Sintering of Ultrafine Silicon Powder

The sintering behavior of compacts of ultrafine silicon powder (0.02 to 0.1 μm particle size) was investigated. Two sintering modes occur: normal sintering associated with densification and subnormal sintering without densification. The micro-structure developed in normal sintering has a fine grain size (0.05 to 0.3 μm) and fine porosity; the grains contain stacking faults and twins. The microstructure developed in subnormal sintering exhibits larger grains (∼1 μm in size) and coarse pores. Green densities >42% of theoretical and temperatures >1100°C are required for densification. Densification follows an exponential time and temperature dependence with an activation energy of 470 kJ/mol, indicating bulk diffusion as the transport mechanism. Grain-boundary diffusion is thought to be inhibited by grain-boundary oxide films. The carbon phase-separates into discrete amorphous regions and is thought to have little effect on sintering behavior.     

Influence of Aluminum Titanate Formation on Sintering of Bimodal Size-Distributed Alumina Powder Mixtures

The sintering behavior of green compacts, in which coarse alumina particles formed a skeletal structure and fine alumina and/or fine titania particles filled the voids of the skeletal structure, was investigated. Sinterability of the green compacts changed according to the titania content in the fine powder. The titania content of 33 mol% was the most effective for the densification. The volume expansion due to the aluminum titanate formation occurred in the voids of the skeletal structure, and the densification of the skeletal structure progressed more because the grain growth between the fine and coarse alumina particles did not proceed. As the titania content decreased, the densification did not progress more than that of the compact with 33 mol% titania content because the grain growth proceeded more. As the titania content increased, the expansion of the compacts was larger, and large grains were formed by the reaction between the titania and coarse alumina particles. Therefore, densification became difficult.     

Compaction and Sintering Behavior of Bimodal Alumina Powder Suspensions by Pressure Filtration

The compaction behavior of fine alumina powders with different particle sizes or bimodal particle-size distributions that are undergoing pressure filtration was investigated. Three alumina powders—average particle sizes of 0.2—0.86 μm—were compacted to a solids fraction of 62—65 vol% from suspensions at pH 3, which was the pH level at which the suspensions showed their lowest viscosity. When the powders of different average sizes were mixed, the suspensions showed better flowability, and the lowest viscosity was obtained when the fraction of fines was ∼30 vol% and pH = 3. The mixed-sized powder suspensions were compacted to higher density than the suspensions of unmixed fine or coarse powders, and the maximum density was obtained for mixed suspensions that had the lowest viscosity, despite the different particle-size ratio. Maximum densities of 72.5% and 75.0% were attained when the size ratios were 2 and 5, respectively. The compacts that were pressure-filtered from mixed suspensions exhibited a single-peaked pore-size distribution and a homogeneous microstructure, whereas the pore-size distributions of dry-pressed compacts were double-peaked. The sintering behavior of the compacts that were pressure-filtrated from bimodal powders exhibited significantly better sinterability and much-less linear shrinkage than the coarser powders and the dry-pressed powder compacts.     

Optimal Composition of Zircon–Fused Silica Ceramic Cores for Casting Superalloys

The dependence of flexural strength on crystalline phase content, residual porosity, and sintering temperature has been investigated for ceramic cores based on a mixture of fused silica and zircon. The rule of binary particle mixture, adopted for three particle sizes of fused silica, predicts a critical weight fraction of fine (zircon) particles of 46.9 wt% for the best packing. However, the optimal composition of a 37.8 wt% zircon–fused silica mixture for the flexural strength of ceramic cores found experimentally is lower in zircon content. The content of α-cristobalite +α-quartz crystallized from infilling silica sols exceeds ∼3 wt% when the strength decreases from ∼10 MPa to ∼6 MPa in samples sintered at 899–927°C. The degradation is ascribed to microcracking and the loss of coherency between the fused silica grains and the infilled silica due to β- →α-phase transformation on cooling and differential densification during sintering.     

