Byeongnam Jo1

Nuclear Professional School,

The University of Tokyo,

2-22 Shirakata,

Tokai-mura, Ibaraki 319-1188, Japan;

Department of Mechanical Engineering,

Texas A&M University,

College Station, TX 77843

e-mail: jo@vis.t.u-tokyo.ac.jp

Debjyoti BanerjeeDepartment of Mechanical Engineering,

Texas A&M University,

College Station, TX 77843

Enhanced Specific Heat Capacityof Molten Salt-Based CarbonNanotubes NanomaterialsThis study aims to investigate the specific heat capacity of a carbonate salt eutectic-based multiwalled carbon nanomaterial (or high temperature nanofluids). The specificheat capacity of the nanomaterials was measured both in solid and liquid phase using adifferential scanning calorimetry (DSC). The effect of the carbon nanotube (CNT) con-centrations on the specific heat capacity was examined in this study. The carbonate mol-ten salt eutectic with a high melting point around 490 �C, which consists of lithiumcarbonate of 62% and potassium carbonate of 38% by the molar ratio, was used as abase material. Multiwalled CNTs were dispersed in the carbonate salt eutectic. A surfac-tant, sodium dodecyl sulfate (SDS) was utilized to obtain homogeneous dispersion ofCNT into the eutectic. Four different concentrations (0.1, 0.5, 1, and 5 wt.%) of CNTwere employed to explore the specific heat capacity enhancement of the nanomaterials asthe concentrations of the nanotubes varies. In result, it was observed that the specificheat capacity was enhanced by doping with the nanotubes in both solid and liquid phase.Additionally, the enhancements in the specific heat capacity were increased with increaseof the CNT concentration. In order to check the uniformity of dispersion of the nanotubesin the salt, scanning electron microscopy (SEM) images were obtained for pre-DSC andpost-DSC samples. Finally, the specific heat capacity results measured in present studywere compared with the theoretical prediction. [DOI: 10.1115/1.4030226]

Keywords: specific heat capacity, nanomaterial, carbon nanotubes, molten salts

1 Introduction

Recently, depletion of fossil fuel and concern about globalenvironment have led to promote and to intensify research intovarious renewable energy such as sunlight, geothermal, wind,tides, and so on. Among these renewable energy sources, solarthermal power generation systems have been considered as anattractive option to generate bulk power equivalent to conven-tional power plants. Currently, concentrating solar power (CSP)plants have been operated in several countries with various typesof concentrating technologies [1].

Synthetic oil, pressurized steam, and molten salt eutectics areused as energy storage medium and heat transfer fluid in the CSPsystems [2,3]. Although the synthetic oil such as Therminol iswidely used in the CSP plants, its high vapor pressure restricts theoperating temperature of the CSP power plants. While molten salteutectics do not have the vapor pressure issue and make the CSPsystems to be operable at higher temperature, their relatively poorthermal properties like specific heat capacity and thermal conduc-tivity are impediment to improve the efficiency of the powerplants and ultimately to reduce the generation cost of electricity.Therefore, enhancing thermal properties of the molten salteutectics is important to improve the overall efficiency ofthe CSP power plants and to reduce the generation cost of theelectricity [3].

With the development of nanoparticle synthesis technique,extensive studies into the nanofluids have been performed in thepast decade. First of all, since anomalously increased thermal con-ductivity of nanofluids were reported in 2001, in which coppernanoparticles and CNTs were dispersed into ethylene glycol andinto oil, respectively [4,5], an immense number of experimentalstudies about the thermal conductivity of nanofluids was per-formed with various nanoparticles [6–9]. Theoretical studies, as

