Showing papers on "Thermal diffusivity published in 2018"

(Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy ceramics with low thermal conductivity

Abstract: A novel high‐entropy carbide ceramic, (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C, with a single‐phase rock salt structure, was synthesized by spark plasma sintering. X‐ray diffraction confirmed the formation of a single‐phase rock salt structure at 26‐1140°C in Argon atmosphere, in which the 5 metal elements may share a cation position while the C element occupies the anion position. (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C exhibits a much lower thermal diffusivity and conductivity than the binary carbides HfC, ZrC, TaC, and TiC, which may result from the significant phonon scattering at its distorted anion sublattice. (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C inherits the high elastic modulus and hardness of the binary carbide ceramics.

Thermal Percolation Threshold and Thermal Properties of Composites with High Loading of Graphene and Boron Nitride Fillers

Abstract: We investigated thermal properties of the epoxy-based composites with the high loading fraction-up to f ≈ 45 vol %-of the randomly oriented electrically conductive graphene fillers and electrically insulating boron nitride fillers. It was found that both types of the composites revealed a distinctive thermal percolation threshold at the loading fraction fT > 20 vol %. The graphene loading required for achieving thermal percolation, fT, was substantially higher than the loading, fE, for electrical percolation. Graphene fillers outperformed boron nitride fillers in the thermal conductivity enhancement. It was established that thermal transport in composites with high filler loadings, f ≥ fT, is dominated by heat conduction via the network of percolating fillers. Unexpectedly, we determined that the thermal transport properties of the high loading composites were influenced strongly by the cross-plane thermal conductivity of the quasi-two-dimensional fillers. The obtained results shed light on the debated mechanism of the thermal percolation, and facilitate the development of the next generation of the efficient thermal interface materials for electronic applications.

Statistical variances of diffusional properties from ab initio molecular dynamics simulations

Abstract: Ab initio molecular dynamics (AIMD) simulation is widely employed in studying diffusion mechanisms and in quantifying diffusional properties of materials. However, AIMD simulations are often limited to a few hundred atoms and a short, sub-nanosecond physical timescale, which leads to models that include only a limited number of diffusion events. As a result, the diffusional properties obtained from AIMD simulations are often plagued by poor statistics. In this paper, we re-examine the process to estimate diffusivity and ionic conductivity from the AIMD simulations and establish the procedure to minimize the fitting errors. In addition, we propose methods for quantifying the statistical variance of the diffusivity and ionic conductivity from the number of diffusion events observed during the AIMD simulation. Since an adequate number of diffusion events must be sampled, AIMD simulations should be sufficiently long and can only be performed on materials with reasonably fast diffusion. We chart the ranges of materials and physical conditions that can be accessible by AIMD simulations in studying diffusional properties. Our work provides the foundation for quantifying the statistical confidence levels of diffusion results from AIMD simulations and for correctly employing this powerful technique. The calculation of ionic diffusivity with ab initio molecular dynamics is often plagued by poor statistics; how accurate are the results? Due to computational limits only small systems and timescales can be modeled, limiting the number of diffusion events sampled. Here, Yifei Mo and colleagues at the University of Maryland outline best practice to obtain ionic diffusivity, as well as how to obtain statistical errors with this approach. They show linear behavior in diffusivity only happens for intermediate time intervals of the simulations. Moreover, variance of diffusivity is related to the total mean squared displacement of all ions, the statistical error reducing as the displacement increases. Accurate ionic diffusion calculations can only be performed for super-ionic conductors, or at high temperature, with implications for the reliability of calculations of diffusivity in other materials.

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

Abstract: 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.

Thermal Transport and Phonon Hydrodynamics in Strontium Titanate.

