Photoacoustic and related photothermal techniques

Many of the renewable and sustainable energy technologies employ novel nanomaterials. For instance, thermal storage and thermoelectric conversion are in constant progress due to the emergence of new structures such as carbon-based materials, bulk nanostructures, 2D novel materials or nanowires. Thermal properties play a significant role to all these energy technologies as key parameters to evaluate the performance and efficiency of those materials in the final device. Understanding the effects of nanostructuring on thermal properties becomes critical, since a reduction in the thermal conductivity due to increased phonon scattering at interfaces is usually expected. Therefore, the determination of the thermal properties remains a critical aspect of material development effort, and measurement techniques are continuously developed or improved. Among those, non-contact heating methods are of importance since they bypass a frequent source of errors characteristic to contact-based thermal measurements, namely the thermal contact resistances, which can be dominant in nanoscale materials. Non-contact heating techniques are usually based on photothermal phenomenon, where heating is generated typically by incident radiation. This paper reviews non-contact heating measurement methods, providing an overview of basic principles for measurement along with associated theoretical model necessary for data reduction and their main applications. The techniques are categorized as time domain and frequency domain techniques, where the thermal response of the sample under study is analyzed as a function of time and frequency, respectively. Both types of methods study the transient response of the sample from a pulsed or modulated heating, and typical measurement output is thermal diffusivity. In addition, other non-contact techniques are also discussed, such as those based on steady-state response, from which the thermal conductivity is directly obtained, or those using AFM probe in the non-contact mode. Finally, main advantages and disadvantages of these techniques are summarized along with their associated uncertainties.

Non-contact methods for thermal properties measurement

Pyroelectricity is the electrical response of a material to a change in temperature. It exists in polymers which contain spontaneous or frozen polarization resulting from oriented dipoles. This review describes the physical basis for the existence of the pyroelectric effect and discusses its behavior in a number of amorphous, semicrystalline, crystalline and liquid crystalline polymers. Some of the techniques for measurement of the pyroelectric effect are presented and its use in pyroelectric relaxation spectroscopy and thermal analysis is described. The spatial distribution of polarization and space charge through the thickness of polymer films can be determined by several thermal pyroelectric techniques. Recent developments in the integration of pyroelectric polymer infrared detectors with solid-state silicon devices are described. One of the most important recent applications of polymers is in photopyroelectric spectroscopy which can be used for the determination of a number of thermal and optical properties of materials. Finally, some miscellaneous applications in physics, chemistry and biology are presented.

Pyroelectric polymer electrets

The study of thermophysical properties is of great importance in several scientific fields. Among them, the heat capacity, for example, is related to the microscopic structure of condensed matter and plays an important role in monitoring the changes in the energy content of a system. Calorimetric techniques are thus of fundamental importance for characterizing physical systems, particularly in the vicinity of phase transitions where energy fluctuations can play an important role. In this work, the ability of the Photopyroelctric calorimetry to study the versus temperature behaviour of the specific heat and of the other thermal parameters in the vicinity of phase transitions is outlined. The working principle, the theoretical basis, the experimental configurations, and the advantages of this technique, with respect to the more conventional ones, have been described and discussed in detail. The integrations in the calorimetric setup giving the possibility to perform, simultaneously with the calorimetric studies, complementary kind of characterizations of optical, structural, and electrical properties are also described. A review of the results obtained with this technique, in all its possible configurations, for the high temperature resolution studies of the thermal parameters over several kinds of phase transitions occurring in different systems is presented and discussed.

Invited Review Article: Photopyroelectric calorimeter for the simultaneous thermal, optical, and structural characterization of samples over phase transitions

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...

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

The use of pyroelectric polyvinylidene fluoride calorimeters for the measurement of thermal diffusivities of thin films is described. The potential of this detection scheme combined with photothermal excitation is demonstrated on polymer films with thickness in the 1 μm range.

H. Coufal

Papers

Thermal diffusivity measurements of thin films with a pyroelectric calorimeter