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Effective Thermal Conductivity of Submicron Powders: A Numerical Study

DOI:10.4028/www.scientific.net/AMM.846.500
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Wei Jing Dai
Wei Jing Dai
Wei Jing Dai
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Yixiang Gan at The University of Sydney
Yixiang Gan
Dorian A H Hanaor at Technische Universität Berlin
Dorian A H Hanaor
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Abstract

Effective thermal conductivity is an important property of granular materials in engineering applications and industrial processes, including the blending and mixing of powders, sintering of ceramics and refractory metals, and electrochemical interactions in fuel cells and Li-ion batteries. The thermo-mechanical properties of granular materials with macroscopic particle sizes (above 1 mm) have been investigated experimentally and theoretically, but knowledge remains limited for materials consisting of micro/nanosized grains. In this work we study the effective thermal conductivity of micro/nanopowders under varying conditions of mechanical stress and gas pressure via the discrete thermal resistance method. In this proposed method, a unit cell of contact structure is regarded as one thermal resistor. Thermal transport between two contacting particles and through the gas phase (including conduction in the gas phase and heat transfer of solid-gas interfaces) are the main mechanisms. Due to the small size of particles, the gas phase is limited to a small volume and a simplified gas heat transfer model is applied considering the Knudsen number. During loading, changes in the gas volume and the contact area between particles are simulated by the finite element method. The thermal resistance of one contact unit is calculated through the combination of the heat transfer mechanisms. A simplified relationship between effective thermal conductivity and loading pressure can be obtained by integrating the contact units of the compacted powders.
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Effective thermal conductivity of submicron powders: A numerical
study
Weijing Dai1,a, Yixiang Gan1,b*, Dorian Hanaor1,c
1The School of Civil Engineering, The University of Sydney, Sydney, Australia
aweijing.dai@sydney.edu.au, byixiang.gan@sydney.edu.au, cdorian.hanaor@sydney.edu.au
Keywords: effective thermal conductivity, gas heat transfer, finite element analysis, compacted
powders
Abstract: Effective thermal conductivity is an important property of granular materials in
engineering applications and industrial processes, including the blending and mixing of powders,
sintering of ceramics and refractory metals, and electrochemical interactions in fuel cells and Li-ion
batteries. The thermo-mechanical properties of granular materials with macroscopic particle sizes
(above 1 mm) have been investigated experimentally and theoretically, but knowledge remains
limited for materials consisting of micro/nano-sized grains. In this work we study the effective
thermal conductivity of micro/nano powders under varying conditions of mechanical stress and gas
pressure via the discrete thermal resistance method. In this proposed method, a unit cell of contact
structure is regarded as one thermal resistor. Thermal transport between two contacting particles
and through the gas phase (including conduction in the gas phase and heat transfer of solid-gas
interfaces) are the main mechanisms. Due to the small size of particles, the gas phase is limited to a
small volume and a simplified gas heat transfer model is applied considering the Knudsen number.
During loading, changes in the gas volume and the contact area between particles are simulated by
the finite element method. The thermal resistance of one contact unit is calculated through the
combination of the heat transfer mechanisms. A simplified relationship between effective thermal
conductivity and loading pressure can be obtained by integrating the contact units of the compacted
powders.
Introduction
In energy systems, granular media are commonly used to store, convert, capture and produce
energy, including lithium-ion batteries, solid oxide fuel cells, and thermal storage systems. Heat
transport in these energy systems is a key issue necessitating extensive research to 1) establish a
fundamental understanding of the thermo-mechanical properties and 2) provide useful knowledge
for process optimisation [1-3]. Due to heterogeneous material properties, complex packing
structures and various inter-particle interactions, different theoretical methods have been developed
to study the heat transport mechanisms and the influencing external factors [4, 5].
