The effect of auxiliary plate on passive heat dissipation of carbon

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The effect of auxiliary plate on passive heat dissipation of carbon nanotube-based materials Wei Yu, Zheng Duan, Guang Zhang, Changhong Liu, and Shoushan Fan Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04933 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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The effect of auxiliary plate on passive heat dissipation of

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carbon nanotube-based materials

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Wei Yu, a Zheng Duan, a Guang Zhang, b Changhong Liu*a and Shoushan Fana

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a.

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Tsinghua University, Beijing 100084, China

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b.

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Technology, China Academy of Space Technology, Beijing 100094, China

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Changhong Liu. Email: [email protected]

Tsinghua-Foxconn Nanotechnology Research Center and Department of Physics,

Energy Conversion Research Center, Qian Xuesen Laboratory of Space

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ACS Paragon Plus Environment

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ABSTRACT

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Carbon nanotubes (CNTs) and other related CNT-based materials with high

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thermal conductivity can be used as promising heat dissipation materials. Meanwhile,

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the miniaturization and high functionality of portable electronics, such as laptops and

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mobile phones, are achieved at the cost of overheating the high power-density

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components. The heat removal for hot spots occurring in a relatively narrow space

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requires simple and effective cooling methods. Here, an auxiliary passive cooling

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approach by the aid of a flat plate (aluminum-magnesium alloy) is investigated to

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accommodate heat dissipation in a narrow space. The cooling efficiency can be raised

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to 43.5%. The cooling performance of several CNT-based samples are compared

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under such circumstances. Heat dissipation analyses show that when there is a nearby

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plate for cooling assistance, the heat radiation is weakened and natural convection is

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largely improved. Thus, improving heat radiation by increasing emissivity without

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reducing natural convection can effectively enhance cooling performance. Moreover,

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the decoration of an auxiliary cooling plate with sprayed CNTs can further improve

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the cooling performance of the entire setup.

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Keywords: Carbon nanotubes, Passive heat dissipation, Thermal management,

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Auxiliary cooling

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With the development of electronic integration technology and microelectronic

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packaging technology, electronic components and electronic equipment have become

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increasingly miniaturized.1-3 The heat removal of the hot spots produced by these

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power-dense components, such as CPUs or backlit LEDs, has become a key problem

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since high temperatures will reduce the reliability and the lifetime of electronic

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devices.2, 4 Consequently, it is urgent to find effective cooling approaches. Meanwhile,

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portable electronics, such as laptops, mobile phones and so on, have become more and

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more compact, which means that the cooling process occurs in a relatively small and

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narrow environment within their enclosed shell. This requires that the cooling

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modules possess great heat dissipation performance as well as a simple structure and

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light weight. Therefore, active cooling devices, such as thermoelectric cooling

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devices5, 6 and forced convective cooling pin fins,7, 8 are restricted because of their

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additional accessories. At present, the effective cooling module is the bottleneck in the

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development of electronic equipment and attracts wide concern from researchers.

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There have been many studies on improving the passive heat dissipation of objects,

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but cooling properties in enclosed or narrow spaces are rarely reported. For example,

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it has been reported that micron fins can improve heat dissipation,9, 10 and the effects

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of the height and the spacing of micron fins on cooling performance were also

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studied.11 Lee et al. demonstrated that an anodic oxide layer can enhance the heat

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dissipation property of aluminum by increasing the emissivity.12 In addition, carbon

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nanotubes (CNTs) have high thermal conductivities13-16 and a low density compared

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with normal metals (copper and aluminum)17,

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. It is believed that CNT-based

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materials will have great applications in heat management. Jiang et al. found that

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carbon nanotube sheets have a high natural convection coefficient of approximately

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69 Wm-2K-1, which is twice that of aluminum foils.19 Multi-layered carbon nanotube

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films also have high heat dissipation due to large emissivity, with a cooling efficiency

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of 12.5% at a heat power of 2.4 W.20 The cooling efficiency of a CNT film-covered

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microchannel surface can reach as high as 25%.21 Furthermore, the cooling

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performance of a well-dispersed CNT suspension used as a molecular cooling fan was

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well studied by the Lin group.22, 23 However, the heat dissipation properties described

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above were measured in open space. There are few reports of cooling performance in

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a narrow closed space.

