I. Industry Pain Points: The Development Challenges of High-Temperature Thermal Storage Materials

Latent heat thermal energy storage (LHTES), with its advantages of being site‑independent and offering high storage efficiency, is a core enabling technology for concentrated solar power (CSP) systems. While commercial photovoltaic panels achieve only 21%–23% photoelectric conversion efficiency, CSP plants equipped with high‑temperature thermal energy storage can theoretically reach conversion efficiencies of 60%–80%.

Different application scenarios impose vastly varying temperature requirements for energy storage: steam generators require 500–700°C, while the optimal operating range for Brayton cycles lies between 700 and 900°C. Thermophotovoltaic converters and advanced metallurgical processes, on the other hand, require operating temperatures exceeding 1200°C. 。

Phase change materials (PCMs) are the core components of latent heat thermal energy storage systems. Conventional materials such as paraffin and molten salts cannot withstand extremely high temperatures. Aluminum-based alloys currently represent the mainstream class of high-temperature metallic PCMs, but their operating temperatures generally remain below 700°C, with a latent heat of fusion ranging only from 180 to 370 J/g, and they exhibit significant performance degradation over long-term use.

Metal-based phase-change materials also suffer from a major drawback: in the molten state, they exhibit high chemical reactivity, readily corroding containment vessels and being degraded by environmental media, which leads to rapid deterioration of their thermal storage performance. Although microencapsulation can isolate external influences and stabilize material properties, this technique has previously been applied only to aluminum-based materials. A ultra-high-temperature phase-change material operating at the 1200°C level has yet to be successfully commercialized. 。

Pure silicon is inherently an ideal candidate for ultra‑high‑temperature thermal energy storage: it has a melting point of 1414°C, a latent heat of fusion as high as 1800 J/g, and excellent thermal conductivity. However, during solidification, pure silicon undergoes a volumetric expansion of about 10%, which can damage the storage system, and its chemical reactivity is too high to allow direct use. To address these challenges, the research team opted to… Silicon-iron alloying Routes, improvement of the comprehensive performance of materials, and the initiation of microencapsulation research.

II. Core Preparation Process: A Two-Step Method for Fabricating Core–Shell Structured Microcapsules

This study selected SiFe43 eutectic silicon-iron alloy As a phase-change core material, by integrating thermodynamic calculations, alloy smelting, powder preparation, and high-temperature encapsulation—four key processes—we have successfully fabricated spherical microcapsules with an SiO₂ shell and a ferrosilicon alloy core.

1. Alloy Design and Spherical Powder Production

• Using FactSage 8.1 software, the Si–Fe phase diagram and thermodynamic parameters were analyzed to determine the composition of the SiFe43 eutectic alloy, which exhibits a theoretical latent heat of fusion of 1010 J/g and a phase transformation temperature of 1213°C.
• A cast‑state ferrosilicon alloy was prepared using an induction melting process under a hydrogen‑protected atmosphere, and its composition was verified to meet the specified requirements by mass spectrometry.
• [Ultrasound-Assisted Powder Atomization Technology] Based on AMAZEMET’s rePOWDER equipment, employing Ultrasonic powder atomization method The bulk cast alloy is processed into spherical powder. This technique enables the production of well‑shaped, defect‑free spherical particles, with an average particle size of 61.6 μm. The particles are intact and exhibit a typical silicon–iron disilicide biphasic microstructure, laying the groundwork for subsequent microencapsulation.

2. A two-step encapsulation process for constructing a core–shell structure

The research team innovatively adopted Two-step impregnation–oxidation encapsulation method Complete packaging:

• Surface Activation: The spherical ferrosilicon powder was immersed in a 40 nm silica nanoparticle suspension and subjected to magnetic stirring at room temperature for 24 hours, allowing the silica to adhere uniformly to the particle surfaces.
• High-temperature shell formation: The filtered and dried powder is placed in air and subjected to high-temperature oxidation at 1200°C for 24 hours, ultimately yielding… A SiO₂ shell with a thickness of 2–3 μm, which is continuous and dense, and exhibits excellent thermal stability. 。

Using characterization techniques such as SEM and XRD, it was confirmed that the final product is intact. SiFe43 core – SiO₂ shell Core–shell microcapsules exhibit an intact, crack-free shell layer, and their phase structure is consistent with the design specifications.

