Drop In Thermal Conductivity Research Articles

Thermal conductivity loss in WC-Co hard metal due to high-temperature cyclic loading damage as a function of microstructure and temperature

Many structures made of WC-Co hard metal are subject to thermo-mechanical loading and have to conduct heat in order to function properly in industrial application. The current work provides results on a significant drop in thermal conductivity of WC-Co hard metals as a function of material volume damage that accumulates during cyclic high-temperature loading of the materials depending on material microstructure. Average WC grain size and Co binder metal content of the investigated grades ranged from submicron to medium and from 10 to 12 wt%, respectively. The hard metals were subjected to uniaxial cyclic loading in a vacuum for different numbers of load cycles at 700 °C and 800 °C. Damage features accumulated in the material volume were documented by means of scanning electron microscopy. Thermal conductivity properties of virgin and damaged materials were determined via laser flash analysis. The results indicated a significant decrease depending on the materials' microstructure, i.e. the defects' predominant location within the microstructure. The damage features that occurred mainly between WC grains in the coarser-grained grade led to larger drops in thermal conductivity with rising temperature compared to damage features that occurred within the Co binder metal in the finer-grained grade. The presented results are of high relevance to the thermo-mechanical load situation of e.g. milling tools since the heat conduction away from their cutting edges is hindered by the documented effect and deemed to lead to a self-acceleration of the damage accumulation.

Determination of thermal conductivity in nano-oxides under the effect of size, dimension and thermal Kapitza resistance

The author has proposed a simple model to determine the effective thermal conductivity variance in nanomaterials with respect to size under Kapitza thermal resistance effect. The formulation is based on the relation of thermal conductivity with specific heat, sound velocity and mean free path in nanosolids. Using the Jiang’s qualitative approach for melting temperature variation in nanomaterials with dimension and size;the thermal conductivity expression for nanosolids is formulated. Effective thermal conductivity of nanosolid is determined on the basis of effective medium approach. The results obtained from formulation depicts the increase in Effective thermal conductivity of nanomaterial with increase in its size. The present model results are seen in good consistency with the available data of previous workers and matches the variation trend as predicted previously. The exponential drop in thermal conductivity of nanomaterials with their size reduction to nanoscale make them efficient material to be use in thermoelectric devices because of their high figure of merit.

Regulating phonon transport in silicon nanofilms by resonant nanopillars

Nanophononic materials have shown significant potential for advancing future thermoelectrics due to their ability to achieve low thermal conductivity. Recent studies have unveiled that thermal conductivity can be further diminished through the phenomenon of phonon resonance, which emerges from the incorporation of additional pillar structures. In this investigation, we delve into the realm of phonon resonance within silicon nanofilms (SiNFs) outfitted with resonant pillars, by employing classical molecular dynamics simulations. Our study reveals that SiNFs featuring resonant pillars exhibit a variable reduction in thermal conductivity contingent upon the thickness of the SiNFs. This reduction is notably influenced by the geometry of the pillars. Remarkably, thermal conductivity can be diminished by up to 20 % for SiNFs endowed with pillars on both sides, characterized by a pillar height of 0.8 nm and a duty cycle of 50 %. This reduction significantly surpasses the corresponding decrease observed in SiNFs equipped with unilateral pillars (13 %).Furthermore, the choice of an appropriate pillar height contributes to a more substantial drop in thermal conductivity. Specifically, a reduction of 14 % is observed for the model with a pillar height of 0.8 nm, while a height of 3.1 nm leads to a more pronounced reduction of 21 %. Intriguingly, we also identified that void defects present on the pillars have the potential to disrupt phonon resonance, thereby leading to an increase in thermal conductivity. The insights garnered from our findings offer valuable guidance for the strategic design of low-dimensional materials, capitalizing on the phenomenon of phonon resonance.

