Introduction to Engineering Materials PDF

Unit - 6 Introduction to engineering materials  Dielectric Dielectric is an insulator (as they do not have any loosely bound or free electrons) when subjected to external electric field get polarized. Dielectric constant, also called relative...

Unit - 6 Introduction to engineering materials  Dielectric Dielectric is an insulator (as they do not have any loosely bound or free electrons) when subjected to external electric field get polarized. Dielectric constant, also called relative 𝐶𝑚 permittivity is define as the ratio of 𝜅= 𝐶𝑜 the capacitance of a capacitor with dielectric to the OR capacitance of an identical capacitor in a vacuum without the dielectric material. ∈ 𝜅= OR ∈𝑜 Define as the ratio of permittivity of material to the permittivity of free space.  Polarizability Polarizability of an atom / molecule is define as ratio of induced dipole moment to applied electric field.  Permittivity Property of every medium which measure the opposition offered against the formation of an electric field. Electric Susceptibility: How easily a material can be polarized in response to applied electric field.  Types of dielectric Dielectric Polar dielectric Non - polar dielectric Polar dielectric  Nonpolar dielectric E=0 E>0 01/25/2025 Magnetic material  Magnetic field  Created by current through a wire. moving charges produce magnetic fields which are proportional to the current, and hence a current carrying conductor produces a magnetic affect around it.  Origin of magnetism  Spin of electrons  Orbital motion of electrons  Nucleus spin (contribution is very small)  Parameters (in magnetism) o Magnetic Moment (m or  ): Measure of the strength of the magnet. Unit – Am2. For current loop m = IA o Magnetic field strength/Magnetizing field (H): Measure of the strength of the externally applied field. Unit - A/m. o Magnetization (M): magnetic moment (m) per unit volume (V). Unit - A/m. When magnetic material is placed in an external magnetic field, it acquires magnetic moment. M measures the materials response to the applied field H. o Magnetic induction/Magnetic flux density (B): Magnetic flux per unit area. Measure no of field lines inside the material per unit area. Units- Tesla or Weber/m2. o Magnetic susceptibility (cm): measure how easily material can be magnetized in presence of external magnetic field. Unitless. permeability unit- weber per ampere-meter (Wb/A∙m) or henrie per meter (H/m)  Diamagnetic material In diamagnetic materials all the electrons are paired so there is no permanent net magnetic moment per atom. e.g. antimony, silver, and gold o diamagnetic materials are repelled by a magnetic field. Get magnetized feebly in the direction opposite to applied magnetic field. o do not retain the magnetic properties when the external field is removed. o magnetic relative permeability is less than one. o have a small, negative magnetic susceptibility (~ - 10-5). o dipole moment of the diamagnetic substances is lower and in the o In non-uniform magnetic filed these substances attracted towards the region where field is weak. o No effect of temperature.  Paramagnetic material Paramagnetic properties are due to the presence of some unpaired electrons, so there is permanent net magnetic moment but randomly orientated. In presence of external magnetic filed magnetic diploes align with the field. e.g. magnesium, molybdenum, olithium, Paramagnetic materials aluminium etcare.. feebly attracted by a magnetic field. o do not retain the magnetic properties when the external field is removed. o The magnetic permeability is more than unity. o have a small, positive magnetic susceptibility (10-5 to 10-2). o In non-uniform magnetic filed these substances move from weaker field to stronger field. o Magnetic susceptibility is inversely proportional to temperature.  Ferromagnetic material Ferromagnetic properties are due to the presence of some unpaired electrons, so there is permanent net magnetic moment. They get their strong magnetic properties due to the presence of magnetic domains. In these domains, large numbers of atom's moments (1012 to 1015) are aligned parallel (coupling interactions cause net spin magnetic moments of adjacent atoms to align with one another, even in the absence of an external field) so that the magnetic force within the domain is strong. e.g. iron, cobalt, nickel etc.. o Ferromagnetic materials are strongly attracted by a magnetic field. o Retain the magnetic properties when the external field is removed. o The magnetic permeability is greater than unity. o have a large, positive magnetic susceptibility (~ 106). o In non-uniform magnetic filed these substances move from weaker parts of field to stronger parts of field. Domain  Temperature dependence of magnetic property o Curie Temperature, temperature at which certain magnetic materials undergo a sharp change in their magnetic properties. o At low temperatures, magnetic dipoles are aligned. Above the curie point, random thermal motions nudge dipoles out of alignment. Paramagnets C is material-specific Curie constant, Tc is curie temperature Ferromagnets  Hard and soft magnetic materials Hysteresis loop of the ferromagnetic materials vary in size and shape. This variation in hysteresis loops leads to a broad classification of all the magnetic materials into hard type and soft type. Hard magnets Soft magnets Cannot be easily magnetized Can be easily magnetized Hysteresis loop area is large Hysteresis loop area is small Susceptibility and Permeability Susceptibility and Permeability values are small. values are high. Retentivity and Coercivity are Retentivity and Coercivity are large small Uses: Electro magnets, Uses: Permanent magnets, DC computer data storage. magnets. Transformer core Alnico, Chromium steel, Iron-silicon alloy, Ferrous nickel tungsten steel, carbon steel. alloy, Ferrites Garnets  Magnetic data storage o Magnetic storage is one of the most widely used digital data storage using a magnetized medium. o Several types of magnetized medium are used to store data such as magnetic tape, floppy disks and hard disk drives. o The basic approach to magnetic data storage is almost similar for the different types of media. o The medium used in magnetic storage devices is coated with ferromagnetic material like iron oxide. o The storage media contains magnetic surface and it is divided into very very small regions of mostly uniform magnetization. o There are two types of magnetic polarities i.e. N-S and S-N each one is used to represent either 0 or 1. o Computer system needs to store data in digital forms consists of binary information i.e. data in the forms of 0 or 1.  