Steep Temperature Research Articles

Analysis of electrokinetic properties of NiFe2O4 nanoparticles synthesized by DC thermal plasma route and its use in adsorption of humic substances

The paper reports the influence of electro-kinetic properties of nanoparticles of NiFe2O4 on the adsorption of humic substances (HS); with an aim of understanding their adsorption behavior. Crystalline single phase magnetic nanoparticles of NiFe2O4 are synthesized by gas phase condensation method using DC thermal plasma reactor. Due to the high temperature and steep temperature gradient this method helps in producing highly crystalline nanoparticles with sufficiently large number of surface states suitable for adsorption. These particles possessed high value of zeta potential, reducing their agglomeration and resulting into an enhanced adsorption capacity. However no significant influence of plasma parameters is observed on their zeta-potential and iso-electric values. The adsorption kinetics is studied and fitted to non-linear kinetic models and the best fit is observed for the pseudo first-order model. The analysis of adsorption isotherms suggests that the non-linear Langmuir model fits better as compared to Freundlich adsorption model. The adsorption of HS onto the nanoparticles is seen to depend on the pH value of the solution. A maximum adsorption of 5.2mg/g is obtained at pH value of 3.64. These results suggest that plasma synthesized nanoparticles of NiFe2O4 have high colloidal stability and can be employed as efficient adsorbents for the removal of humic substances from water sources.

Hot bubbles of planetary nebulae with hydrogen-deficient winds

Heat conduction has been found a plausible solution to explain discrepancies between expected and measured temperatures in hot bubbles of planetary nebulae (PNe). While the heat conduction process depends on the chemical composition, to date it has been exclusively studied for pure hydrogen plasmas in PNe. A smaller population of PNe show hydrogen-deficient and helium- and carbon-enriched surfaces surrounded by bubbles of the same composition; considerable differences are expected in physical properties of these objects in comparison to the pure hydrogen case. The aim of this study is to explore how a chemistry-dependent formulation of the heat conduction affects physical properties and how it affects the X-ray emission from PN bubbles of hydrogen-deficient stars. We extend the description of heat conduction in our radiation hydrodynamics code to work with any chemical composition. We then compare the bubble-formation process with a representative PN model using both the new and the old descriptions. We also compare differences in the resulting X-ray temperature and luminosity observables of the two descriptions. The improved equations show that the heat conduction in our representative model of a hydrogen-deficient PN is nearly as efficient with the chemistry-dependent description; a lower value on the diffusion coefficient is compensated by a slightly steeper temperature gradient. The bubble becomes somewhat hotter with the improved equations, but differences are otherwise minute. The observable properties of the bubble in terms of the X-ray temperature and luminosity are seemingly unaffected.

Boiling Visualization of Two Adjacent Impinging Jets on Hot Steel Plate

The simultaneous measurement between the boiling visualization and the boiling heat transfer characteristics by two adjacent impinging jets on hot steel plate was made by the experimental technique that has a function of high-temperature flat-plate heat flux gauge. The 22 K-type thermocouples were installed at 1 mm below the surface of flat-plate heat flux gauge. The 2-D inverse heat conduction was formulated to solve the surface temperature and heat flux. The boiling visualization was synchronized with a 4K video camera which was meaningful to understand complex boiling heat transfer phenomena. The heat flux gauge was uniformly heated up to 900°C by induction heating. The successive boiling images show where the nucleate boiling starts to occur on hot surface and the film boiling turns to be collapsed. The measured surface temperature and heat flux distribution agrees well with the corresponding boiling visualization: While heat transfer at the stagnation point shows a maximum heat flux, the interaction between two adjacent impinging jets exhibits a relative high heat flux and a steep temperature gradient until the end of boiling heat transfer at which single-phase convection occurs near 200°C.

Contrasting Soil Thermal Regimes in the Forest-Tundra Transition Near Nadym, West Siberia, Russia

Permafrost and varying land surface properties greatly complicate modelling of the thermal response of Arctic soils to climate change. The forest-tundra transition near Nadym in west Siberia provides an excellent study area in which to examine the contrasting thermal properties of soils in a forested ecosystem without permafrost and peatlands with permafrost. We investigated the effects of forest shading, snow cover and variable organic soil horizons in three common ecosystems of the forest-tundra transition zone. Based on the year-round temperature profile data, the most informative annual parameters were: (1) the sum of positive average daily temperatures at depths of 10 and 20 cm; (2) the maximum penetration depth of temperatures above 10 °C; and (3) the number of days with temperatures below 0 °C at a depth of 20 cm. The insulative effect of snow cover in winter was at least twice that of the shading and cooling effect of vegetation in summer. In areas with shallow permafrost, the presence of a thick organic horizon, with an extremely low thermal diffusivity, creates a very steep temperature gradient that limits heat penetration to the top of the permafrost in summer. Copyright © 2015 John Wiley & Sons, Ltd.