Most research in this field has focused on the effective thermal conductivity (ETC) of powder
beds, employing three types of models to predict this parameter. Type I models are based on the
materials’ constitution and porosity and can be used to calculated the ETC by considering powder
beds as dispersion media [4]. Type-II models regard powder beds as thermal circuits in order to
obtain the effective thermal resistance through an analogy with electrical circuits [4, 6-9]. Type-III
models discretise powder beds into many unit cells and integrate the cells’ properties to derive the
ETC [5, 10, 11]. The former two types of models can provide relatively simple equations to predict
the ETC while the type-III models usually involve considerable numerical calculation. However, it
is more convenient to study the effects of external factors on the ETC and to approximate the real
situation in practical application by applying type-III models. To realize a useful type-III model, a
proper description of the unit cell combing different heat transfer mechanism and external factors is
required. An analytical solution for a contact unit involving two spheres was proposed by Batchelor
and O’Brien in order to describe the relationship between ETC and contact area [10]. Baharami et
al. employ the thermal contact resistance model to include the effects of rough contacts in the
Applied Mechanics and Materials Online: 2016-07-25
ISSN: 1662-7482, Vol. 846, pp 500-505
doi:10.4028/www.scientific.net/AMM.846.500
© 2016 Trans Tech Publications, Switzerland
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
Tech Publications, www.ttp.net. (#68764661, Karlsruhe Research Center, Karlsruhe, Germany-22/07/16,06:21:07)
spherical powder beds [7]. The particle size of the powder beds considered in these studies has
typically been above 1 mm, thus the heat transfer through the interparticle gap is regarded as the
same as through a continuum gas phase. Furthermore, Gusarov et al. considered the effect of the
Knudsen number on the ETC of powder beds and the results shows that for millimetre- and
micrometre-sized particles the increase of gas pressure increases the ETC with saturation occurring
above a certain gas pressure [5]. However, these studies have not addressed the powder beds
containing submicron or nano particles which are widely used in sintering process, thermal
insulation, and solid oxide fuel cells. In this study, the finite element method (FEM) is utilised to
calculate heat transport processes in the submicron units with different particle sizes. By altering the
gas phase pressure and mechanical deformation, the relationship between the ETC and loading
condition can be obtained.
Simulation Model
A two-hemisphere contact unit cell is used to evaluate the effective thermal conductivity of
individual element as shown in Figure 1. The hemispheres, with constant material properties, form a
contact along the axial direction concentrically. The gap between the hemispheres is filled with air,
thus completing a cylindrical unit. In this simulation unit, a constant temperature difference
between the top surface and bottom surface is applied, and uniform temperatures of the upper and
lower hemispheres are established at the initial state as and , respectively. An axial
compression is applied from the top surface and the bottom surface is fixed. In this study, the
commercial finite element package ABAQUS is utilised to perform the FEM calculation. The
temperature difference ∆ is maintained to be 1K and −=∆ is created at initial state. The
properties of the air between the gap of the two hemispheres are assumed to be independent of the
temperature distribution of the air phase because ∆T is relatively small. Thus, in the numerical
model, a distance-dependent the gap thermal conductance (*GAP CONDUCTANCE in ABAQUS)
is used to approximate the thermal conductance via the gas gap.
(a) (b) (c)
Figure 1: Unit cell model: (a) schematic drawing of two spherical particles nearly in contact;
(b) Two particles in contact; (c) The FEM model with temperature profile with an arbitrary unit.
Here, ∆T, ∆x, hg and hs denote the temperature difference, particle deformation, heat flux through
the gas phase and through the solid contact, respectively.
Gas heat transfer at submicron-scale depends on the the ratio of the mean free path, , of gas
molecules to the gap distance, as given by the Knudsen number, . For the powder beds
comprised of submicron particles, the gaps between particles become comparable to the mean free
path of the gas molecules, leading to a transition of the heat transfer modes of gas phase. In other
words, the equation ℎ=/ to calculate the heat conductance of gas is invalid in this range,
where is the thermal conductivity of gas and is the gap distance. An approximation based on
∆T hg ∆T
∆x
hg hs
Applied Mechanics and Materials Vol. 846 501
the Knudsen number is used to predict the gas phase heat conductance. The mean free path is
calculated based on the dynamic viscosity
=

 (1)
and the dynamic viscosity is calculated by
=
(
)
(2)
where µ0 is dynamic viscosity at =296.15K. At a certain temperature , the mean free path
depends on gas pressure and gas constitution, where is the molar mass of the gas. When the
Knudsen number  is very large (  ≫100), the heat conductance is approaching the free
molecular limit [5]
ℎ=

( 
)
(3)
Here is the gas constant and is the adiabatic exponent. For 0.01<< 100, the heat
conductance at different Knudsen number is approximated by
ℎ=
 (∗)
⁄+
 ∗)
⁄, (4)
= √

. (5)
It can be concluded that the gas heat conductance at ≫100 depends on not only the
temperature but also the gas pressure while the heat conductance is not affected by the gas pressure
for small values of Knudsen number.
