Small Relaxation Time Research Articles

Polyvinyl alcohol/carboxymethyl cellulose blended polymers doped with PPy/milled MWCNTs filler for Flexible optoelectronic and Energy Storage Applications

Using the solution casting procedure, poly (vinyl alcohol)/carboxymethyl cellulose/polypyrene/milled multiwall carbon nanotubes, PVA/CMC/PPy/x wt% milled MWCNTs blended polymers were formed. X-ray diffraction and scanning electron microscopy were employed to inspect the structure and morphology of the resulted blends. The lowest direct and indirect optical band gaps are (5, 4.3) eV and (4.37, 3.38) eV, respectively, achieved when the MWCNTs content in the doped blend was 0.25 wt %. By incorporating varying quantities of milled MWCNTs into the PVA/CMC/PPy blended polymer, consistent enhancements were observed in the optical dielectric constant and optical conductivity values. The blend with 0.25 wt% MWCNTs exhibited the maximum values of refractive index. The maximum electric dielectric constant and energy density values were attained as x = 0.15. The temperature impacted the dielectric constants and energy storage values. All blends fit with the CBH model. The impact of MWCNTs doping level and the temperature on the impedance spectroscopy and electric modulus of the host blend was explored. The sample with x = 0.15 has the smallest relaxation time. The impact of MWCNTs doping level on the dc conductivity, activation energy and conductivity mechanism of the host blend was explored. The doped blends with x = 0.15 is viable materials for energy storage purposes.

Versailles Santa Barbara-5 derived bifunctional one-dimensional electrocatalysts of hierarchical Ni3S2 nanoprism@nanosheets arrays for self-supporting and binder-free efficient supercapacitor and methanol oxidation reaction

We herein present a structure consisting of two-dimensional (2D) single-crystal Ni3S2 nanosheets designed on one-dimensional (1D) Ni3S2 nanoprism arrays (also known as Ni3S2 nanoprism@nanosheets). These arrays are directly synthesized over nickel foam, which is referred to as Ni3S2/NF. These structures can be used as binderless, self-supporting electrodes for bifunctional methanol oxidation reaction (MOR) applications and high energy density hybrid supercapacitors (HSC). Ni3S2/NF is synthesized through the two steps of hydrothermal processes. In step I, 1D hexagonal Versailles Santa Barbara-5 (VSB-5) nanorod arrays have been prepared onto the surface of NF (denoted as VSB-5/NF). The precursor for step II was 1D VSB-5 nanorod arrays, which were transformed into Ni3S2 using Na2S as a sulfur source via the Kirkendall effect. Electrochemical examination results show that as-synthesized Ni3S2/NF electrode exhibited a high specific capacitance (Cs) of 145.0 mA h g−1 at 5 A g−1, low series resistance (0.18 Ω) and charge-transfer resistance (0.05 Ω), small relaxation time constant (τ0 = 8 ms) and high frequency response at 125.8 Hz. Ni3S2/NF has a superior specific capacity of 145.0 mA h g−1 at 5 A g−1 as a SC electrode. The Ni3S2/NF//CNTs/NF HSC can be operated up to 1.6 V in a 3 M KOH electrolyte, delivering an impressive energy density of 256.35 W h kg−1 at a power density of 6400 W kg−1. The as-assembled Ni3S2/NF//CNTs/NF HSC device exhibited an impressive stability of 81 % even after undergoing 10,000 charge-discharge cycles. When compared to previously reported catalysts, Ni3S2/NF was proved to be an excellent electrocatalyst for MOR due to its ability to generate a current density of 215.0 mA cm−2 at 0.5 VSCE, while maintaining a superior and stable current density of 619.2 mA cm−2 at 0.8 VSCE.

