By Thomas Schliesch, Head of Research & Development | Max Baermann GmbH
In many cases it certainly makes sense to provide permanent magnets with highest flux densities, e.g. for electrical machines to get large torques or good efficiencies. But on the other hand the quest for largest flux densities can originate also a few disadvantages, both for customers as well as for suppliers of permanent magnets. In detail those issues are often increased costs, lower coercivity or increased instability against demagnetization or temperature. Other problems may be higher brittleness, especially for bonded magnets, as well as minor side effects like larger stray fields or increased cogging torques.
In addition, it is not always easy or sometimes even impossible to fulfill the demands of customers for constantly raising flux densities. In my opinion, really large jumps in remanence of basic hard magnetic materials have not been reached during the last 20 years. For sure some minor improvements have been made, but only in the range of 10 percent may be 15 percent at its best for Rare Earth materials. This is not so bad when keeping in mind that forces or torques are determined by squares of flux density components. But this does not really satisfy the hunger for larger and larger flux densities as it is the case e.g. for wind generators. In this field even the use of superconducting systems is discussed to get the highest performance.
Beside enhanced raw material parameters like remanence, increased flux densities can be provided partially by skillful assemblies of permanent magnets as e.g. by Halbach systems or similar configurations. Those can lead to enhancements of several 10 percent when being compared to conventional configurations. Assembling of such structures, when made of homogenously magnetized segments, usually needs some efforts. In bonded magnets it has always been possible to originate respective distributions inherently directly at injection molding, and nowadays even manufacturers of sintered magnets have become able to provide similar distributions in single specimen, at least for a few systems.
Let us get a bit more into detail about injection molded magnets. Here one method to improve flux density, beside the ways described above, is to increase the volume load of magnetic powder. But the boost of filling grade gets difficult above a specific level. As an example, polyamide based Ferrite or Rare Earth magnets can get unpleasant when filling grades are higher than 65 percent by volume. This means, beside increased brittleness and less mechanical strengths, issues like enlarged flow fronts and deviations from specified directions of magnetization take place. Anisotropic powders usually get worse regarding their orientation due to the enlarged internal friction. Additionally visual appearance can get very bad. In their final application those magnets are endangered much more to fail by crack issues, especially when being exposed to temperature variations as it is the case in automotive industry. Things get much worse when trying to attempt filling grades above 70 percent of volume. There it is nearly impossible to make any stable magnets, at least in a failsafe mass production.
Back to the introductory question about the increase of flux density: The demand is certainly understandable for motors and generators and in all other applications where forces or torques are originated. But not for sensor applications! On one hand in this field respective systems have become much better in sensing low magnetic flux densities. In a few of our current projects we partially deal with effective values at sensor location of only 4 or 5mT, whereas a couple of years ago it was nearly standard not to fall below limits like 25mT. Background for this development are the increased use of GMR based systems, Hall or other sensors with flux concentrators, sensors consisting of arrays of single sensor elements, as well as improved methods of error compensation of sensor output. The impact of these and other developments can be well observed in our daily work by a continuous substitution of former Rare Earth systems by cost effective and price stable Ferrite magnets. Very helpful is in addition, that many customers spend more efforts to keep e.g. distance tolerances within reasonable limits.
Consequently the idea rose up to look at the lower level of filling grades, which came incidentally to my mind, when a customer ordered a magnet which by far was stronger than originally demanded, even when using a standard amount of magnet powder only. This let us decide to use a much lower filling grade than usual. The reaction of all collaborators was stunning after getting the first samples. Everybody was excited about the perfect visual appearance, the wide range of machine parameters that could be used without substantially changing the products characteristics, and the flawless distribution of magnetic fields originated around those magnets. The compound recycling was, in contradiction to usual filling grades, nearly possible up to 100 percent without any magnetic or mechanical loss. Any worries about shrinking issues or increases of cooling time before ejection from the machine were unfounded.
In my opinion, the last example shows that it is worth starting some research on the specific behavior of low to very low filled bonded magnets. Many new and exciting subjects could probably be expected, i.e. improved particle orientation of anisotropic materials, higher degrees of freedom by creating new sorts of pole patterns or an easier mechanical combinability, e.g. with steel parts due to the lack of serious cracking issues. Even the use of quite different sorts of polymers with different new characteristics could be possible. The worldwide use of polymers like polyamides or PPS for injection molded magnets is mainly caused by the large amount of powder particles, which can be mixed into respective compounds. With fairly lower volume rates these constrictions would fall. But also suppliers of sintered or other full dense magnets as well as their customers would benefit, if the aim to get always higher flux densities were released. Less volume as well as lower costs are only two aspects to be mentioned.
About the Author
Thomas Schliesch graduated in Physics from university of Hamburg in 1988 and joined the Max Baermann GmbH, which is a well known manufacturer of bonded magnets in Germany, in 1989. Since 1993 he is Head of Research & Development at Max Baermann GmbH. A major part of his work he devoted to electromagnetic design and the development of specific methods for bonded permanent magnets. Thomas can be reached at t.schliesch@max-baermann.de.
