Sintering of stacked pellets for controlling the temperature coefficient of dielectrics

Bearing in mind the excellent dielectric properties at high frequency of the oxides Ba5Nb4O15, BaNb2O6, ZnNb2O6 and Zn3Nb2O8 (ɛr ∼ 20–45, tan(δ) < 10 × 10−4, ρi > 1010 Ω cm), “glass composites” of these materials were investigated in order to control their permittivity temperature coefficients for various applications. In a first method, samples constituted of mixed powders with temperature coefficient of opposite sign have been conventionally sintered. The obtained properties are not the expected ones because of the high reactivity between the mixed phases. In a second method, samples constituted of two stacked pellets with temperature coefficient of opposite sign have been co-sintered by the sinter–forging process, i.e. while maintaining a pressure, the two stacked pellets were co-sintered, leading to a good mechanical behaviour. More importantly we show that this technique avoids the delamination at the interface and that the so obtained dielectric properties are globally in good agreement with the mixing rule. Clearly, it is demonstrated that the temperature coefficient of these dielectrics can be tuned using the sinter–forging process, opening a new route for the optimization of multilayer capacitors.