Zn-doping effect on the energy barrier to magnetization reversal in superparamagnetic nickel ferrite nanoparticles

Measurements of ac-susceptibility and dc-magnetization were carried out on samples of Ni_1-xZn_xFe₂O₄ nanoparticles (x=0, 0.25, 0.5, 0.75) with average diameters 〈D〉≈7 nm. Values of the superparamagnetic blocking temperature T̄_B were obtained from the characteristic temperature behavior of the imaginary susceptibility χ_imag. An Arrhenius-type law, which accurately describes the relationship between the observation time τ_obs and the blocking temperature, was used to determine the effective energy barrier to magnetization reversal U_eff. A Zn-content dependence of the energy barrier is observed, where U_eff changes little for 0≤x≤0.25, it peaks at x=0.5, and decreases back upon further Zn-doping. The large increase of U_eff at x=0.5 is attributed to an enhanced magnetic anisotropy induced by the crossover between two spatial arrangements of spins in the A and B sub-lattices of the ferrimagnetic inverse spinel. PACS 75.50.Bb; 75.50.Gg; 75.30.Et     

Representation of Cubic Lattices by Symmetric Implication Algebras

In this paper a cubic lattice L(S) is endowed with a symmetric implication structure and it is proved that L(S) \ {0} is a power of the three-element simple symmetric implication algebra. The Metropolis–Rota’s symmetries are obtained as partial terms in the language of symmetric implication algebras.     

Correct structures of Diels-Alder adducts from the natural cyclolignan thuriferic acid and its 8-epimer

A detailed analysis of one- and two-dimensional ¹H and ¹³C NMR data for the endo and the exo adducts, obtained by Diels-Alder reaction of thuriferic and epithuriferic acids with cyclopentadiene is described. The unequivocal spectral data assignment of the endo and exo structures was complemented with molecular modelling studies and confirmed through X-ray diffraction studies.     

Thermal behaviour of a polyhedral oligomeric silsesquioxane with epoxy resin cured by diamines

A new material belongs to the family of polyhedral oligomeric silsesquioxanes, the 1-(3-glycidyl) propoxy-3,5,7,9,11,13,15-isobutylpentacyclo-[9.5.1.1(3,9).1(5,15).1(7,13)]octasiloxane (glycidylisobutyl-POSS) is characterized by differential scanning calorimetry, thermogravimetric analysis and atomic force microscopy. Epoxy systems based on diglycidyl ether of bisphenol A (DGEBA) cured with the diamines, 4,4'-diamine-diphenylmethane (DDM) and 1,4-phenylenediamine (pPDA), were kinetically studied by differential scanning calorimetry in isothermal and dynamic modes. The thermal behaviour of these systems as the glycidylisobutyl-POSS was added, is discussed later. This revised version was published online in July 2006 with corrections to the Cover Date.     

Oriented immobilization of Desulfovibrio gigas hydrogenase onto carbon electrodes by covalent bonds for nonmediated oxidation of H2

The orientation of hydrogenase bound covalently to a pyrolytic graphite edge electrode modified with a 4-aminophenyl monolayer can be modulated via electrostatic interactions during the immobilization step. At low ionic strength and when the amino groups of the electrode surface are mostly protonated, the hydrogenase is immobilized with the negatively charged region that surrounds its 4Fe4S cluster nearer to the protein surface facing the electrode. This allows direct electron transfer between the immobilized hydrogenase and the electrode, which is observed by the strong catalytic currents measured in the presence of the H2 substrate. Therefore, a very stable enzymatic electrode is produced that catalyzes nonmediated H2 oxidation.     

A mew method of synthesis of 1,2,3,4-telrahydro-5H-5-oxo-1,4-benzodiazepines

A new one-step facile method for the synthesis of 1,12,3,4-tetrahydro5H-5-oxo-1,4-benzo-diuzepines is described. The method involves the addition of methyl anlhranilates to elhyl-enimine followed by intramolecular cyclizalion of the resulting intermediate compounds. Another attempted method starting from β-anilinoethylisocyanales is also described.     

Analogs of propranolol. Synthesis of 3-alkylamino-1-(3-benzo[b]thienyloxy)-2-propanols

The synthesis by two alternative routes of 1-(3-benzo[b]thienyloxy)-3-isopropylamino-2-propanol hydrochloride ( 1a HCl), a thiophenic isoster of Propranolol, and related compounds, is reported. The protecting and enolizing properties of the 2-methoxycarbonyl group on benzo[b]thiophene-3-one, along with its facile removal, are utilized in the first route. In the second one, conversion of 3-bromobenzo[b]thiophene in 1-(3-benzo[b]thienyloxy)-2,3-o-isopropylidenepropane is the key step. On the other hand, hydrolysis of 1-(3-benzo[b]furanyloxy)-2,3-o-isopropylidenepropane to the corresponding diol, in order to obtain a furanic isoster of Propranolol (17a ), was unsuccessful.     

Degradation Kinetics of an Epoxy/Cycloaliphatic Amine Resin Under Isothermal and Non-isothermal Conditions

A study of an epoxy-cycloaliphatic amine system has been realized using a thermogravimetric technique (TG). Isothermal and non-isothermal (dynamic) methods were employed to determine the kinetic data of this system. Five methods were used for determining the activation energies of this system in the dynamic heating experiments. In two of them (Flynn-Wall-Ozawa, and Kissinger) it is not necessary to have a prior knowledge of the reaction mechanism of the degradation behaviour for this system. In the other ones (Coats and Redfern, Horowitz and Metzger, and Van Krevelen et al.) it is necessary to know this reaction mechanism, besides Criado et al. method was used for determining it. The results have shown that good agreement between the activation energies obtained from all methods can be achieved if it is assumed that the degradation behaviour of this system is of sigmoidal-rate type. This revised version was published online in August 2006 with corrections to the Cover Date.     

Synthesis of 10-(4-methyl-1-piperazinyl)thieno[3,2-b][1,5]benzoxazepines

A series of the title compounds has been synthesized starting from methyl 3-hydroxythiophene-2-carboxylate. Its condensation with 2,4-dinitrochlorobenzene provides through compound 4e a way to introduce 7-substituents by means of diazonium salts reactions.     

Fractal calibration in size-exclusion chromatography I. An introduction

The elution behaviour of different polymer-solvent systems in three types of organic columns for SEC has been compared and interpreted. The experimental data show that the classical universal calibration is not accomplished. Deviations from a unique curve are observed due to the binary and ternary interactions between the components of the system (solvent, polymer and gel) which results on secondary mechanisms accompanying the main pure or "ideal" SEC separation mechanism. Both, enthalpic and entropic effects are interpreted in terms of the swelling and crosslinking degrees of the gel packings, and are also related with the fractal characteristics of their surfaces, such as the fractal dimension and the available pore size. Moreover, a relationship between the fractal dimension of the pore surface and the chromatographic distribution coefficient is proposed.