Here is a compilation of interview question and answers on metallurgy for engineering students.
Q.1. Describe the Crystal Structure of Metals?
Ans. Crystal structure of most metals consists of one atom at each lattice point of the space lattice. These crystals are called monoatomic crystals, though often this adjective is missing. For example, copper is face-centred cubic metal. If one atom (ion) is placed at each lattice point of the FCC space lattice, we get copper crystal lattice, or copper crystal structure as illustrated in Fig. 1.22 (a).
Fig. 1.22 (b) illustrates a copper unit cell taken out from the Fig. (a). Fig. 1.22 (c) illustrates common method of showing a crystal structure unit cell of FCC metal (here copper), where atoms (ions) are drawn much smaller for clarity to see all atoms and their positions.
It is important to note that more than one atom may be placed at each lattice point of the space lattice depending on the material. This criterion of putting one or more atoms at a lattice point is called the basis.
Thus a space lattice with a basis produces a crystal structure:
Space lattice + Basis → Crystal structure …(1.2)
It must be clear by now that an infinite number of crystal structures can be produced by combining different basis and also different lattice parameters with the same space lattice. Fig. 1.24 illustrates three different basis with the same simple cubic lattice (i.e., same lattice parameters) to result in three different crystal structures. Fig. 1.24 (a) is monoatomic crystal structure (one atom at each lattice point), where atoms have been drawn small sized for clarity.
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Fig. 1.24 (b) illustrates a molecular crystal having a diatomic molecule at each lattice point, the centre of the larger atom coincides with the lattice point (such as NaCI). Fig. 1.24 (c) illustrates that corner atoms are of one type and the atom at the body centre is of a different type (look at their sizes).
Here, the basis is two atoms, the larger atom at a lattice point, but the smaller one has been put halfway along the body diagonal at the body-centre, which is not the lattice point. This is not a BCC crystal, where all the atoms are of one type even at the body-centre.
If the basis is varied further, the number of crystal structures increase. Some proteins have a basis of about 10,000 atoms. If the lattice parameters too are varied, the number of crystal structures could be very large. Thus, there are only 14 space lattices, the number of crystal structures could be thousands.
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Lattice parameters describe the size and the shape of the unit cell. Lattice parameters include the dimensions of the sides of the unit cell and the angles between the sides, i.e., the values of a, b, c and α, β and ϒ.
In a cubic crystal system, only the length of one of the sides of the cube is necessary to completely describe the unit cell (angles of 90° are assumed). This length measured at room temperature, is the lattice parameter, or lattice constant, a0.
The length is often given in Angstrom (A°) or nanometers (nm). Several lattice parameters are needed to define the size and shape of complex unit cells. For orthorhombic unit cell, the dimensions of all the three sides of the cell- a0, b0, c0 are required. Hexagonal unit cells require two dimensions, a0 and c0, and the angle of 120° between the a0 axes.
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Q.2. What is Grain Size?
Ans. It is the average ‘diameter’ of a grain revealed in a two dimensional section. The size and shape of grains vary enormously. The ‘diameter’ of a grain in a worked and heat treated metal is usually about 0.01 to 0.1 mm. As-cast metals generally have coarser grains, 0.1 to 10 mm, sometimes even larger.
Grains can occasionally be seen on surfaces of everyday common objects, for example on the zinc coating of galvanised steel sheet. Normally, however, they are invisible because of either oxide film on the surface, or by the mirror glitter of the polished surface. The grain size of a brittle metal can be observed from the just fractured surface.
The bright flat facets, randomly oriented are observed as the fracture follows crystallographic cleavage planes through the grains. An inter-crystalline fracture, i.e., the fracture along the grain boundaries appears as sharp valleys and ridges. Metals and other materials with symmetric crystal structures normally possess equiaxed grains (i.e., with no elongation).
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Elongated grains may be obtained when materials undergo directional solidification, or are subjected to deformation processes.
Measurement of Grain Size:
The measurement of grain size is mainly done by metallography. The metal has to be prepared to reveal the grain boundaries of the grains. A 10 mm x 10 mm x 10 mm size sample is cut from the metal without disturbing the microstructure, either with a saw, or with water-cooled slitting wheel. One surface is then ground repeatedly with successively finer grades of abrasive papers of emery or carborundum down to a fine grade (0000 paper or 600 mesh grit). It is then polished on soft moist velvet cloth with a polishing paste (fine alumina, magnesia, or diamond dust) to a mirror scratch-free Finish. The specimen is then etched with an etching reagent.
