| Doped rare-earth manganeties have been becoming a hot and active topic in the field of magnet-electron functional materials because of their giant magnetoresistance effect and special electromagnetic properties over the years.Owing to the properties, it's possible that such materials become a new type of microwave absorbing materials, but few are reported. The microwave absorbing properties of LaMnO3 system doped by K and Ag at A site respectively were studied in this thesis.The nano-crystalline powder samples of La1-xKxMnO3and La1-xAgxMn03 were prepared by sol-gel method.The formation, composition, crystal structure and particle morphology of these samples were characterized by DSC-TGA,EDS,XRD,SEM. The results showed that the formation of the samples were consist of water evaporation EDTA decomposition, nitrate decomposition and perovskite crystalline growth. Both the crystal structure of La1-xKxMnO3nanocrystalline powders and La1-xAgxMn03 were perovskite type, and A small amount of impurity phase appeared in the samples with the increasing of doping components and temperature. Their crystal powder morphology was irregular ellipsoidal shape or short rod, and the average size of La1-xKxMn03 sample particles was about 60nm and La1-xAgxMn03 about 100nm.Their complex permittivity and complex permeability were measured by microwave vector network analyzer in the frequency range from 2 to 18 GHz, and the reflection coefficient and loss tangent were calculated according to measurements. The effects of doping and thickness on microwave absorption of them and the microwav loss mechanism were discussed. The results showed that the absorption peak of Lai-xKxMn03 with 2.20 mm thickness (x= 0.3) was 27.1dB at 9.9GHz and its bandwidth above lOdB was 10.6GHz,and Lai-xAgxMn03 with 2.60 mm thickness (x=0.15) was 24.9dB at 12.0GHz and bandwidth 7.1 GHz.They had both dielectric loss and magnetic loss, but the former is relatively stronger.lt was possibly caused by a mutual transformation or competetion between ferromagnetic and antiferromagnetic clusters of the matrix in the microwave electromagnetic field that magnetic and dielectric loss tangent changed inversely with the changes of microwave frequency. |
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