High Temperature Gradients Research Articles

Defect recognition in Ti-6Al-4V fabrication by in situ monitoring of directed energy deposition using a laser beam

Purpose This paper aims to investigate the relationship between in situ monitoring characteristics and surface defects in laser-based directed energydeposited Ti-6Al-4V. Design/methodology/approach In situ monitoring was conducted to extract and quantify the monitoring characteristics of each frame. A two-dimensional contour map was generated using the quantified characteristics to determine the defect formation locations. Computational thermal-fluid dynamics software was used to determine which surface tension terms or shielding gas had a significant effect on the depression of the molten pool. Findings This study has made a significant contribution by revealing the direct correlation between the molten pool size and brightness with defect formation in laser-based DED of Ti-6Al-4V. It was found that in regions of reduced height, the molten pool exhibited increased size and brightness, leading to surface depressions due to vapor recoil pressure flattening the molten pool. Moreover, the results highlighted that the enhanced Marangoni forces, caused by a high-temperature gradient, hindered the proper accumulation of molten metal, exacerbating height reductions. This insight provides a deeper understanding of how molten pool dynamics directly influence surface quality, which is a critical factor in DED processes. Originality/value This study contributes to understanding of the relationship between in situ monitoring characteristics and surface defects in laser-based directed energy-deposited Ti-6Al-4V. Additionally, by using in situ monitoring and computational analysis, significant insights were gained into the factors influencing molten pool behavior and subsequent surface defects.

Unique High-Resolution Temperature Mapping of Stage 1 Turbine Vane in a Long-Term Engine Test

Abstract In this study, surface temperature maps resulting from a long-term engine test were evaluated on a stage 1 turbine vane using thermal history coatings (THCs). THCs are ceramic-type sensor coatings doped with a lanthanide ion, giving the coating photoluminescent properties. The THC's structure permanently changes when exposed to high temperatures, which in turn alters its photoluminescent properties. Therefore, historic maximum temperature maps are evaluated by optically probing the THC point-by-point across the vane's surface. The vane was exposed to a non-dedicated (multi-cycle, multi temperature-level) engine test lasting over 8 months. Two passes of THC measurements were taken, termed low resolution (LR) and high resolution (HR), each, respectively, with point pitches of 5 mm and 1 mm. The latter provided a unique insight into the historic maximum temperature profile of the vane, particularly around high-temperature gradient regions, such as those close to effusion cooling holes. The THC temperatures were compared to the engine manufacturer's (Doosan Enerbility) heat transfer code (Doosan Integrated Thermal Analysis for Cooling System (DiTACS)) for validation. The THC temperature mapping was successful with good coverage across the vane. The leading edge and pressure side trailing edge regions were typically the hottest. The HR measurements clearly show sharp thermal gradients around the effusion cooling holes on the vane's airfoil and platforms. Critically, the THC measurements were compared very well to the DiTACS measurements, validating THCs as a credible temperature measurement technique for long-term, non-dedicated engine tests for components operating in some of the most extreme environments of an engine.

High-performance near-substrate heat sink with Tesla-like rotor-wing microchannel for chiplet cooling application

Microchannel heat sink provides a viable solution for enhancing thermal management in high-power chiplets. However, in consideration of cooling efficiency, low pressure drop, cost-effectiveness and reliability, it’s challenging to create an effective heat sink for a million-watt chip. Inspired by Tesla structure, this study introduces a Tesla-like rotor-wing microchannel (TRWM) to boost cooling performance. Due to the features of main channels, branch channels, and micro-islands in TRWM design, fluid in microchannel is effectively mixed and stirred with varied flow patterns, high temperature gradients and significant velocity gradients, which enhances the cooling capacity through precise fluid dynamics adjustment. Three different TRWMs are designed and compared by simulations. And the simulated results show that TRWM of 135° model achieves a maximum heat flux of 1514 W cm−2 @0.1 cm2 under a 60 K temperature rise, improving cooling efficiency by 19.4 % over the traditional channel design with 1221 W cm−2 @0.1 cm2. And then, the TRWM is fabricated using Ultraviolet Lithgraphie Galvanoformung Abformung (UV-LIGA) and bonding processes. Ultimately, the fabricated 135° TRWM sample demonstrated a heat dissipation capacity of up to 1446 W cm-2 @0.1 cm2 with a temperature increase of 60 K, which corresponds well with the simulated results. Moreover, the discrepancy between the actual and simulated values is less than 6.3 % under the same conditions. This performance provides a practical design solution for chiplet cooling and highlights the potential of bioinspired microfluidics.

