as (UFG), Goiânia, Goi as (GO) 74001-970, Brazil

We investigated the negative photoconductivity (NPC) effect that was observed in a p-type Pb1-xSnxTe film for temperatures varying from 300 K down to 85 K. We found that this effect is a consequence of defect states located in the bandgap which act as trapping levels, changing the relation between generation and recombination rates. Theoretical calculations predict contributions to the NPC from both conduction and valence bands, which are in accordance with the experimental observations.

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Recently, some works showed that magnetic fields may reduce the paraffin crystallization and the viscosity of some types of oil. This Article shows the main results obtained in an attempt to determine some factors responsible for the oil interaction with magnetic fields, which caused the rheological properties change in crude oil samples. Under the influence of a magnetic field (1.3 T in 1 min exposure), one of the six brazilian crude oil samples studied (sample 1) showed 39% reduction on its viscosity and a reduction on the viscoelastic properties (loss modulus and storage modulus). However, the other five samples did not show any considerable modification of their rheological properties. We analyzed all six samples using spectroscopy to detect what kind of component was present in sample 1 that could interact with the magnetic field and cause the aforementioned rheological properties change and that was not present in the other samples. The major differences observed in sample 1 were the presence of the Mn 2+ paramagnetic ion (EPR spectroscopy); Sr and Br (XRF spectroscopy); highest aromatic/aliphatic molecules ratio (NMR spectroscopy); and the highest water content (10% v/v, NMR spectroscopy). Thus, the results show that the paraffin could not be the unique factor responsible for the change on the rheological properties of the crude oil samples caused by magnetic fields, as some authors suggested previously.

Poly(vinylpyrrolidone) films containing cobalt chloride or iodine were investigated to obtain information on their possible use as a humidity sensor element. FTIR and UV‐VIS spectroscopies were used to characterize the PVP–I2 and PVP–Co complexes. Infrared spectroscopy revealed a structural change of both shape and intensity of the carbonyl and lactam bands, indicating the formation of an ion‐coordination polymer. The J–E curves for pure PVP, PVP–I2, and PVP–Co films obey ohm's law at low voltages, deviate from the linear response at higher voltages, and finally display breakdown behavior. An increase in current density of the PVP matrix with iodine or cobalt doping is attributed to the formation of charge transfer complexes. The observed hysteresis of the I–V characteristics implies that there was some standing voltage in the film, which could be attributed to a disorientation of polar side groups of PVP. The electrical conductivities of the polymeric complexes were very sensitive to environmental humidity. An explanation of the humidity‐sensing behavior of the PVP–I2 and PVP–Co complexes is presented. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 459–469, 2001