Thermal Diffusivity Measurements in Edible Oils using Transient Thermal Lens

Time resolved thermal lens (TL) spectrometry is applied to the study of the thermal diffusivity of edible oils such as olive, and refined and thermally treated avocado oils. A two laser mismatched-mode experimental configuration was used, with a He–Ne laser as a probe beam and an Ar⁺ laser as the excitation one. The characteristic time constant of the transient thermal lens was obtained by fitting the experimental data to the theoretical expression for a transient thermal lens. The results showed that virgin olive oil has a higher thermal diffusivity than for refined and thermally treated avocado oils. This measured thermal property may contribute to a better understanding of the quality of edible oils, which is very important in the food industry. The thermal diffusivity results for virgin olive oil, obtained from this technique, agree with those reported in the literature.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5-8, 2005, Bratislava, Slovak Republic.     

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 diffusivity of nanofluids containing Au/Pd bimetallic nanoparticles of different compositions

Colloidal suspensions of bimetallic Au/Pd nanoparticles were prepared by simultaneous reduction of the metal ions from their corresponding chloride salts with polymer (PVP) stabilizer. Thermal properties of water containing bimetallic nanoparticles with different nominal compositions (Au/Pd = 12/1, 5/1, 1/1, 1/5) were measured using the mode mismatched dual-beam thermal lens technique to determine the effect of particle composition on the thermal diffusivity of the nanofluids. The characteristic time constant of the transient thermal lens was estimated by fitting the experimental data to the theoretical expression for transient thermal lens. The thermal diffusivity of the nanofluids (water, containing Au/Pd bimetallic nanoparticles) is seen to be strongly dependent on the composition of the particles. The maximum diffusivity was achieved for the nanoparticles with highest Au/Pd molar ratio. A possible mechanism for such high thermal diffusivity of the nanofluids with bimetallic particles is given. UV-Vis spectroscopy, TEM and high-resolution electron microscopy (HREM) techniques were used to characterize the Au/Pd bimetallic nanoparticles.     

Thermal Diffusivity Determination of Protoporphyrin IX Solution Mixed with Gold Metallic Nanoparticles

Nanoparticles appear to be ideally suited for applications in targeted thermal effects in medical therapies and photothermally activated drug delivery; all depend critically on the thermal transport between the nanoparticles and the surrounding liquid. In this work thermal lens spectroscopy (TLS) was used to determine the thermal diffusivity of protoporphyrin IX (PpIX) solutions mixed with gold metallic nanoparticles. PpIX disodium salt (DS) was used in a HCl solution at 25%. Fluids containing gold (Au) nanoparticles at different concentrations were prepared and added to the PpIX solutions. For each solution, UV–Vis spectroscopy was used to obtain the optical absorption spectrum, and transmission electron microscopy (TEM) was used to obtain the gold nanoparticle size. From the TLS signal intensity, it was possible to determine the characteristic time constant of the transient thermal by fitting the theoretical expression to the experimental data. From this characteristic time, the thermal diffusivity was obtained for each solution. The results show that the thermal diffusivity of PpIX mixed with gold nanoparticles increases with an increase of the nanoparticle metallic concentration.     

Thermal Characterization of Solutions Containing Gold Nanoparticles at Different pH Values

The thermal lens spectrometry (TLS) technique was used to obtain the thermal diffusivity of solutions containing gold nanoparticles (15 nm average diameter) at different pH values. TLS, in a mode-mismatched dual beam configuration, provides a reliable alternative to measure, with high sensitivity, the thermal diffusivities of semitransparent materials, and low thermal diffusivities. The results show that the nanofluid thermal diffusivity increases when the pH is increased. These results will be compared with reported studies for nanofluids with variable pH. From this comparison, it can be seen that the pH values of the solutions influenced the superficial density of charges in the nanoparticles. Also, optical absorption spectra for these gold nanoparticle solutions were obtained using a spectrophotometer, and the nanoparticle size was obtained by the TEM technique. The present measurements were performed at room temperature. This study is important for some medical applications such as photothermal cancer therapy.     

Experimental Study on the Effective Thermal Conductivity and Thermal Diffusivity of Nanofluids

This paper reports measurements of the effective thermal conductivity and thermal diffusivity of various nanofluids using the transient short-hot-wire technique. To remove the influences of the static charge and electrical conductance of the nanoparticles on measurement accuracy, the short-hot-wire probes are carefully coated with a pure Al₂O₃ thin film. Using distilled water and toluene as standard liquids of known thermal conductivity and thermal diffusivity, the length and radius of the hot wire and the thickness of the Al₂O₃ film are calibrated before and after application of the coating. The electrical leakage of the short-hot-wire probes is frequently checked, and only those probes that are coated well are used for measurements. In the present study, the effective thermal conductivities and thermal diffusivities of Al₂O₃/water, ZrO₂/water, TiO₂/water, and CuO/water nanofluids are measured and the effects of the volume fractions and thermal conductivities of nanoparticles and temperature are clarified. The average diameters of Al₂O₃, ZrO₂, TiO₂, and CuO particles are 20, 20, 40, and 33 nm, respectively. The uncertainty of the present measurements is estimated to be within 1% for the thermal conductivity and 5% for the thermal diffusivity. The measured results demonstrate that the effective thermal conductivities of the nanofluids show no anomalous enhancement and can be predicted accurately by the model equation of Hamilton and Crosser, when the spherical nanoparticles are dispersed into fluids.     

