Thermal Transport Phenomena Research Articles

Scanning thermal microscopy: A review

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.

Thermal Conductivity Prediction of Thermally Conductive, Electrically Insulating Materials under van der Waals Interactions

The thermal transport phenomenon in small-scale heterogeneous composites is essentially controlled by van der Waals interactions. In this article, thermal conductivity of nanocomposites with 33 wt% crystallized silicon dioxide is four times higher than that of epoxy (EP) resin composites. Nanocomposites with 33 wt% boron carbide exhibit seven times higher thermal conductivity than pure EP. Pal and Lewis-Nielsen multiscale models were used to infer that distance-associated van der Waals interactions vary between composites with different weight fractions. Such variation consequently affects the thermal conductivity of the composites. Scanning electron microscope images of crystallized silicon dioxide/EP composites provide evidence of our reasonable and accurate inferences with regard to the thermal conduction mechanism. Experimental values confirm that the Pal model is superior to the Lewis-Nielsen model. The observed enhancement in thermal conductivity indicates important implications for the development of highly and thermally conductive electrically insulating materials. Results of this study can also be considered to improve modeling for thermal conductivity under van der Waals interactions.

11010 超音波浮遊液滴界面近傍の熱・物質輸送の評価

11010 超音波浮遊液滴界面近傍の熱・物質輸送の評価

Electromagnetohydrodynamic flow of blood and heat transfer in a capillary with thermal radiation

This paper presents a comprehensive theoretical study on heat transfer characteristics together with fully developed electromagnetohydrodynamic flow of blood through a capillary, having electrokinetic effects by considering the constant heat flux at the wall. The effect of thermal radiation and velocity slip condition have been taken into account. A rigorous mathematical model for describing Joule heating in electro-osmotic flow of blood including the Poisson–Boltzmann equation, the momentum equation and the energy equation is developed. The alterations in the thermal transport phenomenon, induced by the variation of imposed electromagnetic effects, are thoroughly explained through an elegant mathematical formalism. Results presented here pertain to the case where the height of the capillary is much greater than the thickness of electrical double layer comprising the stern and diffuse layers. The essential features of the electromagnetohydrodynamic flow of blood and associated heat transfer characteristics through capillary are clearly highlighted by the variations in the non-dimensional parameters for velocity profile, temperature profile and the Nusselt number. The study reveals that the temperature of blood can be controlled by regulating Joule heating parameter.

Investigation of Mixed Convection in a Ventilated Cavity in the Presence of a Heat Conducting Circular Cylinder

The study is aimed to investigate the mixed convective transport within a ventilated square cavity in presence of a heat conducting circular cylinder. The fluid flow is imposed through an opening at the bottom of the left cavity wall and is taken away by a similar opening at the top of the right cavity wall. The cylinder is placed at the center of the cavity. Two cases are considered depending on the thermal conditions of the cavity walls. In the first case, the left and right vertical walls are kept isothermal with different temperatures and the top and bottom horizontal walls are considered as thermally insulated. For the second case, the top and bottom walls are maintained at different constant temperatures and the left and right walls are considered adiabatic. Heat transfer due to forced flow, thermal buoyancy, and conduction within the cylinder are taken into account. Effect of the cylinder size (0.1 ≤ D ≤ 0.5) and the solid–fluid thermal conductivity ratio (0.1 ≤ K ≤ 10) are explored for various values of Richardson number (0 ≤ Ri ≤ 5) at fixed Reynolds (Re = 100) and Prandtl (Pr = 0.71) numbers. The fluid dynamic and thermal transport phenomena are depicted through streamline and isotherm plots. Additionally, the global thermal parameters such as the average Nusselt number and average fluid temperature of the cavity are presented. It is found that the aforementioned parameters have significant influences on the fluid flow and heat transfer characteristics in the cavity.

Effects of Current Collector Positions on Temperature and Reactive Current Distributions in a Micro-Tubular Solid Oxide Electrolysis Cell

A two-dimensional tubular model of a Ni+YSZ electrode supported micro-tubular solid oxide electrolysis cell (SOEC) was developed to clarify its temperature distribution on high temperature steam electrolysis. The entropy change of SOEC is endothermic and the temperature distribution is more complex than that of SOFCs. In addition, in the micro-tubular SOEC, large bias of reaction distributions can be occurred by current collecting position, and it will lead further complicated thermal transport phenomena and big thermal distributions. Therefore, in this model, heat, mass, electric charge transport and electrochemical reaction were calculated simultaneously. The overpotential and the temeprature change were validated by experimental i-V curves and temperature changes of micro-tubular SOEC. The numerical results suggest that effects of current collector position were not small and cannot be ignored in micro-tubular SOEC operation.

