Microwave absorption characteristics of manganese dioxide with different crystalline phase and nanostructures

α-MnO2 nanowires and β-MnO2 nanorods with high crystallinity were obtained successfully by a hydrothermal method at 160 °C for different reaction times. As-synthesized samples were analyzed by X-ray diffraction (XRD) spectra, scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), and high-resolution TEM (HRTEM). The results show that the samples changed from α-MnO2 nanowires with diameters about 50 nm and lengths 2–4 μm to β-MnO2 microrods with diameters 2–5 μm and lengths 5–10 μm under the reaction time prolonged from 2 to 48 h. The dielectric properties and microwave absorbing properties of the as-prepared samples were examined in the frequency range of 2–12 GHz. The samples under the reaction time 2 and 24 h exhibited the most dielectric loss tangent values and microwave absorption performances. The effective absorption bandwidth which is lower than −10 dB is 3.2 and 1.4 GHz, respectively. The results show that the morphology is important to the microwave absorption characteristics of manganese dioxide. It is also found that MnO2 is a kind of materials with both dielectric and diamagnetic loss, and the dielectric loss results from space charge polarization, which plays the dominant role in the total loss.