Preparation and thermal properties research of a novel magnesium chloride hexahydrate-magnesium nitrate hexahydrate/graphite phase carbon nitride composite phase change material

doi:10.11949/0438-1157.20211067

Abstract

Abstract:

The low utilization rate of magnesium resources in Qinghai Salt Lake in China has caused serious resource waste and environmental pollution. If magnesium salt is developed as a phase change heat storage material, its application fields can be improved, thereby promoting the full utilization of magnesium salt resources. In view of the application requirements of medium-low temperature heat preservation and heat insulation, this work selects eutectic salt MgCl₂·6H₂O-Mg(NO₃)₂·6H₂O(MCH-MNH) with mass ratio of 41∶59 as phase change material based on previous work. MgCl₂·6H₂O-Mg(NO₃)₂·6H₂O/g-C₃N₄(MCH-MNH/CN) composite phase change material with low thermal conductivity is prepared by using porous graphite carbon nitride (g-C₃N₄, CN) as the support material in order to reduce its supercooling and improve its thermal insulation performance. The porous CN is obtained by calcination of urea at 550℃, then MCH-MNH/CN composite phase change material is prepared by adsorption method. The morphology, structure and thermal properties of the composite phase change material are characterized and measured. The results show that the eutectic salt phase change material is uniformly adsorbed in the microporous structure of CN, and the recombination with CN is a physical process without chemical reaction. The composite phase change material has a phase change temperature of 55.2℃ and phase change enthalpy of 92.7 J/g without supercooling. The thermal conductivity of the composite phase change material is 0.3 W/(m·K), which is only half of thermal conductivity of the eutectic salt MCH-MNH, and the heat insulation performance is improved. In addition, the composite phase change material has good thermal stability and has application prospects in the field of medium and low temperature thermal insulation.

Key words: thermal storage, MgCl₂·6H₂O, Mg(NO₃)₂·6H₂O, g-C₃N₄, composite phase change material, thermal stability

摘要：

我国青海盐湖镁资源的利用率低，带来严重的资源浪费和环境污染，若将镁盐开发为相变储热材料，则可扩大其应用领域，从而促进镁盐资源的充分利用。针对中低温保温隔热的应用需求，在前期工作基础上，选用MgCl₂·6H₂O-Mg（NO₃）₂·6H₂O（MCH-MNH）质量比为41∶59的共晶盐作为相变材料，为降低其过冷度并提高隔热性能，选用多孔介质石墨相氮化碳（g-C₃N₄，CN）作为支撑材料，制备低热导率的MCH-MNH/CN复合相变材料。先将尿素在550℃高温下煅烧得到多孔的CN，再采用吸附法制备出MCH-MNH/CN复合相变材料，并对复合相变材料的形貌、结构与热性能进行了表征和测量。结果表明，共晶盐相变材料均匀地吸附在CN的微孔结构内，其与CN的复合是一个物理过程，没有发生化学反应；复合相变材料的相变温度为55.2℃，相变焓值为92.7 J/g，几乎没有过冷度，其热导率为0.3 W/（m·K），仅是共晶盐MCH-MNH的一半，提高了隔热性能。此外，复合相变材料还具有良好的热稳定性，在中低温保温隔热领域具有应用前景。

关键词: 储热, 六水氯化镁, 六水硝酸镁, 石墨相氮化碳, 复合相变材料, 热稳定性

CLC Number:

TQ 115

Wenbo ZHANG, Ziye LING, Xiaoming FANG, Zhengguo ZHANG. Preparation and thermal properties research of a novel magnesium chloride hexahydrate-magnesium nitrate hexahydrate/graphite phase carbon nitride composite phase change material[J]. CIESC Journal, 2021, 72(12): 6399-6406.

张文波, 凌子夜, 方晓明, 张正国. 新型六水氯化镁-六水硝酸镁/石墨相氮化碳复合相变材料的制备及其热性能研究[J]. 化工学报, 2021, 72(12): 6399-6406.

Figures/Tables 12

Fig.1 Photographs of the liquid leakage traces of MCH-MNH/CN composite phase change materials with 75%, 80%, and 85% mass fractions of MCH-MNH

Table 1 Mass variation of the filter paper with MCH-MNH/CNcomposite phase change material before and after heating for 3 h

MCH-MNH质量分数	加热前质量/g	加热后质量/g	质量变化/g
75%	0.31	0.32	0.01
80%	0.32	0.32	0
85%	0.32	0.56	0.24

Fig.2 SEM images of CN (a) and MCH-MNH/CN composite phase change material before (b) and after (c) 50 thermal cycles

Fig.3 N2 adsorption/desorption isothermal curve (a) , BJH pore size distribution curve (b) and XRD patterns (c) of CN

Fig.4 FTIR spectrum of CN, MCH-MNH and MCH-MNH/CN composite phase change material

Fig.5 DSC curves of MCH-MNH and MCH-MNH/CN composite phase change material

Table 2 Phase transformation characteristics of MCH-MNH and MCH-MNH/CNcomposite phase change material

样品	熔化温度/ ℃	凝固温度/ ℃	熔化焓值/ (J/g)	凝固焓值/ (J/g)
MCH-MNH	57.7±0.8	38.6	119.8	110.6
MCH-MNH/CN	55.2±0.7	42.8	92.7	88.1

Fig.6 T-history curves of MCH-MNH and MCH-MNH/CN composite phase change material

Table 3 Thermal conductivity of MCH-MNH， MCH-MNH/CN composite phase change material

样品	热导率/(W/(m·K))
MCH-MNH	0.62
MCH-MNH/CN	0.30

Fig.7 TG curves of CN, MCH-MNH and MCH-MNH/CN composite phase change material

Fig.8 DSC curves of MCH-MNH and MCH-MNH/CN composite phase change material before and after 50 thermal cycles

Table 4 Phase transformation characteristics of MCH-MNH and MCH-MNH/CNcomposite phase change material before and after 50 thermal cycles

样品	循环次数	熔化温度/℃	熔化焓值/(J/g)
MCH-MNH	0	57.7	119.8
	50	31.2	59.0
MCH-MNH/CN	0	55.2	92.7
	50	56.7	90.9

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