Grain boundary defects

The piopeities of a ceramic material that make it suitable for a given electronic appHcation are intimately related to such physical properties as crystal stmcture, crystallographic defects, grain boundaries, domain stmcture, microstmcture, and macrostmcture. The development of ceramics that possess desirable electronic properties requires an understanding of the relationship between material stmctural characteristics and electronic properties and how processing conditions maybe manipulated to control stmctural features. 

In addition to mechanical properties, other physical properties of polycrystaUine materials, such as electrical and thermal conduction, are also affected by microstmcture. Although polycrystals are mechanicaUy superior to single crystals, they have inferior transport properties. Point defects (vacancies, impurities) and extended defects (grain boundaries) scatter electrons and phonons, shortening their mean free paths. Owing to... 

Chemical sensors for gas molecules may, in principle, monitor physisorp-tion, chemisorption, surface defects, grain boundaries or bulk defect reactions [40]. Several chemical sensors are available mass-sensitive sensors, conducting polymers and semiconductors. Mass-sensitive sensors include quartz resonators, piezoelectric sensors or surface acoustic wave sensors [41-43]. The basis is a quartz resonator coated with a sensing membrane which works as a chemical sensor. 

The mechanism by which defects concentrate impurities is a subject of research that has important bearing on crystal growth, especially related to formation of crystalline materials for use in the electronics industry. Besides imperfections associated with isolated impurities (i.e., point defects), the other major types of structural defects are line defects (both edge and screw), planar defects, grain boundaries, and structural disorder (Wright 1989). The connection between defect formation and impurity uptake is evident in two of these defects in particular the edge defect and point defect. 

A suitable classification of crystalline defects can be achieved by first considering the so-called point defects and then proceeding to higher-dimensional defects. Point defects are atomic defects whose effect is limited only to their immediate surroundings. Examples are vacancies in the regular lattice, or interstitial atoms. Dislocations are classified as linear or one-dimensional defects. Grain boundaries, phase boundaries, stacking faults, and surfaces are two-dimensional defects. Finally, inclusions or precipitates in the crystal matrix can be classified as three-dimensional defects. 

All of these two dimensional bubble raft demonstrations simulate the arrangement of atoms in crystalline materials. Dislocations, lattice defects, grain boundaries and recrystallization are all phenomena that occur in three dimensional crystalline materials. 

When a dislocation is caused to move across a crystal, one layer will have been displaced relative to its neighbor by one atom spacing. Very large shear displacements are possible when a defect (grain boundary, crack, impurity, etc.) is present. This defect is capable of generating many dislocations one after the other when shear stress is applied. 

The mean free path of electron is a direct function of the electron collision frequency. Electrons can collide with other electrons, lattice, defects, grain boundaries, and surfaces. 

This variation in surface reactivity can likely be attributed to molecular scale heterogeneity of the electrode surface. It is well known that rates of electron transfer on electrode surfaces vary depending on the crystal face (23). For corroding iron systems, corrosion rates are known to vary with location on the iron surface due to crystal defects, grain boundaries, stress differences and surface energy effects (24). 

Potential distribution at sites protected by a crystalline barrier-type oxide (a) at defect-free barrier layer, (b) at defect (grain boundary), and (c) at depassivated site (pit nucleus). (From Marcus, P. et al. Corrosion Sci., 50,2698,2008.)... 

---