Experimental study on microcapsules of Ag doped ZnO nanomaterials enhanced Oleic-Myristic acid eutectic PCM for thermal energy storage

Experimental study on microcapsules of Ag doped ZnO nanomaterials enhanced Oleic-Myristic acid eutectic PCM for thermal energy storage

Accepted Manuscript Title: Experimental study on microcapsules of Ag doped ZnO nanomaterials enhanced Oleic-Myristic acid eutectic PCM for thermal ene...

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Accepted Manuscript Title: Experimental study on microcapsules of Ag doped ZnO nanomaterials enhanced Oleic-Myristic acid eutectic PCM for thermal energy storage Authors: S. Dhivya, S. Imran Hussain, S. Jeyasheela, S. Kalaiselvam PII: DOI: Reference:

S0040-6031(18)30552-5 https://doi.org/10.1016/j.tca.2018.11.010 TCA 78146

To appear in:

Thermochimica Acta

Received date: Revised date: Accepted date:

20 July 2018 9 November 2018 14 November 2018

Please cite this article as: Dhivya S, Hussain SI, Jeyasheela S, Kalaiselvam S, Experimental study on microcapsules of Ag doped ZnO nanomaterials enhanced OleicMyristic acid eutectic PCM for thermal energy storage, Thermochimica Acta (2018), https://doi.org/10.1016/j.tca.2018.11.010 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.

Experimental study on microcapsules of Ag doped ZnO nanomaterials enhanced Oleic-Myristic acid eutectic PCM for thermal energy storage S. Dhivya, S. Imran Hussain, S. Jeyasheela, S. Kalaiselvam *

Chennai 600 025, India

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*Corresponding Author Tel: +91 44 22359220

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Department of Applied Science and Technology, AC Tech Campus, Anna University,

Email addresses: [email protected] (S. Dhivya), [email protected] (S.

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Kalaiselvam)

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Imran Hussain), [email protected] (S. Jeyasheela), [email protected] (S.

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Graphical abstract

Highlights: 

Ag doped ZnO nanomaterials enhanced Oleic-Myristic acid eutectic PCMs were developed.



The synthesized microcapsules have uniform spherical morphology with an average



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size of 3 to 5 µm. The melting and freezing enthalpies of microcapsules are about 75 to 80 J/g and 75 to



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78 J/g.

Microcapsules with highest weight percentage of Ag doped ZnO nanomaterials

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showed high thermal conductivity enhancement.

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Abstract

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Novel microcapsules of Ag doped ZnO nanomaterials enhanced Oleic-Myristic acid eutectic phase change materials (PCMs) were synthesized by in-situ polymerization process.

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X-ray diffraction analysis reveals that Ag doped ZnO nanomaterials has a hexagonal wurtzite

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structure. Nano enhanced composite PCMs are prepared by mixing different weight percentage of nanomaterials with Oleic-Myristic acid eutectic mixture act as core material.

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Microscopic analysis reveals the formation of uniform spherical structure of MelamineFormaldehyde shell over the core material with a perfect core-shell structure. Differential scanning calorimetric analysis showed that microencapsulated PCMs (MEPCMs) melts and

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freezes in the range of 4.37-5.81 °C and 10.84-11.86 °C with the latent heat of 75.39-79.35 J/g and 74.9-77.95 J/g. MEPCMs have better thermal conductivity, stability and reliability determined by thermal conductivity, thermogravimetric and thermal cycling analysis. Based on the results, MEPCMs with Ag doped ZnO nanomaterials could be suggested as the potential core material for low temperature thermal energy storage applications.

Keywords: Microcapsules; Ag doped ZnO nanomaterials; Eutectic mixture; Thermal stability; Thermal energy storage. 1. Introduction Significance of energy storage is being realized with the rapid development of

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economy and industrialization in the world. Conventional energy sources like fossil fuels are limited and their utilization lead to environmental pollution and climatic changes. Past

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literatures have indicated that buildings account for 30% of annual greenhouse gas emissions and 40% of the world’s energy consumption. The reduction of energy consumption in the buildings has been one of the priorities of the recent directives. In order to conserve energy,

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reduce green house gas emission and to minimize the dependency of fossil fuels, thermal

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energy storage is very significant. The storage of thermal energy has become an important

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aspect in science and technology, engineering applications mainly in energy conservation of

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buildings. Latent heat thermal energy storage (LHTES) method is an alternative for thermal

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energy storage with respect to air conditioning system, ventilation and to decrease the operation time of heating [1]. The implementation of LHTES system in a building can reduce

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[2,3].

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temperature fluctuation and hence it enables the passive control of temperature in buildings

Phase Change Materials (PCMs) are recommended as potential Thermal Energy

Storage (TES) medium and have been receiving significant attention as it provides a new

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solution to thermally regulated energy efficient buildings. PCMs absorb and release energy during phase change process almost at constant temperature or within a small range of temperature. Among the various types of PCMs, organic fatty acid PCMs have better properties like non-toxicity, good storage capacity, non-corrosiveness, good chemical stability, melting congruency and low super cooling than other inorganic PCMs [4].

However, organic fatty acid PCMs still have some disadvantages like low thermal conductivity, leakage during phase transition from solid to liquid state. These difficulties limit their applications in reality. To overcome these deficiencies, fatty acid eutectic mixtures can be used as PCMs. These eutectic mixtures are composition of two or more fatty acids with eutectic mass ratio, each of which melts and freezes at the same eutectic temperature

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which can reduce supercooling [5,6].

By the incorporation of nanomaterials with eutectic mixture, the thermal stability and

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thermal conductivity can be enhanced [7]. Semiconducting metal oxides have extensive

applications due to its wide bandgap and large exciton binding energy with unique optical,

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thermal and electrical properties relating to the sustainable development like energy storage

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or conversion, high performance electronics and environmental remediation [8-11]. ZnO

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nanomaterials have magnificent properties such as high refractive index, high thermal

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conductivity, binding energy, antibacterial activity and UV protection capabilities. Hence, ZnO nanomaterials have wide applications in cosmetics, medicine, food, rubber, solar cells,

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supercapacitors, batteries, catalyst and thermal energy storage [12,13].

