Boundary Barrier Research Articles

Effect of Ge−GeO2 co-doping on non-ohmic behaviour of TiO2−V2O5−Y2O3 varistor ceramics**Project supported by the National Natural Science Foundation of China (Nos. 51262017, 51362017).

An investigation was made into the effect of doping with the elemental crystal Ge or/and GeO2 on the TiO2−V2O5−Y2O3 varistor ceramics. The result shows that as the doping contents of V2O5 and Y2O3 are 0.5 mol%, respectively, co-doping with 0.3 mol% Ge and 0.9 mol% GeO2 makes the highest α value (α = 12.8), the lowest breakdown voltage V1mA (V1mA = 15.8 V/mm) and the highest grain boundary barrier ΦB (ΦB = 1.48 eV), which is remarkably superior to the TiO2−V2O5−Y2O3 varistor ceramics undoped with Ge and GeO2 and mono-doped with Ge or GeO2. The TiO2−V2O5−Y2O3−Ge−GeO2 ceramic has the prospect of becoming a novel varistor ceramic with excellent electrical properties.

Charge carrier transport through 3D assemblies of zincblende CdSe and ZnSe quantum dots in weak size-quantization regime

The mechanism of charge carrier transport through 3D assemblies of ZnSe and CdSe quantum dots with zincblende structure in weak size-quantization regime was investigated. The Debye length in the case of ZnSe QDs was found to be 11.5 nm, i.e. almost three times larger than the average diameter of the nanocrystals constituting the films annealed at 250 °C. In CdSe QDs, on the other hand, the Debye’s length of 11.8 nm was almost twice smaller than the average crystal diameter in the films annealed at 300 °C. In the case of ZnSe QD assemblies, it was found that the predominant mechanism governing the charge carrier transport in temperature range from 380 to 650 K is the thermionic emission, with the trap levels taking part in the formation of crystal boundary barrier being located above the Fermi level. Combining temperature-dependent conductivity data with the data from optical absorption studies, the actual position of the trap level was estimated to be at about 0.37 eV (referred to the intrinsic Fermi level at the interface). In contrast to the case of ZnSe, the temperature dependence of conductivity in the case of thin films composed by 3D assemblies of CdSe QDs appeared to be much more complex. In the highest temperature region in which the temperature-dependent conductivity measurements were performed for this system (from 480 to 540 K), it was found that the thermally activated band-to-band electronic transitions govern the conductivity changes, the corresponding thermal band gap energy being 1.85 eV. In the lower-temperature region, down to 300 K, the thermionic emission was found to be predominant charge carrier transport mechanism, with trap levels being positioned above the Fermi level. The two detected trap levels were found to be located at 0.46 and 0.79 eV, corresponding to the measured conductivity activation energies of 0.84 and 0.51 eV.

The effect of grain boundary on the energy storage properties of (Ba0.4Sr0.6M)TiO3 paraelectric ceramics by varying grain sizes.

(Ba0.4Sr0.6)TiO3 (BST) ceramics with various grain sizes (0.3-3.4 μm) were synthesized by the oxalate coprecipitation method and prepared by plasma activated sintering and conventional solid-state sintering process. The effect of grain boundary on the energy storage properties and the dielectric relaxation characteristics of BST paraelectric ceramics (Curie point ≈ -67°C) with various grain sizes were investigated. The dielectric breakdown strength (simplified as BDS) is obviously improved and then deteriorated with decreasing grain size, accounting for the energy density variation. The enhancement of interfacial polarization at grain boundary layers has a negative effect on the BDS, leading to the decreased values for samples with grain size smaller than 0.7 μm. In addition, the insulation effect of grain boundary barriers was discussed based on the complex impedance spectroscopy analysis, which was found to play a dominant role in controlling the BDS with coarser grain size. Among them, the sharply decreased BDS for BST with grain size of 1.8 μm was believed to be attributed to the combination of lower grain boundary density and higher interfacial polarization, due to the significant increase of oxygen vacancies at higher sintering temperature.

Analysis on Characteristics of Installing Environment for Improvement of Boundary Barriers in Rural Villages

ABSTRACT : With the basic research logic stating that the systematic improvement of rural housing plot ′ s boundary barriers should call for the integrated consideration of their on-and-around installing settings, this study tries to analyse characteristics of installing environment of boundary barriers for improvement works of them. Through literature review and then screening process, 3 influencing and appraisal factors were determined: main house-boundary barrier shortest distance as on-site factor, building coverage as inside one, and attached road width as outside one. 21 study villages were subgrouped into structurally improved(9) and not(12). Site investigation and analysis results showed that the relationship between factors has varied very much, i.e. in some cases it is positive or complementary, but in other cases, negative or crossed. Therefore this study would propose that differentiated and flexible strategies with integrated consideration of on-and-around site installing settings be applied in their improvement projects rather than village-wide unified or straight forwarded ones.

Electrical properties of Ta2O5-doped TiO2 varistor ceramics sintered at low-temperature

TiO2-based varistor ceramics doped with 0.25–1.00mol% Ta2O5 were prepared by the conventional solid state reaction technique at 1300°C and the effects of Ta2O5 substitution on their electrical properties have been studied. The results showed that the Ta2O5 dopant played an important role in the formation of the grain boundary barriers. Good electrical properties with E1mA=14.9V/mm, α=4.48, εr=9.68×104, tanδ=0.36 were obtained for 0.50mol% Ta2O5 addition. These results indicate the 99.5mol% TiO2+0.50mol% Ta2O5+2wt% (0.723mol% Bi2O3+1mol% B2O3) composition might be a suitable candidate for capacitor–varistor functional device.

Microstructure and dielectric properties of Ba0.4Sr0.6Ti1−yZryO3 sintered with SrO–B2O3–SiO2 glass additive

Ba0.4Sr0.6Ti1−yZryO3 (0y0.20) (BSTZ) ceramics were prepared via the solid state reaction route, followed by sintering at high temperature. SrO–B2O3–SiO2 glass, as the sintering additive, was introduced to lower the sintering temperature. The effects of the amount of Zr substitution for Ti in Ba0.4Sr0.6Ti1−yZryO3 on the sintering temperature, microstructure and dielectric properties were investigated. Sintering temperature of Ba0.4Sr0.6Ti1−yZryO3 increases gradually with the increasing amount of Zr in Ba0.4Sr0.6Ti1−yZryO3. XRD phase analysis shows that all samples exhibit the desired perovskite structure, where the cell parameter a increases with the increasing amount of Zr substitution. Microstructural studies reveal a slow-down in grain growth by Zr substitution. Dielectric relaxation is observed in the dielectric loss versus temperature plots, where the relaxation time shows an increase with the increasing amount of Zr substitution. The slow-down in grain growth gives rise to the formation of more grain boundaries, which restrict the charge spreading and lead to an increase in the grain boundary barrier Ea, 2.

Legendre spectral collocation method for approximating the solution of shortest path problems

In the current article, we propose an accurate spectral approximation for solving the shortest path problems with boundary and interior barriers. For this goal, the shortest path problems are modelled as variational problems (VPs). Then, the Legendre polynomials are used as a basis for approximating the solution of these problems, and by using the Chebyshev–Gauss–Lobatto collocation points together with the Legendre–Gauss quadrature rule, the VPs will be changed into nonlinear programming problems (NLPs). The resulting NLPs are solved by the NLPSolve command in MAPLE software. Three numerical examples are provided for showing the robustness of the proposed method.

Electrical properties of sprayed Cu2ZnSnS4 thin films and its relation with secondary phase formation and solar cell performance

Cu2ZnSnS4 (CZTS) thin films were grown onto glass substrates using pneumatic spray pyrolysis method under different growth conditions. The electrical properties have been investigated by means of dark and illuminated conductivity measurements in the 293–353K temperature range and Hall measurements at room temperature. The influence of growth parameters on the grain boundary barrier height and other associated grain boundary parameters of the CZTS thin films was determined from electrical, optical, and morphological characterization. The CZTS electrical properties relation with secondary phases formation as well as with transport mechanisms is presented. Finally, CZTS electrical parameters impact on the properties of polycrystalline thin film solar cells is discussed.

