Drop In Electrical Conductivity Research Articles

Ultrahard carbon film from epitaxial two-layer graphene.

Atomically thin graphene exhibits fascinating mechanical properties, although its hardness and transverse stiffness are inferior to those of diamond. So far, there has been no practical demonstration of the transformation of multilayer graphene into diamond-like ultrahard structures. Here we show that at room temperature and after nano-indentation, two-layer graphene on SiC(0001) exhibits a transverse stiffness and hardness comparable to diamond, is resistant to perforation with a diamond indenter and shows a reversible drop in electrical conductivity upon indentation. Density functional theory calculations suggest that, upon compression, the two-layer graphene film transforms into a diamond-like film, producing both elastic deformations and sp 2 to sp 3 chemical changes. Experiments and calculations show that this reversible phase change is not observed for a single buffer layer on SiC or graphene films thicker than three to five layers. Indeed, calculations show that whereas in two-layer graphene layer-stacking configuration controls the conformation of the diamond-like film, in a multilayer film it hinders the phase transformation.

Influence of Fe and Si Addition on the Properties and Structure Conductivity Aluminium

AbstractThe effect of iron and silicon addition on the structure and properties of aluminium wire rod obtained in the laboratory horizontal direct chill casting process has been analysed. In addition, the impact of laboratory wire drawing process has been examined. The addition of iron and velocity of casting increase the strength of aluminium wire rod in as-cast condition while the electrical conductivity drop acceptable. Moreover, the laboratory wire drawing process causes work-hardening wires and increase drawing tension as a result of fragmentation of structure and growth of grain boundaries. It has been shown that iron is beneficial for mechanical and technological properties of aluminium.

Magnesium-doped zircon-type rare-earth orthovanadates: Structural and electrical characterization

Undoped LnVO4 and magnesium-doped Ln0.95V0.95Mg0.10O4-δ (Ln = Pr, Sm, Gd, Dy and Er) orthovanadates were synthesized by solid state reaction method and characterized by XRD, SEM/EDS, electrical conductivity measurements in controlled atmospheres, and modified e.m.f. technique for determination of oxygen-ion transference numbers. XRD analysis showed the formation of phase-pure materials with tetragonal zircon-type structure and a decrease in lattice parameters with a decrease of ionic radius of rare-earth cations. Trace amounts of MgO and Mg-V-O phases revealed by SEM/EDS suggest that the solid solubility limit of magnesium cations in LnVO4 lattice is somewhat lower than the nominal doping level, and that magnesium substitutes preferentially into the vanadium sublattice. LnVO4 and Ln0.95V0.95Mg0.10O4-δ orthovanadates show semiconducting behavior under oxidizing conditions at 450–950°C and are predominantly oxygen-ionic conductors, except PrVO4 that shows mixed conductivity. In the LnVO4 series, electrical conductivity is the highest for PrVO4 and SmVO4 (~4 × 10−4S/cm at 800°C) and decreases with increasing atomic number of rare-earth cation for the other compositions. Additions of magnesium results in a drop of electrical conductivity, by 1.5–2 times for most of compositions. Interstitial oxygen diffusion is discussed as a prevailing mechanism of ionic transport in undoped LnVO4, whilst acceptor-type magnesium doping suppresses the formation of interstitial oxygen ions. Humidity has a rather negligible impact on the electrical properties of substituted ceramics, indicating only minor (if any) protonic contribution to the total electrical transport of the studied orthovanadates.

