Electric field breakdown in borosilicate glass and other amorphous insulators

It is proposed that the intrinsic breakdown field strenght, E_B, can be determined from P=σE²_B where P is the power absorbed from an incident pulse of X-rays and σ is the measured conductivity.     

Excitation and electrical breakdown of solid insulators

Electrical breakdown of solid insulators is a complex process which is governed at every moment by many simultaneous excitation and relaxation effects in the investigated material, by electrotransport in the electron and ion subsystems, and by the formation of a plasma and its explosive action resulting in fracture of the sample.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 32–37, May, 1980.     

Ground-state decay rate for the Zener breakdown in band and Mott insulators

Nonlinear transport of electrons in strong electric fields, as typified by dielectric breakdown, is reformulated in terms of the ground-state decay rate originally studied by Schwinger in nonlinear QED. We discuss the effect of electron interaction on Zener tunneling by comparing the dielectric breakdown of the band insulator and the Mott insulator, where the latter is studied by the time-dependent density-matrix renormalization group. The relation with the Berry's phase theory of polarization is also established.     

Dielectric breakdown of Mott insulators in dynamical mean-field theory

Using nonequilibrium dynamical mean-field theory, we compute the time evolution of the current in a Mott insulator after a strong electric field is turned on. We observe the formation of a quasistationary state in which the current is almost time independent although the system is constantly excited. At moderately strong fields this state is stable for quite long times. The stationary current exhibits a threshold behavior as a function of the field, in which the threshold increases with the Coulomb interaction and vanishes as the metal-insulator transition is approached.     

Ion-induced temperature rise in various types of insulators

Swift heavy ions induce a Gaussian temperature distribution Θ(r) in insulators which depend neither on the physical properties of the solid nor on the kind of the projectiles. In this paper, we show that all experimental data suitable for analysis confirm the validity of Θ(r). The same result is obtained for ZrSiO₄, MgAl₂O₄, KTiOPO₄, Al₂O₃ and Y₂O₃, where systematic experiments have not been performed yet. The analysis demonstrates that Θ(r) may be valid in biomolecular targets and in high-T_c superconductors as well. The Fourier equation cannot reproduce the relation Θ(r); thus, it is not suitable for the estimation of the ion-induced temperatures. The consequences of the uniformity in track formation must also affect other radiation-induced effects.     

Electrical breakdown in solid insulators: Duration of the discharge channel formation

The duration of the discharge channel formation is calculated for 0.01-to 0.5-mm-thick insulators in terms of the avalanche-streamer theory. At this stage of breakdown development, the statistical delay is assumed to be the time from the instant the testing voltage becomes equal to the breakdown value to the instant of streamer origination and that the formation time is the interval between the instant of streamer origination and the instant the electrodes close through the discharge channel.     

Laser-induced breakdown of soda-lime glass microspheres using Nd:YAG laser

The Nd:YAG laser-induced breakdown of 20 μm glass microspheres was investigated using time-resolved optical shadow and Schlieren images. Time-resolved imaging showed the location of the initial breakdown and the shockwave motion over its first 400 μm of expansion. Measured shockwave velocities were in the range of 1–10 km/s and showed a linear dependency on laser fluence within 30 ns.     

Prospects for the LIBS study of films using an axicon optical system

The features of the application of an axicon optical system for the LIBS sampling are described. For the axicon sampling, the crater depth is less than that for a conventional spherical lens. The zone of thermal effect around a sampling point is also reduced. The results obtained indicate that the axicon optical system is promising for the thin-film study.     

Space-charge limited currents in insulators with the Gaussian distribution of traps

A direct mathematical procedure is presented for obtaining the volt-ampere characteristic in an insulating material having Gaussian energy-distribution of traps, under the condition of space-charge-limited emission into the material. The current-voltage dependence is of the V ⁿ type; the exponent n depends on the temperature and the width of the Gaussian distribution. The current-voltage dependence derived for the Gaussian trap energy distribution is formally identical with the case of exponential distribution of the trap density in energy.     

