Synthesis and Properties of BiFeO3–DyMnO3 Solid Solutions

(1 – x)BiFeO₃–xDyMnO₃ solid solutions with x= 0.03–0.30 are synthesized for the first time, and their dielectric and magnetic properties are studied. The solid solutions with x 0.08 have a rhombohedrally distorted perovskite structure, and those with x = 0.10–0.30 have an orthorhombically distorted structure. The 1-kHz electrical conductivity of the solid solutions exhibits semiconducting behavior and rises exponentially with temperature. The dielectric properties of the solid solutions are investigated in the microwave range (0.5–2.5 GHz) between room temperature and 250°C. The real part of their room-temperature dielectric permittivity reaches a maximum at x= 0.20, while the largest tan is observed at x = 0.03. The solid solutions with x= 0.03–0.08 exhibit high dielectric losses and are, therefore, candidate microwave absorbing materials. The magnetic and electrical ordering temperatures in the composition range studied exceed room temperature; that is, these solid solutions are new high-temperature seignettomagnetic materials.     

Synthesis and luminescence properties of GdAlO3 ： Eu3＋

研究了不同退火机制下GdAlO3∶Eu3＋荧光粉的光谱特性。采用高温固相反应法在空气气氛和还原气氛中分别合成了GdAlO3∶Eu3＋荧光粉,讨论了在烧结过程中产生的色心的光谱性质及其对GdAlO3∶Eu3＋发光强度、激发光谱和O2--Eu3＋电荷迁移带位置的影响。研究了后退火对GdAlO3∶Eu3＋光谱特性的影响,进一步解释了VUV激发下的能量传递机制。根据烧结气氛、烧结温度和后退火对色心以及GdAlO3∶Eu3＋光谱特性的影响,找到了一条能有效消除色心获得高荧光强度的两步反应合成路线。     

Preparation of LiTaO3 , LiNbO3 , and NaNbO3 from Peroxide Solutions

A procedure is described for preparing peroxide solutions of LiTaO₃ and LiNbO₃. It is shown that the use of peroxide solutions makes it possible to obtain stoichiometric LiTaO₃, LiNbO₃, and NaNbO₃ and Li _x Na_{1 – x} Nb₃ solid solutions.     

Sintering of pseudo-boehmite and γ-Al2O3

Sintering of pseudo-boehmite, acicular-Al₂O₃ produced by dehydration of pseudo-boehmite, and-Al₂O₃ ex alum was investigated. The sintering process was studied by X-ray diffraction, transmission electron microscopy with selected area electron diffraction and BET surface area measurements. The solid state reaction to-Al₂O₃ causes a steep drop of the surface area to less than 10 m²g^–1. The acicular pseudo-boehmite and-Al₂O₃ supports exhibit an intermediate state where the acicular particles assume a rod-like shape and the surface area falls from about 300 to 100 m²g^–1. It was established that reaction to -Al₂O₃ and, hence, sintering proceeds via a nucleation and growth mechanism. The rate-limiting step is nucleation of -Al₂O₃. Consequently, the contacts between the elementary alumina particles dominate the sinter process. The contact between the acicular elementary particles of pseudoboehmite and-Al₂O₃ studied leads to the reaction to -Al₂O₃ to be almost complete after keeping samples for 145 h at 1050 °C. Decomposition of alum produces very small particles showing negligible mutual contacts. Consequently an elevated thermal stability is exhibited. Treatment of the alumina ex alum with water and drying results in a xerogel in which contact between elementary particles is much more intimate. Accordingly, treatment at 1050 °C causes a sharp drop in surface area.     

