Development of Macro-Encapsulated Phase-Change Material Using Composite of NaCl-Al₂O₃ with Characteristics of Self-Standing

Abstract

Developing thermal storage materials is crucial for the efficient recovery of thermal energy. Salt-based phase-change materials have been widely studied. Despite their high thermal storage density and low cost, they still face issues such as low thermal conductivity and easy leaks. Therefore, a new type of NaCl-Al₂O₃@SiC@Al₂O₃ macrocapsule was developed to address these drawbacks, and it exhibited excellent rapid heat storage and release capabilities and was extremely stable, significantly reducing the risk of leakage at high temperatures for industrial waste heat recovery and in concentrated solar power systems above 800 °C. Thermal storage macrocapsules consisted of a double-layer encapsulation of silicon carbide and alumina and a self-standing core of NaCl-Al₂O_3. After enduring over 1000 h at a high temperature of 850 °C, the encapsulated phase-change material exhibited an extremely low weight loss rate of less than 5% compared with NaCl@Al₂O₃ and NaCl-Al₂O₃@Al₂O₃ macrocapsules, for which the weight loss rate was reduced by 25% and 10%, respectively, proving their excellent leakage prevention. The SiC powder layer, serving as an intermediate coating, further prevented leakage, while the use of Al₂O₃ ceramics for encapsulation enhanced the overall mechanical strength. It was innovatively discovered that the Al₂O₃ particles formed a network structure around the molten NaCl, playing an important role in maintaining the shape and preventing leakage of the composite thermal storage phase-change material. Furthermore, the addition of Al₂O₃ significantly enhanced the rapid heat storage and release rate of NaCl-Al₂O₃ compared to pure NaCl. This encapsulated phase-change material demonstrated outstanding durability and rapid heat storage and release performance, offering an innovative approach to the application of salt phase-change materials in the field of high temperature rapid heat storage and release and encapsulating NaCl as a high-temperature thermal storage material in a packed bed system. Compared with conventional salt-based phase-change materials, the developed product is expected to significantly improve the reliability and thermal efficiency of thermal storage systems.

1. Introduction

With the acceleration of global industrial development since the 1970s, the world has faced challenges such as energy crises, massive energy consumption, and the emission of greenhouse gases [1]. Hence, renewable energy technologies such as solar energy and waste heat recovery are drawing increased attention. Solar and wind energy are ideal choices for renewable energy, with the advantages of being environmentally friendly and having low operational costs. However, both are significantly influenced by geography and climate, and they have intermittency issues that prevent them from providing a stable energy supply [2]. Therefore, developing stable and efficient energy storage technologies to address these challenges is essential. Thermal energy storage (TES) technology solves the inconsistency in heat supply and demand in terms of time and space, enhancing the flexibility of thermal energy utilization [3]. This technology is widely used in various thermal energy utilization fields, such as thermal storage in solar thermal power stations, solar power generation, and waste heat recovery [4,5,6]. Thermal energy storage (TES) technologies are categorized according to their mechanism as sensible heat storage, latent heat storage, and chemical heat storage. Chemical heat storage stores thermal energy by absorbing or releasing heat during chemical reactions, allowing for energy storage and release over a wide temperature range and offering very high energy storage density. However, this process requires catalysts and special conditions, and challenges include the stability of materials and controllability of reactions. Sensible heat storage is a technology that utilizes the temperature change in a material itself to store or release thermal energy. This method is straightforward, cost-effective, and technically simpler, but it has drawbacks such as low energy storage density, a short storage duration, a wide temperature fluctuation range, and non-isothermal heat release. For latent heat storage, phase-change materials store and release heat during the phase-change process. Compared to other technologies, phase-change thermal storage materials are characterized by high thermal density and simple operation. Moreover, the process of heat storage and release maintains a steady temperature, optimizing energy conservation by minimizing energy gradient losses [7,8,9,10].

