Dilatometric studies of Y2W3O12 with added Al2O3

Y₂W₃O₁₂ exhibits negative thermal expansion along the three crystallographic directions due to the transverse thermal vibrations perpendicular to the Y-O-W linkage. It is highly hygroscopic and forms a trihydrate structure at room temperature. Dilatometric studies of Y₂W₃O₁₂ show large thermal expansion hysteresis due to large grain size and a large initial positive thermal expansion due to the removal of water molecules. Al₂O₃ has been added to Y₂W₃O₁₂ upto 10 wt% in an attempt to overcome the hygroscopicity and reduce the particle size and thereby the thermal expansion hysteresis. Thermo gravimetric, dilatometric and electron microscopic studies are presented to support these observations. Dedicated to Professor C N R Rao on his 70th birthday     

Standard free energy of formation of YFeO3, Y3Fe5O12, and a new compound YFe2O4 in the FeFe2O3Y2O3 system at 1200°C

The phase equilibria in the FeFe₂O₃Y₂O₃ system have been established at 1200°C. The following phases were stable: yttria, hematite, magnetite, wüstite, metallic iron, yttrium-iron perovskite, yttrium-iron garnet, and a new phase YFe₂O₄, belonging to a rhombohedral crystal system. The YFe₂O₄ compound has a solid solution from YFe₂O_3.905 to YFe₂O_4.000. The standard free energies of formation of YFe₂O_3.905, YFeO₃, and Y₃Fe₅O₁₂ have been determined to be ?96 800 ± 200 cal, ?59 800 ± 200 cal, and ?143 700 ± 600 cal, respectively, from metallic iron, Y₂O₃, and oxygen.     

Synthesis and magnetic properties of nanocomposite Ni1?xCoxFe2O4–BaTiO3 fibers by organic gel-thermal decomposition process

Nanocomposites of ferrite and ferroelectric phases are attractive functional ceramic materials. In this work, the nanocomposite Ni_1−x Co _x Fe₂O₄–BaTiO₃(x = 0.2, 0.3, 0.4, 0.5) fibers with fine diameters of 3 ~ 7 μm and high aspect ratios were synthesized by the organic gel-thermal decomposition process from the raw materials of citric acid and metal salts. The structure, thermal decomposition process and morphologies of the gel precursors and the resultant fibers derived from thermal decomposition of the gel precursors were characterized by Fourier transform infrared spectroscopy, thermogravimetric differential thermal analysis, X-ray diffraction and scanning electron microscopy. The magnetic properties of the nanocomposite fibers were measured by vibrating sample magnetometer. The nanocomposite fibers of ferrite Ni_1−x Co _x Fe₂O₄ and perovskite BaTiO₃ are formed at the calcination temperature of 900 °C for 2 h. The average grain sizes of Ni_1−x Co _x Fe₂O₄ and BaTiO₃ in the nanocomposite fibers increase from about 15 nm to approximately 67 nm with the increasing calcination temperatures from 900 to 1,180 °C. The saturation magnetization of the nanocomposite Ni_1−x Co _x Fe₂O₄–BaTiO₃(x = 0.2, 0.3, 0.4, 0.5) fibers increases with the increase of grain sizes of Ni_1−x Co _x Fe₂O₄ and Co content, while the coercivity reaches a maximum value at the single-domain size of about 65 nm of Ni_0.5Co_0.5Fe₂O₄ obtained at the calcination temperature of 1,100 °C.     

The compound Cr2TiO5 in the system Cr2O3TiO2

The compound Cr₂TiO₅ could be synthesized as a stoichiometric single phase above 1660°C in air. Application of selected area electron diffraction, high resolution electron microscopy and powder X-ray diffraction studies showed that Cr₂TiO₅ is isomorphous with CrFeTiO₅, with V₃O₅ type structure. It is monoclinic, a = 7.020(1)Å, b = 5.025(1)Å, c = 9.945(2)Å and β = 111.43(2)°. It was found that Cr₂TiO₅ is unstable relative to a mixture of Cr₂O₃ (ss) and a so-called “E” phase, below 1660°C.     

