Dielectric insulating layers, model

Concerning the two-layer model, the thickness and properties of each layer depend on the nature of the electrolyte and the anodisation conditions. For the application, a permanent control of thickness and electrical properties is necessary. In the present chapter, electrochemical impedance spectroscopy (EIS) was used to study the film properties. The EIS measurements can provide accurate information on the dielectric properties and the thickness of the barrier layer [13-14]. The porous layer cannot be studied by impedance measurements because of the high conductivity of the electrolyte in the pores [15]. The total thickness of the aluminium oxide films was determined by scanning electron microscopy. The thickness of the single layers was then calculated. The information on the film properties was confirmed by electrical characterisation performed on metal/insulator/metal (MIM) structures. 

Second, the model of Figure 3.1 is predominantly a dielectric model with dry samples. In bioimpedance theory, the materials are considered to be wet, with double layer and polarization effects at the metal surfaces. Errors are introduced, however, that can be reduced by introducing three- or four-electrode systems (Section 7.10). Accordingly, in dielectric theory, the dielectric is considered as an insulator with dielectric losses in bioimpedance theory, the material is considered as a conductor with capacitive properties. Dry samples can easily be measured with a two-electrode system. Wet, ionic samples are prone to errors and special precautions must be taken. 

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