Theory of capacitive probe method for noncontact characterization of dielectric properties of materials
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The capacitive probe method for noncontact characterization and monitoring of dielectric materials is analyzed theoretically. An analytical method based upon the Hilbert transform technique and a numerical method using the finite element technique for calculating the potential distribution and change in admittance of the probe caused by presence of the dielectric material as a function of liftoff (distance between the probe plane and the surface of the dielectric material) are described. The two methods are compared with each other and their relative advantages discussed. The possibility of extracting useful information about the dielectric constant of the material from experimental data is also discussed in the light of the proposed theory.
I. INTRODUCTION
Electrodes
The capacitive array probe is a device which is quite promising for noncontact characterization of the electrical properties of materials. Recently, we have demonstrated the sensitivity of the device to porosity in thermal barrier coatings and cracks in insulators. The device is also sensitive to the state of cure of a thin graphite-epoxy panel.1 We are currently focusing our efforts on an application of potentially great technological importance: monitoring the sintering of ceramics. An example of possible applications is noncontact monitoring of sintering of superconducting ceramics, where contamination could occur with a contacting device. The basic physics of the operation of the capacitive probe device is illustrated in Fig. l(a). Here a voltage V is impressed across two electrodes which lie in the same plane. The electrodes are called the probe fingers or simply fingers. The electrodes are placed a certain distance d above the dielectric material whose dielectric constant is to be monitored. The distance d is referred to as the liftoff. The admittance Y is obtained by measuring the current / in the receiver electrode (the one which is at -V) and dividing by V; viz. Y = J/V. As shown in Fig. l(a), the electric field "fringes" into the space below (and also behind) the electrode plane. By adding electrodes to the probe electrode configuration, the field can be made to fringe further into space, as shown in Fig. l(b). The electric field pattern can be varied by varying the pattern of the applied voltage over several electrodes. It is thus possible to a)
On attachment from The Ohio State University, Columbus, Ohio 43210. Current address: Department of Civil Engineering, The Colorado State University, Fort Collins, Colorado 80523.
b)
+v liftoff
Dielectric
(a) Electrodes
liftoff
/
Dielectric
(b) FIG. 1. Principle of the capacitive probe method. In (a) equal positive and negative voltages are applied to two probe electrodes kept at a distance d from the dielectric surface. The electric field fringes into the dielectric which changes the admittance of the probes. The penetration of the field in the dielectric can be changed by using a different configuration of the electrodes, as shown in (b).
choose a configuration such that the electric f
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