journal homepage: www.elsevier.com/locate/nanoenergyAvailable
online at www.sciencedirect.comREVIEWReview on nanoencapsulated
phase changematerials: Preparation, characterizationand heat
transfer enhancementChenzhen Liu, Zhonghao Raon, Jiateng Zhao,
Yutao Huo, Yimin LiSchool of Electric Power Engineering, China
University of Mining and Technology, Xuzhou 221116, ChinaReceived
18 January 2015; received in revised form 10 February 2015;
accepted 12 February 2015Available online 20 February
2015KEYWORDSThermal energy sto-rage;Nanoencapsulatedphase change
mate-rial;Nanoencapsulationmethod;Heat transferenhancement;Latent
functionalthermal
uidAbstractInrecentyears,phasechangematerials(PCM)whicharerecommendedaspotentialthermalenergy
storage medium have been receiving signicant attention. The
encapsulation technologyof PCM is an effective way to enhance the
thermal conductivity and solve the issues of leakageand corrosion
during the melting process. As a good choice of thermal energy
storage materials,the nanoencapsulated phase change materials
(NanoPCM) have many advantages, such as smallsize, large specic
surface and high heat transfer rate. Up to now, there has been no
completeliteraturereviewonthepreparation,
characterizationandapplicationof NanoPCM.
Inthispaper,acomprehensivesummaryhasbeenpresentedbasedontheresearchofNanoPCMinrecent
years. The following four aspects have been reviewed in detail:
preparation andcharacterization of NanoPCM, application of
NanoPCMin latent functional thermal uid,dynamics simulationstudy of
NanoPCMandheat transfer enhancement of NanoPCM. It isexpected that
this review article has certain reference value for the further
understanding ofNanoPCM.& 2015 Elsevier Ltd. All rights
reserved.ContentsIntroduction. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 815The preparation methods . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 816Interfacial polymerization . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 817Emulsion polymerization
method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 817Miniemulsion
polymerization method. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 818In situ
polymerization method . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
820http://dx.doi.org/10.1016/j.nanoen.2015.02.0162211-2855/&
2015 Elsevier Ltd. All rights reserved.nCorresponding author. Tel.:
+86 516 83592000.E-mail address: [email protected] (Z.
Rao).Nano Energy (2015) 13, 814826Solgel method. . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 821Applications of NanoPCM in
latent functional thermal uid . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 822Heat transfer enhancement of
NanoPCM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . 823Dynamics simulation study of
NanoPCM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 823Further prospective
research. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 823Conclusions. .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
824Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 824References . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 824IntroductionEnergyshortageis
gettingworsewiththerapiddevelop-ment of economy and industry in the
world. After theenergy crisis inthe1970s, theresearches
onrenewableandsustainableenergyhavebeengainingmoreandmoreattention
[1]. The technology of phase change
energystoragerealizesthestorage,transportationandutilizationofthermalenergywhenthephasechangematerials(PCM)areabsorbingandreleasinglargeamounts
of latent heatundergoing phase change. The phase change energy
storagetechnology, while it is attracting attention gradually,
solvestheproblemthat theenergysupplydoes not matchthedemand in time
and space [2], increases the energy utiliza-tion [3,4] and relieves
energy crisis. Therefore,
phasechangeenergystoragetechnologycanbeappliedtomanyelds such as
waste heat recovery [5,6], solar energystorage [710], intelligent
building [1115], thermal regulat-ing fabric [1618], electronic
devices thermal control[19,20], battery thermal management system
[2123],and so on.The performance of phase change energy
storagedepends on the properties of PCM. According to thematerial
properties, thePCMcanbedividedintoorganicand inorganic [4,24]. With
strong thermal stability, theorganic PCM are commonly used for
energy storage in recentyears [25]. However, someproblems of
organic PCMwillappear in application such as lowthermal
conductivity[26,27] andleakageduringphasetransition[28,29].
