Silicon-on-insulator (SOI) technology has sparked advances in semiconductor and MEMs manufacturing and revolutionized our ability to study phonon transport phenomena by providing single-crystal silicon layers with thickness down to a few tens of nanometers. These nearly perfect crystalline silicon layers are an ideal platform for studying ballistic phonon transport and the coupling of boundary scattering with other mechanisms, including impurities and periodic pores. Early studies showed clear evidence of the size effect on thermal conduction due to phonon boundary scattering in films down to 20 nm thick and provided the first compelling room temperature evidence for the Casimir limit at room temperature. More recent studies on ultrathin films and periodically porous thin films are exploring the possibility of phonon dispersion modifications in confined geometries and porous films.

In this paper, we study the steady state heat transfer process within a spatial domain of the transporting medium whose length is of the same order as the distance traveled by thermal waves. In this study, the thermal conductivity is defined as a function of a spatial variable. This is achieved by analyzing an effective thermal diffusivity that is used to match the transient temperature behavior in the case of heat wave propagation by the result obtained from the Fourier theory. Then, combining the defined size-dependent thermal conductivity with Fourier’s law allows us to study the behavior of the heat flux at nanoscale and predict that a decrease of the size of the transporting medium leads to an increase of the heat transfer coefficient which reaches its finite maximal value, contrary to the infinite value predicted by the classical theory. The upper limit value of the heat transfer coefficient is proportional to the ratio of the bulk value of the thermal conductivity to the characteristic length of thermal waves in the transporting medium.

There have been growing interests in selective control of thermal emission by using micro/nanostructures. The present study describes direct measurements of infrared thermal emission at elevated temperatures of an asymmetric Fabry–Perot resonator at variable angles for each polarization. The multilayered structure mainly contains a SiO 2 optical cavity sandwiched between a thick (opaque) Au film and a thin Au film. Metallic adhesive and diffusion-barrier layers were deposited on a Si substrate before depositing the thick Au film. A dielectric protection layer was deposited atop the thin Au film to prevent oxidation at high temperatures. A SiC wafer was used as the reference to test the emittance measurement facility, which includes a heated sample holder, a blackbody source, mirror assembly, a polarizer, and a Fourier-transform infrared spectrometer with different detectors. The measured emittance spectra of the Fabry–Perot structure exhibit peak broadening and shifting as temperature increases; the mechanisms are elucidated by comparison with theoretical modeling.

Due to the high intrinsic thermal conductivity of carbon allotropes, there have been many attempts to incorporate such structures into existing thermal abatement technologies. In particular, carbon nanotubes (CNTs) and graphitic materials (i.e., graphite and graphene flakes or stacks) have garnered much interest due to the combination of both their thermal and mechanical properties. However, the introduction of these carbon-based nanostructures into thermal abatement technologies greatly increases the number of interfaces per unit length within the resulting composite systems. Consequently, thermal transport in these systems is governed as much by the interfaces between the constituent materials as it is by the materials themselves. This paper reports the behavior of phononic thermal transport across interfaces between isotropic thin films and graphite substrates. Elastic and inelastic diffusive transport models are formulated to aid in the prediction of conductance at a metal-graphite interface. The temperature dependence of the thermal conductance at Au-graphite interfaces is measured via transient thermoreflectance from 78 to 400 K. It is found that different substrate surface preparations prior to thin film deposition have a significant effect on the conductance of the interface between film and substrate.

Film cooling techniques have been successfully applied to gas turbine blades to protect them from the hot flue gas. However, a continuous demand of increasing the turbine inlet temperature to raise the efficiency of the turbine requires continuous improvement in film cooling effectiveness. The concept of injecting mist (tiny water droplets) into the cooling fluid has been proven under laboratory conditions to significantly augment adiabatic cooling effectiveness by up to 50%–800% in convective heat transfer and impingement cooling. The similar concept of injecting mist into air film cooling has not been proven in the laboratory, but computational simulations have been performed on stationary turbine blades. As a continuation of previous research, this paper extends the mist film cooling scheme to the rotating turbine blade. For the convenience of understanding the effect of rotation, the simulation is first conducted with a single pair of cooling holes located near the leading edge at either side of the blade. Then, a row of multiple-hole film cooling jets is put in place under both stationary and rotating conditions. Both the laboratory (baseline) and elevated gas turbine conditions are simulated and compared. Elevated conditions refer to a high temperature and pressure closer to actual gas turbine working conditions. The effects of various parameters including mist concentration, water droplet diameter, droplet wall boundary condition, blowing ratio, and rotational speed are investigated. The results showed that the effect of rotation on droplets under laboratory conditions is minimal. The computational fluid dynamics (CFD) model employed is the discrete phase model (DPM) including both wall film and droplet reflect conditions. The results showed that the droplet-wall interaction is stronger on the pressure side than on the suction side, resulting in a higher mist cooling enhancement on the pressure side. The average rates of mist cooling enhancement of about 15% and 35% were achieved under laboratory and elevated conditions, respectively. This translates to a significant blade surface temperature reduction of 100–125 K with 10% mist injection at elevated conditions.

