ET13.pdf

Properties can also be expressed in terms of the

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properties can also be expressed in terms of the complex permittivity ε *: (18) Typical excitation frequencies range from less than 1 mHz for insulating materials up to several megahertz for semiconducting materials. The sensor geometry is accurately captured by the ratio of the area to the gap ( A · d –1 ) when the electrode widths are much larger than the gap so that the fringing fields at the electrode sides can be neglected. Placing guard electrodes around the sense electrode with their voltage the same as the sense electrode voltage helps minimize the effects of the fringing fields as illustrated in Fig. 27b. A guard electrode can also be placed behind the sense electrode to further reduce extraneous coupling from the fringing fields. The final use of these dielectrometry measurements is to infer related physical properties such as moisture content, density, porosity and impurities. Empirical measurements then can generally map values of the physical variable to values of the material permittivity and conductivity. For simple systems (as in Fig. 28), measurements at a single excitation frequency can be used to determine both the permittivity and conductivity of the material. Most materials are dispersive, however, so the effective properties depend on the excitation frequency. Dispersiveness can be attributed to heterogeneous material properties, such as particles embedded in a matrix or multiple layers of different material properties, or can be attributed to multiple physical processes, such as multiple conduction mechanisms. Different techniques or models are then used to determine the properties of interest. As examples for multiple layered materials, equivalent circuits and expressions for the terminal capacitance or conductance (Eqs. 14 and 15) that account for the properties of each layer ε ε ε ε σ ω * = ′ − ′′ = j j Y Z G j C = = + 1 ω RC = ε σ G R A d = = 1 σ C A d = ε 347 Electromagnetic Techniques for Material Identification F IGURE 27. Parallel plate electrode sensor: (a) basic sensor; (b) sensor with guard electrodes. (a) V 0 cos( ω t ) Sample Fringing fields Electrodes I (b) V 0 cos( ω t ) Drive Sensing electrode Guard electrode driven at potential of sensing electrode I Legend I = terminal current t = time (second) V = voltage ω = angular frequency Guard electrode driven at potential of sensing electrode F IGURE 28. Equivalent circuit for parallel plate electrode sensor and homogeneous dielectric material. Impedance Z Admittance Y Capacitance C
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can be used. 12 For two-phase composite materials, the effective dielectric properties of the composite can be related to the dielectric properties and geometry of the constituent materials. 13 The effects of material heterogeneity can also be displayed graphically. If excited by a sinusoidal voltage in time, as the frequency is varied from zero to infinity, a plot of the imaginary part of the impedance or admittance versus the real part of the impedance or admittance traces out a semicircle for the equivalent circuit of Fig. 28. Such plots are called
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  • Fall '19
  • Magnetism, Magnetic Field, Electrical conductivity

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