Evolution Of Thermal Conductivity Research Articles

Insights into Ag2Mo3SeO12 for photovoltaic and optoelectronic applications: A theoretical exploration of its structural, electronic, and thermoelectric behavior

The theoretical investigation of the newly discovered quadruple perovskite Ag₂Mo₃SeO₁₂ was conducted using density functional theory (DFT) with the generalized gradient approximation (GGA-PBE) for the structural properties. This study examined the compound's structural, electronic, optical, and thermoelectric properties, utilizing the Tran Blaha modified Becke-Johnson exchange potential (mBJ) for accurate band gap measurement to overcome the bandgap underestimation by GGA-PBE. The stable structure was confirmed through energy-volume optimization and fitted with the Birch-Murnaghan equation of state, using PBE-GGA exchange correlation functional. The findings revealed a band structure with direct transition under the TB-mBJ approach with an energy gap of 1.45 eV. Detailed analyses were conducted, including the density of states and charge density distribution maps. Various optical parameters such as the dielectric function, absorption coefficient, refractive index, and reflectivity were computed, with the static dielectric constant measured at 6.3. Notably, a significant evolution of the absorption coefficient in the visible region highlights the potential of Ag₂Mo₃SeO₁₂ for solar cell and optoelectronic applications. These results pave the way for new photovoltaic material designs. Furthermore, the material demonstrates promising thermoelectric properties with an appropriate figure of merit, Seebeck coefficient, and electrical and thermal conductivity evolution under different temperature conditions, indicating its potential for photovoltaic and optoelectronic applications.

The influence of temperature (up to 120 °C) on the thermal conductivity of variably porous andesite

The thermal conductivity of volcanic rock is an essential input parameter in a wide range of models designed to better understand volcanic and geothermal processes. However, although volcanoes and geothermal reservoirs are often characterised by temperatures above ambient, laboratory thermal conductivity measurements are often performed at ambient temperature. In addition, there are currently few data on the temperature dependence of thermal conductivity for andesite, a common volcanic rock. Here, we provide elevated-temperature (up to 120 °C) laboratory measurements of thermal conductivity for variably porous (∼0.05 to ∼0.6) and variably glassy andesites from Mt. Ruapheu (New Zealand) using the transient hot-strip method. Our data show that (1) the thermal conductivity of these andesites has little to no temperature dependence and, therefore, (2) there is also no influence of porosity on the temperature dependence of thermal conductivity. We compare our new data with compiled published data to show that the thermal conductivity of volcanic rocks may decrease, remain constant, or increase as a function of temperature. We show that the thermal conductivity of amorphous glass and crystalline material increase and decrease, respectively, as temperature increases. We therefore interpret the temperature dependence of the thermal conductivity of volcanic rock to be dependent on glass content. The thermal conductivity of the studied andesites, the microstructure of which can be characterised by phenocrysts within a variably glassy groundmass, has little to no temperature dependence because the decrease in the thermal conductivity of the crystalline materials, due to decreases in lattice thermal conductivity, is offset by the increase in the thermal conductivity of the amorphous glass. A simple modelling approach, using the temperature dependence of the thermal conductivity of glass and crystalline material, provides a crystal content of 0.26 for a thermal conductivity independent of temperature, a common crystal content for andesite dome rock. Our findings imply that calculations of heat transfer through partially glassy volcanic rocks need not consider a temperature-dependent thermal conductivity, but that decreases and increases in thermal conductivity with temperature should be expected for fully crystallised or devitrified volcanic rocks and completely glassy volcanic rocks, respectively. We highlight that more experimental studies are now required to assess the evolution of thermal conductivity as a function of temperature in a wide range of volcanic rocks with different crystallinities.