Abstract: We present a study of thermal conductivity, κ, in undoped and doped strontium titanate in a wide temperature range (2-400 K) and detecting different regimes of heat flow In undoped SrTiO_{3}, κ evolves faster than cubic with temperature below its peak and in a narrow temperature window Such behavior, previously observed in a handful of solids, has been attributed to a Poiseuille flow of phonons, expected to arise when momentum-conserving scattering events outweigh momentum-degrading ones The effect disappears in the presence of dopants In SrTi_{1-x}Nb_{x}O_{3}, a significant reduction in lattice thermal conductivity starts below the temperature at which the average inter-dopant distance and the thermal wavelength of acoustic phonons become comparable In the high-temperature regime, thermal diffusivity becomes proportional to the inverse of temperature, with a prefactor set by sound velocity and Planckian time (τ_{p}=(ℏ/k_{B}T))

Concrete as a thermal mass material for building applications - A review

Abstract: Identifying new energy saving methods in the building sector is essential due to limited natural energy sources and the rising population. Thermal mass materials have the ability to absorb and store heat before releasing it later on when necessary. They act as heat sinks during the daytime and as heat sources during the nighttime. Thermal performance is evaluated according to the specific heat capacity and specific latent heat. Applying thermal mass materials such as concrete is deemed a suitable strategy to reduce the energy consumption of buildings. Concrete with low thermal conductivity and high specific heat capacity is desirable in building construction. The aim of this study is to review factors affecting the heat storage capacity of concrete. In addition, common measurement methods of cement-based materials’ thermal conductivity, thermal diffusivity and specific heat capacity are reviewed. Various studies reveal that temperature, humidity, aggregate type, cementitious material type as well as phase change material (PCM) used influence the thermal properties of concrete. The advantages and limitations of PCM-concrete are also summarized in this study.

Synthesis and thermophysical properties of RETa3O9 (RE = Ce, Nd, Sm, Eu, Gd, Dy, Er) as promising thermal barrier coatings

Abstract: Thermal barrier coatings (TBCs) are one of the most important materials in gas turbine to protect the high temperature components. RETa3O9 compounds have a defect-perovskite structure, indicating that they have low thermal conductivity, which is the critical property of TBCs. Herein, dense RETa3O9 bulk ceramics were fabricated via solid-state reaction. The crystal structure was characterized by X-ray diffraction (XRD) and Raman Spectroscope. Scanning electron microscope (SEM) was used to observe the microstructure. The thermal physics properties of RETa3O9 were studied systematically, including specific heat, thermal diffusivity, thermal conductivity, thermal expansion coefficients and high-temperature phase stability. The thermal conductivities of RETa3O9 are very low (1.33-2.37 W/m.K, 373-1073 K), which are much lower than YSZ and La2Zr2O7; and the thermal expansion coefficients range from 4.0×10-6 K−1 to 10.2×10-6 K−1 (1273 K), which is close to La2Zr2O7 and YSZ. According to the differential scanning calorimetry (DSC) curve there is not phase transition at the test temperature. Due to the high melting point and excellent high-temperature phase stability with these oxides, RETa3O9 ceramics were promising candidate materials for TBCs. This article is protected by copyright. All rights reserved.

Impacts of gold nanoparticles on MHD mixed convection Poiseuille flow of nanofluid passing through a porous medium in the presence of thermal radiation, thermal diffusion and chemical reaction

Abstract: Impacts of gold nanoparticles on MHD Poiseuille flow of nanofluid in a porous medium are studied. Mixed convection is induced due to external pressure gradient and buoyancy force. Additional effects of thermal radiation, chemical reaction and thermal diffusion are also considered. Gold nanoparticles of cylindrical shape are considered in kerosene oil taken as conventional base fluid. However, for comparison, four other types of nanoparticles (silver, copper, alumina and magnetite) are also considered. The problem is modeled in terms of partial differential equations with suitable boundary conditions and then computed by perturbation technique. Exact expressions for velocity and temperature are obtained. Graphical results are mapped in order to tackle the physics of the embedded parameters. This study mainly focuses on gold nanoparticles; however, for the sake of comparison, four other types of nanoparticles namely silver, copper, alumina and magnetite are analyzed for the heat transfer rate. The obtained results show that metals have higher rate of heat transfer than metal oxides. Gold nanoparticles have the highest rate of heat transfer followed by alumina and magnetite. Porosity and magnetic field have opposite effects on velocity.