Effect of Mg/Mo Ratio in Stoichiometric Sr2MgMoO6-δ (SMM) Redox-Stable Anode

The state-of-the-art Ni/YSZ anode experiences a linear expansion of up to 1% when oxidized causing an irreversible microstructural change. Hence there is a need to develop redox stable anode materials with multifunctional properties like internal reforming, low temperature operation, and poison resistance. In this work, SMM, an oxygen-deficient, mixed valent perovskite has been studied to exploit its potential as a viable anode material in the SOFC application. The mixed valance of Mo (6+ and 5+) supports the electronic conductivity while losing oxygen. It also provides platform for catalytic activity for the oxidation of hydrogen and other hydrocarbon. It has been found that SMM experiences a volumetric reduction of only 0.03% in reducing atmosphere at 800oC compared to GDC and Ni/YSZ which experiences a volumetric expansion of about 0.2 and 1% respectively. Stoichiometric SMM has a conductivity of 50 S/cm in the reducing atmosphere and with consecutive redox cycling, the conductivity decreases. Effort has been made to understand the reason for the drop in electrical conductivity with increasing redox cycling. It has been found that SMM loses Mo with time at 800oC, accounting for the drop in conductivity. To improve the electrical performance of SMM, Mg/Mo ratio in stoichiometric SMM has been varied to compensate for Mo loss and its effect on thermal stability and electrical conductivity will be discussed in this work. Above all, these compositions will be tested as an anode in an electrolyte supported SOFC and the performance data will be compared with Mo rich SMM compositions. The change in microstructure of the anode after many operating hours will be examined and discussed in this topic. Also EIS analysis of the tested cells will be discussed in this work.

Effect of Mg/Mo Ratio in Stoichiometric Sr2MgMoO6-δ (SMM) Redox-Stable Anode

Stoichiometric SMM has a conductivity of ~50 S/cm in a reducing atmosphere but with consecutive redox cycling, the conductivity decreases. Effort has been made to understand the reason for the drop in electrical conductivity with increasing redox cycling. It has been found that the Mo concentration may change with time at 800oC, accounting for the drop in conductivity. This work investigated the electrical performance of the SMM composition as a function of Mg/Mo ratio and level of excess Mo. In addition, the work investigated the thermal and electrical stability as function of operation and thermal cycling. It was found that at a Mg/Mo ratio of 0.9/1.1, the material achieved both thermal and electrical redox stability. From redox dilatometry, it was found that Sr2Mg0.9Mo1.1O6-δ experienced a reversible volumetric reduction of 0.017% and had a reversible electrical conductivity of 17.5 S/cm in a reducing atmosphere.

Thermodynamic prediction of the effect of CO2 to the stability of (La0.8Sr0.2)0.98MnO3±δ system

Thermodynamic predictions regarding the formation of secondary phases in CO2 containing atmosphere on the (La0.8Sr0.2)0.98MnO3±δ (LSM-20) surface and at the LSM-20 (cathode)/8YSZ (electrolyte) interface have been studied using the CALculation of Phase Diagram (CALPHAD) approach. The effects of temperature, CO2 partial pressure, O2 partial pressure and the cathode composition on formation of secondary phases have been investigated and correlated with the available experimental results found in the literature. Our study predicts that the SrCO3 has the possibility to form on the surface and at triple phase boundaries (TPBs) as a result of CO2 exposure to the system. In addition, our study also indicates that the CO2 exposure does not change the electronic/ionic carriers' concentration in perovskite phase. The observed electrical conductivity drop is predicted to occur due to the formation of secondary phases such as LaZr2O7, SrZrO3 or SrCO3, at the LSM-20/8YSZ interface, resulting in the blocking of the electron/ion transfer paths.

Study on the thermoelectric properties of PVDF/MWCNT and PVDF/GNP composite foam

Thermoelectric effect is defined as the revisable translation between thermal and electrical energy. In this paper, we investigate the properties of p-type poly(vinylidene fluoride) (PVDF) based polymer composite foams that can be used in next generation energy harvesting applications. The composites were created using the continuous melt blending method. Multi-walled carbon nanotubes (MWCNTs) and graphene nano-platelets (GNPs) were used as secondary phases to strengthen the electrical conductivity of the composites. Foam structures were later generated using the super-critical carbon dioxide saturation method. We study the material properties between solid and foam samples; the results indicate a dramatic increase in overall thermoelectric properties for GNP foamed samples. We also report at least an order decrease in thermal conductivity, which is in favor of the thermoelectric effect. An unexpected drop in electrical conductivity was observed after the foaming process and can be explained by the large volumetric expansion of the foam. Finally, we report the Seebeck coefficient for both types of composite foams: 11 μV/K for 5 wt% MWCNT/PVDF foam and 58 μV/K for 15 wt% GNP/PVDF foam.

