Thermal transport crossover from crystalline to partial-crystalline partial-liquid state

Thermal shock protection with scalable heat-absorbing aerogels

Improving thermal insulation is vital for addressing thermal protection and energy efficiency challenges. Though silica aerogel has a record-low thermal conductivity at ambient pressure, its high production cost, due to its nanoscale porous structure, has hindered its widespread use. In this study, we introduce a cost-effective and mild method that enhances insulation by incorporating phase change materials (PCMs) into a micron-porous framework. With a thermal conductivity at 0.041 W m-1K-1 on par with conventional insulation materials, this PCMs aerogel presents additional advantages for thermal protection from transient high-temperature loads by effectively delaying heat propagation through heat absorption. Moreover, the PCMs aerogel remains stable under cyclic deformation and heating up to 300 °C and is self-extinguishing in the presence of fire. Our approach offers a promising alternative for affordable insulation materials with potential wide applications in thermal protection and energy conservation areas.

Ab Initio Driven Exploration on the Thermal Properties of Al-Li Alloy

Al-Li alloys are feasible and promising additives in advanced energy and propellant systems due to the significantly enhanced heat release and increased specific impulse. The thermal properties of Al-Li alloys directly determine the manufacturing, storage safety, and ignition delay of propellants. In this study, a neural network potential (NNP) is developed to investigate the thermal behaviors of Al-Li alloys from an atomistic perspective. The novel NNP demonstrates an excellent predictive ability for energy, atomic force, mechanical behaviors, phonon vibrations, and dynamic evolutions. A series of NNP-based molecular dynamics simulations are performed to investigate the effect of Li doping on the thermal properties of Al-Li alloys. All calculated results for Al-Li alloys are consistent with experimental values for Al, ensuring their validity in predicting Al-Li interactions. The simulation results suggest that a minor increment in the Li content results in a slight change in the melting point, thermal expansion, and radical distribution functions. These three properties are associated with the lattice characteristics; nonetheless, it causes a substantial reduction in thermal conductivity, which is related to the physical properties of the elements. The lower thermal conductivity allows heat accumulation on the particle surface, thereby speeding up the surface premelt and ignition. This provides an alternative atomic explanation for the improved combustion performance of Al-Li alloys. These findings integrate insights from the field of alloy material science into crucial combustion applications, serving as an atomistic guide for developing manufacturing techniques.

Extreme phonon anharmonicity underpins superionic diffusion and ultralow thermal conductivity in argyrodite Ag8SnSe6

Ultralow thermal conductivity and fast ionic diffusion endow superionic materials with excellent performance both as thermoelectric converters and as solid-state electrolytes. Yet the correlation and interdependence between these two features remain unclear owing to a limited understanding of their complex atomic dynamics. Here we investigate ionic diffusion and lattice dynamics in argyrodite Ag8SnSe6 using synchrotron X-ray and neutron scattering techniques along with machine-learned molecular dynamics. We identify a critical interplay of the vibrational dynamics of mobile Ag and a host framework that controls the overdamping of low-energy Ag-dominated phonons into a quasi-elastic response, enabling superionicity. Concomitantly, the persistence of long-wavelength transverse acoustic phonons across the superionic transition challenges a proposed 'liquid-like thermal conduction' picture. Rather, a striking thermal broadening of low-energy phonons, starting even below 50 K, reveals extreme phonon anharmonicity and weak bonding as underlying features of the potential energy surface responsible for the ultralow thermal conductivity (<0.5 W m-1 K-1) and fast diffusion. Our results provide fundamental insights into the complex atomic dynamics in superionic materials for energy conversion and storage.

Anisotropic Phonon Scattering and Thermal Transport Property Induced by the Liquid-like Behavior of AgCrSe2

Superionic conductors exhibiting a periodic crystalline lattice and liquid-like ionic conductivity have emerged as promising materials in energy-conversion devices. Herein, we have investigated the interplay among anharmonic lattice dynamics, thermal conduction, and ultrafast atomic diffusion across the superionic transition of AgCrSe₂. We show that the thermal conductivity (κ) contributions from convection and conduction-convection interactions increase simultaneously due to the gradual fluidization of Ag atoms, leading to a temperature-independent κ in the superionic state. We demonstrate a non-Peierls type thermal transport behavior induced by the strong lattice anharmonicity of Ag atoms, which promotes a nontrivial wave-like phonon tunneling in the normal state of AgCrSe₂. Our current fluctuation analysis demonstrates an anisotropic phonon-liquid scattering behavior that the in-plane nondispersive transverse acoustic (TA) phonons near the zone boundary collapse, while the zone center and boundary TA phonons in the direction perpendicular to the liquid-like flow of Ag atoms survive.

