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  • Magneto-transport Mechanism of Individual Nanostructures via Direct Magnetoresistance Measurement in situ SEM

    Zhang, Junwei; Peng, Yong; Ma, Hongbin; Zhang, Senfu; Hu, Yang; Zeng, Xue; Deng, Xia; Guan, Chaoshuai; Chen, Rongrong; Hu, Yue; Karim, Abdul; Tao, Kun; Zhang, Mingjie; Zhang, Xixiang (ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2020-08-05) [Article]
    The accurate magnetoresistance (MR) measurement of individual nanostructures is essential and important for either the enrichment of fundamental knowledge of magneto-transport mechanism or the facilitation of desired design of magnetic nanostructures for various technological applications. Herein, we report a deep investigation on the magneto-transport mechanism of single CoCu/Cu multilayered nanowire via direct magnetoresistance measurement by using our invented magnetotransport instrument in-situ scanning electron microscope (SEM). Off-axis electron holography experiments united with micromagnetic simulation prove that the CoCu layers in CoCu/Cu multilayered nanowires are formed a single-domain structure, in which the alignment of magnetic moments is mainly determined by shape anisotropy. The MR of the single CoCu/Cu multilayered nanowire is measured to be only 1.14% when the varied external field is applied along nanowire length axis, which matches with the theoretical prediction of Granular Films model. Density functional theory (DFT) calculations further disclose that spin-dependent scattering at the interface between magnetic and nonmagnetic layers is responsible for the intrinsic magnetotransport mechanism
  • Synergetic Contributions in Phase Boundary Engineering to the Piezoelectricity of Potassium Sodium Niobate Lead-Free Piezoceramics

    Lv, Xiang; Zhang, Junwei; Liu, Yao; Li, Fei; Zhang, Xixiang; Wu, Jiagang (ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2020-08-04) [Article]
    Although the pronounced piezoelectricity was obtained in (K, Na)NbO3 piezoceramics with the phase boundary engineering (PBE), the physical mechanisms remain pending. Here we revealed for the first time how PBE influences the piezoelectric properties through synergetic contributions. Cryogenic experiments confirm that PBE constructs a phase coexistence, consisting of rhombohedral (R), orthorhombic (O), and tetragonal (T) phases, with a structural softening, by which a high piezoelectric coefficient d33 of 555 pC/N and the enhanced temperature stability of strain are achieved. The phenomenological theory and transmission electron microscope demonstrate that the superior d33 hinges on the flattened Gibbs free energy and the abundant nano-domains (10-80 nm), which respectively induce the enhanced permittivity and the coexisting single-domain and multi-domain zones. In particular, we disclosed a trade-off relationship between ferroelectric domains and polarnanoregions (PNRs) and found the “double-edged sword” role of PNRs in the piezoelectricity enhancement. Therefore, this work helps understand the physical mechanisms of the piezoelectricity enhancement, benefiting the future research of lead-free piezoceramics.
  • Asthenospheric flow of plume material beneath Arabia inferred from S-wave travel-time tomography

    Lim, Jung-A; Chang, Sung-Joon; Mai, Paul Martin; Zahran, Hani (Journal of Geophysical Research: Solid Earth, American Geophysical Union (AGU), 2020-08-03) [Article]
    Widespread Cenozoic volcanism in the Arabian Peninsula has been attributed to mantle plume activity and/or lithospheric thinning due to rift-related extension. However, there is discrepancy between geochemical and geophysical studies about which mechanism is dominant over the other for post-12 Ma volcanism. Plume signals in some volcanic fields in the Arabian shield are not evident in isotope analyses, but low-velocity anomalies connected to Afar are found beneath the Arabian shield in tomographic studies and interpreted as asthenospheric flow from the Afar plume. To resolve this contradiction, we investigate the upper mantle beneath the Arabian Peninsula and northeastern Africa by inverting relative S- and SKS-wave arrival times recorded at dense seismic networks to derive a high-resolution S-wave velocity model. Our results clearly show a narrow, elongated low-velocity anomaly along the Makkah-Madinah-Nafud (MMN) volcanic line beneath the Arabian shield at 100-300 km depth which extends northward to Harrat Ithanayn and Harrat Lunayyir, but most likely not further north. The limited extent of the low-velocity anomaly and variations in lithospheric thickness of the Arabian shield may explain why geochemical studies did not find plume signals in some harrats. Therefore, the timing and plume signals of volcanism in western Arabia may not be age-progressive from Afar. We also find a possible connection between the low-velocity anomalies beneath Harrat Lunayyir and the MMN line, suggesting that the 2007-2009 seismic swarm may be associated with northward asthenospheric flow of plume material from Afar.
  • Alternating Gyroid Network Structure in an ABC Miktoarm Terpolymer Comprised of Polystyrene and Two Polydienes

