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CHAPTER 16

CHAPTER 16. Magnetic Properties. 16-1. Magnetic Materials. Very important in electrical engineering Soft magnetic materials : Materials that can be easily magnetized and demagnetized. Applications: Transformer cores, stator and rotor materials.

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CHAPTER 16

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  1. CHAPTER 16 Magnetic Properties 16-1

  2. Magnetic Materials • Very important in electrical engineering • Soft magnetic materials: Materials that can be easily magnetized and demagnetized. • Applications: Transformer cores, stator and rotor materials. • Hard magnetic materials: Cannot be easily demagnetized (permanent magnets). • Applications: Loud speakers, telephone receivers. 16-2

  3. Magnetic Fields • Ferromagnetic materials: Iron, cobalt and nickel -provide strong magnetic field when magnetized. • Magnetism is dipolar up to atomic level. • Magnetic fields are also produced by current carrying conductors. • Magnetic field of a solenoid is H = 0.4П n i / l A/m n = number of turns l = length i = current Figure 15.3a 16-3 After C.S. Barrett, W. D. Nix, and A. S. Teteman, “Principles of Engineering Materials,” Prentice-Hall, 1973, p.459.

  4. Magnetic Induction • If demagnetized iron bar is placed inside a solenoid, the magnetic field outside solenoid increases. • The magnetic field due to the bar adds to that of solenoid - Magnetic induction (B) . • Intensity of Magnetization (M) : Induced magnetic moment per unit volume B = μ0H + μ0 M = μ0(H+M) μ0 = permeability of free space = 4π x 10-7 (Tm/A) • In most cases μ0 >μ0 H Therefore B =~ M Figure 15.3b 16-4 After C.S. Barrett, W. D. Nix, and A. S. Teteman, “Principles of Engineering Materials,” Prentice-Hall, 1973, p.459.

  5. Magnetic Permeability • Magnetic permeability = μ = B/H • Magnetic susceptibility = Xm = M/H • For vacuum μ = μ0 = = 4π x 10-7 (Tm/A) • Relative permeability = μr = μ/ μ0 B = μ0 μr H • Relative permeability is measure of induced magnetic field. • Magnetic materials that are easily magnetized have high magnetic permeability. Figure 15.4 16-5

  6. Types of Magnetism • Magnetic fields and forces are due to intrinsic spin of electrons. • Diamagnetism: External magnetic field unbalances orbiting electrons causing dipoles that appose applied field. • very small negative magnetic susceptibility. • Paramagnetism: Materials exhibit small positive magnetic susceptibility. • Paramagnetic effect disappears when the applied magnetic field is removed. • Produced by alignment of individual dipole moments of atoms or molecules. 16-6

  7. Ferromagnetism • Ferromagnetic elements (Fe, Co, Ni and Gd) produce large magnetic fields. • It is due to spin of the 3d electrons of adjacent atoms aligning in parallel directions in microscopic domains by spontaneous magnetization. • Random orientation of domains results in no net magnetization. • The ratio of atomic spacing to diameter of 3d orbit must be 1.4 to 2.7. Figure 15.6 16-7

  8. Magnetic Moments of a Single Unpaired Electron • Each electron spinning about its own axis has dipole moment μB μB = e h / 4 π m • In paired electrons positive and negative moments cancel. • Antiferromagnetism: In presence of magnetic field, magnetic dipoles align in apposite directions . • Examples:- Manganese and Chromium. • Ferrimagnetism: Ions of ceramics have different magnitudes of magnetic moments and are aligned in antiparallel manner creating net magnetic moments. μB = Bohr magneton e = electron charge h = plank’s constant m = electron mass 16-8

  9. Effect of Temperature on Ferromagnetism • Above 0 K, thermal energy causes magnetic dipoles to deviate from parallel arrangement. • At higher temperature, (curie temperature) ferromagnetism is completely lost and material becomes paramagnetic. • On cooling, ferromagnetic domains reform. • Examples: Fe 7700C Co 11230C Ni 3580C Figure 15.9 16-9

  10. Ferromagnetic Domains • Magnetic dipole moments align themselves in parallel direction called magnetic domains. • When demagnetized, domains are rearranged in random order. • When external magnetic field is applied the domains that have moments parallel to applied filed grow. • When domain growth finishes, domain rotation occurs. Figure 15.11 16-10 After R.M. Rosw, L. A. Shepard, and J. Wulff, “Structure and Properties of Materials,” vol. IV: ““ Wiley, 1996, p.193.

