Flash Thermal Diffusivity Research Articles

The Influence of Cyclic Thermal Shocks at High Temperatures on the Microstructure, Hardness and Thermal Diffusivity of the Rene 41 Alloy.

The precipitation-hardenable nickel-based superalloy Rene 41 exhibits remarkable mechanical characteristics and high corrosion resistance at high temperatures, properties that allow it to be used in high-end applications. This research paper presents findings on the influence of thermal shocks on its microstructure, hardness, and thermal diffusivity at temperatures between 700 and 1000 °C. Solar energy was used for cyclic thermal shock tests. The samples were characterized using microhardness measurements, optical microscopic analysis, scanning electron microscopy coupled with EDS elemental chemical analysis, X-ray diffraction, and flash thermal diffusivity measurements. Structural transformations and the variation of properties were observed with an increase in the number of shocks applied at the same temperature and with temperature variation for the same number of thermal shocks.

Pool boiling critical heat flux studies of accident tolerant fuel cladding materials

Pool boiling experiments have been performed at atmospheric pressure on potential accident tolerant fuel (ATF) cladding materials (Zirlo®, Cr coated Zirlo®, FeCrAl coated Zirlo®, and monolithic SiC) to evaluate the Critical Heat Flux (CHF) points. Corrosion resistant coatings of Cr and FeCrAl alloy were deposited on flat Zirlo® samples using cold spray technology. The as-prepared samples after uniform surface polishing were subjected to autoclave tests at 360 °C water and 18.6 MPa for 360 h to simulate prototypic corrosion of the ATF cladding materials in LWR normal operation. Surface characteristics potentially influencing CHF such as surface morphology, roughness, static contact angle, and surface chemistry were characterized using a suite of characterization methods including scanning electron microscopy, 3D optical profilometry, optical contact angle measurements, and x-ray photoelectron spectroscopy (XPS). Thermo-physical properties of the samples such as density, thermal conductivity, and heat capacity were measured by differential scanning calorimetry and laser flash thermal diffusivity measurement. FeCrAl coatings showed CHF values comparable to bare Zirlo® samples, while slightly lower CHF values were observed for Cr coated samples and bulk SiC flats (produced by chemical vapor deposition, CVD). As-deposited Cr coatings showed 67% higher CHF than the as-polished Cr coatings due to its higher surface roughness level. The autoclave testing for the prepared samples generally increased the CHF except for the surface polished Cr coatings. Similar CHF values of bare Zirlo®, FeCrAl coating, and CVD SiC samples were observed after autoclave test but there were negligible change CHF of Cr coatings. XPS studies indicated that the formation or deposition of a few monolayers of hydrophobic carbonaceous species on surface can affect CHF values. The trends in CHF data are discussed in terms of evolution of the surface characteristics of the ATF materials.

Thermophysical properties of multiphase Fe-Al intermetallic-oxide ceramic coatings deposited by gas detonation spraying

While depositing Fe-Al intermetallic powders by applying a gas detonation spraying (GDS), a certain coating structure containing oxide ceramics is created. This multiphase structure exhibits both extreme mechanical resistance and unusual thermophysical properties (TPs). One such property is a relatively low thermal conductivity. A possible GDS structure application as the thermal barrier needs precise determination of TP dependence on temperature and high-temperature stability of the coating structure. In the present study, TPs were investigated for a coating produced from Fe-40Al at.% intermetallic powder in a course of complex measurements including differential scanning calorimetry analyses, laser flash thermal diffusivity measurements, and dilatometric studies complemented with microstructural analyses. The study resulted in a full characterization of the investigated TPs such as the density, thermal expansivity, heat capacity and thermal conductivity. Simultaneously with the TP studies, microstructural analyses were conducted that resulted in the identification of four phases, i.e., Fe3Al, FeAl, Al2O3 and (Fe,Al)3O4, within the investigated coating. During thermal analyses, the phenomenon of a sensitivity of TPs due to the temperature exposure of the investigated coating was revealed and characterized. The obtained results complement the sparse literature data on TPs of the multiphase pseudo-composite FeAl intermetallic/oxide type structures obtained in the GDS.

