1.Key Laboratory of Low-Grade Energy Utilization Technologies and Systems of Ministry of Education, Chongqing University, Chongqing 400044, China 2.School of Energy and Power Engineering, Chongqing University, Chongqing 400044, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 51606017), the China Postdoctoral Science Foundation (Grant No. 2017M612906), and the Postdoctoral Science Foundation of Chongqing, China (Grant No. Xm2016068).
Received Date:21 November 2018
Accepted Date:22 March 2019
Available Online:01 May 2019
Published Online:20 May 2019
Abstract: Latent heat storage mainly uses the latent heat of phase change material (PCM) to realize thermal energy storage and utilization, which is the most important thermal energy storage method at present. However, most of PCMs have the disadvantage of low thermal conductivity, which greatly restricts the thermal response rate and system efficiency of the thermal energy storage system. With the development of nanotechnology, it is expected to improve the thermal conductivity of traditional PCMs by adding high thermal conductivity nanoparticles. In this paper, a novel two-dimensional carbon nanomaterial, graphene is selected as an additive for PCM. In this paper, graphene nanoplatelets-octadecane phase change composite materials are prepared with a two-step method and the mass fractions of graphene nanoplatelets are 0%, 0.5%, 1%, 1.5%, and 2%. Their microstructures, morphologies and thermophysical properties are characterized by scanning electron microscopy (SEM), infrared spectroscopy (IR), differential scanning calorimetry (DSC), and thermal conductivity analysis. The effects of the addition quantity of graphene nanoplatelets on the phase transition temperature, enthalpy, specific heat capacity, thermal conductivity and thermal stability of the composite PCM are compared. The experimental results show that the dispersion stability of the graphene nanoplatelets in the composite system is greatly improved by the addition of dispersant, and the system does not produce obvious agglomeration nor sedimentation after multiple phase transformation cycles. The graphene nanoplatelets still maintain good lamellar structure and homogeneous dispersion in the n-octadecane matrix, and no chemical reaction occurs in the composite process. Comparing with the n-octadecane, the melting point of the composite phase change material decreases slightly, and the freezing point increases slightly. With the increase of graphene nanoplatelets, the latent heat value of graphene nanoplatelets-octadecane composite phase change material decreases gradually. For the composite phase change material with 2.0 wt.% graphene nanoplatelets, the melting enthalpy and solidified enthalpy are reduced by 6.01% and 7.35%, respectively. When the mass fractions of graphene nanoplatelets are 0.5%, 1%, 1.5%, and 2%, the thermal conductivity values of phase change composite materials are nearly 32.4%, 77.4%, 83.1%, and 89.4% higher than the thermal conductivity value of pure octadecane, respectively. Comparing with the significant increase in thermal conductivity, the addition of graphene nanoplatelets has little effect on the phase transition temperature and latent heat of PCM, and still exhibits the good heat storage performance. Keywords:graphene nanoplatelets/ phase change composite materials/ thermophysical properties