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In this work, the passive heat dissipation property was measured in a relatively

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narrow environment, and an auxiliary passive cooling method was proposed. Focus

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has always been on the heat transfer between two objects at close distances, such as

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the near-field radiative heat transfer at a sub-micrometer distance,24,

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convection heat transfer of two parallel vertical plates. This is a frequently appearing

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configuration in the air-cooling fins of mainframe computers and other electrical

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equipment, which has been studied.26-28 However, the plates considered in most of

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these studies are the same size, and little research has been done on the situation of

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millimeters apart. Here, by the aid of a nearby flat plate of aluminum-magnesium

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alloy, the cooling performance of two different sizes of plates which are millimeters

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apart was studied, and a new phenomenon was found. The heat dissipation of

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electrical heaters could be greatly enhanced. The results show that the cooling 4


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as in the

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efficiency can reach as high as 43.5%. Meanwhile, the cooling properties of several

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samples, i.e., the heaters decorated with CNT arrays, with monolayer CNT film, with

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etched CNT arrays and with sprayed CNT film, were studied with and without

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auxiliary cooling plates. Heat dissipation analyses indicate that the heat radiation is

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weakened, but the convection is effectively enhanced with the auxiliary cooling plate.

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The results here show that increasing heat radiation without reducing convection is

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the key to improve the total heat dissipation performance. Consequently, the sprayed

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CNT sample had the greatest heat dissipation enhancement of 65.9% because of its

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high emissivity and special surface structure. Moreover, decoration of the auxiliary

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cooling plate with sprayed CNTs can further increase the heat dissipation. Since

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laptops, mobile phones and other portable electronics mostly use metals, such as an

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aluminum-magnesium alloy, as an outer casing, we believe that the shells can be more

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effectively used as an auxiliary cooling plate for passive heat dissipation.

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Preparation of CNT-decorated chip samples

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(SACNTs) have attracted much attention because of their high thermal conductivity.13,

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(CVD).18, 29-31 Specifically, CNT arrays were grown on silicon wafers with acetylene

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as a precursor under 650−700°C. Most CNTs were multiwalled CNTs with a radius of

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approximately 10-20 nm. In this work, several CNT-based heat dissipation samples

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were prepared, where red copper plates (20×20×1.5 mm3) were used as their

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substrates. The copper plates were immersed in dilute hydrochloric acid to remove the

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oxide layer and rinsed with deionized water in advance.

Superaligned carbon nanotubes

SACNTs used in our experiment were synthesized by chemical vapor deposition

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Cu chip with CNT arrays

It has been reported that the CNT arrays were

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transferred from the silicon substrate to the target substrate by using ice as a binder.32

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Inspired by this, we prepared Cu chips covered with CNT arrays labeled Sample

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Arrays which used the silver paste to transfer the CNT arrays directly to the Cu

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substrate. First, the silver paste was evenly applied on the copper surface. Then, the

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CNT array was stuck to the copper by this thin layer of silver paste. It became tighter

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by heating at 100°C for 30 min. Then, silicon substrate can be easily peeled off.

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Finally, we succeeded in transferring CNT arrays onto the copper substrate. Compared

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with the ice-assisted CNT transfer, the silver paste with a high thermal conductivity

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can greatly reduce the interfacial thermal resistance between the CNTs and the copper

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plate to enhance the heat dissipation of the samples.

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Cu chip with etched CNT arrays

In order to improve the heat dissipation

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performance of the Sample Arrays, we used a laser to etch some microchannels and

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labeled it as Sample Patterned since these micron fins can increase natural

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convection9,

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approximately 250 µm and 1000 µm.

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Cu with monolayer CNT film

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. The microchannel width and the interval of the sample are

Because of the strong van der Waals force

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between carbon nanotubes, the SACNT arrays can be pulled into monolayer CNT

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films using an electric motor at a uniform speed.18, 30, 31 All the individual CNTs in the

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CNT film aligned in almost the same direction, and it has high thermal conductivity in

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this direction.33, 34 In this work, the Cu chip coated with a layer of SACNT film

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denoted as Sample Monolayer was adopted to explore the effect of the monolayer 6


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SACNT film on heat dissipation. The sample was prepared with following methods.

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After the SACNT film had been drawn, the copper chip on the lift table was placed

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under the SACNT film. Then, we adjusted the height of the lift table until the copper

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chip contacted the SACNT film. Afterwards, the extra SACNT film was cut off with

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the laser. The energy and wavelength of the cutting laser are approximately 100 W

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and 1070 nm, respectively. Finally, alcohol was sprayed on the SACNT film to create

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better cohesion between the SACNT film and copper after the alcohol evaporated.

