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This work presents the development of a new photothermal radiometry (PTR) setup using a mix between frequency and spatial domain scans, along with a three-dimensional (3D) heat diffusion model. This newly developed PTR system, with a spatial resolution of 33 μm, is used to measure thermophysical properties of several kinds of materials. These properties include the thermal diffusivity, a, of homogeneous semi-infinite materials, thermal boundary resistance, R t h , and thermal anisotropy of membranes. The measured properties for homogeneous semi-infinite materials and two-layered systems are in good agreement with the literature values. In addition, it was possible to obtain an anisotropic factor of 24.6 between the in-plane and cross-plane thermal diffusivity of a 25 μm anisotropic flexible graphite sheet. Furthermore, it was also possible to measure, both directly and independently, the anisotropic thermal diffusivities for a 1 μm titanium membrane. It is suggested that this new hybrid technique can help us to fill the gap between conventional PTR and other photothermal and thermoreflectance techniques. Using this PTR setup, it is possible to experimentally measure isotropic and anisotropic thermophysical properties of bulk and thin materials, including membranes, with high precision through accurate characterization of the pump beam spots size. This is accomplished without the need for an optical transducer layer. [ABSTRACT FROM AUTHOR]

This work sets the basis for nonlinear homodyne and heterodyne photothermal radiometry (PTR) as a form of active pyrometry. The intrinsic nonlinearity of thermo-optical conversion described by blackbody radiation laws generates intermodulation harmonics, among which the sum and difference frequencies are easily measured by lock-in amplifiers connected in series or in parallel. Double modulation heterodyne PTR avoids superposition with harmonics generated in nonlinear homodyne PTR by laser modulation distortion. Useful expressions are derived for the determination of temperature variations of the target relative to its absolute temperature, independently of emissivity. In order to determine one or the other temperature, calibration of one of them is necessary. The effects of spectral and temperature dependence of emissivity are also discussed. Local self-heating of a glassy carbon target could be estimated using two superposed laser sources modulated at 30 Hz and 40 Hz. This application opens the path to perform temperature-dependent thermophysical properties' investigations in a non-contact manner, with a simple setup. Absolute temperature was determined on the surface of a Peltier element modulated at 0.1 Hz, at the location irradiated by a laser beam modulated at 1 Hz. Three data reduction methods (series, parallel, and transient configurations) yielded concordant results. [ABSTRACT FROM AUTHOR]

Byline: Otto Ganguli (1), Dane Bicanic (1,2), Marija Bonifacic (3), Maria Cristina Nicoli (4), Mihai Chirtoc (5) Keywords: Trans-[beta]-carotene; Oxidation; Vegetable oil; Oxidative stability; Optothermal window Abstract: The optothermal window detection method at 488 nm was used to monitor on-line the concentration of trans-[beta]-carotene that was added to several vegetable oils after treating them at 200 degC in the presence of air for varying amounts of time. Results obtained for extra virgin oil show a direct proportionality between the rate constant describing the disappearance of trans-[beta]-carotene and the duration of thermal treatment. The rate constant for the decay of trans-[beta]-carotene in oils treated under identical conditions was also dependent on the type of oil. Trends and individual data are discussed in the light of a possible application of the method for the determination of the oxidative stability of vegetable oils. Author Affiliation: (1) Laser Laboratory for Photoacoustic and Photothermal Research, Division of Biophysics, Agrotechnology and Food Sciences, Wageningen University and Research Centre, Dreijenlaan 3, 6703 HA, Wageningen, The Netherlands (2) Department of Food Chemistry and Nutrition, Faculty of Food Science and Biotechnology, University of Zagreb, Pierottijeva 6, Zagreb, Croatia (3) Department of Physical Chemistry, RuAer Boskovic Institute, Bijenicka c. 54, 10000, Zagreb, Croatia (4) Dipartimento di Scienze degli Alimenti, Universita degli Studi di Udine, via Marangoni 97, 1 33100, Udine, Italy (5) Institute for Isotopic and Molecular Technology, P.O.Box 700, 3400 , Cluj-Napoca 5, Romania Article History: Received Date: 06/12/2002 Online Date: 12/04/2003

