Direction Of Thermal Gradient Research Articles

Valley-locked thermospin effect in silicene and germanene with asymmetric magnetic field induced by ferromagnetic proximity effect

Silicene and germanene, as graphenelike materials with observable spin-orbit couplings and two distinctive valleys, have potential applications in future low-dissipation spintronics and valleytronics. We here propose a magnetic system of silicene or germanene intercalated between two ferromagetic (FM) dielectric layers, and find that the system with a proximity-induced asymmetric magnetic field supports an attractive phenomenon named the valley-locked spin-dependent Seebeck effect (VL-SSE) driven by a thermal gradient. The VL-SSE indicates that the carries from only one valley could be thermally excited, with opposite spin polarization counterpropagating along the thermal gradient direction, while nearly no carrier from the other insulating valley is excited due to the relatively wide band gap. It is also illustrated that the VL-SSE here does not survive in the usual FM or anti-FM systems, and can be destroyed by the overlarge temperature broadening. Moreover, we prove that the signal for VL-SSE can be weakened gradually with the enhancement of the local interlayer electric field, and be strengthened lineally by increasing the source-drain temperature difference in a caloritronic field effect transistor. Further calculations indicate that the VL-SSE is robust against many perturbations, including the global and local Fermi levels as well as the magnetic strength. These findings about the valley-locked thermospin effect provide a nontrivial and convenient dimension to control the quantum numbers of spin and valley and are expected to be applied in future spin-valley logic circuits and energy-saving devices.

Growth stability of twinned dendrites in directionally solidified Al-4.5 wt% Cu alloy

Based on the twin planes formed at 3000 μm/s, the growth stability of twinned dendrites in Al-4.5 wt% Cu alloy was studied in detail at a low growth rate during Bridgman solidification. Experimental results show that the twinned dendrite growth could still keep stable even after decreasing the rate to 20 μm/s. This is far below the usual rate threshold for the formation of twinned dendrites (∼1000 μm/s). The dominated twinned structures at 20 μm/s have their (1 1 1) twin plane and 〈1 1 0〉 dendritic trunk closest to thermal gradient direction. Importantly, distinct stacking fault (SF) structures directly connecting two adjacent twin boundaries provide a direct experimental evidence for the close correlation between the SFs and the twinned dendrite growth in aluminum alloys.

Effect of cooling rates on the dendritic morphology transition of Mg–6Gd alloy by in situ X-ray radiography

The effect of cooling rate on the transition of dendrite morphology of a Mg-6Gd (wt%) alloy was semiquantitatively analyzed under a constant temperature gradient by using synchrotron X-ray radiographic technique. Results show that equiaxed dendrites, including exotic ‘butterfly-shaped’ dendrite morphology, dominate at high cooling rate (>1 K/s). When the cooling rate decreases in the range of 0.5–1 K/s, the equiaxed-to-columnar transition takes place, and solute segregates at the center of two long dendrite arms (LDA) of the ‘butterfly-shaped’ dendrite. When the cooling rate is lower than 0.3 K/s, directional solidification occurs and the columnar dendritic growth direction gradually rotates from the crystalline axis to the thermal gradient direction with an increase in cooling rate. Meanwhile, interface moves faster but the dendrite arm spacing decreases. Floating, collision and rotation of dendrites under convection were also studied in this work.

Real-time observation of perturbation of a Drosophila embryo's early cleavage cycles with microfluidics

It is of great importance to understand biochemical system's behavior toward environmental perturbation during the development of living organisms. Here a microfluidic platform for Drosophila embryo's online development and observation is presented. The system is capable of developing the embryo's anterior and posterior halves controlled at different temperature environments, and it can be easily coupled with a confocal microscope for real-time image acquisition. The microfluidic chip is consisted of a polymethylmethacrylate (PMMA) substrate with a thickness of 4.0 mm and a polydimethylsiloxane (PDMS) cover designed with a typical ‘Y’ channel with a depth of 400 μm, width of 800 μm. Temperature gradients were created across the anterior half and posterior half of the embryo by utilizing two streams of laminar flow with different temperatures. It was found that thermal gradient would result in asynchronous development of the two halves of the embryos, and the developing difference was related to the direction of thermal gradient. This may result from the presence of an unknown mechanism located in the anterior half of the embryo, which oversees nuclear division synchronicity. These observations would help better understand compensatory mechanisms of Drosophila embryo's development under environmental perturbations.

