Q1Electrical Materials
Question
Q.1. Explain the concept of defects in crystalline solids. How do defects influence the mechanical and electrical properties of materials?
Answer
Crystalline defects — point (vacancies, interstitials, substitutionals), line (dislocations), surface (grain boundaries) and volume defects — disrupt lattice periodicity; they increase electrical resistivity through electron scattering, while strengthening or weakening mechanical properties depending on type and density.
Real crystals deviate from perfect lattice periodicity through defects, classified by dimensionality. Point defects (zero-dimensional) include vacancies (missing atoms), interstitials (extra atoms squeezed between lattice sites), and substitutional/interstitial impurity atoms; their equilibrium concentration rises exponentially with temperature. Line defects (one-dimensional) are dislocations — edge and screw types — where an extra half-plane of atoms or a shear displacement disrupts the lattice along a line. Surface defects (two-dimensional) include grain boundaries between differently-oriented crystallites, twin boundaries, and stacking faults. Volume defects (three-dimensional) include voids, cracks, pores, and second-phase inclusions/precipitates occupying finite volumes within the crystal.
Influence on mechanical properties: dislocations govern plastic deformation — their motion allows metals to deform at stresses far below the ideal lattice strength, while obstacles to dislocation motion (impurity atoms, precipitates, grain boundaries, other dislocations) strengthen the material; this is the basis of solid-solution strengthening, precipitation hardening, grain refinement, and work hardening. Voids and cracks act as stress concentrators, degrading strength and promoting brittle fracture and fatigue failure.
Influence on electrical properties: every defect disturbs the periodic potential seen by conduction electrons and acts as a scattering center, reducing relaxation time and mobility, thereby raising resistivity — this is why cold-worked or heavily alloyed metals conduct worse than pure annealed metal, and why resistivity has a temperature-independent 'residual' component proportional to defect/impurity content (Matthiessen's rule). In semiconductors, point defects and impurities can also introduce energy levels within the band gap, acting as dopants (intentionally, controlling carrier type and density) or as traps/recombination centers (degrading minority-carrier lifetime and device performance), so defect control is central to both the intended doping and the purity requirements of semiconductor technology.