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Cu with sprayed CNT film

In this work, the heat dissipation performance of

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the Cu chip with a sprayed CNT film denoted as Sample Sprayed was also studied.

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The CNT slurry was prepared by dispersing SACNTs in solution. Then, the slurry was

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sprayed evenly on the copper plate with a spray gun. The thickness of the coating

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layer was approximately 10 µm in this experiment. In addition, the auxiliary cooling

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plate was also sprayed in the same way in the control experiment.

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The principle of heat dissipation

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from materials to surroundings, i.e., radiation, convection and conduction. The total

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heat dissipation coefficient, Htotal (W m-2 K-1), can be expressed by the equation:

There are three methods of heat dissipation

= / −

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(1)

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where Th and Ta are the temperatures (K) of the samples and ambient, respectively. A

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is the surface area (m2) of the sample. Pin is input power, and it is equal to total heat

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dissipation power P when thermal equilibrium is established in the system. The

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principle of heat dissipation is discussed below in situations with and without an

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auxiliary cooling plate. 7


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The thermal conductivity of air is very small (kair ≈ 0.026 W m-1 K-1 at room

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temperature).20, 21, 35 In the absence of auxiliary cooling plate, the conductive heat

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dissipation with air can be ascribed to natural convection. Consequently, heat is

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dissipated mainly by radiation and natural convection. That is, = +

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(2)

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where Pr and Pc are thermal radiation power (W) and natural convection power (W),

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respectively.

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Stefan−Boltzmann equation,

Thermal

radiative

properties

can

be

determined

by

the

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= −

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where σ and ε are the Stefan-Boltzmann constant (5.67×10-8 W m-2 K-4) and surface

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emissivity, respectively.

(3)

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The heat dissipation principle of the system with an auxiliary cooling plate is

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slightly different from the above. First, the conductive heat transfer between the

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samples and the plate can be determined. Since the distance between the samples and

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the plate is very close (0~15 mm), the heat conduction between the two cannot be

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negligible. Therefore, the heat conduction power of the entire system, Pd, can be

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described as

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=

!"#$% &

(4)

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where Th, Tplate are the temperatures of the samples and the plate, respectively. d is the

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distance between the samples and the plate. The thermal radiation between the

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samples and the plate can be simplified as a problem of heat radiation between two

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planes, and the thermal radiative power (W), Ps-p, can be calculated by the equation, 36 8


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* ) * !)"#

%$'!( = +,-+

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+ +,-2 / / .+ -+ .+ 0+,2 .2-2

(5)

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where ε1 and A1 are the surface emissivity and upper surface area of the sample,

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respectively. ε2 and A2 are emissivity and area of downward surface of the plate,

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respectively. X1,2 is the radiation view factor. Obviously, when A2 is much larger than

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A1 and X1,2 is almost equal to 1, equation (5) can be simplified as '!( = 3 3 − (4 &

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(6)

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ε2 has little effect on heat radiation in equation (6). In addition, the heat radiation

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power between the plate and the environment, Pp-air, may be determined by the

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equation

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(! = 5 5 −

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It is difficult to calculate Pp-air by this equation because the plate is not isothermal.

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However, a qualitative conclusion can be easily reached according equation (7), that

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is, Pp-air increases with increasing ε2. Normally, the metal surface including the bare

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Al plate has a low emissivity (~0.1).20, 37 Finally, the total heat dissipation power

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(W) of the sample can be described as

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= + + = + 3 3 − (4 &+

(7)

!"#$% &

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9


(8)

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Fig. 1. Schematic diagram of the thermal dissipation measurement setup without (a) and with (b)

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an auxiliary cooling plate.

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As is shown in Fig. 1a, the thermal

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The thermal dissipation measurement setup

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dissipation measurement setup reported in our previous works20 contains a ceramic

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heater (20×20×1.5 mm3), thermal insulators and several different temperature

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monitors. Specifically, the commercial heater was a painted electrical resistor

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encapsulated in the alumina ceramics. Its maximum heating power is approximately

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200 W with the highest temperature of 900 K. In this work, the power of the heater

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was controlled by a KEITHLEY 2410 electric source meter. The heat insulator

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consists of carbon fiber felt (80×80×3 mm3, k ≈ 0.06 W m-1 K-1 at room temperature)

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and a wood block (80×80×20 mm3) joined together by heat-resistant tape. The tested

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samples were combined with the heater using thermal grease in order to reduce the

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interfacial thermal resistance and make almost all of the heat flow upwards.

