A "horseshoe magnet" made of Alnico 5, about 1 inch high. The metal bar (bottom) is a keeper, which is placed across the poles when the magnet is not in use. This helps to preserve the magnetization.

Alnico is a family of iron alloys which, in addition to iron are composed primarily of aluminium (Al), nickel (Ni), and cobalt (Co), hence the acronym[1] al-ni-co. They also include copper, and sometimes titanium. Alnico alloys are ferromagnetic, and are used to make permanent magnets. Before the development of rare-earth magnets in the 1970s, they were the strongest permanent magnet type. Other trade names for alloys in this family are: Alni, Alcomax, Hycomax, Columax, and Ticonal.[2]

The composition of alnico alloys is typically 8–12% Al, 15–26% Ni, 5–24% Co, up to 6% Cu, up to 1% Ti, and the rest is Fe. The development of alnico began in 1931, when T. Mishima in Japan discovered that an alloy of iron, nickel, and aluminum had a coercivity of 400 oersteds (32 kA/m), double that of the best magnet steels of the time.[3]

Properties

Alnico alloys can be magnetised to produce strong magnetic fields and have a high coercivity (resistance to demagnetization), thus making strong permanent magnets. 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 alnico brands are isotropic and can be efficiently magnetized in any direction. Other types, such as alnico 5 and alnico 8, are anisotropic, each having a preferred direction of magnetization, or orientation. Anisotropic alloys generally have greater magnetic capacity in a preferred orientation than isotropic types. Alnico's remanence (Br) may exceed 12,000 G (1.2 T), its coercivity (Hc) can be up to 1000 oersteds (80 kA/m), its maximum energy product ((BH)max) can be up to 5.5 MG·Oe (44 T·A/m). Therefore, alnico can produce a strong magnetic flux in closed magnetic circuits, but has relatively small resistance against demagnetization. The field strength at the poles of any permanent magnet depends very much on the shape and is usually well below the remanence strength of the material.

Alnico alloys have some of the highest Curie temperatures of any magnetic material, around 800 °C (1,470 °F), although the maximal working temperature is typically limited to around 538 °C (1,000 °F).[4] They are the only magnets that have useful magnetism even when heated red-hot.[5] This property, as well as its brittleness and high melting point, resutls from the strong tendency toward order due to intermetallic bonding between aluminum and other constituents. They are also one of the most stable magnets if handled properly. Alnico magnets are electrically conductive, unlike ceramic magnets.

