Permittivity

Permittivity Measurement by High Frequency Eddy Current Testing

High frequency eddy current measurement devices can be used not only for characterizing conductivity and magnetic permeability related properties of electrically conductive materials, but also for permittivity characterization of insulators. This is based on the fact that eddy current systems generate electric and magnetic fields, where the electric field distribution is affected by the permitivity within the generated fields. This effect can be used for non-contact permittivity characterization. Applications include

• Material sorting (plastics, metal oxide, ceramics, etc.)
• Composition assessment (concrete, liquids)
• Assessment of chemical reactions (curing, two compound casting compounds)

SURAGUS offers single point, imaging tools and inline sensors for high frequency characterization. Especially the inline permittivity measurement by high frequency impedance analysis is beneficial

• Realtime
• Non-contact
• High sampling rates
• Measurement through pipes, containers and substrates
• No wearing

SURAGUS offers various sensor types, often customized towards the measurement task that are mostly dedicated for testing on flat substrates or to characterize liquids, powders and other substances in pipes or specific containers. Please feel free to contact our team for discussion of specific testing tasks.

Theory and Background - Influence of Permittivity on Eddy Current Coil Impedance

Maxwell’s equations for constant frequencies (1 - 4) suggest that sample conductivity and permittivity influence the eddy current coil impedance. Here a magnetic field H with the magnetic flux density B, varying in time with the angular frequency ω generates a rotating electric field E independent of the conductivity of the sample (3). The current density J depends on the conductivity of the sample σ (7), whereas the displacement current density J_D depends on the permittivity ε of the material to be tested (6). This effect is not relevant for traditional low frequency eddy current testing solutions but becomes visible with high frequency testing systems. Research and studies using SURAGUS EddyCus high frequency eddy currents systems have be published and peer reviewed for some applications. (cf. S. Gäbler, H. Heuer and G. Heinrich, "Measuring and Imaging Permittivity of Insulators Using High-Frequency Eddy-Current Devices," in IEEE Transactions on Instrumentation and Measurement, vol. 64, no. 8, pp. 2227-2238, Aug. 2015, doi: 10.1109/TIM.2015.2390851. )

What is Permittivity?

Permittivity, is known as a dielectric constant. Permittivity describes the relative ability of a dielectric to store electrostatic energy in an electric field. The smaller the relative dielectric constant of materials, the better the insulation. When a medium is applied with an electric field, the induced charge will be generated to weaken the electric field. The ratio of the original applied electric field (in vacuum) to the final electric field is called permittivity, which is also related to the frequency.

Relevance of Permittivity

The advent of the high-tech era has led to changes and demands for high-tech materials, and the emergence of new technologies has created new performance requirements for materials, including microwave dielectric ceramics, PCB materials, semiconductor materials, mobile phone antenna materials, mobile phone shell materials, electromagnetic shielding materials and dielectric materials, etc. will all change in high-tech applications. This requires higher electromagnetic wave transmission speed and lower signal propagation losses, which means higher dielectric constants and lower dielectric losses of the materials used in the application. Dielectric materials meet these exacting requirements.

What is the Unit?

The standard SI unit for permittivity is Farad per meter (F/m or F·m^-1) . The permittivity in vacuum is called vacuum permittivity ε_0, ε₀ ≈ 8.854187817…×10⁻¹² F/m. The permittivity is often represented by the relative permittivity εr, which is the ratio of the absolute permittivity ε and the vacuum permittivity ε_0.

How can Permittivity be Measured?

There are many methods developed for measuring permittivity or dielectric constant, and each one of them can be conducted to certain frequencies, materials, applications and etc..

Methods	Applicable Scenarios	Advantages	Limits
Open-ended coaxial probe	Liquids, biological specimen, semi-solids	easy to operate	due to low f, not suitable for complex electronics
Transmission Line	mostly applied to test low- and medium loss material with large εr	high accuracy, any frequencies	multi-value problem, request on shape and size, low accuracy on thin film and rough surface
Resonant method	good performance in 1GHz - 10GHz, suitable for lowloss material	good accuracy, easy to operate	Inaccurate in loss tangent, multi error sources
Free space	good performance in 3GHz - 100GHz	No direct contact needed, wide measurement range	errors caused by standing waves

Open ended coaxial probe method has been used for years as a non-destructive testing method. In this method the probe is pressed against a specimen or immersed into the liquids and the reflection coefficient is measured and used to determine the permittivity.

Transmission line method is a popular broadband measurement method. In the method, only the fundamental waveguide mode (TEM mode in coaxial line and TE mode in waveguides) is assumed to propagate. Calibration must be carried out before making measurement.

Resonant method provides high accuracies and assumes the TE or TM propagation mode. Resonant measurements are the most accurate methods of obtaining permittivity and permeability. However, there are limitations on the frequencies and loss characteristics
of the materials that can be measured with the method.

Free space method is for broadband applications and assumed only the TEM propagation mode. Free space measurement allows measurements on material under test (MUT) under high temperatures or hostile environments and generally operates in wide band frequencies.

Industry is applying the following methods in laboratories:

DEA/DSC Analysis

Dielectric Analysis (DEA) or dielectric cure monitoring is a thermal analysis technique for determining cure state. DEA tracks the cure state of a material by measuring the electrical properties of permittivity. Permittivity (ε) is related to energy storage in a material.

Differential scanning calorimetry (DSC), one method for studying polymers, like measuring glass transition temperature Tg, which changes with cure state.

DETA Analysis

Dielectric thermal analysis (DETA) is the process by which analyze the material by measuring the dielectric coefficient, and dielectric loss of a specimen under an alternating electric field as a function of temperature, frequency or time, to evaluate physical and chemical change in the material.

Applications for Permittivity Measurements

The quality assurance and process control of the following applications can be supported permittivity measurements:

• Material sorting (plastics, metal oxide, ceramics, etc.)
• Composition assessment (concrete, liquids)
• Assessment of chemical reactions (two compound casting compounds)