Coefficient Of Thermal Expansion Mismatch Research Articles

Figure‐of‐Merit Compatibility of Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is one of the most efficient energy converters which can produce electricity from reactions of gaseous chemicals. However, a high operating temperature (T o) of 800–1200 °C is required to achieve the highest peak performance. Such high T o triggers miscellaneous problems, that is, mechanical disintegration with further degradation of its electrical performance for a long‐term application. Despite accomplishing various approaches for the improvement of SOFC performance, the standard level to determine SOFC performance has never been achieved. To date, a quantitative analysis is only pinpointing the power output density (P out). The SOFC cannot be simply determined by a mere P since other parameters that is chemical stability between SOFC components, mismatch of thermal expansion coefficient, and area‐specific resistance also contribute to the mechanical durability and electrochemical stability of the SOFC. Hence, herein, a novel idea is proposed to characterize the overall performance of SOFC through the figure‐of‐merit compatibility (χ cp) of SOFC cells. The χ cp can reflect a level of SOFC performance, thus it can be utilized for future improvements.

Thermoelastic fields for a heat exchanger of arbitrary shape in a bi-material infinite plane

When a bi-material with two jointed dissimilar half-planes containing an arbitrarily shaped polygonal inclusion is subjected to heat flow, the thermoelastic fields, including temperature and displacement, can be derived by the Green’s function technique with the integral of the source over the inclusion. Using Hadamard’s regularization, the two-dimensional (2D) thermal, elastic, and thermoelastic Green’s functions of two-jointed dissimilar half-planes are firstly derived from the 3D Green’s functions as the corresponding fundamental solutions. The fundamental solutions for semi-infinite and infinite domains can be recovered by adjusting the material constants. Eshelby’s tensors are derived in terms of the biharmonic, harmonic, and two Boussinesq’s displacement potential functions. When a heat exchanger of arbitrary shape is embedded in a matrix with different thermal and mechanical properties, combining a continuously distributed eigen-temperature gradient and eigenstrain field, the dual equivalent inclusion method (DEIM) is applied to handle the material mismatch of thermal conductivity, stiffness, and thermal expansion coefficient, respectively. Therefore, the full thermoelastic fields can be obtained by the integral over the heat exchanger only. The eigen-fields are expanded in the Taylor series referred to the center of the particle, which exhibits tailorable accuracy with uniform, linear or quadratic terms in comparison with the analytical solution for a circular inhomogeneity in the infinite domain. An exact thermoelastic solution of a circular inhomogeneity embedded within the infinite domain is present. The case study of an electric heat cable in the concrete block demonstrates the capability and exactness of the model. The method can be used for a thin film containing a heat exchanger of arbitrary shape as either a heat sink or source.

An All-Silicon Resonant Pressure Microsensor Based on Eutectic Bonding

In this paper, an all-Si resonant pressure microsensor based on eutectic bonding was developed, which can eliminate thermal expansion coefficient mismatches and residual thermal stresses during the bonding process. More specifically, the resonant pressure microsensor included an SOI wafer with a pressure-sensitive film embedded with resonators, which was eutectically bonded with a silicon cap for vacuum encapsulation. The all-Si resonant pressure microsensor was carefully designed and simulated numerically, where the use of the silicon cap was shown to effectively address temperature disturbances of the microsensor. The microsensor was then fabricated based on MEMS processes where eutectic bonding was adopted to link the SOI wafer and the silicon cap. The characterization results showed that the temperature disturbances of the resonant pressure microsensor encapsulated with the silicon cap were quantified as -0.82 Hz/°C of the central resonator and -2.36 Hz/°C of the side resonator within a temperature range from -40 °C to 80 °C, which were at least eight times lower than that of the microsensor encapsulated with the glass cap. Compared with the microsensor using the glass cap, the all-silicon microsensor demonstrated an accuracy improvement from 0.03% FS to 0.01% FS and a reduction in short-term frequency fluctuations from 3.2 Hz to 1.5 Hz.