Thermal rectification is a phenomenon in which transport is preferred in one direction over the opposite. Although observations of thermal rectification have been elusive, it could be useful in many applications such as thermal management of electronics and improvement of thermoelectric devices. The current work explores the possibility of thermally rectifying devices with the use of nanostructured interfaces. Interfaces can theoretically result in thermally rectifying behavior because of the difference in phonon frequency content between two dissimilar materials. The current work shows an effective rectification of greater than 25% in a device composed of two different materials divided equally by a single planar interface.

Nanocomposites offer unique capabilities of controlling thermal transport through the manipulation of various structural aspects of the material. However, measurements of the thermal properties of these composites are often difficult, especially porous nanomaterials. Optical measurements of these properties, although ideal due to the noncontact nature, are challenging due to the large surface variability of nanoporous structures. In this work, we use a vector-based thermal algorithm to solve for the temperature change and heat transfer in which a thin film subjected to a modulated heat source is sandwiched between two thermally conductive pathways. We validate our solution with time domain thermoreflectance measurements on glass slides and extend the thermal conductivity measurements to SiO 2 -based nanostructured films.

This paper reviews the microscale transport processes that arise in the fabrication of advanced materials. In many cases, the dimensions of the device being fabricated are in the micrometer length scale and, in others, underlying transformations that determine product quality and characteristics are at micro- or nanoscale levels. The basic considerations in these transport phenomena are outlined. A few important materials processing circumstances are considered in detail. These include the fabrication of multilayer and hollow optical fibers, as well as those where micro- and nanoscale dopants are added to achieve desired optical characteristics, thin film fabrication by chemical vapor deposition, and microscale coating of fibers and devices. It is shown that major challenges are posed by the simulation and experimentation, as compared with those for engineering or macroscale dimensions. These include accurate simulation to capture large gradients and variations over relatively small dimensions, simulating high pressures and viscous dissipation effects in microchannels, modeling effects such as surface tension that become dominant at microscale dimensions, and coupling micro- and nanoscale mechanisms with boundary conditions imposed at the macroscale. Similarly, measurements over microscale dimensions are much more involved than those over macro- or industrial scales because of difficult access to the regions of interest, relatively small effects such as tension, buoyancy effects, viscous rupture, bubble entrapment, and other mechanisms that are difficult to measure and that can make the process infeasible. It thus becomes difficult to achieve desired accuracy for validating the mathematical and numerical models. This paper reviews some of the approaches that have been adopted to overcome these difficulties. Comparisons between experimental and numerical results are included to show fairly good agreement, indicating the validity of the modeling of transport.

A semi-empirical model describing the heat transfer characteristics of the pseudo-dropwise condensation of binary vapor on a cooled vertical tube has been formulated. By ignoring the thin film always present on the condensation surface and the intensification of mass transfer caused by the Marangoni effect, the heat transfer characteristics of pseudo-dropwise condensation are tentatively formulated. The model involved an analysis of the diffusion process in the vapor boundary layer along with the heat transfer process through the condensate drops. This model was applied to the condensation of the saturated binary vapor of ethanol and water, and was examined using experimental data at vapor pressure values of 101.33 kPa (provided by Utaka and Wang, 2004, “Characteristic Curves and the Promotion Effect of Ethanol Addition on Steam Condensation Heat Transfer,” Int. J. Heat Mass Transfer, 47, pp. 4507–4516 ), 84.52 kPa and 47.36 kPa. Calculations using the model show a similar trend to the experimental measurements. With the change of the vapor-to-surface temperature difference, the heat transfer coefficients revealed nonlinear characteristics, with the peak values under all ethanol mass fractions of binary vapor. The heat transfer coefficients increased with decreasing ethanol mass fraction.

