SUMAN DAS
RESEARCH INTERESTS
My research interests cover a broad
variety of interdisciplinary topics under the overall
framework of computational design, manufacturing and
materials processing, and materials science. In all of my
research endeavors and collaborations, I strive to
integrate these three elements together to achieve new
fundamental scientific insights, to produce groundbreaking,
high-impact results, and to create innovative and
disruptive manufacturing technologies. The pursuit of these
goals requires the synthesis of diverse disciplines of
science and engineering. These include: (1) the design of
manufacturing technologies and their implementation through
the construction of machines for their physical
realization; (2) the selection, handling, processing,
synthesis, preparation and characterization of materials;
(3) computational modeling, computational geometry, data
structures and digital data processing algorithms; (4)
optics, lasers, light sources, and light-matter
interactions; (5) feedback control systems; (6) materials
analysis, testing, property characterization and theory
validation. Overall, these research efforts are directed
towards applications in the Aerospace, Healthcare, Energy,
and Nanotechnology sectors. Figure 1 below describes the
defining research themes of my research group.
Fig. 1. Defining
Research Themes in Georgia Tech's Direct Digital
Manufacturing Laboratory
Some of the notable results achieved and future research
plans in each of these areas are described below.
1.
AEROSPACE
1.1 Direct Digital
Manufacturing of Airfoils via Large Area Maskless
Photopolymerization
Turbine airfoils
with extremely complex internal cooling passages are
produced through investment casting. The exterior airfoil
shapes are defined by injection-molded wax patterns that
are removed or “lost” after shelling. The internal cooling
passages are defined by injection-molded ceramic cores that
are removed or “lost” after casting. The core and wax
molding operations require sophisticated tooling, leading
to excessive initial and maintenance costs, very slow
fabrication cycles, and low casting yields. Despite the
maturity of current investment casting practices in the
aerospace industry, a major challenge exists in the
affordable, high-yield, production of cooled, single
crystal nickel-superalloy airfoils for turbine engines.
While many improvements in the performance of designs have
been made, no significant improvements have been made to
lower the cost of manufacturing turbine airfoils.
Large Area Maskless Photopolymerization (LAMP) is a rapid
manufacturing technology that is being developed at Georgia
Tech with support from DARPA’s Disruptive Manufacturing
Technologies program in a project titled “Direct Digital
Manufacturing of Airfoils”. LAMP technology directly
produces complex ceramic molds with integral cores for
airfoil investment casting from CAD files by selectively
curing ceramic-loaded photocurable resins layer-by-layer.
Layers are cured through the continuous projection of
high-resolution bitmaps of UV light onto the resin in a
scrolling and seamless manner, allowing for mass
production. The molds are then thermally post-processed to
a foundry-ready state for casting. Figure 2 shows the
overall technical approach. LAMP eliminates 7 major process
steps and nearly all the tooling, handling, and associated
causes for scrap in the investment casting process, leading
to dramatic reductions in lead-time, scrap rate, and
ultimately cost. LAMP thus disrupts the current
state-of-the-art investment casting process for
manufacturing superalloy airfoils by disrupting not only
the cost structure of conventional investment castings, but
also the speed with which components can be fabricated.
Fig. 2. Technical
Approach to Direct Digital Manufacturing of Airfoils
through LAMP technology.
My group at Georgia Tech has designed and built a LAMP
Alpha machine and has worked closely with the group of
Prof. John Halloran at the University of Michigan to
develop both the materials and the process technology. To
date, both uncored and integral-cored ceramic molds have
been built. Equiaxed and single-crystal airfoil castings in
the superalloys Inconel 718 and SC180 have been cast using
these molds at PCC Airfoils Prototype Foundry in Minerva,
Ohio. Figure 3 shows integral cored molds built in cutaway
section, uncored molds, and uncored airfoil castings
produced using these molds. These results illustrate the
pathway that LAMP technology development is taking towards
a disruptive manufacturing technology for producing cored
single-crystal airfoils in a fraction of the time and at a
significantly reduced cost when compared to the
conventional investment casting process.
Fig. 3. Cut-away
section of cored airfoil molds (left), uncored airfoil
molds (middle), and uncored airfoil castings in IN718
superalloy produced through LAMP.
