液体表面张力控制的纳米多孔金属驱动行为探索

The emerging nanoporous metals can act as advanced electrochemical or chemical actuators which are controlled by the surface stress of the metal/electrolyte interface. This type of actuator can operate under appreciable load, with high actuation stoke, high strain energy density and low operating voltage. But its applications are hindered by the need of electrochemical or chemical environment which often involves corrosive or toxic chemicals, the low response time due to the slow ion- and mass- transportation in nanoscale pore channels, the irreversible changes in nanoporous structure induced by surface (electro-)chemistry which also degrade the actuation performance. In this project, we propose to develop a new type of nanoporous metal (or other solid) actuators which is controlled by the liquid surface tension, instead of the solid surface stress as in previous electrochemical actuators. The liquid phase will be injected into the nanopore space, which produces a solid/liquid two phase material with concaved liquid meniscus in the pores on the material surface. The capillarity stress induces stresses and strains in the nanoporous solid skeleton, which gives rise to the reversible elastic strains and actuation strokes. This project will focus on two types of the liquid, the water and liquid metals, and their surface tension induced actuation in nanoporous metals and other solids. Compared with the solid-surface-stress-controlled electrochemical actuators, our new actuators have some advantages: the water-induced actuation does not involves electrochemistry or dangerous chemicals; the liquid metal-induced actuation does not require long distance of mass or ion transport, thus can operate at a high speed; furthermore, both types of actuation does not deteriorate the nanoporous structure, which is beneficial to gain actuation with high stability. We will systematically study the mechanism of this type of actuation, particularly, the roles of the nanoporous structure and the properties of the liquid phase played in actuation. The new actuator with optimized performance may act as artificial muscles in future in MEMS and micro-robots.

纳米多孔金属可作为一种“固体表面应力控制”的电化学或化学驱动材料。它能承受较大载荷、驱动幅度大、能量密度高且驱动电压低，但驱动装置复杂且涉及有害介质，响应速度受限于孔内离子传输速度，反复表面电化学会导致材料结构变化从而影响稳定性。针对这些问题，本项目拟探索一种由“液体表面张力控制”的新型纳米多孔金属驱动器，即将液相注入固相纳米多孔结构并在材料表面孔中形成凹液面，利用毛细力在宏观固体材料上获得可逆弹性变形并实现驱动。本项目将着重研究水和液态金属两种液体表面张力在纳米多孔金属或其它材料上诱发的驱动行为。前者不涉及电化学和有害介质；后者无需孔内离子长距离传输，响应速度快；且两者都不会造成材料结构变化，因此和电化学驱动相比具有明显的优势。本项目将深入研究该驱动的机理，纳米多孔结构参量以及液体表面性能所起作用，并探索其优化途径，目标发展一种适用于微机电系统、微机器人等领域的高性能、新型驱动器。

在电化学控制下，固体表面应力可在纳米多孔金属中产生弹性变形，是一种潜在的新型电化学驱动器材料。该驱动器材料因承载能力强、驱动幅度大、能量密度高与驱动电压低等优势，在微机电系统等领域具有广阔应用前景。但因受制于纳米孔内物质和电荷传输速度较慢，此类驱动器的响应速度一般也较慢；反复表面电化学会导致材料结构变化从而影响驱动性能的稳定。针对这些瓶颈问题，本项目拟从原理上探索一种由“液体表面张力控制”的新型纳米多孔金属驱动器，即利用液体毛细力在宏观固体材料上获得可逆弹性变形并实现驱动。本项目在固液双连续相结构构筑与力学响应，液体表面控制驱动，以及热控制驱动等几个方面开展研究，取得以下成果：1）以纳米多孔金为模型材料研究发现电化学驱动幅度随相对密度降低而降低，且在相对密度高于25%情况下稳定的反常现象，展现了纳米多孔金属表面控制弹性变形的复杂行为。2）探索了Ga在纳米多孔金中的物理填充，以及通过溶体腐蚀构筑C/Ga，Pb/Ga双连续固/液结构的新方法，成功制备获得固/液双连续结构材料，为进一步细化结构，探索其驱动行为提供条件。3）以纳米多孔金为模型材料研究发现在相对密度不变的条件下，纳米多孔金属弹性模量可随粗化发生数量级的变化，为驱动器性能优化提供重要依据。4)系统研究了干燥/充水过程中水表面张力诱发的纳米多孔金属驱动变形行为，发现在纳米多孔金和纳米多孔金/金双金属片产生20MPA以上的应力变化以及>1%巨大弹性变形，展现出优异的驱动性能；5）系统研究了纳米多孔金/石蜡复合材料及金/（纳米多孔金/石蜡）层状复合材料驱动器在石蜡融化凝固过程中产生的热控制驱动行为。发现在此纳米复合结构中，纳米结构有利于抑制石蜡中孔洞形成（纳米孔洞形成需克服表面能），从而利用熔化凝固体积变化实现显著驱动变形。本项目从原理上阐明了固/液双连续结构构筑原理以及液体表面张力在块体材料中诱发驱动行为的技术途径与机制，证实了通过液体表面张力实现驱动器的可行性，为构筑下阶段新型快速、稳定的高性能驱动器提供理论依据和实验证据。本项目研究内容已按计划完成，取得预期成果，部分研究成果已整理发表（SCI论文9篇）申请国家发明专利1项。