well as the experimental works, suggested several possible mecha-nisms to understand and to explain the enhanced thermal conduc-tivity of nanofluids [10]. Additionally, research into the effect ofnanoparticles on pool boiling characteristics and on the criticalheat flux was also actively conducted under various experimentalconditions [11–13]. While many people focused on the thermalconductivity and the boiling heat transfer of nanofluids, relativelya small number of studies was performed to measure specific heatcapacity of the nanofluids. In 2008, Zhou and Ni reported that thespecific heat capacity of water-based Al2O3 nanofluid wasdecreased with increase of the nanoparticle concentration [14].Similarly, Vajjha and Das measured the specific heat capacity forthree different water/ethylene glycol based nanofluids using nano-particles of Al2O3, ZnO, and SiO2 [15]. They also obtaineddecreased specific heat capacity values compared with pure basefluids. Recently, Zhou and collaborators reported the similarresults (decrease) for the specific heat capacity of ethyleneglycol–CuO nanofluids [16]. While it was observed for the spe-cific heat capacity to be decreased in several literatures, therewere previous studies showing the enhanced specific heatcapacity. In 2009, Nelson et al. measured the specific heatcapacity of a polyalphaolefin (PAO) nanofluid with graphite nano-particle fibers [17]. They measured a drastic enhancement of up to50% by adding 0.6 wt.% of the nanoparticle in the PAO nanofluid.Shin and Banerjee observed the enhanced specific heat capacityby doping with silica nanoparticles in some molten salt eutecticsuch as the carbonate salt eutectic and chloride molten salt eutec-tic [18–20]. Bridges and collaborators [21] reported that the volu-metric heat capacity of an ionic liquid was enhanced by dopingwith alumina nanoparticles by up to 45%. Most recently, Jo andBanerjee reported the enhanced specific heat capacity of moltensalt nanomaterials in both liquid and solid phase [22,23]. Theyshowed the effect of base materials [22] and dispersion homoge-neity [23] on the specific heat capacity enhancement.

As introduced above, there is a controversy in the specific heatcapacity of the nanofluids. According to the previous studies, itcan be thought that the base materials and the nanoparticles affect

1Corresponding author.Manuscript received April 24, 2014; final manuscript received October 16, 2014;

published online May 14, 2015. Assoc. Editor: Yogesh Jaluria.

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the specific heat capacity of nanofluids (nanomaterials in liquidphase). Moreover, the effect of nanoparticle concentrations on thespecific heat capacity enhancement such as a threshold concentra-tion has not been sufficiently discussed for molten salt-basednanomaterials. This paper aims to measure the specific heatcapacity of the carbonate salt eutectic-based CNTs nanomaterialusing a DSC. The effect of nanoparticles (CNTs) concentrationson the specific heat capacity of the nanomaterial was examinedfor four concentrations. Three different temperature ranges werechosen to compare the specific heat capacity of the nanomaterialswith that of the pure eutectic: in solid phase (250 �C and 400 �C)and in liquid phase (mean value between 525 �C and 555 �C). TheSEM images were taken for both pre-DSC samples and post-DSCsamples in order to confirm the CNT dispersion of the nanotubesinto the carbonate salt eutectic and to find whether or not aremarkable change in the post-DSC samples was occurred duringseveral thermal cycles in DSC. Finally, the comparison of the spe-cific heat capacity between the experimental measurement in pres-ent study and theoretical predictions by conventionally usedmodels was also discussed.

2 Materials and Methods

2.1 Nanomaterial Synthesis. A carbonate salts eutectic andmultiwalled CNTs (CNT, Meliorum technologies, Rochester, NY)were used as a base material (solvent) and as nanoparticles, respec-tively. A surfactant, SDS (Sigma Aldrich, St. Louis, MO) was cho-sen to be a dispersant. The carbonate salt eutectic was composed oflithium carbonate (Li2CO3, 62 mol. %) and potassium carbonate(K2CO3, 38 mol. %), which were purchased from Sigma Aldrich.All chemicals were used as received in present study. The size ofthe CNT was 10–30 nm in diameter and 1.5 lm in length, accordingto the specification obtained from the manufacturer. Figure 1 showsa transmission electron microscopy (TEM, JEOL JSM-2010) imageof CNT. As shown in the figure, the mean diameter of the

nanotubes is quite similar to their specification. The concentrationof CNT was varied from 0.1% to 5% in mass ratio.

