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Hexagonal Phase (hexagonal + phase)
Selected AbstractsMetastable cubic InN layers on GaAs (001) substrates grown by MBE: Growth condition and crystal structurePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009Sakuntam Sanorpim Abstract Transmission electron microscopy and high resolution X-ray diffraction were applied to characterize the crystal structure and its modification in c-InN layers on GaAs (001) substrates grown by rf-plasma assisted molecular beam epitaxy. The layer quality was shown to depend on growth conditions, namely In- and N-rich conditions. The best quality of c-InN layers was achieved by "stoichiometric" growth under the In-rich condition, resulting in In-rich layers with a small amount of hexagonal-phase inclusion (,8%). On the other hand, nucleation and growth of N-rich layers are shown to result in a high density of stacking faults which drastically decreases toward the InN surface. It is argued that the presence of stacking faults contributes to the structural modification in these layers. We found that the existence of a structural modification from cubic to mixed cubic/hexagonal phase in microstructure of the N-rich layers exhibit higher hexagonal-phase incorporation than that of the In-rich layers. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Curvature properties of novel forms of phosphatidylcholine with branched acyl chainsFEBS JOURNAL, Issue 10 2000Richard M. Epand We studied the properties of a series of phosphatidylcholine molecules with branched acyl chains. These lipids have previously been shown to have marked stimulatory effects on the side-chain cleavage activity of cytochrome P450SCC (CYP11A1), an enzyme of the inner mitochondrial membrane. The synthetic lipids used were diacyl phosphatidylcholines with the decanoyl, dodecanoyl or tetradecanoyl chain having a hexyl, octyl or decyl straight chain aliphatic branch at the 2-position. All three lipids lowered the bilayer to hexagonal phase transition temperature of dielaidoyl phosphatidylethanolamine, the lipids with longer acyl chains being more effective in this regard. As pure lipids all of the forms were found by X-ray diffraction to be predominantly in the hexagonal phase (HII) over the entire temperature range of 7,75 °C. The properties of the HII phase were unusual with regard to the small size of the lattice spacings and the small temperature dependence of the spacings. We used tetradecane to relieve hydrocarbon packing constraints to determine the intrinsic radius of curvature of the lipid monolayer. The elastic bending modulus was measured in the presence of tetradecane by introducing an osmotic gradient across the hexagonal phase cylinders with aqueous solutions of poly(ethylene glycol). The elastic bending modulus was found to be higher than that observed with other lipids and to increase with temperature. Both the small intrinsic radius of curvature and the high elastic bending modulus indicate that the presence of these lipids in bilayer membranes will impose a high degree of negative curvature strain. [source] Reproducible Solvent,Thermal Synthesis, Controlled Microstructure, and Photoluminescence of REPO4:Eu3+, Tb3+ (RE=Y, La, and Gd) NanophosphorsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2010Xiuzhen Xiao By altering the temperature and solvents, we have synthesized hydrated and dehydrated Eu3+/Tb3+ -doped REPO4 (RE=Y, La, and Gd) nanophosphors via a solvo-thermal technology. X-ray powder diffraction and scanning electronic microscopy reveal that they have different structures and different morphologies. REPO4 prepared under subsequent heating at 80°, 120°, and 160°C for 1 day, respectively, present larger particle size than that formed by heating at 160°C for 3 days. Moreover, at the same temperature of 160°C and pH 3, three different solvents (ethanol, N,N -dimethylformamide (DMF), and water) were used, whose influence on the microstructure of LaPO4 has been examined. As a result, LaPO4 samples from anhydrous ethanol solvent show a pure hexagonal phase and nanowire morphology, just like that prepared from the water solvent. On the other hand, the microstructure of LaPO4 samples from DMF,H2O-mixed solvents have been changed: with the increasing volume ratio of DMF to H2O, the crystal phase of LaPO4 has been changed from hexagonal phase to monoclinic phase and the morphology from nanowires to nanoparticles. Finally, the photoluminescence properties of these Eu3+ (Tb3+)-activated rare earth phosphates have been investigated, indicating that the photoluminescent behavior are related to their crystal phases and microstructures. [source] Growth and Mechanism of Network-Like Branched Si3N4 NanostructuresJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2010Zhijian Peng The high-yield synthesis of network-like branched silicon nitride (Si3N4) nanostructures by a simple template catalyst-assisted