Chapter 14. Optical and Magnetic Materials. 경상대학교 Ceramic Design Lab.

Transcription

1 Chapter 14 Optical and Magnetic Materials

2 Magnetic field strength = H H = Ni/l (amp-turns/m) N = # turns i = current, amps l = conductor length B = Magnetic Induction or Magnetic flux density (Wb/m 2 ) Magnetic field lines of force around a current loop and a bar magnet

3 B = mh m is the permeability In a vacuum, B o = m o H where mo is the permeability in a vacuum =4p x 10-7 (1.257 x 10-6 ) H/m or T-m/A Units: T: Tesla 1T = 1 V-s/m 2 Analogous to dielectrics, the relative permeability is m r = m/m o

4 The Magnetization is another field quantity defined by the equation: B = m o H + m o M And M = c m H B = mh = m o H + m o M Or M = (m-m o )/m o * H Thus c m = (m-m o )/m o c m is called the magnetic susceptibility, and is a measure of how easily a material is magnetized Thus B = m o (1+c m )H

5 The Origin of Magnetic Moments Magnetic moments come both from the electrons orbiting the nucleus and its spin The most fundamental magnetic moment is the Bohr magnetron, m B m B = 9.27 x A/m 2 For each electron, the spin magnetic moment is ±m B Furthermore, the orbital magnetic moment contribution is equal to m l *m B, where m l is the magnetic quantum number Only unpaired electrons contribute to the total magnetic moment in an atom

6 FIGURE The electronic structure of the 3d orbital for transition metals. Unpaired electrons contribute to the magnetic nature of these metals.

7 FIGURE The alignment of magnetic moments for adjacent atoms leads to the large net magnetic moment (and B s on a B H plot) for the bulk solid. The example here is pure bcc iron at room temperature.

8 Diamagnetism and Paramagnetism Diamagnetism is extremely small, nonpermanent, opposes external field, and only persists while the external field is applied The atomic dipole configuration for a diamagnetic material with and without a magnetic field. In the absence of an external field, no dipoles exist; in the presence of a field, dipoles are induced that are aligned opposite to the field direction. (b) Atomic dipole configuration with and without an external magnetic field for a paramagnetic material

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10 Ferromagnetism Ferromagnetism is displayed by large and permanent magnetizations. These occur in transition metals (BCC iron, nickel, and cobalt) and some rare earth elements Susceptibility is as high as 10 6 thus, H<<M, and B = m o M Schematic illustration of the mutual alignment of atomic dipoles for a ferromagnetic material, which will exist even in the absence of an external magnetic field In ferromagnets, magnetic moments remain aligned when external fields are removed, resulting in permanent magnetization

11 The maximum possible magnetization, or magnetic saturation, M s of a ferromagnetic material represents the magnetization that results when all the magnetic dipoles in a solid piece are mutually aligned to the external field. There is a corresponding saturation flux density, B s. The saturation magnetization is equal to the product of the net magnetic moment for each atom and the number of atoms present. For iron, cobalt and nickel, the net magnetic moments per atom are 2.22, 1.2 and 0.60 Bohr magnetrons, respectively Example: Calculate the saturation magnetization and saturation flux density for nickel, which has a density of 8.9 g/cm 3 : M s = 0.60m B N N = rn A /A Ni = (8.90 g/cm 3 )*(6.023 x atoms/mol)/58.71 g/mol = 9.13 x atoms/m3 M s = 0.60 x (9.27 x ) x (9.13 x ) = 5.1 x 10 5 A/m B s = m o M s = 4p x 10-7 H/m * 5.1 x 10 5 A/m = 0.64 Tesla

12 Antiferromagnetism Antiparallel alignment of spin magnetic moments in antiferromagnetic manganese oxide results in complete cancellation of magnetic moments and no net magnetism

13 Ferrimagnetism Some ceramics can show permanent magnetism called Ferrimagnetism. Ferrimagnetism is similar to ferromagnetism, though the source of the net magnetic moments is different

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15 FIGURE The unit cell of the normal spinel (MgAl 2 O 4 ) structure. (After F. H. Norton, Elements of Ceramics, 2nd ed., Addison-Wesley Publishing Co., Inc., Reading, MA, 1974.)

16 TABLE 14.8 Some Commercial Ferrite Compositions

17 TABLE 14.9 Some Commercial Garnet Compositions

18 Example: Saturation Magnetization determination for Fe 3 O 4 Calculate the saturation magnetization for Fe 3 O 4 given that each cubic unit cell contains 8 Fe 2+ and 16 Fe 3+ ions, and that the unit cell edge length is nm The saturation magnetization is equal to the product of the number, N, of Bohr magnetrons per cubic meter of Fe 3 O 4 and the magnetic moment per Bohr magnetron, m B : M s = N m B N is the number of Bohr magnetrons per unit cell n B divided by the unit cell volume Vc, or: N = n B /Vc Net magnetization results from the Fe 2+ ions only. Each cell has 8 Fe 2+, and each Fe 2+ has 4 Bohr magnetrons, thus n B = 32, and Vc = a 3, thus M s = (32 Bohr magnetrons/unit cell * 9.27 x A/-m 2 /Bohr magnetron)/(0.839 x 10-9 m) 3 /unit cell M s = 5.0 x 10 5 A/m

19 Temperature and Magnetization Elevated temperatures cause magnetic dipoles to become unaligned. Magnetism is completely destroyed at the Curie Temperature, T c

20 FIGURE Comparison of the critical magnetic field versus temperature for a metallic superconductor (Nb 3 Ge) and two ceramic superconductors.