A novel characterization methodology for vapor-injected compressors: A comparative analysis with existing black-box models

In regions characterized by high temperature gradients, vapor compression systems often necessitate operation at very high pressure ratios resulting in a reduction in system capacity. Economized vapor injection compressors are used to avoid these issues, yet a precise predictive map for various compressor technologies with minimal data and relatively better performance remains unclear. This paper establishes a black-box compressor model to accurately predict compressor power, injection mass ratio, and evaporator mass flow rate in compressors with a single vapor injection port. This model is compared against three legacy models from literature and the ANN model, for reference. All five models are evaluated based on their ability to predict the aforementioned metrics. The proposed black-box model can predict the relevant metrics all within 5 % Mean Absolute Percentage Error (MAPE). Additionally, a refrigerant sensitivity analysis is performed with the black-box models. The model is trained using data from R410A and then used the coefficients to predict the performance of the same compressor when using R454B, and vice versa. It can estimate the evaporator mass flow rate with an accuracy within 3 %, the power within 2 %, and the injection mass ratio with MAPE less than 3 %.

Physical Heat Cycle Measurement of Resistance Spot Welding

Resistance spot welding (RSW) is still the ideal joining method in the automotive industry. Mostly steel sheets are used in the car body, so overlap and layering are required for welding or riveting, as spot welding provides simultaneous clamping force with interfacial welding to ensure the required strength and quality. A fundamental understanding of heating and cooling rates in thermal distributions is essential for predicting microstructure formation in the weld and the heat-affected zones (HAZ) of RSW joints. The ability to measure the heat cycle in the RSW process can be valuable in weld control and welding parameter optimization. RSW parameters can be optimized through tensile shear tests and microscopic investigations. Heat cycle measurement (HCM) demonstrates the welding consequences in terms of the change in mechanical properties and microstructural formations. The accuracy of cooling rate measurements including t8/5 cooling time is very important to predict the microstructural evolution in the HAZ, however, the thermocouple measurement raises numerous challenges due to the high temperature gradient and small weld and HAZ size. During our investigations heat cycle measurement has been conducted experimentally by a K-type thermocouple. The data logger is connected to the output of the thermocouple for recording the voltage to measure the temperature distributions as a function of both time and position during the welding process. Measurement results of 1 mm thick martensitic MS1400 steel overlapped RSW joints are discussed, and the HCM curve of heating and cooling rates of the spot-welding process is presented. The heat cycle during RSW was measured with two different welding parameter combinations. In addition to welding current, welding time, and electrode force, pulsation has shown disparate curves. Numerous experiments have been attempted to measure the heat cycle in HAZ sub-zones due to the difficulty of positioning the thermocouple accurately, uppercritical HAZ, intercritical HAZ, and subcritical HAZ were investigated and measured in both welding parameter combinations. Difficulties were encountered in the experimental work as a result of the instantaneous welding time and the vibration resulting from the passage of alternating electrical current between the two electrodes. A magnetic field is generated that affects the thermocouple measurement and appears as a noisy curve that is filtered out and smoothed. Joule heat, interfacial heat generation, and cooling effects of electrodes are also considered in the experiment.

