Time Constant Of Response Research Articles

Gas sensing properties of indium–gallium–zinc–oxide gas sensors in different light intensity

We have successfully observed the change in indium–gallium–zinc–oxide (IGZO) gas sensor sensitivity by controlling the light emitting diode (LED) power under the same gas concentrations. The light intensity dependence of sensor properties is discussed. Different LED intensities obviously affected the gas sensor sensitivity, which decays with increasing LED intensity. High LED intensity decreases not only gas sensor sensitivity but also the response time (T90), response time constant (τres) and the absorption rate per second. Low intensity irradiated to sensor causes high sensitivity, but it needs larger response time. Similar results were also observed in other kinds of materials such as TiO2. According to the results, the sensing properties of gas sensors can be modulated by controlling the light intensity.

Geobandwidth: comparing time domain electromagnetic waveforms with a wire loop model

We compare time domain systems of different waveform shape, power and receiver sampling times using a wire loop conductor model to define a comprehensive ‘geobandwidth’ that shows the strength of the response over a range of time constants, analogous to a range of conductance. Frequency domain EM responses can also be calculated as a function of time constant for a wire loop model, giving a consistent comparison method for all time domain waveforms and frequency domain. Arbitrary waveforms can be modelled as a sum of simple short ramps, and the geobandwidth determined numerically. Peak time constant (time constant of peak response) or equivalent frequency can be determined analytically or numerically. The frequency content of a time-domain EM system can be characterised by the peak time constant or the equivalent frequency. The results of these calculations are used to compare response amplitude across a wide range of geological target conductance. Systems can be compared on the basis of signal or signal/noise ratio.

Hydrogen sensing characteristics of a Pt/AlGaN/GaN heterostructure field-effect transistor (HFET) prepared by sensitization, activation, and electroless plating (EP) approaches

A Pt/AlGaN/GaN heterostructure field-effect transistor (HFET), based on sensitization, activation, and electroless plating (EP) deposition approaches, is fabricated and studied. Utilizing the sensitization and activation processes, a dense and uniform Pd seed layer could be implanted on the AlGaN layer prior to Pt-gate formation. In addition, a dense Pt-gate morphology and excellent Schottky contact properties are obtained. This causes significant improvement in DC performance and thermal stabilities as compared with a thermal evaporation (TE)-based one. For a used gate dimension of 1×100μm2, the lower gate leakage current of 0.9 (8.4)nA, higher maximum extrinsic transconductance of 90.1 (52.1)mS/mm, and maximum drain saturation current of 325 (178)mA/mm are found for an EP Pt-gate HFET at 300 (600)K. Moreover, as a hydrogen gas sensor, the maximum drain current response (13.7%), high on/off ratio (8×105), and fast response (28s) and recovery (36s) time constants in 10,000, 50, and 5ppm H2/air gases are obtained at 400K, respectively. Therefore, the studied EP Pt-gate HFET shows promise for a high-performance electronic device and hydrogen gas sensing applications.

Enhancement of hydrogen sensing performance of a GaN-based Schottky diode with a hydrogen peroxide surface treatment

In this work, enhanced hydrogen sensing characteristics of a GaN-based Schottky diode-type sensor with a GaOx layer are studied and demonstrated. A thin GaOx layer inserted in Pd/GaN interface is oxidized by the immersion in an H2O2 solution at room temperature. Experimentally, a significantly high hydrogen sensing response of 1.8×105 and a large Schottky barrier height variation ratio of 33.1% are found upon exposure to a 1% H2/air gas at 300K. In addition, a very low detection limit of 0.1ppm H2/air at 300K is obtained. These improved properties could be attributed to the effective dissociation of hydrogen molecules and rougher Pd surface caused by the presence of the GaOx layer. The response (recovery) time constant of 13.3 (23.6)s is obtained upon exposure to a 1% H2/air gas at 300K. The related hydrogen adsorption analysis of the proposed device is also studied and demonstrated.

Improving Signal Dynamics of Fluorescent Protein Voltage Sensors by Optimizing FRET Interactions

FRET (Förster Resonance Energy Transfer)-based voltage sensors can be useful for monitoring neuronal activity in vivo because the ratio of signals between the donor and acceptor pair removes common sources of noise such as heart beat artifacts. We improved the performance of genetically encoded fluorescent voltage sensors by optimizing FRET interactions between donor and acceptor fluorescent proteins (FPs) flanking the voltage sensitive domain of the Ciona intestinalis voltage sensitive phosphatase. The donor FP, UKG, was positioned at 8 different locations downstream of the voltage-sensing domain while the acceptor FP, mKO, was placed at 6 positions upstream of the voltage-sensing domain resulting in 39 different “Nabi1” constructs. Several of these combinations resulted in large signals and fast kinetics. Nabi1 probes responded to changes in membrane potential with improved signal size (up to 11% ΔF/F for a 100 mv depolarization) and fast response time constants both for signal activation (∼2 ms) and signal decay (∼3 ms). Replacement of the mKO and UKG FRET pair with Clover (donor FP) and mRuby2 (acceptor FP) improved expression in cultured neurons enabling the optical resolution of individual action potentials. We are analyzing a series of Nabi2 probes with different insertion sites of FPs. This effort may identify novel probes that may be useful for optical recording of neuronal activities. Supported by US NIH Grants DC005259 and NS054270 and grant WCI 2009-003 from the National Research Foundation of Korea.

Genetically Encoded Protein Sensors of Membrane Potential.

Organic voltage-sensitive dyes offer very high spatial and temporal resolution for imaging neuronal function. However these dyes suffer from the drawbacks of non-specificity of cell staining and low accessibility of the dye to some cell types. Further progress in imaging activity is expected from the development of genetically encoded fluorescent sensors of membrane potential. Cell type specificity of expression of these fluorescent protein (FP) voltage sensors can be obtained via several different mechanisms. One is cell type specificity of infection by individual virus subtypes. A second mechanism is specificity of promoter expression in individual cell types. A third, depends on the offspring of transgenic animals with cell type specific expression of cre recombinase mated with an animal that has the DNA for the FP voltage sensor in all of its cells but its expression is dependent on the recombinase activity. Challenges remain. First, the response time constants of many of the new FP voltage sensors are slower (2-10 ms) than those of organic dyes. This results in a relatively small fractional fluorescence change, ΔF/F, for action potentials. Second, the largest signal presently available is only ~40% for a 100 mV depolarization and many of the new probes have signals that are substantially smaller. Large signals are especially important when attempting to detect fast events because the shorter measurement interval results in a relatively small number of detected photons and therefore a relatively large shot noise (see Chap. 1). Another kind of challenge has occurred when attempts were made to transition from one species to another or from one cell type to another or from cell culture to in vivo measurements.Several laboratories have recently described a number of novel FP voltage sensors. Here we attempt to critically review the current status of these developments in terms of signal size, time course, and in vivo function.

Improvement of a near wake model for trailing vorticity

A near wake model, originally proposed by Beddoes, is further developed. The purpose of the model is to account for the radially dependent time constants of the fast aerodynamic response and to provide a tip loss correction. It is based on lifting line theory and models the downwash due to roughly the first 90 degrees of rotation. This restriction of the model to the near wake allows for using a computationally efficient indicial function algorithm. The aim of this study is to improve the accuracy of the downwash close to the root and tip of the blade and to decrease the sensitivity of the model to temporal discretization, both regarding numerical stability and quality of the results. The modified near wake model is coupled to an aerodynamics model, which consists of a blade element momentum model with dynamic inflow for the far wake and a 2D shed vorticity model that simulates the unsteady buildup of both lift and circulation in the attached flow region. The near wake model is validated against the test case of a finite wing with constant elliptical bound circulation. An unsteady simulation of the NREL 5 MW rotor shows the functionality of the coupled model.

Hydrogen sensing characteristics of Pd/SiO2-nanoparticles (NPs)/AlGaN metal-oxide-semiconductor (MOS) diodes

A Pd/SiO2-nanoparticles (NPs)/AlGaN metal-oxide-semiconductor (MOS) structure is used to fabricate interesting Schottky diode-type hydrogen sensors. The employment of SiO2-NPs could effectively increase the specific surface area of Pd catalytic metal and the Schottky barrier height. Good hydrogen sensing performance is obtained. Experimentally, as compared to a conventional Pd/AlGaN MS diode, a significant 34.5-fold improvement on hydrogen sensing response is obtained under an introduced 1% H2/air gas at 300 K when a 10 wt% concentration of SiO2-NPs is employed in the studied device. Yet, the increase in SiO2-NP concentration relatively deteriorates the ability to detect very low hydrogen concentration levels (≦1 ppm H2/air). In addition, the increase in SiO2-NP concentration creates a decrease and increase on response and recovery time constants of transient behaviors, respectively.

Time-resolved analysis of charge responses determining luminescence properties

Abstract

Light-activated resistive ozone sensing at room temperature utilizing nanoporous In2O3 particles: Influence of particle size

Ordered mesoporous In2O3 particles of variable size synthesized by the nanocasting method are used for preparation of resistive gas-sensing layers. Light activation by a LED (blue light, 460nm) permits room-temperature ozone sensing. Apart from differences in base-line resistance in sensing layers containing small (diameter approx. 170nm) or large particles (approx. 870nm), differences in the response amplitude and response time constant are also observed. Signal stabilization is achieved faster for small particles. In addition, sensors show a particle size-dependent reaction threshold for low ozone concentration. Larger particles show negligible response to 50ppb ozone whereas a significant response is observed for the small-particle sensors. A simple model based on geometrical properties and formation of depletion layers explaining the observed behavior is presented.

Development of a current monitor using a negative impedance circuit

I developed a beam current transformer (CT) for monitoring the beam of a slow-cycling accelerator. The beam monitor is a new type of CT that measures the average current of a circulating beam using a negative impedance circuit (NIC). Adding an NIC can extend the low-frequency cutoff of a CT down to 0Hz (Ninomiya et al., 2006 [1]). This report presents the detail of our proposed beam-monitoring system. The measurable band width of the CT is 30kHz. The stability of the droop time constant of the square pulse response is ~2%/s. This beam monitor was developed for the synchrotron at the Wakasa–Wan Energy Research Center (WERC) and is currently installed there.

Electrical observation of non-radiative recombination in Er doped Si nano-crystals during thermal quenching of intra-4f luminescence

Thermal quenching of luminescence of Er dopants in Si nano-crystals (Si-nc’s) was investigated employing an impedance model for the analysis of photo-injected charges. Relaxation response indicated that Er doping forms not only optical centers but also trapping centers near the Si-nc’s. The response time constant of trapped charges was dependent on temperature, with the dependence correlating to thermal quenching. These findings indicate that quenching occurs by trapping followed by consumption of charges. The complex analyses revealed that the response represents non-radiative recombination at the centers rather than release of confined charges from the Si-nc through the centers. We propose a possible energy diagram for the non-radiative recombination.

Nabi, a Novel FRET-Based Voltage Sensor Protein

FRET (fluorescence resonance energy transfer) has been used to develop genetically encoded biosensors. FRET-based voltage sensors can be useful to monitor change of membrane potential and neuronal activity because the ratio signal removes noise from a common source such as heart beat artifacts. Positions of fluorescent donor and acceptor in voltage sensor proteins may affect plasma membrane expression, signal intensity, and response time constant of FRET. We developed “Nabi” voltage sensors by using the voltage-sensing domain of CiVSP (Ciona intestinalis voltage sensitive phosphatase) as a backbone. We aimed at improving FRET performance of voltage sensors by optimizing interactions between fluorescent donor and acceptor. For this effort, we designed a series of constructs with insertion of two fluorescent proteins (FPs) at different locations in the CiVSP backbone. We engineered a total of 40 different combinatorial constructs to evaluate 6 different locations for one FP and 10 different locations for the other FP of FRET pairs. By examining FRET signals of these voltage sensors in response to changes in membrane potential in HEK293 cells, we identified amino acid residues of CiVSP at which insertion of FP pairs can generate efficient FRET signals. Nabi1 series of probes contained UKG and mKO (Tsutsui H et al., 2008) as FRET donor and acceptor. Nabi2 series contained Clover and mRuby2 (Lam AJ et al., 2012) as FRET pairs. Nabi voltage sensors had improved signal size (up to 15% ΔF/F) and time constant compared to the previously reported FRET based voltage probes. Nabi2 was able to respond to single action potentials in primary cultured neurons. Supported by US NIH Grants DC005259 and grant WCI 2009-003 from the National Research Foundation of Korea.

Object-oriented modelling and simulation for the ALFRED dynamics

In this paper, a control-oriented modelling and simulation tool for the study of the Advanced Lead-cooled Fast Reactor European Demonstrator (ALFRED) plant dynamics is presented. It has been developed in order to perform design-basis transient analyses aimed at providing essential feedbacks for the system design finalization. The simulator has been meant to be modular, open and efficient. In this perspective, an object-oriented modelling approach has been adopted, by employing the reliable, tested and well-documented Modelica language. Simulation of core behaviour is based on point kinetics for neutronics and one-dimensional heat transfer models for thermal-hydraulics, coherently with ALFRED specifications. An effort has been spent to model the bayonet-tube Steam Generator (SG) foreseen to be installed within the reactor vessel. The primary loop model has been built by connecting the above-mentioned components (taking into account suitable time delays) and by incorporating the cold pool, which has revealed to be fundamental for an accurate definition of the time constants characteristic of the system because of its large thermal inertia. The description of the overall plant has been finalized by connecting standard turbine, condenser and other components of the balance of plant. Afterwards, the reactor responses to three typical transient initiators have been simulated (i.e., reduction of feedwater mass flow rate, variation of the turbine admission valve coefficient and transient of overpower). Simulation outcomes confirm the strong coupling between core and SG, besides showing the characteristic time constants of the various component responses. Results of the present study constitute a starting point in the definition of plant control strategies, laying the basis for investigation and development of a model-based control-system design.

Change of short-term memory effect in acute ischemic ventricular myocardium: A computational study

The ionic mechanism of change in short-term memory (STM) during acute myocardial ischemia has not been well understood. In this paper, an advanced guinea pig ventricular model developed by Luo and Rudy was used to investigate STM property of ischemic ventricular myocardium. STM response was calculated by testing the time to reach steady-state action potential duration (APD) after an abrupt shortening of basic cycling length (BCL) in the pacing protocol. Electrical restitution curves (RCs), which can simultaneously visualize multiple aspects of APD restitution and STM, were obtained from dynamic and local S1S2 restitution portrait (RP), which consist of a longer interval stimulus (S1) and a shorter interval stimulus (S2). The angle between dynamic RC and local S1S2 RC reflects the amount of STM. Our results indicated that compared with control (normal) condition, time constant of STM response in the ischemic condition decreased significantly. Meanwhile the angle which reflects STM amount is less in ischemic model than that in control model. By tracking the effect of ischemia on intracellular ion concentration and membrane currents, we declared that changes in membrane currents caused by ischemia exert subtle influences on STM; it is only the decline of intracellular calcium concentration that give rise to the most decrement of STM.

HKUST-1 coated piezoresistive microcantilever array for volatile organic compound sensing

The HKUST-1 metal-organic framework (MOF) was selected because of the large internal surface area, excellent stability and known properties. Mechanical strain is generated upon the adsorption of analytes into the MOF; it is proportional to concentration and is a function of adsorbed species. Piezoresistive microcantilevers serve as a transduction mechanism to convert surface strain into electrical signals. N-doped piezoresistive cantilever arrays were fabricated with ten structures per die. Thin films of HKUST-1 were grown at room temperature using layer-by-layer techniques. Dry nitrogen was used as a carrier gas to expose devices to varying concentrations of 12 different volatile organic compounds (VOCs). Results show that stress-induced piezoresistive microcantilever array sensors with MOF coatings can provide a highly sensitive and reversible sensing mechanism for water vapour and methanol. Characteristic response features allow discrimination based on shape, response time constants and magnitude of response for other VOCs. Devices provided reliable data and proved durable over 18 months of testing. The key advantages of this type of sensor are higher sensitivity with a microporous MOFs, reversible response, α single chip sensing system and low power operation.

Analysis of viscoelasticity of POF gratings in the stress sensing

The time-dependent behavior of polymer optical fiber (POF) grating under constant tensile stresses has been studied. We show that the evident time-dependence is due to the viscoelastic nature of POF grating materials that can be described with the Kelvin Model. Based on the Kelvin Model, the time-dependent relationship between the Bragg wavelength shift and stress has been analyzed in detail. The results show that the viscoelasticity has a great impact upon the stress response of POF gratings. With the increment of stress level, the visco response part increases faster than that of elastic response part. Especially, the response and recovery with and without stress can be fitted with dual exponential decay and the time constant of fast response and relaxation is ∼102s and that of the slow is ∼103s, which corresponds to the fast and slow movements of polymer segments, respectively. Experiments and regressions also show that the stress sensitivity is observed to be 369pm/MPa for the immediate response and 598pm/MPa for response balance, showing high stress sensitivity. All the spring stiffness and dashpot viscosity of Kelvin elements are larger than 1010Pa and 1013Pas. The dashpot viscosity of slow Kelvin element (1014Pas) is around one order larger than that of fast Kelvin element (1013Pas) with stress. Further analysis demonstrate the response and recovery of POF gratings with and without the stress displays an evident non-linear viscoelasticity, which will bring more complexity for POF gratings in the mechanical sensing applications.

Thermal Dynamical Identification of Light-Emitting Diodes by Step-Based Realization and Convex Optimization

A new method for modeling the thermal response of semiconductor devices such as light-emitting diodes (LEDs) from temperature measurements is presented. The method uses a realization-theoretic approach to identification combined with convex optimization methods. Linear matrix inequalities are constructed to guarantee that the identified discrete-time model has strictly real eigenvalues between 0 and 1, so that, when converted to continuous time, the model will have strictly real time constants. Additional optional time-domain constraints are developed to guarantee a predetermined steady-state value, guarantee no undershoot or overshoot in the transient response, and/or guarantee a positive time-constant spectrum. The method is applied to the thermal response of a high-power LED. Temperature measurements of the semiconductor device are used to model the time constants of the thermal response and characterize the relative contribution of each time constant to temperature increase. Experiments indicate how the proposed method can be used to detect thermal defects. It is shown that models with five time constants can model the thermal effects of the LEDs used with high accuracy. The proposed method is applicable to larger order systems with multiple simultaneous temperature measurements.

Dynamics of autodyne response formation in microwave generators

The paper presents results of studying the dynamics of autodyne response formation when switching on a radio-pulse microwave generator which is subject to the influence of its own reflection radiation. Basic relations for a step-wise calculation of autodyne response as a function of time delay, autodyne response time constant, distortion parameter and intrinsic parameters of the self-oscillating system are obtained. Calculation and analysis of peculiarities of autodyne signal generation by radio-pulse oscillator are conducted for the cases of motionless and moving reflecting object under relatively low, medium and high inertia properties of the oscillator if compared to the propagation time of the reflected radiation. Experimental research results that confirm conclusions of theoretical analysis are obtained for a common hybrid-integrated autodyne TIGEL-08 module of the 8-mm frequency-range implemented on a planar two-meza Gunn diodes and the same module stabilized by the external high-Q resonator.

Examining temporal and spatial variations of internal temperature in large-format laminated battery with embedded thermocouples

Information on battery internal temperature is valuable to enhance the understanding of thermo-electrochemical reactions, to validate simulation models, and to refine battery thermal design. In this study, 12 thermocouples are embedded at strategically-chosen locations inside a 25 Ah laminated lithium-ion battery. Another 12 thermocouples are attached at the corresponding locations on the surface. The temporal and spatial variations of the temperature are measured at a series of discharge rates under different thermal conditions. The thermal response of these locations is also analyzed. The major findings include: First, the internal temperatures could differ from the surface for as large as 1.1 °C, even for a thin laminated cell. Second, the time constants of thermal response at the internal locations are generally dozens of seconds larger than on the surface. Third, the internal variation in the plane direction reaches above 10 °C under adiabatic 1.5 C discharge, much larger than in the through-plane direction, indicating the in-plane heat conductivity needs improvement. Finally, forced convection is effective to suppress the temperature rise as well as the variation. The direct measurement of internal temperature initiated in this study paves the way for implanting sensors/microchips in single cell to extract multiple physico-electrochemical signals simultaneously.