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Water impurity is the essential factor of reducing the insulation performance of transformer oil, which directly determines the operating safety and life of a transformer. Molecular dynamics simulations and first-principles electronic-structure calculations are employed to study the diffusion behavior of water molecules and the electrical breakdown mechanism of transformer oil containing water impurities. The molecular dynamics of an oil-water micro-system model demonstrates that the increase of aging acid concentration will exponentially expedite thermal diffusion of water molecules. Density of states (DOS) for a local region model of transformer oil containing water molecules indicates that water molecules can introduce unoccupied localized electron-states with energy levels close to the conduction band minimum of transformer oil, which makes water molecules capable of capturing electrons and transforming them into water ions during thermal diffusion. Subsequently, under a high electric field, water ions collide and impact on oil molecules to break the molecular chain of transformer oil, engendering carbonized components that introduce a conduction electronic-band in the band-gap of oil molecules as a manifestation of forming a conductive region in transformer oil. The conduction channel composed of carbonized components will be eventually formed, connecting two electrodes, with the carbonized components developing rapidly under the impact of water ions, based on which a large number of electron carriers will be produced similar to “avalanche” discharge, leading to an electrical breakdown of transformer oil insulation. The water impurity in oil, as the key factor for forming the carbonized conducting channel, initiates the electric breakdown process of transformer oil, which is dominated by thermal diffusion of water molecules. The increase of aging acid concentration will significantly promote the thermal diffusion of water impurities and the formation of an initial conducting channel, accounting for the degradation in dielectric strength of insulating oil containing water impurities after long-term operation of the transformer.

Iron-modified biochar adsorbent shows its promise for removing Cr(VI) from water. However, the traditional dry-grinding modification methods to prepare iron-modified biochar (DFeBC) usually result in the aggregation of iron oxide particles, thus impeding the adsorption capacity. Herein, blue-green algae waste derived biochar modified in a wet process by K2FeO4 (WFeBC) was developed, and it showed well-distributed iron oxide on the surface of the biochar. As a result, WFeBC displayed a larger theoretical adsorption capacity of 66.22 mg/g than that of DFeBC (48.54 mg/g). It was found that the uniform-distributed iron oxide and abundant oxygen functional groups of WFeBC played an important role in the removal process of Cr(VI). In the adsorption process, the reduction of Cr(VI) to Cr(III) by Fe(II) promoted the removal of hexavalent chromium. The Cr(VI) will be attracted by protonated oxygen functional groups through electrostatic interaction and complexation in an acid environment. All the results show that the wet modification of biochar by K2FeO4 can effectively improve the removal ability of WFeBC to Cr(VI). [ABSTRACT FROM AUTHOR]

The complex configuration, H2 adsorption binding energy, magnetic, and optical properties of FAU zeolites with Ag cations loaded by ion exchange in the vacant dielectric cavities were investigated by employing the first-principles calculations with all-electron-relativistic numerical atom-orbitals scheme and the Metropolis Monte Carlo molecular simulations. The visible absorption spectrum peaked at distinct wavelengths arranging from red or green to blue colors when changing the net charge load, due to the produced various redox states of Ag cations exchanging at multiple Li+-substituted sites. The spin population analyses indicate the ferrimagnetic coupling between Al⁻O⁻Si framework and Ag cations originates from the major ferromagnetic spin polarization in Ag cation cluster coordinating with sodalite cages, with the net spins appreciably depending on the Ag content and exchange site. The H2 adsorption capacities and binding energies represent significant dependence on the content, location, and electronic property of Ag cations introduced into the FAU zeolites. The evident decrease of H2 adsorption binding energy with increased loading concentration demonstrates repulsive interaction between H2 molecules and heterogeneous adsorption configuration on Ag cations. The adsorption sites of H2 sorted by the binding energy with different adsorption configurations were correlated with exchange sites of Ag cations under different Ag loading to comprehend the H2 adsorption mechanism.