摘要:

KI, RbI and K0.8Rb0.2I have been bombarded with high-energy argon ions at temperatures between 20 and 300 K, followed by anin situmeasurement of the optical absorption. In KI and RbI, the rate of F centre production in its broad features is typical of class I alkali halides and follows a pattern similar to that for X-irradiation. The rate is low at 20 K, rises to a maximum at 200 K followed by a decrease at 300 K. Most unusual defect growth behaviour is observed for K0.8Rb0.2I. The measured F centre production is completely suppressed at 20 K and is insignificant at 100 K. This effect appears only in part due to the formation of F+-I centre pairs. The F/V centre ratio is much smaller in K0.8Rb0.2I as compared with KI or RbI. The overall defect production rate in K0.8Rb0.2I is larger at 20 K than at 100 K. Studies of the halogen interstitial centres including annealing and subsequent Raman experiments show that in spite of differing production efficiencies at low temperature in the pure and mixed crystals, their structures appear relatively similar. The results suggest a modified production mechanism in K0.8Rb0.2I under heavy ion bombardment, a conclusion that is supported by comparative X-irradiation experiments at 100 and 200 K. The results are examined in relation to current ideas regarding defect production in alkali halides.

ISSN:1042-0150    

DOI:10.1080/10420159908245930

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Calcium titanite CaTiSiO5undergoes a phase transition at ∼220°C. Extended X-ray Absorption Fine Structure (EXAFS) spectra of both calcium and titanium K-edge have been collected and local structural information extracted from them. The results have been compared with the existing XRD data on this material and the agreement is excellent. In particular, there is good agreement on the changes in bond lengths that occur at the phase transition. A complementary computer simulation study has also been performed and defect formation energies have been calculated.

ISSN:1042-0150    

DOI:10.1080/10420159908245931

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

In this paper we adapt Manning's diffusion kinetics formalism for a random alloy to vacancy pair diffusion in ionic crystals of the CsCl structure. First, we analyse the binary ionic crystal and show that the formalism gives the component tracer correlation factors in closed form and with results in very good agreement with Monte Carlo simulation results. Next, we analyse quaternary mixed systems (two types of homovalent cation and two types of homovalent anion). Again the formalism gives all tracer correlation factors in closed form and the results are in very good agreement with those from Monte Carlo simulation.

ISSN:1042-0150    

DOI:10.1080/10420159908245932

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Tracer diffusivities of both cations in single crystalline calcia stabilized cubic zirconia (11 and 17 mol% CaO) were measured in the temperature range 1130°C <T< 1460°C. The calcium tracer diffusion is more than 10 times faster than the zirconium tracer diffusion. For both elements, the activation energy of tracer diffusion varies between 5.1 and 5.6 eV. The pre-exponential factors are higher than in the case of yttria stabilized zirconia (102−103cm2s−1as compared to 1–10 cm2s−1). There is no significant dependency of the cation diffusivity on the stabilizer content, which indicates a diffusion mechanism via vacancy clusters (V4г, 2V2*o).

ISSN:1042-0150    

DOI:10.1080/10420159908245933

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The comparison of the electrical conductivity measured in NaCl single crystals and in NaCl-Al2O3composites, respectively, shows a strong enhancement due to the presence of the Al2O3particles. The Na+self-diffusion shows the same enhancement in the extrinsic range as well as in the intrinsic high temperature region. The correlation factor is derived from the Nernst-Einstein relation. The valuef= 0.78 × 0.1 shows that the mass transport in the composite proceeds by the same mechanism as in the unmixed ionic crystals, i.e. via Schottky defects in NaCl. The Nernst-Einstein relation is closely verified by involving all the ions of the alkali halide phase, in contrast with the space charge model.

ISSN:1042-0150    

DOI:10.1080/10420159908245934

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

High temperature ionic conductivity of fused silica and thin SiO2films has been investigated in the temperature ranges 700–1400 and 570–830 K respectively by impedance spectroscopy measurements. The results are in agreement with the existence of extrinsic mechanisms in the ionic transport of both types of silicon dioxide, due to the presence of dissociated sodium ions. The numerical analysis of the experimental results led to values of 1.3 and 0.6 eV for the dissociation and migration energies, respectively. No influence of hydrogen content on alkali transport was found by considering wet and dry samples, at variance with results previously obtained on crystalline quartz.

ISSN:1042-0150    

DOI:10.1080/10420159908245935

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The dynamics of light-induced (Kr+-, Ar+-laser) electronic polarons (Nb4+centers with broad absorption band around 0.8 eV) and light-induced centers of other types were investigated in SrxBa1−xNb2O6: Cr (SBN:Cr) and in SBN: Ce using FTIR absorption measurements at low temperature. A theoretical model involving Cr3+/Cr4+, Ce3+/Ce4+, Nb4+electronic polarons and trapping X-centers is proposed. The trapping of polarons at Cr4+/Ce4+centers with subsequent recharging is shown to play an important role in the polaron dynamics. The predictions of the model are in very good agreement with the experimental results.

ISSN:1042-0150    

DOI:10.1080/10420159908245936

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The Thermally Stimulated Depolarization Currents (TSDC) technique was used to characterize a lithological contact in a sedimentary sequence (Cretaceous Northeastern, Venezuela). TSDC spectra in the temperature range from 77 to 240 K were obtained for samples collected near a contact identified in the field as that between Chimana (CH) and Querecual (QC) formations. Deconvolution of the complex spectra, by the Direct Signal Analysis (DSA) method, shows the presence of four overlapping peaks. Drying and rehydration treatments performed on these samples indicate that the dielectric peaks are associated with physisorbed moisture. TSDC results show a progressive change in the mean reorientation energy with the lithology for the four relaxations. In the same lithological zone where this variation is observed, NRM measurements indicate significant changes in remanent magnetization values. The results obtained suggest that TSDC technique can be used as sensitive and alternative means of characterizing lithologies and stratigraphic discontinuities in a sedimentary sequence.

ISSN:1042-0150    

DOI:10.1080/10420159908245937

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Experiments on photoconductivity and thermally stimulated currents were performed on Li-doped KTaO3single crystals. The large enhancement of conductivity, caused by UV excitation at low temperature was found to correlate with a subsequent release of trapped charge between 30 and 40 K. No corresponding release was shown by pure KTaO3, consistently with a very low yield of photoconduction. It is concluded that the presence of hole traps, with the consequent quenching of electron-hole recombination, is at the origin of the photoconductive response of K1−xLixTaO3.

ISSN:1042-0150    

DOI:10.1080/10420159908245938

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

A novel test system based on a thin-walled extruded tubular yttria-stabilised zirconia reactor has been developed, which can be used to investigate the fuel processing catalysis and surface chemistry occurring at nickel/zirconia anodes in solid oxide fuel cells, as well as the durability and electrochemical performance of the fuel cell, under actual operating conditions. In addition to enabling different anode formulations to be evaluated for their catalytic activity, electrical performance and durability, and permitting the surface chemistry to be studied, the system allows simultaneous monitoring of the catalytic activity and surface chemistry, and the cell performance, allowing direct correlation between the fuel cell performance and its reforming characteristics.

ISSN:1042-0150    

DOI:10.1080/10420159908245939

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor