C. Kolinsky

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...

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.

AN APPLICATION OF THE FRONT PHOTOPYROELECTRIC TECHNIQUE FOR MEASURING THE THERMAL EFFUSIVITY OF SOME FOODS

ABSTRACT Photopyroelectric calorimetry, in the front (inverse) configuration (with thermally thick sensor and sample, and optically opaque sensor), was applied to measure the thermal effusivity of some fats and water-based juice products. The information was obtained via a frequency scan of the phase of the pyroelectric signal; the measurements need no calibration. The main advantages of this configuration above the other two previously proposed front schemes are: (i) it does not require the use of very thin foils or expensive semitransparent sensors; (ii) the information is contained in the phase of the signal (and not in the amplitude as usual), which is independent of the power fluctuations of the radiation source and, consequently, the results are more accurate and reproducible.

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.

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.

Pressure effect on the nematic-isotropic transition in dilaterally substituted nematogens

The effect of pressure on the liquid crystal properties of two new dilaterally substituted nematogens has been studied. The method employed involves measurement of the thermal pressure variation of a sample under isochoric conditions. The pressure-temperature phase diagrams were determined. As for unsubstituted compounds, the nematic phase is stabilized upon application of pressure. The values of the (∂T/∂P)NI slopes of the clearing lines for these nematogens are, however, significantly higher than those generally characterizing rod-shaped nematics. The entropy separation method was used to estimate the constant volume and the volume-dependent terms of the entropy changes at the nematic-isotropic transition. The values of these contributions were determined on the basis of thermobarometric data showing that the transition entropy is strongly volume-dependent. This suggests that the higher values of the (∂T/∂P)NI slopes observed for these compounds can be related to the rearrangement of the lateral flexible chains in the nematic phase when the pressure increases, leading to a decrease of the excluded free volume caused by the bulky core of the molecules.

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.