Theleakagecausescertainharmtoenergystoragesystemandenvironment,
whichlimits thefurther applicationof PCM[3032]. In order to solve
these problems, the PCMareencapsulated in a capsule to form the
shell-core compositematerials which can be called as encapsulated
phasechange materials (EPCM)
[3336].TheEPCMaretinycontainerswhichwrapPCMinthecapsule core
[37,38]. The EPCM achieve the solidication ofPCM, not only
enhancing the stability andheat
transferefciencyofPCM[39],butalsofacilitatingitsutilization,storage
and transportation [40]. The EPCM are mainlycomposed of twoparts
[41]:PCM as coreand inorganics orpolymerasshell,
asshowninFigure1.TheEPCMcanbemade in arbitrary shape, either
irregular or regular such astubular, spherical and oval; and they
have single or severalcores within the capsule and multi-walled
capsules [4143],as shown in Figure 2.According toparticlesize,EPCM
canbedividedintothefollowing three types [1]: Nanocapsulated phase
changematerials (NanoPCM) (particlesizeranges between1and1000
nm)[44,45],microcapsulatedphasechangematerials(MicroPCM)
(particlesizeranges between1and1000 m)[16,46] and macrocapsulated
phase change materials(MacroPCM) [47,48] (particle size exceeds 1
mm).MicroPCMtechnologies arematureafter
morethan50yearsofdevelopment.ThestabilityofthePCMcapsulesisinuencedbytheparticlesize.Intheprocessof
uidowthe MicroPCM are easily broken and can increase theviscosity
ofuid, which limits the application of MicroPCM.The relationship
between stability and particle size ofMicroPCMwasstudiedbyYamagishi
etal.[49].TheslurrywhichcontainedtheMicroPCM(sizedistributionsrangingFigure
1 Description of a capsule
[41].MononuclearPolynuclearMatrixMulti-wallFigure 2 Morphology of a
capsule [41].815 Preparation, characterization and heat transfer
enhancementfrom5to1000 m)particles(20
vol%)inwaterwascircu-latedbyapump-circulatingsystem.Itwasobservedthatthe
MicroPCM with particle size of 10001500
mwererapidlybrokenduringthepumpcirculation.Thebreakagerateof
theMicroPCMwithparticlesizeof 75300 mwas40% after 500 times of pump
circulation. The breakage rateof theMicroPCMwithparticlesizeof
20100 mwas lessthan10%after 4000 times of pumpcirculation. But
theMicroPCMwithparticlesizeof 510 mwerealmost
notbrokenduringthepumpcirculationmorethan7000times.The results
demonstrated that the breakage ratesdecreasedas
theMicroPCMparticlesizes decreased,
andsmallersizedMicroPCMweremoredurableandwithstoodstress from the
pump circulation.AddingMicroPCMtofunctionthermal uidcanincreasethe
viscosity ofuid [5052], and MicroPCM easily fracture inthe process
ofuid or pump circulation, which is the obstacleof
long-termcirculation. For thesmall particlesize,
largespecicsurfacearea,suspensionstabilityandlowbreakagerate during
pumping compared with MicroPCM, NanoPCM aregaining attention
gradually [53]. Sukhorukov et al. [54]observed that when the same
forceisapplied thedeforma-tion of 10 nm size capsules is smaller
than that of 10 m
sizecapsules.Besides,theNanoPCMhavesomeuniqueproper-ties, such as
volume effect, surface effect,
macroscopicquantumtunnelingeffectandsoon[55],thatmakethemsteadily
disperse in the thermal uid, and ensure theapplication in energy
storage and thermal management[56]. Therefore, nanocapsule of PCM
has good growthprospects.Currently, there are many researchers who
have summar-izedandreportedresearchprogress,preparationmethodsand
application status of MicroPCM [41,5759]. But compre-hensive
summary for NanoPCM has not been reported yet. Inthis paper,
research progress on preparation method, appli-cation in latent
functionaluid, and heat transfer enhance-ment of NanoPCM in recent
years will be summarized.The preparation methodsThe chemical
method, physical method and physic
chemicalmethodareusuallyadoptedfor microcapsulepreparation[60].
Inorganic andpolymer arecommonly usedas shell[61], and most of the
particle sizes range from 5 to 400
m.TheparticlediameterofNanoPCMissmallerthanthatofMicroPCM,therefore,thetraditionalpreparationtechnolo-giesofMicroPCMarenotsuitableforNanoPCM.Currently,the
main methods for preparation of nanocapsule are listedas
follows.Table 1 Core and shell of NanoPCM.Core Shell Method Capsule
meansizen-Tetradecane[74]Polystyrene Miniemulsion in situ
polymerization 132 nmPalmitic acid[87]SiO2Solgel method 183.7
nmn-Dodecanol[80]Polymethyl methacrylate(PMMA) Miniemulsion
polymerization 150 nmn-Dodecanol[73]Styrene-butyl acrylate
copolymer Miniemulsion polymerization 90100
nm(range)n-Heptadecane[66]Polystyrene Emulsion polymerization 10
nm60 m(range)n-Octadecane[77]St(styrene)MMA
(methylmethacrylate)copolymerMiniemulsion in situ polymerization
102 nmn-Octadecane[81]Methyl methacrylate(PMMA) Direct miniemulsion
method 119 nmn-Octadecane[81]Poly(ethyl methacrylate)(PEMA) Direct
miniemulsion method 140 nmn-Octadecane[83]Methyl
methacrylate-co-allylmethacrylate(MMA-co-AMA)Free radical emulsion
polymerization andin situ polymerization577693
nmn-Octadecane[63]Polystyrene Ultrasonic-assistantminiemulsion
in-situpolymerization124 nmn-Nonadecane[67]Poly(methyl
methacrylate) (PMMA) Via emulsion polymerization 100350
nm(range)n-Dotriacontane[76]Polystyrene(PS) Miniemulsion
polymerizationmethod 168.2 nmn-Tetradecane[84]Urea formaldehyde In
situ polymerization 100 nmParafn [65] Polyurea
Interfacialpolycondensation reaction 498.2 nmParafn [82]
Carboxymethyl cellulose (CMC) in situ polymerization 50 nmC. Liu et
al. 816(i). Interfacial polymerization method.(ii). Emulsion
polymerization method [62,63].(iii). Miniemulsion polymerization
method [64].(iv). In situ polymerization method.(v). Solgel
method.Table1summarizes several NanoPCMpreparedby thefollowing
methods.Interfacial
polymerizationIntheprocessofpreparingphasechangecapsulesthroughinterfacial
polymerization method, the core material isemulsied rstlyafter
theformationof theoil/water orwater/oil emulsionby using
appropriateemulsier. Thenthe polymer as capsule is formed on the
surface of the
corebypolymerizationofthemonomers.Finallythecapsuleisseparated from
oil phase or water phase. When the
methodforpreparingNanoPCMisused,thecoremustbeaddedtothesyringewithacapillarytube.Itrequiresthesettingofhighvoltageanddirectcurrent.Besides,andthedistancebetween
the needle of the syringe and liquid level ofmonomer solution needs
to be as near as possible.Interfacial polymerization method is
suitable for theNanoPCMemployingwater solubleandoil solubleof
PCMcore. Uptonow, NanoPCMhavebeenpreparedbyinter-facial
polymerization method using parafn as core andpolyurea as
shell.Parketal.[65]synthesizedtheNanoPCMviainterfacialpolycondensation
method, whose core and shell wereparafnandpolyurea, respectively.
