Graded compositions and microstructures by infiltration processing

Mullite/alumina particulate composites were fabricated by infiltrating porous alumina preforms with an SiO₂-containing sol followed by a heating step to cause mullite formation and densification. Electron microprobe analysis was performed to obtain concentration profiles across sections of the sintered composites. These analyses revealed the existence of concentration gradients, the mullite content decreasing with increasing distance from the surface of the bodies. Analyses with a scanning electron microscope also indicated a microstructural effect; the alumina grain size in composite bodies tended to increase with distance from the surface of the sample. These two effects (microstructural and compositional) have been related and it has been concluded that while the presence of mullite limits grain growth in alumina, the mullite content must be at least 5 wt % in order for grain growth to occur in a controlled fashion. The results point out the potential of the infiltration approach as a means for tailoring the composition and microstructure of ceramic bodies.     

Microstructural evolution during direct laser sintering in the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system

The microstructural evolution during direct laser sintering of LSD (Layerwise Slurry Deposition)—samples in the Al₂O₃–SiO₂ system has been investigated. Slurries with a water content of 34 wt.% and a SiO₂/Al₂O₃—ratio of about 3:1 have been used to manufacture layers which—after consecutive drying—have been sintered and laminated by laser treatment. Densified samples can be obtained with laser irradiances from 190 to 270 kW/cm² and scan velocities between 35 and 65 mm/s. Elemental mappings of the layers’ cross sections suggest an inhomogeneous phase distribution in the laser sintered LSD samples with a slight alumina concentration gradient. A lower degree of particle melting in the bottom region of the layers is plausible due to attenuation of the laser beam intensity. SEM and HRTEM micrographs show that after a few seconds of laser treatment relictic starting phase, crystalline alumina plus amorphous silica, occur together with needle like mullite, the latter formed within an amorphous aluminosilicate phase. The resulting phase assemblage reflects the non-equilibrium conditions which can be expected for short time laser treatments. Mullite nucleation within the bulk of the liquid phase rather than in the vicinity of the parent alumina phase suggests that dissolution of alumina is the rate controlling step. Subsequent thermal post treatment in air in a conventional sintering furnace causes an increase of density to about 96% and leads to additional phase reactions. Amorphous silica transforms into cristobalite and the amount of alumina is reduced by additional mullite formation. By both coalescence of individual crystals and grain growth the morphology of the newly formed mullite changes during post heat treatment.     

Green photoluminescence of Tb3+ and Ce3+ doped mullite nanoparticles

Mullite have been actively surveyed, but as luminescent host materials, lanthanide doped mullite receive little attention. In this work, terbium and cerium co-doped mullite nanoparticles have been synthesized by co-precipitation process. These as-prepared mullite nanocrystals were characterized by transmission electron microscopy. The results showed that these nanoparticles are roughly spherical in shape and vary in size from 30 to 45 nm. The phase structures of these nanoparticles were investigated by X-ray diffraction. The results indicated that these nanoparticles crystallized well, and the peak positions and intensities agree well with the data for bulk mullite. Their fluorescence properties have been explored in detail and the resulting Al₆Si₂O₁₃: 8%Tb,0.1%Ce nanoparticles showed the typical emission bands centered at 490, 542 and 590 nm.     

Effect of sillimanite beach sand composition on mullitization and properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> system

Mullite was developed by reaction sintering of sillimanite beach sand and calcined alumina. Two varieties of sillimanite beach sand viz. S and Z having different compositions were selected. Synthesis and properties of mullite were very much dependent on the sillimanite beach sand composition. Presence of higher amount of impurities in the Z-variety of sillimanite sand favours the densification by liquid phase formation. Presence of zircon in Z-variety increases the hardness and fracture toughness. Alumina addition improves the mechanical/thermomechanical properties of the samples. Mullite retains the usual orthorhombic habit of sillimanite. Rounded to sub rounded zirconia dispersed within the mullite matrix of the sample ZA is noticed.     

