High temperature polyimide nanocomposites containing twodimensional nanofillers for improved thermal stability and capacitive energy storage performance

Future electronic devices toward high integration and miniaturization demand reliable operation of dielectric materials at high electric fields and elevated temperatures.However,the electrical deterioration caused by Joule heat generation remains a persistent challenge to overcome.Here,the solution-processed polyimide(PI)nanocomposites with unique two-dimensional(2D)alumina nanoplates are reported.Substantial improvements in the breakdown strength,charge–discharge efficiency and discharged energy density at elevated temperatures have been demonstrated in the composites,owing to simultaneously suppressed conduction loss and increased thermal conductivity upon the incorporation of 2D Al_(2)O_(3) nanofillers possessing excellent dielectric insulation and thermophysical properties.The predominance of Al_(2)O_(3) nanoplates in enhancing thermal stability and high-temperature capacitive performance over nanoparticles and nanowires is validated experimentally and is further rationalized via finite element simulations.Notably,the Al_(2)O_(3) nanoplates filled PI nanocomposite exhibits a hightemperature capability up to 200℃ and remarkable efficiency(e.g.≥95% at 200 MV/m)over a wide temperature range,which outperforms commercial dielectric polymers and rivals the state-of-the-art polyimide nanocomposites.

Dual-channel mechano-phosphorescence:a combined locking effect with twisted molecular structures and robust interactions

Organic mechanoluminescence materials,featuring dual emission and ultralong phosphorescence characteristics,exhibit significant potential for applications in real-time stress sensing,pressure-sensitive lighting,advanced security marking techniques,and material breakage monitoring.However,due to immature molecular design strategies and unclear luminescence mechanisms,these materials remain rarely reported.In this study,we propose a valuable molecular design strategy to achieve dual-channel mechano-phosphorescence.By introducing the arylphosphine oxide group into a highly twisted molecular framework,enhanced intra-and intermolecular interactions could be achieved within rigid structures,leading to dual-channel mechanoluminescence with greatly promoted ultralong phosphorescence.Further investigations reveal the substantial boosting effect of intra-and intermolecular interactions on mechanoluminescence and ultralong phosphorescence properties by locking the highly twisted molecular skeleton.This work provides a concise and guiding route to develop novel smart responsive luminescence materials for widespread applications in material science.

1.5-m flat imaging system aligned and phased in real time

Flat optics has been considered promising for constructions of spaceborne imaging systems with apertures in excess of 10 m.Despite recent advances,there are long-existing challenges to perform in-phase stitching of multiple flat optical elements.Phasing the segmented planar instrument has remained at the proof of concept.Here,we achieve autonomous system-level cophasing of a 1.5-m stitching flat device,bridging the gap between the concept and engineering implementation.To do so,we propose a flat element stitching scheme,by manipulating the point spread function,which enables our demonstration of automatically bringing seven flat segments'tip/tilt and piston errors within the tolerance.With phasing done,the 1.5-m system has become the largest phased planar instrument ever built in the world,to our knowledge.The first demonstration of phasing the large practical flat imaging system marks a significant step towards fielding a 10-m class one in space,also paving the way for ultrathin flat imaging in various remote applications.

Space charge dynamics in epoxy resin under voltage polarity reversal at various temperatures

Epoxy resin has been used to cast the core of the dry‐type high voltage direct current bushings and can accumulate space charge under high electric fields at high temperatures,which is believed to be a potential threat to the safe operation of dry bushings,especially under polarity reversal.In this study,results of a study of the evolution of space charge and electric field distribution in epoxy resin,under polarity reversal at 10 and 20 kV/mm,and at temperatures between 40 and 100°C,are presented.The results show that the space charge dynamics in epoxy resin during the test were different at various temperatures.At tem-peratures no more than 60°C,space charge accumulation within the samples was not obvious.Instead,a low‐frequency dielectric relaxation process maybe occurs during the polarity reversal,which made the charge peaks on the electrodes after the reversal were slightly smaller than those before the reversal.At temperatures no less than 80°C,considering that the applied electric field has little effect on the barrier reductions of the traps within the samples,the space charge dynamics were mainly governed by a thermally activated process,and the depths of the deepest occupied traps were obtain by analysing the transient processes.

In-situ observation of electrical tree evolution in epoxy dielectrics with internal cracks

Gas-filled internal crack might appear in thermoset materials like epoxy resin during the equipment manufacturing,which would become a vulnerable local region to initiate the electrical tree,thus prone to cause insulation failure.The withstand voltage test was carried on epoxy samples with artificial cracks based on a rod-plane electrode arrange-ment.Simultaneously,surface state variation and tree evolution with crack were observed by an optical microscope in conjunction with a charge-coupled device camera.The changes in morphology and chemical status of the crack surface were characterized by scanning electron microscopy,laser Raman spectrometer and energy dispersive spec-trometer,respectively.It was found that the erosion and tree started from the borderline of crack under a relatively low electric field strength;however,the area near the electrode had relatively little damage.The breaking of epoxy molecular chains coarsens the crack surface and further forms deep channels on a micro-level,which is the forerunner of the electrical tree inception.Based on these,the initiation mechanisms of the crack-induced electrical tree and the reasons for the erosion near the borderline have been revealed.This study provides a train thought for the polymer degradation opening up into the initial tree channel during tree evolution processes.