Electrical conductivity behaviour of pure and polyblends samples of polyvinyl chloride (PVC) and polymethyl methacrylate (PMMA)

Electrical conductivity of pure PVC, PMMA and their polyblends samples has been studied in detail as a function of polarizing fields at constant temperatures. Different plots were drawn to investigate the nature of mechanism responsible for conduction. The nature of all the thermograms is nonlinear but similar for all temperatures. The plots have two slopes, i.e., ohmic conduction with slope of curve ≈1 at lower voltage region and a non-ohmic conduction with slope ≈1.9 at higher voltage region are observed. The increase in the conductivity and decrease in the activation energy, suggest that plasticization effect is taking place between the polymers when they are blended. Fowler–Nordheim plots are not consistent and showing negative and positive slopes simultaneously for lower as well as higher values of applied voltage. Theoretical and experimental values of β_SR and β_PF were calculated and the β_exp of pure and polyblend samples is in agreement with theoretically β_PF. The β_exp value lies close to β_PF, this shows that Poole–Frenkel mechanism is also effective. The calculated metal electrode potential barrier at a constant voltage suggests that the dominant charge carrier mechanism is Schottky–Richardson type. Hence in the present case both SR and PF mechanisms are seem to be operative.     

Electrical conduction mechanism of polyvinylidenefluoride (PVDF) – polysulfone (PSF) blend film

The electrical conductivity of polyvinylidenefluoride (PVDF) – polysulfone (PSF) blend films have been measured by studying the I–V characteristics in the temperature range of 298–398 K. The results are shown by measuring the dependence of current on field, temperature, and blending compositions in the form of I–V characteristics and analysis has been made by interpretation of Poole–Frenkel, Schottky ln (J) vs. T plots, Richardson and Arrhenius plots. For individual polymers, the conduction mechanism observed to be a Poole–Frenkel type. On blending, the charge conduction appears to be the Schottky emission at lower temperature while Poole–Frenkel mechanism at higher temperature. The analysis of these results suggests that Pool–Frenkel mechanism is mainly responsible for the observed conduction. The conductivity was found to increase with an increase in the polysulfone concentration in the blend; it could be justified in terms of mobility of charge carriers. It is found that mobility of charge carriers increases with the increase in polysulfone concentration in the blend.     

On the Electrical and Optical Properties of Some Poly(Azomethine Sulfone)s in Thin Films

Poly(azomethine sulfone)s were synthesized by reacting 4,4′-sulfonyl bis(4-chlorophenyl) with 2,2-bis(4-hydroxyphenyl)propane and azomethine bisphenol in different molar ratios. Thin films were deposited from solution onto glass substrates. Study of the temperature dependences of the electrical conductivity, σ, and Seebeck coefficient, S, were performed in the temperature range 300 K–500 K. Thermal activation energies of electrical conduction, E_a , calculated from these dependences, ranged between 1.50 eV and 1.85 eV. The values of E_a were smaller for polymers with extended conjugation systems. The possibility to use the polymers in thermistor technology is discussed. The aspect of the temperature dependences of σ and S shows that a model based on the energy band-gap representation can be successfully used for explaining the electronic transport mechanism in the higher temperature range. In the lower temperature range, the mechanism of the electrical conduction is discussed in terms of the Mott variable range hopping conduction. The values of some optical parameters (absorption coefficient, optical band gap, etc.) were determined from transmission spectra.     

Dielectric properties and study of AC electrical conduction mechanisms by non-overlapping small polaron tunneling model in Bis(4-acetylanilinium) tetrachlorocuprate(II) compound

