Photothermal Techniques Applied to the Thermal and Optical Characterization of <Emphasis Type="Italic">Curcuma longa</Emphasis>

In recent years, has been reported an increasing interest to study vegetables and fruits in order to know their benefits to prevent different types of health problems. Curcuma longa has a great potential to prevent diseases as different types of cancer. In the present study, C. longa samples, of different trademarks, were optically and thermally characterized by using photoacoustic spectroscopy and photopyroelectric techniques, obtaining their optical absorption spectra, optical absorption length spectra, the thermal diffusivity, effusivity, conductivity and heat capacity per unit volume, observing optical absorptions corresponding to antioxidant elements, reported such as beneficial for health. These properties also could be important in the food industry for the commercialization of functional foods.     

Thermophysical Properties of Plant Leaves and Their Influence on the Environment Temperature

It is known that the thermal properties of a material influence the temperature around it. Once heated, the rate at which a material transfers the absorbed heat into the surroundings is determined by the thermal effusivity (or thermal inertia) of the material, and it depends on the well-known thermal properties, thermal conductivity, and specific heat capacity. Since a direct measurement of these properties is rather difficult for thin biological specimens such as plant leaves, a photothermal technique is used to measure the thermal effusivity, thermal diffusivity, thermal conductivity, and specific heat capacity for a few representative species of plant leaves. Measurements have been carried out on fresh as well as dry leaves to estimate the differences in their properties. Thermal properties of plant leaves are compared with the corresponding properties of two materials abundant in the environment and discussed. The influence of thermal properties, particularly the thermal effusivity and specific heat capacity, of plant leaves on controlling the temperature of the environment around them is discussed.     

Thermal Effusivity Determination of Metallic Films of Nanometric Thickness by the Electrical Micropulse Method

The thermal effusivity of gold, aluminum, and copper thin films of nanometric thickness (20 nm to 200 nm) was investigated in terms of the films’ thickness. The metallic thin films were deposited onto glass substrates by thermal evaporation, and the thermal effusivity was estimated by using experimental parameters such as the specific heat, thermal conductivity, and thermal diffusivity values obtained at room conditions. The specific heat, thermal conductivity, and thermal diffusivity values of the metallic thin films are determined with a methodology based on the behavior of the thermal profiles of the films when electrical pulses of few microseconds are applied at room conditions. For all the investigated materials, the thermal effusivity decreases with decreased thickness. The thermal effusivity values estimated by the presented methodology are consistent with other reported values obtained under vacuum conditions and more elaborated methodologies.     

Photothermal Techniques for Thermal Characterization of Linear Alcohols

Photothermal techniques were used for the thermal characterization of linear alcohols. This was carried out by measuring the thermal diffusivity (by means of a photopyroelectric technique) and thermal effusivity (by means of a photoacoustic technique) of ten linear alcohols, from methanol to 1-decanol. The thermal conductivity and specific heat for these substances were obtained by means of their relations with the two previous thermal properties, by using the values reported for the densities of the alcohols. The values of thermal effusivity showed a decreasing behavior with the increase in length of the linear molecule, and the values of thermal diffusivity, on the other hand, showed a similar behavior but only up to 1-pentanol, from which these values began to increase; this latter behavior was also observed, although to a lesser extent, for thermal conductivities. This peculiar behavior for thermal diffusivity and thermal conductivities is attributed to the influence of the hydroxyl group, which is strong for low molecular weight alcohols, but it diminishes as the size of alcohol’s molecule increases.     

A Simple Investigation of the Thermal Effusivity of Silver Nanofluid Using Photopyroelectric Technique

We investigated the thermal effusivity of silver nanofluids using a microwave technique. During microwave irradiation, silver nanoparticles with a narrow particle size distribution were formed in water and in ethylene glycol, with a polyvinylpyrrolidone stabilizer. We designed and used a front-photopyroelectric technique that employed a metalized polyvinylidene difluoride (PVDF) pyroelectric sensor, with a thermally thick sensor and sample. Using this technique, we calculated the thermal effusivity of the silver nanofluids at a given frequency using the combination of the signal’s normalized amplitude–phase. The thermal effusivity of the nanofluids increased with the number of microwave irradiation cycles, which increased the nanoparticle concentration in the base fluids. A comparison with reported values illustrates the high accuracy obtained from the results of thermal diffusivity, the thermal effusivity of the PVDF sensor, and the thermal effusivity of ethylene glycol as a base fluid (differing by only 1.7 %, 0.5 %, and 2.3 %, respectively). Our method can therefore be used to study nanofluids with varying nanoparticle properties, such as concentration, size, and shape.     

Thermal properties across threshold compositions in Ge-As-Se glasses

The Ge-As-Se chalcogenide glasses is an interesting system to test the validity of different constraint models proposed for these materials, in view of the fact that this system has a wide glass forming range which makes it possible to measure composition dependent properties over ranges covering most predicted thresholds. We have measured the thermal parameters—thermal diffusivity, thermal effusivity, thermal conductivity and heat capacity—of Ge-As-Se glasses covering composition range in which thresholds in physical properties have been predicted for covalent glasses employing a modified photopyroelectric technique. Our results show that this system exhibits a minimum in heat capacity at an average coordination number Z = 2.4 corresponding to a 3-D network as per the Phillips-Thorpe model, whereas thermal conductivity exhibits a threshold maximum at Z = 2.67 which is in agreement with the Tanaka model.     

Optical and Thermal Analysis of the Time Evolution of Curing in Resins by Photothermal Techniques

Four shades of a commercial visible-light curing dental resin are analyzed using photothermal techniques. The thermal effusivities of the dental resin shades before curing are measured using a variant of the conventional photoacoustic technique. The thermal diffusivities before and after curing are measured using infrared photothermal radiometry in the forward emission configuration. The time evolution process of the photocuring resin is monitored by photothermal radiometry in the forward and backward emission configurations. Inversion of the time evolution signal of the different configurations used permits one to obtain the time evolution of the thermal and optical properties during the photocuring. The thermal effusivity and thermal diffusivity exhibit exponential growth, while the optical absorption decreases exponentially due to the curing process. The relationship of these phenomena with the decrease of monomer concentration induced by the curing is discussed.     

Thermal Characterization of Edible Oils by Using Photopyroelectric Technique

Thermal properties of several edible oils such as olive, sesame, and grape seed oils were obtained by using the photopyroelectric technique. The inverse photopyroelectric configuration was used in order to obtain the thermal effusivity of the oil samples. The theoretical equation for the photopyroelectric signal in this configuration, as a function of the incident light modulation frequency, was fitted to the experimental data in order to obtain the thermal effusivity of these samples. Also, the back photopyroelectric configuration was used to obtain the thermal diffusivity of these oils; this thermal parameter was obtained by fitting the theoretical equation for this configuration, as a function of the sample thickness (called the thermal wave resonator cavity), to the experimental data. All measurements were done at room temperature. A complete thermal characterization of these edible oils was achieved by the relationship between the obtained thermal diffusivities and thermal effusivities with their thermal conductivities and volumetric heat capacities. The obtained results are in agreement with the thermal properties reported for the case of the olive oil.     

Investigating Thermal Parameters of PVDF Sensor in the Front Pyroelectric Configuration

A metalized PVDF pyroelectric (PE) sensor was used as an optically opaque sensor and in a thermally thick regime for both sensor and sample, instead of a very thick sensor in the conventional front PE configuration. From the frequency dependence measurements, the normalized amplitude and phase signal were independently analyzed to obtain the thermal effusivity of the sensor. The differential normalized amplitude measured with water as a substrate was analyzed to determine the sensor thermal diffusivity. The PVDF thermal diffusivity and thermal effusivity agree with literature values. Then, from the known thermal parameters of the sensor, the thermal effusivity of a standard liquid sample, glycerol, and other liquids were obtained by the similar procedure.     

Investigation of Thermal Properties of High-Density Polyethylene/Aluminum Nanocomposites by Photothermal Infrared Radiometry

In this study, thermal properties of high-density polyethylene (HDPE) filled with nanosized Al particles (80 nm) were investigated. Samples were prepared using melt mixing method up to filler volume fraction of 29 %, followed by compression molding. By using modulated photothermal radiometry (PTR) technique, thermal diffusivity and thermal effusivity were obtained. The effective thermal conductivity of nanocomposites was calculated directly from PTR measurements and from the measurements of density, specific heat capacity (by differential scanning calorimetry) and thermal diffusivity (obtained from PTR signal amplitude and phase). It is concluded that the thermal conductivity of HDPE composites increases with increasing Al fraction and the highest effective thermal conductivity enhancement of 205 % is achieved at a filler volume fraction of 29 %. The obtained results were compared with the theoretical models and experimental data given in the literature. The results demonstrate that Agari and Uno, and Cheng and Vachon models can predict well the thermal conductivity of HDPE/Al nanocomposites in the whole range of Al fractions.     

