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Dielectric and Electrical Properties of Materials Prof. Graham Williams: representation can be converted into resonance (ESR) spectroscopy, any other e. g. Y=1/Z, M = 1/ε etc. quasi-elastic light scattering Dielectric and As a result, different approaches (QELS), Fourier transform infra-red electrical to the electrical/dielectric properties spectroscopy (FTIR) and of materials have been made over fluorescence depolarization (FD). properties of the years and their inter- These methods are often materials relationships have been neglected. complementary to DRS for studying Dielectrics Relaxation (using ε), physical processes in materials. The dielectric and electrical Impedance Spectrosocopy (using Z) Fortunately, the situation has properties of insulating and semi- and Electrical Relaxation changed dramatically during the conducting dielectric materials as Spectroscopy (using M) are all past ten years. The arrival of fast measured over wide ranges of inter-related since the experiments accurate impedance analyzers, frequency and temperature have actually measure Z (=1/Y) in all frequency response analyzers and been the subject of considerable cases. This is being appreciated interest in recent years. increasingly so we should accept new technologies In terms of the measurements, a that apparently-different researches now available sample in the frequency range 10-6 are actually inter-related and that to 108 Hz is regarded as a lumped- pulsed-transient equipment for the the electrical quantities used are a circuit element having a frequency- range 10-6 to 109 Hz and fast matter of choice, based on prior dependent complex impedance Z(f) accurate network analyzers and knowledge of the electrical- while at higher frequencies, ranging time-domain reflectometry equip- /dielectric phenomena that may be from the UHF through the ment for the range 108 to 1010 Hz involved in each case. microwave range to the far infra-red together with the computer control Having noted the different re- region, i. e. 108 to 1012 Hz a of measurement, data acquisition presentations that are presently used sample is regarded as a distributed and data-processing and computer for the electrical/dielectric circuit element having a frequency- control of sample temperature has properties of materials, we observe dependent complex propagation enabled DRS, IS and ERS to take that up to about ten years ago most coefficient γ(f). their rightful place alongside NMR, experimental studies of materials Experimental data, in terms of ESR, QELS and FD in the range of were made over limited frequency Z(f) of γ(f), covering the entire modern methods used to study the bands by a manual point-by-point frequency range 10-6 to 1012 Hz physical properties of materials. procedure. may be re-expressed in different Commercial equipment is now Thus, typically, using a low ways. Traditionally, different areas available which provides fast frequency bridge, measurements at of research have used different accurate measurements of the 16 frequencies equally-spaced in derived quantities as is shown in dielectric/electrical properties of the log f scale would take up to 30 table 1. 10 liquid and solid materials over a minutes to complete. While the Thus dielectric properties, range of prescribed temperatures measurements were accurate, the expressed in terms of ε(f), are used and frequencies, where the inability to obtain data quickly when studying molecular motion in measurements are made severely hampered the use of solid polymers while electrical automatically and the data are dielectric relaxation spectroscopy properties expressed in terms of processed on-line into any (DRS) or its equivalents impedance Z(f), M(f) or γ(f) are used when convenient form. spectroscopy (IS) and electrical studying conduction processes in Such equipment is provided, for relaxation spectroscopy (ERS). At ionic solids or liquid electrolytes, example, by the range of the same time rapid advances were etc. etc. It is important to stress that NOVOCONTROL dielectric being made in other physical the electrical/dielectrical properties spectrometers each working over a methods such as nuclear magnetic of a material can be expressed in wide frequency band i. e. 10-5 to resonance (NMR) relaxation 7 6 9 9 10 these different ways and one should 10 Hz, 10 -10 Hz and 10 - 10 spectroscopy, electron-spin remember that a particular Hz. 1 Dielectrics Newsletter march 1994 Measured Quantity Symbol Application Impedance Z = R +iX Conduction in ionic liquids and R = Resistance solids, polymer electrolytes, X = Reactance semiconductors Admittance Y = G + iX' As for impedance G = Conductance X' = inverse reactance Permittivity ε = ε' - iε'' Dielectric relaxation in dipolar ε' = real permittivity liquids, liquid crystals and solids, ε''= loss