Laser Flash Method Research Articles

Thermal conductivity of lead and bismuth-lead eutectic melts up to 1300 K

Using a laser flash method, the thermal conductivity (λ) of Pb and eutectic alloy Bi-Pb (55.2 wt%Bi) melts is measured in the temperature range from the melting point to 1300 K with 3.2–6.0 % uncertainty. The results are compared with the data of other authors. Based on the measurements, the thermal diffusivity (a) and the Lorenz number of melts are calculated. Tables of recommended data for a and λ along with estimated errors are drawn up.

A study on the thermal conductivity of proton irradiated CVD-SiC and sintered SiC, measured using a modified laser flash method with multi-step machining

CVD-SiC and sintered SiC (SPS-SiC) were proton irradiated at 340 ̊C receiving different levels of damage (0.05–0.25 dpa). A novel multi-step machining and measurement method using laser flash analysis (LFA) was developed to derive the thermal conductivity of the irradiated layer (∼46 µm). Before irradiation, the thermal conductivity of SPS-SiC was much lower than CVD-SiC, primarily due to its higher intrinsic defect concentration and smaller grain size which provide a greater density of barriers to phonon transmission. Following irradiation, major thermal conductivity degradation (∼90%) was found to occur to both types of SiC after only a low dose (∼0.1 dpa), with both saturating at a similarly low value (a few W/K⋅m), as the thermal resistivity due to the presence of high density of grain boundaries became less important. Thermal conductivity degradation after irradiation was primarily caused by point defects in both types of SiC, as reflected by Raman spectra.

An improved laser flash method for thermal conductivity measurement of molten salts

Thermal conductivity measurement of high-temperature heat transfer fluids provides a crucial basis for designing utility-scale thermal systems. Molten salts are promising heat transfer and thermal storage fluids in high-temperature thermal energy storage systems, while the molten salt thermal conductivity obtained in existing studies exhibits large deviations due to the high experimental complexity and unstandardized test procedures. In this work, we improve the conventional laser flash analysis method by proposing a theoretical heat transfer model for multi-layer heat conduction and providing a near-optimal molten salt container design. With water as a test sample, the relative error of thermal conductivity measurement using the improved method is 6.3%. The thermal conductivity of Solar Salt from 250 to 400 ​°C, and of Hitec Salt from 160 to 250 ​°C are measured and compared with the previous work. Both results show that the thermal conductivity increases with the temperature rising. This work will promote the technology standardization for accurately acquiring the thermal conductivity of molten salts or other similar high-temperature heat transfer fluids.

Low-carbon shape-stable phase change composite utilizing semi-coke ash for building thermal energy storage

To enhance the efficiency and cost-effectiveness of heating, air-conditioning, and solid waste recycling in buildings, this work proposed the solid waste semi-coke ash as skeleton material and the sodium carbonate as phase-change material to produce shape-stable phase-change composites using the cold compression-hot sintering method for building thermal energy storage. Eight shape-stable phase-change composites were prepared and the detailed chemical compatibility, thermal property, structural and mechanical performance, and micromorphology were investigated by X-ray diffraction, differential scanning calorimetry, laser flash analysis, N2 adsorption and desorption method, constant speed pressurization method, and scanning electron microscopy. Results demonstrated that semi-coke ash is suitable with sodium carbonate for fabricating shape-stable phase-change composites. The optimal mass ratio of semi-coke ash to Na2CO3 powder for sample CC3 was determined to be 52.5:47.5. This particular sample demonstrated a thermal energy storage density of 961.58 J/g and a maximum thermal conductivity of 1.306 W/(m ∙ K). It also exhibited excellent chemical compatibility between its components. Furthermore, sample CC3 displayed a mechanical strength of 23.57 MPa, with elements evenly distributed throughout. Importantly, CO2 emission from the production of sample CC3 was significantly low, indicating great potential for commercialization.

