The stability of permanent magnet materials is a crucial indicator. Stability primarily refers to the process by which the magnetic properties of a magnet change under the influence of internal and external factors after magnetization, and is commonly expressed as the rate of change of performance indicators. Common causes of changes in magnetic properties include temperature, time, electromagnetic fields, radiation, mechanical vibration, and shock. In this article, we will explore the time stability of permanent magnet materials.
[Image] Prolonged operation or placement of a magnet can lead to changes in its physical and chemical properties due to environmental factors (such as temperature, humidity, and corrosive liquids). After magnetization, most of the permanent magnet is magnetized in a specific direction, but some small magnetic domains have disordered magnetization directions (called antimagnetic nuclei). Under the influence of various environmental factors, existing antimagnetic nuclei can grow, and new antimagnetic nuclei can be generated, causing a decay in the magnetic properties of the permanent magnet. This change is generally a slow and irreversible process from the surface inwards, directly affecting the magnet’s main performance parameters such as remanence, intrinsic coercivity, coercivity, or maximum energy product, and may even lead to complete magnet failure. This loss of magnetic properties is irreversible; even if the magnet is remagnetized, it cannot return to the level it was at before prolonged storage. In recent years, with the widespread application of NdFeB permanent magnet materials in fields requiring long service life, such as aerospace, electric vehicles, and high-power wind power generation, application designers have placed increasing emphasis on the time stability of NdFeB permanent magnets.Long-term stability at room temperature: A 2013 study by Finnish researchers showed that sintered NdFeB magnets (HcJ = 15.6 kOe) placed at room temperature for one year (10,000 hours) showed no perceptible magnetization loss in samples with different Pc values (Pc = -0.33, -1.1, -3.3). The Sanhuan Research Institute also conducted a similar measurement study, which lasted for more than 12 years (4441 days). The intrinsic coercivity of the sintered NdFeB magnet used in the experiment was HcJ=18kOe. The sample was an uncoated cube with a side length of 10.2mm and a magnetic permeability of Pc=-2. There were 8 samples, which were directly exposed to the atmospheric environment in the laboratory at a temperature between 22℃ and 28℃. Observations and measurements were carried out once a year for 12 years.
The data above shows that the relative flux loss measured in the first six years was relatively small, with an inflection point around 2208 days (approximately 6 years). Visually, rust spots are visible on the surface of the black sheet magnet after six years, indicating that oxidation and corrosion have begun on the surface and inside the magnet. Over time, the area of oxidation or corrosion will continuously expand, and the rate of performance degradation will accelerate significantly. Furthermore, this experiment extrapolates the flux loss from the currently measured 4441 days (12 years and 2 months) to 30-50 years. The estimated flux loss over 30 years is less than 1%, and over 50 years it is approximately 1.3%. A 2% loss corresponds to approximately 150 years. (Hollow dots in the image above)
This result indicates that if the lifespan of a magnet is defined as the time corresponding to a 5% flux loss rate, even without a corrosion-resistant coating on the surface, the sintered NdFeB magnets measured so far still have a very long lifespan, conservatively estimated at 30-50 years.
Typically, significant flux loss originates from oxidation or corrosion of the magnet surface, resulting in irreversible losses. Among various rare-earth permanent magnet materials, sintered NdFeB exhibits the most severe such losses. However, through composition optimization and surface protection treatments, the oxidation and corrosion resistance of sintered NdFeB magnets has been greatly improved. Therefore, with proper surface protection, the lifespan of sintered NdFeB magnets with sufficiently high HcJ can easily exceed 30-50 years. (This is under the condition of not exceeding the operating temperature.)
Long-term stability at high temperatures: The following figure shows the change in relative flux loss over time for magnets with different Pc values and HcJ=20.1 kOe at 80℃, 120℃, and 150℃.
It is evident from the graph above that, for the same Pc value, the higher the storage temperature of the magnet, the faster the relative flux loss decreases. Magnets with lower absolute Pc values exhibit significantly greater initial and long-term magnetization losses than those with higher Pc values, and both types of losses increase dramatically with increasing temperature. Given that technical and cost constraints prevent further increases in HcJ, increasing the absolute Pc value can effectively suppress magnetization loss.
The time relationship between relative magnetization loss and high-temperature magnetization loss for magnets with different HcJs and Pc values at different temperatures shows that HcJ has a significant impact on high-temperature magnetization loss; higher HcJ results in lower magnetization loss, and high-temperature stability requires magnets to possess high HcJ. Simultaneously, the permeability Pc also determines the long-term high-temperature magnetization loss of the magnet.