High-Performance Carbon Nanotubes Thermal Interface Materials Jamaliah Idris, Tan Win Hon Faculty of Mechanical Engineering, Universiti Teknologi Malaysia Abstract This paper discusses CNT s applications in thermal management. High-performance heat dissipation pastes for thermal interface applications were developed by dispersing CNT powder into a silicone oil that widely used in thermal interface material, Thermal conductivities of the resulting samples were measured. The effects of loading wt% of CNT and size and geometric on the thermal conductivity and performance of paste were also investigated. The results showed that the thermal conductivity of the silicone oil enhanced to.6 W/mK from pure silicone oil with value.65 W/mK. As the loading of CNT increases to 6 wt% the thermal conductivity increase to.237 W/mK. The geometric that long cylindrical in shape improve the ability of CNT to enhance the thermal conductivity properties but will cause losing the conformability of paste. Keywords Carbon Nanotube (CNT), Thermal Interface Materials (TIM), thermal contact resistance, thermal management I. INTRODUCTION The large increased miniaturization of the system now a day, more complex and smaller component in packing results in high density of heat generated. Increase of service temperature will short the service life of an electronic device. So to control the temperature of electronics by design efficient ways of dissipate their generated heat is important. The TIM was applied between heat sink and heat source to provide a better path for heat transfer. The conman materials that use for thermal management have thermal paste, phase changes materials, soft metal foils, elastomer paste and adhesives. When two difference solid surfaces are placed in contact, a large thermal resistance to the heat flow will present at the interface which as thermal contact resistance. The sharp fall of temperature at the interface due to no real surface is perfectly smooth. In reality, when two surface mate together only small amount area will contact each other, and the gap will trap with air where air is the poor heat conductor. [], TIM was used to replace the air to improve heat dissipation. For low bond line application, thermal conductive grease considered as one of the best performances TIM. Normally thermal grease will be added with high loading of high thermal conductivity filler example silver, copper, aluminium, ceramic and zinc oxide. But high fraction of filler will cause higher bond line thickness and lower the TIM performance. Performance of TIM normally affected by three-factor bond line thickness, conformability and thermal conductivity of paste.[2] But in the practical case increase, the thermal conductivity with increase loading will cause high bond line and losing conformability. CNT that has superior thermal properties [3] use as filler of TIM may improve both thermal conductivity and lower the bond line thickness. Compare to other particle spheres in shape, CNT particle with nanosize and. long cylindrical shape that will an easy form path for heat transfer. Many studies have proposed the potential CNT to enhance the thermal conductivity of polymer. Biercuk et al.[] experiment shown the thermal conductivity increase doubles with only % of SWCNT loading. Hu, Jiang, and Goodson tested to improve Ni based TIM with 2.2% volumetric loading of CNT. [5], Xu and Fisher experiment on grew arrays of TIM on silicon to improve conductivity.[6] Techniques have been developed to produce nanotubes in sizeable quantities, including arc discharge, laser ablation, chemical vapor deposition, silane solution method and flame synthesis method.[7] With these methods discover to make CNT possible widely use in application. II. METHODOLOGY A. Sample preparation Silicone oil was chosen as the matrix because of it good wetting [8] and widely use in electronics thermal management applications. The silicone oil (DOW CORNING 2 FLUID) was weighed by the precision electronic balance. After that, a small amount of CNT powder was added to the paste in accordance with the following weight percentage formula: A set of TIMs were created with CNT mixed into a silicone oil with cst viscosity. The CNT powder 2%, 2% and 6% by weight were mixed with silicone oil. Then mechanically mixed for about 3 min and followed by an ultrasonic bath at 6ºC for 2 hour to obtain carbon nanotube dispersion. The ultrasonic bath process also helps in degassing, the micro bauble during mechanically mix will cause poor wetting between silicone oil and CNT. The time for sonication should not be too long to avoid a break CNT in shorter tube length.[9] The long cylindrical shape of CNT will form the path to conduct heat. B. Testing The TIM was tested according to ASTM D57 6 (Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials). [], A steady-state method known as the guarded hot plate method was used to measure the thermal contact conductance of thermal contacts who utilized various thermal pastes at the thermal interface. Various thermal pastes were sandwiched between the 25mm diameter