Nonlinear acoustic equations of fractional higher order at the singular limit

When high-frequency sound waves travel through media with anomalous diffusion, such as biological tissues, their motion can be described by nonlinear acoustic equations of fractional higher order. In this work, we relate them to the classical second-order acoustic equations and, in this sense, justify them as their approximations for small relaxation times. To this end, we perform a singular limit analysis and determine their behavior as the relaxation time tends to zero. We show that, depending on the nonlinearities and assumptions on the data, these models can be seen as approximations of the Westervelt, Blackstock, or Kuznetsov wave equations in nonlinear acoustics. We furthermore establish the convergence rates and thus determine the error one makes when exchanging local and nonlocal models. The analysis rests upon the uniform bounds for the solutions of the acoustic equations with fractional higher-order derivatives, obtained through a testing procedure tailored to the coercivity property of the involved (weakly) singular memory kernel.

Refined Schemes for Computing the Dynamics of Elastoviscoplastic Media

For a stable numerical solution of the system of equations governing an elastoviscoplastic continuous medium model, a second-order explicit-implicit scheme is proposed. An explicit approximation is used for the equations of motion, and an implicit approximation, for the constitutive relations containing a small relaxation time parameter in the denominator of the nonlinear free terms. A second-order implicit approximation for isotropic and anisotropic elastoviscoplastic models is constructed to match the orders of approximation of the explicit elastic and implicit adjustment steps. Refined formulas for correcting the stress deviators after the elastic step are derived for various viscosity function representations. The resulting solutions of the second-order implicit approximation for the stress deviators of the elastoviscoplastic equations admit passage to the limit as the relaxation time tends to zero. The correcting formulas obtained via this passage to the limit can be treated as regularizers of the numerical solutions to the elastoplastic systems.

Role of interfacial flexibility in emulsifying ability of coconut protein isolate remodeled by pulsed light coupled with weak alkali cycling treatment

The interfacial flexibility of proteins is a crucial factor that affects their functional properties. In this study, we investigated the role of interfacial flexibility in the emulsifying ability of coconut protein isolate (CPI) treated by pulsed light coupled with weak alkali cycling (pH 7 → 9 → 7). After treatment, the structure of CPI was unfolded and re-folded, which resulted in the exposure of hydrophobic groups and the formation of smaller aggregates, with the change of secondary structure. Additionally, the S–S bond content increased while the free –SH content decreased. The interfacial flexibility of CPI-based samples at the oil-water interface was evaluated by dissipative quartz crystal microbalance and interfacial dilatational rheology. Compared to CPI, PCPI-9 (CPI treated by pulsed light coupled with pH 9 cycling) possessed a greater viscoelastic modulus and a smaller relaxation time. Meanwhile, the anti-perturbation ability was better than CPI. Then, microstructure, centrifugal stability and emulsifying properties of the O/W emulsions stabilized by CPI-based samples were measured. The results showed that emulsion stabilized by PCPI-9 had significant advantages in long-term stability. The emulsification stability (ESI) increased by 25.45 times compared to CPI. In conclusion, moderate pulsed light coupled with weak alkali cycling treatment could improve the emulsifying ability of CPI by remodeling its structure to change the interfacial flexibility. These results could provide an in-depth insight into the relationship between interfacial flexibility and emulsifying properties of coconut protein isolate.

Amino Acid Mediated Highly Ordered Carbon Dots as Phase Directing Agent for Synthesizing α-Ni(OH)2Decorated with Nitrogen Doped CD as an Electrode for an Efficient Symmetric Supercapacitor

