Theory of correlations in strongly interacting fluids of two-dimensional dipolar bosons

Ground-state properties of a two-dimensional fluid of bosons with repulsive dipole-dipole interactions are studied by means of the Euler-Lagrange hypernetted-chain approximation. We present a self-consistent semi-analytical theory of the pair distribution function $g(r)$ and ground-state energy of this system. Our approach is based on the solution of a zero-energy scattering Schr\"{o}dinger equation for the "pair amplitude" $\sqrt{g(r)}$ with an effective potential from Jastrow-Feenberg correlations. We find excellent agreement with quantum Monte Carlo results over a wide range of coupling strength, nearly up to the critical coupling for the liquid-to-crystal quantum phase transition. We also calculate the one-body density matrix and related quantities, such as the momentum distribution function and the condensate fraction.Comment: 8 pages, 8 figures, submitte

Diamond nucleation from the gas phase onto cold-worked Co-cemented tungsten carbide

Co-cemented tungsten carbide (WC–Co) substrates with fine (1 μm) and coarse (6 μm) grain size were sintered using 6 wt.% Co as a binder. The as-sintered samples were ground to the final geometry (10×10×3 mm3). After the grinding treatment, the full width at half maximum (FWHM) of the WC X-ray diffraction (XRD) peaks indicated a high level of strain in a few micrometers thick surface layer, according to the penetration depth of Cu Kα radiation. The as-ground substrates were submitted to a two-step etching procedure with Murakami's solution, to roughen the surface, and 10 s acid wash to etch surface cobalt out. The Murakami's etching time was varied between 1 and 20 min. Fine- and coarse-grained substrates submitted to different chemical etching times were characterized by scanning electron microscopy and XRD, and then submitted to short diamond nucleation runs in a Hot Filament Chemical Vapour Deposition reactor. Both FWHM of WC peaks and diamond nucleation density decreased by increasing the Murakami's etching duration, providing that the etched layer did not exceed 2 μm thickness. When a layer thicker than a couple of micrometers was removed by etching, diamond nucleation density was very low and no more dependent on etching time. This occurrence suggested that diamond nucleation density correlates well with the amount of residual strain at the substrate surface and can be tailored by a suitable control of strain-related defects produced by mechanical treatments

Sol–gel synthesis and characterization of Co-doped LSGM perovskites

One of the major requirements for the development and commercialization of low-cost SOFCs is the reduction in the operating temperature. One of the methods to reach this aim is the use of solid electrolytes which exhibit superior ionic conductivity at intermediate temperatures (IT, T < 800 °C). Among these ionic conductors, doped LaGaO3 materials show high oxide ionic conductivity in the 600–800 °C range. These perovskites are usually prepared by time- and energy-consuming solid state reaction. In this paper, La0.8Sr0.2Ga0.8Mg0.2O2.8 (LSGM) and La0.8Sr0.20Ga0.8Mg0.2−xCoxO3−δ (LSGMC) powders containing different amounts of Co (x = 0.05, 0.085 and 0.10) were prepared from precursors synthesised by citrate sol–gel method. The precursors were calcined at 1000 °C (10 h) and dense high-purity pellets were obtained by pressing (300 MPa) and sintering in air at 1475 °C (5, 10 and 20 h). Sintered pellets of LSGM and LSGMC contained very small amounts (<1%) of SrLaGa3O7 and SrLaGaO4, respectively, as detected by X-ray diffraction (XRD) and by the combined use of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The data clearly showed the feasibility of sol–gel methods to produce Co-doped LSGM perovskite type oxides

Effect of synthetic route on sintering behaviour, phase purity and conductivity of Sr- and Mg-doped LaGaO3 perovskites

La1-xSrxGa1-yMgyO3-d (LSGM) powders containing different amounts of Sr2+ and Mg2+ were prepared from precursors synthesised by either Pechini or citrate sol-gel method and by subsequent calcination at 1400 °C. Some powders were also submitted to further 10 h firing at 1500 °C. All as-calcined powders contained small amounts of Sr- and Ga-containing phases (namely SrLaGa3O7 and SrLaGaO4), as detected by X-Ray Diffraction (XRD). The relative amounts of these phases depended on x and y, i.e. the dopants’ levels. Nevertheless, powders prepared by the citrate method exhibited systematically higher phase purity than those obtained by the Pechini process. Calcined powders were then sintered at 1500 °C (10 h) in air and the degree of sintering was assessed by scanning electron microscopy (SEM). Phase composition of sintered pellets was different from that of powders. In fact, sintered pellets showed the presence of MgO, as detected by SEM, and of lesser amounts of SrLaGa3O7. Both these phases were less abundant in materials sintered using powders prepared by citrate method, thus suggesting that Pechini method does not represent the best wet chemical process for manufacturing. The conductivity of sintered pellets was measured by impedance spectroscopy in the 600–800 °C interval. Conductivity values of LSGM materials were affected by secondary phase segregation and, therefore, depended on both composition and sol-gel method synthetic route

