First principles calculations of oxygen reduction reaction at fuel cell cathodes

The efficiency of solid oxide fuel cells (SOFC) depends critically on materials, in particular for the cathode where the oxygen reduction reaction (ORR) occurs. Typically, mixed conducting perovskite ABO₃-type materials are used for this purpose. The dominating surface terminations are (001) AO and BO₂, with the relative fractions depending on materials composition and ambient conditions.Here, results of recent large-scale first principles (ab initio) calculations for the two alternative polar (La,Sr)O and MnO₂ (001) terminations of (La,Sr)MnO₃ cathode materials are discussed. The surface oxygen vacancy concentration for the (La,Sr)O termination is more than 5 orders of magnitude smaller compared to MnO₂, which leads to drastically decreased estimated ORR rates. Thus, it is predicted for prototypical SOFC cathode materials that the BO₂ termination largely determines the ORR kinetics, although with Sr surface segregation (long-term degradation) its fraction of the total surface area decreases, which slows down cathode kinetics.     

Detonatability of high-energy-density heterocyclic compounds

The detonatability of 3,4-dinitrofurazanfuroxan (DNTF) and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (?-polymorph, CL-20) and mixtures thereof with polydimethylsiloxane rubber was studied. The dependences of the critical detonation diameter for the mixtures on the HE content and of pure DNTF and CL-20 on their specific surface area within 400–8000 cm²/g were examined. It was established that the detonatabilities of DNTF and CL-20 are close to those of PETN and HMX, respectively.     

Viscosity and temperature transitions in dimethylacetamide solutions of polyamidobenzimidazole and its blends with polysulfone

The rheological properties of 5% solutions of a fiber-forming polyamidobenzimidazole in DMAA containing LiCl additives and polyamidobenzimidazole-polysulfone blends in the same solvent have been studied. The total concentration of polymer blends with various component ratios is 5 wt %. At temperatures below ∼110°C, the systems under study behave as non-Newtonian fluids and their viscosity decreases with temperature. At T > 110°C, the temperature dependence of viscosity passes through a minimum. The position of the minimum on the temperature scale decreases with the concentration of polyamidobenzimidazole. This character of a change in viscosity is associated with the phase separation of polyamidobenzimidazole solution that leads to its gelation. The temperature corresponding to the minimum viscosity coincides with the onset temperature of a sharp turbidity of solution during heating. It is suggested that solutions containing up to 5% polyamidobenzimidazole possess an LCST. The addition of up to 50% polysulfone has almost no effect on the temperature of transition but brings about a marked decline in the viscosity of the system.     

Atomistic theory of mesoscopic pattern formation induced by bimolecular surface reactions between oppositely charged molecules

The kinetics of mesoscopic pattern formation is studied for a reversible A+B?0 reaction between mobile oppositely charged molecules at the interface. Using formalism of the joint correlation functions, non-equilibrium charge screening and reverse Monte Carlo methods, it is shown that labyrinth-like percolation structure induced by (even moderate-rate) reaction is principally non-steady-state one and is associated with permanently growing segregation of dissimilar reactants and aggregation of similar reactants into mesoscopic size domains. A role of short-range and long-range reactant interactions in pattern formation is discussed.     

Compaction and Consolidation of Aramid and Composite Fibers. 2. Consolidation of Composite Fibers

The transverse compaction and consolidation of composite fibers obtained from blends of rigid-chain aromatic polyamides and a thermoplastic polycaproamide PA-6 are investigated with the aim to predict the fiber behavior during their compression and processing into plastics. Introduction of the aliphatic polyamide PA-6 into aramid fibers considerably increases their transverse compliance and promotes their sintering. A method for calculating the viscosity of the thermoplastic in the interfiber space from the ratio between the volumetric rate of compaction and the porosity of the material is proposed. It is found that the effective viscosity of the PA-6 melt, during its flow in the thin interfibrillar layers under compression, grows with decrease in its content in the composite fibers.     

Quantum-chemical approach to defect formation processes in non-metallic crystals

Abstract

Results of the quantum-chemical simulation of the formation of structural and radiation defects are reviewed, using ice, silicon, and silicon dioxide as examples. The relationship between the structural elements of these crystals and the structural defects is analysed. Models of the main defects, their optical characteristics, and the activation energy of their migration are discussed. The relationship between the characteristics obtained by quantum-chemical calculations and the parameters of the macroscopic kinetics of the processes induced by defects in dielectric crystals is considered.     

Structure and plastic deformation of aromatic main-chain mesomorphic copolyesters

Thermodynamic characteristics of inelastic deformation (work W _def, heat Q _def, and stored energy ΔU _def) are studied for aromatic main-chain copolyesters (CPEs) based on p-hydroxybenzoic acid and poly(ethylene terephthalate) (Rodrun and SKB-1), p-hydroxybenzoic acid, naphthalene carboxylic acid, and terephthalic acid with hydroquinone and dioxyphenyl (HX-6000 and HX-7000). The samples are deformed under an active uniaxial compression by ?_def ≈ 50% at room temperature. All CPEs are semicrystalline polymers; their degree of crystallinity is (depending on their prehistory) 5–30%, and the melting temperature of crystallites is 275–350°C. Seemingly, the glassy component of CPEs includes two interpenetrating glassy structures, S-1 and S-2, with different glass-transition temperatures Tg: 90–120 and 250–270°C, respectively. During loading, all coexisting crystalline and glassy structures of CPEs store residual strain ?_res. The kinetics of the temperature-stimulated strain recovery of ?_res is measured. In component S-1, strain recovery occurs in the temperature interval ranging from T _room to 120°C. In the crystalline phase, this process occurs in the melting-temperature interval. In component S-2, strain recovery ?_res commences at T > 120°C. In CPEs, all structural components are involved in deformation at different ?_def. At small strains only component S-1 is deformed; then, at ?_def ≈ 10–15%, component S-2 is involved in the deformation. Crystallites join this process at ?_def > 20–25% (? _y = 8–10%). In CPE, two modes of deformation arise: reversible elastic (retarded elastic) and true plastic irreversible deformation. True plastic permanent strain always exists in the deformed CPEs. Deformation of all CPEs proceeds easier than that of all “common” glassy polymers (polystyrene, poly(methyl methacrylate), etc.). In CPEs, the yield stress and compressive modulus appear to be ≈40–50% lower than in “common” glassy polymers. It seems that the mesomorphic structure of LC CPEs enhances the elementary plastic processes in them. Thermodynamic characteristics of the S-1 phase plasticity are compared with the behavior of “common” glassy polymers. At the early stages of loading, nearly all mechanical work of deformation W _def spent is stored in phase S-1 in the form of δU _def, as in all “common” glassy polymers. This fact implies that the inelastic deformation of LC glasses commences with the nucleation of small-scale and localized intermolecular transformations of the nonconformational type. In both mesomorphic and “common” glassy polymers, the stage of nucleation of such transformations controls the overall kinetics of the inelastic and plastic deformation. Nucleation does not depend on chain rigidity, a circumstance that conflicts with the model of forced elasticity. It seems that crystallites in CPE are deformed according to crystallographic mechanisms. Currently, neither the structure nor the deformation mechanism of component S-2 is known.