Determination of standard Gibbs energies of formation of ternary oxides in the system Co-Sb-O by solid electrolyte emf method

An isothermal section of the phase diagram of the system Co-Sb-O at 873 K was established by isothermal equilibration and XRD analyses of quenched samples. The following galvanic cells were designed to measure the Gibbs energies of formation of the three ternary oxides namely CoSb₂O₄, Co₇Sb₂O₁₂ and CoSb₂O₆ present in the system.

Chemical diffusion of (La,Sr)CoO3 and (La,Sr)FeO3

The present work reports equilibration kinetics for (La_0.8,Sr_0.2)CoO₃(LSC) and (La_0.72,Sr_0.18)FeO₃(LSF) in the temperature range 876–1114 K using a gravimetric method. Chemical diffusion determined in this way that depends on oxygen partial pressure, can be expressed by the following temperature dependence at low and high O₂), respectively, for LSC:

Phase relations in the system Cu-La-O and thermodynamic properties of CuLaO2 and CuLa2O4

Phase relations in the system Cu-La-O at 1200 K have been determined by equilibrating samples of different average composition at 1200 K, and phase analysis of quenched samples using optical microscopy, XRD, SEM and EDX. The equilibration experiments were conducted in evacuated ampoules, and under flowing inert gas and pure oxygen. There is only one stable binary oxide La₂O₃ along the binary La-O, and two oxides Cu₂O and CuO along the binary Cu-O. The Cu-La alloys were found to be in equilibrium with La₂O₃. Two ternary oxides CuLaO₂ and CuLa₂O₄₊ were found to be stable. The value of varies from close to zero at the dissociation partial pressure of oxygen to 0.12 at 0.1 MPa. The ternary oxide CuLaO₂, with copper in monovalent state, coexisted with Cu, Cu₂O, La₂O₃, and/or CuLa₂O₄₊ in different phase fields. The compound CuLa₂O₄₊, with copper in divalent state, equilibrated with Cu₂O, CuO, CuLaO₂, La₂O₃, and/or O₂ gas under different conditions at 1200 K. Thermodynamic properties of the ternary oxides were determined using three solid-state cells based on yttria-stabilized zirconia as the electrolyte in the temperature range from 875 K to 1250 K. The cells essentially measure the oxygen chemical potential in the three-phase fields, Cu + La₂O₃ + CuLaO₂, Cu₂O + CuLaO₂ + CuLa₂O₄ and La₂O₃ + CuLaO₂ + CuLa₂O₄. Although measurements on two cells were sufficient for deriving thermodynamic properties of the two ternary oxides, the third cell was used for independent verification of the derived data. The Gibbs energy of formation of the ternary oxides from their component binary oxides can be represented as a function of temperature by the equations:

Cation distribution investigation of the system Li0.5Fe x Ga2.50-xO4 by X-rays,electrical conductivity and Mossbauer studies

Structural, electrical and Mossbauer studies were carried out for the system Li_0.5Fe _x Ga_2.50-xO₄. All the compounds with 0 ? x? 2.5 crystallised with cubic spinel structure. Lattice constant values calculated from XRD analysis were found to increase with increasing x. X-ray intensity calculations indicated that Li¹⁺ occupies only the octahedral site and Ga³⁺ and Fe³⁺ ions occupy both octahedral and tetrahedral sites. Activation energy and thermoelectric coefficient values decreased with increasing values of x. All the compounds studied were p-type semiconductors and possess low mobility values of 10^?7-10^?9 cm²V^?1 s^?1. Mossbauer data show the presence of iron in the Fe³⁺ state and the isomer shift values for all the compositions of the system are within the range of high spin ferric compounds. The probable ionic configuration for the system is suggested as: $${\text{Ga}}_{{\text{1 - }}\alpha }^{{\text{3 + }}} {\text{Fe}}_\alpha ^{{\text{3 + }}} [Li_{0.5}^1 {\text{Fe}}_{{\text{x - }}\alpha }^{{\text{3 + }}} {\text{Ga}}_{{\text{2}}{\text{.5 - x + }}\alpha }^{\text{3}} ] {\text{O}}_{\text{4}}^{{\text{2 - }}} $$      

Stability and direct conversion of mineral barite crystals in carbonated hydrothermal fluids