Rattling-induced suppression of thermal transport in cubic In2O3 with Sn-Ga diatomic defect

We present the first principles study of cubic In2O3 with a diatomic defect composed of a Sn atom substituting the In atom at the b-site and a Ga atom embedded in the nearest c-site (structural vacancy) with lattice positions according to the Wyckoff notations. Structural, electronic, phononic and thermal properties were investigated within density functional theory formalism. The lattice anharmonicity effects were taken into account for all possible three-phonon scattering processes. The phonon transport was considered within the Peierls-Boltzmann transport equation with relaxation time approximation. In the relaxed lattice, a strong rearrangement of the initial positions of the atoms in the defect vicinity was revealed, which primarily manifests itself in the displacement of the Sn atom toward another interstitial site. Thus, a cage is formed around the defect by 12 O and 12 In atoms. The calculations of elastic constants and mean square displacements of cage region atoms showed the rattling-like behavior of the Sn atom. Bader charge analysis and electron localization function allowed a deeper understanding and explanation of such behavior. Phonon energy spectra as compared to In2O3 and In2O3:(Sn) demonstrated flattening of phonon branches with spatial localization of phonon modes. They also revealed a decrease in average group velocities of phonons, including those of acoustic type, the presence of avoided-crossing features in the low energy range, and an increase of available phase space for three-phonon scattering. Accounting for all these vibrational features due to defect atoms resulted in a thermal conductivity drop at room temperature by more than seven times compared to In2O3.

Влияние высокотемпературного отжига на физико-химические свойства систем на основе FeSi-=SUB=-x-=/SUB=-

It has been established that the physicochemical properties of structures based on iron silicides formed by ion implantation of iron ions into silicon depend significantly on the time of subsequent high-temperature annealing. Objects with different geometric parameters are formed on the surface and the roughness increases. Annealing at 1000 C in an Ar atmosphere is accompanied by a decrease in the content of the Fe-Si chemical bonds during the first 60 seconds. The reason for the drop in thermal conductivity with increasing annealing temperature is the formation of silicide complexes.

Effect of High Temperature Annealing on the Physicochemical Properties of systems based on FeSi-=SUB=-x-=/SUB=-

It has been established that the physicochemical properties of structures based on iron silicides formed by ion implantation of iron ions into silicon depend significantly on the time of subsequent high-temperature annealing. Objects with different geometric parameters are formed on the surface and the roughness increases. Annealing at 1000 o C in an Ar atmosphere is accompanied by a decrease in the content of the Fe-Si chemical bonds during the first 60 seconds. The reason for the drop in thermal conductivity with increasing annealing temperature is the formation of silicide complexes. Keywords: iron silicide, thermoelectric, ion implantation, chemical composition, X-ray photoelectron spectroscopy.

Influence of Generated Defects by Ar Implantation on the Thermoelectric Properties of ScN.

Nowadays, making thermoelectric materials more efficient in energy conversion is still a challenge. In this work, to reduce the thermal conductivity and thus improve the overall thermoelectric performances, point and extended defects were generated in epitaxial 111-ScN thin films by implantation using argon ions. The films were investigated by structural, optical, electrical, and thermoelectric characterization methods. The results demonstrated that argon implantation leads to the formation of stable defects (up to 750 K operating temperature). These were identified as interstitial-type defect clusters and argon vacancy complexes. The insertion of these specific defects induces acceptor-type deep levels in the band gap, yielding a reduction in the free-carrier mobility. With a reduced electrical conductivity, the irradiated sample exhibited a higher Seebeck coefficient while maintaining the power factor of the film. The thermal conductivity is strongly reduced from 12 to 3 W·m–1·K–1 at 300 K, showing the influence of defects in increasing phonon scattering. Subsequent high-temperature annealing at 1573 K leads to the progressive evolution of these defects: the initial clusters of interstitials evolved to the benefit of smaller clusters and the formation of bubbles. Thus, the number of free carriers, the resistivity, and the Seebeck coefficient are almost restored but the mobility of the carriers remains low and a 30% drop in thermal conductivity is still effective (ktotal ∼ 8.5 W·m–1·K–1). This study shows that control defect engineering with defects introduced by irradiation using noble gases in a thermoelectric coating can be an attractive method to enhance the figure of merit of thermoelectric materials.