Commonly, the following chemical reagents are used:
The polished specimen is immersed or swabbed with the reagent till the surface is discoloured. The reagent is then washed off, first with water, then with alcohol. The surface is then dried in warm air. The sample is then observed under microscope. The reagent had attacked the grain boundaries to create grooves there, which are seen us network of dark lines under light-microscope.
Q.3. What are Twin Boundaries in Surface Defects?
Ans. Surface defects, which separate two orientations that are mirror image of one another, are twin boundaries. The volume of the material which has an orientation that is a mirror image of the matrix orientation is called a twin, and this usually is fairly thick, i.e., 103 to 105 atomic layers, each layer being sheared by the same amount over the next one below.
There is a special relative orientation of the twin region with matrix orientation to give mirror image. The mechanism that produces twin orientation requires that a given atom moves only a fraction of lattice spacing, Fig. 4.103 (a). The lattice points on twinning plane are evidently unaffected by the occurrence of twinning.
Twin boundaries occur in pairs, such that the change in orientation (of atoms) introduced by one boundary is restored by the other boundary. Twin boundaries are easily identified under a microscope after etching as they separate regions of different orientations, and each region after etching has different reflectivity of the light back to the microscope. A coincidence lattice with 1/2 density of coincidence points, may exist in the pair of twin crystal. The energy of the twin boundaries is around 0.01 to 0.05 Jm-2.
Twin boundaries could be incoherent or coherent. The twin boundary of Fig. 4.103 (a) is evidently a coherent one. If the twin boundary is not the twinning plane, then, it is an incoherent boundary, as the lattice sites in the boundary are not all common to both regions. Such a boundary has high energy, around half of the ordinary grain boundary.
A twin boundary, Fig. 4.103 (a) becomes a coherent boundary too, if the boundary coincides with the twinning plane, since every lattice site on the boundary is common to both regions. The energy of a coherent twin boundary is around one-tenth of the ordinary grain boundary.
Twins are formed as a result of:
(a) Deformation, i.e., plastic deformation, and then they are called deformation twins. They are commonly observed in HCP metals, and in BCC metals when deformed at high strain rate and or low temperatures;
(b) By the process of growth during annealing, and are then called annealing twins. These are commonly observed in FCC metals.
Q.4. What are the Effects of Cold Work on Properties?
Ans. Cold-working, or strain-hardening is defined as the increase in the stress required to cause slip because of previous plastic deformation, i.e., shear stress required to produce slip continuously increases with increasing shear strain. This is supposed to be caused by interaction of dislocations with each other, and with barriers, which impede their motion through the lattice.
Strain-hardening is a very vital industrial process used to harden the metals and alloys that do not undergo heat treatment. It changes various mechanical, physical and chemical properties of metals and alloys. Fig. 7.5 illustrates that as the amount of cold work increases, there takes place increase in ultimate tensile strength, yield strength, hardness (2-3 times) with decrease in ductility (decrease in percentage elongation and reduction in area).
The cell boundaries and sub-boundaries act as barriers to the motion of dislocations. As with increasing deformation, cells become smaller, the distance between these barriers becomes less, there takes Fig. 7.5. Effect of cold work on mechanical properties of a metal place further strengthening effect in metals and alloys.
In physical properties, increasing the amount of cold deformation, decreases slightly the density of the metal (fraction of a percent), decreases appreciably the electrical conductivity of the material (increase of scattering points of conducting electrons occurs due to increased density of point defects, dislocations and grain boundaries), increases slightly the coefficient of thermal expansion, increases the coercive force but decreases the magnetic permeability.
As the internal energy of the cold worked state is high (due to stored cold worked energy), the chemical activity of the material increases, i.e., the corrosion resistance decreases, and may cause stress-corrosion cracking in certain alloys (such as in brasses). The exposed dislocations on the surfaces of metals serve as centres for metal dissolution in corrosive atmospheres.
A heavily cold worked metal has preferred orientation. This cold worked texture and the mechanical fibering, i.e., flow lines (due to the presence of impurities), both lead to anisotropy in properties of materials. The ductility and the impact toughness is much lower in transverse section than in longitudinal sections.
