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Hexagonal boron nitride (h‐BN) and graphite have similar crystal structures, comparable lattice parameters, and coefficients of thermal expansion, but vastly different electrical and thermal transport. Despite their key differences, it is possible to couple h‐BN and graphite in a bimaterial system allowing the unique properties of both materials to be utilized in a single component. Through a carbothermal reduction of B2O3 in nitrogen, the surface of graphite can be converted to h‐BN. This results in a layered system that is electrically insulating on the surface due to h‐BN, and more compliant as well as conductive within the substrate due to the graphite structural body. We discuss the high‐temperature synthesis and characterization of this layered material, focusing on the processing–microstructure relationship as well as the interface of graphite/h‐BN to assess the chemical and mechanical adhesion of the layers, and to establish how such properties are contingent on the reacting phase of B2O3. This is achieved by investigating the origin of h‐BN formation and the unwanted side reaction of boron carbide formation, through the evaluation of the thermochemistry and kinetics governing the carbothermic reactions. We establish that a reaction temperature and holding time of 1700°C for 18 h produced the thickest h‐BN layers which exhibited the highest fracture toughness over all lower temperature synthesis conditions. [ABSTRACT FROM AUTHOR]

A freezing casting technique is used for constructing ceramic-polymer composites in which the 2 phases are arranged in an electrically parallel configuration. The finished barium-titanate-polymer composites have displayed the high dielectric constant (1000) of ferroelectric ceramics while maintaining the unique properties of polymer materials such as graceful failure, low dielectric loss and high dielectric breakdown.

Pressureless sintering approaches provide a simple avenue to manufacture dense ceramic parts with minimal processing equipment, but current pressureless sintering techniques have yet to demonstrate capabilities of producing dense ceramics while maintaining sub‐50 nm grain sizes. Nanocrystalline yttria stablized zirconia ceramics were process from 4 mol% yttria stablized zirconia (4YSZ) nanopowders with a crystallite size of 7.5 nm using dry cold isostatic pressing (CIP) where powders are dried immediately prior to green compact formation and CIP vacuum bagging. It is shown that CIP pressures >75 000 psi (517 MPa) effectively remove pores larger than 100 nm and that pressureless sintering occurs at reduced temperatures for green densities ≥50%. Though the sintering kinetics are shown to be similar to other zirconia nanopowder sintering studies, the small initial crystallize size and reduced sintering temperature allowed densities as high as 97.2%, while retaining a ceramic grain size at or below 40 nm. Produced nanocrystalline 4YSZ ceramics with a grain size of 30.3 nm and a density of 96.3% had Vicker's hardnesses as high as 14.2 GPa and Vicker's indentation fracture resistance of 3.43 MPa·m, demonstrating that simple processing approaches can be refined to fabricate nanocrystalline ceramics while maintaining high hardness and indentation fracture resistance. [ABSTRACT FROM AUTHOR]

Abstract: Often, addition of BiMO3 to BaTiO3 (BT) leads to improvement in resistivity with a simultaneous shift to n‐type conduction from p‐type for BT. In considering one specific BiMO3 composition, that is, Bi(Zn1/2Ti1/2)O3 (BZT), several prospective candidates for the origin of this n‐type behavior in BT‐BZT were studied—loss of volatile cations, oxygen vacancies, bismuth present in multiple valence states and precipitation of secondary phases. Combined x‐ray and neutron diffraction, prompt gamma neutron activation analysis and electron energy loss spectroscopy suggested much higher oxygen vacancy concentration in BT‐BZT ceramics (>4%) as compared to BT alone. X‐ray photoelectron spectroscopy and x‐ray absorption spectroscopy did not suggest the presence of bismuth in multiple valence states. At the same time, using transmission electron microscopy, some minor secondary phases were observed, whose compositions were such that they could result in effective donor doping in BT‐BZT ceramics. Using experimentally determined thermodynamic parameters for BT and slopes of Kröger‐Vink plots, it has been suggested that an ionic compensation mechanism is prevalent in these ceramics instead of electronic compensation. These ionic defects have an effect of shifting the conductivity minimum in the Kröger‐Vink plots to higher oxygen partial pressure values in BT‐BZT ceramics as compared to BT, resulting in a significantly higher resistivity values in air atmosphere and n‐type behavior. This provides an important tool to tailor transport properties and defects in BT‐BiMO3 ceramics, to make them better suited for dielectric or other applications. [ABSTRACT FROM AUTHOR]

The influence of excess PbO on Pb(Mg1/3Nb2/3)O3–35mol% PbTiO3 {001} single crystal growth by seeded polycrystal conversion (SPC) was studied in the range of 0–10 vol% PbO. As in previous studies, additions of PbO increased the boundary mobility significantly, thus facilitating single crystal growth via SPC. Unlike previous studies, single crystals were grown into pore-free matrices, resulting in different crystal growth kinetics (signifying interface reaction controlled as opposed to diffusion controlled). Porosity was shown to decrease single crystal growth rates by a factor of 2. Additionally, it was found that single crystal and matrix grain growth rates are optimized at 1.5 vol% PbO, as opposed to previous work, which showed that 3 vol% PbO was fastest. Increasing PbO content beyond 1.5 vol% results in growth that is independent of liquid fraction for all annealing times. In addition, the matrix grains were faceted and the growth best-fit parabolic kinetics so interface reaction control was deemed the most likely growth mechanism. [ABSTRACT FROM AUTHOR]

The growth of abnormal grains of PMN-35PT containing ∑3 twin boundaries was compared with that of untwinned abnormal grains of PMN-35PT. It was thought that the twinned abnormal grains might have a growth advantage because of the presence of reentrant edges. The aforementioned types of grains were embedded in a PMN-35PT polycrystalline matrix with 3 vol% excess PbO, and grown by annealing in an oxygen atmosphere at 1150°C for varying times. It was found that abnormal grains containing ∑3 boundaries showed a comparable degree of growth to that of the untwinned grains. Furthermore, for the twinned seed crystals, the extent of growth was independent of whether there was any initial entrapped porosity within the abnormal grain. [ABSTRACT FROM AUTHOR]

The kinetics of {001}-oriented Pb(Mg[sub ⅓]Nb[sub 2/3])O[sub 3]-35 mol% PbTiO[sub 3] (PMN-35PT) single crystals grown by seeded polycrystal conversion were systematically quantified as a function of excess PbO liquid phase. The coarsening behavior of the corresponding matrix grains was similarly quantified. Singlecrystal seed plates were embedded in a matrix of PMN-35PT with varying amounts of liquid phase (PbO) content in the range of 0 to 5 vol% and annealed at 1150°C for 0-10 h. Apparent maxima in the growth rates were observed at a PbO content of ∼3 vol% for both the single crystal and matrix grains. In both cases, the growth data were found to most closely follow cubic growth kinetics. Implications regarding the effect of PbO volume fraction on the matrix and singlecrystal growth mechanisms are discussed. [ABSTRACT FROM AUTHOR]

The influence of additions of excess PbO to Pb(Mg⅓Nb[sub 2/3])O[sub 3]35 mol% PbTiO[sub 3] (PMN-35PT) on {111} single-crystal growth by seeded polycrystal conversion was studied in the range of 0-5 vol% PbO. PbO volatilization and hence weight loss during annealing was controlled effectively by a doublecrucible type of arrangement. PbO additions increased boundary mobility significantly in PMN-35PT, thus facilitating single-crystal growth by seeded polycrystal conversion (SPC). This is attributed to the formation of a boundary wetting PbO-based liquid phase. The growth process occurs very rapidly initially, after which it slows down. This is presumably due to both a decrease in the driving force for boundary migration because of an increase in matrix grain size, and a transition to lower mobility facets. It is also shown that for a given annealing time, the size of the grown crystal scales with the lateral dimensions of the seed crystal. [ABSTRACT FROM AUTHOR]