Fundamental research and continued miniaturization of materials, components and systems have raised the need for the development of thermal-investigation methods enabling ultra-local measurements of surface temperature and thermo-physical properties in many areas of science and applicative fields. Scanning thermal microscopy (SThM) is a promising technique for nanometer-scale thermal measurements, imaging, and study of thermal transport phenomena. This review focuses on fundamentals and applications of SThM methods. It inventories the main scanning probe microscopy-based techniques developed for thermal imaging with nanoscale spatial resolution. It describes the approaches currently used to calibrate the SThM probes in thermometry and for thermal conductivity measurement. In many cases, the link between the nominal measured signal and the investigated parameter is not straightforward due to the complexity of the micro/nanoscale interaction between the probe and the sample. Special attention is given to this interaction that conditions the tip–sample interface temperature. Examples of applications of SThM are presented, which include the characterization of operating devices, the measurements of the effective thermal conductivity of nanomaterials and local phase transition temperatures. Finally, future challenges and opportunities for SThM are discussed.

Scanning thermal microscopy: A review

We study the lattice thermal conductivity $(\ensuremath{\kappa})$ of fully filled skutterudites $\mathrm{YbFe}{}_{4}{\text{Sb}}_{12}$ and ${\text{BaFe}}_{4}{\text{Sb}}_{12}$ from first principles. The calculated $\ensuremath{\kappa}$ of ${\text{YbFe}}_{4}{\text{Sb}}_{12}$ is ten times lower than that calculated for $\mathrm{BaFe}{}_{4}{\text{Sb}}_{12}$, and much lower than any values ever reported for other skutterudites. The ultralow $\ensuremath{\kappa}$ of $\mathrm{YbFe}{}_{4}{\text{Sb}}_{12}$ is closely related to the flat Yb-dominated modes appearing in the frequency range from 5 to 7.2 rad/ps. The flat modes significantly increase the three-phonon scattering channels, which is reflected by the unique characteristics of weighted phase space and leads to much stronger anharmonic scattering of intermediate-frequency optical phonons. Although those flat modes are dominated by the Yb atoms, the filler-related anharmonic interaction does not play a role in the increased phonon scattering. This underlines the importance of the hybridization of the filler and the host matrix in those flat modes, which can be guaranteed by the avoided crossing with acoustic phonons. The depressed phonon spectrum mechanism, common to other skutterudites, also plays a role in the ultralow $\ensuremath{\kappa}$. Our work presents a reduction mechanism of $\ensuremath{\kappa}$ by the filler in cage-like structures such as skutterudites and clathrates, and demonstrates the nonuniqueness of the reduction mechanism in these materials.

Ultralow lattice thermal conductivity of the fully filled skutterudite YbFe 4 Sb 12 due to the flat avoided-crossing filler modes

We investigate the role of rattling guest atoms on the lattice thermal conductivity of a type-I clathrate ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ by first-principles lattice dynamics. Comparing phonon properties of filled and empty clathrates, we show that rattlers cause tenfold reductions in the relaxation time of phonons by increasing the phonon-phonon scattering probability. Contrary to the resonant scattering scenario, the reduction in the relaxation time occurs in a wide frequency range, which is crucial for explaining the unusually low thermal conductivities of clathrates. We also find that the impact of rattlers on the group velocity of phonons is secondary because the flattening of phonon dispersion occurs only in a limited phase space in the Brillouin zone.

Impact of rattlers on thermal conductivity of a thermoelectric clathrate: a first-principles study.

We investigate the role of the quartic anharmonicity in the lattice dynamics and thermal transport of the type-I clathrate ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ based on ab initio self-consistent phonon calculations. We show that the strong quartic anharmonicity of rattling guest atoms causes the hardening of vibrational frequencies of low-lying optical modes and thereby affects calculated lattice thermal conductivities ${\ensuremath{\kappa}}_{L}$ significantly, resulting in an improved agreement with experimental results including the deviation from ${\ensuremath{\kappa}}_{L}\ensuremath{\propto}{T}^{\ensuremath{-}1}$ at high temperature. Moreover, our static simulations with various different cell volumes shows a transition from crystal-like to glasslike ${\ensuremath{\kappa}}_{L}$ around 20 K. Our analyses suggest that the resonance dip of ${\ensuremath{\kappa}}_{L}$ observed in clathrates with large guest free spaces is attributed mainly to the strong three-phonon scattering of acoustic modes along with the presence of higher-frequency dispersive optical modes.

https://link.aps.org/pdf/10.1103/PhysRevLett.120.105901

Quartic Anharmonicity of Rattlers and Its Effect on Lattice Thermal Conductivity of Clathrates from First Principles.

Engineering lattice thermal conductivity requires to control the heat carried by atomic vibration waves, the phonons. The key parameter for quantifying it is the phonon lifetime, limiting the travelling distance, whose determination is however at the limits of instrumental capabilities. Here, we show the achievement of a direct quantitative measurement of phonon lifetimes in a single crystal of the clathrate Ba7.81Ge40.67Au5.33, renowned for its puzzling ‘glass-like’ thermal conductivity. Surprisingly, thermal transport is dominated by acoustic phonons with long lifetimes, travelling over distances of 10 to 100 nm as their wave-vector goes from 0.3 to 0.1 A−1. Considering only low-energy acoustic phonons, and their observed lifetime, leads to a calculated thermal conductivity very close to the experimental one. Our results challenge the current picture of thermal transport in clathrates, underlining the inability of state-of-the-art simulations to reproduce the experimental data, thus representing a crucial experimental input for theoretical developments. Phonon lifetime is a fundamental parameter of thermal transport however its determination is challenging. Using inelastic neutron scattering and the neutron resonant spin-echo technique, Lory et al. determine the acoustic phonon lifetime in a single crystal of clathrate Ba7.81Ge40.67Au5.33.

/pdf/direct-measurement-of-individual-phonon-lifetimes-in-the-48ub05wkvc.pdf

Direct measurement of individual phonon lifetimes in the clathrate compound Ba 7.81 Ge 40.67 Au 5.33

Thermal transport properties in amorphous/nanocrystalline metallic composites: A microscopic insight

Perfectly crystalline solids are excellent heat conductors. Prominent counterexamples are intermetallic clathrates, guest-host systems with a high potential for thermoelectric applications due to their ultralow thermal conductivities. Our combined experimental and theoretical investigation of the lattice dynamics of a particularly simple binary representative, ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}$, identifies the mechanism responsible for the reduction of lattice thermal conductivity intrinsic to the perfect crystal structure. Above a critical wave vector, the purely harmonic guest-host interaction leads to a drastic transfer of spectral weight to the guest atoms, corresponding to a localization of the propagative phonons.

/pdf/localization-of-propagative-phonons-in-a-perfectly-1ix5xc4dg6.pdf

Localization of Propagative Phonons in a Perfectly Crystalline Solid

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Thermal transport properties in amorphous/nanocrystalline metallic composites: A microscopic insight

Localization of Propagative Phonons in a Perfectly Crystalline Solid