A study of the thermal decomposition of Se2O2F8 and Te2O2F8, using molecular beam techniques

The thermal decomposition of the vapor phases of the oxygen bridged dimers Se₂O₂F₈ and Te₂O₂F₈ has been studied by mass spectrometry, electric deflection and flight time analysis on a molecular beam generated directly from the decomposition products. Se₂O₂F₈ begins to decompose at ?250°C; the principal products are SeF₄ and O₂, with SeOF₂ as a minor product. Decomposition is complete by ?500°C. There is some decomposition to monomeric SeOF₄ between 200 and 350°C. Te₂O₂F₈ did not begin to decompose until a temperature of 400°C was reached. Again, the principal products observed were TeF₄, O₂, and TeOF₂ with no evidence for decomposition to the monomeric TeOF₄.     

Thermal behaviour of two new salts of malonic acid: Cu(C3H2O4)·4H2O and Cu(NH4)2(C3H2O4)2

Two new salts of malonic acid have been prepared: the copper(II) malonate tetrahydrate and the copper(II)-ammonium double malonate. Their study by thermal analysis (TG and DTA) leads to the following results:Cu(C₃H₂O₄)·4H₂O: the dehydration is rather complex and it is only under careful conditions that an intermediate hydrate Cu(C₃H₂O₄)·3H₂O could be traced. At about 170°C the dehydration is not ended (the salt holds yet about 0.15H₂O) and the anhydrous salt occurs only at about 240°C. It decomposes immediately leading to residues the composition of which depends upon the surrounding atmosphere; the part played by the gas given off is discussed.Cu(NH₄)₂(C₃H₂O₄)₂: this salt melts and decomposes simultaneously at about 190°C. During the decomposition the copper nitride Cu₃N forms as intermediate compound (as well as copper metal). Concerning the final residues of the decomposition the results and the conclusions are the same as the ones of the previous case.     

Excellent cycling stability of spherical spinel LiMn2O4 by Y2O3 coating for lithium-ion batteries

The Y₂O₃ nano-film is coated on the surface of the spherical spinel LiMn₂O₄ by precipitation method and subsequent heat treatment at 550 °C for 5 h in air. The structure and performance of the bare LiMn₂O₄ and Y₂O₃-coated LiMn₂O₄ are characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive analysis X-ray spectroscopy, galvanostatic charge–discharge, cyclic voltammetry, and impedance spectroscopy. It has been found that the addition of Y₂O₃ does not change the bulk structure of LiMn₂O₄, and the thickness of the Y₂O₃ coating layer is approximate to 3.0 nm. The 1 wt% Y₂O₃-coated LiMn₂O₄ electrode reveals excellent cycling performance with 80.3 % capacity retention after 500 cycles at 1 C at 25 °C. When cycling at elevated temperature 55 °C, the as-prepared sample still shows 76.7 % capacity retention after 500 cycles. These remarkable improvements indicate that thin Y₂O₃ coating on the surface of LiMn₂O₄ is an effective way to improve the electrochemistry performance. Besides, the suppression of Mn dissolution into the electrolyte via the Y₂O₃ coating layer can be accounted for the improved performances.     

Fe2O3/ZnO/ZnFe2O4 composites for the efficient photocatalytic degradation of organic dyes under visible light

In this study, novel ternary Fe₂O₃/ZnO/ZnFe₂O₄ (ZFO) composites were successfully prepared through a simple hydrothermal reaction with subsequent thermal treatment. The as-prepared products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) analysis, Barrett-Joyner-Halenda (BJH) measurement, and UV–vis diffuse reflectance spectroscopy (UV–vis DRS). The photocatalytic degradation of rhodamine B (Rh B) under visible light irradiation indicated that the ZFO composites calcined at 500 °C has the best photocatalytic activity (the photocatalytic degradation efficiency can reach up to 95.7% within 60 min) and can maintain a stable photocatalytic degradation efficiency for at least three cycles. In addition, the photocatalytic activity of ZFO composites toward dye decomposition follows the order cationic Rh B > anionic methyl orange. Finally, using different scavengers, superoxide and hydroxyl radicals were identified as the primary active species during the degradation reaction of Rh B.     

