Dayton Contributes to the History of Magnetic Materials

Dayton has played an important role in the history of magnetic materials. A variety of magnet that you may be using every day was invented here: the modern rare earth permanent magnet. Dr. Sam Liu (Metals and Ceramics) currently is the principal investigator for magnetic materials research in the Research Institute, working with Dr. Ed Kuhl and students Jie Yang and George Doyle to continue the leading-edge magnetic materials research that over the years has led to a new class of magnetic materials.

Everyone knows what a magnet is, but there are actually many different types of magnets, with varying properties and applications. What we normally think of as a magnet, like a refrigerator magnet, is actually a "permanent" magnet, which retains its magnetic properties indefinitely. The first magnet known by humans was lodestone, a form of the mineral magnetite (Fe3O4), which is naturally magnetic. The name "magnetite" comes from its source near the ancient city of Magnesia in Asia Minor. Early mariners used lodestone (or "way stone") to magnetize iron needles for compasses.

After the invention of the electromagnet in 1825 (which requires an electric current for a magnetic force to appear), interest developed in creating new magnetic materials. Various metallic alloys were tested, and it was learned that iron did not need to be present for a substance to exhibit magnetism. In Japan in 1938, it was learned that permanent magnets could be created from powdered oxides, such as iron oxide. This was the first in the class of permanent magnets known as ferrites, which are now commonly used because they are relatively inexpensive to make.

Permanent magnets can be ranked according to their "energy product," or the density of the useful magnetic energy that can be stored in a magnet. Ferrites and AlNiCo (aluminum-nickel-cobalt) magnets have a relatively low energy product but are suitable for many commercial applications. PtCo (platinum-cobalt) has a little higher energy product (between 7 and 9 MGOe), but its application is limited by the cost of platinum. Researchers continued searching for permanent magnets with even higher energy products through the 1940s and 1950s.

The Dayton story begins with Dr. Karl Strnat and Dr. Alden Ray. In the 1960s, while a civilian researcher at the Wright-Patterson AFB Materials Laboratory, Dr. Strnat was attempting to better understand the origins of magnetic properties. UDRI employee Dr. Ray served as the metallurgical consultant to Dr. Strnat’s group. Dr. Strnat was intrigued by some little-noticed observations made during studies of rare earth compounds. Between 1946 and 1952 the study of rare earth metals had been greatly accelerated by advances in chemical separation techniques developed in association with the Manhattan Project. Methods for producing pure rare earth metals in quantity were developed which, in turn, stimulated interest in the use of rare earth metals as alloying additions. Dr. Strnat also took note of the high crystal anisotropy of rare earth-cobalt compounds. An anisotropic substance exhibits different properties depending on which direction across the specimen you are measuring. He knew that this was thought to be an important factor in the presence of magnetic properties in a substance. It had also been reported that GdCo5 (gadolinium-cobalt) exhibited high coercivity (another measure of magnetic force), but this finding did not receive much attention because of the high price of gadolinium.

Dr. Strnat’s critical contribution was to realize that the entire rare earth element family presented an opportunity for developing powerful new magnetic compounds. He started trying many possible compounds, and in 1965, he discovered very favorable magnetic properties in YCo5 (yttrium-cobalt). Then in 1966, he discovered SmCo5 (samarium-cobalt), which had an energy product that turned out to be much higher than any previous permanent magnets. This discovery of the class of magnets known as rare earth permanent magnets prompted a renaissance in research into new magnet materials.

In 1968, Dr. Strnat joined the faculty of the University of Dayton, and continued his work in the area of rare earth permanent magnets. Dr. Strnat and Dr. Ray collaborated on rare earth magnet research through the 1970s, eventually developing the second generation of rare earth permanent magnets, represented by Sm2Co17. These substances had even higher energy products and were commercialized in the early 1980s. Dr. Ray went on to propose a metallurgical model for the Sm2Co17 system, contributing a significant new understanding of this very complex metallurgical system.