In the present work, the synthesis and characterization of the Bis(4-acetylanilinium) tetrachlorocuprate(II) compound are presented. The structure of this compound is analyzed by X-ray diffraction which confirms the formation of single phase and is in good agreement the literature. Indeed, the Thermo gravimetric Analysis (TGA) shows that the decomposition of the compound is observed in the range of 420–520 K. However, the differential thermal analysis (DTA) indicates the presence of a phase transition at T=363 k. Furthermore, the dielectric properties and AC conductivity were studied over a temperature range (338–413 K) and frequency range (200 Hz–5 MHz) using complex impedance spectroscopy. Dielectric measurements confirmed such thermal analyses by exhibiting the presence of an anomaly in the temperature range of 358–373 K. The complex impedance plots are analyzed by an electrical equivalent circuit consisting of resistance, constant phase element (CPE) and capacitance. The activation energy values of two distinct regions are obtained from log σT vs 1000/T plot and are found to be E=1.27 eV (T<363 K) and E=1.09 eV (363 K<T).The frequency dependence of ac conductivity, σ_ac, has been analyzed by Jonscher's universal power law σ(ω)=σ_dc+Aω^s. The value of s is to be temperature-dependent, which has a tendency to increase with temperature and the non-overlapping small polaron tunneling (NSPT) model is the most applicable conduction mechanism in the title compound.     

Electrical properties and conduction mechanism of [N(C2H5)4][N(CH3)4]CuCl4 compound

The [N(CH₃)₄][N(C₂H₅)₄]CuCl₄ single crystal has been synthetized in order to determinate the temperatures transition and to study the electrical properties and the conduction mechanism. At room temperature, this compound crystallizes in the tetragonal system with P-42₁m space group. The calorimetric study shows three anomalies at 248, 284 and 326 K. Electrical conduction and dielectrical relaxation mechanisms at various frequencies and temperatures were analyzed by impedance spectroscopy and the equivalent circuit based on the Z-View-software was proposed. The variation of f_p relaxation determinate by the modulus study and σ_dc specific to the AC conductivity as a function of temperature and confirm the all transitions for our sample. The values of the activation energy are determined and compared by those, which are found in the similar compound. Frequencies dependence of alternative current (AC) conductivity is interpreted in terms of Jonscher's law and the conduction mechanisms for each phase are determined with the Elliot's theory.     

Dielectric-spectroscopic and a.c. conductivity studies on layered Na2-XKXTi3O7 (X=0.2, 0.3, 0.4) ceramics

The dependence of loss tangent (tanδ) and relative permittivity (ε_r) on temperature and frequency has been reported for Na_2-XK_XTi₃O₇ (with X=0.2, 0.3, 0.4) ceramics. The losses are characteristic of dipole mechanism and electrical conduction. The peaks of ε_r at high temperature indicate a possible ferroelectric phase transition for all three compositions. The results of a.c. conductivity studies on the same samples have also been reported. The corresponding ln(σT) versus 1000/T plots have been divided into five regions namely I, II, III, IV and V. The various conduction mechanisms in the different regions have been stressed. Furthermore, the log(σ) versus frequency plots for all the above samples reveal that the electronic hopping (polaron) conduction, which diminishes with the rise in temperature, is dominant in the lower temperature region. The interlayer ionic conduction seems to play a major role in conduction towards higher temperature.     

Frequency domain measurements on Na2B4O7−V2O5 semiconducting glass

Summary Dielectric measurements on Na₂B₄O₇(99.5%)−V₂O₅(0.5%) glass system, in the frequency range 10⁻³ to 10⁴ Hz and temperature range 300 to 500 K, have been carried out. The normalized plots of complex capacitance have shown a single mechanism responsible for conduction for both volume and surface measurements with their close values of activation energies (0.67±0.03) eV and (0.64±0.03) eV, respectively. The low-frequency dispersion (LFD) behaviour has been observed to be perturbed by the presence of more than one competing process. The impedance plots have shown a parallel combination of a capacitor (C) and a resistor (R), with some contribution of a dispersive element due to charge accumulation in the vicinity of the electrodes. The values ofR andC were found to be of the same order of magnitude, for both surface and volume measurements. The observedR has shown a decrease with an increase in temperature due to an increase in mobility of Na⁺ ions, whereasC remains practicaly constant. The complex capacitance surface behaviour is dominated by volume, due to hygroscopy of this glass system.     

Comparative study of (1 0 0) and (1 1 1)B InGaAs single quantum well laser diodes

In this work, a series of identical In_xGa_1−xAs/Al_yGa_1−yAs single quantum well laser diodes, grown on (1 0 0) and (1 1 1)B GaAs substrates, have been thoroughly studied. For all samples, clear evidence of reduced threshold current densities in the (1 1 1)B substrate has been observed in electroluminescence spectra at 17 and 300 K. Modelling of the devices, based on a self-consistent solution of Schrödinger–Poisson's equations, was utilised in order to reproduce the experimental results. The model incorporates strain and piezoelectric effects on the quantum well states, free carrier screening, overlap integral computation, and optical gain calculation. The underlying mechanism, that explains the threshold reduction observed in the (1 1 1)B laser diodes, is discussed based on the results of the modelling.     

