Water condensation on a surface strongly affects its effective emissivity, especially in the atmospheric window, a wavelength range essential for outdoor applications related to energetically passive cooling and heating. We study the evolution of emissivity of a silicon surface during dropwise and filmwise water deposition. The evolution of the spectral radiative properties shows that the increase in effective emissivity due to the growth of a droplet pattern is steeper than for a growing water film of equivalent thickness. The change of surface emissivity takes place in the first moments of condensation where droplets as small as 10 µm drastically impact the reflectance of the pristine surface. The upper limit of effective emissivity is reached for a droplet radius or film thickness of 50 µm. During dropwise condensation, effective emissivity is weighted by the drop surface coverage and then remains within an asymptotic maximum value of 0.8, while in case of filmwise condensation, it is shown to reach 0.9 corresponding to water emissivity. Micrometer-scale spatially-resolved infrared spectral images enable to correlate the spatial variation of spectral properties to the droplet size and localization. Such findings are of interest to the implementation of moisture-controlled emissivity tuning and radiative sky-coolers for dew harvesting. 

Decarbonized and low energy technologies for heat management and conversion are a key milestone for climate change mitigation. Cooling is one of the major fields where such technologies are required. Passive radiative cooling in its nighttime and daytime versions is therefore a promising technology to achieve those goals. We consider in this work a silicon based nanostructured material with outstanding visible and infrared radiation absorption and emission capabilities, Black Silicon (BSi), as a candidate for nighttime passive radiative cooling. We compare BSi with its flat silicon counterpart and different depths of BSi nanostructuration with respect to their radiative cooling power. We show that BSi cooling power is significantly larger than that of flat silicon by a factor up to 1.8 at 30°C with a cooling power of 75 W /m² and 140 W/m² for flat and black Silicon, respectively.

Meta-materials for thermal radiation control and conversion have been a very active field of research during the past decade in relation with several applications such as thermophotovoltaics, radiative cooling, thermal rectification, etc. Such meta-materials are often made of ordered structures such as 1D or 2D periodic structures, photonic crystals, multilayers, resonant cavities, and surface gratings among others. In this contribution, we will focus on a specific class of metamaterials made of randomly micro-structured silicon surfaces alternatively known as black silicon due its light trapping capabilities that make it appear black in the visible range. In recent years, we have studied the morphology [1] of such random structures fabricated using cryogenic deep reactive etching [4] and the influence of this morphology, the doping level, the doping profile [6-9] and surface functionalization [5] on the material radiative properties. We will show how these different parameters enable to significantly enhance the material absorptivity and emissivity over a wide mid-infrared spectral range. These parameters also offer many degrees of freedom to tune the spectral range of the desired property and optimize it for a given application. We will finally illustrate the potential of such meta-materials in different applications of thermal radiation harvesting, conversion and management [2, 3].

In this work, we focus on the impact of UV/vis light on the physical properties of polymers with azo chromophores. We study the relationship between light, the structure of azo molecules, and their thermal and optical properties. Our research objects are functionalized side-chain push–pull mono and bis-azo polymer thin films (80–457 nm). We induced structural changes in the azo molecules by photoisomerization (applying light at 405 nm and 300 mW/cm2), and we observed them by measuring the transmittance spectra. We characterized the local thermal conductivity (κ) of azo polymers by scanning thermal microscopy (SThM). We used quantum chemistry calculations to explain the thermal and optical reactions of the azo polymers to illumination by light. We also used photothermal radiometry to support SThM. We imaged the surfaces of azo polymers in their trans and cis states by atomic force microscopy. The SThM measurements show reversible κ changes for the azo polymer with CN groups, between 0.21 and 0.17 W m–1 K–1 for the trans and cis states, respectively.

Black silicon (BSi) is a synthetic nanomaterial with high aspect ratio nano protrusions inducing several interesting properties such as a very large absorptivity of incident radiation. We have recently shown that heavily doping the BSi in volume enables to significantly enhance its mid-infrared absorptivity and tune its spectral range of interest up to 20 μm. In the present letter, we explore the effect of surface doping on BSi radiative properties and its absorptance in particular since surface doping enables reaching even larger dopant concentrations than volume doping but at more limited penetration depths. We considered 12 different wafers of BSi, fabricated with cryogenic plasma etching on n- and p-type silicon wafers and doped using ion-implantation with different dopant types, dosages, and ion beam energies, leading to different dopant concentrations and profiles. The different wafers radiative properties, reflectance, transmittance, and absorptance are experimentally measured using Fourier transform infrared spectroscopy. We show that doping an n-type BSi wafer with phosphorous with a dose of 1017 atm/cm2 and an energy of 100 keV increases its absorptivity up to 98% in the spectral range of 1–5 μm. We propose a simple phenomenological explanation of the observed results based on the dopant concentration profiles and the corresponding incident radiation penetration depth. Obtained results provide simple design rules and pave the way for using ion-implanted BSi for various applications, such as solar energy harvesting, thermo-photovoltaics, and infrared radiation sensing, where both high absorptance and variable dopant concentration profiles are required.