The rate of strain hardening can be measured by the slope of the flow curve. It is generally lower in HCP metals than in cubic metals. Higher temperatures of deformation also lower the rate of strain-hardening.
The high rate of strain hardening signifies intersections of dislocations on intersecting slip systems, which could give rise to interaction of stress-fields of dislocations, creation of sessile jogs, and, or sessile locks.
Q.5. What are the Properties on Annealing of Cold Worked Metal?
Ans. As the amount of cold work increases, tensile strength and hardness increase, but electrical conductivity and ductility decrease. Small decrease in grain size also takes place. During annealing, all the properties have a reverse trend to what happened in cold deformation i.e. they approach values they had before the deformation.
On recovery, hardness and tensile strength almost remain constant (slight decrease may be noted), but electrical conductivity is restored to a large extent (as it depends mainly on point defects), but ductility and grain size too remain almost constant. Recrystallisation decreases tensile strength and hardness but increases the ductility.
Recrystallisation restores completely the electrical conductivity. New grains nucleate and grow. During grain-growth, this trend in properties continues but at a low rate. Grain size can become very large but there takes place decrease in their number.
Cold deformations, particularly heavy cause the development of preferred orientation.
On recrystallisation, one of the followings may occur:
1. The recrystallisation texture is identical to the deformation texture.
2. The new texture is randomly oriented with no preference.
3. More commonly, the deformed texture transforms into a different recrystallisation texture.
If annealing is performed below a definite temperature limit, the recrystallisation texture is almost identical to the deformation texture except that some texture component may become more significant while others may diminish. The recrystallisation texture of a cold drawn copper wire having the double texture, <111>; <100>, shall be similar but with increasing temperatures of annealing, <111> texture is enhanced and <100> diminishes.
The most common type of annealing texture obtained is the one which is different than the deformation texture. For example, the copper type rolled texture {112} <111> changes on recrystallisation to the cubic texture {100} <100>, Fig. 7.39, i.e. {100} cube-faces in recrystallised grains become parallel to the plane of rolled strip, and <100> edges of cubes are oriented either along, or across the direction of rolling.
A very important aspect of recrystallisation texture is that, it is often crystallographically related to the cold deformation texture. In FCC metals, the lattice of recrystallised grains is tilted through an angle of approximately 30-40° around the common <111> axis relative to the lattice of deformed grain. In BCC metals, it is tilted through an angle of 25-30° about <110>, and in HCP metals, it is tilted through 30° around <0001> directions which is common to both deformed and recrystallised grains.
The development of a different recrystallisation texture is based on the theory of oriented-nucleation and oriented-growth. It is proposed that nuclei of many orientations may form initially. As generally accepted that all recrystallisation nuclei are already present in the deformed matrix in the form of sub-grains, which become more perfect during recovery processes, but before recrystallisation occurs.
A heavily deformed cold rolled copper has deformation texture consisting of mainly {112} <111>, and some traces of {100} <100>. Thus, in such copper when annealed at 600°C, the recrystallised grains in early stages of recrystallisation may have {112} <111> and {110} <112> orientations, and some {100} <100> and {110} <111>, which are identical to the deformation texture.
The theory of oriented-growth proposes that sub-grains bounded by large angle boundaries of definite orientation grow faster and serve as the recrystallisation nuclei. In heavily deformed cold-rolled copper, such nuclei are sub-grains with {100} <100> orientation. Though these cubically oriented nuclei are very less in number than the nuclei with {112} <111> orientations, but the former can grow much faster and ultimately replace all other orientations.
When such copper is annealed for long time or at higher temperatures, say in one hour annealing time, metal has only cubic {100} <100> texture. The theory of oriented grain growth agrees well with the fact that lattice of the recrystallised grain is rotated relative to that of the deformed grains on a common axis through an angle with an interval of 25-30°.
Q.6. What are the Anisotropic Properties in Annealed State of Metals?
Ans. The more perfect is the recrystallisation texture, the properties of the annealed metal become more anisotropic. Normally, it is undesirable to obtain anisotropic properties except in some application such as in transformer steels. It is specially undesirable in sheets to be used for deep drawing.
The sheets produced by heavy rolling and then annealing, having perfect recrystallisation texture, develop anisotropic properties, say in FCC metal as illustrated in Fig. 7.40 (a). The fabric-ability of metal is effected. For example, bending is more difficult when the bend line is parallel to rolling direction.