The pseudomorphic replacement of mineral barite (BaSO₄) crystals into barium carbonate was investigated in the present work by using carbonated alkaline hydrothermal fluids. Hydrothermal treatments were carried out over the temperature range from 150 up to 250 °C for intervals between 1 and 192 h, with different filling ratios (40–70%), and molar ratios of 1, 5, and 10. The reaction products were characterized by XRD and SEM techniques. The chemical reactivity of mineral barite crystals was markedly limited at temperatures below 200 °C, and only a tiny BaCO₃ layer on the surface of the original BaSO₄ crystal was formed on the crystal treated for 192 h. The rate of the pseudomorphic conversion of BaSO₄ into BaCO₃, was accelerated by increasing the reaction temperature and the molar ratio . Powder X-ray diffraction results showed that under hydrothermal conditions the replacement of ions by ions, in barite crystals was completed at 250 °C with a molar ratio = 10 for an interval of 192 h, resulting in the Witherite structure. The morphology of the completely converted BaCO₃ at 250 °C in a Na₂CO₃ solution for 192 h, showed that the conversion proceed without severe changes of the original shape and dimension of the original crystal, similar to that observed in mineral pseudomorphic replacement process.     

Mechanism of the reduction of carbon/alumina powder mixture in a flowing nitrogen stream

The mechanism of the reduction of carbon/alumina powder mixture in a flowing nitrogen stream was studied. Five steps were found to be involved in the overall reaction. $$\begin{gathered} Al_2 O_{3f} (s) + 2C_f (s)\mathop \to \limits^{k_1 } Al_2 O(g) + 2CO(g) \hfill \\ Al_2 O(g) + solid surface\mathop \rightleftharpoons \limits_{k_2^\prime }^{k_2 } [Al_2 O]_s \hfill \\ [Al_2 O]_s + CO(g) + N_2 (g)\mathop \to \limits^{k_3 } 2AlN(s) + CO_2 (g) \hfill \\ CO_2 (g) + C_f (s)\mathop \rightleftharpoons \limits_{k_4^\prime }^{k_4 } CO(g) + [O]_c \hfill \\ [O]_c \mathop \to \limits^{k_5 } CO(g) \hfill \\ \end{gathered}$$ The consumption rates of Al₂O₃ and carbon, and the production rate of AIN, were determined to be $$\begin{gathered} \frac{{d[Al_2 O_3 ]}}{{dt}} = - 143.88(1 + m)exp( - 290 580/RT) [Al_2 O_3 ][C]^2 / \hfill \\ \left\{ {1 + 5.83 x 10^{14} exp( - 427 497/RT)\frac{{[CO_2 ]}}{{[CO]}}} \right\}^2 kg mol s^{ - 1} m^{ - 3} \hfill \\ \frac{{d[C]}}{{dt}} = - 409.504 exp ( - 254 500/RT) [Al_2 O_3 ][C]^2 / \hfill \\ \left\{ {1 + 5.83 x 10^{14} \exp ( - 427 497/RT)\frac{{[CO_2 ]}}{{[CO]}}} \right\}^2 kg mol s^{ - 1} m^{ - 3} \hfill \\ \frac{{d[AlN]}}{{dt}} = 53.24(1 + m) exp( - 290 580/RT) [Al_2 O_3 ][C]^2 / \hfill \\ \left\{ {1 + 5.83 x 10^{14} exp( - 427 497/RT)\frac{{[CO_2 ]}}{{[CO]}}} \right\}^2 kg mol s^{ - 1} m^{ - 3} \hfill \\ \end{gathered}$$ in the temperature range 1648–1825 K.     

Surface and grain-boundary energies of cubic zirconia

Using the multiphase equilibration technique for the measurement of contact angles, the surface and grain-boundary energies of polycrystalline cubic ZrO₂ in the temperature range of 1173 to 1523 K were determined. The temperature coefficients of the linear temperature function obtained, are expressed as $$\frac{{{\text{d}}\gamma }}{{{\text{d}}T}}({\text{ZrO}}_{\text{2}} ){\text{ }} = {\text{ }} - 0.431{\text{ }} \times {\text{ }}10^{ - 3} {\text{ }} \pm {\text{ }}0.004{\text{ }} \times {\text{ }}10^{ - 3} {\text{ Jm}}^{ - {\text{2}}} {\text{ K}}^{ - {\text{1}}} $$ and $$\frac{{{\text{d}}\gamma }}{{{\text{d}}T}}({\text{ZrO}}_{\text{2}} - {\text{ZrO}}_{\text{2}} ){\text{ }} = {\text{ }} - 0.392{\text{ }} \times {\text{ }}10^{ - 3} {\text{ }} \pm {\text{ }}0.126{\text{ }} \times {\text{ }}10^{ - 3} {\text{ Jm}}^{ - {\text{2}}} {\text{ K}}^{ - {\text{1}}} $$ respectively. The surface fracture energy obtained with a Vickers microhardness indenter at room temperature is found to be γ _F=3.1 J m^?2.     