Covalent vs. Non covalent chemical modification of multiwalled carbon nanotubes based-nanofluids: Stability and thermal conductivity steadiness over temperature

Thermal conductivity enhancement coupled to nanoparticle dispersion stability over time are the two main conflicting requirements to overcome to develop carbon nanomaterial-based nanofluids. Both covalent and non-covalent approaches, having their own advantages and disadvantages, are commonly used for carbon nanotube based nanofluids development. However, they are rarely compared regarding their impact on stability and thermal conductivity performances for the same kind of nanofluid. In this work, multiwalled carbon nanotubes have been chemically modified by both covalent and non-covalent functionalization. For the covalent functionalization, a simple oxidation method is applied. It consists of refluxing the powdered MWCNTs in H2SO4/HNO3 at 5 M concentration. For the non-covalent approach, both an anionic (sodium dodecyl sulfate) and a non-ionic (Pluronic P123, comprising poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) in an alternating linear fashion, PEO-PPO-PEO) surfactants were employed at a concentration of 1 wt%. Nanofluids were prepared with multiwalled carbon nanotubes at a fixed concentration of 0.2 wt% for both chemical modification approaches. The oxidation reaction occurs without significant structure damaging as shown by transmission electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. UV–visible spectrophotometry analysis shows a remarkable stability with temperature in the 293.15–303.15 K range over one-week and for a longer period of 49 days for all the three types of nanofluids. Their thermal conductivity has been investigated in the same temperature range. For all the three nanofluid series prepared with a fixed concentration of carbon nanomaterials, a thermal conductivity enhancement compared to that of the base fluid has been observed. The thermal conductivity evolution with temperature is different for each type of nanofluid. The covalently oxidized multiwalled carbon nanotube based nanofluids showed the better thermal conductivity enhancement steadiness (around 7 %) with temperature. The discussion on the surfactant-based nanofluid behavior proposes the possible involved mechanisms as temperature increases for each surfactant.

Quantitative investigation and intelligent forecasting of thermal conductivity in lime-modified red clay.

This paper delves into the engineering applications of lime-stabilized red clay, a highly water-sensitive material, particularly in the context of the climatic conditions prevalent in the Dalian region. We systematically investigate the impact of water content, dry density, and freeze-thaw cycles (with a freezing temperature set at -10°C) on the thermal conductivity of stabilized soil, a crucial parameter for analyzing soil temperature fields that is influenced by numerous factors. By developing and validating both empirical and machine learning prediction models, we unravel the evolution of thermal conductivity in response to these factors: within the range of influencing variables, thermal conductivity exhibits an exponential or linear increase with rising water content and dry density, while it decreases exponentially with increasing freeze-thaw cycles. Furthermore, we quantitatively analyze the specific influence of water content and other factors on the thermal conductivity of stabilized soil and construct a comprehensive prediction model encompassing BP neural network, gradient boosting decision tree, and linear regression models. Comparative analysis highlights the significant enhancement in prediction accuracy achieved by the proposed ensemble model over single machine learning models, with root mean square error (RMSE) values below 0.05 and mean absolute percentage error (MAPE) values remaining under 2.5% in both frozen and unfrozen states. Additionally, a secondary validation using experimental data from other researchers confirms the model's good agreement with previous results, demonstrating its robust generalization ability. Our findings provide valuable insights for engineering studies in the Dalian region and red clay areas subjected to extreme climatic conditions.

Temporospatial variability of snow's thermal conductivity on Arctic sea ice

Abstract. Snow significantly impacts the seasonal growth of Arctic sea ice due to its thermally insulating properties. Various measurements and parameterizations of thermal properties exist, but an assessment of the entire seasonal evolution of thermal conductivity and snow resistance is hitherto lacking. Using the comprehensive snow dataset from the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, we have evaluated for the first time the seasonal evolution of the snow's and denser snow-ice interface layers' thermal conductivity above different ice ages (refrozen leads, first-year ice, and second-year ice) and topographic features (ridges). Our dataset has a density range of snow and ice between 50 and 900 kg m−3, and corresponding anisotropy measurements, meaning we can test the current parameterizations of thermal conductivity for this density range. Combining different measurement parameterizations and assessing the robustness against spatial heterogeneity, we found the average thermal conductivity of snow (<550 kg m−3) on sea ice remains approximately constant (0.26 ± 0.05 WK-1m-1) over time irrespective of underlying ice type, with substantial spatial and vertical variability. Due to this consistency, we can state that the thermal resistance is mainly influenced by snow height, resulting in a 2.7 times higher average thermal resistance on ridges (1.42 m2 K W−1) compared to first-year level ice (0.51 m2 K W−1). Our findings explain how the scatter of thermal conductivity values directly results from structural properties. Now, the only step is to find a quick method to measure snow anisotropy in the field. Suggestions to do this are listed in the discussion.

