Dielectrics

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 (Dielectric Constant) 

Air, Vacuum, Free-space

1
Transformer Oil 2.2
Foam polyethylene (RG-58/U) 1.30
Foam polyethylene (RG-58/AU) 1.37
Foam polyethylene (RG-8/U) 1.16
Foam polyethylene (RG-59/U) 1.20
Foam polyethylene (RG-11/U) 1.20
Polyethylene (RG-174/U) 1.52

Si - Silicon

11.8
Ge - Germanium 16.0
GaAs - Gallium Arsenide 13.1
C - Diamond 5.5
SiC - Silicon Carbide 10.0
Aluminum Oxide 8.80@60Hz, 8.80@1MHz
Barium Titanate 1250@60Hz, 1142@1MHz
Soda-Borosilicate Glass 4.97@60Hz, 4.84@1MHz
Epoxy (Epon RN-48) 4.50@60Hz, 3.52@1MHz
Polycarbonate 3.17@60Hz, 2.96@1MHz
Acrylic 4.0@60Hz, 2.55@1MHz
Polyimide 3.4@60Hz, 3.4@1MHz
Polyvinyl Chloride 3.20@60Hz, 2.88@1MHz
PTFE (Teflon) 2.10@60Hz, 2.10@1MHz
Nylon  8@60Hz, 3.40@1MHz
Polyethylene 2.26@60Hz, 2.26@1MHz
Polypropylene 2.55@60Hz, 2.55@1MHz
Paper 3.30@60Hz, 2.99@1MHz
Water [0C] 87.90
Water [10C] 83.96
Water [20C] 80.20
Water [30C] 76.60
Water [40C] 73.17
Water [50C] 69.88
Water [60C] 66.73
Water [70C] 63.73
Water [80C] 60.86
Water [90C] 58.12
Water [100C] 55.51
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Notes:

1.  Source:  various.
2.  Unless specified, the frequency is probably between DC and 1MHz and the temperature is room temperature.
3.  Dielectric constant = relative permeability.
4.  Index of refraction is always the square root of the dielectric constant at a particular frequency.  However, the term "dielectric constant" usually refers to sub-GHz frequency and "index of refraction" usually refers to visible light frequencies.
5.  The Debye formula can be used to calculate the dielectric constant of water over a wide range of frequencies.