Elsevier

Applied Thermal Engineering

Volume 36, April 2012, Pages 345-352
Applied Thermal Engineering

Studies on thermal properties and thermal control effectiveness of a new shape-stabilized phase change material with high thermal conductivity

https://doi.org/10.1016/j.applthermaleng.2011.10.046Get rights and content

Abstract

In order to overcome the difficulty of conventional phase change materials (PCMs) in packaging, the shape-stabilized PCMs are proposed to be used in the electronic device thermal control. However, the conventional shape-stabilized PCMs have the drawback of lower thermal conductivity, so a new shape-stabilized PCM with high thermal conductivity, which is suitable for thermal control of electronic devices, is prepared. The thermal properties of n-octadecane-based shape-stabilized PCM are tested and analyzed. The heat storage/release performance is studied by numerical simulation. Its thermal control effect for electronic devices is also discussed. The results show that the expanded graphite (EG) can greatly improve the thermal conductivity of the material with little effect on latent heat and phase change temperature. When the mass fraction of EG is 5%, thermal conductivity has reached 1.76 W/(m K), which is over 4 times than that of the original one. Moreover, the material has larger latent heat and good thermal stability. The simulation results show that the material can have good heat storage/release performance. The analysis of the effect of thermal parameters on thermal control effect for electronic devices provides references to the design of phase change thermal control unit.

Highlights

► A new shape-stabilized PCM with higher thermal conductivity is prepared. ► The material overcomes the packaging difficulty of traditional PCMs used in thermal control unit. ► The EG greatly improves thermal conductivity with little effect on latent heat. ► The material has high thermal stability and good heat storage/release performance. ► The effectiveness of the material for electronic device thermal control is proved.

Introduction

Temperature is the main factor affecting the performance of electronic devices. Damaging hot/cold temperatures not only affect the reliability of electronic devices, but also may lead to the failures in them. Studies have shown that a 1 °C increase in the temperature of an electronic device will reduce its reliability by 4%, and an increase of 10–20 °C may double its failure rate [1]. According to statistics, more than 55% of failures in electronic devices have a certain relation with temperatures [2]. Therefore, proper thermal control solutions must be used to control the temperatures of electronic devices within acceptable ranges in order to guarantee the long-term stable operation of the electronic devices. As a passive thermal control solution, phase change thermal control is especially suitable for the electronic device working with periodic bursts of power or in a harsh environment. When the electronic device has a high heat load or is in a high-temperature environment, the phase change material (PCM) will absorb heat to maintain the temperature below a maximum value, which will prevent the device from overheating; whereas when it has a low heat load or is in a low-temperature environment, the PCM will release heat to maintain the temperature above a minimum value, preventing the too low temperature of the electronic device, and also the PCM greatly reduces the temperature fluctuations of the electronic device. Because the phase change thermal control is featured by compact construction, economic energy-saving and so on, it has become the main means of thermal control for the variety of electronic devices.

There are a lot of studies on PCMs for the thermal control of electronic devices. In the civilian fields, Alawadhi and Amon [3] proved the effectiveness of a phase change thermal control unit for portable electronic devices by performing experimental and numerical analyses. Fok et al. [4] conducted an experimental study about the cooling of hand-held electronic devices using a heat sink encapsulated with n-eicosane. They investigate the effect of the PCM, number of fins, orientation of the device, and the power level on the temperature of electronic devices. Tan and Fok [5] also used a heat storage unit filled with n-eicosane to control a mobile phone temperature. It concluded that the use of phase change thermal control can effectively reduce the operating temperature of mobile phone; moreover, the phase change temperature and amount of PCMs are important thermal design parameters in the thermal control unit. Al-Hallaj et al. [6] proposed PCMs for the thermal management of the Li-ion batteries of plug-in hybrid vehicles. By comparing 4 different thermal control solutions for Li-Ion batteries of electric scooters, it was found that using the foam aluminum filled PCMs can reduce the battery temperature rise by about 50% compared with the natural convection cooling [7]. In the fields of spacecraft thermal control, Apollo 15 lunar Rover Vehicle used three phase change thermal control units; n-tridecane was also applied to the cooling loop of space laboratory [8]. In recent years, n-pentadecane PCM units have been used for NASA’s new Orion manned spacecraft and Altair Lander [9], [10]. A phase change thermal storage/radiation heat sink for the extra-vehicular space suits was introduced by Zhao et al. [11]. The heat sink satisfies the design requirements of the cooling system for the spacesuit, and besides it is renewable and closed, which is consistent with development trends in space technology.

