Showing papers in "Inorganic Materials in 2005"

Preparation of Nanocrystalline Alumina under Hydrothermal Conditions

Abstract: Thermodynamic analysis of phase equilibria in the Al2O3-H2O system at temperatures from 120 to 380°C and pressures from 1 to 70 MPa is carried out, and the dehydration of aluminum hydroxide under hydrothermal conditions is studied experimentally. The results indicate that γ-AlOOH (boehmite), which commonly appears in experimental phase diagrams, is a nonequilibrium phase in these temperature and pressure ranges. The dehydration rate and mechanism are shown to strongly depend on the crystallinity of the parent aluminum hydroxide and the pressure in the hydrothermal system.

Synthesis of Nanotubular Mg3Si2O5(OH)4-Ni3Si2O5(OH)4 Silicates at Elevated Temperatures and Pressures

Abstract: Mg3Si2O5(OH)4-Ni3Si2O5(OH)4 nanotubes with the chrysotile structure and MgO : NiO molar ratios of 1 : 2 and 2 : 1 are synthesized by hydrothermal reactions at temperatures from 250 to 450°C and pressures from 30 to 100 MPa. The reaction path and kinetics, as well as the dimensions and morphology of the resulting nanotubes, are shown to depend on the nature of the starting reagents, chemical composition of the reaction system, and hydrothermal synthesis conditions. At higher nickel concentrations in the hydrous silicates, nanotube formation requires higher temperatures, longer hydrothermal treatment times, and higher NaOH concentrations in the reaction system.

Chemical Interaction of Calcium Oxide and Calcium Hydroxide with CO2 during Mechanical Activation

Abstract: The reactions of CaO and Ca(OH)2 with CO2 during mechanical activation are studied by IR spectroscopy, x-ray diffraction, and thermal analysis. The results indicate that the earlier described extensive sorption of CO2 by Ca-containing silicates during grinding is not related to the formation of CaO or Ca(OH)2. Mechanical activation in CO2 converts calcium oxide to amorphous CaCO3 and calcium hydroxide to calcite, whereas Ca-containing silicates under such conditions homogeneously dissolve CO2 to form a material similar to analogous carbonate-containing silicate glasses.

Hydroxyapatite-Carboxymethyl Cellulose Nanocomposite Biomaterial

Abstract: An organic-mineral composite of hydroxyapatite (Ca10(PO4)6(OH)2) nanoparticles and carboxymethyl cellulose (CMC) is synthesized via coprecipitation from a solution containing CaCl2, aqueous ammonia, (NH4)2HPO4, and CMC. The composition and microstructure of the composite and the lattice parameters and particle size of the hydroxyapatite are determined using x-ray diffraction, chemical analysis, IR spectroscopy, scanning and transmission electron microscopy techniques, electron diffraction, and x-ray microanalysis. The hydroxyapatite nanoparticles are shown to form agglomerates about 200 nm in size. The interaction between the nanoparticles and CMC macromolecules leads to the formation of a pore structure potentially attractive for biomedical applications.

Effect of the nature of the metal solvent on the vapor-liquid-solid growth rate of silicon whiskers

Abstract: A relationship is established between the growth rate of silicon whiskers and the solubility of the crystallizing substance in metal solvent particles. The effect of the whisker diameter on the growth rate is analyzed. The solubility of the solid phase is shown to drop as the particle size of the solvent decreases (its specific surface increases). The practical importance of these findings is discussed.

Preparation of ZnCo2O4 spinel whiskers from zinc cobalt oxalate

Abstract: ZnCo2O4 whiskers 2–5 µm in diameter and up to 50 µm in length, retaining their composition and shape up to 800°C, are prepared via decomposition of the mixed oxalate Zn1/3Co2/3C2O4 ⋅ 2H2O in air and are characterized using chemical analysis, x-ray diffraction, IR spectroscopy, and scanning electron microscopy

Microgranules and Nanoparticles on Their Surfaces

Abstract: This review considers advances in the fabrication of metallic, oxide, sulfide, carbon, BN, and other microgranules—the building blocks of porous materials for various applications. It is shown that attaching nanoparticles to the surface of microgranules alters some of the most important properties of the matrix and offers the possibility of creating composite materials with unique physical properties and reactivity.

