Thermal Conductivity Degradation Research Articles

The effect of residual mechanical stresses and vacancy defects on the diffusion expansion of the damaged layer during irradiation of BeO ceramics

The paper presents the results of a study on the application of Raman and UV spectroscopy methods to determine the structural damage kinetics in the near-surface layer of BeO ceramics caused by high-dose irradiation with He2+ ions. Interest in this type of ceramics is due to the combination of its structural and thermophysical parameters, making these ceramics one of the promising classes of materials for microelectronics and structural materials for nuclear reactors, with the possibility of operation in conditions of heightened radiation background. According to the conducted studies, it was established that with the irradiation fluence growth, changes in the nature of deformation structural distortions associated with the accumulation of residual mechanical stresses of tensile and compressive types are observed. At irradiation fluences of 1016-5×1016 Не2+/cm2, tensile stresses play a dominant role in structural distortions, while the value of compressive stresses at fluence growth makes up a small share in the overall nature of the deformations. Moreover, an elevation in the irradiation fluence above 5×1016 He2+/cm2 leads to a rise in the concentration of defects caused by the formation of oxygen vacancies, as well as He-VO type complexes, the presence of which is indicated by the halo intensity growth in the Raman spectra, as well as a change in the intensity of the absorption bands. Analysis of changes in thermophysical parameters revealed that a rise in structural distortions associated with the accumulation of complex defects results in thermal conductivity reduction and a deterioration in heat transfer processes associated with partial amorphization of the damaged layer. Moreover, the established direct relationship between the value of residual mechanical stresses and the degradation of thermal conductivity indicates the cumulative effect of destructive changes caused by irradiation, as well as the influence of diffusion mechanisms on the damaged layer thickness growth.

The low-temperature thermal conduction and millimeter-wave dielectric properties of β-Si3N4 as a heat-dissipation substrate

Increasing operating frequency is required in wireless communication systems. In use, this generates heat, which results in serious problems for semiconductor devices. Silicon nitride (Si3N4) has high thermal conductivity and fracture strength. Therefore, it is expected to be suitable for heat dissipation in active circuit substrates for wireless communication. In this study, Si3N4 with varying thermal conductivities were prepared by controlling starting material and sintering conditions and their low-temperature thermal conductivities were measured. The mean free path of phonons in β-Si3N4 influences its thermal conductivity. The temperature dependence of the Debye–Waller factor obtained by synchrotron radiation scattering supported the high thermal conductivity of the β-Si3N4. The degradation of thermal conductivity was interpreted in terms of the full width at half maximum of Raman spectra. Further, infrared reflectivity and first-principles calculation for β-Si3N4 confirmed their superior dielectric properties compared with other high thermal conductive materials.

Enhancing the thermal cyclic reliability of salt-based shape-stabilized phase change materials by in-situ SiO2–C interconnectivity in rice husk carbon

Utilizing salt-based shape-stabilized phase change materials (ss-PCMs) for high-temperature thermal energy storage stands as a pivotal avenue in realizing carbon neutrality. However, the performance of ss-PCMs significantly deteriorates after thermal cycling. This study utilizes rice husk carbon (RHC) with an in-situ SiO2–C interconnected structure as the encapsulating material, along with expanded graphite (EG) and MgO, to encapsulate ternary chloride (TC). In this approach, RHC simultaneously acts as a thermal conductivity materials (TCM) and a ceramic supporting material (CCM). The results showed that this encapsulation method can effectively encapsulate up to 65% of TC. The 60 wt% TC-10 wt% RHC-10 wt% EG-20 wt% MgO sample named 60T/10R/10E/20M is considered a close to optimal formulation in terms of thermal conductivity and phase change enthalpy. This ss-PCM has a thermal conductivity of 8.86 W m−1 K−1, and phase change enthalpy of 153.6 J g−1. The ss-PCMs maintained shape stability even after 1000 cycles, with minimal mass loss and thermal conductivity degradation compared to the non-RHC-added ss-PCM. This study has prepared high performance and reliability thermal storage materials through a green and low-cost approach.

