Temperature Dependence Of Flux Research Articles

Surface-modified halloysite nanotube-embedded polyvinyl alcohol/polyvinyl amine blended membranes for pervaporation dehydration of water/isopropanol mixtures

Surface modification of a halloysite nanotube (HNT) was performed by piranha etching of the HNT surface to generate hydrophilic moiety on the surface. The HNT and surface-modified HNT (MHNT) were embedded in polyvinyl alcohol/polyvinyl amine (PVA/PVAm) membranes. Surface modification of the HNT was confirmed using X-ray photoelectron spectroscopy, Fourier transform infrared, and transmission electron microscopy analyses. The 5 wt% MHNT-embedded membrane showed the best pervaporation performance (flux = 0.13–0.031 kg/m2 h and separation factor = 427–93,313) among all membranes at 40 °C with 80–90 wt% of isopropanol (IPA) in the feed solution. Up to 5 wt% loading of HNT and MHNT in the membrane, the flux decreased, while the separation factor increased and subsequently decreased continuously with further HNT and MHNT loading. This was attributed to the hydrophilicity arising from HNT inclusion in the membranes. The membrane performance was evaluated under various pervaporation-operating conditions. Based on the solution diffusion model, with increase in water content, flux increased, while separation factor decreased. The temperature dependence of flux followed the Arrhenius equation. The energy of activation for the permeation of water (25.99 kJ/mol) was less than that of IPA (143.49 kJ/mol for the 5 wt% MHNT-loaded membrane at an 85 wt% of feed IPA composition). Overall, the pervaporation data suggested that the presence of MHNT in the PVA/PVAm membrane benefited pervaporation dehydration.

(Invited) Mechanisms of Stress Relaxation and Intensification at the Lithium/Solid Electrolyte Interface

Under electrochemical cycling, stress intensification and relaxation within small volumes at the lithium/solid-state electrolyte (SSE) interface are thought to be critical factors contributing to mechanical failure of the SSE and subsequent short-circuiting of the device. Recent hypotheses independently proposed by Porz et al.,[1] Herbert et al.[2,3] and Wang et al.[4] consider the relationship between stress intensification and local variations in the current density. These arguments are examined in light of nanoindentation studies performed in high-purity vapor deposited lithium films at indentation depths less than 1 μm.[2, 3] These experimental studies examine how lithium responds to stress and stress gradients at length scales and strain rates relevant to the observed SSE failure. Applying the findings of the nanoindentation studies, we present a mathematically tractable and transparent model that relates a two-dimensional state of stress at the lithium/SSE interface to the average current density, the length scale of an interfacial defect and lithium diffusional creep and/or dislocation plasticity stress relaxation mechanisms. Furthermore, the approach presents a common framework to compare and contrast the most recent hypotheses of SSE stress development.[1-4] Accordingly, the model addresses interfacial defects in the form of local deviations from a flat interface (non-planar) and local variations in the overpotential and/or exchange current density (planar interface). Through this simple elastic perturbation model, we describe a defect length scale that is too large for effective Nabarro-Herring diffusional creep relaxation (diffusion length is too long), but too small for efficient dislocation multiplication (insufficient room to physically operate the source), suggesting the possibility of large, deleterious stresses at the lithium/SSE interface. In this instance, the properties of the SSE may become critical in relieving pressure and mitigating the stress intensification enabling mode I or mixed-mode fracture of the SSE. Such concerns have motivated a nanoindentation creep study of Lipon performed with a diamond Berkovich indenter tip. The indentation study shows that Lipon does have a stress relaxation process with a stress exponent, n, of approximately 20, which is found to be effectively independent of temperature and length scale over the respective ranges of 25 to 60 °C and 400 to 600 nm of indentation depth. The uncommonly high magnitude of n suggests molecular units of Lipon (3 Li+ and a unit of PO4 -3) are capable of local rearrangement by short-range diffusion and, thereby, potentially capable of contributing directly to stress relief at the interface. This suggests Lipon may be less susceptible to failure by fracture. [1] L. Porz, T. Swamy, B.W. Sheldon, D. Rettenwander, T. Frömling, H.L. Thaman, S. Berendts, R. Uecker, W.C. Carter, Y.-M. Chiang, Mechanism of Lithium Metal Penetration through Inorganic Solid Electrolytes, Advanced Energy Materials 7(20) (2017). [2] E.G. Herbert, S.A. Hackney, N.J. Dudney, V. Thole, P.S. Phani, Nanoindentation of high purity vapor deposited lithium films: A mechanistic rationalization of diffusion-mediated flow, J. Mater. Res. 33(10) (2018) 1347-1360. [3] E.G. Herbert, S.A. Hackney, N.J. Dudney, V. Thole, P.S. Phani, Nanoindentation of high purity vapor deposited lithium films: A mechanistic rationalization of the transition from diffusion to dislocation-mediated flow, J. Mater. Res. 33(10) (2018) 1361-1368. [4] M. Wang, J. Wolfenstine, J. Sakamoto, Temperature dependent flux balance of the Li/Li7La3Zr2O12 interface, Electrochimica Acta 296 (2019) 842-847.

