Now showing items 1-20 of 11770

    • Controlling Electrochemically Induced Volume Changes in Conjugated Polymers by Chemical Design: from Theory to Devices

      Moser, Maximilian; Gladisch, Johannes; Ghosh, Sarbani; Hidalgo, Tania Cecilia; Ponder, James F.; Sheelamanthula, Rajendar; Thiburce, Quentin; Gasparini, Nicola; Wadsworth, Andrew; Salleo, Alberto; Inal, Sahika; Berggren, Magnus; Zozoulenko, Igor; Stavrinidou, Eleni; McCulloch, Iain (Advanced Functional Materials, Wiley, 2021-04-17) [Article]
      Electrochemically induced volume changes in organic mixed ionic-electronic conductors (OMIECs) are particularly important for their use in dynamic microfiltration systems, biomedical machinery, and electronic devices. Although significant advances have been made to maximize the dimensional changes that can be accomplished by OMIECs, there is currently limited understanding of how changes in their molecular structures impact their underpinning fundamental processes and their performance in electronic devices. Herein, a series of ethylene glycol functionalized conjugated polymers is synthesized, and their electromechanical properties are evaluated through a combined approach of experimental measurements and molecular dynamics simulations. As demonstrated, alterations in the molecular structure of OMIECs impact numerous processes occurring during their electrochemical swelling, with sidechain length shortening decreasing the number of incorporated water molecules, reducing the generated void volumes and promoting the OMIECs to undergo different phase transitions. Ultimately, the impact of these combined molecular processes is assessed in organic electrochemical transistors, revealing that careful balancing of these phenomena is required to maximize device performance.
    • Modulation of electronic and magnetic properties of monolayer chromium trihalides by alloy and strain engineering

      Wang, Qian; Han, Nannan; Zhang, Xuyang; Zhang, Chenhui; Zhang, Xixiang; Cheng, Yingchun (Journal of Applied Physics, AIP Publishing, 2021-04-16) [Article]
      Monolayer CrI3 is a rare ferromagnetic semiconductor with intrinsic long-range magnetic order, which makes it a great potential material in spintronic devices [Song et al., Science 360, 1214 (2018)]. To extend the applications of monolayer CrI3 in flexible devices, the modulation of its electronic and magnetic properties is important. Here, we investigated the combined effect of strain and alloy on the properties of monolayer CrI3 by first-principles calculations. Br is chosen as the alloyed element due to the similar atomic configuration and property of CrX3 (X = Br, I), and the strain is applied by simultaneously changing the in-plane lattice constants (a and b). We find that the bandgap of monolayer Cr2I6−xBrx can be tuned greatly, while the magnetic moment of monolayer Cr2I6−xBrx is regulated very little under different strain and Br concentration. This unique property of monolayer Cr2I6−xBrx under strain makes it a good candidate for the flexible spintronic devices.
    • Skyrmion battery effect via inhomogeneous magnetic anisotropy

      Hao, Xiawei; Zhuo, Fengjun; Manchon, Aurelien; Wang, Xiaolin; Li, Hang; Cheng, Zhenxiang (Applied Physics Reviews, AIP Publishing, 2021-04-14) [Article]
      Magnetic skyrmions are considered a promising candidate for the next-generation information processing technology. Being topologically robust, magnetic skyrmions are swirling spin textures that can be used in a broad range of applications from memory devices and logic circuits to neuromorphic computing. In a magnetic medium lacking inversion symmetry, magnetic skyrmion arises as a result of the interplay among magnetic exchange interaction, Dzyaloshinskii-Moriya interaction, and magnetic anisotropy. Instrumental to the integrated skyrmion-based applications are the creation and manipulation of magnetic skyrmions at a designated location, absent any need of a magnetic field. In this paper, we propose a generic design strategy to achieve that goal and a model system to demonstrate its feasibility. By implementing a disk-shaped thin film heterostructure with an inhomogeneous perpendicular magnetic anisotropy, stable sub-100-nm size skyrmions can be generated without magnetic field. This structure can be etched out via, for example, focused ion beam microscope. Using micromagnetic simulation, we show that such heterostructure not only stabilizes the edge spins of the skyrmion but also protects its rotation symmetry. Furthermore, we may switch the spin texture between skyrmionic and vortex-like ones by tuning the slope of perpendicular anisotropy using a bias voltage. When embedded into a magnetic conductor and under a spin polarized current, such heterostructure emits skyrmions continuously and may function as a skyrmion source. This unique phenomenon is dubbed a skyrmion battery effect. Our proposal may open a novel venue for the realization of all-electric skyrmion-based devices.
    • Can a recipe for the growth of single-layer graphene on copper be used in different chemical vapor deposition reactors?

