Frequency: electrical measurement, 579 probability

Conventional two-electrode dc measurements on ceramics only yield conductivities that are averaged over contributions of bulk, grain boundaries and electrodes. Experimental techniques are therefore required to split the total sample resistance Rtot into its individual contributions. Four-point dc measurements using different electrodes for current supply and voltage measurement can, for example, be applied to avoid the influence of electrode resistances. In 1969 Bauerle [197] showed that impedance spectroscopy (i.e. frequency-dependent ac resistance measurements) facilitates a differentiation between bulk, grain boundary and electrode resistances in doped ZrC>2 samples. Since that time, this technique has become common in the field of solid state ionics and today it is probably the most important tool for investigating electrical transport in and electrochemical properties of ionic solids. Impedance spectroscopy is also widely used in liquid electrochemistry and reviews on this technique be found in Refs. [198 201], In this section, just some basic aspects of impedance spectroscopic studies in solid state ionics are discussed. 

Rf critical magnetic fields of Bent = 35 mT corresponding to peak electric fields of Ep = 19 MV/m were measured. It would be preferable to blame the precipitates with TeS below 4 K for the small Beru obtained so far and not sonie fundamental limitation. Higher Bent can probably be obtained by optimization of the preparation process, as indicated by results achieved at X-band frequencies [ ]. 

The noise spectral density is l/f type in the frequency band 10 mHz to 300 Hz in normal and reverse operation mode. Noise spectral density is a quadratic function of the current, when the electric field strength in isolating layer is so low that avalanche process caimot occur. Measurement performed at very low frequency band 10 mHz to 1 Hz reveals that for some samples noise is l/f type, but it was observed some time instability, which is probably related to the self-healing process. 

The temporal response of the system was also measured. The SLM optical switching time was estimated to be roughly 250 ps, as the sum of the electric storage and FLC material response times (Alarcdn, 2004). We also noticed a damped response when low-frequency switching is carried out this is probably due to relaxation of the FLC molecules. 

For measurements made with low scan speeds, not only is the effect of l/f noise greatest at low wavenumber, but there is also a high probability of interference by harmonics of the electrical line frequency. 

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