Micro Flat Heat Pipes for Microelectronics Cooling

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Micro Flat Heat Pipes for Microelectronics Cooling: Review Lu C. Lv1 and Ji Li2,* 1

College of Physics, University of Chinese Academy Sciences, 19A Yuquanlu Road, Shijingshan District, Beijing 100049, P.R. China; 2Laboratory of Advanced Thermal Management Technologies, College of Physics, University of Chinese Academy Sciences, 19A Yuquanlu Road, Shijingshan District, Beijing 100049, P.R. China Received: August 27, 2013; Accepted: September 13, 2013; Revised: September 13, 2013

Abstract: A review of recent patents and academic articles on micro flat heat pipes over the past decade is presented herein. Firstly, the development history of the mathematical models of the micro flat heat pipes is chronologically summarized. Then, some major investigations in the micro flat heat pipes, both experimental and theoretical, are summed up. Besides, some innovative ideas and novel micro flat heat pipe designs for particular applications are also documented along with comparisons and discussion on various cross-sectional shapes and designs of wick structure, such as grooves, sintered powders, nanotubes and their combinations. Finally, the challenges in the research of micro flat heat pipes are discussed briefly and the development tendency is prospected according to the existing achievements.

Keywords: Electronics cooling, high heat flux, micro flat heat pipe, modeling, novel designs, wick structure. 1. INTRODUCTION The developments of micro-machining and packaging technologies in recent years have brought about a sharp increase in the amount and density of electronic components in very large scale integration chips. Consequently, the thermal energy generated by a microelectronics system is elevated severely and its temperature control faces crucial challenge. Worse yet, the level of temperature uniformity is very low [1, 2]. According to Mochizuki et al. [3], the heat fluxes generated in microchips can reach up to 100W/cm2 in the late 2000s. In fact, much higher heat flux has been generated so far [4]. If not cooled down, the reliability and performance of an electronic product will be significantly and negatively affected. Hence, micro flat heat pipes emerged as a promising solution for cooling electronic equipments [1]. Particularly for aiding uniform heat distribution in electronic chips, the concept of micro heat pipe was first introduced by Cotter [5] in 1984. According to his definition, a MHP does not contain any wick structure, but consists of some small non-circular channels with a diameter of 10500Gm and a length of several centimeters. And "the mean curvature of the vapor-liquid interface is comparable in magnitude to the reciprocal of the hydraulic radius of the total flow channel". The circulation of the fluid in this pipe is caused by the capillary pressure gradient created by the difference in the curvature of the menisci along the liquid-vapor interface, which pumps the condensed liquid back to the evaporator. But one disadvantage of these traditional micro heat pipes is that they cannot handle high heat fluxes [1]. *Address correspondence to this author at the Laboratory of Advanced Thermal Management Technologies, College of Physics, University of Chinese Academy Sciences, 19A Yuquanlu Road, Shijingshan District, Beijing 100049, P.R. China; Tel: +86-13522278866; E-mail: [email protected] 1874-477X/13 $100.00+.00

However, with the technology development in micro heat pipes, specially, micro flat heat pipes (namely, in some other places, thermal ground plane or heat spreader) can handle much higher heat flux than before. In general speaking, a micro flat heat pipe has wider areas for evaporating and condensing and much complicated inner structures compared to a traditional micro heat pipe proposed by Cotter, so it has the ability to transfer high heat fluxes with very small temperature gradients. Thus, the hot spots produced by local heating sources can be effectively removed. Moreover, a micro flat heat pipe is light-weight, easily controlled and has good performance during the startup process and over a long distance. In view of those advantages, micro flat heat pipes were developed rapidly recent years. In the past two decades, a number of theoretical, experimental and numerical investigations have been reported in the literature. Many mathematical models [6-35], both steady and transient, have been shown to accurately predict the performance limitations and operational characteristics. And the determination of operating limits and optimization of designs, along with the visualization of the flow of the working fluid, are the main objectives of experimental investigations. It can be concluded that the performance of a micro flat heat pipe is a complicated function of various factors like the structure, capillary force, heat flux, filling ratio, inclination angle etc. As Sobhan et al. [6] had indicated, "the proper design and fabrication of a micro heat pipe suitable for a particular use, is an art as well as a science, as this involves calculations, intuition and experience". In this article, the development history of the theoretical analysis is reviewed firstly and some fundamental conclusions are obtained. Then, some of important articles and patents addressing the micro flat heat pipes, both experimental and numerical, are presented herein. Various cross-sectional shapes and designs of wick structure, such as grooves, sin© 2013 Bentham Science Publishers

