Rapid progress in the synthesis and processing of materials with structure on nanometer length scales has created a demand for greater scientific understanding of thermal transport in nanoscale devices, individual nanostructures, and nanostructured materials. This review emphasizes developments in experiment, theory, and computation that have occurred in the past ten years and summarizes the present status of the field. Interfaces between materials become increasingly important on small length scales. The thermal conductance of many solid–solid interfaces have been studied experimentally but the range of observed interface properties is much smaller than predicted by simple theory. Classical molecular dynamics simulations are emerging as a powerful tool for calculations of thermal conductance and phonon scattering, and may provide for a lively interplay of experiment and theory in the near term. Fundamental issues remain concerning the correct definitions of temperature in nonequilibrium nanoscale systems. Modern Si microelectronics are now firmly in the nanoscale regime—experiments have demonstrated that the close proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport, thereby aggravating problems of thermal management. Microelectronic devices are too large to yield to atomic-level simulation in the foreseeable future and, therefore, calculations of thermal transport must rely on solutions of the Boltzmann transport equation; microscopic phonon scattering rates needed for predictive models are, even for Si, poorly known. Low-dimensional nanostructures, such as carbon nanotubes, are predicted to have novel transport properties; the first quantitative experiments of the thermal conductivity of nanotubes have recently been achieved using microfabricated measurement systems. Nanoscale porosity decreases the permittivity of amorphous dielectrics but porosity also strongly decreases the thermal conductivity. The promise of improved thermoelectric materials and problems of thermal management of optoelectronic devices have stimulated extensive studies of semiconductor superlattices; agreement between experiment and theory is generally poor. Advances in measurement methods, e.g., the 3ω method, time-domain thermoreflectance, sources of coherent phonons, microfabricated test structures, and the scanning thermal microscope, are enabling new capabilities for nanoscale thermal metrology.

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Nanoscale thermal transport

We review recent advances in experimental methods for high spatial-resolution and high time-resolution thermometry, and the application of these and related methods for measurements of thermal transport in low-dimensional structures. Scanning thermal microscopy (SThM) achieves lateral resolutions of 50 nm and a measurement bandwidth of 100 kHz: SThM has been used to characterize differences in energy dissipation in single-wall and multi-wall carbon nanotubes. Picosecond thermoreflectance enables ultrahigh time-resolution in thermal diffusion experiments and characterization of heat flow across interfaces between materials; the thermal conductance G of interfaces between dissimilar materials spans a relatively small range, 20<G<200 MW m -2 K -1 near room temperature. Scanning thermoreflectance microscopy provides nanosecond time resolution and submicron lateral resolution needed for studies of heat transfer in microelectronic, optoelectronic and micromechanical systems. A fully-micromachined solid immersion lens has been demonstrated and achieves thermal-radiation imaging with lateral resolution at far below the diffraction limit, <2 μm. Microfabricated metal bridges using electrical resistance thermometry and joule heating give precise data for thermal conductivity of single crystal films, multilayer thin films, epitaxial superlattices, polycrystalline films, and interlayer dielectrics. The room temperature thermal conductivity of single crystal films of Si is strongly reduced for layer thickness below 100 nm. The through-thickness thermal conductivity of Si-Ge and GaAs-AlAs superlattices has recently been shown to be smaller than the conductivity of the corresponding alloy. The 3ω method has been recently extended to measurements of anisotropic conduction in polyimide and superlattices. Data for carbon nanotubes measured using micromachined and suspended heaters and thermometers indicate a conductivity near room temperature greater than diamond.

https://nanoheat.stanford.edu/sites/default/files/publications/A57.pdf

Thermometry and Thermal Transport in Micro/Nanoscale Solid-State Devices and Structures

Development of electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBC) aims at low conductivity, increased temperature capability, and longer life. Considerable progress has been achieved by comprehensive understanding of the evolvement of the porous microstructure in columnar ceramic topcoats and its application to tailoring optimized micro-structures. New ceramic compositions such as alternative stabilizers in zirconia, hafnia modified coatings, and pyrochlores are addressed. They have demonstrated their potential for future TBC applications. New results of both microstructure and chemistry are presented together with a summary of recent research results.

Review on Advanced EB-PVD Ceramic Topcoats for TBC Applications

Processing and design methodologies for advanced and novel thermal barrier coatings for engineering applications

This paper discusses microstructural changes produced by two novel approaches using electron beam-physical vapor deposition (EB-PVD) in which periodic strain fields/microporosity was incorporated within the large columnar grains of ZrO 2 –8 wt.% Y 2 O 3 (8YSZ). The traditional columnar microstructure of partially stabilized zirconia has been slightly modified to produce a lower thermal conductive thermal barrier coating (TBC) by periodically interrupting the condensing vapor resulting in microstructural modifications with diffuse or sharp interfaces and morphological changes on the submicron scale without changing the composition of the TBC. These microstructural modifications resulted in a 20–30% reduction in the thermal conductivity, 28–56% increase in hemispherical reflectance, improved oxidation cyclic life (over 100%), and better strain tolerance as compared to standard ZrO 2 –8 wt.% Y 2 O 3 deposited on platinum–nickel–aluminide and CoNiCrAlY bond-coated MAR-M-247 test samples. The TBC with tailored microstructures were examined by various techniques including scanning electron microscopy (SEM), X-ray diffraction (XRD), steady-state laser heat flux technique, hemispherical reflectance and thermal cyclic oxidation tests.

Tailored microstructure of EB-PVD 8YSZ thermal barrier coatings with low thermal conductivity and high thermal reflectivity for turbine applications

Thermal conductivity is one of the most important properties in the design of thermal barrier coatings (TBCs) for high-temperature gas-turbine blades. In the present study, the thermal conductivities of several TBCs were studied. Single-layer and multilayer coatings consisting of Al2O3 and ZrO2 partially stabilized with 8 wt% Y2O3 (8YSZ) were deposited on nickel-base superalloy substrates by electron-beam physical vapor deposition (EB-PVD). A columnar microstructure with (200) texture and [200] direction oriented nearly perpendicularly to the plane of the coating was observed in the single-layer 8YSZ coating. In multilayer coatings with alternating layers of Al2O3 and 8YSZ, the tendency to develop columnar structure, as well as the intensity of (200) texture, decreased as the number of layers increased. In both the single-layer and multilayer coatings, the 8YSZ layer consisted almost entirely of tetragonal (t) phase with a negligible amount of monoclinic (m) phase. This was in contrast to the microstructure of bulk 8YSZ (source material used for deposition), which contained a significant amount of m phase, in addition to t phase. Whereas bulk Al2O3 consisted of α-phase, only γ-phase was found in the Al2O3 layers of coatings. The thermal conductivities of these coatings were measured by the laser flash method. The thermal conductivity data of single-layer 8YSZ coating compared well with the data on bulk ZrO2 stabilized by 5.3 wt% Y2O3 reported in the literature. This was due to the similarity of microstructures between the two. The thermal conductivity data of multilayer coatings were well described by a series heat-transfer model calculation within the limits of experimental errors involved in the measurements.

Microstructure, texture, and thermal conductivity of single-layer and multilayer thermal barrier coatings of Y2O3-Stabilized ZrO2 and Al2O3 made by physical vapor deposition

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Microstructure, texture, and thermal conductivity of single-layer and multilayer thermal barrier coatings of Y2O3-Stabilized ZrO2 and Al2O3 made by physical vapor deposition