Effect of additives on mechanical properties of macroporous silicon carbide ceramics

Macroporous SiC ceramics were fabricated by carbothermal reduction of polysiloxane-derived SiOC containing hollow microspheres, followed by sintering and subsequent annealing. The effects of the additive composition and the annealing temperature on the porosity, microstructure, and mechanical strength of the resulting porous ceramics were investigated. Varying the additive composition was found to result in different porosities, microstructures, and mechanical properties. When the samples were sintered at 1750 °C and then annealed at 1900 °C for 4 h, the SiC prepared with 3% Al₂O₃ and 2% Y₂O₃ showed the highest strength (a flexural strength of 55 MPa and a compressive strength of 289 MPa, at a porosity of 45 %). The present results suggest that judicious selection of the sintering additive composition is very important for improving the mechanical properties of macroporous SiC ceramics.     

Fabrication and thermophysical properties of porous TiB2 ceramics fabricated by reactive spark plasma sintering

Titanium oxide (TiO₂) and boron carbide (B₄C) were added to TiB₂ raw powders to prepare porous TiB₂ ceramics by reactive spark plasma sintering, and the gas escape (such as CO and B₂O₃) resulted in higher porosity. X-ray Diffraction results indicated that the reduction reaction was completed after the reactive spark plasma sintering process. The porosity could be controlled by changing the ratio of synthesized TiB₂ to raw TiB₂ powders. The porosity of porous TiB₂ ceramics with 20 wt.% and 40 wt.% synthsized TiB₂ ceramics are 18.5% and 22.2%, respectively. The thermal diffusivity of the porous TiB₂ ceramics decreased with the porosity due to the low diffusivity behavior of gas and vacuum in pores, and the thermal conductivity for porous TiB₂ ceramics decreased as the temperature increased throughout the measured temperature range. The results here pointed to a potential method for fabricating porous TiB₂ ceramics with controllable thermophysical properties.     

High α-β phase transition and properties of YbF3-added porous Si3N4 ceramics obtained by low temperature pressureless sintering

In this paper, we used YbF₃ as a sintering additive to get a high α-β phase transition in porous Si₃N₄ ceramics. The mechanism of YbF₃ as sintering additives as well as the relationship between microstructure and mechanical properties have been investigated in detail. In addition, we used pressureless sintering to lower the temperature to 1550 °C. YbF₃ makes α-Si₃N₄ completely transform to β-Si₃N₄, whereas only 41.1% β-Si₃N₄ could be obtained with Yb₂O₃. This process yielded ceramics with more flexural strength and increased fracture toughness using less energy. In addition, using YbF₃ substituted for part Yb₂O₃ could promote sintering behaviors of Si₃N₄ ceramics at low temperature to increase α-β phase transition rate and improve the properties of silicon nitride ceramics significantly. In particular, when we used YbF₃-Yb₂O₃ as additives, we obtained a flexural strength of 269.87 MPa and a fracture toughness of 4.59 MPa·m^1/2.     

Effects of adding yttrium nitrate on the mechanical properties of hot-pressed AlN ceramics

Aluminum nitride (AlN) ceramics were prepared by hot-pressing with Y(NO₃)₃·6H₂O as sintering additive. The mechanical properties including flexural strength, Vickers’ hardness, and fracture toughness were studied. The relative density and mechanical property of the monolithic AlN were improved by adding Y(NO₃)₃·6H₂O, which decreased the porosity. At 2 wt% Y₂O₃, the AlN ceramic exhibited the highest strength of 383 MPa, the highest hardness of 15.39 GPa, and the highest fracture toughness of 3.1 MPa m^1/2. However, doping with more additive, the strength, hardness, and toughness of AlN ceramics decreased because of the weak interfacial bonding between AlN matrix and the yttrium aluminates phase.     

Processing and mechanical properties of porous silica-bonded silicon carbide ceramics

A simple processing route for manufacturing highly porous, silica-bonded SiC ceramics with spherical pores has been developed. The strategy adopted for making porous silica-bonded SiC ceramics entails the following steps: (i) fabricating a formed body through a combination of SiC and polymer microbeads (employed as sacrificial templates) and (ii) sintering the formed body in air. SiC particles are bonded to each other by oxidation-derived SiO₂ glass. By controlling the microbead content and the sintering temperature, it was possible to adjust the porosity such that it ranged from 19 to 77%. The flexural and compressive strengths of the porous silica-bonded SiC ceramics with ≈40% porosity were ≈65 MPa and ≈200 MPa, respectively. The superior strengths were attributed to the homogeneous distribution of small (≤30 μm), spherical pores with dense struts in the porous silica-bonded SiC ceramics.     

