Large Thermal Gradients Research Articles

Effect of WC particle size and Ag volume fraction on electrical contact resistance and thermal conductivity of Ag–WC contact materials

Abstract In this work, theoretically dense (> 99%) composites of Ag and WC have been prepared by press–sintering–infiltration for making electrical contacts used as an arc-resistant material in a model switching device. Composites with varying silver content and WC particle size were investigated to get an insight on their electrical contact resistance (Rc) and their ability to withstand enormous thermal stresses during switching. A break-only model switching sequence was used, where the evolution of Rc was measured over 50 cycles and the post-switching microstructures were investigated for thermal stress induced crack formation. A well-established 2D computational microstructure based model, object-oriented finite element analysis version 2 (OOF2), was used to determine the composite thermal conductivity (k) for various grades as a function of temperature. Rc was observed to be consistently low for the coarser WC containing composite and higher silver content composites. This response was attributed to the ductility of the surface layers formed during switching. Crack formation after switching was found to be a direct consequence of large thermal gradients during 50 cycles, which was minimal for coarser WC grained and higher silver content composites which have a higher thermal shock resistance.

Influence of Thermal Conductivity and 2-D Temperature Distribution of Liquid Water Saturation

For use in automotive power, polymer electrolyte fuel cells are being pushed to ever higher levels of performance and efficiency. At these high performance levels, large thermal gradients can exist across the diffusion media. These large gradients can have a strong influence on water transport in the cell via phase-change-induced flow and thermally driven diffusion. The focus of this work is to elucidate the effects of diffusion media thermal conductivity on through-plane 2-D temperature and saturation distributions. Neutron imaging was conducted at the NIST Center for Neutron Research BT-2 neutron imaging facility and used to measure how saturation levels were influenced by the internal heat generation and temperature distributions. A custom 4.8 cm2 fuel cell with precision liquid coolant temperature control was used for the high resolution through-plane radiography. The precision liquid coolant ensures stable, high accuracy thermal boundary conditions for the analysis. Temperature distributions were computed in COMSOL using experimentally measured dry and wet thermal conductivity values for commercial diffusion media. These temperature distributions were then coupled to the measured saturation distributions from neutron imaging to determine the relationships between level of saturation and temperature gradient as shown in Figure 1. Data from this work will build upon an online database of benchmark data for the modeling community made available through this Department of Energy EERE supported program (Award Number DE-EE0000470). Additional funding provided by Department of Energy interagency agreement No. DEAI01-01EE50660. Figure 1

AlSi12 in-situ alloy formation and residual stress reduction using anchorless selective laser melting

Rapid melt pool formation and solidification during the metal powder bed process Selective Laser Melting (SLM) generates large thermal gradients that can in turn lead to increased residual stress formation within a component. Metal anchors or supports are required to be built in-situ and forcibly hold SLM structures in place and minimise geometric distortion/warpage as a result of this thermal residual stress. Anchors are often costly, difficult and time consuming to remove and limit the geometric freedom of this Additive Manufacturing (AM) process. A novel method known as Anchorless Selective Laser Melting (ASLM) maintains processed material within a stress relieved state throughout the duration of a build. As a result metal components formed using ASLM do not require support structures or anchors. ASLM locally melts two or more powdered materials that alloy under the action of the laser and can form into various combinations of eutectic/hypo/hyper eutectic alloys with a new lower solidification temperature. This new alloy is maintained in a semi-solid or stress reduced state throughout the build with the assistance of elevated powder bed pre-heating. In this paper the ASLM methodology is detailed and investigations into processing of a low temperature eutectic Al-Si binary casting alloy is explored. Two types of Al powders were compared; pre-alloyed AlSi12 and elemental mix Al+12wt% Si. The study established an understanding of the laser in-situ alloying process and confirmed successful alloy formation within the process. Differential thermal analysis, microscopy and X-Ray diffraction were used to further understand the nature of alloying within the process. Residual stress reduction was observed within ASLM processed elemental Al+Si12 and geometries produced without the requirement for anchors.

Joule heating in ferromagnetic nanostripes with a notch

The temperature in a ferromagnetic nanostripe with a notch subject to Joule heating has been studied in detail. We first performed an experimental real-time calibration of the temperature versus time as a 100 ns current pulse was injected into a Permalloy nanostripe. This calibration was repeated for different pulse amplitudes and stripe dimensions and the set of experimental curves were fitted with a computer simulation using the Fourier thermal conduction equation. The best fit of these experimental curves was obtained by including the temperature-dependent behavior of the electrical resistivity of the Permalloy and of the thermal conductivity of the substrate $(\mathrm{Si}{\mathrm{O}}_{2})$. Notably, a nonzero interface thermal resistance between the metallic nanostripe and the substrate was also necessary to fit the experimental curves. We found this parameter pivotal to understand our results and the results from previous works. The higher current density in the notch, together with the interface thermal resistance, allows a considerable increase of the temperature in the notch, creating a large horizontal thermal gradient. This gradient, together with the high temperature in the notch and the larger current density close to the edges of the notch, can be very influential in experiments studying the current assisted domain wall motion.