1/f noise from a finite entropy bath: comparison with flux noise in SQUIDs

The primary low-frequency noise in superconducting quantum interference devices (SQUIDs) at low temperature is flux noise with a power spectral density of the form with . Experiments show this noise is due to independent clusters of interacting spins at the metal-insulator interface of the Josephson junction. The temperature dependences of the amplitude and the spectral exponent α are such that the noise spectra of devices taken at different temperatures cross each other at a common crossing frequency fc, so that S(fc) is constant over a wide range of temperatures. Presented here are Monte Carlo simulations of a Heisenberg spin model modified with a type of dynamic constraint that depends on the configurational entropy of clusters of spins. The constraint arises from assuming that coupling between clusters of spins and the thermal reservoir is mediated by a local bath. Noise in the alignment of this model shows similarities to the temperature-dependent flux noise of SQUIDs, reproducing the relationship between α and the amplitude that leads to the existence of a crossing frequency fc of spectra at different temperatures.

Constant Temperature Control of Solar Thermal Power Plant at Transient Variation of Sunlight

The constant temperature control is required for the steady output of photo thermal generator of a solar thermal power plant in the case of transient mutation of sun light . The potential risk incurred by the conventional photo thermal control system based on the temperature dependent flux control is thoroughly explored, with a proposition of a system control technique based on the law of energy conservation for the project. We present in the research that the critical factor for keeping constant temperature during photo thermal conversion is to ensure the equilibrium between input and output energy all the time between different parts of photo thermal conversion. The idea is implemented by the sun light dependent flux control. This research provides solution for overcoming the damage to susceptible system because of the transient mutation of sun light.

Calibration of a surface mass balance model for global-scale applications

Abstract. Global applications of surface mass balance models have large uncertainties, as a result of poor climate input data and limited availability of mass balance measurements. This study addresses several possible consequences of these limitations for the modelled mass balance. This is done by applying a simple surface mass balance model that only requires air temperature and precipitation as input data, to glaciers in different regions. In contrast to other models used in global applications, this model separately calculates the contributions of net solar radiation and the temperature-dependent fluxes to the energy balance. We derive a relation for these temperature-dependent fluxes using automatic weather station (AWS) measurements from glaciers in different climates. With local, hourly input data, the model is well able to simulate the observed seasonal variations in the surface energy and mass balance at the AWS sites. Replacing the hourly local data by monthly gridded climate data removes summer snowfall and winter melt events and, hence, influences the modelled mass balance most on locations with a small seasonal temperature cycle. Modelled winter mass balance profiles are fitted to observations on 82 glaciers in different regions to determine representative values for the multiplication factor and vertical gradient of precipitation. For 75 of the 82 glaciers, the precipitation provided by the climate dataset has to be multiplied with a factor above unity; the median factor is 2.5. The vertical precipitation gradient ranges from negative to positive values, with more positive values for maritime glaciers and a median value of 1.5 mm a−1 m−1. With calibrated precipitation, the modelled annual mass balance gradient closely resembles the observations on the 82 glaciers, the absolute values are matched by adjusting either the incoming solar radiation, the temperature-dependent flux or the air temperature. The mass balance sensitivity to changes in temperature is particularly sensitive to the chosen calibration method. We additionally calculate the mass balance sensitivity to changes in incoming solar radiation, revealing that widely observed variations in irradiance can affect the mass balance by a magnitude comparable to a 1 °C change in temperature or a 10% change in precipitation.