Synthesis and characterization of γ-Al2O3-Water nanofluid with and without surfactant

Nanofluid is an attractive option to be used as a working fluid in various heat transfer applications due to its higher thermal conductivity and heat transfer coefficient compared to a conventional working fluid. One of the major obstacles using nanofluids is their long-term stability. This research aims to synthesize stable nanofluid to evaluate thermophysical properties of nanofluids such as thermal conductivity and viscosity. It also covers the effect of surfactant on stability and thermophysical properties of nanofluids. γ-Al2O3 having 99.9% purity and 20 nm average particle size is used as nanoparticle and deionized water is used as base fluid to synthesize stable nanofluid. Nanoparticles are synthesized in base fluid with various volume fractions of 0.3, 0.5, and 0.75% by two-step method. Effect of SDBS surfactant on stability and thermophysical properties are investigated in current research. Stability of nanofluid is investigated using various techniques such as Visual inspection, DLS, and UV–vis spectrometry. KD2Pro Thermal Analyzer and Anton Paar Rheometer are employed to evaluate thermal conductivity and viscosity of nanofluid. Experimental results depict insignificant improvement in stability of nanofluid with the use of surfactant. Results also show an increase in thermal conductivity and viscosity with rise in volume fraction. Maximum augmentation in thermal conductivity and viscosity is found as 8% and 20% for 0.75% volume fraction nanofluids synthesized without surfactant compared to base fluid. Use of surfactant shows an average drop in thermal conductivity by 3% and an average rise in viscosity by 2% for all volume fractions. Due to overall adverse impact of surfactant, its use is inadvisable in synthesis of nanofluid up to 0.75% volume fraction.

Cyclic thermo-mechanical performance of granular beds: Effect of elastoplasticity

Understanding the coupled thermo-mechanical behaviour of compacted granular beds can benefit various industrial applications, such as pebble bed design in fusion reactors. In this study, a thermo-mechanical discrete element method based on our previous work is improved and adapted to investigate the cyclic thermo-mechanical performance of gas-filled granular materials composed of elastoplastic grains. An interparticle contact model is developed considering the plastic deformation of grains. Through the simulation on a representative volume element of beryllium pebble beds, we provide grain-scale insight into the evolution of thermal conductivity and stress. The simulation results suggest that the network of thermal contacts is impeded by plastic deformation leading to a significant drop of thermal conductivity during cooling. This effect can be suppressed by increasing the initial packing factor. Not limited to pebble bed design, the conclusion of this work can also pave the way for optimizing powder-based manufacturing and energy storage, where combined thermo-mechanical loading conditions and elastoplastic deformation of individual particles are involved.

Thermal Transport Evolution Due to Nanostructural Transformations in Ga-Doped Indium-Tin-Oxide Thin Films.

We report on a comprehensive theoretical and experimental investigation of thermal conductivity in indium-tin-oxide (ITO) thin films with various Ga concentrations (0–30 at. %) deposited by spray pyrolysis technique. X-ray diffraction (XRD) and scanning electron microscopy have shown a structural transformation in the range 15–20 at. % Ga from the nanocrystalline to the amorphous phase. Room temperature femtosecond time domain thermoreflectance measurements showed nonlinear decrease of thermal conductivity in the range 2.0–0.5 Wm−1 K−1 depending on Ga doping level. It was found from a comparison between density functional theory calculations and XRD data that Ga atoms substitute In atoms in the ITO nanocrystals retaining Ia-3 space group symmetry. The calculated phonon dispersion relations revealed that Ga doping leads to the appearance of hybridized metal atom vibrations with avoided-crossing behavior. These hybridized vibrations possess shortened mean free paths and are the main reason behind the thermal conductivity drop in nanocrystalline phase. An evolution from propagative to diffusive phonon thermal transport in ITO:Ga with 15–20 at. % of Ga was established. The suppressed thermal conductivity of ITO:Ga thin films deposited by spray pyrolysis may be crucial for their thermoelectric applications.

