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Three‐point bending tests of the full‐graded concrete under different loading rates were carried out. The cracks were quantitatively analyzed by digital image correlation (DIC). The toughening mechanism during the crack development process was observed, and there are obvious crack branching and aggregate interlocking during the crack propagation. At the same load level, the crack opening width increases with the increase of the loading rate. DIC detects a decrease in the tortuosity of the fracture morphology. And at the same time, the damage of the dam concrete was evaluated by the frequency analysis and the damage type analysis from acoustic emission (AE) signals. The signal strength increases at a higher loading rate as well as the tensile type cracks. The results show that the monitoring results of DIC and AE technique are in agreement with each other, which is very effective for determining the fracture behaviour of concrete materials. Highlights: The aggregate interlocking and crack branching form the toughening mechanism after peakThe interlocking and crack branching decrease with the increase of the loading rate.Signals with different frequency content correspond to different fracture scale.The development type of crack tends to direct tension with the increase of the rate. [ABSTRACT FROM AUTHOR]

Transient stability is the key problem for reliable and secure planning under the new deregulated market conditions. By using immersion and invariance (I&I) method, a nonlinear coordinated generator excitation and steam-valve controller is designed to improve transient stability of power systems. The proposed coordinated I&I controller can assure power angle stability, voltage, and frequency regulations, when a large disturbance occurs on the transmission line or a small perturbation to mechanical power. Compared with the Lyapunov method, the proposed method does not need to construct a Lyapunov energy function. Some numerical simulations are used to validate the proposed controller. Simulation results show that the nonlinear coordinated I&I controller has better control performance than the existing coordinated passivation controller (CPC). [ABSTRACT FROM AUTHOR]

In this paper, we solve an optimal control problem for a class of time-invariant switched stochastic systems with multi-switching times, where the objective is to minimise a cost functional with different costs defined on the states. In particular, we focus on problems in which a pre-specified sequence of active subsystems is given and the switching times are the only control variables. Based on the calculus of variation,we derive the gradient of the cost functional with respect to the switching times on an especially simple form, which can be directly used in gradient descent algorithms to locate the optimal switching instants. Finally, a numerical example is given, highlighting the validity of the proposed methodology. [ABSTRACT FROM AUTHOR]

This work studies the correlation between secondary electron emission (SEE) characteristics and impulse surface flashover in polyethylene nanodielectrics both theoretically and experimentally, and illustrates the enhancement of flashover voltage in low-density polyethylene (LDPE) through incorporating Al2O3 nanoparticles. SEE characteristics play key roles in surface charging and gas desorption during surface flashover. This work demonstrates that the presence of Al2O3 nanoparticles decreases the SEE coefficient of LDPE and enhances the impact energy at the equilibrium state of surface charging. These changes can be explained by the increase of surface roughness and of surface ionization energy, and the strong interaction between nanoparticles and the polymer dielectric matrix. The surface charge and flashover voltage are calculated according to the secondary electron emission avalanche (SEEA) model, which reveals that the positive surface charges are reduced near the cathode triple point, while the presence of more nanoparticles in high loading samples enhances the gas desorption. Consequently, the surface flashover performance of LDPE/Al2O3 nanodielectrics is improved. [ABSTRACT FROM AUTHOR]

A new adaptive control design method based on dynamic scaling and filter for high-order lower triangular system is given, which does not satisfy the certainty equivalence principle. The proposed practically adaptive state feedback controller that ensures all signals of closed-loop systems are globally bounded while keeping the output tracking error to an arbitrary small neighbourhood of the origin simultaneously. The proposed method relies on a novel parameter estimator proposed in Karagiannis & Astolfi (2008, Syst. Control Lett., 57, 733-739). Finally, a numerical example is given. [ABSTRACT FROM AUTHOR]