Synthesis and Processing of Doped Hg1Ba2Ca2Cu3Oy Superconductors

A series of quenching experiments were conducted to understand the sequence of reactions that occur during the synthesis of doped Hg1223, (Hg, A)Ba₂Ca₂Cu₃O _y , A = Re, Bi, and Pb (HgA1223). The formation and decomposition of the intermediate phases during the high-temperature reaction were followed as a function of temperature. HgA1223 phase forms over a wide range of temperatures, 750–950°C, 750–880°C, and 840–880°C for A = Re, Pb, and Bi, respectively. At T<750°C, HgA1212 phase forms for A = Re and Pb. Based on the results of quenching experiments, heat treatment conditions were optimized for the synthesis of pure HgA1223 phase using commercial BaCaCuO precursor powders. A reduced-temperature annealing stage after the high-temperature reaction helps in grain growth and improves the microstructural characteristics of HgA1223 samples. Control of Hg pressure during the reaction is crucial for achieving phase purity, grain growth, and texture in the final products. A novel approach for the control of Hg pressure during the synthesis of HgA1223, which consists of using CaHgO₂ as an external Hg source, is reported. HgA1223 samples synthesized using the new synthesis protocol exhibit improved microstructural and superconducting properties.     

Low temperature needle like mullite grain formation in sol-gel precursors coated on SiC porous substrates

Dense mullite coating having thickness in the range of 3 to 5 μm was produced from sol-gel mullite precursor coated on SiC porous substrates at heat treatment temperatures as low as 1300 °C. Mullite formed in the coating layer was characterised by X-ray diffraction. The precursors have an average particle size of 170 nm and the mullite formed in the coating in situ has a grain size of 3-5 μm. Mullite grains formed on the SiC have needle like morphology. The mullite formation has been explained on the basis of reaction between the silica-alumina nano precursor and the needle like morphology has been similar to that formed from a liquid phase. The gas permeation analysis shows that there is considerable difference between gas pressure while using SiC substrate before and after coating and hence clearly indicated reduction in pore size. This particular approach is good since usual mullite formation is at high temperature and is difficult to attain small grain size. Further, in situ formed mullite, in this investigation covers the SiC surface protecting the SiC from oxidation at high temperature.     

Improvement of alkali corrosion resistance of mullite ceramics at high temperature by depositing Ca0.3Mg0.2Zr2(PO4)3 coating

Ca_0.3Mg_0.2Zr₂(PO₄)₃ coating was deposited on the mullite ceramic to improve its alkali corrosion resistance at high temperatures, using sol–gel method and dip-coating technique. The phase composition and microstructure of the coating were characterized by X-ray diffraction and scanning electron microscopy (SEM). Results show that homogeneous, dense and single-phase Ca_0.3Mg_0.2Zr₂(PO₄)₃ coating was successfully deposited on mullite ceramics. SEM microstructural examination revealed the excellent bonding between Ca_0.3Mg_0.2Zr₂(PO₄)₃ coating and mullite ceramics. The effectiveness of the prepared coating to improve the alkali corrosion resistance of mullite ceramics was assessed through the measurements of mass loss and flexural strength degradation after 96 h and longer exposure time at alkali corrosion condition at 1000 °C. A significant enhancement of the alkali corrosion resistance for Ca_0.3Mg_0.2Zr₂(PO₄)₃-coated mullite samples was observed. Therefore, the effectiveness of the Ca_0.3Mg_0.2Zr₂(PO₄)₃ material as protection coating for mullite ceramic is confirmed.     

Mechanochemically induced amorphous/crystalline phase transition in the Bi4Ti3O12 compound

Concurrent milling of Bi₄Ti₃O₁₂ was carried out in a planetary ball mill with hardened-steel 13 or 6 mm diameter balls with a goal to investigate how the system responds to the constrains imposed by milling. Milling intensity for various milling parameters was derived from the electrical power measurement. The reverse amorphous ↔ crystalline phase transition induced by milling is governed by impact energy and frequency. Below a certain threshold value of impact energy crystallization does not occur, whereas a higher impact frequency accelerates amorphization. Mechanochemical reactions are predominantly discrete processes, which occur at the moment of impact.     