21 Domains and Hysteresis There is a gradual change in magnetic dipole orientation across a domain wall, as shown below: Dipoles are aligned in each domain, but vary from one domain to the other The B-versus-H behavior for a ferromagnetic or ferrimagnetic material that was initially unmagnetized.

22 Hysteresis 1 MGOe = 7.96 kj/m 3 H increases until B s and M s are reached. Upon removal, some magnetism, call remanence, remains at B r. H field must be reversed to H c to eliminate residual magnetism. This is called the coercivity. The area in the hysteresis loop represents work or energy expended in going from (+) to (-) H and back. The product of B*H is measured in kj/m 3 or gaussoersted (MGOe)

23 Magnetization and crystal alignment Magnetization curves for single crystals of iron and nickel. Magnetization varies with crystallographic directions Magnetization curves for single crystals of cobalt.

24 Hard vs. Soft magnets Hard magnetic materials retain magnetism after field is removed; soft magnetic materials do not. Soft magnetic materials require small H to reach B s, and H c is small. This means less energy is wasted in hysteresis loop. Soft magnetic materials are desirable in transformer cores and other applications where residual magnetization is undesirable Defects, such as nonmagnetic phases and voids restrict easy movement of domain walls and are to be avoided in soft magnetic materials. Sometimes Si or Ni is added to Fe to minimize eddy currents that rob energy in hysteresis loops

25 Hard Magnetic Material Have large hysteresis loss. Soft Magnetic Material Have low hysteresis loss. Domain wall moment is difficult Domain wall moment is relatively easier. Coercivity & Retentivity are large Coercivity & Retentivity are small. Cannot be easily magnetized & demagneti zed Magneto static energy is large. Have small values of permeability and sus ceptibility Used to make permanent magnets. Iron-nickel-aluminum alloys, copper-nickle -iron alloys, copper nickel cobalt alloys Can be easily magnetized & demagnetized. Magneto static energy is small. Have large values of susceptibility and permeability. Used to make electromagnets. Iron- silicon alloys, ferrous- nickel alloy s, ferrites, garnets.

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27 Small domains in materials can be magnetically aligned in one of two ways, corresponding to a 0 or a 1 in digital storage. This same technique of magnetic alignment being written and read is used in recording tapes, VCR and other media Magnetic storage Hysteresis loops for particulate magnetic storage media. Saturation flux density is typically Tesla, and the hysteresis loop should be relatively large and square, to ensure that storage will be permanent and magnetization reversal will occur over a narrow range of applied field strengths. For coercivity is typically ~2 x 10 5 A/m.

28 Scanning electron micrograph showing the microstructure of a magnetic storage disk. Needleshaped particles of g-fe 2 O 3 are oriented and embedded within an epoxy phenolic resin. 8000X Each particle is a single domain that may be magnetized only with its magnetic moment lying along this axis. Only two states are allowed, corresponding to digital storage of 1 s and 0 s

29 Application Of Magnetic property In real world there many operation of magnetic property. This property is use as two form as Electromagnetic field and magnetic field. Electronic Motor and Generator An electric motor that uses electromagnets in the spinning stator to turn. There is an electrical 'slip-ring' on the stator that directs the power to a different magnet section of the stator to achieve rotation.

30 Magnetic storage Magnetic storage and magnetic recording are terms from engineering referring to the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetization in a magnetizable material to store data and is a form of non-volatile memory. The information is accessed using one or more read/write heads.

31 Magnetic bearing A magnetic bearing is a bearing which supports a load using magnetic levitation. Magnetic bearings support moving machinery without physical contact, for example, they can levitate a rotating shaft and permit relative motion with very low friction and no mechanical wear.

32 Magnetic separator and Holding Device Magnetic separator for particle size less than 3mm magnetite, pyrrhotite, ilmenite and other materials, wet magnetic separation, but also for coal, nonmetallic minerals, building materials and other materials in addition to iron work.available downstream, semi-reflux, reflux-type and other forms of magnetic separator, cylinder surface magnetic field strength can be produced according to the actual use of the special.

33 Magnetic property in Medical The Attraction of Magnet Therapy Some magnets are multipolar, with both the north and south poles facing the patient/desired body part, often with manufacturers touting that their circular or checkerboard or triangular pattern is in some way superior. But this also further limits how far the magnetic field reaches. Any effect inside the body must be limited to a few millimeters, only skin deep.

34 Magnetic Resonance Angiogram (MRA) A magnetic resonance angiogram (MRA) is a type of magnetic resonance imaging (MRI) scan that uses a magnetic field and pulses of radio wave energy to provide pictures of blood vessels inside the body. In many cases MRA can provide information that can't be obtained from an X-ray, ultrasound, or computed tomography (CT) scan.