Theresults of
differ-entialscanningcalorimeter(DSC)analysisshowedthatthemelting
temperature and latent heat of the NanoPCM weremeasuredtobe56.54
1Cand101.1 J/g,freezingtempera-tureandlatent heat
weremeasuredtobe47.82 1Cand105.6 J/g. Figure 3 shows the scanning
electron microscope(SEM) and transmission electron microscope (TEM)
images ofthe NanoPCM. It is clear that the NanoPCM have a
sphericalstructure. Besides, the particles size is mainly in the
rangeof 400600 nm.Emulsion polymerization methodEmulsion
polymerization method, one of common methods
toproduceorganicNanoPCM,mainlyconsistsofthefollowingsteps. First of
all, the insoluble monomer inthe solventdispersed uniformly in the
reaction mediumunder thefunction of the emulsier, the surfactant
and the mechanicalFigure 3 SEMand TEMimages of NanoPCM(a) with
and(b) without Fe3O4 nanoparticle [65].Figure4
(a)POMimagesofPS/heptadecaneMicro/NanoPCMand (b) SEM images of
PS/heptadecaneMicro/NanoPCM [66].817 Preparation, characterization
and heat transfer enhancementstirring. Thenthepolymer
membraneisgeneratedonthesurfaceofthecorethroughaddingtheinitiatortoinitiatepolymerization
reaction. Eventually the NanoPCM comeinto
being.Emulsionpolymerizationmethodisoftenusedforpoly-mer
polymerization which is suitable for preparation ofNanoPCM using
liquid PCM as core material. So far,
NanoPCMhavebeenpreparedviaemulsionpolymerizationmethodwhich
commonly uses alkane as core and polystyrene or poly(methyl
methacrylate) as shell.Sari et al. [66] synthesized
Micro/Nanoencapsulatedphase change materials (Micro/NanoPCM) with
the n-heptadecane as core and the polystyrene (PS) as
shellthroughemulsionpolymerizationmethod.TheDSCresultsdemonstrated
that the melting temperature and latent heatof PS/heptadecane (1:2)
Micro/NanoPCM were 21.48 1C and136.89 J/g, respectively. The
freezing temperature andlatentheatwere21.37 1Cand134.67
J/g,separately.Thelatent heat of melting decreased from 136.89 J/g
to128.27 J/g due to the damage of several capsules after5000
thermal cycles. The results of thermal
gravimetricanalysis(TGA)showedthattheMicro/NanoPCMrepresentsgood
thermal stability in the preparation process. Thepolarized optical
microscopy (POM) and SEMimages arepresented in Figure 4(a) and (b),
respectively. The
PS/heptadecane(1:2)Micro/NanoPCMisinincompletespheri-cal which can
be clearly seen frommicrographs.
Theparticlesizestestedbytheparticlesizedistribution(PSD)distributed
in the range of 10 nm40 m. And later Sariet al. [67] fabricated
Micro/NanoPCM using n-nonadecane ascoreandpoly (methyl
methacrylate) (PMMA) as shell bymeans of the emulsion
polymerization. The PSD resultsexhibited that the particle sizes
were in the range of10 nm40 m. The melting temperature and latent
heatweremeasuredtobe31.23 1Cand139.20 J/gbyDSC.TheMicro/NanoPCM
have good thermal stability after 5000thermal cycles. All of the
results indicated that
thepreparationofPS/n-heptadecaneandPMMA/n-nonadecaneMicro/NanoPCMhadpromisingpotential
indifferentther-mal energy storage applications such as solar
thermalcontrolling of building envelopes, thermoregulation
tex-tiles,thermal protectingofvehiclebatterysystems,ther-mal
regulating application, and so on.Backet al. [68] andAlkanet al.