Reduction of micro-cracking in nickel superalloys processed by Selective Laser Melting: A fundamental alloy design approach

The Selective Laser Melting (SLM) process generates large thermal gradients during rapid melting of metallic powdered feedstock. During solidification certain alloys suffer from thermally induced micro-cracking which cannot be eliminated by process optimisation. An alloy’s crack susceptibility may reduce by increasing its Thermal Shock Resistance (TSR), potentially achieved through an increase in tensile strength. This hypothesis is investigated with Hastelloy X, a common nickel-base superalloy of known high crack susceptibility when processing SLM. It is demonstrated that through consideration of the imposed rapid solidification conditions, Hastelloy X can be made to form a supersaturated solid solution in the as deposited state. The fundamental solid solution strengthening (SSS) effect is exploited to generate an increase in lattice stress, by increasing the most potent SSS elements present within the alloy, whilst maintaining specification composition. The modified alloy displayed a 65% reduction in cracking and an increase in elevated temperature tensile strength, lending support to the initial hypothesis and identifying a possible approach for developing further SLM crack resistant versions of well-known alloy compositions.

Paramagnetic spin seebeck effect.

We report the observation of the longitudinal spin Seebeck effect in paramagnetic insulators. By using a microscale on-chip local heater, we generate a large thermal gradient confined to the chip surface without a large increase in the total sample temperature. Using this technique at low temperatures (<20 K), we resolve the paramagnetic spin Seebeck effect in the insulating paramagnets Gd3Ga5O12 (gadolinium gallium garnet) and DyScO3 (DSO), using either W or Pt as the spin detector layer. By taking advantage of the strong magnetocrystalline anisotropy of DSO, we eliminate contributions from the Nernst effect in W or Pt, which produces a phenomenologically similar signal.

LES of Stably Stratified Flow with Varying Thermophysical Properties

Large eddy simulations of stably stratified, turbulent channel flow subjected to a large temperature gradient have been performed by considering a broad spectrum of stratification. Due to a large thermal gradient across the channel, temperature-dependent fluid properties like viscosity (µ), density (ρ), and thermal conductivity (κ) are considered as variables. With increased stratification, a cluster of laminar patches appears in the near-wall region, and turbulent momentum and buoyancy fluxes are suppressed drastically in the core of the channel due to the formation of internal gravity waves. Variable viscosity results in flow relaminarization on the hot side of the channel (where viscosity is higher). Density tends to stablize the flow by blocking the upward movement of thermal plumes, while thermal conductivity pays the toll for viscosity at high Reynolds number. A mechanistic model for wall heat transfer is developed for buoyancy-effected flows. We have observed qualitatively and quantitatively the pronounced modifications in turbulent structure and flow statistics.

Long-time Stability of Mg2(Si-Sn-Ge) - Based Thermoelectrics Under Large Temperature Gradient Conditions

We study the thermoelectric properties and stability of Mg2(Si-Sn-Ge) composite, prepared by a combination of ball milling process and heating treatment followed by Spark Plasma Sintering and densification. The goal of this study is to investigate the long-time stability of the efficient n-type thermoelectric material under conditions similar to those in future applications. Rectangular samples with dimensions 6x6x15 mm3 were placed in an Ar-filled chamber, equipped with thermoelectric cooling module and with a heater. The thermoelectric power and electrical resistivity of the sample were measured while temperatures of 800K at the hot side, and 320K at the cold side of the sample were maintained for more than 300hours. Different aspects of the sample stability were investigated: 1. mechanical stability under global and local thermal cycling; 2. chemical stability of the sample-electrode contacts; 3. sample stability at high temperatures; 4. material stability under large thermal gradients.

