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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 3 omega method is one of few reliable measurement techniques for thermal characterization of micro to nanoscale suspended wires or fibers and has been applied for measurements of carbon nanotubes and silicon nanowires. However, the models described in the past were either complicated for analysis or simplified from a more complete solution. In addition, the past models cannot be implemented directly when using a more reliable measurement configuration with a Wheatstone bridge. In this work, a simpler, explicit model, is developed to describe the heat transfer process through a suspended wire for measurement of its thermal properties. Generic trends and values of the 3ω harmonic voltage amplitude and phase responses clearly indicate the frequency limits for thermal conductivity and heat capacity determination and ideal conditions for thermal diffusivity estimation. Based on a sensitivity analysis, these limits are confirmed and appropriate frequency ranges for thermal conductivity and diffusivity are recommended. Radiation influence on the measurement results is quantified and correlated to a dimensionless radiation parameter. Two methods are presented to determine sample thermal properties independent of lateral heat losses and validated by numerical experiments using COMSOL. Uncertainty analysis was also derived by Taylor series expansion with calculated parameter sensitivities.

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 in-depth thermal conductivity $$(k)$$ profile in a proton-irradiated zirconium carbide (ZrC) sample has been directly measured for the first time using scanning thermal microscopy. The measured $$k$$ profile is shown to be independent of imperfect sample topography and constructed by overlaying several thermal line profiles. The shape of the numerically calculated, irradiation damage profile compares well with the resulting profile of $$k$$ degradation. A relatively thick and constant diminished $$k$$ value is revealed through the majority of the damaged zone, consistent with characteristics of proton irradiation. A sharp transition of the thermal signal to the non-irradiated material was found at the rear of the profile, with no indication of the very thinly peaked damage seen in the numerically calculated damage profile. A rudimentary calibration of the measured thermal signal on the sample indicates $$\approx $$ 66 % degradation of $$k$$ in the proton-irradiated zone of the ZrC sample.

This work aimed at thermal transport characterization of high density polyethylene (HDPE) filled with 5 and 50 mu m expanded graphite (EG) particles and with 0.4 mu m unexpanded graphite (UG) particles. Sample platelets were produced by melt mixing followed by compression molding. Thermal conductivity k was determined by combining measurements of density, specific heat capacity and thermal diffusivity, the latter by modulated photothermal radiometry (PTR). Starting from an effective medium approximation model, we derived a linearized expression for the effective k of composites with low particle charge. It explains the unusually high experimental k values (up to four-fold increase) as the effect of strongly non-spherical EG particles (aspect ratio 1/p = 110-290). Larger particle sizes produce higher k enhancement, while the interfacial thermal resistance (R-bd = 21.10(-7) m(2).K/W) has an opposite effect. The same model is consistent with experimental k for low particle charge HDPE/UG composites. At higher particle charge the model fails due to particle interaction leading to validity break of the effective medium approximation. (C) 2012 Elsevier Masson SAS. All rights reserved.

Samples of a titanium–aluminum–vanadium alloy (Ti6Al4V) having different values of roughness are studied by photothermal radiometry. Among the numerous studies carried out on the influence of surface roughness on the photothermal signal, those that relate the air/substrate interfacial volume to an apparent homogeneous layer with effective thermal properties are considered. For such a system, a one-dimensional heat transfer model can successfully be used to describe the response to harmonic excitation up to high frequencies where the thermal diffusion length is of the order of magnitude as the roughness. The breakdown of the effective layer model at high frequencies has prompted some authors to develop more sophisticated methods to describe the complex phenomena encountered. In this study, it is proposed to keep the simple, effective layer model, but also to modify it by including an effective optical absorption coefficient and a reflection coefficient closely related to the surface characteristics. The effective layer becomes semitransparent to the excitation as it turns thermally thick (thickness/thermal diffusion length >1). The apparent thickness of the layer is shown to be directly related to the average difference between the maximum peak and the minimum trough of the surface profile and linearly correlated with the average roughness. This model can be used over a wide range of roughnesses.

