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Sunday, 27 March 2016

INTRODUCTION

          Materials are the driving force behind the technological revolutions and are the key ingredients for manufacturing. Materials are everywhere around us, and we use them in one way or the other. The materials and the manufacturing process employed. could be better appreciated if one understands various types of materials and its properties.

MATERIAL:
          Material is something that consists of matter.

METAL:
          Metal is a hard mineral substance, it is a part of material.

METALLURGY:
          It is the science and technology of metals. It includes extracting metals from ore, production of metals etc., This subject explains the properties, behaviors and internal structure of metals. Metallurgy also teaches us that properties of metals can be changed using various treatments. This allows us to tailor a material's properties to its specific use.



CONCEPT OF ATOM

          All the materials available either in solid, liquid or gaseous form are made up of atoms, the smallest indivisible particles. Atoms of the same element are identical to each other in weight, size and all properties, where as atoms of different elements differ in weight, size and other characteristics. The size of the atoms is of the order of 1 Å (=10-10 m). A material which consists just one type of atoms is called element. Nitrogen, carbon, hydrogen, aluminium, copper, gold, iron etc., are few examples of elements. Group of atoms which tend to exist together in a stable form are called molecules, e.g. H2, O2, N2 etc., Large number of molecules in nature exist as combination of atoms of different elements, e.g water (H2O), etc. Molecules containing one atom (known as monoatomic), two atoms (known as diatomic), three atoms (known as triatomic) or more atoms (known as polyatomic).

          Rutherford and coworkers have shown that the mass of the atom is concentrated at the centre of the atom, called nucleus. The tom essentially has an electrical structure and is made of smaller particles, the principle one being electrons, protons and neutrons.
                                               
          All substances are made up of atoms; each atom consists of the following:
  
                       1. Nucleus                                       2. Electrons

Nucleus: the nucleus is at the centre of the atom and consists of protons and neutrons. Its diameter is 1/10,000th of the atom as a whole. Almost entire mass of a given atom is concentrated in its nucleus. Due to the presence of protons nucleus is positively charged. The number of protons in the nucleus is equal to the atomic number of element. A neutron is an uncharged particle and has same mass as the hydrogen nucleus.

PROTONS:
          The nucleus of hydrogen atom is called the proton. A proton has a unit positive charge of same magnitude as that of electron (= 1.602 ´ 10–19 C). The mass of a proton is 1.672 ´ 10–27 kg. The proton and the neutron are considered to be two different charge states of the same particles which is called a nucleon. The number of protons in a nucleus is called the charge Z of the given nucleus, or the charge number. 

NEUTRONS:
          These are electrically neutral particles and 1.008 times heavier than protons. The mass of each neutron is 1.675 ´ 10–27 kg. Each neutron is composed of one proton and one electron, i.e 

                              Neutron = Proton + Electron
ELECTRONS
          M. Faraday in 1983, in his experiments on the laws of electrolysis, provided the first experimental evidence that electrical charge was not infinitely divisible, but existed in discrete units.
          In 1897, J.J. Thomson, while studying the passage of electricity through gases at low pressure, observed that the rays of light appear to travel in straight lines from the surface of the cathode and move away from it in the discharge tube. These rays are called cathode rays since they start from the cathode of the discharge tube. W.Crookes studied the properties of these cathode rays and showed that the rays,
  1. Travel in straight line and cast shadows
  2. Carried negative charge and sufficient momentum
  3. Possess high kinetic energy and can induce chemical reactions, excite fluorescence on certain substances.
          These properties of the cathode rays were best explained by J.J. Thomson by his hypothesis that the cathode rays consist of a stream of particles, each of mass 'm' and charge 'e' (= 1.602 ´ 10–19 C), originating at the cathode of the discharge tube, These particles are called electrons.

          An electron is a negatively charged particle present in an atom. The number of electrons which surrounds the neutral atom is equal is the number of protons within the nucleus, i.e., the atomic number the electrons move about the nucleus. The orbit or shell nearest to the nucleus known as K-orbit contains 2 electrons, the next (L-orbit) eight electrons and next (M-orbit) eighteen electrons and so on.