Organic semiconductors for opto-electronic devices show several defects which can be enhanced while increasing the operating temperature. Their thermal management and especially the reduction of their temperature are of great interest. For the heat transfer study, one has to measure the thermal conductivity of thin film organic materials. However the major difficulty for this measurement is the very low thickness of the films which needs the use of very specific techniques. In our work, the 3-omega and photothermal radiometric methods were used to measure the thermal conductivity of thin film organic semiconducting material (Alq3). The measurements were performed as function of the thin film thickness from 45 to 785 nm and also of its temperature from 80 to 350 K. With the 3 omega method, a thermal conductivity value of 0.066 W.m−1K−1 was obtained for Alq3 thin film of 200 nm at room temperature, in close agreement with the photothermal value. Both techniques appear to be complementary: the 3 omega method is easier to implement for large temperature range and small thicknesses down to a few tens of nanometers whereas the photothermal method is more suitable for thicknesses over 200nm since it provides additional information such as the thin film volumetric heat capacity.

High-density polyethylene (HDPE)-based hybrid nanocomposites containing graphene nanoplatelets (GnPs) and multiwall carbon nanotubes (MWCNTs) were fabricated using melt mixing followed by compression molding. The influences of size and weight ratio of both carbon-based nanofillers on the electrical, thermal, and mechanical properties of hybrid nanocomposites were evaluated. This study proves that the size and weight ratio of carbon-based nanofillers play a critical role in determining these properties. The optimum size and weight ratio of GnPs and MWCNTs are determined at the maximum achieved enhancement for each property. The HDPE-based nanocomposites containing GnPs with larger surface area and MWCNTs with higher aspect ratio display the highest electrical conductivity at GnPs/MWCNTs weight ratio of 2/3. The combination of GnPs with larger surface area and MWCNTs with lower aspect ratio provides the maximum Young's modulus enhancement of hybrid nanocomposites at 1/4 weight ratio of GnPs and MWCNTs. The nanocomposite containing GnPs with the largest lateral size and MWCNTs with a higher aspect ratio at a 3/2 weight ratio exhibits the highest thermal conductivity. Also, at around the percolation threshold of GnPs, the incorporation of MWCNTs with larger aspect ratio into the HDPE-based nanocomposites containing GnPs with the largest lateral size shows a distinct synergic effect on the thermal conductivity and Young's modulus, while an additive effect on the electrical conductivity and thermal stability. [ABSTRACT FROM AUTHOR]

This work presents the measurements of the Kapitza thermal boundary resistance (TBR) between two types of graphene monolayers epitaxially grown on the silicon face of SiC(0001) substrates by chemical vapor deposition. The studied systems consist of a graphene layer either separated from the bulk SiC by a carbon rich interface layer (called buffer layer BL) exhibiting a 6 3 × 6 3 R 30 ° surface reconstruction or quasifreestanding on the substrate, which will be referred to as QFSMG (for the quasifreestanding monolayer of graphene). The equivalent graphene monolayers' thermal resistances (ratio between the layer thickness and its thermal conductivity) and their respective TBR with the SiC substrates were characterized using a high frequency photothermal radiometry technique in order to distinguish the difference between the two interfaces. The results display a larger TBR through the BL compared to a lower one across the QFSMG. It is suggested that beyond generally used models, the presence of electronic coupling between the QFSMG and the SiC may create new channels for heat conduction at the interface. These results give new insights into the thermal transport at the nanoscale using epitaxial graphene monolayers for better usage in heat management applications (e.g., thermal diodes or thermal transistors). [ABSTRACT FROM AUTHOR]

Four groups of autopolymerizing acrylic resins used for various purposes in current dental practice were investigated by monitoring their temperature changes during the polymerization process. Resins were compared in terms of characteristic parameters (such as the time interval needed to reach mechanical stability and the maximum temperature generated during the polymerization process) and interpretations were made of such temporal temperature profiles. [ABSTRACT FROM AUTHOR]

