Advanced Liquid Metal Cooling for Chip, Device and System

The following sections are included:

• Foreword
• Abstract
• Preface
• About the Author
• Contents

In recent years, with the rapid progress in improving computational speed, thermal management has become a major concern in various high-profile electronic devices [1]. CPU chips are being squeezed into tighter and tighter spaces with almost no room for heat to escape. The total power dissipation levels now reside on the order of 100W with peak power densities reaching 400–500W/cm² and are still steadily climbing. As a result, higher performance and reliability are extremely tough to attain. The development of high-flux devices and large power systems raise serious concerns over efficient cooling [2]. Since the standard conduction and forced-air convection techniques are no longer able to provide adequate cooling for sophisticated electronic systems, new solutions, such as liquid cooling, thermoelectric cooling, heat pipes, and vapor chambers are being intensively looked into. Among these cooling methods, water is perhaps the most widely used working fluid in a wide variety of thermal management areas, especially conventional industrial heat transport engineering [3]. However, it may not be the best option due to various reasons.

To tackle this challenging issue, finding new alternative cooling method remains imperatively important. Over the past few years, it was gradually realized that using liquid metal or its alloy with low melting point as the coolant could significantly lower the chip temperature [4]. This new generation heat transfer enhancement method raised many important fundamental and practical issues to be resolved. In this introductory chapter, we provide a background to the area of high flux cooling. The concept of water-free heat exchanger is illustrated and liquid metals with low melting point are outlined as idealistic fluids which could be used at a wide range of working temperatures in industrial setting. Some liquid metals and their alloy materials, which have received little attention in the past in thermal management area, are examined. Having a superior thermal conductivity and an electromagnetic field drivable while consuming extremely low power, the liquid metal coolant opens up many opportunities to revolutionize modern heat transport processes by serving as a heat transport fluid in industry, administrating thermal management in energy and power systems, and paving the way for innovative enhanced cooling in electronic or optical devices [2]. To explore this new frontier of heat transport technology, studies were conducted to understand the technical barriers encountered by a group of advanced water-based heat transfer strategies. The unique merits that a liquid metal could provide in innovative heat exchanger technologies are detailed. Further, we outline several promising industrial applications, such as heat recovery, chip cooling, thermoelectricity generation, and many other special applications, in which the new technology would play an indispensable role. The technical challenges and scientific issues thus raised are summarized. With the evident capability of meeting various critical requirements of modern advanced energy and power industries, the liquid metal-based technologies are expected to open a new era of chip cooling and thermal management throughout the world.

As new generation mediums for high-performance cooling and thermal management, appropriate candidates for the liquid metal should have a low enough melting point, small viscosity, high thermal conductivity, and large heat capacity. Meanwhile, it must not be poisonous and caustic. The most important prerequisite is that the working liquid metal needs to remain in liquid state when its cooling role is being performed under an appropriate temperature range for computer chips, which is generally below 100°C. It is commonly held that a metal appears as a rigid block. With this impression in mind, the fact is often ignored that those alloys with extremely low melting points, several degrees centigrade above zero, actually stay in the liquid state around room temperature. Among the various liquid metals, it can be found that liquid gallium or its alloys can serve as a perfect candidate for the heat transfer medium in a wide variety of devices and systems. Unfortunately, there have been limited efforts to apply such liquid metals to cool high-power electronic components, especially computer chips, until around 2002 [1].

An in-depth analysis of the thermal properties of liquid metals, such as gallium, strongly suggests that they are well suited for the cooling of computer chips owing to their low melting point. In fact, gallium can generally be kept in a liquid state at a temperature much lower than the room temperature due to its large sub-cooling point. It turns out to be an important merit for gallium-based alloys to be used as the cooling fluid. The low melting point and very low vapor pressure of such a liquid metal make it easy to handle, and its high thermal conductivity guarantees excellent cooling performance. Further, the low kinetic viscosity of liquid metal improves its capability for heat removal, especially at the liquid–solid interface and enhances its attractiveness as a new-generation coolant. The normal (dynamic) viscosity of gallium is about 1.5 times that of water, which means that it can be pumped through small channels with relative ease. The surface tension of liquid metals is much higher than that of water, which makes them immune to the presence of small cracks or channels in case of an imperfect seal, which would be a serious leakage for water as a cooling fluid. Besides, liquid metals are non-toxic and relatively cheap. The two principal advantages lie in their superior thermophysical properties of absorbing heat away from a hot chip and the ability to pump these electrically conductive liquids efficiently with silent, vibration-free, non-moving, magnetofluid-dynamic (MFD) pumps. All these compelling properties warrant their future applications in chip cooling technology.