Extension of the Reduced Integration Scheme to Calculate the Direct Exchange Areas in 3D Rectangular Enclosures with Nonscattering Media

The evaluation of direct exchange areas (DEA) in zonal method is the most important task due to the heavy computer cost of multi-integrals together with the existing of singularities. A technique of variable transformation to reduce the fold of integrals, which was developed originally by Erkku (1959) to calculate the DEAs of uniformly zonal dividing cylindrical system, was extended by Tian and Chiu (2003) for nonuniformly zonal dividing cylindrical system with large thermal gradients. In this paper, we further extend the reduced integration scheme (RIS) to calculate the DEAs in three-dimensional rectangular system. The detail deductions of six-, five-, and fourfold integrals to threefold ones are presented; the DEAs in a rectangular system with assumption of gray medium are computed by the Gaussian quadrature integration (GQI) and the RIS comparatively. The comparisons reveal that the RIS can provide remarkable higher accuracy and efficiency than GQI. More interestingly and practicably, the singularities of DEAs can be decomposed and weakened obviously by RIS.

Visualizing copper assisted graphene growth in nanoscale.

Control synthesis of high quality large-area graphene on transition metals (TMs) by chemical vapor deposition (CVD) is the most fascinating approach for practical device applications. Interaction of carbon atoms and TMs is quite critical to obtain graphene with precise layer number, crystal size and structure. Here, we reveal a solid phase reaction process to achieve Cu assisted graphene growth in nanoscale by in-situ transmission electron microscope (TEM). Significant structural transformation of amorphous carbon nanofiber (CNF) coated with Cu is observed with an applied potential in a two probe system. The coated Cu particle recrystallize and agglomerate toward the cathode with applied potential due to joule heating and large thermal gradient. Consequently, the amorphous carbon start crystallizing and forming sp2 hybridized carbon to form graphene sheet from the tip of Cu surface. We observed structural deformation and breaking of the graphene nanoribbon with a higher applied potential, attributing to saturated current flow and induced Joule heating. The observed graphene formation in nanoscale by the in-situ TEM process can be significant to understand carbon atoms and Cu interaction.

Thermogenic capacity at subzero temperatures: how low can a hibernator go?

Abstract Hibernation in mammals is a physiological and behavioral adaptation to survive intervals of low resource availability through profound decreases in metabolic rate (MR), core body temperature (Tb), and activity. Most small mammalian hibernators thermoconform, with Tb approximating ambient temperature (Ta); arctic species are an exception, since they must actively defend what can be large thermal gradients between Tb and Ta. Here we compare the thermogenic capacity of the arctic ground squirrel (Urocitellus parryii) to that of the golden-mantled ground squirrel (Callospermophilus lateralis), a temperate-zone montane hibernator. We allowed animals to reenter torpor at sequentially lower Ta's and found that arctic ground squirrels maintained steady state torpor at Ta's as low as -26°C, through a 36-fold increase in torpid MR (TMR), compared to their minimum TMR, exhibited at a Ta of 0°C. Golden-mantled ground squirrels are able to maintain steady state torpor at Ta's at least as low as -8°C, through a 13.5-fold increase in MR, compared to their minimum TMR at a Ta of 2°C. In a second experiment, torpid animals were exposed to continuously decreasing Ta's (0.25°C/30 min); individuals of both species increased their metabolism while remaining torpid at low Ta's (as low as -30°C for arctic ground squirrels and -10°C for golden-mantled ground squirrels). Although the capacity to hibernate at subfreezing Ta's is not unique to arctic ground squirrels, their large body size, greater torpid metabolic scope, and previously ascribed capacity to supercool allow them to occupy much colder hibernacula for prolonged seasons of hibernation.

Ultra-high quality factors in superconducting niobium cavities in ambient magnetic fields up to 190 mG

Ambient magnetic field, if trapped in the penetration depth, leads to the residual resistance and therefore sets the limit for the achievable quality factors in superconducting niobium resonators for particle accelerators. Here, we show that a complete expulsion of the magnetic flux can be performed and leads to: (1) record quality factors Q &amp;gt; 2 × 1011 up to accelerating gradient of 22 MV/m; (2) Q ∼ 3 × 1010 at 2 K and 16 MV/m in up to 190 mG magnetic fields. This is achieved by large thermal gradients at the normal/superconducting phase front during the cooldown. Our findings open up a way to ultra-high quality factors at low temperatures and show an alternative to the sophisticated magnetic shielding implemented in modern superconducting accelerators.

