Mismatch Of Coefficients Research Articles

Improving efficiency of silicon solar cells by integrating SiO2-coated lead-free Cs3Bi2Br9 perovskites quantum dots as luminescence down-shifting layer

Lead-based perovskite quantum dots (QDs) have promising applications in optoelectronic devices, but commercial applications are limited by the potential toxicity and poor stability of lead. Meanwhile, there are lattice, polarity, and thermal expansion coefficient mismatches in integrating semiconductor materials onto the surface of photoelectronic circuit devices, resulting in high-density defects limiting the reliability of photoelectronic devices. Therefore, there is a need to investigate perovskite alternatives that are non-toxic, stable, and can be effectively coupled to silicon photonic circuits. In this paper, lead-free perovskite Cs3Bi2Br9/SiO2 QDs were synthesized. The emission wavelength falls within the 400–560 nm range, exhibiting vibrant blue photoluminescence and demonstrating exceptional air stability for a duration exceeding 60 days. Optical tests and density-functional theory (DFT) calculations show that the silica shell layer effectively binds the electron and hole wave functions, prevents photon-generated carriers from escaping, reduces surface defects, and improves the carrier recombination probability. Meanwhile, using Cs3Bi2Br9/SiO2 QDs as a luminescent down-shifting (LDS) layer in combination with a conventional silicon solar cell, the photovoltaic conversion power was increased by 1.82%. Hence, the core-shell structure is designed by the green environmental protection method to control the improvement of the PL characteristics of QDs, which opens a window for the synthesis of new barrier materials in ethanol and water systems, making the Cs3Bi2Br9/SiO2 QDs have great potential for the application of novel optoelectronic devices in the future.

Effect of High Al Content AlGaN Buffer Layer on the Properties of GaN‐on‐Silicon Materials

The lattice parameter and thermal expansion coefficient mismatch between the silicon substrate and GaN lead to high tensile stress, which makes the GaN epitaxial layer prone to cracking. Effective compensation of tensile stress on GaN to prevent cracking is an important issue in GaN epitaxial growth. In this work, GaN‐on‐silicon materials with different AlGaN buffer layer structures are prepared by metal‐organic chemical vapor deposition (MOCVD). The GaN epitaxial material with a smooth and crack‐free surface is fabricated by inserting 7 AlGaN buffer layers. The crystal quality of GaN is characterized using high resolution X‐ray diffraction (HRXRD). The full‐width half maximum (FWHM) value of GaN(002) and GaN(102) crystal plane is 398 and 780 arcsec, respectively. The surface root mean square (RMS) roughness of GaN material is 0.31 nm, and the vertical breakdown voltage (BV) of the epitaxial wafer reaches 918 V. The results show that high Al content of the AlGaN buffer layer can effectively reduce the tensile stress and dislocation density of the GaN layer when the entire AlGaN layer thickness remains constant. Suppressing the generation of surface cracks and improving the crystal quality can improve the vertical breakdown voltage and two‐dimensional electron gas (2‐DEG) characteristics of GaN epitaxial wafers.

Effect of Urban Household Registration Controls on Labor Mismatch in China

This study examines the effect of the degree of household registration control on labor mismatch in China. Based on Chinese urban panel data, we construct a dynamic panel model to empirically examine the effect of the degree of household registration control on labor mismatch in China. The study finds that labor mismatch relatively exists in all regions of China, which differ significantly. Moreover, liberalizing the household registration system in cities with under- and over-allocated labor force can reduce and aggravate the degree of local labor mismatch, and improve and reduce resource allocation efficiency, respectively. This study uses the labor distortion coefficient to replace the original dependent variable, and the labor mismatch coefficient. The authors further conduct a robustness test using the system generalized moment estimation method. These results are consistent. The implication of this study is that, in the context of China’s shift from high-speed to high-quality development, local governments should tailor their household registration system reform programs to the local resource allocation situation to improve the degree of labor mismatch in China, to optimize resource allocation, and to promote economic transformation.