Structural and ionic conductivity studies of solid polymer electrolytes based on poly (vinylchloride) and poly (methylmethacrylate) blends

Thin film of poly (vinylchloride) and poly (methylmethacrylate) blend polymer electrolytes plasticized with a combination of DBP and Li₂SO₄ salts have been prepared by solution casting technique. The prepared films were subjected to a.c. impedance measurements as a function of temperature ranging from 304–373 K. The maximum conductivity at 304 K was found to be 1.24 × 10⁻⁸ S·cm⁻¹ for PVC-PMMA-Li₂SO₄-DBP (7.5-17.5-5-70 mole-%). Temperature dependence studies on the ionic conductivity in the PVC-PMMA-Li₂SO₄-DBP system suggest that the ion conduction follows the Williams-Landel-Ferry (WLF) mechanism, which is further confirmed by Vogel-Tamman-Fulcher (VTF) plots. XRD, FTIR, SEM and thermal studies revealed complex formation in.     

Dielectric and conduction mechanism studies of PVA-orthophosphoric acid polymer electrolyte

Polyvinyl alcohol (PVA)-based proton conducting polymer electrolytes have been prepared by the solution cast technique. The conductivity is observed to increase from 10⁻⁹ to 10⁻⁴ S cm⁻¹ as a result of orthophosphoric acid (H₃PO₄) addition. The plot of conductivity vs temperature shows that a phase transition occurred at 343 K in the sample PVA-33 wt% H₃PO₄. The β-relaxation peak is observed at 313 K. The glass transition temperature of PVA-33 wt% H₃PO₄ is 343 K. Orthophosphoric acid seems to play a dual role, i.e., as a proton source and as a plasticizer. The ac conductivity σ _ac = Aω ^s was also calculated in the temperature range from 303 to 353 K. The conduction mechanism was inferred by plotting the graph of s vs T from which the conduction mechanism for sample PVA-17 wt% H₃PO₄ was inferred to occur by way of the overlapping large polaron tunneling (OLPT) model and the conduction mechanism for the sample PVA-33 wt% H₃PO₄ by way of the correlated barrier height (CBH) model.     

三甲基镓在Pd(111)表面吸附解离及表面预吸附H和O的影响

利用X-射线光电子能谱（XPS）和程序升温脱附谱（TPD）研究了三甲基镓在Pd（111）表面的吸附和解离行为,并考察了表面预吸附H和O的影响。结果表明,在吸附温度为140 K时,三甲基镓在Pd（111）上主要为解离吸附,此时表面物种为Ga（CH₃）_x （x=1,2,3）和CH_x物种。加热将导致Ga的甲基化合物中的Ga-C键发生分步断裂,在不同温度下产生CH₄和H₂从表面脱附。同时,XPS结果证实了在275~325 K的温度区间内存在Ga甲基化合物的分子脱附。退火至更高温度,表面只观察到积碳和金属Ga物种,这二者随着温度的继续升高逐渐向体相扩散。在Pd（111）表面预吸附O和H对上述吸附和解离行为存在显著的影响。当表面预吸附H时,脱附产物CH₄和H₂的脱附主要位于315 K,可归属为一甲基镓的解离脱附。当表面预吸附O时,只在258 K观察到CH₄和H₂的脱附峰,可能来自于Pd-O-Ga（CH₃）₂吸附结构的解离.     