Deletions of chromosomal regulatory boundaries are associated with congenital disease

BackgroundRecent data from genome-wide chromosome conformation capture analysis indicate that the human genome is divided into conserved megabase-sized self-interacting regions called topological domains. These topological domains form the regulatory backbone of the genome and are separated by regulatory boundary elements or barriers. Copy-number variations can potentially alter the topological domain architecture by deleting or duplicating the barriers and thereby allowing enhancers from neighboring domains to ectopically activate genes causing misexpression and disease, a mutational mechanism that has recently been termed enhancer adoption.ResultsWe use the Human Phenotype Ontology database to relate the phenotypes of 922 deletion cases recorded in the DECIPHER database to monogenic diseases associated with genes in or adjacent to the deletions. We identify combinations of tissue-specific enhancers and genes adjacent to the deletion and associated with phenotypes in the corresponding tissue, whereby the phenotype matched that observed in the deletion. We compare this computationally with a gene-dosage pathomechanism that attempts to explain the deletion phenotype based on haploinsufficiency of genes located within the deletions. Up to 11.8% of the deletions could be best explained by enhancer adoption or a combination of enhancer adoption and gene-dosage effects.ConclusionsOur results suggest that enhancer adoption caused by deletions of regulatory boundaries may contribute to a substantial minority of copy-number variation phenotypes and should thus be taken into account in their medical interpretation.Electronic supplementary materialThe online version of this article (doi:10.1186/s13059-014-0423-1) contains supplementary material, which is available to authorized users.

Structurally nanocrystalline-electrically single crystalline ZnO-reduced graphene oxide composites.

ZnO, a wide bandgap semiconductor, has attracted much attention due to its multifunctionality, such as transparent conducting oxide, light-emitting diode, photocatalyst, and so on. To improve its performances in the versatile applications, numerous hybrid strategies of ZnO with graphene have been attempted, and various synergistic effects have been achieved in the ZnO-graphene hybrid nanostructures. Here we report extraordinary charge transport behavior in Al-doped ZnO (AZO)-reduced graphene oxide (RGO) nanocomposites. Although the most challenging issue in semiconductor nanocomposites is their low mobilities, the AZO-RGO nanocomposites exhibit single crystal-like Hall mobility despite the large quantity of nanograin boundaries, which hinder the electron transport by the scattering with trapped charges. Because of the significantly weakened grain boundary barrier and the proper band alignment between the AZO and RGO, freely conducting electrons across the nanograin boundaries can be realized in the nanocomposites. This discovery of the structurally nanocrystalline-electrically single crystalline composite demonstrates a new route for enhancing the electrical properties in nanocomposites based on the hybrid strategy.

Simulation of the Electrical Properties of Semiconductive BaTiO<sub>3</sub> Ceramic Varistors Using Continuum Theory

A continuum field model describing the electrical characteristics of polycrystalline semiconductors ceramics is suggested. Taking into account the continuum theory, a static differential equation about electron level on the base of Poisson equation is established. The one-dimensional quantitative calculation is carried out using the Runge-Kutta method. The results show that as the applied voltage increases, the grain boundary barrier in the nonlinear zone drop drastically. The nonlinear characteristics of high temperature paraelectric phase is larger than that of room temperature ferroelectric phase.

