Presence Of Thermal Gradients Research Articles

Modeling and Analysis of Temperature-Dependent Capacity Degradation in Large Format Lithium-Ion Batteries

The large-format lithium-ion batteries, widely used for electric vehicle applications, are generally characterized by spatial non-uniformities in temperature. The presence of thermal gradients and temperature-dependence of cell parameters are responsible for non-uniform degradation within the battery that leads to poor performance of the battery packs and also poses safety risks. In this work, we aim to develop a detailed physics-based simulation framework to model non-uniform, temperature-dependent capacity degradation within these large format batteries based on the classical pseudo two-dimensional (p2D) battery model. The electrochemical model will be coupled with degradation mechanisms to capture solid-electrolyte interface layer growth, reversible lithium plating, structural disintegration, and volume change within the electrodes on the cell level signatures and usable capacity over charge/discharge cycles. The effects of temperature inhomogeneities will be accounted in the multi-cell simulations at different operating temperatures for individual cells with dynamic currents and temperature and concentration dependent battery parameters as the model inputs.Overall, the primary focus of this study is to capture the effect of spatial temperature variation on battery capacity over cycles and help identify an improved design for the battery module. The model is implemented in an efficient equation-based form for the p2D model and degradation mechanisms with dynamic cell inputs using the mathematical modules in the finite element based solver, COMSOL Multiphysics. This approach enables an easier model development and analysis of the detailed physics and allows an easier implementation of any additional side reaction. The relative contributions of various fade modes have also been compared to understand their competitive interplay over cell cycles. The model predictions are validated with the available experimental data and in-house codes for each case. We believe that this simple yet detailed model can be further developed to be computationally feasible for adoption and implementation in a battery thermal management system.

Modelling and mitigating flux jumps in bulk high-temperature superconductors during quasi-static, high-field magnetisation

(RE)-Ba2Cu3O bulk superconductors acting as permanent trapped field magnet analogues have been shown to trap fields in excess of 17 T. However, the occurrence of thermomagnetic instabilities during their magnetisation process can limit their performance. In 2019, Naito et al trapped 15.1 T in a (Y)-BaCuO bulk pair under applied fields of up to 22 T (2019 J. Appl. Phys. 126 243901), whilst also documenting the mechanical failure of the bulks due to a flux jump. Here, we accurately numerically model this experiment and the observed flux jump, providing insight into the experimental results, such as the role of heating, the presence of thermal gradients, and a possible reason for the location of the bulk-pair fracturing. Extending the study with an investigation into the role of enhanced cooling of the bulk-pair, the influence of cooling power on the calculated trapped field and flux jump is explored. Finally, we propose and numerically investigate a new composite bulk configuration, consisting of stacks of (Y)-BaCuO bulk and interspaced metallic discs, to enhance the thermal stability of the bulk pair, to ultimately improve the trapped field performance.

Investigate of Creep Response in Functionally Graded Material Rotating Disc with variable Thickness

The objective of this paper to investigate analysis of creep in the Functionally Graded Material disc with variable thickness made of aluminum alloy-based metal matrix composite containing silicon carbide particles in presence of thermal gradients in the radial direction. It has been concluded that distributions of the stress and strain rate in an anisotropic disc got affected from the thermal gradients. Thus, the presence of thermal gradients in rotating disc plays a significant role in developing the creep response.

Model Development for Temperature-Dependent Degradation in Large Format Lithium-Ion Batteries

Large-format cells are achieving increasing popularity for Electric Vehicle (EV) applications to maximize volumetric and gravimetric energy densities. These large batteries are often characterized by spatial non-uniformities in temperature and resulting difficulties in maintaining uniform temperature distribution. This poor thermal management is a primary reason for the inefficient performance and safety risks associated with these large format batteries and battery packs. Further, there have been several reported degradation mechanisms which are responsible for the capacity fade within Lithium-ion (Li-ion) batteries over multiple charge/discharge cycles. In particular, the presence of thermal gradients and consequent non-uniformities greatly exacerbates degradation, and poses serious safety risks. Thus, it is imperative to identify and study the degradation mechanisms coupled with the thermal effects by incorporating detailed physics-based models to characterize the cell-level phenomena.This work aims to perform a detailed model-based study on the combined effects of battery degradation and temperature inhomogeneities within large-format Li-ion cells and battery packs. This simultaneous analysis for temperature-dependent degradation will be performed with a state-of-the-art electrochemical model like the classic pseudo-two-dimensional (p2D) model to be able to present a comprehensive analysis of the battery performance parameters. These models will be augmented to include thermal effects and various degradation mechanisms for Solid Electrolyte Interphase (SEI) formation and Lithium plating. The proposed numerical framework will be validated with experimental data for the loss in battery capacity over cycles. In addition, the model development, especially the weak form for model equations and their implementation in a finite-element solver, COMSOL Multiphysics, will be discussed in detail.This study primarily focusses on certain important aspects of temperature-dependent degradation, viz., (a) cell-to-cell temperature variation, (b) operating conditions for current, (c) topology for the cell arrangement within the battery module, and (d) choice of the degradation mechanism. The delineation of the effects of these complex factors on the cell-level signatures will be of significant utility in monitoring and identifying the performance limitations of individual cells, and thereby help maximize the performance of large format cells and battery packs.The ultimate goal is the design of a simple yet efficient numerical model with fast and robust simulations for the analysis of the temperature-dependent degradation of the Li-ion battery packs over a wide range of operating and design conditions. The proposed model will be developed to be computationally feasible for an adoption and implementation in a battery thermal management system.

