Nicolas Horny

Spectrally resolved modulated infrared radiometry of photothermal, photocarrier, and photoluminescence response of CdSe crystals: Determination of optical, thermal, and electronic transport 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...

Thermal Conductivity Profile Determination in Proton-Irradiated ZrC by Spatial and Frequency Scanning Thermal Wave Methods

Using complementary thermal wave methods, the irradiation damaged region of zirconium carbide (ZrC) is characterized by quantifiably profiling the thermophysical property degradation. The ZrC sample was irradiated by a 2.6 MeV proton beam at 600 °C to a dose of 1.75 displacements per atom. Spatial scanning techniques including scanning thermal microscopy (SThM), lock-in infrared thermography (lock-in IRT), and photothermal radiometry (PTR) were used to directly map the in-depth profile of thermal conductivity on a cross section of the ZrC sample. The advantages and limitations of each system are discussed and compared, finding consistent results from all techniques. SThM provides the best resolution finding a very uniform thermal conductivity envelope in the damaged region measuring ∼52 ± 2 μm deep. Frequency-based scanning PTR provides quantification of the thermal parameters of the sample using the SThM measured profile to provide validation of a heating model. Measured irradiated and virgin thermal con...

Kapitza thermal resistance studied by high-frequency photothermal radiometry

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.

Interface resistance in copper coated carbon determined by frequency dependent photothermal radiometry

The heat transfer in copper-carbon flat model systems was studied by frequency dependent photothermal radiometry. A novel approach which relies on the frequency dependence of the photothermal signal phase and amplitude at intermediate frequencies was introduced to determine the thermal interface resistance between the Cu-film and the substrate. The frequency dependent amplitude and phase of the photothermal signals were analyzed in the frame of a model of a one- dimensional heat flow perpendicular to the film plane. The interface resistance of the investigated CuC-sample with a Ti-bonding layer was found to increase by a factor two on heat treatment.

Pyrometry techniques for temperature monitoring in simulated LOCA (Jules Horowitz Reactor)

In many experiments carried out in Material Testing Reactors the temperature is a key parameter for understanding the behavior of nuclear materials. Its estimation, depending on the localization in the reactor, may not be easy. The Light-water One Rod Equipment for LOCA Experimental Investigations (LORELEI) test device which will be implemented on the Jules Horowitz Reactor (MTR) illustrates this difficulty. In this device the CEA will simulate a Loss Of Coolant Accident (LOCA) transient. The aim is essentially to analyze cladding materials behavior in such a hard environment where steam oxidation is enhanced by temperatures exceeding 800°C. The ability to gauge the fuel rod temperature without contact and with a precision of ±10°C and in real time (1Hz) would constitute a great advantage for understanding and modeling the phenomena. This paper aims at describing the different pyrometry techniques that are studied for such a sensor.

Optical pyrometry measurement on oxidized Zircaloy-4 cladding

In order to improve the safety of nuclear power plant, loss-of-coolant accident experiments are implemented in research reactor. In this framework, we develop an optical pyrometry device to measure surface temperature (700-1200°C) of Zircaloy cladding without contact. The whole set-up of the simplified device (under air, without radiation) and the measurement procedure including data treatment based on bichromatic pyrometry are presented, as well as results for various temperature levels. Temperature retrieval based on the hypothesis of emissivity ratio equal to a constant, is scanned over a large wavelength range. A rather constant surface temperature is obtained on the spectral range of measurement, confirming the relevancy of emissivity hypothesis. Differences between this non-contact temperature measurement and a complementary thermocouple temperature measurement are also discussed.

Electronic contribution in heat transfer at metal-semiconductor and metal silicide-semiconductor interfaces

This work presents a direct measurement of the Kapitza thermal boundary resistance Rth, between platinum-silicon and platinum silicide-silicon interfaces. Experimental measurements were made using a frequency domain photothermal radiometry set up at room temperature. The studied samples consist of ≈50 nm of platinum and ≈110 nm of platinum silicide on silicon substrates with different doping levels. The substrate thermal diffusivity was found via a hybrid frequency/spatial domain thermoreflectance set up. The films and the interfaces between the two layers were characterized using scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. X-ray diffraction was also used to determine the atomic and molecular structures of the samples. The results display an effect of the annealing process on the Kapitza resistance and on the thermal diffusivities of the coatings, related to material and interface changes. The influence of the substrate doping levels on the Kapitza resistance is studied to check the correlation between the Schottky barrier and the interfacial heat conduction. It is suggested that the presence of charge carriers in silicon may create new channels for heat conduction at the interface, with an efficiency depending on the difference between the metal’s and substrate’s work functions.

Photothermal spectroscopy of polymer nanocomposites

Abstract 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.

Polymer matrix composites reinforced with expanded and unexpended graphite Particles for electronic packaging applications

Polymer composites with high thermal conductivity are used more frequently in thermal management of electronic packaging systems. In this study, conductive polymer composites were prepared by melt mixing of ethylene-vinyl acetate (EVA) copolymer with graphite at different volumetric concentrations up to 29.3%. Two kinds of graphite were used as reinforcement to prepare composites: untreated natural graphite (UG) having particle sizes ranging from 20 μm to 25 μm and expanded graphite (EG) having originally particle sizes ranging from 5 to 6 μm in length. Upon mixing at high shear forces EG exfoliates in thin sheets of a few nanometers in thickness. Due to this high aspect ratio of graphite sheets, nanocomposites filled with expanded graphite have a lower percolation threshold for electrical conductivity, about (5 to 6) vol.% compared to the composites filled with untreated graphite (UG) which have a percolation threshold of (15 to 17) vol.%. Thermal diffusivity of the samples was measured by photothermal radiometry. At similar concentrations, thermal diffusivity values for the nano-composites, EG-filled EVA, were significantly higher than those composites filled with UG.