用于锂离子电池包热管理的膨胀石墨-相变材料的热力行为.docx

A. Alrashdan et al. / Journal of Materials Processing Technology 210 (2010) 174-179 177

Thermo-mechanical behaviors of the expanded graphite-phase change material matrix used for thermal management of Li-ion battery packs
Abdalla Alrashdan3 Corresponding author. Tel.: [phone]x22174; fax: [phone].
E-mail addresses: [e-mail], alrash@ (A. Alrashdan), [e-mail] (A.T. Mayyas), [e-mail] (S. Al-Hallaj).
, Ahmad Turki Mayyas3, Said Al-Hallajb
a Industrial Engineering Department, Faculty of Engineering, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan b Center for Electrochemical Science and Engineering, Department of Chemical and Environmental Engineering, Illinois Institute of Technology,
10 West33rd Street, Chicago, IL 60616, USA
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Journal ofMaterials Processing Technology
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Article history:
Received 31 December2008 Received in revised form9June 2009 Accepted10 July 2009
Keywords:
Phase change material Thermal conductivity Graphite
Mechanical properties Thermal management Li-ion batteries
In this paper, blocks for the thermal management ofLi-ion battery are prepared. The blocks are made of paraffin wax, which is used as a phase change material (PCM), and graphite flakes. The process starts by compacting expanded graphite into the desired modular shapes and then impregnating it into molten paraffin wax. The modular pieces were assembled together, followed by finishing operations to achieve a desired packaging geometry.
Thermo-mechanical properties of the produced phase change material-expanded graphite (PCM/EG) composites have been studied. The tests include thermal conductivity, tensile compression and bursting test. The results showed that as mass fraction of paraffin wax increases in the composite material, the thermal conductivity, tensile strength, compression strength, and burst strength were improved while tested at low operating temperatures. In contrast, the results showed reverse behaviors when tested at relatively high operating temperature.
© 2009 Published by Elsevier B.V.

abstract

我们制备了电池热管理模块。模块是由作为相变材料(PCM)的石蜡和石墨片组成。流程是首先将膨胀石墨压缩成所需的模块的形状，然后浸渍到熔化的石蜡。将模块片聚集在一起,接着是得到所需的包装形状。
研究了得到的相变材料-膨胀石墨石墨(PCM / EG)复合材料的性质。测试包括导热系数、拉伸压缩和爆破试验。结果表明，在低工作温度下的测试中，随着复合材料中石蜡质量分数的增加，导热系数、抗拉强度、抗压强度和胀破强度提高。相比之下，相对较高的工作温度下的结果却与低温下相反。

1. Introduction
Thermal energy storage systems (TES) have the ability to store high or low temperature energy for later use (Krupa et al., 2007). For example, the solar energy can be stored for overnight heating, the summer heat stored for winter use, etc. Thus, these systems have potential applications in active and passive solar heating, water heating, air conditioning, etc., and are regarded as an economical and safe energy storage technology.
The idea to use phase change materials (PCM) for the purpose of management of thermal energy is to make use of the latent heat of a phase change, usually between the solid and the liquid state. Since a phase change involves a large amount of latent energy at small temperature changes, PCMs are used for temperature stabilization and for storing heat with large energy densities in combination with rather small temperature changes. Passive thermal management using PCMs is suitable for applications where heat dissipation is intermittent or transient. In principle, materials should fulfill different criteria in order to be suitable to serve as a PCM (Kandasamy et al., 2007; Krupa et al., 2007; Mills et al., 2006):
使用相变材料(PCM)于热能管理的目的是利用相变潜热。由于在很小的温度变化范围内相变涉及大量的潜能，PCM被用于在很小的温度变化范围内维持温度的稳定和储存高密度的热量。使用CPCM的热管理系统适用于间歇性或瞬态散热。原则上,材料应满足不同的标准，以满足作为PCM的需求。如下：
合适的熔化温度
高熔化焓/体积单位(kJ/m3)
高比热(kJ /(kg·K))
相变体积变化低
高导热系数
循环稳定性
不易燃
无毒
无腐蚀
在PCM的优点是:高熔化潜热、高能量密度、高比热、可控的温度稳定性,和相变时相变体积小。热量在PCM融化时被储存，在PCM凝固时被释放出来。限定材料的热能存储容量升温物理性质:
The Specific heat capacity ( cp)比热容；
The melting point (Tm )熔点；
The latent heat of fusion (Lf)潜热。