[69]
alsosuccessfullysynthesizedandcharacterizedtheNanoPCMthroughemul-sion
polymerization method.Miniemulsion polymerization
methodMiniemulsionpolymerizationmethodis themost commonmethodfor
preparing NanoPCM at present.In this
method,polymerizationreactionis carriedoutwithintinydroplets,which
are stable, decentralized, size at nanometerlevel under the effect
of high shear force, and containwater, monomer, emulsier
andinitiator, etc. DuringtheFigure5 TEMmicrographsofSBA/n-dodecanol
NanoPCMsynthesizedwithdifferentamountsofemulsierLAS:(a)2%,(b)3%,(c)
4%, and (d) 5% [73].C. Liu et al. 818miniemulsion polymerization
reaction, the monomer deter-mines the chemical composition of
product and the proper-ties of latex [70], and the size and
morphology of
thepolymeremulsionformednallyaresameasthoseoftheoriginal droplets
[71,72].Comparedwithemulsionpolymerization,theminiemul-sion
polymerization method in preparation of NanoPCM hastheadvantagesof
lessenergy inputandhighstability.Thismethodis suitablefor
thecombinationof alkaneas PCMcore and polystyrene, polyurea,
styrene-butyl acrylate(SBA), styrene (St)-methylmethacrylate (MMA)
copolymerand poly (methyl methacrylate) as shell.The NanoPCM
containing n-dodecanol as core andstyrene-butyl acrylate (SBA)
copolymer as shell weresynthesizedthrough
miniemulsionpolymerization methodby Chen et al. [73]. The particle
size, morphology andthermal performancesof
theNanoPCMweremeasuredbyPSD, TEM and DSC, respectively. The results
showed that theencapsulation efciency of NanoPCMhas
reached98.4%.The spherical structure of the NanoPCM can be seen
clearlyfromFigure5.Whenthemassratioofmonomers/n-dode-canol reached
1:1, the average particle size got 100 nm, thethermal
decompositionwas about 195 1C, andthephasechange temperature and
phase change enthalpy were18.4 1C and 109.2 J/g, respectively.Fang
et al. synthesized the NanoPCM, whose shell
ispolystyreneandcoreisn-tetradecane[74],
n-octadecane[63,75]orn-dotriacontane[76]viaminiemulsionpolymer-izationmethod.
ThesynthesizedNanoPCMweresphericalandthez-averageparticlesizewas
132 nm, 124 nmandFigure 6 SEM micrographs of NanoPCM with various
amounts of polyaniline (a) 0 g, (b) 0.5 g, (c) 1.0 g, (d) 1.5 g and
(e) 2.0 g [83].819 Preparation, characterization and heat transfer
enhancement168.2 nm, respectively. Theresults of DSCanalysis
repre-sented that the melting temperature and latent heat of
then-tetradecane/polystyrenenanocapsuleswere4.04 1Cand98.71 J/g,
andthefreezingtemperatureandlatent heatwere 3.43 1C and 91.27 J/g,
respectively. The latent heat
ofthen-octadecane/polystyrenenanocapsulesreachedupto124.4 J/g. The
melting temperature and latent heat of
then-dotriacontane/polystyrenenanocapsules weremeasuredtobe70.9
1Cand174.8 J/g,andthefreezingtemperatureand latent heat were
measured to be 63.3 1C and 177.1 J/g,respectively.Tumirah et al.
[77] fabricated the NanoPCMwith n-octadecane as core and styrene
(St)-methylmethacrylate(MMA) copolymerasshell
usingminiemulsionin-situpoly-merization method. The morphology,
particle size
andthermalpropertiesoftheNanoPCMwerecharacterizedbySEM, dynamic
light scattering (DLS) and DSC. When
theshell/coremassratiowas3:1,themeandiameter,meltingtemperature and
freezing temperature of the sphericalNanoPCMwere102 nm, 29.5
1Cand24.6 1C, respectively.The melting and freezing latent heat
reached 107.9 J/g and104.9 J/g, separately. After 360
heating/cooling cycles, thefabricatedNanoPCMstill hadgoodchemical
stabilityandthermal
reliability.TheNanoPCMwiththeaveragesizebelow200 nmwerealso
synthesized using miniemulsion polymerization methodby Luo et al.
[78], Fuensanta et al. [79], Chen et al. [80] andZhang et al.
[81].In situ polymerization
methodIntheprocessofpreparingphasechangecapsulesthroughinsitupolymerizationmethod,thereactionmonomerandcatalystareall
locatedoutsidethecore.Themonomerissolubleinthecontinuousphase,butthepolymerisimmis-ciblewiththecontinuousphase.
Therefore, thepolymer-ization reaction occurs on the surface of the
core. With thedevelopment of the polymerization, the prepolymer
isgeneratedgraduallyonthesurfaceof thecore, andnallythe capsule
shell is formed [57].Up to now, in the in situ polymerization
method, the organics,which are commonly polymers such as
urea-formaldehyde resin,melamine-formaldehyde, carboxymethyl
cellulose, poly
(methylmethacrylate)andpoly(allylmethacrylate),havebeenmainlycoated
as shell material.Hu et al. [82] synthesized the NanoPCM (parafn as
coreand carboxymethyl cellulose (CMC)-modied MF as shell) viain
situ polymerizationmethod.TheobtainedCMC-modiednanocapsules were
spherical in shape with an averagediameterof 50 nm.
