Liquid metal as energy transportation medium or coolant under harsh environment with temperature below zero centigrade

Abstract

The current highly integrated electronics and energy systems are raising a growing demand for more sophisticated thermal management in harsh environments such as in space or some other cryogenic environment. Recently, it was found that room temperature liquid metals (RTLM) such as gallium or its alloys could significantly reduce the electronics temperature compared with the conventional coolant, like water, oil or more organic fluid. However, most of the works were focused on RTLM which may subject to freeze under low temperature. So far, a systematic interpretation on the preparation and thermal properties of liquid metals under low temperature (here defined as lower than 0°C) has not yet been available and related applications in cryogenic field have been scarce. In this paper, to promote the research along this important direction and to overcome the deficiency of RTLM, a comprehensive evaluation was proposed on the concept of liquid metal with a low melting point below zero centigrade, such as mercury, alkali metal and more additional alloy candidates. With many unique virtues, such liquid metal coolants are expected to open a new technical frontier for heat transfer enhancement, especially in low temperature engineering. Some innovative ways for making low melting temperature liquid metal were outlined to provide a clear theoretical guideline and perform further experiments to discover new materials. Further, a few promising applied situations where low melting temperature liquid metals could play irreplaceable roles were detailed. Finally, some main factors for optimization of low temperature coolant were summarized. Overall, with their evident merits to meet various critical requirements in modern advanced energy and power industries, liquid metals with a low melting temperature below zero centigrade are expected to be the next-generation high-performance heat transfer medium in thermal managements, especially in harsh environment in space.

Graphical abstract

Keywords

liquid metal / cryogenics / low melting point / thermal management / aircraft / liquid cooling / space exploration

1 Introduction

The ever increasing power density of electronic components and more compact package technologies necessitate innovative cooling designs. For example, it becomes increasingly necessary for the high-performance cooling technology to cope with the heat flux of 100−1000 W/cm² so as to fulfill the needs in a variety of newly emerging highly integrated computer chips, high brightness LEDs, and large power solid-state lasers, etc. [1−3]. Clearly, removal of the large amount of heat generated in the electronic components poses a great challenge for current computer system designers and thermal management engineers. All such urgent needs provoke the tremendous efforts toward excellent thermal management solutions. In this regard, thermal management schemes such as air-cooling with fans, liquid cooling [4], thermoelectric cooling [5], heat pipes [6], vapor chambers [7], vapor compression refrigeration [8], impingement cooling [9], micro-channel cooling [10], and liquid metal cooling [11−18] have been successfully investigated. In fact, it is worth emphasizing that the coolants as applied in the above technologies play a most important role. As is well known, compared with the conventional liquid such as water, oil, and many organic fluids, the thermal conductivity of a metal is much higher. Therefore, the liquid metal or its alloys with a low melting point used as the cooling fluid should exhibit a much better cooling capacity than that of traditional fluids. The low melting point liquid metals were proposed as cooling fluids for the thermal management of computer chips in 2002 [18]. Up to the present, much progress has been made along this direction [15−17]. In a word, liquid metals, such as gallium or its alloys, can be considered as excellent coolant for the thermal management of computer chips because of its excellent thermophysical properties [18,19].

Meanwhile, there is also a growing demand for the same heat removal in cryogenics engineering fields related to the harsh environment, such as cryogenic industry and some low temperature electronics situations. Several typical cryogenic fields which work under temperatures below zero degree centigrade can be illustrated in Fig. 1. Thermal management in such harsh environment is vital to the successful design, manufacture, and tactical operation of a variety of electronic systems to meet low temperature, environment, reliability, and cost efficiency requirements.

The main factor that distinguishes harsh environment electronics from conventional commercial electronics is the environment under which they perform. Harsh-environment microelectronics generally operate at a temperature below zero degree centigrade. One of the main fields is in the space exploration program. It is well known that on earth, heat transports via conduction, convection, and radiation. However, convection is almost totally nonexistent in the vacuum of space. Radiation is the primary way for heat transport in space. Space-based electronics that need to be kept cold are attached to radiators which face the deep space and radiate excess heat there. These electronics, such as space based phased-array-radar, laser systems, radiators etc., are thermally insulated from the spacecraft. Cooling is achieved through surface thermal radiation. The thermal management of space-based electronics encompasses not only the removal of waste heat, but also the conservation of heat to provide a benign environment for the instruments and on-board electronic equipment. The harsh environments in space are the main challenge for thermal management of satellites and manned spacecraft. The spacecraft must undergo alternant heating and cooling because of the harsh environment. For example, the extreme temperatures are always falling below −200°C and even become lower for avionics in cold climates. Otherwise, it may reach thousands of degrees centigrade. For satisfying such a drastic temperature change in space, a highly efficient and flexible thermal management is required. In addition, space electronics are steadily becoming more miniaturized, more complex and more powerful than ever before. Increased packaging densities are, therefore, also limiting the volume available for the thermal management systems. Clearly, the operation of instruments and electronics onboard satellites and spacecraft requires efficient cooling systems. In recent years, various novel spacecrafts have been applied rapidly as demonstrated in Fig. 2 [20−22]. When high performance is needed, either for the electronics of control systems or for measuring units or detectors, a controlled lower working temperature may also become indispensable. As a result, the performance, flexibility and versatility of thermal management systems must be improved to meet the growing challenge [23]. The increasing heat dissipation from electronic devices on board satellites makes it urgent to find better cooling solutions.

Space-based radar (SBR) systems have a time-averaged power dissipation level of 50 W/m² to 100W/m². However, because of its stronger function and higher density packaging, the heat dissipation of future radar systems is anticipated to reach 1 to 2 kW/m² [24]. As is well known, one effective thermal solution is to use the large antenna area as a thermal radiator. The other one is to develop new materials with low weight and high thermal conductivity like graphite and/or phase change materials (PCMs). The thermal performance of PCMs can be further improved by filling the PCMs with metallic foams [25]. Moreover, future high power SBR, space-based laser (SBL), integrated power systems (IPSs), and electromagnetic devices will utilize the two-phase thermal management systems [26,27], emerging MEMS based cooling techniques (microchannel cooling and ultra-thin film cooling) [28−30] or even more aggressive use of refrigeration (i.e., cryogenic coolers) [31−33].

Conventional cryogenic fluids are typically categorized as inorganic and organic. Inorganic ones, such as hydrogen [34], nitrogen [35,36] and helium, are most commonly used in various situations. For example, liquid nitrogen, serving as the main and preferred cryo-coolant, has been widely used in aerospace electronic cooling [37,38]. It plays an important role in thermal management of unmanned aerial vehicle systems, infrared search and track sensors, missile warning receivers, satellite tracking systems, and a host of other commercial and military system. As for organic cryogens, liquefied natural gas (LNG) is a common one in recent years, which should be cooled to temperatures as low as 110 K to achieve liquid state at atmospheric pressure [39,40]. All these cryogenic fluids are commonly used in various applications. However, the current coolants as applied in the cooling systems have an array of disadvantages which affect the normal operation in space, such as high pressure for liquefaction, low thermal conductivity, narrow operating temperature and so on. Meanwhile, above room temperature liquid metal will not be able to work since they may freeze in such harsh environments. Hence it is worth mentioning that finding an effective cryogenic-coolant that owns not only a higher thermal conductivity and a higher boiling point, but also a much lower melting point, which may be one of the most efficient and effective solutions to fulfill the above needs for electronics heat transfer in harsh environments, especially for space thermal management of electronics. It is based on the above consideration that a new conceptual class of highly conductive coolants, liquid metals with a temperature below zero degree centigrade are outlined in this paper to fulfill such an emergent need. Significant theoretical and technical issues will be comprehensively interpreted. Future research directions worthy of pursuing will be suggested.

2 Typical low-melting-point metals and alloys

Table 1 lists typical low-melting-point metals and alloys. Theoretically, metals are giant atomic structures held together by metallic bonds. Researchers at Lawrence Livermore National Laboratory have found that liquid sodium initially evolves into a more compact local structure as pressure is increased. In addition, electrons are transformed as well. The electronic cloud gets modified, the electrons sometimes get trapped in interstitial voids of the liquid, and atomic bonds adopt specific directions [48]. Some typical metal coolants and their applications are evaluated as follows.

2.1 Mercury

Mercury is the only metal that stays as liquid under standard conditions for temperature and pressure as presented in Fig. 3(a). It has a freezing point of −38.83 °C and a boiling point of 356.73 °C (see Table 1). Compared to other metals, it is a poor conductor of heat, but a fair conductor of electricity [49]. Mercury has one of the narrowest ranges of its liquid state among all metals [50−52]. The reason for its liquid state below room temperature has not been well disclosed. The most popular explanation is the one using relativistic mechanism. According to the principle of relativity,

(1)m={\displaystyle \frac{m_{\mathrm{0}}}{\sqrt{{(\mathrm{1}-\nu /c)}^{\mathrm{2}}}}},

where m is the relativistic mass; m_{\mathrm{0}}, the rest mass; c, the speed of light; and \nu, the speed of the moving object. The mass of s orbital electron increases with its speed, and 6s orbital contracts and stabilizes. The 6s-6p excitation energy of mercury is much greater than that of zinc and cadmium. So it cannot form a strong bond between mercury atoms unless they have enough energy. The bonding between them is van der Waals force which is so weak that makes mercury appear only in liquid state at room temperature [51].

Mercury is primarily used for the manufacture of industrial chemicals or for electronic applications. For example, Fig. 3(b) and 3(c) present its typical applications for making thermometers [53] and mercury lamp designed by Ross Lovegrove for Artemide in Italy [54]. However, mercury vapors are hazardous to human health. A leakage of mercury can be disastrous, even at room temperature. And it becomes deadly at an elevated temperature. Such kind of metals could easily contaminate the whole system. Hence, it is forbidden in most accelerator environments, and unless specifically requested, its use is not recommended. Therefore, in some applications, mercury is replaced with less toxic but considerably more expensive Galinstan alloy. It is worth mentioning that Amalgam served as a filling material for teeth repair is safe as indicated in Fig. 3(d), which is made of liquid mercury and a powder containing silver, tin, copper, zinc, and other metals [55]. In addition, mercury readily combines with aluminum to form a mercury-aluminum amalgam when the two pure metals come into contact. Since amalgam can produce aluminum oxide when reacting with air, small amounts of mercury can corrode aluminum. For this reason, mercury is not allowed aboard an aircraft under most circumstances because of its risk in forming an amalgam with exposed aluminum parts in the aircraft [56].

2.2 Gallium and gallium alloys

Performances of gallium and gallium alloys are at least as good as or better than mercury in a wide variety of applications, such as high vaporization temperature, similar flow characteristics, nontoxic and the like. Therefore, gallium based metallic materials can be used as a mercury substitute to produce devices and materials such as temperature sensors and thermometers, pressure sensors or pressure activated switches, pumps and filers, liquid mirror telescopes, fluid unions, slip rings, bougies, sphygmomanometers, dental amalgam and a wide variety of other devices. As is listed in Table 1, the melting point of gallium is approximately 29.8°C, which is somewhat higher than needed. In fact, it is expected that a perfect coolant should always stay in liquid state with a wide temperature range, and the low melting point alloy should remain unfrozen at room temperature. According to the phase diagram of GaIn alloys exhibited in Fig. 4, the melting point of GaIn alloy remarkably changes with the addition of In. As is observed from Fig. 4, the eutectic alloy GaIn14.2% (atomic ratio) has a lower melting point of 289 K. In fact, gallium alloys can possibly be made to form eutectic materials that melt below room temperature, as tabulated in Table 2, of which galinstan has the lowest melting point of −19°C [58]. But a reliable running of galinstan below 0°C has remained unsuccessful so far.

Liquid metal gallium or its alloys can be developed as excellent coolant for the thermal management of computer chips and other industrial fields because of their favorable thermal properties superior to those of conventional inorganic and organic coolants, such as higher thermal conductivity, higher electrical conductivity, low vapor pressure, low dissolution in water, higher boiling point above 2200°C, and single-phase liquid cooling [18,19].

