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Utilizing thermochemical reactions for thermal energy storage and solar fuel production has been an emerging research topic. Thermal transport properties of the materials are an important parameter that can determine the kinetics and efficiency of thermochemical reactions. With the increasing number of new thermochemical materials (TCMs); however, there is a lack of reliable techniques to monitor the thermal transport property of the materials and their changes as a function of reactions in real time. In this work, we report the in situ monitoring of thermochemical reactions using modulated photothermal radiometry (MPR). The thermal conductivities of two TCMs, namely, calcium hydroxide (Ca(OH)2) and Ba0.15Sr0.85FeO3−δ (BSF1585), were measured as a function of temperature and time using the MPR technique. The measured thermal conductivities were correlated to the reaction. The work has two significant contributions to the research communities. First, it provides a non-invasive diagnostic tool for monitoring the thermal transport properties of TCMs that can potentially be a high-throughput measurement technique conducive to optimizing TCMs, reactors, and related thermal systems. Second, for TCMs that show observable changes in thermal transport properties, a correlation between the measured thermal conductivity and the conversion fraction of the reaction can be established for monitoring the reaction kinetics based on thermal characterization. [ABSTRACT FROM AUTHOR]

Particle beds are widely used in various systems and processes, such as particle heat exchangers, granular flow reactors, and additive manufacturing. Accurate modeling of the thermal conductivity of particle beds and understanding of their heat transfer mechanisms are important. However, previous models were based on simple cubic packing of particles, which could not accurately represent the actual heat transfer processes under certain conditions. Here, we examine the effect of the packing structure on the thermal conductivity of particle beds. We use monodispersed silica microspheres with average particle sizes ranging from 23 to 330 μm as a model material. We employ a transient hot-wire technique to measure the thermal conductivity of the particle beds with packing density of 43%–57% within a temperature range of room temperature to 500 °C and under N2 gaseous pressures of 20–760 Torr. We then use a discrete element method (DEM) to obtain the realistic packing structure of the particles, which is then fed into a finite-element model (FEM) to calculate the thermal conductivity, with the consideration of solid conduction, gas conduction, and radiation heat transfer. Our results show that the thermal conductivity model based on the more realistic random packing structure derived from the DEM shows better agreement with the experimental data compared to that based on the simple cubic-packing structure. The combined DEM and FEM methodology can serve as a useful tool to predict the effective thermal conductivity of particle beds and to quantify different heat transfer mechanisms under various conditions. [ABSTRACT FROM AUTHOR]

• Black spinel nanoparticle coating turns quartz sand into solar-absorbing and thermal energy storage material. • Solar-weight absorption increases from ∼0.4 to ∼0.9 by the black coating. • Thermal conductivity of the particle bed is similar or increased with the black coating. • Solar absorptance remains unchanged after 100-hr annealing at 800 °C in air and 24-hr abrasion test with ball milling. Solid particles based direct solar absorbing heat transfer fluid (HTF) and thermal energy storage (TES) material is gaining increasing interests for high-temperature concentrating solar power (CSP). However, there is no particulate material that simultaneously possesses the performance and cost metrics needed to achieve the target levelized cost of electricity of future CSP, including high solar absorptance, high thermal conductivity, good mechanical and thermal stability, and importantly, low cost. In this work, we demonstrate a facile and scalable method to turn low-cost quartz sand into a direct solar-absorbing HTF and TES material by coating a thin silica shell containing black spinel nanoparticles (Cu 0.5 Cr 1.1 Mn 1.4 O 4 and CuCr 2 O 4). The black coating increases the solar absorptances of the sand from ∼0.43 to ∼0.89. The coated quartz sand also shows excellent stability and nearly unchanged optical properties after 100-hr of isothermal annealing at 800 °C and up to 24-hr ball mixing with Yttria Stabilized Zirconia (YSZ) grinding balls. The measured effective thermal conductivity of the coated quartz sand increases 0.30 W m−1 K−1 to 0.65 W m−1 K−1 from 20 °C to 700 °C and is comparable to that of uncoated sand as well as Carbo ceramic particle beds currently used in CSP plants. The coating process could represent a cost-effective and generic strategy to enhance the solar absorptance of solid particles, and when applied on low-cost sand, could turn them into low-cost and high-performance HTF and TES media for future CSP systems. [ABSTRACT FROM AUTHOR]