Figure 2: Air heat conductance as a function of gap distance at different gas pressure, at = 298.
The solid line indicate the conductance calculated based on the continuum equation, ℎ=/.
502 Advances of Computational Mechanics in Australia
Further to the heat transfer of gas phase, the heat transfer through the contact region of two
hemispheres in one unit cell needs to be defined. Thermal contact resistance is commonly used to
describe the heat transfer across the solid contact between two relatively large hemispheres. In such
situation, the contact region consists of a finite number of nanoscale contact spots. However, this
contact model is not applicable for situation where the contact region itself falls into nanoscale. Due
to the consistent material properties of the two contact hemispheres, the nano contact region is
considered to have the same thermal properties as the bulk material. Thus the heat conductance of
the contact region is defined as the heat conductance of 1nm thick material in this simulation.
In ABAQUS, the gap conductance is varied with Knudsen number  to simulate the gas heat
transport at different gas pressure. Deformation of the unit cell can be achieved by applying
displacement as the boundary condition. Table 1 summarizes the materials and model parameters
used in the ABAQUS simulations. The hemispheres are assumed to be SiO2 powders and the gas
phase is air. The negative sign of axial deformation ratio means that the two hemispheres are
separated and a finite gap is left between two hemispheres.
Table 1: Materials and model parameters used in ABAQUS simulation
Powder material: SiO2
Thermal conductivity ks=1.3 nW/nm·K, specific heat c=7*1011 nJ/kg·K, density
ρ=2.65*10-24 kg/nm3, Young’s modulus=7*10-8 N/nm2, Poisson’s ratio=0.17.
Gas: Air
Molar mass M=0.028966 kg/mol, dynamic viscosity µ
0
=1.827*10-5 Pa·s,
Axial deformation ratio -1% 0% 1% 2% 5%
Particle diameter (nm) 100 200 500 1000
Gas pressure (Pa) 1 100 10000 100000 1000000 1000000
Result and discussion
In these models, the reaction heat fluxes are calculated according to the temperature boundary
conditions. At the equilibrium state, the effective thermal conductivity of individual unit cells can
be calculated based on the Fourier’s law
=∆
(6)
where is the reaction heat flux, denotes the effective thermal conductivity of unit cell, is the
axial -length of the unit cell and ∆ is the temperature difference.
Figure 3 presents the change of ETC of the unit cells versus the change of the gas pressure for
different particle sizes. When the two hemispheres are not in contact (-1% deformation), the heat
transfer inside the unit cell takes place entirely through the gas phase. It is clear that increasing the
gas pressure continuously increases the ETC of the unit cells which is consistent with the trend in
the heat conductance of air versus pressure in Figure 2. For conditions where the heat transfer occur
solely through the gas phase, larger particle sizes result in a larger ETC of the unit cells. Note that
the gap size is relative to the size of the unit cell, and in Figure 3(a) a displacement of -1% is
considered. When the two hemispheres approach to each other to form a point contact, the effect of
gas pressure diminishes at low gas pressure region. The gas pressure begins to affect ETC of unit
cells at value above 10 kPa and this effect becomes more significant as the gas pressure increases.
However, for conditions of particle contact the relationship between the particles size and ETC is
opposite to the situation of separated hemispheres shown in Figure 3(a). Compared with the ETC of
non-contact group, the ETC of this point contact group is much larger due to the presence of solid-
solid contact conductance. The contact point provides a thermal shortcut for the heat transport
because the heat conductance of the contact region is several orders (103~109) larger than the gas
heat conductance.
