# Unit 01 CRYSTAL STRUCTURE | Materials Engineering Question With Answers.

## With the help of neat sketch explain different types of crystal structure.

Ans. Following are the different types of crystal structures :

a. Body Centred Cubic Structure (BCC) :

1. BCC structure has atoms at its each corner and one atom in its centre

2. The coordination number of BCC arrangement is 8 and packing factor 0.68.

3 The BCC structure can be generally seen in Lithium, Potassium, Sodium etc.

b. Face Centred Cubic Structure (FCC):

1. It consists of atoms at its each corner and one atom at centre of each face

2. The coordination number is 12 and packing factor is 0.74 for FCC arrangement.

3. The FCC structure can be generally seen in Copper, Gold, Silver and Lead etc.

c. Hexagonal Close-Packed Structure (HCP):

1. The unit cell is like a hexagonal prism in HCP.

2. There are two hexagonal faces with one atom in the centre on each, twelve corners with one atom on each, and three atoms in the cell’s body. A HCP unit cell is made up of seventeen atoms in total.

3. This is identical to FCC, having coordination number 12 and packing factor 0.74.

4. This HCP structure is generally seen in Zinc, Magnesium and Beryllium etc.

## NaCl structure has FCC structure. The density of NaCl is 2.18 cm3. Calculate the distance between two adjacent atoms.

Ans. Given: Density of NaCl=2.18 cm3

To Find:The distance between two adjacent atoms

1. Number of effective atoms per unit cell in FCC structure = 4

2. Molecular weight of NaCl = Atomic weight of Na + Atomic weight of Cl

= 23+ 35.5 = 58.5 gm

3. As 58.5 gm of NaCl contains 6.023 x 1023 molecules of NaCl.

∴    Weight of 6.023 x 108 molecules =58.5 gm

This is the weight of unit cell whose volume is a3.

5. The lattice constant ‘a’ is the unit cell dimension. In crystals of NaCl, it is twice the distance between adjacent atoms.

## What are ceramic materials ? What are some common properties and applications of ceramic materials?

Ans.
A. Ceramics:

• 1. The mixture of metallic and non-metallic elements known as ceramics has a predominately ionic interatomic bonding.
• 2. Typically, a high temperature heat treatment method called firing is used to give these materials the desirable characteristics. The mixture of metallic and non-metallic elements known as ceramics has a predominately ionic interatomic bonding.

B. Properties of Ceramics:

• 1. Compared to metals, ceramic materials are more rigid and powerful.
• 2. Ceramic materials are very hard and brittle in nature.
• 3. Due to their fragile nature and fractures, these are very prone to breaking.
• 4. These are excellent insulators and refractories due to their low electrical and thermal conductivity.
• 5. Compared to metals and polymers, they are more resistant to high temperatures and hostile conditions.
• 6. Ceramics also exhibit some magnetic behaviour.

C. Applications of Ceramics:

• 1. For the lining of ovens and furnaces, in firebricks and fireclay.
• 2. In the biomedical/medical profession, as artificial limbs, teeth, etc.
• 3. In the transmission and distribution of electricity, in insulators (dielectrics).
• 4. In crockery in domestic uses, sanitary wares etc.
• 5. As ferrites in memory cores of computers.
• 6. As radiation shield for nuclear reactor.

## Describe the various mechanical properties of ceramics. What are the various electrical properties of ceramics?

Ans.
A. Mechanical Properties of Ceramics :

• 1. Ceramics exhibit brittle fracture at room temperature because, when subjected to a tensile force, they break before deforming plastically.
• 2. When compared to strength in tension, ceramics are stronger in compression.
• 3. Ceramics have porosity, which lessens both their strength and elastic elasticity.
• 4. Ceramics have good hardness, and because of this, they are employed as abrasive materials.
• 5. Creep phenomenon occurs in ceramics at high temperature.
• 6.High temperatures can be tolerated by ceramics.
• 7. Ceramics are not very malleable.
• 8. Frenkel and Schottky defects mostly occur in ceramics.

B. Electrical Properties of Ceramics:

• 1. Ceramics function well as electrical and thermal insulators.
• 2. Ceramics have a high dielectric constant value.
• 3. Dielectric strength of ceramic is high.
• 4. Dielectric losses are low.

## Write short note on volume defects.

Ans. 1. When several point defects combine to create a three-dimensional pore or void, they are referred to as volume or three-dimensional flaws. On the other hand, a group of impurity atoms may combine to form a precipitate that is three dimensional. Ceramics have a high dielectric constant value.

2. A volume defect might be as little as a few nanometers or as vast as a few centimetres.

3. These flaws have a significant impact on the way the material behaves and performs.

## Discuss the tensile test in detail.

Ans. 1. Tensile test is one of the most widely used mechanical tests.

2. This test is frequently used to determine a metal’s strength, ductility, and toughness.

3. The tensile test on a mild steel test piece is described below:

i. The specimen to be tested is in the shape of a circular bar or a flat bar (Fig.)

ii. It has one end that is fixed to the machine’s frame using clamps, and the other end that is similarly fixed to the movable cross-head.

iv. The cross-head is propelled by a mechanical or hydraulic drive system.

v. The magnitude of the load is measured by the load measuring unit.

vi. By fastening an extensometer or gauge to the specimen, elongation is measured.

vii. The gauge length’s indicated points are spaced farther apart as the load increases.

vii. Elongation is thus provided as a function of load by the test. Data on load elongation can be used to compute stress and strain, and a graph of stress against strain for the material can be created.

ix. A lot of testing equipment has the ability to automatically record stress-strain curves for the materials being tested.

4. The varying load and elongation values at various intervals are recorded during the tensile test.

5. The extensometer should be taken out of the gauge length of the specimen just before the pointer on the load scale in the load measurement unit of the testing machine stops moving forward. To avoid harming the extensometer, this is done.

6. At this time, the specimen’s interior structure begins to give way.

7. The load is then increased further, and the final load will be indicated by the load-scale pointer’s maximum travel.

8. To signify necking, the pointer then goes in the other direction.

9. Finally the pointer stops at a point with a noise to indicate fracture.

10. Following fracture, the two pieces of the shattered specimen are positioned (see Fig.) as if they are fixed together, and the length of the specimen is determined by measuring the space between the two gauge marks.

11. Similarly, the average diameter at the place of fracture and the area of fracture calculated.