Magnetic Behavior at Low Temperatures of Ti Oxide Polycrystalline Samples

Ti-oxide polycrystalline samples have been synthesized by the Pechini method and their low temperature magnetic properties have been characterized. The synthesis procedures allow the preparation of high-quality samples ensuring that the observed low temperature magnetic behavior is intrinsic to the system, not due to the polycrystalline nature of the samples. Magnetic susceptibility measurements showed that the anatase and rutile TiO₂ exhibit a paramagnetic behavior. For the CoTiO₃ samples, a totally reversible antiferromagnetic transition at 38 K is reported.     

Addition of ethyl urethane to olefins: A new approach to the synthesis of aliphatic carbamates

Several different palladium-based catalytic systems have been investigated for the addition of urethane (NH₂COOEt) to olefins. A catalytic system based on PdCl₂(CH₃CN)₂/CuCl/O₂ gave good results only in the case of the activated olefin methyl acrylate. Catalytic systems of the type Pd(acac)₂/phosphorus ligand/acid. previously used to catalyse the telomerisation of conjugated olefins with amines, gave the desired adduct. together with the product of addition of one molecule of urethane and two molecules of diolefin, in the case of isoprene and 2.3-dimethylbutadiene. The main by-products were the dimers of the diolefin, whereas the selectivity in urethane appears to be high. In no case could the addition of urethane to an unactivated non-conjugated olefin be achieved.     

Structure determination of CaMnO3 and CaMnO2.5 by X-ray and neutron methods

Low-temperature synthesis of the oxygen-deficient compound CaMnO_2.5 from polycrystalline CaMnO₃ preserves the existing structural framework of the oxidized precursor. The crystal structure of CaMnO_2.5 was determined using neutron powder diffraction data analyzed by the Reitveld profile refinement method. The structure of the reduced phase can be described by the orthorhombic distortion $\text{(a}{}_0\text{+}\text{b}{}_0\text{2}\text{)}\text{2}\text{= a}{}_{\mathrm{<mtext>c</mtext>}}\text{2}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, where a_c is the simple cubic distance (～3.7 Å) characteristic of the Mn⁴⁺O^2?Mn⁴⁺ framework in CaMnO₃. The unique features of the structure are five-coordinate Mn³⁺ cations with nearly square pyramidal (～C_4v) coordination and ordered oxygen vacancies. The preparation, structure refinement, and noncubic distortions of single crystals of CaMnO₃ are also described. Attempts to transform single crystals of CaMnO₃ into CaMnO_2.5 by well-ordered topotactic changes have not been successful.     

R410A condensation inside a commercial brazed plate heat exchanger

This paper presents the heat transfer coefficients and the pressure drop measured during HFC-410A condensation inside a commercial brazed plate heat exchanger: the effects of saturation temperature, refrigerant mass flux and vapour super-heating are investigated. The heat transfer coefficients show weak sensitivity to saturation temperature and great sensitivity to refrigerant mass flux and vapour super-heating. At low refrigerant mass flux (<20 kg/m² s) the saturated vapour condensation heat transfer coefficients are not dependent on mass flux and are well predicted by Nusselt [W. Nusselt, Die oberflachenkondensation des wasserdampfes, Energy 60 (1916) 541–546, 569–575] analysis for vertical surface: the condensation process is gravity controlled. For higher refrigerant mass flux (>20 kg/m²s) the saturated vapour condensation heat transfer coefficients depend on mass flux and are well predicted by Akers et al. [W.W. Akers, H.A. Deans, O.K. Crosser, Condensing heat transfer within horizontal tubes, Chem. Eng. Prog. Symp. Series 55 (1959) 171–176] equation: forced convection condensation occurs. In the forced convection condensation region the heat transfer coefficients show a 30% increase for a doubling of the refrigerant mass flux. The condensation heat transfer coefficients of super-heated vapour are 8–10% higher than those of saturated vapour and are well predicted by Webb [R.L. Webb, Convective condensation of superheated vapor, ASME J. Heat Transfer 120 (1998) 418–421] model. A simple linear equation based on the kinetic energy per unit volume of the refrigerant flow is proposed for the computation of the frictional pressure drop.     

Influence of processing conditions on the thermal decomposition of SrTiO<Subscript>3</Subscript> precursors

The present work investigates the influence of milling and calcination atmosphere on the thermal decomposition of SrTiO3 powder precursors. Both pure and neodymium-modified SrTiO3 samples were studied. Milling did not significantly influence numerical mass loss value, but reduced the number of decomposition steps, modifying the profiles of the TG and DTA curves. On the other hand, milling increases the amount of energy liberated by the system upon combustion of organic matter. It was also observed that the milling process, associated to the calcination in an oxygen atmosphere, considerably decreases the amount of organic matter and increases the final mass loss temperature.     

Resonances of nanocylinders with gap defects

We have investigated the plasmonic resonance characteristics of canonical circular and square cylinders, with gap defects, that are illuminated by a plane wave. The circular and square cylinders have vee shaped gaps and constant width gaps, respectively. The electric and magnetic fields are obtained by solving the Lippmann–Schwinger equation from which we compute the far-field scattering cross-section and near-field local electromagnetic energy density.Numerical results are given for numerous wavelength and gap dimensions to qualitatively present the effects of gap defects on the scattering cross-section and local electromagnetic energy density.     

On the possibility of using model potentials for collision integral calculations of interest for planetary atmospheres

The interaction energy in systems (atom–atom, atom–ion and atom–molecule) involving open-shell species, predicted by a phenomenological method, is used for collision integral calculations. The results are compared with those obtained by different authors by using the complete set of quantum mechanical interaction potentials arizing from the electronic configurations of separate partners. A satisfactory agreement is achieved, implying that the effect of deep potential wells, present in some of the chemical potentials, is cancelled by the effect of strong repulsive potentials.