Photoacoustic Thermal Characterization of Porous Rare-Earth Phosphate Ceramics

The laser induced non-destructive photoacoustic technique has been employed to measure the thermal diffusivity of lanthanum phosphate ceramics prepared by the sol–gel route. The thermal diffusivity value was evaluated by knowing the transition frequency between the thermally thin to thermally thick region from the log–log plot of photoacoustic amplitude versus chopping frequency. Analysis of the data was carried out on the basis of the one-dimensional model of Rosencwaig and Gersho. The present investigation reveals that the sintering temperature has great influence on the propagation of heat carriers and hence on the thermal diffusivity value. The results were interpreted in terms of variations in porosity with sintering temperature as well as with changes in grain size.     

Monitoring the Thermal Parameters of Different Edible Oils by Using Thermal Lens Spectrometry

Several vegetable edible oils (sunflower, canola, soya, and corn) were used to study the thermal diffusivity of edible oils. Thermal lens spectrometry (TLS) was applied to measure the thermal properties. The results showed that the obtained thermal diffusivities with this technique have good agreement when compared with literature values. In this technique an Ar⁺ laser and intensity stabilized He–Ne laser were used as the heating source and probe beam, respectively. These studies may contribute to a better understanding of the physical properties of edible oils and the quality of these important foodstuffs.     

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 Diffusivity of the Aluminum Alloy Al–17Si–4Cu (A390) in the Solid and Liquid States

The thermal diffusivity of the aluminum alloy Al–17Si–4Cu (A390) was measured in the temperature range from room temperature to 730°C using the laser-flash technique. A commercial laser-flash system (Netzsch LFA 427) was used for the measurements. A short laser pulse of 300μs was applied to heat the bottom surface of a disk-shaped specimen, resulting in a time-dependent temperature increase at the top surface. A correction for the laser pulse length as well as the surface radiation and convection was applied in order to evaluate the half time value of the temperature increase. The thermal diffusivity was calculated from the specimen thickness and the half time value. A sapphire crucible was used to contain the specimen in the mushy region and in the liquid state. As the laser is fired from below at the bottom surface of the specimen, the thickness of the melt has to be small to avoid significant buoyancy. The thermal diffusivity of the alloy above the eutectic temperature and in the liquid is drastically lower than in the solid state of the alloy.     

Photothermal Study of Two Different Nanofluids Containing SiO2 and TiO2 Semiconductor Nanoparticles

Thermal and optical properties of two different nanofluids containing SiO₂ and TiO₂ semiconductor nanoparticles were studied by thermal lens spectrometry (TLS) and spectrophotometry. In the case of SiO₂ nanofluids the transmission electron microscopy technique was used to obtain the SiO₂ nanoparticle sizes to investigate the size effect of these nanoparticles on the sample thermal diffusivity which is important in some medical applications such as photothermal-modulated drug delivery systems. On the other hand for the case of TiO₂ nanofluids, the photopyroelectric technique, TLS, scanning electron microscopy, and X-ray diffraction were employed to investigate the concentration effect on the thermal properties of these nanofluids. Thermal diffusivities and effusivities as functions of the TiO₂ nanoparticle concentrations were obtained. From the experimental results, an incremental increase in the thermal diffusivities and effusivities was observed when the nanoparticle concentration was increased, indicating that the nanoparticle concentration is an important factor to be considered to obtain nanofluids with more thermal efficiency which are required for some applications, such as degradation of residual water.     

Thermal Conductivity of Nanofluids – Experimental and Theoretical

The thermal conductivity of nanofluids has been studied experimentally using the transient hot-wire method, and it is shown that a significant increase can be obtained. Existing methods for the prediction and correlation of the thermal conductivity are discussed. It is shown that a lot of work still needs to be done in this area.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

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.     