High-melting-point phase-change materials (PCMs) are essential components in high-temperature heat storage systems [11,12]. Inorganic salt phase-change materials are widely considered for thermal energy storage due to their high latent heat of phase change, stability, and lower costs. However, they exhibit drawbacks such as the subcooling phenomenon, potential corrosion, low thermal conductivity, and possibly insufficient thermal stability at high temperatures [13,14]. A promising solution to these challenges lies in capsule technology. This technique involves the encapsulation of PCMs within a protective shell composed of various organic polymers or inorganic materials, resulting in the formation of phase-change microcapsules. Within these microcapsules, the PCM is encapsulated, effectively mitigating issues related to material leakage, phase separation, and corrosion [15,16]. Chatura et al. propose a novel method of encapsulating high-melting-point chloride-based phase-change materials using cost-effective ceramics. The ceramics demonstrate significant thermal and chemical stability under molten-salt conditions. The encapsulated PCM, tested through over 150 thermal cycles, showed enduring thermo-physical properties without notable degradation. However, the material processing of this study is complex and needs to be determined based on the porosity distribution of the system [17]. Yao et al. developed a composite material combining MgO, a metallic microcapsule (Sn@Void@SiO₂), and eutectic salt (LiNO₃-NaCl) was developed using cold-pressure sintering. Adding 30 wt% Sn@Void@SiO₂ significantly improved the composite’s latent heat capacity and thermal conductivity, with increases of 56.72% and 63.79%, but the durability and stability of the microcapsule at high temperatures have not been studied in depth [18]. Sheng et al. prepared a method for making centimeter-sized macrocapsules with a double-shell coating, consisting of an inner expanded graphite layer and an outer Al₂O₃ layer on NaCl core spheres. These capsules, shaped using organic adhesives, effectively prevent salt leakage and demonstrate high stability and durability, maintaining their integrity even after 100 cyclic tests, but the microcapsule does not significantly improve the low thermal conductivity of NaCl [19]. Yamashita et al. developed a composite phase-change material of NaCl-Al₂O₃, which could keep its shape stable even when it reached the melting point temperature of NaCl, but did not test the salt composite phase-change material for the encapsulation and the durability of this PCM material [20]. Some of these studies require relatively complex processes to manufacture the capsules. From the perspective of thermal storage performance, the use of carrier materials should be minimized. The use of metals and organic binders can introduce issues such as corrosion and increased energy consumption. Additionally, some studies have not comprehensively addressed the issues of NaCl leakage and low thermal conductivity. Therefore, there is an urgent need to develop a salt-based phase-change macrocapsule that is easy to manufacture industrially, cost-effective, resistant to leakage at high temperatures, and has good thermal conductivity.

To both solve the problems of the low thermal conduction rate of NaCl and leakage, this study innovatively develops a macro-encapsulated phase-change material using a composite of NaCl-Al₂O₃, which can maintain a high retention of shape even in the molten state. The addition of Al₂O₃ particles enhances the rapid heat storage and release performance of the composite phase-change material, while a SiC powder layer applied on the outer surface serves as an intermediate coating to prevent leakage. This heat storage material is simple to prepare, inexpensive, and does not require the addition of additional organic carrier materials. This study greatly improves the efficiency of the NaCl-based capsule for rapid heat storage and release, and it is a promising heat storage material for application in the field of high-temperature rapid heat storage and heat release. It is expected to realize the industrial application of high-temperature packed bed heat storage systems.

2. Experimental

2.1. Preparation

The preparation process of the core–shell high-temperature phase-change macrocapsule is shown in Figure 1. For the core material, NaCl and Al₂O₃ powders (AES-12, Sumitomo Chemical, Tokyo, Japan) were mixed in a volume ratio of 8:2, then combined with 1wt% CMC (Granulation Binder, CMF-7, ASONE, Tuttlingen, Germany), stirred, rolled, and kneaded into spheres. These spheres were dried at room temperature and subsequently at 90 °C for one hour. The resultant mixture was then blended with SiC powder (DrySiC-85D-RE, AGC Inc., Tokyo, Japan) and 1wt% CMC, continuing the rolling and kneading process to form spheres with a diameter of approximately 16 mm. After drying at room temperature and further drying at 70 °C for one hour, the spheres were placed inside Al₂O₃ ceramic shells with a diameter of 25 mm. In the processing of these Al₂O₃ ceramic shells, injection molding and dewaxing techniques were adopted. Post sintering, no additional mechanical processing was required, simplifying the methodology and enhancing environmental friendliness. The obtained hemispherical shells featured precise threading at the connecting parts, facilitating the sealing of the PCM core through the rotational engagement of the threads. The hemispherical Al₂O₃ ceramic shells had a wall thickness of 2 mm, an external diameter of 25 mm, and an internal diameter of 21 mm.

