Engineering Materials | Classification Of Engineering Materials


As a student of electrical engineering or Electronics Engineering, you know that a large variety of Engineering materials are used in the manufacture of electrical and electronic equipment.

Broadly speaking Classification Of Engineering Materials as conductors, insulators (dielectrics), magnetic materials, and semiconductors.

It is universally accepted that atoms influence materials properties.

Nevertheless, when atoms form solids, it is basically the electrons that control the nature of bonds between the atoms, the electrical conduction behavior, the magnetic effects, the optical properties, and the chemical reactions between atoms.

In the present chapter, we shall review the Classification Of Engineering Materials in a detailed manner.

Also Read: Bonding In Solids | Types of Bonds in Solids

A large variety of materials are used for the manufacture of electrical and electronic equipment.

These materials are classified as 2 points of view:

  1. Electrical engineering point of view materials are classified, and
  2. Material Scienece point of view materials are classified.

From an Electrical Engineering Point of view, these materials are classified as:

  • Conductors
  • Resistor materials
  • Insulator or dielectrics
  • Magnetic materials
  • Semiconductors
  • Refractory materials
  • Structrual materials

From a material science point of view, materials are classified as:

  • Metals and alloys
  • Ceramics
  • Organic polymers
  • Composite materials

We will now see each material in a detailed manner. So first we will start the classification based on the material science point of view materials.

1Classification of Engineering materials Based on Material Science Point of View

1. Metals and Alloys

  • Metals are elemental substances while alloys are formed when two or more relatively pure metals are melted together to form a new metal.
  • Metals are opaque, lustrous and comparatively heavy.
  • They are strong but can be rolled or hammered in to shape and can be alloyed and welded.
  • All these properties of metals flow from the metallic bond.
  • The metals can be further classified as:
    • ferrous and
    • non-ferrous
  • Similarly, we have ferrous and non-ferrous alloys.
  • Metal alloys have the same properties like metals.
  • Even though alloys are composed of more than one type of atom, their structure is one of the lattices and grains just like as in pure metals.
  • Example of Metals and alloys:
    • steels,
    • aluminium,
    • copper,
    • brasses,
    • bronze,
    • super alloys, etc.

2. Ceramics

  • Ceramic materials contain phases that are compounds of metallic and non metallic elements.
  • A phase is a physically separable and chemically homogeneous constituent of a material.
  • Because there are many possible combi nations of metals and non-metals, there are a multitude of ceramic materials.
  • Most ceramics have crystal structure.
  • However, unlike metals, they do not have large number of free electrons, the electrons being shared covalently as in ionic bonds.
  • Crystal structure of ceramics contain atoms of different sizes.
  • In many cases ceramic crystals grow into chains similar to plastic molecules, but these chains are held together by ionic bonds instead of van der Waal’s forces. These bonds are however, not very strong.
  • Application of Ceramics:
    • Ceramics are used extensively in electrical industry because of their high electrical resistance,
    • they are also used widely for general purpose because of their ability to withstand high temperature.
    • They are also used as coatings over metals.
  • Examples of Ceramics:
    • silica,
    • MgO,
    • CdS,
    • ZnO,
    • SiC,
    • BaTiO3,
    • cement,
    • ferrites,
    • garnets, etc.

3. Organic Polymers

  • Organic materials are a group of materials characterized by large molecules that are built up by joining small molecules, usually artificially.
  • They are natural or synthetic resins, or their compounds that can be moulded, extruded, cast, or used as films or coatings.
  • Most of them are organic substances, usually containing, hydrogen, oxygen, carbon and nitrogen.
  • Their structure in many instances is fairly complex.
  • These materials are called “polymers” because they are formed by polymerization reaction in which relatively simple molecules are chemically combined into massive long chain molecules or three-dimen sional structures.
  • Application: They are extensively used as insulating materials.
  • Examples of Organic Polymers:
    • PVC,
    • PTFE,
    • polyethylene,
    • polystyrene,
    • Terylene,
    • nylon,
    • cotton,
    • natural and synthetic rubbers, etc.

4. Composite Materials

  • In engineering applications large varieties of composite materials are used.
  • They could be a combination of metals and alloys and ceramics, metals and alloys and orgnic polymers, or ceramics and organic polymers.
  • Cermets are combination of metals and ceramics, bonded together.
  • They combine some of the high refractorir.ess of ceramics and the toughness and thermal-shock resistance of metals.
  • They are used as cutting tools, crucibles, nozzles for jet engines and other high temperature devices.

