Examining the Composition of Lead-Free Piezoelectric Ceramics
Currently, Ni is used in the inner electrodes of monolithic ceramic capacitors. In monolithic ceramic capacitors, the compositional transformation of ceramics having BaTiO3
as the main component allows sintering under a reducing atmosphere for achieving co-firing with Ni inner electrodes. A base composition of BaTiO3
of monolithic ceramic capacitors also has piezoelectric properties, but the Curie temperature is 130°C, which is too low for use in piezoelectric actuators. A Curie temperature of 200°C or higher is preferred for use in piezoelectric actuators. A wide variety of composition types are known for lead-free piezoelectric ceramics having a Curie temperature of 200°C or higher and suitable for piezoelectric actuators, including (K,Na) NbO3
-based ferroelectric ceramics, (Bi1/2Na1/2) TiO3
-based ferroelectric ceramics, tungsten-bronze structured ferroelectric ceramics, and bismuth layer-structured ferroelectric ceramics. However, in compositions that include Bi as the main component, the Bi2
becomes metal Bi when fired in a reducing atmosphere, and so, in principle, co-firing with Ni is difficult to achieve. For this reason, we selected (K0.5
-based piezoelectric material, which has relatively favorable piezoelectric properties and does not include Bi, as the lead-free piezoelectric material.
To perform co-firing with Ni, at the very least, Ni must be sintered in a reducing atmosphere where Ni does not oxidize for obtaining the piezoelectric properties. This means that, first, firing was performed in a reducing atmosphere in a single-plate shape not including the inner electrodes, and compositions that produced favorable piezoelectric properties were examined. Firing in a reducing atmosphere was performed for the three compositions below with 5mol% of MnCO3 added as a sintering aid.
・0.96 (K0.5Na0.5) NbO3-0.04CaTiO3+0.05mol MnCO3 (KNN-CT)
・0.96 (K0.5Na0.5) NbO3-0.04CaZrO3+0.05mol MnCO3 (KNN-CZ-1)
・0.96 (K0.5Na0.5) NbO3-0.04CaZrO3+0.03molZrO2+0.05mol MnCO3 (KNN-CZ-2)
The density and insulation resistivity that were obtained in the sintered body are shown in Table 1.
||Sintered body density
||4.5 x 103 (98%)
||7.7 x 104
||4.0 x 103 (87%)
||1.6 x 106
||4.5 x 103 (98%)
||6.3 x 108
*The relative density is indicated in parentheses ( ) .
Table 1 Sintered Body Density and Insulation Resistivity of Single Plate Fired in Reducing Atmosphere