Grain Boundary Thermal Resistance Research Articles

Grain boundary effects on thermal shock responses of yttria-stabilized zirconia

The rapid temperature change or thermal shock is inevitable in many engineering applications of polycrystalline yttria-stabilized zirconia (YSZ). Nevertheless, the thermal shock response of YSZ especially its dependence on the grain boundary (GB) structure in YSZ remains almost unknown to date. In this study, the effect of the GB on thermal shock behaviors of YSZ ceramics was investigated by molecular dynamics (MD) simulations in conjunction with experimental characterizations. According to the distribution of misorientation angles of GBs experimentally extracted from our sintered YSZ samples, four most common GBs with a misorientation angle ranging from 35° to 63° were constructed in MD simulations through the annealing technique. The thermal shock simulations on the bi-crystalline YSZ with such GBs show a weakening effect of the GB on the thermo-elastic strain wave, which is found to be the most significant in the [100]/[111] GB and almost vanish in its [110]/[111] counterpart. Our simulations also reveal that the weakening effect of GB on the thermo-elastic strain wave is majorly attributed to the impedance mismatch between its two adjacent grains, although the grain-boundary thermal resistance observed also can weaken the temperature wave after crossing GBs. These findings provide valuable atomic insights into the thermomechanical behaviors of YSZ under thermal shock, which will guide the design of microstructures of YSZ for some special applications in solid oxide fuel cells and thermal barrier coatings.

Microscale Imaging of Thermal Conductivity Suppression at Grain Boundaries.

Grain-boundary engineering is an effective strategy to tune the thermal conductivity of materials, leading to improved performance in thermoelectric, thermal-barrier coatings and thermal management applications. Despite the central importance to thermal transport, a clear understanding of how grain boundaries modulate the microscale heat flow is missing, owing to the scarcity of local investigations. Here, thermal imaging of individual grain boundaries is demonstrated in thermoelectric SnTe via spatially resolved frequency-domain thermoreflectance. Measurements with microscale resolution reveal local suppressions in thermal conductivity at grain boundaries. Also, the grain-boundary thermal resistance - extracted employing a Gibbs excess approach - is found correlating with the grain-boundary misorientation angle. Extracting thermal properties, including thermal boundary resistances, from microscale imaging can provide comprehensive understanding of how microstructure affects heat transport, crucially impacting the materials design of high-performance thermal-management and energy-conversion devices. This article is protected by copyright. All rights reserved.

Highly Distorted Grain Boundary with an Enhanced Carrier/Phonon Segregation Effect Facilitates High-Performance Thermoelectric Materials.

Grain boundary plays an important role in determining the phonon/carrier transport behaviors of thermoelectric (TE) materials, especially for the polycrystalline materials with the average grain size in nanoscale dimensions. Adjusting the grain size and boundaries of TE materials is considered as an effective approach to decouple TE parameters and thereby synergistically optimize the TE performance. Here, the highly distorted grain boundary with an enhanced carrier/phonon segregation effect is introduced to the commercial n-type Bi2Te2.7Se0.3 matrix. The existence of highly distorted grain boundaries can not only enhance the interfacial Seebeck coefficient without significant detriment to the electrical conductivity but also increase the interfacial thermal resistance of grain boundaries, leading to the synergistic reduction of the thermal conductivity. As a result, a peak figure of merit zTmax ≈ 1.22@425 K and an average zTavg ≈ 1.1(300-500 K) are obtained, which are about 55 and 57% higher than the corresponding values of the commercial Bi2Te2.7Se0.3 matrix, respectively. This work represents a new avenue for improving the commercial Bi2Te2.7Se0.3 TE material, which could further promote the development of the TE technology.

Simulation study of effects of grain boundary and helium bubble on lattice thermal resistance of tungsten

Tungsten (W) as plasma facing material (PFM) has been used in ITER divertor. Many experiments indicate that due to helium (He) ion irradiation the thermal conductivities (TCs) of W can be significantly reduced. In this work, the lattice thermal transport property is studied based on the non-equilibrium molecular dynamics (NEMD) method. Firstly, the Kapitza resistance of grain boundaries (GBs) in W is obtained. It can be confirmed that the Kapitza resistance is positively correlated with GB energies, which reflect the lattice distortion degree in a sense. The result indicates that the distortion energy may play a key role in the thermal properties. Further simulation is performed to study the thermal resistance of GBs with nano He bubble. When the number of He atoms in bubble is less than a value, the thermal resistances are stable and change little. It implies that the effect of GBs is more significant than He bubble to block phonons diffusion. With the number of He atoms increasing, the property of heat conduction deteriorates further because the coupling of He bubble and GBs change the lattice distortion degree.

Revisiting the Reduction of Thermal Conductivity in Nano- to Micro-Grained Bismuth Telluride: The Importance of Grain-Boundary Thermal Resistance

Nanograined bulk alloys based on bismuth telluride (Bi2Te3) are the dominant materials for room-temperature thermoelectric applications. In numerous studies, existing bulk phonon mean free path (MFP) spectra predicted by atomistic simulations suggest sub-100 nm grain sizes are necessary to reduce the lattice thermal conductivity by decreasing phonon MFPs. This is in contrast with available experimental data, where a remarkable thermal conductivity reduction is observed even for micro-grained Bi2Te3 samples. In this work, first-principles phonon MFPs along both the in-plane and cross-plane directions are re-computed for bulk Bi2Te3. These phonon MFPs can explain new and existing experimental data on flake-like Bi2Te3 nanostructures with various thicknesses. For polycrystalline Bi2Te3-based materials, a better explanation of the experimental data requires further consideration of the grain-boundary thermal resistance that can largely suppress the transport of high-frequency optical phonons.

Thermal transport in monocrystalline and polycrystalline lithium cobalt oxide.

Efficient heat dissipation in batteries is important for thermal management against thermal runaway and chemical instability at elevated temperatures. Nevertheless, thermal transport processes in battery materials have not been well understood especially considering their complicated microstructures. In this study, lattice thermal transport in lithium cobalt oxide (LiCoO2), a popular cathode material for lithium ion batteries, is investigated via molecular dynamics-based approaches and thermal resistance models. A LiCoO2 single-crystal is shown to have thermal conductivities in the order of 100 W m-1 K-1 with strong anisotropy, temperature dependence, and size effects. By comparison, polycrystalline LiCoO2 is more isotropic with much lower thermal conductivities. This difference is caused by random grain orientations, the thermal resistance of grain boundaries, and size-dependent intra-grain thermal conductivities that are unique to polycrystals. The grain boundary thermal conductance is calculated to be in the range of 7.16-25.21 GW m-2 K-1. The size effects of the intra-grain thermal conductivities are described by two empirical equations. Considering all of these effects, two thermal resistance models are developed to predict the thermal conductivity of polycrystalline LiCoO2. The two models predict a consistent thermal conductivity-grain size relationship that agrees well with molecular dynamics simulation results. The insights revealed by this study may facilitate future efforts on battery materials design for improved thermal management.

A phase field modeling based study of microstructure evolution and its influence on thermal conductivity in polycrystalline tungsten under irradiation

In this work, we study effects of neutron irradiation and elastic tensile loading on microstructure evolution in polycrystalline tungsten by using a phase field modeling based approach. The proposed model involves the interaction of point defects with grain boundaries (GBs), thermal resistance of GBs, and voids. The void evolution analysis shows that the elastic energy field affects the shape and growth rate of voids. The overall trend of porosity evolution during irradiation follows the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. Voids nucleation and growth can reduce both elastic strain energy and chemical free energy in the system. In addition, increase of porosity could significantly degrade the effective thermal conductivity of analyzed tungsten polycrystalline microstructures. The observations comply with the predictions from the theoretical models.

A molecular dynamics study of tilt grain boundary resistance to slip and heat transfer in nanocrystalline silicon

We present a molecular dynamics study of grain boundary (GB) resistance to dislocation-mediated slip transfer and phonon-mediated heat transfer in nanocrystalline silicon bicrystal. Three most stable ⟨110⟩ tilt GBs in silicon are investigated. Under mechanical loading, the nucleation and growth of hexagonal-shaped shuffle dislocation loops are reproduced. The resistances of different GBs to slip transfer are quantified through their constitutive responses. Results show that the Σ3 coherent twin boundary (CTB) in silicon exhibits significantly higher resistance to dislocation motion than the Σ9 GB in glide symmetry and the Σ19 GB in mirror symmetry. The distinct GB strengths are explained by the atomistic details of the dislocation-GB interaction. Under thermal loading, based on a thermostat-induced heat pulse model, the resistances of the GBs to transient heat conduction in ballistic-diffusive regime are characterized. In contrast to the trend found in the dislocation-GB interaction in bicrystal models with different GBs, the resistances of the same three GBs to heat transfer are strikingly different. The strongest dislocation barrier Σ3 CTB is almost transparent to heat conduction, while the dislocation-permeable Σ9 and Σ19 GBs exhibit larger resistance to heat transfer. In addition, simulation results suggest that the GB thermal resistance not only depends on the GB energy but also on the detailed atomic structure along the GBs.