Applied Mechanics and Materials Vol. 846 503
(a) (b)
(c) (d)
Figure 3: The effective thermal conductivity as a function of air pressure in different sized powders,
at T=298K: (a) non contact, -1% deformation, (b) point contact, 0% deformation,
(c) 2% deformation, and (d) 5% deformation
When the deformation increases as shown in Figure 3 (b-d), with an expanding contact region,
the ETC of the unit cells rises and follows a similar pattern as a function of the gas pressure. The
magnitude of the increase in the ETC due to the change in pressure in the three contact groups (0%,
2%, 5%) actually corresponds to the increment of ETC in the non-contact group (-1%). Only when
the gas heat transfer is comparable to the heat conduction through the contact region, the gas
pressure starts to significantly alter the ETC. This phenomenon indicates these two heat transport
mechanisms, i.e., heat conduction by solid-solid contact and heat transfer through gas phase, are
parallel, which is a general consideration in many other methods used to predict the ETC of powder
beds. Figure 4 shows the effect of deformation on the ETC of unit cells. As the deformation
increases, the ETC of unit cells of different converges. The reason is that the increasing contact area
dominates the heat transport in the unit cell, and the gas gap becomes less significant.
Figure 4: The effective thermal conductivity as a function of axial deformation in different sized
powders, at P=100 kPa and T=298 K
504 Advances of Computational Mechanics in Australia
Conclusion
The effective thermal conductivity of compacted submicron powders was studied for various
loading conditions, including gas pressure, temperature and deformation. A microscopic unit cell
was established to provide the basis for evaluating overall conductivity of the packed beds and was
analysed using finite element method. The Knudsen number has been incorporated in the gas
conduction model to include the influence of gap distance, gas pressure and temperature. The
numerical result shows a clear dependency of the effective thermal conductivity on parameters of
particle size, gas pressure and deformation. Increasing gas pressure tends to elevate the overall
conductivity at high pressure. When interparticles contacts are form, smaller sizes lead to larger
overall conductivity of unit cells. With increasing compaction, the overall conductivity increases
and the solid-solid contact conduction dominates the heat transfer. This study provides a theoretical
basis to consider the heat transfer in compacted beds with submicron-sized particles under a wide
range of loading conditions.
Acknowledgement
Financial support for this research from the Australian Research Council through grants
DE130101639 is greatly appreciated.
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Applied Mechanics and Materials Vol. 846 505
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Heat transfer between contacting solid particles in gas is localized about the points of contact because of the low thermal conductivity of gas. This suggests the model of a network of discrete thermal resistances. Thermal interaction of two particles in the zone of contact between them includes thermal conductivity in the solid phase as well as heat transfer through a gas gap between the particles, where the conductive heat transfer at higher distances from the center of the contact changes to the transition transfer and free molecular transfer near the contact center. In the framework of this approach the effective thermal conductivity depends on three dimensionless morphological parameters: the volume fraction of solid phase, the mean coordination number, and the ratio of the size of necks between particles to the size of particles. The physical properties of the phases are specified by the thermal conductivities of the solid and the gas, the adiabatic exponent of the gas, and the Knudsen number. The proposed model agrees with the experimental S curves of the effective thermal conductivity versus the logarithm of gas pressure. It also describes the experimental tendencies of increasing the effective thermal conductivity with the particle size and the volume fraction of solid. The model can be applied to powder beds with micron-sized particles as well as to packed beds with millimeter-sized particles in gas. Free molecular and transient regimes of heat transfer in the gas filling pores can be important even for millimeter-sized particles at atmospheric pressure when the Knudsen number is as low as 10−5. These rarefied gas phenomena are responsible for such a complicated behavior of the considered heterogeneous media. Their quantitative description gives the model, which does not contain fitting parameters.
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A theoretical model for effective thermal conductivity (ETC) of particulate beds under compression
Article
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  • Oct 2004
Thermal conductivity of particulate beds is an important property for many industrial handling processes as well as for storage of particulate materials. This paper presents a new theoretical model that is based on heat transfer between particles in three modes: heat conduction through contact area, heat conduction through voids and radiation through voids. The model is further adjusted in order to obtain effective thermal conductivity of a particulate bed by using empirical augmentation factors for the heat transfer coefficient of each one of the heat transfer mechanisms. Comparison of the results predicted by the semi-empirical model to our experimental results show good agreement. The theoretical model was investigated to examine the effect of various parameters (such as: particle elasticity and surface roughness, particle and gas thermal conductivity and particle diameter), on the effective thermal conductivity of various particulate beds. Our results show the significant effect of the contact area (that is a clear function of the compression load) between particles on the effective thermal conductivity.
