5 methods to measure the permittivity of a material

Résumé du document

Permittivity of a material describes material response to an electric field.
More precisely, some materials have lattice with dipole moment which creates an electric field. If these materials are subjected to an external electric field then atoms of the lattice will align themselves in such a way that internal electric filed and the external electric field are in opposition.

Extraits

[...] 5 methods to measure the permittivity of a material Introduction What is the permittivity? Permittivity of a material describes material response to an electric field. More precisely, some materials have lattice with dipole moment which creates an electric field. If these materials are subjected to an external electric field then atoms of the lattice will align themselves in such a way that internal electric filed and the external electric field are in opposition Figure 1 Orientation of material's atoms according to the external electric field e Permittivity often depends on the temperature. [...]

[...] Spectrum The method of coaxial probe is preferable for liquids and semi-solid material (powder). Its procedure of calibration is simple, the method is simple, practical, non-destructive test and requires only a single measure Pros/Cons ▪ This method has some benefits: ease to prepare the sample (no machining), fast measure, stable environment (temperature constant). ▪ Nonetheless the air perturbs the value of the capacity measured. Transmission Line Process Transmission line method aims ate putting a sample of the material in two concentric cylinders which measure 10 cm for the one and 4 cm for the other. [...]

[...] This method is based on the resonance phenomenon and it consists of sending electromagnetic waves in the cavity. These waves are generated by a network analyzer, connected to the cavity. The cavity resonates at appropriate frequencies (when the energy electric stored is equal to the magnetic energy stored). When we put a material in the cavity, the study of this resonance can characterize the polarization-dependency loss ( and the losses by conduction and relaxation . With: parameter calculated from electric field and magnetic field f0: resonance frequency in the empty Q0: quality factor in the empty ∆f: resonance frequency in the empty plus the material Q1: quality factor in the empty plus the material Figure 7 resonant cavity principle Spectrum We use this method to low frequency (500 MHz to 5 GHz) for all geometry. [...]

[...] Pos/Con ▪ It makes high precision measure for material with small losing ▪ But, on the other hand, it's only for a small sample, the measures are only done for resonant frequencies. Parallel Plate Process The sample is put into parallel plate to form a condenser. The capacitance is tied to the electric field who is induced in the space between the two electrodes and the conductance is tied to the losses of the material. C and G are measured by RLC meter. Figure 8 process of parallel plate method The permittivity is tied to the tangent of losses of the material. [...]

[...] Reasons for several techniques to measure the permittivity Several techniques exist to measure the permittivity of materials. The choice of the technique depends on the characteristics of the sample (material, geometry, and dimension), the frequency range and desired accuracy No technique can characterize all the materials on a whole frequency range. There is always uncertainty in the measures. Different techniques to measure the permittivity of materials Two major groups exist: analysis methods with low frequencies or with high frequencies. The low frequencies gather resonant cavity, free space, coaxial probe, parallel plate and the high frequencies gather transmission line. [...]