Thermal Management in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) is one of the most promising techniques to extend the recording density in hard disk drives beyond 4 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . This places a range of stringent operating requirements in the head disk interface, including heating and cooling of the media, must occur within the order of nanosecond in order to achieve the necessary data rates, generate a large thermal gradient for sharp bit edge, and ensure that the recorded data are thermally stable during cooling to ambient. Optical energy must be efficiently delivered and confined to a spot in the medium that is much smaller than the diffraction limit so that neighboring tracks are not heated. To shed light on this issue, a mesoscale level model based on kinetic theory called lattice Boltzmann method (LBM) is applied to understand the nanoscale heat transfer in the media layer in HAMR operations. In this paper, we examine the effects of main energy carriers at nansocale (electrons and phonons) via novel LBM-based methodology. We investigate the overall heat transfer contribution of the coupled heat carriers and the electron-phonon coupling issues in the carbon matrix and media grain interface in a representative FePt media layer. Since thermal gradients and grain size are very important for achieving high-density HAMR recording, the in-plane crosstrack temperature gradients of structural effects with two representative grain shapes (hexagonal and circular) and sizes (10 and 5 nm) are investigated. We observed that 10 nm grains of both shapes show lower temperature gradients compared with the 5 nm size, due to the increase of carrier scattering at the grain/carbon matrix interface. Circular grain shape produces larger temperature gradient magnitude compared with hexagonal grain structure. Thus, this paper will produce insight in the mechanism to control the thermal management and temperature gradients for high-density HAMR.

Thermo-mechanical model development and validation of directed energy deposition additive manufacturing of Ti–6Al–4V

A thermo-mechanical model of directed energy deposition additive manufacturing of Ti–6Al–4V is developed using measurements of the surface convection generated by gasses flowing during the deposition. In directed energy deposition, material is injected into a melt pool that is traversed to fill in a cross-section of a part, building it layer-by-layer. This creates large thermal gradients that generate plastic deformation and residual stresses. Finite element analysis (FEA) is often used to study these phenomena using simple assumptions of the surface convection. This work proposes that a detailed knowledge of the surface heat transfer is required to produce more accurate FEA results. The surface convection generated by the deposition process is measured and implemented in the thermo-mechanical model. Three depositions with different geometries and dwell times are used to validate the model using in situ measurements of the temperature and deflection as well as post-process measurements of the residual stress. An additional model is developed using the assumption of free convection on all surfaces. The results show that a measurement-based convection model is required to produce accurate simulation results.

Seismological implications of a lithospheric low seismic velocity zone in Mars

Most seismological models for the interior of Mars lack an upper mantle low velocity zone. However, there is expected to be a large thermal gradient across the stagnant conductive lid (lithosphere) of Mars. This gradient should tend to decrease elastic wave velocities with increasing depth, with this effect dominating the opposing tendency caused by increasing pressure with depth because Mars has low gravity. An upper mantle lithosphere with a low velocity zone (LVZ) beneath a thin high velocity “seismic lid” is thus predicted. The upcoming NASA InSight mission includes a three-component seismometer, which should provide the first opportunity to directly detect any lithospheric LVZ in Mars. Seismic wavefields expected for Mars mantle velocity structures with or without a strong LVZ are very distinct. The LVZ models predict shadow zones for high-frequency seismic body wave phases such as P, S, PP and SS, etc. The most diagnostic waves that can be used to evaluate presence of a lithospheric LVZ given a single seismometer are intermediate-period global surface waves, which travel along the great circle from a seismic source to the seismometer. An LVZ produces distinctive dispersion, with a Rayleigh wave Airy phase around 100s period and very different surface wave seismograms compared to a model with no LVZ. Even a single observation of long-period surface waves from a known range can be diagnostic of the lithospheric structure. Establishing the existence of an LVZ has major implications for thermal evolution, volatile content and internal dynamics of the planet.

Fish growth and degree-days II: selecting a base temperature for an among-population study

The degree-day (DD) is a method of describing the thermal opportunity for growth and development and is becoming increasingly popular when comparing fish growth over large spatial scales (e.g., counter-gradient growth). Temperatures too cold to permit growth are excluded in the DD equation by incorporating a lower temperature threshold (To). However, there is no convention for choosing To, and unknown is the effect of an incorrect Toon how growth is perceived. We simulate data to demonstrate how an incorrect Tomay lead to differences in temperature-corrected growth rates among populations. These differences increase with the error in Toand the thermal range among simulated populations. We then show the same relationships in an analysis of length-at-age data from 81 walleye (Sander vitreus) populations in North America. Together, our results demonstrate that differences in temperature-corrected growth rates among populations can be a statistical artifact rather than a biological phenomenon, especially when populations are distributed over a large thermal gradient.

Generation of photoacoustic transients from optically induced thermal gradients

Irradiation of an absorbing surface in contact with a transparent fluid with a pulsed laser can result in the generation of extremely large thermal gradients. For example, when a laser with a pulse width of 10 ns and a fluence of 1 J/cm2 irradiates a region with an absorption of 1 cm1 having the thermal properties of liquid water, a thermal gradient on the order of 105 K/m at the interface is produced. Here, it is shown that the effect of such thermal gradients on photoacoustic waves from an infinite half space and from a uniformly irradiated sphere is the production of fast transients on the leading edges of the waves. The character of the transients is determined from an additional source term in the wave equation for pressure that obtains when heat conduction is taken into account. Experiments are reported showing the predicted transients on photoacoustic waves from absorbing layers in contact with transparent fluids irradiated with 10 ns laser pulses.

