Dramatic effect of multiwalled carbon nanotubes on the electrical properties of alumina based ceramic nanocomposites

We report recent work on electrical properties of multiwalled carbon nanotubes (MWNTs)/alumina composites. The composites with different contents of MWNTs were consolidated by spark plasma sintering and their temperature dependence dc conductivity was scrutinized in the temperature range from 5 to 300 K. The analysis of the temperature dependence of the conductivity suggests that for temperatures higher than 50 K, conduction can be ascribed to thermal fluctuation induced tunneling of the charge carriers through insulating barriers between MWNTs, while at temperatures below 50 K, the conduction can be attributed to three dimensional variable range hopping through MWNTs network in the alumina matrix. The frequency dependence of the conductivity was studied from 5 to 1.3 × 10⁷ Hz. The universality of the ac conduction in MWNT/alumina composites was examined by construction of master curve.     

填充型聚合物基复合材料的导电和导热性能

研究了高密度聚乙烯为基体、炭黑和炭纤维为填料复合体系的导电和导热性能。发现当导电填料的含量达到渗流阈值时,复合材料的电导率急剧升高;而在渗流阈值附近,其热导率未出现突变。这表明电导渗流现象不完全是由导电粒子通过物理接触生成导电链所致。其导电机制是相当数量的导电粒子相互发生隧道效应。     

On the importance of the structure in the electrical conductivity of fishbone carbon nanofibers

Carbon nanofibers (CNFs) have a remarkable electrical conductivity resulting highly attractive for different applications such as composites or electronics due to their high quality/price ratio. Although it is known that their graphitic character provides a high conductivity, very little is known about the influence of the nanofibers structure on that property. In this study, CNFs characterized by different physical properties are prepared at diverse synthesis temperatures within a range (550–750 °C) in which significant structural and dimensional changes are accomplished and homogeneous nanofiber growth takes place. The electrical conductivity is determined on the powdery as-grown materials modifying the compaction degree by applying pressure. Because of a combination of structural features, the apparent electrical conductivity increases with synthesis temperature of CNFs, ranging from 50 S m^?1 for the worst conducting CNFs at a low compaction degree (25 % of solid volume fraction) to 3 × 10³ S m^?1 for the best conducting CNFs at a high compaction degree (60 % of solid volume fraction). Further analysis is carried out applying the percolation theory to analyze the experimental data and the results suggest that both the orientation of the graphenes and the filament diameter distribution play a determining role in the intrinsic electrical conductivity with values in the interval 1.5 × 10³ to 1.3 × 10⁴ S m^?1. These intrinsic values of electrical conductivity are found between one and two orders of magnitude higher than that of the powder, highlighting the also important effect of porosity.     

Charge transport in CaTiO3: I. Electrical conductivity

The present paper reports the electrical conductivity of polycrystalline undoped CaTiO₃ in the temperature range 973–1323 K under controlled oxygen partial pressure (10–10⁵ Pa). The electrical conductivity data are considered in terms of defect disorder and related semiconducting properties of CaTiO₃. The values of the p(O₂) exponent of electrical conductivity at high p(O₂), that vary between 1/4.3 and 1/6.2 at 973 and 1323 K, respectively, are considered in terms of theoretical defect disorder model of p-type CaTiO₃ and increasing effect of minority charge carriers (electrons) with temperature on p-type conduction. The activation energy of the electrical conductivity, assuming 125.3 kJ/mol at 10 Pa and 94.4 kJ/mol at 72 kPa, has been considered in terms of the formation of defect and their mobility. The band gap, determined from the minimum of electrical conductivity corresponding to the n–p transition is equal to 2.77 eV.     

Characterization and electrical conductivity of La<Subscript>1−x</Subscript>Sr<Subscript>x</Subscript>CrO<Subscript>3</Subscript> NTC ceramics

The La_1?xSr_xCrO₃ (x?=?0–0.1) negative temperature coefficient (NTC) ceramics have been prepared by the traditional solid state reaction method. X-ray diffraction (XRD) analysis has revealed that the as-sintered ceramics crystallize in a single perovskite structure. Scanning Electron Microscope (SEM) images show that the doped Sr²⁺ contributes to in the decrease in porosity. X-ray photoelectron spectroscopy (XPS) analysis indicates the existence of Cr³⁺ and Cr⁶⁺ ions on lattice sites, which result in hopping conduction. The presence of the Cr⁶⁺ is one of the key factors that affect the electrical conductivity of La_1?xSr_xCrO₃. Resistance–temperature characteristics were studied in the range of ?80 to 10?°C for the ceramic samples, the electrical characterizations show that the electrical resistivity and material constant B decrease with the increase of the strontium content.     

