PPE Technique Research Articles

Photopyroelectric spectroscopy and calorimetry

In this Tutorial, we present an overview of the development of the photopyroelectric (PPE) technique from its beginning in 1984 to the present day. The Tutorial is organized into five sections, which explore both theoretical and experimental aspects of PPE detection as well as some important spectroscopic and calorimetric applications. In the “Introduction” section, we present the fundamental basics of photothermal phenomena and the state-of-the-art of photopyroelectric technique. In the “Theoretical aspects” section, we describe some specific cases of experimental interest, with examples in both back and front detection configurations. Several mathematical expressions for the PPE signal in specific detection modes (combined back–front configurations and PPE–thermography methods) are also deduced. The “Instrumentation and experiment” section contains two subsections. The first describes several examples of setups used for both room temperature and temperature-controlled experiments. The second subsection is dedicated to the configuration of detection cells and to the various sensor/sample assemblies that are currently used in spectroscopic and calorimetric experiments for both liquid and solid samples. The “Applications” section is in fact a collection of experimental results dedicated to the thermal characterization of a wide range of solid and liquid samples. At the end of this section, we present some examples that have been selected to convey that the PPE technique is not only useful in the investigation of optical and thermal properties of a variety of condensed matter samples, but also to study physical and chemical processes such as molecular associations, food adulteration, or phase transitions. In “Concluding remarks,” we summarize the advantages of this technique in spectroscopic and calorimetric applications.

Study of the electrocaloric effect in ferroelectric liquid crystals

ABSTRACTElectrocaloric effect (ECE) in two ferroelectric liquid crystalline (FLC) materials has been evaluated by mean of two indirect characterization methods: the photopyroelectric (PPE) technique and the polarization current reversal one. The obtained results show a good correspondence of the adiabatic temperature change associated with the ECE evaluated from both methods. This validates the possibility to use the PPE technique to investigate the ECE in FLCs. This study also demonstrates that FLCs can be used as electrocaloric material. More particularly, it shows that as for their solid homologous, liquid crystals displays more pronounced ECE in the vicinity of a first order transition than that measured near to second-order one.

Photopyroelectric Characterization of Magnetic Nanofluids. Influence of Type and Size of Nanoparticles on the Thermal Parameters

The influence of type and size of nanoparticles on the thermal parameters of some magnetic nanofluids is investigated. Two types of carrier liquids (transformer oil and polypropylene glycol) were combined with two types of iron based magnetic nanoparticles (\(\hbox {Fe}_{3}\hbox {O}_{4}\) and \(\hbox {MnFe}_{2}\hbox {O}_{4})\). Different sizes (10 nm–80 nm) and shapes (spherical, octahedral or cubic) of nanoparticles were obtained depending on the oleic acid/oleylamine molar ratio, which drastically influences the nanocrystals growth rate. This influence is due to the different binding ability of the two stabilizers onto crystal facets. The average size of nanoparticles was 10 nm, 35 nm and 50 nm for \(\hbox {Fe}_{3}\hbox {O}_{4}\) and 10 nm, 20 nm and 80 nm for \(\hbox {MnFe}_{2}\hbox {O}_{4}\) at a concentration of 50 mg\({\cdot }\)ml\(^{-1}\) in all cases. The results obtained by PPE technique indicate that, at this concentration, the presence of the nanoparticles reduces the value of the thermal parameters of pure carrier liquids and both thermal diffusivity and effusivity decrease with increasing nanoparticles size, independently on the carrier liquid. The influence of the nanoparticles size is more pronounced for the thermal effusivity (relative change 24 %) compared with thermal diffusivity (relative change 7 %).

Thermal characterization of ZnBeMnSe mixed compounds by means of photopyroelectric and lock-in thermography methods

In this work, a thermal characterization (measurement of dynamic thermal parameters) of quaternary Zn1−x−yBexMnySe mixed crystals was carried out. The crystals under investigation were grown from the melt by the modified high-pressure Bridgman method with different Be and Mn contents. The effect of Be and Mn contents on thermal properties of Zn1−x−yBexMnySe compounds was analyzed, by using the photopyroelectric (PPE) method in the back configuration (BPPE) for thermal diffusivity measurements and the PPE technique in the front configuration for thermal effusivity investigations. Infrared lock-in thermography was used in order to validate the BPPE results. The measured thermal effusivity and diffusivity allowed the calculation of thermal conductivity of the investigated materials.

