Carbon Nanotube Arrays for Coupled Electromagnetic and Thermal ...

Carbon Nanotube Arrays for Coupled Electromagnetic and Thermal Management in High Power Electronics: Influence of Microstructuration and Stress Investigated by IR Thermography A. Emplit, I. Huynen ICTEAM institute, ELEN department Université catholique de Louvain, 3 Place du Levant 1348 Louvain-la-Neuve, Belgium [email protected] ABSTRACT Multifunctional and multistructured materials are currently developed for high power electronics in transportation and aerospace sectors requiring size and weight reduction. In this work, we investigate laser-machined micro patterns of CNT brushes as an alternative to metallic structures for driving simultaneously EM and heat propagation. The thermal response of the CNT array is observed to be sensitive to the microstructured pattern etched in the CNT brush, and to the mechanical stress induced by an incident air flux. Depending on the induced displacement of nanotubes, a temperature change up to 5°C is measured. The movement of the CNT within the array is correlated to the mechanical stress imposed by the incident air flux. The correlation between thermal fluctuation and air flux is indeed assessed through the proper combination of SEM and interferometry imaging with thermography. KEY WORDS: thermography, carbon nanotubes array, thermomechanic NOMENCLATURE Greek symbols δ height difference (mm) INTRODUCTION The size reduction of the electronic technology i.e. microprocessors, … result in a localization of hot spots (dissipated by specific components) inside the devices. Moreover, intense electromagnetic impulsions (EMI) can be locally emitted in these systems. In this case, the electromagnetic interferences perturb the communication systems in the neighboring devices. Also, there is a growing interest for replacing the dielectric of the busbar in high power electronics by a material combining good absorption of electromagnetic (EM) pulses (preventing interferences by multiple reflections) and good heat propagation. Various periodic, such as arrays of metallic wires, are developed since more than 10 years for EM applications from below 1 GHz up to THz and optical range [1,2]. They belong to a particular class of metamaterials, aiming at driving EM propagation in some unconventional ways. In this context, metallic CNT brushes can be viewed multifunctional (meta)material allowing to control the propagation of thermal and EM wavefronts along particular spatial directions. CNT dispersed in composites are already known as broadband microwave absorbers [3,4]. The 978-1-4799-5267-0/14/$31.00 ©2014 IEEE

absorption depends on the CNT orientation inside the polymer matrix of the composite [5,6]. The pure CNT brushes, similar to a squared array of vertical metal wires, offer new solutions to tailor EM absorption in specific directions and frequency ranges. They are also expected to have a high and spatially anisotropic thermal conductivity (along the CNT axis). These CNT array structures are currently investigated as alternative to cooling systems using circulation of fluids to drive dissipated thermal power outside a system. Some studies are done on CNT array fabrication improvements [7]. The thermal properties of layered structures including these CNT array are measured by different techniques i.e infrared thermography [8] or a phase sensitive transient thermo-reflectance technique [9], respectively to observe the thermography image of pure CNT agglomerates or to obtain the thermal diffusivity of CNT arrays. In this paper, we used a technique combining 3D topography obtained by interferometry imaging and IR thermography to investigate thermomechanical effects in CNT arrays over a large frameview (samples surface in the cm2 range). We applied an incident air flux on fabricated CNT nanobrushes placed above a heating source and demonstrated the correlation between thermal fluctuation measured in presence of air flux and the displacement of CNTs induced by the flux. FABRICATION OF CNT ARRAY GEOMETRY The samples of multiwall CNT brushes come from the Nanocyl SA Company. They are fabricated using the growing of CNT with average length of 50 and 100 µm (with an average diameter of 50nm) on a quartz (SiO2) substrate having a surface of 1cm2 via catalytic CVD (chemical vapor deposition) method. Figure 1 shows SEM micrographies illustrating a view of the supported CNTs after growing (Fig. 1 a) as well as a top view (Fig. 1b) of the boundary between areas where the CNT brush has been grown (left) and bulk quartz substrate (right). The supported CNT brush is then micro-patterned using a picosecond laser etching system (model Oxford J-1064/355) to etch parallel trenches 50 or 100 µm wide into the CNT brush. The wave length of the laser pulse is 355nm. The spot diameter in the focal plane is around 10µm. For CNT brush, the resolution also depends on the interaction between the laser pulse and the sample. This is directly dependent on the focus offset during the etching post processing i.e. with an offset of 400µm, the spot diameter resolution is 20µm. The laser pattern highly depends on the following parameters: the

