International Journal of Thermal Sciences 89 (2015) 79e86
Contents lists avai
International Journal of Thermal Sciences
journal homepage: www.elsevier .com/locate/ i j ts
Thermal management of electronic devices using carbon foam andPCM/nano-composite
W.G. Alshaer a, S.A. Nada a, *, M.A. Rady b, Elena Palomo Del Barrio c, Alain Sommier c
a Mechanical Engineering Department, Benha Faculty of Engineering, Benha University, Benha 13512, Egyptb Mechanical Engineering Department, Faculty of Engineering at Helwan University, Helwan 11421, Egyptc Universit�e de Bordeaux, Laboratoire TREFLE, Esplanade des Arts et M�etiers, 33405 Talence, France
a r t i c l e i n f o
Article history:Received 23 April 2014Received in revised form31 July 2014Accepted 23 October 2014Available online
A detailed experimental study of a hybrid composite system for thermal management (TM) of electronicsdevices was performed. Three different TM modules made of pure carbon foam (CF), a composite of CFand Paraffin wax (RT65) as a phase change material (PCM), and a composite of CF, RT65 and multi wallcarbon nanotubes (MWCNTs) as a thermal conductivity enhancer were developed and tested. Two typesof carbon foam materials of different thermal conductivities, namely CF-20 of low thermal conductivity(3.1 W/m K) and KL1-250 of medium thermal conductivity (40 W/m K) were used in the three Modules.Tests conducted at different power densities showed a reasonable delay in reaching the heater steadystate temperatures using TM module made of CF þ RT65 as compared to pure CF. Heat transferenhancement due to entrapped MWCNTs in the CF micro cells have a significant effect on the thermalresponse of the TM modules. The delay and decrease of heater surface temperature increase with theinclusion of MWCNTs in the TM module made of CF þ RT65/MWCNTs. TM modules with enhancedthermal conductivity of carbon foam KL1-250 was shown to have good capability to control a high powerloads as compared to CF-20. The effectiveness of inclusion of MWCNTs was remarkable in TM modulesbased on CF-20 as compared to KL1-250.
Compactness and high density of electronic circuitry in high-performance chips leads to tremendous heat dissipations rate [1].Overheating of electronic devices and chips reduces system per-formance and may lead to device failure. Mean-time-to-failure,increases exponentially with temperature [2,3] and this meansthat a small difference in operating temperature can result in afailure or a reduction in the life time of electronic devices. This heatdissipated has to be released/offset by efficient cooling system tomaintain safe operating temperatures. A major challenge in thefield of microelectronics and semiconductors is the thermal man-agement (TM) of denser electronics devices/chips to maintain itsperformance and reliability. This challenge leads to a growing needto develop more effective thermal management (dissipation andstorage/offset of heat release) system for such electronic devices.Using thermal energy storage systems for thermal management of
a).
erved.
electronic devices of high heat densities and cyclic temperaturevariations can be considered as a critical issue in the design of suchdevices.
Phase change cooling have emerged as a widely researchedtechnique for thermal management of high heat fluxes in elec-tronics due to its high latent heat storage. In this system thermalenergy during transient high power load can be stored within thephase change material (PCM) and subsequently rejected to theambient over extended periods maintaining a nearly uniformtemperatures of critical components [4e10]. Cooling of outdoortelecommunications enclosures, portable systems and processorchips employing transient power management features arepossible applications of thermal management of electronic systemsusing PCM.
The improvement of operational performance of portable elec-tronics was indicated when such a passive thermal storage devicewas used. Numerical study of natural convection-dominatedmelting of PCM inside a rectangular enclosure from three discreteheat sources was conducted by Binet and Lacroix [11]. Evans et al.[12] analyzed thermal management of power electronic packagesand provided design guidelines relating the materials, geometry,
Table 2Thermo-physical properties of paraffin wax (RT65) [18].
