【作者】 张艳；

【导师】 何毓辉； 缪向水；

【作者基本信息】 华中科技大学 ， 微电子学与固体电子学， 2019， 博士

【摘要】 由于化石燃料的逐渐枯竭和不断增长的能源需求,从环境中收集清洁能源对于人类文明的可持续发展具有重要意义。除了广泛研究的太阳能和生物力学能外,流体中存在的能量因其储备量大和无污染的特性而被广泛认为是最重要的能源之一。同时,以纳米机电系统为代表的新一代纳米尺度功率与电子器件正在蓬勃发展,它们在国防与民用领域的应用前景是无比广大的。而这类新兴的纳米尺度器件与设备通常是在极低的工作电压与极低的功耗下工作,与之匹配的就要求纳米尺度的发电装置,即纳米发电机。因此,基于纳米沟道几何形状和表面材料可调的优势,纳流体沟道体系已被提出作为利用纳流体进行能量转换的新候选者。尽管该领域过去数十年中取得了显著的进步,然而,纳流体能量转换效率的提高仍是当前研究工作面临的重要挑战。掌握单个纳流体器件的性能是提高能量转换效率的基石。本文详细讨论了影响纳流体能量转换的因素,并进一步提出提高能量转换效率的方法。首先,基于纳流体器件的机械能与电能的转换,本文首次提出了短沟道效应并讨论其对纳流体能量转换效率的影响。由于壁面电荷引起的纳米孔离子选择性能够诱导体系流体和离子运动的强烈耦合。这种相互作用促使纳米孔有望作为电动能量转换的纳米器件。然而,当沟道长度非常短时,孔内的流体和离子运动受到孔末端离子耗尽/累积的影响,本文将这种现象称之为短沟道效应。基于三维电动力学建模和仿真,此工作分别讨论了在壁面光滑/不光滑以及存在纳米孔双锥角的条件下,短沟道效应对纳米孔电阻、流体阻抗、流动电导的影响。此工作的研究结果表明,通过利用短沟道效应和制备壁面光滑的纳米孔,在高盐浓度下,能量转换效率可以显着提高至约9%。其次,基于纳流体器件的化学热能转化电能,本文讨论了超薄MoS₂纳米孔体系中的动电输运机制。最近的实验表明,当单原子层MoS₂纳米孔两端施加KCl浓度梯度时,将诱导出巨大的电渗效应,从而展示了超薄纳米孔作为发电机的应用前景。然而相关物理机制并不清楚,此工作就此深入讨论了MoS₂纳米孔体系中离子输运的动电机制。通过考虑膜表面化学反应,此工作发现纳米孔孔内和孔外的表面电荷对于跨孔离子输运具有深远的影响,这揭示了MoS₂体系中较大的开路电压和高电导等有趣实验结果背后的物理图景。该工作建立了一个能够处理超薄膜器件表面电荷的理论模型,同时可以用于评估用二维材料构建的纳米孔发电机的能量转换性能。最后,基于微电子工艺制备了浓差纳流体发电机。电渗能被认为是未来的清洁和可再生能源。当纳米沟道两端施加盐浓度梯度时,盐浓度梯度蕴含的吉布斯自由能将转换为电渗能,因此浓差纳流体发电机对于电渗能的收集起着重要作用。在这部分工作中,制备了基于二氧化硅纳米沟道的浓差纳流体能量收集系统,其输出功率密度在适当的KCl浓度梯度下可达到705 W/m²,超过了之前报道的类似实验结果约两个数量级。输出功率的增强主要归因于适中长度的纳米沟道,其在理想的离子选择性和对输出功率不利的纳米沟道大电阻之间取得了良好的平衡。这种高性能纳流体能量收集器件可满足各种应用需求,包括生物医学微型供电设备和构建未来的清洁能源回收设备。综上所述,本论文从纳米孔/纳米沟道形状、沟道长度、材料、壁面粗糙程度等出发,探讨这些因素对纳流体沟道动电特性以及能量转换效率的影响,提出了提高能量转换效率的方法,同时通过微电子工艺制备了基于盐浓度梯度的纳流体发电机。更多还原