Limitations on the sintering of graded particle systems

Improvement of compact density is commonly achieved by blending coarse and fine particles, but these compacts will not densify without the presence of a significant amount of liquid phase. It was proposed that two step sintering (TSS) could be applied to sinter the fine particle matrix, potentially accommodating the presence of inclusions of large particles. This hypothesis was false. Compacts were prepared with similar green density but with different ratios of coarse, medium, and fine particles and then subjected to TSS. The results indicated that constrained sintering limits densification on both ends of the particle packing spectrum: A fine particle matrix containing large particles fails to densify because the matrix cannot shrink around the inclusion; the densification of fine particle pockets in a skeletal network composed of large particles does not allow sufficient shrinkage in the pockets of small particles.     

Two-Step Sintering of Nanocrystalline ZnO Compacts: Effect of Temperature on Densification and Grain Growth

Two-step sintering (TSS) was applied on nanocrystalline zinc oxide (ZnO) to control the accelerated grain growth occurring during the final stage of sintering. The grain size of a high-density (>98%) ZnO compact produced by the TSS was smaller than 1 μm, while the grain size of those formed by the conventional sintering method was ∼4 μm. The results showed that the temperature of both sintering steps plays a significant role in densification and grain growth of the nanocrystalline ZnO compacts. Several TSS regimes were analyzed. Based on the results obtained, the optimum regime consisted of heating at 800°C (step 1) and 750°C (step 2), resulting in the formation of a structure containing submicrometer grains (0.68 μm). Heating at 850°C (step 1) and then at 750°C (step 2) resulted in densification and grain growth similar to the conventional sintering process. Lower temperatures, e.g., 800°C (step 1) and 700°C (step 2), resulted in exhaustion of the densification at a relative density of 86%, above which the grains continued to grow. Thermogravimetric analysis results were used to propose a mechanism for sintering of the samples with transmission electron micrographs showing the junctions that pin the boundaries of growing grains and the triple-point drags that result in the grain-boundary curvature.     

Effect of Calcination on the Sintering of Gel-Derived, Zirconia-Toughened Alumina

The densification behavior of ZrO₂ (+ 3 mol% Y₂O₃)/85 wt% Al₂O₃ powder compacts, prepared by the hydrolysis of metal chlorides, can be characterized by a transition- and an α-alumina densification stage. The sintering behavior is strongly determined by the densification of the transition alumina aggregates. Intra-aggregate porosity, resulting from calcination at 800°C, partly persists during sintering and alumina phase transformation and negatively influences further macroscopic densification. Calcination at 1200°C, however, densifies the transition alumina aggregates prior to sintering and enables densification to almost full density (96%) within 2 h at 1450°C, thus obtaining a microstructure with an alumina and a zirconia grain size of 1 μm and 0.3–0.4 μm, respectively.     

Effect of Seeding on Phase Development, Densification Behavior, and Microstmcture Evolution in Mullite Fabricated from Microcomposite Particles

An investigation was carried out concerning the effect of mullite seed particles on phase development, densification behavior, and microstructure evolution in powder compacts prepared with silica/alumina microcomposite particles. The incorporation of ∼2 wt% seed particles in the microcomposite powder compacts had relatively little effect on densification, but resulted in significant decreases in the temperature for mullite formation and the grain sizes in mullitized samples. Samples could be sintered to almost full density and subsequently converted to mullite with average grain sizes ≤0.4 μm at temperatures in the range of 1300°-1400°C. The available evidence indicated that mullite formation occurred primarily by nucleaton and growth in the siliceous matrix phase.     

Pressureless densification and mechanical properties of hafnium diboride doped with B4C: From solid state sintering to liquid phase sintering

A pressureless sintering process, using a small amount of boron carbide (≤2 wt%) as sintering aid, was developed for the densification of hafnium diboride. Hafnium diboride ceramics with high relative density were obtained when the sintering temperature changed from 2100 °C to 2350 °C. However, the sintering mechanism was varied from solid state sintering (SSS, below 2300 °C) to liquid phase sintering (LPS, above 2300 °C). Boron carbide addition improved densification by removing the oxide impurities during solid state sintering and by forming a liquid phase which was well wetting hafnium diboride grains during liquid phase sintering process. The different roles of B₄C on the microstructure development and mechanical properties of the sintered ceramics were investigated.     