The drive uses a motor to rotate the media at a high speed (5400 or 7200 rpm). The data is written and read using a small device called head. Each head has a tiny electro- magnetic sensor which consists of an iron core 11 01 wrapped with wire. This head operates very close to the magnetized material. o The data signal is sent through the coil of wire which generates a magnetic flux. At the gap (a few nano-meters), This flux magnetizes small regions of the oxide on the media. o The information is stored on the disk in the forms of 0s and 1s on magnetized regions respectively. Piezoelectric material  Piezoelectric materials Piezoelectric materials are materials that have the ability to generate internal electrical charge from applied mechanical stress.​ The term ​piezo​ is Greek for "squeeze or press.” e.g. bone, proteins, quartz crystal and lead zirconate titanate (ceramics) etc.. o Piezoelectric effect is formed in crystal having no center of symmetry. o Each molecule has polarization (under stress), one end is more negatively charged and other end is positively charged, and that formed dipole. This is a result of the atoms that makeup the molecule and the way the molecules shaped.  Direct piezoelectric effect Piezoelectricity (also called the piezoelectric effect) is the appearance of an electrical potential (a voltage, in other words) across the sides of a crystal when subject it to mechanical stress (by squeezing it). o gas burners o piezoelectric microphones o detection of ultrasound wave (e.g. medical, SONAR)  Inverse / converse piezoelectric effect o Alternative field produce the strain (vibrate back and forth) in the crystal. o used in production of ultrasound (frequency greater than 20K Hz) in medical (ultrasound) and SONAR.  Ultrasonic sensor Ultrasound transducers are used in routine medical imaging. A ​ transducer​ is a piezoelectric device that acts as both a sensor and an actuator. ​Ultrasound transducers​ contain a piezoelectric element that converts an electrical signal into mechanical vibration (transmit mode or actuator component) and mechanical vibration into electric signal (receive mode or sensor component). Superconduct ors  Superconductivity Certain metals and alloys exhibit almost zero resistivity (i.e. infinite conductivity) when they are cooled to certain temperature. This phenomenon is called superconductivity, and material are known as superconductors. This phenomenon was first observed by H.K. Onnes in 1911. He found that when pure mercury was cooled down to 4.12K, the resistivity suddenly dropped to zero. The temperature at which the transition from normal state to superconducting state takes place on cooling in the absence of magnetic field is called the critical temperature (T c ) or the transition temperature. The transition temperature depends on the property of the material. It is found that the superconducting transition is reversible, i.e, above critical temperature (Tc) a super conductor becomes a normal conductor. Resistivity vs Temperature  BCS Theory (Bardeen–Cooper– Schrieffer) The pairing of electrons close to the Fermi level into Cooper pairs through interaction with the crystal lattice (Phonon)  Effect of magnetic field o Below the transition temperature of a material, its superconductivity can be destroyed by the application of a strong magnetic field. The minimum magnetic field strength required to destroy the superconducting property at any temperature is known as critical magnetic field (Hc). o The critical magnetic field (Hc) depends upon the temperature of the superconducting material. The relation between critical magnetic field and temperature is given by  Meissner effect When a superconducting material is placed in a magnetic field of flux density ‘B’ the magnetic lines of force penetrates through the material. When the material is cooled below its critical temperature (transition temperature) then the magnetic lines of force are ejected out from the material. Diamagnetic material have the tendency to expel the magnetic lines of force. Since the super conductor also expels the magnetic lines of forces it behaves as a perfect diamagnet. This behaviour is first observed by Meissner and hence called as Meissner effect.  Types of magnets There are two types of super conductors based on their variation in magnetization, due to external magnetic field applied. Type – 1 superconductor (soft super conductor) Type – 2 superconductor (hard super conductor)  Type – 1 superconductor When the super conductor is kept in the magnetic field and if the field is increased the superconductor becomes normal conductor abruptly at critical magnetic field. E.g. Lead (Pb), Aluminium (Al), Tin (Sn), mercury (Hg), Zinc (Zn) etc.. Below critical field, the specimen Magnetizatio excludes all the magnetic lines of force and exhibit perfect Meissner effect. n Type I superconductors are perfect diamagnet, represented by negative sign in magnetization.  Type – 2 superconductor The materials which lose its superconducting property gradually due to increase on the magnetic field are called Type II superconductor. E.g. niobium nitride (NbN), Niobium titanium (NbTi), Yttrium-Barium- Copper-Oxide etc.. When the super conductor kept in the magnetic field and if the field is increased, below the lower critical field Hc1, the material behaves as a superconductor Magnetizatio (exhibit perfect diamagnetism) and above Hc1, the magnetization decreases and n hence the magnetic flux starts penetrating through the material. The specimen is said to be in a mixed state between Hc1 and Hc2. above Hc2 (upper critical field) it becomes normal conductor. Difference between type – 1 and type - 2 Type - 1 Type - 2  Applications of superconductor o Medical: Used in NMR (Nuclear magnetic resonance), MRI (magnetic resonance imaging) etc.. o Transportation: Magnetic levitation (Maglev trains) etc.. o Electronics: SQUIDS (superconducting quantum interference device), Quantum computing, Sensors etc.. o Research: Particle Accelerators, Magnets, Plasma / fusion research etc..