1.2 Future Research:
Cyber-Enabled Manufacturing of Advanced Castings (CyMAC)
The next step in
the evolution of the direct digital manufacturing of
investment castings is the cyber-enabled manufacturing of
advanced castings (CyMAC). Through CyMAC, my vision is to
revolutionize the design, prototyping, rapid manufacturing,
and rapid qualification of all investment castings made in
the United States. LAMP technology has already demonstrated
the potential to disrupt the state-of-the-art of investment
casting through dramatic increases in the speed of ceramic
mold manufacturing and will serve as the key enabler of
this next revolution. I aim to accelerate the creation of a
new and sustainable digital manufacturing paradigm that
will pervasively reduce costs and lead times associated
with all legacy and new investment castings. While
continuing the research on next-generation LAMP
technologies that promise further increases in build speed
by up to 50X and revolutionary new capability to produce
components with advanced designs considered conventionally
non-manufacturable, I will pursue an integrated and
synergistic technical approach that will combine the
following four elements: Design, Manufacturing and Analysis
Software Tools (CAx), Rapid Manufacturing Technologies,
Rapid Qualification, and a Manufacturing Demonstration
Facility for CyMAC. CyMAC will open up the design space and
will enable the rapid manufacturing of advanced designs
with complex geometries previously considered
non-manufacturable. Figure 4 shows a schematic of CyMAC’s
technical approach.
Fig. 4. Technical
Approach to Cyber-Enabled Manufacturing of Advanced
Castings (CyMAC).
1.3 Direct Digital
Manufacturing of High Value Components via Scanning Laser
Epitaxy
Fabrication
techniques for making nickel-base superalloy components,
particularly turbine blades, have evolved over the last
half century from equiaxed casting to directionally
solidified (DS) casting and more recently to single crystal
(SX) casting with each improved technique resulting in
higher operating temperatures. Both DS and SX casting are
considered exotic techniques and have significant cost
associated with hardware production. For example, SX
turbine blades made using some of the most modern
nickel-base superalloys such as CMSX-4 can cost several
hundreds to thousands of dollars per piece involving
several iterations of prototyping and tooling development
that consume several months to years before final
production. Due to the extreme conditions under which such
engines operate, turbine components undergo loss of
material due to oxidation and subsequent spallation.
Currently employed techniques to rebuild lost material have
several drawbacks: (a) the build-up is often limited to a
thin layer of polycrystalline material, therefore it does
not preserve the directionally solidified (DS) or
single-crystal (SX) microstructure of the base material
that has been eroded away; (b) the material used for
build-up is often of a different, lower performance alloy
composition because the nickel-base superalloy comprising
the hardware to be repaired is non-weldable or
non-joinable; and (c) Lack of adequate thermal control
leads of cracking and the formation of misoriented grains
due to the fact that superalloys are susceptible to
strain-age cracking and liquation cracking. Due to the high
costs associated with making production quality hardware,
turbine engine manufacturers are investigating ways to: (a)
develop techniques to repair and restore damaged equiaxed,
DS, and SX superalloy components to pristine condition, and
(b) produce small lots of functional parts for prototyping
and testing. Figure 5 below describes my group's technical
approach combining physical modeling, experimentation, and
characterization to achieve region-specific microstructure
control in additively manufactured and repaired nickel-base
superalloy components for turbine engines.
Fig. 5. Technical
Approach to Direct Digital Manufacturing of
Heterogeneous Multifunctional Components (HMCs)
through Scanning Laser Epitaxy (SLE).
My group has designed, developed and
implemented a workstation for conducting scanning laser
epitaxy (SLE), a laser materials processing technique
developed as part of ongoing research sponsored by the
Office of Naval Research. This workstation is shown below
in Figure 6. The workstation includes a continuous wave
Nd:YAG laser, controlled atmosphere process chamber with
substrate fixture and thermocouple array, galvanometer
driven scanner and optics, and a process control cabinet.
We have recently added a Ytterbium fiber laser to replace
an older Nd:YAG laser, a high resolution thermal imaging
camera, a video microscope for melt pool imaging. Work is
ongoing to implement a real-time adaptive feedback control
system for melt-pool temperature control.
Fig. 6. Materials
processing workstation for the SLE process developed
with ONR support.
