High-frequency permeability of Ni and Co particle assemblies

Abstract

A coaxial transmission line was constructed, characterized, and calibrated for the frequency dependent measurement of complex relative permeability (l r ) and complex permittivity (e r ). The permeability of Ni powder with a grain size of < 1 lm was measured as a function of packing density to verify the system performance. 8-10 nm diameter Co nanoparticles were synthesized, dried to a powder, and measured. The real part of the permeability for the Co nanoparticles decreased over time as a result of oxidation, and decreased the overall magnetic volume due to the formation of an antiferromagnetic CoO shell. Similarly, the imaginary part of the permeability decreased as a function of oxidation. This was attributed to the insulating CoO shell reducing eddy current losses in the nanoparticle composite. V C 2014 AIP Publishing LLC. [http://dx

Co7Fe3 and Co7Fe3@SiO2 Nanospheres with Tunable Diameters for High-Performance Electromagnetic Wave Absorption

“…Many efforts have been made to develop electromagnetic wave-absorbing (EMA) materials to meet the increasing requirements for absorbing electromagnetic waves. − These efforts have cultivated a great scope wherein various materials have been explored as EMA fillers. − Among these materials, ferromagnetic metal/alloy particles possess unique properties, such as a high saturation magnetization ( M s ), a high Curie temperature ( T c ), and composition controllability, − which endow them with a superior potential to present high and tailorable electromagnetic properties in the gigahertz (GHz) range. − However, these intrinsic superiorities have not been fully revealed because of various obstacles, including the eddy current effect, low filling fraction, or decay. − For instance, eddy current in a single ferromagnetic particle or local aggregations may induce an extremely high permittivity but decreased permeability, leading to deteriorated electromagnetic matching. − On the other hand, the filling ratio in EMA coatings is usually limited to a low level, which restricts improvement of the coatings’ permeability. − Additionally, ferromagnetic nanoparticles are prone to oxidization when exposed to a high temperature or corrosive environment, resulting in a degenerated EMA performance during the serving process. ,, The eddy current effect as well as dielectric loss is related to conductivity ,, and particle size . Ferromagnetic resonance is highly sensitive to particle morphology and particle–particle distance. , The microstructure design of ferromagnetic particles can influence their conductivity, dispersion, and ferromagnetic properties; thus, it would be beneficial to adjust their EMA properties.…”