Dielectric spectroscopy measures the dielectric properties of a medium as a function of frequency. It is based on the interaction of an external field with the electric dipole moment of the sample, often expressed by permittivity. It is also an experimental method of characterizing electrochemical systems. This technique measures the impedance of a system over a range of frequencies, and therefore the frequency response of the system, including the energy storage and dissipation properties, is revealed. Often, data obtained by electrochemical impedance spectroscopy (EIS) is expressed graphically in a Bode plot or a Nyquist plot. Impedance is the opposition to the flow of alternating current (AC) in a complex system. A passive complex electrical system comprises both energy dissipater (resistor) and energy storage (capacitor) elements. If the system is purely resistive, then the opposition to AC or direct current (DC) is simply resistance. Almost any physico-chemical system, such as electrochemical cells, mass-beam oscillators, and even biological tissue possesses energy storage and dissipation properties. EIS examines them.This technique has grown tremendously in stature over the past few years and is now being widely employed in a wide variety of scientific fields such as fuel cell testing, biomolecular interaction, and microstructural characterization. Often, EIS reveals information about the reaction mechanism of an electrochemical process: different reaction steps will dominate at certain frequencies, and the frequency response shown by EIS can help identify the rate limiting step.
The Electrical Characterization Group uses the Agilent 4284A LCR Bridge (20Hz – 1MHz) and the Novocontrol Novocool Alpha analyser system (3mHz – 10MHz) (170K- 520K) in order to perform dielectric spectroscopy measurements aiming at defining the microstructure physical and electrical properties of materials and devices. The Group conducts measurements using the dielectric spectroscopy in the frequency and time domain (method of isothermal charging and discharging following a stepwise driving voltage). The technique of dielectric spectroscopy is applied to a number of materials and devices such as:
- Ceramic materials used for the production of varistors
- Metal-Insulator-Semiconductor (MIS) structures
- Metal-Insulator-Metal (MIM) structures
- geomaterials and cement-based materials.
This technique is also used as a tool for non-destructive testing.