Contributions of Support Point Number to Mirror Assembly Thermal Sensitivity Control.

Due to the extreme environmental temperature variations, solutions that enable ultra-low thermal sensitivity in a mirror assembly are crucial for high-performance aerial optical imaging sensors (AOIS). Strategies such as the elimination of the coefficient of thermal expansion (CTE) mismatch and the employment of a flexure connection at the interface cannot be simply duplicated for the application involved, demanding specific design constraints. The contributions of support point number to the surface thermal sensitivity reduction and support stiffness improvement have been studied. A synthetic six-point support system that integrates equally spaced multiple ultra-low radial stiffness mirror flexure units and assembly external interface flexure units has been demonstrated on a 260 mm apertured annular mirror that involves significant CTE mismatch and demanding support stiffness constraint. The surface deformation RMS, due to the 35 °C temperature variation, is 16.7 nm.

Design and Comparison of Diamond‐ and Sapphire‐Based NbN KIDs for Fusion Plasma Polarimetric Diagnostics

Design and characterization measurements performed on kinetic inductance detectors (KIDs) produced on sapphire and polycrystalline diamond substrates are presented. Designed as part of a nuclear fusion polarimetric diagnostic, the foreseen plasma‐probing frequency of the final devices is 1.3 THz with a maximum response time under 10 ms and cross‐polarization target accuracy of 1%. These detectors are based on superconducting micro‐resonators that undergo de‐tuning upon absorption of radiation. The main characteristics of the devices include polarization sensitiveness and lumped‐elements multi‐pixel configuration produced from photo‐lithographed niobium nitride (NbN) thin films. The direct current (DC) and microwave characterization measurements highlight large differences in the thin film quality, with the superconductor deposited on diamond showing reduced critical temperature, lower critical current density, and increased values of the kinetic inductance. This difference is likely due to the higher lattice constant and thermal expansion coefficient mismatch between film and substrate in the case of diamond, with the different surface finish quality of the crystalline samples available also playing a role. The devices on both substrates show a bolometric response to THz radiation that fulfills the requirement guidelines and represent a good starting point to optimize the design for the application at hand.

Feasibility research of Yb3Al5O12 garnet as environmental barrier coating materials

In this work, Yb3Al5O12 (YbAG) garnet, as a new material for environment barrier coating (EBC) application, was synthesized and prepared by atmospheric plasma spraying (APS). The phases and microstructures of the coatings were characterized by XRD, EDS and SEM, respectively. The thermal stability was measured by TG-DSC. The mechanical and thermal-physical properties, including Vickers hardness (Hv), fracture toughness (KIC), Young's modulus (E), thermal conductivity (κ) and coefficient of thermal expansion (CTE) were also measured. The results showed that the as-sprayed coating was mainly composed of crystalline Yb3Al5O12 and amorphous phase which crystallized at around 917 °C. Moreover, it has a hardness of 6.81 ± 0.23 GPa, fracture toughness of 1.61 ± 0.18 MPa m1/2, as well as low thermal conductivity (0.82–1.37 W/m·K from RT-1000 °C) and an average coefficient of thermal expansion (CTE) (∼6.3 × 10−6 K−1 from RT to 660 °C). In addition, the thermal shock and water-vapor corrosion behaviors of the Yb3Al5O12-EBC systems on the SiCf/SiC substrates were investigated and their failure mechanisms were analyzed in details. The Yb3Al5O12 coating has an average thermal shock lifetime of 72 ± 10 cycles as well as an excellent resistance to steam. These combined properties indicated that the Yb3Al5O12 coating might be a potential EBC material. Both the thermal shock failure and the steam recession of the Yb3Al5O12-EBC systems are primarily associated with the CTE mismatch stress.

In-situ characterization and multiscale finite element analyses for thermomechanical behavior of 3D woven composites

Thermal stress concentration, induced by the mismatch of coefficients of thermal expansion (CTE), contributes significantly to fatigue and failure of composites. We demonstrated the practicality of fiber Bragg grating sensors and high-resolution digital image correlation technique for in-situ characterization of thermal strain field evolution in 3D angle-interlocked woven composites (3DAWC). Multiscale representative volume element (RVE) models were constructed for studying its steady-state and transient-state thermomechanical behaviors. We found the CTE mismatch causes stress localization at the interface and it exhibits a periodic distribution within the 3DAWC, which attributes to its 3D architecture. Stresses and strains concentrate mainly on the sharp interfaces close to the diamond-shaped yarns and the resin-rich areas, respectively. The thermal stress in the resin reaches a maximum of 22 MPa around Tg, close to half of its yield stress. The macroscopic homogenized CTEs calculated from the RVE model in thickness and in warp directions are greater than those of each component. Transient analysis reveals that the non-uniform temperature field induced by heat transfer effects has little effect on maximum stress values in each component.

Improving the durability of cobaltite cathode of solid oxide fuel cells - a review.

Solid oxide fuel cells (SOFCs) are highly efficient, low-emission, and fuel-flexible energy conversion devices. However, their commercialization has lagged due to the lack of long-term durability. Among several performance degradation mechanisms, cathode degradation and elemental inter-diffusion of the electrolyte and cathode has been identified as the predominant factors. In the most common SOFC systems, a cobalt-based perovskite material is used, for example LSC or LSCF. These cobalt-based materials offer mixed conductivity and higher concentration of oxygen vacancies as compared to LSM at lower operating temperature leading to favorable reduction kinetics. However, the presence of cobalt results in higher cost, higher thermal expansion co-efficient (TEC) mismatch and most importantly leads to rapid degradation. Various elements like strontium, cobalt, cerium, chromium, or zirconium accumulate or deposit at the electrode-electrolyte interface, which results in sluggish reaction kinetics of the oxygen reduction reaction (ORR). These elements react to form secondary phases that have lower ionic and electronic conductivity, cover active reaction sites, and eventually lead to cell and system deterioration. Over the past decade, several studies have focused on preventative and protective measures to prolong SOFC lifetime which includes novel fabrication techniques, introduction of new layers, addition of thin films to block the cation transport. Such efforts to prevent the formation of insulating phases and decomposition of the cathode have resulted in a remarkable improvement in long-term stability. In this review paper, current research on leading mechanisms responsible for the degradation of cobaltite cathode of solid oxide fuel cell has been summarized and durability improvement strategies of cobalt-based SOFC cathodes have been discussed.

One-step additive manufacturing of TiCp reinforced Al2O3–ZrO2 eutectic ceramics composites by laser directed energy deposition

With the rapid and adaptable process properties, laser directed energy deposition (LDED) is increasingly becoming popular for the direct additive manufacturing of melt growth eutectic composite ceramics. In this research, LDED is used to fabricate TiCp-reinforced Al2O3–ZrO2 eutectic ceramics. The inhibitory mechanism of various ratios (0 wt % - 50 wt %) of TiCp on crack and porosity defects is investigated. In addition, a summary of the influence of TiCp on the microstructure and mechanical properties is provided. The results indicate that TiCp particles are uniformly distributed throughout the matrix, and the eutectic microstructure around TiCp exhibits an irregular eutectic transition as a result of a decrease in solidification rate. The mismatch of thermal expansion coefficient and elastic modulus between TiCp and Al2O3–ZrO2 eutectic matrix causes the cracks to be pinned and transgranular, which suppresses the cracks effectively. Meanwhile, doping TiCp particles enhances the molten pool impact and increases the molten pool temperature, which accelerates the gas escape rate, leading to the porosity decreasing from 6.37% to 0.29%. In comparison to Al2O3–ZrO2 eutectic ceramics, 50 wt% TiCp content results in the greatest flexural strength and fracture toughness, with increases of 34.18 and 13.42%, respectively. The greatest compressive strength is attained at 30 wt% TiCp doping, which is approximately 83.75% greater than eutectic ceramics.

