共查询到20条相似文献,搜索用时 31 毫秒
1.
M. A. Gaffar A. M. Abousehly A. Abu El‐Fadl M. M. Mostafa 《Crystal Research and Technology》2006,41(11):1120-1130
The dielectric constant (ϵ), dielectric loss (tanδ) and ac conductivity (σac) of virgin and thermally cycled undoped and Ni2+‐doped K2ZnCl4 (KZC) crystals in the ferroelectric‐commensurate (FC), incommensurate (IC) and normal phases (N) have been studied. Anomalous behaviour at the two‐phase transition points was observed while measuring ϵ along the polar a‐axis for the undoped sample. With increasing Ni2+concentration a systematic shift of the phase transition temperature towards lower values and a continuous inhibition of the peak height were detected. ϵ changed linearly with lnf up to f =105 Hz. tanδ along the a‐axis declared the phase transitions by peak changes. After Ni2+‐doping this behaviour was preserved at the FC‐IC phase transition point while the IC‐N phase transition was manifested by a change in the slope of the straight line representing the tanδ‐T dependence. The ac conductivity changed lineally with frequency according to a relation of the form σac = σo f β where 0>β>1.9. σac increased monotonically with increasing temperature and doping concentration in the low‐temperature phases tending to merge in one straight‐line with high activation energy that might be due to superionic dc conduction in the high temperature N‐phase. Doping with Ni2+ pinned stripples, decreased the soliton‐soliton interaction, weakened discommensuration effects, shortened the IC‐phase and strongly affected the ‘chaotic' behaviour at the TC‐IC. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
2.
The electrical properties of carbon nano-balls’ and 4-dimethylaminoazobenzene-2′-carboxylic acid doped dispersed nematic liquid crystal composite were investigated by impedance spectroscopy technique. The conductivity and capacitance were measured in the frequency range 100 kHz-1 MHz and temperature range 300-380 K. The loss peak was observed in the dielectric loss spectra and was identified as nearly-Debye type relaxation. Cole-Cole plots have been used to describe the characteristic changes of electrical properties in mentioned temperature interval. The sample presents monodispersive relaxation behavior with a relaxation time of ∼10−7 s. The relaxation process is attributed to the dipolar rotation of the long molecular axis and the activation energy is found to be 0.097 eV. 相似文献
3.
The dielectric constant and conductivity of 80% V2O5: 20% P2O5 glass has been measured in the frequency range 102 to 109Hz and in the temperature range 80 to 350°K. It is shown that the dielectric behaviour over these ranges is described by a Debye type relaxation process with distribution of relaxation times. A method is proposed to determine the width of distribution from the data at fixed frequencies and different temperatures. The width of distribution increases at frequencies ω > 10/τ, which leads to an a.c. conductivity at these frequencies almost linearly proportional to frequency and independent of temperature. The estimated value of the static dielectric constant of about 30 was found to decrease with temperature while the infinite frequency dielectric constant of 10 was independent of temperature. The carrier concentration calculated from the dielectric relaxation time and the d.c. conductivity through a thermal diffusion model shows reasonable agreement with direct measurement using electron paramagnetic resonance. 相似文献
4.
The results of investigations low frequency dielectric relaxation in layered ferroelectric TlInS2 crystals are presented. The measurements were performed in the temperature range of 180‐230 K and in the frequency range of 5 kHz–1 MHz. Two different relaxation processes were observed in mentioned temperature interval. The crystal has “slow” and “fast” relaxation mechanisms in low and high frequency region, respectively. The presence of two different relaxation mechanisms in TlInS2 is discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
5.
F. A. Mikailov 《Crystal Research and Technology》2009,44(1):82-86
The results of measurements of temperature and time dependencies of the dielectric constant of TlInS2 in the temperature interval of successive incommensurate and commensurate phase transitions have been presented. Considerable decrease of the dielectric constant in a temperature interval including successive phase transitions was observed after annealing the sample at a fixed temperature in the ferroelectric state. Additionally, it has been revealed that the time dependencies of the real part of dielectric susceptibility and dielectric losses angle tangent of TlInS2 crystal in the incommensurate phase exhibits an unusual behaviour, which is accompanied by exponential decreasing of the dielectric constant and increasing of dielectric losses angle tangent by time. The observed effects are qualitatively explained using defect‐density‐wave model and phenomenological theory of incommensurate systems containing discommensurations undergoing nucleation and evolution processes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
6.