Alpha-particle-induced luminescence of rare-earth-doped Y2O3 nanophosphors

The feasibility of utilizing Y₂O₃:Tb³⁺ and Y₂O₃:Eu³⁺ as radioluminescent nanophosphors under alpha-particle excitation is investigated. Materials synthesized by the urea homogeneous precipitation method were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The XRD analysis of as-produced precipitates and nanophosphors fired at temperatures ranging from 950 to 1100 °C indicated the presence of highly crystalline cubic Y₂O₃ with crystallite sizes of ∼40 nm. SEM and TEM analysis revealed that particles with average diameters of ∼200 nm and comprised of ∼40 nm grains were obtained. High-resolution radioluminescence and photoluminescence spectra were used to investigate the unwanted radioluminescence saturation effects associated with the high ionization rate of alpha-particles. Additionally, the radioluminescence intensity as a function of rare-earth ion dopant concentration is investigated for these materials under alpha-particle excitation. The prospect for utilizing these materials as intermediate absorbers in indirect-conversion radioisotope batteries is discussed.     

Synthesis of nano-sized crystalline oxide ion conducting fluorite-type Y2O3-doped CeO2 using perovskite-like BaCe0.9Y0.1O2.95 (BCY) and study of CO2 capture properties of BCY

Formation of nano-sized Y₂O₃-doped CeO₂ (YCO) was observed in the chemical reaction between proton conducting Y₂O₃-doped BaCeO₃ (BCY) and CO₂ in the temperature range 700-1000 °C, which is generally prepared by wet-chemical methods that include sol-gel, hydrothermal, polymerization, combustion, and precipitation reactions. BCY can capture CO₂ of 0.13 g per ceramic gram at 700 °C, which is comparable to that of the well-known Li₂ZrO₃ (0.15 g per ceramic gram at 600 °C). Powder X-ray diffraction (PXRD), energy dispersive X-ray analysis (EDX), laser particle size analysis (LPSA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and ac impedance spectroscopy were employed to characterize the reaction product obtained from reaction between BCY and CO₂ and subsequent acid washing. PXRD study reveals presence of fluorite-like CeO₂ (a=5.410 (1) Å) structure and BaCO₃ in reaction products. TEM investigation of the acid washed product showed the formation of nano-sized material with particle sizes of about 50 nm. The electrical conductivity of acid washed product (YCO) in air was found to be about an order higher than the undoped CeO₂ reported in the literature.     

Thermal behaviour of cobalt oxides doped with ZrO2 and ThO2 and the possible cubic phase stabilization of zirconia

The effects of doping cobalt oxides with different amounts of ZrO₂ and ThO₂ (1.5–9 mol%) on the thermal stability of Co₃O₄ and the re-oxidation of CoO by O₂ to Co₃O₄ were investigated. The techniques employed were DTA, with a controlled rate of heating and cooling, X-ray diffraction, and IR spectrometry.The results obtained by DTA revealed that the addition of both Th⁴⁺ and Zr⁴⁺ (up to 6 mol%) exerted no appreciable effect on the thermal stability of Co₃O₄. Increasing the amount of the dopant ions to 9% resulted in no further change in the thermal stability of Co₃O₄ in the case of Th⁴⁺, and an increase of 16% in case of Zr⁴⁺-doping. However, ThO₂-doping of cobalt oxide was accompanied by an enhancement in the reactivity of CoO towards re-oxidation by O₂ to Co₃O₄ to an extent proportional to the amount of dopant oxide.The X-ray investigation of ZrO₂-doped cobalt oxides calcined in air at 1000°C revealed the presence of highly crystalline and stable zirconia in the cubic form. Such a stable phase could not be obtained at temperatures below 2370°C in the absence of stabilizing agents.X-ray and IR investigations of different solids showed the presence of free thoria and zirconia together with new thorium—cobalt and zirconium—cobalt compounds. However, the slow cooling of Zr-treated cobalt oxides from 1000°C to room temperature led to the decomposition of the newly formed compound. The d-spacings and absorption bands of the newly formed compounds were determined.     

