Thermal Diffusivity Values Research Articles

Study of the Thermal Diffusivity Variation in Thin Duplex Steel Plates Welded by GTAW Process

This study describes the thermal diffusivity of thin duplex steel plates in the thickness direction measured using the laser-flash method after welding. The work reports the experimental efforts in recording temperature profiles of the grade UNS S32304 duplex steel during autogenous welding. The butt weld autogenous joints were carried out by the GTAW (gas tungsten arc welding) process with either argon or argon - 2% nitrogen atmospheres. The amount of nitrogen in the heat affected regions, after welding, was measured and correlated with the variation of the thermal diffusivity of the studied material. The temperature profiles were obtained using k-type thermocouples connected to a digital data acquisition system. Different thermal cycles and thermal diffusivity values were observed in the heat-affected zone (HAZ) for both samples. In the solidified zone (SZ) was observed similar increase of the thermal diffusivity values for the plates welded with pure argon and argon plus nitrogen atmosphere.

Photothermal Techniques for Thermal Characterization of Linear Alcohols

Photothermal techniques were used for the thermal characterization of linear alcohols. This was carried out by measuring the thermal diffusivity (by means of a photopyroelectric technique) and thermal effusivity (by means of a photoacoustic technique) of ten linear alcohols, from methanol to 1-decanol. The thermal conductivity and specific heat for these substances were obtained by means of their relations with the two previous thermal properties, by using the values reported for the densities of the alcohols. The values of thermal effusivity showed a decreasing behavior with the increase in length of the linear molecule, and the values of thermal diffusivity, on the other hand, showed a similar behavior but only up to 1-pentanol, from which these values began to increase; this latter behavior was also observed, although to a lesser extent, for thermal conductivities. This peculiar behavior for thermal diffusivity and thermal conductivities is attributed to the influence of the hydroxyl group, which is strong for low molecular weight alcohols, but it diminishes as the size of alcohol’s molecule increases.

Criteria for accurate measurement of thermal diffusivity of solids using the Angstrom method

This paper presents criteria for accurately measuring the thermal diffusivity of a solid material using the Ångström method. In this technique, a periodic heat load is supplied at one end of the sample and temperatures are measured at different locations along the sample. The thermal diffusivity is estimated from the measured amplitude and phase of the temperature oscillations at various locations along the sample. Criteria are presented for important measurement parameters namely as periodic heat load power, waveform and frequency. Since more than two sensors are used, the accuracy is further improved since the linearity of the phase and amplitude data can be verified. We experimentally establish that a sinusoidal input heat load with a frequency as defined by the criteria established in this paper results in improved measurement accuracy. Based on these criteria, the experiment was performed with a range of materials spanning low to high thermal diffusivities namely Teflon, G10, Titanium Alloy (Ti6Al4V), Stainless steel (SS316) and Aluminium Alloy (Al6061-T6). The results indicate that the measured thermal diffusivity values deviate from the literature data at room temperature by less than 2.1%.

Transient hot wire measures thermophysical properties of organic foam thermal insulation materials

A device based on the transient hot wire (THW) method was developed to measure thermal conductivity and thermal diffusivity of organic foam thermal insulation materials. EPS board and XPS board were used as the experimental samples. The simulation results show that the calculation errors of thermal conductivities with cross hot wire (CHW) method, parallel hot wire (PHW) method and cross-parallel hot wire (CPHW) method are 0%, and the calculation errors of thermal diffusivities with these three methods are small. The experimental results show that for PHW method, the calculation results of thermal conductivity and thermal diffusivity of EPS board fluctuate greatly with the selection of computation time. For CHW method and CPHW method, the measurement errors of thermal conductivities of EPS board are less than 3.5% and that of XPS board are less than 6.5%. The experimental values of thermal diffusivities with CPHW method are close to the reference values, but the experimental values with CHW method are much smaller than the reference values. The measurement errors of thermal diffusivities with these two methods were analyzed theoretically and experimentally. The measurement error with CPHW method is approximately between 27% and 13% with the error of thermal conductivity from −5% to 5%, however, the measurement error with CHW method is more than 99%. The proposed device offers some significant advantages such as economy, flexibility, and the repeatability of measurement accuracy. If applied properly, CPHW method is promising to measure the thermal conductivity and thermal diffusivity of many other types of solid materials.

