However, grain boundaries can affect a material’s behaviour in other ways. This often dictates the degree of scattering of the electrons. The movement of current through materials with significant grain boundary scattering is largely determined by the size of the angles between the grains. These smart devices could well replace the silicon power devices that are currently used for overload detection and control in power grids. These materials undergo a reversible transition from superconducting to ‘normal’ and so switch off any current overload. ![]() Runaway heating effects in these materials are used to limit the current in high power circuits, such as local power grids. This can give rise to ‘thermal runaway’ effects in circuits, which are applied practically in devices known as high temperature superconducting fault current limiters. Other conducting pathways consequently become more highly stressed, as they have to carry more current. Obstructing or diverting the flow of electrons transfers momentum and energy to the source of the obstruction, causing localised heating. Conduction of current in (a) a homogeneous material and (b) a material containing grain boundaries and inclusions.īy careful design of grain boundaries, this scattering can be put to good use. So the movement of electric current is often far from the idealised picture of a swarm of electrons drifting through a homogeneous material under the influence of an applied electric field.įigure 1. However, in a material containing grain boundaries, figure 1b, charge carriers may also be scattered at the interfaces between grains. The value for the mobility of the charge carriers takes into account all inelastic scattering processes by which current flow is impeded. Figure 1a shows current flow through a homogeneous material. Electrical conductivity depends on both the density and the mobility of the charge carriers in a material. Smart attributes have been designed into the grain boundaries of some conductor materials so that they are stable when conducting large currents. High temperature superconductors on flexible substrates, thin film conductor tracks on silicon chips and thick film resistors on printed circuit boards and for surface mount technology are three examples of technologies that could benefit. Once this is better understood, materials and their grain boundary characteristics can be designed with improved electronic performance and, in some cases, smart capabilities to give more reliable electronic devices and systems with improved functionality and speed. These techniques will allow a deeper understanding of, and better modelling of macroscopic behaviour, such as the movement of electric current through polycrystalline materials, which is usually expressed in terms of ‘percolation theory’. This article highlights new techniques for measuring the structure of grain boundaries in polycrystalline materials on a microscopic scale. In some cases, tailoring the structure of the grain boundaries has allowed researchers to design materials with ‘smart’ attributes that make them fit for purpose, stable and self-compensating. ![]() In these materials, grain boundaries play a vital role, affecting the electrical behaviour of the material and determining these compromises. However, there are important exceptions, especially in cases for which compromises must be made between mechanical and electrical properties and costs. Indeed, many of the materials used in today's electronic circuits and devices are monocrystalline. Normally, the ultimate in electronic properties is achieved with single crystal materials. Such is the case with certain high performance electronic materials. In some cases, faults or defects may be exactly what is needed to make a particular material suitable for a particular application especially if those faults or defects can be tailored during processing of the material.
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