Defect Equilibria and Transport in YBa2Cu3O7-x at Elevated Temperatures: III, Defect Model and Conductivity Mechanism

Based on the measurements of electrical conductivity and thermopower (Seebeck coefficient) as a function of oxygen partial pressure, the defect structure and corresponding conduction mechanisms at elevated temperatures are considered for the Y-Ba-Cu-O (1:2:3) system. It has been postulated that the simple hopping model is not applicable to YBa₂Cu₃O_7-x. A modified conduction hopping mechanism has been proposed and equations describing the mobility of charge carriers in the studied system are derived. The most important advantage of the present model, in comparison to previous models, involves considering interactions between both electrons and electron holes and the resulting effect on their mobility terms. The observed departure of experimental data from the Heikes formula is explained by a very high concentration of defects and resulting substantial interactions between the defects which must be taken into account. The proposed transport model exhibits good agreement with experimental data of nonstoichiometry and the presently determined electrical conductivity and thermopower. It has been argued that the charge transfer mechanism depends substantially on the oxygen partial pressure and resulting oxygen content in the oxide lattice.     

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/ k T) 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.     

A Model for Silicon Self-Diffusion in Silicon Carbide: Anti-Site Defect Motion

A simple defect interaction model was developed that explains the identical activation energies observed for carbon and silicon diffusion in single-crystal silicon carbide. In accord with experimental measurement of nonstoichiometry, the model requires a substantial concentration of silicon anti-site defects. The diffusion of silicon is limited by the motion of these defects; this is suggested to occur by their interaction with carbon vacancies. The model predicts that boron doping will increase both carbon and silicon diffusion coefficients.     

Crystal and Defect Structure of Nd1.9Ce0.1CuO4±y

The crystal and defect structure of neodymium cerium cu-prate (Nd_1.9 Ce_0.1 CuO_4±y) has been studied as a function of temperature (1200–1500 K) and oxygen pressure (10⁰–10³ atm) via X-ray diffractometry, powder neutron diffraction, and thermogravimetry methods. The oxygen nonstoichiom-etry changes from oxygen-excess (positive y values) to oxygen- deficient oxides (negative y values). The absolute values of oxygen nonstoichiometry y are calculated from the neu-tron diffraction and thermogravimetry data. The materials adopt the tetragonal structure (space group I 4/ mmm ). A defect model is proposed for Nd_2-x Ce_x CuO_4±y. It is con-cluded that oxygen vacancies V Ö (which occupy O(1) crys-tallographic positions (in planes)) and interstitial oxygen O(i (which occupy O(3) "apical" positions) are present simultaneously. Gaseous oxygen intercalates into the Nd_1.9 Ce_0.1 CuO_4±y crystals and replaces oxygen vacancies in the O(1) position and interstitial oxygen atoms in the O(3) positions. The theoretical model and the experimental data are in good agreement with each other, and the equilibrium constants of defect formation have been calculated.     

Cation Deficiency in Bi2(Sr,Ca)3Cu2O8+d

X-ray lattice parameter measurements of the Bi_{2- x} (Sr,Ca)_{3- y} Cu_{2- z} O_{8+ d} system show that a continuously varying solid-solution cation nonstoichiometry exists within the overall composition ranges x < 0.32, -0.2 < y < 0.32, and z < 0.25. A modulated structure along the b axis remains present over these composition ranges. Defects introduced by cation deficiency have been clarified to be vacancies by density measurements. The oxygen content determined by titration decreases proportionately with increasing cation vacancy concentrations. In Bi-deficient compositions, the interstitial oxygen ion concentration decreased and the oxygen vacancy concentration increased with increasing concentration of Bi vacancies. The critical temperature, T _c , shows a maximum value at the stoichiometric cation composition and decreases to a slightly lower (5 to 10 K) constant value across the nonstoichiometric composition region with both increasing alkaline-earth and Cu deficiency.     

Experimental and Thermodynamic Study of Nonstoichiometry in 〈YBa2Cu3O7-x〉

The dependence of the nonstoichiometry of 〈YBa₂Cu₃O_7−x〉 (solid) has been studied over 5 orders of magnitude in oxygen pressure and from 573 to 1173 K. Hydrogen-reduction methods for determining the absolute oxygen to-metal ratio were developed. The resulting data were used to derive a chemical thermodynamic representation of the experimental variables. The data were used to derive a chemical thermodynamic representaltion of the experimental variables. The data were also compared with several other investigations to indentify the selfconsistent sets of data. The present data and thermodynamic data from the literature were correlated on an Ellingham diagram.     

Role of Point Defects in the Physical Properties of Fluorite Oxides

For the first time, explicit, analytical expressions have been derived for the dependence of electrical conductivity, chemical expansion, and elastic modulus on defect concentration and oxygen partial pressure using a consistent approach. The models have been validated with experimental data for Ce_0.9Gd_0.1O_1.95−δ (GDC) with consistently good fits. In developing the model, expressions were first derived for the functional dependence of defect concentration on the oxygen partial pressure ( P _O2) for n -type fluorite oxides. Expressions for the chemical expansion, elastic modulus, and electrical conductivity as functions of defect concentration are then derived and verified with experimental data.     

