Microscale Transport Phenomena in Materials Processing

[+] Author and Article Information
Yogesh Jaluria

Mechanical and Aerospace Engineering Department, Rutgers University, Piscataway, NJ 08854jaluria@jove.rutgers.edu

J. Heat Transfer 131(3), 033111 (Jan 26, 2009) (17 pages) doi:10.1115/1.3056576 History: Received August 03, 2008; Revised September 27, 2008; Published January 26, 2009

Microscale transport mechanisms play a critical role in the thermal processing of materials because changes in the structure and characteristics of the material largely occur at these or smaller length scales. The heat transfer and fluid flow considerations determine the properties of the final product, such as in a crystal drawn from silicon melt or a gel from the chemical conversion of a biopolymer. Also, a wide variety of material fabrication processes, such as the manufacture of optical glass fiber for telecommunications, fabrication of thin films by chemical vapor deposition, and surface coating, involve microscale length scales due to the requirements on the devices and applications for which they are intended. For example, hollow fibers, which are used for sensors and power delivery, typically need fairly precise microscale wall thicknesses and hole diameters for satisfactory operation. The basic transport mechanisms underlying these processes are discussed in this review paper. The importance of material characterization in accurate modeling and experimentation is brought out, along with the coupling between the process and the resulting properties such as uniformity, concentricity, and diameter. Of particular interest are thermally induced defects and other imperfections that may arise due to the transport phenomena involved at these microscale levels. Additional aspects such as surface tension, stability, and free surface characteristics that affect the material processing at microscale dimensions are also discussed. Some of the important methods to treat these problems and challenges are presented. Characteristic numerical and experimental results are discussed for a few important areas. The implications of such results in improving practical systems and processes, including enhanced process feasibility and product quality, are also discussed.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Sketches of a few thermal materials processing applications that involve microscale transport phenomena. (a) Chemical vapor deposition. (b) Optical fiber drawing. (c) Czochralski crystal growing.

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Figure 2

(a) Structure of a typical optical fiber; (b) sketch of a microstructured optical fiber

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Figure 3

Axisymmetric finite volume zones for the calculation of radiation in the glass preform and fiber

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Figure 4

(a) Iterative convergence of the neck-down profile in optical fiber drawing. Here, r∗=r/R and z∗=z/L, where R is the preform radius and L is the furnace length; (b) feasibility of the optical fiber drawing process, as indicated by regions where drawing is not possible due to viscous rupture resulting from excessive draw tension.

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Figure 5

(a) Calculated temperature field in the glass during drawing of a single-layer optical fiber for three furnace lengths; (b) calculated viscous dissipation and temperature contours in the optical fiber drawing process for typical drawing conditions

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Figure 6

Comparison of the numerical predictions of the neck-down profile with experimental results

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Figure 7

Numerical grid and calculated neck-down profiles for a preform with a core-cladding structure at two values of the refractive index of the outer layer

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Figure 8

(a) Isotherms for various GeO2 concentrations (solid line: pure silica; dashed line: 5.5 mol %GeO2; dashed dotted line: 11.1 mol %GeO2; dotted line: 16.6 mol %GeO2); (b) temperature variation along the centerline for various GeO2 concentrations

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Figure 9

Streamlines and isotherms in the furnace for a typical case of hollow fiber drawing with a parabolic furnace temperature distribution at a drawing speed of 10 m/s

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Figure 10

(a) Variation of collapse ratio along the axis with different pressurizations in the core and (b) with the drawing temperature for different drawing speeds

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Figure 11

(a) Neck-down profile corrections for an infeasible drawing case; (b) feasible domain for hollow fiber drawing in terms of the drawing speed and the drawing temperature

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Figure 12

(a) A typical optical fiber coating applicator; (b) instability and breakdown of the entrance meniscus in the microchannel inlet at high speeds

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Figure 13

Entrance flow in the microchannel inlet of annular gap thickness 103 μm at different pressures and draw speeds

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Figure 14

Meniscus for different imposed pressures, showing breakdown at low pressure

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Figure 15

(a) Dependence of fiber speed, for breakdown for a fiber entering a microchannel inlet, on the imposed pressure; (b) location of the meniscus in the microchannel as function of the pressure

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Figure 16

Calculated flow field in the applicator and exit microchannel, with a prescribed meniscus of height around 100 μm obtained experimentally

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Figure 17

Calculated and measured velocity distributions near the moving optical fiber

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Figure 18

Pressure distribution in the chamber and the exit die, which consists of a converging microchannel, for polymer coating of a moving fiber (a) under isothermal conditions and (b) when thermal effects are included for a fiber speed of 11 m/s

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Figure 19

(a) A sketch of an impingement type CVD reactor; (b) comparison between numerical predictions and experiments for chemical vapor deposition of silicon in a horizontal reactor

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Figure 20

A schematic indicating the mechanisms underlying starch conversion

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Figure 21

(a) Conversion of starch in a tapered screw extruder channel; (b) feasible domain for twin-screw extrusion of starch

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Figure 22

(a) Dependence of average concentration of E′ defects on furnace wall temperature; (b) luminescence versus wavelength of fibers drawn at 2050°C and 80 m/min, indicating concentration of E′ defects

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Figure 23

(a) Neck-down profiles for various GeO2 concentrations; (b) concentration of E′ defects along the centerline for various GeO2 concentrations

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Figure 24

(a) Response surface on deposition rate for an impingement CVD reactor for depositing silicon; (b) variation of deposition rate with location for different inlet velocities and susceptor temperatures



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