Breakdown strength of air

Static charge generation causes an ignition hazard only if the accumulated charges create an electric field sufficient to produce an electrical discharge in a flammable atmosphere. In most processes, this means that the electric field intensity at some location must reach the breakdown strength of air (nominally 3 X lO " V/m). The objective of static-control measures is to ensure that electric field intensities cannot reach this value. 

The maximum charge is reached in a time that depends on the ion concentration as indicated by Eq. (64). Ion concentrations of the order of 108 ions/cm3 are readily achieved by means of corona from wires and ion mobilities are of the order of 2 (centimeters/second)/(volts/centimeter). For these conditions 4c/(/ , n/ Q) is 0.011 sec. It is apparent from Eq. (64), therefore, that the particle will essentially acquire its maximum charge in less than a second. Since i0 can be of the order of 3 V/micron (breakdown strength of air), it is also apparent from Eq. (65), that particle charges corresponding to the range of 3-10 V/micron can be achieved by this means. 

The density of charge that can be obtained on a surface is often limited by the breakdown strength of air, rather than by the basic charging mechanism itself and, as mentioned in Section 1.1, this has often confused the interpretation of electrostatic experiments in the past. Thus, if we take the typical value of 3 MV m-1 for the breakdown strength of air, the maximum uniform charge... 

The intrinsic electrical strength of most plastics is higher by a factor of a thousand than the breakdown strength of air, so that in most practical situations breakdown is dominated by the breakdown of the air. 

Water vapor has been shown to have a marked influenced on contact electrification, acting to increase conductivity as well as to decrease the breakdown strength of air. As a result of adsorbed water on both surfaces, contact potential differences will be lower leading to less transfer of charge. Water vapor also increases polymer conductivity, 0, according to a relationship ... 

For powders, cp/pp is approximately one-half. If we further assume that the outer edge of the volume will go into corona when exceeds 3 V/micron, which is the normal electrical breakdown strength of a flat surface in air, Table II shows the maximum size containers that can be used for various net levels of particle charging without having the charge lost by corona. 

Figure 1.6 shows the electric field breakdown voltage of air as a function of the pressure-separation distance product. It shows that at large values of pressure-distance product, the breakdown electric field strength is approximately equal to 30 kV/cm. At small values of this product, the breakdown electric field strength increases significantly. This phenomenon is due to the rarefaction effect. When the mean free path becomes comparable to... 

Electrical discharge A current flow that occurs when the electrical field strength exceeds the dielectric breakdown value of a medium such as air. 

Side-flashing can also occur below the ground to buried metal pipes or wires and care must be taken in the design and positioning of the grounding electrodes. Typical values of impulse breakdown in soil are 2 to 5kV/cm, which leads to side-flashes of several meters. In air the value is 9kV/cm and brick and concrete has a slightly lower breakdown strength. 

Current dielectric material is dense silica that has a k value of 4. Porous silica has a lower k because of the incorporation of air. Sol-gel silica and mesoporous silica have been studied as possible low-k materials. However, these materials have drawbacks such as low mechanical strength and low heat conductivity. Sol-gel silica also has randomly occurring large pores that can cause electrical breakdown. High mechanical strength is needed for the new low-k materials to be compatible with the CMP process. 

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