Preparation And Electromagnetic Properties Of A Few Kinds Of Dielectric/Ferromagnetic Composite Powder

Al18B4O33w/Co, Al18B4O33w/CoFe and SiCw/Co composite powders were prepared through electroless depositing Co or CoFe coatings on the surface of Al18B4O33 and SiC whiskers, and the prepared composite powders were heat-treated to adjust the composition and microstructure. The morphology, composition and microstructure of composite powders were analyzed and characterized through SEM, XRF, EDS and XRD. The electromagnetic properties of the composite powders were measured by using VSM, PPMS, microhmmeter and VNA, and the electromagnetic wave absorbing (EMA) performance evaluated through calculating the reflection loss (RL) by using the transmission theory. The effect of the composition and microstructure on the electromagnetic properties was investigated and the influence mechanism was studied. A media-reconstruction method was also estabilished to improve the calculation accuracy of the permeability.It is found that process efficiency and rate as well as the composition and the microstructure of the coatings are influenced by the pH value and temperature of the plating solution. The Fe content in CoFe coatings is determined by the relative content of Fe2+ in the plating solution. The efficiency of the CoFe deposition decreases with the increase of [Fe2+] and the deposited CoFe coatings with various Fe contents are all of amorphous state. The coatings deposited through electroless plating present a particle-stacking characteristic. The growth behavior and the microstructure of the coatings are also affected by the surface wettability and the pre-treatment process.Coatings become smoother and denser after heat-treated in a H2 atmosphere through the diffusion of newly reduced Co atoms driven by the surface energy reduction. Contrastively, no obvious morphology change is observed when the coatings are heat-treated in an Ar atmosphere. The amorphous coatings transform quickly to crystalline state when heat-treated at temperatures higher than 400 oC and aα→βtransformation occurs when coatings are thoroughly heat-treated.The whisker/Co composite powders possess high electromagnetic properties because of the introduaction of Co, and the properties can further be elevated through proper heat-treatment. The increase of permittivity and the occurrence of the dielectric relaxation in the composite powder are attributed to the increase of conductivity. The increase of Ms and the enhanced exchange coupling contribute to the increase of permeability. The effect of eddy current is slight when the coatings'resistivity is higher than 5×10-3 ?·cm and a dielectric relaxation takes place when the resistivity is lower than 5×10-2 ?·cm. The spectra characteristic of the composite powder is affected by the crystal structure of the coatings. The electromagnetic properties of Al18B4O33w/CoFe composite powders do not increase when heat-treated at 400 oC. The permeability of the CoFe coatings increases with the increase of temperature but the the increase of permittivity is slight. An electromagnetic resonance is observed in the specimens containing SiC whiskers, which may be related to the shape and unique electrical properties of SiC whiskers.The coatings using proper-treated Al18B4O33w/Co composite powders as fillers present excellent EMA performance with a maximum reflection loss of 78 dB, an effective-absorption bandwidth of 3.5 GHz and a typical thickness of 2 mm. The EMA efficiency first increases and then decreases with the increase of Co content as well as the heat-treatment time. The coatings using SiCw/Co composite powders as fillers present a similar EMA performance as the Al18B4O33w/Co based coatings, but the Al18B4O33w/CoFe based coatings present a low EMA performance.The MRC-BEMT method proposed in the current study can eliminate the mathematical instability that occurs when calculating the permeability in composite media with low filler contents, the calculation consistence as well as prediction accuracy is thus improved. Experimental tests confirm that the MRC-BEMT method is an effective way for permeability calculation, but further verification is required to approve its prediction accuracy.