High-temperature strain-mediated oxidation of 2D MoS2

MoS2 is 2D material applicable for electronics, photo-detectors, light-emitting diodes, and solar cells. Since these applications operate at elevated temperatures, determining and controlling the environmental stability of MoS2 is essential. This study uses Raman spectroscopic experiments at elevated temperatures to observe reversible and irreversible changes in 2D MoS2 films with varying thickness, morphology, and substrate to explore the limits of its thermal stability. Molecular dynamics simulations and in-situ X-ray diffraction confirm that thermal expansion causes the Raman reversible spectral shifts. Further analysis reveals irreversible spectral shifts associated with oxidation and strain effects caused by oxidation with two distinct modes. The former is due to the general homogenous thinning of layers from the top layer down through randomly spaced sulfur vacancies. The latter is due to inhomogeneous oxidation, which known to occur as localized pitting at grain boundaries, void formation from nucleation centers, or stress corrosion cracking starting from edges under strain. Segregation of doping and strain contributions confirms higher strain in the edges. Annealing increases the spread of the strain distribution. These results highlight the critical role of strain, resulting from thermal expansion coefficient mismatch between substrates and 2D MoS2 films, on their environmental stability.

Ablation performance of rare earth oxide (REO)-stabilized tetragonal and cubic zirconia coatings as a thermal protection system (TPS) for carbon/carbon composites

To protect carbon/carbon (C/C) composites from severe oxidation and ablation at temperatures exceeding 500 ℃ during the hypersonic applications, a novel Sm2O3-stabilized ZrO2 coating is applied using atmospheric plasma spray. The surface was pre-treated with an oxyacetylene flame to increase the surface roughness and, therefore, to create geometric textures known as anchors. The non-equilibrium tetragonal (t΄) ZrO2 coating stabilized with 6 mol% Sm2O3 offered the best ablation resistance, with survivability maintained through 120 seconds of ~390 W/cm2 heat flux oxyacetylene ablation heating without any denudation from the C/C substrate. The coating significantly improved the ablation resistance of C/C by reducing the mass ablation rate by ~71% and the linear ablation rate by ~94%. Despite a significant thermal expansion coefficient mismatch between the substrate and the coating, a well-defined mechanical adhesion characterized by the anchors was observed in pre- and post-ablated coating microstructures, indicating its influence in improving ablation resistance.

The Calibration of Soil Simulation Parameters for Wheat Grown after Rice in the Yangtze River Basin of China

In recent years, there have been many studies on the calibration of soil simulation parameters; however, there are few soil parameters for wheat grown after rice that have been calibrated in the lower reaches of the Yangtze River, and the data from such calibrations remain inaccurate. Therefore, using the soil of Jiangsu as our research object, a soil parameter calibration was carried out based on the EEPA (Edinburgh elastoplastic adhesion) model and using the discrete element software EDEM (2020, DEM-Solutions, Edinburgh, UK). The depression depth measured via a uniaxial compression test and the maximum crushing force measured via an unconfined compression test were taken as indexes. The Plackett–Burman test was used to screen seven influencing factors (recovery coefficient, static friction coefficient, rolling friction coefficient, surface energy, contact plastic deformation ratio, tensile exp, and tangential stiff multiplier). The recovery coefficient and static friction coefficient were analyzed using a central composite test with depression depth as an index. The surface energy, plastic deformation ratio, and tangential stiffness factor were analyzed via a Box–Behnken test, with the maximum crushing force as the index. Taking the measured depression depth of 3.36mm and the maximum destructive power of 210 N as the target, the following final data were obtained—recovery coefficient: 0.322, static friction coefficient: 0.676, rolling friction coefficient: 0.5, surface energy: 17.158, contact plastic deformation ratio: 0.358, tensile exp: 2, and tangential stiff multiplier: 0.8. Finally, the simulation value and the actual value of the parameter group were verified and compared. It was found that the mismatch coefficient R2 of the actual value and the simulation value is 93.509%. The mismatch coefficient R2 between the actual and simulated values of unconfined compressive test is 94.2%. This shows that the curves obtained from the real test and simulation test have a high similarity. This study can provide technical support for the simulation and optimization of growing wheat after rice seeding equipment in the lower reaches of Yangtze River in China.

