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  • Shear wave velocity structure beneath Northeast China from joint inversion of receiver functions and Rayleigh wave group velocities: Implications for intraplate volcanism

    Tang, Zheng; Julià, Jordi; Mai, Paul Martin; Mooney, Walter; Wu, Yanqiang (Wiley, 2021-09-17) [Preprint]
    A high-resolution 3-D crustal and upper-mantle shear-wave velocity model of Northeast China is established by joint inversion of receiver functions and Rayleigh wave group velocities. The teleseismic data for obtaining receiver functions are collected from 107 CEA permanent sites and 118 NECESSArray portable stations. Rayleigh wave dispersion measurements are extracted from an independent tomographic study. Our model exhibits unprecedented detail in S-velocity structure. Particularly, we discover a low S-velocity belt at 7.5-12.5 km depth covering entire Northeast China (except the Songliao basin), which is attributed to a combination of anomalous temperature, partial melts and fluid-filled faults related to Cenozoic volcanism. Localized crustal fast S-velocity anomaly under the Songliao basin is imaged and interpreted as late-Mesozoic mafic intrusions. In the upper mantle, our model confirms the presence of low velocity zones below the Changbai mountains and Lesser Xing’an mountain range, which agree with models invoking sub-lithospheric mantle upwellings. We observe a positive S-velocity anomaly at 50-90 km depth under the Songliao basin, which may represent a depleted and more refractory lithosphere inducing the absence of Cenozoic volcanism. Additionally, the average lithosphere-asthenosphere boundary depth increases from 50-70 km under the Changbai mountains to 100 km below the Songliao basin, and exceeds 125 km beneath the Greater Xing’an mountain range in the west. Furthermore, compared with other Precambrian lithospheres, Northeast China likely has a rather warm crust (~480-970 °C) and a slightly warm uppermost mantle (~1200 °C), probably associated with active volcanism. The Songliao basin possesses a moderately warm uppermost mantle (~1080 °C).
  • Crystallization and Morphology of Triple Crystalline Polyethylene-b-poly(ethylene oxide)-b-poly(ε-caprolactone) PE-b-PEO-b-PCL Triblock Terpolymers

    Matxinandiarena, Eider; Múgica, Agurtzane; Zubitur, Manuela; Ladelta, Viko; Zapsas, Georgios; Cavallo, Dario; Hadjichristidis, Nikos; Müller, Alejandro J. (Polymers, MDPI AG, 2021-09-16) [Article]
    The morphology and crystallization behavior of two triblock terpolymers of polymethylene, equivalent to polyethylene (PE), poly (ethylene oxide) (PEO), and poly (ε-caprolactone) (PCL) are studied: PE227.1-b-PEO4615.1-b-PCL3210.4 (T1) and PE379.5-b-PEO348.8-b-PCL297.6 (T2) (superscripts give number average molecular weights in kg/mol and subscripts composition in wt %). The three blocks are potentially crystallizable, and the triple crystalline nature of the samples is investigated. Polyhomologation (C1 polymerization), ring-opening polymerization, and catalyst-switch strategies were combined to synthesize the triblock terpolymers. In addition, the corresponding PE-b-PEO diblock copolymers and PE homopolymers were also analyzed. The crystallization sequence of the blocks was determined via three independent but complementary techniques: differential scanning calorimetry (DSC), in situ SAXS/WAXS (small angle X-ray scattering/wide angle X-ray scattering), and polarized light optical microscopy (PLOM). The two terpolymers (T1 and T2) are weakly phase segregated in the melt according to SAXS. DSC and WAXS results demonstrate that in both triblock terpolymers the crystallization process starts with the PE block, continues with the PCL block, and ends with the PEO block. Hence triple crystalline materials are obtained. The crystallization of the PCL and the PEO block is coincident (i.e., it overlaps); however, WAXS and PLOM experiments can identify both transitions. In addition, PLOM shows a spherulitic morphology for the PE homopolymer and the T1 precursor diblock copolymer, while the other systems appear as non-spherulitic or microspherulitic at the last stage of the crystallization process. The complicated crystallization of tricrystalline triblock terpolymers can only be fully grasped when DSC, WAXS, and PLOM experiments are combined. This knowledge is fundamental to tailor the properties of these complex but fascinating materials.
  • Low-Defect, High Molecular Weight Indacenodithiophene (IDT) Polymers Via a C–H Activation: Evaluation of a Simpler and Greener Approach to Organic Electronic Materials