Whenthemasscontentof
parafninthenanocapsuleswere31.6%,49.1%and63.1%,thecorre-spondingphasechangeenthalpywere41.79
J/g,64.85 J/gand 83.46 J/g, respectively, and the corresponding
crackingratio of the nanocapsules was 17.5%, 10.6% and 11.0%
whenthenanocapsulessuspensionwasshearedmechanicallyat5000 rpm for
10 min.Wanget al. [83] synthesizedtheNanoPCMthroughfreeradical
emulsion polymerization method and insitu polymer-ization method,
using poly (methyl
methacrylate-co-allylmethacrylate)asashellandn-octadecaneascore.Figure6shows
the SEM micrographs of NanoPCM with various
amountsofpolyaniline(PANI).Theinuencesofdifferentcontentsofpolyaniline
as nucleating agent on the surface morphology, thecrystallization
property and the thermal stability of NanoPCMwereinvestigated.
Theresults reectedthat theshapeofcapsule is spherical, the particle
size distributes between 1001000 nm and theaverage size is in the
rangeof577693 nm.There is a little effect of additive amount of the
polyaniline onthe morphology, the particle size and the
encapsulationefciency of capsule, but increasing the amount of
polyanilinewould decrease the degree of supercooling. When the
addingamount of polyaniline were 0 g, 0.5 g, 1.0 g, 1.5 g, and 2.0
g,thecorrespondingsupercoolingdegreeof thecapsulewere2.3 1C, 0.8
1C, 0.4 1C, 1.1 1C and 0.5 1C, respectively.Figure7 SEMmicrographs
of PA/SiO2NanoPCMpreparedatdifferent PH of the solvent:(a) 11, (b)
11.5 and (c) 12 [87].C. Liu et al. 820The NanoPCM, containing
n-tetradecane as the
core,werefabricatedviainsitupolymerizationmethodbyFanget al.[84].
The shellis the polymerizationproductof ureaand formaldehyde. The
results of SEM analysis showed thatthe NanoPCM had mean size of
about 100 nmand n-tetradecanewas well encapsulated. Themass content
ofn-tetradecane exceeded 60%, and the phase changeenthalpy reached
up to 134.16 J/g.Solgel methodSolgel method requires relative mild
condition in thepreparation. The main processes are as follows:
rstly,metal alkoxide as precursor mixes uniformly with thesolvent,
catalyst andcomplexingagent, etc. Secondly,
astableandtransparentcolloidal
solutioncomesintobeingafterhydrolysisandcondensationchemical
reaction.Thenthe gel with three-dimensional network structure
wasformed after aging of the sol. Finally, MacroPCM or NanoPCMcan
be prepared after drying, sintering and curing[41,85,86]. Solgel
methodis suitablefor NanoPCMwithalkane, palmitic acid and indiumas
core material andsilicon dioxide as shell material.Latibari et al.
[87] successfully synthesized the
NanoPCMwhichcontainspalmiticacid(PA)ascoreandSiO2asshellthroughsolgel
method.Threesamples(S1,S2andS3)ofPA/SiO2nanocapsuleswerepreparedatthreedifferentPHvalues(11,11.5and12),respectively.ThemicrographsofPA/SiO2
NanoPCM can be seen from Figure 7, and it is
clearthattheNanoPCMhasasphericalstructure.TheresultsofFourier
transforminfraredspectroscope(FTIR), X-raydif-fractometer (XRD) and
Energy dispersive X-ray
Spectro-metry(EDS)indicatedthattheNanoPCMweresynthesizedsuccessfully,
and they had compact and smooth surface. SEMand TEM tests indicated
that the mean diameters of S1, S2and S3 were 183.7 nm, 466.4 nmand
722.5 nm, respec-tively. The encapsulation ration of PA for S1, S2
and S3 were82.53%, 84.28% and 88.32%, respectively. The
thermalconductivity of theNanoPCMwas improvedcomparedtoFigure 8
Effect of mass concentration of slurry on temperature distribution
[104].821 Preparation, characterization and heat transfer
enhancementthat of pure PA. Atest of 2500 thermal cycling for
S3indicated that the melting and freezing temperatures of
theNanoPCMwerechangedfrom61.6 1Cto60.1 1Candfrom57.08 1C to 56.85
1C, respectively. The latent heats ofmelting and freezing were
changed from180.91 J/g to177.3 J/gandfrom181.22 J/gto178.6 J/g,
respectively.These results indicated that the NanoPCM have
goodthermal properties and reliability and chemical
stability.Hongetal. [88] synthesizedtheNanoPCMthroughsolgelmethod,
whichusedsilicaasshell andindiumascore. Twotypes of silica obtained
from tetraethylorthosilicate (TEOS)
andsodiumsilicatehavebeenusedintheNanoPCM.Theparticlesize and the
degree of super-cooling of the two kinds
ofNanoPCMwereanalyzedandcompared.Theresearchresultsshowed that the
core diameter and shell thickness of NanoPCMusing TEOS-derived
silica as shell were 200 nm and 100 nm andthose of the other
NanoPCM were 200 mm and 50 nm,
respec-tively.Whenthechangerateofthetemperaturewere1 1C/min, 5
1C/min, 10 1C/min, 20 1C/min and 40 1C/min, the super-cooling were
32 1C, 33 1C, 34 1C, 36 1C and 41 1C, respectively.And the
corresponding super-cooling of the other NanoPCM are3.9 1C, 6.1 1C,
8.3 1C, 10.2 1C and 14 1C at the same tempera-ture change rate,
respectively.Amongthemethodsintroducedabove, interfacial
poly-merizationmethodiswidelyusedinencapsuleofdyeandpesticides,
which has the advantages of simple process andwide commercial
application. However, less researchesfocusontheinterfacial
polymerizationandthechoiceofshell material is relatively fewer for
the synthesis
ofNanoPCM.ThepreparationofNanoPCMusinginsitupoly-merization has
good capsule morphology and thermal prop-erties.