In most metal matrix composites (MMCs) interfaces are decisive but hard to manipulate. Especially copper–carbon composites can exhibit excellent mechanical and thermal properties only if the Cu/C interface is modified by an optimised interlayer. Due to the excellent thermal conductivity and mechanical stability of diamond this form of carbon is preferred as reinforcement in heat sink materials (copper–diamond composite) which are often subjected to severe thermal and mechanical loads. In the present case niobium and boron interlayers of various thicknesses were deposited on diamond and vitreous carbon substrates by magnetron sputter deposition. After the coverage of all samples by a copper film, a part of the samples was subjected to heat treatment for 30 min at 800 °C under high vacuum (HV) to simulate the thermal conditions during the production of the composite material by uniaxial hot pressing. De-wetting during heat treatment leads to the formation of holes or humps in the Cu coating. This effect was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). A comparison of time-of-flight secondary ion mass spectroscopy (TOF SIMS) profiles of heat treated samples with those of as deposited ones showed the influence of interdiffusion during the heating process. Diffusion behaviour and chemical composition of the interface were also studied by cross sectional transmission electron microscopy (X-TEM) investigations using focused ion beam (FIB) cut samples. The thermal contact resistance (TCR) of the interface was calculated from results obtained from modulated infrared radiometry (IR). Thin interlayers suppressed de-wetting most effectively and consequently the TCR at the Cu–diamond interface was found to decrease. Therefore they are promising candidates for optimising the Cu–diamond interface., Highlights ► The interface in the Cu–C system was modified by boron and niobium interlayers. ► Thermal treatment of the samples simulated the production process of Cu–diamond MMCs. ► The TCR of the interface was investigated by modulated infrared radiometry. ► Chemical- and diffusion processes were investigated by TEM of interlayer cross sections. ► Very thin interlayers improve the mechanical and thermal interfaces.

The manipulation of mechanical and thermal interfaces is essential for the design of modern composites. Amongst these are copper carbon composites which can exhibit excellent heat conductivities if the Cu/C interface is affected by a suitable interlayer to minimize the Thermal Contact Resistance (TCR) and to maximize the adhesion strength between Cu and C. In this paper we report on the effect of boron based interlayers on wetting, mechanical adhesion and on the TCR of Cu coatings deposited on glassy carbon substrates by magnetron sputtering. The interlayers were 5 nm thick and consisted of pure B and B with additions of the carbide forming metals Mo, Ti and Cr in the range of 5 at.% relative to B. The interlayers were deposited by RF magnetron sputtering from either a pure B target or from a composite target. The interlayer composition was checked by Auger Electron Spectroscopy and found to be homogenous within the whole film. The system C-substrate/interlayer/Cu coating was characterized in as deposited samples and samples heat treated for 30 min at 800 °C under High Vacuum (HV), which mimics typical hot pressing parameters during composite formation. Material transport during heat treatment was investigated by Secondary Ion Mass Spectroscopy (SIMS). The de-wetting and hole formation in the Cu coating upon heat treatment were studied by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). The adhesion of the Cu coating was evaluated by mechanical pull-off testing. The TCR was assessed by infrared photothermal radiometry (PTR). A correlation between the adhesion strength and the value of the TCR which was measured by PTR was determined for as deposited as well as for heat treated samples., Research Highlights ► Investigation of thermomechanical properties of carbon/metal interfaces. ► Application of infrared photothermal radiometry (PTR) for layered media. ► Correlation of mechanical and thermal properties for carbon/metal interfaces.

The state-of-the-art concerning measurements of bulk and localized thermophysical properties with the modulated hot wire and 3ω detection is presented in the introduction. We developed a general theory of ac hot wire method formulated in terms of dimensionless parameters, describing distributed or localized heat exchange mechanisms. Simple analytical solutions are found for special cases of experimental relevance, which are illustrated by novel applications in ac anemometry, thermophysical characterization of nanofluids and SThM investigation of various micro-structured systems.

Thermographic lock-in investigation of thermal interaction between a thermal probe used in scanning thermal microscopy and a thin composite sample is used for estimating the relative fraction of heat flux transferred to the sample by conduction through the air and by direct local contact. The latter is determined by vector subtraction of out-of-contact data from in-contact data, for different modulation frequencies. It represents only 8% of the total modulated heat flux dissipated by the probe, and is frequency-independent. The heat transfer by air conduction varies between 23% and 6% for f = 0.2...40 Hz with a broad equivalent heat source radius of r 0.2 mm.

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.

