Technical aspects of dielectric spectroscopy measurements of liquid crystals

Perkowski, P.; Łada, D.; Ogrodnik, K.; Rutkowska, Jolanta; Piecek, Wiktor; Raszewski, Zbigniew

doi:10.2478/s11772-008-0008-1

Cited by 36 publications

(26 citation statements)

References 2 publications

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“…The imaginary part of the permittivity increases at low frequencies when the MWCNTs are present in the PEDOT-LC system, increasing dielectric losses and reducing the impedance magnitude. This effect has been observed in other studies as well [31,32]. Thus, the change in the impedance seems the result of two combined effects: the electrical interaction of the alignment layer with the MWCNT, and the lack of orientation of a MWCNT fraction in the blend.…”

Section: Bode Plotssupporting

confidence: 77%

The peculiar electrical response of liquid crystal-carbon nanotube systems as seen by impedance spectroscopy

García-García¹,

Vergaz²,

Algorri³

et al. 2015

J. Phys. D: Appl. Phys.

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Conductive nanoparticles, especially elongated ones such as carbon nanotubes, dramatically modify the electrical behavior of liquid crystal cells. These nanoparticles are known to reorient with liquid crystals in electric fields, causing significant variations of conductivity at minute concentrations of tens or hundreds ppm. The above notwithstanding, impedance spectroscopy of doped cells in the frequency range customarily employed by liquid crystal devices, 100 Hz-10kHz, shows a relatively simple resistor/capacitor response where the components of the cell can be univocally assigned to single components of the electrical equivalent circuit. However, widening the frequency range up to 1 MHz or beyond reveals a complex behavior that cannot be explained with the same simple EEC. Moreover, the system impedance varies with the application of electric fields, their effect remaining after removing the field. Carbon nanotubes are reoriented together with liquid crystal reorientation when applying voltage, but barely reoriented back upon liquid crystal relaxation once the voltage is removed. Results demonstrate a remarkable variation in the impedance of the dielectric blend formed by liquid crystal and carbon nanotubes, the irreversible orientation of the carbon nanotubes and possible permanent contacts between electrodes.

show abstract

Section: Bode Plotssupporting

confidence: 77%

The peculiar electrical response of liquid crystal-carbon nanotube systems as seen by impedance spectroscopy

García-García¹,

Vergaz²,

Algorri³

et al. 2015

J. Phys. D: Appl. Phys.

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“…In ITO cells, the main problem is the high elec− trodes resistivity R. When ITO in measuring cell is replaced with gold, the next problem related to the wires inductance L appears. Both effects are mentioned in other papers [1,6]. ITO resistivity effect manifests as a faster decrease of ¢ e EF (as a frequency function) than it can be expected from liquid crystal properties.…”

Section: Introductionmentioning

confidence: 82%

“…In many papers, difficulties with high frequency measure− ments are indicated which are related to finite conductivity of ITO electrodes in measuring cells [1][2][3][4][5][6][7][8][9]. In Ref.…”

Section: Introductionmentioning

confidence: 99%

Dielectric spectroscopy of liquid crystals. Electrodes resistivity and connecting wires inductance influence on dielectric measurements

Perkowski

2012

Opto-Electronics Review

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Dielectric spectroscopy is a very useful experimental method for liquid crystal investigation. Electrodes made from indium tin oxide (ITO) or gold are widely used in measuring cells. During the dielectric spectroscopy measurements performed for smectic liquid crystalline mixture it was found that detection of some important relaxation modes in paraelectric SmA*, ferroelectric SmC* and antiferroelectric SmCA* phases for frequencies higher than 0.2–0.5 MHz is not possible. The measuring setup does not allow us to measure such relaxations due to its own dielectric response covering the dielectric response of liquid crystalline medium. One can observe the spurious contribution for high frequency part of the dielectric spectrum, due to non-zero resistivity of electrode material or non-zero inductivity of connecting wires. In this paper, the new model was introduced. Its final equations show how to calculate parameters of relaxations observed in liquid crystals, from dielectric response of the empty and filled measuring cell. The experimental proof of strong influence of measuring setup properties on effective (measured) values of dielectric permittivities was shown.

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“…This part is propor− tional to the frequency of a measuring signal. It seems that technical parameters of measuring cells are very important [1]. Dielectric response of the cell covers the dielectric re− sponse of the measured liquid crystal.…”

Section: Introductionmentioning

confidence: 99%

“…In many papers difficulties with high frequency measure− ments are indicated which are related to finite resistivity of ITO electrodes in measuring cells [1][2][3][4]. In Ref.…”

Section: Introductionmentioning

confidence: 99%

Dielectric spectroscopy of liquid crystals. Theoretical model of ITO electrodes influence on dielectric measurements

Perkowski

2009

Opto-Electronics Review

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Indium tin oxide (ITO) layers are widely used to make electrodes in measuring cells, because these layers are transparent and electrooptical investigations can be performed using such prepared cells. It was found during the dielectric spectroscopy measurements, performed for smectic liquid crystalline mixture, that it is not possible to detect some important relaxation modes in paraelectric SmA*, ferroelectric SmC*, and antiferroelectric SmCA* phases for the frequencies higher than 300 kHz. The measuring cell does not allow to measure relaxations, because its own dielectric behaviour covers the dielectric response of a liquid crystalline medium. One can observe the spurious contribution for high frequency part of the dielectric spectrum, due to the finite resistance of ITO layers. The theoretical model was introduced, which shows how to calculate relaxations related to liquid crystals from dielectric response of the empty and filled measuring cell. The proof of strong influence of cell properties on effective (measuring) values of dielectric permittivities was shown.

show abstract

Technical aspects of dielectric spectroscopy measurements of liquid crystals

Cited by 36 publications

References 2 publications

The peculiar electrical response of liquid crystal-carbon nanotube systems as seen by impedance spectroscopy

The peculiar electrical response of liquid crystal-carbon nanotube systems as seen by impedance spectroscopy

Dielectric spectroscopy of liquid crystals. Electrodes resistivity and connecting wires inductance influence on dielectric measurements

Dielectric spectroscopy of liquid crystals. Theoretical model of ITO electrodes influence on dielectric measurements

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