• Non-contact technique to handle high-temperature corrosive liquids. • Frequency controlled thermal penetration depth eliminates natural convection. • Front-side sensing and vertical holder orientation minimize salt creeping effect. • Accurate thermal conductivity data obtained for nitrate and chloride salts. • MPR is applicable for in-situ diagnostics of high-temperature thermal system. Molten salts are being used or explored for thermal energy storage and conversion systems in concentrating solar power and nuclear power plants. Thermal conductivity of molten salts is an important thermophysical property dictating the performance and cost of these systems, but its accurate measurement has been challenging, as evidenced by wide scattering of existing data in literature. The corrosive and conducting nature of these fluids also leads to time consuming sample preparation processes of many contact-based measurements. Here, we report the measurement of thermal conductivity of molten salts using a modulated photothermal radiometry (MPR) technique, which is a laser-based, non-contact, frequency-domain method adopted for molten salts for the first time. By unitizing the advantages of front side sensing of frequency-domain measurements and the vertical holder orientation, the technique can minimize the natural convection and salt creeping effects, thus yielding accurate molten salt thermal conductivity. The MPR technique is first calibrated using standard molten materials including paraffin wax and sulfur. It is then applied on measuring pure nitrate salts (NaNO 3 and KNO 3), solar salt (NaNO 3 –KNO 3 mixture), and chloride salt (NaCl–KCl–MgCl 2). The measurement results are compared with data from literature, especially those obtained from laser flash analysis (LFA). Our results demonstrate that the MPR is a convenient and reliable technique of measuring thermal conductivity of molten salts. Accurate thermal conductivity data of molten salts will be valuable in developing the next-generation high-temperature thermal energy storage and conversion systems. [ABSTRACT FROM AUTHOR]

• The first in-situ thermal conductivity measurement of flowing molten chloride salt. • Non-contact modulated photothermal radiometry for high-temperature measurement. • Increased effective thermal conductivity of flowing salt due to convection. • Heat transfer coefficient is obtained from experiments and Gnielinski's correlation. Molten salts are leading candidates for high-temperature heat transfer fluids (HTFs) for thermal energy storage and conversion systems in concentrated solar power (CSP) and nuclear energy power plants. The ability to probe molten salt thermal transport properties in both stationary and flowing status is important for the evaluation of their heat transfer performance under realistic operational conditions, including the temperature range and potential degradation due to corrosion and contamination. However, accurate thermal transport properties are usually challenging to obtain even for stagnant molten salts due to different sources of errors from convection, radiation, and corrosion, let alone at flowing status. To the best of authors' knowledge, there is no available in-situ technique for measuring flowing molten salt thermal conductivity. Here, we report the first in-situ flowing molten salt thermal conductivity measurement using modulated photothermal radiometry (MPR). We could successfully perform the first in-situ thermal conductivity measurement of flowing molten NaCl-KCl-MgCl 2 in the typical operating temperature (520 and 580 °C) with flow velocities ranging from around 0.3 to 1.0 m s−1. The relative change of the molten salt thermal conductivity was measured. Gnielinski's correlation was also used to estimate the heat transfer coefficient h of the flowing NaCl-KCl-MgCl 2 in the given experimental condition. The work showed the potential of the MPR technique serving as an in-situ diagnostics tool to evaluate the heat transfer performance of flowing molten salts and other high-temperature HTFs. [ABSTRACT FROM AUTHOR]