HAMR Medium Structure Design and Its Process for Excellent Thermal Performance

The L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ordered FePt-based medium with a novel thermal design for heat-assisted magnetic recording (HAMR) application has been demonstrated. The proposed seed-then-heat sink approach using the (001) textured Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N as the underlayer to induce the MgO (200) texture at room temperature. The high temperature process for FePtX granular film deposition resulted in the decomposition of the Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N to Cu and made it a suitable heat sink layer. Our approach eliminated the effect of the boundary thermal resistance and enabled a large thermal gradient during the writing process, which is critical to the FePt-based medium for HAMR application.

Thermally Induced Photoacoustic Transients Produced by Laser-Irradiated Fluid Spheres

Pulsed laser irradiation of a weakly absorbing fluid sphere in a transparent medium results in the production of a large thermal gradient at the surface of the sphere. The rapid transfer of heat from the sphere to the surrounding fluid as a result of the thermal gradient generates high frequency photoacoustic transients which affect the leading edge of a photoacoustic wave. Here, the character of the photoacoustic wave is determined by solving a modified wave equation for the photoacoustic effect. A solution to the heat diffusion equation is determined, which, together with the heating function for the optical source, provides the source term for the wave equation for pressure. The wave equation is then solved with appropriate boundary conditions using Laplace transform techniques to give the photoacoustic waveform. The relative magnitude of the transient to the N-shaped wave is shown to be determined, in part, by the laser pulse length.

093 Ultra rapid warming of cryo samples using an IR laser pulse

Our laboratory has been investigating the effect of warming rate on survival of mouse oocytes after very rapid cooling. Cooling and warming rates are affected by sample thermal mass, surface area, thermal impedance, and delta T. To maximize these rates we use 0.1 ul samples placed on Cryotops and plunged directly into LN2 for cooling and into room temperature buffer for warming. The maximum warming rate obtained is ∼ 100,000 C/min and it yields the highest survivals. However, extrapolation of the experimental data suggests faster warming would yield higher survivals. Here we report on the use of an IR laser pulse to achieve warming rates 3.5 to 100 times faster. Our objective was to choose a laser wavelength which would warm the cryo buffer while leaving the cells ‘untouched’, thus leading to indirect warming of the cells. Optical tweezer studies show that a wide variety of cell types can tolerate very high intensities of 1064 nm IR energy emitted by Nd:YAG lasers and so this was the type chosen. Calculations indicate the laser intensity for cryo melting to be significantly less than that used for optical tweezers. Most cryo buffers, including the EAFS we use are relatively transparent in the IR. The buffer IR absorption is easily increased by adding a little India ink. India ink consists primarily of carbon particles from 0.1 to 1 um in diameter, and at the concentration we use, 0.25% (V/V), is harmless to the oocytes. The concentration is carefully chosen to allow most of the IR energy to pass completely through the sample. This yields relatively even sample heating from front to back. We estimate ink-free samples absorb ∼ 9% of the incident energy, and the added ink absorbs ∼ 25% of the remaining energy, yielding a total absorption of ∼ 30%. We have developed a small cryo ‘jig’ which lifts the sample out of an LN2 bath, covers the bath, and fires a laser pulse, all in less than one sec. This assures that the sample is rapidly warmed by the laser pulse rather than by slower air warming. Thus, the samples are warmed from ∼ −180°C to ∼ 0°C by the laser pulse. We have computed the required laser energy for melting, but there are so many variables that we have found it preferable to empirically determine the required laser energy to just melt the samples for a given experiment. This is done by direct visual observation with a 15 x stereo microscope built into the laser system. We use a 40 joule welding laser (for long pulse durations!) with a beam diameter of 2 mm and a pulse length of 0.5 to 30 msec. This yields warming rates from 3.5 x 10 5 to 2 x 10 7 C/min. However, modeling indicates that rates higher than 1 x 10 7 C/min yield unacceptably large thermal gradients across the 70 um diameter oocytes and are not used. Preliminary experiments at a warming rate of 3.5 x 10 5 C/min yield survivals comparable to our traditional methods. Source of Funding: NIH Grant 8R01 OD 011201, Peter Mazur, PI. E. Paredes FPU Research Stays Fellowship from the Spanish Government, 1 November 2012-31 January 2013. Conflict of Interest: None declared. fkleinha@iupui.edu