pyrolysis of a polymer precursor, perhydropolysilazane, was reported. The templates were silicon wafers deposited with Fe films of 5,20 nm in thickness. The processes simply involved thermal cross-linking of the preceramic polymer, crushing of the solidified polymer chunks into fine powder, and thermal pyrolysis of the powder under flowing high-purity nitrogen. The collected white network-like branched nanostructures are ,-Si3N4 of hexagonal phase, and their microstructures, in which the diameters of each linear part of the network-like nanostructure varied in a very wide range from tens of nanometers to hundreds of nanometers, strongly depend on the applied growth parameters, where the key factors are the heating rate and catalyst thickness for change in the diameters. It was proposed that the Si3N4 nanonetworks were formed through "metal-absorption on the surface of nanostructures" model by vapor,liquid,solid mechanism. The reaction mechanism of Si3N4 nanonetworks was also discussed. [source] Aqueous Corrosion of the GeSe4 Chalcogenide Glass: Surface Properties and Corrosion MechanismJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2009Yi-Fan Niu The aqueous corrosion behavior of the GeSe4 glass composition has been studied over time under various conditions (temperature and pH). The evolution of the surface topography by atomic force microscopy and properties such as surface hardness and reduced modulus, as well as the optical transmission in the 1,16 ,m window, have been measured as a function of time spent in the corrosive solution. It was found that even if the glass reacts at room temperature, its optical transparency was barely affected. Nevertheless, the durability of GeSe4 was found to be drastically affected by an increase of both temperature and pH. Furthermore, pure selenium nanoparticles were formed during the corrosion process, and the nature of these nanoparticles,amorphous or crystallized (hexagonal phase),depends on temperature. A reaction mechanism was proposed, and the activation energy of the reaction of corrosion in deionized water (47 kJ/mol) was determined from an original technique that relies on the temporal optical loss variation of a GeSe4 optical fiber placed in water at different temperatures. [source] Hydrothermal Synthesis YbMnO3 and LuMnO3 PlateletsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2008Gangqiang Zhu Single-crystalline YbMnO3 and LuMnO3 platelets have been successfully synthesized via a simple hydrothermal process at 250°C for 24 h. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and selected area electron diffraction patterns were used to characterize the as-synthesized samples. The result reveals that both of the as-synthesized YbMnO3 and LuMnO3 are of hexagonal phase. The effect of treatment time on the phase of the final products has been studied, and a possible formation mechanism of YbMnO3 and LuMnO3 is proposed. [source] Preparation of Unsintered Spherical BaFe12O19 Nanoparticles via an Alcohol-Assisted Spray-Pyrolysis RouteJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2008Hye Moon Lee Unsintered spherical barium ferrite (BaFe12O19) nanoparticles were successfully prepared by the alcohol-assisted spray-pyrolysis (AASP) method without any postannealing processes. Ethyl alcohol (EtOH) in the precursor solution for BaFe12O19 particles was very effective not only in its capacity to decrease the particle size, but also in enhancing crystallization into barium ferrite hexagonal phase. As the vol% of EtOH increased from 0 to 20, a decrease of particle size from 136 to 111 nm and an increase of crystallinity were clearly observed. The AASP method was optimized by adjusting the chemical concentration in the precursor solution, and finally, unsintered spherical BaFe12O19 particles, with a geometric mean diameter of 72 nm, a geometric SD of 1.4, and an intrinsic coercivity of 4510 Oe, were successfully prepared. [source] Nitridation of Silica to an ,-Silicon Nitride Nanorod Using NaNH2 in the Autoclave at 700°CJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2007Lingling Zhu ,-silicon nitride nanorods have been synthesized through solid-state reduction,nitridation of silica using NaNH2 as both a reductant and a nitriding reagent. X-ray powder diffraction patterns show that the products have a hexagonal phase with lattice parameters a=7.767 Å and c=5.630 Å. Transmission electron microscopy reveals that the as-synthesized products are pure nanorods with an average size about 30 nm in diameter and 400 nm in length. X-ray photoelectron spectra indicate that the molar ratio of Si/N is 2.988:4. Fourier-transform infrared spectrum yields a strong Si,N absorption at 926 cm,1 that may be a red shift due to size effect. [source] Morphology of ABCD Tetrablock Copolymers Predicted by Self-Consistent Field TheoryMACROMOLECULAR THEORY AND SIMULATIONS, Issue 4 2005Rong Wang Abstract Summary: We studied the