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1.
The principal dielectric constants of the pentyl to octyl derivatives of 4′-n-alkyl-4-cyanobiphenyl have been measured as functions of temperature in their nematic and isotropic phases. All the compounds
exhibit a strong positive dielectric anisotropy due to the presence of a large dipole moment along the major molecular axis.
The principal dielectric constantsε
‖ andε
⊥ as well as the anisotropy Δε decrease with increasing alkyl chain length. The experimental value of
(ε‖+2ε⊥) decreases with decreasing temperature, and is throughout less than the extrapolated isotropic value, in conformity with
the model of antiferroelectric short range order proposed by Madhusudana and Chandrasekhar. 相似文献
2.
M. C. Rath S. K. Sarkar V. K. Wadhawan R. Verma I. M. L. Das R. Dąbrowski M. Tykarska R. Dhar 《Opto-Electronics Review》2008,16(4):399-403
Electron beam irradiation studies on liquid crystal material 5CB have been carried out at a temperature where the compound
exists in the isotropic liquid phase. In situ time-resolved spectroscopic characterization was carried out during the irradiation.
Three different transients were observed during the 2-μs electron pulse. After about 50 μs, only one transient species was
found to be present, which has an absorption peak at 360 nm. Radiolysed sample exhibits a broad absorption at ∼400 nm. The
dielectric measurements show that even a low level of irradiation results in a dramatic increase in the component of dielectric
permittivity normal to the long axes of the molecules ε⊥′, and a corresponding decrease in the dielectric anisotropy (Δε′=ε∥′−ε⊥′ ). These studies show that 5CB is prone to substantial radiation damage on exposure to the beam of high-energy electrons. 相似文献
3.
B. Lalevic N. Fuschillo B. Kuliyev W. Wang 《Applied Physics A: Materials Science & Processing》1974,5(2):127-132
Transport and dielectric properties of polycrystalline CoO films were studied as functions of the applied field, frequency
and temperature. TheI–V plots showed that the Poole-Frenkel field emission mechanism is responsible for conduction at fields>105 V/cm. The ac conductivity σ(ω), the imaginary part of the dielectric constantε
2, and tan δ plots as functions of frequency revealed three dispersion regions. The σ(ω) andε
2 frequency dependence indicates a non-adiabatic hopping of charge carriers at low frequencies and adiabatic hopping at high
frequencies. The activation energy of a dielectric oscillator is 0.15 eV.
Work supported by the Office of Naval Research. 相似文献
4.
A. Shumelyuk D. Barilov S. Odoulov E. Krätzig 《Applied physics. B, Lasers and optics》2003,76(4):417-421
We apply, for the first time to our knowledge, photorefractive grating spectroscopy to obtain not-yet-known data on the anisotropy
of the dielectric permittivity of Sn2P2S6. Two independent techniques are used, one based on measurements of the amplitude of the space-charge field grating as a function
of grating spacing and the other based on measurements of the grating decay time, also as a function of grating spacing. Both
techniques provide close values for the anisotropy, which appears to be well pronounced, a ratio εxx/εzz≈4 is revealed for two of the three independent components of the dielectric tensor. Our data also allow us to conclude that
the charge mobility is nearly isotropic in the same plane, μxx/μzz≈1.
Received: 2 December 2002 / Published online: 26 March 2003
RID="*"
ID="*"Corresponding author. Fax: +380-44/265-2359, E-mail: odoulov@iop.kiev.ua 相似文献
5.
Vacuum deposited blackish indium oxide films (In-O) as well as the oxidised films (In2O3) were studied for their a.c. behaviour at different temperatures and at various film thicknesses in the audio frequency region.
ε of In-O films was thickness dependent and also showed dielectric relaxation at lower frequencies due to the dipolar orientation
arising from their non-stoichiometric nature. However at liquid nitrogen temperature region ε was thickness independent similar
to the oxidised films which neither showed any relaxation effect nor any thickness dependent ε. The results have been discussed
from the classical theory of dielectric polarisation. 相似文献
6.
