Physical Properties of Permanent Magnets
Physical properties of permanent magnets encompass broader material and structural characteristics, including both magnetic and non-magnetic attributes.
Specifically physical properties of permanent magnet include magnetic properties (like remanence and coercivity) but also:
- Material Composition: Type of material (e.g., neodymium, ferrite, samarium cobalt), which affects magnetic strength and durability.
- Density and Weight: Physical mass and compactness, critical for applications like lightweight badge magnets.
- Temperature Stability: How well the magnet performs under varying temperatures (e.g., Curie temperature, where magnetism is lost).
- Mechanical Strength: Resistance to physical damage, corrosion, or wear, important for durability in products like stainless steel-backed badge magnets.
- Shape and Size: Geometric factors influencing field distribution and application suitability.
1. Maximum Working Temperature:
Maximum Working Temperature is the highest temperature at which a magnet can operate effectively without losing a significant amount of its magnetic strength permanently. Different magnetic materials have different maximum working temperatures. Exceeding this temperature leads to irreversible loss in the magnet's strength due to changes in its microstructure.
2. Curie Temperature:
The temperature coefficient of Br, or remanence, measures how a permanent magnet’s magnetic flux density (Br) changes with temperature. Expressed as a percentage per degree Celsius (%/°C), it indicates the magnet’s sensitivity to temperature variations. For example, neodymium magnets typically have a temperature coefficient of -0.12%/°C, meaning Br decreases as temperature rises. This property is critical for applications like motors, where temperature stability affects performance. Materials like samarium-cobalt exhibit lower coefficients, offering better thermal stability.
4. Temperature Coefficient (Hc)
The temperature coefficient of Hc, or coercivity, quantifies how a permanent magnet’s ability to resist demagnetization (Hc) changes with temperature. Expressed as a percentage per degree Celsius (%/°C), it indicates the magnet’s stability under thermal variations. For instance, neodymium magnets typically have a temperature coefficient of Hc around -0.5 to -0.6%/°C, meaning coercivity decreases as temperature rises. This is crucial for applications like electric motors, where maintaining magnetic strength under heat is essential. Samarium-cobalt magnets show better thermal stability.
5. Recoil Permeability
Recoil permeability measures a magnetic material’s ability to recover magnetization after demagnetization, represented as the slope of the recoil line in the hysteresis loop. It’s the ratio of magnetic flux density (B) to field strength (H), typically ranging from 1 to 10 for permanent magnets. Higher values indicate better magnetic stability.
6. Saturation Field Strength
Saturation field strength is the magnetic field intensity needed to fully magnetize a material, where magnetic flux density (B) reaches its maximum. For ferromagnetic materials, this occurs on the hysteresis loop’s plateau, typically at 10-20 kA/m for soft magnets. Beyond this, magnetization doesn’t increase significantly. The graph below shows a hysteresis loop, highlighting saturation.
7. Coefficient Thermal Expansion ( perpendicular to DOM )
The coefficient of thermal expansion (perpendicular to the direction of magnetization, DOM) of a permanent magnet measures how its dimensions change with temperature, expressed in µm/(m·K). For neodymium magnets, it’s typically 5–7 µm/(m·K), indicating moderate expansion. Ferrite magnets have a higher coefficient, around 10–15 µm/(m·K). This property is critical in applications like motors, where dimensional stability under temperature changes ensures mechanical integrity and consistent magnetic performance perpendicular to the magnetization axis.
8. Coefficient Thermal Expansion ( parallel to DOM )
The coefficient of thermal expansion (parallel to the direction of magnetization, DOM) of a permanent magnet measures its dimensional change along the magnetization axis with temperature, expressed in µm/(m·K). For neodymium magnets, it’s typically around 1–3 µm/(m·K), showing low expansion. Ferrite magnets have a higher coefficient, about 7–10 µm/(m·K). This property is vital in applications like motors, ensuring dimensional stability along the magnetization axis under thermal variations for consistent magnetic performance.
9. Density
The density of a permanent magnet varies depending on its material composition, typically ranging from 7.4 to 8.3 g/cm³ for common types like neodymium-iron-boron (NdFeB) and samarium-cobalt (SmCo). For instance, NdFeB magnets, known for their high magnetic strength, have a density around 7.5–7.6 g/cm³, while ferrite magnets are lighter, around 4.8–5.0 g/cm³. The density is influenced by the magnet’s alloy components and manufacturing process, impacting its weight and suitability for applications like motors or sensors where size and mass are critical.
