Glass/Crystal Interfaces in Liquid-Phase Sintered Materials

This paper presents a review of our studies of interfaces in liquid-phase sintered materials. Recent studies of three different types of interface are discussed, namely, (i) the interface between the free surface of the crystal and the glass, (ii) the interface between the intergranular glass and the crystal and (iii) the interface between the crystallized glass and the substrate crystal. Model systems with relatively well known thermodynamic and crystallographic properties have been chosen. The relationship between the three types of interface is discussed. The dewetting of silicate liquids on free surfaces provides an opportunity to study directly the interface between the free surface and glass. Observations on polycrystalline samples and bicrystals give new understanding of the interplay between intergranular glass layers and the adjoining crystalline grains. Crystallization of the glass on single-crystal substrates directly gives information about the crystallized-glass/crystal interface.     

Injection of single and multiple vortices in an opposed-jet burner

A thorough understanding of turbulent reacting flows is essential to the continued development of practical combustion systems. Combustor codes can be validated using data such as those generated in this study of a vortex interacting with a nonpremixed, opposed-jet hydrogen-air flame. When experimental results are compared with model predictions, the underlying flowfield must be matched carefully. Since the vortex-injection process used in the present experiments can result in many types of vortices, including multiple vortices, restrictions on the experimental operation of the burner are required as well as careful vortex characterization. Vortex-characterization data are acquired using digital, two-color particle-image velocimetry (PIV), and the hydroxyl (OH) layer produced by the flame is imaged using planar laser-induced fluorescence (PLIF). The PIV and OH PLIF measurements are performed simultaneously. Good agreement with previous numerical-modeling predictions is obtained when experiments and computations are performed using similar vortex conditions.     

Ultrasoft spin-dependent pseudopotentials

The use of the spin-dependent pseudopotentials has been shown to markedly enhance the transferability of the commonly used spin-neutral pseudopotential method for the study of the structural and magnetic properties of transition-metal-containing materials. Unfortunately, because the method was based on the rather expensive norm-conserving pseudopotential formalism, the method was limited to the study of fairly small systems. Here we present an extension of the spin-dependent pseudopotential method for the far more computationally advantageous ultrasoft formalism and show that it is very easy to add such a feature to any preexisting computer code. We benchmark our new method by comparing to previously published results and then apply it to the study of several relevant test cases: bulk Ni, Fe, and Co, as well as a Pd atomic wire.     

Silicon-based nanowires from silicon wafers catalyzed by cobalt nanoparticles in a hydrogen environment

We present here the synthesis of silicon-based nanowires directly from silicon wafers at high temperatures and in the presence of cobalt nanoparticles and hydrogen gas. All three ingredients were critical to the growth of Si-based nanowires, which were between 5-60 nm in diameter and microm-mm long. Both heavily coiled and straight Si-based nanowires were made. Experimental evidence suggested that the sources of silicon for the nanowires growth were in the gas phase.     

Structure solution of a novel aluminium methylphosphonate using a new simulated annealing program and powder X-ray diffraction data

The structure of a novel layered aluminium methylphosphonate, formula Al2(CH3PO3)3, has been solved from laboratory X-ray powder diffraction data by simulated annealing of five independent structural sub-units, revealing a combination of four- and five-fold coordinated aluminiums within the inorganic lamellae that is unique for this kind of solid.     

Full dimensional calculations of vibrational energies of H3O+ and D3O+

We report full dimensional calculations of vibrational energies of H3O+ and D3O+ using two implementations of the code MULTIMODE. In one implementation, the reference geometry is the minimum of the potential (the standard choice for MULTIMODE). This implementation is not able to readily describe splittings in the vibrational energies due to motion through the inversion barrier. A second implementation, in which the reference geometry is the inversion saddle point, is able to describe the splittings. These full dimensional calculations are done using the realistic, though not spectroscopically accurate, potential of Ojamae, Singer and Shavitt, and the results are compared with experiment.     

Synthesis of the ABC ring system of azaspiracid. 1. Effect of D ring truncation on bis-spirocyclization

[reaction: see text] Synthesis of a spirocyclization precursor with a truncated D ring has been accomplished. Subsequent bis-spirocyclization induced the formation of equal amounts of the natural transoidal 10R,13R bis-spirocycle and its cisoidal 10R,13S epimer under an apparent thermodynamically controlled process.     

Comparison of S, Pt, and Hf adsorption on NiAl(1 1 0)

First-principles periodic slab density-functional theory (DFT) calculations with a plane-wave basis are used to predict the properties of S, Pt, and Hf adsorption on NiAl(1 1 0). Stable adsorption sites are identified, and adsorbate binding energies and structures are predicted. We find that while S adsorbs in a threefold site, the metals prefer to adsorb in the Ni-Ni twofold bridge site. The latter finding is consistent with scanning tunneling microscopy experiments for adsorption of various transition metals on NiAl(1 1 0) by Ho and coworkers. S is predicted to easily diffuse between threefold sites. We find that Pt and Hf both induce significant changes in the local surface structure, changing twofold bridge sites into fourfold coordination sites by drawing next-nearest-neighbor atoms nearly equidistant with the nearest-neighbor atoms. We find Pt favors interaction with Al slightly more than Ni, while Hf shows a particularly strong affinity for Ni compared to Al. We also predict that Hf may diffuse one-dimensionally along Ni rows with a barrier of ≈0.6 eV.     

Isoscalar quadrupole strength in Ca from the (p,p′α0) reaction at Ep = 100 MeV

The ⁴⁰Ca(p,p′ α) reaction has been studied at an incident proton energy E_p = 99.5 MeV for proton laboratory scattering angles Θ_p^lab = 17°, 23° and 27°. Emission of α particles coincident with the scattered proton has been measured for an angular range Θ_α 0° − 180° relative to the recoil axis. A multipole decomposition for the α₀-decay channel to the ³⁶Ar ground state has been performed from the angular-correlation functions. The energy distribution of the dominating E2 strength deduced in the excitation energy range E_x = 11–21 MeV agrees reasonably well with the results from electron and α-induced α₀-decay investigations. The exhaustion of the E2 energy-weighted sum rule in this channel up to an energy of 17 MeV is 16.1(4.0)%, in accord with the study of the (α, α′ α₀) reaction. However, this value is twice what is found in the (e,e′ α₀) experiment in the same energy region. Thus, the puzzling discrepancy in the E2 strengths derived from electromagnetic and hadronic probes remains unsolved.