2.3 Alkali metal and its alloys

2.3.1 Alkali metals

Lithium, sodium, potassium, rubidium, and cesium constitute the first group of elements in the periodic table and have quite exceptional properties. They have low melting points and can be manipulated readily in a liquid state. Their operating temperature ranges are narrower than those of previously described metals, while sodium is a normal coolant used in large power stations. Lithium has two outstanding advantages in the task of heat transfer, a very high thermal conductivity and a very large specific heat capacity per unit volume. The relatively high melting points of these metals make them suitable for high temperature areas, such as in waste heat recovery plants.

Such liquids are finding ever-increasing use as coolants in, for example, fast nuclear reactors and high-temperature storage batteries. The liquids also have low viscosity, low density and high thermal conductivity, which make them particularly suitable for use as heat-transfer media. Compared with gallium, alkali metals are more active and easily reactive to oxygen and water [62], and, therefore, of potential fire hazards. It is emphasized that the special safety considerations should be entailed when they are really adopted as coolants. In addition, liquid alkali metals can also act as solvents for other elements such as carbon, oxygen, nitrogen, hydrogen, and heavier elements. The presence of these elements in solution renders the liquid metals corrosive, and techniques have now been developed for their monitoring and removal.

2.3.2 Na-K alloys

Multicomponent mutual liquid solutions of alkali metals are excellent coolants and working fluids for power engineering, technology and metallurgy because they possess the widest temperature range of liquid phase as effective both high-temperature and low-temperature coolants simultaneously [63].

NaK alloys, the typical binary system, containing 40% to 90% potassium by weight, are liquid at room temperature. The eutectic mixture with the lowest melting point of −12.6°C has been investigated to contain 76.7% of potassium and 23.3% of sodium. The density of NaK76.7 (indicating the mass fraction of potassium is 76.7%) is 866 kg/m³ at 21°C and 855 kg/m³ at 100°C as listed in Table 1, making it less dense than water, but its specific heat capacity is 953.8 J/(kg·°C), which is roughly one-fourth of that for water [50]. Its thermal conductivity is approximately 23W/(m·°C), which is much higher than the conventional fluid. Thus, if Na-K alloy is adopted as a cooling fluid, a much higher cooling capacity than that of traditional fluids can be obtained. In fact, the first conception of Na-K alloy used as a coolant is that it can be used as the coolant in experimental fast neutron nuclear reactors. Use of lead, pure sodium or the other materials in practical reactors, would require continual heating to maintain the coolant as a liquid, while NaK alloy overcomes this. Figure 5(a) shows the Na-K alloys used as a coolant in the SNAP-10A reactor. The NaK was circulated through the core and thermoelectric converters by a liquid metal direct current conduction-type pump [64]. The Soviet RORSAT radar satellites were just powered by a NaK-cooled reactor [65,66]. As well as its wide temperature range in liquid phase, Na-K has a very low vapor pressure, which is important in the vacuum of space.

In 2009 Xie and Liu [67] proposed for the first time to use NaK77.8 as the cooling fluid for the thermal management of the computer chip and numerically evaluated the working performance and capacity of the chip cooling system. Later, a Danish computer cooling company Dynamics successfully developed Na-K in their CPU coolers. Figure 5(b) shows the case that Na-K alloys (Na-K alloy, 78% K and 22% Na) circulate inside the heat pipes transferring the heat from the cooler base to the heat sink plates and back [68]. In addition, it is worth pointing out that the eutectic mixture with the lowest melting point of −12.6°C is viable when used under harsh environment, especially with a working temperature below zero degree centigrade. It offers much potential in thermal managements, for applications in space, military system and so on.

However, a great trouble is that the Na-K alloy is highly reactive with water and may catch fire when exposed to air. Quantities as small as one gram can be a fire or explosion risk. Therefore, it must be handled with special precautions for safety reasons. For example, NaK alloy used as the cooling fluid must be carefully compacted in cooling modules; it also should be stored in coal oil or liquid paraffin.

2.3.3 Other alkali alloys

Compared with gallium alloys, alkali metal containing alloys, such as binary and ternary systems have pretty low melting point, some of which is even lower than Hg, as given in Table 3.

Apart from the above-mentioned typical Na-K alloys, alkali alloys mainly consist of another five binary systems, such as Cs-K, Cs-Na, K-Rb, Cs-Rb, and Na-Ru. The Cs-K system was studied by Rinck [71] using thermal analyses and electrical conductivities. A detailed thermal analysis of liquid-solid phase equilibria was conducted, which presented both the liquidus and the solidus curves as depicted in Fig. 6. Cs₇₇K₂₃ was reported to have a melting point of −37.5°C [72]. In addition, Shmueli et al. [72] reported the existence of the intermediate phase CsK₂ below −88°C and the compound might have a hexagonal structure. A second intermediate phase Cs₆K₇ reported by Simon et al. [73] has not yet been adequately investigated. The ternary system Cs-K-Na also has the lower melting point. For example, the eutectic point of the ternary system Cs_73.71K_22.14Na_4.14 has a melting point of −78.2°C, which is the lowest-melting metallic liquid of the known on the Earth so far and effectively expands the working temperature scale of liquid metal. Cs_73.71K_22.14Na_4.14 used as a coolant cannot be frozen and flows normally when the environmental temperature is lower than 0°C, which is superior to gallium alloys, especially in some extreme environment.

In a word, low melting temperature liquid metal with a wide operating temperature range can even overcome difficulties caused by the harsh environments of space and play an important role in application of thermal management of electronics, such as unmanned aerial vehicle systems, infrared search and track sensors, missile warning receivers, satellite tracking systems, and a host of other commercial and military systems. However, studies on liquid metal with a melting point below zero degree centigrade are still rather limited.