Field induced anisotropic thermal conductivity of silver nanowire dispersed-magnetic functional fluid

Nanoparticles (NPs) or nanowires (NWs) dispersed suspensions with high thermal conductivity have gained much attention as thermal interface materials in developing thermal engineering applications. Here, a novel silver (Ag) NW - dispersed magnetic functional fluid (NWD-MFF), whose thermal conductivity is actively tunable with an external magnetic field, has been proposed. The NWD-MFFs were prepared by dispersing either Ag NWs with an average diameter of 70nm and aspect ratios ranging between 29 and 162 or Ag NPs with an average diameter of 70nm in water-based magnetic suspension composed of 5.0vol% of magnetic NPs. The influence on thermal conductivity enhancement in the above fluids due to the orientation of non-magnetic Ag NWs and magnetic NPs under external magnetic field was measured using the transient hot-wire method. The Ag NWs performed better than Ag NPs, when their thermal conductivities were measured under an external magnetic field. Furthermore, the thermal conductivities exhibited a linear increase for higher Ag NW concentrations and higher aspect ratios ranging between 0.029–0.110vol% and 29–162, respectively for the external magnetic field applied parallel to the direction of thermal gradient. The maximum thermal enhancement of 7% was recorded for the NWD-MFF dispersing a mere 0.110vol% of Ag NWs with an aspect ratio of 162. On the other hand, a 7% decrease in thermal conductivity was recorded when the external magnetic field was applied perpendicular to the thermal gradient direction. The reason for the enhancement and decrease in thermal conductivity was considered due to the orientation of NWs in the magnetic field direction, which was experimentally confirmed using the dark field microscopy. The above results suggested that the novel NWD-MFF could be used as an efficient thermal interface material for effective heat dissipation as well as heat accumulation that can be easily manipulated using external magnetic field.

Control of thermal gradient using thermoelectric coolers for study of thermal effects

Thermoelectric coolers based on the Peltier effect have been utilized to control temperature gradient to study thermal effects in both bulk and thin film samples. The temperature gradient is controlled by two coolers and the polarity of the thermal gradient can be reversed by reversing an electric driven voltage. With appropriate controlled thermal gradient using this technique, the Nernst and the Seebeck effects can be measured in both bulk and thin film samples free of spurious contributions. In an arbitrary direction of thermal gradient, the Seebeck and the Nernst components can be decomposed from the measured signal based on the symmetry of the effects in a magnetic field.

Evolution of texture during laser surface treatment of an austenitic manganese steel

This work describes the evolution of microstructure and texture of an austenitic manganese steel (1.34C–13.6Mn–0.45Si-Fe, wt.%) during surface modification using a high power diode laser. Strong (002) texture has been observed on the surface of the steel with increase in interaction time. It has been found that columnar type dendrites are responsible for the strong texture which is favorably grown in the 〈001〉 direction perpendicular to the substrate plane. The growth of dendrites with specific crystallographic direction is governed by the interfacial energy anisotropy and thermal gradient direction.

Dendritic Growth Morphologies in Al-Zn Alloys—Part II: Phase-Field Computations

In Part I of this article, the role of the Zn content in the development of solidification microstructures in Al-Zn alloys was investigated experimentally using X-ray tomographic microscopy. The transition region between $${\langle{100}\rangle}$$ dendrites found at low Zn content and $${\langle{110}\rangle}$$ dendrites found at high Zn content was characterized by textured seaweed-type structures. This Dendrite Orientation Transition (DOT) was explained by the effect of the Zn content on the weak anisotropy of the solid–liquid interfacial energy of Al. In order to further support this interpretation and to elucidate the growth mechanisms of the complex structures that form in the DOT region, a detailed phase-field study exploring anisotropy parameters’ space is presented in this paper. For equiaxed growth, our results essentially recapitulate those of Haxhimali et al.[1] in simulations for pure materials. We find distinct regions of the parameter space associated with $${\langle{100}\rangle}$$ and $${\langle{110}\rangle}$$ dendrites, separated by a region where hyperbranched dendrites are observed. In simulations of directional solidification, we find similar behavior at the extrema, but in this case, the anisotropy parameters corresponding to the hyperbranched region produce textured seaweeds. As noted in the experimental work reported in Part I, these structures are actually dendrites that prefer to grow misaligned with respect to the thermal gradient direction. We also show that in this region, the dendrites grow with a blunted tip that oscillates and splits, resulting in an oriented trunk that continuously emits side branches in other directions. We conclude by making a correlation between the alloy composition and surface energy anisotropy parameters.