Paraffinwax
Density,kg/m3
Meltingtemp., �C
Latentheat, kJ/kg
Thermalconductivity,W/m K
Thermaldiffusivity, m2/s
RT65 880 65 152 0.20 1.08E�07
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power input and junction temperature for steady-state conditionsand transient pulses. Andrija Stupar et al. [13] developed an opti-mization procedure for designing a hybrid air cooled heat sinkcontaining PCM for a power electronic device that yielding amaximum possible temperature reduction for a given application.Kamal El Omari et al. [14] numerically analyzed a passive coolingsystem using enclosures with different geometries filled withthermal conductivity-enhanced phase change material (PCM). Thecomputational results showed the high impact of varying geome-try. Sabuj Mallik et al. [15] reviewed the state-of-the-art in thermalmanagement materials which may be applicable to an automotiveelectronic control unit (ECU). This review showed that of thedifferent materials currently available, the Al/SiC composites inparticular had very good potential for ECU application. Yi-HsienWang et al. [16] conducted transient three-dimensional heattransfer numerical simulations to investigate a hybrid phasechange materials (PCM) based multi-fin heat sink showing that theoperating temperature can be controlled well by the attendance ofphase change material and the longer melting time can be con-ducted by using a multi-fin hybrid heat sink respectively.
The above literature showed that most of the TM methods usedPCMs or PCMs with conductive additives as heat transfer en-hancers. The desired temperature control required for the targetedheat management application was achieved using the latent heatstorage ability of PCMs and accordingly a PCM with suitablethermo-physical properties was selected. The present work isfocused on the design of a novel composite material for TM systemwith particular significance for thermal protection of electronicsagainst high density power loads. A composite of paraffinwax (PW)and multi wall carbon nanotubes (MWCNTs) infiltrated in carbonfoam (CF) micro structure hybrid composite has been developed
Fig. 1. CF-20 e general view with magn
and tested for TM of electronic devices under different uniformpower levels. In this composite the CF has been used as a supportstructure for the composite due to its high thermal conductivitywhich leads to an efficient TM system. Two types of carbon foams ofdifferent thermal conductivities have been used as base structure ofthe new composite system. To investigate the performance of thenew composite, pure carbon foam and a composite of carbon foamand Paraffin wax have been also tested as a thermal managementsystem of electronic equipment.
2. Materials and methods
2.1. Basic materials
Three materials have been used to form the composites in thepresent study; namely Carbon Foam (CF) a support structure for thecomposite, pure paraffin wax (RT65) as a PCM and multi walledcarbon nanotubes (MWCNTs) as a heat transfer enhancer. Twotypes of Carbon foam of different thermal conductivities: CF-20 ispartially graphitized carbon foam developed by TouchstoneResearch Laboratory, Ltd. USA, and KL1-250, supplied by KoppersInc., USA. The thermo-physical properties of CF-20, KL1-250 andparaffin wax (RT65) supplied by RUBITHERM Technologies GmbH,used in this study are given in Tables 1 and 2. Thematerials thermo-physical properties are given to us from the suppliers.
Graphistrength® CO1-20 master-batch containing high loading(20 wt.%) of MWCNTs was obtained from Arkema (France) andprovided in pellet form. The degree of purity of those nanotubes is90 wt.%, with length ranging from 0.1 to 10 mm, outer meandiameter in the range of 10e15 nm, and number of walls between10 and 15. Figs.1 and 2 show SEM images of the 3D surface of one ofthe scanned broken pieces for CF-20 and KL1-250. Fig. 3 shows aphoto image of the pellet form of the MWCNTs used in this study.The apparent diameter of the pores in Figs. 1 and 2 looks consistenthaving values of 500 mm and 820 mm respectively as provided bythe supplier.
2.2. RT65/MWCNTs composite
The RT65 was impregnated with MWCNTs to improve its latentheat of fusion and effective thermal conductivity. UsingMWCNTs toimprove RT65 latent heat is a technique approved in a previouswork by Alshaer et al. [19]. The RT65was dopedwithMWCNTswithdifferent mass fractions up to 1%. The effective thermal conduc-tivity enhancement reached by RT65/1 wt.% MWCNTs compositeswas higher than that of the pure RT65 by about 15%. The latent heatof the composites increased significantly and linearly with the
ification �100 left and �500 right.
Fig. 2. KL1-250 e general view with magnification �100 left and �500 right.
W.G. Alshaer et al. / International Journal of Thermal Sciences 89 (2015) 79e86 81
MWCNTs loading leading to an increase in the latent heat by about8.5% [19].
