Dielectric material property measurement methods
Article By : Jean-Jaques (JJ) DeLisle
Here is a brief overview of dielectric material measurement and characterization that engineers should take into account.
Dielectrics are materials that store energy when an external electric field stimulates the material. A common explanation for a dielectric is a material that will store additional charge when placed within a parallel plate capacitor than if there was no material (vacuum) between the plates. The significance of dielectrics ability to do this is the dielectric constant (Dk), which is typically referenced to that of vacuum, or 1. Common relative permittivity ranges are from just above 1 to hundreds of thousand. For RF applications, dielectrics with a range just above 1 to tens are commonly used.
Another key factor of a dielectric is the amount of energy absorbed and lost and heat within the dielectric compared to how much energy is stored when exposed to an external electric field. This phenomenon is known as loss tangent, tan delta, dissipation factor (Df), or dielectric loss. Together, both the dielectric constant and loss tangent make up the complex permittivity of a material, with the real part being the dielectric constant and the imaginary portion of the loss tangent.
A dielectric material is made up of electric charge carriers that become polarized when exposed to an electric field, compensating for the field. This function of a dielectric may involve several different mechanisms of polarization effects that contribute to the dielectric’s permittivity.
Common dielectric mechanisms and polarization effects
- Dipole orientation (orientation polarization)
- Ionic conduction
- Atomic polarization
- Electronic polarization
- Resonant effect (usually associated with atomic or electronic polarization)
- Relaxation effect (usually associated with orientation polarization)
- Interfacial or space charge polarization
For many applications, the exact dielectric mechanisms are not entirely important, but the overall dielectric performance is. However, various potential mechanisms and other aspects of a material may influence methods that are viable for measurement of the dielectric performance of a given material. Moreover, these complexities result in dielectrics performing differently, depending on the measurement configurations in different states.
Many dielectrics also exhibit frequency-dependent dielectric permittivity as well as temperature dependence. Hence, it is often essential to measure the dielectric properties of a material in the state, environment, and form factor in which it will be used to have an “engineering” dielectric property result instead of an “absolute” dielectric property measurement.
That’s why there is a wide array of methods to measure dielectric materials, each with their own range of applicability and limitations. Below are a few of the dielectric measurement considerations.
Dielectric measurement considerations
- Frequency range
- Range of relative permittivity and magnetic permeability
- Loss tangent (Df) measurement
- Accuracy
- Material properties: homogenous, isotropic, etc.
- Sample state: solid volume, sheet, powder, liquid, etc.
- Sample size/volume
- Destructive or non-destructive
- Contacting or non-contacting
- Temperature range
- Traceability
- Standard requirements
- Cost
Some dielectric measurement methods require dedicated measurement instruments and accessories, while others are more generic and may only require some accessories or modified components that can be purchased off-the-shelf. In many cases, a dielectric sample must be prepared, and the exact volume, density, and dimensions must be known in order to accurately perform the dielectric property measurement. The following are a few of the most common dielectric property measurement methods.
Common methods of dielectric measurements
- Impedance analyzers/LCR meters
- Parallel plate capacitor or three terminal method (ASTM D150)
- Open-ended coaxial probe
- Dielectric-loaded waveguide
- Coaxial transmission line
- Planar transmission line
- Focused microwave or millimeter-wave beam (free space)
- Split cylinder resonator
- Split post dielectric resonator
- Cavity perturbation (ASTM D2520)
- Inductance measurement method
LCR meters and impedance analyzers directly stimulate and measure the capacitance, inductance, and loss response of a dielectric sample. The exact dimensions of the sample must be known in advance for these methods to allow for dielectric parameter extraction. These instruments are usually highly accurate, but only good for lower frequencies ranging from MHz to sub-1 GHz. The parallel plate capacitor method is also limited to low frequencies and typically requires a precisely sized dielectric sample that is somewhat limited in thickness by the apparatus. It’s possible to make custom parallel plate capacitors to test larger samples, though ensuring accuracy with a custom approach likely won’t comply with standards or may have accuracy and tolerance issues.
Many of the other methods require a network analyzer, though an impedance analyzer may be applicable for only lower frequencies. The resonant and cavity methods only provide a single frequency result, which is suitable for dielectric materials with a frequency invariant dielectric response or when only a single frequency point is needed. The transmission line methods are broadband and highly accurate but do require significant mathematical analysis to set up.
Waveguide methods are similar to the transmission line methods but are tied to the waveguide bands. The free space method and microwave or millimeter-wave beam methods are similar to the transmission line/waveguide methods, but the sample is floating in free space instead of interacting with the fields of the transmission line or waveguides.
The inductance measurement method is only applicable to materials with relative magnetic permeability greater than 1. Most RF dielectric materials tend to have a magnetic permeability that is effectively 1. Both the transmission line and free space methods can also be used to measure magnetic permeability if that is needed.
There are many more nuances to dielectric material measurement and characterization than is covered in this blog. The goal here was to provide a very brief overview on the subject and inform readers of the factors to take into consideration when approaching dielectric material measurements.
This article was originally published on Planet Analog.
Jean-Jaques (JJ) DeLisle, an electrical engineering graduate (MS) from Rochester Institute of Technology, has a diverse background in analog and RF R&D, as well as technical writing/editing for design engineering publications. He writes about analog and RF for Planet Analog.