TOPIC:Dislocation and Crystal Growth SUBJECT:#SOLID_STATE_PHYSICS #DislocationandCrystalGrowthinppt

1. Dislocation and Crystal
Growth

2. DISLOCATION
 Dislocations are actually one dimensional line defects.
 dislocation is a linear crystallographic defect or irregularity within a crystal
structure which contains an abrupt change in the arrangement of atoms. The
movement of dislocations allow atoms to slide over each other at low stress levels
 Dislocations play an important role in a variety of deformation processes of a
crystal.
 Dislocations can play a constructive role in crystal growth.

3. Understanding the Role of Dislocations in
Material Behavior
Stress fields, strain fields, energy etc.
Free surfaces, grain boundaries etc.
Interactions with other dislocations, interstitials, precipitates etc.
Long range interactions & collective behavior & external constraints
Consider a dislocation in an infinite crystal
Take into account finite crystal effects
Consider interaction of dislocations with other defects
Collective behavior and effects of external constrains

4. Crystals
 A crystal or crystalline solid is a solid material whose constituents (such as atoms
or ions are arranged in a highly ordered microscopic structure, forming a crystal
structure that extends in all directions.
 Crystals are actually grains of a specific material and have a specific size, shape
and arrangement of constituent particles.
 These above mentioned properties are different for different materials and same
for a material under same set of conditions provided for crystal growth.

5. Crystal Growth
 Crystal growth could be defined as the process to producing grains or small seeds
of a material by following different methods.
Growth of crystal
Snow Crystal

6. Continue…
 Crystal growth can be understood in a way by considering an example that we are
going to grow a crystal which have Cubic unit cell when we provide certain
conditions to grow a material units cells start making bonds with similar cells or
the addition of unit cells of same material start with a specific cell in three
dimensions.
 This process of growing crystal continue till a specific size achieved after that a
new crystal of same material start growing.
 The size and shape of crystal of different for different materials is different and for
a specific material same under same set of conditions.
 Above discussion is related to crystals of pure materials.

7. Basic Idea of Crystal Growth

8. Methods of Obtaining Crystals
1. Crystallization
Obtaining crystals by making a saturated solution of material and then by filtration
allow to precipitate crystals.
2. Evaporation
Solve crystals powdered form into solvent and boil solution slightly less then solvent’s
boiling point and then allow solvent to evaporate.
3. Slow cooling
Crystals can be obtained by slowly cooling down a solution of any material.
4. Nucleation

9. Crystal Growth In The Presence of
Dislocation
 Dislocation is a controlling factor which controls crystal growth.
 The theory of dislocation-controlled crystal growth identifies a continuous spiral
step with an emergent lattice displacement on a crystal surface; a mechanistic
corollary is that closely spaced, oppositely winding spirals merge to form
concentric loops.
 In situ atomic force microscopy of step propagation on pathological L-cystine
crystals did indeed show spirals and islands with step heights of one lattice
displacement. We show by analysis of the rates of growth of smaller steps only
one molecule high that the major morphological spirals and loops are actually
consequences of the bunching of the smaller steps.

10.  In the both figures a part indicate the presence of a dislocation.
 Part b of the figures are show the start of formation of crystal in the form of spiral ring.
 Part c of the both figures show a ring formed .
 Part d of both the figures indicate that the process of crystallization remain continuous after the formation of the
first spiral ring.
 Rings after ring are formed with a some minor distance difference in μ meters.

11.  With a difference of 1μm spiral rings are formed during Step or starting point of ring
during crystallization process. formation
 The morphology of the bunched steps actually inverts the predictions of the theory: Spirals arise from pairs of
dislocations, loops from single dislocations. Only through numerical simulation of the growth is it revealed how
normal growth of anisotropic layers of molecules within the highly symmetrical crystals can conspire to create
features in apparent violation of the classic theory

12. Crystal Growth In The Presence of
Dislocation
 Dislocation strongly affects the growth of crystals. And crystal growth is totally
different while we consider dislocation in the crystals.
 Most important change that one can easily observe in growth of crystals is
the process of growth stops after a certain size of a crystal is obtained in
pure materials without any kind of dislocation while in the presence of
dislocation a crystal continue to grow until we stop this process.
 In the presence of dislocation the size and shape of crystals different for
same materials under same set of conditions.
 Both size and shape now only depends on dislocations( number of
dislocations and extent of dislocation)

13. Continue…
 Bigger in size crystals can be obtained by adding dislocation and have different
properties then crystals without dislocation.
 Dislocations are atomic-scale lattice defects that play a central role in determining
the properties of crystalline materials.
 For example, they govern the strength of structural alloys, influence crystal
growth and can be detrimental to the performance of semiconductor devices.
 Fundamental understanding of their structure and behavior is essential if we are
to engineer dislocations to enhance material properties.