Understanding the anharmonic phonon properties of crystal compounds-such as phonon lifetimes and thermal conductivities-is essential for investigating and optimizing their thermal transport behaviors. These properties also impact optical, electronic, and magnetic characteristics through interactions between phonons and other quasiparticles and fields. In this study, we develop an automated first-principles workflow to calculate anharmonic phonon properties and build a comprehensive database encompassing more than 6500 inorganic compounds. Utilizing this dataset, we train a graph neural network model to predict thermal conductivity values and spectra from structural parameters, demonstrating a scaling law in which prediction accuracy improves with increasing training data size. High-throughput screening with the model enables the identification of materials exhibiting extreme thermal conductivities-both high and low. The resulting database offers valuable insights into the anharmonic behavior of phonons, thereby accelerating the design and development of advanced functional materials.

Transition metal oxides are promising candidates in the field of thermoelectricity, which can convert heat and electricity into each other and realize the efficient utilization of waste energy. For the figure of merit ZT = S2σT/(κe + κl), a lower thermal conductivity is desired for an enhanced ZT, and cation doping is an appropriate way to regulate the thermal transport properties. However, because S, σ, and κe are strongly coupled with each other, cation doping for one parameter modification can generate compensation with others, making regulation more difficult. In this work, we demonstrate the effective engineering of the thermal conductivity of SrTiO3 films by partial oxygen isotope substitution with 18O using a straightforward aftergrowth thermal annealing process. The results show that the isotope disorder promotes the scattering of phonons and generates a nearly 20% decreased thermal conductivity of SrTiO3 films. Our work provides a convenient new route for the design of thermoelectric materials with high ZT values.

Point defects can be used to tailor the properties of semiconductors, but can also have undesired effects on electronic and thermal transport, particularly in ultrascaled nanostructures, such as nanowires. Here we use all-atom molecular dynamics to study the effect that different concentrations and spatial distributions of vacancies have on the thermal conductivity of Si nanowires, overcoming the limitations of previous studies. Although vacancies are not as effective as the nanovoids found in e.g. porous Si, they can still reduce the thermal conductivity in ultrathin Si nanowires by more than a factor of two, when found in concentrations smaller than 1%. We also present arguments against the so-called self-purification mechanism, which is sometimes suggested to take place and proposes that vacancies have no influence on transport phenomena in nanowires.

We apply a recently developed method for solving the linearized phonon Boltzmann equation to study the hydrodynamic thermal transport in dielectrics in the collective limit, i.e., when normal collisions dominate resistive ones. The method recovers Guyer and Krumhansl results for a single Debye branch and extends them to general dispersion relations and branches. Specifically, we obtain explicit microscopic expressions for the phonon distribution and for the transport coefficients in this limit. We find that the phonon distribution differs from the commonly used displaced distribution in two terms: one accounting for viscous flow and another one which allows us to solve a long-standing issue on drifting and driftless second-sound velocities. Thus, the new method allows us to generalize previous results and fill some gaps on fundamental aspects of the collective limit through a simple mathematical formalism. We compare the hydrodynamic framework with previous models and discuss its limitations.

Machine learning techniques are used to explore the intrinsic origins of the hydrodynamic thermal transport and to find new materials interesting for science and engineering. The hydrodynamic thermal transport is governed intrinsically by the hydrodynamic scale and the thermal conductivity. The correlations between these intrinsic properties and harmonic and anharmonic properties, and a large number of compositional (290) and structural (1224) descriptors of 131 crystal compound materials are obtained, revealing some of the key descriptors that determines the magnitude of the intrinsic hydrodynamic effects, most of them related with the phonon relaxation times. Then, a trained black-box model is applied to screen more than 5000 materials. The results identify materials with potential technological applications. Understanding the properties correlated to hydrodynamic thermal transport can help to find new thermoelectric materials and on the design of new materials to ease the heat dissipation in electronic devices.

We present a formalism to solve the phonon Boltzmann transport equation (BTE) for finite Knudsen numbers that supplies a hydrodynamic heat transport equation similar to the Navier-Stokes equation for general semiconductors. This generalization of Fourier's law applies in general cases, from systems dominated by momentum-preserving normal collisions, as is well known, to kinetic materials dominated by resistive collisions, where it captures nonlocal effects. The key feature of our framework is that the macrostate is described in terms of the heat flux and its first derivatives. We obtain explicit expressions for the nonequilibrium phonon distribution and for the geometry-independent macroscopic parameters as a function of phonon properties that can be calculated from first principles. Ab initio model predictions are found to agree with a wide range of experiments in silicon. In contrast to approaches directly based on the BTE, the hydrodynamic equation can be solved in arbitrary geometries, thus providing a powerful tool for nanoscale heat modeling at a low computational cost.