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To obtain better crystallization quality, thermal, electrical and optical properties of ZnO nanomaterials, doping has been introduced with transition metal elements. Doping with

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ZnO has been mainly focused on synthesizing p-type conductive ZnO than the n-type conductivity due to some native defects like zinc interstitial and oxygen vacancies. The p-

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type ZnO may be achieved by substitution of I-A group element or I-B group element on the Zn-site, or by substitution of V-A group elements on the O-site. Among these elements I-B group elements (Cu or Ag) has proposed as the best element for p-type doping than I-A group elements as it makes shallow acceptor. It is reported that thermal energy storage can be enhanced by adjusting the energy level surface states of ZnO and it is achieved by doping

with noble metal like Ag and a synergetic effect of surface plasmon was also observed with Ag nanoparticles on doping with ZnO [14,15]. During phase transition, PCM undergo the solid-liquid phase change. Due to the leakage in the liquid state, corrosion and thermal reliability, a technique of utilizing

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microencapsulated phase change material (MEPCM) has been developed. If the synthesized capsules are in the range of micrometers they are called MEPCMs, which consist of a core

made of the thermal energy storage material (PCM) and a polymeric shell as wall material.

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The selection of the wall material for the microencapsulation of PCM plays an important role

in regulating the characteristics of the microcapsule such as heat capacities, thermal stabilities

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and morphologies [16,17]. Few literatures reported that amino-aldehyde cross-linking

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polymeric material act as an excellent shell for MEPCMs as it has better thermal and

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mechanical stability than other shell materials [18,19]. Many methods are developed for

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and suspension-like polymerization.

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microencapsulation including in-situ polymerization, coacervation, interfacial polymerization

The present study has been aimed to achieve low temperature thermal energy storage

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materials for building envelopes. Oleic acid and Myristic acid as eutectic mixture have little supercooling, good thermal reliability and no phase segregation during phase change process.

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The composite PCMs were prepared by dispersing Ag doped ZnO nanomaterials into Pure PCM through ultrasonication in different mass proportions at liquid state act as core material.

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The reason for choosing in-situ polymerization method for synthesizing microencapsulated PCMs is due to the efficient utilization of polymer in the microcapsule shells. The chemical interaction between the shell and core materials of MEPCMs were analyzed using Fourier transform infrared spectroscopy (FTIR). The morphology and particle size of Ag doped ZnO nanomaterials and the MEPCMs were investigated using Field Emission Scanning Electron Microscopy (FESEM), High Resolution Transmission Electron Microscopy (HRTEM) and

Particle Size Distribution (PSD) analysis techniques respectively. The phase change properties and thermal stability of the eutectic mixture and MEPCMs were measured using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). Furthermore, thermal conductivity of the samples was studied extensively.

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2. Experimental section 2.1. Reagents and materials

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Zinc Acetate and Silver Nitrate (AgNO3) used as precursor in nanomaterial synthesis were purchased from Merck Life Science Pvt. Ltd. Urea used as reducing agent, Nitric acid

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(HNO3) and Sodium Hydroxide (NaOH) pellets used as pH regulators were obtained from

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SRL, India. Ethanol and Deionized (DI) water were used as solvents. Melamine and

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formaldehyde used as shell-forming monomers were purchased from Merck Life Science Pvt.

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Ltd. Oleic acid and Myristic acid used as PCM core were supplied by Sigma-Aldrich, USA. Triton X-100 as emulsifier and Ammonium Chloride (NH4Cl) as nucleating agent were

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purchased from SRL, India. All the chemicals used in this experiment were of high purity

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with analytical grade and used without any further purification. 2.2. Synthesis of Ag doped ZnO nanomaterials

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10 g of zinc acetate and 1 wt% of AgNO3 were added to 50 ml of ethanol and the

solution was stirred well using magnetic stirrer. 3ml of HNO3 was added to the solution to

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maintain the pH at 2. To this 8.25 g of urea was added and diluted with 50 ml of DI water. The mixture was heated and kept in a resistance heated furnace at 300 °C for 2 hrs. A fine pale yellow powder of Ag doped ZnO nanomaterials was obtained by combustion method and was further calcinated in a resistance heated furnace at 600 °C for 2 hrs.

2.3. Prediction of eutectic point and preparation of Oleic-Myristic Acid eutectic mixture (Pure PCM) Physico-chemical properties of oleic acid and myristic acid are listed in Table 1. The eutectic point of the binary fatty acids can be predicted using the Schrader’s equation, given

1 1 𝑇𝑚



(1)

𝑅 𝑙𝑛𝑋 ∆𝐻𝑚 ∗ 𝑚𝑜𝑙.𝑤𝑡

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T=

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below.

Where, R is the gas constant 8.314 Jmol-1K-1, T is the phase change temperature of the eutectic mixture, Tm is the melting point, X is the mole fraction, ΔHm is the latent heat

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capacity, and mol.wt is the molecular weight of the major component respectively [6]. From

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the above equation, the eutectic point mole fraction of oleic and myristic acid mixture was

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obtained theoretically and plotted against the composition as shown in Fig. 1 (a). Different

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mass ratio was chosen nearer to the theoretical eutectic point and each sample of Oleic-

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Myristic acid eutectic mixture of different mass ratio (86%-14%, 88%-12%, 90%-10%, 92%08%, 94%-06%) were prepared by fine mixing of the eutectic mixture for 30 minutes of

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ultrasonication at 70 °C using digital ultrasonicator. The eutectic point of Oleic-Myristic acid mixture was determined experimentally using constant temperature bath setup. The freezing

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curves for different mass ratio of eutectic mixture were obtained as shown in Fig. 1 (b). From the above results, the Oleic-Myristic acid mixture with 90%-10% of eutectic mass ratio

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shows a minimal freezing temperature with low supercooling. 2.4. Preparation of the composite PCMs with different wt% of Ag doped ZnO nanomaterials Pure PCM exist in liquid state at ambient temperature and is suitable for low thermal energy storage applications. To prepare composite PCMs, 0.05 g of Ag doped ZnO nanomaterials (for the mass fraction of 0.05 wt% Ag doped ZnO nanomaterials) was added

to100 ml of Pure PCM and ultrasonicated for 3-4 hrs for uniform distribution of nanomaterials. Similarly, 0.1 g, 0.15 g and 0.2 g of Ag doped ZnO nanomaterials were added to 100 ml of Pure PCM and ultrasonicated for 3-4 hrs for the preparation of 0.1 wt%, 0.15 wt% and 0.2 wt% of composite PCMs respectively. Pure PCM and four composite PCMs act

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as core material in MEPCMs. 2.5. Synthesis of microcapsules of Ag doped ZnO-Oleic Acid-Myristic Acid eutectic PCM

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mixtures

Microcapsules of the eutectic mixture were fabricated by in-situ polymerization in three stages. First stage involves the synthesis of prepolymer solution followed by the

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preparation of emulsion and finally the formation of polymeric shell material. The

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prepolymer solution was prepared by mixing 10 ml of formaldehyde and 2 g of melamine

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were added to 10 ml of DI water. NaOH was used to adjust the pH of the solution between