Reaction diffusion in the Nb-Ge system

Reaction diffusion in the Nb-Ge system was studied in the temperature range 1243 to 1723 K for diffusion couples of (pure solid Nb)-(pure liquid Ge) and (pure solid Nb)-(Ge-37.5wt % Nb liquid alloy). Growth of the NbGe₂, Nb₃Ge₂, Nb₅Ge₃ and Nb₃Ge layers was observed, and the growth rates of all except the Nb₃Ge layer were found to conform to the parabolic law. Growth of the Nb₃Ge layer was observed only along the grain boundaries in the Nb₅Ge₃ layer. Interdiffusion coefficients \(\tilde D\) in the NbGe₂, Nb₃Ge₂ and Nb₅Ge₃ phases were determined by Heumann's method, and the temperature dependence of these was expressed by the Arrhenius equations as follows: $$\tilde D_{{\text{NbGe}}_{\text{2}} } = (6.40_{ - 1.66}^{ + 2.25} \times 10^{ - 6} exp [ - (161 \pm 4) kJ mol^{ - 1} {\text{/RT] m}}^{{\text{2 }}} \sec ^{ - 1} $$ $$\tilde D_{{\text{Nb}}_{\text{3}} {\text{Ge}}_{\text{2}} } = (2.27_{ - 0.60}^{ + 0.82} \times 10^{ - 4} exp [ - (282 \pm 4) kJ mol^{ - 1} {\text{/RT] m}}^{{\text{2 }}} \sec ^{ - 1} $$ and $$\tilde D_{{\text{Nb}}_{\text{5}} {\text{Ge}}_{\text{3}} } = (6.28_{ - 1.93}^{ + 2.78} \times 10^{ - 5} exp [ - (238 \pm 5) kJ mol^{ - 1} {\text{/RT] m}}^{{\text{2 }}} \sec ^{ - 1} $$ In addition to the binary Nb-Ge system, the reaction diffusion of (pure solid Nb)-(Cu-13 wt % Ge liquid alloy) couples was also studied. In this case, only growth of the Nb₅Ge₃ layer containing negligible copper content was observed.     

Structural properties of nickel manganite Ni x Mn3−x O4 with 0.5 ⩽x ⩽ 1

Single-phase nickel manganite spinels, Ni _x Mn_3–x O₄, with 0.5 x 1, were prepared by a careful thermal processing of nickel-manganese coprecipitated oxalate precursors. Powder X-ray diffraction analysis of the spinel revealed the presence of cubic single spinel phase with parametera which decreases with nickel content. The lattice parameter variation can be explained in terms of the distribution of Ni²⁺ ions on the octahedral sites. Therefore, a fine analysis of data shows that some Ni²⁺ ions (forx>0.56) are located in tetrahedral sites. The percentage of nickel in A-sites increases with nickel content (x) following the relation % Ni²⁺ in A sites =P = – 82.1x ²+192.4x–81.5 and thus the general formula for cation distribution is

The crystallographic orientation relationship between Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> in the composite material Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Al–Mg–Si alloy

The formation mechanism of spinels on Al₂O₃ particles in the Al₂O₃/Al–1.0 mass% Mg₂Si alloy composite material has been investigated by transmission electron microscopy (TEM) in order to determine the crystallographic orientation relationship. A thin sample of the Al₂O₃/Al–Mg–Si alloy composite material was obtained by the FIB method, and the orientation relationship between Al₂O₃ and MgAl₂O₄, which was formed on the surface of Al₂O₃ particles, was discovered by the TEM technique as follows:

Equilibration kinetics of (La,Sr)MnO3

The present work reports isothermal changes of oxygen non-stoichiometry for the perovskite-type electrode material (La_0.8Sr_0.2)MnO₃ in the temperature range 945–1255 K. A thermogravimetric method was used to monitor the rate of the gas/solid equilibration. For equilibration degrees larger than 0.5, the equilibration kinetic data can be described by a diffusion equation. The determined chemical diffusion coefficient depends essentially on the oxygen partial pressure. Its temperature dependence can be expressed by the following expressions at low and high p(O₂), respectively:

