Sharp Thermal Gradients Research Articles

Pulsed dehydration and garnet growth during subduction revealed by zoned garnet geochronology and thermodynamic modeling, Sifnos, Greece

We use coupled zoned geochronology and thermodynamic modeling of garnet to elucidate the nature and scale of metamorphic dehydration during Eocene subduction of a quartzofeldspathic lithology from Sifnos, Greece. Two large garnet porphyroblasts were microdrilled to sample concentric growth zones, and these were dated using Sm–Nd geochronology. To put results in a geodynamic context and reveal the causes and consequences of garnet growth, we constructed thermodynamic forward models for a series of prescribed pressure–temperature (P–T) paths. Our data reveal three distinct phases of garnet growth: initial growth at 53.4±2.6 Ma (∼0.8 GPa and ∼300 °C), followed by a period of very limited growth until a second phase, at 47.22±0.36 Ma, and then a major pulse of growth, responsible for the majority of the final garnet volume, at 44.96±0.53 Ma (2.06–2.19 GPa and 490–550 °C). This suggests a >2 order of magnitude acceleration in volumetric growth rate from crystal core to rim, with the final growth pulse occurring rapidly (<0.8 My), during a period of nearly isobaric heating at >75 °C/My. This final pulse was accompanied by net bulk rock dehydration of ∼0.5 wt.%. Rapid heating during early stages of exhumation in the subduction channel, or by sharp thermal gradients related to slab-mantle coupling could be causes for this pulsed metamorphism and dehydration. The garnet data thus record a concentrated pulse of dehydration and heating during the otherwise slow and continuous process of subduction.

Optimization of microwave-assisted synthesis of cyclodextrin nanosponges using response surface methodology

The reaction of polymerization of cyclodextrins with suitable polyfunctional agents leads to highly cross linked porous structures, referred to as cyclodextrin nanosponges. The conventional heating approach for the synthesis of nanosponges can lead to nonuniform reaction conditions caused by sharp thermal gradients in the bulk solution. In this work, we present a facile method for the synthesis of cyclodextrin nanosponges by microwave irradiation with significant reduction in reaction time. Response surface methodology and Box–Behnken design were used for the optimization of the process parameters including microwave power level (A), reaction time (B) and stirring speed (C). Two dependent variables practical yield and particle size were measured as responses. Mathematical equations and response surface plots were used to relate the dependent variables with independent variables. The optimization model predicted a yield of 96.5287 % and particle size of about 152.355 nm with A, B and C levels of 508.22, 83.55 and 1,471.55 respectively. The observed responses of the optimized process were in close agreement with the predicted values. Three confirmation batches of cyclodextrin nanosponges were synthesized under optimized conditions. The Fourier transformed infrared spectra of nanosponges showed a characteristic peak of the carbonate group at around 1,750 cm−1 which confirms the formation of nanosponges. TGA revealed the stability of obtained nanosponges up to 325 °C. The nanosponges obtained by microwave irradiation exhibited a higher degree of crystallinity. Dimension of the crystal lattice was found to be equal to 0.70 nm. TEM image confirmed the spherical shape and the particle size range of about 153 nm. Compared to conventional heating method, microwave method resulted in four folds reduction in reaction time and provided with particles of homogeneous size distribution and uniform crystallinity.

Low Core-Mantle Boundary Temperature Inferred from the Solidus of Pyrolite

The melting temperature of Earth's mantle provides key constraints on the thermal structures of both the mantle and the core. Through high-pressure experiments and three-dimensional x-ray microtomographic imaging, we showed that the solidus temperature of a primitive (pyrolitic) mantle is as low as 3570 ± 200 kelvin at pressures expected near the boundary between the mantle and the outer core. Because the lowermost mantle is not globally molten, this provides an upper bound of the temperature at the core-mantle boundary (T(CMB)). Such remarkably low T(CMB) implies that the post-perovskite phase is present in wide areas of the lowermost mantle. The low T(CMB) also requires that the melting temperature of the outer core is depressed largely by impurities such as hydrogen.