Experimental and numerical investigation of direct ammonia solid oxide fuel cells with the implementation of ammonia decomposition source terms in a 3D finite volume-based model

Ammonia and fuel cells have become more popular in the past few years. This is directly associated with the prevailing industrial trend toward reducing emissions. Ammonia is a fuel that can be utilized in solid oxide fuel cells (SOFCs). Nevertheless, ammonia-fueled solid oxide fuel cells (DA-SOFCs) are sensitive to the degradation during operation. This study aims to determine the causes of such incidents as using experimental and numerical methods. Investigation of cells with various thicknesses of anode support layers revealed that the cells fracture during long-term operation under a load of 0.125 A cm−2. The OpenFuelCell (OFC) model developed by the author for the DA-SOFC analysis showed a high temperature gradient (up to 70 °C) in the microstructure. It was found that increasing the amount of ammonia supplied to the cell from 33.3 ml min−1 to 50 ml min−1 while maintaining the same fuel utilization increases the difference in temperature by 55 %. Subsequently, it was shown that the presence of ammonia in the anode functional layer (ASL) of the cell is associated with a decrease in DA-SOFC efficiency. The investigation identifies potential areas for improving DA-SOFCs from both the operational and manufacturing point of view and offers a tool for investigating these areas.

Temperature measurement in laser–metal processing using optical sensing: Radiometric calibration and validation of a multispectral camera

In laser–metal processing such as Laser-based Powder Bed Fusion of Metals (PBF-LB/M), the quality of the final product is significantly influenced by the thermal behaviour of the melt pool. Accurate temperature measurements are challenging due to high temperature gradients and rapid cooling. This study introduces Multispectral Imaging (MSI) as a highly effective thermometry technique to address these demanding conditions. MSI captures multiple bands of radiation, enabling the simultaneous determination of absolute temperature and surface emissivity. This capability is crucial for ensuring high-quality outcomes in laser–metal processing such as PBF-LB/M, contingent upon robust radiometric calibration.To enhance reliability, two radiometric calibration models were developed: an empirical model based on linear sensor data correlations and an analytical model leveraging sensor behaviour. An efficient calibration was found for both models across a range of signal-to-noise ratios in the black body calibrator, where the analytical model even performs robust under a high noise level.Additionally, the temperature accuracy in the solid-state case was tested with a thermo-mechanical simulator. Results showed that the empirical and analytical models achieved mean relative errors of 2.0% and 1.6%, respectively, outperforming the state-of-the-art. Further, laser irradiation experiments highlighted the analytical model’s ability to accurately determine the emissivity decrease during the solid-to-liquid transition, allowing for accurate melt pool temperature estimations. Conversely, the empirical model struggled to estimate temperature effectively in the liquid phase.This study not only proposed a robust methodology of MSI in temperature measurement but also confirmed the accuracy and reliability of the system, broadening the scope for further investigation into the intricate thermal dynamics involved in laser–metal interactions.

Heat pipe-enhanced two-stage thermoelectric harvester based on phase change material

The thermoelectric generator (TEG) technology is important for improving energy efficiency. However, the TEG limited by poor thermoelectric material properties and thermal transfer strategies faces challenges in achieving excellent heat harvesting capacity, especially for large-temperature gradients. Herein, we reported a heat pipe-enhanced two-stage thermoelectric generator (HTTEG) enabled by bismuth alloy-based PCM for effective space waste thermal harvesting. We comprehensively explored the effects of different thermal-enhanced strategies on HTTEG performance. Compared with copper plate and horizontal layout, the effective thermal conductivity obtained by vertical-planar heat pipe is 1.1 × 104 W/mK enhanced by 79.2 and 33.0 times, which has outstanding advantages in achieving considerable temperature difference and excellent thermoelectric harvesting capacity of the HTTEG. We also systematically discussed the thermoelectric properties of heat pipe-enhanced one-stage TEG (HOTEG) and HTTEG by simulation and LED test bench. The results show that the bismuth alloy-based PCM with considerable latent thermal capacity and remarkable heat transfer characteristics can suppress temperature fluctuations and expand thermoelectric harvest interval, achieving excellent power output capacity. Compared with the HOTEG, the thermoelectric power achieved by HTTEG is 4.45 W enhanced by 581.8 %, because optimizing the spatial layout of stage-two thermoelectric modules to the high-temperature gradient interval achieves remarkable heat harvesting efficiency.