Uncertainty of Thermal Diffusivity Measurements by Laser Flash Method

The laser-pulse method is a well-established nonsteady-state measurement technique for measuring the thermal diffusivity, a, of solid homogeneous isotropic opaque materials. BNM-LNE has developed its own bench based on the principle of this method in which the thermal diffusivity is identified according to the “partial time moments method.” Uncertainties of thermal diffusivity by means of this method have been calculated according to the ISO/BIPM “Guide to the Expression of Uncertainty in Measurement.” Results are presented for several cases (Armco iron, Pyroceram 9606) in the temperature range from 20 to 800°C. The relative expanded (k = 2) uncertainty of the thermal diffusivity determination is estimated to be from ±3 to ±5%, depending on the material and the temperature. Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

金属氧化物纳米流体的导热性能研究

采用瞬态热线法测量了4种不同种类、不同体积份额配比的纳米流体的导热系数,分析了纳米颗粒属性、体积分数、悬浮稳定性及温度等因素对纳米流体导热系数的影响.实验结果表明,在流体中加入纳米颗粒将显著提高流体的导热系数.     

Thermal Diffusivity Measurements on Insulation Materials with the Laser Flash Method

An iterative approach is adopted to determine the thermal diffusivity of the xonotlite-type calcium silicate insulation material with very low thermal conductivity. The measurements were performed with a conventional laser flash apparatus by rear-face detection of the temperature response of the three-layered sample, where the insulating material is sandwiched between two iron slices. In the evaluation of the thermal conductivity, the theoretical curve is fitted to the complete temperature–time curve, instead of just using the t _1/2 point. The theoretical model is based on the thermal quadrupole method. The nonlinear parameter estimation technique is used to estimate simultaneously the thermal diffusivity, heat transfer coefficient, and absorbed energy. Based on experimental results, the optimal thickness range of the insulation material in the sample is indicated as 1.6 to 1.9 mm. The effects of the uncertainties of the thicknesses, contact resistance, and thermophysical properties of the three layers on the measurement uncertainty are estimated, giving an overall uncertainty in the thermal conductivity of approximately 7.5%.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

Thermal Diffusivity and Microstructure in API5L-X52 Carbon Steel

The determination of the thermal diffusivity of API5L-X52 carbon steel at room temperature, by means of the photoacoustic technique in a heat transmission configuration, is reported for the first time. Since literature values of thermal diffusivity for this low carbon steel do not exist, comparisons among our thermal diffusivity (α) results for API5L-X52 steel and those reported in the literature for steels with similar compositions are reported. Moreover, a study of the microstructure of this low carbon steel by means of scanning electronic microscopy (SEM) and X-ray diffraction (XRD) is presented. Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

Determination of the Local Thermal Diffusivity of Inhomogeneous Samples by a Modified Laser-Flash Method

The local thermal diffusivity is of special interest for quality control of materials grown by physical vapor transport. A typical specimen of these materials consists of single crystals with sizes up to 1 mm. The conventional laser-flash method delivers only an average value of the thermal diffusivity of these polycrystalline materials. A local sensitive measurement system is desirable to determine the thermal diffusivity of single grains with diameters of 100 μm and above. In this work a modification of a standard laser-flash apparatus is presented. The key feature is the position control of the specimen in the plane perpendicular to the laser beam and the IR-detection unit. The mechanical precision of the position control is better than 100 μm. The IR-detection unit consists of a MCT-detector, a polycrystalline IR-fiber, and a system to focus on the sample surface. To study the experimental potential of the modified laser-flash method, measurements of the local thermal diffusivity of a multiphase specimen with known microscopic thermal properties are presented. The obtained results are discussed with respect to the energy profile of the laser beam and the alignment of the IR-detection unit. It is shown that the thermal diffusivity of a small specimen area with a diameter of 2 mm can be determined with an uncertainty of ±5 %. For a polycrystalline aluminum nitride (AlN) specimen with grain sizes of the order of 1 mm, a mean value for the thermal diffusivity of (72.1 ± 3.6) m² · s⁻¹ at room temperature is determined. A possible local variation of the thermal diffusivity cannot yet be observed. An improvement of the resolution is in progress. Paper presented at the Seventeenth European Conference on Thermopysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Non-destructive Thermal Diagnostics of Porous Materials

Developed mathematical models of apparent thermal conductivity of porous materials are applied to non-destructive methods of thermal diagnostics. The non-destructive thermal diagnostics of porous materials can be used to estimate the size of pores and cracks in the range 10⁻⁹ to 10⁻³ m. A fractal model of porous structure and dependences of thermal conductivity/diffusivity on (experimental) gas pressure are used as a basis for structure parameter calculations. The measuring element (sensor) in this method is the mean free path of gas molecules in pores and cracks (Knudsen number) that is very sensitive to changes in gas pressure. Possible applications of the developed methods include non-destructive thermal diagnostics (NDTD) of nano- and micro-crack sizes; opening, closing and size changes of the cracks at high temperatures in a wide temperature range; evaluation of interfacial and contact heat barrier resistance for coatings; remote laser thermal diagnostics of the cracks; as well as obtaining data on strength, thermal shock behavior, failure and fatigue behavior of coatings and other structures. Examples of several applications of the NDTD method are presented. Invited paper presented at the Fifteenth Symposium on Themophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.