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8.5 to 9 and stirred continuously at 70 °C until a clear transparent solution is obtained. To the

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clear solution another 1 g of melamine was added and stirred at 70 °C till complete dissolution occurs and finally diluted with 10 ml of DI water and stirred well till the

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prepolymer solution was transparent absolutely. In the second stage, the emulsion was prepared by adding 10 g of Pure PCM/ composite PCM and 5 g of Triton X-100 (Triton X-

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100 act as a surfactant and emulsifier, the hydrophobic aromatic hydrocarbon has great affinity for fatty acids which form micelles encapsulating PCM inside as shown in Fig. 2) to

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100 ml of DI water and the pH was adjusted to 4.5-5.0 with 15% acetic acid solution. The mixture was emulsified mechanically at 50 °C with a stirring rate of 1500 rpm for 2 hours. Last stage is the formation of shell over emulsified eutectic PCM mixture. The prepolymer solution was added drop wise into the above emulsion with stirring at the rate of 600 rpm. To the above mixture, 5 wt% of NH4Cl used as a nucleating agent was added and stirred continuously at 60 °C for 90 min. The reaction was terminated by adjusting the pH to 9.0

with 10 wt% of NaOH solution. The schematic fabrication of the microcapsules through insitu polymerization is represented in the Fig. 2. The microcapsules obtained by the above method were filtered and washed with ethanol until the pH reached to 7. To remove the moisture present in the synthesized microcapsule powder, the sample was kept in vacuum

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oven at 100 °C for 24 hrs. The synthesized microencapsulated Pure PCM denoted as MEPCM1 and the

microcapsules of four composite PCMs with different mass fraction (0.05 wt %, 0.1 wt %,

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0.15 wt % and 0.2 wt %) of Ag doped ZnO were represented as MEPCM2, MEPCM3, MEPCM4 and MEPCM5 respectively.

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2.6. Analysis methods

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Bruker’s D2 PHASER X-ray Diffractometer (XRD) with CuKα radiation λ=0.1540

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nm was used to measure the crystalline size and structure of Ag doped ZnO nanomaterials with a step size of 0.02° in the range of 5° to 80°. Fourier Transform Infrared Spectroscopy

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(FTIR) analysis was carried out using Bruker ALPHA FTIR and the spectra were recorded in transmission mode (4000 to 500 cm-1) for Ag doped ZnO nanomaterials, Pure PCM and

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MEPCMs. ZEISS Filed Emission Scanning Electron Microscopy (FESEM) and FEI-

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TECNAI G2-20 TWIN High Resolution Transmission Electron Microscopy (HRTEM) with LaB6 filament were employed for measuring the size and to observe the morphological structure of MEPCMs and Ag doped ZnO nanomaterials before and after calcination. The

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particle size distribution of MEPCMs were performed using CILAS 1180 Liquid carried out in the range of 0.04 μm – 2500 μm/100 classes. The thermal conductivity of Pure PCM and MEPCMs were studied using NETZH laser flash analyzer (LFA). The TGA of Pure PCM and MEPCMs were carried out in the temperature range from 30 °C to 800 °C in nitrogen atmosphere at a heating rate of 10 °C/min using TGA Q50 V20.13 Build 39

thermogravimetric analyzer. Thermal cycling tests were analyzed using applied Biosystems veriti 96 thermal cycler. The thermal property analysis including latent heat and phase change temperature of Pure PCM and MEPCMs were performed using the differential scanning calorimetry (DSC Q200 V23.10 Build 79) instrument in the temperature range of -30 °C to

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+50 °C at a scan rate of 10 °C/min for melting and solidification process. 3. Results and Discussions

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3.1 Crystal structure analysis of Ag doped ZnO nanomaterials

Ag doped ZnO samples were calcinated at 600 °C in furnace to ascertain the

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formation of different nanocrystalline powder. The XRD patterns before and after calcination

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are shown in Fig. 3. It is observed from XRD patterns that clear Ag peaks were observed only

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after calcination at 600 °C. Both the samples were crystalline and has hexagonal wurtzite

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structure of ZnO with diffraction peaks at 31.84°, 34.50°, 36.33°, 47.65°, 56.73°, 63.01°, 66.54°, 68.12°, 69.26°, 72.75° and 77.16° indexed as (100), (002), (101), (102), (110), (103),

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(200), (112), (201), (004) and (202) confirmed by (JCPDS Card no. 075-0576). The additional peaks (asterisk marked) in Fig 3. b at 38.12°, 44.27°, 64.42° and 77.47° indexed as

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(111), (200), (220) and (311) are associated with the face-centered-cubic (fcc) phase of

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metallic Ag nanoparticles (JCPDS Card no. 04-0783). The diffraction peaks of the calcinated samples are sharper and stronger than the non-calcinated one, indicating that there is increase in the particle size and improvement in the crystal quantity of the nanomaterials. Using the

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XRD pattern, the mean crystallinity was calculated from the most intense peak (101) using the Debye-Scherrer formula:

Dh,k,l = 𝛽

0.9𝜆

ℎ,𝑘,𝑙 𝐶𝑜𝑠𝜃

(2)

Where λ is the wavelength in nm, D is the average crystalline diameter, β is the fullwidth at half-maximum (FWHM) and θ is the Bragg’s angle. The crystalline size of before and after calcination samples are found to be 32.1 nm and 33.31 nm respectively. It is observed that after calcination of Ag doped ZnO nanomaterials the average crystalline size

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increases [20, 21]. 3.2. Morphological and elemental analysis of Ag doped ZnO nanoparticles and MEPCMs

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Field Emission Scanning Electron Microscope (FESEM) images of Ag doped ZnO nanomaterials before and after calcination are shown in Fig. 4a and b respectively. The

doping of Ag with ZnO nanoparticles was observed in the HRTEM images as dark portion

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are shown in the Fig. 4(h and i) and it was further inferred by Energy dispersive X-ray

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spectroscopy (EDAX) shown in the Fig. 5a and 5b whereas, the calcinated Ag doped ZnO

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nanomaterials shows sharp peak than the non-calcinated one. The FESEM images reveals

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that the Ag doped ZnO nanomaterials are easily agglomerated because of high aspect ratio

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with nanoscale diameter. The non-calcinated sample form bundles of flakes indicating the presence of moisture, but the calcinated sample shows hexagonal wurtzite structure with

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some Ag nanoparticles suspended on the wurtzite surface. The three-dimensional FESEM images of the calcinated Ag doped ZnO nanomaterials in the range of 31 to 34 nm in size is

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due to the controlled synthesis. HRTEM images of Ag doped ZnO nanomaterials also show the particle size distribution in the nanometer range. By adjusting the synthesis parameters,

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Ag doped ZnO nanomaterials with different shapes and sizes can be produced [22]. The geometric profiles of the synthesized MEPCMs were investigated using FESEM analytical technique. The FESEM images of the encapsulated PCMs shown in Fig. 4c-4g have spherical structure with size around 3 to 5µm. However, some of the microcapsules adhered to each other with uniform structure and compact surfaces and thereby resulted in large particle size distribution than expected. Most of the microcapsule to be resembling indicate the presence

of Ag doped ZnO nanomaterials in composite PCMs has no chemical interaction with melamine-formaldehyde copolymer during encapsulation and better compatibility between the shell and core materials. High Resolution Transmission Electron Microscope (HRTEM) was used for

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observing the core/shell structure of MEPCM5. As seen in Fig. 4(j and k), the core in MEPCM5 is composite PCM (which is the dark part) is located into the shell of Melamine-

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Formaldehyde copolymer (which is the pale part).