Kapitza thermal resistance is determined using high-frequency photothermal radiometry (PTR) extended for modulation up to 10MHz. Interfaces between 50nm thick titanium coatings and silicon or stainless steel substrates are studied. In the used configuration, the PTR signal is not sensitive to the thermal conductivity of the film nor to its optical absorption coefficient, thus the Kapitza resistance is directly determined from single thermal parameter fits. Results of thermal resistances show the significant influence of the nature of the substrate, as well as of the presence of free electrons at the interface. [ABSTRACT FROM AUTHOR]

Several physical techniques were used to study the extent of spoilage in apple juice deliberately inoculated with yeast (concentration of Saccharomyces cerevisiae ranged from 25 cells mL[SUP-1] to 2.5 × 10[SUP6] cells mL[SUP-1], respectively) and their performance compared in terms of detection limit achieved. The optical methods used in this investigation rely on the measurement of either absorption [as is the case for classical spectrophotometry (SP) and the so called optothermal window (OW), a variant of a photothermal method], or scattering [examples are turbidimetry (TB), laser scattering (SC), and laser speckle fluctuation (SF)]. It is shown that the presence of yeast increases both optical absorption and scattering. The most favorable detection limit (25 cells mL[SUP-1]) and a highest (nearly 10[SUP4]) dynamic range, combined with a good linearity, were obtained with the experimental set-up for SC. In addition, the extent of correlation between different methods was determined using two markedly different reference substances, i.e., (i) the mixture of apple and blackcurrant juices, representing a strongly absorbing sample, and (ii) diluted (dilution factor of 10[SUP3]) milk as a strong scatterer. Finally, one has monitored the progress of a spontaneous spoilage process in the inoculated juices stored at 5°C under aerobic conditions. [ABSTRACT FROM AUTHOR]

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 parameters as a function of temperature. We found good agreement with the literature data for the temperature coefficients (Δα/α)/ΔT = 0.267 ± 0.015%/K for water and (Δe/e)/ΔT = 0.10 ± 0.05%/K for ethylene glycol, between 20 and 60 °C. The special case implying the normalization to a reference material and with a glass substrate for the pyroelectric sensor is shown to be the best adapted to the determination of both thermal parameters and of their temperature dependence. [ABSTRACT FROM AUTHOR]

A novel method and instrument capable of either monitoring continuously a condensation process or estimating the thickness of a liquid layer in the μm range are presented. The underlying physical principle is a variant of the photopyroelectric technique in which the changes in the thermal wave propagation due to the presence of the liquid layer are sensed by the pyroelectric sensor. The theoretical model developed supports the experimental results obtained. [ABSTRACT FROM AUTHOR]

Hysteresis loops exhibited by the thermophysical properties of VO2 thin films deposited on either a sapphire or silicon substrate have been experimentally measured using a high frequency photothermal radiometry technique. This is achieved by directly measuring the thermal diffusivity and thermal effusivity of the VO2 films during their heating and cooling across their phase transitions, along with the film-substrate interface thermal boundary resistance. These thermal properties are then used to determine the thermal conductivity and volumetric heat capacity of the VO2 films. A 2.5 enhancement of the VO2 thermal conductivity is observed during the heating process, while its volumetric heat capacity does not show major changes. This sizeable thermal conductivity variation is used to model the operation of a conductive thermal diode, which exhibits a rectification factor about 30% for small temperature differences (≈70 °C) on its terminals. The obtained results grasp thus new insights on the control of heat currents. [ABSTRACT FROM AUTHOR]

