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NANOSCALE HEAT TRANSFER AT SOLID-LIQUID INTERFACE: A MOLECULAR DYNAMICS SIMULATIONS STUDY 원문보기

  • 저자

    팜 트룽 안

  • 학위수여기관

    울산대학교

  • 학위구분

    국내석사

  • 학과

    기계자동차공학과 Nano Heat Transfer

  • 지도교수

    Bohung Kim

  • 발행년도

    2014

  • 총페이지

    73

  • 키워드

    Mechanical Engineering;

  • 언어

    eng

  • 원문 URL

    http://www.riss.kr/link?id=T13540271&outLink=K  

  • 초록

    Nanostructure materials are getting significant impact on addressing science and engineering issues. At the further reduced sizes, heat transfer mechanism requires sophisticated understanding and control thermal transport in the nanoscale. Thermal boundary condition between different material layers can dominant the overall thermal resistance in nanostructure and therefore affect the performance of nanoscale devices. In addition, the solid/liquid interfacial thermal resistance has been an important technical issue in thermal/fluid engineering such as micro electro-mechanical systems and nano electro-mechanical systems with liquid inside. In this thesis, three-dimensional molecular dynamics (MD) simulations are used to investigate the thermal resistance at solid/liquid interfaces. Heat conduction between two parallel solid walls separated by a thin film of liquid is considered. The density distribution of liquid is discussed with the simulation results to further understanding of the dynamic properties of water or argon near solid surfaces. Meanwhile, temperature profiles appear discontinuous between liquid and solid temperatures due to the dissimilarity of thermal transport properties of the two materials, which validate thermal resistance (or Kapitza length) at solid/liquid interfaces. The temperature dependence of Kapitza length at solid/liquid interface is considered. The results of our simulations illustrate the strong adsorption of the silicon wall with liquid molecules, which reduces the effect of wall temperature on the temperature jump at interfaces. Therefore, Kapitza lengths are not affected by the wall temperature and fluctuate around an average value with various temperature walls within the range 283-363 K for liquid water and argon. The independence of Kapitza length on temperature is technologically important for applications where very high heat dissipation is necessary. We also conduct non-equilibrium molecular dynamics simulations (NEMD) to investigate Kapitza length at solid/liquid interfaces under the effects of bulk liquid pressures. Gold and silicon are utilized as hydrophilic and hydrophobic solid walls with different wetting surface behaviors, while the number of confined liquid water molecules is adjusted to obtain different pressures inside the channels. We present a thorough analysis of the structure, normal stress, and temperature distribution of liquid water to elucidate thermal energy transport across interfaces. Our results demonstrate excellent agreement between the pressures of liquid water in nano-channels and published thermodynamics data. The pressures measured as normal stress components are characterized using a long cut-off distance reinforced by a long-range van der Waals tail correction term. To clarify the effects of bulk liquid pressures on water structure at hydrophilic and hydrophobic solid surfaces, we define solid/liquid interface spacing as the distance between the surface and the peak value of the first water density layer. Near the gold surface, we found that interface spacing and peak value of first water density layer were constant and did not depend on bulk liquid pressure; near the silicon surface, those of values depended directly upon bulk liquid. Our results reveal that the pressure dependence of Kapitza length strongly depends on the wettability of the solid surface. In the case of the hydrophilic gold surface, Kapitza length was stable despite increasing bulk liquid pressure, while it varied significantly at the hydrophobic silicon surface. The thermal coupling at water-solid interfaces is a key factor in controlling thermal resistance and the performance of nanoscale devices. This is especially important across the recently engineered nano-composite structures composed of a graphene-coated-metal surface. In this thesis, a series of molecular dynamics simulations were conducted to investigate Kapitza length at the interface of liquid water and nano-composite surfaces of graphene-coated-Cu (111). We found that Kapitza length gradually increased and converged to the value measured on pure graphite surface with the increase of the number of graphene layers inserted on the Cu surface. Different than the earlier hypothesis on the “transparency of graphene,” the Kapitza length at the interface of mono-layer graphene coated Cu and water was found to be 2.5 times larger than the value of bare Cu surface. This drastic change of thermal resistance with the additional of a single graphene is validated by the surface energy calculations indicating that the mono-layer graphene allows only ~18% van der Waals energy of underneath Cu to transmit. We introduced an “overall interaction strength” value for the nano-composites based the quantitative contribution of pair interaction potentials of each material with water into the total surface energy in each case. Similar to earlier studies, results revealed that Kapitza length shows exponentially variation as a function of the estimated interaction strength of the nano-composite surfaces. The effect of Cu/Graphene coupling on thermal behavior between the nano-composite with water was characterized. The Kapitza length was found to decrease significantly with increased Cu/graphene strength in the case of weak coupling, while this behavior becomes negligible with strong coupling of Cu and graphene.


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