太阳风等离子体输出流量的全日面二维结构

本文根据1983年十个 Carrington 周(1733—1742)期间的 K-日冕亮度、行星际闪烁(IPS)观测和光球磁场观测,首次探讨了太阳风等离子体质量、动量和能量输出流量 F_M,F_P 和 F_E 的全日面二维平均结构及其与光球磁场结构的关系.     

锂离子电池硅基负极材料的研究进展

硅基材料是新一代高容量锂离子蓄电池负极材料的典型代表,近年来已成为理论研究和应用研究的热点.本文介绍了锂离子电池硅基负极材料的制备方法、电化学性能及其研究现状,分析了硅材料作为锂离子电池负极材料存在的问题;讨论了硅材料作为锂离子电池负极材料的研究前景.并指出若能克服目前存在问题,将有望成为新一代锂离子电池负极材料.     

Substituent effects and the mechanism of alkene metathesis catalyzed by ruthenium dichloride catalysts

Density functional theory calculations are reported concerning the dissociative mechanism for alkene metathesis by ruthenium dichloride catalysts, including both bisphosphine and diaminocarbene/phosphine complexes. The calculations use a hierarchy of models, ranging from [(L)(PH(3))Ru(Cl)(2)(CH(2))](L=PH(3) or diaminocarbene) through the larger [(L)(PMe(3))Ru(Cl)(2)(CHPh)] to the "real"[(L)(PCy(3))Ru(Cl)(2)(CHPh)]. Calculations show that the rate-limiting step for metathesis is either ring closing from an alkene complex to form a ruthena-cyclobutane, or ring-opening of the latter intermediate to form an isomeric alkene complex. The higher efficiency of the diaminocarbene based catalysts is due to the stabilization of the formal +iv oxidation state of the ruthenium centre in the metallacycle. This effect is partly masked in the smaller model systems due to a previously unnoticed stereoelectronic effect. The calculations do not reproduce the experimental observation whereby the initiation step, phosphine dissociation, is more energetically demanding and hence slower for the diaminocarbene-containing catalyst system than for the bisphosphine. Further calculations on the corresponding bond energies using a variety of DFT and hybrid DFT/molecular mechanics methods all find instead a larger phosphine dissociation energy for the bisphosphine catalyst. This reversed order of binding energies would in fact be the one expected based on the stronger trans influence of the diaminocarbene ligand. The discrepancy with experiment is small and could have a number of causes which are discussed here.     

Grain-boundary states in solid oxide electrolyte ceramics processed using iron oxide sintering aids: a Mössbauer spectroscopy study

Journal of Solid State Electrochemistry - In order to appraise microstructure-determined defect formation processes and minor intergranular states in the solid electrolyte materials sintered...     

Ionic conductivity and thermal expansion of anion-deficient Sr11Mo4O23 perovskite

Transport properties of perovskite-type Sr₁₁Mo₄O₂₃ and composite Sr₁₁Mo₄O₂₃ - 1 wt% Al₂O₃ were studied at 400–1300 K in the oxygen partial pressure range from 0.21 down to 10⁻¹⁹ atm. The electromotive force and faradaic efficiency measurements, in combination with the energy-dispersive spectroscopy of the fractured electrochemical cells, unambiguously showed prevailing role of the oxygen ionic conductivity under oxidizing conditions. At temperatures above 600 K, protonic and cationic transport can be neglected. The oxygen ion transference numbers vary in the range of 0.95–1.00 at 973–1223 K. At temperatures lower than 550 K, the total conductivity of Sr₁₁Mo₄O₂₃ - 1 wt% Al₂O₃ composite measured by impedance spectroscopy tends to increase in wet atmospheres, thus indicating that hydration and protonic transport become significant. Reducing oxygen partial pressure below 10⁻¹⁰–10⁻⁹ atm leads to a significant increase in the n-type electronic conduction. The average thermal expansion coefficients in oxidizing atmospheres are (14.3–15.0) × 10⁻⁶ K⁻¹ at 340–740 K and (18.3–19.2) × 10⁻⁶ K⁻¹ at 870–1370 K.

     

Mechanistic aspects of the dehydration and dehydrohalogenation of halo-hydroxyformaldoxime conformers. A quantum chemical model study