2.3. Thermal management modules and RT65/MWCNTs compositeinfusion in CF
Two sets; each set consists of three thermal managementmodules have been prepared and tested in the present experi-mental investigations. The first set is based on CF-20 carbon foamas a support structure while the second set is based on KL1-250carbon foam as a support structure. The first TM module of eachset consists pure carbon foam core without PCM, the secondmodule is composed of carbon foam infiltrated with pure RT65 asPCM and the third module is composed of carbon foam infiltratedwith RT65/1wt.%MWCNTs composite. Each TMmodules is encasedin an aluminum supporting structure as shown in Fig. 4.
Previous work revealed that the preferred method of infiltratingthe carbon foam pores with a PCM is to melt the PCM and immersethe foam. The pores are then filled by capillary action. Other tech-nique have used vacuum infiltration, in which the foam is evacu-ated before or during infusion, and then the vacuum is released toallow atmospheric pressure to drive the PCM into the foam. Forhigh-density, low-porosity carbon foam, neither evacuation noratmospheric pressure is sufficient to achieve an acceptable PCM fill.A modified version of this method has been used in the presentwork for carbon foam infusions. The vacuum is pulled prior tomelting but not released during solidification with excess pressure
Fig. 3. Pellet form of Graphistrength® CO1-20 masterbatch containing MWCNTs.
up to 6 bar from the infiltration side in order to overcome theproblem of the viscous nano-composite.
Fig. 5 shows a schematic diagram of the infuser components. Itconsists of main cylindrical body (1) connected to a cubical headaluminum assembly (2) through which all PCM composite (8) mustflow. The head assembly holds the sample (3) and also used as amelting chamber that has been mounted on a controlled 1000 Wheater plate (7). A hydraulic cylinder drives the Teflon piston (4) inthe chamber to force PCM through the sample and the head as-sembly. Vacuum pump (5) and control valve (6) are used togenerate and control the vacuum inside the sample. The heater (7)is powered and controlled by a voltage transformer. In a typicalPCM infusion sequence, the chamber is first filled with approxi-mately 150% of the needed PCM; excess material is provided toensure that all air would be expelled out from the sample. Thesample is installed in the head assembly, then the sample/headassembly has been sealed. The sample has been preconditioned bydrawing a partial vacuum and heating to 70 �C for 2 h to removemoisture. After preconditioning, the composite melting sequencestarts until reaching 120 �C. A thermocouple is mounted on thesample head to sense the composite temperature. The meltednano-composite is then forced through the carbon foamwith about6 bar pressure. All infiltrated samples using the above methodshow an infiltration efficiency of about 96%.
3. Experimental setup and procedure
3.1. Experimental setup
The experimental apparatus has been designed to provide aconsistent and controllable/measured set of conditions underwhich TM modules samples have been tested and evaluated. Fig. 6shows a schematic diagram of the test rig. A TM module enclosure
Fig. 4. TM composite sample module (all dimensions in mm).
(7)
(2)(6)
(5)
(1)
(8)(4)
Applied force
(3)
(1) body(2) Head assembly (3) Sample (4) Teflon piston (5) Vacuum pump(6) Control valve(7) Heater plate(8) PCM/nanocomposite
Fig. 5. Infuser, configured for RT65/MWCNTs composite in CF.
Fig. 7. Thermocouples distribution inside the TM modules (all dimensions in mm).
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with a size of 50 � 50 � 40.5 mm has been machined fromaluminum of 1.2 mm wall thickness. Thermal power is supplied tothe sample by a heater block assembly that consists of a 120Wmicaheater element having a size of 40 � 40 mm and 6 mm thick. Thepower to the heater block is supplied by a DC digital power supplywith automatic load sensing. The power supply and DAQ arecomputer controlled via a LabVIEW software. Thermal grease (Heatsink compound RS 554-311) has been applied between the heaterelement and the aluminum block and also between the sampleenclosure and the heater block. Cooling of the sample has beenachieved by constant temperature water circulation unit withcontrolled temperature of 25 �C. A hydraulic press is used to controlan applied force of 1000 N between the elements. The active ele-ments have been wrapped with 50 mm thick expanded cork boardhaving thermal conductivity of 0.037 W/m K. Seven K type ther-mocouples of 0.5 mm diameter have been used for monitoringmodule sample and heater temperatures. All thermocouples areinserted into the center of the sample as shown in Fig. 7. Heaterthermocouple, TH is located just near the heater (between theheater aluminum block and the mica block) using thermal paste.For measuring the junction temperature, TJ thermocouple isattached directly in a grooved channel just near the heateraluminum block and the sample enclosure interface. T1, T2, T3, T4and T5 are located within the center of sample composite formonitoring the temperatures of different points inside the module.All thermocouples were calibrated in a constant temperature pathand a measurement accuracy of ±0.15 �C was obtained. Accuracy ofmeasuring axial distance along thermocouples locations was0.0001 m.