Enhanced thermoelectric performance of solution-grown Bi2Te3 nanorods

The dominant role of trioctylphosphine (TOP) on the precise crystallite size and enhanced thermoelectric (TE) performance of the solution-grown Bi2Te3 nanorods annealed at 250 °C for 5 h under Ar gas flow and pelletization under 1 GPa pressure at 27 °C only has been demonstrated here. This has resulted in systematic reduction of crystallite size (D), emergence of a secondary phase BiTe with increasing TOP, and enhancement in thermopower but drastic drop in thermal conductivity due to atomic-scale control over the grain size and boundaries. The highest ZT and power factor (PF) obtained for nanorods with D = 40 nm are 1.95 and 1.4 times higher than those of D = 63 nm are due to the optimum intergrain energy barrier height for filtering of the charge/heat carriers. Remarkably, this enhanced ZT/PF is promisingly greater than those of the other more sophisticatedly solution-processed samples reported earlier. Moreover, the IR-active A1u modes in Raman spectra of nanorods of centrosymmetric Bi2Te3 have been observed for the first time with TOP-induced reduction of D confirming the breaking of crystal inversion symmetry due to the formation of sub-quintuples. Thus, this grain size alteration strategy for ZT improvement will open a new vista for further development of Bi2Te3-based efficient TE materials near room temperature.

Mode localization and suppressed heat transport in amorphous alloys

Glasses usually represent the lower limit for the thermal conductivity of solids, but a fundamental understanding of lattice heat transport in amorphous materials can provide design rules to beat such a limit. Here we investigate the role of mass disorder in glasses by studying amorphous silicon-germanium alloy (a-Si$_{1-x}$Ge$_x$) over the full range of atomic concentration from $x=0$ to $x=1$, using molecular dynamics and the quasi-harmonic Green-Kubo lattice dynamics formalism. We find that the thermal conductivity of a-Si$_{1-x}$Ge$_x$ as a function of $x$ exhibits a smoother U-shape than in crystalline mass-disordered alloys. The main contribution to the initial drop of thermal conductivity at low Ge concentration stems from the localization of otherwise extended modes that make up the lowest 8\% of the population by frequency. Contributions from intermediate frequency modes are decreased more gradually with increasing Ge to reach a broad minimum thermal conductivity between concentrations of Ge from $x=0.25$ to $0.75$.

Correlating the cellular network, in foamed concrete containing ash, with their physical properties

Cellular concrete is a lightweight solid cementitious foam that is used mainly for thermal insulation and sound absorption. Either partially or wholly, when the Portland cement in the binder is replaced with ash, the environmental impact of the material is reduced. However, the resulting changes in the cell structure and their subsequent impact upon the physical properties of the material have not been well examined. In this study, cellular concrete was prepared from alkali activated fly ash binders and separately, from Portland cement binders blended with wood ash. The cell structure was photographed using laser confocal microscopy and the images were analyzed to quantify the cellular network in terms of the porosity and the fractal dimension of the porous phase. Alongside, the cellular concrete specimens were tested for their thermal constants and sound absorption. The results show that there was an increase in the fractal dimension and an associated drop in thermal conductivity for an increase in the porosity.

