Abstract

The thermal characterization of two binary systems of n-alkanes that can be used as Phase Change Materials (PCMs) for thermal energy storage at low temperatures is reported in this work. The construction of the solid–liquid binary phase diagrams was achieved using differential scanning calorimetry (DSC) and Raman spectroscopy. The solidus and liquidus equilibrium temperatures were determined using DSC for thirty-nine different samples, three for the pure n-alkanes and the remaining for binary mixtures at selected molar compositions and used to acquire the corresponding solid–liquid phase diagrams. The two binary systems of n-octane/n-decane (C₈/C₁₀) and n-decane/n-dodecane (C₁₀/C₁₂) are characterized by a eutectic behavior at low temperatures. The eutectic temperature for the system C₈/C₁₀ was found at 211.95 K and the eutectic composition appeared at the molar fraction x_octane = 0.87. For the system C₁₀/C₁₂, the eutectic temperature was found at 237.85 K, and the eutectic composition appeared for the molar fraction x_decane = 0.78. This work aims to fulfill the lack of available data in the existing literature, considering the potential application of these systems for low-temperature thermal energy storage. Raman spectroscopy was used to complement the DSC data for the construction of the solid–liquid phase equilibrium diagrams, enabling the identification of the solid and liquid phases of the system. Additionally, the liquidus curve of the phase diagram was successfully described using a modified freezing point depression curve as fitting equation, the absolute root mean square deviation for the data correlation of the C₈/C₁₀ and C₁₀/C₁₂ systems being 2.56 K and 1.22 K, respectively. Ultimately, the fitting procedure also enabled a good prediction of the eutectic point for both studied systems.

中文翻译：

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固液相平衡：用于低温储能的烷烃系统

摘要

这项工作报告了两种正构烷烃二元系统的热表征，这些系统可用作低温热能存储的相变材料（PCM）。使用差示扫描量热法（DSC）和拉曼光谱构建固液二元相图。使用 DSC 测定了 39 个不同样品的固相线和液相线平衡温度，其中三个用于纯正烷烃，其余用于选定摩尔组成的二元混合物，并用于获取相应的固-液相图。正辛烷/正癸烷(C ₈ /C ₁₀ )和正癸烷/正十二烷(C ₁₀ /C ₁₂ )的两个二元体系的特征在于低温下的共晶行为。发现系统C ₈ /C ₁₀的共晶温度为211.95 K，并且共晶组成出现在摩尔分数x_辛烷 =0.87处。对于C ₁₀ /C ₁₂体系，共晶温度为237.85 K，共晶组成为摩尔分数x_癸烷 = 0.78。这项工作旨在弥补现有文献中可用数据的缺乏，考虑到这些系统在低温热能存储方面的潜在应用。拉曼光谱用于补充 DSC 数据，以构建固液相平衡图，从而能够识别系统的固相和液相。此外，利用修正的凝固点下降曲线作为拟合方程成功描述了相图的液相线，C 8 _/ C ₁₀和C ₁₀ /C ₁₂体系数据相关性的绝对均方根偏差为2.56 K分别为 1.22 K 和 1.22 K。最终，拟合过程还能够很好地预测两个研究系统的共晶点。