It is expressed in kJ/mole or eV/point imperfection. The work required to be done for creating a point imperfection is called the enthalpy of formation (∆ H f) of the point imperfection. Similarly, an interstitial atom produces strains around the void it is occupying.Īll these factors tend to increase the enthalpy or the potential energy of the crystal. A large atom introduces compressive stresses and corresponding strains around it, while a smaller atom creates a tensile stress- strain field. The elastic strains are present irrespective of whether the impurity atom is larger or smaller than the parent atom. In the case of an impurity atom, as a result of the size difference, elastic strains are created in the region of the crystal immediately surrounding the impurity atom. If the imperfection is a vacancy, the bonds that the missing atom would have formed with its neighbours are not there. The presence of a point imperfection introduces distortions in the crystals. Such defect structures have an appreciable concentration of point imperfections. When an ionic crystal does not correspond to exact stoichiometric formula defect structures are produced. The interstitialicies and Frenkel defects are less in number than vacancies and Schottky defects, because additional energy is required to force the atom into the new position.Ĭlose-packed structures have fewer interstitialicies and Frenkel defects than vacancies and Schottky defects, as additional energy is required to force the atoms in their new positions. The vacancy and interstitialcy are therefore, inverse phenomena.Īn ion dislodged from the lattice into an interstitial site is called Frenkel defect. The foreign atom may form added alloying agent or simply an impurity. It is the addition of an extra atom within a crystal structure particularly if the atomic packing factor is low. They involve vacancies of pair of ions of opposite charges. These are closely related to vacancies but are found in compounds which must maintain a charge balance. They may also arise from thermal vibrations of the atoms at high temperatures. These detects may come up as a result of imperfect packing during the original crystallisation. The various point defects are discussed below:Ī vacancy is the simplest point defect and involves a missing atom within a metal. The number produced in this manner is not dependent on temperature but depends only on the nature of the crystal and on the bombarding particles. The first particle collides with the lattice atoms and displaces them, thereby forming a detect. (iv) By external bombardment by atoms or high-energy particles e.g., from the beam of the cyclotron or the neutrons in a nuclear reactor. While the lattice still retains its general crystalline nature, numerous detects are introduced. (iii) By severe deformation of the crystal lattice e.g., by hammering or rolling. (ii) By quenching (quick cooling) from a higher temperature. The point defects may be created as follows: Point imperfections are always present in crystals and their presence results in a decrease in the free energy. These defects are completely local in effect, e.g., a vacant lattice site. The typical size of a point defect is one or two atomic diameters. Point defects are imperfect point like regions in a crystal. Volume defects or three dimensional defects. Crystal defects occur as points, along lines, or in the form of a surface, and they are called point, line, or plane defects respectively.4. Solid Defects The missing and lacking of atoms or ions in an ideal or imaginary crystal structure or lattice and the misalignment of unit cells in real crystals are called crystal defects or solid defects.Lattice structures (or crystals) are prone to defects especially when their temperature is greater than 0 K. Lattice structures are not perfect in fact most of the time they experience defects. Schottky Defects The Schottky defect is a point defect and occurs when oppositely charged ions vacant their sites.Iron oxide and other metal oxides are used in thermite reactions, and this has been applied in many ways, including welding in The suppressing function has been applied for switching and for protection of random voltage protections. For example, zinc oxide sintered together with other metal oxide additives have been made into nonlinear resistors, which are called Varistors for surge suppressing function. Metal Oxides Metal oxides are very common commodities, widely applied, and have many different varieties.As a result the atom or ion leaves its own lattice site vacant. Frenkel Defect The Frenkel Defect (also known as the Frenkel pair/disorder) is a defect in the lattice crystal where an atom or ion occupies a normally vacant site other than its own.
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