3 Innovative ways to make low-melting-point liquid metals below zero degree centigrade

Broadest applications have been found of the above mentioned low-melting-point liquid metals because of their favorable merits. However, most of the researches are limited to the room temperature liquid metal and its alloys. So far, not many studies have been conducted on the preparation and thermal properties of liquid metals with a melting temperature below zero degree centigrade, which limits their application in low temperature fields. Hence, new liquid metal alloys with even lower melting points should be developed. Compared with the room temperature liquid metal, they are promising in space and some cryogenic field of industry and medicine.

3.1 Phase diagram calculation

For some multi-component alloys, analyses and calculations should be made using related physical and chemical knowledge before starting experiments which may be based on more than one attempt and waste time and energy. Just as the material genome initiative (MGI) which was recently announced by the US administration emphasized, the strong computational analysis will reduce dependence of physical experiments. If one wishes to find new liquid metals with a temperature below zero degree centigrade, it is necessary to make full use of the calculation of phase diagram (CALPHAD) method. Compared with the traditional alloy development approach, CALPHAD can provide a clear guideline for such selections and help avoid large scale experiments with less promising alloys [75,76]. Thus, it is a powerful tool to cut down cost and time during developing different metal alloys combined with key experiments. Indeed, the CALPHAD approach has already been applied to calculate the phase diagram of binary and multicomponent alloys, which provides a theoretical basis and data supports for preparing the liquid metal alloy with a melting point below zero degree centigrade. Figure 7 [77] shows a phase diagram for a fictitious binary chemical mixture (with the two components denoted by α and β) used to depict the eutectic composition, temperature, and point. Von Buch et al. [78], Grobner and Schmid-Fetzer [79] used the CALPHAD approach for the development of creep-resistant Mg-Sc-Mn and Mg-Mn-(Sc, Gd, Y, Zr) alloys. Ohno et al. [80] calculated the Mg-rich corner phase diagram of Mg-Al-Zn system using Pandat software and the result was compared with the literatures and their own experimental data.

Based on the above theory, the phase diagram of NaK alloy is diagramed in Fig. 8. The NaK containing 40% to 90% potassium by weight remains in liquid state at room temperature. Among them, the eutectic mixture consisting of 76.7% potassium and 23.3% sodium has the lowest melting point of −12.62°C, which is in good agreement with the experimental data. Hence phase diagram can effectively forecast the products, transformation temperature and the phase content fractions with different temperature. Drawing the phase diagram is an essential step to prepare alloys with lower melting point effectively.

3.2 Subcooling of metal melt

Generally speaking, the actual crystallization temperature T_n of a kind of metal is lower than the equilibrium crystallization temperatureT_s, and the difference \mathrm{\Delta }T=T_{\mathrm{s}}-T_{\mathrm{n}} is called degree of subcooling. According to nucleation theory [82], the nucleation rate of solid crystals in a mole of liquid is

(2){\displaystyle \frac{d\mathrm{\Delta }T}{dt}}={\displaystyle \frac{NkT}{h}}\exp \left(-{\displaystyle \frac{\mathrm{\Delta }{F}^{*}}{kT}}\right),

(3)\mathrm{\Delta }{F}^{*}=\left({\displaystyle \frac{\mathrm{16}\pi }{\mathrm{3}}}\right){\displaystyle \frac{{\sigma }^{\mathrm{3}}}{\mathrm{\Delta }{F}_{\upsilon }^{\mathrm{2}}}},

where \mathrm{\Delta }{F}^{\ast }is the local free-energy change; \sigma, the surface tension; and \mathrm{\Delta }{F}_{\upsilon }, the free-energy change per unit volume.

In fact, many researches have been devoted to the structural analysis [83−85] of liquid metals, and the molecular dynamics (MD) or Monte Carlo simulation is a favorable tool in investigating the liquid structure theoretically [86−88]. X-ray diffraction and neutron diffraction are two experimental methods to obtain the information of structures in liquid metals [89]. Table 4 gives the maximum subcooling obtained in bulk and in small droplets. In Table 4, T_s is the equilibrium crystallization temperature;T_n, the actual crystallization temperature; and \sigma, the liquid-solid interfacial energy. The maximum subcooling of the liquid metal can be gained with the help of both nucleation theory, the molecular dynamics and Monte Carlo simulation and some experiments such as X-ray diffraction, neutron diffraction and so on.

It is observed from Table 4 that gallium can generally be kept in a liquid state at a temperature much lower than room temperature, due to its large sub-cooling point. In fact, the subcooling of gallium has been studied before [91]. Figure 9 shows the typical curve of the gallium temperature transients during the cooling and heating process. It indicates that gallium remains in liquid state even when the temperature is as low as 0.89°C in the cooling process. In fact, it is surprising that gallium encapsulated in carbon nanotubes may keep in liquid state even below −80°C [92].

Many effective methods can be used to increase the subcooling of the liquid metal, which is helpful to maintain the metal in liquid state at a lower temperature, such as electromagnetic levitation [93], molten glass slag purification [94], recycle superheating under Ar atmosphere, and so on. The conventional purificant is B₂O₃, and it is found that the cycle number, temperature and denucleating time can have effect on the undercooling of melt. At present, great advances have been made in subcooling technologies of liquid metal with a bulk mass. Some metal and alloys, such as Ni, Fe, Cu and alloys, have a higher subcooling of 150−500 K [94,95].

In a word, the above methods can be taken into account to achieve a higher subcooling for liquid metal with a melting temperature below zero degree centigrade, and there is no doubt that it is another effective and important method to decrease the freezing point of liquid metals. The liquid metal with a higher subcooling can remain in a liquid state at a lower temperature which is helpful to realize thermal management of electronics or other materials under cryogenic state. However, the subcooling is an unstable state for liquid metals or molten metals and easily subject to many experimental conditions, such as mechanical vibration, impurity diffusion, rapid cooling rate, and etc. It sometimes requires even more extreme or exotic conditions, such as confined geometry or microgravity [96]. Some researchers indicate that micrometer-sized [97] or submicrometer-sized [98] liquid gallium can also be undercooled down to 150 K. Besides, Ga droplets with sizes of 3−15 nm deposited onto amorphous substrates can be kept at 90 K for times of the order of days without showing the onset of crystallization [99]. The liquid metal nanoparticle with a higher subcooling can remain in its liquid state in harsh environments and be used as the coolant for heat transfer of microelectronics.