10. Vickers Hardness
The Vickers hardness of a permanent magnet measures its resistance to surface indentation and wear, expressed in HV units. Neodymium magnets typically have a Vickers hardness of 600–700 HV, indicating good resistance to scratching but brittleness. Ferrite magnets are slightly lower, around 400–500 HV. This property is crucial in applications like motors or sensors, where surface durability impacts longevity and performance under mechanical contact or abrasive conditions, despite the magnet’s inherent fragility.
11. Bending or Flexural Strength
Flexural strength of a permanent magnet measures its ability to withstand bending forces before fracturing, typically expressed in MPa. It reflects the magnet’s mechanical durability under stress. For example, neodymium magnets, with a flexural strength around 200–400 MPa, are brittle and prone to cracking under bending. Ferrite magnets, with lower values (~100 MPa), are similarly fragile. This property is crucial for applications involving mechanical loads, like motors, where structural integrity impacts performance and longevity.
12. Compressive Strength
The compressive strength of a permanent magnet measures its ability to withstand forces that attempt to crush or compact it, typically expressed in MPa. For example, neodymium magnets exhibit high compressive strength, around 800–1000 MPa, but remain brittle under other stresses. Ferrite magnets have lower compressive strength, approximately 600–700 MPa. This property is critical in applications like industrial motors, where magnets face compressive forces, ensuring they maintain structural integrity under high-pressure conditions.
13. Tensile Strength
The tensile strength of a permanent magnet measures its ability to resist breaking under tension, typically expressed in MPa. Neodymium magnets, despite strong magnetic properties, have low tensile strength, around 30–80 MPa, making them brittle and prone to fracture under pulling forces. Ferrite magnets are similarly weak, with tensile strength around 20–50 MPa. This property is crucial in applications like magnetic assemblies, where tensile stresses could compromise the magnet’s structural integrity and performance.
14. Youns Modulus
Young’s modulus of a permanent magnet measures its stiffness, indicating how much it deforms under tensile or compressive stress, expressed in GPa. For neodymium magnets, Young’s modulus is typically around 150–170 GPa, reflecting high stiffness but brittleness. Ferrite magnets have a lower modulus, about 100–130 GPa, indicating slightly more flexibility but still fragility. This property is key in applications like motors, where mechanical stability under stress impacts the magnet’s durability and performance.
15. Possion's Ratio
Poisson’s ratio for magnetic materials measures transverse strain relative to axial strain during magnetostriction, typically ranging from 0.2 to 0.5 for ferromagnetic materials. It quantifies how materials deform under magnetic fields. For example, in nickel, applying a magnetic field causes dimensional changes, with transverse contraction proportional to axial expansion. The graph below plots axial and transverse strain against magnetic field strength.
16. Thermal Conductivity
The thermal conductivity of a permanent magnet measures its ability to conduct heat, typically expressed in W/(m·K). Neodymium magnets have moderate thermal conductivity, around 7–9 W/(m·K), while ferrite magnets are lower, at 3–5 W/(m·K). This property affects how magnets dissipate heat in applications like electric motors, where high temperatures can degrade performance. Higher thermal conductivity helps maintain magnetic stability by preventing overheating, making it critical for high-performance, heat-intensive environments.
17. Electrical Resistivity
The electrical resistivity of a permanent magnet measures its ability to resist the flow of electric current, typically expressed in μΩ·cm. Neodymium magnets have low resistivity, around 120–160 μΩ·cm, making them moderately conductive. Ferrite magnets, being ceramic-based, have much higher resistivity, around 10^6 μΩ·cm, acting as insulators. This property is critical in applications like electric motors, where low resistivity can lead to eddy current losses, affecting efficiency, while high resistivity minimizes such losses.
18. Specific Heat
The specific heat of a permanent magnet measures the amount of heat energy required to raise the temperature of a unit mass by one degree, typically expressed in J/(kg·K). For neodymium magnets, specific heat is around 460–500 J/(kg·K), while ferrite magnets are higher, about 600–800 J/(kg·K). This property is crucial in applications like motors, where it affects how magnets respond to thermal loads, influencing their temperature stability and performance under operating conditions.