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)

Table of Materials and their Dielectric Constants

 

Substance	Dielectric Constant
Acetal	3,8
Acetaldehyde	15
Acetamide	59,2
Acetoacetic acid ethyl ester	15
Acetone	21,5
Acetophenone	18
Acetylacetone	23
Acetyl bromide	16,2
Acetyl chloride	15,9
Acetylene dibromide	7,2
Acetylene tetrabromide	5,6
Aconite acid ester	6,3
Adipic Acid	1,8
Aerosile	1
Activated carbon	12
Alum	4,2
Allyl alcohol	20,6
Allyl chloride	8,2
Allyl iodide	6,1
Aluminium bromide	3,4
Aluminium foil	10,8
Aluminium hydroxide	2,5
Aluminium splinters	7,3
Aluminium sulfate	2,6
Triethylaluminium	2,9
Formic acid	57,9
Ammonia	15
Ammonia solution (25%)	31,6
Ammonia salt	4,3
Pentanol	14,8
Amyl amine	4,5
Aniline	7
Anisealdehyde	22,3
Anisole	4,5
Anthracite/hard coal	3,2
Antimony hydride	1,8
Malic acid diethylester	10
Argon	1,5
Arsine	2,1
Arsole	2,3
Asbestos	10
Ascorbic acid (vitamin C)	2,1
Azelaic acid diethylester	5
Azoxybenzene	5,2
Basalt	2,5
Cotton fibre flour	3,2
Bauxite	2,5
Bentonite	8,1
Benzal chloride	6,9
Benzaldehyd	17,6
Benzil (80°C)	10
Gas	2
Benzene	2,3
Benzene, heavy	3,2
Benzyl alcohol	13,5
Benzyl^amine	4,6
Benzyl chloride	7
Beer brew	25
Bitumen	2,8
Hydrogen cyanide	158
Bore oil emulsion	25
Bornylacetat	4,6
Bromine	3,1
Butanoic acid	3
Camphene	2,3
Caproic acid	2,6
Caprylic acid	2,5
Carbazole	1,3
Carbonylcyanid	10,7
Cellit	1,6
Cetyl alcohol (60°C)	3,6
Quinoline	8,8
Chlor, fluid	2,1
Chloral	6,7
Chlor benzene	5,7
Chloroacetic acid	33,4
Chloorhydrin	31
Chlorinated lime	2,3
Chloroform (trichlormethane)	4,8
Cola essence	17,3
Cream (skin)	19
Cuminaldehyde	10,7
Cyanogen	2,5
Decalin	2,1
Degalan	3,1
Desmodur	10
Diacetone alcohol	18,2
Diamylether	3
Dibenzofuran (100°C)	3
Dibenzyl (60°C)	2,5
Diesel Fuel	2,1
Diethylamine	3,8
Dimethylether (methyl ether)	5
Diofan	32
Dioxane	2
Diphenyl	2,5
Printing ink	4,6
Ice cream	16,5
Iron(III)oxide red	1,9
Emulphor	4
Epichlorhydrin	23
Peanuts, dried	3,1
Peatnut expeller	2,4
Vinegar	24
Acetic acid	6,2
Cement asbestos	3,2
Ethanol (ethyl alkohol)	16,2
Aether	4
Ethayl acetate	6
Ethylamine	6,9
Ethyl benzoate	6
Ethyl benzene	2,4
Ethylene chlorhydrin	25
Ethylene chloride	10,6
Ethylenediamine	15
Etylene oxide	13,9
Ethyl mercaptan	6,9
Fenchone	12,8
Ferrite pellets	21
Ferrosilicon	10
Green vitriol	32,4
Ferrozell	18,3
Fat coal	3,4
Fatty acid	1,7
Fish oil	2,6
Flax pellets	1,4
Meat and bone meal	1,9
Tankage	1,9
Fly ash	3,3
Fluorine	1,5
Fluorbenzene	6,4
Hydrogen Fluoride	83,6
Calcium fluoride	2,5
Formamide	109
Furan	3
Furfurol	41,7
Animal feed grist	2,4
Germanium tetrachloride	2,4
Grain grist	3
Gypsum	1,8
Fiber glass powder	1,1
Glass granulate	4
Cullet	2
Glucose (50%)	30
Glycerol	13,2
Glycerol water	37
Glycol	37
Glysantin	25
Granuform	4
Guaiacol	11
Guano	2,5
Oat	4,9
Urea	2,9
Resin	1,5
Hazels	2
Hot glue	2,3
Heating oil	2,1
Helium	1,1
Heptane	1,9
Heptanal	9,1
Heptanoic acid	2,6
Heptene	2,1
Hexane	1,9
Hexene	2,1
Hexanol	12,5
Hibiscus	2,8
Wood chips	2,3
Charcoal	1,3
Wood swarf	1,5
Splints	1,1
Honey	24
Hydrazine	58
Imidazole, pure	23
Isoamyl acetate	4,8
Isoamyl alcohol	15,6
Isoamyl bromide	6
Isoamyl chloride	6,1
Isoamyl ether	2,8
Isoamyl iodide	5,6
Isobutanoic acid	2,6
Isobutyl alcohol	18,1
Isobutyl amine	4,4
Isobutyl benzene	2,3
Isobutyl bromide	7,2
Isobutyl chloride	6,5
Isobutyl cyanide	18
Isobutyl iodide	6,5
Isobutyl nitrate	11,7
Isobutyl silane	2,5
Isoquinoline	10,7
Isocyanate	6,1
Isoprene	2,1
Isopropanol	18
Isosafrol	3,3
Iodine	11,1
Iodobenzene	4,6
Methyl iodide	7,1
Hydrogen iodide	2,9
Coffee beans	1,5
Cacao beans	1,8
Caustic potash	3,3