Thermal characterization of II–VI binary crystals by photopyroelectric calorimetry and infrared lock-in thermography

In this paper, a complete thermal characterization (measurement of all static and dynamic thermal parameters) of some selected II–VI binary crystals was carried out. The semiconductors under investigation were grown from the melt by high-pressure/high-temperature-modified Bridgman method. The contact photopyroelectric (PPE) method in back configuration and non-contact infrared lock-in thermography technique were used in order to get the thermal diffusivity of the investigated crystals. The thermal effusivity of the samples was obtained using the PPE technique in the front configuration, together with the thermal wave resonator cavity method. Knowing the values of the thermal effusivity and thermal diffusivity, the remaining two thermal parameters, i.e., thermal conductivity and specific heat were calculated.

Characterization of thermal properties of Cd1−x−yZnxMgySe mixed crystals by means of photopyroelectric and infrared imaging techniques

Abstract In this work a complete thermal characterization of Cd 1− x − y Zn x Mg y Se mixed crystals was carried out. Bulk Cd 1− x − y Zn x Mg y Se semiconductors with different x and y contents were grown from the melt by the modified high pressure Bridgman method. The photopyroelectric (PPE) method in the back configuration (BPPE) and the infrared (IR) lock-in thermography were applied to measure the thermal diffusivity. Values of thermal effusivity of the samples were obtained with the PPE technique in the front configuration (FPPE), coupled with a thickness thermal wave resonator cavity (TWRC) scanning procedure. Measured thermal effusivity together with the thermal diffusivity allowed calculating the thermal conductivity of the investigated materials. For the calculation of the specific heat, the densities of the samples were calculated from their weight and geometry. The effect of Mg/Zn molar ratio on thermal properties of these quaternary Cd 1− x − y Zn x Mg y Se compounds was analyzed and discussed.

Photothermal Techniques Applied to the Thermal Characterization of l–Cysteine Nanofluids

Thermal-diffusivity (D) and thermal-effusivity (e) measurements were carried out in l-cysteine nanoliquids l-cysteine in combination with Au nanoparticles and protoporphyrin IX (PpIX) nanofluid) by using thermal lens spectrometry (TLS) and photopyroelectric (PPE) techniques. The TLS technique was used in the two mismatched mode experimental configuration to obtain the thermal-diffusivity of the samples. On the other hand, the sample thermal effusivity (e) was obtained by using the PPE technique where the temperature variation of a sample, exposed to modulated radiation, is measured with a pyrolectric sensor. From the obtained thermal-diffusivity and thermal-effusivity values, the thermal conductivity and specific heat capacity of the sample were calculated. The obtained thermal parameters were compared with the thermal parameters of water. The results of this study could be applied to the detection of tumors by using the l-cysteine in combination with Au nanoparticles and PpIX nanofluid, called conjugated in this study.

Generalized 1D photopyroelectric technique for optical and thermal characterization of liquids

The analytical solution for the one-dimensional heat diffusion problem for a three-layer system, in the Beer–Lambert model for light absorption, is used for the implementation of a photopyroelectric (PPE) methodology for thermal and optical characterization of pigments in liquid solution, even for those ones potentially harmful to the pyroelectric sensor, taking the liquid sample's thickness as the only variable. Exponential decay of the PPE amplitude followed by a constant PPE phase for solutions at low pigment concentration, and exponential decay of the PPE amplitude but a linear decrease of the PPE phase for the concentrated ones are theoretically shown, allowing measurements of the optical absorption coefficient (at the wavelength used for the analysis) and the thermal diffusivity for the liquid sample, respectively. This PPE methodology was tested by measuring the thermal diffusivity of a concentrated solution of methylene blue in distilled water and the optical absorption coefficient, at two wavelengths (658 and 785 nm), of water solutions of copper sulfate at various concentrations. These optical parameters were used for measuring the molar absorption coefficient of this last pigment in water solution at these two wavelengths. This last optical property was also measured using a commercial spectrometer, finding very good agreement with the corresponding ones using this PPE technique.

Combined FPPE–PTR Calorimetry Involving TWRC Technique II. Experimental: Application to Thermal Effusivity Measurements of Solids

Photopyroelectric calorimetry in the front detection configuration (FPPE) and photothermal radiometry (PTR) were simultaneously used, together with the thermal-wave resonator cavity method (TWRC), in order to investigate the thermal effusivity of solids inserted as backing layers in a detection cell. A new combined FPPE–PTR–TWRC setup was designed. It was demonstrated experimentally that the PTR technique, combined with the TWRC method, is able to provide calorimetric information about the third layer of a detection cell. Applications on solids with different values of the thermal effusivity (starting from metals, down to thermal isolators) are presented. The values of the thermal effusivity obtained with the PTR technique are similar to those obtained with the PPE technique, and in agreement with literature values; the two methods reciprocally support each other. The accuracy of both methods is higher when the values of the thermal effusivity of the backing layer and coupling fluid are close.