A detailed exploration of the configurational and conformational space of chloro- and bromo-hydroxyformaldoximes, Xhfaox (X = Cl, Br) has been carried out with the aid of the B3LYP level of density functional theory, using the 6-31G(d,p) basis set. The most stable configuration in each series of the Clhfaox and Brhfaox conformers corresponds to the Z-s-cis, s-trans configuration, while the highest energy Z-(s-trans, s-cis) conformers were found at 7.0(7.6) and 6.0(6.6) kcal mol(-1), respectively, at the B3LYP(QCISD(T)) levels of theory. Saddle points were also located on the PES of the Clhfaox and Brhfaox compounds corresponding to Z-(s-cis, s-cis) conformers at 13.8(14.9) and 13.6(14.6) kcal mol(-1), respectively, at the B3LYP(QCISD(T)) levels. Upon dehydration Xhfaox could afford a number of isomeric CXNO species. The dehydration processes of Xhfaox are predicted to be endothermic, the computed heats of reactions found in the range of 20.5 to 86.2 kcal mol(-1) and 15.9 to 100.4 kcal mol(-1) at the B3LYP and QCISD(T) levels, respectively. The reaction pathways for the addition of water to halo-fulminates yielding the most stable Xhfaox conformers was predicted to be concerted with a single transition structure, but are asynchronous with activation barriers of 32.8 and 43.0 kcal mol(-1) for the chloro- and bromo-derivatives, respectively. The PES governing the isomerization reactions of the CXNO isomers have also been calculated, and possible isomerization pathways have been delineated. Upon dehydrohalogenation the Xhfaox conformers yield hydroxy-isocyanate or hydroxy-fulminate, the former being more stable by 31.8(18.8) kcal mol(-1) at the B3LYP(QCISD(T)) levels of theory. The reaction pathways for the addition of HX to hydroxy-isocyanate were predicted to be slightly exothermic, the heats of reactions being -3.2 and -5.5 kcal mol(-1), respectively, and have to surmount high activation barriers of 39.7 and 35.0 kcal mol(-1), respectively. Similarly, the addition of HX to hydroxy-fulminate was predicted to be much more exothermic, the heats of reactions being -34.7 and -37.3 kcal mol(-1), respectively, and have to surmount much lower activation barriers of only 10.5 and 7.5 kcal mol(-1) respectively, at the B3LYP level. Finally, calculated structures, relative stability, and bonding properties of all stationary points located on the PES of the systems and reactions studied are thoroughly discussed with respect to computed electronic properties.     

Mechanism of a chemical classic: quantum chemical investigation of the autocatalyzed reaction of the serendipitous wöhler synthesis of urea

The detailed reaction pathways for the ammonium cyanate transformation into urea (W?hler's reaction) in the gas phase, in solution, and in the solid state have exhaustively been explored by means of first-principles quantum chemical calculations at the B3LYP level of theory using the 6-31G(d,p) basis set. This serendipitous synthesis of urea is predicted to proceed in two steps; the first step involves the decomposition of the ammonium cyanate to ammonia and isocyanic or cyanic acid, and the second one, which is the main reaction step (and probably the rate-determining step), involves the interaction of NH(3) with either isocyanic or cyanic acid. Several alternative pathways were envisaged for the main reaction step of W?hler's reaction in a vacuum involving the formation of "four-center" transition states. Modeling W?hler's reaction in aqueous solution and in the solid state, it was found that the addition of NH(3) to both acids is assisted (autocatalyzed) by the active participation of extra H(2)O and/or NH(3) molecules, through a preassociative, cooperative, and hydrogen-transfer relay mechanism involving the formation of "six-center" or even "eight-center" transition states. The most energetically economic path of the rate-determining step of W?hler's reaction is that of the addition of NH(3) to the C=N double bond of isocyanic acid, directly affording urea. An alternative pathway corresponding to the anti-addition of ammonia to the Ctbd1;N triple bond of cyanic acid, yielding urea's tautomer HN=C(OH)NH(2), seems to be another possibility. In the last case, urea is formed through a prototropic tautomerization of its enolic form. The energies of the reactants, products, and all intermediates along with the barrier heights for each reaction path have been calculated at the B3LYP/6-31G(d,p) level of theory. The geometry optimization and characterization of all of the stationary points found on the potential energy hypersurfaces was performed at the same level of theory.     

Oxygen non-stoichiometry and defect thermodynamics in La2Ni0.9Fe0.1O4+δ

The oxygen hyperstoichiometry of K₂NiF₄-type La₂Ni_0.9Fe_0.1O_4+δ, studied by thermogravimetric analysis and coulometric titration in the oxygen partial pressure range 6×10⁻⁵-0.7 atm at 923-1223 K, is considerably higher than that of undoped lanthanum nickelate. The p(O₂)-T-δ diagram of iron-doped lanthanum nickelate can be adequately described by introducing point-defect interaction energy in the concentration-dependent part of defect chemical potentials and accounting for the site-exclusion effects. The critical factors affecting the equilibrium oxygen incorporation process include coulombic repulsion of interstitial anions, trapping of the p-type electronic charge carriers by iron, and interaction between Fe³⁺ and holes localized on nickel cations. Due to low chemical expansion of La₂Ni_0.9Fe_0.1O_4+δ lattice, the thermodynamic functions governing oxygen intercalation, site-blocking factors and hole mobility are all independent of the defect concentrations. The predominant 3+ state of iron cations under oxidizing conditions was confirmed by the Mössbauer spectroscopy. The stability of La₂NiO₄-based phase in reducing atmospheres is essentially unaffected by doping.