Bulk Single Crystal (bulk + single_crystal)

Distribution by Scientific Domains
Distribution within Physics and Astronomy

Selected Abstracts

Crystal growth features and properties of layered rare earth and barium cobaltates

G. L. Bychkov
Abstract High quality single crystals of LnBaCo2O5+, (0 < , < 1) (Ln = Pr, Eu, Gd, Tb, Dy) with rectangular shape and average dimensions 5 mm 5 mm 1 mm were grown for the first time from overstoichiometric flux melt. The data on the primary crystallization field of most cobaltate phases, which appear in parts of corresponding Gibbs triangles in the range 1373 , 1523 K are discussed. Bulk single crystals of new phases HoBaCo4O7 up to 300 mm3, YBaCo4O7 and TbBaCo4O7 up to 10 mm3 in volume were grown first. A solution of these compounds structure has been found in the space group P63mc. For the Ln = Pr, Gd, Sm, Tb, Dy, Ho based systems under the liquidus line there are several other new cobaltate phases of both perovskite, and hexagonal crystal structure. Stability of the double perovskite phase and temperature of the antiferromagnetic , ferromagnetic phase transition in relation to the radius of rare earth ions are compared. The data on the exact analysis of chemical composition, including EDX and iodometric titration, as well as results on magnetic susceptibility of EuBaCo1- xAlxO5+, (0 < x < 0.3) are discussed. ( 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Corrosion of Ceramics in Aqueous Hydrofluoric Acid

Kurt R. Mikeska
A variety of commercially available ceramic-based oxides, carbides, nitrides, and borides were evaluated for chemical attack in an azeotropic aqueous hydrofluoric acid (HF) test protocol at 90C. Weight change measurements and microstructure analysis showed that HF corrosion in polycrystalline ceramics generally occurred at grain boundaries by the dissolution of grain boundary phases although the bulk single crystal may inherently resist attack. Virtually all commercially prepared polycrystalline oxide ceramics (i.e., Al2O3, TiO2, ZrO2) and nonoxide ceramics (i.e., Si3N4, AlN, BN) were extensively corroded while polycrystalline pure carbides (i.e., SiC, TiC, B4C, WC) resisted corrosion. Equilibrium thermodynamic calculations show that these materials are soluble in HF; however, the kinetics of dissolution are slow enough in some cases to permit useful engineering lifetimes. [source]

The ,-particle excited scintillation response of YAG:Ce thin films grown by liquid phase epitaxy

Petr Prusa
Abstract Y3Al5O12:Ce (YAG:Ce) thin films were grown from PbO-, BaO-, and MoO3 -based fluxes using the liquid phase epitaxy (LPE) method. Photoelectron yield, its time dependence within 0.5,10 ,s shaping time, and energy resolution of these samples were measured under ,-particle excitation. For comparison a sample of the Czochralski grown bulk YAG:Ce single crystal was measured as well. Photoelectron yield values of samples grown from the BaO-based flux were found superior to other LPE films and comparable with that of the bulk single crystal. The same is valid also for the time dependence of photoelectron yield. Obtained results are discussed taking into account the influence of the flux and technology used. Additionally, , particle energy deposition in very thin films is modelled and discussed. ( 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

The antisite LuAl defect-related trap in Lu3Al5O12:Ce single crystal

M. Nikl
Abstract Absorption, radioluminescence and thermoluminescence spectra were measured for a set of Lu3Al5O12:Ce samples consisting of a bulk single crystal and Liquid Phase Epitaxy-grown films prepared from the same raw materials. The triple peak structure within 120,200 K distinguished in thermolu- minescence glow curves of the bulk crystals was ascribed to an electron trap arising due to the LuAl antisite defect. The depth of the trap associated with the dominant peak at 142 K was evaluated using the initial rise method. ( 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Intensified spin-dependent-transport and localized-spin freezing in magnetite sinter made from low size-dispersion hematite nanoparticles with low temperature calcination