    Moschovas, Dimitrios; Manesi, Gkreti-Maria; Karydis-Messinis, Andreas; Zapsas, Georgios; Ntetsikas, Konstantinos; Zafeiropoulos, Nikolaos E.; Piryazev, Alexey A.; Thomas, Edwin L.; Hadjichristidis, Nikos; Ivanov, Dimitri A.; Avgeropoulos, Apostolos (Nanomaterials, MDPI AG, 2020-07-31) [Article]
    The synthesis, molecular and morphological characterization of a 3-miktoarm star terpolymer of polystyrene (PS, M¯n = 61.0 kg/mol), polybutadiene (PB, M¯n = 38.2 kg/mol) and polyisoprene (PI, M¯n = 29.2 kg/mol), corresponding to volume fractions (φ) of 0.46, 0.31 and 0.23 respectively, was studied. The major difference of the present material from previous ABC miktoarm stars (which is a star architecture bearing three different segments, all connected to a single junction point) with the same block components is the high 3,4-microstructure (55%) of the PI chains. The interaction parameter and the degree of polymerization of the two polydienes is sufficiently positive to create a three-phase microdomain structure as evidenced by differential scanning calorimetry and transmission electron microscopy (TEM). These results in combination with small-angle X-ray scattering (SAXS) and birefringence experiments suggest a cubic tricontinuous network structure, based on the I4132 space group never reported previously for such an architecture.
  • In Situ Plasma-Grown Silicon-Oxide for Polysilicon Passivating Contacts

    Alzahrani, Areej A.; Allen, Thomas; de Bastiani, Michele; Van Kerschaver, Emmanuel; Harrison, George T.; Liu, Wenzhu; De Wolf, Stefaan (Advanced Materials Interfaces, Wiley, 2020-07-29) [Article]
    Large-scale manufacturing of polysilicon-based passivating contacts for high-efficiency crystalline silicon (c-Si) solar cells demands simple fabrication of thermally stable SiOx films with well controlled microstructure and nanoscale thickness to enable quantum-mechanical tunneling. Here, plasma-dissociated CO2 is investigated to grow in situ thin (<2 nm) SiOx films on c-Si wafers as tunnel-oxides for plasma-deposited, hole-collecting (i.e., p-type) polysilicon contacts. It is found that such plasma processing offers excellent thickness control and superior structural integrity upon thermal annealing at 1000 °C, compared to state-of-the-art wet-chemical oxides. As a result, p-type polysilicon contacts are achieved on n-type c-Si wafers that combine excellent surface passivation, resulting in an implied open-circuit voltage exceeding 700 mV, with a contact resistance as low as 0.02 Ω cm2.
  • Three-stage auto-ignition of n-heptane and methyl-cyclohexane mixtures at lean conditions in a flat piston rapid compression machine