A global model study of processes controlling aerosol size distributions in the Arctic spring and summer

We use a global chemical transport model (CTM) with size‐resolved aerosol microphysics to evaluate our understanding of the processes that control Arctic aerosol, focussing on the seasonal changes in the particle size distribution during the transition from Arctic haze in spring to cleaner conditions in summer. This period presents several challenges for a global model simulation because of changes in meteorology, which affect transport pathways and precipitation scavenging rates, changes in the ocean‐atmosphere flux of trace gases and particulates associated with sea ice break‐up and increased biological activity, and changes in photolysis and oxidation rates which can affect particle nucleation and growth rates. Observations show that these changes result in a transition from an accumulation mode‐dominated aerosol in spring to one dominated by Aitken and nucleation mode particles in summer. We find that remote Arctic aerosol size distribution is very sensitive to the model treatment of wet removal. In order to simulate the high accumulation mode concentrations typical of winter and spring it was necessary to substantially reduce the scavenging of these particles during transport. The resulting increases in accumulation mode lead to improvement in the modeled Aitken mode particle concentrations (which fall, due to increased scavenging in the free troposphere) and produce aerosol optical depths in good agreement with observations. The summertime increase in nucleation and Aitken mode particles is consistent with changes in local aerosol nucleation rates driven mainly by increased photochemical production of sulphuric acid vapor and, to a lesser extent, by decreases in the condensation sink as Arctic haze decreases. Alternatively, to explain the observed summertime Aitken mode particle concentrations in terms of ultrafine sea spray particles requires a sea‐air flux a factor 5–25greater than predicted by current wind speed and sea surface temperature dependent flux parameterizations. The enhanced total flux is clearly higher than measured in the Arctic and cannot explain the observed nucleation mode in the high Arctic. The model suggests that the summertime source of Aitken particles has very little effect on the accumulation mode and aerosol optical depth but they may contribute to cloud condensation nuclei in clouds with updraught velocities greater than about 15 cm/s. From a global aerosol modeling perspective, our understanding of Arctic aerosol is poor. We suggest several processes that currently limit our ability to simulate this challenging environment.

Mixed matrix membranes of sodium alginate and poly(vinyl alcohol) for pervaporation dehydration of isopropanol at different temperatures

Mixed matrix membranes of sodium alginate (NaAlg) and poly(vinyl alcohol) (PVA) containing 5 and 10wt.% silicalite-1 particles were fabricated by solution casting method and the cured membranes were crosslinked with glutaraldehyde. These membranes were used in pervaporation (PV) dehydration of isopropanol at 30, 40, 50 and 60°C. Membrane morphology was studied by scanning electron microscopy and universal testing machine to assess their mechanical strengths. Swelling results of the pristine and mixed matrix membranes were correlated with their PV performances. Selectivities of the mixed matrix membranes of NaAlg were 11,241 and 17,991 with the fluxes of 0.039 and 0.027kg/m2h, respectively, for 5 and 10wt.% silicalite-1 loadings. Corresponding values for mixed matrix membranes of PVA were 1295 and 2241, and 0.084 and 0.069kg/m2h, respectively, for 10wt.% water-containing feed at 30°C. Pristine membranes of NaAlg and PVA exhibited lower selectivities of 653 and 77 with increased fluxes of 0.067 and 0.095kg/m2h, respectively. From the temperature dependence of flux and diffusivity data with 10wt.% water-containing feed, Arrhenius plots were constructed to compute heat of sorption, ΔHs values. Mixed matrix membranes of NaAlg were better than PVA mixed matrix membranes at all compositions (10–40wt.%) of water. Molecular dynamics (MD) simulation was employed to compute the interfacial interaction energies of NaAlg and PVA polymers with silicalite-1 filler; also sorption of liquid molecules was computed. Simulated diffusivities compared well with the experimental data. Thermodynamic treatment of sorption, diffusion and permeation processes was attempted based on the Flory–Huggins theory to explain the PV performances of the membranes.

The role of molecular interactions and interfaces in diffusion: Permeation through single-crystal and polycrystalline microporous membranes

In this second paper of a two part series, we investigate the implications of the interfacial phenomenon, caused by adsorbate-adsorbate interactions coupled with the difference in adsorbate density between the zeolite and the gas phase, upon benzene permeation through single-crystal and polycrystalline microporous NaX membranes. The high flux predicted for thin single-crystal membranes reveals that substantially enhanced flux should be expected in submicron films. Simulations also indicate that the standard local equilibrium assumption made for larger scale membranes is inapplicable at the submicron scale associated with nanometer size grains of thin and/or polycrystalline membranes. Apparent activation energies predicted for benzene permeation through NaX membranes via kinetic Monte Carlo (KMC) simulations are in good agreement with laboratory experiments. The simulations also uncover temperature-dependent flux pathways leading to non-Arrhenius behavior observed experimentally. The failure of the Darken approximation, especially in the presence of the interfacial phenomenon, leads to a substantial overprediction of the flux. Simulations of polycrystalline membranes suggest that this same interfacial phenomenon leads to resistance that can reduce flux by an order of a magnitude with only moderate polycrystallinity.