3.3 Experimental approaches

Some researchers have conducted experiments to decrease the melting point of alloys. Taylor and Rancourt [100] proposed to add small quantities (less than 5%) of other elements such as lithium, sodium, rubidium, silver, antimony, gold, platinum, cesium and bismuth to liquid metal alloys to provide a mechanism for depressing their freezing point. Table 5 presents the compositions of a plurality of alloys and their physical state at 4°C. Gallium alloys and other metals such as silver, bismuth, tin, indium and zinc generally have lower melting points. A gallium alloy with 8% of tin will melt at 20°C, while a gallium alloy with 25% of indium will melt at 16°C. Moreover, among the compositions of a plurality of alloys listed in Table 5, the gallium-indium-tin alloys with a small quantity of Ag and Bi can stay in liquid state at 4°C. Especially, one particular formation (68%Ga, 20%Sn, 10.5%In, 0.75%Bi, 0.75%Ag) is found to have a freezing point near −4°C. This determination was made in a salt/ice water bath.

Taylor et al. also inferred that the surface oxidation of liquid metal could affect the depressing of freezing point. Therefore, the 30% NaOH is used to clean the metals and enable the pure metals to interact. To further prevent the oxidation of metallic components, the experiments are operated under an Argon atmosphere.

In addition, two ways to decrease the melting point were found by Wu et al. [101]. One is to form the low melting point compounds, and the other is to refine the eutectic cluster. For example, they found that adding Bi into Cu-8P would lower the melting point of binary Cu-8P eutectic alloy because Bi and Cu can create compounds with low melting point distributing at the boundaries. They also found that the eutectic cluster can be refined with the addition of Fe, and that was the reason why the addition of Fe could decrease the melting point of Cu-8P alloy. Hence finding an element which can be added into the low melting point alloys, such as alkali alloys, Hg, gallium alloys, etc., to form the compound with a lower melting point or to refine the eutectic cluster may be a useful solution to reduce the melting point of alloys.

For the present discussion, the methods of preparation and analysis of low melting temperature eutectic liquid alloys is mainly focused on, which is helpful to establish the basic theory and experiment and to further realize the applications of low melting temperature liquid metal used as a coolant in space or other cryogenic field including industry, medicine and so on. Hence, combining with the above theory and the experiment results, finding the proper metal elements and drawing corresponding phase diagram are critical steps to prepare low temperature liquid alloys.

4 Promising applications for liquid metal with a melting temperature below zero degree centigrade

So far, devices using liquid metal as coolants for high heat flux cooling have already been commercially available. In the near future, progress will be made significantly in thermal management of electronics in low temperature field. Liquid metals with melting temperatures below zero degree centigrade could present many advantages and potentialities. For example, when the ambient temperature is lower than 0°C, the conventional organic/inorganic fluid and room temperature liquid metal will freeze and cannot work normally, while liquid metals with extremely low melting points and higher thermal conductivities can still flow very well and perform heat transfer. In addition, low melting temperature liquid metal with a wide operating temperature range can even overcome difficulties caused by the harsh environments of space and play an important role in application of thermal management of electronics, such as unmanned aerial vehicle systems [102], infrared search and track sensors [103], missile warning receivers [104], satellite tracking systems [105], and a host of other commercial and military system as shown in Fig. 10. Some low melting point metals or alloys including mercury and the alkali alloy cannot only be used to produce devices and materials, such as temperature sensors and cryogenic thermometers, pressure sensors or pressure activated switches, pumps and a wide variety of other devices, but also be used as coolants to improve heat transfer. Low melting temperature liquid metal with perfect characteristics may possibly be used on a large scale in high power electronics devices and quickly spread all over the temperature range of a wide scope of applications.

In a word, liquid metals with melting temperatures below zero degree centigrade are considered to be the next-generation high-performance heat transfer medium and are believed to have remarkable advantages over the conventional organic/inorganic fluid with regard to higher thermal conductivity, lower thermal resistance, and better fluidity.

But there are some things that should be taken into consideration. Environmental degradation and limited energy are two major issues in this century. Finding a coolant with perfect characteristics must be a scientific and technical challenge which should be overcome as quickly as possible. Figure 11 shows the main factors which should be considered in choosing an optimized coolant, which are safety, environmental-friendliness and applicability.

Gallium based alloys can satisfy the requirement of safety. However, their melting points are still high. To decrease the melting point, the Na-K alloys or mercury may be added to the gallium based alloys to form the eutectic metal. However, the controlling of the flammability of Na-K alloys and remove of the toxicity of Hg are top-priority problems.

Environment-friendly is another external requirement which should not be neglected. The weighing standard includes ozone depletion potential (ODP), global warming potential (GWP), etc. The preparation of liquid metal with a melting temperature below zero degree centigrade should fulfill such requirements.

Products with higher coefficient of performance (COP) at lower cost are always preferred. Most liquid metals are more expensive than conventional working fluids. The cost of liquid metals with a melting temperature below zero degree centigrade applied in space or any other cryogenic field should be reduced. For example, adding some low-cost nano-particles to liquid metals not only improves its thermal conductivity, but also reduces the cost.

In summary, the preparation of liquid metals with melting temperatures below zero degree centigrade, which is a relatively new field, has a broad application prospects and is worthy of further in-depth research.

5 Conclusions

Continued electronic miniaturization, coupled with increased performance requirements and severe environmental constraints in harsh environment systems, has resulted in ever increasing circuit power densities and dissipation levels. Moreover, the future electrical generation for the all-electric ships, more-electric aircraft (MEA), and more-electric vehicles (EV) will require higher levels of power generation and increased thermal management challenges. The low temperature liquid metal with better thermal properties compared with existing coolants would offer attractive opportunities for thermal energy transportation and heat transfer enhancement. In this paper, the feasibility of innovating liquid metals with a melting temperature below zero degree centigrade, such as mercury, alkali alloys, gallium alloys and even more novel low-melting-temperature alloys, is vigorously evaluated from three aspects including theory, experiments and applications. Furthermore, considering the three basic and necessary material fabrication criteria including safety, environmental-friendliness and applicability, the optimized low-melting-temperature liquid metals are proposed which should satisfy the widest and most urgent requirements of high density electronics heat transfer. More importantly, it may finally lead to a brand new way of making advanced heat transfer approach in space and will accelerate future exploration of the solar system effectively.

