticeable enhancements with the addition of SiC and sintering temperatures, where the microhardness and apparent strengthening efficiency of nanocomposites containing 8wt% SiC and sintered at 875°C were 958.7 MPa and 1.07 vol%?1, respectively. The electrical conductivity of the sample slightly decreased with additional SiC and increased with sintering temperature. The prepared Cu/SiC nanocomposites possessed good electrical conductivity, high thermal stability, and excellent mechanical properties.     

Sintering behaviour of feldspar and influence of electric charge effects

The characterization of feldspar for electric porcelain and the behaviour of these materials after heating at 1230℃ were studied. X-ray diffraction (XRD) and scanning electronic microscopy (SEM) were used to identify the present phases and the densification level. Feldspar sand was treated by flotation. The floated feldspar is constituted by microcline, quartz, and minor amounts of albite. The microstructure of sintered feldspar at 1230℃ is essentially vitreous with open microporosities. The dielectrical properties of composites were characterized by using the induced courant method (ICM), which indicates that the charge trapping capacity depends on the mineralogical and chemical composition of feldspar.     

Synthesis of glass ceramics from kaolin and dolomite mixture

Cordierite-and anorthite-based binary glass ceramics of the CaO-MgO-Al₂O₃-SiO₂ (CMAS) system were synthesized by mixing local and abundant raw minerals (kaolin and doloma by mass ratio of 82/18). A kinetics study reveals that the activation energy of crystallization (Ea) calculated by the methods of Kissinger and Marotta are 438 kJ·mol-1 and 459 kJ·mol-1, respectively. The Avrami parameter (n) is estimated to be approximately equal to 1, corresponding to the surface crystallization mechanism. X-ray diffraction (XRD) analysis shows that the anorthite and cordierite crystals are precipitated from the parent glass as major phases. Anorthite crystals first form at 850℃, whereas the μ-cordierite phase appears after heat treatment at 950℃. Thereafter, the cordierite allotropically transforms to α-cordierite at 1000℃. Complete densification is achieved at 950℃; however, the density slightly decreases at higher temperatures, reaching a stable value of 2.63 kg·m-3 between 1000℃ and 1100℃. The highest Vickers hardness of 6 GPa is also obtained at 950℃. However, a substantial decrease in hardness is recorded at 1000℃; at higher sintering temperatures, it slightly increases with increasing temperature as the α-cordierite crystallizes.     

ZTA—SiCw—TiC复相陶瓷材料强韧化机理研究

四方氧化锆相变增韧、碳化硅晶须补强、碳化钛粒子弥散强化，同时引入一个新的陶瓷基复合材料系统中产生了叠加的强韧化效果。热压ＺＴＡ－ＳｉＣｗ－ＴｉＣ复相陶瓷材料与Ａｌ２Ｏ３－ＳｉＣｗ－ＴｉＣ相比具有更高的综合机械性能。碳化钛在基体中形成连续的骨架阻碍晶粒的长大，随着碳化钛含量的增加，基体的硬度明显增加。主要强韧化机理有相变增韧、晶须拔出、载荷转移、裂纹偏转等。基体的主要断裂方式为穿晶解理断裂。     

Studies on induction hardening of powder-metallurgy-processed Fe-Cr/Mo alloys

Induction hardening of dense Fe-Cr/Mo alloys processed via the powder-metallurgy route was studied. The Fe-3Cr-0.5Mo, Fe-1.5Cr-0.2Mo, and Fe-0.85Mo pre-alloyed powders were mixed with 0.4wt%, 0.6wt%, and 0.8wt% C and compacted at 500, 600, and 700 MPa, respectively. The compacts were sintered at 1473 K for 1 h and then cooled at 6 K/min. Ferrite with pearlite was mostly observed in the sintered alloys with 0.4wt% C, whereas a carbide network was also present in the alloys with 0.8wt% C. Graphite at prior particle boundaries led to deterioration of the mechanical properties of alloys with 0.8wt% C, whereas no significant induction hardening was achieved in alloys with 0.4wt% C. Among the investigated samples, alloys with 0.6wt% C exhibited the highest strength and ductility and were found to be suitable for induction hardening. The hardening was carried out at a frequency of 2.0 kHz for 2-3 s. A case depth of 2.5 mm was achieved while maintaining the bulk (interior) hardness of approximately HV 230. A martensitic structure was observed on the outer periphery of the samples. The hardness varied from HV 600 to HV 375 from the sample surface to the interior of the case hardened region. The best combination of properties and hardening depth was achieved in case of the Fe-1.5Cr-0.2Mo alloy with 0.6wt% C.     

SiC的固相热压烧结

以α－SiC粉末为起始原料，B和C粉末为助烧剂，采用适当的热压烧结工艺，在1950℃，25MPa条件下获得了相对密度为97.8％的α－SiC块体陶瓷．其抗弯强度和断裂韧性值分别为383MPa，4.95MPa·m1／2．并初步研究了致密化机理．     

烧结气氛对ZTM／SiC陶瓷致密化的影响

ＺＴＭ／ＳｉＣ陶瓷基复合材料的致密化取决于烧结温度、烧结压力和烧结气氛。本文主要研究烧结气氛对ＺＴＭ／ＳｉＣ陶瓷致密化的影响。实验结果表明：该陶瓷不能在空气中实现常压烧结，而在气氛保护下实现了常压烧结致密化；中性或弱还原性气氛可该复合陶瓷的烧结温度，提高致密化速率，改善陶瓷材料性能，另，还从理论上分析了烧结气氛在该陶瓷致密化过程中的作用。     

配料Z值对常压烧结Sialon结构与性能的影响

由于Si     

Al_2O_3／nano－SiC复相陶瓷力学性能及显微结构

研究了热压烧结Ａｌ２Ｏ３／ｎａｎｏ－ＳｉＣ复相陶瓷的力学性能及显微结构。研究表明,纳米ＳｉＣ的引入显著地改善了材料的力学性能,在ＳｉＣ添加体积分数为１０％时,Ａｌ２Ｏ３／ｎａｎｏ－ＳｉＣ复相陶瓷抗弯强度σｆ达峰值为８６９ＭＰａ,断裂韧性ＫＩｃ也达峰值为６．７ＭＰａ·ｍ０．５,比纯Ａｌ２Ｏ３基体材料分别提高１３８％和８１％。ＴＥＭ观察表明：纳米ＳｉＣ晶粒主要存在于Ａｌ２Ｏ３基体晶粒内部,形成独特的“晶内型”结构。当受外力作用时,既能因弥散的纳米颗粒诱发穿晶断裂,且穿晶断裂时,还能因晶内存在第二相颗粒而引起裂纹偏转,起到增强增韧作用。