The back photopyroelectric (PPE) configuration, with opaque sample and thermally thick sample and sensor, was applied in order to obtain room temperature values of the thermal diffusivity of some liquid mixtures. The methodology is based on a sample's thickness scan, and not on a frequency scan as is usually the case. The possibility of precisely controlling the sample's thickness variation leads to very accurate and reproducible values for the thermal diffusivity. The method is proposed as an alternative for evidencing molecular associations in binary liquid mixtures.

We introduce the theoretical description of 3ω signal from the Wollaston probe of a scanning thermal microscope (SThM) in terms of an equivalent low-pass filter. We performed thermal conductivity k measurements with lateral resolution of about 100 nm. The first application concerns NiTi shape memory alloys microstructured by focused ion beam implantation. Local martensite to austenite structural phase transition has been identified upon heating the sample from room temperature to 100°C The 3w signal changes were -1.95% in amplitude and 0.6° in phase, corresponding to thermal conductivity k increase of 3.5%. The second application consists of static measurement of local k on points situated on flat faces of bare diamond crystallites 300 μm in diameter, and on crystallites coated with Cr, Cu and Cu/Cr layers with thickness in the range 0.5-30 μm. The high k advantage of bare crystallites is lost upon coating the particles, but the thermal barrier depends also on the specific configuration when the particles are in contact to one another in materials obtained from such powders.

We propose a novel approach in scanning thermal microscopy of layered samples. The thermal probe (ThP) (Wollaston wire) acts as a local a.c. heat source at the front of a sample layer deposited on a pyroelectric (PE) sensor. The PE signal is proportional to the heat wave transmitted through the sample. The ThP and PE signals can be used to generate complementary thermal conductivity maps and with some restrictions, thermal diffusivity maps of the sample. Additionally, the topography map is obtained in the usual way from the atomic force microscope. We give the theoretical background for the interpretation of PE signal obtained at low and at high frequency, and we demonstrate that it carries information on the thermal diffusivity of a test sample (12 μm thick PET polymer sheet). Finally, we discuss the contributions of heat transfer channels between ThP and sample, and the role of contact thermal resistance.

In this work, we give a theoretical description of and experimental results on the thermal probe frequency response, and we determine its time constant and the calibration factor. We identify two operation modes (isothermal and adiabatic) separated by a crossover frequency and we discuss the conditions for maximum sensitivity to sample thermal properties. Finally, the theoretical temperature distribution along the sensing wire is compared with the experimental profile acquired by infrared thermography on an upscaled wire.

We describe the order parameter S(T) of liquid crystals in the nematic state by a four-parameter expression which is consistent with mean-field theory. A two-step fitting method is applied to high resolution refractive index data for the nCB (n=5–8) liquid crystals, processed according to Vuks and Neugebauer models. We find quasi-universal parameter values. The average critical exponent is β=0.241±0.012, in agreement with the mean-field prediction for a tricritical point. Thus the temperature dependence of the nematic order parameter is quasi-tricritical in all cases. The order parameter at the effective transition point T** is S**=0.143±0.05 and the relative polarizability anisotropy Δα/⟨α⟩=0.69±0.03. We show that earlier results using the Haller approximation were too low (β=0.16–0.19) because of the assumption S**=0 and T**=T NI. The chosen fitting function provides the best description of S(T) both close and far from T**. For all parameters, the alternating odd–even effect with the number of C atoms i...

Using different photopyroelectric configurations, we have experimentally studied the frequency dependence of the thermal properties of the glass former glycerol near its glass transition. With our method, an unambiguous and separate determination of the frequency dependence of the specific heat capacity and the thermal conductivity is possible. Within experimental uncertainty, in the frequency range from 0.1 Hz to 1100 Hz, unlike the specific heat capacity, the thermal conductivity of glycerol does not show any frequency dependence through the glass transition region. We have also extended the thermal spectroscopy frequency range to 100 kHz, which is one decade higher than other methods.

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(-1) to 2.5 x 10(6) cells mL(-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(-1)) and a highest (nearly 10(4)) 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 103) milk as a strong scatterer. Finally, one has monitored the progress of a spontaneous spoilage process in the inoculated juices stored at 5degreesC under aerobic conditions.

