Ceramics breakdown voltage

For a spark to occur U must exceed the breakdown voltage of the gap. It is assumed that the voltage given by Eq. (6.74) is established when breakdown occurs. At this stage the field in the ceramic is in the same direction as the poling field and assists in maintaining the domain orientations in the poled sense. 

The electrical properties of ceramic substrates perform an important task in the operation of electronic circuits. Depending on the applications, the electrical parameters may be advantageous or detrimental to circuit function. Of most interest are the resistivity, the breakdown voltage or dielectric strength, and the dielectric properties, including the dielectric constant and the loss tangent. 

The surface of most ceramics is highly "wettable," in that moisture tends to spread rapidly. Under conditions of high humidity, coupled with surface contamination, the effective breakdown voltage is much lower than the intrinsic value. 

In denominated "smart ceramics," the ceramic varistor acts as variable resistors, with resistive behavior at low voltages and conductive behavior starting from a specific voltage value, known as the breakdown voltage (V ) or breakdown electric field ( p,) [9,10],... 

Lasdy, the importance of electroceramic substrates and insulators should not be overlooked. Here one strives to raise the breakdown strength by eliminating the interesting conduction mechanisms just described. Spark plugs, high voltage insulators, and electronic substrates and packages are made from ceramics like alumina, mullite [55964-99-3] and porcelain [1332-58-7]. 

In static operations, the lifetime is mainly limited by the humidity, which penetrates through the external insulation layer and leads to a leakage current increase. A larger leakage current can lead to an electrical breakdown. Due to the dielectric and mechanical losses, the piezoelectric actuator warms up under continuous excitation. Losses are mainly non-linear and depend on the excitation frequency, the voltage amplitude and the humidity. To avoid a depoling effect of the ceramic, the temperature in the actuator should be monitored to ensure that it stays well below the ceramics Curie temperature. So a typical range of temperatures is —40°C to 80 °C. 

The fimction of the substrate is to provide the base onto which thin-film circuits and/or thick-film circuits, which make up the electrical circuits, are fabricated and various multilayer films are deposited. In addition, the substrate provides the necessary mechanical support and rigidity needed to produce a reliable functional circuit. It must have adequate thermal management ability to ensure proper temperature operation, and it must possess a proper electrical insulation to withstand circuit voltages without breakdown. One can think of the substrate as the foimdation on which the circuit traces and components are mounted and supported. Ceramic materials are often used for thin- and thick-film applications, because ceramics have high thermal conductivity, good chemical stability, and are also resistant to thermal and mechanical shock [4]. 

In common with other insulating materials, ceramic insulators must have low dielectric constants so as to avoid capacitance effects, adequate dielectric strength to withstand the applied voltage without breakdown, low dissipation in order to avoid excessive electrical losses, and mechanical strength sufficient to withstand service conditions. 

As discussed in Section n. A, the presence of both electrons and ions is required in a high-voltage vacuum gap to initiate breakdown and estabUsh a vacuum arc. These can be conveniently injected into the vacuum gap in the form of a plasma to give rapid breakdown with a minimum of jitter. One such arrangement for plasma injection is shown in Fig. 9. A ceramic cyhnder is coated with titanium hydride only a few millimeters thick. A V-notched groove is then cut through the metallic hydride into the ceramic, forming a ceramic gap. A shield cap is placed on one end of the ceramic cylinder, and a lead wire is attached to the cap. This assembly is inserted into a conical recess in the cathode electrode of the main vacuum gap as shown. 

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