Monodisperse Na2SO4Â·10H2O@SiO2 Microparticles against

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

Article

Monodisperse Na2SO4·10H2O@SiO2 microparticles against super-cooling and phase separation during phase change for efficient energy storage Ming Li, Wei Wang, Zhengguo Zhang, Fan He, Shan Yan, PeiJie Yan, Rui Xie, Xiao-Jie Ju, Zhuang Liu, and Liang-Yin Chu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00231 • Publication Date (Web): 03 Mar 2017 Downloaded from http://pubs.acs.org on March 5, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Monodisperse Na2SO4·10H2O@SiO2 microparticles against super-cooling and phase separation during phase change for efficient energy storage ∥

Ming Li,† Wei Wang,*,†,‡ Zhengguo Zhang, Fan He,† Shan Yan,† Pei-Jie Yan,† Rui Xie,†,‡ XiaoJie Ju,†,‡ Zhuang Liu,† and Liang-Yin Chu*,†,‡,§ †

School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, P. R. China

‡

State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, Sichuan

610065, P. R. China ∥

Key Laboratory of Enhanced Heat Transfer and Energy Conservation, The Ministry of

Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, P. R. China §

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing,

Jiangsu 211816, P. R. China

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Uniform SiO2 microparticles containing controllable content of Na2SO4·10H2O against super-cooling and phase separation are developed for efficient energy storage at mild temperatures.

Na2SO4 solution with 3-aminopropyltriethoxysilane and silicone oil with

tetraethylorthosilicate are emulsified into monodisperse W/O emulsions from microfluidics for template fabrication of the microparticles via hydrolysis and condensation. During the reaction process, Na2SO4 in the emulsion droplets crystallizes in the microparticles. Incorporation of sodium borate and sodium hexametaphosphate, combined with the confined distribution of Na2SO4·10H2O in the mesoporous microparticles, successfully avoids the phase separation of Na2SO4·10H2O and dramatically reduces its super-cooling. This allows the microparticles to achieve repeatable energy storage/release property at mild temperatures for thermo-regulation. Such a thermo-regulating performance is demonstrated by incorporating the microparticles into a model house for repeatedly regulating its surface and inside temperatures. These microparticles show great potential for developing advanced materials for myriad fields such as energy, architecture and healthcare. KEYWORDS: phase change materials; Na2SO4·10H2O; microparticles; thermo-regulation; microfluidics


Page 2 of 37

Page 3 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Phase change materials (PCMs), which can absorb and release energy during their phase transition for effective regulation of temperature and enhancement of energy utilization,1-5 are widely used in various fields such as solar energy storage systems,6-9 thermo-regulating textiles,10-14 and energy saving buildings.15-17 Typically, PCMs can be classified into inorganic, organic and inorganic/organic composite materials.18-21 Among these materials, inorganic salt hydrates show great potential for myriad applications due to their low phase transition temperature, high thermal conductivity, high storage density, low cost and non-flammability.22-25 Especially, Na2SO4·10H2O with a mild phase transition temperature of 32.4 ºC for practical use,26 is promising PCMs for use in fields of energy, architecture and healthcare. However, due to the fluidity of melted Na2SO4·10H2O during phase transition, it needs to be encapsulated inside other materials for further use. Therefore, effective encapsulation of Na2SO4·10H2O in materials is crucial for its use in myriad fields. Up to now, several materials such as expanded graphite and SiO2,27-32 have been used for Na2SO4·10H2O encapsulation. By vigorously mixing the expanded graphite with a solution containing Na2SO4·10H2O and Na2HPO4·12H2O, expanded graphite mixed with encapsulated Na2SO4·10H2O and Na2HPO4·12H2O can be obtained.

Further coating of the hydrated

salts/expanded graphite composites with paraffin can produce centimeter-scale PCMs with high thermal conductivity.27

Moreover, SiO2 is widely used as encapsulation material for

Na2SO4·10H2O due to its good thermal conductivity and chemical stability, low cost and low toxicity.33-35 The SiO2-based PCMs are usually fabricated by immersing SiO2 blocks in solution of Na2SO4 and Na2HPO4 and then coating them with polyvinylpyrrolidone to produce centimeter-scale PCMs.28,30 As compared with the above-mentioned bulky PCMs, encapsulation



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 37

of Na2SO4·10H2O in micro-scale particles can provide larger area for heat transfer and smaller volume for mixing with other materials for more flexible use. Usually, SiO2 microparticles are used for Na2SO4·10H2O encapsulation.29,31

For example, SiO2 microparticles containing

Na2SO4·10H2O can be derived from ultrasonically produced Pickering emulsions, and then incorporated in polymeric films for textile applications.31 However, these microparticles usually suffer from poorly-controlled encapsulating content of Na2SO4·10H2O, non-uniform microparticle sizes, and problems of super-cooling and phase separation. Especially, supercooling can lead to a crystallization temperature of Na2SO4·10H2O much lower (usually below 0 ºC) than the melting temperature, thus the energy release cannot be achieved at mild temperatures.29 Meanwhile, phase separation can lead to stratification of Na2SO4 solution, thus largely reduce the storage capacity and the life of PCMs.36 Due to the problems of super-cooling and phase separation, it is difficult to achieve repeated energy storage/release at mild temperatures, which limits the practical applications. Although nucleating agents, crystal habit modifiers and thickening agents are usually used to reduce the super-cooling and phase separation of Na2SO4·10H2O in industrial-scale crystallization,37,38 there have not been any reports using such agents in Na2SO4·10H2O encapsulated systems. Moreover, although porous particles

encapsulated

with Na2SO4·10H2O show reduced

super-cooling and

phase

separation,28,29 their high degree of super-cooling (~50 °C) limits their further use. Therefore, development of uniform SiO2 microparticles with controllable Na2SO4·10H2O content and repeated energy storage/release at mild temperature range is highly desired. Here we report on the fabrication of uniform SiO2 microparticles containing controllable content of Na2SO4·10H2O against super-cooling and phase separation in phase change process for efficient and repeated energy storage/release at mild temperatures. Synergistic effects based


Page 5 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


on nucleating agent and crystal habit modifier for crystallization adjustment and mesoporous SiO2 matrix for crystallization confinement are used to reduce the super-cooling and eliminate the phase separation. For microparticle fabrication, microfluidic techniques, with excellent manipulation on emulsion droplets,39-43 provide highly-controllable emulsions as templates. A glass-capillary microfluidic device is used to generate monodisperse W/O emulsions as templates for the microparticle fabrication (Figure 1a).

