Synthesis and study of TlHBIVUO6·nH2O (BIV=Si, Ge)

Previously unknown thallium uranosilicate TlHSiUO₆·H₂O and its germ anium analog TlHGeUO₆·3H₂O were prepared by ion exchange under hydrothermal conditions. The structure and properties of the compounds obtained were studied by X-ray diffraction, IR spectroscopy, and thermal analysis. Crystal hydrates formed in the system TlHSi(Ge)UO₆-H₂O were described. The thermal stability of the compounds and pathways of their thermal decomposition were studied.     

Some new aspects of the phase diagram of the NH 4 + /H+-W-O system

The phase composition of partially-reduced products of ammonium paratungstate (APT) was investigated. The samples were prepared by reducing the APT in a H₂ + N₂/129 volume ratio/atomosphere in the temperature range 610 to 860 K, and studied by means of chemical analysis, by X-ray diffraction analysis and by thermoanalytical methods. The reduction products can be described by the general formula of M _xWO_3–y+x/2 where M can be either H⁺ and/or NH ₄ ⁺ . Products considered as tungsten bronzes (TB) can be formed in a fairly narrow temperature range. Another product, the so-called decomposed APT (DAPT) is a mixture of bronzes and of some other compounds characterized by y/2 and considered as some intermediate phases between TB and tungstates.     

Characterization of M(H2PO4)2·2H2O (M = Mn,Co, Ni) and their in situ thermal decomposition by magnetic measurements

It has been established that M(H₂PO₄)₂·2H₂O (M = Mn, Co, Ni) are paramagnetics between 173 and 353 K with weak antiferromagnetic exchange interaction between the metal ions. In situ magnetic measurements during the thermal decomposition of the salts show that the oxidation state and the octahedral coordination of M²⁺ are preserved. From the data obtained it could be supposed that in M(H₂PO₄)₂·2H₂O (M = Co, Ni) this process is topotactic with no long-range diffusion transport. In Mn(H₂PO₄)₂·2H₂O, the formation of the large variety of intermediate products probably requires more drastic rearrangement and diffusion of the manganese ions during the complex transformations, which reflect on both the value and the sign of the θ constants. M₂P₄O₁₂ (M = Mn, Co, Ni), which are the final decomposition products of the corresponding dihydrogen phosphates are paramagnetics in the temperature range of 295–573 K with antiferomagnetic interactions between the metal ions. The lattice parameters of Ni(H₂PO₄)₂·2H₂O have been calculated. It crystallizes in the monoclinic system with a = 7.228(1) Å; b = 9.778(1) Å; c = 5.306(1) Å; β = 94.50(1)°, SG P2₁/n with Z = 2.     

CaCu3Ti4O12 ceramics from different methods: microstructure and dielectric

CaCu₃Ti₄O₁₂ (CCTO) ceramics were synthesized by methods of sol–gel, traditional solid-state reaction, and thermal decomposition of organic solution. The results exhibit that the microstructures and electric characteristics are affected by the methods of synthesis. The X-ray diffraction patterns show that all samples have a perovskite-like CCTO phase. Moreover, CCTO ceramic from traditional solid-state reaction have the phases of TiO₂ and CuO. The scanning electron microscopy images show that CCTO ceramics from different methods have different grain sizes, grain boundaries, and densities. Dielectric properties of the CCTO ceramics were characterized in a broad frequency range (10–10⁷ Hz) at room temperature. The CCTO ceramics from thermal decomposition of organic solution have the dielectric constant of more than 5 × 10⁴ at 10 Hz. The nonlinear relationship between the current density and the electric field strength can be observed in all the three samples.     