    AlRamadan, Abdullah; Houidi, Moez Ben; Sotton, Julien; Bellenoue, Marc; Johansson, Bengt; Sarathy, Mani (Proceedings of the Combustion Institute, Elsevier BV, 2020-07-28) [Article]
    One approach to enhancing the thermal efficiency of combustion systems is to burn fuels at ultra-lean conditions (equivalence ratio below 0.5). It has been recently reported that the auto-ignition of some hydrocarbon fuels, under specific temperature, pressure, and mixture conditions, releases heat in three distinctive stages. The three auto-ignition stages can be divided as a first low-temperature auto-ignition stage with conventional low temperature, and a high-temperature stage separated into two sub-stages. This study presents ignition delay time measurements of n-heptane and methyl-cyclohexane (MCH) mixtures in a flat piston rapid compression machine (RCM) under ultra-lean conditions. It provides experimental evidence of three-stage auto-ignition. This phenomenon of delayed high-temperature heat release is seldom reported in the literature and this is the first time to be reported for these types of fuels. The experiments cover two binary n-heptane/MCH mixtures of 15/85 and 70/30 by volume, pressures of 11 bar and 16 bar, temperature range of 700 to 900 K, and equivalence ratio of 0.4. The RCM optical access was utilized for high-speed chemiluminescence imaging. Detailed chemical kinetic simulations in a homogenous batch reactor with variable volume were conducted to further interrogate the three-stage auto-ignition phenomenon. Chemiluminescence shows that three-stage auto-ignition occurs in the adiabatically compressed end-gas, which indicates that this phenomenon is chemically-driven and is not induced by a thermal stratification in the RCM experiments. The model predicts the features of three-stage auto-ignition, which were experimentally observed at temperatures approximately below 750 K. As expected, significant discrepancies are observed in the ignition delays of experiment and simulation in the negative temperature coefficient (NTC) region. The simulation of the n-heptane/MCH 70/30 mixture shows better agreement with experiments in the Positive Temperature Coefficient (PTC) region compared to the 15/85 mixture.
  • Chemical structure of atmospheric pressure premixed laminar formic acid/hydrogen flames

    Osipova, K.N.; Sarathy, Mani; Korobeinichev, O.P.; Shmakov, A.G. (Proceedings of the Combustion Institute, Elsevier BV, 2020-07-28) [Article]
    The work presents an experimental and kinetic modeling study of laminar premixed formic acid [HC(O)OH]/H2/O2/Ar flames at different equivalence ratios (φ=0.85, 1.1 and 1.3) stabilized on a flat burner at atmospheric pressure, as well as laminar flame speed of HC(O)OH/O2/Ar flames (φ=0.5–1.5) at 1 atm. Flame structure as well as laminar flame speed were simulated using three different detailed chemical kinetic mechanisms proposed for formic acid oxidation. The components in the fuel blends show different consumption profiles, namely, hydrogen is consumed slower than formic acid. According to kinetic analysis, the reason of the observed phenomenon is that the studied flames have hydrogen as a fuel but also as an intermediate product formed from HC(O)OH decomposition. Comparison of the measured and simulated flame structure shows that all the mechanisms satisfactorily predict the mole fraction profiles of the reactants, main products, and intermediates. It is noteworthy that the mechanisms proposed by Glarborg et al., Konnov et al. and the updated AramcoMech2.0 adequately predict the spatial variations in the mole fractions of free radicals, such as H, OH O and HO2. However, some drawbacks of the mechanisms used were identified; in particular, they predict different concentrations of CH2O. As for laminar flame speed simulations, the Konnov et al. mechanism predicts around two times higher values than in experiment, while the Glarborg et al. and updated AramcoMech2.0 show good agreement with the experimental data.
  • Removal of Organic Micropollutants from Water by Macrocycle-Containing Covalent Polymer Networks.

    Sessler, Jonathan L; Ji, Xiaofan; Wang, Hu; Wang, Hongyu; Zhao, Tian; Page, Zachariah A; Khashab, Niveen M. (Angewandte Chemie (International ed. in English), Wiley, 2020-07-28) [Article]
    Access to clean drinking water is a recognized societal need that touches on the health and livelihood of millions of people worldwide. This is providing an incentive to develop new water-treatment technologies. Traditional technologies, while widespread, are usually inefficient at removing organic pollutants from sewage or so-called grey water. Macrocycle-containing covalent polymer networks have begun to attract attention in the context of water treatment owing to the inherent stability provided by the polymer backbones and their ability to capture micropollutant guests as the result of tunable macrocycle-based host-guest interactions. In this minireview, we summarize recent advances (from 2016 to mid-2020) involving the removal of organic micropollutants from water using macrocycle-containing covalent polymer networks. An overview of future challenges within this subfield is also provided.
  • Methanol as the Hydrogen Source in the Selective Transfer Hydrogenation of Alkynes Enabled by a Manganese Pincer Complex