A unique graphene composite coating suitable for ultra-low temperature and thermal shock environments

Traditional epoxy resin is prone to cracking and losing its protective properties under ultra-low temperature and thermal shock environments. The reason for coating failure is thermal stress damage caused by the mismatch of the thermal expansion coefficient (CTE) between the coating and substrate. Moreover, the brittleness of epoxy resin (EP) increases in ultra-low temperature environments. In order to reduce the CTE of the composite and improve the toughness of the coating, negative expansion graphene (GP) modified by a silane coupling agent (MP) is used as the filler of EP. The thermal stress of the coating under ultra-low temperature of −196 °C was analyzed by finite element simulation and the stress damage mechanism was explained. The simulation results show that the thermal stress of the coating decreases by 10–15 MPa after adding MP@GP. Simultaneously, MP@GP can act as a lamellar physical barrier, extend the penetration path of corrosive media, and inhibit the electrochemical corrosion of the metal surface. After 20 thermal shock tests, the MP@GP/EP coating remains intact without cracks and the |Z|0.01 Hz value of the 1 % MP@GP/EP coating reaches the highest at 3.99 × 1010 Ω·cm2. Therefore, the coating has unique thermal shock and corrosion resistance under multiple synergistic effects.

Microstructure and mechanical properties of in-situ Ti5Si3/TC4 composites via spark plasma sintering and hot rolling

In this study, low energy ball milling and spark plasma sintering techniques prepared the in-situ synthesized Ti5Si3 reinforced Ti-6Al-4V (TC4) matrix composites and then rolled them at high temperatures. In the composites with high Si content, the Ti5Si3 particles distributed at the interface of α-phase and β-phase agglomerated and then formed a quasi-continuous network structure, which will deteriorate the plasticity of the composites. After hot rolling, the grains of the composite were significantly refined, and more Ti5Si3 particles were precipitated. With the increase in Si content, the tensile strength of sintered composites increased slightly, but the elongation decreased greatly. After hot rolling, the tensile strength of the composite increased significantly with an increment of 359.4 MPa, among which the composite with 0.6 wt% Si achieved a synchronous increase in strength and plasticity. The strengthening mechanism of sintered composites included dislocation strengthening, solution strengthening, and grain refinement, among which dislocation strengthening and grain refinement were the main strengthening mechanisms. The strengthening mechanism of rolled composites was mainly the dislocation strengthening caused by rolling, the grain refinement, and the mismatch of thermal expansion coefficient.

Micro-additives and their impact on tensile and fracture performance of solder

High-performance components and sustainable electronics require new and durable interconnection technologies. In addition to Solid-Liquid Interdiffusion (SLID) Bonding and silver sintering, established solders are also advancing. Especially in the field of power electronics, where high-temperature applications require high demands on assembly and interconnection technology, the lead-free SnAgCu solders has established a good track record. This basic solder is then improved and optimized for the requirements of its specific application, for example by adding micro-additives. This paper deals with the tensile strength of micro-alloyed SnAg0.4Cu0.5 (SAC) solders, building on our previous work considering shear tests and phase formation behaviour published in Steenmann et al. (2021). In power electronics, solders as joining partners between component and base plate have to accommodate high mechanical and thermal loads resulting, e.g., from a mismatch in coefficients of thermal expansion of the joined materials. The cracking behaviour of the solders in tensile tests provides information on its mechanical resistance and load-bearing capacity. In this work, we examine SAC based solders with small amounts of Sb (3 wt%), Bi (6 wt%) and Ni (0.2 wt%) to investigate the effects of the micro-additives on tensile strength. The micro-additives are studied individually in quaternary systems “SAC + X”, but also in the combination SnAg4Cu0.7Bi3Ni0.1Sb1.5 and SnAg0.5Bi3Sb5 and compared respectively to SAC solder. We achieve this by melting the solders using a standard soldering process at different times and temperatures to replicate the solders' phase formation behaviour during soldering. We vary the peak temperature and holding time of the process to investigate the respective effects on phase growth and the associated change of solder behaviour under mechanical load. The specimens prepared at different parameters are tested under identical conditions on a tensile testing machine based on DIN EN ISO 6892 1 “Metallic materials — Tensile testing”. We investigate the intermetallic phases on scanning electron microscope images by energy dispersive X-ray (EDX) spectroscopy and derive the relationships with the micro-additives. In this paper, we discuss in detail the differences in tensile behaviour depending on the micro-additives. By considering all results holistically, the influence of the individual and combined micro-additives in the solder can be deduced.

