Synthesis And Microwave Absorbing Properties Of Low Frequency Absorber

With the rapidly development of economic, technology and electronic industry, electromagnetic radiation has become to be the fourthly pollution following the atmospheric pollution, water pollution and noise pollution and imperil the health of human beings serious. Using absorbing composites would be one of the effective methods to protect the electromagnetic radiation. Besides, the perperities of microwave absorption and thermogenesis are more demanded for heat seed biomaterials. it is needed at low frequency stage. Therefore it would have great significance to study the absorbing materials in civil utilization.inorganic-organic absorbing composite materials are based on the design principle that the absorbing capability of inorganic-organic absorbing composite materials depend greatly on the absorbing agents. Series ferrites and ferrite coated WMNTs were synthesized by a sol-gel process. Other inorgnaic powder was prepared as well. Those absorbing agents were composited with organic materials. The microwave elecromagnetic properties were studied. The results show these absorbing agents have good absorbing capability in some frequency range.M-type Barium Ferrite (BaFe₁₂O₁₉) nanopowder was synthesized by a sol-gel process using stoichiometric amounts of Ba(NO₃)₂·6H_O2, Fe(NO₃)₃·9H₂O, and citric acid as the starting materials. The reaction mechanism and the effects of calcinations temperature and calcinations time on the grain size were studied. M-type Barium ferrite powder with the average grain size range of 40-70 nm can be prepared above 800°C and the calcinations temperature has obvious influence on crystalline process, the particle size and its distribution. The BaFe₁₂O₁₉/ unsaturated polyester composite has been prepared. The complex permeability and permittivity of the BaFe₁₂O₁₉ / unsaturated polyester composite were measured. Base on the theoretical calculation of reflection loss, the microwave absorption propertie of BaFe₁₂O₁₉/unsaturated polyester composite has been made in the frequency range from 0.5-5.0 GHz. A maximum reflection loss value of - 52.7 dB has been obtained for absorber thickness of 11.2 mm at 4.4 GHz. The result indicates that this composite has significant potential for electromagnetic wave absorption application in the future. These materials are relevant in engineering fields related with specific wireless communications between electronic equipments (e.g. wireless LAN systems).Ce-substituted lithium ferrite, Li_0.5Ce_xFe_2.5-xO₄ (x=0, 0.015 and 0.15), was prepared from metal nitrates and citric acid by citrate sol-gel method. The thermal decomposition process was investigated by TG-DSC. The phase composition and microstructure of Li_0.5Ce_xFe_2.5-xO₄ was characterized by X-ray powder diffraction analysis (XRD) and transmission electron microscope (TEM). The complex permittivity and complex permeability and microwave absorption properties of Li_0.5Ce_xFe_2.5-xO₄/paraffin wax composite were measured by the transmission /reflection coaxial line method in the range of 2-18 GHz. It is shown that the substitution of Ce had a close effect on properties of Li_0.5Ce_xFe_2.5-xO₄ ferrite. Also the present investigation demonstrates that microwave absorbers for applications over 15 GHz, with satisfactory reflection loss more than -20 dB for specific frequency, could be obtained by controlling the substituted Ce elementPowder of Mg-doped BaFe₁₂O₁₉ was prepared from powders of BaFe₁₂O₁₉ and MgO by sintering process combinated with high-energy ball milling. The phase composition and microstructure of the as prepared powder was characterized by X-ray powder diffraction analysis (XRD) and scaning electron microscope (SEM). The complex permittivity and complex permeability and microwave absorption properties of Mg-doped BaFe₁₂O₁₉/paraffin wax composite were measured by the transmission/reflection coaxial line method in the range of 0.5-6.0 GHz. The composite exhibited efficient microwave absorption (reflection loss < -20 dB) in the frequency range 4.07-4.57 GHz with the respective thickness from 3.3 to 1.9 mm. The results also show that the Mg-doped BaFe₁₂O₁₉/paraffin wax composite has a minimum reflection loss value of - 41 dB at 4.27 GHz with a matching thickness of 2.6 mm. It is concluded that Mg-doped BaFe₁₂O₁₉ can be used for microwave absorber in military and commercial applications.Ce-substituted BaFe₁₂O₁₉ (BaCe_xFe_12-xO₁₉, x=0.05) was prepared by citratesol-gel method. The average crystallite size calculated from the XRD line broadening is about 30.1 nm. The complex permittivity and complex permeability and microwave absorption properties of Ce-substituted BaFe₁₂O₁₉ (x=0.05)/paraffin wax composite was measured by the transmission/reflection coaxial line method in the range of 8-13 GHz. The results show that the BaCe_xFe_12-xO₁₉ (x=0.05)/paraffin wax composite has a minimum reflection loss value of-37.4 dB at 12.8 GHz with a matching thickness of 3.5 mm. It is concluded that Ce-doped BaFe₁₂O₁₉ can be used for microwave absorber in military and civil applications.Lithium-zinc (LiZn) ferrites and magnesium-substituted LiZn ferrites were prepared by citrate sol-gel method at different temperature. The thermal decomposition process was investigated by DTA-TG The phase composition of the LiZn ferrite powder was characterized by XRD. The morphology of the LiZn ferrites powder was observed by TEM. The particle size was found to increase with the increase of the sintering temperature. The particles of samples obtained after heat-treatment at 750, 900, 1050, 1200°C are uniform and their crystalline sizes are in the range of 50-90 nm. The complex permittivity and complex permeability and microwave absorption properties of magnesium-substituted LiZn ferrite/paraffin wax composite were measured by the transmission/reflection coaxial line method in the range of 0.5-12.0 GHz. A minimum reflection loss of -38 dB is obtained at 5.1GHz for composite with 60 wt% of ferrite powder and absorber thick of 5.5 mm. The results show that Magnesium-substituted LiZn ferrite powder composties can be applied as microwave absorber in 2.5-7.5 GHz.A novel CNTs-based composite, LiZn ferrite (Li_0.435Zn_0.195Fe_2.37O₄) coated CNTs composite, was successfully prepared by a sol-gel method. The nanostructure have confirmed by TEM and XRD. The microwave absorbing property of the LiZn ferrite coated CNTs composite was studied, and it was found that the composite materials exhibited good microwave absorbing property in S-band.Dynamics analysis of sol-gel process of ferrite powder, and the relavent equations have also been developent. The microwave absorption mechanisms of those composites were analyzed.This paper was supported by The State of 863 (2003AA302232), the NaturalScience Foundation of the People's Republic of China (Grant No. 30540061 and 50672051) and Jinan Key Subjects of The Science and Technology (054042).