The synthesis of the α-phase of layered Ni(OH)2 is desirable owing to its efficient charge storage applications. The hydrothermal route of synthesis usually leads to formation of mixed of α- and β-phases of Ni(OH)2. We present here a novel hydrothermal approach for synthesizing predominantly the α-phase of Ni(OH)2 using a nitrogen doped crystalline carbon dot ([N-CD]BA) as a phase directing agent. The reaction medium comprising [N-CD]BA led to the formation of a microflower-like structure denoted as Ni(OH)2/[N-CD]BA with more petal density than in pristine α/β mixed phases of Ni(OH)2. The structure, composition, texture, and morphology of the as-synthesized batches of Ni(OH)2/[N-CD]BA and pristine Ni(OH)2 are thoroughly characterized. The electrochemical studies recorded by cyclic voltammetry and galvanostatic charging–discharging measurements demonstrated a battery-type supercapacitor. The batch of Ni(OH)2 prepared with 12.5 vol % [N-CD]BA, denoted as Ni(OH)2/12.5%[N-CD]BA, exhibited an optimum specific capacity of 482 C g–1 recorded at 1.0 A g–1, which is 4.5 times higher than that of pristine Ni(OH)2. The enhanced charge storage and transport mechanism are explained from electrochemical impedance spectroscopy (EIS). The high power density is reflected from a small relaxation time (0.4 s), and the charging–discharging efficiency is also improved for α-Ni(OH)2/12.5%[N-CD]BA. The application of the materials as supercapacitors has been demonstrated by fabricating a symmetric supercapacitor (SSC) device, i.e., Ni(OH)2/[N-CD]BA//Ni(OH)2/[N-CD]BA, using 2 M KOH as the electrolyte. The SSC exhibited a maximum energy density of 24 Wh kg–1 and a power density of 1.5 kW kg–1. The real-time application of the symmetric supercapacitor device has been demonstrated by successfully illuminating red LED lamps and powering a motor driven 1.6 mW fan.

Диэлектрическо-спектроскопическая сепарация характеристик механизмов проводимости в нанкристаллическх пленках AgI

The study of the frequency dependence of the components of the complex permittivity of thin (100 nm) nanocrystal silver iodide (AgI) films revealed the presence of two different types of relaxation processes, sharply differing in their relaxation times: tau1<<tau2. It is established that the small relaxation time of tau1 is the time of formation of a stable configuration of the internal electric field when the external high-frequency sinusoidal electric field is shielded by a system of free conduction electrons. On the contrary, a long relaxation time of tau2 characterizes the process of shielding a low-frequency external field by a system of quasi-free silver ions. It is shown that the temperature dependence of the numerical value of tau1(T) has a thermal hysteresis with a loop, the position of the heating and cooling branches of which coincides with the temperature region of the forward and reverse phase transitions in the AgI film: semiconductor-superionic and superionic-semi-conductor. The results of calculating the parameters of dielectric spectra in the framework of Debye theory are presented, demonstrating good agreement with the parameters of the proposed electrical circuit of the film sample calculated by the complex impedance method

Theoretical Understanding of thermoelectric energy conversion efficiency in Lead-Free halide double perovskites showing intrinsic defect tolerance

Thermoelectric devices based on the Seebeck effect can directly convert waste heat to electricity, but the demand for better thermoelectric materials remains unfulfilled. The lead-free halide double perovskites have recently shown promises for their stability and environment-friendly nature, but impacts from intrinsic defects (vacancies and anti-sites) on their thermoelectric performance remains elusive. Combing first-principle calculations, the Boltzmann transport theory, and the defect formation theory, we investigate the thermoelectric properties of lead-free double perovskite Cs2NaInCl6, taking the intrinsic defects into consideration. Our results demonstrate that the pristine Cs2NaInCl6 presents thermodynamic, mechanical, and dynamic stability. The band edges are mainly comprised of the electrons from In and Cl atoms. Furthermore, the double perovskite has an ultra-low lattice thermal conductivity and a high Seebeck coefficient, while showing a small charge carrier relaxation time and electric conductivity. Promisingly, the maximum ZT values can reach as high as ∼ 1.36 and ∼ 1.44 at 800 K temperature, respectively, with optimal extrinsic carrier concentrations of ∼ 1.9 × 1019 and 5.8 × 1020 cm−3 for N- and P-type carriers (holes and electrons), and the maximum power generation efficiency can reach ∼ 14% (for P-type) when the hot-side temperature is 800 K. Among various intrinsic defect types, we determine the preferable defect types in Cs2NaInCl6 based on the minimal defect formation energy criteria. The free carriers can be switched from N- to P-type under these two preferable defects, respectively, with Cl and Na vacancies. Even under a very high defect concentration of 2.1 × 1019/cm3 considered here, the extra carrier concentrations induced by the defects are only ∼ 1017 cm−3 at 800 K, showing strong defect tolerance in carrier concentration. This work suggests that lead-free halide double perovskites are promising thermoelectric materials, and they show strong intrinsic defect tolerance that prevents a negative impact on the extrinsic doping-controlled carrier concentrations.

