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PROOF COPY [NANO-12-1053]

I. T. BarneyCenter for Nanoscale Multifunctional Materials,

Department of Mechanical and

Materials Engineering,

Wright State University,

3640 Colonel Glenn Hwy,

Dayton, OH 45435

S. Ganguli

A. K. Roy

Thermal Sciences & Materials Branch,

Air Force Research Laboratory,

AFRL/RXBT Bldg 654,

2941 Hobson Way,

WPAFB, OH 45433-7750

S. M. MukhopadhyayCenter for Nanoscale Multifunctional Materials,

Department of Mechanical and

Materials Engineering,

Wright State University,

3640 Colonel Glenn Hwy,

Dayton, OH 45435

Improved Thermal Response1 in Encapsulated Phase Change

Materials by Nanotube Attachment2 on Encapsulating Solid3

4 This paper demonstrates greatly improved specific power (W/g) for encapsulated phasechange materials (EPCM) as a result of modified interface morphology. Carbon nano-tubes are strongly attached to the interior walls of the graphitic foam encapsulation.Microstructure analysis using scanning electron microscopy (SEM) indicates that thewax infiltrates into the CNTAQ1 forest and creates an intimate contact with increasedinterfacial area between the two phases. Specific power has been calculated by meas-uring thermal response times of the phase change materials using a custom system. Thecarbon nanotubes increase the specific power of the encapsulated phase change materi-als by about 27% during heating and over 146% during the more important stage oflatent heat storage. Moreover, SEM images of the interface after repeated thermalcycling indicate that the presence of CNT may also improve durability of the EPCM bypreventing interfacial gaps and maintaining improved contact between the graphite andPCM phases. [DOI: 10.1115/1.4007327]

Keywords: carbon nanotubes, carbon foam, paraffin wax, latent heat storage, specific5 power, phase change materials

6 Introduction

7 Thermal management is a crucial engineering challenge in the8 development of compact and mobile electronic systems. Most9 failures in power electronics are thermally related. For many

10 electrical components, failure rates have a near exponential de-11 pendence on the operating temperature, and thermal cycling can12 cause 8 times higher failure rates for regular swings over 20 �C13 [1]. Therefore, new materials and manufacturing techniques are14 needed to keep pace with demands for controlling waste heat and15 maintaining optimal operating temperatures in systems without16 compromising device performance [1–3].17 Many next generation electronic systems need thermal solu-18 tions that can accept high power outputs in a compact design19 while maintaining a narrow range of operating temperatures. For20 instance, laser diode bars are of interest in many aerospace21 applications. However, they have high transient power usage and22 optical efficiencies in the range of 30–50% [4,5]. They also23 require extremely uniform operating temperatures to maintain24 their efficiency and prevent dramatic reductions in component25 lifetime [3,5]. This and related applications will need better ther-26 mal interfaces between multiple components through complex27 thermal management systems. A better investigation in multi-28 phase thermal physics on the macro, micro, and nanoscales is29 needed on many issues including interfacial tension and wetting,30 surface roughness, and how these relate to interfacial thermal31 resistances [3,6].32 Phase change materials (PCM) offer several advantages in the33 packaging of some high power electronic systems including laser34 diodes. They can store a large amount of thermal energy as latent35 heat during the solid–liquid transition while maintaining tempera-36 ture within a narrow operating range. However, there are several37 challenges that must be addressed, major ones being relatively38 low thermal conductivity and cyclical volume changes. Com-39 monly studied phase change materials include both inorganic