Enhanced strength and ductility in nanocarbon hybrid reinforced B4C/Al laminated composites fabricated by vacuum hot pressing

In order to develop B4C/Al composites with good strength-toughness balance, nanocarbon hybrid-reinforced B4C/Al composites with obvious laminar structure were fabricated by the flake powder metallurgy and vacuum hot pressing. Al-30B4C-0.75CNTs-0.25GNPs composites achieved the best balance of tensile strength and toughness, with average values of 324.37 MPa and 10.14%, respectively. The interconnected network structure created by CNTs and GNPs has a more effective load transfer effect than that of single CNTs, which is attributed to the synergistic strengthening effect of CNTs and GNPs. The geometrically necessary dislocation strengthening caused by the mismatch of thermal expansion coefficients between B4C and Al matrix is the main contribution of B4C to the composite. Additionally, the γ-Al2O3 uniformly distributed in the Al matrix is beneficial for the strength enhancement of the composite due to the Orowan mechanism, and sintering and pressing under vacuum prevents excessive formation of Al2O3.

Influence of Design Parameters on Mechanical Behavior of Multi-Bolt, Countersunk C/SiC Composite Joint Structure.

Aerospace vehicle connection constructions are in urgent need of joint structures with excellent aerodynamic profiles and environmental adaptability. To address issues such as poor aerodynamic profile, material thermal expansion coefficient mismatch, and limited joint structure evaluation indexes, a multi-bolt, countersunk C/SiC composite joint structure is presented in this study. The development of a 3D Hashin progressive damage model and its dedicated solver code is presented. The validity of the model is confirmed by comparing simulation results with experimental data. Three evaluation indexes are proposed, peak load, weight increment efficiency, and bolt load distribution, to thoroughly evaluate the mechanical performance of multi-bolt, countersunk C/SiC composite joint structures. Using the proposed model and evaluation indices, we evaluate sixteen different designs of multi-bolt, countersunk C/SiC composite joint structures and analyze how design parameters affect their mechanical properties and damage patterns. The results show that the best mechanical properties of the joint structure are achieved when the ratio of bolt pitch to through hole diameter is 3, the ratio of bolt spacing between columns to through hole diameter is 4, the ratio of the distance between the free edge of the substrate to through hole diameter is 1.5, the ratio of through hole diameter to specimen thickness is 1.7, and the ratio of the distance between the edge of the substrate to through hole diameter is 1.5.

Effect of Top-Coat Thickness and Interface Fluctuation on the Residual Stress in APS-TBCs

This study focused on the numerical simulation of the distribution of residual stress in yttria-stabilized zirconia (YSZ) coatings prepared with atmospheric plasma spraying (APS). We particularly investigated the stress distribution around the interface between the top coat and bond coat. During thermal spray deposition, droplets and particles deposit on the substrate in a complex manner, causing interface waviness and non-uniform stress distribution. Therefore, residual stress is an important consideration when preparing thermal barrier coatings (TBCs). Residual stresses directly affect the performance of bond coats (BCs) and ceramic top coats (TCs). To accurately evaluate residual stress, we considered interface waviness and the thickness of the ceramic top coat and conducted a detailed analysis of stress distribution. The results show that compressive stress exists at the TC/BC interface, which may be caused by the mismatch in the thermal expansion coefficient between the YSZ top coat and the substrate, potentially leading to coating delamination. Moreover, the residual stress at the TC/BC interface significantly increases with an increasing YSZ thickness. When the top-coat thickness exceeds 300 μm, stress concentration and failure of the coating are likely to occur. Meanwhile, the optimized thermal spray experiment results confirm that the residual stress at the BC/YSZ interface of the thermal barrier coating is tensile stress, with a maximum value of 155 MPa, which is consistent with the finite element calculation results. Furthermore, the failure modes of TBCs with rough interface conditions are discussed in detail. Our research provides important guidance for TBC design and optimizing their performance.

