Phase Transformations in Dicalcium Silicate: II, TEM Studies of Crystallography, Microstructure, and Mechanisms

The crystallography, microstructures, and phase transformation mechanisms in dicalcium silicate (Ca₂SiO₄) were studied by TEM. Three types of superlattice structures were observed in the α'_L and β phases. Almost all β grains were twinned and strained. Symmetry-related domain structures inherited from previous high-temperature transformations were observed in β grains. Both the α→α'_H and α'_L→β transformations were considered to be ferroelastic, and spontaneous strains were calculated. In terms of the crystal structures, the major driving force for the β→γ transformation is proposed to be strains and cation charge repulsions in the β structure. This mechanism can be displacive, but it needs to overcome a comparatively high energy barrier.     

SiCf/O′-SiAlON composite: properties and oxidation retained properties

A composite of O′SiAlON (Si_2-xAl_xN_2-xO_1+x, with x 0.14) reinforced with 20 vol.% SiC monofilaments was fabricated by hot-pressing, at 1600°C, for 2 h under 34 MPa pressure. The mechanical and interfacial properties of the composites, as-fabricated as well as post-oxidized, were, investigated. The composite exhibited a significant improvement in ultimate flexure strength (640 MPa) and work of fracture (42 kJ m⁻²) compared with that (350 MPa and 1.8 kJ m⁻², respectively) of the monolithic material. These mechanical properties were slightly increased after the composite was heat treated for 24 h in air at 1200 and 1300°C. However, the composite exhibited a significant degradation in ultimate strength, while the work of fracture (WOF) remained unchanged after exposure in air at temperatures beyond 1400°C. The as-fabricated composite revealed a low interfacial shear strength (6.2 MPa) and a frictional sliding stress (3.2 MPa). After the composite was oxidized at elevated temperatures, the interfacial bonding and sliding stresses were reduced to noticeable extents, resulting from the degradation of the carbon coating layer of the SiC monofilaments.     

Complete Elastic Tensor for Mullite (∼2.5Al2O3·SiO2) to High Temperatures Measured from Textured Fibers

Directionally solidified mullite fibers have been grown by the laser-heated, float-zone method from starting materials with a nominal composition of 3Al₂O₃·2SiO₂. The fibers used in this study have large single-crystal regions with composition 2.5Al₂O₃·SiO₂ and (001) fiber axis orientation. The complete elastic tensor of these samples has been determined by Brillouin spectroscopy at room temperature and elevated temperatures up to 1200°C. Isotropic moduli (bulk, shear, and Young's) have been calculated using the Voigt–Reuss–Hill averaging scheme. The room-temperature values obtained are K _VRH= 173.5 ± 6.9 GPa, G _VRH= 88.0 ± 3.5 GPa, E _VRH= 225.9 ± 9.0 GPa. All moduli show gradual, linear decreases with temperature. The temperature derivatives obtained for the equivalent, isotropic moduli are d K _VRH/d T =−17.5 ± 2.5 MPa/°C, d G _VRH/d T =−8.8 ± 1.4 MPa/°C, d E _VRH/d T =−22.6 ± 2.8 MPa/°C. Substantial differences between bulk properties calculated from the single–crystal measurements in this study and the properties reported in the literature for polycrystalline sintered mullite are identified, indicating the importance of factors such as microstructure, intergranular phases, and composition to the elasticity of mullite ceramics.     

Formation and Properties of 2Tb2O3· Al2O3

The compound 2Tb₂O₃· Al₂O₃ was fabricated, and selected properties were investigated. The room-temperature X-ray diffraction pattern was indexed and the lattice parameters were calculated. Powder density was determined and used to calculate the number of molecules per unit cell (Z). This allowed calculation of a theoretical density. The material was characterized with regard to thermal expansion and indentation hardness and toughness. It was found to exhibit a rapid and reversible polymorphic transformation at high temperature.     

Preparation, Microstructure, and Mechanical Properties of Silicon Carbide–Dysprosia Composites

Composites of silicon carbide (SiC) with up to 30 vol% of dysprosia (Dy₂O₃) were fabricated by hot pressing and hot isostatic pressing. The effects of Dy₂O₃ dispersions on the microstructure and on selected mechanical properties of the composites were investigated. When 10-15 vol% of Dy₂O₃ was dispersed in the SiC matrix, the fracture toughness increased by ∼40%, whereas the flexural strength was comparable to that of unreinforced SiC. The increased fracture toughness was due to crack deflection, in conjunction with crack-interface grain bridging, and was not related to a phase transformation of Dy₂O₃ in the matrix.     

Tailorable thermal expansion in leucite-pollucite materials derived from geopolymers for environmental barrier coatings

The K[AlSi₂O₆]-Cs[AlSi₂O₆] pseudo-binary system was synthesized by geopolymer crystallization. The thermal expansion properties of these materials were studied by in situ high-temperature X-ray diffraction to characterize thermal expansion behavior for potential application as environmental barrier coatings. Tailorable thermal expansion through changing cation stoichiometry allowed reduced thermal expansion mismatch with SiC_f/SiC composites compared to rare-earth-based coatings.     