Growth of Sillenite-Structure Single Crystals

Abstract: The main processes for preparing bulk single crystals and films of photorefractive and piezoelectric Bi 12 M x O 20 ± δ (M = Group II-VIII elements) sillenite compounds are considered. Experimental data are sum- marized on the crystal growth of Bi 12 M x O 20 ± δ from the melt and under hydrothermal conditions, and the key morphological features of sillenites are analyzed. Various types of macroscopic growth defects in sillenite-type crystals are described, and their origin is discussed. The compositions of second-phase inclusions in undoped and doped (Group I-VIII elements) Bi 12 SiO 20 , Bi 12 GeO 20 , and Bi 12 TiO 20 single crystals are presented, and the main physicochemical properties of various Bi 12 M x O 20 ± δ crystals are summarized.

Influence of cations on the vibrational spectra and structure of [WO4] complexes in molten tungstates

Abstract: The vibrational spectra and structure of isolated [WO4]2− anions in molten alkali, alkaline-earth, rare-earth, Pb, Bi, Zn, Sc, and Cd tungstates are studied by high-temperature Raman spectroscopy. The degenerate modes of the [WO4]2− anion in K-Gd tungstate melts are found to be split, which is interpreted in terms of the symmetry of the cation environment of the [WO4]2− anion in the melts. The frequency of the fully symmetric stretching mode (ν1) of the [WO4]2− group in molten tungstates is examined in relation to the polarizing power P and electronegativity γ of the cation. The origin of the nonmonotonic variation of ν1 with P and γ for the tungstate melts studied is discussed.

Effects of oxygen and carbon impurities on the optical transmission of As2Se3 glass

Abstract: The IR spectra of As2Se3 glass samples containing known amounts of oxygen (4 × 10-3 to 7.7 × 10-2 wt %) or carbon (≤5.8 × 10-3 wt %) are measured, and the parameters of impurity-related absorption bands and extinction coefficients are determined. The effects of oxygen, carbon, sulfur, and hydrogen on the transmission of As2Se3 glass are analyzed. At an oxygen content on the order of 10-5 wt %, this impurity causes optical losses in the range 900–1100 cm-1 comparable to the intrinsic losses. The permissible carbon content of As2Se3 glass is 10-6 to 10-5 wt %. Carbon inclusions 0.07 µm in diameter cause optical losses comparable to the intrinsic losses in the spectral window of As2Se3 glass when present in a concentration of ∼104 cm-3.

High-temperature thermodynamic properties of the Al2O3-SiO2 system

Abstract: High-temperature mass spectrometry is used to investigate vaporization processes and determine SiO2 activity in the Al2O3-SiO2 system between 1850 and 1970 K. The results are consistent with the known phase equilibria in this system.

Hybrid Intercalative Nanocomposites

Abstract: This review focuses on organic-inorganic hybrid nanocomposites, a research area that has made rapid progress in recent years. Inorganic components (hosts) include both natural materials (clays, silicates, smectites, layered phosphates, and others) and compounds prepared by different synthetic techniques. Into their interlayer spaces, various organic guests—solvents, monomers, and polymers—can be intercalated. Among the hybrid nanocomposites analyzed in detail are those based on polyconjugated electrically conducting polymers, such as poly(aniline) and poly(pyrrole), and various mineral matrices. Particular attention is paid to polymer-metal chalcogenide nanocomposites and their applications as semiconducting materials. One of the most common and practically important intracrystalline processes in the fabrication of hybrid nanocomposites is the incorporation of monomer molecules into pores of the host, followed by controlled internal transformations into polymer, oligomer, or hybrid-sandwich products (in situ postintercalative transformations). This approach is often called “ship-in-the-bottle” polymerization. Another widely used approach is the incorporation of macromolecules into layered host lattices from solutions or melts. This process offers the possibility of producing graphite intercalation compounds and inorganic-organic multilayer composites, including self-assembled nanocomposites in the form of (P/M)n multilayers, where M and P are oppositely charged inorganic and polymer nanolayers.

Doping of Optical Fiber Preforms via Porous Silica Layer Infiltration with Salt Solutions

Abstract: A process is described for reproducible deposition of porous layers uniform along the preform axis, and the effect of the nature of the solvent on the infiltration of salt solutions into the porous layer is analyzed in relation to the fabrication of fiber preforms with controlled doping level. Data are presented on the variation of the retention volume in the porous layer with sintering temperature.