Build orientation optimization considering thermal distortion in additive manufacturing

Additive manufacturing (AM) technology enables the fabrication of three-dimensional objects with complex shapes and has been extensively applied in various industries. AM is a layer-wise fabrication process where a variety of factors affect manufacturing performance and product quality. One of the most important factor is the thermal distortion, which is caused by the high temperature gradients in the fabrication process. The thermal distortion is also influenced by the support structure of the printing object, and this distortion varies depending on the chosen build orientation. Additionally, for the overhang regions, extra supports are required for printing and will be removed in the post-processing. For the 3D printing process, the thermal distortion and support requirements are interconnected and linked to build orientation. To investigate suitable build orientation, the thermal distortion and support are quantified, and a multi-objective build orientation optimization method is proposed to obtain representative orientations. Based on the proposed method, 9 typical 3D shapes are evaluated. In addition, the single-objective build orientation optimization problem is studied and compared, and the influence of slicing layers per stage on the simulation accuracy and efficiency is discussed. The effectiveness and applicability of the method are verified, and representative directions can be obtained for different fabrication purposes.

Establishment of Multi-Physics coupling model and analysis on thermal stress and crack risk in directional growth of TiAl alloys under electromagnetic confinement

The electromagnetic confinement directional solidification (EMCDS) technique is an optimal method for preparing large-size and non-contamination directional TiAl alloy crystals. Despite its advantages, the high temperature gradient inherent to this process induces thermal stress within the ingot, which increases the risk of cracking. To address this challenge, an innovative Integrated Multi-Physics Coupling Model was established to map and study the thermal stress field during EMCDS in this study. It synchronized the computation of the electromagnetic field, temperature field, solute field, flow field, and stress field during the crystal growth of TiAl alloy, and its high accuracy was proved by the micro-indentation experiment. Our analysis reveals that transverse temperature differences are crucial in inducing thermal stresses, and identifies that hot cracks and cold cracks are prone to occur respectively at the area of radial 23R along the sample (X = 23R) and on the sample surface, which aligns with experimental observations impressively. This model can more accurately and efficiently optimize the process parameters of the EMCDS process to avoid cracks and promote its industrial application.

A robust and high-throughput superhydrophobic-superoleophilic Zn/Ni composite coating prepared on stainless steel mesh for oil-water separation

Frequent oil spills and the discharge of oily wastewater have adverse effects on the environment and ecosystems. Against the backdrop of escalating environmental concerns, developing high-performing and eco-friendly oil-water separation techniques is crucial. In this study, we successfully created a PFOTS-Zn/Ni superhydrophobic-superoleophilic coating, abbreviated as PZNM, by electroplating Zn2+ and Ni2+ on the stainless steel mesh, followed by modification with perfluorooctyltriethoxysilane-ethanol solution. The wettability test results show that the water contact angle (WCA) reaches 161.3°, with a water sliding angle (WSA) of 2°, and the oil contact angle (OCA) is approximately 0°. Subsequently, the PZNM samples were subjected to tests including blade scratching combined with sandpaper abrasion, tape peeling, bending tests, submerging in acidic and alkaline solutions, high-temperature gradient exposure, and electrochemical corrosion to verify their outstanding long-term durability. Most importantly, it can be shaped into a practical continuous oil-water separation device. In the oil-water separation test, it was found that the superhydrophobic-superoleophilic PZNM achieves highly efficient oil-water separation, with a water/n-hexadecane separation efficiency of up to 93.2 % and a water-chloroform separation flux of 6450±30 L·m−2·h−1. The preparation process is straightforward and green, providing an economical solution. This design holds considerable prospects for applications in the domain of oil-water separation.