A Temperature-Insensitive Resonant Low-Pressure Microsensor Based on Au–Si Eutectic Wafer Bonding

This article shows a temperature-insensitive resonant low-pressure microsensor based on Au–Si eutectic wafer bonding. The microsensor comprised an silicon-on-insulator (SOI) wafer as the sensitive part, and a silicon cap packaging the SOI wafer in vacuum through Au–Si eutectic bonding to lower the thermal stresses and reduce the temperature sensitivity as no coefficient of thermal expansion mismatch exists between the SOI wafer and the silicon cap. Meanwhile, the silicon cap can counterbalance the thermal stresses induced by Au layer and Ti getter, lowering the temperature sensitivity of the microsensor from 7 to 2 Hz/°C with the thickness increasing from 300 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1500 \,\mu \text{m}$ </tex-math></inline-formula> based on numerical simulation. Characterization results showed that the low-pressure sensitivities of the microsensors were about ±6 Hz/Pa, and the temperature sensitivities were about 5 Hz/°C, indicating the microsensor was insensitive to temperature based on Au–Si eutectic wafer bonding. Furthermore, high accuracies (measurement errors within ±3.5 Pa) and satisfactory long-term stability (measurement errors within ±3 Pa after one month) were obtained under the low pressures from 0.1 to 1 kPa and temperature from −40 °C to 80 °C, indicating the better performances than previous works for the low-pressure microsensors.

Measurement and simulation of thermal-induced residual stresses within friction stir lapped Al/steel plate

Welding residual stress and distortion usually affect the assembly accuracy, product performance, and service life, with no exception for the Al/steel hybrid structure. The lapped joint of 5052 Al alloy and DP590 steel was made by friction stir welding (FSW) at various rotation speeds. A subsequentially coupled computational modeling approach based on the in-house software JWRIAN combining an efficient hybrid solution method was developed to predict the temperature field, the residual stress distribution and the warpage deformation. Based on the simulation, a higher welding temperature with subsequently a higher average cooling rate was created at a high rotation velocity, leading to the area extension of the longitudinal tensile stress and transverse compressive stress near the weld zone. The thermal expansion coefficient mismatch leads to the mirroring distribution of the near interfacial residual stress on both sides, particularly for transverse residual stress. With the increasing rotation speed, the longitudinal residual stress on the steel side near the interface was changed from compressive to the tensile state. The maximum warpage deformation in the Al plate border became larger at a higher rotation speed. The surface residual stress was measured by X-ray diffusion method and the internal residual stress was reconstructed by the contour method with the aid of elastic FE analysis. The comparisons of temperature history, residual stress and deformation indicated satisfactory agreements between simulation and experiment ones.

Novel machine learning-based prediction approach for nanoindentation load-deformation in a thin film: Applications to electronic industries

Electronics industries, notably device fabrication industries, deposit thin films on thick substrates to create various devices such as sensors, actuators, LED Multi-Quantum Well (MQW), and High Electron Mobility Transistors (HEMT). The lattice and thermal expansion coefficient mismatch between the thin film and the substrates causes misfit strain generation. The misfit strain can not only cause defect formation but can also change the local deformation pattern. The altered deformation pattern can have an impact on electronic device performance. Therefore, the load-deformation behaviour of thin films deposited on a thick substrate has been investigated using the nanoindentation modelling approach. Two alternative modelling strategies have been employed.The finite element analysis-based nanoindentation simulation was carried out to evaluate the load required for a fixed deformation in the thin film with changing misfit strain between the thin film and the substrate. The next stage was further predicting the load required for indentation deformation in the thin film at higher misfit strain using a machine learning-based linear regression model. Gallium nitride thin filmlayer on silicon and sapphire substrate were considered as case studies for this purpose. It was discovered that as the misfit strain between the substrate and the thin film increases, so does the thin film's elastic recovery and brittleness. Both the FEM and the machine learning model results were validated against published experimental findings available in the literature. The machine learning model presented in this work can be utilized to evaluate the performance of devices like MEMS, NEMS, LEDs, etc. without carrying out iterative nanoindentation experiments or FE simulations, which will aid in reducing the overall cost of these optoelectronic devices and increase the overall profit of the electronic industries.