Suitable melting temperature
High melting enthalpy per volume unit (kJ/m3)
Highspecificheat[kJ/(kgK)]
Low volume change due to the phase change
High thermal conductivity
Cycling stability
Not flammable
Not poisonous
Not corrosive
Among the advantages of PCM are: high latent heat of fusion giving high energy density, high specific heat, controllable temperature stability, and small volume change on phase change. Heat is stored (withdrawn from the hot component) during melting and is released to the ambient during the freezing period (Kandasamy et al., 2007). Some of the physical properties that determine the thermal energy storage capacity of materials are (Krupa et al., 2007; Kandasamy et al., 2007):
• The Specific heat capacity (cp)
• The melting point (Tm)
• The latent heat of fusion (Lf)
Thermal management is recognized as one of the most significant bottle necks in the development of advanced microprocessors for mobile electronic devices (Luyt and Krupa, 2007), such as
personal digital assistants (PDAs), mobile phones, notebooks, digital cameras, etc. Considering the effects of temperature on the reliability of the electronic components, the thermal design must be able to keep the working temperature of such devices below their respective allowable maximum temperatures (generally ranging from 85 to 1200C) at all times during normal operation (Wang et al., 2008).
Reliability of electronic packages is another challenging factor in the design of mobile electronic devices. However, thermal management of Li-ion batteries plays a significant role in large power applications in addressing the thermal safety apart from improving the performance and extending the cycle life. The electrochemical performance of the Li-ion battery chemistry, charge acceptance, power and energy capability, cycle life and cycle life cost are very much controlled by the operating temperature (Mills et al., 2006). One of the side effects of exposure to high temperature is fast aging and accelerated capacity fades (Khateeb et al., 2005).
Sari and Karaipekli (2007) found that thermal conductivity of paraffin wax could be improved by incorporating expanded graphite into the wax matrix. They found that thermal conductivities of the composite PCMs with mass fraction of 2%, 4%, 7%, and 10% EG indicated that the thermal conductivity of paraffin (0.22 W/m K) increased as 81.2%, 136.3%, 209.1%, and 272.7%, respectively. This was attributed to high thermal conductivity of the EG (Sari and Karaipekli, 2007). Zhang et al. (2006) investigated the effect of additives on thermal conductivity of shape-stabilized phase change material. Their results showed that incorporating graphite into molten wax tend to improve thermal conductivity of paraffin wax about 52.7%. These results were obtained by Mills et al. (2006) as they pressed EG into sponge-like texture, which they then impregnated into molten paraffin wax at 800C for about 60 min. Thermal conductivity of the produced PCM/EG composites produced by this method have tremendous thermal conductivity up to 16.6 W/m K (Mills et al., 2006; Mills and Al-Hallaj, 2005).
Although many thermal properties of PCM/EG are discussed, no results are available about mechanical properties of these composites or their behaviors at relatively elevated temperatures. Hence, the aim of this work is to perform a systematic experimental study to analyze the important effects of the thermo-mechanical properties of the produced samples, like tensile and compression behaviors, bursting and thermal conductivity.

2.2. Manufacturing process
The manufacturing method was based on the presented by Mills et al. (2006). The only main difference is that the EG was initially compacted into desired modular pieces (dimensions are shown in Fig. 1) which reduces the amount of waste material during the drilling processes. The green compacts density is 0.04375 g/cm Materials and methods
2.1. Expanded graphite
The graphite matrix is made by compacting expanded graphite (EG) to a desired bulk density. EG is easily produced from flake graphite. Flake graphite is characterized by stacked sheets of carbon where the carbon making up the sheet is held together by strong covalent bonds and the stacked sheets are held together by weak van der Waals bonds (Mills et al., 2006). Detailed description of expanded graphite production method is discussed comprehensively in literature (e.g. Mills and Al-Hallaj, 2005; Sari and Karaipekli, 2007). The following is a brief description of the production method of EG, as outlined by Sari and Karaipekli (2007). In this process, EG was prepared from graphite to maximize mass fraction of paraffin to be absorbed into its porous structure. The graphite sample was first converted to intercalated, or expandable, graphite through chemical oxidation in the presence of a mixture of sulfuric and nitric acid, and then dried in a vacuum oven at 650C for 24 h. EG was then obtained by rapid expansion and exfoliation of expandable graphite in a furnace over 9000C for 60 s. In this study, EG was supplied by All Cell Technologies, LLC (Chicago, IL).
. Technical grade paraffin (melting point = 58-600C and thermal conductivity = 0.2 W/m K) was used as PCM. The produced green compacts were then impregnated in the molten paraffin wax at 800C for about 12 h. The density of the composite is about 0.83g/cm3, which is 17-18x, the density of green compacts in order to get a high thermal conductive matrix (Mills and Al-Hallaj, 2005). The modular pieces were assembled using commercial binder available in the markets (widely used for joining PVC pipes). The inscribed hole diameter was about 16 mm which was enlarged using CNC drilling center to produce holes with diameter of 18.2 mm (the diameter of Li-ion battery). After drilling the holes, the complete block was subjected to finishing operation before usage. Final dimensions of block are 3.7 cm x 5 cm x 6.5 cm. The complete block is then used as a Li-ion battery case in which commercial, type 18650 Li-ion cells could be placed (see Fig. 2).

