Alexander, S.J.; (2012) Discovery of heterometallic layered oxides using solid-state reactions of nano-Precursors. Doctoral thesis , UCL (University College London).

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Abstract

The synthesis of new solid-state materials is often a laborious task due to the low speed of diffusion in bulk solids, meaning each reaction requires hightemperatures and multiple steps. Shortening diffusion distances has been shown to increase reaction rates and lower reaction temperatures. This thesis addresses the need to increase the rate of solid-state materials discovery, by heat-treatment of nanosized precursors. The nano-precursors were synthesised using continuous hydrothermal flow synthesis, CHFS. In CHFS a flow of metal nitrate salts are brought into contact with a flow of supercritical water to precipitate metal oxides and/or hydroxides. The reaction between La(OH)3 and Ni(OH)2 co-precipitated using CHFS was investigated using in-situ X-ray diffraction. This resulted in the formation of La2NiO4 in 78 minutes, an order of magnitude faster than when using more traditional routes, highlighting the effectiveness of this approach. A high-throughput CHFS reactor was then used to synthesise La4Ni2.7M0.3O10-δ (where M = V, Cr, Mn, Fe, Co, Cu and Al). By calcining the nanoprecursors for these compositions in parallel it was possible to reduce the synthesis time to make twenty-four solid-state compounds to 12 hours. Structure and properties were screened and, La4Ni2.7V0.3O10-δ, La4Ni2.7Cr0.3O10-δ, La4Ni2.7Mn0.3O10-δ and La4Ni2.7Al0.3O10-δ were characterised. Subsequently this process was carried out using automation to increase the number of compositions synthesised. Firstly, for the La4Ni3-xFexO10-δ system (x = 0.0 – 3.0 and Δx = 0.1), 62 samples were synthesised, resulting in identifying a greatly increased phase boundary, up to a maximum Fe content of La4Ni2.06Fe0.94O10. Secondly for the La4Ni3-xMxO10-δ and La3Ni2-xMxO7-δ systems (x = 0.0 – 2.0. Δx = 0.2 and M = Mn, Al, Pd, Ga) in which 240 samples were synthesised. La4Ni3-xGaxO10-δ was isolated up to a maximum Ga content of x = 0.6, and La2Ni1-xPdxO4 could be synthesised with a maximum Pd content of x = 0.4.

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