We have developed a nanofabricated resistance thermometer device to measure the thermal conductivity of graphene monolayers exfoliated onto silicon dioxide. The measurement results show that the thermal conductivity of the supported graphene is approximately 600 W / m K at room temperature. While this value is lower than the reported basal plane values for graphite and suspended graphene because of phonon leakage across the graphene-support interface, it is still considerably higher than the values for common thin film electronic materials. Here, we present a detailed discussion of the design and fabrication of the measurement device. Analytical and numerical heat transfer solutions are developed to evaluate the accuracy and uncertainty of this method for thermal conductivity measurement of high-thermal conductivity ultrathin films.

In this paper, we optimized the topology of a thin-film resistive heater as well as the electrical potential of the electrodes on the boundaries. The objective was to minimize the difference between the actual and prescribed temperature profiles. The thin-film thickness was represented by 100 design variables, and the electrical potential at each electrode were also design variables. The topology optimization problem (inverse problem) has been solved with two methods, i.e., with a genetic algorithm (GA) and with a conjugate gradient method using adjoint and sensitivity problems (CGA). The genetic algorithm used here was modified in order to prevent nonconvergence due to the nonuniqueness of topology representation. The conjugate gradient method used in inverse conduction was extended to cope with our electrothermal problem. The GA and CGA methods started with random topologies and random electrical potential values at electrodes. Both the CGA and GA succeeded in finding optimal thin-film thickness distributions and electrode potential values, even with 100 topology design variables. For most cases, the maximum discrepancy between the optimized and prescribed temperature profiles was under 0.5 ° C , relative to temperature profiles of the order of 70 ° C . The CGA method was faster to converge, but was more complex to implement and sometimes led to local minima. The GA was easier to implement and was more unlikely to lead to a local minimum, but was much slower to converge.

This work considers heat transfer in fin-thin film systems. Two types of these systems are analyzed: (A) a thin film sandwiched between two identical fin halves, and (B) a fin sandwiched between two identical thin films. The corresponding coupled energy equations are solved numerically by an implicit, iterative, finite-difference scheme. Comparisons with derived approximate closed-form solutions are performed and good agreement is obtained. A parametric study of all involved parameters is conducted and presented graphically. Useful correlations containing the various physical parameters for both types are reported. It is found that thermal efficiencies of fins can be increased significantly by introducing an internal flow inside the fin material. Moreover, factors producing more internal convections are found to increase the fin-thin film thermal efficiency. In addition, thermal efficiencies of type A systems are found to be higher than those of type B systems. Moreover, the resulting system thermal efficiency is found to have at most one local maximum and one local minimum over the whole relative heights ratio spectrum. Finally, this work paves a way for an effective combined passive and active method for enhancing heat transfer.

In this work, the analytical solution of a fundamental problem of heat conduction in anisotropic medium is derived. The steady-state temperature field in an anisotropic trimaterial subject to an arbitrary heat source is analyzed. “Trimaterial” denotes an infinite body composed of three dissimilar materials bonded along two parallel interfaces. The method of analytical continuation is applied across the two parallel interfaces in order to derive the trimaterial solution in a series form from the corresponding homogeneous solution. A variety of problems, e.g., bimaterial, a finite thin film on a semi-infinite substrate, and a finite strip, can be analyzed as special cases of the present study. The numerical results of the temperature distributions for some practical examples are provided in graphic form and discussed in details.

Pump-probe transient thermoreflectance (TTR) techniques are powerful tools for measuring the thermophysical properties of thin films, such as thermal conductivity, Λ , or thermal boundary conductance, G . This paper examines the assumption of one-dimensional heating on, Λ and G , determination in nanostructures using a pump-probe transient thermoreflectance technique. The traditionally used one-dimensional and axially symmetric cylindrical conduction models for thermal transport are reviewed. To test the assumptions of the thermal models, experimental data from Al films on bulk substrates (Si and glass) are taken with the TTR technique. This analysis is extended to thin film multilayer structures. The results show that at 11 MHz modulation frequency, thermal transport is indeed one dimensional. Error among the various models arises due to pulse accumulation and not accounting for residual heating.

