季戊四醇固固相变材料导热性能的分子动力学研究

Study on molecular dynamics simulation of thermal conductivity of pentaerythritol solid-solid phase change materials

火力发电中余热耗散现象不可避免,有效地利用电厂余热不仅带来直接的经济效益,而且也是实现双碳目标的重大举措。季戊四醇具有稳定性好、安全性高,并且在一定温度时会发生体心立方结构(α相)与面心立方结构(γ相)相互转化的优点,因而在固固相变领域具有良好的应用场景.研究季戊四醇在不同温度时的导热性能对工业余热回收利用十分必要.本文利用分子动力学模拟方法,分别对季戊四醇相变前后的两种相态的导热性能开展了研究.为了揭示季戊四醇热导率变化的物理机理,进一步对X射线衍射图谱、声子态密度、不同频率的声子贡献以及不同类型的作用力贡献开展分析.研究结果表明,随着体系温度升高,α相季戊四醇的热导率逐渐下降.当温度超过相变温度,季戊四醇由α相转变为γ相,热导率大幅下降.相比于α相,γ相季戊四醇热导率的降低归因于:(1)γ相季戊四醇晶胞结构发生了变化,分子间氢键遭到破坏;(2)γ相季戊四醇晶体内声子散射增强;(3)主导声子频率的不同导致了α相和γ相季戊四醇热导率差异;(4)γ相季戊四醇晶体中对热导率贡献最大的键拉伸作用力占比较小.研究结果可为工业余热回收利用发展及固固相变材料的开发和设计提供理论指导.

The waste heat dissipation phenomenon is inevitable in thermal power generation.Effective utilization of power plant waste heat not only brings direct economic benefits,but also is an important measure to achieve the goal of dual carbon.Pentaerythritol has the advantages of good stability,high safety,and mutual transformation of body-centered cubic structure(αphase)and face-centered tetrahedral molecular structure(γphase)at a certain temperature,so it has a good application scenario in the field of solid-solid phase transformation.It is necessary to study the thermal conductivity of pentaerythritol at different temperatures for industrial waste heat recovery and utilization.In this paper,the thermal conductivity of pentaerythritol before and after phase transformation was studied by molecular dynamics simulations.In order to further analyze the physical mechanisms of thermal conductivity change,the X-ray diffraction pattern,the phonon vibrational density of states,the contributions of phonons with different frequencies,and the contributions of different types of forces to the thermal conductivity were further analyzed.The results show that the thermal conductivity of pentaerythritol decreases with the increase of temperature.When the temperature exceeds the phase transition temperature,pentaerythritol changes fromαphase toγphase.Compared with theαphase pentaerythritol,the decrease of thermal conductivity ofγphase pentaerythritol is attributed to the following reasons.(1)The change of crystal cell structure ofγphase pentaerythritol and the destruction of intermolecular hydrogen bonds.(2)Phonon scattering enhancement inγphase pentaerythritol crystal.(3)The thermal conductivities of pentaerythritol inαandγphases are contributed by phonons with different frequencies.(4)In theγphase pentaerythritol crystal,the tensile force of the bond which contributes most to the thermal conductivity is relatively small.Our findings can provide theoretical guidance for the industrial waste heat recovery and utilization,and the development and design of solid-solid phase change material.