Isomorphism Between Sillimanite and Mullite

It is concluded that a continuous isomorphous series can be expected between sillimanite and 2/1 mullite. The formation of individual members of this series depends on the temperature and pressure conditions used. At 1 atm pressure the members between 3/2 and 2/1 mullite are stable; 2/1 mullite is not a structurally distinct mineral, so the separate name praguite for it is unnecessary.     

Solid Solution Range and Microstructures of Melt-Grown Mullite

The solid solution range of melt-grown mullite was examined by crystal-chemical methods. The maximum Al₂O₃ content as determined by EDX was ∼83.6 wt%, 75 mol%, or the nominal composition 3Al₂O₃.SiO₂. For samples of overall composition 81 to 83 wt% Al₂O₃, extra lines indicating crystallographic superstructure appeared in Guinier X-ray patterns. The corresponding TEM microstructure consisted of a mullite matrix finely twinned on (001), the twins being 0.02 to 0.10 μm wide, with oriented exsolution of α-A1₂O₃, often twinned, also being present. The analogy between mullite superstructure and that of plagioclase feldspars, as well as the relevance of these findings to the SiO₂-Al₂O₃ metastable phase equilibria are discussed.     

Formation and Densification Behavior of Mullite Aggregates from Beach Sand Sillimanite

Dense mullite aggregates with 60% and 70% Al₂O₃ have been prepared from precursor mixtures consisting of beach sand sillimanite and a high-purity aluminum hydroxide following conventional single- and double-stage firing processes. The bulk density (BD), apparent porosity (AP), and water absorption (WA) capacity of sintered mullite aggregates were found to be strongly influenced by the premullitization step of this precursor mixture. Mullite aggregates formed in a double-stage firing process exhibited higher BD and mullite content and lower AP and WA capacity in comparison with those obtained by the single-stage firing process. The values of coefficient of thermal expansion of sintered mullite aggregates are close to those found in the literature reports for high-purity stoichiometric mullite.     

Structural Control of Elastic Constants of Mullite in Comparison to Sillimanite

Nine elastic stiffness coefficients, c_ij, of a mullite single crystal (2Al₂O₃·SiO₂) are measured using acoustic resonance spectroscopy. The obtained values are similar to those of the structurally related aluminosilicate phase sillimanite (Al₂O₃·SiO₂). Characteristic elastic properties of the two minerals are interpreted with the help of their crystal structures and atomic force constants for sillimanite. The high longitudinal stiffness coefficients, c₃₃, of mullite (∼352 GPa) and sillimanite (∼388 GPa) are caused by continuous “stiff” load-bearing tetrahedral chains parallel to c-axis, while the “soft” octahedral chains have minor direct influence. They stabilize the tetrahedal chains against tilting. The lower c₃₃ value of mullite in comparison to the sillimanite value may be caused by a weakening of the load-bearing tetrahedral chains which are parallel to c-axis because of partial replacement of silicon by the weaker-bonded aluminum. The longitudinal stiffness coefficients perpendicular to c-axis are significantly lower, because of sequences of alternating “soft” octahedral and “stiff” tetrahedral units. Within the plane (001), the compliant octahedra exhibit stiffness-controlling influence with coefficients parallel to b-axis (c₂₂∼ 233 GPa) being somewhat lower than parallel to a-axis (c₁₁∼ 291 GPa). This is explained with the occurrence of compliant octahedral Al(1)–O(D) bonds, which are more effective parallel to b-axis rather than to a-axis. Because octahedra are unaffected by the aluminum to silicon substitution, c₁₁ and c₂₂ coefficients of mullite and sillimanite are very similar. Shear stiffness coefficients of mullite increase from c₅₅ (∼77 GPa) to c₆₆ (∼80 GPa) to c₄₄ (∼110 GPa), indicating increasing resistance against shear deformation within the planes (010), (001), and (100). The lattice plane of the highest shear stiffness (100) is built up of an oxygen-oxygen network, diagonally braced along 〈011〉 (“Jägerzaun”). This network with short oxygen–oxygen distances can be sheared by compression and elongation along oxygen–oxygen interaction lines only which is rather unlikely. Because of the lack of such networks in the planes (010) and (001), bending and deformation of structural units become easier, and consequently c₅₅ and c₆₆ are <c₄₄. All three shear stiffness coefficients of mullite are slightly lower than those of sillimanite because of the reduction of the mean tetrahedral bond strength in mullite caused by partial substitution of silicon by aluminum.     

Solid Solutions of Silicon Oxide in Mullite

An explanation is proposed to account for the possibility of formation of solid solutions of silicon oxide in mullite. The difference in the diffusion coefficients of cations in complex oxides leads to the formation of a solid solution of the oxide containing the cation with the lower diffusion coefficient in the complex oxide and a phase containing the cation with the higher diffusion coefficient or rich in this oxide. Using yttrium or scandium additives, which are chemisorbed on the surface of aerosil (SiO₂ source), it is possible to make the rate of diffusion mass transfer of aluminum cations higher than that of silicon cations and thus obtain a solid solution of silicon oxide in mullite.     