two-dimensional (2D) microphase-separated morphology of linear ABCD tetrablock copolymers by self-consistent field theory. By varying the interaction parameters and the compositions, we found at least twelve structures, two of which , "four-color" lamellae and "three-color" core-shell hexagonal phase , prove the existing experimental observations. These morphologies were discussed in correlation with the volume fraction of the components and the interaction parameters. A specific behavior of symmetrical tetrablock copolymers, i.e., fA,=,fD and fB,=,fC, is that the stable phases are lamellae, which is different from symmetrical ABC triblock copolymer having order-to-order transition. These results are helpful for the design of new block copolymer-based nanomaterials. [source] Identification of van Hove singularities in the GaN dielectric function: a comparison of the cubic and hexagonal phasePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2009C. Cobet Abstract We present a detailed analysis of interband transition structures in the dielectric function of GaN. The dielectric function of the stable wurtzite and the metastable zinc blende phase were determined by means of synchrotron spectroscopic ellipsometry in the spectral range between 3 eV and 20 eV where the most significant structures of the dielectric function are located. In the hexagonal case, both the ordinary and extraordinary dielectric tensor component was measured on GaN films with M -plane/[1 00] orientation. In a comparative discussion of the two hexagonal tensor components and the zinc blende dielectric function, all prominent absorption structures were assigned to specific interband transitions at high symmetry points in the Brillouin zone. The assignment considers the individual dipole transition probabilities depending on the crystal symmetry and the geometry of the measurement. Furthermore, a detailed theoretical band-to-band analysis of dielectric function features, published by Lambrecht et al. [1], was considered. In conclusion, we suggest a new labeling of absorption structures as used in classical III,V materials like GaAs, which reflects the origin of transition structures from specific points in the respective Brillouin zones. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Phase transitions and transport phenomena in Li0.25Cu1.75Se superionic compoundPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 15 2004M. Kh. Abstract Phase transformation points in Li0.25Cu1.75Se mixed electronic,ionic conductor have been determined by calorimetric, conductometric and thermoelectric measurements. The phase transformation (PT) from triclinic to monoclinic occurs at 403,413 K. At 503,515 K the monoclinic phase is followed by a rhombohedral modification. Both of these PTs are accompanied by drops on the calorimetric curve. At about 653 K observed anomalies in the temperature dependencies of the ionic conductivity, of the chemical diffusion coefficient and the jump of the ionic Seebeck coefficient have been induced by the PT to hexagonal phase. Neutron diffraction studies reveal the cubic structure of Li0.25Cu1.75Se compound (with space group Fm3m) at 773 K. The corresponding PT causes anomalies in the electrical and diffusion properties at 703,713 K. Cu ions are statistically distributed over tetrahedral and trigonal voids in an Fm3m cage; lithium ions randomly occupy 32(f) positions. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Calculated high pressure crystal structure transformations for phosphorusPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003Rajeev Ahuja Abstract In this paper we have studied the structural stability of the sp bonded element, P, by means of the first principles calculations. The theoretical calculations made use of a full potential linear muffin-tin orbital (FPLMTO) method adopting the local density approximation to the density functional theory. We reproduce the observed crystallographic phase stability of P as a function of compression. Our results confirm the recent experimental finding of Akahama et al. We have also proposed a new structure for an experimentally reported unidentified intermediate phase in between simple cubic and simple hexagonal phase. This new structure is similar to what has been observed for Si. We have explained the stability of different phases under pressure using our calculated density of states (DOS). [source] Effect of annealing temperature on the crystalline quality and phase transformation of Chemically Deposited CdSe filmsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2005M. Zapata-Torres Abstract Polycrystalline CdSe thin films were grown on glass substrates by chemical bath deposition at 50 ºC. The samples were annealed in air atmosphere at different temperatures and characterized by X-ray diffraction and Raman spectroscopy. It was found that the as-grown films have cubic structure. These samples maintain their