Large Increase In Conductivity Research Articles

Ion Transport and Switching Speed in Redox-Gated 3-Terminal Organic Memory Devices

The dynamics of redox gated organic memory devices based on dynamic doping of polythiophene were examined in detail in order to improve “write” and “erase” speed and determine ultimate performance. A 3-terminal geometry similar to a field effect transistor provided a source/gate circuit which reversibly oxidized a polythiophene polymer to cause a large increase in conductance between the source and drain electrodes. The devices were cycled for >1000 complete R/W/R/E cycles, and operated at relatively low voltage compared to commercial “flash” memory. The “write” and “erase” speeds were improved by a factor of >100 by using a spin-coated electrolyte layer and by small increases in device temperature. The influence of charging current and polaron propagation on response time were determined to be minor, with the rate limiting process being identified as the rate of conducting polaron generation. The main factor determining the W/E time was the mobility of ions in the polyethylene oxide electrolyte layer, which resulted in resistance losses during the application of the S-G “write” pulse. Response time was strongly dependent on the atmosphere, with water or acetonitrile vapor significantly increasing the rate of polaron generation. The results are important for design of molecular memory devices based on dynamic doping, and indicate likely avenues for further performance improvements.

Dramatically improved dielectric properties of polymer composites by controlling the alignment of carbon nanotubes in matrix

Aligned multi-walled carbon nanotubes (MWCNTs)/polyvinyl alcohol composite films were prepared by using an easy and controllable electrospinning-in situ film-forming (EF) technique. A high dielectric constant (k), a low dielectric loss, a consistently high breakdown strength, and a high energy density were obtained by using this technique. The dramatically improved dielectric properties are ascribed to the good dispersion and alignment of MWCNTs in the matrix, facilitating the formation of a large number of separated nano-capacitors (high k and low direct current (DC) conductance). For comparison purposes, the same composite films were prepared by solution casting (SC). At the same MWCNT content, the SC method yielded a higher k, but a significantly higher dielectric loss and much lower breakdown strength and energy density because of the random dispersion of MWCNTs in the matrix and the formation of a MWCNT network, which result in a large increase in DC conductance. The formation mechanism of the different microstructures and the relationships between the microstructures and dielectric properties are clarified. Our results indicate that high-performance MWCNTs/polymer dielectric composites can be obtained by controlling the microstructure of the composites by using the EF technique, which widens the applications of dielectric materials.

Ceramic/polymer interpenetrating networks exhibiting increased ionic conductivity with temperature control of ion conduction for thermal runaway protection

A novel polymer electrolyte nanostructure consisting of poly(ethylene oxide), PEO, complexed with lithium triflate placed in a nanoporous ceramic membrane was fabricated. The PEO electrolyte was placed in 200 nm pores of AAO filtration membranes and took the form of “sleeves” of polymer lining the 200 nm pores. The confinement of the electrolyte in this nanostructure somewhat increased the ion conduction of the polymer electrolyte compared to a non-confined PEO electrolyte. This increase was attributed to reduction in PEO crystallinity and polymer chain ordering resulting from interaction with the alumina walls of the nanoporous membranes. As expected, when the non-confined PEO sample was heated to the melting temperature, a large increase in ion conduction, resulting from the molten phase, was seen. Interestingly, the polymer confined in nanopores did not exhibit a large increase in conductivity. This was once again attributed to interaction with the alumina walls of the nanoporous membranes, hindering ion motion as compared to molten pure PEO polymer. The value to thermal runaway prevention of lower conductivities after going through the melt will be discussed. TGA studies indicate that the nanocomposite electrolyte is more thermally stable at high temperatures than pure PEO electrolyte. At 450 °C, 80% of the pure PEO was lost to thermal degradation while the nanocomposite electrolyte lost only 30% of its weight by this degradation process. This increased thermal stability was attributed to polymer confining wall interactions and a nanoscale passive thermal management system created by the heat absorbing AAO matrix and the transfer of heat to the gas in the open tubes of the polymer electrolyte.