Structural and electrical properties of La-modified BiFeO3–BaTiO3 composites

In the present investigation, La-modified solid solutions of BiFeO₃ (BFO) and BaTiO₃ (BT) in different molar ratios [i.e., (Bi_0.5?x La _x Ba_0.5)(Fe_0.5Ti_0.5)O₃, with x = 0.0, 0.05, 0.10 and 0.15)] have been synthesized using a solid-state reaction route. Structural and electrical properties of single phase (with minor secondary phase) of BFO–BT system have been studied in details to understand their ferroelectric and other properties. Preliminary X-ray diffraction analysis confirms the formation of a new system, which is different from that of its parent compounds. Substitution of a small amount BaTiO₃ into BiFeO₃ enhances dielectric and ferroelectric responses and reduces electrical leakage or tangent loss. The ac conductivity obeys Jonscher’s universal power law. The electrical behavior of the samples was investigated by impedance spectroscopy in a wide temperature range (25–525 °C) at different frequency (1 kHz–1 MHz). The impedance spectroscopy of the materials also confirms the origin of the relaxation mechanism in the system.     

Effects of CuO、MoO3 and WO3 dopping on magnetic properties of NiZn ferrites

研究了CuO、MoO3和WO3掺杂对NiZn铁氧体电磁性能的影响.研究表明,适量的CuO掺杂能提高材料烧结密度并降低磁晶各向异性常数,从而提高材料的起始磁导率,但居里温度也有一定程度的下降.当主配方中CuO含量(摩尔分数)为4%时能最好的兼顾材料高磁导率和高居里温度的要求.而MoO3和WO3掺杂则均能引起晶界附近阳离子空位增多,从而加速晶界移动,促进晶粒尺寸增大,进而提高材料的起始磁导率.同时,由于W离子具有较强的占据铁氧体A位替代Fe3 的趋势,需要更大的掺杂量才能达到磁导率的峰值,其居里温度和饱和磁感应强度也低于相应MoO3掺杂的材料.     

Morphology-controllable synthesis and gas-sensing properties of α-MoO3

Different morphologies of orthorhombic molybdenum trioxide (MoO₃) were successfully synthesized through a facile hydrothermal method followed by a subsequent heat treatment. We found that the surfactants played a critical role in the formation process and the possible formation mechanism was discussed. Especially when chromic chloride hexahydrate was added as a surfactant, the unique net-like structure was prepared. Furthermore, the gas-sensing properties of the sample were tested towards the reducing ethanol gas. The results indicated that the MoO₃ with a net-like structure shows the largest response than the other structures.     

Synthesis and some properties of Ti3SiC2 by hot pressing of titanium,silicon, and carbon powders Part 1 – Effect of starting composition on formation of Ti3SiC2 and observation of Ti3SiC2 crystal morphology

Abstract

Dense Ti₃SiC₂ bulk ceramic was synthesised by hot pressing of elemental titanium, silicon, and carbon powders. The effects of starting composition of the powders on the synthesis of pure Ti₃SiC₂ were examined. Phase identification was carried out by X-ray diffraction. Silicon content in the starting composition played an important role in formation of the final constituent phases in the composite. It was difficult to obtain the pure phase Ti₃SiC₂ because other thermodynamically stable reaction products such as TiC, TiSi₂, or Ti₅Si₃ were always present together with Ti₃SiC₂. The microstructure of samples was examined using scanning electron microscopy and transmission electron microscopy. Observations showed that the Ti₃SiC₂ matrix was composed of elongated, platelike, and equiaxial grains. It is suggested that the hexagonal crystal Ti₃SiC₂ exhibits anisotropic grain growth behaviour. The relative growth rates on different planes, therefore, endow Ti₃SiC₂ with several morphologies.     

Conduction mechanism and dielectric properties of BiFeO3–BaTiO3 solid solutions

The first measurements are reported for the frequency-dependent conductivity of (1?x)BiFeO₃–xBaTiO₃ (x = 0.10, 0.15 and 0.30) solid solutions in the frequency range of 10⁰–10⁶ Hz and in the temperature range of 50–300 °C. Powder X-ray diffraction confirms the formation of solid solutions. The dielectric properties were seen to improve with increasing BaTiO₃ (BT) content. The conductivity (AC and DC) measurements reveal an inverse variation of the frequency exponent ‘s’ with temperature, high density of states and thermally activated process. The calculated density of states was found to be N(E_f) = 80.2 × 10³² eV^?1 cm^?1 at 1 kHz and 50 °C for BiFeO₃–10 % BaTiO₃ (BFO–10 % BT) solid solution. The impedance spectroscopy analysis confirms the presence of grain and grain boundary affecting the conductivity. Our results provide the first unambiguous evidence of conduction in crystallite BFO–BT solid solutions through correlated-barrier-hopping model.     

