A. Hadj Sahraoui

New methodology for thermal parameter measurements in solids using photothermal radiometry

The photothermal radiometry (PTR) signal is analyzed in order to simultaneously obtain the thermal diffusivity and effusivity of solid materials. Analytical procedures that allow the determination of the thermal parameters via a frequency scan of the amplitude or the phase of the PTR signal are presented. The measurement procedures do not involve a multiparameter-fit optimization algorithm. The methods have been used for the measurement of thermophysical properties of vitreous carbon and lead-itanate-zirconate ceramic samples.

On the accurate determination of thermal diffusivity of liquids by using the photopyroelectric thickness scanning method

An enhanced accurate method of measuring the thermal diffusivity of liquids by the samples thickness scan of the phase of the photopyroelectric signal is presented. The method, making use of the absolute values of the phase and sample thickness, leads to very accurate results for the room temperature values of thermal diffusivity (about +/-0.3%). The high accuracy of the method is due to a very precise control of the samples thickness variation (0.1 microm step), to a proper localization of the thickness scan range, and to a new procedure of data analysis. The high accuracy of the method recommends it for the study of processes associated with small changes of the thermal parameters.

The application of the photopyroelectric method for measuring the thermal parameters of pyroelectric materials

The photopyroelectric calorimetry, in the standard (back) configuration, is applied in order to measure the thermal parameters of some pyroelectric materials. It is demonstrated that the method is able to simultaneously measure the thermal diffusivity and effusivity of a pyroelectric material. The information is obtained via a frequency scan of the amplitude or the phase of the pyroelectric signal; the measurements need no calibration. A combined amplitude-phase procedure, at a single frequency, leads to the same results. In the mean time, if the thermal parameters of the pyroelectric sensor are known, one can get the thermal effusivity of a sample acting in the experimental cell as a substrate. Investigations and theoretical simulations were performed on a well known pyroelectric material, LiTaO3, with various liquid substrates.

Analysis of the photopyroelectric signal for investigating thermal parameters of pyroelectric materials

The photopyroelectric signal is analyzed in order to simultaneously obtain the thermal diffusivity and effusivity of pyroelectric materials. Two different experimental configurations are described and compared in terms of accuracy and sensitivity. The information is obtained via a frequency scan of the amplitude or the phase of the pyroelectric signal. The methods have been used for the measurement of thermophysical properties of a lead–titanate–zirconate ceramic sample.

Study of thermal parameter temperature dependence of pyroelectric materials

A simplified photopyroelectric configuration that allows the determination of the temperature-dependent thermal parameters of pyroelectric materials is described. The procedure is based on a combination of phase and amplitude signal data obtained at a single frequency. Experimental results obtained on the thermal parameters of LiTaO3 single crystal are presented.

The photothermoelectric technique (PTE), an alternative photothermal calorimetry

The recently introduced photothermoelectric (PTE) effect is proposed as an alternative for measuring dynamic thermal parameters of solid samples. The front PTE configuration, together with the thermal-wave resonator cavity method as a scanning procedure, was used to measure the value of thermal effusivity. The back PTE configuration, together with the chopping frequency of incident radiation as a scanning parameter, leads to the direct measurement of thermal diffusivity. A theory based on the above two detection configurations was developed and its application to solids, covering a large range of typical values of thermal parameters (aluminum and copper alloys, glass, teflon, polyethylene, LiTaO3), was described in order to demonstrate the suitability of the method. Experimental support for other well-known techniques (photopyroelectric and infrared lock-in thermography) has validated the results obtained with the novel method.

Photothermal imaging as a tool to study phase transitions in binary mixtures of liquid crystals

The photothermal beam deflection technique is used to study a binary mixture of liquid crystals in a contact preparation. The photothermal signal is generated while scanning the contact preparation. The interphase boundaries between crystal-smectic A and smectic A-nematic are detected, and their displacement is monitored as a function of temperature. The theoretical analysis of the numerically simulated signal is presented. In particular the influence of the modulation frequency on the sensitivity in the detection of interphase boundaries is investigated.

An active thermography approach for thermal and electrical characterization of thermoelectric materials

The enhancement of figure of merit (ZT) of thermoelectrics is becoming extremely important for an efficient conversion of thermal energy into electrical energy. In this respect, reliable measurements of thermal and electrical parameters are of paramount importance in order to characterize thermoelectric materials in terms of their efficiency. In this work, a combined theoretical-experimental active thermography approach is presented. The method consists of selecting the right sequential interdependence between the excitation frequency and the sampling rate of the infrared camera, by computing a temporal Fourier analysis of each pixel of the recorded IR image. The method is validated by using a reference sample which is then applied to a recent synthesized titanium trisulphide thermoelectric material (TiS3). By combining AC and steady-state experiments, one can obtain information on both thermal and electrical parameters of TE materials (namely thermal diffusivity, Seebeck coefficient). The thermal diffusivity and thermal conductivity of TiS3 are also measured using photothermal radiometry technique (PTR) and the resulting values of these parameters are α = 9.7*10−7 m2 s−1 and k = 2.2 W m−1 K, respectively. The results obtained with the two techniques are in good agreement. In the case of TE materials, the main benefit of the proposed method is related to its non-contact nature and the possibility of obtaining the electric potential and temperature at the same probes. The Seebeck coefficient obtained by active IR thermography (S = −554 μV K−1) is consistent with the one obtained using an ULVAC-ZEM3 system (S = −570 μV K−1). For a large number of users of thermographic cameras, which are not equipped with a lock-in thermography module, the present approach provides an affordable and cheaper solution.

Study of thermal parameters' temperature dependence in solids using photothermal radiometry

A photothermal radiometry configuration that allows the measurement of the temperature dependence of thermal parameters of solid materials is described. Two procedures are proposed. The first one is based on a combination of phase and amplitude signal data collected at a single frequency and the second one makes use of the information contained in the phase signal data, obtained at two different chopping frequencies. The methods are recommended for calorimetric studies requiring temperature scans at a constant chopping frequency.

Photopyroelectric Study of Thermal and Pyroelectric Parameters Temperature Dependence of Pyroelectric Materials

A simplified photopyroelectric (PPE) configuration that allows the simultaneous determination of the temperature dependent thermal and pyroelectric parameters of pyroelectric materials is described. The thermal parameters (thermal diffusivity, conductivity, effusivity and volumic specific heat) are obtained from the analysis of the phase of the complex photopyroelectric (PPE) signal generated from the pyroelectric sample itself and measured for two different modulation frequencies. The pyroelectric coefficient is obtained from the amplitude of the signal. Experimental results on thermal and pyroelectric parameters of LiTaO 3 single crystal in the 25C-70C temperature range are reported.