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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.
Laijun Liu Yanmin Huang Congxue Su Liang Fang Meixia Wu Changzheng Hu Huiqing Fan 《Applied Physics A: Materials Science & Processing》2011,104(4):1047-1051
Sodium potassium niobate K0.5Na0.5NbO3(KNN) ceramic was synthesized by a solid-state technique. The X-ray diffraction of the sample at room temperature showed a
monoclinic phase. The real part (ε′) and imaginary part (ε″) of dielectric permittivity of the sample were measured in a frequency range from 40 Hz to 1 MHz and in a temperature range
from 350 to 850 K. The ε′ deviated from Curie–Weiss law above 702 K, due to additional dielectric contributions resulting from universal dielectric
response and thermally activated space charges at high temperatures. This anomaly arose from a Debye dielectric dispersion
that slowed down following an Arrhenius law. We have established a link between the dielectric relaxation and the conductivity. 相似文献
3.
Observation of relaxation modes in room temperature ferroelectric liquid crystal mixtures 总被引:1,自引:0,他引:1
The dielectric measurements in SmC* and SmA phases of a room temperature ferroelectric liquid crystal mixture FLC-6980 in
the cells of different thickness in planer alignment have been carried out in the frequency range 100 Hz to 1 MHz. A relaxation
mode (called NRM) whose dielectric increment is less than the Goldstone mode has been observed in the SmC* phase. This mode
appears due to the surface effect. Goldstone mode and the soft mode was observable in the vicinity of SmC*-SmA transition
temperature (T
C*A). The dielectric parameters of the Goldstone mode, new mode and the soft mode have been studied as a function of frequency
and temperature. The calculated values for fNRM, δεNRM and distribution parameter αNRM are found to be 325 kHz, 6 and 0.156 for 5μm thick planer cell at 37°C. It is seen that in the vicinity of theT
C*A, soft mode obeys the Curie-Weiss law given by mean field theory. The results have been compared with materials of large spontaneous
polarization. 相似文献
4.
R. Rianyoi R. Potong N. Jaitanong R. Yimnirun A. Chaipanich 《Applied Physics A: Materials Science & Processing》2011,104(2):661-666
In this work, barium titanate (BT) and cement composites of 0-3 connectivity were produced with BT concentrations of 30%,
50% and 70% by volume using the mixing and pressing method. The dielectric constant (ε
r
) and the dielectric loss (tan δ) at room temperature and at various frequencies (0.1–20 kHz) of the ferroelectric BT-Portland cement composites with different
BT concentrations were investigated. The results show that the dielectric constant of BT-PC composites was found to increase
as BT concentration increases, and that the highest value for ε
r
—of 436—was obtained for a BT concentration of 70%. In addition, the dielectric loss tangent decreased with increasing BT
concentration. Moreover, several mathematical models were used; the experimental values of the dielectric constants are closest
to those calculated from the cube model. The 0-3 cement-based piezoelectric composites show typical ferroelectric hysteresis
loops at room temperature. The instantaneous remnant polarization (P
ir
), at an applied external electrical field (E
0) of 20 kV/cm (90 Hz) of 70% barium titanate composite, was found to have a value ≈3.42 μC/cm2. Furthermore, the piezoelectric coefficient (d
33) was also found to increase as BT concentration increases, as expected. The highest value for d
33 was 16 pC/N for 70% BT composite. 相似文献
5.
Recent experimental studies show that the dielectric constant of lead fluoride increases rapidly as the superionic transition
temperatureT
c is approached. In this work the static dielectric constant (ε) is calculated theoretically for low and high temperature limits. Assuming conduction to take place through a percolation
mechanism,ε is found to diverge at the threshold temperatureT
th where ionic conduction starts. 相似文献
6.
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
7.
Sandwich and planar structures were fabricated using
manganese phthalocyanine (MnPc) as active layer and gold (Au) as electrodes
by thermal evaporation method. The permittivity ε of MnPc was determined from
the dependence of capacitance on film thickness. J-V
characteristics of Au/MnPc/Au structure at room temperature were performed.
Thermally generated hole concentration p0, hole
mobility μp, total trap concentration
Nt and depth of the trap level were estimated. The
activation energies of MnPc films were determined from the Arrhenius plots
of ln σ versus 1000/T. The absorption and reflectance spectra of MnPc thin film deposited at room
temperature were recorded in the spectral range 300–900 nm. The optical band
gap of MnPc thin film was determined from the α2 versus hν graph. The optical constants n and k were found. The real and imaginary parts of the optical dielectric constant
ε1
and ε2 were calculated. 相似文献
8.