High-pressure phase transition in Ho2O3

High-pressure X-ray diffraction and Raman studies on holmium sesquioxide (Ho₂O₃) have been carried out up to a pressure of ∼17 GPa in a diamond-anvil cell at room temperature. Holmium oxide, which has a cubic or bixbyite structure under ambient conditions, undergoes an irreversible structural phase transition at around 9.5 GPa. The high-pressure phase has been identified to be low symmetry monoclinic type. The two phases coexist to up to about 16 GPa, above which the parent phase disappears. The high-pressure laser-Raman studies have revealed that the prominent Raman band ∼370 cm⁻¹ disappears around the similar transition pressure. The bulk modulus of the parent phase is reported.     

Phase formation and texture development in mullite/zirconia composites fabricated by templated grain growth

Mullite is a promising candidate for advanced ceramic applications but its low fracture toughness and difficulties in sintering are the main limitations for more widespread industrial applications. Therefore, mullite/zirconia composites were prepared from a reactive mixture of alumina and zircon powders. Additives, TiO₂ and MgO, were used to modify aluminosilicate glass to increase densification and <001> aluminum borate templates were incorporated to texture mullite in [001] by templated grain growth. Mullite/zirconia phase formation was complete at 1450°C in the presence of both templates and additives, as compared to 1500°C for the samples with only additives and to 1600°C for the samples with only templates. Dense mullite/zirconia composites with highly <001>-textured mullite grains (Lotgering factor ∼1) and a retention of ∼13% tetragonal ZrO₂ were fabricated after sintering at 1450°C for 2 h. A high quality of mullite texture with a degree of orientation parameter of 0.22 and a narrow distribution of elongated mullite grains within 8.8° around [001] were successfully obtained in the composites.     

Kaolinite-mullite reaction series: a TEM study

The kaolinite-mullite reaction series in single crystal kaolinite has been characterized by transmission electron microscopy. The exotherm observed at 980° C is attributed to the formation of a spinel phase. Mullite crystallites have also been observed with the spinel phase and both phases have a composition near that of 32 mullite. Subsequent heat treatment leads to the growth of mullite crystallites on the original kaolinite plates with the c-axis perpendicular to the plate.     

莫来石含量对钙长石/莫来石复相多孔陶瓷组织结构与性能的影响

以CaCO₃、SiO₂、α-Al₂O₃为原料, 采用泡沫注凝法制备了不同莫来石含量的钙长石/莫来石复相多孔陶瓷, 研究了莫来石含量对复相多孔陶瓷的体积密度、气孔率、抗压强度、热导率及微观组织和结构的影响. 结果表明, 莫来石含量对气孔率有很大的影响, 烧结过程中液相出现引起的收缩是气孔率下降的主要原因; 在气孔率相近的情况下, 莫来石含量较高试样的抗压强度和热导率也较高, 致密的孔壁、长柱状的莫来石晶粒使得复相多孔陶瓷的抗压强度提高. 所制备的钙长石/莫来石复相多孔陶瓷的开口气孔率介于60.8%~75.2%, 抗压强度为12.94~36.95 MPa, 热导率为0.30~1.33 W/(m·K).     

砷化镓单晶中微晶和非晶的形成机制

利用高分辨电子显微镜对0.0049N和0.049N荷载Vickers压痕锈发砷化镓单晶的相转变进行了观察和研究,结果表明,在大小压痕作用下分别发生了单晶向和微晶的转变,微晶的结构由小于10nm,取向各异的纳米晶和非晶组成,在完全非晶化的结构中存在少量由几个原子组成的原子簇,在非晶和晶体的交界区能观察到许多晶体缺陷以及沿这些缺陷产生的晶格扭曲和非晶相岛,对这种非晶化现象提出了两种可能的诱发机制,高压力诱导非晶化和剪切诱导非晶化。     