Electron–Phonon Coupling and Properties of Doped BaBiO3

Density functional calculations based on local density approximation (LDA) of the properties of doped barium bismuthates are reported. Using a linear-response approach within the linear-muffin-tin-orbital method the phonon spectrum of Ba_0.6K_0.4BiO₃ is calculated. The electron–phonon coupling constant is then evaluated for a grid of phonon wavevectors using the self-consistent change in the potential due to phonon distortion. Anharmonic contributions to from the tilting of oxygen octahedra are also evaluated on the basis of the frozen-phonon approach.     

Synthesis and some properties of Ti3SiC2 by hot pressing of Ti,Si and C powders Part 2 – Mechanical and other properties of Ti3SiC2

Abstract

Some properties of the remarkable Ti₃SiC₂ based ceramic synthesised by hot pressing of elemental Ti, Si, and C powders have been investigated. Its flexural strength by using three point bending tests and fracture toughness by using single edge notched beam tests were measured at room temperature to be in the range 310–427 MPa and about 7·MPa m^1/2, respectively. This material is a relative 'soft' ceramic with a low hardness of 4 GPa. Ti₃SiC₂ is similar to the soft metals and is a damage tolerant material that is able to contain the extent of microdamage. An oxidation test has been performed in the temperature range 1000–1400°C in air for 20 h. The oxidation resistance below 1100°C was good. Two oxidized layers were formed, the outer layer consisting of pure rutile-type TiO₂, and the inner layer a mixture of SiO₂ and TiO₂. The average coefficient of thermal expansion (CTE) of Ti₃SiC₂ was measured to be 9·29 × 10^?6 K^?1 in the temperature range 25–1400°C. The thermal shock resistance of Ti₃SiC₂ was evaluated by quenching the samples from 800°C, 1200°C, and 1400°C, respectively. The retained flexural strength drops dramatically at quenching temperature, but shows a slight increase after quenching from 1400°C compared with quenching from 800°C and 1200°C.     

Synthesis and Properties of Laβ-Al_2O_3

Laβ-Al2O3 solid electrolytes were synthesized directly and their conductivities were measured in terms of ac impedance spectroscopy.The magnitude of conductivities is 10-6 to 10-2 S·cm-1.Based on these data, the dependence of the conductivity of Laβ-Al2O3 upon the temperature and the dopant concentration in the electrolytes was studied, and the activation energy for conduction was calculated, which is 0.89 eV to 1.02 eV from 450℃ to 1000℃. The ion transference numbers were also determined.     

Mechanical Behaviour and Structure Instability of Al_3Ti Alloy

Trialuminide alloys of elements such as Ti. Nb or Zr are of particular interest as materials for high temperature usage because their density is very low and specific strength and elastic rnoduli are then very high. This report concentrates on recent work on Al3Ti alloys which have been alloyed with ternary elements such that the higher symmetry ordered cubic structure is obtained, leading to somewhat easier operation of deformation mechan isms and hence improved ductility and toughness.Fine details of the crystal structure of cubic trialuminides are considered here and it is shown that the materials generally possess some remnant tetragonal chemical ordering which can affect their me chanical behaviour. In addition the compositional range over which a stable single phase is retained is shown to be extremely small, such that in most cases the materials examined show some form of microstructural instability. These instabilities affect the mechanical behaviour of the materials, for exarnple producing general strengthening. leading to precipitation hardening du ring hig h temperature testing, and causing age hardening instabilities during high temperature static or dynamic testing.Such structural instabifity feads to significant modifications at superdislocations, affecting both the dislocation cores and their associated APB's. Failure for these cubic materials still occurs at very small plastic strains and seems to be determined by difficulties of superdislocation creation near a propagating crack rather than by problems of suitable dislocation configuration and mobility. Possible ways to enhance ductility and toughness by alloying and microstructural modification will be discussed.     