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focus offset, the scan repetition, the pulse average power, the scan rate and the repetition rate of the pulse. The pattern repeatability and accuracy can be controlled according to the etching parameters and the quality result of the CNT brush process (flat quartz substrate, hardness of the CNT attachment at the surface …). The resulting pattern is observed to be reproducible and the accuracy depends on the sample and the etching parameters. After laser microstructuration two families of patterns are observed by OM microscopy (Figure 2): (1) 1D array of micro-lines (50µm wide) of CNT brush separated by laser-etched trenches (2) 2D array of micro-lines forming a kind of micro-lattice (CNTs are indeed present in the dark areas). The laser etching parameters were: no focus offset, an average power of 142mW, 25 scan repetitions, a scan rate of 200mm/s, and a repetition rate of 20 kHz. Fig. 2 OM micrography (Olympus U-PS Provis Ax70) by light transmission of the transition between two patterns obtained by laser ablating in the CNT brush: 2D micro-lattice (top) and 1D array of micro-lines. Light zones correspond to substrate areas without CNT.

(a)

(b) Fig. 1 SEM micrography of the CNT brush grown on the quartz substrate (a) side view, courtesy of Nanocyl SA (b) top view obtained using SEM LEO 982 GmbH

Figure 3 shows the corresponding SEM image of a microlattice similar to that of Figure 2 (top). The darker zone in SEM images is now the substrate. Such lattice was obtained by etching parallel trenches 100 µm wide, and equally spaced by 50µm, in two successive orthogonal directions. As a result a 2D array formed by 50x50 µm squared micro-pillars would be expected. But during the etching of trenches some of the laser-ablated CNTs “fall back” on the sample between the unetched parts, making some walls or bridges between the micro-lines of CNT. This results surprisingly in a quite regular micro-squared lattice, as also observed on the upper part of Figure 2. Depending of the quality of the laser process, the etch goes through the CNT brush until the quartz substrate (as the sample on the figure 2) or cut the CNT a few micrometers above the quartz substrate (as for the other sample on Figure 3).

Fig. 3 SEM imagery (LEO 982 GmbH) of 2D micro-lattice (CNTs are not present in dark areas). EXPERIMENTAL SETUPS To quantify the 3-D topography profile before and during the experiment, we apply an interferometry imaging technique, using a Micro System Analyser (Polytec’s MSA500, white light interferometry). This technique gives accurate

quantitative values from 1 nm to 200 µm for the deformation measurement, depending of the used objective. The setup is illustrated in Fig 4.

Fig. 4 Setup of the 3-D topography measurement. Topographic images are provided by the system within the scale range -100µm up to 100µm. To obtain comparable measurements, the system is calibrated in order to reference the +100 µm level at the top end of the longer CNT. This calibration remains unchanged for all the measurements, independently of the applied stimuli on the sample, in order to be able to observe and quantify the resulting changes of position of these top ends. As a consequence, the 0 µm level shown in our scale bars at Figs. 5, 12 and 13 does not correspond to a particular reference value. The method enables first us to further assess the quality of the laser etching. This can be confirmed by the 3D topography realized on the sample of Figure 3 and illustrated on Figure 5, which indicates that the darker zones on Figure 3 correspond to a depth close to the quartz substrate (where CNT are etched) and the CNT bridges between the micro-pillars are at the same height than the top end of the CNTs.

Fig. 5 3D topography image of one of the obtained microlattice.

The thermography measurement setup (Figure 6) consists in an NIR camera (model FLIR SC660), a thermal source (a thermal resistance in this case), an incident nitrogen air flux (linked to a thermal mass flow meter model Mass View).

Fig. 6 Setup for the thermal measurements, showing the IR camera, the sample placed on the heating source and the tip supplying the air flux. The air flux flow is controlled by a meter model Mass View. The air flux consists of nitrogen gas at room temperature which is sent on the sample. The diameter of the tip is also a critical parameter. The tip diameter was 3mm for this series of measurements and the flow was in the 0 to 6 liters per minute range. From measurements without sample, we observe no change in the IR thermography results depending from the air flux applied. Depending of the setup used during the measurement, the air flux source is just fixed or stays on a Newport rail system (figure 7 right) which allows to accurately control both horizontal and vertical position of the tip and imposes the same configuration for the different measurements.