Effect of crystallographic orientation on dendrite growth in directional solidification

Dendrite is a typical pattern in directional solidification, attracting many theoretical and experimental researches. However, the effect of crystallographic orientation on dendrite growth is less considered in these researches. In this paper, using a transparent model alloy SCN-1.0 wt% Eth, the effects of crystallographic orientation on the incubation time of planar interface instability, the dendritical morphological feature, and the primary dendrite spacing in directional solidification are investigated. Three crystal grains with different angles between the dendrite axis direction and thermal gradient direction are chosen in our experimental. The experimental results show that the incubation time of planar interface instability increases with the deviation of dendrite axis from thermal gradient direction, which suggests that the deviation of dendrite axis favors the planar interface stability. And with the increase of deviation angle of dendrite axis, the side branches of dendrite become more asymmetric and the preferred side-branches suppress neighboring dendrite growth. Also it is found that the primary dendrite spacing becomes larger as the angle between the dendrite axis direction and thermal gradient direction increases.

Effect of interface energy anisotropy on the dendritic growth in directional solidification

The dendritic growth patterns in directional solidification with different amplitudes of solid-liquid interface energy anisotropy were investigated using the two-dimensional cellular automata (CA) model. It is shown that when the preferred growth direction of the crystal was the same as the direction of thermal gradient, the solidification pattern would transform from seaweed to dendrite with the increase of amplitude of interface energy anisotropy. The amplitude of interface energy anisotropy could also influence the morphology of dendritic tips. As the amplitude of interface energy anisotropy increased, the dendritic tip radius, the liquid concentration ahead of the tip and the tip undercooling decreased. A power law relationship evisted between the stability parameter of dendritic tip and the amplitude of interface energy anisotropy in directional solidification. The primary arm spacing changed little with the increase of interface energy anisotropy. When the angle between the preferred growth direction of the crystals and the direction of thermal gradient was-40, and the amplitude of interface energy anisotropy increased, the solidification pattern would transform from seaweed to degenerated dendrite and finally to tilted dendrite.

Analysis of high angle boundaries in directionally solidified turbine blade made of CMSX-4® superalloy

High angle boundary (HAB) and low angle boundary (LAB) are casting defects of single crystal (SX) and directionally solidified (DS) turbine blades and decrease the lifetime of these blades during service. During directional solidification, primary dendrite arms grow in the opposite direction of the thermal gradient direction and perpendicular to the mushy zone interface. When this interface is not flat, primary dendrite arms growing in various areas of the mushy zone are characterized by different growth directions. Then, after the primary dendrite tips contact each other, LABs or HABs form (depending on the angle between the directions of the primary dendrite arms). This paper presents characterization studies of HABs in a DS turbine blade made of CMSX-4® superalloy. The blade was characterized by three columnar grains with HABs. Qualitative and quantitative analyses of the HABs using electron backscatter diffraction in a scanning electron microscope (SEM-EBSD) were carried out. The EBSD technique helped to determine the crystallographic orientation of the grains near the HAB, misorientation angles between grains (and inside each grain), and the angle of deviation between the [001] direction and the blade axis.

Enhancement and Retardation of Crack Initiation and Growth Due to Thermomechanical Stresses

A thermomechanical energy density factor S is derived in terms of the mechanical and thermal stress intensity factors and , respectively. While both and can change sign depending on the directions of the applied mechanical stress and thermal gradient, the quadratic form , however, is positive definite. The mixed term with A 12 can be either positive or negative depending on the sign of and . Hence, S can increase or decrease with the crack length that is contained in the expressions for the intensity factors. Based on the criterion of crack growth similarity with the proportion S/r being equal to the area under the true stress and true strain curve, it is found that crack growth initiation can be enhanced or retarded. If the mechanical stress is fixed in tension, crack growth is enhanced when the thermal gradient is in the direction of crack initiation while the opposite prevails when the direction of thermal gradient is against the direction of crack initiation induced by the mechanical tension. The formulation provides a clear interaction of how the mechanical and thermal effects would compete. Results are obtained for 38Cr2Mo2VA steel, LY12 aluminum and TC6 titanium alloys which are the sequential order of crack growth enhancement and retardation. Such information can assist the correlation of crack growth data when comparing the results for different materials.