Na2SO4 solution with 3-

aminopropyltriethoxysilane (APTS) and silicone oil with tetraethylorthosilicate (TEOS) are respectively used as the inner and outer fluids. The amphiphilic TEOS and partially hydrolyzed APTS assemble at the droplet interfaces (Figure 1b1 and 1b2), and then hydrolyze and condense to form SiO2 microparticles (Figure 1b3 and 1b4).

Meanwhile, Na2SO4 in the droplets

crystallizes into Na2SO4·10H2O during the reaction process, leading to encapsulation of Na2SO4·10H2O in the SiO2 matrix of the resultant microparticles (Figure 1b5 and 1b6). For such microparticles, super-cooling is reduced by adding sodium borate (nucleating agents) and sodium hexametaphosphate (crystal habit modifiers) in the inner fluid,37,38 to achieve a crystallization temperature at room temperature. Meanwhile, the encapsulated Na2SO4·10H2O shows no phase separation due to the confined distribution in the mesoporous SiO2 matrix, which also benefits the reducing of super-cooling. These microparticles can repeatedly absorb and release energy at mild temperatures during the phase transition of Na2SO4·10H2O upon heating/cooling cycles, thus providing excellent performances for thermo-regulation.

This is demonstrated by

incorporating the microparticles into the walls and roofs of a model house for repeatedly regulating its surface and inside temperatures under repeated heating/cooling cycles. These Na2SO4·10H2O@SiO2 microparticles are highly potential for developing advanced materials for myriad fields such as energy, architecture and healthcare.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 37

EXPERIMENTAL SECTION Materials. Tetraethylorthosilicate (TEOS, 98%), 3-aminopropyltriethoxysilane (APTS, 98%), and sodium sulfate (Na2SO4, ≥99%), were purchased from Energy Chemicals. Silicone oil (20 cSt) was purchased from Jinan Yingchuang Chemicals. Dow Corning® 749 (DC-749), used as the droplet stabilizer, was purchased from Dow Corning Corporation.

Sodium borax

pentahydrate (Na2B4O7·5H2O, ≥99.7%) and sodium hexametaphosphate (SHMP, ≥99.7%), used as nucleating agent and crystal habit modifier for reducing the super-cooling respectively, were purchased from Chengdu Kelong Chemicals. Sodium sulfate decahydrate (Na2SO4·10H2O, ≥99.99%) and cyclohexane (C6H12, ≥99.5%) were respectively purchased from Xiya Reagent and Chengdu Kelong Chemicals. Ethoxylated trimethylolpropane triacrylate (ETPTA) and 2hydroxy-2-methyl-1-phenyl-1-propanone (HMPP) used for constructing the model house were purchased from Sigma-Aldrich. Polydimethylsiloxane (PDMS, Sylgard184) was purchased from Dow Corning. Deionized water produced by a water purification system (Elix 10, Millipore) was used throughout the experiments. Microfluidic Fabrication of Na2SO4·10H2O@SiO2 Microparticles.

W/O emulsions

generated from microfluidics were used as templates for the microparticle fabrication. A glasscapillary microfluidic device, with cylindrical capillaries coaxially assembled in a square tube for constructing the microchannels (Figure 1a), was fabricated according to our previous work for emulsion generation.44 The diameters of the cylindrical injection tube and collection tube were 550 µm, and the inner diameter of the tapered front-end of the injection tube was 100 µm. All the glass capillaries were used without surface modifications.


Page 7 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Briefly, silicone oil containing 45 wt% silica precursors TEOS and 1 wt% stabilizer DC-749 was used as the outer fluid; while an aqueous solution containing 14.3 vol% APTS, 3 % (w/v) Na2B4O7·5H2O, 0.1 % (w/v) SHMP, and different contents of Na2SO4, was used as the inner fluid for all microparticle fabrication. For emulsion generation, the outer and inner fluids were injected into the microfluidic device by using syringe pumps (LSP01-1A, Baoding Longer Precision Pumps). The generated W/O emulsions were collected in a vessel containing silicone oil with 45 wt% TEOS and 1 wt% DC-749 at 10 ºC to allow hydrolysis and condensation reaction of TEOS and APTS at the droplet interfaces for forming the SiO2 matrix. Meanwhile, the hydrolysis reaction also consumed the water in the W/O emulsion droplets and gradually increased the Na2SO4 concentration for reaching saturation.

This process facilitated the

crystallization of Na2SO4 to form Na2SO4·10H2O in the SiO2 matrix.

Next, the resultant

Na2SO4·10H2O@SiO2 microparticles were washed with cyclohexane to remove the silicone oil and excess TEOS, and then air-dried. At last, the dried microparticles were frozen by liquid nitrogen for further crystallization of Na2SO4. Morphological Characterization of Emulsions and Microparticles. The generation process of W/O emulsion templates in the microfluidic device was observed using a high-speed digital camera (Phantom Miro3, Vision Research). The morphologies of the resultant emulsions before and after reaction for 55 h were observed with an optical microscope (DM IL LED, Leica). The size and size distribution of these emulsions and the resultant microparticles were determined using an automatic analytic software based on their optical micrographs. To determine the size monodispersity of the emulsion droplets and microparticles, coefficient of variation (CV), defined as the ratio of the standard deviation of size distribution to its arithmetic mean, was used. The CV value can be calculated as follows:



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 37

1

 N ( Di − Dn ) 2  2 CV =100% ×  ∑   i =1 N − 1 

Dn

(1)

where Di is the diameter of the i th sample (µm), Dn is the arithmetic average diameter of the samples (µm), and N is the total number of measured samples. Samples with a CV value less than 5% are defined as monodisperse. The structures and element compositions of the Na2SO4·10H2O@SiO2 microparticles were characterized with a scanning electron microscope (SEM; JSM-7500F, JEOL), equipped with energy dispersion X-ray (EDX). The effects of reaction time and content of Na2SO4 in the inner fluid on the morphology of the Na2SO4·10H2O@SiO2 microparticles were systematically investigated by using SEM technique.

The porous structures of the Na2SO4·10H2O@SiO2

microparticles were characterized by a nitrogen adsorption analyzer (ASAP 2020, Micromeritcs). The pore size distribution and surface area of Na2SO4·10H2O@SiO2 microparticle were obtained respectively by the Barrett Joyner and Halenda (BJH) method based on the adsorption data, and the Brunauer-Emmett-Teller (BET) method.