In this paper, a new insulation system of ZnO varistor ceramic-Al2O3 ceramic-ZnO varistor ceramic insulators (defined as A-B-A insulators) is proposed to improve surface flashover performance in vacuum so that it is possible to reduce the electric field at the vacuum-insulator-cathode triple junction (CTJ) by changing the conductivity and permittivity of the ZnO varistor ceramic layer of the A-B-A insulators. Then, the influences of the conductivity and permittivity of ZnO varistor ceramics on the dc and impulse surface flashover voltage in vacuum were experimentally investigated, respectively. It was found that the dc surface flashover voltage of the samples increases greatly with a decrease of the dc conductivity of the ZnO varistor ceramic layer and is improved by 81% compared with the Al2O3 ceramic sample. It is attributed to the decrease of the electric field at the vacuum-layer A-CTJ caused by the conductivity of layer A larger than that of layer B and, furthermore, by the nonlinear current-voltage characteristics of layer A, i.e., ZnO varistor ceramics. Additionally, the impulse surface flashover voltage first increases and then decreases with a decrease of the permittivity of the ZnO varistor ceramic layer and is increased by 136%. It is explained by the decrease of the electric field at the triple junction by the permittivity of layer A larger than that of layer B and, furthermore, by the nonlinear current-voltage characteristics of layer A. These results demonstrate the insulation system of the ZnO varistor ceramic-Al2O3 ceramic-ZnO varistor ceramic insulators as an effective way to improve surface flashover performance in vacuum. [ABSTRACT FROM AUTHOR]

In the present paper, we study the effect of trap depth for shallow traps on the surface flashover performance in vacuum; that is on deepening the trap depth, the flashover performance is firstly enhanced and then weakened, only when the trap depth changes in the certain range, the best surface flashover performance in vacuum is exhibited. Further to a previous study, we extensively study wide range of trap depths by changing the aggregation structure of semi-crystalline cross-linked polyethylene (XLPE). In the present study, we firstly annealed the XLPE at 135 °C for 10 minutes then cooled it at different conditions to get samples with different trap depths. We found that the trap depth deepened on increasing the grain size. Furthermore, we found that the flashover performances of the samples (Cooled at a rate of 0.5 °C /min) were enhanced by 76% and 19%, respectively under dc and impulse voltage. These results demonstrate heat-treatment as an effective way to enhance the flashover performance in vacuum for the semi-crystalline polymers. [ABSTRACT FROM AUTHOR]

With the existence of depletion layer, the intrinsic donor defect structure of ZnO ceramics has been investigated by broadband dielectric spectroscopy in a wide temperature range. Two loss peaks originating from electronic relaxation of oxygen vacancy and zinc interstitial, respectively, are observed in ZnO ceramics simultaneously, implying the coexistence of these two intrinsic defects. The corresponding activation energy for electronic relaxation of zinc interstitial and oxygen vacancy are 0.26 and 0.36 eV, respectively, which is consistent with other reports [ABSTRACT FROM AUTHOR]

Studies the degradation of epoxy resin-coated ZnO varistors at elevated temperatures and ambient humidity conditions. Change in the leakage current; Ionization of the moisture at the interface.

Charge transport properties in nanodielectrics present different tendencies for different loading concentrations. The exact mechanisms that are responsible for charge transport in nanodielectrics are not detailed, especially for high loading concentration. A charge transport model in nanodielectrics has been proposed based on quantum tunneling mechanism and dual-level traps. In the model, the thermally assisted hopping (TAH) process for the shallow traps and the tunnelling process for the deep traps are considered. For different loading concentrations, the dominant charge transport mechanisms are different. The quantum tunneling mechanism plays a major role in determining the charge conduction in nanodielectrics with high loading concentrations. While for low loading concentrations, the thermal hopping mechanism will dominate the charge conduction process. The model can explain the observed conductivity property in nanodielectrics with different loading concentrations. [ABSTRACT FROM AUTHOR]

This paper considers an optimal control problem for switched stochastic systems. The controlled parameter consists of the switching times, and the cost criterion has the form of the mixed integral and endpoint cost terms of performance functions defined on the system's state trajectory. An approach to finding the optimal switching times is proposed. First, we derive the derivative of the cost with respect to the switching time for one switching time case. Then, for the problem with multi-switching times, we present the corresponding gradient formula. The obtained formulas have an especially simple form and can be directly used in gradient descent algorithms to locate the optimal switching instants. Finally, the validity of the proposed method is demonstrated by two numerical examples. [ABSTRACT FROM AUTHOR]

