Thermal Diffusivity Measurements Research Articles

Analytical model for laser-induced transient grating measurements of thermal diffusivity in non-opaque materials

The thermal transport and elastic properties of materials are often measured using the laser-induced transient grating spectroscopy (TGS) technique. The analysis of the TGS signal usually involves fitting well-known expressions, derived assuming the limiting cases of opaque or transparent materials, to the measured data. In this paper, the system of thermoelastic equations is analytically solved for an isotropic homogeneous material assuming finite laser penetration depth, which is an important consideration when the penetration depth is on the order of the acoustic wavelength. The need to use such a solution is discussed and compared to the expression for opaque material. The solution is benchmarked against TGS signal measured on single-crystal silicon with {100} surface orientation and is found to significantly improve the accuracy of the calculated thermal diffusivity as compared to using the expression for opaque material.

The investigation of synthesis and textured properties of in situ formed hBN with spark plasma sintering

Hexagonal boron nitride (hBN) exhibits high thermal conductivity within the in-plane direction and significantly lower thermal conductivity in the cross-plane direction because of its anisotropic nature. When the texture properties of bulk hBN are strong, it is a potentially promising application area for the insulating heat spreaders and heat sinks. In this study, hBN was synthesized in situ with the spark plasma sintering (SPS) technique and the effects of sintering temperature on the orientation of hBN grains were investigated. The precursor was prepared by chemically reacting urea and boric acid in heat treatment at 850 °C. The hBN was produced using the previous precursor in a SPS at 1500, 1700, and 1900 °C. The samples were characterized using XRD, FTIR, SEM and Raman spectroscopy. The properties of the samples were further determined through measurements of density, DSC and thermal diffusivity. The hBN samples obtained through the SPS process have nano and micron-size grains. The hBN was highly crystalline and homogeneous, as determined by the XRD pattern, Raman spectra and Raman maps. The orientation of hBN grains has been revealed in the c-axis of hBN crystallites and was preferably arranged within a plane parallel to the SPS pressing direction. The hBN orientation preference index (IOP) value of −19925.5 at 1700 °C indicates the highest quality texture orientation. Increasing the temperature to 1900 °C decreased the orientation in the direction parallel to the pressing direction. The highest thermal conductivity calculated according to the thermal diffusivity perpendicular to the sintering pressure direction was 8.86 W m−1K−1 at 1900 °C.

Thermal diffusivity measurements of polycaprolactone membranes by means of photoacoustic techniques

Thermal diffusivity measurements of polycaprolactone membranes by means of photoacoustic techniques

Laser-spot lock-in thermography with a non-research-grade thermal camera: From thermal diffusivity measurements to visualizing free convection

The widespread use of Lock-in Thermography (LIT) has been hindered by the high cost of the thermographic cameras available for such applications. We show that a low-cost thermographic camera (we used the QIANLI MEGA-IDEA Super iR Cam 2S) can be straightforwardly used in advanced applications, particularly for measuring the thermal diffusivity of solids using the Laser Spot LIT Technique and the Slope Method. The results of measurements performed in test samples were compared with those obtained using the FLIR SC5000 research-grade camera, whose usefulness for this kind of application has been demonstrated before. The Super iR Cam 2S is almost two orders of magnitude cheaper than the FLIR SC5000 but has an inferior performance. However, a good agreement between the thermal diffusivities of materials, with values between 10 and 100 mm2/s, obtained using both cameras and those reported in the literature demonstrates inexpensive instrumentation usefulness for conducting high-quality research work. The visualization of cracks between samples with different diffusivities was demonstrated too. For the first time, we also show the possibility of using Laser Spot LIT to directly observe a convective effect in a fluid layer closest to the sample’s surface.

