# Sodium nitrite-sodium nitrate eutectic based composite phase change material (CPCM) for medium-temperature thermal energy storage (TES): Exploring the relationship between microstructures and thermal properties

Benjamin Grégoire, Hongkun Ma, Anabel Palacios, Song Yang, Mengqi Bai, Tongtong Zhang, Yulong Ding

► To cite this version:

Benjamin Grégoire, Hongkun Ma, Anabel Palacios, Song Yang, Mengqi Bai, et al.. Sodium nitrite-sodium nitrate eutectic based composite phase change material (CPCM) for medium-temperature thermal energy storage (TES): Exploring the relationship between microstructures and thermal properties. Journal of Energy Storage, 2024, 101, pp.113922. 10.1016/j.est.2024.113922. hal-04777915

HAL Id: hal-04777915

https://hal.science/hal-04777915v1

Submitted on 2 Jul 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

Research papers

# Sodium nitrite-sodium nitrate eutectic based composite phase change material (CPCM) for medium-temperature thermal energy storage (TES): Exploring the relationship between microstructures and thermal properties

Benjamin Grégoire*,<fnref n="1" />, Hongkun Ma, Anabel Palacios<fnref n="2" />, Song Yang<fnref n="3" />, Mengqi Bai<fnref n="4" />, Tongtong Zhang, Yulong Ding*

Birmingham Centre for Energy Storage (BCES), School of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

- Latent heat

- Energy density

- Microstructure

- Molten salt

- Thermal decomposition

- Mobile heat

## ABSTRACT

This paper is concerned with a novel medium-temperature composite phase change material (CPCM). More specifically, the CPCM contains a sodium nitrite-sodium nitrate phase change material for latent and sensible heat storage, magnesium oxide as a ceramic matrix material for shape-stabilisation and sensible heat storage, and expanded graphite as a thermal conductivity enhancer material (TCEM) to improve heat transfer. The focus is on understanding the relationship between the CPCM microstructures and their thermal properties. This CPCM was found to be unique, exhibiting both a reversible solid-solid transition (≈175 °C) and a reversible solid-liquid transition (≈223 °C), with a total latent heat of fusion of ~120 J/g. In-depth microstructural characterisation of the CPCM indicated adequate thermal stability during thermal cycling in air up to 350 °C, which was also confirmed by their stable latent heat and thermal conductivity. The results highlighted a correlation between microstructures of the CPCMs and their thermal properties, as well as potential thermal decomposition reactions in high temperature industrial applications of molten salts such as concentrating solar power.

### Abbreviations

- BSE back-scattered electrons

- CPCM composite phase change material

- DSC differential scanning calorimetry

- EDS energy-dispersive x-ray spectroscopy

- EG expanded graphite

- LFA laser flash analysis

- PCM phase change material

- SEM scanning electron microscope

- TCEM thermal conductivity enhancer material

- TES thermal energy storage

- XRD x-ray diffraction

- XRT x-ray tomography

## 1. Introduction

The majority (73 %) of the UK's industrial energy demand is for heating, with 35 % for steam generation alone [1]. Besides the associated emissions of greenhouse gases, industrial heating processes give rise to a significant amount of waste heat, as high as 50 % + in sectors like steel and glass [2]. Thus, there is a significant potential for the emission reduction through waste heat recovery, which is estimated at between 11 and 48 TWh/year in the UK with values up to £90/MWh at today's energy prices [3]. This underscores the potential to enhance energy efficiency and decrease environmental impacts in industrial heat production and consumption. Various strategies have been proposed to decarbonise industrial thermal processes, such as direct renewable energy utilisation, e.g., concentrating solar power generation and thermochemical fuel production, or coupled with thermal energy storage

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: b.gregoire@bham.ac.uk (B. Grégoire), y.ding@bham.ac.uk (Y. Ding).</footnote>

<footnote n="1">¹ Currently with Laboratoire des Sciences de l'Ingénieur pour l'Environnement, LaSIE UMR 7356 CNRS, La Rochelle Université, Avenue Michel Crépeau, 17042, La Rochelle Cedex 1, France.</footnote>

<footnote n="2">² Currently with Competence Centre Thermal Energy Storage, Lucerne University of Applied Sciences and Arts, Technikumstrasse 21, 6048, Horw, Switzerland.</footnote>

<footnote n="3">³ Currently with School of Engineering, The University of Manchester, Manchester M13 9PL, UK.</footnote>

<footnote n="4">⁴ Currently with School of Nuclear Science and Engineering, North China Electric Power University, Changping district, Beijing, 102206, China.</footnote>
<table><caption>**Table 1**

Composition, manufacturing method and thermal properties of medium temperature CPCMs reported in the literature with phase change temperature within 200–310 °C.</caption><thead><tr><th>PCM (wt%)</th><th>Skeleton material</th><th>TCEM</th><th>Optimal mass fraction of PCM (%)</th><th>Manufacturing method</th><th>Melting temperature (°C)</th><th>Latent heat of melting (J/g)</th><th>Solidification temperature (°C)</th><th>Latent heat of freezing (J/g)</th><th>Year</th><th>Ref.</th></tr></thead><tbody><tr><td>87LiNO<sub>3</sub>-13NaCl</td><td>EG</td><td>–</td><td>77.8</td><td>Solution impregnation and drying at T<sub>m</sub> + 30 °C</td><td>227.7</td><td>231.5</td><td>212.7</td><td>-205.7</td><td>2014</td><td>Zhong et al. [16]</td></tr><tr><td>50NaNO<sub>3</sub>-50KNO<sub>3</sub></td><td>EG</td><td>EG</td><td>80-95</td><td>Mixing and cold compression</td><td>218-228</td><td>108.97</td><td>215-225</td><td>–</td><td>2015</td><td>Xiao et al. [17]</td></tr><tr><td>NaNO<sub>3</sub></td><td>Silica</td><td>–</td><td>60</td><td>Gelatinization, drying at 100 °C and heating at 500 °C</td><td>302</td><td>108</td><td>300</td><td>-110</td><td>2015</td><td>Guo and Wang [18]</td></tr><tr><td>NaNO<sub>3</sub></td><td>Expanded vermiculite</td><td>–</td><td>83.3-90.9</td><td>Direct impregnation at 350 °C</td><td>306.1-306.6</td><td>149.3-161.5</td><td>304.8-305.8</td><td>-146.9 to -158.9</td><td>2016</td><td>Li et al. [19]</td></tr><tr><td>NaNO<sub>3</sub></td><td>Diatomite</td><td>Graphite</td><td>70</td><td>Mixing, cold compression, drying at 120 °C and sintering at 370 °C</td><td>307.8</td><td>115.79</td><td>–</td><td>–</td><td>2017</td><td>Xu et al. [20]</td></tr><tr><td>NaNO<sub>3</sub></td><td>Ca(OH)<sub>2</sub></td><td>–</td><td>60</td><td>Mixing, moistening, compression/ sintering at 120 °C and drying at 100 °C</td><td>306.5</td><td>102.8</td><td>–</td><td>–</td><td>2019</td><td>Yu et al. [21]</td></tr><tr><td>NaNO<sub>3</sub></td><td>Diatomite</td><td>–</td><td>52.35</td><td>Mixing, compression, sintering and direct impregnation at 340 °C</td><td>302.8</td><td>92.5</td><td>301.2</td><td>-93.1</td><td>2021</td><td>Jiang et al. [22]</td></tr><tr><td>NaNO<sub>3</sub></td><td>CaCO<sub>3</sub>-modified</td><td>–</td><td>58.67</td><td></td><td>304.0</td><td>103.8</td><td>302.5</td><td>-104.4</td><td>2021</td><td>Jiang et al. [22]</td></tr><tr><td>NaNO<sub>3</sub></td><td>Diatomite</td><td>SiC</td><td>56.6</td><td></td><td>≈302</td><td>≈100</td><td>–</td><td>–</td><td>2021</td><td>Jiang et al. [23]</td></tr><tr><td>NaNO<sub>3</sub></td><td></td><td>–</td><td>55.36</td><td></td><td>≈306</td><td>101.83</td><td>≈300</td><td>-100.72</td><td>2021</td><td>Jiang et al. [24]</td></tr><tr><td>60NaNO<sub>3</sub>-40KNO<sub>3</sub></td><td>Red mud</td><td>–</td><td>30-60</td><td>Mixing, cold compression and sintering at 400 °C</td><td>226.23-230.63</td><td>28.02-58.14</td><td>–</td><td>–</td><td>2021</td><td>Anagnostopoulos et al. [25]</td></tr><tr><td></td><td>Red mud</td><td>Graphite</td><td>47.5-60</td><td>Mixing, cold compression and sintering at 500 °C</td><td>218.33-230.15</td><td>46.25-65.47</td><td>–</td><td>–</td><td>2021</td><td>Anagnostopoulos et al. [26]</td></tr><tr><td>NaNO<sub>3</sub></td><td>Diatomite</td><td>Nano EG</td><td>62.91</td><td>Mixing, cold compression and sintering at 320 °C</td><td>304.64</td><td>87.32</td><td>302.03</td><td>-87.32</td><td>2022</td><td>Soleimanpour et al. [27]</td></tr><tr><td>NaNO<sub>3</sub></td><td>Diatomite</td><td>Nano Diamond</td><td>62.91</td><td>Mixing, cold compression and sintering at 320 °C</td><td>304.98</td><td>90.51</td><td>302.14</td><td>-91.02</td><td>2022</td><td>Soleimanpour et al. [27]</td></tr><tr><td>95NaNO<sub>3</sub>-5NaCl</td><td>EG</td><td>EG</td><td>80</td><td>Water-soluble dispersion, drying at 80 °C and sintering at 400 °C</td><td>295.0-297.1</td><td>169.1-189.4</td><td>281.9-282.2</td><td>-168.1 to -186.4</td><td>2022</td><td>Li et al. [28]</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>Fig. 1. Manufacturing process of the composite phase change materials into cylindrical modules.</figcaption></figure>

(TES) systems to address intermittency issues. Among TES technologies, shape-stabilised composite phase change materials (CPCMs) have attracted wide attention [4–6]. An ideal shape-stabilised CPCM should meet the following requirements [7]: (i) suitable melting point for working conditions, (ii) high specific heat capacity and phase change enthalpy to deliver high energy density and compact TES system, (iii) reproducible and consistent phase change temperature, (iv) good thermal/chemical stability and long cycle life, (v) high thermal conductivity, which can meet the requirements in terms of charging/discharging time of the heat storage system, and (vi) low environmental impact.

