Prasit Thongbai

Dielectric relaxations and dielectric response in multiferroic BiFeO3 ceramics

Single-phase multiferroic BiFeO3 ceramics were fabricated using pure precipitation-prepared BiFeO3 powder. Dielectric response of BiFeO3 ceramics was investigated over a wide range of temperature and frequency. Our results reveal that the BiFeO3 ceramic sintered at 700 °C exhibited high dielectric permittivity, and three dielectric relaxations were observed. A Debye-type dielectric relaxation at low temperatures (−50 to 20 °C) is attributed to the carrier hopping process between Fe2+ and Fe3+. The other two dielectric relaxations at the temperature ranges 30–130 °C and 140–200 °C could be due to the grain boundary effect and the defect ordering and/or the conductivity, respectively.

Dielectric relaxation and dielectric response mechanism in (Li, Ti)-doped NiO ceramics

Giant dielectric permittivity (Li, Ti)-doped NiO (LTNO) ceramics are prepared by a simple PVA sol–gel method. The dielectric properties are investigated as a function of frequency (102–106 Hz) at different temperatures (233–473 K). The concentration of Li has a remarkable effect on the dielectric properties of the LTNO ceramics. The modified Cole–Cole equation, including the conductivity term, is used to describe the experimental dielectric spectra of a high permittivity response with excellent agreement over a wide range of frequencies (103–106 Hz) and temperatures (233–313 K). A frequency dielectric dispersion phenomenon in an LTNO ceramic is also analyzed by impedance spectroscopy. A separation of the grain and grain boundary properties is achieved using an equivalent circuit model. The grain and grain boundary conduction and the dielectric relaxation time of the Li0.05Ti0.02Ni0.93O follows the Arrhenius law associated with estimated activation energies of 0.216, 0.369 and 0.391 eV, respectively. Through the analysis by the modified relaxation model and impedance spectroscopy, it is strongly believed that the high dielectric permittivity response of the LTNO is not only contributed by the space charge polarization (Maxwell–Wagner polarization) mechanism at low frequency regions, but also by the defect-dipole polarization mechanism at high frequency regions.

The origin of giant dielectric relaxation and electrical responses of grains and grain boundaries of W-doped CaCu3Ti4O12 ceramics

The origin of giant dielectric relaxation behavior and related electrical properties of grains and grain boundaries (GBs) of W6+-doped CaCu3Ti4O12 ceramics were studied using admittance and impedance spectroscopy analyses based on the brick–work layer model. Substitution of 1.0 at. % W6+ caused a slight decrease in GB capacitance, leading to a small decrease in the low-frequency dielectric constant. Surprisingly, W6+ doping ions have remarkable effects on the macroscopic dielectric relaxation and electrical properties of grains. X-ray photoelectron spectroscopy analysis suggested that the large enhancements of grain resistance and conduction activation energy of grains for the W6+-doped CaCu3Ti4O12 ceramic are caused by reductions in concentrations of Cu3+ and Ti3+ ions. Considering variation of dielectric properties together with changes in electrical properties of the W6+-doped CaCu3Ti4O12 ceramic, correlation between giant dielectric properties and electrical responses of grains and GBs can be describe...

Effects of grain, grain boundary, and dc electric field on giant dielectric response in high purity CuO ceramics

Giant dielectric constant e′ of ∼(2.8–3.7)×104 was observed in high purity CuO (99.999%) ceramics with grain sizes of 4.57±1.71 and 9.57±3.01 μm. The e′ and Ea increase with an increase in grain size due to the different electrical properties in the grains. The high dielectric response observed in the CuO ceramics can be described by the internal barrier layer capacitance model. The resistance of grain boundaries (Rgb) and the dielectric constant of the CuO samples decrease with increasing dc bias due to the decrease in grain boundaries capacitance, whereas the resistance of grains (Rg) remains constant.

The sintering temperature effects on the electrical and dielectric properties of Li0.05Ti0.02Ni0.93O ceramics prepared by a direct thermal decomposition method

We reported the effects of grain size on high dielectric and related electrical properties of Li0.05Ti0.02Ni0.93O (LTNO) ceramics, which were prepared by a direct thermal decomposition method. The analysis of complex impedance indicated that these LTNO ceramics were electrically heterogeneous consisting of conducting grains and insulating grain boundaries (GBs). Interestingly, our results revealed that the dielectric permittivity (e′) increases with the increase in grain size, which can be well described by Maxwell–Wagner relaxation model. Furthermore, we also found that the activation energy required for relaxation process (Ea) and related activation energy of the conductivity in the grain interior (Eg) decreased with the increase in grain size. These results suggested that the different microstructures resulted in chemical change (e.g., oxygen vacancies) inside the grains, leading to the changes in electrical properties of the LTNO ceramics.