(Invited) Construction of Soft Materials Composed of Hierarchically Assembled Filamentous Viruses

Recent studies have revealed that viruses such as filamentous M13 bacteriophages (phages), with a specific affinity for artificial materials or biomolecules, can be utilized to produce various materials. To widen the applicability, the control of phage assemblies is an essential requirement. Herein, we demonstrate solid films composed of regularly assembled phages with high thermal diffusivities. The oriented structures of the phages in the films were characterized in detail. It was revealed that thermal diffusivity of the films with oriented structures prepared by the flow-induced method was higher values than that with the disordered structure. Furthermore, the thermal diffusivity values of the films with nematic- and smectic-liquid crystalline orientated phage structures were slightly different, suggesting phage oriented structures were essential for the thermal diffusivity. The results indicated that thermophysical property of phage films could be controlled by their assembly.

Chaotic Rayleigh-Bénard convection with finite sidewalls.

We explore the role of finite sidewalls on chaotic Rayleigh-Bénard convection. We use large-scale parallel spectral-element numerical simulations for the precise conditions of experiment for cylindrical convection domains. We solve the Boussinesq equations for thermal convection and the conjugate heat transfer problem for the energy transfer at the solid sidewalls of the cylindrical domain. The solid sidewall of the convection domain has finite values of thickness, thermal conductivity, and thermal diffusivity. We compute the Lyapunov vectors and exponents for the entire fluid-solid coupled problem. We quantify the chaotic dynamics of convection over a range of thermal sidewall boundary conditions. We find that the dynamics become less chaotic as the thermal conductivity of the sidewalls increases as measured by the value of the fractal dimension of the dynamics. The thermal conductivity of the sidewall is a stabilizing influence; the heat transfer between the fluid and solid regions is always in the direction to reduce the fluid motion near the sidewalls. Although the heat interaction for strongly conducting sidewalls is only about 1% of the heat transfer through the fluid layer, it is sufficient to reduce the fractal dimension of the dynamics by approximately 25% in our computations.

Thermal diffusivity of tungsten irradiated with protons up to 5.8 dpa

Thermal properties of pure tungsten, irradiated in the Swiss neutron spallation source at the Paul Scherrer Institut, have been studied in the temperature range 25–500 °C. Disk-shaped specimens were prepared from a tungsten sheet which was irradiated with high energy protons and spallation neutrons. The specimens tested in this work received total damages of maximum 3.9 and 5.8 dpa at average irradiation temperatures of 115 and 140 °C, respectively. The thermal diffusivity of the irradiated tungsten was measured using the conventional flash method. For both specimens, the results show a significant decrease in thermal diffusivity after irradiation. Relative to unirradiated tungsten, the irradiated samples show thermal diffusivity values which are 28–51% lower, depending on temperature. Annealing of the irradiated specimen of 3.9 dpa at 1000 °C for 1 h resulted in a slight recovery of thermal diffusivity. In addition, thermal conductivity values were calculated from the thermal diffusivity data.

Influence of measurement parameters of laser flash analysis on the observed thermal diffusivity and the choice of parameters to get repeatable measurements

Sugar alcohols are food grade, low cost and have very high latent heat capacities. They are therefore an attractive candidate for energy storage applications. However, to be able to utilize their potential, thermal conductivity is an important constraint. Laser flash analysis (LFA 447) was used for the measurement of thermal diffusivity. Influence of sample thickness, measurement time, voltage, pulse width and choice of curve fitting model on the measured thermal diffusivity values is studied. These parameters are then carefully chosen to ensure repeatability in the diffusivity measurement of sugar alcohols. Diffusivity results obtained using these criteria for several sugar alcohols are found to be in good agreement with consecutive measurements.

Thermal Diffusivity of Compounds Loaded with Carbon Nanofibers

In this work, the thermal enhancement in silicone grease-based compounds as a function of carbon nanofiber (CNF) volume fraction was investigated. The thermal diffusivity of the samples was determined by a photopyroelectrical technique with a sample thickness scan. The results show that heat transport on these compounds strongly depends on the CNF volume fraction, due to the high thermal conductivity of CNF compared to the matrix; hence, a low loading percentage of the fibers produce a significant growth in the thermal diffusivity of the composite. The results show that the thermal diffusivity values of the CNF-silicone composite are comparable with commercial thermal compounds based on diamond and Ag microparticle fillers. The thermal conductivity of the samples was calculated, and its enhancement was analyzed using a modified Lewis–Nielsen model, taking into account the dependence of the maximum packing fraction and the form factor with the aspect ratio of the CNF. The influence of the Kapitza thermal resistance was discussed. These materials might find practical applications in systems in which the CNF improves the ebbing of heat away from semiconductor devices or in any other application in which heat dissipation is needed.

