Photoluminescence and magnetic properties of β-Ni(OH)2 nanoplates and NiO nanostructures

Single-crystalline β-nickel hydroxide (β-Ni(OH)₂) nanoplates of hexagonal structure have been synthesized through hydrothermal process. The β-Ni(OH)₂ nanoplates possess well-defined hexagonal shapes with landscape dimension of 45–140 nm and thickness of 20–50 nm. Post-thermal decomposition of the β-Ni(OH)₂ nanoplates led to the formation of single-crystalline NiO nanostructures with landscape dimension of 25–120 nm including nanorolls, nanotroughs and nanoplates. The sizes of the central hole in NiO nanorolls and the low-lying ground in NiO nanotroughs are in the range of 10–24 nm. Two photoluminescence emission peaks appear at 390.5 nm and 467 nm in the photoluminescence spectrum of NiO nanostructures and were assigned to the ¹T_{1 g} (G) → ³A_{2 g} and ¹T_{2 g} (D) → ³A_{2 g} transitions of Ni²⁺ in oxygen octahedral sites, respectively. Temperature-dependent magnetic measurement results show that an antiferromagnetic-paramagnetic transition occur at 26.3 K in β-Ni(OH)₂ nanoplates.     

Precipitation Processes in Al-Mg-(Mn,Cu) Type Alloy Sheets Evaluated Through Electrical Resistivity Variations

Precipitation/dissolution processes were followed by electrical resistivity variations in Al-Mg-(Mn)-Cu and Al-Mg-Mn type alloy sheets after different thermo-mechanical treatments (TMTs). In order to get an insight into the precipitation processes during processing of Al-Mg type alloys, some samples were solution treated at 535°C/1 h, and some of them were recrystallization annealed at 320°÷350°C/3 h. After that all the samples were treated in a same manner: cold rolling to 50–60% and final annealing at temperatures in the range of 220°÷470°C/3 h. It was supposed that the structure of samples pre-treated by recrystallization annealing at 320°/350°C is characterized with S type (Al₂MgCu) phases in the Al-Mg-(Mn)-Cu alloys and with β′/β (Mg₅Al₈) phases in the Al-Mg-Mn alloys. These structure features are rather unchangeable during the subsequent cold rolling and annealing treatments in the temperature range of 220°÷320°C. During annealing at higher temperatures (such as 470°C) precipitation of Mn-bearing particles (MnAl₆ or (Fe,Mn)Al₆) might occur in both type of alloys. After solution treatment at 535°C most of the alloying elements are dissolved into solid solution, thus inducing a high potential for precipitation processes during annealing at 220°÷470°C. The cold deformation was found to contribute the electrical resistivity intensively at the beginning of deformation (in the range up to ∼20%), and it was not influenced by the chemical composition of tested alloys. The resistivity variations with the cold deformation were fitted by power-law equations.     

Thermoanalytical study of the polymorphism and melting behavior of Cu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>

We have developed a procedure for the synthesis of phase-pure α- and β-Cu₂V₂O₇. Thermal analysis and X-ray diffraction demonstrate that the β-phase (monoclinic structure) exists at low temperatures (stability range 25–610°C), while α-Cu₂V₂O₇ (orthorhombic structure) is stable in the range 610–704°C. The α-phase observed during cooling, in particular at room temperature, is in a metastable state. The melting of the high-temperature phase γ-Cu₂V₂O₇, which forms between 704 and 716°C, has the highest rate in the range 770–785°S and is accompanied by peritectic decomposition and oxygen gas release. Subsequent cooling gives rise to four exothermic peaks, one of which (780.9°C) is attributable to the crystallization of the peritectic melt, one (620.1°C) is due to the γ → α → β phase transformations of Cu₂V₂O₇, and the other two arise from the crystallization of multicomponent low-melting-point eutectics containing α- and β-Cu₂V₂O₇, CuVO₃, and other compounds.     

Sonochemical synthesis of calcium phosphate powders

β-tricalcium phosphate (β-TCP) and biphasic calcium phosphate powders (BCP), consisting of hydroxyapatite (HA) and β-TCP, were synthesized by thermal decomposition of precursor powders obtained from neutralization method. The precursor powders with a Ca/P molar ratio of 1.5 were prepared by adding an orthophosphoric acid (H₃PO₄) solution to an aqueous suspension containing calcium hydroxide (Ca(OH)₂). Mixing was carried out by vigorous stirring and under sonochemical irradiation at 50 kHz, respectively. Glycerol and D-glucose were added to evaluate their influence on the precipitation of the resulting calcium phosphate powders. After calcination at 1000°C for 3 h BCP nanopowders of various HA/β-TCP ratio were obtained.     