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Moreover, there is a k-type thermocouple inlaid in the upper side of the heater to

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measure the heater temperatures. Another thermocouple was attached on the upper

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surface of the samples by high-purity silver paste. This thermocouple, combined with 10


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an infrared (IR) thermometer, can determine the emissivity of the samples.20,

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Specifically, the IR thermometer was used to detect surface temperatures of the

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samples in real time. By adjusting the emissivity setting of the IR thermometer until

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the IR thermometer reading was the same as the thermocouple, the emissivity setting

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at that time was the surface emissivity of the samples. The temperature range and

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spectral range of the IR thermometer (OPTRIS LS) are 240-1170 K and 8–14 µm,

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respectively. In addition, the real-time temperatures measured by the k-type

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thermocouple can be recorded by an 8-channel data collector (ART DAM 3039F)

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with a precision of 0.1 K connected to the computer by an RS-485 bus.

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The entire thermal device described above was placed on a z-axis lift table with

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accuracy of 0.01 mm. The auxiliary cooling plate was placed over it by supporting it

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on two wood blocks (200×100×20 mm3), as shown in Fig. 1b. Specifically, the tested

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auxiliary

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(96%Al-2.2~2.8%Mg-0.4%Fe-0.25%Si) with dimensions of 200×200×2 mm3. This

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kind of aluminum plate was selected in our experiment because most electronics

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currently use it as their shells. Additionally, the thermocouple and IR thermometer

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were applied to measure the emissivity of the plate. To avoid the effects of airflow, the

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entire setup was placed in an airtight environment.

cooling

plate

was

a

plate

of

aluminum-magnesium

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alloy

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Fig. 2. The increasing temperature curves of the bare Cu chip with and without an auxiliary

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cooling plate at the power of 2 W.

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In our experiment, the heat

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The influence of auxiliary plate on heat dissipation

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dissipation properties of copper with and without an auxiliary cooling plate were both

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measured. The temperature versus time curves are shown in Fig. 2. The distance d

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between the plate and the copper chip was 0.5 mm, and the heating power was

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approximately 2 W. To ensure that the entire system reached thermal equilibrium, the

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heating process is maintained for approximately 40 minutes. The equilibrium

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temperature of the heater with the auxiliary plate is significantly lower than that

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without the plate, which are 82.0°C and 126.7°C, respectively. This indicates that the

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heat dissipation properties of the entire device have been greatly improved with the

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aid of the plate. To quantitatively describe the effect of the auxiliary plate, the cooling

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efficiency η is introduced, which is defined as η= (∆T/∆Ta) × 100%,

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the difference between the equilibrium temperatures with and without the plate. ∆Ta is

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the temperature difference between the ambient and bare Cu without the plate. The

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calculated cooling efficiency is 43.5%. It is significantly larger than that of the 12


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where ∆Τ is

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reported level for the disordered SWCNT composite39 and multilayer CNT films20,

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which are approximately 5.4% and 12.5%, respectively. Therefore, the auxiliary

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cooling plate makes a leap of the heat dissipation property of the system. Laptops and

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other electronics currently on the market mostly use aluminum alloy casings to protect

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their bodies. Therefore, the metal shell may have a great influence on the heat

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dissipation of devices inside.

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Fig. 3. (a) The cooling efficiencies and (b) the percentages of heat radiative power, natural

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convective power and heat conductive power of bare Cu with an auxiliary cooling plate at

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different distances between the bare Cu and Al plate.

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The effect of the distance 7 on heat dissipation

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auxiliary plate had a significant effect on heat dissipation, the relationship between

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heat dissipation and d (distance between the bare Cu and Al plate) at the range of

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0~15 mm was explored. The changes of cooling efficiency with d are shown in Fig.

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3a. As d increases, the corresponding cooling efficiency decreases, and the cooling

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efficiency at d of 3 mm is almost the same as the values without an auxiliary plate. In

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other words, when d is greater than 3 mm, the plate does not have positive effect on

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the heat removal, and the cooling efficiency continues to decline until d increases to 6 13


After determining that the

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mm to reach the minimum. After that, the cooling efficiency gradually approaches

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those without an auxiliary plate. This phenomenon provides a guide for the

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installation of shells in electronic devices such as laptops and mobile phones. That is,

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when the distance between the shell and the heat source such as a CPU is in a certain

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range, the metal shell can play a cooling effect; otherwise, it will have no cooling

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effect or can even hinder heat dissipation. Furthermore, we compared the cases of

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CNT buckypaper in contact and not in contact with the auxiliary plate and found that

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an auxiliary plate with a small d has a great effect on heat dissipation. Detailed results

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are shown in the Supporting Information.