H. Kobori
Abstract The spin-dependent-transport (SDT) has been studied for magnetite (Fe3O4) nano-particle sinter (MNPS) made from low size-dispersion hematite (,-Fe2O3) nanoparticles (LSDHN's) with low temperature calcination. Two kinds of LSDHN's are grown by the hydrothermal synthesis. The average sizes of them are 30 nm and 60 nm. The MNPS is produced by calcining the LSDHN's at 500 C for 5 hours in the atmosphere of Ar(90%)/H2(10%) mixed gases. As compared with a bulk single crystal, the considerable intensification of negative-differential-magnetoresistance (ND-MR) has been observed for the MNPS. We have not observed abrupt change of the electrical resistivity in the vicinity of the temperature of the Verwey transition (which is the metal-insulator transition) appeared for a bulk single crystal. The ND-MR for 30 nm shows larger values than that of 60 nm on the temperature dependence. From the X-ray diffraction experiment, the MNPS is found to include crystalline magnetite regions. We consider that the MNPS is composed of large amorphous-like grain-boundaries and small crystalline grains. The electrical current is inferred to flow in grain-boundary regions. In grain-boundary regions, since the localized spins are relatively random distributed, the spin-polarized conduction electrons show the SDT. Below the Verwey temperature, we have observed the magnetization difference between zero-field cooling (ZFC) and field-cooling (FC). This phenomenon indicates that the localized spins in the amorphous-like grain-boundaries are frozen in some degree. Above the Verwey temperature, the magnetoresistance is well fitted by the square of the Langevin function. We consider that the localized spins in the amorphous-like grain-boundaries do not form perfectly random configuration and are somewhat ordered in a short range region. ( 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Reversal magnetoresistance and unusual localized-spin freezing in magnetite sinter made from low size-dispersion hematite nano-particles with high temperature calcination

H. Kobori
Abstract The magneto-resistance and magnetization have been studied for magnetite (Fe3O4) sinter made from low size-dispersion hematite (,-Fe2O3) nanoparticles (LSDHN's) with high temperature calcination. Two kinds of LSDHN's were grown by the hydrothermal synthesis. The average particle sizes of them are 30 nm and 60 nm. The magnetite sinter was produced by calcining the LSDHN's at 800 C for 5 hours in the atmosphere of Ar(90%)/H2(10%) mixed gases. We have observed an abrupt change of the electrical resistivity by one order of magnitude in the vicinity of the Verwey temperature (123 K) of a bulk single crystal. From the X-ray diffraction experiment, we have found that the magnetite sinter includes crystalline region. The magnetite sinter is considered to be composed of relatively narrow grain-boundary regions of amorphous-like magnetite and large grain regions of crystalline magnetite. It is regarded that the grain-boundary-conduction is dominant below the Verwey temperature and the inter-grain-conduction is dominant above the Verwey temperature. We have observed the positive differential magnetoresistance (PD-MR) in low temperature regions and the negative differential magnetoresistance (ND-MR) in high temperature regions. The ND-MR is an ordinary phenomenon for magnetite, but the PD-MR is a peculiar one. In addition, an unusual localized-spin-freezing phenomenon has been also observed. Below the Verwey temperature, the magnetization difference between zero-field-cooling and field-cooling has been observed. The magnetization difference shows a sudden change in the vicinity of the Verwey temperature. ( 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Spin-dependent-magnetoresistance control by regulation of heat treatment temperature for magnetite nano-particle sinter

H. Kobori
Abstract The control of spin-dependent-magnetoresistance by regulation of the heat treatment (HT) temperature for magnetite (Fe3O3) nano-particle sinter (MNPS) has been studied. The average nano-particle size in the MNPS is 30nm and the HT was carried out from 400C to 800C. The HT of the MNPS varies the coupling form between adjacent magnetite nano-particles and the crystallinity of that. The measurements on electrical resistance (ER), magnetoresistance (MR) and magnetization were performed between 4K and 300K. The behavior of the ER and MR considerably changes at the HT temperature of ,600C. Below ,600C the ER indicates the variable-range-hopping conduction behavior and the MR shows the large intensity in a wide temperature region. Above ,600C the ER shows the indication of the Verwey transition near 110K like a bulk single crystal and the MR designates the smaller intensity. We consider that below ,600C the ER and MR are dominated by the grain-boundary conduction and above ,600C those are determined by the inter-grain conduction. The magnetic field application to the grain-boundary region is inferred to cause the large enhancement of the MR. [source]

Structural, Compositional and Electrochemical Characterization of Pt,Co Oxygen-Reduction Catalysts