Through thickness changes to stiffness and thermal conductivity in thermal barrier coatings subjected to gradient exposure

Sintering-induced increases in the elastic modulus and thermal conductivity of thermally sprayed Thermal Barrier Coatings (TBCs) under constraint by a bond-coated substrate due to thermal gradient exposure have been measured. The results suggest the degree of sintering due to this constrained thermal exposure to be significantly lower when compared to free standing exposed specimens. Overall, a ∼30% increase in elastic modulus and ∼12% increase in thermal conductivity have been measured in a ∼ 450 µm thick TBC after 12 h of gradient exposure. Here, the top surface of the TBC was kept at 1250 °C, and the gradient was determined based on a constant backside cooling air flow rate. The elastic modulus, measured across the thickness of coating, reveals that in the region near the top surface, the elastic modulus increased by ∼ 50%. Contrastingly, no significant increase in elastic modulus was observed near the metal/coating interface. The dynamic evolution of thermal conductivity and sintering under gradient exposure and constraint was calculated using an analytical model which was then used to rationalize the experimental observations.

Thermal conductivity of Zr52.5Ti5Cu17.9Ni14.6Al10 alloy in as-cast glassy, relaxed and crystallized states

The thermal transport property of amorphous alloys is significant to understand the glass-forming ability and developing the thermal insulation coating. However, systematic studies on the thermal conductivity of alloys in as-cast glassy, relaxed and crystalline states are still scarce. Here, we systematically investigated the thermal conductivity behaviors in the temperature range of 10–300 K for as-cast glassy, relaxed, partially-crystallized and well-crystallized Zr52.5Ti5Cu17.9Ni14.6Al10 alloys. The total thermal conductivity (κ) of the as-cast Zr52.5Ti5Cu17.9Ni14.6Al10 metallic glass is approximately 5.66 Wm−1K−1 at room temperature and is slightly lower than that of the structurally relaxed sample, which can be attributed to the opposite changes in electronic thermal conductivity (κe) and phonon thermal conductivity (κp). In contrast, the total thermal conductivity of the partially-crystallized and well-crystallized Zr52.5Ti5Cu17.9Ni14.6Al10 alloys improves significantly owing to the abrupt decrease of electrical resistivity. In addition, the well-crystallized sample exhibits larger grains than the partially-crystallized sample, which will further enhance the phonon thermal conductivity. Our results lay a foundation for further understanding the mechanism of thermal conductivity evolution with the microstructure of metallic alloys.

Evolution of stress-induced thermal anisotropy in granular materials: A directed network perspective

External stress can induce anisotropic effective thermal conductivity λ eff in granular assemblies. However, few studies have used a proper microstructural parameter to study the evolution of thermal anisotropy. In this paper, deformed particle assemblies under different stress paths were generated using discrete element method. By considering each particle as a node, and each heat transfer path via interparticle contact or particle-air-particle as a directed edge, a novel directed thermal network was constructed and a sample-scale parameter named “directed network thermal resistance R" was introduced. Results show contact attributes (i.e., connectivity, quality and orientation) not only dominate the value but also the evolution of anisotropic λ eff that calculated using thermal network models. R has the merit of considering the three contact attributes and shows a general inverse linear relationship to anisotropic λ eff regardless of the dominant heat transfer direction, and its ratio can capture the evolution of thermal anisotropy. • Stress variation does not always synchronize with thermal conductivity evolution. • However, thermal conductivity reaches a common terminal value at large strain. • Interparticle contacts orientation contributes to high thermal conductivity. • New directed network thermal resistance R considers three contact attributes. • R is inversely proportional to anisotropic thermal conductivity.