For the above research findings, it has been proved that the use of phase change thermal control units for thermal management of electronic devices is effective. Unfortunately, these phase change thermal control units all used the large latent heat which is given off during solid to liquid change of PCMs to maintain temperature stability of electronic devices; hence the thermal control units must be tight packaged to prevent leakage. Furthermore, the solid to liquid phase change of PCMs is often accompanied with volume change, so it makes the packaging very difficult. The package not only increases the cost and weight of a thermal control unit but also brings the risk of leakage which reduces the operation reliability. For example, the researchers at Johnson Space Center performed a freezing/thawing test of three encapsulated PCMs in a thermal vacuum chamber, and found that two of the samples shown signs of leaking [12].

In order to make better use of PCMs for thermal control, the idea of applying the shape-stabilized PCMs to electronic device thermal control is proposed in the study. When the shape-stabilized PCM melts, phase change material is enclosed in micro-network structures of support material, not easy to flow out, so it always remains in the solid state. Furthermore, the support role of the support material makes little change in the PCM volume. Therefore, the shape-stabilized PCMs can overcome the difficulties of packaging in conventional PCMs, as well as to help to improve the safety and reliability of the phase change thermal control. However, the conventional shape-stabilized PCMs generally were applied in the building energy storage [13], [14], [15], [16]. It is unsuitable to be used in electronic device thermal control because that, 1) poor heat transfer performance lowers the impact resistance of electronic devices under transient high heat flow; 2) phase change temperatures of paraffin mixtures used in building storage are usually so high that it can not meet the suitable temperature requirements of electronic devices.

To solve above problems, in this study a series of shape-stabilized PCMs with high thermal conductivity, which are suitable for the thermal management of electronic devices, were prepared. Those shape-stabilized PCMs use organic n-alkanes as phase change material, high-density polyethylene (HDPE) as support material, and expanded graphite (EG) as thermal conductivity enhancer. The material has the following desirable characteristics: 1) it can always keep in a solid state before and after the phase change, so that it does not require tight packaging, which reduces the weight of the thermal control unit and removes the possible danger caused by leakage; 2) the thermal conductivity of the shape-stabilized PCM is increased significantly by adding a small amount of EG in the case of causing little effect on the phase change temperature and latent heat; 3) it has a larger latent heat compared with paraffin mixtures, which can better satisfy the requirements of compact and lightweight structure for the thermal control unit; 4) the phase change temperatures of organic n-alkanes range from −5 °C to 66 °C, so it is easy to match with electronic devices having different temperature control requirements. The thermal properties of n-octadecane-based shape-stabilized PCM are tested and analyzed. The effect of thermal conductivity on heat storage/release performance of the material is studied by numerical simulation. The thermal control effect of the material for electronic devices is also analyzed.

Section snippets

Preparation of shape-stabilized PCMs with high thermal conductivity

A simple preparation process of the material is as follows: firstly, expandable graphite (average particle size: 270 μm, expansion ratio: 350 ml/g) was dried for 24 h at 65 °C, and then expanded to EG in a muffle furnace at about 800 °C for 60 s, thirdly, a kind of organic n-alkane was melted by heating at 170 °C, fourthly, the HDPE (the mass proportion of paraffin and HDPE was 80:20) was mixed into the organic n-alkane and stirred until uniform in a vacuum container, and the fifth step was

Thermal conductivity

The thermal conductivities of the shape-stabilized PCMs were measured by probe thermal properties analyzer (Quickline™-30), accuracy of which is 5% of reading +0.003W/(m K) over the range of 0.015–0.050W/(m K), 5% of reading +0.001W/(m K) over the range of 0.050–0.70W/(m K) and 5% of reading over the range of 0.70–6.0W/(m K).

The shape-stabilized PCMs containing 0%, 1%, 2%, 4%and 5% in weigh of EG were measured at room temperature (19 °C). The test results of five different samples are given in

The mathematical model

As the impact of convection can be ignored in phase change heat transfer process of the shape-stabilized PCM and the latent heat of PCM is converted into heat capacity in a small and limited temperature range using apparent heat capacity method [18], the phase change heat transfer is considered as the problem of nonlinear heat conduction with temperature-dependent material properties in a “single-phase”. The governing equation is:ρcTt=x(kTx)+y(kTy)+z(kTz)+SDiscretize the Eq. (1)

Conclusions

  • (1)

    In this paper, using organic n-alkanes as phase change material, HDPE as support material, and EG as thermal conductivity enhancer, a new shape-stabilized PCM with high thermal conductivity is prepared. The problem of packaging difficulty in conventional phase change materials is solved by the use of the new material, which can help to reduce the quality of the thermal control unit and improve the stability and reliability of the thermal control systems.

  • (2)

    The test results show that adding a small

Acknowledgements

The authors would like to thank the Fundament Research Funds for the Central Universities and National Science Foundation of China (grant no. 50876096) for the financial supports.

References (18)

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