Polymorphism of Tricalcium Silicate and Its Solid Solutions

In tricalcium silicate and its solid solutions six modifications were established: one rhombohedral ( R ), two monoclinic ( M _I, M _II), and three triclinic ( T _I, T _II, T _III). All these forms are rhombohedral or pseudorhombohedral and extremely similar; the variations in the X-ray diagrams and the transformation enthalpies are minute. Only three of the five transitions were observed by both X rays and DTA: T _I- T _II (600°), T _III– M _I (980°), and M _I– M _II (990°); one was established by DTA ( T _II– T _III, 920°) and one by X rays only ( M _II– R , 1050°C). This scheme of polymorphism is compared critically with results of other authors, and the nature of the various displacive transformations is discussed. X-ray and DTA investigations of solid solutions of Ca₃SiO₅ with Al₂O₃ and ZnO established the phase relations as functions of temperature and composition. With Al₂O₃, two modifications ( T _I and T _II) were stabilized; and with ZnO, five polymorphs ( T _I T _II, M_I, M _II, and R ). With both pure C₃S and the solid solutions, the results are consistent and have a close bearing on the constitution of alites in portland cement clinker.     

Synthesis and Structural Characterization by X-ray Absorption Spectroscopy of Tin-Doped Mullite Solid Solutions

The formation of solid solutions between tin cations and mullite by calcination at 1400°C of amorphous precursors prepared by pyrolysis of aerosols is reported. The oxidation state of the tin cations and the position that they occupy in the mullite structure have been analyzed using XAS (XANES and EXAFS) spectroscopy, which shows that the tetravalent tin cations are located at the octahedral positions of the Al³⁺ ions, which induces cell expansion. The limit of tin incorporation under the experimental conditions reported here correspond to a tin/mullite mole ratio of ∼5%, which is within the range previously reported for other tetravalent cations (4%–6%).     

Solid Solubility of Na2O in Mullite

Incorporation of up to 0.4 wt% Na₂O was found in the mullites of fused-cast mullite brick. Sodium oxide incorporation probably requires very high temperatures and persists only when the material is quenched rapidly to room temperature; it may be stabilized by simultaneous entry ofZr⁴⁺ into the mullite structure.     

Cyclic Infiltration of Porous Zirconia Preforms with a Liquid Solution of Mullite Precursor

Composites of mullite and zirconia were fabricated via the cyclic infiltration of porous zirconia-based preforms with a liquid mullite precursor. The maximal amount of mullite precursor that could be infiltrated was dependent primarily on the initial open porosity of the preforms. When a zirconia preform with an initial open porosity of ∼58% was cyclically infiltrated to saturation, the open porosity was reduced to ∼43%, with a median pore diameter of 15 nm. After sintering at a temperature of 1500°C for 2 h, the saturation-infiltrated zirconia preforms could be densified to ∼98% of the theoretical density. In zirconia samples, infiltrated mullite had a tendency to coalesce into large, elongated grains as the sintering temperature was increased. The presence of infiltrated mullite did not have a significant effect on the zirconia grain structure. The distribution of mullite in the samples was nonuniform, and the distribution profiles varied as the number of infiltration cycles varied. Although the sintered density and hardness showed small improvements after saturation infiltration, the fracture toughness did not increase.     

关于固溶体生成的讨论

讨论了不同类型固溶体生成的影响因素,为判断固溶体类型提供了依据。     

Solid Solution of Boron in Graphite

The solubility of boron in graphite was determined in the temperature interval of 1800° to 2500°C. The maximum solubility was 2.35 at. % at 2350°C; the solution was shown to be substitutional.     

Tape Casting and Properties of Mullite and Zirconia–Mullite Ceramics

Mullite and ZrO₂-mullite ceramics have been prepared by tape casting mixtures of Al₂O₃, quartz, and ZrO₂ powders and subsequent reaction sintering. Tape casting leads to homogeneous, high-density green materials with good sinterability. The design of a thermal cycle which favors densification with respect to mullitization allows the preparation of nearly dense, nearly fully reacted materials at sintering temperatures below 1600°C. ZrO₂ additions limit grain growth, but the ZrO₂ content must not be too high when a high tetragonal:monoclinic ratio is required.     

Gas Recirculation at the Hydrogen Electrode of Solid Oxide Fuel Cell and Solid Oxide Electrolysis Cell Systems

In this study it is theoretically analyzed how flue gas recirculation at the hydrogen electrode of solid oxide cells (SOC) systems effects fuel utilization and carbon formation. Interdepence between cell fuel utilization, system fuel utilization and gas recirculation is investigated numerically. Tendency towards carbon deposits is evaluated via thermodynamic equilibrium calculations. It is quantified which gas recirculation rates are necessary to achieve high values of system fuel utilization even if the cell fuel utilization is kept at a moderate level. Furthermore, tendency towards carbon deposition strongly depends on temperature, pressure and feed gas composition and can be reduced by adequate recirculation rates. The presented results can be used for the configuration of gas recirculation in SOC systems.     

