Distribution of Grain Boundary Properties

About 30 individual grain boundaries, i.e. 30 different electrode configurations were investigated by microcontact impedance spectroscopy. The resulting histograms of the resistance, capacitance and relaxation frequency obtained from an equivalent circuit fit of the low-frequency arc are shown in Fig. 38. For comparison, a conventional (macroscopic) impedance measurement was performed on an identically prepared sample. The relaxation frequency of the grain boundary semicircle is indicated in Fig. 38c by a solid line. 

It is particularly interesting to test whether an analysis of a conventional macroscopic impedance measurement based on a brick layer model leads to the same results as obtained with microcontacts. The relaxation frequency of the grain boundary arc measured in the conventional impedance experiment (1.2 Hz) is similar to the mean value (3.0 Hz) deduced from the microelectrode experiments, although not identical. One possible reason for the moderate discrepancy is the inaccuracy with respect to the temperature measurement, which is somewhat difficult in the case of the microelectrode set-up. A temperature error of about 20 K could already explain the difference. 

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Fig. 13. (a) Sketch of the microelectrode configuration used to investigate the distribution of grain boundary properties, (b) Typical impedance spectrum calculated for a model sample (inset) consisting of 24 cubic grains and two microelectrodes on adjacent grains. An equivalent circuit consisting of two serial RC-elements (inset) can be used to fit the spectrum. 

A quantitative analysis of grain boundary impedances measured with macroscopic electrodes can be rather problematic if grain boundary properties vary from boundary to boundary (cf. Sec. 3.2). Hence, additional information on the distribution of grain boundary resistivities is often desired. Microelectrode measurements can yield such additional information (Sec. 4.2) and below a microcontact impedance spectroscopic study of grain boundaries in a polycrystal is exemplarily presented. The material of choice is again SrTiCE (0.2 mol % Fe-doped), which represents a model material for the technologically highly important class of perovskite-type titanates (see also above). 

The well-known nonequilibrium process of plasma chemical synthesis has found practical application for a large number of compounds and compositions. However, in recent years more attractive and well-developed processes of synthesis become gas phase condensation under quasi-equilibrium conditions of moderate heat and mass transfer. This process becomes preferable over latter plasma chemical synthesis due to its ability to control the thermal regime and more flexibility in regard to dispersion and purity of the synthesized product. Uniformity of particle size and chemical composition (powder purity) are essential for fabrication of nanocomposites or dense nanocrystalline materials with improved physical, chemical and mechanical properties. This is because the particle size distribution determines the stability of grains during consolidation of a polycrystal while the concentration of impurities affects properties of grain boundaries and entire material (Table 5.1). 

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