Measurements of Temperature and Humidity Responsive Swelling of Thin Hydrogel Films by Interferometry in an Environmental Chamber.

Thin film thermo-responsive hydrogels have become a huge interest in applications such as smart drug-delivery systems or sensor/actuator technology. So far, mostly, the response of such hydrogels has been measured only by varying the temperature in a liquid environment, but studies of the response towards humidity and temperature are rare because of experimental limitations. Often the swelling measurements are performed on samples placed on a stage that can be heated/cooled, while vapors enter the permeation chamber at their own temperature. This thermal difference leads to some uncertainties on the exact relative humidity to which the sample is exposed to. In this study, we explored the possibility of performing swelling measurements under thermal equilibrium by placing the sample and an interferometer, as a detector, in an environmental chamber and therefore exposing the smart hydrogel to adjustable temperatures and relative humidity conditions while measuring the hydrogel’s thin film thickness changes. As a case study, we used thin films of the thermo-responsive hydrogel, poly N-vinylcaprolactam deposited by initiated chemical vapor deposition (iCVD). Similar thin films were previously characterized by in situ ellipsometry while the sample was heated on a stage and exposed to humid air produced at room temperature. The comparison between the two measurement methods showed that while measurements in the presence of thermal gradients are limited mostly to low humidity, measurements in thermal equilibrium are restricted only by the operation limits of the used environmental chamber.

Modelling stress evolution and voiding in advanced copper nano-interconnects under thermal gradients

As the chip power densities increase radically with scaling, thermal gradients are predicted to become a major reliability concern in interconnects compatible with the 3 nm technology node and beyond. In this study, the impact of temperature gradients on the reliability of Cu nano-interconnects with highly scaled linewidth of 10 nm is simulated using a Korhonen-type modelling framework previously calibrated for electromigration predictions. ∆T dictates the stress distribution at steady state while dT/dx along the interconnect governs the stress evolution rate. It is shown that a linear temperature profile with a ∆T of >25 °C can generate voiding by thermomigration alone in these narrow lines. The adverse impact of thermal gradients by thermomigration and also by thermally induced flux divergence points are explored, showing a 20% drop of jmax in the presence of a ∆T of 10 °C and secondary nucleation sites along the line depending on temperature distribution. Immortality of Cu nano-interconnects to electromigration in the presence of thermal gradients was assessed whereby a thermally adjusted jLc is found relevant depending on the direction of electron flow with respect to temperature gradient.

Thermal-mechanical behavior of a SiC/SiC CMC subjected to laser heating

With ceramic matrix composites (CMCs) being targeted for use as hot section components in gas turbine engines, it is important to understand the thermal-mechanical behavior of these materials within extreme application environments. This work focused on characterization of the in-situ response of a melt-infiltrated (MI) composite (i.e., a laminated Hi-Nicalon™, BN-interphase, melt-infiltrated (MI) SiC matrix composite) subjected to combined thermal-mechanical loads representative of intended operational environments. Laboratory scale test specimens were loaded in uniaxial tension at room temperature and at elevated temperature under both isothermal and non-isothermal conditions. During laser heating experiments, a number of in-situ techniques enabled monitoring of damage formation, strain evolution, and temperature up to failure at 1150 °C. Results indicate that applying mechanical loads axially in the presence of thermal gradients may induce interply delamination cracks not observed after CMC test specimens are loaded axially under isothermal conditions.