Estimation of thermophysical properties of a film coated on a substrate using pulsed transient analysis

The thermophysical properties (thermal diffusivity, effusivity) of a film coated on a substrate have been measured by a pulsed transient analysis. The experimental approach is to utilize the film surface temperature decay following a heating pulse from a Q-switched Nd:glass laser. The temperature decay was measured using a HgCdTe infrared detector. Following the collection of data, a nonlinear least-squares regression was performed to estimate the optimal values of three separate thermal parameters by fitting the data to the semiinfinite substrate model solution. The model was checked systematically by analysis of the sensitivity and correlation of the three parameters, and the thermal diffusivity and effusivity ratio of the film and substrate were obtained from the optimal values of the estimated parameters.     

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.     

Optical and dielectric properties of nitrophenolate based nonlinear optical crystal

A semi-organic nonlinear optical (NLO) material, lithium-p-nitrophenolate trihydrate (LPNP) was synthesized. Single crystals of dimensions 20 × 7 × 3 mm³ were harvested following the solvent evaporation technique. The functional groups present in the compound were identified from FT-IR and FT-Raman spectral analyses, and its molecular structure was confirmed. Identification of the compound was accomplished by X-ray diffraction technique (powder and single crystal XRD). The unit-cell dimensions and the morphology of the grown crystals were identified from single crystal XRD measurements. The thermal transport properties, thermal effusivity (e), thermal diffusivity (α), thermal conductivity (k) and heat capacity (C _p) were measured by the photopyroelectric technique at room temperature. Dielectric constant and dielectric loss were also measured as a function of frequency between 42 Hz and 5 MHz, and temperature between 32 and 100 °C. From optical transmittance measurements, the direct optical band gap of the LPNP crystal was estimated to be 2.47 eV. Laser damage threshold is 60.91 GW cm⁻². Powder second harmonic generation (SHG) measurement was carried out using a modified Kurtz–Perry technique. Third order nonlinear response was studied using Z-scan technique with a He–Ne laser (632.8 nm, 35 mW). The magnitude and the sign of the nonlinear absorption and nonlinear refraction are derived from a transmittance curve. The NLO parameters Intensity dependent refractive index n ₂, nonlinear absorption coefficient β and third order susceptibility χ⁽³⁾ were estimated.     

Paraffin core-polymer shell micro-encapsulated phase change materials and expanded graphite particles as an enhanced energy storage medium in heat exchangers

The Micro-Encapsulated Phase Change Materials (MEPCMs) with the melting point temperature of 28 °C was used as an energy storage medium to control the thermal behaviour of a heat exchanger. The NEPCM particles were mixed with Expanded Graphite (EG) microparticles to improve the heat transfer rate in the heat exchanger. The MEPCM particles are made of paraffin cores with a particle average size of 45 μm. The paraffin core of the particles can undergo a solid/liquid phase change and store a tremendous amount of thermal energy due to the latent heat of phase change. The heat exchanger is a copper pipe radiator with aluminium fins confined in either a bed of MEPCMs or a composite mixture of MEPCMs + Expanded Graphite (EG) microparticles. The thermophysical properties of MEPCMs and MEPCM + EG were measured. The outcomes show 132% enhancement in thermal conductivity and a 28% decrease in the sensible specific heat of the MEPCM + EG composite with the weight ratio of 70% MEPCM and 30% EG compared to pure MEPCM. The charging and discharging behaviour of heat exchanger for various flow rates and two cases of MEPCM and MEPCM + EG were studied experimentally. The results reveal that using EG notably enhances the heat transfer and capability of the heat exchanger during the charging and discharging process. Using MEPCM + EG reduces the temperature drop at the heat exchanger outlet about 15 °C compared to the case of pure MEPCM.     

Photoacoustic measurement of internal quantum efficiency and observation of the exciton effect in GaSe with photoacoustic phase spectra

We studied the room temperature photoacoustic spectra of GaSe single crystals in the vicinity of the energy gap. Exciton formation was observed in both amplitude and phase spectra. The thermal source that arises in the illuminated sample because of optical absorption without free-carrier generation was incorporated in the heat diffusion equation in order to extend the theoretical approach of photoacoustic signal generation. We calculated the optical absorption coefficient, which shows the exciton formation, and the electron-hole generation quantum efficiency eta(G) using an extended model from the phase and amplitude photoacoustic spectra, respectively.     