Shape-stabilised CPCMs usually contain [6] (i) a phase change material (PCM) for latent heat storage, (ii) a three-dimensional structural material or ‘skeleton material’ to confine the PCM and (iii) a thermal conductivity enhancer material (TCEM) dispersed within the PCM to enhance heat transfer. CPCMs can store energy as TES coming from renewables by storing heat directly from concentrated solar power (CSP) plants or, for instance, by using off-peak electricity from wind or solar farms that would be otherwise curtailed [8]. This technology, due to its PCM component, can also store and deliver heat from waste heat sources at specific temperatures for end-user applications [9]. Since most industrial waste heat is in the low to medium temperature range in terms of energy content [8], but their recycling rate is very low [10], there is significant interest to formulate, characterise and develop cost-effective CPCMs with a careful screening and selection of its constituents as well as an optimisation of the manufacturing process [7]. For waste heat recovery up to 400 °C, PCMs with phase change temperature at about 200–300 °C are particularly attractive to cover the heat demand for low-to-medium-temperature applications [8,11]. There is also a drive to design, model and build innovative storage concepts utilising such materials.

Medium-temperature CPCMs with solid-liquid transition within 200–310 °C are essentially based on nitrate salts and their mixtures as PCMs, a ceramic as skeleton material and expanded graphite as TCEM as shown in Table 1. The optimal mass fraction of PCM in published CPCMs formulations is usually claimed to be between 50 and 80 wt% although, in some cases, leakage occurred. The most common manufacturing method of salt-based CPCMs follows (i) milling/mixing of raw materials,

(ii) cold uniaxial compression and (iii) sintering in an electric furnace at a temperature greater than the melting point of the PCM [6]. While research on CPCMs is quickly growing, there are still some fundamental and technical challenges to overcome before successful integration for large-scale applications. Considerations of the chemical and physical compatibility between the CPCMs constituents are key to providing durable and reliable long-duration energy storage, so is their compatibility with containment materials (e.g. carbon steels or stainless steels). CPCMs must maintain a stable three-dimensional structure to ensure that the heat transfer properties are met for efficient charging/discharging cycles and accommodate volume changes, the latter having implications on the structural durability of the storage system. In this regard, ceramic materials such as magnesium oxide often have a relatively high interfacial energy meaning that a small contact angle is expected between the liquid PCM and the ceramic skeleton material [12]. Consequently, the liquid PCM phase can wet the skeleton material and spread on its surface while the wetting liquid provides a capillary force to pull the ceramic particles together and rearrange them to form a dense composite [12]. This occurs during the sintering treatment and increases the rigidity of the CPCM’s structure consisting of grains of skeleton material bound by solidified channels of salt. Such a microstructure can thus prevent PCM leakage during repeated solid-liquid phase change cycles. Nevertheless, a degradation of the mechanical stability of the CPCM could result in the loss of its isotropic structure, resulting in uneven heat fluxes across the TES system. This may have a detrimental effect on the CPCM thermal properties and the overall efficiency of the TES system. It is therefore key to control the CPCM microstructure and ensure that repeated charging/discharging cycles do not significantly affect its mechanical integrity and thermal properties.

In this work, a CPCM based on NaNO₂-NaNO₃ eutectic mixture was developed to facilitate efficient heat recovery and transport, achieving a maximum operating temperature of 350 °C. This PCM was selected for its suitable solid-liquid phase change temperature and its combination of high latent heat and low price compared to other medium-temperature PCM [11]. Light MgO was selected as the matrix material due to its reported ability to prepare dense shape-stabilised CPCM modules [6], its good chemical compatibility with nitrate salts [13,14] and its relatively
high thermal conductivity for a ceramic material [15]. The study focused on the thermal characterisation of the CPCM (latent heat, phase change temperature, specific heat capacity and thermal diffusivity) and their relationship with microstructural aspects (phase distribution, porosity, stability). The experimental methodology was based on a systematic approach with at least three parallel samples for each thermophysical property tested. The main novelty of our work lies with in-depth microstructural investigation by scanning electron microscopy (SEM) and systematic X-ray diffraction (XRD) analyses conducted to better relate thermal and microstructural properties of the CPCM. Our work is expected to guide research groups working on the formulation, development, and characterisation of TES materials as well as potential users of the technology. A key objective is to propose and characterise medium-to-high temperature CPCM formulations with high potential for large-scale manufacturing to promote TES deployment and industrial decarbonisation [8].

## 2. Materials and methods

### 2.1. Materials and manufacturing process of composite phase change material

Sodium nitrate (NaNO₃, 99.5 % purity), sodium nitrite (NaNO₂, 99.1 % purity) and magnesium oxide (MgO, 97.4 % purity) were purchased from Plater Group, UK. All three materials were industrial grade, more cost-effective for large-scale application compared to lab-grade materials usually employed for formulation studies in the literature. Expanded graphite was purchased from Graphit Kropfmühl GmbH, Germany. The manufacturing process of the CPCM modules involves four main steps, (i) milling of the salts, (ii) mixing of the raw materials, (iii) shaping into cylindrical modules by uniaxial compression and (iv) sintering (Fig. 1). Milling of the salts was performed with a Retsch rotor beater mill SR 200. After milling, sodium nitrate and sodium nitrite were mixed with a Cyclomix® to the desired eutectic composition (55NaNO₂-45NaNO₃, wt%). It should be noted that the PCM mixture was not premelted, unlike many other studies using eutectic salt mixtures, to simplify the manufacturing process and reduce the preparation time. The PCM salt mixture, magnesium oxide and expanded graphite were then mixed again with a Cyclomix® to obtain a homogeneous CPCM powder mixture as shown after step (ii) in Fig. 1. Please note that the exact mass fraction of the respective components cannot be disclosed at the time of writing. Compacting of the CPCM powder mixture was carried out with a LLOYD LS100 compression machine at a pressure of 50 MPa into ≈50 g cylindrical modules (≈50 mm diameter and ≈15.7 mm thickness).

After manufacturing, the diameter and thickness of the cylindrical CPCM modules were systematically measured with a digital calliper with a precision of 10 µm. The modules were also weighed with a precision balance accurate to 10⁻³ g. Their apparent density was then calculated as the ratio of the mass to the volume occupied by the module and expressed in g.cm⁻³. For the sintering of the CPCM, the samples were placed into an electric furnace and heated in air at a rate of 5 °C/min until reaching 300 °C. The temperature was maintained at 300 °C for 1 h before cooling down to room temperature. The samples were left to cool down below 50 °C before their removal from the furnace and were kept in air-tight containers before further use. The sintering temperature was selected after post-microstructural characterisation of the CPCM modules and preliminary thermal cycling tests that highlighted their stability (i.e. negligible dimensional change and absence of leakage).

### 2.2. Thermal cycling tests

The thermal stability of the CPCM was investigated through thermal cycling tests conducted in an electric furnace. Each cycle comprised the following steps: (i) heating at a rate of 5 °C/min until reaching the

maximum temperature, (ii) maintaining an isothermal step at the maximum temperature for 1 h, (iii) natural cooling of the furnace down to 120 °C and (iv) maintaining an isothermal step at 120 °C for 1 h. Two independent experiments were conducted with the same furnace and only differed by the maximum temperature reached of either 350 or 400 °C. At regular cycle intervals during the cycling tests (i.e. 4, 8, 12, 16, 24, 32, 40 and 48 cycles), the CPCM modules were removed from the furnace for visual examination and check for potential leakage and assessing any loss of shape-stabilisation effect. The cycled CPCM samples were systematically measured with a digital calliper with a precision of 10 µm and weighed with a 10⁻³ g precision balance to calculate their apparent density and weight. The evolution of the apparent density and weight loss as a function of the number of thermal cycles was then used as criteria to determine the thermal cycling stability of the CPCM. The results were presented as the average of at least three parallel samples with associated standard deviation values. The same procedure was followed for CPCM modules cycled at a maximum temperature of either 350 or 400 °C. After 4, 12, 24 and 48 thermal cycles, representative CPCM modules were taken out of the furnace for further microstructural and thermal characterisation. The mechanical properties of the CPCM were not evaluated in this study since it was not aimed to be used as structural support and that no external mechanical stress is applied onto the CPCM modules during application.

### 2.3. Thermal characterisation

To determine the thermal properties of the optimised CPCM formulation, samples were prepared and analysed by differential scanning calorimetry (Mettler Toledo DSC3+) in aluminium crucibles to obtain the heat flow curves upon heating/cooling between 35 and 400 °C at a heating rate of 10 °C/min and cooling rate of 5 °C/min. The measurements were performed using nitrogen as purge gas with a flow of 50 mL min⁻¹. Sample size was 20 ± 1 mg. The temperature and enthalpy were calibrated using the melting points of pure In (156.7 °C, 27.5 J/g) and pure Zn (419.5 °C, 106.3 J/g) samples. Three samples were tested for each testing condition and two melting-solidification cycles were performed for repeatability. For analysis of the sintered and cycled CPCM modules, the samples were prepared by mechanical grinding into a fine powder using a mortar and pestle. Unless otherwise mentioned, the DSC curves displayed and associated data correspond to the second melting-solidification cycle.