factor polymers and glass forming liquids Dielectric Modules M = M' + iM'' Electrical relaxation in ionic liquids and solids, electrolytes, molten salts, semiconductors Electrical Conductivity σ = σ' + iσ'' Conduction in ionic liquids and σ' = real conductivity solids, semiconductors Complex Propagation γ = α + iβ Dielectric and far infra-red Coefficient at wavelength λo, α = attenuation factor absorption in dipolar liquids and β π λ solids = 2 n'/ o Complex refractive index n n = n'+ in'' Table 1 The availability of such modern continual sweeping and recording liquids, molten salts and glassy equipment makes possible high the time of each measurement, ionic solids. quality research into the yielding ε'(f,t) and ε''(f,t) throughout 2. Liquid-crystalline Materials dielectrical/electrical properties of a the time-dependent process. Post- Low molar mass and polymeric wide range of organic and inorganic processing of data allows (ε',ε'') thermotropic liquid crystals materials - as an extension of earlier values to be determined as a including nematic, smectic, work - and of new functional function of frequency at given times chiral-nematic and ferroelectric materials that are of importance in t. Such studies open up an entirely materials, lytropic liquid crystals relation to their electrical, dielectric, new domain of application of including rod-like polymers or electro-optical and conduction modern dielectric-relaxation surfactants with solvents, hybrid properties. spectroscopy, especially for liquid-crystalline polymer net- In addition, measurements are polymerizing organic systems. works, polymer-dispersed liquid now possible, through the speed Similar time-dependent crystals. and practical convenience of the dielectric/electrical phenomena may modern techniques, with systems be envisaged for inorganic materials 3. Crystalline Material that were not feasible using the (e. g. the dynamics of phase- Ionic crystals as insulators, traditional manual point-by-point transformations in inorganic solids). semiconductors and electrolytes, methods. Table 2 shows examples of molecular crystals including materials and t-dependent system rotator-phase crystals and real-time dielectric that are being studied using modern crystalline polymers, ferro- spectroscopy techniques: electric crystals, crystalline semi- In particular, the techniques of real- conductors and photo- time dielectric spectroscopy is 1. Amorphous Organic And conductors. Inorganic Materials emerging as a means of studying the 4. Time-dependent Systems Molecular liquids including dielectric/electrical behaviour of Systems undergoing bulk poly- glass-forming liquids, materials whose physical or merization e. g. thermosets, bulk amorphous polymers, chemical properties are changing addition polymerization by ther- semiconductors and photo- with time. In this case the dielectric mal or photochemical initiation, conductors, liquid, solid and permittivity ε(f) is determined systems undergoing crystal- polymer electrolytes, ionic across a frequency range by lization e. g. crystallizing 2 Dielectrics Newsletter march 1994 polymers, systems undergoing using the traditional point-by-point Prof. Friedrich Kremer: phase separation e.g. polymer method of measurement. His former mixtures at elevated tem- supervisor, Professor Mansel Dielectric peratures. Davies, remarked in the early 1960's spectroscopy: that any expansion of dielectrics Old spectroscopic research would only occur when Particular interest in the future is commercial dielectric measuring technique gains likely to be given to the dielectric assemblies became available. and electro-optical properties of new actuality It has taken over 30 years for that liquid crystals, novel hybrid organic to be achieved and we can now say Dielectric spectroscopy is an old liquid-crystal-polymer-networks with complete confidence that the spectroscopic technique, which has and polymer-dispersed liquid modern broadband dielectric roots going back to the last century. crystalline display materials spectrometers will enable research Claurius, Mosotti, Lorenz, Maxwell what comes next ? into the dielectric and conduction - just to mention some of the properties of materials of all kinds pioneers - made important where the dielectric properties will to be conducted quickly, accurately contributions to the understanding vary with an applied AC or DC and conveniently at the highest of electrical and dielectric biasing field. Such behaviour has professional level to yield infor- phenomena. been studied extensively by Kremer mation that is crucial to our under- In the second decade of this (Mainz / Leipzig) for ferroelectric standing of polarization and charge century Debye formulated the crystals and has given important transport processes in both theory of
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