SWAG #2: non-uniformities in materials + multiple breakdown mechanisms make it difficult to model. Mica shows up in 1940's vintage literature. Here are some interesting papers that I perused after Rich posed the question:
https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9079498 Space change behavior in cross-linked polymers https://onlinelibrary.wiley.com/doi/10.1002/app.49379 Touches on different breakdown mechanisms (see Figure 3; also the Figure 2 reference may be worthwhile but I haven't tried accessing it) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7764431/ electron injection and avalanche breakdown process https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=65f577afe99e3253e7e3f38054ce9ea49b16a636 Electromechanical breakdown mechanism but also states "The exact cause for the observed behavior remains to be investigated" https://apps.dtic.mil/sti/pdfs/ADA635433.pdf Paschen Curve anomalies (for consideration of gas dielectrics) https://www.osti.gov/servlets/purl/1656858 Describes influence of polymer chain ends (Figure 1 is a good illustration) Cheers, Adam in Atlanta adam.di...@ieee.org On Mon, Jun 24, 2024 at 11:00 PM Patrick <conwa...@gmail.com> wrote: > just a SWAG... perhaps dielectric strength is dependent on volume, and > the increased 'thickness' is assumed to be > thickness-for-a-constant-surface-area. If that's the case then an > increasing thickness is also an increased volume which also increases > available charge carriers, reducing breakdown voltage. > > A test of my SWAG would be to incrementally increase dielectric thickness > and determine if breakdown voltage eventually finds a minimum and then > begins to increase with thickness. > > interesting question. > > On Sun, Jun 23, 2024, 13:32 Richard Nute <ri...@ieee.org> wrote: > >> >> Why does air (or any insulating material) have decreasing dielectric >> strength, kV/mm, with increasing distance through the dielectric >> substance? Assume homogenous field. (I have assumed the dielectric >> strength was constant for the material.) In other words, what is the >> physical basis for the non-constant dielectric strength clearance tables >> in various safety standards? (I have yet to find the answer from the >> web.) How can I predict the dielectric constant for a given distance >> through air (or any insulation)? >> >> Charles J. Fraser, in Mechanical Engineer's Reference Book (Twelfth >> Edition) >> <https://www.sciencedirect.com/book/9780750611954/mechanical-engineers-reference-book>, >> 1994: >> >> If the potential difference across opposite faces of a dielectric >> material is increased above a particular value, the material breaks down. >> The failure of the material takes the form of a small puncture, which >> renders the material useless as an insulator. The *potential gradient* >> <https://www.sciencedirect.com/topics/engineering/potential-gradient> >> necessary >> to cause break down is normally expressed in kilovolts/millimetre and is >> termed the ‘dielectric strength’. The dielectric strength of a given >> material decreases with increases in the thickness. Table 2.2 gives >> approximate values for some of the more common dielectric materials. >> >> Table 2.2. Dielectric strength of some common insulators >> >> *Material* * Thickness (mm)* * Dielectric strength (kV/mm)* >> Air 0.2 5.75 >> 0.6 4.92 >> 1.0 4.36 >> 10.0 2.98 >> Mica 0.01 200 >> 0.10 115 >> 1.00 61 >> >> Thanks, and best regards, >> >> Rich >> >> >> ------------------------------ >> >> This message is from the IEEE Product Safety Engineering Society emc-pstc >> discussion list. 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