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] Arik M, Becker C, Weaver S, Petroski J. Thermal management of LEDs: package to system. In: 3rd International Conference on Solid State Lighting. San Diego, CA, 2003, 64 −75

[3] Tzuk Y, Tal A, Goldring S, Glick Y, Lebiush E, Kaufman G, Lavi R. Diamond cooling of high-power diode-pumped solid-state lasers. IEEE Journal of Quantum Electronics, 2004, 40(3): 262−269

[4] Strassberg D. Cooling hot microprocessors. EDN (European Edition), 1994, 39: 40−48

[5] Lundquist C, Carey V P. Microprocessor-based adaptive thermal control for an air-cooled computer CPU module. In: Proceedings of the 17th Annual IEEE Semiconductor Thermal Measurement and Management Symposium. San Jose, USA, 2001, 168−173

[6] Xie H, Ali A, Bhatia R. Use of heat pipes in personal computers. In: Proceedings of the Intersociety Conference—Thermo Mechanical Phenomena in Electronic Systems. Seattle, USA, 1998, 442−448

[7] Nquyen T, Mochizuki M, Mashiko K, Saito Y, Sauciuc I. Use of heat pipe/heat sink for thermal management of high performance CPUs. In: Proceedings of the 16th Annual IEEE Semiconductor Thermal Measurement and Management Symposium. San Jose, USA, 2000, 76−79

[8] Rao W, Zhou Y X, Liu J, Deng Z S, Ma K Q, Xiang S H. Vapor-compression-refrigerator enabled thermal management of high performance computer. International Congress of Refrigeration, Beijing, China, 2007

[9] Amon C, Murthy J, Yao S C, Narumanchi S, Wu C F, Hsieh C C. MEMS-enabled thermal management of high-heat-flux devices EDIFICE embedded droplet impingement for integrated cooling of electronics. Experimental Thermal and Fluid Science, 2001, 25(5): 231−242

[10] Tuckerman D B, Pease R F W. High-performance heat sinking for VLSI. IEEE Electron Device Letters, 1981, 2(5): 126−129

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

[12] Deng Y G, Liu J. Hybrid liquid metal-water cooling system for heat dissipation of high power density microdevices. Heat and Mass Transfer, 2010, 46(11−12): 1327−1334

[13] Deng Y G, 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

[14] Ma K Q, Liu J, Xiang S H, Xie K W, Zhou Y X. Study of thawing behavior of liquid metal used as computer chip coolant. International Journal of Thermal Sciences, 2009, 48(5): 964−974

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

[16] Deng Y G, Liu J. Heat spreader based on room-temperature liquid metal. ASME Journal of Thermal Science and Engineering Applications, 2012, 4(2): 024501

[17] Li P P, Liu J. Harvesting low grade heat to generate electricity with thermosyphon effect of room temperature liquid metal. Applied Physics Letters, 2011, 99(9): 094106–3

[18] Liu J, Zhou Y X. A computer chip cooling method which uses low melting point metal and its alloys as the cooling fluid. China Patent 02131419.5. 2002

[19] Deng Y G, Liu J. Design of practical liquid metal cooling device for heat dissipation of high performance CPUs. ASME Journal of Electronic Packaging, 2010, 132(3): 031009

[20] Ryall J. Space probe set to “collide” with earth to simulate approaching asteroid. 2009-06-11

[21] Weinberger S. Lockheed trumps boeing for new GPS. 2008-05-16

[22] Coppinger R. ESA’s manned ARV team despondent over cash.

[23] THERMACORE. Satellite thermal control: unique products for unique challenges. 2013-05-26

[24] Zuo Z J, North M T, Wert K L. High heat flux heat pipe mechanism for cooling of electronics. IEEE Transactions on Components and Packaging Technologies, 2001, 24(2): 220−225

[25] Haws J. Short E. Method and apparatus for cooling with phase change materials and heat pipes. European Patent 00965034.2–2220–US0025297. 2002

[26] Meyer L, Dasgupta S, Shaddock D, Tucker J. Fillion R. A silicon-carbide micro-capillary pumped loop for cooling high power devices. In: 9th Annual IEEE Symposium on Semiconductor Thermal Measurement and Management. Austin, USA, 1993, 364−368

[27] Butler D, Ku J. Swanson T. Loop heat pipes and capilary pump loops—an application perspective. In: Space Technology and Applications International Forum-STAIF. Albuquerque, USA, 2002, 49−56

[28] Golliher E L. Microscale technology electronics cooling overview. In: Space Technology and Applications International Forum-STAIF. Albuquerque, USA, 2002, 250−257

[29] Ohadi M, Qi J. Thermal management of harsh environment electronics. Microscale Heat Transfer Fundamentals and Applications, 2005, 193: 479−498

[30] Heffington S N, Black W Z, Glezer A. Vibration-induced droplet atomization heat transfer cell for high-heat flux dissipation. Thermal Challenges in Next Generation Electronic Systems (THERMES-2002). Santa Fe, USA, 2002

[31] Fan X, Zeng G, LaBounty C, Croke E, Vashaee D, Shakouri A, Ahn C, Bowers J E. High cooling power density SiGe/Si micro coolers. Electronics Letters, 2001, 37(2): 126−127

[32] Zimm C, Jastrab A, Sternberg A, Pecharsky V Jr, Gschneidner K, Osborne M, Anderson I. Description and performance of a near-room temperature magnetic refrigerator. Advances in Cryogenic Engineering, 1998, 43: 1759−1766

[33] Swfit G W. Thermoacoustics: A Unifying Perspective for Some Engines and Refrigerators. New York: Acoustical Society of America (ASA) Publications, 2002

