Templated grain growth of textured mullite/zirconia composites

Mullite is an attractive material for advanced ceramic applications, but its low fracture toughness prevents it from widespread industrial applications. Therefore, mullite/zirconia composites were prepared from a reactive mixture of alumina and zircon with additives of TiO₂ and MgO to increase mechanical properties and densification. <001> aluminum borate templates were used to nucleate, and texture mullite in [001]. Mullite/zirconia formation started at 1350 °C and was complete at 1450 °C. Dense mullite/zirconia composites with highly textured mullite were produced after sintering at 1450 °C. A relatively constant tetragonal ZrO₂ content of 11±2 wt.% was retained at room temperature after sintering between 1350 and 1550 °C. A high quality of texture with an orientation parameter of 0.22 and a very narrow distribution of elongated mullite grains within 8.8° around [001] were successfully produced.     

High temperature mechanical properties of reaction-sintered mullite/zirconia and mullite/alumina/zirconia composites

Strength and fracture toughness of reaction-sintered mullite/zirconia composites (RSMZ) and reaction-sintered mullite/alumina/zirconia composites have been investigated as a function of temperature. Thermal shock resistance has also been determined. It was found that dispersion of zirconia particles and the particular microstructure of mullite obtained by means of anin situ reaction process leads to improved properties, with a room temperature fracture toughness of about 5.25 MPa m^1/2. Up to 1000° C fracture strength and toughness values are quite high, which make these materials potential candidates for high temperature applications.     

Alumina–mullite–zirconia composites: Part II Microstructural development and toughening

Densification characteristics and fracture toughness of the alumina–mullite–zirconia (AMZ) composites fabricated by the colloidal mixing route were examined. The densification of boehmite–silica–zirconia precursor compacts was characterized by three distinct stages. Stage I was represented by a rapid shrinkage between 1100 and 1300°C and was caused by the viscous flow sintering of the SiO2 component. Stage II was characterized by a temporary cessation of the shrinkage caused by the mullitization. Stage III was represented by a restoration of the shrinkage for temperatures above 1450 °C. The α-Al2O3 seeding facilitated the formation of elongated grains in the AMZ composites, and these elongated grains correspond to α-Al2O3. The toughening caused by the microcrack nucleation was comparable to that by the t → m martensitic transformation and increased with increasing total content of zirconia.     

Microstructural development of woven mullite fibre-reinforced mullite ceramic matrix composites by infiltration processing

Mullite fibre (Nextel 720?)-reinforced mullite ceramic matrix composites (CMCs) with zirconia weak interface were fabricated from heterocoagulated nano-size boehmite/amorphous silica powder particles dispersed in water, using electrophoretic deposition (EPD) and pressure filtration (PF). The nano-size mullite precursor was first prepared and characterised in terms of short-range particle–particle interactions and particle size distribution. Woven Nextel 720 mullite fibres were first desized and then coated with hydrothermally derived zirconia using dip-coating. EPD was performed under constant voltage conditions with varying deposition times, to infiltrate the dispersed powder suspensions into mullite fibre preforms, enabling the parameters necessary for good deposition of stoichiometric mullite to be established. EPD formed bodies were further consolitated using PF. The EPD/PF prepared green body specimens were dried under controlled atmosphere conditions before being sintered at 1200°C for 2 h in air. Mullite fibre mats were fully infiltrated using EPD parameters of 12 V DC applied voltage with 4 min deposition time, then eight EPD infiltrated fibre mats were further consolidated together using PF. The resulting CMC produced contained 35 vol% fibre loading and showed 81% theoretical density aftersintering at 1200°C for 2 h.     

Fracture toughness,strength and Vickers hardness of yttria-ceria-doped tetragonal zirconia/alumina composites fabricated by hot isostatic pressing

Yttria-ceria-doped tetragonal zirconia ((Y, Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing (HIP) at 1400–1600 °C and 147 MPa for 30 min in Ar gas using fine powders prepared by hydrolysis of ZrOCl₂ solution. The mechanical properties of these ceramic composites were evaluated. The fracture toughness and bending strength of the composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5-4 mol% CeO₂-ZrO₂, 2.5 mol% YO_1.5-4 mol% CeO₂-ZrO₂ and 2.5 mol% YO_1.5-5.5 mol% CeO₂-ZrO₂ fabricated by HIP at 1400 °C were 6–7 MPa m^1/2 and 1700–1800 MPa. Fracture toughness, strength and hardness of (Y, Ce)-TZP/Al₂O₃ composites were strongly dependent on HIP temperature. The fracture strength and hardness were increased, and grain growth of zirconia grains and phase transformation from the tetragonal to the monoclinic structure of (Y, Ce)-TZP during HIP in Ar at high temperature (1600 °C) were suppressed by the dispersion of Al₂O₃ into (Y, Ce)-TZP.     