Reduced Graphene Oxide-Based Dielectric Nanocomposites with Small Dielectric Relaxation Times for Emerging Dielectric Electronics with High-Frequency Performance Demands

Reduced Graphene Oxide-Based Dielectric Nanocomposites with Small Dielectric Relaxation Times for Emerging Dielectric Electronics with High-Frequency Performance Demands

Effect of vibrational modes on electron transfer directionality: Photosynthetic reaction centers.

It is shown in the example of the photosynthetic reaction centers (RCs) that the electron transfer can be directed by vibrational modes into the needed site where it is localized. In the case of the RC, it is the low vibrational mode that produces such an effect. We find that the electron transfer unidirectionality in the photosynthetic reaction center can be determined by the asymmetry in the reorganization energy of the vibrational modes at high temperatures. We also numerically solve generalized master equationsfor various vibration relaxation times. The results are compared with the solution of master equations. It is shown that for small relaxation times, the non-Markovian electron transfer kinetics gives similar results as the Markovian approximation, but the results are significantly different for the long vibration relaxation times.

Significantly suppressed thermal transport by doping In and Al atoms in gallium nitride.

Thermal transport plays a key role in the working stability of gallium nitride (GaN) based optoelectronic devices, where doping has been widely employed for practical applications. However, it remains unclear how doping affects thermal transport. In this study, based on first-principles calculations, we studied the doping effect on the thermal transport properties of GaN by substituting Ga with In/Al atoms. The thermal conductivities at 300 K along the in-plane(out-of-plane) directions of In- and Al-doped GaN are calculated to be 7.3(8.62) and 12.45(11.80) W m-1 K-1, respectively, which are more than one order of magnitude lower compared to that of GaN [242(239) W m-1 K-1]. From the analysis of phonon transport properties, we find that the low phonon group velocity and small phonon relaxation time dominate the degenerated thermal conductivity, which originated from the strong phonon anharmonicity of In/Al-doped GaN. Furthermore, by examining the crystal structure and electronic properties, the lowered thermal conductivity is revealed lying in the strong polarization of In-N and Al-N bonds, which is due to the large difference in electronegativity of In/Al and N atoms. The results achieved in this study have guiding significance to the thermal transport design of GaN-based optoelectronic devices.

Spinel-MgMn2O4 nanofibers: An attractive material for high performance aqueous symmetric supercapacitor

It is a known fact that morphology greatly influences the energy storage performance of a material. In this work, spinel magnesium manganese oxide (MgMn2O4) having nanofibric morphology has successfully been prepared for the first time using a versatile and cost-effective electrospinning technique. The morphology and the pore size are altered and optimized by varying the process parameters in the electrospinning process, and their corresponding effect on supercapacitive properties are also studied thoroughly. The aligned, mesoporous, and high aspect ratio of spinel-MgMn2O4 nanofiber exhibit the specific capacitance of ∼345 Fg−1 at 1 Ag−1 and 250 Fg−1 at 4 Ag−1 as well as demonstrate an outstanding rate capability and good capacity retention (∼90%) after 10,000 successive cycles at 4 Ag−1 in a three-electrode configuration. The improved energy storage performance of MgMn2O4 active material on graphite substrate is ascribed to its nanofibric morphology with interconnected particles having pores/voids in between, which impart a higher diffusion coefficient (D= 3.226 × 10−10 cm2s−1) and a small relaxation time constant (τ = 0.1 s). The quantitative capacitive contribution generating from EDLC and/or the surface pseudocapacitance reactions and the contribution from diffusion-controlled redox reactions to the total current have also been determined. The assembled symmetric all-solid-state supercapacitor (ASSSC) shows the high specific energy of ∼30 W h kg−1 at the specific power of ∼510 W kg−1 in the potential window of 0.0 V to 2 V with PVA-H2SO4 gel electrolyte. This performance is superior to that exhibited by many ASSSC reported recently. Two ASSSC devices connected in series can light up parallelly connected 55 red LEDs for more than 3 min, indicating practical applicability of our designed symmetric supercapacitor in electronics appliances. The outstanding electrochemical performance of the MgMn2O4 nanofiber reveals its potential to be a promising electrode material for supercapacitors.

