WEEK 3NONCRYSTALLINE MATERIALS

NONCRYSTALLINE MATERIALSGasesLiquidsGlassy solids

Metals form crystalline solids, but some ceramic materials are Crystalline, whereas others the inorganic glasses are amorphous.

Atoms in crystalline solids are positioned in an orderly and repeatedPattern. Noncrystallines are random and disordered atomic distribution.

GasesInert gases

Inert gases have no regular ordering of atoms.Very weak covalent and Van der Waals bonding forces.

LiquidsThe molecules have no order and are bonded together by stronger covalent and hydrogen bonding forces.

HHGlassy solidsThe atoms or molecules are bonded together mainly by strong ionic, covalent or metallic bonding forces.

IMPERFECTIONS IN SOLIDSAll solids contain large number of imperfections or deviation from crystalline perfection.Point Defects VacanciesThe simplest of the point defects is a vacancy or vacant lattice site, which is formed when an atom missing from a normal site during solidification from high temperatures or radiation damage.

How to calculate NvThe equilibrium number of vacancies (Nv) increases with temperature. QvNv = N exp (- ) kT NA x densityN = total number of atomic sites = Atomic weightQv = energy required to form vacancyk = Boltzmanns constant (1.38 x 10-23J/atom-K)

Self Interstitials A self Interstitial is in an atom from the crystal that is crowded into an interstitial site. Host atoms that interstitial sites and their presence can cause large distortions in the surrounding lattice.

ImpuritiesForeign atoms that occupy the lattice or interstitial sites of metals to form alloys.The addition of impurity atoms to a metal will result in the formation of a solid solution and/or a new second phase.

Impurities in SolidsSolid Solutions A solid solution is formed when foreign atoms are added to the host material.The crystal structure of the host material is maintained and nonew structures or phases are formed.

Self InterstitialsSubstititional solid solution Interstitial solid solution

Substititional solid solutionForeign atoms replace or substitute for the host atoms.The degree of substitution depends on (a) the atomic size(b) crystal size(c) electronegativity (d) valency

Interstitial solid solution Small foreign atoms (eg C, N, H) eg carbon 2% with Iron steelRFe = 0.124 nm RC = 0.071 nm

Line Defects Which is one of the kinds of Defects.Three types again1/ Edge dislocation2/ Screw dislocation 3/ Mixed dislocation