Dielectric properties, lattice‐ and microstructure of ceramic BaTiO3 showing grain sizes of 0.3–100 μm were studied. At grain sizes <10 μm the width of ferroelectric 90° domains decreases proportionally to the square root of the grain diameter. The decreasing width of the domains can be theoretically explained by the equilibrium of elastic field energy and domain wall energy. The smaller the grains, the more the dielectric and the elastic constants are determined by the contribution of 90° domain walls. The permittivity below the Curie point shows a pronounced maximum er ≊5000 at grain sizes 0.8–1 μm. At grain sizes <0.7 μm the permittivity strongly decreases and the lattice gradually changes from tetragonal to pseudocubic.

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Dielectric properties of fine‐grained barium titanate ceramics

The processing, electromechanical properties, and microstructure of lead zirconate titanate (PZT) ceramics over the grain-size range of 0.1-10 μm were studied. Using measurements over a large temperature range (15-600 K), the relative role of extrinsic contribution (i.e., domain-wall motion) was deduced to be influenced strongly by the grain size, particularly for donor-doped PZT. Analytical transmission electron microscopy studies were conducted to investigate the trend in domain configurations with the reduction of grain size. The correlations between domain density, domain variants, domain configurations (before and after poling), spontaneous deformation, and the elastodielectric properties were qualitatively discussed, leading to new insights into the intrinsic and extrinsic effects and relevant size effects in ferroelectric polycrystalline materials.

Intrinsic and Extrinsic Size Effects in Fine-Grained Morphotropic-Phase-Boundary Lead Zirconate Titanate Ceramics

Measurements of the dielectric constant κ in high‐purity BaTiO3 ceramics as a function of temperature show that κ is quite dependent on grain size in the ferroelectric state while it is almost independent of grain size in the paraelectric state. In the ferroelectric orthorhombic and rhombohedral phases, κ decreases rapidly with lowering temperature when grain size is reduced smaller than 3 μm. It is also observed that the Curie temperature Tc is decreased very slightly, but the lower two transition points are shifted to higher temperatures as grain size decreases.

Grain‐size effects on dielectric properties in barium titanate ceramics

High strain piezoelectric ceramics are the state-of-the-art materials for high precision, positioning devices. A comprehensive review of the latest developments of the various types of perovskite piezoelectric ceramic systems is presented herein, with special attention given to three promising families of lead-free perovskite ferroelectrics: the barium titanate, alkaline niobate and bismuth perovskites. Included in this review are details of phase transition behavior, strain enhancement approaches, material reliabilities as well as the status of some promising applications. This current review describes both compositional and structural engineering approaches that are intended to achieve enhanced strain properties in perovskite piezoelectric ceramics. The factors that affect the strain behavior of high-strain perovskite piezoelectric ceramics are addressed. The reliability characteristics of these high-strain ferroelectrics as well as the recent approaches to the long-term electrical, thermal and time-stability enhancement are summarized. Several promising applications of high-strain perovskite materials are introduced, which take advantages of their characteristics; examples include high-energy storage, pyroelectric and electro-caloric effect and luminescent properties.

Progress in high-strain perovskite piezoelectric ceramics

Using three different sintering methods: spark plasma sintering, two-step sintering, and normal sintering (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT) lead-free piezoelectric ceramics with grain sizes in the range of 0.4–32.2 μm were prepared. The effects of grain size on the electrical properties and temperature stability of BCZT ceramics were systematically investigated. Results showed that reducing grain size shifted both the Tc and TT-R to higher temperatures, and tended to enhance the relaxor behavior. A strong dependence of piezoelectric properties on the grain size was observed, and ~10 μm was a critical point for fabricating high-performance BCZT ceramics. For samples with grain sizes >10 μm, excellent piezoelectric properties of kp > 0.48, kt > 0.46, d33 > 470 pC/N and d33* > 950 pm/V were obtained. Moreover, no evident relationship between the grain size and temperature stability existed in this material, and all samples exhibited thermal instability below the Curie temperature. However, increasing grain size was helpful for improving the resistance to thermal depoling. The depolarization was assisted by internal mechanical stresses and the movement of 180° and 90° domain walls, which explained the increased resistance to thermal depoling in coarse-grained samples.

Correlation Between the Microstructure and Electrical Properties in High‐Performance (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 Lead‐Free Piezoelectric Ceramics

Hot-pressed ceramics of the lead titanate zirconate system with different grain sizes were prepared and some of their electrical properties measured. The values of the dielectric and piezoelectric properties were analysed together with information on barium titanate ceramics already published in the literature. Many similarities were found between both systems. There are two main contributions to the properties; the single-crystal single-domain intrinsic effect and the contribution of the wall motion. The intrinsic effect is analysed considering the grains as monodomains. A model is developed which proposes that each grain has a different transition temperature Tc. The Tc values are taken as following a Gaussian distribution and the behaviour of the dielectric constant against temperature is computed and checked with experimental results. All the results can be explained qualitatively with the model; a connection between findings and the idea of diffuse phase transitions is outlined.

https://iopscience.iop.org/article/10.1088/0022-3719/7/17/024/pdf

H T Martirena

Papers

Grain-size effects on properties of some ferroelectric ceramics