A thermodynamically consistent phase field model for brittle fracture in graded coatings under thermo-mechanical loading

This paper presents a thermodynamically consistent phase field formulation designed for investigating quasi-static brittle fracture under thermo-mechanical loading conditions in functionally graded materials. The governing equations for linear momentum and crack evolution are derived by minimizing the free energy functional, which includes elastic strain energy and surface energy, with respect to displacement field and the phase field variable, respectively. A thermodynamically consistent formulation is employed to establish the governing equation for heat conduction. The proposed formulation is converted into a set of finite element equations using the Ritz method and implemented in COMSOL Multiphysics, a commercial finite element software which allows for writing governing equations in their strong form. A segregated solver is utilized to solve iteratively the coupled problem. The proposed model is verified against existing mechanical and thermo-mechanical phase field models, demonstrating good agreement with the literature. To explore the model’s capabilities, a graded layer composed of Zirconia and a Titanium alloy with a surface crack undergoing thermo-mechanical loading is considered as a case study for this investigation. Additionally, the model incorporates a thermal conductivity degradation proposed. A parametric study is performed to investigate the effect of varying parameters like volume fraction, thermal loading rate, and the thermal conductivity degradation on the load–displacement curve of the graded layer. These findings contribute to the understanding of thermo-mechanical behavior of surface graded layers and serve as a foundation for addressing more complex thermo-mechanical problems.

A study on the thermal conductivity of proton irradiated CVD-SiC and sintered SiC, measured using a modified laser flash method with multi-step machining

CVD-SiC and sintered SiC (SPS-SiC) were proton irradiated at 340 ̊C receiving different levels of damage (0.05–0.25 dpa). A novel multi-step machining and measurement method using laser flash analysis (LFA) was developed to derive the thermal conductivity of the irradiated layer (∼46 µm). Before irradiation, the thermal conductivity of SPS-SiC was much lower than CVD-SiC, primarily due to its higher intrinsic defect concentration and smaller grain size which provide a greater density of barriers to phonon transmission. Following irradiation, major thermal conductivity degradation (∼90%) was found to occur to both types of SiC after only a low dose (∼0.1 dpa), with both saturating at a similarly low value (a few W/K⋅m), as the thermal resistivity due to the presence of high density of grain boundaries became less important. Thermal conductivity degradation after irradiation was primarily caused by point defects in both types of SiC, as reflected by Raman spectra.

Finite element models for radiation effects in nuclear fusion applications

Deuterium-tritium fusion reactions produce energy in the form of 14.1 MeV neutrons, and hence fusion reactor components will be exposed to high energy neutron irradiation while also being subjected to thermal, mechanical and magnetic loads. Exposure to neutron irradiation has numerous consequences, including swelling and dimensional changes, comparable in magnitude to the peak transient thermal deformations occurring in plasma-facing components. Irradiation also dynamically alters the various thermo-mechanical properties, relating temperature, stress and swelling in a strongly non-linear way. Experimental data on the effect of neutron exposure spanning the design parameter space are very sparse and this highlights the relevance of computer simulations. In this study we explore the equivalence between the body force/surface traction approach and the eigenstrain formalism for treating anisotropic irradiation-induced swelling. We find that both commercial and massively parallelised open source software for finite element method (FEM) simulations are suitable for assessing the effect of neutron exposure on the mechanically loaded reactor components. We demonstrate how two primary effects of irradiation, radiation swelling and the degradation of thermal conductivity, affect the distributions of stress and temperature in the divertor of the ITER tokamak. Significant uncertainties characterising the magnitude of swelling and models for treating it suggest that on the basis of the presently available data, only an order of magnitude estimate can be given to the stress developing in reactor components most exposed to irradiation during service.

Investigation of Nano-Heat-Transfer Variability of AlGaN/GaN-Heterostructure-Based High-Electron-Mobility Transistors

The aim of this work is to propose an electrothermal model for predicting the electron mobility, the effective thermal conductivity, and the operating temperature of AlGaN/GaN HEMT devices. The suggested model comprises an enhanced ballistic-diffusive model (BDE) coupled with a drift-diffusion model (D-D). Furthermore, the given model considers total electron mobility, which depends on mobility degradation caused by phonon interactions, surface imperfections, and carrier mobility inside the bulk GaN material. The model is validated based on available experimental and numerical results, and good concordance is observed. It is found that the degradation of the drain current is due to electron mobility and effective thermal conductivity degradation. The output characteristic’s degradation due to changing device temperature is analyzed. We demonstrate that for gate biases of −1 V, 0 V, and 1 V, operating temperatures of 390 K, 470 K, and 570 K are obtained when the drain currents are 0.1 A, 0.24 A, and 0.38 A, respectively. Furthermore, we demonstrate that the temperature is maximal in the active region. The temporal temperature evolution presents the same trends with the same amplitude compared to the experimental data, and the error does not exceed 5%.

