Collaborative Research: A Low-Pressure Plasma Process for Nano-Coating of Micron- and Nano-Sized Particles

各种材料的微米和纳米级颗粒是许多化学品、陶瓷和金属复合材料、医药和食品、固体燃料和电池等能源相关产品以及电子相关产品的基石和重要组成部分。这些颗粒的表面可以通过涂覆其他材料来改变，以改善其附着力、疏水性、亲水性、印刷性和耐腐蚀性等性能。该建议的目标是通过同时进行计算和实验研究，设计、分析和优化一种连续的低压等离子体工艺，用于在纳米和微米尺寸的颗粒上沉积纳米涂层。低压等离子体的独特之处在于它能够处理各种衬底材料、颗粒大小和形状以及气相前驱体。它们具有低温加工(300K至600K)、可进行的化学成分范围广、与液相加工相比具有卓越的纯度控制、以及能够在纳米范围内产生表面特征的优势。此外，这些等离子体的非平衡性质产生了大量的高能电子，并导致了带负电荷的尘埃粒子。高度带电的结果是这些颗粒对聚集的抵抗力，这是一个通常困扰液体和气相加工的问题。等离子体中的电荷稳定对小到50 nm的颗粒是有效的，从而使加工的尺寸远低于传统流态化的极限而不会受到团聚的不利影响。Co-Pi的研究小组最近证明了在微米和亚微米颗粒上使用低压等离子体工艺沉积厚度在几到几百纳米量级的薄膜的可行性。然而，现有的装置有一些缺点：(A)它的特点是沉积的薄膜不均匀，这是由于在低反应性区域固定颗粒而引起的，(B)它可以处理的颗粒物质的数量有限，以及(C)由于提供稳定的颗粒约束所需的不对称电极设计，它不容易进行优化。在这里，我们建议使用径向对称的等离子体进行连续的薄膜沉积，而不需要粒子被困在鞘中。在这种配置中，重力与其他捕获力的解耦防止了捕获，使颗粒能够连续地通过反应堆。数值研究考虑了等离子体(离子和电子)和尘埃粒子的拉格朗日方程的解，以及电磁场、流体运动和等离子体中前体和其他物质的输运的欧拉方程。还模拟了导致颗粒表面包覆的化学反应过程，并将其包含在模拟中。为了充分解决涂层不均匀的问题，将使用粒子在单元(PIC)的直接方法以及更一般的方法来模拟等离子体粒子，该方法包括求解离子和电子的欧拉方程以及用于尘埃粒子充电的随机方法。所提出的等离子体反应器的成功设计和优化需要模拟和实验的协同。本文数值部分的目标是建立尘埃等离子体的真实模型，探索尘埃等离子体过程的物理过程，探索实验的最佳操作和设计。实验的目的是建立一个连续的生产过程，为模拟提供输入和验证数据，并改善沉积薄膜的控制和质量。通过对该反应器的初步计算研究，证明了该反应器用于颗粒均匀包覆的可行性。研究小组涵盖了跨学科的范围，拥有广泛的建模和实验专业知识。拟议研究的更广泛影响包括在几个技术和教育前沿的潜在重大进步。开发一种将纳米厚层沉积到小颗粒上的连续生产方法对工业和

ABSTRACT - 0422900Micrometer and nanometer size particles of various materials are building blocks and important constituents of many chemicals, ceramics and metal composites, pharmaceutical and food products, energy related products such as solid fuels and batteries, and electronics related products. The surfaces of these particles can be altered by coating them with other materials for improving properties such as, adhesion, hydrophobicity, hydrophilicity, printability, corrosion resistance, etc. The goal of this proposal is to design, analyze and optimize a continuous, low-pressure plasma process for the deposition of nanocoatings on nano- and micron-sized particles, by conducting concurrent computational and experimental studies.Low-pressure plasmas are unique in their ability to handle a broad variety of substrate materials, particle sizes and shapes, and gas-phase precursors. They offer the advantage of low temperature processing (300 K to 600 K), wide range of chemistries that can be conducted, excellent purity control compared to liquid-phase processing, and ability to produce surface features in the nanometer range. Further, the non-equilibrium nature of these plasmas produces a population of highly energetic electrons and results in negatively charged dust particles. A consequence of the high degree of charging is the resistance of such particles against aggregation, a problem that usually plagues both liquid and gas-phase processing. Charge stabilization in the plasma is effective for particles as small as 50 nm thus making it possible to process sizes well below the limits of traditional fluidization without the detrimental effects of aggregation.The Co-PI's group has recently demonstrated the feasibility of low-pressure plasma process for depositing films with thickness of the order of a few to several hundred nanometers on micron and sub-micron particles. The existing setup, however, has certain shortcomings: (a) it is characterized by non-uniformities of the deposited film that arise from the immobilization of particles in areas of low reactivity, (b) it is limited in the amount of particulate matter that it can process, and (c) it is not easily amenable to optimization because of the asymmetric electrode design needed to provide stable particle confinement. Here, we propose to use a radially symmetric plasma for continuous film deposition that does not require particles to become trapped in the sheath. In this configuration, the decoupling of gravity from other trapping forces prevents trapping, allowing particles to move continuously through the reactor. The numerical study considers the solution to the Lagrangian equations for plasma (ions and electrons) and dust particles in conjunction with the Eulerian equations for electromagnetic fields, fluid motion, and transport of the precursor and other species in the plasma. The chemical reaction process, leading to particle surface coating, is also modeled and included in the simulations. To adequately address the issue of coating nonuniformity, the plasma particles will be simulated using both the direct method of particle-in-cell (PIC) as well as a more general method involving the solution of the Eulerian equations for ions and electrons in conjunction with a stochastic approach for dust particle charging. The successful design and optimization of the proposed plasma reactor requires the synergy between simulation and experiments. The goal of the numerical part of this work is to establish a realistic model of the dusty plasma, to probe the physics of the process and to explore optimal operation and design for experiments. The goal of the experiments is to establish a continuous production process, to provide input and validation data for the simulation, and to improve the control and quality of the deposited films. The feasibility of the proposed reactor for uniform coating of particles has been demonstrated by conducting a preliminary computational study. The research team covers an interdisciplinary spectrum with extensive modeling and experimental expertise.The broader impacts of the proposed study include potential significant advances in several technological and educational fronts. The development of a continuous production method for the deposition of nanometer-thick layers onto small particles is significant for industrial and