40compounds like hydrated salts, and organic compounds such as41paraffin wax and fatty acids. Hydrated salts have some advantages42since they have roughly twice the thermal conductivity, twice the43volumetric latent heat storages (kJ/m3), and 25% greater latent44heat storage per kg when compared to organic PCM [7]. However,45they have a major drawback because they do not maintain their46properties as well with cycling [7].47Organic PCM such as paraffin wax is therefore the more dura-48ble option. It is chemically inert, undergoes minimal subcooling,49has low vapor pressure, no phase separation, stable melting50temperature, and storage capacity over many cycles, and can be51operated at wide range of temperatures depending on the size of52the molecules used [7–11]. However, the thermal conductivities53of paraffin waxes are only 0.25–0.35 W/m K for the solid phase54and roughly half this for the liquid [7,9,12,13]. Poor conductivity55makes the thermal response of pure paraffin wax slow. This needs56enhancement in order to effectively manage high power thermal57spikes or large continuous outputs.58Two broad approaches are used to improve the response time59for organic PCM: encapsulation in thermally conductive material,60and mixing thermally conductive additives into the PCM matrix.61Thermally conductive microencapsulation involves encapsulat-62ing the PCM within a conductive matrix, which can increase the63interfacial contact between the phase change materials and the64heat source. This can increase thermal conductivity, control volu-65metric changes, and prevent chemical or physical interactions66with their environment [7]. Microencapsulation reduces the total67latent heat of the composite material and can also decrease the68melting temperature [14].69One of the most important considerations for choosing an70encapsulation material is the wettability of the PCM with the71interface of the encapsulation. Poor interfacial contact will72increase the thermal resistance of the interface considerably. This73is exacerbated by repeated large volume changes of the PCM74between the liquid and solid phase during thermal cycling. The75expansion and contraction of the PCM can continually open and76close voids along the interface with the encapsulation. Graphitic77materials are suitable for wax-based PCM in this regard because

Manuscript received April 11, 2012; final manuscript received July 30, 2012; pub-lished online xx xx, xxxx. Assoc. Editor: Debjyoti Banerjee.

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78 of their good thermal conductivity, relatively low density, and79 good interfacial wetting with the PCM [11–16]. They have been80 shown to perform well together for some designs of latent heat81 storage systems [12–17].82 Thermally conducting additives suitable for paraffin wax PCMs83 include expanded graphite and carbon nanotubes. Expanded84 graphite can increase the thermal conductivity of the composite85 by 1–2 orders of magnitude [16]. Conductivity increases fairly lin-86 early for low volume content but becomes less predictable as87 loading approaches 10 wt. % [9,13,14,16]. Multiwalled carbon88 nanotubes (MWNT) decrease the melting temperature slightly but89 increase the total latent heat by up to 10% for 1% loading by90 volume [12,15]. MWNT loaded to 2 wt. % can increase the ther-91 mal conductivity of paraffin wax by 35% and 45% in the solid and92 liquid phases, respectively [15]. Additionally, MWNT have been93 shown to increase thermal transport across some solid–solid94 interfaces especially when chemically bonded at the tip [17,18].95 The increase can be up to two orders of magnitude larger if the96 nanotubes are chemically bonded to the interfaces as opposed to a97 pressure based contact [17].98 The goal of this project is to build upon the existing knowl-99 edge and create a new class of encapsulating material that com-

100 bines the advantages of PCM containment with additive heat101 transport through a single hierarchical structure. The base start-102 ing material is multicellular graphitic foam selected due to its103 high crystallinity, good strength to weight ratio, and isotropic104 properties [19]. The surface of this material that interfaces with105 the PCM is altered by well attached carbon nanotubes. The car-106 bon nanotubes are chemically bonded to the interior walls of the107 multicellular graphitic foam encapsulation and extend outward108 into the solid/liquid paraffin phase change material. This greatly109 increases the surface area of the microencapsulation and110 improves the interface and thermal transport between the two111 materials. In addition, the highly conductive nanotubes extend-112 ing into the PCM increase the overall conductivity of the com-113 posite region. The net result is a two-prong approach to114 enhancing the response time of PCM-encapsulation hybrid115 device.

116This paper discusses the fabrication, characterization, and117thermal testing of these materials, which show this to be a very118promising approach of enhancing PCM-incorporated structures.

