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Thermal conductivity enhancement of lauric acid phase change nanocomposite with graphene nanoplatelets
Accepted Manuscript Thermal conductivity enhancement of lauric acid phase change nanocomposite with graphene nanoplatelets Sivasankaran Harish, Daniel Orejon, Yasuyuki Takata, Masamichi Kohno PII:
S1359-4311(15)00071-X
DOI:
10.1016/j.applthermaleng.2015.01.056
Reference:
ATE 6325
To appear in:
Applied Thermal Engineering
Received Date: 1 September 2014 Revised Date:
4 December 2014
Accepted Date: 22 January 2015
Please cite this article as: S. Harish, D. Orejon, Y. Takata, M. Kohno, Thermal conductivity enhancement of lauric acid phase change nanocomposite with graphene nanoplatelets, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2015.01.056. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Thermal conductivity enhancement of lauric acid phase change nanocomposite with graphene nanoplatelets
a
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Sivasankaran Harisha, Daniel Orejona,b, Yasuyuki Takataa,b, Masamichi Kohnoa,b* Department of Mechanical Engineering, Thermofluid Physics Laboratory, Kyushu University,
744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan.
International Institute for Carbon-Neutral Energy Research (WPI - I2CNER), Kyushu
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b
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University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. Abstract
In this work, we prepared lauric acid based phase change nanocomposites with chemically functionalized graphene nanoplatelets and measured its thermal conductivity using
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transient hot wire method. We show that inclusion of graphene nanoplatelets increases the thermal conductivity of phase change material by 230 % at a loading of 1 vol %. Comparing the experimental results with the model calculations based on the effective medium theory suggests
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that graphene based nanocomposites outperforms those with carbon nanotubes or metal nanoparticles reported in the literature. High thermal conductivity, high aspect ratio and low
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thermal interface resistance at the graphene - host matrix interface makes it the most suitable nano filler candidate to enhance the thermal conductivity of low conductive materials. Differential scanning calorimetry study of the nanocomposites show that the phase change enthalpy and the melting temperature remains similar to that of pristine material, which makes graphene a promising candidate for thermal energy storage applications. Keywords: phase change material, graphene, thermal conductivity, carbon nanotube, lauric acid *Corresponding author E-mail address: [email protected] (M. Kohno)
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d l
k pcm
phase change material thermal conductivity, W m-1 K-1
kMLG
thermal conductivity graphene nanoplatelets
T
t q
k
c ii
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k keff
diameter, m length, m temperature, °C time, s heat flux per unit of length, W m-1 thermal conductivity, W m-1 K-1 effective thermal conductivity, W m-1 K-1
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Nomenclature
equivalent thermal conductivity along the symmetric axis, W m-1 K-1
equivalent thermal conductivity along the transverse direction, W m-1 K-1
k 33c kp
equivalent thermal conductivity along the longitudinal direction, W m-1 K-1 thermal conductivity of spheroid, W m-1 K-1
Lii
geometric shape factor of ellipsoidal particle along symmetric axis
L11
geometric shape factor of ellipsoidal particle along transverse direction
L33 a R
geometric shape factor of ellipsoidal particle along longitudinal direction aspect ratio length to diameter thermal boundary resistance, m2 K W-1 volume fraction phase change enthalpy, J kg-1
∆H fus
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1. Introduction
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φ
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k11c
Thermal energy storage using phase change materials (PCM) are often employed in waste
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heat recovery and solar energy storage [1, 2]. Thermal energy storage is also an attractive way to cool micro-electronic devices during peak loads [3]. At present, many PCMs which can be classified as inorganic, organic and their mixtures have been used for this purpose due to its large latent heat storage capabilities and wide range of melting temperatures [4, 5]. Inorganic PCMs possess higher latent heat storage capabilities when compared to organic PCMs however their applications are rather limited due to high super cooling. Among organic PCMs, fatty acids and
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their eutectic mixtures are considered promising for storage applications due to its excellent properties such as high phase change enthalpy, non-toxicity, low vapour pressure, little or no
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super cooling, good thermal and chemical stability, wide range of melting point and low cost [6]. The rate of energy storage and release of PCMs is directly proportional to its thermal conductivity. Fatty acids based PCMs like other organic materials possess low thermal
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conductivity, which remains a great challenge for practical applications. Recently this has led to increasing interests in the use of high conductive nano inclusions to enhance the thermal
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conductivity of such PCMs [7]. High conductive nanoparticles such as Ni, Al2O3, TiO2 [8-12], silver nanowires [13] or carbon nanoadditives such as nanofibers [14], carbon nanotubes (CNT) [15-18] and graphite nanoplatelets [19-23] have also been used for enhancing the thermal conductivity of PCMs.
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Utilization of metallic nanoparticles would decrease the energy storage capabilities of the PCMs. Carbon based nanoadditives have shown tremendous potential to enhance the thermal conductivity of organic PCMs due to its high thermal conductivity. Carbon nanotubes (CNT), a
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one-dimensional allotrope of carbon is widely used for this purpose. Wang et al. [15, 16] reported a thermal conductivity enhancement of 51 % at a loading of 1 wt % for chemically
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modified CNTs dispersed in palmitic acid. Cui et al. [17] investigated the effect of carbon nanofibers and CNTs in soy wax based nanocomposites. They reported a significant improvement in thermal conductivity upon nano inclusions. However, they showed that CNTs performed poorly compared to carbon nano fibers which is in direct contradiction to the results of Wang et al. [15, 16]. Recent thermal conductivity measurements of graphene nanosheets, a two-dimensional allotrope of carbon shows a much higher thermal conductivity than CNTs [19]. Hence it is anticipated that the use of graphene will significantly enhance the thermal
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conductivity of organic fatty acids based PCMs, the latter being of significant interest for energy storage applications. Yavari et al. [19] utilized graphene nanosheets and reported a thermal conductivity enhancement by a factor of ~1.8 for fatty acid based ester nanocomposites at a
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loading of 1 vol %. Similar thermal conductivity enhancement was reported for paraffin/graphene nanoplatelets based nanocomposites in the literature [19-23].