MRI and x-ray CT study of spatial distribution of core breakdown in 'Conference' pears

Two non-destructive tomographic techniques, X-ray CT imaging and magnetic resonance imaging (MRI), were applied to study the development of core breakdown disorder in 'Conference' pears (Pyrus communis cv. Conference). This disorder, which is characterized by brown discoloration of the tissue and development of cavities, is induced by elevated CO(2) and decreased O(2) levels during controlled atmosphere storage. Tomographic images of pears stored for 10 months under disorder inducing conditions, were acquired with both techniques and compared to the actual slices. Both X-ray and MRI were able to differentiate between unaffected tissue, brown tissue and cavities. A simple image-processing program, based on threshold values, was developed to determine the area percentage of affected and unaffected tissue as well as the cavity and core area per slice. For all three imaging techniques the area percentage brown tissue per slice increased with the diameter of the pear, but was systematically underestimated by 12% and 6% for, respectively, X-ray and MRI, compared to the actual slices. The area percentage cavity corresponded very well for all techniques. It was also found that the contours of the brown tissue were parallel to the fruit boundaries, suggesting a relation between the disorder symptoms and gas diffusion properties of the fruit. It was concluded that MRI is the most appropriate technique to study the development of core breakdown disorder during postharvest storage in future experiments.     

Laser-induced breakdown threshold and fracture versus the nanostructure of two-phase glass

The radiation hardness and mechanical strength of single-and two-phase glasses are studied for the case when nanosecond laser pulses (λ= 1.06 μm, τ_0.5 ≈ 12.5 ns) are focused inside the material. Laser interferometry is applied to measure the displacement of the free surface, find optical breakage thresholds, and carry out the fractographic analysis of damaged regions. It is shown that breakdown channels and damage regions develop in a nonlinear manner according to optical breakdown mechanisms, changing each other with an increase in the laser energy. The strength of the two-phase glass is found to be more than four times that of the single-phase glass, although their elastic properties differ insignificantly. Such a considerable difference in the hardness of these materials with chemically similar constitutents is attributed to the presence of the double-lattice nanometer-scale structure of the two-phase glass.     

Feasibility study for detecting copper contaminants in transformer insulation using laser-induced breakdown spectroscopy

In recent times, copper sulphide (Cu₂S) diffusion in the transformer insulation is a major problem reducing the life of transformers. It is therefore essential to identify a simple methodology to understand the diffusion of Cu₂S into the solid insulation [oil impregnated pressboard (OIP)]. In the present work, laser-induced breakdown spectroscopy (LIBS) was adopted to study the diffusion of Cu₂S into the pressboard insulation and to determine the depth of diffusion. The diffusion of Cu₂S in pressboard was confirmed by electrical discharge studies. In general, flashover voltage and increase in ageing duration of pressboard insulation/Cu concentration had inverse relationship. The characteristic emission lines were also studied through optical emission spectroscopy. Based on LIBS studies with Cu powder dispersed pressboard samples, Cu I emission lines were found to be resolvable up to a lowest concentration of 5 μg/cm². The LIBS intensity ratio of Cu I–Ca II emission lines were found to increase with increase in the ageing duration of the OIP sample. LIBS studies with OIP samples showed an increase in the optical emission lifetime. LIBS results were in agreement with the electrical discharge studies.     

Luminescence characterisation of alumina substrates using cathodoluminescence microscopy and spectroscopy

Polycrystalline alumina (Al₂O₃) substrates, found in many electronic devices and proposed as dosemeters in emergency situations, were invstigated using a scanning electron microscope (SEM) equipped with cathodoluminescence (CL) and elemental analysis probes. The characteristics of the CL spectra, surface morphology, and impurity content of the Al₂O₃ substrates were examined and compared with those of single crystal dosimetry-grade Al₂O₃:C. Whereas the CL spectrum, measured from 250 to 800 nm, for the Al₂O₃:C, contained resolved bands located at ∼340 nm and at ∼410 nm, the spectrum measured with the Al₂O₃ substrate was significantly broader, extending from ∼250 to ∼450 nm, and also included a narrow band at 695 nm. While it is likely that the accepted model of recombination at F⁺ (∼340 nm) and F (∼410 nm) in Al₂O₃:C also applies to the substrate, it is suggested that the presence of impurities within the alumina give rise to additional recombination centres. The 695 nm emission has been assigned to a Cr³⁺ ion impurity in previous work on alumina and a band indicated at ∼300 nm may be associated with Mg²⁺ or Ca²⁺_, the presence of which was confirmed by elemental mapping. Comparison of the spatial distribution of CL with the surface morphology and elemental composition of the samples indicates that the components of the emission spectrum can be qualitatively correlated with impurity content and morphological features of the samples.     

