Hideaki Niimi

High-Power Piezoelectric Vibration Characteristics of Textured SrBi2Nb2O9 Ceramics

The high-power piezoelectric vibration characteristics of textured SrBi2Nb2O9 (SBN) ceramics, that is bismuth-layer-structured ferroelectrics, were studied in the longitudinal mode (33-mode) by constant current driving method and compared with those of ordinary randomly oriented SBN and widely used Pb(Ti,Zr)O3 (PZT) ceramics. In the case of textured SBN ceramics, resonant properties are stable up to a vibration velocity of 2.6 m/s. Vibration velocity at resonant frequency increases proportionally with the applied electric field, and resonant frequency is almost constant in high-vibration-velocity driving. On the other hand, in the case of randomly oriented SBN and PZT ceramics, the increase in vibration velocity is not proportional to the applied high electric field, and resonant frequency decreases with increasing vibration velocity. The resonant sharpness Q of textured SBN ceramics is about 2000, even at a vibration velocity of 2.6 m/s. Therefore, textured SBN ceramics are good candidates for high-power piezoelectric applications.

Preparation of Multilayer Semiconducting BaTiO3 Ceramics Co-Fired with Ni Inner Electrodes

A multilayer semiconducting BaTiO3 ceramics having positive temperature coefficient of resistivity (PTCR) characteristics is investigated with the aim of lowering its resistance. The multilayer BaTiO3 ceramics was fired in reducing atmosphere and reoxidized at 700 °C in air. The result reveals that the multilayer semiconducting BaTiO3 ceramics with Ni inner electrodes shows high resistance even when the thickness is lowered; however, excessive donor doping markedly decreases the resistance of BaTiO3 ceramics to a very low value. The donor in excessive amounts is considered to cancel the Ni acceptors occupying the Ti sites; these acceptors diffuse from the Ni inner electrode and increase resistivity. As a result, a multilayer BaTiO3 ceramics element (3.2×2.5 mm2) with a low resistance of 0.05 Ω is realized by using excess donor doping (Sm=0.35 mol %).

Effects of Ba/Ti Ratio on Positive Temperature Coefficient of Resistivity Characteristics of Donor-doped BaTiO3 Fired in Reducing Atmosphere

The positive temperature coefficient (PTC) of resistivity characteristics of donor-doped BaTiO3 fired at 1350 °C in a reducing atmosphere of 1×10-7 MPa at the partial pressure of oxygen is investigated. The result reveals that BaTiO3 ceramics fired in such a reducing atmosphere usually exhibit low PTC characteristics, whereas La-doped BaTiO3 with compositions in the range of (Ba+La)/Ti1.02 exhibit pronounced PTC characteristics. A La content of less than 0.4 mol % is suitable for the PTC characteristics, while samples doped with more than 0.8 mol % La do not show the PTC characteristics. The PTC characteristics of semiconducting BaTiO3 fired in a reducing atmosphere are enhanced with a decrease in the ion radius of a donor ion in the order La<Nd<Sm, therefore, semiconducting BaTiO3 doped with Sm shows pronounced PTC characteristics with a resistivity increase of more than 6 orders of magnitude. However, the resistivity of BaTiO3 ceramics doped with Y, whose ion radius is less than that of Sm, is extremely high resistivity that decreases with increasing temperature, thereby indicating only a negative temperature coefficient (NTC) characteristics.

Influence of distribution of additives on electrical potential barrier at grain boundaries in ZnO-based multilayered chip varistor

The influence of the distribution of additives on the electrical potential barrier at grain boundaries in a ZnO-based multilayered chip varistor was investigated by scanning surface potential microscopy (SSPM) and transmission electron microscope (TEM). The leakage current strongly depended on the distribution of additives, and a more uniform distribution improved the electrical nonlinearity. SSPM images revealed that the grain-boundary resistance of a uniformly distributed sample was higher than that of a poorly distributed sample. The decrease in the electrical nonlinearity due to the poor distribution was attributed to a decrease in the potential barrier height instead of an increase in the inactive grain boundary. The TEM analysis indicated that the poor distribution of Pr ions interfered with the formation of the potential barrier and resulted in the decrease in its height.

Effect of Sn2+ Ion Substitution on Dielectric Properties of (Ba,Ca)TiO3 Ferroelectric Ceramics

In this study, we investigated the sintering atmosphere dependence of the electric properties of Sn-doped (Ba0.82Ca0.13)TiO3 ceramics to clarify the effect of Sn2+ ions. The temperature of the dielectric constant peak for Sn-doped (Ba0.82Ca0.13)TiO3 ceramics sintered in a low P(O2) (1×10-11 MPa) atmosphere is higher than that for the sample sintered in a high P(O2) (2×10-2 MPa) atmosphere. The remanent polarization and coercive field for the Sn-doped (Ba0.82Ca0.13)TiO3 ceramics sintered at low P(O2) (1×10-11 MPa) also increased compared with those of the sample sintered at high P(O2) (2×10-2 MPa) at room temperature. The low P(O2) sintering atmosphere for Sn-doped (Ba0.82Ca0.13)TiO3 ceramics leads to a high tetragonality and the existence of Sn2+ ions. In addition, by X-ray absorption spectroscopy, it was verified that Sn2+ ions occupy Ba2+ sites in the (Ba0.82Ca0.13)TiO3 ceramics. Consequently, it is concluded that the high ferroelectric-to-paraelectric phase transition temperature and the high remanent polarization on the ceramics originate from Sn2+ ions substituted for Ba2+ sites in Sn-doped (Ba0.82Ca0.13)TiO3 ceramics.