Microstructure, Microchemistry, and Flexural Strength of Mullite Ceramics

The microstructure of mullite ceramics hot-pressed and sintered at different temperatures was studied using transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) with EDS, and electron probe microanalysis (EPMA). The specimens, consisting of stoichiometric mullite grains without glassy phase, are obtained by hot-pressing stoichiometric mullite powder at 1575°C for 1 h. Silica-rich glassy phases are observed using TEM at three-grain junctions of mullite grains in specimens heated at and above 1600°C. However, high-resolution transmission electron micrographs show no glassy phase at two-grain boundaries in all specimens. SEM with EDS analyses show that the average value of Al₂O₃ contents of mullite grains increases slightly with increasing temperature. These results are consistent with a published Al₂O₃–SiO₂ phase diagram. The flexural strength of mullite specimens at room temperature depends on their microstructure, such as the grain size and grain size distribution of mullite grains. The strength is high at room temperature and up to 1200°C, and it decreases at and above 1350°C, irrespective of the presence of the glassy phase.     

Mullite for Structural, Electronic, and Optical Applications

Mullite (3Al₂O₃·2SiO₂) is becoming increasingly important in electronic, optical, and high-temperature structural applications. This paper reviews the current state of mullite-related research at a fundamental level, within the framework of phase equilibria, crystal structure, synthesis, processing, and properties. Phase equilibria are discussed in terms of the problems associated with the nucleation kinetics of mullite and the large variations observed in the solid-solution range. The incongruent melting behavior of mullite is now widely accepted. Large variations in the solid solubility from 58 to 76 mol% alumina are related to the ordering/disordering of oxygen vacancies and are strongly coupled with the method of synthesis used to form mullite. Similarly, reaction sequences which lead to the formation of mullite upon heating depend on the spatial scale at which the components are mixed. Mixing at the atomic level is useful for low-temperature (<1000°C) synthesis of mullite but not for low-temperature sintering. In contrast, precursors that are segregated are better suited for low-temperature (1250° to 1500°C) densification through viscous deformation. Flexural strength and creep resistance at elevated temperatures are significantly affected by the presence of glassy boundary inclusions; in the absence of glassy inclusions, polycrystalline mullite retains >90% of its room-temperature strength to 1500°C and displays very high creep resistance. Because of its low dielectric constant, mullite has now emerged as a substrate material in high-performance packaging applications. Interest in optical applications mainly centers on its applicability as a window material within the mid-infrared range.     

Synthesis,Characterization, and Consolidation of Cr2(C,N) Solid Solution Powders

Single‐phase Cr₂(C,N) powders, which are mainly composed of large worm‐like particles with the diameter of 1–3 μm, have been successfully synthesized by carbothermal reduction nitridation (CRN) method at 1200°C for 1 h in N₂ atmosphere for the first time. The phase evolution and morphology were investigated using XRD, TG‐DSC, SEM, and TEM techniques. The reactions during the synthesis process were proposed and analyzed. The surface of Cr₂(C,N) determined by XPS is mainly composed of four elements, Cr, N, C, and O. Furthermore, the attempts to consolidate Cr₂(C,N) powders were carried out using spark plasma sintering (SPS), and the fully densified bulk ceramics were obtained at 1200°C. The effect of sintering temperature on phase composition and mechanical properties were also discussed.     

硅线石材料的预处理及其莫来石化行为

研究了硅线石材料的预处理及其莫来石化行为。结果表明：本文设计的少污染、无需专用设备的酸洗工艺能满足除杂的要求。采用搅拌球磨的方法获得的微细硅线石粉料有利于莫来石的转化;借助于X-射线衍射图详细地分析了三种温度下硅线石的莫来石化行为,在低于理论温虔1545℃的情况下,已有部分莫来石生成,在1650℃×4h的条件下,可彻底地转化为莫来石和玻璃相,并用HF萃取法测定这一条件下的莫来石转化率。最后,通过合适工艺获得了平均粒径为2.8μm的合成莫来石粉料。     

固体氧化物燃料电池系统及其应用

对固体氧化物燃料电池系统和单体电池及其工作原理、材料组成等作了简要介绍,并介绍了固体氧化物燃料电池在电厂混合发电方面与燃气轮机组成的联合系统技术以及以天然气为燃料家庭热电联产方面的应用。并指出固体氧化物燃料电池由于其高效、环保清洁将是未来能源利用的主要方式。     

高温环境下碳硫硅钙石-钙矾石固溶体的研究

由碳硫硅钙石引起的硫酸盐侵蚀破坏在许多国家都有相关工程案例.铝相可以加速碳硫硅钙石的生成,并能够与钙矾石形成固溶体,但铝相的作用机理目前尚无定论.在不同温度下合成了一系列固溶体,并利用XRD,SEM-EDS及红外对固溶体进行了测试分析,结果表明:40℃温度下亦可以生成碳硫硅钙石/固溶体;20℃与40℃均能够形成碳硫硅钙石型与钙矾石型两种形态固溶体;a轴固溶体间隙随温度升高而增大,20℃环境下固溶体间隙为1.1076～1.1182nm,40℃时固溶体间隙为1.1069～1.1189nm;随温度升高碳硫硅钙石-钙矾石固溶体晶体形貌更加细长.