Thermal and Thermoelectric Transport in Semiconductor Nanostructures

During the last decade or so we have witnessed tremendous advancement in characterization and understanding of thermal transport in nanostructures, particularly in the ‘phonon boundary scattering’ regime where the characteristic size of nanostructures ranges from tens to hundreds nanometers and is comparable to phonon mean free path. This understanding is the key underlying reason for improved performance observed in nanostructured thermoelectric materials. However, it is increasingly clear that the boundary scattering mechanism may soon reach its limit and new mechanisms for improving thermoelectric performance need to be explored. One of the opportunities may arise from the study of thermal transport in the so-called ‘confinement’ regime, where the characteristic size of nanostructures is below 10-20 nanometers and is comparable to phonon wavelength. In this work, we will discuss our recent work on using rationally designed nanostructures to study nanoscale thermal and thermoelectric transport phenomena in this regime. We will describe the instrumentation we have been developing that enables the characterization of thermal transport and related properties of these nanostructures and discuss and interpret the measurement results.

Thermal transport phenomena and limitations in heterogeneous polymer composites containing carbon nanotubes and inorganic nanoparticles

Abstract An Off-Lattice Monte Carlo model was developed to investigate effective thermal conductivities ( K eff ) and thermal transport limitations of polymer composites containing carbon nanotubes (CNTs) and inorganic nanoparticles. The simulation results agree with experimental data for poly(ether ether ketone) (PEEK) with inclusions of CNTs and tungsten disulfide (WS 2 ) nanoparticles. The developed model can predict the thermal conductivities of multiphase composite systems more accurately than previous models by taking into account interfacial thermal resistance ( R bd ) between the nanofillers and the polymer matrix, and the nanofiller orientation and morphology. The effects of (i) R bd of CNT–PEEK and WS 2 –PEEK (0.0232–115.8 × 10 −8 m 2 K/W), (ii) CNT concentration (0.1–0.5 wt%), (iii) CNT morphology (aspect ratio of 50–450, and diameter of 2–8 nm), and (iv) CNT orientation (parallel, random and perpendicular to the heat flux) on K eff of a multi-phase composite are quantified. The simulation results show that K eff of multiphase composites increases when the CNT concentration increases, and when the R bd of CNT–PEEK and WS 2 –PEEK interfaces decrease. The thermal conductivity of composites with CNTs parallel to the heat flux can be enhanced ∼2.7 times relative to that of composites with randomly-dispersed CNTs. CNTs with larger aspect ratio and smaller diameter can significantly improve the thermal conductivity of a multiphase polymer composite.

Thermal characteristics of electromagnetohydrodynamic flows in narrow channels with viscous dissipation and Joule heating under constant wall heat flux

The present paper critically analyzes, for the first time, the heat transfer characteristics associated with thermally fully developed combined electromagnetohydrodynamic flows through narrow flow conduits, considering electrokinetics effects, for the constant wall heat flux condition. In order to maintain the generality of the problem under consideration, the liquid flow is actuated by a combination of imposed pressure-gradient, electrokinetic effects and additional electromagnetic interactions. The alterations in the thermal transport phenomenon, induced by the variations in the imposed electromagnetic effects are thoroughly explained, in a pin-pointed manner, through an elegant analytical formalism. The essential features of the electromagnetohydrodynamic flow, and associated heat transfer characteristics, through the narrow confinement, are clearly highlighted by the variations in the non-dimensional flow velocity, non-dimensional temperature profile and the Nusselt number. Moreover, the influence of the imposed electromagnetic effects on micro/nano-scale thermal transport characteristics is clearly differentiated from other inherent effects like Joule heating and viscous dissipation. The implications of the constant wall temperature condition are also briefly discussed. The present endeavor has far-reaching implications towards the development of state-of-the-art electro-magneto-mechanical devices, with improved efficiency and functionality, for applications in the field of micro-scale thermal management.

Thermal transport in S-shaped graphene nano-junctions

By using nonequilibrium Green's function method, we study the phonon transport properties in S-shaped graphene nano-junctions (GNJs). Interesting transmission phenomenon is found. The transmission spectrum of low frequency phonon shows forbidden frequency band, by changing the width W2 of the S-shaped GNJs. These low frequency forbidden bands are sensitive to their periodic geometric shape of GNJs. These thermal transport phenomena can be explained by analyzing the phonon transmission coefficient. This paper illustrates the thermal transport mechanisms in the different S-shaped GNJs, and the results could provide significant physical models and theoretical validity in designing the thermal devices based on the GNJs.

Brillouin light scattering spectra as local temperature sensors for thermal magnons and acoustic phonons

We demonstrate the use of the micro-Brillouin light scattering (micro-BLS) technique as a local temperature sensor for magnons in a permalloy (Py) thin film and phonons in the glass substrate. When the Py film is uniformly heated, we observe a systematic shift in the frequencies of two thermally excited perpendicular standing spin wave modes. Fitting the temperature dependent magnon spectra allows us to achieve a temperature resolution better than 2.5 K. In addition, we demonstrate that the micro-BLS spectra can be used to measure the local temperature of magnons and the relative temperature shift of phonons across a thermal gradient. Such local temperature sensors are useful for investigating spin caloritronic and thermal transport phenomena in general.

