Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 35

Volume resistivity is a bulk material property based on the resistance of a cube of material with sides of unit length, with a voltage applied to two opposing faces of the cube (Figure 1.3).

The volume resistance R_v measured through a material of volume resistivity ρ_v using electrodes of area A is given by

where t = A = 1, or t/A = 1, which reduces to ρ_v = R_v.

The unit of volume resistivity is ohm meter (Ωm). The volume resistivity of a material is often simply referred to as its resistivity.

In practice, standards exist for measuring volume resistivity using concentric ring electrodes (IEC 62631‐3‐1 (International Electrotechnical Commission 2016c), IEC 61340‐2‐3 (International Electrotechnical Commission 2016b), ANSI/ESD STM 11.12 (EOS/ESD Association Inc. 2015b))

Figure 1.3 Definition of volume resistivity.

Volume resistance, R_v, is a resistance measured between opposing faces of a material. The electrodes may be of any convenient form. Often this measurement is made using electrodes designed for volume resistivity measurement so that conversion from volume resistance to volume resistivity is a simple calculation. In ESD work, conversion to volume resistivity is often not needed. The volume resistance obtained with defined standard electrodes is used directly, saving the effort of calculation. Examples of surface and volume resistance measurement methods are given in Chapter 11.

The conductivity, σ, of the material is simply the inverse of its resistivity.

The units of conductivity are siemens per meter (Sm⁻¹).

The resistivity of materials can vary by many orders of magnitude from 10⁻⁸ Ωm (e.g. copper) to more than 10¹⁵ Ωm (e.g. mica, quartz, polytetrafluoroethylene, polyethylene).