List of Commonly Used Electrical Engineering Materials

Here is a list of eleven commonly used electrical engineering materials.

1. Dielectric Materials:

Dielectrics do not possess free electric charges and hence do not conduct electric current, this means that the dielectric materials are usually insulating materials which possess a very low electrical conductivity. However while the function of an insulating materials is to obstruct the flow of current, the function of dielectric material is to store electrical energy.

The dielectric materials which provide electrical insulation between conductors are called insultant. The molecules of a polar dielectric material possess a dipole moment because the centres of gravity of the positive and negative charges do not coincide in these molecules.

There are also non-polar dielectrics in which the centre of gravity of the +ve and -ve charges normally coincide in the absence of an external field and the dipole moment of the molecule is zero. In an external field, the molecules are deformed and an induced dipole moment appears which is proportional to the electric field intensity.

In the absence of an external electric field, the dipole moments of the molecules of a dielectric are randomly oriented. Consequently the resultant dipole moment equals zero. In an external electric field the dielectrics are polarized i.e. there is a net resultant dipole moment in the direction of the applied electric field.

Dielectrics are used in capacitors an as electrical insulation. The dielectric constant or relative permittivity of material can vary with temperature and frequency, the bonding crystal structure, phase constitution, and structural defect of the dielectric. All these factors influence the response of the induced or permanent electric dipoles in the dielectric to a steady or alternating electric field.

If the polarization lags the applied field strength, it leads to an electric field. If the polarization lags the applied field strength, it leads to an electrical energy loss which appears as heat and is proportional to the product of the relative permittivity and the tangent of the lag angle d.

Another undesirable energy loss in dielectric arises from ion and electron migration. Overheating or cyclic heating leads to degradation of the dielectric and breakdown. The electric field strength at which an insulator breaks down is called its dielectric strength.

Most dielectrics are, therefore, rated by three factors:

(1) Relative permittivity,

(2) Tangent of lag angle and

(3) Dielectric strength.

2. Insulating Material:

When insulation is designed every attempt is made to avoid the existence of air space in it.

The air spaces exercise harmful effects in the following way:

When a solid insulation containing air spaces is subjected to voltage, ionisation occurs. Consequences of ionisation, include (i) a great power loss in the insulation, (ii) thermal instability, and (iii) lowering of the breakdown voltage of the insulation. In addition to it there is carbonisation; decomposition and mechanical damage to the insulating material. Thus when there are spaces in the insulation, it should not be overstressed.

Requirements of Insulating Materials:

Main requirements of electric insulating materials can be broadly classified as:

1. Electrical,

2. Mechanical,

3. Thermal,

4. Chemical.

1. Electrical Requirements:

(i) Resistivity:

This is usually measured as insulation resistance and is based on measurement of flow of direct current into the insulation after electrification of one minute. For a good insulating material, the current should be low and the insulation resistance large. Materials which have low insulation resistance deteriorate quickly under weathering conditions.

(ii) Dielectric Strength:

It determines the capacity of an insulating material to resist puncture initiated by electrical potentials. It is adversely affected by high temperature, by weathering or ageing, by continued heat, and by ingress of moisture.

(iii) Power Factor:

Power factor is a measure of the power loss in the insulation and should be low. It varies with the temperature and usually increased with the rise in temperature of the insulation. A rapid increase indicates danger.

2. Mechanical Requirements:

(i) Density:

Electrical insulations are used on the basis of volume and net weight. Insulating material of low density is especially suitable for small portable equipment and aircraft components.

(ii) Viscosity:

It is of importance in liquid dielectrics. Uniform viscosity provides uniform electrical and thermal properties.

(iii) Moisture Absorption:

Water lowers the electrical resistance and dielectric strength. With its absorption certain chemical and mechanical effects may result, e.g., swelling, wrapping and corrosion.