Thermo-mechanical response of poly(methyl methacrylate) (PMMA) large volumes exposed to time-dependent environmental conditions

Low thermal conductivity and elevated absorbance of large bulky volumes of poly(methyl methacrylate) (PMMA) exposed to moderately aggressive environmental conditions may cooperate to determine critical mechanical conditions, kindling unexpected high thermal stresses values which lead the material to failure. From the engineering point of view, this can be explained as the result of two concomitant phenomena which activate a cascade of events: very sharp thermal gradients engendered by transient thermal processes induced by cyclic environmental conditions, combined with significant bulk heat generation due to the high thermal inertia of massive PMMA volumes, in turn aggravating the steepness of the thermal gradients, may in fact ingenerate severe stress regimes, potentially undermining the structural stability of the material. Moving from these considerations, the present study is aimed to investigate possible rupture of PMMA blocks experiencing heating processes as a consequence of their exposure to outdoor cyclic environmental conditions. The problem is approached by means of both rigorous analytical arguments and the Finite Element based numerical methods, finally exploiting the theoretical outcomes to formulate a hypothesis which might explain the still unclear phenomenon of the sudden breaking of the PMMA structure, named Huge Wine Glass and designed by the world famous Japanese architect Toyo Ito, which occurred in Pescara (Italy) in 2009.

Nanoparticle‐Driven Orientation Transition and Soft‐Shear Alignment in Diblock Copolymer Films via Dynamic Thermal Gradient Field

Sharp dynamic thermal gradient (∇T ≈ 45 °C mm(-1)) field-driven assembly of cylinder-forming block copolymer (c-BCP) films filled with PS-coated gold nanoparticles (AuNPs; dNP ≈ 3.6 nm, φNP ≈ 0-0.1) is studied. The influence of increasing AuNP loading fraction on dispersion and assembly of AuNPs within c-BCP (PS-PMMA) films is investigated via both static and dynamic thermal gradient fields. With φNP increasing, a sharp transition from vertical to random in-plane horizontal cylinder orientation is observed due to enrichment of AuNPs at the substrate side and favorable interaction of PMMA chains with gold cores. Furthermore, a detachable capping elastomer layer can self-align these random oriented PMMA microdomains into unidirectional hybrid AuNP/c-BCP nanolines, quantified with an alignment order parameter, S.

Small eddies observed in Lake Superior using SAR and sea surface temperature imagery

Making use of the fine resolution of satellite SAR imagery, we observe small eddies during the spring and summer months in several locations in Lake Superior. During these months there is a thermal gradient between warmer nearshore waters and colder offshore waters which enhances cyclonic coastal currents. Using spaceborne SAR imagery from the European Space Agency's ERS-1 and ERS-2 missions from 1992 to 1998, we observe small eddies, identifying and mapping basic eddy characteristics including diameter, location, and rotational sense. In total, 45 eddies were located, of which 41 were cyclonic and 4 anticyclonic. Average diameter was 9.8km and average distance to shore was 8.1km. Based on sea surface temperature data from AVHRR, the eddies are located within the region of sharp thermal gradients of order 3–5°C per 3km. Spatial and temporal coverage was uneven, however, more eddies were seen in SAR images taken in late summer along the southern and eastern shores as well as areas where the boundary current interacts with topographic features including islands and promontories.

Tuning Molecular Relaxation for Vertical Orientation in Cylindrical Block Copolymer Films via Sharp Dynamic Zone Annealing

Fabricating vertically ordered and etchable high aspect ratio nanodomains of block copolymer (BCP) thin films on diverse substrates via continuous processing dynamic cold zone annealing (CZA) is particularly attractive for nanomanufacturing of next-generation electronics. Previously, we reported dynamic CZA studies with a shallow thermal gradient (maximum ∇T ∼ 14 °C/mm) that produced only BCP cylinders oriented parallel to substrate. Here, we report a CZA utilizing a dynamic sharp thermal gradient (∇T ∼ 45 °C/mm) (i.e., CZA-S). This method allows for production of etchable and vertically oriented cylindrical domains of poly(styrene-b-methyl methacrylate) in 100–1000 nm thick films on low thermal conductivity rigid (quartz) and flexible (PDMS, Kapton) substrates. Competing substrate wetting interactions dominate BCP orientation in films below 100 nm while broadening of the thermal gradient profile in films thicker than 1000 nm leads to loss of vertical orientation. An optimal dynamic sweep rate (∼5 μm/s) p...

