Carbothermal synthesis of TiB<Subscript>2</Subscript> powders of micron size

The characteristic details of the carbothermal synthesis of TiB₂ powders from the stoichiometric mixture TiO₂–H₃BO₃–C at temperatures lower 1700 K are investigated using thermal analysis (ТG—thermogravimetry and DSC—differential scanning calorimetry), as well as X-ray diffraction and scanning electron microscopy. In the temperature interval 300 K → 1673 K → 1273 K and at a heating rate of 10 K/min, the reaction in the powder mixture begins at approximately 1300 K and ends at 1470 K during cooling. After 3 h of isothermal synthesis at 1473 K, the TiB₂ yield is more than 90%. The resulting products are hexagonal plate-like crystals 5–10 μm across with thickness of 3 to 4 μm. Kinetic analysis showed that in the temperature range of 1330 to 1673 K the TiB₂ synthesis reaction is of the first-order, and the calculated activation energy of the process is 315 ± 24 kJ/mol.     

Synthesis of nanocrystalline TiB<Subscript>2</Subscript> powder from TiO<Subscript>2</Subscript>, B<Subscript>2</Subscript>O<Subscript>3</Subscript> and Mg reactants through microwave-assisted self-propagating high-temperature synthesis method

In this research work, microwave-assisted self-propagating high-temperature synthesis (SHS) process was employed for the fabrication of titanium diboride (TiB₂) compound from TiO₂–B₂O₃–Mg mixtures. Thermodynamic evaluations of this system and its relevant subsystems revealed that TiB₂–MgO composite powder can be easily produced by a SHS reaction. However, experimental results of a TiO₂ : B₂O₃ : 5Mg mixture heated in a domestic oven showed the formation of some intermediate compounds such as Mg₃B₂O₆, presumably due to some degree of Mg loss. The optimum amount of Mg in TiO₂ : B₂O₃ : xMg mixtures, yielding the highest amount of TiB₂ phase, was found to be around 7 mol, i.e., 40 mol% more than the stoichiometric amount. Experimental results revealed that a pure TiB₂ compound could be obtained by leaching the unwanted by-products in an HCl acid solution. Scanning electron microscopic observations and Scherrer calculations showed that the produced TiB₂ contains sub-micron (150–200 nm) particles, where each particle consists of a number of nanosized (32 nm) crystallites.     

The relationship between TiB<Subscript>2</Subscript> volume fraction and fatigue crack growth behavior in the in situ TiB<Subscript>2</Subscript>/A356 composites

The relationship between TiB₂ volume fraction and fatigue crack growth behavior in the A356 alloy matrix composites reinforced with 3, 5.6, and 7.8 vol% in situ TiB₂ particles has been investigated. The mechanisms of crack propagation in the TiB₂/A356 composites were also discussed. The results show that the 3 vol% TiB₂/A356 composite has nearly the same crack growth behavior as the matrix alloy, while the 5.6 vol% TiB₂/A356 composite exhibits a little bit faster crack growth rate. The 7.8 vol% TiB₂/A356 composite presents the lowest resistance to crack growth, indicating that the crack growth is accelerated by increasing TiB₂ volume fraction. Fractographies reveal that an increase in TiB₂ volume fraction results in a change from the formation of striation and slip to the failure of voids nucleation, growth, and coalescence. Cracks tend to propagate within the matrix and avoid eutectic silicon and TiB₂ particles in the intermediate ΔK region, while prefer to propagate along interfaces of eutectic silicon and TiB₂ particles and link the fractured eutectic silicon particles in the near fractured ΔK region. Furthermore, the propensity for the separation of TiB₂ increases with the increase in TiB₂ volume fraction. The massive voids caused by fractured eutectic silicon and separated TiB₂ particles propagate and coalesce, and then accelerates the crack growth in TiB₂/A356 composites.     

Oxidation behavior of TiB<Subscript>2</Subscript> micro- and nanoparticles

The oxidation of TiB₂ particles (75 to 1500 nm in size) has been studied at temperatures of up to 1000°C by thermogravimetry, X-ray diffraction, X-ray photoelectron spectroscopy, IR frustrated total internal reflection spectroscopy, and energy dispersive X-ray analysis. The oxidation onset was observed between 210 and 475°C, depending on the particle size. This distinction can presumably be accounted for in terms of the deformation produced by the Laplace pressure. Oxidation at temperatures under 1000°C leads to the formation of the rutile phase of TiO₂ and boron oxide (B₂O₃). Moreover, at a temperature of ? 1000°C titanium borate, TiBO₃, was observed to form. Under all of the conditions examined, the oxidation reaction does not reach completion and the oxidation products contain unreacted TiB₂.     

