Changes In Dc Resistance Research Articles

Response Surface Modeling of the Steady-State Impedance Responses of Gas Sensor Arrays Comprising Functionalized Carbon Nanotubes to Detect Ozone and Nitrogen Dioxide.

Carbon nanotube (CNT) sensors provide a versatile chemical platform for ambient monitoring of ozone (O3) and nitrogen dioxide (NO2), two important airborne pollutants known to cause acute respiratory and cardiovascular health problems. CNTs have shown great potential for use as sensing layers due to their unique properties, including high surface to volume ratio, numerous active sites and crystal facets with high surface reactivity, and high thermal and electrical conductivity. With operational advantages such as compactness, low-power operation, and easy integration with electronics devices, nanotechnology is expected to have a significant impact on portable low-cost environmental sensors. Enhanced sensitivity is feasible by functionalizing the CNTs with polymers, metals, and metal oxides. This paper focuses on the design and performance of a two-element array of O3 and NO2 sensors comprising single-walled CNTs functionalized by covalent modification with organic functional groups. Unlike the conventional chemiresistor in which the change in DC resistance across the sensor terminals is measured, we characterize the sensor array response by measuring both the magnitude and phase of the AC impedance. Multivariate response provides higher degrees of freedom in sensor array data processing. The complex impedance of each sensor is measured at 5 kHz in a controlled gas-flow chamber using gas mixtures with O3 in the 60-120 ppb range and NO2 between 20 and 80 ppb. The measured data reveal response change in the 26-36% range for the O3 sensor and 5-31% for the NO2 sensor. Multivariate optimization is used to fit the laboratory measurements to a response surface mathematical model, from which sensitivity and selectivity are calculated. The ozone sensor exhibits high sensitivity (e.g., 5 to 6 MΩ/ppb for the impedance magnitude) and high selectivity (0.8 to 0.9) for interferent (NO2) levels below 30 ppb. However, the NO2 sensor is not selective.

Capacitive Mode Vapor Sensing Phenomenon in ZnO Homojunction: An Insight Through Space Charge Model and Electrical Equivalent Circuit

Conventionally employed resistive mode, vis-à-vis relatively non-conventional capacitive mode measurement technique for ZnO homojunction based alcohol vapor sensor is reported in this paper. ZnO homojunction comprising of sol-gel grown p-ZnO (sodium doped) nanoparticles and electrochemically derived undoped n-ZnO nanotubes was fabricated and after structural (FESEM) and electrical (I-V and C-V) characterizations, the sensing study (in both resistive and in capacitive mode) was carried out at room temperature (~25 °C). Compared to the DC resistance change based measurement, three distinctive features were noticed in capacitive mode sensing; (i) For methanol, the fabricated p-n homojunction device showed higher (~2415%) selectivity window (response magnitude difference with the nearest interfering species) in the capacitive mode, (ii) Capacitive measurement offered a dramatic improvement in the response magnitude (RM ~3021%) compared to its resistive counterpart (RM ~121%), (iii) Transient characteristics were sluggish in capacitive mode due to diffusion limited interactions. An innovative approach was adopted to understand the underlying sensing mechanism through correlation of an ambient dependent (in inert, in air and in vapor environment) space charge model of the homojunction device with the corresponding quantitative equivalent circuit.

Analysis method of a spin-torque oscillator using dc resistance change during injection locking to an external microwave magnetic field

Spin torque oscillators (STOs) that generate a persistent magnetization oscillation by spin-transfer torque are promising spintronic devices for various applications. In this study, we propose and experimentally demonstrate a method to characterize the magnetization dynamics of STOs using injection locking. By placing the STO near an antenna that generates a microwave magnetic field of a variable frequency (fMW), injection locking of the STO to the microwave field occurs when fMW is in the locking range around the intrinsic oscillation frequency. During injection locking, the dc resistance of the STO exhibits a peak-and-valley dependence on fMW, which originates from the modification of the magnetization trajectory induced by injection locking. Based on this principle, the oscillation frequency can be estimated by measuring the dc resistance change. Because this method does not require measuring the high-frequency magnetoresistance signal from the STO, which is used in the conventional method, it is advantageous in cases where the high-frequency magnetoresistance signal is undetectable or exhibits additional frequency components different from the oscillation frequency.

Substrate‐Versatile Direct‐Write Printing of Carbon Nanotube‐Based Flexible Conductors, Circuits, and Sensors

AbstractManufacturing of printed electronics relies on the deposition of conductive liquid inks, typically onto polymeric or paper substrates. Among available conductive fillers for use in electronic inks, carbon nanotubes (CNTs) have high conductivity, low density, processability at low temperatures, and intrinsic mechanical flexibility. However, the electrical conductivity of printed CNT structures has been limited by CNT quality and concentration, and by the need for nonconductive modifiers to make the ink stable and extrudable. This study introduces a polymer‐free, printable aqueous CNT ink, and, via an ambient direct‐write printing process, presents the relationships between printing resolution, ink rheology, and ink‐substrate interactions. A model is constructed to predict printed feature sizes on impermeable substrates based on Wenzel wetting. Printed lines have conductivity up to 10 000 S m−1. The lines are flexible, with <5% change in DC resistance after 1000 bending cycles, and <3% change in DC resistance with a bending radius down to 1 mm. Demonstrations focus on i) conformality, via printing CNTs onto stickers that can be applied to curved surfaces, ii) interactivity using a CNT‐based button printed onto folded paper structure, and iii) capacitive sensing of liquid wicking into the substrate itself. Facile integration of surface mount components on printed circuits is enabled by the intrinsic adhesion of the wet ink.

Assessment of the electrical conductivity of thermally activated water jets containing injections of inhibiting salt used to extinguish electrical equipment at gas compressor stations

Introduction. One of the main problems of fire extinguishing at gas compressor stations is fires in confined spaces where electrical installations can be under voltage. To extinguish these fires, the co-authors propose to use jets of thermally activated water containing salts that inhibit burning. It is necessary to assess the electrical conductivity of water jets to prevent the exposure of fire-fighting units to electrical injuries.Materials and methods. In the experimental study, the principal electricity measuring instrument is digital dc megaohmmeter M4122U, which takes readings if operated from a laptop. The flow rate was measured using IT 2518 measurement complex and TDR14–2–3 primary flow converter of the turbine type. Termit T-35 hardness converter performed electromagnetic water treatment.Theoretical fundamentals. Measurements of leakage currents in jets of thermally activated water, fed through fire nozzles mounted onto a multi-purpose fire truck, were taken, and the maximum permissible distance to an electrical installation was determined experimentally and analytically using an experimental stand.Results and discussions. Averaged values of resistance of a jet of thermally activated water fed from a long-range nozzle at control points do not leave the confidence corridor with a probability of 0.95, which means that they do not cause a significant change in dc resistance. Inhibiting salt, injected into the water, reduces the resistance of jets of thermally activated water by no more than 2–3 % compared to under-heated water. The treatment of the inhibiting salt, injected into the under-heated water, using Termit T-35 hardness converter, does not significantly change the resistance of thermally activated water jets.Conclusions. It is theoretically and experimentally proved that the injection of inhibiting salts into the underheated water, as well as the electromagnetic treatment of water using low-power stiffness converters, do not significantly affect the electrical conductivity of jets of thermally activated water if compared to jets that contain no inhibiting admixtures. The extinguishing of surfaces of electrical installations using jets of thermally activated water containing flame inhibiting injections of salts, as well as the development of an inerting concentration in confined spaces of gas compressor stations is electrically safe for any person engaged in fire extinguishing.

Polaronic Relaxation and Variable-Range-Hopping Conductivity in Fe3O4 Nanoparticles

We report the polaronic relaxation and Variable Range Hopping (VRH) conductivity in 100nm average particle size cold pressed Fe3O4 nanoparticles. The crystal structure was studied using XRD showing spinel structure with (311) intense peak. The AC transport in the cold pressed powder was studied from 10 K to room temperature and from 100 Hz to 5.5 MHz. It was observed that the solid couples capacitively with the perturbing electric field initially and as the frequency increases it becomes more inductive. The impedance data was modelled using modified Havriliak–Negami relaxation model. It was observed that the cold pressing introduces an asymmetric broadening in the relaxation process. The change in impedance with temperature showed a drastic drop at around 120 K due to Verwey transition in real as well as imaginary part. The change in DC resistance with temperature indicated VRH mechanism of charge transport in the solid with a metal insulator transition at around 120 K. The relaxation times calculated indicated that the hopping process has an attempt frequency of the order of 30 microseconds. The activation energy change across the phase transition with relaxation time and resistance indicated that the charge transport is coherent. The activation energy of ∼0.05eV above Varway transition indicated that the transport is small polaronic in nature.

Chemoresistive CO2 Gas Sensors Based On La2O2CO3: Sensing Mechanism Insights Provided by Operando Characterization.

Our previous studies demonstrated that rare-earth oxycarbonates Ln2O2CO3 (Ln = La, Nd, and Sm) and rare-earth oxides Ln2O3 (Ln = Nd, Sm, Gd, Dy, Er, and Yb) are sensitive to CO2 and that hexagonal La2O2CO3 is the best among them in terms of sensitivity, stability, and selectivity. In this study, we have conducted a comprehensive operando characterization on a hexagonal La2O2CO3 based sensor for the basic understanding of the sensing mechanism. This was done by performing under actual operating conditions simultaneous DC resistance and work function changes measurements, AC impedance spectroscopy measurements, and simultaneous DC resistance and DRIFT spectroscopy measurements. The results demonstrate that the double Schottky barriers at grain-grain boundaries are dominant contribution to sensor resistance; there is a competitive adsorption between carbonate species and hydroxyl groups, which depends on both CO2 concentration and humidity and leads to the change in height of the Schottky barriers. Finally, we propose a reaction model stating that there are three types of adsorbates, -CO32-, -OH-, and -O2-, and the relative concentration of which is controlled by a reaction with ambient humidity and CO2. This model is able to consistently explain all our experimental findings.

Realization of Absolute Phase and AC Resistance of Current Shunts by Ratio Measurements

This paper presents a simplified method for realizing absolute phase and ac resistance for current shunts using only impedance ratio measurements. The method is based on three geometrically identical current shunts with different resistances, but with the same inductance, capacitance, and ac–dc resistance change. We demonstrate how the inductance, capacitance, and ac resistance can be calculated from the complex impedance ratio measurements, therefore realizing absolute current shunt impedance. The method gives competitive uncertainties of around $200~ \mu \Omega / \Omega $ for amplitude and $400~ \mu $ rad for phase at 1 MHz in the 1- $\Omega $ range.

Kirigami Strain Sensors Microfabricated From Thin-Film Parylene C

A kirigami-based strategy was investigated for strain transduction, using serpentine gold traces embedded in thin-film Parylene C. Multiple kirigami slit designs were evaluated via COMSOL simulation and mechanical force testing; sensors having tightly packed slits stretched up to 17.5 mm (350% strain) before mechanical failure and 9 mm (180% strain) before electrical failure. Strain transduction was achieved by monitoring DC resistance changes during stretching. DC resistance linearly increased with strain, with sensitivities up to 0.16 $\Omega$ /mm (gauge factor = 0.007) and minimal hysteresis. High-frequency trace impedance and inter-trace capacitance were also investigated during strain cycling. Capacitance increased with strain and high-frequency impedance show a nonlinear strain relationship. The biocompatible construction and extremely low profiles (20 $\mu \text{m}$ thick) of these sensors are attractive for minimally invasive in vivo strain sensing applications. [2018-0130]

Extremely high magnetic-field sensitivity of charge transport in the Mn/SiO2/p-Si hybrid structure

We report on abrupt changes in dc resistance and impedance of a diode with the Schottky barrier based on the Mn/SiO2/p-Si structure in a magnetic field. It was observed that at low temperatures the dc and ac resistances of the device change by a factor of more than 106 with an increase in a magnetic field to 200 mT. The strong effect of the magnetic field is observed only above the threshold forward bias across the diode. The ratios between ac and dc magnetoresistances can be tuned from almost zero to 108% by varying the bias. To explain the diversity of magnetotransport phenomena observed in the Mn/SiO2/p-Si structure, it is necessary to attract several mechanisms, which possibly work in different regions of the structure. The anomalously strong magnetotransport effects are attributed to the magnetic-field-dependent impact ionization in the bulk of a Si substrate. At the same time, the conditions for this process are specified by structure composition, which, in turn, affects the current through each structure region. The effect of magnetic field attributed to suppression of impact ionization via two mechanisms leads to an increase in the carrier energy required for initiation of impact ionization. The first mechanism is related to displacement of acceptor levels toward higher energies relative to the top of the valence band and the other mechanism is associated with the Lorentz forces affecting carrier trajectories between scatterings events. The estimated contributions of these two mechanisms are similar. The proposed structure is a good candidate for application in CMOS technology-compatible magnetic- and electric-field sensors and switching devices.

Reliability of Self-Assembly Vertical Interconnects in Magnetically Aligned Anisotropic Conductive Adhesive for Electronic Packaging Applications

A self-assembly process in a magnetically aligned anisotropic conductive adhesive (MA-ACA) is a novel approach to selectively create high-density vertical interconnects by magnetic interaction between ferromagnetic input-output pads and particles. In order to characterize self-assembly vertical interconnects created in an MA-ACA, quad-flat no-lead chips with 40 daisy chain pads were assembled on a printed circuit board. In reliability tests, the dc resistance of self-assembly test samples was observed to be 35.9% and -12.1% changed at temperatures of 85°C and -55°C, respectively, after 700 thermal shock cycles and 0.4% changed after an 8-h vibration test. In contrast, the dc resistance was observed to be significantly 41.4 Ω increased due to humidity absorption after an 85°C temperature-85% humidity test for 500 h. Thus, in order to prevent humidity percolation in the self-assembly vertical interconnects, a Parylene-C material was used as a barrier layer. The dc resistance change of Parylene-deposited test samples with a thickness of 10 μm was observed to be greatly 85.2% reduced after an 85°C temperature-85% humidity test. In addition, based on a Weibull distribution model, the mean time to failure of self-assembly vertical interconnects was estimated to be importantly 94.7% improved. Consequently, the proposed self-assembly packaging using a Parylene material is applicable to cost-effective electronic applications.

Amorphous InGaZnO4 films: Gas sensor response and stability

Abstract The response characteristics of amorphous-InGaZnO 4 (a-IGZO 4 ) thin films toward reducing/oxidizing gases (H 2 /NO 2 ), at sensor operating temperatures, are reported for the first time. The lack of grain boundaries eliminates a major source of electrical, microstructural and chemical inhomogeneities associated with polycrystalline semiconducting metal oxides (SMOs), rendering a-IGZO 4 a highly promising model sensor system. Gas sensor tests were carried out in the temperature range of 200–400 °C by monitoring changes in DC resistance during cyclic exposure to trace concentrations (between 1.25 and 50 ppm) of H 2 or NO 2 in dry air. The response ( S ) to H 2 was found to go through a temperature maximum (e.g. S ∼ 0.7 at 350 °C for pH 2 = 12.5 ppm) that value being a function of pH 2 . The response to NO 2 , on the other hand, decreased with increasing temperatures with the highest recorded values at 200 °C (e.g. S ∼ 33 at 200 °C for pNO 2 = 5 ppm). The response followed an approximate power law dependence on gas partial pressure ( p ), S = Ap β , with β taking on values of ∼0.5–1.0 as temperature increased from 200 to 400 °C. Response times were found to range from 10 s to greater than 1000 s as temperature decreased. The hysteretic behavior exhibited by a-IGZO films between 150 and 400 °C, under temperature sweep conditions, is attributed to kinetically limited adsorption/desorption and reaction rates.

An Au clusters related spill-over sensitization mechanism in SnO2-based gas sensors identified by operando HERFD-XAS, work function changes, DC resistance and catalytic conversion studies

The role of Au additives in SnO(2)-based thick film gas sensors was investigated by a combination of operando investigation techniques, namely spectroscopic high energy resolved fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) and simultaneous DC resistance and work function change measurements. The results have shown that the Au is present in the form of small metallic particles at the surface of the host metal oxide without changing its bulk or surface electronic properties. The sensitization effect of Au can therefore be attributed to the "spill-over effect", meaning that the Au particles enrich the surface of the active metal oxide with oxygen species which consequently react with reducing gases such as CO and H(2). This is in contrast to the effect of Pd and Pt promoters which were found to be distributed at an atomic level on the surface and in the bulk of the supporting sensing material and therefore have a tremendous effect on its bulk and surface electronic properties.

Electrical detection of the ferromagnetic resonance: Spin-rectification versus bolometric effect

The authors have investigated the dc resistance change of a Permalloy microstrip under microwave irradiation. The experimental results demonstrate that both the spin-rectification and the bolometric effects significantly affect the dc resistance change, and the contribution of each can be precisely determined due to their different dependences on the modulation frequency of the microwave. Therefore, both the cone angle of magnetization precession and the thermal relaxation time following microwave heating are obtained.

Dual-Chemiresistor GC Detector Employing Monolayer-Protected Metal Nanocluster Interfaces

The synthesis and testing of two gold-thiolate monolayer-protected (nano)clusters as interfacial layers on a dual-chemiresistor vapor sensor array are described. Responses (changes in dc resistance) to each of 11 organic solvent vapors are rapid, reversible, and linear with concentration at low vapor concentrations, becoming sublinear at higher concentrations. Limits of detection (LODs) range from 0.1 to 24 parts per million and vary inversely with solvent vapor pressure. When configured as a GC detector and used to analyze 0.5-L preconcentrated samples of the 11-vapor mixture, the array provides LODs of < or = 700 parts per trillion for most vapors, comparing favorably with those from an integrated array of polymer-coated surface acoustic wave sensors configured and tested similarly. This first report on the use of such an array as a GC detector shows that the combination of response patterns and GC retention times improves capabilities for vapor recognition compared to the sensor array alone or to single-detector GC systems. Spray-coated nanocluster thin films can be deposited reproducibly and exhibit response stability in air that ranges from fair to excellent for up to several months. Scaling the active device area down by a factor of 16 has no significant effect on sensitivity. Implications of these results for portable vapor sensing systems are discussed.

CO interaction with co-adsorbed C 2H 4 on Cu(111) as revealed by friction with the conduction electrons

Simultaneous measurements of the DC resistance and IR reflectivity changes of thin epitaxially grown copper (111) films have been performed during co-adsorption of CO with C 2H 4. The linear dependence of these two quantities, which has previously been observed for CO on Cu(111) films, also holds for this coadsorbed system. Importantly, we have shown that the linearity factor is independent of the adsorbate type, as predicted by the scattering model of the conduction electrons. Co-adsorption of CO with C 2H 4 results in markedly more pronounced changes in IR reflectance and in resistance; these changes are higher than the sum of the individual changes of each species. The results are interpreted as a consequence of covalent through-metal bonding of the adsorbates via the lowest unoccupied molecular orbitals.

Relationships among resonant frequency changes on a coated quartz crystal microbalance, thickness changes, and resistance responses of polymer-carbon black composite chemiresistors

The relationships among frequency changes on a film-coated quartz crystal microbalance, thickness changes, and dc resistance changes have been investigated for carbon black-insulating polymer composite vapor detectors. Quartz crystal microbalance (QCM) measurements and ellipsometry measurements have been performed simultaneously on polymer films that do not contain carbon black filler to relate the QCM frequency change and the ellipsometrically determined thickness change to the analyte concentration in the vapor phase. In addition, quartz crystal microbalance measurements and dc resistance measurements on carbon black composites of these same polymers have been performed simultaneously to relate the QCM frequency change and dc electrical resistance response to the analyte concentration in the vapor phase. The data indicate that the dc resistance change is directly relatable to the thickness change of the polymers and that a variety of analytes that produce a given thickness change produce a constant resistance change for each member of the test set of polymers investigated in this work.

Trace metal determination by dc resistance changes of microstructured thin gold film electrodes

The deposition of adatoms onto the surface of a thin metal film can increase its lateral dc resistance. The magnitude of the increase is related to the surface coverage. We have utilized this phenomenon for the determination of heavy metal concentrations in aqueous solution. Using thin gold films as working electrodes in a three electrode arrangement, we deposited cadmium, lead, nickel, thallium and zinc ions as well as Cd-EDTA complexes and measured the corresponding resistance changes. Different species could be distinguished from each other. For cadmium ions a linear relationship between the induced resistance change and the cadmium concentration has been achieved over a concentration range between 4 ng/g and 1 μg/g.

Testing method for the determination of the crack growth rate and crack growth resistance under cyclic loading

The length of a growing crack can be measured by different methods that provide reliable and stable determination of an increment {Delta}a in it equal to 0.1 mm or 0.002 W. If necessary, measurements may be carried out with lower accuracy, provided that the restrictions on the increment in the crack length {Delta}a between two successive measurements are followed. This will increase measured increments and, hence, the duration of tests and the required number and/or size of specimens, which may result in a change in the variance of the FCG rate. An important characteristic of the method is its applicability to different materials and shapes of specimens in a broad range of FCG rates as well as its applicability under different test conditions (temperature, environment, irradiation, etc.), in particular, in cases where visual detection is impossible. The most promising methods will be those allowing one to perform automatic measurement without stopping the machine and to generate data in a form suitable for computer processing. In practice, procedures based on elastic compliance, changes in dc or ac resistance (potential drop), rupture of electrical resistance wires attached along the presumed crack path, ultrasound, induced eddy current, and magnetism are used for measurementmore » of crack length. Attempts have also been made to estimate the FCG rate by acoustic emission or a fractographic technique. But the most common procedure is visual measurement of the distance between the crack tip and reference marks on the specimen surface with a 5-50-fold magnification device (a cathetometer or a traveling microscope or telescope).« less

Acoustic study of YBa2Cu3Ox thin films.

The attenuation of surface acoustic waves by epitaxial ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ films has been studied for x\ensuremath{\simeq}6 to 7. For fully oxygenated samples, the acoustic attenuation as a function of temperature shows two broad peaks at about 135 and 240 K, and decreases monotonically below the lower peak temperature. The cause of attenuation peaks is attributed to scattering by optical phonons. Our data do not show any gap structure at ${\mathit{T}}_{\mathit{c}}$ due to relatively weak electron-phonon interactions at the acoustic frequencies. As the oxygen deficiency increases, the temperature dependence of the dc resistance changes from metallic to semiconducting and finally to insulating behavior. Acoustic attenuation data correspondingly show a drastic change due to different attenuation mechanisms: from the phonon scattering loss in the metallic regime to the electric-field coupling loss in the semiconducting and insulating regimes. In the latter regimes, the temperature dependence of low-frequency resistance calculated from the attenuation data can be fitted to a three-dimensional Mott variable-range-hopping model.