十八烷酸热传导机制的尺度效应研究

十八烷酸的热导率具有显著的尺寸依赖性，然而只有宏观热输运性质得到充分研究,限制了微尺度特征结构的复合材料热性能的进一步提高。因此，为满足新型复合相变材料的研究需求，十八烷酸的纳米尺度热输运特性亟待解决。基于分子动力学模拟，系统性地研究了块体、纳米线和纳米链三种形态的十八烷酸热导率的演变规律，通过平衡法模拟的热导率分别为0.4546、0.2213和0.0085 W·m^-1·K^-1；结合声子态密度和重叠能分析，发现在较低频率声子振动的衰减导致纳米线热导率低于块体，显著降低的声子重叠能严重阻碍纳米链的声子输运导致极低热导率。

Size effect of heat conduction mechanism on stearic acid

The thermophysical properties of steaic acid (SA) has a significant size dependence. However, only the macroscopic heat transport properties have been fully studied, which limits the further improvement of the thermal properties of composites phase change materials (PCMs) with microscale structure features, such as nanocapsule, aerogel materials and porous material. Therefore, in order to meet the research needs of materials design of the high-performance composite PCMs, the nanoscale heat transport properties of SA need to be discovered urgently. In this work, based on molecular dynamics simulation, the difference of thermal conductivity and the thermal transport mechanisms of SA in three forms of bulk, nanowire and nanochain were systematically studied. The thermal conductivity of three forms simulated by equilibrium method dynamics was 0.4546, 0.2213 and 0.0085 W·m^-1·K^-1, respectively. And the vibration density of states and overlap energy were used for the further exploration of the heat transport mechanism. The results show that the thermal conductivity of bulk is the largest, followed by the nanowire, and the nanochain has the lowest thermal conductivity. Due to the size effect, which results in the more boundary scattering events, the attenuation of phonon vibration at lower frequencies causes the nanowire thermal conductivity to be lower than that of the bulk. Besides the boundary scattering, the curled SA molecules in the nanochain cause the serious decrease of phono overlap energy between bonding atoms on the end of molecules. The high mismatch level of atoms increases the phonon scattering events happen in the molecule and hinders the heat transport between two molecules, and results in the lowest thermal conductivity in the SA nanochain.