Anthony R. West

CaCu3Ti4O12: One-step internal barrier layer capacitor

There has been much recent interest in a so-called “giant-dielectric phenomenon” displayed by an unusual cubic perovskite-type material, CaCu3Ti4O12; however, the origin of the high permittivity has been unclear [M. A. Subramanian, L. Dong, N. Duan, B. A. Reisner, and A. W. Sleight, J. Solid State Chem. 151, 323 (2000); C. C. Homes, T. Vogt, S. M. Shapiro, S. Wakimoto, and A. P. Ramirez, Science 293, 673 (2001); A. P. Ramirez, M. A. Subramanian, M. Gardel, G. Blumberg, D. Li, T. Vogt, and S. M. Shapiro, Solid State Commun. 115, 217 (2000)]. Impedance spectroscopy on CaCu3Ti4O12 ceramics demonstrates that they are electrically heterogeneous and consist of semiconducting grains with insulating grain boundaries. The giant-dielectric phenomenon is therefore attributed to a grain boundary (internal) barrier layer capacitance (IBLC) instead of an intrinsic property associated with the crystal structure. This barrier layer electrical microstructure with effective permittivity values in excess of 10 000 can be fa...

Impedance and modulus spectroscopy of semiconducting BaTiO3 showing positive temperature coefficient of resistance

Polycrystalline barium titanate that has been doped to give a positive temperature coefficient of resistance (PTCR) effect is an inhomogeneous material electrically. Analysis of ac impedance data using the complex impedance plane representation gives the dc resistance of PTCR ceramics. By additional use of the complex electric modulus formalism to analyze the same data, the inhomogeneous nature of the ceramics may be probed. This reveals the presence of two, sometimes three elements in the equivalent circuit. Grain‐boundary and bulk effects may be distinguished from capacitance data: grain‐boundary effects have temperature‐independent capacitances, whereas bulk effects show a capacitance maximum at the Curie point and Curie–Weiss behavior above the Curie point. Both grain‐boundary and bulk effects appear to contribute to the PTCR effect. These results reveal limitations in current theories of the PTCR effect.

Impedance and modulus spectroscopy of polycrystalline solid electrolytes

A method of characterising the electrical properties of polycrystalline electrolytes is described which enables grain boundary (intergranular) and bulk (intragranular) impedances to be separated and identified, by reference to an equivalent circuit which contains a series array of parallel RC elements. In the simple case of the “ideal solid electrolyte”, the equivalent circuit contains a single RC element. The impedance and modulus spectra, i.e. plots of Z″ and M″ versus log ω, are simple “Debye” peaks whose peak maxima coincide at an angular frequency ωmax=(τσ)−1, where τσ is the “conductivity relaxation time” and the complex modulus is the inverse complex perimittivity. For real solid electrolytes there is usually a distribution of relaxation times, in which case the maxima in the impedance and modulus spectra no longer coincide. An assignment of peaks in these more complex spectra is possible in principle, since the modulus spectrum effectively suppresses information concerning grain boundary (and electrode) effects. Experimental results are presented for cold-pressed lithium orthosilicate and germanate, and for sintered β-alumina. Some advantages of this new approach are demonstrated by comparison with conventional impedance and admittance plane methods of analysis.

The determination of hopping rates and carrier concentrations in ionic conductors by a new analysis of ac conductivity

Abstract The a.c. conductivity σ(ω) of ionic materials takes the form, σ(ω) = σ(0) + Aωn. The carrier hopping rate, ωp, is obtained from the new expression σ(0) = A ωpn and the carrier concentration is estimated from σ(0). The contribution of creation and migration terms to the activation energy for conduction may be determined from the thermal activation of σ(0) and ωp and the corresponding entropy terms quantified. Data have been analyzed for four widely different ionic materials: single crystal Na β-alumina, polycrystalline Li4SiO4, Ag7l4AsO4 glass and Ca(NO3)2/KNO3 glass and melt. For each, the carrier concentration and hopping rates have been obtained.

