Microstructure and Conduction of Composites Bi<Subscript>2</Subscript>CuO<Subscript>4</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> Near the Eutectic Melting Point

Microstructure and conduction of ceramic composites Bi₂CuO₄ + xBi₂O₃ (x = 5, 10, 15, 20 wt %) near the eutectic melting point (770°C) are studied. Bismuth oxide, initially randomly distributed over the ceramics bulk, after quenching from temperatures exceeding the eutectic melting point, becomes localized at triple junctions and grain boundaries in Bi₂CuO₄, which is caused by wetting grain boundaries and forming a liquid-channel structure. The jumpwise change in the composites’ conductivity near 730 and 770°C caused by polymorphic transformation of Bi₂O₃ and the eutectic melting with simultaneous formation of a liquid-channel structure. Transport numbers of the oxygen ion are measured at 770°C by coulomb-volumetric method. The conduction by oxygen ions increases in the composites with decreasing average size of Bi₂CuO₄ crystallites.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 596–601.Original Russian Text Copyright © 2005 by Lyskov, Metlin, Belousov, Tret’yakov.     

Electrical conductivity studies in the system Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript>-Li<Subscript>1.33</Subscript>Ti<Subscript>1.67</Subscript>O<Subscript>4</Subscript>

Electrical conductivity in the monoclinic Li₂TiO₃, cubic Li_1.33Ti_1.67O₄, and in their mixture has been studied by impedance spectroscopy in the temperature range 20–730 °C. Li₂TiO₃ shows low lithium ion conductivity, σ₃₀₀≈10^–6 S/cm at 300 °C, whereas Li_1.33Ti_1.67O₄ has 3×10^–8 at 20 °C and 3×10^–4 S/cm at 300 °C. Structural properties are used to discuss the observed conductivity features. The conductivity dependences on temperature in the coordinates of 1000/T versus log_e(σT) are not linear, as the conductivity mechanism changes. Extrinsic and intrinsic conductivity regions are observed. The change in the conductivity mechanism in Li₂TiO₃ at around 500–600 °C is observed and considered as an effect of the first-order phase transition, not reported before. Formation of solid solutions of Li_{2– x} Ti_{1+ x} O₃ above 900 °C significantly increases the conductivity. Irradiation by high-energy (5 MeV) electrons causes defects and the conductivity in Li₂TiO₃ increases exponentially. A dose of 144 MGy yields an increase in conductivity of about 100 times at room temperature. Electronic Publication     

Conductivity of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-NiO composites

The conductivity and transport number of oxygen ions of Bi₂O₃-(10, 30, 50) vol % NiO composites are measured using the four-probe and coulomb-volumetric methods at various temperatures. It is shown that the Bi₂O₃-50 vol % NiO composite exhibits a high mixed ionic-electronic conductivity in the temperature range from 730 to 800°C.     

Conductivity and thermal expansion of the Ce<Subscript>0.8</Subscript>Gd<Subscript>0.2</Subscript>O<Subscript>1.9</Subscript> solid electrolyte in the oxidizing and reducing atmospheres

Highly compact (99%) solid electrolyte Ce_0.8Gd_0.2O_1.9 with submicron (0.3 μm) grains is synthesized. The dilatometric (20–850°C) and conductivity (180–350°C) measurements are performed on the electrolyte in air and as a function of the partial oxygen pressure \(p_{O_2 } \) (0.21?1×10^?25 atm) at 600, 700, and 800°C. An inflection is found in the temperature dependences of the thermal coefficient of linear expansion and conductivity (impedance measurements) at ～230°C, which is the evidence for a phase transition. The activation energies for conduction in the grain bulk and boundaries differ only slightly, indicating that the grain boundaries’ resistance is caused not by the precipitation of the second phase at the boundaries, but most probably by the presence of intergranular nanopores. The dilatometric measurements confirm a significant increase in the linear dimensions of Ce_0.8Gd_0.2O_1.9 in the reducing atmospheres with a parallel increase in its electron conductivity. The electron conductivity and specific elongation increase proportionally to \(p_{O_2 }^{ - 1/4} \) at all temperatures. The \(p_{O_2 } \) values, at which the transport numbers of ions t _i = 0.5, are determined. They are 10^?22.5, 10^?20, and 10^?18 atm at 600, 700, and 800°C, respectively.     

