用凝胶法制备片状莫来石粉及其应用

用拟薄水铝石和非晶态ＳｉＯ_２微粉为原料，用溶胶－凝胶工艺制取了莫来石凝胶。通过干燥、煅烧，制得片状结晶莫来石粉．初步研究了凝胶法莫来石粉在钛酸铝陶瓷和烧结刚玉砖中的应用。实验结果表明，片状莫来石起了促进烧结、降低烧成温度的作用，并使钛酸铝－莫来石复相陶瓷和刚玉砖的强度得到显著提高．     

Microstructural Characterization of Reactively Sintered Mullites

Mullite ceramics have been produced by reaction sintering of powders prepared using pseudoboehmite–colloidal silica and aluminum sulfate–colloidal silica mixtures. The microstructural development of these mullites was studied by a number of transmission electron microscopy based techniques including diffuse dark field, Fresnel fringe defocus imaging, and high-resolution transmission electron microscopy. This characterization procedure showed that mullite ceramics free from glassy phases at triple junctions and grain boundaries could be produced from both mixtures using suitable sintering temperatures and alumina/silica ratios. The wetting of grain boundaries by glass, occurring in the mullite ceramics from either incomplete reaction between alumina and silica components or release of silica from the mullite structure with increasing temperature, was found to depend on the prior thermal history of the ceramics.     

Transient liquid phase diffusion process for porous mullite ceramics with excellent mechanical properties

Porous mullite ceramics were fabricated by the transient liquid phase diffusion process, using quartz and fly-ash floating bead (FABA) particles and corundum fines as starting materials. The effects of sintering temperatures on the evolution of phase composition and microstructure, linear shrinkage, porosity and compressive strength of ceramics were investigated. It is found that a large amount of quartz and FABA particles can be transformed into SiO₂-rich liquid phase during the sintering process, and the liquid phase is transient in the Al₂O₃-SiO₂ system, which can accelerate the mullitization rate and promote the growth of mullite grains. A large number of closed pores in the mullite ceramics are formed due to the transient liquid phase diffusion at elevated temperatures. The porous mullite ceramics with high closed porosity (about 30%) and excellent compressive strength (maximum 105?MPa) have been obtained after fried at 1700?°C.     

Properties of multiple oxide-bonded porous SiC ceramics prepared by an infiltration technique

Multiple oxide-bonded porous SiC ceramics were fabricated by infiltrating a porous powder compact of SiC and alumina with cordierite sol followed by sintering at 1300-1400°C in air for 3 hours. The microstructures, phase components, mechanical properties, and air permeation behavior of the developed porous ceramics were examined and compared with materials obtained by the traditional powder processing route. The porosity, average pore diameter, and flexural strength of the ceramics varied from 33 to 37 vol%, ~12-14 μm and ~23-39.6 MPa, respectively, with variation in sintering temperature. The X-ray diffraction results reveal that both the amount of cordierite and mullite as the binder increased with increase in sintering temperature. In addition, it was found that the addition of alumina in powder form effectively enhanced the strength due to formation of mullite in the bond phase in contrast to the samples prepared without alumina additive. To determine the suitability of the material in particulate filtration application, particle collection efficiency of the filter material was evaluated theoretically using single collector efficiency model.     

Microstructure and mechanical properties of in-situ grown mullite toughened 3Y-TZP zirconia ceramics fabricated by gelcasting

In-situ grown mullite toughened zirconia ceramics (mullite-zirconia ceramics) with excellent mechanical properties for potential applications in dental materials were fabricated by gelcasting combined with pressureless sintering. The effect of sintering temperature on the microstructure and mechanical properties of mullite-zirconia ceramics was investigated. The results indicated that the columnar mullite produced by reaction was evenly distributed in the zirconia matrix and the content and size of that increased with the increase of sintering temperature. Mullite-zirconia ceramics sintered at 1500 °C had the optimum content and size of the columnar mullite phase, generating the excellent mechanical properties (the bend strength of 890.4 MPa, the fracture toughness of 10.2 MPa.m^1/2, the Vickers hardness of 13.2 GPa and the highest densification). On the other hand, zirconia particles were evenly distributed inside the columnar mullite, which improved the mechanical properties of columnar mullite because of pinning effect. All of this clearly confirmed that zirconia grains strengthened columnar mullite, and thus the columnar mullite was more effective in enhancing the zirconia-based ceramics. Simultaneously, the residual alumina after reaction was distributed evenly in the form of particle, which improved the mechanical properties of the sample because of pinning effect. Overall, the synergistic effect of zirconia phase transformation toughening with mullite and alumina secondary toughening improved the mechanical properties of zirconia ceramics.     