Fig. 7 Setup for the cumulated thermal measurements and 3D topography measurements, showing the IR camera and the Polytec system (left); and the both Newport rail and support controlling accurately the horizontal and the vertical position of the air flux tip (right).

COUPLED SENSITVITY OF CNT ARRAY TO MECHANICAL STRESS AND THERMAL STIMULI The correlation between thermal fluctuation and air flux is assessed through the proper combination of SEM and interferometry imaging with thermography. The thermography using IR camera allows studying the thermal behavior of CNT array having a specific periodic microstructure. The thermomechanical measurement setup consists in a tip which sends the air flux directly on the sample under a defined angle or direction (figures 7 and 8). This direction influences the way the CNT move within the micro-pattern as well as the flux intensity.

applied nitrogen air flux. The static thermal heat flux is sufficient to characterize the relation between the emissivity measured by the camera, the orientation of the CNT and the temperature of the applied air flux. On Figure 9, the incident air source is at the left of the sample. The sample is deposited on a static heat source at 30°C. Without air flux (0 l/m), the temperature is observed to be uniform over the whole sample. Under 6 l/min, the measured difference can reach up to 5°C. An enlarged view of the thermographic response of the various micro-patterns under air flux is provided at the figures 10 and 11. Figure 10 shows the thermography image obtained with the following parameters: 30°C+/-1°C as temperature of the heat source below the sample, and without external incident air flux (this correspond to the same conditions than for the thermography on Figure 9a). Figure 11 shows the corresponding thermography image when an incident air flux is applied (5 l/min).

Fig. 8 Close view of the sample (dark square) and the tip sending the air flux (on the left). This topology is used with both NIR camera and the Micro System Analyser.

Fig. 10 Thermography for 30°C +/- 1°C for an incident air flux of 0 l/min

Fig. 11 Thermography for 30°C +/- 1°C for an incident air flux of 5 l/min. Fig. 9 Thermography image obtained for a heat source at 30°C (a) without incident air flux (0 l /min N2) (b) with an incident air flux of 6 l/min N2. The different parameters which influence the heat emissivity and the mechanical stress are: the angle between the incident nitrogen air flux and the sample, the CNT micropattern within the sample, the temperature of the heat source, and the temperature and magnitude of the applied nitrogen air flux. Figure 9a and 9b show the IR thermography results for a static thermal heat flux and respectively without and with an

On Figure 11, the areas corresponding to 4 different patterns present on the sample are clearly visible: the pure CNT brush with some squared areas created by preminary laser tests made to define the process conditions (top right), the 2D micro-lattices resulting from the etching of trenches spaced by 50µm and having a width of either 50 µm (bottom right) or 100µm (top left), and finally a 1D array of 50µm wide CNT micro-lines equally spaced by trenches of 50µm width (bottom left). A decrease of temperature is observed in presence of the air flux: it is more important for the two micro-lattices patterns than of the two other patterns.

DEFORMATION OF THE CNT UNDER STRESS AND PHYSICAL MECHANISMS OF HEAT TRANSPORT The heat propagation across the sample results from different mechanisms: electron thermal conduction inside the CNT and phonon conduction inside the quartz substrate. The exchange mechanism between the environment and the sample is the air convection. This convection depends on the microstructuration. The CNT array deformation can be described using previous mechanic studies done in the literature as out of plane shear tests[10] or indentation stress-strain response[1112]. The CNT stay attached to the substrate and the deflection induces a curvature due to the elastic mechanical properties of these CNT. The curvature shape depends on the stress orientation. Depending from the microstructuration, we observed induced deformation resulting from the CNT mutual mechanical interactions.

Figure 14 shows the histogram of the results of figure 13, i.e. the number of occurrences of each measured value of shift δ. It is concluded that the mean value is 1.8µm, with the maximum value and the minimum value at 108.1µm and 119.7µm respectively. The maximum of occurrences is obtained for a shift of 3.6µm. It is concluded that the average z-position of top ends of the CNTs in the micro-lattice of Fig. 3 is shifted by about 1.8 m under presence of an air flux density of 6 l/min. Incident air flux applied on the sample results in a local stress on CNT bridges forming the micro-lattice, which induces their deflection, and affects significantly their heat dissipation capability and emissivity. This explains finally the change in temperature observed for such micro-lattice patterns.