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Effective thermal conductivity of disperse materials. II. Effect of external load
Article
Using the steady-state heat flux method, the effective thermal conductivity λ of a particle bed is measured at room temperature, varying the external load P on the bed in the range of 3–30 kPa. Grains of various forms (debris, pebble, cubo-octahedral) and balls of different materials (polypropylene, sodium chloride, ceramics, diamonds, metals) of which the thermal conductivity of the solid measures 0.2–1500 W/(m K) are used. From the experimental dependencies λ(P) = λ0 + Δλ(P), values for effective thermal conductivity in the absence of contact heat conduction λ0 and contact conductivity Δλ(P) are found. The analysis allowed us (a) to refine further our earlier evaluation (in the first part of this study Abyzov et al. (2013)) of the adequacy of models which do not take account of contact thermal conductivity (the differential effective medium model and those of Gusarov et al., Raghavan–Martin, Chiew–Glandt/Gonzo, Kunii et al., Zehner–Schlunder) and (b) to assess the capabilities provided by a number of models of contact thermal conductivity (those of Dul’nev–Sigalova, Kaganer, Gusarov and others). A model with nominally flat rough contacts between particles in the bed is proposed, which describes the observed effects for contact thermal conductivity.
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Effective thermal conductivity of disperse materials. I. Compliance of common models with experimental data
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The thermal conductivity of packed beds of various materials (metals, ceramics, superhard materials, polymers, etc.) is measured by the steady-state heat flux method at room temperature in air. Powders and granular materials are used, with particle sizes ranging mainly from 50 mu m to 1.5 mm, with various particle shapes (debris, pebble, cubo-octahedral single-crystals, regular spheres), the thermal conductivity of the solid ranging widely from 0.15 to 1500 W/(m K). Experimental data and model calculations of the effective thermal conductivity of the bed are compared, including dependences on bed porosity and particle size. Both classical and more recent models are involved, such as the differential effective medium model and those of Raghavan-Martin, Kunii et al., Zehner-Schlunder, Gusarov et al., Hsu et al., Batchelor-O'Brein, and some others, which allow analytical calculations. The adequacy of the models is reviewed in connection with the properties of the disperse materials (particle shape etc.), based on the concepts of the absence or presence of contact thermal conductivity in the bed. C) 2013 Elsevier Ltd. All rights reserved.
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A DEM study on the effective thermal conductivity of granular assemblies
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A Prediction of the Bounds on the Effective Thermal Conductivity of Granular Materials
Article
Two models are developed to describe the effective thermal conductivity of randomly packed granular systems based on a one dimensional Ohm's Law method. These models are shown to represent upper and lower bounds on the effective conductivity of all normally distributed stochastic mixtures. An empirical factor has been obtained to account for three dimensional thermal effects. Comparisons to experimental data indicate that the modified correlation is generally accurate to within ±20% over a wide range of constituent materials.
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Thermal Conductivity of Packed Beds: A Review
Article
The voluminous literature available on thermal conductivities of packed beds without fluid flow is reviewed. The discussion includes experimental methods as well as theoretical approaches. A classification of predictive models is attempted. One of these models is analysed in detail and recommended for practical use. The predictions of the model are compared with a large amount of experimental data. In the course of this comparison phenomena of physical interest and of technical significance are discussed. Most of the methods presented in this review retain validity for dispersed systems other than gas-filled packed beds and for properties other than the thermal conductivity.ZusammenfassungDie umfangreiche Literatur über die Wärmeleitfähigkeit nicht durchströmter Festbetten wird zusammenfassend dargestellt. Experimentelle Methoden sowie auch theoretische Ansẗze werden diskutiert. Der Versuch einer klassifizierenden Darstellung der Berechnungsmodelle wird unternommen. Eins dieser Modelle wird eingehend analysiert und für den praktischen Gebrauch empfohlen. Modellrechungen werden mit einer groβen Anzahl experimenteller Daten verglichen. Im Rahmen dieses Vergleichs werden physikalisch interessante bzw. praktisch relevante Effekte diskutiert. Die meisten der in dieser Übersicht angegebenen Methoden können auf andere disperse Systeme (Suspensionen, Emulsionen, feste Dispersionen), sowie auch auf andere Eigenschaften angewendet werden.
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