Synthesis and study the physical properties of the nanosized Li0.5AlxFe2.5-xO4 samples

Nanoparticles of Li0.5AlxFe2.5-xO4 (x = 0.00, 0.15, 0.25, 0.35, 0.45) have been synthesized by using a chemical auto-combustion method. The effect of varying Al3+ ions content on the microstructure, magnetic, thermal, and dc electrical conductivity, mobility and activation energy of the as-prepared nanosamples have been studied. The obviously low loss thin hysteresis curve of the M-H relation reveals that the present materials are classified soft magnetic. The dependence of magnetic properties on adding Al3+ ions is explained as due to many factors such as crystallite size, measured density, porosity, expected cation distribution, and magnetocrystalline anisotropy. The cation distributions for all samples have been suggested such that the experimental and theoretical values of magnetic moments and lattice constants are in good consistency. Moreover, the pure Li-ferrite sample of (x = 0) has lower values of thermal conductivity, specific heat capacity, and thermal effusivity; and higher values of thermal diffusivity than those of the Al substituted Liferrite samples. The reasons of this behavior have been discussed. Also, the temperature and the chemical composition dependence of dc conductivity and drift mobility of the prepared samples have been studied.

Numerical analysis of the heat transfer properties of z-pinned composites

The controlled increase of the through-thickness heat transfer properties of fibre reinforced polymer composites using z-pins is demonstrated using finite element (FE) modelling. The modelling reveals that the through-thickness thermal diffusivity of composite materials can be tailored to a specific value via the judicious choice of the volume content and thermal conductivity of z-pins. The effects of the volume content (up to 5%) and material type (carbon fibre composite, steel, titanium, copper) of z-pins on the through-thickness and in-plane thermal diffusivities of a carbon-epoxy composite is analysed. The through-thickness thermal diffusivity of the composite increases linearly with the z-pin content, and this is because the pins act as a thermally conductive pathway. The through-thickness thermal diffusivity value of the composite also increases with the thermal conductivity of the z-pin, however with an accompanying increase to the amount of lateral heat transfer. FE modelling reveals the thermal properties of composites in the in-plane directions are not affected significantly by z-pins. This study reveals that z-pinning is a novel approach to increase the through-thickness heat transfer properties of composites.

Effect of carbon nanoparticle reinforcement on mechanical and thermal properties of silicon carbide ceramics

This research presents an analysis of the influence of graphene reinforcement on the thermal and mechanical properties of silicon carbide ceramics, at 2.5% (wt%) graphene content. The SiC composites, containing various carbon nanofillers (graphene oxide and graphene nanoparticles), were sintered by the classical two stage spark plasma sintering method. Two current modes were used, the continuous mode and the pulsed current mode. The results from photothermal radiometry and investigations of the mechanical properties showed that graphene additives significantly improve the thermal properties and toughness of material, sintered from a SiC powder. An 45% growth in the toughness was observed, which increased from 1.21 to 1.75 MPa/m1/2. The thermal diffusivity value also increased from 0.60 to 0.71 cm2/s and giving an improvement in thermal properties of 18%. The friction coefficient reached 7% giving an increase in value from 0.62 to 0.66. Microscopic investigations supported the photothermal radiometry (PTR) results. Whilst, thermal imaging revealed homogeneity of the local thermal properties of the products fabricated from the starting SiC powder.

Rheological and Thermal Transport Characteristics of a Transformer Oil Based Ferrofluid

Ferrofluids based on insulating liquids are intensively studied as a potential substitute of liquid dielectric in high voltage technologies. In this work we focus on the experimental investigation of flow and thermal transport characteristics of a ferrofluid based on transformer oil (Mogul) and iron oxide nanoparticles. The magneto-rheological behavior of the ferrofluid was studied by a rotational rheometer in the shear rate range from 1 to 1000 s-1 and magnetic field up to 1 T. By means of a thermal constants analyzer and a transient plane source method we obtained the thermal conductivity, specific heat and thermal diffusivity values for the studied oil and the ferrofluid. It is shown that the Newtonian character of the ferrofluid changes to a non-Newtonian with application of the magnetic field. The notable magneto-viscous effect has been observed especially at low shear rates. We found that the doping of the transformer oil by 3 wt% of the nanoparticles results in a thermal conductivity enhancement by about 3.2%. (Less)