L. Yankova T. I. Milenov P. M. Rafailov G. V. Avdeev M. N. Veleva M. M. Gosopodinov 《Crystal Research and Technology》2013,48(7):439-445
Crystals of La2CoMnO6 doped with Pb were successfully grown by the high temperature solution growth method and their magnetic and transport properties were studied. The examined crystal is found to have predominantly ordered Co2+/Mn4+ structure with randomly distributed Mn3+ substituting Co2+. A relaxor‐like temperature dependence of the dielectric constant with relaxation maximum is established in the temperature interval 180–210 K. On the base of the dc‐conductivity data, it is assumed that the charge transport in the interval 180–350 K is governed by small‐polaron hopping, whose onset coincides with the Curie temperature. 相似文献
7.
R. Rizwana T. Radha Krishna A. R. James P. Sarah 《Crystal Research and Technology》2007,42(7):699-706
Sr0.2Na0.4Nd0.4Bi4Ti4O15 is prepared by Sol‐Gel route using the modified polymeric precursor method based on the Pechini process and its electrical properties are investigated by Complex Impedance Spectroscopy (CIS). Results are presented in two ways, complex plane plots (real versus imaginary) and spectroscopic plots (real and imaginary against frequency). CIS has provided convincing evidence for the existence of both grain (bulk) and grain‐boundary effects that were separated in the frequency domain of the impedance spectrum. Experimental data are fitted to two parallel RC equivalent circuits in series. The values of resistance and capacitance of bulk and grain boundary contributions are also calculated. Relaxation times are calculated for both grains and grainboundaries. Conduction and charge carrier behaviors are studied from frequency and temperature dependence of ac conductivity. The ac conductivity follows a two‐power law behavior at different temperatures with frequency exponents s1 and s2. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
8.
M. F. Mostafa A. A. Youssef Th. Sh. El Dean A. M Mostafa I. S. Ahmed Farag 《Crystal Research and Technology》2008,43(5):547-560
The crystal structure of bis‐(methyltriphenylphosphonium) tetrabromocobaltate (II), [(C19H18P)2 CoBr4] is determined: Mr = 933.203, monoclinic, P21, a = 9. 6977 (3) Å, b = 12.5547 (4)Å, c = 16.4503 (6)Å, β = 105.603 (2)°, V = 1929.04 (11)Å3, Z = 2, Dx = 1.607 Mg m‐3, T = 298 K. Differential thermal analysis at high temperatures shows three endothermic peaks characterizing four phases, with onset temperatures at T1= 313±2 K, T2 = 320±4 K and T3= 360±1 K. The structural instability detected via the temperature dependence of permittivity at T1 is ascribed to order‐disorder transition associated with cation dipole reorientation. Permittivity and ac conductivity studies as a function of temperature (295 K‐375 K) and frequency (0.11 kHz < f <100 kHz) are presented. The results indicate the importance of the cation size and shape on the phase transitions in the system. Bulk conductivity behavior is thermally activated. The associated activation energies are in the range 2.9 to 1.0 eV depending on the temperature regime. Two contributions to the ac conductivity, one dominating at low temperatures and high frequencies which are characterized by superlinear frequency exponent and the second dominates at high temperatures characterized by a sublinear frequency exponent. The behavior is interpreted in terms of the jump relaxation model. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
9.
The frequency (10-108 Hz) and temperature (100–400 K) dependence of the ac conductivity of anthracene carbons is explained within two theoretical models. The properties of carbon heat treated up to 600°C are correctly explained by means of a model where the current flow is due to electron hops over a potential barrier between localized sites.The conductivity of the carbon heat-treated up to 650°C is interpreted using a theory proposed by Mott and Davis. The spatial densities of states are determined in both cases. 相似文献
10.
《Journal of Non》1986,79(3):333-351
The ac and dc electrical properties of the P2O5BaOCuO glass system have been measured.TSPC and TSDC experiments and dc conductivity as a function of time indicate the predominantly electronic character of these glasses. The conduction process can be basically explained by a polaron hopping model in an adiabatic regime. Conductivity values which depend on the glass microstructure and switching phenomena are observed. The filamentary-feature of this process suggests a Poole-Frenkel mechanism.A Debye dielectric relaxation non-simple process is deduced from the frequency and temperature dependence of loss tangent and dielectric constant. The activation energies agree with those determined from dc measurements, suggesting a unique electronic hopping conduction mechanism in both regimes.The ac and dc electrical properties are strongly affected by the glass composition and essentially by the redox Cu+/Cut ratio.A conduction model accounting for ac and dc behaviour is finally proposed. 相似文献
11.
