Conductivity behaviour of a cubic/tetragonal phase stabilized nanocrystalline La2O3-ZrO2

La₂O₃ doped nanocrystalline zirconia (ZrO₂) was prepared by chemical co-precipitation method for the 3, 5, 8, 10, 15, 20 and 30 mol.% concentrations of La₂O₃. Structural studies were performed using X-ray diffraction (XRD). All the as-synthesized samples were found to be in monoclinic phase. As-synthesized samples were given heat treatment at higher temperatures for tetragonal/cubic structural phase stabilization. Sintering the samples at temperature 1173 K stabilized the tetragonal and cubic phases. A slight shift in the 100% peak of the cubic phase was observed towards the low diffraction angle indicating the substitution of the bigger La³⁺ ion into the ZrO₂ lattice. Grain sizes were found to lie between 10 and 13 nm. Electrical conductivity studies were performed on the cubic phase stabilized La₂O₃-ZrO₂ by complex impedance spectroscopy. The conductivity increases up to the dopant concentration 10 mol.% and then decreases with further increase in La₂O₃ concentration. Initial increase in conductivity is correlated to the stabilization of the cubic phase and the subsequent decrease in the conductivity with the dopant content is interpreted on the basis of the oxygen-ion movement model. Electrical conductivity has contributions from grain and grain boundary regions. But the grain boundary conductivity is slightly higher than the corresponding grain conductivity. Higher grain boundary conductivity shows higher diffusion coefficient for the atoms on the surface of the ZrO₂ grains. The possible mechanism of the oxygen ion conduction in the La₂O₃ stabilized zirconia (LSZ) is reported. The Barton, Nakajima and Namikawa (BNN) relation has been applied to the conductivity data and found that the d.c. and a.c. conductions have been correlated to each other by the same mechanism.     

Grain Boundary Diffusion in C<Subscript>60</Subscript> Thin Films

This paper focuses on the effect of grain boundaries on the diffusion processes in polycrystalline C₆₀ thin films. Electrically induced diffusion of Au was investigated by in situ measurements of the film conductivity. Electron Paramagnetic Resonance (EPR) spectroscopy was used to study diffusion of oxygen. Increase in grain sizes in polycrystalline C₆₀ thin films was found to result in the acceleration of gold and oxygen diffusion. The results are interpreted assuming that these impurities diffuse in C₆₀ films dominantly along grain boundaries.     

Electronic conductivity and chemical diffusion in n-conducting barium titanate ceramics at high temperatures

The electronic conductivity as well as the chemical diffusion coefficient of barium titanate ceramics doped with Y and Mn (donor-doped and acceptor co-doped) have been determined by application of conductivity relaxation experiments. The equilibrium values of the electronic conductivity of n-conducting BaTiO₃ have been analyzed by application of a defect chemical model involving electrons and cation vacancies as the predominant defect species at oxidizing conditions (fairly high oxygen partial pressures). The relaxation curves of the electronic conductivity yield the chemical diffusion coefficient of the bulk by employing a spherical grain model where the appropriate diffusion length is the radius of grains (average grain size). The conductivity relaxation experiments have been performed as a function of temperature ranging from 1100 to 1250 °C at oxygen partial pressures between 0.01 and 1 bar. The kinetics of the oxygen exchange process can be interpreted in terms of extremely fast diffusion of oxygen via oxygen vacancies along the grain boundaries and slow diffusion of Ti (cation)-vacancies from the grain boundaries into the grains. The Ti-vacancy diffusion coefficients were extracted from the chemical diffusion coefficients as a function of temperature. Typical values for the Ti-vacancy diffusivity are around 10^− 15 cm² s^− 1 with an activation energy of 3.9 ± 0.7 eV.     