    Ponder, James F.; Chen, Hung-Yang; Luci, Alexander M. T.; Moro, Stefania; Turano, Marco; Hobson, Archie L.; Collier, Graham S.; Perdigão, Luís M. A.; Moser, Maximilian; Zhang, Weimin; Costantini, Giovanni; Reynolds, John R.; McCulloch, Iain (ACS Materials Letters, American Chemical Society (ACS), 2021-09-16) [Article]
    The development, optimization, and assessment of new methods for the preparation of conjugated materials is key to the continued progress of organic electronics. Direct C–H activation methods have emerged and developed over the last 10 years to become an invaluable synthetic tool for the preparation of conjugated polymers for both redox-active and solid-state applications. Here, we evaluate direct (hetero)arylation polymerization (DHAP) methods for the synthesis of indaceno[1,2-b:5,6-b′]dithiophene-based polymers. We demonstrate, using a range of techniques, including direct visualization of individual polymer chains via high-resolution scanning tunneling microscopy, that DHAP can produce polymers with a high degree of regularity and purity that subsequently perform in organic thin-film transistors comparably to those made by other cross-coupling polymerizations that require increased synthetic complexity. Ultimately, this work results in an improved atom economy by reducing the number of synthetic steps to access high-performance molecular and polymeric materials.
  • Capturing 3D atomic defects and phonon localization at the 2D heterostructure interface

    Tian, Xuezeng; Yan, Xingxu; Varnavides, Georgios; Yuan, Yakun; Kim, Dennis S.; Ciccarino, Christopher J.; Anikeeva, Polina; Li, Ming-yang; Li, Lain-Jong; Narang, Prineha; Pan, Xiaoqing; Miao, Jianwei (Science Advances, American Association for the Advancement of Science (AAAS), 2021-09-15) [Article]
    The three-dimensional (3D) local atomic structures and crystal defects at the interfaces of heterostructures control their electronic, magnetic, optical, catalytic, and topological quantum properties but have thus far eluded any direct experimental determination. Here, we use atomic electron tomography to determine the 3D local atomic positions at the interface of a MoS2-WSe2 heterojunction with picometer precision and correlate 3D atomic defects with localized vibrational properties at the epitaxial interface. We observe point defects, bond distortion, and atomic-scale ripples and measure the full 3D strain tensor at the heterointerface. By using the experimental 3D atomic coordinates as direct input to first-principles calculations, we reveal new phonon modes localized at the interface, which are corroborated by spatially resolved electron energy-loss spectroscopy. We expect that this work will pave the way for correlating structure-property relationships of a wide range of heterostructure interfaces at the single-atom level.
  • Two-Dimensional TiO2/TiS2 Hybrid Nanosheet Anodes for High-Rate Sodium-Ion Batteries

    Bayhan, Zahra; Huang, Gang; Yin, Jian; Xu, Xiangming; Lei, Yongjiu; Liu, Zhixiong; Alshareef, Husam N. (ACS Applied Energy Materials, American Chemical Society (ACS), 2021-09-15) [Article]
    The sodium-ion battery (NIB) is promising for next-generation energy storage systems. One promising anode material is titanium dioxide (TiO2). However, the sluggish sodiation/desodiation kinetics of TiO2 hinders its application in NIBs. Herein, we converted TiO2 into a two-dimensional (2D) TiO2/TiS2 hybrid to improve its sodium storage capability. The 2D TiO2/TiS2 hybrid nanosheet electrode provides high kinetics and excellent cycling performance for sodium-ion storage. This work provides a promising strategy to develop 2D hybrid nanomaterials for high-performance sodium storage devices.
  • High Throughput Printing of Two-Dimensional Materials into Wafer-scale Three-dimensional Architectures