Thermal transport of graphene nanoribbons embedding linear defects

Using nonequilibrium Green's function method, the thermal transport properties of zigzag graphene nanoribbons (ZGNR) embedding a finite (semi-infinite or infinite) long linear defect are investigated in this paper. The results show that defect type and defect length have significant influence on the thermal conductance of ZGNR. When the embedded linear defects have the same lengths, thermal conductance of ZGNR embedding t5t7 defect is lower than that of ZGNR embedding Stone-Wales defect. As for the ZGNR embedding finite and the same type defects, their thermal conductance reduce with the increase of the defect length. However, as the linear defect is long enough, the thermal conductance is insensitive to the change of length. By comparing the ZGNRs embedding finite, semi-infinite and infinite long defects, we find that the thermal conductance of ZGNR embedding an infinite long defect is higher than that of ZGNR embedding a semi-infinite defect, while the thermal conductance of the latter is higher than that of ZGNR embedding a finite long defect. This is due to the fact that different structures possess different numbers of scattering interfaces in the phonon transmission direction. The more the scattering interfaces, the lower the thermal conductance is. These thermal transport phenomena are explained by analyzing transmission coefficient and local density of states. These results indicate that linear defects can tune thermal transport property of ZGNR efficiently.

Study on the Thermal Transport Phenomenon in the Transition Region of Natural Convection along a Vertical Smooth Flat Plate Uniformly Heated

遷移域は層流から乱流に移行する過程において必然的に発生する現象であるが，自然対流の遷移域における熱流動現象やそれが熱伝達におよぼす影響については必ずしも解明されていない．そこで，本研究では等熱流束で加熱された垂直な平滑平板に沿って発達する定常状態下の自然対流に焦点を当て，伝熱面の二次元温度場を赤外線サーモグラフィで観測しながら遷移域に出現する特徴的な熱輸送機構を実験的に考察した．すなわち，遷移域に出現する温度パターンや温度変動についてさまざまな画像・統計処理を加えて温度場の輸送機構を考察した．壁面温度場を赤外線サーモグラフィで可視化した結果，遷移域には特徴的な温度パターン（低温の馬蹄様形状温度パターン）が幾つか観察された．また，遷移域の温度変動には，比較的短い周期の変動成分だけではなく，比較的長い周期の変動現象が内在することも初めて明らかとなった．この周期的な変動現象は，乱流域へ移行するにつれて消滅する．なお，前者の短周期変動に基づいた遷移域は，従来から提唱されている線形安定理論による周波数領域と良好に対応した．

Nanoscale heat transfer – from computation to experiment

Heat transfer can differ distinctly at the nanoscale from that at the macroscale. Recent advancement in computational and experimental techniques has enabled a large number of interesting observations and understanding of heat transfer processes at the nanoscale. In this review, we will first discuss recent advances in computational and experimental methods used in nanoscale thermal transport studies, followed by reviews of novel thermal transport phenomena at the nanoscale observed in both computational and experimental studies, and discussion on current understanding of these novel phenomena. Our perspectives on challenges and opportunities on computational and experimental methods are also presented.

THERMAL TRANSPORT MEASUREMENT TECHNIQUES FOR NANOWIRES AND NANOTUBES

Recent advances in the synthesis of inorganic and organic nanowires and nanotubes have provided both components for various functional devices and platforms for the study of low- dimensional transport phenomena. However, tremendous challenges have remained not only for the integration of these building blocks into functional devices, but also in the characterization of the fundamental transport properties in these nanoscale model systems. In particular, thermal and thermoelectric transport measurements can be considerably more complicated than electron transport measurements, especially for individual nanostructures. During the past decade, a number of experimental methods for measuring the thermal and thermoelectric properties of individual nanowires and nanotubes have been devised to address these challenges, some of which are reviewed and analyzed in this chapter. Although the Seebeck coefficient and electrical conductivity can also be obtained from some of the measurement methods, this chapter is focused on measurement techniques of the thermal conductivity and thermal diffusivity of nanowires and nanotubes. It is suggested that the limitations in the current experimental capability will provide abundant opportunities for innovative approaches to probing fundamental thermal and thermoelectric transport phenomena in individual nanostructures.

