Synthesis and Magnetic Properties of SrO–Fe2O3–B2O3 Glass-Ceramics

Glasses with nominal compositions of SrFe₁₂O₁₉ + 8SrB₂O₄ (I) and SrFe₁₂O₁₉ + 12Sr₂B₂O₅ (II) are prepared by rapid quenching from the liquid state and are converted to glass-ceramics containing fine magnetic particles of SrFe₁₂O₁₉ by heat treatment between 600 and 950°C. The materials are characterized by x-ray diffraction, differential thermal analysis, electron microscopy, and magnetic measurements. The phase transformations accompanying glass crystallization are identified. The glass composition and heat-treatment conditions are shown to influence the aspect ratio of the forming submicron-sized strontium hexaferrite particles. The strongest coercive fields reached in glass-ceramics I and II are 504 and 456 kA/m, respectively.     

Study on particle-stabilized Si3N4 ceramic foams

The stability of bubbles and the microstructures of sintered Si₃N₄ ceramic foams produced by direct foaming method were investigated. The bubbles produced by short-chain amphiphiles (propyl gallate) have higher stability as compared with that produced by long-chain surfactants (TritonX-114). Si₃N₄ ceramic foams using short-chain amphiphile are particle-stabilized one, the pore cells are spherical and closed, and cell surfaces are smooth and dense. The pore cells of sintered Si₃N₄ ceramic foams using TritonX-114 foaming are coarse and large, and pore cells are polyhedral. High gas-pressure sintering is conducive to the development of the whisker-like microstructures in Si₃N₄ ceramic foams. The sintered Si₃N₄ ceramic foams with the whisker-like microstructure are quite promising for improving the mechanical strength of the ceramics by a simple and safe way.     

Synthesis and characterization of nanostructured ferroelectric compounds

Nanosized BaTiO₃, PbTiO₃, and LiNb₂O₃ have been synthesized by ligand–precursor route using tartaric acid, TEA, and EDTA. The average particle sizes are 15, 18, and 21 nm for BaTiO₃, PbTiO₃, and LiNb₂O₃, respectively. The transition temperatures are 55, 127, and 223 °C for BaTiO₃, PbTiO₃, and LiNb₂O₃, respectively, and corresponding values of the dielectric constant are 5394, 6150, and 1400, respectively. The transition temperature and dielectric constant are lowered from the values of polycrystalline samples due to the high degree of defects in nanostructured materials.     

The effect of the number of Ag layers on some physical properties of Bi1.8Pb0.4Ca2.2Sr2Cu3Ox

Bi_1.8Pb_0.4Ca_2.2Sr₂Cu₃O_x/Ag multilayered samples with 2, 4 and 8 layers are prepared by powder in tube (PIT) method and the effect of multilayering with Ag on the superconducting and the microstructural properties of Bi_1.8Pb_0.4Ca_2.2Sr₂Cu₃O_x is studied. The investigations consist of XRD, SEM, EDS, magnetoresistivity, and I–V measurements. In the magnetoresistivity measurements, magnetic fields up to 0.5 T are applied both parallel and perpendicular to the current flow direction. The superconducting and microstructure properties of the samples are enhanced as the number of Ag layers are increased. The samples are composed of a highly oriented Bi-2223 phase in the regions near the Ag layers. The formation of the dense oriented structure is near the interface between oxide and the Ag layer. T_c and J_c values are enhanced from 107 to 109 K and from 240 to 600 A/cm² by increasing the number of Ag layers, respectively. The results suggest that Ag plays an important role in the improvement of superconducting properties. The possible mechanisms of the enhancement of superconducting properties due to Ag layering are discussed.     

Characteristics of GaN thin films by inductively coupled plasma etching with Cl2/BCl3 and Cl2/Ar

GaN thin films were etched by inductively coupled plasma (ICP). The effects of BCl₃ and Ar with different Cl₂ fraction are studied and compared. The ICP power and RF power are also altered to investigate the different effects by using Cl₂/BCl₃ or Cl₂/Ar as etching gases. The etch rate and surface morphology of the etched surface are characterized by using surface profiler, scanning electron microscopy and atomic force microscopy. The root-mean-square roughness values are systematically compared. It is found that the etch rates of Cl₂/Ar are higher than that of the Cl₂/BCl₃ in the Cl₂ fraction ranging from 10 to 90%. When the ICP power is increased, the RMS roughness of GaN surface after ICP etching shows reverse trend between Cl₂/BCl₃ and Cl₂/Ar gas mixture. The results indicate quite different features using Cl₂/BCl₃ and Cl₂/Ar for GaN ICP etcing under the same conditions.     