Figure 2 shows the schematic of the procedures used to synthe-size the carbonate molten salt eutectic-based CNT nanomaterialsfor specific heat capacity measurements. Initially, the CNT wasdispersed in distilled water with the SDS (at a mass concentrationof 1%) with respect to the eutectic to avoid agglomeration of theCNT. Then the colloid was subject to sonication in an ultrasonicbath for 2 hr. The aqueous salt solution was mixed with additionalvolumes of water and the aqueous CNT nanofluid. Then the sus-pension was sonicated again for 3 hr. Immediately after the soni-cation process, the water was evaporated using a syringe pump toobtain dry nanomaterial samples for the specific heat capacitymeasurements in DSC: The suspension was dispersed drop-by-drop on a beaker placed on a hot-plate at 250 �C. Since SDS iseasily decomposed by prolonged heating at a certain temperatureover 40 �C [24], and moreover it was obtained that fast waterevaporation favorably affects the enhancement of the specific heatcapacity in our previous study [23]. Hence, this syringe methodwas employed to minimize the water evaporation time, to avoidagglomeration of the CNT, and finally to augment the enhance-ment of the specific heat capacity of the nanomaterials. In order toconfirm whether the nanotubes were well mixed with the salteutectic, TEM (JEOL JEM-2010) images were taken for pre-DSCsamples as show in Fig. 3. From this image, it can be observedthat the salt eutectic mixed homogeneously with the CNTs. Inaddition, SEM (JEOL JSM-7500F) for pre-DSC samples andpost-DSC samples was also taken to examine differences in theCNT dispersion after the thermal cycles in DSC.

2.2 Specific Heat Measurements. The specific heat capacityvalues of the nanomaterials and pure eutectic were measuredusing DSC (TA instrument, model: Q20) for both solid and liquidstates. Based on the measured difference in heat flow, the specificheat capacity was determined by following a standard protocol

Fig. 1 TEM image of multiwalled CNTs

Fig. 2 Schematic of procedures for synthesizing nanomaterials

Fig. 3 TEM image of nanomaterials synthesized by mixing car-bonate salt eutectic with CNT

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established by the American standard test method (ASTM E1269)[25]. According to this protocol, the difference in heat flow valuesbetween an empty pan (Tzero aluminum hermetic pans, TA instru-ments) and a reference material (a sapphire disk) was recorded as afunction of temperature from the DSC instrument. The measure-ments were repeated using the same thermocycling protocol multi-ple times in every test for the individual samples (pure eutectic ornanomaterials). The temperature range was varied from 150 �C upto 560 �C. A ramp rate of 20 �C/min was employed in the heatingprocess. An isothermal process was used at the beginning and at theend of each thermal cycle (i.e., at 150 �C and 560 �C). The heatflow data of the first thermal cycle in the DSC were discarded incalculation of the specific heat capacity because the first heat flowcurve was often observed to be skewed, probably due to thermalnonequilibrium issues. For the DSC test runs of the empty pans andthe sapphire standard, the heat flow measurements from the firstrun were also discarded. After the first cycle, the measurements inDSC were repeated five times in order to ensure reproducibility ofthe experimental results and to verify if significant variationoccurred in the measurements. The specific heat capacity valueswere compared in both solid and liquid phase: at 250 �C and at400 �C for solid phase and mean values between 525 �C and 555 �Cfor liquid phase. Prior to the measurement of the specific heatcapacity of nanomaterials, the specific heat capacity of pure eutec-tic was measured to compare with a reference value from a litera-ture [26] in order to verify the experimental conditions and thematerials themselves. The results for the nanomaterials were com-pared with that of the pure eutectic to examine the enhancements.

