Effect of fabrication conditions on I-V properties for ZnO varistor with high concentration additives by sol-gel technique

Polycrystalline nano-grain-boundary multi-doping ZnO-based nonlinear varistors with higher concentration additives have been fabricated by sol-gel and standard solid-state reaction method, of which the best sample has a very high threshold voltage of E-b = 3300 V/mm. The effect of sintering processes, sintering temperature and sintering time, and that of additive concentration of Bi2O3 on E-b of the samples are systematically investigated. The results show that the great merit of sol-gel method is its high threshold voltage obtained by a lower sintering temperature than the solid-state reaction method. The present work also shows that five phases including solid-state sintering, rich Bi liquid phase formation and ZnO as well as other additive dissolution, ZnO grain growth, the secondary phase sufficient formation and evolution have been experienced at different sintering temperatures. The hole type defect and nonhomogeneity of the microstructure will lead to the decrease of threshold voltage, i.e., the grain size and the homogeneity of the material will be important factors and directly affect the characteristic of the varistor. The sintering characteristic and the influence of Bi2O3 content on the threshold voltage are also discussed. (c) 2004 Elsevier B.V. All rights reserved.

Recent advances in synthesis and properties of cubic boron nitride films

Cubic boron nitride (c-BN) attracts widespread interest as a promising material for many potential applications because of its unique physical and chemical properties. Since the 1980's the research in c-BN thin films has been carried out, which reached its summit in the mid of 1990's, then turned into a downward period. In the past few years, however, important progress was achieved in synthesis and properties of cubic boron nitride films, such as obtaining > 1 mu m thick c-BN films, epitaxial growth of single crystalline c-BN films, and advances in mechanics properties and microstructures of the interlayer of c-BN films. The present article reviews the current status of the synthesis and properties of c-BN thin films.

Effect of GaAS(100) 2 degrees surface misorientation on the formation and optical properties of MOCVD grown InAs quantum dots

The influence of GaAS(1 0 0)2 degrees substrate misorientation on the formation and optical properties of InAs quantum dots (QDs) has been studied in compare with dots on exact GaAs(1 0 0) substrates. It is shown that, while QDs on exact substrates have only one dominant size, dots on misoriented substrates are formed in lines with a clear bimodal size distribution. Room temperature photoluminescence measurements show that QDs on misoriented substrates have narrower FWHM, longer emission wavelength and much larger PL intensity relative to those of dots on exact substrates. However, our rapid thermal annealing (RTA) experiments indicate that annealing shows a stronger effect on dots with misoriented substrates by greatly accelerating the degradation of material quality. (c) 2005 Elsevier B.V All rights reserved.

Optical properties of GaN wurtzite quantum wires

The electronic structure and optical properties of freestanding GaN wurtzite quantum wires are studied in the framework of six-band effective-mass envelope function theory. It is found that the electron states are either twofold or fourfold degenerate. There is a dark exciton effect when the radius R of GaN wurtzite quantum wires is in the range of [0.7, 10.9] nm. The linear polarization factors are calculated in three cases, the quantum confinement effect (finite long wire), the dielectric effect and both effects (infinitely long wire). It is found that the linear polarization factor of a finite long wire whose length is much less than the electromagnetic wavelength decreases as R increases, is very close to unity (0.979) at R = I nm, and changes from a positive value to a negative value around R = 4.1 nm. The linear polarization factor of the dielectric effect is 0.934, independent of radius, as long as the radius remains much less than the electromagnetic wavelength. The result for the two effects shows that the quantum confinement effect gives a correction to the dielectric effect result. It is found that the linear polarization factor of very long (treated approximately as infinitely long) quantum wires is in the range of [0.8, 1]. The linear polarization factors of the quantum confinement effect of CdSe wurtzite quantum wires are calculated for comparison. In the CdSe case, the linear polarization factor of R = I nm is 0.857, in agreement with the experimental results (Hu et al 2001 Science 292 2060). This value is much smaller than unity, unlike 0.979 in the GaN case, mainly due to the big spin-orbit splitting energy Delta(so) of CdSe material with wurtzite structure.

Optical properties of self-assembled InAs/InAlAs/InP quantum wires with different InAs deposited thickness

We report on the photoluminescence (PL) properties of InAs/InAlAs/InP quantum wires (QWRs) with various InAs deposited thickness. The PL linewidth of the QWRs decreases with increasing InAs deposited thickness due to the different thicknesses of the QWRs and defects in the samples. The defects and lateral composition modulation of the InAlAs layers play an important role in the temperature-dependent PL properties of the samples. (c) 2005 Elsevier B.V. All rights reserved.

A study of the growth and optical properties of AlInGaN alloys

We have studied the growth and optical properties of AlInGaN alloys in this article. By the measurement of three samples, we found that the incorporation of In decreases with the increase of temperature, while there is nearly no change for the incorporation of Al. The sample grown at the lowest temperature had the best material and optical properties, which owes to the high In component, because the In component can reduce defects and improve the material quality. We also used the time-resolved photoluminescence(PL) to study the mechanism of recombination of carriers, and found that the time dependence of PL intensity was not in exponential decay, but in stretched-exponential decay. Through the study of the character of this decay, we come to the conclusion that the emission comes from the recombination of localized excitons. Once more, this localization exhibites the character of quantum dots, and the stretched, exponential decay results from the hopping of carriers between different localized states. In addition, we have used the relation of emission energy dependence of carrier's lifetime and the character of radiative recombination and non-radiative combination to confirm our conclusion.

Dielectric properties of Bi2Ti2O7 films grown on Si(100) substrate by APMOCVD

The growth of Bi2Ti2O7 films with (111) orientation on Si(100) substrate by atmospheric pressure metal-organic chemical vapor deposition(APMOCVD) technique at 480similar to550 degreesC is presented. The films were characterized by X-ray diffraction analysis, atomic force microscopy and electron diffraction. The results show high quality Bi2Ti2O7 films with smooth shinning surface. The dielectric properties and C-V characterization of the films were studied. The dielectric constant (epsilon) and loss tangent (tgdelta) were found to be 180 and 0.01 respectively. The charge storage density was 31.9fC/mum(2). The resistivity is higher than 1x10(12) Omega. .cm under the applied voltage of 5V. The Bi2Ti2O7 films are suitable to be used as a new insulating gate material in dynamic random access memory (DRAM).

Blue Luminescent Properties of Silicon Nanowires Grown by a Solid-Liquid-Solid Method

Silicon nanowires (SiNWs) were grown directly from n-(111) single-crystal silicon (c-Si) substrate based on a solid-liquid-solid mechanism, and Au film was used as a metallic catalyst. The room temperature photoluminescence properties of SiNWs were observed by an Xe lamp with an exciting wavelength of 350 nm. The results show that the SiNWs exhibit a strongly blue luminescent band in the wavelength range 400-480 nm at an emission peak position of 420 nm. The luminescent mechanism of SiNWs indicates that the blue luminescence is attributed to the oxygen-related defects, which are in SiOx amorphous oxide shells around the crystalline core of SiNWs.

Tunable optical properties of nano-Au on vanadium dioxide

The optical properties of Au nanoparticles deposited on thermochromic thin films of VO2 are investigated using spectroscopy. A localized modification on the transmittance spectrum of VO2 film is formed due to the presence of Au nanoparticles which exhibit localized surface plasmon resonance (LSPR) in the visible-near IR region. The position of the modification wavelength region shows a strong dependence on the Au mass thickness and shifts toward the red as it increases. On the other hand, it was found that the LSPR of Au nanoparticles can be thermally tunable because of the thermochromism of the supporting material of VO2. The LSPR wavelength, lambda(SPR), shifts to the blue with increasing temperature, and shifts back to the red as temperature decreases. A fine tuning is achieved when the temperature is increased in a stepwise manner.

Electronic structure and magnetic coupling properties of Gd-doped AlN: first-principles calculations

In this work, the electronic structure and magnetic coupling properties of Gd doped AlN have been investigated using first-principles method. We found that in the AlN:Gd system, due to the s-f coupling allowed by the symmetry, the exchange splitting of the conduction band is much larger than that of the valence band, which makes the electron-mediated ferromagnetism possible in this material. This property is also confirmed by the energy differences between anti-ferromagnetic and ferromagnetic phase for Al14Gd2N16 with different concentrations of electrons (holes), as well as by the calculated exchange constants. The result indicates that Gd-doped AlN is a promising candidate for the applications in future spintronic devices.

Optical properties of AIInGaN quaternary alloys

Carrier recombination dynamics in AlInGaN alloy has been studied by photoluminescence (PL) and time-resolved photoluminescence (TRPL). The fast redshift of PL peak energy is observed and well fitted by a physical model considering the thermal activation and transfer processes. This result provides evidence for the exciton localization in the quantum dot (QD)-like potentials in our AlInGaN alloy. The TRPL signals are found to be described by a stretched exponential function of exp[(-t/tau)(beta)], indicating the presence of a significant disorder in the material. The disorder is attributed to a randomly distributed quantum dots or clusters caused by indium fluctuations. By studying the dependence of the dispersive exponent 8 on the temperature and emission energy, we suggest that the exciton hopping dominate the diffusion of carriers localized in the disordered quantum dots. Furthermore, the localized states are found to have OD density of states up to 250 K, since the radiative lifetime remains almost unchanged with increasing temperature.