Potash salt	2
Lime	2
Potato starch	1,7
Ceramic compound	17
Ketchup	24
Gravel	2,6
Diatomaceous earth	1,4
Silicic acid	2
Bone fat	2,7
Bonemeal	1,7
Sodium chloride	23
Coal, 15 % moisture	4
Diethyl carbonate	2,8
Coal dust	2,5
Coconut oil (refined)	2,9
Coke	3
Cork powder	1,7
Concentrated feed	3,2
Chalk	2,1
Cresol	11
Cresol resin	18,3
Crystal sugar	2
Fertiliser	4,3
Plastic pellets	1,2
Copper ore	5,6
Lanolin	4,2
Latex	24
Lauric acid ethyl ester	3,4
Glue	2
Linoleic acid	2,7
Solvent	18
Laughing gas	1,5
Skim milk powder	2,3
Corn	3,6
Corn grist	2,1
Corn starch sirup	18,4
Malt	2,7
Mandelic acid nitril	18
Marble stones small (2-3 mm)	2,5
Mice feed	2,3
Flour	2,5
Molasses	31,3
Menthol	4
Mesityl oxide	15
Metal powder	6
Methanol (methyl alkohol)	33
Methyl acetate	8
Methylene bromide	7
Methylene chloride	9
Methylene chloride	9,1
Metylene iodide	5,3
Methyl nitrate	23,5
Methyl cellulose	3
Mono chlormethane	9,8
Morpholine	7,3
Naphthenic acid	2,6
Naphtalene	2,5
Soda	3
Sodium methylate	1,5
Sodium perborate	2,2
Sodium peroxide	2,7
Sodium sulfate	2,7
Nitrobenzene	35
Nitroethane	29
Nitroglycol	28,3
Nitroglycerin	19,3
Nitro varnish	5,2
Nitromethane	39
Nitro phoska	5,4
Nitrosyl bromide	15,2
Nitrosyl chloride	19
Pasta	1,9
Octane	2
Octene	2,1
Octyl bromide	5
Oil	2
Oleic acid	2,5
Water-in-oil-emulsion	24,2
Oxalo ethyl acetate	6
Palm tree nut	2,2
Palmitic acid	2,3
Palm nut/kernel/seed	2,8
Palm seed oil	1,8
Paper scraps	1,2
Paraffin	1,6
Paraldehyde	15,1
Pelargon	2,8
Penta borane	21
Penta ethyl chloride	3,8
Penta chlortoluene	4,8
Pentane	1,8
Pentanal	11,8
Pentene	2
Perchlorate	3,6
Hexachlorobutadiene	2,6
Perlite	1,7
PET powder	1,5
Phenetole	4,2
Phenol	8
Phenol resin	7,4
Phosgene	4,3
Phosphate	4
Phosphorus, liquid	3,9
Phosphorus salt	4
Pinane	2,1
Piperidine	5,8
Polyamide pellets	1,7
Polyethylene	1,2
Polypropylene	1,6
Polyrol	2,8
Polyvinyl acetals	2,8
Popcorn	1,1
Pril	1,2
Propionaldehyde	14,4
Propanoic acid	3,2
Propanol (propyl alcohol)	2,2
Propylamine	3
Propylene, liquid	1,9
Propylene chloride	9
Propylether	3,3
PVC powder, pure	1,3
Pyridine	13,2
Pyrroles	8
Silica sand	2
Quartz stone meal	2,7
Mercury diethyl	2,1
Rapeseed	3,3
Rapeseed grist	2,1
Rice	3
Rye	6
Rye bran	2,2
Beets seeds	3,5
Beets cuttings	7,3
Carbon black	18,8
Saccharose solution	20
Sawdust	1,3
Nitric acid (98%)	19
Hydrochloric acid	5
Salt water	32
Oxygen	1,5
Chamotte	1,8
Foam flakes	1,1
Lard (80°C)	2,1
Soft soap	32
Chocolate powder	2
Black liquor	32
Sulphur	3,5
Sulphur dioxide	14
Carbon disulphide	2,6
Sulfuric acide	21,9
Sulfuric acide (17%)	31
Sulfuric acide (97%)	8,6
Sulfur trioxide	3,1
Hydrogen sulfide	6
Heavy fuel oil	2,2
Soap flakes	9,2
Soap pellets	3,5
Mustard	24
Grain of mustard seed	3,6
Silicone oil	2,7
Silicone rubber	2,9
Soy flour	4,5
Grain of soy	2,9
Sunflower seeds	2
Chaff	1,5
Stearic acid	2,3
Rock salt (0-25 mm)	4,3
Styrene	2,4
Tobacco dust	1,8
Talcum	1,5
Tea powder	2
Tar	4
Terephthalic acid	1,5
White spirit	2
Terpinene	2,7
Terpinolene	2,3
Tetrachlorethylene	2,5
Carbon tetrachloride	2,3
Thomaskali dust	3,4
Thujone (0°C)	10,8
Meat and bone meal	2,2
Titan tetrachloride	2,8
Toluene	2,4
Clay	2,3
Transformer oil	2,1
Trichloroethylene	3,2
Triptan	1,9
Dry yeast	2
Ultrasil	1,4
Undecan	2
Valeric acid	2,7
Viscose	34,5
Wax	1,8
Benzine	2
Water	80,3
Water (360°C)	10
Water, demineralisiert	29,3
Water, heavy	78,3
Sodium silicate	16
Hydrogen	1,2
Hydrogen peroxide	84,2
Wine	25
Tartaric acid	35,9
Wheat	4
Wheat starch	2,5
Xylitol	40
Xylene	2,3
Tooth paste	18,3
Cellulose	1,2
Cement	2,2
Zinc oxide	1,5
Zinc powder	4,4
Sugar	1,8
Tinder	12