Unleashing the Potential of Boron Nitride Spheres for High‐Performance Thermal Management

AbstractHighly integrated and miniaturized electronic devices require advanced thermal management techniques to improve reliability and performance. Thanks to their high thermal conductivity and electrical insulation, boron nitride nanosheets (BNNSs) are commomly used as fillers to construct thermally conductive polymer composites for heat dissipation. However, the BNNS reinforced composites exhibit anisotropic thermal conductivity due to the anisotropic structure of BNNSs. Micro‐sized boron nitride spheres (BNSs) with isotropic thermal conductivity are considered one of the best solutions. Nevertheless, precisely measuring the thermal conductivity of BNSs remains a challenge, limiting the understanding of the thermal transport mechanism. Herein, we have successfully estimated the thermal conductivity of BNSs using the laser flash method. Factors influencing BNSs’ thermal conductivity, including precursor, polymer binder and sintering temperature, are also investigated. Under optimized conditions, BNSs exhibit high, isotropic thermal conductivity of 37.2±2.5 W/(m ⋅ K), and the BNS pellet outperforms its h‐BN counterpart in heat dissipation for an LED light. This superiority is attributed to outstanding heat transfer performance in the cross‐plane direction, in addition to high in‐plane thermal conductivity. This study provides a feasible method to estimate the thermal conductivity of spherical materials and highlights promising boron nitride materials with isotropic thermal conductivity for heat dissipation in advanced electronics.

Thermodynamic and Kinetic Simulations Used for the Study of the Influence of Precipitates on Thermophysical Properties in NiTiCu Alloys Obtained by Spark Plasma Sintering.

The thermodynamic and kinetic simulations based on the re-assessment of the thermodynamic and kinetic database of the Ni-Ti-Cu system were employed to predict the phenomena of mechanical alloying, spark plasma sintering and thermal properties of the intriguing Ni-Ti-Cu system. Thermodynamic calculations are presented for the stable and unstable phases of NiTiCu materials and support a correlation with the evolving microstructure during the technological process. Also, the thermal conductivity, the thermal diffusivity and the specific heat of spark plasma sintered and aged Cu-alloyed NiTi-based shape memory alloys (NiTiCu) with two compositions, Ni45Ti50Cu5 and Ni40Ti50Cu10, are evaluated and the influence of mechanical alloying and precipitates on thermal properties is discussed. Measurements of these thermal properties were carried out from 25 °C up to 175 °C using the laser flash method, as well as differential scanning calorimetry. The thermal hysteresis of the 20 mm diameter samples was between 8.8 and 24.5 °C. The observed T0 temperatures from DSC experimental transformation features are in reasonable accordance with the thermodynamic predictions. The determined k values are between 20.04 and 26.87 W/m K and in agreement with the literature results. Moreover, this paper can provide some suggestions for the preparation of NiTiCu shape memory alloys and their applications.

Experimental Investigation of the Anisotropic Thermal Conductivity of C/SiC Composite Thin Slab

Fiber-reinforced composites possess anisotropic mechanical and heat transfer properties due to their anisotropic fibers and structure distribution. In C/SiC composites, the out-of-plane thermal conductivity has mainly been studied, whereas the in-plane thermal conductivity has received less attention due to their limited thickness. In this study, the slab module of a transient plane source method is adopted to measure the in-plane thermal conductivity of a 2D plain woven C/SiC composite slab, and the test uncertainty is analyzed numerically. The numerical investigation proves that the slab module is reliable for measuring the isotropic and anisotropic slabs with in-plane thermal conductivity greater than 10 W⋅m−1⋅K−1. The anisotropic thermal conductivity of the 2D plain woven C/SiC composite slab is obtained within the temperature range of 20–900 °C by combining with a laser flash analysis method to measure the out-of-plane thermal conductivity. The results demonstrate that the out-of-plane thermal conductivity of C/SiC composite decreases with temperature, while its in-plane thermal conductivity first increases with temperature and then decreases, and the ratio of in-plane thermal conductivity to out-of-plane thermal conductivity is within 2.2–3.1.

Role of microstructure in the exploitation of self-healing potential in form-stable composite phase change materials based on immiscible alloys