Phase and structural evolution of dysprosia stabilized zirconia ceramics under CMAS corrosion environment

In this study, 10 wt % dysprosia stabilized zirconia (10DySZ) ceramics were prepared by pressureless sintering method. The CMAS corrosion behaviors of DySZ ceramics were investigated and the corrosion mechanism was also discussed in detail. Results show that the CMAS corrosive agent can induce phase transformation of DySZ ceramic due to the stronger ability to capture Dy stabilizers than Zr ions. During the corrosion process, molten CMAS would preferentially corrode the grain boundaries and then infiltrated into the interior of DySZ ceramic, resulting in the formation of loose structure on the ceramic surface. Sandy-shaped DySZ particles were also formed in the residual CMAS layers based on dissolution and precipitation mechanism. The corrosion temperature plays a crucial role in accelerating the phase transformation and thermal-mechanical damage of DySZ ceramic. Mismatch of thermal expansion coefficient between CMAS and DySZ ceramics as well as composition and structural difference between dense layer and porous layer are all responsible to the formation of destructive cracks.

(Invited) Integration of InP Heterojunction Bipolar Transistors on Silicon Substrates for 6G Networks

The fast expansion of the global datasphere creates a huge demand on the future wireless communication technologies. Although 5G, the 5th generation of the cellular network technology, is still being rolled out, the potential of 6G is already intensively explored. The vision of 6G implies not only massive connectivity and higher data rate (up to 100 Gb/s and more) in ultra-reliable low-latency communication, but machine-to-machine interaction will dominate over those involving humans. AI (artificial intelligence) and machine learning will be omnipresent.6G network will address a frequency spectrum beyond 100 GHz, which provides wider bandwidths and, therefore, potentially simpler modulation schemes and eventually lower power consumption. However, a clear disadvantage of these mm-waves or sub-THz frequencies are the atmospheric absorption and blockage effect by physical obstacles or rain. These challenges must be tackled by a sufficiently high transceiver device density per area together with advanced beam steering and AI empowered networks.The power amplifier (PA) is one of the most critical components of the RF front-end module. Here, InP based heterojunction bipolar transistors (HBT) clearly outperform PAs based on CMOS or SiGe BiCMOS concerning saturated output power, energy efficiency and breakdown voltage. However, the limiting downside of InP HBTs is the missing cost-efficient and scalable fabrication, which hampers its adoption in high-volume 6G applications. In this paper we review various InP HBT fabrication concepts with the aim of lowering the production costs and maintaining high device performance.The best HBT device performance is demonstrated for the lattice-matched deposition on native small-diameter (≤ 150 mm) InP substrates due to perfect III-V crystal quality. Although device fabrication on these wafers cannot profit from the advanced tool park available in 200/300 mm fabs, the main drawbacks are the high cost and brittleness of InP substrates. Realizing InP HBTs on Si substrates allows to benefit from their robustness, low cost, and the access to a mature device fabrication on large-diameter wafers.In wafer reconstitution, numerous diced InP wafer tiles, which already contain the grown HBT layer stack, are bonded to a Si carrier to populate the wafer surface. After planarization, the reconstructed Si wafer can be further fabricated in a CMOS compatible 200/300 mm fab. In transfer printing a comparable approach is followed while only the active device stack is transferred to the Si carrier. Transferring only a thin InP layer to the Si carrier, which serves as a starting surface for the HBT deposition, leads to a so-called engineered substrate. Compared to the previous transfer methods, the III-V device deposition is carried out after bonding. The key advantage of all these heterogenous integration concepts is the access to a CMOS compatible and scalable device fabrication while maintaining the high III-V crystal quality. Nevertheless, the HBT production costs can only be significantly reduced with a certain re-usability of the InP substrates.The monolithic deposition directly on Si substrates would be the most cost-efficient integration approach. Unfortunately, the mismatch in lattice constant and thermal expansion coefficient between Si and III-V materials leads to the formation of relaxation defects, which degrade the HBT device performance. The growth of thick strain relaxation buffers allows to reduce the defect density in the device layers but bears new challenges such as wafer warpage or III-V crack formation.Nano-ridge engineering (NRE) is based on selective area growth (SAG) applying trench-patterned Si wafers. The III-V deposition starts inside very narrow trenches to achieve efficient aspect ratio trapping (ART) of misfit defects. After the trench is filled with III-V material, the growth is continued to engineer a broad and pristine nano-ridge (NR) which holds the device stack. A second oxide pattern prevents sidewall deposition around the NRs during the deposition of the HBT heterolayers.In the first part of this paper, pros, and cons of the most common InP HBT integration scenarios as summarized in figure will be compared in detail. Among all concepts, NRE holds the potential of being the most cost-efficient integration approach due to SAG in line with very limited III-V material deposition. However, the key open question is what NR crystal quality, and hence, device performance can be realized. This question will be addressed in the second part of the paper, reporting about the latest developments at imec. Figure 1

