Type Electronic Conductivity Research Articles

The influence of excess K2O on the electrical properties of (K,Na)1/2Bi1/2TiO3 ceramics

The solid solution (KxNa0.50-x)Bi0.50TiO3 (KNBT) between Na1/2Bi1/2TiO3 and K1/2Bi1/2TiO3 (KBT) has been extensively researched as a candidate lead-free piezoelectric material because of its relatively high Curie temperature and good piezoelectric properties, especially near the morphotropic phase boundary (MPB) at x ∼ 0.10 (20 mol. % KBT). Here, we show that low levels of excess K2O in the starting compositions, i.e., (Ky+0.03Na0.50-y)Bi0.50TiO3.015 (y-series), can significantly change the conduction mechanism and electrical properties compared to a nominally stoichiometric KNBT series (KxNa0.50-x)Bi0.50TiO3 (x-series). Impedance spectroscopy measurements reveal significantly higher bulk conductivity (σb) values for y ≥ 0.10 samples [activation energy (Ea) ≤ 0.95 eV] compared to the corresponding x-series samples which possess bandgap type electronic conduction (Ea ∼ 1.26–1.85 eV). The largest difference in electrical properties occurs close to the MPB composition (20 mol. % KBT) where y = 0.10 ceramics possess σb (at 300 °C) that is 4 orders of magnitude higher than that of x = 0.10 and the oxide-ion transport number in the former is ∼0.70–0.75 compared to <0.05 in the latter (between 600 and 800 °C). The effect of excess K2O can be rationalised on the basis of the (K + Na):Bi ratio in the starting composition prior to ceramic processing. This demonstrates the electrical properties of KNBT to be sensitive to low levels of A-site nonstoichiometry and indicates that excess K2O in KNBT starting compositions to compensate for volatilisation can lead to undesirable high dielectric loss and leakage currents at elevated temperatures.

Hydration of Rutile TiO2: Thermodynamics and Effects on n- and p-Type Electronic Conduction

The bulk conductivity of polycrystalline 1 mol-% Fe-doped rutile TiO2 has been measured as a function of pH2O and temperature under oxidizing and reducing conditions. From the pH2O-dependency of the conductivity, it is concluded that protons are significant positive defects and, furthermore, that mixed p-type electronic and protonic, and n-type electronic conduction dominate under oxidizing and reducing conditions, respectively. H2O/D2O isotope exchange confirmed that protons are significant charge carriers under wet oxidizing conditions below approximately 600 °C. Thermodynamic parameters for the hydration reaction were obtained by modeling the experimental pH2O and temperature dependencies, assuming that the acceptor dopant (Fe3+) is charge compensated by protons and oxygen vacancies, and from ab initio density functional theory (DFT) calculations. The experimental data yield standard enthalpy changes of hydration of −130 ± 16 kJ/mol, whereas the calculated values are somewhat more negative; −155 to −162 kJ/mol. Based on such favorable thermodynamics of hydration, it is concluded that protons will be the dominating positive defect in TiO2 under most conditions of practical interest.

Defect formation inLaGa(Mg,Ni)O3−δ: A statistical thermodynamic analysis validated by mixed conductivity and magnetic susceptibility measurements

A statistical thermodynamic approach to analyze defect thermodynamics in strongly nonideal solid solutions was proposed and validated by a case study focused on the oxygen intercalation processes in mixed-conducting $\mathrm{La}{\mathrm{Ga}}_{0.65}{\mathrm{Mg}}_{0.15}{\mathrm{Ni}}_{0.20}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ perovskite. The oxygen nonstoichiometry of Ni-doped lanthanum gallate, measured by coulometric titration and thermogravimetric analysis at $923\char21{}1223\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the oxygen partial pressure range $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}0.9\phantom{\rule{0.3em}{0ex}}\mathrm{atm}$, indicates the coexistence of ${\mathrm{Ni}}^{2+}$, ${\mathrm{Ni}}^{3+}$, and ${\mathrm{Ni}}^{4+}$ oxidation states. The formation of tetravalent nickel was also confirmed by the magnetic susceptibility data at $77\char21{}600\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and by the analysis of $p$-type electronic conductivity and Seebeck coefficient as function of the oxygen pressure at $1023\char21{}1223\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The oxygen thermodynamics and the partial ionic and hole conductivities are strongly affected by the point-defect interactions, primarily the Coulombic repulsion between oxygen vacancies and/or electron holes and the vacancy association with ${\mathrm{Mg}}^{2+}$ cations. These factors can be analyzed by introducing the defect interaction energy in the concentration-dependent part of defect chemical potentials expressed by the discrete Fermi-Dirac distribution, and taking into account the probabilities of local configurations calculated via binomial distributions.