Preparation of mullite by the reaction sintering of kaolinite and alumina

In the present study, mullite specimens and mullite/alumina composites are prepared by reaction sintering kaolinite and alumina at a temperature above 1000°C. The phase and microstructural evolution of the specimens and their mechanical properties are investigated. Primary mullite appears at a temperature around 1200°C. The alumina particles are inert to the formation of primary mullite. Alumina starts to react with the silica in glassy phase to form secondary mullite above 1300°C. The formation of secondary mullite decreases the amount of glassy phase. Furthermore, the addition of alumina reduces the size of mullite grains and their aspect ratio. The strength and toughness of the resulting mullite increase with the increase of alumina content; however, the mechanical properties of the mullite and mullite/alumina composites are lower than those of alumina for their relatively low density.     

锆莫来石-碳化硅材料反应烧结过程的研究

研究了用锆英石、氧化铝和炭黑的混合物制备原位SiC颗料复合锆莫来石材料的反应烧结进程和显微结构特征。结果表明：反应烧过程中,SiC、莫来石的生成反应滞后于锆莫来石的分解反应;反应前期,SiC的生成占主导作用。反应后期,莫来石的形成及致密化进程占主导作用;材料中存在大量气孔,ZrO3以均匀分布和聚集体两种形式分布于莫来石、SiC及玻璃相构成的基质中。     

Sintering and Microstructure Modification of Mullite/Zirconia Composites Derived from Silica-Coated Alumina Powders

This paper addresses the densification and microstructure development during firing of mullite/zirconia composites made from silica-coated-alumina (SCA) microcomposite powders. Densification occurs in two stages: in the presence of a silica–alumina mixture and after conversion to mullite. The first stage of densification occurs through transient viscous phase sintering (TVS). This is best promoted by rapid heating, which delays the crystallization of silica to higher temperatures. A further sintering stage is observed following mullitization. The introduction of seeds promotes solid-state sintering, most probably due to refinement of the mullite matrix. For seed concentrations up to about 1% the sintering kinetics depend on seed concentration. This suggests that nucleation still remains the rate-controlling mullitization step. Above this concentration the reaction becomes growth controlled. Introduction of seeds also promotes direct mullitization without transient zircon formation that was observed in a previous study of the same process without seeding. Seeding also promotes the development of elongated grains by way of a solid-state recrystallization process.     

热压烧结细晶粒氧化铝陶瓷(英文)

以沉淀法制各的商业α-Al2O3粉体为原料,自制镁铝硅玻璃为烧结助剂,采用热压烧结工艺低温制备高性能氧化铝陶瓷.用Archimedes法、电子探针和三点弯曲法研究了氧化铝陶瓷的致密化行为、显微结构和力学性能.结果表明:在1400℃烧结的氧化铝陶瓷的相对密度高达98.9%,晶粒细小,平均晶粒尺寸约为0.6μm,晶界上有莫来石相析出,样品的抗弯强度和断裂韧性分别达442MPa和4.7MPa·m1/2.     

Translucent Yttria‐ and Silica‐Doped Mullite Ceramics with Anisotropic Grains Produced by Spark Plasma Sintering

Translucent, high‐performance, mullite ceramics with anisotropic grains were prepared by the spark plasma sintering (SPS) of a powder mixture consisting of commercial mullite powder, which already contained small amounts of alumina (θ and α) and silica (cristobalite) (≤3 wt% in total), to which 2 and 1 wt% of yttria and amorphous silica was admixed, respectively. The combination of low‐viscosity Y₂O₃–Al₂O₃–SiO₂ transient liquid formation and SPS sintering provided enhanced densification, also provoking anisotropic grain growth (which became exaggerated after 20 min of SPS dwell time), at a relatively low sintering temperature of 1370°C. In this way, it was possible to meet the conflicting demands for obtaining a dense mullite ceramic with anisotropic grains, ensuring good mechanical properties, while preserving a noticeable light transmittance. In terms of mechanical and optical properties, the best results were obtained when SPS dwell times of 5 and 10 min were employed. The as‐sintered samples possessed densities in the range 3.16–3.18 g/cm³, anisotropic grains with an aspect ratio (AR) of 7 and a grain thickness of approximately 0.45 μm, a flexural strength between 350 and 420 MPa, a Vickers indentation toughness and a hardness of approximately 2.45 MPa·m^1/2 and 15 GPa, respectively, and an optical transmittance of between 30% and almost 50% in the IR range.     