Fabrication of a new SThM probe

We present a novel method to determine the three-dimensional (3D) temperature field of a radio frequency (RF) chip based on a 3D heterogeneous integration of GaN HEMT and RF-SOI technologies combining the advantages of both. It is composed of a stack of multiple layers of different materials on top of a SOI substrate. A RF GaN transistor is located at the top of the stack and acts as a heat source. We use several resistance temperature detectors (RTDs) embedded at different key locations in the stack coupled to an infrared (IR) thermography. When combined to a 3D numerical model, such methodology enables us to extract the necessary information in order to retrieve the temperature 3D distribution in the whole sample.

n order to elucidate the mechanism of enhancement of heat transfer in polymer composites, in this work, we investigated two types of polymer-carbon filler composites. This investigation was made using scanning thermal microscopy (SThM) with the Wollaston microprobe operated in active mode as a function of the carbon filler weight fraction within the polymer matrix. Samples consist of high-density polyethylene (HDPE) filled with 50 µm expanded graphite (EG) and polyvinylidene difluoride (PVDF) containing multiwall carbon nanotubes (MWCNTs). For HDPE/EG samples, SThM images allow the detection of zones with a thermal conductance larger than that of the matrix for the highest studied filler concentration. These zones correspond to EG filler agglomerations within the polymer and explain the observed enhancement of the thermal conductivity k of the HDPE/EG composite. For PVDF/MWCNTs samples, it is found that k increases from 0.25 W m−1 K−1 for pristine PVDF to 0.37 W m−1 K−1 for PVDF nanocomposites filled with 8 wt. % MWCNTs. This k variation vs filler concentration is found in good correspondence with that of the β phase relative percentage in the PVDF nanocomposites. This suggests that the observed heat transfer enhancement is rather due to the formation of β phase for PVDF/MWCNTs samples, resulting from the addition of MWCNTs than the addition of MWCNTs itself. Thus, tuning the thermophysical properties of polymer-based nanocomposites can establish new design laws to confer them specific thermal properties.

Thermal characterization of morphologically diverse copper phthalocyanine thin layers by Scanning Thermal Microscopy

The microstructural changes induced by ion implantation may lead to advantageous modifications of chromium disilicide’s (CrSi2) electrical and thermal prop- erties. As a potential thermoelectricmaterial, CrSi2 has attracted attention due to its semiconductor properties and high thermal stability. This contribution investigates the influence of different ion species and implantation conditions on the microstructure, electrical resistivity q and thermal conductivity j behaviors in amorphous CrSi2 thin films. * 260-nm-thick CrSi2 films were produced by mag- netron sputtering and deposited onto a SiO2/Si substrate. Samples were implanted at room temperature either with Ne or Al ions to form a concentration–depth pla- teau reaching a concentration of & 1.0 at.% (Ne), or & 0.008 at.% (Al). Ne and Al implantations were also performed with the targets heated at 250 °C. The microstructural modifications were characterized via TEM and STEM-EDX. The electrical resistivity q was measured by the van der Pauwmethod, and the thermal conductivity j measurements were obtained with SThM. The results obtained show that room temperature Al and Ne implantations cause the reduction of q as comparedto thepristinefilm. Incontrast, theqvalues are significantlyhigher forNe and Al implantations in heated substrates. The microstructure evolution, electrical and thermal behaviors are discussed considering the effects of radiation damage and the formation of dense nanocrystallite arrays during the implantation process.

We present in this study a novel way to determine the three-dimensional (3D) temperature field o f a R adio Fre- quency Silicon On Insulator (RF SOI) electronic chip, using several resistance temperature detectors (RTDs) embedded at different locations of the chip. The RTDs are designed and placed at different locations to experimentally obtain the temperature at key locations of the chip enabling the calibration of a multiphysical numerical model that provides the 3D temperature field in the whole chip under operating conditions. The obtained results provide useful insights on the role of different parameters (e.g. used materials properties, heat source power, substrate, boundary conditions, etc.) to engineers interested in the modelling and optimization of heat transport and thermal management of electronic chips for RF applications.