Tough-interface-enabled stretchable electronics using non-stretchable polymer semiconductors and conductors.

Semiconducting polymer thin films are essential elements of soft electronics for both wearable and biomedical applications1-11. However, high-mobility semiconducting polymers are usually brittle and can be easily fractured under small strains (<10%)12-14. Recently, the improved intrinsic mechanical properties of semiconducting polymer films have been reported through molecular design15-18 and nanoconfinement19. Here we show that engineering the interfacial properties between a semiconducting thin film and a substrate can notably delay microcrack formation in the film. We present a universal design strategy that involves covalently bonding a dissipative interfacial polymer layer, consisting of dynamic non-covalent crosslinks, between a semiconducting thin film and a substrate. This enables high interfacial toughness between the layers, suppression of delamination and delocalization of strain. As a result, crack initiation and propagation are notably delayed to much higher strains. Specifically, the crack-onset strain of a high-mobility semiconducting polymer thin film improved from 30% to 110% strain without any noticeable microcracks. Despite the presence of a large mismatch in strain between the plastic semiconducting thin film and elastic substrate after unloading, the tough interface layer helped maintain bonding and exceptional cyclic durability and robustness. Furthermore, we found that our interfacial layer reduces the mismatch of thermal expansion coefficients between the different layers. This approach can improve the crack-onset strain of various semiconducting polymers, conducting polymers and even metal thin films.

Nanoscale bimetallic strip: Atomistic bending mechanisms of AuPd bimetallic nanowhiskers

The bimetallic strips combining two metals with different thermal expansion coefficients are widely employed for temperature sensing, actuation and control. In the present study, we fabricated nanoscale bimetallic strip by one-sided coating with Pd of ultra-strong single crystalline Au nanowhiskers grown by molecular beam epitaxy. The as-fabricated bimetallic nanowhiskers were slightly bent due to heteroepitaxial stresses in the single crystalline Pd coating. The temperature-induced shape changes of these bimetallic Au-Pd nanowhiskers were studied via in-situ thermal actuation in scanning electron microscope. Annealing of the bimetallic nanowhisker resulted in a change in the amplitude and sign of the curvature. At low temperatures, the bending was limited and reversible, indicating that, like in the classical bimetallic strip, it is caused by thermal stresses due to the mismatch of thermal expansion coefficients of Au and Pd. At higher temperatures, the bending was irreversible and in opposite direction as compared to thermal bending. This type of bending was attributed to the coherency loss at the semi-coherent Au-Pd interface, and to the interdiffusion of Au and Pd and concomitant change of the lattice parameter of the formed Au-Pd alloy. Our results demonstrate that single crystalline metallic nanowhiskers with controlled curvature, chemical composition and thermal behavior can be produced employing interface engineering and chemical interdiffusion.

Reliability Assessment of Thermocompressed Epoxy Molding Compound through Glass via Interposer Architecture by the Submodeling Simulation Approach.

In glass interposer architecture and its assembly process, the mechanical responses of interposer structure under thermocompression process-induced thermal loading and generated shrinkage of molding material are regarded as a major reliability issue. Thousands of metal-filled via are involved in glass interposers and are regarded as a potential risk that can lead to cracking and the failure of an entire vehicle. In this study, a finite element-based submodeling approach is demonstrated to overcome the complexity of modeling and the relevant convergence issue of interposer architecture. Convergence analysis results revealed that at least four via pitch-wide regions of a local simulation model were needed to obtain the stable results enabled by the submodeling simulation approach. The stress-generation mechanism during thermocompression, the coefficient of thermal expansion mismatch, and the curing process-induced shrinkage were separately investigated. The critical stress location was explored as the outer corner of the chip, and the maximum first principal stress during the thermocompression process generated on the chip and glass interposer were 34 and 120 MPa, respectively.