The aim of this work is to investigate a simple on-line control methodology applicable to press hardening. Short production runs were performed in a laboratory plant, using a pyrometer to measure sheet and die temperatures with varying processing conditions. Sheets thus treated were studied in terms of microstructure and mechanical properties. Different closing die time and refrigeration conditions were employed to force OK and Not OK conditions. The experimental data including the process variables as a well as the resultant temperatures have been analysed and modelled by means of statistical analysis and Machine Learning algorithms, to discover hidden correlations that can lead to actionable predicting models. The results show a direct link of the final temperature with the microstructure and its hardness. The outcome of this paper can be used for efficient process design and detection of anomalous temperature meanwhile an industrial hot stamping process take part. In addition, the analysis performed can help productivity and quality assurance while leading towards a smarter and more efficient manufacturing scenario.

According to Fourier’s law, a temperature difference across a material results in a linear temperature profile and a thermal conductance that decreases inversely proportional to the system length. These are the hallmarks of diffusive heat flow. Here, we report heat flow in ultrathin (25 nm) GaP nanowires in the absence of a temperature gradient within the wire and find that the heat conductance is independent of wire length. These observations deviate from Fourier’s law and are direct proof of ballistic heat flow, persisting for wire lengths up to at least 15 μm at room temperature. When doubling the wire diameter, a remarkably sudden transition to diffusive heat flow is observed. The ballistic heat flow in the ultrathin wires can be modeled within Landauer’s formalism by ballistic phonons with an extraordinarily long mean free path.

The diameter dependence of the thermal conductivity of nanowires is usually modeled using Matthiessen's rule, by putting the mean free path of phonons equal to the diameter d of the nanowire. This results in a thermal conductivity kappa that decreases with decreasing d, due to the increase in boundary scattering. Recent molecular dynamics studies of heat transport in thin silicon nanowires have shown a nonmonotonic diameter dependence of kappa, where a decrease with decreasing d is followed by an increase to a value of kappa exceeding the bulk thermal conductivity. This increase of kappa was explained by an increase of the importance of hydrodynamic transport effects in the thinner wires, where the normal scattering by phonon-phonon interaction increases, but the Umklapp scattering decreases [Y. Zhou, X. Zhang, and M. Hu, Nano Lett. 17, 1269 (2017)]. Here, we study heat transport in thin nanowires of the compound semiconductor gallium-phosphide in the wurtzite crystal structure, using molecular dynamics simulations. A similar nonmonotonic d dependence of kappa is found as in silicon nanowires, but with a minimum in kappa occurring at a much larger diameter of d approximate to 8 nm instead of 2-3 nm.

At short length scales phonon transport is ballistic: the thermal resistance of semiconductors and insulators is quantized and length independent. At long length scales, on the other hand, transport is diffusive and resistance arises as a result of the scattering processes experienced by phonons. In many cases of interest, however, these two transport regimes coexist. Here we propose a first-principles approach to treat quasiballistic phonon transport where diffusive and ballistic phonons receive separate theoretical treatments. Partitioning the overall phonon population for a given transport length is performed examining the mean free paths obtained from the solution of the Boltzmann transport equation and allowing only diffusive phonons to participate in anharmonic phonon-phonon scattering processes. We present results for Si and diamond, discussing the crossover from ballistic to diffusive transport as the length scale and/or the temperature increases and compute the relative contribution of ballistic and diffusive phonons to the thermal conductance in each transport condition.

We combine first-principles electronic structure calculated thermal conductivity data with a numerical solution of the one-dimensional heat equation to show that an asymmetric distribution of impurity scattering, if suitably designed, yields the conditions for a low-temperature thermal rectification. This happens as a result of the differences in the peaks of the temperature dependence of the thermal conductivity. We demonstrate the effectiveness of the method by probing the thermal rectification rendered by a silicon slab with a steplike position-dependent isotopic composition. The same conclusions are obtained by using experimentally measured values of the thermal conductivity of Si samples with different isotope distributions.

