Wenwei Ge

Crystal growth and high piezoelectric performance of 0.95Na0.5Bi0.5TiO3?0.05BaTiO3 lead-free ferroelectric materials

A large single crystal of 0.95Na0.5Bi0.5TiO3?0.05BaTiO3 (0.95NBT?0.05BT) with dimensions of 35?mm diameter ? 10?mm length was grown by the top-seeded-solution growth method (TSSG). X-ray powder diffraction results indicate that the as-grown 0.95NBT?0.05BT crystal is of a perovskite structure and belongs to the rhombohedral system. The dielectric, ferroelectric and piezoelectric properties were investigated in detail. The room temperature dielectric constants for unpoled 0?0?1, 1?1?0 and 1?1?1 oriented crystal samples are 1450, 1650 and 1750 at 1?kHz and decrease to 1050, 800 and 480 after poling. The remanent polarizations Pr of 0?0?1, 1?1?0 and 1?1?1 oriented crystal samples are 16.44??C?cm?2, 23.69??C?cm?2 and 27.63??C?cm?2 with the coercive fields Ec of 32.72?kV?cm?1, 31.20?kV?cm?1 and 28.15?kV?cm?1, respectively. Interestingly, the piezoelectric constant d33 of the 0.95NBT?0.05BT crystal shows apparent anisotropy along its pseudocubic 0?0?1, 1?1?0 and 1?1?1 directions. The 0?0?1 poled 0.95NBT?0.05BT crystals show excellent piezoelectric properties with d33 = 280?pC?N?1. However, the d33 values of the 1?1?1 oriented 0.95NBT?0.05BT crystal are only 90?pC?N?1. The origin of apparent anisotropy in piezoelectric properties has been discussed in detail.

Growth and pyroelectric properties of high Curie temperature relaxor-based ferroelectric Pb(In1∕2Nb1∕2)O3–Pb(Mg1∕3Nb2∕3)O3–PbTiO3 ternary single crystal

To enhance the service temperature of relaxor-PbTiO3 pyroelectric single crystals, high quality ternary perovskite single crystal was grown by a modified Bridgman technique. Analyzed by x-ray fluorescence, the as-grown crystal is 0.41Pb(In1∕2Nb1∕2)O3–0.17Pb(Mg1∕3Nb2∕3)O3–0.42PbTiO3 [PIMNT(41/17/42)], which appears to be a tetragonal ferroelectric phase with relatively high Curie temperature of 253°C. It exhibits the relative permittivity of 487 and low dielectric loss of 0.3% at 50Hz and room temperature. The pyroelectric properties with a pyroelectric coefficient of 5.7×10−4C∕m2K and a detectivity of 6.34×10−5Pa−1∕2 would satisfy the needs of operation as a high Curie temperature material. The results show that PIMNT crystal with better temperature stability, compared with the pure PMNT single crystals, is a good candidate as an infrared detector material.

Multilayer Rosen-type piezoelectric transformer prepared with Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystal

A multilayer Rosen-type piezoelectric transformer with the size of 20 mm length, 4 mm width and 2.8 mm thickness has been fabricated using ferroelectric single crystal Pb(Mg1/3Nb2/3)O3–PbTiO3. The electrical properties of the prepared transformer were systematically investigated. Results show that the efficiency of this transformer can reach 92% and the voltage step-up ratio under open circuit exceeds 100. Comparing this with the single-layer Rosen-type transformer, we reported lower output impedance and larger step-up voltage ratios have been obtained under the same load resistance. Near the resonance frequency, this transformer has a much purer vibration mode and wider working bandwidth. Furthermore, this transformer can successfully drive a 0.5 W LCD backlight under a lower input voltage without obvious temperature rise. These global properties make this piezoelectric transformer a promising candidate for small power inverters and cryogenic applications.

Wavelength dependence of electro-optic effect in tetragonal lead magnesium niobate lead titanate single crystals

The refractive indices, birefringence, and linear electro-optic (EO) effect of 0.62Pb(Mg1∕3Nb2∕3)O3–0.38PbTiO3 single crystals have been characterized as a function of wavelength. The dispersions of refractive indices for both ordinary and extraordinary rays were accurately described by a two-term Sellmeier dispersion equation, in which the parameters connected to the energy band structure were determined. A strong dispersion was found for birefringence and EO coefficient γc=γ33−(no∕ne)3γ13. With the increasing wavelengths, both of them decrease fast. The two-term Sellmeier dispersion model, in which one oscillator frequency is polarization dependent, was shown to represent closely all the results. The polarization potential, which specifies the magnitude of the oscillator frequency shift, was 0.40eVm4C−2.

Effect of substrate photoexcitation on channel conduction in a modulation‐doped AlxGa1−xAs/GaAs heterostructure

If a modulation‐doped AlGaAs/GaAs heterostructure is illuminated by light, photoexcitation of deep levels in the GaAs substrate leads to some interesting effects. Below 100 K, the heterostructure shows a persistent photoconductivity effect. Moreover, a strong persistent channel depletion is observed at low temperatures when a small negative voltage is applied to the substrate contact (backgate). The latter effect is explained by a double‐layer model of GaAs where the GaAs side of the heterostructure consists of (1) a buffer layer and (2) a semi‐insulating substrate. Under illumination, most of the applied negative voltage drops across the very thin buffer layer, and the enhanced electric field in the layer exerts a very strong influence on the conducting channel.