Thermal lens spectrometry (TLS) and photopyroelectric (PPE) techniques were used to obtain the thermal diffusivity and effusivity of different nanofluid samples. The thermal effusivity of these samples was obtained by the PPE technique in a front detection configuration. In the case of the determination of the thermal diffusivity, TLS was used for the different solvents in the presence of gold nanoparticles (nanofluids). In this technique, an Ar + laser and intensity stabilized He-Ne laser were used as the heating source and probe beam, respectively. The experimental results showed that thermal diffusivity values of the studied solvents (water, ethanol, and ethylene glycol) were enhanced by the presence of gold nanoparticles. Comparisons with literature values show good agreement with pure solvents. These techniques are applicable for all kind of liquid samples, including semitransparent ones.
Photoluminescence properties of cubic zinc blende ZnO thin films grown on glass substrates prepared by the spray pyrolysis method are discussed. X-ray diffraction spectra show the crystalline wurtzite with preferential growth in the (002) orientation and a metastable cubic zinc blende phase highly oriented in the (004) direction. Raman measurements support the ZnO cubic modification growth of the films. Photoluminescence (PL) spectra of zinc blende films are characterized by a new PL band centerd at 2.70 eV, the blue emission, in addition there are two principal bands that are also found in hexagonal ZnO films with the peak positions at 2.83 eV and 2.35 eV. The origin of the 2.70 eV band can be attributed to transitions from Zn-interstitial to Zn-vacancies. It is also important to mention that the PL intensity of the 2.35 eV band of the zinc blende thin films is relatively higher than in the band present in hexagonal ZnO films, which means that zinc blende films have more oxygen vacancies, as was corroborated by means of the energy dispersion spectroscopy (EDS) measurements. PL spectra at 77 °K were measured and the 2.70 eV band was confirmed for the zinc blende films. Some PL bands of cubic films also appeared for the hexagonal phase, which is due, to a certain extent, to the similar ions stacking of both wurtzite and zinc blende symmetries.
A self-normalized photoacoustic technique for thermo-optical characterization of materials, mounted between transparent media, is presented. It involves a complex ratio of photoacoustic signals in transmission and front configurations, taking the modulation frequency as the only variable. The analytical solutions for the corresponding 1D heat diffusion problems are analyzed to provide suitable methodologies for measuring the optical absorption coefficients and thermal diffusivity of such samples. This methodology was tested by measuring the optical absorption coefficient, at 660 nm, of methylene blue solutions at various concentrations and the thermal diffusivity of a black drawing ink sample. In addition, an approximated range of optical absorption coefficients, where this photoacoustic methodology is adequate, was established.
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