Grain boundaries


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Grain boundaries

The internal interfaces that separate neighboring misoriented single crystals in a polycrystalline solid. Most solids such as metals, ceramics, and semiconductors have a crystalline structure, which means that they are made of atoms which are arranged in a three-dimensional periodic manner within the constituent crystals. Most engineering materials are polycrystalline in nature in that they are made of many small single crystals which are misoriented with respect to each other and meet at internal interfaces called grain boundaries. These interfaces, which are frequently planar, have a two-dimensionally periodic atomic structure. A polycrystalline cube 1 cm on edge, with grains 0.0001 cm in diameter, would contain 1012 crystals with a grain boundary area of several square meters. Thus, grain boundaries play an important role in controlling the electrical and mechanical properties of the polycrystalline solid. It is believed that the properties are influenced by the detailed atomic structure of the grain boundaries, as well as by the defects that are present, such as dislocations and ledges. Grain boundaries generally have very different atomic configurations and local atomic densities than those of the perfect crystal, and so they act as sinks for impurity atoms which tend to segregate to interfaces. See Crystal defects, Crystal structure

Using electron microscopy and x-ray diffraction, it was determined that the grain boundary structure is frequently periodic in two dimensions. The geometry of a grain boundary is described by the rotation axis and angle, Θ, that relate the orientations of the two crystals neighboring the interface, and the interface plane (or plane of contact) between the two crystals. Grain boundaries are typically divided into categories characterized by the magnitude of Θ and the orientation of the rotation axis with respect to the interface plane. When Θ is less than (arbitrarily) 15°, the boundary is called small-angle, and when Θ is greater than 15°, the boundary is large-angle. See X-ray diffraction

Because of the large differences in atomic structure and density between the grain boundary region and the bulk solid, the properties of the boundary are also quite different from those of the bulk, and have a strong influence on the bulk properties of the polycrystalline solid. The mechanical behavior of a solid, that is, its response to an applied stress, often involves the movement of dislocations in the bulk, and the presence of boundaries impedes their motion since, in order for deformation to be transmitted from one crystal to its neighbor, the dislocations must transfer across the boundary and change direction. The detailed structure at the interface influences the ease or difficulty with which the dislocations accomplish this change in direction.

Since grain boundaries in engineering materials are not in a high-purity environment, the presence of impurities dissolved in the solid may have a strong influence on their behavior. The presence of one-half of a monolayer of impurity atoms, such as sulfur or antimony in iron, at the grain boundary can have a drastic effect on mechanical properties, making iron, which is ductile in the high-purity state, extremely brittle, so that it fractures along grain boundaries. The segregation of the impurity atoms to the boundaries has been well documented by the use of Auger electron spectroscopy, and studies have led to the suggestion that the change in properties may be related to a change in the dislocation structure of the grain boundary induced by the presence of these impurities.

Since modern electronic devices are fabricated from semiconductors, which may be polycrystalline, the presence of grain boundaries and their effect on electrical properties is of great technological interest. In a semiconductor such as silicon the local change in structure at the interface gives rise to disruption of the normal crystal bonding, or sharing of valence electrons. One consequence can be the charging of the grain boundaries, which produces a barrier to current flowing across them and thus raises the overall resistance of the sample. This polycrystalline effect is exploited in devices such as zinc oxide varistors, which are used as voltage regulators and surge protectors.

Grain boundaries

The internal interfaces that separate neighboring misoriented single crystals in a polycrystalline solid. Most solids such as metals, ceramics, and semiconductors have a crystalline structure, which means that they are made of atoms which are arranged in a three-dimensional periodic manner within the constituent crystals. Most engineering materials are polycrystalline in nature in that they are made of many small single crystals which are misoriented with respect to each other and meet at internal interfaces called grain boundaries. These interfaces, which are frequently planar, have a two-dimensionally periodic atomic structure. A polycrystalline cube 1 cm on edge, with grains 0.0001 cm in diameter, would contain 1012 crystals with a grain boundary area of several square meters. Thus, grain boundaries play an important role in controlling the electrical and mechanical properties of the polycrystalline solid. It is believed that the properties are influenced by the detailed atomic structure of the grain boundaries, as well as by the defects that are present, such as dislocations and ledges. Grain boundaries generally have very different atomic configurations and local atomic densities than those of the perfect crystal, and so they act as sinks for impurity atoms which tend to segregate to interfaces.

Using electron microscopy and x-ray diffraction, it was determined that the grain boundary structure is frequently periodic in two dimensions. The geometry of a grain boundary is described by the rotation axis and angle, Θ, that relate the orientations of the two crystals neighboring the interface, and the interface plane (or plane of contact) between the two crystals. Grain boundaries are typically divided into categories characterized by the magnitude of Θ and the orientation of the rotation axis with respect to the interface plane. When Θ is less than (arbitrarily) 15°, the boundary is called small-angle, and when Θ is greater than 15°, the boundary is large-angle.

Because of the large differences in atomic structure and density between the grain boundary region and the bulk solid, the properties of the boundary are also quite different from those of the bulk, and have a strong influence on the bulk properties of the polycrystalline solid. The mechanical behavior of a solid, that is, its response to an applied stress, often involves the movement of dislocations in the bulk, and the presence of boundaries impedes their motion since, in order for deformation to be transmitted from one crystal to its neighbor, the dislocations must transfer across the boundary and change direction. The detailed structure at the interface influences the ease or difficulty with which the dislocations accomplish this change in direction.

Since grain boundaries in engineering materials are not in a high-purity environment, the presence of impurities dissolved in the solid may have a strong influence on their behavior. The presence of one-half of a monolayer of impurity atoms, such as sulfur or antimony in iron, at the grain boundary can have a drastic effect on mechanical properties, making iron, which is ductile in the high-purity state, extremely brittle, so that it fractures along grain boundaries. The segregation of the impurity atoms to the boundaries has been well documented by the use of Auger electron spectroscopy, and studies have led to the suggestion that the change in properties may be related to a change in the dislocation structure of the grain boundary induced by the presence of these impurities. See Metal, mechanical properties of, Plastic deformation of metal

Since modern electronic devices are fabricated from semiconductors, which may be polycrystalline, the presence of grain boundaries and their effect on electrical properties is of great technological interest. In a semiconductor such as silicon the local change in structure at the interface gives rise to disruption of the normal crystal bonding, or sharing of valence electrons. One consequence can be the charging of the grain boundaries, which produces a barrier to current flowing across them and thus raises the overall resistance of the sample. This polycrystalline effect is exploited in devices such as zinc oxide varistors, which are used as voltage regulators and surge protectors. See Surge suppressor, Varistor

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