Thermal characterization becomes challenging as the material size is reduced to micro/nanoscales. Based on scanning probe microscopy (SPM), scanning thermal microscopy (SThM) is able to collect thermophysical characteristics of the microscopic domain with high spatial resolution. Starting from its development history, this review introduces the operation mechanism of the instrument in detail, including working principles, thermal probes, quantitative study, and applications. As the core principle of SThM, the heat transfer mechanism section is discussed emphatically. Additionally, the emerging technologies based on the SThM platform are clearly reviewed and corresponding examples are presented in detail. Finally, the current challenges and future opportunities of SThM are discussed.

Accurately and directly characterizing the thermal properties of graphene and thin-graphite films (GFs) is of fundamental importance for understanding the heat transport mechanism and of practical interest in possible applications of thermal management. However, due to the lack of experiment data, the mechanism of the thickness dependence of GFs thermal properties has not been fully understood yet. In this study, a 90-nm-thick GF is characterized by the time-domain thermoreflectance method, and the obtained GFs in-plane thermal conductivity and interfacial thermal conductance between GFs and gold are (1354±297) W/(m·K) and (38±6) MW/(m2·K), respectively. Two theoretical models are also applied for comparison and discussion, and we conclude that the influence from the surface perturbation by supporting materials on the phonon transport of graphite nano-films will beyond the near surface layers to the more inner ones. This work not only provides a better understanding of the fundamental mechanisms of the thermal transport size effect in GFs, but also facilitates the possible applications of GFs as heat spreaders in the future.

In this paper, a non-contact method based on Raman spectroscopy was used to measure the thermal conductivity of an individual single-walled carbon nanotube (SWCNT) and a multi-walled carbon nanotube (MWCNT). The effect of laser-induced heating on carbon nanotubes (CNTs) was considered. The local temperatures along the longitudinal direction of carbon nanotube were determined by Raman shift, combined with one-dimensional heat conduction model, and the thermal conductivity was finally obtained. The thermal conductivity of the SWCNT with a length of 25 μm and a diameter of 1.34 nm decreases as the temperature increases in the measuring temperature range (316 K–378 K). The corresponding thermal conductivities change from 1651 W/(m·K) to 2423 W/(m·K); the thermal conductivities of the MWCNT with 40 μm length and 9.2 nm diameter are within 1109–1700 W/(m·K) at 316 K–445 K. To further analyze the size effect on the thermal conductivity, molecular dynamics simulation has been carried out. The result shows that the thermal conductivity of an individual carbon nanotube increases with increasing nanotube length and eventually converges. This work is expected to provide some reference data for the studies of thermal properties of individual CNTs.

The hot strip method, as one typical transient method, is widely used to measure the effective thermal conductivity of thermal insulation materials at various temperatures. Since the test theory is based on solving the energy equation via heat conduction, the test result will be questionable when measuring thermal insulation materials, such as silica aerogel and photovoltaic glazing, in which the participating thermal radiation is a dominant heat transfer mode at high temperature. In this study, numerical investigation is employed to reveal the measurement reliability of hot strip method when applied to translucent thermal insulation materials. By reproducing the dynamic conduction-radiation coupled heat transfer process within the translucent materials numerically, the effective thermal conductivity of translucent materials with varying extinction coefficients are obtained at various temperatures. Comparisons are made for the effective thermal conductivity of translucent materials determined by the hot strip method, one-dimensional steady state method, transient plane source method and Rosseland model. Large discrepancies are found among the effective thermal conductivity determined by different methods for translucent materials with low extinction coefficient. The thermal conductivity obtained from the hot strip method is overestimated at elevated temperature when compared with that from one-dimensional steady state method. In order to measure the effective thermal conductivity of translucent materials accurately, the effect of thermal radiation should be considered for different transient methods.

