Studies on the V2O5-TiO2 system part 1. Thermoelectric power and electrical conductivity

Summary The thermoelectric power and the resistivity of V₂O₅- TiO₂ mixtures over ranges of composition and temperature from 20° to 500° in air have been measured, The mixtures were obtained by coprecipitation of aqueous solutions of NH₄VO₃ and TiCl₄. calcined during 90 h at 550° and then sintered. Resistances were measured by the four points method.At the VO_5/2 35–100% M range, the activation energies for conductivity change from 0,36 to 0.62eV, while for the thermoelectric power they change from 0,18 to 0.24eV. It can be assumed from these values that the conduction mechanism over this concentration range is due to the hopping of small polarons, arising principally, from the presence of V⁴⁺ ions.In the samples with a high TiO₂ content, the activation energies for conductivity were 0.82 and 0.36eV, for the different samples.From the variation of thermoelectric power with temperature, it can be assumed that the ionization energy of the donors centers lies at 0.83eV under the conducting band. A mechanism for band conduction is inferred from the results, being the V⁴⁺ donating centers and the V⁵⁺ receptive centers.Author to whom all correspondence should be addressed     

Bose-Einstein condensation of metastable helium

We have observed a Bose-Einstein condensate in a dilute gas of 4He in the (3)2S(1) metastable state. We find a critical temperature of (4.7+/-0.5) microK and a typical number of atoms at the threshold of 8 x 10(6). The maximum number of atoms in our condensate is about 5 x 10(5). An approximate value for the scattering length a = (16+/-8) nm is measured. The mean elastic collision rate at threshold is then estimated to be about 2 x 10(4) s(-1), indicating that we are deeply in the hydrodynamic regime. The typical decay time of the condensate is 2 s, which places an upper bound on the rate constants for two-body and three-body inelastic collisions.     

Study of Polymorphism From DSC Melting Curves; Polymorphs of Terfenadine

Terfenadine samples prepared by crystallization in different media and supersaturation conditions were used to investigate the polymorphism of the substance. The study was based on DSC melting curves. An empirical parametric equation was used for modelling the experimental data. The signal recorded was resolved into the corresponding overlapping peak components by fitting analysis. Four polymorphic phases were identified. This revised version was published online in August 2006 with corrections to the Cover Date.     

The effect of the expansion ratio on a turbulent non-Newtonian recirculating flow

 Measurements of the mean and turbulent flow characteristics of shear-thinning moderately elastic 0.1% and 0.2% xanthan gum aqueous solutions were carried out in a sudden expansion having a diameter ratio of 2. The inlet flow was turbulent and fully developed, and the results were compared with data for water in the same geometry and with previous published Newtonian and non-Newtonian data in a smaller expansion of diameter ratio equal to 1.538. An increase in expansion ratio led to an increase in the recirculation length and in the axial normal Reynolds stress at identical normalised locations, but the difference between Newtonian and non-Newtonian characteristics was less intense than in the smaller expansion. An extensive comparison of mean and turbulent flow characteristics was carried out in order to understand the variation of flow features. Received: 31 July 2000 / Accepted: 27 August 2001     

Determination of methanol and ethanol by gas chromatrography following air sampling onto florisil cartridges and their concentrations at urban sites in the three largest cities in Brazil

A new sampling protocol was developed to determine methanol and ethanol in the gas phase, at low concentration levels, in urban atmospheres. The procedure involves collection of air samples (20.0-30.0 l) with three florisil cartridges connected in series, at a flow rate ranging from 1.0 to 2.0 l min(-1) and subsequent elution of the alcohols with water. Separation and quantification were done by gas chromatography (GC) coupled with a flame ionization detector, 'SPI' injector and column DB WAX (30 mx0.53 mmx1 mum). The minimum mass detected by the method, based on two times the average background mass on the blank cartridges, was 0.3 mug for both alcohols which, for a sampled volume of 30 l, resulted in detection limits of 7.6 and 5.3 ppbV for methanol and ethanol, respectively. The determined alcohol concentrations, in 42 different samples from the three largest cities in Brazil-S?o Paulo, Rio de Janeiro and Salvador-ranged from 72 ppbV to below the detection limit for methanol and from 355 to 12 ppbV for ethanol.     

Homogeneous Submanifolds of Codimension Two

We study codimension 2 homogeneous submanifolds of Euclidean space for which the index of minimum relative nullity is small. We prove that if min_xMf(x)n-5, where (x) denotes the nullity of the second fundamental form of the immersion f at the point x, then the manifold M ⁿ is either isometric to a sphere or to a product of two spheres S²×S ^n–2 or covered by the Riemannian product S ^n–1 ×R. As a consequence, we obtain a classification of compact codimension 2 homogeneous submanifolds of dimension at least 5.     

Indium(III) bromide catalyzed one-pot synthesis of trichloromethylated tetrahydropyrimidinones

One-pot condensation of ethyl trichloroacetoacetate 1 with p-substituted aromatic aldehydes (Ph, 4-Me-C₆H₄, 4-Cl-C₆H₄, 4-MeO-C₆H₄) or furfural, and urea or thiourea catalyzed by indium(III) bromide affords eight trichloromethylated tetrahydropyrimidinones, in high yields.     

Influence of lasers propagation delay on the sensitivity of atom interferometers

In atom interferometers based on two photon transitions, the delay induced by the difference of the laser beams paths makes the interferometer sensitive to the fluctuations of the frequency of the lasers. We first study, in the general case, how the laser frequency noise affects the performance of the interferometer measurement. Our calculations are compared with the measurements performed on our cold atom gravimeter based on stimulated Raman transitions. We finally extend this study to the case of cold atom gradiometers.     

Analysis of FSS with Dielectric Losses at Millimeter Wave Band Using the Transverse Transmission Line Method

The analysis of periodic structures on isotropic substrates with dielectric losses is presented. Frequency selective surface structures composed by an array of conducting patch elements are considered. This analysis uses the Galerkin method in combination with the transverse transmission line method, in the Fourier domain, to determine the reflection and transmission characteristics of the considered structures. The analysis provides very accurate results compared to those presented by other authors.