Radiation-induced thermal conductivity degradation modeling of zirconium

This study presents a radiation-induced thermal conductivity degradation (TCD) model of zirconium as compared to the conventional UO2 TCD model. We derived the governing factors of the radiation-induced TCD model, such as maximum TCD value and temperature range of TCD. The maximum TCD value was derived by two methods, in which 1) experimental result of 32 % TCD was directly utilized as the maximum TCD value and 2) a theoretical approach based on dislocation was applied to derive the maximum TCD value. Further, the temperature range of TCD was determined to be 437–837 K by 1) experimental results of post-annealing of irradiation hardening as compared to 2) the rate theory and thermal equilibrium. Consequently, the radiation-induced TCD model of zirconium was derived to be fr=1−0.321+exp⁡{(T−637)/45} . Because the thermal conductivity of zirconium is one of the factors determining the storage and transport system, this newly proposed model could improve the safety analysis of spent fuel storage systems.

Thermal conductivity degradation due to radiation-induced amorphization in U3Si2: A pilot study

In this study, we investigate the thermal conductivity of U3Si2 amorphized by ion irradiation using 84 MeV 136Xe ions at 190 °C. The suspended-bridge method was utilized to measure the thermal conductivity, allowing for a detailed analysis of the specimen while minimizing interference from other crystalline phases. Our results indicate that the thermal conductivity of amorphous U3Si2 is significantly lower than that of unirradiated crystalline U3Si2. These findings are consistent with recent studies on in-pile-irradiated U3Si2 samples that consider the effects of U3Si2 amorphization, fission gas bubbles, and other impurities.

Effect of ionizing radiation on thermal and mechanical properties of filled-epoxy adhesives

Stave assemblies at the Large Hadron Collider and other high-energy physics experiments require radiation-resistant adhesives composed of light elements with high thermal conductivity and low interfacial resistance. Porous carbon foams are currently bonded to carbon fiber faceplates with graphite-filled epoxy. Low-viscosity epoxies are used to increase the amount of filler in order to double the thermal conductivity without impacting the properties of the material under irradiation. Promising new adhesive formulations showed no degradation in thermal conductivity after exposure to a fluence of 1016 n/cm2 (>0.1 MeV neutrons) or 10 Gy gamma radiation, but showed variable degradation, compared to the currently used adhesive, after exposure to a fluence of 1015 p/cm2 (1 MGy, 67.5 MeV protons). Adhesives with more than double the irradiated thermal conductivity of the baseline material showed marked improvement in thermal transport in graphite/epoxy/foam/epoxy/graphite structures based on thermal diffusivity measurements. Silicon carbide particles can be added to graphite to stiffen the modulus of the filled epoxy. When spherical AlN was used as a filler, the degradation in thermal conductivity under proton irradiation varied based on the size of the ceramic filler. Options for improving the thermal performance of irradiated adhesives for high-energy physics experiments are discussed.

Effect of Strain on the Resistivity and Thermal Conductivity of High Purity Niobium

High purity niobium (Nb) is a technologically important material for large-scale accelerator and nano-scale quantum computing applications in microwave frequency range. The high thermal conductivity and low resistivity of Nb are critical to the high performance at temperature range of 0.01 - 4.0 K. The presence of interstitials such as O, N, H, and C act as scattering centers and alter the mean free path, reducing resistivity and thermal conductivity, and contributing significantly to Nb's thermal performance for temperatures of 2.0 K and above. The residual resistivity ratio (RRR), defined as the ratio of the normal state resistivity at 300 K to that at 4.2 K (Nb, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$T_{c}$</tex-math></inline-formula> = 9.2 K), is an accepted direct estimate of the impurity content of fully recrystallized Nb. Complete re-crystallization of Nb is challenging unless very high temperatures are employed, which is often impractical, hence, in practice, dislocation and dislocation structures impact the thermal performance of Nb due to strong phonon scattering contributions. This paper reports on the degradation of thermal conductivity and RRR of high purity Nb large grain, single crystal with fixed impurity, varying strain, and dislocation content levels. Experimental thermal conductivity data fits the Boltzmann transport equation incorporating dislocation density.