Fig. 2. PCM/EG block.

2.3. Impregnation of the graphite matrix into paraffin wax
The next step after producing the modular pieces is the impregnation into molten paraffin wax. The matrices were submerged in a liquid paraffin bath at 800C. The small samples were removed from the paraffin bath after 1, 3, 6, 9, and 12 h to measure their weights. The bulk density, and therefore the porosity, determines the total volume available for PCM storage.
2.4. Thermal conductivity
The thermal conductivity of the samples was measured using a steady state apparatus illustrated in Fig. 3 Grade 304 stainless steel block (kss = 16.3 W/mK, 10 mm thick) with a cross-sectional area equal to the area of the graphite matrix composites was placed on a heater at about 800C. To enhance diffusion of heat, continuous cold water was maintained through a heat exchanger placed over the sample. The temperature drop across the stainless steel and graphite matrix was measured using three K-type thermocouples, placed at the center of each surface (Mills et al. 2006). The apparatus was insulated to ensure one-dimensional heat transfer. Fourier's law of heat conduction was then used to determine the thermal conductivity of the graphite sample. No attempt was made to quantify the effects of thermal resistance between each unit. Thermal conductivity of sample was calculated according to the following formula (Mehling et al., 2000): where 入 is thermal conductivity of sample (W/mK), Qtherm is the heat flux from the surface of steel block, Ax is the sample thickness (m), A is the area of sample (m2) and AT is the temperature difference between two sides of the sample (K).
2.5. Tensile and compression tests
Commercial Li-ion battery packs are usually packed with a separator made of thin film placed between cells. Different commercial materials are used with a tensile strength generally ranges between 5 and 10 MPa. From manufacturing point of view, the tensile strength of the separator is 6.8 MPa (Newman et al., 2006).
Tensile strength was performed only on the PCM/EG block shown on Fig. 2. A sample cut from the block in the direction parallel to the compaction is used. The squared cross-sectional specimens were cut to the required dimensions as shown in Fig. 4. The gage length was 60 mm and cross-sectional area was 100 mm2. The sample was drilled from both ends and fixed in their fixtures vertically to ensure uniaxial loading. A universal Instron testing machine was used to obtain the load-deflection diagrams which then converted to their corresponding stress-strain diagrams. The cross-head speed was set to 1 mm/min.
The compression test was performed at room temperature and at45 0CtostudytheeffectoftemperatureonthestrengthofPCM/EGcomposites. Cylindrical samples with 20 mm in length and 10 mm in diameter were subjected to uniaxial compressive load using a universal Instron testing machine. Also, the cross-head speed was set to 1 mm/min.
2.6. Burst test
The specimens were turned into small cylinders to simulate the single Li-ion case with an inner diameter of 18.2 mm and a wall thickness of 2.5 mm (2.5 mm represents the average value of wall thickness in x- and y-directions). At the open side of the specimen, a Teflon inlet valve was used to pressurize the selected specimens. The internal pressure was gradually increased to different levels with compressed air and tested at the two different temperature settings (i.e. room temperature 22 士 10C and 45 0C). Burst testing was performed using a sustained gas pressurization system capable of gas pressurization to around 12 bar.
3. Results and discussions
3.1. Impregnation process
.^suo*Do>》«a(r;

Fig. 4. Tensile specimen with squared cross-section.
Paraffin has an excellent stability concerning the thermal cycling, i.e. a very high number of phase changes can be performed without a change of the material's characteristics (Heinz and Streicher, 2006; Zalba et al., 2003). The PCM is loaded into the graphite matrix through capillary forces between liquid PCM and the graphite (Mills et al., 2006). For example, the produced composite impregnated for about 12 h consists of roughly 80 vol.% PCM, 10 vol.% highly porous EG and 10 vol.% remaining air. Experimental impregnation curve could be established by weighing the total weight of the impregnated sample at different time periods. Such curve is shown in Fig. 5.

0 2 4 6 8 10 12 14 16
Time (hours)
Fig. 5. Typical saturation curves for PCM impregnation process.
16141210
8 6 4 2 c
5101&01& o 3 3 2 2 1 1 {SEBJS15

A. Alrashdan et al. /Journal ofMaterials Processing Technology210 (2010) 174-179 179

Fig. 8. Stress-strain diagram for different samples impregnated fordifferent times andtestedatroomtemperaratureandat45 0C.
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