Furtherresearchshouldbecommittedtonotonlysimplifytheprocess,butalsoreducethecostofindustrialscale
production. The miniemulsion polymerization
methodisagreatwaytopreparecore/shell polymers[89].Therehave been
many studies that using this method successfullysynthesized NanoPCM
which have good thermal perfor-mance and stability. In the
preparation process of NanoPCMusing miniemulsion polymerization
method, the demands ofhigher stability of system and polymerization
rate moderatecan be easily met [90]. Besides, the size of the
capsule canbe adjusted by controlling the stabilizer dosage.
Theadvantages of miniemulsion polymerization indicate
thattheprocessofpreparationNanoPCMiseasytocontrolandconducive to
the implementation of industrial production.Applications of NanoPCM
in latent functionalthermal uidThelatentfunctional thermal uid,
composedof thermaluid as continuous phase and PCMparticle additives
asdispersion, is a kind of solidliquid two phase liquid [91,92].Due
to the fact that the latent functional thermal uid
hashigherheatstoragecapacityandheattransfercoefcientthantraditional
singlephase uid, researchers
havepaidmoreandmoreattentiontoitinrecentyears.TheEPCMslurryisakindoflatentfunctional
thermal uidwhichiscomposed of encapsulated PCM and single phase
heattransfer uid[93]. EPCMslurry, as efcient heat
transfermediumintheheat storage, not onlyimproves
theheatstoragecapacityandheattransfer rate,
butalsoreducestheheatexchangersize,
uidconveyingpipelinesizeandtransportenergyconsumption.
EPCMslurrycanbewidelyused in various energy storage systems to
achieve thestorage and transportation of
energy.CurrentlyMicroPCMslurry, as latent functional
thermaluid,hasbeenstudiedbymanyresearchers[9498].HeattransfercoefcientsofMicroPCMslurryweremeasuredbyWangetal.[99].TheMicroPCMslurryconsistedofmicro-encapsulated
1-bromohexadecane and water, with the massfraction of MicroPCM
varying from 5% to 27.6%. The resultsshowed that the thermal
storage capacity and heat transfercoefcients were both improved
after adding MicroPCM.
ThesameresultscanalsobefoundintheworkofDelgado[100]andRao[101].AlthoughEPCMslurryhasmoreadvantagesinheat
storage capacity and heat transfer performance comparedwith
traditional heat transferuid, it has some disadvantages,such as
easily fracturing in the process ofow during
pumping,increasingtheuid'sviscosity,makingthepipeeasytowearand jam
[102,103] and so on. NanoPCM has the advantages ofsmall size and
larger specic surface area. It was more stableFigure 9 The typical
structures of NanoPCM [107].C. Liu et al.
822thanMicroPCMinstructure.Thefracturerate,theeffectofincreasing
theuid's viscosity and wearing the pipeline are allsmaller
thanthoseof MicroPCM[39,54]. NanoPCMslurryaslatent functional
thermaluid has broad application
prospectsintheeldofintelligentbuilding, thermal
regulatingfabricandelectronicdevices thermal control, etc.
