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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]

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]

Quantitative thermal measurements with spatial resolution allowing the examination of objects of submicron dimensions are still a challenging task. The quantity of methods providing spatial resolution better than 100 nm is very limited. One of them is scanning thermal microscopy (SThM). This method is a variant of atomic force microscopy which uses a probe equipped with a temperature sensor near the apex. Depending on the sensor current, either the temperature or the thermal conductivity distribution at the sample surface can be measured. However, like all microscopy methods, the SThM gives only qualitative information. Quantitative measuring methods using SThM equipment are still under development. In this paper, a method based on simultaneous registration of the static and the dynamic electrical resistances of the probe driven by the sum of dc and ac currents, and examples of its applications are described. Special attention is paid to the investigation of thin films deposited on thick substrates. The influence of substrate thermal properties on the measured signal and its dependence on thin film thermal conductivity and film thickness are analyzed. It is shown that in the case where layer thicknesses are comparable or smaller than the probe-sample contact diameter, a correction procedure is required to obtain actual thermal conductivity of the layer. Experimental results obtained for thin SiO $$_{\mathrm {2}}$$ and BaTiO $$_{\mathrm {3 }}$$ layers with thicknesses in the range from 11 nm to 100 nm are correctly confirmed with this approach. [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]