Metallic Phase Change Materials (PCMs), based on solid-liquid transitions, represents one of the most promising technologies for efficient Thermal Energy Storage (TES), due to their superior thermal conductivity and energy storability per unit volume, but suffer of limited solutions for their handling at the molten state. The use of Miscibility Gap Alloys (MGAs) allows to manage PCM volume expansion and keep it confined when molten, preventing interaction with the environment. A relevant example is provided by the Al-Sn system, where Al covers the role of the high-temperature stable and highly thermal-conductive passive matrix and Sn the active PCM. The alloy can thus be considered a Composite PCM (C-PCM). The response fastness of these systems depends on their thermal diffusivity, subjected to abrupt variations under the presence of discontinuities and damages. In this sense, the authors investigated the possibility to employ molten Sn mobility in a potentially damaged C-PCM for self-healing purposes, aimed to restore, at least partially, the material continuity and thus its thermal diffusivity. Exudation heat treatments above the melting temperature of Sn were performed on sets of Al-40%wt. Sn metallic composites, produced either with powder metallurgy or liquid metal routes, in order to quantify and assess the mobility of the Sn under simulated operating conditions. Exudation tests assess Simple Mixed powders and liquid metal routes sample as the ones with the highest healing potential. Al dissolution and re-deposition was established by EDS analyses as one of the principal Sn mobility mechanisms. Laser Flash Analysis tests, as well as microstructural investigations, were performed on the samples before and after both healing-focused and simulated service heat treatments to evaluate the changes of thermal diffusivity. Healing-focused treatment at 250°C for 1 hour generally displayed a moderate thermal diffusivity recovery and simulated service by shorter cycles between 170°C and 270°C slightly reduce it. The beneficial role of healing focused heat treatments at 250°C for 1 hour suggests that the presence of fully molten Sn phase during service for relatively long time could be beneficial for functional healing. The requirements of suitable Al-Sn microstructures for self-healing purposes, granting at the same time the C-PCM functionalities, i.e., thermal energy storage and form-stability, were set.

Thermal Performance and Reliability of Liquid Metal Pastes with Solid Metal Particles as Thermal Interface Materials for the Cooling of Computing Electronics

In recent years, we have developed a series of gallium-based liquid metal pastes containing low content solid metal particle additives (LMPMPs) to enhance thermal performance and control bondline thickness (BLT) of prepared liquid metal composites as thermal interface materials (TIMs). This new material is synthesized via in-situ by introducing gallium oxide into liquid metal alloys. In our recent research, we intensively investigated thermal properties of different primary liquid metal alloys containing small amounts of metal additives, and ran extensive reliability tests including power cycling, thermal cycling, and aging tests on the synthesized LMPMPs. In this paper, the thermal properties of the synthesized LMPMPs with different liquid metal alloys are studied with reliability tests that focus on power cycling by monitoring thermal resistance of LMPMPs over time using a custom power cycler. We have found power density, BLT, and different liquid metal alloys have a significant impact on thermal properties and reliability of LMPMPs. We also conducted thermal aging tests on Cu-LMPMPs-Cu sandwich specimens at 85°C in an ambient atmosphere to study the deterioration of thermal conductivity of LMPMPs over time, measured by laser flash analyzer using the layered structure method. Highly accelerated stress test (HAST) with a profile of 110°C, 85RH%, and 264 hours was used to study the decomposition of liquid metal alloys. Thermal cycling test with a profile of -40 to 125°C was employed to investigate the pump-out of LMPMPs. It has been proven that LMPMPs have much better thermal performances than polymer-based thermal grease.

High temperature oxidation and thermal properties of laminated Ti3Al(Si)C2-TiC/Nb based composites obtained by spark plasma sintering

This paper describes high-temperature oxidation behavior and thermal properties of metal-ceramic laminated composites fabricated by spark plasma sintering of Ti3Al(Si)C2-filled preceramic papers (TAC) and Nb foils. The high-temperature oxidation tests were carried out in air at 800–1300 °C. The phase composition and microstructure of the oxide layers were analyzed by X-ray diffraction and scanning electron microscopy, respectively. The thermal properties were measured by the laser flash method in the temperature range of 30–800 °C. The evolution of the microstructure of the reaction layer (RL) formed at TAC/Nb interfaces was studied. The passive oxidation of the surface TAC layers results in the formation of outer TiO2 and inner continuous Al2O3 phase at temperatures up to 1100 °C, while local accelerated oxidation occurs above 1100 °C. As a result of breakaway oxidation, a multilayer oxide structure consisting of TiO2, Al2TiO5, Al2O3 and SiO2 phases were formed. The oxidation of inner Nb layers from the side surface can lead to high volume expansion and generation of stresses in the ceramic and metal layers due to the formation of Nb2O5 oxide phase. The stress relaxation occurred by complex mechanisms, including plastic deformation of the Nb layers, delamination at the interfaces, and formation of cracks in the reaction and TAC layers. The high temperature exposure results in slight increase in columnar grains in the reaction layers at 1200 °C, while the thickness of RL remains unchanged. The TAC/Nb composites exhibit higher thermal conductivity and diffusivity compared to monolithic TAC ceramic composites.