Review and Chemical Thermodynamic Representation of 〈U1–zCezO2±x〉 and 〈U1–zLnzO2±x); Ln = Y, La; Nd, Gd

The entire data base for the dependence of the nonstoichiometry, x , on temperature and chemical potential of oxygen (oxygen potential) in 〈U_{1– z} Ce _z O_{2+ x} 〉 and 〈U_{1– z} Ln _z O_{2+ x} 〉 was retrieved from the literature and represented by a thermodynamic method. The method reproduces the behavior of the experimental data and also results in partial molal Gibbs free energy quantities that are useful for any thermodynamic calculation involving these nonstoichiometric phases. The behavior of these systems is also compared with that for 〈U_{1– z} Pu _z O_{2± x} 〉.     

High-Temperature Defect Structure of Lanthanum Cuprate

High-temperature (650° to 8507deg;C) electrical conductivity and Seebeck coefficient measurements as functions of oxygen partial pressure and temperature in polycrystalline lanthanum cuprate support a defect model consisting of oxygen interstitials charge compensated by electron holes. The La:Cu ratio was therefore estimated to be 2.000 ± 0.001. By comparison with existing oxygen nonstoichiometry data, the high-temperature electron hole mobility and density-of-states were estimated.     

Electrical Conductivity and Lattice Defects in Nanocrystalline Cerium Oxide Thin Films

The results of the electrical conductivity and Raman scattering measurements of CeO₂ thin films obtained by a polymeric precursor spin-coating technique are presented. The electrical conductivity has been studied as a function of temperature and oxygen activity and correlated with the grain size. When compared with microcrystalline samples, nanocrystalline materials show enhanced electronic conductivity. The transition from extrinsic to intrinsic type of conductivity has been observed as the grain size decreases to <100 nm, which appears to be related to a decrease in the enthalpy of oxygen vacancy formation in CeO₂. Raman spectroscopy has been used to analyze the crystalline quality as a function of grain size. A direct comparison has been made between the defect concentration calculated from coherence length and nonstoichiometry determined from electrical measurements.     

Oxygen Nonstoichiometry and Defects in Mn-Doped Gd2Ti2O7+x

The oxygen nonstoichiometry in Mn-doped Gd₂Ti₂O₇, Gd₂(Ti_0.975Mn_0.025)₂O_7+x, was measured electrochemically, as a function of temperature and oxygen partial pressure, with the aid of an oxygen titration cell. The analysis of the data shows that the defect equilibrium can be described by considering the dominant point defects to be neutral oxygen interstitials, doubly charged oxygen vacancies, and trivalent and quadrivalent Mn ions substituted in the Ti sites. The enthalpies for the formation of neutral oxygen interstitials and trivalent Mn are determined.     

Low-Temperature Densification of Lanthanum Strontium Manganite (La1−xSrxMnO3+δ), x=0.0–0.20

Intermediate-stage sintering of lanthanum strontium manganite (LSM, where Sr=0.00, 0.05, 0.10, 0.15, and 0.20) was shown in dilatometry studies to be accelerated when subjected to alternating flows of air and nitrogen. The extent of rate enhancement decreased with increased Sr content, and decreased with increased temperature, which coincides with diminished oxygen nonstoichiometry. Shrinkage rates were further shown to be sensitive to the difference in oxygen content in the alternating gas flows. Baseline air sintering rates were measured using stepwise isothermal dilatometry, from which kinetic parameters were calculated using the Makipirtti–Meng model. Activation energies for sintering in air were determined to be 255 ± 26, 258 ± 28, 308 ± 32, 373 ± 37, and 417 ± 41 kJ/mol for Sr=0.0, 0.05, 0.10, 0.15, and 0.20, respectively. A diffusion-based model is proposed that is consistent with trends in accelerated shrinkage versus temperature. Transient cation vacancy gradients, which lead to higher cation mobility, were calculated from established oxygen diffusivities and oxygen nonstoichiometry as a function of temperature and time. A potential application of this approach is the processing of LSM-based cathode-side contact pastes in solid oxide fuel cells.     

氧气在醋酸水溶液中的溶解度估算

将水作为溶剂主体,通过热力学分析导出了氧气在纯水中的溶解度模型。将醋酸作为有机溶剂添加剂,用Stechenov关联式表征了醋酸对氧气在水中的溶解度的影响。模型参数由文献数据回归得到,模型计算得到的溶解度值与文献报导的实验值进行比较吻合良好。建立的氧气在醋酸水溶液中的溶解度机理模型可用于关联和预测不同醋酸浓度下不同温度和压力时氧气在醋酸水溶液中的溶解度。     

Development of a kinetic model for the oxidation of methane over Pd/Al2O3 at dry and wet conditions

The kinetics of the oxidation of methane over a commercial 0.5% Pd on γ-Al₂O₃ catalyst has been studied in a lab-scale fixed-bed reactor, the effect of temperature, and methane, oxygen and water partial pressures being investigated, in a range of interest for environmental applications. Different Eley–Rideal, Langmuir–Hinshelwood and Mars–van Krevelen models were fitted to the experimental results, the best fitting being obtained for a Mars–van Krevelen model that considers slow desorption of the reaction products. The model parameters obtained both from differential and integral treatment of the experimental data are in good agreement with each other. A modification of the proposed model, taking into account that water is adsorbed over oxidised sites, is also able to model the inhibition produced by steam.     