Preparation and Enhanced Thermal Performance of Novel (Solid to Gel) Form-Stable Eutectic PCM Modified by Nano-Graphene Platelets

Abstract: This study presents the development of form-stable eutectic mixtures, modified with nanoscale structures for enhanced thermal performance. These additives may result in the next generation of phase change materials (PCMs) for thermal energy storage systems. An appropriate gelling or thickening agent (2-hydroxypropyl ether cellulose) is introduced so that the PCM will lose its fluidity, become form-stable, and the liquid leakage problem will be overcome. Nano-graphene platelets (NGPs) are added in order to enhance the thermal properties and overall heat transfer. Differential scanning calorimetry (DSC) was carried out for the thermal analysis of the PCMs. The paper experimentally studied in detail the enhanced thermo-physical properties required for stimulating and modelling the PCM in energy storage applications such as specific heat, thermal diffusivity, thermal conductivity, enthalpy, and density. The principle of the T-history method was applied using a parallel plate heating/cooling guarded plate apparatus to determine the true phase transition temperatures of bulk PCM. The supercooling of the enhanced shape stable mixture was found to be less than 0.1 °C. The thermal reliability test indicated that the enhanced form-stable eutectic mixture had reliable thermal performance over a postulated lifetime of 80 years. As a result, the developed form stable PCM eutectic mixture is a promising material for thermal energy storage.

Thermal Conductivity during Phase Transitions.

Abstract: Thermal conductivity is a very basic property that determines how fast a material conducts heat, which plays an important and sometimes a dominant role in many fields. However, because materials with phase transitions have been widely used recently, understanding and measuring temperature-dependent thermal conductivity during phase transitions are important and sometimes even questionable. Here, the thermal transport equation is corrected by including heat absorption due to phase transitions to reveal how a phase transition affects the measured thermal conductivity. In addition to the enhanced heat capacity that is well known, it is found that thermal diffusivity can be abnormally lowered from the true value, which is also dependent on the speed of phase transitions. The extraction of the true thermal conductivity requires removing the contributions from both altered heat capacity and thermal diffusivity during phase transitions, which is well demonstrated in four selected kinds of phase transition materials (Cu2 Se, Cu2 S, Ag2 S, and Ag2 Se) in experiment. This study also explains the lowered abnormal thermal diffusivity during phase transitions in other materials and thus provides a novel strategy to engineer thermal conductivity for various applications.

The effects of thermal treatments on the subcritical crack growth of Pingdingshan sandstone at elevated high temperatures

Abstract: Increasing the deeper understanding of the thermal damages and failure mechanisms of sandstone undergoing thermal treatments at different temperatures is a key concern for deep-mining and underground coal gasification processes. In this research study, a scanning electron microscope (SEM) apparatus, JSM-5410LV, which was equipped with a built-in digital electro-hydraulic servo loading system, was applied to carry out a series of three-point bending tests on Pingdingshan sandstone following heat treatments at elevated and high temperatures ranging from 25 to 600 °C. The subcritical crack initiation load, peak load, and elastic modulus were found to increase with the increases in the thermal treatment temperatures until a maximum was achieved at 125 °C, after which decreases were observed. However, it should be noted that there were sudden drops observed for the specimens after the thermal treatment temperature reached 150 °C due to the thermal diffusivity of the cement. The subcritical crack growth length was theoretically calculated, and a digital speckle correlation method (DSCM) was applied to verify the initial load and subcritical crack growth length. It was found that the fracture toughness fluctuated significantly when the thermal treatment temperature ranged from 25 to 125 °C, and reached a peak of 47.45 MPa mm0.5. It was also observed that, as the temperature was raised from 175 to 600 °C, the fracture toughness gradually decreased. The subcritical crack growth mode was determined to be intra-granular cracking following the thermal treatments below 125 °C, while a mixture of intra-granular and trans-granular cracking occurred in the specimens which experienced thermal treatments of 175 °C. The relationship between heat treatment temperature and subcritical crack growth was derived, which could be used to develop geothermal energy extraction from the critical temperature resources.

