Nobuhiro Okabe

On the appearance of antiferroelectricity: The effect of polarization and conjugation in antiferroelectric liquid crystals

Abstract In order to clarify the role of polarization and the conjugation in antiferroelectric liquid crystal molecules (AFLC), systematic series of AFLC compounds containing fluoro-substituent have been synthesized. The polarization of peripheral region of the chiral center and the conjugation along the long axis of an AFLC molecule are responsible for the appearance of the antiferroelectricity. Based on these experimental findings, the conjugated system expected for AFLC molecules is tentatively proposed.

Influence of Optical Purity on the Smectic-Cβ* Phase of Antiferroelectric Liquid Crystals

The Smectic- Cβ* (SmCβ*) phase, which appears between the SmA and SmCA* phases in an optically pure liquid crystal of MHPOBC, has been studied by means of simultaneous measurements of the dielectric constant e and the electroclinic coefficient κ under a dc bias electric field. The result of electroclinic coefficient measurement shows that the SmCβ* phase is obviously different from the SmC* phase and can be considered as a ferrielectric phase. From the simultaneous measurement of e and κ, we can observe the difference in the phase transition behaviors between pure and slightly racemized MHPOBC samples and confirm that the decrease of optical purity causes the change from the SmCβ* phase to the SmC* phase.

Simultaneous improvement in both exhaust emissions and fuel consumption by means of Fischer–Tropsch diesel fuels

In recent years, paraffinic fuels have attracted attention because of their potential for reducing diesel exhaust emissions, mainly smoke or particulate matter emissions. One of the paraffinic fuels, a Fischer–Tropsch diesel fuel, was selected to demonstrate the lower exhaust emissions while improving fuel economy in this study. To examine the detailed effects of fuel specifications on diesel combustion and emissions, preliminarily tests for three Fischer–Tropsch fuels and a baseline diesel fuel were carried out with three diesel engines having different engine displacements. In addition, differences in combustion phenomena between Fischer–Tropsch fuels and the baseline diesel fuel were observed by means of a single-cylinder engine with optical access. From these findings, one of the tested engines was modified to improve both exhaust emissions and fuel consumption, simultaneously, dedicated to the use of neat Fischer–Tropsch fuels. The conversion efficiency of an oxides of nitrogen reduction catalyst has also been improved. The desirable properties of Fischer–Tropsch fuels for diesel combustion, namely high cetane number and absence of poly-aromatic hydrocarbon contents, have been fully utilized to enhance the conventional diesel combustion limits to show the possibility to achieve very low exhaust emissions with substantial improvement in fuel economy. The results of this study indicate not only the superior emission characteristics of the Fischer–Tropsch fuels, but also evidence that higher exhaust gas recirculation and lower excess air ratios will be a key concept of both engine and aftertreatment optimization for further fuel consumption improvement.

Bilayered Super-Structures of Antiferroelectric Mesogens

Abstract Crystal structures of two antiferroelectric mesogens, TFMHPBC and MHPBC-10, were analysed by an X-ray diffraction method. In both crystals, mesogen molecules formed a herringbone structure which was essentially the same as that proposed for the antiferroelectric liquid crystal phase. Because of the crystallographic 21-symmetry along the b-axis, only the b-axis component of the dipole moment remains in a smectic layer. Since the dipole moment in the next layer has the same magnitude but the opposite direction, both crystals show no dipole moment as a whole. These structural features observed in their crystal states seem to be conserved in their antiferroelectric liquid crystal phases which are just above their crystal phases.

Dielectric and Electrooptical Responses of Antiferroelectric Liquid Crystals in the Smectic-A Phase

The Smectic-A (SmA) phases of the antiferroelectric liquid crystals (R)-MHPOBC, (R)-MHPOCBC and slightly racemized MHPOBC (R:S=17:3) have been studied by means of simultaneous measurements of the electric displacement and the tilt angle induced by the electric field. The SmA–SmCα* phase transition was observed in (R)-MHPOBC and (R)-MHPOCBC, while a phase transition similar to the SmA–SmC* one was observed in slightly racemized MHPOBC. In both cases, the Landau-type phenomenological theory explains well the critical behavior in the SmA phase, and the coefficients related to the theory can be determined.

Nonlinear dielectric relaxation spectroscopy of antiferroelectric liquid crystals

The nonlinear dielectric relaxation spectra of ferroelectric liquid crystals (FLCs) have been studied in the chiral smectic-C phase. The linear and third-order nonlinear dielectric spectra show the relaxation corresponding to the fluctuation in the azimuthal angle of directors called the Goldstone mode. We calculated the nonlinear dielectric spectra of the Goldstone mode theoretically by the torque balance equation which describes the dynamics of FLCs under the electric field. The calculated spectra make good agreement with the measured ones. We also evaluated the material constants of FLCs from the best-fitted values of the linear and nonlinear dielectric increment and relaxation time.

Electrical and Optical Responses of an Antiferroelectric Liquid Crystal in SmA, SmCα* and SmC* Phases

Abstract The SmA, SmCα* and SmC* phases of an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl) phenyl 4‘-octyloxybiphenyl-4-carboxylate (MHPOBC) have been studied by means of simultaneous measurements of the dielectric constant and the electroclinic coefficient under a dc bias electric field. From the three dimensional representation of the electroclinic coefficient as a function of dc electric field and temperature, it is confirmed that there is no helix with a long pitch in the SmCα* phase and the electric field-temperature phase diagram for these phases is obtained. In the SmA phase near the phase transition temperature, the temperature dependences of the dielectric constant and the electroclinic coefficient are analyzed in terms of the phenomenological theory of Landau type and the parameters appearing in the theory are determined.