Non-destructive estimation of mechanical properties in Usibor® 1500 via thermal diffusivity measurements: A thermographic procedure

The study investigated the anti-correlation between thermal diffusivity and Ultimate Tensile Strength (UTS) in Usibor® 1500 steel. The non-destructive pulsed laser spot thermography technique was used to analyze fifteen boron steel specimens with varying bainite/martensite phase percentages, while the UTS was measured through uniaxial tensile tests. A 23 % thermal diffusivity difference was found between fully martensitic and fully bainitic structures, with UTS varying by around 90 %. The strong anti-correlation was confirmed (Spearman coefficient −0.98) and an empirical power-law equation was derived to estimate UTS based on thermal diffusivity variations. The approach showed an R-squared value over 0.84, providing a non-destructive thermographic procedure for UTS estimation in Usibor® 1500 steel, offering valuable material property insights.

Preparation and Characteristics of High-Performance, Low-Density Metallo-Ceramics Composite.

By applying the physical vapour deposition method, hollow ceramic microspheres were coated with titanium, and subsequently, they were sintered using the spark plasma sintering technique to create a porous ceramic material that is lightweight and devoid of a matrix. The sintering process was carried out at temperatures ranging from 1050 to 1200 °C, with a holding time of 2 min. The samples were subjected to conventional thermal analyses (differential scanning calorimetry, thermogravimetry, dilatometry), oxidation resistance tests, and thermal diffusivity measurements. Phase analysis of the samples was performed using the XRD and the microstructure of the prepared specimens was examined using electron microscopy. The titanium coating on the microspheres increased the compressive strength and density of the resulting ceramic material as the sintering temperature increased. The morphology of the samples was carefully examined, and phase transitions were also identified during the analysis of the samples.

Rapid and nondestructive testing for simultaneous measurement of thermal conductivity and thermal diffusivity of flat materials based on thermography

This study presents a novel experimental method that simultaneously supplies the thermal conductivity and thermal diffusivity of flat materials, which solves the limitations of traditional methods that have strict demands for thermal excitation, boundary, and size of samples. In this method, IR thermal imaging is used to collect data from the material surface temperature field caused by a laser excitation, and the differential terms in the basic heat-conduction differential equation are estimated by curve fitting. The differential equation is so transformed in an algebraic equation, which allows to simultaneously determine the thermal conductivity and thermal diffusivity. The measurement experiment does not require a strict control of the boundary or initial conditions, or thermal excitation modalities. The hypothesis this method is based on is that the spatial derivative of temperature in the Z-direction is linear with the temperature difference between the material surface and environment. Using this constraint the optimal operation conditions can be identified. The influence of operation conditions, including the heat transfer distance, excitation duration, material thickness, and number of sampling points on the measured surface, together with the results, were analysed by numerical simulation. The optimal operation conditions when testing two 304 stainless steel samples were found from this simulation analysis. Results of 304 stainless steel samples 1 and 2 mm thick show a deviation between the reference and measured values of the two thermal properties within 4.2 %, and repeatability within 5.5 %. Therefore, this method can realise rapid, nondestructive, and simultaneous measurements of thermal conductivity and thermal diffusivity.

Application of machine learning algorithms to model soil thermal diffusivity

Soil thermal diffusivity (k) is an important thermal property that significantly affects ground energy storage and heat transfer. Direct measurements of soil thermal diffusivity are challenging due to sensor limitations and variable soil physical properties, such as water content, bulk density, mineralogy and texture. Therefore, indirect estimations of k are commonly used. In this study, the abilities of six machine learning (ML) models, including k-nearest neighbours (KNN), multilayer perceptron (MLP), support vector regression (SVR), decision tree (DT), random forest (RF) and gradient boosting decision tree (GBDT), to estimate k values are evaluated based on a compiled database consisting of 999 samples. The ML model performances are evaluated by three model performance indicators (i.e., coefficient of determination-R2, mean absolute error-MAE and root mean square error-RMSE). The GDBT model best estimates soil thermal diffusivity, showing the highest fitness (R2 = 0.99). Model estimation accuracies are determined for varying numbers of available model inputs, and recommended minimum numbers of model inputs needed to accurately estimate thermal diffusivity are tabulated. This study demonstrates the ability of ML models to estimate values of soil thermal diffusivity, and it provides reference information for future thermo-related soil science and soil engineering applications.