Influence of filament thickness, thermal process and magnetic field on misalignment angles in multifilament Bi2212 tapes

Misalignment angles in multifilament Bi2212 tapes have been studied by transport current density ( J c) measurements. Transversal misalignment angles depend on the filaments thickness due to the crystal growth along the Bi2212/silver interface. Therefore, the thinner the filaments, the higher the J c. A dynamic thermal treatment improves grain alignment inside the tapes due to preferential crystal growth along the thermal gradient direction. Furthermore, melting in high magnetic field reduces the misalignment angle due to magnetic anisotropy effects. Therefore, good transport J c performances can be expected for thicker multifilament Bi2212 tapes prepared using a combination of the dynamic process and melting in a high magnetic field.

Experimental crystallization of gallium: ultrasonic measurements of elastic anisotropy and implications for the inner core

We present ultrasonic measurements of elastic anisotropy in gallium undergoing directional solidification in the presence of imposed thermal gradients, rotation, convection, turbulence, and magnetic fields. Simultaneous in situ measurements of temperature and compressional wave speed are used to track the crystallization front during solidification. We find that individual solidified gallium samples are always polycrystalline and elastically anisotropic, with grains elongated in the solidification direction. The measured compressional wave anisotropy in individual solid samples ranges from 20 to 80% of the single crystal values, depending on experimental conditions. We also find the amount of elastic anisotropy varies with position in an individual sample. Based on ensemble averages from multiple experiments made under similar environmental conditions, we find the direction of elastic anisotropy in the solid is sensitive to the thermal gradient direction, while the amount of anisotropy is most sensitive to the presence or absence of initial nucleation in the melt. Experiments that show average anisotropy have the ultrasonically fast axis aligned with gravity and the thermal gradient. Strongly anisotropic solids result when nucleation grains are present in the initial melt, whereas smaller or zero average anisotropy results when nucleation grains are initially absent. Other externally imposed factors we have examined, such as turbulence and magnetic fields, have either no measurable influence or tend to reduce the amount of anisotropy of the solid. Our results suggest that during crystallization of Earth’s inner core, the orientation of average anisotropy in the newly formed solid is controlled primarily by radial solidification, while the amount of anisotropy may be influenced by pre-existing inner core texture.

Nonisothermal Gravitational Composition Variation in a Large Deep-water Field

Summary This paper examines the oil composition distribution of a large deep-water field which the temperature variation, because of the water depth (800 to 2000 m), is in the opposite direction of the Earth's thermal gradient. The main practical objective is to investigate if the oil distribution would suggest fluid connectivity along this 40 km long and 15 km wide oil field. This could significantly impact the appraisal and development phases of the field. The methodology adopted here includes reservoir geochemistry, fitting of a single equation of state, compositional simulation, and thermodynamic equilibrium. The results from reservoir geochemistry indicate a common oil source, different migration pulses, a lateral migration path and some hydrocarbon mixing after filling process. A single equation of state is set to represent the oil behavior for the whole field. The successful matching is achieved by means of splitting the heaviest component and so reducing the molar concentration of the pseudocomponent on a single regression procedure. Compositional simulation runs, representing the secondary migration, suggest spots of gas caused by phase changes. During the filling process, the results show heavier oil in front of the upward flow. During the mixing process, the gravitational segregation seeks to reverse this trend. A new thermodynamic equilibrium model that accounts for the effect of temperature shows an increase of oil segregation with a temperature gradient opposite to the thermal's gradient. The results suggest thermodynamic equilibrium of oils in an area of the reservoir. This may suggest reservoir connectivity. In the area the oils are in thermodynamic equilibrium, fluid composition and properties are extrapolated to different depths while function of their respective temperatures.

Heat and sound transmission attenuated framing structure, particularly door or window framing

To improve thermal and acoustic insulation between the outside (O) and the inside (I) of a structure having windows or doors therethrough, stationary frames (10, 12; 52, 54) are secured to the building, retaining movable window or door frames (2). The stationary frames include two metal rails (10, 12; 52, 54), one (10, 52) of the frames being located at the outside (O) of the building and the other (12, 54) at the inside (I) of the building. The inner and outer rails are separated from each other by a double-walled chambered strip (14) having first and second elongated strip portions connected by cross ribs (21, 22, 23) which, therebetween, define chambers (17-20). The chambers are serially located in a direction of thermal gradient between the outside and the inside of the building and arranged in stepped or Z-shaped configuration. The chambered strips are rigidly connected at lateral portions thereof with the inside and outside rails, for example by interengaging, interlocking arrangements including interfitting dovetail-shaped portions.