Investigation on dispersion stability and thermal-physical properties of hybrid graphene and MEPCM suspensions

Microencapsulated phase change materials (MEPCMs) were prepared by microfluidic technology. The MEPCMs, measured by scanning electron microscope, exhibited a uniform particle size distribution and good monodispersity. Glycerin and pure water were mixed in definite proportion as the base fluid with the same density as the MEPCM. Consequently, stable suspension of MEPCMs was easily obtained by mixing the MEPCMs into the base fluid. Graphenes were added to the suspension due to the super thermal conductivity of this material. The hybrid suspension, measured by dispersion stability analyzer, showed good stability, suggesting that the MEPCMs could impede the sedimentation of graphenes effectively. The viscosity and thermal conductivity of the hybrid suspension were investigated with rotational rheometer and flash thermal diffusivity instrument, respectively. The results showed that in the hybrid suspension, the thermal conductivity exhibited an obvious improvement in comparison with that of the MEPCM suspension, and the increase in viscosity caused by the addition of graphenes was negligible.

A Phenomenological Model of Unconventional Heat Transport Induced by Phase Transition in Cu2−xSe

Research in copper selenide thermoelectric (TE) alloys has raised the possibility of a significant enhancement of the TE figure-of-merit ZT when the Seebeck coefficient is affected by a concurrent phase transformation. This ZT increase has also been related to a radical reduction of the thermal conductivity evaluated by transient laser flash thermal diffusivity measurements. In contrast, steady-state Harman-based measurements do not support a significant ZT increase only a modest one, because the thermal conductivity instead of decreasing goes through a sharp maximum as it approaches the critical phase transformation temperature of 407 K. The nature of this sharp increase of heat transfer has not been related to the well-known electronic or phononic contributions. Below the critical temperature, when the alloy is exposed to a steady-state temperature gradient, an additional heat transfer phenomenon takes place, induced by the ongoing gradual phase transition. We show that the enthalpy associated to the α to β and β to α phase transformations can lead to heat flow in the direction of the temperature gradient above and beyond conventional heat conduction. This unconventional heat transfer mechanism disappears when the temperature rises above the critical temperature where only a stable β phase remains. We propose a model of such a heat transport which leads to the sharp maximum of the related thermal conductivity. Numerical results obtained from the model compare favorably to the experimentally measured thermal conductivity.

Electrical and thermal properties of surface passivated carbon nanotube/polyvinylidene fluoride composites

The composites reported here were prepared through a melt-rolling mixing process, in which surface passivated multi-wall carbon nanotube and poly(vinylidene fluoride) were used as the filler and the matrix, respectively. X-ray diffraction, precision inductance-capacitance-resistance, and laser flash thermal-diffusivity analyses were employed to characterise the structural, electrical and thermal properties. Results indicate that through the surface passivation of carbon nanotubes, the electrical resistivities of the composites were greatly enhanced, while the thermal conductivities do not change significantly. The electrical properties were discussed in comparison with the Maxwell-Wagner polarisation, universal law, and Gerhardt's method.

Additive manufacturing of Inconel 718—Copper alloy bimetallic structure using laser engineered net shaping (LENS™)

