Conductivity Decay Research Articles

Persistent decay of fresh xylem hydraulic conductivity varies with pressure gradient and marks plant responses to injury.

Defining plant hydraulic traits is central to the quantification of ecohydrological processes ranging from land-atmosphere interactions, to tree mortality and water-carbon budgets. A key plant trait is the xylem specific hydraulic conductivity (Kx ), that describes the plant's vascular system capacity to transport water. While xylem's vessels and tracheids are dead upon maturity, the xylem is neither inert nor deadwood, various components of the sapwood and surrounding tissue remaining alive and functional. Moreover, the established definition of Kx assumes linear relations between water flux and pressure gradient by tacitly considering the xylem as a "passive conduit". Here, we re-examine this notion of an inert xylem by systematically characterizing xylem flow in several woody plants using Kx measurements under constant and cyclic pressure gradients. Results show a temporal and pressure gradient dependence of Kx . Additionally, microscopic features in "living branches" are irreversibly modified upon drying of the xylem, thus differentiating the macroscopic definition of Kx for living and dead xylem. The findings highlight the picture of the xylem as a complex and delicate conductive system whose hydraulic behaviour transcends a passive gradient-based flow. The study sheds new light on xylem conceptualization, conductivity measurement protocols, in situ long-distance water transport and ecosystem modelling.

Enhancing Short-Term Plasticity by Inserting a Thin TiO2 Layer in WOx-Based Resistive Switching Memory

In this work, we emulate biological synaptic properties such as long-term plasticity (LTP) and short-term plasticity (STP) in an artificial synaptic device with a TiN/TiO2/WOx/Pt structure. The graded WOx layer with oxygen vacancies is confirmed via X-ray photoelectron spectroscopy (XPS) analysis. The control TiN/WOx/Pt device shows filamentary switching with abrupt set and gradual reset processes in DC sweep mode. The TiN/WOx/Pt device is vulnerable to set stuck because of negative set behavior, as verified by both DC sweep and pulse modes. The TiN/WOx/Pt device has good retention and can mimic long-term memory (LTM), including potentiation and depression, given repeated pulses. On the other hand, TiN/TiO2/WOx/Pt devices show non-filamentary type switching that is suitable for fine conductance modulation. Potentiation and depression are demonstrated in the TiN/TiO2 (2 nm)/WOx/Pt device with moderate conductance decay by application of identical repeated pulses. Short-term memory (STM) is demonstrated by varying the interval time of pulse inputs for the TiN/TiO2 (6 nm)/WOx/Pt device with a quick decay in conductance.

Interfacial Charge Transfer and Gate-Induced Hysteresis in Monochalcogenide InSe/GaSe Heterostructures.

Heterostructures of two-dimensional (2D) van der Waals semiconductor materials offer a diverse playground for exploring fundamental physics and potential device applications. In InSe/GaSe heterostructures formed by sequential mechanical exfoliation and stacking of 2D monochalcogenides InSe and GaSe, we observe charge transfer between InSe and GaSe because of the 2D van der Waals interface formation and a strong hysteresis effect in the electron transport through the InSe layer when a gate voltage is applied through the GaSe layer. A gate voltage-dependent conductance decay rate is also observed. We relate these observations to the gate voltage-dependent dynamical charge transfer between InSe and GaSe layers.

Acid–base core–shell microspheres are incorporated into proton exchange membranes to effectively alleviate the rapid decline in proton conductivity at low humidity

The development of a proton exchange membrane (PEM) that can avoid rapid decay of proton conductivity under low humidity is of great significance for the practical application of PEMFC. In this study, acid–base core–shell microspheres (PCSMs-MA@TAC) with a carboxylic acid core and a triazine shell were synthesized by distillation-precipitation polymerization using cross-linked carboxylic acid microspheres (PMAA) as seeds. These PCSMs were then incorporated into a sulfonated poly(ether ether ketone) matrix to make hybrid membranes. Incorporation of PCSMs microspheres can not only strengthen the vehicle mechanism by increasing the water uptake of the membrane, but also the acid–base pairs formed at the SPEEK/PCSMs interface provide a new low-energy barrier pathway for proton hopping, thereby enhancing the proton conduction of the Grotthuss mechanism. The results show that when the content is 10 wt%, the proton conductivity of the SPEEK/PCSMs-MA@TAC composite membrane can reach 0.161 S cm−1 at 80°C and 100% RH, which is 19.3% higher than the SPEEK control membrane (0.135 S cm−1). In particular, even at 60% RH, the proton conductivity of the SPEEK/PCSMs-MA@TAC-10 composite membrane is still 67 mS cm−1, which is 3.16 times higher than that of the SPEEK membrane. Therefore, the SPEEK/PCSMs-MA@TAC composite membrane can maintain superior performance even under high temperature and low humidity conditions.

