Micro/Nanoscale Heat Transfer: Interfacial Effects Dominate the Heat Transfer

2012 ◽  
Author(s):  
X. Zhang ◽  
Z. Y. Guo
Keyword(s):  
Heat Transfer ◽  
Heat Conduction ◽  
Near Field ◽  
Mean Free Path ◽  
Viscous Force ◽  
Inertia Force ◽  
Relative Importance ◽  
Interfacial Effects ◽  
Nanoscale Heat Transfer ◽  
The Continuum

This paper describes the effects of size on heat conduction in nanofilms, convective heat transfer in micro/nanochannels, and near-field radiation in nanogaps. As the size is reduced, the ratio of the surface area to the volume increases; therefore, the relative importance of the interfacial effects also increases. The physical mechanisms for these size effects have been classified into two classes. When the scale is reduced to the order of micrometers (except for gases), the interfaces only affect the macro parameters and the continuum assumption still holds, but the relative importance of the various forces (inertia force, viscous force, buoyancy, etc.) and effects (interfacial effect, axial heat conduction in the tube wall, etc.) changes, resulting in changes in the heat transfer characteristics from normal conditions. As the size is further reduced to the order of submicrometers or nanometers, the interface affects not only the macro parameters but also the micro parameters (mean free path, relaxation time, etc.) so the continuum assumption breaks down and Newton’s viscosity law and Fourier’s heat conduction law are no longer applicable. Thus, the major characteristic of micro/nanoscale heat transfer is that the interfacial effects dominate the heat transfer.

Effects of Interfacial Interactions Between Fluid and Solid Wall on Microscale Flows

2006 ◽  
Author(s):  
Xingang Liang
Keyword(s):  
Heat Transfer ◽  
Solid Wall ◽  
Mean Free Path ◽  
Characteristic Size ◽  
Interfacial Interactions ◽  
Interfacial Effects ◽  
Flow And Heat Transfer ◽  
Microscale Flow ◽  
Flow Phenomena ◽  
Microscale Flows

This work discusses the interfacial effects on flow and heat transfer at micro/nano scale. Different from bulk cases where interfaces can be simply treated as a boundary, the interfacial effects are not limited to the interface at microscale but extend into a significant, even the whole domain of the flow and heat transfer field when the characteristic size of the domain is close to the mean free path (MFP) of fluid particles. Most of microscale flow phenomena result from interfacial interactions. Any changes in the interactions between the fluid and solid wall particles could affect the flow and heat transfer characteristics, such as flow and temperature profiles, friction coefficient. The interactions depend on many parameters, such as the force between fluid and solid wall particles, microstructure of interfaces. The flow and heat transfer features does not only depend on the fluid itself, but also on the interaction with the solid wall because the interface impact can go deep inside the flow. Same fluid, same channel shape but different wall materials could have different flow characters.

A Numerical Study of the Nanoscale Heat Transfer in the Head-Disk Interface for Heat Assisted Magnetic Recording

2016 ◽  
Author(s):  
Haoyu Wu ◽  
David Bogy
Keyword(s):  
Heat Transfer ◽  
Magnetic Recording ◽  
Transfer Problem ◽  
Numerical Study ◽  
Near Field ◽  
Experimental Result ◽  
Head Disk Interface ◽  
Heat Assisted Magnetic Recording ◽  
Disk Interface ◽  
Nanoscale Heat Transfer

The near field transducer (NFT) overheating problem is an issue the hard disk drive (HDD) industry has faced since heat-assisted magnetic recording (HAMR) technology was first introduced. In this paper, a numerical study of the head disk interface (HDI) is performed to predict the significance of the nanoscale heat transfer due to the back heating from the disk. A steady-state heat transfer problem is first solved to get the disk temperature profile. Then an iterative simulation of the entire HDI system is performed. It shows that the heat transfer coefficient in the HDI increases to about 6:49 × 106 W/(m2K) when the clearance is 0:83 nm. It also shows that in the free space laser scenario, the simulation result is close to the experimental result.