Starch is one of the most important carbohydrate sources in human nutrition. For the thermal analysis of starch, techniques such as differential scanning calorimetry have been extensively used. As an alternative, we have applied a photopyroelectric (PPE) configuration and adiabatic scanning calorimetry (ASC) to study the thermal properties of starch–water systems. For this study we used nixtamalized corn flour and potato starch with different quantities of distilled water, in order to obtain samples with different moisture content. By using PPE and ASC methods we have measured, for each technique separately, the heat capacity by unit volume (ρcp) at room temperature for a corn flour sample at 90% moisture. The obtained values agree within experimental uncertainty. By using these techniques we also studied the thermal behavior of potato starch, at 80% moisture, in the temperature range where phase transitions occur. In this case the PPE signal phase could be used as a sensitive and versatile monitor for ph...

The optothermal window detection method at 488 nm was used to monitor on-line the concentration of trans-ß-carotene that was added to several vegetable oils after treating them at 200 °C 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-ß-carotene and the duration of thermal treatment. The rate constant for the decay of trans-ß-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.

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 parameter...

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 −1 K −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.

We report on the first frequency-dependent measurements of thermal effusivity of glass-forming liquids using the photopyroelectric (PPE) method. The advantage is higher modulation frequency capability than with previously reported methods. On the basis of our present results (up to 30 kHz), we find in the dispersion temperature region of glycerol a clear frequency-dependence of the real part and a non-zero, frequency-dependent imaginary part of the thermal effusivity, which is consistent with literature data. The experimental data reveal that the full exploitation of the high-frequency experimental results requires to take into account also the piezoelectric component of the signal caused by the thermoelastic response of the sample. The high-frequency limit of this PPE experiment may be extended by a modified cell design and by using another pyroelectric material instead of a lithium tantalate crystalline sensor.

We report on a new optoelectronic device for monitoring human blood sedimentation and for the determination of the erythrocyte sedimentation rate (ESR), based on spectroscopic contrast between plasma and sediment. The radiation generated by a yellow light-emitting diode (LED) at 570 nm is transmitted through the plasma volume formed during the sedimentation process, while the sediment itself is practically opaque at this wavelength. The transmitted light flux measured by a photodiode is proportional to the height of the plasma column in the cell, so that complete time evolution of plasma height can be obtained. †On leave from the National R&D Inst. for Isotopic and Molecular Technology, P.O. Box 700, 3400, Cluj-Napoca 5, Romania The time derivative of these curves yields the sedimentation velocity that passes through a maximum. The device was tested using an aqueous suspension of red latex spheres. The ESR obtained for human blood correlates with that determined by the conventional Westergreen method, but...

A simple pyroelectric sensor is presented for on-line monitoring of Rayleigh-Benard -Marangoni convective instabilities in methanol, ethanol and isopropanol aqueous solutions, caused by alcohol evaporation from the free surface of the liquid. The signals measured at the liquid-substrate interface represent thermal fluctuations of the order of 10−2 K, having a frequency spectrum between 0.08 Hz and 0.4 Hz. The numerical estimation of the Rayleigh and Marangoni numbers leads to the conclusion that the latter is dominant in the conditions of the experiment. The amplitude and duration of the thermal noise are investigated as a function of alcohol concentration. An attempt is made to determine characteristic invariant quantities.

Introduction Fruits and vegetables used in various processed products, as well as in fresh market sales, are expected to comply with the certain quality standards. However. quality is everything but a single parameter state and, therefore, a variety of subjective and objective measurements are usually performed. In particular, the firmness and maturity are considered important attributes to the quality of fruits and vegetable crops. Presently. these parameters are usually determined either destructively (by a pressure tester) or by means of ultrasound; a development of alternative methods has been strongly encouraged in the recent years.

The photopyroelectric (PPE) technique is usually used as a contact photothermal method to characterize thin solid materials. We performed theoretical and experimental investigations of a non-contact PPE configuration to analyse the effects of an air gap between the thermal source and a PVDF sensor using a modulated laser beam as optical excitation. The detection of the thermal wave developed within the sample was possible across an air gap of up to 15 mm. The PPE signal was described well thanks to a two-dimensional (2D) model with axial symmetry, which could be reduced in some particular cases to a simpler 1D propagation model. Border effects can be artificially taken into account in the calculations by increasing the adiabatic lateral limits in the model, keeping the active sensor area constant. Despite there being strong attenuation with increasing modulation frequency, the PPE method can be used as well as can other non-contact photothermal techniques. New perspectives for the application of the PPE technique have been opened.