The EDAX data mentioned in Table 2 show almost same peaks for Ag doped ZnO nanomaterials before and after calcination samples. The Fig. 5a and 5b represents EDAX

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pattern indicate that both the samples are composed of only Zn, O, Ag and no evidence of

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other impurities. The EDAX of MEPCM1 as shown in Fig. 5c shows the presence of

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elements like carbon, nitrogen and oxygen. Carbon and oxygen are the components of Pure

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PCM and the presence of nitrogen peak confirms the encapsulation of melamine-

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formaldehyde copolymer over the Pure PCM. In the Fig. 5d and 4l presence of Zn and Ag element peak along with carbon, nitrogen and oxygen peaks confirms the presence of Ag

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doped ZnO nanomaterials with the Pure PCM in the core of MEPCM5.

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3.3. Particle size distribution (PSD) investigation of microcapsules The PSD analysis results are shown in Fig. 6 for MEPCM1, MEPCM2, MEPCM3,

MEPCM4 and MEPCM5. From the PSD curves, it can be noticed that the diameter size of

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MEPCM1, MEPCM2, MEPCM3, MEPCM4 and MEPCM5 are distributed in the intervals of 0-142 μm, 0-131 μm, 0-128 μm, 0-110 μm and 0-92 μm respectively. In addition, the average particle size of the microcapsules were determined as 73.9 μm, 69.8 μm, 67.2 μm, 56.3 μm and 53.1 μm. The mean diameter and average particle size decreases from MEPCM1 to MEPCM5 on increasing the weight percentage of Ag doped ZnO nanomaterials. The average

particle size of the microcapsules were slightly larger than that observed from FESEM analysis. It may due to the cluster structures of the microcapsule being considered as single capsule with larger particle size by PSD analysis. 3.4. Spectroscopic characterization of the eutectic mixture and microencapsulated eutectic

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mixture FTIR spectra of the Pure PCM, MEPCM1, MEPCM2, MEPCM3, MEPCM4,

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MEPCM5 and Ag doped ZnO nanomaterials as shown in Fig. 7a and b were examined to

prove the polymerization reaction regarding the formation of melamine-formaldehyde shell around the PCMs. The FTIR spectra of the Pure PCM show peaks at 2921-2935 cm-1 and

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2855-2858 cm-1 signify the asymmetric and symmetric stretching vibration of CH2 group

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[23]. The peaks at 1454 cm-1 and 933 cm-1 are assigned to the in-plane and out of plane

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bending vibration of the –OH group of the eutectic mixture [24]. The spectra at 1707 cm-1

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and 1284 cm-1 refer to the C=O and CO stretching vibrations. The band at low frequency

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between 500-700 cm-1 is correlated to the bending vibration of metal oxide bond. The spectral band at 590 cm-1 and 724 cm-1 clearly shows the presence of ZnO and Ag ions [25].

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The spectra at around 1510 and 1160 cm-1 corresponds to the vibration of phenyl ring and the stretching vibration of the CO in the phenyl ring of Triton-X 100. Meanwhile, the peak at

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around 3390 cm-1 is overlapped by the amino and imino stretching vibration and also due to the phenolic hydroxyl stretching vibrations. The CH bending vibrations at 1490 and 1360

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cm-1 corresponds to the presence of methylene bridges in the melamine-formaldehyde resin. The spectra of the microcapsules at 717 cm-1 also reveal the in-plane rocking vibration of the methylene group [19]. The spectra of the microencapsulated PCMs include most of the characteristic peaks of the Pure PCM, which confirms that the core material has no chemical interaction with melamine-formaldehyde copolymer.

3.5. Thermal properties of Pure PCM and MEPCMs Thermal properties of Pure PCM and MEPCMs were evaluated by DSC analysis. The phase change curves of Pure PCM and MEPCMs are shown in Fig. 8. Pure PCM has a melting and freezing temperature of 4.22 and 10.72 °C with a latent heat of 147.4 J/g and

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146.73 J/g. For the synthesized MEPCMs, the melting and freezing temperatures along with latent heat are illustrated in Table 3. During the encapsulation of Pure PCM with the shell

material, the interface between the core and the shell acts as the nucleus to promote the phase

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transition from melting to a triclinic phase and thus reduces their crystallization temperature. The endothermic peak temperature of Pure PCM and the encapsulated PCMs are very close

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to each other. This may infer that the encapsulated PCMs have little influence on the melting

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process when compared with pure PCM.

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Using DSC results, the essential phase change parameters like encapsulation ratio,

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efficiency and thermal energy storage capacity were evaluated and are summarized in Table

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3. For microcapsules, releasing/storing thermal energy through the cooling/heating process is mainly done by the core material. According to the present study, as the wt% of Ag doped

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ZnO nanomaterials increases latent heat storage capacity also increases. The encapsulation ratio (R) and the encapsulation efficiency (E) are the two important parameters for MEPCMs

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in studying the thermal properties and they are calculated using the equations given below: ∆𝐻

R = ∆𝐻 𝑚,𝑀𝐸𝑃𝐶𝑀 x 100%

(3)

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𝑚,𝑃𝑢𝑟𝑒𝑃𝐶𝑀

∆𝐻𝑚,𝑀𝐸𝑃𝐶𝑀 +∆𝐻𝑓,𝑀𝐸𝑃𝐶𝑀

E = ∆𝐻

η=

𝑚,𝑃𝑢𝑟𝑒𝑃𝐶𝑀 +∆𝐻𝑓,𝑃𝑢𝑟𝑒𝑃𝐶𝑀

∆𝐻𝑚,𝑀𝐸𝑃𝐶𝑀 +∆𝐻𝑓,𝑀𝐸𝑃𝐶𝑀 𝑅

∆𝐻𝑚,𝑃𝑢𝑟𝑒𝑃𝐶𝑀 +∆𝐻𝑓,𝑃𝑢𝑟𝑒𝑃𝐶𝑀

x 100%

x 100%

(4)

(5)

Where, ΔHm,MEPCM and ΔHf,MEPCM represents the melting and freezing latent heat values of MEPCMs, and ΔHm,PurePCM and ΔHf,PurePCM are melting and freezing latent heat values of Pure PCM [26] . In addition to this, the thermal energy storage capacity (η) was calculated using the above equation and noted in Table 3, which shows that thermal energy storage capacity of MEPCMs increases with increasing in the wt (%) of Ag doped ZnO

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nanomaterials. The latent heat values of MEPCMs were lesser than the Pure PCM due to the presence of shell materials, but it shows good thermal conductivity and prevents leakage

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during phase change process.