Specific heat capacity measurements were obtained following the sapphire method, which was conducted by DSC (Netzsch 204 F1 Phoenix) using small CPCM pellets (5 mm diameter). The measurements were performed upon heating at a constant heating rate of 10 °C/min. The 5 mm diameter CPCM pellets were prepared with a LLOYD LS100 compression machine at a pressure of 50 MPa. The experimental data was then used to calculate the energy density of the novel CPCM within its operating temperature range (80–350 °C) following Eq. (1):

$$Q = \int_{T_{\min}}^{T_{\text{pc}}} c_{p,s} dT + \Delta H_{\text{pc}} + \int_{T_{\text{pc}}}^{T_{\text{max}}} c_{p,l} dT \quad (1)$$

With Q the energy density in J·g⁻¹; Tmin the lower operating temperature of the CPCM (fixed at 80 °C in this work); Tpc the phase change temperature of the PCM (as determined by DSC); Tmax the maximum operating temperature of the CPCM (350 °C in this work); ΔHpc is the latent heat of phase change of the CPCM in J/g, cp,s is the average specific heat capacity in J/(g·K) of the CPCM below the melting point of the PCM and cp,l is the average specific heat capacity in J/(g·K) of the CPCM above the melting point of the PCM. Since the CPCM developed in this study exhibits both a reversible solid-solid transition and a reversible solid-liquid transition, the latent heat of phase change is taken as the total latent heat. The heat capacity measurements were done so as to subtract the influence of both phase changes and make sure that the 'thermal energy is not counted twice'.
<figure><img src="image_5.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 2. (a) The XRT scanning of the sample; (b) the binary image of one slice, where white pixels represent particles, and black pixels represent pores.</figcaption></figure>

Thermal diffusivity and thermal conductivity values were obtained through the Laser Flash Analysis (LFA) with a Netzsch LFA 427 device. The measurements were done under a flow of nitrogen (100 mL/min). Samples with a diameter of 12.7 mm were prepared with a LLOYD LS100 compression machine at a pressure of 50 MPa to obtain parallel surfaces and then coated with a graphite layer (GRAPHIT 33, Kontakt Chemie) to foster absorption of the laser and heat emissivity [29]. Four samples for each testing condition were measured for repeatability. The thickness of each sample was precisely measured with a digital calliper (10⁻³ mm) and taken as a reference for thermal diffusivity calculations. The thermal conductivity of the CPCM samples is then calculated using the following equation:

$$ \lambda(T) = \alpha(T) \times \rho(T) \times c_p(T) \qquad (2) $$

where α is the thermal diffusivity (mm²/s), ρ is the density (g/cm³) and cₚ is the specific heat capacity (J/(g.K)) of the CPCM at a given temperature.

## 2.4. Microstructural characterisation

The structural analysis of the CPCMs before and after thermal cycling tests was conducted using X-Ray Diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer equipped using CuKα₁ radiation (λ = 1.5418 Å) and θ-2θ geometry. Data were collected at room temperature

<figure><img src="image_7.png" /><figcaption>Fig. 3. XRD analyses of the CPCM after (a) mixing and (b) sintering treatment.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 4. BSE images and corresponding X-ray distribution maps of C, N, O, Na and Mg for the sintered CPCM at (a) x500 and (b) x3000 magnification.</figcaption></figure>

between 20° and 70° in 2θ with a step size of 0.02° and a counting time of 0.5 s per step. The DIFFRAC.EVA software was used to identify the crystal structures by referencing the ICDD database. Scanning Electron Microscopy (SEM) was used to analyse the microstructure of the sintered and cycled CPCM modules. Observations were performed with a desktop SEM from Hitachi Inc. (TM3030) at 15 kV. Imaging was done in Back-Scattered Electron (BSE) mode to reveal chemical composition gradients in the observed samples. A Bruker Inc. (Quantax 70, Bruker) energy dispersive x-ray spectrometer (EDS) with a built-in silicon drift detector (XFlash®430H detector, deepest penetration can be 3.1 µm) coupled with the SEM, was used to perform semi-quantitative chemical analyses, and obtain X-ray elemental distribution maps.

The pore size distribution of the CPCMs before and after thermal cycling at different temperatures was investigated by X-ray tomography (XRT) with the Bruker SKYSCAN™ 2211. Sharper images were generated by the flat panel detector using a 0.5 mm Ti filter at room temperature. The voltage and current were set as 110 kV and 100 µA, respectively. The rotation step degree was 0.2° and the exposure time was 280 ms. The scanned area of the sample is revealed in Fig. 2a. After reconstructing the raw X-ray tomography (XRT) images, binary images (see Fig. 2b) containing only black and white pixels are generated through thresholding using the CTAn software. The subsequent pore size

measurements and porosity analysis are performed using CTAn's quantitative morphometric assessments. CTAn identifies pores within selected white pixels (referred to as “objects”) in the image. Pores are recognised as clusters of black pixels surrounded by white pixels, and the software distinguishes between closed pores (bubbles) and open pores (connecting channels) in the 3D space, providing information about the pore size distribution and porosity network. Due to limitations associated with the sample size, only pores larger than 35 µm could be detected.

## 3. Results

### 3.1. *Microstructural characterisation of the CPCM*

Fig. 3a presents the XRD analyses of the CPCM mixture after mixing of raw components. As expected, it is composed of NaNO₂ (orthorhombic, main peak at 30.1°), NaNO₃ (trigonal, main peak at 29.5°), MgO (cubic, main peak at 43.0°) and graphite (hexagonal, main peak at 26.6°). The powder sample prepared from the sintered CPCM shows a similar diffractogram (Fig. 3b) although the diffraction peaks associated with both NaNO₂ and NaNO₃ crystal structures are enlarged after sintering. This can be explained by the melting process and the consequent
<caption>**Table 2**</caption>

Semi-quantitative chemical analyses of the two phases observed in Fig. 4. Results given as the average of three EDS spot analyses performed in different regions of the sample for each phase.

<table><thead><tr><th>EDS spot location</th><th>N (at.%)</th><th>O (at.%)</th><th>Na (at.%)</th><th>Mg (at.%)</th><th>Possible compound (s)</th></tr></thead><tbody><tr><td>Bright phase</td><td>11.8 ± 1.6</td><td>52.3 ± 1.7</td><td>13.3 ± 0.5</td><td>22.6 ± 2.1</td><td>MgO, NaNO<sub>2</sub>/NaNO<sub>3</sub></td></tr><tr><td>Dark phase</td><td>18.5 ± 0.1</td><td>53.9 ± 0.1</td><td>22.1 ± 0.5</td><td>5.5 ± 0.4</td><td>NaNO<sub>2</sub>/NaNO<sub>3</sub>, MgO</td></tr></tbody></table>

reduction in crystallite size (non-sintered CPCM mixture contains relatively large salt crystals). The XRD analysis of the sintered CPCM will be taken as a reference to study microstructural changes within the CPCM during cycling tests (section 3.3).

Fig. 4a shows a BSE image of the sintered CPCM with corresponding X-ray elemental distribution maps. The image was taken in a representative area of the sample. The sintered CPCM module shows a dense microstructure with good cohesion between the expanded graphite particles (appearing dark in BSE, see C-map) and the rest of the material. Interestingly, the salt mixture penetrated the porous structure of expanded graphite as highlighted with the Na-map. Some porosity can also be observed but with little effect on the homogeneity of the material. Such porosity is likely to result from interparticle voids after axial compression and volume variation during the solid-liquid phase transition. At higher magnification (Fig. 4b), the CPCM shows a fine eutectic microstructure with two apparent phases in BSE imaging mode. The brighter phase is rich in Mg while the darker phase is rich in Na and N (see corresponding X-ray maps). Semi-quantitative EDS analyses of both phases were performed and are reported in Table 2. It should be

noted that a precise chemical composition for each phase cannot be given due to the limited spatial resolution of the EDS and the fine microstructure of the CPCM but can still give useful information on the elemental partitioning between the two phases. The concentration of N and Na in both phases is almost identical in agreement with the stoichiometry of both NaNO₂ and NaNO₃ compounds. EDS spot analyses also confirm that the bright phase is enriched in Mg while the dark phase contains little Mg. O was found to be slightly enriched in the dark phase but remains at about 51–53 at.%, which agrees with its respective stoichiometry in MgO (50 at.%) and equimolar NaNO₂-NaNO₃ (56 at.%). The fine microstructure of the sintered CPCM observed by SEM suggests that the MgO powder selected as ceramic skeleton material provides an efficient containment for the binary salt mixture.

## 3.2. Thermal characterisation of the CPCM

The DSC curves for the first and second melting/solidification cycles of the sintered CPCM are given in Fig. 5a and b, respectively. Three sample repetitions were performed for each cycle and the thermodynamic data extracted from the curves is presented on the graphs (average values with standard deviation). The DSC results indicate that, while this CPCM formulation has a decent latent heat value (≈65 J/g for the solid-liquid transition at about 223 °C), its energy density is further increased by the occurrence of a solid-solid transition (≈55 J/g) in the temperature range 175–189 °C upon heating and 124–160 °C upon cooling. The CPCM formulation shows an overall high latent heat of about 120 J/g. It is worth noting that the DSC curves obtained were similar between the three samples. Both solid-solid and solid-liquid transitions were systematically observed with a very narrow onset temperature of phase change and latent heat value as highlighted by the associated small standard deviation values presented in Fig. 5a and b. The observations suggest that a good compositional homogeneity of the CPCM was achieved after the manufacturing process. In addition, no

<figure><img src="image_9.png" /><figcaption>**Fig. 5.** DSC curves of the sintered CPCM based on NaNO₂-NaNO₃ eutectic. Three samples were tested and two cycles were performed for each sample for repeatability. Onset temperature of phase change and associated latent heat values are indicated on the graphs.</figcaption></figure>

<caption>**Table 3**</caption>

Thermal events and DSC data were obtained for the sintered CPCM based on NaNO₂-NaNO₃ eutectic for the first (1:) and second (2:) heating-cooling cycles. Comparison with data from the literature for NaNO₂-NaNO₃ eutectic is also provided (data highlighted with asterisks).

