Electrostatic potential barriers at doped ZnO-ZnO interfaces can be modified by stress-induced polarization charges. This concept was enhanced by preparing ZnO-based single crystal-polycrystal-single crystal structures by diffusion bonding. Increasing time for epitaxial solid-state transformation results in structures with a decreasing thickness of residual polycrystalline material in between two well-oriented single crystals. Microstructural and electrical analysis quantifies the influence of high-temperature treatment during epitaxial growth on the stress sensitivity of the prepared structures. The orientation of the single crystals is defined to maximize the interaction between stress-induced polarization charges and the potential barriers at doped ZnO-ZnO interfaces. With decreasing thickness of residual polycrystalline material, the percentage of grain boundaries with favorably aligned polarization vectors is increased resulting in a higher stress sensitivity. This effect is compensated by an adverse effect of the high-temperature treatment on the initial potential barrier height. Hence, a maximum in stress sensitivity can be observed for intermediate times of epitaxial growth. The prepared structures close the gap between the varistor piezotronics based on bulk ceramics with random orientation of the polarization vector and the bicrystal piezotronics with perfect orientation of the polarization vector, demonstrating the capability of microstructural engineering for varistor-based piezotronic devices. 相似文献
Studies of the electrochemical oxidation of a series of straight-chain terminal alkenoic acids adsorbed at a Pt(111) electrode surface are reported. Compounds adsorbed were: propenoic acid (acrylic acid, PPA); 3-butenoic acid (vinylacetic acid, 3BTA); 4-pentenoic acid (allylacetic acid, 4PTA); 6-heptenoic acid (6HPA); and 10-undecenoic acid (10UDA). Vibrational spectra of adsorbed layers were obtained by use of electron energy-loss spectroscopy (EELS). Molecular packing densities were measured by use of Auger spectroscopy. Electrochemical oxidation of each adsorbed layer was explored by means of cyclic voltammetry in aqueous inert electrolyte (KF/HF). As the analogous aliphatic acids are not chemisorbed at Pt under the same conditions, the alkenoic acids evidently adsorb at Pt(111) predominantly through the C=C double bond. Molecular packing densities indicate that the carboxylic acid moiety is in contact with the Pt surface only in the case of PPA. EELS spectra also indicate that the carboxylate groups (other than in PPA) are present as pendants. The carboxylic acid O-H stretching bands of most of the adsorbed acids are red-shifted and broadened, evidently due to extensive intermolecular hydrogen bonding; the exceptions are PPA, for which the interaction is primarily with the Pt surface, and 3BTA, for which intermolecular interaction between the carboxylic acid pendants is apparently prevented by steric considerations. The surface-attached carboxylic acid moieties react with KOH solution, leading to retention of K+ ions, detected by Auger spectroscopy, and to changes in the vibrational spectra indicative of carboxylate anions; reactivity toward KOH decreases with chain length. Adsorbed alkenoic acids at Pt(111) surfaces are stable in water and in vacuum. Oxidation of the adsorbed short-chain acids PPA and 3BTA proceeds to completion, forming CO2 as the principal product. Oxidation of the adsorbed long-chain acids converts the C=C moiety to 2CO2, and transforms the remainder of the molecule to an unadsorbed diacid (likely possibilities are malonic acid from 4PTA; glutaric acid from 6HPA; and heptane-1,7-dioic acid from 10UDA). 相似文献
Functionally reconfigurable general purpose parallel machines (FRPM) could be reconfigured during the operation from SIMD to MIMD mode or vice versa (first aspect) and from one interconnection network to another according to the data storing order (second aspect). General purpose machines are considered in order to obtain an arbitrary data exchange between the processing elements they are built of. A model for describing such interconnection networks is presented. A full-information exchange network in introduced which is reconfigurable in a programming way to tree-, matrix-, cube-, linear-neighbourhood and FFT-network. Some schemes for constructing SIMD/MIMD reconfigurable machines are given. The usefullness of using FRMP for image processing and pattern recognition is discussed. 相似文献
Here, we present the performance of a thermoelectric (TE) module consisting of n-type (La0.12Sr0.88)0.95TiO3 and p-type Ca3Co4?xO9+δ materials. The main challenge in this investigation was operating the TE module in different atmospheric conditions, since n-type has optimum TE performance at reducing conditions, while p-type has optimum at oxidizing conditions. The TE module was exposed to two different atmospheres and demonstrated higher stability in N2 atmosphere than in air. The maximum electrical power output decreased after 40 h when the hot side was exposed to N2 at 600°C, while only 1 h at 400°C in ambient air was enough to oxidize (La0.12Sr0.88)0.95TiO3 followed by a reduced electrical power output. The module generated maximum electrical power of 0.9 mW (~?4.7 mW/cm2) at 600°C hot side and δT?~?570 K in N2, and 0.15 mW (~?0.8 mW/cm2) at 400°C hot side and δT?~?370 K in air. A stability limit of Ca3Co3.93O9+δ at ~?700°C in N2 was determined by in situ high-temperature x-ray diffraction.