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Furthermore, three methods of heat dissipation power can be calculated by

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equations (4), (6) and (8). Then, their proportions, Pr/P, Pd/P and Pc/P, can be easily

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determined, which are shown in Fig. 3b. The distance has little effect on radiative heat

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dissipation, and conductive heat dissipation decreases with increasing d, which is

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consistent with equation (5). More importantly, natural convection plays a dominant

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role in the heat dissipation.

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The original phenomenon described above may be explained by the following. In

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open space, for a bare Cu plate, natural convection is the main way to dissipate heat

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because heat radiation and thermal conduction can be ignored due to the low

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emissivity (ε～0.02) and low thermal conductivity of air, respectively. However,

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when there is an auxiliary plate, heat dissipated by thermal conduction is inversely

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proportional to distance d. Therefore, it cannot be neglected with a small d, and we

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did find that the temperature of auxiliary plate, TAl, has increased, as shown in the 14


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Supporting Information Fig. S2a. This weakens natural convection, according to

2

equation S1, because the temperature difference between the auxiliary plate and heat

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source is reduced. Nevertheless, heat dissipation by conduction is far greater than the

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decrease in convection, so the cooling effect is significantly improved compared with

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those in open space. As d gets larger, the cooling effect is reduced because of the

6

decrease of thermal conduction, and the cooling efficiency might be negative when

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the heat dissipated by conduction is smaller than the reduction in convection. When d

8

continues to increase, convection improves due to the reduction of TAl, and the total

9

cooling performance improves again. It can be easily concluded that η with a large

10

enough distance is close to the value in open space. That is why η decreases first and

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then increases with increasing d and has a minimum value, ηmin, just as shown in Fig.

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3a. In general, the change of heat dissipation with an auxiliary plate is caused by the

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competition of air conduction and convection. In addition, the calculated trend of heat

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cooling performance is consistent with Fig. 3a. The detailed calculation is in the

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Supporting Information.

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Fig. 4. (a) The increasing temperature curves of bare Cu and Sample Arrays in the cases with (d

18

=1 mm) and without an auxiliary cooling plate. (b) The total heat dissipation coefficients of bare

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Cu and three other samples in the cases with (d =1 mm) and without an auxiliary cooling plate. (c) 15


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The radiative and convective heat dissipation powers of Sample Patterned and Sprayed in the

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cases with and without an auxiliary cooling plate.

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Different heat dissipation in open space and enclosed space

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between the plate and the samples is very small, the heat removal occurs in the

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relatively narrow space. To compare the sample difference in cooling properties in an

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open space and a narrow space, the equilibrium temperatures of bare Cu and Sample

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Arrays were measured in the cases with and without auxiliary plates. The increasing

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temperature curves are shown in Fig. 4a. In the absence of the auxiliary cooling plate,

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the cooling effect of Sample Arrays is obviously better than bare Cu, and the

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corresponding equilibrium temperatures are 119.2°C and 130.3°C, respectively. This

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phenomenon is mainly due to the CNT arrays having higher radiative heat dissipation

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with the higher emissivity of 0.95. When there is an auxiliary plate at the distance of 1

13

mm, the equilibrium temperatures of Sample Arrays and bare Cu were 102.0°C,

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100.0°C, respectively. It means that the heat dissipation properties of the two are

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almost the same or even the heat dissipation performance of Sample Arrays is slightly

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worse than bare Cu. This is different from the phenomenon without an auxiliary plate,

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which may indicate that there is something new worth studying in the relatively

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enclosed space compared with those in an open space.