Electrically Conducting Ceramics Based on Al–AlN–TiB<Subscript>2</Subscript>

The high-temperature conductivity of ceramics based on Al–AlN–TiB₂ obtained by self-propagating high-temperature synthesis has been studied. The dependences of DC resistivity were measured in vacuum (1 Pa) by a four-point technique in a temperature range of 293–1273 K. The evolution of the phase composition of the material upon heating was studied by time-resolved diffraction in real-time mode. The measurements show a “metallic” behavior of conductivity when the Al–AlN–TiB₂ composite is heated, and there is also an abrupt change of resistivity in a temperature range of 870–970 K that is linked with Al melting. In this case, the crystalline structure of ceramic phases does not undergo changes in the studied temperature range. The same temperature coefficient of resistance has been observed for all studied compositions up to the melting point of aluminum.     

Fabrication of TiC and TiB<Subscript>2</Subscript> locally reinforced steel matrix composites using a Fe–Ti–B<Subscript>4</Subscript>C–C system by an SHS-casting route

Steel matrix composites locally reinforced by in situ TiC and TiB₂ particulates were successfully fabricated using self-propagating high-temperature synthesis (SHS) in a Fe–Ti–B₄C–C system during casting. The locally reinforced steel matrix composites consist of three distinct regions: (i) a TiC and TiB₂ particulate-reinforced region, (ii) a transition region, and (iii) a steel matrix region. The TiC and TiB₂ particulates in the locally reinforced regions display a relatively uniform distribution, and their sizes decrease with the increase in Fe content from 10 wt.% to 40 wt.%. The wear resistance of the locally reinforced region of the steel matrix composites is much higher than that of the unreinforced steel matrix.     

Reaction synthesis of TiC–TiB<Subscript>2</Subscript>/Al composites from an Al–Ti–B<Subscript>4</Subscript>C system

The TiC–TiB₂/Al composites were fabricated by self-propagating high-temperature synthesis (SHS) from Al–Ti–B₄C compacts. The addition of Al to the Ti–B₄C reactants facilitates the ignition occurrence, lowers the reaction exothermicity, and modifies the resultant microstructure. The maximum combustion temperature and combustion wave velocity decrease with the increase in the Al amount. The B₄C particle size exerts a significant effect on the combustion wave velocity and the extent of the reaction, while that of Ti has only a limited influence. The reaction products are primarily dependent on the B₄C particle size and the Al content in the reactants. Desired products consisting of only the TiC, TiB₂, and Al phases could be obtained by a cooperative control of the B₄C particle size and the Al content.     

Vibration milling of TiB<Subscript>2</Subscript> + TiNi powder mixtures

We have studied how the duration of the vibration comilling of a 80 vol % TiB₂ + 20 vol % TiNi powder mixture influences the particle size, morphology, and fine-structure parameters of its components. At a milling time of 60 h, we obtained a mixture containing 27 vol % nanoparticles, in which the cubic TiNi phase had a crystallite size of 1.1 nm. We believe that vibration-milled TiB₂ + TiNi mixtures are potentially attractive for the fabrication of composite materials by powder metallurgy methods.     

Thermodynamic Properties of FeNb<Subscript>2</Subscript>O<Subscript>6</Subscript> and FeTa<Subscript>2</Subscript>O<Subscript>6</Subscript>

Empirical calculational approaches have been used to evaluate the enthalpy, entropy, heat capacity, and melting point of iron(II) niobate and iron(II) tantalate and the coefficients A, B, and C in an equation for the temperature dependence of their heat capacity. The melting point of FeTa₂O₆ has been experimentally determined to be 1891 ± 5 K. The calculated heat capacity (C°_p (298.15 K)) of iron tantalate and the Gibbs energies of formation of FeN₂O₆ and FeTa₂O₆ have been compared to previously reported data.     

The Thermochemistry of Crystalline M<Subscript>3</Subscript>AlF<Subscript>6</Subscript>, NaAlF<Subscript>4</Subscript>, KAlF<Subscript>4</Subscript> and Gaseous MAlF<Subscript>4</Subscript> Fluoroaluminates of Alkali Metals,M

The formation enthalpies of crystalline M₃AlF₆ and gaseous MAlF₄ are confirmed and refined and the formation enthalpy of KAlF₄(cryst.) is estimated for MF–AlF₃ systems (M is Li, Na, or K) with a minimum in the total saturated vapor pressure.     

