Open Access
Review
Issue
Manufacturing Rev.
Volume 5, 2018
Article Number 8
Number of page(s) 10
DOI https://doi.org/10.1051/mfreview/2018003
Published online 01 June 2018

© M.D. Romero-Sanchez et al., Published by EDP Sciences 2018

Licence Creative Commons
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction

Thermal energy storage (TES) using phase change materials (PCMs, for latent heat storage) is a key technology in improving efficiency of Concentrated Solar Power Plant (CSP) where solar heat can be stored for electricity production when sunlight is not available or to recover waste heat in industrial processes. However, the use of PCMs for TES is currently facing different problems, such as the lack of thermal stability of the energy storage materials, loss of effectiveness and/or serious corrosion problems when working at high temperatures (in this context, high temperature is considered when storage is performed between 120 and 600 °C) [13]. Therefore, incorporation of PCMs into TES systems requires its encapsulation into containers or support materials to avoid incompatibility or reactivity problems within the media where incorporated (i.e. corrosion in metal storage tanks).

Macro- (>1 mm) and microencapsulation (μm or nm particle size) procedures have been successfully developed and patented for low temperature PCMs (organic and inorganic materials), mainly using polymeric shells [49], which cannot be used for high temperature applications. There are also some literature and even TES pilot installations using macroencapsulated inorganic salts. Zn, NaNO3, MgCl2, and eutectic mixtures with melting temperatures higher than 300 °C have been encapsulated according to US2011/0259544 [10] using Ni or carbon and stainless steel materials in the form of cylinders as containers, with mm to cm size. Patent US2012/0055661 [11] addresses to the encapsulation of molten salt nitrates in metal tubes which are sealed off for permanent containment. Recently, a procedure has been patented (US 2015/0284616) [12] for the encapsulation of molten salts such as NaNO3 or KNO3. This method is based on the coating of the PCM pellet (27 mm size) with a flexible polymer followed by metal coating by electroless and electroplating processes (Ni, Cu, Zn, Zn-Fe alloys, etc.). Using electroplating method, other authors (Maruoka et al.) have obtained particles (3 mm diameter) with lead-nickel core-shell structure suitable for heat recovery of high temperature waste heat [13]. More simple methods can also be used for the encapsulation of metal PCMs such as indium (melting temperature = 156 °C) by using silica as shell by sol–gel procedure and obtaining nm size particles [14]. However, this method requires the melting of the metal, with increased difficultness for handling when using metals with higher melting temperature. Authors have proposed sol–gel processes using organic polymers with high melting temperature (i.e. polyimide) and organic-inorganic hybrids shells for the microencapsulation of PCMs. In this sense, stainless steel has been coated by sol–gel procedure of hybrid coatings for corrosion protection with interesting results, forming a physical barrier towards corrosion [15,16]. However, although interesting studies have been carried out, neither suitable solutions for microencapsulation of inorganic PCMs in the range of 300–500 °C have been successfully encountered with the requirements for TES applications nor any product can be currently found in the market.

NaNO3 has been identified in literature by several authors with thermal properties suitable for energy storage in CSP systems [1722]. In this study, the objective is its microencapsulation using in situ synthetized SiO2 shell by sol–gel procedure as a soft chemical synthesis procedure. Influence of NaNO3 and SiO2 precursors molar ratio on the TES capacity of microencapsulated NaNO3 has been evaluated, as well as thermal stability through thermal cycles, as a method to evaluate the durability of synthetized materials and feasibility for TES [21].

2 Experimental

2.1 Materials and experimental procedure

Sodium nitrate salt (NaNO3, 99.99% purity, Merck) has been selected as PCM to be microencapsulated, due to its convenient melting temperature for TES in CSP. NaNO3 is water soluble (2 g/10 mL).

Sol–gel technique has been used for the microencapsulation of NaNO3 into SiO2 shells. Tetraethylorthosilicate (TEOS, Sigma-Aldrich) has been used as SiO2 precursor.