Effect of gamma irradiation on the electrical and optical properties and conduction mechanism of α-SiO2

The electrical and optical properties of gamma-irradiated α-SiO₂ crystals are studied in broad dose and temperature ranges, and their lattice dynamics is analyzed using IR absorption spectroscopy. The radiation effect on the electrical conductivity of the crystals is shown to be a nonlinear function of temperature and gamma dose, characteristic of percolation systems at low temperatures. The conductivity of the crystals is shown to be proportional to T ^?1/2 in a broad temperature range. The mechanism of conduction in the crystals is discussed in terms of percolation theory.     

Effect of Gamma Irradiation on the Electrical Properties of Synthetic Quartz Crystals

The effect of gamma irradiation on the electrical conductivity, dielectric losses, and dielectric permittivity of synthetic quartz crystals was studied in a broad temperature range (295–670 K). The conductivity of the crystals was shown to exhibit Arrhenius behavior above 420 K and power-law behavior (T ^1/4) below 420 K. These findings are interpreted as evidence that, at elevated temperatures, thermally activated transport prevails, whereas at low temperatures, the dominant mechanism of conduction is hopping transport. The conductivity of quartz passes through a maximum at a gamma dose of about 10⁶ rad. Gamma irradiation slightly reduces the activation energy for low-temperature conduction, whereas the high-temperature activation energy remains virtually unchanged.     

Electrical conduction in manganese molybdate

Electrical conduction in manganese molybdate (MnMoO₄) has been explained on the basis of electrical transport data obtained by measuring electrical conductivity (a.c. and d.c.), thermoelectric power and the dielectric constant of MnMoO₄ in the temperature range 300–1000K. MnMoO₄ has been found to be a semiconductor with the energy gap 3.5 eV. A marked change at ~ 600K occurs due to transition from extrinsic to the intrinsic conduction. Depending upon the electrical transport data, electrical conduction in the extrinsic region has been explained in terms of the small polaron hopping mechanism and in the intrinsic region it has been explained in terms of the localized model of of electrical transport.     

DC electrical conductivity of poly(methyl methacrylate)/carbon black composites at low temperatures

The study deals with the dc electrical conduction of poly(methyl methacrylate)/carbon black composites of different carbon black (CB) filler concentrations (2, 6, 12 wt%). The dc electrical conductivity was studied as a function of filler concentration, and temperature in the range (20–290 K). It was found that the composites exhibit negative temperature coefficient of resistivity (NTCR) at low temperatures and enhancement in the dc electrical conductivity with both temperature and CB concentration. The observed increase of conductivity with CB concentration was interpreted through the percolation theory. The dependence of the electrical conductivity of the composites in low temperatures was analyzed in term of a formula in consistence with Mott variable rang hopping (VRH) mechanism. The observed overall mechanism of electrical conduction has been related to the transfer of electrons through the carbon black aggregations distributed in the polymer matrix.     

Direct current conductivity of carbon nanofiber-based conductive polymer composites: effects of temperature and electric field

Polymer composites based on high density polyethylene (HDPE) and carbon nanofiber (CNF) were fabricated by melt compounding. The dependences of electrical conductivity of HDPE-CNF composites on filler concentration, temperature, and applied electric field were investigated. The results showed that the conductivity of the HDPE-CNF composites follows the scaling law of percolation theory. Increasing temperature caused a sharp increase in the resistivity of HDPE-CNF composites near the melting temperature of HDPE, yielding a positive temperature coefficient (PTC) effect of resistance. The potential mechanisms involved in the PTC effect of such composites were analyzed. An investigation of the effect of electric field on the conductivity of HDPE-CNF composites revealed the presence of tunneling conduction. The tunneling conductivity increased with increasing filler content because of high tunneling frequency, and decreased with rising temperature as a result of gap widening between conducting CNF fillers.     

Interstitial Cd doping CdTe films by co-sputtering

A series of CdTe thin films were grown in a Cd enriched ambient by co-sputtering a Cd–CdTe target. Different concentrations were obtained by changing the relative area occupied by the metallic Cd pieces placed onto the CdTe target. It was observed that the process of electrical conduction is not the same for all the samples. For samples with Cd < 50 at% the electrical conductivity is due to the direct activation of charge carriers from the valence band or from impurity states to the conduction band. For samples with Cd > 50 at% electrical conduction is due to the hopping of carriers from state to state by optimizing the distance of tunneling. Conductivity (σ) vs temperature (T ) measurements indicate that electrical conductivity behaves as ln σ α (T₀/T )1, 4, the Mott law for variable range hopping (VRH) in disordered systems. We think that this change in conductivity occurs because Cd atoms place in interstitial sites, doping the CdTe lattice, after the Cd vacancy (V_cd) are filled.     

