Thermal Transport in Nanotube Composites for Large-Area Macroelectronics

2005 ◽  
Author(s):  
Satish Kumar ◽  
Muhammad A. Alam ◽  
Jayathi Y. Murthy
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Thermal Transport ◽  
Channel Length ◽  
Tube Length ◽  
Finite Volume Scheme ◽  
Large Area ◽  
Contact Conductance ◽  
Channel Lengths ◽  
Contact Parameters

Thermal transport in a new class of nanocomposites composed of isotropic 2D ensembles of nanotubes or nanowires in a substrate is considered for use as the channel region of thin film transistors. The random ensemble is generated numerically and simulated using a finite volume scheme. The effective thermal conductivity of a nanotube network embedded in a thin substrate is computed. Percolating conduction in the composite is studied as a function of wire/tube densities and channel lengths. The conductance exponents are validated against available experimental data for long channels devices. The effect of tube-tube contact conductance, tube-substrate contact conductance and substrate-tube conductivity ratio is analyzed for various channel lengths. It is found that beyond a certain limiting value, contact parameters do not result in any significant change in the effective thermal conductivity of the composite. It is also observed that the effective thermal conductivity of the composite saturates beyond a limiting channel-length/tube length ratio for the range of contact parameters under consideration.

Numerical Model for Thermal Transport in 3-D Nanotube Composites

2007 ◽  
Author(s):  
Satish Kumar ◽  
Jayathi Y. Murthy
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Finite Volume ◽  
Thermal Transport ◽  
Three Dimensional ◽  
Finite Volume Scheme ◽  
Interfacial Resistance ◽  
Volume Scheme ◽  
Numerical Predictions ◽  
Nanotube Composites

The effective thermal conductivity of three dimensional (3-D) nanocomposites composed of carbon nanotube (CNT) dispersions is computed using Fourier conduction theory. The random ensemble of nanotubes is generated numerically and each nanotube is discretized using a finite volume scheme. The background substrate mesh is also discretized using a finite volume scheme. We incorporate all parameters crucial for thermal transport studies, i.e. tube aspect ratio, tube density, composite sample size, substrate-CNT conductivity ratio and the interfacial resistance due to tube-tube and tube-substrate contact. Two-dimensional (thin film) nanocomposites are also simulated for comparison. Numerical predictions of effective thermal conductivity are in excellent agreement with the effective medium approximation (EMA) for both 2-D and 3-D nanocomposites at low tube densities, but depart significantly from EMA predictions when tube-tube interaction becomes significant. It is found that the effect of tube-tube contact on effective thermal conductivity is more significant for 2-D composites than 3-D composites. Hence percolation effects may play a more significant role in thermal transport in 2-D nano-composites.

Numerical Simulations of Thermal Transport in Random Porous Materials and Powder Systems Using the Smoothed Particle Hydrodynamics Method

2017 ◽  
Author(s):  
Deepak Shah ◽  
Alexey N. Volkov
Keyword(s):  
Thermal Conductivity ◽  
Porous Materials ◽  
Effective Thermal Conductivity ◽  
Smoothed Particle Hydrodynamics ◽  
Thermal Transport ◽  
Thermal Contact Conductance ◽  
Powder Bed ◽  
Particle Hydrodynamics ◽  
Smoothed Particle ◽  
Powder Particles

A numerical method to solve thermal transport problems in powder bed systems and porous materials with finite thermal contact conductance at interfaces between individual powder particles or grains is developed based on the Smoothed Particle Hydrodynamics approach. The developed method is applied to study the effective thermal conductivity of two-dimensional random powder bed systems with binary distribution of powder particles radii. The effects of particle size distribution parameters, density parameter, and effective interface area between particles on the effective thermal conductivity are studied. It is found that at finite Biot number, which characterizes the ratio of the interfacial conductance to the conductance of the bulk powder material, the effective thermal conductivity of porous samples increases with increasing fraction of particles of larger size.

A Numerical Technique for Computing Effective Thermal Conductivity of Fluid-Particle Mixtures

2004 ◽  
Author(s):  
Satish Kumar ◽  
Jayathi Y. Murthy
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Aspect Ratio ◽  
Effective Thermal Conductivity ◽  
Volume Fraction ◽  
Numerical Technique ◽  
Fluid Particle ◽  
Finite Volume Scheme ◽  
Ratio Distribution ◽  
Periodic Arrays

Periodic arrays of particles, foams, and other structures impregnated with a static fluid play an important role in heat transfer enhancement. In this paper, we develop a numerical method for computing conduction heat transfer in periodic beds by exploiting the periodicity of heat flux and the resulting linear variation of mean temperature. The numerical technique is developed within the framework of an unstructured finite volume scheme in order to enable the computation of effective thermal conductivity for complex fluid-particle arrangements. The method is applied to the computation of effective thermal conductivity of ordered as well as random beds of spheres and rods. The effect of varying surface area, aspect ratio, volume fraction, orientation, and distribution is studied for various solid-to-fluid conductivity ratios. Unlike classical theories which predict only a dependence on volume fraction, these direct simulations show that aspect ratio, distribution, and alignment of particles have an important influence on the effective thermal conductivity of the bed.