Firstly, an aqueous phase containing hexadecyltrimethylammonium bromide (CTAB, Sigma-Aldrich) and NaNO3 is prepared. An organic phase containing cyclohexane (CH, analysis grade, Merck) as solvent and Span 80 (Fluka) as surfactant is added drop by drop to the aqueous phase. An emulsion is obtained after vigorous stirring and sonication (2 min, 50% intensity, Branson Digital Sonication). Subsequently, corresponding amounts of TEOS (NaNO3:SiO2 molar ratio 1:0.5 and 1:0.25) are added drop by drop to the emulsion. NaOH 1 N was used to adjust pH to 11 for hydrolysis of TEOS to SiO2 in alkaline media. After 12 h stirring, solution is twice centrifuged and washed with CH. Product obtained was dried in an oven at 40 °C for 24 h and subsequently at 150 °C for 24 h (Scheme 1).

thumbnail Scheme 1

Experimental procedure for NaNO3-SiO2 microparticles.

2.2 Experimental techniques

Different experimental techniques have been used for the thermal, chemical and morphological characterization of the materials obtained.

2.2.1 Fourier Transform Infrared Spectroscopy (FT-IR)

It has been used to detect the chemical groups present in the particles and compared to the chemical composition of the raw materials.

The FT-IR spectra of the samples have been obtained using a Varian 660 FT-IR spectrophotometer (Varian, Inc.) with dry KBr pellets prepared using a manual hydraulic press. FT-IR spectra in absorbance mode were recorded (100 scans with 4 cm−1 resolution) among the range of 600–4000 cm−1.

2.2.2 Scanning Electron Microscopy (SEM)

The surface morphology of the microparticles was obtained using a Phenom TM Electron Microscope system (SEM). For the SEM imaging, a drop of the corresponding microparticles dispersed in absolute ethanol was deposited on the sample holder and allowed to dry. The energy of the electron beam was 15 kV. This technique has been also used to estimate particle size. Field emission scanning electron microscopy (FESEM) has also been used for the characterization of morphology (Merlin VP Compact, Zeiss) working at higher resolution and reduced voltage (2 kV) minimizing charging effects.

2.2.3 Thermal Gravimetry Analysis (TGA)

TGA (TGA/Differential scanning calorimetry (DSC) 1 SF Stare System, Mettler Toledo) has been used to evaluate thermal stability of the microparticles. Samples have been heated till 400 °C at 10 °C/min under N2 atmosphere.

2.2.4 Differential Scanning Calorimetry (DSC)

Thermal properties of the microparticles obtained with the inorganic shell have been measured by using DSC. A DSC1/700 Stare system, Mettler Toledo) Differential Scanning Calorimeter has been used at a heating rate of 10 °C/min among the range of 50–400 °C, with a sample weight of about 15 mg and under N2 atmosphere.

Thermal analysis of the particles has been carried out during heating and cooling processes. Melting and crystallization temperatures and enthalpies have been calculated. Moreover, to evaluate thermal stability and durability of the microparticles, several thermal cycles have been carried out and melting and crystallization enthalpies have been calculated for comparison.

3 Results and discussion

NaNO3-SiO2 microparticles have been synthetized by sol–gel procedure using 1:0.5 and 1:0.25 molar ratios for NaNO3:TEOS (as SiO2 precursor), with the corresponding nomenclature Na-0.5Si and Na-0.25Si, respectively. The objective is to optimize this ratio to avoid the leakage of NaNO3 when melted or the shielding of NaNO3 by the SiO2 shell losing heat storage capacity.

The analysis of the chemical composition of the synthetized Na-0.5Si and Na-0.25Si is shown in the IR spectra in Figure 1.

IR spectrum of NaNO3 (Fig. 1) shows main peaks at 1385 cm−1 and 838 cm−1 due to N-O stretching vibration in NO3 and 2400–2500 cm−1 due to N = O stretching vibration.

IR spectra of samples Na-0.5Si and Na-0.25Si are very similar and show the characteristic bands of NaNO3 at 1385 and 838 cm−1 (confirming the presence of nitrate) and the band at 1090 cm−1 ascribed to the Si-O bond in SiO2. The band at 474 cm−1 may correspond to O-Si-O bending in SiO2 and/or asymmetric and Si-O-Si bending vibrations of the SiO2.

The band at 3440 cm−1 can be ascribed to the stretching vibration of O-H, Si-OH and/or N-H groups.