(iv) Hardness of Surface:

Hardness of surface enables the dielectric to resist surface scratching and abrasion while lower surface resistivity permits irregular moisture films to form and also contribute to corona and other surface determining effects. Surface roughness is objectionable.

(v) Surface Tension:

In liquid dielectrics low surface tension is desirable as it causes greater wetting of the electrical components and thus gives better cooling, impregnation and greater voltage uniformity. To improve this property some wetting agent may be added.

(vi) In case of solid insulators, tensile strength, compressive strength, shear strength, bending strength and impact strength are all important. Machinability and resistance to splitting are also equally important.

(vii) Uniformity:

Dielectric should be uniform throughout as it keeps the electrical losses as low as possible and electric stresses uniform under high voltage differences.

3. Thermal Requirements:

(i) Specific Heat and Thermal Conductivity:

In solids there is wide variation of these properties although in solids electrical and thermal conductivity go together especially very closely in metals. In case of liquids used as coolants in transformers, these properties are of little importance due to small range of variation available in all insulating materials. Among gases, hydrogen and helium have five to ten times the thermal conductivity and specific heat of other gases and inspite of their lower density, they are very good coolants. Hydrogen is sometimes employed for cooling large generators.

(ii) Thermal Expansion:

Thermal expansion is important because of the mechanical effects caused by thermal expansions due to temperature changes. In insulating materials it should be very small.

(iii) Ignitability:

Insulating materials exposed to arcing should be non-ignitable. In case they are ignitable, they should be self-extinguishing, resistant to cracking.

(iv) The softening point of solid insulating material should be above the temperature occurring in practice.

4. Chemical Requirements:

(i) Resistance to External Chemical Affects:

Insulating materials should be resistant to oil or liquids, gas fumes, acids and alkalis. The materials should not undergo oxidation and hydrolysis even under adverse conditions.

(ii) Resistance to Chemical in Soils:

Cables laid in the soil can deteriorate by the action of chemicals in soils. The suitability of insulating material for such conditions can be decided by a long experience.

(iii) Effect on Other Materials:

An insulating material should not have a deteriorative effect on the materials in contact with it. It should not have any direct solvent acting on these materials.

3. Piezoelectric Material:

An applied electric field inducing an electric dipole moment in a dielectric, displaces ion relative to each other. Figure 6.11 shows-an ion array with and without an applied field. The dimensions of the crystal have increased in the field direction. This physical property is called electrostriction.

Now let us consider the effect on the polarization by changing the dimensions of the crystal by means of a mechanical strain. We consider two kinds of crystals, namely, those with a center of symmetry and those without. First a word about the nature of a center of symmetry. If the unit cell of a dielectric is such that we can draw from a central point a vector to one charged ion, and on drawing an equal and opposite sector from the point we also find a similar ion, then the structure has a center of symmetry at the central point.

We will now consider the effect of mechanically changing the spacing in very simple two-dimensional ion arrays with and without centers of symmetry.

i. Array with a Centre of Symmetry:

Figure 6.12 shows an array of ions in which the position of the ion A is obviously a center of symmetry. In Fig. 6.12 (b) the array has been compressed by a mechanical force. Ion B has been moved closer to ion A, thus decreasing the BA dipole moment. However, atom C has also been moved closer to atoms A, which decreases the CA dipole moment by the same amount and, moreover, in the opposite direction to the decrease in the BA moment. Thus no net change in polarization results from mechanical deformation of a crystal with a center of symmetry.

ii. Array without a Center of Symmetry:

Figure 6.13 (a) shows a very simple two-dimensional array of ions with no center of symmetry. A compressive force is applied to this array in 6.13 (b) and a tensile force in 6.13 (c). The total electric dipole moment has been decreased in (b) and increased in (c). Thus, a net change in polarization results from a mechanical deformation of a crystal having no center of symmetry and is known as piezoelectric effect. Materials that exhibit this phenomenon are called piezoelectric.