《Journal of Non》1986,79(3):353-366
The ac and dc electrical properties of the P2O5BaOCuO glass system have been measured.TSPC and TSDC experiments and dc conductivity as a function of time indicate the predominantly electronic character of these glasses. The conduction process can be basically explained by a polaron hopping model in an adiabatic regime. Conductivity values which depend on the glass microstructure and switching phenomena are observed. The filamentary-feature of this process suggests a Poole-Frenkel mechanism.A Debye dielectric relaxation non-simple process is deduced from the frequency and temperature dependence of loss tangent and dielectric constant. The activation energies agree with those determinated from dc measurements, suggesting a unique electronic hopping conduction mechanism in both regimes.The ac and dc electrical properties are strongly affected by the glass composition and essentially by the redox Cu+/Cut ratio.A conduction model accounting for ac and dc behaviour is finally proposed. 相似文献
12.
The conductivity, mobility, photoconductivity and photo response measurements in GaS0.75Se0.25 mixed crystals were carried out in the temperature range of 150‐450 K. The room temperature conductivity, mobility and electron concentration values were 10‐9 (Ω‐cm)‐1, 48 cm2V‐1s‐1 and ∼109 cm‐3, respectively. Two donor levels were obtained from temperature‐dependent conductivity and carrier concentration, located at energies of about 755 and 465 meV below the conduction band. Single donor‐single acceptor analysis yields the same donor level at 465 meV with donor and acceptor concentrations of 8.7 × 1014 and 5.3 × 1013 cm‐3, respectively. The mobility‐temperature dependence shows that ionized impurity scattering dominates the conduction up to the temperature 310 K with different temperature exponent, while above this critical temperature; the phonon scattering is dominant conduction mechanism. From the photo‐response spectra, the maximum photocurrent was observed for all the samples at 2.42 eV, and varied slightly with temperature. Moreover, the photocurrent‐light intensity dependence in these crystals obeys the power law, Iph ∝ ϕγ with γ between 1.7 and 2.0 for various applied fields and temperatures. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
13.
O. Karabulut M. Parlak K. Yilmaz R. Turan B. G. Akinoglu 《Crystal Research and Technology》2003,38(12):1071-1076
GaSe single crystals grown by Bridgman method have been doped by ion implantation technique. The samples were bombarded in the direction parallel to c‐axis by Si ion beam of about 100 keV to doses of 1 × 1016 ions/cm2 at room temperature. The effects of Si implantation with annealing at 500 and 600 °C on the electrical properties have been studied by measuring the temperature dependent conductivity and photoconductivity under different illumination intensities in the temperature range of 100–320 K. It is observed that Si implantation increases the room temperature conductivity 10−7 to 10−3 (Ω‐cm)−1 depending on the post annealing temperature. The analysis of temperature dependent conductivity shows that at high temperature region above 200 K, the transport mechanism is dominated by thermal excitation in the doped and undoped GaSe samples. At lower temperatures, the conduction of carriers is dominated by variable range hopping mechanism in the implanted samples. Annealing of the samples at and above 600 °C weakens the temperature dependence of the conductivity and photoconductivity. This indicates that annealing of the implanted samples activates Si‐atoms and increases structural deformations and stacking faults. The same behavior was observed from photoconductivity measurements. Hence, photocurrent‐illumination intensity dependence in the implanted samples obeys the power low Ipc ∝ Φn with n between 1 and 2 which is an indication of continuous distribution of localized states in the band gap. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
14.
E. entürk 《Crystal Research and Technology》2004,39(2):157-160
In this paper, we have investigated two‐relaxator mechanism and dielectric characteristics of Ce3+ doped Sr0.61Ba0.39Nb2O6 with dielectric spectroscopy measurements. The crystal undergoes a ferroelectric phase transition at 340 K. The temperature dependence of the real and imaginer part of the complex dielectric susceptibility in vicinity of ferroelectric‐paraelectric phase transition has been studied in the frequency region 0.1 kHz–10 MHz. The measurements of the dielectric constant of the real and imaginer parts show strongly frequency dependence. The investigations of the dielectric constant revealed a non‐Debye type dielectric relaxation for Ce+3 doped SBN61 by using Cole‐Cole plots. It reveals the coexistence of the two dielectric relaxators in vicinity of the phase transition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
15.
The conductivity of liquid chalcogenides has been measured at frequencies 2 Hz - 2×105 Hz and 1010 Hz in the temperature range 500 K to 830 K. While in liquid Se the conductivity increases with increasing frequency no frequency dependence of the conductivity is found in liquid As2Se3. A simple dielectric model explains the change of conductivity with frequency and temperature. 相似文献
16.