Dielectric and electromechanical properties of barium titanate single crystals grown by templated grain growth

Single crystals of BaTiO(3) were grown by templated grain growth (TGG). TGG involves contacting a single crystal "template" to a sintered polycrystalline matrix, then heating the assemblage to a temperature that promotes the migration of the single crystal boundary through the matrix. In this investigation the properties of millimeter-sized, plate-shaped and bar-shaped, single crystals of BaTiO(3) grown by TGG were examined in order to compare the results to single crystals grown by conventional methods. A dielectric constant, epsilon(33)(T)/epsilon(0), of 260, polarizations of P(R)~16 muC/cm(2) and P(sat)~21.5 muC/cm(2), electromechanical coupling coefficients of k(33)~0.51 and k(31)~0.18, and a piezoelectric coefficient of d(33)~140 pC/N were achieved. Entrapped porosity in the crystals made fully poling the crystals difficult.     

模板晶粒生长技术制备SrBi2Nb2O9织构陶瓷

以熔盐法制备的片状SrBi2Nb2O9晶体为模板剂,采用模板晶粒生长技术和流延法制备了Sr-Bi2Nb2O9织构陶瓷,研究了模板含量对SrBi2Nb2O9织构陶瓷烧结行为、织构度、显微结构的影响。结果表明:模板含量为10wt%时,1200℃保温2h烧结可获得体积密度最大的SrBi2Nb2O9织构陶瓷,模板含量继续增加,体积密度降低;织构陶瓷的晶粒尺寸随模板含量的增加而逐渐增大,且晶粒取向性生长趋于明显,当模板含量为10wt%时,织构化SrBi2Nb2O9陶瓷的晶粒取向率f达到最大值,为0.81;织构化陶瓷的压电常数d33达到13pC/N,高于固相法制备的陶瓷的压电常数。     

炭/炭复合材料SiC/SiC+mullite/mullite涂层制备及其高温性能

为提高炭/炭复合材料的防氧化性能,采用包埋法与超音速等离子喷涂法相结合,在其表面制备了SiC/SiC+mullite/mullite多层防氧化涂层.外涂层主要组成是莫来石(mullite)相.对涂层试样进行了1 500℃恒温氧化和1 500℃～室温热震测试.实验结果表明,涂层试样经1 500℃恒温氧化150 h后,失重率仅为0.26%;经1 500℃～室温15次热震后,失重率仅为0.25%,显示出较优异的防氧化、抗热震性能.莫来石(mullite)具有良好的耐高温性能、低的氧扩散率,且SiC涂层氧化生成的SiO2在高温下能够愈合裂纹等缺陷,这是SiC/SiC+mullite/mullite涂层较好防氧化能力的主要原因.     

放电等离子烧结制备Diamond/Al复合材料

采用放电等离子烧结法（SPS）制备了Diamond/Al复合材料,研究了金刚石粒径、成分配比、工艺参数等对复合材料的导热性能的影响。结果表明,SPS可以得到导热性能较好的Diamond/Al复合材料,致密度是影响该材料导热性能的最重要因素。在实验确定的金刚石体积分数50%,金刚石粒径70 μm,温度550℃、压力30 MPa的工艺条件下,所制备的材料致密度较高,热导率为182 W/(m·K),比相同条件下纯铝粉烧结体的热导率提高了34.8%,表明金刚石的添加对烧结铝基材料导热性能有明显的改善作用。      

Properties of multilayered mullite/Mo functionally graded materials fabricated by powder metallurgy processing

Mullite/Mo functionally graded material (FGM) was developed with a powder metallurgy process. Microscopic observations revealed that the microstructure of the mullite/Mo FGM had a gradual stepwise variation. The thermomechanical properties of the mullite/Mo system had graded distributions that depend on the composition variation across the thickness. Thermal shock tests were done on the FGMs and on monolithic mullite. The FGM specimens had better thermal shock resistance than did monolithic mullite. The thermal shock resistance of FGMs was influenced by the sintering-induced residual thermal stress. Cracks caused by thermal shock were observed, and the mechanism of crack formation was discussed.     