Influence of Casting Velocity on Structure and Properties of AlFe0,5 Alloy

The effect of iron addition on the structure and properties of aluminium wire rod obtained in the laboratory horizontal direct chill casting process has been analyzed. In addition, the impact of the casting speed on the properties of aluminium bars, containing the 0.5 wt% of Fe, and laboratory wire drawing process has been examined. The addition of iron increase the strength and plasticity of aluminium wire rod in as-cast condition and after wire drawing process, while the electrical conductivity drop acceptable. Moreover, by improving the plasticity of wires became possible to increase the deformability limit, measured by the reduction in the wire diameter by drawing to a level of less than 0.5 mm. Small-diameter wires are used for the construction of automotive wire harnesses, cables battery or winding wires. It has been shown that iron is beneficial for mechanical and technological properties of aluminium, and the horizontal direct chill casting process by graphite crystallizer may be an alternative solution in comparison with industrial scale of continuous casting and rolling by Properzi method (CCR) or Southwire SCR process in the context of the preparation of smaller amounts of material with scrap metals.

Electrically conductive ceramics and new joining technology for applications in HTR engineering

Ceramic constructional components are quite extensively required for operation of high-temperature nuclear reactors. Functional ceramics, in addition to constructional ceramics, are increasingly coming into the focus of research. Ceramic materials are predestined for use at high temperatures and in corrosive atmospheres. Modification of silicon carbide (SiC) by targeted doping, for instance, produces a suitable material for the production of heating conductors and thermoelectric generators. As a construction material, silicon carbide (SiC) is especially interesting due to its very good thermal, mechanical and radiological properties. SiC, furthermore, performs well when activated by neutron irradiation, with the induced activation subsiding after only a few hours (Hurtado, 1996). This property vector makes it an ideal starting material for use in a wide range of functional elements in high-temperature power engineering, particularly in high-temperature nuclear reactor engineering (e.g. V/HTR) including thermochemical plants for hydrogen generation or Synfuel production. In principle, it is possible to produce all-ceramic assemblies consisting of a thermoelectric generator and a sensor that can provide reliable measurement signals under extreme conditions in the high-temperature range without external power supply. This paper explains the feasibility of laser-joining such modified non-oxide ceramics, how to make electrically conductive joints, and thus, how to design complex assemblies. The parameters required for an optimal laser process to join ceramic materials were determined in extensive preliminary experiments. These investigations focused on the specific electrical resistances and optical properties. Specifically developed brazing fillers were fine-tuned so that the joints of the ceramics improved in terms of their physical interactions, chemical reactions and ability to bond or key chemically and mechanically with the ceramic surfaces. Thereby, the electrical conductivity required could be maintained up to temperatures above 950°C. Laser processing was carried out in a high-performance laser laboratory with diode lasers utilized in power ranges from 3kW to 10kW (continuous wave – cw). The measured four-point bending strength of the joined samples was between 120 and 170MPa (approx. 60% that of the starting material). No significant electrical conductivity drop was found in the joint area during resistance measurements. The results underline that the high-temperature laser joining technology used here is well-suited to provide electrically conductive ceramics with high-temperature resistant properties. This opens up new possibilities for the cost-effective and efficient manufacture of an entire range of high-temperature sensors as required in power engineering and, above all, in the field of high-temperature reactor engineering.