Incorporation of polyethylene fillers in all-polymer high-thermal-conductivity composites

This work presents a lightweight electrically insulating composite material for high thermal conductivity: all polymer composites created by mixing ultra-high molecular weight polyethylene flakes into a matrix of low-density polyethylene or ethylene vinyl acetate. The ultra-high molecular weight polyethylene flakes have a reported thermal conductivity of 40 W m−1 K−1, and when 30 volume percent is compounded in an ethylene vinyl acetate matrix, the thermal conductivity is measured to be 2.5 W m−1 K−1 at 52 °C and the notched impact strength is 350 J/m. The temperature stability of the polyethylene flakes is investigated using polarized Raman spectrometry and thermal conductivity measurements, and it is discovered that flakes compounded at temperatures of 115 °C and above show a dramatic drop in thermal conductivity due to melting and reduced polymer chain alignment. This study highlights the possibility of using high-thermal-conductivity polymer fillers in bulk composites for near-room-temperature applications.

Nitrene functionalization as a new approach for reducing the interfacial thermal resistance in graphene nanoplatelets/epoxy nanocomposites

Conventional functionalization methods, such as oxidation, can be very damaging for the nanocarbons microstructure and therefore detrimental for their intrinsic thermal conductivity. In this study, nitrene chemistry was investigated as a non-disruptive approach for the covalent functionalization of graphene nanoplatelets (GNP). The nitrene precursor was synthesized and thermally decomposed in situ to functionalize GNP (cm-GNP). Temperature and solvents of reaction were found to have profound impact on the functionalization yield. Epoxy nanocomposites were then prepared with pristine, oxidized and cm-GNP. Influence of dispersion methodology was crucial for cm-GNP, as sonication was found to damage the functionalization. Oxidation caused a dramatic drop in thermal conductivity compared to pristine GNP. By contrast, nitrene chemistry produced the highest thermal conductivity enhancement. Finally, epoxy nanocomposites thermal conductivity results were correlated and discussed in light of SEM and micro-computed X-ray tomography (μCT) analyses. The μCT highlighted new features, such as micro-voids surrounding pristine GNP in epoxy nanocomposites, which are invisible with conventional methods. It was found that functionalization not only enhanced the dispersion but also improved GNP/polymer interactions. Moreover, μCT clearly demonstrated that nitrene functionalization eliminated the micro voids surrounding the fillers in the epoxy nanocomposites.

Development and Performance of Tungsten-Coated Graphitic Foam for Plasma-Facing Components

High-density graphitic foam is an ideal low-Z plasma-facing material for deuterium-deuterium plasma experiments where tritium codeposition is not an issue. However, like all carbon, graphitic foam suffers from a precipitous drop in thermal conductivity at high temperatures, >600°C. To mitigate these problems, functionally graded layers of tungsten can be deposited to a thickness of 2 to 4 mm onto the plasma side of the foam using chemical vapor deposition. The graphitic foam then acts as a high-conductivity heat sink at temperatures below 600°C for the thin high-Z armor coating. The overall component weighs 18 times less than a comparable volume of tungsten and lacks the coefficient of thermal expansion joining issues between the CuCrZr tubing and the tungsten. This paper discusses the coating development and characterization and presents the results of recent plasma exposures in W7-X. It also reports on computational fluid dynamics heat transfer modeling and preparations for high heat flux testing of mock-ups. This hybrid plasma-facing component (PFC) consisting of innovative engineered materials may be a cost-effective, actively cooled solution for the divertors and other PFCs in long-pulse machines like W7-X and WEST.

High heat-flux response of high-conductivity graphitic foam monoblocks

Plasma-facing components based on the so-called monoblock design are planned for use in the divertor region of long-pulse plasma devices such as ITER and JT-60SA due to their capacity to handle high heat fluxes with active water cooling. The plasma-facing materials that are preferred for these monoblocks are tungsten for ITER or carbon-carbon fiber reinforced carbon (CFC) for JT-60SA. The requirements for the plasma-facing components include the ability to handle high plasma fluxes resulting in high temperatures. Therefore, reasonably high thermal conductivity is required. In this study, high thermal conductivity graphite foam is explored as a monoblock material. Four different test articles are created, two with the foam brazed to CuCrZr tubes, and two with the foam press fit to the tubes. These monoblocks are then thermally cycled at 8 MW/m2 in the high heat-flux test facility GLADIS to examine their robustness for long-pulse divertor applications. Experimental results are compared with computational fluid dynamics simulation. Graphite foam shows promise for use in a plasma-facing component, but further development is necessary to address the significant drop in thermal conductivity at high temperatures.

