뮤-메탈

뮤-메탈은 투과성이 매우 높은 니켈 철 연성 강자 합금으로, 정적 또는 저주파 자기장으로부터 민감한 전자장비를 차폐하는 데 사용된다. 그것은 여러 가지 구성을 가지고 있다. 그러한 구성 중 하나는 약 77% 니켈, 16% 철, 5% 구리, 2% 크롬 또는 몰리브덴이다.^[1][2] 보다 최근에는 mu-metal이 ASTM A753 합금 4로 간주되며 약 80% 니켈, 5% 몰리브덴, 실리콘과 같은 다양한 다른 원소 소량, 나머지 12~15% 철로 구성되어 있다.^[3] 그 이름은 물리학과 공학 공식에서 투과성을 나타내는 그리스 문자 mu(μ)에서 유래했다. 이 합금의 여러 다른 특허 공식들은 MuMETal, Mumetall, Mumetal2와 같은 상표명으로 판매된다.

Mu-metal은 일반적으로 일반 강철의 수천 개에 비해 상대 투과성 값이 8만–100,000이다. '부드러운' 강자성 물질로, 낮은 자성 음이소트로피와 자성을 가지고 ^[1]있어 낮은 자기장에 포화되도록 강제성이 있다. 이것은 AC 자성 회로에 사용될 때 낮은 이력 손실을 준다. permalloy와 같은 다른 높은 내구성의 니켈-철 합금은 유사한 자기 특성을 가지고 있다; mu-metal의 장점은 더 연성, 유연성, 작동성이 뛰어나서 자석 실드에 필요한 얇은 시트로 쉽게 형성될 수 있다는 것이다.^[1]

뮤-메탈 물체는 최종 형태인 후 열처리를 해야 하는데, 수소 대기의 자기장에 들어감으로써 자기 투과성이 약 40배 증가한다.^[4] 어닐링은 물질의 결정 구조를 변경하여 곡물을 정렬하고 일부 불순물, 특히 탄소를 제거하여 자기 영역 경계의 자유로운 움직임을 방해한다. 아닐링 후 휨이나 기계적 충격은 물질의 곡물 정렬에 지장을 주어 영향을 받는 부위의 투과성이 저하될 수 있으며, 이는 수소 아닐링 단계를 반복하면 회복될 수 있다.

Magnetic shielding

Mu-metal is a soft magnetic alloy with exceptionally high magnetic permeability. The high permeability of mu-metal provides a low reluctance path for magnetic flux, leading to its use in magnetic shields against static or slowly varying magnetic fields. Magnetic shielding made with high-permeability alloys like mu-metal works not by blocking magnetic fields but by providing a path for the magnetic field lines around the shielded area. Thus, the best shape for shields is a closed container surrounding the shielded space. The effectiveness of mu-metal shielding decreases with the alloy's permeability, which drops off at both low field strengths and, due to saturation, at high field strengths. Thus, mu-metal shields are often made of several enclosures one inside the other, each of which successively reduces the field inside it. Because mu-metal saturates at such low fields, sometimes the outer layer in such multilayer shields is made of ordinary steel. Its higher saturation value allows it to handle stronger magnetic fields, reducing them to a lower level that can be shielded effectively by the inner mu-metal layers.

RF magnetic fields above about 100 kHz can be shielded by Faraday shields: ordinary conductive metal sheets or screens which are used to shield against electric fields.^[5] Superconducting materials can also expel magnetic fields by the Meissner effect, but require cryogenic temperatures.

The alloy has a low coercivity, near zero magnetostriction, and significant anisotropic magnetoresistance. The low magnetostriction is critical for industrial applications, where variable stresses in thin films would otherwise cause a ruinously large variation in magnetic properties.

History

Mu-metal was developed by British scientists Willoughby S. Smith and Henry J. Garnett^[6][7][8] and patented in 1923 for inductive loading of submarine telegraph cables by The Telegraph Construction and Maintenance Co. Ltd. (now Telcon Metals Ltd.), a British firm that built the Atlantic undersea telegraph cables.^[9] The conductive seawater surrounding an undersea cable added a significant capacitance to the cable, causing distortion of the signal, which limited the bandwidth and slowed signaling speed to 10–12 words per minute. The bandwidth could be increased by adding inductance to compensate. This was first done by wrapping the conductors with a helical wrapping of metal tape or wire of high magnetic permeability, which confined the magnetic field. Telcon invented mu-metal to compete with permalloy, the first high-permeability alloy used for cable compensation, whose patent rights were held by competitor Western Electric. Mu-metal was developed by adding copper to permalloy to improve ductility. 80 kilometres (50 mi) of fine mu-metal wire were needed for each 1.6 km of cable, creating a great demand for the alloy. The first year of production Telcon was making 30 tons per week. In the 1930s this use for mu-metal declined, but by World War II many other uses were found in the electronics industry (particularly shielding for transformers and cathode ray tubes), as well as the fuzes inside magnetic mines. Telcon Metals Ltd. abandoned the trademark "MUMETAL" in 1985.^[10] The last listed owner of the mark "MUMETAL" is Magnetic Shield Corporation, Illinois.^[11]

Uses and properties

Mu-metal is used to shield equipment from magnetic fields. For example:

• Electric power transformers, which are built with mu-metal shells to prevent them from affecting nearby circuitry.
• Hard disks, which have mu-metal backings to the magnets found in the drive to keep the magnetic field away from the disk.^{[citation needed]}
• Cathode-ray tubes used in analogue oscilloscopes, which have mu-metal shields to prevent stray magnetic fields from deflecting the electron beam.
• Magnetic phonograph cartridges, which have a mu-metal case to reduce interference when LPs are played back.
• Magnetic resonance imaging (MRI) equipment.
• The magnetometers used in magnetoencephalography and magnetocardiography.
• Photomultiplier tubes.
• Vacuum chambers for experiments with low-energy electrons, for example, photoelectron spectroscopy.
• Superconducting circuits and especially Josephson junction circuits.
• Fluxgate magnetometers and compasses as part of the sensor.
• Proximity sensors (inductive type)

Similar materials

Other materials with similar magnetic properties include Co-Netic, supermalloy, supermumetal, nilomag, sanbold, molybdenum permalloy, Sendust, M-1040, Hipernom, HyMu-80 and Amumetal. In recent times pyrolytic graphite (also incidentally used in some OLED panels as a heat sink) has been used as it also shows useful magnetic field exclusion properties.^{[citation needed]}

References

External links

• Mu-Metal Properties
• Zero gauss chambers Archived 2013-02-17 at the Wayback Machine
• mu 금속 실드 사용에 대한 정보