    Wei, Xuan; Lin, Chia-Ching; Wu, Christine; Lu, Ang-Yu; Qaiser, Nadeem; Cai, Yichen; Fu, Jui-Han; Chiang, Yu-Hsiang; ding, lianhui; Ali, Ola; Xu, Wei; Zhang, Wenli; Kong, Jing; Chen, Han-Yi; Tung, Vincent (Research Square Platform LLC, 2021-09-15) [Preprint]
    Architected materials that actively respond to external stimuli hold tantalizing prospects for applications in energy storage, harvesting, wearable electronics and bioengineering. Transition metal dichalcogenides (TMDs) which represent the three-atom-thick, two-dimensional (2D) building blocks, are excellent candidates but have found limited success compared to metallic, inorganic, and organic counterparts due to the lack of up-scalable manufacturing. Here we report the high-throughput printing of 2D TMDs into wafer-scale 3D architectures with structural hierarchy across seven orders of magnitude between critical feature sizes. Anode made of 3D MoS2 architectures comprises the concentric vortex-like intricacy that unites technological merits from architecture, mechanical engineering, and electrochemistry not found in its bulk or exfoliated/epitaxy counterparts. The result is, contrary to expectation, the high-rate, high-capacity, and high-loading lithium (Li)-storage, surpassing those state-of-the-art anode designs while the technique offers an evaporation-like simplicity for industrial scalability.
  • Janus monolayers of magnetic transition metal dichalcogenides as an all-in-one platform for spin-orbit torque

    Smaili, Idris; Laref, Slimane; Garcia, Jose H.; Schwingenschlögl, Udo; Roche, Stephan; Manchon, Aurelien (Physical Review B, American Physical Society (APS), 2021-09-15) [Article]
    We theoretically predict that vanadium-based Janus dichalcogenide monolayers constitute an ideal platform for spin-orbit torque memories. Using first-principles calculations, we demonstrate that magnetic exchange and magnetic anisotropy energies are higher for heavier chalcogen atoms, while the broken inversion symmetry in the Janus form leads to the emergence of Rashba-like spin-orbit coupling. The spin-orbit torque efficiency is evaluated using optimized quantum transport methodology and found to be comparable to heavy nonmagnetic metals. The coexistence of magnetism and spin-orbit coupling in such materials with tunable Fermi-level opens new possibilities for monitoring magnetization dynamics in the perspective of nonvolatile magnetic random access memories.
  • Fracture Permeability Estimation Under Complex Physics: A Data-Driven Model Using Machine Learning

