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Research Papers: Porous Media

Simulated Microstructural Evolution and Design of Porous Sintered Wicks

[+] Author and Article Information
Karthik K. Bodla

Cooling Technologies Research Center,
an NSF IUCRC,
School of Mechanical Engineering and
Birck Nanotechnology Center,
Purdue University,
West Lafayette, IN 47907

Suresh V. Garimella

Cooling Technologies Research Center,
an NSF IUCRC,
School of Mechanical Engineering and
Birck Nanotechnology Center,
Purdue University,
West Lafayette, IN 47907
e-mail: sureshg@purdue.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received August 12, 2013; final manuscript received February 25, 2014; published online March 26, 2014. Assoc. Editor: Bruce L. Drolen.

J. Heat Transfer 136(7), 072601 (Mar 26, 2014) (10 pages) Paper No: HT-13-1410; doi: 10.1115/1.4026969 History: Received August 12, 2013; Revised February 25, 2014

Porous structures formed by sintering of powders, which involves material-bonding under the application of heat, are commonly employed as capillary wicks in two-phase heat transport devices such as heat pipes. These sintered wicks are often fabricated in an ad hoc manner, and their microstructure is not optimized for fluid and thermal performance. Understanding the role of sintering kinetics—and the resulting microstructural evolution—on wick transport properties is important for fabrication of structures with optimal performance. A cellular automaton model is developed in this work for predicting microstructural evolution during sintering. The model, which determines mass transport during sintering based on curvature gradients in digital images, is first verified against benchmark cases, such as the evolution of a square shape into an area-preserving circle. The model is then employed to predict the sintering dynamics of a side-by-side, two-particle configuration conventionally used for the study of sintering. Results from previously published studies on sintering of cylindrical wires are used for validation. Randomly packed multiparticle configurations are then considered in two and three dimensions. Sintering kinetics are described by the relative change in overall surface area of the compact compared to the initial random packing. The effect of sintering parameters, particle size, and porosity on fundamental transport properties, viz., effective thermal conductivity and permeability, is analyzed. The effective thermal conductivity increases monotonically as either the sintering time or temperature is increased. Permeability is observed to increase with particle size and porosity. As sintering progresses, the slight increase observed in the permeability of the microstructure is attributed to a reduction in the surface area.

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Figures

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Fig. 1

Microstructure of a sintered wick: (a) a 2D cross section obtained via X-ray microtomography, and (b) corresponding 3D reconstructed microstructure, shown for a 250–355 μm particle size wick from Ref. [9]

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Fig. 2

The workflow employed in the current work for estimating transport characteristics for a wick with user-defined particle size distribution is shown

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Fig. 3

Microstructure evolution of a square to an area-preserved circle, shown for a square with 30 × 30 pixels

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Fig. 4

Sintering kinetics for a two-cylinder sintering scenario: (a) number of iterations required to reach a constant neck size, and (b) relative neck size growth as a function of sintering iterations for various values of n. Also shown in (c) is the typical power law behavior of neck size growth, as observed in experiments and predicted by theory (Eq. (1)).

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Fig. 5

Effective thermal conductivity calculation for the two-cylinder sintering scenario: (a) problem setup, and (b) computed values for initial and intermediate stages

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Fig. 6

(a) Microstructure evolution and comparison with phase-field simulations of [15], (b) predictions of effective thermal conductivity, and (c) sintering kinetics for a random 2D circular particle bed

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Fig. 7

Effect of particle size on sintering of random, 3D spherical powder compacts: (a) microstructure evolution for sample made up of 170 μm particles, and (b) effective thermal conductivity as a function of change in surface area relative to the initial microstructure

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Fig. 8

Convergence history of the 3D finite difference code from Ref. [25] employed in this work for calculating effective thermal conductivity of 3D microstructures directly. The solution may be assumed converged after approximately 100 iterations for all cases considered in this study.

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Fig. 9

Effect of porosity on sintering of random, 3D spherical powder compacts: (a) microstructure evolution for the case of 57% porosity, and (b) effective thermal conductivity as a function of relative change in surface area

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