This study aimed to improve the thermal properties of some biopolymers composites. Two aliphatic polyesters were used as matrix, the polylactic acid (PLA) and the polybutylene succinate (PBS). The filler used in the composites was expanded graphite (EG) with three different sizes, 5, 50 and 200 μm. Thermal diffusivities were measured by infrared photothermal radiometry (PTR). Model of effective medium approximation (EMA) was used to study the effect on effective thermal conductivity k e of thermal properties of matrix and fillers, the effect of the interfacial thermal resistance ITR between matrix and fillers or the effect of aspect ratio of fillers. Measurement of particles sizes of fillers before and after preparation process shows the presence of small particles, creating lower thermal conductivities than predicted by EMA model. Filtration of EG before process does not allows to increase k e more than 6% because of mechanical stresses of the process. Finally silane functionalization is used to improve interactions between the fillers and the matrices in order to obtain better thermal conductivities. Mechanical studies on composites are also presented. [ABSTRACT FROM AUTHOR]

The examined samples were W-Re and Mo-Re thin alloy layers with high content of Rhenium, applied in industry as protective coatings. The 550 nm of W-50.4%Re and 580 nm of Mo-39.5%Re alloys were deposited on crystalline silicon and amorphous silica substrates by magnetron sputtering method. For thermal characterization of investigated layers the scanning thermal microscopy (SThM) and high frequency photothermal radiometry (HF-PTR) measurements were carried out. The thermal methods were supported by the microstructural studies performed by the atomic force microscopy (AFM) and X-ray diffraction (XRD). The SThM method allowed determining the thermal conductivity (κ) of the alloy layers. From the HF-PTR the effective thermal conductivity (κ eff) was determined directly. Estimation of the thermal boundary resistance (R th) between particular alloy layer and its substrate enabled determination of the real κ of the layer with its thermal diffusivity (α) and effusivity (ε) from the HF-PTR fitting. The results showed, that W-Re and Mo-Re layers deposited on Si are characterized by higher κ , α , and ε values, and lower R th in comparison to those deposited on SiO 2. The κ values for W-Re and Mo-Re layers deposited on Si and SiO 2 substrates were estimated in the range from 7 W·m−1 K−1 to 12 W·m−1 K−1, and 1.6 W·m−1 K−1 to 4 W·m−1 K−1, respectively. The correlation between the alloy layer structure, it's thermophysical properties and substrate structure was observed. The AFM and XRD measurements confirmed the short range order formation in Mo-Re layers deposited on Si. • Thermophysical characterization of W-Re and Mo-Re layers deposited on Si and SiO 2. • Thermal conductivity determination by SThM and PTR methods. • Thermal diffusivity and effusivity determination of W-Re and Mo-Re films. • Estimation of interfacial thermal boundary resistance. • Influence of different substrates on thermophysical properties of W-Re and Mo-Re. [ABSTRACT FROM AUTHOR]

High-density polyethylene (HDPE)-based nanocomposites incorporating three different types of graphene nanoplatelets (GnPs) were fabricated to investigate the size effects of GnPs in terms of both lateral size and thickness on the morphological, thermal, electrical, and mechanical properties. The results show that the inclusion of GnPs enhance the thermal, electrical, and mechanical properties of HDPE-based nanocomposites regardless of GnP size. Nevertheless, the most significant enhancement of the thermal and electrical conductivities and the lowest electrical percolation threshold were achieved with GnPs of a larger lateral size. This could have been attributed to the fact that the GnPs of larger lateral size exhibited a better dispersion in HDPE and formed conductive pathways easily observable in scanning electron microscope (SEM) images. Our results show that the lateral size of GnPs was a more regulating factor for the above-mentioned nanocomposite properties compared to their thickness. For a given lateral size, thinner GnPs showed significantly higher electrical conductivity and a lower percolation threshold than thicker ones. On the other hand, in terms of thermal conductivity, a remarkable amount of enhancement was observed only above a certain filler concentration. The results demonstrate that GnPs with smaller lateral size and larger thickness lead to lower enhancement of the samples' mechanical properties due to poorer dispersion compared to the others. In addition, the size of the GnPs had no considerable effect on the melting and crystallization properties of the HDPE/GnP nanocomposites., (Copyright © 2020, Evgin et al.; licensee Beilstein-Institut.)