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Based on the previous studies, it is evident that many high conductive nanomaterials were utilized to enhance the thermal conductivity of organic PCMs. However, it still remains unclear
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in selecting the appropriate material and dimensionality of nano inclusion to enhance the thermal transport of organic PCMs without affecting the energy storage capability. In this work, using an organic fatty acid based phase change material we show remarkable enhancement in thermal conductivity by a factor of approximately ~2.3 using chemically functionalized graphene loading of 1 vol %. Besides, we compare the experimental results with model calculations based on
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effective medium theory for nano inclusions of different dimensionalities considering the role of interfacial thermal transport into account. Based on model calculations, we show that planar structure, high thermal conductivity and low Kapitza resistance between graphene-host matrix
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interface make graphene nanoplatelets a promising nano filler candidate. Furthermore, this
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material offers better thermal performance to enhance the thermal properties of organic materials than carbon nanotubes or other nanoparticles reported in the literature. Moreover, differential scanning calorimetry investigations show that the energy storage capability and the phase transition temperature of the PCMs remain unaltered at small loadings which makes graphene a promising filler candidate for thermal energy storage applications.
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2. Materials and Methods 2.1 Material characterization and sample preparation
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The phase change material used in the present work is n-Dodecanoic acid (Lauric acid, C12H24O2) with a melting temperature of ~44 °C and with an excellent phase transition enthalpy of ~180 kJ/kg. Liquid phase exfoliated multilayer graphene nanoplatelets (MLG) were purchased
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from XG Sciences with an average thickness of 5 - 10 nm (Grade M, Mean particle diameter of 15 µm, Density 2.2 g/cm3). A scanning electron microscopy image (SEM, FEI Versa 3D Dual
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Beam, LoVac detector) of pristine lauric acid is shown in figure 1.
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Figure 1: SEM visualization of pristine lauric acid Figure 2 (a) shows the SEM visualization of MLG nanoplatelets and figure 2 (b) shows
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the typical Raman spectra (HORIBA micro-Raman system HR-800) of the MLG nanoplatelets measured using a laser wavelength of 488 nm. The G-band observed around 1582 cm-1 corresponds to the in-plane vibrations of the carbon atoms. The D-band which is also known as disorder band observed around 1350 cm-1 corresponds to the defects in the graphitic structure. The 2D-band observed around 2750 cm-1 is considered to be the overtone of D-band arising as a result of two phonon lattice vibrations [24].
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G−band
Intensity (arb. u)
(a)
(b)
D−band
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2D−band
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1200 1500 1800 2100 2400 2700 −1 Raman Shift (cm )
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Figure 2: (a) SEM visualization of MLG nanoplatelets. (b) Typical Raman spectra of MLG nanoplatelets.
MLG nanoplatelets of 0.5 g were dispersed in 25 mL of concentrated nitric acid (68 wt %) and then refluxed at a temperature of 100 °C for 2h. The mixture was filtered, washed with deionized water and dried at 150 °C in vacuum for 6 h. The nanocomposite samples were
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prepared by adding MLG nanoplatelets with molten lauric acid kept on a hot plate maintained at a temperature of 100 °C under rigorous stirring using a magnetic stirrer at 250 rpm for 30 min.
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After that, the samples were subjected to intensive sonication using an ultrasonic processor (Hielscher GmbH, UP-400S with H3/Micro Tip 3) for 30 min at 50 % amplification. Samples
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with MLG nanoplatelets loading of 0.1 vol %, 0.25 vol %, 0.5 vol %, 0.75 vol % and 1 vol % were prepared along with a reference sample of pristine lauric acid.
2.2 Thermal conductivity measurements Thermal conductivity of the nanocomposites was measured using a custom built transient hot wire (THW) method developed by Nagasaka and Nagashima [25]. Details of the experimental setup are reported elsewhere [26]. In brief, a platinum wire of diameter (d) 25.4 µm
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with an electrically insulating isonel coating of 2 µm and length (l) 50 mm is used in present experiments. The hot wire acts as both the heating element and as the electrical resistance thermometer. During measurement, the transient hot wire is heated by a constant DC power
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supply. The temperature (T) rise of hot wire was determined from the change in resistance of hot wire, which can be measured as a function of time (t) using a Wheatstone-bridge circuit. From a known electric power supply (q) and the slope of the curve ln(t) versus T, the thermal
and measured temperature T can be written as follows [27]. q d ln t 4π dT
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k=
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conductivity (k) was estimated using equation (1). The relation between thermal conductivity k
(1)
In equation (1), q is the heat flux per unit length, T and t are temperature and time respectively. Uncertainty of experiments was found to be in the range of ± 2.5 %. For every
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sample, at least 10 measurements were performed and the mean value was considered as the resulting thermal conductivity.
In our experiments, nanocomposite sample was melted and poured into a cylindrical copper
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test cell of length 70 mm and diameter 20 mm. A water proof lid with pre-positioned
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thermocouple and platinum hot wire was used to seal the container after the sample encapsulation. The copper cylinder was then placed in a temperature-controlled thermostatic bath with a temperature variation of ± 0.1 °C. During the experiments, the samples were solidified to room temperature from the liquid state and the thermal conductivity measurements were performed at a temperature of 293 K. Furthermore, detailed thermal characterization of latent heat of fusion and the phase transition temperature of the nanocomposite samples were performed using a differential
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scanning calorimeter (DSC8230, Rigaku). DSC tests were performed over a temperature range of 30-60 °C at a ramping rate of 0.5 °C/min. For every sample, DSC experiments were performed for 5 cycles and the mean value was reported here. The uncertainty of the experiments was found
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to be in the range of ± 3 %. 3. Results and discussions
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To validate the THW setup, the thermal conductivity of deionized water and ethylene glycol were measured in liquid state at different temperatures, and n-octadecane was used for
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calibrating solid and liquid state at different temperatures. For the thermal conductivity calculations, data recorded between 0.1 and 2 s after the step input was used. A comparison of measured and reference thermal conductivity data is shown in figure 3. Measured results were in
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0.6
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Thermal conductivity (W m
−1
−1
K )
good agreement with the reference data reported in the literature [28-30].
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0.4
Water − Present Water − Ref [28] EG − Present EG − Ref [29] n−Octadecane − Present n−Octadecane − Ref [30]
0.2
290
300
310 320 Temperature (K)
330
Figure 3: Calibration of THW setup with water, ethylene glycol and n - octadecane.
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Thermal conductivity results of pristine lauric acid and nanocomposites are shown in figure 4. The thermal conductivity of pristine lauric acid at 293 K (solid state) is measured as
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0.215 ± 0.01 W m-1 K-1. The addition of MLG nanoplatelets is expected to increase the thermal conductivity of the nanocomposite due to the high thermal conductivity of MLG nanoplatelets. The thermal conductivity of nanocomposite increases gradually with increasing loading of MLG
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nanoplatelets. A maximum thermal conductivity of 0.489 ± 0.01 W m-1 K-1 is achieved at 293 K for MLG nanoplatelets loading of 1 vol %. The present thermal conductivity is higher by a factor
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of ~2.3 compared to the pristine lauric acid. The present thermal conductivity enhancement is superior to the results of carbon nanofibers [14], MWCNT based phase change nanocomposites [15 - 17], silver nanowires [13] and nanoparticles [8-12].