    He, Xupeng; Zhu, Weiwei; Santoso, Ryan; Alsinan, Marwa; Kwak, Hyung; Hoteit, Hussein (SPE, 2021-09-15) [Conference Paper]
    Abstract The permeability of fractures, including natural and hydraulic, are essential parameters for the modeling of fluid flow in conventional and unconventional fractured reservoirs. However, traditional analytical cubic law (CL-based) models used to estimate fracture permeability show unsatisfactory performance when dealing with different dynamic complexities of fractures. This work presents a data-driven, physics-included model based on machine learning as an alternative to traditional methods. The workflow for the development of the data-driven model includes four steps. Step 1: Identify uncertain parameters and perform Latin Hypercube Sampling (LHS). We first identify the uncertain parameters which affect the fracture permeability. We then generate training samples using LHS. Step 2: Perform training simulations and collect inputs and outputs. In this step, high-resolution simulations with parallel computing for the Navier-Stokes equations (NSEs) are run for each of the training samples. We then collect the inputs and outputs from the simulations. Step 3: Construct an optimized data-driven surrogate model. A data-driven model based on machine learning is then built to model the nonlinear mapping between the inputs and outputs collected from Step 2. Herein, Artificial Neural Network (ANN) coupling with Bayesian optimization algorithm is implemented to obtain the optimized surrogate model. Step 4: Validate the proposed data-driven model. In this step, we conduct blind validation on the proposed model with high-fidelity simulations. We further test the developed surrogate model with newly generated fracture cases with a broad range of roughness and tortuosity under different Reynolds numbers. We then compare its performance to the reference NSEs solutions. Results show that the developed data-driven model delivers good accuracy exceeding 90% for all training, validation, and test samples. This work introduces an integrated workflow for developing a data-driven, physics-included model using machine learning to estimate fracture permeability under complex physics (e.g., inertial effect). To our knowledge, this technique is introduced for the first time for the upscaling of rock fractures. The proposed model offers an efficient and accurate alternative to the traditional upscaling methods that can be readily implemented in reservoir characterization and modeling workflows.
  • Recent Progress on Polymers of Intrinsic Microporosity and Thermally Modified Analogue Materials for Membrane-Based Fluid Separations

    Wang, Yingge; Ghanem, Bader; Ali, Zain; Hazazi, Khalid; Han, Yu; Pinnau, Ingo (Small Structures, Wiley, 2021-09-14) [Article]
    Solution-processable amorphous glassy polymers of intrinsic microporosity (PIMs) are promising microporous organic materials for membrane-based gas and liquid separations due to their high surface area and internal free volume, thermal and chemical stability, and excellent separation performance. This review provides an overview of the most recent developments in the design and transport properties of novel ladder PIM materials, polyimides of intrinsic microporosity (PIM–PIs), functionalized PIMs and PIM–PIs, PIM-derived thermally rearranged (TR), and carbon molecular sieve (CMS) membrane materials as well as PIM-based thin film composite membranes for a wide range of energy-intensive gas and liquid separations. In less than two decades, PIMs have significantly lifted the performance upper bounds in H2/N2, H2/CH4, O2/N2, CO2/N2, and CO2/CH4 separations. However, PIMs are still limited by their insufficient gas-pair selectivity to be considered as promising materials for challenging industrial separations such as olefin/paraffin separations. An optimum pore size distribution is required to further improve the selectivity of a PIM for a given application. Specific attention is given to the potential use of PIM-based CMS membranes for energy-intensive CO2/CH4, N2/CH4, C2H4/C2H6, and C3H6/C3H8 separations, and thin film composite membranes containing PIM motifs for liquid separations.
  • An Aqueous Mg 2+ -Based Dual-Ion Battery with High Power Density

    Zhu, Yunpei; Yin, Jun; Emwas, Abdul-Hamid; Mohammed, Omar F.; Alshareef, Husam N. (Advanced Functional Materials, Wiley, 2021-09-13) [Article]
    Rechargeable Mg batteries promise low-cost, safe, and high-energy alternatives to Li-ion batteries. However, the high polarization strength of Mg2+ leads to its strong interaction with electrode materials and electrolyte molecules, resulting in sluggish Mg2+ dissociation and diffusion as well as insufficient power density and cycling stability. Here an aqueous Mg2+-based dual-ion battery is reported to bypass the penalties of slow dissociation and solid-state diffusion. This battery chemistry utilizes fast redox reactions on the polymer electrodes, i.e., anion (de)doping on the polyaniline (PANI) cathode and (de)enolization upon incorporating Mg2+ on the polyimide anode. The kinetically favored and stable electrodes depend on designing a saturated aqueous electrolyte of 4.5 m Mg(NO3)2. The concentrated electrolyte suppresses the irreversible deprotonation reaction of the PANI cathode to enable excellent stability (a lifespan of over 10 000 cycles) and rate performance (33% capacity retention at 500 C) and avoids the anodic parasitic reaction of nitrate reduction to deliver the stable polyimide anode (86.2% capacity retention after 6000 cycles). The resultant full Mg2+-based dual-ion battery shows a high specific power of 10 826 W kg−1, competitive with electrochemical supercapacitors. The electrolyte and electrode chemistries elucidated in this study provide an alternative approach to developing better-performing Mg-based batteries.
  • Design of experiment optimization of aligned polymer thermoelectrics doped by ion-exchange