A theoretical model (thermal equilibrium model) shown inEq. (1) which is based on the mass fraction of nanoparticles added

was proposed to calculate the specific heat capacity of mixtures[27], and it has been used to validate the experimental results forthe specific heat capacity of several nanofluids in previous litera-tures [14,16,28]. This theoretical model was employed in thisstudy to predict the specific heat capacity of the molten salt nano-materials and to compare them with the experimental results

cP;nf ¼mn � cP;n þ mf � cP;f

mn þ mf

(1)

where m is the mass fraction, cp is the specific heat capacity, sub-script n indicates the nanoparticle, subscript nf indicates nano-fluids, and subscript f indicates the base fluid (solvent).

2.3 Measurement Uncertainty. As mentioned above, thespecific heat capacity was determined by ASTM method accord-ing to the following equation:

cp;s ¼ cp;st

Dqs � mst

Dqst � ms

(2)

where cp is the specific heat, Dq is the heat flow differencebetween the specimen and the empty pan, and m is the mass. Sub-scripts, s and st, indicate the samples (pure salt eutectic or nano-materials) and the standard materials (sapphire in this study),respectively. The heat flow differences were obtained by subtract-ing baseline (empty pan, subscript b) heat flow from the heat flowof the sapphire and the sample. The measurement uncertainty isrepresented as follows [29]:

Ucs

cs

¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiU cstð Þ

cst

� �2

þ U Dqbð Þd qbð Þ

� �2

þ U Dqsð Þd qsð Þ

� �2

þ U Dqstð Þd qstð Þ

� �2

þ U msð Þms

� �2

þ U mstð Þmst

� �2s

(3)

The uncertainty was resulted from curve fitting of the specificheat capacity of the sapphire, the heat flow of the sample and thesapphire, and the mass of the sample and the sapphire. The uncer-tainties of the curve fitted specific heat capacity of the sapphireand the heat flow are 60.3% and 62%, respectively. The maxi-mum measurement uncertainty in the determination of the specificheat capacity is estimated to be 63.5%.

3 Results

3.1 Specific Heat Capacity of Nanocomposites (Nanomaterialin Solid Phase). Figure 4 shows the heat flow curves of a pure salteutectic and a nanomaterial sample (containing CNTs of 5 wt.%).Comparing the two curves, it is observed that there is only a mar-ginal difference in the magnitude of the heat flow curves for thetwo samples. It should be noted that the magnitude of heat flowcurves depends on the amount of samples loaded in the test pan.Accordingly, it can be presumed no chemical reaction occurredbetween the salt eutectic and the CNT (including SDS) over thewhole temperature range in which measurements were performed.

The specific heat capacity of the nanocomposites and theenhancement at 250 �C for four different concentrations of CNTare shown in Fig. 5. A simple correlation for the specific heatcapacity of the pure carbonate salt eutectic at a certain tempera-ture range suggested by Araki et al. was presented in the followingequation [26]:

cP ¼ 0:562þ 1:16� 10�3 � T J=g Kð Þ 365 � T � 631Kð Þ (4)

where T indicated the absolute temperature. At 250 �C, the litera-ture value is 1.169 (J/g K), and similarly the experimental value

obtained in our previous study was 1.19 (J/g K), which is a margin-ally different from the literature data but it is in reasonable agree-ment [30]. Compared with the specific heat capacity of the pureeutectic (1.19 (J/g K)), the measured specific heat capacity for thevarious concentrations of CNT was observed to be enhanced. Asshown in Fig. 5, the specific heat capacity at the lowest concentra-tion of CNT explored in this study (0.1 wt.%) was 1.24 (J/g K),which corresponds to the enhancement of about 4.0%. Taking themeasurement uncertainty into consideration, the enhancement at

Fig. 4 Heat flow curves of a pure eutectic sample and a nano-material sample obtained from DSC

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0.1 wt.% is within the margins of the measurement uncertainty(�3%). As the concentration of CNT was increased to 5%, the spe-cific heat capacity was also increased up to 12.3%.