Influence of the conditions of formation on the structure and phase composition of subsurface layers of uranium coated with the high-temperature oxide

It is established by x-ray structure analysis that the oxide film formed on the surface of uranium during oxidation by dry oxygen at pressures of 0.1–0.001 Pa and temperatures of 500–700 °C for several hours consists of UO₂ with U₄O₉ inclusions. Near the surface of the uranium the temperature of the α→β phase transition is lowered by 150–160 °C in comparison with the transition in the bulk of the metal, and the low-temperature stabilization of the β phase of uranium is observed. Pis’ma Zh. Tekh. Fiz. 24, 7–11 (June 12, 1998)     

Low temperature preparation ofβ-alumina by an alkoxide route

β-alumina has been prepared by the thermal decomposition of a mixture of sodium and aluminium isopropoxides followed by heating up to 1000°C. It has been found necessary to use sodium isopropoxide in excess (25–30%) for the complete formation ofβ-alumina at 1000°C. On the other hand one obtains a mixture ofβ-alumina andα-alumina when the starting materials are taken in the stoichiometric ratio Na₂O:11Al₂O₃. DTA, TG and DTG studies of a mixture of sodium and aluminium isopropoxide showedβ-alumina formation at 1000°C. NCL Communication No. 4446.     

Properties of reaction bonded silicon nitride obtained from slip cast preforms

Fabrication of silicon preforms of high green density (>1·2 g/cm³) by slip casting of silicon (in aqueous medium) has been studied. The nitridation product consists of 59–85% α-Si₃N₄, 7–22%β-Si₃N₄ and 7–23% Si₂N₂O phase. The amounts of un-nitrided silicon were negligible. The microstructure is either granular or consists of needle-like grains (α-Si₃N₄) and whiskers deposited in the large pores. MOR values of the specimens are almost constant up to 1000°C or 1400°C or show slight increase up to 1000°C or 1200°C. In some cases a little dip around 1200°C, then a sharp increase in MOR up to 1400°C was observed.K _ic values are almost constant up to 1000°C, and thereafter increase sharply. Pore size distribution, existence of Si₂N₂O phase and oxidation of RBSN at high temperatures have been considered for the explanation of the observed behaviour.     

Deformation and transformation mechanisms of poly(vinylidene fluoride) (PVF2)

The deformation process and the accompanyingα→β phase transformation of poly(vinylidene fluoride), drawn at 82 to 90 and 130° C, has been characterized by electron microscopy and X-ray and electron diffraction. Micronecking occurs at both draw temperatures, fibrils and mosaic blocks being drawn off the edges of the micronecks. The degree of phase transformation, at the same elongation, is dependent on the draw temperature; at 130° C the majority of the sample remains in theα phase at the natural draw ratio. The phase transformation at both draw temperatures accompanies the transformation from lamellar (block) structure to fibril.     

KMg2AlSi4O12 phyllosiloxide as potential interphase material for ceramic matrix composites: Part 1 Chemical compatibility

KMg₂AlSi₄O₁₂ is a phyllosiloxide isostructural with phlogopite mica, but totally free of OH^- ions. It decomposes at ≈ 950 °C at atmospheric pressure but remains stable up to at least 1350 °C under high pressures. Its chemical compatibility with α-alumina, MgAl₂O₄ spinel, forsterite, β-SiC and borosilicate glass selected as representative of fibres and matrices in ceramic matrix composites (CMCs), has been assessed via annealing experiments on multilayers and particulate composites at 900–1200 °C. At T = 900 °C and P = 100 MPa, the phyllosiloxide is chemically stable with respect to all the ceramics. At higher temperatures, interdiffusion occurs with the formation of various reaction products. At T = 1050 °C and P = 2 GPa, the extent of the reaction zone is larger for both α-alumina and forsterite than for spinel and β-SiC, whereas at 1200 °C, the reactivity of the phyllosiloxide with all the ceramics becomes about the same. Borosilicate glass with a softening point lower than the decomposition onset of KMg₂AlSi₄O₁₂ at relatively low pressures seems to be an ideal model matrix material for assessing the potential of the phyllosiloxide as an interphase material in CMCs. This revised version was published online in November 2006 with corrections to the Cover Date.     

Preparation of boron microcrystals via high-pressure,high-temperature pyrolysis of decaborane,B<Subscript>10</Subscript>H<Subscript>14</Subscript>

Crystalline boron has been prepared via high-pressure, high-temperature pyrolysis of decaborane, B₁₀H₁₄. We obtained α-tetragonal boron crystals at a pressure of 8–9 GPa and temperatures in the range 1100–1600°C and β-rhombohedral boron intergrowths at 3 GPa and 1200°C.     