Parallelized discrete exterior calculus for three-dimensional elliptic problems

A formulation of elliptic boundary value problems is used to develop the first discrete exterior calculus (DEC) library for massively parallel computations with 3D domains. This can be used for steady-state analysis of any physical process driven by the gradient of a scalar quantity, e.g. temperature, concentration, pressure or electric potential, and is easily extendable to transient analysis. In addition to offering this library to the community, we demonstrate one important benefit from the DEC formulation: effortless introduction of strong heterogeneities and discontinuities. These are typical for real materials, but challenging for widely used domain discretization schemes, such as finite elements. Specifically, we demonstrate the efficiency of the method for calculating the evolution of thermal conductivity of a solid with a growing crack population. Future development of the library will deal with transient problems, and more importantly with processes driven by gradients of vector quantities. Program summaryProgram Title: ParaGEMSCPC Library link to program files:https://doi.org/10.17632/t8rpkc7xnx.1Developer's repository link:https://bitbucket.org/pieterboom/paragems/Licensing provisions: BSD 2-clauseProgramming language: Fortran 90External routines/libraries: BLAS/LAPACK, MPI, PETSc [1], hypre ParaSails [2,3]Nature of problem: Large scale simulation of 3D elliptic boundary value problems in discrete media with heterogeneous or discontinuous properties.Solution method: ParaGEMS is a library implementing discrete exterior calculus (DEC) for distributed memory architectures. It is distributed with mini-applications for solving 3D elliptic boundary value problems, which describe a number of physical laws (Fourier's law, Fick's law, Darcy's law and Ohm's law), in discrete media with heterogeneous or discontinuous properties.Additional comments including restrictions and unusual features: ParaGEMS requires meshes generated in the default format used by Triangle [4] and TetGen [5].

Thermal Conductivity Evolution of Carbon-Fiber Ablators Submitted to High Temperatures

Carbon-phenolic ablators are widely used as thermal protection materials for atmospheric entry capsules. The knowledge of their thermophysical properties is therefore of primary importance to correctly simulate their thermal response, eventually reducing design safety margins without the risk of compromising the integrity of the underlying vehicle structure and payload. In this work, we present the results of an experimental work that aimed to evaluate the effective thermal conductivity of the carbon-phenolic ablator ZURAM® in its charred state. The material thermal conductivity was obtained, up to 2790°C, using the available heat-capacity law for this material, ad hoc laser flash analysis thermal diffusivity measurements, and material density estimates. The results suggest a strong impact of the material heating history on the thermal conductivity. As a consequence of this hysteresis-like behavior, measurements taken during the heating or the cooling phases differ sensibly, with discrepancies far beyond the quantified experimental uncertainties. Heating-phase measurements follow a specific trend and could be reduced to a fitting polynomial law with associated error bounds. Measurements performed during the cooling phase seem to be affected by a modification process of the material intrinsic properties that is speculated to be related to the graphitization of the carbon fibers composing the material.

Thermal conductivity of salvaged fused cast alumina used in the glass industry

AbstractFused cast alumina (FCA) has been and continues to be used as a refractory material in energy intensive industries such as glass melting and chemical processing. In‐service degradation due to high temperature exposure in harsh environment affects the designed furnace thermal profiles and energy consumption. Phase transformation may occur at the refractory hot face during glass melting altering the properties. Three FCA blocks recovered from industrial furnaces were investigated in this study. The as‐received FCA consists primarily of a mixture of alpha (α) and beta (β) alumina that has a thermal conductivity value of 5–6 W/mK. The Hot Disk method was used to obtain thermal conductivity directly on the refractory blocks. At the hot face, a transformation from β to α alumina occurred and was confirmed by an X‐ray diffraction study. Thermal conductivity measurements as a function of position also showed a clear transition from β to α alumina at both ends of a complete block with no voids. Thermal conductivity of the α alumina tripled compared to β alumina. This study provides important information of heat transfer and thermal conductivity evolution to refractory manufacturers and users.

Ab initio calculations on thermal conductivity of Fe-Ni-O fluid: Constraints on the thermal evolution of Earth's core

Thermal conductivity is a crucial parameter for the thermal evolution of Earth's core. In this paper, the thermal conductivity of Fe-Ni-O fluid was calculated by ab initio simulations, and then age of inner core and thickness of thermal stratification were derived. The results indicate that the thermal conductivity of Fe-Ni-O fluid along the core adiabatic curve ranges from 111.68 to 182.33 W/m/K. The thermal conductivity at the core-mantle boundary increases monotonically with temperature as expected. The thermal conductivity of Fe-Ni-O fluid is lower than that of pure Fe and Fe-Ni fluids. The age of the inner core is determined to be 0.868 Ga in our fiducial model by calculating the energy budget in the core and time evolution of thermal conductivity of Fe-Ni-O fluid. The thickness of thermal stratification is 454 km when dTc/dt=100 K/Ga by calculating the temperature gradient.

