Mullite has achieved outstanding importance as a material for both traditional and advanced ceramics because of its favourable thermal and mechanical properties. Mullite displays various Al to Si ratios referring to the solid solution Al 4+2 x Si 2−2 x O 10− x , with x ranging between about 0.2 and 0.9 (about 55 to 90 mol% Al 2 O 3 ). Depending on the synthesis temperature and atmosphere mullite is able to incorporate a number of transition metal cations and other foreign atoms. The crystal structure of mullite is closely related to that of sillimanite, which is characterized by chains of edge-connected AlO 6 octahedra running parallel to the crystallographic c -axis. These very stiff chains are cross-linked by tetrahedral chains consisting of (Al,Si)O 4 tetrahedra. In more detail: Parallel to a the tetrahedra are linked to the relatively short more stiff Al–O(A, B) bonds, whereas parallel b they are linked parallel to the relatively long more compliant Al–O(D) bonds. In mullite some of the oxygen atoms bridging the tetrahedra are removed for charge compensation. This gives rise to the formation of oxygen vacancies and of T 3 O groups (so-called tetrahedral triclusters). The anisotropy of the bonding system of mullite has a major influence on the anisotropy of its physical properties. For example: • the highest longitudinal elastic stiffness is observed parallel c , but lower ones parallel a and especially parallel b , • the maximum of the thermal conductivity occurs parallel c , but maller ones parallel a and especially parallel b , • large thermal expansion especially parallel b , • fastest crystal growth and highest corrosion parallel c . Heat capacity and thermal expansion measurements of mullite display reversible anomalies in the temperature range between about 1000 and 1200 °C. It is believed that tetrahedral cations, bridging O atoms, and O vacancies undergo dynamical site exchange processes at high temperatures. At lower temperatures the dynamic disorder may transform to a static one. Diffraction experiments revealed that also partially ordered states may exist.

Structure and properties of mullite—A review

Transparent ceramics: Processing, materials and applications

Solid Li-ion electrolytes used in all-solid-state lithium-ion batteries (LIBs) are being considered to replace conventional liquid electrolytes that have leakage, flammability, and poor chemical stability issues, which represents one major challenge and opportunity for next-generation high-energy-density batteries. However, the low mobility of lithium ions in solid electrolytes limits their practical applications. Here, we report a solid composite polymer electrolyte with Y2O3-doped ZrO2 (YSZ) nanowires that are enriched with positive-charged oxygen vacancies. The morphologies and ionic conductivities have been studied systemically according to concentration of Y2O3 dopant in the nanowires. In comparison to the conventional filler-free electrolyte with a conductivity of 3.62 × 10–7 S cm–1, the composite polymer electrolytes with the YSZ nanowires show much higher ionic conductivity. It indicates that incorporation of 7 mol % of Y2O3-doped ZrO2 nanowires results in the highest ionic conductivity of 1.07 × ...

Improved Lithium Ionic Conductivity in Composite Polymer Electrolytes with Oxide-Ion Conducting Nanowires

This paper presents a study on the thermal properties of a range of geopolymers in order to assess their suitability for high temperature applications such as thermal barriers, refractories and fire resistant structural members. Geopolymers were synthesised from five different fly ashes using sodium silicate and sodium aluminate solutions to achieve a set range of Si:Al compositional ratios. The thermo-physical, mechanical and microstructural properties of the geopolymers are presented and the effect of the source fly ash characteristics on the hardened product is discussed, as well as implications for high temperature applications. The amount and composition of the amorphous component (glass) of each of the fly ashes was determined by combining XRD and XRF results. It was found that the Si:Al ratio in the glass of the fly ashes strongly influenced the thermal performance of the geopolymers. Geopolymers synthesised from fly ashes with a high Si:Al (≥ 5) in the glass exhibited compressive strength gains and greater dimensional stability upon exposure to 1000 °C, whereas geopolymers synthesised from fly ashes with low Si:Al (

Thermal analysis of geopolymer pastes synthesised from five fly ashes of variable composition

Transparent ceramics have various potential applications such as infrared (IR) windows/domes, lamp envelopes, opto-electric components/devices, composite armors, and screens for smartphones and they can be used as host materials for solid-state lasers. Transparent ceramics were initially developed to replace single crystals because of their simple processing route, variability in composition, high yield productivity, and shape control, among other factors. Optical transparency is one of the most important properties of transparent ceramics. In order to achieve transparency, ceramics must have highly symmetric crystal structures; therefore, the majority of the transparent ceramics have cubic structures, while tetragonal and hexagonal structures have also been reported in the open literature. Moreover, the optical transparency of ceramics is determined by their purity and density; the production of high-purity ceramics requires high-purity starting materials, and the production of high-density ceramics requires sophisticated sintering techniques and optimized sintering aids. Furthermore, specific mechanical properties are required for some applications, such as window materials and composite armor. This review aims to summarize recent progress in the fabrication and application of various transparent ceramics.

