Optical Properties

Introduction By “optical property” is meant a material’s response to exposure to electromagnetic radiation and, in particular, to visible light. In classical sense, electromagnetic radiation is considered to be wave-like, consisting of electric and magnetic field components that are perpendicular to each other and also to the direction of propagation.

Spectrum of the electromagnetic radiation

Light and Electromagnetic Spectrum Visible light: Electromagnetic radiation with wavelength 0.4 to 0.75 micrometers. Ultraviolet : 0.01 – 0.4 micrometers Infrared: 0.75 – 1000 micrometers Light is in form of waves and consist of particles called photons. ΔE = hν = hC/λ ΔE = Energy λ = wavelength ν = frequency C = speed of light = 3 x108 m/s H = plank’s constant = 6.62 x 10-34 J.s 15-2

Electron Transitions Δ E = hν The absorption and emission of electromagnetic radiation may involve electron transitions from one energy state to another. An electron may be excited from an occupied state at energy E2 to a vacant and higher-lying one, denoted E4, by the absorption of a photon of energy. The change in energy experienced by the electron, ΔE, depends on the radiation frequency as follows: Δ E = hν

Important concepts: Since the energy states for the atom are discrete, only specific ΔE exist between the energy levels; thus, only photons of frequencies corresponding to the possible ΔE for the atom can be absorbed by electron transitions. A stimulated electron cannot remain in an excited state indefinitely; after a short time, it falls or decays back into its ground state, or unexcited level, with a reemission of electromagnetic radiation.

The energy difference =3.54eV-1.38eV= 2.16eV A photon in a ZnS semiconductor drops from an impurity energy level at 1.38eV below its conduction band to its valence band. If ZnS has an energy band gap of 3.54eV, what is the wavelength of the radiation given off by the photon? What is the color of the radiation? The energy difference =3.54eV-1.38eV= 2.16eV =hc/E =574.7nm Visible yellow region

Wavelength vs. Band Gap Example: What is the minimum wavelength absorbed by Ge? (Given Eg = 0.67 eV, h = 6.62 x 10-34 J.s, c = 3.0 x 108 m/s) Answer:

Magnetic Properties

Introduction Magnetism – a phenomenon by which materials assert an attractive or repulsive force or influence on other materials, has been known for thousands of years. Many of our modern technological devices rely on magnetism and magnetic materials; these include electrical power generators and transformers, electric motors, radio, television, telephones, computers, and components of sound and video reproduction systems. Iron, some steels and the naturally occurring mineral are well-known examples of materials that exhibit magnetic properties.

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

Basic Concepts Magnetic Dipoles • Magnetic forces are generated by moving electrically charged particles. • Magnetic dipoles are found to exist in magnetic materials, which, in some respects, are analogous to electric dipoles. • Magnetic dipoles are influenced by magnetic field in a manner similar to the way in which electric dipoles are affected by electric fields.

Applied Magnetic Field • Created by current through a coil: Applied magnetic field H current I N = total number of turns L = length of each turn

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 SI unit: A/m n = number of turns l = length i = current 16-3 After C.S. Barrett, W. D. Nix, and A. S. Teteman, “Principles of Engineering Materials,” Prentice-Hall, 1973, p.459.

Response to a Magnetic Field The magnetic induction, or magnetic flux density, denoted by B, represents the magnitude of the internal field strength within a substance that is subjected to an H field. • Magnetic induction results in the material current I B = Magnetic Induction inside the material Unit for B: tesla or Wb/m2

Magnetic flux density in a material – dependence on permeability and magnetic field strength The magnetic field strength and flux density are related according to B = μH The parameter μ is called the permeability, which is a property of the specific medium through which the H field passes and in which B is measured. The permeability has dimensions of webers per ampere-meter (Wb/A-m) or henries per meter (H/m) or Tesla.meters per ampere (T.m/A).

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 μ0M > 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.

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

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

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. Unpaired inner 3d electrons are responsible for ferromagnetism. 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. Parallel alignment of magnetic dipole due to positive exchange energy, e.g. Fe, Co, Ni. 16-7

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 opposite 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. E.g. Fe3O4. μB = Bohr magneton Unit SI μB=9.27x10-24 A.m2 e = electron charge h = plank’s constant m = electron mass 16-8

Alignment of Magnetic Dipoles: a)Ferromagnetism b)Antiferromagnetism c)Ferrimagnetism