To understand processing ability and measure resultant interfacial and thermal properties of Inconel 718 and copper alloy GRCop-84, bimetallic structures were fabricated using laser engineering net shaping (LENS™), a commercially available additive manufacturing technique. It was hypothesized that additively combining the two aerospace alloys would form a unique bimetallic structure with improved thermophysical properties compared to the Inconel 718 alloy. Two approaches were used: the direct deposition of GRCop-84 on Inconel 718 and the compositional gradation of the two alloys. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray Diffraction (XRD), Vickers microhardness and flash thermal diffusivity were used to characterize these bimetallic structures to validate our hypothesis. The compositional gradation approach showed a gradual transition of Inconel 718 and GRCop-84 elements at the interface, which was also reflected in the cross-sectional hardness profile across the bimetallic interface. SEM images showed columnar grain structures at the interfaces with Cr2Nb precipitate accumulation along grain boundaries and the substrate-deposit interface. The average thermal diffusivity of the bimetallic structure was measured at 11.33 mm2/s for the temperature range of 50 °C–300 °C; a 250% increase in diffusivity when compared to the pure Inconel 718 alloy at 3.20 mm2/s. Conductivity of the bimetallic structures increased by almost 300% compared to Inconel 718 as well. Such structures with designed compositional gradation and tailored thermal properties opens up the possibilities of multi-material metal additive manufacturing for next generation of aerospace structures.

Preparation and thermal performance of form-stable expanded graphite/stearic acid composite phase change materials with high thermal conductivity

Thermal energy storage plays an important role in improving the working reliability and utilization efficiency of renewable energy systems. Thermal energy storage includes sensible heat storage, latent heat storage using phase change material (PCM) and thermochemical heat storage. In comparison with sensible heat storage, latent heat storage has the distinct advantages of its high energy storage density and stable temperature during the energy storage and release processes. However, low thermal conductivity of PCM is the common drawback for various latent heat energy storage systems. The purpose of the research is to enhance the thermal conductivity of PCM by developing new form-stable composite phase change materials using expanded graphite matrix. Firstly, a preparation method “heating adsorption of liquid SA into expanded graphite (EG) matrix and compressing stable-shape composite” is employed to synthesis form- stable composite phase change material. Twenty samples with different EG contents and packed densities are prepared using stearic acid (SA) as phase change material and two kinds of EG with different dilatation as porous matrix. Secondly, the microstructures, thermal properties and thermal stabilities of different EG/SA samples are tested to analyze the influence of the dilatation of expanded graphite on the thermal performance of form-stable composite phase change materials. Moreover, EG/SA form-stable heat storage units with different EG contents and packed densities are fabricated to analyze the thermal storage/release performance through heating/cooling experiment. The microstructures of EG and EG/SA samples are observed using scanning electron microscopy (SEM) and the results show there exist obvious differences between two kinds of EG and SA distribution in the samples. The EG/SA samples with high dilatation EG are even more uniform because the high dilatation EG has larger pore size and pore volume. The data obtained from differential scanning calorimetry (DSC) show the addition of EG and compressing operation have negligible effect on the latent heat and slightly increase the phase transition temperature of pure SA. The form-stable EG/SA composite PCMs have lower supercooling degree than pure SA. LFA447 flash thermal diffusivity instrument is used to evaluate the effect of EG’s dilatation on the thermal conductivity of EG/SA samples. The results show that the use of high dilatation EG is more effective to improve the thermal conductivity of form-stable EG/SA composite PCMs when packed density is large. The radial thermal conductivity of form-stable 20 wt.%EG/SA phase change composite with packed density of 950 kg m−3 is as high as 19.6 W m−1 K−1 when high dilatation EG is utilized. This figure is more than 110 times higher than that of pure SA. Finally, the thermal stability of EG/SA samples is conducted by repeating the charging and discharging processes of SA. These samples prepared by high dilatation EG have better thermal stability than the samples prepared by low dilatation EG. The EG with high dilatation can assure the uniform distribution of SA inside EG/SA sample and thus prevent the leakage of liquid SA during the phase change transformation process due to its dilated microstructure. The heating/cooling test results show that the heat storage/release time of EG/SA composite units is about 1/8–1/4 of that of pure SA unit, and this means the heat transfer can be significantly enhanced. It suggests that the form-stable EG/SA composite phase change materials prepared by high dilatation EG has good comprehensive performance due to its high thermal conductivity, good thermal stability and no leakage problem.