Selective laser melting of 2507 duplex stainless steel: Effect of energy density on microstructure and corrosion resistance

Duplex stainless steel (DSS) is widely used in the marine, petroleum, chemical, automotive, and other fields owing to its excellent mechanical properties and corrosion resistance. However, the research on duplex stainless steel prepared by additive manufacturing is still limited. In this paper, high-density 2507 DSS was successfully prepared by selective laser melting (SLM) additive manufacturing. The effects of energy density on the formability, phase composition, microstructure and corrosion properties of SLM 2507 DSS were investigated. The results showed that with the decrease of the energy density, the density of the specimen increases first and then decreases, and the density achieves 99.18% with the energy density of 190.5 J/mm3. The types of phases are not affected by the energy density, i.e., all 2507 DSS samples prepared by SLM showed a ferrite phase. The YOZ (parallel to the building direction) plane of the SLM 2507 DSS samples showed predominantly columnar grains attributed to the high temperature gradient and epitaxial growth characteristics. With the increase of energy density, the average grain size decreases slightly from 16.97 μm to 15.78 μm, the KAM value decreases slightly from 1.15 to 1.05, and the low angle grain boundaries (LAGBs) increase significantly from 68.4% to 74.8%. The SLM 2507 DSS sample exhibited excellent corrosion resistance. The self-corrosion potential of the sample is 136 mV and the self-corrosion current density is 2.066 × 10−8 A/cm2 at the maximum density. This investigation provides a new approach for the preparation of super duplex stainless steel, which can provide a theoretical basis and guidance for industrialized application.

Effect of Hot Isostatic Pressing and Solution Heat Treatment on the Microstructure and Mechanical Properties of Ti-6Al-4V Alloy Manufactured by Selective Laser Melting

A powder-bed-based additive manufacturing process called electron beam melting (EBM) is defined by high temperature gradients during solidification, which produces an extremely fine microstructure compared to the traditional cast material. However, porosity and segregation defects are still present on a smaller scale which may lead to a reduction in mechanical properties. It is important to have a better knowledge of the influence of post-fabrication treatments on the microstructure and mechanical characteristics before the use of additive manufacturing parts in specific applications. In this study, the effects of solution heat treatment (SHT) and hot isostatic pressing (HIP) on the microstructure and mechanical properties of Ti-6Al-4V alloy fabricated by the EBM process have been investigated. The SHT and HIP treatments can significantly improve the ductility of EBM Ti-6Al-4V due to the coarsening of α laths and the formation of β grains.

Effect of stress-relief heat treatment on the stress-rupture properties of a nickel-based superalloy manufactured by laser powder bed fusion

Residual stress is a ubiquitous phenomenon in laser powder bed fusion (LPBF) metal parts due to high temperature gradients and uneven heating during building processes. Subsequent stress-relief (SR) heat treatment is usually conducted to reduce the geometrical distortion and microscopic cracks induced by residual stress after the LPBF process. Using electron microscopy, X-ray diffraction, and stress-rupture tests, the experimental results revealed that the stress-rupture life of a nickel-based superalloy, IN625 alloy, subjected to SR treatment at 870°C and 980°C is inferior to that of the as-built (AB) alloy. The fundamental cause for this phenomenon is that more δ phase precipitates in the grains of the AB alloy during the stress-rupture test at 750°C/230 MPa, resulting in an increased dispersion hardening effect and a decreased ductility. Furthermore, the Laves and δ phases along grain boundaries in the SR treated alloys are more likely to coarsen, which is the second reason for the decreased stress-rupture life of the SR treated alloys.