    Huang, Yuxuan; Lukito Tjhe, Dionisius Hardjo; Jacobs, Ian; Jiao, Xuechen; He, Qiao; Statz, Martin; Ren, Xinglong; Huang, Xinyi; McCulloch, Iain; Heeney, Martin; McNeill, Christopher R.; Sirringhaus, Henning (Applied Physics Letters, AIP Publishing, 2021-09-13) [Article]
    Organic thermoelectrics offer the potential to deliver flexible, low-cost devices that can directly convert heat to electricity. Previous studies have reported high conductivity and thermoelectric power factor in the conjugated polymer poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT). Here, we investigate the thermoelectric properties of PBTTT films in which the polymer chains were aligned uniaxially by mechanical rubbing, and the films were doped by a recently developed ion exchange technique that provides a choice over the counterions incorporated into the film, allowing for more optimized morphology and better stability than conventional charge transfer doping. To optimize the polymer alignment process, we took advantage of two Design of Experiment (DOE) techniques: regular two-level factorial design and central composite design. Rubbing temperature Trub and post-alignment annealing temperature Tanneal were the two factors that were most strongly correlated with conductivity. We were able to achieve high polymer alignment with a dichroic ratio >15 and high electrical conductivities of up to 4345 S/cm for transport parallel to the polymer chains, demonstrating that the ion exchange method can achieve conductivities comparable/higher than conventional charge transfer doping. While the conductivity of aligned films increased by a factor of 4 compared to unaligned films, the Seebeck coefficient (S) remained nearly unchanged. The combination of DOE methodology, high-temperature rubbing, and ion exchange doping provides a systematic, controllable strategy to tune structure–thermoelectric property relationships in semiconducting polymers
  • Interfacial Model Deciphering High-Voltage Electrolytes for High Energy Density, High Safety, and Fast-Charging Lithium-Ion Batteries

    Zou, Yeguo; Cao, Zhen; Zhang, Junli; Wahyudi, Wandi; Wu, Yingqiang; Liu, Gang; Li, Qian; Cheng, Haoran; Zhang, Dongyu; Park, Geon-Tae; Cavallo, Luigi; Anthopoulos, Thomas D.; Wang, Limin; Sun, Yang-Kook; Ming, Jun (Advanced Materials, Wiley, 2021-09-12) [Article]
    High-voltage lithium-ion batteries (LIBs) enabled by high-voltage electrolytes can effectively boost energy density and power density, which are critical requirements to achieve long travel distances, fast-charging, and reliable safety performance for electric vehicles. However, operating these batteries beyond the typical conditions of LIBs (4.3 V vs Li/Li+) leads to severe electrolyte decomposition, while interfacial side reactions remain elusive. These critical issues have become a bottleneck for developing electrolytes for applications in extreme conditions. Herein, an additive-free electrolyte is presented that affords high stability at high voltage (4.5 V vs Li/Li+), lithium-dendrite-free features upon fast-charging operations (e.g., 162 mAh g−1 at 3 C), and superior long-term battery performance at low temperature. More importantly, a new solvation structure-related interfacial model is presented, incorporating molecular-scale interactions between the lithium-ion, anion, and solvents at the electrolyte–electrode interfaces to help interpret battery performance. This report is a pioneering study that explores the dynamic mutual-interaction interfacial behaviors on the lithium layered oxide cathode and graphite anode simultaneously in the battery. This interfacial model enables new insights into electrode performances that differ from the known solid electrolyte interphase approach to be revealed, and sets new guidelines for the design of versatile electrolytes for metal-ion batteries.
  • A numerical method for self-similar solutions of the ideal magnetohydrodynamics