Fabrication of thin films by sputtering deposition using (Ba<Subscript>0.3</Subscript>Sr<Subscript>0.7</Subscript>)(Zn<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> ceramic as target

Dielectric ceramic thin films were fabricated on SiO₂ (110) substrates by the radio frequency (RF) magnetron sputtering method using (Ba_0.3Sr_0.7)(Zn_1/3Nb_2/3)O₃ microwave dielectric ceramic as target. The microstructure, components, and morphology of the thin films were investigated thoroughly. The results reveal that the experimental conditions can affect the growth of the thin films significantly. The main phases of the thin films are Ba_0.5Sr_0.5Nb₂O₆ and Ba_0.27Sr_0.75Nb₂O_5.78, which are of different composition from that of the ceramic target due to Zn loss. The thin films are polycrystalline with high-quality crystalline and are made up of dense rod-like structures. The growth mechanism of the thin films is discussed in particular.     

Growth of praseodymium tartrate crystals in silica gel

The growth of praseodymium tartrate crystals in the system Pr(NO₃)₃-Na₂ SiO₃-C₄H₆O₆, using a single-tube-single-gel technique is described. The growth conditions are delineated and a spherulitic morphology is reported. The spikes attached to the spherulites are single crystals of praseodymium tartrate. The mechanisms of crystallization for various types of spherulites are described. The information presented contributes to the understanding of spherulitic growth in general, and that of praseodymium tartrate in particular.     

Fe-containing nanoparticles on the surface of silica microgranules

We have prepared nanomaterials consisting of SiO₂ microgranules and Fe-containing nanoparticles on their surface and determined the size and composition of the nanoparticles. The nanoparticles are shown to have a core-shell structure (α-Fe core and γ-Fe₂O₃ shell). The α-Fe core and Fe₂O₃ shell are responsible, respectively, for the broad and narrow lines in the EMR spectrum of the nanoparticles. The magnetic properties of larger nanoparticles are dominated by the core, while those of smaller nanoparticles are dominated by the oxide shell.     

The structure of chemical vapor deposited silicon carbide

The morphologies of SiC deposited by the thermal decomposition of CH₃SiCl₃ are presented. These are non-fluid bed deposits prepared in an induction-heated reactor. The morphologies of the deposits were found to vary systematically with the substrate temperature, chamber pressure and gas composition. Deposits at a low temperature, a moderate chamber pressure and with low H₂:CH₃SiCl₃ ratio are generally fine grained. Macrograins composed of finer micrograins are typical in intermediate temperature deposits. The surface contours of the macrograins are smoothly rounded. As temperatures or pressures are increased, the edges of grains become sharper, the surfaces become flatter and faceting becomes common. At very high temperatures and high H₂: CH₃SiCl₃ ratios, large single crystals are observed. Fracture surfaces containing faceted crystals no longer show the preponderance of transgranular fracture noted in deposits at lower temperature and lower H₂:CH₃SiCl₃ ratio. X-ray diffraction showed that these deposits are cubic β-SiC and disordered hexagonal 2H-SiC. The plane of preferred orientation was (111) for the low temperature and (220) for the higher temperature deposits. Microprobe analysis showed that carbon-rich, stoichiometric or silicon-rich SiC was deposited depending on the temperature, pressure and gas composition.     

Properties of aurivillius phases in the Bi4Ti3O12-BiFeO3 system

A number of new Bi _m+1Fe _m?3TiO_3m+3 compounds with integer and fractional m values are identified in the Bi₄Ti₃O₁₂-BiFeO₃ system. All of the compounds are shown to have Aurivillius-type structures. The phase-transition and decomposition temperatures of the new and earlier known compounds of the Bi₄Ti₃O₁₂-BiFeO₃ system are determined. The linear thermal expansion coefficients of the synthesized compounds are evaluated using dilatometry, and their sintering onset temperatures are determined.     

Effect of C6H14N2O7 and reaction temperature on sensing properties of hierarchical WO3 for H2S sensor

In this study, WO₃ nanoparticles with hierarchical structures are prepared by a simple hydrothermal method using diammonium hydrogen citrate (C₆H₁₄N₂O₇) as an additive, and the effects of reaction temperature and addition of C₆H₁₄N₂O₇ on their morphology are investigated in detail. The physical properties and morphology of the prepared WO₃ materials are analyzed by scanning electron microscopy, X-ray diffraction , transmission electron microscopy, and Brunauer–Emmett–Teller method, and a possible growth mechanism is proposed. The results show that C₆H₁₄N₂O₇ is adsorbed on the edges of the nanosheets and inhibits their anisotropic growth. In addition, based on this material, sensors are self-assembled for detecting the decomposition products of SF₆, and the gas-sensitive properties of the material are investigated. The experimental results show that the sensors based on WO₃ with hierarchical structures are highly sensitive to H₂S and exhibit a low operating temperature. This indicates that such sensors can be effectively used to detect the main decomposition products of SF₆.