      Hakami, Marim A.; Deokar, Geetanjali Baliram; Smajic, Jasmin; Batra, Nitinkumar; Costa, Pedro Miguel Ferreira Joaquim (Chemistry, an Asian journal, Wiley, 2021-04-13) [Article]
      In the last decade, catalytic chemical vapor deposition (CVD) has been intensively explored for the growth of single-layer graphene (SLG). Despite the scattering of guidelines and procedures, variables such as the surface texture/chemistry of catalyst metal foils, carbon feedstock, and growth process parameters have been well-scrutinized. Still, questions remain on how best to standardize the growth procedure. The possible correlation of recipes between different CVD setups is an example. Here, two thermal CVD reactors were explored to grow graphene on Cu foil. The design of these setups was entirely distinct, one being a "showerhead" cold-wall type, whereas the other represented the popular "tubular" hot-wall type. Upon standardizing the Cu foil surface, it was possible to develop a recipe for cm 2 -scale SLG growth that differed only on the carrier gas flow rate used in the two reactors.
    • The Geological Potential of the Arabian Plate for CCS and CCUS - An Overview

      Vahrenkamp, Volker; Alafifi, Abdulkader Musa; Tasianas, Alexandros; Hoteit, Hussein (SSRN Electronic Journal, Elsevier BV, 2021-04-09) [Article]
      Given allowable carbon emissions for reaching climate targets, CCS and CCUS are without alternatives to simultaneously maintain a supply of sufficient energy for the world and preventing stranded subsurface assets for hydrocarbon producing countries. Permanent storage of carbon dioxide (CO2) in deep subsurface formations is acknowledged as a scalable and achievable technology to contribute to the ongoing efforts of limiting CO2 emissions and possibly lead to the use of stored CO2 for geothermal energy generation. The sequestration processes include entrapping CO2 in saline aquifers and hydrocarbon reservoirs in its mobile phase and in basalts as carbonate minerals. So, what are then the geological subsurface opportunities in Arabia for CO2 sequestration? A high level assessment has been conducted to identify geological formations suitable for storing and utilizing CO2 on a large scale. Over the Arabian peninsula four significantly different geological terrains are likely suitable for CCS & CCUS: (1) An Eastern section of stacked Mesozoic aquifers along the coast and inland of the Arabian Gulf, (2) rift basins with deep saline aquifers along the Red Sea, (3) Cenozoic volcanic rocks inland of the Red Sea coast, and Proterozoic ultramafic rocks in the Arabian Shield, and (4) a fringe of Cretaceous obducted marine crust (ophiolites) in Northeastern Oman and the UAE.
    • Calcium fluoride as high-k dielectric for 2D electronics

      Wen, Chao; Lanza, Mario (Applied Physics Reviews, AIP Publishing, 2021-04-09) [Article]
      Calcium fluoride is a dielectric material with a wide bandgap (∼12.1 eV) and a relatively high dielectric constant (∼6.8) that forms a van der Waals interface with two-dimensional (2D) materials, meaning that it contains a very low amount of defects. Thin calcium fluoride films can be synthesized using multiple techniques that are scalable to the wafer level, including molecular beam epitaxy, atomic layer deposition, and chemical vapor deposition. However, the consolidation of calcium fluoride as dielectric for 2D electronics requires overcoming some fundamental challenges related to material quality and integration, as well as carrying out advanced characterization and computational studies to evaluate its real potential. Here, we review the status of calcium fluoride dielectric films in terms of material synthesis, fundamental electrical properties, and future applications; we also discuss the most important challenges of calcium fluoride integration in 2D materials-based, solid-state nano/micro-electronic devices, and propose several potential routes to overcome them. Our manuscript may serve as a useful guide for other scientists working on 2D electronics in general, and provides a clear pathway for calcium fluoride research in the future.
    • Optical solitons supported by finite waveguide lattices with diffusive nonlocal nonlinearity