Before N2 adsorption measurements, the

Na2SO4·10H2O@SiO2 microparticles were immersed in deionized water repeatedly until the Na2SO4·10H2O were completely removed and then dried at 30 ºC in vacuum for 20 h. The samples were degassed in vacuum at 60 °C for at least 4 h. Investigation on the Thermal Property of Na2SO4·10H2O@SiO2 Microparticles. The phase transition temperatures and enthalpies of Na2SO4·10H2O@SiO2 microparticles with different Na2SO4 contents upon heating and cooling were measured by a Differential Scanning Calorimeter (DSC; Q2000, TA Instruments), with pure Na2SO410H2O as the control group. For the heating process, the temperature was increased from -50 ºC to 50 ºC at a rate of 10 ºC min-1. Considering the super-cooling may occur during the cooling process, the temperature was


Page 9 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


decreased back to -50 ºC at a slower rate of 5 ºC min-1. Such a slower rate for cooling process could benefit complete heat transfer between the inside and outside of the sample-containing pan, thus the measured crystallization temperature could be more precise and close to the real crystallization temperature.

Both the heating and cooling processes were conducted under

nitrogen atmosphere. To evaluate the thermal property, a super-cooling temperature (Ts) is used, which can be calculated as follows: (2)

Ts = Tm, p − Tc, p

where Tm,p and Tc,p are respectively the temperatures at which the highest endothermic and exothermic peaks are obtained during the heating and cooling processes in the DSC curve. Meanwhile, enthalpy ratio (R) is used to show the thermal property of NaSO4·10H2O@SiO2 microparticles as compared to pure Na2SO4·10H2O, which can be calculated as follows based on literatures:45-47 R =100% ×

∆H m, microparticles

(3)

∆H m, Na 2SO4 ⋅10H2 O

where ∆H m, microparticles and ∆H m, Na 2SO 4 ⋅10H 2 O are the melting enthalpies (J g-1) of the Na2SO4·10H2O@SiO2 microparticles and pure Na2SO4·10H2O respectively.

Investigation

on

the

Thermo-Regulating

Performance

of

Model

House

with

Na2SO4·10H2O@SiO2 Microparticles. To investigate the thermo-regulating performance of the Na2SO4·10H2O@SiO2 microparticles, the microparticles were incorporated into the walls and roofs of the model house for measuring the changes of its surface and inside temperatures under heating and cooling processes using artificial sunlight.

For constructing the model house,

ETPTA plates, used for building the walls and roofs, were fabricated by using self-made PDMS



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 37

molds for shaping the plates. Briefly, ETPTA containing 0.3 g mL-1 Na2SO4·10H2O@SiO2 microparticles and 1% (v/v) photo-initiator HMPP were added into the PDMS mold, and then exposed to UV light for 40 s for constructing the ETPTA plates. Then, a wall frame of polylactic acid (thickness: 0.5 mm), fabricated by a 3D printer (Replicator Z18, Makerbot), was used for assembling four ETPTA plates to construct the walls. With another two ETPTA plates as roofs covered on the walls, the model house was built. Meanwhile, SiO2 microparticles without Na2SO4·10H2O were incorporated in the same model house as the control group. To simulate solar irradiation, a high-voltage Xenon short arc lamp (250 W, Napu Photoelectricity) was used to provide artificial sunlight with an irradiation intensity of 800 W m-2 on the roofs. The surface and inside temperatures of the two model houses were respectively monitored by infrared camera (E40, FLIR system) and thermocouple thermometer (8620, TASI) placed inside the house, with an accuracy of 0.1 ºC. When the inside temperatures of the two model houses both reached equilibrium states, the lamp was turned off for cooling down the model houses to the environmental temperature (~15 ºC). Such a heating/cooling cycle was repeated for ten times to test the stability and repeatability of the thermo-regulating capacity.

Investigation on the Thermal Cycling Property of Na2SO4·10H2O@SiO2 Microparticles. To test the thermal properties of Na2SO4·10H2O@SiO2 microparticles during 100 heating/cooling cycles, a cylindrical ETPTA mini-plate (diameter: 5.4 mm, height: 1.2 mm) with

0.3

g mL-1

of

such

microparticles

was

fabricated

instead

of

using pure

Na2SO4·10H2O@SiO2 microparticles for DSC measurement, because the application of these Na2SO4·10H2O@SiO2 microparticles usually requires supporting materials.

During each

heating/cooling cycle, the sample was heated from 5 ºC to 45 ºC at a rate of 12 ºC min-1 and kept for 30 s by using a thermal cycler (Cycler V, Maple-lab); Then the sample was cooled down to 5


Page 11 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ºC at the same rate and kept for 30 s again. The phase transition temperatures and enthalpies of the mini-plate before and after 100 heating/cooling cycles were measured by DSC.

RESULTS AND DISCUSSION Template Synthesis of Na2SO4·10H2O@SiO2 Microparticles from W/O Emulsions. W/O emulsions generated from glass-capillary microfluidic device are used as templates for fabrication of the Na2SO4·10H2O@SiO2 microparticles (Figure 1a). After emulsion generation, the TEOS and APTS molecules tend to assemble at the droplet interfaces, due to the amphiphilicity of TEOS and partially hydrolyzed APTS (Figure 1b1 and 1b2). After that, the TEOS and APTS further hydrolyze and condense at the droplet interfaces because of the alkalinity of inner fluid caused by the partially hydrolyzed APTS (Figure 1b3 and 1b4). Thus, SiO2 matrix first forms at the droplet interfaces and then grows inward, with TEOS diffused inwards for further hydrolysis reaction. This can produce microparticles with SiO2 matrix. Meanwhile, during the hydrolysis, the water content in the emulsion droplets decreases, leading to a gradual increase of the Na2SO4 concentration to reaches saturation. This facilitates the crystallization of Na2SO4 in the SiO2 matrix to form encapsulated Na2SO4·10H2O (Figure 1b5 and 1b6). The W/O emulsion droplets generated from the microfluidic device (Figure 2a) provide excellent templates for synthesis of uniform Na2SO4·10H2O@SiO2 microparticles. As a typical example, Figures 2b and 2c show the optical micrographs of the primary W/O emulsions and the resultant microparticles after reaction for 55 h. The CV values of the W/O emulsions and microparticles are 1.29 % and 2.79 % respectively, both showing good size monodispersity (inset Figures 2b and 2c).

With monodisperse size and shape, the microparticles can achieve



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 37

controllable encapsulation of phase change materials to obtain well-tailored heat transfer kinetics and quantitative energy storage/release for rational use.

Moreover, incorporation of such

monodisperse microparticles in materials also allows the composite materials to achieve homogeneous properties such as mechanical strength and thermo-conductivity.