<table><thead><tr><th>Transition</th><th>T<sub>onset</sub> (°C)</th><th>T<sub>peak</sub> (°C)</th><th>T<sub>endset</sub> (°C)</th><th>Latent heat (J/g)</th></tr></thead><tbody><tr><td>Solid-solid (heating)</td><td>1: 174.9 ± 0.1<br>2: 175.9 ± 1.1<br>*173.0 [30]</td><td>1: 183.0 ± 0.2<br>2: 183.1 ± 0.8<br>*183.0 [31]</td><td>1: 197.4 ± 0.2<br>2: 192.6 ± 3.0</td><td>1: 54.4 ± 0.3<br>2: 54.6 ± 1.5</td></tr><tr><td>Melting (heating)</td><td>1: 225.0 ± 0.1<br>2: 223.5 ± 0.5<br>*224.5 [30]</td><td>1: 227.9 ± 0.1<br>2: 226.9 ± 0.5<br>*228.0 [31]</td><td>1: 232.2 ± 0.4<br>2: 230.5 ± 0.9</td><td>1: 70.3 ± 1.6<br>2: 65.3 ± 1.3</td></tr><tr><td>Solidification (cooling)</td><td>1: 223.9 ± 0.1<br>2: 223.1 ± 0.1<br>*224.5 [30]</td><td>1: 222.6 ± 0.1<br>2: 221.8 ± 0.3</td><td>1: 219.4 ± 0.1<br>2: 218.2 ± 0.4</td><td>1: -66.1 ± 0.2<br>2: -65.3 ± 0.8</td></tr><tr><td>Solid-solid (cooling)</td><td>1: 160.4 ± 0.2<br>2: 159.9 ± 0.5</td><td>1: 144.2 ± 0.5<br>2: 143.7 ± 0.4</td><td>1: 123.1 ± 0.2<br>2: 123.2 ± 0.1</td><td>1: -56.8 ± 0.2<br>2: -56.1 ± 0.9</td></tr></tbody></table>
<table><caption>**Table 4**
EDS quantification results (at.%) for the spot analyses shown in Fig. 11.</caption><thead><tr><th>EDS spot</th><th>C</th><th>N</th><th>O</th><th>Na</th><th>Mg</th><th>Possible compound(s)</th></tr></thead><tbody><tr><td>1</td><td>74.2</td><td>8.3</td><td>15.0</td><td>1.5</td><td>1.0</td><td>Graphite</td></tr><tr><td>2</td><td>-</td><td>20.2</td><td>53.8</td><td>22.8</td><td>3.2</td><td>NaNO<sub>2</sub>/NaNO<sub>3</sub></td></tr><tr><td>3</td><td>-</td><td>18.9</td><td>54.0</td><td>23.4</td><td>3.8</td><td>NaNO<sub>2</sub>/NaNO<sub>3</sub></td></tr><tr><td>4</td><td>-</td><td>12.7</td><td>53.0</td><td>14.8</td><td>19.5</td><td>MgO, NaNO<sub>2</sub>/NaNO<sub>3</sub></td></tr><tr><td>5</td><td>-</td><td>11.0</td><td>51.3</td><td>13.9</td><td>23.8</td><td>MgO, NaNO<sub>2</sub>/NaNO<sub>3</sub></td></tr><tr><td>6</td><td>77.8</td><td>5.7</td><td>13.6</td><td>1.7</td><td>1.2</td><td>Graphite</td></tr><tr><td>7</td><td>18.6</td><td>1.9</td><td>50.1</td><td>26.2</td><td>3.3</td><td>Na<sub>2</sub>CO<sub>3</sub></td></tr><tr><td>8</td><td>7.8</td><td>11.3</td><td>51.8</td><td>12.2</td><td>17.0</td><td>MgO, NaNO<sub>2</sub>/NaNO<sub>3</sub>, Na<sub>2</sub>CO<sub>3</sub></td></tr><tr><td>9</td><td>11.6</td><td>7.9</td><td>51.9</td><td>8.6</td><td>20.0</td><td>MgO, NaNO<sub>2</sub>/NaNO<sub>3</sub>, Na<sub>2</sub>CO<sub>3</sub></td></tr></tbody></table>

<caption>**Fig. 6.** Thermal diffusivity and effective thermal conductivity results of the sintered CPCM (average of four individual samples) as a function of temperature.</caption>

major difference can be seen between the two melting-solidification cycles in terms of latent heat values and phase change temperatures. This indicates that no decomposition of the salt occurred during testing of the CPCM samples under the nitrogen purge gas, which is an important observation to ensure the reliability of the DSC analyses performed. Further experimental data extracted from the DSC analyses of the sintered CPCM are presented in Table 3 and compared to literature data on NaNO₂-NaNO₃ eutectic. It should be noted that the experimental data of the CPCM samples are also in good agreement with that of the NaNO₂-NaNO₃ eutectic mixture, which was characterised as baseline, taking into account its respective mass fraction in the CPCM formulation.

Comparative DSC analyses were performed and showed a solid-solid transition (77.9 ± 2.8 J/g; 170.8 ± 2.7 °C) and solid-liquid transition (106.5 ± 3.5 J/g; 225.1 ± 0.2 °C) for the second heating.

Heat capacity measurements were also conducted to get experimental data on the specific heat capacity of the CPCM within its operating temperature range (80–350 °C). The average specific heat capacity values are 1.46 ± 0.09 J/(g·K) and 1.48 ± 0.06 J/(g·K) in solid and liquid states, respectively. The calculated energy density of the novel CPCM between 80 and 350 °C was, therefore, 517 ± 13 J/g.

Thermal diffusivity of the sintered CPCM was measured at 25 °C and between 100 and 350 °C with a 50 °C step increment. The corresponding results are shown in Fig. 6. The evolution of the thermal diffusivity values with temperature is common for a CPCM with (i) a maximum value at 25 °C, (ii) a progressive decrease up to the melting point of the PCM and (iii) a stable thermal diffusivity value above the melting point. The trend between 200 and 250 °C is explained by the occurrence of the solid-liquid phase change transition of the PCM at about 223 °C. As such, the PCM was in solid state when performing the measurements at 200 °C and in liquid state when performing the measurements at 250 °C. This is a common behaviour for PCMs (and by extensions CPCMs) made of inorganic salts as shown, for instance, by Anagnostopoulos et al. [25]. Following Eq. (1), the effective thermal conductivity of the sintered CPCM was calculated for the same temperature increments (Fig. 6). The associated error was about 10 % for measurements at 25 °C but only about 5 % for all other temperatures.

### 3.3. Thermal cycling results – Microstructural characterisation of the CPCM

Fig. 7 presents the evolution of the apparent density and weight loss of the sintered CPCM modules during thermal cycling tests at a maximum temperature of 350 and 400 °C. The apparent density of the CPCM modules cycled at 350 °C is constant throughout the thermal cycling tests at 1.57–1.58 g.cm⁻³ (Fig. 7a) while the weight loss did not exceed 0.6 % after 48 thermal cycles (Fig. 7b). This indicates that the CPCM is thermally stable at 350 °C and that it provides adequate shape-stabilisation effect. On the other hand, the apparent density of the CPCM samples cycled at 400 °C is decreasing as a function of the total number of cycles and reached 1.39 g.cm⁻³ after 48 thermal cycles (a 14 % decrease). This was essentially caused by an important weight loss during heat treatment of the CPCM with a maximum average value of 9.26 % after 48 thermal cycles (Fig. 7b).

Fig. 8 presents the XRD results of the CPCM cycled at 350 °C for 4, 12, 24 and 48 thermal cycles and of the sintered CPCM (i.e. 0 cycle) for comparison. No phase transformation was noticed after thermal cycling tests up to 350 °C with the systematic identification of the crystal

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Evolution of (a) apparent density and (b) weight loss of the CPCM during thermal cycling at a maximum temperature of 350 and 400 °C. Average and standard deviation values of at least 3 individual samples.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 8. XRD analyses of the CPCM after sintering treatment and thermal cycling tests at a maximum temperature of 350 °C.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 9. XRD analyses of the CPCM after sintering treatment and thermal cycling tests at a maximum temperature of 400 °C.</figcaption></figure>

structures associated with NaNO₂, NaNO₃, MgO and graphite (Fig. 8b-e). In addition, there was very little variation of the diffraction peaks intensity as a function of thermal cycling duration for all compounds. The thermal cycling tests at a maximum temperature of 350 °C had therefore little effect on the structural integrity of the CPCM. Fig. 9

shows the XRD analyses of the CPCM modules cycled at 400 °C for 4, 12, 24 and 48 thermal cycles. After 4 thermal cycles, the composition of the CPCM is similar with no additional diffraction peak observed (Fig. 9b). However, the composition of the CPCM changed after 12 and more thermal cycles as highlighted by the variation of the peak intensity of the
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** BSE images and corresponding X-ray maps of Na and C after 4, 12, 24 and 48 thermal cycles at a maximum temperature of (a) 350 °C and (b) 400 °C.</figcaption></figure>

respective compounds and the identification of sodium carbonate (Na₂CO₃, natrite) crystal structure (Fig. 9c-e). The intensity of the diffraction peaks of Na₂CO₃ increases as a function of the thermal cycles while that of NaNO₂ decreases to be almost nil after 48 thermal cycles. It can therefore be anticipated that the formation of Na₂CO₃ into the CPCM occurs at the expense of NaNO₂. The relative intensity of the diffraction peaks associated with NaNO₃ was, however, very similar between all tested samples (Fig. 9a-e). The associated mechanisms and likely explanation are detailed in the Discussion section of this paper.

Fig. 10a and Fig. 10b are BSE images and corresponding X-ray maps

of Na and C for the CPCM after thermal cycling tests at a maximum temperature of 350 and 400 °C, respectively. The samples cycled at 350 °C (Fig. 10a-1 to a-4) show the distribution of expanded graphite particles, appearing dark in BSE mode, in a dense Na-rich matrix. Little porosity is observed within the CPCM while no major microstructural changes can be noticed as a function of the number of thermal cycles. These observations confirm that the CPCM is thermally stable at 350 °C. The samples cycled at 400 °C (Fig. 10b-1 to b-4) show, however, a higher level of porosity. While expanded graphite particles are still observed in the CPCM (see X-ray map of C), it is clear from the X-ray map of Na that
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** BSE images and corresponding X-ray maps of C, N, O, Na and Mg after 48 thermal cycles at a maximum temperature of (a) 350 °C and (b) 400 °C. EDS spot locations are shown on the BSE images and results reported in Table 4.</figcaption></figure>

microstructural changes occurred during thermal cycling at 400 °C. While Na is randomly distributed in the CPCM after 4 cycles (Fig. 10b-1), it is found to segregate after 12 or more cycles (Fig. 10b-2 to b-4). After 48 thermal cycles at 400 °C (Fig. 10b-4), the segregation of Na within the CPCM is obvious as highlighted by the white arrows.

SEM analyses were conducted at higher magnification to further characterise the microstructure of the CPCM after 48 thermal cycles (Fig. 11). After cycling at 350 °C (Fig. 11a), the microstructural features are similar to the ones observed after sintering treatment shown in Fig. 4. Expanded graphite particles are randomly distributed while the CPCM matrix shows two distinct phases in BSE mode. The dark phase (EDS spot locations 2 and 3) contains little Mg but comparably high N

and Na concentrations and corresponds to the $NaNO_2/NaNO_3$ mixture. The bright phase (EDS spot locations 4 and 5) is however rich in Mg and corresponds to the ceramic skeleton material (MgO) which effectively contained the $NaNO_2/NaNO_3$ salt mixture.

After cycling tests at 400 °C (Fig. 11b), some voids appearing black in the BSE image are visible. The image was taken in a region showing a Na-enrichment (see Fig. 10b-4). The X-ray maps indicate that the compound rich in Na is also rich in O and C. The EDS spot performed in location 6 confirms this and it can be concluded that this compound is in fact $Na_2CO_3$ as also observed by XRD.

XRT was used as a non-destructive technique to investigate the pore size distribution of the sintered CPCM before and after thermal cycling
<figure><img src="image_15.png" /><figcaption>Fig. 12. (a) Pore size distribution of the sintered CPCM before and after 48 thermal cycles at 350 or 400 °C; (b) Evolution of the total porosity for the CPCM during thermal cycling tests at 350 or 400 °C.</figcaption></figure>

tests. Fig. 12a shows the pore size range distribution of various CPCM samples in percent volume. One can see that the pore size distribution for the CPCM cycled at 350 °C is almost identical to the sintered CPCM. This reveals that the CPCM maintained a stable microstructure after 48 thermal cycles at 350 °C. However, the pore size percent volume in the 35.00–2065.00 µm of the CPCM cycled at 400 °C nearly doubled that of the sintered CPCM. This suggests that thermal cycling at 400 °C

enhanced the formation of small pores within the material. Correspondingly, with a higher percent volume of the small pore size range, other pore size ranges had lower percent volume compared to the sintered CPCM and the CPCM cycled at 350 °C.

Fig. 12b displays the total porosity values of various CPCMs before and after thermal cycling tests. Before thermal cycling, the CPCM shows the smallest total porosity with a value of about 22 %. After thermal
<figure><img src="image_16.png" /><figcaption>Fig. 13. DSC curves of CPCM samples after sintering and thermal cycling tests at a maximum temperature of 350 °C and corresponding latent heat values upon (a), (b) heating and (c), (d) cooling.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 14. Evolution of the phase change temperature as a function of thermal cycling at (a) 350 and (b) 400 °C for both solid-solid and solid-liquid transitions. Phase change temperatures obtained during both heating (solid lines) and cooling (dashed lines) are presented.</figcaption></figure>

cycling tests at 350 °C, the total porosity values increased slightly but remained quite stable as a function of the number of thermal cycles at about 22.3 ± 0.2 %. The total porosity of the CPCM cycled at 400 °C increased with thermal cycling with the highest value of 24.09 % after 48 thermal cycles. This trend might be explained by microstructural changes and the associated formation of small pores within the CPCM as shown in Fig. 11b. Nevertheless, pores smaller than 35 µm could not be detected with the flat-panel mode of the XRT and the total porosity of the CPCM cycled at 400 °C is therefore likely to be underestimated.

### 3.4. Thermal cycling results – Thermal characterisation of the CPCM

Fig. 13 presents the DSC curves and associated latent heat values for CPCM samples after thermal cycling tests at 350 °C. Please note that the

latent heat data presented in Fig. 13b and d is the average of three individual samples with standard deviation. However, the DSC curves of only one of these three samples for each cycling step are presented in Fig. 13a and c for clarity. Upon heating (Fig. 13a), two main thermal events associated with the solid-solid transition and the melting of the PCM can be noticed. The CPCM shows a stable behaviour with constant latent heat values as a function of thermal cycling (Fig. 13b). The average latent heat values associated with the solid-solid transition range between 54.6 and 58.1 J/g while the average latent heat of melting ranges between 64.8 and 66.8 J/g. The total latent heat stored by the CPCM upon heating remained therefore at about 120 J/g. Likewise, the phase change temperature for both solid-solid and solid-liquid transitions remained unchanged at about 176 and 223 °C, respectively (Fig. 14a).
<figure><img src="image_18.png" /><figcaption>**Fig. 15.** DSC curves of CPCM samples after sintering and thermal cycling tests at a maximum temperature of 400 °C and corresponding latent heat values upon (a), (b) heating and (c), (d) cooling.</figcaption></figure>

<caption>**Table 5**</caption>

Average specific heat capacity of various CPCM samples when the PCM is either in solid or in liquid states. Three samples for each condition were analysed.

<table><thead><tr><th rowspan="2">PCM state</th><th colspan="3">Average c<sub>p</sub> (J/(g·K))</th></tr><tr><th>CPCM-sintered</th><th>CPCM-350 °C<br>(48 cycles)</th><th>CPCM-400 °C<br>(48 cycles)</th></tr></thead><tbody><tr><td>Solid state</td><td>1.46 ± 0.09</td><td>1.41 ± 0.08</td><td>1.42 ± 0.09</td></tr><tr><td>Liquid state</td><td>1.48 ± 0.06</td><td>1.45 ± 0.04</td><td>1.46 ± 0.02</td></tr></tbody></table>

The DSC curves of the CPCM samples upon cooling are presented in Fig. 13c. The two thermal events observed correspond to the solidification process of the PCM and the solid-solid transition at lower temperatures. Both transitions gave stable latent heat values, as shown in Fig. 13d, with no noticeable effect of the thermal cycling. Upon cooling, the average latent heat values associated with the solid-solid transition range between 55.6 and 60.1 J/g while the average latent heat of solidification ranges between 65.3 and 68.1 J/g. The total latent heat released by the CPCM upon cooling was systematically above 120 J/g, regardless of the number of thermal cycles. As observed upon heating, the phase change temperature for both solid-solid and solid-liquid transitions remained unchanged at about 159 and 223 °C, respectively (Fig. 14a).

Fig. 15 presents the DSC curves and associated latent heat values for CPCM samples after thermal cycling tests at 400 °C. Unlike the samples cycled at 350 °C, the thermal cycling exposure at 400 °C impacted the thermal performance of the CPCM. This effect is visible both on the DSC heating curves (Fig. 15a) and cooling curves (Fig. 15c) with (i) a reduction in intensity of the thermal peaks and (ii) a shift of the melting/ solidification temperature of the PCM to higher temperatures as a function of thermal cycling. After 4 thermal cycles, the total latent heat decreased by about 6–7 % compared to the sintered CPCM. The total latent heat of the CPCM further decreased after 12 and 24 thermal cycles

at 400 °C. While the solid-solid transition was still observed for these samples, it appears to no longer be completely reversible as shown by the difference in latent heat between the heating and cooling procedures (compare Fig. 15b and d). This resulted in the total latent heat of the CPCM after 12 and 24 thermal cycles being lower during cooling than during heating. After 48 thermal cycles, the solid-solid transition is no longer observed and only the solid-liquid transition accounts for the total latent heat (about 57 % of the original value).

Table 5 summarises the average $c_p$ values of three kinds of samples when the PCM is in both solid and liquid states. The CPCM sample after sintering (CPCM-sintered) exhibits the highest $c_p$ in the solid state. However, the deviation is less than 4 % and 3 % for the samples cycled at 350 °C and 400 °C, respectively. Above the phase change temperature, the deviation is even lower with a difference less than 2 % and 1 % for the samples cycled at 350 and 400 °C, respectively. These differences are within the experimental uncertainty of the DSC measurements. Thermal cycling at either 350 or 400 °C has therefore limited effect on the heat capacity of the CPCM. Fig. 16 better illustrates the limited effect of thermal cycling on the heat capacity of the CPCM (average of three samples for each testing condition). Deviations are essentially due to the contribution of the latent heat of phase change for both solid-solid and solid-liquid transitions. Near the phase change temperature, specific heat capacity values are overestimated and need to be corrected as illustrated in Fig. 16. The evolution of the specific heat capacity of the CPCM with the increase of temperature is in agreement with that of $NaNO_2$ [32] and $NaNO_3$ [33,34], the main compounds of the CPCM as well as the data reported by Anagnostopoulos et al. for a CPCM based on solar salt [25,26].

Fig. 17 presents the thermal diffusivity and thermal conductivity results of the sintered CPCM before and after thermal cycling. Thermal cycling at 350 °C had limited effect on these values compared to the sintered CPCM with an average increase of about 4 % for thermal diffusivity and less than 1 % for thermal conductivity, respectively. This
<figure><img src="image_19.png" /><figcaption>Fig. 16. Evolution of specific heat capacity (c_p) of the sintered CPCM before and after 48 thermal cycles at 350 or 400 °C with 10 °C step and corresponding data at specific temperatures. Values under brackets are corrected to remove the influence of phase changes.</figcaption></figure>

<table><thead><tr><th>T (°C)</th><th>Sintered</th><th>350°C- 48 cycles</th><th>400°C- 48 cycles</th></tr></thead><tbody><tr><td>25</td><td>0.99 ± 0.02</td><td>0.99 ± 0.02</td><td>1.02 ± 0.02</td></tr><tr><td>100</td><td>1.28 ± 0.03</td><td>1.24 ± 0.02</td><td>1.27 ± 0.01</td></tr><tr><td>150</td><td>1.59 ± 0.02<br>(1.45 ± 0.02)</td><td>1.55 ± 0.02<br>(1.40 ± 0.02)</td><td>1.33 ± 0.01</td></tr><tr><td>200</td><td>2.14 ± 0.04<br>(1.63 ± 0.04)</td><td>1.91 ± 0.02<br>(1.56 ± 0.02)</td><td>1.48 ± 0.02</td></tr><tr><td>250</td><td>1.57 ± 0.05</td><td>1.47 ± 0.02</td><td>2.09 ± 0.02<br>(1.58 ± 0.02)</td></tr><tr><td>300</td><td>1.48 ± 0.02</td><td>1.45 ± 0.04</td><td>1.44 ± 0.03</td></tr><tr><td>350</td><td>1.39 ± 0.04</td><td>1.40 ± 0.04</td><td>1.48 ± 0.02</td></tr></tbody></table>

is in good agreement with microstructural observations and other thermal data confirming the stability of the CPCM up to 350 °C. Thermal cycling at 400 °C resulted, however, in a drastic reduction in thermal diffusivity by 21 % and in thermal conductivity by 32 %. This is explained by the thermal decomposition of sodium nitrite and the consequential formation of pores within the CPCM (Fig. 11). The partial decomposition of expanded graphite is not excluded at 400 °C although thermogravimetric analyses of pure expanded graphite indicated good thermal stability in air. The thermal decomposition of sodium nitrite may also have contributed to this. The associated mechanisms are discussed in section 4.2 and in more details in Appendix A.