α-Alumina and spinel react into single-phase high-alumina spinel in <3 seconds during flash sintering

In situ X-ray diffraction measurements at the Advanced Photon Source show that α-Al₂O₃ and MgAl₂O₄ react nearly instantaneously and completely, and nearly completely to form single-phase high-alumina spinel during voltage-to-current type of flash sintering experiments. The initial sample was constituted from powders of α-Al₂O₃, MgAl₂O₄ spinel, and cubic 8 mol% Y₂O₃-stabilized ZrO₂ (8YSZ) mixed in equal volume fractions, the spinel to alumina molar ratio being 1:1.5. Specimen temperature was measured by thermal expansion of the platinum standard. These measurements correlated well with a black-body radiation model, using appropriate values for the emissivity of the constituents. Temperatures of 1600-1736°C were reached during the flash, which promoted the formation of alumina-rich spinel. In a second set of experiments, the flash was induced in a current-rate method where the current flowing through the specimen is controlled and increased at a constant rate. In these experiments, we observed the formation of two different compositions of spinel, MgO•3Al₂O₃ and MgO•1.5Al₂O₃, which evolved into a single composition of MgO•2.5Al₂O₃ as the current continued to increase. In summary, flash sintering is an expedient way to create single-phase, alumina-rich spinel.     

The Indianization of Colonial Medicine

ZUSAMMENFASSUNG: Anders als die weitgehend in der Geschichtsschreibung belegte psychiatrische Anstalt für Europ?er und Europ?erinnen mit ihrem englischen Leiter Owen Berkeley-Hill ist die weitaus gr??ere Institution für indische Patienten und Patientinnen im nordindischen Ranchi bisher nicht untersucht worden. Im Mittelpunkt dieses Beitrags steht die Karriere des Leiters dieser Institution, Jal E. Dhunjibhoy, zu Beginn des 20. Jahrhunderts als von der britischen Kolonialregierung eine Indianisierung der medizinischen Einrichtungen angestrebt wurde. Im Gegensatz zu bisherigen Studien über intermediaries und middles konzentriert sich dieser Aufsatz auf einen hochrangigen einheimischen Mitarbeiter. Die verbreitete Annahme zwangsl?ufiger historischer Prozesse wird dabei differenziert, eine regionale Kontextualisierung vorgenommen und die Kontinuit?t offener und versteckter Diskriminierung indischer Mediziner in Bezug auf Entlohnung, gesellschaftliche Stellung, fachliche Anerkennung, posthume Würdigung und historiographischer Berücksichtigung herausgearbeitet. Es wird verdeutlicht, in welcher Weise koloniale Akteure in bestehende gesellschaftliche Disparit?ten und soziopolitische Prozessen verstrickt waren und wie die Karriere eines leitenden Mediziners von einer Vielzahl von au?ermedizinischen Zusammenh?ngen bestimmt wurde. Gleichzeitig wird hervorgehoben, dass ihre strukturelle Positionierung als inbetweens, Kollaborateure oder Repr?sentanten einer erfolgreichen Dekolonisierung für ein nuancierteres Verst?ndnis ihrer beruflichen und pers?nlichen Identit?tsformierung nicht ausreicht.     

Highly Porous Geopolymers Through Templating and Surface Interactions

The natural porosity of geopolymers and the simplicity of geopolymer synthesis make them a potential candidate for the formation of highly porous ceramics. Here, the synthesis of highly porous (≈70 vol% or more) metakaolin geopolymer is demonstrated using a novel emulsion preparation with one‐pot curing and hydrophobization of the interior pores with alkylalkoxysilanes. Using mercury intrusion porosimetry, tailoring of the characteristic percolation pore size is demonstrated over a range of ≈200 nm to 10 μm. Using powder X‐ray diffraction, reactivity was shown to be decreased versus typical geopolymers, but substantial geopolymerization still occurred and the samples formed cohesive monoliths. Optional calcination under inert conditions allowed for formation of a glassy ceramic with a notable SiC phase, as well as further increasing the porosity by removing the hydrophobic pore coatings.     

The Change of X‐ray Diffraction Peak Width During in situ Conventional Sintering of Nanoscale Powders

Diffraction peaks of nanoscale particles of 3 mol% yttria‐stabilized zirconia become sharper as the powder sinters. The reduction in the peak width is correlated with the increase in density. The sharpening of the peak agrees reasonably well with the remaining free surface area as the sample sinters. Therefore, high curvature of the free surface of the pores is assumed to lead to peak broadening (the grain boundaries that grow at the expense of the free surfaces of the pores do not have this curvature). The change in the grain size during sintering does not make a significant contribution to peak width.