A new series of 20-component fluorite-based compositionally complex oxides (20CCFBO x Nb/Ta) with the general chemical formula (15RE 1/15) 2 x +1 (Ce 1/3 Zr 1/3 Hf 1/3) 3–3 x (Nb 1/2 Ta 1/2) x O 8- δ (0 ≤ x ≤ 1, where 15RE 1/15 = La 1/15 Pr 1/15 Nd 1/15 Sm 1/15 Eu 1/15 Gd 1/15 Tb 1/15 Dy 1/15 Y 1/15 Ho 1/15 Er 1/15 Tm 1/15 Yb 1/15 Lu 1/15 Sc 1/15) are synthesized. Despite that the Gibbs phase rule allows for the existence of up to 20 phases at the thermodynamic equilibrium, 17 of the 20CCFBO x Nb/Ta compositions synthesized in this study all possess single ultrahigh-entropy phases in fluorite, pyrochlore, or weberite structure, as shown by X-ray diffraction (XRD). Only < 1 vol% of secondary phases are observed in two compositions near the phase-transition points. With changing compositional variable x , this series of 20CCFBO x Nb/Ta undergoes an abrupt fluorite-pyrochlore transition at x = ∼0.27 and an abrupt pyrochlore-weberite transition at x = ∼0.87. Careful characterization reveals abrupt changes of order parameters at both phase transitions. In addition, weberite short-range ordering can persist into the long-range pyrochlore phase, which leads to the lowest thermal conductivities. [ABSTRACT FROM AUTHOR]

• First non-contact in-situ thermal conductivity measurement of flowing fluid. • Identify experimental conditions to obtain intrinsic thermal conductivity of fluid. • Demonstrate the technique on water, ethanol, and two types of oils. • Measure oil from room temperature to 170 °C. In situ thermal transport measurement of flowing fluid could be useful for the characterization and diagnosis of practical thermal systems such as fluid heat exchangers and thermal energy storage systems. Despite abundant reports on the ex-situ thermal conductivity measurement of stagnant fluids, a suitable technique for the thermal conductivity measurement of flowing fluid has been rarely reported. This paper presents the thermal conductivity measurement of flowing fluid within a pipe using a non-contact modulated photothermal radiometry (MPR) technique, where the surface of the pipe is heated by an intensity-modulated laser and the heat diffuses into the fluid with suitable modulation frequency. We design a tube section with small wall thickness suitable for the MPR measurements to maximize the sensitivity of the thermal response to the fluid properties while minimizing the lateral heat spreading effect. Intrinsic thermal conductivity of different fluids was obtained within a proper range of frequency and flow velocity where the forced convection effect is negligible. The forced convection effect became prominent at high flowing velocity and at low modulation frequency, leading to higher effective thermal conductivity of the fluid. It is found that the intrinsic thermal conductivity could be obtained when the flow velocity is less than 100 mm/sec and Re D 1/2Pr1/3 < 100 for DI water and Xceltherm oil under the specified experimental conditions, where R e D is the Reynolds number and P r is the Prandtl number. [ABSTRACT FROM AUTHOR]

A systematic study was performed to measure the effective thermal conductivity of ceramic particle beds, a promising heat transfer and thermal energy storage medium for concentrating solar power (CSP). The thermal conductivity of the ceramic particle beds was measured using a transient hot-wire (THW) method within a temperature range of room temperature to 700 °C, the target operating temperature of the next-generation CSP systems. Two different types of ceramic particles were examined: (1) CARBOBEAD HSP 40/70 and (2) CARBOBEAD CP 40/100 (also known as ACCUCAST ID 50) with the average particle sizes of ~405 μm and ~275 μm, respectively, and thermal conductivities ranging from ~0.25 W m−1 K−1 to ~0.50 W m−1 K−1 from 20 °C to 700 °C in both air and N 2 gas. The gaseous pressure dependence of the thermal conductivity of the ceramic particle beds was also studied in the N 2 environment to differentiate the contributions from gas conduction, solid conduction, and radiation. Calculations using the Zehner, Bauer, and Schlünder (ZBS) model showed good agreements with the measurements. Based on the model, it is concluded that the effective thermal conductivity of the packed particle beds is dominated by the gas conduction while the solid conduction and radiation contributes to about 20% of the effective thermal conductivity at high temperature. • First thermal conductivity measurement of CARBOBEAD HSP 40/70 and CP 40/100 up to 700 °C. • Analysis of thermal conductivity from temperature and gaseous pressure dependence. • Quantification of relative contributions from gas, solid contacts, and radiation at various temperature. • Gaseous conduction is found to be the dominant mechanism. [ABSTRACT FROM AUTHOR]