J. Rutkowska P. Perkowski W. Piecek Z. Raszewski J. Kędzierski 《Opto-Electronics Review》2008,16(3):262-266
Dielectric measurements were performed for two smectogens, being structurally analogous with the opposite space orientation
of the-COO ester group in the molecular core, using a HP4192A impedance analyser. Gold coated electrode cells of different
thicknesses were used. Temperature dependences of the relaxation frequency fR and the inverse of the dielectric strength Δε′┴
−1 obtained by fitting experimental values of perpendicular components, i.e., the real ε′┴ and the imaginary ε″┴ parts of the complex dielectric permittivity to the Cole-Cole equation as well as the investigation of modification of relaxation
processes under bias were determined for the SmC*, SmA*, and N* phases of studied compounds. One can conclude, on the basis of the above results, that dielectric relaxation processes observed
by us in the studied compounds are similar to those of the soft and Goldstone mode typically observed by others in the SmC*, SmA*, and N* phases. It is concluded from acomparison of their properties with other related compounds that the link between the biphenyl
moiety and-COO ester group is closely related to the stability of smectic phases. 相似文献
7.
Empirical formula suggested by Kita and Koizumi for evaluation of relaxation intensity in a limited range of frequency around
the relaxation frequency for the Cole-Cole type distribution has been tested for poly butyl acrylate (PBA), Poly butyl methacrylate
(PBMA) and poly isobutyl methacrylate (PiBMA).
The relaxation intensity Δε is expressed in terms ofε″
M
, the dielectric loss maxima andW, the frequency separation for half, two thirds or three quarters ofε″
M
, in the form Δε=ε″
M
/[(C
1/W)+C
2+C
3
W], where the numerical constantsC
1,C
2,C
3 are given for the respective type of relaxation. 相似文献
8.
L. Gao X. P. Yu 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,55(4):403-409
We derive expressions for the effective nonlinear susceptibility
tensors for both the second harmonic generation (SHG) and induced
third harmonic generation (THG) of nonlinear composite materials,
in which nondilute coated particles with radial dielectric
anisotropy are randomly embedded in the linear host. Two types of
coated particles are considered. The first is that the core
possesses a second order nonlinear susceptibility and the shell is
linear and radially anisotropic, while the second is that the
core is linear with radial anisotropy and the shell has a second
order nonlinear susceptibility. We observe greatly enhanced SHG
and THG susceptibilities at several surface plasmon resonant
frequencies. For the second model, due to the coating material
being metallic, there exists two fundamental resonant frequencies
ωc1 and ωc2, whose difference
ωc2-ωc1 is strongly dependent on the interfacial
parameter and the radial dielectric anisotropy. Furthermore, in
both systems, the adjustment of the dielectric anisotropy results
in larger enhancement of both SHG and induced THG susceptibilities
at surface plasmon resonant frequencies than the corresponding
isotropic systems. Therefore, both the core-shell structure and
the dielectric anisotropy play important roles in determining the
nonlinear enhancement and the surface resonant frequencies. 相似文献
9.
Heng Ma Jun Xu Hiroyuki Okada Hiroyoshi Onnagawa Sigeru Sugimori Kazuhisa Toriyama 《Current Applied Physics》2013,13(5):814-818
This paper describes an experimental investigation for physical properties of fluorinated phenyl bicyclohexane liquid crystals (LCs) mixtures. Measurements are reported of the large anisotropic dielectric properties of two structurally similar LCs (1,2-difluoro-4-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl] benzene and 1-cyano-2-fluoro-4-[trans-4-(trans-4-propylcyclohexyl) cyclohexyl] benzene) mixed with a zero dielectric anisotropic LC (1-methyl-2-fluoro-4-[trans-4-(trans-4-ethylcyclohexyl) cyclohexyl] benzene). The results indicate that the mixing proportions strongly influence the dielectric constants along with the longer molecular axis. For two samples which is made from meta-para- or meta-fluorine substituted molecules, the temperature dependence of the anisotropic dielectric striking resemble that of a unitary compound with para-fluorine substituted. The threshold voltage of the mixtures relies on the element that possesses a large dielectric anisotropic. The mixing of the materials leads to a slight increase for splay elastic constants in an anti-parallel measurement cell. 相似文献
10.
The dispersion and anisotropy of the permittivity of the liquid crystal 5CB in the solid, nematic, and isotropic phases are
investigated at frequencies ranging from 50 to 1000 MHz using discretely tunable hybrid microstrip cavities. Resonance features
which grow with increasing temperature are observed in the dielectric spectra. It is found that the section of the dispersion
which is due to oriented vibrations of the mesophase molecules covers the frequency range almost up to 300 MHz, and the resonances
found could be due to conformational vibrations of the molecules.
Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 251–253 (25 August 1997) 相似文献
11.
