Tag: dysprosium

[Paper] “Thermal Activation in Permanent Magnets” published in JOM

Explanation of method for calculating the thermally activated coercivity of using micromagnetics.

Explanation of method for calculating the thermally activated coercivity of using micromagnetics.

This week our new paper titled “Thermal Activation in Permanent Magnets” has been published in JOM (Springer). The invited paper is under a special topic, “Permanent Magnets beyond Nd-Dy-Fe-B“.  An author manuscript (reprint) is available here.

In the paper we provide a more detailed overview of the micromagnetic methods we have developed to model the thermal activation of permanent magnets. These methods allow us to simulate and understand the behaviour of permanent magnets at finite temperatures, which is important since the generators in wind turbines and electric motors in green cars operate at higher temperatures. For example, in electric cars the typical operation temperature of the motors can be around 450ºK (177º C).

Using two examples from our work with Toyota and the ROMEO project we highlight the importance of reversal mechanisms in explaining the observed performance (for example, coercivity) of the magnets.

The paper is initially published “online first” here with the permanent DOI 10.1007/s11837-015-1415-7. It can be cited as follows:

S. Bance, J. Fischbacher, A. Kovacs, H. Oezelt, F. Reichel, T. Schrefl, “Thermal Activation in Permanent Magnets”, JOM  2015 DOI:10.1007/s11837-015-1415-7

Abstract

The coercive field of permanent magnets decays with temperature. At non-zero temperatures, the system can overcome a finite energy barrier through thermal fluctuations. Using finite element micromagnetic simulations, we quantify this effect, which reduces coercivity in addition to the decrease of the coercive field associated with the temperature dependence of the anisotropy field, and validate the method through comparison with existing experimental data.




New paper; Thermally-activated coercivity in core-shell permanent magnets

Our new paper titled “Thermally-activated coercivity in core-shell permanent magnets” has been accepted for publication in Journal of Applied Physics. Click here to download the paper as a PDF file.

In the paper we describe recent micromagnetics simulations on NdFeB grains that have undergone a dysprosium (Dy) grain boundary diffusion process (GBDP). The super hard (Dy,Nd)FeB shell that is formed during this process stabilizes the grains against thermal fluctuations that can be detrimental to the coercivity of the magnet in high-temperature situations. NdFeB permanent magnets are usually doped with Dy to increase their performance at high temperatures but the GBDP allows us to target the Dy at the most important locations i.e. the grain surface, thus reducing the overall required amount of Dy, which is expensive and in short supply.

Such magnets are critical components of wind turbines, electric/hybrid vehicles, marine propulsion and manufacturing machinery. By working to improve the performance of permanent magnets we can make such machines lighter and more energy efficient.

Fig3small

Simulations of the reversal process in NdFeB grains with (i) a pure NdFeB grain, (ii) a NdFeB grain with a soft outer defect and (iii) NdFeB core, (Dy, Nd)FeB shell and an outer soft defect. Thermally-activated coercive field values are indicated with the field direction (red arrows). The saddle point image is the configuration with the highest total energy, forming the peak of the energy barrier.