Nonmonotonic Size Dependence in the Hole Mobility of Methoxide-Stabilized PbSe Quantum Dot Solids

We present a facile procedure to fabricate p-type PbSe-based quantum dot solids with mobilities as large as 0.3 cm(2) V(-1)s(-1). Upon partial ligand exchange of oleate-capped PbSe quantum dots with the methoxide ion, we observe a pronounced red shift in the excitonic transition in conjunction with a large increase in conductivity. We show that there is little correlation between these two phenomena and that the electronic coupling energy in PbSe quantum dot solids is much smaller than often assumed. However, we observe for the first time a nonmonotonic size dependence of the hole mobility, illustrating that coupling can nonetheless be dominant in determining the transport characteristics. We attribute these effects to a decrease in charging energy and interparticle spacing, leading to enhanced electronic coupling on one hand and enhanced dipole interactions on the other hand, which is held responsible for the majority of the red shift.

AlGaN/GaN HEMT based hydrogen sensor with platinum nanonetwork gate electrode

AlGaN/GaN high electron mobility transistor (HEMT) based hydrogen sensors incorporating platinum nanonetworks in the gate region were demonstrated. Pt nanonetworks with 2–3 nm diameter were synthesized by a simple and low-cost solution phase method, and applied to the gate electrode of transistor sensor. The HEMT with physically and electrically connected Pt nanonetwork gate showed good pinch-off and modulation of drain current characteristics. Compared to conventional Pt thin film AlGaN/GaN HEMT sensor, the Pt nanonetwork sensor has dramatically improved current response to hydrogen. Relative current change of Pt nanonetwork gated sensor in 500 ppm H2 balanced with Air ambient was 3.3 × 106% at VGS of −3.3 V, while 2.5 × 102% at VGS of −2.9 V for Pt film. This results from large increase in channel conductance induced by huge catalytic surface area of nanostructured Pt networks.

Electrical conductivity of Sr2−xVMoO6−y (x = 0.0, 0.1, 0.2) double perovskites

Electrical conductivity of Sr2-xVMoO6-y (x = 0.0, 0.1, 0.2) double perovskites has been investigated in a reducing atmosphere at temperatures up to 800 °C. This material has a key application in solid oxide fuel cell anodes as a mixed ion and electron conductor. A solid state synthesis technique was used to fabricate materials and crystal structure was verified through x-ray diffraction. Subsequent to conventional sintering in a reducing environment, elemental valence states were indentified through x-ray photoemission spectroscopy on the double perovskite material before and after annealing in a hydrogen environment. Samples exhibited metallic like conduction with electrical conductivities of 1250 S/cm (Sr2VMoO6-y′), 2530 S/cm (Sr1.8VMoO6-y″), and 3610 S/cm (Sr1.9VMoO6-y‴) at 800 °C in 5% H2/95% N2, with a substantial increase in conductivity upon cooling to room temperature. Room temperature electrical conductivity values for Sr1.9VMoO6-y‴ make it a candidate as the highest electrically conductive oxide known. Highly insulating secondary surface phases, Sr3V2O8, and SrMoO4, begin to reduce at 400 °C in a hydrogen environment, as confirmed by X-ray photoemission and thermal gravimetric analysis. This reduction, from V5+ and Mo6+ to lower valence states, leads to a large increase in sample electrical conductivity.

Programmable ZnO nanowire transistors using switchable polarization of ferroelectric liquid crystal

We demonstrate modulations of electrical conductance and hysteresis behavior in ZnO nanowire transistors via electrically polarized switching of ferroelectric liquid crystal (FLC). After coating a nanowire channel in the transistors with FLCs, we observed large increases in channel conductance and hysteresis width, and a strong dependence of hysteresis loops on the polarization states associated with the orientation of electric dipole moments along the direction of the gate electric field. Furthermore, the reversible switching and retention characteristics provide the feasibility of creating a hybrid system with switch and memory functions.