119Experimental

120The multicellular graphitic foam selected for aerospace applica-121tions is a pitch derived open cellular structure with porosity122about 70%. A nanolayer of amorphous silica is deposited onto the123foam by microwave plasma enhanced chemical vapor deposition124using precursors of hexamethyl-disiloxane (HMDSO) in O2 gas.125MWNT are grown attached to the silica nanolayer using a floating126catalyst chemical vapor deposition with ferrocene and xylene in127an Ar/H2 environment. Details of both the silica deposition128[20–24] and nanotube growth [25] processes have been previously129published.130Microcellular graphitic foam disks of 30 mm diameter and1318 mm thick are used as encapsulation material for paraffin wax132(Aldrich, mp 53–57 �C). Encapsulation with and without nano-133tubes is used for comparison and referred to as CNTfoam and134MGfoam, respectively, from here on.135The hybrid encapsulated phase change material (EPCM) was136created by melting wax and heating to 70 �C in a steel bowl. Sub-137sequently, the encapsulation material, MGfoam or CNTfoam, is138submerged. All EPCM were weighed and found to be between13967–71 wt. % paraffin wax. The variation was mostly due to the140irregularity of the foam structure. After wax infusion, the EPCM141was wrapped in a layer of Al foil to prevent loss of wax during142cycling.143Differential scanning calorimetry (DSC) was performed on the144paraffin wax, MGfoam EPCM, and CNTfoam EPCM to measure145latent heat storage between 0 and 100 �C at a rate of 3 �C per146minute. Three samples of each type were tested to get an average147since porosity can vary the weight percent of wax in small148samples.149Thermal testing was performed using a custom apparatus as150shown in Fig. 1. A resistive heater is attached below a copper151cylinder and the EPCM sample is placed in the chamber above the

Fig. 1 Thermal testing apparatus designed and built at Air Force Research Laboratory. The Cu cylinder isheated with 10 W from below. The input temperature is measured with TC1. The thermal response of the sampleis measured with TC2.

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152 copper. A thermocouple (TC1) is used to measure the input tem-153 perature between the copper cylinder and the Al foil on the154 EPCM. A second thermocouple (TC2) is inserted 4 mm down into155 the center of the EPCM to measure the response as a function of156 time. A 2 kg weight is placed on top of the lid to the sample cham-157 ber to ensure good and consistent thermal contact between the Cu158 and Al. The heater is given constant power of 10 W and tempera-159 tures for both TC1 and TC2 are recorded every 0.71 s. Thermal160 testing is done from room temperature (20 �C) until the EPCM has161 fully melted.162 Microstructural images are taken using a JEOL 7401 F field163 emission scanning electron microscope. Images of the MGfoam164 and CNTfoam interfaces with wax were taken after careful prepa-165 ration to ensure that there was no contamination of the FE–SEM166 (scanning electron microscopy) chamber with paraffin wax. The167 EPCM samples were first frozen in liquid nitrogen and then168 shattered. A shard having the full cross section was placed on a169 sample stub with carbon paint and then allowed to thaw to room170 temperature at 10�3 Torr for 24 h to remove any volatiles. The171 sample was then frozen in liquid nitrogen again and then placed172 immediately into the insertion chamber for the SEM. Images were173 taken at low power (1 kV and 10 lA) to maximize imaging time174 before the wax began to soften.

175 Results and Discussion

176 Structure Analysis of Encapsulation Material. The foam177 selected for this study had 70% porosity, which was determined to178 provide a good compromise between mechanical strength and179 thermal properties. Figures 2 and 3 show the structure and inter-180 face of the foam. There is significant statistical variation within181 the structure which is not uncommon in foams created by passing182 gas bubbles through semiviscous precursors. For this grade of183 foam, pore diameters were in the several hundred micrometer184 range, and interconnectivity among pores could be clearly seen.185 The surface of the CNTfoam is shown in Figs. 4 and 5. The186 carbon nanotubes grown onto the pore walls are seen to be multi-187 walled with spaghetti like random distribution over the surface. A188 large fraction forms a dense entangled carpet-like nanotube layer189 that extends only about 3–5 lm out from the pore walls [25] into190 the PCM matrix. As the nanotubes grow longer, aligned bundles191 of CNT snake out from the walls and penetrate much further into192 the interior of the pores. This implies that in the future, it is possi-193 ble to increase the PCM permeation with CNT by increasing the194 number of nanoropes, unless the overall nanocarpet thickness can195 be uniformly increased.