基于流体模型和非平衡态电子能量分布函数的高功率微波气体击穿研究

用流体模型研究高功率微波气体击穿时, 电子能量分布函数常被假设为麦克斯韦分布形式, 此假设可能将给模拟结果带来较大的误差. 通过求解玻尔兹曼方程, 得到非平衡状态下的电子能量分布函数. 分别将上述两类分布函数引入到流体模型中, 对氩气击穿进行了数值模拟. 结果表明, 基于非平衡分布函数得到的击穿时间与粒子模拟结果符合得很好, 而当平均电子能量较低时, 麦克斯韦分布函数的高能尾部导致了较短的击穿时间. 最后, 采用非平衡分布函数计算了不同压强下的氩气击穿阈值, 发现其与实验结果基本符合. 关键词： 微波气体击穿 电子能量分布函数 流体模型 玻尔兹曼方程     

Laser-induced breakdown spectroscopy for on-line control of laser cleaning of sandstone and stained glass

Laser-induced breakdown spectroscopy (LIBS) was used to control the cleaning process of polluted sandstone and medieval stained glass. A combination of a KrF Excimer and LIBS was used to clean areas of the artwork. The spectroscopic study of the plasma emission was used to determine the elemental composition of the crust and the underlying material. The on-line implementation of the spectroscopic technique LIBS to the cleaning process provides important information about the optimal cleaning parameters of artworks from glass and stone in order to avoid over-cleaning. Received: 26 January 1999 / Accepted: 22 April 1999 / Published online: 7 July 1999     

A study on cathodoluminescence intensities in the range below 3 kV

It has been shown that the dependence of cathodoluminescence intensities on accelerating voltage, in the range below 3 kV, is considerably changed with the conditions of the screen made from the same phosphor. The experimental results suggest that the threshold voltage of the curve obtained is correlated with neither the depth of the surface recombination layer covering the crystal nor the conductivity of the crystal.     

Influence of electron-beam irradiation in SEM on the cathodoluminescence and electron-beam-induced current in InGaN/GaN light-emitting diodes with a buried active region

The influence of irradiation in a scanning electron microscope on the optical properties inherent to light-emitting diodes (LEDs) with multiple InGaN/GaN quantum wells, assembled by means of the flip-chip mounting technique, has been investigated via the cathodoluminescence (CL) and electron-beam-induced current methods. It is demonstrated that the action of an electron beam qualitatively varies both these LEDs and structures with a thin upper GaN layer only at large beam energies. It has been revealed that irradiation not only leads to changes in the spectrum and intensity of CL but also decreases the energy corresponding to the excitation of emission associated with quantum wells. A similar effect is also observed in structures whose external quantum efficiency has been decreased several times due to long-running tests performed at an injection current density of 35 A/cm² and a temperature of 100°C.     

Low-temperature spectrally resolved cathodoluminescence study of degradation in opto-electronic and microelectronic devices

This study reports on the microcharacterization of devices for optoelectronic and for microelectronic applications using low temperature (T = 5 and 77 K) spectrally resolved cathodoluminescence (SCL). The mechanisms leading to compositional inhomogeneities in the regrowth regions of InP-based butt-coupled laser-waveguide devices for semiconducting optical amplifiers (SOAs) and for defect generation in the active and cladding layers of GaAs based pump lasers for erbium-doped optical fibre amplifiers (EDFAs) were studied. Beryllium outdiffusion in the base regions of GaAs-based heterojunction bipolar transistors (HBTs) after bias ageing was also studied. By comparing the CL results with TEM, SIMS and HRXRD studies and with the existing literature, the observed growth and operation induced defects were attributed, respectively, to the following mechanisms: recombination-enhanced defect glide (REDG) in the pump lasers, recombination enhanced impurity diffusion (REID) in the HBTs and electrostatically induced growth flux instabilities in the butt-coupled laser-waveguide devices.