• Modulated photothermal radiometry for bulk and coatings. • Measured bulk materials with thermal conductivity 1 - 30 W m−1 K−1 up to 973 K. • First thermal conductivity measurement of thin solar-absorbing coatings. • Thermal conductivity of the coatings of 0.4 ~ 0.8 W m−1 K−1 up to 973 K. This paper presents the development of instrumentation for the measurement of high-temperature thermal conductivity of bulk and coatings using a modulated photothermal radiometry (MPR) method, where a sample is heated by an intensity-modulated laser to probe into different layers of the sample. While MPR has been previously established, most of the prior studies only focus on the measurement at room temperature. The MPR has not been well studied for measurements of bulk and coating materials at high temperatures, which are increasingly important for a multitude of applications, such as materials used in the concentrating solar power (CSP) plants and the nuclear reactors. MPR is a non-contact technique that utilizes the intrinsic thermal emission from the specimens for thermometry, which is favorable for measurements at high temperatures in harsh environment. The authors designed and utilized a sample holder suitable for high temperature measurement up to 973 K with good temperature uniformity within the sample. The high-temperature MPR setup was validated by measuring bulk materials with known thermal conductivity. The setup and technique were then extended to the measurement of black solar-absorbing coatings of 10 to 50 μm thick on various substrates by modulating the frequency of the laser heating beam and the thermal penetration depth. The studies showed that thermal conductivities of typical solar-absorbing coatings are 0.4 ~ 0.8 W m−1 K−1, indicating a possibly large temperature drop within the coating under high solar irradiation flux, such as over 1000-sun for central solar towers in CSP plants. [ABSTRACT FROM AUTHOR]

We engineered the thermal conductivity of nano-grained Bi0.5Sb1.5Te3 (BST) by embedding SiO2 and diamond nanoparticles (NPs) with concentration ranging from 0.5 to 5 vol. %. The embedded NPs work as additional scattering centers for long mean free path phonons that are not effectively scattered by the grain boundaries. We found that both the SiO2 and diamond NPs materially reduced the lattice thermal conductivity (κ lat ) within the temperature range of 50–300 K, with stronger reduction occurring at a lower temperature. Furthermore, the diamond NPs were found to cause large reduction in κ lat compared to the SiO2 NPs at the same concentrations. Further theoretical analysis showed that the diamond NPs possess about tenfold higher interfacial thermal resistance with the BST matrix compared to that of SiO2 NPs, due to the larger acoustic mismatch between diamond and BST as compared to SiO2 and BST. As a result of the large reduction of κ lat , the thermoelectric figure of merit (ZT) was enhanced by 15% at room temperature with 0.5 vol. % diamond NP relative to the pristine nano-grained samples without the NPs. [ABSTRACT FROM AUTHOR]

• 18 compositionally-complex (medium- and high-entropy) pyrochlore oxides fabricated. • Mixing of cations at A- and/or B-sites leads to reduced thermal conductivity. • Thermal conductivity correlates well with a modified size disorder parameter. • A new descriptor proposed for designing low- k medium- and high-entropy ceramics. • Medium-entropy ceramics can outperform their high-entropy counterparts. High-entropy ceramics generally exhibit reduced thermal conductivity, but little is known about what controls this suppression and which descriptor can predict it. Herein, 22 single-phase pyrochlores were examined. Up to 35% reductions in thermal conductivity were achieved in medium- and high-entropy compositions with retained moduli, thereby attaining insulative yet stiff properties for thermal barrier coating applications. Notably, the measured thermal conductivity correlates well with a modified size disorder parameter δ S i z e * , thereby suggesting the importance of severe lattice distortion. Thus, this δ S i z e * is suggested as a useful descriptor for designing thermally-insulative compositionally-complex ceramics, where medium-entropy ceramics can outperform their high-entropy counterparts. Image, graphical abstract [ABSTRACT FROM AUTHOR]