Characterization of Ln4Al2O9 (Ln=Y, Sm, Eu, Gd, Tb) rare-earth aluminates as novel high-temperature barrier materials

A family of cuspidine-type rare-earth (RE) aluminates with the general formula Ln4Al2O9 (Ln= Y, Sm, Eu, Gd, Tb) was prepared for potential use as thermal-barrier coating (TBC) materials with appropriate properties. Various trivalent lanthanides were applied to tailor the properties of the oxides for use as ceramic top coat (TC) materials intended for high-temperature applications. Following various heat treatments, the X-ray diffraction (XRD) results obtained demonstrated that Eu4Al2O9 (EuAM) possessed the greatest structural stability of all the samples at 1200 and 1300 °C. Moreover, Y4Al2O9 (YAM) had a long lifetime at 1000 °C, and was stable at 1100 °C. At 1200 °C, Sm4Al2O9 (SmAM) and Gd4Al2O9 (GdAM) were more stable than Tb4Al2O9 (TbAM). However, at 300–1000 °C, the TbAM exhibited the highest thermal expansion coefficient (TEC) of all the samples. At 600 °C, the thermal diffusivity values of the five compositions were favourable, and were lower than that of yttria-stabilised zirconia (YSZ) oxides.

Annealing study and thermal investigation on bismuth sulfide thin films prepared by chemical bath deposition in basic medium

Bismuth sulfide thin films were prepared by chemical bath deposition using thiourea as sulfide ion source in basic medium. First, the effects of both the deposition parameters on film growth as well as the annealing effect under argon and sulfur atmosphere on as-deposited thin films were studied. The parameters were found to be influential using the Doehlert matrix experimental design methodology. Ranges for a maximum surface mass of films (3 mg cm−2) were determined. A well-crystallized major phase of bismuth sulfide with stoichiometric composition was achieved at 190 °C for 3 h. The prepared thin films were characterized using grazing incidence X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis. Second, the bandgap energy value was found to be 1.5 eV. Finally, the thermal properties have been studied for the first time by means of the electropyroelectric (EPE) technique. Indeed, the thermal conductivity varied in the range of 1.20–0.60 W m−1 K−1, while the thermal diffusivity values increased in terms of the annealing effect ranging from 1.8 to 3.5 10−7 m2 s−1.

A propane burner test for passive fire protection (PFP) formulations containing added halloysite, carbon nanotubes and graphene

Passive fire protection materials (PFP) have been characterised quantitatively using a constant heat flux propane burner test, with the PFP attached to a steel substrate. The burner test was able to produce a large, constant heat flux, to simulate a severe fire condition. The heat transferred through the PFP was calculated from the temperature rise of the steel. A simplified model is discussed, to account for different combinations of substrate thickness, PFP thickness, heat flux and exposure time. It was found that, despite the complex processes of resin decomposition and intumescence, heat transmission could be modelled, to a reasonable approximation, by treating the PFP as a material with single point values of apparent thermal diffusivity and conductivity. This leads to an equation that can be used to characterise PFPs and indeed to specify their required thickness.The burner test was employed to interpret the effects of adding small quantities of three nano-materials: halloysite nanotubes (HNT), multi-walled carbon nanotubes (MWCNT) and graphene nanoplatelets (GNP), to a standard PFP formulation. It was found that HNT addition resulted in a significant performance improvement, whereas the other nano-materials did not produce an improvement.

Understanding the temperature dependence of thermo-physical properties of nanofluid suspensions using non-intrusive dynamic measurements

