The synthesis and characterisation of ceramic composites and A-site deficient perovskites as dielectrics for multi-layer ceramic capacitors

This thesis will regard two differing mechanisms for the enhancement of temperature stable dielectrics in NaNbO3 related compositions, with the aim of synthesising dielectrics meeting X7R and X8R rated Temperature Coefficient of Capacitance (TCC) and/or dielectrics with prospective use for energy storage applications. The first of these mechanisms will be the formation of NaxBa1-xNbxTi1-xO3 (XNNBT) based ceramic-ceramic composites with 3-0 dimensional interconnectivities between its BaTiO3 (BT) matrix phase and homogeneously distributed relaxor NNBT secondary phases. The formation of this composite is based on the ability to retain compositional heterogeneity post sintering due to the thermodynamic stability of relaxor components of the NNBT solid solution. X7R rated TCC is achieved for a BT-60NNBT composite with a 80:20 wt.% phase fraction, with a room temperature permittivity magnitude over 2000 and tan δ <0.05 across a -150 to 400 °C temperature range. This TCC specification can be extended to X8R rating with the doping of 7 mol% Ca onto the A site of the BT matrix pre sintering. These B(Ca)T-60NNBT ceramic-ceramic biphasic composites exhibit a linear electric field dependence of polarisation, with moderate energy storage efficiencies ~ 77 % and Wrec ~ 0.56 Jcm-3 when 2 wt.% glass sintering additive is used. The formation of the composite is reproducible from hand-mixing of its component phases to ball milling of larger batches, whilst X8R TCC rating can be replicated with a ball milling of a stoichiometric mix of NN, BT and BCT commercial nano-powders.
The second mechanism for enhancing temperature stability of NaNbO3 dielectrics is the increase in A site vacancy concentration through aliovalent La3+ A site doping, along the NaNbO3 – La1/3NbO3 solid solution, LaxNa1-3xNbO3 (XLNN). X-ray Diffraction and Analytical Scanning Electron Microscopy analyses verify A-site vacancies to form as charge compensating defects, accommodated through a series of compositionally driven phase transitions induced by the competing influence and variation in the cooperative tilt network of the [NbO6] octahedra, cationic displacements and ordering of A-site species. A range of dielectric behaviours is observed across the solid solution. For 7.5LNN and above, an order-disorder transition in Nb displacements induces a suppression in TCC from that associated with NaNbO3, with dielectric behaviour retained. TCC is, however, unsatisfactory for X7R or X8R rating. Mixed Na+/n-type conductivity is induced for 15LNN and above, which is optimised in 25LNN, with a conductivity 8.0 x 10-5 Scm-1 at 400 °C. The La1/3NbO3 end member shows comparable mixed ionic-electronic conduction, where the ionic species is undetermined. Linearisation of polarisation electric field dependence occurs above the order-disorder transition at ~ 7.5LNN, for which an energy storage efficiency ~ 72 % and recoverable energy density ~ 0.4 Jcm-3 is achieved for 10LNN. The solid solution offers a range of properties suitable for diverse dielectric and non-dielectric applications.