Definitions

Alnico

Alnico

[al-ni-koh]
Alnico is an acronym referring to alloys which are composed primarily of aluminium (symbol Al), nickel (symbol Ni) and cobalt (symbol Co), hence al-ni-co, with the addition of iron, copper, and sometimes titanium, typically 8-12% Al, 15-26% Ni, 5-24% Co, up to 6% Cu, up to 1% Ti, rest is Fe. The primary use of alnico alloys is magnet applications.

Properties

Alnico alloys make strong permanent magnets, and can be magnetized to produce strong magnetic fields. Of the more commonly available magnets, only rare-earth magnets such as neodymium and samarium-cobalt are stronger. Alnico magnets produce magnetic field strength at their poles as high as 1500 gauss (0.15 tesla), or about 3000 times the strength of Earth's magnetic field. Some types of Alnico are isotropic and can be efficiently magnetized in any direction. Other types, such as Alnico 5 and Alnico 8, are anisotropic, with each having a preferred direction of magnetization, or orientation. Anisotropic alloys generally have greater magnetic capacity in a preferred orientation than isotropic types. Alnico remanence (Br) may exceed 12000 gauss (1.2T), its coercion force (Hc) can be up to 1000 oersted (80kA/m), its energy product ((B*H)max) can be up to 5.5M Gs*Oe (44 T*A/m) - this means Alnico can produce high magnetic flux in closed magnetic circuit, but has relatively small resistance against demagnetization.

Alnico is produced by casting or sintering processes. Anisotropic alnico magnets are oriented by heating above a critical temperature, and cooling in the presence of a magnetic field. Both isotropic and anisotropic Alnico require proper heat treatment to get its magnetic properties - without it its coercion is about 10 Oe - comparable to technical iron, which is a soft magnetic material. After the heat treatment alnico becomes kind of composite material, named "precipitation material" - it consist of iron and cobalt rich precipitates in rich-NiAl matrix. Alnico anisotropy is oriented if it is in external magnetic field during the precipitate particle nucleation, which occur when cooling from 900 to 800 C, near Curie point. Without external field there are local anisotropies of different orientations, due to spontaneous magnetization. The precipitate structure is a "barrier" against magnetization changes, as it prefers few magnetization states requiring high energy to get the material into any intermediate state. Also, small magnetic field change magnetization of matrix phase only, and it is reversible.

Alnico alloys have some of the highest Curie points of any magnetic material, around 800 °C, although the maximum working temperature is normally limited to around 538 °C. This property, as well as its brittleness and high melting point, is the result of the strong tendency toward order due to intermetallic bonding between aluminium and its other constituents. They are also one of the most stable magnets if they are handled properly.

As of 2008, Alnico magnets cost about $20 per lb, $4.30 per BHmax. magnetic characteristics.

Use

Alnico magnets are used in electric motors, electric guitar pickups, microphones, sensors, loudspeakers, and cow magnets.

References

  1. Physical Review: 77p839_2, 80p112_2, 80p302_1, 81p478_1;
  2. Review of Modern Physics 17p15, 25p316_1;
  3. Journal of Applied Physics 29p299, 31pS82, 40p1308

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