2.2. Characterization

2.2.1. Microstructure and Phase Analysis of Sample

The morphology of the entire thermal energy storage sphere was examined using X-ray computed tomography (X-CT, SKYSCAN, BRUKER, Billerica, MA, USA). The phase composition of the NaCl-Al₂O₃ core was characterized using X-ray diffraction (XRD, Ultima IV, Rigaku, Tokyo, Japan). The micro-morphology of the contact surface between the SiC coating with the NaCl-Al₂O₃ core material and NaCl-Al₂O₃ core material was observed through scanning electron microscopy (SEM, S-4300, Hitachi, Tokyo, Japan) and energy-dispersive X-ray spectroscopy (EDX; JEOL, JED-2300T, Peabody, MA, USA).

Figure 1. The preparation process of NaCl-Al₂O₃@SiC@Al₂O₃ macrocapsules.

Figure 1. The preparation process of NaCl-Al₂O₃@SiC@Al₂O₃ macrocapsules.

2.2.2. Performance Testing of Samples

To verify the exceptional durability of the core–shell high-temperature phase-change macrocapsule under prolonged high-temperature conditions, comparative experiments were carried out with two types of phase-change material spheres. Both spheres were encapsulated by the same Al₂O₃ ceramic shell but featured different internal structures. One sphere was designed with a core consisting solely of NaCl powder, directly enveloped by the ceramic shell. In contrast, the other sphere incorporated a core composed of a blended powder of NaCl and Al₂O₃ at a volume ratio of 8:2, also encapsulated by the ceramic shell. The weights of three different types of thermal energy storage spheres were initially recorded. These spheres were then placed in a high-temperature furnace, exposed to 850 °C in an air atmosphere for 1000 h. The measurement of weight loss began after the initial 100 h of heating, allowing for the assessment of each sphere’s durability and stability.

To assess the durability of the core–shell high-temperature phase-change macrocapsule, comparative experiments were conducted on the heat transfer rates of samples before and after enduring high-temperature treatment at 850 °C for 1000 h. Thermocouples (TAKAHASHI-thermo Co., Ltd., Abashiri, Japan; k-type; 0.5mm diameter) were inserted into the core of two samples. Subsequently, they were placed in a high-temperature furnace at 850 °C for heating. When the thermocouples indicated a temperature nearing 850 °C, the samples were promptly removed and allowed to cool to 400 °C at room temperature and then reinserted into the high-temperature furnace for additional heating. This process was repeated for two cycles. A schematic diagram of the rapid heat storage and release performance test is shown in Figure 2. In order to compare the thermal conductivity rates of NaCl and NaCl-Al₂O₃, operations were also conducted following the aforementioned method.

3. Results and Discussion

3.1. The Rapid Heat Storage and Release Performance

The Al₂O₃ ceramic spheres filled with the same volume of core material NaCl and core material NaCl-Al₂O₃ heated at 850 °C for 24 h were put into a high-temperature furnace at 850 °C, and the melting point of both of them was about 800 °C. It can be found that the heating rate of NaCl-Al₂O₃ is faster than that of NaCl, and it is the first to reach its melting point. Meanwhile, when the samples are cooled down from the melting point, the cooling rate of NaCl-Al₂O₃ is also obviously faster than that of the NaCl, as shown in Figure 3. This indicates that the rapid heat storage and release rate of NaCl-Al₂O₃, augmented by the addition of Al₂O₃ particles, is notably higher than that of pure NaCl, leading to an improvement in thermal conductivity and effectively addressing the issue of NaCl’s lower thermal conductivity.