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.

Polystyrene (PS) thin films spin coated on either silicon or quartz substrates were prepared to investigate the thin film's thickness effects on both the heat transport and glass phase transition temperature of the PS. This characterisation has been achieved through a scanning thermal microscope setup (SThM) using doped silicon thermoresistive nanoprobe. Thermal conductance and thermomechanical analyses were performed. Our first results show that thermomechanical analysis measurement on ultrathin films on a substrate depends on the substrate thermal properties and confirm that the PS phase transition temperature is dependent on the interactions between PS ultrathin films and their respective substrates.

Spatially localized measurement of isotropic and anisotropic thermophysical properties by photothermal radiometry

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 conduc- tive 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 concentra- tion. 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.

The irradiation of polycrystalline isotropic graphite with swift heavy ions (4.8 and 5.9 MeV/u Au and 4.8 MeV/u U) in the elec- tronic energy-loss regime leads to significant structural damage and modifications of thermophysical properties. Analysis of the irradiated sample cross-sections by Raman spectroscopy and SEM provide clear evidence of defect production and at high fluences of a transition towards a glassy carbon-like material. To quantify the radiation-induced effects on thermal effusivity and thermal conductivity, the technique of frequency domain photothermal radiometry in combination with a three-layer model was success- fully applied. The analysis of the relative effusivity degradation shows a single ion impact mechanism of the accumulated ra- diation damage with a track diameter of 5–6 nm. Such a tremendous degradation can lead to inefficient heat dissipation and finally to thermomechanical failure of graphite beam dumps and production targets in ion accelerators. The risk due to largely reduced thermal conductivity values in high-dose environments have to be considered when designing and esti- mating the long-term operation conditions of beam-intercepting devices for the new generations of high power accelerators such as the Facility for Antiproton and Ion Research (FAIR) in Darmstadt, the High Luminosity Large Hadron Collider (HL–LHC) and the Future Circular Collider (FCC), both at CERN in Geneva.

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.

In summary, this work presented the thermal characterization of the interfaces resistances for two types of graphene monolayers epitaxially grown on a SiC(0001) substrate. Each sample denotes a type of graphene/SiC interface with either a buffer layer or without a buffer layer. The TBRs measured values indicate that the presence of an insulating BL reduces the interfacial heat conduction, while the presence of more mobile charge carriers in the QFSMG slightly enhance, via electronic contributions, the heat transfer between the free-standing graphene monolayer and the substrate. In other words, despite a higher phonon-phonon interaction though the BL/SiC interface, the electronic contribution between the QFSMG and SiC takes over the interfacial heat transfer as in the case of metal/metal contact compared to the semiconductor/semiconductor ones, where the phononic contribution decreases but the interface conductance increases with the electronic one58. Furthermore, the accordance of the present thermal results with previous electrical ones proves the capability of epitaxial monolayer graphene with hydrogen intercalation (without a BL) to enhance the thermal performance of graphene-based thermal devices. Therefore, these materials can also be used as cornerstone for epitaxial graphene-based nanoelectronics for better heat management applications like in thermal diodes with rectifying properties or thermal transistors. 

Thermophysical parameters of azobenzene-containing polymer thin films with different substituents in the side-chain, and guest-host sys- tems were investigated. In this work, the PTR and the SThM were proposed as suitable methods for thermophysical characterization of organic samples. The combination of these thermal techniques with AFM imaging provides complementary information about the heat transport properties of azobenzene polymer thin films. These observations lead to the conclusion, that the polymer system, into which the azobenzenes are incorporated, can affect the heat transport in azo dye thin films. This is accomplished through the following ef- fects: the first one is related to thermophysical properties of the polymer matrix itself, into which azobenzenes are embedded, and the second one is caused by the spatial organization of the active residues, which have higher stability and restriction of the rotation in side-chain position than in the guest-host system. The obtained thermophysical properties, which are responsible for the heat transport and its storage, can be very useful in designing azobenzene polymer thin films based devices through accurate knowledge of their thermal behavior in the system.

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 di- rectly 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.

Passionné, possédant une curiosité scientifique et une capacité d’apprentissage et d’adaptation rapide, j'aime collaborer avec des scientifiques ou enseignants de différentes disciplines, pour développer de nouvelles compétences et résoudre de nouveaux défis. Par mon travail je vise à encourager le changement afin d'aider la communauté scientifique dans sa recherche vers un avenir meilleur.