Stefan Wolfe wrote:

I think that when considering plastic and mica type materials for high
capacitance values, component engineers do not necessarily look at
dielectric constant as the most important property. Mylar, mica and
polyethylene are common capacitor materials and have rather low dielctic
constants just a little higher than teflon. What makes them most attractive
is their ability to be manufactured in very thin sheets (increased C) and
their high dielectric strengths (increased V ratings). Teflon is higher cost
but is very good for high voltage ratings due to superior dielectric
strength.

Aluminum and tantalum oxides tend to have very high dielectric constants due
to the capability of these metal oxide molecules to store electrons. What is
more desirable however is their property of being very thin. What makes them
a problem is their relatively low dielectric strengths thus their low
voltage ratings.

Actually, there are a lot more considerations yet. Some plastics are
self-healing, so a momentary arc won't permanently destroy the
capacitor. And some have exceptionally low leakage current. Various
materials also have widely different temperature coefficients. In
general, the very high k (dielectric constant) ceramics have higher
temperature coefficients than lower k materials. Some ceramics, like the
very high k ceramics used for Z5U and similar capacitors, are also
hygroscopic, microphonic, piezoelectric, and their k varies with
frequency and voltage. Many capacitors, depending largely on the
dielectric, also have nonlinear properties such as "soak" (dielectric
absorption) and "hook". Loss, expressed as loss tangent, ESR, or power
factor, is also often an important consideration, and it can be very
different for different dielectrics. Choosing the right capacitor for a
particular job can be pretty demanding. This one, fortunately, is easier
than some.

Roy Lewallen, W7EL