Cool PPT on Crystal Defects

I have found a cool ppt on Crystal Defects. Please do go through the same.

Please note, the ppt is not mine. I found it floating on the web and don't claim any copyright of the same. I am sorry that I couldn't post the source as I forgot the same.

Atomic Packing Factor

         In crystallography, atomic packing factor (APF) or packing fraction is the fraction of volume in a crystal structure that is occupied by atoms. It is dimensionless and always less than unity. For practical purposes, the APF of a crystal structure is determined by assuming that atoms are rigid spheres. The radius of the spheres is taken to be the maximal value such that the atoms do not overlap. For one-component crystals (those that contain only one type of atom), the APF is represented mathematically by

where N_atoms is the number of atoms in the unit cell, V_atom is the volume of an atom, and V_{unit cell} is the volume occupied by the unit cell. It can be proven mathematically that for one-component structures, the most dense arrangement of atoms has an APF of about 0.74. In reality, this number can be higher due to specific intermolecular factors. For multiple-component structures, the APF can exceed 0.74.

Worked out example

Body-centered cubic crystal structure

BCC structure

       The primitive unit cell for the body-centered cubic (BCC) crystal structure contains nine atoms: one on each corner of the cube and one atom in the center. Because the volume of each corner atom is shared between adjacent cells, each BCC cell contains two atoms.

Each corner atom touches the center atom. A line that is drawn from one corner of the cube through the center and to the other corner passes through 4r, where r is the radius of an atom. By geometry, the length of the diagonal is a√3. Therefore, the length of each side of the BCC structure can be related to the radius of the atom by

     Knowing this and the formula for the volume of a sphere((4 / 3)pi r³), it becomes possible to calculate the APF as follows:

  The above information is taken from Wikipedia. Please do refer to the same for further info.

 with warm regards

allmyposts

Diffusion In Solids

I was studying about diffusion in solids. The simplest definition of diffusion is "movement of atoms in solids under thermal energy and a gradient" is called diffusion. Where the gradient can be concentration or Electric / Magnetic field ...

This is relatively simple topic and can be easily understood. But the topic is a little important as questions regarding time taken for carburization are repeatedly asked in the question papers.

The important topics to study include:

• Diffusion mechanisms (vacency, interstitial ..)
• Rate of diffusion in steady state and non steady state (Ficks first law and second law)
• Kirkendall effect

Perfect resource for this topic is study material by IEI. But the book by Raghavan is also very very good. The book is available here

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

with warm regards

allmyposts

Line defects and Surface defects

    Yeah, I was reading about these topics only from last Saturday. Please head to my advice, this is the wrong way of spending your weekend. So basically there is a lot to study, but everything is quite easy to understand. Video tutorials can be of great help in this issue, but alas, I couldn't surf for the same till now. I shall update the same here at the earliest.

What all needs to be covered:

1. What are line defects, what are dislocations? ( both are same basically )
2. Types of line defects viz screw dislocation & edge dislocation.
3. How to find out the burgers vector for a given defect. What is burgers circuit, how to find out the direction of burgers vector. What is Burgers Vector?
4. What are various types of surface defects? 
5. Study about grain boundary and interface, tilt boundary, twinning, stacking faults & such.

Note:

• Twinning is important concept as the same is asked many many times in the old question papers.
• For simple definitions of defects and classification please refer to material science by RS Khurmi & RS Sedha 
• For brief and clear cut explanation please refer to the  study material provided by AMIE

Resources:

• A good pdf which helps to visualize the dislocations is here
• A good image to help visualize point defects is here
• Another point defect explanation here

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

with warm regards

allmyposts

Prerequisites for understanding defects in crystals

    Hello EveryOne, So about crystal defects, I am not sure if I can complete the chapter by this weekend or not. I had to go through a lot of old question papers to clearly understand what all must be studied to complete the portion of this chapter. All I can tell you is, the notes from study material is not sufficient at all. 

    The prerequisites  for studying the material in the study material are as follows as per me:

1. What are crystalline structures? What is lattice? What is unit cell?
2. Study about the 7 crystal systems. Study about the 14 Bravias lattices
3. Understand the concept of Miller indices. Find miller indice for a given plane. Sketch a plane when miller indices are given.
4. Understand about crystal directions. Find crystal directions when miller indices are given.
5. Find out the relation between radius of atom and parameter of lattice in a Bravias lattice. Find out the no. of atoms in a given lattice. Find out the no. of atoms in sq. mm. of cube
6. Find out Atomic Packing Factor. Know how to calculate the density of cube using Avagordo's Number.

    Yeah, I must say I made some progress in this chapter anyway. I know a little about everything I listed above and practiced some problems on the same.

     A note about questions from these topics in old papers. The questions are given here. Do try to solve them.

1. Calculate volume of FCC unit cell in terms of atomic radius. (W-05)
2. Show that Atomic Packing Factor of FCC unit cell is greater than the Atomic Packing Factor of BCC unit cell. (W-05)
3. Calculate the Atomic Packing Factor of a FCC unit cell. (W-06)
4. If lattice parameter of alpha iron is 286 pm, find out the atomic radius (W-06).

    There is a lot more to study including types of defects and the effects of these defects on the structure intensive properties of crystalline materials .... Everything happens at a pace. All we can do is, improve the pace. Thats what I intend to do this time.

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

Shall keep you posted

with warm regards

allmyposts