3.2. Leakage Prevention Test and Improvement Mechanism

Figure 4 and

Table 1. The rate of weight loss (wt%) under high temperature exposure at 850 °C.

Sample	25 h	100 h	150 h	200 h	400 h	700 h	1000 h
A	0	2.2	27.9	28.1	28.1	-	-
B	0	4	13.8	13.9	14.1	14.1	-
C	0	0	0	0	0	2.3	3.6

compare the weight change in three different types of heat storage spheres after high-temperature treatment. Sample A consists solely of NaCl powder as the core material and is encapsulated by an Al₂O₃ ceramic shell. Sample B uses a mixture of NaCl and Al₂O₃ powder in a volume ratio of 8:2 as the core material, also enveloped by an Al₂O₃ ceramic shell of the same specification. Sample C, the target of this study, has the same core material specifications as sample B, but with an additional layer of SiC powder as a coating on the outer surface, and is finally encapsulated by an Al₂O₃ ceramic shell. After the three test samples were heated at 850 °C for 100 h, the samples were removed, and their weights were recorded separately, calculating the weight changes. It was found that both samples A and B had a certain degree of weight loss, less than 5%, while sample C showed no change. However, between 100 and 150 h of heating, the weight loss rate of Samples A and B sharply increased. The weight reduction rate of sample A increased from 2.2% to 27.9%. This is due to the excellent wettability of molten NaCl on the surface of Al₂O₃ in the core–shell structure using NaCl as the core material and Al₂O₃ ceramic as the shell, which seems impossible to prevent the leakage of molten NaCl. Additionally, another reason for the difficulty in successfully creating an encapsulated structure in sample A is the significant volumetric expansion of NaCl during the phase change from solid to liquid, approximately 39%, which greatly affects the leakage of NaCl and the strength damage of the shell material. After 150 h, the weight loss ceased, showing a trend of stabilization. For sample B, the weight loss rate increased from 4% to 13.8% between 100 and 150 h of heating, and no significant weight changes were observed beyond 150 h. Compared to sample A, sample B had a much lower rate of weight loss and less core material leakage. It is speculated that the addition of Al₂O₃ powder could mitigate the leakage of core materials during high-temperature heating processes. For sample C, between the initial heating and 400 h, the weight loss rate was 0, indicating that no core material leakage occurred. From 400 to 1100 h, the weight began to decrease slowly, the rate of weight loss after 700 and 1000 h of high temperature exposure was 2.3% and 3.6%, respectively, and after 1100 h of extended heating, the weight loss rate was below 5%. Compared with samples A and B, the durability performance of sample C significantly improved. This further confirms that after adding a SiC coating layer to the NaCl-Al₂O₃ core material, the SiC layer can prevent the leakage of core materials.

An X-ray computed tomography image of the core–shell high-temperature phase-change macrocapsule at 850 °C for 1000 h is shown in Figure 5. Figure 5a shows the appearance of the core–shell high-temperature phase-change macrocapsule after prolonged high-temperature exposure, revealing an Al₂O₃ ceramic shell as the outermost black layer, a gray SiC coating in the middle, and the NaCl-Al₂O₃ core material appearing in white. Figure 5b–d shows the tri-layer structure of the thermal storage sphere with clarity. There exists a noticeable gap between the SiC coating and the Al₂O₃ ceramic shell, which are not in contact. The Al₂O₃ ceramic shell exhibits stability in its physical properties after extended thermal treatment, evidenced by the absence of cracks and defects. In contrast, the SiC layer shows some degree of cracking, potentially due to the rapid expansion of NaCl during melting, forcing the SiC layer to expand. A composite layer structure of SiC-NaCl-Al₂O₃ is also observed at the interface between the SiC coating and the core material. The NaCl-Al₂O₃ core maintains its spherical shape with negligible deformation when compared to its pre-heating state, even after enduring temperatures of 850 °C for over 1000 h. This high retention of form suggests that the incorporation of a minor proportion of Al₂O₃ particles fosters a new NaCl-Al₂O₃ composite structure. This composite configuration remains stable in shape and performance despite long-duration thermal exposure. By combining NaCl with Al₂O₃ in an 8:2 volumetric ratio, the formulation not only ensures a high latent heat density attributable to the considerable content of NaCl but also stabilizes the core material’s properties, culminating in a robust phase-change composite structure.