Low-temperature sintering of coarse alumina powder compact with sufficient mechanical strength

Coarse alumina powder compacts doped with various amounts of titania and copper oxide were pressurelessly sintered from 900 °C to 1600 °C. Their phase assemblages and microstructural evolution, as well as their properties, were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry/thermogravimetric (DSC/TG) analysis, and three-point bending and wetting test. The role of TiO₂ and CuO during the sintering is discussed in detail. The experimental results show that the liquid phase from the copper oxide appeared at approximately 1200 °C, so the solid-state reaction between alumina and titania took place at a lower temperature. Such solid state-reaction sintering had a strong impact on the grain growth and greatly promoted the densification of the alumina compact. In addition, the liquid phase inhibited the abnormal grain growth and microcracking. As a result, the coarse alumina powder compacts doped with 5 wt% TiO₂–CuO were fully densified and exhibited sufficient flexural strength (342±21 MPa) when sintered at a temperature of 1450 °C for 2 h.     

Influence of solid loading and particle size distribution on the porosity development of green alumina ceramic mouldings

Alumina ceramic mouldings with different solid contents ranging from 55 to 70 vol% and different ratios of coarse/fine powders, i.e. 0.4 μm (fine) and 3 μm (coarse), respectively, were prepared by compression moulding at 75 °C under a compressive stress of 10 MPa. The porous parameters, such as porosity, pore size and pore size distribution, of the green compacts were evaluated after removal of organic vehicles. Experimental evidence showed that the green density, as well as the sintered density, of the moulded alumina increased linearly with increased solid loading to an optimum of 65 vol% and decreased roughly linearly with increased coarse/fine ratio. Further increase in solid loading reduced particle packing efficiency, resulting in lower green and fired densities. No considerable improvement in green and sintered density of the moulded alumina was achieved by adjusting the coarse/fine ratio, which is due to the fact that coarse particles suppress the driving force of densification. The green compacts generally showed a bimodal pore size distribution character which may be the most important factor in dominating the densification of the powder compacts. The peak frequency at larger pore region is approximately 20–35 μm in diameter and at the smaller pore region is ˜50–95 nm in diameter. The larger pores are believed to be due to the presence of internal voids originating from entrapped gas and are probably caused by the removal of organic vehicles.     

Influences of Particle Size of Alumina Filler in an LTCC System

A low temperature co-fired ceramics system consisting of a typical calcium aluminoborosilicate glass and alumina filler was used to investigate the effects of four different sizes, 13 nm, 0.5, 3, and 39 μm, of a commercially available alumina filler on the resultant densification, crystallization, and dielectric properties. There was definitely a proper range of alumina particle size, which leads to desirable densification and enhanced dielectric properties. The onset temperatures of densification and crystallization depended strongly on the filler particle size. The 3 μm sample as an optimum filler size exhibited a promising performance of k ∼8.1 and Q ∼160 at a resonant frequency of 14.8 GHz, which results from early densification and intensive crystallization of the anorthite CaAl₂Si₂O₈ phase. Particularly, the use of nano-sized alumina (13 nm) retarded both densification by ∼200°C and crystallization by ∼80°C compared with the results of the 3 μm alumina case. The dependence of the filler particle size was postulated as being related to the wetting and connectivity behavior of glass through consequent inter-reactions between glass and ceramic.     