Since both the strain and the polarization are characteristically directional in a crystal, complex relationships exist between these quantities. For example, quartz crystals cut in wafers whose faces are parallel to the basal plane (X cut) will contract or expand in a direction parallel to an electric field perpendicular to the faces.

In general, any mechanical stress can produce an electric polarization in a piezoelectric crystal; that is, it does not matter whether the stress is applied in compression, dilation or shear. For example, consider a piezoelectric crystal placed between two metal plates, as shown in Fig. 6.13 (d) if the crystal is compressed by an applied mechanical stress σ, a mechanical strain _m is produced in the crystal.

m = σ/Y = Δ ω/ ω

Where, Y is Young’s modulus and Δ is the change in the width on application of mechanical stress.

Piezoelectric materials include titanates of barium and lead, lead zirconate (PbZrO₃), ammonium dihydrogen phosphate (NH₄H₂PO₄), Rochelle salt (KNaC₄H₄O₆, 4H₂O), tourmaline [(FeCrNaKl)₄ Mg₁₂B₆Al₁₆H₈Si₁₂O₆₃], ethylene dihydrogen tartrate (C₆H₁₄N₂O₆) and quartz.

4. Ferroelectric Material:

Ferroelectricity is a phenomenon which was discovered in 1921. The name refers to certain magnetic analogies, though it is somewhat misleading as it has no connection with iron at all. Ferroelectricity has been called Seignette electricity, as Signette or Rochelle salt (RS) was the first material found to show ferroelectric properties such as a spontaneous polarization on cooling below the ferroelectric Curie point T, ferroelectric domains and a ferroelectric hysteresis loop.

A huge leap in the research on ferroelectric materials came in the 1950’s leading to the widespread use of barium titanate (BaTiO₃) based ceramics in capacitor applications and piezoelectric transducer devices. Since then, many other ferroelectric ceramics including lead titanate (PbTiO₃), lead zirconate titanate (PZT) lead lanthanum zirconate titanate (PLZT), Guanidine aluminium sulphate hexahydrate (GAHS), potassium niobite (KNbO₃), potassium dihydrogen phosphate (KH₂PO₄), sodium nitrite (NaNO₂) and relaxer ferroelectric like lead magnesium niobite (PMN) have been developed and utilized for a variety of applications.

With the development of ceramic processing and thin film technology, many new applications have emerged. The biggest use of ferroelectric ceramics has been in the areas such as dielectric ceramics for capacitor applications, ferroelectric thin films for non-volatile memories and electro-optic materials for data storage and displays.

Some interactions in ionic crystals lead to a type of spontaneous polarization termed antiferroelectric. Unlike the ferroelectric, anti-ferroelectric has no permanent moment, because of the geometry of the spontaneous polarization. As a possible model, imagine lines of ions polarized in one direction, with adjacent lines of ions polarized oppositely.

If the polarization in one direction is equal to that in the other the net polarization is zero. These solids have an antiferroelectric curie temperature T_c above which the effect does not exist. They have a relative capacitivity somewhat larger than that of the normal dielectric. For the common antiferroelectrics, ԑ_r is order of 50 to 150.

5. Conducting Materials:

On the basis of resistivity of the materials, conducting materials are broadly classified into two categories:

1. Low resistivity materials—copper, aluminium, steel, silver etc.

2. High resistivity materials—tungsten, nichrome, platinum, manganin, constantan etc.

Properties of Low Resistivity Materials:

A low resistivity material, such as copper, besides possessing a low value of resistivity, should also possess the following additional properties:

i. Low Temperature Coefficient of Resistance:

This means that the change of resistance with change in temperature should be low. This is necessary to avoid variation in voltage drop and power loss with changes in temperature. With the rise in temperature, due to the flow of electrical current in the transmission lines and windings of electric machines, etc., the resistance increases, and hence the power loss and voltage drop increases. To keep these losses low, the conducting material should have low temperature coefficient of resistance.