Role of the microstructure on the transport properties of Y-doped zirconia and Gd-doped ceria

Transmission electron microscopy characterizations and XPS analyses have allowed us to show the influence of the microstructure and nanochemistry on the transport properties of Y₂O₃-(9 mol%)-stabilized zirconia (YSZ) and Gd₂O₃ (10 mol%)-doped ceria (GDC). The grain boundary electrical conductivity (σ_gb) and oxygen diffusion coefficient (D_o) of conventional YSZ ceramics increase with the grain size, while an opposite behavior was found for GDC samples. This difference was attributed to glassy precipitates present at YSZ grain boundaries. Furthermore, it was shown that kinetic demixing processes take place during cooling, at the end of sintering. This causes important changes in the cationic species distribution at interfaces and plays an important role on the transport properties of these two materials. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Ambipolar diffusion phenomena in BaTiO3 and SrTiO3

The electrical conductivity of resistive oxygen sensors based on semiconducting titanates (BaTiO₃, SrTiO₃) is determined by the amount and the ratio of the different concentrations of atomic defects in the sensor material. At sufficiently high temperatures the sensor exchanges oxygen with the surrounding gas atmosphere resulting in variations of the concentration of defects. The kinetics of these oxygen exchange processes are determined by the diffusion of the defects in the solid. In this paper the diffusion coefficients of the defects, which are decisive for the electrical conductivity, are determined by measurement of the conductivity during the diffusion processes. The evaluation of these results by an analytical model which considers the interaction between all simultaneously diffusing types of defects, allows a survey of the effects influencing the diffusion rate. With this knowledge, it is possible to estimate the influence of grain boundaries, acceptors, temperature and oxygen pressure on the response time of the sensor.     

纳米CaF2和纳米Ca0.75La0.25F2.25的结构对离子电导率的影响

用惰性气体蒸发和真空原位加压方法制备了两种具有清洁界面的纳米离子固体CaF₂(平均粒度为16nm)和Ca_0.75La_0.25F_2.25(平均粒度为11nm),在31℃至530℃详细测量了其复阻抗谱。结果表明:1)在300—530℃两种纳米离子导体都很好地遵从Arrhenius方程式;2)纳米CaF₂的离子电导率比多晶CaF₂约高1个数量级、比单晶CaF₂     

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)     

Effect of granularity and inhomogeneity in excess conductivity of YBa2Cu3O7−δ+xBaTiO3 superconductor

Synthesis of polycrystalline YBCO+xBaTiO₃ (x=1.0, 2.5, 5.0) superconductor has been done and the effects of granularity and inhomogeneities due to inclusions of nano-BaTiO₃ in excess conductivity are reported in this work. The phase formation, texture and grain alignments were analyzed through XRD and SEM techniques. SEM results reveal that the grain size is reduced and morphology is improved with the incorporation of nano-BaTiO₃ particles. Superconducting order parameter fluctuation (SCOPF) studies on the electrical conductivity were investigated from the resistivity vs. temperature data in the experimental domain relatively above T_c. Log(Δσ) vs. log(ε) plots show that the 2D to 3D crossover temperature (T_LD) that demarcates dimensional nature of fluctuation inside the grains is influenced by BaTiO₃ incorporation in YBCO matrix. An upward shift of T_LD in the mean field region has been observed as a consequent dominance of 3D region with increase in 1 wt% BaTiO₃ in the composites as compared to higher inclusions. It has been analyzed that microscopic inhomogeneities produced as a result of diffusion of a fraction of Ti ions into the grains affect fluctuations in the excess conductive region. The interplay of microscopic inhomogeneities produced inside the grains and mesoscopic inhomogeneities in the grain boundaries on the excess conductivity has been explained in terms of thermal fluctuations for the composites.     

The effect of cation nonstoichiometry on the electrical conductivity of acceptor-doped CaZrO3

The electrical properties of acceptor-doped Ca_1−xZr_0.99M_0.01O_3−δ (M = Mg²⁺, In³⁺) systems were investigated as a function of cation nonstoichiometry (0 ≤ x ≤ 0.05). The characterization was carried out using the impedance spectroscopy between 550 °C and 1100 °C in dry air. The contributions of the grain and grain boundary conductivity to the total conductivity were obtained from the impedance data. When the Ca deficiency (x) increased, the total conductivity rapidly decreased with the corresponding increase in activation energy. Although the grain conductivity increased slightly with increasing x, the total conductivity is mostly determined by the highly resistive grain boundary. With varying x, the activation energy of total conductivity showed the percolation behavior. The percolation threshold values vary according to the doped species. It may be due to the difference in concentration of oxygen vacancies of the specimens.     