III. Performance Testing: Energy Storage Density Far Exceeds That of Conventional Batteries and Existing Thermal Storage Systems

The research team employed differential scanning calorimetry (DSC) to conduct thermal property measurements over the temperature range of 20–1450°C, yielding exceptionally promising key data:

1. Heat storage capacity : The measured latent heat of fusion for the unencapsulated ferrosilicon powder is 988 J/g, in excellent agreement with the theoretical value; after encapsulation, the microcapsule’s latent heat of fusion is 860 J/g, a slight reduction attributable to surface alloy oxidation that forms an outer shell, with the decrease falling within an acceptable range.
2. Volumetric energy density : After conversion to a volume-based unit, the energy storage density of this microcapsule can reach 1.09~1.25 MWh/m³ By comparison, this is 10 to 20 times that of lead–acid batteries and 2 to 6 times that of lithium-ion batteries, while also being 5 to 10 times the capacity of the mainstream latent‑heat thermal energy storage systems currently used in concentrated solar power applications.
3. High-temperature stability : Place the sample in an environment at 1280°C (50°C above the phase transformation temperature) and hold it for 1 hour. The morphology and structure of the microcapsules with an SiO₂ shell remain intact. In contrast, the unencapsulated ferrosilicon powder exhibits significant melting and agglomeration. This demonstrates that the silica shell can effectively shield the core phase-change material from high-temperature degradation.

Image and Text Guide

Figure 1. FactSage calculation results: (a) Si–Fe binary phase diagram; (b) Enthalpy–temperature relationship for the SiFe₄₃ eutectic alloy.

Figure 2. SEM/EBSD analysis results of the SiFe43 alloy in the as-cast condition.

Figure 3. Scanning electron microscope (SEM) image of atomized SiFe₄³ spherical powder.

Figure 4. Results of the characterization of SiFe43/silica microcapsules by SEM/EDS (a) and XRD (b).

Figure 5. DSC characterization results of SiFe43 powder in the atomized state (a) and the encapsulated state (b). The ΔHf values, expressed in MWh/m³, were calculated based on the experimentally determined room-temperature density of 4.5821 g/cm³.

Figure 6. Scanning electron microscope (SEM) images of SiFe43 powder after air thermal treatment at 1280°C for 1 hour, in the encapsulated state (a) and in the atomized state (b). 。

IV. Summary and Future Prospects

This study For the first time, microencapsulation of an ultra-high-temperature ferrosilicon phase-change material has been achieved. , has overcome the technical barriers that have limited microencapsulation to medium-temperature aluminum-based materials, enabling its application to ultra‑high‑temperature metal alloys exceeding 1200°C. The key findings are summarized as follows:

1. Utilize Ultrasonic powder atomization technology It can stably produce high‑quality spherical silicon–iron alloy powders, meeting the requirements for microcapsule fabrication.
2. The two-step oxidation encapsulation process successfully constructs a continuous and stable SiO₂ protective shell, endowing the material with excellent high-temperature erosion resistance.
3. SiFe43@SiO₂ microcapsules exhibit a phase-change temperature exceeding 1200°C, with an energy-storage density that significantly outperforms commercial batteries and conventional high-temperature thermal storage materials, making them well-suited for ultra-high-temperature applications such as concentrated solar power, thermophotovoltaic conversion, and advanced metallurgy.

At present, this research remains at the basic exploratory stage; in the next phase, the team will focus on conducting… Thermal Cycling Endurance Test , to verify the long-term stability of the microcapsules under repeated melting–solidification cycling, thereby advancing this novel ultra‑high‑temperature thermal energy storage material from the laboratory to practical engineering applications. In the future, this technology is expected to significantly enhance the operational efficiency of ultra‑high‑temperature energy systems, opening up new avenues in concentrated solar power and high‑end industrial energy storage.