Observation of distinct, temperature dependent flux noise near bicrystal grain boundaries in YBa 2Cu 3O 7− x films

The characteristics of the magnetic flux noise in high temperature superconducting thin-films of yttrium-barium-copper-oxide (YBa 2Cu 3O 7) in the vicinity of artificial grain boundaries have been studied by means of a low critical temperature superconducting quantum interference device (SQUID) magnetometer. The measured power spectrum of the flux noise typically shows 1/ f-behaviour at low frequencies. This is, in particular, evident near the transition temperature. When measuring the flux noise near an artificial grain boundary we observe in selected samples additional Lorentzian type spectra in narrow temperature ranges indicative for well-defined barriers for flux hopping. Such spectra are not observed when measuring away from the grain boundary. The distinct fluctuators are seen in the 60–80 K interval corresponding to barrier heights of order 0.1–0.4 eV. If present in Josephson junctions patterned across such artificial grain boundaries, they could lead to serious limitations in the junction performance in certain temperature ranges.

Low temperature oxidation of SiGe in ozone: Ultrathin oxides

X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and spectral ellipsometry have been used to study ozone/atomic oxygen assisted direct oxidation of single crystalline SiGe. In situ Fourier transform infrared spectroscopy is used to monitor changes in the gas phase composition of O3/O2 gas mixtures. At all temperatures studied, 125–530 °C, XPS results indicate that Ge is incorporated into the growing oxide as GeO2. Increasing the oxidation temperature causes a decrease in the percentage of Ge incorporated in the growing oxide and an increase in the silicon content. Further, at oxidation temperatures of 400 °C and above, substantial Ge segregation is observed using SIMS. The change in composition is described using a temperature dependent flux model. Also, as the oxidation temperature increases, the oxide/SiGe interface becomes more abrupt with 1.9 monolayer (ML) of suboxide (Si+1, Si+2, Si+3) detected at 125 °C and 0.8 ML of suboxide at 530 °C. Based on thickness measurements from XPS and spectral ellipsometry, initial growth rates of 5 and ∼1 Å/min were achieved for ozone oxidation of Si0.85Ge0.15 at 530 and 125 °C, respectively. For the ultrathin regime (oxide thickness <25 Å), the effective activation energy for SiGe oxidation in 950 ppm of O3 is determined to be approximately 0.12 eV.

Reversible-magnetization derived London penetration depth of κ-(BEDT-TTF)2X for X=Cu(NCS)2 and X=Cu[N(CN)2]Br: No indication for unconventional superconductivity

In this paper we summarize our recent results on the magnetic penetration depth λ of the title organic superconductors. Using the London model, we determine the temperature dependence of λ from the slope of the linear M(lnH) dependence of the reversible isothermal magnetization. For both salts the in-plane London penetration depth shows only a weak T dependence at low temperature in excellent agreement with a conventional order parameter. Our results are incompatible with any of the unconventional pairing schemes proposed by Kanoda et al. and Le et al.. It is also the intention of this paper to address experimental difficulties in determining the magnetic penetration depth, in particular, the influence of extrinsic factors as temperature dependent flux pinning and demagnetization effects on the various experimental techniques.

LINUS cycle calculations including plasma transport and resistive flux loss

In the LINUS fusion reactor concept, a rotating liquid-metal liner is used for reversible mechanical compression of thermonuclear plasmas, where a vacuum-field ‘buffer’ zone is used between the plasma and wall to reduce transport losses. The concept has the particular attractiveness of operating in a self-sustaining cycle if the alpha-particle burn energy equals all losses incurred during the liner implosion and re-expansion. A minimum LINUS size can be derived by requiring this burn energy to equal the energy lost to Ohmic dissipation associated with flux diffusion into the linear material and mechanical losses associated with the liner fluid mechanics. A one-dimensional plasma transport and burn code, including incompressible liner dynamics with heat transfer and temperature-dependent flux diffusion in the liquid metal, is used to model various LINUS configurations. Corrections due to compressibility are discussed using a simplified analytical model. Numerical coefficients are derived for simple LINUS scaling laws. The particular case of plasma contact with the liquid metal is studied to determine the effect on LINUS performance. Some thermal contact with the liner before peak compression does not kill the plasma burn, and can thus be allowed in compression scenarios. The tendency of the field to remain imbedded in the hot plasma is, however, not sufficient to significantly retard diffusive flux loss into the liner and, thus, does not reduce the minimum LINUS size. Particular emphasis is placed on evaluating, from a physics standpoint, the relative merits of the lithium-plus-steel shell and the Pb-Li + inner-Li-layer configurations proposed in the literature.