Water distribution across the mantle transition zone and its implications for global material circulation

Various methods for inferring the water distribution in Earth's mantle are reviewed including geochemical and geophysical methods. The geochemical approach using the water contents of basalts shows that the water content in the source regions of ocean island basalt is generally larger than that of the source region of mid-ocean ridge basalt, but the location of the source regions of ocean island basalts is poorly constrained. Geophysical approaches have potential of providing constraints on the spatial distribution of water but their usefulness depends critically on the sensitivity of geophysical observations to water content relative to other factors, in addition to the resolution of geophysical observations. Existing experimental data on the influence of water on seismologically observable properties and on electrical conductivity are reviewed. Frequently used seismological observations such as the anomalies in seismic wave velocities and of the topography on the mantle discontinuities are only weakly sensitive to water content but more sensitive to other factors such as the major element chemistry and temperature for a typical range of water contents. In contrast, electrical conductivity is highly sensitive to water content and only modestly sensitive to other factors such as temperature, oxygen fugacity and major element chemistry. Models of electrical conductivity–depth profiles are constructed where the influence of hydrogen and iron partitioning among coexisting minerals and of the depth variation in oxygen fugacity are incorporated. It is shown (i) that the electrical conductivity varies more than two orders of magnitude for a plausible range of water content in the mantle (~ 10 ppm wt to ~ 1 wt.%) and (ii) that if water content is constant with depth, there will be a drop in electrical conductivity at ~ 410-km. Although the resolution is not as high as seismological observations, geophysically inferred electrical conductivity distributions generally show higher conductivity in the mantle transition zone than the upper mantle, suggesting that the water content in the transition zone is higher than that in the upper mantle with some lateral variations. Implications of inferred water distribution are discussed including the possible partial melting near 410-km and its role in global water circulation.

Different effects of grain boundary scattering on charge and heat transport in polycrystalline platinum and gold nanofilms

The in-plane electrical and thermal conductivities of several polycrystalline platinum and gold nanofilms with different thicknesses are measured in a temperature range between the boiling point of liquid nitrogen (77 K) and room temperature by using the direct current heating method. The result shows that both the electrical and thermal conductivities of the nanofilms reduce greatly compared with their corresponding bulk values. However, the electrical conductivity drop is considerably greater than the thermal conductivity drop, which indicates that the influence of the internal grain boundary on heat transport is different from that of charge transport, hence leading to the violation of the Wiedemann–Franz law. We build an electron relaxation model based on Matthiessen's rule to analyse the thermal conductivity and employ the Mayadas & Shatzkes theory to analyse the electrical conductivity. Moreover, a modified Wiedemann–Franz law is provided in this paper, the obtained results from which are in good agreement with the experimental data.

The Effect of Amendment of Vegetable Waste Compost Used as Substrate in Soilless Culture on Yield and Quality of Melon Crops

One of the main environmental impacts of forced systems in horticulture — such as plastic covered and soilless culture — is the generation of organic plant residues and substrate waste. Many people are keen on research and development of ecologically friendly substrates. Thus, leaching experiments have been carried out with distilled water to determine whether compost — from horticultural greenhouse crop waste — can be used as growing media in vegetable crop production. The compost was examined and compared to other substrate alternatives like coconut coir waste. Two experiments were conducted in order to compare the two composts with coconut coir waste in terms of yield and fruit quality of melon crops (Cucumis melo). Electrical conductivity decreased sharply with leaching and eventually reached acceptable levels despite the high initial value of this parameter in the composts. This drop in electrical conductivity was parallel to that experienced in the contents of soluble mineral elements, with mainly high levels in SO42−, K+, Cl−, Mg2+, Ca2+ and Na+- concentrations. Most of the soluble mineral elements in compost were removed applying six times the amount of water. The comparison between two environmentally sound substrates, show that compost seems to be an acceptable ecological growing medium and similar to commercial coconut coir in soilless culture, it does not affect yield and quality of melon crops.

