Sample cell for the microwave spectroscopy of ionic liquids. The cell having a shape of coaxial cable is composed of a platinum tube serving as an outer conductor, a platinum wire as an inner conductor, and two thick-walled fused quartz tubes as insulators. The liquid sample is introduced to a gap between the two quartz tubes through the sample entrance and the clearance between the Pt tube and quartz tube. The cell is sealed by indium O-rings to keep it air-tight. The microwave generated by a vector network analyzer (VNA) enters the cell and returns to VNA via the coaxial connectors located at both ends of the cell. The central part of the cell is heated up by a resistance heater, into which a bored copper block is inserted to minimize the temperature gradient.
The amplitude of a transmission signal (a) and that of a reflection signal (b) in the frequency domain for at . The real part of the transmission signal (c) and that of the reflection signal (d) in the time domain, obtained by Fourier transformation of and , respectively. In (d), the reflection from the sample appears around 1 ns, and the reflection from the electrode around 0 ns.
Amplitude of the transmission spectra measured at every from toward a high temperature indicated in each panel for (a) , (b) , (c) , and (d) .
Temperature dependence of the electrical conductivity for , , and . Present experimental data are denoted by black closed symbols with error bars, and the published data are plotted by various colored symbols without error bars. The symbol due to Ref. 35 at is indistinguishable from that due to Ref. 32.
Real part of the complex dielectric permittivity, , deduced approximately by Eqs. (4)–(7) for at . Using a raw spectrum, a crude form of is obtained (dashed line). When the effect of electrodes is partially eliminated with the help of back Fourier transformation, is improved (solid line), from which the initial values of the fitting parameters are determined.
Temperature dependence of for , , , and . Present experimental data are denoted by closed symbols, and the published data are plotted by various open symbols.
Imaginary part of the complex dielectric function, (solid line), and the dc conductivity term (chain line) for at . The dashed line denotes a Debye term in (a), and a Cole–Davidson term in (b). The dotted line denotes a high frequency Debye term in both (a) and (b). The thin line in (b) denotes a Cole–Davidson term reported in Ref. 12 and used as initial values of fitting parameters in this study.
The relation between and predicted by EMA for with , and for (inset) at . In the inset, the double circle denotes the central values for , and the closed circle denotes the values for evaluated by assuming that of is the same as that of , as shown by the dashed lines.
Best-fit parameters of , , , , and are listed together with the experimentally obtained values of (in the first column) and the resulting (in the last column). Except for (Ref. 29), are present results.
Molar volumes of the polar and nonpolar domain for , , , and . in the third row is the volume fraction of the polar domain.
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