Dielectric properties of Bi2Ti2O7 films grown on Si(100) substrate by APMOCVD

The growth of Bi2Ti2O7 films with (111) orientation on Si(100) substrate by atmospheric pressure metal-organic chemical vapor deposition(APMOCVD) technique at 480similar to550 degreesC is presented. The films were characterized by X-ray diffraction analysis, atomic force microscopy and electron diffraction. The results show high quality Bi2Ti2O7 films with smooth shinning surface. The dielectric properties and C-V characterization of the films were studied. The dielectric constant (epsilon) and loss tangent (tgdelta) were found to be 180 and 0.01 respectively. The charge storage density was 31.9fC/mum(2). The resistivity is higher than 1x10(12) Omega. .cm under the applied voltage of 5V. The Bi2Ti2O7 films are suitable to be used as a new insulating gate material in dynamic random access memory (DRAM).

The main factor influencing the tensile properties of surface nano-crystallized graded materials

The improved mechanical properties of surface nano-crystallized graded materials produced by surface severe plastic deformation ((SPD)-P-2) are generally owing to the effects of the refined structure, work-hardened region and compressive residual stress. However, during the (SPD)-P-2 process, residual stress is produced simultaneously with work-hardened region, the individual contribution of these two factors to the improved mechanical properties remains unclear. Numerical simulations are carried out in order to answer this question. It is found that work hardening predominates in improving the yield strength and the ultimate tensile strength of the surface nano-crystallized graded materials, while the influence of the residual stress mainly emerges at the initial stage of deformation and decreases the apparent elastic modulus of the surface nano-crystallized graded materials, which agrees well with the experimental results. (C) 2010 Elsevier B.V. All rights reserved.

Microencapsulation of n-hexadecane as a phase change material in polyurea

For thermal energy storage application, polyurea microcapsules about 2.5 mum in diameter containing phase change material were prepared using interfacial polycondensation method. In the system droplets in microns are first formed by emulsifying an organic phase consisting of a core material ( n-hexadecane) and an oil-soluble reactive monomer, toluene-2, 4-diisocyanate (TDI), in an aqueous phase. By adding water-soluble reactive monomer, diamine, monomers TDI and diamine react with each other at the interface of micelles to become a shell. Ethylenediamine (EDA), 1, 6-hexane diamine (HDA) and their mixture were employed as water-soluble reactive monomers. The effects of diamine type on chemical structure and thermal properties of the microcapsules were investigated by FT-IR and thermal analysis respectively. The infrared spectra indicate that polyurea microcapsules have been successfully synthesized; all the TG thermographs show microcapsules containing n-hexadecane can sustain high temperature about 300 degreesC without broken and the DSC measurements display that all samples possess a moderate heat of phase transition; thermal cyclic tests show that the encapsulated paraffin kept its energy storage capacity even after 50 cycles of operation. The results obtained from experiments show that the encapsulated n-hexadecane possesses a good potential as a thermal energy storage material.

Stratigraphic Model Predictions of Geoacoustic Properties

Geoacoustic properties of the seabed have a controlling role in the propagation and reverberation of sound in shallow-water environments. Several techniques are available to quantify the important properties but are usually unable to adequately sample the region of interest. In this paper, we explore the potential for obtaining geotechnical properties from a process-based stratigraphic model. Grain-size predictions from the stratigraphic model are combined with two acoustic models to estimate sound speed with distance across the New Jersey continental shelf and with depth below the seabed. Model predictions are compared to two independent sets of data: 1) Surficial sound speeds obtained through direct measurement using in situ compressional wave probes, and 2) sound speed as a function of depth obtained through inversion of seabed reflection measurements. In water depths less than 100 m, the model predictions produce a trend of decreasing grain-size and sound speed with increasing water depth as similarly observed in the measured surficial data. In water depths between 100 and 130 m, the model predictions exhibit an increase in sound speed that was not observed in the measured surficial data. A closer comparison indicates that the grain-sizes predicted for the surficial sediments are generally too small producing sound speeds that are too slow. The predicted sound speeds also tend to be too slow for sediments 0.5-20 m below the seabed in water depths greater than 100 m. However, in water depths less than 100 m, the sound speeds between 0.5-20-m subbottom depth are generally too fast. There are several reasons for the discrepancies including the stratigraphic model was limited to two dimensions, the model was unable to simulate biologic processes responsible for the high sound-speed shell material common in the model area, and incomplete geological records necessary to accurately predict grain-size