Thermal diffusivity dependence on temperature and concentration of aqueous colloidal suspensions of Al2O3 in deionized water has been investigated by conducting transient heat transfer experiments. The study has been performed in a purely non-intrusive manner using Mach Zehnder interferometry. Through the dependence of refractive index on temperature, the time sequence of recorded interferograms has been used to generate highly sensitive data of evolving temperature distribution in the thermal diffusion field. Determination of thermal diffusivity from temperature-position data has been carried out by the method of least square. In comparison with deionized water, substantial enhancement in thermal diffusivity with increasing temperature and concentration of nanofluid suspension of Al2O3 is observed through the time sequence of the recorded interferograms as well as through analytical measurements. Additionally, the measurement of ζ-potential, pH and effective viscosity of dilute Al2O3 suspension at various temperatures explains the possible mechanisms at micro-scale levels that are responsible for such an anomalous behaviour of thermal diffusivity. The competing interaction of these factors reveals dependence of thermal diffusivity on particle cluster formation and thereby in the resultant stability of the suspension. The experimental findings indicate that, at specific size and shape, the higher conductivity of solid alumina particles and their cluster formation plays a major role in enhancing the thermophysical properties of the nanofluid Al2O3 suspension over the considered range of temperature and concentration. Also, a reasonably close agreement of the obtained trends of thermal diffusivity values with those reported in the literature and clear visualization of enhancement in thermal transport through interferograms signifies the importance of the proposed non-intrusive measurement technique for reliable measurement of thermal properties of nanofluids.

Multi-layered thermal barrier coatings on BMI polyimide matrix composite

Al/YSZ and (Cu-6Sn)/YSZ as the double layer thermal barrier coatings (TBCs) and Cu-6Sn/NiCrAlY/YSZ as the triple layer TBC with 100μm thick for bonding layers and ceramic top coat were deposited on bismaleimide (BMI) matrix composite substrates. The results showed the coherent and clean substrate/coating interface in the triple layer coating system. On the other hand, significant pores, cavities and cracks were appeared throughout substrate/bond coat interfaces when double layer coating systems were created on the polymeric substrate. Fourier Transform Infra-Red (FTIR) spectroscopy showed that decomposition of polymeric substrate took place for double layer coating system in spite of that for the triple layer coating system. The results of laser flash analysis showed that lower thermal diffusivity value of Cu-6Sn/NiCrAlY bonding layer in the triple layer coating system than the two other bonding layers in double layer coating systems may result in lower heat flux transmitted to the substrate during ceramic top coat deposition. NiCrAlY intermediate layer reduced the differences of microhardness and coefficient of thermal expansion (CTE) values between YSZ top coat and metallic inner layer resulting in reduced mismatch in the thermal and mechanical properties at the two interfaces of the triple layer coating system as compared to double layer coating systems. It can be inferred that the triple layer coating system with 300μm thick YSZ might increase the working temperature of polymeric substrate due to significant reduction in its thermal conductivity value. Thermal gradient cycling test at the coating surface temperature between 300°C to 450°C showed that the increase of surface temperature is followed by the significant decrease of TBC lifetime. The failure mode was the crack formation and delamination at the bond coat/substrate interface.

Determining and Modelling of Thermal and Rheological Properties of Cashew Apple By-Products

This study aims to determine thermal and rheological properties of blended cashew apple juice (cajuína), nectar, and pulp. The effects of temperature on rheological properties were evaluated and the behavior was adjusted to rheological models. Comparing the thermal properties of cashew products, cajuína showed lower density, and higher values of thermal conductivity, thermal diffusivity and specific heat. Equations providing these properties in function of temperature were obtained, and presented good fits. Cajuína and cashew apple pulp were well characterized by the Ostwald-de-Waelle and Hershel-Bulkley models, respectively, while for cashew apple nectar, two behaviors were observed depending on the temperature used.

Thermal Wavelength Measurement of Nanofluid in an Optical-Fiber Thermal Wave Cavity Technique to Determine the Thermal Diffusivity.

The application of optical-fiber thermal wave cavity (OF-TWC) technique was investigated to measure the thermal diffusivity of Ag nanofluids. The thermal diffusivity was obtained by measuring the thermal wavelength of sample in a cavity scan mode. The spherical Ag nanoparticles samples were prepared at various sizes using the microwave method. Applying the thermal wavelength measurement in a flexible OF-TWC technique requires only two experimental data sets. It can be used to estimate thermal diffusivity of a small amount of liquid samples (0.3 ml) in a brief period. UV-Vis spectroscopy and transmission electron microscopy were used to measure the characterization of the Ag nanoparticles. The thermal diffusivity of distilled water, glycerol, and two different types of cooking oil was measured and has an excellent agreement with the reported results in the literature (difference of only 0.3%–2.4%). The nanofluids showed that the highest value of thermal diffusivity was achieved for smaller sized nanoparticles. The results of this method confirmed that the thermal wavelength measurement method using the OF-TWC technique had potential as a tool to measure the thermal diffusivity of nanofluids with different variables such as the size, shape, and concentration of the nanoparticles.