Structural and Thermal Diffusivity Analysis of an Organoferroelastic Crystal Showing Scissor-Like Two-Directional Deformation Induced by Uniaxial Compression.

A two-directional ferroelastic deformation in organic crystals is unprecedented owing to its anisotropic crystal packing, in contrast to isotropic symmetrical packing in inorganic compounds and polymers. Thereby, finding and constructing multidirectional ferroelastic deformations in organic compounds is undoubtedly complex and at once calls for deep comprehension. Herein, we demonstrate the first example of a two-directional ferroelastic deformation with a unique scissor-like movement in single crystals of trans-3-hexenedioic acid by the application of uniaxial compression stress. A detailed structural investigation of the mechanical deformation at the macroscopic and microscopic levels by three distinct force measurement techniques (including shear and three-point bending test), single crystal X-ray diffraction techniques, and polarized synchrotron-FTIR microspectroscopy highlighted that mechanical twinning promoted the deformation. The presence of two crystallographically equivalent faces and the herringbone arrangement promoted the two-directional ferroelastic deformation. In addition, anisotropic heat transfer properties in the parent and the deformed domains were investigated by thermal diffusivity measurement on all three axes using microscale temperature-wave analysis (μ-TWA). A correlation between the anisotropic structural arrangement and the difference in thermal diffusivity and mechanical behavior in the two-directional organoferroelastic deformation could be established. The structural and molecular level information from this two-directional ferroelastic deformation would lead to a more profound understanding of the structure-property relationship in multidirectional deformation in organic crystals.

Thermal Diffusivity Measurement of Yttrium Oxide Powder by a Fabricated Photoacoustic Spectrometer

Photoacoustic (PA) is an infrequent form of spectroscopy where electromagnetic light waves directly interact with the molecules and an acoustic wave is produced. The absorbed energy is measured by detecting the pressure fluctuations in enclosed air volume. For this experiment, we have fabricated a PA cell and executed parametric studies for maximum sensitivity of its performance. The optimum inert cavity volume is found to be 84.3 × 10−7 m3 for maximum signal. Also, we have designed a pre-amplifier circuit and optimized it for desired frequency response by changing its basic components. Whole setup of the spectrosmeter was optimized using lamp carbon black as a reference sample. Finally, we have measured the thermal diffusivity of Yttrium oxide using PA spectroscopy and it is found to be 6.52 × 10−6 m2/s.

Thermophysical properties of pressure-sensitive paint

The paper deals with the problem of the thermophysical properties of pressure-sensitive paint (PSP). The paint was used to determine the pressure distribution on the surface of models examined in wind tunnel aerodynamic tests. The aim of the study was to provide reliable data on the values of thermophysical parameters and their dependence on temperature. PSP is a thin-film structure that is applied layer by layer a specific number of times on a metallic substrate, and determining the thermal properties requires non-standard test procedures to obtain reliable results. The measurements were carried out on paint fragments separated from the flat substrate of a metallic thin film (molybdenum), from which samples were obtained for complementary studies of the thermophysical properties. The study includes weight measurements, thermogravimetric analysis, micro-calorimetric analysis, and measurement of effective thermal diffusivity using the temperature oscillation technique. The findings are complemented with structural test data collected by means of scanning electron microscopy, surface characterisation, and density determination by the displacement method. The analysis of the structural test data indicates the micro-structural isotropy of the examined material with geometric anisotropy of the layer as a whole. In this way, the results of thermal diffusivity tests can be properly evaluated, and reliable data can be obtained on the complete thermophysical properties of the PSP layer. The indirectly determined thermal conductivity, i.e., by recalculating the specific heat, density, and thermal diffusivity data, allows the test material to be classified as an insulator. Apart from the presentation of the test results and calculations, this paper also contains a brief discussion of the test procedures.

