Effect of Dynamic Surface Polarization on the Oxidative Stability of Solvents in Nonaqueous Li-O2_2 Batteries

Polarization-induced renormalization of the frontier energy levels of interacting molecules and surfaces can cause significant shifts in the excitation and transport behavior of electrons. This phenomenon is crucial in determining the oxidative stability of nonaqeous electrolytes in high energy density electrochemical systems such as the Li-O$_2$ battery. On the basis of partially self-consistent first-principles ScGW0 calculations, we systematically study how the electronic energy levels of four commonly used solvent molecules, namely dimethylsulfoxide (DMSO), dimethoxyethane (DME), tetrahydrofuran (THF) and acetonitrile (ACN), renormalize when physisorbed on the different stable surfaces of Li$_2$O$_2$, the main discharge product. Using band level alignment arguments, we propose that the difference between the solvent's highest occupied molecular orbital (HOMO) level and the surface's valence band maximum (VBM) is a refined metric of oxidative stability. This metric and a previously used descriptor, solvent's gas phase HOMO level, agree quite well for physisorbed cases on pristine surfaces where ACN is oxidatively most stable followed by DME, THF and DMSO. However, this effect is intrinsically linked to the surface chemistry of solvent's interaction with the surfaces states and defects, and depends strongly on their nature. We conclusively show that the propensity of solvent molecules to oxidize will be significantly higher on Li$_2$O$_2$ surfaces with defects as compared to pristine surfaces. This suggests that the oxidatively stability of solvent is dynamic and is a strong function of surface electronic properties. Thus, while gas phase HOMO levels could be used for preliminary solvent candidate screening, a more refined picture of solvent stability requires mapping out the solvent stability as a function of the state of the surface under the operating conditions.Comment: 10 Pages, 8 Figure

Thermochemical properties of polycyclic aromatic hydrocarbons (PAH) from G3MP2B3 calculations

In this article, we present a new database of thermodynamic properties for polycyclic aromatic hydrocarbons (PAH). These large aromatic species are formed in very rich premixed flames and in diffusion flames as part of the gas-phase chemistry. PAH are commonly assumed to be the intermediates leading to soot formation. Therefore, accurate prediction of their thermodynamic properties is required for modeling soot formation. The present database consists of 46 species ranging from benzene (C_6H_6) to coronene (C_(24)H_(12)) and includes all the species usually present in chemical mechanisms for soot formation. Geometric molecular structures are optimized at the B3LYP/6-31++G(d,p) level of theory. Heat capacity, entropy, and energy content are calculated from these optimized structures. Corrections for hindered rotor are applied on the basis of torsional potentials obtained from second-order Møller-Plesset perturbation (MP2) and Dunning's consistent basis sets (cc-pVDZ). Enthalpies of formation are calculated using the mixed G3MP2//B3 method. Finally, a group correction is applied to account for systematic errors in the G3MP2//B3 computations. The thermodynamic properties for all species are available in NASA polynomial form at the following address: http://www.stanford.edu/group/pitsch/

Scalar mass conservation in turbulent mixture fraction based combustion models through consistent local flow parameters

Mixture fraction-based models are widely employed for predicting turbulent non-premixed combustion processes due to their cost-effectiveness and well-established subfilter closure. In these models, the transport of reactive scalars in physical space is decomposed into two components: scalar transport relative to mixture fraction and transport of mixture fraction in physical space. Conventional flamelet models do not consider that these two processes have to be formulated consistently, which can lead to scalar mass conservation errors. In the context of multiphase flows, scalar transport in mixture fraction space is governed by three conditional flow-dependent parameters: the conditional scalar dissipation rate, the conditional scalar diffusion rate, and the conditional spray source term. The evolution of mixture fraction in physical space is typically modeled using the presumed Filtered Density Function (FDF) approach. This paper introduces a novel formulation for the conditional flow parameters that aligns with the presumed FDF approach, thereby ensuring scalar mass conservation. The proposed model is applied to a Large-Eddy Simulation (LES) of the inert ECN Spray A case, with a comparison against a conventional flow parameter model that employs an inverse error function shape for the scalar dissipation rate. The results indicate that the conventional model produces similar conditional dissipation rates to the new model in regions where combustion takes place. However, significant discrepancies are observed in the conditional diffusion rate, highlighting the susceptibility of the conventional model to scalar mass conservation errors for non-unity Lewis number scalars

Время-амплитудный конвертор-хронотрон

Рассматривается структурная схема время-амплитудного преобразователя на основе многоканального хронотрона. Используется двойное преобразование: время - числовой код - амплитуда. Прилагаемая блок-схема преобразователя обеспечивает повышенное быстродействие устройства (~30 МГц)

In-situ temperature and major species measurements of sooting flames based on short-gated spontaneous Raman scattering

Spontaneous Raman spectroscopy (SRS) is a conventional in-situ laser diagnostic method that has been widely used for measurements of temperature and major species. However, SRS in sooting flames suffers from strong interference including laser-induced fluorescence, laser-induced incandescence, and flame luminosity, which is a long-lasting challenge. This work introduces a low-cost, easy-to-implement, and calibration-free SRS thermometry in sooting flames based on a 355-nm nanosecond laser beam. Several strategies were utilized to increase the signal-to-noise ratio and suppress the interference: (1) nanosecond ICCD gate width; (2) optimized ICCD gate delay; (3) specially designed focusing shape of the laser beam; (4) ultraviolet polarizer filter. The temperature was obtained by fitting the contour of Stokes-Raman spectra of N2 molecules, which does not require additional calibration. Based on the measured temperature, the mole fraction of major species can be obtained with calibration. This method was used in the temperature and major species measurements of an ethylene-based counterflow diffusion flame. The experimental results show an excellent agreement with the simulation results, demonstrating the feasibility of performing non-intrusive laser diagnostics of sooting and other particle-laden flames accurately