Since first establishing thermal measurement techniques for micrometer-scale wires, various methods have been devised and improved upon. However, the uncertainty of different measurements on the same sample has not yet been discussed. In this work, a round robin test was performed to compare the thermal conductivity and thermal diffusivity measurement methods for a fine metal wire. The tested material was a pure gold wire, with a diameter of 30 µm. The wire was cut into certain lengths and distributed to four institutions using five different measurement methods: the direct current (DC) self-heating method, the DC heating T-type method, the 3 ω method for thermal conductivity, the scanning laser heating alternating current (AC) method, and the spot periodic heating radiation thermometry method for thermal diffusivity. After completing the measurements, the reported thermal conductivity and thermal diffusivity at room temperature, i.e., 317 W∙m⁻¹∙K⁻¹ and 128×10⁻⁶ m²∙s⁻¹, respectively, were adopted as references for comparison with the measurement results. The advantages and disadvantages of each method are described in terms of the effect of electrical and thermal junctions fabricated on a wire, such as an electrode, a thermocouple, and a heat bath. The knowledge obtained from the tested methods will be useful for selecting and designing a measurement technique for various wire-like materials.

Amorphous hafnium dioxide (a-HfO₂) has attracted increasing interest in the application of semiconductor devices due to its high dielectric constant. However, the thermal transport properties of a-HfO₂ are not well understood, which hinders its potential application in electronics. In this work, we systematically investigate the thermal transport property of a-HfO₂ using the molecular dynamics method. The non-equilibrium molecular dynamics simulations reveal that the thermal conductivity of a-HfO₂ is length-dependent below 100 nm. Spectrally decomposed heat current further proves that the thermal transport of propagons and diffusons is sensitive to the system length. The thermal conductivity is found to increase with temperature using Green-Kubo mode analysis. We also quantify the contribution of each carrier to the thermal conductivity at different temperatures. We find that propagons are more important than diffusons in thermal transport at low temperatures (<100 K). In comparison, diffusons dominate heat transport at high temperatures. Locons have negligible contribution to the total thermal conductivity.

In polymers, heat could transfer efficiently along the long polymer chains; however due to the finite length of polymer chains, such heat eventually has to pass across the chain-chain boundary which is less effective in heat transfer. This paper investigated the thermal transport across polyethylene chains with molecular dynamics (MD) simulations. Thermal transport across two polymer chains overlapping with each other is studied with different chain length (75 nm, 150 nm and 251 nm) and chain-chain overlapping length. The results show that with increasing overlapping length, the total thermal conductance across the two chains exhibits maximum value, which is due to the increasing thermal resistance along the chains and the decreasing inter-chain thermal boundary resistance. Mathematically, we show that the total thermal resistance can be decomposed into two terms. The coupling term related to the inter-chain thermal resistance tends to saturate even with long overlapping length.

Engineering nanostructured superlattices provides an effective solution toward the realization of high-performance thermoelectric device and thermal management materials, where the anisotropic thermal conductivity is critical for designing orientation-dependent thermal devices. Herein, the lattice thermal conductivity anisotropy of Al/Ag superlattices as one typical example of superlattice materials is investigated utilizing non-equilibrium molecular dynamics simulations. The cross-plane and in-plane lattice thermal conductivities of one-dimensional superlattices are in the ranges of 0.5–3.2 W/(m·K) and 1.8–5.1 W/(m·K) at different period lengths, respectively, both of which are smaller than those of bulk materials. More specifically, the cross-plane lattice thermal conductivity of superlattices increases with the period length, while the in-plane phonon thermal conductivity first increases and then trends to convergence, resulting in the non-monotonic thermal anisotropy value. To further reveal the microscopic phonon transport mechanism, the interfacial phonon thermal resistance, density of states and spectral phonon transmission coefficient including anharmonic phonon properties under different period lengths are calculated. Our results can be helpful for understanding phonon transport in low-dimensional materials and provide guidance for optimizing the thermal conductivity anisotropy of superlattice materials in the application ranging from thermoelectric devices to thermal management in micro/nano systems.

In this work, the effects of electric field on the microstructure and transport property of [bmim][Tf₂N] were simulated by molecular dynamics method to provide regulating strategy of required ionic liquid. The simulation results showed that [bmim]+ and [Tf₂N]– move slowly along the positive and negative direction of the electric field, respectively, and anions and cations are still arranging alternatively under weak electric field which has slight influence on the electrostatic force in [bmim][Tf₂N]. When the electric field is strong, it has significant influence on the electrostatic force of [bmim][Tf₂N], which results the aggregation of [bmim]+ and [Tf₂N]– and the appearance of large hole inside [bmim][Tf²N]. In addition, with the increase of electric field intensity, the density of [bmim][Tf₂N] increases, which means the free volume inside [bmim][Tf₂N] become smaller. Meanwhile, the thermal conductivity and viscosity exhibit anisotropy.

