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 Wave Resonator Cavity Applied to the Study of the Thermal Diffusivity of Coffee Infusions

Among the photothermal methods, the photopyroelectric technique, in its several experimental configurations, has been extensively used to measure the thermal properties of liquids, mainly the thermal effusivity and diffusivity. In this paper, the use of the so-called thermal wave resonator cavity method, in the cavity-length-scan mode, to measure the thermal diffusivity of commercial coffee infusions with samples at different concentrations and degrees of degradation induced by heating cycles is reported. A linear relationship between the logarithm of the pyroelectric signal amplitude and the sample thickness was observed, in agreement with the basic theory for the experimental configuration used here, from which the thermal diffusivity values of the samples were obtained. The thermal diffusivity was found to be almost independent of the coffee concentration in water but that this parameter is sensitive to sample modifications induced by degradation. This work represents another step to demonstrate the capability of the used method for characterization of the thermal properties of liquids.     

Photoacoustic determination of thermophysical properties of thin metallic plates using parameter estimation

The phase and the amplitude of the photoacoustic signal were measured as a function of chopping frequency for several kinds of widely used thin metallic plates (stainless steel 304, brass, aluminum, and copper) attached to plexiglass backing. The experimental data have been analyzed systematically by parameter estimation technique based on the two-layer model developed from Rosencwaig-Gersho (R-G) theory. Using this analysis, the values of thermal diffusivity and thermal effusivity of the materials have been determined.     

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.     

Combined FPPE–PTR Calorimetry Involving TWRC Technique. Theory and Mathematical Simulations

Photopyroelectric calorimetry in the front detection configuration (FPPE) was combined with photothermal radiometry (PTR), in order to investigate dynamic thermal parameters of different layers of a detection cell. The layout of the detection cell consists of three layers: directly irradiated pyroelectric sensor, liquid layer, and solid backing material; and the scanning parameter is the thickness of the liquid layer (thermal-wave resonator cavity method). The theory developed for the two techniques indicates that both FPPE and PTR signals can lead, in the thermally thin regime for the sensor and liquid layer, to the direct measurement of the thermal diffusivity or effusivity of the sensor and/or liquid layer, or the thermal effusivity of the backing material. The two methods offer complementary results and/or reciprocally support each other.     

Experimental Investigation on the Reliability of Thermal Wave Interferometry in the Thermophysical Characterization of Plasma Sprayed Coatings

The main focus of this work is to compare thermal diffusivity and effusivity data resulting from thermal wave interferometry (TWI) experiments on tungsten coatings of different thicknesses with those obtained using reference techniques, namely, the laser flash method and scanning electron microscopy (SEM). The deviations between TWI and the latter techniques are discussed in terms of lack of data in the low frequency range. The investigation shows that the lack of data at low frequencies does not affect diffusivity measurements, while it has a strong effect on effusivity measurements for thermally thick coatings. The conclusions of this experimental study are in good agreement with theoretical predictions resulting from a sensitivity analysis reported in a previous study.     

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.     

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.     

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.     

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.     

Evaluation of risk and benefit in thermal effusivity sensor for monitoring lubrication process in pharmaceutical product manufacturing

In the process design of tablet manufacturing, understanding and control of the lubrication process is important from various viewpoints. A detailed analysis of thermal effusivity data in the lubrication process was conducted in this study. In addition, we evaluated the risk and benefit in the lubrication process by a detailed investigation. It was found that monitoring of thermal effusivity detected mainly the physical change of bulk density, which was changed by dispersal of the lubricant and the coating powder particle by the lubricant. The monitoring of thermal effusivity was almost the monitoring of bulk density, thermal effusivity could have a high correlation with tablet hardness. Moreover, as thermal effusivity sensor could detect not only the change of the conventional bulk density but also the fractional change of thermal conductivity and thermal capacity, two-phase progress of lubrication process could be revealed. However, each contribution of density, thermal conductivity, or heat capacity to thermal effusivity has the risk of fluctuation by formulation. After carefully considering the change factor with the risk to be changed by formulation, thermal effusivity sensor can be a useful tool for monitoring as process analytical technology, estimating tablet hardness and investigating the detailed mechanism of the lubrication process.     

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.     

Influence of Rolling on the Thermal Diffusivity of Metal Alloys by Photothermal Infrared Radiometry

The thermal diffusivities of metallic foils subjected to cold-rolling processes have been studied by photothermal radiometry in a thermal transmission and reflection configuration. In this work, measurements were conducted on foils of Al, Cu, and stainless steel (V2A) that were subjected stepwise to cold-rolling process, reducing the sample of around 1 mm, as prepared, to approximately 0.1 mm. It was found that the effect of the cold-rolling is manifested as a relationship between the relative diffusivity and the relative thickness, both with respect to their corresponding initial values. This empirical relationship consists of a linear decrease of the relative diffusivity with the negative of the logarithm of the relative thickness of the sample. Within the approximation of small deformations, this behavior is consistent with linear diminishing of the thermal diffusivity with the degree of rolling. An influence of rolling on both the thermal diffusivity and effusivity was previously observed for a polycrystalline NiTi shape memory alloy with a nearly equi-atomic composition. Due to the thermal-diffusivity behavior of metal alloys due to rolling, a simple microscopic model is proposed to explain this influence upon the effective thermal parameters within the framework of a one-dimensional heat flow perpendicular to the foil surface. The model assumes the reduction of grain sizes and the consequent increase of grain interfaces during rolling as responsible for a larger effective thermal resistance. Numerical results are shown using the available polycrystalline NiTi for both the thermal-diffusivity and thermal-effusivity values.     