Do dislocations always decrease thermal conductivity?

Dislocations play a significant role in introducing disorder and distortion within an ideal bulk crystal lattice, resulting in the degradation of phonon thermal conductivity. In this study, we employ non-equilibrium Molecular Dynamics (NEMD) simulations to investigate the thermal conductivity of two spherical, single-crystal face-centered cubic (fcc) nanoparticles (NP) with a radius denoted as R. Our analysis encompasses pristine interfaces as well as interfaces containing dislocations, at the NP contact. Surprisingly, our findings reveal that the presence of dislocations leads to an increase in thermal conductivity, contrary to the expectations derived from bulk models and simulations. This counterintuitive behavior can be attributed to the expansion of the contact radius (ac) between the nanoparticles, facilitated by the dislocations. Notably, the thermal conductivity demonstrates a substantial reduction of approximately 90% compared to bulk values, and in all simulated cases, it scales effectively as a function of (ac/R). We explore various models to elucidate the impact of dislocations on thermal conductivity and ascertain that, under our specific conditions, only a marginal decrease would be anticipated. In alignment with these models, our results indicate that the introduction of localized dislocation arrays spanning nearly ten lattice parameters into a bulk sample elicits a thermal conductivity decrease of less than 10%. These findings provide compelling evidence that dislocations per se do not significantly affect thermal conductivity. Instead, the enhanced contact area generated by dislocations governs the heat flux within the simulated nanostructures. Notably, the analysis of thermal boundary conductance, which evaluates the conductivity across the interfaces, does exhibit a decrease in interfaces featuring dislocations. Considering that the dislocation content can be influenced by various synthesis conditions of the nanoparticles, these findings hold promising implications for tailoring the thermal conductivity of nanoparticle beds or coatings.

Analysis of thermal fuel behaviour under steady-state irradiation using selected cases of the IAEA CRP FUMEX

Results of the project “Comprehensive Verification of the FALCON Code for Calculation of Nuclear Fuel Temperature”, relating to thermal fuel behaviour under steady-state (SST) irradiation and slow power ramps are presented. Two groups of irradiation experiments carried out in the Halden Boiling Water Reactor (HBWR) are analyzed. Improvement of the prediction capability with respect to fuel temperature under SST irradiation after the earlier calibration of the selected models is confirmed. The data from the on-line measurement of fuel temperature are interpreted in consideration of the effects of Fuel Thermal Conductivity (FTC) degradation with burnup, Fission Gas Release (FGR), reduction of FTC due to development of gaseous fuel porosity, and Pellet-Cladding Mechanical Interaction (PCMI). Relevance of the analyzed data to current new concepts of Advanced Technology Fuel (ATF) is noted.

Enhancement of thermoelectric properties of transition metals, nickel and copper dually doped ZnO

Thermoelectricity is one of the potential sustainable energy sources, that can be treated as a remedy for the present energy scarcity as it is a clean-green technology. ZnO is a promising multifunctional metal oxide and its thermoelectric conversion property is highly exploited in the field of waste heat recovery. Herein, the impact of nickel and copper substitution on the thermoelectric properties of ZnO is investigated. The structural and morphological analysis of the doped materials confirmed the hexagonal wurtzite structure of ZnO. The strain induced in the lattice and the variation of crystallite size established the substitution of the dopants into the host matrix. The optical band gap energy shows a reduction from 3.1 eV to 2.15 eV with a redshift in the absorption edge. The electrical conductivity and Seebeck coefficient of ZnO increased upon doping. The most remarkable result of nickel and copper doping is the enhancement of the Seebeck coefficient to 1318 µV/K. As a result of the lattice distortion created due to the dual substitution of the dopants, the thermal conductivity is considerably reduced to 8.36 W/mK. The power factor enhancement and degradation of thermal conductivity give rise to a six-fold increase in the thermoelectric figure of merit of the material.

