Re: dielectric material
Bismuth is not a good choice as a dielectric. It is a low melting point
metal and conducts electricity.
The best choices for capacitors in TC work are: Mica, Teflon and
Polyethylene. Cost effective choices are: Polyethylene, Polystyrene,
Polypropylene, Quartz Glass, Mineral oil, Mineral wax, etc.
The hands-down winner, would probably be mica (if you are just
interested from an academic point of view). It has the highest puncture
voltage, with one of the highest dielectric constants, and has very low
When you say "the best dielectric" it has to be qualified with what you
intend to do with it. Capacitor dielectrics are what most folks on the
list are talking about. There you want good insulation resistance, high
dielectric constant, high puncture voltage, and low dissipation factor.
If you just need to insulate against high voltage flash over, or
puncture, glazed ceramic or glass are good choices.