Theoretical analysis of non-linear Joule heating effects on an electroosmotic flow with patterned surface charges

In this work, non-linear Joule heating effects induced on an electroosmotic flow with patterned surface charges driven inside of a slit microchannel are analyzed. Here, the movement of the fluid is controlled by placing electro-thermal forces, which are induced through an imposed longitudinal electric field, E0, and the wall electric potential generated by electrodes inserted along the surface of the microchannel wall. For this analysis, the physical properties of the fluid are included as known functions, which depend on the temperature. Therefore, in order to determine the flow, temperature, and electric potential fields together with their simultaneous interactions, the governing equations have to be solved in a coupled manner. For a strong Joule heating, the non-isothermal flow regime reveals that with the presence of thermal gradients, the local electro-thermal and viscous forces, F¯χ and F¯v,χ, are affected in a sensible manner, which results in changes in the flow pattern causing the interruption or intensification of recirculations along the microchannel. In parallel, increments of flow rate or pressure are observed due to electro-thermal interactions associated with the mentioned thermal dependence of the physical properties; here the corresponding effects induced in those regions where thermal gradients are located prevail. The present analytical formulation allows determining the local electro-thermal rotational moment, Mχ, which clearly reveals the fundamental importance of when thermal gradients interact with the electric and flow fields. Thus, the local electro-thermal rotational moment can be used to interpret alterations in the flow pattern linked to the temperature rise.

Effect of blisters and rims on radial hydride precipitation in fuel elements under dry storage conditions

ABSTRACT Hydrogen enters in Zry clads during fuel element life in service; when hydrogen exceeds the terminal solid solubility, circumferential hydrides are precipitated. In the presence of thermal gradients, produced for instance by oxide spallation, localized concentration of hydrides - known as blisters - can be formed. Due to the fact that zirconium hydrides have a lower density that zirconium alloys, stresses are produced around blisters. These stresses may overcome the necessary threshold to precipitate radial hydrides. Moreover, cracks inside blisters may act as initiators of delayed hydride cracking (DHC). In the dry storage facilities developed for Atucha 1, Zry - 4 clads of spent fuel elements (SFE) must withstand environmental conditions which include maximum temperature of 200°C and internal SFE pressure of ~ 75 bar (value at end of cycle). During dry storage, if blisters are present, stresses around them are superposed to circumferential stresses generated by the internal pressure due to fission gasses helping precipitation and/or growth of radial hydrides and DHC process. In order to assess this possibility, in the present work sections of Zry-4 cladding were hydrided. Blisters have been grown by thermal gradient under different conditions of temperature and shapes of thermal contact in order to obtain different blister dimensions and fraction of radial hydrides. After blister formation, some specimens were subjected to Hydride Reorientation Test (HRT) by applying an internal pressure of 75 bar at 200°C. Finally, tubes were sectioned and metallographically analyzed looking for radial hydrides. In some samples radial hydrides were observed, which effectively grew by the internal pressure of Zry-cladding during HRT.

Motion control in free-standing shape-memory actuators

In this work, free-standing shape-memory thermally triggered actuators are developed by laminating ‘thiol-epoxy’-based glassy thermoset (GT) and stretched liquid-crystalline network (LCN) films. A sequential curing process was used to obtain GTs with tailored thermomechanical properties and network relaxation dynamics, and also to assemble the final actuator. The actuation extent, rate and time were studied by varying the GT and the heating rate in thermo-actuation with an experimental approach. The results demonstrate that it is possible to tailor the actuation rate and time by designing GT materials with a glass transition temperature close to that of the liquid-crystalline-to-isotropic phase transition of the LCN, thus making it possible to couple the two processes. Such coupling is also possible in rapid heating processes even when the glass transition temperature of the GT is clearly lower than the isotropization temperature of the LCN, depending on the network relaxation dynamics of the GT and the presence of thermal gradients within the actuators. Interestingly, varying the GT network relaxation dynamics does not affect the actuation extent. As predicted by the analytical model developed in our previous work, the modulus of the GT layer is mainly responsible for the actuation extent. Finally, to demonstrate the enhanced control of the actuation, specifically designed actuators were assembled in a three-dimensional actuating device able to make complex motions (including ‘S-type’ bending). This approach makes it possible to engineer advanced functional materials for application in self-adaptable structures and soft robotics.

Issues and characterization of fiber Bragg grating based temperature sensors in the presence of thermal gradients

This paper reviews the main issues arising from the use of fiber Bragg gratings as temperature sensors in the presence of significant thermal gradients. These conditions occur for example during laser thermal ablation of tumors. In particular, the paper focuses on the identification of the grating position along the fiber, which represents one of the main uncertainty contributions. A novel experimental setup for the grating center localization is proposed and the corresponding characterization procedure has been devised. The setup is built in such a way as to generate reproducible linear temperature distributions. Tests carried out using a sensor prototype have shown that the grating position can be found with a standard uncertainty of 0.3 mm.