Study on the behavior of elastic-plastic fracture under mixed I+II mode loading in aluminum alloy Ly12-J-integral analysis

The elastic-plastic fracture behavior of aluminum alloy Ly12 under mixed I+II mode loading was studied by finite element method and fracture test. A mixed mode elastic-plastic fracture criterion of J-integral was proposed by using the J-resistance curve, and the maximum fracture effective plastic strain _p max of different mixed ratios at crack tip were also calculated. The results show that(1) the initiation J-integral values of different mixed ratios have the equation

Oxidation Behavior of C/SiC Composite with CVD SiC-B4C Coating in a Wet Oxygen Environment

A two-layered self healing coating with a B₄C internal layer and a SiC external layer is prepared on C/SiC composite by chemical vapor deposition (CVD). Microstructure and component of the coating was analyzed by SEM, EDS, and XRD. Oxidation behavior of SiC-B₄C coated C/SiC composite was compared with SiC-SiC coated C/SiC in an environment of at 700°C, 1,000°C and 1,200°C for 100 h, respectively. It is demonstrated that the SiC-B₄C coating is more efficient to protect the composite from oxidation than SiC-SiC coating below 1,000°C due to the self healing behavior. After oxidized at 700°C for 100 h, the residual flexural strength of SiC-B₄C coated C/SiC is about 86%, and that of SiC-SiC coated is about 64%. While after oxidized at 1,200°C, the former is about 86% and the later is about 89%. This is due to the enhanced evaporation of B₂O₃ at higher temperature.     

Electrical, magnetic and Mössbauer properties of cadmium - cobalt ferrites prepared by the tartarate precursor method

Six samples of the system Cd_1–x Co _x Fe₂O₄ were prepared by the tartarate precursor method with x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0. The formation of ferrispinels were studied by X-ray powder diffraction, infrared spectroscopy, electrical conductivity, thermoelectric power, magnetic hysteresis, initial magnetic susceptibility and Mössbauer spectroscopy. The data of the temperature variation of the direct current electrical conductivity showed a definite kink (390°C) except x = 0.0 and 0.2, which corresponds to the ferrimagnetic to paramagnetic transitions. Magnetic properties of the samples with x 0.6 showed definite hysteresis loops. The observed low magnetic moment can be explained in terms of the non collinear spin arrangement. A well defined hyperfine Zeeman spectra are observed for samples with x 0.6 at room temperature and resolved into two sextets corresponding to octahedral and tetrahedral sites. The electrical, magnetic and Mössbauer properties suggest that, a canted spin arrangement upto x = 0.8 and Néel's configuration above this composition. The probable ionic configuration for the system is suggested as

Determination of diffusivity of sodium in liquid tin using Na-ß″ alumina

Diffusivity of sodium in molten tin was determined using an electrochemical cell of the type Na/Na-β″-Al₂O₃/(Na)Sn where Na-β″-Al₂O₃, which is a sodium-ion conductor, was the solid electrolyte. Using the above cell in which a small amount of sodium dissolved in tin was transported through β″-Al₂O₃ upon application of an external voltage, and using a known solution to Fick's second law for appropriate boundary conditions, the diffusivity was determined to be $$D{\text{ = 4}}{\text{.4 }}\left( {\begin{array}{*{20}c} {{\text{ + 1}}{\text{.0}}} \\ {{\text{ - 0}}{\text{.5}}} \\ \end{array} } \right){\text{ x 10}}^{{\text{ - 4}}} {\text{ exp }}\left( {{\text{ - }}\frac{Q}{{RT}}} \right){\text{ cm}}^{\text{2}} {\text{ sec}}^{{\text{ - 1}}} $$ withQ = 16320 J mol^?1 over a range of temperatures from 240 to 440° C. From the solution to Fick's second law, it was shown that first term approximations, which have often been used in the past, lead to an estimate of diffusivity which is about a factor of 2 too high indicating that more terms should be considered. The diffusivity was also determined using a transient technique in which the decay in voltage upon removal of externally applied voltage was recorded as a function of time. The diffusivity so determined, in which it was assumed that the only polarization was the concentration polarization, was higher than the previous method. The difference between the two diffusivities became smaller with increasing temperature. These experiments thus suggested that interfacial or activation polarization must also be present.     

