Note: Descriptions are shown in the official language in which they were submitted.
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COMPLIANT, STRAIN TOLERANT INTERCONNECTS
FOR SOLID OXIDE FUEL CELL STACK
CROSS-REFERENCE TO RELATED CASES
[0001] This application claims the benefit of earlier-filed provisional patent
applications
entitled, "Compliant, Strain Tolerant Interconnects and Seals for Solid Oxide
Fuel Cell
Stack," Application No. 60/444,025, filed January 31, 2003, and "Compliant
Interconnects and Seals for Solid Oxide Fuel Cell Stack," Application No.
60/454,899,
filed March 14, 2003, and is a continuation-in-part of U.S. Application No.
10/307,008,
Filed November 27, 2002.
BACKGROUND OF THE INVENTION
[0002] The invention relates to solid oxide fuel cell (SOFC) stacks and, more
particularly, to an interconnect that enhances the lifetime of SOFC stacks.
[0003] A fuel cell is a device which electrochemically reacts a fuel with an
oxidant to
generate a direct current. The fuel cell typically includes a cathode, an
electrolyte and an
anode, with the electrolyte being a non-porous material positioned between the
cathode
and anode materials. In order to achieve desired voltage levels, such fuel
cells are
typically connected together using interconnects or bipolar plates to form a
stack, or fuel
cell stack, through which fuel and oxidant fluids are passed. Electrochemical
conversion
occurs, with the fuel being electrochemically reacted with the oxidant, to
produce a DC
electrical output.
[0004] The basic and most important requirements for the interconnect
materials on the
cathode side of a SOFC stack are sufficient oxidation and corrosion resistance
in air at
the stack operating temperatures; sufficient electron conductance; and close
matching of
thermal expansion behavior to that of the ceramic cell. ~ In the case of
metallic
interconnects, the requirement of sufficient electron conductance is
essentially equivalent
to the electron conductance of the oxide scale that forms on the metal surface
because the
oxide scale tends to be the limiting resistance. Currently, the lack of
stable, long-life
(>40,000 hours), metallic interconnects for the cathode side of the stack, is
a serious
weakness of planar solid oxide fuel cells, because existing metal alloys
cannot meet the
thermal expansion, oxidation resistance, and electron conductance requirements
simultaneously.
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[0005] Cathode interconnect materials that have been used to date include
perovskite-
based ceramics, e.g. lanthanum chromite, high temperature chromium-based
alloys or
composites thereof, and nickel-based alloys or intermetallics have been used
typically for
cells operating in the 800-1000 °C range.
(0006] The prior art on ceramic-based interconnects such as lanthanum chromite
indicates that this material exhibits both usable high temperature
conductivity and
thermal expansion behavior that matches the cell. However the ceramic is very
expensive, has low toughness and is difficult to manufacture as a suitable
interconnector.
Chromium-based interconnector materials have similar drawbacks.
[0007] Lower operating temperatures, (650-800°C) with planar anode-
supported cells,
permit use of lower cost materials such as ferritic stainless steels that have
a better
coefficient of thermal expansion (CTE) match with the cell than Ni-based
alloys.
Commercial grades of ferritic steels may have suitable oxidation resistance at
temperatures less than about 600°C or for short lifetimes, but do not
have the required
oxidation resistance to last for 40,000 hours, or longer, due to the
increasing ohmic
resistance across the oxide scale with time under load.
(0008] The majority of prior art on these issues has attempted to prevent or
ameliorate
the degradation caused by oxide scale. Specifically, to take advantage of the
lower cost
and favorable CTE of ferntic steels, minor alloying additions and/or surface
coatings
have been researched to improve the oxidation resistance and conductivity.
Certain
elements such as Mn appear beneficial in forming manganese chromite which
increases
the conductivity of the oxide scale, but more data is needed to determine
whether both
conductivity and oxidation resistance are sufficient for long-term
applications.
However, elements known to improve oxidation resistance, such as A1 and Si,
also tend
to disadvantageously reduce the oxide conductivity and increase the CTE of the
alloy. In
Fe-Cr-AI-Y type steels, excellent oxidation performance is traded for the high
resistivity
of the resulting alumina film. Hence, the current state-of the-art with regard
to low cost
Fe-Cr-based steels, has not fully resolved the long-term contact and oxidation
issues.
