Vanadium-doped and mixed Si/Ge-based apatites: synthesis, crystal chemistry and ionic conductivity
Date of Issue2014
School of Materials Science and Engineering
Apatite-type oxides taking the general formula [AI4][AII6][(BO4)6][X2±y], particularly those of the rare earth silicate and germanate systems, are among the more promising materials being considered as alternative solid oxide fuel cell electrolytes. Nonstoichiometric lanthanum silicate and germanate apatites display pure ionic conductivities exceeding those of yttria-stabilized zirconia at moderate temperatures (500 – 800 ºC). Indeed, it is the underlying complexity and flexibility of these substances that endows them with such promise as ion conductors. Many factors influence the ionic mobility, the most important being the concentration of cation vacancies and excess anions incorporated in the structure, the valence and atomic size of the substitutional elements, crystal/grain orientations, and phase purity. In this thesis, the main focus falls on the synthesis and characterization of various apatite electrolyte materials and establishing underlying correlations between oxygen ion percolation and crystal chemistry. Apatites can be synthesized in various ways, including solid state sintering, sol-gel methods and hydrothermal reactions. The former is favored since large quantities are readily fabricated for division and multi-technique characterization, while the latter has the advantage of lower synthesis temperature. This thesis examined two synthesis methods (solid state and hydrothermal) for powdered apatites, and the growth of single-crystal apatites for investigating anisotropic ion migration. Laboratory X-ray, synchrotron, and neutron diffraction were employed to refine crystal structures by the Rietveld method. The microstructures, local structures and chemical compositions were analyzed by electron diffraction, spectroscopic and microscopic methods. Vanadium-doped apatites of nominal composition [La8AE2][Ge6-xVx]O26+x/2 (AE = Ca, Sr, Ba; 0 ≤ x ≤ 1.5) were prepared by conventional solid state sintering. Single-phase products were obtained for x ≤ 0.5, with a combination of powder synchrotron X-ray and neutron diffraction confirming these products to be P63/m apatites. The ionic conductivities extracted by complex impedance spectroscopy showed that small vanadium amendments enhanced oxygen mobility at intermediate temperatures (500 – 800 ºC) by more than one order of magnitude, as the incorporation of V5+ through displacement of Ge4+ is charge balanced with interstitial O2- that improves ionic conduction. The most promising composition was La7.88Ca2Ge5.35V0.65O26.15 that delivered σ = 3.44×10-4 S•cm-1 at 500 ºC. The superstoichiometric oxygen was delocalised, without fixed X-ray or neutron scattering centres. Crystal chemistry systematics demonstrate that the Ca-apatite was superior because the relatively small framework expanded through (La/AE)O6 metaprism twisting (φ) that widened the tunnel, ensuring P63/m symmetry was adopted, which favours the passage of O2- with lower activation energy. Mixed Si/Ge-based apatites were prepared by hydrothermal synthesis under mild conditions, rather than the conventional solid-state method at high temperatures. Single-phase and highly-crystalline nanosized apatite powders were obtained with the morphology changing across the series from spheres for the Si-based end-member to hexagonal rods for the Ge-based end-member. Powder X-ray and neutron analysis found all these apatites to be hexagonal (P63/m). Quantitative X-ray microanalysis established the partial (< 15 atomic percent) substitution of La3+ by Na+ (introduced from the NaOH hydrothermal reagent) showed a slight preference to enter the AI 4f framework position over the AII 6h tunnel site. Moreover, retention of hydroxide (OH-) was confirmed by infrared spectroscopy and thermogravimetric analysis, and these apatites are best described as oxy-hydroxyapatites. To prepare dense pellets for conductivity measurements, both conventional heat treatment and spark plasma sintering methods were compared, with the peculiar features of hydrothermally synthesized apatites and influence of sodium on ionic conductivity considered. Growth of single-crystal apatites [Nd9.33-x+y/3Lax][Si6-yGay]O26 (0 ≤ x ≤ 2; 0 ≤ y ≤ 0.2) was successful by the floating-zone method and preliminary work undertaken on these materials. It was shown that the growth direction of the single crystals without a seed was distinct, and the ionic conductivity was anisotropic and higher, than that for powdered apatites. Future investigations of the correlations between crystal chemistry and ionic conductivity on this apatite series will be necessary to establish the optimal electrolyte composition.
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