The phenomenon of earing during deep-drawing indicates directionality. Earing is the formation of wavy edge on the top of a drawn cup which necessitates extensive trimming to produce a uniform top. Earing, Fig. 7.40 (b) is correlated with the planar anisotropy measured by
∆R = (R0 + R90 – 2 R45) / 2
When aluminium sheet (cold rolled and annealed) is stamped, it develops wavy edges, or ears (four or six symmetrical), which increases the scrap and the production cost. The common method recommended for obtaining almost isotropic copper, or brass sheets is to use a low cold reduction in the last rolling operation and also a low annealing temperature in the final stage.
Q.7. How does Ductile Fracture Occurs?
Ans. A ductile fracture, takes place with plastic deformation occurring to initiate the crack and with intense plastic deformation occurring at the tip of the propagating crack. In the absence of second phase inclusions, ductile fracture is described as rupture as it is a slow separation process. The cup-and-cone fracture often seen when most polycrystalline ductile material fail.
It is now well established that such a ductile fracture is initiated by the nucleation of voids at second phase particles (carbide, sulphide or silicate) either by cracking of the particles, or by decohesion at the particle/matrix interface. Thus, the volume fractions, distribution and morphology of both carbides and inclusions are important in determining the ductile behaviour.
The cup-and-cone fracture is closely associated with the formation of a neck in a tensile specimen. Necking starts at the point of plastic instability when the increase in strength due to strain- hardening is unable to compensate for the decrease in cross- sectional area. This happens normally at maximum load in the engineering stress-strain curve.
Necking produces a triaxial state of stress in the region. Many voids are seen to be present in this region. The formation of voids begins very early in a tensile test as a result of high stresses produced by pile-up of dislocations on individual particles. In steels, the bonding of inclusions is usually weak, so that voids form there at low plastic strains.
In high purity metals, voids can form at grain boundary triple point. Particles as small as 5nm have been seen to nucleate voids. In the tensile test, voids form prior to necking, but are too small to be seen metallo- graphically. After the necking, when the hydrostatic tensile stresses develop, the microvoids coalesce and become prominent to be seen, Fig. 15.17 (a). The density of these voids depends strongly on the amount of deformation; increasing with increasing deformation.
The coalescence of these voids, as a result of their growth in size and with elongation of the bridges of material between the voids under the applied stress, can be assumed to lead to the formation of a ductile crack as shown in Fig. 15.17 (b). Such a crack can then propagate by the void-sheet mechanism.
In Fig. 15.17 (c), the stress-concentration at the ends of the original ductile crack causes the localised plastic deformation in regions that make angles of 30°-40° with the tensile stress-axis. These slip bands, which undergo extensive deformation and thus, have high density of voids, are called void-sheets. As the elongation of the voids and the bridges of material between the voids occurs, voids coalesce.
The void-sheet splits into two to propagate the crack as illustrated in one of the void- sheets in Fig. 15.17 (d). At the end of this crack, two new void-sheets can form as illustrated, one which is extension of the original void-sheet that has split, and the second is inclined in the opposite direction.
If the crack propagates in the first void-sheet i.e., in the original direction, it is then moving away from centre of the necked region, and into a region of decreasing stress. Normally, the crack moves into the other void-sheet i.e., it propagates back into region of maximum stress-concentration. This process is repeated to spread the crack across the cross-section until it approaches the surface of the specimen.
So far, the average direction of crack growth is radially outward in the direction transverse to the tensile axis. On a finer scale, the crack zig-zags back and forth across the transverse plane. That is why the characteristic appearance is rough with spongy texture at low magnification, whereas at high magnification, the fractured surface consists of ‘dimples’ formed from voids which were separated by thin walls until it fractured.
The final fracture in which the shear-lip of the cup-and-cone fracture forms, also appears to take place by void-sheet mechanism. As the central fracture propagates towards the surface, the cross-sectional area of the rim progressively decreases until the void-sheet shear band extend to the surface as illustrated in Fig. 15.17 (e). The sudden splitting up of the shear bands results at roughly 45° to the tensile axis in the formation of shear-lip.
