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1.
The influence of silver myristate used as a precursor of silver nanoparticles on the direct current conductivity σ dc of epoxy polymer within the concentration range of ≤0.8 wt % was investigated. The value of direct current conductivity was determined on the basis of analysis of the frequency dependence of complex permittivity within the frequency range of 10?2–105 Hz. The temperature dependence of σ dc is composed of two regions. The dependence corresponds to the Vogel-Fulcher-Tammann empirical law σ dc = σ dc0exp{?DT 0/(T-T 0)} (where T 0 is the Vogel temperature and D is the strength parameter) at temperatures higher than the glass transition temperature T g. At the same time, T 0 does not depend on the concentration of nanoparticles. The Arrhenius temperature dependence characterized by activation energy about 1.2 eV is observed at temperatures lower than T g. The observed shape of the temperature dependence is related to the change in the mechanism of conductivity after “freezing” of ionic mobility at temperatures lower than T g. The value of σ dc is increased as the concentration of nanoparticles is raised within the temperature range of T > T g. The obtained dependence of σ dc on silver myristate concentration is similar to the root one, indicating the absence of percolation within the studied range of concentrations.  相似文献   

2.
The conductivity and modulus formulation in lithium modified bismuth zinc borate glasses with compositions xLi2O–(50-x) Bi2O3–10ZnO–40B2O3 has been studied in the frequency range 0.1 Hz–1.5 × 105 Hz in the temperature range 573 K–693 K. The temperature and frequency dependent conductivity is found to obey Jonscher's universal power law for all the studied compositions, the dc conductivity (σdc), crossover frequency (ωH), and frequency exponent (s) have been estimated from the fitting of the experimental data of ac conductivity with Jonscher's universal power law. Enthalpy to dissociate the cation from its original site next to a charge compensating centre (Hf) and enthalpy of migration (Hm) have been estimated. It has been observed that number of charge carriers and ac conductivity in the lithium modified bismuth zinc borate glasses increases with increase in Li2O content. Further, the conduction mechanism in the glass sample with x = 0 may be due to overlapping large polaron tunneling, whereas, conduction mechanism in other studied glass samples more or less follows diffusion controlled relaxation model. The ac conductivity is scaled using σdc and ωH as the scaling parameter and is found that these are suitable scaling parameter for conductivity scaling. Non-Debye type relaxation is found prevalent in the studied glass system. Scaling of ac conductivity as well as electric modulus confirms the presence of different type of conduction mechanism in the glass samples with x = 0 and 5 from other studied samples. The activation energy of relaxation (ER) and dc conductivity (Edc) are almost equal, suggesting that polarons/ions have to overcome same barrier while relaxing and conducting.  相似文献   

3.
The electrical conductivity process in a new class of ion-containing polymers—highly concentrated solid solutions of hydrated perchlorate salts in polyacrylonitrile (PAN)—is described (σdc = 10?7?10?2Ω?1cm?1). A low-ac instrument (70 cps) is used to measure electrical conductivity. We present a cryogenic system in which the temperature dependence of the conductivity is studied (78–340°K). The ionic character of the conductivity process is established. The conductivity both above and below the glass-transition (Tg) point is thermally activated with an activation energy of 0.7–0.9 e V for the glassy state (?g) and 0.12–0.6 eV for the rubber-like state (?r). The systems described exhibit a compensation effect between the pre-exponential factor for the conductivity in the glassy state σ0g and the difference in activation energy ?g – ?r   相似文献   

4.
The electrical conductivity of a series of semiconducting polyazophenylenes was studied as a function of temperature and molecular weight in the temperature range 293–493°K and for molecular weights between 9500 and 62800. The compounds studied included poly-2,4-diminotoluene and poly-2,5-diaminotoluene. The temperature dependence of conductivity obeys the exponential law σ = σ0 exp {–Ea/kT}. All of the compounds studied show a break in the conductivity curve at a temperature of about 380°K with the slope of the curve greater at temperatures above 380°K. The activation energies Ea determined from the higher slopes are in the range 0.63–0.89 eV, and the corresponding σ0 values are in the range 10?4-10° (ohm-cm)?1. Both Ea and σ0 show a tendency to increase with increasing molecular weight such that the compensation law was valid. Activation energies and σ0 values determined from the (smaller) slope below the temperature of the break point showed a tendency to decrease with successive measurements on new samples. This indicates some kind of orientation effect, and conclusions concerning the properties of the material on basis of results below the break point are therefore difficult.  相似文献   