For low temperature thermal energy storage systems, thermal conductivity of

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MEPCMs plays an essential parameter. Pure PCM is an shows low thermal conductivity of

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0.2513 Wm-1K-1 which slows down the thermal property during phase transition. The thermal

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conductivity value of MEPCM1 is 0.2539 Wm-1K-1, which is very nearer to that of Pure

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PCM. Whereas, for MEPCM2, MEPCM3, MEPCM4 and MEPCM5 the thermal conductivity values are 0.2853 Wm-1K-1, 0.3196 Wm-1K-1, 0.3457 Wm-1K-1 and 0.3735 Wm-1K-1

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respectively. The increase in thermal conductivity of microencapsulated composite PCMs with increasing mass fraction of Ag doped ZnO nanomaterials is due to the interaction

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between the Pure PCM and Ag doped ZnO nanomaterials [8]. The shape and size of

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nanomaterials also influence the above nature. From Table 3, it can be seen that among the five MEPCMs, MEPCM5 with 0.2 wt% of Ag doped ZnO nanomaterials shows highest

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thermal conductivity.

Thermal reliability of MEPCM5 is considered as an important parameter for

determining the long term stability of MEPCMs. Thermal cycling test for MEPCM5 has been carried for 0, 200, 500 and 1000 thermal cycles are shown in Fig. 8b. The change in melting temperature was observed from 5.81 to 5.67 ºC with latent heat from 75.39 J/g to 75.21 J/g and the change in freezing temperature was observed from 11.86 to 11.73 ºC with a decrease

in latent heat from 74.90 J/g and 74.74 J/g. Using equation 6, thermal cycling performance of MEPCM5 were calculated and illustrated in Table 4.

Thermal cycling performance =

∆𝐻´𝑚,𝑀𝐸𝑃𝐶𝑀5 ∆𝐻𝑚,𝑀𝐸𝑃𝐶𝑀5

x 100%

(6)

Where, ∆𝐻𝑚,𝑀𝐸𝑃𝐶𝑀5 and ∆𝐻´𝑚,𝑀𝐸𝑃𝐶𝑀5 represents the melting latent heat values

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before and after thermal cycling performance of MEPCM5. Since there is very little deviation for melting and freezing of MEPCM5 even after 1000 heating and melting thermal cycles,

SC R

will neither reduce the heat storage capacity nor affect the storage or release rates.

The photographs of Pure PCM and MEPCM5 at different temperatures are presented

-10 ºC. MEPCM5 was made into small pellet using hydraulic press pelletizer.

N

reduced to

U

in Fig. 9. The Pure PCM was filled in a pellet shaped pattern and the temperature was

A

The frozen Pure PCM and the pelletized MEPCM5 were mounted on constant temperature

PCM has melted at

M

chamber with the temperature ranging from -20 ºC to +50 ºC. It was observed that the Pure 5 º C, but the MEPCM5 has maintained the initial shape without any

ED

leakage even at high temperature.

PT

Comparison of MEPCM5 with the other MEPCMs from literature is presented in

CC E

Table 5, it can be observed that MEPCM5 has higher latent heat with lower melting point. 3.6. Thermal degradation analysis of Pure PCM and MEPCMs Thermal degradation properties are evaluated using thermogravimetric analysis. TGA

A

thermograms of pure PCM, MEPCM1, MEPCM2, MEPCM3, MEPCM4, MEPCM5 and MEPCM5 after 1000 cycles are shown in the Fig. 10 and illustrated in the Table 6. The TGA curve of Pure PCM shows no weight change till 150 °C and has one degradation step between 160-260 °C with ̴ 95% weight loss corresponding to the decomposition of Pure PCM. The thermogram of MEPCM1 show degradation in two steps with ̴ 24% weight loss

related to the decomposition or evaporation of core material (Pure PCM) around the temperature range of 340-390 °C and followed by the second weight loss above 400 °C corresponds to the degradation of shell materials. On the other hand, the thermograms of MEPCM2 to MEPCM5 show degradation in three steps; these steps are related to the decomposition or evaporation of the core material (composite PCMs) around the temperature

IP T

range of 340-390 °C with ̴ 25% weight loss, shell materials (Triton-X and melamine-

formaldehyde resin) around 410-540 °C with ̴ 29% weight loss and followed by the third

SC R

degradation above 550 °C corresponding to Ag doped ZnO nanomaterials respectively [27]. This indicates encapsulation significantly lower the evaporation of core material due to the

U

dense Melamine-Formaldehyde shell, which prevents the leakages of core material in

N

MEPCMs even under repeated heating and cooling as shown for MEPCM5 after 1000 cycles

A

[35]. The non-corrosive nature of MEPCMs was noticed while testing the microcapsules in

M

the testing pans as no destruction was observed. Hence, the synthesized MEPCMs have good thermal stability and can be incorporated as low temperature thermal energy storage materials

ED

in buildings.

PT

4. Conclusions

MEPCMs were synthesized with Pure PCM/Ag doped ZnO nanomaterials enhanced

CC E

composite PCMs as core and melamine-formaldehyde copolymer as rigid shells by in-situ polymerization. The effect of Triton-X 100 as surfactant as well as cross-linking agent and

A

thermo physical behavior of shell composition in MEPCMs were studied comprehensively. 1) The FESEM images and XRD analysis confirmed good agreement with the average particle and crystalline size of Ag doped ZnO nanomaterials, which were synthesized by combustion method. The morphological results revealed that all synthesized

MEPCMs has uniform spherical form with size around 1 to 5 µm, but the capsules adhered to each other and hence the average particle size was greater in PSD. 2) The FTIR spectra of Pure PCM and MEPCMs confirmed that the melamineformaldehyde shell has no chemical interaction between the core and shell material. 3) The thermal conductivity of MEPCMs increase from MEPCM1 to MEPCM5. TGA

IP T

analysis showed that the MEPCM2, MEPCM3, MEPCM4 and MEPCM5 had good

thermal stability and they could be used as better thermal energy storage devices than

SC R

MEPCM1.