The local structural behavior of PbZr0.5Ti0.5O3 (PZT 50/50) ceramics during application of an electric field was investigated using pair distribution function (PDF) analysis. In situ synchrotron total scattering was conducted, and the PDFs were calculated from the Fourier transform of the total scattering data. The PDF refinement of the zero-field data suggests a local-structure model with [001] Ti-displacement and negligible Zr-displacement. The directional PDFs at different field amplitudes indicate the bond-length distribution of the nearest Pb-B (B¼Zr/Ti) pair changes significantly with the field. The radial distribution functions (RDFs) of a model for polarization rotation were calculated. The calculated and the experimental RDFs are consistent. This result suggests the changes in Pb-B bond-length distribution could be dominantly caused by polarization rotation. Peak fitting of the experimental RDFs was also conducted. The peak position trends with increasing field are mostly in agreement with the calculation result of the polarization rotation model. The area ratio of the peaks in the experimental RDFs also changed with field amplitude, indicating that Zr atoms have a detectable displacement driven by the electric field. Our study provides an experimental observation of the behaviors of PZT 50/50 under field at a local scale which supports the polarization rotation mechanism. [ABSTRACT FROM AUTHOR]

Using a computer-aided image recording monitoring system, extensive measurements have been performed in the inner layer of 66 kV cross-linked polyethylene (XLPE) cables. It has been found that there are three kinds of electrical trees in the samples, the branch-like tree, the bush-like tree and the mixed tree that is a mixture of the above two kinds. When the applied voltage frequency is less than or equal to 250 Hz, only the mixed tree appears in XLPE samples, when the frequency is greater than or equal to 500 Hz, only the dense branch-like tree develops, both of which are attributed to the coexistence of non-uniform crystallization and internal residual stress in semicrystalline XLPE cables during the process of manufacturing. Through the fractal analyses of these electrical trees, it has been found that both the propagation and structure characteristics can be described by fractal dimension directly or indirectly. It is suggested that the propagation and structural characteristics of electrical trees are closely related to the morphology and the residual stress in material at low frequency, i.e., the propagation characteristics of electrical trees depends upon not only the boundaries between big spherulites and amorphous region, but also the impurity, micropore concentration and the relative position of needle electrode tip with respect to spherulites or amorphous region in the low frequency range. However, at high frequency, it has nothing to do with the morphology of material. It is suggested that the injection and extraction process of charge from and to dielectrics via the needle electrode are more intense at high frequency than in low frequency. Thus, it can form relatively uniform dielectric weak region in front of needle electrode, which leads to similar initiation and propagation characteristics of electrical trees at high frequency. [ABSTRACT FROM AUTHOR]

We employ transmission electron microscopy to explore the reason for large piezoelectricity (d33≈400pC/N) in a Pb-free 0.96(K0.5Na0.5)0.95Li0.05Nb0.93Sb0.07O3 -0.04BaZrO3 ceramic from microstructure. The result shows that the high piezoelectricity corresponds to a miniaturized nanodomain configuration in a domain hierarchy. The nanodomains disappear on heating accompanied by a reduction in d33 value. Further convergent beam electron diffraction study reveals a coexistence of tetragonal and orthorhombic phase, which indicates that large piezoelectricity of KNLNS0.07-BZ may stem from easy polarization rotation due to low polarization anisotropy on the tetragonal-orthorhombic phase boundary. [ABSTRACT FROM AUTHOR]

SrTiO3/La0.9Sr0.1MnO3 (STO/LSMO) multilayer was fabricated on n-Si (100) substrate by using a computer-controlled laser molecular-beam epitaxy technique. A rectifying behavior was observed in the multilayer. Meanwhile, the capancitance-voltage (C-V) characteristics were investigated, which reveals a hysteresis memory effect in forward and backward bias regions. A detail study suggests that the hysteresis behaviors originate from the cooperation and competition between the interfacial polarization and trapping/detrapping mechanisms. Our results are expected to meet the high desire of the optimization and device design of random access memory. [ABSTRACT FROM AUTHOR]