Influence of point defects on the electrical conductivity and dielectric properties of langasite

Abstract: The temperature-dependent electrical conductivity and dielectric permittivity and room-temperature optical absorption spectra of La3Ga5SiO14 (langasite) crystals grown under different conditions are measured. The resistivity and peak-loss temperature t tanδ of the crystals are shown to be determined by the concentration of oxygen vacancies, which originate from changes in melt composition during crystal growth. The t tanδ of langasite is shown for the first time to be anisotropic (measurements on Z- and X-cuts). The properties of the crystals are suitable for the fabrication of stable piezoelectric elements capable of operating above 600°C.

Luminescence Study of LiY1 − xEux(MoO4)2

Abstract: The luminescence spectra of LiY1 − x Eux(MoO4)2 (x = 0.0005, 0.001, 0.01, 0.05, 0.1, 0.5, 1) scheelite solid solutions are measured under laser excitation at 337.1 nm. The effect of Eu3+ concentration on the luminescence behavior of the solid solutions is examined. The highest integrated emission intensity is offered by LiY0.5Eu0.5(MoO4)2. Eu3+ substitution for Y3+ has no effect on the symmetry of the emission centers involved. Eu3+ is shown to occupy only one site in the structure of the solid solutions. X-ray diffraction and luminescence data indicate that all of the solid solutions have an undistorted scheelite structure.

Preparation and properties of fine BaCeO3 powders for low-temperature sintering

Abstract: Detailed characterization of BaCeO3 powders prepared through different chemical homogenization processes demonstrates that pyrolysis of coprecipitated oxalates ensures the smallest crystallite size and the highest rate of phase formation among the processes examined. Increasing the heating rate during the thermal decomposition of oxalates reduces the average crystallite size and the strength of aggregates in the powder. Sintering of the resultant barium cerate powders at 1000°C in the presence of cupric oxide enables the preparation of ceramics with a density above 90% of theoretical density. The low-temperature liquid-phase sinterability of BaCeO3 powders depends crucially on the presence of residual impurity phases, which notably reduce the shrinkage rate.

Effect of core glass composition on the optical properties of active fibers

Abstract: We optimized the composition of core glass for active fibers. Using solution doping, we obtained aluminogermanosilicate core fibers containing up to 0.6 mol % Yb2O3 with optical losses within 10 dB/km.

Ultrafast mechanochemical synthesis of mixed oxides

Abstract: Based on experimental data on the structure of disordered milling products in MO-M’O3 systems, a theory is proposed for ultrafast mechanochemical synthesis in oxide systems, considered as a threshold process. The key points of the theory are (1) the development of a transient dynamic state (D)* in contact regions via mass transfer by the roll mechanism in primary loading events and the formation of rotation regions (D)*2 in secondary events; (2) the formation of ordered reaction products supersaturated with vacancies as a result of the relaxation of the (D)* and (D)*2 states under unloading and quenching conditions; (3) the formation, as a result of secondary events, of a nanocomposite through mechanochemical equilibrium due to diffusion between the ordered and disordered states of different compositions; and (4) rapid charge separation and recombination, leading to reduction of oxides during milling concurrently with the disintegration of particles and mechanochemical interactions. The principal factors influencing the dynamics of mechanochemical synthesis (established by linearizing the dependence of the chemical response of the system on mechanical load) are the molecular weight of the simple oxides involved, the enthalpy of the reaction, and the difference in Mohs’ hardness between the reactants. The typical structures of mechanochemical synthesis products (solid solutions) are stable to disordering and compositional changes. The theory is supported by the trimodal mechanochemical synthesis rate distribution for mixed oxides and the mechanically induced electron-hole ferromagnetism. Ways of controlling the dynamics of mechanochemical synthesis and the structure of the products are discussed.

In Situ X-ray Diffraction Study of the Phase Transition of Nanocrystalline In(OH)3 to In2O3

Abstract: Single-crystalline indium oxide (In2O3) nanocubes were successfully prepared via dehydration of indium hydroxide (In(OH)3) nanocubes synthesized by a hydrothermal method. In situ x-ray diffraction patterns of the phase transition were obtained. Transmission electron microscopy indicated that the morphology of the In2O3 nanocrystallites was the same as that of their precursors, In(OH)3 nanocrystallites. The reported method, the dehydration of In(OH)3 nanocrystallites prepared by a hydrothermal method, shows a simple and effective synthesis route for In2O3 nanostructured materials.