Efficient analysis of transient heat transfer problems exhibiting sharp thermal gradients

In this paper, heat transfer problems with sharp spatial gradients are analyzed using the Generalized Finite Element Method with global-local enrichment functions (GFEM gl). With this approach, scale-bridging enrichment functions are generated on the fly, providing specially-tailored enrichment functions for the problem to be analyzed with no a-priori knowledge of the exact solution. In this work, a decomposition of the linear system of equations is formulated for both steady-state and transient heat transfer problems, allowing for a much more computationally efficient analysis of the problems of interest. With this algorithm, only a small portion of the global system of equations, i.e., the hierarchically added enrichments, need to be re-computed for each loading configuration or time-step. Numerical studies confirm that the condensation scheme does not impact the solution quality, while allowing for more computationally efficient simulations when large problems are considered. We also extend the GFEM gl to allow for the use of hexahedral elements in the global domain, while still using tetrahedral elements in the local domain, to allow for automatic localized mesh refinement without the use of constrained approximations. Simulations are run with the use of linear and quadratic hexahedral and tetrahedral elements in the global domain. Convergence studies indicate that the use of a different partition of unity (PoU) in the global (hexahedral elements) and local (tetrahedral elements) domains does not adversely impact the solution quality.

An improved nonintrusive global–local approach for sharp thermal gradients in a standard FEA platform

SUMMARYSeveral classes of important engineering problems—in this case, problems exhibiting sharp thermal gradients—have solution features spanning multiple spatial scales and, therefore, necessitate advancedhpfinite element discretizations. Althoughhp‐FEM is unavailable off‐the‐shelf in many predominant commercial analysis software packages, the authors herein propose a novel method to introduce these capabilities via a generalized FEM nonintrusively in a standard finite element analysis (FEA) platform. The methodology is demonstrated on two verification problems as well as a representative, industrial‐scale problem. Numerical results show that the techniques utilized allow for accurate resolution of localized thermal features on structural‐scale meshes withouthp‐adaptivity or the ability to account for complex and very localized loads in the FEA code itself. This methodology enables the user to take advantage of all the benefits of bothhp‐FEM discretizations and the appealing features of many available computer‐aided engineering /FEA software packages to obtain optimal convergence for challenging multiscale problems. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Generalized finite element analysis of three-dimensional heat transfer problems exhibiting sharp thermal gradients

In this paper, heat transfer problems exhibiting sharp thermal gradients are analyzed using the classical and generalized finite element methods. The effect of solution roughness on the ability of the methods to obtain accurate approximations is investigated. Convergence studies show that low order (linear and quadratic) elements require strongly refined meshes for acceptable accuracy. We propose a generalized FEM with global–local enrichments for the class of problems investigated in the paper. In this procedure, a global solution space defined on a coarse mesh is enriched through the partition of unity framework of the generalized FEM with solutions of local boundary value problems. The local problems are defined using the same procedure as in the global–local FEM, where boundary conditions are provided by a coarse scale global solution. Coarse, uniform, global meshes are acceptable even at regions with thermal spikes that are orders of magnitude smaller than the element size. Convergence on these discretizations was achieved even when no or limited convergence was observed in the local problems. Two approaches are proposed to improve the boundary conditions prescribed on local problems and their convergence. The use of the corresponding improved local solutions as enrichments for the global problem extends the range of target error level for the enriched global problem. The two-way information transfer provided by the proposed generalized FEM is appealing to several classes of problems, especially those involving multiple spatial scales. The proposed methodology brings the benefits of generalized FEM to problems where limited or no information about the solution is known a priori.