Li+ ion conductivity in the system Li4SiO4Li3VO4

Two ranges of solid solutions were prepared in the system Li₄SiO₄Li₃VO₄: Li_4?xSi_1?xV_xO₄, 0 < x ? 0.37 with the Li₄SiO₄ structure and Li_3+yV_1?ySi_yO₄, 0.18 ? y ? 0.53 with a γ structure. The conductivity of both solid solutions is much higher than that of the end members and passes through a maximum at ～40Li₄SiO₄ · 60Li₃VO₄ with values of ～1 × 10^?5 ohm^?1 cm^?1 at 20°C, rising to ～4 × 10^?2 ohm^?1 cm^?1 at 300°C. These conductivities are several times higher than in the corresponding Li₄SiO₄Li₃(P,As)O₄ systems, especially at room temperature. The solid solutions are easy to prepare, are stable in air, and maintain their conductivity with time. The mechanism of conduction is discussed in terms of the random-walk equation for conductivity and the significance of the term c(1 ? c) in the preexponential factor is assessed. Data for the three systems Li₄SiO₄Li₃YO₄ (Y = P, As. V) are compared.     

Synthesis and structural characterization of perovskite YFeO<Subscript>3</Subscript> by thermal decomposition of a cyano complex precursor,Y[Fe(CN)<Subscript>6</Subscript>]·4H<Subscript>2</Subscript>O

The thermal decomposition of Y[Fe(CN)₆]·4H₂O has been studied in order to investigate the formation of the multi-ferroic oxide YFeO₃. The starting material (Y[Fe(CN)₆]·4H₂O) and the decomposition products were characterized by IR spectroscopy, thermal analysis, X-ray powder diffraction (PXRD), and scanning electron microscopy. Metastable YFeO₃ with hexagonal structure, space group P6 ₃ /mmc, was obtained by thermal decomposition of Y[Fe(CN)₆]·4H₂O at 600 °C in air. Orthorhombic YFeO₃ was obtained by the same method at T ≥ 800 °C in air. The crystal structure of orthorhombic YFeO₃ was refined by Rietveld analysis using PXRD data. We found that it was slightly deficient in Y³⁺, which is in agreement with the small amount of Y₂O₃ found as impurity in the sample. The formula of the orthorhombic phase is Y_0.986FeO₃.     

Effect of metal oxide additives on the thermal decomposition of KMnO4

The study of the thermal decomposition of potassium permanganate in air has been studied using DTA, TG and isothermal decomposition between 150 and 400°C. Rate constants were evaluated and found to be affected by the presence of foreign ions. Oxides such as Fe₂O₃ and Cr₂O₃ resulted in a lowering of the decomposition temperature. The modified catalytic mechanisms were proposed by considering the electron transfer and the oxygen-abstraction models.     

Vanadium- and molybdenum-containing compositions prepared by mechanochemical and following thermal treatments of V2O5/(NH4)2Mo2O7 (V/Mo = 0.7/0.3)

X-ray powder diffraction, DTA, FT-IR spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), nitrogen adsorption, and mercury porometry were used to characterize samples prepared as a result of mechanochemical treatment (MCT) of a V₂O₅/(NH₄)₂Mo₂O₇ (V/Mo = 0.7/0.3) composition in water, ethanol, and air, as well as after calcining them at temperatures from the range 300–700°C. The MCT of nonporous powders in water yields porous materials with definite meso- and macropore sizes. Heat treatment in air at 300–450°C enhances the formation of a molybdenum substitutional solid solution in V₂O₅ and conserves rather high values of specific surface areas and pore volumes. An increase in heat treatment temperature is accompanied by the degradation of the solid solution and the formation of a V₂MoO₈ phase.     

Polymorphism in Bi2WO6

The polymorphism of Bi₂WO₆ has been studied using differential dilatometry and differential thermal analysis with polycrystalline specimens prepared by sintering the oxides Bi₂O₃ and WO₃. Two reversible polymorphic transitions were observed, one at 662°C and one at 962°C. The former transition showed a very small change of enthalpy and very little dimensional change, while the latter showed a large thermal hysteresis, had a large change of enthalpy, and was accompanied by a sizable volume change. The high-temperature powder X-ray data indicated that the intermediate phase as well as the low-temperature form had orthorhombic symmetry and the high-temperature form had monoclinic symmetry. The 662°C transition is displacive and the 962°C transition reconstructive. A crystal structure of the high-temperature form is proposed and discussed in comparison with that of the low-temperature form.     

