What are Solids?
Material have specific used depending upon their characteristics and properties, such as hardness, ductility,malleability, conducting or magnetic etc. What makes steel hard, lead soft, iron magnetic and copper electrically conducting? It depends upon the structure- the particular order and bonding of atoms in a material. this clue has made it possible to design and create materials with new and unusual properties and uses for the modern technology.
Classification of Solids
In crystalline solids there is a regular arrangement of molecules. The neighbours of every molecules are arranged in a regular pattern that is constant throughout the crystal. There is, thus an ordered structure in crystalline solids.
The vast majority of solids, e.g., metals such as copper,iron and zinc, ionic compounds such as sodium chloride ceramics such as zirconia are crystalline. The arrangement of molecules, atoms or ions within all types of crystalline solids can be studied using various X-ray techniques. should be noted that atoms, molecules or ions in a crystalline solid are not static. For example, each atom in a metal crystal vibrates about a fixed point with an amplitude that increases with rise in temperature. it is the average atomic position which are perfectly ordered over large distances.
The cohesive forces between atoms molecules or ions crystalline solids maintain the strict long-range order inspite temperature at which the vibration become so greater that the structure suddenly breaks up, and the solid melts. The transition from solid (order) to liquid (disorder) is, therefore abrupt or discontinuous. Every crystalline solid has a definite melting point.
Amorphous or Galaxy solids:
The word amorphous means without form or structure. Thus in amorphous solids there is no regular arrangement of molecules like that bin crystalline solids. We can, therefore say that amorphous solids are more like liquid with the disordered structure frozen in.
For example ordinary glass, which is a solid at ordinary temperature, has no regular arrangement of molecules. On heating, it gradually soften into a paste like state before it becomes a very viscous liquid at almost 800°c. Thus amorphous solids are also called glassy solids. This type of solids have no definite melting point.
Polymers may be said to be more or less solid materials with a structure that is intermediate between order and disorder. They can be classified as partially or poorly crystalline solids.
Polymers from a large group of naturally occurring and synthetic materials. Plastic and synthetic rubbers are termed ‘Polymers’ because they are formed by polymerization reactions in which relative simple molecules are chemically combined into massive long chain molecules or “three dimensional” structures. These material have rather low specific gravity compared with even the lightest of metals,and yet exhibit strength-to-weight ratio.
Polymers consists wholly or in part of chemical combinations of carbon with oxygen ,hydrogen, nitrogen and other metallic or non-metallic elements. Polythene, polystrene and nylon,etc.,are examples of polymers. Natural rubber is composed in the pure state entirely of a hydrocarbon with the formula (C5H6).
A crystalline solid consists of three dimensional pattern that repeats itself over and over again. The smallest three dimensional basic structure is called unit cell. The whole structure obtained by the repetition of unit cell is known as crystal lattice. For example, the pattern of Na Cl particles have a cube shape. The cube shape of the sodium chloride is just one of several crystal shape. In a cubic crystal all the sides meet at right angles. Other crystal shapes have corners in which one or more of the angles are not right angles.
Mechanical Properties of solids:
Deformation in solids:
If we hold a soft rubber ball in our hand and then squeeze it, the shape or volume of the ball will change. However, if we stop squeezing the ball, and open our hand, the ball will return to its original spherical shape. This has been illustrated schematically.
Similarly, if we hold two ends of a rubber strings in our hands, and move our hands apart to some extent, the length of the string will increase under the action of the applied force exerted by our hands. Greater the applied force larger will be the increase in length. Now on removing the applied force, the string will return to its original length. From this example, it is concluded that deformation (i.e., change in shape length or volume) is produced when a body is subjected to some external force.
In crystalline solids atoms are usually arranged in a certain order. these atoms are held about their equilibrium position, which depends on the strength of the inter-atomic cohesive force between them. When external force is applied on such a body, a distortion results because of the displacement of the atom from their equilibrium position and the body is said to be in a state of stress. After the removal of external force, the atoms return to their equilibrium position, and the body regains its original shape, provided that external applied force was not too great.The ability of the body to return to its original shape is called elasticity. Illustrated deformation produced in a unit cell of a crystal subjected to an external applied force.
It is defined as the force applied on unit area to produce change in the shape,volume or length of a body.Mathematically it is expressed as:
The SI unit of stress (σ) is newton per square meter (Nm-2),which is given the name pascal (pa).
Stress may cause a change in length,volume and shape.
When a stress changes the length,it is called tensile stress.
When a stress changes its volume, it is called compressional stress.
When stress changes the shape, it is called shear stress.
Strain is a measure of the deformation of a solid when stress is applied to it.In the case of deformation in one dimensional strain is defined as the frictional change in length.If Δl is the change in length and l is the original length,then strain is given by:
Since strain is ratio of lengths,it is dimensionless,and therefore,has no units.
If strain is ε is due to tensile stress σ,it is called tensile strain.
If strain is produced as a result of compressive stress,it is called compressive strain.
When the applied stress changes the volume,the change in volume per unit volume is known as volumetric strain.
When the opposite faces of a rigid cube are subjected to shear stress.
Electrical properties of solids:
The fundamental electrical property of a solid is its response to an applied electric field i.e,its ability to conduct electric current.The electrical behaviors of various materials are diverse.Some are very good conductors,e.g,metals with conductivities of the order of 107 (Ωm).At the other extreme,some solids ,e.g wood,diamond etc.,have very low conductivities ranging between 10-10 and 10-20 (Ωm)-1,these are called insulators.Solids with intermediate conductivities,generally from 10-6 to 10-4 (Ωm)-1,are termed semiconductors,e.g.,silicon,germanium etc.The conventional free electron theory based on Bohr model of electron distribution in an atom failed to explain completely the vast diversity in the electrical behavior of these three types of materials.
On the other hand ,energy band theory based on wave mechanical model has been found successful in resolving the problem.
Experiments have revealed that the ratio of stress to strain is a constant for a given material,provided the external applied force is not too great.This is called modulus of elasticity,and can be mathematically described as:
Since strain is a dimensional quantity,the units of modulus of elasticity are the same as those of stress,i.e.,Nm-2or pa.
The ratio of tensile (or compressive) stress to tensile (or compressive) strain is called young’s modulus.Mathematically it is expressed as:
For three dimensional deformation,when volume is involved,then the ratio of applied stress to volumetric strain is called Bulk modulus.Mathematically it is expressed as:
Where ΔV is the change in original volume V.
The ratio of shear stress and shear strain is called shear modulus.Mathematically it is expressed as:
Elastic constants for some of the materials are given in table: