Oscillations of linear integro-differential equations

Sufficient conditions which guarantee that certain linear integro-differential equation cannot have a positive solution are established.     

Advances in constant-velocity Mössbauer instrumentation

A prototype of a programmable constant-velocity scaler is presented. This instrument allows the acquisition of partial Mössbauer spectra in selected energy regions using standard drivers and transducers. It can be fully operated by a remote application, thus data acquisition can be automated. The instrument consists of a programmable counter and a constant-velocity reference. The reference waveform generator is amplitude modulated with 13-bit resolution, and is programmable in a wide range of frequencies and waveforms in order to optimize the performance of the transducer. The counter is compatible with most standard SCA, and is configured as a rate-meter that provides counts per selectable time slice at the programmed velocity. As a demonstration of the instrument applications, a partial Mössbauer spectrum of a natural iron foil was taken. Only positive energies were studied in 512 channels, accumulating 20 s per channel. A line width of 0.20 mm/s was achieved, performing with an efficiency of 80%.     

Immobilization of catalase on membranes of poly(ethylene)-g-co-acrylic acid and poly(tetrafluoroethylene)-g-co-acrylic acid and their application in hydrogen peroxide electrochemical sensors

The immobilization of catalase on grafted membranes of poly(ethylene)-g-co-acrylic acid and poly(tetrafluoroethylene)-g-co-acrylic acid and their application in hydrogen peroxide electrochemical sensors is described. The introduction of carboxylic acid groups onto a hydrophobic support provides a good environment for subsequent enzyme immobilization. This single membrane, hydrogen peroxide sensor showed significant improvement with respect to the double membrane versions. The response is very rapid, the linear range being from 10 μM up to 6 mM, with a detection limit of 4.7 μM, and a lifetime of more than 4 months.     

On some Nikolski - and Oswald-type inequalities

Inequalities of Nikolski (Trudy Mat. Inst. Steklova 38 (1951), 2.3, p. 255) in L_2π^p, p 1, and of Oswald (Izv. Vyssh. Uchebn. Zaved. Mat. 7 (1976), (3.4), p. 71; Theorem 1, p. 69), 0 < p < 1, are extended to the case of Orlicz spaces L_2π.     

Yttrium aluminate crystals for slabs

The methods of growing YAG:Nd crystals with a flat interface and YAP:Nd crystals with a sharp conical interface are described. The form of the interface was controlled by the He and H₂O content in the reducing atmosphere composed mainly of Ar and H₂ as well as by axial temperature gradient above the melt level. The crystals of 120–180 mm in length were machined to the high-quality slabs.     

Silica hybrid nanocomposites

In this work we present experimental results about the formation, properties and structure of sol — gel silica based biocomposite containing Calcium alginate as an organic compound. Two different types of silicon precursors have been used in the synthesis: tetramethylortosilicate (TMOS) and ethyltrimethoxysilane (ETMS). The samples have been prepared at room temperature. The hybrids have been synthesized by replacing different quantitis of the inorganic precursor with alginate. The structure of the obtained hybrid materials has been studied by XRD, IR Spectroscopy, EDS, BET and AFM. The results proved that all samples are amorphous possessing a surface area from 70 to 290 m²/g. It has also been established by FT IR spectra that the hybrids containing TMOS display Van der Walls and Hydrogen bonding or electrostatic interactions between the organic and inorganic components. Strong chemical bonds between the inorganic and organic components in the samples with ETMS are present. A self-organized nanostructure has been observed by AFM. In the obtained hybrids the nanobuilding blocks average in size at about 8–14 nm for the particles.     

Particle shape in thixotropic aluminium and thorium molybdates gels

Summary Electron microscope examination of thixotropic aluminium and thorium molybdate gels shows that one hour after formation they are constituted by short fibrils of small axial ratio. The fibrils of the aluminium molybdate gels, with ageing at room temperature or with boiling, increase in diameter and length; the fibrous shape of the particles is maintained after two and a half years of ageing; all fibrils are crystalline by electron diffraction. The fibrils of the thorium molybdate gels, except in the gels containing hydrochloric acid, change with ageing at room temperature or with boiling, into plates of hexagonal, elliptical or rectangular profile; the fibrils and plates are crystalline and have the same electron diffraction pattern as the fibrils.This work was aided, in part, by grants from Conselho Nacional de Pesquisas and the Rockefeller Foundation.     