5.
Moti Ram 《Solid State Sciences》2009,11(7):1206-1210
LiCo3/5Fe2/5VO4 has been synthesized by solution-based chemical method. X-ray diffraction study at room temperature reveals an orthorhombic phase of the compound. Electrical properties are studied using complex impedance spectroscopy (CIS) technique in a range of frequency and temperature. The complex impedance plots identify the grain interior, grain boundary and electrode–material interface contributions to electrical response in the material. Temperature dependence of a.c. and d.c. conductivity indicates that electrical conduction in the material is a thermally activated process. The value of activation energy computed from the Arrhenius plot of σdc with 103/T is ~(0.356 ± 0.012) eV (25–275 °C). Frequency dependence of a.c. conductivity at different temperatures is found to obey Jonscher's universal law.  相似文献   

6.
The temperature dependence of T1 for 3He gas in the range 0–4°K is calculated for a Lennard-Jones (12,6) potential. The relaxation of the nuclear spins is assumed to be due to a dipolar interaction between the nuclei. A minimum value in the relaxation time, T1,min, is found to occur at a temperature denoted by Tmin. By repeating the calculation for different pairs of values of the potential parameters ? and σ, we have found that for a density of 10?2 g/cm32Tmin = 13.0?1.12 × 1032, T1,min2(Tmin)12 = 17.4?6.56 × 1022, with ?, σ, Tmin and T1,min in eV, Å, °K and minutes, respectively. From measurements of Tmin and T1,min, ? and σ can be determined.  相似文献   

7.
This work deals with the dielectric properties of conductive composite materials, which consist of thermoplastic polypropylene (PP) matrix filled with carbon black (CB). The CB concentration was systematically varied in a wide range. Our main interest is focused on the investigation of electrical conductivity mechanism and related percolation phenomena in these materials. To study the electrical and dielectric properties of composites we used broadband ac dielectric relaxation spectroscopy (DRS) techniques in a wide temperature range. By measurements of complex dielectric permittivity, ϵ*, the dependence of ac conductivity, σac, and dc conductivity, σdc, on the frequency, the temperature and the concentration of the conductive filler was investigated. The behavior of this system is described by means of percolation theory. The percolation threshold, PC, value was calculated to be 6.2 wt.% CB. Both, dielectric constant and dc conductivity follow power‐law behavior, yielding values for the critical exponents, which are in good agreement with the theoretical ones. Indications for tunneling effect in the charge carriers transport through the composites are presented. The temperature dependence of dc conductivity gives evidence for the presence of positive temperature coefficient (PTC) effect.  相似文献   

8.
9.
The temperature dependence of heat capacity C p o = f (T) of second generation hard poly(phenylene-pyridyl) dendrimer (G2-24Py) was measured by a adiabatic vacuum calorimeter over the temperature range 6–320 K for the first time. The experimental results were used to calculate the standard thermodynamic functions: heat capacity C p o (T), enthalpy H o(T)–H o(0), entropy S o(T)–S o(0) and Gibbs function G o(T)–H o(0) over the range from T → 0 K to 320 K. The standard entropy of formation at T = 298.15 K of G2-24Py was calculated. The low-temperature heat capacity was analyzed based on Debye’s heat capacity theory of solids. Fractal treatment of the heat capacity was performed and the values of the temperature characteristics and fractal dimension D were determined. Some conclusions regarding structure topology are given.  相似文献   

10.
Synthesis of fluoro-substituted substances based on brownmillerite Ba2In2O5 is carried out. The width of the homogeneity region of the Ba2In2O5?0.5x F x (0 < x ≤ 0.25) solid solution was established using X-ray analysis. Measurement of temperature dependences of conductivity in atmospheres with different partial pressure of water vapor (pH2O = 3.3 and 2 × 103 Pa) showed an increase in conductivity at T ≤ 550°C in a humid atmosphere, which is due to appearance of proton transport. The dependence of conductivity on partial oxygen pressure (pO2 = 0.21 × 105 to 10?15 Pa) is studied in the temperature range of 500–1000°C; ion transport numbers are calculated. The method of polarization measurements was used to determine transport numbers of fluoride. Total conductivity is divided into ion (proton, oxygen, and fluoride ion) and electron components. Analysis of concentration dependences of conductivities showed that low concentrations of fluoride allow increasing both the total and partial conductivities (oxygen-ion and proton) and, besides, allow shifting the “order-disorder” phase transition by 100°C to the low temperature range.  相似文献   