The presence of both compounds, NaNO3 as PCM and SiO2 as shell material have been identified in the FTIR spectra of the products synthetized, with different relative intensity of the typical bands for NaNO3 and SiO2 at 1385 and 1090 cm−1, being higher for the Na-0.25Si microparticles (relative intensity = 2.88) compared to the Na-0.5Si microparticles (relative intensity = 2.37), indicating the higher proportion of SiO2 in the Na-0.5Si sample. Moreover, this also indicates that the bonding between NaNO3 and SiO2 is a physical bonding. Other authors have previously obtained similar results when preparing mixed systems of NaNO3 and Sr(NO3)2 single crystals in SiO2 media [23].

The influence of the presence of SiO2 on the thermal properties of NaNO3 has been evaluated by DSC. Thermal properties of the samples synthetized have been analyzed during heating and cooling processes from 25 to 400 °C and 400 to 25 °C at 10 °C/min under N2 atmosphere. Figure 2 includes the DSC thermogram for raw NaNO3 used as PCM for microencapsulation. Melting and crystallization temperatures as well as the enthalpy of the respective processes (calculated by peak integration) have been included in Table 1. Values obtained for NaNO3 agree with those obtained in literature [24,25] with some slight differences due to the different conditions used for the experiments (gas for atmosphere, heating and cooling speed rate, etc). Figure 2 shows two endothermic peaks for NaNO3 during the heating process, at 278 °C due to a solid–solid phase transition and at 306.8 °C ascribed to the melting of NaNO3 (melting enthalpy = 158.1 J/g). During the cooling process, an exothermic peak releasing the energy previously stored is observed at 304 °C and a solid phase transition at 270 °C. These values of thermal properties for NaNO3 indicate the feasibility to be used as TES material. Figure 3 and Table 1 include the DSC thermograms and the quantified melting and crystallization temperatures (Tm, Tc) and enthalpies (ΔHm, ΔHc), respectively, for the samples Na-0.5Si and Na-0.25Si.

As observed in Figure 3, the presence of SiO2 produces some differences in the thermal behavior of synthetized Na-0.5Si and Na-0.25Si microparticles. In both cases, the melting temperature of NaNO3 has decreased respect to the melting temperature in the raw NaNO3. This could be explained because of the smaller particle size of the composites compared to the raw NaNO3 and the fact that ΔHm and ΔHc (J/g) for the composites have been calculated including the SiO2 weight [2628].

Na-0.5Si microparticles show an endothermic peak at 146.0 °C, which may be due to impurities, as this peak does not appear when carrying out thermal cycles to Na-0.5Si microparticles and explained below.

On the other hand, the melting and cooling enthalpies of Na-0.5Si and Na-0.25Si microparticles are also affected by the presence of SiO2, with a decrease in the crystallization temperature and enthalpy respect to the raw NaNO3 due to the presence of the SiO2 which is possible to restrict the crystallization of the NaNO3 in the microparticles.

The microparticles prepared with the lower SiO2 proportion (Na-0.25Si) show the highest melting and crystallization enthalpy values. The higher the enthalpy of this peak, the higher amount of energy stored.

The effect of the heating and cooling process on the chemical composition of the microparticles has been evaluated by FTIR spectroscopy. Figure 4 includes the FTIR spectra of Na-0.5Si microparticles before and after DSC experiment.

Na-0.5Si microparticles after DSC experiment show similar bands in the FTIR spectrum as same microparticles before DSC. The typical band for the NaNO3 appears at 1385 cm−1 and the Si-O band at 1090 cm−1. However, the relative intensity of these bands changes after the heating and cooling process in the DSC experiment. Relative intensity values of 2.33 and 1.77 (relative intensity of the bands at 1385 and 1090 cm−1) have been obtained for the microparticles of Na-0.5Si and Na-0.5Si after DSC, respectively, indicating a reduction of the NaNO3 proportion respect to the SiO2 in the Na-0.5Si microparticles after DSC experiment. This result is in agreement with the TGA analysis carried out to the microparticles, as observed in Figure 5, for thermal stability evaluation.