MMPA
class 		IEC
code
ref. 		Composition
by weight
(Fe comprises remainder) 		Magnetic properties Physical properties Thermal properties
Max. energy
product,
(BH)max 		Residual
induction, Br 		Coercive
force, Hc 		Intrinsic
coercive
force, Hci 		Density Tensile
strength 		Transverse
modulus of
rupture 		HRC Thermal
expansion
coefficient
(10−6 per °C) 		Electrical
resistivity,
at 20 °C
(μΩ·cm) 		Reversible temp.
coefficient,
(% per °C) 		Curie
temp. 		Max.
service
temp.
Al Ni Co Cu Ti (MGOe) (kJ/m3) (gauss) (mT) (Oe) (kA/m) (Oe) (kA/m) (lb/in3) (g/cm3) (psi) (MPa) (psi) (MPa) Near
Br 		Near
max.
energy
prod. 		Near
Hc 		(°C) (°F) (°C) (°F)
Isotropic cast AlNiCo
Alnico 1 R1-0-1 12 21 5 3 - 1.4 11.1 7200 720 470 37 480 38 0.249 6.9 4000 28 14000 97 45 12.6 75
Alnico 2 R1-0-4 10 19 13 3 - 1.7 13.5 7500 750 560 45 580 46 0.256 7.1 3000 21 7000 48 45 12.4 65 -0.03 -0.02 -0.02 810 1490 450 840
Alnico 3 R1-0-2 12 25 - 3 - 1.35 10.7 7000 700 480 38 500 40 0.249 6.9 12000 83 23000 158 45 13.0 60
Anisotropic cast AlNiCo
Alnico 5 R1-1-1 8 14 24 3 - 5.5 43.8 12800 1280 640 51 640 51 0.264 7.3 5400 37 10500 72 50 11.4 47 -0.02 -0.015 +0.01 860 1580 525 975
Alnico 5DG R1-1-2 8 14 24 3 - 6.5 57.7 13300 1330 670 53 670 53 0.264 7.3 5200 36 9000 62 50 11.4 47
Alnico 5-7 R1-1-3 8 14 24 3 - 7.5 59.7 13500 1350 740 59 740 59 0.264 7.3 5000 34 8000 55 50 11.4 47
Alnico 6 R1-1-4 8 16 24 3 1 3.9 31.0 10500 1050 780 62 800 64 0.265 7.3 23000 158 45000 310 50 11.4 50 -0.02 -0.015 +0.03 860 1580 525 975
Alnico 8 R1-1-5 7 15 35 4 5 5.3 42.2 8200 820 1650 131 1860 148 0.262 7.3 10000 59 30000 207 55 11.0 53 -0.025 -0.01 +0.01 860 1580 550 1020
Alnico 8HC R1-1-7 8 14 38 3 8 5.0 39.8 7200 720 1900 151 2170 173 0.262 7.3 10000 59 30000 207 55 11.0 54 -0.025 -0.01 +0.01 860 1580 550 1020
Alnico 9 R1-1-6 7 15 35 4 5 9.0 71.6 10600 1060 1500 119 1500 119 0.262 7.3 7000 48 8000 55 55 110. 53 -0.025 -0.01 +0.01 860 1580 550 1020
Isotropic sintered AlNiCo
Alnico 2 R1-0-4 10 19 13 3 - 1.5 11.9 7100 710 550 44 570 45 0.246 6.8 65000 448 70000 483 45 123.4 68
Anisotropic sintered AlNiCo
Alnico 5 R1-1-10 8 14 24 3 - 3.9 31.0 10900 1090 620 49 630 50 0.250 6.9 50000 345 55000 379 45 11.3 50
Alnico 6 R1-1-11 8 15 24 3 1 2.9 23.1 9400 940 790 63 820 65 0.250 6.9 55000 379 100000 689 45 11.4 54
Alnico 8 R1-1-12 7 15 35 4 5 4.0 31.8 7400 740 1500 119 1690 134 0.252 7.0 50000 345 55000 379 45 11.0 54
Alnico 8HC R1-1-13 7 14 38 3 8 4.5 35.8 6700 670 1800 143 2020 161 0.252 7.0 55000 379 45 11.0 54

As of 2018, Alnico magnets cost about 44 USD/kg (US$20/lb) or US$4.30/BHmax.[6]

Alnico 5 magnet used in a magnetron tube in an early microwave oven. About 3 in (8 cm) long.

Classification

Alnico magnets are traditionally classified using numbers assigned by the Magnetic Materials Producers Association (MMPA), for example, alnico 3 or alnico 5. These classifications indicate chemical composition and magnetic properties. (The classification numbers themselves do not directly relate to the magnet's properties; for instance, a higher number does not necessarily indicate a stronger magnet.)[7]

These classification numbers, while still in use, have been deprecated in favor of a new system by the MMPA, which designates Alnico magnets based on maximum energy product in megagauss-oersteds and intrinsic coercive force as kilo oersted, as well as an IEC classification system.[7]

Manufacturing process

Advertisement by Jensen Radio Manufacturing Co. for Alnico 5 loudspeakers in 1945. As illustrated, Alnico 5 allowed a dramatic reduction in size and weight of magnet needed to produce a given flux, from 90 oz in 1930 to 4.6 oz.

