A new evaluation method for Nusselt numbers in naphthalene sublimation experiments in rotating-disk systems

A comparative analysis of theoretical and empirical relations, as well as experimental data for mass transfer of a rotating disk in laminar, transitional and turbulent flows for naphthalene sublimation in air was done. New correlations between local and average Sherwood numbers for the entire disk were offered. A new evaluation approach for Nusselt numbers based on the experimental data for naphthalene sublimation in laminar, transitional and turbulent flows was developed.     

Site-differentiated solid solution in (Na(1-x)Cu(x))2Ta4O11 and its electronic structure and optical properties

The (Na(1-x)Cu(x))(2)Ta(4)O(11) (0 ≤ x ≤ 0.78) solid-solution was synthesized within evacuated fused-silica vessels and characterized by powder X-ray diffraction techniques (space group: R3c (#167), Z = 6, a = 6.2061(2)-6.2131(2) ?, c = 36.712(1)-36.861(1) ?, for x = 0.37, 0.57, and 0.78). The structure consists of single layers of TaO(7) pentagonal bipyramids as well as layers of isolated TaO(6) octahedra surrounded by Na(+) and Cu(+) cations. Full-profile Rietveld refinements revealed a site-differentiated substitution of Na(+) cations located in the 12c (Wyckoff) crystallographic site for Cu(+) cations in the 18d crystallographic site. This site differentiation is driven by the linear coordination geometry afforded at the Cu(+) site compared to the distorted seven-coordinate geometry of the Na(+) site. Compositions more Cu-rich than x ~ 0.78, that is, closer to "Cu(2)Ta(4)O(11)", could not be synthesized owing to the destabilizing Na(+)/Cu(+) vacancies that increase with x up to the highest attainable value of ~26%. The UV-vis diffuse reflectance spectra show a significant red-shift of the bandgap size from ~4.0 eV to ~2.65 eV with increasing Cu(+) content across the series. Electronic structure calculations using the TB-LMTO-ASA approach show that the reduction in bandgap size arises from the introduction of Cu 3d(10) orbitals and the formation of a new higher-energy valence band. A direct bandgap transition emerges at k = Γ that is derived from the filled Cu 3d(10) and the empty Ta 5d(0) orbitals, including a small amount of mixing with the O 2p orbitals. The resulting conduction and valence band energies are determined to favorably bracket the redox potentials for water reduction and oxidation, meeting the thermodynamic requirement for photocatalytic water-splitting reactions.     

Role of Polyalanine Domains in β‐Sheet Formation in Spider Silk Block Copolymers

Genetically engineered spider silk‐like block copolymers were studied to determine the influence of polyalanine domain size on secondary structure. The role of polyalanine block distribution on β‐sheet formation was explored using FT‐IR and WAXS. The number of polyalanine blocks had a direct effect on the formation of crystalline β‐sheets, reflected in the change in crystallinity index as the blocks of polyalanines increased. WAXS analysis confirmed the crystalline nature of the sample with the largest number of polyalanine blocks. This approach provides a platform for further exploration of the role of specific amino acid chemistries in regulating the assembly of β‐sheet secondary structures, leading to options to regulate material properties through manipulation of this key component in spider silks.

     

Oxidative coupling of methane on alkali metal-containing catalysts

MgO, MgHPO₂, MgCl₂, silica gel, SiO₂ (aerogel), zeolite H-ZSM-5 and various modified compounds of Li, Na, K, Be and Mn have been examined. Effect of temperature, space velocity and partial pressure of oxygen on the selectivity to C₂ and carbon oxides formation was studied in detail in the presence of NaCl-MnO/MgO. Paper presented at the II European Congress on Catalysis (EUROPACAT-II), Maastricht (The Netherlands), 3–8 September, 1995 and the VIII International Symposium on Relations between Homogeneous and Heterogeneous Catalysis, Balatonfüred (Hungary), 10–14 September, 1995.     

Structure,magnetic, and electrical properties of bismuth niobates doped with <Emphasis Type="Italic">d</Emphasis>-elements: XVI. Magnetic properties of manganese-containing solid solutions of bismuth orthoniobate BiNiO<Subscript>4</Subscript>

Magnetic susceptibility and ESR properties of manganese-containing solid solutions of bismuth orthoniobate BiNb_1–x Mn_xO_4–δ have been studied. Mn(II), Mn(III), and Mn(IV) atoms as well as their dimers exhibiting antiferro- and ferromagnetic types of exchange have been found in the solid solutions. Parameters of the exchange interactions in the clusters and the distribution of monomers and dimers of manganese atoms in the solid solutions have been calculated as functions of the paramagnetic atom fraction.     

Characterizing compact Clifford semigroups that embed into convolution and functor-semigroups

We study algebraic and topological properties of the convolution semigroup of probability measures on a topological groups and show that a compact Clifford topological semigroup S embeds into the convolution semigroup P(G) over some topological group G if and only if S embeds into the semigroup exp(G)\exp(G) of compact subsets of G if and only if S is an inverse semigroup and has zero-dimensional maximal semilattice. We also show that such a Clifford semigroup S embeds into the functor-semigroup F(G) over a suitable compact topological group G for each weakly normal monadic functor F in the category of compacta such that F(G) contains a G-invariant element (which is an analogue of the Haar measure on G).     

Multiple hindered rotators in a gyroscope-inspired tribenzylamine hemicryptophane

A gyroscope-inspired tribenzylamine hemicryptophane provides a vehicle for exploring the structure and properties of multiple p-phenylene rotators within one molecule. The hemicryptophane was synthesized in three steps in good overall yield using mild conditions. Three rotator-forming linkers were cyclized to form a rigid cyclotriveratrylene (CTV) stator framework, which was then closed with an amine. The gyroscope-like molecule was characterized by (1)H NMR and (13)C NMR spectroscopy, and the structure was solved by X-ray crystallography. The rigidity of the two-component CTV-trismethylamine stator was investigated by (1)H variable-temperature (VT) NMR experiments and molecular dynamics simulations. These techniques identified gyration of the three p-phenylene rotators on the millisecond time scale at -93 °C, with more dynamic but still hindered motion at room temperature (27 °C). The activation energy for the p-phenylene rotation was determined to be ~10 kcal mol(-1). Due to the propeller arrangement of the p-phenylenes, their rotation is hindered but not strongly correlated. The compact size, simple synthetic route, and molecular motions of this gyroscope-inspired tribenzylamine hemicryptophane make it an attractive starting point for controlling the direction and coupling of rotators within molecular systems.     

Temperature and thermal stresses in material of a pad during braking

The transient temperature field and corresponding quasi-static thermal stresses are analysed in a system consisting of a semi-space and a strip. The strip is heated on its outer surface by a heat flux with the intensity equal to the specific power of friction during braking with a uniform retardation. The evolution and distribution in depth from a surface of friction for temperatures and thermal stresses were investigated for the metal-ceramic FMK-11 material of the strip.     

Characterization of classes of finite groups with the use of generalized subnormal Sylow subgroups

A characterization of the classes of all π-nilpotent, all π-closed, and all π-decomposable finite groups is obtained by using generalized subnormal Sylow subgroups.