The long-term stability of magnets is a concern of every user. The stability of samarium cobalt (SmCo) magnets is more important for their harsh application environment. In 2000, Chen[1] and Liu[2] et al., studiy the composition and structure of high-temperature SmCo, and developed high-temperature-resistant samarium-cobalt magnets. The maximum operating temperature (Tmax) of SmCo magnets was increased from 350°C to 550°C. After that, Chen et al. improved the oxidation resistance of SmCo by depositing nickel, aluminum and other coatings on the SmCo magnets.
In 2014, Dr. Mao Shoudong, the founder of “MagnetPower”, systematically studied the stability of SmCo at high temperatures, and the results were published in JAP[3]. The general results are as follows:
1. When SmCo is in a high-temperature state (500°C, air), it is easy to form a degradation layer on the surface. The degradation layer is mainly composed of an external scale (Samarium is depleted) and an internal layer (a lot of oxides). The basic structure of the SmCo magnets was completely destroyed in the degradation layer. As shown in Figure 1 and Figure 2.
Fig.1. The optical micrographs of the Sm2Co17 magnets isothermal treated in air at 500 °C for different times. The degradation layers under surfaces which are (a) parallel and (b) perpendicular to the c-axis.
Fig.2. BSE micrograph and EDS elements line-scan across the Sm2Co17 magnets isothermal treated in air at 500 °C for 192 h.
2. The main formation of the degradation layer significantly affects the magnetic properties of SmCo, as shown in Figure 3. The degradation layers were mainly composed of Co(Fe) solid solution, CoFe2O4, Sm2O3, and ZrOx in the internal layers and Fe3O4, CoFe2O4, and CuO in the external scales. The Co(Fe), CoFe2O4, and Fe3O4 acted as soft magnetic phases compared to the hard magnetic phase of the central unaffected Sm2Co17 magnets. The degradation behavior should be controlled.
Fig. 3. The magnetization curves of Sm2Co17 magnets isothermal treated in air at 500 °C for different times. The test temperature of the magnetization curves is 298 K. The external field H parallels to the c-axis alignment of the Sm2Co17 magnets.
3. If coatings with high oxidation resistance is deposited on SmCo to replace the original electroplating coatings, the degradation process of SmCo can be more significantly inhibited and the stability of SmCo can be improved, as shown in Figure 4. The application of OR coating significantly inhibit the weight increase of the SmCo and the loss of magnetic properties.
Fig.4 the structure of the oxidation resistance OR coating on the Sm2Co17 magnet.
“MagnetPower” has since carried out experiments of long-term stability (~4000hours) at high-temperature, which can provide a stability reference of SmCo magnets for the future use at high temperatures.
In 2021, based on the maximum operating temperature requirement, “MagnetPower” has developed a series of grades from 350°C to 550°C （T series）. These grades can provide sufficient choices for high-temperature SmCo application, and the magnetic properties are more advantageous. As shown in Figure 5. Please refer to the web page for details: https://www.magnetpower-tech.com/t-series-sm2co17-smco-magnet-supplier-product/
Fig.5 The high temperature SmCo magnets (T series) of “MagnetPower”
CONCLUSIONS
1. As a highly stable rare earth permanent magnets, SmCo can be used at high temperature (≥350°C) for a short period of time. The high temperature SmCo (T series) can be applied at 550°C without irreversible demagnetization.
2. However, if the SmCo magnets were used at high temperature (≥350°C) for a long time, the surface is prone to produce a degradation layer. The use of anti-oxidation coating can ensure the stability of the SmCo at high temperature.
Reference
[1] C.H.Chen, IEEE Transactions on Magnetics, 36, 3291-3293, (2000);
[2] J.F. Liu, Journal of Applied Physics, 85, 2800-2804, (1999);
[3] Shoudong Mao, Journal of Applied Physics, 115, 043912,1-6 (2014)
Post time: Jul-08-2023