Large Thermal Conductivity Switching in Ferroelectrics by Electric Field-Triggered Crystal Symmetry Engineering

A convenient, reversible, fast, and wide-range switching of thermal conductivity is desired for efficient heat energy management. However, traditional methods, such as temperature-induced phase transition and chemical doping, have many limitations, e.g., the lack of continuous tunability over a wide temperature range and low switching speed. In this work, a strategy of electric field-driven crystal symmetry engineering to efficiently modulate thermal conductivity is reported with first-principles calculations. By simply changing the direction of an external electric field loaded in ferroelectric PbZr_0.5Ti_0.5O₃, near the morphotropic phase boundary composition, we obtain the largest switching of thermal conductivity for ferroelectric materials at room temperature based on the dual-phonon theory, i.e., normal and diffuson-like phonons, with three different criteria. The calculation results indicate that with decreasing crystal symmetry, the degeneracy of phonon modes reduces and the avoid-crossing behavior of phonon branches enhances, leading to the increase of diffuson-like phonons and weighted phonon-phonon scattering phase space. A thermal switch prototype based on PbZr_0.5Ti_0.5O₃ is further shown that can protect the Li-ion battery by modulating its temperature up to 17.5 °C. Our studies would pave the way for designing next-generation thermal switch with high speed, a wide temperature range, and a large switching ratio.

Broadband Wide-Angle VElocity Selector (BWAVES) neutron spectrometer designed for the SNS Second Target Station

A recently proposed wide-angle velocity selector (WAVES) device for choosing the velocity of detected neutrons after they have been scattered by the sample paves the way for inverted geometry neutron spectrometers with continuously adjustable final neutron wavelength. BWAVES broadband inverted geometry spectrometer proposed for the Second Target Station at the Spallation Neutron Source at Oak Ridge National Laboratory is designed using WAVES to simultaneously probe dynamic processes spanning 4.5 decades in time (energy transfer). This makes BWAVES a uniquely flexible instrument which can be viewed as either a quasielasitc neutron scattering (QENS) spectrometer with a practically unlimited (overlapping with the vibrational excitations) range of energy transfers, or a broadband inelastic vibrational neutron spectrometer with QENS capabilities, including a range of accessible momentum transfer (Q) and a sufficiently high energy resolution at the elastic line. The new capabilities offered by BWAVES will expand the application of neutron scattering in ways not possible with existing neutron spectrometers.

Perspective on multi-scale simulation of thermal transport in solids and interfaces

Phonon-mediated thermal transport is inherently multi-scale. The wave-length of phonons (considering phonons as waves) is typically at the nanometer scale; the typical size of a phonon wave energy packet is tens of nanometers, while the phonon mean free path (MFP) can be as long as microns. At different length scales, the phonons will interact with structures of different feature sizes, which can be as small as 0D defects (point defects), short to medium range linear defects (dislocations), medium to large range 2D planar defects (stacking faults and twin boundaries), and large scale 3D defects (voids, inclusions, and various microstructures). The nature of multi-scale thermal transport is that there are different heat transfer physics across different length scales and in the meantime the physics crossing the different scales is interdependent and coupled. Since phonon behavior is usually mode dependent, thermal transport in materials with a combined micro-/nano-structure complexity becomes complicated, making modeling this kind of transport process very challenging. In this perspective, we first summarize the advantages and disadvantages of computational methods for mono-scale heat transfer and the state-of-the-art multi-scale thermal transport modeling. We then discuss a few important aspects of the future development of multi-scale modeling, in particular with the aid of modern machine learning and uncertainty quantification techniques. As more sophisticated theoretical and computational methods continue to advance thermal transport predictions, novel heat transfer physics and thermally functional materials will be discovered for the pertaining energy systems and technologies.

Optimization of Synthesizing Upright ZnO Rod Arrays with Large Diameters through Response Surface Methodology

The deposition parameters involved in chemical bath deposition were optimized by a response surface methodology to synthesize upright ZnO rod arrays with large diameters. The effects of the factors on the preferential orientation, aspect ratio, and diameter were determined systematically and efficiently. The results demonstrated that an increased concentration, elevated reaction temperature, prolonged reaction time, and reduced molar ratio of Zn2+ to tri-sodium citrate could increase the diameter and promote the preferential oriented growth along the [002] direction. With the optimized parameters, the ZnO rods were grown almost perfectly vertically with the texture coefficient of 99.62. In the meanwhile, the largest diameter could reach 1.77 μm. The obtained rods were merged together on this condition, and a dense ZnO thin film was formed.