Figure 5 includes the TGA thermograms for Na-0.5Si and Na-0.25Si microparticles. The Na-0.5Si microparticles suffer a first weight loss at 135 °C (2.62 wt.%), which agrees with the endothermic peak observed in the DSC thermogram in Figure 3a and explained due to the presence of impurities of the synthesis of NaNO3 microparticles. Additionally, a slight weight reduction is also produced at 295 °C (1.40 wt.%), which corresponds to the melting temperature of NaNO3 in the microparticles determined by DSC (Tab. 1). Therefore, a decrease in the amount of NaNO3 in Na-0.5Si microparticles is produced during melting, which may be due to the transformation into other compounds such as NaNO2. Moreover, results obtained by DSC (Tab. 1) also show that the melting temperature of NaNO3 in the Na-0.5Si microparticles has decreased respect to the raw NaNO3. It is important to notice that the NaNO3 weight loss has been produced in spite of the higher proportion of SiO2 as shell material, compared to Na-0.25Si microparticles, which do not show any appreciable weight loss in the temperature range analyzed.

Figure 6 shows the morphology of the Na-0.5Si and Na-0.25Si microparticles obtained by SEM. As observed in Figure 6a for Na-0.5Si, NaNO3 has crystallized as bundles coated with SiO2 microparticles (wide particle size distribution: 0.2–0.5 μm), leading to NaNO3 and SiO2 composites. However, micrographs obtained for Na-0.25Si microparticles show prismatic shape NaNO3 crystals coated with SiO2 microspheres (particle size 0.5–1 μm), showing an entrapment of NaNO3 crystals in SiO2 microparticles agglomerates. Similar results have been obtained by using FESEM, as included in the micrographs in Figure 7. The different morphology of the NaNO3 crystals may be explained due to the different crystal growth in the presence of different ratio of SiO2 microparticles. Many studies in literature indicate that the crystal growth strongly depends on impurities and additives in the crystallization media or solution, affecting the composition, structure and final properties of the crystal [29]. The different morphology of the NaNO3 particles leads to a different thermal behaviour, as previously observed by DSC and TGA, with better results for the Na-0.25Si microparticles, with the lower SiO2 proportion.

Thermal stability of the Na-0.5Si and Na-0.25Si microparticles through thermal cycles has also been evaluated. Heating and cooling thermal cycles by DSC have been carried out from 50 to 400 °C and 400 to 50 °C at 10 °C/min. Melting and crystallization temperatures and enthalpies have been analyzed and included in Figure 8a and b, respectively for Na-0.5Si and Na-0.25Si and Table 2.

Data obtained for Na-0.5Si microparticles indicate that there is a loss of heating and cooling enthalpies with thermal cycles, i.e. 5.4 and 5.9 J/g, respectively, after 16 thermal cycles. This indicates a poor thermal stability of the Na-0.5Si microparticles, which agrees with the TGA results, showing a decrease in the amount of NaNO3 in the microparticles, due to the gradual transformation of NaNO3–NaNO2 as observed with the continuous decrease in the melting temperature when increasing the number of thermal cycles (NaNO2 melting temperature = 271 °C).

On the other hand, Na-0.25Si microparticles prepared with a lower proportion of SiO2, show very good thermal stability even after 20 thermal cycles from 50 to 400 °C and 400 to 50 °C, with constant melting and cooling enthalpies (values higher than 20 and 22 J/g, respectively). Main melting and crystallization temperatures are around 288 °C and 275 °C, respectively (Tab. 2b). Moreover, it is important to notice in both the heating and cooling processes, the small peak, respectively at 270.5 °C and 267.0 °C. These peaks may correspond to the melting and crystallization of NaNO2. The peak at 267.0 °C increases its intensity with thermal cycles, appearing as an intense crystallization peak at 270.5 °C in cycle 16. This could be explained due to the decomposition of NaNO3 to NaNO2 at the DSC temperature range to 400 °C. Commonly, NaNO3 decomposition takes place at temperatures higher than 450 °C [30,31], therefore at 400 °C, a slow decomposition is produced, which gradually increases after each thermal cycle. As a difference, Na-0.5Si microparticles only show one melting and crystallization peak, which are shifted to lower melting temperatures and higher crystallization temperatures, respectively, when increasing thermal cycles. Na-0.5Si microparticles do not show the melting and crystallization peaks at 270.5 and 267.0 °C, as observed for the Na-0.25Si microparticles. Some authors have also shown the NaNO2 formation when using NaNO3 as PCM material [32]. Bauer et al. [33] have demonstrated that the presence of NaNO2 in molten NaNO3 leads to a reduction of the NaNO3 melting temperature compared to raw NaNO3. The thermal dissociation is reversible, which may explain the similar intensity of the melting peaks at 270.5 °C when increasing thermal cycles.

