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

The phase transition process of the MnO 2 phase with increases in Co 2+ doping can be described by the Ouroboros symbol. The undoped sample is pure δ-MnO 2 with nanosheets structures. Then a small amount of Co 2+ ions changes the final products and generates α-MnO 2 nanofibres in the δ-MnO 2 matrix. The products become pure α-MnO 2 with an appropriate amount of Co added. However, when the Co amount continues to increase, the amount of α-MnO 2 decreases in the products and turns back to form pure δ-MnO 2 in the end. Analysing the electromagnetic absorption performance, the relationship between the properties and the proportion of δ-MnO 2 /α-MnO 2 in the powders adjusted by Co 2+ doping has been explored, and the composites of δ-/α-MnO 2 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.