Failure mechanism of parallel gap resistance welding joint between Ag foil and GaAs solar cell by temperature cycling

Reliability of joints in solar arrays significantly influences the service life of satellites. Interface between solar cell and interconnector experiences serious temperature cycling during space service which would further lead to failure. To further improve the interface joining thermal reliability, elucidation of the interface formation and corresponding microstructure evolution during thermal fatigue is necessary. In this study, parallel gap resistance welded (PGRW) multi-layered joint between GaAs solar cell and Ag foil are subjected to different temperature cycling tests (−160–120 °C, −165–160 °C) with various cycles. Obtained results confirm the joining mechanism of the joint as solid-solution interdiffusion between Ag foil and Au surface of solar cell electrode. Also, conducted temperature cycling essentially lead to thermal fatigue process at Ag/Au interface, therefore more serious interface strength degradation is generated by larger temperature cycling range. Joint failure is initiated by thermal fatigue induced dislocation and residual strain concentrations around dissimilar interface. And the large mismatch in coefficients of thermal expansion (CTE) of the multilayer structure amplifies the thermal fatigue effect.

Effect of mold on curing deformation of resin transfer molding‐made textile composites

AbstractIn the process of preparing textile composites by resin transfer molding (RTM) method, both the upper and lower surface of the mold will interact with the composite parts due to the mismatch of thermal expansion coefficient between the mold and the part, resulting in warpage deformation. To address this key technical problem, this article first studied the effect of mold on the warpage deformation of composite material under different fiber volume fractions. Then a mold–part interaction modeling method for RTM process with nonthermoelastic deformation is proposed. By introducing shear layers between the upper and lower surfaces of the mold and the composite part, a finite element simulation model for predicting the curing deformation of the component is established and experimentally verified. The results show that the effect of mold–part interaction on the warpage deformation of the composite decreases with increasing fiber volume fraction. Meanwhile, the proposed modeling method can avoid complete material characterization, and the comparison between experimental and simulation results proves that the model can accurately simulate the curing deformation of composite components under the same process conditions. Finally, the analysis reveals that the interaction caused by thermal mismatch between the composites and the mold is less related to the intermediate layup, but mainly related to the fiber orientation of the layup layer in contact with the mold.Highlights The warpage deformation law of the mold on the composites with different fiber volume fractions is investigated. A mold–part interaction modeling method for a resin transfer molding process with nonthermoelastic deformation is proposed, where the mold stretching effect is represented by the cumulative effect of the interaction between the two layers on the upper and lower surfaces of the part.

Alumina scale buckling during high temperature oxidation of Cr2AlC MAX Phase

Single-crystal and fine-grained polycrystalline samples of Cr2AlC were oxidized under dry air flow at temperature in the 1000–1400 °C range during 100 h. A continuous alumina layer forms on top of the Cr2AlC surface whereas a Cr7C3 sublayer also appears. In-lab characterization of oxidized Cr2AlC samples shows strong damaging at the free surface, resulting from the buckling of the alumina scale. In-situ X-ray diffraction measurements under synchrotron radiations were performed to measure the lattice strain during the first hours of oxidation process and further calculate the internal stress in the Al2O3 layers. Alumina layers undergo tensile stress during isothermal oxidation, showing that the buckling of the alumina scale does not result from the oxide growth. Such a tensile stress likely results from the Cr2AlC to Cr7C3 phase transformation. During cooling, the tensile stress decreases down to compressive values, due to the thermal expansion coefficient mismatch between the film and the substrate, leading to buckling of the alumina layer. It is demonstrated that the dimensions of the buckles cannot be explained either by gas pressure or by the magnitude of the internal compressive stress in the alumina scale after cooling. The discrepancy between the experimental maximum deflection and the one predicted by the elastic theory can only be explained by a significant plastic deformation occurring in the alumina scale.