Scale-to-scale energy and enstrophy transport in two-dimensional Rayleigh–Taylor turbulence

We apply a recently developed filtering approach, i.e. filter-space technique (FST), to study the scale-to-scale transport of kinetic energy, thermal energy, and enstrophy in two-dimensional (2D) Rayleigh–Taylor (RT) turbulence. Although the scaling laws of the energy cascades in 2D RT systems follow the Bolgiano–Obukhov (BO59) scenario due to buoyancy forces, the kinetic energy is still found to be, on average, dynamically transferred to large scales by an inverse cascade, while both the mean thermal energy and the mean enstrophy move towards small scales by forward cascades. In particular, there is a reasonably extended range over which the transfer rate of thermal energy is scale-independent and equals the corresponding thermal dissipation rate at different times. This range functions similarly to the inertial range for the kinetic energy in the homogeneous and isotropic turbulence. Our results further show that at small scales the fluctuations of the three instantaneous local fluxes are highly asymmetrically distributed and there is a strong correlation between any two fluxes. These small-scale features are signatures of the mixing and dissipation of fluids with steep temperature gradients at the fluid interfaces.

Recognition of shear heating on a long‐lived major fault using Raman carbonaceous material thermometry: implications for strength and displacement history of the MTL, SW Japan

AbstractThe extent to which movement on major faults causes long term shear heating is a contentious issue and an important aspect in the debate about the strength of major faults in the crust. Comparing the results of experimental work on the kinetics of crystallization of carbonaceous material with results of thermal modeling show that the Raman carbonaceous material (CM) geothermometer is well suited to studying shear heating on geological time scales in suitable lithologies exposed around exhumed major fault zones. The Median Tectonic Line (MTL), SW Japan, is the largest on‐land fault in Japan with a length of > 800 km. Application of Raman CM thermometry to pelitic schist adjacent to the fault reveals the presence of a rise in peak temperature of around 60 °C over a distance of around 150 m perpendicular to the MTL fault plane. The spatial association of this thermal anomaly with the fault implies it is due to shear heating. Thermal modeling shows the recorded thermal anomaly and steep temperature gradient is compatible with very high rates of displacement over time scales of a few thousand years. However, the implied displacement rates lie outside those generally observed. An alternative explanation is that an originally broader thermal anomaly that developed during strike slip faulting was shortened due to the effects of normal faulting. Constraints on displacement rate, width of the original anomaly, duration of heating and peak temperature imply a coefficient of friction, μ, greater than 0.4.

Ionization balance of impurities in turbulent scrape-off layer plasmas I: local ionization-recombination equilibrium

In magnetized fusion devices, cross field impurity transport is often dominated by turbulence, in particular in the scrape-off layer. In these outer regions of the plasma, fluctuations of plasma parameters can be comparable to mean values, and the way ionization and recombination sources are treated in transport codes becomes questionnable. In fact, sources are calculated using the mean density and temperature values, with no account of fluctuations. In this work we investigate the modeling uncertainties introduced by this approximation, both qualitatively and quantitatively for the local ionization equilibrium. As a first step transport effects are neglected, and their role will be discussed in a companion paper. We show that temperature fluctuations shift the ionization balance towards lower temperatures, essentially because of the very steep temperature dependence of the ionization rate coefficients near the threshold. To reach this conclusion, a thorough analysis of the time scales involved is carried out, in order to devise a proper way of averaging over fluctuations. The effects are found to be substantial only for fairly large relative fluctuation levels for temperature, that is of the order of a few tens of percents.

Sensitivity of online coupled model to extreme pollution event over a mega city Delhi

Sensitivity of interactive Weather-Chemistry model has been examined to predict the air quality (1 and 3 days in advance) of Indian mega city Delhi during two identical extreme events of Diwali in 2012 and 2013. Analysis is conducted 3 days prior to 3 days later of Diwali day for both events to verify the rapid changes in fine particulate matter (PM2.5) due to widespread display of Diwali fireworks. The model successfully predicted the variability in PM2.5 during 2012 for the entire period of analysis with reasonable accuracy. Although model performed reasonably well until Diwali day in 2013 but it was unable to simulate rapid built up of PM2.5 (1500 μg−3 hourly average) during post Diwali day as it failed to capture unusual mid-night steep temperature gradient followed by a record lowering of boundary layer height. The predictability of the model and its limitation to micrometeorological processes are discussed in detail.