  11. Types of Energies that Determine the Structure • Most stable structure is attained when overall potential energy is minimum. • Potential energy with a domain is minimized when all atomic dipoles are aligned in single direction. • Magnetostatic energy: Potential energy produced by its external field. • Formation of multiple domain reduces magnetostatic energy. Figure 15.13 16-11

  12. Magnetocrystalline Anisotropy Energy • Magnetization with applied field for a single crystal varies with crystal orientation. • Saturation magnetization occurs most easily for the <100> direction of BCC iron. • Saturation magnetization occurs with highest applied field for <111> direction. • Some grains of polycrystalline materials need some energy to rotate their resultant moment. • This energy is magnetocrystalline anisotropy energy. Figure 15.14 16-12

  13. Domain Wall Energy • Domain wall is the region through which the orientation of the magnetic moment changes gradually. • 300 atoms wide due to balance between exchange force and magnetocrystalline anisotropy. • Equilibrium wall width is width at which sum of two energies are minimum. Figure 15.15 16-13 After C.S. Barrett, W. D. Nix, and A. S. Teteman, “Principles of Engineering Materials,” Prentice-Hall, 1973, p.485.

  14. Magnetostrictive Energy • Magnetostriction: Magnetically induced reversible elastic strain. • Energy due to mechanical stress created by magnetostriction is called magnetostriction energy. • It is due to change in bond length caused by rotation of dipole moments. • Equilibrium domain configuration is reached when sum of magnetostrictive and domain wall energies are minimum. Figure 15.16 Figure 15.17 16-14

  15. Magnetization and Demagnetization • Magnetization and demagnetization do not follow same loop. • Once magnetized, remnant induction ‘Br’ remains even after demagnetization. • Negative field Hc (coercive force) must be applied to completely demagnetize. • Magnetization loop is called hysteresis loop. • Area inside the loop is a measure of work done in magnetizing and demagnetizing. Figure 15.18 16-15

  16. Soft Magnetic Materials • Easily magnetized and demagnetized. • Low coercive force and high saturation induction are desirable properties. • Hysteresis energy losses: Due to dissipated energy required to push the domain back and forth. • Imperfections increases hysteresis. • Eddy current energy losses: Induced electric current causes some stray electric currents resulting from transient voltage. • Source of energy loss by electrical resistance healing. 16-16

  17. Iron Silicon Alloys • Iron – 3 to 4% Si alloys are commonly used soft magnetic materials. • Silicon reduces electrical resistivity, decreases hysteresis, decreases magnetostriction. • Silicon decreases saturation induction and curie temperature. • Laminated core further reduces eddy current losses. • Decrease in energy loss is also achieved by using grain oriented silicon sheet. 16-17

  18. Metallic Glasses • Noncrystalline domains. • Soft magnetic properties, have combination of ferromagnetic materials with metalloids B and Si, • Used in low energy core-loss transformers, magnetic sensors and recording heads. • Produced by rapid cooling as a thin film on a rotating copper surface mold. • Strong, hard, flexible and corrosion resistant. • Easy movement of domain walls due to absence of grain boundaries. • Low hysteresis loss. 16-18

  19. Nickel-Iron Alloys • Higher permeability at lower field. • Used in highly sensitive communication. • 50% Ni alloy : moderate permeability, high saturation induction. • 79% Ni alloy: High permeability, low saturation induction. • Low magnetoanisotropy and magnetostrictive energy. • Initial permeability is increased by annealing in presence of magnetic field. 16-19

