J.L. Zhang

Polymorphic phase transition and excellent piezoelectric performance of (K0.55Na0.45)0.965Li0.035Nb0.80Ta0.20O3 lead-free ceramics

A lead-free ceramic with the composition (K0.55Na0.45)0.965Li0.035Nb0.80Ta0.20O3 was found having an outstanding piezoelectric performance. It possesses high piezoelectric properties of d33=262 pC/N, kp=0.53, k33=0.63, and k31=0.31 with e′=1290 and tan δ=0.019 at room temperature. In spite of the orthorhombic-tetragonal polymorphic phase transition around 30 °C, temperature stability of the electromechanical coupling coefficients is very good over the common usage temperature interval between −40 and 85 °C. Furthermore, the piezoelectric properties remain almost unchanged in the severe thermal aging test down to −150 °C and up to about 300 °C. It is suggested that the outstanding piezoelectric performance of this ceramic can be largely ascribed to the phase coexistence in a wide temperature range and the Ta-rich composition.

Influence of compositional ratio K/Na on physical properties in (KxNa1−x)NbO3 ceramics

Lead-free KxNa1−xNbO3 (KNN) ceramics with x=0.10–0.80 were prepared by a conventional solid-state reaction method to investigate the influence of the K/Na ratio on their physical properties. It has been found that the KNN ceramics with x=0.45–0.60 exhibit quite different microstructures from those with other alkali compositional ratios. The lattice parameter discontinuity previously reported at x=0.475 has not been confirmed. The existence of a change in the line slopes of lattice parameter variation with x is verified in the vicinity of x=0.35. The permittivity e′, piezoelectric constant d33, and planar electromechanical coupling coefficient kp show broad peaks in the compositional range of x=0.40–0.60, where d33 and kp are nearly independent of the K/Na ratio and all the ceramics have the high piezoelectric values of d33∼125 pC/N and kp∼0.41, respectively. From the analyses, we suggest that the high piezoelectric properties for the KNN ceramics near x=0.50 are closely related to their microstructures of...

Piezoelectric properties and stabilities of CuO-modified Ba(Ti,Zr)O3 ceramics

Due to the orthorhombic-tetragonal polymorphic phase transition near room temperature, undesirable large temperature dependence of piezoelectric properties is observed over common usage temperature range in BaTiO3 ceramics with high d33 values. Whereas shifting the phase transition temperature upward by partially substituting Ti with Zr is effective in reducing the piezoelectric temperature dependence, serious long-term degradation occurs. However, it is found that this could be overcome by incorporating a small amount of CuO additive. CuO-modified Ba(Ti0.9625Zr0.0375)O3 ceramics possess excellent piezoelectric properties of d33=300 pC/N, kp=0.493, and k33=0.651 with tan δ=0.011, and its kp remains larger than 0.40 in the broad temperature range from −43 to 73 °C and is almost constant between −25 and 55 °C. The results indicate that CuO-modified Ba(Ti,Zr)O3 ceramics are a promising low-cost lead-free material for practical applications.

Enhancement of thermoelectric efficiency in oxygen-deficient Sr1−xLaxTiO3−δ ceramics

We report that the Seebeck coefficient (S) is remarkably enhanced in oxygen-deficient Sr1−xLaxTiO3−δ ceramics. The S values of all oxygen-deficient samples are larger than those of the near-stoichiometric ones and are temperature-independent at high temperatures, showing a narrow band behavior. This indicates that the introduction of oxygen vacancy changes the density of electronic states around the Fermi energy. The maximum for the figure of merit (ZT) of Sr0.9La0.1TiO3−δ ceramic reaches 0.21 at about 750 K, demonstrating enhancement by a factor of more than 1.3 over that of the near-stoichiometric materials.

Extremely temperature-stable piezoelectric properties of orthorhombic phase in (K,Na)NbO3-based ceramics

Piezoelectric temperature stabilities are often a critical issue for many piezoelectric materials in practical applications. We report an important physical phenomenon recently discovered in a variety of (K,Na)NbO3-based ceramics. Piezoelectric properties are extremely temperature-stable in orthorhombic phase. No or only weak piezoelectric temperature dependence and excellent thermal aging stability are observed over considerably wide temperature intervals, which are broader than 325 °C in (K0.48Na0.52)NbO3 ceramic and 215 °C in (K0.50Na0.50)0.98Li0.02Nb0.82Ta0.18O3 ceramic, respectively. In general, the orthorhombic phase possesses much better piezoelectric temperature stabilities than the tetragonal phase. Thus, we suggest that future compositional designs of (K,Na)NbO3-based lead-free piezoelectric ceramics should take this phenomenon into full consideration.