[0009] Other materials, such as Ni-Cr or Ni-Cr-Fe-based alloys, while having
good
oxidation/corrosion resistance by design, typically have CTE values in the IS-
18 parts
per million (ppm)1°C compared to the about 12 ppm/°C of ferritic
steels which better
match the CTE of the ceramic cell.
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(0010] Preferential removal of the oxide and/or coating/doping of the alloy
surface with
noble metals such as Ag, Au, Pt, Pd, and Rh has been used to mitigate
conductivity loss
by reducing oxygen diffusion into the contact points of the interconnect, but
noble metals
are too costly to use in power plants and commercial applications.
[0011] The oxidation resistance is clearly a concern on the cathode/oxidant
side of the
interconnect. However, the partial pressure of oxygen at the anode/fuel
electrode may
also be high enough to form Crz03 and the oxide may be even thicker (viz. the
presence
of electrochemically formed water) than on the cathode side of the
interconnect, so the
resistivity of the interconnect may increase an both sides. The construction
materials on
the anode side of the interconnect could be the same as the cathode, although
prior art has
shown that, in the case of a ferntic steel interconnect in contact with a
nickel anodic
contact, weld points that formed between the steel and the nickel still formed
a thin
electrically insulating Cr203 layer over time which degraded performance.
[0012] ~ It is clear, from the above review of background art, that the need
remains for a
substantially improved interconnect between adjacent cells, whereby interface
strains,
caused by CTE mismatch during.thermal cycling, are substantially eliminated,
while the
material provides long-term oxidation resistance and high electron conductance
across
the oxide scale.
[0013] It is therefore the primary object of the present invention to provide
an
interconnect or bipolar plate that meets the aforementioned needs_
[0014] Other objects and advantages of the present invention will appear
hereinbelow.
SUMMARY OF THE INVENTION
[0015] In accordance with the present invention, the foregoing objects and
advantages
have been readily attained.
[0016] The present invention provides a solid oxide fuel cell design having a
compliant
porous interconnect which alleviates the thermal expansion mismatch stresses
which are
typically generated by higher thermal expansion oxidation resistant
interconnect metals
and/or alloys for the cathode.
[0017] The interconnect of the present invention advantageously allows for the
use of
higher thermal expansion oxidation resistant metals or alloys for the
separator plate.
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[0018] The interconnect of the present invention further advantageously allows
for less
stringent dimensional tolerances of the stack components since the
interconnect is
compliant in all three dimensions and permits displacement with minimal
increase in
stress to accommodate dimensional variations.
[0019] According to the invention, an interconnect is provided which comprises
a
compliant porous member, compliant in all three-dimensions and having first
portions
defining a separator plate contact zone and second portions spaced from said
first
portions and defining an.electrode contact zone.
[0020] In further accordance with the invention, an interconnect assembly is
provided
for solid oxide fuel cells, which assembly comprisesa separator plate having
two opposed
surfaces; and at least one interconnect positioned adjacent to at least one of
said two
opposed surfaces and comprising a compliant porous member, compliant in all
three-
dimensions and having first portions defining a separator plate contact zone
and second
portions spaced from said first portions and defining an electrode contact
zone.
[0021] Still further according to the invention, the interconnect assembly
could be
comprised of one or more layers, at least one of which is compliant as
described herein.
In this embodiment, the compliant layer may or may not be in contact with the
sepazator
plate or the electrodes.
[0022] Still further according to the invention, a solid oxide fuel cell
assembly is
provided which comprises a plurality of fuel cells arranged in a stack; and a
plurality of
interconnect assemblies positioned between adjacent cells of said-staek,~
said~interconneet
assemblies comprising a separator plate having two opposed surfaces and at
least one
interconnect positioned adjacent to at least one of said two opposed surfaces
and
comprising a compliant porous member, compliant in all three dimensions and
having
first portions defining a separator plate contact zone and second portions
spaced from
said first portions and defining an electrode contact zone.
[0023] Particularly desirable configurations of the interconnect involve three-
dimensional compliant superstructures made out of wire weaves or other
compliant sub-
structures.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0024] A detailed description of preferred embodiments of the present
invention
follows, with reference to the attached drawings, wherein:
[0025) Figure 1 schematically illustrates a fuel cell stack assembly in
accordance with
the present invention;
(0026] Figure 2 schematically illustrates a portion of the fuel cell stack
assembly of
claim 1;
[0027] Figure 3 illustrates a preferred embodiment of an interconnect of the
present
invention;
(0028] Figures 4 and 5 illustrate another preferred embodiment of an
interconnect of the
present invention;
(0029] Figure 6 illustrates an alternate embodiment of an interconnect of the
present
invention;
[0030] Figure 7 illustrates an alternate embodiment of an interconnect of the
present
invention;
[0031] Figure 8 illustrates another alternate embodiment of an interconnect of
the
present invention;
Figure 9 illustrates a wire structure with compliance loops according to the
invention; and
[0032] - Figure 10 illustrates-another preferred embodiment of an-interconnect
of the-
present invention.