    Sklyaruk, Jan; Zubar, Viktoriia; Borghs, Jannik C.; Rueping, Magnus (Organic Letters, American Chemical Society (ACS), 2020-07-28) [Article]
    The first base metal-catalyzed transfer hydrogenation of alkynes with methanol is described. An air and moisture stable manganese pincer complex catalyzes the reduction of a variety of different alkynes to the corresponding (Z)-olefins in high yields. The reaction is stereo- and chemoselective and scalable.
  • Enhanced Thermoelectric Performance and Lifetime in Acid-Doped PEDOT:PSS Films via Work Function Modification

    Villalva, Diego Rosas; Haque, Mohammed; Nugraha, Mohamad; Baran, Derya (ACS Applied Energy Materials, American Chemical Society (ACS), 2020-07-28) [Article]
    In recent years, most of the work on p-type organic thermoelectrics focus on improving the thermoelectric properties of PEDOT:PSS through a sequential doping-dedoping process. However, the air-stability of thermoelectric parameters of these systems, which is essential for the realization of reliable devices remains largely unexplored. In this study, Poly (ethyleneimine)-ethoxylate (PEIE) acts as a work function modification agent and encapsulation layer to improve the thermoelectric performance and air-stability of nitric acid (HNO3) doped PEDOT:PSS films. The evaporation of HNO3 is responsible for a simultaneous decrease in electrical conductivity and an increase in the Seebeck coefficient leading to the degradation of the power factor. PEIE reduces the evaporation of HNO3 from PEDOT:PSS, and increases the power factor from 72 to 168 μW m-1K-2. After a week of exposure to air, the films show a power factor of 124 μW m-1K-2, retaining 74% of its initial thermoelectric merits. These results underscore the importance of PEIE as a material for enhancing thermoelectric performance and air-stability in the development of polymer-based thermoelectrics.
  • A statistical analysis of developing knock intensity in a mixture with temperature inhomogeneities

    Luong, Minh Bau; Desai, Swapnil; Pérez, Francisco E. Hernández; Sankaran, Ramanan; Johansson, Bengt; Im, Hong G. (Proceedings of the Combustion Institute, Elsevier BV, 2020-07-27) [Article]
    Knock formation and its intensity for a stoichiometric ethanol/air mixture under a representative endgas auto-ignition condition in IC engines with temperature inhomogeneities are investigated using multidimensional direct numerical simulations (DNS) with a 40-species skeletal mechanism of ethanol. Two- and three-dimensional simulations are performed by systematically varying temperature fluctuations and its most energetic length scale, lT. The volumetric fraction of the mixture regions that have the propensity to detonation development, FD, is proposed as a metric to predict the amplitude of knock intensity.It isfound that with increasing lT, FD shows a good agreement with the heat release fraction of the mixture regions with pressure greater than equilibrium pressure, FH. The detonation peninsula is well captured by FD and FH when plotting them as a function of the volume-averaged ξ , ξ, (ξ = a/Ssp is the ratio of the acoustic speed, a to the ignition front speed, Ssp). Decreasing lT is found to significantly reduce the super-knock intensity. The results suggest that decreasing lT, as in engines with tumble desig
  • Fluorophosphates: Next Generation Cathode Materials for Rechargeable Batteries

    Sharma, Lalit; Adiga, Shashishekar P.; Alshareef, Husam N.; Barpanda, Prabeer (Advanced Energy Materials, Wiley, 2020-07-27) [Article]
    Cost, safety, and cycle life have emerged as prime concerns to build robust batteries to cater to the global energy demand. These concerns are impacted by all battery components, but the realizable energy density of lithium-ion batteries (LIBs) is limited by the performance of cathodes. Thus, cathode materials have a significant role to play in advancing the performance and economics of secondary batteries. To realize next generation Li-ion and post Li-ion batteries, a variety of cathode insertion materials have been explored, but finding a cost effective and stable cathode material that can deliver high energy density has been a daunting task. Oxide cathode materials are ubiquitous in commercial applications, as they can deliver high capacity. In comparison, polyanionic insertion materials can offer tuneable (high) redox potential, operational safety, and structural as well as thermal stability. Indeed, a wide range of polyanionic materials like phosphates, borates, sulfates, and their complexes have been reported. In this article, the alkali metal fluorophosphates class of polyanionic cathodes for secondary batteries is discussed. The various reported fluorophosphate insertion materials are discussed in terms of their electrochemical and electrocatalytic properties. The historical overview, recent progress, and remaining challenges for polyanionic fluorophosphates are presented along with suggested future research directions and potential application.
  • The Role of Adding Bi0.5A0.5ZrO3 in Affecting Orthorhombic-Tetragonal Phase Transition Temperature and Electrical Properties in Potassium Sodium Niobate Ceramics

