Grain size effects on the physical properties of polycrystalline ferroelectrics have been extensively studied for decades; however there are still major controversies regarding the dependence of the piezoelectric and ferroelectric properties on the grain size. Dense BaTiO3 ceramics with different grain sizes were fabricated by either conventional sintering or spark plasma sintering using micro- and nano-sized powders. The results show that the grain size effect on the dielectric permittivity is nearly independent of the sintering method and starting powder used. A peak in the permittivity is observed in all the ceramics with a grain size near 1 μm and can be attributed to a maximum domain wall density and mobility. The piezoelectric coefficient d33 and remnant polarization Pr show diverse grain size effects depending on the particle size of the starting powder and sintering temperature. This suggests that besides domain wall density, other factors such as back fields and point defects, which influence the domain wall mobility, could be responsible for the different grain size dependence observed in the dielectric and piezoelectric/ferroelectric properties. In cases where point defects are not the dominant contributor, the piezoelectric constant d33 and the remnant polarization Pr increase with increasing grain size.

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Unfolding grain size effects in barium titanate ferroelectric ceramics

Lead-free (1 − x)BaTiO3–xBi(Zn2/3Nb1/3)O3 (x = 0.05–0.20) materials were fabricated via solid-state reactions. A pure perovskite pseudocubic structure is obtained for all compositions. Dielectric measurements reveal an intensified diffusion and relaxor-like characteristics from 5 mol% to 20 mol% Bi(Zn2/3Nb1/3)O3. Weakly coupled relaxor behavior is concluded from the exceptionally high activation energies of ∼0.20–0.22 eV from the Vogel–Fulcher model for x ≥ 0.10, which possibly results in the extremely low dielectric nonlinearity and extra slim polarization–electric field loops. An optimal discharged energy density of 0.79 J cm−3 with a high energy efficiency of 93.5% is achieved at 131 kV cm−1 for x = 0.15, which proves that the BaTiO3–Bi(Zn2/3Nb1/3)O3 material is a promising candidate for high energy storage applications.

Lead-free BaTiO3–Bi(Zn2/3Nb1/3)O3 weakly coupled relaxor ferroelectric materials for energy storage

Broadband dielectric spectroscopy is applied to investigate the electrical properties of disordered perovskite-like ceramics in a wide temperature range. From the x-ray diffraction analysis it was found that the newly obtained (Na0.75Bi0.25) (Mn0.25Nb0.75)O3 ceramics consist of two chemically different phases. The major perovskite one has an orthorhombic structure described by the Pbcm space group (No 57, in yxz setting). The minor phase shows an orthorhombic symmetry, all-face-centred lattice F, with the lattice parameters a = 10.797(4) A, b = 7.601(3) A and c = 7.691(3) A. The electric modulus M* formalism used in the analysis enabled us to distinguish and separate the relaxation processes, dominated by marked conductivity in the e*(ω) representation. In the ceramics studied, the relaxation times are thermally activated and the dipole process has a clearly non-Debye behaviour. The relaxation process described with the use of the activation energy of approximately 0.4 eV and the characteristic relaxation time, τ0 = 1 × 10−11 s, was found to be related to oxygen vacancies. The low frequency relaxation shows Debye behaviour with a slightly lower activation energy and a longer characteristic time.

https://iopscience.iop.org/article/10.1088/0022-3727/38/9/019/pdf

Electric modulus approach to the analysis of electric relaxation in highly conducting (Na 0.75 Bi 0.25 )(Mn 0.25 Nb 0.75 )O 3 ceramics

We herein present comparative investigation on the dielectric properties of both ceramic and single crystal SrTiO3 samples in the temperature from room temperature to 1073 K. Two relaxations were observed in both samples. They behave as Debye-like and relaxor-like relaxations in ceramic and single crystal samples, respectively. These relaxations were found to be bulk effect related to oxygen-vacancy. In single crystal sample, the relaxations result from the long-range conduction associated with singly and doubly charged oxygen vacancies. In ceramic sample, the oxygen vacancies are more strongly localized in relation to the crystal. This leads to a new phenomenon of formation and dissociation of oxygen vacancy clusters before the vacancies make contribution to the long-range conduction. The low-temperature relaxation in ceramic sample was determined by the clustering and dissociating processes of the oxygen vacancies. The high-temperature relaxation in ceramic sample was found to share the same mechanism as that in the single crystal sample.

Oxygen-vacancy-related dielectric relaxations in SrTiO3 at high temperatures

The electrical conductivity behaviour in the paraelectric state has been investigated in lanthanum modified lead zirconate titanate (PLZT) ceramics, considering the universal relaxation law. The contribution of conductive processes to the dielectric response for the low-frequency range was discussed taking into account the oxygen vacancies mechanism. The activation energies for the thermally hopping and conductivity processes were obtained. The results for the frequency dependence of the conductivity suggest at the oxygen vacancy hopping processes, due to relaxations in oxygen vacancy-related dipoles, being mainly responsible for the conduction behaviour in the studied system.

https://iopscience.iop.org/article/10.1088/0022-3727/41/21/215503/pdf

Ionized oxygen vacancy-related electrical conductivity in (Pb(1-x)La(x)) (Zr(0.90)Ti(0.10))(1-x/4)O(3) ceramics

The diffuse dielectric anomaly by the dielectric relaxation found at the high-temperature region of 400–700 °C was investigated in perovskite-type ferroelectric oxides such as BaTiO3, (Pb,La)TiO3, and (Pb,La)(Zr,Ti)O3 ceramics. We observed that the diffuse dielectric anomaly in perovskite oxides was strongly affected by oxygen-related processing parameters. We have modified the Debye relaxation equation by introducing the mobile dipole of the thermal motion of oxygen vacancies in order to explain the temperature-dependent behavior of the diffuse dielectric anomaly. A relationship between the dielectric polarization/relaxation and the electrical conduction by the thermal motion of oxygen vacancies was microscopically suggested to explain the origin of the diffuse dielectric anomaly found at 400–700 °C in perovskite-type ferroelectric oxides.

S. K. Choi

Papers

Diffuse dielectric anomaly in perovskite-type ferroelectric oxides in the temperature range of 400–700 °C