Characterisation of two chemical compounds formed between hydrated portland cement and benzene-1,2-diol (pyrocatechol)

Exposure to benzene-1,2-diol (pyrocatechol), a common soil contaminant, has been shown to cause loss of strength in concrete. Synthesis and characterisation of two compounds formed when Portland cement comes in contact with benzene-1,2-diol has been conducted. These compounds are (benzene-1,2-diolato(1-))hydroxidocalcium(II) (formula: [Ca(C₆O₂H₅)OH]) and triaqua(benzene-1,2-diol)(benzene-1,2-diolato(1-))hydroxidocalcium(II) (formula: [Ca(C₆O₂H₅)(C₆O₂H₆)(H₂O)₃(OH)]). The compounds may play a role in the deterioration process. Characterisation was conducted in terms of thermal decomposition behaviour, infra-red spectra and crystal structure (using powder X-ray diffraction). The likely thermal decomposition reactions of both compounds have been determined, and features in the infra-red spectra assigned to molecular vibrations. Both structures have been solved, with the exception of the location of some hydrogen atoms, and the structures refined using Rietveld refinement methods. It is anticipated that both the thermal analysis data obtained, and crystal structures deduced, in this article can now be used to quantify these phases in cement and concrete exposed to benzene-1,2-diol.     

The phases formed by heating Mn(II) and Zn(II) exchanged α-zirconium phosphate

The phases formed upon heating α-zirconium phosphate, Zr(HPO₄)₂·H₂O, loaded to different levels of Mn(II) and Zn(II) were determined by a combination of thermal and X-ray powder methods. The fully exchanged phases, ZrM(PO₄)₂·4H₂O lose water stepwise at progressively higher temperatures until anhydrous. Above 500°C the X-ray pattern indicates a triphosphate-like phase of the type M_0.5Zr₂(PO₄)₃ is formed. At lower loadings, solid solutions with relatively broad composition regions are observed. The data are summarized in phase diagrams which, however, are not equilibrium diagrams.     

Proton Conductivity and Thermal Properties of Ba(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We have grown single crystals of barium dihydrogen phosphate and studied its thermal transformations during heating to 500°C and its electrotransport properties. Ba(H₂PO₄)₂ (Pccn) has been shown to undergo no phase transitions up to its dehydration temperature. The thermal decomposition of Ba(H₂PO₄)₂, accompanied by dehydration, involves two steps, with maximum rates at ~265 and 370°C, and results in the formation of barium dihydrogen pyrophosphate and barium metaphosphate, respectively. The total enthalpy of the endothermic dehydration events is–244.6 J/g. Using impedance spectroscopy, we have studied in detail the proton conductivity of polycrystalline and single-crystal Ba(H₂PO₄)₂ samples in a controlled atmosphere. Adsorbed water has been shown to have a significant effect on the proton conductivity of Ba(H₂PO₄)₂ up to 130°C. The proton conductivity of the Ba(H₂PO₄)₂ single crystals has been shown to be anisotropic. The conductivity anisotropy correlates with specific structural features of the salt. Higher conductivity values, 3 × 10^–9 to 2 × 10^–7 S/cm in the range 60–160°C, have been observed in the [100] crystallographic direction, exceeding the conductivity along [010] by an order of magnitude. The activation energy for proton conduction is 0.80 eV.     

Fine-grained high-T c superconducting YBa2Cu3O7−δ powders through a modified citrate method

Synthesis and thermal decomposition characteristics of acetate-modified citrate precursor have been investigated. The intermediates of thermal decomposition exhibit enhanced reactivity. Quantitative estimation of the intermediate phases above 1125 K has been carried out and a directed reaction between Y₂Cu₂O₅ and BaCuO₂ phases is postulated for the formation of pure 1–2–3 phase. Isothermal calcination of the precursor at 1175 K for 9 h is sufficient for the complete conversion of the intermediate phases to SmBa₂Cu₃O_7−δ. The particles thus obtained are nearly spherical and monosized to give a BET surface area of 3 m²/g. The oxygenated powder showsT _c onset at 97 K for magnetic susceptibility and a narrow hysteresis for magnetization withH _c1 andJ _c values of 105Oe and 6×10⁴ A/cm² respectively at 77 K. EPR and EDAX techniques confirm the phase purity. Electron diffraction studies confirm XRD results. For oxygenated powder the excess charge (p ⁺) on square planar copper site [Cu-O] ^p * is estimated to be 0.28.     