Microwave dielectric properties of flexible silicone rubber – Ba(Zn1/3Ta2/3)O3 composite substrates

Silicone rubber composites filled with Ba(Zn_1/3Ta_2/3)O₃ (BZT) were prepared by hot pressing and the effect of filler content on the microwave dielectric, mechanical and thermal properties as well as on moisture absorption were investigated. The observed relative permittivity (ɛ_r) was compared with different theoretical models. Among the different theoretical models Jayasundere Smith and Modified Lichtenecker were in good agreement with experimental values of ɛ_r. The study of the mechanical property showed that the silicone rubber – BZT composites were flexible and stretchable. The coefficient of thermal expansion and specific heat capacity decreased whereas thermal conductivity, thermal diffusivity and the moisture absorption increased with increase in filler loading.     

Thermal Characterization, Using the Photopyroelectric Technique, of Liquids Used in the Automobile Industry

Thermal properties of liquids used in the automobile industry such as engine oil, antifreeze, and a liquid for windshield wipers were obtained using the photopyroelectric (PPE) technique. The inverse PPE configuration was used in order to obtain the thermal effusivity of the liquid samples. The theoretical equation for the PPE signal in this configuration, as a function of the incident light modulation frequency, was fitted to the experimental data in order to obtain the thermal effusivity of these samples. Also, the back PPE configuration was used to obtain the thermal diffusivity of these liquids; this thermal parameter was obtained by fitting the theoretical equation for this configuration, as a function of the sample thickness (called the thermal wave resonator cavity), to the experimental data. All measurements were done at room temperature. A complete thermal characterization of these liquids used in the automobile industry was achieved by the relationship between the obtained thermal diffusivities and thermal effusivities with their thermal conductivities and volumetric heat capacities. The obtained results are compared with the thermal properties of similar liquids.     

Experimental verification of localized defect states in Ga-Ge-Se nano colloidal solutions

Linear optical studies on Ga₉Ge₂₇Se₆₄ nanocolloidal solutions were carried out. Evidence for the existence of defect states in these nanoclusters are obtained from absorption spectra, and its occurrence is emphasized by analyzing the fluorescence and thermal diffusivity studies. An intermediate peak in the band tail of the absorption spectrum is observed at high concentration, and it is interpreted as the absorption due to localized defect states. Fluorescence emission corresponding to this peak confirms the same. The thermal diffusivity studies show that nanocolloidal solutions with less solute concentration have high thermal diffusivity. This indicates the existence of high defect states density which increases with the increase in cluster size, leads to scattering of phonons from the defect centers and in turn results in lowering of thermal diffusivity.     

无机纳米流体的热物性及其测试新方法的研究

研究了一种能同时测量纳米流体导热系数λ、导温系数α和比热C_p的非稳态多功能测试新方法,它结合了单丝法准确测量导热系数和双丝法准确测量导温系数的优点.对几种液体的导热系数和导温系数的实测值与TPRC推荐值进行比较,最大偏差分别为-0.4％和-2.7％.导热系数和导温系数测试均方根误差分别<±0.5％和±3％.并用此法测量了纳米Al₂O₃流体和纳米TiO₂流体的热物性参数,结果表明在流体中加入无机纳米粉体后其入和α较分散介质均有明显提高,加和原理不完全适用于纳米流体比热计算.     

Thermal Characterization of Calcium Phosphates for Biomedical Use

Photoacoustic spectroscopy (PAS) has been used for thermal characterization of diverse materials. The use of PAS has become an important tool since it is a nondestructive technique. Furthermore, its use to measure also optical absorption spectra has advantages over the usual transmission measurements due to important features such as the fact that scattered light does not disturb the measurements significantly and also the sample does not need to be prepared to have good quality surfaces. By using a heat transmission configuration of the photoacoustic technique, the thermal properties of biological samples have been investigated. In the present study, the photoacoustic technique is applied to obtain the thermal diffusivity of hydroxyapatite (HA) [Ca₁₀(PO₄)₆(OH)₂]. Complementary studies of X-ray diffraction and energy dispersion spectroscopy (EDS) were performed.