surfaces of two aluminium blocks. The surface for meter measured was average.29 micron. Each aluminium block had a height of 2 mm. Two. 5mm diameter small holes were drilled to about 2.5mm depth to place the thermal couple. Small holes were measured 5mm from both surfaces. The two surface contacts with TIM were mechanically polishing. The thickness is control by using three difference diameter copper wires. The testing procedure is as explained following. Turn on the heating and cooling units and let stabilize at the specified set points to give an average sample temperature of 5 C. The temperatures of the meter bars are recorded when at equilibrium. Equilibrium is attained when, at constant power, 2 sets of temperature readings taken at 5 minute intervals differ by less than ±. C, or if the thermal impedance has changed by less than % of the current thermal impedance over a 5 minute time interval. Calculate the mean specimen temperature and the thermal impedance. The thermal impedances of three specimens with difference thickness are tested. The thickness of the whole sandwiched aluminium bars and thermal paste were measured before and after a set of test to make sure the wire was not deformed. Solvent was used to clean the surface every test with material change. C. Microstructural examination The powder of CNT produces by flame synthesis method [] was observed by using a scanning electron microscope (SEM) and the EDX analysis. gradient of the graph and the y-intercept of the graph is considered as sum of thermal contact resistance for both hot and cool meter bar with the paste under no-load condition. The results show as in Table. A. Microstructure of CNT Figure CNT SEM micrograph III. RESULTS AND DISCUSSION The readings at the meter bars were collected at a steady state then the thermal impedance calculated by the data was plotted in the graph and fit by linear regression. Based on linear equation associated with the plot of the thermal impedance versus the paste thickness, the thermal conductivity of the paste was determined by reciprocal of the Figure 2 diameter of CNT measured with average 7nm Table - Thermal conductivity and thermal interfacial resistance of selected pastes with no load Filler wt% Linear fit of the dependence of the thermal impedance(y) on the bond line thickness(x) Thermal interfacial resistance Thermal conductivity (W/mK) (m 2 K/W) CNT y=6.73x+3.5-6 3.5 x -6.65 2 y=.696x+.3-5.3 x -5.6 2 y=.32x+6.8-5 6.8 x -5 3.22 6 y=.236x+9.2-5 9.2 x -5.237 ZnO DC3 6 y=.622x+9.8-6 9.8 x -6.62
Thermal Impedance (m 2 K/W) Thermal conductivity (W/mK) Thermal Contanct Resistance (m 2 K/W).5 Thermal Conductivity versus Loading of Filler x -.9.8 Thermal Contact Resistance versus Loading of CNT 3.5 3 silicone oil + CNT shape-preserving.7.6 2.5.5 2..5.3.5 2 3 5 6 Loading %wt Figure 3 -Graph of thermal conductivity of paste against loading of CNT B. Thermal conductivity The thermal conductivities of the silicone oil and CNT composites as a function of CNT loading measured are presented in Figure 3. It can be observed from Figure.3 that the thermal conductivity of the silicone oil + CNT composites increases with the increase of the wt.% of CNT. With the high thermal conductivity of the CNT as filler, silicone oil thermal conductivity has been large enhanced. Compare to the sample of DC 3 commercial thermal paste with silicone oil and zinc oxide as filler. the low thermal conductivity of zinc oxide powder only increased the thermal conductivity of the paste to about.62 W/mK with high loading as 69 wt%. The pure silicone oil thermal conductivity was measured about.65w/mk. 65W / mk It thermal conductivity enchanted to about.237w/mk with loading of 6wt.% CNT. Beside thermal conductivity of filler, filler size and shape are also important factors for the thermal conductivity. With the small loading of CNT, the thermal conductivity of pure silicone has great enchantment. This is because the long cylindrical shape of CNT easier form thermal conducting pathway. With only 2 wt.% of CNT added the thermal conductivity of silicone oil has the great increase from.62 W/mK to.6 W/mK. From the experimental values of CNT composites show that thermal conductivity is not increased as expected with the extremely high thermal conductivity of CNT and with high loading. Presence of an interfacial thermal resistance between carbon nanotubes and the polymer matrix, which is known as Kapitza resistance [2] The nanotube/matrix interfacial thermal contact resistance can increasing from poor wetting of the matrix to CNT, and the presence of the CNT/matrix interfacial thermal resistance could lead to drop in the effective thermal conductivity.[3] Due to the large specific surface area of carbon nanotubes, the presence of this thermal boundary resistance can significantly reduce thermal conductivity of CNT composites..2. 2 3 5 6 Loading of CNT (%wt) Figure - Thermal contact resistance against loading of CNT of paste C. Thermal contact resistance The thermal contact resistance increase with the increasing of loading of filler.as shows in Figure. This mean with the increasing the loading of filler the paste will be losing conformability to the microscopic valley at the proximate surface. Thermal contact resistance measured for pure silicone oil was 3.5 x -6 m 2 K/W and increase to.3 x -5 m 2 K/W with 2 wt% CNT added. With the increasing of loading the increasing of thermal contact resistance become lesser. This thermal contact resistance can be decreased by increasing the pressure. The lower contact resistance paste was desired for low bond line thickness application. These values of thermal contact resistance will be less if applied in real application. Because in real application, reasonable pressure will apply and will improve conformability of the paste. In this test, the pressure only acts to the wire not on the thermal paste..9.8.7.6.5..3.2. x -3 Thermal Impedance versus Thickness for silicone oil 6%wt CNT and DC 3 silicone oil with 6%wt CNT DC 3 2 3 Thickness (m) 5 6 x - Figure 5 - Thermal impedance against thickness for DC 3 and silicone oil with 6%wt CNT