How Long Is Otosclerosis Surgery Effective? Hearing Results after a 22-Year Follow-Up

Introduction: Stapes surgery is a safe procedure, with favourable hearing outcome. The objective of the study is to assess the long-term hearing results, addressing the bone conduction (BC) decay and the need for hearing aids in otosclerosis patients. Methods: We enrolled patients who underwent stapes surgery by means of stapedectomy or stapedotomy between 1991 and 2001. All enrolled patients underwent pure-tone audiometry (PTA) between September 2017 and June 2018. A set of questions was administered to record the prevalence of subjective symptoms and the need for hearing aids. Results: Seventy patients were enrolled for a long-term evaluation; 37 patients underwent bilateral surgery; therefore, 107 ears were included in the analysis. The average follow-up period was 22 years. No statistically significant difference was found between early and late post-operative air conduction (AC) PTA (41 vs. 49 dB; p > 0.05) nor between early and late post-operative BC-PTA (29 vs. 37 dB; p > 0.05). A significant difference was observed for AC at 8 kHz (65 vs. 78 dB; p < 0.05) and BC at 2 and 4 kHz (28 vs. 40 dB and 45 vs. 58 dB, respectively; p < 0.05). Conclusions: This is, to our knowledge, the longest mean follow-up time in the literature. A mild decrease in both AC and BC threshold can be expected and the sensorineural decay is more pronounced on the high frequencies. The subjective hearing symptoms and overall sound perception are satisfactory.

Full‐scale evaluation of critical control points and monitors at a reuse facility

AbstractThe hazard analysis and critical control point (HACCP) process is being widely adopted as a design, control, and operational methodology to maintain the reliability of delivering water quality that is safe for public health. This study applied the HACCP methodology to identify and assess the reliability of critical control points and critical monitors to manage acute and chronic health risks in potable reuse treatment trains. Specifically, a failure analysis was performed for full‐scale ultrafiltration and reverse osmosis critical control points to determine the reliability of critical monitors and their associated impacts on finished water quality. The results supported the use of ultrafiltration and reverse osmosis membranes as critical control points in potable reuse and identified the sensitivities of both current and emerging critical monitoring parameters, including turbidity, total organic carbon, ultraviolet absorbance at 254 nm, conductivity, fluorescence, calcium, sulfate, sucralose, and pressure decay tests.

Effect of Aging under AC Superimposed Harmonic Voltage on Trap and Carrier Transport Properties of Silicone Rubber

The rapid growth of power grid capacity and the widespread use of a large number of power electronics and non-linear loads have led to harmonics in the power system. Harmonics in the power system will cause safety hazards to the normal operation of power equipment, exacerbate the aging of insulation materials, and reducing the overall operation reliability of the system. In the present work, we used power frequency ac voltage superimposed harmonics to carry out ageing experiments on power cable terminals. Then, we tested the infrared spectra, dielectric spectra, electrical conductivity, and surface potential decay characteristics of silicone rubber insulation materials on the cable terminals aged for different times. The experimental results show that the dielectric constant and dielectric loss of silicone rubber gradually increase with the aging time. In particular, the dielectric loss of silicone rubber changed greatly at low frequencies. The effect of dc conductance of aged silicone rubber on dielectric loss is significantly enhanced at low frequencies, which causes the dielectric loss to increase as the frequency decreases following an inverse power law. The surface potential decay rates of silicone rubber insulation after positive and negative corona charging accelerate with increasing the aging time, which is consistent with the experimental results of electrical conductivity. By analyzing the distribution characteristics of electron and hole traps in silicone rubbers, it is found that the trap energy levels of electron and hole traps become shallower as the operating time increases. The calculation of the carrier hopping conduction model shows that the shallow trap formed with increasing the aging time will lead to increases in both carrier mobility and conductivity. When the conductivity rises to a certain value, the silicone rubber will lose its insulation performance, resulting in insulation failure.