The sensitivity toward mechanical stress of barium titanate-based positive temperature coefficient resistor material was investigated by determining the resistance change with application of uniaxial stress from room temperature to 200 °C, which is well above the Curie temperature TC. Using the Landau–Ginsburg–Devonshire theory the resistance increases in the paraelectric state, the negligible impact of stress close to TC and the observed increase in TC with increasing stress could be rationalized. For the ferroelectric state, the stress-related resistance increase was attributed to ferroelasticity, a change in bulk permittivity and interfacial stress inducing a piezoelectric potential. The obtained results are also discussed with respect to recent endeavors to tune properties of potential barriers in piezoelectric semiconductors by mechanical stress. 相似文献
Electrocaloric device structures have been developed as multilayer ceramics (MLCs) based on fundamental research carried out on PMN‐8PT bulk ceramics. Two different MLC structures were prepared with nine layers each and layer thicknesses of 86 μm and 39 μm. The influence of the device design on its properties has been characterized by microstructural, dielectric, ferroelectric, and direct electrocaloric measurement. For direct characterization two different methods, ie temperature reading (thermistor and thermocouple) and heat flow measurement (differential scanning calorimetry), were used. A comparison of results revealed a highly satisfactory agreement between the different methods. This study confirms that MLCs are promising candidates for implementation into energy‐efficient electrocaloric cooling systems providing large refrigerant volume and high electrocaloric effect. Due to their micron‐sized active layers, they allow for the application of high electric fields under low operation voltages. We measured a maximum electrocaloric temperature change of ΔT=2.67 K under application/withdrawal of an electric field of ΔE=16 kV mm?1, which corresponds to operation voltages below 1.5 kV. 相似文献
The safety control of dams is based on measurements of parameters of interest such as seepage flows, seepage water clarity, piezometric levels, water levels, pressures, deformations or movements, temperature variations, loading conditions, etc. Interpretation of these large sets of available data is very important for dam health monitoring and it is based on mathematical models. Modelling seepage through geological formations located near the dam site or dam bodies is a challenging task in dam engineering. The objective of this study is to develop a feedforward neural network (FNN) model to predict the piezometric water level in dams. An improved resilient propagation algorithm has been used to train the FNN. The measured data have been compared with the results of FNN models and multiple linear regression (MLR) models that have been widely used in analysis of the structural dam behaviour. The FNN and MLR models have been developed and tested using experimental data collected during 9 years. The results of this study show that FNN models can be a powerful and important tool which can be used to assess dams. 相似文献
A conceptual workflow model specifies the control flow of a workflow together with abstract data information. This model is later on refined by adding specific data information, resulting in an executable workflow which is then run on an information system. It is desirable that correctness properties of the conceptual workflow are transferable to its refinements. In this paper, we present classical workflow nets extended with data operations as a conceptual workflow model. For these nets, we develop a novel technique to verify soundness. An executable workflow is sound if from every reachable state it is always possible to terminate properly. Our technique allows us to analyze a conceptual workflow and to conclude whether there exists at least one sound refinement of it, and whether any refinement of a conceptual workflow model is sound. The positive answer to the first question in combination with the negative answer to the second question means that sound and unsound refinements for the conceptual workflow in question are possible. 相似文献