Spherical Particles of Zirconia-Titania of Hexagonal Structure from a Neutral Amine Route

Through the sol–gel process, using the so-called neutral amine route, spherical particles of 1:1 zirconia–titania were synthesized from zirconium(IV) and titanium(IV) butoxides as well as 1,12-diaminododecane as precursor species. The obtained product exhibited a hexagonal structure, as determinated by X-ray diffraction data. The obtained material was also characterized by thermogravimetry, differential scanning calorimetry, infrared spectroscopy, scanning electron microscopy, and surface area measurements. Despite the release of template molecules on heating, the spherical morphology was retained up to about 1200°C, at which the disruption of the spheres took place.     

Effects of Adsorbed Polyaniline on Redox Process on MoO3 Surface

The effects exhibited by adsorbed conducting polyaniline on the redox process on a molybdenum oxide surface were studied. Thermogravimetric results indicate a 4% polyaniline deposition. Cyclic voltammograms of the adsorbed polymer on MoO3 show that polyaniline exerts remarkable effects on the molybdenum blue oxidation-reduction process, with oxidation and reduction potentials of 0.33 and 0.18 V, respectively. This effect strongly enhances the electrode response, and can be used as an important tool in qualitative and/or quantitative determinations of molybdenum in solution as well as in any substrate. Copyright 1999 Academic Press.     

Surface Extension in Layered Structures with the Use of Metal Meshes for Heat-Transfer Enhancement

The properties of multilayer metal-fiber coverings superimposed on plane base layers, which consist of various combinations of a copper mesh and different-length copper wires, are examined. Coverings from discrete copper microwires, two-layer coverings including a mesh and microwires, and three-layer coverings consisting of a mesh and two layers of microwire are investigated. The contacts between the fibers, the copper mesh, and the base layer are analyzed by using the metallography technique. Quantitative estimates for these contacts are presented. The side effects connected with the hydrogen disease of copper are discussed. The thermal properties of the porous structures are analyzed with the use of a termographic chamber. It is shown that the porous metal-fiber structures have good mechanical and heat-transfer properties.     

Superbulky metallocene complexes of the heavier alkaline-earth metals strontium and barium

Superbulky sandwich complexes with the cyclopentadienyl ligand (4-nBu-C(6)H(4))(5)Cp (abbreviated here as Cp(BIG)) have been prepared by reaction of Cp(BIG)H with benzylic strontium or barium reagents. Both metallocenes have been structurally characterized by single crystal X-ray diffraction. Even for the larger metallocene (Cp(BIG))(2)Ba a highly symmetric sandwich complex with parallel Cp rings was isolated (Cp(center)-Ba = 2.667(1) A). Both structures show evidence for a C-HC(pi) network between the Cp ligands. These attractive forces induce an inward out-of-plane bending of the aryl substituents (Sr 3.4(2) degrees ; Ba 5.3(2) degrees ). A linear correlation between this bending angle and metal size has been found.     

On the phase transition to sheet percolation in random Cantor sets

Thed-dimensional random Cantor set is a generalization of the classical middle-thirds Cantor set. Starting with the unit cube [0, 1] ^d , at every stage of the construction we divide each cube remaining intoM ^d equal subcubes, and select each of these at random with probabilityp. The resulting limit set is a random fractal, which may be crossed by paths or (d–1)-dimensional sheets. We examine the critical probabilityp _s(M, d) marking the existence of these sheet crossings, and show that p_s(M,d)1–p_c(M ^d) asM, where p_c(M ^d) is the critical probability of site percolation on the lattice (M ^d) obtained by adding the diagonal edges to the hypercubic lattice ^d. This result is then used to show that, at least for sufficiently large values ofM, the phases corresponding to the existence of path and sheet crossings are distinct.     