Microwave Sintering of Alumina at 2.45 GHz

The sintering kinetics and microstructural evolution of alumina tubes (∼17 mm length, ∼9 mm inner diameter, and ∼11 mm outer diameter) were studied by conventional and microwave heating at 2.45 GHz. Temperature during microwave heating was measured with an infrared pyrometer and was calibrated to ±10°C. With no hold at sintering temperature, microwave-sintered samples reached 95% density at 1350°C versus 1600°C for conventionally heated samples. The activation energy for microwave sintering was 85 ± 10 kJ/mol, whereas the activation energy for conventionally sintered samples was 520 ± 14 kJ/mol. Despite the difference in temperature, grains grew from ∼1.0 μm at 86% density to ∼2.6 μm at 98% density for both conventionally sintered and microwave-sintered samples. The grain size/density trajectory was independent of the heating source. It is concluded that the enhanced densification with microwave heating is not a consequence of fast-firing and therefore is not a result in the change in the relative rates of surface and grain boundary diffusion in the presence of microwave energy.     

Role of Particle Substructure in the Sintering of Monosized Titania

Monosized titania particles (∼0.35-μ diameter) prepared by controlled hydrolysis of titanium tetraethoxide in ethanol were found to be porous agglomerates of ∼6-nm primary particles. The sintering behavior of compacts constituted of monodispersed agglomerates was evaluated, and changes in macroscopic dimensions were correlated with changes in particle microstructure and chemistry. The total volume shrinkage during sintering was ≥87%. Five contributions to the total shrinkage and the temperature ranges for the associated processes were identified: removal of chemisorbed water (from ambient to 250°C), crystallization to anatase (between 250° to 425°C), intra-agglomerate densification (425° to 800°C), conversion of anatase to rutile (600° to 800°C), and inter-agglomerate densification (>800°C). Approximately one-half the compact shrinkage was the result of agglomerate substructure changes. Studies of the agglomerate structural evolution indicated the intra-agglomerate densification and crystallite growth rates are the secondary factors, after compact packing, that influenced microstructure development.     

Pressureless Sintering of Boron Carbide

B₄C powder compacts were sintered using a graphite dilatometer in flowing He under constant heating rates. Densification started at 1800°C. The rate of densification increased rapidly in the range 1870°–2010°C, which was attributed to direct B₄C–B₄C contact between particles permitted via volatilization of B₂O₃ particle coatings. Limited particle coarsening, attributed to the presence or evolution of the oxide coatings, occurred in the range 1870°–1950°C. In the temperature range 2010°–2140°C, densification continued at a slower rate while particles simultaneously coarsened by evaporation–condensation of B₄C. Above 2140°C, rapid densification ensued, which was interpreted to be the result of the formation of a eutectic grain boundary liquid, or activated sintering facilitated by nonstoichiometric volatilization of B₄C, leaving carbon behind. Rapid heating through temperature ranges in which coarsening occurred fostered increased densities. Carbon doping (3 wt%) in the form of phenolic resin resulted in more dense sintered compacts. Carbon reacted with B₂O₃ to form B₄C and CO gas, thereby extracting the B₂O₃ coatings, permitting sintering to start at ∼1350°C.     

Pressureless Sintering of Zirconium Diboride: Particle Size and Additive Effects

Zirconium diboride (ZrB₂) was densified by pressureless sintering using <4-wt% boron carbide and/or carbon as sintering aids. As-received ZrB₂ with an average particle size of ∼2 μm could be sintered to ∼100% density at 1900°C using a combination of boron carbide and carbon to react with and remove the surface oxide impurities. Even though particle size reduction increased the oxygen content of the powders from ∼0.9 wt% for the as-received powder to ∼2.0 wt%, the reduction in particle size enhanced the sinterability of the powder. Attrition-milled ZrB₂ with an average particle size of <0.5 μm was sintered to nearly full density at 1850°C using either boron carbide or a combination of boride carbide and carbon. Regardless of the starting particle size, densification of ZrB₂ was not possible without the removal of oxygen-based impurities on the particle surfaces by a chemical reaction.     