    Lv, Xiang; Zhang, Nan; Wu, Jiagang; Zhang, Xixiang (Acta Materialia, Elsevier BV, 2020-07-25) [Article]
    By studying the alone and synergetic effects of Zr4+ and Bi3+ on potassium sodium niobate ((K, Na)NbO3, KNN) ceramics, we revealed how Bi0.5A0.5ZrO3 (A = K, Na, Ag, and (Na0.82K0.18)) reduces the orthorhombic-tetragonal phase transition temperature (TO–T) of KNN ceramics. Investigations into the alone effects reveal that aliovalent substitutions on K+/Na+ (Nb5+) with Bi3+ (Zr4+) inevitably destroy long-range ordering (LRO) and thus worsen piezo/ferroelectric properties. Despite this, Zr4+ can replace Nb5+ within a high content, remain an orthorhombic (O) phase, and slightly increase TO–T. Although substituting on K+/Na+ with Bi3+ decreases TO–T, it already significantly destroyed LRO before shifting TO–T to room temperature. Then, investigations into the synergetic effects show that Zr4+ acts as a buffer, Bi3+ is an accelerator, and A+ is a stabilizer. The buffer can exist in KNN ceramics within a high content and neutralizes the charges caused by the accelerator that concentrates on decreasing TO–T, and the stabilizer compensates for the stability of the perovskite phase. Their synergetic effects explain why Bi0.5A0.5ZrO3 can gradually reduce the TO–T of KNN ceramics without significantly destroying LRO. Therefore, this work helps understand how Bi0.5A0.5ZrO3 decreases TO–T and further design the phase boundary for KNN ceramics.
  • Light-Harvesting Two-Photon-Absorbing Polymers

    Goswami, Subhadip; Cekli, Seda; Alarousu, Erkki; Winkel, Russell W.; Younus, Muhammad; Mohammed, Omar F.; Schanze, Kirk (Macromolecules, American Chemical Society (ACS), 2020-07-24) [Article]
    A series of atactic polystyrene-based polymers was synthesized that contains grafted π-conjugated organic and organometallic chromophores to investigate two-photon light-harvesting properties. The polymers feature 4-(diphenylamino)fluorene (DPAF) and Pt−DPAF as π-conjugated units which are known to be moderately efficient two-photon absorption (2PA) chromophores. The polymers were synthesized by reversible addition−fragmentation transfer (RAFT) polymerization of 4-chloromethylstyrene, and following substitution of Cl by N3, the DPAF and Pt−DPAF chromophores were grafted onto the polymer via the coppercatalyzed azide−alkyne click reaction. The loading of Pt−DPAF units in the polymers was varied from 0 to 20% by varying the feed ratio in the click reactions. With an increase in the Pt−DPAF content, the fluorescence quantum yield from the DPAF singlet excited state decreases and fluorescence is replaced by phosphorescence characteristic of the Pt−DPAF units at 530 nm. The emission lifetime and ultrafast transient absorption spectroscopy confirm that rapid and efficient singlet energy transfer occurs from DPAF to DPAF− Pt. Excitation of the polymers with 100 fs, near-infrared pulses gives rise to upconverted emission, and the observed emission spectra are similar to those under one-photon excitation. The results indicate that the DPAF units effectively function as 2-photon absorption light-harvesting units, transferring the excitation to the Pt−DPAF units where intersystem crossing occurs efficiently. Taken together, the results point the way to development of novel polymer-based optical power-limiting materials for ultrashort and long optical pulses.
  • Tuning the Surface Structure of Polyamide Membranes Using Porous Carbon Nitride Nanoparticles for High-Performance Seawater Desalination

