Search

Morphologically diverse copper phthalocyanine (CuPc) thin layers were thermally characterized by Scanning Thermal Microscopy (SThM). The organic layers with thicknesses below 1 µm were deposited by physical vapor deposition in a high vacuum on the N-BK 7 glass substrates. Four set of samples were fabricated and studied. Atomic Force Microscopy imaging revealed strong differences in the surface roughness, mean grain size/height, as well as distances between grains for the CuPc layers. For quantitative thermal investigations, three active SThM operating modes were applied using either a Wollaston thermal probe (ThP) or KNT ThP as thermal probe heated with a DC, an AC (3ω-SThM) current or their combination (DC/AC SThM). Meanwhile, qualitative analysis was performed by thermal surface imaging. The results of this study revealed a correlation between the morphology and the local thermophysical properties of the examined CuPc thin layers. It was found that the heat transport properties in such layers will deteriorate with the increase of the surface roughness and porosity. Those results can be a valuable contribution to the further development of phthalocyanine-based devices.

International audience; 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, Rth, 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.

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.

The goal of this work is to establish the heat flux sharing among different heat exchange channels in scanning thermal microscopy (SThM), basing on experimental data. To this end, dc or modulated excitation with 3-omega detection at the third harmonic is applied to two types of resistive (Wollaston-, WThP and nanofabricated-, NThP) thermal probes. The main difference between the WThP and the NThP is the absence of a thermal coupling zone between the active zone and the heat sink in the WThP. Probe responsivities in dc or ac regime are defined, which depend only on total thermal conductance of probe-sample system. It is shown that in SThM it is not possible to discriminate between heat fluxes leaking across the air to the ambient and to the sample. Because of their interdependence, only their sum can be determined with the ThP alone. Replacing the sample by a pyroelectric heat flux sensor (PES), its 2-omega signal yields the ratio of the flux received through the contact to that received through the air. Eventually, the complete heat flux sharing among different channels is determined, for the studied high conductivity sample. For both probes, about 60 % of the dissipated power leaks through the cantilever to the probe holder. The heat flux through the contact represents 19.5 % for the WThP and 11.4 % for the NThP. The remaining heat flux is lost to the ambient or to the sample via the air. The corresponding thermal conductance values are determined in successive steps in vacuum (V), in air (A), out of contact (N) and in contact (C) using thermal quadrupole representations. The procedure requires only the calibration of the ThP and not that of the PES. For thermal conductivity measurements, the reference configuration (N) offers superior dynamic range to (A) one.

Acta materialia 184, 187 - 198 (2020). doi:10.1016/j.actamat.2019.11.037, Published by Elsevier Science, Amsterdam [u.a.]

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 (κeff) was determined directly. Estimation of the thermal boundary resistance (Rth) between particular alloy layer and its substrate enabled determination of the real κ of the layer with its thermal diffusivity (α) and effusivity (e) 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 e values, and lower Rth in comparison to those deposited on SiO2. The κ values for W-Re and Mo-Re layers deposited on Si and SiO2 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.

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., Science Grant Agency VEGAVedecka grantova agentura MSVVaS SR a SAV (VEGA) [2/0010/18, 2/0093/16], This work was partially supported by the Science Grant Agency VEGA, project no. 2/0010/18, and 2/0093/16 (Slovakia).

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).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 electro...

This work focuses on the nonlinear electrochemical (EC) activity of fluids and its impact on the accuracy of their thermophysical property measurements using the transient or periodically modulated hot wire method (THW or 3ωHW). This underestimated source of errors becomes crucial for nanofluids, where the thermal conductivity enhancement coefficient (k/k0 − 1) is small, requiring high-resolution measurements (below 1%). Theoretical predictions combining thermal and EC nonlinearities are confronted to experimental results obtained with two variants of 3ω cells. Water-based nanofluids (TiO2 and Al2O3) and water solutions of acetic acid, sodium hydroxide and sodium chloride were tested. The two classes of samples show different 3ω amplitude and phase behaviors, which is attributed to additional electrokinetic effects in nanofluids. For all studied samples an anti-correlation between the magnitude of the harmonics and the dc electrical resistivity of the samples was observed. A simple estimation method of 3ωHW errors is proposed consisting in measuring the 2nd harmonic in addition to the 3rd one. The combination of 2nd and 3rd harmonics allowed determining the nonlinearity coefficient which quantitatively characterizes the EC activity of samples. Eventually, the radical solution to avoid the discussed sources of errors is to use an electrically insulated hot wire sensor.