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Thermal Conductivity (W m
−1
−1
K )
0.6
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0.4
0.2
0
0.2 0.4 0.6 0.8 Volume Fraction (vol %)
1
Figure 4: Thermal conductivity measurements of LA/MLG nanocomposite as a function of MLG loading at a temperature of 293 K.
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Recent molecular dynamics simulations show that the presence of graphene/CNT in alkane induces a more orderly structural arrangement while reducing nucleation time upon crystallization [31, 32]. When lauric acid undergoes phase change from liquid to solid state, it
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forms needle-like crystalline structures whose aspect ratio can vary from micro-millimeter scales depending on the freezing speed [33]. During the crystallization process, it is possible that the MLG nanoplatelets are entrapped within the grain boundaries of the crystalline structures thereby
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creating “nano rich” inter-crystalline region [18, 34]. We hypothesize that these inter-crystalline region provide additional heat transport pathways resulting in the enhanced thermal conductivity
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of the nanocomposite.
The thermal conductivity enhancement is also analyzed based on effective medium theory (EMT) of heterogeneous composites considering the influence of thermal boundary resistance formulated by Nan et al. [35]. The effective medium model assumes that the nano
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inclusions as randomly oriented spheroidal particles and the interaction effects between the particles are not taken into account. For such a system, the thermal conductivity enhancement
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can be calculated using the following equations [2-8]: k eff
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k pcm
=
3 + φ[2 β11 (1 − L11 ) + β 33 (1 − L33 )] 3 − φ[2β11 L11 + β 33 L33 ]
(2)
Where
β11 =
k11c − k pcm
k pcm + L11 (k11c − k pcm )
L11 =
,
β 33 =
k33c − k pcm k pcm + L33 (k11c − k pcm )
a2 a − Cosh −1a, for a>1 2 2 2(a − 1) 2(a − 1) 3 / 2
,
(3)
(4)
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L11 =
a2 a + Cosh −1a, for a < 1, 2 2(a − 1) 2(1 − a 2 ) 3 / 2
and L33 = 1 − 2 L11
(5)
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(6)
Where k eff and k pcm correspond to effective thermal conductivity of nanocomposite and phase change material respectively, φ is the volume fraction of nano inclusions, L11 and L33 are
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geometric shape factors of the ellipsoidal particle. a (a = l d ) is the aspect ratio of ellipsoid (l and d are length and diameter of nano inclusion respectively), where a>1 is for prolate spheroid
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(CNTs) and a <1 for oblate spheroid (graphene) respectively. k11c and k 33c correspond to the equivalent thermal conductivity along the transverse and longitudinal directions of the composite respectively.
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The equivalent thermal conductivities along the symmetric axis of the composite cell by incorporating the effect of thermal interface resistance can be calculated as follows:
kiic =
γ = (2 + 1 a )
Rk pcm
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With
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1+
l
for a ≥ 1
kp i=1,3 γLii k p
(7)
k pcm
and
γ = (1 + 2 a )
Rk pcm d
for a ≤ 1
(8)
Where k p and R correspond to the thermal conductivity of spheroid and the thermal boundary resistance between the spheroid and host-matrix respectively. For model calculations, length of MLG nanoplatelets is considered to be ~1 µm after sonication with a thickness of 10 nm. The thermal conductivity of the MLG nanoplatelets was
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assumed to be 3000 W m-1 K-1 [36] and pristine lauric acid as 0.21 W m-1 K-1. The thermal boundary resistance between graphene - host matrix was considered as an unknown parameter and extracted from the EMT calculations. Besides, we also performed EMT calculations for
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CNT/LA nanocomposites to compare the performance of graphene and CNTs. For the calculation purpose, CNTs were assumed to have a mean length and diameter of ~1 µm and 1 nm respectively. CNTs were also assumed to have similar thermal conductivity as graphene for the
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present calculations. Although, literature results show that graphene possesses higher thermal conductivity than CNTs, we show later in the article that high thermal conductivity has less
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impact than the thermal boundary resistance. The thermal boundary resistance between CNT host matrix was assumed to be ~10-8 m2 K W-1 based on existing experimental and molecular
10
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Present Experiments 1
−2
K
2
−2
K
3
−2
K
TBC − 10 MW m
TBC − 10 MW m TBC − 10 MW m
−1 −1 −1
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Thermal conductivity ratio (K eff / kpcm )
dynamics results [37, 38].
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5
Increasing TBC
0
0.25 0.5 0.75 Volume Fraction (vol %)
1
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Figure 5: Comparison of EMT model predictions along with the experimental data as a function
of thermal boundary conductance.
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Figure 5 shows that the EMT model prediction as a function of thermal boundary conductance (inverse of thermal boundary resistance) along with the experimental results. It can be clearly seen that the present experimental results fall on the prediction line corresponding to
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the thermal boundary conductance (TBC) of ~100 MW m-2 K-1. Molecular dynamics simulations of Konatham et al. [39] predicts the TBC of functionalized graphene/oil interfaces to be in the
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range of 50 - 270 MW m-2 K-1. The atomistic simulations show that the TBC decreases as the size of the graphene sheet increases. The present interface conductance extracted from the EMT calculations agrees well with previous atomistic simulations results. The reasonably low interface conductance obtained from the EMT model calculation is possibly due to the few
5
Present Experiments −2
−1
TBC − 100 MW m K (MLG)
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Thermal conductivity ratio (K eff / kpcm )
simulations.
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micrometer long graphene sheets used in the experiments which is consistent with the atomistic
4
−2
−1
TBC − 100 MW m K (CNT) −2
−1
TBC − 25 MW m K (CNT)
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Spherical nanoparticles limit
3
2
1 0
0.25 0.5 0.75 Volume Fraction (vol %)
1
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Figure 6: EMT calculations of thermal conductivity enhancement for various carbon
nanoadditives along with experimental results.
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Graphene/organic interfaces have a higher TBC compared to CNT/organic interfaces [3739]. Experimental and atomistic simulations show that the TBC between CNT/octane interfaces to be in the range of 5-32 MW m-2 K-1. Figure 6 shows the EMT predictions of CNT/LA nanocomposites along with the present experimental results for TBC of 25 MW m-2 K-1 and 100
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MW m-2 K-1. TBC of 100 MW m-2 K-1 is unrealistic for CNT/organic interfaces compared to the
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existing experimental results [37]. However, EMT calculations show that even with such high TBC values, CNT inclusions underperform compared to graphene nanosheets. This is consistent with the experimental results of Wang et al [15, 16], for palmitic acid/MWCNT nanocomposites. Wang et al. [15, 16] reported a factor of ~1.4 enhancement in thermal conductivity of palmitic
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acid/multi-walled carbon nanotube (MWCNT) nanocomposites at a loading of 0.5 vol %. Figure 6 also shows the estimate for carbon spherical particles of 1 nm diameter (aspect ratio of 1). The influence of spherical particles in the thermal conductivity enhancement is much
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less pronounced compared to CNTs and graphene which is consistent with the experimental results reported in the literature for nanoparticles [8-12] and carbon nanospheres [40]. This
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shows that graphene nanoplatelets are better filler candidate than CNTs or any other nanoparticles reported in the literature to achieve substantial thermal conductivity enhancement due to its high aspect ratio, high thermal boundary conductance, high thermal conductivity, and also due to its low cost.
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0
4
kMLG / kMLG / kMLG / kMLG / kMLG /
kpcm − 10 1 kpcm − 10 2 kpcm − 10 3 kpcm − 10 4 kpcm − 10
3
Present Experiments Increasing kMLG /kpcm
1 0
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2
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Thermal conductivity ratio (K eff / kpcm)
5
0.5
1
Volume Fraction (Vol %)
Figure 7: Thermal conductivity enhancement as a function of kMLG/kpcm.
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For the calculations in figure 6 we assumed the thermal conductivity of MLG nanoplatelets as 3000 W m-1 K-1. However, there exist discrepancy in the thermal conductivity
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values reported in the literature for graphene [36]. Hence, we performed EMT calculations by choosing the thermal conductivity of graphene as an unknown parameter. The thermal boundary
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conductance was set as 100 MW m-2 K-1 based on previous estimates. Figure 7 shows the effective thermal conductivity enhancement as a function of the ratio of the thermal conductivity of graphene nanoplatelet (kMLG) to the thermal conductivity of phase change material (kpcm). It can be seen from the figure the thermal conductivity enhancement increases when the ratio of kMLG/kpcm increases. However, when the ratio of kMLG/kpcm becomes ≥ 103, the thermal conductivity enhancement becomes saturated and remains insensitive for further increase in the thermal conductivity of graphene nanosheets. Similar results were obtained for CNT based
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nanocomposites (not shown in figure). Organic PCMs usually have thermal conductivity in the range of 0.15-0.4 W m-1 K-1. Considering the threshold limit of kMLG/kpcm = 103, and assuming the organic PCM thermal conductivity to be ~0.215 W m-1 K-1 , a simple estimate shows that it is
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sufficient for the nano inclusion to have a thermal conductivity 215 W m-1 K-1 to achieve substantial thermal conductivity enhancement. Graphene, CNTs and metallic nanoparticles have higher thermal conductivity than the value obtained from the simple estimate. However, CNTs
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and graphene have advantage over other nanoparticles owing to their dimensionality, while the planar structure of the graphene sheets and low Kaptiza resistance at the graphene-host matrix
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makes it the ideal candidate than CNTs to enhance the thermal properties of materials.
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LA + 0.25 vol % MLG LA + 0.5 vol % MLG LA + 1 vol % MLG
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Heat flow (mW)
Pristine LA
40
42
44
46
48
50
Tempearture (°C)
Figure 8: DSC results of melting transition of pristine lauric acid and the nanocomposites.
Besides thermal conductivity, another critical factor for the PCM is its phase change enthalpy. Hence DSC tests were performed to measure the phase change enthalpy (
Hfus) of the
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PCM and the nanocomposites. The DSC measurement results of the melting cycle is shown in figure 8. It can be seen from figure 8 that the melting point and the
Hfus is measured as 181±3 J/g for the
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compared to that of pristine lauric acid. In this work,
Hfus remains similar
PCM and the nanocomposites and melting temperature is 43.8 ± 0.1 °C. Some literature results show a noticeable decrease in the phase change enthalpy upon inclusion of the nano-
reduction in
Hfus. Yavari et al. [19] show a
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inclusion while other studies show a negligible change in the
Hfus by 15 % at a graphene loading of 4 wt % and Fan et al. [22] show a Hfus by 10 % for CNT and graphene nanoplatelets. On the other hand,
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maximum reduction in
Kim et al. [20] show a contrary result of negligible change in
Hfus for paraffin/graphene
nanoplatelets composites even at a higher loading of 5 wt %. The negligible change in
Hfus
noticed in this work is possibly attributed to the absence of chemical reaction between the PCM
Conclusions
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and the graphene nanoplatelets and also a small MLG/PCM interface area.
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Organic fatty acid based phase change nanocomposites with graphene nanoplatelets were prepared and its thermal conductivity was measured using transient hot wire method. The
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thermal conductivity of the nanocomposites was found to increase gradually with increasing the loading of graphene nanoplatelets. A maximum thermal conductivity enhancement of ~230 % was measured in this work at a graphene loading of 1 vol %. The experimental results were also compared with the effective medium theory calculations. Model calculations show that thermal interface conductance between the graphene/PCM interface was ~100 MW m2 K-1, which is higher compared to CNT/organic interfaces reported in the literature. Further calculations show that the ratio of kMLG/kpcm need to be atleast 103 or higher to achieve substantial thermal
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conductivity enhancement. These results will be helpful in selecting the appropriate nano filler candidate to enhance the thermal properties of materials for various applications like nanofluids and thermal interface materials. High aspect ratio, high thermal conductivity and high thermal
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boundary conductance makes graphene nanoplatelets a better nano filler candidate compared to CNTs and other existing nanoparticles. Differential scanning calorimetry results shows insignificant change in the phase transition enthalpy and melting temperature compared to that of
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pristine lauric acid. Such a graphene based nanocomposite with enhanced thermal transport may find promising applications in solar energy harvesting and thermal management of electronic
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devices. Acknowledgements
SH was financially supported by postdoctoral fellowship from Japan Society of
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Highlights of the manuscript Thermal conductivity of lauric acid with graphene nano inclusions were measured.