Functional flexibility: The potential of morphing composites

From plants tracking the sun to the aerodynamics of bird wings, shape change is key to the performance of natural structures. After years of reliance on mechanical joints, human engineering now focuses on improving aerodynamic efficiency through smooth, full form changes in material geometry, achieved using technologies such as morphing composites. Promising improved power generation and efficiency in wind turbines and safer more sustainable aircraft and cars, these materials can achieve both large geometric changes with low energy requirements by cycling between several stable physical states and more gradual changes in geometry by exploiting coefficient of thermal expansion mismatch and structural anisotropy, shape memory polymers and 4D printing. The merits and limitations of these various shape change systems are the subject of extensive and ongoing academic research and both commercial and defence industry trials to improve the viability of these technologies for widespread adoption. Shape change capabilities are often associated with problems in material cost, mass, mechanical properties, manufacturability, and energy requirements. Nonetheless, the considerable and rapid advances in this technology, already resulting in successful trials in advanced civilian and military aircraft and high-performance cars, indicate that future research and development of this materials platform could revolutionise many of our most critical power generation, defence and transport systems.

Ultra-Narrow Linewidth Photo-Emitters in Polymorphic Selenium Nanoflakes.

Photoluminescence (PL) in state-of-the-art 2D materials suffers from narrow spectral coverage, relatively broad linewidths, and poor room-temperature (RT) functionality. The authors report ultra-narrow linewidth photo-emitters (ULPs) across the visible to near-infrared wavelength at RT in polymorphic selenium nanoflakes (SeNFs), synthesized via a hot-pressing strategy. Photo-emitters in NIR exhibit full width at half maximum (Γ) of 330 ± 90 µeV, an order of magnitude narrower than the reported ULPs in 2D materials at 300 K, and decrease to 82 ± 70 µeV at 100 K, with coherence time (τc ) of 21.3 ps. The capping substrate enforced spatial confinement during thermal expansion at 250 °C is believed to trigger a localized crystal symmetry breaking in SeNFs, causing a polymorphic transition from the semiconducting trigonal (t) to quasi-metallic orthorhombic (orth) phase. Fine structure splitting in orth-Se causes degeneracy in defect-associated bright excitons, resulting in ultra-sharp emission. Combined theoretical and experimental findings, an optimal biaxial compressive strain of -0.45% cm-1 in t-Se is uncovered, induced by the coefficient of thermal expansion mismatch at the selenium/sapphire interface, resulting in bandgap widening from 1.74 to 2.23 ± 0.1 eV. This report underpins the underlying correlation between crystal symmetry breaking induced polymorphism and RT ULPs in SeNFs, and their phase change characteristics.