My group has demonstrated the ability to successfully
produce fully dense, crack-free and defect-free monolithic
deposits from nickel superalloy powders on substrates of
the same composition in both polycrystalline MARM-247 and
single-crystal CMSX-4 superalloys through SLE. A thorough
investigation of SLE process parameters including laser
power and scan speed has been conducted to determine their
influence on the rebuilt microstructure and the degree of
melt-back in the parent substrate. In the CMSX-4 single
crystal deposits, the primary dendrite arm spacing is
approximately 10-30 times finer than that of the parent
substrate (shown in Figure 5). Particularly in the case of
single-crystal components, such a fine microstructure is
unattainable in components manufactured through traditional
investment casting. The benefit of such a fine
microstructure is that the overall component is generally
more homogenous from the standpoint of the distribution of
the alloying species and should thus result in superior
performance and properties over that of the “as cast”
component. Figures 7 and 8 below show representative
specimens on MARM-247 and CMSX-4 deposited through SLE
processing.
Fig. 7.
Experimental Results on Scanning Laser Epitaxy of
MARM-247 and CMSX-4.
Fig. 8. Optical
microscopy and electron-backscattered diffraction
images of the longitudinal cross-section of a typical
SLE-processed CMSX-4 sample. The coarser
microstructure in the lower half is that of the
investment-cast CMSX-4 plate substrate. The highly
refined microstructure in the upper half is that of
the laser-melted and resolidified CMSX-4 powder
forming a fully dense deposit with a microstructure
derived from epitaxial solidification. The majority of
CMSX-4 deposited through SLE has [001] crystal
orientation consistent with the investment-cast
substrate. Some stray grains are present. The top of
the deposit is encrusted with an equiaxed region due
to high cooling rates and loss of thermal gradient.
Further investigations are ongoing to fully understand the
origins of microstructure formation in SLE and its linkage
to processing parameters. Further technological efforts are
also ongoing in the areas of robust feedback control, SLE
equipment design, and process automation to transition the
SLE process to the DoD and its suppliers. The primary areas
of investigation and development include: 1) Solidification
modeling to obtain a deeper fundamental understanding of
the mechanisms contributing to the microstructural
transitions (e.g. columnar to equiaxed, oriented to
misoriented). 2) Enhanced thermal control over the
workpiece utilizing temperature distribution measurements
from an infrared thermal imaging camera and an instrumented
thermocouple array, coupled with knowledge of the
mechanisms of microstructure formation, leading to better
process control. 3) Utilization of electron microscopy
tools (EBSD and TEM) to investigate grain orientation as a
function of process parameters and the observation of gamma
prime precipitates to further optimize the SLE process. 4)
Development of mature SLE manufacturing process, equipment,
and documentation for technology transition to DoD
suppliers and repair facilities, with process knowledge
tailored to the components and alloys of interest.
During my doctoral and post-doctoral work at UT, I designed
and built two machines and was awarded two patents on
direct laser fabrication techniques for making functional,
fully dense components in high performance materials
including nickel superalloys, titanium alloys, and
superalloy cermets. Components produced on these machines
were shown to exhibit equivalent or superior
microstructures, chemistries, and mechanical properties in
comparison to conventionally cast materials. Figure 9 shows
examples of produced components: a titanium alloy
(Ti-6Al-4V) guidance section housing for the Sidewinder
missile and a MARM-247 superalloy-cermet abrasive turbine
blade tip for a next generation fighter aircraft engine.
Interestingly, the additive manufacturing of metal
components through the layer-by-layer melting of metal
powders using powerful scanning lasers has once again
become an area of strong commercial interest, as all of the
aerospace and industrial gas turbine companies strive to
speed up the prototype, test and development cycle for new,
higher efficiency engines. Thus, with funding from the
Office of Naval Research and PCC Airfoils, my group is
initiating a research program to investigate the processing
of nickel-base superalloys to produce functional fully
dense components such as turbine airfoils through SLE
conducted layer-by-layer.
Fig. 9. (a) A
titanium alloy Ti-6Al-4V guidance section housing for
the AIM-9 Sidewinder missile, (b) Microstructure of
fully dense Ti-6Al-4V, (c) MARM-247/CBN/Alumina
superalloy cermet abrasive turbine blade tip, (d)
Microstructure of fully dense cermet with uniform
dispersion of CBN and Alumina particles in MARM-247
superalloy matrix.