Electrical,thermal and infrared studies of cadmium metavanadate

Cadmium(II) metavanadate has crystal structure related to brannerite (ThTi₂O₆) structure. The high temperatureβ-CdV₂O₆ phase isn-type semiconductor between 185 and 750°C. The electrical conduction in theβ-CdV₂O₆ occurs due to deviation from oxygen stoichiometric composition of the lattice. The seebeck coefficient (α) of the sample is negative and constant in the entire range of investigation. The mechanism of transport in cadmium metavanadate lattice is via thermally activated hopping of localized electrons on vanadium (V⁵⁺) sites of the lattice. The DTA result indicated that CdV₂O₆ undergoes phase transition at 185°C and not at 670°C as reported earlier. There is no DTA evidence to show the possibility ofβ →α phase reverse transition. The XRD powder patterns of the two modifications are nearly similar indicating brannerite related structures. The infrared absorption band of vanadium-oxygen stretching vibration modes of distorted VO₆ octahedra ofβ-CdV₂O₆ is exhibited at 855 cm⁻¹.     

Strength and fracture toughness of in situ-toughened silicon carbide

Fine β-SiC powders either pure or with the addition of 1 wt % of α-SiC particles acting as a seeding medium, were hot-pressed at 1800 °C for 1 h using Y₂O₃ and Al₂O₃ as sintering aids and were subsequently annealed at 1900 °C for 2, 4 and 8 h. During the subsequent heat treatment, the β → α phase transformation of SiC produced a microstructure of “in situ composites” as a result of the growth of elongated large α-SiC grains. The introduction of α-SiC seeds into the β-SiC accelerated the grain growth of elongated large grains during annealing which led to a coarser microstructure. The sample strength values decreased as the grain size and fracture toughness continued to increase beyond the level where clusters of grains act as fracture origins. The average strength of the in situ-toughened SiC materials was in the range of 468–667 MPa at room temperature and 476–592 MPa at 900 °C. Typical fracture toughness values of 8 h annealed materials were 6.0 MPa m^1/2 for materials containing α-SiC seeds and 5.8 MPa m^1/2 for pure β-SiC samples. This revised version was published online in November 2006 with corrections to the Cover Date.     

Phase transition in copper(II) pyrovanadate

A new phase Cu₂V₂O₇ synthesized, exhibits phase transitions between 475°C and 500°C. These phase transitions are reversible with ease in contrast toα →β phase transition at 712°C of Cu₂V₂O₇ phase reported earlier. These phase transitions are identified by DTA technique and characterized by detailed XRD investigations at different temperatures. The crystal structures of these Cu₂V₂O₇ phases are related to either thortveitite (Sc₂Si₂O₇) type or a modification of it.     

Synthesis of Mg-α-sialon from the mixture of silicon, aluminum and magnesia powders in a flowing nitrogen atmosphere

Synthesis of Mg-α-Sialon has been investigated by the mixture of silicon, aluminum and magnesia powders in a flowing nitrogen atmosphere in the range of 1300–1600 °C, when Mg-α-Sialon is designed with a chemical formulation of Mg _x Si_12−3x Al_3x O _x N_16−x in present work. The results showed that Mg-α-sialon initially occurred at 1400 °C and basically increased with elevated temperatures. For the samples of x = 0.6, 0.8 and 1.0 the products mainly consisted of Mg-α-Sialon with small amounts of Si, AlN and 21R AlN-polytypoid phases at 1600° C. However, in final products of x = 1.2, 1.4 and 1.6 only a little of Mg-α-Sialon formed and a great amount of Si remained in these samples at all the fired temperatures. Fortunately, the content of Mg-α-Sialon in these samples were obviously increased by adding a small amount of α-Si₃N₄ as seeds before nitridation.     

Alpha-tricalcium phosphate (α-TCP): solid state synthesis from different calcium precursors and the hydraulic reactivity

The effects of solid state synthesis process parameters and primary calcium precursor on the cement-type hydration efficiency (at 37°C) of α-tricalcium phosphate (Ca₃(PO₄)₂ or α-TCP) into hydroxyapatite (Ca_10−xHPO₄(PO₄)_6−x(OH)_2−x x = 0–1, or HAp) have been investigated. α-TCP was synthesized by firing of stoichiometric amount of calcium carbonate (CaCO₃) and monetite (CaHPO₄) at 1150–1350°C for 2 h. Three commercial grade CaCO₃ powders of different purity were used as the starting material and the resultant α-TCP products for all synthesis routes were compared in terms of the material properties and the reactivity. The reactant CaHPO₄ was also custom synthesized from the respective CaCO₃ source. A low firing temperature in the range of 1150–1350°C promoted formation of β-polymorph as a second phase in the resultant TCP. Meanwhile, higher firing temperatures resulted in phase pure α-TCP with poor hydraulic reactivity. The extension of firing operation also led to a decrease in the reactivity. It was found that identical synthesis history, morphology, particle size and crystallinity match between the α-TCPs produced from different CaCO₃ sources do not essentially culminate in products exhibiting similar hydraulic reactivity. The changes in reactivity are arising from differences in the trace amount of impurities found in the CaCO₃ precursors. In this regard, a correlation between the observed hydraulic reactivities and the impurity content of the CaCO₃ powders—as determined by inductively coupled plasma mass spectrometry—has been established. A high level of magnesium impurity in the CaCO₃ almost completely hampers the hydration of α-TCP. This impurity also favors formation of β- instead of α-polymorph in the product of TCP upon firing.     