Development of Fourier Transform Ultrafast Laser Flash Method for Simultaneous Measurement of Thermal Diffusivity and Interfacial Thermal Resistance

The thermoreflectance technique is one of the few methods which can measure thermal diffusivity of thin films as thin as 100 nm or thinner in the cross-plane direction. The thermoreflectance method under rear-heat front-detect configuration is sometimes called ultrafast laser flash method because of its similarity to laser flash method. Up to now it has typically only been possible to attempt to evaluate the interfacial thermal resistance between the thin films by preparing and measuring several samples with different thicknesses. In this study, a method to directly determine interfacial thermal resistance by a single measurement of a thin film on substrate is represented, by analyzing the shape of thermoreflectance signals with analytical solutions in frequency domain and time domain. Thermoreflectance signals observed from metallic thin films on sapphire substrate with different thickness steps were analyzed by Fourier analysis and fitted by analytical equations with four parameters: heat diffusion time across the first layer, ratio of virtual heat sources, characteristic time of cooling determined by interfacial thermal resistance and relative amplitude of the signal. Interface thermal resistance between the thin film and substrate was able to be determined reliably with smaller uncertainty.

Biomass- and Carbon Dioxide-Derived Polyurethane Networks for Thermal Interface Material Applications.

Recent environmental concerns have increased demand for renewable polymers and sustainable green resource usage, such as biomass-derived components and carbon dioxide (CO2). Herein, we present crosslinked polyurethanes (CPUs) fabricated from CO2- and biomass-derived monomers via a facile solvent-free ball milling process. Furan-containing bis(cyclic carbonate)s were synthesized through CO2 fixation and further transformed to tetraols, denoted FCTs, by aminolysis and utilized in CPU synthesis. Highly dispersed polyurethane-based hybrid composites (CPU-Ag) were also manufactured using a similar ball milling process. Due to the malleability of the CPU matrix, enabled by transcarbamoylation (dynamic covalent chemistry), CPU-based composites are expected to present very low interfacial thermal resistance between the heat sink and heat source. The characteristics of the dynamic covalent bond (i.e., urethane exchange reaction) were confirmed by the results of dynamic mechanical thermal analysis and stress relaxation analysis. Importantly, the high thermal conductivity of the CPU-based hybrid material was confirmed using laser flash analysis (up to 51.1 W/m·K). Our mechanochemical approach enables the facile preparation of sustainable polymers and hybrid composites for functional application.

Thermal Conductivity Calculation in Organic Liquids: Application to Poly-α-Olefin.

In this work, we aim to understand and predict the thermal properties of automotive lubricants using non-equilibrium molecular dynamics. After a previous study on model materials for the mechanical parts of a car engine, we now focus on the thermal conductivity κ of the poly-α-olefin base oil (PAO4) using the well-known sink and source method to study the response of the system to an imposed heat flux. We present a detailed methodology for the calculation of κ, taking into account specific constraints related to the system under study, such as large steady-state fluctuations and rapidly growing stationarization times. We provide thermal conductivity results using four different force fields, including OPLS-AA, PCFF and COMPASS, in a temperature range of 300 to 500 K, which corresponds to the typical operating range of a car engine. The results are compared to experimental measurements performed on the commercial compound using the laser flash method. Agreement at room temperature is shown to be excellent for our in-house force field.

Research on experimental parameter selection for measuring thermal diffusivity by laser flash method at room temperature

Based on the laser flash method, the selection of experimental parameters on the accuracy of measurement results for measuring thermal diffusivity is investigated in this paper. High purity graphite is employed as the experimental material. Three experimental parameters are taken into consideration, including specimen thickness, laser pulse power, and laser pulse width. Firstly, the principle of the laser flash method is introduced. Then, a numerical simulation model is established and independence tests are performed. In order to investigate the impact of different experimental parameters to the precision of the measurement results using the laser flash method, six thicknesses of the specimens, five laser pulse powers, and five laser pulse widths are selected for numerical simulation and LFA 427 measurement experiments. Finally, an orthogonal design method with three-factor and three-level is constructed to investigate the influence degree of these three factors on the measurement results. It is found that the laser pulse width has the most significant influence, while the laser pulse power has the least impact. For high purity graphite specimens, it is required to choose a thicker specimen, a lower laser pulse power, and a smaller laser pulse width to ensure better measurement accuracy.