Processing Parameter Effects and Thermal Properties of Y2Si2O7 Nanostructured Environmental Barrier Coatings Synthesized by Solution Precursor Induction Plasma Spraying

The solution precursor plasma spray process, in which a solution of metal salts is axially injected into an induction thermal plasma, is suitable for deposition of nanostructured environmental barrier coatings. The effects of main processing parameters, namely the solution precursor concentration, spraying distance, reactor pressure, and atomization gas flow rate, have been analyzed using D-optimal design of experiments regarding the deposition rate and coating porosity responses. Among these four parameters, the solution precursor concentration had the greatest influent on the coating structure, followed by the spraying distance and reactor pressure, and finally the atomization gas flow rate with a small contribution. It is pointed out that the species that impact on the substrate are agglomerates of nanoparticles. The equivalent thermal conductivity of selected coatings was computed from experimental temperature evolution curves obtained by laser flash thermal diffusivity analysis, using two methods: a multilayer finite-element model with optimization, and a multilayer thermal diffusion model. The results of the two models agree, with coatings exhibiting low thermal conductivity between 0.7 and 1 W/(m K) at 800 °C.

Characterization of structure and thermophysical properties of three ESR slags

The structure and properties of electroslag remelting (ESR) slags were characterized. Slags samples of three compositions were obtained from industrial remelting processes at Special Metals Corporation and from casting in a laboratory vacuum induction melter. The structure of the slag samples was observed using optical and electron microscopy, and phases were identified and their relative amounts quantified using X-ray diffraction. Laser flash thermal diffusivity, density, and differential scanning calorimetry measurements for specific heat were performed to determine the bulk thermal conductivity of the samples. Sample porosity was measured as a function of depth using a serial sectioning technique, and a onedimensional computational model was developed to estimate the thermal conductivity of the fully dense slags. These results are discussed in context with previous studies, and opportunities for future research are identified. AFRL Case Number: 88ABW-2015-1871.

A modified evaluation method to reduce finite pulse time effects in flash diffusivity measurement.

A modified evaluation method for laser flash is proposed. In this method, the moment of laser-heating cutoff time is considered as zero point. The penetration depth and formula equation of the sample temperature distribution are obtained with the approximate analytical solution before time zero (during laser heating) for the physical model of a continuously heated half-infinite, well-distributed sample. The weighted-average and approximate-equation methods are then used to quantitatively determine the laser effect depth, which leads to the formulation of a modified evaluation method in flash thermal diffusivity measurement. Results of the simulation calculations and experiments confirm the correctness of the modified method, which remarkably increases flash method applications. The modified method is applicable only to cases in which δ(x) does not exceed the sample thickness (√(12ατ0)≤L) during laser heating.

Thermal Conducting Silicone Rubber Composites Filled with Aligned Nickel Nanoparticles Induced by Magnetic Field

Thermal conducting silicone rubber composites filled with aligned nickel (Ni) nanoparticles induced by magnetic field were prepared by the solution mixing process. The structure, thermal, dielectric and magnetic properties of the silicone rubber composites were investigated by optical microscopy, laser flash thermal diffusivity analyzer, LCR digital meter and vibrating sample magnetometer. The results show that with the induction of magnetic field, Ni nanoparticles aligned form thermal conducting networks resulting in increase of the thermal diffusivity and relative dielectric constant of the silicone rubber composites.

In Situ Measurement of the Thermal Contact Resistance of an Al Lap Joint During Braze Processing

A technique was developed, using a laser flash thermal diffusivity apparatus, to measure the thermal contact resistance, Rc, of an Al lap joint during braze processing. The method required a determination of the temperature dependence of the thermal diffusivity of the individual Al braze sheets, as well as a two-layer lap joint made from these sheets, under identical conditions. A two-layer modeling method was then used to analyze the above data to determine Rc for the lap joint as a function of processing temperature. This in situ analysis of Rc for a developing joint during brazing was able to distinguish both solid-state and liquid-phase reactions occurring at the faying surfaces of the joint. Consequently, it represents a powerful, new experimental tool that can be used to investigate the mechanisms of braze joint formation and the thermal properties of a joint as a function of processing condition. In the particular case of this study, the technique demonstrates that the use of a Ni-based fluxless brazing process to join aluminum (Al) sheets, reduced the Rc of the lap joint by 100-folds from 1.3 × 10−4 m2 K/W before brazing to 1.35 × 10−6 m2 K/W after brazing.