Electrical and structural characteristics of lanthanum-doped barium titanate ceramics

Single phase La-doped BaTiO3 with the formula Ba1−xLaxTi1−x/4O3: 0⩽x⩽0.20 was prepared by solid state reaction of oxide mixtures at 1350 °C, 3 days, in O2. The tetragonal distortion in undoped BaTiO3 decreased with x and samples were cubic for x⩾0.05. Both the tetragonal/cubic and orthorhombic/tetragonal transition temperatures decreased with x, but at different rates, and appeared to coalesce at x∼0.08. The value of the permittivity maximum at the tetragonal/cubic phase transition in ceramic samples increased from ∼10 000 for x=0 at 130 °C to ∼25 000 for x=0.06 at ∼−9 °C. At larger x, the permittivity maximum broadened, showed “relaxor”-type frequency dependent permittivity characteristics and continued to move to lower temperatures. Samples fired in O2 were insulating and showed no signs of donor doping whereas air-fired samples were semiconducting, attributable to oxygen loss.

Characterization of Electrical Materials, Especially Ferroelectrics, by Impedance Spectroscopy

A review is given of some of the problems encountered in the analysis and interpretation of impedance data. The importance of choosing the correct equivalent circuit to represent the data is emphasized and it is shown how ferroelectric materials, with their characteristic temperature-dependent capacitance, are particularly suited to discriminating between plausible equivalent circuits. Results are discussed for two materials, LiTaO3 single crystal and BaTiO3 ceramics.

Review of crystalline lithium-ion conductors suitable for high temperature battery applications

Abstract A review of crystalline lithium-ion conducting materials is presented with a focus on those suitable for high temperature battery applications. A wide range of materials is considered, both oxides and non-oxides, and several systems are recommended as being of particular interest. Factors considered include high ionic conductivity at operating temperatures (300–600°C), compatibility with electrode materials and ease of preparation.

Temperature dependence of the a.c. conductivity of Naβ-alumina

Abstract Low temperature a.c. conductivity of β-alumina is of the form σ ( ω ) = σ ( ω ) + Aω n . Both σ(0) and A are found to be thermally activated. Simple relationships, dependent on the value of n , are shown to exist between (i) the relative magnitudes of σ(0) and A and (ii) their respective activation energies.

The extraction of ionic conductivities and hopping rates from a.c. conductivity data

Over a wide range of frequencies, the a.c. conductivity of ionic materials shows two regions of frequency-dependent conductivity. These are each characterized by a term Kωp1−nωn where K, n are constants, ωp is a fundamental frequency identified with the hopping rate and ω is the measuring frequency. This behaviour is an example of Jonschers Law of Dielectric Response for ionic conductors. In many cases, the region of low-frequency dispersion approximates to a frequency-independent plateau which may be taken as the d.c. conductivity. In others, a significant low-frequency dispersion is present and cannot be ignored in determining the effective d.c. conductivity. A method for the extraction of d.c. conductivities, hopping rates and for estimating carrier concentration effects is described. Data for three different types of material, single-crystal LiGaO2, β″-alumina and Na/Ag β-alumina are used to illustrate the method.

Impedance and modulus spectroscopy of “real” dispersive conductors

Abstract Power law dispersions in the bulk, intragranular, ac conductivity and permittivity of a non-ideal solid electrolyte are modelled by a frequency-dependent admittance in parallel with the R, C elements of a conventional equivalent circuit. Expressions are derived for imaginary parts of the complex impedance Z ″ and electric modulus M ″ for such a dispersive conductor. The dependance of the shapes of the Z ″ and M ″ versus frequency peaks on the equivalent-circuit parameters are illustrated. Z ″ and M ″ are predicted to peak at different frequencies. An analysis of Na β-alumina data is presented. It is shown that the hopping rate of ions in a solid electrolyte can be obtained from the functional form of Z ″ and M ″ peaks and the frequency separation of their peak maxima.