In this dissertation, by seamlessly tailoring nanoporous materials with functional liquids, we have demonstrated in experiments and simulations that the formed liquid-nanoporous composite material system provides an ideal platform for energy conversion with high efficiency, including energy absorption, actuation and harvesting.
We start with the energy absorption protection mechanism based on nanofluidics through MD simulations. Upon an impact loading or blast wave on the formed liquid-nanoporous composite material system, a new protection mechanism of energy capture to the impact and blast energy wave is discovered. That is, the incoming kinetic energy can be quickly converted to the potential energy of water molecules and is captured, and then confined into slightly hydrophobic nanopores for a while, leading to an obvious reduction in both transmitted force and energy. The captured energy will be released gradually at unloading stage due to the inherence of non-wetting surface wall of nanopores through the diffusion of liquid molecules out of nanopores. The characteristic time of the process is around 10-9-10-7 second, making it ideal for mitigating a highly nonlinear stress wave such as blast energy wave. Since the captured energy is not necessarily converted to other forms of energy (e.g. heat) and can be released gradually upon unloading, it is totally distinct from conventional energy absorption or dissipation mechanism, thus making the system reusable. MD simulations further demonstrate that the captured energy will not always increase with impacting velocity, and depends on the material parameters, such as nanopore size, solid composition, liquid phase, and charged state of nanoporous wall, and
external conditions, such as impacting velocity, which have been studied systematically. Under the same impacting velocity, the transmitted force and energy reduction, energy capture and duration show an overall increase with the increase of nanopore length and diameter, but the energy capture per volume decreases for an increased nanopore diameter. Given a certain nanopore, the transmitted force and energy reduction, energy capture and duration increase till arriving at a maximum first and then decrease due to the filling up of the nanopore. For the same pore size and impacting velocity, if the carbon nanopore in the system is replaced by a silica nanopore, the transmitted force reduction will increase, while the energy reduction will show a little drop; if the surface wall of the carbon nanopore is uniformly charged, independent of the sign of charge, or the liquid phase of water molecules changes to a electrolyte solution (NaCl or KCl solution), both transmitted force and energy reduction will decrease. The density profiles of water molecules at the axial and radial directions of nanopores are examined to understand the mechanism of energy capture. The axial density shows approximately the same during all stages of infiltration and defiltration process, and the relative uniformly layered-structure distribution in radial direction is observed, implying a “cluster-like” energy trapping and releasing mechanism. These MD findings are verified through a parallel blast experiment on a zeolite-water composite material system.
We then study the energy dissipation during transport of confined liquid molecules through nanopores, where the kinetic energy is dissipated due to the transport “friction” resistance exerted to the nearest liquid molecules by solid wall atoms. The temperature of confined liquids and wall roughness of nanopores are considered two key factors of affecting friction resistance, and investigated. The effective shear stress and nominal viscoity which represent an “overall” resistance the wall exerting to the flow are employed to characterize the
flow resistance. As the temperature of confined liquids rises, both the effective viscosity and the nominal viscosity decrease. The coupling effects of thermal effect between the nanopore size and transport rate effects are further studied, showing that the transport resistance is more sensitive to temperature in a smaller nanopore or higher transport rate. In addition, when the ions are added to the confined liquids, due to the enhanced liquid-solid wall interaction, the effective shear stress shows an enhancement compared with that of pure water molecules. When water molecules transport through a rough nanopore, MD simulations indicate that both the effective shear stress and the nominal viscosity increase with the decrease of wavelength of roughness or the increase magnitude of roughness. Besides, the roughness effect becomes more prominent in a smaller nanopore. Velocity profile, radial density and hydrogen bond of water molecules inside nanopores are employed to understand these findings and provide an underlying molecular mechanism. Either a lower temperature or a rougher nanopore shows a stronger solid wall-water interaction and an enhancement in the radial movement of water molecules near the wall, thus leading to a higher transport resistance. These findings are qualitatively verified by a pressure- induced infiltration experiment on a nanoporous carbon-water system.