This chapter is dedicated to illustrate the typical liquid metal medium and related properties regarding advanced cooling. Unlike most of the conventional cooling fluids [2], a liquid metal offers tremendous opportunities for the coming society.

As explained in previous chapters, the low-melting point liquid metals are attracting increasing attention over a wide variety of important areas owing to their excellent rheological properties at around room temperature. It is well known that the pure metals which remain in liquid state at low temperatures are mercury (Hg, −39°C), cesium (Cs, 28.40°C), gallium (Ga, 29.8°C), and rubidium (Rb, 38.89°C). Clearly, mercury, cesium, and rubidium cannot be widely used because of their unavoidable toxicity or high chemical activity. Therefore, more and more studies have centered on gallium, which is non-toxic and stable in air and water, and many distinctive findings have revealed a series of novel properties of this material in various areas [1, 2].

Gallium element is widely distributed in the Earth’s crust. It does not exist as a pure metal but is usually found in the ores of bauxite and sphalerite. Therefore, it is often extracted as a by-product during the processing of the ores of other metals. Starting from its low-melting point character, other properties of the gallium are also distinctive from the traditional rigid metals. It is used commonly in the analog integrated circuits, high-temperature thermometers, nuclear reactors, focused ion beam, biomedical sensors, and so on. Moreover, based on this fundamental element, it is possible to fabricate its liquid metal state into different forms and characteristics.

So far, there have already been quite a few methods available to deal with the liquid metals through which the material characteristics, including melting point, thermal and electrical conductivity, viscosity, fluidity, and plasticity, could be adjusted to meet specific needs. These low-melting alloys and materials derived from the room temperature liquid metals could collectively be called functional liquid metal materials which constitute the next big wave in research.

As new-generation thermal management materials, the preparations of low-melting liquid metals become more and more increasingly important. So far, various methods, such as alloying, oxidizing, and adding metals or non-metallic materials, have been developed to prepare desirable functional materials based on gallium or other metals [1]. These methods could not only change the form of the materials but also endow the original liquid metals with rather diversified performances, which have further expanded the application range of the low-melting liquid metals to meet various needs. This chapter summarizes the fabrication methods and characterizations of the liquid metal thermal management materials. The future outlook, including challenges, routes, and related efforts, are illustrated and interpreted.

Liquid metals with low melting point around room temperature are emerging as powerful coolants for driving away heat due to their superior thermo-physical properties and the unique ability to be driven efficiently by a completely silent electromagnetic pump. However, the adoption of gallium, one of the best candidates as a metal coolant so far, may cause serious corrosion to some structure materials and subsequently affect the performance or even result in dangerous running of a cooling device. In high-temperature cases, this issue becomes more serious. Fortunately, so far, we have had appropriate solutions to prevent such complications. To address this critical issue, this chapter illustrates the potential ways from a practical perspective through a discussion of several typical examples [1]. Particularly, the compatibility of gallium with four representative metal substrates (6063 aluminum alloy, T2 copper alloy, anodic coloring 6063 aluminum alloy, and 1Cr18Ni9 stainless steel) is comprehensively discussed in order to understand the corrosion mechanisms better and help find the most suitable structure material for making a liquid metal cooling device. An image acquisition and contrasting method to quantify the dynamic corrosion behavior in detail is explained. Moreover, corrosion morphology analyses are performed by means of scanning electron microscope (SEM). The chemical compositions of the corroded layers are evaluated using energy dispersive spectrometry (EDS). Based on the experiments, it is found that the corrosion of 6063 aluminum alloy is rather evident and serious under the temperature range for chip cooling. The loose corrosion product will not only result in no protection for the inner substrate but also accelerate the corrosion process. Compared to 6063 aluminum alloy, T2 copper alloy shows a slow and general corrosion, but part of the corrosion product can shed from the substrate, which will accelerate corrosion action and may block the flowing channel. Anodic coloring 6063 aluminum alloy and 1Cr18Ni9 stainless steel are found to have excellent corrosion resistance among these four specimens. No evident corrosion phenomena are found under the examination of SEM and EDS when exposed for 30 days at the temperature of 60°C, which suggests their suitability as structure material for the flow of liquid metal. However, as for the anodic coloring 6063 aluminum alloy, surface treatment and protection are of vital importance. The knowledge presented in this chapter is of significance for making a liquid metal chip cooling device, which can actually be used in the industry.