2 Classification of Engineering materials Based on Electrical Engineering Point of View

From an electrical engineering point of view, the materials can be classified as Conductor, Insulator(dielectric), magnetic materials, and semiconductors.

1. Conductors

  • Conductors of electricity are materials which afford continuous passage of an electric current when subjected to a difference of electric potential.
  • The greater the density of current for a given potential difference, the more efficient the conductor is said to be.
  • High conductivity in metals is associated with the presence of free or conduction electrons. These electrons are able to move freely in the lattice and do not belong to any particular atom (metallic bonds).
  • Some of the good conductors of electricity are silver, copper, aluminium, etc.
  • The current flow in a conductor is governed by ohm’s law. Due to flow of current in a conductor some heat is developed which is given by Joule’s Law.
  • Just like electrical conductivity, the heat conduction in conductors is mostly through free electrons.
  • There exists a relationship between the electrical conductivity and thermal conductivity which is called Wiedemann Franz Law.
  • Low resistivity metals and alloys are used as conductors and for electrical contacts.
  • High resistivity alloys are used for resistors and as heating elements.
  • A special class of materials called super-conductors exhibit almost zero resis tivity when they operate below certain temperature-transition temperature. These materials have typical properties and find application as supermagnets and others.

2. Dielectric Materials (Insulating Materials)

  • These Materials provide electrical insulation between two media which are at different potential and also act as stores of electrical charges (in capacitors).
  • When the main function is insulation, the materials are called insulating materials, and when charge storage is the main function they are termed as dielectrics.
  • In these materials the electrons are bound so tightly to their respective atomic nuclei that electrical conduction by electrons cannot occur.
  • hese materials show their effect under the influence of an electric field, and they give rise to a phenomenon called polarization, which helps in storing electrostatic energy (in dielectrics).
  • Under the influence of an alternating field, their be haviour can be understood by a complex dielectric constant.
  • A large number of gaseous, liquid, and solid insulating materials are avail able these days with excellent properties.
  • One has to choose a specific material for an application after checking all the properties and making a cost com parison of different alternatives.
  • The solid insulating materials are selected on the basis of their thermal withstand rating.

3. Magnetic Materials

  • Magnetic materials are materials that can become magnets or are attracted towards magnets.
  • The peculiar properties of magnets have been known since loadstones were recognized 3,000 years ago.
  • Magnetic materials generate electric power, energize electric motors, reproduce sound and visual images and store information in computers, etc.
  • Though all materials are magnetic in the sense that they display some degree of reaction to the magnetic field, ferromagnetic materials show a strong tendency to concentrate magnetic lines of flux or to line up with the magnetic field.
  • Of the pure elements, only iron, cobalt, nickel, and gadolinium are known to be magnetic in the sense of ordinary temperatures.
  • The desired properties in magnetic materials can be obtained by a combination of these metals with other elements to form alloys which generally have to be subjected to certain heat treatment to bring about the desired result.
  • Magnetic materials can, in general, be divided into two primary classes:
    1. permanent or hard magnetic materials
    2. non-permanent or soft magnetic materials
  • permanent or hard magnetic materials those materials which on the removal of an external magnetic field show a strong tendency to remain magnetized in the direction of an initially applied field.
  • non-permanent or soft magnetic materials, the direction of magnetization can be readily changed and these materials become essentially demagnetized on the removal of an applied field.
  • Hard materials are used for permanent magnets while soft materials are for electromagnets.
  • Some special magnetic materials are used for specific applications.

4. Semiconductors

  • It is usual to define a semiconductor as a material whose electrical conductivity is intermediate between that of a metal and an insulator.
  • This valid definition is not sufficiently precise, however. Semiconductors have resistivities that are highly sensitive to temperature and impurity content.
  • Semiconductors, in general, are crystalline materials.
  • In semiconductors, the motion of valence electrons of one atom is coordinated with the motion of those of an adjacent atom so that a covalent bond is imposed between them.
  • Silicon and Germanium are the best-known semiconductor materials, have structures that are almost perfect.
  • Some other semiconducting materials are selenium, gray tin, tellurium, etc.
  • A large number of compounds are also semiconductors, like GaAs, InP, InAs, AlSb, GaP, LiSb, etc.
  • Examples of magnetic semiconductors: NiO, LaMnO3, CdCr2Se4, etc.
  • The present-day world is dominated by semiconducting materials.
  • They are in almost all electronic circuits, solid-state rectifiers, photovoltaic cells, transistors, strain gauges, thyristors, etc.
  • Most of the engineering applications of semi-conductors involve semiconductors that have been doped intentionally with specific impurities.
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