International audience; The methacrylic polymer films containing azobenzenes with different peripheral substituents in the form of side-chain (P) and guest-host (A) systems were thermally investigated. The azo polymers with thicknesses below 1 μm were deposited on glass substrates using spin coating method. Photothermal radiometry (PTR) and scanning thermal microscopy (SThM) were used for determination of thermophysical properties of polymer samples. The topography of studied compounds was examined by atomic force microscopy (AFM). The results showed the influence of peripheral substituents as well as that of the polymer system on the thermophysical properties of studied azo polymers. Thermal diffusivity (α) extracted from PTR measurements is between 5.56 10−8 m2s−1 and 11.3‧10−8 m2s−1 for A samples, 2.76‧10−8 m2s−1 and 7.62‧10−8 m2s−1 for P samples depending on the substituent. Likewise, thermal effusivity (ε) changes from 474 Ws1/2m−2K−1 to 532 Ws1/2m−2K−1 for A, and from 473 Ws1/2m−2K−1 to 564 Ws1/2m−2K−1 for P samples. The volumetric heat capacity (Cv) and in-depth thermal conductivity (κ) were calculated on the base of PTR results. Local κ obtained from SThM measurements is between 0.15 Wm−1K−1 and 0.22 Wm−1K−1 due to different substituents and polymer system. Thermal studies performed in this work provide valuable information for engineers designing azobenzene polymer thin films based devices and supplement the knowledge of their thermal behavior in the system.

This study aims to investigate experimentally the effects of aspect ratio (length/diameter ratio) and concentration of multiwalled carbon nanotubes (MWCNTs) on thermal properties of high density polyethylene (HDPE) based composites. The aspect ratios of two types of MWCNT fillers are in the range of 200-400 and 500-3000. Composite samples were prepared by melt mixing up to weight fraction of 19% filler content, followed by a compression molding. Measurements of density, specific heat and thermal diffusivity (by modulated photothermal radiometry, PTR) were performed and effective thermal conductivities k(e) of nanocomposites were calculated using these values. The results show that the composites containing MWCNTs with higher aspect ratio have higher thermal conductivities than the ones with lower aspect ratio. In terms of conductivity enhancement k(e)/k(m) - 1, the results indicate that MWCNTs with higher aspect ratio provide three to fourfold larger enhancement than the ones with lower aspect ratio, at low filler concentrations. (C) 2015 Elsevier Ltd. All rights reserved.

Selecting a heat transfer fluid (HTF) depends on the value of thermophysical parameters. In response to the need for innovative heat transfer fluids in concentrated solar power (CSP) plants, vegetable oil offers a promising solution. The relevant thermophysical properties are density, thermal conductivity and specific heat capacity. Moreover, a rheological property, the dynamic viscosity is also relevant. The objective of this work was to study and compare these properties for seven different vegetable oils (rapeseed, soybean, sunflower, palm, copra, cotton and jatropha) in the temperature range from ambient to 250 °C. For all the properties studied, the values evolved with increasing temperature and were influenced by the fatty acid composition of each vegetable oil. Experimental results are compared with those in the literature and are found to be consistent. The temperature dependencies are correlated to temperature using polynomial equations. The correlations presented here may be useful as a database for the selection of innovative heat transfer fluids.

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.

The metal phthalocyanines (MePc) were thermally investigated using photothermal radiometry (PTR) and scanning thermal microscopy (SThM). Copper phthalocyanine and lead phthalocyanine layers with different thicknesses were deposited on Si (111) wafer and on glass substrates by physical vapor deposition in a high vacuum. SThM measurements allowed the determination of local thermal conductivity (κ) of studied MePcs. This method did not require any preliminary sample preparation. The influence of the substrate thermal properties on SThM thermal signal was taken into account for estimating the true value of κ for the organic layer. For PTR investigations MePcs were coated with an opaque aluminum film. Photothermal measurements enabled determination of the in-depth κ. To complement thermal investigations of MePc layers, quantitative analysis of surface morphology properties was performed by an atomic force microscopy. The results of thermal investigations were explained on the base of different surface morphologies of examined organic layers.

International audience; In this paper, the simultaneous measurement of out-of-plane thermal diffusivity and effective infrared absorption coefficient of an IR semi-transparent GaAs wafer using infrared lock-in thermography technique (LIT) is presented. The method relies on analysis of the generated LIT phase images recorded at different modulation frequencies, using the thermal wave model in the transmission configuration. The out-of-plane thermal diffusivity and effective infrared absorption coefficient are estimated from the best fit of the theoretical model to the experimental data. The obtained values are in good agreement with those obtained by supplementary measurement using the modulated photothermal infrared radiometry technique (PTR) in the reflection mode, and also with data reported in the literature. In addition, simple modification of the LIT experiment set up allows one to determinate the in-plane thermal diffusivity of n-GaAs wafer. It was found that in-plane and out-of-plane thermal diffusivities of the GaAs wafer are very close, as expected, within the limit of measurement errors. The results show that the LIT technique in transmission configuration can provide spatial information about both the (effective) infrared absorption coefficient and thermal diffusivity of semiconductor crystals.