The newly emerging functional materials of room temperature liquid metal and its alloy are playing increasing roles in advanced cooling. However, the currently available liquid metal mediums could still not completely fulfill the needs. Considering that the thermal physical properties of the previously developed liquid metals were somewhat limited due to their inherent characteristics, we proposed a new conceptual strategy to revolutionize existing technologies which has led to the materials termed as nano liquid metals.

As is known to all, conventional nanofluids or liquids are generally made by loading nanoparticles into base fluids such as water, oil, or other engineering liquids to form various dilute functionalized suspensions [1]. Such components are critical in a wide range of important areas [2–6]. So far, extensive efforts have been made toward investigating the fundamental and practical issues of nanofluids, including the following aspects: effects of species [3], concentrations [4], shapes [5, 6], and sizes [7, 8] of nanoparticles, base fluids [9–11], working temperature [7], coating [8], and mechanism interpretations [12–19]. Although addition of nanoparticles did enhance the otherwise poor capabilities of the original fluids, the extent of this improvement is somewhat limited due to inherent characteristics of the adopted conventional base fluids. As an alternative, the new conceptual strategy of nano liquid metal [20] would revolutionize existing nanofluids. Through suspending nanometer-sized particles into liquid metal (Fig. 5.1), a group of different unconventional capabilities for the fabricated nanofluids can be enabled.

Although initially introduced for the cooling of high heat flux computer CPUs [21], the room-temperature liquid metal and its alloys have been extended to rather diverse areas, such as thermal management [22–24], waste heat recovery [25, 26], kinetic energy harvesting [27], thermal interface material [28], printed electronics [29–31], 3D printing [32, 33], biomedical technologies [34], and soft machines [35]. The multimode capabilities of nano liquid metals lie in the outstanding properties of the base fluids of liquid metals, including low melting point, large thermal and electrical conductivity, as well as tremendous desirable physical and chemical properties (Figs. 5.1(a–c)). By making full use of nanotechnologies, liquid metal can be molded into many outstanding nanomaterials. According to the specific needs and preparation processes, these new conceptual liquid composites could exhibit superior fluidic, thermal, electrical, magnetic, chemical, mechanical, and biomedical properties, which are much better than existing nanofluids (Fig. 5.1(d)). This guarantees the versatile adaptability of nano liquid metal materials. This chapter illustrates the basic features of nano liquid metals, their fabrication strategies, perspective on applications, as well as scientific challenges.

Heat transfer between solid interfaces can be of great importance in many applications [1]. However, solid–solid interfaces usually have relatively high thermal resistance, especially the high contact resistance which is due to the surface roughness of both surfaces preventing perfect contact between the surfaces (Fig. 6.1). For example, the contact resistance of a thermal interface consisting of bare silicon to silicon contact is on the order of 100mm² kW⁻¹ [2, 3]. To solve this problem, thermal interface materials (TIMs) with high performance are generally used to form a thermal joint between an electronic component, such as a chip, and a cooling system [4–6]. It can significantly reduce the contact resistance between solid contacts. Generally, a thermal interface material should have high thermal conductivity and sufficient compliance to assure great conformability to the solid surfaces. Such thermal interface materials can significantly enhance the contact between solid surfaces and thus reduce the contact resistance.