DETAILED DESCRIPTION
[0033) The invention relates to a fuel cell assembly and, more particularly,
to a solid
oxide fuel cell (SOFC) stack having improved metallic interconnect which
decouples the
need for good coefficient of thermal expansion (CTE) match with other stack
components
from other requirements such as oxidation resistance and oxide scale electron
conductance.
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[0034] The invention relates further to a fuel cell stack and, more
particularly, to a solid
oxide fuel cell stack having an improved interconnect, whereby stresses due to
difference
in thermal expansion coefficient between adjacent fuel cell stack components,
and
specifically between the cathode or anode interconnect and adjacent fuel cell
or separator
plate, are minimized so as to provide for enhanced fuel cell stack lifetime
and robustness
under steady state and thermal cycling.
[0035] Reduction in stress is accomplished through a compliant interconnect
superstructure provided from a compliant sub-structure, wherein the
interconnect
superstructure is provided having contours so as to define spaced contact
zones for
contacting a separator plate on one side and an electrode of a fuel cell on
the other side,
and the sub-structure is provided by highly compliant pre-buckled architecture
as present
in a wire mesh. The combination of the two, compliant superstructure and
compliant
sub-structure, providing spaced contact zones in accordance with the present
invention
advantageously allows for CTE mismatch between various components of the stack
without subjecting such components, or bonds or other types of connection
therebetween,
to excessive stress, while improving fluid flow and electrical functionality
of the fuel cell.
[0036] The compliant interconnects described herein are designed such that
high values
of both in-plane and out-of plane compliance are achieved. One skilled in the
art will
recognize that any such interconnect that provides for acceptable levels of
either in-plane
compliance or out-of plane compliance, or both, will be within the broad scope
of the
invention. Preferably;-the compliant superstructure is compliant in at least
three-
orthogonal axes, and is compliant with respect to a load applied from any
direction.
Various approaches to achieve this include wire weave based superstructures as
described
above, 3-dimensional knitted wire structures, helical coils in various
configurations
including slanted helical coils provided by pre-buckled highly compliant sub-
structures,
wires with in-built highly compliant compliance loops, similar interconnects
made from
sheet metal, foil, foam, or expanded metals formed into superstructures, etc.
Preferred
compliance values of the interconnects are SxIO'~ mm2/N (in strain/stress
units) and
higher for typical interconnects at room temperature. More preferred
compliancy values
are 5x10'$ mmz/N and higher for typical interconnects. Most preferred
compliancy
values are Sxl O~ mm2/N and higher for typical interconnects, but one of
ordinary skill in
the art will recognize that other compliancy values can be acceptable and are
within the
scope of the invention.
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[0037] Turning to Figure 1, a fuel cell stack assembly 10 in accordance with
the present
invention is schematically illustrated. Assembly 10 preferably includes a
plurality of fuel
cells 12 arranged in a stack with bipolar plates 14 positioned therebetween.
(0038] Fuel cells 12 typically include an electrolyte 16, a cathode layer 18
positioned
on one side of electrolyte 16, and an anode layer 20 positioned on the other
side of
electrolyte 16. Bonding or current carrying layers 22 may be used on the two
sides.
(0039] Bipolar plate 14 in accordance with the present invention
advantageously
includes a separator plate 24 having a cathode facing surface 26 and an anode
facing
surface 28, a cathode-side interconnect 30 positioned between cathode facing
surface 26
and a cathode layer 18 (or layer 22) of an adjacent fuel cell 12, and an anode-
side
interconnect 32 positioned between anode facing surface 28 and an anode layer
20 (or
layer 22) of an adjacent fuel cell 12. Interconnects 30, 32 are advantageously
provided of
an electron conducting material and are in electrical communication with
separator plate
24.