[34] Dawson V P, Bowles M D. Taming Liquid Hydrogen: The Centaur Upper Stage Rocket 1958−2002. Washington, DC: NASA Office of External Relations, 2004

[35] Kwon D W, Sedwick R J. Cryogenic heat pipe for cooling high temperature superconductors. Cryogenics, 2009, 49(9): 514−523

[36] Purvis T, Vaughn J M, Rogers T L, Chen X, Overhoff K A, Sinswat P, Hu J, McConville J T, Johnston K P, Williams R O 3rd. Cryogenic liquids, nanoparticles, and microencapsulation. International Journal of Pharmaceutics, 2006, 324(1): 43−50

[37] Yildiz Y, Nalbant M. A review of cryogenic cooling in machining processes. International Journal of Machine Tools & Manufacture, 2008, 48(9): 947−964

[38] Hong S Y. Economical and ecological cryogenic machining. Journal of Manufacturing Science and Engineering, 2001, 123(2): 331−338

[39] Pacio J C, Dorao C A. A review on heat exchanger thermal hydraulic models for cryogenic applications. Cryogenics, 2011, 51(7): 366−379

[40] Gorla R S R. Rapid calculation procedure to determine the pressurizing period for stored cryogenic fluids. Applied Thermal Engineering, 2010, 30(14−15): 1997−2002

[41] Liu J. Development of new generation miniaturized chip-cooling device using metal with low melting point or its alloy as the cooling fluid. In: Proceedings of the International Conference on Micro Energy Systems. Sanya, China, 2005, 89−97

[42] Smither R K. Liquid metal cooling of synchrotron optics. In: Society of Photo-Optical Instrumentation Engineers (SPIE) International Symposium on Optical Applied Science and Engineering, San Diego, USA, 1992, 116−134

[43] Iida T, Guthrie R I L. The Physical Properties of Liquid Metals. Oxford: Clarendon Press, 1993

[44] Shimoji M. Liquid Metals: An Introduction to the Physics and Chemistry of Metals in the Liquid State. New York: Academic Press, 1977

[45] Karcher C, Kocourek V, Schulze D. Experimental investigations of electromagnetic instabilities of free surfaces in a liquid metal drop. In: International Scientific Colloquium Modelling for Electromagnetic Processing. Hannover, Germany, 2003, 105−110

[46] Wikipedia. NaK. 2013-05-26

[47] Bradhurst D H, Buchanan A S. Surface properties of liquid sodium and sodium potassium alloys in contact with metal-oxide surfaces. Australian Journal of Chemistry, 1961, 14(3): 397−408

[48] Chu K Y. Sodium loses its luster: A liquid metal that's not really metallic.

[49] Wikipedia. Mercury (element). 2013-05-26

[50] Senese F. Why is mercury a liquid at STP? 2013-05-26

[51] Norrby L J. Why is mercury liquid? Or, why do relativistic effects not get into chemistry textbooks? Journal of Chemical Education, 1991, 68(2): 110−113

[52] Lide D R. CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. 2005, 4.125−4.126

[53] MDCH. Mercury and Your Health.

[54] Lovegrove R. Artemide Mercury Suspension. 2013-05-26

[55] Dental Amalgam P E I. 2013-05-26

[56] Vargel C, Jacques M, Schmidt M P. Corrosion of Aluminium. Elsevier, 2004, 158

[57] Anderson T J, Ansara I. The Ga-Sn (Gallium-Tin) system. Journal of Phase Equilibria, 1992, 13(2): 181−189

[58] Surmann P, Zeyat H. Voltammetric analysis using a self-renewable non-mercury electrode. Analytical and Bioanalytical Chemistry, 2005, 383(6): 1009−1013

[59] Ghoshal U, Grimm D, Ibrani S, Johnston C, Miner A. High-performance liquid metal cooling loops. In: Proceedings of the 21th IEEE Semiconductor Thermal Measurement and Management Symposium. San Jose, USA, 2005, 16−19

[60] Liu G Y, Tan H D. Gallium and gallium compounds. In: Cyclopaedia of Chemical Engineering: Metallurgy and Metallic Materials. Beijing: Chemical Industry Press, 1994, 329−335(in Chinese)

[61] Schormann M, Klimek K S, Hatop H, Varkey S P, Roesky H W, Lehmann C, Röpken C, Herbst-Irmer R, Noltemeyer M. Sodium-potassium alloy for the reduction of monoalkyl aluminum (III) compounds. Journal of Solid State Chemistry, 2001, 162(2): 225−236

[62] Li H Y, 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

[63] Kagan D N, Krechetova G A, Shpilrain E E. Elaborating and applying a new method of Gibbs energy determination for multicomponent alkali-metal coolants. Journal of Physics: Conference Series, 2008, 98(3): 032007

[64] EnviroReporter.com. SSFL Area IV-SNAP. 2013-05-27

[65] Dyson R W, Penswick B, Robbie M, Geng S M. Investigation of liquid metal heat exchanger designs for fission surface power. In: Sixth International Energy Conversion Engineering Conference (IECEC). Cleveland, USA, 2008, 1−6

[66] Klinkrad H. Space Debris: Models and Risk Analysis. Springer, 2006, 83

[67] Xie K W. Study on the liquid metal cooling method for thermal management of computer. Dissertation for the Master′s Degree. Beijing: the Chinese Academy of Science, 2009: 58−76

[68] Butler H. Danamics LMX Superleggera Cooler Review. 2013-05-28

[69] Oshe R W. Handbook of Thermodynamic and Transport Properties of Alkali Metals. Oxford. UK: Blackwell Scientific Publications Ltd, 1985, 987

[70] Wikipedia. Fusible alloy. 2013-05-28

[71] Rinck E. Diagram of solidification and electric conductivityof the potassium-cesium alloys. Comptes Rendus Hebdomadaires Des Seances De L'Academie Des Science, 1936, 203: 255−257