As also observed by these authors, the presence of NO2 in the NaNO3 solution may lead to the changes in the NaNO3 behaviour during melting and cristallyzation processes.

As previously observed in Figure 3 and Table 1, Na-0.25Si microparticles show higher TES capacity (higher melting and crystallization enthalpies) than the microparticles with the higher SiO2 proportion (Na-0.5Si). This could be expected due to the lower amount of SiO2 shielding the core NaNO3. Moreover, and according to the TGA (Fig. 5) and DSC thermograms (Fig. 8, Tab. 2), the thermal stability through thermal cycles of Na-0.5Si microparticles is reduced compared to the microparticles prepared with lower SiO2 ratio (Na-0.25Si). This is an unexpected result because of the higher amount of SiO2 encapsulating and protecting the NaNO3 used as PCM in the Na-0.5Si microparticles.

In real CSP applications, PCMs are not allowed to heat or cool after each thermal cycle in a wide range of temperature around its melting and crystallization temperature. To evaluate the TES capacity of Na-0.25Si microparticles when subjected to thermal cycles in narrower temperature ranges, DSC technique has also been used. Melting and crystallization enthalpies have been determined to analyze the NaNO3 capacity to melt and crystallize in these narrow temperature ranges. Considering that the melting temperature of NaNO3 in the microparticles is around 290 °C, thermal cycles between 240 and 340 °C have been carried out to Na-0.25Si microparticles (Fig. 9). Results included in Table 3 indicate the TES capacity of the encapsulated product even after 60 thermal cycles.

Results obtained with the thermal cycles in the temperature range between 240 and 340 °C indicate the TES capacity of NaNO3 as PCM in the microparticles even after 60 thermal cycles, with constant melting and crystallization enthalpies higher than 26 and 25 J/g, respectively, as well as melting and crystallization temperatures (291.7, 283 °C, respectively). As a difference with the thermal cycles in Figure 8 (50–400 °C), in this case, for the 240–340 °C thermal cycles, the crystallization peak at 270 °C only appears as a shoulder of the main crystallization peak at around 283 °C. This may be explained due to the lower maximum temperature applied, 340 °C, avoiding the transformation of NaNO3–NaNO2, previously observed in the thermal cycles carried out to 400 °C.

Hence, the effectiveness of NaNO3 microparticles to be used as PCMs is determined by the maximum temperature during use. Temperatures higher than 400 °C slightly lead to the formation of NaNO2 and therefore, modifications on the melting and crystallization temperatures and enthalpies respect to the raw NaNO3.

thumbnail Fig. 1

FTIR spectra of NaNO3 and synthetized Na-0.5Si and Na-0.25Si microparticles.

thumbnail Fig. 2

DSC thermogram for NaNO3.

Table 1

Melting and cooling temperatures (Tm, Tc) and enthalpies (ΔHm, ΔHc) for raw NaNO3 and Na-0.5Si and Na-0.25Si microparticles. Data obtained from DSC thermograms for heating and cooling processes.

thumbnail Fig. 3

DSC thermograms for (a) Na-0.5Si and (b) Na-0.25Si.

thumbnail Fig. 4

FT-IR spectra of Na-0.5Si before and after DSC experiment.

thumbnail Fig. 5

TGA thermograms for raw NaNO3 and Na-0.5Si and Na-0.25Si microparticles.

thumbnail Fig. 6

SEM micrographs of (a) Na-0.5Si and (b) Na-0.25Si microparticles.

thumbnail Fig. 7

FESEM micrographs of (a) Na-0.5Si and (b) Na-0.25Si microparticles.

thumbnail Fig. 8

DSC thermograms of thermal cycles for (a) Na-0.5Si and (b) Na-0.25Si microparticles.

Table 2

Melting and cooling temperatures (Tm, Tc) and enthalpies (ΔHm, ΔHc) during thermal cycles for a) Na-0.5Si b)Na-0.25Si microparticles. Data obtained from DSC thermograms for heating and cooling processes.

thumbnail Fig. 9

DSC thermograms of thermal cycles in narrow temperature range (240–340 °C) for Na-0.25Si microparticles.