Growth and Thermal Conductivity Study of CuCr2Se4-CuCrSe2 Hetero-Composite Crystals

The CuCrSe2 shows attractive physical properties, such as thermoelectric and multiferroic properties, but pure-phase CuCrSe2 crystal is still quite challenging to obtain because CuCr2Se4 can be easily precipitated from a CuCrSe2 matrix. Here, taking the advantage of this precipitation reaction, we grew a series of CuCrSe2-CuCr2Se4 hetero-composites by adjusting growth parameters and explored their thermal conductivity property. Determined by electron-diffraction, the orientation relationship between these two compounds is [001] (100) CuCrSe2‖[111] (220) CuCr2Se4. The out-of-plane thermal conductivity κ of these hetero-composites was measured by a time-domain thermo-reflectance method. Fitting experimental κ by the Boltzmann-Callaway model, we verify that interface scattering plays significant role to κ in CuCrSe2-CuCr2Se4 hetero-composites, while in a CuCrSe2-dominated hetero-composite, both interface scattering and anharmonic three-phonon interaction lead to the lowest κ therein. Our results reveal the thermal conductivity evolution in CuCr2Se4-CuCrSe2 hetero-composites.

A combined method for analyzing the effective thermal conductivity evolution of satin weave thermoset prepregs during preforming process

The macro effective thermal conductivity of satin weave thermoset prepregs evolves dynamically in the preforming and demonstrates itself as in function of yarn angle and temperature. Accurate knowledge of it is fundamental for the high-fidelity heat transfer and deformation analysis in the preforming simulation. In this paper, a method combing virtual deformation modeling and multiscale thermal homogenization was proposed to analyze the macro effective thermal conductivity evolution of satin weave thermoset prepregs during preforming. The virtual deformation modeling based on the transverse Neo-Hookean hyper-elastic material law was used to acquire yarn geometry in the meso thermal homogenization analysis. Predicted yarn shapes were validated with the X-ray μCT images. The macro effective thermal conductivities at different temperatures and yarn angles were computed with the Macro/Meso thermal homogenization. It shows that the in-plane macro effective thermal conductivities evolve nonlinearly and greatly with respect to both temperature and yarn angle variation, while the transverse macro effective thermal conductivities evolve almost only with temperature variation. The predicted thermal conductivities were compared with specifically designed thermal experiments. Good agreement is noted, demonstrating the effectiveness of the proposed method. This work provides a new alternative way for predicting thermal conductivities of woven thermoset prepregs.

Influence of water-rock interaction on permeability and heat conductivity of granite under high temperature and pressure conditions

Water-rock interactions exist between the geothermal host rock (GHR) and the cold injected water for Enhanced (or Engineered) Geothermal Systems (EGS), changing permeability and thermal conductivity of GHR, influencing the heat extraction of EGS. Therefore, a water-rock interaction experiment is performed under high temperature and pressure conditions for heat-treated granite. The variation in permeability and thermal conductivity, mineral composition, and solution composition of the rock matrix is consequently analyzed, and their influence on the heat extraction of EGS is discussed. The thermodynamic analysis of the chemical processes of the experiment is presented using batch chemical simulators, PHREEQC 3.0, reproducing the chemical time series of the water-rock reactions. Water-rock reaction changes the composition of the rock and the circulating fluid. For primary minerals, albite, anorthite, and k-feldspar dissolve, biotite tends to be stable, and quartz gradually increases. Secondary minerals, muscovite, calcite, kaolinite, and chlorite, are formed in our simulation results, and their source and generation mechanisms are discussed. However, the kaolinite and chlorite were not detected in the experiment due to their low quantity. Both the dissolution of primary mineral and the formation of secondary mineral change the permeability and the thermal conductivity of the granite. The permeability exhibits a transition from increasing to decreasing while the thermal conductivity shows an opposite variation. Both approach constant values but only at 60% of the initial value. However, from the long-term perspective, the change in permeability and thermal conductivity of the granitic host rock around the fracture will not exert a strong impact on the heat extraction of EGS. The test results in this context could facilitate our understanding of permeability characteristics and the evolution of thermal conductivity of a granitic host rock during the water-rock interaction, and their influence on the heat extraction of EGS during the operation process.