LARGE‐SCALE DIVERSITY PATTERNS OF CEPHALOPODS IN THE ATLANTIC OPEN OCEAN AND DEEP SEA

Although the oceans cover 70% of the Earth's surface and the open ocean is by far the largest ecosystem on the planet, our knowledge regarding diversity patterns of pelagic fauna is very scarce. Here, we examine large-scale latitudinal and depth-related patterns of pelagic cephalopod richness in the Atlantic Ocean in relation to ambient thermal and productive energy availability. Diversity, across 17 biogeochemical regions in the open ocean, does not decline monotonically with latitude, but is positively correlated to the availability of oceanic resources. Mean net primary productivity (NPP), determined from ocean color satellite imagery, explains 37% of the variance in species richness. Outside the poles, the range in NPP explains over 40% of the variability. This suggests that cephalopods are well adapted to the spatial patchiness and seasonality of open-ocean resources. Pelagic richness is also correlated to sea surface temperature, with maximum richness occurring around 15 degrees C and decreasing with both colder and warmer temperatures. Both pelagic and benthos-associated diversities decline sharply from sublittoral and epipelagic regions to the slope and bathypelagic habitats and then steadily to abyssal depths. Thus, higher energy availability at shallow depths seems to promote diversification rates. This strong depth-related trend in diversity also emphasizes the greater influence of the sharp vertical thermal gradient than the smoother and more seasonal horizontal (latitudinal) one on marine diversity.

One-step catalyst-free generation of carbon nanospheres via laser-induced pyrolysis of anthracene

Carbon nanospheres with diameters between 100 and 400 nm have been successfully synthesized via low-power laser-assisted pyrolysis of anthracene in a nitrogen atmosphere. The developed facile route yields homogeneous nanoparticles and requires no supplementary carbon feedstock or catalyst. The sharp thermal gradient afforded by the laser results in two kinds of carbon products that differ in crystallinity and mean particle size. Our detailed findings point to the carbon nanospheres being comprised of small-unclosed aromatic layers that are connected together by simple organic linkers. C–H bonds in the anthracene molecules are partially broken by the laser beam energy, and as the newly created large radicals aggregate, carbon nanospheres are formed.

Root zone of the sheeted dike complex in the Oman ophiolite

In the Oman ophiolite crustal section, a contact zone between the gabbro unit and the volcanics and diabase sheeted dikes, called the root zone of the sheeted dike complex, has been recently mapped at a fine scale in a selected area. The Oman ophiolite is derived from a fast spreading ridge which had a melt lens located between the main gabbro unit and the root zone of the sheeted dike complex. With a few exceptions accounted for, this horizon has a fairly constant thickness, ∼100 m, and a crude internal pseudo‐stratigraphy. At the base of the root zone are isotropic ophitic gabbros interpreted as a thermal boundary layer. This layer is transitional between the magmatic system of the melt lens, convecting at 1200°C, and a high‐temperature (&lt;1100°C) hydrothermal system, convecting within the root zone. Above this level, the isotropic gabbros have been, locally, largely molten due to an influx of seawater, at ∼1100°C, thus generating varitextured ophitic and pegmatitic gabbros. These latter gabbros constitute the upper part of the root zone and are associated with trondjhemitic intrusions as screens in the lower sheeted dikes. Diorites and trondjhemites were also generated by hydrous melting, at temperatures below 1000°C. The whole root zone is a domain of very sharp average thermal gradient (∼7°C/m). At the top of the root zone, a new thermal boundary layer, with diabase dikes hydrated in amphibolite facies conditions, separates the preceding high‐temperature convective system from the well‐known greenschist facies (&lt;450°C) hydrothermal system operating throughout the sheeted dike complex, up to the seafloor. The isotropic gabbros near the base of the root zone are intruded by protodikes with distinctive microgranular margins and an ophitic center. Protodike swarms are exceptional because, intruding a medium at ∼1100°C, they are largely destroyed by dike‐in‐dike intrusions and by hydrous melting. However, they demonstrate that this zone was generated by melt conduits issued from the underlying melt lens. Each dike of the sheeted dike complex is thus fed by one protodike. As this zone has been recently drilled by IODP in the eastern Pacific Ocean, a brief comparison is proposed.