Diffusion of 22Na and 45Ca and ionic conduction in two standard soda-lime glasses

The tracer diffusivities of ²²Na and ⁴⁵Ca in two high-quality silica glasses produced by the Deutsche Glastechnische Gesellschaft as standard glasses I and II have been measured in the temperature range between 473 K and 783 K. The temperature dependences of the tracer diffusion coefficients in both glasses follow Arrhenius laws. The diffusion of ²²Na is more than six orders of magnitude faster than the diffusion of ⁴⁵Ca. The ionic conductivity was determined by frequency-dependent impedance spectroscopy and the conductivity diffusion coefficient D_σ was deduced from the dc conductivity via the Nernst–Einstein relation. The temperature dependences of D_σ for both glasses follow also Arrhenius functions. The activation parameters and pre-exponential factors for tracer diffusion and for conductivity diffusion were determined. The activation enthalpy of ²²Na and the activation enthalpy of the dc conductivity are equal, showing that the conductivity of standard glasses is due to the motion of Na ions. The viscosity diffusivities D_η were determined from available viscosity data using the Stokes–Einstein relation. They are considerably slower than both tracer diffusivities. The Haven ratios H_R are temperature independent for both glasses. The diffusivities of ²²Na and ⁴⁵Ca in soda-lime glasses increase with increasing Na₂O content.     

Different conduction behaviors of grain boundaries in SiO2-containing 8YSZ and CGO20 electrolytes

Both doped zirconia and ceria have been widely recognized as promising electrolytes in solid oxide fuel cells (SOFC). Total conductivity is an important parameter to evaluate solid electrolytes. It is well know that the contribution to the total conductivity by grain boundaries is especially pronounced for SiO₂-contaminated electrolytes. In this study, we report on the different conduction behaviors of grain boundaries (GB) found in SiO₂-containing (impure) 8YSZ (8 mol% Y₂O₃-doped ZrO₂) and CGO20 (10 mol% Gd₂O₃-doped CeO₂) ceramics. In the grain size range (∼ 0.5–10 μm) studied, the GB conductivity of impure CGO20 ceramics constantly decreases with increasing grain size, in contrast to that observed in impure 8YSZ electrolytes whose GB conductivity increases almost linearly with grain size. It is also found that the variation in GB conductivity versus grain size is different from case to case, depending on the sintering/annealing conditions used to fabricate the ceramics. Two mechanisms were proposed to explain the GB behaviors of the impure 8YSZ and CGO20 ceramics. For doped ceria, the GB phases are supposed to be inert, which do not react with or dissolve into the matrix. Increasing sintering temperature leads to not only grain growth but also change in viscosity and wetting nature of the GB phases. These two factors promote further propagation of the GB phases along the grain boundaries, leading to an increased GB coverage fraction. For doped zirconia, however, the major factor dominating the GB conduction is the further dissolution of SiO₂ into zirconia lattice as a result of increase in sintering temperature or/and time. In addition, we will also evaluate and discuss the validities of the three models that are widely used to analyze the GB conduction in solid electrolytes.     

The relationship between chemical composition distributions and specific grain boundary conductivity in Y-doped BaZrO3 proton conductors

The relationship between chemical composition at a grain boundary and specific grain boundary conductivity is studied in Y-doped BaZrO₃ ceramics sintered at 1800 °C. Y enrichment at the grain boundary as yttrium concentration increased and Ba deficiency in ceramics as an increase of sintering time are observed. At the grain boundary, phase segregation, i.e. existence of secondary phase and amorphous phase which prevent proton migration, is not observed. The results indicate that at the grain boundary, concentration of Ba vacancy and Y substituted into Zr site is changed. Grain boundary conductivity shows significant dependences to Ba deficiency and Y concentration in ceramics. It is due to significant dependences of specific grain boundary conductivity to Ba deficiency and Y concentration in ceramics. The results indicate that proton migration across the grain boundary is sensitive to concentration of those effectively charged defect, i.e. Y substituted into Zr site and Ba vacancy at grain boundary.     

