William T. Petuskey

Icosahedral packing of B12 icosahedra in boron suboxide (B6O)

Objects with icosahedral symmetry (Ih) bear a special fascination; natural examples are rare, but include radiolaria and virus particles (virions). The discovery of C60, a molecule in the shape of a truncated icosahedron with Ih symmetry, has aroused widespread interest. In 1962, Mackay described a radiating packing of spheres in Ih symmetry, in which the centres of successive shells of spheres lie on the surfaces of icosahedra. There has been extensive investigation of the conditions under which such packing might be realized in assemblies of atoms or of molecules such as C60 (ref. 5). Here we report the preparation, at high temperatures and pressures, of boron suboxide (B6O) in which the preferred form of the material is as macroscopic, near-perfect, regular icosahedra, similar to the multiply-twinned particles observed in some cubic materials. A major difference is that B6O has a rhombohedral structure that nearly exactly fits the geometrical requirements needed to obtain icosahedral twins. These icosahedral particles have a structure that can be described as a Mackay packing of icosahedral B12 units, and thus has long-ranged order without translational symmetry.

A structural investigation of CaOAl2O3 glasses via 27Al MAS-NMR

Abstract 27Al isotropic chemical shifts and quadrupolar coupling constants have been obtained for glasses along the CaOAl2O3 join, prepared by fast (splat) quench techniques and slow cooling in containerless levitation experiments. Quench rates with the two techniques were 106-107°/s and 200–300°/s. Fast-quenched glasses with CaO:Al2O3

Energetics and electronic structure of the hypothetical cubic zincblende form of GeC

The authors have performed the first ab-initio investigation of the hypothetical IV-VI compound GeC with zincblende structure, and have compared its properties with those of SiC. The calculations are performed using a planewave expansion within the local density approximation and the pseudopotential approximation. It is found that GeC, like SiC, is a wide-gap semiconductor with an indirect bandgap. Unlike SiC, they find that GeC is thermodynamically unstable toward decomposition into its segregated components under zero pressure. However, it becomes significantly more stable under pressure up to the phase transition of Ge to the beta -tin structure. The effect of different choices of the form and generating scheme for the pseudopotential and the form for the correlation energy functional on the results of such calculations is also discussed.

Mechanical Properties of Titanium Nitride Nanocomposites Produced by Chemical Precursor Synthesis Followed by High-P,T Treatment

We investigated the high-P,T annealing and mechanical properties of nanocomposite materials with a highly nitrided bulk composition close to Ti3N4. Amorphous solids were precipitated from solution by ammonolysis of metal dialkylamide precursors followed by heating at 400–700 °C in flowing NH3 to produce reddish-brown amorphous/nanocrystalline materials. The precursors were then densified at 2 GPa and 200–700 °C to form monolithic ceramics. There was no evidence for N2 loss during the high-P,T treatment. Micro- and nanoindentation experiments indicate hardness values between 4–20 GPa for loads ranging between 0.005–3 N. Youngs modulus values were measured to lie in the range 200–650 GPa. Palmqvist cracks determined from microindentation experiments indicate fracture toughness values between 2–4 MPa·m1/2 similar to Si3N4, SiC and Al2O3. Significant variations in the hardness may be associated with the distribution of amorphous/crystalline regions and the very fine grained nature (~3 nm grain sizes) of the crystalline component in these materials.

Hardness, elasticity, and fracture toughness of polycrystalline spinel germanium nitride and tin nitride

The hardness, elastic moduli, and fracture toughness of the spinel phases, γ–Ge 3 N 4 and γ–Sn 3 N 4 , were determined using indentation data and theoretical calculations. Measurements were performed on polycrystalline specimens using the technique of nanoindentation to determine the reduced moduli and hardnesses from the unloading portion of the indent curves. Reduced moduli of γ–Ge 3 N 4 and γ–Sn 3 N 4 were found to be 295 and 167 GPa, respectively. The nanohardnesses of γ–Ge 3 N 4 and γ–Sn 3 N 4 were found to be 31 and 13 GPa, respectively. The shear moduli G 0 and Poisson’s ratios ν 0 were derived using theoretical bulk moduli B 0 obtained from density-functional theory calculations. The calculated values were B 0 = 260 GPa, G 0 = 146 GPa, ν 0 = 0.26 for γ–Ge 3 N 4 , and B 0 = 186 GPa, G 0 = 64 GPa, ν 0 = 0.34 for γ–Sn 3 N 4 . Fracture toughness was estimated by direct measurement of radial cracks emanating from Vickers microindents. It was determined that for γ–Ge 3 N 4 , K I C = 2.3 MPa(m) 1/2 , while for γ–Sn 3 N 4 , K I C = 1.4 MPa(m). 1/2

Oxygen self-diffusion in single-crystal MgO: secondary-ion mass spectrometric analysis with comparison of results from gas–solid and solid–solid exchange

Abstract Self-diffusion coefficients for 18 O in single-crystal MgO have been determined from a novel specimen comprising an epitaxial layer of high-purity Mg 18 O upon a single crystal substrate of normal MgO. Heating the specimen in air produced a gas–solid exchange gradient at the sample surface as 18 O in the epitaxial layer exchanged with 16 O in air. A solid–solid interdiffusion gradient was produced between the substrate crystal and the 18 O-enriched epitaxial layer. SIMS analysis of gas–solid exchange gradients prepared in the temperature range 1000–1650 °C provided diffusion coefficients that could be described as D/( m 2 s −1 )= 1.8 −1.1 +2.9 ×10 −10 exp [−(3.24±0.13) eV /kT] . Interdiffusion gradients produced by annealing at 1100 and 1200 °C yielded the self-diffusion coefficients that were comparable to those obtained from gas–solid exchange, indicating that the surface exchange reaction is fast enough. The results are interpreted in terms of a defect model in which oxygen diffusion occurs by an interstitial type of defect as a result of suppression of anion vacancy concentration by large concentrations of extrinsic cation vacancies.

On the dehydration mechanism of Mg(OH)2 by a high-energy electron beam

The dehydration process in Mg(OH)2 induced by high-energy electron irradiation is studied by in situ electron energy loss spectroscopy. During dehydration, both the low energy-loss spectra and the Mg L23 edge show the existence of partially oxidized Mg- or O-deficient MgO in the dehydrated products, which is not seen in the thermally dehydrated MgO. This indicates that the dehydration mechanism under the electron beam may be different from the mechanism involved in a thermal process.

Solutions to Fick's second law of diffusion with a sinusoidal excitation

Ficks second law of diffusion was solved using the Laplace transformation for a homogeneous linear sample and a bisectional linear sample, for the boundary conditions of one end of the sample being impressed with a sinusoidal excitation of concentration or of mass flow, and the other end being a reflective or an infinitive boundary.

Interfacial effects on Ag:S nonstoichiometry in silver sulfide/alumina composites

Abstract The relative stoichiometry of Ag2S containing dispersions of 0.2 μm Al2O3 particles was measured as a function of the chemical potential of silver between 158 and 220°C using solid state coulometric titration. For composites consisting of 3.5 m2 of interface area per 1 cm3 of Al2O3Ag2S composite, the effective homogeneity range in comparison to the pure material was as much as 120 times greater for the low temperature phase (α-Ag2S) and 50% greater for the high temperature phase (β-Ag2S). The process was described as that of silver absorption at solid—solid interfaces. Absorption isotherms and partial molar quantities for the interface were determined at 158 and 200°C.

Layered perovskites as 'soft-ceramics'

Abstract The layered perovskite KCa 2 Nb 3 O 10 is shown to possess large elastic and fracture anisotropies which are explained on the basis of its structure. Its fracture toughness was measured relative to the in-basal and basal-normal directions. Combined with previously determined elastic moduli, the fracture energy release rates and associated anisotropy were determined and discussed in terms of crack deflection characteristics. The general mechanical characteristics of KCa 2 Nb 3 O 10 were discussed relating to what is described as a ‘soft’ ceramic capable of sustaining considerable mechanical damage. Its high thermal stability, T m =1464°C, and its oxidation resistance are additional features that make it attractive for high temperature applications. Anecdotal demonstrations of its softness and low damage thresholds are presented, such as might be important for cerarnic–matrix composites or machinable ceramic applications.