     

Etching of MgO crystals in acids: kinetics and mechanism of dissolution

Kinetics of etching of MgO crystals have been studied in H₂SO₄, HNO₃ and HCl. The effects of etching time, acid concentration and temperature on the growth of hillocks, on the selective etch rate and on the rate of overall dissolution are demonstrated. It is observed that etch rates are independent of time, but are determined by the temperature and concentration of the acid. The etch rate-concentration curves show maxima which are characteristic of an acid. The values of activation energy for the processes of dissolution, selective etching and hillock growth and the corresponding frequency factors are computed. It is established that the process of dissolution in concentrated H₂SO₄ is diffusion controlled, while in H₂SO₄ with concentrations below 18 N and in HNO₃ and HCl it is reaction rate controlled. The pre-exponential factor is found to be a function of acid concentration. The results are discussed from the standpoint of chemistry. A comment on the data published on MgO by previous workers is made.     

Microwave and photoluminesent characterizations of (Ca<Subscript>2</Subscript>Mg<Subscript>3</Subscript>)(X<Subscript>1.75</Subscript>Sb<Subscript>0.25</Subscript>)TiO<Subscript>12</Subscript> [X = Nb and Ta] ceramics

(Ca₂Mg₃)(X_1.75Sb_0.25)TiO₁₂ [X = Nb and Ta] ceramics are prepared through the conventional solid-state route. The samples are calcined at 1,100 and 1,180 °C, and are sintered at 1,250 and 1,375 °C. The substitution of Sb decreases the calcination and sintering temperatures of pure (Ca₂Mg₃)(Nb/Ta)₂TiO₁₂. The structure of the samples is analyzed using X-ray diffraction method. The microstructure of the sintered pellet is studied using scanning electron microscopy. The dielectric properties such as dielectric constant (ε_r), quality factor (Q_uxf) and temperature coefficient of resonant frequency (τ_f) are measured in the microwave frequency region. By Sb substitution, thermal stability is achieved, with the increase in dielectric constant, without much change in the quality factor. The materials have intense emission lines in the wavelength region 500–700 nm. The compositions have good microwave dielectric properties and photoluminescence and hence are suitable for dielectric resonator and ceramic laser applications.     

Stability,thermal and mechanical properties of PtxAly compounds

The stability, thermal and mechanical properties of Pt_xAl_y intermetallic compounds are investigated by density functional theory (DFT). The cohesive energy and formation enthalpy of Pt_xAl_y phases show that they are thermodynamically stable structures and these are in good agreement with the experiments. The heat capacity of the compounds is calculated by quasi-harmonic approximation (QHA) method. The thermal expansion coefficient as a function of temperature for each compound is also discussed. The elastic properties such as bulk modulus, Young’s modulus are evaluated by Viogt–Reuss–Hill approximation. The anisotropic properties of sound velocities for the Pt_xAl_y compounds are explored. The calculated Poisson’s ratio varies from 0.26 to 0.39 for Pt_xAl_y phases and the bonds in the compounds are mainly metallic and covalent types.     

Glass formation range in the SeO2-TeO2-V2O5-MoO3 system

The glass forming region in the quaternary system under increased oxygen pressure and at a slow melt cooling rate (2 to 2.5° C min^-1) has been determined. The stable glasses are located in the central part of the system but nearer to the SeO₂-TeO₂ side. The structural units of these two glass formers are of decisive importance in building up the glass lattice. Infrared spectra of selected compositions from the glass forming region are taken. From the data obtained for the binary glasses in the TeO₂-V₂O₅, TeO₂-SeO₂, TeO₂-MoO₃, V₂O₅-MoO₃ systems and the spectra of the four component compositions, it is shown that the basic structural units participating in the glass lattice formation are the SeO₃, VO₅, TeO₄ and TeO₃ groups. Structural models are proposed: glasses in the SeO₂ direction possess laminar and chain structure, while with increase of TeO₂ concentration, a three-dimensional structure is built up.     

A ReaxFF-based molecular dynamics study of the pyrolysis mechanism of HFO-1336mzz(Z)

A series of ReaxFF molecular dynamics simulations are performed to investigate the pyrolysis mechanisms of HFO-1336mzz(Z). Five initiation reaction pathways are observed in the process of pyrolysis. By comparing the activation energy of the reactions, the ground state CF₃CH=CHCF₃ exciting into the triplet state CF₃CH-CHCF₃ is the main initial reaction pathway with the activation energy of 238.553 kJ mol⁻¹. HF and C₂F₂ are the dominant products and other final products including CF₄, CHF₃ and C₂F₄ are detected in the process of pyrolysis as well. The intra-molecular elimination (1, 2-elimination), radical attacking mechanism and radical combination reaction are three formation mechanisms of HF. The formation mechanism of C₂F₂ is slightly different from HF, including fluorine transfer reaction, radical combination reaction and defluorination. By the kinetic analysis of HFO-1336mzz(Z) pyrolysis, the pre-exponential factor and activation energy are obtained from ReaxFF simulations.     