4) The phase change processes of MEPCMs are very nearer to those of the Pure PCM.

U

The latent heat of MEPCMs decreases with the increasing content of Ag doped ZnO

N

nanomaterials. The leakage test confirmed the shape stability of MEPCM5. The

A

thermal cycling test was also found to have acceptable thermal properties even after

energy storage processes.

M

1000 cycles evidences good thermal reliability and stability for long term thermal

ED

This work confirmed that the use of Ag doped ZnO nanomaterials can be feasible and good solution to improve the thermo physical properties of the encapsulated PCMs. From the

PT

above all results, it can be concluded that the MEPCMs with Ag doped ZnO nanomaterials

CC E

can be incorporated with various building components like bricks, window blinds, paints etc., to reduce indoor temperature swing especially for low temperature thermal energy storage applications in buildings.

A

Acknowledgments

The authors gratefully acknowledge DST, New Delhi for providing financial support to carry out this research work under DST - CERI (DST File No. TMD/CERI/BEE/2016/038 (G)) and DST – Women Scientist Scheme A (WOS-A) (DST File No. SR/WOS-A/CS-

5/2017 (G)). One of the authors, Ms. S. Dhivya is thankful to DST, New Delhi for the award

A

CC E

PT

ED

M

A

N

U

SC R

IP T

of DST-Women Scientist fellowship.

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IP T

storage in buildings, Energy Build. 90 (2015) 106-113. [2] Y. Tang, G. Ailva, X. Huang, D. Su, L. Liu, G. Fang, Thermal properties and

SC R

morphologies of MA-SA eutectics CNTs as composite PCMs in thermal energy storage, Energy Build. 127 (2016) 603-610.

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SiO2 nanoparticles on structure and property of form-stable PCMs made of cellulose

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acetate phase inversion membrane absorbed with capric-myristic-stearic acid ternary

A

eutectic mixture, Thermochim. Acta 653 (2017) 49-58.

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[4] X. Huang, G. Alva, L. Liu, G. Fang, Preparation, characterization and thermal

ED

properties of fatty acid eutectics/bentonite/expanded graphite composite as novel form-stable thermal energy storage materials, Sol. Energy Mater. Sol. Cells 166

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CC E

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[6] S. I. Hussain, R. Dinesh, A. Ameelia Roseline, S. Dhivya, S. Kalaiselvam, Enhanced thermal performance and study the influence of sub cooling on activated carbon dispersed eutectic PCM for storage applications, Energy Build. 143 (2017) 17-24. [7] S. Harikrishnan, S. Magesh, S. Kalaiselvam, Preparation and thermal energy storage behaviour of stearic acid - TiO2 nanofluids as a phase change material for solar heating systems, Thermochim. Acta 565 (2013) 137-145.

[8] S. Harikrishnan, S. Kalaiselvam, Preparation and thermal characteristics of CuOOleic acid Nanofluids as a phase change material, Thermochim. Acta 533 (2012) 4655. [9] G. Wang, Y. Yang, D. Han, Y. Li, Oxygen defective metal oxide for energy conservation and storage, Nano Today 13 (2017) 23-29.

IP T

[10] Z. Zhang, J. Liu, J. Gu, L. Su, L. Cheng, An overview of metal oxide materials as

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semiconductor metal oxide photocatalysis, Mater. Horiz. 1 (2014) 400-410.

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[12] M. Gancheva, M. Markova-Velich Kova, G. Atanasova, D. Kovacheva, I. Uznuov, R.

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Cukeva, Design and photocatalytic activity of nanosized zinc oxide, Appl. Surf. Sci.

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368 (2016) 258-266.

[13] M. Hasanpoor, M. Aliofkhazraei, H. Delvari, In-situ study mass and current density

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for electrophoretic deposition of ZnO nanoparticles, Ceram. Inter. 42(6) (2016) 69066913.

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[14] R. Parameshwaran, R. Jayavel, S. Kalaiselvam, Study on thermal properties of organic ester phase-change material embedded with silver nanoparticles, J. Therm.

CC E

Anal. Calorim. 114 (2013) 845-858.

[15] L. Ma, X. Yang, Z. Q. Zhou, M. Lu, A synergetic effect of surface Plasmon and

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ammoniation on enhancing photocatalytic activity of ZnO nanorods, RSC Adv. 6 (2016) 97808-97817.

[16] T. Kousksou, A. Arid, A. Jamil, Y. Zeraouli, Thermal behaviour of building material containing microencapsulated PCM, Thermochim. Acta 550 (2012) 42-47.

[17] Y. Konuklu, M. Unal, H. O. Paksoy, Microencapsulation of caprylic acid with different wall materials as phase change material for thermal energy storage, Sol. Energy Mater. Sol. Cells 120 (2014) 536-542. [18] M. Huang, Y. Luo, Y. Zhong, M. Xiao, J. Hu, Preparation and Characterization of Microencapsulated Phase Change Materials with Binary Cores and Poly (allyl

IP T

methacrylate) (PALMA) Shells Used for Thermo-regulated Fibers, Thermochim. Acta 655 (2017) 262-268.

SC R

[19] H. Zhang, X. Wang, Fabrication and performance of microencapsulated phase change materials based on n-octadecane core and resorcinol- modified melamine-

U

formaldehyde shell, Colloids Surf., A Physicochem. Eng. Asp. 332 (2009) 129-138.

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[20] S. K. Gandomani, R. Yousefi, F. J. Sheini, N. M. Huang, Optical and electrical

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properties of p-type Ag doped ZnO nanostructures, Ceram. Int. 40 (2014) 7957-7963.

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[21] B. D. Ahn, H. S. Kang, J. H. Kim, G. H. Kim, H.W. Chang, S. Y. Lee, Synthesis and analysis of Ag-doped ZnO, J. Appl. Phys. 100 (2006) 093701.

ED

[22] K. R. S. Kumar, S. Kalaiselvam, Experimental investigations on the thermophysical properties of CuO-palmitic acid phase change material for heating applications, J.

PT

Thermal. Anal. Calorim. 129 (2017) 1647-1657. [23] D. K. Doguscu, A. Altinas, A. Sari, C. Alkan, Polystyrene microcapsules with

CC E

palmitic-capric acid eutectic mixture as building thermal energy storage materials, Energy Build. 150 (2017) 376-382.

