Study of phase transition of TiO<Subscript>2</Subscript>–CaO system

Electrical conductivity of TiO₂ doped with CaO has been measured at different temperatures for various molar ratios. The conductivity after initially remaining constant till about 140 °C increases with temperature due to the migration of vacancies created by doping. After attaining a maximum value at 240 °C, conductivity decreases due to the collapse of fluorite framework. A second rise in conductivity at high temperature beyond 400 °C indicates the phase transition of TiO₂, from anatase to rutile, which is confirmed by the differential scanning calorimetry results. X-ray powder diffraction, impedance measurements, and Fourier transform infrared spectral studies were also carried out for confirming the doping effect and phase transitions in TiO₂. Doping of TiO₂ with CaO shifts the transition to lower temperatures.     

Electrical properties of CoO,NiO, CuO and ZnO doped beta″-alumina

The measurements of ionic conductivity of sintered beta″-alumina samples doped with CoO, NiO, CuO and ZnO were caried out. It was found that conductivities of these samples are lower than conductivity of Li₂O stabilized beta″-alumina. For CoO, NiO, CuO as well as Li₂O stabilized beta″-alumina the bending of Arrhenius plots was observed. For samples doped with ZnO the plots were linear in whole 20°C–450°C temperature range. The doping effect on bulk conductivity was stronger than on grain boundary conductivity.     

Low-temperature sintering effects on NASICON-structured LiSn<Subscript>2</Subscript>P<Subscript>3</Subscript>O<Subscript>12</Subscript> solid electrolytes prepared via citric acid-assisted sol-gel method

LiSn₂P₃O₁₂ with sodium (Na) super ionic conductor (NASICON)-type rhombohedral structure was successfully obtained at low sintering temperature, 600 °C via citric acid-assisted sol-gel method. However, when the sintering temperature increased to 650 °C, triclinic structure coexisted with the rhombohedral structure as confirmed by X-ray diffraction analysis. Conductivity–temperature dependence of all samples were studied using impedance spectroscopy in the temperature range 30 to 500 °C, and bulk, grain boundary and total conductivity increased as the temperature increased. The highest bulk conductivity found was 3.64?×?10^?5 S/cm at 500 °C for LiSn₂P₃O₁₂ sample sintered at 650 °C, and the lowest bulk activation energy at low temperature was 0.008 eV, showing that sintering temperature affect the conductivity value. The voltage stability window for LiSn₂P₃O₁₂ sample sintered at 600 °C at ambient temperature was up to 4.4 V. These results indicated the suitability of the LiSn₂P₃O₁₂ to be exploiting further for potential applications as solid electrolytes in electrochemical devices.     

Electrical characterization of porous La-doped BaSnO<Subscript>3</Subscript> using impedance spectroscopy

A few compositions in the system Ba_1???x La _x SnO₃ (x?=?0.00, 0.01, 0.05, and 0.10) have been synthesized via the solid state ceramic route. The synthesized powders have been characterized using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis, Raman spectroscopy, Fourier transformation infrared, thermogravimetrical analysis, and differential thermal analysis techniques. The powder X-ray diffraction pattern of the samples confirms the formation of a single-phase solid solution only up to 0.50?≤?x. It was found that all the samples have a cubic crystal structure. The electrical properties of La-modified BaSnO₃ were studied using ac impedance spectroscopy technique over a wide range of temperatures (50–650 °C) in the frequency range of 10 Hz–13 MHz. The complex impedance plots above 300 °C show that total impedance is due to the contributions of grain and grain boundaries. The resistance of these contributions has been determined. Variation of these resistances with temperature shows the presence of two different regions with different slopes. The nature of the variation of conductivity of the grain and grain boundaries is different in different regions. Based on the value of activation energy, it is proposed that conduction via hopping of doubly ionized oxygen vacancies (V_O ^??) is taking place in the temperature region of 300–450 °C, whereas in the temperature region of 450–650 °C, it is due to proton, i.e., OH^? ions, hopping.     