Solvent directed fabrication of Bi2WO6 nanostructures with different morphologies: Synthesis and their shape-dependent photocatalytic properties

In this work, Bi₂WO₆ with complex morphologies, namely, flower-like, pancake-like, and tubular shapes have been controllably synthesized by a facile solvothermal process. The as-obtained samples are systematically investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM). The effects of solvents on the morphologies of Bi₂WO₆ nanostructures are systematically investigated. According to the time-dependent experiments, a two-step growth mode basing on Ostwald ripening process and self-assembly has been proposed for the formation of the flower-like and pancake-like Bi₂WO₆ nanostructures. The photocatalytic properties of Bi₂WO₆ nanostructures are strongly dependent on their shapes, sizes, and structures for the degradation of rhodamine B (RhB) under visible-light irradiation. The deduced reasons for the differences in the photocatalytic activities of these Bi₂WO₆ nanostructures are further discussed.     

First principles study on the elastic properties and electronic structures of (Fe, Cr)3C

First principles calculations are performed to investigate the elastic properties and electronic structures of Cr doped Fe₃C carbides, the obtained results are compared with Cr₃C and Fe₃C. The calculated bulk modulus of Fe₁₁CrC₄ and Fe₁₀Cr₂C₄ is 260GPa and 270GPa, respectively, larger than Fe₃C. So the hardness of Fe₃C phase can be enhanced by doped with an appropriate amount of Cr, however, the calculated formation enthalpy and defect formation enthalpy of Fe₁₁CrC₄ and Fe₁₀Cr₂C₄ are positive. On the other hand, the electronic calculations reveal that the ground states of Fe₁₁CrC₄ and Fe₁₀Cr₂C₄ are ferromagnetic. The evaluated local magnetic moments of Fe at 4c sites are larger than that of 8d sites, which is analogous to Fe₃C. Milliken population results indicate that the stabilities of Fe₁₁CrC₄ and Fe₁₀Cr₂C₄ are reduced mainly due to the strong repulsive bonds among metal atoms.     

Tricalcium aluminate hydration: Microstructural observations by in-situ electron microscopy

Environmental scanning electron microscopy (ESEM) was used to study the microstructural changes that take place during the hydration of tricalcium aluminate (C₃A) in the absence and presence of gypsum (CS¯H₂; where A = Al₂O₃, C = CaO, H = H₂O, S¯ = SO₃). The ESEM proves to be a valuable tool in the observation of cement hydration and no specialised equipment other than the ESEM is required. The hydration process can be observed at any time without the need to halt the hydration process prior to specimen preparation. Subsequently, artefacts associated with specimen preparation, such as water loss and desiccation, are now avoided. In the absence of sulphate, amorphous gel, poorly crystalline hexagonal calcium aluminate hydrate (? C₄AH₁₉) and cubic calcium aluminate hydrate (C₃AH₆) are observed on the surface of C₃A grains. When small amounts of sulphate (2% gypsum) are present the same phases are observed. If larger amounts of sulphate (25% gypsum) are added to the system amorphous gel products, crystalline ettringite (C₆AS¯₃H₃₂) and monosulphate (C₄AS¯H₁₂) are observed. The crystalline products grow both within the amorphous gel and, where space allows, in interstices suggesting a through solution mechanism of transport.     

Silicon-nitride-based nanoceramic materials

Based on the principle of maximum bond energy, a structural model is proposed for Si₃N₄ + 60 wt % MoSi₂ + 5 wt % Y₂O₃ + 1.5 wt % Al₂O₃ ceramics. In this model, Si₃N₄ clusters form a star-like structure with columnar pores 0.45 and 1.5 nm in size. The smaller pores are filled with MoSi₂ molecules, and the larger pores are filled with Y₂O₃ and Al₂O₃ clusters. The solubility of MoSi₂ molecules in silicon nitride is 70 wt %, and those of Y₂O₃ and Al₂O₃ are 5.45 wt %. The energies of intercluster interaction in two mutually perpendicular directions are 3.2 and 5.5 eV, which is comparable to bond energies in structural metallic materials. The lattice parameters of molybdenum disilicide are determined by x-ray diffraction. The formation of MoO₃ and SiO₂ molecules during thermal cycling has an insignificant effect on the structure of the material.Translated from Neorganicheskie Materialy, Vol. 41, No. 2, 2005, pp. 185–192.Original Russian Text Copyright© 2005 by Grechikhin, Golubtsova.