Measurement and modeling of partial discharge arising from different cavity geometries at very low frequency

In this paper, a comparative study including measurement and modeling of partial discharge for various void geometries (cylinder, block, and prisms) at very low frequency (VLF) and power frequency (PF) voltage is presented. Results from experiments and simulations are presented with phase-resolved discharge patterns and integrated parameters (maximum/average discharge magnitudes, and repetition rates). Dynamic simulation, achieved using a combination of finite element analysis and Matlab, matches the experimental results. Differences of discharge characteristics between VLF and PF may be attributed to the physical mechanisms in the modeling including cavity surface conductivity and space charge decay rate. Compared with discharge activities at PF, the VLF excitation in general exhibits relatively lower discharge magnitude and repetition rate. Also, the inception voltage is found to be lower at VLF.

Stretchable electrothermochromic fibers based on hierarchical porous structures with electrically conductive dual-pathways

Stretchable color-changing fibers are urgently demanded for smart textiles/clothing due to their perfect implantability, permeability of vapor and heat, and flexibility/ stretchability. Herein, stretchable electrothermochromic fibers were fabricated with unconventional stretchable conductive fibers as core layers and thermochromic coatings as shell layers. In the stretchable conductive fibers, hierarchical porous structures with percolative one-dimensional (1D) conductive networks were constructed through phase inversion of carbon nanotube/polyurethane (CNT/PU) solutions. With the deposition of silver nanoparticles (AgNPs) on the surface of micro-pores, electrically conductive dual-pathways consisting of 0D AgNPs and 1D CNTs were formed to significantly enhance the electric conductivity and thus improve the electrothermal performance of the fibers. More importantly, because of the connective CNTs and AgNPs, such dual-pathways ensured the electron transport under the stretching state, preventing the sharp decay of conductivity and electrothermal performance. Through the continuous wet-spinning method, the stretchable conductive fibers can be easily obtained with the length up to several meters. At last, stretchable electrothermochromic fibers were prepared with two color-changing modes and implanted into textile perfectly, advancing their applications in wearable display and military adaptive camouflage of smart clothing.

Recent Advances of Volatile Memristors: Devices, Mechanisms, and Applications

Due to the rapid development of artificial intelligence (AI) and internet of things (IoTs), neuromorphic computing and hardware security are becoming more and more important. The volatile memristors, which feature spontaneous decay of device conductance, own the distinct combination of high similarity to the biological neurons and synapses and unique physical mechanisms. They are excellent candidates for mimicking the synaptic functions and ideal randomness source of the entropy for hardware‐based security. Herein, the recent advances of volatile memristors in devices, mechanisms, and application aspects are summarized. First, a brief introduction is presented to describe the switching type, materials, and temporal response of volatile memristors. Second, the volatile switching mechanisms are discussed and grouped into ion effects, thermal effects, and electrical effects. Third, attention is focused on the applications of volatile memristors for access devices, neuromorphic computing (artificial neurons and synapses), and hardware security (true random number generators and physical unclonable functions). Finally, major challenges and future outlook of volatile memristors for neuromorphic computing and hardware security are discussed.

Electrical Characterization of a New Crosslinked Copolymer Blend for DC Cable Insulation

To design reliable high voltage cables, clean materials with superior insulating properties capable of operating at high electric field levels at elevated temperatures are required. This study aims at the electrical characterization of a byproduct-free crosslinked copolymer blend, which is seen as a promising alternative to conventional peroxide crosslinked polyethylene currently used for high voltage direct current cable insulation. The characterization entails direct current (DC) conductivity, dielectric response and surface potential decay measurements at different temperatures and electric field levels. In order to quantify the insulating performance of the new material, the electrical properties of the copolymer blend are compared with those of two reference materials; i.e., low-density polyethylene (LDPE) and peroxide crosslinked polyethylene (XLPE). It is found that, for electric fields of 10–50 kV/mm and temperatures varying from 30 °C to 70 °C, the DC conductivity of the copolymer blend is in the range of 10−17–10−13 S/m, which is close to the conductivity of crosslinked polyethylene. Furthermore, the loss tangent of the copolymer blend is about three to four times lower than that of crosslinked polyethylene and its magnitude is on the level of 0.01 at 50 °C and 0.12 at 70 °C (measured at 0.1 mHz and 6.66 kV/mm). The apparent conductivity and trap density distributions deduced from surface potential decay measurements also confirmed that the new material has electrical properties at least as good as currently used insulation materials based on XLPE (not byproduct-free). Thus, the proposed byproduct-free crosslinked copolymer blend has a high potential as a prospective insulation medium for extruded high voltage DC cables.