Electron-electron interactions in decoupled graphene layers

Multi-layer graphene on the carbon face of silicon carbide is an intriguing electronic system which typically consists of a stack of ten or more layers. Rotational stacking faults in this system dramatically reduce inter-layer coherence. In this article we report on the influence of inter-layer interactions, which remain strong even when coherence is negligible, on the Fermi liquid properties of charged graphene layers. We find that inter-layer interactions increase the magnitudes of correlation energies and decrease quasiparticle velocities, even when remote-layer carrier densities are small, and that they lessen the influence of exchange and correlation on the distribution of carriers across layers.Comment: 8 pages, 4 figures, submitte

Dry turning of alumina/aluminum composites with CVD diamond coated Co-cemented tungsten carbide tools

Triangular (TPGN 160308) WC-6 wt.%Co inserts having different average grain sizes (1 and 3 µm) were submitted to surface roughening either by wet etching with Murakami's reagent or by a heat treatment in the hot filament chemical vapour deposition (HFCVD) reactor. The heat treatment was performed in a monohydrogen-rich atmosphere at substrate temperatures as high as 1000 degrees C. Scanning electron microscopy and energy-dispersive spectroscopy showed that this pre-treatment led to surface roughening of the as-ground inserts and to a lower surface Co concentration. Prior to deposition, all inserts were etched with an acid solution of hydrogen peroxide. Diamond coatings were deposited by HFCVD. The coated inserts were tested by dry machining of aluminum-matrix composite (Al-10%Al2O3) bars. Turning test results indicated that a proper combination of substrate pretreatment and microstructure can significantly improve tool life

Quantitative determination of the adhesive fracture toughness of CVD diamond to WC-Co cemented carbide

Well-separated diamond particles were nucleated and grown by hot filament chemical vapor deposition (HFCVD) onto WC-Co cemented carbide pretreated by Murakami's reagent and H2O2 + H2SO4 solution. The adhesive strength of diamond particles to WC-Co cemented carbide was quantitatively determined in terms of interface toughness by directly applying an external load to the CVD diamond particles. From the measurement of the maximum load required to scratch off the particles, we determined that the adhesive toughness was 14 J/m(2). This value is more than twice as high as that of CVD diamond on smooth silicon substrate and comparable to the cleavage fracture energy of diamond. The newly developed procedure will allow to check the effectiveness of substrate surface pretreatments for further improving the adhesion level of diamond films on WC-Co. (C) 2000 Elsevier Science S.A. All rights reserved

Analytical expressions for the charge-charge local-field factor and the exchange-correlation kernel of a two-dimensional electron gas

We present an analytical expression for the static many-body local field factor $G_{+}(q)$ of a homogeneous two-dimensional electron gas, which reproduces Diffusion Monte Carlo data and embodies the exact asymptotic behaviors at both small and large wave number $q$. This allows us to also provide a closed-form expression for the exchange and correlation kernel $K_{xc}(r)$, which represents a key input for density functional studies of inhomogeneous systems.Comment: 5 pages, 3 figure

A Raman study of diamond film growth on Co-cemented tungsten carbide

Phase purity and crystallinity of diamond films grown by hot filament chemical vapor deposition on ISO-grade K10 cemented carbide [94.2 weight percent (w/o) WC-5.8 w/o Co] were studied by Raman spectroscopy as a function of substrate temperature, gas phase composition, and substrate pretreatments. High-quality diamond films were grown using 0.5% CH4/H-2 in a rather narrow range of substrate temperatures (750 to 760 degrees C). In all the deposited coatings, the first-order Raman band of diamond is detected at 1337 cm. This fact indicates that a 2 GPa residual compressive stress is present in the diamond phase. The linewidth of the diamond Raman peak increases with deposition temperature. This effect has been ascribed to a higher density of defects in diamond crystallites. It has been observed that Co removal from the substrate surface by wet chemical etching before deposition is less effective than a careful selection of deposition parameters to reduce the codeposition of nondiamond carbon phases. This finding has been attributed to the fast diffusion of the binder from the bulk to the substrate surface, even for the etched substrates