Using photothermal radiometry to enable real-time sintering density measurements

Measurements of thermal diffusivity can increase speed, efficiency, and reliability of electronic field assisted sintering techniques.

Measuring Thermal Diffusivity of Azoheteroarene Thin Layers by Photothermal Beam Deflection and Photothermal Lens Methods.

Measurement of thermal properties of thin films is challenging. In particular, thermal characterization is very difficult in semi-transparent samples. Here, we use two photothermal methods to obtain information about the thermal diffusivity as well as thermal conductivity of azoheteroarene functionalized polymer thin layers. The photothermal beam deflection (PBD) method is employed to gather data directly on thermal conductivity and thermal diffusivity, while the thermal lens (TL) method is employed to measure the effective thermal diffusivity. Consequently, the thermal diffusivity of the layers is indirectly estimated from the effective thermal diffusivity using a well-established theoretical relationship. Despite the utilization of distinct methods, our study reveals a remarkable consistency in the highly accurate results obtained from both approaches. This remarkable agreement reaffirms the reliability and mutual compatibility of the employed methods, highlighting their shared ability to provide accurate and congruent outcomes.

NiTiCu shape memory alloys with ultra-low phase transformation range as solid-state phase change materials

Shape memory alloys (SMAs) have been demonstrated as effective phase change materials (PCMs) for thermal energy storage (TES) applications. NiTi and NiTiHf SMAs have shown high TES performance, as quantified by PCM figure of merit (FOM) but their use in applications requiring narrow operation temperature windows is limited by large overall phase transformation ranges (OTR). This work investigates NiTiCu SMAs as PCMs with high FOM and low OTR. A full-factorial design of experiments is used to examine 24 NiTiCu compositions. The compositions were fabricated using vacuum arc melting and their phase transformation and thermophysical properties were characterized using calorimetry, thermal diffusivity, and density measurements. The NiTiCu compositions spanned martensitic transformation temperatures between -22 and 84 °C and exhibited greater FOM (250–1050 106J2K−1s−1m−4), compared to traditional PCMs (typically <100 106J2K−1s−1m−4), with major benefits associated with higher density and higher thermal conductivity values. In addition, the NiTiCu compositions in this study show ultra-low OTR (12–20 °C) compared to NiTi and NiTiHf SMAs (>50 °C), enabling utility in narrow operating temperature windows. Thermal cycling was also performed revealing extreme stability of martensitic transformation with only 0.04 °C shift in transformation temperatures after 80 thermal cycles, which is the lowest reported to date in SMA literature.

Thermal diffusivity and conductivity measurement of undoped La2O3 powder by homemade photoacoustic spectrometer

Thermal diffusivity and conductivity measurement of undoped La2O3 powder by homemade photoacoustic spectrometer

Microstructure characterization of electric field assisted sintering (EFAS) sintered metallic and ceramic materials using local thermal diffusivity measurement

Electric Field Assisted Sintering (EFAS, also referred to as spark plasma sintering) is a powerful technology for the consolidation of powder materials. The high heating rate during the sintering process is critical for minimizing energy consumption, but it can also cause microstructure heterogeneities in sintered parts, such as spatially varied porosity. The examination of localized porosity usually requires the use of a scanning electron microscope with a carefully prepared surface. In this paper, photothermal radiometry is used to measure local thermal diffusivity and extract localized porosity of EFAS-sintered parts by using a percolation-threshold model. Applying this approach, we identified the radial position-dependent porosity variation in EFAS parts, which is likely formed due to the large temperature gradient during the sintering process. This approach has a unique advantage because it can measure samples with minimal or no surface preparation, enabling the possibility of in situ characterization in EFAS with proper system modification. Necessary modifications on the measurement approach for EFAS deployment and in situ characterization are also discussed.