Complex mullite structures fabricated via digital light processing of a preceramic polysiloxane with active alumina fillers

By taking advantage of the multi-functional properties of preceramic polymers, their transformation into ceramic material at low sintering temperatures and the processing capabilities of polymer manufacturing processes, mullite components were fabricated by additive manufacturing. A photocurable silicone preceramic polymer resin containing alumina particles was shaped into complex structures via Digital Light Processing. Dense and crack-free, highly complex porous mullite ceramics were produced by firing a mixture of a commercially available photosensitive polysiloxane as the silica source, containing alumina powder as active filler, in air at a low sintering temperature (1300 °C). In particular, the developed formulations, coupled with the additive manufacturing approach, allow for precise control of the architecture of the porous ceramic components, providing better properties compared to parts with stochastic porosity.     

Properties and mechanism of mullite whisker toughened ceramics

High-strength ceramics were prepared from high alumina fly ash (HAFA) and activated alumina as raw materials with magnesia as a sintering additive. The growth kinetics and influence mechanism of secondary mullite whiskers were investigated. Meanwhile, the effects of the Al₂O₃/SiO₂ mass ratio (A/S) and the amount of magnesia on the content and morphology of mullite in the green body were investigated, so as to emphasize the effect of the liquid phase in the sintering process on the growth of secondary mullite whiskers. The results showed that the aspect ratio of secondary mullite whiskers increased significantly after adding activated alumina to increase the A/S ratio of raw materials. When 30 wt% activated alumina was added, the mullite content increased by 5.39%, and the whisker length increased from 1.36 μm to 4.18 μm. The addition of magnesia improved the liquid phase formed during the sintering process and the K value method was used to determine the sintering liquid phase content under various conditions. It was observed that increasing the magnesia level by 1 wt% could raise the liquid phase content by 5–7%. When the total liquid content of the system was 30–40%, the growth activation energy in the diameter direction of the whisker reduced significantly, promoting the growth of secondary mullite whiskers along the C axis. The morphology of mullite gradually developed from fibrous to long columnar crystal, making it combine more densely with the green body matrix. Furthermore, the staggered long columnar mullite crystal structure changes the fracture mode of ceramics from intergranular to transgranular fracture, which fully uses the high mechanical strength of mullite. As a result, the fracture energy and strength of ceramics are significantly improved.     

Fabrication of mullite ceramics by rotary forging and pressureless sintering

Mullite ceramics were fabricated from boehmite/silica diphasic gels using a rotary forging compaction technique and pressureless sintering. Almost fully dense mullite samples were obtained after sintering at 1350°C for 2 h. The microstructure of sintered samples comprised fine (average size <1 μm), equiaxed grains. The samples showed superior sintering behaviour in comparison to those fabricated using conventional uniaxial and isostatic pressing. Powder compaction by rotary forging is thought to generate viscous deformation of the contact points increasing the interparticle contact area as well as increasing the packing density by breaking down hard agglomerates more effectively with concomitant rearrangement of the primary powder particles, thus promoting greater densification at relatively low temperatures. ©     

双相铝硅凝胶Al2O3／SiO2比对莫来石陶瓷显微结构的影响

对具有不同Ａｌ２Ｏ３／ＳｉＯ３比的双相铝硅凝胶的烧结行为及陶瓷材料的显微结构进行了研究，结果表明，少量液相的存在对先驱粉末的致密率和材料显微结构有显著的影响。富硅莫来石先驱粉末成型体能在较低的温度下常压烧结，相对密度高达９８％以上，试样有柱状莫来石晶粒存在。     

Novel design of elongated mullite reinforced highly porous alumina ceramics using carbonized rice husk as pore-forming agent

A facile strategy for the fabrication of elongated mullite reinforced porous alumina ceramics (PACs) using carbonized rice husk (CRH) as pore-forming agent and silica source is reported for the first time. A large amount of elongated mullite is synthesized in pores due to the reaction of amorphous silica in CRH skeleton and alumina ceramic powder. Elongated mullite acts as the bridges between pore walls, enhancing the compressive strength of PACs. Furthermore, secondary pores from the intersection of elongated mullite is favor of decreasing of the thermal conductivity. High performance PAC with porosity of 74.3% has been fabricated by employing 25 wt% CRH, which possesses relatively low thermal conductivity of 0.189 W/(m•K) and ultra-high compressive strength of 45 MPa. Its comprehensive performance is much better than that of existing ceramic materials. Our findings present a facile, eco-friendly and effective approach to fabricate high performance PACs as the high-temperature thermal insulation materials.     