Structure Characterization of Na2SO4·10H2O@SiO2 Microparticles.

The effects of

reaction time and Na2SO4 content in the inner fluid on the microparticle morphology are systematically investigated. As shown in Figure 3, with fixed Na2SO4 content, the microparticle morphology becomes smoother with increasing the reaction time, because condensation with longer time can lead to denser SiO2 matrix, which restricts further growth of the Na2SO4·10H2O crystals.37 For microparticles fabricated with Na2SO4 contents of 10 % (w/v), 11.43 % (w/v), and 12.86 % (w/v), spherical shapes and smooth surfaces can be achieved when their reaction times reach 35 h, 45 h and 55 h, respectively. However, with fixed reaction time, increase of the Na2SO4 content can result in microparticles with poor morphology.

For example, for

microparticles fabricated with reaction time of 55 h, when the Na2SO4 content increases from 12.86 % (w/v) to 17.14 % (w/v), their morphology become poor with wrinkled surface. The reason is that the higher crystallization pressure associated with higher Na2SO4 content breaks the microstructures of the SiO2 matrix.48 The SEM images (Figure 4) of the Na2SO4·10H2O@SiO2 microparticles with different Na2SO4 contents and reaction time show the uniform structures of the resultant microparticles. For the microparticles with Na2SO4 contents of 0 % (w/v) (Figure 4a) and 7.14 % (w/v) (Figure 4b) and reaction time of 20 h, they show spherical shape with smooth surface. When the Na2SO4 content and reaction time are increased, the resultant uniform microparticles exhibit truncated sphere shapes (Figure 4c-f), due to the much higher density of the W/O emulsion droplets as compared


Page 13 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with the continuous phase (0.92 g cm-3). For example, with increasing the Na2SO4 content from 7.14 % (w/v) to 10 % (w/v), the density of the W/O emulsion droplets can increase from 1.06 g cm-3 to 1.12 g cm-3. When the collected droplets are stacked at the bottom of the container, such a high density can lead to droplet deformation during the reaction process due to the compression by the container bottom and the adjacent droplets. Thus, Na2SO4·10H2O@SiO2 microparticles with uniform truncated sphere shape are obtained.

Na2SO4·10H2O

Distribution

and

Thermal

Property

of

Na2SO4·10H2O@SiO2

Microparticles. To confirm the successful encapsulation and distribution of Na2SO4·10H2O in the resultant microparticles, the microparticles are used for element analysis, N2 adsorption analysis and DSC characterization. Typically, microparticles fabricated with Na2SO4 content of 12.86 % (w/v) and reaction time of 55 h are used for the element analysis (Figure 5). As shown in Figure 5a and 5b, elements including Na, Si, and S (Figure 5a3-a5) are all distributed at the wrinkled surface of the microparticle (Figure 5a2 and 5b). Meanwhile, as shown in Figure 5c and 5d, these elements (Figure 5c3-c5) also exist at the cross-section of the microparticle (Figure 5c2 and 5d), which exhibits nanoparticle-cluster-containing nanostructures with nano-voids (Figure 5d). Moreover, the N2 adsorption isotherms of the SiO2 microparticles with and without Na2SO4·10H2O (Figure 6a), which exhibit typical type-IV isotherms, and their pore size distribution (Figure 6b) confirm the mesoporous structure of the SiO2 microparticles. The Na2SO4·10H2O@SiO2 microparticles show pore diameter of 8.40 nm and surface area of 115.79 m2 g-1, while the microparticles without Na2SO4·10H2O show pore diameter of 7.72 nm and surface area of 224.56 m2 g-1.

The expanded pore diameter for Na2SO4·10H2O@SiO2

microparticles is due to the formation of Na2SO4·10H2O crystals in the pores of SiO2 matrix during microparticle fabrication. Such internal structures benefit the uniform distribution and



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 37

confinement of Na2SO4·10H2O inside the SiO2 matrix against homogeneous nucleation and further crystal growth for avoiding super-cooling and phase separation. Moreover, the intensities of Na and S distributed at the cross-section are higher than those at the surface, indicating a higher content of Na2SO4 inside the microparticle for effective encapsulation. The presence of Na2SO4·10H2O in the SiO2 microparticles are further confirmed by DSC characterization.

The DSC curves of Na2SO4·10H2O@SiO2 microparticles fabricated with

different contents of Na2SO4 and reaction time of 55 h upon heating and cooling are respectively shown in Figure 7a and 7b. As shown in Figure 7a, when increasing temperature from -50 ºC to 50 ºC, for microparticles fabricated with Na2SO4 contents of 10.00 % (w/v), 11.43 % (w/v), 12.86 % (w/v) and 17.14 % (w/v), their Tm,p values are 32.45 ºC, 31.71 ºC, 34.71 ºC and 34.88 ºC, respectively.

These Tm,p values are close to the phase transition temperature (~32 ºC) of

Na2SO4·10H2O,24 confirming the encapsulation of Na2SO4·10H2O inside the microparticle. Meanwhile, the Tc,p values are 17.62 ºC, 21.99 ºC, 25.89 ºC and 26.23 ºC respectively for microparticles with Na2SO4 contents of 10.00 % (w/v), 11.43 % (w/v), 12.86 % (w/v) and 17.14 % (w/v) (Figure 7b). Thus, the values of Ts for these SiO2 microparticles are in the range from 8.65 ºC to 14.83 ºC, which are much lower than the Ts values reported in literatures.28,29 Such lower Ts values are resulted from the effects of nucleating agent and crystal habit modifiers37 in the inner fluid, and the well distribution of Na2SO4·10H2O inside the confined nano-spaces of the SiO2 matrix. The results indicate the successful reducing of the super-cooling to achieve energy storage and release at mild temperatures. Moreover, in Figure 7a and 7b, no peaks are observed at 0 ºC for the microparticles with Na2B4O7·5H2O and SHMP during the heating and cooling processes, indicating no excessive water existed and thus no phase separation. As a comparison, the Tm,p and Tc,p of the microparticles that containing 12.86 % Na2SO4 but without


Page 15 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Na2B4O7·5H2O and SHMP are about 35 oC and -15 oC respectively. Therefore, the Ts value is 50 o

C, thus indicating a serious super-cooling problem.

