Dielectrics, thick-film dielectric strength

Material Thickness of film plus adhesive (mm) Tensile strength (kg/cm width) Elongation at break (%) Moisture absorp- tion (%) Moisture- vapour transmission rate (g/m per 24 h Dielectric strength (V)... 

The importance of developing pinhole-free, electrolyte films of nanometer thickness is potentially useful for all 3-D battery designs. For this reason, most of this section reviews the synthesis and characterization of this ultrathin polymer electrolyte with an emphasis on topics such as leakage currents and dielectric strength, which become critically important at the nanoscale. A few comments concerning the packaging of 3-D batteries are made at the end. 

Figure 12. Dielectric strength of the normal mode versus film thickness at 332.5 K for an Mw of 52, 000 g/mol. Inset Dielectric loss s" versus frequency at 330 K showing the normal mode of thin PIP films (Mw — 52,000 g/mol) of different thicknesses, as indicated. The dotted line indicates the position of the peak maxima.

Figure 27. Dielectric strength of the alpha relaxation process of POHOAc at 500 K as a function of film thickness. Inset Refractive index of POHOAc at X — 630 nm in dependence on the film thickness.

Dielectric Breakdown Measurements. The dielectric breakdown strength was measured on the freely standing BCB-2 films as described above. The breakdown strength was 2.5 x 10 v/cm for 22 mm thick films. This value is -50% of the breakdown strength measured for 1.5 mm polyimide films on Si (8). However, the breakdown strength does not vary proportionally to the film thickness. The breakdown strength was also measured for 1.7 mm BCB-2 films deposited on a Si substrate. The value obtained from this measurement was 4.0 x 106 v/cm. The latter value was much closer to breakdown strengths observed in polyimide films of comparable thickness. 

The reliability of dynamic film thickness determinations by capacitance measurements is an open question, principally because there are no absolute standards by which to judge. In their investigation of elastohydrodynamic lubrication, Archard and Kirk [6] determined film thicknesses over the range 20 to 1000 nm the uncertainty of their determinations ranged from 20 nm for films of the order of 20-50 nm thick to 250 nm for films 750-1000 nm thick. Crook s comparisons of film thicknesses determined by his two capacitance methods indicated an uncertainty of 200 nm [8]. Orcutt reported that the voltages required for his capacitance measurements exceeded the dielectric strength of the oil film for thicknesses less than 150 nm [10]. 

The short-term dielectric strength of PFA is 80 kV/mm at a thickness of 0.25 mm, measured by ASTM D149. FEP films give similar results while PTFE has at5q)ical dielectric strength of 47 kV/mm. The dielectric strength of PFA decreases in the presence of corona discharge, a behavior similar to other fluoropolymers. 

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