This paper describes an experimental investigation on the infrared radiative properties of heavily doped Si at room temperature. Lightly doped Si wafers were ion-implanted with either boron or phosphorus atoms, with dosages corresponding to as-implanted peak doping concentrations of 10 20 and 10 21 cm − 3 ; the peak doping concentrations after annealing are 3.1 × 10 19 and 2.8 × 10 20 cm − 3 , respectively. Rapid thermal annealing was performed to activate the implanted dopants. A Fourier-transform infrared spectrometer was employed to measure the transmittance and reflectance of the samples in the wavelength range from 2 μ m to 20 μ m . Accurate carrier mobility and ionization models were identified after carefully reviewing the available literature, and then incorporated into the Drude model to predict the dielectric function of doped Si. The radiative properties of doped Si samples were calculated by treating the doped region as multilayer thin films of different doping concentrations on a thick lightly doped Si substrate. The measured spectral transmittance and reflectance agree well with the model predictions. The knowledge gained from this study will aid future design and fabrication of doped Si microstructures as wavelength selective emitters and absorbers in the midinfrared region.

The topology and geometry of microstructures play a crucial role in determining their heat transfer performance in passive cooling devices such as heat pipes. It is therefore important to characterize microstructures based on their wicking performance, the thermal conduction resistance of the liquid filling the microstructure, and the thin-film characteristics of the liquid meniscus. In the present study, the free-surface shapes of the static liquid meniscus in common microstructures are modeled using SURFACE EVOLVER for zero Bond number. Four well-defined topologies, viz., surfaces with parallel rectangular ribs, horizontal parallel cylinders, vertically aligned cylinders, and spheres (the latter two in both square and hexagonal packing arrangements), are considered. Nondimensional capillary pressure, average distance of the liquid free-surface from solid walls (a measure of the conduction resistance of the liquid), total exposed area, and thin-film area are computed. These performance parameters are presented as functions of the nondimensional geometrical parameters characterizing the microstructures, the volume of the liquid filling the structure, and the contact angle between the liquid and solid. Based on these performance parameters, hexagonally-packed spheres on a surface are identified to be the most efficient microstructure geometry for wicking and thin-film evaporation. The solid-liquid contact angle and the nondimensional liquid volume that yield the best performance are also identified. The optimum liquid level in the wick pore that yields the highest capillary pressure and heat transfer is obtained by analyzing the variation in capillary pressure and heat transfer with liquid level and using an effective thermal resistance model for the wick.

With the ever decreasing characteristic lengths of nanomaterials, nonequilibrium electron-phonon scattering can be affected by additional scattering processes at the interface of two materials. Electron-interface scattering would lead to another path of energy flow for the high-energy electrons other than electron-phonon coupling in a single material. Traditionally, electron-phonon coupling in transport is analyzed with a diffusion (Fourier) based model, such as the two temperature model (TTM). However, in thin films with thicknesses less than the electron mean free path, ballistic electron transport could lead to electron-interface scattering, which is not taken into account in the TTM. The ballistic component of electron transport, leading to electron-interface scattering during ultrashort pulsed laser heating, is studied here by a ballistic-diffusive approximation of the Boltzmann transport equation. The results for electron-phonon equilibration times are compared with calculations with TTM based approximations and experimental data on Au thin films.

Microscale transport mechanisms play a critical role in the thermal processing of materials because changes in the structure and characteristics of the material largely occur at these or smaller length scales. The heat transfer and fluid flow considerations determine the properties of the final product, such as in a crystal drawn from silicon melt or a gel from the chemical conversion of a biopolymer. Also, a wide variety of material fabrication processes, such as the manufacture of optical glass fiber for telecommunications, fabrication of thin films by chemical vapor deposition, and surface coating, involve microscale length scales due to the requirements on the devices and applications for which they are intended. For example, hollow fibers, which are used for sensors and power delivery, typically need fairly precise microscale wall thicknesses and hole diameters for satisfactory operation. The basic transport mechanisms underlying these processes are discussed in this review paper. The importance of material characterization in accurate modeling and experimentation is brought out, along with the coupling between the process and the resulting properties such as uniformity, concentricity, and diameter. Of particular interest are thermally induced defects and other imperfections that may arise due to the transport phenomena involved at these microscale levels. Additional aspects such as surface tension, stability, and free surface characteristics that affect the material processing at microscale dimensions are also discussed. Some of the important methods to treat these problems and challenges are presented. Characteristic numerical and experimental results are discussed for a few important areas. The implications of such results in improving practical systems and processes, including enhanced process feasibility and product quality, are also discussed.