170 Recent Patents on Mechanical Engineering 2013, Vol. 6, No. 3

tered powders, nanotubes and their combinations, are also summarized and compared. Some significant conclusions are drawn too. Furthermore, some innovative ideas and novel designs for particular applications are also documented, for example, adverse-gravity and flexible micro flat heat pipes. Finally, the challenges in the research of the micro flat heat pipes are also discussed briefly and the development tendency is prospected in the conclusion section. 2. ANALYSIS AND OPTIMIZATION A mathematical model is a description of the essence of a system. It may help to study the effects of different critical components on the performance of a heat pipe and to make predictions about its behavior. Due to the universal applicability of models, we can use the mathematical model to assess the precision of the experimental results and to provide theoretical foundation for developing the prototype and optimizing its structure. Even though there have been numerous models for micro heat pipes, the thermal and hydrodynamic performance of micro heat pipes have not been studied enough, such as the exact nature of liquid evaporation from the corners of a micro heat pipe and the mass flux during evaporation and condensation. Most of the mathematical models are based on the simplest micro heat pipe configuration which has a single noncircular channel for both the liquid and the vapor phases in the pipe. This is mainly because of the simplicity of these types of micro heat pipes and the accessibility to use the mathematical concepts and language. Lots of works have been published for the thermal and hydrodynamic modeling of a single micro heat pipe or arrays of micro heat pipes. Among them, Longtin et al. [7] developed a one-dimensional steady-state model of a micro heat pipe with triangular cross section to calculate the working fluid pressure, velocity and film thickness along the length of the micro heat pipe. Then, Peterson and his coworkers [8-11] also proposed some onedimensional models to analyze the thermal performance of micro heat pipes. In 2000, Sartre et al. [12] considered the high heat transfer in the contract area between the meniscus and the wall, coupling the one-dimensional model to a thermal two-dimensional conduction model. And in the same year, Tio et al. [13] and Sobhan et al. [14] studied the micro heat pipes with triangular grooves theoretically. And Sugumar and Tio [15], in consideration of the effect of gravity, applied the porous medium model [13] to inclined micro heat pipes with triangular and cusped-diamond-shaped grooves. Moreover, Wang and Peterson et al. [16] improved the Longtin equations to model the micro flat heat pipes made of several aluminum wires sandwiched between aluminum sheets, taking into account the effect of the liquidvapor phase interaction. Suh and Park [17], considering the interfacial shear stress, numerically investigated the flow of liquid and vapor in a micro heat pipe with trapezoidal grooves. Launay et al. [18] developed a detailed mathematical model for a 55-micro-heat-pipe array to predict its heat transport capability. Then a detailed analysis on the maximum heat transfer capabilities of a micro flat heat pipe was presented by Tzanova et al. [19]. In 2005, Suman and his coworkers [20, 21], utilizing a macroscopic approach, presented a one-dimensional steady-

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state model of a micro heat pipe for any geometrical shape. And two different wick designs, namely, rectangular and triangular, were considered for case study. At the same time, one-dimensional transient model was developed for a micro heat pipe with triangular grooves [22, 23]. But their models neglected the shear stress at the liquid-vapor interface. Lately, Suman and Hoda [24] improved the above models in view of the shear stress, the disjoining pressure used in the momentum equation and the sensible heat used by the substrate. The obtained results gave a qualitative description of the transient phenomena in the fluid flow and mass transfer processes in the micro-grooved heat pipe. In 2006, Lefevre et al. [25], considering both the liquid and the vapor flows inside a micro flat heat pipe and the heat conduction inside the wall, coupled a two-dimensional hydrodynamic model for pressures and velocities of both the liquid and the vapor to a three-dimensional thermal model for the temperature field of the wall. Continuously, Lefevre et al. [26] applied this model to a flat plate heat pipe with rectangular micro-grooves. Do et al. [27] proposed a steadystate model for predicting the thermal performance of a micro flat heat pipe with a rectangular wick structure. The effects of liquid-vapor interfacial shear stress, the contact angle, and the amount of liquid charge were counted in this model. Taking into account the impact of evaporation or condensation on the equivalent thermal conductivity of the capillary structure, Revellin et al. [28] modified the model from Lefevre et al. [25] by superposing two independent solutions. In 2009, Shukla [29] studied the heat transfer limitation of a micro heat pipe with polygonal channels by developing a one-dimensional steady-state model. In the patent [30], a mathematical mode for the fluid flow and heat transfer inside heat pipes with rectangular grooves is developed, with considering the effect of shear force on the vapor-liquid interface in the grooves on the thermal performance. In 2012, Tian and Zhu [31] developed a threedimensional steady-state model to study the thermal performance of a micro flat heat pipe with fiber wick and to optimize its structure. Hung and Tio [32] used a onedimensional model [33] for analyzing the effect of gravity on the thermal characteristics of inclined micro heat pipes. Then, they coupled this model with the phase-change interfacial resistance model to study a water-filled micro heat pipe [34]. Most recently, in order to understand the thermal performance limitations and structural optimization, Jiang et al. [35] made a detailed three-dimensional analysis of the heat and mass transfer inside a vapor chamber heat spreader. A transient model was developed by Liu and Chen [36] for a triangular micro heat pipe. A detailed analysis of the transient start-up performance was showcased, with a consideration of the effects of groove dimension and working fluid. These aforementioned models enable us to study the transient or steady performance and the limitations of a micro flat heat pipe and guide us to optimize the process of the fabrication of such a device to improve its work performance. A summary of some models is presented in Table 1 and some fundamental conclusions can be obtained from these models:

Micro Flat Heat Pipe: Review

Table 1.

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A Summary of Some Mathematical Models. Authors

Nature of Model

Wick Structure

Main Aims

Longtin et al. (1994) [7]

1D steady

Triangular groove

Maximum heat transfer and Optimization of design

Peterson and Ma (1996) [9]

1D steady

Triangular groove

Maximum heat transfer and minimum meniscus radius

Tio et al. (2000) [13]

1D steady

Triangular groove

Maximum heat transfer

Sobhan et al. (2000) [14]

1D transient and steady

Triangular groove

Parameters of the vapor and liquid flow

Wang and Peterson [16]

1D steady

Aluminum wires

Heat transfer performance optimum design parameters

Suh and Park [17]

1D steady

Trapezoidal groove

Heat transfer performance

Launay et al. (2004) [18]

1D steady

Triangular groove

Heat transfer and fluid flow characteristics

Tzanova et al. (2004) [19]

1D steady

Rectangular groove

Maximum heat transfer

Suman and Kumar (2005) [20]

1D steady

Triangular and rectangular groove

Fluid flow and heat transfer characteristics

Suman et al. (2005) [21]

1D steady

Triangular and rectangular groove

Capillary limit and dry out length

Suman et al. (2005) [22]

1D transient

Triangular groove


Suman and Hoda [24]

1D transient

V-shaped groove


Do et al. (2008) [27]

1D steady

Rectangular groove

Heat transfer performance and optimum dimensions of the groove

Shukla (2009) [29]

1D steady

Polygonal channel

Heat transfer limitation

Hung and Tio (2010) [33]

1D steady

Triangular groove

Effect of solid wall on heat transfer capillary

Hung and Seng [66]

1D steady

Star-groove

Effect of geometric design on thermal performance


1D steady

Triangular groove

Effect of gravity on thermal performance

Liu and Chen (2013) [36]

1D transient

Triangular groove

Effect of groove dimension and working fluid on start-up process

Sartre et al. (2000) [12]

3D steady

Triangular groove

Effect of interfacial evaporation on the thermal performance

Lefevre et al. (2008) [26]

3D steady

Rectangular groove

Maximum heat transfer and temperature field

Revellin et al. (2009) [28]

3D steady

Rectangular groove

Thermal conductivity


3D steady

Triangular groove

Heat transfer performance

Tian and Zhu (2012) [31]

3D steady

Glass fibers

Heat transfer performance and optimum structure design

Jiang et al. (2013) [35]

3D steady

Metal powder

Comprehensive numerical simulation

(i). The charging level of the working fluid has a significant influence on the fluid flow and heat transfer characteristics in a micro heat pipe.