A similar behavior in the specific heat capacity of the nanocom-posites was obtained at 400 �C as shown in 250 �C. Figure 6shows the specific heat capacity of the nanocomposites and thecorresponding enhancements at 400 �C. At the CNT concentrationof 0.1 wt.%, almost the same specific heat capacity as that of thepure eutectic was obtained. At higher concentration of CNT,larger enhancement in the specific heat capacity values wasobserved. The highest enhancement obtained at 5 wt.% of CNTwas 12.0% (which is similar to 12.3% at 250 �C).

3.2 Specific Heat Capacity of Nanofluids (Nanomaterialsin Liquid Phase). The specific heat capacity of the nanofluidsamples (in liquid phase) was also measured in present study. Inthe literature, Araki and collaborators experimentally measuredthe heat capacity of the pure carbonate salt eutectic and theyreported a constant value of 1.6 (J/g K) [26]. The specific heatcapacity measured in this study was 1.612 (J/g K), and it was uti-lized to calculate the enhancements in the specific heat capacityfor nanofluids [30]. Figure 7 shows the results for the enhance-ments of the specific heat capacity in liquid phase. The concentra-tion dependence of the specific heat capacity of nanofluids wassignificant. The specific heat capacity results were significantlyenhanced with increasing concentration of CNT. The mostremarkable feature in the liquid phase was the relatively largerenhancements than those at both 250 �C and 400 �C (solid phase).In liquid phase, the specific heat capacity of the nanomaterial wassignificantly increased by up to 14.7% on mixing with CNT atmass concentration at 5%.

3.3 Theoretical Prediction of Specific Heat. Using the ther-mal equilibrium model in Eq. (1), the specific heat capacity of thenanofluids (nanomaterial samples in the liquid phase) was pre-dicted as a function of the nanoparticle concentration. The specificheat capacity values of the eutectic, the graphite, and the CNT atthree different temperatures are listed in Table 1. The specific heatcapacity for the nanoparticles (CNT) which is needed to theoreti-cally estimate the specific heat capacity of the nanofluid wasobtained from a graphite value [31]. The theoretical predictionsfor the specific heat capacity values were plotted as a function ofthe temperature and the concentration of CNT in Fig. 8(a). Asexpected, larger enhancements in the specific heat capacity of thenanofluids with increasing amounts of CNT were observed. How-ever, the theoretical predictions did not match the experimentaldata. As shown in Fig. 8(a), the maximum enhancement predictedby the thermal equilibrium model was less than 1% even at theCNT concentration of 5 wt.%. However, the enhancement in theexperiments was obtained to be more than 14% at the same con-centration of the CNT. The prediction from the thermal equilib-rium model did not match the experimental results, even when thespecific heat capacity of CNT (generally referred to as cv) wasreplaced with the higher value for single walled CNTs network[32]. The maximum prediction value in Fig. 8(b) was about 2.5%,but it is still much less than the experimental results (12.3%).

4 Discussion

The lack of agreement between predictions from the theoreticalmodel and the experimental data can be explained to be due toseveral factors. First, the specific heat capacity of the nanoparticle(CNTs in this study) is likely to be drastically increased.

Fig. 5 (a) Specific heat capacity and (b) enhancements of thenanomaterials in solid phase (250 �C) as a function of CNTmass concentration

Fig. 6 (a) Specific heat capacity and (b) enhancements of thenanomaterials in solid phase (400 �C) as a function of CNTmass concentration

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Wang et al. calculated the specific heat capacity of a nanoparticleand also investigated the effect of the particle size and the temper-ature [33]. In their paper, they showed that the specific heatcapacity can be enhanced with decrease in the size of the nanopar-ticle and with increase in temperature. Nelson et al. pointed outthis effect as a possible reason for the enhanced specific heatcapacity of the PAO-graphite nanofluids [17]. However, the spe-cific heat capacity of the CNTs needs to be up to 350% at the tem-perature where the eutectic exists in liquid, for the theoreticalmodel to match the experimental results. Therefore, although theenhanced specific heat capacity of the nanoparticles might be asignificant contributor, it does not account for the observedenhancements for the specific heat capacity of the nanofluids.

