Flaky carbonyl iron particles with both small grain size and low internal strain for broadband microwave absorption

• We propose a heating wet ball-milling method to fabricate flaky carbonyl iron powders (CIPs).
• Controlling milling temperature (T) can produce the CIPs with small 〈D〉 and low 〈ε〉.
• The flaky CIPs obtained at T of 250 °C can serve for broadband microwave absorbing materials.

Controlling the microstructures such as particle shape, grain size (〈D〉) and internal strain (〈ε〉) of magnetic absorbents are of importance for the construction of broadband microwave absorbing materials (MAMs). Here we have developed a heating wet ball-milling method to tune the aspect ratio (ar), 〈D〉 and 〈ε〉 of the flaky carbonyl iron powders (CIPs) so as to achieve optimized microwave absorbing properties. In our protocol, when the milling temperature (T) exceeds 250 °C, the heat effects dominate over the deformation effects, deteriorating the electromagnetic performances due to the rapid growth up of 〈D〉. When T is below 250 °C, the deformation effects play a dominant part. In this condition, the 〈ε〉 constantly accumulate and ar decrease with T, decreasing the relative complex permeability. As a result, controlling T cannot enlarge 〈D〉 and 〈ε〉, but only increase ar. This comes from the competition of the grain growth effect and deformation effect at different temperatures. The as-obtained flaky CIPs are a good candidate absorbent for broadband MAMs of thin thickness. The heating wet ball-milling method proposed here, as a simple and effective way to control the particle shape, grain size and internal strain of CIPs, can be easily extended to the fabrication of other high performance magnetic absorbents.

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