Meanwhile, for these microparticles

without Na2B4O7·5H2O and SHMP, the peak around 0 oC during cooling process in Figure 7b indicates that the phase separation problem still exists. Such results of the control group further confirm the effects of the nucleating agent and crystal habit modifier on reduction of supercooling and elimination of phase separation. To evaluate the enthalpy of Na2SO4·10H2O encapsulated in the microparticles with nucleating agent and crystal habit modifier, the phase change enthalpies during melting and crystallization processes are calculated by the integral of endothermic (Figure 7a) and exothermic (Figure 7b) peaks for determining the enthalpy ratio (R).

The enthalpies of the Na2SO4·10H2O@SiO2

microparticles are strongly dependent on the Na2SO4 content used in the inner fluid. As shown in Figure 7c, with increasing the Na2SO4 content, the enthalpies increase for both heating and cooling processes.

From these enthalpy values, the R values of Na2SO4·10H2O@SiO2

microparticles can be obtained based on the enthalpy of pure Na2SO4·10H2O during heating process (201 J g-1) (inset Figure 7d). The calculated R values increase with increasing the Na2SO4 content in the inner fluid (Figure 7d), indicating an adjustable thermal property of the microparticles. As a typical example, the microparticles with Na2SO4 content of 12.86 % (w/v) show a R value of 47.07%, indicating effective encapsulation of Na2SO4·10H2O. Therefore, based on the successful avoiding of phase separation and dramatic reducing of super-cooling, and effective encapsulation of Na2SO4·10H2O, excellent and repeatable energy storage/release at mild temperatures can be achieved for these microparticles for thermo-regulation.

Model House with Na2SO4·10H2O@SiO2 Microparticles for Thermo-Regulation. The Na2SO4·10H2O@SiO2 microparticles with good phase transition behaviors at temperatures



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 37

around 30 ºC show great potential as energy storage/release systems for thermo-regulation. We demonstrate this by fabricating a model house containing walls and roofs incorporated with Na2SO4·10H2O@SiO2 microparticles (Figure 8a1) for thermo-regulation.

Since the

microparticles with Na2SO4 content of 12.86 % (w/v) (Figure 4e) show smoother surface morphology as compared with the ones with Na2SO4 content of 17.14% (w/v) (Figure 4f), the microparticles containing Na2SO4 content of 12.86 % (w/v), with Tm,p of 34.71 ºC and Tc,p of 25.89 ºC, are incorporated into ETPTA plates for constructing the house. For the two model houses containing SiO2 microparticles with (A in Figure 8a2) and without (B in Figure 8a2) Na2SO4·10H2O, their surface temperature distributions and inside temperatures during heating and cooling processes are respectively monitored. As shown in Figure 8b, at t = 0 min, the surface temperatures of both model houses are uniformly distributed, and are nearly the same as the environmental temperature. After heating for 1 min, their surface temperatures increase, but are still with nearly the same value. Then, at t = 9 min, model house A shows a lower surface temperature as compared with that of the model house B, because the Na2SO4·10H2O contained in model house A melts upon heating and absorbs heat to resist temperature increase. After complete melting of the Na2SO4·10H2O, the surface temperatures of both model houses become the same at t = 45 min. Similarly, during the cooling process, the decrease of the surface temperature of model house A is slower than that of model house B (Figure 8c), because the Na2SO4 contained in model house A crystallizes and releases heat to resist temperature decrease. The average temperatures of the selected surface regions of the two model houses (Figure 8a2) during heating and cooling processes are analyzed from the infrared images to show the change behaviors of the surface temperatures (Figure 8d). The maximum differences between the surface temperatures of the two model houses are 5.9 ºC (at t = 9 min) for the heating process,


Page 17 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and 4.0 ºC (at t = 55 min) for the cooling process. Such maximum differences appear when the surface temperatures of model house A reach 31.6 ºC upon heating and 24.8 ºC upon cooling, which are close to the Tm,p and Tc,p of the used Na2SO4·10H2O@SiO2 microparticles. This further confirms the slight degree of super-cooling (6.8 ºC) for the embedded microparticles, which can benefit their further practical use. The time-dependent changes of the inside temperatures of the two model houses during heating and cooling processes are also monitored (Figure 8a1). As shown in Figure 8e, similar to the change behaviors of the surface temperature, the increase and decrease of the inside temperatures of model house A are slower than those of model house B.

The maximum

differences between the inside temperatures of the two model houses are 3.2 ºC (at t = 12 min) for the heating, and 2.6 ºC (at t = 58 min) for the cooling. When reaching such maximum differences, the inside temperatures of model house A are 33.4 ºC upon heating and 23.4 ºC upon cooling, which are also close to the Tm,p and Tc,p of the used Na2SO4·10H2O@SiO2 microparticles.

All

these

results

confirm

the

excellent

performances

of

the

Na2SO4·10H2O@SiO2 microparticles in the house materials for thermo-regulation. Such thermo-regulating performances during the heating and cooling processes exhibit excellent repeatability. We demonstrate this by treating the two model houses with repeated heating/cooling cycles. As shown in Figure 9, both the inside temperatures of model house A (Figure 9a) and the temperature differences between model houses A and B (Figure 9b) show excellent repeatability. This also confirms no phase separation problems for the microparticles. For each heating/cooling cycle, the maximum and minimum values respectively appear at temperatures close to the Tm,p and Tc,p of the used Na2SO4·10H2O@SiO2 microparticles. Moreover, to further investigate the repeatable thermo-regulating performance of the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 37

microparticles inside the ETPTA walls and roofs of the model house, an ETPTA mini-plate containing Na2SO4·10H2O@SiO2 microparticles before and after 100 heating/cooling cycles is used for DSC characterization.

As shown in Figure 10, before and after 100 cycles, the

enthalpies during the heating process are 14.50 J g-1 and 12.54 J g-1 respectively. Thus, 86.48 % of the entire enthalpy is maintained after 100 heating/cooling cycles. The low enthalpy of the composite plate is due to that the ETPTA has a higher density than the microparticles, resulting in only 15.98 wt% of microparticle content in the ETPTA plate for low enthalpy. However, it is worth noting that, when only considering the encapsulated microparticles, these microparticles can still show a high enthalpy of 90.74 J g-1, which is almost the same as that of the unencapsulated microparticles (94.62 J g-1), indicating a good performance of these microparticles to be encapsulated for application. Moreover, the enthalpy value can be further adjusted by tuning the Na2SO4·10H2O content in the microparticles during their microfluidic fabrication process, and using materials with lower density for encapsulating the microparticles. All the results

show

the

stable

and

repeatable

thermo-regulating

performance

of

the

Na2SO4·10H2O@SiO2 microparticles, which is highly desirable for their long use for applications.