Thermal conductivity evolution of early-age concrete under variable curing temperature: Effect mechanism and prediction model

The thermal conductivity and microevolution mechansim of early-age concrete (EAC) under variable curing temperature environments significantly influence the temperature cracks in massive concrete. An experiment was carried out to compare thermal conductivity evolution differences of EAC under variable curing temperature (VCT) and standard curing temperature (SCT) conditions. The concrete specimens of moisture and dry states were used to consider the effect of moisture content on thermal conductivity. Results showed that the moisture concrete thermal conductivity under VCT condition decreases with curing ages and is higher than SCT condition. For the dry samples, the thermal conductivity of VCT condition increases with curing ages and is lower than the SCT condition. The difference in thermal conductivity between VCT and SCT increases from zero to seven days and decreases after seven days. The scanning electron microscope (SEM) tests showed that the VCT leads to slower growth of pore structure and larger porosity of EAC due to the slower hydration rate under VCT condition. The thermal conductivity difference between VCT and SCT is mainly caused by the differences in water content, chemical components in solid phase, and porosity (solid-phase volume fraction). Meanwhile, the relation among thermal conductivity, porosity and most probable pore size shows that curing temperature has a great effect on hydration degree and controls the thermal conductivity evolution. Besides, a prediction model of EAC thermal conductivity under VCT condition was built based on hydration kinetics.

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 conductivity evolution of a compressed expanded natural graphite – Phase change material composite after thermal cycling

Compressed Expanded Natural Graphite (CENG) impregnated with Phase Change Material (PCM) is an interesting material for latent heat storage application requiring a high heat transfer rate. This composite material, that has orthotropic properties, undergoes expansion while the PCM changes phase that might affect its thermal properties and its mechanical integrity. In order to assess this possible evolution, the CENG-PCM thermal conductivities in the two principal directions have been measured before and after a 1500 thermal cycling test. The thermal characterisation was performed using two methods: a classic hot guarded plate method and a more originally inverse method. The results show no significant change of the thermal conductivity values after ageing cycles in spite of the appearance of cracks in the planar direction of the composite. We can conclude that the delamination of the CENG was not sufficient to affect the thermal conductivities. In addition, a DSC analysis shows that the latent heat is also almost unchanged after the thermal cycling.

High power continuous wave laser heating of graphite in a high temperature range up to 3800 K

As graphite is an important material for several applications, the evaluation of its properties at very high temperatures is of major importance. Conducting studies at temperatures >2800 K is, however, very challenging, and the amount of available data in the literature is, therefore, very limited. This paper presents a methodology that compares experimental data with simulation results following a progressive increase of the temperature range investigated. The study was conducted on a commercial polycrystalline graphite produced by sintering (EDM-3 from POCO) under high power continuous laser heating (kW ytterbium at a wavelength of 1080 nm). Experiments were done inside a vacuum chamber equipped with pyrometers and cameras allowing fine monitoring of the temperature of the samples. A 3D numerical model has been developed based on the finite element method to analyze the experimental results. The evolution of the thermal and optical properties of EDM-3 with temperature is required for correctly modeling laser/material interactions. By running a sequential comparison of calculation with a dedicated experiment, we achieve better knowledge of such properties. From this study, we obtain estimations of the evolution of thermal conductivity, emissivity, and evaporation rate up to 3800 K of amorphous graphite.

Thermal conductivity of concrete at high temperatures for thermal energy storage applications: Experimental analysis

Thermal conductivity plays an important role in energy storage when the materials are charging and discharging. This paper presents an experimental investigation of the evolution of thermal conductivity up to 600 °C in different concretes. Moreover, the thermal conductivity was measured during thermal fatigue cycles when temperature ranged between 300 and 600 °C, simulating the operation conditions in a storage system of molten salts in a Concentrating Solar Power Plant (CSP). Five concrete compositions were analysed using diverse types of aggregates with different thermal response, covering a wide range of the initial thermal conductivity. The results confirm that the loss of thermal conductivity with temperature during the first heating is mainly due to the free water loss. Moreover, the type of aggregate influences the overall thermal performance of concrete due to its thermal conductivity and the volumetric differences with the cement paste. Siliceous aggregates underwent the highest decrease of thermal conductivity of concrete (+50%) with regard to room temperature. Regarding the cooling phase, thermal conductivity recovers between 20% and 40% depending on the type of aggregate. The outcomes of the present study demonstrate that the assumption of a constant thermal conductivity value in numerical simulations to predict its thermal capacity for energy storage is not appropriate.