D.F. Zhang

Enhanced dielectric responses in Mg-doped CaCu3Ti4O12

We report the effects of the Mg doping on the dielectric properties of CaCu3Ti4O12 in the frequency range of 200 Hz–200 kHz and in the temperature range of 58–473 K. It is found that the incorporations of Mg2+ result in increases in the dielectric constant by 12%–20% and decreases in the dielectric loss by 41%–64% (with the minimum of 0.042 for CaCu2.7Mg0.3Ti4O12) at room temperature and at 1 kHz. The x-ray photoemission spectroscopy measurements reveal that the content of Cu3+ increases with the increasing concentration of Mg2+. The enhanced dielectric response may be related to the influence of Cu3+ and/or Mg2+. In other words, Mg2+ is an effective ion to optimize the dielectric properties of CaCu3Ti4O12.

Enhancement of ferroelectric properties of Na1∕2Bi1∕2TiO3-BaTiO3 single crystals by Ce dopings

Ferroelectric single crystal of Na1∕2Bi1∕2TiO3-BaTiO3 (NBT-BT) and Ce doped NBT-BT have been grown by flux technique. It is found that the addition of Ce plays a significant role in improving the ferroelectric properties of NBT-BT crystals, (i) by improving the value of the dielectric constant at room temperature and at Tm (the phase transition temperature between antiferroelectric and paraelectric phases with dielectric maximum), (ii) by increasing the depolarization temperature (Td) and Tm, (iii) by increasing the degree of diffuseness, and (iv) by increasing the remnant polarization (Pr) and coercive field (Ec). The reasons behind these enhancements of ferroelectric properties are discussed in detail.

The effects of grain boundary response and electrode contact response on the dielectric properties of CaCu3Ti4O12

The influences of dc bias on complex impedance, complex module and dielectric response of CaCu 3 Ti 4 O 12 for different sintering durations have been investigated to elucidate the origin of its dielectric properties. It is found that both the electrode contact response and grain boundary response contribute to the dielectric constant of CaCu 3 Ti 4 O 12 . The origins of the semicircles in complex impedance plots of CaCu 3 Ti 4 O 12 have also been investigated. At 120 °C, the mid-frequency semicircle in complex impedance plots represents the electrode contact response and the low-frequency semicircle may correspond to the oxygen-vacancy relevant response. The semicircle of the grain boundary response in the complex impedance plot is covered by the semicircle of the electrode contact response at this temperature.

Photovoltaic Effects of InGaN/GaN Double Heterojunctions With p-GaN Nanorod Arrays

The p-GaN/In0.06Ga0.94N/n-GaN double heterojunctional solar cells with solely formed nanorod arrays of p-GaN have been fabricated on sapphire (0001). The p-GaN nanorod arrays are demonstrated to significantly reduce the reflectance loss of light incidence. A stress relief of the intrinsic InGaN region is observed from high-resolution X-ray diffraction analyses. The electroluminescence emission peak is blue shifted compared with the conventional solar cells. These results are reflected by the spectral dependences of the external quantum efficiency (EQE) that show a shorter cutoff wavelength response. The maximum EQE value is 55.5%, which is an enhancement of 10% as compared with the conventional devices.

Nonlinear electrical characteristics and dielectric properties of (Ca, Ta)-doped TiO2 ceramics

TiO2 ceramics doped with 1.0 mol% Ca and different concentrations of Ta were obtained by sintering processing at 1450°C. The microstructures, nonlinear electrical behavior and dielectric properties of the ceramics were investigated. The samples have nonlinear coefficients of α = 2.0–5.0 and ultrahigh relative dielectric constants which is up to 105. Especially, the effects of Ta dopant on the nonlinear electrical characteristics and dielectric properties of the (Ca, Ta)-doped TiO2 ceramics were studied in detail. When the concentration of Ta is 2.0 mol%, the sample exhibits the highest nonlinear coefficient and a comparatively lower dielectric constant. By analogy to a grain-boundary atomic defect model, the effects of Ta and the nonlinear electrical behavior of the TiO2 system were explained.

InGaN/GaN Multiple Quantum Well Solar Cells with Good Open-Circuit Voltage and Concentrator Action

The photovoltaic properties of large-chip-size (2.5×2.5 mm2) InGaN/GaN multiple quantum well (MQW) solar cells grown by metal organic chemical vapor deposition were studied under concentrated AM1.5G sun irradiation. We demonstrate a high open-circuit voltage of 2.31 V for blue-light-emitting InGaN/GaN MQW solar cells under 1 sun. The higher open-circuit voltage is mainly ascribed to the extremely low reversed saturation current density of approximately 10-19 mA/cm2. The open-circuit voltage and short-circuit current density were found to increase as sunlight intensity increases, with a peak value of 2.50 V observed at 190 suns, showing a great potential for concentrator applications.

Nonlinear electrical and dielectric properties of (Ca, Ta)-doped TiO2 varistors

(Ca, Ta)-doped TiO2 varistors with high nonlinear coefficients were obtained by ceramic sintering process. The nonlinear electrical and dielectric properties of the samples doped with 1.0 mol% Ca and different concentrations of Ta (0.05-2.0 mol%) were investigated. Especially, the effects of Ta dopant on the nonlinear and dielectric properties of the (Ca, Ta)-doped TiO2 varistors were studied in detail. When the concentration of Ta is 0.5 mol%, the sample possesses the highest nonlinear coefficient (16.6). By analogy to a, grain-boundary atomic defect model, the effects of Ta and the nonlinear electrical behavior of the TiO2 system were explained.

InGaN-based multiple quantum well photovoltaic cells with good open-circuit voltage and concentration behavior

We report photovoltaic properties and concentration action of InGaN/GaN multi-quantum well (MQW) solar cells on c-plane sapphire substrate grown by metal-organic vapor phase epitaxy (MOVPE). The fabricated QW solar cells show a comparatively high open-circuit voltage of 2.06 at one sun and good concentration properties. The open circuit voltage (Voc) keeps increasing logarithmically with concentration ratio until 60 suns. The peak Voc of InGaN/GaN MQW solar cells, which have a predominant peak energy of 2.7 eV from electroluminescence (EL) measurements, is found to be 2.45 V when the concentration ratio reaches 333x. It was found that after an optimization of InGaN/GaN MQWs, the Voc can reach 2.31 V, displaying a bandgap to Voc difference (Eg/q-Voc) of 0.45 V. The obtained Eg/q-Voc is comparable to that of single-crystalline silicon or GaAs solar cells. The higher open-circuit voltage is mainly ascribed to extremely very low reversed saturation current density of around 10-19 mA/cm2, which could be attributed to the good crystal quality evidenced by TEM and HRXRD measurements. In addition, we give some discussions upon dependences of conversion efficiency and fill factor on concentration ratio for the fabricated MQW solar cells.

Negative refraction: an intrinsic property of uniaxial crystals

We theoretically and experimentally show that negative refraction can be realized at the surface of conventional uniaxial crystals by orientating the crystals with their optic axes at a certain angle 0 to the normal of the light incoming surface. The concept of negative refraction can be extended to be an intrinsic property of all uinaxial crystals. It is revealed that the angular range for incident light to yield negative refraction attains its maximum that only depends on the difference of