      Huang, Changming; Deng, Hanying; Dong, Liangwei; Shang, Ce; Zhao, Bo; Suo, Qiangbo; Zhou, Xiaofang (Chinese Physics B, IOP Publishing, 2021-04-07) [Article]
      We investigate the properties of fundamental, multi-peak, and multi-peaked twisted solitons in three types of finite waveguide lattices imprinted in photorefractive media with asymmetrical diffusion nonlinearity. Two opposite soliton self-bending signals are considered for different families of solitons. Power thresholdless fundamental and multi-peaked solitons are stable in the low power region. The existence domain of two-peaked twisted solitons can be changed by the soliton self-bending signals. When solitons tend to self-bend toward the waveguide lattice, stable two-peaked twisted solitons can be found in a larger region in the middle of their existence region. Three-peaked twisted solitons are stable in the lower (upper) cutoff region for a shallow (deep) lattice depth. Our results provide effective guidance for revealing the soliton characteristics supported by a finite waveguide lattice with diffusive nonlocal nonlinearity.
    • Synthesis of core-shell copper-graphite submicronic particles and carbon nano-onions by spark discharges in liquid hydrocarbons.

      Glad, X; Gorry, J; Cha, Min Suk; Hamdan, A (Scientific reports, Springer Nature, 2021-04-07) [Article]
      Spark discharge in hydrocarbon liquids is considered a promising method for the synthesis of various nanomaterials, including nanocomposites. In this study, copper-carbon particles were synthesized by generating spark discharges between two Cu electrodes immersed in heptane, cyclohexane, or toluene. The synthesized particles were characterized using scanning electron microscopy, high-resolution transmission electron microscopy, and selected area electron diffraction. Overall, two families of particles were observed: Cu particles (diameter
    • Synthesis of gold(I)-trifluoromethyl complexes and their role in generating spectroscopic evidence for a gold(I)-difluorocarbene species.

      Nolan, Steven Patrick; Vanden Broeck, Sofie M P; Nelson, David J; Collado, Alba; Falivene, Laura; Cavallo, Luigi; Cordes, David B; Slawin, Alexandra M Z; Van Hecke, Kristof; Nahra, Fady; Cazin, Catherine S J (Chemistry (Weinheim an der Bergstrasse, Germany), Wiley, 2021-04-06) [Article]
      Readily-prepared and bench-stable [Au(CF 3 )(NHC)] compounds were synthesized using new methodologies, starting from [Au(OH)(NHC)], [Au(Cl)(NHC)] or [Au(L)(NHC)]HF 2 precursors (NHC = N-heterocyclic carbene). The mechanism of formation of these species was investigated. Consequently, a new and straightforward strategy for the mild and selective cleavage of a single carbon-fluorine bond from [Au(CF 3 )(NHC)] complexes was attempted and found to be reversible in the presence of an additional nucleophilic fluoride source. This straightforward technique has led to the unprecedented spectroscopic observation of a gold(I)-NHC difluorocarbene species.
    • Rock Triaxial Tests: Global Deformation vs Local Strain Measurements—Implications

      Perbawa, Andika; Gramajo, Eduardo; Finkbeiner, Thomas; Santamarina, Carlos (Rock Mechanics and Rock Engineering, Springer Nature, 2021-04-05) [Article]
      Accurate stress–strain measurements in triaxial tests are critical to compute reliable mechanical parameters. We focus on compliance at the interfaces between the specimen and endcaps, and test specimens under various triaxial conditions using different instrumentation protocols. The tested materials include aluminum, Eagle Ford shale, Berea sandstone, and Jubaila carbonate. Results obtained following common practice reveal that surface roughness at the specimen-endcap interfaces leads to marked seating effects, affects all cap-to-cap based measurements and hinders ultrasonic energy transmission. In particular, cap-to-cap deformation measurements accentuate hysteretic behavior, magnify biases caused by bending and tilting (triggered by uneven surfaces and misalignment), and affect the estimation of all rock parameters, from stiffness to Biot’s α-parameter. Higher confining pressure diminishes seating effects. Local measurements using specimen-bonded strain gauges are preferred (Note: mounting strain gauges on sleeves is ill-advised). We confirm that elastic moduli derived from wave propagation measurements are higher than quasi-static moduli determined from local strain measurements using specimen-bonded strain gauges, probably due to the lower strain level in wave propagation and preferential high-velocity travel path for first arrivals.
    • PINNtomo: Seismic tomography using physics-informed neural networks