Coordination modes of 2-hydroxynicotinic acid in second- and third-row transition metal complexes

The new Pd(II), Pt(II), Re(V), Mo(VI) and W(VI) complexes of 2-hydroxynicotinic acid (H₂nicO), trans-[PdCl(HnicO)(PPh₃)₂]·0.75CH₃CN (1), K[PdCl(HnicO)₂]·H₂O (2), [Pd(HnicO)₂(bipy)] (3), cis-[PtCl(HnicO)(PPh₃)₂]·0.75CH₃OH·0.5H₂O (4), [PtCl(HnicO)(bipy)] (5), cis-[ReOI₂(HnicO)(PPh₃)] (6), Na₂[Mo₂O₆(HnicO)₂]·5H₂O (7), Na₂[Mo₄O₁₂(HnicO)₂]·2H₂O (8) and Na₂[W₂O₆(HnicO)₂]·5H₂O (9) have been prepared. The crystal structures of 1 and 4, were determined by X-ray diffraction and show the HnicO⁻ ligand coordinated to palladium or platinum through the nitrogen atom only. Infrared, Raman, ¹H and ¹³C{¹H} NMR spectroscopic data for the complexes are presented and are in agreement with the crystallographic results.     

Carboxylate anions binding and sensing by a novel tetraazamacrocycle containing ferrocene as receptor

A tetraazamacrocycle containing ferrocene moieties has been synthesized and characterized. The tetraprotonated form of this compound was evaluated as a receptor (R) for anion recognition of several substrates (S), Cl(-), PF(6)(-), HSO(4)(-), H(2)PO(4)(-) and carboxylates, such as p-nitrobenzoate (p-nbz(-)), phthalate (ph(2-)), isophthalate (iph(2-)) and dipicolinate (dipic(2-)). (1)H NMR titrations in CD(3)OD indicated that this receptor is not suitable for recognizing HSO(4)(-) and H(2)PO(4)(-), but weakly binds p-nbz(-), and strongly interacts with ph(2-), dipic(2-), and iph(2-) anions forming 1 : 2 assembled species. The largest beta(2) binding constant was determined for ph(2-), followed by dipic(2-) and finally iph(2-). The effect of the anionic substrates on the electron-transfer process of the ferrocene units of R was evaluated using cyclic voltammetry (CV) and square wave voltammetry (SWV) in methanol solution and 0.1 mol dm(-3)(CH(3))(4)NCl as the supporting electrolyte. Titrations of the receptor were undertaken by addition of anion solutions in their tetrabutylammonium or tetramethylammonium forms. The protonated ligand exhibits a reversible voltammogram, which shifts cathodically in the presence of the substrates. The data revealed kinetic constraints in the formation of the receptor/substrate entity for dipic(2-), ph(2-) and iph(2-) anions, but not for p-nbz(-). In spite of the slow kinetics of assembled species formation with the ph(2-) substrate, this anion provides the largest redox-response when the supramolecular entity is formed, followed by dipic(2-), iph(2-) and finally p-nbz(-) anions. This trend is in agreement with the (1)H NMR results and the values of the binding constants. Single crystal X-ray structures of the receptor with PF(6)(-), ph(2-), iph(2-) and p-nbz(-) were carried out and showed that supermolecules with a RS(2) stoichiometry are formed with the first three anions, but RS(4) with p-nbz(-). In all cases the binding occurs outside the macrocyclic cavity via N-H...O=C hydrogen bonds for carboxylate anions and N-H...F hydrogen bonds for the PF(6)(-) anion, which is in agreement with the solution results. The macrocyclic framework adopts different conformations in order to interact with each substrate having Fe...Fe intramolecular distances ranging from 10.125(14) to 12.783(15)A.