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Inclusion of 1 vol % of graphene enhances the thermal conductivity by 230 %. Model calculations show graphene performs superiorly than other nano materials.
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Phase change enthalpy and melting temperature remains unaltered at 1 vol % loading.
S1359-4311(15)00071-X
DOI:
10.1016/j.applthermaleng.2015.01.056
Reference:
ATE 6325
To appear in:
Applied Thermal Engineering
Received Date: 1 September 2014 Revised Date:
4 December 2014
Accepted Date: 22 January 2015
Please cite this article as: S. Harish, D. Orejon, Y. Takata, M. Kohno, Thermal conductivity enhancement of lauric acid phase change nanocomposite with graphene nanoplatelets, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2015.01.056. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Thermal conductivity enhancement of lauric acid phase change nanocomposite with graphene nanoplatelets
a
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Sivasankaran Harisha, Daniel Orejona,b, Yasuyuki Takataa,b, Masamichi Kohnoa,b* Department of Mechanical Engineering, Thermofluid Physics Laboratory, Kyushu University,
744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan.
International Institute for Carbon-Neutral Energy Research (WPI - I2CNER), Kyushu
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b
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University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. Abstract
In this work, we prepared lauric acid based phase change nanocomposites with chemically functionalized graphene nanoplatelets and measured its thermal conductivity using
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transient hot wire method. We show that inclusion of graphene nanoplatelets increases the thermal conductivity of phase change material by 230 % at a loading of 1 vol %. Comparing the experimental results with the model calculations based on the effective medium theory suggests
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that graphene based nanocomposites outperforms those with carbon nanotubes or metal nanoparticles reported in the literature. High thermal conductivity, high aspect ratio and low
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thermal interface resistance at the graphene - host matrix interface makes it the most suitable nano filler candidate to enhance the thermal conductivity of low conductive materials. Differential scanning calorimetry study of the nanocomposites show that the phase change enthalpy and the melting temperature remains similar to that of pristine material, which makes graphene a promising candidate for thermal energy storage applications. Keywords: phase change material, graphene, thermal conductivity, carbon nanotube, lauric acid *Corresponding author E-mail address: [email protected] (M. Kohno)
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d l
k pcm
phase change material thermal conductivity, W m-1 K-1
kMLG
thermal conductivity graphene nanoplatelets
T
t q
k
c ii
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k keff
diameter, m length, m temperature, °C time, s heat flux per unit of length, W m-1 thermal conductivity, W m-1 K-1 effective thermal conductivity, W m-1 K-1
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Nomenclature
equivalent thermal conductivity along the symmetric axis, W m-1 K-1
equivalent thermal conductivity along the transverse direction, W m-1 K-1
k 33c kp
equivalent thermal conductivity along the longitudinal direction, W m-1 K-1 thermal conductivity of spheroid, W m-1 K-1
Lii
geometric shape factor of ellipsoidal particle along symmetric axis
L11
geometric shape factor of ellipsoidal particle along transverse direction
L33 a R
geometric shape factor of ellipsoidal particle along longitudinal direction aspect ratio length to diameter thermal boundary resistance, m2 K W-1 volume fraction phase change enthalpy, J kg-1
∆H fus
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1. Introduction
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φ
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k11c
Thermal energy storage using phase change materials (PCM) are often employed in waste
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heat recovery and solar energy storage [1, 2]. Thermal energy storage is also an attractive way to cool micro-electronic devices during peak loads [3]. At present, many PCMs which can be classified as inorganic, organic and their mixtures have been used for this purpose due to its large latent heat storage capabilities and wide range of melting temperatures [4, 5]. Inorganic PCMs possess higher latent heat storage capabilities when compared to organic PCMs however their applications are rather limited due to high super cooling. Among organic PCMs, fatty acids and
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their eutectic mixtures are considered promising for storage applications due to its excellent properties such as high phase change enthalpy, non-toxicity, low vapour pressure, little or no
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super cooling, good thermal and chemical stability, wide range of melting point and low cost [6]. The rate of energy storage and release of PCMs is directly proportional to its thermal conductivity. Fatty acids based PCMs like other organic materials possess low thermal
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conductivity, which remains a great challenge for practical applications. Recently this has led to increasing interests in the use of high conductive nano inclusions to enhance the thermal
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conductivity of such PCMs [7]. High conductive nanoparticles such as Ni, Al2O3, TiO2 [8-12], silver nanowires [13] or carbon nanoadditives such as nanofibers [14], carbon nanotubes (CNT) [15-18] and graphite nanoplatelets [19-23] have also been used for enhancing the thermal conductivity of PCMs.
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Utilization of metallic nanoparticles would decrease the energy storage capabilities of the PCMs. Carbon based nanoadditives have shown tremendous potential to enhance the thermal conductivity of organic PCMs due to its high thermal conductivity. Carbon nanotubes (CNT), a
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one-dimensional allotrope of carbon is widely used for this purpose. Wang et al. [15, 16] reported a thermal conductivity enhancement of 51 % at a loading of 1 wt % for chemically
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modified CNTs dispersed in palmitic acid. Cui et al. [17] investigated the effect of carbon nanofibers and CNTs in soy wax based nanocomposites. They reported a significant improvement in thermal conductivity upon nano inclusions. However, they showed that CNTs performed poorly compared to carbon nano fibers which is in direct contradiction to the results of Wang et al. [15, 16]. Recent thermal conductivity measurements of graphene nanosheets, a two-dimensional allotrope of carbon shows a much higher thermal conductivity than CNTs [19]. Hence it is anticipated that the use of graphene will significantly enhance the thermal
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conductivity of organic fatty acids based PCMs, the latter being of significant interest for energy storage applications. Yavari et al. [19] utilized graphene nanosheets and reported a thermal conductivity enhancement by a factor of ~1.8 for fatty acid based ester nanocomposites at a
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loading of 1 vol %. Similar thermal conductivity enhancement was reported for paraffin/graphene nanoplatelets based nanocomposites in the literature [19-23].
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Based on the previous studies, it is evident that many high conductive nanomaterials were utilized to enhance the thermal conductivity of organic PCMs. However, it still remains unclear
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in selecting the appropriate material and dimensionality of nano inclusion to enhance the thermal transport of organic PCMs without affecting the energy storage capability. In this work, using an organic fatty acid based phase change material we show remarkable enhancement in thermal conductivity by a factor of approximately ~2.3 using chemically functionalized graphene loading of 1 vol %. Besides, we compare the experimental results with model calculations based on
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effective medium theory for nano inclusions of different dimensionalities considering the role of interfacial thermal transport into account. Based on model calculations, we show that planar structure, high thermal conductivity and low Kapitza resistance between graphene-host matrix
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interface make graphene nanoplatelets a promising nano filler candidate. Furthermore, this
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material offers better thermal performance to enhance the thermal properties of organic materials than carbon nanotubes or other nanoparticles reported in the literature. Moreover, differential scanning calorimetry investigations show that the energy storage capability and the phase transition temperature of the PCMs remain unaltered at small loadings which makes graphene a promising filler candidate for thermal energy storage applications.