      Waheed, Umair bin; Alkhalifah, Tariq Ali; Haghighat, Ehsan; Song, Chao; Virieux, Jean (arXiv, 2021-04-04) [Preprint]
      Seismic traveltime tomography using transmission data is widely used to image the Earth's interior from global to local scales. In seismic imaging, it is used to obtain velocity models for subsequent depth-migration or full-waveform inversion. In addition, cross-hole tomography has been successfully applied for a variety of applications, including mineral exploration, reservoir monitoring, and CO2 injection and sequestration. Conventional tomography techniques suffer from a number of limitations, including the use of a smoothing regularizer that is agnostic to the physics of wave propagation. Here, we propose a novel tomography method to address these challenges using developments in the field of scientific machine learning. Using seismic traveltimes observed at seismic stations covering part of the computational model, we train neural networks to approximate the traveltime factor and the velocity fields, subject to the physics-informed regularizer formed by the factored eikonal equation. This allows us to better compensate for the ill-posedness of the tomography problem compared to conventional methods and results in a number of other attractive features, including computational efficiency. We show the efficacy of the proposed method and its capabilities through synthetic tests for surface seismic and cross-hole geometries. Contrary to conventional techniques, we find the performance of the proposed method to be agnostic to the choice of the initial velocity model.
    • Development of catalysts for sulfuric acid decomposition in the sulfur–iodine cycle: a review

      Khan, Hassnain Abbas; Jaleel, Ahsan; Mahmoud, Eyas; Ahmed, Shoaib; Bhatti, Umair Hassan; Bilal, Muhammad; Hussain, (Catalysis Reviews - Science and Engineering, Informa UK Limited, 2021-04-04) [Article]
      To achieve carbon-neutral energy vectors, researchers have investigated various sulfur-based thermochemical cycles. The sulfur–iodine cycle has emerged as a cost-effective global process with massive hydrogen production potentials. However, all sulfur-based thermochemical cycles involve sulfuric acid decomposition reaction, which is highly corrosive and energy intensive. The activation energy of this reaction can be reduced using catalysts that decrease the onset temperature of the reaction. Renewable heat sources such as solar and waste nuclear heat demand high stability to operate within a wide temperature window (650°C–900°C). Several metal/metal oxide systems based on noble and transition metals have been investigated over the last twenty years. In the literature, supported Pt-based catalysts are regarded as the prime choice for stable operations. However, during catalytic operations, noble metals are degraded owing to sintering, oxidation, leaching, and other processes. Transition metal oxides such as Fe, Cu, Cr, and Ni exhibit promising catalytic activity at high temperatures; however, at low temperatures (>600°C), their activation is reduced owing to poisoning and the formation of stable sulfate species. The catalytic activity of transition metal oxides is determined by the decomposition temperature of its corresponding metal sulfate; thus, the metal sulfate formation is considered as the rate-limiting step. Herein, the catalytic systems studied over the last decade are summarized, and recommendations for designing robust catalysts for commercial applications are presented.
    • Lithium-Ion Desolvation Induced by Nitrate Additives Reveals New Insights into High Performance Lithium Batteries

      Wahyudi, Wandi; Ladelta, Viko; Tsetseris, Leonidas; Alsabban, Merfat; Guo, Xianrong; Yengel, Emre; Faber, Hendrik; Adilbekova, Begimai; Seitkhan, Akmaral; Emwas, Abdul-Hamid; Hedhili, Mohamed N.; Li, Lain-Jong; Tung, Vincent; Hadjichristidis, Nikos; Anthopoulos, Thomas D.; Ming, Jun (Advanced Functional Materials, Wiley, 2021-04-02) [Article]
      Electrolyte additives have been widely used to address critical issues in current metal (ion) battery technologies. While their functions as solid electrolyte interface forming agents are reasonably well-understood, their interactions in the liquid electrolyte environment remain rather elusive. This lack of knowledge represents a significant bottleneck that hinders the development of improved electrolyte systems. Here, the key role of additives in promoting cation (e.g., Li+) desolvation is unraveled. In particular, nitrate anions (NO3−) are found to incorporate into the solvation shells, change the local environment of cations (e.g., Li+) as well as their coordination in the electrolytes. The combination of these effects leads to effective Li+ desolvation and enhanced battery performance. Remarkably, the inexpensive NaNO3 can successfully substitute the widely used LiNO3 offering superior long-term stability of Li+ (de-)intercalation at the graphite anode and suppressed polysulfide shuttle effect at the sulfur cathode, while enhancing the performance of lithium–sulfur full batteries (initial capacity of 1153 mAh g−1 at 0.25C) with Coulombic efficiency of ≈100% over 300 cycles. This work provides important new insights into the unexplored effects of additives and paves the way to developing improved electrolytes for electrochemical energy storage applications.
    • A tangent linear approximation of the ignition delay time. I: Sensitivity to rate parameters