Rare earth permanent magnets are used in a variety of settings. The Sm-Co magnets are most often only used in military and space applications because of their expense, and because they can hold large magnetic fields at high temperatures. Other magnets lose their field strength at elevated temperatures and would need to be cooled; cooling apparatus would contribute unwanted weight and take up space. Applications include magnetrons, paths for microwave energy such as circulators and isolators, inertial devices such as gyroscopes and accelerometers, reaction and momentum wheels (to control and stabilize satellites), motors, actuators, generators, holding and coupling devices, and magnetic bearings.

A "third generation" of rare earth permanent magnets was announced in 1983: Nd-Fe-B (neodymium-iron-boron) compounds. These compounds have an advantage over the Sm-Co compounds in that the elements are less scarce, and higher energy densities can be created. However, these compounds are not as suitable for high temperature applications, as they tend to lose their magnetic properties at high temperatures. They are also not as stable chemically, and are more susceptible to corrosion. Most of the fundamental research for this system was done in the US in the 1970s and the new magnets were commercialized in Japan and in the US in 1980s. Because of its relative advantages and disadvantages, this third generation of rare earth magnets sees more commercial application. They are used in motors in hard and floppy disk drives, and in printers. Optical CD drives use rare earth permanent magnets. One can buy "high end" speakers and microphones that contain rare earth permanent magnets. They also see application in microwave amplifiers and medical MRI units. Gold-plated rare earth permanent magnets are even being sold today to "naturally" heal a variety of ailments.

The current home for UDRI permanent magnet research is Dr. Liu’s Magnetics Laboratory, with facilities located in the Annex and the fifth floor of Kettering Laboratories. Dr. Liu has been intrigued by the mystery of magnetism since his childhood in China, and grew up to do rare earth permanent magnet research there. Dr. Liu took the opportunity to come to the home of the Wright Brothers and rare earth magnets (both of which were synonymous in his mind with Dayton) in 1980, when he received sponsorship from the Chinese government to be a visiting scholar at the University of Dayton. After two years he returned to China to teach and do further research. In 1986 he returned to UD to obtain a doctoral degree, which he completed in 1989, with Dr. Ray serving as Dr. Liu’s committee chair.

Dr. Liu was hired by UDRI to search for further improvements in the magnetic properties of Sm2Co17. Guided by Dr. Ray’s model, he successfully developed new Sm2Co17-type permanent magnets with high saturation magnetization and obtained a record energy product of 34 MGOe in 1989 (UDRI still holds this world record). Another record energy product was obtained in 1990 for temperature-compensated (Sm,Gd)2Co17 magnets. This material retains a high energy product with increasing temperature. UDRI was also the first to develop sintered light rare earth substituted (Sm,Pr,Nd)2Co17 magnets that have even higher saturation magnetization.

Dr. Liu is also proficient in computer programming. Among other programs he developed to help in his ongoing research projects, Sam successfully developed a computer-aided alloy composition selection method that can accurately predict temperature coefficients of magnetization for temperature-compensated rare earth magnet alloys. This tool saves a tremendous amount of laboratory time.

More recently, Dr. Liu has been involved in research to develop new high-temperature magnetic materials. The US Air Force has proposed a "more electric aircraft" initiative, in which the conventional hydraulic system will be replaced by an electromagnetic system. This application would require that magnetic materials operate at temperatures higher than before, increasing from the current 300°C to greater than 400°C. Cooperating with the Electron Energy Corporation, Sam and his coworkers have been developing new magnet materials with significantly improved high temperature stability.

Dr. Liu has upheld the University of Dayton reputation by making 24 presentations at international conferences since he joined UDRI in 1989. His recent presentation on high temperature magnets at the 7th Joint MMM-INTERMAG Conference in January 1998 in San Francisco drew a great deal of attention from magnetics researchers. Worldwide, Dayton continues to be recognized for its significant role in permanent magnet research.

June 1998
by Julia Phelps

For more information about magnetics research at UDRI, contact Dr. Sam Liu at (937) 229-3527.