Electrostatic turbulence in electron temperature jumps of the solar atmosphere

We discuss the connection of the formation and properties of solar atmosphere transition region characterized by a steep electron temperature gradient with electrostatic turbulence, which provides a high effective electron collision frequency and a low thermal conductivity of the medium. A simple dependence of the noise electric field in the transition region on the effective collision frequency has been derived. Based on known experimental models of the solar atmosphere, we have estimated the height dependence of the thermal conductivity and the strength of the noise electric fields for a tube with a relatively weak magnetic field passing from the chromosphere into the corona.

MD Simulations of Nanomachining Monocrystalline Silicon

Molecular dynamics (MD) simulations of nanomachining of monocrystalline silicon were performed with the aid of Tersoff potential. The effects of machining conditions on the nature of heat distribution and corresponding phase transformation during nanomachining were investigated. It is clearly demonstrated that heat distribution shows a roughly concentric shape around the shear zone. A steep temperature gradient is observed in diamond tool and the highest temperature lies in chip. Stress distribution presents dual annular shapes, the highest compressive stress and tensile stress lay in shear zone and machined surface, respectively. Phase transformation mainly occurred in chips, shear zone and machined surface. Additionally, atoms in the machined surface are transformed from diamond cubic structure (Si-I) to β-tin structure (Si-II) and bct5-Si.

MD Simulation and Optimization Analysis for Nanoscale Material Removal Process

To understand the thermal effects on material removal at atomic level, molecular dynamics (MD) simulation and optimization method are performed with the aid of Morse, EAM and Tersoff potential. The heat distribution is showed in 3D images under various parameters. The simulation results reveal that the heat distribution is roughly concentric around the tool edge and a steep temperature gradient is observed between diamond tool and chip. During material removal process, there is a narrow region with high temperature in shear zone where most of heat generated due to plastic deformation of workpiece material, the high temperature extends from here to chip, diamond tool and workpiece, but the highest temperature lies in chip. Compared with low speed, a higher temperature region below the tool edge implied a larger shear stress is built up in a local region and a rougher machined surface is generated at high cutting speed.

Dielectric relaxation in pure and Co-doped Bi12GeO20 single crystals

We report the results of investigation of dielectric spectroscopy study of single crystals of [Formula: see text] and [Formula: see text] doped with Co nanoparticles. The complex dielectric constant was measured in the temperature interval from 5 to 450 K and frequencies from 1 Hz to 1 MHz. The electrical conductivity of both samples was thermally activated with potential barriers of 0.55 eV and 0.59 eV, respectively. Doped samples had bigger complex dielectric constants and [Formula: see text] exhibited slightly steeper temperature dependence than in the pure sample. The dielectric relaxation was observed in pure and doped single crystals and relaxation frequencies showed similar activation energies as electrical conductivities.

Simultaneous multi-scale and multi-instrument observations of Saturn’s aurorae during the 2013 observing campaign

On 21 April 2013, during a co-ordinated Saturn auroral observing campaign, the northern and southern poles of the planet were observed from the Earth using the NASA Infrared Telescope Facility (IRTF), Keck, and Hubble Space Telescope (HST) simultaneously with the Cassini infrared, visible, and ultraviolet remote sensing instruments. We present simultaneous multi-scale and multi-wavelength analysis of the morphology of auroral emissions at Saturn. The visible main auroral emission vary between ∼2 and 10kR on timescales of minutes and across spatial scales of down to ∼14km on the planet. The H2 Far Ultraviolet (FUV) brightness varies by a factor of ∼10, from ∼4–40kR, over timescales of 1min and spatial scales of 720km. H3+ infrared emissions vary less than the H2 emissions, from ∼5–10μWm−2sr−1, over similar spatial scales (∼300km) and timescales of a few seconds to a few hours. The fine-scale temporal and spatial features seen in the main oval show that complex structures are present even during quiet solar wind conditions. Diffuse ultraviolet emissions southward of the southern midnight main oval that are not seen in the infrared, implying a steep temperature gradient of ∼50K over 2–4° latitude equatorward of the main oval. Dynamics on scales of ∼100km at the poles are revealed by lower spatial resolution observations, the morphologies of which are partly consistent with overlapping local-time fixed and co-rotating current systems. We also present the first direct comparison of simultaneous infrared, visible, and ultraviolet auroral emissions at Saturn. Finally, the main auroral emissions are found to be approximately co-located in the midnight sector, forming an arc with a width of ∼0.5–1°, at 72–74° southern latitude, moving slightly equatorward with increasing local-time.