1.4 Future Research:
Cyber-Enabled Direct Digital Manufacturing of Heterogeneous
Multifunctional Components (DDM of HMCs)
The next step in
the evolution of SLE is the direct digital manufacturing of
heterogeneous multifunctional components (HMCs) wherein
material, microstructural, and property heterogeneity
enables the performance of at least one function but more
often multiple functions in addition to structural
integrity. Multifunctional performance through
heterogeneity can be imparted to a component through graded
microstructure, graded composition, graded physical and
chemical properties, or a combination of the above
attributes. An example of a HMC is a jet engine turbine
blade made of a nickel base superalloy with equiaxed,
directionally solidified, or single-crystal microstructure
having a ceramic thermal barrier coating (TBC) covering the
airfoil surfaces and a ceramic-metal matrix composite
material on the airfoil tip to serve as an abrasion
resistant and oxidation resistant coating. A second example
of a HMC is a turbine disk made of a monolithic superalloy
composition with equiaxed microstructure but with radially
graded grain sizes, with smaller grains at the hub enabling
better tensile properties and larger grains towards the rim
to enable superior creep strength and resistance to fatigue
crack growth. A third example is a turbine disk made of two
distinct superalloys and with a radial compositional
gradient. A fourth example of a HMC is an integrally bladed
rotor (IBR) with single crystal airfoils metallurgically
bonded to a disc with equiaxed microstructure. A fifth and
final example is a turbine blade with primarily
single-crystal superalloy microstructure throughout the
airfoil section but with a dual alloy attachment section,
with a portion of the attachment section made of a
polycrystalline superalloy to provide superior low cycle
fatigue life at the attachment section. As indicated
through the five examples cited above, transitions in
microstructure, texture, composition, and properties in a
HMC can be engineered over relatively small or large length
scales compared to characteristic dimensions of the part,
depending upon the functional gradient characteristics
desired in a particular application. The direct digital
printing of dry powders and well-dispersed nanomaterials to
form smooth layers with spatially patterned heterogeneous
materials followed by their consolidation through laser
melting (DDM of HMCs) has the potential to create 3-D HMCs
with entirely new functionality and characteristics that
can meet or exceed the performance requirements being
demanded of next-generation advanced high-performance
materials. I plan to pursue research in the DDM of HMCs
over the next several years.
2.
HEALTHCARE
2.1 Tissue Engineering
and Regenerative Medicine
Tissue
engineering is an interdisciplinary field that combines
engineering and the life sciences to develop techniques
that enable the restoration, maintenance, or enhancement of
living tissues and organs. A majority of these techniques
utilize three-dimensional scaffold structures composed of
natural or synthetic bioresorbable polymers. These scaffold
structures are endowed with complex internal architecture,
channels and porosity that provide sites for cell
attachment and proliferation, as well as for conveying
cells, growth factors and biomolecular signals to promote
tissue regeneration at an implantation site. Tissue
engineering has the potential to facilitate breakthrough
therapies for repair of whole joints and organs. However,
present fabrication methods have limited control over the
geometry and porosity of scaffolds. In order to reconstruct
complex joints such as the temporomandibular joint (TMJ),
novel fabrication methods are needed to build complex,
three-dimensional anatomically shaped scaffolds
incorporating multiple biomaterials and porosity gradients
that can enable the simultaneous growth and regeneration of
multiple tissues (bone, ligament, cartilage or tendon),
tissue interfaces and blood vessels.
My research in tissue engineering centers on devising novel
additive manufacturing methods that can construct tissue
engineering scaffolds with functionally tailored
characteristics, notably geometry, material composition,
and porosity. To date, we have investigated selective laser
sintering to fabricate scaffolds in nylon-6,
polycaprolactone (PCL), and polycaprolactone/hydroxyapatite
composites. Figure 10 shows the technical approach to
creating microarchitectured tissue engineering scaffolds
using selective laser sintering of polymers and
polymer-ceramic composites.
Fig. 10. Technical
Approach to Digital Manufacturing of Tissue
Engineering Scaffolds Through Selective Laser
Sintering.
Figures 11-13 below illustrate some of the tissue
engineering scaffolds that have been constructed and tested
to date. Our investigations aim to design customized
scaffolds by combining computational design techniques,
patient specific computed tomography (CT) or magnetic
resonance imaging (MRI) data and surgeon inputs, and then
to fabricate the scaffolds using additive manufacturing
techniques. We have successfully demonstrated in vivo bone
regeneration on polycaprolactone scaffolds built by
selective laser sintering combined with a gene therapy
approach. Our future investigations aim to design, build
and test multifunctional scaffolds for whole joints
structures incorporating a bone scaffold, combined with a
cartilage scaffold, and a ligament scaffold with different
functional architectures and compositions, and seeded as
needed with gradients of biofactors.