Since the distance

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Furthermore, the cooling performance of Sample Monolayer, Sample Patterned

20

and Sample Sprayed were also measured and are shown in Fig. 4b. The cooling effect

21

of all samples with the help of the auxiliary plate has greatly improved. The total heat

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dissipation for Sample Monolayer with an auxiliary plate or not is slightly higher than 16


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that of the corresponding bare Cu because the monolayer CNT film with a very high

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transparency17, 40 has little effect on radiative heat dissipation. However, convective

3

heat dissipation is slightly improved, which is consistent with the previous work. 20

4

Comparing Sample patterned with Sample sprayed in Fig. 4b, Sample Patterned

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has a better cooling performance in open space, but the total heat dissipation

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coefficient (Htotal) of the Sample Sprayed is larger in the narrow space. To explain this

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phenomenon, the cooling powers by the three methods of heat dissipation were

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calculated and are shown in Fig. 4c. The conductive powers of two samples with the

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auxiliary plate is almost the same, which is consistent with equation (5). It is worth

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noting that the conductive power is included in the convective power for an easy

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comparison with the samples without the auxiliary plate. The radiative cooling part of

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the two samples is weakened, but the convection heat dissipation is enhanced

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effectively in the narrow space. More importantly, the enhancement of convection is

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much larger than the reduction of heat radiation, so the total cooling property can be

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improved with the help of the auxiliary cooling plate. Furthermore, although Sample

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Patterned has a higher emissivity (εpatterned=0.9) than Sample Sprayed (εsprayed=0.85),

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and Sample Sprayed has a much higher natural convective power than Sample

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Patterned. Therefore, Sample Sprayed has a greater cooling performance, and the

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enhancement of its total heat dissipation coefficient is approximately 65.9%. It can be

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concluded that it is key to improve the heat dissipation performance by effectively

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improving the emissivity of the samples without weakening convection.

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The different convective cooling performance of Sample Patterned and Sample 17


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Sprayed can be explained in terms of air molecule motion. For Sample Arrays, the

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closely packed SACNT arrays have an average diameter of 15 nm and a height of 0.3

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mm, which are shown in Fig. S3a. This kind of geometry may trap the ambient gases

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and prevent heat exchange easily with the arrays because of the spatial confinement

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effect,41, 42 as shown in Fig. S3d. However, from Fig. S3c, Sample Sprayed with a thin

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layer (10 µm) of a fluffy CNT film does not have this kind of spatial confinement and

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has a better convective performance.

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Fig. 5. The increasing temperature curves of bare Cu with a bare auxiliary plate and an auxiliary

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plate sprayed with a CNT film at d=1 mm. Inset: photograph of Al plate before (left) and after (right)

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spraying with the CNT film.

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The cooling performance of auxiliary plate sprayed with CNT film

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auxiliary cooling plate was further sprayed with the CNT slurry to improve the heat

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dissipation. The emissivity of auxiliary plate with CNT film can reach as high as 0.85.

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As shown in Fig. 5, when the upper surface of the auxiliary plate is sprayed with the

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CNT film, the equilibrium temperature of Cu is 97°C, which is lower than that with

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the bare Al plate (100°C). This further improvement can be reasonably ascribed to the

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enhancement of radiation heat dissipation of the plate. From that, we can infer that 18


The

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both the Cu chip and auxiliary plate sprayed with the CNT film can exhibit superior

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heat dissipation performance.

3

In summary, we have investigated the heat dissipation properties of several

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CNT-based materials in the narrow closed space, which is confined using an auxiliary

5

plate. Our results indicate that these CNT materials have a very different heat

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dissipation performance than those in the open space. We find that the auxiliary plate

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can play diverse roles in heat dissipation. The auxiliary cooling plate can get the

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cooling efficiency of bare Cu as high as 43.5%. Heat dissipation analyses indicate that

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heat dissipates mainly by natural convection. The auxiliary cooling plate weakens the

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radiative power but improves the natural convection part of the CNT-decorated chip.

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Consequently, it can be concluded that the heat dissipation performance can be

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improved by increasing the heat radiation without weakening the convection. The

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sprayed CNT film shows an outstanding cooling performance in this study owing to

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its special surface structure and high emissivity of 0.85. Moreover, the auxiliary plate

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sprayed by a CNT film can further improve the cooling property of the system. This

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work suggests that auxiliary passive cooling has a high value for research and

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applications.

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Supporting Information

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1. Comparison of buckypaper in contact and not in contact with the auxiliary plate

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2. An analysis of different cooling performances at varied d

19


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3. Figures for the explanation of the spatial confinement effect

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3

Acknowledgements

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This work was supported by the Natural Science Foundation of China (51572146,

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51702357).

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Corresponding Author

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*E-mail: [email protected]

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Notes

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The authors declare no competing financial interests.

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References

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The effect of auxiliary plate on passive heat dissipation of carbon

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