Rosalba Castañeda-Guzmán

Ferroparaelectric transitions in relaxor materials studied by a photoacoustic technique

Photoacoustic experiments were carried out in two ceramic compounds to present a promising way to study relaxor materials. This technique has been shown to have some advantages in the investigation of phase transitions in a variety of materials. We use the temperature-dependent data of both dielectric and photoacoustic responses from the ceramic compound Bi4−xRxTi3O12, with Rx=Pr, x=0 and 1.6, to illustrate this work. We establish definitely that Bi4Ti3O12 is a relaxor.

Optical absorption photoacoustic measurements for determination of molecular symmetries in a dichroic organic-film

A novel approach to specify symmetries and main optical axes in anisotropic polymeric films is proposed. This method is based on the analysis of the optical absorption via the pulsed laser photoacoustic (PLPA)- technique in a common polarizer film, while rotating the polarizer axis at normal incidence. Since the PLPA-signals are directly proportional to the optical absorption, it is shown that a symmetric and complementary Maluss law can be obtained over full root mean square (RMS)- and correlation (CA)-analysis of the PLPA-signals. Such data processing reveals the main material directions of the constituting film molecules defining the symmetry structure of the sample. PLPA-results were compared to the pure optical transmission experiments and show unambiguous information, allowing this technique to be used in nonstandard and opaque polymeric films, where the analysis of the optical measurements represents a difficult task, and in general, in anisotropic media.

Pulsed Photoacoustic: A Reliable Technique to Investigate Diffuse Phase Transitions and Associated Phenomena in Ferroelectrics

Often permittivity against T curves do not provide a conclusive characterisation of dielectric features of ferroelectric materials. Particularly, when permittivity is obtained from impedance measurements, where to experimental data is gradually lost at high frequencies when the temperature rises. Here we show how a combination of permittivity measurements and photoacoustic experiments can give a much more complete characterization of the ferro-paraelectric phase transitions and associated phenomena than those obtained from permittivity data alone. Peaked correlation curves, obtained from photoacoustic experiments were interpreted to be a manifestation of large enough microscopic variations of the compressibility as the temperature changed. On one hand, using a classical ferroelectric (BaTiO 3 ) and a well-recognized relaxor (Ba(Ti 0.65 Zr 0.35 )O 3 ) the main differences between their photoacoustic response are established. On the other hand, dielectric and photoacoustic results from the compounds Bi 2 WO 6 and Bi 2 [Ta 0.1 W 0.9 ]O 6 − δ , were combined to characterize the temperature dependence of their dielectric behavior. Results provide evidence of the presence of a diffuse phase transition in Bi 2 WO 6 at 660°C similar to that corresponding to relaxor behavior. Additionally the occurrence of a classical ferro-paraelectric phase transition above 900°C was found. This last phenomenon is detectable only by photoacoustic experiments. In Bi 2 [Ta 0.1 W 0.9 ]O 6 − δ the presence of a well defined transition was not detected. Dielectric determinations throughout ac measurements were carried out in the frequency interval 5 Hz-13 MHz. A pulsed Nd:YAG laser (10 Hz, 5 ns pulse width) was used to perform the photoacoustic experiments, where the out-coming signal was detected by a piezoelectric transducer. Dielectric and photoacoustic experiments were performed in a temperature interval from 250°C up to 1000°C.

Photoacoustic and Dielectric Study of Lead Zirconate Titanate Nanoparticles

Using Sol-Gel method lead titanate zirconate (PZT) nanoparticles with an average size of 13.07 ± 2.8 nm, estimated from transmission electron microscopy (TEM) images, were obtained. Their X-ray powder diffraction (XRD) pattern shows a single phase of Pb(Zr 0.52 Ti 0.48 )O 3 nanocrystallites. Photoacoustic and dielectric experiments on these PZT nanoparticles were performed in a wide temperature range; from room temperature up to 660°C. Photoacoustic experiments performed in as synthesized powders reveal an extraordinary dispersion in a large temperature interval, attributable to the size of the PZT-nanoparticles. Combining photoacoustic and dielectric results, obtained from sintered samples it was possible to establish the existence of a dispersive ferro-paraelectric transition temperature at around 470°C. Impedance curves were used to extract both frequency dependent (ϵ ′(ω)) and frequency independent (ϵ ′) dielectric constant. The temperature dependence of (ϵ ′), behaves in a quite dispersive manner, although a peak around 470°C is suggested. A much more elaborate situation is found when (ϵ ′(ω)) is plotted against temperature. At selected temperatures, frequency dependent curves of ϵ ′(ω) show that at low frequencies (< 100 Hz) there is a strong dispersion, but as the frequency increases ϵ ′(ω) tends to be frequency independent. The magnitude of ϵ ′(ω) depends slightly on the temperature.

