C.L. Wang

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.

Influence of rare earth doping on thermoelectric properties of SrTiO3 ceramics

Thermoelectric properties of SrTiO3 ceramics, doped with different rare earth elements, were investigated in this work. Its found that the ionic radius of doping elements plays an important role on thermoelectric properties: SrTiO3 ceramics doped with large rare earth ions (such as La, Nd, and Sm) exhibit large power factors, and those doped with small ions (such as Gd, Dy, Er, and Y) exhibit low thermal conductivities. Therefore, a simple approach for enhancing the thermoelectric performance of SrTiO3 ceramics is proposed: mainly doped with large ions to obtain a large power factor and, simultaneously, slightly co-doped with small ions to obtain a low thermal conductivity. Based on this rule, Sr0.8La0.18Yb0.02TiO3 ceramics were prepared, whose ZT value at 1 023 K reaches 0.31, increasing by a factor of 19% compared with the single-doped counterpart Sr0.8La0.2TiO3 (ZT = 0.26).

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-...

Piezoelectricity and local structural distortions in (Na0.5Bi0.5)1−xSrxTiO3-Bi12TiO20 flexoelectric-type polar ceramics

We have previously described sintered Na0.5Bi0.5TiO3-Bi12TiO20 composites as flexoelectric-type polar ceramics because they have a net macroscopic flexoelectric polarization. Here, we report on the universal existence of the macroscopic flexoelectric polarization in the (Na0.5Bi0.5)1−xSrxTiO3-Bi12TiO20 system, in which enhanced piezoelectricity is observed. By combining Raman spectroscopy and x-ray photoelectron spectroscopy techniques, we have identified the local flexoelectric polarization as distorted BiO5 polyhedra and TiO6 octahedra in the SrTiO3-Bi12TiO20 ceramic. The macroscopic polarization may be due to the partial alignment of these distorted units located within the grain boundary amorphous phases. Bi12TiO20 could have an important role in these flexoelectric-type polar ceramics.

Vibrational and thermal properties of small diameter silicon nanowires

We present the results of vibrational and thermal properties for small diameter silicon nanowires (Si-NWs) from first principles calculations. Phonon spectrums of the Si-NWs are obtained based on the density functional perturbation theory. We found that heat-carrying acoustic branches exhibit “bending,” which results from the strong interaction between acoustic and no-zero-frequency flexural modes. The bending of acoustic branches implies that the phonon group velocity (V=dω/dq) of Si-NWs is less than that of corresponding bulk silicon. Therefore, a lower lattice thermal conductivity of Si-NWs can be caused by the bending of acoustic phonon. In comparison with bulk silicon, optical branches of Si-NWs exhibit “blueshift,” which is due to the high frequency vibration of silicon atoms at the edge of Si-NWs. From the obtained phonon spectrums, specific heat is calculated. The specific heat of Si-NWs is also lower than that of bulk silicon crystal. The reduction in the specific heat is due to the small magnitu...

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.

High temperature thermoelectric performance of Ca1−xDyxMn1−xNbxO3 ceramics (x≤0·10)

Abstract Perovskite type Ca1−xDyxMn1−xNbxO3 ceramics (x≤0·10) are synthesised by solid state reaction. Microstructures are characterised, and the thermoelectric properties are evaluated between 300 and 1100 K. The samples obtained in this work have high relative densities. The electrical resistivity shows a typical metallic conductivity behaviour and decreases with substitution, the variation of which is not significant in whole measured temperature range. The Seebeck coefficients are negative, indicating n type conduction, and the largest value is obtained, −174 μV K−1, at x = 0·02. The highest power factor is 258 μW (K2 m)−1 for the x = 0·02 sample. A maximum ZT value reaches 0·08 for electron doped Ca0·98Dy0·02Mn0·98Nb0·02O3 at 973 K, which is about twice as high as that of undoped CaMnO3.

Enhancement of thermoelectric figure of merit by doping Dy in La{sub 0.1}Sr{sub 0.9}TiO{sub 3} ceramic

Ceramic samples of La0.1Sr0.9−xDyxTiO3 (x = 0.01, 0.03, 0.07, 0.10) have been prepared by the solid-state reaction method. Characterization from the powder X-ray diffraction indicates that their crystal structure changes from cubic to tetragonal phase. Their electrical and thermal transport properties are measured in the temperature range of 300–1100 K. n-Type thermoelectric is obtained with large Seebeck coefficient. The figure of merit is markedly improved, due to relatively lower electrical resistivity and thermal conductivity by Dy doping effect. A much lower electrical resistivity of 0.8 mΩ cm at room temperature is obtained in La0.1Sr0.8Dy0.1TiO3, and with a relatively lower thermal conductivity of 2.5 W/m K at 1075 K. The maximum figure of merit reaches ∼0.36 at 1045 K for La0.1Sr0.83Dy0.07TiO3, which is the largest value among n-type oxide thermoelectric ceramics.