In this study, by using the nonequilibrium molecular dynamics and the kinetic theory, we examine the tailored nanoscale thermal transport via a gas-filled nanogap structure with mechanically-controllable nanopillars in one surface only, i.e., changing nanopillar height. It is found that both the thermal rectification and negative differential thermal resistance (NDTR) effects can be substantially enhanced by controlling the nanopillar height. The maximum thermal rectification ratio can reach 340% and the T range with NDTR can be significantly enlarged, which can be attributed to the tailored asymmetric thermal resistance via controlled adsorption in height-changing nanopillars, especially at a large temperature difference. These tunable thermal rectification and NDTR mechanisms provide insights for the design of thermal management systems.

The thin-film thermoelectric cooler (TEC) is a promising solid-state heat pump that can remove the high local heat flux of chips utilizing the Peltier effect. When an electric current pulse is applied to the thin-film TEC, the TEC can achieve an instantaneous lower temperature compared to that created by a steady current. In this paper, we developed a novel strategy to reduce the peak temperature of the chip working under dynamic power, thus making the semiconductor chip operate reliably and efficiently. A three-dimensional numerical model was built to study the transient cooling performance of the thin-film TEC on chips. The effects of parameters, such as the current pulse, the heat flux, the thermoelement length, the number of thermoelements, and the contact resistance on the performance of the thin-film TEC, were investigated. The results showed that when a current pulse of 0.6 A was applied to the thin-film TEC before the peak power of the chip, the peak temperature of the chip was reduced by more than 10°C, making the thin-film thermoelectric cooler a promising technology for the temperature control of modern chips with high peak powers.

Nanowires exhibit excellent thermoelectric performance, due to the stronger quantum confinement and phonon scattering effect compared to bulk materials. However, it is a challenge to accurately evaluate the thermoelectric performance of nanowires. In this paper, the thermoelectric properties of an individual suspended Sb₂Se₃ nanowire have been characterized by comprehensive T-type method, including thermal conductivity, electrical conductivity, Seebeck coefficient and figure of merit. The thermal conductivity increases from 0.57 W/(m∙K) to 3.69 W/(m∙K) with temperature increasing from 80 K to 320 K. The lattice vibration dominates the heat conduction process, and due to its flawless crystal structure, the thermal conductivity is not lower than the reported values of bulk Sb₂Se₃. The electrical conductivity increases from 7.83 S/m to 688 S/m in the temperature range of 50 K–320 K, which is a great improvement compared with the corresponding bulk value. At 294 K, the Seebeck coefficient of the Sb2Se3 nanowire is –1120 μV/K and the corresponding figure of merit is 0.064.

Fast fluid transport on graphene has attracted a growing body of research due to a wide range of potential applications including thermal management, water desalination, energy harvesting, and lab-on-a-chip. Here, we critically review the theoretical, simulational, and experimental progress regarding the fluid slippage on graphene. Based on the summary of the past studies, we give perspectives on future research directions towards complete understanding and practical applications of slip flow on graphene.

The kinetics of Al-Ni and Cu-Ni nanodroplets spreading over a Cu substrate in the presence of a temperature difference between them is studied via molecular dynamics simulations. The simulations show that significant dissolution reactions occur for the two systems and there is no precursor film generated during spreading. The spreading rate significantly increases when nanodroplets contain less Ni atoms in the Al-Ni/Cu wetting systems. However, a different trend is observed in the Cu-Ni/Cu wetting systems. The spreading rate remains unchanged regardless of the ratio of Cu to Ni atoms owing to the fact that Cu and Ni have almost the same lattice constants. The simulations also demonstrate that, because of the temperature gradient between the nanodroplet and substrate, local solidification takes place in the later spreading stage, which significantly hinders spreading. Due to the mismatch of lattice constants between Al and the Cu atoms in the Al-Ni/Cu wetting systems, hexagonal closest packed (hcp), body centered cubic (bcc), and face centered cubic (fcc) arrangements of atoms are observed when the Al-Ni nanodroplets solidify completely, whereas there is only a fcc arrangement in the Cu-Ni/Cu wetting systems.

