Mechanism and kinetics of formation of La<Subscript>2</Subscript>SrFe<Subscript>2</Subscript>O<Subscript>7</Subscript> and Nb<Subscript>2</Subscript>SrFe<Subscript>2</Subscript>O<Subscript>7</Subscript>

Phase formation processes in the systems Ln₂O₃-SrO-Fe₂O₃ (Ln = La, Nd) in air in the temperature range 1200–1500°C were studied. The synthesis of the complex ferrites La₂SrFe₂O₇ and Nb₂SrFe₂O₇ involves the formation of the intermediate compounds LnFeO₃ and LnSrFeO₄ and occurs by the same mechanism as the synthesis of the corresponding aluminates, but much faster.     

Influence of B<Subscript>2</Subscript>O<Subscript>3</Subscript> on the structure and crystallization of soil active glasses

Glasses of the SiO₂–P₂O₅–K₂O–MgO–CaO–B₂O₃ system acting as nutrients carriers in the soil environment were synthesised by the melt-quenching technique. Thermal properties were studied using DTA/DSC methods and the influence of B₂O₃ and P₂O₅ content on thermal stability and crystallization process of these glasses was examined. The structure of the glass network was characterized by FTIR, ³¹P, and ¹¹B MAS NMR. The chemical activity of the glasses in the 2 mass% citric acid solution was measured by the ICP-AES method. The analysis indicated that the formation of P–O–B units with chemically stable tetrahedral borate groups decreases the glass solubility in conditions simulating the soil environment.     

Synthesis and characterization of ferroelectric SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> nanotubes arrays

Ferroelectric SrBi₂Ta₂O₉ nanotubes were fabricated by sol–gel dipping template technique and characterized by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. They had a single orthorhombic perovskite structure, and most of SBT nanotubes showed highly preferential crystal growth along the [115] orientation. FE-SEM and TEM investigations showed that nanotubes have smooth wall morphologies and well-defined diameters corresponding to the diameter of the applied template. From HRTEM results, the clear lattice fringes indicated that the nanotubes are structurally uniform and well crystallized. The growth mechanisms of SBT nanotubes into the AAO templates were explored.     

Thermodynamic properties of silicate glasses and melts: V. Systems CaB<Subscript>2</Subscript>O<Subscript>4</Subscript>-CaSiO<Subscript>3</Subscript> and Ca<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>5</Subscript>-CaSiO<Subscript>3</Subscript>

Thermodynamic properties of melts of the CaB₂O₄-CaSiO₃ and Ca₂B₂O₅-CaSiO₃ systems were determined by the method of high-temperature mass-spectrometry. The melts of these systems are characterized by negative deviations from the ideal behavior at 1800 K.     

Specific heat on Sr<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>7</Subscript> and Sr<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>7</Subscript>

Summary Specific heats on the single crystals of Sr₂Nb₂O₇, Sr₂Ta₂O₇ and (Sr_1-xBa_x)₂Nb₂O₇ were measured in a wide temperature range of 2-600 K. Heat anomalies of a λ-type were observed at the incommensurate phase transition of T_INC (=495 K) on Sr₂Nb₂O₇ and at the super-lattice phase transition of T_SL (=443 K) on Sr₂Ta₂O₇; the transition enthalpies and the transition entropies were estimated. Furthermore, a small heat anomaly was observed at the low temperature ferroelectric phase transition of T_LOW (=95 K) on Sr₂Nb₂O₇. The transition temperature T_LOW decreases with increasing Ba content x and it vanishes for samples of x>2%.     

Study on the non-isothermal kinetics of decomposition of 4Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>·2H<Subscript>2</Subscript>O<Subscript>2</Subscript>·NaCl

The non-isothermal decomposition kinetics of 4Na₂SO₄·2H₂O₂·NaCl have been investigated by simultaneous TG-DSC in nitrogen atmosphere and in air. The decomposition processes undergo a single step reaction. The multivariate nonlinear regression technique is used to distinguish kinetic model of 4Na₂SO₄·2H₂O₂·NaCl. Results indicate that the reaction type Cn can well describe the decomposition process, the decomposition mechanism is n-dimensional autocatalysis. The kinetic parameters, n, A and E are obtained via multivariate nonlinear regression. The n ^th-order with autocatalysis model is used to simulate the thermal decomposition of 4Na₂SO₄·2H₂O₂·NaCl under isothermal conditions at various temperatures. The flow rate of gas has little effect on the decomposition of 4Na₂SO₄·2H₂O₂·NaCl.     