X-ray diffraction study of La3Ga5.5Ta0.5O14 and La3Ga5.5Nb0.5O14 langasite-type single crystals

Abstract: The structure of langatate (as-grown and vacuum-annealed: LGT-I and LGT-II, respectively) and langanite (seed- and tail-end portions: LGN-III and LGN-IV, respectively) single crystals grown by the Czochralski technique from charges of nominal composition La3Ga5.5M0.5O14 = La3(Ga0.5M0.5)(1)Ga3(2)Ga2(3)O14 (M=Ta5+, Nb5+) is studied by x-ray diffraction. The LGT-I and LGT-II crystals are shown to differ in the Ga and Ta distributions over crystallographic sites: La3(Ga0.52)Ta0.48(2) 5+Ga3(Ga0.94Ta0.06(1) 3+)2O14 in LGT-I (Ta5+ in the octahedral site Ga(1) and Ta3+ in the trigonal-pyramidal site Ga(3)) and La3(Ga0.55Ta0.45(2) 5+)Ga3Ga2O13.93(2)□0.07 in LGT-II (Ta5+ in the octahedral site Ga(1) and oxygen vacancies in O(1)). The increased Ta content is responsible for the lower structural perfection of LGT-I. LGN-III and LGN-IV have essentially identical compositions, La3(Ga0.47Nb0.53(1) 5+)Ga5O14 and La3(Ga0.48Nb0.52(1) 5+)Ga5O14, respectively, but differ in polarity.

Ionic conductivity of ln2 + xzr2 - xo7 - x/2 (ln = sm-gd) solid solutions

Abstract: The electrical conductivity of Ln2 + x Zr2 − x O7 − x/2 (Ln = Sm-Gd) solid solutions prepared from mechanically activated Ln2O3 and ZrO2 is shown to correlate with their structural properties. In the three systems, the x-T regions are determined in which electrical transport is dominated by oxygen-ion conduction. In the Sm2O3-ZrO2 system, ionic conductivities from 5 × 10−4 to 6 × 10−3 S/cm at 740°C are found in Sm2 + x Zr2 − x O7 − x/2 with 26.6, 33.3, 35.5, 37, and 40 mol % Sm2O3 prepared at 1450, 1530, and 1600°C. Eu2 + x Zr2 − x O7 − x/2 and Gd2 + x Zr2 − x O7 − x/2 containing 33.3 to 37 mol % Ln2O3 have 740°C ionic conductivities of 10−3 to ∼7.5 × 10−3 and 10−3 to 7 × 10−3 S/cm, respectively. The activation energy of conduction in Ln2 + x Zr2 − x O7 − x/2 (Ln = Sm-Gd), E a = 0.84–1.04 eV, increases with the atomic number of Ln and x. The highest ionic conductivity is offered by the stoichiometric Ln2Zr2O7 (Ln = Sm-Gd) pyrochlores prepared at 1600°C, owing to the optimal concentration of LnZr + ZrLn antistructure pairs (∼5–22%). The grains in the ceramic samples studied range in size from 0.5 to 2 µm.

Preparation and Properties of Thin HfO2 Films

Abstract: HfO2 layers were grown on silicon by metalorganic chemical vapor deposition using (C5H5)2Hf(CH3)2, (C5H5)2Hf(N(C2H5)2)2, and Hf(dpm)4 as volatile precursors and were characterized by x-ray diffraction, x-ray photoelectron spectroscopy, and IR spectroscopy. The films were shown to consist of monoclinic HfO2 and to contain hafnium silicide and silicate at the HfO2/Si interface. The presence of hafnium silicide was attributed to oxygen deficiency produced by argon ion milling. Hafnium silicate was formed as a result of the reaction between hafnium and silicon oxides during annealing. Current-voltage and capacitance-voltage measurements on Al/HfO2/Si test structures were used to determine the dielectric permittivity and electrical resistivity of the films: ɛ = 15–20, ρ ∼ 1015 cm.

Annealing effect on the luminescent properties and native defects of ZnO

Abstract: The effect of air annealing on the luminescence, native defects, and morphology of ZnO powder prepared through pyrolysis of a zinc nitrate solution is studied at annealing temperatures from 100 to 1000°C. The results indicate that annealed ZnO is only enriched in oxygen at annealing temperatures below 200°C. During annealing at higher temperatures, ZnO loses predominantly oxygen, which gives rise to green luminescence. The green band is shown to consist of several components. According to electron microscopy results, high-temperature annealing drastically increases the particle size of the ZnO powder.

