Characterization Techniques for Materials’ Properties Measurement

The electromagnetic properties of materials are defined from the two following constitutive parameters: permittivity, ε, and permeability, μ; ‘ε’ indicates how the medium reacts when an electric field is applied (field E of the electromagnetic wave), whereas μ indicates how the material behaves further to a magnetic excitation (field H of the electromagnetic wave). To take into account the losses that occur in any material, permittivity and permeability need both to be expressed by complex values: ε= ε'-j ε", μ=μ'-jμ”. But already, what represent these parameters? As, during this chapter we will mostly be interested by the measurement of the permittivity of materials, we’ll going to explain the dielectric properties and similar explanation can be mounted for the magnetic properties. In general, a material is classified as “dielectric” if it has the ability to store energy when an external electric field is applied. Lets imagine a DC voltage source placed across a parallel plate capacitor, we’ll have the ability to store an electric charge and called the capacitance ‘C’ defined to be the charge ‘q’ per applied voltage ‘V’ that is C0 = q/V, where C is given in coulombs per volt or farad. The capacitance is higher, the larger the area A of the plates and the smaller the distance L between them. And so, C0 = ε0 (A/L), where ε0 is a universal constant having the value of 8.85 × 10−12 F/m (farad per meter) and is known by the name permittivity of empty space (or of vacuum). Now lets insert a material between the plates, the new value of the capacitance ‘C’ is increased by a factor ε = C/C0, which lead to C = εε0 (A/L), where ‘ε’ represents the magnitude of the added storage capability. It is called the (unit less) dielectric constant (or occasionally the relative permittivity εr). So, for the complex representation εr= εr'-j εr", the real part of permittivity (εr') is a measure of how much energy from an external electric field is stored in a material. The imaginary part of permittivity (εr") is called the loss factor and is a measure of how dissipative or lossy a material is to an external electric field. The imaginary part of permittivity (εr") is always greater than zero and is usually much smaller than (εr'). the loss is usually denoted by: the loss tangent or ‘tan δ’ which is defined as the ratio of the imaginary part of the dielectric constant to the real part. The loss tangent ‘tan δ’ is called tan delta, tangent loss or dissipation factor. 13