Chemical diffusion in calcium titanate

This work reports the gas/solid equilibration kinetics for the O₂/CaTiO₃ system. The electrical conductivity measurement was applied for monitoring the kinetics in the ranges of temperature 973-1323 K and oxygen partial pressure 10 Pa-72 kPa. It was found that the gas/solid equilibration kinetics for the polycrystalline CaTiO₃ specimen in the above experimental conditions is determined by bulk diffusion rather than by grain boundary conditions. The obtained data of the electrical conductivity vs. time were used for the determination of the chemical diffusion coefficient as a function of temperature at low and high p(O₂), respectively:

(1)     

Thermoelectric power of mixed electronic-ionic conductors III. Case of calcium titanate

The primary purpose of the present work is the determination of the thermopower component related to electronic charge carriers for undoped calcium titanate. The second purpose of this work is establishment of the relationship between this thermopower component and the electronic component of electrical conductivity. An essential part of the present study includes the determination of the thermopower components corresponding to different charge carriers (electrons, electron holes and ions). The determination procedures are based on the following three models:

Symmetrical model. This model assumes consistency between thermopower and electrical conuctivity in terms of the n-p transition (this model assumes that minimum of electrical conductivity corresponds to the electronic component of the thermopower equal zero). It was shown that this model does not apply for CaTiO₃.

The Heikes model. This model is based on Heikes formula and also hopping mechanism of the transport of electrons. It was shown that thermopower of CaTiO₃ cannot be described by this model and, consequently, thermopower vs. electrical conductivity cannot be considered within the Jonker formalism.

General model. This model is based on a general thermopower equation for mixed conductors without any simplifying assumptions. Application of this model indicates that the electronic component of thermopower is not consistent with the minimum of electrical conductivity.

     

Solitons in the one-dimensional forest fire model

Fires in the one-dimensional Bak-Chen-Tang forest fire model propagate as solitons, resembling shocks in Burgers turbulence. The branching of solitons, creating new fires, is balanced by the pairwise annihilation of oppositely moving solitons. Two distinct, diverging length scales appear in the limit where the growth rate of trees, p, vanishes. The width of the solitons, w, diverges as a power law, 1/p, while the average distance between solitons diverges much faster as d approximately exp(pi2/12p).     

Synthesis of D-Glucose-based Azacrown Ethers with Phosphinoxidoalkyl Side Chains and Their Application to an Enantioselective Reaction

Five chiral -D-glucose-based monoaza-15-crown-5 ethers with phosphinoxidoalkyl side chains of one to five carbon atoms (5a–e)have been synthesised. The cation binding ability of the new lariat ethers was evaluated bythe picrate extraction method. The substituents at the nitrogen atom were not a major influenceon the cation extraction ability of the azacrown ether; the compounds showed, however, a significant asymmetric induction as phase transfer catalysts in the Michael addition of2-nitropropane to chalcone (95% ee).     

Indirect magnetic interaction in the “net fractal” systems

A localized spin system of fractal symmetry with indirect exchange between them is considered. We define a specific class of fractals as the “net fractals” which display multidimensional logarithmic periodicity. Basing on this property we model the effective indirect exchange interaction by the conventional RKKY exchange with the logarithmic coordinates playing role of the real space ones. Finally, we discuss the case of non-ideal “net fractals” in which fractional dynamics of the electrons is expected. In this case we show that RKKY exchange integrals are given by the formulas derived under assumption that a system has a fractional spectral dimension.     

Electrical properties of TiO<Subscript>2</Subscript>: equilibrium vs dynamic electrical conductivity

The present work reports semiconducting properties of high purity TiO₂ determined in the gas/solid equilibrium, as well as during controlled heating and cooling in the range 300–1,273 K. The activation energy of the electrical conductivity is considered in terms of the activation enthalpy of the formation of ionic defects and the activation enthalpy of the mobility of electronic defects. These data, determined from the dynamic electrical conductivity experiments, are compared to the electrical conductivity data determined in equilibrium. It is shown that only the equilibrium electrical conductivity data for high-purity TiO₂ are well defined. It is shown that the activation energy of the electrical conductivity determined in equilibrium differs substantially from that for the dynamic electrical conductivity data during cooling and heating. It is concluded that the formation enthalpy term determined from the dynamic conductivity data is determined by the heating/cooling rate rather than materials’ properties.     

Interactions of oxygen with yttria-stabilized zirconia at room temperature

This paper reports the results of work function and EPR studies of yttria-stabilized zirconia (10Y-ZrO₂). The experimental data are considered in terms of the formation of oxygen chemisorbed species and subsequent oxygen incorporation as well as related charge transfer. It is concluded that oxygen chemisorption on 10Y-ZrO₂ at room temperature in its initial stage results in the formation of O ₂ ⁻ species. These species are then transformed into O ₂ ²⁻ , O⁻ species and, finally, are slowly incorporated into the oxide lattice. These processes take place without presence of Pt or any other electrode material.