Studies on grain boundary effects in spray deposited BICOVOX 0.1 films on platinum-coated stainless steel substrate

Thin film of Bi₂Co_0.1V_0.9O_5.35 is deposited by using spray pyrolysis technique on platinum-coated stainless steel substrate. Impedance measurements done in the frequency range 1 to 10 MHz and in the temperature range 502 to 720 K revealed two relaxation processes with distinguishable time constants. The first corresponds to the grain interior charge transfer while the second could be due to grain boundary. The change in polarization seems to be associated with hopping of charge carriers showing Arrhenius behavior with increase in temperature. The relaxation frequency of grain interior transport for the thin film ranges from 96 kHz to 2.59 MHz. The blocking factor was found to be increasing with increase in temperature at low temperature region from 502 to 640 K. At higher temperature above 640 K, diffusive nature of grain boundaries is inferred with the decrease in blocking factor. The same inference is derived by specific grain boundary conductivity calculations since specific grain boundary conductivity decreases in low temperature region while it increases rapidly at higher temperature. These observations prove the grain boundaries to be blocking in lower temperatures while at higher temperatures above 640 K they turn diffusive. These changes are attributed to structural phase transition, or ordering of vacancies in Bi₂Co_0.1V_0.9O_5.35 films.     

Lithium conductivity and lithium diffusion in NASICON-type Li<Subscript>1+x</Subscript>Ti<Subscript>2–x</Subscript>Al<Subscript>x</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> (x= 0; 0.3) prepared by mechanical activation

LiTi₂(PO₄)₃ (LTP) and Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) (S. g. R-3c) have been prepared using conventional ceramic and mechanical activation (MA) methods. It has been shown that preliminary mechanical activation of initial mixtures leads to different nature and amount of dielectric admixtures in the final product after heat treatment at 800–1000 °C as compared with ceramic method. Transport properties of as prepared materials have been studied by lithium ionic conductivity at d.c. and a.c. (complex impedance method), and ⁷Li NMR spin-lattice relaxation rate T₁ ^–1 measurements. Lithium ionic conductivity of mechanochemically prepared LTP and LATP was characterized by significant reduction of grain boundary resistance, especially for LTP, while the bulk conductivity and Li ion diffusion does not noticeably change. The activation energy of bulk conductivity and Li ion diffusion, i.e. short-range motion, appeared to be almost the same for all samples and was equal to ~0.20 eV. On contrary, the activation energy of d.c.-conductivity, i.e. long-range Li ion motion decreases from ~0.6 eV for ceramic samples to ~0.4 eV for samples prepared via mechanochemical route. It was proposed that MA leads to formation of nano-particulate high-conductive grain boundaries both in LTP and LATP. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Effect of alumina addition on the microstructure and grain boundary resistance of magnesia partially-stabilized zirconia

The electrical properties of 9 mol% MgO–ZrO₂ (Mg-PSZ) with 1 mol% Al₂O₃ and the mechanisms for electrical degradation were investigated using structural, morphological, and electrochemical analyses. The addition of Al₂O₃ caused an increase in both the monoclinic and the Mg-rich phases at the grain boundaries in the Mg-PSZ. Coarse grains larger than 20 μm and an intergranular layer composed of the Mg-rich phase were identified in a specimen sintered at 1600 °C. This specimen exhibited a minimum of ionic conductivity (4.98 × 10⁻⁴ S cm⁻¹ at 700 °C) due to the grain boundary resistance (245 Ω cm²), which dominated the overall resistance. A similar trend was observed over the entire temperature range (600–1500 °C). An intergranular siliceous impurity (SiO₂) was present in conjunction with the Mg-rich phase. This impurity and the Mg-rich phase acted as a barrier layer for oxygen ion diffusion. The presence of the intergranular phases (i.e. the monoclinic and Mg-rich phases) contributed to the degradation of the ionic conductivity in Mg-PSZ with an Al₂O₃ addition.     