The collision between the nanoparticle and wall surface is supposed to cause the escape of nanoparticle molecules which indicates the potential phase change of the nanoparticle. It is significant to understand the mechanism of the collision process involved with phase change for applications of nanoparticles in energy and mass transfer. In this study, the collision process between nanoparticle made of monatomic argon molecule and wall surface made of nickel metal crystal is simulated by molecular dynamics method. The travelling behavior and energy transformation of escaped molecules are respectively analyzed. The effects of the intermolecular force and initial temperature on the collision process are further discussed. The results show that the nanoparticle can be accelerated by the wall surface with the intermolecular force and finally collide with it. The molecules escape from the nanoparticle either by bouncing off the wall surface or the intermolecular energy exchange with the energy transformation between the potential energy and kinetic energy. The molecules far from the nanoparticle center are more likely to escape, while the velocity distributions of the escaped molecules follow the Maxwell distribution. More escaped molecules, namely higher phase change potential, are observed with lower intermolecular force and higher initial temperature. As a fundamental study on nanoparticle phase change in the vicinity of wall surface, the present investigation will be helpful for further study on the heat transfer characteristics and phase change mechanisms of nanoparticles.

Heat conduction of nanoconfined liquid may differ from its bulk because of the effects of size, geometry, interface, temperature, etc. In this study, the roles of some critical factors for the heat conduction of nanoconfined water film are systematically analyzed by using the molecular dynamics method. With decreasing thickness, the normal thermal conductivity of nanoconfined water film between two copper plates decreases exponentially, while the thermal resistance, peak of the radial distribution function, and atomistic heat path increase exponentially. The average bond order, radial distribution function, mean squared displacement, and vibrational density of states are calculated to analyze the effects of structure, distribution, molecular diffusion, and vibration of water molecules on heat conduction especially in a region having no oxygen atoms (which is observed by the near-wall density profile). The results show that phonon scattering is dominant for determining the reduced thermal conductivity in this near-wall region. The thermal conductivity ratio of confined water film to bulk water has a roughly linear relationship with the logarithm of the proportion of the near-wall region. Moreover, the high interfacial thermal resistance is positively correlated to the film thickness, but it has a negligible impact on heat conduction. This work provides insights into the contribution of water molecules near the solid/liquid interface to the heat conduction of nanoconfined liquid for process intensification.

The solid-liquid interfacial thermal transport depends on the physical properties of the interfaces, which have been studied extensively in open literature. However, the fundamental understanding on the mechanism of the solid-liquid interfacial thermal transport is far from clear. In the present paper, heat transfer through solid-liquid interfaces is studied based on the non-equilibrium molecular dynamics simulations. It is shown that the interfacial heat transfer can be enhanced by increasing interfacial coupling strength or introducing the nanostructured surfaces. The underlying mechanism of the interfacial thermal transport is analyzed based on the calculation results of the heat flux distribution, potential mean force, and the vibrational density of states at the interfacial region. It is found that the interfacial thermal transport is dominated by the kinetic and virial contributions in the interface region. The enhancement of the interfacial heat transfer can be attributed to the fluid adsorption on the solid surface under a strong interfacial interaction or by the nanostructured solid surfaces, which reduce the mismatch of the vibrational density of states at the solid-liquid interface region.

Addition of graphene nanoplatelet (GNP) into water is a promising method for improving cold storage system performance, and its application requires comprehensive understanding of solidification behavior of GNP-water nanofluid. In the present study, the influences of GNP mass concentration, cold storage cavity size and shape on solid-liquid interface evolution, temperature distribution, streamline profile as well as solidification rate are numerically analyzed. The enthalpy-porosity technique is adopted to track solid-liquid interface. The results show that the enhancement effect of GNPs on solidification is mainly reflected in the final stage in which heat conduction is predominant; the solidification occurs at the bottom of cavity in the early stage, and the solid-liquid interface is similar to the shape of cavity itself and then tends to be circular in the middle and final stages respectively; the reduction degree of solidification time reaches 30.1% at GNP mass concentration of 1.2 wt% under present simulation conditions; decreasing cavity size and adopting triangular cavity are beneficial for promoting the solidification, but they will suppress the enhancement effect of GNPs on solidification.