11.
The thermal conductivity and heat capacity of high-purity single crystals of yttrium titanate, Y2Ti2O7, have been determined over the temperature range 2 K?T?300 K. The experimental heat capacity is in very good agreement with an analysis based on three acoustic modes per unit cell (with the Debye characteristic temperature, θD, of ca. 970 K) and an assignment of the remaining 63 optic modes, as well as a correction for CpCv. From the integrated heat capacity data, the enthalpy and entropy relative to absolute zero, are, respectively, H(T=298.15 K)−H0=34.69 kJ mol−1 and S(T=298.15 K)−S0=211.2 J K−1 mol−1. The thermal conductivity shows a peak at ca. θD/50, characteristic of a highly purified crystal in which the phonon mean free path is about 10 μm in the defect/boundary low-temperature limit. The room-temperature thermal conductivity of Y2Ti2O7 is 2.8 W m−1 K−1, close to the calculated theoretical thermal conductivity, κmin, for fully coupled phonons at high temperatures.  相似文献   

12.
Temperature dependences of the heat capacity of G-3(D4) and G-6(D4) carbosilanecyclosiloxane dendrimers are studied for the first time by precision adiabatic vacuum and differential scanning calorimetry in the range of 6 to 350–450 K. Physical transformations in the investigated temperature range are observed and their standard thermodynamic characteristics are determined and discussed. Standard thermodynamic functions for a mole unit are calculated from the experimental data: C p (T), H (T), ? H (0), S (T) ? S (0), and G (T) ? H (0) in the range of T → 0 to (350–449) K and standard entropies of formation at 298.15 K. Low-temperature (T ≤ 50 K) heat capacity is analyzed using the Debye theory of heat capacity of solids and the multifractal model. The values of fractal dimensionality D are determined and some conclusions on the topology of the investigated structures are drawn. The corresponding thermodynamic properties of the investigated carbosilanecyclosiloxane dendrimers under study are compared.  相似文献   

13.
Measurements of dc conductivity of Cd-stearate multilayer films below room tenperature down to liquid nitrogen temperature have revealed the temperature dependence σ = σ0 exp(?AT?12). This relation, together with the values of σ0 and A, confirms the theoretical predictions for hopping conduction in a multilayer system associated with interface states.  相似文献   

14.
The deuterium NMR lineshape and spin—lattice relaxation time, T1, have been measured in deuterium molybdenum bronze, D1.6MoO3, over the temperature range 166–400 K. The 2D quadrupole coupling constant is 21 kHz at room temperature. The temperature dependence of the 2D T1 has been interpreted in terms of two independent motional processes for deuterium. The data suggest that one of the processes corresponds to diffusion of the 2D nuclei whereas the other may arise from a 180° flipping of the OD2 moieties. This specific interpretation agrees with the results obtained for proton T1 and proton lineshape data reported earlier.  相似文献   

15.
The temperature dependence of the heat capacity of LiZr2(PO4)3 crystal phosphate is studied in an adiabatic vacuum calorimeter in the temperature range of 6 to 358 K. A phase transition caused by the transition of a low-temperature (triclinic) modification to a high-temperature (rhombohedral) modification is observed in the temperature range of 290–338 K and its standard thermodynamic characteristics are estimated and analyzed. Standard thermodynamic functions are calculated from experimental data: heat capacity, enthalpy, entropy, and Gibbs function in the range of T → 0 to 358 K. Fractal dimensionality D is calculated from the data on low-temperature (20 K ≤ T ≤ 50 K) heat capacity and the topology of the phosphate’s structure is estimated.  相似文献   

