The thermoelastic bending and thermal diffusion processes influence on photoacoustic signal generation using open photoacoustic cell technique

Abstract

Thermal diffusion and thermoelastic bending are two consequences of heating generated on the sample surface. Both are employed in Open Photoacoustic Cell (OPC) technique to measure the thermal diffusivity of the sample. In this work, we explore the potential use of the OPC technique to study the effectiveness of thermoelastic bending process and thermal diffusion process on photoacoustic signal (S) generation in solids. More specifically, it is observed that if the thermoelastic bending process becomes more effective while the sample thickness is decreased, this information can be used to obtain a method to self-check the value of the thermal diffusivity parameter measured. The method is based on the measurement of the thermoelastic bending parameter as a function of the sample thickness (ls). The expected dependence of the thermoelastic bending parameter (C2) with the sample thickness, according to the theoretical model, is C2 ∝ ls−3. Our results for aluminum metallic samples give a C2 ∝ ls−2.8 dependence. Also, a thermal diffusivity value of αexp = (8.4 ± 0.3) × 10−5 m2/s was measured for metallic aluminum. This value is in good agreement when compared with the theoretical value αAl = 8.6 × 10−5 m2/s.

On the thermal characterization of solids by photoacoustic calorimetry: thermal diffusivity and linear thermal expansion coefficient

“…when the ideal thermally thick sample condition, f >>fc, is fulfilled), αT=a1+a2 takes a value of (1.9±0.6)×10 -5 °K -1 (black circle in the figure), which is in agreement with the value reported for the linear thermal expansion coefficient of stainless steel. Thus, for measurement of this parameter with accuracy an adequate method should involve measurements for different material thicknesses following the above methodology.The above result confirms that of a previous reported work of Somer et al17 , where it was demonstrated that in the so-called Open Photoacoustic Cell (OPC) set-up, the actual value of the thermoelastic bending parameter (determined by the sample´s thermal expansion coefficient) approaches a constant value as the sample thickness becomes thicker, i.e. as the sample goes towards the ideal thermally thick regime (see Fig.…”

Effect of the absorption coefficient of aluminium plates on their thermoelastic bending in photoacoustic experiments

“…2,4,17,23 However, this model 22 breaks down for flexible materials, with strong thermoelastic bending, as is the case of Al and semi-transparent materials like glasses. 24,25,26 Recently, Sommer et al 27 found sizeable differences between Rousset et al 22…”

“…This drawback of the theoretical model was overcome with the introduction of an empirical power law to correlate the TD and TE components of the PA signal. Even though this method, 27 based on the measurement of the thermoelastic bending parameter (fitting parameter) as a function of the sample thickness, 4 yields reliable results for the Al thermal diffusivity, it may not retrieve the correct units of each PA signal.…”

“…Scheme of the open-cell photoacoustic setup used in our experiments.PA measurements have been conducted for three Al plates, which have been cut with a radius Rs = 4 mm, and thicknesses L = 280 μm, 197 μm, and 112 μm (with a relative error of about 1%), from a commercial aluminum billet. The bulk thermal conductivity, thermal diffusivity, density, specific heat capacity, and coefficient of linear expansion of the samples are:27,Furthermore, to reduce the high reflectivity and improve the optical absorption of the Al samples,35 their illuminated surface has been coated with a layer of black pigment, whose uniform thickness is within the micrometer resolution of 4 mm, which is quite small in comparison to the one of the samples. This value stands for the average coating thickness measured with a micrometer at the center and edges of the samples.…”

Electro-acustic influence of the measuring system on the photoacoustic signal amplitude and phase in frequency domain

“…In the open-cell photoacoustic spectroscopy, the microphone with associated electronics and the lock-in amplifier Somer et al, 2013) play the major role in the experiments. A microphone is an acoustic-to-electric transducer that converts sound into an electrical signal.…”