Thermodynamic properties of SrAl<Subscript>12</Subscript>O<Subscript>19</Subscript> and SrAl<Subscript>4</Subscript>O<Subscript>7</Subscript>

Strontium aluminates are important compounds with interesting properties such as long-duration phosphorescence and elastico-deformation luminescence. They have potential application in flexible light emitting panels. Since there are serious discrepancies in available thermodynamic data for these compounds, a redetermination of their Gibbs energies of formation was undertaken using solid-state electrochemical cells incorporating single-crystal SrF₂ as the electrolyte in the temperature range from 1000 to 1300 K. However, the measurements were restricted to SrAl₁₂O₁₉ and SrAl₄O₇ because of the formation of strontium oxyfluoride phase between SrAl₂O₄ and SrF₂. For the reactions, SrO + 6 Al₂O₃ → SrAl₁₂O₁₉, ΔG ^o/J mol^?1 (± 280) = ?83386 ? 25.744 (T/K), and SrO + 2Al₂O₃ → SrAl₄O₇, ΔG ^o/J mol^?1 (± 240) = ?80187 ? 25.376 (T/K). The high entropy of SrAl₄O₇ and SrAl₁₂O₁₉ can be partly related to their complex structures. The results of this study are consistent with calorimetric data on enthalpy of formation of other Sr-rich aluminates and indicate only marginal stability for SrAl₄O₇ relative to its neighbours, SrAl₁₂O₁₉ and SrAl₂O₄. The thermodynamic data explain the difficulty in direct synthesis of phase pure SrAl₄O₇ and the formation of SrAl₂O₄ as the initial ternary phase when reacting SrO and Al₂O₃ or crystallizing from amorphous state, irrespective of composition.     

Particle distribution and interfacial reactions of Al–7%Si–10%B<Subscript>4</Subscript>C die casting composite

Aluminum–boron carbide particle reinforced composite is an advanced material which can be used in applications such as neutron-shielding components, aircraft, and aerospace structures. In the microstructural characterization of an Al–7%Si–10%B₄C die casting, attention is particularly focused on particle distribution and interface reaction products between B₄C particles and the aluminum matrix. The quantitative analysis results show that, in a cross-section of the cast part, more particles concentrate in the center and fewer particles are present in the wall regions. Moreover, some particle segregation bands have been observed. The mechanisms of the particle migration are proposed to describe the phenomenon. However, the average particle fraction in any cross-section of the cast part is almost the same. A barrier layer consisting of several sublayers was detected on the surface of B₄C particles. Using electron diffraction in selected areas, it is found that these sublayers are composed of Al₃BC crystals, TiB₂ crystals, Si crystals, and coarse stick-shaped TiB₂ particles. In addition, it is observed that Si plays an important role in the formation of a dense barrier layer. The barrier layer can limit B₄C decomposition and improve B₄C stability in the aluminum melt.     

Low-Temperature Heat Capacities and Standard Molar Enthalpy of Formation of Potassium Benzoate C<Subscript>7</Subscript>H<Subscript>5</Subscript>O<Subscript>2</Subscript>K(s)

Potassium benzoate C₇H₅O₂K (CAS Registry No. 582-25-2) was synthesized by the method of liquid phase reaction. Chemical and elemental analyses, FTIR, and X-ray powder diffraction (XRD) techniques were applied to characterize the composition and structure of the compound. Low-temperature heat capacities of the compound were measured by a precision automated adiabatic calorimeter over the temperature range from 78 K to 398 K. A polynomial equation of the heat capacities as a function of temperature was fitted by the least-squares method. Smoothed heat capacities and thermodynamic functions of the compound were calculated based on the fitted polynomial. In accordance with Hess’s law, a reasonable thermochemical cycle was designed, and 100 mL of 1 mol · dm⁻³ NaOH solution was chosen as the calorimetric solvent. The standard molar enthalpies of dissolution for the reactants and products of the supposed reaction in the selected solvent were measured by an isoperibol solution-reaction calorimeter. Finally, the standard molar enthalpy of formation of the title compound C₇H₅O₂K (s) was derived to be -(610.94 ± 0.77) kJ · mol⁻¹.     

Dielectric properties and electrical conductivity of ZrO<Subscript>2</Subscript>-CeO<Subscript>2</Subscript> ceramics

ZrO₂-CeO₂ (10, 18, and 23 mol % CeO₂) solid solutions have been synthesized via coprecipitation. The powders have been sintered at 1875 K, and the electrical conductivity and dielectric properties of the resultant ceramics have been studied. The dielectric relaxation observed in the ceramics can be understood in terms of ionic transport accompanied by the formation of dipoles relaxing in an ac electric field.     

A physicochemical study of the perovskites M<Superscript>II</Superscript>(In<Subscript>2/3</Subscript>U<Subscript>1/3</Subscript>)O<Subscript>3</Subscript> (M<Superscript>II</Superscript> = Sr,Ba) at low temperatures

The heat capacity of crystalline Sr(In_2/3U_1/3)O₃ and Ba(In_2/3U_1/3)O₃ in the range 80–350 K was determined by adiabatic vacuum calorimetry, and the thermodynamic functions of these compounds in the range from T → 0 to 350 K were calculated. The standard entropies of formation of these compounds at 298.15 K were calculated. The absolute entropies and standard entropies of formation of perovskites M^II(A^III _2/3U_1/3)O₃ (M^II = Sr, A^III = Sc, In, Fe; M^II = Ba, A^III = Sc, In, Y, Nd-Lu) were estimated.     