Materials development and potential applications of transparent ceramics: A review

Elastic constants and their temperature derivatives of orthorhombic
2/1-mullite single crystals were determined using resonant
ultrasound spectroscopy between ambient temperature
and 14001C in air. The values at room temperature given
in GPa were c11 = 279.5(4), c22 = 234.9(2), c33 = 360.6(3),
c44 = 109.49(7), c55 = 74.94(4), c66 = 79.89(4), c12 = 103.1(2),
c13 = 96.1(3), and c23 = 135.6(1). The high temperature ultrasound
data sets were corrected for thermal expansion effects
using the coefficients, which were measured in the range -170
to 1400°C by means of dilatometry. Up to about 1000°C, the
temperature evolution of the elastic stiffnesses was linear and the
corresponding thermoelastic constants Tij=d log cij/dT given in
10 exp-6 °C exp-1 were T11=-100(2), T22=-126(2), T33=-78.2(6),
T44=-75.3(7), T55=-81.3(9), T66=-113(1), T12=-153(3),
T13=-94(4), and T23=-47(2). While the temperature derivatives
of the longitudinal stiffnesses and the transverse interaction
coefficients remained linear, the shear resistances c44, c55,
and c66 displayed an increased softening above about 1000°C,
which was accompanied by a rapidly increasing ultrasound dissipation.
In the same temperature region, small, but clearly recognizable
discontinuities in the longitudinal and in the volume
thermal expansion coefficients occurred. Taking into account the
behavior of the heat capacity, the observed anomalies may indicate
a glass-like transition of mullite above about 1000°C. An
unexpected peak in the ultrasound dissipation at about 70°C was
most likely caused by anelastic point defect relaxations.

Elastic Properties of Mullite Single Crystals up to 1400°C

The heat capacity at constant pressure (Cp) of mullite (single crystalline and dense and powdered polycrystalline) was determined between −125° and 1400°C. Measurements were performed in three laboratories using two different disk-type differential scanning calorimeters (DLR and Netzsch) and one cylinder-type (Setaram). On the basis of these experimental data a new master curve Cp (T) is presented. Up to about 1100°C the Cp curve agrees well with those from previously published results: The Cp master curve displays a parabolic shape with Cp values of about 0.415 J/(g·K) at −125°C, 0.78 J/(g·K) at 25°C, and 1.25 J/(g·K) at 1000°C. At temperatures above 1100°C a yet unknown, steplike, weak Cp increase is observed with ΔC°p step values ranging between 0.03 and 0.10 J/(g·K). This Cp anomaly of mullite is reproducible under various heating conditions and is reversible both on heating and on cooling. The extrapolated onset temperature of the anomaly (Ton.) is controlled by the crystalline state of the samples (single crystal, Ton. above 1100°C; polycrystal, Ton. above 1250°C) but does not depend on the composition of mullite. Sillimanite, which is structurally closely related to mullite, displays no Cp anomaly. The absence of latent heat (ΔH= 0) and the shape of the Cp anomaly of mullite are comparable to the behavior of glass-forming materials at the glass transition. The observations can best be interpreted in terms of an onset of ion-jumping between adjacent structural sites T and T* and O(C) and O(C)*. Other interpretations, i.e., clockwise and counterclockwise rotations of structural units, or minor decomposition of mullite to α-Al2O3 plus SiO2 glass observed at the surface of the single crystals, are less probable.

Heat Capacity of Mullite - New Data and Evidence for a High-Temperature Phase Transformation

Nine elastic stiffness coefficients, cij, 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〉 (“Jagerzaun”). 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.

Structural Control of Elastic Constants of Mullite in Comparison to Sillimanite

Yttria stabilized zirconia (YSZ) layers, suited for thermal barrier coatings (TBCs) in turbine engines, were produced by electron-beam physical vapour deposition (EB-PVD) on metallic (Ni-based alloys) ceramic (Al 2 O 3 ) substrates (typically at 1000 °C) using ingot compositions of about 4 mol% Y 2 O 3 –ZrO 2 . Employing powdered samples, removed from the surface of the as-deposited YSZ layers, precision X-ray diffraction data sets were recorded between 60 and 900 °C and the lattice parameters of the metastable, tetragonal (t′) and the cubic (c) phases were refined using Rietveld's method. It was unambiguously verified that phase (c) was present in all investigated samples as a minority phase already in the as-deposited state. The thermal expansion coefficients of the tetragonal phase (t′) ( α 11  = 9.3(2) × 10 −6  K −1 , α 33  = 10.8(1) × 10 −6  K −1 ) and the cubic phase (c) ( α 11  = 8.5(2) × 10 −6  K −1 ) were evaluated from the refined temperature dependent lattice parameter values of this study, and turned out to be close to each other, but significantly different from the coefficient of the monoclinic phase (m) ( α 11  = 9.0 × 10 −6  K −1 , α 22  = 1.2 × 10 −6  K −1 , α 33  = 11.9 × 10 −6  K −1 , α 13  = 0.0 × 10 −6  K −1 ), which was derived from temperature dependent lattice parameter measurements published by Touloukian et al. 5 The expansion coefficient of the monoclinic phase (m) exhibited a very pronounced anisotropy in contrast to the tetragonal phases (t′) and (t). Our values for (t′) and (c) were in excellent agreement with that of other tetragonal and cubic phases from the literature with quite different Y 2 O 3 contents.

Bernd Hildmann

Papers

Structure and properties of mullite—A review

Elastic Properties of Mullite Single Crystals up to 1400°C

Heat Capacity of Mullite - New Data and Evidence for a High-Temperature Phase Transformation

Structural Control of Elastic Constants of Mullite in Comparison to Sillimanite

Thermal expansion of EB-PVD yttria stabilized zirconia