Practical Coilgun Design

Insulating Materials

The larger the plate area and the smaller the spacing between the plates, the greater the capacitance. The capacitance also depends on the kind of insulating material between the plates; it is smallest with air insulation. Substituting other insulating materials for air may increase the capacitance many times.

The ratio of the capacitance with some material other than air between the plates, to the capacitance with air between the plates, is called the dielectric constant of the insulating material. The material itself is called a dielectric.

The dielectric constants of a number of common materials in capacitors are shown in this table. For example, if a sheet of polystyrene is substituted for air between the plates of a capacitor, the capacitance will be increased 2.6 times.

Material Dielectric Constant* Puncture Voltage**
Air 1.0 21
Alsimag 196 5.7 240
Bakelite 4.4 - 5.4 300
Bakelite, mica filled 4.7 325-375
Cellulose acetate 3.3 - 3.9 250 - 600
Fiber 5 - 7.5 150 - 180
Formica 4.6 - 4.9 450
Glass, window 7.6 - 8 200 - 250
Glass, Pyrex 4.8 335
Mica, ruby 5.4 3800 - 5600
Mycalex 7.4 250
Paper, Royalgrey 3.0 200
Plexiglass 2.8 990
Polyethylene 2.3 1200
Polystyrene 2.6 500 - 700
Porcelain 5.1 - 5.9 40 - 100
Quartz, fused 3.8 1000
Steatite, low loss 5.8 150 - 315
Teflon 2.1 1000 - 2000
  *At 1MHz **In volts/mil (0.001 inch)
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