      Almohammadi, Saja Mohammad; Hantouche, Mireille; Le Maître, Olivier P.; Knio, Omar (Combustion and Flame, Elsevier BV, 2021-04-02) [Article]
      A tangent linear approximation is developed to estimate the sensitivity of the ignition delay time with respect to individual rate parameters in a detailed chemical mechanism. Attention is focused on a gas mixture reacting under adiabatic, constant-volume conditions. The uncertainty in the rates of elementary reactions is described in terms of uncertainty factors, and are parameterized using independent canonical random variables. The approach is based on integrating the linearized system of equations governing the evolution of the partial derivatives of the state vector with respect to individual random variables, and a linearized approximation is developed to relate the ignition delay sensitivity to the scaled partial derivatives of temperature. The efficiency of the approach is demonstrated through applications to chemical mechanisms of different sizes. In particular, the computations indicate that for detailed reaction mechanisms the TLA leads to robust local sensitivity predictions at a computational cost that is order-of-magnitude smaller than that incurred by finite-difference approaches based on one-at-a-time rate perturbations.
    • Theory-Guided Synthesis of Highly Luminescent Colloidal Cesium Tin Halide Perovskite Nanocrystals

      Liu, Qi; Yin, Jun; Zhang, Bin-Bin; Chen, Jia-Kai; Zhou, Yang; Zhang, Lu-Min; Wang, Lu-Ming; Zhao, Qing; Hou, Jingshan; Shu, Jie; Song, Bo; Shirahata, Naoto; Bakr, Osman; Mohammed, Omar F.; Sun, Hong-Tao (Journal of the American Chemical Society, American Chemical Society (ACS), 2021-04-01) [Article]
      The synthesis of highly luminescent colloidal CsSnX<sub>3</sub> (X = halogen) perovskite nanocrystals (NCs) remains a long-standing challenge due to the lack of a fundamental understanding of how to rationally suppress the formation of structural defects that significantly influence the radiative carrier recombination processes. Here, we develop a theory-guided, general synthetic concept for highly luminescent CsSnX<sub>3</sub> NCs. Guided by density functional theory calculations and molecular dynamics simulations, we predict that, although there is an opposing trend in the chemical potential-dependent formation energies of various defects, highly luminescent CsSnI<sub>3</sub> NCs with narrow emission could be obtained through decreasing the density of tin vacancies. We then develop a colloidal synthesis strategy that allows for rational fine-tuning of the reactant ratio in a wide range but still leads to the formation of CsSnI<sub>3</sub> NCs. By judiciously adopting a tin-rich reaction condition, we obtain narrow-band-emissive CsSnI<sub>3</sub> NCs with a record emission quantum yield of 18.4%, which is over 50 times larger than those previously reported. Systematic surface-state characterizations reveal that these NCs possess a Cs/I-lean surface and are capped with a low density of organic ligands, making them an excellent candidate for optoelectronic devices without any postsynthesis ligand management. We showcase the generalizability of our concept by further demonstrating the synthesis of highly luminescent CsSnI<sub>2.5</sub>Br<sub>0.5</sub> and CsSnI<sub>2.25</sub>Br<sub>0.75</sub> NCs. Our findings not only highlight the value of computation in guiding the synthesis of high-quality colloidal perovskite NCs but also could stimulate intense efforts on tin-based perovskite NCs and accelerate their potential applications in a range of high-performance optoelectronic devices.
    • Machine-driven earth exploration: Artificial intelligence in oil and gas