Ionic and electronic conductivity of Yb2+xTi2−xO7−x/2 materials

Bulk and grain boundary conductivities of Yb_2+xTi_2−xO_7−x/2 (x = 0, 0.1, 0.18 and 0.29) materials were studied by impedance spectroscopy in the range 300–900 °C in air. Ionic and electronic conductivities were separated by both ion blocking Hebb–Wagner measurements and total conductivity measurements as a function of oxygen partial pressure in the temperature range 700–1000 °C. The oxygen partial pressure dependence of the total conductivity shows that these materials are nearly pure ionic conductors in air and that the ionic conductivity decreases for Yb-rich compositions. This was interpreted as a predominant effect of a decrease in mobility of ionic charge carriers, opposing the expected increase in concentration of oxygen vacancies with increasing Yb content. The studied materials become mixed conductors under typical fuel conditions, except possibly at temperatures below about 700 °C. Yb-excess slightly suppresses the electronic conductivity.     

The oxygen ion conductivity of mullite prepared using a wet chemical process

The oxygen ion conductivities of mullite solid solution samples were measured in air by use of the ac complex impedance spectroscopy technique in the frequency range 20 Hz–100 kHz and the temperature range 450–950°C. The results showed that both thermal history and the composition of specimens significantly influenced the conductivity values. These results are discussed in terms of the interaction between oxygen vacancies and aliovalent defect species and the crystal structure and microstructure of mullite samples. As a result of these experiments, it appears that mullite solid solutions would be promising materials for use as oxygen sensors in elevated temperature systems.     

A study of the electrical conductivity and transition points of potassium carbonate

The electrical conductivity of K₂CO₃ was measured between 230°C and 840°C in both air and carbon dioxide. The conductivity depends upon the atmosphere because of the decomposition of Na₂CO₃ at the surface of the sample. There exists only one polymorphic transformation at 430°C, which is probably a second-order one. From conductivity measurements carried out on samples doped with CaCO₃, one can derive the migration energy of cation vacancies and the energy of formation of defects.     

Trap characterization for Bi4Ti3O12 thin films by thermally stimulated currents

Thermally stimulated current (TSC) measurements performed in the 100 K–400 K temperature range on Bi₄Ti₃O₁₂ (BiT) thin films annealed at 550 °C and 700 °C had revealed two trapping levels having activation energies of 0.55 eV and 0.6 eV. The total trap concentration was estimated at 10¹⁵ cm⁻³ for the samples annealed at 550 °C and 3×10¹⁵ cm⁻³ for a 700 °C annealing and the trap capture cross-section was estimated about 10⁻¹⁸ cm². From the temperature dependence of the dark current in the temperature range 20 °C–120 °C the conduction mechanism activation energy was found to be about 0.956–0.978 eV. The electrical conductivity depends not only on the sample annealing temperature but also whether the measurement is performed in vacuum or air. The results on the dark conductivity are discussed considering the influence of oxygen atoms and oxygen vacancies. Received: 28 January 1998 / Accepted: 8 January 1999 / Published online: 5 May 1999     

Influence of sintering conditions on the electrical properties of 10% ZrO2–10% Y2O3–CeO2 (mol%)

Yttria–zirconia doped ceria, 10% ZrO₂–10% Y₂O₃–CeO₂ (mol%) (CZY) and 0.5 mol% alumina-doped CZY (CZYA), prepared through oxide mixture process, were sintered by isothermal sintering (IS) and two-step sintering (TSS) having as variable the temperature and soaking time. The electrical conductivity of sintered samples was investigated in the 250 to 600 °C temperature range by impedance spectroscopy in air atmosphere. The microstructure was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Alumina, as additive, improves the grain boundary conductivity of samples sintered at temperatures lower than 1500 °C. Concerning the sintering mode, two-step sintering (TSS) proved to be a good procedure to obtain CZYA samples with high electrical conductivity and density (> 95%) at relatively low sintering temperature and long soaking time.     

Enhancement of conductivity in La2Mo2O9 ceramics fabricated by a novel three-stage thermal processing method

Pure and dense La₂Mo₂O₉ ceramic electrolytes with grain sizes of 1–3 μm were fabricated from nanocrystalline powders by a novel three-stage, one-cycle, pressureless thermal processing method at temperatures as low as 600 °C. Phase formation, microstructure and grain size of the samples were examined using X-ray diffraction and scanning electron microscopy. Density of the sintered samples was determined as in the range of 94–96% of the theoretical density by weight/geometric measurements. Impedance spectroscopy was used to characterize the electrical properties of the sintered samples. The conductivity of the three-stage sintered samples reaches a value of 0.018 S/cm at 600 °C and 0.05 S/cm at 700 °C, much higher than that of the samples fabricated by conventional solid-state reaction method, but similar to that of the samples sintered at 950 °C for 12 h from the same nanocrystalline powders. The high conductivity of these samples was attributed to the co-operation of the excellent performance of nanocrystalline powders and the advantages of the novel three-stage low-temperature thermal processing.     