Thermal regime of the NW shelf of the Gulf of Mexico. Part A: Thermal and pressure fields

Abstract The thermal field of the Gulf of Mexico (GoM) is analyzed from a comprehensive temperature-depth database of about 8500 bottom hole temperatures and reservoir temperatures. Our stochastic analysis reveals a widespread, systematic sharp thermal gradient increase between 2500 and 4000 m. The analysis of the pressure regime indicates a systematic correlation between the pressure and temperature fields.

A Lagrangian Objective Analysis Technique for Assimilating In Situ Observations with Multiple-Radar-Derived Airflow

Abstract A new Lagrangian analysis technique is developed to assimilate in situ boundary layer measurements using multi-Doppler-derived wind fields, providing output fields of water vapor mixing ratio, potential temperature, and virtual potential temperature from which the lifting condensation level (LCL) and relative humidity (RH) fields are derived. The Lagrangian analysis employs a continuity principle to bidirectionally distribute observed values of conservative variables with the 3D, evolving boundary layer airflow, followed by temporal and spatial interpolation to an analysis grid. Cloud is inferred at any grid point whose height z &amp;gt; zLCL or equivalently where RH ≥ 100%. Lagrangian analysis of the cumulus field is placed in the context of gridded analyses of visible satellite imagery and photogrammetric cloud-base area analyses. Brief illustrative examples of boundary layer morphology derived with the Lagrangian analysis are presented based on data collected during the International H2O Project (IHOP): 1) a dryline on 22 May 2002; 2) a cold-frontal–dryline “triple point” intersection on 24 May 2002. The Lagrangian analysis preserves the sharp thermal gradients across the cold front and drylines and reveals the presence of undulations and plumes of water vapor mixing ratio and virtual potential temperature associated with deep penetrative updraft cells and convective roll circulations. Derived cloud fields are consistent with satellite-inferred cloud cover and cloud-base locations.

LGM Summer Climate on the Southern Margin of the Laurentide Ice Sheet: Wet or Dry?*

Abstract Regional climate simulations are conducted using the Polar fifth-generation Pennsylvania State University (PSU)–NCAR Mesoscale Model (MM5) with a 60-km horizontal resolution domain over North America to explore the summer climate of the Last Glacial Maximum (LGM: 21 000 calendar years ago), when much of the continent was covered by the Laurentide Ice Sheet (LIS). Output from a tailored NCAR Community Climate Model version 3 (CCM3) simulation of the LGM climate is used to provide the initial and lateral boundary conditions for Polar MM5. LGM boundary conditions include continental ice sheets, appropriate orbital forcing, reduced CO2 concentration, paleovegetation, modified sea surface temperatures, and lowered sea level. The simulated LGM summer climate is characterized by a pronounced low-level thermal gradient along the southern margin of the LIS resulting from the juxtaposition of the cold ice sheet and adjacent warm ice-free land surface. This sharp thermal gradient anchors the midtropospheric jet stream and facilitates the development of synoptic cyclones that track over the ice sheet, some of which produce copious liquid precipitation along and south of the LIS terminus. Precipitation on the southern margin is orographically enhanced as moist southerly low-level flow (resembling a contemporary Great Plains low-level jet configuration) in advance of the cyclone is drawn up the ice sheet slope. Composites of wet and dry periods on the LIS southern margin illustrate two distinctly different atmospheric flow regimes. Given the episodic nature of the summer rain events, it may be possible to reconcile the model depiction of wet conditions on the LIS southern margin during the LGM summer with the widely accepted interpretation of aridity across the Great Plains based on geological proxy evidence.