Complex impedance spectroscopy studies of (La0.70−xNdx)Sr0.30Mn0.70Cr0.30O3 (x≤0.30) perovskite compounds

The transport properties of Nd-doped perovskite materials (La_0.7−xNd_x)Sr_0.3Mn_0.7Cr_0.3O₃ (x≤0.30) were investigated using impedance spectroscopy techniques over a wide range of temperatures and frequencies. AC conductance analyses indicate that the conduction mechanism is strongly dependent on temperature and frequency. The DC conductance plots can be described using the small polaron hopping (SPH) model, with an apparent reduction of the polaron activation energy below the Curie temperature T_C. Complex impedance plots exhibit semicircular arcs described by an electrical equivalent circuit. Off-centered semicircular impedance plots show that the Nd-doped compounds obey to a non-Debye relaxation process. The conductivity of grains and grain-boundaries has been estimated. The activation energies calculated from the conductance and from time relaxation analyses are comparable. This indicates that the same type of charge carriers is responsible for both the electrical conduction and relaxation phenomena.     

Colossal Magnetoresistance of Layered Manganite La<Subscript>1.2</Subscript>Sr<Subscript>1.8</Subscript>Mn<Subscript>2</Subscript>O<Subscript>7</Subscript> and Its Description by a “Spin–Polaron” Conduction Mechanism

The resistance of a La_1.2Sr_1.8Mn_2(1–z)O₇ single crystal has been studied in magnetic fields from 0 to 90 kOe. The magnetoresistance at temperature T = 75 K, near which a colossal magnetoresistance maximum is observed, has been successfully described in terms of the “spin–polaron” electric conduction mechanism. This value of the colossal magnetoresistance is due to a three-fold increase in the polaron size. The method of separating contributions of various conduction mechanisms to the magnetoresistance developed for materials with activation type of conduction is generalized to compounds in which a metal–insulator transition is observed. It is found that, at a temperature of 75 K, the contribution of the “orientation” mechanism is maximum (≈20%) in a magnetic field of 5 kOe and almost disappears in fields higher than 50 kOe.     

Phase transitions and electrical conduction in thermal energy storage compound (n-C12H25NH3)2CdCl4

Differential scanning calorimetry (DSC) and differential thermal analysis (DTA) are performed for the compound (n-C₁₂H₂₅NH₃)₂CdCl₄. The ac conductivity σ_(ω,T), and the complex dielectric permittivity ?*_(ω,T) are measured as a function of temperature (100 K < T < 375 K) and at some selected frequencies (3 → 100 kHz). Two structural phase transitions are detected at T = (330 ± 1) K and T = (343 ± 1) K as minor and major transitions, respectively. The analysis of the measured electrical parameters reveals that the frequency-dependent conductivity obeys the power law, and the quantum mechanical tunneling (QMT) model is the main conduction mechanism in the low-temperature phase (LTP; phase III). The role of hydrogen bond N–H…Cl as a trigger force for phase transitions has been discussed. While the LTP is of the order–disorder type, the high-temperature phase (HTP) or phase I seems to be conformational and represents the main transition.     

Features of the conductivity of the quasi-one-dimensional compound TiS3

The structure and transport properties of single crystal whiskers of the TiS₃ quasi-one-dimensional semiconductor have been investigated. The anisotropy of the conductivity in the plane of layers (ab) has been measured as a function of the temperature. The anisotropy at 300 K is 5 and increases with a decrease in the temperature. Features on the temperature dependences of the conductivity along and across the chains are observed at 59 and 17 K. Near the same temperatures the form of the current-voltage characteristics measured along the chains is qualitatively changed. The current-voltage characteristics below 60 K exhibit nonlinearity and have a threshold form below 10 K. The results indicate possible phase transitions and the collective conduction mechanism at low temperatures.     

Investigation of low-temperature electrical conduction mechanisms in highly resistive GaN bulk layers extracted with Simple Parallel Conduction Extraction Method

The electrical conduction mechanisms in various highly resistive GaN layers of Al _x Ga_1−x N/AlN/GaN/AlN heterostructures are investigated in a temperature range between T=40 K and 185 K. Temperature-dependent conductivities of the bulk GaN layers are extracted from Hall measurements with implementing simple parallel conduction extraction method (SPCEM). It is observed that the resistivity (ρ) increases with decreasing carrier density in the insulating side of the metal–insulator transition for highly resistive GaN layers. Then the conduction mechanism of highly resistive GaN layers changes from an activated conduction to variable range hopping conduction (VRH). In the studied temperature range, ln (ρ) is proportional to T ^−1/4 for the insulating sample and proportional to T ^−1/2 for the more highly insulating sample, indicating that the transport mechanism is due to VRH.     