Atomistic simulations of highly conductive molecular transport junctions under realistic conditions

We report state-of-the-art atomistic simulations combined with high-fidelity conductance calculations to probe structure-conductance relationships in Au-benzenedithiolate (BDT)-Au junctions under elongation. Our results demonstrate that large increases in conductance are associated with the formation of monatomic chains (MACs) of Au atoms directly connected to BDT. An analysis of the electronic structure of the simulated junctions reveals that enhancement in the s-like states in Au MACs causes the increases in conductance. Other structures also result in increased conductance but are too short-lived to be detected in experiment, while MACs remain stable for long simulation times. Examinations of thermally evolved junctions with and without MACs show negligible overlap between conductance histograms, indicating that the increase in conductance is related to this unique structural change and not thermal fluctuation. These results, which provide an excellent explanation for a recently observed anomalous experimental result [Bruot et al., Nat. Nanotechnol., 2012, 7, 35-40], should aid in the development of mechanically responsive molecular electronic devices.

Origin of temperature plateaus in laser-heated diamond anvil cell experiments

Many high-pressure high-temperature studies using laser-heated diamond cells have documented plateaus in the increase of temperature with increasing laser power or with time. By modeling heat transfer in typical laser-heated diamond anvil cell experiments, we demonstrate that latent heat due to melting or other phase transformation is unlikely to be the source of observed plateaus in any previously published studies, regardless of whether pulsed or continuous lasers were used. Rather, large increases (∼10-fold) in thermal conductivity can explain some of the plateaus, and modest increases in reflectivity (tens of percent) can explain any or all of them. Modeling also shows that the sub-microsecond timescale of heating employed in recent pulsed heating experiments is fast enough compared to heat transport into and through typical insulations, but too slow compared to heat transport into metallic laser absorbers themselves to allow the detection of a large plateau due to latent heat of fusion. Four new designs are suggested for future experiments that could use the simple observation of a latent heat-induced plateau to provide reliable high-pressure melting data.

Role of the Skeletal Muscle Pump in Exercise Onset Hyperemia in the Human Forearm

Blood flow to active skeletal muscle increases with the first contraction, but the mechanism responsible for this exercise onset hyperemia is unknown. We tested the hypothesis that the muscle pump is necessary for onset hyperemia. Blood pressure and brachial artery blood flow velocity (Doppler ultrasound) were measured in 6 young males in three different trials, each performed with the arm at heart level (HL) and with the arm hanging below heart level (BH) in the dependent position, perpendicular to the floor : 1) a single dynamic handgrip contraction (20% MVC) (HG), 2) following a 10‐minute arm occlusion combined with a 2‐minute isometric handgrip (from 7:30 to 9:30 of the occlusion period) to induce maximal vasodilation in the forearm (OC), and 3) arm occlusion and handgrip followed by a dynamic handgrip performed 5 seconds after cuff release (OC+HG). HG elicited an increase in forearm conductance that was not different between arm positions (247±77 vs 308±35%). OC elicited a large increase in conductance that was not different between arm positions (0.100±0.010 vs. 0.101±0.008 units). OC+HG (.093±0.008 at heart level and 0.086±0.010 units below heart level) did not cause a greater increase in conductance than either the OC trial or the pre‐handgrip value in the OC+HG trial. These data suggest that the muscle pump does not play an important role in immediate exercise hyperemia in the forearm.

