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Intermediate Phase (intermediate + phase)
Selected Abstractso -ZrW1.6Mo0.4O8: A Novel Orthorhombic Intermediate Phase Formed During the Synthesis of the Negative Thermal Expansion Cubic ZrW1.6Mo0.4O8 Material by the Precursor Dehydration RouteCHINESE JOURNAL OF CHEMISTRY, Issue 12 2003Xin-Hua Zhao Abstract The mechanism of the precursor dehydration route was revealed for the synthesis of NTE c-ZrW1.6Mo0.4O8. The hydrate precursor was dehydrated at 473 K and transformed to a NTE cubic compound above 800 K. A novel intermediate phase o -ZrW1.6Mo0.4O8 occurs between the temperature range of 573,800 K. The XRD pattern of novel intermediate was refined with the structural model of LT-ZrMo2O8 by using Rietveld method. The residuals are Rwp = 7.80% and Rp = 5.79%. The space group is Pmn21 and the lattice parameters are a = 0.5917(4) nm, b =0.7273(4) nm, c =0.9148(6) nm, and Z = 2. [source] Thermal dehydration kinetics of a rare earth hydroxide, Gd(OH)3INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 2 2007Chengkang Chang This paper reports the synthesis, characterization, and dehydration kinetics of a rare earth hydroxide, Gd(OH)3. Uniform rod-like Gd(OH)3 powder was prepared by a colloidal hydrothermal method. The powder thus obtained dehydrated into its oxide form in a two-step process, where crystalline GdOOH was obtained as the intermediate phase. Crystal structure study revealed a monoclinic structure for GdOOH, with space group P2/1m and lattice parameters a = 6.0633, b = 3.7107, c = 4.3266, and , = 108.669. The first-step dehydration follows the F2 mechanism, while the second step follows the F1 model, indicating that both the steps are controlled by nucleation/growth mechanism. The activation energy Ea and frequency factor A are 231±12 kJ/mol and 2.08 × 1018 s,1 for the first step and 496 ± 32 kJ/mol and 7.88 × 1033 s,1 for the second step, respectively. Such high activation energy calculated from the experimental data can be ascribed to the high bonding energy of GdO bond, and the difference in activation energy for the two steps is due to the change in the bond length of hexagonal Gd(OH)3 and monoclinic GdOOH. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 75,81, 2007 [source] Chagas' disease reactivation with skin symptoms in a patient with kidney transplantINTERNATIONAL JOURNAL OF DERMATOLOGY, Issue 6 2007Verónica Gallerano MD Immunodepressed patients in the intermediate phase of Chagas' disease may undergo reactivation of the disease together with atypical symptoms. The case of an immunodepressed kidney transplant patient with reactivation of Chagas' disease with skin symptoms is reported. A 65-year-old man presented with infiltrated erythematous lesions on the anterior aspect of the right thigh of 2 weeks' duration. The lesions later extended to the abdomen, thorax, and lower limbs. In the histologic skin examination, amastigotes and Trypanosoma cruzi trypoamastigotes were observed. A fresh smear showed positive parasitemia. Using the Strout hemoconcentration method, multiple Trypanosoma cruzi trypoamastigotes with motility could be seen. Polymerase chain reaction was positive for Trypanosoma cruzi. An immunofluorescence test was positive (1 : 64) and there was hemoagglutination (1 : 32). Treatment was started with benznidazole, 7 mg/kg/day. The patient did not evolve favorably and died 20 days after hospitalization. Skin lesions may be a manifestation of the reactivation of Chagas' disease in immunosuppressed patients. All patients with positive Chagas' serology who require immunosuppressant drugs should receive specific treatment for Chagas' disease. [source] A new route for the direct synthesis of Al2O3/SiC nanopowder mixtures by the carbothermal reduction of parent oxidesAICHE JOURNAL, Issue 5 2010Cavus Falamaki Abstract The direct synthesis of Al2O3/SiC nanopowders from the parent oxides Al2O3 and SiO2 through mullite carbothermal reduction as intermediate phase has been investigated. The effect of the amount of excess stoichiometric carbon (active charcoal, AC) as sole carbon source on the microstructure evolution has been studied. The effect of type of carbon source (AC, graphite (G), 50 wt % AC 50 wt % G mixture, 57 wt % AC 43 wt % G mixture) on the microstructure evolution was investigated using 30 wt % excess stoichiometric carbon. The effect of reaction temperature, reaction duration, initial green compact thickness, and carbon source on the mullite conversion, morphology, and surface area of the final powders has been thoroughly investigated. The calculated activation energy is in the range of 203,230 kJ mol,1, depending on the carbon source used. The synergetic effect observed for the AC/G mixtures has been accordingly explained. © 2010 American Institute of Chemical Engineers AIChE J, 2010 [source] The effect of a finite mass reservoir on the collapse of spherical isothermal clouds and the evolution of protostellar accretionMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2005E. I. Vorobyov ABSTRACT Motivated by recent observations that detect an outer boundary for starless cores, and evidence for time-dependent mass accretion in the Class 0 and Class I protostellar phases, we re-examine the case of spherical isothermal collapse in the case of a finite mass reservoir. The presence of a core boundary, implemented through a constant-volume approximation in our simulation, results in the generation of an inward-propagating rarefaction wave. This steepens the gas density profile from r,2 (self-similar value) to r,3 or steeper. After a protostar forms, the mass accretion rate evolves through three distinct phases: (1) an early phase of decline in , which is a non-self-similar effect due to rapid and spatially non-uniform infall in the pre-stellar phase; (2) for large cores, an intermediate phase of near-constant from the infall of the outer part of the self-similar density profile, which has low (subsonic) infall speed in the pre-stellar phase; and (3) a late phase of rapid decline in when accretion occurs from the region affected by the inward-propagating rarefaction wave. Our model clouds of small to intermediate size make a direct transition from phase (1) to phase (3) above. Both the first and second phase (if the latter is indeed present) are characterized by a temporally increasing bolometric luminosity Lbol, while Lbol is decreasing in the third (final) phase. We identify the period of temporally increasing Lbol with the Class 0 phase, and the later period of terminal accretion and decreasing Lbol with the Class I phase. The peak in Lbol corresponds to the evolutionary time when 50 ± 15 per cent of the cloud mass has been accreted by the protostar. This is in agreement with the classification scheme proposed in the early 1990s by André et al.; our model adds a physical context to their interpretation. We show how our results can be used to explain tracks of envelope mass Menv versus Lbol for protostars in Taurus and Ophiuchus. We also develop an analytic formalism that successfully reproduces the protostellar accretion rate from profiles of density and infall speed in the pre-stellar phase. It shows that the spatial gradient of infall speed that develops in the pre-stellar phase is a primary cause of the temporal decline in during the early phase of protostellar accretion. [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] Natural oxidation of InN quantum dots: the role of cubic InNPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2010David González Abstract The natural aging process occured in indium nitride quantum dots (QDs) heterostructures as a consequence of exposure to the atmosphere has been studied by means of transmission electron microscopy and electron beam related techniques. The comparison between GaN-capped and uncapped InN QDs kept at room conditions during 36 months indicates the structural changes that take place. While the capping layer seems to act in a protective way avoiding any change in the QDs, the uncapped structures suffer a series of phase transformations, where the original wurtzite structure is replaced by a layer of cubic phases. The main constituent of this layer is shown to be bcc-In2O3 formed by the substitution of the nitrogen atoms by oxygen from the atmosphere. This supposes a transformation from a hexagonal to a cubic structure, explained by the existence of an oxygen-rich cubic InN acting as an intermediate phase. The difference in the formation enthalpy between the original and the final product, together with the good match between the crystals would explain this transformation that shows the high instability of InN at environmental conditions. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] On the mechanism of some first-order enantiotropic solid-state phase transitions: from Simon through Ubbelohde to MnyukhACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2006Frank H. Herbstein The first (so-called) lambda transition in solids was found in the specific heat measurements for NH4Cl at 242,K by F. Simon in 1922 [Simon (1922). Ann. Phys.68, 241,280]. Analogous phenomena found in many other solids gave rise to doubts (expressed most clearly by A. R. Ubbelohde some 50 years ago) about the applicability of classical thermodynamics to some phase transitions [Ubbelohde (1956). Brit. J. Appl. Phys.7, 313,321]. However, Y. Mnyukh's studies of enantiotropic phase transitions in eight organic crystals showed that all proceed by a nucleation-and-growth mechanism [summarized in Mnyukh (2001), Fundamentals of Solid State Phase Transitions, Ferromagnetism and Ferroelectricity. 