石蜡/ZIF-8@ZIF-67定形相变材料的储热性能

    Thermal storage properties of paraffin/ZIF-8@ZIF-67 shape-stable phase change materials

    • 摘要: 针对固-液相变材料在相变过程中由于体积变化产生的芯材泄漏现象,该研究以ZIF-8@ZIF-67核壳结构为载体,以石蜡(paraffin)为相变芯材,采用溶剂蒸发法制备了质量分数为50%、60%、70%和75%的石蜡/ZIF-8@ZIF-67定形相变材料。通过扫描电镜、透射电镜、X射线衍射、傅里叶红外光谱、BET比表面积、热重分析、差式扫描量热和瞬态平板热源等测试手段对定形相变材料的形貌、结构、热稳定性、相变潜热、热导率进行表征;采用Fluent软件对填充相变材料的相变蓄热供暖系统进行了数值模拟分析。结果表明:ZIF-8@ZIF-67形貌为大小均匀的菱形十二面体,石蜡的最高负载量为70%,且石蜡与ZIF-8@ZIF-67 MOF载体之间没有发生化学变化。质量分数为70%的石蜡/ZIF-8@ZIF-67经50次热循环后,熔化焓为54.36 J/g,与循环前相比无明显下降(循环前熔化焓为59.59 J/g),说明其具有良好的热循环稳定性。研究表明质量分数为70%的石蜡/ZIF-8@ZIF-67相变材料可以很好的保持整体系统的温度稳定性,具有良好的蓄-放热效果。

       

      Abstract: Phase change materials (PCMs) have been widely used for energy storage in various fields. They can also absorb and release heat during phase transition, leading to narrowing the gap between energy supply and demand. Among them, the solid-liquid phase change materials have received considerable attention, due to their low energy loss and small volume change during phase transition. However, the liquid leakage has remained a great challenge in their practical application. Therefore, the PCMs are often incorporated into a porous matrix to form a shape-stabilized composite. Particularly, metal-organic frameworks (MOFs) can be expected to serve as the matrices for PCMs, such as the tunable pore size, high specific surface area, and customizable chemical properties. One representative type of MOFs, zeolitic imidazole frameworks (ZIFs) can be used to encapsulate the PCMs, due mainly to their large pore size and excellent thermal stability. At the same time, paraffin is a type of organic solid-liquid phase change material that is widely used in various fields, due to its excellent chemical stability and high energy storage capacity. In this study, a series of shape-stable phase change materials were prepared with paraffin as the core material and ZIFs as the supporting matrix using solvent evaporation. The resulting materials shared a mass fraction of 50% to 70% paraffin, named paraffin/ZIF-8@ZIF-67. A physical model was established for the PCMs thermal storage and release system. The morphology and chemical structure of the shape-stable PCMs were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and BET surface area analysis. The thermal stability, energy storage capacity, and thermal conductivity of the materials were measured by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and transient plane source (TPS), respectively. The thermal storage and release performance of the shape-stable PCMs were simulated using Fluent software. The results showed that ZIF-8@ZIF-67 presented a diamond-shaped dodecahedral structure with a high specific surface area and porosity, where the load reached up to 70% paraffin without a chemical reaction between paraffin and ZIF-8@ZIF-67. The melting enthalpy of the 70% paraffin/ZIF-8@ZIF-67 was 59.59 J/g, and the thermal conductivity was 0.25 W/(m·℃). Furthermore, there was no significant change in the experimental curves after 50 thermal cycles, but the enthalpy value slightly decreased to 54.36 J/g, indicating excellent cycling stability. The Fluent simulation showed that the 70% paraffin/ZIF-8@ZIF-67 maintained the system temperature stability with excellent thermal storage and release performance. Therefore, the shape-stable PCMs of paraffin/ZIF-8@ZIF-67 showed broad application prospects in greenhouse heating.

       

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