Influence of calcite on the electrokinetic treatment of a natural clay

After presenting a geochemical model for the interaction between calcite and varying environmental conditions, the paper discusses the experimental results of long duration electrokinetic tests, run on a natural clayey soil in unbuffered conditions. Local measurements of electrical potential, temperature and water flow were performed during the tests, while pH and fluid conductivity were measured locally once the tests had been dismantled. Sharp change of pH and reduction of the soil electrical conductivity, that in pure clays usually occur in the proximity of the cathode, were observed in the region close to the anode. As well, the soil in the anode area systematically tended to develop fractures, that mostly persisted until the end of the experiments. The features observed, that are not adequately taken into account by existing models, are interpreted in the light of the proposed calcite–water interaction. The onset of fractures, soil desaturation and electrical conductivity drop seem to be justified as a consequence of severe CO2 pressures arising in the anode area. Calcite dissolution and precipitation is held responsible for the changes of void ratio along the samples.

Ageing Behavior of Cu-Ag Alloys

To develop Cu alloy with tensile strength of 800 MPa and electrical conductivity of 80 %IACS (International Annealed Copper Standard), the variation of mechanical strength and electrical conductivity in Cu-Ag alloy during fabrication processes including casting, solid solution and ageing treatment were investigated. Solid solution hardening leads to a large drop in electrical conductivity of Cu-Ag alloys due to super-saturation of Ag solute in Cu matrix. Ageing hardening gives rise to enhance both of the mechanical strength and the electrical conductivity. Therefore, it can be mentioned that the electrical conductivity of Cu-Ag alloys was affected dominantly by Ag solute in Cu matrix.

Growth and characterization of vanadium dioxide thin films prepared by reactive-biased target ion beam deposition

Using a novel growth technique called reactive bias target ion beam deposition, the authors have prepared highly oriented VO2 thin films on Al2O3 (0001) substrates at various growth temperatures ranging from 250to550°C. The influence of the growth parameters on the microstructure and transport properties of VO2 thin films was systematically investigated. A change in electrical conductivity of 103 was measured at 341K associated with the well known metal-insulator transition (MIT). It was observed that the MIT temperature can be tuned to higher temperatures by mixing VO2 and other vanadium oxide phases. In addition, a current/electric-field induced MIT was observed at room temperature with a drop in electrical conductivity by a factor of 8. The current densities required to induce the MIT in VO2 are about 3×104A∕cm2. The switching time of the MIT, as measured by voltage pulsed measurements, was determined to be no more than 10ns.

Electronic and structural transitions in dense liquid sodium

At ambient conditions, the light alkali metals are free-electron-like crystals with a highly symmetric structure. However, they were found recently to exhibit unexpected complexity under pressure. It was predicted from theory--and later confirmed by experiment--that lithium and sodium undergo a sequence of symmetry-breaking transitions, driven by a Peierls mechanism, at high pressures. Measurements of the sodium melting curve have subsequently revealed an unprecedented (and still unexplained) pressure-induced drop in melting temperature from 1,000 K at 30 GPa down to room temperature at 120 GPa. Here we report results from ab initio calculations that explain the unusual melting behaviour in dense sodium. We show that molten sodium undergoes a series of pressure-induced structural and electronic transitions, analogous to those observed in solid sodium but commencing at much lower pressure in the presence of liquid disorder. As pressure is increased, liquid sodium initially evolves by assuming a more compact local structure. However, a transition to a lower-coordinated liquid takes place at a pressure of around 65 GPa, accompanied by a threefold drop in electrical conductivity. This transition is driven by the opening of a pseudogap, at the Fermi level, in the electronic density of states--an effect that has not hitherto been observed in a liquid metal. The lower-coordinated liquid emerges at high temperatures and above the stability region of a close-packed free-electron-like metal. We predict that similar exotic behaviour is possible in other materials as well.

