Book: Ceramics - Art or Science? Author: Dr. Stan Jones

16. Characteristics and Applications of Advanced Ceramic Materials

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16.28 Capacitive Pressure Sensors

Capacitive pressure sensors contain a ceramic structure that transforms the pressure applied into an electrical signal. A capacitor can be formed between two alumina plates, one of which is thick and acts as the support while the other is thin and acts as a flexible diaphragm. Parts of one side of each plate are metalised, and they are supported close together with a glass bond so that any deflection under pressure will measurably alter the capacitance between them and can be measured. Their wide uses include automotive, for monitoring the engine, brakes and fuel, for power steering and transmission oil pressure and manifold air pressure, and industrial applications needing accuracy and reliability.

16.29 Varistors

Electronic and electrical equipment can be subject to severe impulse voltage transients that can destroy sensitive equipment. These transients or surges can be generated by lightning, switching and electrostatic discharge from the human body. As a solution the designer can over-specify the equipment, potentially at huge extra cost, or incorporate protection. The latter is usually the most economic solution. Crude protection at higher voltages such as lightning strikes can be spark gaps, where the breakdown voltage of the air in a gap between two metal rods is low enough so that the equipment is not damaged.

Ultra high voltage artificial lightning strike - source NAREC

Ultra high voltage artificial lightning
strike - source NAREC

At low voltages, zener diodes can be used. However, varistors, which were discovered in 1968, can be designed to limit voltage surges from 5 volts to 1MV, protecting both electronic and electrical equipment so they do not become overstressed.

Varistors are novel ceramic semi-conductive devices that are insulators at low voltages and conductors at higher voltages, with an abrupt change between. So they are used to drain off current when the applied voltage exceeds a certain value. One of their most important applications is to protect transformers on the electricity grid from surges of voltages caused by lightning strikes. Lightning strikes happen far more frequently than imagined, and 400 kV grid transformers can cost several million pounds each! However, much smaller varistors are used to protect more modest electronic equipment including ones in the home. Studies have shown that domestic electricity supplies can suffer brief surges up to 1000 Volts several times a day, which would destroy unprotected semiconductor-based equipment.

Zinc oxide discs and complete high voltage surge arrestor - source Meidensha Corp

Zinc oxide discs and complete high voltage
surge arrestor - source Meidensha Corp

The most common ceramic used is based on zinc oxide, with 2 to 20% additives of up to eight other metal oxides such as antimony, bismuth, cobalt and manganese, each contributing to improved performance. As with many electronic ceramics the active region is in the barrier region between the grains of the zinc oxide ceramic formed during the firing process, so this is not an effect found in single crystals. Silicon carbide is also used, as a matrix of 50-150 micron particles in silica glass. The theory of operation is far from exact because of the huge complexity of the characteristics of these grain boundaries, and some additives are included by “trial and error”. The manufacturing process is familiar; the constituents are ground finely in a ball mill, mixed with water and a binder, dried into granules, usually pressed into discs, then sintered at 1100 to 1400 ºC and the flat surfaces metalised.

Hyundai domestic surge protector

Hyundai domestic surge protector

The higher voltage surge arrestors are built up from a stack of ZnO discs enclosed in a porcelain insulator. As an example, for a medium voltage 24kV distribution transformer, six 32mm diameter, 32mm thick ZnO discs are used which will withstand a peak current of 65kA. The largest devices for grid system surge arrestors are made up of discs 80 mm diameter and 25 mm thick.

Lower voltage applications include protecting high-value computers, sometimes with the varistor built into electrical outlet sockets and cylindrical ones as part of connector pins. Miniature ones are also used within electronic equipment, including those made for surface mount printed circuit boards. They are also used to protect inductive loads that cause surges when switched, such as electromechanical relay contacts and the electric motor and windscreen wipers in automobiles.

Although the number of units made each year fluctuates, it is estimated that two billion varistors were made in 1988.  Another estimate was that over a billion varistors were in use around 2004, protecting equipment from ten to a million Volts.

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