〈110〉 dendrite growth in aluminum feathery grains

Automatic indexing of electron backscattered diffraction patterns, scanning electron microscopy, and optical microscopy observations have been carried out on aluminum-magnesium-silicon, aluminum-copper, and aluminum-silicon alloys directionally solidified or semicontinuously cast using the direct chill casting process. From these combined observations, it is shown that the feathery grains are made of 〈110〉 primary dendrite trunks (e.g., [01\(\bar 1\)]) split in their centers by a coherent (111) twin plane. The average spacing of the dendrite trunks in the twin plane (about 10 to 20 µm) is typically one order of magnitude smaller than that separating successive rows of trunks (or twin planes). The [01\(\bar 1\)] orientation of these trunks is close to the thermal gradient direction (typically within 15 deg)—a feature probably resulting from a growth competition mechanism similar to that occurring during normal 〈100〉 columnar dendrite growth. On both sides of these trunks, secondary dendrite arms also grow along 〈110〉 directions. Their impingement creates wavy noncoherent twin boundaries between the coherent twin planes. In the twin plane, evidence is shown that 〈110〉 branching mechanisms lead to the propagation of the twinned regions, to the regular arrangement of the primary dendrite trunks along a [\(\bar 2\)11] direction, and to coherent planar twin boundaries. From these observations, it is concluded that the feathery grains are probably the result of a change from a normal 〈100〉 to a 〈110〉 surface tension/attachment kinetics anisotropy growth mode. This change might be induced by the added solute elements, by the local solidification conditions (thermal gradient, growth rate, and melt convection), and possibly by the help of the twin plane itself. Convection in the melt could also play a role in the symmetrization of the 〈110〉 growth directions of the side arms. Finally, the proposed mechanisms of feathery grain growth are further supported by the observation of 〈110〉 dendrite growth morphologies in thin aluminum-zinc coatings.

Electron backscattered diffraction investigation of the texture of feathery crystals in aluminum alloys

Scanning electron microscopy (SEM), metallographic observations, and automated electron back-scattered diffraction (EBSD) experiments were carried out on “feathery crystals” of a unidirectionally solidified (1D) Al-Cu alloy and of a direct-chill (DC) cast Al-Mg-Si alloy. The results clearly show that the “feathery grains” are made of twinned lamellae, which are parallel to a (111) twin plane. The contrast seen in the metallographic sections after a Barker etching or observed in an SEM is perfectly corroborated with the EBSD reconstructed microstructure. The lamellae are separated by an alternance of straight and wavy lines. Some equiaxed grains are also observed occasionally in the specimens. From the 〈111〉 pole figures of the various grains, it is concluded that the thermal gradient direction is close to, but not necessarily within, the (111) twin plane: its direction is in between a [011] and a [112] direction. Within a given feathery grain, small variations of the crystallographic orientations (subgrain boundaries) are observed. The lamellae of one grain can sometimes penetrate into another one. Based upon this information, the mechanism of feathery grain growth previously proposed by Eady and Hogan is ruled out. Although no other growth mechanism is proposed yet, it is believed that feathery grains are the result of a competition similar to that occurring in normal dendritic alloys, but with twinned dendrites.

Orientation dependence of primary dendrite spacing

The orientation dependence of the primary dendrite spacing is examined through the solidification of a succinonitrile-3.61 wt pct acetone alloy with a Bridgman-type device. Primary dendrite spacing has been studied as a function of the primary dendrite trunk orientation with respect to the thermal gradient direction under different growth rate conditions. The observations show that the orientation dependence of the primary dendrite spacing can be related to the formation of new primary dendrites at divergent grain boundariesvia the branching mechanism. A branching-based model is developed to study the interaction between secondary and tertiary dendrite arms. Based on this model, a simple analytical relationship is proposed to account for the orientation dependence of the primary dendrite spacing.

Analytical and numerical predictions of dendritic grain envelopes

An analytical model is developed for the prediction of the shape of dendritic grain envelopes during solidification of a metallic alloy in a Bridgman configuration (i.e. constant thermal gradient and cooling rate). The assumptions built into the model allow a direct comparison of the results with those obtained from a previously developed cellular automaton-finite element (CAFE) model. After this comparison, the CAFE model is applied to the study of the extension of a single grain into an open region of liquid after passing a re-entrant corner. The simulation results are compared with experimental observations made on a directionally solidified succinonitrile-acetone alloy. Good agreement is found for the shape of the grain envelopes when varying the orientation of the primary dendrites with respect to the thermal gradient direction, the velocity of the isotherms or the thermal gradient.