S. G. C. Moreira

Characterization of Buriti (Mauritia flexuosa L.) oil by absorption and emission spectroscopies

Amostras de oleo obtido do fruto do Buriti (Mauritia flexuosa L.) foram caracterizadas por espectroscopia de absorcao e emissao. O espectro de absorcao foi obtido no intervalo de 300 a 2000 nm, enquanto o espectro de emissao foi analisado entre 400 e 800 nm, onde observamos varias bandas. Para melhor entender a complexidade destes espectros, tambem obtivemos os espectros de absorcao e emissao dos componentes majoritarios do oleo de Buriti. Correlacionando estes dados, apresentamos uma discussao sobre a origem das bandas observadas.

Thermal and electrical properties of starch–graphene oxide nanocomposites improved by photochemical treatment

Bionanocomposite films have been prepared by casting an aqueous suspension of acetylated starch (ST) and poly(vinyl alcohol) (PVA) loaded with graphene oxide (GO). A photochemical and reagentless method has been successfully performed to convert the GO phase into reduced graphene oxide (RGO). The nanocomposites have displayed improved thermal and electrical properties when the amount of the GO phase is increased and properly converted to RGO. The molecular-level interactions between components are mainly hydrogen-bonding type according to attenuated total reflectance-Fourier transform infrared (ATR-FTIR) and Raman spectroscopies, as well as thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) has confirmed the effective mixing between the GO and the ST-PVA matrix. The thermal diffusivity and electrical resistivity of ST-GO nanocomposites have increased one order and decreased two orders of magnitude, respectively, after the photochemical treatment. These findings have confirmed the effectiveness of the proposed approach to produce starch-based nanocomposites with improved thermal and electrical properties.

Observation of negative differential resistance and hysteretic effect on buriti oil:polystyrene organic devices

The authors have observed hysteretic behavior and negative differential resistance in single-layer devices made from buriti oil mixed with polystyrene and sandwiched between poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) and silver (Ag) electrodes. The hysteresis curve shows not only two different values of current but different behaviors as the increasing voltage curve shows negative differential resistance and the decreasing voltage curve does not. The fact that a simple fabrication technique yields reproducible conductance switching and hysteresis is relevant for the development of theories of organic electronic device behavior.

Temperature induced stress phase transition in CdTe quantum dots observed by dielectric constant and thermal diffusivity measurements

The authors measured the dielectric constant by capacitance method and the thermal diffusivity by thermal lens technique in the temperature range from 20to300K for CdTe quantum dot doped borosilicate glass samples. Results show a huge difference between the thermal behavior of the pure glass matrix, without quantum dots, and of the doped glass, especially around 90 and 250K. The authors attributed this difference to the phase transition experienced by the CdTe nanocrystals due to the high pressure exerted by the glass matrix over the CdTe quantum dots. The temperature induced stress is caused by the thermal expansion coefficient mismatch between the quantum dot and the glass matrix.

Low-temperature phase transformation studies in the stearic acid: C form.

This paper reports the temperature-dependent measurements in the C form of stearic acid. Raman scattering, X-ray diffraction, and differential scanning calorimetry measurements were performed at low temperatures. The polarized Raman spectra were measured for temperatures ranging from 8 to 300 K over the spectral range of 30-3000 cm(-1). The spectral changes observed in both the lattice vibrational modes and the internal vibrational modes regions of the Raman spectrum, allowed to identify a phase transition undergone by the stearic acid crystal occurring between 210 and 170 K and a change in the structure continues to be observed down to 8 K. The anharmonicity of some vibrational modes and the possible space groups presented by the crystal at low temperatures were also discussed. Low-temperature X-ray diffraction measurements were performed from 290 to 80 K and the results showed slight changes in the lattice parameters at ∼200 K. Furthermore, the evidence of the phase transformation was provided by the differential scanning calorimetry measurements, which identified an enthalpic anomaly at about 160 K.

Thermal lens spectrometry: Optimizing amplitude and shortening the transient time

Based on a model introduced by Shen et al. for cw laser induced mode-mismatched dual-beam thermal lens spectrometry (TLS), we explore the parameters related with the geometry of the laser beams and the experimental apparatus that influence the amplitude and time evolution of the transient thermal lens (TL) signal. By keeping the sample cell at the minimum waist of the excitation beam, our results show that high amplitude TL signals, very close to the optimized value, combined with short transient times may be obtained by reducing the curvature radius of the probe beam and the distance between the sample cell and the detector. We also derive an expression for the thermal diffusivity which is independent of the excitation laser beam waist, considerably improving the accuracy of the measurements. The sample used in the experiments was oleic acid, which is present in most of the vegetable oils and is very transparent in the visible spectral range.

Conformational change in the C form of palmitic acid investigated by Raman spectroscopy and X-ray diffraction

Fatty acids are substances found in most living beings in nature. Here we report the effect of the low temperature in the vibrational and structural properties of the C form of palmitic acid, a fatty acid with 16 carbon atoms. The Raman spectra were obtained in the temperature interval from 300 to 18K in the spectral range between 30 and 3100 cm(-1). The assignment of the duly observed bands was done based on the density functional theory. On cooling, the main changes observed in the lattice mode region of the Raman spectra were interpreted as a conformational modification undergone by the palmitic acid molecules in the unit cell. The X-ray diffraction measurements were obtained from 290 to 80K showing a slight modification in the lattice parameters at about 210K. Differential scanning calorimetry (DSC) measurements were recorded between 150 and 300K and no enthalpic anomaly in the DSC thermogram was observed. These techniques provided strong evidence of the conformational change in the molecules of palmitic acid at low temperatures.

New approach for piezoelectric resonances and phase transitions in KDP and ADP crystals

In this work we introduce a new experimental setup, called here the three-electrode system (TES), which is very useful for analyzing and comparing piezoelectric crystals, as well as for studying their phase transitions and measuring their piezoelectric coefficients. ADP and KDP, which are very well-known crystals, were used in this work as good examples of piezo-crystals. The principal characteristic of TES is the application of an electric field to a small region of the crystal and the observation of the electric field at another region, usually located at the extremes of the crystal. In this way we can see several interesting effects, like piezoelectric resonances, phase transitions and harmonic generation, which are dependent on voltage and frequency conditions. We consider the piezoelectric concept in developing a mathematical model for the generated elastic wave, starting at the excitation electrode and traveling to the second electrode, where the signal depends on the piezoelectric coefficient. Our results have shown that this technique is very efficient for the determination of phase transitions.

EFFECT OF IMPURITIES ON THE DIELECTRIC PROPERTIES OF OLEIC ACID

Temperature dependence of the dielectric constant of oleic acid with different degrees of purity was measured in the 233–313 K range. It was observed that the presence of impurities (palmitic and linolenic acids) strongly influences the dielectric constant, melting point and dipole moment of the oleic acid. The values of apparent dipole moment and the Kirkwood correlation parameters were evaluated using the Kirkwoods model and were estimated to be about 1.20, 1.61 and 2.27 D, respectively for 60, 90 and 97 wt% oleic acid.

KDP:Mn piezoelectric coefficients obtained by X-ray diffraction.

Crystals of pure potassium dihydrogen phosphate KH(2)PO(4) (KDP) and Mn-doped KDP (KDP:Mn) were grown from a water solution by the slow evaporation method and their piezoelectric properties were studied by X-ray diffraction methods. The results have shown an increase in the piezoelectric coefficients with the doping.