Space Solar Power Station (SSPS) will operate in geosynchronous orbit. Its operating environment is rather complicated, including space plasma, energetic particles radiation, extreme temperature, and cosmic rays. To reduce power loss, SSPS has to apply high-voltage (HV) power generation and transmission technique. Deep dielectric charging will be a major problem in the design of insulation structure in SSPS. In this paper, the hopping conductivity of polyimide was studied. Hopping distance and hopping barrier height were obtained from surface potential decay analysis. Second, a deep dielectric charging model of polyimide under FLUMIC spectrum electron irradiation was established, in which the hopping conductivity parameters were applied. The effects of electron flux enhancement and operating voltage on the charging characteristics were discussed in four cases, i.e., A) suspended-HV; B) HV-suspended; C) grounded-HV; and D) HV-grounded. When the electron flux enhancement increases, the influence on case A is much larger than the other three cases. When the operating voltage increases, the influence on cases A and B is small, whereas the influence on cases C and D increases fast. Thus, when designing the SSPS, the effect of operating voltage and electron flux enhancement on the insulation system must be carefully considered. [ABSTRACT FROM AUTHOR]

Silicone rubber (SiR) is used as an insulating material for cables installed in a nuclear power plant. Gamma rays irradiated SiR sheets for various periods at temperatures of 145 and 185 °C, and the resultant changes were analyzed by examining complex permittivity spectra and surface potential decay characteristics. Three different processes, namely, instantaneous polarization, electrode polarization due to the accumulation of ions to form double charge layers at dielectric/electrode interfaces, and DC conduction caused by directional hopping of ions, contribute to the complex permittivity. By fitting the spectra to theoretical equations, we can obtain the dielectric constant at high frequencies, concentration and diffusion coefficient of ions and DC conductivity for the pristine and degraded samples. The instantaneous polarization becomes active with an increase of dose and ageing temperature. The thermal expansion coefficient estimated from the temperature dependence of dielectric constant at high frequencies becomes smaller with an increase in dose, which is in good agreement with the experimental results of the swelling ratio. Additionally, trap distributions are calculated from surface potential decay measurements and analyzed to explain the variation in conductivity. Trap energy increases firstly, and then decreases with an increase in dose, leading to a similar change in DC conductivity. It is concluded that generations of both oxidative products and mobile ions, as well as the occurrence of chain scission and crosslinking are simultaneously induced by gamma rays. [ABSTRACT FROM AUTHOR]

MOEA/D is a promising multi-objective evolutionary algorithm based on decomposition, and it has been used to solve many multi-objective optimization problems very well. However, there is a class of multi-objective problems, called many-objective optimization problems, but the original MOEA/D cannot solve them well. In this paper, an improved MOEA/D with optimal differential evolution (oDE) schemes is proposed, called MOEA/D-oDE, aiming to solve many-objective optimization problems. Compared with MOEA/D, MOEA/D-oDE has two distinguishing points. On the one hand, MOEA/D-oDE adopts a newly-introduced decomposition approach to decompose the many-objective optimization problems, which combines the advantages of the weighted sum approach and the Tchebycheff approach. On the other hand, a kind of combination mechanism for DE operators is designed for finding the best child solution so as to do the a posteriori computing. In our experimental study, six continuous test instances with 4-6 objectives comparing NSGA-II (nondominated sorting genetic algorithm II) and MOEA/D as accompanying experiments are applied. Additionally, the final results indicate that MOEA/D-oDE outperforms NSGA-II and MOEA/D in almost all cases, particularly in those problems that have complicated Pareto shapes and higher dimensional objectives, where its advantages are more obvious. [ABSTRACT FROM AUTHOR]