Photoacoustic effect applied to sound speed measurement.

We present a simple experiment to show the photoacoustic effect, a well established but not widely known effect which has many applications. The photoacoustic effect consists of the generation of acoustic waves by pulsed radiation incident on a sample. In our case, we used a homemade Nitrogen laser as a source of pulsed light for many samples in order to measure the speed of sound. The Nitrogen laser is easy to build by undergraduate students, it is a transversal discharge laser at atmospheric pressure (TEA), excited by a Blumlein circuit, emitting nanosecond pulses in the ultraviolet region of the spectrum ((lambda) equals 337.1 nm) and has been previously reported. The acoustic waves generated on the surface of the samples travel through the material and are detected with a piezoelectric sensor. The transducer is also easy to build using the piezoelectric of cigar lighters. The electric signals are registered by a 100 Mhz oscilloscope triggered by the light produced at the laser discharge. Knowing the thickness of the sample and the arrival time of the acoustic wave we can precisely measure the speed of sound.

Zinc blende phase detection in ZnO thin films grown with low doping Mn concentration by double-beam pulsed laser deposition

Manganese-doped zinc oxide thin films (ZnO:Mn) were grown on silicon and corning glass using double beam pulsed laser deposition. In this configuration, two synchronized pulsed-laser beams were employed to ablate independently ZnO and Mn targets. The presence of the zinc blende phase was investigated by means of X-ray diffraction, pulsed laser photoacoustic analysis and the calculation of the lattice parameter a. The crystallography plane (110) of the cubic zinc blende was found in all the films. Energy dispersive X-ray spectroscopy and different statistical analysis were employed to analyze the effect of the relative delay between plasma plumes on the average incorporation of Manganese. The minimum content of Mn—0.176 at%—was found for a relative delay of 10 µs, this result suggest that this delay is the inflection point to be considered in relation to a significant decrease in the incorporation of the dopant element. A significant positive Correlation analysis—r (4) = 0.98, p < 0.05—between the thickness and the average Mn incorporation was found, this means that as the percentage of manganese in the structure increases the thickness also increases.

Thermomechanical and Photophysical Properties of Crystal-Violet-Dye/H2O Based Dissolutions via the Pulsed Laser Photoacoustic Technique

Different thermoelastic parameters, for example, the acoustic attenuation and the speed of sound, are fundamental for instrumental calibration and quantitative characterization of organic-based dissolutions. In this work, these parameters as functions of the concentration of an organic dye (crystal-violet: CV) in distillated water (H2O) based dissolutions are investigated. The speed of sound was measured by the pulsed-laser photoacoustic technique (PLPA), which consists in the generation of acoustic-waves by the optical absorption of pulsed light in a given material (in this case a liquid sample). The thermally generated sound-waves traveling through a fluid are detected with two piezoelectric sensors separated by a known distance. An appropriate processing of the photoacoustic signals allows an adequate data analysis of the generated waves within the system, providing an accurate determination of the speed of sound as function of the dye-concentration. The acoustic attenuation was calculated based on the distance of the two PZT-microphones to an acoustic-source point and performing linear-fitting of the experimental data (RMS-amplitudes) as function of the dye-concentration. An important advantage of the PLPA-method is that it can be implemented with poor or null optical transmitting materials permitting the characterization of the mechanical and concentration/aggregate properties of dissolved organic compounds.

Phase transitions on pure and alloy metals by photoacoustic technique

We present the results of phase transitions measurements (state and structural) in pure metals (In and Sn) and in Zn- 21Al-5Cu alloy, using a photoacoustic technique. It is based on real time monitoring of the acoustic signal with PZT sensors after an excitation of the sample with laser pulses.