In Situ Synthesis Aluminum Borate Whiskers Reinforced TiB2 Matrix Composites for Application in Aluminum Reduction Cells

The TiB₂ matrix ceramics reinforced by aluminum borate whiskers (Al₁₈B₄O₃₃ w) had been prepared by the pressureless sintering method. The mechanical properties and densification behavior of the TiB₂ matrix ceramics were investigated. The results showed that Al₁₈B₄O₃₃ w was in situ synthesized by the reaction of boehmite (AlOOH) and TiB₂ powders during the sintering process. Increasing the sintering temperature had benefited for densification of the TiB₂ matrix ceramics. Al₁₈B₄O₃₃ w could increase the flexural strength and Vicker’s hardness. It is obtained that the maximum value Vicker’s hardness with 1.81 GPa and flexural strength with 82 MPa for samples sintered at 1600°C.     

Role of Zr(OH)4 hard agglomerates in fabricating porous ZrO2 ceramics and the reinforcing mechanisms

Porous ZrO₂ ceramics were fabricated by adding Zr(OH)₄ hard agglomerates to ZrO₂ powder, followed by pressureless sintering. The mechanical properties of porous ceramics sintered from pure ZrO₂ powder were poor. The addition of Zr(OH)₄ increased the strength and fracture toughness of the porous ZrO₂ ceramics for sintered specimens containing lower porosity. However, the Young’s modulus had little change so that the strain to failure of porous ZrO₂ ceramics increased with the incorporation of Zr(OH)₄. Scanning electron microscopy (SEM) observations revealed that microstructures of the green compacts prepared from pure ZrO₂ powder were nonuniform due to the ZrO₂ soft agglomeration, which resulted in a localized nonuniform shrinkage during densification. The localized nonuniform shrinkage led to a weaker grain bonding and degraded the mechanical properties of porous ZrO₂ ceramics. In this work, we found that this microstructure nonuniformity could be eliminated by the addition of Zr(OH)₄, because the bimodal particle size distribution confined the formation of ZrO₂ soft agglomerates due to a space constraint and an internal friction between the Zr(OH)₄ hard agglomerates during compaction. As Zr(OH)₄ decomposed into ZrO₂ grains during heating, the Zr(OH)₄ hard agglomerates disappeared before sintering occurred. The present study indicates that Zr(OH)₄ hard agglomerate is a unique agent to improve the mechanical properties of porous ZrO₂ ceramics.     

In situ synthesis and phase analysis of low density O＇-sialon-based multiphase ceramics

In situ formed low density O'-sialon-based multiphase ceramics were prepared by liquid-phase sintering method at 1400°C with Si3N4, SiO2 and Al2O3 as raw materials.Crystalline phases were identified by X-ray diffraction(XRD).The quantitative phase analysis was finished by matrix-flushing method and the substitution parameter x value of O'-sialon was estimated.The effects of sintering additives on the phase composition of the material were studied.The results show that, when using Y2O3 alone, Al6Si2O13 phase can be formed in the material, but when using Y2O3 and MgO, MgAl2O4 phase can be preferentially formed and the Al6Si2O13 is not observed.The mechanical properties of the material were measured and the relationships between microstructure and mechanical properties were discussed.The sample with Y2O3 and MgO sintering additives, using fused quartz alone as SiO2 source, displays a combination of high bending strength(163 MPa) and good fracture toughness(3.11 MPa·m1/2).Bending strength and fracture toughness of the samples increase with the increase of the content and aspect ratio of elongated grains and decrease with the increase of the porosity.     

Comparison of ZrB2–ZrO2f ceramics prepared by hot pressing and pressureless sintering

Zirconium diboride based ultra-high temperature ceramics toughened by zirconia fiber (ZrB₂–ZrO_2f) were prepared by hot pressing and pressureless sintering, and the effect of two sintering techniques on the phase composition, microstructures and mechanical properties of ZrB₂–ZrO_2f ceramics were studied in detail. The densification behavior was investigated through the analysis of the density curves. The microstructures and mechanical properties of ZrB₂–ZrO_2f ceramics were analyzed and compared in order to research the influence of the two sintering techniques. Results indicated that the hot-pressing process was more suitable for preparing ZrB₂–ZrO_2f ceramics than pressureless sintering process. The comprehensive properties of ZrB₂ plus 30 vol.% ZrO_2f ceramics obtained at temperature 1950 °C by hot pressing for 2 h were optimal, the flexural strength and fracture toughness reached 633 MPa and 5.6 MPa·m^1/2, respectively. The higher flexural strength was attributed to the smaller size of grains and higher relative density, furthermore, the toughening mechanisms were fiber debonding, fiber pull-out, crack deflection and transformation toughening.     