Active junction temperature control of IGBTs for electric vehicle drive system based on hybrid control strategy

Junction temperature swing is the most important factor to induce the IGBTs failure due to the associated internal temperature gradient acting on the mismatch of coefficients of thermal expansion between adjacent layers and internal temperature gradients, and smoothing the junction temperature fluctuation is a valid way to prolong the lifespan of IGBT modules. This paper investigates the dependency of the junction temperature fluctuation smoothing effect on the stator current and switching frequency, and presents a hybrid control strategy to smooth the junction temperature swing. Depending on the heating up or cooling down stages of IGBTs, the proposed strategy adjusts the stator current and switching frequency respectively. For simulation, Urban dynamometer driving schedule (UDDS) for the VEH_SMCAR model in ADVISOR is used to verify the effectiveness, where the evaluated result shows that the proposed method can effectively reduce the number of the large junction temperature fluctuation cycles and the life-span increases by 90 %. Meanwhile, the experimental results also validate the effectiveness of the proposed hybrid control strategy in smoothing junction temperature fluctuation.

A review on new cobalt-free cathode materials for reversible solid oxide fuel cells

The exponential growth in the requirement of fuel cells and batteries leads to increased demand for cobalt due to its common use in high-performance Li-ion batteries and high-temperature fuel cells/electrolyzers. This sharp increment in demand raises concern about the availability of limited reserves of cobalt which can impact the price of cobalt. Moreover, the geographic limitations of cobalt resources may endanger the whole supply chain. In addition to all those, huge moral issues of cobalt mining are also another problem. Hence, leading battery, fuel cells and electrolyzer manufacturers are looking for sustainable alternatives to reduce cobalt dependency. A more specific limitation is shown in Solid Oxide Fuel Cells (SOFCs) cathode materials that contain cobalt. Incompatibilities have already been observed between the cathode materials containing cobalt and the electrolytes in terms of the thermal expansion coefficient mismatch during the transition of the operating temperature from high to low. An advantage of low operating temperatures is the reduction of material costs compared to high temperature. Increasing the electrochemical performance of the cell and eliminating thermal expansion coefficient difference problems are in concert aimed at the development of cobalt-free cathode materials. Therefore, cobalt-free cathode materials are vital for the sustainability of SOFCs and green transition of the energy sector since they can be used as cathode and anode material in symmetrical SOFCs which is also known as reversible SOFC (RSOFC). In this review, we comprehensively summarize the recent advances of cobalt-free perovskite cathode materials for intermediate temperature RSOFCs.

Thermal expansion and mechanical properties of urethane-modified epoxy bonded CFRP/steel joints at low and high temperatures for automotive

Long-term reliability is critical in automobiles owing to their frequent exposure to the external environment and lengthy operational durations. Using dissimilar materials as adhesives to reduce the weight of vehicles results in a mismatch of coefficients of thermal expansion (CTE). Difference in CTE causes constant internal stress at the adhesive interface when the temperature changes, leading to delamination of the adhesive at the interfaces, microcrack initiations, and crack propagations. Therefore, a blend of linear urethane prepolymer and epoxy was tested to alleviate stress concentration. Dynamic mechanical analysis (DMA) was performed to determine the modulus according to the urethane content, while tensile strength was measured to evaluate changes in the physical properties. It was observed that increasing the urethane content decreased the modulus but progressively enhanced the tensile strength elongation rate. The lap shear strength specimen was thermally shocked to validate the decrease relative to the initial value based on the cycles. Consequently, combining urethane and epoxy successfully enhanced the elongation and reduced the internal stress caused by temperature changes.