A low frequency dielectric relaxation process of CaCu3Ti4O12 ceramics, which is generally overlapped by dc conduction in traditional dielectric spectra, is detected and clarified based on an improved analytical methodology in this letter. This relaxation process remarkably contributes to instability of low frequency permittivity and it clearly demonstrates the correlation between charge migration and space charge polarization. It is revealed that conduction will be achieved if carriers migrate and combine in the electrodes, while space charge polarization will be achieved if carriers migrate to the electrodes without combination. [ABSTRACT FROM AUTHOR]

Nitric oxide NO is one of the major targets for environmental monitoring, but the existing NO sensors are limited by their low sensitivity and narrow test range. Here, a NO gas sensor employing multiwalled carbon nanotubes (MWCNTs) was fabricated, and its properties in NO-N2 mixture were investigated from both emission and ionization. The current I e passing through the nanotubes cathode was found to decrease with increasing NO concentration and increase linearly in different slopes with the extracting voltage U e. It is shown that the Schottky barrier of the MWCNTs calculated by I e increased with NO concentration due to the adsorption of NO gas, which restrained the electron emission and consequently weakened the ionization. The positive ion currents I c passing through the collecting electrode at different voltages of U e were found to monotonically decrease with increasing NO concentration, which was induced by both of the reduced electron emission and the consumption of the two excited metastable states N2(A³u +) and N2(a'¹u -) by NO. The sensor exhibited high sensitivity at the low temperature of 30 °C. The calculated conductivity was found to be able to take place of Ic for NO detection in a wide voltage range of 80-150 V U e. [ABSTRACT FROM AUTHOR]

The piezoceramic 0.96(K0.5Na0.5)0.95Li0.05Nb0.93Sb0.07O3-0.04BaZrO3 (KNLNS0.07-BZ), which shows large piezoelectric response (d 33 ≈ 425 pC/N), has been considered as one of the promising Pb-free substitutions for Pb(Zr,Ti)O3. In this paper, we investigate the phase transition sequence for KNLNS0.07-BZ by employing the dielectric measurement, mechanical spectroscopy, as well as Raman spectroscopy. Two ferroelectric-ferroelectric transitions have been detected by inspecting anomalies in the spectra, indicating the existence of three ferroelectric phases. Moreover, in-situ X-ray diffraction study has been further performed on KNLNS0.07-BZ to identify the crystal structure for each phase. The result reveals that the phase sequence for KNLNS0.07-BZ evolves from tetragonal (T) to rhombohedral (R) via an intermediate orthorhombic (O) phase. And the piezoelectric-optimal region for KNLNS0.07-BZ locates on a T-O boundary rather than the previously reported T-R boundary. Strong piezoelectricity may stem from the easier polarization rotation on the T-O boundary with reduced polarization anisotropy. [ABSTRACT FROM AUTHOR]

The piezoceramic 0.96(K0.5Na0.5)0.95Li0.05Nb0.93Sb0.07O3-0.04BaZrO3 (KNLNS0.07-BZ), which shows large piezoelectric response (d33 ≈ 425 pC/N), has been considered as one of the promising Pb-free substitutions for Pb(Zr,Ti)O3. In this paper, we investigate the phase transition sequence for KNLNS0.07-BZ by employing the dielectric measurement, mechanical spectroscopy, as well as Raman spectroscopy. Two ferroelectric-ferroelectric transitions have been detected by inspecting anomalies in the spectra, indicating the existence of three ferroelectric phases. Moreover, in-situ X-ray diffraction study has been further performed on KNLNS0.07-BZ to identify the crystal structure for each phase. The result reveals that the phase sequence for KNLNS0.07-BZ evolves from tetragonal (T) to rhombohedral (R) via an intermediate orthorhombic (O) phase. And the piezoelectric-optimal region for KNLNS0.07-BZ locates on a T-O boundary rather than the previously reported T-R boundary. Strong piezoelectricity may stem from the easier polarization rotation on the T-O boundary with reduced polarization anisotropy. [ABSTRACT FROM AUTHOR]