Theoretical calculation of the coiled-coil stability in water in the context of its possible use as a molecular rack

The coiled-coil stability and rigidity may be of importance for molecular electronics (electronically bistable molecules). The coiled-coil binding free energy has been calculated using molecular dynamics (MD). The energy has been computed as a difference of the appropriate free energies; derived for the coiled-coil and isolated alpha-helices separately. All MD simulations have been performed using an explicit model of the solvent, whereas the continuum solvent approach has been applied to analyze the MD trajectories. The computed stability of the coiled-coil is of the order of -87 kcal/mol, i.e., about -1.2 kcal/mol per amino acid residue, and arises mainly from the electrostatic interactions and hydrophobic effect. The entropy term has been roughly estimated to be of the order of -22 kcal/mol. This assures that coiled-coil polypeptides may be used as a stable molecular scaffolding.     

Synthesis, structure, and characterization of two new layered mixed-metal phosphates, BaTeMO4(PO4) (M = Nb5+ or Ta5+

Two new isostructural mixed-metal phosphates, BaTeMO(4)(PO(4)) (M = Nb(5+) or Ta(5+)), have been synthesized as bulk phase powders and single crystals by standard solid-state techniques using BaCO(3), TeO(2), Nb(2)O(5) (or Ta(2)O(5)), and NH(4)H(2)PO(4) as reagents. The materials have novel layered crystal structures consisting of [M(5+)O(6/2)](-) corner-sharing octahedral chains that are connected to [Te(4+)O(4/2)](0) polyhedra and [P(5+)O(2/1)O(2/2)](-) tetrahedra. The Ba(2+) cations reside between the layers and maintain charge balance. The Te(4+) cations are in asymmetric coordination environments attributable to their lone pairs. The Nb(5+) distorts along the local C(4) direction of its octahedron resulting in a "short-long-short-long" Nb-O-Nb bond motif. The Nb(5+) cation displaces away from the oxide ligands that are bonded to Te(4+) or P(5+) cations, attributable to the structural rigidity of the TeO(4) and PO(4) polyhedra. Thus, the TeO(4) and PO(4) polyhedra support and reinforce the intraoctahedral distortion observed within the NbO(6) octahedra. Infrared and Raman spectroscopy, thermogravimetric analysis, and ion-exchange experiments are also presented. Crystal data: BaTeNbO(4)(PO(4)), orthorhombic, space group Pbca (No. 61), with a = 6.7351(9) A, b = 7.5540(10) A, c = 27.455(4) A, V = 1396.8(3) A(3), and Z = 8; BaTeTaO(4)(PO(4)), orthorhombic, space group Pbca (No. 61), with a = 6.734(2) A, b = 7.565(3) A, c = 27.435(9) A, V = 1372.6(8) A(3), and Z = 8.     

Dielectric dispersion in water + 2-hydroxypyridine solid mixtures

The complex electric permittivity was measured in water + 2-hydroxypyridine (2HP) solid mixtures as a function of concentration, temperature and frequency. Just after freezing of diluted mixtures, (mole fraction of 2HP < 0.2) pronounced dielectric dispersion in the MHz region was observed. The dispersion disappears on cooling between -30 and -40 degrees C in a first-order phase transition. The dispersion was explained in terms of the movement of a guest molecule (2HP) in a clathratelike structure of ice.     

Study of the effect of director fluctuations on liquid-crystalline molecular reorientation

Initial phase of field-induced molecular reorientation in threshold geometry of a liquid-crystalline structure is essentially influenced by thermal orientational fluctuations. Still not quite clear is the way in which the reorientation starts and how it runs over an LC-structure in its initial phase. Fluctuations are stochastic events in time and also in space. This feature suggests, that the reorientation process may be induced by director fluctuations locally in a particular part of the LC-structure. In this work we have studied the impact of fluctuations on the molecular reorientation in nematics involving optical nonlinear interaction. The restricted area of a nematic layer was excited by light beam that amplified director fluctuations locally. By manipulation of optical excitation we were able to affect the dynamics of the effect. Since the threshold-type configuration of nematics is used in a number of optical devices, the obtained results may have practical aspect contributing to a new way of controlling the field-induced transition.