Characterizations of Fe Doping on B‐Site of (La0.8Ca0.2)(Cr0.9Co0.1)O3−δ Interconnect Materials for SOFCs

The microstructure, lattice parameters, and electrical conductivity mechanisms for Fe doping on B‐site of (La_0.8Ca_0.2)(Cr_0.9Co_0.1)O_3?δ were systematically investigated. The oxygen nonstoichiometry was measured by means of thermogravimetry as a function of oxygen partial pressure. In this study, the concept of defect chemistry is used to explain the relationship between the concentration of electron hole with the electrical conductivity. Based on the result of electrical conductivity in air, it is concluded that the concentration of electron hole at high oxygen activity is larger than that at low oxygen activity. This is due to the fact that (La_0.8Ca_0.2)CrO_3?δ‐based ceramics are p‐type conductors, the electrical conductivity is dominated by the concentration of hole. At higher Fe‐doping level, the compensation mechanism at high oxygen activity is significantly dominated by the formation of oxygen vacancy, that is, ionic compensation. The compensation mechanism at low oxygen activity is significantly dominated by the formation of the formation of Cr⁴⁺, that is, electrical compensation at lower Fe‐doping level. Based on oxygen nonstoichiometry data, it is found that with increasing Fe‐doping amount on B‐site of (La_0.8Ca_0.2)(Cr_0.9Co_0.1)O_3?δ specimens, the initial weight‐stable temperature shifted to lower temperature which might be highly related with the change in compensation mechanism at the temperature.     

Thermogravimetric Analysis and Defect Models of the Oxygen Nonstoichiometry in La2–xSrxCuO4–y

The oxygen nonstoichiometry, y, of La_{2– x} Sr _x CuO_{4– y} (0 ≤ x ≤ 1) was determined as a function of temperature (800–1050°C) and oxygen partial pressure (10^–4 to 1 atm) by a thermogravimetric technique. The results were interpreted in terms of a number of defect models. It was found that a model based on the defect reaction 4Cu^×_Cu+ 2 V ^¨_o+ O_2(g)= 4Cu^._Cu+ 2O^×_o described the results well when the x and y dependences of the partial molar enthalpy of oxygen in the solid were considered. The large defect concentrations observed in this system lead to defect interactions, possibly ordering on the oxygen sublattice.     

Kinetic model for autoxidation of β-carotene in organic solutions

Autoxidation of β-carotene was studied experimentally using n-decane as a solvent under various reaction conditions of temperature and dissolved oxygen concentration A novel kinetic model was proposed on the basis of an autocatalytic free-radical chain reaction mechanism. A secondary initiation reaction by decomposition of hydroperoxide and reactions concerned with a β-carotene-derived C-centered radical in propagation and termination processes were taken into consideration in the model. There were four unknown kinetic constants, and the constants were estimated by fitting the model with the experimental data. The fitted results are in good agreement with the experimental data in all stages of the kinetics of autoxidation and over a wide range of oxygen concentrations. The model described not only the appearance of the induction stage but also the effect of the oxygen concentration on the autoxidation rate. In addition, the model predicted the behavior of autoxidation in another solvent at low temperature that had been reported by other researchers.     

Nonstoichiometry and Mixed Conduction in α-Ta2O5

Electrical conductivity of single-crystal and polycrystalline Ta₂O₅ was measured as a function of composition, temperature, and oxygen partial pressure ( P _O2) by both dc and ac complex impedance methods. A defect model based on the assumption of predominant disorder on the oxygen sublattice is proposed to explain the observed nonstoichiometry of α-Ta₂O₅. The defect model is consistent with the observed P _O2-independent ionic conductivity and the P ^−1/4_O2-dependent electronic conductivity. Blocking of the ionic component was observed at low P _O2. Both the electronic and ionic components of conductivity were found to decrease with time. This aging phenomena is related to destabilization of the a phase.     

Defect Equilibria and Transport in YBa2Cu3O7-x at Elevated Temperatures: II, Nonstoichiometry

Aspects of nonstoichiometry for the Y-Ba-Cu (1: 2: 3) system are considered. The general formula YBa₂Cu₃O_7-x has been assumed for considerations of nonstoichiometry in 1: 2:3 oxide cuprates. Assuming that copper ions may occupy different lattice positions (independently of their valency), the equilibrium constants for oxygen intercalation were determined: