miércoles, 18 de marzo de 2009

Sponge-like nanoparticles absorb microwaves

Conventional microwave absorbers are severely inhibited by their high specific gravity and consequently new absorbers that are relatively lightweight, structurally sound, flexible and absorb strongly across a wide band range are required.
Reporting their results in Nanotechnology, researchers at Lanzhou University, China, have revealed the excellent microwave absorption properties of manganese oxide nanostructures, which the team believes can easily satisfy the above demands. In addition, the group has developed a simple and facile hydrothermal method of fabricating the material. Factors affecting their properties and the absorption mechanism are also discussed in detail.
Hydrohausmannite nanoparticles of about 10 nm were prepared by a hydrothermal method at 100 °C for 72 h. Annealing of the precursor was carried out in air at 400 and 800 °C for 10 h, and the products are γ-Mn3O4 nanoparticles of about 25 nm and a 3D porous Mn2O3 network with a ligament size of about 100 nm, respectively.
Research team
Time-dependent experiments were carried out to understand the formation process of the sponge-like 3D porous network. The microwave absorption of all samples can be attributed to dielectric loss. The precursor shows strong absorption at 16.6 GHz with a minimum reflection loss of –17.6 dB and the absorption bandwidth lower than –10 dB (90% absorbed) is about 2 GHz. The sample annealed at 400 °C for 10 h shows a strongest absorption peak of –27.1 dB at 3.1 GHz with a bandwidth lower than –10 dB of 0.8 GHz and a broad peak with a bandwidth lower than –10 dB of about 2.6 GHz. With annealing time increased at 800 °C, the absorption becomes weaker. This is because the annealing process can greatly reduce the number of point defects and dangling bonds, and can also reduce the surface volume ratio.
Future work includes the preparation of composite nanomaterials composed of different nanoparticles, which may result in even better microwave absorption properties. Corresponding studies of the absorption mechanism and complementary modeling are also being undertaken by the team.
About the author
D Yan, S Cheng, R F Zhuo, J T Chen, J J Feng, H T Feng and H J Li are PhD students from the Institute of Plasma and Metal Materials, which is part of the School of Physical Science and Technology based at Lanzhou University, China, and they are all majoring in condensed matter physics. Z G Wu and J Wang are lecturers at the institute. Prof. P X Yan is the head of the institute and also a member of academic committee of the State Key Laboratory of Solid Lubrication, Institute of Chemistry and Physics, Chinese Academy of Science in Lanzhou. D Yan is now studying the preparation and application of low dimensional manganese oxide-based nanomaterials.

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