由于纳米石墨烯片的比表面积非常大, 界面之间存在很强的范德华力, 在使用过程中很容易重新堆叠形成不可逆转的石墨, 产生团聚或沉降, 因此, 纳米石墨烯片在复合相变材料体系中能否均匀分散并且稳定存在是制备成功与否的关键. 为了提高纳米石墨烯片在正十八烷中的分散性, 需要向复合体系加入分散剂, 分散剂可分为离子型分散剂和非离子型分散剂, 离子型分散剂可以增大纳米石墨烯片表面电量, 进而增大颗粒之间的斥力, 而非离子型分散剂可吸附于纳米石墨烯片表面, 形成覆盖层, 阻止片层之间发生团聚. 本文选用的两种分散剂—超分散剂WinSperse 3050和超分散剂Disuper S35都是非离子型分散剂. 图1为未添加分散剂与添加不同种类分散剂的复合相变材料(graphene nanoplatelets, GNP 0.5 wt.%)分散稳定性实验结果. 可以看出, 静置15 min后, 未添加分散剂的A样品开始出现沉淀, 而添加了分散剂的B样品和C样品中却未出现相分离. 图2为上述三个样品经过5次、10次熔化-凝固循环后的分散情况, 同样可以看出添加了分散剂的两个样品具有良好的分散稳定性. 这是因为仅采用磁力搅拌或超声振荡等物理分散方法虽然能够较好地实现纳米石墨烯片在液相正十八烷中的分散, 但外力作用一旦停止, 纳米石墨烯片就会在范德瓦尔斯力的作用下将会重新团聚. 而加入分散剂后, 纳米石墨烯片表面形成了吸附层, 这样的吸附层会产生空间位阻, 起到剥离纳以及分散米石墨烯片的作用, 当分散剂表面的大分子足够长并且延展充分时, 纳米石墨烯片层层间距将远大于范德瓦尔斯力的作用距离(0.5 nm)[22], 另外, 分散剂还会使片层之间产生排斥力, 阻止了纳米石墨烯片之间的接触, 从而减少了团聚[23], 对比B, C两个样品发现, C样品的样品瓶内壁附着着较多石墨烯颗粒, 证明仍然有部分纳米石墨烯片发生了团聚, 而B样品分散效果更佳, 因此可认为B样品中使用的超分散剂Disuper S35更适用于该体系, 这是由于超分散剂Disuper S35除了可在纳米石墨烯片的表面形成吸附层, 其自身的亲油性长链还可以与液相十八烷接触, 有利于纳米石墨烯片在液相十八烷中长期均匀稳定地分散. 因此将其作为后续研究纳米石墨烯片-正十八烷复合相变材料热物时使用的分散剂. 图 1 未添加分散剂与添加不同种类分散剂的复合相变材料(GNP 0.5 wt.%)分散稳定性 (a)初始状态; (b)静置15 min; (c)静置30 min; (d)凝固状态 Figure1. Dispersion stability of composite phase change material (GNP 0.5 wt.%) without addition of dispersant and with adding different kinds of dispersants: (a) Initial state; (b) let the mixture stand for 15 min; (c) let the mixture stand for 30 min; (c) solidification state.
图 2 复合相变材料(GNP 0.5 wt.%)经历不同次数熔化-凝固循环后的状态 (a) 5次; (b) 10次 Figure2. Statuses of composite phase change material (GNP 0.5 wt.%) after different melting-solidification cycles: (a) After 5 cycles; (b) after 10 cycles.
图3是实验制备的含有不同质量分数的纳米石墨烯片复合相变材料, 可以看出, 当纳米石墨烯片添加量不超过2%时, 均能够稳定分散于十八烷体系中而没有出现明显的团聚和相分离. 图 3 含有不同纳米石墨烯片质量分数的复合相变材料 Figure3. Composite phase change materials with different mass fractions of graphene nanoplatelets.
相变温度、相变焓和比热容都是相变材料应用时重要的选择标准, 因而有必要知道添加纳米石墨烯片对复合相变材料相变特性的影响. 图6为纯正十八烷以及不同质量分数纳米石墨烯片-正十八烷复合相变材料在熔化和凝固过程中的DSC差示扫描量热测试曲线. 从图中可以观察到, 复合相变材料具有与纯十八烷类似的曲线形状, 即在熔化和凝固过程中均只有一个明显的固-液相变峰, 这表明纳米石墨烯片的添加不会改变基体的性质. 与纯正十八烷相比, 复合相变材料的DSC曲线在熔化和凝固阶段都发生了前移. 为了定量研究纳米石墨烯片质量分数对复合相变材料相变温度及相变焓的影响, 采用Proteus Analysis软件对实验测得的DSC曲线进行分析, 并将熔化过程以及凝固过程相关数据分别列于表1和表2中. 图 6 正十八烷及其复合相变材料的DSC曲线 Figure6. The DSC curves of n-octadecane and composite phase change materials.
材料
起始温度 Tms/℃
峰值 Tmp /℃
终止温度 Tme/℃
相变焓 Hm/J·g–1
正十八烷
28.1
33.3
35.9
241.4
0.5%纳米石墨烯片/正十八烷
27.9
33.5
36.5
237.4
1.0%纳米石墨烯片/正十八烷
27.9
32.9
36.0
237.0
1.5%纳米石墨烯片/正十八烷
27.5
33.4
36.2
234.8
2.0%纳米石墨烯片/正十八烷
27.9
33.3
35.7
226.9
表1正十八烷及其复合相变材料熔化过程的相变温度及相变焓 Table1.Phase transition temperature and enthalpy of n-octadecane and composite phase change materials during melting process.
材料
起始温度 Tss/℃
峰值 Tsp/℃
终止温度 Tse/℃
相变焓 Hs /J·g–1
正十八烷
26.1
21.5
19.8
–240.7
0.5%纳米石墨烯片/正十八烷
26.3
20.9
19.0
–237.8
1.0%纳米石墨烯片/正十八烷
26.5
21.1
19.3
–237.2
1.5%纳米石墨烯片/正十八烷
26.4
21.5
20.0
–233.5
2.0%纳米石墨烯片/正十八烷
26.5
21.1
19.5
–223.0
表2正十八烷及其复合相变材料凝固过程的相变温度及相变焓 Table2.Phase transition temperature and enthalpy of n-octadecane and composite phase change materials during solidification process.