Space-charge-limited currents and carrier trapping in plasma-polymerized malononitrile films

The dark electrical conductivity of plasma-polymerized malononitrile (PPMa) films in an Ag/PPMa/Ag sandwich structure was investigated over the temperature range 300–525 K at a reduced pressure of 10^-5 Torr. The room temperature current-voltage characteristics indicate space-charge-limited currents. The results for the conductivity as a function of inverse temperature are in accordance with the band model proposed by Barbe and Westgate, and the thermal energy gap between the top of the valence band and the bottom of the conduction band was determined to be 0.86±0.01 eV. At temperatures below about 380±5 K the conduction process is consistent with the presence of an electron trapping level situated 0.34±0.05 eV below the conduction band edge with a density of (5.02±0.05) × 10¹⁶ cm^-3. It is assumed that above about 435±5 K the conduction process is intrinsic. Experiments on the chemisorption of oxygen suggest that electrons are the majority carriers in PPMa films.     

Zn incorporation in CuInSe2: Particle size and strain effects on microstructural and electrical properties

Incorporation of the doping element Zn in the temperature range (550–700 °C) and the impact on structural and electrical properties of CuInSe₂ material are investigated. X-ray diffraction patterns showed the chalcopyrite nature of the pure and doped CuInSe₂ and revealed that diffusion temperature governs particle size as well as tensile strain. The calculated lattice parameters and cell volumes revealed that Zn diffuses in CuInSe₂ by substitution on Cu sites. Electrical properties of the material have been investigated using a contact-less technique based on high frequency microwave (HF). It is found that Zn atoms influence the defect equilibrium resulting in the conversion of the conduction type. The conductivity of the samples has been found increasing as the diffusion temperature increases.     

Electrical conductivity of zinc ferrites near stoichiometry and manganese-zinc ferrites under vacuum or in the presence of oxygen

The electrical conductivity of zinc ferrites near stoichiometry and of manganese-zinc ferrites has been investigated as a function of temperature under vacuum and in the presence of oxygen. Under vacuum, the conductivity of these ferrites with iron excess is explained by the hopping mechanism, and with ZnO excess by the development of vacancies in octahedral sites of cation-deficient spinel. Activation energies and the transition temperatures are presented. During the oxidation in oxygen of Mn-Zn ferrites, the profile of the log σ= f(T) curves shows that the mechanism of electrical conduction in the temperature range 100 to 350 °C can be explained in terms of the oxidation of Fe²⁺ to Fe³⁺ ions at octahedral sites. For the temperature range 300 to 450 °C, the conductivity involves the hopping of electrons from octahedral sites of Mn³⁺ ions to octahedral sites of Mn⁴⁺ ions. Above 550 °C the oxidation of Mn²⁺ ions leads to a marked change in conductivity with the generation of new phases.     

Influence of temperature and frequency on electrical properties of electron beam evaporated bromoaluminum phthalocyanine, BrAlPc, thin films

The AC electrical properties of electron beam evaporated Bromoaluminum phthalocyanine (BrAlPc) thin films have been studied in the frequency range 10²–10⁵ Hz and in the temperature range of 303–413 K. The BrAlPc thin films are characterized by field emission scanning electron microscopy (FESEM). The capacitance is found to be sensitive to the frequency and increases with increasing temperature and decreases with increasing frequency. A loss minimum has been observed in the frequency dependence of the dissipation factor. Such behavior is found to be in good qualitative agreement with the model of Goswami and Goswami. The AC conductivity $\sigma \left( \omega \right)$ σ ( ω ) is found to vary as $\omega^{s}$ ω s in the studied frequency range. At frequencies 10–10² Hz, s is less than unity and decreases with increase in temperature indicating a dominant hopping process. At frequency ranges 10²–10⁴ Hz, exponent s lies very close to the unity and is independent of temperature, which shows the quantum mechanical tunneling is dominated conduction mechanism. At higher frequencies 10⁴–10⁵ Hz, s is found to be temperature independent. The temperature dependence of AC conductivity shows a linear increase with the increase in temperature. Moreover, the activation energies of device are determined as a function of frequency.     