Origin of the αc relaxation in poly(ethylene-terephthalate) by thermally stimulated currents

The thermally stimulated depolarization currents of poly (ethylene-terephthalate) electrets with a reduced degree of crystallinity (≃4%), corona-charged at polarization temperatures between 65 and 100 °C, show a heteropolar α^* relaxation placed between the α relaxation (heteropolar) and the ρ relaxation (homopolar). This α^* relaxation is associated with a uniform mechanism and has been observed in the discharge of electrets with shorted evaporated electrodes, which have been formed using the windowing polarization method at polarization temperatures between 76 and 79 °C. The intensity and position of α^* depend on the degree of crystallinity and the morphology. The application of the method of thermal stimulation by steps, which leads to a gradual crystallization of the sample, shows that the α_c relaxation of crystalline PET has its origin in α^*.     

Electrochemical properties of Co(OH)<Subscript>2</Subscript> powders as an anode in an alkaline battery

The structure and electrochemical properties of β-Co(OH)₂ powders, synthesized by a chemical precipitation method, were investigated. The results of X-ray diffraction show that a reversible reaction between β-Co(OH)₂ and hcp Co occurs. The cycling induces a capacity loss of β-Co(OH)₂ electrode, which is probably attributed to the dissolution of β-Co(OH)₂ and the subsequent formation of α-Co(OH)₂ and CoOOH. The results of electrochemical impedance spectra indicate that the electrochemical discharge process of the as-prepared Co(OH)₂ powders consists of three steps, namely the charge-transfer reactions of Co/CoH _x and Co/Co(OH)₂, and the hydrogen diffusion within Co, depending on the depth of discharge.     

Microstructures and high temperature oxidation resistance of alloys from Nb–Cr–Si system

Oxidation behavior of Nb–30Si–(10,20)Cr alloys have been evaluated in air from 700 to 1400 °C by heating for 24 h and furnace cooling them. The lower weight gain per unit area has been observed for 20Cr alloy at 1200, 1300, and 1400 °C. Pesting has been observed at lower temperatures (700, 800, 900 °C). Analysis indicates that the powder formation at 900, 100, 1100 °C may be associated with β form of Nb₂O₅ (base centered monoclinic form). However the m-monoclinic form of Nb₂O₅ evolves at temperatures above 900 °C while o-orthorhombic Nb₂O₅ forms at below this temperature. The phases in the alloys have been calculated using the Pandat^TM software program at different temperatures using calculated Nb–Cr–Si phase diagrams.     

Effect of titanium on the structure and mechanical properties of Cu-Al-Ti alloys

The structure and mechanical properties of Cu10 wt% Al base alloys with 0–2.5 wt% Ti additions were investigated using transmission electron microscopy, optical microscopy and tensile tests. Addition of titanium has a decreasing effect on the grain size after quenching fromα + β region and causes significant strengthening of alloys. Alloy containing 1 wt%Ti quenched from 900° C shows mixture ofα, retainedβ (DO₃), disorderedβ′ (3R) and orderedβ′ ₁ (18R) martensites. Alloy with 2.5 wt% Ti addition after quenching containsα, retainedβ (DO₃), ordered T₁ phase of L2₁ superlattice and orderedβ′ ₁ martensite with either R18 or L1₀ structure indicating different stacking of ordered planes as the effect of titanium addition.     

Effect of annealing on phase transition in poly(vinylidene fluoride) films prepared using polar solvent

The γ-phase poly (vinylidene fluoride) (PVDF) films are usually prepared using dimethyl sulfoxide (DMSO) solvent, regardless of preparation temperature. Here we report the crystallization of both α and γ-phase PVDF films by varying preparation temperature using DMSO solvent. The γ-phase PVDF films were annealed at 70, 90, 110, 130 and 160°C for five hours. The changes in the phase contents in the PVDF at different annealing conditions have been described. When thin films were annealed at 90°C for 5 h, maximum percentage of β-phase appears in PVDF thin films. The γ-phase PVDF films completely converted to α-phase when they were annealed at 160°C for 5 h. From X-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR), differential scanning calorimetry (DSC) and Raman studies, it is confirmed that the PVDF thin films, cast from solution and annealed at 90°C for 5 h, have maximum percentage of β-phase. The β-phase PVDF shows a remnant polarization of 4.9 μC/cm² at 1400 kV/cm at 1 Hz.