[en] Integrated effect and evolution of streamer dynamics were investigated under long-term repetitive sub-microsecond pulses (RMP) based on the pulse-sequence resolved electrical and optical measurements in 0.1–0.4 MPa nitrogen. New statistical methods were proposed for characterizing the discharge evolution process, including the repetitive working coefficient and the ‘sequence-phase-intensity-density’ diagram. A three-stage discharge evolution pattern is applicable for positive and negative RMP. A periodical feature of intermittent-mode corona discharges appears with increasing the pulse repetition frequency (PRF) under positive RMP, where the interval number increases and interval time gradually saturates with PRF. The breakdown occurs at high PRF following successive-mode corona discharges. The breakdown envelope curve consists of a rapidly falling stage followed by a saturation stage at low pressure or a rising stage at high pressure. Subsequent corona discharges are facilitated under negative RMP based on decrements in inception voltage and phase. Low-amplitude DC sweeping voltage and electric field nonuniformity illustrate dramatic and nonlinear influences on the evolution of streamer dynamics and breakdown characteristics under RMP. The integrated effects under long-term RMP in high-pressure nitrogen are not effectively supported by the traditional metastable-species-dominated mechanism and the uniform pre-ionization assumption in repetitively pulsed discharges. The space-charge-dominated memory effect mechanism is proposed based on the electron depletions at the voltage pulse front/tail and the effect of space charges on the spatial distribution of the electric field strength. (paper)

[en] Evolution of the surface streamer and the discharge mode transition from the corona discharge to the surface flashover were investigated by pulse sequence resolved electrical and optical measurements under long-term repetitive nanosecond pulses (RNP). The test sample was a cylindrical epoxy resin insulator attached with a needle electrode in 0.1–0.4 MPa nitrogen. Under positive RNP, the inception phase of subsequent streamer discharges decreases with increasing the pulse repetition frequency (PRF), and a periodical streamer stagnation phenomenon appears at high gas pressure. Under negative RNP, the surface streamer does not illustrate symbolic decreasing tendencies in light intensity and corona inception phase in gas gap. The dependence of the evolution of surface streamer velocity on PRF is correlated with the repulsive force from surface charges and the accessibility of seed electrons. Statistical characteristics of back discharges under negative RNP are qualitatively explained by the local electric field around the triple junction. The reversal phenomenon of polarity effect of the allowable repetitive working coefficient is probably resulted from the corona stabilization effect, the surface electron detrapping process, and the assistance of background electric field. Principal results qualitatively support that behaviors of surface streamer and flashover are dominated by the electric field distribution, volume and surface memory effects as well as their interactions and bidirectional transformations, which are different from the monotonically facilitative tendency predicted by the metastable-species-dominated memory effect mechanism. (paper)

[en] Streamer dynamics under repetitive nanosecond pulse (RNP) is of great importance to understand the streamer essence and breakdown phenomenon in non-equilibrium-plasma-based applications. However, the evolution of streamer dynamics, discharge mode transition, and influential mechanisms in high-pressure gas under long-term RNP were not sufficiently clear and required further investigations. We presented the pulse-sequence resolved analysis on streamer dynamics, discharge mode transition, and polarity effect in high-pressure nitrogen under long-term RNP of pulse width from 15 to 800 ns and RNP superimposed by DC sweep voltage. Under positive RNP, the corona discharge probability in a pulse train was affected by the pulse repetition frequency (PRF) and a transformation from the intermittent mode to continuous mode appeared under high PRF. The envelope curve of the number of applied pulses before breakdown followed the shape transformation from capital ‘L’ to ‘U’ with pressure from 0.1 to 0.4 MPa. The streamer channel contraction and the spatial shift of streamer initiation position under high PRF were observed. Under negative RNP, the corona discharge would be in the continuous mode from a low PRF. The average inception voltage and phase of following corona discharges were lower and earlier than those of the first one, respectively. The auxiliary sweep voltage illustrated dramatic and nonlinear effects on discharge characteristics under RNP, which was determined by the sweep voltage amplitude, pulse polarity, and PRF. The inception oscillation stage and steady-state stage were distinct under short-width RNP. An influential mechanism in high-pressure nitrogen was proposed with emphasis on the spatial electric field distortion caused by remaining space charges, which was affected by the pressure-dependent nonlinear diffusion and time-dependent space charge drift under a sweep voltage. The evolution of streamer dynamics and discharge mode transition would help in better understanding of the streamer essence and insulation capability criteria under long-term RNP. (paper)