Mechanochemical Synthesis of Crystalline Compounds in the Bi-V-O System

Abstract: Crystalline mechanochemical synthesis products in the Bi2O3–GeO2 system are studied by x-ray diffraction. The results indicate the formation of sillenite (Bi12GeO20), eulytite (Bi4Ge3O12), and Aurivillius (Bi2GeO5) phases. The Aurivillius phase is shown to be in mechanochemical equilibrium with the sillenite phase in the 2Bi2O3 + GeO2 system and with the eulytite phase in the Bi2O 3 + GeO 2 system. The structural parameters of the synthesized metastable solid solutions are determined. The three phases contain high concen-trations of vacancies. In addition, the sillenite and Aurivillius phases are characterized by compositional disordering. Structural and ESR data point to partial reduction of the oxides, which accounts for the formation of the Aurivillius phase. According to x-ray photoelectron spectroscopy results, mechanical activation of bismuth oxide produces reduced binding energy states of Bi and O, which is tentatively attributed to clustering and the formation of complex radicals.

Synthesis and Structure of Mn-Doped CdGeAs2 Single Crystals

Abstract: Large, perfect CdGeAs2 single crystals doped with 0.006, 0.49, and 0.89 wt % Mn are grown by the Bridgman method. The Mn concentration in the crystals is determined by atomic absorption spectrophotometry. X-ray diffraction results indicate that Mn doping influences the bond distances in CdGeAs2 and its lattice parameters. Based on the observed structural changes, a model is proposed for the Mn incorporation into the structure of CdGeAs2.

Thermoelectric Properties of (Bi 2 Te 3 ) 1-x-y (Sb 2 Te 3 ) x (Sb 2 Se 3 ) y Single Crystals

Abstract: Homogeneous and graded n- and p-type (Bi2Te3)1 − x − y (Sb2Te3) x (Sb2Se3) y crystals are grown by the Czochralski technique with melt supply through a floating crucible. The dimensionless thermoelectric figure of merit of the n- and p-type crystals is ZT = 1.1 (350 K) and 1.0 (375 K), respectively. It is shown that (Bi2Te3)1 − x − y (Sb2Te3) x (Sb2Se3) y pseudoternary solid solutions can be used to produce monolithic n- and p- type graded and segmented thermoelectric materials by Czochralski growth. The thermoelectric power distribution across the seed-crystal interface is studied using scanning hot microprobe measurements.

Nanostructured Films of Vanadium Borides

Abstract: The elemental and phase compositions of films produced by rf magnetron sputtering of a VB2 target are determined by x-ray diffraction, electron microscopy, and secondary ion mass spectrometry. The composition-depth profiles of the films are analyzed.

A simple microwave-assisted route to prepare black cobalt, Co3O4

Abstract: In this communication, we report a novel microwave-assisted decomposition reaction of cobalt nitrate hexahydrate, Co(NO3)2 · 6H2O. Using microwave processing (10 min, 2.45 GHz), phase-pure tricobalt tetroxide (black cobalt, Co3O4) was obtained. The compound was characterized by x-ray powder diffraction and infrared spectroscopy.

Metastable iron sulfides

Abstract: Data on various iron sulfides are analyzed with special reference to the presence of a ferrimagnetic phase with a Curie temperature of 580°C in pyrrhotine. X-ray diffraction studies and magnetic measurements provide substantial evidence for the existence of a metastable phase Fe2S3 with a hexagonal structure, embedded in the structure of pyrrhotine. The exchange integrals and temperature-dependent relative magnetization of iron sulfides are calculated in the molecular-field approximation for two-sublattice ferrimagnets. The calculation results agree well with experimental data.

Ordering of Fe-Si phases

Abstract: The microstructure of Fe-Si alloys containing 8 and 15.5 at % Si and heat-treated between 550 and 1200°C is studied by transmission electron microscopy, and the phase composition of alloys containing 19 and 23 at % Si is determined by x-ray diffraction. The Fe-15.5 at % Si alloy heat-treated above 700°C is found to consist of a disordered solid solution and B2 phase. The B2 particles can be thought of as portions of {100} layers consisting entirely of Si atoms and sandwiched between {100} layers of Fe atoms, that is, as a two dimensional phase. At t ≤ 675°C, a compositionally modulated microstructure develops in which the Si-enriched zones have the Fe3 Si stoichiometry and DO3 structure. At high temperatures, the Fe-19 at % Si alloy consists of the αand B2 phases, and the Fe-23 at % Si alloy consists of the αand DO3 phases. These findings are at variance with the generally accepted Fe-Si phase diagram.