Visualizing Quantum Interference in Molecular Junctions.

Electron transport across a molecular junction is characterized by an energy-dependent transmission function. The transmission function accounts for electrons tunneling through multiple molecular orbitals (MOs) with different phases, which gives rise to quantum interference (QI) effects. Because the transmission function comprises both interfering and noninterfering effects, individual interferences between MOs cannot be deduced from the transmission function directly. Herein, we demonstrate how the transmission function can be deconstructed into its constituent interfering and noninterfering contributions for any model molecular junction. These contributions are arranged in a matrix and displayed pictorially as a QI map, which allows one to easily identify individual QI effects. Importantly, we show that exponential conductance decay with increasing oligomer length is primarily due to an increase in destructive QI. With an ability to "see" QI effects using the QI map, we find that QI is vital to all molecular-scale electron transport.

Electron-Transport Characteristics through Aluminum Oxide (100) and (012) in a Metal-Insulator-Metal Junction System: Density Functional Theory-Nonequilibrium Green Function Approach.

Al2O3 is commonly used in modern electronic devices because of its good mechanical properties and excellent electrical insulating property. Although fundamental understanding of the electron transport in Al2O3 is essential for its use in electronic device applications, a thorough investigation for the electron-transport mechanism has not been conducted on the structures of Al2O3, especially in nanometer-scale electronic device settings. In this work, electron transport via Al2O3 for two crystallographic facets, (100) and (012), in a metal–insulator–metal junction configuration is investigated using a density functional theory-based nonequilibrium Green function method. First, it is confirmed that the transmission function, T(E), decreases as a function of energy in (E – EF) < 0 regime, which is an intuitively expected trend. On the other hand, in the (E – EF) > 0 regime, Al2O3(100) and Al2O3(012) show their own characteristic behaviors of T(E), presenting that major peaks are shifted toward lower energy levels under a finite bias voltage. Second, the overall conductance decay rates under zero bias are similar regardless of the crystallographic orientation, so that the contact interface seemingly has only a minor contribution to the overall conductance. A noteworthy feature at the finite bias condition is that the electrical current drastically increases as a function of bias potential (>0.7 V) in Al2O3(012)-based junction compared with the Al2O3(100) counterpart. It is elucidated that such a difference is due to the well-developed eigenchannels for electron transport in the Al2O3(012)-based junction. Therefore, it is evidently demonstrated that at finite bias condition, the contact interface plays a key role in determining insulating properties of Al2O3–Pt junctions.

Impact of the dangling bond defects and grain boundaries on trapping recombination process in polycrystalline 3C SiC

The bulk polycrystalline (pc) 3C SiC of n- and p-type obtained by thermal decomposition of methyltrichlorosilane vapor have been studied by applying the electron paramagnetic resonance (EPR), direct current (DC) conductivity, photoconductivity (PC) and PC time-resolved decay methods. The energy level of the two donor-like minority carrier traps at ε1 = 77 meV and ε2 = 92.6 meV located at the grain boundaries (GB) of pc-3C SiC has been obtained using the measurements of the temperature dependence of DC conductivity and PC in the range of 80–600 K. The minority carrier traps assigned to carbon and silicon dangling bonds with the carbon back bonds were observed in the EPR spectra of pc-3C SiC of n- and p-type at g = 2.0029(3), g = 20042(3), respectively. The PC time decay after the termination of the photo-excitation was studied in monocrystalline and pc-3C SiC of n- and p-type at 80 K. The persistent relaxation of PC has been described by kinetic equations accounting the trapping, ionization, and recombination processes of non-equilibrium charge carriers bound dynamically to shallow donors and acceptors. We have concluded that the main process responsible for the long-lived relaxation of the PC is trap-assisted electron-hole recombination in n-type pc-3C SiC and ionization of boron acceptors, as well as the hole escape/capture at the boron level in p-type pc-3C SiC. The differences in the relaxation process of the PC in n- and p-type pc-3C SiC were explained by the presence of the potential barrier height of about 8.6 meV at GB for the capture of the majority carriers in p-type pc-3C SiC.