ii. Mechanical Strength:

Mechanical stresses are often produced in overhead line conductors used for transmission and distribution of electrical power due to the wind and their own weight. Mechanical stresses are also produced in conducting materials used for the windings of motors, generator and transformers when loaded. Therefore, to withstand the mechanical stresses in such applications, the conducting material should possess sufficient mechanical strength.

iii. Ductility:

Different sizes and shapes of conductors are required for different applications. To fulfil this requirements the conducting material should be ductile enough to enable it to be drawn into different sizes and shapes.

iv. Solderability:

Conductors are required to be joined very often. The joint should offer minimum contact resistance.

v. Resistance to Corrosion:

Conducting materials should be such that they are used easily corroded or rusted when used without insulation in outdoor atmosphere.

6. Solder Materials:

Solder is an alloy which is used to join two or more pieces of materials. The melting point of a solder is lower than the materials to be joined. The molten solder joins the pieces of metals and the process is known as soldering.

Solders used for electrical purposes can be divided into two groups:

(i) Soft solder (melting point lower than 400°C)

(ii) Hard solders (melting point higher than 400°C).

A soft solder in an alloy of tin and lead. The most popular composition is 50% tin and lead. The tin-lead solder serves to join copper, bronze, brass, lead, tinned iron, zinc, etc.

A hard solder is an alloy of copper and zinc. It melts at a very high temperature. It is used for joining brass, copper, iron and steel. There are two varieties of hard solder namely brazing solder and silver solder. Brazing is a soldering at high temperature. The non-ferrous filler metal is used and has a melting point less than the base metals. The filler metal is distributed in the joint by capillary attraction.

Electrical Contact Materials:

The term electrical contact means a releasable junction between two conductors which is apt to carry current. Materials used for making contacts operate under the severest conditions when the contacts serve to make and break electrical circuits very frequently.

For electrical contacts used in switches, bushes and relays, the material must possess high electrical conductivity, high thermal conductivity, high melting point and good oxidation resistance. High thermal conductivity helps to dissipate the heat effectively. High melting point is desirable so that any accidental over heating does not fuse together the contact points. Good oxidation resistance is necessary to keep the contact clean and free of insulating oxides, Ag and Cu largely satisfy the above requirements.

Contacts operating on D.C. circuits are subjected to material transfer, i.e., the transfer of metal from the face of one contact to the face of its matching contact. In A.C. circuits, this type of material transfer is not usually encountered within the frequency range of about 25 to 400 Hz.

The other factors which affect the contact performance are- frequency of operation (number of operations a pair of contacts may be required to perform in a unit time), speed of contact separation, type of electric load (capacitive, inductive or resistive), and the medium in which the contact operators.

Circuit breaking contacts have to withstand arcing or spark over, whenever, they are separated or brought together. They deteriorate with time because of – (i) mechanical wear; (ii) corrosion resulting from oxidation and other chemical reactions due to contact with surrounding media and other factors; (iii) erosion from fusing, evaporation, and wear of working surfaces during service. Due to corrosion, contact surfaces usually acquire a film of oxide which has low conductivity and reduces the effectiveness of contacts.

7. Electrode Materials:

When in an electrical circuit, there is metal rod dipped in a solution of its salt, the metal rod is called electrode. The widespread use of these electrodes is in the emf cells and in electroplating industry. The electrode at which electrons leave the cell is called the anode and the one at which electrons center the cell is called the cathode.

Alkali and alkaline earth elements can give up electrons more easily than the transition series elements and are said to be more anodic than the transition series elements. Zn and Mg are generally used as anode materials. A more widely used material is magnesium alloys because of low cost.