CONCLUSIONS In summary, uniform SiO2 microparticles containing controllable content of Na2SO4·10H2O against super-cooling and phase separation during phase change are developed for efficient and repeated energy storage/release at mild temperatures. Monodisperse W/O emulsion droplets generated

from

microfluidics

are

Na2SO4·10H2O@SiO2 microparticles.

used

for

template

synthesis

of

the

uniform

Incorporation of sodium borate and sodium


Page 19 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hexametaphosphate in the droplets for adjusting the Na2SO4·10H2O crystallization, associated with the well distribution of Na2SO4·10H2O inside the mesoporous SiO2 matrix for confining Na2SO4·10H2O crystallization, successfully eliminates the phase separation problem and dramatically reduces super-cooling. Thus, the resultant Na2SO4·10H2O@SiO2 microparticles can exhibit excellent energy storage/release property at mild temperatures with good repeatability for thermo-regulation. This is demonstrated by incorporating the microparticles into the walls and roofs of a model house for repeatedly regulating its surface and inside temperatures under heating and cooling cycles. Such monodisperse microparticles may also be incorporated into fibers to prepare medical textiles and space suits for storing/releasing energy for aerospace and medical use, or be packaged for preservation of food and actives.49,50 Moreover, by combining the Na2SO4·10H2O with other salts, the phase transition temperature of Na2SO4·10H2O can be flexibly adjusted to meet the different needs in various applications. The Na2SO4·10H2O@SiO2 microparticles with repeatable energy storing/releasing for thermoregulation show great potential for developing advanced materials for application in various fields such as energy, architecture and healthcare.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (L.-Y.C.); * E-mail: [email protected] (W.W.).

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge support from the National Natural Science Foundation of China (91434202, 81321002), the Program for Changjiang Scholars and Innovative Research Team in University (IRT15R48) and State Key Laboratory of Polymer Materials Engineering (sklpme2014-1-01).

REFERENCES (1) Su, W. G.; Darkwa, J.; Kokogiannakis, G. Review of Solid-Liquid Phase Change Materials and Their Encapsulation Technologies. Renew. Sust. Energ. Rev. 2015, 48, 373-391. (2) Liang, S. E.; Li, Q. B.; Zhu, Y. L.; Chen, K. P.; Tian, C. R.; Wang, J. H.; Bai, R. K. Nanoencapsulation of n-Octadecane Phase Change Material with Silica Shell through Interfacial Hydrolysis and Polycondensation in Miniemulsion. Energy 2015, 93, 1684-1692. (3) Wang, Y. M.; Mi, H. Y.; Zheng, Q. F.; Ma, Z. Q.; Gong, S. Q. Flexible Infrared Responsive Multi-Walled Carbon Nanotube/Form-Stable Phase Change Material Nanocomposites. ACS Appl. Mater. Interfaces 2015, 7, 21602-21609. (4) Phadungphatthanakoon, S.; Poompradub, S.; Wanichwecharungruang, S. P. Increasing the Thermal Storage Capacity of a Phase Change Material by Encapsulation: Preparation and Application in Natural Rubber. ACS Appl. Mater. Interfaces 2011, 3, 3691-3696.


Page 20 of 37

Page 21 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(5) Akhiani, A. R.; Mehrali, M.; Tahan Latibari, S.; Mehrali, M.; Mahlia, T. M. I.; Sadeghinezhad, E.; Metselaar, H. S. C. One-Step Preparation of Form-Stable Phase Change Material through Self-Assembly of Fatty Acid and Graphene. J. Phys. Chem. C 2015, 119, 22787-22796. (6) Hou, Y.; Vidu, R.; Stroeve, P. Solar Energy Storage Methods. Ind. Eng. Chem. Res. 2011, 50, 8954-8964. (7) Kenisarin, M.; Mahkamov, K. Solar Energy Storage Using Phase Change Materials. Renew. Sust. Energ. Rev. 2007, 11, 1913-1965. (8) Xu, B.; Li, P. W.; Chan, C.; Tumilowicz, E. General Volume Sizing Strategy for Thermal Storage System Using Phase Change Material for Concentrated Solar Thermal Power Plant. Appl. Energy 2015, 140, 256-268. (9) Xiao, X.; Zhang, P.; Li, M. Thermal Characterization of Nitrates and Nitrates/Expanded Graphite Mixture Phase Change Materials for Solar Energy Storage. Energy Convers. Manage. 2013, 73, 86-94. (10) Gao, X. Y.; Han, N.; Zhang, X. X.; Yu, W. Y. Melt-Processable Acrylonitrile–Methyl Acrylate Copolymers and Melt-Spun Fibers Containing MicroPCMs. J. Mater. Sci. 2009, 44, 5877-5884. (11) Nelson, G. Application of Microencapsulation in Textiles. Int. J. Pharm. 2002, 242, 55-62. (12) Li, W.; Ma, Y. J.; Tang, X. F.; Jiang, N.; Zhang, R.; Han, N.; Zhang, X. X. Composition and Characterization of Thermoregulated Fiber Containing Acrylic-Based Copolymer Microencapsulated Phase-Change Materials (MicroPCMs). Ind. Eng. Chem. Res. 2014, 53, 5413-5420.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 37

(13) McCann, J. T.; Marquez, M.; Xia, Y. Melt Coaxial Electrospinning: A Versatile Method for the Encapsulation of Solid Materials and Fabrication of Phase Change Nanofibers. Nano Lett. 2006, 6, 2868-2872. (14) Wen, G. Q.; Xie, R.; Liang, W. G.; He, X. H.; Wang, W.; Ju, X. J.; Chu, L. Y. Microfluidic Fabrication and Thermal Characteristics of Core-Shell Phase Change Microfibers with High Paraffin Content. Appl. Therm. Eng. 2015, 87, 471-480. (15) Kuznik, F.; David, D.; Johannes, K.; Roux, J. J. A Review on Phase Change Materials Integrated in Building Walls. Renew. Sust. Energ. Rev. 2011, 15, 379-391. (16) Memon, S. A. Phase Change Materials Integrated in Building Walls: A State of the Art Review. Renew. Sust. Energ. Rev. 2014, 31, 870-906. (17) Sun, S. X.; Xie, R.; Wang, X. X.; Wen, G. Q.; Liu, Z.; Wang, W.; Ju, X. J.; Chu, L. Y. Fabrication of Nanofibers with Phase-Change Core and Hydrophobic Shell, via Coaxial Electrospinning Using Nontoxic Solvent. J. Mater. Sci. 2015, 50, 5729-5738. (18) Hyun, D. C.; Levinson, N. S.; Jeong, U.; Xia, Y. Emerging Applications of Phase-Change Materials (PCMs): Teaching an Old Dog New Tricks. Angew. Chem. Int. Ed. 2014, 53, 3780-3795. (19) Sarı, A. Form-Stable Paraffin/High Density Polyethylene Composites as Solid-Liquid Phase Change Material for Thermal Energy Storage: Preparation and Thermal Properties. Energy Convers. Manage. 2004, 45, 2033-2042. (20) Zhang, Z. G.; Zhang, N.; Peng, J.; Fang, X. M.; Gao, X. N.; Fang, Y. T. Preparation and Thermal Energy Storage Properties of Paraffin/Expanded Graphite Composite Phase Change Material. Appl. Energy 2012, 91, 426-431.


