NANOTECHNOLOGY MATERIALS FOR HEAT SINK

Nathan Otundo Onsare

1* Department of Electrical and Computer Engineering, IUPUI, 420 University Blvd. Indianapolis, IN

Email: nonsare@iupui.edu

ABSTRACT

The power density of electronic packages has substantially increased. The thermal interface resistance involves

more than 50% of the total thermal resistance in current high-power packages. The portion of the thermal budget

spent on interface resistance is growing because die-level power dissipation densities are projected to exceed 100

W/cm2 in near future. There is an urgent need for advanced thermal interface materials (TIMs) that would achieve

order-of-magnitude improvement in performance.

Carbon nanotubes have received significant attention in the past because of its small diameter and high thermal

conductivity. The present study is intended to overcome the shortcomings of commercially used thermal interface

materials by introducing a compliant material which would conform to the mating surfaces and operate at higher

temperatures.

Keywords: Thermal Interface Materials (TIM), Nanotechnology.

1 INTRODUCTION

Thermal management is an important design consideration for a number of engineered devices,

components and packages. The need for integrating complex functions in a single circuit,

together with a demand for thinner, light weight, and most efficient products, which have been

massively growing. High power and faster speeds are the major requirements of the global

industry that results in excessive heating of the systems. The modern electronic systems are

characterized by increased density of circuits in that circuits become denser, and the high power

is required is generated throughout the system that should be dissipated. Rising amount of power

has to be offset by efficient cooling in order for a system to be efficient. If the heat is not

dissipated, the lifetime and reliability of the electronics will be at great risk. This is a problem

that requires external thermal solutions, including heat sinks, fans, heat exchanger etc. A major

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challenge in the field is, therefore, the ability to manage the heat without compromising on the

performance of the system.

2. Basics of CNTs

Carbon nanotubes (CNTs) are honeycomb-like (i.e., hexagonally shaped) arrangements of

carbon atoms that are rolled into cylindrical tubes with diameters as small as a few atoms wide

and aspect ratios as high as 105.

Figure 1: Three possible carbon nanotube atomic structures

CNTs can be produced from a wide variety of processes, such as arc-discharge, laser ablation

and chemical vapor deposition (CVD) methods (Figure 2).

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Figure 2

However, for device applications, growth of CNTs by CVD methods is particularly attractive,

due to features such as selective spatial growth, large area deposition capabilities, and aligned

CNT growth.

Improved Properties

Such cylindrical graphitic polymeric structures have novel or improved properties that make

them potentially useful in a wide variety of applications in electronics, optics and other fields of

materials science. Carbon nanotubes are endowed with exceptionally high material properties,

very close to their theoretical limits, such as electrical and thermal conductivity, strength,

stiffness, and toughness.

There are three properties of CNTs that are specifically interesting for the industry: the electrical

conductivity, mechanical strength, and their thermal conductivity. A combination of these

properties enables a whole new variety of useful and beneficial applications.

CNT ARRAY TIMS

The three CNTs in Figure 3 have some of the most promising thermal performance

characteristics. The first is the one-sided interface structure discussed above. The second is the

two-sided configuration, which consists of CNT arrays adhered to surfaces on both sides of the

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interface and brought together mechanically using van der Waals. The third structure comprises

vertically oriented CNT arrays directly and simultaneously synthesized on both sides of thin foil

substrates that are inserted into an interface. The CNT-coated foil structures serve as a method

for applying CNT arrays to interfaces between heat sinks and devices that would experience

damage from exposure to the high temperatures normally required for high-quality CNT growth

(above 700°C).

Chemical Vapor Deposition (CVD) processes that are common in the electronics industry

enables the growth of various substrates such as silicon, silicon carbide, copper, and aluminum

that are important for thermal management applications [8] on CNT arrays interfaces as shown in

Figure 3.

Figure 3. (a) One-sided interface; (b) Two-sided interface, (c) example of CNT-

coated foil interface, (d) CNT arrays on both sides of 25 µm-thick Al foil.

Advanced TIMs utilize extraordinarily high axial thermal conductivity of CNTs in which

theoretical predictions suggest values as high as 3000 W/m-K [3] and 6600 W/m-K [4] for

individual multiwalled CNTs and single-wall CNTs, respectively.

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HEAT TRANSFER THROUGH CNTs

Figure 4

One-sided CNTs are most active and they are grown directly on one substrate with CNT free

ends in contact with an opposing substrate.

Several CNT contacts at both substrates form parallel heat flow paths within the framework of

the thermal network for heat flow. This network shows thermal resistances resolved at the

individual nanotube level for true CNT-substrate interfaces, both at the growth substrate and at

the opposing interface.

Means of Simulations

1. Molecular dynamics simulations

The approach is used to get the thermal conductivity λ, with a periodic array of hot and cold

regions along the nanotube achieved by velocity. Nanotubes exhibit high degree of long range

order over hundreds of nanometers. The uneasiness imposed by the heat transfer reduce the

effective phonon mean free path to below the unit cell size where it is hard to achieve

convergence, since the phonon mean free path in nanotubes is significantly larger than unit cell

sizes tractable in molecular dynamics simulations.