    Chen, Fang; Samtaney, Ravi (Journal of Computational Physics, Elsevier BV, 2021-09-10) [Article]
    We present a numerical method to obtain self-similar solutions of the ideal magnetohydrodynamics (MHD) equations. Under a self-similar transformation, the initial value problem (IVP) is converted into a boundary value prob1 lem (BVP) by eliminating time and transforming the system to self-similar coordinates (ξ ≡ x/t, η ≡ y/t). The ideal MHD system of equations is augmented by a generalized Lagrange multiplier (GLM) to maintain the solenoidal condition on the magnetic field. The self-similar solution to the BVP is solved using an iterative method, and implemented using the p4est adaptive mesh refinement (AMR) framework. Existing Riemann solvers (e.g., Roe, HLLD etc.) can be modified in a relatively straightforward manner and used in the present method. Numerical tests numerical tests illustrate that the present self-similar solution to the BVP exhibits sharper discontinuities than the corresponding one solved by the IVP. We compare and contrast the IVP and BVP solutions in several one dimensional shock-tube test problem and two dimensional test cases include shock wave refraction at a contact discontinuity, reflection at a solid wall, and shock wave diffraction over a right angle corner.
  • Thermo-Responsive Membranes from Blends of PVDF and PNIPAM-b-PVDF Block Copolymers with Linear and Star Architectures

    Algarni, Fatimah; Musteata, Valentina-Elena; Falca, Gheorghe; Chisca, Stefan; Hadjichristidis, Nikos; Nunes, Suzana Pereira (Macromolecules, American Chemical Society (ACS), 2021-09-10) [Article]
    We report the synthesis of poly(n-isopropylacrylamide)-b-poly(vinylidene fluoride), (PNIPAM-b-PVDF), copolymers with linear and star structures, as well as the self-assembly and fabrication of thermo-responsive membranes from blends of these block copolymers and a linear PVDF homopolymer. The synthesis was achieved by reversible addition–fragmentation chain-transfer sequential copolymerization using mono- or multifunctional transfer agents. The self-assembly in bulk and selective solvents was investigated. The PVDF blocks are crystallizable and hydrophobic and the PNIPAM thermo-responsive in water. The morphology is dominated by the breakout crystallization of the PVDF block. Nanoporous membranes were fabricated by non-solvent-induced phase-separation method. The membranes revealed a macroscale zig–zag morphology, which is dependent on the block copolymer architecture. Due to the presence of PNIPAM, these membranes exhibited thermo-responsive behavior with water permeability and rejection alternately varying with the operating temperature, which is reversible in multiple heating–cooling cycles.
  • Effects of ammonia and hydrogen on the sooting characteristics of laminar coflow flames of ethylene and methane

    Steinmetz, S.A.; Ahmed, H.A.; Boyette, Wesley; Dunn, M.J.; Roberts, William L.; Masri, A.R. (Fuel, Elsevier BV, 2021-09-10) [Article]
    Hydrogen and its derivatives, including ammonia, are gaining increasing attention as carbon-neutral fuel alternatives. An intermediate step in the transition to hydrogen and ammonia is the blending of these fuels with hydrocarbons, introducing the challenge of soot formation. The impact of ammonia on soot formation has recently been the focus of several studies, but a complete understanding of its chemical effects is lacking. Hydrogen, by comparison, has received significant attention from the soot community. However, controversy remains with regards to hydrogen’s chemical impact, and the dependence of this impact on fuel and flame configuration. This work investigates the effect of both hydrogen and ammonia on soot formation in laminar coflow flames of both ethylene and methane. Hydrogen or ammonia are introduced either by addition or substitution, with parallel studies of helium and argon, in order to isolate their chemical effects. Time- and spectrally-resolved laser-induced emissions from UV and IR excitation are used to quantify differences in soot and soot precursor formation. Additionally, chemical kinetics calculations and analyses are used to elucidate the effects of ammonia introduction to ethylene flames. Ammonia is found to chemically inhibit soot when mixed with either ethylene or methane, with increasing effects on larger precursors. Calculations suggest that this suppression is due to carbon consumption in the formation of HCN and CN. Hydrogen is found to chemically enhance soot formation in both ethylene and methane flames.
  • Cycle-skipping mitigation using misfit measurements based on differentiable dynamic time warping