MAXIMUM NUMBER OF ELECTRONS IN SUB-SHELLS:

In a sub-shell, the maximum number, of electrons is = 2(2l+1)
Where, l=number of sub-shell
Thus, for first sub-shell: l=0
Maximum number of electrons = 2(2*0 +1)
                                                        = 2 
The sub-shell is known as S-sub-shell. The symbols 2S, 3S, 4S ........ denoted that S-sub-shell of the first, second, third, and fourth energy levels respectively.
For second sub-shell: l=1, Maximum number of electrons = 6
The sub-shell is known as P-sub-shell. The symbols 2P, 3P, 4P ........ denoted that P-sub-shell of the first, second, third, and fourth energy levels respectively.
For third sub-shell: l=2, Maximum number of electrons = 10
The sub-shell is known as d-sub-shell. The symbols 2d, 3d, 4d ......... denoted that d-sub-shell of the first, second, third, and fourth energy levels respectively.
For fourth sub-shell: l=3, Maximum number of electrons = 14
The sub-shell is known as f-sub-shell. The symbols 2f, 3f, 4f    .......... denoted that f-sub-shell of the first, second, third, and fourth energy levels respectively.

ISOTOPES

          All atoms having different atomic weights but belonging to the same element are termed as isotopes, i.e. atomic number of isotopes of an element remains the same. Obviously, isotopes contain same number of protons and electrons. Thus the isotopes are atoms of different weight belonging to the same element and having the same atomic number. The difference in the masses of the isotopes of the same element is due tot he different number of neutrons contained in the nucleii. For example, hydrogen exists in three isotopic forms. Atomic number of hydrogen is 1. Three isotopes of hydrogen are:

a) Ordinary hydrogen (1H1) with atomic mass equal to 1.
b) Deuterium (1D2) with atomic mass equal to 2.
c) Tritium (1T3) with atomic number equal to 3.

similarly, chlorine has two isotopes, 17Cl35 and 17Cl37. Those isotopes are available in the ratio of 3;1. Their average atomic weight is 






ISOBARS

          Atoms with the same mass but belonging to different chemical elements are called isobars. Obviously, isobars possess different number of protons and electrons in their atoms. Total number of protons and neutrons in each of their nuclei is also same. The example of first pair of isobars is argon and calcium. Argon (atomic number 18) has 18p, 18e and 22n in its atom. Calcium (atomic number 20) has 20p, 20e and 20n in its atom.


ISOTONES

          These are the nuclides having the same numbers of neutrons (N) but a different Z and A. Example of Isotones are 136 C and 147 N. Isotones having a given value of N, obviously do not all corresponding to the same chemical element.
          The analysis of the properties of isotones and isobars helps us to disclose several features of atomic nuclei. Such analysis helps us to predict that what will happen to the stability of a nucleus when an extra n or p is needed to the nucleus.


Saturday, 26 March 2016

CRYSTAL STRUCTURE

          Materials exist in nature in two principal forms as crystalline and non-crystalline (amorphous) solids, which differ substantially in their properties. In a crystal, the arrangement of atoms is in a periodically repeating pattern where as no such regularity of arrangement is found in a non-crystalline material. A crystalline solid can either be a single crystal, where the entire solid consists of only one crystal. or an aggregate of many crystal separated by well defined boundaries, in the latter form, the solid is said to be poly-crystalline. For crystalline solids sharp melting point, these are an-isotropic solids i.e., their properties are different in different directions. Whereas non-crystalline or amorphous solids are isotropic.



Crystal's:

          These are solids which have a regular periodic arrangement in their component parties, bounded by flat faces, orderly arranged in reference to one an other, which converge at the edges and vertices. A crystal is symmetrical about its certain elements like points, lines or planes and if it rotated about these elements, it is not possible to distinguish its new position from the original position. This symmetry is an important characteristic based on internal structure of crystal. Symmetry helps one to classify crystals and describing their behavior. At temperatures below that of crystallization, the crystalline state is stable for all solids.