3.2. Experimental program and procedure
As mentioned above, two sets of TM modules with CF-20 andKL1-250 as base structure have been used. For each set, three
Fig. 6. Schematic of the experimental setup.
different modules of the following composition have been testedunder different power inputs:
▪ TM module of pure CF.▪ TM module of CF þ RT65 composite.▪ TM module of CF þ RT65/MWCNTs composite with 1% massfraction of MWCNTs.
In all tests the initial temperature of the TM composite sampleshas been adjusted to 25 �C approximately. Three different uniformpower levels of 18, 24, 30Whave been investigated for TMmodulesbased on CF-20 foam. These operating conditions are typical valuesfor electronic cooling applications [20,21].
4. Results and discussion
4.1. TM modules with CF-20 foam as a base structure
Fig. 8 shows the transient and steady state temperatureresponse of the three TM Modules (Pure CF-20, CF þ RT65 com-posite and CF þ RT65/MWCNTs composite) at a uniform powerinput of 30 W. Typical trends were obtained for other power levels,namely 18 and 24 W. The thermal response of pure CF-20 Module(Fig. 8(a)) is characterized by two stages. The first stage is an initialtransient stage with an approach to the steady state condition untilreaching the steady state condition in the second stage. On theother hand, for CF-20 þ RT65 and CF-20 þ RT65/MWCNTs modules(Fig. 8(b) and (c), respectively), the thermal response is character-ized by four stages. The initial transient stage is similar to thetransient response of a semi-infinite body heated from belowwithout phase change. The second stage starts when the first layerattached to the heater plate starts melting. This is indicated by adeflection of layer temperature response (T1). In this stage, thedownstream solid beyond the melting front behaves like a semi-infinite body with negligible downstream conduction. In the thirdstage, the melting front advances and downstream heat conductionin the solid phase becomes important. This is manifested by avariation in the slope of the thermal response of layers above themelt layer as shown in Fig. 8(b) and (c). In the final stage, amonotonic approach to steady state temperature is observed for alllayers.
As shown in Fig. 8, there is a time lag for CF-20 þ RT65 and CF-20 þ RT65/MWCNTs modules in reaching the steady state
Fig. 8. Temperature histories of thermocouples of different TM modules(power ¼ 30 W).
Fig. 9. Temperature histories at different thermocouple locations for CF-20 þ RT65/MWCNTs.
W.G. Alshaer et al. / International Journal of Thermal Sciences 89 (2015) 79e86 83
temperature. For examples, the first 450 s show a sharper rise inthe heater and junction temperatures of CF-20 module untilreaching 60 �C (Fig. 8(a)). The time needed to reach the sametemperature is 600 s for CF-20 þ RT65 and CF-20 þ RT65/MWCNTs modules (Fig. 8(b) and (c)). For CF-20, the time neededto reach the temperature of 80 �C is 1100 s. On the other handthere is a time lag of 800 s for CF-20 þ RT65 module and 1700 s forCF-20 þ RT65/MWCNTs module to reach the same temperaturelevel of 80 �C. This means that the transient response in CF-20 þ RT65 and CF-20 þ RT65/MWCNTs modules as shown inFig. 8(b) and (c) have been affected by the large increase in theapparent volumetric specific heat of the composite due to theinfiltration of the RT65 and RT65/MWCNTs composite inside theCF structure. In addition, the energy absorbed during phasechange of the infiltrated PCM dampened the increase of theheater and junction temperatures until reaching the steady state
condition. The anomalous transient response of the heater andjunction temperatures observed in the plot of Fig. 8(c) shows astrange constant response at time of 2650 s during about 300 sfollowed by a slow increase until reaching the steady state con-dition. Fig. 8(c) shows that the full melting of the RT65/MWCNTsnear the location of thermocouple T1 occurs at a temperature of69.5 �C and time of 2650 s. The same behavior of T2 and T3 wereobserved with a time lag of 150 s and 50 s, respectively. Thisbehavior of T1, T2, and T3 has not been observed at lower powerlevels of 18 and 24 W for CF-20 þ RT65/MWCNTs (see Fig. 9 as anexample) because of the full melting of RT65/MWCNTs nearlocation T1 has not been reached yet until steady state condition.