Synthesis and study of solid acid materials SnO<Subscript>2</Subscript>/B<Subscript>2</Subscript>O<Subscript>3</Subscript>

SnO₂/B₂O₃ samples were produced by a reaction between SnCl₄, H₃BO₃, and (NH₂)₂CO in a boiling aqueous solution. The Sn: B molar ratio in these samples was 1: 1, 1: 2, and 1: 3. The phase composition and degree of crystallinity of these materials was studied. The surface acidity of the samples was analyzed by the method based on a temperature-programmed reaction of dehydration of 2-methyl-3-butyn-2-ol. Thermal transformations of SnO₂/B₂O₃ samples were examined by means of differential-thermal analysis.     

High-temperature heat capacity of stannates Er<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript> and Tm<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

Erbium stannate Er₂Sn₂O₇ and thulium stannate Tm₂Sn₂O₇ with a pyrochlore-type structure were produced by solid-phase synthesis by calcining stoichiometric mixtures of the respective oxides in air at 1473 K for 240 and 200 h. The high-temperature heat capacity of Er2Sn2O7 and Tm₂Sn₂O₇ was studied by differential thermal calorimetry at 353–1000 K. From the experimental dependences C _P = f(T), the thermodynamic functions (enthalpy change, entropy change, and reduced Gibbs free energy) of oxide compounds were calculated.     

Effect of nucleating agents on the crystallization of Li<Subscript>2</Subscript>O-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> system glass

The processes of nucleation of Li₂O-Al₂O₃-SiO₂ glasses with TiO₂ and TiO₂+ZrO₂ as nucleating agents were discussed. The DTA peak temperature and DTA peak height shown a strong dependence on the nucleation temperature in the glass with TiO₂, while in the glass with TiO₂+ZrO₂ this tendency was small. The optimum nucleation temperatures were 745 and 760°C for two glasses. It suggested that with TiO₂+ZrO₂ as nucleating agents, the crystallization had lower sensitivity for nucleation temperature, and the glass had higher nucleation efficiency than with TiO₂.     

Effects of the Chemical Modifier on the Thermal Evolution of SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> Precursor Powders

The effects of the chelating agent on the thermal evolution of SrBi₂Ta₂O₉ precursor powders were investigated. The precursor solutions were prepared from non-hydrolyzing precursors of bismuth and strontium and a tantalum alkoxide. The utilization of diethanolamine or triethanolamine as chelating agent was found to produce the segregation of metallic bismuth in the as-prepared powders, which led to the formation of a multiphase system. On the other hand, acetoin, one of the α-hydroxyketones, showed outstanding characteristics for the low-temperature synthesis of SrBi₂Ta₂O₉: elimination of residual organics at low temperature, an earlier onset of crystallization, and no segregation of secondary phases during the whole crystallization process.     

Controlled synthesis,characterization and thermal properties of Mg<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>5</Subscript>

Optimum conditions for synthesizing monoclinic and triclinic Mg₂B₂O₅ compounds by high-temperature solid-state reactions were investigated. Mixtures composed of boric acid and magnesium oxide at MgO:B₂O₃ mole ratios of 1:0.25, 1:0.5 and 1:1.5 were heated for 1 hour at temperatures between 600–1050°C and the formed phases were identified by XRD analysis. Monoclinic Mg₂B₂O₅ was formed by heating at 850°C for 4 hours together with minimum amounts of triclinic Mg₂B₂O₅, while triclinic Mg₂B₂O₅ was formed as a single phase at 1050°C for the same reaction time. The products obtained at optimum conditions were subjected to a series of tests to determine their chemical compositions, particle size distributions, surface area values, IR spectra and TG/DTA patterns.      

