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Characterization and Performance of DotPS Nanoencapsulated Phase Change Materials as Latent Functionally Thermal Fluid

By Jeanne Hawkins,2014-09-10 21:19
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Characterization and Performance of DotPS Nanoencapsulated Phase Change Materials as Latent Functionally Thermal Fluid

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    Characterization and Performance of Dot/PS

    Nanoencapsulated Phase Change Materials as Latent

    Functionally Thermal Fluid

    5 FANG Yutang, YANG Guo, LIU Hong, GAO Xuenong, ZHANG Zhengguo

    (The Key Laboratory of Enhanced Heat Transfer & Energy Conservation, Ministry of

    Education,South China University of Technology, GuangZhou 501640)

    Abstract: Latent functionally thermal fluid (LFTF) with encapsulated phase change materials (PCMs) is widely used in thermal control, the cooling of electronic equipments, fluidized beds, and other

    10 systems that require high heat transfer efficiency. A novel LFTF with nanoencapsulated phase change material (NEPCM) composed of polystyrene (PS) as shell and n-dotriacontane(Dot) as core was synthesized by ultrasonically initiated miniemulsion polymerization. The composition, morphology and the thermal properties of NEPCM were characterized by particle size analyzer, TEM, FT-IRDSC and

     TG. The results showed that the prepared capsules were regularly spherical with average diameter of-115 163.4 nm and latent heat of 158.4kJ kg. The fluid performance showed that the synthesized latex was

    of high specific heat capacity, excellent freeze-thaw resistance, mechanical stability and low viscosity, thus it is very suitable for being used as latent functionally thermal fluid.

    Keywords: Nanoencapsulated phase change material; Latent functionally thermal fluid; Ultrasonic initiated miniemulsion polymerization

    20

    0 Introduction

    Latent functionally thermal fluid (LFTF) is a special multiphase fluid with encapsulated phase change materials (PCMs, e.g. microencapsulated PCM)) as disperse phase and heat transfer fluid as continuous phase. Compared with conventional single-phase fluid, LFTF has various

    25 advantages such as high-density thermal energy storage, high-speed transportation, low flow drag and less heat loss in the pipe transportation, high specific heat capacity and so on. Therefore, LFTF is a promising material for the applications of thermal control, the cooling of electronic

    [1]equipments, fluidized beds, and other systems that require high heat transfer efficiency .

    There are various preparation methods for the microencapsulated phase change materials

    [2][3] [4]30 (MEPCMs), such as in-situ polymerization , interface polymerization and coacervation , etc.

    The earlier experimental studies mainly concentrated on enhancing heat conduction performance

    [5-8]. [9, 10] of the LFTF that contains MEPCM Recent publications based on numerical simulations

    showed that laminar convective heat transfer of MEPCM slurry could be enhanced. Alvarado et al [8] presented thermo-physical property data of MEPCM slurry. They also presented the impact of

    35 using enhanced surface tube in combination with MEPCM slurry under constant heat flux and turbulent conditions. Chen et al [11] studied the behaviors of the convective heat transfer of MEPCM suspension for laminar flow in a circular tube under a constant heat flux. The results showed that the heat transfer enhancement ratio of 15.8 wt% MPCM suspensions can reach 1.42 times of that of water, and the pump consumption of the MPCM suspension system decreased

    40 greatly with a larger heat transfer rate compared with water.

    However, the performance of MEPCM turned bad after repeated cycling. The large particles of the microencapsulated PCM not only increased the fluid's viscosity and transfer resistance, but also crushed each other during pump delivery process. Therefore, it is necessary to develop

     nanoencap -sulated PCM (NEPCM) with smaller particle size than MEPCM.There is limited

    Foundations: National High Technology Research and Development Program (No.2009AA05Z203), Research Fund for the Doctoral Program of Higher Education (No.20090172110015)

     Brief author introduction:FANG Yutang, (1964-),Male, ProfessorPhase change storage materials. E-mail:

    ppytfang@scut.edu.cn

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    45 progress for producing NEPCM. Zhang et al [13] synthesized a kind of nanoencapsulated PCM by

    in-situ polymerization, in which melamine-formaldehyde resin was used as the shell, n-octadecane

    and cyclopean as the core. Similarly, Momoda et al [14] prepared nanocapsules with arachidic and

    trimethlolethane as core and organicsilicon polymer as shell, then dispersed NEPCM in low

    viscosity hydrocarbon as fuel cells coolant. Miniemulsion polymerization was a convenient 50 one-step encapsulation technique for preparing nanocapsules. Luo et al[15] studied the

    nanoencapsulation of hydrophobic compounds by miniemulsion polymerization, and it was found

    that the thermodynamic factors and the kinetic factors, as well as the nucleation modes all had a

    great influence on the latex morphology, but the thermo-physical properties of synthesized

    [16], [17,18] nanocapsules were not mentioned. Parket alFang et al prepared polystyrene (PS)

    55 nanoparticles containing paraffin wax as PCM using the ultrasonic-assistant miniemulsion

    polymerization.

    Generally, the miniemulsion polymerization time is more than 4 h, and the initiator residues

    could affect the stability of the NEPCMs emulsion. Utilization of the cavitation and the non-linear

    acoustic streaming of ultrasonic radiation, it can effectively synthesize core-shell structure

    [19]60 nanocomposites.