cubic structure for annealing temperatures between 60 ºC and 300 ºC. For annealing temperatures higher than 300 ºC we obtain a mixture of cubic and hexagonal phases. The analysis made by X-ray diffraction and Raman dispersion show that the samples annealed at temperatures under the phase-transition temperature increase their crystalline quality. In order to determinate the temperature for the complete transition of the cubic phase, we used the precipitated material obtained during the grown of the CdSe films. This material was annealed on air atmosphere between 300 °C and 500 °C with 50 ° intervals. The samples were measured by X-ray diffraction. The samples maintained the cubic structure if the annealing temperature is under 300 °C. For temperatures between 300 °C and 450 °C we found a mixture of cubic and hexagonal phase. For an annealing temperature of 500°C we obtain only the hexagonal phase. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Star-Shaped Oligobenzoates: Non-conventional Mesogens Forming Columnar Helical MesophasesCHEMISTRY - A EUROPEAN JOURNAL, Issue 12 2008Matthias Lehmann Dr. Abstract Star-shaped mesogens with a phloroglucinol or a trimesic acid core and oligobenzoate arms with up to five repeating units have been synthesised. These non-conventional mesogens form various columnar mesophases over a broad temperature range. The liquid-crystal phases were characterised by optical microscopy, differential scanning calorimetry, X-ray diffraction, dilatometry and solid-state NMR spectroscopy. In addition to the high-temperature hexagonal columnar phases, the columnar self-assemblies undulate upon cooling and consequently form higher-ordered body-centred orthorhombic columnar 3D structures. A model of E -shaped folded conformers helically displaced along the columns is proposed. Helical preorganisation in the hexagonal phase precedes the transition to the low-temperature phases. Space filling and nano-segregation compete in the self-organisation process, thus aliphatic chains and the polar oligobenzoate scaffold are not perfectly separated in these star-shaped mesogens. [source] Use of micro-Raman spectroscopy to study reaction kinetics in blended white cement pastes containing metakaolinJOURNAL OF RAMAN SPECTROSCOPY, Issue 12 2009Moisés Frías Abstract Curing temperature is known to play an important role in the formation, development, and stability of the hydrated phases appearing during pozzolanic reactions (chemical reaction between puzzolanic addition, metakaolin (MK), and calcium hydroxide from cement hydration). A typical example of this important reaction is to be found in metakaolin-bearing cement pastes, characterized by hexagonal phases whose thermodynamic stability declines with rising temperature. These phases cannot be exhaustively researched with traditional techniques (such as X-ray diffraction) due to their poor crystallinity. Consequently, micro-Raman spectroscopy was used in the present study to explore the behavior of white cement paste blends containing 0, 10, and 25% MK at two curing temperatures (20 and 60 °C). This led to the identification, for the first time using Raman spectroscopy, of phases C2ASH81 (stratlingite) and C3ASH6, which appear in the MK,white cement reaction. The CSH gel formed was characterized by Q1 dimers and a C/S ratio of 1.3,1.5. Raising the curing temperature favored the formation of C4AH13. Copyright © 2009 John Wiley & Sons, Ltd. [source] Effect of annealing temperature on the crystalline quality and phase transformation of Chemically Deposited CdSe filmsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2005M. Zapata-Torres Abstract Polycrystalline CdSe thin films were grown on glass substrates by chemical bath deposition at 50 ºC. The samples were annealed in air atmosphere at different temperatures and characterized by X-ray diffraction and Raman spectroscopy. It was found that the as-grown films have cubic structure. These samples maintain their cubic structure for annealing temperatures between 60 ºC and 300 ºC. For annealing temperatures higher than 300 ºC we obtain a mixture of cubic and hexagonal phases. The analysis made by X-ray diffraction and Raman dispersion show that the samples annealed at temperatures under the phase-transition temperature increase their crystalline quality. In order to determinate the temperature for the complete transition of the cubic phase, we used the precipitated material obtained during the grown of the CdSe films. This material was annealed on air atmosphere between 300 °C and 500 °C with 50 ° intervals. The samples were measured by X-ray diffraction. The samples maintained the cubic structure if the annealing temperature is under 300 °C. For temperatures between 300 °C and 450 °C we found a mixture of cubic and hexagonal phase. For an annealing temperature of 500°C we obtain only the hexagonal phase. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||||||||