Mat. Res. Soc. Symp. Proc. Vol. 672 © 2001 Materials Research Society
ELECTRON BEAM - DIRECTED VAPOR DEPOSITION OF MULTIFUNCTIONAL
STRUCTURES
D. T. QUEHEILLALT, Y. KATSUMI, H. N. G. WADLEY
University of Virginia, Department of Materials Science & Engineering, 116 Engineers Way
Charlottesville, Virginia, U.S.A. 22904-4745, [email protected]
ABSTRACT
Multifunctional structures are those that combine load bearing support in addition to supplemental functions such as
actuation, electrochemical energy storage or thermal management. Electron beam - directed vapor deposition (EBDVD) technology has been used for the deposition of templated cellular structures for micro heat-pipe structures and
porous electrode coatings for rechargeable nickel - metal hydride cells. In addition to load bearing support, the templated cellular structures exhibit enhanced thermal management characteristics and the electrochemical cells can be
integrated into the load bearing supports of linear and truss based structures leading to their multifunctionality. During EB-DVD, the electron beam evaporated vapor flux is encompassed by a rarefied transonic inert gas jet, entraining
the vapor in a non-reactive gas flow and transporting it onto a polymer or metal template structure. Here, EB-DVD
technology has been used to synthesize copper based templated cellular structures for thermal management systems
and porous nickel coatings for the positive electrode of rechargeable nickel - metal hydride cells.
INTRODUCTION
Cellular metals, either stochastic or periodic, exhibit property characteristics that suggest their
use as multifunctional structures. Multifunctional structures are engineered such that they incorporate structural performance along with an additional function such as electrochemical energy
storage, actuation, or thermal management [1]. Cellular metals in such applications are topology sensitive: that is, the functional properties are sensitive to the micro-architecture of the cells [2].
Stochastic open cell metal foams have a unique structure that enables their use as heat dissipation
media. Their attributes include the intrinsic high thermal conductivity of the material comprising
the foam, gas/fluid transport through the reticulated structure and hollow ligaments, which can
serve as micro heat exchangers. Periodic micro-truss based structures possess attributes which
enable them to serve as load bearing electrodes for nickel - metal hydride structural batteries.
Electron beam - physical vapor deposition (EB-PVD) is a widely used method for the highrate production of atomic and molecular vapor [3]. Normally, EB-PVD vapor is ballistically transported to a substrate under high vacuum conditions where it condenses on surfaces that are in the
line-of-sight of the flux source. However, by intersecting the vapor plume with a rarefied trans- or
supersonic inert gas jet, it is possible to entrain the evaporated flux in a non reacting gas flow and
transport it to a substrate under low vacuum (10-5 - 0.5 mbar) conditions, electron beam - directed
vapor deposition (EB-DVD) [4,5]. During EB-DVD deposition of the atomistic flux, transport
occurs by gas phase scattering from the stream lines of the flow. When the gas jet intersects an
object, the stream lines pass around the object. Scattering of the condensable flux from this flow
then enables non line-of-sight deposition. The degree to which blind surfaces can be coated is a
complicated function of the objects shape, the flow field near it, the composition of the flow, and
the collision cross section of the material to be deposited. As a result, there exists an optimal set of
process conditions that are best suited for each objects coating. These are controlled by the electron beam power and scan pattern on the metal source, the gas inlet nozzle flow pressure, and the
operating chamber pressure. Here, we explore the application of an EB-DVD approach for synthesizing open cell metal foams for compact heat exchangers and porous micro-truss structures
for nickel - metal hydride structural batteries.
O5.6.1
ELECTRON BEAM - DIRECTED VAPOR DEPOSITION
Plasma assisted EB-DVD integrates four key process technologies: sophisticated electron
beam evaporation; low vacuum, flowing gas vapor transport; high density gas and vapor plasma
activation and pulsed or constant substrate biasing. The EB-DVD system generates vapor by heating one or more surfaces with a state-of-the-art electron beam gun. The electron beam gun technology (maximum power = 10 kW, beam accelerating voltage = 70 kV) includes a high speed ebeam scanning system (up to 100 kHz) with a small beam spot size (<0.5 mm). A multi-pump
vacuum system allows operation at pressures ranging from high to low vacuum (10-5 - 0.5 mbar),
while a flowing gas vapor transport system (0 - 20 slm) allows for supersonic transport of both a
non-reacting carrier gas and/or a reactive carrier gas. The plasma activation is performed by a hollow-cathode plasma unit capable of producing a high density plasma in the system’s gas and
vapor stream, with an integrated substrate bias system capable of applying either a constant electrical potential to the substrate or an alternating, positive then negative, bias (0 - ±300 V).
