Dissertation/Thesis Abstract

Optimization of Iron Cobalt-based Nanocomposite Alloys for High Induction and Increased Resistivity
by Shen, Shen, Ph.D., Carnegie Mellon University, 2013, 141; 3574866
Abstract (Summary)

FeCo-based nanocrystalline soft magnetic materials are promising to provide high saturation induction, high Curie temperature and excellent soft magnetic properties for electric vehicle and high frequency power conversion applications. The increasing operation frequency of various inductive applications requires nanocomposite alloys with higher resistivity to suppress power losses.

In this thesis, the method of measuring as-cast and annealed resistivity of melt-spun ribbon alloys by obtaining alloy densities was established. Archimedes method with deionized water as a medium was used to determine the density of crystalline alloys. A gas pycnometer using dry Helium gas as the medium exhibited improved accuracy in measuring the density of amorphous ribbon alloys compared to the conventional Archimedes method using a liquid medium. This method was applied to previously developed HITPERM (FeCoZrBCu) and HTX002 (FeCoBSiCu) type of alloys as well as carbon-containing (FeCoBCCu) alloys to guide composition adjustments pursuing for improved magnetic properties.

In the HITPERM type of alloys, the composition dependence of as-cast resistivity was studied and simulated by Mott's two-current model with a rigid-band assumption which provided guidance for further adjusting alloy composition looking for higher resistivity. An alloy designed with the Fe:Co ratio for maximum as-cast resistivity and Hf as glass former exhibits low power loss values being approximately 1/4 of those measured on the alloy with the original HITPERM composition for a range of frequencies. The Al and Si additions were found effective to achieve a high resistivity of 151.9 μΩ·cm in the as-cast alloys but also lead to embrittlement of melt-spun ribbons.

Composition adjustments on the HTX002 type of alloys which are castable in air and available for larger-scale production were also explored. Increasing the ferromagnetic late transition metal content by reducing glass formers was found effective to achieve a high saturation induction above 1.75 T for the electric vehicle applications where high induction is prioritized to reduce size and weight. Due to the insufficient glass former contents in these alloys, however, some unique issues such as castability limits, trade-off between high induction and low power losses, high temperature stability, etc. were studied. As-cast resistivity measurements were utilized to quantify the castability limits of maintaining amorphous nature in the as-cast state. The significant effects of Nb content on the trade-off between high induction and low power losses and on the high temperature crystallization processes were investigated.

For the high frequency power conversion applications where high saturation induction can be sacrificed to some extent, HTX002 type of alloys were adjusted in composition pursuing increased resistivity to reduce power losses. Mixed early transition metal content was investigated and the combination of Ta and Nb was found optimal to suppress power losses. High early transition metal content up to 6 at% based on this combination was studied and resulted in larger-scale production.

The effect of Co substitution for Fe was explored in carbon-containing alloys which are promising as high induction soft magnetic materials with acceptable low losses and low costs. Higher saturation induction, lower power losses and better high temperature stability of magnetization were exhibited when the Co content was properly chosen. However, these alloys exhibited low annealed resistivity which resulted in dramatic increase of eddy current loss with operation frequency and hence limited for low frequency applications.

Indexing (document details)
School: Carnegie Mellon University
Department: Materials Science Engineering
School Location: United States -- Pennsylvania
Source: DAI-B 75/02(E), Dissertation Abstracts International
Subjects: Electrical engineering, Materials science
Keywords: Iron cobalt alloys, Power conversion, Resistivity, Soft magnets
Publication Number: 3574866
ISBN: 978-1-303-52031-0
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