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139 results on '"Kresse, H."'

44. Dielectric characterization of the B[sub 7] and a B[sub X] phase.

Author

Salfetnikova, J., Nadasi, H., Weissflog, W., Hauser, A., and Kresse, H.

Subjects
DIELECTRIC relaxation, FERROELECTRIC crystals
Abstract

The first clear experimental evidence of a dielectric relaxation in the B[sub 7] phase is given. The associated molecular process is probably the reorientation of the bent-shaped molecules about the long molecular axis. It is also shown that the dielectric constant strongly increases at the phase transition I-B[sub 7] pointing to a co-operative motion of ferroelectric clusters. Furthermore, a phase transition B[sub 7]-B[sub X] is observed. The values of about 10 for the high frequency limit of the dielectric constants of the B[sub 7] and B[sub X] phases and the static dielectric constants of the isotropic phase are in agreement. Thus, in all the phases the main dipoles can reorient faster than 10[sup -8]s, i.e. the experimental limit. Only in the supercooled B[sub X] phase were the dynamics of the transversal dipoles measured. Using atomic force microscopy, focal-conic domains modulated by parallel lines have been observed at room temperature. [ABSTRACT FROM AUTHOR]

Published
2002
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45. Characterization of the B[sub 4] phase by dielectric and AFM measurements.

Author

Kresse, H., Salfetnikova, J., Nadasi, H., Weissflog, W., and Hauser, A.

Subjects
*PHASE transitions, *ATOMIC force microscopy, *DIELECTRIC measurements
Abstract

Dielectric measurements on a sample consisting of banana-shaped molecules were carried out in a frequency range between 0.1 Hz and 10 MHz. The sample exhibited B[sub 2] and B[sub 4] phases. As usual, two ranges of relaxation were detected in the B[sub 2] phase, the fast reorientation about the long axes of the molecules and a slow collective process. In the B[sub 4] phase, only one dielectric active process at low frequencies was found; this does not differ from the low frequency relaxation of the B[sub 2] modification. This relaxation is probably related to the dynamics of superstructures. Crystallization could be observed after keeping the sample for a longer time at higher temperature. Thus, it was possible to differentiate clearly between the crystalline and the B[sub 4] phases. AFM investigations prove the existence of focal-conic domains and periodic superstructures in the B[sub 4] phase; then do not appear in the crystalline state. For this reason the B[sub 4] phase is regarded as different from a classical crystalline phase. [ABSTRACT FROM AUTHOR]

Published
2001
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46. Dielectric characterization of B[sub 3] and B[sub 4] phases.

Author

Salfetnikova, J., Schmalfuss, H., Nadasi, H., Weissflog, W., and Kresse, H.

Subjects
POLYMORPHISM (Crystallography), LIQUID crystals, ELECTRONICS
Abstract

Dielectric measurements on two samples formed from banana-shaped molecules with terminal alkyl or alkyloxy groups were carried out in a frequency range between 10[sup -3] Hz and 10 MHz. Both samples exhibit B[sub 2] and B[sub 3] phases; one of them has a B[sub 4] state as well. As usual, two ranges of relaxation were detected in the B[sub 2] phase, the fast reorientation about the long axes of the molecules and a slow collective process. Only one dielectrically active low frequency process could be separated in the B[sub 3] and B[sub 4] phases; this is probably related to the dynamics of superstructures. The high frequency limit of the dielectric constants gives a hint that neither phase is a classical solid. A quite strong increase in the conductivity at the transition into the B[sub 3] phase is critically discussed. [ABSTRACT FROM AUTHOR]

Published
2000
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47. SmA-SmC-B[sub 2] polymorphism in an achiral cyano substituted banana-shaped mesogen.

Author

Weissflog, W., Kovalenko, L., Wirth, I., Diele, S., Pelzl, G., Schmalfuss, H., and Kresse, H.

Subjects
LIQUID crystals, POLYMORPHISM (Crystallography)
Abstract

A new achiral five-ring banana-shaped mesogen is presented which has a cyano substituent in the 4-position of the central core. Microscopical, X-ray, dielectric and electro-optical investigations give evidence for the unusual phase sequence B[sub 2], SmC, SmA. The B[sub 2] phase shows an antiferroelectric switching behaviour; the spontaneous polarization was found to be 330 nCcm[sup -2]. [ABSTRACT FROM AUTHOR]

Published
2000
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48. Structural and electro-optical investigations of the smectic phase of chlorine-substituted banana-shaped compounds.

Author

Pelzl, G., Diele, S., Grande, S., Jakli, A., Lischka, Ch., Kresse, H., Schmalfuss, H., Wirth, I., and Weissflog, W.

Subjects
MOLECULES, ELECTROOPTICS, ELECTRONICS
Abstract

Six members of a new homologous series of achiral banana-shaped molecules have been synthesized and studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy and X-ray diffraction. According to X-ray diffraction measurements on oriented samples, four homologues form an XB2 phase behaviour. From electro-optical studies the spontaneous polarization and the tilt angle could be measured. An orientational order parameter of 0.8 was determined by 13C NMR and this is nearly independent of the temperature. NMR investigations also give information about the real conformation of the molecules in the XB2 phase. Dielectric measurements indicate that the rotation around the molecular long axis is clearly hindered because of the packing of the bent molecules within the smectic layers. which exhibits antiferroelectric switching. [ABSTRACT FROM AUTHOR]

Published
1999
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