Fig. 2 Pore structure of the multicellular graphitic foam encap-sulation material Fig. 3 Surface of a pore wall in the multicellular foam showing

graphitic planes perpendicular to the interface

Fig. 4 Carbon nanotubes grown over the multicellulargraphitic foam. Aligned snakes of carbon nanotubes can beseen extending into the pores.

Fig. 5 Spaghetti like distribution of randomly oriented carbonnanotubes attached to the surface of the multicellular graphiticfoam

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196 Wettability of the Encapsulation Surface. It is important to197 determine the nature of the contact between the carbon phase and198 the paraffin wax in all cases. Intimate contact is needed to ensure199 rapid diffusion of thermal energy between the encapsulation mate-200 rial and the phase change material. It has been discussed earlier201 that previous studies have reported that graphitic materials in gen-202 eral provide good interfacial wetting for paraffin wax PCMs203 [12–17]. Those results will apply to interfaces between wax and204 planar graphitic surfaces. However, the CNT grafted surface in205 this study has unprecedented hierarchical morphology, and wett-206 ability of molten wax or any fluid will be dominated by the degree207 of permeation of the fluid phase into the CNT forest.208 Wettability was tested by melting a beaker of wax to 10 �C209 above its melting temperature and submerging CNTfoam samples210 in it. The sample was left in the liquid wax until bubbles ceased211 appearing (�1 min) and then removed and allowed to cool. The212 encapsulated phase change material was then cut open and ana-213 lyzed in SEM using the procedure outlined earlier. Figure 6 shows214 the microstructure of the interfacial region. It can be seen that215 within the temperature studied, the foam was fully infiltrated.216 SEM micrographs show good wettability between the CNT and217 the wax region. This should assure efficient thermal transport218 between the nanotube radiators and the wax phase.

219Thermal Properties of Encapsulated Phase Change220Materials. Table 1 gives the DSC results for neat paraffin wax221and three samples each of MGfoam and CNTfoam encapsulated222phase change materials. The paraffin wax has the highest value223at 203.6 J/g, followed by the CNTfoam at 139.1 J/g, and the224MGfoam at 117.3 J/g. The peak melting point is at 54.5 6 0.2 �C225for all three samples. The CNTfoam has about 15% higher226average latent heat storage than the MGfoam which agrees with227literature results that adding MWNT into paraffin wax increases228the latent heat of fusion [12,15].229Table 2 shows the comparative Response Time and Specific230Power of these materials. It must be noted that CNTfoam samples231have a slightly lower wt. % of paraffin wax than MGfoam. This is232due to the 2.5 wt. % increase of the CNTfoam encapsulation after233CNT are added. The total weight of the EPCM and the weight234percentage of paraffin wax are given in the first two columns of235Table 2. Table 2 also gives the elapsed time to heat the sample236from room temperature to the melting point (DtH), the elapsed237time to complete the phase change (DtPCM), the specific power238during heating from room temperature to the melting point (QH),239and the specific power during phase change (QPCM). Specific240power for heating is calculated using Eq. (1) and specific power241during phase change is calculated using Eq. (2)

QH ¼cp � DTH

DtH(1)

QPCM ¼L

DtPCM

(2)