Synthesis and Characterization of N3P3(O2C12H8)2(OC6H4Si(CH3)3)(OC6H4Br) and Its Conversion to Nanostructured Si Material

A new silicated cyclotriphosphazene N₃P₃(O₂C₁₂H₈)₂(OC₆H₄Si(CH₃)₃)(OC₆H₄Br) 1 has been synthesized and characterized. The solid state pyrolysis of 1 in air gives a nanostructured SiP₂O₇ 3D network. The morphology of the network strongly depends on the temperature of the pyrolysis. Spinal-like columns and ring-shaped SiP₂O₇ are formed at 800 °C, while, at 600 °C, fused grains of about 300 nm were observed. Based on air TG and DSC thermal studies, we propose the mechanism of formation for the nanostructured network.     

Thermal decomposition of ammonium metavanadate supported on Al2O3

The thermal decomposition of ammonium metavanadate supported on aluminium oxide was investigated using DTA, TG and X-ray diffraction techniques.The results obtained revealed that ammonium vanadate decomposed at 225–250°C giving an intermediate compound ((NH₄)₂V₆O₁₆) which decomposed readily at 335–360°C producing V₂O₅. Alumina was found to chance the formation of the intermediate compound and retard its decomposition. Some of the V⁵⁺ ions of V₂O₅ lattice seemed to be reduced into V⁴⁺ and V³⁺ ions by heating in air at 450°C in the presence of Al₂O₃. Such a reaction was attributed to dissolution of some Al³⁺ ions in the V₂O₅ lattice via location in interstitial positions and/or in cationic vacancies. Al₂O₃ was found to interact with V₂O₅ at 650° C giving well-crystalline A1VO₄ which decomposed at about 750°C forming well-crystalline δ-Al₂O₃ and V₂O₅,. Pure Al₂O₃, heated in air at 1000°C, existed in the form of the κ-phase which, on mixing with V₂O₅ (0.5 V₂O₅:1 Al₂O₃) and heating in air at 1000°C, was converted entirely to the well-crystalline α-Al₂O₃ phase.     

Synthesis and properties of M2V8O21 (M = K, Tl) octavanadates and K2?xTlxV8O21 (0 ≤ x ≤ 2) solid solutions

Formation parameters and properties of M₂V₈O₂₁ octavanadates where M = K and Tl were studied using X-ray powder diffraction, microscopy, thermogravimetry, vibrational spectroscopy, EPR, and voltammetry. Original synthesis processes for these compounds were developed; their thermal stability parameters were determined. Potassium and thallium(I) octavanadates were shown to form complete solid solutions with each other. Potassium octavanadate in air is stable to 450°C; above this temperature, it transforms, on account of partial reduction of vanadium, to vanadium bronze by the reaction K₂V₈O₂₁ ↔ K₂V₈O_20.8 + 0.1O₂. The K₂V₈O_20.8 percentage in the sample increases with rising temperature. The substitution of small thallium amounts for potassium (x ≥ 0.025) enhances the stability of the phase until it melts at ∼525°C.     

Crystal structure and electrical conductivity of cubic fluorite-based (YO1.5) x (WO3)0.15(BiO1.5)0.85- x (0?≤ ?x ≤?0.4) solid solutions

(WO₃)_0.15(BiO_1.5)_0.85 exhibits a tetragonal structure derived from the fluorite subcell. The electrical conductivity of (WO₃)_0.15(BiO_1.5)_0.85 is lower than that of Y₂O₃-doped Bi₂O₃. The structure and electrical conductivity of samples formulated as (YO_1.5) _x (WO₃)_0.15(BiO_1.5)_{0.85- x} (x = 0.1, 0.2, 0.3, and 0.4) were investigated. The as-sintered (YO_1.5)_0.1(WO₃)_0.15(BiO_1.5)_0.75 exhibited a single cubic structure that is isostructural with δ-Bi₂O₃. For x = 0.2, 0.3, and 0.4, the as-sintered samples consisted of a cubic fluorite structure and rhombohedral Y₆WO₁₂. After heat treatment at 600 °C for 200 h, the cubic structures are stable for x = 0.1, 0.3, and 0.4. A transformation from cubic to rhombohedral phase after heat treatment at 600 °C for 200 h was observed in the sample originally formulated as (YO_1.5)_0.2(WO₃)_0.15(BiO_1.5)_0.65.