Dielectric Measurement Techniques

The microdielectrometer has been used to monitor polymer curing in-situ 22.23. The CFT device can make measurements at lower frequencies than could be achieved by conventional dielectric measurement techniques. Measurements at multiple frequencies can be made in real-time. A Fourier transform equivalent of the microdielectrometer has been developed to extend the frequency range to as low as 0.005 Hz 24. 

The chapter is organized as follows. In the first part broadband dielectric spectroscopy is introduced. A brief review of the theoretical background of dielectric spectroscopy is provided. This will include the basics of dielectric spectroscopy, dielectric measurement techniques, and also data analysis. This section is followed by a discussion of the dielectric behavior of homopolymers to provide the basics to understand the dielectric properties of blends. In the second part of the chapter, the dielectric behavior of polymeric blends is reviewed where both miscible and immiscible systems are discussed. The chapter is restricted to binary systems. 

F. Kremer, A. Schonhals, Broadband dielectric measurement techniques (Chapter 2), in Broadband Dielectric Spectroscopy, ed. by F. Kremer, A. Schdnhals (Springer, Berlin, 2003b), pp. 35-58... 

We assume that if many of the liquids of interest, such as propylene carbonate, were studied by higher frequency (measurement techniques, new, high frequency components would be discovered which would account at least partially for the short time scale dynamics we see in the solvation C(f) data. Indeed, the apparent observation of a single Debye time is inconsistent with theories of liquids that take into account dipole-dipole interactions (see Kivelson [109]). Furthermore, some of the liquids studied have extraordinarily large apparent infinite frequency dielectric constants (e.g., = 10... 

Other important works containing copious references on dielectric theory, measurement techniques and data tabulation have been published. Pioneering work was done by von Hippel (1954 a b and c). Buckley and Maryott (1958) have tabulated data on liquids. Nelson (1991) and with Trnga (Tinga and Nelson 1973) and El-Rays and Ulaby (1987) have tabulated dielectric information on agricultural as well as other materials. Ohlsson and Bengtsson (1975) and Kent (1987) have published data on foods. 

Dielectric measurements have been established for nearly three decades as a technique for monitoring the cure of polymeric resins. Dramatic changes in the dielectric properties of the material accompany the transformation of the resin from a viscous liquid to a solid. 

This article give an overview about the microwave properties of insulating and superconducting oxide dielectrics. In addition, microwave measurement techniques for bulk and thin film oxides will be reviewed. 

The microwave properties of oxide based dielectric bulk material, thin film nonlinear dielectric materials and oxide high temperature superconducting materials were reviewed in this article. In addition, the most important microwave measurement techniques have been discussed. Important future directions of related material research aiming towards further integration both on chip and subsystem level, increase of performance and cost reduction are ... 

Polymer properties are very often dependent on the polymer preparation. So, a good monitoring of the polymerization process is the key step to obtaining good and reproducible materials. The extent of the polymerization can be controlled in different ways. IR is the most usual [27,30] but is not very accurate and requires the extraction of samples to analyze. Recently, an in situ monitoring of PMR-15 processing has been provided by means of frequency-dependent dielectric measurements [33,34]. This non-destructive technique allows the characterization of all the steps of the curing process and thus they can be optimized. 

The effects of temperature and frequency on the permittivity and dissipation factor of a high-purity alumina ceramic are shown in Fig. 5.24. The discrepancies between the permittivity levels in Fig. 5.24 and values given elsewhere are probably due to differences in microstructure and measurement technique. Reliable room temperature values for er for single-crystal sapphire at 3.4GHz are 9.39 perpendicular to the c axis and 11.584 parallel to it, which are close to the values measured optically. The average er to be expected for a fully dense ceramic form is therefore 10.12, and values close to this have been determined. Nothwithstanding the uncertainties there is no doubt that the general behavioural pattern indicated by Fig. 5.24 is correct and typical of ceramic dielectrics. 

A schematic view of a microdielectrometer sensor is shown in Fig. 8 and illustrates the electrode array, the field-effect transistors and a silicon diode temperature indicator 15) which functions as a moderate accuracy ( 2 °C) thermometer between room temperature and 250 °C. The sensor is used either by placing a small sample of resin over the electrodes, or by embedding the sensor in a reaction vessel or laminate. Since all dielectric and conductivity properties are temperature dependent, the ability to make a temperature measurement at the same point as the dielectric measurement is a useful feature of this technique. 