(vi). More attention should be paid to the effect of the wettability on the fluid flow and thermal performance as the surface forces dominate in such micro systems.

(ii). The tilt angle is relevant to the working performance of a micro heat pipe, especially to the micro heat pipes with axial grooves.

3. INNOVATION AND TESTS

(iii).The dimensions of the micro grooves and the micro heat pipe strongly affect the thermal performance and the types of the wick structure are also very influential. (iv). The conduction of the wall cannot be neglected, especially when the working temperature is low. (v). The thermal resistances of the interfacial region and the vapor flow play a dominant role in the total resistance of a micro heat pipe.

During the past decade, abundant experimental and theoretical researches have been done on the micro flat heat pipes and most of them are to assess the thermal performance of the micro flat heat pipes with various wick structures. In fact, for the micro heat pipes, the wick is crucial to their thermal performance because it plays an important role in the evaporation and condensation of the working fluid as well as the flow of the condensed liquid back to the evaporator. Most practical types of wick can be grooves, sintered powders, meshes, nanotubes (e.g., CNTs) and their combinations. The


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micro flat heat pipes with grooves are the most widely used concept to undergo a research. Apart from these types of micro flat heat pipes, the adverse-gravity micro flat heat pipes and some other innovative designs emerging during the past decade are also presented in this part. 3.1. Micro-grooved flat heat pipe 3.1.1. Triangular Cross-section Micro flat heat pipes with triangular cross-sections are always the first choice for the researchers to study the steady-state and transient characteristics of a micro heat pipe. This is mainly because those triangular grooves can be obtained by the easiest fabrication methods, like anisotropic etching process [37-39]. Besides, it is easy to analyze and to develop the mathematical models for this design. Therefore, the investigations of this type of micro flat heat pipe are abundant. The early summary of the fabrication and the testing of micro heat pipes can be found in the reference [1] before 1994 and [6] from 1994 to 2003.

Fig. (1). Cross-section of the triangular channels with arteries [38].

In 2003, Berre et al. [38] fabricated two different micro heat pipe arrays, one with 55 triangular grooves (20mm 230m 170m), and the other with 25 triangular grooves (20mm500m340m) coupled with liquid arteries, as depicted in Fig. (1). The micro heat pipe array has a thermal conductivity of 133W/(m·K) for an input power of 3W, which is close to the silicon conductivity. The micro heat pipe array with liquid arteries represented an increase of 300% of the Si thermal conductivity. Then, Launay et al. [39] improved the experiment of Berre et al. [38] and verified the effectiveness of the micro heat pipe array with arteries further. In 2008, Qu et al. [40] analyzed the effect of a functional surface on the thermal performance of a triangular micro heat pipe. The functional surface can obviously increase the liquid capillary force by augmenting the difference of the contact angles between the evaporator and the condenser. As shown in Fig. (2), the surface has a smaller contact angle in the evaporating section than that in the condenser. This leads to a larger gradient of curvature radius along the axis, and as a result, the mass rate of the condensate back to the evaporator increases. Eventually, an obvious improvement of the thermal performance is obtained. Simulation results verify that this device really remove a greater amount of heat than the one without a functional surface under the same condition. In 2010, Rahmat and Hubert [41] used the finite element software to simulate a three-dimensional network of triangular channels in a micro heat pipe. Moon and Hwang [42] presented a flat plate heat spreader with triangular multi-grooves both in the evaporator and condenser. The heat spreader consists of three plate metals: the upper plate and the lower plate are used as condenser and evaporator, the middle one is functioned as paths for evaporated vapor and condensed fluid. Recently, Liu et al. [43] proposed a novel design of micro flat heat pipes with implanted arterial paths. The novel device consists of one vein triangular groove which has two neighbor arterial grooves distributed on both side of the vein, and these channels are connected together at both ends, as seen in Fig. (3). Experimental results showed that this device can work effectively when the temperature is between 37.3°C and 44.1°C, but for traditional micro flat heat pipes from 36.8°C to 40.2°C.

Fig. (2). Distribution of contact angles along the axial direction and droplet distributions on the functional surface [40].

Fig. (3). Working principle of the artery MHP [43].

3.1.2. Trapezoidal Cross-section Even though the study of the flow characteristics in the trapezoidal grooves [44-46] is abundant, there are not too many experimental investigations on the micro flat heat pipe with axial trapezoidal grooves as the heat pipe wick structure yet.


In 2010, Qu and Wu [47] conducted the flow visualization of FC-72 in two different silicon-based micro pulsating heat pipes with trapezoidal cross section. Schematic diagram of the cross section is shown in Fig. (4). Then, in order to study the start-up, heat transfer and flow characteristics of the device [47], Qu and his coworkers [48, 49] supplemented and improved the experimental process. Recently, Liu and Huang [50] simulated three kinds of micro flat heat pipes with different cross-sections, namely, rectangular, trapezoidal and triangular. Results showed the trapezoidal one had the best heat transfer characteristics. In addition, the thermal performance of two different heat pipes, one with smooth grooves and the other with a thin porous layer on the grooves’ surface, was tested and compared by Vasiliev et al. [51]. Because of the special inner structure of the porous layer, the wettability of the grooves’ surface is largely improved and the heat transfer ability is also improved due to the following enhancement mechanism: evaporation at the top of the porous layer and evaporation and nucleation at the bottom of the groove. Results indicated that these experiments provide us a promising approach to enhance the operation performance of micro-grooved heat pipes.


