11.1: Permittivity of Some Common Materials

The values below are relative permittivity \(\epsilon_r \triangleq \epsilon/\epsilon_0\) for a few materials that are commonly encountered in electrical engineering applications, and for which permittivity emerges as a consideration. Note that “relative permittivity” is sometimes referred to as dielectric constant.

Here we consider only the physical (real-valued) permittivity, which is the real part of the complex permittivity (typically indicated as \(\epsilon'\) or \(\epsilon_r'\)) for materials exhibiting significant loss.

Permittivity varies significantly as a function of frequency. The values below are representative of frequencies from a few kHz to about 1 GHz. The values given are also representative of optical frequencies for materials such as silica that are used in optical applications. Permittivity also varies as a function of temperature. In applications where precision better than about 10% is required, primary references accounting for frequency and temperature should be consulted. The values presented here are gathered from a variety of references, including those indicated in “Additional References.”

Free Space (vacuum): \(\epsilon_r \triangleq 1\)

Table \(\PageIndex{1}\): Solid Dielectrics:

Material	\(\epsilon_r\)	Common uses
Styrofoam\(^1\)	1.1
Teflon\(^2\)	2.1
Polyethylene	2.3	coaxial cable
Polypropylene	2.3
Silica	2.4	optical fiber\(^3\)
Polystyrene	2.6
Polycarbonate	2.8
Rogers RO3003	3.0	PCB substrate
FR4 (glass epoxy laminate)	4.5	PCB substrate

\(^1\) Properly known as extruded polystyrene foam (XPS).
\(^2\) Properly known as polytetrafluoroethylene (PTFE).
\(^3\) Typically doped with small amounts of other materials to slightly raise or lower the index of refraction (\(=\sqrt{\epsilon_r}\)).

Non-conducting spacing materials used in discrete capacitors exhibit \(\epsilon_r\) ranging from about 5 to 50.

• Semiconductors commonly appearing in electronics – including carbon, silicon, geranium, indium phosphide, and so on – typically exhibit \(\epsilon_r\) in the range 5–15.
• Glass exhibits \(\epsilon_r\) in the range 4–10, depending on composition.
• Gasses, including air, typically exhibit \(\epsilon_r\cong 1\) to within a tiny fraction of a percent.
• Liquid water typically exhibits \(\epsilon_r\) in the range 72–81. Distilled water exhibits \(\epsilon_r \approx 81\) at room temperature, whereas sea water tends to be at the lower end of the range.
• Other liquids typically exhibit \(\epsilon_r\) in the range 10–90, with considerable variation as a function of temperature and frequency. Animal flesh and blood consists primarily of liquid matter and so also exhibits permittivity in this range.
• Soil typically exhibits \(\epsilon_r\) in the range 2.5–3.5 when dry and higher when wet. The permittivity of soil varies considerably depending on composition.

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