Non-vacuum, single-step conductive transparent ZnO patterning by ultra-short pulsed laser annealing of solution-deposited nanoparticles

A solution-processable, high-concentration transparent ZnO nanoparticle (NP) solution was successfully synthesized in a new process. A highly transparent ZnO thin film was fabricated by spin coating without vacuum deposition. Subsequent ultra-short-pulsed laser annealing at room temperature was performed to change the film properties without using a blanket high temperature heating process. Although the as-deposited NP thin film was not electrically conductive, laser annealing imparted a large conductivity increase and furthermore enabled selective annealing to write conductive patterns directly on the NP thin film without a photolithographic process. Conductivity enhancement could be obtained by altering the laser annealing parameters. Parametric studies including the sheet resistance and optical transmittance of the annealed ZnO NP thin film were conducted for various laser powers, scanning speeds and background gas conditions. The lowest resistivity from laser-annealed ZnO thin film was about 4.75×10−2 Ω cm, exhibiting a factor of 105 higher conductivity than the previously reported furnace-annealed ZnO NP film and is even comparable to that of vacuum-deposited, impurity-doped ZnO films within a factor of 10. The process developed in this work was applied to the fabrication of a thin film transistor (TFT) device that showed enhanced performance compared with furnace-annealed devices. A ZnO TFT performance test revealed that by just changing the laser parameters, the solution-deposited ZnO thin film can also perform as a semiconductor, demonstrating that laser annealing offers tunability of ZnO thin film properties for both transparent conductors and semiconductors.

Antidromic potential spread modulates the receptor responses in the stretch receptor neurons of the crayfish

The effects of antidromic potential spread were investigated in the stretch receptor neurons of the crayfish. Current and potential responses to conductance changes were recorded in the dynamic clamp condition and compared to those obtained by using some conventional clamp methods and a compartmental neuron model. An analogue circuit was used for dynamic calculation of the injected receptor current as a function of the membrane potential and the given conductance change. Alternatively, receptor current responses to a mechanical stimulus were recorded and compared when the cell was voltage clamped to a previously recorded impulse wave form and the resting potential, respectively. Under dynamic clamp, the receptor current had an oscillating waveform which contrasts with the conventional recordings. Frequency, amplitude and sign of the oscillations were dependent on the applied conductance level, reversal potential and electrotonic attenuation. Mean current amplitude and frequency of the evoked impulse responses were smaller under dynamic clamp, especially for large conductance increases. However, firing frequency was larger if plotted against the mean current response. Recorded responses were similar to those calculated in the model. It was not possible to evoke any adaptation in the slowly adapting neuron by using the dynamic clamp. Evoked potential change served as a self limiting response, preventing the depolarization block. However, impulse duration was significantly shorter in the rapidly adapting neuron when the dynamic clamp was used. It was concluded that, in the stretch receptor neurons during a conductance increase, antidromic potential spread modulates the receptor responses and contributes to adaptation.

The distribution of blood flow in the carotid and vertebral arteries during dynamic exercise in humans

The mechanism underlying the plateau or relative decrease in cerebral blood flow (CBF) during maximal incremental dynamic exercise remains unclear. We hypothesized that cerebral perfusion is limited during high-intensity dynamic exercise due to a redistribution of carotid artery blood flow. To identify the distribution of blood flow among the arteries supplying the head and brain, we evaluated common carotid artery (CCA), internal carotid artery (ICA), external carotid artery (ECA) and vertebral artery (VA) blood flow during dynamic exercise using Doppler ultrasound. Ten subjects performed graded cycling exercise in a semi-supine position at 40, 60 and 80% of peak oxygen uptake (VO2 peak) for 5 min at each workload. The ICA blood flow increased by 23.0 ± 4.6% (mean ± SE) from rest to exercise at 60% (VO2 peak). However, at 80% (VO2 peak), ICA blood flow returned towards near resting levels (9.6 ± 4.7% vs. rest). In contrast, ECA, CCA and VA blood flow increased proportionally with workload. The change in ICA blood flow during graded exercise was correlated with end-tidal partial pressure of CO2 (r = 0.72). The change in ICA blood flow from 60% (VO2 peak) to 80% (VO2 peak) was negatively correlated with the change in ECA blood flow (r = −0.77). Moreover, there was a significant correlation between forehead cutaneous vascular conductance and ECA blood flow during exercise (r = 0.79). These results suggest that during high-intensity dynamic exercise the plateau or decrease in ICA blood flow is partly due to a large increase in ECA blood flow, which is selectively increased to prioritize thermoregulation.

Investigation of the electrical conductivity of propylene glycol-based ZnO nanofluids

Electrical conductivity is an important property for technological applications of nanofluids that has not been widely studied. Conventional descriptions such as the Maxwell model do not account for surface charge effects that play an important role in electrical conductivity, particularly at higher nanoparticle volume fractions. Here, we perform electrical characterizations of propylene glycol-based ZnO nanofluids with volume fractions as high as 7%, measuring up to a 100-fold increase in electrical conductivity over the base fluid. We observe a large increase in electrical conductivity with increasing volume fraction and decreasing particle size as well as a leveling off of the increase at high volume fractions. These experimental trends are shown to be consistent with an electrical conductivity model previously developed for colloidal suspensions in salt-free media. In particular, the leveling off of electrical conductivity at high volume fractions, which we attribute to counter-ion condensation, represents a significant departure from the "linear fit" models previously used to describe the electrical conductivity of nanofluids.

Development of a Thermal Rectifier Usable at High Temperature

By using Al-based metallic alloys characterized by a disordered structure and a narrow pseudogap of a few hundred meV in energy width persisting at the Fermi level, we succeeded in preparing materials possessing a large increase of thermal conductivity with increasing temperature. This unusual increase of thermal conductivity is caused by the electronic structure effect known as the bipolar diffusion effect (BDE) in the context of the two-band model. A thermal rectifier was constructed using materials exhibiting the BDE. By showing the thermal rectification of the bulk sample prepared in this study, we demonstrate that our newly proposed idea of a thermal rectifier using the BDE is applicable for practical use.

Ultrasonic-aided fabrication of gold nanofluids

A novel ultrasonic-aided one-step method for the fabrication of gold nanofluids is proposed in this study. Both spherical- and plate-shaped gold nanoparticles (GNPs) in the size range of 10-300 nm are synthesized. Subsequent purification produces well-controlled nanofluids with known solid and liquid contents. The morphology and properties of the nanoparticle and nanofluids are characterized by transmission electron microscopy, scanning electron microscope, energy dispersive X-ray spectroscope, X-ray diffraction spectroscopy, and dynamic light scattering, as well as effective thermal conductivities. The ultrasonication technique is found to be a very powerful tool in engineering the size and shape of GNPs. Subsequent property measurement shows that both particle size and particle shape play significant roles in determining the effective thermal conductivity. A large increase in effective thermal conductivity can be achieved (approximately 65%) for gold nanofluids using plate-shaped particles under low particle concentrations (i.e.764 μM/L).

Leaf Transpiration Efficiency of Sweet Corn Varieties from Three Eras of Breeding

ABSTRACTWhen measured under midday field conditions, modern varieties of maize (Zea mays L.) often have substomatal carbon dioxide concentration (Ci) values in excess of those required to saturate the photosynthetic carbon dioxide assimilation rate (A). This results in lower leaf transpiration efficiency (TE), the ratio of photosynthesis to transpiration, than potentially achievable for a given rate of photosynthesis. In some other crops, breeding has indirectly resulted in a large increase in stomatal conductance (gs), which would decrease TE. I tested whether this occurred in sweet corn by comparing five open‐pollinated varieties released before 1903 with four standard hybrid varieties released in the mid‐1900s and four varieties released after 1990. Leaf gas exchange of each variety was measured under ambient midday field conditions on eight sunny days over two seasons in Beltsville, MD. Although there were significant differences among varieties in gs, A, Ci, and TE, no effect of era was significant for any variable except A. Of the four modern varieties tested, two had the highest gs and two had the lowest gs of any of the varieties compared, with high gs associated with low TE. These four varieties were further compared in a third year and were found to differ consistently in leaf TE but not in A. It is concluded that breeding over the last century has not increased gs or decreased leaf TE in sweet corn but that there is significant variation in TE among modern sweet corn varieties.