[72] Shmueli U, Steinberg V, Sverbilova T, Voronel A. New crystalline phases of an equiatomic K-Cs alloy at low temperature. Journal of Physics and Chemistry of Solids, 1981, 42(1): 19−22

[73] Simon A, Brumer W, Hillenkotter B, Kullmann H J. Novel compounds between potassium and cesium. Zeitschrift fur Anorganische und Allgemeine Chemie, 1976, 419: 253−274

[74] Ren X, Li C R, Du Z M, Guo C P. Thermodynamic assessments of six binary systems of alkali metals. Calphad, 2011, 35(3): 446−454

[75] Saunders N, Miodownik A P. CALPHAD (Calculation of Phase Diagrams—A Comprehensive Guide. Elsevier Science Ltd., 1998

[76] Kaufman L, Bernstein H. Computer Calculation of Phase Diagrams. New York: Academic Press, 1970

[77] Wikipedia. Eutectic system. 2013-05-28

[78] Von Buch F, Lietzau J, Mordike B L, Pisch A, Schmid-Fetzer R. Development of Mg-Sc-Mn alloys. Materials Science and Engineering A, 1999, 263(1): 1−7

[79] Grobner J, Schmid-Fetzer R. Selection of promising quaternary candidates from Mg–Mn–(Sc, Gd, Y, Zr) for development of creep-resistant magnesium alloys. Journal of Alloys and Compounds, 2001, 320(2): 296−301

[80] Ohno M, Mirkovic D, Schmid-Fetzer R. Phase equilibria and solidification of Mg-rich Mg-Al-Zn alloys. Materials Science and Engineering A, 2006, 421(1−2): 328−337

[81] Tang R Z, Tian R Z. Binary eutectic phase diagram and the crystal structures of intermediate phase. Changsha:Zhongnan University Press, 2009, 736 (in Chinese)

[82] Newhouse W H, Hagner A F, Devore G W. Structural control in the formation of gneisses and metamorphic rocks. Science, 1949, 109(2825): 168−169

[83] Wang L, Liu J. Discontinuous structural phase transition of liquid metal and alloys. Physics Letters [Part A], 2004, 328(2−3): 241−245

[84] Zhang Y N, Wang L, Wang W M, Zhou J K. Structural transition of sheared-liquid metal in quenching state. Physics Letters [Part A], 2006, 355(2): 142−147

[85] Prabhu K N, Ravishankar B N. Effect of modification metal treatment on casting/chill interfacial heat transfer and electrical conductivity of Al-13% Si alloy. Materials Science and Engineering A, 2003, 360(1−2): 293−298

[86] Shim J H, Lee S C, Lee B J, Suh J Y, Cho Y W. Molecular dynamics simulation of the crystallization of a liquid gold nanoparticle. Journal of Crystal Growth, 2003, 250(3−4): 558−564

[87] Li H, Bian X F, Wang G H. Molecular dynamics computation of the liquid structure of Fe₅₀Al₅₀ alloy. Materials Science and Engineering A, 2001, 298(1−2): 245−250

[88] Chen X S, Zhao J J, Sun Q, Liu F, Wang G, Shen X C. Surface thermal stability of nickel clusters. Physica Status Solidi. B, Basic Research, 1996, 193(2): 355−361

[89] Hattori T, Kinoshita T, Taga N, Takasugi Y, Mori T, Tsuji K. Pressure and temperature dependence of the structure of liquid InSb. Physical Review B: Condensed Matter and Materials Physics, 2005, 72(6): 064205

[90] Turnbull D. The Subcooling of liquid metals. Journal of Applied Physics, 1949, 20(8): 817

[91] Li T, Lv Y G, Liu J, Zhou Y X. A powerful way of cooling computer chip using liquid metal with low melting point as the cooling fluid. Forschung im Ingenieurwesen, 2005, 70(4): 243−251

[92] Liu Z, Bando Y, Mitome M, Zhan J H. Unusual freezing and melting of gallium encapsulated in carbon nanotubes. Physical Review Letters, 2004, 93(9): 095504

[93] Platzek D. Liquid metal undercooled below its Curie temperature. Physical Review Letters, 1994, 65(13): 1723−1724

[94] Wei B B, Yang G C, Zhon Y H. High undercooling and rapid solidification of Ni 32.5%Sn eutectic alloy. Acta Metallurgica et Materialia, 1991, 39(6): 1249−1258

[95] Liu R P, Volkraann T, Herlach D M. Undereooling and solidification of Si by electromagnetic levitation. Acta Materialia, 2001, 49(3): 439−444

[96] Hofmeister W H, Robinson M B, Bayuzick R J. Undercooling of pure metals in a containerless, microgravity environment. Applied Physics Letters, 1986, 49(20): 1342−1344

[97] Bosio L, Windsor C G. Observation of a metastability limit in liquid gallium. Physical Review Letters, 1975, 35(24): 1652−1655

[98] Cicco A D. Phase transitions in confined gallium droplets. Physical Review Letters, 1998, 81(14): 2942−2945

[99] Parravicini G B, Stella A, Ghignaa P, Spinolo G, Migliori A, d’Acapito F, Kofman R. Extreme undercooling (down to 90 K) of liquid metal nanoparticles. Applied Physics Letters, 2006, 89(3): 033123

[100] Taylor L T, Rancourt J. Non-toxic liquid metal composition for use as a mercury substitute. United States Patent No. 5,792, 236. 1998-08-11

[101] Wu Y Y, Liu X F, Liu X J, Bian X F. Effect of Sb, Bi and Fe on melting points and microstructures of eutectic Cu-8P alloys. Chinese Journal of Nonferrous Metals, 2004, 14(7): 1206−1210

[102] ARMY. Hermes® 90 UAS unmanned aircraft system. 2013-05-28

[103] LOCKHEED MARTIN. IRST sensor system. 2013-06-01

[104] NORTHROP GRUMMAN. 2008photo archive.2013-06-01

[105] Cortney. China Launches Beidou GPS System, Set to Rival US GPS. 2012-01-03

RIGHTS & PERMISSIONS

Higher Education Press and Springer-Verlag Berlin Heidelberg