A detailed temperature history for all thermocouple locationshas been discussed above for the 30 W uniform power condition tocompare the response between the three TM samples. For all thefurther coming analyses, the presentation and discussion will befocused based on the behavior of themaximum heater temperature(TH).
Fig. 10(a)e(c) shows heater temperature response for CF-20, CF-20 þ RT65 and CF-20 þ RT65/MWCNTs modules at different uni-form powers 18, 24 and 30 W, respectively. Steady state tempera-tures for three TM modules at different power levels are given inTable 3.
As shown in Fig. 10(a)e(c), CF-20 þ RT65/MWCNTs has betterreduction in the heater temperatures as compared to CF-20 and CF-20 þ RT65. The behavior is the same for all power levels withincreasing the percentage reduction of heater temperature withincreasing the power level. For example the reduction of the heatertemperatures for CF-20 þ RT65/MWCNTs is 4.83, 6.15 and 6.78 �Ccorresponding to 18, 24, and 30 W power loads respectively. Theobserved reduction in the heater temperatures can be attributed tothe thermal conductivity enhancement of the composite due to theMWCNTs dispersed in RT65 and entrapped in the carbon foammicro cells upon the infiltration procedure. The increase of tem-perature reduction of RT65/MWCNTs over RT65 with the increaseof power (18e30 W) is related to the thermal conductivityenhancement of the CNTs inside the CF micro cells. This reductionin temperature values is more pronounced for high uniform powerload (30 W) than the low order 18 W power load. The thermalconductivity enhancement for CF-20 þ RT65/MWCNTs may bepossibly due to the formation of a conductive network by the CNTsinside the carbon foam micro cells. Fig. 10(a)e(c) also shows thatthe observed decrease in the heater steady state temperaturevalues for CF-20 þ RT65 module over CF-20 module is negligible.
Fig. 10. Heater temperatures for different TM modules at different uniform powerlevels.
Table 4Uniform power steady state heater temperatures (�C) corresponding to the threesamples for each foam type.
Carbon foam type CF-20 KL1-250
TM structure (18 W power input) (58 W power input)Pure CF 66.22 66.25CF þ RT65 65.83 66.14CF þ RT65/MWCNTs 61.37 65.03
W.G. Alshaer et al. / International Journal of Thermal Sciences 89 (2015) 79e8684
4.2. KL1-250 foam as a base structure
The interest now is directed to study the effect of thermalconductivity of the carbon foam on the performance of the TMmodules to enable the selection of appropriate CF to work withspecified levels of powers of electronic devices. Composites TMmodules based on KL1-250 foam with thermal conductivity of40 W/m K have been considered for this investigation. Similar toCF-20, three TM modules based on KL1-250 foam have been
Table 3Steady state heater temperatures (�C) corresponding to the three TM modules.
prepared and tested; namely Pure KL1-250, KL1-250 þ RT65 andKL1-250þ RT65/MWCNTs TMmodules. The tested power levels forKL1-250 composites module will be magnified over the ones usedfor CF-20 to achieve the same maximum steady state heater tem-peratures levels of CF-20. In order to reach the same steady statetemperature level for KL1-250 foam type as that of the CF-20 foamtype, a trial and error procedures have been applied.
Table 4 represents the steady state heater temperature valuesfor each CF type corresponding for the three studied TM compos-ites. These temperatures are measured using 18Wand 58W powerinputs for CF-20 and KL1-250 respectively to achieve the sametemperature for the case of pure carbon foam. The reduction inheater temperature for other composites is then attributed only tothe TM composite construction. A power input of 58 W is neededfor the pure KL1-250 module to reach the same steady state heatertemperature level of 66.25 �C that obtained for pure CF-20 moduleat power input of 18W. This means that KL1-250modules can carrymore power compared to that of CF-20 for the same operatingtemperature and vise versa. For the same input power, KL1-250modules reveals lower operating temperature compared to thatof CF-20 due to its higher effective thermal conductivity.
