Transformation between nanosheets and nanowires structure in MnO₂ upon providing Co²⁺ ions and applications for microwave absorption

The phase transition process of the MnO₂ phase with increases in Co²⁺ doping can be described by the Ouroboros symbol. The undoped sample is pure δ-MnO₂ with nanosheets structures. Then a small amount of Co²⁺ ions changes the final products and generates α-MnO₂ nanofibres in the δ-MnO₂ matrix. The products become pure α-MnO₂ with an appropriate amount of Co added. However, when the Co amount continues to increase, the amount of α-MnO₂ decreases in the products and turns back to form pure δ-MnO₂ in the end. Analysing the electromagnetic absorption performance, the relationship between the properties and the proportion of δ-MnO₂/α-MnO₂ in the powders adjusted by Co²⁺ doping has been explored, and the composites of δ-/α-MnO₂ show better absorption ability than the single-phase samples. As a result, the optimal reflection loss (RL) is -54.8 dB, and the effective absorption bandwidth can cover the Ku band at a thickness of 2.2 mm and the X band at a thickness of 3.1 mm with 50 wt.% filler loading ratios. This research might shed new light on the improvement of novel microwave absorption materials.