Oxyapatites as low temperature solid electrolytes : influence of crystal chemistry and physical form

• Main Author: An, Tao
• Other Authors: Christian Leo Kloc
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Science::Chemistry::Crystallography::Chemical crystallography; DRNTU::Engineering::Materials::Energy materials; DRNTU::Science::Chemistry::Crystallography::Crystal structure and growth; DRNTU::Science::Chemistry::Crystallography::X-ray crystallography; DRNTU::Science::Chemistry::Crystallography::Electron microscopy
• Online Access: http://hdl.handle.net/10356/60488
• Institution: Nanyang Technological University

Lanthanoid silicates (Ln9.33Si6O26) adopt the hexagonal apatite structure and are under investigation as solid oxide fuel cell (SOFC) electrolytes due to their high oxide ion conductivity at intermediate temperatures (500 – 700°C). This superior property is attributed to an interstitial mechanism of ion migration, rather than the vacancy mediated diffusion found in other electrolyte families. In a formal sense, the cation-deficient Ln9.33+2x/3Si6O26+x apatites can be described as polysomes, where Ln vacancies are accommodated with the co-creation of oxygen interstitials. These defect clusters play a crucial role in O2- transport, and result in highly anisotropic mobility along the large-diameter c axis channels. In addition, conduction through the framework walls surrounding the channels is now seen as significant. To better understand the relationship between these structural defects and conductivity, large single crystals of pure Nd9.33Si6O26 apatite and chemical analogues containing Al, Sr, and Cr were grown. Specifically, neodymium silicates, Nd9.33+x/3AlxSi6-xO26 (0 ≤ x ≤ 1.5), Nd8+xSr2-xSi6O26+x/2 (0 ≤ x ≤ 0.2), and Nd9.33CrxSi6-xO26+x/2 (0.1 ≤ x ≤ 0.4) were for the first time synthesised as appreciable crystals, using the floating-zone method. The products were homogeneous, transparent, and crack-free for Nd9.33+x/3AlxSi6-xO26 with 0 ≤ x ≤ 1.0, while minor fracture was present in Nd9.83Al1.5Si4.5O26, Nd8+xSr2-xSi6O26+x/2 (0 ≤ x ≤ 0.2), and Nd9.33CrxSi6-xO26+x/2 (0.1 ≤ x ≤ 0.4) due to internal stress generated by differences in metallic ionic radii or the insertion of interstitial oxygen. Crystal quality was validated by neutron diffraction and synchrotron X-ray rocking curve diffraction. The incorporation of aluminium into apatite was independently confirmed by energy dispersive X-ray spectroscopy (EDX) microanalysis, while for the Al3+, Sr2+ and Cr5+ series, lattice parameter dilation with increasing dopant content was established by powder X-ray diffraction (PXRD). The crystallographic orientation of these crystals was confirmed by polarising optical microscopy and electron backscatter diffraction (EBSD), with ionic conductivity measured over a wide range of temperature both parallel to and perpendicular to the c-axis. Conductivity was anisotropic and an order of magnitude higher parallel to the c axis. Oxygen transport in undoped Nd9.33Si6O26 at an intermediate temperature of 500°C surpasses other oxide electrolytes by an order of magnitude, with a slight negative inflection observed in the conductivity plot at ~500°C, reflecting a change in conduction mechanism. Al-doping improved the O2- mobility across the ab plane, but degraded performance along the c axis, and removed the inflection observed in the pure apatite. In contrast, Sr-doped stoichiometric Nd8Sr2Si6O26 has lower conductivity both //c and ┴c, while extra-stoichiometric oxygen in Nd8.2Sr1.8Si6O26.1 improved conductivity //c. Undoped Nd9.33Si6O26 and Al-doped Nd9.67Al0.5Si5.5O26 crystals subjected to long-term annealing (950°C/3 months), to mimic deployment in an SOFC, displayed poorer O2- mobility //c and removed the inflection in Nd9.33Si6O26. The conductivity of Nd9.67Al0.5Si5.5O26 also decreased after extended heat treatment, but to a lesser extent. Crystallographic analysis with X-ray and neutron diffraction confirmed P63/m symmetry for all single crystals, and the latter technique also established the distribution and concentration of oxygen interstitials, where greater abundance need not always correlate with improved ionic conductivities, due to the interplay between activation energy and diffusion pathways. For the undoped Nd9.33Si6O26 crystal, a relatively low concentration of interstitial oxygen enabled a dual-conduction path – either inter-tunnel or intra-tunnel path – whose relative contributions were temperature dependent, and proved to be the origin of the conductivity inflection observed in [001] Arrhenius plots. Both Al-doping and annealing caused the tunnel interstitials to shift towards the framework, decreasing the conductivity along [001] and in the former case, higher framework interstitial concentration boosted O2- mobility across the ab plane. The lowest conductivity, found in stoichiometric Nd8Sr2Si6O26, can be attributed to the elimination of tunnel interstitials together with the absence of extra-stoichiometric oxygen. Nd framework vacancies and the distribution of Sr were found to be homogeneous at the atomic scale by scanning transmission electron microscopy (STEM) that verified X-ray and neutron structure solutions. Dense polycrystalline ceramics with similar compositions to the single crystals were fabricated by spark plasma sintering (SPS) to quantitatively investigate the role of grain boundaries in oxygen migration. The phase assemblages were inspected by PXRD, which also confirmed >90% relative density, while EBSD was used to study microstructures and grain size distributions. Impedance measurements showed that for Al-doped samples, the decrease in ionic conductivity along c was more than offset by the combination of increased conductivity across ab and reduced steric hindrance, resulting in higher overall mobility. Alkaline earth doping was also found to be beneficial, but less effective than Al-doping, due to the elimination of cation vacancies in the former. A valuable future study would be the fabrication of a prototype SOFC using membranes of these dense polycrystalline ceramics to evaluate their efficiency and durability.