ABSTRACT
High-voltage and high-power devices are indispensable in spacecraft for outer space explorations, whose operations require aerospace materials with adequate vacuum surface insulation performance. Despite persistent attempts to fabricate such materials, current efforts are restricted to trial-and-error methods and a universal design guideline is missing. The present work proposes to improve the vacuum surface insulation by tailoring the surface trap state density and energy level of the metal oxides with varied bandgaps, using coating on a polyimide (PI) substrate, aiming for a more systematical workflow for the insulation material design. First-principle calculations and trap diagnostics are employed to evaluate the material properties and reveal the interplay between trap states and the flashover threshold, supported by dedicated analyses of the flashover voltage, secondary electron emission (SEE) from insulators, and surface charging behaviors. Experimental results suggest that the coated PI (i.e., CuO@PI, SrO@PI, MgO@PI, and Al2O3@PI) can effectively increase the trap density and alter the trap energy levels. Elevated trap density is demonstrated to always yield lower SEE. In addition, increasing shallow trap density accelerates surface charge dissipation, which is favorable for improving surface insulation. CuO@PI exhibits the most remarkable increase in shallow trap density, and accordingly, the highest flashover voltage is 42.5% higher than that of pristine PI. This study reveals the critical role played by surface trap states in flashover mitigation and offers a novel strategy to optimize the surface insulation of materials.
ABSTRACT
With the development of vacuum electronic devices toward high power, high frequency, and miniaturization, the voltage holdoff capacity of the insulation materials in devices has also been raised to a higher demand. Cr/Mn/Ti-doped Al2O3 ceramics were prepared, and the bulk density, micromorphology, phase composition, resistivity, secondary electron emission coefficient, and surface flashover threshold in the vacuum of the Al2O3 were characterized. The results show that the addition of TiO2 to the Al2O3 ceramic can promote the sintering of the ceramic. The Cr/Mn/Ti-doped Al2O3 ceramic with a homogeneous microstructure can be obtained by an appropriate amount of TiO2 addition. In the process of the heat treatment, the TiO2 in the ceramics was reduced to a certain degree, which had an impact on the microstructure of the Al2O3 ceramic. Adding a small amount of TiO2 can improve the voltage holdoff performance in the vacuum. The value of the surface flashover threshold in the vacuum of the Cr/Mn/Ti-doped Al2O3 ceramic containing 1 wt.% TiO2 reached a value of 33 kV, which is 32% higher than that of the basic Al2O3 ceramic. The preparation of Al2O3 ceramics with a high voltage holdoff capacity in a vacuum provides fundamental technical support for the development of vacuum electronic devices.
ABSTRACT
With the rapid development of photovoltaic industry, the recycling of waste solar photovoltaic (PV) panels is becoming a critical and global challenge. Considering PV panels recycling is significantly effective and worthwhile to save natural resources and reduce the cost of production, how to selectively recycle valuable components of PV panels is the hot and dominant topic. Different from current mechanical crushing, heat treatment and chemical operation processes, novel and environment-friendly recycling approaches by using high voltage pulse discharge in water, called high voltage fragmentation (HVF), was discussed under different discharge conditions. The results showed that discharging across surface and interior of PV panels produced ablation round holes, sputter metal particles and dendritic channels. The average particle size decreased with the ascent of pulse number and voltage amplitude. Considering the energy consumption, the optimal condition of HVF in this paper was 160â¯kV for 300 pulses with the energy consumption of 192.99â¯J/g, crushing the PV panels into particles of 4.1â¯mm in average (13.7% of the initial size). More particle was distributed among the 0.1-2â¯mm size fractions as the energy increased. Selective fragmented products, such as Cu, Al, Pb, Ag and Sn, are concentrated on the fractions under 1â¯mm. Finally, hybrid crushing energy consumption model combined with fractal theory was discussed, which presented close relationship between energy and average particle size. Walker's model (nâ¯=â¯2.047 determined by fractal theory) had the best fitting effect.