Sintering behavior and ionic conductivity of Ce<Subscript>0.8</Subscript>Gd<Subscript>0.2</Subscript>O<Subscript>1.9</Subscript> with a small amount of MnO<Subscript>2</Subscript> doping

A 20% GdO_1.5 doped ceria solid solution with a small amount of MnO₂ doping (≤5% molar ratio) was prepared via the mixed oxide method from high-purity commercial powders with grain size around 0.2–0.5 μm. X-ray diffraction analysis indicated that all the samples exhibited the fluorite structure, and no new phase was found. The data from dilatometeric measurements and scanning electron microscopy observations revealed that 1% Mn doping reduced the sintering temperature by over 150 °C, and enhanced the densification and grain growth. Mn doping has little effect on grain interior conductivity, but a marked deterioration in grain boundary behavior is observed. This leads to a lower total conductivity in comparison with the undoped Ce_0.8Gd_0.2O_2–δ. Therefore, for solid oxide fuel cells (SOFCs) with Mn-containing compounds as electrodes, optimization of electrode fabrication conditions is needed to prevent the formation of a lower conductivity layer at the electrode/electrolyte interface since Mn will diffuse from the electrode side to the electrolyte during fabrication and operation of SOFCs. Electronic Publication     

Physicochemical properties and mixed electronic and ionic conduction of composite ceramics in the system ZrO<Subscript>2</Subscript>-Bi<Subscript>2</Subscript>CuO<Subscript>4</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>

Composites ZrO₂-(Bi₂CuO₄+ 20 wt % Bi₂O₃) (50–80 vol % ZrO₂) are synthesized and their physicochemical properties are studied. It is demonstrated that the composites comprise triple-phase mixtures of ZrO₂ of a monoclinic modification, Bi₂CuO₄, and solid solution Bi_2?x Zr _x O_{3 + x/2} and retain their mechanical strength up to 800°C. Impedance spectroscopy is used to examine their electroconductivity at 700–800°C in the interval of partial oxygen pressures extending from 37 to 2.1 × 10⁴ Pa. Contributions made by electronic and ionic constituents to their overall conductivity are evaluated. The best specimens’ conductivity is ～0.01 S cm^?1, with the electronic and ionic transport numbers nearly equal. The composite consisting of 50 vol % ZrO₂ and 50 vol % (Bi₂CuO₄ + 20 wt % Bi₂CuO₄) is tested in the role of an oxygen-separating membrane. The selective flux of oxygen in the temperature interval 750–800°C amounts to (2.2–6.3) × 10^?8 mol cm^?2 s^?1, testifying that these materials may be used as gas-separating membranes.     

Surface and bulk conduction and thermodynamic properties of ionic salt CsH<Subscript>5</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

Transport properties of ionic salt CsH₅(PO₄)₂ are studied by the impedance method. The salt’s bulk conductivity ranges from 10^?8 to 10^?4 S cm^?1 in the temperature interval 90 to 145°C. The apparent activation energy is high (1.6–2.0 eV). The conductivity is slightly anisotropic: it is maximum in the [001] direction and minimum in the [100] direction (～5.6 and 1 times × 10^?6 S cm^?1, respectively, at 130°C). The conductivity of polycrystalline samples is higher by 1–2 orders of magnitude, and the activation energy drops to 1.05 eV due to the formation of a pseudoliquid layer with a high proton mobility at the intercrystallite boundary. The salt’s thermodynamic properties are examined by differential scanning calorimetry and thermogravimetry. No phase transitions are discovered in the salt up to the melting point (151.6°C), with the melting enthalpy equal to ～34 kJ mol^?1. The crystallization occurs at lower temperatures (107°C) and the crystallization enthalpy (?18 kJ mol^?1) is lower than the melting enthalpy. The melting is accompanied by slow decomposition of the salt. Factors affecting the proton transport in the salt are analyzed.     

Influence of the Conditions for Preparing LaPO<Subscript>4</Subscript>-Based Materials with Inclusions of the LaP<Subscript>3</Subscript>O<Subscript>9</Subscript> Phase on Their Thermal and Mechanical Properties

A material based on lanthanum orthophosphate LaPO₄ with inclusion of particles of lanthanum metaphosphate LaP³O₉ was synthesized. The influence of the process parameters of the synthesis on the structure and properties of the material was determined. Heat treatment of the coprecipitated lanthanum phosphates at 700°C leads to the formation of a nanopowder with the LaPO₄crystallite size of approximately 17 nm. Heat treatment of the nanopowder at temperatures from 1100 to 1500°C yields compact materials based on the LaPO₄–LaP₃O₉ system. The heat treatment of the nanopowder at 1100°C leads to a sharp decrease in the porosity of the material (to ~5%) at insignificant grain growth (200–400 nm); under these conditions, the thermal conductivity [λ(25°C) = 3.2 W m–1 K^–1], microhardness [Hv(25°C) = 4.6 ± 0.4 GPa], Young’s modulus [E(25°C) = 132 ± 9 GPa], and cracking resistance [K_1c(25°C) = 1.6 ± 0.1 MPa m^1/2] pass through maxima. The thermal expansion coefficient of the material depends on the heat treatment conditions only slightly and amounts to (8.2 ± 0.2) × 10^–6 K^–1.     