Traditionally, thermal grease has been extensively applied as a typical thermal interface material in electronic packaging. It generally consists of highly thermally conductive solid fillers and matrix materials that provide good surface wettability and compliance of the material during application. However, the thermal conductivity of traditional thermal greases, such as silicon oil-based medium, is still somewhat low which restricted its further use in thermal management. As an alternative, the low melting point metals, alloys, and their allied materials are an improvement owing to their intrinsically high conductivity. This chapter illustrates the basics of liquid metal-based thermal interface material. Some typical examples such as gallium-based TIM was considered to explain the material preparation, characterization, and application. The wettability of this material with other substrates is disclosed and compared with each other. Further, some desired shapes to enhance thermal transfer, such as semi-liquid paste and thermal pads which can be cut to the required shape, were illustrated. Some fundamental phenomena such as bulk expansion during application of liquid metal TIM were introduced and interpreted.

Liquid metal thermal interface materials are considered to be the next-generation high-performance heat transfer medium and are believed to have remarkable advantages over already commercialized TIMs with regard to higher thermal conductivity, lower thermal resistance, and better adhesivity.

Phase change material (PCM) cooling technique is a kind of passive cooling technique that uses phase change material as the coolant. When facing a thermal shock, PCM absorbs the heat and melts, while its temperature nearly keeps constant over the melting process and thus prevents the power devices from overheating. After the thermal shock, heat is dissipated from the PCM to the ambient, the PCM solidifies and prepares for next thermal shock. PCM cooling technique is suitable for power devices which generate heat intermittently, such as portable electronics and power battery pack. It can also be used in the cases where unstable thermal shock exists.

The application of PCMs grew rapidly in the last few years, especially in those areas like solar energy, thermal comfort control, green building, environmental conservation, and electronic cooling [1]. Conventionally, organic PCMs (typically paraffin) are widely used for thermal management of power devices. The main drawback of paraffin PCMs lie in their low thermal conductivity, which seriously hinders the heat conduction inside the PCMs and thus decreases the heat transfer efficiency. There are generally two methods to improve this situation as follows: (1) increasing the thermal conductivity of the PCM via modification or nanoparticle inclusion; (2) providing high conductive paths into the PCM to enhance the heat transfer inside, such as internal fin and metallic foam.

So far, tremendous efforts have been made on finding new powerful PCMs or improving performance of the currently available PCMs which generally subject to inherent defects, such as low thermal conductivity, poor stability after millions of repeated solidifying and melting processes, easy phase separation during transition, and narrow temperature span between the melting point and the evaporation state. Many works have been done to investigate and optimize the performance of internally finned PCM heat sinks. It was shown that a large number of uniformly distributed pin fins were much helpful for enhancing the heat transfer effects of PCMs.

To better serve the stringent requirements of many emerging utilization systems, a new class of high performance PCMs, the low melting point liquid metals or their alloys, have gradually come into the use recently [1]. Continuous research demonstrated its much superior thermal performance than that of conventional PCMs in the cooling of a group of important applications, such as high-power portable electronics, lithium-ion battery pack, and microprocessor chip. This chapter illustrates the unique merits, application features, and potential values of these highly conductive liquid like materials with their basic properties interpreted. Some latest advancement made in the area is discussed. Comparative evaluation on the fundamental mechanisms and practical issues between conventional PCMs and the low melting point metal PCM is carried out. Further, some relevant scientific and technical challenges were raised. It is expected to incubate an emerging frontier toward studying and utilizing metal PCMs in the near future, which is rather useful for a broad range of areas.