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2. Materials and Methods 2.1 Material characterization and sample preparation
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The phase change material used in the present work is n-Dodecanoic acid (Lauric acid, C12H24O2) with a melting temperature of ~44 °C and with an excellent phase transition enthalpy of ~180 kJ/kg. Liquid phase exfoliated multilayer graphene nanoplatelets (MLG) were purchased
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from XG Sciences with an average thickness of 5 - 10 nm (Grade M, Mean particle diameter of 15 µm, Density 2.2 g/cm3). A scanning electron microscopy image (SEM, FEI Versa 3D Dual
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Beam, LoVac detector) of pristine lauric acid is shown in figure 1.
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Figure 1: SEM visualization of pristine lauric acid Figure 2 (a) shows the SEM visualization of MLG nanoplatelets and figure 2 (b) shows
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the typical Raman spectra (HORIBA micro-Raman system HR-800) of the MLG nanoplatelets measured using a laser wavelength of 488 nm. The G-band observed around 1582 cm-1 corresponds to the in-plane vibrations of the carbon atoms. The D-band which is also known as disorder band observed around 1350 cm-1 corresponds to the defects in the graphitic structure. The 2D-band observed around 2750 cm-1 is considered to be the overtone of D-band arising as a result of two phonon lattice vibrations [24].
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G−band
Intensity (arb. u)
(a)
(b)
D−band
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2D−band
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1200 1500 1800 2100 2400 2700 −1 Raman Shift (cm )
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Figure 2: (a) SEM visualization of MLG nanoplatelets. (b) Typical Raman spectra of MLG nanoplatelets.
MLG nanoplatelets of 0.5 g were dispersed in 25 mL of concentrated nitric acid (68 wt %) and then refluxed at a temperature of 100 °C for 2h. The mixture was filtered, washed with deionized water and dried at 150 °C in vacuum for 6 h. The nanocomposite samples were
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prepared by adding MLG nanoplatelets with molten lauric acid kept on a hot plate maintained at a temperature of 100 °C under rigorous stirring using a magnetic stirrer at 250 rpm for 30 min.
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After that, the samples were subjected to intensive sonication using an ultrasonic processor (Hielscher GmbH, UP-400S with H3/Micro Tip 3) for 30 min at 50 % amplification. Samples
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with MLG nanoplatelets loading of 0.1 vol %, 0.25 vol %, 0.5 vol %, 0.75 vol % and 1 vol % were prepared along with a reference sample of pristine lauric acid.
2.2 Thermal conductivity measurements Thermal conductivity of the nanocomposites was measured using a custom built transient hot wire (THW) method developed by Nagasaka and Nagashima [25]. Details of the experimental setup are reported elsewhere [26]. In brief, a platinum wire of diameter (d) 25.4 µm
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with an electrically insulating isonel coating of 2 µm and length (l) 50 mm is used in present experiments. The hot wire acts as both the heating element and as the electrical resistance thermometer. During measurement, the transient hot wire is heated by a constant DC power
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supply. The temperature (T) rise of hot wire was determined from the change in resistance of hot wire, which can be measured as a function of time (t) using a Wheatstone-bridge circuit. From a known electric power supply (q) and the slope of the curve ln(t) versus T, the thermal
and measured temperature T can be written as follows [27]. q d ln t 4π dT
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k=
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conductivity (k) was estimated using equation (1). The relation between thermal conductivity k
(1)
In equation (1), q is the heat flux per unit length, T and t are temperature and time respectively. Uncertainty of experiments was found to be in the range of ± 2.5 %. For every
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sample, at least 10 measurements were performed and the mean value was considered as the resulting thermal conductivity.
In our experiments, nanocomposite sample was melted and poured into a cylindrical copper
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test cell of length 70 mm and diameter 20 mm. A water proof lid with pre-positioned
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thermocouple and platinum hot wire was used to seal the container after the sample encapsulation. The copper cylinder was then placed in a temperature-controlled thermostatic bath with a temperature variation of ± 0.1 °C. During the experiments, the samples were solidified to room temperature from the liquid state and the thermal conductivity measurements were performed at a temperature of 293 K. Furthermore, detailed thermal characterization of latent heat of fusion and the phase transition temperature of the nanocomposite samples were performed using a differential
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scanning calorimeter (DSC8230, Rigaku). DSC tests were performed over a temperature range of 30-60 °C at a ramping rate of 0.5 °C/min. For every sample, DSC experiments were performed for 5 cycles and the mean value was reported here. The uncertainty of the experiments was found
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to be in the range of ± 3 %. 3. Results and discussions
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To validate the THW setup, the thermal conductivity of deionized water and ethylene glycol were measured in liquid state at different temperatures, and n-octadecane was used for
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calibrating solid and liquid state at different temperatures. For the thermal conductivity calculations, data recorded between 0.1 and 2 s after the step input was used. A comparison of measured and reference thermal conductivity data is shown in figure 3. Measured results were in
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0.6
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Thermal conductivity (W m
−1
−1
K )
good agreement with the reference data reported in the literature [28-30].
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0.4
Water − Present Water − Ref [28] EG − Present EG − Ref [29] n−Octadecane − Present n−Octadecane − Ref [30]
0.2
290
300
310 320 Temperature (K)
330
Figure 3: Calibration of THW setup with water, ethylene glycol and n - octadecane.
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Thermal conductivity results of pristine lauric acid and nanocomposites are shown in figure 4. The thermal conductivity of pristine lauric acid at 293 K (solid state) is measured as
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0.215 ± 0.01 W m-1 K-1. The addition of MLG nanoplatelets is expected to increase the thermal conductivity of the nanocomposite due to the high thermal conductivity of MLG nanoplatelets. The thermal conductivity of nanocomposite increases gradually with increasing loading of MLG
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nanoplatelets. A maximum thermal conductivity of 0.489 ± 0.01 W m-1 K-1 is achieved at 293 K for MLG nanoplatelets loading of 1 vol %. The present thermal conductivity is higher by a factor
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of ~2.3 compared to the pristine lauric acid. The present thermal conductivity enhancement is superior to the results of carbon nanofibers [14], MWCNT based phase change nanocomposites [15 - 17], silver nanowires [13] and nanoparticles [8-12].