Solid electrolytes with cesium cation conductivity in the Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript>-Cs<Subscript>2</Subscript>O system

Solid solutions based on cesium monogallate CsGaO₂ are synthesized in the Ga₂O₃-TiO₂-Cs₂O system. Their crystalline structure and also temperature and concentration conductivity dependences are studied. The cesium cation character of conductivity is confirmed. The most conducting samples contain an excess of cesium oxide and have the structure of high-temperature γ-modification of KAlO₂. Their specific conductivity is (5.0–6.7) × 10^?3 S cm^?1 at 400 °C, (2.5–5.0) × 10^?2 S cm^?1 at 700°C at the activation energy of 33–35 kJ/mol^?1.     

Microwave-assisted combustion synthesis of nanocrystalline La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> oxide-ion conductor and its characterization

Nanocrystalline La₂Mo₂O₉ oxide-ion conductor has been successfully synthesized by microwave-assisted combustion method within a very short time duration using aspartic acid as the newer fuel in a domestic microwave oven. The synthesized nanocrystalline powder showed good sinterability and reached more than 97% of theoretical density even at low temperature of 800 °C for 5 h. The sintered La₂Mo₂O₉ sample exhibited a conductivity of 0.159 S/cm in air at 750 °C.     

Moderate-temperature protonic conductors based on CsH<Subscript>2</Subscript>PO<Subscript>4</Subscript> and modified silicon dioxide

With the aim of creating an inert matrix for composite in the (1 ? x) CsH₂PO₄ ? xSiO₂ system, the surface of finely divided dioxide is modified and matrices with more acid centers are produced. Physicochemical properties of composite systems on the basis of these are studied. The role played by acid centers in the formation of more stable and better conductive composite systems in the temperature interval 150 to 250°C is demonstrated. The conductivity of composites based on silicon dioxide modified with CsHSO₄ or H₃PO₄ at 130–230°C is shown to exceed that of the initial phase CsH₂PO₄ and the composite systems based on uniformly porous silicon dioxide with the surface acidity pH ～7 studied previously. The composites are of interest for further research. Compositions of phases under formation and conditions for their existence are determined.     

Lithium-conducting solid electrolytes of Li<Subscript>4</Subscript>GeO<Subscript>4</Subscript>-Li<Subscript>3</Subscript>PO<Subscript>4</Subscript> system with additions of zirconium ions

The effect of partial substitution of Zr⁴⁺ ions for Ge4+ ions in highly conducting lithium-cationic solid electrolyte Li_3.75Ge_0.75P_0.25O₄ is studied. It is found that the introduction of zirconium ions considerably raises the conductivity of basic electrolyte in the high-temperature range. For the optimal composition, the conductivity is 2.82 × 10⁻¹ S cm⁻¹ at 400°C and 1.55 S cm⁻¹ at 700°C. Possible reasons for the effects are discussed.     

Electrical properties of triple molybdates Rb<Subscript>5</Subscript>LnHf(MoO<Subscript>4</Subscript>)<Subscript>6</Subscript>

Triple molybdates of the compositions Rb₅LnHf(MoO₄)₆ (5:1:2) and Rb₂LnHf₂(MoO₄)_6.5 (2:1:4), Ln = Ce-Lu, were prepared by solid-phase reactions. The temperature dependence of the electrical conductivity of the compounds Rb₅LnHf(MoO₄)₆ (5:1:2) at 200–500°C was studied.     

Physicochemical Properties of Li<Subscript>6</Subscript>V<Subscript>5</Subscript>O<Subscript>15</Subscript> as the Cathode Material for Lithium-Ion Batteries

Lithium-vanadium oxide with the formal composition Li₆V₅O₁₅, uniform microsctructure, and the particle size of 100 nm is synthesized by a solution method. The synthesized compound is characterized by the methods of X-ray diffraction analysis, Raman spectroscopy, and synchronous thermal analysis. The total electric conductivity is measured by the method of impedance spectroscopy and its electronic component is estimated by dc method. In the temperature range of 200–400°C, Li₆V₅O₁₅ represents a mixed electronic- ionic conductor with predomination of the ionic component and is thermally stable up to 550°С. Preliminary tests of a laboratory model of electrochemical cell Li|LiPF₆|Li₆V₅O₁₅ are carried out.     