    chen, fuqiang; Peter, Daniel; Ravasi, Matteo (arXiv, 2021-09-09) [Preprint]
    The dynamic time warping (DTW) distance has been used as a misfit function for wave-equation inversion to mitigate the local minima issue. However, the original DTW distance is not smooth; therefore it can yield a strong discontinuity in the adjoint source. Such a weakness does not help nonlinear inverse problems converge to a plausible minimum by any means. We therefore introduce for the first time in geophysics the smooth DTW distance, which has demonstrated its performance in time series classification, clustering, and prediction as the loss function. The fundamental idea of such a distance is to replace the $\min$ operator with its smooth relaxation. Then it becomes possible to define the analytic derivative of DTW distance. The new misfit function is entitled to the differentiable DTW distance. Moreover, considering that the warping path is an indicator of the traveltime difference between the observed and synthetic trace, a penalization term is constructed based on the warping path such that the misfit accumulation by the penalized differentiable DTW distance is weighted in favor of the traveltime difference. Numerical examples demonstrate the advantage of the penalized differentiable DTW misfit function over the conventional non-differentiable one.
  • 3-D Modeling of Ultrathin Solar Cells with Nanostructured Dielectric Passivation: Case Study of Chalcogenide Solar Cells

    Raja, Waseem; Aydin, Erkan; Allen, Thomas; De Wolf, Stefaan (Advanced Theory and Simulations, Wiley, 2021-09-09) [Article]
    Ultrathin solar cells can be a path forward to low-cost photovoltaics due to their reduced material consumption and shorter required deposition times. With excellent surface passivation, such devices may feature higher open-circuit voltages (VOC). However, their short-circuit current density (JSC) may be reduced due to decreased light absorption. This mandates implementation of efficient light-trapping structures. To design efficient ultrathin solar cells that combine surface-passivation and light-trapping features, accurate 3-D modeling is necessary. To this end, a novel 3-D optoelectrical finite-element model is developed to analyze the performance of ultrathin solar cells. The model is applied to the case of ultrathin (<500 nm) chalcogenide solar cells (copper indium gallium (di) selenide, CIGSe), rear-passivated with nanostructured Al2O3 to circumvent optical and electrical losses. It is found that such a nanopatterned dielectric passivation scheme enhances broadband light-trapping with reduced rear-surface recombination, resulting in an absolute power conversion efficiency enhancement of 3.9%, compared to cells without passivation structure. Overall, the work shows how 3-D finite element modeling can aid in analyzing and developing new optical and electrical solar cell designs for ultrathin solar cells such as those based on chalcogenides and perovskites.
  • Flame edge dynamics in counterflow nonpremixed flames of CH4/He versus air at low strain rates: An experimental and numerical study

