Hidden Domains

Electric steels are indispensible elements of electric machinery and constitute with about 12 million tons per year 95% of the produced soft magnetic materials. They are produced as GRO, grain oriented, and NOR, non-grain oriented steel sheets for applications with a well defined magnetisation direction, like in transformers, or isotropic magnetisation behavior as required in motors or generators, respectively. The key magnetic properties of these materials include their magnetic hysteresis, remanence, saturation and losses, all of which are tied to their domain structure. Thus, the design and conservation of advantageous domain structures throughout the production process of electric machines are of outstanding importance for high energy efficiency. Neutron imaging provides unique insights to the underlying domain structure as it can access the bulk even through the typical coatings that electrical steel sheets require for isolation between each other when assembled.

In particular neutron dark-field contrast imaging [1,2] resolves either individual domain walls, if domains are sufficiently large to be resolved directly, as is the case e.g. in GRO sheets, and if domain walls are aligned close to parallel to the beam. In materials with smaller domains neutron dark-field contrast imaging probes the local density of domain walls. Based on these capabilities we have investigated in the past the domain structure dependence in GRO steel sheets on laser treatments [3] as well as tensile stresses, such as those caused by coatings, as well as their response to alternating external fields [4-6]. It could be visualized at which applied fields and frequencies the domains freeze and cease to adjust to the external stimulus. In a subsequent step we where able to observe in detail the repetitive movement of domain walls in alternating external fields, the speed of domain growth and shrinkage and the point of domain annihilation depending on the applied field parameters. This study enabled sub-millisecond time resolution through a strobo-kinetic imaging approach [7].  

In contrast, in NOR steels we investigated the influence of different cutting techniques on the local deterioration of the domain structure in the vicinity of the cutting edge [8]. The background of such studies is that electric steels are produced as sheet material in large dimensions. From these sheets the specific shapes required for a particular electric machine have to be cut, The strain induced at the edges by cutting reduces the magnetic conductivity in these areas and leads to losses impacting the machine performance. Thus, such observations enabling an optimisation of production parameters spurred immediate interest in the respective industry and applied research.