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Chapter 2 Literature Review of Dielectric Characterization Methods
Introduction
In order to fully characterize a material, numerous methods should be used to verify the results and cover the entire desired frequency range. The complex permittivity of a material is partially dependent upon frequency, and methods covering a large span of
frequencies are necessary to fully and effectively characterize the material. A large amount of research has been conducted involving the interaction of electromagnetic waves with a medium. By observing the response of a material under test to an electromagnetic excitation, the data can then be compared to the predicted theoretical response, which is given by the application of Maxwell’s equations to the geometry of interest. From this comparison, the electrical properties can be determined. This thesis deals only with the characterization of the complex permittivity of a material and assumes the material is non-magnetic with a relative permeability μr of 1. The real part of the permittivity is most directly related to the propagation velocity of waves in a material, while the imaginary part is directly linked to the losses incurred as electromagnetic waves travel through a medium. It is assumed that the material is homogeneous, isotropic, linear, and time-invariant.
Dielectric Permittivity Overview
The permittivity of a dielectric material is best explained by viewing the electrical response of the material at a microscopic level. Dielectrics are defined as materials in which the primary charges in the atoms and molecules are bound charges held in place by atomic and molecular forces [2]. Since the bound charges are constrained to a given region within the material, they are not able to conduct freely, as are free charges within a conductor. Rather, the electrons shift slightly in response to applied electrical fields. This phenomenon creates a large number of electric dipoles within the material.Figure 2-1 shows the atomic behavior of an atom in the absence and presence of an applied electric field. Under the applied field, the charges within the atom align themselves to the electric field in a process known as orientational polarization. A dipole moment is created, since the positive and negative charges are separated in space by a finite distance. Note that in the case of free charges, there are no restraining forces to retain the charges within a given region, so they will continue to move in opposite directions. This dipole moment can be compared to the stretching of a rubber band, in which potential energy is stored. In the case of polar molecules, potential energy is stored in the process of rotational alignment. To analyze the material on a macroscopic level, the total polarization vector P can be approximately quantified as the product of the material volume by the estimated number of dipoles per unit volume.To further explain the idea of bound charge, we now observe the effects of a partially-dielectric-filled parallel-plate capacitor in the presence of a finite voltage potential difference between the capacitor plates. A Voltage source is applied to the capacitor as shown in Figure 2-2. In the area containing free space as the medium, the electric field is constant between the plates and its magnitude is represented by V/d,where d is the distance separating the plates. In the area containing the dielectric medium, the majority of the dipoles in the material align to the electric field, while the oppositely charged adjacent bound charges cancel one another out. This leaves a certain amount of bound charge on each plate, which is neutralized by the free charges on the capacitor plates. The voltage on the plates, however, must remain constant, which leads to the introduction of additional free charges on the capacitor plates.
List of Figures
List of Tables
Chapter 1: Introduction
Chapter 2: Literature Review of Characterization Methods
2.1 Introduction
2.2 Dielectric Permittivity Overview
2.3 Dielectric Characterization Considerations
2.4 Commonly-Used Methods
2.4.1 Parallel-Plate Capacitor
2.4.2 Coaxial Cavity
2.4.3 Rectangular Metallic Waveguide
2.4.4 Resonators.
2.4.5 Open-Ended Coaxial Probe
2.4.6 Free Space Methods
2.5 ASTM Standards
Chapter 3: Capacitor Dielectric Characterization
3.1 Introduction
3.2 Overview of Capacitance
3.3 The Guarded Electrode
3.4 Sample Measurement
Chapter 4: X-Band Waveguide Characterization of Thin Dielectric Sheets .
4.1 Introduction
4.2 Systematic Error
4.3 De-Embedding of Material S-parameters from Simulation
4.4 Theoretical S-parameters
4.5 Comparison of Theoretical and Simulated S-parameters
4.6 Inverse Solution
4.7 Sensitivity Analysis
4.8 Material Measurement
4.8.1 VNA TRL Calibration
4.8.2 Sample Preparation
4.8.3 8510C Measurement
4.8.4 Measurement Data
4.8.5 Measurement Solution
Chapter 5: Conclusion
Summary
Recommended Future Work
Conclusions
References
