Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 52

Point defects are imperfections that arise from a single central site, but cause strain in a small volume around them and can impact the properties of the bulk. They include a variety of imperfections, such as those shown in Figure 2.5: (i) vacancies – empty lattice sites that can impact diffusion; (ii) interstitials – generally smaller atoms located in between normal lattice sites that impact a variety of properties, such as strength, hardness ductility, and yield strength; and (iii) substitutions – solute atoms located at a lattice site that can also impact strength and hardness.

Point defects can often occur in pairs or sets. For example, in ionic crystals, point defects can occur in pairs that maintain stoichiometry or charge neutrality, such as Schottky defects (vacancies of ions with opposite charge) or Frenkel defects (a vacancy and an interstitial). A high density of point defects, particularly vacancies, can give rise to nonstoichiometric compounds. Such compounds often consist of transition metals, lanthanides, and actinides paired with polarizable anions, such as oxides and sulfides, e.g. Fe_xO and Cu_xS. Point defects can be naturally occurring, such as the substitutional impurities that give rise to beautiful gems, e.g. sapphires and rubies. Point defects can also be designed into the material to tailor its properties, such as doping semiconductors to control their electrical conductivity.

Figure 2.5 Schematic of common point defects in crystals, including a vacancy, an interstitial, and a substitution.