Engineering of a FGM Interlayer to Reduce the Thermal Stresses Inside the PFCs

A substantial contribution of the stresses that arise inside the Plasma-Facing Components (PFCs) when a heat load is applied is caused by the mismatch of the Coefficient of Thermal Expansion (CTE) between the armor, usually made of tungsten (W), and the heat sink. A potential way to reduce such contribution to the secondary stresses is the use of an interlayer made with a Functionally Graded Material (FGM), to be interposed between the two sub-components. By tailoring the W concentration in the volume of the FGM, one can engineer the CTE in such a way that the thermal stresses are reduced inside the PFC. To minimize and, theoretically, reduce to zero the stresses due to the CTE mismatch, the FGM should ensure kinematic continuity between the armor and the heat sink, in a configuration where they deform into exactly the shape they would assume if they were detached from each other. We will show how this condition occurs when the mean thermal strain of each sub-component is the same. This work provides a methodology to determine the thickness and the spatial concentration function of the FGM able to ensure the necessary kinematic continuity between the two sub-components subjected to a generic temperature field monotonously varying in the thickness, while remaining stress-free itself. A method for the stratification of such ideal FGM is also presented. Additionally, it will be shown that the bending of the PFC, if allowed by the kinematic boundary conditions, does not permit, at least generally, the coupling of the expansion of the armor and of the heat sink. As an example of our methodology, a study case of the thermomechanical design of a steel-based PFC with an engineered W/steel FGM interlayer is presented. In such an exercise, we show that our procedure of engineering a FGM interlayer is able to reduce the linearized secondary stress of more than 24% in the most critical section of the heat sink, satisfying all the design criteria.

(Digital Presentation) Design of High Barrier Property and Flexible Inorganic/Polymer Hybrid Film for Encapsulation of Flexible Electronics

Designing encapsulation film with both high barrier property and excellent flexibility has been challenging for the current brittle inorganic films owing to the strain-induced cracking. Inorganic/organic multilayer has been proposed due to the organic layer can function as buffers of the inorganic layer and defect-decoupling layer, realizing the improvement of flexibility and maintaining barrier property. Polydimethylsiloxane (PDMS) was widely used as an organic encapsulation layer due to its high stability, elasticity, and transparency. The introduction of PDMS helps the barrier film to resist distorting under the bending motions. However, the delamination and cracking occurred under bending motion in a small curvature radius due to the mismatched strain and coefficient of thermal expansion (CTE) between PDMS and inorganic films such as Al2O3. Realization of improving their strain and CTE mismatch is possible by doping nanoparticles into PDMS. The Al2O3/PDMS hybrid film with the doping of SiO2 fabricated via atomic layer infiltration (ALI) method showed mechanical stability and endure 1000 repetitive stretch tests with 1%, but the water vapor transmission rate (WVTR) was only 10-3 g/m2/day, which was attributed to the lower crosslinking density of PDMS result in higher free volume and chain mobility. It’s a challenge to develop a strategy for improving the crosslink density of PDMS. In this work, the PDMS-based composite substrate with engineered stiffness was developed by employing the addition of silane coupling agent and SiO2. Atomic layer infiltration (ALI) coupled with UV-curing was introduced for sub-surface cross-linking of PDMS in Al2O3/SiO2/PDMS hybrid film, and the resulted Al2O3/SiO2/PDMS hybrid film showed highly stretchable and barrier properties. The result indicates that Al2O3/SiO2/PDMS hybrid film is a promising encapsulation film applicable to flexible electronics.Keywords: atomic layer infiltration (ALI), flexible film, encapsulation, UV-curing method, water vapor transmission rate (WVTR)Refs. Y. Li, D. Wen, Y. Zhang, Y. Lin, K. Cao, F. Yang, R. Chen, Highly-stable PEN as a gas-barrier substrate for flexible displays via atomic layer infiltration, Dalt. Trans. (2021) 16166–16175. https://doi.org/10.1039/d1dt02764f.Zhang, D. Wen, M. Liu, Y. Li, Y. Lin, K. Cao, F. Yang, R. Chen, Stretchable PDMS Encapsulation via SiO2 Doping and Atomic Layer Infiltration for Flexible Displays, Adv. Mater. Interfaces. 9 (2022) 1–9. https://doi.org/10.1002/admi.202101857.