Superior corrosion resistance and thermal/electro properties of graphene−epoxy composite coating on Mg alloy with biomimetic interface and orientation

Biomimetic graphene−epoxy composite coating was fabricated with a “brick and mud” structure via a spin-coating strategy. Imidazolium-based ionic liquids modified graphene (GI) was synthesized to improve the interfacial compatibility between the graphene (G) layers and epoxy resin. The surface morphology and chemical structures were observed and analyzed through scanning electron microscopy (FE-SEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) as well as Fourier transform infrared spectroscopy (FTIR). The thermal imager was used to record the temperature change on the surface. The surface resistivity and thermal conductivity of the coating were determined via volumetric surface area resistivity tester and laser flash method, respectively. The corrosion resistance was investigated through electrochemical impedance spectroscopy (EIS) and salt spray testing. The GI layers in the composite coating exhibited a near-parallel orientation, which led to improved heat transfer and electrostatic dissipative efficiency. The through-plane thermal conductivity and surface resistivity were 0.77 W·m−1·K−1 and 1.6×106 Ω·cm, respectively. Electrochemical impedance spectroscopy tests revealed that the low-frequency modulus of the coating reached 6.76×1010 Ω·cm2 after 40 d of immersion, which was about two orders of the magnitude higher than that of the pure epoxy coating, showing a superior corrosion resistance.

Powder sintering kinetics during electron beam based additive manufacturing

Electron Beam Powder Bed Fusion (PBF-EB) allows not only for the additive manufacturing of fully dense metallic components via melting but also parts with defined local pore structures and tailored functional properties through selective sintering. The hybrid PBF-EB technology based on the combination of selective sintering and melting is, e.g., of great interest for the fabrication of porous electrodes composed of titanium-based alloys with high specific surface area, which are widely used for electrochemical processes (e.g., as anode for water electrolysis cells). This study aims to provide a comprehensive understanding of the sintering kinetics of Ti6Al4V (especially in the first and second sintering stages) during PBF-EB. The sintering degrees of the PBF-EB-processed Ti6Al4V samples are characterized by determining the thermal diffusivity measured by laser flash analysis. At the same time, the thermal history during PBF-EB is analyzed using a pyrometer. The correlation between the sintering degrees and the process parameters utilized in the PBF-EB procedure is discussed by calculating the activation energy of sintering and described using a master sintering curve. Based on the findings of this study, it is possible to control the sintering process of Ti6Al4V powder in a targeted manner by utilizing various PBF-EB parameters.

Simulation study on the coupled heat and mass transfer of CO2 in a heavy oil-water liquid system based on an improved lattice Boltzmann (LB) method

This study investigates the heat and mass transfer when low-temperature CO2 diffuses in a heavy oil–water liquid system based on an improved Lattice Boltzmann (LB) method. First, we determine the CO2 diffusion coefficients in heavy oil and water and the heat transfer coefficients of heavy oil and water using the pressure drop (DP) method and the laser flash (LF) method, respectively. Then, a numerical model that couples the heat and mass transfer in the system is established based on the improved LB method. The effect of low-temperature CO2 on the temperature field of the system and the effect of changes in the temperature field on CO2 diffusion are investigated. Finally, we develop a micro-model reproducing the heat and mass transfer of CO2 in water and heavy oil-saturated porous media to validate the accuracy of the numerical model. The results demonstrate that the CO2 diffusion coefficients in heavy oil and water linearly increase with the temperature increases. The heat transfer coefficients of heavy oil and water are insensitive to the changes in temperature. The heat transfer process has a great impact on the mass transfer of CO2 in the liquid system. The CO2 diffusion velocity slows down as the system temperature decreases because of the heat transfer (from 400 K to 300 K), leading to a reduction percentage of 300 % in the CO2 front movement velocity. Compared to the scenarios neglecting heat transfer, CO2 diffusion distance and amount decrease by 38 % and 50 %, respectively because of the changes in the temperature field of the system.

How the Effective Thermal Conductivity of Lithium-Ion Cell Electrodes Is Impacted By the Compression during Calendering