Photothermal Characterization of Nanocomposites Based on High Density Polyethylene (HDPE) Filled with Expanded Graphite

The effective thermophysical and optical properties of high density polyethylene (HDPE) filled with 50???m and 5???m particle sizes of expanded graphite (EG50, EG5) are characterized. The methods used were front- and back-detection modulated photothermal radiometry (FD-, BD-PTR) and BD-flash IR thermography. Results were interpreted according to one-dimensional heat diffusion models. The absolute thermal diffusivity was determined at low frequency from FD- and BD-PTR spectra, while the volumetric heat capacity, the thermal effusivity, and the optical absorption coefficient were determined from broad-band FD-PTR spectra. The directly obtained diffusivity values compare well with those calculated from the heat capacity and thermal effusivity, and with BD-flash results. The errors caused by the finite absorption coefficient of diluted samples are also evaluated and corrected for. A particle-size effect with the opposite influence on thermal and optical properties has been observed. Heat transport parameters of HDPE/EG composites are significantly enhanced (factor of 3 to 4 in thermal diffusivity) at low particle charge before reaching saturation above a 0.10 particle volume fraction. These features are explained in the framework of effective medium models by strongly non-spherical EG particles.     

Polymer Nanofibers Incorporated with Silver Nanoparticles: Thermal Properties

In this study, photothermal techniques were used to investigate the thermal diffusivity, effusivity, and conductivity of samples based on polyvinylidene difluoride (PVDF) polymeric nanofibers incorporated with silver nanoparticles (Ag-NPs). Different amounts were investigated to analyze the thermal effect of Ag-NPs on the polymeric matrix. The Ag-NPs were synthesized by sol–gel and microwave-assisted methods, which have advantages over conventional synthesis methods. The composite of PVDF nanofibers and Ag-NPs was obtained by electrospinning technique while varying the processing parameters. The UV–Vis characteristic spectrum of the nanoparticles was obtained. The hydrodynamic radius of the Ag-NPs was about 16 nm, which was determined by a nanozetasizer. A ζ potential of about 0.03 mV was also measured in this system. This parameter is a measure of the magnitude of the repulsion or electrical attraction between particles and is one of the main measurements to determine the stability of nanoparticles. The morphologies were observed by scanning electron microscopy and showed cylindrical fibers with diameters ranging from 159 nm to 658 nm. Transmission electron microscopy was used to observe the incorporation and distribution of Ag-NPs in the PVDF nanofibers. The thermal effects of Ag-NPs on the polymeric matrix were determined from the thermal properties. The thermal conductivity increased from 0.12 W·m^?1·K^?1 to 0.34 W·m^?1·K^?1 when the Ag-NP amount was increased from 4 % to 12 % in the polymeric matrix.     

Stabilization of Joule Heating in the Electropyroelectric Method

Recently the so-called electropyroelectric technique for thermal characterization of liquids has been proposed (Ivanov et?al., J. Phys. D: Appl. Phys. 43, 225501 (2010)). In this method a pyroelectric sensor, in good thermal contact with the investigated sample, is heated by passing an amplitude-modulated electrical current through the electrical contacts. As a result of the heat dissipated to the sample, the pyroelectric signal measured as a voltage drop across the electrical contacts changes in a periodical way. The amplitude and phase of this signal can be measured by lock-in detection as a function of the electrical current modulation frequency. Because the signal amplitude and phase depend on the thermal properties of the sample, these can be determined straightforwardly by fitting the experimental data to a theoretical model based on the solution of the heat diffusion equation with proper boundary conditions. In general, the experimental conditions are selected so that the thermal effusivity becomes the measured magnitude. The technique has the following handicap. As the result of heating and wear of the metal coating layers (previously etched to achieve a serpentine form) with time, their electrical resistance changes with time, so that the heat power dissipated by the Joule effect can vary, and thermal effusivity measurement can become inaccurate. To avoid this problem in this study, a method is proposed that allows maintaining stable the Joule dissipated power. An electronic circuit is designed whose stability and characteristics are investigated and discussed.     

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.     

Photothermal gas analyzer for simultaneous measurements of thermal diffusivity and thermal effusivity

In this paper, we describe a new, simple, and fast photothermal method for simultaneous measurements of two important gas thermal properties: thermal diffusivity and thermal effusivity. The method consists essentially in combining a photoacoustic cell and a thermal wave pyroelectric cell enclosed in a single compact gas analyzer. The photoacoustic cell is kept filled with synthetic air and sealed. The pyroelectric cell is also filled with synthetic air, and after some warm up time, the synthetic air is exchanged to the gas of interest. It is shown that the analysis of the transient and saturation signals of both photoacoustic and pyroelectric cells is capable of measuring the thermal properties with an accuracy of 3%. This particular capability of performing simultaneously the measurements of thermal diffusivity and thermal effusivity allows us to carry on the complete characterization of the thermal properties of gases.     

Evaluation of the lubrication mechanism at various rotation speeds and granule filling levels in a container mixer using a thermal effusivity sensor

To research the detailed mechanism of the lubrication process using the thermal effusivity sensor, the relationships of the lubrication progress with the pattern of powder flow, the rotation speed and the filling level were investigated. The thermal effusivity profile was studied as a function of the number of rotations at various rotation speeds. It was observed that at lower rotation speeds, the profiles of the lubrication progress were almost the same, regardless of the rotation speed. In this region, the highest speed was defined as the critical rotation speed (CRS), which was found to be one of the important factors. The CRS had close relations with avalanche flow in the blender. The first and the second phases were observed in the lubrication process. The first phase was influenced by the CRS and the filling level in the blender. The second phase was influenced by the rotation speed. The mechanism of two-phase process was proposed as a macro progression of the dispersion of the lubricant (first phase) and micro progression of the coating of the powder particles with lubricant (second phase). The accurate monitoring by the thermal effusivity sensor was able to help a better understanding in the lubrication process.     

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.