Laser powder bed fusion in high-pressure atmospheres

Abstract: High-speed imaging and schlieren imaging were used to investigate the interaction of the laser beam with the powder bed at pressures up to 5 bar, in argon and helium atmospheres. The entrainment of powder particles in the flow of shielding gas generated by the laser plume, and hence denudation, was reduced at high pressure for both gases. However, for argon, high pressure increased the temperature of both the melt pool and the laser plume, which significantly increased the generation of spatter and ionisation of the metal vapour with degraded surface smoothness and continuity. For helium, the formation of spatter and plasma did not increase with the increase in pressure above that observed at atmospheric pressure: its higher thermal conductivity and thermal diffusivity limited the laser plume temperature. Layers built at 5 bar in helium had a surface smoothness and continuity comparable to those built in argon at atmospheric pressure, but achieved at a higher laser scan speed, suggesting that a high-pressure helium atmosphere may be used to enhance the build rate.

Two-dimensional heat and mass transfer and thermodynamic analyses of porous microreactors with Soret and thermal radiation effects—An analytical approach

Abstract: Transport of heat and mass and the thermodynamics of porous microreactors with thermal diffusion and radiation effects are investigated analytically. The examined configuration includes an axisymmetric, thick-wall microchannel with an iso-flux thermal boundary condition imposed on the external surfaces. The microchannel is filled with porous materials and accommodates a zeroth order homogenous chemical reaction. Internal radiative heat transfer is modelled in addition to heat convection and conduction, while the local thermal non-equilibrium approach is taken within the porous section of the system. The transport of species is coupled with that of heat via the inclusion of thermodiffusion or Soret effect. Two-dimensional heat and mass transfer differential equations are solved analytically. The results are subsequently used to predict the thermodynamic irreversibilities inside the reactor and a thorough analysis of local and total entropy generation rates is performed. Also, the changes in Nusselt number, calculated on the internal walls of the microreactor, versus various parameters are reported. It is shown that the radiation effects can impact the temperature of the solid phase of the porous medium and lead to alteration of Nusselt number. It is further observed that the transfer of mass is the main source of irreversibility in the system. The findings are of particular use for the design and analysis of the microreactors with homogenous chemical reactions and can be also used for the validation of computational models.

Oxidation of Al 2 O 3 Scale-Forming MAX Phases in Turbine Environments

Abstract: High temperature oxidation of alumina-forming MAX phases, Ti2AlC and Cr2AlC, were examined under turbine engine environments and coating configurations. Thermogravimetric furnace tests of Ti2AlC showed a rapid initial transient due to non-protective TiO2 growth. Subsequent well-behaved cubic kinetics for alumina scale growth were shown from 1273 K to 1673 K (1000 °C to 1400 °C). These possessed an activation energy of 335 kJ/mol, consistent with estimates of grain boundary diffusivity of oxygen (~375 kJ/mol). The durability of Ti2AlC under combustion conditions was demonstrated by high pressure burner rig testing at 1373 K to 1573 K (1100 °C to 1300 °C). Here good stability and cubic kinetics also applied, but produced lower weight gains due to volatile TiO(OH)2 formation in water vapor combustion gas. Excellent thermal stability was also shown for yttria-stabilized zirconia thermal barrier coatings deposited on Ti2AlC substrates in 2500-hour furnace tests at 1373 K to 1573 K (1100 °C to 1300 °C). These sustained a record 35 µm of scale as compared to 7 μm observed at failure for typical superalloy systems. In contrast, scale and TBC spallation became prevalent on Cr2AlC substrates above 1423 K (1150 °C). Cr2AlC diffusion couples with superalloys exhibited good long-term mechanical/oxidative stability at 1073 K (800 °C), as would be needed for corrosion-resistant coatings. However, diffusion zones containing a NiAl-Cr7C3 matrix with MC and M3B2 particulates were commonly formed and became extensive at 1423 K (1150 °C).