WHAT DO IRIS OBSERVATIONS OF Mg II k TELL US ABOUT THE SOLAR PLAGE CHROMOSPHERE?

We analyze observations from the Interface Region Imaging Spectrograph of the Mg II k line, the Mg II UV subordinate lines, and the O I 135.6 nm line to better understand the solar plage chromosphere. We also make comparisons with observations from the Swedish 1 m Solar Telescope of the H{\alpha} line, the Ca II 8542 line, and Solar Dynamics Observatory/Atmospheric Imaging Assembly observations of the coronal 19.3 nm line. To understand the observed Mg II profiles, we compare these observations to the results of numerical experiments. The single-peaked or flat-topped Mg II k profiles found in plage imply a transition region at a high column mass and a hot and dense chromosphere of about 6500 K. This scenario is supported by the observed large-scale correlation between moss brightness and filled-in profiles with very little or absent self-reversal. The large wing width found in plage also implies a hot and dense chromosphere with a steep chromospheric temperature rise. The absence of emission in the Mg II subordinate lines constrain the chromospheric temperature and the height of the temperature rise while the width of the O I 135.6 nm line sets a limit to the non-thermal velocities to around 7 km/s.

Densification and Microhardness of Spark Plasma Sintered ZrB2+SiC Nano-Composites

Ultra-High-Temperature Ceramics (UHTCs) such as ZrB2 and HfB2 with incorporation of SiC nanofiller are useful as structural materials for applications in propulsion and thermal protection systems such as turbine-engine hot section components, leading edge of hypersonic vehicles, where extremely high heat fluxes generate very high temperatures and steep temperature gradients [1]. Spark plasma sintering (SPS) technique is used for densifying the UHTCs under the influence of uniaxial pressure and pulsed direct current [2]. Here, we study the densification, grain growth, and microhardness of ZrB2 nanocomposites with 15% and 20% SiC consolidated using SPS.

Chromospheric seismology above sunspot umbrae

The acoustic resonator is an important model for explaining the three-minute oscillations in the chromosphere above sunspot umbrae. The steep temperature gradients at the photosphere and transition region provide the cavity for the acoustic resonator, which allows waves to be both partially transmitted and partially reflected. In this paper, a new method of estimating the size and temperature profile of the chromospheric cavity above a sunspot umbra is developed. The magnetic field above umbrae is modelled numerically in 1.5D with slow magnetoacoustic wave trains travelling along magnetic fieldlines. Resonances are driven by applying the random noise of three different colours---white, pink and brown---as small velocity perturbations to the upper convection zone. Energy escapes the resonating cavity and generates wave trains moving into the corona. Line of sight (LOS) integration is also performed to determine the observable spectra through SDO/AIA. The numerical results show that the gradient of the coronal spectra is directly correlated with the chromosperic temperature configuration. As the chromospheric cavity size increases, the spectral gradient becomes shallower. When LOS integrations is performed, the resulting spectra demonstrate a broadband of excited frequencies that is correlated with the chromospheric cavity size. The broadband of excited frequencies becomes narrower as the chromospheric cavity size increases. These two results provide a potentially useful diagnostic for the chromospheric temperature profile by considering coronal velocity oscillations.

A revised paleomagnetic pole from the mid-Neoproterozoic Liantuo Formation in the Yangtze block and its paleogeographic implications

The paleogeographic relationship between South China and Australia during the Neoproterozoic is still hotly debated. Although a series of propositions for the close proximity between South China and Australia are suggested, their relative positions are varied. To better constrain the paleoposition of South China, we have carried out a new paleomagnetic study from the Neoproterozoic Liantuo Formation in two localities, Yichang and Changyang, in Hubei province, China. Stepwise thermal demagnetization reveals that a late Cretaceous remagnetization (component ‘L’, 350–500°C) was removed from most of the samples. A medium component (‘M’) isolated mainly between 350°C and 600°C, but in some cases with an upper limit of 640°C, yields a mean direction of Dg=59.5°, Ig=64.6°, α95=3.7° in situ; and Ds=81.6°, Is=61.5°, α95=4.1° after tilt correction. A steeper medium-high temperature component, ‘B’, was separated from 71 specimens with almost the same temperature range of ‘M’ (Dg=341.3°, Ig=78°, α95=5.6° in situ; and Ds=78.3°, Is=83.8°, α95=6.9° after tilt correction) and is similar to the ‘B’ component obtained by Evans et al. (2000). In addition, a high temperature component ‘H’ (Ds=102.5°, Is=62.4°, α95=3.7° after tilt correction) with both reversed and normal polarity was revealed up to 690°C, yielding a pole at 12.7°N, 157.4°E, with dp/dm=4.5°/5.8°. A change of magnetic polarity, with 5 magnetozones defined, is revealed in our sampling section, in which a reversed-normal polarity succession in the upper part of the section may be correlative with that obtained in a section 6.5km away by Evans et al. (2000), demonstrating the primary nature of the ‘H’ component. Combining our results with those of Evans et al. (2000) yields a grand mean pole from the upper Liantuo Formation at 13.2°N and 155.2°E with A95=5.3°. Considering the distribution of our sampling sites, and together with the results of a detailed recent SIMS U–Pb zircon study on the Liantuo Formation in the Three Gorges area (Lan et al., 2015), an age of 720Ma was assigned to component ‘H’ and its corresponding pole. After carefully reevaluating the Neoproterozoic apparent polar wander paths (APWP) from Australia and South China, we suggest that the South China block was located to the northwest of Northern Australia during the Ediacaran period. Subsequently, the South China block moved to the northwest of Western Australia through counter-clockwise rotation of the Northern Australian craton at the later time of 550Ma, or in the middle Cambrian.

