Novel materials and routes for rare-earth-free BaTiO3-based ceramics for MLCC applications.

The NaNbO3-BaTiO3 (NNBT) solid solution was studied as a novel RE-free material for MLCC applications. Relaxor behaviour was found for NaNbO3 (NN)-concentrations as low as 2 mol%. The solid solution changes its behaviour with increasing NN-concentration from ferroelectric, to mixed ferroelectric-relaxor, to relaxor, to mixed behaviour again and finally ferroelectric. Broad permittivity profiles could therefore be obtained for a number of compositions with a wide range of Tmax, Na0.9Ba0.1Nb0.9Ti0.1O3 (90NNBT) possessed an industry standard (X7R) of TCC = ±15 % from -55 to 125 °C with low dielectric loss and a RT permittivity of ~ 800.
Bilayers were then used to imitate CS microstructures and improve TCC. Optimisation of ‘core’-like material, i.e. BT, and a ‘shell’-like material, i.e. 2.5NNBT, in a bilayer at a volume ratio of 0.67 2.5NNBT with 0.33 BT resulted in a TCC of ±6% over the temperature range of 25 to 125 °C whilst maintaining a RT permittivity ~3000 and low dielectric loss. Utilising simulations of bilayer permittivity profiles reduced the number of trial and error compositions required to achieve permittivity and TCC targets. One limitation, however, was the interfaces that form, as they add an additional unaccounted component to the series model used. Their impact was reduced through careful processing.
BT-2.5NNBT-90NNBT trilayers resulted in extended temperature range for low TCC applications, pushing the upper temperature up to over 150 °C. 0.33(BT)-0.33(2.5NNBT)-0.33(90NNBT) maintains a TCC of ±15 % to over 150 °C, with RT permittivity values above 100 and low dielectric loss. Adapted ternary phase diagrams were used to identify compositions that led to lower TCCs.
Several important observations were drawn from the bi- and trilayer systems which suggested that that low TCC capacitors may be developed for any temperature range by the following protocols: (i) choose a temperature range, i.e. 100-200 °C; (ii) choose a material that possesses a Tmax of around 100 °C; (iii) choose a material with Tmax a little above 200 °C and (iv) choose a third material that possesses a Tmax that sits in the middle of the previous two materials, or has a broad shoulder that spans the gap between the other two Tmaxs. The number of materials are varied depending on the required temperature range. In general, the lowest number of materials that gives the required TCC should be chosen. This concept was tested for the creation of a temperature stable plateau
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between 100 and 200 °C by a BT-85NNBT-90NNBT trilayer. The permittivity-temperature profile shows a plateau between ~100 and ~200 °C with permittivity changes of ~ ±10 % in that temperature range.
Industrial MLCC prototypes based on the hypotheses from this work were made by AVX Ltd in Coleraine. The devices possessed comparable TCC and better lifetimes compared with equivalent commercial products.