DC electrical conductivity of CoSiF6·6H2O and (NH4)2SO4

D.C. electrical conductivity of single crystals of (NH₄)₂SO₄ and CoSiF₆·6H₂O have shown conductivity jump near their respective structural transition temperature. Activation energy of (NH₄)₂SO₄ has been found to be consistent with the earlier data. However, CoSiF₆·6H₂O has given prolonged aging effect due to dipolar relaxation and formation of space charge polarisation.σ _true andP _max have been measured.P _max has shown a negative maximum at the transition point. Activation energy of ZnSiF₆·6H₂O has been found to be comparable with CoSiF₆·6H₂O.     

Optical properties of SnO2 nanostructures prepared via one-step thermal decomposition of tin (II) chloride dihydrate

In this study we report on a simple method to synthesize tin dioxide (SnO₂) nanostructures through one-step thermal decomposition of tin (II) chloride dihydrate (SnCl₂·2H₂O) under ambient conditions. The size and morphology of the SnO₂ nanostructures can be controlled by changing the temperature and the time of the decomposition process. The structural properties of the products were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) equipped with energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The optical properties of the samples were studied by UV–vis spectroscopy. The optical gap of the samples was determined from the analysis of the absorbance versus wavelength curves. The results showed that the morphology, the size and the optical gap of the samples varied with the temperature and treatment duration.     

Synthesis and characterization of (Cu1−x Znx)3(PO4)2 · H2O (0 < x ≤ 0.19) solid solutions

New substitutional solid solutions with the general formula (Cu_1?x Zn_x)₃(PO₄)₂ · H₂O have been synthesized in the range 0 < x ≤ 0.19 (monoclinic structure, sp. gr. C2/c). The solid solutions have been characterized by chemical analysis, thermal analysis, x-ray diffraction, and IR spectroscopy. At higher Zn contents, no solid solutions have been obtained.     

Low-temperature fabrication and electrical property of 10 mol% Sm2O3-doped CeO2 ceramics

Ten mol% Sm₂O₃-doped CeO₂ solid-solution (20SDC) powders have been synthesized via carbonate coprecipitation using ammonium hydrogen carbonate (AHC) and urea as the precipitants, respectively. Characterizations were achieved by elemental analysis, X-ray diffractometry, differential thermal analysis/thermogravimetry, and FESEM. An amorphous hydroxyl carbonate precursor (Ce,Sm)(OH)CO₃·2H₂O having nanosized (~10 nm) spherical particles was formed with AHC, while a mixture of crystalline (Ce,Sm)₂(CO₃)₂(OH)₂·H₂O and (Ce,Sm)₂O(CO₃)₂·H₂O phases exhibiting irregular particle morphologies was obtained with urea. Both the precursors convert to oxide solid solutions without any phase detected corresponding to Sm₂O₃ during calcination. The oxide powder processed via the AHC method can be sintered to >99% of the theoretical at a low temperature of 1200 ?C, due to the good dispersion and ultrafine size (~15 nm) of the particles, while that from the urea method can only reach ,67.2% dense at the same temperature. Electrical conductivity of the densified ceramic was measured in air in the range 400—700 ?C by the DC three-point method, and an activation energy of ~60.5 kJ/mol was derived from the experimental data.

© 2003 Elsevier Ltd. All rights reserved.     

Experimental measurement of thermophysical properties of H2O/KCOOH (potassium formate) desiccant

This paper presents the measurement of the thermal conductivity and the dynamic viscosity of H₂O/KCOOH (potassium formate) desiccant with a salt concentration from 60 to 80% in the temperature range 1–80 °C. The thermal conductivity measurement gives evidence of a great sensitivity to salt concentration and lower sensitivity to temperature: H₂O/KCOOH desiccant shows a thermal conductivity from 23 to 33% lower than water at the same temperature. H₂O/KCOOH desiccant exhibits a Newtonian behaviour in all the investigated ranges of temperature and concentration. The relative viscosity shows a great sensitivity to salt concentration and weak or no sensitivity to temperature up to a solution concentration of salt around 70%. For higher solution concentration of salt (75 and 80%) the relative viscosity shows a great sensitivity also to temperature. H₂O/KCOOH desiccant presents a dynamic viscosity from 4 to 30 times higher than water at the same temperature.     