242where cp is the specific heat capacity of the EPCM, L is the latent243heat of fusion for the EPCM, and DTH is the temperature change244during heating (34 K). Figure 6 shows the average specific power245of the EPCM during heating and during the phase change with24610 W of input power. Error bars represent the measured range of247the three samples for that point. The heater power was chosen as24810 W because it was a significantly higher power than 5 g of either249the MGfoam or CNTfoam EPCM could absorb.250As seen in Table 1 and Fig. 7, the specific power during heating251is 0.104 W/g for MGfoam and 0.132 W/g for the CNTfoam. This252is a 27% improvement for the CNTfoam which is significant but253of less interest for electrical applications since the PCM will be254chosen to transition near the operating temperature. The specific255power during phase change was 0.223 W/g for MGfoam and2560.548 W/g for the CNTfoam. This is an improvement in specific257power of 146% for the CNTfoam EPCM during latent heat258storage.259The sample to sample variations in measured quantities are260worth noting. For MGfoam EPCM, power appears to be constant261across samples. The CNTfoam samples have higher sample to262sample variations in specific power. This likely stems from batch263to batch variations in nanotube growth. In order to determine the264statistical range or impact of these variations, a larger sample set265may be needed in the future. It is clear from the data that despite

Fig. 6 Paraffin wax is found to penetrate the nanotubes all theway to the silica interface

Table 1 DSC results

Sample Heat storage (total range)

Pure paraffin wax 203.6 J/g (202.4–204.8)MGfoam EPCM 117.3 J/g (110.6–121.9)CNTfoam EPCM 139.1 J/g (137.0–141.1)

Table 2 Thermal response times and specific power of EPCM

Weight ofEPCM (g)

Wt. % paraffinin EPCM

Heating timeDtH (s)

Heating powerQH (W/g)

Phase changetime DtPCM (s)

Phase changepower QPCM (W/g)

MGfoamMGfoam1 5.00 71 519 0.105 524 0.224MGfoam2 4.98 70 484 0.113 532 0.220MGfoam3 5.33 68 566 0.096 519 0.226MGfoam average 5.10 69.7 523 0.104 525 0.223

CNTfoamCNTfoam1 4.84 70 411 0.133 288 0.483CNTfoam2 5.32 68 463 0.118 269 0.517CNTfoam3 4.80 67 367 0.149 204 0.682CNTfoam average 4.99 68.3 413.7 0.132 253.7 0.548

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266 such variations, the power of any CNTfoam EPCM device is seen267 to be over twice that of any MGfoam EPCM device.

268 Influence of Thermal Cycling. In order to study the influence269 of thermal cycling on the EPCM structures, PCM infused270 MGfoam3 and CNTfoam3 were rapidly cycled 15 times using a271 furnace at 120 �C and a refrigerator. After this cycling, the sam-272 ples were weighted to account for any loss of wax. It was seen273 that there was minimal, but detectable wax loss (about 1%) after274 15 cycles. This may be attributed to vaporization and/or leaks. For275 future use in larger-scale engineering applications, such losses can276 be prevented by encapsulating these structures in an impervious277 conducting shell.278 The response times were measured after 15 cycles. The279 response of the MGfoam remained unchanged. It was noted that280 the heating rate of CNTfoam3 actually improved by about 20%281 after 15 cycles, probably due to improved infiltration within282 nanotubes with repeated cycling. The specific power during latent283 heat storage (QPCM) remained constant for both MGfoam3 and284 CNTfoam3. This indicates that thermal cycling is not expected to285 diminish the heat storage capacity for these EPCM structures.286 Long term performance involving larger number of cycles will be287 conducted in future.288 After rapid thermal cycle tests, these samples were cut open289 to image the encapsulation/wax interface. Figure 8 shows a mag-290 nified region of MGfoam3 along the graphite/wax interface and291 gaps and fissures are visible along that interface. Figure 9 shows292 the interface for the CNTfoam, and it can be seen that any visible293 cracks are within the wax phase, away from the interface. It is294 clear that the wax is much better bound to the interface of the gra-295 phitic encapsulation after the CNT are added.296 This study clearly indicates that the nanotubes attached on the297 foam surface are preventing interfacial delamination of the PCM298 from the encapsulation surface. Additionally, it appears the299 CNTfoam EPCM is likely to retain the significantly enhanced300 contact area between the two phases even after repeated thermal301 cycling.302 The very significant improvement in specific power demon-303 strated by the CNTfoam EPCM over the MGfoam structure can304 be attributed to two effects. The first is increased interfacial con-305 tact area, resulting in reduced interfacial resistance between the306 encapsulation and the wax phases. The second is the increased307 thermal conductivity of the part of the wax that is infiltrated with308 carbon nanotubes.309 Earlier geometric calculations using very conservative esti-310 mates for CNT density and length had predicted that growth of