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Fig. (4). Schematic view of the cross section [47].

3.1.3. Rectangular Cross-section

Fig. (5). Cross section of the flat micro heat pipe [54].

A novel flat heat pipe with rectangular or triangular grooves in all inner surfaces is presented in the patent [52]. The equivalent diameters of the grooves are not equal and the inner porous media as the support for the heat pipe body has different producer price index values on the inner surfaces of upper and lower plates. As a result, the capillary limitation, boiling limitation and the structural strength of the heat pipe are greatly promoted. But it is expensive and hard to fabricate such a complicated wick structure. In the patent [53], a flat heat pipe is applied to the heat dissipation of the high-power LED. The primary innovation is that the integral metal sheet used as wick structure is a hollow rectangular type and the metal solid part among through holes can serve as the support for the core body.

tor and tested its heat transfer characteristics. In the patent [58], the nanofluid enhanced heat pipe exhibited high heat transfer performance, while being small-sized and lightweighted. Such a heat transfer device is realized by having organic fine particles contained in a working fluid through the optimizations of particle size, mixing ratio, composition and surface characteristics. In the patent [59], a polymer micro-grooved heat pipe fabricated by standard polymer surface micromachining techniques is presented. Polymer is a CMOS compatible material which makes it possible to fabricate the microchannels on the same wafer as the chip itself. In addition, various coolants like water and nanofluids can be charged into this device as the working fluid.

Although, the performance of micro flat heat pipes with rectangular grooves is worse than that of a triangular one [20, 21], some researchers also prefer to choose the rectangular micro flat heat pipes as the experimental objectives in some particular cases, such as nanofluids and liquid metal. In 2010, Do and Jang [54] analyzed the effect of water-based Al2O3 nanofluids as working fluid on the thermal performance of a micro flat heat pipe with rectangular grooves as depicted in Fig. (5). By comparing with DI water, the nanofluid was capable of enhancing the thermal performance up to 100% when the concentration was less than 1.0%. It was also found that the thermal resistance of the nanofluid heat pipe tended to decrease with increasing the nanoparticle size, as shown in Fig. (6). Harris et al. [55] fabricated and measured a silicon-based micro heat pipe array with 22 square channels and a thermal conductivity as high as 324W/(m·K) was reported. Dean et al. [56] successfully fabricated and evaluated a micro flat heat pipe with mercury as the working fluid. Experimental results showed this device had a thermal conductivity of approximately 790W/(m·K). It was much higher than the thermal conductivity of 290W/(m·K) for a water-filled micro flat heat pipe Fig. (7). Recently, Deng et al. [57] applied a micro heat pipe array to a flat solar collec-

Nanofluids are a relatively new generation of coolants comprised of a base fluid with nano-sized particles, generally a metal or meta oxide, suspended within the base fluid. Even though a great number of investigations on nanofluids have been performed because of their thermal conductivity enhancement, the qualitative agreement of the mechanism has not been reached. For instance, one explanation [60] emphasizes that the enhancement of thermal conductivity is a function of nanoparticle aggregation, but its validation needs further careful examination. In fact, nanofluids do change the surface properties of the wall of such a device and the stability of the dispersion is also critical. 3.1.4. Radial Grooves The micro flat heat pipes with radial grooves can be categorized to vapor chambers. They have wide area for the evaporator and condenser of such devices that makes them an effective approach to take away the thermal energy generated by electronic components. Actually, numerous investigations have been reported in the literature. In 2004, Kang et al. [61] designed a novel micro heat pipe heat spreader with three copper foil layers to allow liquid and vapor flow separation to reduce viscous sheer force. This device has two wick designs, one using 200Gm wide radial trapezoidal


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Fig. (6). Effect of the nanoparticle size on thermal resistance [54].

Fig. (7). Test results showing the temperature difference between evaporator and condenser with respect to power input, for various tests conducted on all four test specimens: Hg-filled, water-filled, and empty arrays and solid Si [56].

grooves and the other with 100-mesh copper screens. Experimental results showed that the first one performed better than the latter one. And a thermal resistance of 1.1K/W was obtained. Go [62] tested an acetone-charged vapor chamber with metal-etched micro wick structure and it was found that this device had a heat removal capacity of 80W/cm2 with an overall thermal resistance of 0.76K/W. Ivanova et al. [63] introduced a flat silicon heat pipe with rectangular radial grooves, as shown in Fig. (8). Experimental investigations showed that the micro flat heat pipe only had a thermal resistance of 0.9K/W. Zhang et al. [64] experimentally and theoretically studied the effect of heat flux and filling amount on the thermal performance of a vapor chamber with radial rectangular grooves. In 2013, Chen et al. [65] studied an aluminum vapor chamber with different wick structures, radical grooves and sintered aluminum powders, respectively giving the thermal resistance of the grooved one as 0.72K/W and the other as 0.69K/W.

Fig. (8). Capillary design of the heat pipe [64].

In the patent [66], a vapor chamber has been proposed to improve the base conductivity of the heat sink, as depicted in Fig. (9). Boiling enhancement features are adapted to the vapor chamber through a boiling enhanced multi-wick structure. With this structure, the condensate is collected from the



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lary limitation and enlarge the superficial area for boiling heat transfer. 3.1.5. Other Cross-section

Fig. (9). A sectional side view of a vapor chamber implemented as a flat plate [66].

Fig. (10). Geometry of cross-sectional shapes of the cuspeddiamond-shaped grooves [15].