Unlike earlier academic endeavors on a single fluid, liquid metals are found to be uniquely important when used along with other solutions which are especially critical in many applications. In this sense, the hydrodynamic properties of liquid metal and allied fluids made of liquid metal/aqueous solution are elementary in the design and operation of various functional devices or systems involved. In terms of the general physical and chemical properties, such as density, thermal conductivity, and electrical conductivity, the huge differences between the two fluidic phases of liquid metal and conventional fluid raise a big challenge for quantifying the hybrid flow behaviors [1]. Interesting enough, the liquid metal immersed in the solution would easily move and deform when administrated with external non-contact electromagnetic force, or even induced by redox reaction, which is entirely different from the cases of a single fluid or conventional contacting force. Owing to its remarkable capability for flow and deformation, liquid metal immersed in the solution is apt to deform on an extremely large scale, resulting in marked changes on its boundary and interface. However, until now, the working mechanisms of the movement and deformation of liquid metal in the allied solution environment still lack appropriate models to describe such scientific issues via a set of well-established unified theories. To promote deep understanding of liquid metal in future biomedical applications where hybrid or even multiple phase fluids are often involved, this chapter is dedicated to illustrate the unconventional hydrodynamics from experiment, theory, and simulation aspects. Typical phenomena and basic working mechanisms are explained. Some representative simulation methods are incorporated to tackle the governing functions of the electrohydrodynamics. Further, prospects and challenges are raised, which is to offer a startup insight into the new physics of hybrid fluids under applied fields.

The last five decades have witnessed a great prosperity and development of very large-scale integrated circuits (VLSI) and personal computers in microelectronic industry. Meanwhile, the ever-increasing power density of electronic components and more compact package technology have led to the challenging issue of thermal management becoming rather hard to solve.

So far, many convective liquid cooling methods have been proposed and investigated to meet the needs of various kinds of microelectronic devices. As for the typical applications which have the heat flux density below 100W/cm², such as LED lamp and advanced CPUs, conventional fan heat sink and heat pipe are most widely used because these cooling methods can effectively meet the heat dissipation requirements and has the evident advantage of low cost. However, if the heat flux density goes beyond 100W/cm² or even reaches 1000W/cm², typical liquid-based cooling methods, such as micro channel and jet impingement, are necessary and become the mainstream in industry. Apart from those mature technologies, some novel and advanced cooling methods, such as electrohydrodynamic approach, thermoelectric method, nano fluid or phase change microcapsule fluid cooling, and piezoelectric fan also have been proposed and widely investigated. However, these innovative cooling methods often involve complex fabrication process, high cost, reliability issues, or insufficient cooling capability, which are the main obstacles for their large-scale commercialization and utilization.

Broad societal needs have focused attention on technologies that can effectively dissipate huge amount of heat from high power density electronic devices. Liquid metal cooling which has been proposed in recent years is fast emerging as a novel and promising solution to meet the requirements of high heat flux optoelectronic devices due to the superior thermophysical properties and the unique electromagnetic-driven characteristic of liquid metal. Up to now, a lot of researches have been performed to investigate the cooling capability and typical applications of liquid metal, such as the heat-driven liquid metal cooling device, nano liquid metal fluid, and liquid metal injection.

In this chapter, a comprehensive description of liquid metal cooling system design theory, device fabrication, performance evaluation, and economic feasibility is presented. Several critical fundamental design principles are given. Typical liquid metal cooling prototypes were fabricated and cooling capability comparison with typical cooling devices in the market is performed. The compared results and economic feasibility are presented and discussed. This chapter also presents theoretical optimization and experimental investigations on a practical liquid metal CPU cooling product. A series of critical parameters was identified and the optimization criterion was established. Then, all the critical parameters for the electromagnetic pump and fin radiator were investigated and optimized using related theoretical sub-models. With appropriate industrial design, a practical liquid metal CPU cooling product was fabricated and compared with six typical commercial cooling products. The results demonstrated that the liquid metal product could serve as one of the best CPU cooling devices in the market.