W. Huang R. Richert 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,66(2):217-221
We investigate the nonlinear dielectric effects in a polar viscous liquid, propylene carbonate, by analyzing the voltage and
current traces obtained for a sinusoidal electric field at a frequency of 1 kHz and field amplitudes as high as 162 kV/cm.
The main source of non-linear behavior results from the energy absorbed from the field and is understood quantitatively. However,
there is a qualitative difference in the behavior of the field induced change, Δε′′(E), and the third harmonic amplitude of the current, I3ω. Although both Δε′′(E) and I3ω are considered reliable measures of non-linear behavior, we show here that the third harmonic signal reflects only those
non-linear responses that are instantaneous on the time scale of the test frequency. 相似文献
12.
V. R. Zakamov L. M. Kukin S. V. Krivov V. I. Shashkin 《Radiophysics and Quantum Electronics》2008,51(10):780-786
We consider the possibility of manufacturing a planar antenna on the dielectric layer with high permittivity, which is intended
for a monolithic planar detector of the millimeter-wave range. The previous design of the slot antenna on a foil-cladded dielectric
with low permittivity (ε = 2.2) is analyzed. The simulation results show that a transition from the substrate with low dielectric permittivity to
a semi-insulating GaAs substrate (ε ≈ 13) with the directivity kept at a level of about ∼10 at the resonant frequency is possible for antennas with external
sizes comparable with the wavelength. The parameters of the planar detectors on semi-insulating GaAs substrates in the 3-mm
wavelength range (94–97 GHz) were measured. Directional patterns of the planar antennas at the resonant frequencies reasonably
coincide with the calculated data.
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 10, pp. 864–871, October 2008. 相似文献
13.
R. M. A. Azzam 《Applied Physics A: Materials Science & Processing》1979,20(2):193-195
A new angle of incidence of significance, when considering the reflection of electromagnetic waves at interfaces between transparent
media, is defined. At this angle, denoted by φe, the reflection coefficient of parallel-polarized radiation is equal in magnitude and opposite in sign to the reflection
coefficient at normal incidence. No similar angle exists for the perpendicular polarization. If ε is the relative dielectric
constant, i.e., the ratio of the dielectric constant of the medium of refraction to that of the medium of incidence, we find
that tan φe=(ε2+ε)1/2. Measurement of φe, by equalization of the absolute (intensity) reflectances at normal and oblique incidence, allows ε to be determined using
the inverse relation ε=(tan2φe+1/4)1/2−1/2. 相似文献
14.
Eighteen binary mixtures of 4-pentyloxy-4′-cyanobiphenyl (5OCB) and 4, 4′-di(alkoxy)azoxybenzenes (nOAB) with n = 5, 6, and
7 at different concentrations have been studied in detail. Over a wide concentration range, induced SmA phase was observed
in all the mixtures except in one. In most of the mixtures, the refractive indices no, ne, and Δn lie between the values of
pure components. The optical birefringence of the mixtures is found to decrease non-additively with concentration. The molecular
polarizability anisotropy and the orientational order parameter <P2> of the mixtures were calculated. Nature of variation of polarizability anisotropy with concentration is similar to that
of Δn. In allmixtures <P2> is more in smectic phase than in nematic phase and it also shows non-additive behaviour. Observed <P2> values of the mixtures were compared with McMillan’s and Maier-Saupe theoretical values and good agreement is found in some
mixtures. The transition temperatures obtained by fitting <P2> data to McMillan’s theory agree with the observed values within a few degrees. Ratio of elastic constants K33 and K11 is determined for mixtures having nematic phase and found to diverge at TNA transition point. 相似文献
15.
Temperature-dependent dielectric characterization of 4-hexyl-4′-[2-(4-isothiocyanatophenyl)ethyl]-1,1′-biphenyl (HIEB), which exhibits smectic B and nematic phases, has been carried out over the frequency range of 1 Hz to 10 MHz for homeotropic and planar alignment of sample molecules. This compound shows positive dielectric anisotropy (Δε = ε ∥ ? ε ⊥ > 0) in the nematic (N) phase. One mode of dielectric relaxation showing Arrhenius behaviour has been detected in the hexatic smectic B (SmB) phase. Various electrical parameters, namely, dielectric anisotropy, relaxation frequency and activation energy, have been determined in the N and SmB phases. 相似文献
16.