Defect interactions, statistical thermodynamic and electronic transport in ionic nonstoichiometric oxides

Abstract In this paper, we discuss the effects of the defect interactions on the thermodynamic properties and electronic transport of nonstoichiometric oxides. We first show that two limited thermodynamic behaviours can be distinguished depending on whether the oxide has a metallic type electronic conduction or a hopping electronic conduction. In the first case, the interactions are expected to be short ranged because of the screening of the conduction electrons, while they are Coulombic in the second case. This paper is limited to this last case, exemplified mainly by M1−xO(M=Co, Mn,…) cubic oxides. Next, we analyse the two approximated methods usally used in this field, the ideal mass action law method and the Debye Huckel theory, and we show that they are both insufficient to accurately describe these compounds. We finally show by means of Monte Carlo simulations that a simple model in which point defects interact through a unique dielectric constant allows us to render an account of the main features ...

Structure and electrochemical properties of intercalation compounds in the system silver-titanium disulfide, Ag x TiS 2

In the system AgTiS 2 three structurally related phases exist, a dilute gas phase and second- and first-stage intercalates. Single crystals of Ag 0.35TiS 2, grown by vapour transport, were studied by X-ray diffraction. A superstructure due to three-dimensional order of silver, a √ 3 × a √ 3 × 2 c, space group P 31 c , is present below T c = 301 K. The transition to the disordered substructure, a = 3.428, c = 6.398 A ̊ , space group group P 3 m1 , is of second-order nature. Kinetic parameters of the mobile ions (silver) were measured at temperatures above 450 K on powder compacts in an electrochemical cell using small signal a.c. frequency response and the galvanostic intermittent titration technique (GITT). The first- and second-stage phases show fast ionic conduction of silver; the chemical diffusion coefficient ▪ is about 10 −5 cm 2 s −1A at 460 K and the activation energy is about 0.26 eV. The free enthalpy of intercalation is small, −2.9 kJ mol −1 for stage II Ag 0.19TiS 2 and −4.6 kJ mol −1 for stage I Ag 0.42TiS 2 (at 480 K). The temperature coefficient of the e.m.f. is positive. The intercalated phases show metallic type electronic conduction.

ChemInform Abstract: ELECTRICAL CONDUCTIVITY OF PURE AND YTTRIA‐DOPED URANIUM DIOXIDE

AbstractDie Leitfähigkeiten für reines und mit <10 mol‐% Y dotiertes UO2+x werden als Funktion des O2‐Partialdruckes zwischen 800 und 1400°C gemessen.

The small anomaly in the temperature dependence of the electrical conductivity of iron containing phosphate glasses

The dc conductivities, σ,of 2 CaO·K 2 O· x Fe 2 O 3 ·(5 - x )P 2 O 5 glasses (1.4 ⩽ x ⩽2.2) [Fe 3+ /(Fe 2+ + Fe 3+ )= constant (0.88±0.015)] have been measured in the temperature range from room temperature to 300°C. It has been found that the glass transition temperature increases rapidly with increasing Fe/P ratio and that mixed-valence hopping type electronic conduction takes place in these glasses and every log σ versus 1/ T curve obtained shows a change in slope near 100°C, which produces a small increase, 0.14 (for x =2.2)−0.18 eV (for x =1.4), in activation energy at higher temperatures. Both the activation energy and pre-exponential factor for conduction display different dependences on the Fe/P ratio in the different temperature ranges on both sides of the anomaly temperature. These results are discussed in terms of the Mott's equation for small polaron hopping.

Electrical Conductivity of Pure and Yttria‐Doped Uranium Dioxide

The conductivities of both pure urania and urania doped with <10 mol% yttrium were measured as a function of the oxygen partial pressure between 800° and 1400°C. Yttrium enhances the p ‐type electronic conductivity for near‐stoichiometric oxygen concentrations, allowing the hole mobility to be determined as μ=(5547/T) exp(‐0.032 aJ/kT) cm2/Vs, confirming a small polaron transport model. Using this mobility, the hole concentration in pure urania was calculated as a function of the degree of nonstoichiometry and temperature and then compared to models for the formation of defect clusters. Increasing numbers of mobile holes created per excess oxygen ion with increasing temperature indicate that the holes must dissociate from the oxygen defect clusters at high temperature and suggest that the oxygen defects may dissociate as well.

Oxygen permeability of calcia-stabilized zirconia

Oxygen permeability of commercial calcia-stabilized zirconia has been measured at 1673 to 1823 K by the following cell; O2 ZrO2(CaO) N2 - O2 (P’tO2 = 1 atm) solid electrolyte (P″O2 = 0.39 - 1010-3 atm). Oxygen permeability of calcia-stabilized zirconia is proportional to (1 - P″O21/4). From the permeability measurement, the conduction properties of the electrolyte were log σ-‡b+ = 0.28- 5100/T and logPb+ =-0.87 + 15,400/T where σ-‡b+ is the þ-@#@ type electronic conductivity at PO2 = 1 atm, and Pb+ is the oxygen activity at which the þ-@#@ type electronic conductivity and the ionic conductivity are equal.