介电泳辅助水溶解KDP晶体高效超精密抛光方法研究

KDP crystal is currently the only non-linear optical material suitable for frequency conversion elements and electro-optic switches in laser-induced inertial confinement fusion. According to the problems that polishing fluid is difficult to enter the center of workpiece and lead to nonuniform materials removal, a novel dielectrophoretic assisted water dissolution high efficiency ultra-precision polishing method is proposed. The novel method utilizes dielectrophoretic effect caused by non-uniform electric field in polishing area to polarize the polishing fluid which is made up of organic solvent and nano-scale water droplets wrapped by surfactant. The polishing fluid move toward the electrode under the dielectrophoretic force and adsorb on a polishing pad near the electrode to ensure that polishing fluid in center of workpiece is adequate, therefore the material removal of whole workpiece surface is uniform. The polishing efficiency and accuracy are both significantly improved. Meanwhile a near-lossless super smooth KDP crystal surface is obtained by the interaction between mechanical friction and nano-scale dissolution. This project plans to develop new kinds of polishing fluid by analyzing the dielectrophoretic effect on polishing interface for different fluid components; control the diameter of nano-scale water droplets, and investigate the formation mechanism of super-smooth surface under the dielectrophoretic force; establish the dielectrophoretic force controlling model and material removal function, and achieve uniform distribution of polishing fluid film in machining area to improve polishing accuracy. This research has considerably important theoretical significance and engineering value for solving the machining challenges of KDP optical elements and other water-soluble materials.

KDP晶体是目前唯一可用于惯性约束核聚变中大口径倍频和光电开关的光学材料。针对大尺寸KDP晶体抛光时抛光液不易进入工件中心区域，导致材料去除不均匀进而影响面形精度的难题，提出一种介电泳辅助水溶解高效超精密抛光方法：在加工区增加非均匀电场，使抛光液中母液和被表面活性剂包裹的纳米尺度水滴极化，在介电泳力作用下抛光液吸附于工件表面，保证工件中心供液充足，实现工件全表面的均匀性去除，从而提高面形精度，并大幅减少加工时间；通过纳米尺度的溶解及机械去除作用，获得KDP晶体超光滑表面。本项目拟揭示介电泳效应对不同抛光液成分在水溶解抛光界面处的作用机理，研制新型抛光液；控制纳米级水滴尺寸，探索介电泳力作用下水溶解抛光超光滑表面形成机制；建立介电泳力控制模型和材料去除函数，通过控制介电泳力实现抛光区域液膜均匀分布，获得高面形精度。研究成果对解决KDP元件及其它水溶性材料的加工难题具有理论意义和工程应用价值。

KDP晶体是目前唯一可用于惯性约束核聚变中大口径倍频和光电开关的光学材料。针对大尺寸KDP晶体抛光时抛光液不易进入工件中心区域，导致材料去除不均匀进而影响面形精度的难题，提出一种介电泳辅助水溶解高效超精密抛光方法：在加工区增加非均匀电场，使抛光液中母液和被表面活性剂包裹的纳米尺度水滴极化，在介电泳力作用下抛光液吸附于工件表面，保证工件中心供液充足，实现工件全表面的均匀性去除，从而提高面形精度，并大幅减少加工时间。本项目从介电泳辅助水溶解抛光方法的基本原理、材料去除机理、介电泳效应产生机理等几个方面开展了数值仿真以及实验验证等研究工作。研究内容如下：搭建验证性平台观测介电泳力作用下抛光液的吸附行为，验证介电泳辅助水溶解抛光原理，证实了水溶解抛光液中微水滴会在介电泳力作用下发生形变，并聚集在晶体表面附近，从而提高抛光过程中参与溶解的微水滴数量；同时上电极也会对抛光液产生“吸附”作用，延长抛光液在晶体表面作用时间，减小抛光液甩出率。分析并推导了介电泳辅助水溶解抛光方法的材料去除模型，确定了材料去除的关键因素。对单颗微水滴进行动力学分析，模拟极化后微水滴的运动行为，分别研究电压、极板间距、电极布置方式对介电泳特性的影响，并数值模拟不同形状电极对微水滴受到介电泳力的影响规律，认为双螺旋结构电极具有最大的电场梯度。根据介电泳辅助水溶解抛光原理，设计了介电泳效应产生装置，并搭建试验平台。经过20分钟抛光，介电泳辅助水溶解抛光后KDP晶体表面粗糙度由Ra 590 nm降低至Ra 1.365 nm，优于传统水溶解抛光的Ra 1.637 nm，且面形精度更高，同时抛光效率提升24%。并通过介电泳辅助水溶解抛光加工实验，验证了材料去除模型中的关键参数对抛光结果的影响规律，得到了最优的加工工艺参数组合：双螺旋电极布置，电源电压10 kV，抛光液流量20 ml/min，抛光压力30 kPa，抛光盘转速90 r/min。研究成果对解决KDP元件及其它水溶性材料的加工难题具有理论意义和工程应用价值。