Page 23 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(21) Giro-Paloma, J.; Oncins, G.; Barreneche, C.; Martínez, M.; Fernández, A. I.; Cabeza, L. F. Physico-Chemical and Mechanical Properties of Microencapsulated Phase Change Material. Appl. Energy 2013, 109, 441-448. (22) Duan, Z. J.; Zhang, H. Z.; Sun, L. X.; Cao, Z.; Xu, F.; Zou, Y. J.; Chu, H. L.; Qiu, S. J.; Xiang, C. L.; Zhou, H. Y. CaCl2·6H2O/Expanded Graphite Composite as Form-Stable Phase Change Materials for Thermal Energy Storage. J. Therm. Anal. Calorim. 2014, 115, 111-117. (23) Wang, T. Y.; Huang, J.; Zhu, P. P.; Xiao, J. B. Fabrication and Characterization of MicroEncapsulated Sodium Phosphate Dodecahydrate with Different Crosslinked Polymer Shells. Colloid Polym. Sci. 2013, 291, 2463-2468. (24) Kenisarin, M. M. High-Temperature Phase Change Materials for Thermal Energy Storage. Renew. Sust. Energ. Rev. 2010, 14, 955-970. (25) Guo, Q.; Wang, T. Study on Preparation and Thermal Properties of Sodium Nitrate/Silica Composite as Shape-Stabilized Phase Change Material. Thermochim. Acta 2015, 613, 66-70. (26) Kenisarin, M.; Mahkamov, K. Salt Hydrates as Latent Heat Storage Materials: Thermophysical Properties and Costs. Sol. Energy Mater. Sol. Cells 2016, 145, 255-286. (27) Wu, Y. P.; Wang, T. Hydrated Salts/Expanded Graphite Composite with High Thermal Conductivity as a Shape-Stabilized Phase Change Material for Thermal Energy Storage. Energy Convers. Manage. 2015, 101, 164-171. (28) Wu, Y. P.; Wang, T. Preparation and Characterization of Hydrated Salts/Silica Composite as Shape-Stabilized Phase Change Material via Sol-Gel Process. Thermochim. Acta 2014, 591, 10-15.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

(29) Zhang, J.; Wang, S. S.; Zhang, S. D.; Tao, Q. H.; Pan, L.; Wang, Z. Y.; Zhang, Z. P.; Lei, Y.; Yang, S. K.; Zhao, H. P. In Situ Synthesis and Phase Change Properties of Na2SO4·10H2O@SiO2 Solid Nanobowls toward Smart Heat Storage. J. Phys. Chem. C 2011, 115, 20061-20066. (30) Wu, Y. P.; Wang, T. The Dependence of Phase Change Enthalpy on the Pore Structure and Interfacial Groups in Hydrated Salts/Silica Composites via Sol-Gel. J. Colloid Interface Sci.

2015, 448, 100-105. (31) Hassabo, A. G.; Mohamed, A. L.; Wang, H.; Popescu, C.; Moller, M. Metal Salts Rented in Silica Microcapsules as Inorganic Phase Change Materials for Textile Usage. Inorg. Chem.: Indian J. 2015, 10, 59-65. (32) Kazemi, Z.; Mortazavi, S. M. A New Method of Application of Hydrated Salts on Textiles to Achieve Thermoregulating Properties. Thermochim. Acta 2014, 589, 56-62. (33) Zhao, W. R.; Gu, J. L.; Zhang, L. X.; Chen, H. R.; Shi, J. L. Fabrication of Uniform Magnetic Nanocomposite Spheres with a Magnetic Core/Mesoporous Silica Shell Structure. J. Am. Chem. Soc. 2005, 127, 8916-8917. (34) Szczepanowicz, K.; Podgórna, K.; Szyk-Warszyńska, L.; Warszyński, P. Formation of Oil Filled Nanocapsules with Silica Shells Modified by Sequential Adsorption of Polyelectrolytes. Colloids Surf., A 2014, 441, 885-889. (35) Wu, C. B.; Wu, G.; Yang, X.; Liu, Y. J.; Gao, C. X.; Ji, Q. H.; Wang, M.; Chen, H. Z. Preparation of Mannitol@Silica Core-Shell Capsules via an Interfacial Polymerization Process from Water-in-Oil Emulsion. Colloids Surf., A 2014, 457, 487-494. (36) Marks, S. An Investigation of the Thermal Energy Storage Capacity of Glauber's Salt with Respect to Thermal Cycling. Sol. Energy 1980, 25, 255-258.


Page 25 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(37) Marks, S. B. The Effect of Crystal Size on the Thermal Energy Storage Capacity of Thickened Glauber's Salt. Sol. Energy 1983, 30, 45-49. (38) Levy, H. A.; Lisensky, G. C. Crystal Structures of Sodium Sulfate Decahydrate (Glauber's Salt) and Sodium Tetraborate Decahydrate (Borax). Redetermination by Neutron Diffraction. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1978, 34, 35023510. (39) Liang, W. G.; Yang, C.; Wen, G. Q.; Wang, W.; Ju, X. J.; Xie, R.; Chu, L. Y. A Facile and Controllable Method to Encapsulate Phase Change Materials with Non-Toxic and Biocompatible Chemicals. Appl. Therm. Eng. 2014, 70, 817-826. (40) Li, D. Y.; Guan, Z. C.; Zhang, W. H.; Zhou, X.; Zhang, W. Y.; Zhuang, Z. X.; Wang, X. R.; Yang, C. J. Synthesis of Uniform-Size Hollow Silica Microspheres through Interfacial Polymerization in Monodisperse Water-in-Oil Droplets. ACS Appl. Mater. Interfaces 2010, 2, 2711-2714. (41) Utada, A.; Lorenceau, E.; Link, D.; Kaplan, P.; Stone, H.; Weitz, D. Monodisperse Double Emulsions Generated from a Microcapillary Device. Science 2005, 308, 537-541. (42) Wang, W.; Zhang, M. J.; Chu, L. Y. Functional Polymeric Microparticles Engineered from Controllable Microfluidic Emulsions. Acc. Chem. Res. 2013, 47, 373-384. (43) Wang, W.; Zhang, M. J.; Xie, R.; Ju, X. J.; Yang, C.; Mou, C. L.; Weitz, D. A.; Chu, L. Y. Hole-Shell Microparticles from Controllably Evolved Double Emulsions. Angew. Chem. Int. Ed. 2013, 52, 8084-8087. (44) Chu, L. Y.; Utada, A. S.; Shah, R. K.; Kim, J. W.; Weitz, D. A. Controllable Monodisperse Multiple Emulsions. Angew. Chem. Int. Ed. 2007, 46, 8970-8974.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 37

