Xiang Ming Chen

Dielectric relaxations and formation mechanism of giant dielectric constant step in CaCu3Ti4O12 ceramics

The dielectric relaxation behaviors of CaCu3Ti4O12 ceramics were evaluated together with the mixed-valence structure, and the formation mechanism of a giant dielectric constant step was discussed. The giant dielectric constant step was bounded by two dielectric relaxations in low and high temperature ranges. The low-temperature dielectric relaxation was intrinsic, and it was very similar to the electronic ferroelectricity, while the high temperature relaxorlike dielectric peak was assigned to be the result of defect ordering since it could be suppressed by O2 annealing. Both the low and high temperature dielectric relaxations were the thermal activated relaxation process following the Arrhenius law. Moreover, it was supposed that the giant dielectric constant step resulted from the competing balance between the low and high temperature dielectric relaxations.

Dielectric relaxations in Ba(Fe1∕2Ta1∕2)O3 giant dielectric constant ceramics

Dielectric relaxations of Ba(Fe1∕2Ta1∕2)O3 ceramics were investigated and discussed over a broad temperature and frequency range. Two dielectric relaxations following Arrhenius law were observed at 153–382 and 440–623K, where there was a giant dielectric constant step between them. The frequency dependent rapid drop of dielectric constant at 153–382K was nearly a Debye relaxation with the intrinsic nature, while the high temperature dielectric relaxation with an extremely high dielectric constant peak and very strong frequency dispersion was attributed to the defect ordering but not a typical relaxor ferroelectric behavior. The O2 annealing almost completely suppressed the dielectric peak and subsequently extended the giant dielectric step, while the low temperature dielectric relaxation and the magnitude of such step were not obviously affected.

Giant dielectric response and relaxor behaviors induced by charge and defect ordering in Sr(Fe1∕2Nb1∕2)O3 ceramics

Dielectric spectra of Sr(Fe1∕2Nb1∕2)O3 ceramics were characterized in wide temperature (123–623K) and frequency (100Hz–1MHz) ranges. Two dielectric relaxations with strong frequency dispersion and following the Arrhenius law were detected in low and high temperature ranges, respectively, and between them there was a giant dielectric constant step. The high temperature relaxorlike dielectric peak could be almost completely removed by annealing in O2, and it should be assigned to be a defect ordering induced relaxor behavior, while the low temperature dielectric relaxation was proposed to stem from the electronic ferroelectricity. Moreover, it was speculated that the giant dielectric constant step resulted from the competing balance between the low and high temperature dielectric relaxations.

Dielectric characteristics of ceramics in BaO-Nd2O3-TiO2-Ta2O5 system

Abstract Dielectric ceramics in the BaO–Nd 2 O 3 –TiO 2 –Ta 2 O 5 system were prepared and characterized. The ceramics with tungsten–bronze structure based on the compositions Ba 2 NdTi 2 Ta 3 O 15 and Ba 5 NdTi 3 Ta 7 O 30 had a high dielectric constant (>100) with a lower frequency-dependency when complete densification was achieved; a low dielectric loss was obtained in the former.

Complex-permittivity measurement on high-Q materials via combined numerical approaches

A modified indirect algorithm for extracting real permittivities from measured resonant frequencies has been developed. It is based on the perturbation theory along with the Newton iteration method, and improves the speed of the calculation with a factor of 2-4. It is superior, especially when system matrices under calculation are large. Based on the complementary advantages of the modified algorithm and a direct algorithm, a mixed algorithm of high speed and accuracy has been described. A real measurement system incorporating the linear fractional curve-fitting method and the mixed algorithm has been built to measure complex permittivities of high-Q materials.