Nonlocal and Nonequilibrium Heat Conduction in the Vicinity of Nanoparticles

1996 ◽  
Vol 118 (3) ◽  
pp. 539-545 ◽  
Author(s):  
G. Chen
Keyword(s):  
Heat Transfer ◽  
Heat Conduction ◽  
Particle Temperature ◽  
Approximate Expression ◽  
Mean Free Path ◽  
Boltzmann Transport ◽  
Host Medium ◽  
Conduction Theory ◽  
Fourier Heat Conduction ◽  
Thermal Phenomena

Heat transfer around nanometer-scale particles plays an important role in a number of contemporary technologies such as nanofabrication and diagnosis. The prevailing method for modeling thermal phenomena involving nanoparticles is based on the Fourier heat conduction theory. This work questions the applicability of the Fourier heat conduction theory to these cases and answers the question by solving the Boltzmann transport equation. The solution approaches the prediction of the Fourier law when the particle radius is much larger than the heat-carrier mean free path of the host medium. In the opposite limit, however, the heat transfer rate from the particle is significantly smaller, and thus the particle temperature rise is much larger than the prediction of the Fourier conduction theory. The differences are attributed to the nonlocal and nonequilibrium nature of the heat transfer processes around nanoparticles. This work also establishes a criterion to determine the applicability of the Fourier heat conduction theory and constructs a simple approximate expression for calculating the effective thermal conductivity of the host medium around a nanoparticle. Possible experimental evidence is discussed.

Ultrasensitive Bimaterial Cantilevers Optimized for Nanowire Heat Conduction Measurement

2012 ◽  
Author(s):  
Carlo Canetta ◽  
Ning Gu ◽  
Arvind Narayanaswamy
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Conduction ◽  
Silicon Nitride ◽  
Flat Plate ◽  
Radiative Heat Transfer ◽  
Radiative Heat ◽  
Near Field ◽  
Aluminum Coating ◽  
Thermal Sensors

We have developed a microcantilever-based technique for measurement of heat conduction through individual nanowires. We fabricated silicon nitride cantilevers with nominal dimensions of length 100 μm, width 2–6 μm, and thickness 130 nm. Cantilever chips are designed with multiple cantilevers spaced at varying distances. With a reflective aluminum coating of optimized thickness, these bimaterial cantilevers can be used as ultrasensitive thermal sensors capable of measuring very small heat flux through a nanostructure fixed between two cantilevers. The ultrasensitive bimaterial cantilevers designed in this work are not limited to heat conduction measurements, but will also be useful for measuring near-field radiative heat transfer between a sphere, attached to the tip of the cantilever, and a flat plate.

Some Flow and Thermal Phenomena at Microscale

2003 ◽  
Author(s):  
Zhi-Xin Li ◽  
Zeng-Yuan Guo
Keyword(s):  
Heat Transfer ◽  
Heat Conduction ◽  
Characteristic Length ◽  
Vapor Interface ◽  
Flow And Heat Transfer ◽  
Electrostatic Charges ◽  
Thermal Phenomena ◽  
Vapor Nucleation ◽  
The Continuum ◽  
Liquid Vapor

The physical mechanisms for the size effects on the flow and heat transfer have been divided into two classifications: (a) variations of the predominant factors influence the relative importance of various phenomena in the flow and heat transfer as the characteristic length decrease, even if the continuum assumption is still valid; (b) the continuum assumption breaks down as the characteristic length of the flow becomes comparable to the mean free path of the molecules. The departure of most flow and thermal phenomena in the MEMS from conventional ones is due to the variation of predominant factors in the flow and heat transfer problems, rather than that Navier-Stokes equation and Fourier heat conduction equation etc are no longer valid. Due to the larger surface to volume ratio for microchannels and microdevices, factors related to surface effects have more impact to microscale flow and heat transfer. For example, surface friction induced flow compressibility makes the fluid velocity profiles flatter and leads to higher friction factors and Nusselt numbers; surface roughness is likely responsible for the increased friction factor, the early transition from laminar to turbulent flow and Nusselt number; and other effects, such as the axial heat conduction in the channel wall, the channel surface geometry, surface electrostatic charges, and measurement errors, could lead to different flow and heat transfer behaviors from that at conventional scales. The condensation/evaporation across the liquid-vapor interface and the liquid-vapor nucleation are processes at nanometer scale. In these cases the macroscopic approach is hard to reveal the details at nanometer scales, while the molecular dynamic simulation is a powerful tool to describe such microscopic processes, and has been applied to investigate the density and temperature profiles across the liquid-vapor interface. The condensation coefficient on the liquid-vapor interface under thermodynamic equilibrium condition has been well predicted based on the characteristic time method, which can distinguish the condensed particles from the reflected particles. Molecular dynamics simulations show that liquid-vapor nucleation undergoes three stages, namely, cavity growth, cavity coalescence and bubble formation.

Heat Conduction Across Nanoscale Interfaces and Nanomaterials for Thermal Management and Thermoelectric Energy Conversion

2010 ◽  
Author(s):  
Theodorian Borca-Tasciuc
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Conduction ◽  
Thermal Management ◽  
Contact Area ◽  
Silicon Substrate ◽  
Mean Free Path ◽  
Contact Mode ◽  
Nanoscale Interfaces ◽  
Air Conduction

Nanoscale heat conduction plays a critical role in applications ranging from thermal management of nanodevices to nanostructured thermoelectric materials for solid state refrigeration and power generation. This lecture presents recent investigations in our group. The first part of the lecture demonstrates heat conduction across nanoscale interfaces formed between individual nanoscale heaters and the silicon substrate [1]. A systematic experimental study was performed of thermal transport from individual nanoscale heaters with widths ranging between 77nm-250nm to bulk silicon substrates in the temperature range of 80–300K. The effective substrate thermal conductivity was measured by joule heating thermometry. We report up to two orders of magnitude reductions in the measured effective thermal conductivity of the silicon substrate when the heater widths are smaller than the mean free path of the heat carriers in the substrate, as summarized in Fig. 1. The effective mean free path of the silicon substrate was extracted from the measurements and was found to be comparable with recent molecular dynamics simulations. A proof of concept demonstration of a novel Thermal Interface Material (TIM) is presented next. The high thermal conductivity TIM is based on a highly connected high thermal conductivity nanostructured filler network embedded in a polymer matrix where the contribution of filler-matrix interfaces to thermal resistance is minimized. It was found [2] that the thermal conductivity could be varied from ∼0.2 to 20 W/mK when the volume fraction of metallic nanoparticles was varied from 0–20%. For similar volume fractions and filler composition, microparticle based composites have two orders of magnitude lower thermal conductivities. SEM characterization and thermal transport modeling are employed to support the conclusion that morphological changes in the nano-TIM are responsible for the thermal conductivity reduction. Thermoelectric transport investigations are discussed for a novel class of highly scalable nanostructured bulk chalcogenides developed at Rensselaer Polytechnic Institute [3]. Un-optimized, single-component bulk assemblies of Bi2Te3 and Sb2Te3 single crystal nanoplates show large enhancements (25–60%) in the room temperature thermoelectric figure of merit compared with individual bulk counterparts (Table 1). Nanostructuring was found to lead to strong thermal conductivity reduction without significantly affecting the mobility of the charge carriers, as shown in Table 2. A scanning thermal microprobe technique developed for simultaneous thermal conductivity (κ) and Seebeck coefficient (α) measurements in thermoelectric films is also presented [4]. In this technique, an AC alternative current joule-heated V-shaped microwire that serves as heater, thermometer and voltage electrode, locally heats the thin film when contacted with the surface (Fig. 2). The κ is extracted from the average DC temperature rise thermal resistance of the microprobe and α from the DC Seebeck voltage measured between the probe and unheated regions of the film by modeling the heat transfer in the probe, sample and their contact area, and by calibrations with standard reference samples. Application of the technique on sulfur-doped porous Bi2Te3 and Bi2Se3 films reveals α = −105.4 and 1.96 μV/K, respectively, which are within 2% of the values obtained by independent measurements carried out using microfabricated test structures. The respective κ values are 0.36 and 0.52 W/mK, which are significantly lower than the bulk values due to film porosity, and are consistent with effective media theory. The dominance of air conduction at the probe-sample contact area determines the microscale spatial resolution of the technique and allows probing samples with rough surfaces. Non-contact mode measurement of thermal conductivity was also demonstrated and confirmed by independent characterization [5]. In non-contact mode the technique utilizes ballistic air conduction as the dominant heat transfer mechanism between the thermal probe and the sample and thus eliminates uncertainties due to solid contact and liquid meniscus conduction.

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