311CNT using this process should increase the surface area of the312underlying foam substrate by over 2 orders of magnitude [25].313This, combined with excellent wettability between the graphite314and PCM phases, can result in very significant enhancement of315thermal transport between the graphite and PCM regions. Regard-316ing the thermal conductivity of the nanotube wax composite317region, it is expected that the nanoscale size and large interfacial318area of carbon nanotubes can cause improved thermal transport in319wax-CNT composite [26]. However, previous work has shown320that even relatively high loading of loose nanotubes completely321dispersed in wax would only improve the thermal conductivity of322paraffin wax by only 35–45% [15]. In this study, the CNT-323dispersed wax region with the increased thermal conductivity is a324small fraction of the entire PCM phase, and improvement in over-325all PCM conductivity is expected to be lower than that. It there-326fore appears that the more significant contribution to the strong327reduction in response time should be from the increase in interfa-328cial contact area. It should be pointed out that there is scope for329additional improvement in response time if the CNT ropes and330carpet can be made longer for increased penetration into the PCM331phase.

Fig. 7 Average specific power at 10 W input during heatingand during storage of latent heat for the multicellular encapsu-lation without CNT (light gray 5 MGfoam) and with CNT (darkgray 5 CNTfoam). Error bars represent the full range ofmeasurements.

Fig. 8 Nano fissure along interface of multicellular graphiticfoam (bottom region) with paraffin wax PCM (top region)

Fig. 9 Interface of CNTfoam3 showing graphite (bottom), CNTand wax (middle-bright lines), and the paraffin wax beyond(top). Second blurry line of silica and CNT are from a shiftedsheet of the graphite in the foreground.

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332 Conclusions

333 This project demonstrates significantly faster thermal response334 times, and thus higher specific power (W/g), for microencapsu-335 lated PCM resulting from carbon nanotubes attached on the sur-336 face of the microencapsulation. The carbon nanotubes are337 strongly bonded to the interior walls of the multicellular graphitic338 foam encapsulation and extend outward into the solid/liquid339 paraffin phase change material. The carbon nanotubes increase the340 specific power of the encapsulated phase change materials by341 27% during heating and over 146% during the more important342 storage of latent heat. This large improvement is attributed to343 improved interfacial contact area between the graphitic encapsula-344 tion and the paraffin PCM as well as the increased thermal con-345 ductivity of the portion of PCM that has CNT. For the current346 samples, improved interfacial contact is expected to be the larger347 contributor given the orders of magnitude increase in interfacial348 contact area and limited volume penetration of the carbon nano-349 tubes. However, further improvements can be realized by extend-350 ing the nanotube layer deeper into the pores, hence increasing351 penetration into the PCM phase. The improved specific power of352 the encapsulated phase change material makes it a viable candi-353 date for thermal management in high power, pulsed energy354 electronics.

355 Acknowledgment

356 The authors would like to thank the Dayton Area Graduate357 Studies Institute, Air Force Research Laboratory, Wright State358 University, and the Ohio Third Frontier Program for their support.

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359 sient Spike Power Applications,” Int. J. Therm. Sci., 53, pp. 1–7.[2] Alrashdan, A., Mayyas, A. T., and Al-Hallaj, S., 2010, “Thermo-Mechanical

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J_ID: NANO DOI: 10.1115/1.4007327 Date: 7-August-12 Stage: Page: 6 Total Pages: 7

ID: veeraragavanb Time: 19:05 I Path: //xinchnasjn/ASME/3b2/NANO/Vol00000/120029/APPFile/AS-NANO120029

000000-6 / Vol. 00, MONTH 2012 Transactions of the ASME