Effects of Gas Adsorption on the Electrical Conductivity of Single-Wall Carbon Nanohorns

We present significant electrical conductivity responses of the pelletized as-prepared and oxidized (ox-) single-wall carbon nanohorns (SWNHs) on adsorption of CO(2) and O(2). The morphological and pore structures of both pelletized SWNHs were examined by transmission electron microscopy (TEM) and nitrogen adsorption isotherm, leading to explicit evidences of the formation of nanoscale windows on the wall by oxidation. The SWNH and ox-SWNH induced a semiconducting behavior, strongly responded to CO(2) and O(2) adsorptions, and each exhibited n-type- and p-type-like conductivities. The electrical conductivity increase and decrease for CO(2) and O(2) adsorption, respectively, were observed for SWNH, whereas ox-SWNH showed a marked electrical conductivity drop on CO(2) adsorption and almost no change on O(2) adsorption. The dramatically different electrical conductivity response of ox-SWNH is presumed to be ascribed to the annihilation of pentagons in the single graphene wall by oxidation.

Temperature-induced barium de-trapping from a double-well potential inBa6Ge25

The crystal structure of barium–germanium clathrateBa6Ge25 was studied using neutron powder diffraction in the temperaturerange 20–300 K. The compound was found to be cubic (spacegroup P4132) in the entire temperature range. However, the fully ordered model of the crystal structure(no split sites) is marginal at room temperature, and clearly fails at low temperature. Amuch better description of the crystal structure below 250 K is given in terms of two splitBa sites, with random occupancies, for two out of three types of cages present in theBa6Ge25 structure. The Ba atoms were found to interact strongly with the Ge host. The separationof the split Ba sites grows with decreasing temperature, with a sudden increase on coolingthrough the 200–250 K temperature range, accompanied by an expansion of the entirecrystal structure. The ‘locking-in’ of Ba atoms into split sites was originally suggested byPaschen et al (2002 Phys. Rev. B 65 134435) as a plausible scenario behind anomalies in thetransport and magnetic properties. Our data prompt us to favour a simple model for thistransition, based on temperature-induced de-trapping of Ba from a deep double-wellpotential. The most significant of the transport anomalies, that is, the drop in electricalconductivity on cooling, can be easily explained within this model through the enhancedstructural disorder, which would affect the relaxation time for all portions of the Fermisurface. We suggest that the other anomalies (increase in the absolute value ofthe negative Seebeck coefficient, decrease in the magnetic susceptibility) can beexplained within the framework of the one-electron semi-classical model, without anyneed to invoke exotic bi-polaron-driven charge carrier interaction mechanisms.

Absence of intrinsic electric conductivity in single dsDNA molecules

The intrinsic dc conductivity of long, individual lambda phage dsDNA molecules has been investigated by ultrasensitive low current–voltage-spectroscopy (IV) under ambient conditions and controlled low humidity inert gas atmosphere on microfabricated metal–insulator–metal gap structures. We found a strong dependence of the measured conductivity on the apparent humidity, which we attribute to capillary condensation of water to the immobilized DNA molecules, giving rise to additional ionic currents. Additional IV-spectroscopy experiments under controlled argon atmosphere always revealed a significant drop in electrical conductivity to 4 × 10 −15 A V −1 μm −1, indicating almost no considerable contribution of electrical long range charge transport.

Correlation between the composition, structure and properties of dual ion beam deposited SiN x films

Silicon nitride (SiN x ) films were prepared by dual ion beam deposition at room temperature. An assisted N 2 + ion beam (current I b=0–45 mA) was directed to bombard the substrate surface to control the N content x, which saturated at x≈1.36 when I b⩾25 mA. The presence of SiN bonds was indicated by the appearance of a Si 2p photoelectron peak at 101.9 eV and an infrared absorption peak at 850 cm −1. As x increases from 0 to 1.36, the hardness, elastic modulus and compressive stress increase from 12.2 to 21.5 GPa, 191 to 256 GPa and 0.52 to 1.4 GPa and the friction coefficient against stainless steel ball decreases from 0.65 to 0.37. The optical band gap increases remarkably with a concomitant drop in electrical conductivity (σ RT) by more than 10 7 times. Ion bombardment induces defects and trap states in the mid-gap, such that the transport mechanism is dominated by hopping of charge carriers through the trap states. Consequently, the activation energy of electrical conductivity is much lower than the optical band gap.