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Loaded and floats to a sensed voltage v s or is

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loaded and floats to a sensed voltage V S or is virtually grounded with a terminal current I S . The sensed signal amplitude and phase with respect to the driven voltage depend on the complex permittivity of the adjacent dielectric. With these interdigitated structures, the inversion between the measured sensor terminal variables and the dielectric properties is more complicated than with the parallel plate structures. 17-19 The spatially periodic variation of electric potential along the surface (in the Y direction) produces an electric field that penetrates into the medium (in the Z direction). The potential obeys Laplace’s equation and can be represented using a fourier series. This model shows that the electric field for each spatial mode at a given frequency decays exponentially (in the Z direction) with a decay proportional to the spatial wavelength of the periodic electrodes (Fig. 31b). Consequently, sensors with longer wavelengths have larger penetration depths and will respond to changes of material properties far from the interface of sensor to dielectric material. Smaller wavelength sensors will primarily respond to changes near the interface. Thus, multiple wavelength sensors provide spatial profile information about the test material and permit, for example, simultaneous measurement of the dielectric properties and thickness of any air gaps that may be present between the test material and the sensor. These multiple wavelength sensors can use multiple sets of interdigitated spatially periodic electrodes attached to a common substrate or colocated designs that interweave the electrodes for a multiple wavelength measurement in a single sensor footprint. 20 Segmenting of the electrodes into arrays also permits wide area imaging of dielectric properties. Multiple frequency techniques are also used to study interfacial electrochemical processes such as surface corrosion and energy storage in battery components and for electrochemical impedance spectroscopy. 21 In addition to the bulk response of the materials, these techniques allow interfacial processes, such as reaction rates and transport of charged species, to be isolated. These measurements provide insight into physical phenomena in the materials and are used to optimize performance characteristics. 349 Electromagnetic Techniques for Material Identification F IGURE 31. Single-sided electrode format: (a) interdigital dielectrometry sensor in one-sided contact with material under test; (b) electrode spatial wavelength limits electric field intensity depth of penetration into test material. (a) Test material λ z x y V S + + V D (b) Sensor 1 λ 1 d 1 ≅ λ 1 ÷ 3 Legend d = electric field penetration depth V D = sinusoidally varying time signal V S = sensed voltage x , y , z = directional coordinates λ = wavelength of spatially periodic electrodes Sensor 2 λ 2 d 2 ≅ λ 2 ÷ 3 Electric field lines
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1. “Metal and Alloy Identification.” Nondestructive Testing Handbook , second edition: Vol. 9, Special Nondestructive Testing Methods.
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  • Fall '19
  • Magnetism, Magnetic Field, Electrical conductivity

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