Numerical calculation of transient electric field considering space charge in oil-paper insulation of converter transformers

Purpose The purpose of this paper is to develop a numerical simulation method based on the transient upstream finite element method (FEM) and Schottky emission theory to reveal the distribution characteristics of space charge in oil-paper insulation. Design/methodology/approach The main insulation medium of the converter transformer in high voltage direct current transmission is oil-paper insulation. However, the influence of space charge is difficult to be fully considered in the insulation design and simulation of converter transformers. To reveal the influence characteristics of the space charge, this paper proposes a numerical simulation method based on Schottky emission theory and the transient upstream FEM. This method considers the influence of factors, such as carrier mobility, carrier recombination coefficient, trap capture coefficient and diffusion coefficient on the basis of multi-physics field coupling calculation of the electric field and fluid field. Findings A numerical simulation method considering multiple charge states is proposed for the space charge problem in oil-paper insulation. Meanwhile, a space charge measurement platform based on the electrostatic capacitance probe method for oil-paper insulation structure is built, and the effectiveness and accuracy of the numerical simulation method is verified. Originality/value A variety of models are calculated and analyzed by the numerical simulation method in this paper, and the distribution characteristics of the space charge and total electric field in oil-paper insulation medium with single-layer, polarity reversal of plate voltage and double-layer are obtained. The research results of this paper have the guiding significance for the engineering application of oil-paper insulation and the optimal design of converter transformer insulation.

Ionic Transport Triggered by Asymmetric Illumination on 2D Nano-Membrane

Ionic transport and ion sieving are important in the field of separation science and engineering. Based on the rapid development of nanomaterials and nano-devices, more and more phenomena occur on the nanoscale devices in the field of thermology, optics, mechanics, etc. Recently, we experimentally observed a novel ion transport phenomenon in nanostructured graphene oxide membrane (GOM) under asymmetric illumination. We first build a light-induced carriers’ diffusion model based on our previous experimental results. This model can reveal the light-induced ion transport mechanism and predict the carriers’ diffusion behavior under different operational situations and material characters. The voltage difference increases with the rise of illuminate asymmetry, photoresponsivity, recombination coefficient, and carriers’ diffusion coefficient ratio. Finally, we discuss the ion transport behavior with different surface charge densities using MD simulation. Moderate surface charge decreases the ion transport with the same type of charge due to the electrostatic repulsion; however, excess surface charge blocks both cation and anion because a thicker electrical double layer decreases effective channel height. Research here provides referenced operational and material conditions to obtain a greater voltage difference between the membrane sides. Also, the mechanism of ion transport and ion sieving can guide us to modify membrane material according to different aims.

Photocatalytic water splitting of nanocomposite materials based on TiO2 and rGO nanorods

The paper presents the results of a study of films formed by titanium dioxide nanorods and deposited on their surface of reduced graphene oxide by electrochemical deposition. Nanostructured films based on TiO2 nanorods were prepared in a 100 ml stainless steel autoclave with a fluoroplastic insert from a solution containing 35 ml of deionized water (H2O), 35 ml of hydrochloric acid (HCl) (36.5 %, Sigma–Aldrich) and 0.25 ml of titanium butylate C16H36O4Ti (97 %, Sigma–Aldrich). The addition of reduced graphene oxide to the structure of titanium dioxide nanorods increases the specific surface area of nanostructures from 29.3 m2 /g to 63.1 m2 /g. Calculations based on the film impedance spectra have shown that the optimal deposition time of reduced graphene oxide on the surface of TiO2 nanorods is 3 minutes, since it has a low recombination coefficient and a long electron lifetime. Studies of the photocatalytic activity of nanomaterials and registration of the released hydrogen and oxygen gases have shown that when the films are irradiated for 5 hours, the amount of hydrogen released varies from 50 to 225 mmol/cm2 .

Strong self-trapping by deformation potential limits photovoltaic performance in bismuth double perovskite

Bismuth-based double perovskite Cs2AgBiBr6 is regarded as a potential candidate for low-toxicity, high-stability perovskite solar cells. However, its performance is far from satisfactory. Albeit being an indirect bandgap semiconductor, we observe bright emission with large bimolecular recombination coefficient (reaching 4.5 ± 0.1 × 10−11 cm3 s−1) and low charge carrier mobility (around 0.05 cm2 s−1 V−1). Besides intermediate Fröhlich couplings present in both Pb-based perovskites and Cs2AgBiBr6, we uncover evidence of strong deformation potential by acoustic phonons in the latter through transient reflection, time-resolved terahertz measurements, and density functional theory calculations. The Fröhlich and deformation potentials synergistically lead to ultrafast self-trapping of free carriers forming polarons highly localized on a few units of the lattice within a few picoseconds, which also breaks down the electronic band picture, leading to efficient radiative recombination. The strong self-trapping in Cs2AgBiBr6 could impose intrinsic limitations for its application in photovoltaics.

Using the effective recombination coefficient to constrain the positive ion composition in Saturn's ionosphere

<p>We combine RPWS/LP and INMS data from Cassini's Grand Finale orbits into Saturn's lower ionosphere to calculate the effective recombination coefficient α<sub>300</sub> at a reference electron temperature of 300 K. Assuming photochemical equilibrium at altitudes below 2500 km and using an established method to determine the electron production rate, we derive upper limits for α<sub>300</sub> of ∼ 2.5∗10<sup>-7</sup> cm<sup>3 </sup>s<sup>-1</sup>, which suggest that Saturn's ionospheric positive ions are dominated by species with low recombination rate coefficients.<br />An ionosphere dominated by water group ions or complex hydrocarbons, as previously suggested, is incompatible with this result, as these species have recombination rate coefficients > 5∗10<sup>-7</sup> cm<sup>3 </sup>s<sup>-1</sup> at an electron temperature of 300 K. The results do not give constraints on the nature of the negative ions.</p>

Particle generation and recombination

This chapter introduces a semi-classical interpretation of particle generation and recombination using the bimolecular recombination coefficient and radiative lifetime. Particles—electrons and holes in the semiconductor—can be generated and recombine because of the multitude of energetic interactions. Radiative recombination and generation arise in the interaction with photons and can be spontaneous or stimulated. Important non-radiative processes such as the Hall-Shockley-Read process and the Auger process, which arise in multiparticle interactions, are discussed. Auger recombination is common at small bandgaps and high concentrations but also appears in large bandgap materials under high injection conditions. Impact ionization is an example of Auger generation arising from high fields. The Auger process is analyzed quantum-mechanically to show how energy and momentum conservation equations and quantum restrictions lead to the observed behavior. The chapter also discusses recombination at surfaces, which is inevitably present because of the defects and confined states arising from symmetry breaking.