Thermal Impedance (m 2 K/W) Thermal Impedance (m 2 K/W) Thermal Impedance (m 2 K/W) The thermal contact resistance of commercial thermal paste DC 3 was much lower compare with the silicone oil with 6%wt CNT paste as shown in Figure 5 although both paste has almost same loading. The contact resistance for silicone oil with 6%wt CNT paste was 9.2 x -5 m 2 K/W and commercial thermal paste DC 3 was 9.8 x -6 m 2 K/W. The contact resistance of the paste was the important factor that influences the performance of the paste. The lower the value will be desired. This can explain by the geometric of the filler for the pastes. The DC 3 was the mixer of silicone oil with the zinc oxide that sphere and in nano size. The particle can easy filling in the microscopic valleys of the surface and form the pathway for heat transfer as shown in Figure.7 a. Compare to CNT that have long cylindrical shape the length in micron size have less conformability to microscopic valley at proximate surface as shows in Figure 6. This problem may be able to solve with use CNT and other filler in nano size and sphere in shape. With this the thermal conductivity can be enchanted and not decrease the conformability of the paste. But the fraction need well study to make sure the performance at the maximum. a) b) Figure 6 - a) paste with sphere shape filler b) paste with long cylinder shape filler a) 8 x - 7 5 micron micron 6 5 3 2 a b 2 3 5 6 x -5 5 micr b) c) mic 3 x - 5 m 2..5 9 8 7 6.95 5.9.5 2 2.5 3 3.5 5 6 7 8 9 2 2 22 23 2 Figure 7 a) Thermal impedance of the paste against loading of CNT for bond line thickness 5 and micron b) zoom in for graph (a) at region b c) zoom in for graph (a) at region b
D. Performance of Thermal Paste Figure 7 a) shows the graph of thermal impedance against loading of CNT for thickness 5 and micron. The graph great decrease of thermal impedance with small amount of CNT was added. The minimum thermal impedance for 5 micron bond line is about 2% wt loading of CNT as shows in Figure 7.b). For micron bond line after 2% wt loading of CNT thermal impedance continuous small decrease until reach the minimum at about 2% wt loading of CNT as shows in Figure 7 c). For both bond line thicknesses after the loading with minimum of thermal impedance, increase the loading will lead to increase thermal impedance. Large enhance of thermal conductivity for the paste with small loading of CNT lower the thermal impedance of the paste. For the low bond line application the factor dominates lower the thermal impedance is thermal contact resistance between the pastes with mating surface. Conformal and wetting of the pastes were important for low bond line application. For thick bond line application the thermal conductivity will dominates. The pastes have high thermal conductivity with high loading is suitable for the thick bond line application. In real application reasonable pressure will apply to increase conformal of the paste and reduce the bond line thickness. When applied the thermal paste should avoid trap of air because air bubble trapped will reduce the performance and cause hot spot. The correct way should use when to apply the thermal interface material at mate the surface of heat sink and heat source. IV. CONCLUSIONS This paper presented the results of experimental approaches to use the CNT powder as filler for high performance TIM. The thermal conductivity of the silicone oil enhanced to.6 W/mK which about % of enhancement with only 2 wt% of CNT shown that the CNT have highly potential to use as filler of the TIM. The performance of the thermal paste shows maximum at thin bond line with about 2 wt% of CNT and also at middle bond line with about 2 wt% of CNT. The long cylindrical shape and high thermal conductivity of CNT will from network for heat transfer. But in other hand, paste with high loading of CNT the thermal conductivity not increase as high as expected. The CNT paste has higher thermal contact resistance compare to paste with filler sphere in shape. This was due to CNT has less conformability compare to sphere particle. The suspension of low loading CNT in silicone oil also observed poor. This may can improve by functionalized the CNT. ACKNOWLEDGMENT I am obliged to staff members of UTM, for the valuable information provided by them in their respective fields. I am grateful for their cooperation during the period of my assignment. REFERENCES [] Prasher, Ravi. "Thermal interface materials: historical perspective, status, and future directions." 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