# 4. Discussion

## 4.1. Thermal performance and microstructural stability of the CPCM

The reversibility of both solid-solid and solid-liquid transitions upon heating/cooling indicates that the CPCM can be used to harvest heat and later deliver this heat. In addition, no degradation of the thermal properties of the CPCM was noticed after thermal cycling tests at a maximum temperature of 350 °C. The CPCM shows a stable latent heat of about 120 J/g and an energy density as high as 506 ± 11 J/g after undergoing 48 thermal cycles at 350 °C. The decrease in energy density compared to the sintered CPCM is insignificant (0.21 %) and mainly attributed to experimental errors for heat capacity measurements. Besides, thermal cycling at 350 °C has minimal impact on the thermal diffusivity and thermal conductivity values of the CPCM. The thermal performance and stability of the CPCM at 350 °C is further supported by microstructural observations. In fact, the microstructure of the cycled CPCM is identical to the sintered CPCM, which supports the role of MgO as ceramic skeleton material. SEM observations at high magnification (Fig. 4b) highlights the formation of PCM-rich channels within the MgO matrix (µm-range), which provided a sufficient surface tension [12] to prevent leakage of the PCM during thermal cycling tests. Very fine PCM-rich channels (nm-range) can also be observed in the CPCM and probably contributed to the effective encapsulation of the PCM by capillary force [35]. These microstructural features can be explained by the use of light MgO as ceramic skeleton material, which presents a certain degree of porosity (although not evaluated in this work). Some of the PCM mixture can indeed be expected to flow into the pores of MgO due to the solid-liquid phase change transition occurring during the sintering

process [36]. Nonetheless, most of the PCM mixture will remain in the interparticle voids as described by Jiang et al. [36], which can explain the uneven distribution of Mg within the CPCM. Likewise, this can be expected to affect the thermal properties of the CPCM although specific thermal characterisation techniques would be necessary to investigate local thermal properties and their relationship with microstructural features of the CPCM.

Fig. 18 compares the latent heat of the developed CPCM with other medium-temperature CPCMs with similar phase change temperature (about 220 °C). Results highlight the superior latent heat values of NaNO₂-NaNO₃ eutectic compared to solar salt. This is explained by the rather low latent heat value of KNO₃ compared to both NaNO₂ and NaNO₃ [37,38]. It is also worth noting that our CPCM formulation has comparable latent heat to the solar salt/expanded graphite formulation of Xiao et al. [39] although the later do not confer shape-stabilisation effect and is prone to leakage.

## 4.2. Thermal decomposition of sodium nitrite

Both microstructural and thermal analyses indicate that the main degradation mechanism of the CPCM is associated with the thermal decomposition of sodium nitrite. However, observations reveal that the decomposition of the salt was not significant after thermal cycling at 350 °C while it was clearly observed after thermal cycling at 400 °C. At 400 °C, the decreasing concentration of sodium nitrite as a function of thermal cycling increases the melting temperature of the PCM as shown by progressively higher solidus and liquidus temperatures of the PCM in Fig. 14b. In other words, the decomposition of sodium nitrite shifts the NaNO₂-NaNO₃ eutectic salt composition towards a NaNO₃-rich mixture. This effect has also clear consequences on the solid-solid transition with a decreased latent heat as a function of thermal cycling at 400 °C. However, despite the overall decrease in latent heat of the CPCM this implies that the main degradation mechanism is not linked to the oxidation of NaNO₂ into NaNO₃ only. In fact, should it be the case, one could expect an increased latent heat value for the solid-liquid transition at the expense of the solid-solid transition. The oxidation of NaNO₂ into NaNO₃ would also lead to a mass increase of the CPCM samples, however, it is obvious that the decomposition of NaNO₂ is rather associated with a weight loss (Fig. 7b). Besides, XRD analyses did not show any significant increase in concentration for sodium nitrate in the CPCM cycled at 400 °C (see Fig. 9). It therefore appears that reaction (3) has
<figure><img src="image_20.png" /><figcaption>Fig. 17. Comparison of (a) thermal diffusivity and (b) effective thermal conductivity of the sintered CPCM before and after 48 thermal cycles at 350 or 400 °C.</figcaption></figure>

limited effect (if at all) on the thermal decomposition of NaNO₂ here. Our findings agree with the pioneer work of Stern on the thermal decomposition of nitrites/nitrates [40] in that only reaction (4) is to be expected if the gaseous product species are continuously removed such as in an open system. In a closed system, the reaction products would rather stabilise NaNO₃ and N₂ [40] such that the decomposition of NaNO₂ may be managed, at least at 400 °C. This agrees well with the DSC studies of HITEC® salt mixture in nitrogen atmospheres reported in the literature [41,42].

$$2\text{NaNO}_2\text{(l)} + \text{O}_2\text{(g)} \rightleftharpoons 2\text{NaNO}_3\text{(l)} \quad (3)$$

$$2\text{NaNO}_2\text{(l)} \rightleftharpoons \text{Na}_2\text{O(diss.)} + \text{NO}_2\text{(g)} + \text{NO(g)} \quad (4)$$

In addition, XRD and SEM/EDS analyses revealed the formation of Na₂CO₃ into the CPCM after thermal cycling up to 400 °C. The formation of Na₂CO₃ can be expected by the absorption of CO₂ into the molten salt and its reaction with dissolved sodium oxide following the Lux-Flood acid-base reaction [43,44]:

$$\text{Na}_2\text{O(diss.)} + \text{CO}_2\text{(diss.)} \rightleftharpoons \text{Na}_2\text{CO}_3\text{(s)} \quad (5)$$

While the storage capacity of the CPCM could be increased by increasing the maximum application temperature, this may not lead to the desired effect due to thermal decomposition of the salt. According to our experimental data, an increase of 20 °C of the maximum application temperature would only result in an increase of about 30 J/g of the
<figure><img src="image_21.png" /><figcaption>Fig. 18. Comparison of the latent heat values for different medium-temperature CPCMs with phase change at about 220–225 °C.</figcaption></figure>

CPCM's energy density (i.e. 5 %). The benefits of increasing the temperature for such an increase in energy density would be compromised by the associated risk of thermal decomposition due to higher operating temperatures. Therefore, a safety margin is considered by limiting the maximum application temperature to 350 °C, which strikes a balance between preventing salt decomposition and optimising the CPCM's energy density. It is therefore recommended that the CPCM developed in this study should be used up to 350 °C in an open system while, in principle, it could be used at higher temperature in a sealed system. Besides, this CPCM would be particularly suited to use in combination with a CPCM containing sodium nitrate for cascade latent heat storage systems. For instance, this CPCM could be used up to 300 °C and the CPCM containing sodium nitrate at above 300 °C thereby mitigating potential decomposition reactions of sodium nitrite.

### 4.3. Can solid-solid phase change of inorganic salt compounds be exploited for TES?

The originality of the developed CPCM lies in the occurrence of the solid-solid phase change of the sodium nitrite-sodium nitrate PCM. To the best of the authors' knowledge, exploration of solid-solid phase change of inorganic salt compounds has not been pursued in the literature as a TES material [45], although a few studies have reported latent heat data for CPCMs based on $KNO_3$ [36] or solar salt [39]. Therefore, we aim to bridge this gap. The reasons underlying the occurrence of a solid-solid transition in the developed CPCM can best be understood by referring to the phase diagram of $NaNO_2-NaNO_3$ [31]. In fact, the equimolar compound $Na_2(NO_2, NO_3)$ with an orthorhombic structure is expected to form during the cooling of a $NaNO_2-NaNO_3$ liquid phase. This compound has previously been identified by DSC by Zamaly and Jamal [30]. In our DSC experiments, we found that this compound could not form during the first heating curve, which could explain why studies showing only the heating curves could not detect it by DSC for similar $NaNO_2-NaNO_3$ mixture [37]. However, it could be detected during the first cooling scan as predicted by the $NaNO_2-NaNO_3$ phase diagram [31]. After the first heating-cooling cycle, the solid-solid transition could be observed by DSC in all tested samples with excellent reversibility. It is worth noting that the latent heat associated with this solid-solid transition is far from being negligible for the sintered CPCM (about 55 J/g) compared to about 65 J/g for the solid-liquid transition. This corresponds to about 46 % of the overall latent heat stored by the CPCM and contribute for 11 % to the overall energy density of the CPCM in the temperature range 80–350 °C. Thermal cycling results at 350 °C indicated that this transition is completely reversible in terms of latent heat

value although its onset temperature differs between heating (173 °C) and cooling (160 °C). As such, the 'pure' PCM used only for solid-solid transition shows a latent heat of 87.0 ± 1.5 J/g (average of 3 samples), which is higher than leakage-free CPCMs using solar salt [25,26] or $KNO_3$ [36,46] as PCMs.

### 5. Conclusions

Our work highlights the benefits of combining microstructural and thermal characterisation of a composite phase change material to gain a better understanding of its thermal stability and potential degradation mechanisms. In-depth microstructural characterisation of the CPCM indicates adequate thermal stability during thermal cycling tests in air up to 350 °C, which is also confirmed by stable latent heat and thermal conductivity values. This medium-temperature CPCM formulation based on $NaNO_2-NaNO_3$ eutectic salt mixture was developed for efficient waste heat recovery and heat transport. Here are the main findings of our work:

(1) The PCM investigated in this study presents both a solid-solid transition and a solid-liquid transition. This means that to be fully reversible, the minimum and maximum operating temperatures of the PCM should be lower than the solid-solid phase change temperature and higher than the solid-liquid phase change temperature, respectively.

(2) The solid-solid phase change of inorganic salt compounds can be considered as a credible contender for TES. Solid-solid PCMs offer several advantages over conventional solid-liquid PCMs, including (i) absence of leakage, (ii) no need for encapsulation and (iii) smaller volume variation. Future research should be dedicated to taking advantage of salt compounds presenting a solid-solid phase change to develop a new class of TES material.

(3) Special attention should be paid to salt mixtures containing sodium nitrite (e.g. HITEC®) and exposed to open systems to prevent its thermal decomposition. In practice, this restricts the use of such mixtures in air and open systems to 350 °C. The decomposition products are sodium oxide ($Na_2O$), nitric oxide (NO) and nitrogen dioxide ($NO_2$), which were found to react with the CPCM compounds in our work. For example, the precipitation of sodium carbonate within the CPCM when thermally cycled at 400 °C occurred due to acid-base reactions between dissolved $Na_2O$ and $CO_2$.