When a steel contains pearlite, voids are created by the cracking of the cementite. Fig. 15.18 illustrates the mechanism. The cementite plates of pearlite are parallel to the tensile axis. One of the carbide plates cracks as illustrated in Fig. 15.18 (a). As the shear stress is maximum at 45° to the tensile axis, a concentrated shear zone at about 45- 50° to the tensile axis causes the cracking of the adjacent carbide plates of pearlite, Fig. 15.18 (b).
Under the tensile stress, the voids grow, Fig. 15.18 (c) and coalesce to form a crack to cause ductile fracture as illustrated in Fig. 15.18 (d). Spheroidised cementite particle in spheroidised pearlite resists this cracking. Because dislocations in pile-up in ferrite in such a steel can easily cross slip, and as the area of contact is smaller, tensile stresses developed in the particles are less. That is why quenched and tempered steels having very fine spheroidised carbide particles have good ductility and toughness as these resist the void formation, and thus, the crack formation.
Q.8. Explain the Hardenability of Steel and its Tool.
Ans. Hardenability is defined as the ability of steel to develop its maximum hardness when subjected to the normal hardening heating and quenching cycle. High hardenability means that the steel hardens more nearly throughout rather than just at the surface. In other words, good hardenability is indicated by a greater depth of hardening below the surface.
Often a confusion arises about the meaning of hardening power and hardenability. It may be remembered that hardening power refers to the maximum hardness obtainable with steel of given composition while hardenability refers to the depth of hardening in a given thickness of steel.
Only the carbon content affects the maximum hardness that can be realised with any steel. Higher carbon content makes more carbon and iron carbide available to distort the martensitic structure and consequently allows higher hardness. The hardenability of a steel becomes greater as the percentage of carbon increases and is further improved by the addition of such alloying elements as manganese, nickel, chromium, molybdenum and vanadium, and by increase in the austenite grain size.
The factors which affect the depth of hardening of steel components are given below:
1. Hardenability of the steel from which the component is made.
2. Severity of quench used.
3. Size and shape of the piece.
4. Surface condition and the austenite grain size.
The term critical diameters and ruling sections are used to indicate hardenability. The critical diameter of a steel may be defined as the maximum diameter of bar in which, when quenched in a specified medium, the steel will develop at the axis or at some other specified position, a specified hardness or a specified proportion of martensite in its structure.
The ruling section is the maximum size of bar in which specified mechanical properties such as ultimate tensile strength, yield point stress, elongation, izod value etc., can be developed at the centre by quenching in a specified manner.
There are a number of methods available for measuring the hardening response of a steel to heat treatment. One of the most useful and widely accepted and the only discussed here is the Jominy test. Here all the factors which effect the depth of hardening of steel, except composition are fixed or held constant.
Any variation in the depth of hardening, therefore, reflects a variation in hardenability. The data obtained from this test are relatively easy to interpret, appropriate plotting the hardenability of different steels may be readily compared. Furthermore Jominy test results are useful in predicting the hardness at any location within a quenched steel part without a specific test, it is done on the basis of the composition of the steel.
Refer Fig. to 5.22. In the Jominy test, the standard specimen consists of a cylindrical rod 4″ long and 1″ diameter. In making test the specimen is first heated to a suitable austentising temperature and held there long enough to obtain a uniform austenite structure. It is then placed in a jig and stream of water allowed to strike one end of specimen.
The cooling conditions within the Jominy bar during quenching change very little across the diameter but vary rapidly at that end and progressively less rapidly at points towards the opposite end. After being thoroughly cooled, shallow flats are ground on the bar along the flats at 1/16″ intervals for the first inch followed by reading at 1/18″ intervals for the next 1 1/2 “. A plot is made of the hardness reading against the distance from the quenched end of the bar. Such a plot is shown in Fig. 5.23.
Q.9. What is the Procedure for Machining of Plastics?
Ans. Two type of plastics are being used for fabrication. These are plane plastics in cast conditions or laminated plastics. Plastics present such problems as heating and edge fretting. During machining material is removed in fine particles and in some case gases may also evolve in chips and gases are harmful and hence must be removed immediately. Plastics may be subjected to almost all machining operations with the difference in cutting speed, feed, tool parameters etc.
Brief suggestions are given below:
1. Filing:
Both plastics can be filed and surface finishes of Rough (∇) and Fine (∇ ∇) types are obtainable.
a. For rough filing use rasp or bastard file.
b. For fine filing use smooth or dead smooth file.