Steric-effect-induced alteration of thermal transport phenomenon for mixed electroosmotic and pressure driven flows through narrow confinements

The present paper addresses, for the first time, the thermal transport process for mixed electroosmotic and pressure-drive flows of electrolyte solutions, through nanoscopic confinements with step-change in the wall temperature, by going beyond the prevalent simplifying assumption of non-interacting, point charge behaviour of the ions in the electrolyte solution. An attempt is made here to delineate the alterations in the heat transfer characteristics induced by the inclusion of the finite ionic size effect or the ‘steric effect’, in the electrokinetic framework, which becomes very significant for electro-hydrodynamic flows through narrow-confinements with high surface charge density. Under such situations the finite size of the ions cannot be trivially precluded from the analysis, thereby rendering the point charge assumption erroneous. The distinctive influence of the ‘steric’ effect of ions on the heat transfer characteristics is numerically investigated by delineating the variations in the local liquid temperature, local Nusselt number and the thermal entrance length for the thermally developing regime. The observed significant influences of the bulk volume fraction of ions, as represented by the steric factor, on the thermal transport phenomenon are physically explained by simultaneously highlighting their intrinsic differences with the effects of other existing parameters, like Joule heating and viscous dissipation. The utilitarian scope of the work lies in the fact that it proposes a more comprehensive methodology for analyzing the heat transfer characteristics in state-of-the-art nanoscale electromechanical devices.

Effect of Plate Shape and Size on Thermal Fluid Flow in Channel

An experimental study is performed on thermal and fluid-flow transport phenomenon in a single narrow channel of plate heat exchanger. Emphasis is placed on enhancement of heat transfer performance and attenuation of pressure drop in a plate heat exchanger with the different plate shapes. It is found that 1) the frictional pressure drop and heat transfer performance for the herringbone plate is substantially intensified, whereas the separated herringbone plate causes an attenuation pressure drop and heat transfer enhancement; 2) its trend is affected by the gap of middle line g; and 3) the optimum size of g is g 10.

Thermal Modeling and Analysis of a Thermal Barrier Coating Structure Using Non-Fourier Heat Conduction

This paper presents a numerical solution of the hyperbolic heat conduction equation in a thermal barrier coating (TBC) structure under an imposed heat flux on the exterior of the TBC. The non-Fourier heat conduction equation is used to model the heat conduction in the TBC system that predicts the heat flux and the temperature distribution. This study presents a more realistic approach to evaluate in-service performance of thin layers of TBCs typically found in hot sections of land based and aircraft gas turbine engines. In such ultrafast heat conduction systems, the orders of magnitude of the time and space dimensions are extremely short which renders the traditional Fourier conduction law, with its implicit assumption of infinite speed of thermal propagation, inaccurate. There is, therefore, the need for an advanced modeling approach for the thermal transport phenomenon taking place in microscale systems. A hyperbolic heat conduction model can be used to predict accurately the transient temperature distribution of thermal barrier structures of turbine blades. The hyperbolic heat conduction equations are solved numerically using a new numerical scheme codenamed the mean value finite volume method (MVFVM). The numerical method yields minimal numerical dissipation and dispersion errors and captures the discontinuities such as the thermal wave front in the solution with reliable accuracy. Compared with some traditional numerical methods, the MVFVM method provides the ability to model the behavior of the single phase lag thermal wave following its reflection from domain boundary surfaces. In addition, parametric studies of properties of the substrate on the temperature and the heat flux distributions in the TBC revealed that relaxation time of the substrate material, unlike the thermal diffusivity and thermal conductivity has very little effect on the transient thermal response in the TBC. The study further showed that for thin film structures subject to short time durations of heat flux, the hyperbolic model yields more realistic results than the parabolic model.

Thermal transport in four-terminal graphene nano-junctions

The thermal transport properties of four-terminal graphene nano-junctions (FGNJs) consisting of semi-infinite armchair-edged nanoribbon and zigzag-edged nanoribbon were calculated. The thermal transport in FGNJs is sensitive to their geometric shape. The thermal conductance of FGNJs depends on the width of semi-infinite graphene nanoribbons and center region. These thermal transport phenomena can be explained by analyzing the phonon transmission coefficient. Compared with previous thermal rectifiers, reverse modulation can be obtained by changing the width of the thermal terminal. The results provide significant physical models and theoretical validity in designing the thermal devices based on the graphene nano-junctions.

Extrinsic Spin Nernst Effect from First Principles

We present an abinitio description of the thermal transport phenomenon called the spin Nernst effect. It refers to generation of a spin accumulation or a pure spin current transverse to an applied temperature gradient. This is similar to the intensively studied spin Hall effect described by intrinsic and extrinsic mechanisms due to an applied electric field. Analogously, several contributions are present for the spin Nernst effect. Here we investigate the extrinsic skew scattering mechanism which is dominant in the limit of dilute alloys. Our calculations are based on a fully relativistic Korringa-Kohn-Rostoker method and a solution of the linearized Boltzmann equation. As a first application, we consider a Cu host with Au, Ti, and Bi impurities.