Dielectric systems, high-precision

Network analysis (6.3.3) 100 MHz-10 GHz Both reflected wave and wave transmitted through sample are analyzed in terms of phase and amplitude using common network analyzers High precision attained for materials of sufficient conductivity and dielectric strength The Rhode Schwarz ZVRE vector network analyzer in combination with a dielectric sample holder and a compatible temperature system... 

Unfortunately, other experimental factors, such as contact capacitance at the junction of the cell leads and the measurement system, lead capacitance, and capacitance due to the dielectric properties of the thermostatting medium, may contribute substantially to the parallel capacitance. These effects may be minimized by proper choice of cell design and use of oil rather than water in the thermostatting bath. The art of making ac conductance measurements has been refined to a high degree of precision and accuracy, and detailed discussions of the rather elaborate procedures that are often necessary are available [9,10]. 

The relative dielectric permittivity of open porous materials (e.g. aerogels) especially its variation with ambient conditions is very important for materials application. A precise measurement has to be assured because of highly porous materials has a value close to unity in the limit of 100 % porosity. Also the variation of due to changes in the environmental parameters (humidity etc.) might be small on a absolute scale, but large on a relative one. This had been the case in previous studies who focused on the relationship between the adsorption of water and/or chemical compounds and of porous systems, e.g. zeolithes[l] and Si02 aerogels [2]. To avoid misinterpretation of the data the measurement has to be checked for the influence of cables and of the electronic devices etc. 

Electrochemical methods have played an important role in the recognition of cation radicals as intermediates in organic chemistry and in the study of their properties. An electrode is fundamentally an electron-transfer agent so that, given the proper solvent system, anodic oxidation allows formation of the cation radical without any associated proton or other atom transfer and without the formation of a reduced form in the immediate vicinity of the cation radical. Moreover, because the potential of the electrode can be adjusted precisely, its oxidizing power can be controlled, and further oxidation of the cation radical can often be avoided. Finally, the electrochemical experiment can involve both production of the cation radical and an analysis of its behavior, so that information about the thermodynamics of its formation and the kinetics of its reaction can be obtained, even if the cation radical lifetime is as short as a few milliseconds. There are some limitations, however, in the anodic production of cation radicals. The choice of solvent is limited to those that show reasonable conductivity with a supporting electrolyte (e.g. tetra-n-butylammonium perchlorate, TBAP). Acetonitrile, methylene chloride and nitrobenzene have been employed as solvents, but other favorites, such as benzene and cyclohexane, cannot be used. The relatively high dielectric constant of the suitable... 

Control of the Sample Environment High-vacuum conditions or constant inert gas flow are necessary for studying intrinsic relaxation phenomena in hygroscopic materials. A chemically inert atmosphere is also desirable in studies of polymer solutions, biomacromolecules, and other biological substances, where precise control of the hydration levels is vital. In isothermal scans fluctuations of the sample temperature should be as small as possible (below 0.1 K), such fluctuations are usually controlled by most commercial temperature stabilization/regulation systems (see Section 6.4.1.2). The same recommendation applies for the heating rates typically used in nonisothermal dielectric techniques. 

---