One of the most commonly asked questions about rare earth permanent magnets is the effects of thermal cycling, for which there is no rule of thumb. Initially, thermal cycling will degrade the performance of both Neodymium Iron Boron (NdFeB) and Samarium Cobalt magnets (SmCo). There are two primary mechanisms which drive this degradation:
- Reorientation of weak domains – occurs rapidly
- Grain boundary oxidation – occurs over a longer period
The Primary Mechanisms Driving Degradation
The first demagnetisation effect, reorientation of weak domains, is a result of some of the magnetic domains in the alloy reorienting and no longer contributing to the overall net magnetic field of the magnet. These reorient magnetic domains are considered “weak”, as they are the most prone to external affects. Since there are always “weak magnetic domains”, this type of loss will always occur in magnets. However, thermal cycling can make the loss worse.
Although the initial magnet degradation occurs within the first few thermal cycles, it is an advantage. Mainly because the magnets can be easily thermally cycled prior to integration into the next assembly. This pre-cycling removes the inconsistency between magnets and pre-degrades the magnets to a level that they would eventually achieve in operation.
The second demagnetisation effect goes to grain boundary oxidation and thermal creep. Basically, the shift regions between some of the domains start to breakdown. The weaker domains, now no longer fully constrained, partially reorient and contribute less to the net field generated by the magnet. This type of degradation will always occur, but thermal cycling can make it worse. For high quality, fully saturated magnets, this effect usually takes years to happen.
Both effects are usually minor, however, the degree of loss depends on the quality of the magnet. The effects are difficult to anticipate and measure because experimental data is lacking. There have been numerous IEEE papers on this subject, but they fail to offer a functional correlation between the experimental data and an actual rare earth magnet’s net degradation over a lifetime. However, it does show that most losses occur within the initial thermal cycles, which is already known and accepted within the magnet industry. It is important to note that longer term effects from thermal cycling are less definite, thus there are no concrete conclusions regarding long term effects.
Thermal cycling must always reside in the temperature extremes tolerated by the magnet. Meaning that, regardless of the number of thermal cycles, the temperature cycle range must not exceed the magnet’s heat tolerance capability. If a magnet is exposed to heat higher than its upper threshold, a different demagnetising loss is experienced. The magnet will then no longer be an exclusive loss from thermal cycling. Other key considerations include:
- There is no general rule of thumb for estimating the magnetic loss in a magnet from thermal cycling
- Most of the loss will occur within the first few thermal cycles
- Grain boundary oxidation “magnetic creep” is a very long-term effect and proves difficult to estimate
- Generally, higher quality magnets are less likely to suffer thermal demagnetising affects than lower quality magnets
Higher quality magnets:
- Contain less contaminates in their matrix
- Have better control of heat distribution during the sintering process
- Have better magnetic orientation during the pressing
- Commonly, magnets with higher a Hci will resist all magnetising effects better than magnets with having a lower Hci
Thermal cycling will reduce a magnet’s magnetic output, but it is difficult to anticipate the degree of degradation. The higher the quality of the magnet, the greater likelihood the magnet will resist degradation from thermal cycling. At Goudsmit UK we specialise in the sub-contract manufacturing of high quality magnets and magnetic assemblies. For more information contact us today at email@example.com, or if you’d like to speak to a member of our team call us on +44 (0) 2890 271 001.