In order to study the operating characteristics and heat transfer limit, Moon et al. [68] manufactured and tested two micro heat pipes. One had a triangular cross-section with curved sides and the other had a rectangular cross-section with curved sides. The details are presented in the patent [69]. Experimental results showed the heat transfer limit of the triangular micro heat pipe was 1.6 times larger than the rectangular one. Similarly, Sugumar and Tio [15] investigated two micro heat pipes with triangular grooves and cusped-diamond-shaped grooves, as seen in Fig. (10), respectively. Results showed that the latter had a higher allowable heat transport rate than the former with the effect of gravity taken into consideration. In 2011, Hung and Seng [70] theoretically studied the effect of geometric design on the thermal performance of a star-grooved micro flat heat pipe shown in Fig. (11). It was found that the performance of a micro flat heat pipe with star-grooved cross section outperformed the one with regular polygonal cross section because of the higher capillarity provided by the smaller corner apex angle. It was also observed that the performance of a micro heat pipe deteriorated with increase in number of corners, and the heat transport capacity increased with the crosssectional area. Recently, Wong and Chen [71] experimentally measured the evaporator resistance and observed the evaporation characteristics of a flat-plate heat pipe with parallel U-shaped grooves. In the patent [72], a thinned flat plate heat pipe is fabricated by using extrusion process to be applied to electronic components having small sized or thin-film structure. Meanwhile, a though-hole is formed in the longitudinal direction of the heat pipe. It is worth pointing out that most grooves in the through-hole are only formed on a part of one side or both side surfaces of the through-hole so as to ensure a relatively large steam flowing space. The capillary force generated by this wick structure is large enough to ensure the condensate to flow. According to the inventor, the thickness of the flat heat pipe can be reached 1mm or less and high heat transfer performance can also be obtained in such a thickness. 3.2. Sintered Micro Wicks

Fig. (11). Geometry of different cross-sectional shapes [70].

condensation sites using a wicking structure with a spatiallyvarying wicking power. Various boiling enhancement structures can be adapted at the heating zone to simultaneously provide capillary force and boiling enhancement. In this manner, the boiling enhancement structure is not totally submerged inside a pool of liquid, and thus can operate in anti-gravity orientations. In the patent [67], a flat heat pipe with radical triangular grooves is fabricated to augment the heat transfer rate. Meanwhile, sintered cylindrical copper powders are disposed on the central portion of the plate bellow and the depth and width of the grooves are becoming larger and larger in the radial direction. This design can greatly promote the capil-

With the development of the powder metallurgy, greater capillary pumping force and larger porosity can be obtained by sintering powders or meshes. More importantly, many researchers have applied this technique to the manufacturing of the wick structure in micro flat heat pipes. Koito et al. [73] investigated a copper vapor chamber with sintered sheets and columns made of copper powder and found that the heat source size had an important effect on the thermal resistance of the evaporator section. Then, Koito and his coworkers [74] further studied the thermal characteristics of this device numerically and experimentally. In the patent [75], Li presented a plate type heat pipe with novel wick structure and its fabrication method. The heat pipe body is flattened by stamping process. As shown in Fig. (12), the wick structure consists of grooves with po-


Fig. (12). Cross section of the flat heat pipe [75].

Fig. (13). Average boiling thermal resistance for all test samples as a function of the sample wick thickness [80].

lygonal cross section and sintered copper or aluminum powders, which ensures the good heat transfer performance. Similarly, a phase-change flattening technology is adopted to get such a device in the patent [76]. Chamarthy et al. [77], using a novel fluorescent visualization method, assessed the effect of acceleration on the wicking ability of the wick structure made of 75>m diameter sintered copper particles. Semenic and Catton [78]compared the biporous wick and the monoporous wick, both made of copper powders. It was concluded that the critical heat flux of the biporous wick was almost three times higher than that of the monoporous wick. In the patent [79], Huang et al. proposed a flat heat pipe structure including a flat pipe, a radial capillary structure and an axial capillary structure in detail. The radial capillary structure can be metal mesh or sintered powder and is disposed on all the inner surfaces in the evaporator section. The axial capillary structure can also be metal mesh or sintered powder but is attached onto one of the sidewalls and extended to the evaporator section.

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In 2010, Weibel et al. [80] studied the effect of particle size and wick thickness on the thermal performance of a vapor chamber with sintered copper powder wick surfaces, as seen in Fig. (13). Liou et al. [81] investigated the evaporation characteristics of water in sintered multilayer copper mesh by visualization and conducted evaporator resistance measurements. Then a composite wick of mesh and powder was further studied by Wong et al. [82]. Both of the investigations observed the absence of nucleate boiling for the heat flux up to 100W/cm2, in spite of the fact that there are abundant nucleation sites on the sintered surface. Recently, Wong and his coworkers [83] also studied the effect of the working fluids on the evaporation process and found that the maximum heat fluxes for water were much higher than that of methanol and acetone, but the minimum evaporator resistances were almost the same. Moreover, very weak and localized nucleate boiling was observed for acetone and methanol. Weibel et al. [84] studied a novel vapor chamber heat spreader with 1mm thick wicks composed of 100m sintered copper particles. Three variants of micro wicks were investigated and their dimensions are shown in Fig. (14). In the patent [85], a thin heat pipe comprised of a thin hollow tube and a wick structure formed in at least half of an inner wall of the tube by chemical etching process was presented in detail. The capillary structure can be grooves or porous structure including a number of micro-holes and the surface can also be hydrophilic in the evaporating section and hydrophobic in the condensing section. 3.3. Nano or Micro Wick Structures In recent years, carbon nanotubes (CNTs) have captured great scientific and engineering interest owing to their extraordinary thermal, electrical and mechanical properties. They are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). At room temperature, a SWNT has thermal conductivity along its axis of about 3500W/(m·K) [86] and thermal conductivity of an MWNT is more than 3000W/(m·K) [87]. This alone can make CNTs be a promising approach in cooling applications. The feasibility of using a CNTs array as the wick for heat pipes or vapor chambers was assessed by Vadakkan et al. [88] experimentally and numerically. In the patent [89], a nanostructured composite wick adopted in a heat dissipation system comprising a vapor chamber, a heat sink, a heat source and a nanostructure array extending from the heat source. As shown in Fig. (15), the nanostructured composite wick comprises a channel, a plurality of nanostructures,

Fig. (14). Dimensions of the (a) Homogeneous, (b) Grid-patterned, and (c) Wedge-pattered sintered copper powder micro wick layers [84].


wherein the nanostructures have a differentially-spaced apart gradient along the length of the channel so as to promote capillary fluid flow.