Effect of burn-up on the thermal conductivity of light water reactor fuel: Results of investigations employing the laser flash technique

Analysing data drawn from three of EPRI's NFIR Programmes and the High Burn-up Rim Project (HBRP) managed by CRIEPI of Japan, expressions for the degradation of thermal conductivity with burn-up up to 103.5 MWd/kgHM are presented. These indicate that at high burn-up, above 70 MWd/kgHM, the thermal conductivity of the matrix of the high burn-up structure (HBS) increases by 25 - 30% concomitant with recrystallization of the fuel grains. This increase is partially or completely off-set, however, by a decrease in conductivity associated with the porosity of the HBS which contains the fission gas gathered from the fuel matrix during transformation of the microstructure. Below 70 MWd/kgHM, in the absence of the HBS, the conductivity is about 15% lower than that currently accepted. It is also found that the conductivity of UO2 fuel falls sharply immediately when irradiation begins. This is attributed to radiation damage rather than the incorporation of fission products in the fuel lattice. The data further reveal that at burn-ups below 70 MWd/kgHM the conductivity of UO2 containing 10 wt% Gd2O3 is markedly lower than that of UO2, but once the HBS has formed its conductivity is indistinguishable from that of conventional UO2, UO2 additive fuel or MOX. It is concluded that for accurate determination of the radial temperature profile in a high burn-up fuel pellet, knowledge of the volume fraction of the HBS in the pellet and the radial distribution of the porosity in the affected region is required.

Experimental and Numerical Investigations of a Thermal Management System Using Phase-Change Materials and Forced-Air Cooling for High-Power Li-Ion Battery Packs

The thermal management system of a power battery is crucial to the safety of battery operation; however, for the phase-change material (PCM) thermal management system of a battery, the thermal cycling of phase-change material under large discharge rate conditions will lead to thermal conductivity degradation and thermal stress problems. A method of manufacturing PCM containers with metal fins to package pure phase-change material is put forward to solve the problem. The system temperature under different conditions is studied using numerical and experimental methods. A thermal resistance model is built to analyze the thermal transfer performance of PCM containers with fins. The results show that the PCM container structure can effectively control the battery temperature within the suitable temperature range under the low discharge rate, but the maximum temperature of the battery pack at the high discharge rate of 3 C will exceed the optimum operating temperature range. Adding fins can reduce the maximum temperature and improve the system temperature uniformity. By combining fins with forced-air cooling, the maximum temperature and maximum temperature difference of the battery pack at a high discharge rate can be effectively reduced.

New recommendation for the thermal conductivity of irradiated (U, Pu)O2 fuels under fast reactor conditions. Comparison with recent experimental data

New experimental results obtained at the Joint Research Centre in Karlsruhe (JRC-Karlsruhe) within the European Sustainable Nuclear Industrial Initiative Plus project (ESNII+) were used to develop an updated recommendation for the thermal conductivity of (U, Pu)O2 fuels irradiated in fast reactor (FR) conditions. Fresh and irradiated FR fuels were characterised, and their thermal diffusivity and heat capacity were measured by the laser flash technique. The thermal conductivity of those fuels was calculated, using hydrostatic density measurements when available or using calculated density.The thermal conductivity model, developed in this work, describes different effects induced by irradiation on the degradation of thermal conductivity: soluble fission products (FPs), precipitated FPs, radiation damage, and porosity. Those effects are considered in separate corrective factors, which depend on the chemical composition of the irradiated fuel, its microstructure, and the temperature. To estimate these parameters, thermomechanical calculations (PLEIADES-GERMINAL V2) and thermochemical calculations (Thermo-Calc V4.1 with the TAF-ID V11 database) were carried out in conjunction. The data provided for each fuel sample consisted of fuel temperature, burn-up, end-of-life (EOL) isotopic composition, molar fraction of the dissolved FPs in the matrix, molar fraction of the oxide, and the metallic precipitated phases, density, produced and released fission gases.In this work, we showed a clear discrepancy between the experimental data and the most widely used thermal conductivity model for irradiated fuels (Lucuta et al., 1996). Lucuta's model was then modified and a new recommendation for FR (U, Pu)O2 fuels was provided. This work demonstrates that by introducing the neodymium content as the parameter driving the thermal conductivity of the matrix, a better agreement between the model and the experimental data can be achieved. This paper discusses the uncertainties related to the irradiation conditions and the data provided by the GERMINAL-Thermo-Calc calculation.