STRUCTURE PLUS THERMAL MANAGEMENT
A conventional heat pipe consists of a sealed canister containing a “wicking” material, which
is evacuated and partially back-filled with a fluid (enough to saturate the wick). It functions
because of the existence of three distinct regions: an evaporator or heat addition region, a condenser or heat rejection region, and an adiabatic or isothermal region. The operating principles of
micro-heat pipes are essentially the same as those operating in larger, more conventional heat
pipes. Heat is applied to one end of the heat pipe, the fluid vaporizes in that region and forces it to
be transported to the cooler end where it condenses and gives up the latent heat of vaporization
[6]. This vaporization and condensation process causes the liquid-vapor interface in the liquid
arteries to change continually along the pipe and results in a capillary pressure difference between
the evaporator and condenser regions. This capillary pressure difference promotes the flow of the
working fluid from the condenser back to the evaporator through the triangular shaped corner
regions. These types of structures can increase the heat dissipation characteristics compared to a
conventional solid “finned” heat sinks made from materials of high thermal conductivity.
Here we explore using EB-DVD for the synthesis of copper based open cell, reticulated
micro-heat pipe structures, Fig. 1. For the deposition experiments reported here, parallel-piped
3
( 40 × 40 × 15 mm ) samples were used as the deposition template. They consisted of an open
cell, reticulated polyurethane foam with a pore size of ~1.25 mm (20 pores per inch). The polymer
foam was positioned 17.0 cm from the nozzle/vapor source. The polymer foam templates were
placed stationary in the vapor flux region to investigate the deposition and infiltration characteristics. Various deposition experiments were conducted using different combinations of electron
beam power, upstream and process chamber pressures and gas flow rates to investigate the resultant copper coating quality. At high chamber pressures (0.25 mbar) and gas flow rates (15.0 slm)
He, flux scattering from the stream lines resulted in thick coatings at the exterior of the sample.
Low chamber pressures (0.05 mbar) and gas flow rates (2.5 slm He) enhanced line-of-site coating
and incomplete coverage of ligaments. Intermediate chamber pressures of 0.1 mbar and a gas
flow rate of 7.5 slm resulted in the best ligament coating uniformity and infiltration. A post deposition, thermal decomposition treatment was used to remove the polymer template resulting in an
open cell, reticulated metal based foam possessing hollow ligaments, Fig. 2. The final operating
conditions selected for the deposition used an electron beam power = 4.2 kW, He gas flow = 7.5
slm, chamber pressure = 0.14 mbar, upstream nozzle pressure = 0.67 mbar.
O5.6.2
Cooling fluid
crossflow
Heat source
Reticulated Foam
with Hollow Ligaments
"Wicking"
liquid
High thermal
conducting material
(e.g. Al, Cu, AlN, SiC, etc.)
A schematic illustrations of an isotropic three-dimensional, reticulated foam micro-heat pipe
array used for the development of super thermal conducting materials.
Heat pipe
Figure 1
a) Copper foam
b)
~ 40 x 40 x 15 mm3
c)
e)
d)
100 µm
500 µm
Figure 2
5 mm
100 µm
Micrographs showing the structure of (a) an open cell, reticulated copper foam, (b) uniform
deposition characteristics and (c) - (e) hollow cusp shaped ligaments following thermal
decomposition of the parent polyurethane template.
O5.6.3
STRUCTURE PLUS ELECTROCHEMICAL STORAGE
In addition to the structural efficiency of periodic truss structures, they have the potential to be
manufactured with electrochemical storage capabilities. It has been identified that several variations of primarily nickel - metal hydride batteries, can be incorporated into lattice based microtruss structures for multifunctional applications. Vapor deposition of porous nickel surfaces, for
the positive electrode, when combined with the large surface areas of the lattice based truss structures, makes them ideal candidates for the integration of battery technology for multifunctional
applications. Two types of metallic alloys are generally used as the hydrogen storage metal alloy.