High-temperature proton conductivity in acceptor-doped LaNbO4

The conductivity of acceptor-doped LaNbO₄ has been investigated in the temperature range 300 to 1200 °C as a function of the oxygen pressure and water vapor pressure by means of impedance spectroscopy and EMF measurements. The conductivity is predominantly ionic below 800 °C in air and for higher temperatures under reducing conditions. Protons are the major ionic charge carrier in the presence of water vapor. A maximum in proton conductivity of ∼ 0.001 S/cm was obtained at 950 °C in atmospheres containing ca 2% H₂O. At high temperatures (> 1000 °C) under oxidizing conditions, electron hole conduction prevails. The conductivity has been modeled assuming that oxygen vacancies and protons compensate the acceptor doping. Transport coefficients describing mobility of defects and thermodynamic constants for the incorporation of protons have been derived.     

Effect of Zn–Ti double substitution on the electrical properties of Bi4V2O11

New samples of the Bi₂Zn_0.1–xTi_xV_0.9O_5.35+x; 0.02 ≤ x ≤ 0.08 system have been synthesized through a standard solid-state reaction route. XRD analysis and differential thermal analysis have been used to characterize the phase structure of samples. The γ′ phase is stabilized to room temperature in all investigated samples. The electrical properties of the BIZNTIVOX system have been studied by using AC impedance spectroscopy. An AC impedance response as a function of frequency (20 Hz–1 MHz) has been used to investigate the electrical conductivity and the dielectric permittivity in the temperature range of 150 °C–700 °C. In this temperature range, the phase transition γ′ to γ has been observed in all the compositions studied. AC impedance spectroscopy indicates that the resistance of samples decreases with increase of temperature. The ionic conductivity of samples appeared as a two-line region in Arrhenius dependence. At 300 °C, the highest ionic conductivity is shown by the composition x = 0.05 (σ₃₀₀ = 1.35 × 10^?4 S cm^?1).     

Study of electrical conductivity changes and phase transitions in Co3O4 doped ZrO2

The electrical conductivity of ZrO₂ doped with Co₃O₄ has been measured at various temperatures for different molar ratios. The conductivity increases due to the migration of vacancies created by doping. The conductivity is also found to increase with rise in temperature up to 120°C, and after attaining a maximum the conductivity decreases due to a collapse of the lattice framework. A second rise in conductivity around 460°C in all the compositions confirms the phase transition in ZrO₂ from monoclinic to tetragonal symmetry. X-ray powder diffraction and DTA studies were carried out for confirming the doping effects and the transition in ZrO₂.     

ac conductivity of Na-Ag β” -alumina polycrystalline samples

The results of ac conductivity measurements carried out on ceramic samples of Li₂O and NiO-Li₂O-doped β7rdquo;-alumina with Na⁺, Ag⁺ and Na⁺ -Ag⁺ mobile ions are presented. The modifications of doping only slightly influenced either bulk or grain boundary conductivity in Na β” -alumina. The activation energies of conduction in Na β”-alumina equal to 0.28 eV at low temperatures decreased to 0.14–0.15 eV at temperatures above 200°C. The Arrhenius plots for Ag β” -alumina were found to be linear in whole 20–450°C temperature range with the activation energies of 0.19 eV (Li₂O) and 0.24 eV (NiO-Li₂O). The conductivity measurements done on partially exchanged samples revealed the presence of the mixed alkali effect.     

AC conductivity of BiFeO<Subscript>3</Subscript> ceramics obtained by spark plasma sintering of nanopowder

The structure and electrical properties of BiFeO₃ ceramics obtained by spark plasma sintering of a nanopowder are investigated. The nanopowder was synthesized by burning of an organic nitrate precursor. The ac conductivity was measured in a frequency range of 1 kHz–10 MHz in a temperature interval of 25–500°C. It is established that the temperature conductivity coefficients above and below ~350°C significantly differ with both alternating and direct currents. The frequency dependence of the conductivity obeys the Jonscher power law σ ~ ω ^s , where s < 1. The interpretation of this behavior is given in the framework of the model of correlated hops of charge carriers over potential barriers. It is assumed that the hopping mechanism is realized between Fe²⁺ and Fe³⁺ ions in ceramic grains. The role of oxygen vacancies in the conduction is also discussed.     

