Liquids, dielectric breakdown

Irradiated liquids do possess electrical conductivity, clearly indicating the existence of ionic species as in gaseous systems. Some results for cyclohexane23 are shown in Fig. 2. Dielectric breakdown of the liquid occurs, however, before a saturation value of the current is reached. 

The dielectric properties of oligodimethylsiloxanes and their dependence on temperature point to good dielectric characteristics of PMS liquids. Taking into consideration that PMS do not form conductive carbon particles in case of electric breakdown or sparking, it is obvious why they are used as liquid dielectrics in transformers and other electric devices. There is a law in Japan and the USA which forbids the use of nonflammable yet toxic pentachlorodiphenyl and leaves room for oligodimethylsiloxanes. This has stimulated a much more active production of PMS liquids in Japan, Germany and other countries. 

Fig. 2.70. (a) The field between the plates is notmal and the water dielectric is chemically stable. Its property as an insulator depends on the concentration of ions but it is highly resistive, (b) When the field applied is sufficiently high, dielectric breakdown occurs the liquid no longer supports the field and the charges flow away, forming streamers. ... 

On the other hand, most chemists and physicists who have discussed this phenomenon in the 1980s and 1990s observed that the streamers come from the electrode and that light is emitted from the electrode. The interfacial region undoubtedly plays the determinative role in the dielectric breakdown of liquids. 

SURFACE TENSION AS A GUIDE TO THE PREDICTION OF BREAKDOWN IN LIQUID DIELECTRICS. 

Like the ferroelectric liquid crystal polymers,the co-polymers of the PVDF type require poling. By contrast, the polarization of an LB film is solidly built into the structure, and is only removed,if fields in excess of the dielectric breakdown field are applied. The main practical disadvantage associated with VF2/VF3 co-polymers is their microphonic activity. Fortunately, the LB films studied to date, with the exception of the azobenzene compounds, have piezoelectric coefficients low enough to be undetectable. 

ASTM Standard D877-G7, "Dielectric Breakdown of Insulating Liquids Using Disk Electrodes," in 1975 Book of ASTM Standards Part 40, American Society for Testing and Materials, Philadelphia, Pennsylvania (1975), p. 168. 

Electric-Field-Induced Orientation. Despite the fact that electric field orientation of liquid crystals is an important technology used, for example, in liquid crystal displays, there has been almost no work on the electric field alignment of LCTs. This may be because electric fields are effective only for thin films the high field strengths required, on the order of 10 V/cm or greater, can lead to dielectric breakdown in thick samples. Nevertheless, although bulk orientation cannot be obtained via electric fields, use of electric fields does provide a complement to the surface-induced techniques described above. 

Reducing the size of the macropores in the PZT microstructure by pressure enhanced liquid-phase sintering improves the dielectric breakdown strength of the PZT however, presumably due to the minor concentration of non-PZT phase present in the microstructure, the piezoelectric properties are not deleteri-ously affected. 

Arii, K., Hayashi, K., Kitani, L, and Inuishi, Y., Breakdown time lag and time of flight measurement in liquid dielectrics. V, Int. Conf. Cond. Breakdown Did. Liqu., Noordwijk-erhout, Goldschvartz, J. M., Niessen, A. K., and Boone, W., Eds., Delft University Press, Delft, 1975. 

Brueck, S. R. J. and Kildal, H., Laser-induced dielectric breakdown in cryogenic liquids, /. Appl Phys., 52, 1004, 1981. 

Yoshino, K., Ohseko, K., Shiraishi, M., Terauchi, M., and Inuishi, Y, Dielectric breakdown of cryogenic liquids in terms of pressure, polarity, pulse width and impurity, J. Electrostatics, 12, 305, 1982. 

---