Phase change effect of low melting point metal for an automatic cooling of USB flash memory

Introduction

The USB flash memory and solid-state disk are increasingly used in recent years and the data transfer speed has developed from USB 1.1, USB 2.0 to USB 3.0. This brings about the high power generation and thus evident temperature increase during the high-speed data transfer between computer and the USB flash memory. In general, a lower temperature would allow for a higher data transfer speed. For example, the gate leakage current in transistors increases exponentially with temperature. Its reliability and performance are also heavily gated by thermal factors [1]. Consequent problems caused by the overheating, especially the hot cover of the USB flash memory in use, were frequently reported, which is the major reason to result in thermal discomfort for the user. Such temperature would exceed 40°C after data transmission of a few minutes, if no thermal management was administrated [2,3]. It is, therefore, inconvenient for the user to hold such a hot USB flash memory in hand. Technologically, the heat produced in the flash memory chip and its main control unit during working was hard to dissipate due to the extremely small space in the computer slot. Conventional thermal management approaches such as heat pipe, forced air or water cooling, therefore, are not practical for such use on account of requiring bulky and massive equipment. Clearly, an alternative cooling is urgently needed in the area.

Except for the resulted discomfort to the user, the high temperature also brings chip damage and thus data loss. So far, no research has been found on thermal management of the USB flash memory. Here, to tackle the above ever tough issue, a brand new thermal management approach was proposed in this research communication to maintain a cool state of the USB flash memory, which is based on using the high conductivity and phase change behavior of low melting point metals around room temperature to efficiently absorb the heat of USB flash memory. The liquid metal cooling has recently be invented for the thermal management in computer chips [4,5], harvesting low grade heat to generate electricity [6,7], as well as cooling of high power LED, etc. [8]. And the application fields are still increasingly being expanded [9].

To disclose the basic features of using phase change material (PCM) based on gallium or its alloy to regulate the temperature of the USB flash memory automatically, conceptual experiments were conducted in this research communication. For such cooling, the heat produced in the USB flash memory can be quickly absorbed by gallium during the melting phase. The molten gallium can then be re-solidified by releasing heat to the surroundings when the USB flash memory is extracted from the computer USB port. This method offers an idealistic thermal management approach for the safe running of the USB flash memory as it is seldom used continuously for more than twenty minutes during data transmission and its idle times are usually long enough for the molten gallium to re-solidify.

Experimental results

In order to investigate the thermal phenomenon of the USB flash memory during data transmission, a kind of USB flash memory called EAGET CM981-4 G was tested, which was commercially available in the market. Before the test, a thermal couple was fixed on the back cover of the USB flash memory with cyanoacrylate superglue, and then connected it to the Agilent and the computer (Fig. 1) to measure the temperature of the memory. After that the USB flash memory was inserted into the computer USB port, and copy data files with a size of 3.63 G into the EAGET CM981-4G from the desktop computer. During data transmission, the temperature of the USB flash memory cover was acquired by the Agilent LXI Data Acquisition/Switch Unit 34972A device (USA) and then saved in the personal computer for later analysis. The data transmission process lasted about twenty minutes. However, the temperature measurement was prolonged ten minutes after data transmission was finished in order to test the whole thermal phenomenon under no data transmission situation.

For the case with PCM cooling, the modified USB flash memory was also inserted into the USB port, in which the space between the USB flash memory and the container was filled with gallium. The dimensions of the USB flash memory and the container are 12 mm×28 mm×5 mm, 14.4 mm×16.4 mm×7.4 mm, and the wall thickness of the container is 0.2 mm. The volume of the gallium confined between the USB flash memory and container was 0.668 mL. The temperature transients before and after the flash was filled with gallium were then compared to evaluate the cooling performance of the PCM.

During the test, the ambient temperature was around 24.5°C. The test and data transmission began at the same time. The test results were plotted in Fig. 2, where it was observed that the temperature of the USB flash memory without thermal management reached 42°C quickly during data transmission and the time consumed was transitory. The temperature finally stayed at 37°C even no data transmission occurs. It is uncomfortable for the user to hold such a hot USB flash memory whose highest temperature was above 40°C. Therefore, developing thermal management for the USB flash memory in use is very necessary [10].

To evaluate the cooling performance of the low melting point metal gallium on the USB flash memory, additional experiment was conducted. For such case, the USB flash memory was packaged in the container, and then the container was filled with solid gallium. Finally, the container was sealed with a cover to make sure that the gallium would not leak. In consideration of the volumetric expansion of the gallium during the phase change, the container which was made of stainless steel was not completely filled with gallium, thus the tumefaction was allowed in the margin of the container. The temperature data were acquired by the Agilent LXI Data Acquisition/Switch Unit 34972A device (USA) and then saved in the personal computer for analysis. The experimental prototype was also depicted in Fig. 1.