[en] A simple way of fabricating AlN-SiC solid solutions (AlN-SiCss) through the combustion reaction of aluminum, carbon and Si₃N₄ powder mixtures in air is reported. X-ray diffraction analysis, scanning electron microscopy and energy dispersive spectroscopy are employed to analyze the phase composition and microstructure of the as-synthesized products. It is found that the phase composition of the product changes gradually with the increase in air infiltration distance, accompanied by variation in the partial pressure of N₂/O₂ at different locations of the powder compacts. Homogeneous single-phase AlN-SiCss powders with well-crystallized hexagonal morphologies and a fine particle size of 3 μm can be obtained within a suitable distance range of N₂ infiltration. Thermodynamic analysis of the Al-C-Si₃N₄-air combustion reaction system was conducted based on the N₂/O₂ diffusion kinetic model, and the calculated results are in good agreement with the experimental phenomenon

[en] All-ceramic multilayer composite ytterbium doped yttrium aluminum garnet (Yb:YAG) laser gain medium with doping concentration distribution of 0-5-10-15-20-15-10-5-0 at.% Yb ions was successfully fabrication by the technique of tape casting and simple vacuum sintering process. Full dense microstructure is achieved, and excellent optical properties are gained. The obtained result shows that the optical transmittance of >80% is reached when the wavelength is larger than 500 nm. The emission cross section is 4.03×10^-20 cm² at the wavelength of 1030 nm. Continuous wave (CW) laser performance is further demonstrated when the sample is pumped by 940 nm fiber-coupler diode laser. The threshold absorbed pump power is 5.9 W, and the slope efficiency attains to 27% with transmission of output coupler of 6%

[en] Permanent scatterers differential interferometric synthetic aperture radar (PS-DInSAR) is a technique for detecting surface micro-deformation, with an accuracy at the centimeter to millimeter level. However, its performance is limited by the number of SAR images available (normally more than 20 are needed). Compressive sensing (CS) has been proven to be an effective signal recovery method with only a very limited number of measurements. Applying CS to PS-DInSAR, a novel CS-PS-DInSAR method is proposed to estimate the deformation with fewer SAR images. By analyzing the PS-DInSAR process in detail, first the sparsity representation of deformation velocity difference is obtained; then, the mathematical model of CS-PS-DInSAR is derived and the restricted isometry property (RIP) of the measurement matrix is discussed to validate the proposed CS-PS-DInSAR in theory. The implementation of CS-PS-DInSAR is achieved by employing basis pursuit algorithms to estimate the deformation velocity. With the proposed method, DInSAR deformation estimation can be achieved by a much smaller number of SAR images, as demonstrated by simulation results. (paper)

[en] By employing vapor-phase transport method, ZnO microrods are fabricated and directly assembled on p-GaN substrate to form a heterostructural microlaser array, which avoids of the relatively complicated etching process comparing previous work. Under applied forward bias, whispering gallery mode ZnO ultraviolet lasing is obtained from the as-fabricated heterostructural microlaser array. The device's electroluminescence originates from three distinct electron-hole recombination processes in the heterojunction interface, and whispering gallery mode ultraviolet lasing is obtained when the applied voltage is beyond the lasing threshold. This work may present a significant step towards future fabrication of a facile technique for micro/nanolasers

[en] Conducting poly aniline (PANI) was in situ polymerized at the surface of electro spun poly(methyl methacrylate) (PMMA) fibers to obtain flexible composite fibers. The electrical conductivity of an individual PANI/PMMA composite fiber was estimated to be 2.0×10"-"1S cm"-"1 at room temperature. The ammonia sensing properties of the samples were tested by impedance analysis. The PANI/PMMA fibers could obviously respond to low concentration of ammonia at ppb level and could respond to relatively high concentration of ammonia at 10 ppm level quickly. In addition, the sensitivity exhibited a good linear relationship to the ammonia concentration. Particularly, the flexible PANI/PMMA fibers showed a reversible change in electrical resistance with repeated cycles of bending and relaxing, and the electrical resistance decreased with the increase of curvature. These results indicate that the flexible PANI/PMMA composite fibers may be used in toxic ammonia gas detection, strain sensing, and flexible electronic devices.

[en] We studied the photoluminescence (PL) and structural properties of Ce and Yb co-doped silicon oxide films after high temperature annealing. The PL spectra of Ce³⁺ and Yb³⁺ ions were sensitive to the structural variation of the films, and the Yb PL intensities were significantly enhanced especially upon 1200 °C annealing. X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy, indicated that rare earth silicates and the CeO₂ phase had formed in the oxides. The proportions of the phases varied with the “nominal Si-richness” of the films. Energy transfer from the excited Ce³⁺ to Yb³⁺ can be inferred from both PL excitation and decay spectra