Some studies on the phase formation and kinetics in TiO<Subscript>2</Subscript> containing lithium aluminum silicate glasses nucleated by P<Subscript>2</Subscript>O<Subscript>5</Subscript>

Lithium aluminum silicate (LAS) glasses of compositions (wt%) 10.6Li₂O–71.7SiO₂–7.1Al₂O₃–4.9K₂O–3.2B₂O₃–1.25P₂O₅–1.25TiO₂ were prepared by the melt quench technique. Crystallization kinetics was investigated by the method of Kissinger and Augis–Bennett using differential thermal analysis (DTA). Based on the DTA data, glass ceramics were prepared by single-, two-, and three-step heat treatment schedules. The interdependence of different phases formed, microstructure, thermal expansion coefficient (TEC) and microhardness (MH) was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermo-mechanical analysis (TMA), and microhardness (MH) measurements. Crystallization kinetics revealed that Li₂SiO₃ is the kinetically favored phase with activation energy of 91.10 kJ/mol. An Avrami exponent of n = 3.33 indicated the dominance of bulk crystallization. Based upon the formation of phases, it was observed that the two-stage heat treatment results in highest TEC glass ceramics. The single-step heat treatment yielded glass ceramics with the highest MH.     

Enthalpies of dilution of aqueous mixed solutions of LiCl and Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> at 298.15 K

The enthalpies of dilution, Δ_dil H _m, have been measured for LiCl+Li₂B₄O₇+H₂O system at T=298.15 K by using a RD496-III microcalorimeter. A suitable measurement method was used to obtain the better data of the enthalpies of dilution for the ternary mixing solutions to low concentrations. The relative apparent molar enthalpies, L _ϕ, have been determined and the relationships between L _ϕ and ionic strength I at different molal fractions of Li₂B₄O₇ were obtained. The effect of the borate Li₂B₄O₇ on the heat properties for the studied system was discussed.     

Phase relations in the Zn<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>-Cu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript> system from room temperature to melting

Phase relations in the Zn₂V₂O₇-Cu₂V₂O₇ system were studied by high-temperature X-ray diffraction and differential thermal analysis. The major phase constituents of the system are solid solutions based on Zn₂V₂O₇ and Cu₂V₂O₇ polymorphs and their coexistence regions. The generation of α-Zn_{2 − 2x} Cu_2x V₂O₇ solid solution, where 0 ≤ x ≤ 0.30, leaves almost unchanged the stabilization temperature of the high-temperature zinc pyrovanadate phase. The α-Cu_{2 − 2x} Zn_2x V₂O₇ homogeneity range is 5 mol % Zn₂V₂O₇. In the range 0.050 ≤ x ≤ 0.09 from 20 to ∼ 620°C, there is the two-phase field of α-Cu₂V₂O₇ and β-Cu₂V₂O₇ base solid solutions. At still higher temperatures, β-Zn_{2 − 2x} Cu_2x V₂O₇ and α-Cu_{2 − 2x} Zn_2x V₂O₇ coexist in the mixed-phase region. β-Zn_{2 − 2x} Cu_2x V₂O₇ solid solution, where 0 ≤ x ≤ 0.30, exists above 610 ± 5°C. The extent of the β′-Cu₂V₂O₇-base solid solution is 9 to 65 mol % Zn₂V₂O₇ at 615 ± 5°C, expanding to 0 mol % Zn₂V₂O₇ with rising temperature. Original Russian Text ￠ T.I. Krasnenko, M.V. Rotermel’, S.A. Petrova, R.G. Zakharov, O.V. Sivtsova, A.N. Chvanova, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 10, pp. 1755–1762.     

Glass formation in the CaO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> and SrO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> systems

A physicochemical study of glasses based on the MO-Bi₂O₃-B₂O₃ and SrO-Bi₂O₃-B₂O₃ systems was performed. Glass formation regions were found. The structural and optical properties, as well as the thermal behavior of the glasses, were studied.     

Chemical durability of MoO<Subscript>3</Subscript>-P<Subscript>2</Subscript>O<Subscript>5</Subscript> and K<Subscript>2</Subscript>O-MoO<Subscript>3</Subscript>-P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

Thermal and chemical durability studies of the phosphate glasses belonging to the binary MoO₃-P₂O₅ and the ternary K₂O-MoO₃-P₂O₅ systems are reported. The chemical resistant attack tests carried out on the free alkaline MoO₃-P₂O₅ glasses show that the glass associated with the P/Mo ratio 2 has the high chemical durability. It shows also a high glass transition temperature value. The above findings are interpreted in terms of the cross-link density of the glasses and the strength of the M-O bonds (M=P, Mo). The influence of K₂O addition on the properties (density, T _g, durability) of this binary high water resistant glass is studied. It is found that the chemical durability along with the other physical properties are reduced by the incroporation of K₂O in the glass matrix. The results were explained by assuming the formation of non-bridging oxygens and weak bonds. The mechanism of the dissolution of these glasses is proposed.     