Investigation of Heat Distribution in SOC-Stacks By Multiphysical Modeling

In the scope of clean energy, the solid oxide cell (SOC) offers a promising solution for different applications. The electrolyte, the air electrode and the fuel electrode of an SOC are typically based on yttrium stabilized zirconia (YSZ), lanthanum strontium cobaltite (LSC) or manganite (LSM) and nickel cermet, respectively. It typically operates in a temperature range of 600 – 1000 oC and can be operated as a fuel cell (SOFC), an electrolysis cell (SOEC) or reversibly (rSOC).An SOC-stacks consists of several repeating units, each of which is assembled by various components. Among them, the sealants are important to maintain the reliability and long-term operations of a stack. Several issues lead to high stress in the sealants, including the mechanical load, mismatch of thermal expansion coefficients, and thermal gradients 1. It was indicated that a small temperature gradient near the manifolds results in excessively high tensile stress. This may lead to glass ceramics sealing failure, which is critical in SOC designs 2. Therefore, the authors carry out the elastic stress simulations in an operating SOC-stack due to non-uniform temperature distributions.A previously introduced CFD model 3 is further developed to address coupled chemical and electrochemical reactions in rSOC-stacks. This model has been implemented in the open-source library, OpenFOAM. It considers the major multiphysical phenomena e.g. heat, mass, momentum and charge transport as well as heat radiation and electrochemical reactions and employs a volume-averaged approach 4. The computational requirements can be greatly reduced, which offers the possibility to simulate large stacks in reduced computing time. The temperature distributions derived from the stack simulation are used as inputs for thermal stress analysis, by mapping the interconnect temperatures to the detailed repeating unit. An open-source, finite element method (FEM) based software, Calculix, is applied to simulate the stress distributions. It is assumed all layers are tightly connected without slipping.The SOC-stack design considered in the multiphysical simulation as well as the FEM stress analysis is an in-house design, Mark-F20. It consists of 18 repeating units, each with an active area of approximately 320 cm2. The stack operates in the furnace condition, at a temperature of 700 oC, with CH4 mixture being supplied to the anode in the SOFC-mode (i=0.5 A/cm², U=0.8 V). The flow is in a counter-flow regime. A comparison of temperature distributions in interconnect is conducted between simulation and experiment. A maximum deviation of 10 K can be observed near the fuel inlet. It is acceptable providing the overall temperature variations of 80 K and the uncertainties in the measurements. This low-temperature zone results from the fast endothermic methane steam reforming reaction. The zone near the fuel outlet is cooled down by the inlet airflow on the cathode side. The stress-free temperature is 800 oC. It can be found that the maximum stress appears around the fuel inlet manifold with a magnitude of 100 MPa. The stress is also large near the air inlet manifold with a magnitude of 70 MPa. These areas show higher possibilities of mechanical failure.Consequentially, the stack model that has been developed enables to investigate the heat distribution in SOC-stacks. By comparing temperature distribution and the IV-curve predicted by the model with experimental measurements, good agreements were found.In the future, the sealant failure will be investigated 2 and attention has to be paid to future stack designs. References(1) Blum, L.; Groß, S. M.; Malzbender, J.; Pabst, U.; Peksen, M.; Peters, R.; Vinke, I. C. Investigation of Solid Oxide Fuel Cell Sealing Behavior under Stack Relevant Conditions at Forschungszentrum Jülich. Journal of Power Sources 2011, 196 (17), 7175–7181.(2) Bremm, S.; Dölling, S.; Becker, W.; Blum, L.; Peters, Ro.; Malzbender, J.; Stolten, D. A Methodological Contribution to Failure Prediction of Glass Ceramics Sealings in High-Temperature Solid Oxide Fuel Cell Stacks. Journal of Power Sources 2021, 507, 230301.(3) Zhang, S.; Peters, R.; Varghese, B. A.; Deja, R.; Kruse, N.; Beale, S. B.; Blum, L.; Peters, R. Modeling of Reversible Solid Oxide Cell Stacks with an Open-Source Library. ECS Trans. 2021, 103 (1), 569.(4) Beale, S. B.; Zhubrin, S. V. A Distributed Resistance Analogy for Solid Oxide Fuel Cells. Numerical Heat Transfer, Part B: Fundamentals 2005, 47, 573–591. Figure 1