【Abstract】 The exploitation of clean energy from ambient environment has become paramount to the sustainable development of human civilization because of the ongoing depletion of fossil fuels and ever-growing energy demands.In addition to the extensively studied solar and biomechanical energy,energy that exists in the fluidic system is one of the most popular energy resources due to its large reserves and non-polluting properties.Meanwhile,a new generation of nanoscale power and electronic devices represented by nanoelectromechanical systems are booming,and their application prospects in defense and civilian fields are extremely promising.Such emerging nanoscale devices typically operate at very low operating voltages and very low power consumption,the nano-scale power device can meet this requirement,namely nano-generators.Therefore,based on the engineered advantages of nanochannel geometry and material properties,nanofluidic channel systems have been proposed as new candidates for energy harvesting.Despite significant improvements have been achieved in the past few years,improving nanofluid energy conversion efficiency is still an important challenge for current research work.Mastering the performance of a single nanofluidic device is the cornerstone for improving energy conversion efficiency.In the thesis,the factors affecting the energy conversion of nanofluids are discussed in detail,and the methods to enhance/optimize the energy conversion efficiency are proposed.First,based on energy conversion from mechanical to electric,we described the concept of short channel effect and discussed its effect on electrokinetic energy conversion of nanofluids.The ion selectivity of nanopores due to the wall surface charges is capable of inducing strong coupling between fluidic and ionic motion within the system.This interaction opens up the prospect of operating nanopores as nanoscale devices for electrokinetic energy conversion.However,the very short channel lengths make the ionic movement and fluidics inside the pore to be substantially affected by the ion depletion/accumulation around the pore ends,we define this phenomenon as short channel effect.Based on three-dimensional electrokinetic modeling and simulation,we present a systematic theoretical study of nanopore electrical resistance,fluidic impedance,and streaming conductance under conditions of slippery/non-slippery wall or nanopore with round corner.Our results show that by utilizing the short channel effect and preparing slippery nanopores the energy conversion efciency can be dramatically increased to about9%under large salt concentrations.Secondly,based on energy conversion from Gibbs free energy to electric,we discussed the electrokinetic transport mechanisms in ultra-thin MoS₂ nanopore systems.Recent experiments demonstrated giant osmotic effects induced in a single-atomic-layer MoS₂nanopore by imposing a KCl concentration bias,thereby highlighting the prospect of ultrathin nanopores as power generators.However,the relevant physical mechanism is not clear,thus we discussed the electrokinetic mechanism of ion transport in the MoS₂ nanopore system.By taking membrane surface chemistry into account,we found profound roles of surface charges in and out of the nanopore on the cross-pore ion transport,which shed light on the intriguing experimental observations of a high pore conductance with a large open-circuit voltage in the MoS₂ system.The present work establishes a theoretical model capable of dealing with ultrathin membrane surface charges for evaluating the energy conversion performance of nanopore power generators constructed with two-dimensional materials.Finally,a silica-nanochannel based nanofluidic generator was fabricated based on standard semiconductor manufacturing process.The development of nanofluidic energy harvesting system plays a fundamental role in harvesting osmotic power from Gibbs free energy within salt concentration gradient,which is considered as a future clean and renewable energy source.In this study,a silica-nanochannel based nanofluidic energy harvesting system was fabricated and its output power density could reach 705 W/m² under suitable KCl concentration bias which exceeded—by almost two orders of magnitude—the results obtained by previous work.The enhancement of energy harvesting was mainly ascribed to the appropriate length of nanochannel that makes a good balance between the desirable ion selectivity and the unfavorable large resistance of nanochannel.This high-performance nanofluidic energy devices could be used in a variety of applications,including power biomedical tiny devices or constructing future clean-energy recovery plants.In an all,based on the shape of nanopore/nanochannel,channel length,material and wall roughness,the effects of these factors on electrokinetic properties of nanofluidic channels and energy conversion efficiency were systematic discussed,and we proposed effective strategies for enhancing/optimizing the energy conversion efciency.Meanwhile,a silica-nanochannel based nanofluidic generator was fabricated based on standard semiconductor manufacturing process.更多还原