Size effects of nano-rutile TiO2 on latent heat recovered of binary eutectic hydrate salt phase change material
Introduction
Nanotechnology and nanomaterials, spanning many subject areas, have become an active topic over the past decade [1,2]. In recent years inorganic hydrate salts phase change materials (PCMs) used for thermal energy storage have attracted considerable attention because of their low cost, nontoxicity and nonflammable [[3], [4], [5]]. Meanwhile, for the purpose of performance improvement, nanoparticles modified hydrate salts PCMs have gained research popularity. For example, the use of AlN nanoparticles to inhibit supercooling of sodium acetate trihydrate (SAT) [6]; study on the effect of nano-copper (nano-Cu) content on the degree of supercooling of sodium acetate trihydrate [7]; an explanation on the mechanism of nucleation based on the interaction between Al2O3 nanoparticles and CH3COONa·3H2O-KCl composites salt system [8]; the improvement of the sunlight-harvesting and light-thermal conversion efficiency by the addition of multi-walled carbon nanotube (CNT) and nanographite (NG) into magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) [9], and so forth.
Remarkably, the chemical and physical properties of nanoparticles, such as chemical reactivity [10], surface adsorption capacity [11] and surface energy density [12], etc, exhibit a distinctly particles size-dependent behavior. Based on the characteristics, the nanoparticles with different sizes could result in different interaction of nanoparticles with base hydrate salt PCMs to further influence the thermal properties of PCMs. This is of great significance to the enhanced thermal properties of hydrate salt PCMs incorporated with nanoparticles as well as the optimum size selection of the corresponding nanoparticles in terms of thermal property improvement of hydrate salt PCMs. Dudda et al. [13] investigated the specific heat capacity of NaNO3-KNO3 eutectic with four different SiO2 nanoparticles (1 wt%), in which the size of the SiO2 nanoparticles were 5 nm, 10 nm, 30 nm and 60 nm, respectively. The results indicated that the specific heat capacity of the base salt increases with increase of SiO2 nanoparticle sizes. Zabalegui et al. [14] prepared a nanofluid by liquid paraffin, where multi-wall carbon nanotubes (MWCNT) with different diameters (15.5−400 nm) were used as nano-fillers. The DSC tests suggested that the smaller particle diameter, the greater the latent heat of fusion reduction at the same volume fraction of nano-fillers. Although some efforts have been made to study the effect of size effects of nanoparticles on the thermal properties of PCMs, to date, the attempts on size effects of nanoparticles to low temperature hydrate salt PCMs are hardly found in the literatures.
Referring to the former work [15], the concept of Latent heat recovered (LHR) is introduced to the evaluation of the thermal properties. The fraction of recovered latent heat is estimated by using the equation: LHR = × 100 %, where ΔHc and ΔHm represent the crystallization enthalpy (the heat released by PCM during solidification) and melting enthalpy (the heat absorbed by PCM in the melting process), respectively. LHR refers to the relative heat release capability, which is adopted as an important indicator to describe the endothermic/exothermic characteristics of PCM in this paper.
To demonstrate the size effect of nanoparticle and disclose the specific influences of the nanoparticles with different size on thermal properties of hydrate salt PCM, we adopted the Na2CO3·10H2O-Na2HPO4·12H2O eutectic hydrate salt (EHS) as the model hydrate salt PCM (sodium carbonate decahydrate (Na2CO3·10H2O) and disodium hydrogenphosphate dodecahydrate (Na2HPO4·12H2O) in the mass ratio of 2 : 3) and the rutile TiO2 nanoparticles with average particle diameters of 25 nm and 100 nm as the model nanoparticle fillers, because the rich hydrophilic groups of TiO2 nanoparticles would be very beneficial to the dispersion in the EHS solution and make it easier for hydrate salt to stick to the surface of TiO2 nanoparticles. Eventually, the TiO2 nanoparticles modified eutectic hydrate salt phase change composites were prepared. Specific to the LHR of phase change composites as the main evaluation indicator of thermal properties, combining with the crystallinity and microstructure evolution of TiO2 nanoparticles modified EHS composites, to make a thorough discussion for the size effect of TiO2 nanoparticles in this work.
Section snippets
Materials
Disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O, purity>99 %) and sodium carbonate decahydrate (Na2CO3·10H2O, AR) were adopted as inorganic hydrated salts PCMs; Rutile TiO2 nanoparticles with average particle sizes of about 25 nm and 100 nm, which are denoted as nano-TiO2-25 nm and nano-TiO2-100 nm, respectively, were provided by Shanghai Macklin Biochemical Technology Co., Ltd.
Sample preparation for thermal properties
The Na2CO3·10H2O-Na2HPO4·12H2O eutectic hydrate salt (EHS) was prepared according to the method provided by
Characterization of TiO2 nanoparticles
Fig. 1a and b show that the XRD patterns of nano-TiO2-25 nm and nano-TiO2-100 nm, respectively. It can be seen that there are no obvious differences in the XRD patterns of nano-TiO2-25 nm and nano-TiO2-100 nm. In addition, all peaks of the two XRD patterns can correspond to PDF (No. 21-1276) of rutile TiO2 and no new peaks are appeared, illustrating the high purity of the obtained rutile TiO2.
Fig. 2a and b show TEM imagines and the particle size distribution curves of nano-TiO2-25 nm and
Conclusions
In this article, the size effect of rutile TiO2 nanoparticles on the latent heat recovered of eutectic hydrate salt (EHS) phase change material (PCM) were studied. The 0.1 vol%, 0.3 vol% and 0.5 vol% TiO2 nanoparticles with two different sizes of 25 and 100 nm were incorporated into EHS to prepare the nanoparticles modified EHS composites. In order to evaluate the size effect of TiO2 nanoparticles, the factorial experiment analysis has been performed. The results show that the TiO2
Declaration of Competing Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.
Acknowledgments
The financial support from National Key R&D Program of China (No. 2017YFB0309901) and China Postdoctoral Science Foundation (2018M631936) for current research is gratefully acknowledged.
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