Development of new Sodium Niobate-Barium Titanate (NNBT) solid-solution formulations to improve dielectric-properties of Multi-Layer Ceramic Capacitors (MLCC) for X9R-grade applications

Solid solutions of NaNbO3(NN) and BaTiO3(BT) having four different NN-mol% (60, 70, 80, 87.5) were studied to widen the range of application temperature (-55°C to 200°C) with an improved temperature coefficient of capacitance. These novel NN-based compositions not only help reduce the dependence on scarce RE-dopants (Dy, Ho, etc.) but also broaden the permittivity peak over the relevant temperature range. Multiple characterisation techniques were used to establish the performance and to confirm the structure-property relationship for permittivity and dielectric loss at different field frequencies.
Single-phase NNBT-perovskite composition was achieved by sintering at 1250°C post-calcination. Dense ceramic-microstructure with rounded grain boundaries indicated initiation of melting. 60NNBT and 70NNBT with a cubic lattice at RT exhibited relaxor-type behaviour showing pronounced frequency-dependence. At the low-temperature regime, the dielectric loss for these relaxors were above the acceptable 5%-mark. 80NNBT and 87.5NNBT displayed conventional ferroelectric behaviour with sharp permittivity peaks while dissipation factor remained below 0.035 across the temperature range of interest. Thermal characterisation confirmed that ferroelectric to paraelectric transformation temperatures for 80NNBT and 87.5NNBT are at 52 and 178oC, respectively. These transitions were associated with hysteresis displayed through the heating-cooling cycle. With increasing NN-mol%, a contraction in unit-cell volume is observed.
A composite pellet made from 70NNBT and 87.5NNBT was studied. It exhibited high dielectric loss at low frequencies (1 and 10kHz) beyond 200oC. At higher frequencies (100, 250, and 1000kHz), the tanδ-value remained ⁓0.04. An increase in the loss at >300oC irrespective of frequency, was due to long-range migration of charge-carriers. The Tmax-value (85°C) remained between the Tmax-values of individual components. A single peak in the permittivity profile signifies interdiffusion and homogenization of 70NNBT and 87.5NNBT. This was corroborated by XRD since all the peaks could be indexed using a tetragonal scheme. To broaden the permittivity peak, the individual ferroelectric responses of 70NNBT and 87.5NNBT need to be preserved by preventing the interdiffusion.