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To determine the thermal conductivity, equilibrium molecular dynamics simulations is used

based on the Green-Kubo expression that relates this quantity to the integral over time t of the

heat flux autocorrelation function by;

Outcomes of simulations

In Figure 5 the results of in-equilibrium molecular dynamics simulations for the thermal

conductance of a nanotube aligned along the z axis. Results suggested that Jz(t) converges within

the first few picoseconds to its limiting value in the temperatures range below 400 K.

Figure 5

In Figure 6, the average of which is proportional to the thermal conductivity λ. Molecular

dynamics simulations are performed for a total time length of 25 ps to represent well the long-

time behavior.

Results for the temperature dependence of the thermal conductivity of a carbon nanotube, shown

in Fig. 6, show the fact that is proportional to the heat capacity C and the phonon mean free path.

Calculations suggest that at T = 100 K, carbon nanotubes show an unusually high thermal

conductivity value of 37,000 W/m-K.

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Figure 6.

Results of calculations, shown in Figure 7, suggest that the nanotube shows a very similar

thermal transport behavior as a hypothetical isolated graphene mono-layer, whereas even larger

thermal conductivity should be expected for a monolayer than for a nanotube.

Figure 7.

Combined results of equilibrium and in-equilibrium molecular dynamics simulations with

accurate carbon potentials determine the thermal conductivity λ of carbon nanotubes and its

dependence on temperature. Results show high value λ≈6,600 W/m-K for a nanotube at room

temperature, compared to the thermal conductivity of a hypothetical graphene monolayer or

graphite. These high values of λ are associated with the large phonon mean free paths in the

systems.

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Figure 8

Other Materials

Aluminum

It has a thermal conductivity of 226W/m-K. The production of aluminum heat sinks is

inexpensive; they can be made using extrusion because of its softness, aluminum can also be

milled quickly; die-casting and even cold forging are also possible. Aluminum is also very light

thus, an aluminum heat sink will put less stress on its mounting when the unit is moved around.

Copper

Thermal conductivity is about twice as high as aluminum - about 402W/m-K. This makes it a

good material for heat-sinks. Its disadvantages include high weight, high price, and less choice as

far as production methods are concerned. Copper heat-sinks can be milled, die-cast, or made of

copper plates bonded together; extrusion is not possible.

To combine the advantages of aluminum and copper, heat-sinks can be made of aluminum and

copper bonded together. Here, the area in contact with the heat source is made of copper, which

helps lead the heat away to the outer parts of the heat-sink. If the thermal transfer between the

copper and the aluminum is poor, the copper embedding may do more harm than good.

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Silver

Has an even higher thermal conductivity than copper, but only by about 10%. This does not

justify the much higher price for heat-sink production - however, pulverized silver is a common

ingredient in high-end thermal compounds.

CONCLUSION

Thermal interface materials are important for efficient removal of heat in electronics packaging

applications. Improvement in TIM performance is dependent on developing materials that have

both high thermal conductivity and high compliance. The introduction of CNTs into thermal

interface materials has the potential for improving the bulk thermal conductivity. The

performance change can be attributed to the increase in thickness due to difficulty in spreading

the mixture.

CNTs possess excellent electrical, thermal and mechanical properties. They are proposed as

candidate for a lot of applications in electronics. This paper reviewed the state of the art of three

particular applications: (i) CNT as TIMs, (ii) CNT as interconnect material. The main challenge

to use CNTs in TIMs is the large contact resistance at the CNT ends.

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As for the interconnect application, CNTs have been proved to have better thermal stability over

traditional metals. However the obstacles are the non-ideal crystal structure of CNTs resulting

from current CVD technologies, which lead to relatively high resistivity of such, interconnects.

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References

1. Cola, B.A., “Photoacoustic Characterization and Optimization of Carbon Nanotube Array

Thermal Interfaces,” Ph.D. Dissertation, Purdue University, West Lafayette, IN, 2008.

2. Panzer, M., Zhang, G., Mann, D., Hu, X., Pop, E., Dai, H., Goodson, K.E., “Thermal

Properties of Metal-Coated Vertically Aligned Single-Wall Nanotube Arrays,” ASME

Journal of Heat Transfer, Vol. 130, 2008, p. 052401.

3. . Xu, Y., Zhang, Y., Suhir, E., Wang, X., “Thermal Properties of Carbon Nanotube Array

Used for Integrated Circuit Cooling,” Journal of Applied Physics, Vol. 100, 2006, p.

074302.

4. Berber, S., Kwon, Y.K., Tomanek, D., “Unusually High Thermal Conductivity of

Carbon Nanotubes,” Physical Review Letters, Vol. 84, 2000, pp. 4613-4617.

5. Biercuk, M.J., Llaguno, M.C., Radosavljevic, M., Hyun, J.K., Johnson, A.T., Fischer,

J.E., “Carbon Nanotube Composites for Thermal Management,” Applied Physics Letters,

Vol. 80, 2002, pp. 2767-2769

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