热辅助超声波增材制造设备设计与分析

超声波增材制造技术在功能、复合材料的快速制备领域应用广泛,为克服现有超声波增材制造设备功率的限制,提出了一种大功率热辅助超声波增量制造设备,并采用COMSOL5.0有限元模拟软件对超声波振动系统进行辅助设计.该设备由超声波振动系统、压力机构、支撑行走机构、加热模块构成,焊接压头采用两侧对称结构,采用双换能器串联推-挽技术,显著增加超声焊接功率.并通过辅助加热模块提供额外的热量输入,提高被焊金属温度.并进行多层铜箔的超声波增量滚压焊接试验测试.结果表明,该热辅助超声波增材制造设备的焊接性能和焊接质量良好.     

Effect of point heat source on growth of subsurface crack induced in brittle material machining

Based on the indentation fracture mechanics, a thermal-assisted grinding model is established to investigate the effect of heat source on subsurface crack propagation. Combining the complex function method with the continuous distribution technique of dislocations, the stress intensity factors (SIFs) near the crack tip under various thermal and mechanical loads are calculated numerically. Results show that whether the median crack propagation is inhibited or promoted by the heat source mainly depends on the relative positions between thermal loading and mechanical loading. The stronger heat source in front of the abrasive grain can inhibit the growth of subsurface crack. When the heat source is located behind the abrasive grain, a minimum value of SIF can be obtained if the grinding parameters are well controlled. Especially, with the increase in heat source intensity, the maximum and minimum SIFs occur when the heat source is located at 1.5 times the distance of half crack length behind and in front of the abrasive grain, respectively.     

聚合物微结构热辅助超声波压印成形

提出了基于硅模具的热辅助超声波压印成形方法,用于高效率、高精度地复制热塑性聚合物微结构。该方法利用作用于聚合物基片与模具间的超声波振动,快速升高界面温度,以达到聚合物的成形温度。为了降低破坏模具的风险,将模具预热到低于玻璃点转化温度(Tg)以下35℃至50℃之后再施加超声波进行成形。最后,通过正交实验研究了超声振幅、超声波压力、超声波时间、热辅助温度以及聚合物基片厚度对压印结果的影响,揭示了超声波压印工艺的成形机理。实验结果表明,热辅助温度对压印影响最大,其次为超声波振幅,而超声波压力是影响复制均匀性的最重要的参数;薄的聚合物基片在同样的超声波参数和模具结构下更容易成形。通过优化参数,对聚甲基丙烯酸甲酯(PMMA)基片的深度复制精度达到了99%,复制周期小于50s。研究表明,热辅助超声波压印成形效率高,是一种具有批量制造潜力的聚合物微结构成形方法。     

Revisiting Hole Injection in Quantum Dot Light-Emitting Diodes

Injecting holes from the hole transport layer (HTL) into the quantum dot (QD) emitting layer in quantum dot light-emitting diodes (QLEDs) is considered challenging due to the presence of a relatively high hole injection barrier at the HTL/QD interface. However, QLEDs with exceptional brightness and efficiency are achieved, prompting a reevaluation of the traditional hole injection mechanisms. This study examines the hole injection mechanism in QLEDs using a combination of experiments and simulations. The results demonstrate that the applied bias significantly reduces the barrier height between the highest occupied molecular orbital level of the HTL and the valence band (VB) of the QDs, facilitating hole injection. The bending of the lowest unoccupied molecular orbital energy level of the HTL at the HTL/QD interface confines electrons within the QD, effectively minimizing leakage current. Additionally, the triangle-shaped potential barrier arising from the bending of the VB energy level of the QDs creates favorable conditions for hole–tunneling injection. Moreover, both simulations and experiments consistently demonstrate that the predominant pathway for hole injection from the HTL to the QDs in the QLED device involved thermally assisted tunneling. This study is important to understand the hole injection mechanism in QLEDs.