Quantification of factors influencing the thermal conductivity of cement-based foam

In this experimental study cement-based foams were examined to quantify the influence of density, moisture content, porosity and pozzolanic admixture on thermal conductivity. The mixtures were cast for three densities between 400 and 800 kg/m3. For each density series, the cement was blended, up to 20% by mass, with three different pozzolanic admixtures, namely fly ash, silica fume and metakaolin. Thermal conductivity was measured at the hydration age of 60–300 days, using the transient plane source (TPS) technique. The results revealed that among the three pozzolanic admixtures, silica fume was the most efficient in reducing the conductivity. In addition, all series of mixtures reported a drop in thermal conductivity with the change in density (35% on an average from 800 kg/m3 to 400 kg/m3). It was also found that moisture influences the thermal conductivity significantly, especially at higher densities. Finally, this study presents a predictive model for evaluating the thermal conductivity of cement-based foam. On examining the validity of this model with the experimental and predicted results, it was found that the predicted values were within 7% of the experimentally obtained values.

Erosion dynamics of tungsten fuzz during ELM-like heat loading

Transient heat loading and high-flux particle loading on plasma facing components in fusion reactors can lead to surface melting and possible erosion. Helium-induced fuzz formation is expected to exacerbate thermal excursions, due to a significant drop in thermal conductivity. The effect of heating in edge-localized modes (ELMs) on the degradation and erosion of a tungsten (W) fuzz surface was examined experimentally in the Ultra High Flux Irradiation-II facility at the Center for Materials Under Extreme Environment. W foils were first exposed to low-energy He+ ion irradiation at a fluence of 2.6 × 1024 ions m−2 and a steady-state temperature of 1223 K. Then, samples were exposed to 1000 pulses of ELM-like heat loading, at power densities between 0.38 and 1.51 GW m−2 and at a steady-state temperature of 1223 K. Comprehensive erosion analysis measured clear material loss of the fuzz nanostructure above 0.76 GW m−2 due to melting and splashing of the exposed surface. Imaging of the surface via scanning electron microscopy revealed that sufficient heating at 0.76 GW m−2 and above caused fibers to form tendrils to conglomerate and form droplets. Repetitive thermal loading on molten surfaces then led to eventual splashing. In situ erosion measurements taken using a witness plate and a quartz crystal microbalance showed an exponential increase in mass loss with energy density. Compositional analysis of the witness plates revealed an increase in the W 4f signal with increasing energy density above 0.76 GW m−2. The reduced thermal stability of the fuzz nanostructure puts current erosion predictions into question and strengthens the importance of mitigation techniques.

Thermal Conductivity of Hydrated Paste in Cement-Based Foam Microstructure

Abstract This article presents the thermal constants of hydrated cement paste, which constitute the air-void wall of cement-based foam microstructure, and examines its influence on moisture, age, pozzolanic admixture type, and content. Along with the reference mix, containing portland cement only, six other mixes were prepared by replacing cement with fly ash, silica fume, and metakaolin, up to 20 % by mass in the binder. The Transient Plane Source, a thermal analyzer that conforms to ISO test standards, was employed to evaluate the thermal constants. Here, the measurements were made at 60, 120, 210, and 300 days. The results revealed that the drop in thermal conductivity for the cement paste was significant in the earlier age but progressively became insignificant as the hydration advanced. It was also found that adding fly ash and silica fume in higher dosages resulted in reducing the conductivity, while metakaolin exhibits a reverse trend. Furthermore, the reduction in the moisture content due to pozzolanic admixture was also noticed. Finally, an empirical formulation was developed to predict the thermal conductivity of the hydrated cement paste, which was also validated against the results of past research.