Understanding the p-type defect chemistry of CuCrO2

CuCrO2 is the most promising Cu-based delafossite for p-type optoelectronic devices. Despite this, little is known about the p-type conduction mechanism of this material, with both CuI/CuII and CrIII/CrIV hole mechanisms being proposed. In this article we examine the electronic structure, thermodynamic stability and the p-type defect chemistry of this ternary compound using density functional theory with three different approaches to the exchange and correlation; the generalized-gradient-approximation of Perdew, Burke and Ernzerhof (PBE), PBE with an additional correction for on-site Coulombic interactions (PBE + U) and the nonlocal, screened-exchange hybrid functional HSE06. The fundamental band gap of CuCrO2 is demonstrated to be indirect in nature. Under all growth conditions, the dominant intrinsic p-type defect will be the Cu vacancy, with hole formation centered solely on the Cu sublattice. Mg doping is found to be significantly lower in energy than intrinsic defect formation, explaining the large increases in conductivity seen experimentally. Cu-rich/Cr-poor growth conditions are found to be optimal for both intrinsic and extrinsic (Mg doping) defect formation, and should be adopted to maximize performance.

Electrical conduction mechanisms in piezoelectric ceramics under harsh operating conditions

Piezoelectric ceramics are widely used in industry for sensor and actuator applications. Under an applied electric field over a long duration, the conductivity of the ceramic increases resulting in leakage currents and increased power consumption. This process is accelerated by harsh environments and operating conditions such as high temperature, high humidity and high electric field. In this study, a piezoelectric ceramic was stressed by exposure to high relative humidity, at an elevated temperature, with a continuously applied d.c. electric bias field. Periodically during the test, the bias field was removed and a low frequency a.c. electrical cycle applied, during which current and voltage were measured. Results show that the conductivity is both time and voltage dependent and indicative of a complex breakdown process in the ceramic. The breakdown field strength was observed to vary only slightly during the exposure to humidity and d.c. bias despite large increases in conductivity. The reestablishment of the conduction process on re-application of the d.c. bias field was also studied. Whilst these results are specific to the piezoelectric material system studied, the mechanisms and insights into the interaction of electrodes with a ceramic material under conditions of high humidity and high electric field are likely to have much wider relevance for electro-ceramics in general, including technologically important materials such as dielectrics for energy storage and electronics, electronic oxides, and ferroelectric memory devices.

Predicted thermal performance of triple vacuum glazing

The simulated triple vacuum glazing (TVG) consists of three 4 mm thick glass panes with two vacuum gaps, with each internal glass surface coated with a low-emittance coating with an emittance of 0.03. The two vacuum gaps are sealed by an indium based sealant and separated by a stainless steel pillar array with a height of 0.12 mm and a pillar diameter of 0.3 mm spaced at 25 mm. The thermal transmission at the centre-of-glazing area of the TVG was predicted to be 0.26 W m −2 K −1. The simulation results show that although the thermal conductivity of solder glass (1 W m −1 K −1) and indium (83.7 W m −1 K −1) are very different, the difference in thermal transmission of TVGs resulting from the use of an indium and a solder glass edge seal was 0.01 W m −2 K −1. This is because the edge seal is so thin (0.12 mm), consequently there is a negligible temperature drop across it irrespective of the material that the seal is made from relative to the total temperature difference across the glazing. The results also show that there is a relatively large increase in the overall thermal conductance of glazings without a frame when the width of the indium edge seal is increased. Increasing the rebate depth in a solid wood frame decreased the heat transmission of the TVG. The overall heat transmission of the simulated 0.5 m by 0.5 m TVG was 32.6% greater than that of the 1 m by 1 m TVG, since heat conduction through the edge seal of the small glazing has a larger contribution to the total glazing heat transfer than that of the larger glazing system.