Chemistry - Dielectric constant

Overview

Dielectrics are usually insulators. Examples include porcelain (ceramic), mica, glass, plastics, and the oxides of various metals. Some liquids and gases can serve as good dielectric materials. Dry air is an excellent dielectric, and is used in variable capacitors and some types of transmission lines. Distilled water is a fair dielectric.

Dielectrics have the property of making space seem bigger or smaller than it is dimensionally. For example, when you put a dialectric between two electric charges it reduces the force acting between them, just as if you had moved them apart. When an electromagnetic wave travels through a dielectric, the velocity of the wave will be slowed down. The wave will behave as if it had a shorter wavelength. The measurement of these effects is known as the dielectric constant.

Electrically the dielectric constant is a measure of the extent to which a substance concentrates the electrostatic lines of flux. More specifically it is the ratio of the amount of electrical energy stored in an insulator, when an electrical field is imposed across it, relative to a vacuum (which has a dielectric constant of 1). Thus, the dielectric constant is also known as a relative permittivity.

The dielectric constant ε_r is defined as the ratio:

$\epsilon_{r} = \frac{\epsilon}{\epsilon_{0}}$

where ε is the permittivity of the material in question, and ε₀ is the permittivity of free space. ε₀ is derived from Maxwell's Equations relating the electric field intensity E to the electric flux density D.

$\epsilon_{0} = \frac{1}{36\pi}\times10^{-9} = 8.85\times 10^{-12} C^{2} / Nm^{2}$

In vacuum (free space), the permittivity ε is just ε₀, so the dielectric constant is unity:

$\epsilon_{r} = \frac{\epsilon_{0}}{\epsilon_{0}} = 1$

Measurement

The relative dielectric constant ε_r can be measured for static electromagnetic fields as follows: First, the capacitance of a test capacitor C₀ is measured with air as dielectric. Then using the same capacitor with the same distance between the plates we measure its capacitance C_x using the a dielectric between the plates. The relative dielectric constant can be then calculated as:

$\epsilon_{r} = \frac{C_{x}} {C_{0}}$

For time-varying electromagnetic fields the dielectric constant of materials will be frequency dependant.

Applications

The dielectric constant is an essential piece of information when designing capacitors, and in other circumstances where a material might be expected to introduce capacitance into a circuit. If a material with a high dielectric constant is placed in an electric field, the magnitude of that field will be measurably reduced within the volume of the dielectric. This fact is commonly exploited to increase the rating of capacitors.

Dielectrics are used in RF transmission lines. In coax polyethylene can be used between the center conductor and outside shield. It can also be placed inside of waveguides to form dielectric waveguides. Dielectric waveguides are seldom used because the dielectric losses for all known dielectric materials are too great to transfer the electric and magnetic fields efficiently, however they can have specialty applications such as forming filters.

Optical fibers are purposely doped with impurities so as to control the precise value of ε_r within the cross-section. This will control the refractive index of the material and control the optical modes of transmission. Doped fiber can also be configured to form an optical amplifier.

Dielectrics are also used in Printed Wiring Boards, PWBs in layers beneath etched conductors.

Categories: Electricity