NASA Glenn is conducting in-house research on rare earth permanent magnets to assist in developing a free-piston Stirling convertor for the Advanced Stirling Radioisotope Generator (ASRG) and Fission Surface Power (FSP) applications. The permanent magnet research efforts include magnet characterization and magnet aging tests.
Magnet characterization is typically performed on 1-cm cube neodymium-iron-boron rare earth permanent magnets obtained from various vendors. The remanence (Br), intrinsic coercivity (Hci) and the magnetization are measured for each of the magnet samples over the temperature range of 20 ºC to 140 ºC.
Based on the results of the magnet characterization tests, several magnet grades are selected for a short-term magnet aging test. The short-term magnet aging test is a 200 hour test, with the magnet samples exposed to elevated temperatures and external demagnetizing fields. Magnet grades that survive the short-term magnet aging test are then subjected to the long-term magnet aging test. The long-term magnet aging test is an 18,000 hour test, with the magnet samples exposed to elevated temperatures and external demagnetizing fields.
A 1-cm cubic magnet is normally the sample geometry used to characterize the magnetic properties. The first step is to magnetize the sample in a process called charging. The magnetic properties can then be measured using an electromagnet and a pair of sensing coils. The B-H curve (normal induction curve) can then be calculated from the M-H curve (intrinsic induction curve).
Finally, magnet sample’s magnetic properties such as remanence, intrinsic coercivity, and coercivity are determined. Magnet characterization is normally performed at GRC for one of three reasons: 1) to gain insight into how a magnet will perform at various temperatures, 2) to measure the magnetic properties prior to conducting a magnet aging test, or 3) to measure the magnetic properties at the conclusion of a magnet aging test.
Magnet aging is determined by comparing a magnet’s pre-aging characterization data ( Br and Hci) with its post-aging characterization data. The important magnetization loss data is the decrease in Br while the important measure of loss in resistance to demagnetization is the decrease in Hci. After post-aging magnet characterization, the magnet samples are recharged, and then re-characterized to see if any permanent changes occur during testing.