The electric breakdown field of CaCu3Ti4O12 ceramics has been enhanced by one order, i.e. from the conventional 1.0-2.0 kV cm-1 to 21 kV cm-1. Such great enhancement, associated with lower and relatively flat dielectric loss at low frequency, has arisen from optimum Al2O3 addition and sintering process. It is indicated that the addition of Al2O3 can affect the grain growth and the grain boundary characteristics via the distribution of a secondary phase of CuAl2O4, which is confirmed by x-ray diffraction and electron dispersive spectroscopy. The performance of grain boundary can be described well by the Schottky barrier model. It is found that the activation energy of hopping conduction at grain boundary is increased from 0.60 to 0.81 eV due to the secondary phase, which consequently leads to enhanced breakdown field. [ABSTRACT FROM AUTHOR]

Research on the vacuum surface flashover mechanisms of insulating material under nanosecond pulses is important for the design and manufacture of high-power pulse generators. Here, the vacuum surface flashover characteristic of epoxy composites filled with different microfillers Al2O3·3H2O, Al2O3, BaTiO3, and TiO2 is investigated using a unipolar pulse generator (50-ns rise time and 600-ns full-width at half-maximum). In addition, their thermally stimulated depolarization current and secondary electron emission (SEE) coefficients are measured. The results show that the flashover performance does not depend just on the SEE coefficient, but that the different effects of shallow and deep traps must also to be taken into account. [ABSTRACT FROM AUTHOR]

In the present letter, we report that through a reversible domain switching mechanism, a large electro-shape-memory effect (up to 0.23% under the electric field of 5 kV/mm) is achieved in the aged Mn-doped (Pb,Sr) TiO3 ceramics. We also provide the microscopic understanding on the relationship between the strain (&eh;) and field (E) curve with aging phenomenon. Different from the common analogue response (linear converse piezoelectric effect), such electrostrain exhibits a digital characteristic and consequently enables a promising 'on-off' application. Owing to the domain switching nature, the key parameters (e.g., maximum strain, coercive field and working temperature range) of the present electrostrain can be customized by changing the ratio of Pb/Sr and thus the ratio of c/a to facilitate the possible practical applications. [ABSTRACT FROM AUTHOR]

Acceptor doping and donor doping have been known to result in opposite ferroelectric aging effects, but the aging effect of hybrid-doped (acceptor+donor) ferroelectrics has remained unclear. In this letter the authors report the aging effect in Mn3++Nb5+ hybrid-doped BaTiO3 ceramics with acceptor fixed at 1 mol % but with donor varied from 0.5 to 2 mol %. The authors found surprisingly that ferroelectric aging existed in all the samples, no matter which of the two, acceptor or donor, was dominant: all the samples showed a double hysteresis loop after aging, but with increasing donor (Nb5+) concentration the coercive field and hysteresis decrease. Meanwhile, a large nonlinear recoverable electrostrain up to 0.17% (due to reversible domain switching) was observed at 3 kV/mm, which exceeded that of acceptor-monodoped ceramics and was about twice the piezoelectric strain of hard lead zirconate titanate. These results demonstrate that hybrid doping is an effective way to enhance the domain-switching-related electrostrain properties. Finally, the aging effect of hybrid-doped ferroelectrics was explained. [ABSTRACT FROM AUTHOR]

Molecular chains with chemical defects acting as deep traps can be orientated by an electric field after trapping charges. Electric field force acts for a long period on a ‘charged’ molecular chain with occupied deep traps because of the long residence time of trapped charges. Based on the bipolar charge transport model, and the fact of thermally-activated motion of molecular chains and expansion of free volume when above the glass transition temperature, numerical simulation of coupling charge transport and molecular chain dynamics for polymeric insulating material under electro-thermal stress was conducted. Simulation results confirmed that temperature effects could enhance charge injection and charge energy, strengthening impact ionization, while also extending molecular chain displacement and reducing molecular chain fracture strength at high temperatures, thus synergistically forming chain scission and thereby the conductive paths. Other than the single factor of the space charge effect, the extended molecular chain displacement with increased temperature and thickness should be responsible for temperature- and thickness-dependent breakdown phenomena. [ABSTRACT FROM AUTHOR]