      Alkhalifah, Tariq Ali; Almomin, Ali; Naamani, Ali (The Leading Edge, Society of Exploration Geophysicists, 2021-04) [Article]
      Artificial intelligence (AI), specifically machine learning (ML), has emerged as a powerful tool to address many of the challenges we face as we try to illuminate the earth and make the proper prediction of its content. From fault detection, to salt boundary mapping, to image resolution enhancements, the quest to teach our computing devices how to perform these tasks accurately, as well as quantify the accuracy, has become a feasible and sought-after objective. Recent advances in ML algorithms and availability of the modules to apply such algorithms enabled geoscientists to focus on potential applications of such tools. As a result, we held the virtual workshop, Artificially Intelligent Earth Exploration Workshop: Teaching the Machine How to Characterize the Subsurface, 23–26 November 2020.
    • Synergy processing of diverse ground-based remote sensing and in situ data using the GRASP algorithm: applications to radiometer, lidar and radiosonde observations

      Lopatin, Anton; Dubovik, Oleg; Fuertes, David; Stenchikov, Georgiy L.; Lapyonok, Tatyana; Veselovskii, Igor; Wienhold, Frank G.; Shevchenko, Illia; Hu, Qiaoyun; Parajuli, Sagar (Atmospheric Measurement Techniques, Copernicus GmbH, 2021-04-01) [Article]
      Abstract. The exploration of aerosol retrieval synergies from diverse combinations of ground-based passive Sun-photometric measurements with collocated active lidar ground-based and radiosonde observations using versatile Generalized Retrieval of Atmosphere and Surface Properties (GRASP) algorithm is presented. Several potentially fruitful aspects of observation synergy were considered. First, a set of passive and active ground-based observations collected during both day- and nighttime was inverted simultaneously under the assumption of temporal continuity of aerosol properties. Such an approach explores the complementarity of the information in different observations and results in a robust and consistent processing of all observations. For example, the interpretation of the nighttime active observations usually suffers from the lack of information about aerosol particles sizes, shapes and complex refractive index. In the realized synergy retrievals, the information propagating from the nearby Sun-photometric observations provides sufficient constraints for reliable interpretation of both day- and nighttime lidar observations. Second, the synergetic processing of such complementary observations with enhanced information content allows for optimizing the aerosol model used in the retrieval. Specifically, the external mixture of several aerosol components with predetermined sizes, shapes and composition has been identified as an efficient approach for achieving reliable retrieval of aerosol properties in several situations. This approach allows for achieving consistent and accurate aerosol retrievals from processing stand-alone advanced lidar observations with reduced information content about aerosol columnar properties. Third, the potential of synergy processing of the ground-based Sun-photometric and lidar observations, with the in situ backscatter sonde measurements was explored using the data from KAUST.15 and KAUST.16 field campaigns held at King Abdullah University of Science and Technology (KAUST) in the August of 2015 and 2016. The inclusion of radiosonde data has been demonstrated to provide significant additional constraints to validate and improve the accuracy and scope of aerosol profiling. The results of all retrieval setups used for processing both synergy and stand-alone observation data sets are discussed and intercompared.
    • Computational characterization of hydrogen direct injection and nonpremixed combustion in a compression-ignition engine

      Babayev, Rafig; Andersson, Arne; Dalmau, Albert Serra; Im, Hong G.; Johansson, Bengt (International Journal of Hydrogen Energy, Elsevier BV, 2021-04) [Article]
      With the revived interest in hydrogen (H2) as a direct combustion fuel for engine applications, a computational study is conducted to assess the characteristics of H2 direct-injection (DI) compression-ignition (CI) non-premixed combustion concept. Development of a CFD modeling using CONVERGE CFD solver focuses on hydrogen's unique characteristics by utilizing a suitable numerical method to reproduce the direct H2 injection phenomena. A grid sensitivity study is performed to ensure the fidelity of results with optimal cost, and the models are validated against constant-volume optical chamber and diesel engine experimental data. The present study aims to contribute to the future development of DICI H2 combustion engines, providing detailed characterization of the combustion cycle, and highlighting several distinct aspects of CI nonpremixed H2 versus diesel combustion. First, unlike the common description of diesel sprays, hydrogen jets do not exhibit significant flame lift-off and air entrainment near injector nozzle, and the fuel-air interface is drastically more stratified with no sign of premixing. It is also found that the DICI H2 combustion concept is governed first by a free turbulent jet mixing phase, then by an in-cylinder global mixing phase. The former is drastically more dominant with the DICI H2 engine compared to conventional diesel engines. The free-jet mixing is also found to be more effective that the global mixing, which indicates the need to completely rethink the optimization strategies for CI engines when using H2 as fuel.
    • Experimental and theoretical study of PAH and incipient soot formation in laminar flames

