Magnets
- Most people are familiar with some of the uses of magnetic fields and forces. Magnets can be used to see inside a person's skull with medical technology, they can be used to create an electric current in a dynamo, and they are particularly handy for sticking things to a refrigerator. But what it is that makes a magnet work, and which gives it all of the unique properties, is called its magnetic field. Explaining what a magnetic field is, how it's generated, and what effect that can have on certain metals, though, is a little more difficult to explain.
Magnetic Fields
- All magnets create a magnetic field. This field can be thought of as an invisible sphere around a magnet that is the limits of its powers of attraction. A magnetic field is created by a flow of electrons, which are negatively charged particles. For instance, a magnetic field can be created by running a charge through a coil of copper wire wrapped around a nail. This sort of magnet is called an electromagnet, since the field is being created by an electric current. Natural magnets need no current, but they create this field of moving electrons naturally.
Attraction
- While it's commonly thought that magnets attract metals in general, there is a select group of metals that responds to magnetic fields. Those metals include nickel, iron, steel, and cobalt. Due to the makeup of these metals when they're placed in an electric field they're attracted to the magnet that's the source. If a metal is left in contact with a magnet and its magnetic field long enough, then that metal may take on temporary magentic properties as well. This is because of how a magnet aligns the atoms and electrons of the magnetic metals... but once they return to their former layout, then the temporary magnetism goes away.
Magnetic properties
Some important properties used to compare permanent magnets are: remanence (Mr), which measures the strength of the magnetic field;coercivity (Hci), the material's resistance to becoming demagnetized; energy product (BHmax), the density of magnetic energy; and Curie temperature (TC), the temperature at which the material loses its magnetism. Neodymium magnets have higher remanence, much higher coercivity and energy product, but often lower Curie temperature than other types. Neodymium is alloyed with terbium and dysprosium in order to preserve its magnetic properties at high temperatures.[3] The table below compares the magnetic performance of neodymium magnets with other types of permanent magnets.Magnet Mr (T) Hci (kA/m) BHmax (kJ/m3) TC (°C) Nd2Fe14B (sintered) 1.0–1.4 750–2000 200–440 310–400 Nd2Fe14B (bonded) 0.6–0.7 600–1200 60–100 310–400 SmCo5 (sintered) 0.8–1.1 600–2000 120–200 720 Sm(Co, Fe, Cu, Zr)7 (sintered) 0.9–1.15 450–1300 150–240 800 Alnico (sintered) 0.6–1.4 275 10–88 700–860 Sr-ferrite (sintered) 0.2–0.4 100–300 10–40 450
Physical and mechanical properties
Comparison of physical properties of sintered neodymium and Sm-Co magnets[4] Property Neodymium Sm-Co Remanence (T) 1–1.3 0.82–1.16 Coercivity (MA/m) 0.875–1.99 0.493–1.59 Permeability 1.05 1.05 Temperature coefficient of remanence (%/K) −0.12 −0.03 Temperature coefficient of coercivity (%/K) −0.55..–0.65 −0.15..–0.30 Curie temperature (°C) 320 800 Density (g/cm3) 7.3–7.5 8.2–8.4 CTE, magnetizing direction (1/K) 5.2×10−6 5.2×10−6 CTE, normal to magnetizing direction (1/K) −0.8×10−6 11×10−6 Flexural strength (N/mm2) 250 150 Compressive strength (N/mm2) 1100 800 Tensile strength (N/mm2) 75 35 Vickers hardness (HV) 550–650 500–550 Electrical resistivity (Ω·cm) (110–170)×10−6 86×10−6
Applications
[edit]In technology
Neodymium magnets have replaced alnico and ferrite magnets in many of the myriad applications in modern technology where strong permanent magnets are required, because their greater strength allows the use of smaller, lighter magnets for a given application. Some examples are
- head actuators for computer hard disks
- magnetic resonance imaging (MRI)
- magnetic guitar pickups
- loudspeakers and headphones
- magnetic bearings and couplings
- electric motors:
- cordless tools
- servo motors
- lifting and compressor motors
- synchronous motors
- spindle and stepper motors
- electrical power steering
- drive motors for hybrid and electric vehicles. The electric motors of each Toyota Prius require 1 kilogram (2.2 pounds) of neodymium.[3]
- actuators
- electric generators for wind turbines; up to 600 kg of PM material per megawatt (Neodymium content is estimated to be 31% of magnet weight).[1]
Demand for neodymium in electric vehicles is estimated to be 5 times larger than that in wind turbines.
Other applications
In addition, the greater strength of neodymium magnets has inspired new applications in areas where magnets were not used before, such as magnetic jewelry clasps, children's magnetic building sets (and other neodymium magnet toys) and as part of the closing mechanism of modern sport parachute equipment.[7] The strength and magnetic field homogeneity on neodymium magnets has also opened new applications in the medical field with the introduction of open magnetic resonance imaging (MRI) scanners used to image the body in radiology departments as an alternative to superconducting magnets that use a coil of superconducting wire to produce the magnetic field. As with most solid-based magnets, the magnetic field gradient of neodymium magnets decreases towards the centers of their surfaces, thus there is a force that attracts metallic objects to the edges.