Prospects of Hybrid Recording Materials

Thermally assisted or hybrid magnetic recording system is expected to be a data storage device with high areal density beyond 1 Tb/in2. Several features of the hybrid recording are treated, such as high resolution recording capability by sharp thermal gradient, and pico-second order high-speed magnetization reversal. Since the hybrid recording solves the head field limit problem, the high magnetic anisotropy (Ku) materials with order of 107 erg/cm3 are used for the hybrid recording media. We discuss the candidates of the high Ku materials, FePt and CoPt L10 ordered alloy, Tb-Fe-Co amorphous, and Co/Pd multilayer thin films.

Relationship between spatial distribution of the Patagonian stock of Argentine anchovy, Engraulis anchoita, and sea temperatures during late spring to early summer

The relative abundance of adult Argentine anchovies (Engraulis anchoita) and the thermal structure of the sea between 41° and 45°30′S during four research surveys were compared. Acoustic data were collected while making parallel transects along which CTD (conductivity, temperature, depth) stations were regularly distributed. Anchovy abundance was related to both the sea surface temperature and the stratification of the water column, as classified according to the φ parameter of stability (Simpson, 1981). Regarding the whole water column, the sea temperatures where adult anchovies were recorded ranged from 8.5 to 16.5°C, but anchovy echo traces from waters above 14°C were obtained mainly at night. The sea surface temperature bounds for anchovy distribution were 11 and 17°C, with a preferential range between 12.5 and 16°C, but no absolute value of sea surface temperature was found to be most favourable for the species. The highest fish abundances were related to sharp thermal gradients, either horizontally recorded in frontal zones (≈ 0.02°C or more per km) or vertically associated to the occurrence of a thermocline (stability &gt; 10 J m–3). Sea fronts seemed to be the axes for the distribution of anchovy shoals, and the annual changes in the positions of the fronts seemed to be followed by analogous changes in the locations of the main anchovy concentrations. Within thermally stratified areas and during the daytime, anchovies apparently preferred the thermocline layer or the layer immediately above that, although a few fish shoals were located below the thermocline.

Influence of beam geometry and tissue boundaries on thermal ultrasonic lesions

The size and shape of thermal lesions induced by high-intensity focused ultrasound (HIFU) are key factors in treating disease. Studies are being conducted to determine how HIFU beams and exposures can be designed to produce useful lesion attributes in specific target tissues. The studies include radially asymmetric beams and tissues with different acoustic absorption coefficients; sharp thermal gradients at such boundaries can significantly affect lesion geometry. Ocular tumor therapy uses asymmetric beams that produce lesions with ellipsoidal cross sections to facilitate treatments with closely packed lesion matrices. Three-dimensional simulations have analyzed heat patterns and lesions induced by these beams in irregular tumors whose geometry is derived from clinical serial-plane scans using 50-MHz diagnostic ultrasound. The simulations documented difficulties in treating boundary regions adjacent to the waterlike vitreous humor. Lesion results were consistent with in vitro and in vivo animal experiments. A related study involves an annular-array therapy system with a central rectangular aperture, housing a diagnostic array. The system (Spectrasonics, Inc.) is intended for treatments in various organs, including liver. Since the therapy beams are not radially symmetric, 3-D simulations and initial animal experiments have examined the asymmetry in induced temperature patterns and lesions. These investigations also addressed spurious grating peaks in axial intensity.

Technical Note Parasitic thermovoltage in the multithermocouple probe, its explanation and elimination

The parasitic thermovoltage can influence the temperature measurement during hyperthermia using the miniature multithermocouple probes with a common wire. It was noticed that, when a thermocouple junction is placed in a sharp thermal gradient, a parasitic thermovoltage is added to the voltage of all remaining thermocouples situated in the direction to the tip of the probe. This article gives the theoretical explanation, experimental verification, and practical elimination of this phenomenon.