Table 3

Melting and cooling temperatures (Tm, Tc) and enthalpies (ΔHm, ΔHc) during thermal cycles in narrow temperature range (240–340 °C) for Na-0.25Si microparticles. Data obtained from DSC thermograms for heating and cooling processes.

4 Conclusions

Sol–gel has been demonstrated as a feasible technology for the microencapsulation of NaNO3 using SiO2 as shell material.

Effectiveness of microencapsulated NaNO3 as TES material greatly depends on the morphology of microparticles and therefore, on the NaNO3:SiO2 ratio. Results have shown that Na-0.25Si microparticles have higher energy storage capacity even with a lower proportion of SiO2 respect to the NaNO3 core material compared with the higher ratio in Na-0.5Si microparticles. The SiO2 shell may affect the NaNO3 crystal growth. This indicates the great influence of experimental parameters on the effectiveness of microencapsulated materials. In this sense, deeper work is being done by the authors to analyze the influence of the NaNO3 crystal phase on its energy storage capacity when microencapsulated within SiO2 shells.

TES stability of microencapsulated NaNO3 with SiO2 depends among other factors on the maximum temperature during use. Temperatures higher than 400 °C lead to the dissociation of NO3 to NO2 and therefore to a modification of the TES properties of NaNO3.

Acknowledgments

The authors gratefully acknowledge the financial support received from European Commission and Romanian Government-Management Authority from Ministry of Research and Innovation, in the frame of Competiveness Operational Programme, Action A1.1.4-E-2015, project ID P_37_776, SMIS code 104730, Acronym ENERHIGH.

Albert Ioan Tudor also gratefully acknowledges the financial support received from Romanian Ministry of Research and Innovation in the frame of the project PN 16 20 0302.

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Cite this article as: Maria Dolores Romero-Sanchez, Radu-Robert Piticescu, Adrian Mihail Motoc, Francisca Aran-Ais, Albert Ioan Tudor, Green chemistry solutions for sol–gel micro-encapsulation of phase change materials for high-temperature thermal energy storage, Manufacturing Rev. 5, 8 (2018)

All Tables

Table 1

Melting and cooling temperatures (Tm, Tc) and enthalpies (ΔHm, ΔHc) for raw NaNO3 and Na-0.5Si and Na-0.25Si microparticles. Data obtained from DSC thermograms for heating and cooling processes.

Table 2

Melting and cooling temperatures (Tm, Tc) and enthalpies (ΔHm, ΔHc) during thermal cycles for a) Na-0.5Si b)Na-0.25Si microparticles. Data obtained from DSC thermograms for heating and cooling processes.

Table 3

Melting and cooling temperatures (Tm, Tc) and enthalpies (ΔHm, ΔHc) during thermal cycles in narrow temperature range (240–340 °C) for Na-0.25Si microparticles. Data obtained from DSC thermograms for heating and cooling processes.

All Figures

thumbnail Scheme 1

Experimental procedure for NaNO3-SiO2 microparticles.

In the text
thumbnail Fig. 1

FTIR spectra of NaNO3 and synthetized Na-0.5Si and Na-0.25Si microparticles.

In the text
thumbnail Fig. 2

DSC thermogram for NaNO3.

In the text
thumbnail Fig. 3

DSC thermograms for (a) Na-0.5Si and (b) Na-0.25Si.

In the text
thumbnail Fig. 4

FT-IR spectra of Na-0.5Si before and after DSC experiment.

In the text
thumbnail Fig. 5

TGA thermograms for raw NaNO3 and Na-0.5Si and Na-0.25Si microparticles.

In the text
thumbnail Fig. 6

SEM micrographs of (a) Na-0.5Si and (b) Na-0.25Si microparticles.

In the text
thumbnail Fig. 7

FESEM micrographs of (a) Na-0.5Si and (b) Na-0.25Si microparticles.

In the text
thumbnail Fig. 8

DSC thermograms of thermal cycles for (a) Na-0.5Si and (b) Na-0.25Si microparticles.

In the text
thumbnail Fig. 9

DSC thermograms of thermal cycles in narrow temperature range (240–340 °C) for Na-0.25Si microparticles.

In the text

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