Experimental study of thermo-mechanical behavior of SiC composite tubing under high temperature gradient using solid surrogate

Nuclear grade silicon carbide fiber (SiCf) reinforced silicon carbide matrix (SiCm) composite is a promising candidate material for accident tolerance fuel (ATF) cladding. A major challenge is ensuring the mechanical robustness of the ceramic cladding under accident conditions. In this work the high temperature mechanical response of a SiCf–SiCm composite tubing is studied using a novel thermo-mechanical test method. A solid surrogate tube is placed within and bonded to the SiCf–SiCm sample tube using a ceramic adhesive. The bonded tube pair is heated from the center using a ceramic glower. During testing, the outer surface temperature of the SiC sample tube rises up to 1274 K, and a steep temperature gradient develops through the thickness of the tube pair. Due to CTE mismatch and the temperature gradient, the solid surrogate tube induces high tensile stress in the SiC sample. During testing, 3D digital image correlation (DIC) method is used to map the strains on the outer surface of the SiC-composite, and acoustic emissions (AE) are monitored to detect the onset and progress of material damage. The thermo-mechanical behavior of SiC-composite sample is compared with that of monolithic SiC samples. Finite element models are developed to estimate stress–strain distribution within the tube assembly. Model predicted surface strain matches the measured surface strain using the DIC method. AE activities indicated a progressive damage process for SiCf–SiCm composite samples. For the composites tested in this study, the threshold mechanical hoop strain for matrix micro-cracking to initiate in SiCf–SiCm sample is found to be ∼300 microstrain.

Triangular Shell Heat Transfer Element for the Thermal Analysis of Nonuniformly Heated Structures

AbstractThis paper presents a triangular shell heat transfer element that is used to simulate the thermal response of nonuniformly heated structures. The formulation uses a combination of finite-element and control volume techniques to arrive at a layered shell element that is used to solve the 3D conduction heat transfer problem in a highly efficient manner. This paper considers a 3-node linear element and a 6-node quadratic element. The element formulation is verified against a converged continuum heat transfer model for a thick steel plate exposed to a concentrated heat flux, a thick steel pipe exposed to a concentrated heat flux, and a concrete slab exposed to a localized fire. The verification studies demonstrate that the linear and quadratic elements consistently converge to the continuum model and require a fraction of the computational cost. The verification study involving the concrete slab exposed to a localized fire demonstrates that the formulation can readily handle steep temperature gradient...

THE FORMATION OFIRISDIAGNOSTICS. IV. THE Mg ii TRIPLET LINES AS A NEW DIAGNOSTIC FOR LOWER CHROMOSPHERIC HEATING

A triplet of subordinate lines of Mg II exists in the region around the h&k lines. In solar spectra these lines are seen mostly in absorption, but in some cases can become emission lines. The aim of this work is to study the formation of this triplet, and investigate any diagnostic value they can bring. Using 3D radiative magnetohydrodynamic simulations of quiet Sun and flaring flux emergence, we synthesize spectra and investigate how spectral features respond to the underlying atmosphere. We find that emission in the lines is rare and is typically caused by a steep temperature increase in the lower chromosphere (above 1500 K, with electron densities above 10$^{18}$ m$^{-3}$). In both simulations the lines are sensitive to temperature increases taking place at column masses >= 5e-4 g cm$^{-2}$. Additional information can also be inferred from the peak-to-wing ratio and shape of the line profiles. Using observations from NASA's Interface Region Imaging Spectrograph we find both absorption and emission line profiles with similar shapes to the synthetic spectra, which suggests that these lines represent a useful diagnostic that complements the MgII h&k lines.