Glenn C. Mather

Ionic conductivity of La(Sr)Ga(Mg,M)O3−δ (M=Ti, Cr, Fe, Co, Ni): effects of transition metal dopants

Abstract Oxygen-ion conductivity of the perovskite-type solid solutions (La,Sr)Ga1−zM2O3−δ (M=Ti, Cr, Fe, Co; z=0–0.20), LaGa1−y−zMgyMzO3−δ (M=Cr, Fe, Co; y=0.10–0.20, z=0.35–0.60) and LaGa1−zNizO3−δ (z=0.20–0.50) was studied using the techniques of oxygen permeation, Faradaic efficiency, ion-blocking electrode and the e.m.f. of oxygen concentration cells. Oxygen-ion transference numbers vary from 2×10−6 to 0.98 throughout the series and p-type electronic conductivity increases with increasing transition metal content. Substitution of Ga with higher valence cations (Ti, Cr) decreases ionic conductivity whereas small amounts of Fe or Co (∼5%) increase ionic conductivity. For higher transition metal contents, lower levels of oxygen-ion conductivity and an increase in the activation energy, EA, for ionic transport, from 60 (5%-doped) to 230 kJ/mol (>40%-doped) are observed. In heavily doped phases, EA tends to decrease with temperature and, above 1170 K, values are similar to the undoped phase suggesting that an order–disorder transition takes place. Factors affecting the observed ionic conductivity trends are discussed.

A review of cation-ordered rock salt superstructureoxides

The types of structures formed when two or more cations occupy the sodium sites in the NaCl unit cell in a non-random manner are reviewed with reference to the cation arrangement in layers of close-packed octahedra and the cation arrangement in the NaCl subcell. Factors influencing the stability of particular structure types, including bond valence and radius ratios, are discussed.

Stoichiometry, structures and polymorphism of spinel-like phases, Li1.33xZn2 – 2xTi1 + 0.67xO4

A complete range of spinel-like solid solutions forms between Li4Ti5O12 and Zn2TiO4. Over a range of intermediate compositions, Li1.33xZn2 – 2xTi1 + 0.67xO4(0.50 ⩽ x ⩽ 0.90), the solid solutions are primitive cubic, but for compositions to either side, they are face centred. Rietveld refinement of powder X-ray diffraction data shows that ordering of Li and Ti onto two sets of octahedral sites is responsible for the primitive structure; Li and Zn are disordered over the tetrahedral sites. A fully ordered, ideal composition occurs at x = 0.75, Li2ZnTi3O8, which is isostructural with LiFe5O8. The phase diagram of the spinel join shows that the ordered, primitive cubic solid solutions transform to disordered, face-centred cubic solid solutions at high temperatures, with a maximum in the transition temperature at 1150 °C for x = 0.75. The subsolidus phase diagram Li2TiO3–TiO2–ZnO at 1150 °C has been determined.

Synthesis and characterisation of Ni–SrCe0.9Yb0.1O3−δ cermet anodes for protonic ceramic fuel cells

Abstract Cermet anode material with a proton-conducting ceramic phase, Ni–SrCe 0.9 Yb 0.1 O 3− δ , was synthesised by combustion from mixtures of molten nitrates and urea followed by sintering and reduction. Co-pressing the anode combustion powder on “green” SrCe 0.9 Yb 0.1 O 3− δ electrolyte and co-firing produced symmetrical anode/electrolyte/anode assemblies with planar electrodes of thickness ca. 100 μm. The addition of Co(NO 3 ) 2 ·6H 2 O to the electrolyte as sintering aid lowered the sintering temperature to 1250 °C, thereby allowing high anode porosity while retaining good anode/electrolyte adherence. The anode microstructure is composed of a uniform distribution of submicron Ni and perovskite particles. Analysis of the symmetrical assemblies by a.c. impedance spectroscopy indicates that the electrode polarisation resistances are composed of at least two contributions. Stability issues concerning the Ni–SrCe 0.9 Yb 0.1 O 3− δ anodes in reducing atmospheres are discussed.

Crystal structure of the Li+ ion-conducting phases, Li0.5 – 3x Re0.5 +x TiO3: RE = Pr, Nd; x≈ 0.05

The crystal structure of the C polymorph of the solid solutions, Li0.5 – 3x RE0.5 +x TiO3(RE = Pr, Nd) determined from powder neutron diffraction using Rietveld refinement, is an ordered perovskite of the GdFeO3 type. The structures of both phases are similar: the A sites contain (0.5 +x) RE cations and (0.5 – 3x) off-centre lithium ions, with 2x sites vacant. Orthorhombic unit cell, RE = Nd: a= 5.43661(7)A, b= 7.6647(1)A, c= 5.39752(7)A; RE = Pr: a= 5.43793(5)A, b= 7.66828(8)A, c= 5.40769(5)A; space group Pnma(no. 62). Conduction occurs by a Li+ vacancy mechanism through interconnecting A sites, avoiding those sites that are occupied by the RE cations.