Microstructure forming mechanism of inconel 625 alloy fabricated by laser/ultra-high (UHF) induction hybrid deposition method

To reveal the microstructure forming mechanism of laser/ultra-high frequency (UHF) induction deposition, this paper developed a microscopic phase-field (PF) model to numerically investigate dendrite growth during solidification. The macroscopic model of molten pool evolution is adopted to provide the solidification conditions for the microscopic PF model. The dendrite growth during laser deposition is simulated to evaluate the effect of UHF induction heat on the dendrite growth. Results show that because of the high temperature gradient and cooling rate, the PDAS of laser-UHF induction hybrid deposited layer is less than that of the laser deposited layer. The UHF induction heat also leads to a high flow velocity of the molten metal during laser-UHF induction hybrid deposition. The high flow velocity contributes to the decrease in PDAS by inhibiting the interdendritic enrichment of solute. During laser-UHF induction hybrid deposition, a higher solute gradient is present in the tip region of dendrite arm, leading to a faster dendrite growth rate. The UHF induction heat also increases the solute distribution coefficient during deposition, which further inhibits the element segregation. Under the action of UHF induction heat, a low interdendritic solute gradient and an evenly distributed solute can be obtained, thus helping increase interdendritic undercooling degrees and decreasing the PDAS. The simulated PDAS and solute distribution have good consistency with the experimental results. The spectral analysis of EDS line detection indicates that the laser-UHF induction hybrid deposited layer has a more refined microstructure and weaker element segregation than the laser deposited layer does.

Effect of deep cryogenic treatment on microstructure and mechanical properties of Fe50Mn30Co10Cr10 high entropy alloy fabricated by laser metal deposition

As a typical additive manufacturing technology, laser metal deposition (LMD) has been widely used to process high entropy alloy (HEA). However, deposited products usually have poor mechanical properties. In this study, deep cryogenic treatment (DCT) is proposed to enhance the mechanical performance of the laser melting deposited Fe50Mn30Co10Cr10 high entropy alloy. The tensile strength, hardness and strength-plasticity product of the HEA after 48 h of immersion by DCT processing were 901 MPa, 286HV and 33.3 GPa%, respectively. Compared with the as-deposited state, the HCP ε phase content and tensile strength of the deep cryogenic treated (DCTed) sample increased by 2.2 times and 30.8 %, respectively, while the average grain size decreased by 58.0 %. Furthermore, the friction coefficient and wear rate of the DCTed samples reduced by 25.6 % and 30.3 %, respectively. The high temperature gradient soaking in liquid nitrogen induced dynamic recrystallization, refining the FCC γ grain structures in the HEA. Meanwhile, the samples contained more HCP ε phase due to the transformation-induced plasticity (TRIP) effect triggered by thermal stress. The results indicate that the effects of fine-grain strengthening and second-phase particle dispersion strengthening contribute to the excellent mechanical performance. DCT processing provides a simple and effective way for overcoming the strength-ductility trade-off in the additive manufactured Fe50Mn30Co10Cr10 HEA.

Analysis of Soot Deposition Effects on Exhaust Heat Exchanger for Waste Heat Recovery System

This study investigates the thermal–hydraulic behavior and deposition characteristics of a shell and tube exhaust heat exchanger using a CFD-based predictive model of soot deposition. Firstly, considering the influences of thermophoretic, wall shear stress, and other deposition and removal mechanisms, a predictive model is developed for long-term performance of heat exchangers under soot deposition. Then, the variations in exhaust heat exchanger performance during a 4 h deposition period are simulated based on the model. Subsequently, the variation of deposition distribution and different deposition velocities are also evaluated. Finally, an analysis of the long-term performance of the exhaust heat exchanger under varying gas velocities and temperature gradients is conducted, revealing the performance variations under all engine-operating conditions. Results show that the deterioration in normalized relative j/f1/2 varies from 5.26% to 24.91% under different work conditions, and the exhaust heat exchanger with high gas velocity and low temperature gradient exhibits optimal long-term performance.