Next, we develop a thermal actuation system based on nanofluidics with high energy output and conversion efficiency, allowing converting thermal energy to mechanical energy. When liquid molecules invade a hydrophobic nanopore, the infiltration pressure shows a thermal dependence. By using MD simulation, we systemically investigate the fundamental thermal infiltration behaviors of liquid molecules into hydrophobic nanopores. When the applied pressure arrives at a critical value beyond which the capillary resistance is overcome, water molecules start invading the nanopores, where the critical infiltration pressure decreases with the increase of temperature. The thermal dependence of infiltration pressure results from reduced interfacial
tensions between water molecules and nanopores, and is elucidated by studying the distribution of confined water molecular structures through the radial density profile, contact angle, and surface tension. In addition, the infiltration pressure is also a functional of pore size and solid phase as well as liquid phase. The infiltration pressure decreases with the increase of pore size; while increases when either solid phase carbon atoms or liquid phase water molecules are substituted by silica and NaCl-solution counterpart due to a stronger interfacial tension in comparison to carbon-water system. Based on the thermally tunable infiltration pressure of liquid molecules into CNTs, a conceptual design of thermal actuation system is proposed and realized through MD simulations. The working principle is summarized here: as environmental temperature varies, the system transits between a relatively stronger and weaker hydrophobicity, through which the liquids outside nanopores can be compressed into or the confined liquids inside nanopores can be repelled out of, leading to a variation of system volume. When an external mechanical pressure is applied on the system, the variation of system volume can lead to an output of mechanical work, similar to a conventional thermal machine. As temperature increases, the output power density and efficiency increase. The energy density can be as high as about 10J/g. The system performance can be further improved by optimizing systemic and material parameters such as the pore size, solid phase, and liquid phase. Parallel with MD simulations, a thermally controlled infiltration experiment on a zeolite-water system is performed. Both the thermal infiltration behavior of liquids into nanopores and the working principle of proposed thermal actuation are qualitatively verified.
Closely following the strategy of designing the thermal actuation system, we study the electric infiltration characteristics of liquid molecules into hydrophobic nanopores and conclude that it can also be adjusted by an external electric field. As the applied electric intensity
increases, the critical infiltration pressure decreases. Similar with the mechanism at evaluated temperatures, it can also be attributed to a reduced hydrophobicity which has been elucidated through the employment of surface tension, contact angle, as well as radial density profile of confined liquid molecules. Besides, further MD simulations indicate an asymmetric response to the sign of the applied electric field with the same magnitude: the critical infiltration pressure in the presence of a positive electric field is a little higher than that in the presence of a negative electric field, indicating that the conventional electrochemical theory may break down in nanoenvironments. The electric infiltration behavior is also coupled with the pore size, polar silica nanopore and electrolyte solution. Generally, in a smaller nanopore, the electric infiltration behavior is more sensitive, and the employment of polar silica nanopore and electrolyte solution shows the same trend. Given this clear molecular adjustable mechanism of electrically controlled nanofluidic infiltration behavior, a concept of electrically controlled actuation system is proposed. As the external electric intensity changes, the infiltration pressure changes, leading to a variation of system volume during the liquid into or out of nanopores, and thus the mechanical work can output. Both the output power density and the efficiency increase with the increase of electric intensity. The energy density is about 10J/g with the present parameters, and can be higher by optimizing the pore size, solid and liquid phase of systemic parameters. These MD findings are verified qualitatively by an electrically controllable infiltration experiment on a zeolite-KCl solution material system, providing a solid fundament on the application of electroactuation in practice.
Using MD simulations in the last chapter, we explore voltage generation mechanism by passing hydrogen chloride (HCl) solution through nanopores. MD simulations show that the accumulated displacement of Cl- ions nearest the nanoporous wall increases linearly with
simulation time, and a constant drifting velocity is obtained, responsible for voltage generation. The contribution from H3O+ can be neglected due to a quite uniform and stable distribution. The
voltage generation mechanism of the ion hopping and moving forward nearest nanoporous wall is confirmed. Given an ion density distribution nearest the nanoporous wall and their drifting velocity, the generated voltage along the flow direction can be deduced. Further MD calculations show a nonlinear increase of voltage with the increase of the applied flowing velocity, consistent well with experiments in literatures. More importantly, the induced voltage is very sensitive with a small temperature fluctuation, which can be employed to harvest energy from low-grade heat. In addition, other critical factors of such as pore size and liquid concentration are also studied, through which an enhanced system performance can be achieved.