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In the present work, we demonstrate the ability of a technique based on Monte Carlo resolution of the Boltzmann transport equation associated to the Green–Kubo autocorrelation of the phonon heat flux to predict, at thermal equilibrium, the thermal conductivity tensor of nanoporous structures. This methodology, which is derived from a former work [D. Lacroix, M. Isaiev, G. Pernot, Phys. Rev. B 104, 165 202 (2021)] developed in the case of bulk systems, is used to predict thermal transport properties of Si porous matrices and Si phononic membranes at room temperature. A broad range of porosities and different pore network organizations are considered. Our results are compared to available experimental data and former modeling techniques. In addition, analytical models based on the phonon mean free path are detailed and compared to numerical simulations. [ABSTRACT FROM AUTHOR]

In this study, we conducted calculations to determine the thermal conductivity of silicon nanowires with various shapes and cross-sectional designs using the Monte Carlo method combined with Green–Kubo heat flow autocorrelation. This computational approach is known for its reliability in predicting thermal properties of complex nanostructured devices. We specifically examined nanowires with circular or rectangular sections combined to complex serpentine and fishbone modulations. Our findings indicate that the nanowire's geometry significantly impacts the phonon mean free path and thermal transport. More specifically, patterns with serpentine modulations exhibit a pronounced reduction in thermal conductivity attributable to enhanced phonon boundary scatterings. Our simulation results are compared with the available experimental data, highlighting the potential of our method for structural optimization in thermal management applications, particularly in devices like thermoelectric converters. [ABSTRACT FROM AUTHOR]

Scanning Thermal Microscopy (SThM) has become an important measurement technique for characterizing the thermal properties of materials at the nanometer scale. This technique requires a SThM probe that combines an Atomic Force Microscopy (AFM) probe and a very sensitive resistive thermometer; the thermometer being located at the apex of the probe tip allows for the mapping of temperature or thermal properties of nanostructured materials with very high spatial resolution. The high interest of the SThM technique in the field of thermal nanoscience currently suffers from a low temperature sensitivity despite its high spatial resolution. To address this challenge, we developed a high vacuum-based AFM system hosting a highly sensitive niobium nitride (NbN) SThM probe to demonstrate its unique performance. As a proof of concept, we utilized this custom-built system to carry out thermal measurements using the 3ω method. By measuring the V3ω voltage on the NbN resistive thermometer under vacuum conditions, we were able to determine the SThM probe's thermal conductance and thermal time constant. The performance of the probe is demonstrated by performing thermal measurements in-contact with a sapphire sample. [ABSTRACT FROM AUTHOR]

The characterization of material thermal properties at nanoscales remains a challenge even if progress was achieved in developing outstanding characterization techniques like scanning thermal microscopy (SThM). In the present work, we propose a detailed procedure based on the combined use of a SThM probe characterization and its Finite Element Method (FEM) modeling to recover in operando 3ω measurements achieved under high vacuum. This approach is based on a two-step methodology: (i) a fine description of the probe's electrical and frequency behaviors in "out of contact" mode to determine the intrinsic parameters of the SThM tip and (ii) a minimization of the free parameter of our model, i.e., the contact thermal resistance, by comparing 3ω measurements with the simulations of the probe operating "in contact mode." Such an approach allows us to measure thermal interface resistances between the tip and the surface. We applied our methodology to three different materials with known thermal properties: Si, SiO2 bulk materials, and a gold thin film. In addition, the FEM modeling provides insights into SThM thermal probes sensitivity, as a function of probe/sample interface resistance and the contact area to measure material thermal conductivity paving the way to quantitative SThM measurements. [ABSTRACT FROM AUTHOR]