An experimental study on thermal efficiency of hybrid GO/MWCNTs nanoparticles suspended in a binary mixture of ethylene glycol and water

Energy consumption has a demand growth across the globe due to modernization. However, the use of fossil fuels is creating a huge impact on climate change and has made it essential to harvest solar renewable energy technology. The heat transfer fluid plays an important role in maintaining the optimum temperature of the panel by removing the heat absorbed and transferring to the thermal energy storage medium. Due to the poor thermal conductivity and thermal degradation of conventional heat transfer fluids such as water, ethylene glycol and oil, these pose limitations to be utilised in solar energy technology systems. Thus, to overcome this problem, a hybrid nanofluid is used to boost the thermal conductivity and degradation temperature. However, the addition of hybrid nanoparticles further enhances the thermo-physical properties of the fluids. In this present study, GO/MWCNTs hybrid nanoparticles (0.01 – 0.2 wt%) were dispersed in hybrid base fluid (EG/water) in the ratio of 40:60. The finding shows that for visual stability, MWCNT sediment was faster compared to GO while, hybrid GO/MWCNT, showed higher stability compared to the single nanoparticle. The TGA results show the higher thermal degradation temperature at 0.05 wt% of pure GO, pure MWCNT and hybrid GO/MWCNT with thermal degradation of 122.74 ℃, 118.93 ℃ and 119.26 ℃ respectively. The use of nanofluids acts as a clean fuel for electricity production and decreases CO2 emission, which can further promote sustainable development goals.

Influence of point defect accumulation on in-pile thermal conductivity degradation: Fuel rod defect distribution and deviation between in-pile and post irradiation thermal conductivity

As nuclear fuel burn-up increases, its thermal conductivity degrades due to the accumulation of defects that lead to increased phonon scattering rates. This results in a rise in the centerline temperature of the fuel rod, whereby heat generation must be decreased to avoid undesired behavior such as fuel melting and extensive fission gas release. Fuel performance codes are utilized to optimize the fuel's operational conditions; while they are based on established physical principles, their empirical nature limits their predictive capabilities. Recently, an effort has been made to develop predictive fuel performance codes for commonly used nuclear fuels, as well as accelerated qualification of advanced nuclear fuels. In this report, we elaborate on the importance of careful analysis of point defects’ impact on thermal conductivity in fuel performance analysis. A model is presented where point defect concentration is estimated based on Rate Theory modeling and used as input to the Klemens-Callaway model to calculate their contribution to the degradation of thermal conductivity in UO2 under prototypical irradiation conditions. This analysis suggests that point defect concentration is significant at the rim of the fuel pellet and neglecting this leads to underestimation of the centerline temperature, which may have consequences on fission gas behavior.

Three-dimensional microstructural characterization of FBR MOX fuel and the contribution of microstructural features to the thermal conductivity of the fuel

Combination of microstructural characterization, property measurements, and phase field modeling is used to investigate fast breeder reactor (FBR) mixed oxide (MOX) fuel irradiated to burnup of 13.7% fissions per initial metal atom (FIMA). The fuel was characterized at different radial locations, which revealed that grey phase can be present in the central region if it nucleates on five metal precipitates (FMPs). In addition, in the mid-radial region FMPs do not diffuse out of the region once formed and the size of Pd–Te precipitates is dictated by the porosity present in the region. Thermal conductivity measurements (TCM) were conducted as a function of radial location and the microstructure of the fuel was correlated with the observed trend. Reconstructions of the 3D solid and gaseous fission product structures in different regions of the fuel were used to simulate the effective thermal conductivity (ETC) of the respective regions and determine which microstructural feature has the strongest impact on thermal conductivity. Based on conducted assessment, FMPs and Pd-Te precipitates improve local conductivity of central and mid-radial regions even in the presence of grey phase, but defects are primary contributor to the degradation of thermal conductivity on the periphery of the fuel.