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10.1: Permittivity of Some Common Materials

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  • 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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