Imperfection in Solid
In last topic I introduced you to the concept of crystalline materials and amorphous materials and how there is a long range ordering in crystalline materials. Now with this idea you might think that should a crystalline material have ordered throughout the material or in a part of the material a part of the specimen. So a single crystal is one where there is the complete specimen has ordered throughout the specimen, whereas poly crystal has different crystals throughout the specimen. But now imagine, you have even if you have a single crystal or if you have a poly crystal. Then is it possible that there is a uninformative within the material or is it that even for a single crystal or for a poly crystal. There has to be exact ordering throughout the material. What I mean is that all the atoms should be at exact position. No atoms should be missing from anywhere, no additional atoms should be anywhere is that the case as we will see in today's topic that is not the case.
So our discussion will be on imperfections in solid. The case we have studied till now are perfect solids are crystalline materials perfectly ordered that is the question and as I said no. So if the crystalline material is not perfectly order then what is the exact scenario? What is the practical scenario there are actually a large number of imperfections present in any crystalline material. Be it single crystal, be it polycrystals. There the type of imperfection present may be different in single crystal and poly crystalline material. As we will see and also within the polycrystalline material. There may be different kind of imperfection trivalent.
What are Defects
So this particular kind of imperfections which are present in a crystalline material are known as defects. So as the term suggests defect is kind of a deviation from perfection. That's why defects and this defects you might think that the defects which are there hamper the properties. But as a matter of fact the defects are sometimes very important for property enhancement too, because all the applications have different requirements and different applications requirements can be met by tailoring the amount of defect and the types of defect we have in the material. So the properties are profoundly affected by that defects that are present in the material and defects in crystalline material or crystalline lattice is irregularity in one or more dimension. So what this means is that defects can be rather not just one or more dimension. It can even be in zero dimension as well see it can even be zero dimension. So crystalline or lattice irregularity which is known as the imperfections or defects can be in zero dimension or point affect one dimension or line defect two dimension or surface defect and three dimensional or what is known as volume defect?
What are Point Defects
So to begin with our study let us see what are the point defects that are present in a solid are the zero dimensional defects. In metals we can have either vacancy or self. We'll see in details about each of the individual type of defect in the topic. But just to give you a brief glimpse Vacancy means atom is absent. Where there should be an atom where a self interstitial means extra atom is present in the void between atoms. So this is effectively less atom than there should be this is effectively more atom. Than there should be then point effects in the case of ceramics can be Frenkel defect and Schottky defect. Frenkel defect is basically at the missing from its location and is located at a different location. Schottky defect is basically two atoms missing. We'll see in details later then we have impurities that can be both in metal case and ceramic case.
Now what do I mean by impurities, see the idea is that for a particular metal or for a particular ceramic the composition is defined. Let's say for example iron iron has Fe atoms present at its lattice sites. But suppose that we add something to iron. Let's say we add carbon that makes steel. So in addition to the iron that was initially there. We now have additional element which was not supposed to be there that is carbon. So this thing which is externally added or which is externally present which should not be there are known as impurities. Now the impurities can be present both in the metal case as well as the ceramic case and what are the type of defects that can be under the categorization of impurities.
The type of defect will be substitutional or interstitial. As the name suggests substitutional impurities will be impurities which will substitute a metal or ceramic atom from its initial position. So the impurity atom will displace our original atom and takes its position that is substitutional interstitial. The impurity atom will go and sit at a interstitial site, it will not displace any atom. So these all are broadly speaking the point defects that can be present in a solid. Now that we have a idea about the point effects. We'll see all of them in details and their importance their significance. How they affect the properties of solid?
Line Defects
Let us see line defect, line defect as an if this line is a one dimension so line defect are basically one dimensional defects. So instead of atoms missing there will be line of atom missing or line of atom additional atoms present where there should not be. So the line defect is basically known as dislocation. Dislocation can be of two categories. It can either be a edge dislocation or it can be as through screw dislocation. A third category is basically a combination of edge and screw dislocation which is known as mixed dislocation. So now we know what is line defect.
Surface Defects
Let us see what are the surface defects. Possible surface defect as the name suggests any surface is a two-dimensional thing. So a surface defect is a two dimensional defect or two-dimensional imperfection in solid. So what are the possible surface defects? Some of the possible surface defects are the external surface. External surface is actually a discontinuity. So ideally there should not be any external surface. The atoms should keep on arranging till infinity for the perfect crystalline material to exist. But obviously that is not a practical scenario. The specimen need to end somewhere because it is a finite specimen. So it will obviously have a external surface and that is a discontinuity there by that is a imperfection.
Grain Boundaries
Then secondly we have what is known as grain boundaries? This grain boundaries comes into picture in the case of polycrystalline materials. As I have discussed in the previous topic polycrystalline materials are materials which have different regions of crystallinity and each crystal region. What is known as each grain has a orientation mismatch with its surrounding grain. Thereby these grains have are separated by a discontinuity which is known as the grain boundary and this grain boundary being a boundary is a two dimensional defect. Then we will see something which is known as twin boundary. So these three are examples of surface defects. There are other surface defects too but our point of concern will be mainly discussing these three surface defect.
Let's see the three dimensional defects. So imperfections in solid can be in three dimension to what is the possible examples. Some of the possible examples can be that it can be a pore. There can be porosity within the material and what material is completely absent from region. So that region is a three-dimensional region. So pore is a three dimensional defect then there can be cracks, there can be foreign inclusions, there can be inclusion particles or impurity particles present rather than atoms. So those things will cover three dimensional region thereby it will be a volume defect also. There can be different phase completely other phase present which is not supposed to be there. So this kind of makes the three dimensional defect. So today's topic was a basic introduction to the imperfections in the solid present. We saw the different kinds of zero dimensional or point effects, one-dimensional or line defect, 2d or surface defect and three-dimensional defects or volume defects.
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