BaTiO3-NaNbO3 based solid solutions for high field, temperature-stable multilayer ceramic capacitors

Barium titanate (BT) and sodium niobate (NN) form a complete solid solution across the compositional range. Ti4+ and Nb5+ are known as ‘d0 ions in which the outer s-shells are full, but the d-shells empty which engenders a large ionic polarizability and therefore permittivity when in octahedral coordination. Previous studies have focused on the low field dielectric performance or the piezoelectric/strain behaviour of this solid solution. To date, there have been few systematic studies of the structure - high field/energy storage performance of the BT-NN solid solution. In this thesis, the BT and NN rich ends are explored with a view to developing next generation, high field and high energy density multilayer ceramic capacitors (MLCCs).
For the BT rich end, a heterogeneous microstructure was present based on two broad anomalies in the permittivity vs. temperature curves. BT10NN was selected to investigate the role of donor and acceptor doping on conductivity. Mg2+ (0≤x≤0.05) was used as an acceptor dopant. However, XRD patterns suggest that Mg2+ at least in part substituted on the A rather than B-site compensating for Na+ loss at low concentrations before entering the B-site at higher values of x (>0.02). As x increased, the conduction mechanism changed from n to p-type for x = 0.02, before reverting to n-type as x increased. Doping with Mg2+ further enhanced the temperature-stability to achieve X6R specification in the Electronic Industry Alliance codes. At x > 0.02, a single broad peak was observed in the permittivity vs. temperature data but impedance spectra nonetheless revealed two components in the electrical response. An energy density (Wrec) of 3.4 J/cm3 was achieved with efficiency (η) of 82.6% for x = 0.01 but higher concentrations of Mg2+ promoted the formation of oxygen vacancies (n-type behaviour) and led to inferior dielectric performance.
For the NN rich end of the solid solution, the crystal structure transformed from orthorhombic to tetragonal phase with increasing BT concentration (0≤x≤0.25). In addition, the oxygen transport number (tion) was reduced by two orders of magnitude. Polarisation – Electric field (P-E) response and permittivity-temperature data was obtained for all compositions and based on these measurements, NN15BT and NN25BT were chosen as suitable materials for further optimization. Based on previous work, a third end member (Nd(Mg2/3Nb1/3)O3, NMN) was added to the solid solution to decrease the correlation length of polar order.1 Compositions were pseudocubic for x > 0.02 and phase transitions shifted to below room temperatures. NMN generally suppressed the polarisation but increased Emax (maximum pulsed field before breakdown) giving rise to Wrec = 3.95 J/cm2 for NN15BT10NMN ceramics with η = 70.67 % and Wrec = 1.62 J/cm2 and η = 81.43 % for NN25BT10NMN.
Within the explored compositions, Mg doped BT-NN (x = 0.01) showed the highest energy density and best temperature stability and therefore was chosen to fabricate multilayers. Multilayers showed good temperature stability (X6R) and energy storage performance with Wrec of 3.36 J/cm3 and η of 80.6 %, confirming the potential of BT-NN solid solutions for high field MLCC applications, particularly considering that they are Bi, rare-earth as well as Pb free.