On a parabolic–elliptic chemotactic model with coupled boundary conditions

This paper deals with a nonlinear system of parabolic–elliptic type with a logistic source term and coupled boundary conditions related to pattern formation. We prove the existence of a unique positive global in time classical solution. We also analyze the associated stationary problem. Moreover it is proved, under the assumption of sufficiently strong logistic damping, that there is only one nonzero homogeneous equilibrium, and all the solutions to the nonstationary problem tend to this steady state for large times.     

The use of apatite fission track thermochronology to constrain fault movements and sedimentary basin evolution in northeastern Brazil

An apatite fission track study of crystalline rocks underlying sedimentary basins in northeastern Brazil indicate that crustal blocks that occur on opposite sides of a geological fault experienced different thermal histories. Samples collected on the West block yielded corrected fission-track ages from 140 to 375 Ma, whereas samples collected on the East block yielded ages between 90 and 125 Ma. The thermal models suggest that each block experienced two cooling events separated by a heating event at different times. We concluded that the West block moved downward relative to the East block ca. 140 Ma ago, when sediments eroded from the East side were deposited on the West side. This process represents the early stage of sedimentary basin formation and the opening of the South Atlantic Ocean in the region. Downward and upward movements related to heating and cooling events of these crustal blocks at different periods until recent times are proposed.     

Fission-track dating of Macusanite glasses with plateau and size correction methods

Two Macusanite pebbles (MB1 and MB2) were dated with the fission-track method. Six irradiations were carried out in different nuclear reactors: Pavia (Italy), IPEN-CNEN (Brazil) and IPEN-Lima (Peru). Measurements of the thorium and uranium induced-fission per target nucleus using natural thorium thin films and natural U-doped glasses calibrated against natural uranium thin films, together with a λ_F of 8.46×10⁻¹⁷ a⁻¹ were used to determine the ages. The apparent ages were corrected using the plateau and size correction methods. Track measurements were performed by different analysts, using different counting criteria. In addition, tracks were measured on samples which had been submitted to thermal treatment as well as on samples which had not been heated. Thermal treatments were carried out to erase the fossil tracks before neutron irradiation. No significant differences have been found in individual results, using the two Macusanite pebbles and the different nuclear reactors, age correction techniques, analysts, track-counting criteria, and thermal treatments before neutron irradiation. The great majority of the results (14/17) is compatible with the Ar–Ar ages of 5.12±0.11 and 5.10±0.11 Ma, Macusanite MB1 and MB2, respectively. However, the fission-track ages are systematically less (8%) than the Ar–Ar ages of the two Macusanite samples studied.     

U and Th thin film neutron dosimetry for fission-track dating: application to the age standard Moldavite

Neutron dosimetry based on U and Th thin films was used for fission-track dating of the age standard Moldavite, the central European tektite, from the Middle Miocene deposit of Jankov (southern Bohemia, Czech Republic). Our fission-track age () agrees with a recent ⁴⁰Ar/³⁹Ar age, , based on several determinations on Moldavites from different sediments, including the Jankov deposit. This result indicates that the U and Th thin film neutron dosimetry represents a reliable alternative for an absolute approach in fission-track dating.     

Logarithmic correction to the specific heat in tris-sarcosine calcium chloride

Specific heat experimental results in the 3x10^?2 ≦ | t | ≦5.5x10^?5 temperature range ($\text{t =}\text{(T?T}{}_{\mathrm{C}}\text{)}\text{T}{}_{\mathrm{C}}$) were fitted by means of the logarithmically corrected Landau theory of uniaxial dipolar ferroelectrics. The non-universal parameters b and t_O as well as the transverse and longitudinal correlation lengths at zero temperature are determined.     

The effect of anatase crystal morphology on the photocatalytic conversion of NO by TiO<Subscript>2</Subscript>-based nanomaterials

Hydrogen titanate nanotubes (H-TTNT) were synthesized by the alkali hydrothermal method followed by proton exchange and then submitted either to thermal treatment or to acid hydrothermal reaction to generate TiO₂-anatase nanocrystals of different morphologies. The samples were characterized by XRPD, TGA, sulfur analysis, N₂ physisorption, UV-Vis spectroscopy and TEM. Their photocatalytic activities were determined by measuring the NO conversion in inert gas stream passed through the powder catalyst bed under UV radiation. Incomplete transformation into anatase resulted in nanomaterials with low activity due to coexistence with H-TTNT or TiO₂-B precursors. Anatase specimens derived from H-TTNT aged in strong sulfuric acid media contained equidimensional nanoparticles, but retention of sulfate negatively affected their photocatalytic activity. Combining milder acidic pH with higher aging temperature, allowed synthesis of a sulfate free anatase with the same optical properties and specific surface area as the counterpart produced by calcination of H-TTNT at 550°C; however, the former exhibited truncated bi-pyramid nanocrystals and the other adopted the form of nanorods. This latter showed the highest photocatalytic activity for NO abatement, outperforming the benchmark photocatatyst TiO₂-P25; this improved activity was tentatively ascribed to the maximization of high energy {001} facets in anatase nanorods formed during calcination of H-TTNT.

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Electroclinic effect near a smectic A-chiral smectic I transition

The electroclinic effect in the S_A phase of (S) (4'-decyloxy-2'-hydroxybenzylidene)-4-amino(1-ethoxycarbonylethyl) cinnamate has been studied. The induced tilt angles have been measured as a function of temperature near the S_A-S*_I transition at 87°C. The response of the material to different voltages and frequencies has been analysed by using two complementary optical techniques. At low A.C. probe fields the amplitude of the induced tilt shows a relaxational behaviour which can be described by a single relaxation time. The relaxation frequencies have been found to be linear in temperature near the transition, in a similar way to the S_A-S*_C transition.     

Influence of the mass and heating rate in the kinetic parameters in solid=solid+gas reactions

A study about the influence of the mass and heating rate on the kinetic parameters in solid = solid +gas reactions shows that the low dispersion of the values for activation energy and order parameter support the idea that the use of a small mass is consistent with the theoretical suppositions. This should be taken into account in the diffusion phenomena and temperature gradient involved in the experimental process.     

Uranium and thorium thin film calibrations by particle track techniques

Particle track techniques, which enable estimation of the uranium and thorium contents in films where energy self-absorption is negligible, are presented. These techniques allow calibration of uranium and thorium thin films with adequate precision to be used in neutron fluence determinations for fission-track dating. Calibration via a particles is relatively simple for the case of uranium films, whereas for thorium films it is necessary to use the spectrometric characteristics of the employed track detector. Besides those based on a particles, calibration procedures via fission fragments are presented both for uranium as well as for thorium films. This revised version was published online in July 2006 with corrections to the Cover Date.