The synthesis and characterisation of Co2X (Ba2Co2Fe28O46) and Co2U (Ba4Co2Fe36O60) ferrite fibres, manufactured from a sol-gel process

The X and U phases of hexagonal ferrites are notoriously difficult to make as pure materials, and what little data there is published regarding their physical or magnetic properties relates only to single crystals. Stoichiometric Co₂X and Co₂U fibres were made from a sol-gel based process, and the characteristics of the sols and fibres were studied using PCS, XRD, SEM and VSM. The evolution of the X and U fibres was studied, and shown to be similar that of the W and Z fibres whose structures they respectively resemble. Co₂X and Co₂U had been formed at 1200°C, and the morphology of the fibres was different from those of the W and Z phases. The magnetic properties, again while resembling the W and Z phases, were different to any of the other hexagonal ferrite fibres. The magnetic hysteresis loops of both fibres were magnetically as was, the Co₂X fibres having M _s = 45.0 emu g^–1 and H _c = 0.085 T, and the Co₂U fibres having M _s = 51.5 emu g^–1 and H _c = 0.059 T.     

Synthesis and structural study of new potassium uranyl selenates K2(H5O2)(H3O)[(UO2)2(SeO4)4(H2O)2](H2O)4 and K3(H3O)[(UO2)2(SeO4)4(H2O)2](H2O)5

Single crystals of new uranyl selenates K₂(H₅O₂)(H₃O)[(UO₂)₂(SeO₄)₄(H₂O)₂](H₂O)₄ (1) and K₃(H₃O)[(UO₂)₂(SeO₄)₄(H₂O)₂](H₂O)₅ (2) were prepared by isothermal evaporation at room temperature. The crystal structure of 1 was solved by the direct method [C2/c, a = 17.879(5), b = 8.152(5), c = 17.872(5) Å, β = 96.943(5)°, V = 2585.7(19) ų, Z = 4] and refined to R ₁ = 0.0449 (wR ₂ = 0.0952) for 2600 reflections with |F _o| ≥ 4σ _F . The structure of 2 was solved by the direct method [P2₁/c, a = 17.8377(5), b = 8.1478(5), c = 23.696(1) Å, β = 131.622(2)°, V = 2574.5(2) ų, Z = 4] and refined to R ₁ = 0.0516 (wR ₂ = 0.1233) for 4075 reflections with |F _o| ≥ 4σ _F . The structures of 1 and 2 are based on [(UO₂)₂(SeO₄)₄(H₂O)₂]^4? layers. The charge of the inorganic layer is compensated by potassium and oxonium ions arranged in the interlayer space. Each K ion is surrounded by seven O atoms belonging to uranyl selenate layers and water molecules, so that it binds with each other the adjacent uranyl selenate structural elements.     

Pressure derivatives of elastic constants of monoclinic M(NO3)3·9H2O (M=Al,Fe, Cr) and third-order elastic constants of Al(NO3)3·9H2O

The elastic constants and their pressure and temperature derivatives of monoclinic M(NO₃)₃·9H₂O (M=Al, Fe, Cr) have been determined from ultrasonic resonance frequencies of thick plates and from their temperature- and pressure-induced shifts. In addition, the complete third-order elasticity tensor of Al(NO₃)₃·9H₂O has been evaluated from a combination of measurements of shifts of ultrasonic resonance frequencies produced by hydrostatic pressure and uniaxial stresses in various directions. The elasticity tensors of the three isotypic salts and their temperature and pressure derivatives are almost identical. The elasticity tensor exhibits a distinct anisotropy as well as the tensor of thermal expansion. The thermoelastic constants behave quite normally, whereas the pressure derivatives show anomalous features, i.e. some of them are negative similar to the situation in cubic Ba(NO₃)₂ and α-ammonium or α-selenate alums. These effects are accompanied by a very small, even negative, thermal expansion. The third-order elastic constants of Al(NO₃)₃·9H₂O also reflect this anomalous behaviour.     

