RESUMO
Photoacoustic spectroscopy is applied to evaluate the impact of Moringa at different concentrations (0, 1.25, 2.5, 5 and 10%) on the elaboration, sanity, texture, and color of wheat bread. It was found that: i) Photoacoustic signal amplitude values of bread significantly increase from 37 to 90% when moringa powder concentration raises from 1.25% to 10%, at 300 nm wavelength. ii) Comparing the photoacoustic signal values at 300, 330, and 370 nm wavelengths, produced by the different bread types, there were statistically significant differences. iii) The sanitary quality of bread mixed with a 2.5% of moringa is relatively higher than the ones obtained for other concentrations, such that the number of fungal colonies were reduced by 99% in comparison with the control bread without moringa, after six days of storage. Moringa at 2.5% of concentration could thus improve the sanitary quality of wheat bread. iv) The addition of moringa for making bread slows down its textural changes (hardness, elasticity, cohesiveness, resilience, and chewiness) during storage. v) Finally, the highest correlation between the photoacoustic amplitude and the moringa concentration occurs at the wavelengths of 300 and 330 nm, which could be related to significant changes in the content of flavonoids and phenolic acids.
RESUMO
Hysteresis loops exhibited by the thermal properties of undoped and 0.8 at.% W-doped nanocrystalline powders of VO2 synthesized by means of the solution combustion method and compacted in pellets, are experimentally measured by photothermal radiometry. It is shown that: (i) the W doping reduces both the hysteresis loops of VO2 and its transition temperature up to 15 °C. (ii) The thermal diffusivity decreases (increases) until (after) the metallic domains become dominant in the VO2 insulating matrix, such that its variation across the metal-insulation transition is enhanced by 23.5% with W-0.8 at.% doping. By contrast, thermal conductivity (thermal effusivity) increases up to 45% (40%) as the metallic phase emerges in the VO2 structure due to the insulator-to-metal transition, and it enhances up to 11% (25%) in the insulator state when the local rutile phase is induced by the tungsten doping. (iii) The characteristic peak of the VO2 specific heat capacity is observed in both heating and cooling processes, such that the phase transition of the 0.8 at.% W-doped sample requires about 24% less thermal energy than the undoped one. (iv) The impact of the W doping on the four above-mentioned thermal properties of VO2 mainly shows up in its insulator phase, as a result of the distortion of the local lattice induced by the electrons of tungsten. W doping at 0.8 at.% thus enhances the VO2 capability to transport heat but diminishes its thermal switching efficiency.
RESUMO
Hysteresis loops in the emissivity of VO2 thin films grown on sapphire and silicon substrates by a pulsed laser deposition process are experimentally measured through the thermal-wave resonant cavity technique. Remarkable variations of about 43% are observed in the emissivity of both VO2 films, within their insulator-to-metal and metal-to-insulator transitions. It is shown that: i) The principal hysteresis width (maximum slope) in the VO2 emissivity of the VO2 + silicon sample is around 3 times higher (lower) than the corresponding one of the VO2 + sapphire sample. VO2 synthesized on silicon thus exhibits a wider principal hysteresis loop with slower MIT than VO2 on sapphire, as a result of the significant differences on the VO2 film microstructures induced by the silicon or sapphire substrates. ii) The hysteresis width along with the rate of change of the VO2 emissivity in a VO2 + substrate sample can be tuned with its secondary hysteresis loop. iii) VO2 samples can be used to build a radiative thermal diode able to operate with a rectification factor as high as 87%, when the temperature difference of its two terminals is around 17 °C. This record-breaking rectification constitutes the highest one reported in literature, for a relatively small temperature change of diode terminals.