As a unique feature of nanofluids, it was reported that a denselayer of liquid molecules (compressed liquid layer) is formed onthe surface of the nanoparticles in experimental and computa-tional studies. Oh et al. reported the evidence for ordering of liq-uid atoms adjacent to crystalline interface [34]. Moleculardynamics simulations, as well as the experimental proof by

Oh et al., showed the formation of a compressed layer with a cer-tain thickness at the interface between a solid nanoparticle and liq-uid molecules, for instance in the molten salt eutectic–CNTsnanofluids [22] and in the water–CNTs nanofluids [35]. This moreordered liquid regions which can be regarded to acquire a quasi-crystalline structure can amplify the capability of thermal energystorage in the nanofluid. Hence, the specific heat capacity of thenanofluid can be modified by doping with a minute concentrationof the nanoparticles in the salt eutectic samples. The compressedregion of the liquid molecules is expected to play a significantrole for the enhancements of the specific heat capacity of thenanofluids. For the specific heat capacity enhancements in solidphase, the increase in the specific heat capacity of the nanotubesthemselves with temperature might play a more dominant role inraising the specific heat capacity. Relatively small enhancementsof the specific heat capacity in solid phases can support this partic-ular hypothesis.

Based on the contribution from the interfacial compressed layeron the specific heat capacity, higher enhancements are expected tooccur in the liquid phase. Figure 9 shows the SEM images of apre-DSC sample and a post-DSC sample (with 5 wt.% of CNT).While some of the nanotubes are observed to exist individually inthe eutectic, a larger amount of agglomerated CNT was observedin the images. Hence, better dispersion of the CNT can enablelarger enhancement in the specific heat capacity of the nanomate-rial [23]. In previous studies, the effect of the liquid layering onthe solid particle interface has been discussed to be a significantcontributor to the dramatically enhanced thermal conductivity ofnanofluids [36–38]. Accordingly, it can be expected that the ther-mal conductivity of the molten salt eutectic can also be augmented

Fig. 7 (a) Specific heat capacity and (b) enhancements of thenanomaterials in liquid phase as a function of CNT massconcentration

Table 1 Specific heat capacity of pure carbonate salt eutectic,graphite, and CNTs

Temperature(�C)

Carbonateeutectic(J/g K)

Graphite[28]

(J/g K)

CNTnetwork [29]

(J/g K)

250 1.188 1.25 1.79400 1.287 1.50 1.88525–555 (liquid phase) 1.612 1.66 1.97

Fig. 8 Theoretical prediction of specific heat capacity in thenanomaterials using thermal equilibrium model: (a) using thespecific heat capacity of graphite [28] and (b) using the specificheat capacity of CNT network [29]

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by dispersing nanoparticles. Additionally, percolated tube struc-tures were also suggested as a significant factor for the drasticallyenhanced thermal conductivity of nanofluids [39]. This percolat-ing network may also increase the thermal conductivity of themolten salt eutectic-based CNT nanomaterials, because thesechainlike structures can enhance heat transport in the nanomateri-als. However, it is presumed that the percolated CNTs do not playan important role in the enhanced specific heat capacity of thenanomaterials. As mentioned above, the effect of the compressedlayers was much more dominant on the specific heat capacity ofthe nanomaterials. Thus, the percolating network structures canreduce the total volume of the compressed liquid layer due toincrease of the effective nanoparticle size.