X. G. Tang Q. X. Liu J. Wang H. L. W. Chan 《Applied Physics A: Materials Science & Processing》2009,96(4):945-952
The effect of a dc bias field on the diffuse phase transition and nonlinear dielectric properties of sol-gel derived Ba(Zr0.2Ti0.8)O3 (BZT) ceramics are investigated. Diffuse phase transitions were observed in BZT ceramics and the Curie–Weiss exponent (CWE)
was γ∼2.0. The dielectric constant versus temperature characteristics and the γ in the modified Curie–Weiss law, ε
−1=ε
m
−1[1+(T−T
m
)
γ
/C1](1≤γ≤2), as a function of the dc bias field was obtained for BZT ceramics. The results indicated that γ is a function of dc bias field, and the γ value decreased from 2.04 to 1.73 with dc bias field increasing from 0 to 20 kV/cm. The dielectric constant decreases with
increasing dc bias field, indicating a field-induced phase transition. The dc bias field has a strong effect on the position
of the dielectric peak and affects the magnitude of the dielectric properties over a rather wide temperature range. The peak
temperature of the dielectric loss does not coincide with the dielectric peak and an obvious minimum value for the dielectric
loss at the temperature of the dielectric peaks is observed. At room temperature, 300 K, the high tunability (K=80%), the low loss tangent (≈0.01) and the large FOM (74), clearly imply that these ceramics are promising materials for tunable capacitor-device applications. 相似文献
9.
In this paper we report the effect of Pb incorporation in the dielectric properties of a-Se75In25 glassy alloy. The temperature and frequency dependence of the dielectric constants and the dielectric losses in glassy Se75In25
−x
Pb
x
(x = 0, 5, 10 and 15) alloys in the frequency range (1 kHz-5 MHz) and temperature range (300–340 K) have been measured. A detailed
analysis shows that the dielectric losses are dipolar in nature and can be understood in terms of hopping of charge carriers
over a potential barrier as suggested by Elliott for the case of chalcogenide glasses. It has been found that both dielectric
constant and the dielectric loss are highly dependent on frequency and temperature and also found to increase with increasing
concentration of Pb in binary a-Se75In25 glassy system. The results have been interpreted in terms of increase in the density of defect states by the incorporation
of Pb as a metallic additive in the aforesaid glassy system. 相似文献
10.
C. Fanggao G. A. Saunders E. F. Lambson R. N. Hampton G. Carini A. Bartolotta M. Lanza 《Il Nuovo Cimento D》1994,16(7):855-864
Summary Electrical-impedance measurements have been made in the frequency range 5 Hz to10 MHz in pure poly(ethylene oxide) having
a molecular weight of 600 000 from 254 K nearly up to the melting point of the crystalline phase (about 330 K). As the temperature
approaches the melting point there are large increases in the realε′ and imaginaryε″ parts of the dielectric constant. The frequency dependence ofε′ is characterized by a primary-relaxation process, whose frequency increases with increasing temperature as a consequence
of decrease of the average structural relaxation time. There is strong evidence that this low-frequency dispersion rises mainly
from the diffusive transport of localised charge carriers rather than a purely orientation relaxation process. In addition
the effects of hydrostatic pressure (0–25 Gpa) on the frequency dependences of the realε′ and imaginaryε″ parts of the dielectric constant have been measured in the same temperature range.
Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994. 相似文献
11.
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
12.
A. H. Reshak Z. Charifi H. Baaziz 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,60(4):463-468
We present the results of the ab initio theoretical study of the optical properties for PbFX (X = Cl, Br, I) compounds in its matlockite-type structure using the
full potential linearized augmented plane wave (FP-LAPW) method as
implemented in the WIEN2K code. We employed generalized gradient
approximation (GGA), which is based on exchange-correlation energy
optimization to calculate the total energy. Also we have used the
Engel-Vosko GGA formalism, which optimizes the corresponding potential for
band structure calculations. Our calculations show that the valence band
maximum (VBM) and conduction band minimum (CBM) are located at Z resulting
in a direct energy gap. We present calculations of the frequency-dependent
complex dielectric function ε( ω) and its
zero-frequency limit ε1 ( 0 ). We find that the
values of ε1 ( 0 ) increases with decreasing the
energy gap. The reflectivity spectra and absorption coefficient has been
calculated and compared with the available experimental data. The optical
properties are analyzed and the origin of some of the peaks in the spectra
is discussed in terms of the calculated electronic structure. 相似文献
13.