Ripple formation on TA2 pure titanium surface induced by annular laser beam at low pressure environment

Formation of the micron-scale circular ripples on metal surfaces produced by continuous wave laser irradiation in low-pressure environment was investigated experimentally and numerically in this manuscript. It is found that splashing of the molten material and oxidation of the metal surface, which usually happened in the atmospheric environment, were avoided in low-pressure environment. Therefore, circular ripples were formed during the solidification process of the molten pool. Numerical simulations based on the finite element method (FEM) and the moving mesh method were applied to investigate the evolution of the flow field behavior of the molten pool. It is found that the Marangoni effect and capillary force during solidification process dominate the formation of the micron-scale circular ripples. In addition, the ripple spacing can be controlled by the temperature gradient. At the same power density, a higher temperature gradient on the molten surface can be produced by an annular laser beam, compared to a Gaussian beam, thus a denser circular ripple structure were produced, which were confirmed by the experimental results. The results of this paper may enrich the investigations on the metal surface texturing methods, laser vacuum processing and laser material engineering.

Performance evaluation of GH3536/GH4169 functionally graded materials with linear gradient changes fabricated through direct energy deposition

This study employs the direct energy deposition method to fabricate functionally graded materials (FGMs) using GH3536 and GH4169 alloys. The approach achieves a seamless linear gradient transition in the FGMs, progressing from GH3536 alloy to GH4169 alloy. Additionally, the impact of a solid solution twin-aging process on the properties of the FGMs was investigated. The findings reveal that variations in the powder content of GH3536/GH4169 alloy exhibit minimal impact on the microstructure of the deposited FGMs, with columnar crystals predominantly growing along the <001> direction. The direct energy deposition process induces an exceptionally high temperature gradient and cooling rate, preventing the timely precipitation of the second strengthening phase (γ", γ', δ), as well as M6C and M23C6 in the FGMs. Furthermore, the formation of the Laves phase (Ni2Nb, Cr2Mo) diminishes the overall performance of the FGMs. In correspondence with the linear variation of GH3536/GH4169 alloy powder content, the hardness of each layer within the deposited FGMs exhibits an initial increase followed by a subsequent decrease. Following the solution aging treatment, complete dissolution of the Laves phase and δ phase occurs, leading to the formation of a secondary chain phase (M23C6) along the grain boundaries. Simultaneously, needle and granular δ phases disperse within the grain structure, contributing to enhanced performance in the FGMs and ensuring uniform hardness throughout.

Model-Based Interpretation of Thermal-Gradient Effects in Li-Ion Cells

The present study develops a combined experimental and modeling approach to study temperature gradients in Li-ion batteries. Although through-plane temperature differences may be small (ΔT ≈ 1-10 °C), large interelectrode thermal gradients (∇T ≈ 1,000-10,000 °C/m) can be present and have been found to significantly affect Li-ion battery performance and degradation [1,2]. Figure 1 illustrates a thin pouch cell (order of 300 microns thick) sandwiched between cool and warm plates. The cell uses a graphite anode and a NMC622 cathode. By maintaining the temperature of the cool and warm blocks, a thermal gradient can be enforced across the cell. Depending on how the cell is positioned, the cell’s cathode or anode can be on the cool or warm side. Experiments include overpotential analysis and galvanostatic intermittent titration technique (GITT) at different states of charge. Results show different behaviors as the thermal gradient is applied and the directionality of the thermal gradient changes.This presentation presents a pseudo-two-dimensional (P2D) model [3] that has been modified to represent the experimental conditions, including the thermal gradients. The model considers the temperature dependencies of thermodynamic properties, electrolyte conductivities, electrode diffusion coefficients, intercalation rates, etc. [4]. The model captures some, but not all, of the measured behaviors. Thus, an important objective is to investigate physical and chemical behaviors that are not included in typical P2D models. For example, off-diagonal Onsager contributions could be significant in the presence of high temperature gradients. The results predict how thermal gradients can contribute to Li-ion concentration fluxes (Soret contribution), and thus affect battery performance. The scientific intent is to understand the thermal gradient behaviors and thus assist battery design guidelines and battery-management control considerations. Acknowledgments The authors thank the Office of Naval Research (Dr. Michele Anderson) for financial support (ONR award numbers: N00014-23-1-2694 and N00014-22-1-2411) of this work.