      Li, Zepeng (2021-04) [Dissertation]
      Advisor: Roberts, William L.
      Committee members: Lacoste, Deanna; Chung, Suk Ho; Knio, Omar; Thomson, Murray J.
      Emissions of soot and polycyclic aromatic hydrocarbons (PAHs) from incomplete burning of hydrocarbon fuels pose a great threat to the environment and human health. To reduce such emissions, a comprehensive understanding of their evolution process is essential. In this work, a series of research studies were conducted to evaluate sooting tendencies and to experimentally and theoretically develop PAH mechanisms. The sooting tendencies of oxygenated fuels were quantitively investigated in counterflow diffusion flames. Sooting limits are described by critical fuel and oxygen mole fractions, measured with a laser scattering technique. The addition of dimethyl ether displays non-monotonic behavior on sooting tendencies at elevated pressures, which is attributed to the chemical effect from kinetic simulations. The tendency of incipient soot formation of other oxygenated fuels (e.g., alcohol, acid, ether, ketone, and carbonate ester) was also assessed, using a similar approach. As the precursor of soot, PAH measurement using laser induced fluoresecnce was implemented to track the evolution processes from PAHs to incipient soot. Developing a PAH mechanism is essential to the understanding of soot formation; however, PAH formation and its growth process are not well understood. Based on previous research, PAHs with 5-membered rings are abundant in flames. Therefore, the growth of PAHs with 5-membered rings was investigated, using acenaphthylene (A2R5) as the example. The density functional theory (DFT) and the transition state theory (TST) were adopted to calculate potential energy surfaces and reaction rate coefficients. The existence of 5-membered rings appreciably impacts PAH production by facilitating the formation of planar PAHs with C2H substitution, thereby improving existing PAH mechanisms. In PAH mechanisms, the thermochemistry properties are not all calculated, but are hypothesized to be equal to those of a similar structure. The simulation accuracy of the hypothesis is explored here by discussing the sensitivity of the thermochemistry parameters in flame simulations. The group additivity method utilizing THERM codes is used to calculate thermochemistry properties. PAH loading affects the sensitivity of thermochemistry properties to both flame temperature and product yields. These results show that either accurate thermochemistry properties, or reverse reaction rates should be provided in the mechanism to improve simulation accuracy.
    • Broadband Elastic Wave Propagation in Intact Rocks (Quasi-static to MegaHertz)

      Perbawa, Andika (2021-04) [Dissertation]
      Advisor: Santamarina, Carlos
      Committee members: Hoteit, Ibrahim; Santamarina, Carlos; Finkbeiner, Thomas; Lubineau, Gilles; Cascante, Giovanni
      Elastic wave propagation in saturated porous rocks reflects the fluid and mineral stiffness and their frequency-dependent interaction. Seismic imaging and borehole measurements in the field use low-frequency, long-wavelength signals (Hz-to-kHz), while standard laboratory-measurements operate in the MHz range. This thesis advances broadband elastic wave propagation methods (quasi-static, cyclic loading, first-mode resonance, and ultrasonic) to characterize intact rocks in order to gather laboratory data relevant to field conditions. Results show the critical effect of surface roughness at the specimen-endcap interfaces on stiffness measured under quasi-static conditions; local strain measurements using specimen-bonded strain gauges avoid seating effects. Multi-mode low-frequency resonant column testing provides the most reliable assessment of attenuation; attenuation increases and resonant frequency decreases with vibration amplitude for all vibration modes (longitudinal, torsional, and flexural). Ultrasonic P and S-wave velocities increase as a function of conf fining pressure and during early stages of deviatoric loading; trends follow a Hertzian power law. The corresponding -factors and -exponents exhibit a strong correlation with specimen type. The combination of ultrasonic measurement and coda wave analysis allows us to detect minute velocity changes during fluid invasion and damage evolution. Differences in P-wave velocity in specimens saturated with brine and supercritical CO2 are higher at seismic frequencies than in ultrasonic frequencies. 5 The new experimental methods implemented in this research and the comprehensive characterization studies provide new tools into intact rock characterization and contribute new insights on the physical properties of intact rocks and fluid-matrix interaction. Results highlight critical differences between field values and standard laboratory measurements