以石墨烯懸浮液提升石蠟相變化儲能材料之導熱性

Enhancing Thermal Conductivity of Paraffin as a Phase Change Material Through Graphene Dispersion

因應全球暖化是本世紀人類最重要的課題之一，造成這種環境危機的主因之一是文明社會過度消耗化石燃料能源，導致溫室氣體的過量排放所致。因此，吾人必須嚴格調控能源供應，有效利用熱能並力行節能減碳以對抗暖化危機。從熱能管理的角度來看，儲熱技術一直是熱能應用研發的焦點，包含家用冷暖空調、太陽能集熱器、儲熱系統和工業餘熱回收利用等領域。在各種熱能儲存技術中，潛熱的儲能技術深受重視，藉由提高單位質量的儲能能力，可以實現更多的熱能應用場域。
相變化材料（Phase change materials, PCMs）是潛熱儲能系統中不可或缺的存儲介質。石蠟（Paraffin）具有高熔化熱、低蒸氣壓（熔融態時）、化學安定性佳等諸多優良特性，近年受到廣泛的關注，許多研究學者視其為最具發展潛能的理想儲熱材料之一。然而，石蠟的導熱率（~0.2 Wm-1K-1）很低，不利於後續的商品化發展。為了緩解這一缺點，在本研究中，吾人於石蠟相中加入具良好導熱性質的石墨烯粉體，以增加石蠟相變化材料的儲熱效率。
初步研究顯示，若直接投入石墨烯粉末，則易形成分散不佳的狀態。若改以石墨烯懸浮液為原料，再與熔融態石蠟摻混，則可順利形成分散均勻的石蠟-石墨烯複合材料。若在上述體系中，加入少量的表面活性劑，則可以進一步提升石墨烯於石蠟相中的分散性。此點可由以下實驗中加以證明：使用10、20和30wt.-%等3種不同濃度的石墨烯懸浮液，於相同的石墨烯負載量下，所形成的石墨烯/石蠟PCMs複合材料，其導熱性增強且數值相似。如同熱重分析（Thermogravimetric analysis, TGA）所證實的，此三個樣品都表現出良好的分散性。其實際樣品重量與理論值的偏差僅為0.05%。
石墨烯/石蠟PCMs複合材料的熱導率以熱盤法（hot-disk method）測定。結果顯示：石蠟/石墨烯複合材料於3 wt.-%石墨烯負載量時，具有最大的導熱係數，0.2792 Wm-1K-1；與純石蠟相比，導熱率提高了2.8%。以示差掃描量熱分析儀（Differential scanning calorimetry, DSC）量測樣品的熔化潛熱，結果顯示：石蠟/石墨烯複合材料較純石蠟的熔化潛熱降低了5~8%，對實際應用而言，此一增量是合理的。若改以其他導熱性替代品，如多壁奈米碳管（multi-walled carbon nanotubes, MWNT）與氮化硼（boron nitride, BN），在相同條件下進行評估。則這些樣品的熔化潛熱大幅下降超過8%，對實際應用而言，改質效果較不理想。

Global warming is a serious concern in the 21st century and is primarily caused by excessive energy consumption. In particular, thermal energy is a key means of ensuring that our energy consumption is sustainable. Energy storage, particularly thermal energy storage, has emerged as a key area of research. It has found-applications in various fields, including household heating/cooling systems, solar energy collectors, power storage（Li-ion battery systems）, and industrial waste heat recovery. Among the available thermal energy storage techniques, latent thermal energy storage based on phase change materials（PCMs）is a practical technique that offers a high storage capacity per unit mass. Paraffin, a type of PCM, has gained significant attention due to its desirable characteristics, such as high heat of fusion, low vapor pressure in the melt, chemical inertness, and chemical stability. However, a major drawback of paraffin is its low thermal conductivity（approximately 0.2 Wm-1K-1）, which limits its applications in thermal energy storage systems. To overcome this limitation, the present study explored the use of commercial graphene incorporated into pure paraffin to enhance its thermal conductivity.
Preliminary studies have revealed that the direct addition of graphene powder to a paraffin matrix does not result in an even distribution of graphene within the composite material. Therefore, in this study, homogeneous paraffin-graphene composites were formed by mixing a graphene suspension with the paraffin matrix. Moreover, the addition of a trace amount of surfactant to the system substantially enhanced the dispersion of graphene. By incorporating the surfactant and ensuring uniform graphene dispersion, graphene/paraffin PCM composites were derived at graphene loadings of 10, 20, and 30 wt%. These composites demonstrated similar enhancements in thermal conductivity compared with pure paraffin. To assess the uniformity of the dispersion within the composite samples, thermos gravimetric analysis（TGA）was performed. The results of TGA revealed that the graphene was extraordinarily uniformly dispersed within the composite samples. The deviation in weight for the composite samples was only 0.05% compared with the theoretical values.
The thermal conductivity of the composite samples was determined using the hot-disk method. The results revealed that the paraffin/graphene composite achieved a maximum thermal conductivity of 0.2792 W m-1K-1at a graphene loading of 3 wt.%, a 2.8% improvement compared with that of pristine paraffin. The latent heat of fusion for the composite samples was determined using differential scanning calorimetry, and the latent heat of fusion for the composite samples was observed to decline by 5%–8% compared with that of pristine paraffin. This decrease is considered