Dielectric properties of high-permittivity A2/3CuTa4O12 ceramics

Purpose – This paper aims to present the comparative study on the composition, microstructure and dielectric behavior of a group of new nonferroelectric high-permittivity A2/3CuTa4O12 (A = Y, Nd, Sm, Gd, Dy or Bi) ceramics. Design/methodology/approach – The materials under investigation were synthesized by solid-state reaction method and sintered at 1,120-1,230°C. Dielectric properties were investigated in the temperature range from −55 to 740°C at frequencies 10 Hz to 2 MHz. Microstructure, elemental composition and phase composition of the ceramics were examined by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) methods. DC conductivity was studied in the temperature range 20-740°C. Findings – XRD analysis revealed peaks corresponding to Cu2Ta4O12 along with small amounts of secondary phases based on tantalum oxides. Impedance spectroscopic data and the results of SEM and EDS studies imply the spontaneous formation of internal barrier layer capacitors in the investigated materials. Two steps can be distinguished in the dielectric permittivity versus frequency plots. The low-frequency step of 1,000-100,000 is assigned to grain boundary barrier layer effect, while the high-frequency one of 34-46 is related to intrinsic properties of grains. Originality/value – Search for new high-permittivity capacitor materials is important for the progress in miniaturization and integration scale of electronic passive components. The paper reports on processing, microstructure, microanalysis studies and dielectric properties of a group of novel nonferroelectric materials with the perovskite structure of CaCu3Ti4O12 and the general formula A2/3CuTa4O12, being spontaneously formed internal barrier layer capacitors.

A Differential Evolution Stereo Matching Method in Digital Image Correlation

Stereo matching is widely used in three-dimensional (3D) reconstruction, stereo machine vision and digital image correlation. The aim of stereo matching process is to solve the well-known correspondence problem, which tries to match points or features from one image with the same points or features in another image from the same 3D scene. There are two basic ways, correlation-based and feature-based, are used to find the correspondences between two images. The correlation-based way is to determine if one location in one image looks/seems like another in another image, and the feature-based way to find if a subset of features in one image is similar in the another image. In stereo matching, a simple algorithm is to compare small patches between two rectified images by correlation search. For the pair images acquired from two cameras inevitably exists some rotation transformation, the algorithm first runs a preprocessing step to rectify the images with the epipolar rectification to simplify the problem of finding matching points between images. The epipolar rectification is to determine a transformation of each image plane such that pairs of conjugate epipolar lines become collinear and parallel to one of the image axes. It will lead the loss of gray information of images. The effect is dependent on the amount of angle. When the angle is big enough, the correlation search may yield error results because of retrograded correlation effect. In order to solve the problem, the paper presents an improved stereo matching algorithm with differential evolution to solve the correspondence problem. Our method doesn’t need to runs the preprocessing step to rectify the images with the epipolar rectification. It uses a differential evolution algorithm to minimize the correlation function which contains the angle information after acquiring the epipoar geometry constraint of two image pairs. Then it utilizes a flood-fill algorithm to search correspondence sub-region in the area around the epipolar line. The flood-fill algorithm can overcome the problem of the traditional row-column scanning search method, which will encounter boundary barrier where exists concave polygons or cavities. The Experimental results show that the proposed method can be easily implemented in stereo matching without loss of information of image features with large rotation angle transformation. In the paper, we will introduce the stereo matching principle and its algorithms, including the differential evolution algorithm for finding the correspondences with large rotation transformation between stereo image pairs and the flood-fill traversal strategy for matching large area with complex concave polygons or cavities. In the end of the paper, some experimental results will be given to illustrate the method effectiveness. Keywords: digital image correlation, stereo matching algorithm, epipolar geometry, flood fill algorithm, differential evolution, rotation angle

Grain boundary barrier modification due to coupling effect of crystal polar field and water molecular dipole in ZnO-based structures

Surface water molecules induced grain boundaries (GBs) barrier modification was investigated in ZnO and ZnMgO/ZnO films. Tunable electronic transport properties of the samples by water were characterized via a field effect transistor (FET) device structure. The FETs fabricated from polar C-plane ZnO and ZnMgO/ZnO films that have lots of GBs exhibited obvious double Schottky-like current-voltage property, whereas that fabricated from nonpolar M-plane samples with GBs and ZnO bulk single-crystal had no obvious conduction modulation effects. Physically, these hallmark properties are supposed to be caused by the electrostatical coupling effect of crystal polar field and molecular dipole on GBs barrier.