CHEMPHYSCHEM, Issue 7 2010
S. Axnanda
Abstract Pt,Co thin-film electrocatalysts have been characterized using low-energy ion-scattering spectroscopy (LEISS), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), temperature-programmed desorption (TPD) and electrochemistry (EC). For comparative purposes, LEISS and EC were also carried out on a bulk Pt3Co(111) single crystal. The extensive experimental work resulted in the establishment of the surface phase diagram of the alloy film marked by a substantial divergence between the composition at the interface and that in the interior. When a dual-layer deposit of Pt and Co was annealed at high temperatures, alloy formation transpired in which the outermost layer became single-crystalline and enriched in Pt. The preferential surface segregation of Pt, however, was not sufficient to generate a platinum-only overlayer or "skin". Invariably, Co was found to co-exist with Pt, independent of the substrate [Mo(110) or Ru(0001)] employed; Pt3Co was the most favored composition. The same result, the absence of a Pt skin, was likewise indicated at the post-thermally-annealed surface of the bulk Pt3Co(111) monocrystal. For alloy-film surfaces more enriched in Pt than Pt3Co, the topmost layer was constituted primarily, but not exclusively, of Pt(111) domains. The proclivities of the alloys towards enhanced catalysis of the oxygen-reduction reaction were assessed in terms of their voltage efficiencies, as manifested by the open-circuit potential (OCP) in O2 -saturated sulfuric acid electrolyte. The Pt3Co surface, whether from the thin film or the bulk single crystal, exhibited the highest OCP, a significant improvement over pure Pt but still appreciably lower than the thermodynamic limit. The degradation of the Pt3Co thin-film surface was predominantly due to Co corrosion. A minimal amount was spontaneously dissolved upon simple immersion in solution; slightly higher dissolution occurred at potentials above the OCP. The fraction that was not immediately corroded proved to be stable even after prolonged periods at potentials more positive than the OCP. [source]

Recent developments in modelling of liquid phase electroepitaxy under applied magnetic field

S. Dost
Abstract Growth of single crystals under magnetic field is of interest for suppressing the adverse effect of natural convection and for better mixing in the liquid solution, which are the favorable conditions for prolonged growth and high quality crystals. In this short review article, recent developments in the modelling of electroepitaxy under magnetic field are presented. An application is given for the liquid phase electroepitaxial growth of GaAs bulk single crystals under a static magnetic field. Experimental results, that have shown that the growth rate under an applied static magnetic field is proportional to the applied magnetic field, and increases with the field intensity level, are predicted from the present model. The model also predicts growth interface shapes accurately. ( 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Perfect Bi4Ti3O12 Single-Crystal Films via Flux-Mediated Epitaxy,

R. Takahashi
Abstract Excellent crystallinity of material films and atomic control of their surface/interface, sufficient for the realization of their optimal physical properties, are technological premises for modern functional-device applications. Bi4Ti3O12 and related compounds attract much interest as highly insulating, ferroelectric materials for use in ferroelectric random-access memories. However, it has been difficult thus far for Bi4Ti3O12 films to satisfy such requirements when formed using vapor-phase epitaxy, owing to the high volatility of Bi in a vacuum. Here, we demonstrate that flux-mediated epitaxy is one of the most promising and widely applicable concepts to overcome this inevitable problem. The key point of this process is the appropriate selection of a multi-component flux system. A combinatorial approach has led to the successful discovery of the novel flux composition of Bi,Cu,O for Bi4Ti3O12 single-crystal film growth. The perfect single-crystal nature of the stoichiometric Bi4Ti3O12 film formed has been verified through its giant grain size and electric properties, equivalent to those of bulk single crystals. This demonstration has broad implications, opening up the possibility of preparing stoichiometric single-crystal oxide films via vapor-phase epitaxy, even if volatile constituents are required. [source]

Sublimation growth of bulk crystals of AlN-rich (AlN)x(SiC)1,x solid solutions

Matthias Bickermann
Abstract (AlN)x(SiC)1,x bulk single crystals are prepared by sublimation growth employing an AlN source placed in a tantalum carbide container. A 6H -SiC substrate acts both as seed and as source for silicon and carbon species. As the growth temperature is increased from 1900C to 2050C or the seed surface orientation is changed from (0115) to (0001), crystal coloration changes from yellowish to greenish to blackish. We attribute the change in coloration to an increasing Si/C content in the samples and estimate that up to 10% at of Si and C are incorporated. Accompanying changes in below band-gap optical absorption and cathodoluminescence are discussed. ( 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

CuIn2n+1Se3n+2 single dot structures: creation and photosensitivity

Yu. V. Rud
Abstract Photosensitive single dot structures based on CuIn2n+1Se3n+2 (n = 0, 1, 2) bulk single crystals have been creation for the first time by means of electric-discharge welding (EDW). The stationary current-voltage characteristics and the photovoltaic properties of the structures based on CuInSe2, CuIn3Se5 and CuIn5Se8 ternary semiconductors have been studied, which show evidence for the rectification effect and photoconversion. The character of interband transitions is established and the energy bandgap variation in this type of ternary compounds is traced. It is concluded that EDW can be successfully used for the creation of photoconverters based on multi-component semiconductors. ( 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]