During the design process of lithium-ion batteries the simultaneous optimization of the mechanical, electrical and thermal properties of the components and their interfaces is crucial. As these are strongly influenced by the production steps, a thorough understanding of their impact on said properties is key to reaching optimized material properties of the cells. While many researchers have already investigated the impact of the calendering process on the electrical and mechanical behavior [1][2][3][4], this study covers the alteration of the thermal material properties due to calendering.In this context, different material coatings consisting of two graphite anodes with different starting thicknesses and two cathodes with NMC622 and NMC811 as active materials were investigated. The electrodes were calendered with different forces to reach a broad spectrum of porosities and layer thicknesses. For each material the specific heat capacity and density of the solid phase were measured with differential scanning calorimetry and gas pycnometry, respectively. The thermal diffusivity was measured with laser flash analysis for different compression rates for each active material. With the derived values, the effective thermal conductivity of the different electrodes was determined and interesting indications were found.Considering those results, the values of the effective thermal conductivity of one-sided coated electrodes, plotted vs. the compression rate do not show a steadily increasing slope as suggested by results of common models for their dependence on a decreasing porosity [5][6]. Instead, the thermal conductivity stays at an almost constant value or even decreases at small to medium compression rates before it increases again for a strong compression. This observation indicates that more complex changes and effects happen within the particulate coating layer and at its interfaces to the substrate during the process of calendering than just a change in the porosity. As an example: In a mechanical context, similar variations of the adhesion strength between coating and current collector have been observed [1][2]. It may thus be assumed that the calendering process affects the thermal connection between coating and substrate. The contribution will discuss this and more potentially happening effects and according modifications and extensions of existing models for the thermal conductivity of electrode stacks.

Mechanical properties of medium-temperature thermoelectric materials based on tin and lead tellurides

The strength and thermoelectric properties of PbTe and Sn0.9Pb0.1Te medium-temperature polycrystalline specimens with p and n conductivity types, respectively, have been studied. The specimens have been produced using extrusion and spark plasma sintering. The strength parameters of the materials were studied using uniaxial compression at 20 to 500 °C. The structure of the materials was studied using X-ray diffraction and electron microscopy. The electrical conductivity and the Seebeck coefficient were measured simultaneously using the four-probe and differential methods. The temperature conductivity and the specific heat capacity were measured using the laser flash and differential scanning calorimetry methods. The PbTe and Sn0.9Pb0.1Te materials produced using extrusion and spark plasma sintering prove to be single-phase and have homogeneous compositions. For comparable synthesis methods, the dislocation density in the Sn0.9Pb0.1Te specimens is by an order of magnitude lower than in the PbTe ones. Study of the mechanical properties of n and p conductivity type specimens over a wide temperature range from 20 to 500 °C has shown that their deformation is plastic and has no traces of brittle fracture. For these plastic materials, the strength criterion has been accepted to be the arbitrary yield stress corresponding to the stress at a 0.2% deformation. The 20 °C yield stress of PbTe and Sn0.9Pb0.1Te is far higher for the specimens produced by extrusion. For all the test temperatures and synthesis methods the Sn0.9Pb0.1Te specimens have a higher strength than the PbTe ones. The PbTe and Sn0.9Pb0.1Te specimens produced by extrusion have better thermoelectric properties than the spark plasma sintered ones. The heat conductivity of the PbTe and Sn0.9Pb0.1Te specimens is almost the same regardless of compaction method.

Accelerated thermal property mapping of TRISO advanced nuclear fuel

TRistructural ISOtropic (TRISO) fuel is a leading-edge nuclear fuel form representing a departure from the more traditional nuclear fuel forms utilized in the reactor fleet of today. Rather than a monolithic fuel pellet of uranium dioxide, integral fuel forms containing TRISO fuel are composed of thousands of microencapsulated uranium-bearing fuel kernels and individually coated with multiple layers of pyrolytic carbon and silicon carbide. These multilayered ceramic coatings serve as an environmental barrier to ensure radioactive and chemically reactive fission products are contained within the reactor fuel elements, but also participate in the transfer of heat generated in the nuclear fuel to the coolant – the primary purpose of a nuclear reactor. Since traditional thermal property measurement techniques, such as laser flash analysis, would be unable to resolve the thermal properties of the individual TRISO coating layers, a simplified frequency-domain thermoreflectance technique has been developed to rapidly map the thermal properties of TRISO particles. Using this technique, the thermal properties of TRISO particles have been mapped from room temperature up to 1000 °C to examine the spatial variation and temperature-dependency of the thermal properties within each layer. Additionally, spatial-domain thermoreflectance was used to examine the anisotropy of the thermal properties for each layer at different locations within a single TRISO particle, and across multiple TRISO particles to assess the intra- and inter-particle uniformity of thermal properties, respectively. To elucidate the underlying causes for the measured variations in thermal properties, scanning electron microscopy and Raman spectroscopy were used to examine variations in microstructure and chemical bonding within the different coating layers. Results from this work are then compared with previous examinations of TRISO fuel particles and microstructurally driven mechanisms for the variations in the measured thermal properties of the different carbonaceous layers are discussed.