Derek C. Sinclair

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.

Effect of atmosphere on the PTCR properties of BaTiO3 ceramics

The influence of low-temperature annealing, at < 360 °C, in various reducing and oxidizing atmospheres for a series of BaTiO3 ceramics with a positive temperature coefficient of resistance (PTCR) is discussed. Combined impedance and modulus spectroscopy is used to analyse a.c. impedance data and shows that the total resistance of the sample can be composed of up to three components, dependent on the cooling rate from the sintering temperature. For quickly cooled samples the PTCR response is dominated by an outer shell on individuals grains, whereas for slowly cooled samples the grain boundary resistance dominates. Annealing in reducing atmospheres destroys the grain boundary PTCR effect whereas the outer-shell grain PTCR effect is relatively insensitive to the reducing atmosphere. It is proposed that the acceptor states responsible for the outer-grain and grain-boundary PTCR effects are predominantly intrinsic metal vacancies, i.e. Ba and/or Ti, and adsorbed oxygen, respectively.

Stoichiometry and stability of bismuth vanadate, Bi4V2O11, solid solutions

The phase reported in the literature as Bi4V2O11 is a solid solution phase whose composition covers the range ∼66.7 to 70.4% Bi2O3. The phase diagram of the Bi2O3ue5f8V2O5 system in the region of this phase has been determined and electrical properties of the Bi4V2O11 solid solutions measured. The high temperature γ polymorph is an excellent oxide ion conductor, whose conductivity decreases gradually with increasing Bi2O3 content. The low temperature α polymorph is a ferroelectric whose permittivity appears to increase to a maximum at the α→β transition temperature.

Dielectric properties of spark-plasma-sintered BaTiO3

Spark-plasma-sintering (SPS) has been applied to BaTiO3 to prepare dense ceramics consisting of submicrometre-sized powder. Relatively dense (typically 97% of the theoretical X-ray density) pellets with an average grain size remaining similar to that of the starting powder, approximately 0.6 μm, were obtained by the SPS process. Fixed frequency (1 kHz) measurements show the room temperature permittivity of SPS ceramics to be relatively high, approximately 3500, and at least double the value of conventionally sintered ceramics, approximately 1500. Alternating current (a.c.) impedance spectroscopy measurements show that SPS is an effective process to reduce the influence of intergranular (grain boundary) effects on the permittivity and direct current (d.c.) resistance characteristics of BaTiO3 ceramics substantially.

Ionic and electronic conduction in La0.95Sr0.05GaO3−δ, La0.95Sr0.05AlO3−δ and Y0.95Sr0.05AlO3−δ

Abstract The syntheses of La 0.95 Sr 0.05 GaO 3− δ (LSG), La 0.95 Sr 0.05 AlO 3− δ (LSA) and Y 0.95 Sr 0.05 AlO 3− δ (YSA), and their electrical properties using constant frequency (10 kHz) a.c. conductivity measurements as a function of oxygen partial pressure ( P O 2 ranging from air to 10 −15 Pa) and temperature (800–1000°C) are described. Ion-blocking measurements were also performed on some compositions. All materials showed dominant ionic conductivity within limited ranges of working conditions, although their ionic domains differed considerably in magnitude. LSG samples had the highest ionic conductivity, with only a minor p-type contribution in air. LSA had intermediate conductivity, with p-type conductivity dominant in air. YSA samples had the lowest conductivity of all those measured, again with dominant p-type conductivity in air. All materials were stable within large P O 2 domains, and exhibited negligible n-type conductivities even under reducing conditions typical of fuel-rich atmospheres.

Superconductivity in La-doped Bi “2201”

Abstract La 3+ substitution into the Bi 2201 structure, of the “ideal” composition Bi 2 Sr 2 CuO δ , has been investigated by means of an extensive phase diagram study into the solid solution formation at 885°C in air. Single phase solid solutions may be presented by the formula Bi 2+ x Sr 2- x - y La y CuO δ , where y has a maximum value of 0.6. With increasing La content, the crystal symmetry changes from orthorhombic to tetragonal and the incommensurate supercell becomes less pronounced, as indicated by a decrease in intensity of the supercell X-ray powder reflections. Superconducting T conset values vary systematically with cation contents x,y and oxygen content δ; a maximum T c of 36.5 K is observed for composition x =-0.05, y =0.4.