Electronic and Thermoelectric Properties of V2O5, MgV2O5, and CaV2O5

Developing new thermoelectric materials with high performance can broaden the thermoelectric family and is the key to fulfill extreme condition applications. In this work, we proposed two new high-temperature thermoelectric materials—MgV2O5 and CaV2O5—which are derived from the interface engineered V2O5. The electronic and thermoelectric properties of V2O5, MgV2O5, and CaV2O5 were calculated based on first principles and Boltzmann semi-classical transport equations. It was found that although V2O5 possessed a large Seebeck coefficient, its large band gap strongly limited the electrical conductivity, hence hindering it from being good thermoelectric material. With the intercalation of Mg and Ca atoms into the van der Waals interfaces of V2O5, i.e., forming MgV2O5 and CaV2O5, the electronic band gaps could be dramatically reduced down to below 0.1 eV, which is beneficial for electrical conductivity. In MgV2O5 and CaV2O5, the Seebeck coefficient was not largely affected compared to V2O5. Consequently, the thermoelectric figure of merit was expected to be improved noticeably. Moreover, the intercalation of Mg and Ca atoms into the V2O5 van der Waals interfaces enhanced the anisotropic transport and thus provided a possible way for further engineering of their thermoelectric performance by nanostructuring. Our work provided theoretical guidelines for the improvement of thermoelectric performance in layered oxide materials.

Thermal transport in amorphous small organic materials: a mechanistic study

Understanding the thermal transport mechanisms in amorphous organic materials is of great importance to solve hot-spot issues in organic-electronics nanodevices. Here we studied thermal transport in two popular molecular electronic materials, N,N-dicarbazolyl-3,5-benzene (mCP) and N,N'-diphenyl-N,N'-di(3-methylphenyl)-(1,1'-biphenyl)-4,4'diamine (TPD), in the amorphous state by molecular dynamics simulations. We found that due to the softness of organic materials, the low thermal conductivity of both systems can be greatly enhanced under pressure. Notably, in such systems, the convective term of heat flux provides an important contribution to thermal transport as it cross-correlates with the Virial term in the Green-Kubo formula. Mode diffusivity calculations reveal that low-frequency modes can contribute significantly to thermal transport in both mCP and TPD. By increasing the pressure, the sound velocity and relaxation time of such low-frequency modes can be enhanced, and a part of these modes converts from diffusons to propagons. The cooperation of these three effects is responsible for the strong pressure dependence of thermal transport in amorphous organic systems. Molecular pair heat flux calculations demonstrate that heat transfer mainly happens between pairs of molecules with distances below 1.4 nm. This work paves the way for the optimization of thermal transport in amorphous organic materials widely used in opto-electronics, e.g. as OLED and OPV.

Highly Durable and Thermally Conductive Shell-Coated Phase-Change Capsule as a Thermal Energy Battery

Robust and thermally conductive phase-change capsules (PCCs) can be effectively used as dispersoids for heat transfer fluids (HTFs) to utilize waste heat. Here, we demonstrate PCCs encapsulated with a cross-linked poly(2-hydroxyethyl methacrylate) shell that showed high durability and low thermal hysteresis for effective heat uptake and release. The circulation system was manufactured by mimicking the 4th Generation District Heating (4GDH) system to confirm the heat delivery efficiencies of PCC-dispersed slurries (PCSs) as the HTFs. The enthalpy change of water after it received heat from the PCS improved by up to 41.1% on increasing the amount of PCCs in the PCS. Furthermore, a high PCC recovery of 92 wt % was achieved after 1500 cycles, which accompanied a phase transition. The PCC developed by us can thus enable effective storage/delivery of waste heat-driven energy for zero-energy buildings and a 4GDH system, as well as thermal management of electronics.

Thermal conductivity of V2O5 nanowires and their contact thermal conductance

Vanadium pentoxide (V2O5)-based composites show outstanding performances as cathode materials in lithium-ion batteries. However, their inferior thermal conductivity restricts the heat dissipation through the cathode electrode. In this study, we measured the thermal conductivity of V2O5 nanowires using the thermal bridge method and found that their thermal conductivity is 3.84 ± 0.38 W m-1 K-1 at T = 300 K. The contact thermal resistance between two nanowires with the same size was measured to be up to 50%-80% of the total thermal resistance in the measured samples, indicating that their contact is the bottleneck for thermal dissipation.

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.