Picosecond thermoreflectance is an unprecedented powerful technique for nanoscale heat transfer analysis and metrology, but different sources of artifacts were reported in the literature making this technique difficult to use for long delay (several ns) thermal analysis. We present in this paper a new heterodyne picosecond thermoreflectance (HPTR) technique. As it uses two slightly frequency shifted lasers instead of a mechanical translation stage, it is possible to avoid all artifacts leading to erroneous thermal parameter identifications. The principle and set-up are described as well as the model. The signal delivered by the HPTR experiment is calculated for each excitation configurations, modulating or not the pump beam. We demonstrate the accuracy of the technique in the identification of the thermal conductivity of a 50 nm thick SiO2 layer. Then, we discuss the role of the modulation frequency for nanoscale heat transfer analysis. [ABSTRACT FROM AUTHOR]

Carbon nanotube (CNT) networks embedded in a polymer matrix have been extensively studied as a flexible thermoelectric transport medium over the recent years. However, their power factor has been largely limited by the relatively inefficient tunneling transport at junctions between CNTs and the low-density conducting channels throughout the networks. This work demonstrates that significant power factor enhancements can be achieved by adding electrically insulating microscale particles in three-dimensional CNT networks embedded in the polymer matrix. When silica particles of a few μm diameters were co-embedded in single-walled CNT (SWCNT)-polydimethylsiloxane (PDMS) composites, both the electrical conductivity and the Seebeck coefficient were simultaneously enhanced, thereby boosting the power factor by more than a factor of six. We found that the silica microparticles excluded a large volume of the composite from the access of CNTs and caused CNT networks to form around them with the polymer as a binder, resulting in improved network connectivity and alignment of CNTs. Our theoretical calculations based on junction tunneling transport for three-dimensional CNT networks show that the significant power factor enhancement can be attributed to the enhanced tunneling with reduced junction distance between CNTs. Additional power factor enhancement by a factor of three was achieved by sample compression, which further reduced the mean junction distance to enhance tunneling but also reduced the geometric factor at the same time, limiting the enhancement of electrical conductivity.

In this work we present a molecular dynamics investigation of thermal transport in a silica-gallium nitride nanocomposite. A surprising enhancement of the thermal conductivity for crystalline volume fractions larger than 5% is found, which cannot be predicted by an effective medium approach, not even including percolation effects, the model systematically leading to an underestimation of the effective thermal conductivity. The behavior can instead be reproduced if an effective volume fraction twice larger than the real one is assumed, which translates into a percolation effect surprisingly stronger than the usual one. Such a scenario can be understood in terms of a phonon tunneling between inclusions, enhanced by the iso-orientation of all particles. Indeed, if a misorientation is introduced, the thermal conductivity strongly decreases. We also show that a percolating nanocomposite clearly stands in a different position than other nanocomposites, where thermal transport is dominated by the interface scattering and where parameters such as the interface density play a major role, differently from our case.

A High-Throughput Time-Domain ThermoReflectance (HT-TDTR) technique was developed to perform fast thermal conductivity measurements with minimum user actions required. This new setup is based on a heterodyne picosecond thermoreflectance system. The use of two different laser oscillators has been proven to reduce the acquisition time by two orders of magnitude and avoid the experimental artefacts usually induced by moving the elements present in TDTR systems. An amplitude modulation associated to a lock-in detection scheme is included to maintain a high sensitivity to thermal properties. We demonstrate the capabilities of the HT-TDTR setup to perform high-throughput thermal analysis by mapping thermal conductivity and interface resistances of a ternary thin film silicide library Fe x Si y Ge 100-x-y (20

Ti-based silicide quantum dot superlattices (QDSLs) are grown by reduced-pressure chemical vapor deposition. They are made of titanium-based silicide nanodots scattered in an n-doped SiGe matrix. This is the first time that such nanostructured materials have been grown in both monocrystalline and polycrystalline QDSLs. We studied their crystallographic structures and chemical properties, as well as the size and the density of the quantum dots. The thermoelectric properties of the QDSLs are measured and compared to equivalent SiGe thin films to evaluate the influence of the nanodots. Our studies revealed an increase in their thermoelectric properties-specifically, up to a trifold increase in the power factor, with a decrease in the thermal conductivity-making them very good candidates for further thermoelectric applications in cooling or energy-harvesting fields.