Thermal conductivity and diffusivity of fine-grained sedimentary rocks

Abstract: Thermal conductivity is an important thermophysical rock property, needed for heat flow determination, deep thermal regime determination, and reconstruction of thermal history of sedimentary basin. It is generally accepted that the thermal conductivity of rocks decreases with increasing temperature and increases with increasing pressure, and the effects of temperature and pressure counteract each other. Although many measurements of thermal conductivity of the igneous and metamorphic rocks have been taken, little attention has been paid to sedimentary rocks and heat flow in sedimentary basins. Samples selected for the investigation, representing sedimentary rocks of aleuritic structure, were collected from boreholes (from deep formation, approx. 3000 m) and outcrops. The mineral composition was determined with the use of TG/DSC analysis, under inert and oxidizing atmosphere. Thermal conductivity measurements were taken with the use of C-Therm analyser, in directions parallel and perpendicular to the bedding in rock. Thermal conductivity of the examined rocks ranges from 0.96 to 6.06 W m−1 K−1 and is strongly dependent on mineral content and bedding direction.

The Edwards–Wilkinson Limit of the Random Heat Equation in Dimensions Three and Higher

Abstract: We consider the heat equation with a multiplicative Gaussian potential in dimensions d ≥ 3. We show that the renormalized solution converges to the solution of a deterministic diffusion equation with an effective diffusivity. We also prove that the renormalized large scale random fluctuations are described by the Edwards–Wilkinson model, that is, the stochastic heat equation (SHE) with additive white noise, with an effective variance.

Microstructure evolution and thermal properties of an additively manufactured, solution treatable AlSi10Mg part

Abstract: Because of rapid solidification involved in the laser or e-beam based additive manufacturing (AM) process, solution treatable metallic parts made by these methods usually possess a unique nonequilibrium microstructure which changes significantly during subsequent thermal treatment. Such evolution alters the size, morphology, length scale, and distribution of microstructural features and has a substantial impact on thermal properties and possibly on electrical properties as well. This study focuses on effects of microstructural evolution on thermal properties of an additively manufactured AlSi10Mg part. The changes of thermal properties such as thermal expansion, heat capacity, thermal diffusivity, and thermal conductivity as a function of thermal treatment are reported. The results show that the formation of supersaturated primary α aluminum and unique cellular structure imparted by fast solidification in the AM process are the major cause for the low thermal diffusivity and low thermal conductivity observed in this solution treatable, as-built part. A correlation between microstructural evolution and changes in thermal properties is established. Advantages and tailoring of the thermal properties of additively built parts are discussed. Implications of these results are important for other additively manufactured components based on popular solution treatable alloys.

Thermal Pattern Contrast Diagnostic of Microcracks With Induction Thermography for Aircraft Braking Components

Abstract: Reciprocating impact load leads to plastic deformation on the surface of the kinematic chains in an aircraft brake system. As a result, this causes fatigue and various complex natural damages. Due to the complex surface conditions and the coexistence damages, it is extremely difficult to diagnose microcracks by using conventional thermography inspection methods. In this paper, the thermal pattern contrast method is proposed for weak thermal signal detection using eddy current pulsed thermography. In this process, the extraction and subsequent separation differentiate a maximum of the thermal spatial-transient pattern between defect and nondefect areas. Specifically, a successive optical flow is established to conduct a projection of the thermal diffusion. This directly gains the benefits of capturing the thermal propagation characteristics. It enables us to build the motion context connected between the local and the global thermal spatial pattern. Principal component analysis is constructed to further mine the spatial-transient patterns to enhance the detectability and sensitivity in microcrack detection. Finally, experimental studies have been conducted on an artificial crack in a steel sample and on natural fatigue cracks in aircraft brake components in order to validate the proposed method.