PbSbBiS4-Sb2S3 and PbSbBiS4-Bi2S3 joins in the PbS-Sb2S3-Bi2S3 system

Phase equilibria along the PbSbBiS₄-Sb₂S₃ and PbSbBiS₄-Bi₂S₃ joins of the PbS-Sb₂S₃-Bi₂S₃ system have been studied for the first time using differential thermal analysis, X-ray diffraction, microstructural analysis, microhardness tests, and density measurements, and the phase diagrams of the joins have been mapped out. The joins are shown to be pseudobinary with limited series of terminal solid solutions. The solid solutions are p-type semiconductors.     

Nanocrystallization of ferroelectric lithium niobate in LiNbO3-SiO2 glasses

Transparent glasses in the (100-x)LiNbO₃-xSiO₂ system where 20 ≤ × ≤ 35, were produced by conventional melt-quenching technique. The quenched samples were amorphous as proved by X-ray diffraction (XRD) technique. Annealing of the quenched samples at temperatures ranging from 580 to 975 °C resulted in the precipitation of lithium niobate nano-crystals. Scanning electron microscopy (SEM) showed the presence of randomly oriented LiNbO₃ nano-crystals dispersed in a continuous glass matrix. The relative dielectric constant (ε_r) was in the range of 80 to 180 and increased with increasing LiNbO₃ concentration. The glass ceramic samples annealed at temperatures up to 600 °C are fully transparent.     

Transparent glass ceramic containing Er:CaF2 nano-crystals prepared by sol-gel method

Er³⁺ doped SiO₂-CaF₂ transparent glass ceramic was prepared by sol-gel method. The microstructural evolution of the samples was studied with X-ray diffraction (XRD), transmission electron microscopy (TEM), and infrared spectra (IR). After heat-treatment at 900 °C, the Si-OH bonds and other organic groups were basically eliminated. The CaF₂ crystallites in the sample heat-treated at 900 °C are 10-20 nm in size, distributed homogeneously among the amorphous silica matrix. The efficient upconversion emission for Er^3+.4F_9/2 → ⁴I_15/2 transition was recorded under 980 nm excitation, which could be ascribed to the incorporation of Er³⁺ ions into the CaF₂ nano-crystals with low phonon energy.     

Phase transition in V3O5

A phase transition at 420°K previously unreported, has been discovered in V₃O₅. The transition is characterized by an endothermic DTA peak on heating and an abrupt change in the electrical resistivity at the same temperature. Results from X-ray analysis of the V₃O₅ samples exhibiting the transition agree very well with the values reported by Andersson (1). Powder diffraction analysis of the high temperature phase indicates that there is a small change in the cell volume but the crystal apparently conserves its symmetry through the transition.     

Study of transition metal ion doped mullite by positron annihilation techniques

Mullite, an extremely useful ceramic material, is doped with transition metal ions. The changes in the electronic properties of these doped materials have been studied by positron annihilation lifetime spectroscopy as well as Doppler broadened line shape analysis. The results on the positron annihilation parameters are characteristic of ionic size, oxidation state and the d-electron configuration of the respective transition metals doped in the parent lattice of the mullite. These results, along with the resistivity measurements are suggestive of transition of the parent mullite from an insulator to a semimetal in the modified structure.     

Anisotropic mullitization in CuO-doped oxide mixture activated by high-energy ball milling

Mullite phase formation and grain growth, in a CuO-doped Al₂O₃ and silica mixture, has been investigated. The oxide mixture was activated with both stainless steel and tungsten carbide milling media. The milled powders demonstrated a much lower mullite formation temperature when compared to the conventional solid-state reaction process. Anisotropic grain growth was observed in the powders milled with stainless steel media, while well-shaped mullite whiskers were produced in the WC milled samples. The lowered mullitization temperature together with the anisotropic grain growth behavior was attributed to the refined microstructure of the powders as a result of the high-energy ball milling process.     

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.