Figure 6(a1–a4) shows the cross-sectional SEM and EDS images of the NaCl-Al₂O₃ core material of the EPCM after heating at 850 °C for 2 h. From Figure 6(a1), it is observed that the surface exhibits no pores or defects, and the structure is very dense. From Figure 6(a2–a4), it is visible that Al₂O₃ particles form variably shaped networked chain structures by partitioning the NaCl into multiple sections. The portions of NaCl surrounded by the Al₂O₃ networked chain structure display various shapes, including hexagonal and rectangular. Figure 6(a5) shows the elemental weight composition of the sample. The weight percentages of Na, Cl, Al, and O are 25.43%, 34.49%, 14.65%, and 25.44%, respectively, leading to a volumetric ratio of NaCl and Al₂O₃ at approximately 82% and 18%.

Figure 6(b1–b4) presents the cross-sectional SEM and EDS images of the NaCl-Al₂O₃ core material of the EPCM after heating at 850 °C for 1000 h. From Figure 6(b1–b4), it is noted that the structure remains dense with no pores or defects after undergoing high-temperature treatment for 1000 h, and the networked chain structure formed by Al₂O₃ particles around NaCl remains intact and stable. From the elemental weight composition shown in Figure 6(b5), the weight percentages of Na, Cl, Al, and O are 25.59%, 33.73%, 15.83%, and 24.86%, respectively. This suggests that the volumetric ratios of NaCl and Al₂O₃ are approximately 80% and 20%. Comparing the volumetric ratios of NaCl and Al₂O₃ in the samples heated for just 2 h at 850 °C, the change is minimal, indicating that the EPCM, even after prolonged high-temperature exposure, almost does not undergo leakage or structural changes. Combined with the morphology of the NaCl-Al₂O₃ core material shown in Figure 5 X-CT result, it can be inferred that the networked chain structure formed by Al₂O₃ particles around NaCl is crucial for maintaining the stability and durability of the core material. Even when NaCl is in a molten state at temperatures above its melting point for extended periods, it acts like a chain that fixes NaCl in place, thus preventing leakage and maintaining a stable structure, and the schematic is shown in Figure 6c.

Meanwhile, the rapid heat storage and release rate of NaCl-Al₂O₃ is significantly improved, and there may be two other reasons for this. One is because Al₂O₃ itself has high thermal conductivity [21], which enhances the overall thermal conductivity. The second is because, due to the addition of Al₂O₃, a mesh bridging network is formed as shown in Figure 6. This network improves the pathways for thermal conduction within the material. Pure NaCl lacks this conductive network, resulting in a relatively lower thermal conductivity [22,23]. Furthermore, the high thermal conductivity of Al₂O₃ and its distribution within the core material provide more efficient channels for heat propagation [24], significantly enhancing the overall thermal conductivity of the core material. Therefore, the introduction of Al₂O₃ particles into NaCl not only maintains its shape but also constructs a more effective thermal conduction network, significantly enhancing the rate of heat transfer. This underlines the considerable potential of the NaCl-Al₂O₃ core material for applications in high-performance thermal management.

Figure 7(a1,a2) shows the appearance of NaCl-Al₂O₃@SiC and SiC coatings after heating at 850 °C for 1000 h. It can be observed that the core material NaCl-Al₂O₃ is closely bonded to the SiC coating. After removing NaCl-Al₂O₃, some white substances were found attached to the surface of the SiC coating. Figure 7(a3) presents a schematic of the contact surface between the core material NaCl-Al₂O₃ and the SiC coating. The SEM image and EDS mapping image of the contact surface between the SiC coating with the NaCl-Al₂O₃ core material after heating at 850 °C for 1000 h are shown in Figure 7(b1–b4). It can be found that the NaCl-Al₂O₃ core material only adheres to the contact surface with the SiC coating and does not penetrate into the SiC coating. This suggests that even after prolonged high-temperature heating, the molten NaCl-Al₂O₃ core material does not permeate the SiC coating.