The retest was started when the USB flash memory was inserted into the USB port, and copy 3.63 G files into the EAGET CM981-4G from the desktop computer. The retest and data transmission were initiated at the same time. The ambient temperature was around 24.5°C. In order to guarantee that the working condition is uniform, the same computer did not run any applications except copying the file during the test.

The temperature responses of the modified USB flash memory and the original one were demonstrated in Fig. 3, where it was noticed that the maximum temperature of the modified USB flash memory was kept around 29°C, which was dramatically lower than that of the original one. Further, for the no data transmission stage, the temperatures for the USB flash memory filled with gallium PCM was 28°C, which was also much lower than 37°C for the original case without thermal management. Thus a conclusion was reached that the temperature of the modified USB flash memory was significantly reduced to the level well acceptable by users.

Discussion and conclusions

Recently, phase change materials are gradually used as an alternative cooling method for various situations such as spacecraft and avionics thermal control, personal computing and communication equipment, wearable computers, power electronic equipment and thermal management in portable phones etc. The use of PCM-based thermal management system in mobile electronic devices for power levels ranging from 6 to 12 W was investigated by Kandasamy et al. [11]. Shen and Tan researched the N-eicosane PCM in mobile phone cooling [12]. Meanwhile conventional PCMs also displayed various drawbacks such as low conductivity, large specific volumetric dilatation, inflammability, phase separation and so on. In the present work, the conceptual experiments have demonstrated that the liquid metal used as phase change material owns many prominent advantages over existing approaches such as: (i) The thermal conductivity of gallium is considerably larger than that of traditional PCMs, which would enhance heat transfer and further heat dissipation to the environment. (ii) The larger enthalpy of fusion per unit volume of gallium makes it possible to absorb much more heat and thus prolongs the temperature holding time during at work, especially for small-size USB flash memories. (iii) The specific volumetric dilatation during phase change of gallium is relatively small, thus more gallium can be filled in the container, which also prolongs the temperature holding time.

Based on the results, the conclusion can be made that metals or metal alloys with low melting point such as gallium used as phase change material to maintain a cool USB flash memory and the burgeoning solid state disk are highly feasible and effective, which offers a new PCM cooling strategy for electronic devices. In the present case, about 0.668 mL gallium would hold the temperature of the USB flash memory at 29°C for more than 18 minutes, which is sufficient enough to keep the USB flash memory work at a safe temperature. Clearly, the new thermal management has generalized significance and can be extended to more areas such as transitory power, energy, optical or even chemical device etc.

References

[1] Chowdhury I, Prasher R, Lofgreen K, Chrysler G, Narasimhan S, Mahajan R, Koester D, Alley R, Venkatasubramanian R. On-chip cooling by superlattice-based thin-film thermoelectrics. Nature Nanotechnology, 2009, 4(4): 235–238

[2] Prada L, Garcia J D, Carretero J. Garcia F. Saving power in flash and disk hybrid storage system. In: 2009 IEEE International Symposium on Modeling, Analysis & Simulation of Computer and Telecommunication Systems, Arlington, Virginia, USA, 2009, 632–634

[3] Leo H L, Sinclair G B. Flash temperature distributions at the head-disk interface in hard disk drives. ASME Journal of Tribology, 1998, 120(3): 536–541

[4] Liu J, Zhou Y. Chip cooling device using low melting point metals or their alloys as the flow of refrigerant. China Patent, CN02131419.5. 2004

[5] Ma K, Liu J. Liquid metal cooling in thermal management of computer chips. Frontiers of Energy and Power Engineering, 2007, 1(4): 384–402

[6] Li P, Liu J. Harvesting low grade heat to generate electricity with thermosyphon effect of room temperature liquid metal. Applied Physics Letters, 2011, 99(9): 094106

[7] Dai D, Zhou Y, Liu J. Liquid metal based thermoelectric generation system for waste heat recovery. Renewable Energy, 2011, 36(12): 3530–3536

[8] Deng Y, Liu J. A liquid metal cooling system for the thermal management of high power LEDs. International Communications in Heat and Mass Transfer, 2010, 37(7): 788–791

[9] Li H, Liu J. Revolutionizing heat transport enhancement with liquid metals: Proposal of a new industry of water-free heat exchangers. Frontiers in Energy, 2011, 5(1): 20–42

[10] Lv Y, Liu J. Interpretation on thermal comfort mechanisms of human bodies by combining Hodgkin-Huxley neuron model and Pennes bioheat equation. Forschung im Ingenieurwesen, 2005, 69(2): 101–114

[11] Kandasamy R, Wang X Q, Mujumdar A S. Application of phase change materials in thermal management of electronics. Applied Thermal Engineering, 2007, 27(17,18): 2822–2832

[12] Shen W J, Tan F L. Thermal management of mobile devices. Thermal Science, 2010, 14(1): 115–124

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