These are rare earth (Misch metal) alloys based around lanthanum nickel, known as the AB5 class
alloys, and alloys consisting of titanium and zirconium, known as the AB2 class of alloys [7]. In
both cases, some of the base metals are alloyed with other elements to achieve the desired energy
storage and charge retention characteristics.
Several electron beam - directed vapor deposition methodologies were investigated for the
deposition of porous nickel coatings used for the construction of nickel metal-hydride batteries,
Fig. 3. The two basic battery designs will incorporate the original metal lattice as either the anode
or cathode and the other electrode (with separator) inserted inside of the free space of the truss
structure. These structures can then be placed into a container and back filled with the active KOH
electrolyte to complete the battery construction. A minimum amount of electrolyte is used in this
sealed cell design, with most of the liquid absorbed by the separator and the electrodes.
For the deposition experiments reported here, both a corrugated linear nickel substrate and a
304 stainless steel micro-truss structure were used as the deposition templates, Fig. 4. Details for
the synthesis of the lattice based micro-truss structures can be found elsewhere [8]. The substrates
was positioned 17.0 cm from the nozzle/vapor source. Various deposition experiments were conducted using different combinations of electron beam power, upstream and process chamber pressures and gas flow rates to investigate the resultant nickel coating quality. The final operating
conditions selected for the deposition used an electron beam power = 2.1 kW, He gas flow = 5.0
and 7.5 slm. The chamber pressure and upstream nozzle pressure were 0.1 and 0.48 mbar for a gas
flow of 5.0 slm and 0.14 and 0.62 mbar for a gas flow of 7.5 slm. For the lattice based micro-truss
structures, nickel was deposited from one direction on the first sample and from opposite directions on the second sample to examine the infiltration characteristics of these structures.
Negative Electrode
Positive Electrode
e-beam
e-beam
Integrate components
Wrap electrodes
with separator
Vapor deposit
porous Ni
Vapor flux
(AB2 / AB5)
He
Vapor flux
(Ni)
He
Skull
melt
Skull
melt
Electrode
rod array
Ni
Figure 3
A schematic illustration of the EB-DVD processes used for the synthesis of the negative/positive
electrodes and the construction of a micro-truss based structural battery.
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a) linear structure
b) micro-truss structure
10 mm
10 mm
Figure 4
Photomicrographs showing the structure of (a) the corrugated nickel electrode and (b) the 304
stainless steel micro-truss based electrode for nickel - metal hydride structural batteries.
a)
b)
2 µm
Figure 5
2 µm
Micrographs showing the (a) the macrostructure and (b) - (d) the EB-DVD porous nickel coating
deposited on a corrugated nickel electrode structure.
b)
a)
2 µm
2 µm
d)
c)
2 µm
Figure 6
2 µm
Micrographs showing the EB-DVD porous nickel coatings deposited on 304 stainless steel microtruss structure for (a) one sided deposition and (b) two sided deposition.
O5.6.5
CONCLUSIONS
A multifunctional structure is one which combines load bearing support in addition to supplemental functions such as thermal management or electrochemical energy storage. Electron beam directed vapor deposition (EB-DVD) has been used to synthesize copper based templated cellular
structures with hollow ligaments for open cell, reticulated micro heat-pipe structures and porous
nickel coatings on linear and micro-truss based structures for the positive electrode of rechargeable nickel - metal hydride structural batteries. In addition, EB-DVD deposition conditions such
as electron beam power and rate of helium gas flow have been varied to evaluate the microstructural characteristics of the deposited copper and nickel films.
ACKNOWLEDGEMENTS
This work has been performed as part of the research of the Multidisciplinary University
Research Initiative (MURI) program on Ultralight Metal Structures conducted by a consortium of
Universities consisting of Harvard University, the Massachusetts Institute of Technology, the University of Virginia and Cambridge University led by Anthony Evans of Princeton University. We
are grateful for the many helpful discussions with our colleagues in these organizations. The consortium’s work has been supported by DARPA/DSO under contract N00014-96-I-1028 monitored
by Steve Wax (DARPA) and Steve Fishman (ONR).
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