The X‐ray diffraction and Infrared (IR) spectral studies of (NH4)2 CuCl4.2H2O single crystals reveals that these crystals contains tetragonal crystal structure with the unit cell dimensions of a = 7.58Å, c = 7.95Å, z= 2, β =90° and two water molecules in the unit cell. The temperature dependence of thermally stimulated depolarization current (TSDC) and dc electrical conductivity (σ) studies of this two‐dimensional (NH4)2 CuCl4.2H2O single crystal have been carried out in 77K–300K temperature region. The TSDC thermograph shows only one sharp peak at 248K with a peak current of 130nA, which is attributed to the Maxwell‐Wagner peak. The activation energy (U), relaxation time (τ) are calculated as 0.78eV and 3.44×10‐15 s respectively. Dc electrical conductivity studies of these crystals show a first order phase transition at about 248K. 相似文献
17.
《Journal of Non》2007,353(13-15):1322-1325
In the present paper the effect of Bi impurity (low ∼4 at.% and high ∼10 at.%) on the ac conductivity (σac) of a-Ge20Se80 glassy alloy is studied and the experimentally deduced values are fitted with theoretically deduced values by using correlated barrier hopping model (CBH). Frequency dependent ac conductance of the samples over a frequency range of 100–50 kHz has been taken in the temperature range (268–360 K). At frequency 2 kHz and temperature 298 K, the value of ac conductivity (σac) decreases at low concentration of Bi (4 at.%). However, the value of σac increases at higher concentration of Bi (10 at.%). The ac conductivity is proportional to ωs for undoped and doped samples. The value of frequency exponent (s) decreases as the temperature increases. These results have been explained on the basis of some structural changes at low and high concentration of Bi impurity. 相似文献
18.
The low field conduction mechanism in amorphous Nb2O5 doped with Nb is investigated by measurements of the ac conductivity as a function of frequency (3 Hz?6 × 106 Hz), dc conductivity as a function of temperature (100–400 K), capacitance as a function of frequency (3 Hz?6 × 106 Hz) and conductance G as a function of voltage at 103 Hz. Loss tangent and quality factor data are also given because of their technical and scientific relevance. Evidence for hopping conduction at low applied fields is presented by the following results: (1) a monotonic increase in ac conductivity σ(ω)αωn where 0.5 < n < 1.0 in the range 3 Hz?6 × 106 Hz; (2) a linear dependence of current on voltage at low fields; and (3) low activation energy for dc conduction with a transition at 210 K to a still lower activation energy; and (4) a decrease in polarizability with frequency. At high fields, E > 105 V/cm, dc conductivity is dominated by the field emission mechanism of the Poole-Frenkel or Poole type. 相似文献
19.
A new method to determine ac conductivity of amorphous Ge using Al-amorphous Ge–SiO2–P+Si tunnel junctions is presented. Frequency dependence of ac conductivity is found to satisfy the power law in the frequency range between 1 and 50 kHz and the density of localized states at the Fermi level is estimated to be ~ 1.7 × 1020 cm?3 eV?1 which decreases to ~ 4.5 × 1019 cm?3 eV?1 after annealing at 175°C.Temperature dependence of tunneling conductance of Al-amorphous Ge–SiO2–P+Si junctions is appreciable only near zero bias. Zero bias conductance of the junctions obeys the law of Mott; the density of localized states obtained from the law is one order of magnitude smaller than that obtained by ac conductivity measurements, being insensitive to annealing. This behavior of the tunnel junctions differes in many respects from those of Al–Al2O3-amorphous Ge tunnel juntions. 相似文献
20.
To identify the impurity levels in CuIn5S8 single crystals, the dark electrical conductivity and photoconductivity measurements were carried out in the temperature range of 50–460 K. The data reflect the intrinsic and extrinsic nature of the crystals above and below 300 K, respectively. Energy band gaps of 1.35 and 1.31 eV at 0 K and 300 K, were defined from the dark conductivity measurements and the photocurrent spectra, respectively. The dark and photoconductivity data in the extrinsic temperature region reflect the existence of two independent donor energy levels located at 130 and 16 meV. The photocurrent‐illumination intensity dependence (F) follows the law IphαFγ, with γ being 1.0, 0.5 and 1.0 at low, moderate and high intensities indicating the domination of monomolecular, bimolecular and strong recombination at the surface, respectively. In the intrinsic region and in the temperature region where the shallow donor energy level 16 meV is dominant, the free electron life time, τn, is found to be constant with increasing F. In the temperature region 140 K < T < 210 K, the free electron life time increases with increasing illumination intensity showing the supralinear character. Below 140 K, τn decrease with decreasing illumination intensity. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献