Fig. 11. (a) An
8mm cubic cellular scaffold design, (b) An 8mm
diameter, 6mm high cylindrical cellular scaffold
design (both with 800μm orthogonal channels and 1200μm
pillars), (c) and (d) Nylon-6 scaffolds fabricated by
SLS (scale in mm), (e) Biomimetic scaffold design
derived from human proximal femur trabecular bone
micro-CT data, and (f) Biomimetic scaffold fabricated
in Nylon-6 by SLS.
Fig. 12. (a)
Implantation of cylindrical Nylon-6 scaffold into a
Yucatan minipig mandible, (b) Volumetric rendering of
scaffold micro-CT after 6 weeks showing bone ingrowth,
(c) A cylindrical scaffold design with periodic
interconnected porous architecture (12.5 mm diameter,
25.4 mm length, 2 mm pores) for mechanical tests, (d)
Corresponding scaffold built in polycaprolactone by
SLS (e) A scaffold design (5.0 mm diameter, 4.5 mm
height, 1.5mm diameter pore) loaded with BMP-7 and
implanted in immunocompromised mice to test bone
regeneration, and (f) Micro-CT image of corresponding
polycaprolactone scaffold removed after 4 weeks
showing significant amounts of newly generated bone
penetrating and encapsulating the scaffold.
Fig. 13. 3D
renditions and actual SLS processed PCL scaffolds. Top
row: (a) a minipig mandibular condyle scaffold design,
(b) & (c) two different human condyle scaffold
designs incorporating 700 micron struts and 1.1-2mm
pores. Bottom rows: Rectangular and cylindrical
microarchitectured scaffolds for tensile and
compressive mechanical property measurements as a
function of scaffold pore architecture. Bottom far
right: A micro-computed tomography reconstruction of a
cylindrical scaffold for non-desctuctive visualization
of interior architecture.
Figures 14 and
15 below illustrate some of our results on computational
design and digital manufacturing of anatomically shaped
microarchitectured scaffolds
Fig. 14.
Comparison between computationally predicted and
experimentally measured mechanical properties of
monolithic PCL scaffolds.
Fig. 15.
Comparison between computationally predicted and
experimentally measured mechanical properties of
PCL/TCP scaffolds.
Figure 16
illustrates the applications and the impact of tissue
engineering scaffolds with external anatomic shapes and
internal microarchitecture with reference to specific
example pertaining to repair and reconstruction of the
temporo-mandibular joint.
Fig.
16. Applications and impact of tissue engineering scaffolds
with external anatomic shapes and internal micro
architecture manufactured through selective laser
sintering.
2.2 Future Research:
Scaffolds with Encapsulated Cells for Cartilage Tissue
Engineering
We aim to use
LAMP technology to build hydrogel scaffolds with embedded
microfluidic networks and encapsulated cells for cartilage
tissue engineering. Previous attempts at the fabrication of
large cartilage constructs have been plagued by poor
nutrient transport to the interior of the tissue construct
resulting in poor tissue growth and necrosis of the
embedded cells. A microfluidic network embedded within the
bulk of the construct may be able to meet the transport
requirements of the interior cells but conventional
manufacturing methods for scaffolds are—in most
cases—limited to simple geometries and large feature sizes.
LAMP is a cost-efficient, scalable technique for the
fabrication of cartilage tissue scaffolds with a
computationally designed microarchitecture. To fully
realize its potential, the following research tasks will be
pursued: 1) Systematic studies linking the hydrogel
material composition and LAMP processing parameters to
spatial resolution and material properties. 2) Development
of computational design methods for cartilage tissue
constructs taking into consideration mechanical and
transport requirements. 3) Experimental characterization
and validation of specimens made by computational design
and digital manufacturing. 4) Implantation and testing of
scaffolds in animal models.
Fig. 17.
Fabrication of PEG-DA hydrogel scaffolds for cartilage
tissue engineering using LAMP
technology.
