"Sub-Hertz" Dielectric Spectroscopy

Dielectric spectroscopy measurements below 1 Hz are often dominated by “conduction-like” effects. For this reason, they often appear to be dismissed as being of little interest. In this paper two “sub-hertz” responses are considered that give insights into the insulating sys-tems concerned. The first system is that of cross-linked polyethylene, taken from a power cable system. Measurements at temperatures between 60°C and close to melting at 100°C show a change in characteristic from a percolation process to a “true” DC conduction at close to the melting point. Using DC conductivities, it appears to be possible to show whether the cable has been subjected to thermo-electric ageing. This might give insights into where the conduction and hence the ageing in the XLPE is occurring. The second system is an epoxy composite. By considering the sub-hertz response, it is possible to demonstrate the effect of the interface between the filler and the epoxy matrix. In this system, ageing, resulting in delamination between the glass fiber filler and the epoxy, is clearly detected by sub-hertz dielectric spectroscopy. This process is likely to be facilitated by the presence of water, which is known to lead to mechanical failure in such systems, and which can also be detected by "sub-hertz" dielectric spectroscopy. The implications for nano-dielectrics are then briefly considered

Dielectric spectroscopy of epoxy/glass composite materials

Glass fibre reinforced epoxy (GFRE) material is used in pressboard transformers for optical telecommunication systems, typically at voltages between 1 to 2kV. A programme has been set up to follow the electrical ageing of the GFRE through dielectric and space charge (PEA) measurements. Here we report on the characterisation of the GFRE prior to ageing made by means of linear dielectric spectroscopy. Preliminary results for the aged samples show differences in dielectric response that could be related to de-bonding at the epoxy-fibre interfaces observed in some failed samples

230 s room-temperature storage time and 1.14 eV hole localization energy in In0.5Ga0.5As quantum dots on a GaAs interlayer in GaP with an AlP barrier

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 106, 042102 (2015) and may be found at https://doi.org/10.1063/1.4906994.A GaP n+p-diode containing In0.5Ga0.5As quantum dots (QDs) and an AlP barrier is characterized electrically, together with two reference samples: a simple n+p-diode and an n+p-diode with AlP barrier. Localization energy, capture cross-section, and storage time for holes in the QDs are determined using deep-level transient spectroscopy. The localization energy is 1.14(±0.04) eV, yielding a storage time at room temperature of 230(±60) s, which marks an improvement of 2 orders of magnitude compared to the former record value in QDs. Alternative material systems are proposed for still higher localization energies and longer storage times

Space charge behaviour in epoxy laminates under high constant electric field

The development of space charge in insulating materials is one of the main causes of their electrical ageing. The pulsed electro-acoustic method is often used to determine space charge distribution, but the signal analysis in the case of laminate structures is much more complex to analyse. In this paper the authors describe and use a simulated signal in order to study laminates made of epoxy resin and fibre mat. The relatively large conductivity of the fibres compared with that of the resin seems to produce a rapid charge dissociation and recombination in the fibres. Under voltage the presence of fibres close to an electrode seems to promote charge injection

Materials for Future Quantum Dot-Based Memories

The present paper investigates the current status of the storage times in self-organized QDs, surveying a variety of heterostructures advantageous for strong electron and/or hole confinement. Experimental data for the electronic properties, such as localization energies and capture cross-sections, are listed. Based on the theory of thermal emission of carriers from QDs, we extrapolate the values for materials that would increase the storage time at room temperature to more than millions of years. For electron storage, GaSb/AlSb, GaN/AlN, and InAs/AlSb are proposed. For hole storage, GaSb/Al0.9Ga0.1As, GaSb/GaP, and GaSb/AlP are promising candidates

Intelligent machine learning with evolutionary algorithm based short term load forecasting in power systems

Electricity demand forecasting remains a challenging issue for power system scheduling at varying stages of energy sectors. Short Term load forecasting (STLF) plays a vital part in regulated power systems and electricity markets, which is commonly employed to predict the outcomes power failures. This paper presents an intelligent machine learning with evolutionary algorithm based STLF model, called (IMLEA-STLF) for power systems which involves different stages of operations such as data decomposition, data preprocessing, feature selection, prediction, and parameter tuning. Wavelet transform (WT) is used for the decomposition of the time series and Oppositional Artificial Fish Swarm Optimization algorithm (OAFSA) based feature selection technique to elect an optimal set of features. In order to improvise the convergence rate of AFSA, oppositional based learning (OBL) concept is integrated into it. Then, the water wave optimization (WWO) with Elman neural networks (ENN) model is employed for the predictive process. Finally, inverse WT is applied and obtained the hourly load forecasting data. To validate the effective predictive outcome of the IMLEA-STLF model, an extensive set of simulations take place on benchmark dataset. The resultant values ensured the promising results of the IMLEA-STLF model over the other compared methods

Electronic Properties and Density of States of Self-Assembled GaSb/GaAs Quantum Dots

The electronic properties of a self-assembled GaSb/GaAs QD ensemble are determined by capacitance-voltage (C-V) and deep-level transient spectroscopy (DLTS). The charging and discharging bias regions of the QDs are determined for different temperatures. With a value of 335 (±15) meV the localization energy is rather small compared to values previously determined for the same material system. Similarly, a very small apparent capture cross section is measured (1·10−16 cm2). DLTS signal analysis yields an equivalent to the ensemble density of states for the individual energies as well as the density function of the confinement energies of the QDs in the ensemble