In recent years, driven by the global surge in demand for clean energy, high‑efficiency energy storage technologies have emerged as a major research focus in the energy sector. Among these, latent heat thermal energy storage (LHTES) has garnered significant attention due to its high energy density and excellent thermal stability.

Recently, a joint team comprising researchers from the Norwegian University of Science and Technology and Poland’s Warsaw University of Technology published a groundbreaking study in the journal Results in Engineering: for the first time, they have successfully developed microcapsules containing an ultra‑high‑temperature phase change material (PCM) based on a silicon–iron eutectic alloy, with an operating temperature exceeding 1,200°C and an energy storage density far surpassing that of existing technologies. This article will explore this cutting‑edge advancement, covering its background, underlying technical principles, experimental results, and potential future applications.

Why are ultra-high-temperature phase-change materials needed?

1. The Urgent Need for High-Temperature Energy Systems

In the energy sector, energy storage under high-temperature conditions is key to improving efficiency. For example:

• Concentrated solar power (CSP): Solar energy is converted into thermal energy using concentrating mirrors, which then drives a generator to produce electricity. If the temperature of the energy storage system is increased from 500°C to 1200°C, the power generation efficiency can jump from 20% to over 60%.
• Industrial Metallurgy and Thermophotovoltaics (TPV): Metallurgical processes require sustained high-temperature heat supply, while thermophotovoltaic technology necessitates ultra‑high temperatures (>1200°C) to efficiently convert thermal energy into electricity.

However, conventional thermal storage materials—such as paraffin and molten salts—tend to decompose at high temperatures and exhibit poor thermal conductivity, making them inadequate for meeting performance requirements. Metal-based phase-change materials, including aluminum and silicon alloys, stand out due to their high thermal conductivity and high energy-storage density; yet their highly reactive molten states can readily cause container corrosion, thereby limiting their practical applications.

2. Core Challenges of Phase-Change Materials

Phase-change materials store and release energy through solid–liquid phase transitions, but they must meet three key requirements:

• High latent heat of phase change (ΔHf): determines the energy storage capacity;
• Chemical stability: Avoid reactions with the container or the environment at elevated temperatures.
• Volume stability: Volume changes during phase transitions must be controllable to prevent container rupture.

Although conventional aluminum-based alloys, such as Al–Si, can operate below 700°C, their energy storage density is only 180–370 J/g, and they cannot withstand higher temperatures. Silicon (Si), with its exceptionally high melting point (1414°C) and latent heat of fusion (1800 J/g), is an ideal candidate; however, pure silicon undergoes a volume expansion of approximately 10% upon solidification, which can lead to structural failure if directly encapsulated. Consequently, addressing these limitations through alloying and microencapsulation has become a key research focus.

Ferrosilicon Eutectic Alloy: A Breakthrough in Materials Design

1. Advantages of Eutectic Alloys

The research team selected a silicon–iron (Si–Fe) eutectic alloy as the core material, owing to its advantages:

• Ultra-high temperature resistance: The eutectic point reaches 1213°C, making it suitable for environments exceeding 1200°C.
• High energy storage density: the theoretical latent heat value reaches 1010 J/g, more than three times that of aluminum-based alloys.
• Volume stability: Alloying effectively suppresses the drastic volume changes of pure silicon.

Through thermodynamic simulations (using FactSage software) and experimental validation, the team identified the optimal composition as SiFe43 (43% Fe, 57% Si, by mass), whose melting–solidification process is accompanied only by minor volume fluctuations, thereby addressing the key limitations of pure silicon in practical applications.

2. Microencapsulation Technology: Dressing Materials in a “Protective Coat”

Microencapsulation isolates phase-change materials from direct contact with the environment by forming a protective layer—such as an oxide shell—on their surface. This technique has previously been widely applied to aluminum-based alloys; however, encapsulating silicon–iron alloys presents significantly greater challenges:

• High-temperature oxidation stability: The housing must remain intact at 1200°C.
• Interface bonding strength: prevents the molten alloy from breaching the shell.

The research team has developed a two-step microencapsulation process:

• Ultrasonic atomization powder production: The SiFe43 alloy melt is atomized into spherical powder (average particle size 61.6 μm) using the rePOWDER ultrasonic atomization powder‑making machine from AMAZEMET, ensuring particle uniformity.
• Oxidative coating: The powder is impregnated with a nano‑silica suspension and subjected to oxidative treatment at 1200°C for 24 hours, resulting in the formation of a continuous, dense SiO₂ shell with a thickness of 2–3 μm.

Through scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses, the team confirmed the successful construction of the SiFe43/SiO₂ core–shell structure (Fig. 1). The SiO₂ shell remains intact at elevated temperatures, providing reliable protection for the alloy.

Performance Verification: Excellent Both Energy Storage Density and Stability

1. Thermal Property Testing

DSC measurements reveal that the phase-change latent heat of the unencapsulated SiFe43 powder is close to the theoretical value; even after microencapsulation, it still exceeds that of conventional materials, and its microcapsule‑based volumetric energy storage density far surpasses that of many batteries and LHTES systems.

2. High-Temperature Stability Test

After heating the microcapsules at 1280°C (50°C above their melting point) for 1 hour, SEM analysis revealed that the SiO₂ shell remained intact and the particle morphology was preserved, whereas the unencapsulated alloy powder exhibited melt‑induced agglomeration (Figure 3). This demonstrates that the SiO₂ shell can effectively prevent the alloy from reacting with the external environment even under extreme temperatures.

Future Applications and Challenges

• Concentrated solar power (CSP): When paired with a Brayton cycle or a Stirling engine, it enables efficient, round-the-clock electricity generation.
• Industrial waste heat recovery: stores high-temperature waste heat from industries such as metallurgy and chemical processing, thereby reducing energy waste.
• Thermophotovoltaic (TPV) systems: They directly convert ultra-high-temperature thermal energy into electricity, thereby enhancing conversion efficiency.

Graphic Diagram

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 as-cast SiFe43 alloy.

Figure 3. SEM image of SiFe43 spherical powder after atomization-based powder production.

Figure 4. SEM characterization results of SiFe43/SiO2 microcapsules, along with EDS (a) and XRD (b) analyses.

Figure 5. DSC characterization results of SiFe43 powder in the atomized (a) and encapsulated (b) states. ΔHf values are calculated in units of MWh/m³, using an experimentally determined room-temperature density of 4.5821 g/cm³.

Figure 6. SEM images of SiFe43 powder after heat treatment at 1280°C for 1 hour in air, showing the as‑packaged (a) and atomized (b) states.

A breakthrough in the microencapsulation technology for ferrosilicon eutectic alloys has opened up a new avenue in the field of ultra‑high‑temperature energy storage. With its exceptionally high energy density and outstanding stability, this technology holds great promise for accelerating the commercialization of next‑generation clean energy systems. As further research progresses, it could become a key enabler in achieving the “dual carbon” goals.