Phase diagram of the relaxor (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 investigated by dielectric and Raman spectroscopies

Lead zinc niobate-lead titanate[(1−x)Pb(Zn_1/3Nb_2/3)O₃-xPbTiO₃] (PZN-PT) crystals with x=4.5% and x=12% have been investigated using dielectric and Raman measurements over a range of temperatures. Above room temperature, dielectric measurements show that both compositions exhibit structural phase transitions according to the phase diagram proposed by Kuwata et al. [Ferroelectrics 387 (1981) 579]. Below room temperature, an anomaly at around 180 K for the x=12% sample is observed, suggesting another phase transition. Raman measurements are used to study all phase transitions.     

Size-controllable synthesis of nanosized-TiO2 anatase using porous Vycor glass as template

In this paper we report the synthesis and characterization of TiO₂ nanocrystal dispersed into a porous Vycor glass. We have obtained very small TiO₂ nanocrystals in the anatase form. The nanocrystal size is controlled via the mass increment only thus preventing the growth through the coalescence process. The nanocrystal size was monitored through transmission electron microscope and Raman scattering. The␣coalescence control is attributed due to the obtention of nanocrystals dispersed into the host and to the terminal bonds present in the porous which act as an anchor thus resulting in a low diffusion of the nanocrystals through the porous network.     

Characterization of methylcellulose produced from sugar cane bagasse cellulose: Crystallinity and thermal properties

In the present work, methylcellulose produced from sugar cane bagasse was characterized by FTIR, WAXD, DTA and TGA techniques. Two samples were synthesized: methylcellulose A and methylcellulose B. The only difference in the process was the addition of fresh reactants during the preparation of methylcellulose B. The ratio between the absorption intensities of the C-H stretching band at around 2900 cm⁻¹ and O-H stretching at around 3400 cm⁻¹ for methylcellulose B is higher than for methylcellulose A, indicating that methylcellulose B showed an increase in the degree of substitution (DS). Methylcellulose A presents a more heterogeneous structure, which is similar to the original cellulose as seen through FTIR and DTA. Methylcellulose B showed thermal properties similar to commercial methylcellulose. The modification of methylcellulose preparation method allows the production of a material with higher DS, crystallinity and thermal stability in relation to the original cellulose and to methylcellulose A.     

Magnetic transitions in NiBr2 6NH3 and NiCl2 6NH3 up to 75 koe

The field dependence of the magnetic susceptibility of powdered NiBr₂ 6NH₃ and NiCl₂ 6NH₃ was measured up to 75 koe at temperatures above and below T_N. The anomalies found are discussed in terms of other magnetic data known for these salts.     

N- and P-channel transport behavior in thin film transistors based on tricyanovinyl-capped oligothiophenes

We report the structural and electrical characterization of thin films of organic semiconductor molecules consisting of an oligothiophene core capped with electron-withdrawing tricyanovinyl (TCV) groups. X-ray diffraction and atomic force microscopy of evaporated films of three different TCV-capped oligothiophenes showed that the films were highly crystalline. Electrical transport was measured in thin film transistors employing silver source and drain contacts and channel probes to correct for contact resistance. Three compounds exhibited n-channel (electron) conduction consistent with cyclic voltametry data that indicated they undergo facile reduction. Maximum electron mobilities were 0.02 cm2/V.s with an on/off current ratio of 10(6). A fourth end-capped molecule, TCV-6T-TCV, which had six thiophene rings, exhibited both p- and n-channel transport. Overall, these results confirm that substitution of oligothiophene cores with electron-withdrawing groups is a useful strategy to achieve electron-transporting materials.     

Structural Organization of syndiotactic polystyrene films crystallized in the β form

Syndiotactic polystyrene (sPS) films were obtained either by casting or by melt crystallization in the β form. Different thermal treatments of the melt led to mixed α and β forms. The structural organization was investigated by x-rays, transport properties, and Fourier transform infrared analysis. The β form is characterized by chains in zigzag planar conformation. For samples prevalently in β form, the fraction of impermeable phase, derived from sorption measurements, was found coincident with the fraction of crystalline phase derived from x-rays. In samples with mixed α and β forms, however, a fraction of mesophase is also present. The β form of sPS is stable to dichloromethane, whereas the α form is transformed into a crystalline form with the chains in helical conformation. Nevertheless, when the relative fraction of β form is lower than a critical value, it also dissolves in liquid dichloromethane, undergoing the conformational transition into the helical form.