Effect of dual liquid phase sintering aids on the densification and microstructure of low temperature sintered alumina ceramics

Alumina ceramics was prepared by pressureless sintering technology in which a CuO–TiO₂–Bi₂O₃ mixture (0–4.0 wt% Bi₂O₃ and 4.0 wt% CuO and TiO₂) was added as dual liquid phase sintering aids. The phase compositions, microstructural feature, and sintering behaviour of the alumina ceramics were analyzed. The results showed that adding 2.5 wt% Bi₂O₃ to alumina ceramics can increase the contribution rate of initial stage of sintering to the sintering process. The relative density of the sample reached 97.63% after sintering at 1200 °C for 90 min. Measurements from differential scanning calorimetry, with the addition of CuO–TiO₂–Bi₂O_3, demonstrated the formation of two liquid phase points, 827.4 and 936.8 °C. Notably, the solid solution temperature of TiO₂ and Al₂O₃ ceramics diminished thanks to the dual liquid phase sintering aids, and at the same time the activation energy required also dropped from 368.96 to 137.31 kJ/mol. Research indicates that the combined action of dual liquid phase sintering and solid-state reaction sintering has promoted the densification of alumina ceramics during the sintering process while at the same time inhibiting the growth of abnormal grains so that a homogeneous microstructure can be formed.     

Sintering behaviour of a ZnO waste powder obtained as by-product from brass smelting

The effect of two sintering methods (conventional sintering and two-step sintering) and of alumina addition on the sintering behaviour of a ZnO-rich waste powder (ZnO > 95 wt%), a by-product from brass smelting industry, was studied aiming to improve the sintered density and grain size. Both conventional sintering and two-step sintering methods did not lead to fully dense powder compacts, as densification was conditioned by abnormal grain growth and the particle size of the ZnO-rich residue. When two-step sintering was used the grain growth was reduced comparatively to conventional sintering method. The highest relative sintered density (about 90%) was achieved when samples of ZnO waste and samples of ZnO waste with 2 wt% added Al₂O₃ were processed by two-step sintering and corresponded to a mean grain size of around 18 µm and 7 µm, respectively. XRD and SEM results indicated that alumina addition helped to inhibit grain growth due to the formation of gahnite spinel (ZnAl₂O₄) precipitates in the grain boundaries of zincite (ZnO) grains.     

Differential Sintering by Improper Selection of Sintering Parameters during Pulse Electric Current Sintering

Recently, we reported on the retention of fine-grained micro-structure in Al₂O₃/3 vol% 3Y-ZrO₂ composites using the pulse electric current sintering (PECS) technique. It was demonstrated that a high heating rate is beneficial for the retention of fine grains and homogeneous microstructure. As there are few reports on microstructural inhomogeneity and excessive grain growth in compacts densified by the PECS technique, we carried out a series of experiments on monolithic alumina by varying the sintering parameters and discussed the characteristic results. All specimens that were densified under selective sintering conditions attained high density (∼99% of the theoretical density) at 1250°3C in > 5 min. The average fracture strength of monolithic alumina was observed to be 741 ± 25 MPa and the fracture toughness was 2.2 MPa.m <^1/2, and these were reasoned out to small grains. However, compacts sintered under very low compaction pressure attained ∼92%-93% of the theoretical density, and these specimens had undesirable microstructural inhomogeneity owing to differential sintering. Hence, in the present study, we address the problem of differential sintering.     

CuO+TiO2复相添加剂对氧化铝陶瓷烧结性能和显微结构的影响及其烧结动力学分析

研究了CuO＋TiO2复相添加剂对氧化铝陶瓷烧结性能，显微结构的影响以及形成液相时氧化铝陶瓷烧结动力学。添加剂的加入极大的促进了氧化铝陶瓷的烧结。当CuO与TiO2质量比为1：2的时候，氧化铝样品致密度最高。液相含量对致密度有明显影响，液相含量越高，烧结速率越快。添加剂的存在使氧化铝晶粒细化，晶粒形貌为等轴状。利用等温烧结的实验方法研究了烧结动力学，结果表明，是由氧离子和铝离子的扩散作用控制了烧结过程。