Dielectric strengths measurement

All electrical property values are strongly dependent on water content for water, the dielectric constant is approximately 81 and resistivity is about 106 2 cm. The dielectric constant has been used as a measure of moisture in coal (Speight, 1994, and references cited therein). However, it should be noted that the effect is not considered to be additive due to the different electrical properties of physically and chemically bound water. With an increase in moisture content, electrical conductivity and dielectric constant increase, whereas resistivity and dielectric strength decrease. Hence, except for special purposes (e.g., dielectric strength measurements of underground coal blocks), electrical measurements require the meticulous drying of coal prior to experiments. 

Thermal Expansion. Early BNL dielectric strength measurements were made with eight to ten layers of tape helically wound on a 1.27 cm diameter stainless steel mandrel. Most nonoriented polyethylene and polypropylene tape candidates fractured during these tests, which were made at 6 to 8 K. A comparison of the 293 to... 

The (relative) dielectric constant is the ratio of the capacitance of a condenser formed by two metal electrodes separated by a suitable layer of the material considered and the same separated by dry air. The dielectric strength measures the dielectric breakdown resistance of a material imder an applied voltage. The applied voltage value just before breakdown is divided by the specimen thickness. Thus, because the result depends on thickness, this value must be specified. The... 

On sheet samples, dielectric strength measurements can be used to spot poor dispersion, particularly of carbon black. The applied potential can be AC or DC, depending on anticipated service. With AC testing, fiequency influences the values obtained. In both cases, the potential may be steadily increased until failure occurs (the rapid rise method). Alternatively, the potential may be inereased in steps per UL and ASTM protocols. Rapid rise tests generally yield higher values of dielee-tric strength. 

Electrica.1 Properties. The bulk electrical properties of the parylenes make them excellent candidates for use in electronic constmction. The dielectric constants and dielectric losses are low and unaffected by absorption of atmospheric water. The dielectric strength is quoted for specimens of 25 p.m thickness because substantially thicker specimens cannot be prepared by VDP. If the value appears to be high in comparison with other materials, however, it should be noted that the usual thickness for such a measurement is 3.18 mm. Dielectric strength declines with the square root of increasing... 

Dielectric strength is measured by determining the minimum voltage which will puncture a sample of known thickness placed between electrodes of specified size and shape. Because both the magnitude and duration of the appHed voltage influence the results, this property can be measured in three ways. 

It has been shown that the polarizability of a substance containing no dipoles will indicate the strength o/any dispersive interactions that might take place with another molecule. In comparison, due to self-association or internal compensation that can take place with polar materials, the dipole moment determined from bulk dielectric constant measurements will often not give a true indication of the strength of any polar interaction that might take place with another molecule. An impression of a dipole-dipole interaction is depicted in Figure 11. 

The efficiency of transformer oils as dielectrics is measured by electric strength tests. These give an indication of the voltage at which, under the test conditions, the oil will break down. Various national standards exist that all measure the same basic property of the oil. There is an international specification, lEC 296/1982, which may be quoted by equipment manufacturers in their oil recommendations. 

The resistance of most plastics to the flow of direct current is very high. Both surface and volume electrical resistivities are important properties for applications of plastics insulating materials. The volume resistivity is the electrical resistance of the material measured in ohms as though the material was a conductor. Insulators will not sustain an indefinitely high voltage as the applied voltage is increased, a point is reached where a drastic decrease in resistance takes place accompanied by a physical breakdown of the insulator. This is known as the dielectric strength, which is the electric potential in volts, which would be necessary to cause the failure of a 1/8-in. thick insulator (Chapter 4, ELEC-TRICAL/ELECTR ONICS PRODUCT). 

What do resistivity, capacitance and dielectric strength properties depend on How do we measure each of these electrical properties ... 

Dielectric strength - The measure of a product s ability to resist passage of a disruptive discharge produced by an electric stress. 

Dielectric relaxation measurements for the adsorbed water have been reported by Jansen (44) the dielectric relaxation time is essentially 3r where r is the rotational jump time of the water molecule. From Figure 5 it can be seen that the dielectric and NMR mobility estimates agree rather well. All is not quite in order, however, for Jensen estimates from relaxation strength that he sees only one-third of the water molecules. 

Dielectric strength indicates the voltage gradient at which dielectric failure occurs is generally measured (ASTM D-149) at commercial power frequencies, and is... 

The latter are the dielectric disparities measures of the strength of dielectric images across the two interfaces. 

Dielectric strength. The electric stress, measured in volts, at which a given thickness of dielectric material breaks down and is punctured by an electric discharge. 

In order to predict absolute dielectric strengths we need to have more detailed information than is yet available about electronic states and mobilities in polymers. For the present we can only conclude that there is satisfactory agreement between the form of the theoretical results, based on a rather general electronic model, and the best experimental results. To the extent that the model is a very reasonable one, we can say that we can understand intrinsic breakdown behaviour. Measurement of pre-breakdown currents, especially with pointed electrodes which impose regions of very high field strength at their tips when embedded in the material, suggests that electronic carrier production either by injection from the electrodes (Schottky emission) or from impurities (Poole-Frenkel effect) may play a part in the breakdown process. More work is required, however, before this can be fully understood. 

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