Austin Fleming

Optical fiber sensors (OFS) are a potential candidate for monitoring physical parameters in nuclear environments. However, under an irradiation field the optical response of the OFS is modified via three primary mechanisms: (i) radiation-induced attenuation (RIA), (ii) radiation-induced emission (RIE), and (iii) radiation-induced compaction (RIC). For resonance-based sensors, RIC plays a significant role in modifying their performance characteristics. In this paper, we numerically investigate independently the effects of RIC and RIA on three types of OFS widely considered for radiation environments: fiber Bragg grating (FBG), long-period grating (LPG), and Fabry-Perot (F-P) sensors. In our RIC modeling, experimentally calculated refractive index (RI) changes due to low-dose radiation are extrapolated using a power law to calculate density changes at high doses. The changes in RI and length are subsequently calculated using the Lorentz–Lorenz relation and an established empirical equation, respectively. The effects of both the change in the RI and length contraction on OFS are modeled for both low and high doses using FIMMWAVE, a commercially available vectorial mode solver. An in-depth understanding of how radiation affects OFS may reveal various potential OFS applications in several types of radiation environments, such as nuclear reactors or in space.

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.

This work presents a direct measurement of the Kapitza thermal boundary resistance R_th, 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.

A new fiber-based modulated optical reflectance configuration is developed in this work. The technique maintains the fiber-based heating laser (pump) and detection laser (probe) in close proximity at a fixed separation distance in a ceramic ferrule. The pump beam periodically heats the sample inducing thermal waves into the sample. The probe beam measures the temperature response at a known distance from the pump beam over a range of heating modulation frequencies. The thermal diffusivity of the sample may be calculated from the phase response between the input heat flux and the temperature response of a sample having a reflective surface. The unique measurement configuration is ideal for in situ measurements and has many advantages for laboratory-based systems. The design and development of the system are reported along with theoretical justification for the experimental design. The thermal diffusivities of Ge and SiC are measured and found to be within 10% of reported literature values. The diffusivity for SiO2 is measured with a relative difference of approximately 100% from the literature value when the ferrule is in contact with the sample. An additional measurement was made on the SiO2 sample with the ferrule not in contact resulting in a difference of less than 2% from the literature value. The difference in the SiO2 measurement when the ferrule is in contact with the sample is likely due to a parallel heat transfer path through the dual-fiber ferrule assembly.

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.

Concentrating solar power (CSP) plants can provide dispatchable power with the thermal energy storage (TES) capability for greater renewable-energy grid penetration. To increase the market competitiveness, CSP technology needs to increase the solar-to-electric efficiency and reduce costs in the areas of solar collection from the heliostat field to the receiver, energy conversion systems, and TES. The current state-of-the-art molten-salt systems have limitations regarding both the potential for cost reduction and improvements in performance. Even with significant improvements in operating performance, these systems face major challenges to satisfy the performance targets, which include high-temperature stability (>650°C), low freezing point (<0°C), and material compatibility with high-temperature metals (>650°C) at a reduced cost. The fluidized-bed CSP (FB-CSP) plant being developed by the National Renewable Energy Laboratory (NREL) has the potential to overcome the above issues with substantially lower cost. The particle receiver is a critical component to enable the FB-CSP system.

This paper introduces the development of an innovative receiver design using the blackbody design mechanism by collecting solar heat with absorber tubes that transfer the radiant heat to flowing particles. The particle and receiver materials can withstand temperatures of >1000°C because the receiver can use low-cost materials, such as ceramics and stainless steel, and the solid particles can be any low-cost, stable materials such as sand or ash for particle containment and TES. The heated particles can be stored in containers for TES or supply heat for power generation. This study investigated the performance of convection, reflection, and infrared (IR) re-radiation losses on the absorber solar receiving side. We developed a flux model to predict the reflection losses from the absorber tubes based on the NREL SolTrace program, and conducted thermal modeling by using the Fluent Software. This paper presents the thermal modeling and results on the receiver performance. The receiver configuration may have broad applications for different heattransfer fluids (HTFs), including gas, liquid, or the solid particle-based system in our receiver development.