2. Sawing:
Both plastics can be cut by a circular saw using cutting speed of 50 m/min and hand feed. Pitch of the saw should be close to 10 mm. However, the thickness of saw will vary with thickness of plastic to be cut.
a. For thickness 0.5 o 4 mm use 3 mm thick saw.
b. For thickness of 5 to 8 mm use 4 mm thick saw.
c. For thickness 19mm and above use 5 mm thick saw.
3. Drilling:
Drilling a plastic may pose such problems as damaged hole periphery on surfaces where drill enters and exists and also striking of drill in the hole.
By using sharp cutting edges and by carefully setting them on the surface before starting the operation, by putting a wooden support directly below the hole where drill will exist, and by frequently lifting the drill from hole the above problems are overcome. The drill may also be dipped in mineral oil for obtaining a smooth surface finish of hole.
a. For plain plastic use- point angle – 90° to 116°C.
b. However, if section is thin this angle may be 50°C.
i. Flute angle – 60°
ii. Cutting speed – 70 m/min
iii. Feed – 0.2 to 0.3 mm/rev.
c. For laminated plastics- use cutting speed 60 to 90 m/min and feed 0.2 to 0.5 mm/rev.
d. However, drilling may be done parallel or perpendicular to laminated plastics. If parallel use point angle of 70° and rake angle of 0-20°.
If perpendicular use point angle 100-110°, rake angle 10 to 12°.
4. Planning:
For both plain and laminated plastics use well sharpened and lapped carbide tipped tool at cutting speed of 10 to 20 m/min and feed of 0.2 to 0.8 mm/stroke.
5. Turning:
Carefully the tool must be set at the centre of work piece, and the tool tip must be rounded to a radius of 1.5 to 2 mm.
For both materials use clearance angle of 3° and rake angle of 15°. Chips should be removed by suction.
Number of teeth on milling cutter must be 1/2 to 2/3 of the number of a corresponding steel job. Clearance angle of 20° and rake angle of 20 to 25° is recommended on carbide tipped tool. While feed for both types of plastic as 0.5 to 0.8 mm/rev is recommended the plain plastic should be cut with a cutting speed of 300 m/min and laminated plastic with a cutting speed of 120 to 250 m/min.
Grinding of plastics is possible with belt, sand paper and grinding wheels. For rough grinding grit 20 wheels and for finish grinding grit 60 silicon carbide wheels are used. For belt grinding V = 6 m/min, sand paper V = 25 m/min.
For surface grinding water may be used as coolant.
Tapping, turning or milling can be used. In both turning and milling use HSS cutter with cutting speed 20 to 40 m/min.
Milling will result in better surface.
When using taps use wax and grease to lubricate with taps of wide flute.
Laminated plastics can be sheared upto thickness of 3 mm. However, if thickness greater than 3 mm is to be sheared the material is recommended to be warmed.
Q.10. What are Transistors?
Ans. A ‘transistor’ is a semiconductor device in which current flows in semiconductor materials. It was invented in 1948 by J. Bardeen and W.H. Brattain of Bell Telephone Laboratories. It is an amplifying device in which an input signal is transmitted at an increased magnitude.
When a thin layer of P-type or N-type semiconductor is between a pair of opposite types it constitutes a transistor.
A triode transistor consists of two P-N junction diodes placed back to back.
Types of Junction Transistors:
The following are the two common types of junction transistors:
1. Grown junction type.
2. Alloy-junction type.
Fig. 7.30. (a) shows a grown P-N-P junction triode transistor.
Fig. 7.30 (b) shows the form of N-P-N junction transistor.
In the manufacture of grown junction transistors the single crystal-growing process is employed.
The left hand section or region is called the emitter whereas right hand section is known as collector. The middle section called base region or base is extremely thin as compared to either the emitter or collector and is lightly doped. Function of emitter is to inject majority charge carriers into the base and that of the collector is to collect or attract these carriers through the base.
Fig. 7.28 (c) shows an alloy-junction transistor which consists of two leads of indium metal alloyed on the opposite sides of a thin slice of an N-type germanium. Collector is larger in size than the emitter.
These transistor may be also of P-N-P type or N-P-N type.
P-N-P Transistor:
Following facts may be kept in mind while understanding the basic mechanism of transistor operation:
(i) Since emitter is to provide charge carriers, it is always forward biased.
(ii) First letter of transistor type indicates the polarity of the emitter voltage with respect to base.