The preparation of a core/shell structure with alumina coated spherical silica powder

In this work, a SiO2@Al2O3 core/shell structure was prepared through a sol–gel approach. Various techniques were used to characterize the as-prepared products, including thermogravimetric analysis and differential scanning calorimeter (TG-DSC), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and laser flash thermal diffusivity apparatus. The TG-DSC and XRD results showed a SiO2@Al2O3 structure comprised of amorphous silica and α-Al2O3 at a calcination temperature of 1000°C. In addition, the FESEM images and EDS results revealed that the α-Al2O3 shell had a thickness of around 90nm. The presence of SiO and AlO was confirmed through FTIR analysis. Different modification surfactants were investigated, revealing the superior properties of polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) for dispersing and coating spherical silica powders. We obtained smooth spherical SiO2@Al2O3 particles by changing Al2O3 sol adding rate. The results confirmed that spherical silica powders had been uniformly coated by alumina powders with the size less than 50nm. The SiO2@Al2O3 powders demonstrated higher thermal conductivity than raw spherical silica powders. The thermal conductivity of spherical silica powders was 0.24Wm−1K−1, while that of SiO2@Al2O3 powders reached 0.46Wm−1K−1.

Fit for purpose models for metrology: a model selection methodology

We describe a model selection methodology for partial differential equation (PDE) based models and show the results of applying it to a test problem derived from a model of the laser flash thermal diffusivity measurement technique. A methodology for comparing and choosing simplified models is of benefit to the model development process in metrology. It is assumed that the computational aim is not only to solve the model to obtain the results that the metrologist requires, but to ensure that the model is no more complex than necessary to achieve this and that results can be obtained in a reasonable time using the available computing resources. The advantage of the proposed method is that it avoids the need to solve directly the underlying complex model. We present the results of comparisons of four models of the laser flash problem and identify the further work needed to extend the approach to a wider range of problems and to identify suitable measures for comparing residuals.

Tuning the thermal diffusivity of silver based nanofluids by controlling nanoparticle aggregation

With the aim of preparing stable nanofluids for heat exchange applications and to study the effect of surfactant on the aggregation of nanoparticles and thermal diffusivity, stable silver colloids were synthesized in water by a green method, reducing AgNO3 with fructose in the presence of poly-vinylpyrollidone (PVP) of various molecular weights. A silver nanopowder was precipitated from the colloids and re-dispersed at 4 vol% in deionized water. The Ag colloids were characterized by UV–visible spectroscopy, combined dynamic light scattering and ζ-potential measurements, and laser flash thermal diffusivity. The Ag nanopowders were characterized by scanning electron microscopy and thermal gravimetric analysis. It was found that the molecular weight of PVP strongly affects the ζ-potential and the aggregation of nanoparticles, thereby affecting the thermal diffusivity of the obtained colloids. In particular, it was observed that on increasing the molecular weight of PVP the absolute value of the ζ-potential is reduced, leading to increased aggregation of nanoparticles. A clear relation was identified between thermal diffusivity and aggregation, showing higher thermal diffusivity for nanofluids having higher aggregation. A maximum improvement of thermal diffusivity by about 12% was found for nanofluids prepared with PVP having higher molecular weight.