By making full use of the double merits of the liquid metal, i.e., superior heat transfer performance and external field drivable ability, the present lab demonstrated a group of liquid cooling concept for the self-adaptable thermal management of computer chip, such as using waste heat to drive flow of the liquid metal. Some are based on thermoelectric generator (TEG) while others are based on thermos or fluids. Such devices consume no external net energy, which warrants it a self-supporting and completely silent liquid cooling module. Experiments on devices driven by one or two stage TEGs indicate that a dramatic temperature drop on the simulating chip has been realized without aid of any fans. The higher the heat load, the larger will be the temperature decrease caused by the cooling device. Further, the two TEGs generate a larger current if a copper plate is sandwiched between them to enhance heat dissipation. This method is expected to be significant in future thermal management of a desk or notebook computer, where both efficient cooling and extremely low energy consumption are of major concern. Apart from this strategy, thermosyphon effect was also found very useful to drive room temperature liquid metal for electronic cooling. This may lead to a self-supported cooling which utilizes only the waste heat produced by the hot chip to drive the flow of liquid metal. In addition, the device thus fabricated will be without any external pump and moving elements inside. This chapter illustrates a series of basic ways to develop self-adaptable liquid metal cooling.

Heat dissipation systems generally require as powerful as possible heat transfer capacity however sufficiently small enough size in many thermal management applications [1], especially for high-performance CPU, large power light-emitting diode (LED), mobile electronic devices, etc. As the processes for the integration and miniaturization of opto-electronic devices are being accelerated, the cooling devices are quickly developing toward the goals of high efficiency, compactness, and celerity. So far, typical heat dissipation systems for small electric devices, such as fan cooling, heat pipe, thermoelectric cooling, and mini-channel, have been intensively studied which greatly promoted the progress of thermal management science and technology. Up to now, conventional coolants, such as water, have received much attention and were widely used in a variety of heat transfer fields. However, due to the existing deficiencies in these classical heat-removal strategies, many challenging issues encountered in thermal management of devices need to be adequately resolved. For example, the coolant systems employing forced air or water to take heat away are generally bulky and inefficient, while water has a thermal conductivity of only 0.599W/(m ·°C) and may become boiled above 100°C with the working pressure building up considerably. This makes the heat transfer capacity of such fluids rather insufficient. Therefore, a compact heat dissipator with higher performance urgently needs investigation.

Mini-/micro-channel cooling has long been identified as an effective way for heat dissipation of many cutting-edge electronic devices with high heat flux. That is mainly because it has many evident merits, such as smaller size, lesser coolant inventory, higher convection coefficient, and larger heat transfer area when compared to traditional convections. In recent years, numerous researches have been performed to understand the cooling mechanism better and improve the cooling capability of mini-/micro-channel. However, most of the works try to raise the cooling capability of mini-/micro-channel from the view point of structure optimization. Researches on better and superior coolants for mini-/micro-channel were rarely reported. A majority of the mini-/ micro-channels used traditional coolants, such as air, water, Freon, and ethanol solution. In addition, two-phase coolants, such as water–vapor, nitrogen–water, and ethanol–CO₂ were also widely studied. However, all of these coolants lack the ability to cope with even higher heat dissipation requirements due to the limitation of their thermo-physical properties. Some novel coolants, such as nanofluids was proposed for mini-/microchannel cooling but their improvement of cooling capability was also limited and might involve deposition problems. Therefore, searching for better coolant with superior thermo-physical properties is very important to greatly improve the cooling capability of mini-/micro-channel devices.

Recently, liquid metals, with low melting point around room temperature and much higher thermal conductivity than that of common liquids, is introduced as very promising working fluids in mini-channel heat exchanger [2]. Liquid metals are superior owing to their high efficiency, low energy consumption, high reliability, and ease of manufacture. In addition, liquid metals possess excellent electro-physical properties and can be easily driven by a pump with no moving parts. This chapter is dedicated to illustrate the cooling performance and flow resistance characteristic of a liquid metal-based micro-channel heat exchanger. The experiments and theoretical analysis indicated that micro-channel with liquid metal as the coolant is a powerful way for heat dissipation of high heat flux density electronic devices.

Conventional cooling methods can be classified into many types, such as fan heat sink, heat pipe, vapor compression, micro channel, and jet impingement. The coolants are also abundant and various, for instance air, water, fluorocarbon, and ethanol, even the nanofluids have been widely studied. Nevertheless, up to now, the most effective and practical way for heat dissipation of high-power density devices is still water-based cooling due to its high heat capacity and the lowest cost.