As shown in Figs. 5–7, the specific heat capacity of the nano-materials was proportional to the concentration of CNT. The peakpoint where the specific heat capacity has the maximum valuewas not obtained in present study. Based on our speculation thatthe compressed liquid layer adjacent to nanoparticles increasesthe specific heat capacity of the nanomaterials, the agglomerationof CNT unfavorably affects the specific heat capacity enhance-ment, and the agglomerated CNT should increase with the CNTconcentrations. Therefore, the specific heat capacity of the nano-materials would have the maximum value at a certain concentra-tion, and it could decrease with increase of the CNTconcentrations.

In the present study, the specific heat capacity of the carbonatesalt eutectic was enhanced by doping them with CNT, and thelevel of enhancement was much higher than the theoretical predic-tions. Since the thermal equilibrium model for determining thespecific heat capacity of mixtures is a function of the specific heatcapacity and mass fractions of the constituents, variations in thespecific heat capacity of the nanofluids are entirely dependent onthe amount of nanoparticles and their properties. Thus, the addi-tions of CNT at small amounts concentration cannot enable thelevel of the specific heat capacity enhancement to be significant.Hence, there is a contradiction between the experimental resultsand the theoretical predictions. Although the theoretical model

was in good agreement with the data for the specific heat capacityof aqueous nanofluids [14], this study reported degradation in thespecific heat capacity of the aqueous nanofluids. Consequently, itis expected that the theoretical model may be applicable to limitedcases such as liquid–liquid mixtures or liquid–vapor mixtures.Therefore, a new theoretical model which can be applicable for allnanofluids is needed.

5 Conclusions

The specific heat capacity enhancements for the carbonate salteutectic-based CNT nanomaterial were observed for both solidand liquid phase. The effect of the CNT concentrations was inves-tigated in this study. In addition, the experimental results werecompared with the theoretical predictions. The knowledge gainedfrom these experiments is summarized as follows:

(a) The specific heat capacity was significantly enhanced by upto about 15% in liquid phase by dispersing multiwalledCNTs at 5% mass concentration in the carbonate salt eutec-tic. Also, even at a minute concentration of CNT(0.1 wt.%), the specific heat capacity was enhanced in liq-uid phase. Moreover, in solid phase, the specific heatcapacity enhancement was up to �12% for the nanofluidwith CNT at 5 wt.%.

(b) The specific heat capacity of the nanomaterial was gradu-ally increased with the concentration of CNT. The enhance-ment was higher in liquid phase than in solid phase. Thisimplies that a possible transport mechanism exists exclu-sively in liquid phase.

(c) The CNT dispersion was confirmed by TEM images forpre-DSC samples. In addition, no significant difference inSEM images between the pre-DSC samples and the post-DSC samples was observed.

(d) Theoretical prediction for the specific heat capacity valueswas obtained using a conventional analytical model. How-ever, the prediction from the theoretical model did not

Fig. 9 SEM images of a pre-DSC sample (a) and (b) and a post-DSC sample (c) and (d)

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match the enhancement for the specific heat capacity val-ues, as observed in the experimental measurements. Thisshows that the literature model (which is based on mixingproperties of immiscible substances or phases) is limited inapplicability. Better theoretical models are required to esti-mate the specific heat capacity of the nanomaterials.

Acknowledgment

For this study, the authors acknowledge the support of theDepartment of Energy (DOE) Solar Energy Program (Golden,CO) under Grant No. DE-FG36-08GO18154; Amd. M001 (Title:“Molten Salt-Carbon Nanotube Thermal Energy Storage for Con-centrating Solar Power Systems”).

Nomenclature

cp ¼ specific heat capacity (J/g K)m ¼ mass (g)T ¼ temperature (K or �C)

Dq ¼ heat flow difference (mW)

Subscripts

b ¼ baseline (empty pan)f ¼ base fluid (base material)n ¼ nanoparticle

nf ¼ nanofluids ¼ sample

st ¼ standard material

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