    Zhou, Zongyao; Li, Xiang; Shinde, Digambar; Sheng, Guan; Lu, Dongwei; Li, Peipei; Lai, Zhiping (Membranes, MDPI AG, 2020-07-24) [Article]
    Enhancing the water flux while maintaining the high salt rejection of existing reverse osmosis membranes remains a considerable challenge. Herein, we report the use of a porous carbon nitride (C3N4) nanoparticle to potentially improve both the water flux and salt rejection of the state-of-the-art polyamide (PA) thin film composite (TFC) membranes. The organic–organic covalent bonds endowed C3N4 with great compatibility with the PA layer, which positively influenced the customization of interfacial polymerization (IP). Benefitting from the positive effects of C3N4, a more hydrophilic, more crumpled thin film nanocomposite (TFN) membrane with a larger surface area, and an increased cross-linking degree of PA layer was achieved. Moreover, the uniform porous structure of the C3N4 embedded in the ”ridge” sections of the PA layer potentially provided additional water channels. All these factors combined provided unprecedented performance for seawater desalination among all the PA-TFC membranes reported thus far. The water permeance of the optimized TFN membrane is 2.1-folds higher than that of the pristine PA-TFC membrane, while the NaCl rejection increased to 99.5% from 98.0%. Our method provided a promising way to improve the performance of the state-of-art PA-TFC membranes in seawater desalination.
  • Highly UV Resistant Inch-Scale Hybrid Perovskite Quantum Dot Papers

    Li, Ting-You; Xu, Xuezhu; Lin, Chun-Ho; Guan, Xinwei; Hsu, Wei-Hao; Tsai, Meng-Lin; Fang, Xiaosheng; Wu, Tom; He, Jr-Hau (Advanced Science, Wiley, 2020-07-24) [Article]
    Halide perovskite quantum dots (PQDs) are promising materials for diverse applications including displays, light-emitting diodes, and solar cells due to their intriguing properties such as tunable bandgap, high photoluminescence quantum yield, high absorbance, and narrow emission peaks. Despite the prosperous achievements over the past several years, PQDs face severe challenges in terms of stability under different circumstances. Currently, researchers have overcome part of the stability problem, making PQDs sustainable in water, oxygen, and polar solvents for long-term use. However, halide PQDs are easily degraded under continuous irradiation, which significantly limits their potential for conventional applications. In this study, an oleic acid/oleylamine (traditional surface ligands)-free method to fabricate perovskite quantum dot papers (PQDP) is developed by adding cellulose nanocrystals as long-chain binding ligands that stabilize the PQD structure. As a result, the relative photoluminescence intensity of PQDP remains over ≈90% under continuous ultraviolet (UV, 16 W) irradiation for 2 months, showing negligible photodegradation. This proposed method paves the way for the fabrication of ultrastable PQDs and the future development of related applications.
  • Model-Based Design of Graphite-Compatible Electrolytes in Potassium-Ion Batteries

    Zhang, Jiao; Cao, Zhen; Zhou, Lin; Liu, Gang; Park, Geon-Tae; Cavallo, Luigi; Wang, Limin; Alshareef, Husam N.; Sun, Yang-Kook; Ming, Jun (ACS Energy Letters, American Chemical Society (ACS), 2020-07-24) [Article]
    Potassium-ion batteries (KIBs) are attractive alternatives to lithium-ion batteries (LIBs) because of their lower cost and global potassium sustainability. However, designing compatible electrolytes with graphite anode remains challenging. This is because the electrolyte decomposition and/or graphite exfoliation (due to K+–solvent co-insertion) always exist, which is much harder to overcome compared to the case of LIBs because of the higher activities of K+. Herein, we report a general principle to design compatible electrolytes with the graphite anode, where the K+ can be reversibly (de)intercalated. We find that the electrolyte composition is critical to determining the graphite performance, which can be tuned by the kind of solvent, anion, additives, and concentration. We present a new interfacial model to understand the variation in performance (i.e., K+ (de)intercalation or K+–solvent co-insertion or decomposition). Our model is distinctly different from the solid electrolyte interphase interpretation. This work offers new opportunities to design high-performance KIBs and potassium-ion sulfur batteries. Particularly, we present new guideline to design electrolytes for KIBs and other advanced mobile (ion) batteries.
  • Understanding multi-stage HCCI combustion caused by thermal stratification and chemical three-stage auto-ignition

    Ben Houidi, Moez; AlRamadan, Abdullah; Sotton, Julien; Bellenoue, Marc; Sarathy, Mani; Johansson, Bengt (Proceedings of the Combustion Institute, Elsevier BV, 2020-07-23) [Article]
    The Homogeneous Charge Compression Ignition (HCCI) concept shows great potential for improving engine efficiency and reducing pollutant emissions. However, the operation with this concept in Internal Combustion (IC) engines is still limited to low speed and load conditions, as excessive Pressure Rise Rates (PRR) are generated with its fast auto-ignition. To overcome this limitation, the use of moderate thermal and charge stratification has been promoted. This leads to multi-stage ignition, and thus a potentially acceptable PRR. Recently Sarathy et al. (2019), three-stage auto-ignition has been emphasized as a chemical phenomenon where the thermal runaway is inhibited during the main ignition event. The current paper demonstrates experimental evidence on this phenomenon observed during n-heptane and n-hexane auto-ignition at lean diluted conditions in a flat piston Rapid Compression Machine (RCM). Multi-stage ignition events caused by either chemical kinetics or by the well-known thermal stratification of this type of RCM are clearly identified and differentiated. The combination of these two factors seems to be a suitable solution to overcome PRR limitations.
  • Thermally Induced Formation of HF4TCNQ- in F4TCNQ-Doped Regioregular P3HT

    Watts, Kristen E; Neelamraju, Bharati; Moser, Maximilian; McCulloch, Iain; Ratcliff, Erin L.; Pemberton, Jeanne E (The Journal of Physical Chemistry Letters, American Chemical Society (ACS), 2020-07-23) [Article]
    The prototypical system for understanding doping in solution-processed organic electronics has been poly(3-hexylthiophene) (P3HT) p-doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Multiple charge transfer states, defined by the fraction of electron transfer to F4TCNQ, are known to coexist and are dependent on polymer molecular weight, crystallinity, and processing. Less well understood is the loss of conductivity after thermal annealing of these materials. Specifically, in thermoelectrics, F4TCNQ-doped regioregular (rr) P3HT exhibits significant conductivity losses at temperatures lower than other thiophene-based polymers. Through detailed spectroscopic investigation of progressively heated P3HT films co-processed with F4TCNQ, we demonstrate that this diminished conductivity is due to formation of the non-chromophoric, weak dopant HF4TCNQ-. This species is likely formed through hydrogen abstraction from the alpha aliphatic carbon of the hexyl chain at the 3-position of thiophene rings of rr-P3HT. This reaction is eliminated for polymers with ethylene glycol-containing side chains, which retain conductivity at higher operating temperatures. In total, these results provide a critical materials design guideline for organic electronics.
  • Dual solution of boundary-layer flow driven by variable plate and streaming-free velocity

    Ferdows, M.; Alzahrani, Faris; Sun, Shuyu (Advances in Mechanical Engineering, SAGE Publications, 2020-07-23) [Article]
    This article presents a numerical study to investigate boundary-layer heat transfer fluid associated with a moving flat body in cooperation of variable plate and streaming-free velocity along the boundary surface in the laminar flow. The thermal conductivity is supposed to vary linearly with temperature. Similarity transformations are applied to render the governing partial differential equations for mass, momentum and energy into a system of ordinary differential equations to reveal the possible existence of dual solutions. MATLAB package has been used to solve the boundary value problem numerically. We present the effects of various parameters such as velocity ratio, thermal conductivity and variable viscosity on velocity and temperature distribution. The analysis of the results concerning Skin friction and Nusselt number near the wall is also presented. It is focused on the detection and description of the dual solutions. The study reveals that the undertaken problem admits dual solutions in particular range of values of different physical parameters. It can be seen that for the first branch solution, the fluid velocity decreases near the sheet, but it increases far away from the sheet for velocity ratio parameter, whereas the opposite effect is induced for second branch solution. Skin friction coefficient and rate of heat transfer increase due to increase in thermal conductivity parameter.

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