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Electrophoretic deposition (EPD) is a method of producing coatings or films on electrically conducting objects or, in some cases, creating standalone components and materials using a process called electrophoresis. This term describes the migration of electrically charged particles in a liquid toward an electrode under the influence of an electric current. Small particles suspended in a liquid often have a positive or negative electric charge because of the way their molecules interact with those of the medium. If a direct current is applied across the suspension using electrodes, the particles will move toward the electrode with the opposite charge. Electrophoresis is commonly used in biochemical analysis and has become an important part of many industrial processes.
Also known as electrophoretic coating or e-coating, electrophoretic deposition uses the object to be coated as an electrode onto which oppositely charged particles are deposited, forming a layer. Depending on whether the positive or negative electrode is used, the process might be referred to as anodic or cathodic electrodeposition, respectively. Where the particles would normally be electrically neutral, a compound might be bonded to them to give them an electrical charge in suspension. The resulting electrical repulsion between the particles also prevents them from clumping together.
This process has many applications, particularly in nanotechnology and materials science. Unlike electroplating, EPD can be used to deposit a wide range of non-metallic substances, as well as metals, and is a relatively quick and low-cost method of applying an insulating or protective coating to small electrical components. Normally, however, the coating has a higher electrical resistance than the electrode, so that as the process continues, the current decreases with increasing resistance. This might impose limits on its use.
It also is possible to remove the deposit from the electrode to form a separate object. For example, electrophoretic deposition can be employed to deposit suspended carbon nanotubes onto a flat electrode, forming a thin film of nanotubes that can then be detached. Carbon nanotube films have many applications, including thin film solar cells, fuel cells and touch screens.
Electrophoretic deposition also allows the manufacture of new types of ceramic material. Composite materials can be synthesized by using a mixture of nanoparticles in a suspension, resulting in, for example, improved strength or useful electrical properties. Another important application is in functionally graded materials (FGMs); it is possible to create objects composed of two materials that normally are incompatible, perhaps because of differing thermal expansion rates, by ensuring a steady gradient between the two rather than a sharp boundary. This is achieved by varying the composition of the suspension during electrophoresis. Laminates composed of alternating layers of different materials also can be manufactured simply by switching between different suspensions.