Electrode Potential:

When a metal is dipped into a solution containing ions of the metal, a potential is developed between the metal and the solution. This is called the single electrode potential. This potential develops because of the following reaction-

M ⇌ Mⁿ⁺ + ne

where M is the metal and n is the number of electrons released. In the forward direction, a metal atom gets into solution as metal ion Mⁿ⁺ releasing n electrons. In the reverse direction the metal ion is deposited on the metal. An equilibrium is reached after some time. This potential exists even when the ions in solution are of different type. Some standard electrode potentials are given in Table 6.9.

Gold at the bottom of the table is the most noble metal and will not dissolve easily. Lithium at the top of the table is the most active and base metal; it will go into solution readily.

8. Thermal Insulators:

Thermal insulating materials are substances of extremely low thermal conductivities. The purpose of a thermal insulator is to prevent or retard the loss of heat, which takes place by convention, conduction or radiation. The insulating capacity is inversely proportional to conductivity. The body to be protected from heat losses is covered with the insulating material, so that the latter intervenes between the hot body and the environment.

The situations, where thermal insulator’s use arises are:

(a) Where the flow of heat has to be stopped from the outside environment to the equipment of plant, which has necessarily to be operated at low temperature;

(b) Where the flow of heat has to be stopped from a furnace or heat generating plant to the outer environment.

Uses:

Insulating materials are used in refrigerators cold storage rooms, brine pipelines, steam-carrying pipes, ovens, boilers, etc.

Thermal property of an insulator depends upon:

(1) Pores:

Most of the common insulating materials can be regarded as mixtures of fibrous or cellular granular bodies. The entrapped or any other gas in such relative proportions as to make the flow of heat very difficult. Though air and most gases have low thermal conductivities, but the heat transfer by convection increases with the pore volume, because greater convection currents are formed in larger enclosed spaces. Hence, the thermal conductivity of material can be decreased by changing the structure of pores. Consequently, a multitude of fine pores is preferable to having a few large ones.

(2) Presence of Moisture:

Presence of Moisture in the pores increases the thermal conductivity, because the air in the pores is replaced by nearly twenty-times more conducting water vapours. Consequently, every care should be taken that insulator surfaces are water-proofed. Alternatively, the insulating material pores should be of closed-type, so that the moisture cannot enter them. Moreover, the insulating material should be unreactive to water.

Characteristics of an Ideal Insulator:

(1) Its thermal conductivity should be low.

(2) It should be fire-proof.

(3) It should resist moisture absorption.

(4) It should be chemically stable to surrounding conditions.

(5) Its cost should be low.

(6) It should be odourless in use.

(7) It should stable physically, and mechanically at temperatures it attains.

Classification of Thermal Insulator:

Thermal insulating materials are either organic or inorganic. The former are suitable for low temperature works (upto 150°C). On the other hand, for higher temperature insulation works, inorganic materials are used.

(i) Organic Thermal Insulators:

Are naturally occurring materials of quite low apparent densities and they possess very large number of small cotton, wool, cattle hair, wood pulp, silk, saw-dust, paper, leather, charcoal powder, code powder, cellular rubber, etc. Their thermal conductivities and apparent specific gravities are given in Table 6.10.

(ii) Organic Thermal Insulators:

Some of the important organic thermal insulating materials and the maximum temperature upto which they can be used are given in Table 6.11.

9. Bimetals:

A bimetal is made of two metallic strips of unlike metal alloys with different coefficients of thermal expansion. At a certain temperature, the strip will bend and actuate a switch or a lever of a switch. The bimetal can be heated directly or indirectly. When heated, the element bends so that the metal with the greater coefficient of expansion is on the outside of the arc formed while that with smaller coefficient of expansion is on the inside.

When cooled, the element bends in the other direction. Alloys of iron and nickel with low coefficients of thermal expansion are used as one elements of the bimetallic strip. The other element consists of materials having high value of coefficient of thermal expansion e.g. iron, nickel, constantan, brass etc.

Bimetallic strips are used in electrical apparatus and in devices such as relays and regulators. For example, in order to maintain a constant temperature in a heater a simple bimetallic regulator may be used.