Phase equilibria in the system Li<Subscript>2</Subscript>O-MgO-B<Subscript>2</Subscript>O<Subscript>3</Subscript>

The subsolidus region of the Li₂O-MgO-B₂O₃ system has been studied by X-ray powder diffraction and differential thermal analysis. Isothermal sections at 500–550 and 650–700°C have been designed. The following complex borates have been found to form: at 500–550°C, Li₂MgB₂O₅ and LiMgBO₃ are formed; at 650–700°C, a new phase Li₄MgB₂O₅ is formed along with LiMgBO₃; and at 5500–600°, Li₂MgB₂O₅ is formed.     

Solid vitreous inorganic electrolytes in Li<Subscript>2</Subscript>O-LiF-P<Subscript>2</Subscript>O<Subscript>5</Subscript> system for rechargeable lithium power sources: Ionic conductivity and activation energy

The effect of chemical composition to ionic conductivity and activation energy of vitreous solid electrolytes (SE) based on Li₂O-P₂O₅-LiF system (Li₂O ≥ 45.4 mol %) was detected. The temperature effect to conductivity and activation energy was studied. An original technology was designed to prepare vitreous SEs in Li₂O-P₂O₅-LiF system containing up to 20 mol % LiF and characterized with ionic conductivity up to 4.4 × 10^?7 S cm^?1 (24°C) and activation energy about 0.567 eV. The synthesized materials are characterized with high X-ray amorphism and technological performance.     

Solid electrolytes with cesium-cationic conduction in the Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript>-Cs<Subscript>2</Subscript>O system

New cesium-conducting solid electrolytes based on cesium monoferrite in the Fe₂O₃-TiO₂-Cs₂O system are synthesized and studied. It is found that the introduction of titanium dioxide significantly reduces the electronic component of conductivity, which prevails in pure CsFeO₂, and raises the ionic conductivity. The latter becomes predominant with increasing concentration of TiO₂. The effect of dimensional factor on the characteristics of electrolyte is shown. The optimal compositions studied have very high cesium-cationic conductivity: it is above 10⁻² S cm ⁻¹ at 300°C.     

Phase diagrams of the KF-K<Subscript>2</Subscript>TaF<Subscript>7</Subscript> and KF-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> systems

The phase diagrams of the systems KF-K₂TaF₇ and KF-Ta₂O₅ were determined using the thermal analysis method. The phase diagrams were described by suitable thermodynamic model. In the system KF-K₂TaF₇ eutectic points at x _KF=0.716 and t=725.4°C and at x _KF=0.214 and t=712.2°C has been calculated. It was suggested that K₂TaF₇ melts incongruently at around 743°C forming two immiscible liquids. The system KF-Ta₂O₅ have been measured up to 8 mol% of Ta₂O₅. The eutectic point was estimated to be at x _KF∼0.9 and t∼816°C. The formation of KTaO₃ and K₃TaO₂F₄ compounds has been observed in the solidified samples.     

Solid-state synthesis of undoped and Sr-doped K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript>

The solid-state synthesis of undoped K_0.5Na_0.5NbO₃ (KNN) and KNN doped with 1, 2 and 6 mol% Sr, from potassium, sodium and strontium carbonates with niobium pentoxide, was studied using thermal analysis and in situ high-temperature X-ray diffraction (HT-XRD). The thermogravimetry and the differential thermal analyses with evolved-gas analyses showed that the carbonates, which were previously reacted with the moisture in the air to form hydrogen carbonates, partly decomposed when heated to 200 °C. In the temperature interval where the reaction was observed, i.e., between 200 and 750 °C, all the samples exhibited the main mass loss in two steps. The first step starts at around 400 °C and finishes at 540 °C, and the second step has an onset at 540 °C and finishes with the end of the reaction between 630 and 675 °C, depending on the particle size distribution of the Nb₂O₅ precursor. According to the HT-XRD analysis, the perovskite phase is formed at 450 °C for all the samples, regardless of the Sr content. The formation of a polyniobate phase with a tetragonal tungsten bronze structure was detected by HT-XRD in the KNN with the largest amount of Sr dopant, i.e., 6 mol% of Sr, at 600 °C.     

Solid-phase synthesis of intermetallic compounds Al<Subscript>13</Subscript>Co<Subscript>4</Subscript>, Al<Subscript>13</Subscript>Fe<Subscript>4</Subscript>, and Al<Subscript>13</Subscript>Co<Subscript>2</Subscript>Fe<Subscript>2</Subscript>

Intermetallic compounds Al₁₃Co₄, Al₁₃Fe₄, and Al₁₃Co₂Fe₂ were obtained by solid-phase synthesis in air at temperatures below 600°C using precursor metals subjected to mechanochemical preactivation. The phase composition of the synthesized aluminides and composites Al₁₃Co₄/SiO₂ and Al₁₃Fe₄/SiO₂ was analyzed.