3. ENERGY
3.1 Fuel Cells
Concern for
energy conservation and for renewable energy sources has
renewed interest in design and fabrication of fuel cells,
micro-batteries, micro-combustors, micro-chemical reactors,
, micro-channel based heat exchangers, solvent extractors,
gas adsorption media, thermoelectric power devices, and
catalytic structures. These devices have the potential for
improved efficiency, energy density, compactness,
portability and the integration of multiple functions. Each
could portray significant commercial, military, societal
and environmental value. With appropriate additive
manufacturing methods, such devices could be fabricated
with locally tailored material compositions optimized with
respect to thermal, fluid, optical, chemical reaction or
catalytic performance considerations.
My research in the area of fuel cells has focused on
investigating novel methods for prototyping and
manufacturing fuel cell components. In particular, my
collaboration with Prof. Pravansu Mohanty (ME Department,
UM-Dearborn) led to low cost rapid prototyping and rapid
manufacturing techniques to make components for proton
exchange membrane fuel cells (PEMFC). In a hybrid two-step
process combining rapid prototyping and thermal spray
deposition, first 3D printing and selective laser sintering
techniques were used to produce low cost ceramic patterns
with complex channel architectures. Second, thermal spray
deposition techniques were used to deposit multiple
materials layer-by-layer onto the sacrificial patterns for
making functionally graded metallic bipolar plates for
PEMFCs with improved corrosion resistance and electrical
conductivity. Shown in figure 18(a) is the 3D CAD model of
a pattern for a bipolar plate. Figure 18(b) is the actual
pattern made in gypsum by 3D printing. Figure 10(c) shows
the actual metallic bipolar plate with a Ni-Al to
Molybdenum functional gradient, produced by thermal spray
deposition onto the 3DP pattern.
Fig. 18. (a) 3D
CAD model of bipolar plate pattern, (b) Actual gypsum
pattern made by 3D printing, and (c) Actual bipolar
plate produced by thermal spray deposition of graded
multilayers of fully dense Ni-Al and Molybdenum. The
plate is 95mm square, with a 1mm square serpentine
channel, 2.9mm inter-channel spacing and 3mm diameter
gas inlet holes at each end.
Solid oxide fuel cells (SOFCs) have the highest power
density, and are thus considered the most energy efficient
power generation devices amongst all fuel cells. SOFC
electrodes must be adequately porous to enable rapid mass
transport of reactant and product gases to and from the
triple phase boundary, and sufficiently conductive to
perform efficient electron transfer. State-of-the-art SOFCs
cannot adequately address these conflicting requirements
due to limited microarchitecture control achieved through
conventional manufacturing techniques. SOFC performance
could be greatly improved with computationally designed
microarchitectures that simultaneously optimize mass
transport and electron transfer. Comprehensive macroscopic
multiphysics modeling of SOFC unit cells endowed with such
microarchitectures based on the LSM-YSZ-NiYSZ
electrochemistry promise to increase the current generation
efficiency by up to 20% over the state-of-the-art.
My group has conducted computational design optimizations
utilizing multiphysics simulations in COMSOL® software
using published parameters. These simulations capture the
underlying physics and chemistry of fuel and oxidant gas
mass transport through the electrodes, electrochemical
charge transfer reactions at the triple-phase boundary,
electronic conduction through the electrodes and the
interconnects, and ionic conduction through the electrolyte
with the goal of optimizing electrode microarchitecture for
simultaneous enhancement of electronic conductivity and
mass transport. Unit cells of anode-supported SOFCs of type
NiYSZ-YSZ-LSM with designed microarchitectures of porous
channels of different sizes, geometries, and array
distributions have been computationally tested and compared
with a baseline model based on standard planar SOFC
geometry.
A typical planar SOFC architecture is shown in figure
19(a). Due to considerations of computational complexity,
we restricted our modeling effort to a unit cell shown in
figure 19(b).
Fig. 19. (a)
Planar SOFC architecture, and (b) unit cell used in
the SOFC computational design.
Shown in figure 20 is the baseline unit cell, and unit
cells of computationally optimized SOFCs with porous array
microarchitectures in the anode, cathode, and cathode
interconnect. The unit cell is 1mm x 1mm (l) x 1mm (w) x
1.06mm (h) with 1mm thick NiYSZ anode, 10µm thick YSZ
electrolyte, and 50µm thick LSM cathode layers. The
interconnects are 250µm wide, 1mm long and several mm
thick. In these particular designs, the anode side has a
vertical array of cylindrical pores terminating close to
the anode-electrolyte interface, and the cathode side has a
horizontal rectangular pore channel at the
cathode-interconnect interface.