The effect of infiltrating RT65 in KL1-250 and infiltrating RT65/MWCNTs in KL1-250 is shown in Fig. 11. As shown in this figure, thesteady state heater temperature for KL1-250 þ RT65/MWCNTs islower than that of KL1-250 þ RT65 and KL1-250 by about 1.2 �C.This can be attributed to the improvement of the thermal con-ductivity of the TMmodules by the addition of MWCNTs. The figurealso shows that in the transient part, the temperature level of forKL1-250 þ RT65/MWCNTs and KL1-250 þ RT65 is lower than thatof pure KL1-250. This can be attributed to the heat stored in thePCM (RT65) in the transient part.
Fig. 12 compares the transient temperature profiles and theinput powers for the two CF foam types (CF-20 and KL1-250) TMcomposites modules. The figure shows that for all the three TMcomposites, the steady state temperature has been reached in KL1-250 quicker than that obtained for CF-20 foam. This can be
Fig. 11. Heater temperatures for the three different modules of KL1-250 based com-posite at 58 W uniform power.
Fig. 12. Transient response of heater temperature for CF-20 and KL1-250 basedstructure modules at different values of input power.
W.G. Alshaer et al. / International Journal of Thermal Sciences 89 (2015) 79e86 85
attributed to the high thermal conductivity of KL1-250 foam. Themost interesting observation in the two CF types infiltrated withRT65/MWCNTs composite is the large decrease in the steady stateheater temperature by about 4.83 �C as shown in Fig. 12(a) for theCF-20 module in comparison with 1.22 �C decrease in the heatersteady state temperature for KL1-250 module. The main reason ofthis observation is related to the more conductivity of KL1-250foam. As the hosting structure becomes more conductive, theshare of any conductive additives becomes smaller.
5. Conclusion
A detailed experimental study of hybrid thermal managementcomposite systems was performed for thermal control and pro-tection of electronic devices. Carbon foam was used as a base
structure for TMmodules due to its high thermal conductivity. Twotypes of carbon foam samples of different thermal conductivities,namely CF-20 of low thermal conductivity (3.1 W/m K) and KL1-250 of medium thermal conductivity (40 W/m K) were tested.Three TM modules of pure CF, CF þ RT65 and CF þ RT65/MWCNTscomposites were prepared and tested for each of CF-20 and KL1-250. RT65 þ 1 wt.% MWCNTs composite was used as a phasechange material and thermal conductivity enhancers. For betterunderstanding of the detailed thermal analysis of the differenttypes of TM composite modules, the thermal response of themodules was analyzed for different input power levels. Resultsshowed a decrease in the module temperature for the modulecomposed of CF-20þ RT65 as compared to pure CF-20 module. Thereduction in module temperature increased with the inclusion ofMWCNTs in the CF-20þ RT65 composite. This enhancement can beexplained by the increase of thermal conductivity of the module asdue to the formation of conductive network by MWCNTs in thecarbon foam micro cells.
Analysis for CF-20 þ RT65/MWCNTs indicated a decrease inheater temperature values for each power level as compared withCF-20 þ RT65 and pure CF-20. Investigation of TM systems usingTMmodules based on KL1-250 foam of higher thermal conductivityinstead of CF-20 was carried out. Similar to CF-20 base structure,the analysis for KL1-250 þ RT65/MWCNTs indicated a decrease inheater temperature value as compared to KL1-250þ RT65 and pureKL1-250. The results showed that, (i) KL1-250 based modules cancarrymore power compared to that of CF-20 for the same operatingtemperature and vice versa for the same input power, KL1-250modules reveals lower operating temperature compared to thatof CF-20, and (ii) for all the three TM composites, the steady statetemperaturewas reached in KL1-250 quicker than that obtained forCF-20 foam. The addition of MWCNTs to the TM composites wasshown to be more effective for carbon foams of relatively lowthermal conductivity.
Acknowledgment
This work was supported by the French government via thecultural section of the French Embassy in Egypt and the Institut deM�ecanique et d'Ing�enierie e Bordeaux e France.
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