Imperfections in Solid Materials Band theory, insulators, semiconductors p-and n- type, superconductors

“Crystals are like people, it is the defects in them which tend to make them interesting!” - Colin Humphreys.

In real materials we find: Crystalline Defects or lattice irregularity Most real materials have one or more “errors in perfection” with dimensions on the order of an atomic diameter to many lattice sites Defects can be classification: 1. according to geometry (point, line or plane) 2. dimensions of the defect

Imperfections in Solids Solidification- result of casting of molten material 2 steps Nuclei form Nuclei grow to form crystals – grain structure Start with a molten material – all liquid nuclei crystals growing grain structure liquid Adapted from Fig.4.14 (b), Callister 7e. Crystals grow until they meet each other

Point Defects Vacancy self- interstitial • Vacancies: -vacant atomic sites in a structure. Vacancy distortion of planes • Self-Interstitials: -"extra" atoms positioned between atomic sites. self- interstitial distortion of planes

Point Defects in Alloys Two outcomes if impurity (B) added to host (A): • Solid solution of B in A (i.e., random dist. of point defects) OR Substitutional solid soln. (e.g., Cu in Ni) Interstitial solid soln. (e.g., C in Fe) • Solid solution of B in A plus particles of a new phase (usually for a larger amount of B) Second phase particle --different composition --often different structure.

Line Defects Are called Dislocations: Schematic of Zinc (HCP): And: • slip between crystal planes result when dislocations move, • this motion produces permanent (plastic) deformation. Schematic of Zinc (HCP): • before deformation • after tensile elongation slip steps which are the physical evidence of large numbers of dislocations slipping along the close packed plane {0001} Adapted from Fig. 7.8, Callister 7e.

Edge Dislocation Edge Dislocation Fig. 4.3, Callister 7e.

Motion of Edge Dislocation • Dislocation motion requires the successive bumping of a half plane of atoms (from left to right here). • Bonds across the slipping planes are broken and remade in succession. Atomic view of edge dislocation motion from left to right as a crystal is sheared. (Courtesy P.M. Anderson)

Screw Dislocations b Dislocation line (b) Burgers vector b (a) Adapted from Fig. 4.4, Callister 7e.

Edge, Screw, and Mixed Dislocations Adapted from Fig. 4.5, Callister 7e.

Imperfections in Solids Dislocations are visible in (T) electron micrographs Adapted from Fig. 4.6, Callister 7e.

MICROSCOPIC EXAMINATION Applications To Examine the structural elements and defects that influence the properties of materials. Ensure that the associations between the properties and structure (and defects) are properly understood. Predict the properties of materials once these relationships have been established. Structural elements exist in ‘macroscopic’ and ‘microscopic’ dimensions

Optical Microscopy • Useful up to 2000X magnification (?). • Polishing removes surface features (e.g., scratches) • Etching changes reflectance, depending on crystal orientation since different Xtal planes have different reactivity. 0.75mm crystallographic planes Adapted from Fig. 4.13(b) and (c), Callister 7e. (Fig. 4.13(c) is courtesy of J.E. Burke, General Electric Co. Micrograph of brass (a Cu-Zn alloy)

Band Theory of Solids A useful way to visualize the difference between conductors,insulators and semiconductors is to plot the available energies for electrons in the materials.

Electrical Conductivity in Diamond sp3 hybridization localized s orbitals insulator

Conductor, Semiconductor, and Insulator

Doped Semiconductors n-type p-type

Chemistry In Action: High-Temperature Superconductors

Crystal structure determination

C6H6O3S