Formation of high electrical-resistivity thin surface layer on carbonyl-iron powder (CIP) and thermal stability of nanocrystalline structure and vortex magnetic structure of CIP

Abstract

This study focuses on the carbonyl-iron powder (CIP) used in the metal composite bulk magnetic core for high-efficient/light-weight SiC/GaN power device MHz switching dc-dc converter, where the fine CIP with a mean diameter of 1.1 μm is used to suppress the MHz band eddy current inside the CIP body. When applying the CIP to composite core together with the resin matrix, high electrical resistivity layer must be formed on the CIP-surface in order to suppress the overlapped eddy current between adjacent CIPs. In this study, tens nm thick silica (SiO2) was successfully deposited on the CIP-surface by using hydrolysis of TEOS (Si(OC2H5)4). Also tens nm thick oxidized layer of the CIP-surface was successfully formed by using CIP annealing in dry air. The SiC/GaN power device can operate at ambient temperature over 200 degree-C, and the composite magnetic core is required to operate at such ambient temperature. The as-made CIP had small coercivity below 800 A/m (10 Oe) due to its nanocrystalline-structure and had a single vortex magnetic structure. From the experimental results, both nanocrystalline and single vortex magnetic structure were maintained after heat-exposure of 250 degree-C, and the powder coercivity after same heat-exposure was nearly same as that of the as-made CIP. Therefore, the CIP with thermally stable nanocrystalline-structure and vortex magnetic state was considered to be heat-resistant magnetic powder used in the iron-based composite core for SiC/GaN power electronics.

Low permeability composite magnetic core transformer with high coupling coefficient and its application to PFM controlled quasi-resonant mode flyback-type DC-DC converter

“…Although not measured below 1 MHz in frequency, the relative permeability r' of about 6 was considered to be constant at hundreds kHz band and the magnetic loss r" was considered to be small enough at same hundreds kHz band. The volume resistivity of the surface-oxidized carbonyl-iron powder/epoxy composite magnetic core was about 100 m 4) , which was 100 times higher than that of Mn-Zn ferrite core. Therefore, it was considered that the eddy current loss of the transformer core could be significantly suppressed at least up to 10 MHz.…”

“…The authors are developing a novel magnetic core as a candidate substitute for the Ni-Zn ferrite core, for example, the carbonyl-iron powder/epoxy composite magnetic core was developed 4) and was applied to switching power converters 5), 6) . In order to increase the volume resistivity of the composite magnetic core body, the surface-oxidation for the carbonyl-iron powder was very effective 4) .…”

“…In order to increase the volume resistivity of the composite magnetic core body, the surface-oxidation for the carbonyl-iron powder was very effective 4) . Though the surface-oxidized carbonyl-iron powder/epoxy composite magnetic core had a low relative permeability of about 6, the magnetic loss was very small up to 10 MHz in frequency.…”

“…Surface-oxidized carbonyl-iron powder/epoxy composite magnetic core transformer 2.1 Properties of surface-oxidized carbonyl-iron powder/epoxy composite magnetic core As the soft magnetic metal powder for the composite magnetic core, non-reduction carbonyl-iron powder with a mean diameter of 1.6 m was used. In order to prevent the electrical contact between the adjacent powders in the composite magnetic core and to suppress the over-lapped eddy current loss, high resistive oxidized layer of several tens of nm was introduced on the surface of the powders by thermal oxidation in air at 200 degrees-C for 6h 4) . The composite magnetic core was fabricated by the casting and post-curing using the composite slurry with the surface-oxidized carbonyl-iron powder and epoxy precursor solution.…”

The role of microstructural evolution during spark plasma sintering on the soft magnetic and electronic properties of a CoFe–Al2O3 soft magnetic composite

“…Organic polymer SMCs yielded moderate resistivity, on the order of 100 lXÁm, but typically are not thermally stable above 300 °C and have low J s , less than 1.0 T [11,12]. In other approaches, consolidated SMCs made with coatings of Al 2 O 3 , SiO 2 , or even CaF 2 on ferromagnetic powder particles reached high resistivities on the order of 100-1000 lXÁm, and moderate J s , approximately 1.0-1.5 T [13][14][15][16][17][18][19][20][21][22]. The high resistivity of Al 2 O 3 containing SMCs is attributed to the high electrical resistivity of Al 2 O 3 , on the order of 10 17 lXÁm.…”