Study of Nonlinear Current-Voltage Characteristic of Semiconductor Ceramic by the Method of Tilted Energy Band

A analytic method for the calculation of the electrical characteristics of semiconducting ceramics is suggested. This paper put forward the concept of effective carrier concentration. Electrical characteristics under extra electric field have been calculated by the method of tilted energy band. The non-linear current-voltage characteristics with different grain sizes has been obtained. The results pointed out that the current-voltage characteristics divide into three regions: Linear region before breakdown field, nonlinear region near breakdown field and upturn region after breakdown field; The grain boundary barrier leads to the strong non-linear characteristics, which has nothing to do with the grain size. With the grain size decreasing, the breakdown field increases. The results are compared with experimental data.

An Analytical Subthreshold Model of Polycrystalline Silicon Thin-Film Transistors Based on Meyer-Neldel Rule

It is demonstrated that the subthreshold region of polycrystalline Si thin-film transistors contains both the channel full inversion and channel partial inversion regimes. Channel inversion charge sheet density is modeled based on the gate-to-channel capacitance measurement. Following the observation of the Meyer-Neldel rule for effective channel mobility (μeff) in the subthreshold region, a μeff model is constructed with the physical origin of multiphonon excited carrier thermionic emission over the grain boundary (GB) barriers. Then, by considering the carrier drift, an analytical subthreshold drain current model is developed. Subsequently, an analytical expression for subthreshold swing (SS) is derived. The gate voltage (Vg) induced GB barrier lowering effect is determined to be the key mechanism for subthreshold behavior. Besides, the Vg point of channel full inversion is demonstrated to be a key point of the subthreshold region, where SS reaches a minimum.

Extrinsic mechanism for colossal dielectric constant in CaCu3Ti4O12 ceramics evidenced by nanodomain

We studied the effect of various sintering temperature and dwell time on phase formation, microstructure and dielectric properties of CaCu3Ti4O12 ceramic synthesized by sol–gel method. The dielectric property was greatly influenced by sintering temperature and dwell time. We found colossal dielectric constant (∼153 816) with low dielectric loss (∼0.20) for 1100 °C/4 h sintered sample at room temperature and 1 kHz frequency. Impedance spectroscopy results support the grain boundary barrier layer capacitor model. Observation of nanostripe structure domains inside the grains of CaCu3Ti4O12 confirms the extrinsic mechanism for colossal dielectric response of this material.

The modulation of grain boundary barrier in ZnMgO/ZnO heterostructure by surface polar liquid

Modulation of grain boundary barrier in ZnO layer by polar liquid, was investigated in ZnMgO/ZnO heterostructures grown by plasma-assisted molecular beam epitaxy. Traditionally, surface adsorbates can only affect the surface atoms or surface electronic states. However, it was found that the electronic conduction property of ZnO far from the surface could be tailored obviously by the polar liquid adsorbed on the ZnMgO surface. Physically, this phenomenon is supposed to be caused by the electrostatical couple between the liquid polarity and the grain boundary barrier in the ZnO layer through crystal polarization field.

다요소의사결정법에 의한 농촌마을담장정비의 기본방향

다요소의사결정법에 의한 농촌마을담장정비의 기본방향

Dielectric properties of CaCu3Ti4O12 ceramics with Zn substitution for Cu

CaCu3–xZnxTi4O12 (x is from 0 to 1·0) polycrystalline samples were fabricated via a two-step solid state reaction process. The lattice parameter of the monophasic CaCu3Ti4O12 phase increased as Zn content increased. Scanning electron microscopy (SEM) images of the CCTO ceramic show bimodal grain size distribution and the grain size decrease largely with the appearance of Zn2TiO4 second phase. The dielectric permittivity of pure CCTO ceramic is ∼1·5×104 at f = 100 Hz. The dielectric constant of the sample largely increased with Zn substitution in the frequency range f<104 Hz. The highest dielectric constant was 6·2×104 at f = 100 Hz with Zn substitution of x = 0·8. The improved dielectric properties are believed to be related to the presence of a thin grain boundary barrier layer. The resistivity of the grain boundary decreased largely with Zn substitution as evidenced from the impedance plots.