S. Delenclos

Assessment of calibration procedures for accurate determination of thermal parameters of liquids and their temperature dependence using the photopyroelectric method

We present a systematic theoretical and experimental investigation on the accuracy of thermal diffusivity α and thermal effusivity e of liquids measured by the photopyroelectric (PPE) method in back-detection configuration (BPPE). Special cases corresponding to different cell structures are analyzed in terms of error determination of α and e for water and ethylene glycol. We propose a new normalization procedure allowing for estimation of these parameters with accuracy of 2% on α and 5% on e over extended frequency range. The normalization eliminates the frequency-dependent influence of the transducer impedance and associated electronics, reduces the errors due to coupling fluid between cell components, and reduces the number of temperature-dependent parameters that must be known in order to characterize the sample. Technical solutions for improving the performances are suggested. Another goal of the study was to demonstrate the possibility of the BPPE method to yield small variations of thermal parameter...

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.

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.

Current mode versus voltage mode measurement of signals from pyroelectric sensors

Usually, in photothermal experiments using pyroelectric sensors, the instrumental transfer function is cancelled by normalization to reference measurements, regardless of current (C) or voltage (V) mode processing of signals. Nevertheless, there are several advantages when using a current preamplifier, instead of a high-impedance voltage preamplifier. Due to the low input impedance of the former, the capacitance of the sensor (and its temperature dependence), the capacitance of the connection cable, and the loss resistance of the pyroelectric crystal do not influence the signal. Moreover, the whole input circuitry is less prone to electromagnetic pick up through stray capacitances. In (C) mode, the frequency characteristic is linear (with −180° constant phase) up to a rather high frequency, instead of having (in V mode) a 1/f dependence above a certain frequency and a variable phase shift from 0° to −90°.

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.

Heat transport in polymer-dispersed liquid crystals under electric field

The concepts of effective thermal conductivity and interfacial thermal contact resistance in composite media are applied to study heat transport in polymer-dispersed liquid crystals (PDLC). In these systems, the thermal properties of liquid crystal inclusions are changed by an imposed electric field. The photopyroelectric (PPE) technique with a cell allowing the application of an electric field to the sample is used to measure the thermal parameters. A model based on effective medium approximation is used to assess the impact of interfaces on the flow of heat through the determination of the Kapitza radius. It was found that the effect of interfaces becomes dominant compared to the volume conduction of the droplet when the liquid crystal (LC) droplet radius becomes smaller than 1 micron. The comparison of the thermal behavior of LC in the droplets with that of bulk liquid crystal allowed to evaluate the effect of confinement on the LC nematic phase.

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.

Deux methodes photothermiques a l'etude d'une preparation de contact d'un melange binaire de cristaux liquides

The photopyroelectric (PPE) method is proposed as a sensitive technique to study a binary mixture of liquid crystals in a contact preparation. The photothermal signal is generated while scanning the contact preparation. The crystal-smectic A, smectic A-nematic, and nematic-isotropic interphase boundaries are detected. The displacement of these boundaries due to the variation of the temperature is monitored. The study of these displacements allows us to draw the complete temperature-concentration phase diagram of a binary mixtures.

Thermal analysis of Polymer Dispersed Liquid Crystal under electric field using photopyroelectric technique

In this paper the first measurement of effective thermal parameters (thermal diffusivity, effusivity, conductivity and heat capacity) of Polymer Dispersed Liquid Crystal (PDLC) composites using the photopyroelectric (PPE) calorimetry is reported. The PPE technique is used in the standard “back” configuration and the cell has been designed for allowing the application of an electric field to the sample. Results show a dependence of the thermal parameters on the applied electric field which is explained by the reorientation of the liquid crystal molecules inside the droplets.