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and several pillars are fixed in its groove to function as upholders, along as the backflow channels. On the surface of the plate below, there exist a layer of nanotubes manufactured by GLAD or ECD to enlarge the capillary force. This wick structure can dramatically improve the heat transfer rate, especially in high flux. In the patent [94], carbon nanotubes are coated on a metal wick structure to promote incipient boiling of the working fluid. Beside the high thermal conductivity, the application of the CNTs can greatly increase the number of bubble nucleation sites, thus increasing boiling heat transfer coefficient.

Fig. (15). Cross-section of a nanostructured heat pipe [89].

Similarly, patent [90] employed a micro heat pipe with nanostructured wick on order of approximately 10 to 400nm with spacing between the nanoelements in the approximate range of 20 to 600nm. The proposed wicks enable more efficient heat exchange in the heat pipe. Khanikar et al. [91] experimentally explored the heat transfer enhancement benefits of coating the bottom of a micro rectangular channel(44.8mm 10mm 0.371mm) with CNTs. It was found that the critical heat flux was fairly constant at low mass velocities but degraded following repeated tests at high mass velocities. Under varying heat fluxes, Hashimoto et al. [92] performed several experiments with different wick structures. Results showed the vapor chamber with sintered microstructure made of copper powders of 50m could support 8.5W/mm2 without dry-out, four times higher than that of the one with a screen mesh. Moreover, the thermal resistance of the wick structure made of sintered copper powder coated with CNTs could be further reduced 20-37% compared with the bare sintered surface. In the patent [93], two plate metals are sealed together to form a vapor chamber. The top plate is used as the condenser

In 2011, Weibel et al. [95] developed a numerical model to assess the thermal resistance of a vapor chamber coated with CNTs in the evaporator region and sintered copper powder in other regions. In the same year, Ranjan et al. [96] numerically studied the enhancement in thermal performance in the evaporator section by using nanowicks made of CNTs and metallic nanowires grown on a copper substrate. In 2012, Weibel and his coworkers [97, 98] fabricated four different wick samples to independently explore the heat transfer enhancement due to the proposed augmentation features. Their images are shown in Fig. (16). To further improve such evaporator structures, more samples were fabricated and compared by Weibel et al. [99, 84], and the evaporation and boiling mechanisms were studied carefully under heat fluxes exceeding 500W/cm2. The effect of the density of the CNTs was also explored and the results were presented in Fig. (17). Recently, by using microwave plasma enhanced chemical vapor deposition technique, Kousalya et al. [100] coated the CNTs on a 200m thick sintered copper powder layer. A considerable increase in dry-out heat flux up to 457W/cm2 and a notable reduction in the boiling incipience superheat were observed.

Fig. (16). Schematic operation of a vapor chamber heat spreading device showing the internal transport processes and the evaporator micro geometry of the four fabricated test surfaces [98].


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surface force will be more important than the gravity in such micro systems, but the effect of gravity on the operation performance should not be ignored in practice. Though lots of advances in design, most micro flat heat pipes reported in literatures can only work functionally during horizontal operation. So creating a micro flat heat pipe that can work functionally under the diverse-gravity orientation has been becoming a hot topic now.

Fig. (17). Effect of the density of the CNTs on the minimum thermal resistance [100].

Fig. (18). Various heat transfer rates with respect to the tilt angle of the FMHP [103].

In the patent [101], the thermal conductivity and phase transition heat transfer mechanism are applied for the cooling of active optical element, such as LED and photodiode. The wick structure includes of nanowire N-type semiconductor and nanowire P-type semiconductor. The application of nanowires make this device has a high boiling heat transfer rate, which can be attributed to the high nucleate site density, excellent wettability and enhanced capillary force. 3.4. Adverse-gravity Micro Flat Heat Pipes As for a micro flat heat pipe, if oriented horizontally, onsets of dry-out and flooding take place simultaneously at the evaporator end and condenser end respectively [13]. However, if negatively inclined, the backflow of the condensed working fluid in the channel will be impeded because of the effect of gravity. Then the device will not be able to take away the thermal energy generated by the heater. Eventually, it will be destroyed due to overheating. In theory, the

Moon et al. [68] reported that a maximum heat transfer limit of 10W could be achieved using the triangular micro heat pipe with curved sides. And a significant decrease in the heat transfer limit under adverse-gravity was also indicated, approximately to 30% of that during horizontal operation. The effect of gravity was more prominent for a triangular micro heat pipe than a cusped-diamond one [15]. In 2007, Rulliere et al. [102] studied the thermal behavior of a microgrooved two-phase heat spreader (TPHS), which is flat with a wide evaporating area (190 90mm^2) and a small condenser area (30 90mm^2). Tests were performed in both horizontal and vertical orientations. It was shown that the temperature field was similar in both cases for heat transfer rates lower than 155W. Lim et al. [103] proposed a micro flat heat pipe that adopts fin-shaped micro grooves as a wick structure and water as the working fluid. Subsequent test results showed that the micro flat heat pipe was able to operate under adverse-gravity conditions with little degradation in heat transfer performance, as seen in Fig. (18). The effect of gravity on the flow-related properties of the sintered powder wicking structure was visualized by Chamarthy et al. [77]. In the patent [104], a sintered wick or a fabric wick is inserted into the evaporator section of the flat plate heat pipe to achieve high heat transfer performance in an inclination or vertical direction. To have better anti-gravity ability and bear greater thermal power input, a flat heat pipe with novel wick structure is presented in the patent [105]. The main body of the heat pipe has four inner sides and at least one containing the working fluid guaranteed by a very complicated wick design. Recently, Hung and Tio [32] found that gravity could enhance the heat transport capability and the rate of circulation of the working fluid for a positively inclined micro heat pipe, and vice versa. However, a micro-scale titanium pillar array coated with nanostructures was fabricated by Ding et al. [106], and they found that the bi-textured titanium structures were capable of holding water under a radial acceleration of 12.13g. Then, Thompson et al. [107] proposed a flat-plate oscillating heat pipe and tested its thermal performance under high-gravity conditions. Charged with acetone this device had a neatly constant effective thermal conductivity of about 700W/(m·K) from 0g to 10g acceleration at a heat input of 95W. Similar experiments were performed by Oshman et al. [108] for a micro flat heat pipe shown in Fig. (19) with an inner chamber of 30 30 1.0 mm3. It was observed that the effective thermal conductivity of the device ranged from 1653W/(m·K) at 0g to 541W/(m·K) at 10g. They used a hybrid wicking structure consisting of rectangular grooves formed with copper pillars coated with a woven copper mesh.



