Aluminum Oxide Coating for Carbonyl Iron Absorption Performance

By Muhammad OsamaReviewed by Frances BriggsSep 30 2025

Using an ultra-thin aluminum oxide coating, researchers have dramatically enhanced the high-temperature microwave absorption performance of carbonyl iron.

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As wireless technologies have become widespread, demand for materials capable of absorbing high-frequency electromagnetic radiation has increased rapidly, especially in extreme, high-temperature environments. 

Flaky carbonyl iron powders (CIPs) are well-known for their excellent magnetic properties, but they face two major limitations: poor oxidation resistance and high eddy current losses at elevated frequencies. These factors reduce their efficiency and reliability, particularly in electromagnetic interference (EMI) shielding and stealth applications.

Published in the Journal of Alloys and Compounds, researchers have now demonstrated that coating CIPs with aluminum oxide (Al₂O₃) using atomic layer deposition protects the material from oxidation and improves impedance matching and overall absorption capacity.

The Study

The team synthesized flaky CIP@Al₂O₃ composite powders using a two-step process of ball milling followed by atomic layer deposition. Flakes of carbonyl iron were produced using ball milling, which preserved the sheet-like morphology of the iron particles that is key to high in-plane anisotropy.

The atomic layer deposition process was carried out using high-purity trimethylaluminum (TMA) and deionized water as precursors to deposit atomically thin coatings of aluminum oxide with sub-nanometer control of layer thickness. 

The coatings formed a distinct interface between the conductive iron core and the insulating oxide shell. This structure was designed to improve dielectric loss, suppress eddy currents, and significantly enhance thermal and oxidation resistance without sacrificing the CIP's intrinsic magnetic behavior.

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Results: Better Absorption, Higher Temperatures

The coated CIP@Al₂O₃ powders significantly improved microwave absorption across key frequency bands. The effective absorption bandwidth (defined as reflection loss of greater than -10 dB) was broadened to 8.95 GHz in the X/Ku bands, far exceeding the performance of uncoated CIP. The minimum reflection loss reached -49.84 dB, indicating excellent absorption capability.

The materials also demonstrated thermal resilience. The oxidation resistance temperature increased by 300 °C, with magnetic properties (μ’ = 1.5, μ” = 2.2) remaining stable even at 500 °C. Dielectric loss was improved by 80 % over the four to 18 GHz frequency range, primarily due to interfacial polarization between the iron and Al₂O₃ layers.

These improvements were directly attributed to the uniform, pinhole-free atomic layer deposition coatings, which allowed precise control over surface properties and minimized performance trade-offs.

Applications of the Enhanced Material

The enhanced CIP@Al₂O₃ composites are particularly well-suited for radar absorption and EMI shielding. Their thermal and oxidation stability make the composites well-suited for use in extreme environments, including aerospace, defense, and automotive systems.

In addition, the potential for scalable, tunable atomic layer deposition coatings opens the door to tailored material designs, allowing researchers to fine-tune electromagnetic properties for specific application requirements.

What’s Next?

Looking ahead, further optimization of the atomic layer deposition process could yield even better performance and wider material compatibility. Exploring alternative coating materials and testing other surface engineering methods could further enhance thermal and electromagnetic behavior.

This work reinforces the importance of precise surface engineering in developing next-generation materials for high-frequency and high-temperature applications.

Journal Reference

Gong, W., et al. (2025, September). Atomic Al₂O₃ Coating and Interfacial Polarization to Enhance High Temperature Electromagnetic Absorption of Flaky Carbonyl Iron. Journal of Alloys and Compounds, 183969. DOI: 10.1016/j.jallcom.2025.183969, https://www.sciencedirect.com/science/article/abs/pii/S0925838825055306

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