The electrical and thermal conductivities of dilute copper-chromium alloys at low temperatures

Electrical and thermal conductivity measurements are reported on dilute copper-chromium alloys containing 32 and 50 at. ppm chromium, in their unannealed state and also after annealing at 530–550°C for 16 and 66 h in each case, under fore-vacuum. From the electrical resistivity measurements evidence of a possible cluster formation is obtained in the annealed sample. The electronic Lorenz number of the alloys increases with decrease of temperature in the temperature range 4.2–1.6 K. Lower temperature measurements might show a maximum around the Kondo temperature (～1 K) as predicted by the available theories on dilute magnetic alloys.     

Microstructure and electrical property of copper films on a flexible substrate formed by an organic ink with 9.6 % of Cu content

An organic copper ink with 9.6 wt% of Cu content derived from a short carbon chain organic copper precursor was successfully applied on a modified PI substrate and easily formed a favorable conductive copper film by self-reduction in the sintering process, which showed excellent conductivity. The effects of sintering temperature and time on the microstructure and conductivity action of the copper films were studied by XRD, EDS and SEM and electrical measurements, respectively. The sheet resistance and resistivity were determined to be as low as 0.11 Ω/□ and 2.2 × 10^?5 Ω·cm. The conduction mechanism is discussed in terms of the percolation theory.     

Temperature dependence of electrical conductivity in double-wall and multi-wall carbon nanotube/polyester nanocomposites

The aim of this study is to investigate temperature dependence of electrical conductivity of carbon nanotube (CNT)/polyester nanocomposites from room temperature to 77 K using four-point probe test method. To produce nanocomposites, various types and amounts of CNTs (0.1, 0.3 and 0.5 wt.%) were dispersed via 3-roll mill technique within a specially formulized resin blend of thermoset polyesters. CNTs used in the study include multi walled carbon nanotubes (MWCNT) and double-walled carbon nanotubes (DWCNT) with and without amine functional groups (–NH₂). It was observed that the incorporation of carbon nanotubes into resin blend yields electrically percolating networks and electrical conductivity of the resulting nanocomposites increases with increasing amount of nanotubes. However, nanocomposites containing amino functionalized carbon nanotubes exhibit relatively lower electrical conductivity compared to those with non-functionalized carbon nanotubes. To get better interpretation of the mechanism leading to conductive network via CNTs with and without amine functional groups, the experimental results were fitted to fluctuation-induced tunneling through the barriers between the metallic regions model. It was found that the results are in good agreement with prediction of proposed model.     

Synthesis of Dy doped Yb0.1Ca0.9MnO3 ceramics with a high relative density and their thermoelectric properties

Perovskite-type Yb_0.1Ca_0.9?xDy_xMnO₃ ceramics were synthesized by solid state reaction. Their microstructures were characterized and the thermoelectric properties were evaluated between 300 K and 1100 K. Each of sample exhibits single phase with orthorhombic structure. All samples have high relative densities, and their values are between 95% and 97%, which is consistent with the SEM image. The electrical resistivity shows a typical metallic conductivity behavior. Lowest electrical resistivity 2.36 mΩ cm is achieved at room temperature, and the variation of electrical resistivity is not evident in whole measured temperature range. The Seebeck coefficients are negative, indicating an n-type conduction. The highest power factor 310 μW/(K² m) is obtained for the sample with x = 0.02. The thermal conductivity is decreased by the difference in the mass between the Ca²⁺ and Dy³⁺ ions, especially in the heavy doped samples. The highest figure of merit is 0.11 at 1069 K for the sample with x = 0.02.     

Effect of sintering temperature on structural and electrical properties of gadolinium doped ceria (Ce<Subscript>0·9</Subscript>Gd<Subscript>0·1</Subscript>O<Subscript>1·95</Subscript>)

Gadolinium doped ceria oxide is one of the promising materials as an electrolyte for IT-SOFCs. Ce_0·9Gd_0·1O_1·95 (GDC10) powder was prepared by solid state reaction and sintered at 1473 K, 1573 K, 1673 K and 1773 K. All samples were studied using X-ray diffraction, scanning electron micrograph and d.c. conductivity measurement. The crystallinity and surface morphology of the samples improved with sintering temperature. Further, the electrical conductivity measurement indicated that the conduction mechanism is mainly ionic. The conductivity of samples sintered at 1673 K and 1773 K at 800°C are of the order of 0·1 S-cm⁻¹. The activation energies decreased from 1·25–0·82 eV with increase in sintering temperature.