(iii) Collector’s job is to collect or attract those carriers through the base, hence it is always reverse-biased.
(iv) Second letter of transistor type indicates the polarity of collector voltage with respect to the base.
The above points apply both to P-N-P and N-P-N transistors.
Working:
Fig. 7.31 shows a P-N-P transistor connected in the common-base (or grounded- base) configuration (it is so called because both the emitter and collector are returned to the base terminals. The emitter junction is forward-biased whereas the collector junction is reverse- biased.
The holes in the emitter are repelled by the positive battery terminal towards the P-N or emitter junction. The potential barrier at the junction is reduced due to the forward-bias, hence holes cross the junction and enter the N-type base.
Because the base is thin and lightly- doped, majority of the holes (about 95%) are able to drift across the base without meeting electrons to combine with. The balance of 5% of holes are lost in the base region due to recombination with electrons. The holes which after crossing the N-P collector junction enter the collector region are swept up by the negative collector voltage Vc.
The following points need attention:
(i) In a P-N-P transistor majority charge carriers are holes.
(ii) The collector current is always less than the emitter current because some recombination of holes and electrons take place (Ic = Ie – Ib).
(iii) The current amplification (α) (or gain of P-N-P transistor) for steady conditions when connected in common base configuration is expressed as:
α = [Ic (collector current)/Ie (emitter current)] < 1
(iv) Emitter arrow shows the direction of flow of conventional current. Evidently, electron flow will be in the opposite direction.
N-P-N Transistor:
Fig. 7.32 shows a N-P-N junction transistor. The emitter is forward-biased and the collector reverse-biased. The electrons in the emitter region are repelled by the negative battery terminal towards the emitter or N-P junction. The electrons crossover into the P-type base region because potential barrier is reduced due to forward bias.
Since the base is thin and lightly doped, most of the electrons (about 95%) cross over to the collector junction and enter the collector region where they are readily swept up by the positive collector voltage Vc. Only about 5% of the emitter electrons combine with the holes in the base and are lost as charge carriers.
The following points need particular attention:
(i) In a N-P-N transistor, majority charge carriers are electrons.
(ii) Ic (collector current) is less, than Ie (emitter current) so that α < 1.
(iii) Emitter arrow shows the direction of flow of conventional current.
Note:
The junction transistors have been made in power ranges from a few mill watts to tens of watts. The tiny junction transistor is unparalleled in that it can be made to work at power levels as 1 micro-watt.
Advantages and Disadvantages of Transistors as Compared to Thermionic Tubes:
Advantages:
(i) Compact size, light in weight, more resistant to shocks and vibrations than a vacuum tube.
(ii) Instantaneous operation, as no heating up is required.
(iii) Long life if operated within the permissible limits of temperature.
(iv) No energy loss in heating.
(v) Operating voltage quite low.
Disadvantages:
(i) It can be used upto a few mega-Hz only.
(ii) Operation beyond temperature of 75°C is not possible.
(iii) Loud hum noise as compared to thermionic tubes under similar operating conditions.
Q.11. What are Superconductors?
Ans. Superconductors are those elements, compounds and alloys of metals and non-metals which exhibit extraordinary magnetic and electric behaviour at extremely low temperatures (near absolute zero).
a. Superconductors exhibit the following extraordinary properties below critical temperature (Tc):
(i) r (specific resistance) = 0
(ii) μr (relative permeability) = 0
(iii) B (magnetic flux density) = 0
(iv) x (magnetic susceptibility =M/H) = -1
b. Superconductors do not obey ohm’s law below Tc.
Properties of Superconductors:
The properties of semiconductors are as follows:
1. The value of r of superconducting materials at room temperature is greater than other elements.
2. When current is passed through the superconducting materials, the heating value (I2 R) is zero [since r ® very small (zero at Tc) as such R = 0)].
3. When a superconductor is subjected to a sufficient strong magnetic field below Tc, its superconducting property is destroyed.
4. All thermoelectric effects disappear in superconducting state.
Type-I and Type-II Superconductors:
Based on their magnetic response superconducting materials can be divided into the following two types, designated as:
1. Type-I or Ideal superconductors.
2. Type-II or hard superconductors.
1. Type-I or Ideal Superconductors:
a. These superconductors, while in the superconducting state are completely diamagnetic (i.e. they show Meissner effect).