Thermal Characterization of Nanofluids with Different Solvents

Thermal lens spectrometry (TLS) and photopyroelectric (PPE) techniques were used to obtain the thermal diffusivity and effusivity of different nanofluid samples. The thermal effusivity of these samples was obtained by the PPE technique in a front detection configuration. In the case of the determination of the thermal diffusivity, TLS was used for the different solvents in the presence of gold nanoparticles (nanofluids). In this technique, an Ar+ laser and intensity stabilized He–Ne laser were used as the heating source and probe beam, respectively. The experimental results showed that thermal diffusivity values of the studied solvents (water, ethanol, and ethylene glycol) were enhanced by the presence of gold nanoparticles. Comparisons with literature values show good agreement with pure solvents. These techniques are applicable for all kind of liquid samples, including semitransparent ones.

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.

On the influence of the coupling fluid in photopyroelectric measurements

In this work, the influence of the coupling fluid in the measurement of the thermal properties of solid samples using a photopyroelectric (PPE) setup in the standard back configuration is analyzed. It is demonstrated theoretically and experimentally that due to the coupling fluid, the thermal diffusivity of the sample is underestimated. However, its influence disappears when performing relative measurements as a function of the temperature. That is the reason why the PPE technique is very appropriate for the thermal characterization of phase transitions.

Deconvolution and measurement of bulk and surface optical absorptions in Ti:Al2O3 crystals using photopyroelectric interferometry

The extension of our earlier single-layer (monolithic) photopyroelectric (PPE) interferometric theory to include surface and bulk optical absorptions has allowed the measurement of both bulk absorption coefficient and surface absorptance in one single experiment. Based on purely thermal-wave interferometry, the thermal-wave cavity lengths of a PPE interferometer were scanned using pairs of Ti: sapphire crystals with appropriate combinations of figure of merit, surface polish, and thickness. In the conventional single-ended (noninterferometric) PPE technique, the surface reflectivity, surface absorptance, and bulk absorption coefficient are always coupled together. However, PPE destructive interferometry provides a method for extracting highly precise values of one of these optical parameters, without the requirement of equally precise knowledge of the values of the others.

A photopyroelectric method with air as the thermal coupling fluid

The photopyroelectric (PPE) technique is usually used as a contact photothermal method to characterize thin solid materials. We performed theoretical and experimental investigations of a non-contact PPE configuration to analyse the effects of an air gap between the thermal source and a PVDF sensor using a modulated laser beam as optical excitation. The detection of the thermal wave developed within the sample was possible across an air gap of up to 15 mm. The PPE signal was described well thanks to a two-dimensional (2D) model with axial symmetry, which could be reduced in some particular cases to a simpler 1D propagation model. Border effects can be artificially taken into account in the calculations by increasing the adiabatic lateral limits in the model, keeping the active sensor area constant. Despite there being strong attenuation with increasing modulation frequency, the PPE method can be used as well as can other non-contact photothermal techniques. New perspectives for the application of the PPE technique have been opened.

Dual-modulation-frequency photopyroelectric technique: application to thermophysical investigation of (NH<sub>4</sub>)HSO<sub>4</sub> and (ND<sub>4</sub>)<sub>2</sub>TeCl<sub>6</sub> crystals at low temperature

The photopyroelectric (PPE) method for thermo-optic materials investigation uses a pyroelectric sensor to measure the photothermal heating of the sample. We report on a self-calibrated PPE technique consisting of simultaneous excitation of the PPE cell at two frequencies. The corresponding signals are retrieved from the multiplexed pyroelectric response with two lock-in amplifiers. Their ratio depends only on the sensor’s specific heat and the sample’s thermal effusivity, as temperature-dependent quantities. This scheme has the advantage that it cancels out possible fluctuations of excitation intensity and of the pyroelectric coefficient. Experimental results with (NH4)HSO4 and (ND4)2TeCl6 crystals show that this feature is particularly important when studying structural phase transitions at low temperature (20 to 300 K). Finally it is found that below 240 K epoxy resin is a better coupling medium than silicon grease.

Evidence for the role of the surface potential and native oxide on thermal properties of crystalline Si

Thermal properties of crystalline Si have been studied using the photopyroelectric (PPE) technique in the temperature range 100–300 K. It has been observed that the generation and propagation of the thermal waves in Si depends upon the thermal history of the sample. The native amorphous silicon dioxide layer on Si and the surface potential affect the propagation of the thermal waves and this effect is not reversible in the temperature range 150 K<T<260 K. It is observed that the contactless PPE technique can also be used to study recombination processes of excess carriers in semiconductors.