Crystallization of AlPo4-5 from a system-1.5 (C3H7)3N.1.0Al2O3.1.0 P2O5. 40.0 H2O : Characterization of the products of crystallization

Crystallization of AlPO₄-5 from a gel of composition 1.5 Pr₃N (tripropylamine)· 1.0 Al₂O₃· 1.0 P₂O₅· 40.0 H₂O has been investigated by carrying out the hydrothermal synthesis of the crystalline aluminophosphate in a closed stainless steel pressure bomb at 423 K for different durations of the crystallization (0–48 h). The solid products obtained during the course of crystallization have been characterized thoroughly for their crystallinity (as AlPO₄-5), crystal size and morphology, N₂-sorption capacity (at 78 K and relative pressure of 0.3), thermal analysis, site energy distribution by TPD of pyridine, acidity by chemisorption of pyridine at 673 K and also for their catalytic activity in cumene cracking and o-xylene isomerization reactions at 673 K. The aluminophosphate formed at the different crystallization periods differ from each other largely in these properties. The optimum crystallization period for the synthesis of AlPO₄-5 is about 24 h.     

Synthesis and characterization of γ-Bi2O3 based solid electrolyte doped with Nb2O5

γ-phase bismuth oxide is a well known high oxygen ion conductor and can be used as an electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). This study aims to determine new phases of Bi₂O₃-Nb₂O₅ binary system and the temperature dependence of the electrical transport properties. The reaction products obtained in open air atmosphere were characterized by X-ray powder diffractions (XRD). The unit cell parameters were defined from the indexes of the powder diffraction patterns. The γ-Bi₂O₃ crystal system were obtained by doping 0.01 < mole% Nb₂O₅ < 0.04 at 750 °C for 48 and 96 h. Thermal behaviour and thermal stability of the phases were investigated by thermal analysis techniques. Surface and grain properties of the related phases were determined by SEM analysis. The temperature dependence of the electrical properties of γ-Bi₂O₃ solid solution was measured by four-point probe d.c. conductivity method. In the investigated system, the highest value of conductivity was observed for σ _T = 0.016 ohm^?1 cm^?1 at 650 °C on 4 mole% Nb₂O₅ addition. The electrical conductivity curves of studied materials revealed regular increase with temperature in the form of the Arrhenius type conductivity behaviour.     

Magnetic properties of Ni1−x Cu x Cr2O4(0⩽x⩽1) compounds

The compounds Ni_1−x Cu _x Cr₂O₄ (0⩽x⩽1) have been synthesised by solid-state reaction between basic nickel(II) carbonate, basic copper(II) carbonate and chromium (III) carbonate in required molar ratios at 800±10°C for 20 hr. The reaction products have been characterized by chemical analyses and powder x-ray diffraction patterns. Magnetic susceptibility has been measured in the temperature range of 300–900 K at 10 kOe. All the products show ferrimagnetic behaviour with the ferrimagnetic Curie temperature (T _c) in the range of 50–150 K. The curie temperature increases when copper(II) ion is substituted for nickel(II) ion in NiCr₂O₄. The experimental values of the average effective magneton number (p-0304;) agree with theoretical values.     

Electrical conduction in some (CnH2n) (NH 3 + )2 FeCl4 compounds

The d.c. conductivity, a.c. conductivity and thermoelectric power of the compounds H₃N₊(CH₂)₊ NH₃FeCl₄, wheren=2, 3, 7 and 10, have been studied over a temperature range of 150–500 K. The conductivity results confirm the presence of more than one structural phase transition for each compound investigated. The thermoelectric power measurements showed that electrons are the main charge carriers in all crystal phases. The conductivity results were explained on the basis of an electron hopping mechanism over the whole temperature range.