According to the results shown in Figure 4, sample C (composed of NaCl-Al₂O₃ core material with an added SiC coating) exhibited a weight loss rate reduced to below 5% after heating at 850 °C for over 1000 h. This represents a significant reduction in weight loss compared to sample B (NaCl-Al₂O₃ core material). Therefore, it can be concluded that during high-temperature exposure, the NaCl-Al₂O₃ core material does not infiltrate the SiC coating. The addition of the SiC coating effectively isolates the molten NaCl-Al₂O₃ core material from direct contact with the Al₂O₃ ceramic shell, forming a stable and hard protective layer. This layer reduces the leakage of the core material and further enhances the durability and stability of the high-temperature phase-change capsules. The addition of the SiC coating effectively isolates the molten liquid NaCl from direct contact with the Al₂O₃ ceramic shell, mitigating the leakage of the core material and further enhancing the durability and stability of the core–shell high-temperature phase-change macrocapsules.

3.3. Durability Performance after over 1000 h of High-Temperature Treatment

A comparison of the rapid heat storage and release performance of NaCl-Al₂O₃@SiC@Al₂O₃ macrocapsules before and after heating at 850 °C for 1000 h was conducted, as shown in Figure 8. The melting points at the core center of the samples, both before and after being heated for 1000 h at 850 °C, remained at 800 °C without any difference. This is attributed to the melting point of NaCl being 801 °C and that of Al₂O₃ being 2054 °C. In two heating–cooling cycle experiments, the heating rates of the two samples from room temperature to the core center temperature were almost identical. However, when the samples were cooled from the melting point of 800 °C to 500 °C at room temperature, the cooling rate of the sample before heat treatment was slightly faster than that of the sample after heat treatment, but the difference was minimal. Thus, it was generally considered that after enduring high-temperature treatment exceeding 1000 h at 850 °C, no changes occurred in the rapid heat storage and release performance of samples, validating the excellent stability of the NaCl-Al₂O₃@SiC@Al₂O₃ macrocapsules.

The XRD patterns of the NaCl-Al₂O₃ core material after heating at 850 °C for 1000 h are shown in Figure 8. From the XRD pattern in Figure 9, only the phases of NaCl and Al₂O₃ are observed, and no other substances are detected, indicating that after high-temperature treatment, no new compounds are generated through reactions between the core materials NaCl and Al₂O₃, demonstrating chemical stability.

4. Conclusions

This research developed novel high-concentration NaCl-Al₂O₃@SiC@Al₂O₃ microcapsules to address the issues of the leakage and low thermal conductivity of NaCl at high temperatures. After undergoing heating at 850 °C for over 1000 h, the material exhibited a weight loss rate of less than 5% compared with NaCl@Al₂O₃ and NaCl-Al₂O₃@Al₂O₃ macrocapsules, for which the weight loss rate was reduced by 25% and 10%, respectively, proving their excellent leakage prevention, and maintained its rapid heat storage and release performance, unaffected by prolonged high-temperature exposure. The core material NaCl-Al₂O₃ did not react to form new phases, and the Al₂O₃ particles within the molten NaCl formed a networked chain structure, which was key to maintaining its shape and preventing leakage, thus demonstrating its excellent durability and stability. The addition of a SiC coating also contributed to leakage prevention. The rapid heat storage and release performance of NaCl-Al₂O₃ was notably faster than that of NaCl. The incorporation of Al₂O₃ particles, with their inherent high thermal conductivity, improved the overall thermal conductivity and formed a network improving the internal thermal conduction routes in the material. This provided more effective channels for heat propagation, significantly enhancing the overall rapid heat storage and release performance of the core material. The self-standing network structure of Al₂O₃ particles within the molten NaCl phase of the PCM composite structure represents an innovative approach for high-temperature PCM. The newly developed core–shell-structured phase-change thermal storage macrocapsules are simple to prepare and cost-effective, with excellent rapid heat storage and release rates and stability. These macrocapsules show great potential in high-temperature rapid heat storage and release applications, such as regenerative burner systems and solar thermal storage.