    Since it can remove the post-treatment processes (free initiator) and significantly reduce

    reaction timeabout 35min, so ultrasonically initiated polymerization is a synthesis process with

    high efficient and environmentally friendly features. In this paper, the nanocapsules with

    polystyrene as shell and n-dotriacontane as core were synthesized by ultrasonically initiated free 65 radical-catalyzed miniemulsion in-situ polymerization, and the characterization and performance

    of LFTF with NEPCM were also discussed.

    1 EXPERIMENT

    1.1 Materials

    The monomer styrene (St, AR, from Guangdong Guanghua Chemical Reagent Co. Ltd., 70 China) was firstly washed three times with sodium hydroxide aqueous solution of 10wt%, then

    with deionized water before being used. The comonomer acrylonitrile (AN, AR, from Tianjin

    Kermel Chemical Reagent Co. Ltd., China) was used as received. n-dotriacontane (Dot, AR, from

    Shanghai Pinchun Chemical Reagent Co. Ltd., China) was used as the core material. Sodium

    dodecylsulfate (SDS, AR, from Guangdong Xilong Chemical Reagent Co. Ltd., China) and 75 poly-(ethyleneglycol) monooctyl-phenylether (OP-10, AR, from Shanghai Lingfeng Chemical

    Reagent Co. Ltd., China) were used as emulsifiers.

    1.2 Preparation of NEPCM

    Typically, under 70 water bath and magnetic stirring, 10g ? St, 10g Dot ,1g AN and 0.25g

    OP-10 were mixed to obtain oily mixture. Aqueous medium was prepared by mixing together 80 200g deionized water and 0.25g SDS. The oily mixture and the aqueous medium were placed into

    a 500ml con-shape flask and pre-emulsified by homogenizer (model FJ200-S, Shanghai Specimen

    and Model Factory, China) with 6000 RPM for 10 min. Pre-emulsion was transported into a

    500ml three-necked flask with draintube, nitrogen inlet and ultrasonic generator ( Model JYD-900,

    Shanghai Zhisun Instruments Co., Ltd , China).After removal of the oxygen in the system with 85 nitrogen for 10min, ultrasonically reacted at 75% amplitude for 35min under the steady

    temperature of 55 and reflux ? condensation to obtain nanoencapsulated PCM emulsion, then

    naturally cooled to room temperature. Demulsification was accomplished by washing the

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     emulsion with 10wt% sodium chloride solution. After the crude white solid was washed three times by petroleum ether and deionized water to remove unencapsulated Dot, the target product

    90 was obtained under vacuum drying at 50? for 24h.

     1.3 Characterization and performance of NEPCM The particle size of NEPCM was measured with NPA150 (Microtrac Co., Ltd, USA) nanoparticle size analyzer. The latex was diluted to 0.01wt% before measurement. The

    morphology of the nanocapsule was observed with H-7500(Hitachi Co., Ltd, Japan) transmission

    electron microscopy at an accelerating voltage of 80 kV. The latex was diluted to 1wt %, then 95

     mounted on carbon-coated copper grids and left dried at room temperature before analysis. The FT-IR spectra of the samples were recorded on TENSON 27(Bruker Co., Ltd, Germany) in wave

     number range from 400 cm-1 to 4000cm-1 and using potassium bromide tablet. DSC measurements of the NEPCM and its latex were carried out on Q20 (TA Instrument, USA)

     100 differential scanning calorimeter under N2 atmosphere and 5?min-1 heating or cooling rate. The

     thermal stabilities of the dried nanocapsules were evaluated using STA 449C thermo gravimetric analyzer (Netzsch Co., Ltd, Germany) under N2 atmosphere and 10?min-1 scanning rate. The

     viscosity of the latex was determined using Brookfield DV-?+ rotation viscometer (Brookfield Co., Ltd, USA) with S61 rotor type at 100RPM and temperature range of 25 to 65?. Identical

     samples were tested three times and the average was recorded. The resistance freezing-thaw cycle 105 test was completed by putting the emulsion into the breaker and sealing, then transferring it into a high-low temperature constant temperature chamber (Model QA-FC-40, Young Chenn Instrument

     Co., Ltd, China) at temperature range from 0 to 80?. The heating and cooling rate was 3?min-1, the constant temperature time was 30min.The mechanical stability of latex was tested by

     centrifugation. Put into the centrifuge tube, the latex was centrifuged for 30min at 1500-3000RPM 110 by Model 800 tabletop centrifuger (Suzhou Weier Laboratory Supplies Co., Ltd. China), then filtrated and vacuum-dried at 50? for 24h to obtain solid product. The mechanical stability of latex was evaluated by the ratio (R) of the solid product after centrifugation to the solid content of latex. The bigger the ratio R was, the worse the mechanical stability of latex was.

     2 RESULTS AND DISCUSSION115

     2.1 Characterization of NEPCM Particle size and morphology

     Fig.1 displays the particle size and distribution of the nanoencapsulated PCM. It can be seen that the particle size of NEPCM varied from 50 nm to 300 nm, exhibiting a narrow size

    distribution. The Z-average particle size of the nanoencapsulated PCM was 162.4 nm. 120

     Fig.2a shows TEM image (Mag.50k) of NEPCM. Fig 2b is the partial enlarged image (Mag.150k).

     It can be seen that most of the nanocapsules were regular spherical, and the core of n-dotriacontane (pale part) was located in the shell of polystyrene (dark part). The diameter of the

    nanocapsules was about 130~150nm, consistent with the result of the particle size analysis. 125

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     Fig. 1 Particle size and distribution of NEPCM