atomic structure and crystal structure & its defects
- 1. 1st Presentation session of Independent study (Structure-Property Correlation in Materials) Presented by Under the guidance of Sumit Singh Rajput PhD scholar Dr. Jose Immanuel R Assistant professor IIT Bhilai MECHANICAL ENGINEERING DEPARTMENT
- 2. TOPIC OF DISCUSSION Atomic structure Types and nature of atomic bonding Crystal structure of metals Crystal defects
- 3. Atom comes from the Greek word atomos, which means something invisible and cannot be cut or divided. John Dalton suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity called them atom. Joseph John Thompson found that atoms could sometimes eject a far smaller negative particle which he called an electron. Ernest Rutherford Gold foil experiment. Niels Bohr Proposed that energy levels of electrons are discrete and that the electrons revolve in stable orbits around the atomic nucleus but can jump from one energy level to another Atoms
- 4. Particle Mass (g) Charge (Coulombs) Charge (units) Electron (e- ) 9.1 x 10-28 -1.6 x 10-19 -1 Proton (p) 1.67 x 10-24 +1.6 x 10-19 +1 Neutron (n) 1.67 x 10-24 0 0 Subatomic Particles
- 5. 1. Ionic Bonding 2. Covalent Bonding 3. Metallic Bonding 4. Van Der Wall’s Bonding TYPES OF ATOMIC BONDING IN SOLIDS An ionic bonding is the Attractive Force existing between positive ion and a negative ion when they are brought into close proximity or surrounding. They are formed when atoms of different elements lose or gain their electrons in order to achieve stabilized outermost electronic configuration. Ionic Bonding
- 6. Covalent bonding A covalent bond is formed, when two or more electrons of an atom, in its outermost energy level, are shared by other atoms. e.g.-nitrogen molecule. • Ionic solids are rigid. They have high melting and boiling points. • Good insulators of electricity in their solid state and good conductor of electricity in their molten state. • Soluble in water and slightly soluble in organic solvents. • Bad conductors of electricity. • low melting and boiling points. • Insoluble in water, Soluble in organic solvents
- 7. Metallic bonding is the strong attraction between closely packed positive metal ions and a 'sea' of delocalized electrons. e.g. – any metals • High thermal and electrical conductivity • Increase mechanical properties • Have a bright lusters • Metallic solids are malleable and ductile Metallic bonding
- 8. Weak and temporary bonds between molecules of the same or different substance are known as Van der Walls bonding. Types of Van der walls forces 1) dipole-dipole attraction force 2) dipole-induced dipole 3) London dispersion force 4) Hydrogen bonding Van der walls bonding
- 9. Crystalline solids: A solid is said to be crystalline if the various constituent particles like atoms, ions, or molecules are arranged in a definite geometric pattern within the solid, e.g. NaCl , KNO3 Amorphous solids: A solids is said to be amorphous if the various constituent particles are not arranged in any regular fashion. They are also called pseudo-solid or super cooled liquid. They are intermediate state between liquids and solids. e.g., glass and rubber Solids Basic Of Crystal Structure Lattice:- An infinite periodic array of points in a space. The arrangement of points defines the lattice symmetry Basis(Motif):- A group of one or more atoms, located in a particular way with respect to each other and associated with each point. Crystals are classified into two types 1. Poly crystal 2. Single crystal
- 10. A primitive cell is a unit cell that contains exactly one lattice point. It is the smallest possible cell. Primitive Cell A repetitive arrangement of them can build up the whole crystal without overlaps/gaps. There is no further partition of the unit cell that could itself be used as a unit cell. Unit cell
- 13. Crystals are like people: it is the defects in them which tend to make them interesting - Dr. .J. HUMPHREYS DEPARTMENT OF PHYSICS, ARIZONA STATE' UNIVERSITY • Crystalline material has a crystal structure in which the atoms are positioned in perfect ordered pattern which is repetitive over large atomic distance. • Defects have a profound impact on the macroscopic properties of materials. • Sometimes defects are deliberately created to improve properties of crystal Crystal defects
- 14. Point defects (0D) Surface / Interface (2D) Line defects (1D) Volume defects (3D) Vacancy Interstitial Frenkel defect Schottky defect Edge Dislocation Screw Dislocation Grain boundary Twin boundary Stacking faults Dispersant Precipitates Inclusion Voids / Cracks
- 15. Point Defects are the irregularities or deviations from ideal arrangement around a point or an atom in a crystalline substance. Point defects Non-ionic crystals Ionic crystals a) vacancy b) Interstitial a) Frenkel defects b) schottky defects
- 16. • A dislocation is a line discontinuity in the regular crystal structure. • Line defects are the irregularities or deviations from ideal arrangement in entire rows of lattice point. • Dislocation affects the mechanical properties • It has three types Edge dislocation Edge dislocation line Burgers vector As the dislocation moves, the extra half plane will break its existing bonds and form new bonds with its neighbor opposite of the dislocation motion
- 17. Screw dislocation The spiral stacking of crystal planes leads to the burgers vector being parallel to the dislocation line. Mixed dislocation This dislocation has both edge and screw character with a single Burgers vector consistent with the pure edge and pure screw regions. Screw dislocation Mixed dislocation
- 18. It is a two dimensional defect. Planar defects during mechanical and thermal treatments. The change may be of the orientation or of the stacking sequence of the planes. Planar defects are of the following types: • Grain boundaries • Tilt boundaries • Twin boundaries • Stacking faults PLANAR OR SURFACE DEFECTS Grain boundaries
- 19. PERCIPITATES : precipitates, which are a fraction of a micron in size and decorate the crystal DISPERSANTS :Dispersants, which vary in size from a fraction of a micron to the normal grain size (10-100µm), some time they are intentionally introduced into the microstructure INCLUSIONS : Inclusions vary in size from a few microns to macroscopic dimensions, and are relatively large, undesirable particles that entered the system as dirt or formed by precipitation VOIDS : voids are holes in the solid formed by trapped gases or by the accumulation of vacancies. Bulk or Volume Defects
- 20. • C. J. Humphreys, Stem Imaging of Crystals and Defects, springer link Books • Callister’s materials science and engineering second edition , William D. Callister,Jr, David G. Rethwisch, wiley chapter -5 page 123-130 • Introduction to physical metallurgy,second edition , Sidney H Avner, chapter 2, page 81-97