M. Navarrete

Signal processing in photoacoustic detection of phase transitions by means of the autospectra correlation-based method: Evaluation with ceramic BaTiO3

This work describes a simple numerical procedure which, when applied to digitally recorded photoacoustic (PA) signals, allows the construction of thermal profiles (rS,drS/dT) to determine: the transition order, the phase transition temperature (Tc), and the phase transformation temperature range (ΔT), on samples, which undergo low–high transitions. During continuous heating of the sample, the ultrasonic signal was generated using a pulsed laser beam incident on a surface and detected on the opposite surface of the sample using a long quartz bar attached to a piezoelectric sensor. The thermal profile, rS, is built from a sequence of the ordinary correlation coefficients ri associated with an interval of temperature. The ri coefficients are calculated from amplitude spectra pairs. The amplitude spectra are obtained via fast Fourier transforms from original PA records. This procedure is applied on samples of bulk ceramic BaTiO3 to obtain their thermal behavior. The PA signal and temperature sample were regis...

Studies of the thermal dissolution process of the Suzuki phase of the Eu2+ ion in KBr single crystals by analysis of photoacoustic signals

An experimental investigation of the thermal behaviour of the dissolution process of the Suzuki phase (SP) by continuous heating (1 °C min−1) of KBr:Eu2+ crystals is reported in this work. The thermal profiles were determined by the correlation functions between subsequent photoacoustic (PA) signals registered during the dissolution process. The behaviour of the thermal profile is directly related to the absorption coefficient of the Eu2+ ion in precipitated states that are present in the crystal. The PA signal is detected as a consequence of the non-radiative processes that take place after the excitation of the low-energy band of the Eu2+ ion by means of a focused laser pulse at 355 nm. The results obtained by this method are compared with those simultaneously obtained by the photoluminescence (PL) technique. The samples were heated from room temperature to 205 °C. The PA signal and PL spectrum were obtained every 6 °C. The temperature range of the SP dissolution process was from 77 to 115 °C. These results are in agreement with those obtained by the PL technique and with the data reported in the literature.

Photoacoustic determination of phase transition in BaTiO3 induced by high pressure at room temperature

The high-pressure behavior of BaTiO3 was investigated by the photoacoustic technique in a diamond anvil cell at room temperature up to 3 GPa. Only one phase transition was observed with an increase in pressure: BaTiO3 tetragonal→cubic. The phase transition induced by high pressure was detected by sequential measurements of photoacoustic signals generated by short laser pulses applied to the diamond front face for each pressure applied. A correlation analysis of the photoacoustic data signal sequence values clearly shows a phase transition within an uncertainty of ±5%.

Phase transition of polycrystalline BaTiO3 at high-pressure detected by a pulsed photoacoustic technique

Sound velocity changes of unpolarized polycrystalline BaTiO3, in the diamond anvil cell (DAC) up to 6.8 GPa, across the tetragonal to cubic phase transition at room temperature (RT) have been detected by a photoacoustic technique. The light absorption before, during, and after the structural phase transformation shows changes. This is very clear using a photoacoustic method where a pulsed laser is used as a standard source of ultrasound. The phase transition is recognized by a profile generated through sequence values of the temporal peak positions acquired from sequential measurements of the pulsed photoacoustic signal (PA) obtained for each pressure applied. The results are in agreement with those obtained by high-pressure Raman spectroscopy carried out under similar conditions. In both techniques a 4:1 methanol–ethanol mixture was used as a pressure transmitting medium, and the pressure was calibrated using the ruby fluorescence technique.

Spectroscopic and thermodynamic features of conical bubble luminescence

The influence on luminescence from conical bubble collapse (CBL) with varying Ar gas content while perturbing the liquid 1,2-Propanediol (PD) has been investigated. The temporal, spatial, and spectral features were analysed with regards to the dynamics of collapse and liquid degradation. Sulphuric acid and sodium chloride were added to disturb the liquid. The following three cases were studied: PD/Ar, (I), (PD + H(2)SO(4))/Ar, (II), and (PD + H(2)SO(4) + NaCl)/Ar, (III). The intensities of those cases decrease as III > II > I. Temporally, single and multiple light emissions were found to occur. The pulse shape exhibited a large variety of profiles with a main maximum and up to two local maxima around the main maximum. These local maxima resembled those generated by laser cavitation. Spatially, no radial symmetry was detected in the light emissions. Spectrally, the Swan, CH and CN lines were observed at low volumes of gas and driving pressure. The ·OH radical and OH-Ar bands, as well as the Na and K lines, consistently appeared superimposed on an underlying continuum that almost disappeared in (III). The Na line was observed with two satellite diffuse bands representing Na-Ar complexes in (I) and (II), whereas in (III), only the line of sodium could be seen. Weak and diffuse emission lines from the Ar atom in the near-IR region were observed in (I) and (II). The proposed mechanism of bright CBL was based on the energy transfer from electron-excited homolytic cleavage products to the chromophore molecules generated during the collapse-rebound time line (~8200 K and ~1 ms of collapse time from model), which had accumulated inside the liquid and remained on the walls of cavity during the repetition of the collapse. A general mechanism for the bright CBL is broached.