A

[24] M. V. Limaye, S. B. Singh, S. K. Date, D. Kothari, V. R. Reddy, A. Gupta, V. Sathe, R. J. Choudhary, S. K. Kulkarni, High coercivity of oleic acid capped CoFe2O4 nanoparticles at room temperature, J. Phys. Chem. B 113 (2009) 9070-9076.

[25] O. A. Yildirium, H. Emrah, C. Durucan, Highly efficient room temperature synthesis of Silver-Doped Zinc Oxide (ZnO:Ag) Nanoparticles: Structural, Optical and Photocatalytic properties, J. Am. Ceram. Soc. 96 (2013) 766-773. [26] S. I. Hussain, A. A. Roseline, S. Kalaiselvam, Bifunctional nanoencapsulated eutectic

energy storage, Materials and Design 154 (2018) 291-301.

IP T

phase change material core with SiO2/SnO2 nanosphere shell for thermal and electric

[27] X. Qiu, L. Lu, J. Wang, G. Tang, G. Song, Fabrication, thermal properties and

SC R

thermal stabilities of microencapsulated n-alkane with poly(lauryl methacrylate) as shell, Thermochim. Acta 620 (2015) 10-17.

U

[28] C. Alkan, A. Sari, A. Karaipekli, O. Ozun, Preparation, characterization, and thermal

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properties of microencapsulated phase change material for thermal energy storage,

A

Solar Energy Materials and Solar Cells, 93 (2009) 143-147.

M

[29] Y. Fang, T. Zou, X. Liang, S. Wang, X. Liu, X. Gao, Z. Zhang, Self-assembly synthesis and properties of microencapsulated n-tetradecane phase change materials

3074-3080.

ED

with a calcium carbonate shell for cold energy storage, Sustain. Chem. Eng. 5 (2017)

PT

[30] R. Yang, H. Xu, Y. Zhang, Preparation, physical property and thermal physical property of phase change emulsion, Sol. Energy Mater. Sol. Cells, 80 (2003) 405-416.

CC E

[31] A. M. Borreguero, M. V. Sanchez, M. L. Sanchez-Silva, M. S. Carmona, J. F. Rodriguez, Development of microcapsules containing phase change materials for

A

refrigeration, Refrigeration Science and Technology, 5 (2010) 29-36.

[32] I. Cao, F. Tang, G. Fang, Preparation and characterization of microencapsulated palmitic acid with TiO2 shell as shape-stabilized thermal energy storage materials, Sol. Energy Mater. Sol. Cells, 123 (2014) 183-188.

[33] J. G. Paloma, Y. Konuklu, A. I. Fernandez, Preparation and exhaustive characterization of paraffin or palmitic acid microcapsules as novel phase change material, Solar Energy 112 (2015) 300-309. [34] C. Alkan, A. Sari, A. Karaipekli, Preparation, thermal properties and thermal reliability of microencapsulated n-eicosane as novel phase change material for

IP T

thermal energy storage, Energy Conservation and Management 52 (2011) 687-692.

[35] A. Zhao, J. An, J.Yang, E.H. Yang, Microencapsulated phase change materials with

A

CC E

PT

ED

M

A

N

U

SC R

composite titania-polyurea (TiO2-PUA) shell, Applied Energy 215 (2018) 468-478.

Figure captions Fig. 1. (a) Theoretical enthalpy graph for binary mixture, (b) Freezing graph of OleicMyristic acid eutectic mixture (Pure PCM). Fig. 2. Schematic formation of microencapsulated PCMs through in-situ polymerization.

IP T

Fig. 3. XRD patterns of Ag doped ZnO nanomaterials (a) before and (b) after calcination at

SC R

600°C, (c) standard XRD pattern of ZnO and (d) standard XRD pattern of Ag.

Fig.4. Morphologies and EDAX result of the samples: (a) before and (b) after calcination of Ag doped ZnO, (c) MEPCM1, (d) MEPCM2, (e) MEPCM3, (f) MEPCM4 and (g) MEPCM5

U

are FESEM images, (h and i) Ag doped ZnO and (j and k) MEPCM5 are HRTEM images, (l)

N

EDAX of MEPCM5

A

Fig. 5. Energy dispersive X-ray spectra of Ag doped ZnO

(a) before and

(b) after

M

calcination, (c) MEPCM1 and (d) MEPCM5.

ED

Fig. 6. PSD graph of MEPCM1, MEPCM2, MEPCM3, MEPCM4 and MEPCM5.

PT

Fig. 7. (a and b) FTIR of Ag doped ZnO nanomaterials, Pure PCM and MEPCMs. Fig. 8. (a) DSC analysis of Pure PCM and MEPCMs and (b) Thermal cycling curve of

CC E

MEPCM5 for 0, 200, 500, 1000 cycles. Fig. 9. Photographs of Pure PCM (Oleic-Myristic acid eutectic mixture) and MEPCM5 at

A

different temperatures. Fig.10. TGA curve of Pure PCM, MEPCMs and MEPCM5 after 1000 cycles.

IP T SC R U N A M ED PT CC E A

Fig. 1. (a)Theoretical enthalpy graph for binary mixture (b) Freezing graph of Oleic-Myristic acid

eutectic

mixture

(Pure

PCM).

IP T SC R U N A M ED PT

A

CC E

Fig. 2. Schematic formation of microencapsulated PCMs through in-situ polymerization.

IP T SC R U N A M

standard

XRD

pattern

for

A

CC E

PT

(c)

ED

Fig. 3. XRD patterns of Ag doped ZnO nanomaterials (a) before and (b) after calcination at 600°C, ZnO

and

(d)

standard

XRD

pattern

for

Ag.

A ED

PT

CC E A

M

N U SC R

I

I N U SC R

Fig.4. Morphologies and EDAX result of the samples: (a) before and (b) after calcination of Ag doped ZnO, (c) MEPCM1, (d) MEPCM2, (e) MEPCM3, (f) MEPCM4 and (g) MEPCM5 are FESEM images, (h and i) Ag doped ZnO and (j and k) MEPCM5 are HRTEM images, (l) EDAX

A

CC E

PT

ED

M

A

of

MEPCM5.

IP T SC R U N A M

MEPCM1

A

CC E

PT

(c)

ED

Fig. 5. Energy dispersive X-ray spectra of Ag doped ZnO (a) before and (b) after calcination, and

(d)

MEPCM5.

IP T SC R U N A M ED

A

CC E

PT

Fig. 6. PSD of MEPCM1, MEPCM2, MEPCM3, MEPCM4 and MEPCM5.

IP T SC R U N A M ED PT CC E A Fig. 7. (a and b) FTIR of Ag doped ZnO nanomaterials, Pure PCM and MEPCMs.

