Breakdown, dielectric

Small inclusions or cavities in the dielectric can intensify local fields because of their higher curvature. High electric fields can cause arcing across such a cavity ions and electrons in the cavity can be accelerated by these fields to further erode the walls of the cavity causing it to grow and eventually break down the dielectric. 

Ionic conductivity, although small, can transport conductive impurities from the surface of the dielectric to the interior. This process can be aided by high humidity or an acidic environment. These impimties can eventually form a conductive path through the dielectric, resulting in thermal runaway or electrolytic breakdown. 

Finally, even if all of the above mechanisms can be avoided, if the electric field is increased beyond a certain point, some electrons may be pulled out of the valence band (or from impimty sites) and accelerated by the field so that they gain sufficient energy to knock other electrons out of the valence band to form an avalanche breakdown. Polystyrene has one of the highest dielectric strengths with 140 MV/m. Ceramics such as alumina, BN, MgO, and ordinary window glass breakdown at 10-20 MV/m. 

The very thin dielectric layer that separates the gate from the charmel in a FET is very susceptible to breakdown from static charge before the chip is installed in the device. For this reason such chips are shipped in conductive static-proof packages and extreme care in handling such components must be exercised. 

Unlike the Zener effect in which the breakdown voltage is engineered into the design of the dielectric, many of the above mechanisms are unpredictable and time-dependent, so the chances of breakdown increases with age. 

When a dielectric is subjected to an ever-increasing electric field, at some point a short circuit develops across it. Dielectric breakdown is defined as the voltage gradient or electric field sufficient to cause the short circuit. This phenomenon depends on many factors, such as sample thickness, temperature, electrode composition and shape, and porosity. 

In ceramics, there are two basic types of breakdown intrinsic and thermal. 

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Highly protective layers can also fonn in gaseous environments at ambient temperatures by a redox reaction similar to that in an aqueous electrolyte, i.e. by oxygen reduction combined with metal oxidation. The thickness of spontaneously fonned oxide films is typically in the range of 1-3 nm, i.e., of similar thickness to electrochemical passive films. Substantially thicker anodic films can be fonned on so-called valve metals (Ti, Ta, Zr,. ..), which allow the application of anodizing potentials (high electric fields) without dielectric breakdown. 

Electrical. Glasses are used in the electrical and electronic industries as insulators, lamp envelopes, cathode ray tubes, and encapsulators and protectors for microcircuit components, etc. Besides their abiUty to seal to metals and other glasses and to hold a vacuum and resist chemical attack, their electrical properties can be tailored to meet a wide range of needs. Generally, a glass has a high electrical resistivity, a high resistance to dielectric breakdown, and a low power factor and dielectric loss. 

In actual practice, mechanical and electrical design factors usually require the cables to have layers of a certain thickness such that the electrical stress is far below the dielectric breakdown poiat. 

Water as an impurity accelerates the oxidation rate. Figure 4 compares growth curves for Si02 under dry and steam conditions. Halogens can also be introduced to the oxidation process, thereby reducing sodium ion contamination. This improves dielectric breakdown strength, and reduces interface trap density (15). 

Grade 1—T1 Flex strength, MPa Water absorption, % Permittivity, 1 MHz Dissipation factor, 1 MHz Impact strength, J/m Dielectric breakdown parallel to laminations, kV... 

ASTM D149, Test Methodsfor Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies, Vol. 8.01, ASTM, Philadelphia, Pa., 1987. 

The dielectric breakdown strength in vitreous siUca depends on its impurity content, its surface texture, and the concentration of stmctural defects, such as cord and bubbles. Good quaUty glasses have room temperature breakdown strength in the range of 200—400 kV/cm. 

Ferroelectrics. Ferroelectrics, materials that display a spontaneous polarization ia the abseace of an appHed electric field, also display pyroelectric and piezoelectric behavior. The distinguishing characteristic of ferroelectrics, however, is that the spontaneous polarization must be re-orientable with the appHcation of an electric field of a magnitude lower than the dielectric breakdown strength of the material. 

These lead-based materials (PZT, PLZT, PMN) form a class of ceramics with either important dielectric, relaxor, pie2oelectric, or electrooptic properties, and are thus used for appHcations ia actuator and sensor devices. Resistive properties of these materials ia film form mirror the conduction processes ia the bulk material. Common problems associated with their use are low dielectric breakdown, iacreased aging, and electrode iajection, decreasiag the resistivity and degrading the properties. 

Higher voltage stresses, (equation (23.1)) may lead to dielectric breakdown. 

Dielectric Strength lEC 243-1. This is a measure of the dielectric breakdown resistance of a material under an applied voltage. The applied voltage just before breakdown is divided by the specimen thickness to give the value in kV/mm. Since, however, the result depends on the thickness this should also be specified. 

The piezoelectric response investigation also provides direct evidence that significant inelastic deformation and defect generation can occur well within the elastic range as determined by the Hugoniot elastic limit. In quartz, the Hugoniot elastic limit is 6 GPa, but there is clear evidence for strong nonideal mechanical and electrical effects between 2.5 and 6 GPa. The unusual dielectric breakdown phenomenon that occurs at 800 MPa under certain... 

Sioux and Teissie [203] loaded propidium iodide in 70% leukocytes in whole blood using the dielectric breakdown method. The entrapped drug showed a half-life of longer than 4 hours at 4 and 37°C. When compared with the nonpulsed cells, leukocytes loaded with the drug showed 10 times more accumulation in the inflammation area than in control areas. 

At a certain anodic potential, the compact film breaks down and lets electrons pass through without much resistance, causing oxygen evolution at a high rate. This dielectric breakdown is discussed in more detail in Section V. 

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

Static charge generation causes an ignition hazard only if the accumulated charges create an electric field that is 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 dielectric breakdown strength of air (nominally 3 x 106 V/m). The objective of static control measures is to ensure that electric field intensities cannot reach this value. 

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