Phase Transformation and Deformation Mechanism of Ti_3Al-base Alloy

The phase transformation and deformationmechanism of the alloy based on composition Ti_3Alwith addition of Nb,V,Mo have been studied by useof transmission electron microscopy (TEM).It hasbeen shown that the orientation relationship ofα_2 phase transformed from β phase is:(0001)α_2//(l10)β,[1210]α_2//[111]β.The present dislocationslip systems in α_2 phase are (1100)[0001] and(1100)<1120>.There also exist α_2 twins whichhave new twin relationship and the twin plane is(2021).     

Improvement of structural and mechanical properties of alumina coatings by incorporation of TiO2 and α-Al2O3 nanoadditives

Abstract

Alumina coatings embedded with different nanoadditives were fabricated on aluminium alloy by microarc oxidation (MAO). Incorporation of nanograins into the prepared coatings was accomplished by dispersing nanoadditives into different electrolytes during the MAO process. Our results show that nanograins are successfully embedded in the ceramic coatings, and the embedded coatings are compact and have lower porosity. The mechanical properties of the nanograin embedded coatings such as hardness, adhesion and wear resistance are consequently improved, and the samples prepared in aluminate electrolyte with α-Al₂O₃ nanoadditive have better mechanical properties than those prepared in other electrolytes. Our results also show that the mechanical properties of MAO coatings are closely related to the surface structure. The introduction mechanism of nanograins into the ceramic coatings resulted from the reactions occurring in the microarc discharge channels such as diffusion and electrophoresis, which is believed to improve the structure of the prepared coatings.     

Crystal Structure and SHG Characterization of γ-BiSeO3Cl

-BiSeO₃Cl crystals were grown and characterized by x-ray microanalysis and second harmonic generation. The structure of -BiSeO₃Cl was determined by single-crystal x-ray diffraction: sp. gr. Pna2₁, a = 11.707(4) Å, b = 7.047(5) Å, c = 5.315(1) Å, Z = 4, R ₁ = 0.564, wR ₂ = 0.1272. The structures and second-order nonlinear responses of - and -BiSeO₃Cl are discussed in comparison with those of MTeO₃X (M = Bi, rare earth; X = Cl, Br, I) oxyhalides.     

Superplasticity and Diffusion Bonding of Ti_3AI Intermetallic Compounds

The superplasticity of Ti_3Al intermetallic compounds has been investigated in this paper.TheTi-14Al-21Nb ternary alloy showed 477% elongation at the strain rate of 1.49×10~(-5) s~(-1) and950℃.The elongation of Ti-14Al-21 Nb-3Mo-1V quinary alloy approached to 573% at the strainrate of 4.52×10~(-5) s~(-1) and the same temperature,and it was found that the elongation value in-creased to 1096.4%as temperature was raised up to 980℃ at the same strain rate.Ti_3Al base al-loys were bonded by diffusion bonding technology and good joints were created,the simulatedspecimens were performed by SPF/DB process.     

Synthesis of β-MoO3 by vacuum drying and its structural and electrochemical characterisation

The β-MoO₃ was obtained successfully free of α-MoO₃ through soft chemistry methods. The formation of β-MoO₃ with high purity was determined by the formation of the precursor MoO₃·2H₂O when a solution of Na₂MoO₄·2H₂O was passed through a cation-exchange resin. A structural, spectroscopic and thermal study of the polymorph synthesised was made by XRD, electron dispersion spectroscopy (EDS), FTIR and TGA/DTA techniques, respectively, in order to make a study about the possibilities of β-MoO₃ as active material in a lithium battery. Electrochemical experiments showed a high ability of the β-MoO₃ to form lithium molybdenum bronzes via a lithium insertion reaction.