Alnico magnets are produced by casting or sintering processes.[8] Cast alnico is produced by conventional methods using resin bonded sand molds. Sintered alnico magnets are formed using powdered metal manufacturing methods. Sintering alnico is suitable for complex geometries.[9]

Most alnico produced is anisotropic, meaning that the magnetic direction of the grains is oriented in one direction. 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 develop optimal magnetic properties. Without it, alnico's coercivity is about 10 Oe, comparable to technical iron, a soft magnetic material. After the heat treatment alnico becomes a composite material, named "precipitation material"—it consists of iron- and cobalt-rich[10] precipitates in a rich-NiAl matrix.

Assortment of Alnico magnets in 1956. Alnico 5, developed during World War 2, led to a new generation of compact permanent magnet motors and loudspeakers.

Alnico's anisotropy is oriented along the desired magnetic axis by applying an external magnetic field to it during the precipitate particle nucleation, which occurs when cooling from 900 °C (1,650 °F) to 800 °C (1,470 °F), near the Curie point. There are local anisotropies of different orientations without an external field due to spontaneous magnetization. The precipitate structure is a "barrier" against magnetization changes, as it prefers few magnetization states requiring much energy to get the material into any intermediate state. Also, a weak magnetic field shifts the magnetization of the matrix phase only and is reversible.

Uses

Alnico cow magnet, used to bind sharp metal wire and other iron objects that may be ingested by the animal and otherwise cause damage to the digestive tract

Alnico magnets are widely used in industrial and consumer applications where strong permanent magnets are needed. Examples are electric motors, electric guitar pickups, microphones, sensors, loudspeakers, magnetron tubes, and cow magnets. In many applications they are being superseded by rare-earth magnets, whose stronger fields (Br) and larger energy products (B·Hmax) allow smaller-size magnets to be used for a given application.

The high-temperature resistance of alnico magnets leads to many uses that cannot be filled by less resistant magnets, such as in magnetic stirring hotplates.

References

  1. ^ Hellweg, Paul (1986). The Insomniac's Dictionary. Facts On File Publications. p. 115. ISBN 978-0-8160-1364-7.
  2. ^ Brady, George Stuart; Clauser, Henry R.; Vaccari, John A. (2002). Materials Handbook: An Encyclopedia for Managers. McGraw-Hill Professional. p. 577. ISBN 978-0-07-136076-0.
  3. ^ Cullity, B. D.; Graham, C. D. (2008). Introduction to Magnetic Materials. Wiley-IEEE. p. 485. ISBN 978-0-471-47741-9.
  4. ^ Arnold-Alnico Magnets. Arnoldmagnetics.com. Retrieved on 2011-07-30.
  5. ^ Hubert, Alex; Rudolf Schäfer (1998). Magnetic domains: the analysis of magnetic microstructures. Springer. p. 557. ISBN 978-3-540-64108-7.
  6. ^ Frequently Asked Questions Archived 2019-03-12 at the Wayback Machine. Magnetsales.com. Retrieved on 2011-07-30.
  7. ^ a b "Standard Specifications for Permanent Magnet Materials (MMPA Standard No. 0100-00)" (PDF). Magnetic Materials Producers Association. Retrieved 9 September 2015.
  8. ^ Campbell, Peter (1996). Permanent magnet materials and their application. UK: Cambridge University Press. pp. 35–38. Bibcode:1996pmma.book.....C. ISBN 978-0-521-56688-9.
  9. ^ [1]. thomas-skinner.com. Thomas & Skinner, Inc. High Performance Magnetic Materials. Extracted from website 01 August 2019
  10. ^ Chu, W.G; Fei, W.D; Li, X.H; Yang, D.Z; Wang, J.L (2000). "Evolution of Fe-Co rich particles in Alnico 8 alloy thermomagnetically treated at 800 °C". Materials Science and Technology. 16 (9): 1023–1028. Bibcode:2000MatST..16.1023C. doi:10.1179/026708300101508810. S2CID 137015369.

Further reading

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