Microstructure and mechanical properties of mullite/ZrO2 composites

The sintering, microstructure and toughness of mullite/ZrO₂ composites with increasing amount of ZrO₂ (0 to 20 vol %) have been studied. A very active premullite powder has been used as matrix. TheK _IC values increase from 2.1 to 3.2 MN m^?3/2 as the volume fraction of zirconia increases from 0 to 0.2. The realtive fraction of tetragonal zirconia decreases as the volume fraction of ZrO₂ increases to reach ～0.1 in the sample with 0.2 volume fraction of ZrO₂. The presence of ZrO₂ enhances the sintering rate and end-point density of the composites. Finally, the increasing toughness in mullite/ZrO₂ composites has been explained by a grain boundary strengthening mechanism produced by a metastable solid solution (～0.5 wt %) of ZrO₂ in mullite.     

Suspension characteristics and rheological properties of mullite/zirconia powder in methyl isobutyl ketone

Particle size and its distribution, sedimentation bulk density and rheology of mullite, zirconia, and mullite/zirconia mixed suspensions have been studied in terms of oxide loading (20, 30 vol%), and types of additives (dispersant, dispersant/plasticizer, dispersant/plasticizer/binder). Polyester/polyamine, dibutyl phthalate, poly(vinyl butyral), and methyl isobutyl ketone have been used as the dispersant, plasticizer, binder, and liquid medium, respectively. Sedimentation density significantly increased upon adding dispersant; the effect was more pronounced with zirconia suspension most likely due to the fine and hence high specific surface area of zirconia. With further addition of plasticizer and plasticizer/binder, the sedimentation density decreased. The suspension viscosity generally behaved in an opposite manner to the sedimentation density, i.e., low sedimentation density gave high low-shear viscosity, indicative of high order structure formation in the suspended particles. High shear rate rheology showed a shear thinning and its onset began at lower shear rate with higher solid loading. Mullite/zirconia mixed suspension gave intermediate sedimentation and rheological behavior, implying the two types of particles are non-interacting.     

Templated grain growth of SrBi2Ta2O9 ceramics: Mechanism of texture development

Textured SrBi₂Ta₂O₉ (SBT) ceramics were fabricated via templated grain growth (TGG) technique using platelet-like SBT single crystal templates. The templates (5 wt%) were embedded in a fine-grain SBT powder matrix containing 3 wt% of Bi₂O₃ excess that were subjected to uniaxial pressing and sintering at 1000–1250 °C for up to 24 h. Microstructural characterization by SEM was performed to establish the effect of sintering parameters on the grain growth and texture development. It was found that the ceramics developed a bimodal microstructure with notable concentration of large (longer than 90 μm) aligned grains with c-axis oriented parallel to the pressing direction. The mechanism controlling the texture development and grain growth in SBT ceramics is discussed.     

Microstructure and texture of copper/niobium composites processed by ECAE

The structure refinement is a challenge for conductors used for the winding of resistive coils producing non-destructive pulsed magnetic fields over 80T. These nanocomposite conductors composed of a conducting multiscale Cu matrix embedding Nb reinforcing nanofilaments are usually manufactured by using a Severe Plastic Deformation (SPD) process based on hot extrusion, accumulative cold drawing and bundling (ADB) steps [Thilly et al. Philos Mag A 82:925, 2002]. Equal-channel angular extrusion (ECAE) is investigated here as an alternative route since it may provide faster refinement to obtain the ultrafine structure needed for optimized electrical and mechanical properties of the conductors. Therefore, copper-niobium specimens obtained by hot extrusion were processed by ECAE at room temperature. The specific die of the ECAE tool used here is constituted by a round channel with three angles corresponding to a total equivalent strain of about 2.5. Deformed samples were examined by optical microscopy and characterized by hardness profiles and x-ray diffraction (texture pole figures). After one ECAE-pass, the shape of the samples is modified but no trace of damage appeared at the Cu-Nb interfaces. An increase of the hardness values localized in the copper matrix is revealed whereas the hardness of the niobium remains unchanged. Prior to ECAE, the hot extrusion process induced a fibre texture for both copper and niobium. Two fibre texture components were observed for copper: <111> and <200>. A single <110> fibre texture component is evidenced for the niobium. After ECAE a significant variation of the texture is observed in relation with the strong shear induced by this process.     

Evaluation of Weibull master curves of zirconia ceramics and zirconia/alumina composites

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.