Basics of Measuring the Dielectric Properties of Materials
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Introduction
A wide variety of industries need a better understanding of the materials they are working with to shorten design cycles, improve incoming inspection, process monitoring, and quality assurance. Every material has a unique set of electrical characteristics that are dependent on its dielectric properties. Accurate measurements of these properties can provide scientists and engineers with valuable information to properly incorporate the material into its intended application for more solid designs or to monitor a manufacturing process for improved quality control.
A dielectric materials measurement can provide critical design parameter information for many electronics applications. For example, the loss of a cable insulator, the impedance of a substrate, or the frequency of a dielectric resonator can be related to its dielectric properties. The information is also useful for improving ferrite, absorber and packaging designs. More recent applications in the area of aerospace, automotive, food and medical industries have also been found to benefit from knowledge of dielectric properties.
Keysight Technologies, Inc. offers a variety of instruments, fixtures, and software to measure the dielectric properties of materials. Keysight measurement instruments, such as network analyzers, impedance analyzers, and LCR meters range in frequency up to 1.1 THz. Fixtures to hold the material under test (MUT) are available that are based on coaxial probe, parallel plate, coaxial/waveguide transmission lines, free space, and resonant cavity methods. The table below shows product examples that can be measured by Keysight’s material test solutions.
Dielectric Theory
The material properties that will be discussed here are permittivity and permeability. Resistivity is another material property that will not be discussed here. Information about resistivity and its measurement can be found in the Solutions for Measuring Permittivity and Permeability with LCR Meters and Impedance Analyzers application note. It is important to note that permittivity and permeability are not constant. They can change with frequency, temperature, orientation, mixture, pressure, and molecular structure of the material.
Dielectric constant
A material is classified as “dielectric” if it has the ability to store energy when an external electric field is applied. If a DC voltage source is placed across a parallel plate capacitor, more charge is stored when a dielectric material is between the plates than if no material (a vacuum) is between the plates. The dielectric material increases the storage capacity of the capacitor by neutralizing charges at the electrodes, which ordinarily would contribute to the external field. The capacitance with the dielectric material is related to the dielectric constant. If a DC voltage source V is placed across a parallel plate capacitor (Figure 1), more charge is stored when a dielectric material is between the plates than if no material (a vacuum) is between the plates.
Permeability
Permeability (μ) describes the interaction of a material with a magnetic field. A similar analysis can be performed for permeability using an inductor with resistance to represent core losses in a magnetic material (Figure 4). If a DC current source is placed across an inductor, the inductance with the core material can be related to permeability.
Some materials such as iron (ferrites), cobalt, nickel, and their alloys have appreciable magnetic properties; however, many materials are nonmagnetic, making the permeability very close to the permeability of free space (μr = 1). All materials, on the other hand, have dielectric properties, so the focus of this discussion will mostly be on permittivity measurements.