SINGLE CRYSTAL

Most of the materials exist in polycrystalline form, but there are some materials, which exist in the form of single crystals, e.g., sugar, sodium chloride (common salt), diamond, etc.,
Single crystals represent a material in its ideal condition and are produced artificially from their vapor or liquid state. These crystals help us in studying behavior and defects of the material in ideal conditions.

UNIT CELL AND SPACE LATTICE

          The 3-dimensional network of imaginary lines connecting atoms is called space lattice. The metallic crystals can be considered as consisting of tiny blocks which are represented in three dimensional patterns. The tiny block formed by the arrangement of a small group of atoms is called the unit cell. It is that volume of solid from which the entire crystal can be constructed by translational repetition in three dimensions.
          If each atom in a lattice is replaced by a point; then each point is called a lattice point and the arrangement of the points is referred to as the (three dimensional) lattice array. Thus a space lattice is. defined as an array of points in three dimensions in which every point has surroundings identical to that every point in the array. If all the atoms, molecules or ions at the lattice points are identical, the lattice is called a Bravias lattice.

          The distance between the atoms-points is called inter atomic or lattice spacing. A space lattice is a conventional geometrical basis by which crystal structure can be described. Every point of a space lattice has identical surroundings.
          The length of the side of a unit cell is the distance between the atoms of the same kind. In the case of pure metals whose crystals have simple cubic structure, it is equal to the basic distance 'a' only.
          There are 14 types of possible lattices and they fall into 7 crystal systems.

LATTICE PERAMETER AND CRYSTALLOGRAPHIC PLANES

          Lattice parameter means the dimensions of the unit cell in any of the crystallographic arrangements. In case of a cubic symmetry, the size of the lattice is fixed by the length of the cubic unit.
          Each atom has a site defined by its geometrical centre of relationship with the crystal lattice and this is the mean position of the atom. The position of the atom is less well defined, because it undergoes thermal vibrations within the space available to it between its neighbours.
          The crystal structure can be regarded as an array of atoms built of layers one over the other, each layer consisting of simple arrangement of uniform rows of atoms, which mate or key with the atoms in adjacent layers. Such layers of atoms are called crystallographic planes and the inter atomic forces in a solid vary greatly with geometry of these planes.

CRYSTAL STRUCTURE FOR METALLIC ELEMENTS

          The crystals of most metals have highly closed symmetrical structure with closed packed atoms. The most common types of space lattice (or unit cell) with which metallic elements crystallise are given below:
4. Hexagonal closed pack structure (HCP).


Crystal type
Example
(1) Simple cubic
(2) B.C.C 

     (3) F.C.C  
(4) H.C.P
     Polonium
    α-iron, γ-iron, Na, vanadium, Chromium, Molybdenum
     γ-iron, Al, Cu, Ag, gold, Lead, Pt
     Mg, Zn, Cd, zirconium, titanium

Simple cubic

Here the atoms are located at the corners of the cube. 
It consist of 8 atoms at corners
Number of atoms per unit cell = 8 x 1/8 =1 atom
Atomic radius = r = a/2





Body centred cubic (BCC)

The unit cell of BCC system has an atom at each corner, which are shared by the adjoining eight cubes and one at the body centre. Obviously, each unit cell share 8 atoms one on each of its corners in additions to one atom at the body centre. Hence

The share of each cube = 8 corners x 1/8 + 1 centre x 1 = 2
Number of atoms per unit cell = 2 atoms
Atomic radius = r = 

EXCr,Fe(α), W, Tantalum, Molybdenum 



FACE CENTERED CUBIC STRUCTURE (F.C.C)

          Here the atoms are present at the 8 corners of the cube and also at each side of the face (6 faces). 