[0040] Referring to Figure 2, a particular aspect of the present invention is
the design of
cathode-side and anode-side interconnects 30, 32, wherein the interconnects
are provided
as a sheet of woven wire material formed to have a plurality of first portions
34 or 38
defining an electrode contact zone, and a plurality of second portions 36
defining a
separator plate contact zone which is spaced from the electrode contact zone.-
[0041] Still referring to Figure 2, interconnects 30, 32 in accordance with
the present
invention, especially cathode-side interconnect 30, consists of compliant sub-
structure,
preferably wire weaves, material as described above which is formed, for
example
through die stamping, rolling, bending or the like, to have a three-
dimensional
superstructure defining first and second portions 34, 36.
(0042] The compliant wire weave sub-structure of interconnects 30, 32 in
accordance
with the present invention is advantageously a wire weave such as that
illustrated in
Figure 2, which advantageously provides a pre-buckled architecture that
increases
compliance of interconnect 30, 32. This compliance allows for movement or
deflection
without stressing of first portions 34, relative to second portions 36 during
thermal
cycling and the like, which advantageously serves to eliminate stresses caused
by CTE
mismatch between various components. The compliant interconnects formed from
such
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sub-structures and superstructures also advantageously allows for movement
between
first portion 34 with respect to second portion 36 without stressing during
assembly,
thereby permitting larger dimensional tolerance variations.
[0043] The wire weave as shown in Figure 2 may include a first plurality of
wires or
substructures disposed in one direction, and a second plurality of wires or
substructures
disposed in a different direction, so as to define a woven wire structure
which is porous to
operating fuel cell gaseous materials and compliant as desired in different
directions, in
accordance withthe presentinvention.
[0044] Figure 3 shows a perspective view of an interconnect 30, 32 to further
illustrate
a preferred sub-structure and superstructure thereof.
[0045] Figures 1, 2 and 3 illustrate interconnects 30, 32 as members having a
substantially sinusoidal cross section, wherein peaks 38 on one side of a
centerline 40
define the electrode contact zone, and peaks 42 on the other side of
centerline 40 define
the separator plate contact zone. In accordance with a preferred aspect of the
present
invention, and as illustrated in Figure 3, the undulating or vertically
contoured shape of
interconnect 30, 32 extends in the transverse direction to the cross section
illustrated in
Figures 1 and 2 so as to define a series of spaced peaks 38, 42, each
extending in opposite
directions from centerline 40, so as to define the spaced contact zones
discussed above.
[0046] It should of course be appreciated that other architectures could be
provided for
interconnects 30, 32, within the broad scope of the present invention, which
could equally
provide for the spaced contact zones. connected bycompliant~members which
provide-for
advantageous reduction in stresses between components as desired in accordance
with the
present invention.
[0047] Figure 4, for example, illustrates interconnect 30, 32 with compliant
superstructures shaped in a substantially orthogonal, for example square or
retangular
channel pattern, made from a compliant sub-structure maferial, preferably wire
weave,
wherein the interconnects form spaced contact zones in the cross sectional
view. Figure
4 further illustrates a preferred wire weave structure according to the
invention. Figure 5
shows a perspective view of such an interconnect 30, 32 on bipolar plate 14.
[0048] Another example, Figure 6, shows a substantially square channeled
superstructure interconnect 30, 32 with spaced contact zones present in both
the cross
sectional and the transverse direction.
[0049] Another example, Figure 7, shows a substantially trapezoidal
superstructure
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interconnect 30, 32 made from compliant sub-structures.
[0050] Another example, Figure 8, illustrates a superstructure interconnect
30, 32 made
into a circular or a helical, preferably slanted, structure wherein a
compliant sub-structure
such as a pre-buckled wire or wire weave forms the three-dimensional
superstructure.
[0051] Figure 9 illustrates an embodiment wherein wires 52 are provided with
compliance loops 54 as described above. This structure serves to enhance the
ability of
the wire to resiliently deform as needed to respond to different CTE, and also
to provide
desired manufacturing tolerances. This compliance loop structure can be
incorporated
into the substructure .andlor the superstructure of the interconnect of the
present
invention.
[0052] Figure 10 shows a substantially hour-glass shaped superstructure
interconnect
30, 32 made from compliant sub-structures.
[0053] Interconnect 30, 32 in these examples can be positioned between
components of
the stack in similar fashion to the embodiment described above in connection
with
Figures 1-4.
[0054] Clearly, those skilled in the art will realize that a large number of
patterns and
arrangements of such compliant sub-structures as well as superstructures
exist, and are all
within the broad scope of the present invention.
[0055] Different materials and architectures may be desirable for cathode-side
interconnect 30 than for anode-side interconnect 32.