(45) Yu, S. Y.; Wang, X. D.; Wu, D. Z. Microencapsulation of n-Octadecane Phase Change Material with Calcium Carbonate Shell for Enhancement of Thermal Conductivity and Serving Durability: Synthesis, Microstructure, and Performance Evaluation. Appl. Energy

2014, 114, 632-643. (46) Do, T.; Ko, Y. G.; Chun, Y.; Choi, U. S. Encapsulation of Phase Change Material with Water-Absorbable Shell for Thermal Energy Storage. ACS Sustainable Chem. Eng. 2015, 3, 2874-2881. (47) Wang, H.; Wang, J. P.; Wang, X. C.; Li, W.; Zhang, X. X. Preparation and Properties of Microencapsulated Phase Change Materials Containing Two-Phase Core Materials. Ind. Eng. Chem. Res. 2013, 52, 14706-14712. (48) Rodriguez-Navarro C; Doehne E; Sebastian E. How does Sodium Sulfate Crystallize? Implications for the Decay and Testing of Building Materials. Cem. Concr. Res. 2000, 30: 1527-1534. (49) Mondal S. Phase Change Materials for Smart Textiles–An Overview. Appl. Therm. Eng.

2008, 28: 1536-1550. (50) Zalba B; Marı́n J M; Cabeza L F; Mehling H. Review on Thermal Energy Storage with Phase Change: Materials, Heat Transfer Analysis and Applications. Appl. Therm. Eng.

2003, 23: 251-283.


Page 27 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figures

Figure 1. Schematic illustration of microfluidic fabrication of Na2SO4·10H2O@SiO2 microparticles from W/O emulsions. (a) Microfluidic device for generating W/O emulsion templates.

(b) Formation of Na2SO4·10H2O@SiO2 microparticles via hydrolysis and

condensation (b1-b4), and freezing (b5) for constructing SiO2 matrix and crystallizing Na2SO4 into Na2SO4·10H2O (b6).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 37

Figure 2. W/O emulsion templates for fabricating Na2SO4·10H2O@SiO2 microparticles. (a) High-speed snapshots showing the generation process of W/O emulsions.

(b,c) Optical

micrographs and size distributions of the primary W/O emulsions (b) and the resultant microparticles (c) after reaction for 55 h. Scale bars are 400 µm.


Page 29 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Effects of reaction time and Na2SO4 content in the inner fluid on the morphology of the Na2SO4·10H2O@SiO2 microparticles.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 37

Figure 4. SEM images of the Na2SO4·10H2O@SiO2 microparticles containing different contents of Na2SO4 with optimized reaction time. The Na2SO4 content and reaction time are respectively 0 % (w/v) and 20 h (a), 7.14 % (w/v) and 20 h (b), 10 % (w/v) and 35 h (c), 11.43 % (w/v) and 45 h (d), 12.86 % (w/v) and 55 h (e), and 17.14 % (w/v) and 55 h (f). Scale bars are 200 µm.


Page 31 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Morphology and element analysis of Na2SO4·10H2O@SiO2 microparticles with Na2SO4 content of 12.86 % (w/v) and reaction time of 55 h.

(a) SEM images of the

microparticle (a1) with magnified surface (a2) for analyzing the surface distributions of Na (a3), Si (a4) and S (a5). (b) Magnified SEM image of the microparticle surface. (c) SEM images of the cracked microparticle (c1) with magnified cross-section (c2) for analyzing the inside distributions of Na (c3), Si (c4) and S (c5). (d) Magnified SEM image of the cross-section of the microparticle. Scale bars are 100 μm in (a1,c1), 4 μm in (a2,c2), and 100 nm in (b,d).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

Figure 6. Characterization of the mesoporous structures of Na2SO4·10H2O@SiO2 microparticles. (a) N2 adsorption isotherms and (b) pore size distribution of SiO2 microparticles with Na2SO4 contents of 12.86 % (w/v) and 0 % (w/v). The reaction time for both types of microparticles are 55 h.


Page 33 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Thermal properties of Na2SO4·10H2O@SiO2 microparticles containing different contents of Na2SO4 during phase transitions. (a,b) DSC curves of the microparticles during heating (a) and cooling (b) processes. (c,d) Effects of the Na2SO4 content on the enthalpies of the microparticles with Na2B4O7·5H2O and SHMP upon heating and cooling (c) and the enthalpy ratio (R) (d). The inset in (d) shows the DSC curves of pure Na2SO4·10H2O during heating and cooling processes.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 37

Figure 8. Model house incorporated with Na2SO4·10H2O@SiO2 microparticles for thermoregulation under simulated solar radiation. (a) Schematic illustration (a1) and photo (a2) of the model houses containing SiO2 microparticles with (A) and without (B) Na2SO4·10H2O. Scale bar is 1 cm. (b,c) Infrared thermal images showing the surface temperature distributions of the two model houses during heating (b) and cooling (c) processes. (d,e) Time-dependent changes of the surface (d) and inside (e) temperatures of the model houses. 34 ACS Paragon Plus Environment

Page 35 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. Thermo-regulating performance of the model houses containing SiO2 microparticles with and without Na2SO4·10H2O under periodically "on-off" simulated solar radiation. (a) Time-dependent change of inside temperatures of the model houses during repeated heating and cooling. (b) Time-dependent change of the inside temperature difference between the two model houses during repeated heating and cooling.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 37

Figure 10. Thermal property of ETPTA mini-plate containing Na2SO4·10H2O@SiO2 microparticles before and after 100 heating and cooling cycles.


Page 37 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Only


Monodisperse Na2SO4Â·10H2O@SiO2 Microparticles against

Recommend Documents