Giant dielectric permittivity observed in CaCu3Ti4O12∕(Li,Ti)-doped NiO composites

The giant values of the dielectric permittivity as high as e′∼105 at various frequencies (f=100kHz–1MHz) over a broad temperature range of −50–190°C have been observed in polycrystalline CaCu3Ti4O12 ceramics that are reinforced with small amount of Li0.3Ti0.02Ni0.68O nanoparticles of 39nm. This enormous dielectric permittivity is even higher than that of CaCu3Ti4O12 single crystals. The dielectric behavior of CaCu3Ti4O12 and CaCu3Ti4O12–Li0.3Ti0.02Ni0.68O composites exhibits Debye-like relaxation which can be interpreted based on Maxwell-Wagner model. The dielectric dispersion of the composites was discussed based on internal boundary layer capacitor effect.

Investigation on temperature stability performance of giant permittivity (In + Nb) in co-doped TiO2 ceramic: a crucial aspect for practical electronic applications

In this work, it was shown that the crucial aspect for practical applications of a newly discovered (In + Nb) co-doped TiO2 material is the temperature stability of its dielectric permittivity (e′). Despite an extremely large e′ value of ≈5.1 × 104 and a low loss tangent (tan δ ≈ 0.03) successfully obtained in 10% (In + Nb) co-doped TiO2, careful inspection revealed that e′ was largely changed below room temperature (RT) as a result of an ambient-RT dielectric relaxation, giving rise to a large value of the temperature coefficient. However, this result can be effectively improved by decreasing the co-doping concentration. Although this dielectric relaxation also occurred in 2.5% (In + Nb) co-doped TiO2, its e′ variation below RT was slight. Notably, very high e′ ≈ 1.57 × 104 and ultra-low tan δ ≈ 0.006 (at 30 °C and 102 Hz) with an excellent temperature coefficient of less than ±7% in the range of −70 to 180 °C were achieved. The giant e′ response over a broad temperature range in (In + Nb) co-doped TiO2 was primarily due to the polarization of highly localized electrons in large defect-dipole clusters. The additional polarization relaxation near the RT range might be associated with interfacial polarization of delocalized electrons originating from uncorrelated Nb25+Ti3+ATi defect dipoles.

Electrical responses in high permittivity dielectric (Li, Fe)-doped NiO ceramics

Electrical properties of giant-permittivity core/shell structured Li0.05FexNi0.95−xO (LFNO) are studied as functions of frequency, temperature, and dc bias. Three electrical responses of depletion surface (DS), grain boundary (GB), and bulk grain are detected in the LFNO ceramics. The DS and GB effects can be separated by removing the surface samples, whereas the grain effect is extracted by applying dc bias. It is found that the interfacial polarizations at the DSs and GBs are suppressed by applied voltages. Our results suggest that the polarization relaxation in the LFNO ceramics is closely related to the electrical response inside the grains.

Effects of Fe, Ti, and V doping on the microstructure and electrical properties of grain and grain boundary of giant dielectric NiO-based ceramics

We report the giant dielectric response and electrical properties of Li0.05B0.02Ni0.93O (B=Fe, Ti, and V) ceramics prepared by a polymer pyrolysis route. The giant dielectric response in these materials can be ascribed based on the Maxwell–Wagner polarization and thermally activated mechanisms. It is found that Fe, Ti, and V doping has a strong effect on the microstructure and the conduction of grains and grain boundaries of these NiO-based ceramic systems, which make large contribution to their dielectric properties.

Origin(s) of the apparent colossal permittivity in (In1/2Nb1/2)xTi1−xO2: clarification on the strongly induced Maxwell–Wagner polarization relaxation by DC bias

The effects of DC bias on the dielectric and electrical properties of co-doped (In1/2Nb1/2)xTi1−xO2 (IN-T), where x = 0.05 and 0.1, and single-doped Ti0.975Nb0.025O2 ceramics are investigated. The low-frequency dielectric permittivity (e′) and loss tangent of IN-T ceramics with x = 0.05 and 0.1 are greatly enhanced by applying a DC bias at 40 and 20 V, respectively, whereas the relatively high-frequency e′ remains unchanged. The induced low-frequency Maxwell–Wagner polarization completely vanishes by immediately applying no DC bias. After overload limited measurement, this polarization permanently emerges without DC bias, whereas the primary polarization remains unchanged. Using combined Z′′ and M′′ spectroscopic plots, it is found that the strongly induced-polarizations are contributed from the combination effects of the sample–electrode contact and resistive outer surface. Very high performance of the colossal permittivity in IN-T ceramics is attributed to the formation of a resistive outer-surface layer and insulating grain boundaries. These results not only provide important insights into the origins of the colossal dielectric response in the IN-T ceramic system, but are also important for deciding the doping conditions of TiO2-based materials for practical applications.