Lock-in photothermal method for in-plane thermal diffusivity measurements using arrayed temperature sensors on suspended SiNx membranes.

A device consisting of a line- or spiral-shaped temperature sensor array on a two-dimensional (2D) silicon nitride (SiNx) membrane of thickness 50 or 150nm is developed for use in the lock-in photothermal method to determine the in-plane thermal diffusivity of SiNx membranes in air and in vacuum. The results of 2D heat diffusion are analyzed by the quadrupole method, and the system is approximated to the one-dimensional (1D) fin standing in a surrounding media (the fin approximation). The results show that 2D thermal diffusion on the membrane is affected not only by heat exchange with the surrounding environment but also by parallel thermal diffusion caused by heat conduction in the air along the membrane surface. The measurement using photothermal heating and contact detection of the temperature response enables the phenomenon to be detected consistently at a wide frequency range of temperature waves (50-1000Hz). The measured thermal diffusivity values of the SiNx membrane are much smaller than those of bulk material, which can be reasonably considered an effect of the confined state of the phonon in the nanoscale geometry of the membrane.

Effect of ionizing radiation on thermal and mechanical properties of filled-epoxy adhesives

Stave assemblies at the Large Hadron Collider and other high-energy physics experiments require radiation-resistant adhesives composed of light elements with high thermal conductivity and low interfacial resistance. Porous carbon foams are currently bonded to carbon fiber faceplates with graphite-filled epoxy. Low-viscosity epoxies are used to increase the amount of filler in order to double the thermal conductivity without impacting the properties of the material under irradiation. Promising new adhesive formulations showed no degradation in thermal conductivity after exposure to a fluence of 1016 n/cm2 (>0.1 MeV neutrons) or 10 Gy gamma radiation, but showed variable degradation, compared to the currently used adhesive, after exposure to a fluence of 1015 p/cm2 (1 MGy, 67.5 MeV protons). Adhesives with more than double the irradiated thermal conductivity of the baseline material showed marked improvement in thermal transport in graphite/epoxy/foam/epoxy/graphite structures based on thermal diffusivity measurements. Silicon carbide particles can be added to graphite to stiffen the modulus of the filled epoxy. When spherical AlN was used as a filler, the degradation in thermal conductivity under proton irradiation varied based on the size of the ceramic filler. Options for improving the thermal performance of irradiated adhesives for high-energy physics experiments are discussed.

Structure–property relations in graphitic pebbles for nuclear applications

This work presents an analytical approach for holistically characterizing graphitic matrix pebbles for nuclear applications whereby the macrostructure, microstructure, and thermophysical properties of pebbles are determined. A systematic sectioning method was applied to several pebbles to describe the regional properties of the samples. Intact matrix-only spheres and sections of spheres fabricated by Kairos Power were characterized via optical imaging, x-ray computed tomography, x-ray diffraction, and ellipsometry to determine 2D and 3D macrostructure and anisotropy. The thermophysical properties of these materials were determined via measurements of density, specific heat, thermal expansion, and thermal diffusivity. The results of this study indicate that the pebble fabrication methods and their resultant effect on microstructure have a nontrivial effect on thermophysical properties, confirming the importance of robust characterization of these components. A discussion of the characterization approach and its applicability to nuclear fuel development activities is also included.

Thermal diffusivity measurement of undoped Yb2O3 oxide by self-made photoacoustic cell

Photoacoustic spectroscopy is an essential technique for material characterization by measuring acoustic noise inside a cell. In this work, we have fabricated a photoacoustic cell and a pre-amplifier circuit and optimized both for maximum sensitivity. The cavity volume is measured ∼4.90 × 10−6 m3 at optimum condition. After that, the thermal diffusivity of undoped ytterbium oxide is measured by recording photoacoustic voltages. It is found 42.22–44.90 mm2/s for the excitation pump power 150–300 mW range by 980 nm diode laser source.