Thermal effect on the creep in a rotating disc by using Sherby’s law

The effect of imposing linear thermal gradient on the steady state creep behavior of a rotating functionally graded Al-SiCp disc is investigated in the present study by using Sherby’s law. Mathematical model to describe steady state creep behavior in rotating disc made of isotropic Al-SiC composite in presence of linear thermal gradient in the radial direction has been formulated. The distributions of stresses and strain rates have been obtained. The creep response of a composite disc with uniform temperature has also been computed for comparison with the results obtained for thermally graded discs. The creep rates in a rotating thermally graded disc can be significantly reduced in presence of thermal gradients.

Characterisation and modelling of thermal expansion coefficient of woven carbon/epoxy composite and its application to the determination of spring-in

Properties of resin and composite, especially anisotropic coefficients of thermal expansion, are very crucial to precisely determine residual stress generated in a composite part. No comprehensive study is available in the literature to determine these properties for woven composites and then its application to model residual stress in woven carbon epoxy composite parts. In the present article, experimental results on thermal coefficients of RTM6 epoxy resin as well carbon/epoxy woven composites obtained using different experimental techniques are compared with homogenised coefficients of thermal expansion results. Evolution of spring-in angle of L-shaped carbon/epoxy woven composite (during and after cure) with three different thicknesses is modelled by simultaneously solving the thermal-kinetics and thermal-chemical-mechanics coupling by using finite element code COMSOL Multiphysics. Objective was to quantify the contribution of curing and cooling to the formation of residual stress. Anisotropic properties of composite, during and after cure, required for numerical simulation are obtained using an analytical method. Variation in properties with degree of cure and thermal gradients induced in the part during fabrication are considered while modelling. Modelled properties of cured composites were compared with experimental values and were found in agreement. The spring-in angle values obtained by numerical simulation are compared with the results of the analytical model as well as experiments. Effect of variation of fibre volume fraction and presence of thermal gradients on spring-in was studied as well.

Fission gas bubble percolation on crystallographically consistent grain boundary networks

Fission gas release in nuclear fuels can be modeled in the framework of percolation theory, where each grain boundary is classified as open or closed to the release of the fission gas. In the present work, two-dimensional grain boundary networks were assembled both at random and in a crystallographically consistent manner resembling a general textured microstructure. In the crystallographically consistent networks, grain boundaries were classified according to its misorientation. The percolation behavior of the grain boundary networks was evaluated as a function of radial cracks and radial thermal gradients in the fuel pellet. Percolation thresholds tend to shift to the left with increasing length and number of cracks, especially in the presence of thermal gradients. In general, the topology and percolation behavior of the crystallographically consistent networks differs from those of the random network.

Thermodynamic magnon recoil for domain wall motion

We predict a thermodynamic magnon recoil effect for domain wall motions in the presence of temperature gradients. All current thermodynamic theories assert that a magnetic domain wall must move toward the hotter side, based on equilibrium thermodynamic arguments. Microscopic calculations on the other hand show that a domain wall can move either along or against the direction of heat currents, depending on how strong the heat currents are reflected by the domain wall. We have resolved the inconsistency between these two approaches by augmenting the theory in the presence of thermal gradients by incorporating in the free energy of domain walls by a heat current term present in nonequilibrium steady states. The condition to observe a domain wall propagation toward the colder regime is derived analytically and can be tested by future experiments.

Noise spectroscopy of CoFeB/MgO/CoFeB magnetic tunnel junctions in the presence of thermal gradients

We present experimental data of the precessional dynamics of the free layer of CoFeB/MgO/CoFeB based magnetic tunnel junctions (MTJ) in the presence of thermal gradients across the MTJ. The free layer precession is investigated by noise spectroscopy. Thermal gradients of the order of tens of mK/nm across the MTJ are generated by electrical heating. Without applied thermal gradients we find spin transfer torque modified magnetization precession. With increasing thermal gradients we generally observe a decrease of the precession frequency which could be related to an increasing overall free layer temperature. However an asymmetry of the line width behavior for parallel and antiparallel orientation points towards additional effects beyond thermal activation. This could be a hint for the modification of the precessional dynamics in magnetic tunnel junctions by thermal spin torques.