scholarly journals Direct Simulation of Thermal Transport Through Sintered Wick Microstructures

2011 ◽  
Vol 134 (1) ◽  
Author(s):  
Karthik K. Bodla ◽  
Jayathi Y. Murthy ◽  
Suresh V. Garimella
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Effective Thermal Conductivity ◽  
Thermal Transport ◽  
Heat Pipes ◽  
Interfacial Heat Transfer Coefficient ◽  
Interfacial Heat Transfer ◽  
Particle Beds

Porous sintered microstructures are critical to the functioning of passive heat transport devices such as heat pipes. The topology and microstructure of the porous wick play a crucial role in determining the thermal performance of such devices. Three sintered copper wick samples employed in commercial heat pipes are characterized in this work in terms of their thermal transport properties––porosity, effective thermal conductivity, permeability, and interfacial heat transfer coefficient. The commercially available samples of nearly identical porosities (∼61% open volume) are CT scanned at 5.5 μm resolution, and the resulting image stack is reconstructed to produce high-quality finite volume meshes representing the solid and interstitial pore regions, with a conformal mesh at the interface separating these two regions. The resulting mesh is then employed for numerical analysis of thermal transport through fluid-saturated porous sintered beds. Multiple realizations are employed for statistically averaging out the randomness exhibited by the samples under consideration. The effective thermal conductivity and permeability data are compared with analytical models developed for spherical particle beds. The dependence of effective thermal conductivity of sintered samples on the extent of sintering is quantified. The interfacial heat transfer coefficient is compared against a correlation from the literature based on experimental data obtained with spherical particle beds. A modified correlation is proposed to match the results obtained.

Percolation Effects on the Thermal Conductivity of 3D Nanotube Composites

2008 ◽  
Author(s):  
Satish Kumar ◽  
Muhammad A. Alam ◽  
Jayathi Y. Murthy
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Thermal Transport ◽  
Three Dimensional ◽  
Linear Response Theory ◽  
Critical Parameters ◽  
Conductivity Ratio ◽  
Interfacial Resistance ◽  
Numerical Predictions ◽  
Nanotube Composites

We analyze thermal transport in three-dimensional (3D) nano-composites composed of carbon nanotube (CNT) dispersions to investigate percolation effects on the effective thermal conductivity of these composites. Thermal transport simulations for the randomly distributed nanotubes inside the host substrate are based on the diffusive Fourier conduction theory. The numerical model incorporates the effect of substrate-CNT conductivity ratio and the interfacial resistance due to tube-tube and tube-substrate contact, which are the most critical parameters governing thermal transport properties. Numerical predictions of effective thermal conductivity are in excellent agreement with the linear response theory and effective medium approximation (EMA) when assumptions of theory are incorporated in the model. The trends for the variation of effective thermal conductivity with increasing nanotube density are in broad agreement with previous experimental observations. Our numerical results also show that the onset of thermal percolation is gradual and largely dependent on the tube-to-substrate conductivity ratio and interfacial resistance at tube-tube and tube-substrate contact.

An Improved Efficient Network Model for Determining the Effective Thermal Conductivity of Particulate Thermal Interface Materials

2013 ◽  
Vol 135 (3) ◽  
Author(s):  
B. Dan ◽  
B. G. Sammakia ◽  
G. Subbarayan ◽  
S. Kanuparthi
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Network Model ◽  
Thermal Transport ◽  
Volume Fraction ◽  
Spherical Particles ◽  
Thermal Interface Materials ◽  
Interparticle Interactions ◽  
Cylindrical Region ◽  
Interface Materials

Thermal interface materials (TIMs) are particulate composite materials widely used in the microelectronics industry to reduce the thermal resistance between the device and heat sink. Predictive modeling using fundamental physical principles is critical to developing new TIMs since it can be used to quantify the effect of particle volume fraction and arrangements on the effective thermal conductivity. The existing analytical descriptions of thermal transport in particulate systems do not accurately account for the effect of interparticle interactions, especially in the intermediate volume fractions of 30–80%. An efficient random network model (RNM) that captures the near-percolation transport in these particle-filled systems, taking into account the interparticle interactions and random size distributions, was previously developed by Kanuparthi et al. The RNM approach uses a cylindrical region to approximate the thermal transport within the filler particles and to capture the interparticle interactions. However, this approximation is less accurate when the polydispersivity of the particulate system increases. In addition, the accuracy of the RNM is dependent on the parameters inherent in an analytical description of thermal transport between two spherical particles and their numerical approximation into the network model. In the current paper, a novel semispherical approximation to the conductance of the fillers is presented as an alternative to the cylindrical region approximation used earlier. Compared with the cylindrical model, the thermal conductivities of the semispherical model are more closely to the finite element (FE) results. Based on the FE analysis, the network model is improved by developing an approximation of the critical cylindrical region between two spherical particles over which energy is transported. Comparing the RNM results with FE results and experimental data, a linear relationship of the critical parameter with the thermal conductivity ratio and the volume fraction was found that provides a more accurate prediction of the effective thermal conductivity of the particulate TIMs.

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