Therefore, thedevelopment of NanoPCM slurry is inevitable.Fang et
al. [74] synthesized
polystyrene/n-tetradecaneNanoPCMusingultrasonicassistantminiemulsioninsitupoly-merization,
and added the NanoPCM to water as latentfunctional thermal
uidwhichwas utilizedincoldthermalenergy storage. The research
results showed that the
thermalconductivityofthewaterwasimprovedfrom0.6226 W/(m K)to0.63806
W/(m K) andfrom0.7296 W/(m K) to0.84676 W/(m K) at the temperature
of 5 1C and 25 1C after addingNanoPCMwiththemass fractionof 15%,
respectively. Whenthetemperatureis 5 1Candthemass concentrationof
theNanoPCMare15%, 7.5%, 3.75%and0%, theviscosities are16.16 mPa
s,12.18 mPa s,8.56 mPa s,and4.68 mPa s,respec-tively. It is visible
that the NanoPCM slurry in the low viscositiesrange is suitable for
using as latent functional thermal uid.Seyf et al. [104]
studiedthethermal characteristicof amicrotube (NanoPCM slurry as
coolant) heat sink with tangen-tial impingement through three
dimensional numerical simula-tion. The NanoPCM slurry consists of
polyalphaolen (PAO) asbase uid and octadecane as nanoparticle. As
shown inFigure 8, when the Reynolds number (Re) is 400,
thetemperature boundary layer growth for NanoPCM slurry cool-ant is
slower than that of pure PAO, and increasing the massconcentration
of slurry can reduce the wall temperature anduniformthe temperature
distribution in solid and liquidphases. Andlater Seyf et al. [105]
analyzedtheeffects
ofmassconcentrationandReynoldsnumberofNanoPCMslurryon convection
heat transfer of steady laminarowing past anisothermal
squarecylinder bymeans of numerical solution.The NanoPCM slurry
consists of water and n-octadecaneNanoPCMwith an mean diameter of
100 nm. The resultsshowed that the NanoPCM has signicant effect on
enhancingthe heat transfer ability and the increases in volume
fractionandReynoldsnumberleadtoenhancementofheattransferand shear
stress over the cylinder.Wu et al. [103] synthesized the
polymer/parafn NanoPCMusing miniemulsion polymerization method. The
NanoPCMwereaddedinwater toformNanoPCMslurrywhichcouldenhance the
heat transfer coefcient of jet impingement andspray cooling. The
results showed that the volume fraction ofthe NanoPCM has a great
effect on heat transfer coefcient.Compared to water, slurry with
28% volume fraction ofNanoPCM enhances heat transfer coefcient by
50% and 70%,for jet impingement and spray cooling,
respectively.Heat transfer enhancement of
NanoPCMAlthoughNanoPCMslurryaslatentfunctionalthermal
uidhashigherheatstoragecapacityandheattransfercoef-cientthantraditional
singlephaseuid,theheattransfercoefcient is still low. And PCMhas a
larger degree
ofsupercoolinginphasechange,soitsapplicationislimited[106].
Therefore, theheat transfer of NanoPCMmust beenhanced in order to
reduce the degree of supercooling andimprove the heat transfer
efciency.Park et al. [65] synthesized the magnetic Fe3O4
nanopar-ticles(NPs)-embeddedPCMnanocapsules(Mag-PCM)basedonaparafncoreandpolyureashell
viainterfacial poly-condensationmethod. Threeweightpercentagesof
Fe3O4were added to PCM nanocapsules, respectively. The
weightpercentagesofFe3O4nanoparticlesinMag-PCMweremea-sured to be
3.1%, 5.7% and 6.6% by TGA. The
Fe3O4nanoparticleswereembeddedinthepolyureashell.Whenthe amount of
Fe3O4 NPs is added, the thermal conductivityof the nanocapsules
increased and the supercooling degreeof parafn decreased.Dynamics
simulation study of NanoPCMWiththeunceasingdevelopmentof
computertechnology,the computer simulation has been widely used in
studies ofPCM. Some researchers studied thermal properties,
selfdiffusion, mesoscopic morphology and evolution mechanismof the
NanoPCM by dynamics simulation.Rao et al. [107] studied the self
diffusion of the NanoPCM bymolecular dynamic simulation. In this
study two NanoPCMmodels were fabricated by using n-octadecane as
core mate-rial and SiO2 as shell materials: one with constrained
shell andanother with free shell. Figure 9 shows the typical
structuresof NanoPCM. The molecular dynamic simulation
resultsshowedthat thickness andhardness of NanoPCMshell hasimpact
on the self diffusion properties of NanoPCM.
TheNanoPCMwithconstrainedshellwillrestrainthetorsionandstretch
strength of molecular chain outside the core materials,and the
diffusion coefcient of NanoPCM will decreased.