Strong piezoelectricity exhibited by large-grained BaTiO3 ceramics

A good understanding of the strong piezoelectric mechanism of BaTiO3 ceramics is very important from both scientific and technological viewpoints. This Letter reports an unusual piezoelectric phenomenon observed in a group of large-grained dense BaTiO3 ceramics with average grain sizes ranging from 5.1 to 16.3 μm, which are prepared with a hydrothermally synthesized BaTiO3 fine powder by hot-press sintering. Among these BaTiO3 ceramics, the one possessing an average grain size of 5.6 μm exhibits a particularly prominent piezoelectricity with piezoelectric constants d33 = 485 pC/N and d33* = 725 pm/V at room temperature and a peak d33 value of 574 pC/N around 8.5 °C. This ceramic is further investigated from the aspects of domain patterns and polarization versus electric field (P-E) loops and compared with a conventionally sintered BaTiO3 ceramic prepared using a solid-state reaction method. The investigation shows that it has the domain patterns of simple parallel stripes inside the grains and the square-...

Interface-induced piezoelectricity in an unpoled Na0.5Bi0.5TiO3-based composite ceramic

Composite ceramics containing Bi12TiO20 and Na0.5Bi0.5TiO3 grains were fabricated by an ordinary sintering technique. An anomalous piezoelectric response consistent with triclinic symmetry was found in unpoled samples and could be detected up to at least 765 °C. This piezoelectric effect appears concurrently with macroscopic cellular interfaces in as-sintered ceramics, suggesting that it may originate from an interface effect. Since these kinds of composites also have near-zero temperature coefficients of the resonance frequencies over the temperature range from 400 to 520 °C, they have potential as high-temperature piezoelectric materials.

Aging behaviours of CuO modified BaTiO3 ceramics

Abstract In order to reduce the sintering temperature and enhance the piezoelectricity, a small amount of CuO was added into BaTiO3 during sintering. The aging behaviours of these CuO modified BaTiO3 ceramics were studied systematically. For unpoled ceramic, the P–E loop changes gradually from a normal single-loop curve to a double-loop one with aging. Meanwhile, the S–E loop develops gradually from a normal strain curve to the one with almost zero strain under electric field below 10 kV mm–1. For poled ceramic, it builds a much stronger and more stable internal bias after aging. The P–E loops shift significantly along the electric field and the strain loop shows high asymmetry with one side of high linearity. A dipolar defect model was proposed to explain these aging behaviours. The defect dipoles consisting of Cu2+ occupying B site and oxygen vacancy can reorientate gradually with time to align with the spontaneous polarisation. Then the aligned defect dipoles could provide a strong restoring force on the domain wall motion and thus lead to all the observed aging behaviours.

Piezoelectric properties and structural phase transitions of naturally polarized (Na0.75K0.25Bi)0.5TiO3 crystal

Electrical properties of a naturally polarized (Na0.75K0.25Bi)0.5TiO3 crystal have been investigated over a broad range of temperatures. Noticeable piezoelectric resonance signals have been observed up to 540°C. Based on the piezoelectric measurement, we suggest a R3c-P4bm-Pm3¯m sequence of structural phase transitions in this crystal. Moreover, at temperatures above 520°C, mobile sodium ions in the crystal are proposed to contribute to the electrical conduction; they have an activation energy of 2.06eV. This migration of Na+ at high temperatures can explain the naturally generated piezoelectricity in (Na0.75K0.25Bi)0.5TiO3 crystal.

Growth, Crystal Structure, Theoretical Analysis and Properties of Te 4+ -Doped KTiOPO 4

A single crystal of Te4+-doped KTiOPO4(Te:KTP) has been grown by the flux method. The electronic structure and density of states of KTiOPO4 (KTP) and Te:KTP were calculated from first principles. As the results reveal, there is no change in the space group or lattice structure of Te:KTP, but that some increase in lattice parameters occurred. The chemical composition of Te:KTP was analyzed using x-ray photoelectron spectroscopy (XPS). The possible existence of Ti3+ has been evaluated by measuring the electron paramagnetic resonance spectrum, and the results reveal that the ion is absent from this crystal. It was observed that Te4+ doping reduces the conductivity of the crystal from measurements of its conductivity at different temperatures and frequencies, indicating that Te:KTP has excellent electro-optical properties. The effect of Te4+ doping on the second harmonic generation in KTP was also studied. The thermal expansion, thermal diffusivity, thermal conductivity and specific heat capacity of KTP and Te:KTP were determined.