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.     

Fabrication of Mullite Body Using Superplastic Transient Phase

Mullite, an extremely creep-resistant ceramic, has been fabricated using a novel processing/forming approach taking advantage of superplastic transitional phases. Starting with a mixture of alumina, silica, and a small amount of lithia additive (0.8 wt%), a processing window of about 50°C around 1350°C has been found within which the material can be densified and superplastically deformed with negligible mullitization. The lithia additive promotes a transient lithium aluminosilicate glassy phase that greatly enhances sintering and deformation. The superplastic premullite maintains a nearly constant grain size during deformation between 1250° and 1400°C, over a strain rate from 6 × 10⁻⁷ to 10⁻¹ s⁻¹, and has unusually high activation energy values in the range of 1150 to 2086 kJ/mol. An increase in the transient glassy phase content due to the increased matrix dissolution at higher temperatures contributes in part to this anomaly. The mullite work pieces thus shaped become creep resistant again after a postforming annealing/mullitization treatment which decreases the creep rate by 6 orders of magnitude. The mechanical properties (hardness, toughness, and strength) of the finished mullite are compared to those of conventionally processed mullite.     

Biaxial strength and slow crack growth in porous alumina with silica sintering aid

Biaxial strength, fracture toughness and subcritical crack growth are reported for coarse grained porous alumina ceramics. The materials were prepared with a varying amount of a silica sintering aid, which resulted in the formation of a glassy secondary phase at the grain boundaries. Crystalline mullite was additionally found in the material with the highest silica content. The biaxial strength, measured by Ball-on-Ring and Ball-on-3-Balls, was highest for the material without mullite at the grain boundaries, and the biaxial strength decreased with increasing porosity. The fracture toughness of the materials was in the range of 1.7–1.9 MPa m^0.5. Measurements of subcritical crack growth by a modified lifetime method in air and aqueous environments demonstrated a higher crack growth rate in water and acid relative to in air. The effect of porosity and grain boundary phase were discussed in relation to subcritical crack growth and fracture mode in the coarse grained alumina ceramics.     

Phase formation,microstructure development,and mechanical properties of kaolin-based mullite ceramics added with Fe2O3

The effects of Fe₂O₃ on phase evolution, density, microstructural development, and mechanical properties of mullite ceramics from kaolin and alumina were systematically studied. X-ray diffraction results suggested that the ceramics consisted of mullite, sillimanite, and corundum, in the sintering range of 1450°C–1580°C. However, as the sintering was raised to 1580°C, mullite is the main phase with a content of 94%, and the corundum phase content is 5.9%. Simultaneously, high-temperature sintering had a positive effect on the densification of the mullite ceramics, where both the bulk density and flexural strength could be optimized by adjusting the content of Fe₂O₃. It was found that 6 wt% Fe₂O₃ was optimal for the formation of rod-shaped mullite after sintering at 1550°C for 3 h. The sample's maximum bulk density was 2.84 g/cm³, with a flexural strength of 112 MPa. Meanwhile, rod-shaped mullite grains with an aspect ratio of ~9 were formed. As a result, a dense network structure was developed, thus leading to mullite ceramics with excellent mechanical properties. The effect of Fe₂O₃ on the properties might be attributed to the fact that Al³⁺ ions in the [AlO₆] octahedron were replaced by Fe³⁺ ions, resulting in lattice distortion.     

The Effect of Sintering Temperature on Porous Silica Composite Strength

The effect of sintering temperature on porous silica composite strength was studied by discussing three factors, namely crystal phase, glassy phase and porosity. The fired products of clay and silica are composed of crystalline phase and glassy phase. The crystalline phases consist of alpha-quartz and mullite and the glassy phase contains a disordered silica network. With the increase of sintering temperature up to 1360°C, the crystalline silica decreased gradually. The disappearing silica dissolved into the glass and became a part of glass network and resulted in the enhancement of glass strength. This change in glass played an important role in the improvement of sample strength. At the same time, the increase of sintering temperature promoted the densification of samples and reduced the porosity of products, which also contribute to the increase of sample strength. When the sintering temperature is up to 1390°C, the silica in glass tended to convert to cristobalite with the expansion of glassy phase. This expansion weakened the connection of atoms in glass network and brought some closed pores into products, which led to the decrease of sample strength.