Electronic devices lately have followed a trend toward smaller size, necessitating integration and multilayer technology to improve their efficiency. Piezoelectric multilayer actuator represents one typical example of these new, smaller electronic devices with ever-increasing applications. A multilayer actuator inevitably uses an internal electrode- generally Ag/Pd alloy and is sintered with ceramics. The Pd in internal electrode reacts with Pb in ceramics during sintering at high temperature (above 1200℃), and that reaction causes defects in the device. High temperature sintering also promotes the loss of Pb in Pb-based perovskite materials. The reaction between Pd and Pb and the loss of Pb make the application of Pb-based perovskite materials problematic, although they generally have excellent piezoelectric and dielectric properties. To stabilize internal electrode and suppress the Pb loss, it therefore is necessary to lower the sintering temperature. A low sintering temperature can be attained by adding suitable dopants in conventional solid-state sintering.
The composition $Pb(Ni_{1/3}Nb_{2/3})O_3-PbZrO_3-PbTiO_3$ (PNN-PZT), which has excellent piezo- electric properties, have been applied to commercial multilayer actuator. PNN-PZT has perovskite structure ($ABO_3$) in which Pb is located at the A-sites and Ni, Nb, Ti, and Zr are randomly distributed over the B-sites. $Cd^{2+}$ has been investigated as sintering additive to obtain a low sintering temperature of PNN-PZT ceramics. It has been reported that the presence of a small amount of CdO (- 2mol%) remarkably increased the relative dielectric permittivity (1 kHz) and the piezoelectric constant and that high densities and improved properties were obtained in PNN-PZT ceramics doped with CdO. It has been also found that $Cd^{2+}$ plays a key role in low temperature sintering of ceramics both by lowering the sintering temperature and improving the piezoelectric properties.
Complex perovskite structure materials, such as PNN, contains two groups of doubly ionized cations. One group, consisting of Ba, Pb, and Sr etc., is located on A-site; the other, including Mn, Fe, Zn, Co, Ni, and Mg etc., may be located on B-site in complex perovskite structure. Based on ionic radius, $Cd^{2+}$ is located on the boundary between the two groups and thus assumed to be substituted both into A-site $Pb^{2+}$ and B-site $Ni^{2+}$. The most studies on Cd-doped PNN-PZT ceramics, however, have been assumed that $Cd^{2+}$ substituted A-site of perovskite structure without any evidence and have not explained the effect of the substitution site on properties, nevertheless the substitution site of $Cd^{2+}$ can influence significantly on sintering behavior and electrical properties. The purpose of this study was to investigate the effect of Cd-substitution site on sintering behavior and electrical properties in PNN-PZT ceramics.
By substituting $Cd^{2+}$ into both A-site and B-site in PNN-PZT ternary perovskite material, it is possible to determine the effects of the substitution site of $Cd^{2+}$ on sintering behavior. Sintering was performed in the temperature range from 1000℃ to 1300℃. Although $Cd^{2+}$ is substituted into both A-site and B-site in PNN-PZT, $Cd^{2+}$ prefers A-site to B-site. If $Cd^{2+}$ replaces $Pb^{2+}$, weight gain is observed during sintering process. On the contrary, if $Cd^{2+}$ replaces $Ni^{2+}$, weight loss is promoted during sintering. From these weight changes, it is believed that $Cd^{2+}$ changes the bonding strength between B-site cation and oxygen of octahedron in perovskite structure. The changes of lattice parameters as a function of $Cd^{2+}$ content were consistent with those of the bonding strength. One of the purposes of this study was to make up the equivalent circuit for bulk relaxation in order to interpret the electric and ferroelectric properties with a impedance spectrum technology. As ac field applied to dielectric or ferroelectric materials, the dipole displacement and the charge carrier migration occur simultaneously. With constructing the Debye equivalent circuit and a single RC circuit in parallel (briefly DR Circuit), two dispersions in only grain may be interpreted. The most relaxation behaviors of dielectrics and ferroelectrics can be represented with a combination of parameters in DR Circuit, not including the complicate mathematics. The effects of $Cd^{2+}$ substitution site on sintering behavior and electrical properties were also investigated. The densification mechanism were influenced by substitution site of $Cd^{2+}$, and A-Sample (specimen doped into A-site) and B-Sample (specimen doped into B-site) was sintered by liquid phase sintering and solid state sintering, respectively. Whereas $Cd^{2+}$ thus promoted the densification and grain growth in A-Sample, it retarded densification and grain growth in B-Sample. The substitution site of the $Cd^{2+}$ also influenced the electric and ferroelectric properties significantly. Whereas both of a grain and grain boundary influenced the resistivities in A-Sample, Only a grain effect on the resistivities was shown in B-Sample. When sintering time increased, the resistivities of a grain and grain boundary in A-Sample decreased and those of B-Sample revealed a drastical increases. The movement of charge carrier in A-Sample then was more free than that in B-Sample, and this was the cause for the shift of the Curie temperature with the Cd-doping.