Different Effects of Mg and Si Doping on the Thermal Transport of Gallium Nitride

Mg and Si as the typical dopants for p- and n-type gallium nitride (GaN), respectively, are widely used in GaN-based photoelectric devices. The thermal transport properties play a key role in the thermal stability and lifetime of photoelectric devices, which are of significant urgency to be studied, especially for the Mg- and Si-doped GaN. In this paper, the thermal conductivities of Mg- and Si-doped GaN were investigated based on first-principles calculations and phonon Boltzmann transport equation. The thermal conductivities of Mg-doped GaN are found to be 5.11 and 4.77 W/mK for in-plane and cross-plane directions, respectively. While for the Si-doped GaN, the thermal conductivity reaches the smaller value, which are 0.41 and 0.51 W/mK for in-plane and cross-plane directions, respectively. The decrease in thermal conductivity of Mg-doped GaN is attributed to the combined effect of low group velocities of optical phonon branches and small phonon relaxation time. In contrast, the sharp decrease of the thermal conductivity of Si-doped GaN is mainly attributed to the extremely small phonon relaxation time. Besides, the contribution of acoustic and optical phonon modes to the thermal conductivity has changed after GaN being doped with Mg and Si. Further analysis from the orbital projected electronic density of states and the electron localization function indicates that the strong polarization of Mg-N and Si-N bonds and the distortion of the local structures together lead to the low thermal conductivity. Our results would provide important information for the thermal management of GaN-based photoelectric devices.

Unveiling the origins of low lattice thermal conductivity in 122-phase Zintl compounds

The intrinsically low lattice thermal conductivity κL is essentially important for thermoelectric technology to maintain large working temperature difference, in order to enlarge the output power. In principle, high density and complex structure at atomic and nano scale are typical features for thermoelectric materials with low lattice thermal conductivities (κLs). However, many 122-phase Zintl compounds with promising thermoelectric properties defy these paradigms. They adopt three relatively simple structures with comparably small mean atomic mass, while these compounds exhibit considerably low lattice thermal conductivities κLs. Herein we unveil the origins of low κLs in 122-phase Zintl compounds, ascribed from the important aspects including atomic constituents, their arrangements and chemical bonding. The coupling between the acoustic branches and low-frequency optical branches, as well as the anisotropic chemical bonding are responsible for the high anharmonicity, favoring for the low phonon velocity and small relaxation time. This work offers a guidance to design high-performance Zintl thermoelectric materials with ultralow lattice thermal conductivity.

A macroscopic traffic model based on relaxation time

A macroscopic model based on analogies with Little’s Law is proposed. The relaxation time is employed to characterize changes in density. This change is large for a small relaxation time and large interactions between vehicles occur. The relaxation time encompasses driver behavior which includes the time to perceive and process traffic situations and the resulting actions. Thus, it incorporates the perception and reaction time of a driver. The proposed model is evaluated for a transition caused by a bottleneck on a road and is compared with Zhang model. Performance results are presented which show that traffic evolution with the proposed model is more realistic than with the Zhang model.

An ultra-compressible piezoresistive strain and pressure sensor based on RGO-CNT-Melamine foam composite for biomedical sensing

Many heitherto reported compressive piezoresistive foam sensors suffer from variable sensitivity over large compression range, limiting the applicability of the sensor for a narrow range of motion. This is because the area of contact between conductive foam struts is very small and varies linearly over only a small compression range. We resolved this problem by fabricating an ultracompressive foam sensor with large area conductive flakes using facile technique of soak-drying Melamine foam with the conducting aqueous RGO-CNT ink. Sensor exhibited a constant gauge factor (GF) 1.6 over a large and highly linear compression range (∼2% to 60 %) and detected both small and large body movements with superior detectability. Besides that all vital features of a tiny signal such as pulse waveform were delineated due to the excellent low pressure (<5 kPa) sensitivity 0.22 kPa−1 and the small relaxation time of 19 ms. The sensor showed a stable and repeatable performance over 11000 cycles. Its operation is also tested over a wide range of ambient temperature (-7 ℃ to 140 ℃) and humidity (25%–82%, RH) and even under water with no major deterioration in performance.