As the most classical coolant in a heat transfer process, water cooling is widely adopted which however inherits limited thermal conductivity and relies heavily on mechanical pump. As an alternative, the room-temperature liquid metal was increasingly emerging as an important coolant to realize much stronger enhanced heat transfer. However, its thermal capacity is somewhat lower than that of water, which may restrict the overall cooling performance. In addition, the high cost because of taking too much amount of liquid metal into the device also turns out to be a big concern for practical purpose. From this consideration, a new trend [1] in the area is to develop hybrid or multiple coolants for thermal management through combining the individual merits from both the liquid metal with high conductivity and water with large heat capacity. Such a system not only has more excellent cooling capability than that based on water alone but also has much lower initial cost compared with absolute liquid metal cooling system. A series of experiments under different operation conditions was thus performed to evaluate the cooling performance of such hybrid systems. The compared results with absolute water cooling and liquid metal cooling system showed that the cooling capability of the new system is competitive with absolute liquid metal cooling, but the initial cost could be much lower. The theoretical thermal resistance model and economic feasibility analysis shows that the hybrid liquid metal–water cooling system is quite feasible and useful.

Further, given specific design, the electrically or electromagnetically induced actuation effect of liquid metal can be applied as the flow-driving strategy, which may significantly simplify the system design [1]. This enables the liquid metal pump and its surrounding aqueous solution to be quickly accelerated to a large speed under only a very low electric voltage. This kind of work suggests an important way to make highly compact chip cooling device, which can be flexibly extended into a wide variety of engineering areas. It can be anticipated that the hybrid coolants enabled cooling device will be very useful for a wide variety of thermal management areas where quite confined space is critically required in the near future.

This chapter is dedicated to explain the basics of the hybrid liquid metal cooling from the aspects spanning from materials, driving, device designing to applications.

Liquid metal has significant value in harvesting various kinds of energy, especially low-grade heat. Energy is the core supporting element of economic and social progress of human society. The recent energy crisis and environmental burden are becoming increasingly urgent and drawing an ever-growing attention on energy harvesting [1]. As an important component of energy technology, harvesting waste heat at low temperature (or termed as low-grade heat) to generate electricity is of enormous importance for energy sustainability. Currently, the available waste heat sources generally include recoverable industrial heat-generating process, exhausted waste heat of transportation vehicles, solar energy, combustion of solid waste, and geothermal energy. Industrial waste heat takes up a major part of the waste energy in the whole society (Fig. 13.1). It was reported that 33% of the manufacturing industrial energy was released directly to the atmosphere or cooling systems as waste heat because many industries were not able to recycle the excessive energy. A range of 0.9TWh to 2.8TWh of electricity might be produced by waste heat each year if thermoelectric materials with average ZT (Z: figure-of-merit of thermoelectric materials, T : absolute operating temperature of thermoelectric materials) values ranging from 1 to 2 were available. In addition, extremely large amounts of waste heat energy are generated from inefficient transportation vehicles. Generally, a cooling system is required to guarantee the functioning of the engine because of the thermal limit of automotive engine. Typically, heat produced by automotive engine is removed by cooling loop which is full of coolant (water, oil) primarily, then dissipates to the ambient through air cooling. Researches pointed out that for internal combustion engine, only 25% of the energy generated by fuel combustion was used to run the vehicle, while 40% of the energy was discarded with exhausted gas, 30% of the energy was rejected through coolant, and 5% of the energy was dissipated as friction.

To make full use of the low-grade heat, various heat-to-electricity generators, such as steam turbine, thermoelectric (TE) and thermoacoustic machines have been developed [2]. However, it is often difficult to connect the generator directly to the heat source. The major reasons come from the limited surface area or abnormal shape of the heat source. Clearly, heat needs to be extracted before generating electricity. So far, a number of difficulties related to the current techniques, such as the poor performance of materials, low level of encapsulation techniques, and limitation of heat exchangers, have been hampering its progress to some extent. To resolve such problems, sufficient utilization of low-grade waste heat is becoming increasingly important. The room-temperature liquid metal is offering a superior solution toward this need, owing to its excellent heat transfer performance and low-power driving property, by partly replacing water which is usually selected as candidate in conventional waste heat recovery process. In this way, a new-generation device with high efficiency in recovery of waste heat and power generation can be developed.With the investigation of liquid metal heat exchanger technology, it is expected that a new field of industrial waste heat utilization could be established. The researches carried out will bring about a deep impact in many respects, such as energy and water conservation, energy consumption reduction, and environmental protection.