    Jung, Ki Sung; Kwon, Soon Hyeong; Chung, Suk Ho; Park, Jeong; Yoo, Chun Sang (Combustion and Flame, Elsevier BV, 2021-09-09) [Article]
    The characteristics of the flame structure, stabilization, and extinction of counterflow nonpremixed flames of CH4/He versus air at low strain rates are investigated by performing a series of experiments and twodimensional (2-D) numerical simulations. By adopting an experimental methodology using He curtain flow, we can locate the flames near the center of the counterflow burner and measure the critical He mole fraction in the fuel stream, XHe,cr, for flame extinction at very-low strain rates. XHe,cr obtained from 2-D numerical simulations in normal and zero gravity show a good agreement with those from the experiments, which substantiates that the experimental methodology can effectively reduce the buoyancy effect at low strain rates. It is found from various steady and unsteady 2-D numerical simulations that the dynamics of flame edge plays a critical role in determining the flame stabilization and extinction, and the edge flame is stabilized at a location where negative edge flame propagation speed, Se, balances positive local flow velocity, Ue. The transport budget analysis reveals that despite the negative Se by the diffusive loss of heat and radicals, the edge flame can survive by the help of the convective gain of heat and radicals from the trailing diffusion flame. It is also found that the counterflow flame can survive the increase of He mole fraction in the fuel stream, XHe, by shrinking its flame length since the local chemical reaction at the flame edge is enhanced with decreasing the flame length. However, as XHe exceeds XHe,cr, a slight inward movement of the edge flame induces a large magnitude of negative Se compared to positive Ue such that the counterflow flame is totally extinguished by the shrinkage of the outer edge flame toward the flame center.
  • Computational comparison of the conventional diesel and hydrogen direct-injection compression-ignition combustion engines

    Babayev, Rafig; Andersson, Arne; Serra Dalmau, Albert; Im, Hong G.; Johansson, Bengt (Fuel, Elsevier BV, 2021-09-09) [Article]
    Most research and development on hydrogen (H2) internal combustion engines focus on premixed-charge spark ignition (SI) or diesel-hydrogen dual-fuel technologies. Premixed charge limits the engine efficiency, power density, and safety, while diesel injections give rise to CO2 and particulate emissions. This paper demonstrates a non-premixed compression-ignition (CI) neat H2 engine concept that uses H2 pilots for ignition. It compares the CI H2 engine to an equivalent diesel engine to draw fundamental insights about the mixing and combustion processes. The Converge computational fluid dynamics solver is used for all simulations. The results show that the brake thermal efficiency of the CI H2 engine is comparable or higher than diesel, and the molar expansion with H2 injections at TDC constitutes 5–10 % of the total useful work. Fuel-air mixing in the free-jet phase of combustion is substantially higher with H2 due to hydrogen's gaseous state, low density, high injection velocity, and transient vortices, which contribute to the 3 times higher air entrainment into the quasi-steady-state jet regions. However, the H2 jet momentum is up to 4 times lower than for diesel, which leads to not only ineffective momentum-driven global mixing but also reduced heat transfer losses with H2. The short H2 flame quenching distance may also be inconsequential for heat transfer in CI engines. Finally, this research enables future improvements in CI H2 engine efficiency by hypothesizing a new optimization path, which maximizes the free-jet phase of combustion, hence is totally different from that for conventional diesel engines.
  • Redox-Active Polymers Designed for the Circular Economy of Energy Storage Devices

    Tan, Siew Ting Melissa; Quill, Tyler J.; Moser, Maximilian; LeCroy, Garrett; Chen, Xingxing; Wu, Yilei; Takacs, Christopher J.; Salleo, Alberto; Giovannitti, Alexander (ACS Energy Letters, American Chemical Society (ACS), 2021-09-08) [Article]
    Electrochemical energy storage is a keystone to support the rapid transition to a low-carbon-emission future for grid storage and transportation. While research on electrochemical energy storage devices has mostly dealt with performance improvements (energy density and power density), little attention has been paid to designing devices that can be recycled with low cost and low environmental impact. Thus, next-generation energy storage devices should also address the integration of recyclability into the device design. Here, we demonstrate recyclable energy storage devices based on solution-processable redox-active conjugated polymers. The high electronic and ionic charge transport in these polymers enables the operation of single-phase electrodes in aqueous electrolytes with C-rates >100 with good electrochemical stability when the cell is charged to 1.2 V. Finally, we demonstrate the recyclability of these devices, achieving >85% capacity retention in each recycling step. Our work provides a framework for developing recyclable devices for sustainable energy storage technologies.

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