179

Fig. (19). Geometry of the polymer heat spreader and location of the evaporator and condenser regions [108].

Fig. (20). Layout of the FPHP [114].

3.5. Micro Flat Flexible Heat Pipe In the aforementioned investigations, the micro flat heat pipes are relatively constructed with rigid materials and fixed in stationary conditions. However, in some special situations where the configurations are flexible or the heat sources are oscillating, a flexible, lightweight and high-performance device will be a promising thermal management solution [109]. Owing to the elasticity, a micro flat heat pipe can be directly integrated into narrow spaces like printed circuit board. If flexible enough, this device is able to actually wrap around small heating elements to cool them effectively [110, 111]. In recent years, some researchers have been undergoing investigations on the micro flexible heat pipe. Larson et al. [110] and Rosenfeld et al. [111] had been issued a patent for

flexible heat pipe, respectively. A flat, flexible, polymer heat pipe with a grooved wicking structure was proposed by McDaniels and Peterson [112] and a thermal conductivity of about 740W/(m·K) was obtained. In 2004, a ultra-thin micro heat pipe was introduced by Furukawa Electric Co. [113]. When the adiabatic section temperature was kept at 50°C, the 0.7mm thick device displayed a thermal resistance of about 1K/W with 4W power input in all inclination angles. Recently, Oshman et al. [109] fabricated a micro flat heat pipe on a flexible liquid crystal polymer substrate with a hybrid wicking structure composed of a single layer of woven copper mesh bonded to the top of 200-m-wide grooves. This device could endure a heat flux of 11.94W/cm^2 and had effective thermal conductivities varying from 650 to 830W/(m·K). Then, new layout [108] was further arranged and a maximum thermal conductivity of 1653W/(m·K) was achieved.


Table 2.

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Comparison of Heat Transfer Performance of Some Micro Flat Heat Pipes. Heat Flux (W/cm2)

Authors

Material

Working Fluid

Wick Structure

Berre et al. (2003) [38]

Silicon

Ethanol methanol

Triangular grooves

Launay et al. (2004) [39]

Silicon

Ethanol Methanol

Triangular grooves

Liu et al. (2011) [43]

Silicon Glass

Ethanol

Triangular grooves

Qu and Wu (2010) [47]

Silicon

FC-72

Trapezoidal grooves

2.7/4.5/6.8

55-58%

Qu et al. (2012) [48]

Silicon

FC-72 R113

Trapezoidal grooves

10.7

80-100°C / 41-58% / 0-45-90°

Harris et al. (2010) [49]

Silicon

Water

Square grooves

32.23-35.01

63.8-82°C / 0.638-0.793K/W / 261-324W/(m·K)

Dean et al. (2012) [56]

Silicon

Mercury

Rectangular grooves

Deng et al. (2013) [57]

Aluminum

Acetone

Rectangular grooves

0.04-0.24

Kang et al. (2004) [61]

Copper

Methanol

Trapezoidal grooves and mesh copper screen

18

Go (2005) [62]

Aluminum

Acetone

Metal-etched channels

80

85°C / 0.76K/W

Ivanova et al. (2006) [63]

Silicon

Water

Rectangular grooves

50-70

100-130°C / 1K/W

Zhang et al. (2009) [64]

Copper

Water

Rectangular grooves

5-35

50-70°C / 33.3% / 0-90°

Chen et al. (2013) [65]

Aluminum Alloy

Acetone

Sintered aluminum powders and grooves

6.2-24.7

45-85°C / 10-45-70% / 0.72/0.69K/W

Moon et al. (2004) [59]

Copper

Water

Curved-side triangular and rectangular grooves

9.1/6.4

Wong et al. (2012) [71]

Copper and glass

Water

U-shaped grooves

Koito et al. (2006) [73]

Copper

Water

Sintered copper sheets and columns

Semenic et al. (2009) [78]

Copper

Water

Sintered copper powders

Weibel et al. (2010) [80]

Copper

Water

Sintered copper powders

Liou et al. (2010) [81]

Copper and glass

Water

Sintered copper meshes

Wong et al. (2012) [82]

Copper and glass

Water Methanol Acetone

Sintered copper meshes

Khanikar et al. (2009) [91]

copper

Water

CNT-coated rectangular grooves

75-105

Hashimoto et al. (2010) [92]

copper

Water

CNT-coated sintered copper powders

150-500


Cu/Mo/Cu

Water


437


Cu/Mo/Cu

Water

CNT-coated copper meshes

500

Kousalya et al. (2013) [100]

Cu/Mo/Cu

Water


457

2.1-4.3

0-5

17

Other Indicators 47-58°C / 0-66% / 600W/(m·K)

0-66% / 900W/(m·K)

54.6-78.3°C

261-789.2W/(m·K)

10-25

16-32

300/244-990 50-535 16-100

100/35/25

45-85°C / 30-45-60-90° 0-57-82% / 1.1-1.6K/W

95-100°C / 20% / -90-90°

0.1-0.15K/W

40-100°C / 0.266K/W

132-145W/(m·K)

0.2K/W 30-62°C

0.05/0.11/0.13K/W

0.28-0.45K/W



181

Table (2) contd….