Apart from industrial waste heat recovery, harvesting solar energy based on thermoelectric generator (TEG) is another important way to capture low-grade heat [2]. A typical TEG driven by solar power consists of a thermal collector, TEG, and groups of flow pipes. In this type of TEG, thermal collector is used to absorb heat from the solar radiation, then the heat is carried to TEG by flow pipes. Building Scientific Research Center (BSRC) presented a new concept of roof design named “Thermoelectric Roof Solar Collector (TE-RSC)”, which was composed of thermoelectric modules, a rectangular fin heat sink, a copper plate, a transparent acrylic sheet, and air gap. Research results indicated that about 1.2W power could be obtained by such a generator with 10 thermoelectric modules of 0.0525m² surface area, under a solar radiation intensity of about 800W/m² at ambient temperature between 30–35°C. Though the electrical conversion efficiency of TE-RSC system was reported to be as low as 1%–4%, this kind of generator is still popular in remote areas or some special fields. Overall, TEG plays an even more significant role in recovering the waste heat. Nonetheless, the application of TEG is limited in a small domain where the cost is not the main issue to be considered because of the low energy conversion efficiency of thermoelectric devices and units. In fact, direct solar thermal power generation forms can be many, such as thermoelectric, thermionic, magnetohydrodynamic, and alkali metal thermoelectric, and these methods are very attractive ways to provide electric energy from solar heat. On the one hand, these methods have the potential to be more efficient than traditional ways since they can convert heat to electricity directly without experiencing the conventional intermediate mechanical energy conversion process. On the other hand, these electricity generators are generally silent, reliable, and scalable, making them very suitable to serve as distributed power generation system and for certain specialized fields, such as space applications. In recent years, a new kind of technology based on liquid metal was presented as an effective way in harvesting the solar energy. Considerable efforts have been devoted to investigate the energy conversion theory and practical applications. Under such conception, a group of demonstrating experiments were carried out, which indicated the feasibility of the new methods.

This chapter is dedicated to presenting a thorough illustration on typical advances in developing the liquid metal-based heat harvesting and utilization strategies. Both the fundamental issues and latest application researches are discussed. Future developments are prospected.

Every material has its merits and demerits and liquid metals are no exception. However, through collaboration with existing strategies, liquid metals could still achieve superior thermal management. As a class of newly emerging material, liquid metals exhibit many outstanding performances in a wide variety of thermal management areas, such as thermal interface material, heat spreader, convective cooling, and phase change material (PCM) for thermal buffering. To help mold the next-generation unconventional cooling technologies and further advance the liquid metal cooling to an even higher level in tackling more extreme, complex, and critical thermal issues and energy utilizations, we proposed a novel conceptual scientific category, which could be termed as combinatorial liquid metal heat transfer science [1]. This chapter is dedicated to present the basic ideas of the combinatorial liquid metal heat transfer science and technologies, their potential impacts and applications. Through comprehensive interpretations on a group of representative liquid metal thermal management strategies, a series of basic ways were outlined for developing liquid metal-enabled combined cooling systems. The main scientific and technical features of the proposed hybrid cooling systems were illustrated. Particularly, five abstractive segments toward constructing the combinatorial liquid metal heat transfer systems were clarified. The most common methods of innovating liquid metal combined cooling systems based on this classification principle were discussed, and their potential utilization forms were proposed [1]. For illustration purpose, several typical examples such as low melting point metal PCM combined cooling systems and liquid metal convection combined cooling systems, were specifically introduced. Finally, future prospects to search for and make full use of the liquid metal combined high-performance cooling systems were discussed. It is expected that in practical application in the future, more unconventional combination forms of liquid metal cooling can be obtained based on the current fundamental principles.

The following section is included:

• Index