Impedance spectroscopy

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The impedance spectroscopy is to determine the alternating current resistance, and impedance is called, depending on the frequency of the alternating current . For this purpose, the impedance is determined at several frequencies over a defined frequency range ( spectrum ).

Impedance spectroscopy has several different main areas of application: It is used in physics and materials science to study materials, e.g. B. used by solids such as ion conductors or plastics , and by conductivity mechanisms; then it is also called dielectric spectroscopy . Another large area of ​​application is electrochemistry , which makes impedance spectroscopy z. B. is used to examine batteries and fuel cells as well as corrosion processes , see electrochemical impedance spectroscopy . The electrical uses corresponding spectra - the here frequency response of the impedance are called - for the investigation and characterization of electrical and electronic components and assemblies such. B. of conductors , resistors , capacitors , coils and their combinations.

Differences and similarities between different types of impedance spectroscopy

The dielectric spectroscopy , the electrochemical impedance spectroscopy , and an experimental determination of the frequency response of the impedance in the electronics are closely related measurement methods. They all have the advantage that complete and also closed systems (such as batteries in their steel housing) can be characterized quickly and non-destructively. In any case, the spectrum obtained can be presented as a Bode diagram or a Nyquist diagram . The differences lie primarily in the systems examined. Electrochemical impedance spectroscopy considers systems in which (electro) chemical reactions can occur and in which the transport of substances plays an essential role. The material conversions of the (electro) chemical processes can be endothermic or exothermic, which also means that an electrochemical cell generally develops a DC voltage. In dielectric spectroscopy, there is typically no DC voltage component, and higher AC voltages are used. While the electrochemical investigations focus on mass transfer and reactions, dielectric spectroscopy, for example, investigates the relaxation mechanisms in solids. One application of dielectric spectroscopy in biology is the examination of tissues and cell suspensions. It can be used in biotechnology to monitor fermentation processes.

Using impedance spectroscopy, frequency-dependent system phenomena can be analyzed without having to take measurements inside the system.

application areas

Electrochemical Impedance Spectroscopy :

Dielectric Spectroscopy:

Presentation, evaluation and interpretation of the measurement results

The complex alternating current calculation is used to evaluate the impedance spectra .

The impedance spectrum describes the transfer function of the system and can be represented as a function of the frequency ( Bode diagram ) or as a locus curve ( Nyquist diagram , also known as the Cole-Cole diagram when showing the permittivity ). Since this mainly occurs with capacitance and less often with inductance , the negative imaginary axis is usually plotted upwards. If typical curve courses are known for certain states in a system, a graphic evaluation of the diagrams is often already possible.

If the graphic interpretation of the impedance spectrum (for example in the Nyquist diagram) is not sufficient, an equivalent circuit diagram of the system to be examined can be created for a more detailed analysis . The equivalent circuit depicts the assumed chemical and physical processes relevant for the investigation. For example, a capacitor can represent a possibly present electrochemical double layer . In addition to the impedances common in electrical engineering ( resistances , capacitances and inductances), other phenomena can also occur, for example diffusion processes in electrochemical systems. Additional elements such as the Warburg impedance or the Nernst impedance are used to depict diffusion phenomena in the model .

The parameters of the equivalent circuit diagram can be adapted to the measured values with an adjustment calculation. For this calculation, there is software specially tailored to the issues of impedance spectroscopy, which adapts the parameters using methods of non-linear optimization. The parameters of the adapted model or their change between different operating states allow an interpretation of states and processes in the system.

The impedance spectra can be checked using the Kramers-Kronig relationships . The ZHIT algorithm or the test proposed by Boukamp with linear adaptation and transformation can be used for this.

Web links

Individual evidence

  1. ^ Vadim F. Lvovich: Impedance Spectroscopy: Applications to Electrochemical and Dielectric Phenomena . John Wiley & Sons, Hoboken, New Jersey 2012, ISBN 978-0-470-62778-5 , Chapter 1: Fundamentals of Electrochemical Impedance Spectroscopy , pp. 1-21 .
  2. ↑ Koji Asami: Characterization of biological cells by dielectric spectroscopy . Section 5. Dielectric spectroscopy of biological materials. In: Journal of Non-Crystalline Solids . tape 305 , no. 1-3 , July 2002, ISSN  0022-3093 , pp. 268-277 , doi : 10.1016 / S0022-3093 (02) 01110-9 .
  3. ↑ Bernard A. Boukamp: A Linear Kronig-Kramer's Transform Test for Immittance Data Validation . In: Journal of The Electrochemical Society . tape 142 , no. 6 , June 1995, ISSN  1945-7111 , p. 1885-1894 , doi : 10.1149 / 1.2044210 .