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Thermal Conductivity (W m
−1
−1
K )
0.6
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0.4
0.2
0
0.2 0.4 0.6 0.8 Volume Fraction (vol %)
1
Figure 4: Thermal conductivity measurements of LA/MLG nanocomposite as a function of MLG loading at a temperature of 293 K.
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Recent molecular dynamics simulations show that the presence of graphene/CNT in alkane induces a more orderly structural arrangement while reducing nucleation time upon crystallization [31, 32]. When lauric acid undergoes phase change from liquid to solid state, it
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forms needle-like crystalline structures whose aspect ratio can vary from micro-millimeter scales depending on the freezing speed [33]. During the crystallization process, it is possible that the MLG nanoplatelets are entrapped within the grain boundaries of the crystalline structures thereby
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creating “nano rich” inter-crystalline region [18, 34]. We hypothesize that these inter-crystalline region provide additional heat transport pathways resulting in the enhanced thermal conductivity
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of the nanocomposite.
The thermal conductivity enhancement is also analyzed based on effective medium theory (EMT) of heterogeneous composites considering the influence of thermal boundary resistance formulated by Nan et al. [35]. The effective medium model assumes that the nano
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inclusions as randomly oriented spheroidal particles and the interaction effects between the particles are not taken into account. For such a system, the thermal conductivity enhancement
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can be calculated using the following equations [2-8]: k eff
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k pcm
=
3 + φ[2 β11 (1 − L11 ) + β 33 (1 − L33 )] 3 − φ[2β11 L11 + β 33 L33 ]
(2)
Where
β11 =
k11c − k pcm
k pcm + L11 (k11c − k pcm )
L11 =
,
β 33 =
k33c − k pcm k pcm + L33 (k11c − k pcm )
a2 a − Cosh −1a, for a>1 2 2 2(a − 1) 2(a − 1) 3 / 2
,
(3)
(4)
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L11 =
a2 a + Cosh −1a, for a < 1, 2 2(a − 1) 2(1 − a 2 ) 3 / 2
and L33 = 1 − 2 L11
(5)
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(6)
Where k eff and k pcm correspond to effective thermal conductivity of nanocomposite and phase change material respectively, φ is the volume fraction of nano inclusions, L11 and L33 are
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geometric shape factors of the ellipsoidal particle. a (a = l d ) is the aspect ratio of ellipsoid (l and d are length and diameter of nano inclusion respectively), where a>1 is for prolate spheroid
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(CNTs) and a <1 for oblate spheroid (graphene) respectively. k11c and k 33c correspond to the equivalent thermal conductivity along the transverse and longitudinal directions of the composite respectively.
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The equivalent thermal conductivities along the symmetric axis of the composite cell by incorporating the effect of thermal interface resistance can be calculated as follows:
kiic =
γ = (2 + 1 a )
Rk pcm
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With
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1+
l
for a ≥ 1
kp i=1,3 γLii k p
(7)
k pcm
and
γ = (1 + 2 a )
Rk pcm d
for a ≤ 1
(8)
Where k p and R correspond to the thermal conductivity of spheroid and the thermal boundary resistance between the spheroid and host-matrix respectively. For model calculations, length of MLG nanoplatelets is considered to be ~1 µm after sonication with a thickness of 10 nm. The thermal conductivity of the MLG nanoplatelets was
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assumed to be 3000 W m-1 K-1 [36] and pristine lauric acid as 0.21 W m-1 K-1. The thermal boundary resistance between graphene - host matrix was considered as an unknown parameter and extracted from the EMT calculations. Besides, we also performed EMT calculations for
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CNT/LA nanocomposites to compare the performance of graphene and CNTs. For the calculation purpose, CNTs were assumed to have a mean length and diameter of ~1 µm and 1 nm respectively. CNTs were also assumed to have similar thermal conductivity as graphene for the
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present calculations. Although, literature results show that graphene possesses higher thermal conductivity than CNTs, we show later in the article that high thermal conductivity has less
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impact than the thermal boundary resistance. The thermal boundary resistance between CNT host matrix was assumed to be ~10-8 m2 K W-1 based on existing experimental and molecular
10
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Present Experiments 1
−2
K
2
−2
K
3
−2
K
TBC − 10 MW m
TBC − 10 MW m TBC − 10 MW m
−1 −1 −1
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Thermal conductivity ratio (K eff / kpcm )
dynamics results [37, 38].
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5
Increasing TBC
0
0.25 0.5 0.75 Volume Fraction (vol %)
1
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Figure 5: Comparison of EMT model predictions along with the experimental data as a function
of thermal boundary conductance.
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Figure 5 shows that the EMT model prediction as a function of thermal boundary conductance (inverse of thermal boundary resistance) along with the experimental results. It can be clearly seen that the present experimental results fall on the prediction line corresponding to
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the thermal boundary conductance (TBC) of ~100 MW m-2 K-1. Molecular dynamics simulations of Konatham et al. [39] predicts the TBC of functionalized graphene/oil interfaces to be in the
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range of 50 - 270 MW m-2 K-1. The atomistic simulations show that the TBC decreases as the size of the graphene sheet increases. The present interface conductance extracted from the EMT calculations agrees well with previous atomistic simulations results. The reasonably low interface conductance obtained from the EMT model calculation is possibly due to the few
5
Present Experiments −2
−1
TBC − 100 MW m K (MLG)
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Thermal conductivity ratio (K eff / kpcm )
simulations.
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micrometer long graphene sheets used in the experiments which is consistent with the atomistic
4
−2
−1
TBC − 100 MW m K (CNT) −2
−1
TBC − 25 MW m K (CNT)
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Spherical nanoparticles limit
3
2
1 0
0.25 0.5 0.75 Volume Fraction (vol %)
1
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Figure 6: EMT calculations of thermal conductivity enhancement for various carbon
nanoadditives along with experimental results.