TheNanoPCMwithfreeshell will increasethe uidity of corematerial,
reduce the thermal contact resistance betweencapsules and enhance
the heat transfer of the whole capsule.Later, Rao et al. [108]
studied the evolution mechanismsand mesoscopic morphologies of the
NanoPCM by dissipativeparticle dynamics simulation method. The
simulation resultsshowedthat thetwo types ofNanoPCM can
besynthesizedusing n-docosaneas a corematerials andethyl
acrylate(EA),styrene(St)orallyloxynonyl-phenoxypropanolpoly-oxyethylene
ether ammoniumsulfate (DNS-86) as shellmaterials, respectively. A
typical coreshell structure ofthe NanoPCM can be synthesized when
the proportion of
thecomponentsissuitable.TheNanoPCMfailedtobesynthe-sized when the
surfactant and shell materials is excess. Andthe optimal
encapsulation rate of n-docosane is
54.51%analyzedbydissipativeparticledynamics simulation. Theresults
of particledynamics simulationresearchcouldbeuseful in the design
and experiment of the NanoPCM.Further prospective researchMicroPCM
has been studied for many years, but theresearches on NanoPCM start
in recent years. Manyresearchers
studiedthepreparationandcharacterizationof the NanoPCM, but there
are still many aspects worth
ourfurtherinvestigation.Suggestionsforthefutureworkscanbe
summarized into the following several aspects:(1)
Raisetheefciencyof preparationof NanoPCM:
sincetheproductionefciencyof NanoPCMis quite low, it isdifcult
tomeet theneeds of industrial applications.823 Preparation,
characterization and heat transfer
enhancementTherefore,increasingtheproductivityoftheNanoPCMis
inevitable.(2) Molten salt and hydrated salt encapsulation: molten
saltand hydratedsalthavehighlatentwhenthey occurredin
phasetransition,sotheyareverysuitableasenergystoragematerials. But
at present theresearchof thecoreof NanoPCMmainlyfocuses
onorganicmaterial.Therefore, molten salt and hydrated salt as core
ofNanoPCM need further research.(3) Applications of NanoPCM:
uptonow, therehas
beenlittleresearchworkofNanoPCMappliedinwasteheatrecovery, solar
energy storage, intelligent building,thermal regulating fabric,
electronic devices thermalcontrol andbatterythermal
managementsystem,etc.Thereforefurther studies of
NanoPCMapplicationareneeded.(4) Further researches on preparation
of NanoPCM with higherencapsulationefciency,betterstability,
betterthermalperformanceandmoreuniformparticlesizedistributionneed
to be conducted. The diversity in choice of core andshell material,
theoptimal process conditions andthereduction of production cost
can not be neglected.ConclusionsThis paper mainly reviewed the
research progress ofNanoPCM in recent years. This review consists
of four parts:preparationandcharacterizationof NanoPCM,
applicationof NanoPCM in latent functional thermaluid, heat
transferenhancement of NanoPCM and dynamics simulation study
ofNanoPCM. Fivekindsof
availablepreparationmethodsonNanoPCMhavebeenintroduced, that is,
interfacial poly-merization method, emulsion polymerization method,
mini-emulsion polymerization method, in situ polymerizationmethod
and solgel method. The characterizations ofNanoPCM preparation
using the above methods weredescribed. Applications of NanoPCMin
latent functionalthermal uid were summarized. Not only the
thermalstorage capacity of the thermal transfer uid can
beincreased, but also the performance of thermal
conductivityoftheuidcanbeenhancedbyaddingtheNanoPCM.Andthen, this
paper introducedthe self diffusion,
evolutionmechanismsandmesoscopicmorphologiesoftheNanoPCMbydynamicssimulation.Finally,thelimitationsofcurrentresearch
of NanoPCM were explained, and the furtherprospective research of
NanoPCM were discussed.AcknowledgmentsThis workwas
supportedbytheNational Natural ScienceFoundation of China (Nos.
51406223 and U1407125) and theNational Natural Science Foundation
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Y.T. Huo, X.J. Liul, RSCAdv. 4(2014)3955239557.Chenzhen Liu
received his Bachelor degreeinThermal EnergyandPower
EngineeringfromHebei University of Engineering in2010, and Master
degree in Power Engineer-ingfromGuangdongUniversityofTechnol-ogy in
2014. Heis currently pursuing
hisPh.D.underthesupervisionofProf.Zhon-ghaoRaoandProf. YiminLi
atChinaUni-versity of Mining and Technology. Hisresearch mainly
focuses on nanouid.Zhonghao Rao received his Bachelor degreein
Thermal and Power Engineering andMaster degree in Thermal
Engineering fromGuangdong University of Technology in 2008and2010,
andPh.D. in Chemical ProcessEquipment
fromSouthChinaUniversityofTechnologyin2013.Heiscurrentlyapro-fessor
working at China University of Miningand Technology. His research
mainly focusesonthermal energyconversionandstorageespecially by
using phase change
materials.JiatengZhaoreceivedhisBachelordegreeinThermal
EnergyandPower EngineeringfromChinaUniversityof
MiningandTech-nology in 2013. He iscurrently pursuing hisMaster
degree under the supervision of Prof.ZhonghaoRaoandProf. YiminLi at
ChinaUniversity of Mining and Technology. Hisresearch mainly
focuses on thermal physicalproperties of
nanouid.YutaoHuoreceivedhisBachelordegreeinThermal Energy and Power
Engineering fromChina University of Mining and Technology in2014.
Heis currently pursuing his Masterdegree under the supervision of
Prof.
Zhon-ghaoRaoatChinaUniversityofMiningandTechnology.Hisresearchmainlyfocusesonheat
and mass transfer and nanouid.YiminLi receivedhisPh.D.
inEngineeringMechanicsfromChinaUniversityof Miningand Technology in
1998. He worked atSchool of Electric Power Engineering,
ChinaUniversity of Mining andTechnology. Cur-rently he is Professor
and Dean of School ofElectric Power Engineering,
ChinaUniver-sityofMiningandTechnology.Hisresearchmainly focuses on
the theory ofuid.C. Liu et al. 826