A bimetal relay or release can be used for overload protection of electric motors or any electric circuit. In such an arrangement the circuit current is passed through the bimetallic strip. If the current rises above its setting (many bimetal devices are adjustable for a particular current), the strip will be heated enough to bend and break the circuit either directly or through an intermediate relay.

10. Fuse Material:

A fuse is a protective device, which consists of a thin wire or strip. This wire or strip is placed in series with the circuit it has to protect, so that the circuit-current flows through it. When this current is too large, the temperature of the wire or strip will increase till the wire or strip melts thus breaking the circuit and interrupting the supply.

The current is cut off by the fuse as follows:

Upon melting of the wire, the metal-ions from an arc constitute a conducting path through which the current continues to flow. In order to quench the arc, the resistance in the arc-path must rise to such an extent that the available voltage is no longer able to sustain the arc.

For a proper quenching of the arc in a fuse the following measures can be taken:

(i) The material chosen for a fuse wire is that whose metal-ions after melting have low conductivity. Silver or an alloy of silver is such materials.

(ii) If the temperature of the arc- path is kept low, its resistance and therefore the voltage-drop along the arc will be high due to less mobility of the metal-ions and electrons. The cooling of the arc in a fuse can be achieved by embedding the fuse wire in quartz sand as is done in cartridge and HRC fuses.

(iii) The higher the pressure of the gas in which the arcing takes place, the higher will be the required voltage to sustain the arc. This is achieved by putting the melting wire in an explosion tight enclosure, in which a pressure is automatically built up by the heat produced by the arc.

A fuse material should possess the following properties:

(a) Low resistivity. This means, thin wires can be used, which will give less metal vapour after melting of the wire. Less metal vapour in the arc gives lower conductivity and thus makes quenching of the arc easier.

(b) Low conductivity of the metal vapour itself.

(c) Low melting point. This means that the temperature of the fuse material for normal currents stays at a low value.

Originally, only lead was used as fuse material because of its low melting point. But as the resistivity of lead is high, thick wires are required. For rewirable fuses alloys of tin and lead or tinned copper wires are commonly used.

In cartridge fuses silver and silver alloys are used in fuses of lower ratings and copper, alloy are used in fuses of higher ratings.

An important characteristic of a fuse material is its current-time relationship. For wire in which the heat is dissipated largely by radiation (rather than by conduction at the ends), the fusing current (Fusing current is the minimum current to fuse the wire in such a time interval as shall be necessary for the wire to have attained its steady temperature) can be calculated using the relation,

I = adⁿ

where I is the fusing current in amperes, d is wire diameter in cm and ‘a’ is a constant whose value varies from material to material. The values of constant ‘a’ for some materials are- Copper 2530; Aluminium 1870; Iron 777; Tin 405.5; Lead 304.6. The approximate value of n is 1.5.

The rating of any fuse depends entirely on its dimensions, mounting, surrounding powders or liquids, enclosure and other factors which affect its heat dissipating capacity and its ability to extinguish arcs after fusing.

11. Dehydrating Material:

Silica gel is an inorganic chemical, a colloidal, highly absorbent silica used as a dehumidifying and dehydrating agent, as a catalyst carrier and sometimes as a catalyst. Calcium chloride and silica gel are used in dehydrating breathers to remove moisture from the air entering a transformer as it breathes.

Silica gel breathers are a more recent development and are replacing the calcium chloride breathers. Its main advantage is that when it becomes saturated with moisture it does not restrict breathing as does calcium chloride. Silica gel, when dry, is blue in colour and the color changes to pale pink as it becomes saturated with moisture.

If it is found to contain excessive moisture, it must be replaced or reconditioned. It can be dried by heating it in an open container at a temperature of between 150°C and 200°C for two hours. Calcium chloride can be dried by heating it in the same manner at a temperature of between 180°C and 200°C until completely dry.