The presence of hard and soft domains in segmented polyurethanes also has been confirmed by experimental results using pulsed NMR and low-frequency dielectric measurements. Assink (55) recently has shown that the nuclear-magnetic, free-induction decay of these thermoplastic elastomers consists of a fast Gaussian component attributable to the glassy hard domains and a slow exponential component associated with the rubbery domains. Furthermore, the NMR technique also can be used to determine the relative amounts of material in each domain. 

Dielectric relaxation study of two-phase microstructures in segmented copolymers was first attempted by North and his co-workers (55, 57,58,59). Dielectric measurements down to 10 5Hz were made on MDI-based segmented polyether- and polyester-urethanes using a dc transient technique. These materials displayed large, low-frequency... 

As mentioned previously, the complex dielectric permittivity (g>) can be measured by DS in the extremely broad frequency range 10-6-1012 Hz (see Fig. 1). However, no single technique can characterize materials over all frequencies. Each frequency band and loss regime requires a different method. In addition to the intrinsic properties of dielectrics, their aggregate state, and dielectric permittivity and losses, the extrinsic quantities of the measurement tools must be taken into account. In this respect, most dielectric measurement methods and sample cells fall into three broad classes [3,4,91] ... 

Several comprehensive reviews on the BDS measurement technique and its application have been published recently [3,4,95,98], and the details of experimental tools, sample holders for solids, powders, thin films, and liquids were described there. Note that in the frequency range 10 6-3 x 1010 Hz the complex dielectric permittivity e (co) can be also evaluated from time-domain measurements of the dielectric relaxation function (t) which is related to ( ) by (14). In the frequency range 10-6-105 Hz the experimental approach is simple and less time-consuming than measurement in the frequency domain [3,99-102], However, the evaluation of complex dielectric permittivity in the frequency domain requires the Fourier transform. The details of this technique and different approaches including electrical modulus M oo) = 1/ ( ) measurements in the low-frequency range were presented recently in a very detailed review [3]. Here we will concentrate more on the time-domain measurements in the high-frequency range 105—3 x 1010, usually called time-domain reflectometry (TDR) methods. These will still be called TDS methods. 

Since the late nineteenth century, dielectric spectroscopy has been used to monitor dynamical properties of solid and liquid materials. At that time, dielectric measurements were performed either at a single frequency or in a very limited frequency range now, however, measurement technique and instrumentation have developed to such an extent that dielectric spectroscopy is today a well-established method to probe molecular dynamics over a broad range in frequency or time (cf. reviews by Johari [1], Bottcher and Bordewijk [34], Williams [35,36], and Kremer and Schonhals [37]), even with commercially available equipment. Including the latest developments, one can even say that nowadays dielectric spectroscopy is the only method that is fully able to realize the idea of 0- to 1-THz spectroscopy. In data sets that cover the range of up to 10 6—1013 Hz—that is, from ultra-low frequencies up to the far infrared—the full range of reorientational dynamics in... 

In order to actually cover 19 decades in frequency, dielectric spectroscopy makes use of different measurement techniques each working at its optimum in a particular frequency range. The techniques most commonly applied include time-domain spectroscopy, frequency response analysis, coaxial reflection and transmission methods, and at the highest frequencies quasi-optical and Fourier transform infrared spectroscopy (cf. Fig. 2). A detailed review of these techniques can be found in Kremer and Schonhals [37] and in Lunkenheimer [45], so that in the present context only a few aspects will be summarized. 

To close this section, two main aspects of PD measuring technique shall be explained in more detail. The first point is to determine the maximum permissible PD intensities in an adequate way. The general aim is to ensure an appropriate quality of dielectrics in electrical apparatus and components or to predict the residual operational lifetime of such parts after a certain time on duty. There is absolutely no algorithm for this, i.e. all values for maximum permissible PD intensities are based on experience in practice. So, the values given in Table 8.5 are near to practice, but may be reduced accordingly for special applications. 

The limitations of capacitance measurements in undoped a-Si H have resulted in a greater emphasis on measurement techniques which use the shift of with a trapped space charge (Eq. (4.18)). An example is the field effect experiment, which is of special interest because it was the first technique used to obtain N E) in a-Si H (Madan, LeComber and Spear 1976). The experimental configuration is shown in Fig. 4.19. A voltage across the dielectric layer induces a space charge Q = C Vj in the a-Si H film, where is the capacitance of the dielectric. The Fermi energy in the a-Si H near the interface... 

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