Thermal conductivity at the amorphous-nanocrystalline phase transition in beech wood biocarbon

High-porosity samples of beech wood biocarbon (BE-C) were prepared by pyrolysis at carbonization temperatures T _carb = 650, 1300, and 1600°C, and their resistivity ρ and thermal conductivity κ were studied in the 5–300 and 80–300 K temperature intervals. The experimental results obtained were evaluated by invoking X-ray diffraction data and information on the temperature dependences ρ(T) and κ(T) for BE-C samples prepared at T _carb = 800, 1000, and 2400°C, which were collected by the authors earlier. An analysis of the κ(T _carb) behavior led to the conclusion that the samples under study undergo an amorphous-nanocrystalline phase transition in the interval 800°C < T _carb < 1000°C. Evaluation of the electronic component of the thermal conductivity revealed that the Lorentz number of the sample prepared at T _carb = 2400°C exceeds by far the classical Sommerfeld value, which is characteristic of metals and highly degenerate semiconductors.     

Evolution of thermal conductivity of In3Sbβ Teγ thin films up to 550 °C

The temperature dependent thermal conductivity of In–Sb–Te thin films has been measured by modulated photothermal radiometry in the 20–550 °C range for samples with different Te content. Significant changes with temperature are observed and ascribed to a sequence of structural transformations on the basis of in‐situ Raman spectra. The data suggest that the as‐deposited material consisting of a mixture of polycrystalline InSb_0.8Te_0.2and amorphous Te first undergoes a progressive crystallization of the amorphous part, mostly above 300 °C. Further increase in temperature above 460 °C leads, for higher Te content in the alloy, to the formation of crystalline In₃SbTe₂, intertwined with a less conductive compound, possibly InTe and/or InSb. Upon cooling to room temperature, the initial polycrystalline InSb_0.8Te_0.2phase is mostly recovered along with other compounds, with a slightly higher thermal conductivity than that of the as deposited material. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Ionic conductivity of double sodium-scandium and cesium-zirconium molybdates

The electrical properties of the Na₃Sc(MoO₄)₃ and Cs₂Zr(MoO₄)₃ compounds are investigated using impedance spectroscopy (1–10⁶ Hz) in the temperature range 100–650°C. Double molybdates in the form of a fine-crystalline powder are obtained by solid-phase synthesis in air at 450–600°C for 20–50 h. It is found that the temperature dependence of the ionic conductivity of ceramic samples exhibits anomalies at temperatures of 605 ± 5°C for Na₃Sc(MoO₄)₃ and 425 ± 15°C for Cs₂Zr(MoO₄)₃ due to the phase transitions, which are confirmed by the data of thermal analysis. Above the superionic transitions, the ionic conductivity reaches 0.084 S/cm (650°C) for Na₃Sc(MoO₄)₃ and 0.002 S/cm (462°C) for Cs₂Zr(MoO₄)₃.     

Proton and native-ion conductivities in Y2O3 at high temperatures

The ionic conductivity of hot-pressed samples of undoped Y₂O₃ has been studied by the emf-method in atmospheres of controlled oxygen and water-vapor pressures. The variation in the ionic conductivity was studied as a function of time (7 months at 1200°C), temperature (600–1300°C), water-vapor pressure (3–1400 Pa), and oxygen pressure (10⁻¹⁰ −10⁵ Pa). The overall conductivity can be divided into contributions from electronic carriers (mainly electron holes), native ionic defects, and hydrogen defects. The transport of charged hydrogen species is dominated by migration of “free” protons. The hydrogen-ion conductivity is detectable under all conditions and becomes the dominant ionic-conductivity contribution at high water-vapor pressures and low temperatures. The ionic contributions are discussed in terms of grain-boundary and bulk transport properties. Native-ion and proton-diffusion coefficients in yttria are estimated. Equations for the emf of oxide specimens containing charged hydrogen defects have been derived.     

Electrical conductivity of Na2SO4(I)

The electrical conductivity of the solid phase Na₂SO₄(I) has been measured between the melting point at 884°C and the first order phase transition at about 240°C. The measurements have been performed using complex impedance measurements on pellet samples as well as on U-cells. The electrical conductivity is strongly dependent on sample at low temperatures and the activation energy ranges from 0.5 eV to 1.7 eV over the measured temperature range.