Spark Plasma Sintering of α/β Si<Subscript>3</Subscript>N<Subscript>4</Subscript> Ceramics with MgO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and MgO-Y<Subscript>2</Subscript>O<Subscript>3</Subscript> as Sintering Additives

In the present work, the α/β Si₃N₄ ceramics were fabricated by spark plasma sintering (SPS) at 1400-1500 °C for 6 min with 3wt.%MgO + 5wt.%Al₂O₃ and 3wt.%MgO + 5wt.%Y₂O₃ as sintering additives. The results showed that the phase composition, microstructure and mechanical properties of α/β Si₃N₄ ceramics were highly dependent on the type of sintering additive. The incomplete phase transformation from α to β occurred in the presence of an oxynitride (Mg-Al(Y)-Si-O-N) liquid phase. Compared with MgO-Al₂O₃, MgO-Y₂O₃ can significantly improve the β conversion rate of as-sintered α/β Si₃N₄ ceramics. And the as-sintered ceramics using MgO + Al₂O₃ as sintering additives had higher mechanical properties.     

Rapid sintering of nanocrystalline 8 mol.%Y2O3-stabilized ZrO2 by high-frequency induction heating method

Using a high-frequency induction-heating sintering (HFIHS) method, nanocrystalline 8 mol.%Y₂O₃-stabilized ZrO₂ was obtained from ultra fine powders. The observed advantages of this process include very quick densification to near theoretical density and prohibition of grain growth in nano-structured materials. Nearly fully dense nanocrystalline 8 mol.%Y₂O₃-stabilized ZrO₂, with a relative density of up to 99.8%, could be obtained with simultaneous application of 100 MPa pressure and an induced current within 10 min of sintering time without significant change in grain size. The influences of the sintering temperature and the mechanical pressure on the final density and grain size of the products were investigated. The hardness and fracture toughness of the dense ZrO₂ ceramics produced by HFIHS were investigated.     

Nanostructural Free-Volume Effects in Humidity-Sensitive MgO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Ceramics for Sensor Applications

Technologically modified spinel MgO-Al₂O₃ ceramics were prepared from Al₂O₃ and 4MgCO₃·Mg(OH)₂·5H₂O powders at sintering temperatures of 1200, 1300, and 1400 °C. Free-volume structural effects in MgO-Al₂O₃ ceramics and their electrophysical properties were studied using combined x-ray diffraction, scanning electron microscopy, Hg-porosimetry, and positron annihilation lifetime spectroscopy. It is shown that increasing of sintering temperature from 1200 to 1400 °C results in the transformation of pore size distribution in ceramics from tri- to bi-modal including open macro- and meso(micro)pores with sizes from ten to hundreds nm and nanopores with sizes up to a few nm. Microstructure of these ceramics is improved with the increase of sintering temperature, which results in decreased amount of additional phases located near grain boundaries. These phase extractions serve as specific trapping centers for positrons penetrating the ceramics. The positron trapping and ortho-positronium decaying components are considered in the mathematical treatment of the measured spectra. Classic Tao-Eldrup model is used to draw the correlation between the ortho-positronium lifetime and the size of nanopores, which is complementary to porosimetry data. The studied ceramics with optimal nanoporous structure are highly sensitive to humidity changes in the region of 31-96% with minimal hysteresis in adsorption-desorption cycles.     

烧结工艺对粉末烧结多孔材料微观结构与力学性能影响

本文进行了烧结气氛和烧结温度对支撑体孔径、透过性能影响的研究。环拉实验结果表明,氢气烧结样品的抗拉强度87.56MPa,氢气+氯化铵活化烧结样品115.20MPa,强度提高了30%。通过烧结温度对支撑体力学性能影响的研究,得到了支撑体致密度和力学性能之间的关系,可实现通过测试多孔材料的密度来预测多孔材料的强度。     

NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript>-based cermet inert anodes for aluminum electrolysis

Recent developments in the preparation, sintering process, mechanical properties, and thermal shock resistance of cermet inert anodes for aluminum electrolysis are reviewed in this paper. To obtain the desired technologies of low-temperature activated sintering of cermet inert anodes, the effects of material composition, sintering atmosphere, sintering temperature, and sintering aids on the densifi cation and microstructure of NiFe₂O₄-10NiO- based ceramics and cermets were studied. To obtain the toughening and strengthening technology of the cermet, the effects of material composition including ceramic and metallic phases are discussed. The cermet inert anodes with high density and mechanical properties were prepared through adjustment of material composition and sintering technology and selection of feasible sintering aids.     

凝胶注模成型制备微多孔氮化硅陶瓷

采用凝胶注模法,在无其它添加剂的条件下,通过提高单体含量,成功制备出高性能微多孔氮化硅陶瓷,陶瓷抗弯强度高达137 MPa以上,气孔率高达50%以上,孔中径小于1 μm.结果表明:随着有机单体含量的增加,氮化硅微多孔陶瓷气孔率单调增加;随着固含量的增大,氮化硅微多孔陶瓷气孔率单调下降,抗弯强度先上升然后又下降,固含量有一优化值,此时陶瓷体抗弯强度最大;随着烧结温度的增加,氮化硅陶瓷强度单调增加,而气孔率单调下降.     