Heat Flux (W/cm2)

Other Indicators

Authors

Material

Working Fluid

Wick Structure

Rulliere et al. (2007) [102]

Copper

Water Methanol N-pentane

Rectangular grooves

Lim et al. (2008) [103]

Copper

Water

Fan-shaped grooves

Thompson et al. (2011) [107]

Copper

Acetone

Square grooves

Oshman et al. (2012) [108]

LCP

Water

Woven copper meshes and square grooves

Furukawa Electric Co. [113]

copper

Water

Oshman et al. (2011) [109]

LCP

Water

Copper pillars coated with woven copper meshes

2.85-11.94

42-57°C / -90-90°/ 650830W/(m·K)

Oshman et al. (2013) [114]

PAVVF4W

Water

Copper meshes coated with AI2O3/SiO 2

0.8-4.8

0/45/90°/ 1.2-3.0K/W / 1520W/(m·K)

1.8-14.3 33

31-63

2.5-7.5

In 2013, Oshman et al. [114] fabricated a micro flat flexible heat pipe with a novel wicking structure consisting of three layers of 200 copper meshes coated with an atomic layer deposited AI2O3/SiO2. Its layout is shown in Fig. (20). Flexed at 0o, 45o and 90o, the device was tested under heat input ranging from 5 to 30W with water as the working fluid. It was found that the minimum thermal resistance of the micro flat flexible heat pipe was about 1.2K/W, approximately one-fourth that of a geometrically equivalent copper reference sample. In addition, the mass of the device was only 9.5g, about one-fifth that of the copper reference. 3.6. Performance Comparison In order to compare the performance of micro flat heat pipes depicted in the above sections, some performance indicators like the heat flux, as well as the wick structure, are listed in Table 2. The working fluid and the material are also presented in this table. Through comparisons, some important conclusions can be drawn: (i).

0.5-1.3

The micro flat heat pipe has been shown to be an effective device in various heat transfer applications.

(ii). The amount and the type of the working fluid strongly influence the thermal performance of a micro flat heat pipe. An extra large amount leads to condenser flooding, while too small evaporator dryout. There exists an optimal filling ratio regarding the best thermal performance of a micro flat heat pipe. (iii). Most of the micro flat heat pipes can be deemed to be effective devices only in comparatively high heat input or high working temperature. Under low heat input, the performance of some micro flat heat pipes is worse than that of copper. (iv). As for the micro-grooved heat pipes, their good performance is related to their geometric parameters, such as the corner apex angle and the roughness. It is found

60-100°C

40-120°C / 33.5% / -90-90° 84°C / 0-10 g / 729.7W/(m·K)

120°C / 0-10 g / 5411653W/(m·K) -90-90°/ 1K/W

that the performance of a micro heat pipe deteriorates with increase in the number of corners and increases with the cross-sectional area. The triangular micro flat heat pipe performs better than the rectangular one. (v). Compared with a grooved micro heat pipe, the sintered one has better temperature uniformity and can work well under higher heat flux. The cause is that the sintered powders or meshes have a bigger porosity factor and can provide greater capillary force. But it is harder and more expensive to fabricate a micro flat heat pipe with sintered wick structure. (vi). Augmented with CNTs, the traditional wick structures, such as grooves, sintered powders and meshes, have been shown to handle very high heat fluxes. This integrated wick alters the surface nucleation cavities and reduces the required superheat for incipience, thus obviously lowering the overall thermal resistance and enhancing the heat transfer coefficient. The density of CNTs and the thickness of coatings of the CNTs are also important issues. Both of them can be optimized for maximum heat transport. (vii). By using special material, it is feasible to fabricate a micro flat heat pipe to change its shape in response to some special requirements. This device has many advantages, such as high performance, flexible, lightweight, and inexpensive. The flexibility can obviously reduce the thermal contact resistance between the wall and the heater, as well as the wall and capillary wick. (viii).The influence of the inclination angle of a micro flat heat pipe cannot be neglected. When inclined positively, gravity can enhance the heat transfer capacity, and vice versa. The wick structure is a key element to fabricate an adverse-gravity micro flat heat pipe. 4. CURRENT & FUTURE DEVELOPMENTS Micro flat heat pipes are effective heat transport components utilizing the phase change of the working fluid in their


cavities. Besides the common characteristics of the micro heat pipes, such as compact structure, light weight, high thermal conductivity and constant temperature, a micro flat heat pipe has its own unique merits. It can work functionally with flexible heat fluxes and in more complex cases. Meanwhile, it also has good performance under adverse-gravity orientation. In view of these advantages, the micro flat heat pipes are being applied to many fields rapidly. In order to make better use of the micro flat heat pipes, researchers have carried out numerous investigations on various geometric designs of the devices, using different working fluids and different materials. However, there are still many challenges waiting for the researches and engineers to develop smaller, more effective and more reliable micro flat heat pipes. No matter what material is chosen, metal, polymer or silicon, the cost of fabrication is still very high. This is the bottleneck of the wide development and implementation of micro flat heat pipes. In addition, the heat transfer mechanism, especially the process of the phase change and the two-phase flow inside the micro flat heat pipe, is not understood thoroughly. Therefore, more effective mathematical models and cheaper fabrication methods need to be developed. Meanwhile, the application of novel materials, like nanofluids, functional surfaces and nanotubes, should be considered seriously too, which make it possible to produce simpler and more effective micro flat heat pipes. There is no doubt that the micro flat heat pipes will be widely used in microelectronic devices. CONFLICT OF INTEREST The authors confirm that this article content has no conflicts of interest. ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (Project 51176202).

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