Heat-resistant neo magnet breakthrough

Researchers from the U.S. Department of Energy's Critical Materials Institute, led by Ames National Laboratory, have developed a "Hot-roll Nano Neo Magnet" method for manufacturing high-performance neodymium-iron-boron permanent magnets that are heat-resistant, easier, and greener to produce.

Neodymium is a rare earth metal that tarnishes rapidly when exposed to air. In 1983, it was discovered that when alloyed with iron and boron, it would create a magnetic force 10 times stronger than all other ferrite, ceramic or alnico (aluminum-nickel-cobalt) magnets of the same size.

Neodymium magnets continue to be the most powerful commercially produced magnet on the market, with a single magnet weighing only a few grams can hold up to a thousand times its own weight.

However, not all magnets are created equal. Neodymium-iron-boron permanent magnets need additional coating so as not to tarnish and have a low-temperature resistance to prevent their magnetic properties from degrading permanently.

As a rule, excess heat speeds up the rate at which the particles within the magnet move, causing misalignment and loss of magnetism. These magnets lose 0.11% of their magnetism for every degree Celsius rise in temperature. This loss in magnetic strength can recover when temperatures fall back to normal levels as long as the maximum operating temperature is not exceeded; otherwise, degradation is permanent and successive hot-cold cycles will deteriorate its performance further. They can be remagnetized, but their strength is irreversibly compromised.

Special grades are available that can withstand higher temperatures of up to 446°F (230°C) but lose other properties. All grades are affected by magnetic fields and radiation, limiting the environment in which the magnet can be used (for example, a neo magnet on its own will demagnetize at a lower temperature than the same magnet connected to metal).

In industrial applications, these magnets need to function in much higher temperatures and resist performance degradation. The need to prevent heat-induced demagnetization is especially important for wind turbines and electric vehicle motors but is also important in everyday technology applications like hard drives, smartphones, and speakers.

Critical Materials Institute researchers have developed a new commercially-scalable manufacturing method that results in a nanograin neodymium magnet that is encased in stainless steel to help prevent demagnetization. As a bonus, the process they developed is semi-continuous compared to the batches currently produced by the industry, making it more cost-effective and energy efficient.

A new neo magnet

Jun Cui, associate professor of materials science and engineering and senior scientist at DOE Ames Laboratory, explains that there are two traditional ways to address the heat-induced demagnetization challenge – add a bit of dysprosium, a rare earth in very limited supply, to the neodymium magnet, or begin the process with much smaller particles of the magnetic material, resulting in finer-grained magnets.

In addition to the rarity of dysprosium, the process of making magnets with this rare earth involves heating the material to extremely high temperatures to increase density.

The fine-grained magnet method traditionally involves packing the grains extremely tightly before being subjected to two further phases of hot deformation to densify the magnet. Once these magnets are formed, they undergo a final nickel coating process to resist tarnishing.

The Ames Lab researchers steered away from dysprosium and instead developed a streamlined fine-grain neodymium magnet-making process.

This process begins with grains measured in nanometers rather than microns – a human hair being typically 70 microns in diameter, with one micron being equal to 1,000 nanometers.

The micron-sized particles are exposed to a magnetic field to uniformly align the magnetic poles of each particle, which are then compacted to fuse into a dense solid.

"We end up just starting out as powders and then we pack them into a stainless-steel tube. We pack them really dense and then we just hot roll them," Cui said. "We heat it up and then send it to the rolling mill and then the whole thing just goes."

The inherent benefits to this process are that the materials remain completely sealed in a stainless-steel casing throughout the process, negating the need for a vacuum furnace to protect the magnet materials from the air or a coating process afterward. This also allows for the creation of thinner magnets that maintain their structural integrity and properties.

This method is also expected to be easier to scale than the multi-step batch process traditionally used.

"We can make very long magnets continuously, which can be sliced into numerous smaller magnets," Cui said. "So now you are suddenly looking at a completely new way of making magnets that is cost effective."