(4) While magnesium oxide was selected as the ceramic skeleton material in our study, other more cost-effective alternatives could
be considered. Recent works demonstrated that recycled materials or by-products (e.g. red mud residues, coal fly ash, demolition waste) could be a viable option to increase the economic profitability and sustainability of manufactured CPCMs. The manufacturing process of the CPCM modules is easily scalable and unexpensive and has therefore strong potential for commercialisation.

(5) We also encourage the community to systematically perform two heating-cooling cycles by DSC of at least three independent samples to give accurate and reliable latent heat data. In fact, some phase transitions may not be visible during the first heating scan hiding the true behaviour of the PCM for TES applications where materials are exposed to repeated charging/discharging cycles.

### CRediT authorship contribution statement

**Benjamin Grégoire:** Writing – original draft, Methodology, Investigation, Data curation, Conceptualization.
**Hongkun Ma:** Writing – original draft, Methodology, Investigation, Data curation.
**Anabel Palacios:** Writing – review & editing, Methodology, Investigation.
**Song Yang:** Writing – review & editing.
**Mengqi Bai:** Writing – review & editing.
**Tongtong Zhang:** Writing – review & editing, Supervision, Funding acquisition.
**Yulong Ding:** Writing – review & editing,

Supervision, Funding acquisition.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Yulong Ding reports financial support was provided by Engineering and Physical Sciences Research Council. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

### Acknowledgments

The work was partially supported by UKRI Engineering and Physical Sciences Research Council (EPSRC) under the Industrial Decarbonisation Challenge funds through a UK Industrial Decarbonisation Research and Innovation Centre (IDRIC) under EP/V027050/1 and Impact Acceleration Account (IAA).

## Appendix A. Thermal decomposition of sodium nitrite and associated mechanisms

Sodium nitrite is known to decompose in air in an open system at temperatures above 330-350 °C [40,41]. The thermal decomposition reactions usually reported in the literature follow either (i) the oxidation of sodium nitrite into sodium nitrate [41] and (ii) the dissociation of sodium nitrite into sodium oxide and nitrogen oxides [40,42,47] see reactions (A.1) and (A.2).

$$2\text{NaNO}_2(l) + \text{O}_2(g) \rightleftharpoons 2\text{NaNO}_3(l) \quad (A.1)$$

$$2\text{NaNO}_2(l) \rightleftharpoons \text{Na}_2\text{O(diss.)} + \text{NO}_2(g) + \text{NO(g)} \quad (A.2)$$

To prevent the oxidation of sodium nitrite, nitrogen is typically used as a cover gas in DSC experiments to obtain thermal data of this compound or salt mixtures containing sodium nitrite [41,48]. However, it is essential to note that the reverse reaction (sodium nitrate decomposition into sodium nitrite) is also expected to occur as shown by Bauer et al. at 450 °C [33]. Consequently, using a salt mixture containing both compounds (e.g. 55NaNO₂-45NaNO₃ in our study) would mean that reaction (A.1) is close to equilibrium either in air or under nitrogen [48]. Besides, while reaction (A.1) can be in principle controlled in nitrogen atmosphere, reaction (A.2) remains a concern even in a sealed nitrogen atmosphere due to the formation of gaseous nitrogen oxides until an equilibrium is reached and the associated pressure build-up [47]. In this regard, the influence of the cover gas (composition and open/sealed system) on the thermal decomposition of sodium nitrite and its compounds behaves similarly to solar salt, extensively studied by Bonk and co-authors [47,49]. They indeed reported that a certain partial pressure of gaseous nitrogen oxides is necessary for the stabilisation of solar salt at high temperature (i.e. higher than 565 °C, its current operating temperature in CSP plants) under semi-closed vessels [49]. The thermal decomposition reactions are expected to proceed to the products side for as long as the products are either consumed (i.e. further chemical reaction) or removed (venting of gaseous nitrogen oxides in an open system) following Le Chatelier's principle. Even though the CPCM based on NaNO₂-NaNO₃ eutectic is not expected to reach such high temperatures, it is likely that similar mechanisms operate. In an open system, it can therefore be expected that the thermal decomposition of sodium nitrite is a limiting factor for the maximum operating temperature of the CPCM developed in our study. This rationale can also be extended to the maximum operating temperature of the HITEC® salt mixture (53KNO₃-40NaNO₂-7NaNO₃, weight %) since both HITEC® and the PCM in this study (55NaNO₂-45NaNO₃) have a close mass fraction of sodium nitrite.

Following the decomposition reactions of sodium nitrite, the formation of Na₂CO₃ into the CPCM after thermal cycling up to 400 °C is expected by the absorption of CO₂ into the molten salt and its reaction with dissolved sodium oxide following the Lux-Flood acid-base reaction [43,44]:

$$\text{Na}_2\text{O(diss.)} + \text{CO}_2\text{(diss.)} \rightleftharpoons \text{Na}_2\text{CO}_3\text{(s)} \quad (A.3)$$

Based on our experimental conditions, CO₂ can have two different origins as (i) atmospheric CO₂ or (ii) as a decomposition product of the oxidation of expanded graphite. While we may not be able to differentiate both origins, it appeared by SEM that Na₂CO₃ tend to precipitate in the vicinity of expanded graphite particles as illustrated in Fig. 11b. Taking into account that NO can interact with graphite surfaces at 400 °C releasing CO and CO₂ [50], it is reasonable to assume that the presence of graphite may lead to an increased concentration of CO₂. The formation of CO₂ could be further promoted by the Boudouard reaction favoured to the generation of CO₂ at 400 °C (A.5). This can be summarised into the following two interdependent reactions:

$$2\text{C(s)} + 3\text{NO(g)} \rightleftharpoons \text{CO}_2\text{(g)} + \text{CO(g)} + \frac{3}{2}\text{N}_2\text{(g)} \quad (A.4)$$

$$2\text{CO(g)} \rightleftharpoons \text{CO}_2\text{(g)} + \text{C(s)} \quad (A.5)$$

This two-stage mechanism could contribute both to the consumption of NO in reaction (A.4) and Na₂O in reaction (A.3), which in turn would shift
the reaction (A.2) to the decomposition of NaNO₂ following Le Chatelier's principle. Our findings would therefore suggest that the presence of graphite may accelerate the decomposition of the PCM above 350 °C, which highlights the importance of material compatibility to achieve performant and durable CPCMs. To the best of the authors' knowledge, this has not been reported despite the extensive use of sodium nitrate and graphite in CPCM formulations. The thermal decomposition of sodium nitrite at 400 °C and possibly the partial decomposition of expanded graphite is also responsible for the decreased thermal conductivity of the CPCM as shown in Fig. 17b. This effect can be explained by the associated increased porosity, reduced density and progressive loss of the TCEM (i.e. expanded graphite).

## References

[1] J.I. Chowdhury, Y. Hu, I. Haltas, N. Balta-Ozkan, G. Matthew, L. Varga, Reducing industrial energy demand in the UK: a review of energy efficiency technologies and energy saving potential in selected sectors, Renew. Sust. Energ. Rev. 94 (2018) 1153-1178, https://doi.org/10.1016/j.rser.2018.06.040.

[2] H. Jouhara, N. Khordehgha, S. Almahmoud, B. Delpech, A. Chauhan, S.A. Tassou, Waste heat recovery technologies and applications, Thermal Science and Engineering Progress 6 (2018) 268-289, https://doi.org/10.1016/j. tsep.2018.04.017.

[3] Element Energy Limited, Ecofys, Imperial College London, Stevenson P, Hyde R, The potential for recovering and using surplus heat from industry, 2014.

[4] P. Zhang, X. Xiao, Z.W. Ma, A review of the composite phase change materials: fabrication, characterization, mathematical modeling and application to performance enhancement, Appl. Energy 165 (2016) 472-510, https://doi.org/ 10.1016/j.apenergy.2015.12.043.

[5] C. Cárdenas-Ramírez, F. Jaramillo, M. Gómez, Systematic review of encapsulation and shape-stabilization of phase change materials, J Energy Storage (2020) 30, https://doi.org/10.1016/j.est.2020.101495.

[6] Z. Jiang, A. Palacios, B. Zou, Y. Zhao, W. Deng, X. Zhang, et al., A review on the fabrication methods for structurally stabilised composite phase change materials and their impacts on the properties of materials, Renew. Sust. Energ. Rev. (2022) 159, https://doi.org/10.1016/j.rser.2022.112134.

[7] Navarro ME, Trujillo AP, Jiang Z, Jin Y, Zhang Y, Ding Y. Chapter 7. Manufacture of Thermal Energy Storage Materials, 2021, p. 121-90. doi: https://doi.org/ 10.1039/9781788019842-0012.

[8] IRENA. Innovation Outlook: Thermal Energy Storage. 2020.

[9] K. Du, J. Calautit, P. Eames, Y. Wu, A state-of-the-art review of the application of phase change materials (PCM) in mobilized-thermal energy storage (M-TES) for recovering low-temperature industrial waste heat (IWH) for distributed heat supply, Renew. Energy 168 (2021) 1040-1057, https://doi.org/10.1016/j. rene.2020.12.057.

[10] Y. Xiao, K. Zhang, Y. Zhang, W. Zhou, W. Liu, F. Wan, Research on control method of waste heat utilization system based on multi-parameter coupling, Sci. Rep. (2022) 12, https://doi.org/10.1038/s41598-022-15808-0.

[11] J. Pereira da Cunha, P. Eames, Thermal energy storage for low and medium temperature applications using phase change materials - a review, Appl. Energy 177 (2016) 227-238, https://doi.org/10.1016/j.apenergy.2016.05.097.

[12] Z. Ge, F. Ye, H. Cao, G. Leng, Y. Qin, Y. Ding, Carbonate-salt-based composite materials for medium- and high-temperature thermal energy storage, Particuology 15 (2014) 77-81, https://doi.org/10.1016/j.partic.2013.09.002.

[13] M. Liu, Q. Jin, P. Shen, Cold sintering of NaNO₃/MgO heat-storage composite, Ceram. Int. 46 (2020) 28955-28960, https://doi.org/10.1016/j. ceramint.2020.08.066.