This paper seeks to decipher the exact relationship between the liquid film thickness and the hydrodynamics of gas-liquid slug flows. An instantaneous measurement system is developed by integrating the laser focus displacement meter (LFDM) and high-speed camera to characterize the temporal evolution of the liquid film and the dynamic characteristics of continuous slug flows. A glass tube with internal diameter of 0.75 mm is used and the tested ranges of superficial gas and liquid velocities are 0.01–1.2 m/s and 0.01–0.09 m/s respectively. The non-zero signals of LFDM representing the bubble slug flows changed from regular periodic intervals to chaotic fluctuations when slug-annular flow pattern appears. The dominant frequencies of the periodic intermittent slug flows increased from about 0.5–2 Hz to nearly 10–20 Hz as the superficial gas velocity rised from 0.025 to 0.78 m/s. The bubble and liquid slug lengths calculated by the time interval of liquid film thickness and bubble velocity correlated well with the empirical model. Meantime, the average value of void fraction derived from the calculation of transient liquid film thickness shows a linear growth with the gas holdup ratio.

Polyimide (PI) films are the common and important components on spacecraft for thermal control. Their parameter design accuracy has an important impact on the spacecraft thermal balance but there are few studies focusing on the effect of film surface defects. In the present study, photothermal properties of composite films made of PI films and germanium/indium tin oxide/aluminum coatings are obtained by UV-VIS-NIR spectrophotometer. The film surface morphology of composite films is obtained by the atomic force microscope. The present study creatively introduces surface roughness into modeling from the perspective of finite-difference time-domain (FDTD) model. The FDTD method considering surface roughness is used to present a comprehensive investigation on the coating effect on the photothermal properties including transmittance and reflectance of composite films. Based on the random rough surface generated by root-mean-square (RMS) roughness and correlation length, the FDTD method is applied to characterize the electromagnetic field of composite films. It has been found that the numerical accuracy is greatly improved and the numerical results agree well with the experimental data when surface roughness is considered. The results also show that coating material and thickness have remarkable impacts on photothermal properties of the composite films.

Photothermal therapy is emerging as a very promising way for minimally invasive cancer treatment. To enhance thermal energy deposition of laser in target malignant tissues, liquid metal nanoparticles (LMNPs) have been recently identified as completely unprecedented photothermal sensitizers due to their unique physicochemical properties and superior photothermal conversion rate under near-infrared (NIR) laser irradiation. However, there is currently a strong lack of understanding of the laser energy distribution and the transient temperature field within the biological tissues, which would seriously hinder the development of LMNPs assisted photothermal therapy. Therefore, this paper focused on the distinctive photothermal effect of LMNPs embedded in biological tissues under NIR laser irradiation. The mathematical model coupling the Monte-Carlo photon transport model with Penne’s bioheat transfer model has been established. Simulation studies have shown that LMNPs play an important role in enhancing the absorption of NIR laser, which contributes to local temperature rise and improves the temperature distribution. Comparing with the control case without LMNPs, the maximum temperature increases by nearly 1.0 time, the local temperature rise reaches 30°C in 1.0 second. When the diameter and concentration of LMNPs are 40 nm and 1012/mm3, the resulting temperature variation and distribution is best for the effective killing of tumors without damaging normal tissues. In addition, the simulation results are meaningful for guiding the selection of laser irradiation time in conjunction with the cooling time, ensuring the controllable accuracy of treatment. To the best of our knowledge, the present study is one of the first attempts to quantify the influence of transformable LMNPs on the temperature distributions inside the biological tissues, showing important academic significance for guiding LMNPs assisted photothermal treatment.

Considering the issues of energy saving and environment protection, the performance of refrigeration systems requires to be improved. In recent years, nano-fluids have attracted greatly attention from the researchers due to their outstanding thermal characteristics. In this work, the published investigations on the preparation and characterization of nano-fluids have been discussed at first. Furthermore, the key thermo-physical properties of nano-fluids, such as thermal conductivity, viscosity, specific heat and density have been summarized. Finally, the performance enhancements in different types of refrigeration systems by using nano-fluids have been reviewed. It is concluded that nano-fluids as refrigerant, lubricant or secondary fluid have wide potential application in refrigeration systems.