Effect of Sm2O3 content on microstructure and thermal conductivity of Spark Plasma Sintered AlN ceramics

Dense aluminum nitride ceramics were prepared by Spark Plasma Sintering at a lower sintering temperature of 1700 °C with Sm₂O₃ as sintering additives. The effect of Sm₂O₃ content on the density, phase composition, microstructure and thermal conductivity of AlN ceramics was investigated. The results showed that Spark Plasma Sintering could fabricate dense AlN ceramics with superior thermal properties in a very short time. Sm₂O₃ not only facilitated the densification via the liquid-phase sintering mechanism but also improved thermal conductivity by decreasing oxygen impurity. Thermal conductivity decreased with increasing amount of Sm₂O₃ and the highest thermal conductivity was obtained for the AlN ceramics with 2 wt.% Sm₂O₃ content. During Spark Plasma Sintering process, only 2–3 wt.% sintering additives was enough to fabricate dense AlN ceramics, and the microstructures played a key role in controlling the thermal conductivity of AlN ceramics.     

烧结助剂对高气孔率SiC陶瓷结构和性能的影响

由叔丁醇、丙烯酰胺和SiC粉及烧结助剂组成固相含量为10%(体积分数)的陶瓷浆料,采用凝胶注模成型和无压烧结工艺制备多孔SiC陶瓷,研究Al2O3和Al2O3+SiO2这两种烧结助剂体系对多孔SiC陶瓷的气孔率、显微结构和力学性能的影响。结果表明:Al2O3+SiO2复合烧结助剂明显改善SiC陶瓷的烧结性能,与采用单一的Al2O3烧结助剂相比,SiC样品的烧结温度和莫来石的生成温度均降低50℃左右;两种不同的烧结助剂制成的试样中的气孔均呈很窄的单峰分布,中位孔径为2μm左右;随烧结温度的升高压缩强度增大,而气孔率变化不大;以Al2O3+SiO2为烧结助剂、在1 400℃烧结的试样的气孔率和强度分别达到70.57%和17.74 MPa。     

Mechanical behaviors of alumina ceramics doped with rare-earth oxides

The effects of three types of additives Y₂O₃, La₂O₃, and Sm₂O₃ on the sintering and mechanical behaviors of alumina ceramics were investigated. The bending strengths of alumina ceramics with Sm₂O₃ and Y₂O₃ additions were 455 and 439 MPa, respectively, higher than that with La₂O₃ addition. The fracture toughness of the ceramics with Sm₂O₃ and Y₂O₃ were also higher than that with La₂O₃ addition. The fracture mode of rare earth oxides doped alumina ceramics exhibited obvious transgranular fractures as well as intergranular fracture. The results of research show that the improvement of bending strength and fracture toughness of alumina ceramics with rare earth oxides was achieved by refining the grain size and strengthening the grain boundary.     

超细晶粉末冶金CoCrFeMnNiGd0.15高熵合金的组织性能

采用机械合金化和放电等离子烧结的方法制备出超细晶CoCrFeMnNiGd_0.15合金,研究了CoCrFeMnNiGd_0.15高熵合金的组织与性能。结果表明,其组织为多相结构,基体为FCC固溶体相,析出相为稀土氧化物(Gd₂O₃)和富Gd、Ni、Mn的四方结构相。随着烧结温度的提高,析出相的含量不断增加且尺寸不断增大,合金的压缩屈服强度不断下降而塑性则不断上升。在900℃烧结时材料具有最优的综合力学性能,其压缩屈服强度(σ_0.2)、抗压强度(σ_max)、断裂时的塑性应变(ε_p)和维氏硬度分别达到1662 MPa、2518 MPa、30.6%和458 Hv。     

Effects of Kaolinite addition on the densification and dielectric properties of BaTiO3 ceramics

Commercial Kaolinite was employed as sintering aid to reduce the sintering temperature of BaTiO₃ ceramics. The effects of Kaolinite content and sintering temperature on the densification, microstructure and dielectric properties of BaTiO₃ ceramics have been investigated. The density characterization results show that the addition of Kaolinite significantly lowered the sintering temperature of BaTiO₃ ceramics to about 1200 °C. XRD results show BaTiO₃ ceramics with a low amount of Kaolinite exhibited perovskite structure, but 10.0 wt% Kaolinite additions resulted in the formation of a secondary phase, BaAl₂Si₂O₈. BaO-TiO₂-Al₂O₃-SiO₂ glass phase was formed and improved the average breakdown strength of ceramics, which was supported by SEM-EDX results. The Kaolinite content had shown a strong influence on the dielectric constant and the diffuse transition. BaTiO₃ ceramic with 4.0 wt% Kaolinite addition possessed well temperature stability of dielectric constant.