摘要:

Here we discuss a phenomenological approach for the understanding of the anomalous point defect formation in ionic crystals prior to the phase transition as well as the prediction of the phase transition temperatures. Quantitative agreement was achieved using a simple cube root law for the defect-defect interaction potential. This approach was applied to materials (AgCl, AgBr, AgI, PbF2) with different structures (rock salt, fluorite, wurtzite), different types of disorder (Frenkel and anti-Frenkel) and different types of phase transition (solid-liquid and solid-solid, first and second order). The computed defect-defect interaction leads to a quantitative description of the conductivity anomalies and to the prediction of a phase instability at a temperature which is very close to the actual transition point.

ISSN:1042-0150    

DOI:10.1080/10420159508222733

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

In the following paper we report the results of a study, by means of impedance spectroscopy, of the conducting properties of thermochemically blackened Yttria Stabilized Zirconia (YSZ) samples, previously investigated by means of optical and paramagnetic resonance spectroscopies. The work focuses on the relation between the characterized electron traps and the semiconducting properties of YSZ. On the light of the results we propose a level diagram for the electrons that accounts for the optical properties of the defects as well as for the electronic conduction of thermochemically reduced samples. This conduction would take place through a band of defects situated about 1.3 eV below the conduction band.

ISSN:1042-0150    

DOI:10.1080/10420159508222734

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The effect of prolonged annealing treatments at elevated temperatures (900–1700 K) on the a.c. ionic conductivity of Sawyer PQ ‘hydrogen swept’ quartz has been investigated by impedance spectroscopy measurements. Transient and irreversible effects have been observed during the annealings: specifically, a monotonic enhancement of the conductivity is detected in the 900–1300 K temperature range, while at higher annealing temperatures an opposite effect is evidenced. The experimental results can be interpreted in the frame of a phenomenological model featuring two equilibrium reactions; an indirect role of hydrogen impurity in the ionic transport dynamics is evidenced, also by means of infrared spectroscopy measurements.

ISSN:1042-0150    

DOI:10.1080/10420159508222735

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The lattice and grain boundary diffusion coefficients of18O(at 0.1 atm) were determined at 900°C in bulk Cr2O3, and in Cr2O3scales developed on a Ni70Cr30alloy. The diffusion profiles were established by SIMS and analyzed considering two domains, the first one relative to apparent diffusion and the second to grain boundary diffusion. With such an analysis and taking a f value = 0.0006, modified from a ridge model, the oxygen lattice diffusion coefficients determined in Cr2O3scales are then in very good agreement with those in bulk Cr2O3. With some assumptions, our diffusion data lead to a parabolic oxidation constant equal to the experimental one. The scale growth is ensured by counter-current diffusion of oxygen and chromium, mainly by grain boundary diffusion.

ISSN:1042-0150    

DOI:10.1080/10420159508222736

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The influence of yttrium doping on the ionic and electronic conductivities of α alumina developed on a β NiAl alloy by oxidation at 1100[ddot]C was investigated by electrochemical measurements, in order to point out the possible effect of this element on the transport properties of alumina scales. It appears that the yttrium influence is small. It slightly reduces the ionic transport number at low oxygen pressure and increases it at high pO2: the alumina scale behaves as an ionic conductor near the substrate-oxide interface and progressively becomes a mixed conductor or an electronic conductor near the outer oxide-gas interface. The ionic-electronic transition is sharper for undoped

ISSN:1042-0150    

DOI:10.1080/10420159508222737

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The expressions for effective diffusion coefficient are obtained in the mean field approximation for two-phase system for spatial dimensions of 1, 2 and 3. The existence of potential barrier for diffusion on the phase boundary was taken into account via the boundary conditions. Obtained formulae could be applied in the theory of diffusion-controlled reactions and for interpreting the experimental data on defect diffusion in two-phase media.

ISSN:1042-0150    

DOI:10.1080/10420159508222738

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

In this work, we perform a study of the off-center effect on the diffusion coefficient and we propose a new mechanism for fast diffusion. The model assumes that the equilibrium positions of the cationic impurities are noncentral and that the diffusion proceeds by hopping across the potential barrier along the nonlinear paths with the highest probability [1]. The nonlinear paths between two consecutive positions characterize even the self-diffusion and furthermore, in the off-center diffusion this path is appropriate. The main results is that the off-center position considerably enhances the diffusion. The theoretical diffusion coefficients have been obtained by modeling the potential barrier. Changes of the configuration entropy and the vibration spectra due to the presence of the noncentral impurity are included in the model. We emphasize the good agreement of the model with the experimental data for KCl:Cu+and show that if the impurity is placed close to the central site, the appropriate diffusion coefficient is close to that for the cationic self-diffusion. We proceeded in the Li+cation case as in the case of Cu+cation.

ISSN:1042-0150    

DOI:10.1080/10420159508222739

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Chromium precipitates generated by controlled internal reduction of (Al, Cr)2O3polycrystals are used as indicator for the occurrence of DIGM (diffusion-induced grain boundary migration) in this material. Asymmetric precipitate-free zones limited at one side by a grain boundary are observed inside the reduction scale. This observation is ascribed to diffusion induced migration of the grain boundaries, defect relaxation in the oxygen potential gradient being the driving force.

ISSN:1042-0150    

DOI:10.1080/10420159508222740

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The second-order elastic constants of AgBr have been measured from 20[ddot]C to 400[ddot]C. Over this range, C'11decreased by 50%, C’ decreased by 86%, C44decreased by 23%, C11decreased by 61%, C12decreased by 43%, and Bsdecreased by 51%. The decreases in the elastic constants are linear, as expected, until approximately 200[ddot]C, whereupon they all begin to decrease more rapidly than linearly. The variations from linearity range from 4% to 31% at 400[ddot]C. This anomalous behaviour near the melting point is similar to other silver halide physical properties and may be attributed to the unusually high defect concentration. Similar elastic constant changes are seen in superionic conductors near the superionic transition, indicating that the silver halides may be starting the transition to a superionic state when the halide sublattice melts and the transition is frustrated.

ISSN:1042-0150    

DOI:10.1080/10420159508222741

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The phenomena of Schottky barrier creation in RbAg4I5are studied. Luminescence with high spatial resolution is used as a tool for investigation of the process on the blocking electrode-solid electrolyte boundary. The profile of the distribution of electronic centers near the contact region is measured. The presence of electrons and ions in the depletion region at the interface between the RbAg4I5and the graphite electrode is established and interpreted within the framework of the modified Schottky model. The kinetics of the creation of Schottky barriers is measured and described by taking into account the presence of self-trapped electrons and mobile silver ions in RbAg4I5. It is shown that the small value of the self-trapped electron diffusion coefficient (Dst≃ 1 × 10−8cm2/sec) limit the kinetics of the process of creation of Schottky barriers.

ISSN:1042-0150    

DOI:10.1080/10420159508222742

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor