Constant Measurements Research Articles

Strength-duration time constant and rheobase measurements: Comparison of the threshold tracking method and a manual procedure

ObjectiveTo compare the strength-duration time constant (SDTC) and rheobase measurements obtained by the threshold tracking method (TT) and by a non-automated method (MM). MethodsThe MM procedure involved measuring, using a routine electrodiagnostic device, the intensity required to evoke a motor response whose amplitude corresponds to 40% of the maximum amplitude for four stimulus duration (1.0, 0.7, 0.5, 0.2 ms), and studying the linear relationship between stimulus charge and stimulus duration (slope = rheobase, intercept on the x-axis = SDTC). Using TT and MM, 30 successive healthy subjects (mean age = 38 years old) underwent a prospective evaluation of SDTC and rheobase of the median nerve motor axons at the wrist. Nerve stimulation and bipolar recording of evoked motor responses were performed with disposable self-adhesive surface electrodes. ResultsThe Spearman correlations between the two methods were 0.78 (p < 0.0001) for SDTC and 0.96 (p < 0.0001) for the rheobase. The Bland-Altman analysis did not reveal any systematic bias of MM compared to TT. ConclusionsThe MM procedure was reliable for strength-duration relationship analysis. SignificanceWe encourage neurophysiologists, who do not have dedicated threshold tracking equipment, not to hesitate to use these simple tools to assess peripheral nerve excitability.

Quantifying Ligand Binding to the Surface of Metal-Organic Frameworks.

The binding of molecules to the exterior surface of metal-organic frameworks (MOFs) is not a well-understood phenomenon. Herein, the surface chemistry of three MOFs, UiO-66, MIL-88B-NH2, and ZIF-8, is investigated using dye-displacement experiments. MOF particle surfaces were modified with ligand-appended BODIPY dyes. The ability of the coordinated dyes to be displaced by a variety of exogenous ligands was measured by ultraviolet-visible spectroscopy. This method allowed for measurement of apparent binding constants for different ligands to the MOF surface. As might be expected, ligand affinity was dependent on the nature of the underlying metal-ligand composition of the MOF. This work provides a quantitative evaluation of ligand binding to MOF surfaces and important insights for the modulation, modification, and manipulation of MOFs.

Dissolution–precipitation kinetics during C[formula omitted]S hydration: A holographic interferometry study

Tricalcium silicate C3S has been widely employed in model systems to interpret the complex mechanisms of cement hydration. The present study aims at providing an accurate description of the dissolution rate of C3S by implementing a new approach based on holographic interferometry. We propose a protocol that allows the measurement of the pure dissolution rate constant of the C3S phase by experiments performed in supersaturated conditions with respect to C–S–H. In these experiments, a flat polycrystalline surface of C3S was monitored in a quiescent solution. The amount of calcium released within the holographic cell was measured and the aqueous solution composition was constrained to the C–S–H stoichiometry at the interface. An equilibrium approach was applied to assess interfacial pH, saturation indexes, and C–S–H precipitation at the experimental conditions. The rate law of the pure dissolution of C3S was assessed in conditions ranging from far to close-to-equilibrium.

Measurement of spring constant by means of Arduino: A STEM teaching proposal

This study advances a STEM teaching proposal and aims to determine spring constant of a spring pendulum by means of an Arduino microprocessor. The measurements are managed by simply letting the spring pendulum to oscillate freely and recording the distance perceived by the distance sensor-Arduino system as a function of time. The mean periods are estimated by using displacement-time plots of the harmonic motion and the results are used to estimate the spring constant. The spring constant is also determined conventionally by employing Hooke's law a number of times. The relative error rate between the two results is found to be about % 6.00 which is pretty acceptable. This approach is important in the sense that it is inexpensive and also encourages students to learn how to use the Arduino microprocessor. The approach adds to physics education efforts due to creating an enjoyable and beneficial teaching-learning environment.

OPTICAL AND DIELECTRIC PROPERTIES OF EPOXY RESIN FILLED WITH TITANIUM DIOXIDE PARTICLES

This work studies the electrical properties for pure epoxy, and epoxy with (0, 1, 1.5, 2 wt%) titanium dioxide powder composites. The effects of titanium dioxide contents on optical and dielectrically properties of the epoxy/ titanium dioxide have investigated by several techniques were used to characterize the epoxy/titanium dioxide: UV-visible spectrophotometer reveals a new absorption band in the wavelength range (350-600) nm. The findings of this investigation show that when the proportion of titanium dioxide added increases, the absorbance increases which are ascribed to interchain interaction. The absorption coefficient (α), extinction coefficient (k), refractive index (n), the real and imaginary permittivity (εr, εi), and energy gap (Eg) of epoxy/titanium dioxide samples were effectively determined from the recorded data optical transmission technique. In addition, these epoxy have been examined in the wavenumber range (4000-500 cm-1) using an FT-IR spectrometer and also a dielectric constant measurement. The thickness of all the samples is 1.2 mm ± 0.1 mm.

Measuring the Hubble constant with cosmic chronometers: a machine learning approach

Local measurements of the Hubble constant (H_0) based on Cepheids e Type Ia supernova differ by approx 5 sigma from the estimated value of H_0 from Planck CMB observations under Lambda CDM assumptions. In order to better understand this H_0 tension, the comparison of different methods of analysis will be fundamental to interpret the data sets provided by the next generation of surveys. In this paper, we deploy machine learning algorithms to measure the H_0 through a regression analysis on synthetic data of the expansion rate assuming different values of redshift and different levels of uncertainty. We compare the performance of different regression algorithms as Extra-Trees, Artificial Neural Network, Gradient Boosting, Support Vector Machines, and we find that the Support Vector Machine exhibits the best performance in terms of bias-variance tradeoff in most cases, showing itself a competitive cross-check to non-supervised regression methods such as Gaussian Processes.

Work-based measurement of k with a spring-mass system: a demo of the work done by a variable force

This article details the use of the concept of the work done by a variable force in measuring the spring constant of the spring in a spring-mass system. The raw data used in the experiment is similar to the one used in measuring the same quantity, but by directly invoking Hooke’s law, in which the slope of the linear plot of the applied force as a function of the spring elongation directly yields the spring constant. Slightly different, the work done approach showed in this article allows for the measurement of the spring constant by plotting the work done of the applied force as a function of the square of the elongation. The result is accurate to within less than 1.00%. Aside from a work-based determination of a spring constant, the current lab activity can be regarded as a demonstration of the concept of the work done by a variable force as well. Moreover, the concept of the integral of a function—as well as that of the area of the triangle—being equal to the area bounded by the graph of the same function has an appearance in this paper. Therefore, the potential utility of this work as a physics-based illustration of the area of the triangle and of one of the fundamentals of integral calculus.

Empty space and the (positive) cosmological constant

I discuss empty space, as it appears in the physical foundations of relativistic field theories and in the semiclassical study of isolated systems. Of particular interest is the relationship between empirical measurements of the cosmological constant and the question of appropriate representation of empty space by spacetimes, or models of general relativity. Also considered is a speculative move that shows up in one corner of quantum gravity research. In pursuit of holographic quantum cosmology given a positive cosmological constant, there is evidently some freedom available for theoretical physicists to pick between two physically inequivalent spacetime representations of empty space, moving forward: de Sitter spacetime or its ‘elliptic’ cousin.

Broadband Measurements of Soil Complex Permittivity.

Agriculture is a major consumer of freshwater and is often associated with low water productivity. To prevent drought, farmers tend to over-irrigate, putting a strain on the ever-depleting groundwater resources. To improve modern agricultural techniques and conserve water, quick and accurate estimates of soil water content (SWC) should be made, and irrigation timed correctly in order to optimize crop yield and water use. In this study, soil samples common to the Maltese Islands having different clay, sand, and silt contents were, primarily, investigated to: (a) deduce whether the dielectric constant can be considered as a viable indicator of the SWC for the soils of Malta; (b) determine how soil compaction affects the dielectric constant measurements; and (c) to create calibration curves to directly relate the dielectric constant and the SWC for two different soil types of low and high density. The measurements, which were carried out in the X-band, were facilitated by an experimental setup comprising a two-port Vector Network Analyzer (VNA) connected to a rectangular waveguide system. From data analysis, it was found that for each soil investigated, the dielectric constant increases notably with an increase in both density and SWC. Our findings are expected to aid in future numerical analysis and simulations aimed at developing low-cost, minimally invasive Microwave (MW) systems for localized SWC sensing, and hence, in agricultural water conservation. However, it should be noted that a statistically significant relationship between soil texture and the dielectric constant could not be determined at this stage.

Direct observation of coherence transfer and rotational-to-vibrational energy exchange in optically centrifuged CO2 super-rotors

Optical centrifuges are laser-based molecular traps that can rotationally accelerate molecules to energies rivalling or exceeding molecular bond energies. Here we report time and frequency-resolved ultrafast coherent Raman measurements of optically centrifuged CO2 at 380 Torr spun to energies beyond its bond dissociation energy of 5.5 eV (Jmax = 364, Erot = 6.14 eV, Erot/kB = 71, 200 K). The entire rotational ladder from J = 24 to J = 364 was resolved simultaneously which enabled a more accurate measurement of the centrifugal distortion constants for CO2. Remarkably, coherence transfer was directly observed, and time-resolved, during the field-free relaxation of the trap as rotational energy flowed into bending-mode vibrational excitation. Vibrationally excited CO2 (ν2 > 3) was observed in the time-resolved spectra to populate after 3 mean collision times as a result of rotational-to-vibrational (R-V) energy transfer. Trajectory simulations show an optimal range of J for R-V energy transfer. Dephasing rates for molecules rotating up to 5.5 times during one collision were quantified. Very slow decays of the vibrational hot band rotational coherences suggest that they are sustained by coherence transfer and line mixing.

Measurements of Rate Constants of Energy Transfer Processes in Ar/He Plasma of Pulse-Periodic Discharge

Measurements of Rate Constants of Energy Transfer Processes in Ar/He Plasma of Pulse-Periodic Discharge

Harnessing Unresolved Lensed Quasars: The Mathematical Foundation of the Fluctuation Curve

Strong gravitational lensed quasars (QSOs) have emerged as powerful and novel cosmic probes as they can deliver crucial cosmological information, such as a measurement of the Hubble constant, independent of other probes. Although the upcoming LSST survey is expected to discover 103–104 lensed QSOs, a large fraction will remain unresolved due to seeing. The stochastic nature of the quasar intrinsic flux makes it challenging to identify lensed quasars and measure the time delays using unresolved light-curve data only. In this regard, Bag et al. introduced a data-driven technique based on the minimization of the fluctuation in the reconstructed image light curves. In this article, we delve deeper into the mathematical foundation of this approach. We show that the lensing signal in the fluctuation curve is dominated by the autocorrelation function (ACF) of the derivative of the joint light curve. This explains why the fluctuation curve enables the detection of the lensed QSOs only using the joint light curve, without making assumptions about QSO flux variability, nor requiring any additional information. We show that the ACF of the derivative of the joint light curve is more reliable than the ACF of the joint light curve itself because intrinsic quasar flux variability shows significant autocorrelation up to a few hundred days (as they follow a red power spectrum). In addition, we show that the minimization of fluctuation approach provides even better precision and recall as compared to the ACF of the derivative of the joint light curve when the data have significant observational noise.

Reference-free self-calibrating tip-based scattering-type THz near-field microscopy

In this work, we present a quantitative analysis model based on reference-free self-calibration to analyze scattered fields and approach curves on a dielectric substrate for terahertz scattering-type scanning near-field optical microscopy. The results of our model are compared with experimentally measured data and a fully numerical analysis based on a line dipole image method and a quasi-electrostatic approximation. The model is used to extract the effective radius of the tip and the relative permittivity of the silicon substrate to the near-field scattering signal. The measured approach curves on Au and silicon substrates show good agreement with the calculated approach curves, and the refractive index for silicon is precisely determined to be 3.42. For a tip-based THz scattering-type scanning near-field optical microscope, the proposed analysis model allows for the extraction of the effective probe radius and dielectric functions, thereby enabling conclusive measurements of geometric parameters and optical constants.

Non-Aqueous Zn-Ion Hybrid Supercapacitors: Acetonitrile vs Propylene Carbonate Based Electrolyte

The development of high efficiency energy storage systems is increasingly important as these systems enable utilize energy from renewable sources and reduce greenhouse gas evolution caused by fuel combustion technologies at the same time. Electrochemical characteristics of Zn-ion hybrid supercapacitor (ZIHS) cells based on 1 M acetonitrile and propylene carbonate electrolytes in zinc tetrafluoroborate (Zn(BF4)2), zinc di[bis(trifluoromethylsulfonyl)imide] (Zn(TFSI)2) and zinc trifluoromethanesulfonate (Zn(OTf)2) have been studied using cyclic voltammetry, constant current charge/discharge and electrochemical impedance methods. The Ragone plots have been calculated from constant power measurement data. Very high energy and power densities (80 Wh kg−1 and 21.2 kW kg−1) have been calculated for 1 M Zn(BF4)2/AN based Zn-ion hybrid supercapacitor. Some assembled ZIHSs had shown excellent cycling and energy stability over 20000 cycles.

Long-Range Kinetic Effects on the Alternating Ring Opening Metathesis of Bicyclo[4.2.0]oct-6-ene-7-carboxamides and Cyclohexene.

We report an investigation of rates of ruthenium-catalyzed alternating ring opening metathesis (AROM) of cyclohexene with two different Ru-cyclohexylidene carbenes derived from bicyclo[4.2.0]oct-6-ene-7-carboxamides (A monomer) that bear different side chains. These monomers are propylbicyclo[4.2.0]oct-6-ene-7-carboxamide and N-(2-(2-ethoxyethoxy)ethanylbicyclo[4.2.0]oct-6-ene-7-carboxamide. The amide substitution of these monomers directly affects both the rate of the bicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening and the rate of reaction of the resulting carbene with cyclohexene (B monomer). The resulting Ru-cyclohexylidenes underwent reversible ring opening metathesis with cyclohexene. However, the thermodynamic equilibrium disfavored cyclohexene ring opening. Utilization of triphenylphosphine forms a more stable PPh3 ligated complex, which suppresses the reverse ring closing reaction and allowed direct measurements of the forward rate constants for formation of various A-B and A-B-A' complexes through carbene-catalyzed ring-opening metathesis and thus gradient polymer structure-determining steps. The relative rate of the propylbicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening is 3-fold faster than that of the N-(2-(2-ethoxyethoxy)ethanylbicyclo[4.2.0]oct-6-ene-7-carboxamide. In addition, the rate of cyclohexene ring-opening catalyzed by the propyl bicyclooctene is 1.4 times faster than when catalyzed by the ethoxyethoxy bicyclooctene. Also, the subsequent rates of bicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening by propyl-based Ru-hexylidene are 1.6-fold faster than ethoxyethoxy-based Ru-hexylidene. Incorporation of the rate constants into reactivity ratios of bicyclo[4.2.0]amide-cyclohexene provides prediction of copolymerization kinetics and gradient copolymer structures.

Effects of aluminum and oxygen additions on quenched-in compositional fluctuations, dynamic atomic shuffling, and their resultant diffusionless isothermal [formula omitted] transformation in ternary Ti–V-based alloys with bcc structure

Diffusionless isothermal ω transformation is a recently discovered ω transformation that occurs isothermally during aging near room temperature in Ti alloys with a bcc (β-phase) structure. Unlike conventional displacive phase transitions, the diffusionless isothermal ω transformation occurs in locally unstable nanoscale β-phase regions formed by quenched-in statistical compositional fluctuations. In the present study, the effects of Al and oxygen additions in ternary β-phase Ti–V-based alloys on the diffusionless isothermal ω transformation and its attempt process, called dynamic atomic shuffling, were studied. Elastic constant and internal friction measurements for Ti–16.6V–2.1Al and Ti–17.5V–2.5O (at.%) alloy single crystals and analysis of quenched-in compositional fluctuations using the thermodynamic theory of fluctuations indicated that the oxygen addition increases the β-phase stability, and it selectively increases the activation energy of dynamic atomic shuffling in locally unstable Ti-rich regions, which is caused by the strong attractive interaction between Ti and oxygen elements. As a result, oxygen addition suppresses the diffusionless isothermal ω transformation that occurs during aging at 298 K. In contrast, low β-phase stability, which promotes ω-phase nucleation, is retained by Al addition. Furthermore, dynamic atomic shuffling with a low activation energy is retained even though the average activation energy is enhanced by the Al addition, originating from the relatively weak atomic interaction between Ti and Al. Therefore, the diffusionless isothermal ω transformation cannot be suppressed by Al addition.

Flexible microwave sensor films based on nested-complementary split ring resonator for liquid dielectric constant detection

This paper proposes a flexible double-sided attached microwave sensor film for real-time measurement of the liquid dielectric constant, and proposes a nested-complementary split ring resonator（NCSRR）structure with high sensitivity and quality factor values that is based on the structure evolution of a complementary split ring resonator（CSRR）. The sensor films are composed of a ground film with an NCSRR structure and a U-shaped microstrip line film. By taking a borosilicate glass beaker and a polypropylene beaker as examples, two types of microwave sensors are formed by attaching films to both sides of the beakers. Lubricating oil with 0%− 25% water content is added to the beakers separately, and the dielectric constants of the mixture are characterized nondestructively by the transmission coefficient (S21) of the device. According to the mathematical model established from the measurement results, the resonant frequencies of both sensors are linearly and negatively correlated with the dielectric constant of the liquid to be measured, which proves that the sensor film can be attached to a substrate with arbitrary curvature to form a microwave resonant sensor.

Imaging the water content of rammed earth materials with induced polarization

Rammed earth is a traditional building material characterized by its low environmental impact. The variation of the moisture content of rammed earth can affect its mechanical properties. Therefore, the non-intrusive evaluation of the water content and its distribution inside rammed earth structures is of high interest to anticipate the risk of failure. A non-destructive geophysical method called induced polarization is used to evaluate its sensitivity to the water content distribution. Measurements are first performed on small cylindrical core samples of rammed earth in order to test a physical model developed to connect induced polarization properties to saturation. Then, the model is used to estimate the water content at 3 distinct heights in 3 cylindrical core samples undergoing capillarity rise. Finally, these samples were cut and their water contents were independently determined and compared to that estimated from induced polarization measurements. A good correlation between the two estimates was obtained. In the second part of this study, induced polarization is used to image the water content distribution and its evolution over time in a rammed earth structural element undergoing desiccation. The monitoring is done for a period of 25 days starting from saturated conditions. We assess the evolution of the 3D distribution of the water content and we demonstrate that the water content obtained through induced polarization tomography is in agreement with independent data from a Time Domain Reflectometry (TDR) probe. Such a probe provides a local measurement of the water content thanks to the measurement of the dielectric constant of the material.

Lewis Acids and Electron-Withdrawing Ligands Accelerate CO Coordination to Dinuclear CuI Compounds.

A series of dinuclear molecular copper complexes were prepared and used to model the binding and Lewis acid stabilization of CO in heterogeneous copper CO2 reduction electrocatalysts. Experimental studies (including measurement of rate and equilibrium constants) and electronic structure calculations suggest that the key kinetic barrier for CO binding may be a σ-interaction between CuI and the incoming CO ligand. The rate of CO coordination can be increased upon the addition of Lewis acids or electron-withdrawing substituents on the ligand backbone. Conversely, Keq for CO coordination can be increased by adding electron density to the metal centers of the compound, consistent with stronger π-backbonding. Finally, the electrochemically measured kinetic results were mapped onto an electrochemical zone diagram to illustrate how these system changes enabled access to each zone.

Quickly form stable cathode/electrolyte interface of LiNi0.5Mn1.5O4 (LNMO)/graphite high-voltage lithium ion cells by using tosylmethyl isocyanide (TosMIC) as electrolyte additive

Tosylmethyl isocyanide (TosMIC) is used as electrolyte additive to study the effect on the cycling performance of LiNi0.5Mn1.5O4 (LNMO)/graphite 5V lithium-ion cell. The constant current charge and discharge measurement results shows that the capacity retention rate of the cell containing 0.3 wt% TosMIC increases by 18% after 200 cycles, while 1.0 wt% TosMIC worsens the cycling performance of the cell. The decomposition product PF5 of electrolyte salt LiPF6 may interact with TosMIC to form TosMIC-PF5 complex. The results of surface and electrochemical analysis indicate that the complex can be oxidized at the cathode and reduced at the anode simultaneously prior to the solvent molecules of the electrolyte. Theoretical calculation further shows that TosMIC-PF5 can be oxidized to form a dimer (TosMIC-PF5)2 at 4.46V (vs. Li/Li+), which may be deposited on the LNMO cathode to help form a coating layer. This coating layer can be rapidly formed in the first cycle and can significantly alleviate the oxidation of the electrolyte, but it cannot inhibit the decomposition of excessive additive. Two proposed reduction reactions of TosMIC-PF5 occur at 1.66 V and 2.00 V (vs. Li/Li+). Unfortunately, the reduction of this additive has a negative impact on the cycling performance of the cell.