16.
We show that the magnetic field dependence of the proton spin–lattice relaxation rates 1/T1=-b0 in non-crystalline proteins may be quantitatively related to structural fluctuations localized along the backbone that modulate proton–proton dipolar couplings. The parameter A is related to the temperature, the dipolar coupling strength and the energy for the highest vibrational frequency in the polymer backbone. The parameter b is related to the fractal dimensionality of the spatial distribution of protons and to the spectral dimensionality that characterizes the anomalous diffusion. Extension of the theory is presented to treat the case of hydrated proteins. This theory is satisfactorily compared with field cycling experiments realized on lysozyme protein.  相似文献   

17.
The interaction of Fe2O3 nanoparticles emphasized between poly(propylene glycol) (PPG 4000) and silver triflate (AgCF3SO3) on the conformal changes of coordination sites and the electrochemical properties have been investigated. On the influence of Fe2O3 nanoparticles distribution, the interactions between the ether oxygen in C–O–C of the polymer chain with Ag+ ion as a result of bond strength of the C–O–C stretching vibration, the end group effect has been examined by Fourier transform infrared (FT-IR) spectroscopy. The formation of transient cross-links between polymer chains and filler particles appears to be a characteristic change in the glass transition temperature (T g) and enhance the effective number of cations as well. The strength of ion–polymer interactions was revealed by the transport of ions, t Ag+, and found to be in the range of 0.42–0.50, and the ionic conductivity was ascertained by complex impedance analysis with a maximum of 9.2?×?10?4 S cm?1 at 298 K with a corresponding concentration of 10 wt% Fe2O3 nanoparticles. The temperature dependence of conductivity has been examined based on the Vogel–Tammann–Fulcher (VTF) equation, thereby suggesting the segmental chain motion and free volume changes. From the impedance data, both the dielectric and modulus behaviours have been revealed and both were well correlated as a function of frequency.  相似文献   

18.
The spin-lattice relaxation time T1 of 129 Xe gas is calculated with the kinetic theory due to Chem and Snider. A Lennard-Jones (12,6) potential functions is employed as a model for the spherical potential while the transient spin-rotation interaction is assumed to be responsible for the relaxation of the nuclei. Cross sections for spin transitions on collisions are calculated either quantum mechanically or semiclassically depending on the relative energy. The temperature dependence of T1 is determined in the range 200–450 K. The calculated value of T1 at 298 K and 1 amagat is 2.8 x 05 s while the value measured by Hund and Carr is (2.0 ± 0.2) x 105s.  相似文献   

19.
Three reactive epoxy–amine systems based on diglycidyl ether of bisphenol A (DGEBA) with 4,4′-diaminodiphenylsulfone (DDS), 4,4′-methylenebis [3-chloro 2,6-diethylaniline] (MCDEA), and 4,4′-methylenebis [2,6-diethylaniline] (MDEA), were studied during isothermal curings at 140 and 160°C. The simultaneous kinetic and dielectric studies allow to express conductivity, σ, in terms of conversion, x, and of glass transition temperature, Tg. The conductivity, σ0, of the initial monomer mixture and, σ of the fully cured network are measured. It is found that:
  • The glass transition temperature, Tg, versus conversion, x, curves follows the equation of Di Benedetto modified by Pascault and Williams
  • There exists a linear relation between log σ/log σ0 and Tg.
So, it is possible to predict both kinetic and dielectric behaviors of these epoxy-amine systems by the knowledge of Tg0, ΔCp0, and σ0, respectively, glass transition temperature, heat capacity, and conductivity of initial monomer mixture, Tg and ΔCp, and σ, respectively, glass transition temperature and heat capacity and conductivity of fully cured network. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2911–2921, 1998  相似文献   

20.
The temperature dependence of the heat capacity C p o = f(T) of crystalline calcium-zirconium phosphate was studied over the temperature range 7–650 K by precision adiabatic vacuum and dynamic scanning calorimetry. The experimental data were used to calculate the standard thermodynamic functions C p o (T), H o(T) ? H o(0), S o(T), and G o(T) ? H o(0) at temperatures from T → 0 to 650 K and the standard entropy of formation of Ca0.5Zr2(PO4)3 at T = 298.15 K. The data on low-temperature (30 K ≤ T ≤ 50 K) heat capacity were used to calculate fractal dimension D. Conclusions about the character of the topology of the structure of the phosphate were drawn.  相似文献   

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