Thermal Gradients in Solid Oxide Fuel Cell Anodes: X-Ray Diffraction, Thermal Imaging and Model Prediction

Solid Oxide fuel cells (SOFC) are a widely discussed future power option due to their high energy efficiency and fuel flexibility in addition to low maintenance requirements. However, the long term mechanical stability of the stack can be compromised due to simple operation as a result thermally mismatched materials (1). The commonly used Ni / YSZ anode – electrolyte combination provides a high degree of electrochemical performance; however, the presence of the Ni catalyst which aids the performance of the cell results in a significant mismatch in the coefficient of thermal expansion (CTE) between the anode and electrolyte of cells (2). Issues arising from mismatched CTE can be further exacerbated by the presence of thermal gradients within SOFCs. Thermal gradients which can arise from simple operation, internal reforming and inhomogeneous current distribution amongst other factors (3-5) have been noted to significantly increase the prospect of cell failure (6). In this study the effect of thermal gradients on SOFC anodes is investigated using both experimental and computational methods. Experiments conducted at Diamond Light Source using combined synchrotron X-ray diffraction and infrared thermal imaging (7) are detailed highlighting the presence of non-uniform thermally derived stresses at operationally relevant temperatures within the Ni phase of the anode of cells. Computationally derived thermal gradients have been generated using finite element method modelling in order to study the effect of cell operation utilising a three dimensional electrochemical model. The results obtained from the model have been coupled with the experimental results in order to investigate the effect of various parameters upon the cell. Operational conditions including cell polarization and fuel flow conditions and configurations are examined to highlight the importance of cell optimisation in minimising stresses within the cell anode to prolong cell lifetime whilst maximising performance.

Thermally driven domain-wall motion in Fe on W(110)

It has recently been shown that domain walls (DWs) in ferromagnets can be moved in the presence of thermal gradients. In this work we study the motion of narrow domain walls in low-dimensional systems when subjected to thermal gradients. The system chosen is a monolayer of Fe on W(110) which is known to exhibit a large anisotropy while having a soft exchange, resulting in a very narrow domain wall. The study is performed by means of atomistic spin dynamics simulations coupled to first-principles calculations. By subjecting this system to thermal gradients we observe a temperature-dependent movement of the domain wall. The thermal gradient always makes the domain wall move towards the hotter region of the sample with a velocity proportional to the gradient. Our material specific study is complemented by model simulations to discern the interplay between the thermal gradient, magnetic anisotropy, and the exchange interaction and shows that the larger DW velocities are found for materials with broader domain-wall width. The relatively slow DW motion of the Fe/W(110) system is hence primarily caused by its narrow domain-wall width, which results from its large magnetic anisotropy and soft exchange.

Advances in deployable structures and surfaces for large apertures in space

Large apertures in space have applications for telecommunications, Earth observation and scientific missions. This paper reviews advances in mechanical architectures and technologies for large deployable apertures for space antennas and telescopes. Two complementary approaches are described to address this challenge: the deployment of structures based on quasi-rigid members and highly flexible structures. Regarding the first approach, deployable articulated structures are classified in terms of their kinematics as 3D or planar linkages in multiple variants, resulting in different architectures of radial, peripheral or modular constructions. A dedicated discussion on the number of degrees of freedom and constraints addresses the deployment reliability and thermo-elastic stability of large elastic structures in the presence of thermal gradients. This aspect has been identified as a design driver for new developments of peripheral ring and modular structures. Meanwhile, other design drivers are maintained, such as the optimization of mass and stiffness, overall accuracy and stability, and pragmatic aspects including controlled industrial development and a commitment to operators’ needs. Furthermore, reflecting surface technologies and concepts are addressed with a view to the future, presenting advances in technical solutions for increasing apertures and reducing areal mass densities to affordable levels for future missions. Highly flexible materials capable of producing ultra-stable shells are described with reference to the state of the art and new developments. These concepts may enable large deployable surfaces for antennas and telescopes, as well as innovative optical concepts such as photon sieves. Shape adjustment and shape control of these surfaces are described in terms of available technologies and future needs, particularly for the reconfiguration of telecommunications antennas. In summary, the two complementary approaches described and reviewed cover the domain of present and foreseeable space applications. Recent European developments are discussed within a global context and a critical review of the state of the art and recent advances taking into account the reliability and structural stability as design drivers.

Experimental Investigation of Drying in a Model Porous Medium: Influence of Thermal Gradients

In this study, isothermal and non-isothermal drying experiments with imposed constant temperature gradients have been conducted in a two-dimensional square pore network of borosilicate glass (SiO2) with interconnected etched channels. In experiments with temperature decreasing from the open surface of the pore network, a travelling stabilized gas-liquid region could be established, while in isothermal experiments with uniform temperature faster breakthrough and an extended two-phase zone were observed. Both findings are in good agreement with pore network simulations. However, numerical underestimation of drying rates (especially in the second period of drying) indicates that liquid films might play a major role in the experimental pore networks, even in the presence of thermal gradients.