[14] J. Lu, Z. Zhang, W. Wang, J. Ding, Effects of MgO nanoparticles on Thermo-physical properties of LiNiO₂-NaNO₃-KNO₃ for thermal energy storage, Energies (Basel) 14 (2021) 677, https://doi.org/10.3390/en14030677.

[15] A.J. Slifka, B.J. Filla, J.M. Phelps, Thermal conductivity of magnesium oxide from absolute, steady-state measurements, J Res Natl Inst Stand Technol 103 (1998) 357, https://doi.org/10.6028/jres.103.021.

[16] L. Zhong, X. Zhang, Y. Luan, G. Wang, Y. Feng, D. Feng, Preparation and thermal properties of porous heterogeneous composite phase change materials based on molten salts/expanded graphite, Sol. Energy 107 (2014) 63-73, https://doi.org/ 10.1016/j.solener.2014.05.019.

[17] X. Xiao, P. Zhang, M. Li, Experimental and numerical study of heat transfer performance of nitrate/expanded graphite composite PCM for solar energy storage, Energy Convers. Manag. 105 (2015) 272-284, https://doi.org/10.1016/j. enconman.2015.07.074.

[18] Q. Guo, T. Wang, Study on preparation and thermal properties of sodium nitrate/silica composite as shape-stabilized phase change material, Thermochim. Acta 613 (2015) 66-70, https://doi.org/10.1016/j.tca.2015.05.023.

[19] R. Li, J. Zhu, W. Zhou, X. Cheng, Y. Li, Thermal properties of sodium nitrate- expanded vermiculite form-stable composite phase change materials, Mater. Des. 104 (2016) 190-196, https://doi.org/10.1016/j.matdes.2016.05.039.

[20] G. Xu, G. Leng, C. Yang, Y. Qin, Y. Wu, H. Chen, et al., Sodium nitrate - diatomite composite materials for thermal energy storage, Sol. Energy 146 (2017) 494-502, https://doi.org/10.1016/j.solener.2017.03.003.

[21] Q. Yu, Z. Jiang, L. Cong, T. Lu, B. Suleiman, G. Leng, et al., A novel low-temperature fabrication approach of composite phase change materials for high temperature thermal energy storage, Appl. Energy 237 (2019) 367-377, https://doi.org/10.1016/j.apenergy.2018.12.072.

[22] F. Jiang, Z. Ge, X. Ling, D. Cang, L. Zhang, Y. Ding, Improved thermophysical properties of shape-stabilized NaNO₃ using a modified diatomite-based porous ceramic for solar thermal energy storage, Renew. Energy 179 (2021) 327-338, https://doi.org/10.1016/j.renene.2021.07.023.

[23] F. Jiang, X. Ling, L. Zhang, D. Cang, Y. Ding, Improved thermal conductivity of form-stable NaNO₃: using the skeleton of porous ceramic modified by SiC, Sol. Energy Mater. Sol. Cells (2021) 231, https://doi.org/10.1016/j.solmat.2021.111310.

[24] F. Jiang, X. Ling, L. Zhang, D. Cang, Y. Ding, Modified diatomite-based porous ceramic to develop shape-stabilized NaNO₃ salt with enhanced thermal conductivity for thermal energy storage, Sol. Energy Mater. Sol. Cells (2021) 231, https://doi.org/10.1016/j.solmat.2021.111328.

[25] A. Anagnostopoulos, M.E. Navarro, M. Stefanidou, Y. Ding, G. Gaidajis, Red mud-molten salt composites for medium-high temperature thermal energy storage and waste heat recovery applications, J. Hazard. Mater. (2021) 413, https://doi.org/ 10.1016/j.jhazmat.2021.125407.

[26] A. Anagnostopoulos, M.E. Navarro, M. Stefanidou, P. Seferlis, G. Gaidajis, Y. Ding, Effect of carbon on the performance of red mud-molten salt composites for thermal management and waste heat recovery applications, J Energy Storage 44 (2021) 103363, https://doi.org/10.1016/j.est.2021.103363.

[27] S. Soleimanpour, S.M. Sadrameli, S.A.H.S. Mousavi, M. Jafaripour, Preparation and characterization of high temperature shape stable NaNO₃/diatomite phase change materials with nanoparticles for solar energy storage applications, J Energy Storage 45 (2022) 103735, https://doi.org/10.1016/j.est.2021.103735.

[28] Y. Li, S.L. Jiang, C.G. Wang, Q.Z. Zhu, Effect of EG particle size on the thermal properties of NaNO₃-NaCl/EG shaped composite phase change materials, Energy (2022) 239, https://doi.org/10.1016/j energy.2021.122062.

[29] G. Boissonnet, C. Chalk, J.R. Nicholls, G. Bonnet, F. Pedraza, Thermal insulation of YSZ and Erbia-doped Yttria-stabilised zirconia EB-PVD thermal barrier coating systems after CMAS attack, Materials 13 (2020) 4382, https://doi.org/10.3390/ ma13194382.

[30] H. Zamaly, M. Jemal, Diagramme de phases du système binaire NaNO₂-NaNO₃, Journal de La Société Chimique de Tunisie 8 (1982) 17-21.

[31] R.W. Berg, D.H. Kerridge, P.H. Larsen, NaNO₂ + NaNO₃ phase diagram: new data from DSC and raman spectroscopy, J. Chem. Eng. Data 51 (2006) 34-39, https:// doi.org/10.1021/je050105n.

[32] L. Kourkova, R. Svoboda, G. Sadovska, V. Podzemna, A. Kohutova, Heat capacity of NaNO₂, Thermochim. Acta 491 (2009) 80-83, https://doi.org/10.1016/j. tca.2009.03.005.

[33] T. Bauer, D. Laing, R. Tamme, Characterization of sodium nitrate as phase change material, Int. J. Thermophys. 33 (2012) 91-104, https://doi.org/10.1007/s10765- 011-1113-9.

[34] B. D'Aguanno, M. Karthik, A.N. Grace, A. Floris, Thermostatic properties of nitrate molten salts and their solar and eutectic mixtures, Sci. Rep. (2018) 8, https://doi. org/10.1038/s41598-018-28641-1.

[35] X. Chen, P. Cheng, Z. Tang, X. Xu, H. Gao, G. Wang, Carbon-based composite phase change materials for thermal energy storage, transfer, and conversion. Advanced, Science (2021) 8, https://doi.org/10.1002/advs.202001274.

[36] Z. Jiang, M.E. Navarro Rivero, A. Anagnostopoulos, X. She, X. Liu, Y. Xuan, et al., Fabrication of form stable composite phase change materials for thermal energy storage by direct powder incorporation with a preheating process, Powder Technol. 391 (2021) 544-556, https://doi.org/10.1016/j.powtec.2021.06.030.

[37] M.A. Orozco, K. Acurio, F. Vásquez-Aza, J. Martínez-Gómez, A. Chico-Proano, Thermal storage of nitrate salts as phase change materials (PCMs), Materials (2021) 14, https://doi.org/10.3390/ma14237223.

[38] M.M. Kenisarin, High-temperature phase change materials for thermal energy storage, Renew. Sust. Energ. Rev. 14 (2010) 955-970, https://doi.org/10.1016/j. rser.2009.11.011.

[39] Z. Xiao, J. Huang, P. Zhu, C. Wang, X. Li, Preparation, characterization and thermal properties of binary nitrate salts/expanded graphite as composite phase change material, Thermochim. Acta 587 (2014) 52-58, https://doi.org/10.1016/j. tca.2014.04.021.

[40] K.H. Stern, High temperature properties and decomposition of inorganic salts part 3, nitrates and nitrites, J. Phys. Chem. Ref. Data Monogr. 1 (1972) 747-772, https://doi.org/10.1063/1.3253104.

[41] A.G. Fernández, H. Galleguillos, E. Fuentealba, F.J. Pérez, Thermal characterization of HITEC molten salt for energy storage in solar linear concentrated technology, J. Therm. Anal. Calorim. 122 (2015) 3-9, https://doi. org/10.1007/s10973-015-4715-9.

[42] C. Villada, A. Bonk, T. Bauer, F. Bolívar, High-temperature stability of nitrate/ nitrite molten salt mixtures under different atmospheres, Appl. Energy 226 (2018) 107-115, https://doi.org/10.1016/j.apenergy.2018.05.101.

[43] R.I. Olivares, The thermal stability of molten nitrite/nitrates salt for solar thermal energy storage in different atmospheres, Sol. Energy 86 (2012) 2576-2583, https://doi.org/10.1016/j.solener.2012.05.025.

[44] T. Yokokawa, Gas solubilities in molten salts and silicates. 58 (1986), https://doi. org/10.1351/pac198658121547.

[45] A. Fallahi, G. Guldenops, M. Tao, S. Granados-Focil, S. Van Dessel, Review on solid-solid phase change materials for thermal energy storage: molecular structure
and thermal properties, Appl. Therm. Eng. 127 (2017) 1427-1441, https://doi.org/10.1016/j.applthermaleng.2017.08.161.

[46] F. Jiang, L. Zhang, D. Cang, X. Ling, Y. Ding, Preparation and characterization of a heat storage material: shape-stabilized KNO3 using a modified diatomite-based porous ceramic as the skeleton, Ceram. Int. 47 (2021) 26301-26309, https://doi.org/10.1016/j.ceramint.2021.06.040.

[47] Bonk A, Braun M, Sötz VA, Bauer T. Solar Salt - Pushing an old material for energy storage to a new limit. Appl. Energy 2020;262. doi:https://doi.org/10.1016/j.apenergy.2020.114535.

[48] C. Villada, F. Jaramillo, J.G. Castaño, F. Echeverría, F. Bolívar, Design and development of nitrate-nitrite based molten salts for concentrating solar power applications, Sol. Energy 188 (2019) 291-299, https://doi.org/10.1016/j.solener.2019.06.010.

[49] J. Steinbrecher, A. Bonk, V.A. Sötz, T. Bauer, Investigation of regeneration mechanisms of aged solar salt, Materials 14 (2021) 5664, https://doi.org/10.3390/ma14195664.

[50] C.E. Brown, P.G. Hall, Reaction of nitric oxide with a graphite surface, and the adsorptive properties of the resultant surface, Surf. Sci. (1972) 30, https://doi.org/10.1016/0039-6028(72)90008-8.