Examples- Hg, Pb, In etc.
b. These are unsuitable as magnets because of their low Hc values (Hc < 105 A/m corresponding to about 0.1 tesla).
2. Type-II or Hard Superconductors:
a. These superconductors are of magnetic grade.
Examples:
Nb-Ti alloy and intermetallic compound Nb3Sn. The bismuth lead strontium calcium copper oxide (Bi Pb Sr Ca CuO) tape is a type-II semiconductor; it is silver-sheathed. Fig. 7.14 shows the behaviour of the above superconductors (type-I and type-II) as a function of -M and H.
Applications of Superconductors:
Some of the important applications of superconductors are as follows:
1. The cryotron.
2. Electrical Machines:
Possible to manufacture electrical generation and transformers in exceptionally small sizes having efficiency of 99.99 percent.
3. Electromagnet:
It has been possible to produce superconducting solenoids which do not product heat during operation. By the use of superconductivity, it is possible to design electromagnets for use in laboratories for lower temperature devices (like maser).
4. Power cables.
Cryogenic Magnets:
These days the superconductors are employed for creation of strong magnetic fields for-
1. Switching;
2. Memory elements of computers;
3. Hollow resonators;
4. Transformer;
5. Rectifiers;
6. Bearings;
7. Rotating electrical machines;
8. Magnetic screens;
9. Electric transmission lines.
Q.12. Explain the Elastic and Plastic Behaviour of Solids.
Ans. When a material is subjected to an external force (load), its response is always a deformation which will be classified as either recoverable (elastic) or non-recoverable (plastic or viscous). All materials show both types of deformation with one or other predominating under a given set of conditions.
Metals show less elastic but more plastic deformation at even room temperatures while steady flow (creep) becomes especially predominant at elevated temperatures. Amorphous materials are usually visco-elastic in that viscous flow and elasticity are equally prevalent at certain temperature ranges. Glass, for example, exhibits both elasticity and viscous flow at 500 to 600°C, but at room temperature it is a brittle, low deforming solid.
I. Experiments show that metal deformation is elastic if it is small enough. The word elastic implies springing back a behaviour readily observable in springs and rubber bands. An ideal elastic deformation takes place instantaneously when load is removed.
Elastic strain of metal crystal is directly proportional to the applied load and the maximum amount that can occur is normally no more than 0.10 to 1.0 percent of the original length of the material. The elastic strain in materials other than metals is frequently not proportional to stress and may involve as high as several hundred percent elongation.
II. Elasticity of solids has its origin in the existence and stability of interatomic and intermolecular bonding (forces). The deformation (displacement) caused by small elastic changes of shape and volume are proportional to the applied force, in accordance with Hooke’s law. This law can be mathematically formulated in terms of stress and strain in which case coefficient of proportionality is called the modulus of elasticity.
III. The recoverable nature of elastic deformation enables a metal to store energy and to release it under controlled conditions. However, the amount of this energy is too small to serve as a source of energy for prime mover in competition with other forms of energy.
Deviation from Perfect Elastic Behaviour:
Many engineering applications involve large deformations where Hooke’s law usually does not apply. The linear relation between stress and strain no longer exists. Metals such as cast iron and non-ferrous metals exhibit a curve rather than a straight line from the very beginning on their stress-strain diagram.
In the tensile stress strain curve for a mild steel (Fig. 2.38) stress beyond the point M (proportional limit) is no longer proportional to strain and deformation is non-recoverable and this deformation is called plastic deformation.
This non-recoverable deformation is due to an internal flow of materials. If the material is ductile, a yield point is reached beyond which solid will begin to flow without further increase in stress. In polycrystalline solids this ductile region is then followed by a region of strain hardening, in which it becomes difficult to further deform the material. Continued deformation will ultimately lead to fracture.
Plastic deformation in crystals is the result of relative deformation along specific crystallographic planes and is caused by shear stress parallel to the planes. The pure stretching or compressing of atomic beds can only result in elastic deformation or brittle fracture, but a shearing stress can cause one layer of atoms to be permanently displaced relative to another layer.
Crystals have close packed crystal planes in which atomic density is the highest. Thus the separation of the parallel or slip planes in the same set should be maximum. The utility of metals is closely associated with the number of these slip planes in the structure. The plastic deformation of a crystal is also accompanied by the appearance of slip bands, in the microstructure.