Relaxorlike dielectric behavior and weak ferromagnetism in YFeO3 ceramics

Dielectric characteristics of YFeO3 antiferromagnetic ceramics were evaluated over broad temperature and frequency ranges. Two dielectric relaxations were observed at low and high temperatures, respectively, and a dielectric constant step was detected between them. The low temperature dielectric relaxation was an intrinsic thermally activated process following the Arrhenius law with the activation energy very close to that for electronic ferroelectrics, while the high temperature dielectric relaxation was related to the point defect since it could be significantly suppressed by O2 annealing. M-H hysteresis loop was detected at room temperature, and this indicated the weak ferromagnetism in YFeO3 ceramics.

Enhanced multiferroic characteristics in NaNbO3-modified BiFeO3 ceramics

Modification of BiFeO3 multiferroic ceramics were investigated by introducing NaNbO3 to form (Bi1−xNax)(Fe1−xNbx)O3 (x=0.1, 0.3, and 0.5) solid solutions. With increasing NaNbO3 content, the crystalline structure changed and the ferroelectric properties were improved. The significantly reduced leakage resulted in more regular ferroelectric hysteresis loop for all compositions. Dielectric characteristics of (Bi1−xNax)(Fe1−xNbx)O3 (x=0.1, 0.3, and 0.5) ceramics were evaluated. Two dielectric relaxations were observed in the temperature ranges of 550–600 and 650–710 K in (Bi0.9Na0.1)(Fe0.9Nb0.1)O3 ceramics, and the higher-temperature dielectric relaxation was related to the antiferromagnetism transition, which indicated the coupling between the ferroelectric and magnetic orders essential for the multiferroic materials. Moreover, weak ferromagnetism with remnant magnetization (Mr) of 0.14 emu/g and coercive field (Hc) of 6.48 kOe were observed in the sample with x=0.1, which suggested that the present modifie...

Dielectric anomalies in (BaxSr1−x)4Nd2Ti4Nb6O30 ceramics with various radius differences between A1- and A2-site ions

Dielectric response of tetragonal tungsten bronze dielectrics (BaxSr1−x)4Nd2Ti4Nb6O30 was investigated over a broad temperature and frequency range, and the obvious composition-dependent dielectric anomalies with respect to x value were discussed in detail in association with the radius differences between A1- and A2-site ions. With decreasing the magnitude of radius difference between A1- and A2-site ions, the normal ferroelectric peak above 400K became weaker, and two relaxor peaks at lower temperatures became obvious. The low- and high-temperature relaxor behaviors, which followed well the Vogel-Fulcher relationship, were associated with the polar clusters caused by the off-center Nb∕Ti displacements and the incommensurate tilting modulation, respectively.

Giant dielectric response in two-dimensional charge-ordered nickelate ceramics

Dielectric relaxations of charge-ordered Ln1.5Sr0.5NiO4 (Ln=La and Nd) ceramics were investigated over a broad temperature range. The giant dielectric constant (over 70 000) with a low dielectric loss of ∼0.1 was determined at high frequencies (up to 5 MHz) over a broad temperature range. There are two dielectric relaxations in the vicinity of charge ordering temperatures. The thermal activated small polaronic hopping between two charge ordering temperatures should contribute to the giant dielectric response in the present ceramics. Compared to other giant dielectric constant materials, the present materials have the notable advantage for high frequency applications.

Dielectric relaxations of yttrium iron garnet ceramics over a broad temperature range

Dielectric response of Y3Fe5O12 ceramics was investigated over broad temperature and frequency ranges. Three dielectric relaxations were identified in the temperature range from 125to620K. A Debye-type dielectric relaxation at low temperatures (125–320K) with an activation energy of 0.29eV is shown to originate from the carrier hopping process between Fe2+ and Fe3+. In a higher temperature range (320–620K), a low frequency (f≦10kHz) dielectric relaxation with an activation energy of 0.84eV most likely arises from the inhomogeneous structure, such as grain boundaries. Another dielectric relaxation in a wider frequency range has a similar activation energy with conduction (Ea=1.00eV) suggests that it can be attributed to the conduction.