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 JD 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. )

 

Formel_permittivity.png

 

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 ε0, ε≈ 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 ε0.

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
   

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