Volume resistivities

The volume resistivity of a material is its resistance to the flow of electrical current. Specifically it is the ratio of the potential gradient in the direction of the current to the current density. It is dependent upon moisture content and temperature and the values given in Table 4.3 are typical room temperature values. 

ASTM D2739-97 Standard test method for volume resistivity of conductive adhesives. 

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Electrical Properties. The low polarizabiHty of perfluorinated Hquids makes them exceUent insulators. Theh dielectric strengths are about 40 kV (ASTM D877) dissipation factors are about 0.0001 at 1 MH2 dielectric constants are about 1.8 volume resistivities are about 1 x 10 ohm-cm (ASTM D257) (17). 

Electrical Properties. Polytetrafluoroethylene is an excellent electrical insulator because of its mechanical strength and chemical and thermal stabihty as well as excellent electrical properties (Table 6). It does not absorb water and volume resistivity remains unchanged even after prolonged soaking. The dielectric constant remains constant at 2.1 for a temperature range of —40 to 250°C and a frequency range of 5 Hz to 10 GHz. 

Electrica.1 Properties. Because of excellent electrical properties, FEP is a valuable and versatile electrical insulator. Within the recommended service temperature range, PTFE and EEP have identical properties as electrical insulators. Volume resistivity, which is >10 H/cm, remains unchanged even after prolonged soaking in water surface resistivity is >10 H/sq. 

Electrically conductive mbber (13) can be achieved by incorporation of conductive fillers, eg, use of carbon or metal powders. These mbbers exhibit volume resistivities as low as lO " H-cm. Apphcations include use in dissipation of static charge and in conductive bridging between dissimilar electronic materials under harsh operating conditions. 

The resistance of a segment of the path described above is proportional to its length, iaversely proportional to its cross-sectional area, and proportional to a specific property of the material of the segment called resistivity or volume resistivity, ie ... 

The cables designed for use at voltages over 49 kV require that the conductor and insulation shields be firmly bonded to the insulation in order to avoid any possibiUty of generating corona at interfaces strippable insulation shields are not accepted. The A ETC specifications for cables rated for 59—138 kV require a volume resistivity of one order of magnitude lower than for the medium voltage cables. 

To minimize electrical losses, especially at high frequencies, a low dissipation factor is required. High volume resistance provides good insulation to prevent... 

Electromagnetic interference (EMI) testing has become more prevalent for materials that either emit or are affected by EMI. Shielding efficiency (SE) of materials is deterrnined by measuring electric field strength between a transmitter and receiver with or without the presence of the material under test. Several researchers have suggested a correlation between volume resistivity and SE values (300,301). 

Electrical, electronic, and technical appHcations use polycarbonates for a variety of purposes. The woddwide market is about 156,000 t aimuaHy. Because of exceHent electrical properties (dielectric strength, volume resistivity), and resistance to heat and humidity, polycarbonate is used for electrical connectors (qv), telephone network devices, oudet boxes, etc. Polycarbonate had been popular for use in computer and business machine housings, but the use of neat resin has been largely supplanted by blends of polycarbonate with ABS. OveraH, however, the total use of polycarbonate continues to increase. 

Vulcani2ed mbber is an insulator (volume resistivity is 10 Q-cm), and the static generated by mbber tires created serious problems in vehicles until the introduction of electrically conductive carbon black as a reinforcing pigment. An excellent correlation was found between the potential generated and the resistivity of the tires (127,128) (see Rubber natural). 

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