Sodium diffusion in cryolite at elevated temperatures studied by quasielastic neutron scattering

Quasielastic neutron scattering (QENS) has been applied to study the sodium mobility on nanosecond time scales in the perovskite fluoride cryolite, Na₃AlF₆, at high temperatures. Up to T = 1153 K the diffusion of Na ions is well described by a diffusion process of jumps between six and eight-fold coordinated sites. Above this temperature, where a step-like increase in the electrical conductivity occurs, the jump length increases, which indicates additional jumps over larger distances. The electrical conductivity derived from the self-diffusion coefficient via the Nernst–Einstein relation and the corresponding activation energy are in excellent agreement with the previous conductivity measurements. We conclude that the jump diffusion of sodium ions is the dominant mechanism for the electrical conductivity in cryolite at high temperatures up to T = 1153 K.     

Rietveld refinement, microstructure, conductivity and impedance properties of Ba[Zr0.25Ti0.75]O3 ceramic

In this work, we report the Rietveld refinement, microstructure, conductivity and impedance properties of Ba[Zr_0.25Ti_0.75]O₃ ceramic synthesized by solid state reaction. This ceramic was characterized by X-ray diffraction, Rietveld refinement, scanning electron microscopy and energy dispersive X-ray spectrometry. Impedance spectroscopy analyses reveals a non-Debye relaxation phenomenon being its relaxation frequency moving toward to positive side with increase of temperature. A significant shift in impedance loss peaks toward higher frequency side indicates conduction in material and favoring the long range motion of mobile charge carriers. The frequency dependent ac conductivity at different temperatures indicates that the conduction process is thermally activated. The variation of dc conductivity exhibited a negative temperature coefficient of resistance behavior. The ac conductivity data are used to evaluate the density of states at Fermi level and activation energy of this ceramic. The dc electrical and thermal conductivities of grain and grain boundary have been discussed.     

The kinetics of barium precipitation at dislocations in NaCl monocrystals

The kinetics of barium precipitation at dislocations in NaCl monocrystals has been studied in thermally and mechanically treated NaCl + 4 ppm BaCl₂ samples by investigating the isothermic variation of ionic conductivity as a function of time. The course of precipitation which takes place at dislocations located at grain boundaries can be divided into three time regions characterized by diffusion of impurities to dislocation cores at grain boundaries, nucleation, formation of new grain boundaries, etc. At higher number of dislocations an interruption of the precipitation appears due to a local free energy of nucleation minimum at radiusr ₀=6·92×10^–8 cm.     

Thermal and electrical conductivity of CdSnAs<Subscript>2</Subscript> in solid and liquid states

The mechanisms of CdSnAs₂ heat and charge transfer in solid and liquid states were investigated. It was demonstrated that heat transfer in solid state is accomplished by phonons, electrons, diffusion, and the recombination of electron-hole pairs. A sharp increase in thermal and electrical conductivity up to the values characteristic of metals is observed upon melting. In contrast to metals, the electrical conductivity of CdSnAs₂ melt increases with temperature.     

The influence of grain boundary on the conductivity of donor doped SrTiO3

The electrical conductivity of polycrystalline Sr(Ti_0.999Nb_0.001)O₃ was investigated. The conductivity was smaller by 1–2 order than that of the single crystal. The conductivity increased with temperature with the activation energy of 0.61 eV. The distribution of grain boundary nature of the polycrystalline sample was determined by Orientation Imaging Microscope (OIM) analysis. The ratio of coincident lattice boundaries was determined to be approximately 20%. The impedance of bicrystals across the grain boundary with different grain boundary type was measured. The grain boundary impedance was found to consist of two RC parallel components in series. The activation energies of them were 0.56–0.71 eV and 1.73–1.97 eV, respectively. These two processes were assigned to the grain boundary or annealed surface layer and the Schottky barrier between the bulk and the surface or the grain boundary layer. A possible conduction mechanism of polycrystalline material was considered that of the three dimensional network of the grain boundary layer.