Number of atoms per unit cell = 6 each side x 1/2 + 8 corners x 1/8 =4 atoms
Atomic radius = r = 

EX: Al, Cu, Pb, Ni, Pt, Ag





HEXAGONAL CLOSED PACK STRUCTURE

          Here 12-atoms at each corner and 2-atoms at face centre 3-atoms in middle of crystal. Each atom at corner shared by 6 adjacent unit cells, 2 face centered are shared by another 2 unit cells and 3 atoms in centre so hence,

The Number of atoms per unit cell =2 each face x 1/2 +12 corners x 1/6 + 3 = 6 atoms

EX: Cd, Mg, Ti, Zn 


Friday, 25 March 2016

NaCl crystal structure

          The NaCl crystal is face centered cubic (F.C.C) unit cell with the counter ion filling the octahedral holes in the structure. It does not matter which ion is taken to be at the vertices of the cell and which in the holes; the same pattern is obtained, as can be seen in the figure below:



Some other crystal structures

AX Crystal structure


Zinc Blend (ZNS) Structure


MgO and FeO Structure


AmXp Crystal Structure


ABX3 Crystal Structure (BaTiO3)


Cesium Chloride Structure


Zinc Blend Structure


AmBnXp - Type Crystal Structure (Barium Titanate BaTiO3)


MILLER INDICES:- Definition:

Definition:
          Miller indices is a system of notation of planes within a crystal of space lattice. They are based on the intercepts of plane with thee three crystal axes, i.e. edges of the unit cell. The intercepts are measured in terms of the edge lengths or dimensions of the unit cell which are unit distance from the origin along three axes.
          Miller evolved a method to designate a set of parallel planes in a crystal by three numbers h, k, and l, usually written within brackets thus (h, k, l) known as Miller indices of the plane.

Procedure for finding miller indices

The miller indices of a crystal plane are determined as follows:

STEP 1    :  Find the intercepts of the plane along the axes x, y, z ; Ex: 2, 2, 3.
STEP 2    :  Take reciprocals of the same ratio. Ex: 1/2, 1/2, 1/3.
STEP 3    :  Convert into smallest integers in the same ratio. Ex: 3, 3, 2.
STEP 4    :  Enclose in parentheses. Those are in the form of (h, k, l). Ex: (3, 3, 2).

Salient features of miller indices of crystal planes

  1. All the parallel planes have the same miller indices. Thus the Miller indices defined a set of parallel planes.
  2. A plane parallel to one of the co-ordinate axes has an intercept of infinity.
  3. If the miller indices of the two planes have the same ratio [i.e., 841 and 422 or 211] then the planes are parallel to each other.



Tuesday, 22 March 2016

IMPERFECTS (DEFECTS) IN CRYSTALS

          Up to now, we have described perfectly regular crystal structures, called ideal crystals and obtained by combining a basis with an infinite space lattice. In ideal crystals atoms were arranged in a regular way. In actual crystals, however, imperfections or defects are always present and their nature and effects are very important in understanding the properties of crystals. These imperfections, electrical properties etc, to a great extent.
          Natural crystals always contain defects, often in abundance, due to the uncontrolled conditions under which they were formed. The presence of defects which affect the color can make these crystals valuable as gems, as in ruby (chromium replacing a small fraction of the aluminium in aluminium oxide: Al2O3). Crystal prepared in laboratory will also always contain defects.
          Imperfections are found in all crystals unless some special means are used to reduce them to a low level. The atoms do not have their full quota of electrons in the lowest energy level. But the atoms vibrate due to thermal effect and the electrons also change their positions. There is much other type of defects found in the structure of the crystals.
          The crystallographic defects are classified on the basis of their geometry as follows:

(a) Substitutional impurity
(b) Interstitial impurity
        a. External Defects
        b. Internal Defetcs
     D. stacking fault

Point Defects

          The defects which take place due to imperfect packing of atom during crystallization are known as point defects. The point defect also takes place due to vibrations of atoms at high temperatures, quenching, by severe deformation of the crystal lattice; e.g., By hammering or rolling. Point imperfections are completely local in effect, e.g., A vacant lattice site. Point defects are always present in crystals and their present results in a decrease in the free energy.

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