In a previous work, a complete model has been presented for the thermal property characterization of suspended wires using the 3ω technique. As validation of the model and comparison of the effects of measurement configuration, several samples are measured in a vacuum chamber (

The aim of this study is to build an active auto-zeroing circuit for the three omega thermal conductivity measurement method to overcome the disadvantage of the method. For this purpose an auto-zeroing algorithm is implemented which consists of two stages; simply reading the voltage drops on lower hand side resistances with high resolution and setting the resistance of the digital potentiometer to the value calculated through proposed algorithm using the difference of acquired voltages. As a result, a probe with auto-zeroing capability is developed successfully. Usage of the new probe will increase the reliability of three omega measurement results by reducing the effects of uncertainties arising from manual zeroing routines.

International audience; Following the growing need for innovative heat transfer fluids in concentrated solar power (CSP) plants, the thermal conductivities of different vegetable oils (rapeseed, soybean, sunflower, palm, copra, cotton and jatropha) were measured in the temperature range from ambient to 230 °C relative to a reference oil. The small differences in the obtained thermal conductivities are influenced by the fatty acid composition. For balanced saturated/unsaturated fatty acids composition, the average thermal conductivity decreases from 0.167 W m−1 K−1 at 20 °C to 0.137 W m−1 K−1 at 230 °C. The use of a reference synthetic oil makes the calibration of the thermal probe unnecessary. The used method is based on a hot wire thermal probe with ac excitation and 3ω lock-in detection and has a long-term relative error of 1.2% and absolute accuracy of 2%. It allows measuring in real-time, continuously and independently, the thermophysical properties of oils for thermal applications.

International audience; Kapitza thermal resistance is determined using high-frequency photothermal radiometry (PTR) extended for modulation up to 10 MHz. Interfaces between 50 nm 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.

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 ke 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 ke 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.

Photothermal (PT) methods are able to characterize effective thermooptical properties of particulate two-phase polymer nanocomposites. Modulated laser light is used to generate a thermal response of the sample which contains information on its thermophysical and optical properties. Thus frequency- or time-domain PT spectra can be acquired by various PT methods. The sample structure, which is beyond the reach of pure optical methods, becomes accessible to a depth known as the thermal diffusion length. This frequency-dependent parameter is the basis of PT tomography. After the presentation of a literature survey on PT studies on composite materials, the authors’ results on polymer-based (high- and low-density polyethylene, EVA, PLA, PBS) composites with carbon (expanded and unexpanded graphite, carbon nanotubes), montmorillonite clay, and Al particles are presented. Thermooptical parameters of nanocomposites are then extracted and correlations are made with the different aspect ratios of the fillers in the frame of mean field theory models, allowing for the estimation of interfacial thermal resistance.

The influence of defective areas in the interface on the correlation between the thermal interface conductance and the mechanical adhesion strength was investigated on as-prepared and heat-treated samples of copper-coated carbon flat specimens with different bonding layers between the copper film and the substrate. The thermal interface conductance was determined by frequency-domain photothermal radiometry. The mechanical adhesion strength of the film coating was deduced from pull-off experiments. The imperfect interfaces were modeled by two different values for the thermal interface conductance, $$G_{1}$$ and $$G_{2}$$ , which co-exist at different areas on the interface and are weighted according to their areas, $$A_{1}$$ and $$A_{2}$$ . The model parameters were determined by adjusting the frequency dependence of the normalized phases and phase differences of the PTR signals from as-prepared and heat-treated samples. The total thermal conductance of the interface was found to exhibit a correlation with the adhesion strength for most of the heat-treated samples whereas, among the as-prepared samples, considerable deviations from such a trend exist. The observations are explained by the impact of supplementary stress on the adhesion strength measurements which are due to the strain developed during the preparation process at the interface. The interfacial stress and strain are mostly released during thermal annealing. A semi-empirical formula was developed that describes the impact of the defective areas on the adhesion strength using the experimentally determined thermal model parameters.

Spectrally resolved modulated infrared radiometry (SR-MIRR) with super-band gap photoexcitation is introduced as a self-consistent method for semiconductor characterization (CdSe crystals grown under different conditions). Starting from a theoretical model combining the contributions of the photothermal (PT) and photocarrier (PC) signal components, an expression is derived for the thermal-to-plasma wave transition frequency ftc which is found to be wavelength-independent. The deviation of the PC component from the model at high frequency is quantitatively explained by a quasi-continuous distribution of carrier recombination lifetimes. The integral, broad frequency band (0.1 Hz–1 MHz) MIRR measurements simultaneously yielded the thermal diffusivity a, the effective IR optical absorption coefficient βeff, and the bulk carrier lifetime τc. Spectrally resolved frequency scans were conducted with interchangeable IR bandpass filters (2.2–11.3 μm) in front of the detector. The perfect spectral match of the PT an...