Ultrafast THz Conductivity Dynamics of a Novel Fe-Doped InGaAs Quantum Photoconductor

We propose an Fe-doped, InGaAs-based semiconductor heterostructure optimized for the application as an ultrafast photoconductor and investigate its optical as well as THz properties. A sample series with varying doping concentration is grown by molecular-beam epitaxy. After examination of its static optical properties, the pump-induced intraband carrier dynamics are investigated via optical pump-THz probe measurements. Here, we observe conductivity decay times as low as 0.23 ps which are attributed to electron capture into Fe-related defects. These results are corroborated by monitoring the corresponding interband dynamics via all-optical pump-probe measurements. In addition, the competitiveness of THz detectors fabricated from a subset of these samples is demonstrated by integrating them into a standard time-domain spectrometer.

Stability of tungsten-doped δ-Bi3YO6

The stability of bismuth yttrium oxide doped with tungsten, Bi3Y1−xWxO6+3x/2, on annealing in air for up to 1000 h at 650 °C was investigated using a.c. impedance spectroscopy and neutron diffraction. Compositions up to x = 0.2 all show some degree of conductivity decay, but only in the case of the x = 0.10 composition is this decay relatively small and not associated with a significant structural change. Details of the short range structure of this composition were investigated by reverse Monte Carlo modelling of total neutron scattering data and again reveal no significant changes prior to and post annealing. A preference for 〈100〉 oxide ion vacancy pair alignment is found in the structure, consistent with models for oxide ion conduction in these systems. Additionally, the total scattering analysis reveals a preferred vacancy association with the W6+ cations, which adopt a four coordinate geometry in this system.

The Performance of Zr-Doped Al-Zn-Sn-O Thin Film Transistor Prepared by Co-Sputtering

In this work, a thin film transistor (TFT) with Zr-doped aluminum-zinc-tin oxide (Zr-AZTO) semiconductor as active layer was investigated. The Zr-AZTO thin films were co-sputtered by ZrO2 and AZTO targets (RF-Sputter) in Ar, and annealed at 350 °C in air atmosphere. With the discharge power of AZTO increasing from 100 W to 120 W, the content of Zr element decreases from 0.63 ± 0.01 at.% to 0.34 ± 0.01 at.%, and the oxygen vacancy decreases from (19.0 ± 0.1)% to (17.3 ± 0.8)%. The results of Zr-AZTO thin film show that the main factor is the co-sputter power of ZrO2 target. With the co-sputter power of ZrO2 increasing from 15 W to 45 W, the content of Zr element increases from 0.63 ± 0.01 at.% to 2.79 ± 0.01 at.%, the content of oxygen vacancy decreases from (19.0 ± 0.1)% to (14.1 ± 0.1)%, Eg increases from 2.76 eV to 2.86 eV, and the root mean square (RMS) roughness firstly decreases from 0.402 nm to 0.387 nm and then increases to 0.490 nm. The Micro Wave Photo Conductivity Decay (μ-PCD, LTA-1620SP) was used to measure the quality of Zr-AZTO thin film and the mean peak and D value decreases from 139.3 mV to 80.9 mV and from 1.54 to 0.77 as the co-sputter power of ZrO2 increases from 15 W to 45 W, which means it has highest localized states and defects in high ZrO2 co-sputter power. The devices prepared at 15 W (ZrO2)/100 W (AZTO) co-sputter show a best performance, with a μsat of 8.0 ± 0.6 cm2/(V∙S), an Ion/Ioff of (2.01 ± 0.34) × 106, and a SS of 0.18 ± 0.03 V/dec. The device of Sample B has a 0.5 V negative shift under −20 V NBS and 9.6 V positive shift under 20 V PBS.

Photocatalytic Degradation Dynamics of Methyl Orange Using Coprecipitation Synthesized Fe3O4 Nanoparticles

This study aims to investigate the photocatalytic degradation performance, mechanism, and dynamics of methyl orange (MO) which is a widely used organic dye in textile industries as well a hazardous wastewater pollutant. The degradation process was catalyzed by employing a synthesized Fe3O4 magnetic nanoparticle (NP) using the coprecipitation method. The structural and morphological properties of the synthesized Fe3O4 NPs were investigated by employing XRD, HR-SEM, and XPS, which proved that acquired Fe3O4 NPs were in a pure phase. Moreover, the crystallite sizes fall in the range of 28–31.8 nm and were estimated by applying the Scherrer equation on the XRD spectrum as well as calculated independently by applying a statistical approach on the SEM micrographs. The UV–Vis maximum in the visible range at 468.8 nm consists of two absorption frequency bands due to the effect of the hydrogen-bond interaction between water and the azo nitrogens in the MO. A non-monotonic spectral dynamic accompanied by peak wavelength shifts, as well as the absolute signal amplitude and signal area of the MO band, suggests that a cleavage of the azo bond is not the only and/or the dominant process in the photocatalytic oxidization of the MO in a protic solvent. The overall absorbance process is a complicated response to a combination of nonspecific and specific solute-solvent interactions, dipole-dipole interactions, hydrogen-bonding networks, and other possible intermolecular interactions such as hydrophobic/hydrophilic interactions. A bi-exponential decay was found to be the best fitting function to model the decay of the time-dependent electrical conductivity of the MO aqueous solution under photocatalytic oxidization. The Fe3O4 NPs exhibited a 98.3% removal of MO within 110 min. Photocatalytic degradation of methyl orange can be modeled to the first-order model with a rate constant k of 0.037 min−1 taking into account the initial concentration of 1175 ppm of MO. The degradation/decolorization efficiency deduced from the low-frequency band of the visible spectra is around 99.4% after 110 min. The real-time degradation/decolorization efficiencies deduced from the overall absorbance maxima and the low-frequency band have a discrepancy of 50.1% at 20 min and 12.3% at 60 min representing the progressive attenuation of the H-bond impact dissociation of MO (degradation/decolorization).

Features of superexchange nonresonant tunneling conductance in anchored molecular wires

A modified superexchange model is used to clarify the physical mechanisms for the formation of nonresonant tunneling conductance in terminated molecular wires. Due to the specific relationship between its key parameters, this model has wider areas of applicability compared to the flat-barrier model and the standard superexchange model, which are widely involved for the physical interpretation of experimental results. Moreover, the results obtained in the two latest models appear in the modified model as characteristic limiting cases. Our estimates show that the exponential decay of conductance, characterized by an attenuation factor β (per repeating unit), is limited by the conditions β ≤ 1.2 and β ≥ 3.7 for the flat-barrier and standard models, respectively. At the same time, the modified superexchange model yields β &amp;gt; 0, which, thus, allows us to analyze the tunneling conductance in molecular wires containing both saturated and conjugated bonds. We also show that for a small number of N repeating wire units (about 3–6 depending on the value of β), the exponential dependence of conductance on N is violated and, accordingly, contact conductance is not identical to conductance at N = 0. Formulas are found which, on the basis of experimental data, make it possible to establish the values of superexchange parameters as well as indicate the conditions of possible hybridization between the orbitals of the anchor groups and the adjacent end units belonging to the interior wire region. One example is the establishment of features in the tunneling conductance of terminated alkane chains caused by the nature of their anchor groups.

Scattering theory of transport through disordered magnets

We present a scattering theory of transport through noncollinear disordered magnetic insulators. For concreteness, we study and compare the random field model (RFM) and the random anisotropy model (RAM). The RFM and RAM are used to model random spin disorder systems and amorphous materials, respectively. We utilize the Landauer-Buttiker formalism to compute the transmission probability and spin conductance of one-dimensional disordered spin chains. The RFM and the RAM both exhibit Anderson localization, which means that the transmission probability and spin conductance decay exponentially with the system length. We define two localization lengths based on the transmission probability and the spin conductance, respectively. Next, we numerically determine the relationship between the localization lengths and the strength of the disorder. In the limit of weak disorder, we find that the localization lengths obey power laws and determine the critical exponents. Our results are expressed via the universal exchange length and are therefore expected to be general.