Fig. 20.
Computationally designed unit cells with (a) single
pore, (b) two by two pore array, and (c) three by
three pore array in the anode layer, and a horizontal
pore channel at the cathode/interconnect interface
Shown in Figure 21 is a SOFC button cell endowed with a
porous micro architecture in the anode layer obtained
through femtosecond laser ablation.
Fig. 21. (a) A
10mm SOFC button cell endowed with porous
microarchitecture in the anode layer through hole
drilling using femtosecond laser ablation. (b) A
top-view SEM image of the hole drilled in the anode
layer. (c) A SEM image of the fracture surface through
the holes, showing the depth and profile of drilled
holes.
3.2 Future Research:
Direct Digital Manufacturing of Computationally Optimized
SOFCs
Future research
efforts will be directed towards prototyping and scale-up
manufacturing to make commercial size SOFC plates with
sizes on the order of 10cm x 10cm. Microarchitectured SOFC
designs provide opportunities for further improvement
through a hybrid approach involving incorporation of
nanoparticles or thin films of catalysts inside our
designed porous channels. Such an approach has the
potential for simultaneously increasing reactant gas mass
transport through the electrodes and overall surface area
of catalyst thereby increasing the number of sites for
electrochemical reaction, and the reaction rate.
4.
NANOTECHNOLOGY
4.1 Large-Area Maskless
Nanofabrication Through Laser Interference Patterning
Progress in
nanotechnology requires versatile nanostructuring
techniques capable of patterning diverse materials
including those not traditionally allowed in cleanrooms.
Large-area maskless nanofabrication techniques that achieve
patterning through photochemical or photophysical means can
dramatically reduce or even eliminate the high costs and
long processing times associated with cleanroom technology.
Such techniques can directly produce nanostructures on
functional materials or can produce templates for
subsequent synthesis or growth of nanostructured materials.
My group has conducted research on one group of such
techniques, known as laser interference patterning (LIP),
which is an excellent technique for producing periodic
micro- and nanostructures over large areas. A schematic of
a LIP experimental setup is shown in Figure 22. As shown on
the right of this figure, two-beam interference leads to
periodic line-like intensity distribution while three-beam
interference leads to an intensity distribution resembling
periodic peaks and valleys.
Fig. 22. Schematic
setup for a three-beam laser interference system:
nanosecond pulsed Nd:YAG laser with wavelengths of
1064nm, 532nm, 355nm, and 266nm (1) lens, (2)
beam-splitters, (3) mirrors, (4) sample. The simulated
intensity distributions of two-beam and three-beam
interference are shown on the right.
When LIP is used for patterning photosensitive materials
such as photoresists, it is known as laser interference
lithography (LIL). When used for melting, it is termed
laser interference melting (LIM), and when it used for
ablation, it is known as laser interference ablation (LIA).
My group has investigated all the above three modes of LIP
to pattern photoresists, semiconductors, and metals to
create diverse nanostructures. Some of the key examples are
shown in figures 23-25 below.
Fig. 23. (A) LIL
of pentaerythritol triacrylate (PETIA) with three-beam
interference. (B) LIL of polyethylene glycol
diacrylate (PEGDA) hydrogel.
Fig. 24. Periodic
ZnO nanowires grown through periodic holes produced in
a photoresist through LIL.
Fig. 25. Periodic
bundles of carbon nanotubes grown on islands of iron
catalyst patterned through LIA.
4.2
Future Research: Large Area Multiscale and Hierarchical
Maskless Micro- and Nanomanufacturing
Future advances
in nanotechnology will require the ability to pattern and
process materials at the micro- and nanoscale over large
areas rapidly, without masks in non-cleanroom type of
setting. These advances will also require the capability of
patterning hierarchical multiscale structures in diverse
materials. This will require the design of sophisticated
processes and material systems. In the future, I aim to
investigate hybrid approaches that combine UV
photopatterning, laser interference patterning, and
block-copolymer lithography to create a unique methodology
for generating hierarchical micro- and nanostructures with
dual and triple length scale feature size and periodicity
over very large areas, from wafer-scale up to areas
approaching those for roll-to-roll processing.