1st Books]. Nucleation is localized at defects in the parent phase; growth can be epitaxic and oriented if parent and daughter phases have closely similar structures, or random (not oriented) if there are substantial structural differences. This conclusion is supported by a critical review of Mnyukh's eight examples and other results published in the interim. It seems that Ubbelohde's invocation of `hybrid crystals' and `smeared transitions' can mostly be accounted for by lack of equilibrium in the phase-transition studies cited by him. However, the intermediate phase in 4,4,-dichlorobenzophenone appears to have structural resemblances to Ubbelohde's' `hybrid crystal'. [source] Interplay Between Kinetically Slow Thermal Spin-Crossover and Metastable High-Spin State Relaxation in an Iron(II) Complex with Similar T1/2 and T(LIESST)CHEMISTRY - A EUROPEAN JOURNAL, Issue 19 2007Victoria Abstract This paper describes the first material to show the well-known light-induced excited spin-state trapping (LIESST) effect, the metastable excited state of which relaxes at a temperature approaching its thermal spin-crossover. Cooling polycrystalline [FeL2][BF4]2,x,H2O (L=2,6-bis{3-methylpyrazol-1-yl}pyridine; x=0,1/3) at 1,K,min,1 leads to a cooperative spin transition, taking place in two steps centered at 147 and 105,K, that is only 54,% complete by magnetic susceptibility. Annealing the sample at 100,K for 2,h results in a slow decrease in ,MT to zero, showing that the remainder of the spin-crossover can proceed, but is kinetically slow. The crystalline high- and fully low-spin phases of [FeL2][BF4]2,x,H2O are isostructural (C2/c, Z=8), but the spin-crossover proceeds via a mixed-spin intermediate phase that has a triple unit cell (C2/c, Z=24). The water content of the crystals is slowly lost on exposure to air without causing decomposition. However, the high-spin/mixed-spin transition in the crystal proceeds at 110±20,K when x=1/3 and 155±5,K when x=0, which correspond to the two spin-crossover steps seen in the bulk material. The high-spin state of the compound is generated quantitatively by irradiation of the low-spin or the mixed-spin phase at 10,K, and in approximately 70,% yield by rapidly quenching the sample to 10,K. This metastable high-spin state relaxes back to the low-spin ground state at 87±1,K in one, not two, steps, and without passing through the intermediate phase. This implies that thermal spin-crossover and thermally activated high-spin,low-spin relaxation in this material become decoupled, thus avoiding the physical impossibility of T(LIESST) being greater than T1/2. [source] In-situ Energy-Dispersive X-ray Diffraction Studies of Crystal Growth and Compound Conversion Under Solvothermal ConditionsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 3 2003Lars Engelke Abstract The results of in-situ energy-dispersive X-ray diffraction under solvothermal conditions performed on isostructural, layered thioantimonates Mn2Sb2S5·L (L = amine) demonstrate the great potential of the method. When the synthesis was carried out at low temperatures with L being 1,3-diaminopropane (DAP), two crystalline intermediate phases were detected which then grew and disappeared when product growth started. Surprisingly, when N -methyl-1,3-diaminopropane (MDAP) was used, no crystalline intermediates could be detected and the induction time was significantly shorter than for DAP. For reactions up to 100 °C and for higher temperatures with , < 0.8 (, is the extent of reaction), the crystallisation is predominantly controlled by nucleation. Further progress of crystallisation (, > 0.8) leads to a change of the dominant process and a diffusion-controlled mechanism is observed. During the reaction with diethylenetriamine (DIEN), three crystalline intermediates were detected prior to product growth. The induction time is longer than for the other two compounds. The crystallisation seems to be diffusion-controlled and is faster than for the DAP and MDAP compounds. In a solution of DIEN, the crystalline phases Mn2Sb2S5·L (L = DAP or MDAP) are transformed into the DIEN product under solvothermal conditions, and a rigorous analysis of the intensities of the reflections suggests a partial dissolution of the crystalline starting materials followed by crystallisation of the DIEN material. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] In situ X-ray diffraction analysis of iron ore sinter phasesJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2004Nicola V. Y. Scarlett Owing to the depletion of world lump iron ore stocks, pre-treated agglomerates of fine ores are making up a growing proportion of blast-furnace feedstock (,80%). These agglomerations, or `sinters', are generally composed of iron oxides, ferrites (most of which are silicoferrites of calcium and aluminium, SFCAs), glasses and dicalcium silicates (C2S). SFCA is the most important bonding phase in iron ore sinter, and its composition, structural type and texture greatly affect its physical properties. Despite its prevalence and importance, the mechanism of SFCA formation is not fully understood. In situ powder X-ray diffraction investigations have been conducted into the formation of SFCA, allowing the study of the mechanism of its formation and the observation of intermediate phases with respect to time and temperature. Studies have been carried out to investigate the effects of changing the substitution levels of aluminium for iron. The use of the Rietveld method for phase quantification gives an indication of the order and comparative rates of phase formation throughout the experiments. [source] Preparation of Ba6,3xNd8+2xTi18O54 via Ethylenediaminetetraacetic Acid PrecursorJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2000Yebin Xu Ba6,3xNd8+2xTi18O54 ceramic powders were synthesized by the modified Pechini method using ethylenediaminetetraacetic acid (EDTA) as a chelating agent. A purplish red, molecular-level, homogeneously mixed gel was prepared, and transferred into a porous resin intermediate through charring. Single-phase and well-crystallized Ba6,3xNd8+2xTi18O54 powders were obtained from pulverized resin at a temperature of 900°C for 3 h, without formation of any intermediate phases. Meanwhile, the molar ratio of EDTA to total metal cation concentration had a significant influence on the crystallization behavior of Ba6,3xNd8+2xTi18O54. The Ba6,3xNd8+2xTi18O54 (x= 2/3) ceramics prepared via EDTA precursor have excellent microwave dielectric characteristics: ,= 87, Qf= 8710 GHz. [source] ,-Ca3N2, a Metastable Nitride in the System Ca,NCHEMISTRY - A EUROPEAN JOURNAL, Issue 14 2009Peter Höhn Dr. Abstract A new page in the phase book: The metastable nitride ,-Ca3N2, the synthesis, crystal structure (see picture), and physical properties of which are reported, is an isotype of corundum (,-Al2O3). Vacancies in the crystal structure of ,-Ca3N2 are discussed by using full-potential local orbital methods and taking into account both the physical properties and the density of states. The structural chemistry of calcium nitrides is represented by just two different intermediate phases: cubic Ca3N2 (referred to as ,-Ca3N2, Mn2O3 type structure) and Ca2N (CdCl2 type structure). Other binary phases (",-Ca3N2", "Ca11N8") have been proven to represent cyanamide nitrides, Ca4N2[CN2] and Ca11N6[CN2]2, respectively. The metastable nitride ,-Ca3N2 is prepared either by reaction of freshly distilled calcium metal with nitrogen at 700,K, or by reaction of Ca2N with N2 at 500,K. ,-Ca3N2 transforms monotropically (Ttrans,810,K) to cubic ,-Ca3N2. The crystal structure of ,-Ca3N2 (rhombohedral Rc, no. 167, a=619.884(3),pm, c=1662.958(10),pm) is an isotype of the corundum structure (,-Al2O3) and comprises a network of NCa6/4 octahedra that share common faces and edges. Magnetic susceptibility and electrical resistivity measurements confirm that ,-Ca3N2 is a diamagnetic semiconductor. Die Strukturchemie der Calciumnitride wird von lediglich zwei intermediären Verbindungen bestimmt: kubisches Ca3N2 (nachfolgend als , -Ca3N2 bezeichnet; Mn2O3 -Strukturtyp) und das im CdCl2 -Strukturtyp kristallisierende Ca2N. Die binären Phasen ,, -Ca3N2" und ,Ca11N8" konnten inzwischen als die Cyanamid-Nitride Ca4N2[CN2] und Ca11N6[CN2]2 identifiziert werden. Metastabiles , -Ca3N2 kann entweder durch Reaktion von frisch destilliertem Calcium-Metall mit Stickstoff bei 700,K oder durch Umsetzung von Ca2N mit N2 bei 500,K erhalten werden. , -Ca3N2 transformiert monotrop (Ttrans,810,K) zur kubischen Phase , -Ca3N2. Die Kristallstruktur von , -Ca3N2 (rhomboedrisch Rc, No. 167, a=619.884(3),pm, c=1662.958(3),pm) ist isotyp zur Korundstruktur (, -Al2O3) und besteht aus einem Netzwerk von NCa6/4 -Oktaedern, die über gemeinsame Flächen und Kanten verknüpft sind. Messungen der magnetischen Suszeptibilität und des elektrischen Widerstands zeigen, daß , -Ca3N2 ein diamagnetischer Halbleiter ist. [source] | |||||||||||||||||||||||||