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Graphene/organic interfaces have a higher TBC compared to CNT/organic interfaces [3739]. Experimental and atomistic simulations show that the TBC between CNT/octane interfaces to be in the range of 5-32 MW m-2 K-1. Figure 6 shows the EMT predictions of CNT/LA nanocomposites along with the present experimental results for TBC of 25 MW m-2 K-1 and 100
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MW m-2 K-1. TBC of 100 MW m-2 K-1 is unrealistic for CNT/organic interfaces compared to the
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existing experimental results [37]. However, EMT calculations show that even with such high TBC values, CNT inclusions underperform compared to graphene nanosheets. This is consistent with the experimental results of Wang et al [15, 16], for palmitic acid/MWCNT nanocomposites. Wang et al. [15, 16] reported a factor of ~1.4 enhancement in thermal conductivity of palmitic
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acid/multi-walled carbon nanotube (MWCNT) nanocomposites at a loading of 0.5 vol %. Figure 6 also shows the estimate for carbon spherical particles of 1 nm diameter (aspect ratio of 1). The influence of spherical particles in the thermal conductivity enhancement is much
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less pronounced compared to CNTs and graphene which is consistent with the experimental results reported in the literature for nanoparticles [8-12] and carbon nanospheres [40]. This
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shows that graphene nanoplatelets are better filler candidate than CNTs or any other nanoparticles reported in the literature to achieve substantial thermal conductivity enhancement due to its high aspect ratio, high thermal boundary conductance, high thermal conductivity, and also due to its low cost.
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0
4
kMLG / kMLG / kMLG / kMLG / kMLG /
kpcm − 10 1 kpcm − 10 2 kpcm − 10 3 kpcm − 10 4 kpcm − 10
3
Present Experiments Increasing kMLG /kpcm
1 0
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2
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Thermal conductivity ratio (K eff / kpcm)
5
0.5
1
Volume Fraction (Vol %)
Figure 7: Thermal conductivity enhancement as a function of kMLG/kpcm.
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For the calculations in figure 6 we assumed the thermal conductivity of MLG nanoplatelets as 3000 W m-1 K-1. However, there exist discrepancy in the thermal conductivity
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values reported in the literature for graphene [36]. Hence, we performed EMT calculations by choosing the thermal conductivity of graphene as an unknown parameter. The thermal boundary
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conductance was set as 100 MW m-2 K-1 based on previous estimates. Figure 7 shows the effective thermal conductivity enhancement as a function of the ratio of the thermal conductivity of graphene nanoplatelet (kMLG) to the thermal conductivity of phase change material (kpcm). It can be seen from the figure the thermal conductivity enhancement increases when the ratio of kMLG/kpcm increases. However, when the ratio of kMLG/kpcm becomes ≥ 103, the thermal conductivity enhancement becomes saturated and remains insensitive for further increase in the thermal conductivity of graphene nanosheets. Similar results were obtained for CNT based
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nanocomposites (not shown in figure). Organic PCMs usually have thermal conductivity in the range of 0.15-0.4 W m-1 K-1. Considering the threshold limit of kMLG/kpcm = 103, and assuming the organic PCM thermal conductivity to be ~0.215 W m-1 K-1 , a simple estimate shows that it is
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sufficient for the nano inclusion to have a thermal conductivity 215 W m-1 K-1 to achieve substantial thermal conductivity enhancement. Graphene, CNTs and metallic nanoparticles have higher thermal conductivity than the value obtained from the simple estimate. However, CNTs
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and graphene have advantage over other nanoparticles owing to their dimensionality, while the planar structure of the graphene sheets and low Kaptiza resistance at the graphene-host matrix
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makes it the ideal candidate than CNTs to enhance the thermal properties of materials.
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LA + 0.25 vol % MLG LA + 0.5 vol % MLG LA + 1 vol % MLG
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Heat flow (mW)
Pristine LA
40
42
44
46
48
50
Tempearture (°C)
Figure 8: DSC results of melting transition of pristine lauric acid and the nanocomposites.
Besides thermal conductivity, another critical factor for the PCM is its phase change enthalpy. Hence DSC tests were performed to measure the phase change enthalpy (
Hfus) of the
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PCM and the nanocomposites. The DSC measurement results of the melting cycle is shown in figure 8. It can be seen from figure 8 that the melting point and the
Hfus is measured as 181±3 J/g for the
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compared to that of pristine lauric acid. In this work,
Hfus remains similar
PCM and the nanocomposites and melting temperature is 43.8 ± 0.1 °C. Some literature results show a noticeable decrease in the phase change enthalpy upon inclusion of the nano-
reduction in
Hfus. Yavari et al. [19] show a
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inclusion while other studies show a negligible change in the
Hfus by 15 % at a graphene loading of 4 wt % and Fan et al. [22] show a Hfus by 10 % for CNT and graphene nanoplatelets. On the other hand,
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maximum reduction in
Kim et al. [20] show a contrary result of negligible change in
Hfus for paraffin/graphene
nanoplatelets composites even at a higher loading of 5 wt %. The negligible change in
Hfus
noticed in this work is possibly attributed to the absence of chemical reaction between the PCM
Conclusions
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and the graphene nanoplatelets and also a small MLG/PCM interface area.
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Organic fatty acid based phase change nanocomposites with graphene nanoplatelets were prepared and its thermal conductivity was measured using transient hot wire method. The
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thermal conductivity of the nanocomposites was found to increase gradually with increasing the loading of graphene nanoplatelets. A maximum thermal conductivity enhancement of ~230 % was measured in this work at a graphene loading of 1 vol %. The experimental results were also compared with the effective medium theory calculations. Model calculations show that thermal interface conductance between the graphene/PCM interface was ~100 MW m2 K-1, which is higher compared to CNT/organic interfaces reported in the literature. Further calculations show that the ratio of kMLG/kpcm need to be atleast 103 or higher to achieve substantial thermal
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conductivity enhancement. These results will be helpful in selecting the appropriate nano filler candidate to enhance the thermal properties of materials for various applications like nanofluids and thermal interface materials. High aspect ratio, high thermal conductivity and high thermal
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boundary conductance makes graphene nanoplatelets a better nano filler candidate compared to CNTs and other existing nanoparticles. Differential scanning calorimetry results shows insignificant change in the phase transition enthalpy and melting temperature compared to that of
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pristine lauric acid. Such a graphene based nanocomposite with enhanced thermal transport may find promising applications in solar energy harvesting and thermal management of electronic
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devices. Acknowledgements
SH was financially supported by postdoctoral fellowship from Japan Society of
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Highlights of the manuscript Thermal conductivity of lauric acid with graphene nano inclusions were measured.
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Inclusion of 1 vol % of graphene enhances the thermal conductivity by 230 %. Model calculations show graphene performs superiorly than other nano materials.
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Phase change enthalpy and melting temperature remains unaltered at 1 vol % loading.