The importance of phase purity in Ni–BaZr0.85Y0.15O3−δ cermet anodes – novel nitrate-free combustion route and electrochemical study

A novel, nitrate-free, combustion method has been developed to prepare Ni–BaZr0.85Y0.15O3−δ (Ni–BZY) cermet anodes for proton ceramic fuel cells. Nickel acetate and 30% H2O2 were used as starting precursors for the combustion reaction into which pre-prepared BZY powders were dispersed. The advantages of this nitrate-free combustion method have been demonstrated by comparison to a more common nitrate/glycine combustion route. The results demonstrate that use of the nitrate-free precursors is essential to avoid partial decomposition of the pre-prepared BZY phase. Employment of the more common nitrate-based precursors, due to their acidic nature, results in removal of Ba from the perovskite phase and formation of Ba(NO3)2, subsequently leading to the presence of BaY2NiO5 in the final product. The impact of Ni–BZY phase purity on resultant polarisation behaviour has been assessed as a function of water vapour and oxygen partial pressures for electrodes of comparable microstructure. Partial decomposition of the perovskite phase limits performance by increasing the higher frequency polarisation resistance and this phenomenon is suggested to be associated with impaired proton transport in the oxide cermet phase. The novel actetate–H2O2 combustion method here described may be of interest for the formation of other ceramic oxide materials, offering economical advantages over more classical nitrate-based combustion routes, as well as significant environmental benefits due to the avoidance of releasing NOx gases.

Synthesis and structures of the partially ordered rock salt phases, Li3M2XO6: MMg, Co, Ni; XNb, Ta, Sb

The crystal structures of four rock salt superstructure phases, Li3M2XO6(M2X Ni2Nb, Ni2Ta, Co2Ta and Mg2Nb) have been refined by X-ray and/or neutron powder diffraction; three members of this family were synthesized for the first time, M2X Ni2Nb, Ni2Sb and Co2Ta. All have similar rock salt superstructures but with partial order only, over three sets of octahedral sites, of the cations M2X. Differences in the distributions of site occupancies are discussed.

KLaF4 nanocrystallisation in oxyfluoride glass-ceramics

Nanocrystallisation of the cubic and hexagonal polymorphs of KLaF4 in a 70SiO2–7Al2O3–16K2O–7LaF3 (mol%) glass has been achieved by heat treatment above the glass transition temperature. For treatment at 580 °C, only the cubic structure crystallises, with a maximum crystallite size of ~9 nm. At higher temperatures, crystallisation of the hexagonal structure also takes place. The crystallisation process has been analysed using several thermal and structural techniques and is revealed to occur from a constant number of nuclei. The formation of a viscous barrier which inhibits further crystal growth and limits the crystal size to the nanometric range is observed. The title materials doped with lanthanide ions may be good candidates for optical applications.

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.

Surface proton conductivity of dense nanocrystalline YSZ

Ionic transport in nanocrystalline solid oxides is of considerable current interest. Several studies have reported room-temperature proton conductivity in nanoscaled 8 mol% Y2O3-doped zirconia (YSZ), although the location of the transport species is not clear. In this study, nanocrystalline YSZ with a grain size of ∼50 nm was prepared by spark-plasma sintering of nanoscaled commercial powder to a density of >97% of the theoretical value. Impedance spectroscopy was employed to analyze the electrical behavior in the temperature range 25–600 °C in H2O- and D2O-wetted atmospheres (air, O2 and 10% H2:90% N2). Transport in wet conditions below 50 °C is limited to the sample surface and occurs via proton hopping (Grotthus mechanism), as demonstrated by a conductive H+/D+ isotope effect. The impedance in these conditions is dominated by a single arc which can be modelled with parallel paths for proton transport on the lateral sample surface and a capacitance with values of the order of those of the grain interior. Surface proton transport is considerable, exhibiting a resistance at 26 °C in wet atmospheres comparable to that obtained at ∼300 °C. In contrast, transport by volumetric processes (grain, grain boundary or nanopores) was demonstrated to be insignificant by experiments involving conductivity measurements with a coated lateral surface, with electrode configurations of different areas, and emf measurements in a water-vapour concentration cell.