Synthesis And Microwave Absorption Properties Of Nanostructured Manganese Dioxides

Microwave absorption materials are of crucial importance to solve the expanding electromagnetic interference problems caused by the development of wireless communications and high-frequency circuit devices in gigahertz range. Conventional microwave absorbents, such as magnetic ferrites, usually have large densities, which limit their widespread applications. Besides, Carbon nanotubes and conducting polymers display good abilities for microwave absorption, but the fabrication of these materials involves complex processes. Therefore, it is desirable to exploit new microwave absorption materials that are lightweight, easy to synthesize and of strong absorption in a wide range.Recently, considerable research attention has been focused on nanostuctured transition metal oxides, which show highly efficient microwave absorption properties due to their strong dielectric loss. Microwave absorption properties of transition metal oxides often depend strongly on their morphologies. In particular, enhanced microwave absorption properties can be obtained from hierarchical nanomaterials with complicated geometrical morphologies. Manganese oxides with hierarchical nanostructures may be a good candidate for microwave absorbent, yet their microwave absorption properties have not been extensively studied. Moreover, morphology-controlled synthesis of manganese oxide nanostructures may offer avenues to understand the role that the morphology plays in affecting the corresponding microwave absorption performance. In this thesis, we synthesized urchinlike manganese dioxides with different morphology and crystalline structure. The microwave absorption properties of the urchins were investigated in detail. The main research is listed as follow.1. Hollow urchinlike α-MnO2nanostructures were synthesized via simple hydrothermal route. By tuning the reaction conditions, three types of hollow urchinlike α-MnO2nanostructures, namely, columnar nanorod clusters, tetragonal nanorod clusters, and tetragonal nanotube clusters were prepared. The formation mechanism of the nanourchins were investigated, from which the Ostwald repening process and the etching by H+should be attributed to the formation and morphology control of the hollow urchinlike α-MnO2nanostructures.2. Microwave absorption properties of the α-MnO2nanourchins were studied in detail. Manganese oxide urchins consisting of tetragonal nanorods exhibit the best microwave absorption performance among the three products, from which a reflection loss of-41dB and a match thickness of1.9mm were obtained. Furthermore, obvious magnetic loss was found in the the loss mechanisms. A microcircuit model was introduced to interpret the difference of microwave absorption properties between different samples. This model was futher confirmed by a comparison of microwave absorption properties between nanourchins and dispersed nanorods.3. A low-temperature hydrothermal method was developed to synthesize urchinlike y-MnO2nanostructures. Time-dependent evolutions of morphology and crystallinity were investigated to explore the growth mechanism of the y-MnO2urchins. The results show that the growth process of the γ-MnO2urchins occurs in two main stages, which are the generation of y-MnO2microspheres and the following epitaxial growth of y-MnO2nanorods on the surface of the initial microspheres. Microwave absorption properties of the urchinlike y-MnO2nanostructures were studied in terms of complex permittivity and permeability. An effective absorption bandwidth (reflection loss lower than-10dB) of8.8GHz was achieved from the y-MnO2/paraffin wax composite.