Effect of particle size on the conductive and electrochemical properties of Li<Subscript>2</Subscript>ZnTi<Subscript>3</Subscript>O<Subscript>8</Subscript>

We have studied the effect of final annealing temperature on the formation of lithium zinc titanate, its electrical conductivity, and its electrochemical performance. Li₂ZnTi₃O₈ has been shown to form in a wide range of annealing temperatures, from 673 to 1073 K. Its particle size increases systematically with increasing annealing temperature, whereas its conductivity decreases. The highest electrochemical capacity at low currents is offered by the materials annealed at 773 and 873 K, and the highest cycling stability is offered by the material prepared at 873 K.     

Excitation of the uranyl ion in U(IV) oxidation with xenon difluoride in frozen aqueous sulfuric acid solutions: II. Effect of solvent and H<Subscript>2</Subscript>SO<Subscript>4</Subscript> concentration

The effect of solvent (aqueous solutions of HClO₄ and H₂SO₄) on the low-temperature (T < 273 K) reaction of U(IV) with XeF₂, accompanied by formation of uranyl ion in the electronically excited state *(UO ₂ ²⁺ ), was examined. In the course of heating of 0.2–8.31 M HClO₄ solutions after their quick cooling to 77 K, the chemiluminescent reaction occurs at a detectable rate only at T > 220 K. In the similar experiments performed with 0.05–0.5 M H₂SO₄ as solvent, the emission appears at a considerably lower temperature, 165–175 K. The chemiluminescence (CL) intensity increases in the course of exothermic phase transitions occurring when the temperature of H₂SO₄ solutions (at [H₂SO₄] > 0.05 M) is changed. The increase in the CL intensity in the course of a phase transition is associated with the appearance in a multicomponent frozen system of a juvenile surface of H₂SO₄ crystal hydrates. The lack of CL at T < 220 K in HClO₄ solutions is associated with the fact that heating of the samples after their quick cooling to 77 K is not accompanied by formation of crystalline phases exhibiting catalytic properties toward the oxidation of U(IV) with XeF₂.     

Thermochemical and luminescent properties of the K<Subscript>2</Subscript>MgV<Subscript>2</Subscript>O<Subscript>7</Subscript> and M<Subscript>2</Subscript>CaV<Subscript>2</Subscript>O<Subscript>7</Subscript> (M = K,Rb, Cs) vanadates

We have studied the compounds K₂MgV₂O₇ and M₂CaV₂O₇ with M = K, Rb, and Cs. These vanadates melt incongruently in the range 635–717°C. Cooling their decomposition products to room temperature leads to the formation of nonequilibrium phase assemblages characteristic of the corresponding oxide systems. The compounds offer broadband photo- and radioluminescence with an essentially white (to the human eye) emission spectrum. A model is proposed for luminescence centers in the vanadates, which involves the formation of defects in vanadium-oxygen groups, and an energy level diagram of the emission centers is constructed in the form of configuration curves in the harmonic oscillator approximation. The luminescent properties of these compounds suggest that they can be used as basic components of cathodo- and roentgenoluminescent screens and white-light-emitting diodes with improved color performance.     

Thermodynamic properties of the SnSb<Subscript>2</Subscript>Te<Subscript>4</Subscript> compound

The SnTe–Sb₂Te₃–Te system has been studied in the temperature range 300–430 K using emf measurements on reversible concentration cells of the type (–)SnTe(s) | liquid electrolyte, Sn²⁺ |(Sn–Sb–Te)(s)(+). The system has been shown to consist of two three-phase regions, separated by the SnSb₂Te₄–Te tie line. The best fit equation for the temperature-dependent emf data has been used to evaluate the partial thermodynamic functions of the SnTe and Sn in the alloys. Using these data, subsolidus phase diagram data for the SnTe–Sb₂Te₃–Te system, and relevant thermodynamic functions for SnTe and Sb₂Te₃, we calculated the standard Gibbs energy of formation, standard enthalpy of formation, and standard entropy of the SnSb₂Te₄ compound.     

Growth and structure of PbWO<Subscript>4</Subscript> films

Lead tungstate films have been grown on silicon by magnetron sputtering followed by heat treatment at various temperatures. The thermal oxidation of metal-oxide (Pb/WO₃/Si and W/PbO₂/Si) and metal-metal (Pb/W/Si) bilayer systems at temperatures above 870 K yields films that are dominated by monoclinic PbWO₄ and contain WO₃, also monoclinic. The optimal configuration for PbWO₄ synthesis is Pb/WO₃/Si because, even during lead deposition onto tungsten oxide, we observe the formation of lead tungstate, PbWO₄, and subsequent heat treatment increases the percentage of this phase in the film.