Inhomogeneity measurement techniques

The definition of inhomogeneous thermoelectric properties and the measurement techniques of FGM element also include several technological problems to be solved. 

The 10 A—1 jum size region is a difficult one in which to detect inhomogeneities. Direct techniques include the observation of the inhomogeneities by electron microscopy, and the observation of radiation (optical. X-ray or electron) scattered by the inhomogeneities In a less direct approach, the presence of inhomogeneities is inferred from their anticipated effect on measured properties thermal, optical, and electrical (discussed in Chapter 5) for example. These techniques are now briefly reviewed. 

Strictly speaking, the laser Doppler technique is an indirect measuring technique because it measures the velocity inhomogenities in the flow, typically tracer particles. These tracers represent the fluid flow velocity only if no appreciable slip velocity is present. Otherwise the slip velocity must also be determined. In particular, if these tracer particles are small, in the order of microns, they can be assumed to be good flow tracers following the flow and thus their velocity corresponds to the fluid velocity. 

Preliminary measurements with space-resolved PMC techniques have shown that PMC images can be obtained from nanostructured dye sensitization cells. They showed a chaotic distribution of PMC intensities that indicate that local inhomogeneities in the preparation of the nanostructured layer affect photoinduced electron injection. A comparison of photocurrent maps taken at different electrode potentials with corresponding PMC maps promises new insight into the function of this unconventional solar cell type. 

Minimisation of sample preparation is the main bottleneck in polymer/additive analysis. The importance of sample preparation increases with miniaturisation of the separation techniques. However, there is no point in improving instrumentation when the true sources of errors in measurement are sampling, sample inhomogeneity or sample instability. 

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