IP T SC R U N A M ED PT CC E A

Fig. 8. (a) DSC analysis of Pure PCM and MEPCMs and (b) Thermal cycling curve of MEPCM5 for 0, 200, 500, 1000 cycles.

IP T SC R U N A M ED PT

CC E

Fig. 9. Photographs of Pure PCM (Oleic-Myristic acid eutectic mixture) and MEPCM5 at

A

different temperatures.

IP T SC R U N A M

A

CC E

PT

ED

Fig. 10. TGA curve of Pure PCM, MEPCMs and MEPCM5 after 1000 cycles.

Table captions Table 1 Physico-chemical properties of Oleic Acid and Myristic Acid phase change material.

IP T

Table 2 EDAX of Ag doped ZnO before and after calcination, MEPCM1 and MEPCM5.

SC R

Table 3 Thermal Properties of Pure PCM and MEPCMs.

U

Table 4

A

N

Thermal cycling performance of MEPCM5 for 0, 200, 500, 1000 cycles.

M

Table 5

ED

Comparison of present study results with other MEPCMs from the literature. Table 6

A

CC E

PT

TGA data of Pure PCM and MEPCMs

Table 1 Physico-chemical properties of Oleic Acid and Myristic Acid phase change material. Oleic Acid

Myristic Acid

Molecular Formula

C18H34O2

C14H28O2

Molar Mass

282.47 g/mol

228.37 g/mol

Colour

Light Yellow

Appearance

Clear, Viscous liquid

Melting Point

4-8 °C

Density

0.889-0.895 g/ml

Latent heat of fusion

140.2 J/g

CC E

Solubility in Water

A

SC R Colourless

White, Solid

U

N A

M

ED

PT

Refractive Index

IP T

Description

53-56 °C 0.99 g/ml 181 J/g

1.459

1.4723

Insoluble

Soluble

I N U SC R

Table 2

EDAX of Ag doped ZnO before and after calcination, MEPCM1 and MEPCM5.

Ag doped ZnO before

%

44.87

CC E

OK

-

PT

CK

MEPCM5

Atomic

Weight

Atomic

Weight

Atomic

Weight

Atomic

%

%

%

%

%

%

%

-

-

-

38.34

42.44

38.40

42.59

75.26

27.97

52.29

53.54

50.82

51.81

49.31

ED

Weight

MEPCM1

calcination

M

calcination

Ag doped ZnO after

A

Element

-

-

-

-

8.11

6.74

9.69

8.07

Zn K

54.39

24.58

71.52

47.56

-

-

0.09

0.02

Ag L

0.74

0.16

0.51

0.15

-

-

0.01

0.01

Totals

100.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

A

NK

I Thermal Properties of Pure PCM and MEPCMs. Freezing

Melting

Freezing

J/g

J/g

(°C)

(°C)

(latent heat)

ED

(latent heat)

A

Melting

M

Description

N U SC R

Table 3

Encapsulation Encapsulation

Thermal

Thermal

ratio

efficiency

storage

conductivity

R(%)

E(%)

capacity

(Wm-1K-1)

η (%)

147.4

146.73

4.22

10.72

-

-

-

0.2513

MEPCM1

4.37

10.84

53.83%

53.47%

99.34%

0.2539

PT

Pure PCM

77.95

MEPCM2

78.7

77.53

4.89

11.37

53.39%

53.25%

99.48%

0.2853

MEPCM3

77.8

76.90

5.15

11.53

52.78%

52.78%

99.65%

0.3196

MEPCM4

76.6

75.95

5.54

11.69

52.03%

51.89%

99.74%

0.3457

MEPCM5

75.39

74.90

5.81

11.86

51.14%

51.09%

99.91%

0.3735

A

CC E

79.35

Table 4 Thermal cycling performance of MEPCM5 for 0, 200, 500, 1000 cycles.

Freezing

Thermal cycling performance

ΔHm

Ts

ΔHs

(°C)

(J/g)

(°C)

(J/g)

0

5.81

75.39

11.86

74.90

-

200

5.78

75.32

11.81

74.83

99.90

500

5.72

75.28

11.76

99.85

1000

5.67

75.21

11.73

74.74

99.76

SC R

Tm

U

cycles

Melting

IP T

Thermal

A

CC E

PT

ED

M

A

N

74.78

(%)

I N U SC R

Table 5

Comparison of present study results with other MEPCMs from the literature. Encapsulation

Melting

Melting

method

(°C)

latent heat

A

Shell material

ED

M

Core material

Calcium carbonate

CC E

n-Tetradecane

PMMA

PT

Docosane

A

n-Tetradecane

Petrepar n-C13,

PS, PMMA, PEMA

References

(J/g) Emulsion

41

54.6

polymerization

Thermal energy storage, solar

[28]

space heating

Self-assembly

5.35

58.54

Cold thermal energy storage

[29]

In-situ polymerization

2.06-5.97

66.26-80.62

Heating, ventilating, air-

[30]

conditioning, refrigeration and heat exchange PS

Petrepar n- C14 Palmitic acid

Applications

Suspension-like

-

58.6,79.0

Refrigeration

[31]

61.7

63.3

Thermal energy storage

[32]

polymerization TiO2

Sol-gel

I N U SC R

n-octadecane

PScEA

Emulsion co-

42.39

49.03

Thermal energy storage systems

[33]

54.2

35.2

Thermal energy storage devices

[34]

3.11

58.79

Coating material for electrical

[26]

polymerization Eicosane

PMMA

Emulsion

SiO2/SnO2

interfacial hydrolysis

ED

polyethylene glycol

Melamine-

PT

Oleic-Myristic acid-Ag doped

CC E

ZnO (MEPCM5)

A

In-situ emulsion

M

Oleic acid-

A

polymerization

Formaldehyde

conduction in electronic chips

polycondensation In-situ polymerization

5.81

75.39

Building components like bricks, window blinds, paints

Present study

Table 6 TGA data of Pure PCM and MEPCMs. Maximum second

Maximum third

weight loss

weight loss

weight loss

residual char

temperature

temperature

temperature

(%)

(°C)

(°C)

Pure PCM

249.7

-

MEPCM1

375.1

450.2

MEPCM2

378.3

MEPCM3

381.3

MEPCM4 MEPCM5

-

1.73

-

9.98

A

605.2

20.8

481.6

608.7

21.6

389.4

500.7

611.5

22.1

TE

SC R

(at 800 °C)

508.3

624.9

22.8

408.4

507.6

623.7

22.7

N

M

475.4

D

408.9

U

(°C)

EP

MEPCM5 after

Amount of

IP T

Maximum first

Sample Code

A

CC

1000 cycles

44