QUANTIFY THE CNT DEFLECTION IN THE MICROPATTERNS To quantify the deflection in the micropatterns, a 3D topography is done using interferometry. Figure 12 shows the 3D topography image obtained for the micro-lattice pattern 100µmx100µm (as shown on the SEM image, Figure 3).

Fig. 13 Topography image of the difference between positions detected for 6 l/min and 0 l/min over the same local area of the CNT micro-lattice as in Figure 11. The color bar represents the shift δ expressed in mm.

Fig. 12 Topography image of a local area of the CNT microlattice. The color bar represents the z axis (vertical position) in mm. To quantify the deflection in the micro-patterns induced by the air flux, 3D topography images without and with air flux are compared. Each topography image provides the absolute z (vertical)-position measurement at each x and y abscissa in the pattern. The difference between z-values measurements with and without air flux gives further the absolute displacement (δ) of CNT tips occurring at each (x,y) location and induced by the external air flux stimulus applied on the sample. The figure 13 shows the δ shift at the surface of the micro-squared 100µmx100µm pattern when an air flux density of 6 l/min is applied at the surface with an angle of 60° between the surface vector and the axis of the tip.

Fig. 14 Histogram associated to the shift δ between 6 l/min and 0 l/min for the same local area of the CNT micro-lattice investigated at figure 13.

CONCLUSION This work provides preliminary investigations of the sensitivity of micro-patterns of CNT brushes to a mechanical stress induced by an air flux. Their thermal emissivity is affected as evidenced by the significant change in temperature measured with an IR camera. It is shown to be strongly dependent on the topology of the micro-pattern considered, and induced by the spatial deflection of CNTs inside the pattern. IR thermography is such demonstrated to be able to detect the influence of any air flux over such nanostructures. This knowledge is of potential interest for the design of flow sensors and the development of new techniques for heat dissipation combining air cooling with optimized topologies of multifunctional CNT composites. Other aspects in progress in this work include the determination of additional thermal parameters such the true thermal emissivity and the thermal diffusivity. In this latter case, we will need to use the laser flash method [8] or a source depending of the frequency and apply the phase sensitive transient thermo-reflectance technique [9]. Acknowledgments The authors are grateful to the National Fund for Scientific Research (F.R.S.-FNRS) and the Walloon Region through Plan Marshall PIT ATAC-Concept project for funding this research. This work is also supported by the MINATIS project co-funded by the European Regional Development Fund (ERDF) and the Walloon Region. Authors are also indebted to Nanocyl Company for proving samples of CNT brushes, especially to Mr. D. Bonduel and to Mrs. A. Usoltseva who realized SEM micrographies. We also thank at UCL Profs. Chr. Bailly and Th. Pardoen for fruitful discussions, and the technical staff of the Welcome and Winfab facilities for their help with respectively measurements and clean room processes. References [1] K. Kordás, G. Tóth, P. Moilanen, M. Kumpumäki, J. Vähäkangas, and A. Uusimäki, “Chip cooling with integrated carbon nanotube microfin architectures”. Applied Physics Letter 2007, vol 90, pp 123105. [2] Z. Qi, Proceedings of Asia-Pacific Microwave Conference. 2006. [3] A. Saib, Lukasz Bednarz, Raphael Daussin, Christian Bailly, Xudong Lou, Jean-Michel Thomassin, Christophe Pagnoulle, Christophe Detrembleur, Robert Jérôme, and Isabelle Huynen, “Carbon Nanotube Composites for Broadband Microwave Absorbing Materials”, IEEE Transactions on microwave theory and techniques, Vol 54, No 6, 2006. [4] A Emplit, F Tao, C Bailly, I Huynen, “Novel family of broadband nanocomposite microwave absorbers with carbon nanotubes in solid polymer film” 38th European Microwave Conference, 2013. [5] Y. Danlée, Huynen, I., Bailly, C., « Thin smart multilayer microwave absorber based on hybrid structure of polymer and carbon nanotubes », Appl. Phys. Lett. 100, 213105 (2012) [6] Fangming Du, John E. Fischer, and Karen I. Winey, “Effect of nanotube alignment on percolation

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