Thermal Diffusivity and Dielectric Property of Silicone Rubber/Alumina Microparticles Composites Prepared by Solution Mixing Process

The silicone rubber composites filled with various contents of alumina (Al2O3) microparticles were prepared by the solution mixing process. The fractured surface morphology, thermal diffusivity and dielectric property of the composites were investigated by scanning electron microscopy (SEM), the laser flash thermal diffusivity analyzer, a LCR digital meter and a resistor tester, respectively. The results show that with Al2O3 content increasing from 8 vol. % to 35 vol. %, the thermal diffusivity increases from 0.13 mm2/s to 0.31 mm2/s, while the dielectric property reduces slightly. At a given Al2O3 content, the thermal diffusivity of the composites exhibits a reduction tendency with the increasing temperature. The Nielsen model is generally suitable to predict the thermal diffusivity of the composites.

In-pile irradiation induced defects and the effect on thermal diffusivity of MgO

The effects of neutron irradiation temperature and dose on thermal diffusivity are investigated by comparing non-irradiated and in-pile irradiated MgO samples. MgO pellets were irradiated in-pile of the Advanced Test Reactor at Idaho National Laboratory. Samples were irradiated at 623 and 973K to fast neutron fluences of ∼1×1025 (1.5dpa) and ∼2×1025n/m2 (3dpa). Post irradiation examination included X-ray diffraction, scanning electron microscopy, laser flash thermal diffusivity, and transmission electron microscopy. Neutron irradiation of MgO causes a significant reduction in the thermal diffusivity (46%–72% at room temperature) due to irradiation-induced defects. The radiation induced thermophysical and structural evolution of MgO is reported.

Study of Glow Wire Ignition Temperature (GWIT) and Comparative Tracking Index (CTI) performances of engineering thermoplastics and correlation with material properties

Recent regulation IEC 60335-1 ed.4 (2008) was introduced for materials used in electric appliance, establishing new limits in Glow Wire Ignition Temperature (GWIT) performance for materials used for electric connectors. Development of new products with high GWIT is possible, but the main issue is to keep good mechanical properties and processability, as well as tracking resistance (Comparative Tracking Index-CTI). Only a few patents and scientific publications exist about glow wire test performance of polymers. In this work we report GWIT and CTI properties for three engineering thermoplastic polymers (PBT, PET and PC). We have also studied the phenomena involved in this test, treating the phenomena with the parameterisation approach already used in the studies of the fire behaviour of polymers. PC, PBT and PET filled with 30% w/w glass fibres have been tested, and material properties that can be related to GWIT and CTI performance have been measured by TGA, Laser Flash Thermal Diffusivity (LFTD), Pyrolysis-GC/MS. CTI seems to be correlated with the char formation tendency of the materials, so PBT show a higher tracking resistance than PET and PC. Polycarbonate was the only material that passed the glow wire test (GWIT higher than 775°C) but generally GWIT performance is not directly related with degradation temperature, since PET is thermally more stable compared with PBT, but less stable in glow wire test. The ignition process, together with the unsteady heat and mass transfer process characteristic of glow wire testing, are affected by many parameters at the same time. That’s why it is not easy to relate results of TGA, Laser flash, Pyrolysis-GC/MS with the glow wire ignition temperature of the materials tested, but the whole of these properties can give useful indication.

Mechanical and thermal properties of SiC f/SiC composites irradiated with neutrons at high temperatures

A high fluence irradiation of SiC f/SiC composites has been performed to study the effect of neutron irradiation on the mechanical and physical behaviour of those composites. The fibre reinforced silicon carbide composites have been irradiated at two temperature levels of 600 °C and 900 °C up to a fluence of 3.5 × 10 25 n/m 2 ( E > 0.1 MeV). The stiffness of the bending bars after irradiation changed with factors between 0.75 and 1.04 for the different composites while the bending strength after irradiation was reduced with a factor of 2 in some cases. The laser flash thermal diffusivity ratio's ( α irr/ α o) of the composites measured at 600 °C are from 0.1 to 0.5 and from 0.25 to 0.75 for an irradiation temperature of 600 °C and 900 °C, respectively. The dimensional changes observed are small.