Resistance switching and retention behaviors in polycrystalline La-doped SrTiO3 ceramics chip devices

Resistance switching and retention behaviors in polycrystalline La-doped SrTiO3 ceramics have been investigated. La-doped SrTiO3 ceramics exhibits resistance switching as large as that of thin-film devices and exhibits a long-term memory effect of over 5 h. By means of a complex impedance analysis, it was clarified that these resistance changes can be reasonably attributed to the change in the electrical potential barrier at the interfaces and the resistance of the grain boundary changes remarkably by the application of voltage pulses. From the resistance retention properties at various temperatures, it was found that the high-resistance state is very stable even at 125 °C, whereas the resistance in the low-resistance state increases with time and its relaxation speed becomes remarkably faster with increasing temperatures. These results imply that the migration of the point defects could change the distribution of the space charge near the interface, resulting in a change in the interface resistances.

Effect of local electrical properties on the electrostatic discharge withstand capability of multilayered chip ZnO varistors

The local electrical properties at individual grain boundaries of multilayered chip varistors composed of ZnO–Bi2O3 (Bi–ZnO) and ZnO–Pr6O11 (Pr–ZnO) ceramics have been investigated using a scanning probe microscope (SPM) to clarify their effect on the electrostatic discharge (ESD) withstand capabilities. Pr–ZnO varistors exhibit a higher ESD withstand capability compared to Bi–ZnO varistors, although both types of devices exhibit similar electrical nonlinearity and surge current withstand capabilities. Bi–ZnO varistors exhibit asymmetric current-voltage (I-V) characteristics after the application of ESD pulses; their breakdown voltage decreases from 9 to 4 V and the leakage current increases. This indicates that the ESD pulses destroy some electrical potential barriers. Scanning surface potential microscopy (SSPM) measurements of a Bi–ZnO varistor reveal the existence of high electrical potential barriers at grain boundaries and electrode interfaces, and high-resistance secondary phases. In contrast, SSPM...

Degradation of potential barriers in ZnO-based chip varistors due to electrostatic discharge

Degradation of potential barriers in ZnO-based varistors due to the electrostatic discharge (ESD) was investigated using scanning probe microanalysis and capacitance-time and isothermal capacitance transient spectroscopies. Pr6O11-ZnO (Pr-ZnO) varistors exhibit excellent ESD withstand capability compared with Bi2O3-ZnO (Bi-ZnO) varistors. After the application of ESD, asymmetrically degraded double Schottky barriers were observed in both Pr-ZnO and Bi-ZnO varistors, and the Schottky barrier in Bi-ZnO was found to be destroyed. The potential barriers of both types of varistors can respond to an ESD pulse, whose rise time is ∼1 ns, but after application of the ESD pulse, the Bi-ZnO varistor takes more time to recover its initial capacitance than does the Pr-ZnO varistor. Such difference in the transient behaviors of potential barriers is attributed to differences in the energy and distribution of interfacial states of Pr-ZnO and Bi-ZnO varistors. Experimental results clearly indicated a strong correlation b...

Improvement in Resistance Switching and Retention Properties of Pt ∕ TiO2 Schottky Junction Devices

Resistance switching and retention properties of Pt/TiO 2 Schottky junction devices are improved by doping anatase Ti0 2 thin films with Co ions. Co-doped TiO 2 devices exhibit excellent rectifying current―voltage characteristics and resistance switching compared to undoped TiO 2 devices. Conventional Schottky junction devices such as Pt/Nb:SrTi0 3 and Pt/TiO 2 devices exhibit very poor resistance retention properties in low resistance state (LRS); the resistance in LRS increases with time and the resistance relaxation accelerates with an increase in temperature. Co-doped Ti0 2 devices show excellent resistance retention properties even at 100°C. Experimental results indicate that in Schottky junction devices, resistance switching originates from the formation of local tunneling paths, and the oxygen vacancies near the electrode interface play an important role in resistance switching. These results are very important from the point of view of understanding the resistance switching mechanism and improving the resistance switching properties of Schottky junction devices.

Resistance switching effect in Nb-doped SrTiO3 (100) bicrystal with (100) ∼45° twist boundary

We have fabricated Nb-doped SrTiO3 bicrystals with (100) ∼45° twist boundary by changing the fabricating conditions and demonstrated that they can exhibit a large resistance switching effect. Nb:SrTiO3 bicrystals cooled at the rate of −300 K/h show an asymmetric large hysteresis in current-voltage characteristics and the magnitude of resistance change attains two orders of magnitude. The resistance state can also be switched by the application of voltage pulses with different polarity and both states can be memorized for over 5 h at room temperature. From experimental results, it can be speculated that the twist boundary could be an origin of large resistance switching and memory effects and it behaves like an electrode interface in SrRuO3/Nb:SrTiO3 Schottky junction devices.