In the framework of the Keating model with allowance made for the anharmonic constant of the central interaction between the
nearest neighbors μ, analytical expressions have been obtained for three third-order independent elastic constants c
ijk
(μ, ζ) of single-layer graphene, where ζ = (2α − β)/(4α + β) is the Kleinman internal displacement parameter and α and β are
the harmonic constants of the central interaction between the nearest neighbors and the noncentral interaction between the
next-nearest neighbors, respectively. The dependences of the second-order elastic constants on the pressure p have been determined. It has been shown that the moduli c
11 and c
22 differently respond to the pressure. Therefore, graphene is isotropic in the harmonic approximation, whereas the inclusion
of anharmonicity leads to the appearance of the anisotropy. 相似文献
14.
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
15.
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
16.
Rare earth element substituted bismuth ferrites (BiFeO3) are of enormous importance as magnetoelectric materials. The polycrystalline samples of Bi
x
La1−x
FeO3 (x=0, 0.2, 0.4, 0.6, 0.8) were prepared by solid-state reaction using standard ceramic method. The single-phase formation of
these compounds was confirmed by X-ray diffraction (XRD) studies. The samples with x=0, 0.2, 0.4, 0.6 are found to be orthorhombic while the sample with x=0.8 is triclinic. The dielectric constant (ε′) and dissipation factor (tan δ) were measured in the frequency range 100 Hz
to 1 MHz at room temperature and as a function of temperature at certain fixed frequencies (1 kHz, 10 kHz, 100 kHz, 1 MHz).
All the samples showed dielectric dispersion. The dielectric constant with temperature shows a broad peak; the peak temperature
shifts with frequency which reflects the relaxor-type behavior. The peak above 600 K in the measured temperature range corresponds
to antiferromagnetic ordering temperature (Néel temperature). The broadness of the peak changes with composition. The ac conductivity
as well as ε′ are found to be maximum for the sample x=0.2 at room temperature. 相似文献
17.
The measurements of electrical conductivity (σ) from 300 to 1200 K and dielectric constant (ε′) from 4·2 to 1200 K of A-type Nd2O2 pellets are reported here. Electrical conductivity (σ) data can be explained in terms of impurity. The dielectric constant (ε′) increases slowly up to 500 K as is expected for ionic solids. The increase ofε′ becomes much faster above 500 K, which is attributed to space charge polarization of thermally generated charge carriers. 相似文献
18.
C.S. MenonR. Ramji Rao 《Journal of Physics and Chemistry of Solids》1972,33(11):2129-2139
The lattice dynamics, second and third order elastic constants, pressure derivatives of the second order elastic constants and the temperature variation of the lattice thermal expansion of Cadmium have been worked out utilising Keating's approach. The ten TOE constants are calculated using four anharmonic parameters. The model used here reproduces the measured pressure derivatives of the SOE constants of Cadmium extremely well. The low temperature limit of the volume Gruneisen function γL is found to be nearer to the value of Andres. The high temperature limit γv(T) is in good agreement with the value calculated by Gschneidner Jr. from the Cv data for Cadmium. The lattice dynamics of Cadmium is found to be essentially similar to that of Zinc. 相似文献
19.
A. I. Burkhanov K. P. Guzhakovskaya L. I. Ivleva 《Bulletin of the Russian Academy of Sciences: Physics》2011,75(10):1399-1402
The dielectric nonlinearity in an SBN-75 single crystal doped with 0.01 at % Cr was investigated in the vicinity of a diffuse
phase transition by studying the reverse dielectric dependences ε′(E
=). The influence of illumination and exposure (aging) on the behavior of ε′(E
=) was demonstrated. A reduction in the dielectric memory effect after sample illumination was detected. 相似文献
20.
M. Paasch M. Winterlich R. Böhmer R. Sonntag G. J. McIntyre A. Loidl 《Zeitschrift für Physik B Condensed Matter》1995,99(1):333-338
The χ,T phase diagram of (NH4I)x(KI)1−x has been determined using neutron diffraction experiments and dielectric spectroscopy. At low temperatures and with decreasing
χ, the sequence γ, β, ε and glass phase has been detected. The critical concentration χc≈0.55 separates the glassy phase with frozen-in orientational disorder from the ε phase which reveals long-range orientational
order. Close to χc our experiments reveal evidence for two subsequent glass transitions. 相似文献