Multiscale development of a fission gas thermal conductivity model: Coupling atomic, meso and continuum level simulations

Fission gas production and evolution significantly impact the fuel performance, causing swelling, a reduction in the thermal conductivity and fission gas release. However, typical empirical models of fuel properties treat each of these effects separately and uncoupled. Here, we couple a fission gas release model to a model of the impact of fission gas on the fuel thermal conductivity. To quantify the specific impact of grain boundary (GB) bubbles on the thermal conductivity, we use atomistic and mesoscale simulations. Atomistic molecular dynamic simulations were employed to determine the GB thermal resistance. These values were then used in mesoscale heat conduction simulations to develop a mechanistic expression for the effective GB thermal resistance of a GB containing gas bubbles, as a function of the percentage of the GB covered by fission gas. The coupled fission gas release and thermal conductivity model was implemented in Idaho National Laboratory’s BISON fuel performance code to model the behavior of a 10-pellet LWR fuel rodlet, showing how the fission gas impacts the UO2 thermal conductivity. Furthermore, additional BISON simulations were conducted to demonstrate the impact of average grain size on both the fuel thermal conductivity and the fission gas release. Published by Elsevier B.V.

Investigation on interfacial thermal resistance and phonon scattering at twist boundary of silicon

Grain interfaces in nanocrystalline materials play a critical role in thermal transport. A series of twist boundary thermal resistances in silicon is investigated by the nonequilibrium molecular dynamics simulation so as to find the relationship between the boundary resistance, the twist angle, the boundary energy and temperature. The results indicate that the magnitude of the twist grain boundary (GB) thermal resistance is on the order of 10−9 m2 KW−1, and the GB thermal resistance becomes larger with increasing GB energy at most twist angles, and it drops obviously with increasing temperature. The phonon wave packet dynamic simulation shows that the transmission coefficient of the low frequency phonons with long wavelength is close to 100% at the boundary with different twist angles. The transmission coefficient of the longitudinal phonon wave packet decreases with increasing frequency and transverse phonons are produced due to the scattering. In most cases, higher grain boundary energy corresponds to lower transmission coefficient, leading to larger GB thermal resistance.

Influence of structure disorder on the lattice thermal conductivity of polycrystals: A frequency-dependent phonon-transport study

It is widely accepted that the lattice thermal conductivity of a polycrystal mainly depends on its grain sizes, phonon mean free paths, and grain-boundary thermal resistance. However, uncertainties always exist on how much grain misalignment and a wide grain size distribution in a real polycrystal could affect the thermal analysis. Considering frequency-dependent phonon mean free paths, the influence of these factors is carefully examined by phonon Monte Carlo simulations for a series of disordered silicon polycrystals with grain sizes ranging from 1 to 400 nm. More generally, simulations are also performed on thermally anisotropic polycrystals. Despite all structure variation, this work suggests that the “direction-averaged” lattice thermal conductivity of a polycrystal is always close to that of an aligned polycrystal, with an effective grain size matching the interface density of the studied polycrystal.

Thermal Resistance of Grain Boundaries in Alumina Ceramics and Refractories

The influence of grain boundaries on heat transfer through polycrystalline alumina has been investigated between 20° and 500°C. The thermal conductivities of small‐grained porous ceramics and large‐grained dense ceramics have been measured using the laser‐flash technique. Two methods have been developed to assess the average thermal resistance of a grain boundary. The first method is based on the comparison of room‐temperature thermal conductivities for dense ceramics that have various average grain sizes. This method yields a value of 0.9 × 10−8 m2·K·W−1. The second method, particularly suitable for porous ceramics, is based on the extrapolation of the inverse of the thermal conductivity versus temperature to give an intercept with the axis at T= 0 K. This value is attributed to the thermal resistance of grain boundaries. By taking into account the influence of the pore content using an effective medium theory, the average thermal resistance of a grain boundary has been evaluated to be 1.3 × 10−8 m2·K·W−1 in dense alumina and 2.2 × 10−8 m2·K·W−1 in alumina containing a pore volume fraction of 0.3.

Grain-boundary thermal resistance in polycrystalline oxides: alumina, tin oxide, and magnesia

The influence of grain boundaries on heat transfer through polycrystalline oxides has been investigated. The approach is based on comparison of macroscopic thermal conductivity values for materials which have different grain sizes. Samples of alumina, magnesia, and tin oxide were made in the form of discs by standard ceramic processing of pressed powders. By control of the firing conditions, strong variations in the grain size and pore content were achieved in all three materials. The thermal conductivity was measured via the thermal diffusivity by the laser-flash technique. The average grain-boundary thermal resistances at room temperature in dense ceramics were deduced to be 0.9 x 10 - 8 m 2 K W - 1 for alumina, 0.7 x 10 - 8 m 2 K W - 1 for magnesia, and 1.2 × 10 - 8 m 2 .K W - 1 for tin oxide. Moreover, the grain-boundary thermal resistance in tin oxide is shown to be virtually constant in the temperature range 20-400°C. An alternative method was used to confirm these values, based on the measurement of the thermal conductivity as a function of temperature. The role of the effective heat-carrying cross section for a grain-boundary plane in a porous ceramic is also discussed.