E.I. Efremova A.A. Shiryaev Z.A. Kydryashova L.A. Nosikova S.A. Syrbue V.V. Chernyshev 《Phase Transitions》2015,88(5):503-512
The system of hydrogen-bonded liquid crystals formed from binary mixtures p-n-heptyloxybenzoic (I) acid and p-n-undecloxybenzoic (II) acid has been investigated by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction, and scattering and dielectric measurements. The T–X phase diagram was obtained for this system. All mixtures show enantiotropic smectic and nematic phases. The crystalline phase represents a (α, β) solid solution. At the 2:1 ratio of I to II, an intermediate phase – co-crystal γ – is formed. The co-crystal γ possesses a much wider mesophase range than the corresponding initial components. In contrast to mixtures based on the solid solutions having a positive anisotropy, the co-crystal shows a negative dielectric anisotropy in the mesophase. 相似文献
17.
Values of dielectric constant (ε′) and loss factor (ε″) have been experimentally determined for binary liquid mixtures of ethyl methyl ketone+ethylenediamine and methyl isobutyl
ketone+ethylenediamine at 9.44 GHz microwave frequencies at 30°C. The values ofε′ andε″ have been used to evaluate the molar polarization, apparent polarization and the excess permittivities. Excess refractive
index, viscosity and activation energy of viscous flow have also been estimated. These parameters have been used to explain
the formation of 1:1 complexes for both the systems. 相似文献
18.
Ruiting Sun Xiaoli Wang Jing Shi Le Wang 《Applied Physics A: Materials Science & Processing》2011,104(1):129-133
New perovskite solid solution ceramics of (1−x)BaTiO3-xBi(Mg1/2Ti1/2)O3 ((1−x)BT-xBMT, x≤0.09) were synthesized by the solid-state reaction technique. X-ray diffraction studies have revealed a stable single perovskite
structure for all samples. Dielectric measurements were carried out at different frequencies and temperatures. The polarization
evolutions with temperatures were measured to investigate the ferroelectric properties. All the compositions show features
of ferroelectrics with diffuse phase transition, though the temperature T
m
of their dielectric constant maximum ε
m
is frequency dependent. The dielectric constant peak ε(T) of (1−x)BT-xBMT ceramics become broad with increasing BMT content. During the temperature range of ε(T) peak summit, (1−x)BT-xBMT ceramics present quasi-linear dielectric phenomenon under high electric field with very high dielectric constant. 相似文献
19.
P. Kula A. Spadło J. Dziaduszek M. Filipowicz R. Dąbrowski J. Czub S. Urban 《Opto-Electronics Review》2008,16(4):379-385
Compounds with moderate and large negative dielectric anisotropy (Δɛ) are very attractive liquid crystal (LC) for vertical
alignment mode (VA). Materials with such properties can be achieved by lateral substitution of a polar group into a mesogenic
molecule. We synthesized some new LC materials with a negative value of Δɛ, a moderately high birefringence (Δn), and a low
viscosity. The mesomorphic and physical behaviour of the novel biphenyls, terphenyls and quaterphenyls fluorosubstituted in
the rigid core and also with fluorinated alkyl and alkoxy chains are investigated. The prepared series of four LC compounds
are promising for new LC mixtures for various applications. Examples of nematic mixtures with Δɛ∼−3.25 will be presented. 相似文献
20.
S. A. Pikin E. I. Demikhov E. S. Pikina M. V. Gorkunov 《Journal of Experimental and Theoretical Physics》1997,84(3):506-515
The dynamics of spontaneous polarization switching of the ferroelectric smectic C* in a variable electric field are examined theoretically and experimentally with the help of polarized light scattering.
The observed effect of quasiresonant scattering both in freely suspended smectic films and in ordinary electro-optical cells
is interpreted within the framework of the nonlinear model of isolated movable kinks in the director orientation distribution.
It is shown that the maximum of the scattering intensity at the characteristic frequency of the applied electric field disappears
at low temperatures and for small thicknesses of the smectic film. The dependence of the “resonant” frequency on the electric
field amplitude, the proximity to the phase transition temperature, the film thickness and thickness of the ferroelectric
domains, and also various material parameters is found. Estimates are made of such important characteristics as the dielectric
anisotropy, viscosity, and elasticity of the smectic films. The effect of film thickness on the density distribution of the
polar anisotropy energy in the film and on the corresponding shape of the moving orientation front within the film are discussed.
Zh. éksp. Teor. Fiz. 111, 919–937 (March 1997) 相似文献