Effect of oxygen on the electrophysical properties of some carbon allotropes (graphite and C60 fullerene)

The electrophysical properties of powder samples of graphite and C₆₀ fullerene have been studied in vacuum, oxygen ambient, and air. Room-temperature conduction of the samples has a percolative character. An increase of temperature brings about an increase of sample conductivity caused by oxygen desorption from the bulk. A peak-shaped feature not observed before has been found in the temperature behavior of the resistance of finely dispersed C₆₀ and graphite powder samples in the T=310–340 K interval. Oxygen atoms were found to be involved in charge transport in powder samples of graphite and C₆₀ fullerene through their electrical activity on the surface of grains differing in size. Fiz. Tverd. Tela (St. Petersburg) 39, 1703–1705 (September 1997)     

DYNAMICS IN POLY(OXYMETHYLENE) AND POLY(OXYMETHYLENE-CO-OXYETHYLENE) AS STUDIED VIA A COMBINED CREEP RATE SPECTROSCOPY/DIFFERENTIAL SCANNING CALORIMETRY APPROACH

The peculiarities and kinetics of segmental dynamics in a few semi-crystalline poly(oxymethylene) (POM) samples and in poly(oxymethylene-co-oxyethylene) with 1.5% ethylene oxide units were studied over the temperature range from 110 to 430K. Differential scanning calorimetry (DSC) and laser-interferometric creep rate spectroscopy (CRS) were used. The latter was operated under uniaxial tension or compression. A number of dynamic anomalies were observed. These included a suppressed glass transition (T _g) with its transformation into segmental relaxations below and above T _g, and a pronounced dynamic heterogeneity, with the dispersion of activation energies of segmental motion ranging from 60 to 500 kJ mol^?1. Formation of anomalous long folds in POM and the copolymer structure is assumed from DSC data, indicating a predominant contribution of “straightened out” tie chains to the structure of disordered regions in these isotropic polymers. Discrete high-resolution CRS analysis showed that numerous peaks (separate types of segmental motion) constituted dynamics in the interlamellar layers of the polymers under study. Considerable influence of comonomer or small additives, or preliminary treatments (quenching, small pre-straining) on discrete CR spectra was observed and are discussed in the text. All the anomalies observed could be treated in terms of the concept of the common segmental nature of α and β relaxations in flexible-chain polymers; as the breakdown of intermolecular motional cooperativity due to nanoscale confinement effect, and as a different constraining influence of crystallites on dynamics in the intercrystalline layers.     

An NMR thermometer for cryogenic magic-angle spinning NMR: the spin-lattice relaxation of (127)I in cesium iodide

The accurate temperature measurement of solid samples under magic-angle spinning (MAS) is difficult in the cryogenic regime. It has been demonstrated by Thurber et al. (J. Magn. Reson., 196 (2009) 84-87) [10] that the temperature dependent spin-lattice relaxation time constant of ⁷⁹Br in KBr powder can be useful for measuring sample temperature under MAS over a wide temperature range (20–296 K). However the value of T₁ exceeds 3 min at temperatures below 20 K, which is inconveniently long. In this communication, we show that the spin-lattice relaxation time constant of ¹²⁷I in CsI powder can be used to accurately measure sample temperature under MAS within a reasonable experimental time down to 10 K.     

Lattice site occupation and precipitate dissolution of implanted and irradiated Al(Rh) and Al(Ir)

Abstract

The apparent substitutional fraction, f_s , of Rh and Ir implanted into Al single crystals at 293 K with peak concentrations of 0.17±0.01 at.% is 0.57 ± 0.02 and 0.47±0.02, respectively. Upon annealing to 593 K f_s decreases and the critical angles of the impurities narrow due to partially coherent precipitate formation. Irradiation with 300 keV Ar ions at 77 K with deposited energy densities up to 20 dpa leads to precipitate dissolution accompanied by an increase of f_s up to 0.70±0.02 for Al(Rh) and 0.82±0.02 for Al(Ir). The relatively high f_s values do not decrease during annealing from 77 K to 293 K, indicating that vacancy trapping is not a dominant process. Precipitate formation seems therefore to limit the maximum obtainable f_s values during implantation at 293 K for these systems.