Emergent traveling waves in spatially extended system with finite memory of transport

Standard diffusion equation is formalized on the basis of Brownian movement of dispersing species in absence of any persistence in the transport motion of individuals. As a result, this description accounts for instantaneous spreading of dispersing species over an arbitrarily large distance from their original location predicting infinite velocities. This feature appears to be unrealistic, particularly while considering invasion dynamics in biological systems. Therefore, a better description demands the consideration of dispersal with inertia. Here, we investigate the spatiotemporal dynamics of a reaction-transport system including Cattaneo's modification of flux and a source term with cubic nonlinearity, reminiscent of population dynamics or flame propagation models. We show that the spatially extended system admits a traveling wave solution. The presence of a small finite relaxation time of the diffusive flux modifies the speed of the traveling wave, specifically, resulting decay in speed with an increase of relaxation time of the flux. Our investigation reveals a complex interplay of reaction parameters and the finite memory of transport. The observed traveling wave solution reveals that there are conditions inbuilt in the choice of the reaction rate parameters, which governs the evolution of the wave solution. Our analytical predictions are qualitatively in good agreement with the numerically simulated results.

Dielectric, Electrical Conductivity Behavior and Molecular Modeling of Some Pyrimidine and Purine Compounds

The dielectric and electrical conductivity measurements for biologically active nucleic acid compounds reveal semiconducting properties and small relaxation times. On the basis of electronic transition within molecules, two pathways for the conduction of electricity may be expected. The first conducting process occurring in the lower temperature region is attributed to n→π* transitions which require less energy to be performed. In the upper temperature region, conduction could be attributed to π→π* transitions which need more energy to participate in electronic conduction. The observed increment of conduction in the upper temperature region may be attributed to interactions between n→π* and π→π* transitions. Quantum chemical parameters such as the highest occupied molecular orbital energy (EHOMO) and the lowest unoccupied molecular orbital energy (ELUMO) were given using molecular modeling. Energy gap (ΔE) and parameters which give information about the reactive chemical behavior of compounds such as electronegativity (χ), chemical potential (µ), global hardness (η), softness (σ) and electrophilicity index (ω) were calculated.

Thermo-Fluid Mechanics of gas-Solid Particle Flows over horizontal Flat Plate

Mathematical model has been developed to protect fluid and solid particle homogeneous mixture velocity concentration and temperature for a heated horizontal flat plate. Conversation equation based on Eulerian scale are approximated for small relaxation times through stream function and similarity transformations. Parametric database generated through computer program for arbitrary constants on comparison with clear fluid reveals the particle concentration has pronounced effect on velocity and temperature profiles.

Fabrication of Binder-Free TiO2 Nanofiber Electrodes via Electrophoretic Deposition for Low-Power Electronic Applications

In this article, binder-free TiO2 nanofiber-based electrodes have been fabricated via fast, cost-effective, and environmental-friendly electrophoretic deposition (EPD) technique on a graphite substrate for energy storage devices such as supercapacitor for low-power electronic applications. The experimental studies confirm that the surface morphology of the fabricated electrodes is greatly dependent on the EPD voltage, which is one of the key factors, which controls the electrochemical performance of the TiO2 nanofibers-based electrodes. The binder-free, TiO2 nanofiber-based electrode exhibits a specific capacitance of 408 Fg−1 ± 5 Fg−1 at 1 Ag−1 current density. The fabricated electrode shows a higher diffusion coefficient $\sim 2.81\times 10^{-{10}}$ , which facilitates an easy diffusion of electrolytic ions inside the sample and a very small relaxation time constant ( $\tau _{\text {o}} = {98}$ ms), which allows the system to deliver its energy in a quick manner. Moreover, the fabricated electrode retains 85 % of its initial capacitance value after 1000 successive cycles. It can be believed that the reported EPD technique can be used to fabricate the electrodes for energy storage devices for portable low-power electronic applications.