The interface microstructure in alumina (FP) fibre/magnesium alloy composite

The objective of this work was to characterize the interfacial reaction zone in the metal matrix composite system-Al₂O₃(FP)/Mg (ZE41A). The composite was fabricated by liquid infiltration method. The reaction zone, a result of the reaction between magnesium in the alloy and the alumina fibres, was analysed for its morphology, chemistry, and crystallographic orientation using transmission electron microscopy. The results of this study showed the reaction zone to be, on average, 100nm wide and composed of MgO. The grains of the reaction zone ranged from less than 10 nm at the fibre/reaction zone interface to greater than 100nm at the matrix/reaction zone interface. It is proposed that the growth of the reaction zone was controlled by a seepage mechanism involving infiltration of liquid magnesium between MgO crystalS. Finally, it was observed that the MgO grains have the following crystallographic orientation relationship with the alumina grains from which they grew:

High-field ESR on Light-Induced Transition of Spin Multiplicity in FeCo Complex

Cyanide-bridged Fe-Co complex [Fe(Tp)(CN)₃]₂Co(bpe)?5H₂O (1?5H₂O; Tp = hydro-tris(pyrazolyl)borate; bpe = 1,2-bis(4-pyridyl)ethane) shows temperature- and light- induced metal-to-metal charge transfer (MMCT) involving spin state changes between magnetic $\mathrm{Fe}^{\mathrm{III}}_{\phantom{\mathrm{III}}\mathrm{LS}}\mbox{--}\mathrm{Co}^{\mathrm{II}}_{\phantom{\mathrm{II}}\mathrm{HS}}$ (HS = high spin, LS = low spin) state and nonmagnetic $\mathrm{Fe}^{\mathrm{II}}_{\phantom{\mathrm{II}}\mathrm{LS}}\mbox{--}\mathrm{Co}^{\mathrm{III}}_{\phantom{\mathrm{III}}\mathrm{LS}}$ state, while the dehydrated material 1 does not show any MMCT and holds $\mathrm{Fe}^{\mathrm{III}}_{\phantom{\mathrm{III}}\mathrm{LS}}\mbox{--}\mathrm{Co}^{\mathrm{II}}_{\phantom{\mathrm{II}}\mathrm{HS}}$ state. We have investigated the magnetic properties of each spin state in 1 and 1?5H₂O by means of magnetization and ESR measurement under pulsed high magnetic field. At low temperature below T _N, in both 1 and 1?5H₂O, the saturation magnetization in the induced ferromagnetic phase is well explained by S and g values derived from the magnetic susceptibility study. In the ESR of 1, we observed characteristic modes corresponding to a spin excitation in the induced ferromagnetic phase where its temperature dependence shows an evolution of spin correlation in the $\mathrm{Fe}^{\mathrm{III}}_{\phantom{\mathrm{III}}\mathrm{LS}}\mbox{--}\mathrm{Co}^{\mathrm{II}}_{\phantom{\mathrm{II}}\mathrm{HS}}$ state at low temperature. We further found that the similar ESR modes grow in the light-induced state of 1?5H₂O. The results strongly suggest that the light-induced magnetization in 1?5H₂O is driven by a light-induced MMCT, which involves transition of spin multiplicity from the nonmagnetic $\mathrm{Fe}^{\mathrm{II}}_{\phantom{\mathrm{II}}\mathrm{LS}}\mbox{--}\mathrm{Co}^{\mathrm{III}}_{\phantom{\mathrm{III}}\mathrm{LS}}$ to the magnetic $\mathrm{Fe}^{\mathrm{III}}_{\phantom{\mathrm{III}}\mathrm{LS}}\mbox{--}\mathrm{Co}^{\mathrm{II}}_{\phantom{\mathrm{\mathrm{II}}}\mathrm{HS}}$ pair.     

Defect structure of acceptor-doped calcium titanate at elevated temperatures

The defect structure of acceptor (Al or Cr)-doped polycrystalline calcium titanate was investigated by measuring the oxygen partial pressure dependence (at 10° to 10–18 atm) of the electrical conductivity at 1000 and 1050° C. The observed electrical conductivity data were proportional to for the oxygen pressure range < 10^–10 atm and proportional to for the oxygen pressure range ( 10^–7 atm. The conductivity values were observed to increase with the acceptor concentration in the p-type region with the shift in the conductivity minima towards lower oxygen partial pressure. The absolute value of the electrical conductivity in the acceptor-doped samples were lower in the n-type region compared to the values in the undoped CaTiO₃. Aluminium and chromium were found to be equally effective in acting as acceptor impurities in CaTiO₃. The defect chemistry of CaTiO₃ is dominated by the added acceptor impurities for the entire oxygen partial pressure range used in this investigation.