Thermal behaviour and properties of Na<Subscript>2</Subscript>O-TiO<Subscript>2</Subscript>-P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

Differential scanning calorimetry (DSC) and thermomechanical analysis (TMA) were used to study the thermal behaviour of (50-x)Na₂O-xTiO₂-50P₂O₅ and 45Na₂O-yTiO₂-(55-y)P₂O₅ glasses. The addition of TiO₂ to the starting glasses (x=0 and y=5 mol% TiO₂) resulted in a nonlinear increase of glass transition temperature and dilatation softening temperature, whereas the thermal expansion coefficient decreased. All prepared glasses crystallize under heating within the temperature range of 300–610°C. The contribution of the surface crystallization mechanism over the internal one increases with increasing TiO₂ content. With increasing TiO₂ content the temperature of maximum nucleation rate is also gradually shifted from a value close to the glass transition temperature towards the crystallization temperature. X-ray diffraction measurements showed that the major compounds formed by glass crystallization were NaPO₃, TiP₂O₇ and NaTi₂(PO₄)₃. The chemical durability of the glasses without titanium oxide is very poor, but with the replacement of Na₂O or P₂O₅ by TiO₂, it increases sharply.     

Characterization of NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles synthesized by various methods

Microwave-induced combustion with glycine, CTAB-assisted hydrothermal process with NaOH and NH3, EDTA assisted-hydrothermal methods have been applied to prepare NiFe₂O₄ nanoparticles for the first time. Structural and magnetic properties of the products were investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmison electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and electron spin resonance spectrometry (EPR). TEM measurements showed that morphology of the product depends on the synthesis method employed. The average cystallite size of NiFe₂O₄ nanoparticles was in the range of 14–59 nm as measured by XRD. The uncoated sample (Method A) had an EPR linewidth of 1973 Oe, the coated samples reached lower values. The magnetic dipolar interactions existing among the Ni ferrite nanoparticles are reduced by the coatings, which could cause the decrease in the linewidth of the EPR signals. Additionally, the linewidth increases with an increase in the size and the size distribution of nanoparticles.     

Crystallization kinetics and thermal stability of an amorphous Fe<Subscript>77</Subscript>C<Subscript>5</Subscript>B<Subscript>4</Subscript>Al<Subscript>2</Subscript>GaP<Subscript>9</Subscript>Si<Subscript>2</Subscript> bulk metallic glass

The thermal stability, kinetics and glass forming ability of an Fe₇₇C₅B₄Al₂GaP₉Si₂ bulk amorphous alloy have been studied by differential scanning calorimetry. The activation energy, frequency factor and rate constant corresponding to the multiple crystallization steps were determined by the Kissinger method. X-ray diffraction and transmission electron microscopy studies revealed that the crystallization starts with the primary precipitation of α-Fe from the amorphous matrix. The kinetics of nucleation of the α-Fe nanoparticles was investigated by two different methods, i.e. isothermal annealing and continuous heating after partial annealing.     

Synthesis of IR phosphors based on germanatoborate Gd<Subscript>14</Subscript>Ge<Subscript>2</Subscript>B<Subscript>6</Subscript>O<Subscript>34</Subscript>

Processes of the formation of germanatoborates Gd₁₄B₆Ge₂O₃₄ and Gd_13.02Nd_0.98B₆Ge₂O₃₄ have been studied using different methods of synthesis (solid-state interaction, direct and inverse co-precipitation, self-propagating high-temperature synthesis (SHS)). It has been established that the synthesis of germanatoborates Gd₁₄B₆Ge₂O₃₄ and Gd_13.02Nd_0.98B₆Ge₂O₃₄ using the inverse precipitation and SHS methods occurs with the formation of an intermediate apatite-like phase, which upon heating to above 1100°С is reconstructed into the Ln₁₄B₆Ge₂O₃₄ (Ln = Gd, Nd) structure. The germanatoborates synthesized crystallize in the trigonal system (space group P3₁). The lattice parameters of Gd_13.02Nd_0.98B₆Ge₂O₃₄ are a = 9.746(4) Å and c = 25.795(13) Å. The thermal stability of the Gd₁₄B₆Ge₂O₃₄ and Gd_13.02Nd_0.98B₆Ge₂O₃₄ germanatoborates has been studied. The obtained materials of composition Gd_13.02Nd_0.98B₆Ge₂O₃₄ show luminescence properties and can be employed as infrared phosphors.