Synthesis and characterisation of La0.95Sr0.05GaO3-δ, La0.95Sr0.05AlO3-δ and Y0.95Sr0.05AlO3-δ

Abstract The oxide-ion conducting phases La 0·95 Sr 0·05 GaO 3−δ (LSG), La 0·95 Sr 0·05 AlO 3−δ (LSA) and Y 0·95 Sr 0·05 AlO 3−δ (YSA) have been synthesised, either by solid state reaction or through a chemical route. Structural and microstructural characterisation was carried out using XRD and SEM/EDS techniques and a.c. impedance spectroscopy (250–1000°C) was employed to determine the electrical properties. Although low-temperature impedance results were strongly dependent on phase purity and microstructure, high temperature electrical conductivity data could be used to compare the electrical conductivities of LSG, LSA and YSA. Chemical modification from the well-known Sr-doped lanthanum gallate via replacement of different cations on the A and/or B site decreased the conductivity, increased the activation energies and decreased the ionic conductivities. From the results, there is no simple correlation between oxygen ion conductivity and geometric aspects related to unit cell parameters and cation radii. Instead, cation polarisability seems to play an extremely important role in designing improved oxygen ion conductors based on the perovskite structure.

Electrical properties of Cu2P3I2

Abstract Conductivity data on single crystal and cold-pressed polycrystalline samples of Cu 2 P 3 I 2 demonstrate that Cu + ion migration dominates the electrical conductivity over the temperature range 25–300 °C in argon. Both sets of samples exhibit fully reversible Arrhenius-type conductivity behavior with activation energy values of ~0.45eV and typical conductivity values of ca. 1.90 × 10 −3 S.cm −1 at 186 °C. The conductivity appears to be one-dimensional.

Densification and conductivity enhancement of Na4Zr2Si3O12-based solid electrolytes using TiO2 as a sintering aid

Abstract Densification of Na 4 Zr 2 Si 3 O 12 (NZS) solid electrolytes was performed by dispersing TiO 2 (0.8–5.9 wt.xa0%, corresponding to 5–30 molxa0%) in NZS powders prior to sintering at 1200°C. Increases in pellet density, from ca. 65 to 94% of the theoretical (X-ray density) value, and in electrical conductivity from 10 −7 to 10 −6 S/cm at 50°C were observed for small additions of TiO 2 , which acts as a sintering aid. AC impedance spectroscopy reveals that the enhancement is not a bulk effect but instead is associated with a reduction in inter-granular constriction resistances within porous NZS ceramics. The presence of adsorbed water species in NZS powders prepared via a sol-gel route is found to have a dramatic effect on the conductivity enhancement.

A high pressure-high temperature study of Bi2Sr2MnO6+δ(0.4 ≤ δ ≤ 0.7)

A high pressure-high temperature study of Bi{sub 2}Sr{sub 2}MnO{sub 6+{delta}} samples were subjected to a high pressure-high temperature process and the effects on crystal structure and magnetic properties recorded. The unit cell was found to expand anisotropically with the c axis length increasing at four times the rate of the a and b axes where (da/dP) = (db/dP) = 1.0 {+-} 0.2 {times} 10{sup {minus}3} {angstrom}MPa{sup {minus}1} and (dc/dP) = 3.5 {+-} 0.5 {times} 10{sup {minus}3} {angstrom}MPa{sup {minus}1} with an overall unit cell volume expansion of 1%. The net oxidation state of the manganese decreased from +3 towards +2 with a resultant increase in T{sub N}. These changes can be attributed to the effect of removing interstitial oxygen from the structure which primarily changes the oxidation state of the manganese.