  20. Hard Magnetic Materials - Properties • High coercive force Hc and Induction Br. • High hysteresis loss and difficult to demagnetize. • Some energy of the field is converted to potential energy. • Maximum energy product is a measure of magnetic potential energy = Max (B x H). • Max (B x H) = area of largest rectangle that can be inscribed in the second quadrant of the hysteresis loop. Figure 15.25 16-20

  21. Alnico Alloys • Alnico : Aluminum + Nickel + Cobalt • High energy product, high remnant induction and moderate coercivity. • Produced by casting or powder metallurgy. • Structure: Single phase BCC at 12500C but decomposes to α and α’ at 750 to 8500C. • α’ is highly magnetic. • If heat treated in magnetic field, α’ becomes elongated and hence is difficult to rotate – High coercivity. Figure 15.26 16-21 After B. D. Cullity, “Introduction to Magnetic Materials,” Addision- Wesley, 1972, p. 566

  22. Rare Earth Alloys • Very high maximum energy product and coercivity due to unpaired 4f electrons. • SmCo5 single phase magnets: Coercivity is based on nucleation and pinning down of domain walls at surfaces and grain boundaries. • Powder metallurgy fabrication: Particles pressed in magnetic field and sintered. • Precipitation hardened Sm(Co,Cu)2.5 alloy: Part of Co substituted by Cu. • Precipitate produced at low temperatures and domain walls are pinned at precipitates. • Used in medical devices. 16-22

  23. Neodymium-Iron-Boron Magnetic Alloys • Produced by powder metallurgy and rapid solidification melt-spun ribbon process. • Highly ferromagnetic Nd2Fe14B grains are surrounded by nonferromagnetic Nd rich intergranular phase. • High coercivity and energy product due to difficulty in reverse nucleating. • Used in automotive starting motors. Figure 15.29 16-23 After J. J. Croat and J. F. Herbst, MRS Bull., June 1988, p.37.

  24. Iron-Chromium-Cobalt Magnetic Alloys • Structure and properties analogues to Alnico. • 16% Fe, 28% Cr, 11% Co. • Single phase at high temperature (12000C). • Precipitates of chromium rich α2 phase forms below 6500C. • Domain walls gets pinned into precipitate particles. • Particles are elongated by forming to increase coercivity. • Can be cold formed. • Used in permanent magnets of modern telephones. Figure 15.30 16-24 After S. Jin et al., J. Appl.Phys., 53:4300 (1982).

  25. Ferrites • Magnetic ceramics made by mixing Fe2O3 with other oxides and carbonates in powder form. • Domain structure and hysteresis loop similar to ferromagnets but low magnetic saturation. • Soft ferrites: MO Fe2O3 where M is Fe 2+, Mn 2+, Ni 2+ or Zn 2+. • Inverse spinel structure. • Cubic unit cells with 8 subcells. • Only 1/8th of tetrahedral sites are occupied in normal spinal structure. Figure 15.34 16-25

  26. Net Magnetic Moments in Inverse Spinel Ferrites • Fe 2+ ions 4 unpaired 3d electrons. • Fe 3+ ions 5 unpaired 3d electrons. • Each unpaired 3d electron has one Bohr magneton 16-26

  27. Properties and Applications of Soft Ferrites • Useful magnetic properties, good insulators. • High electrical resistivity – low eddy current losses. • Applications: Low-signal, Memory-core, audiovisual and recording head applications. • Recording heads are made up of Mn-Zn and Ni-Zn spinel ferrites. • Magnetic core memories are used in some computers. 16-27

  28. Magnetically Hard Ferrites • General formula: MOFe2O3 (hexagonal crystal structure). • Examples: Barium Ferrite (BaO.6Fe2O3) and Strontium Ferrite (SrO.6Fe2O3 ). • Low cost, low density, high coercive force. • High magnetocrystalline anisotropy. • Magnetization takes place by domain wall nucleation and motion. • Applications: Generators, relays, motors, loudspeakers and door closers. 16-28

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