We carry out a systematic study of the thermal conductivity of four single-layer transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se) from first-principles by solving the Boltzmann transport equation (BTE). We compare three different theoretical frameworks to solve the BTE beyond the relaxation time approximation (RTA), using the same set of interatomic force constants computed within density functional theory (DFT), finding that the RTA severely underpredicts the thermal conductivity of MS2 materials. Calculations of the different phonon scattering relaxation times of the main collision mechanisms and their corresponding mean free paths (MFP) allow evaluating the expected hydrodynamic behaviour in the heat transport of such monolayers. These calculations indicate that despite of their low thermal conductivity, the present TMDs can exhibit large hydrodynamic effects, being comparable to those of graphene, especially for WSe2 at high temperatures.

We present a theoretical study of the lattice thermal conductivity of ${\mathrm{SrTiO}}_{3}$ in its antiferrodistortive ferroelastic phase and of its dependence on an applied external electric field, via electrophononic couplings. The calculations are done by using second-principles density-functional theory and the full solution of the Boltzmann transport equation. Our results allow, on one hand, to identify and explain deviations from the usual temperature dependence of the thermal conductivity, revealing Poiseuille flow and a rare umklapp transport regime, in agreement with recent experimental results [Martelli et al., Phys. Rev. Lett. 120, 125901 (2018)]; on the other hand, they show that an external electric field, by reducing the symmetry of the lattice, activates different phonon-phonon scattering processes and thus yields a reduction of the thermal conductivity, supporting the generality of a heat control strategy previously reported by some of us [Seijas-Bellido et al., Phys. Rev. B 97, 184306 (2018)].

Amultiscale hydrodynamic-heat-transportmodel applicable to arbitrary geometries using finite-element methods is compared with the experimental effective thermal conductivity of silicon thin films and periodic holey membranes for different sizes and temperatures. The range of system length scales and temperatures in which the model predictions agree with experimental data is discussed and quantitatively determined. The model agrees with experimental results when the smallest system size is larger than twice the nonlocal length, an intrinsic property of the material that depends only on temperature. These results open the door to the use of the hydrodynamic equation instead of an effective Fourier model to interpret current heat-transport experimental data.

We compute the thermal conductivity of the most common polymorphs of titanium dioxide (TiO2), rutile and anatase, using a first-principles A approach and solving the phonon Boltzmann transport equation beyond the relaxation time approximation. We find that both polytypes are anisotropic, as expected from their crystal structure; however, while kappa(xx) = kappa(yy) < kappa(zz) for rutile, the opposite holds for anatase. The modal decomposition of the thermal conductivity provides insight in this inversion in the anisotropy of the two polytypes.

The Guyer–Krumhansl equation is an extension to the classical Fourier law that is particularly appealing from a theoretical point of view because it provides a link between kinetic and continuum models and is based on well-defined physical parameters. Here we show how, subjected to a specific boundary condition analogous to the slip conditions for fluids, the Guyer–Krumhansl equation yields promising results in predicting the effective thermal conductivity of nanowires with circular and rectangular cross-sections.

We demonstrate theoretically how, by imposing epitaxial strain in a ferroelectric perovskite, it is possible to achieve a dynamical control of phonon propagation by means of external electric fields, which yields a giant electrophononic response, i.e., the dependence of the lattice thermal conductivity on external electric fields. Specifically, we study the strain-induced manipulation of the lattice structure and analyze its interplay with the electrophononic response. We show that tensile biaxial strain can drive the system to a regime where the electrical polarization can be effortlessly rotated and thus yield giant electrophononic responses that are at least one order of magnitude larger than in the unstrained system. These results derive directly from the almost divergent behavior of the electrical susceptibility at those critical strains that drive the polarization on the verge of a spontaneous rotation.

Several recent experiments have demonstrated the failure of the Fourier heat equation in semiconductors at the nanoscale. We show that a generalized heat transport equation including a hydrodynamic term can explain three of these experiments on a silicon substrate: heater lines experiments both in stationary and nonstationary settings and the nonstationary response in grating experiments. To this end, we solve the hydrodynamic heat equation either numerically or analytically. The non-Fourier response observed in those experimental situations can be easily explained in terms of hydrodynamic concepts, such as friction and vorticity. For instance, the experimentally observed increase of thermal boundary resistance as size decreases can be simply understood as an increase of friction. We conclude that, contrary to common belief, hydrodynamics is a fundamental ingredient of semiconductor heat transport in the nanoscale at room temperature, providing physical insight and a unifying framework on the observed non-Fourier response. The relations between hydrodynamic heat transport with phonon momentum conservation and the Boltzmann transport equation are also provided.