[0056] Cathode-side interconnect 30 is preferably provided having the-
architecture as -
described above and illustrated in Figures 1 and 2.
[0057] Anode-side interconnect 32 can advantageously be provided having the
same
architecture, or having a foam architecture defining foam cells which,
themselves, define
the contact zones for contact on one. side with separator plate 24 and on the
other side
with the anode of a fuel cell 12.
(0058] Further, in the cathode environment, it is desirable to provide cathode-
side
interconnect 30 of an oxidation resistant conductive material, preferably of a
material
selected from the group consisting of selected stainless steels, stainless
steel alloys and
super-alloys comprising Ni-Cr-, Ni-Cr-Fe-, Fe-Cr-, Fe-Cr-Ni and Co-based
alloys as well
as Cr-based alloys and noble metallalloys. Such super-alloys include HAYNES~
alloy
230, HAYNES~ alloy 230-W, and Hastelloy X, which have been found preferable in
the
present invention. Other materials include composites of at least 2 materials,
for example
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metals and ceramics containing any of the above mentioned metals and alloys.
Another
set of materials include noble metal coated super-alloys.
' [0059] Anode-side interconnect 32 is advantageously provided of a material
selected
from the group consisting of Ni, Ni-Cu, Ni-Cr-, Ni-Cr-Fe-, Fe-Cr-, Fe-Cr-Ni
and Co-
based alloys as well as Cr-based alloys and noble metal/alloys and including
such alloys
coated with Ni, Cu or Ni-Cu as well as noble metals. Other materials include
composites
of metals and ceramics containing any of the above mentioned metals and
alloys.
[0060] In accordance with the present invention, interconnects 30, 32 when
provided
having the configuration of Figures 1 and 2 preferably define a superstructure
wherein
peals 38, 42 define a superstructure wavelength of between 0.1 mm and 100 mm,
a
superstructure amplitude of between 0.1 mm and 50 mm, and a superstructure
periodicity
which may be uniform or random.
[0061] Further, the wire weave sub-structure of interconnect 30, 32 in
accordance with
the present invention is preferably provided having a wire diameter of between
0.05 mm
and 5 mm,a sub-structure weave wavelength of between 0.45 mm and 50 mm, a
weave
amplitude of between 0.05 mm and SO mm, a weave pattern which may be square,
plain,
satin, twill or other patterns, and a weave periodicity which may be uniform
or random.
[0062] In addition, the wire weave sub-structure of interconnect 30, 32 may be
composed of wires of different diameters and/or alloys in different places to
facilitate
functionality.
[0063] In accordance with the present invention, separator-plate~24 can
advantageously-
be bonded to anode-side interconnect 32 and cathode-side interconnect 30
through
various methods to produce high-strength interfaces therebetween. For example,
such
joints or components can be bonded, welded or brazed together, or can be
secured
together in other manners which would be well known to a person of ordinary
skill in the
art. Furthermore, it is within the broad scope of the present invention to
position these
components adjacent to each other without any bonding therebetween.
[0064] The wire weave sub-structure and three-dimensional superstructure of
the
interconnects in accordance with the present invention advantageously serves
to alleviate
stresses at the anode and cathode interfaces, and minimizes fracture of the
interface and
the cells themselves.
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[0065] In further accordance with the present invention, and as illustrated in
Figure 1, a
compliant seal is further advantageously provided for sealing between edges of
bipolar
plate 14 and adjacent fuel cells 12.
[0066] In accordance with the present invention, the seal design is provided
in the form
of a rail or spacer 44 defining therein a groove 4G, and a seal member 48
positioned in
groove 46 and compressed between bipolar plate 14 and adjacent fuel cells 12
to provide
the desired seal therebetween. A compression stop 50 is provided to control
the amount
of deflection of the compliant seals and to advantageously assemble compliant
interconnects, compliant seals and all other elements of the stack.
[0067] In further accordance with the present invention, seal member 48 is
advantageously provided as a compliant or compressible member formed from a
suitable
material, preferably alumina fibers. Alumina is most desirable in accordance
with the
present invention because alumina does not contaminate the fuel cell as do
other seal
materials which have conventionally been used, such as glass, glass-ceramics
and the
like.
[0068] Thus, in accordance with the present invention, seal member 48 is
advantageously provided as compliant alumina fibers which can preferably be
impregnated with another material selected so as to provide substantial gas
impermeability of seal member 48 while nevertheless allowing for compliance or
compressibility thereof.
[0069] The seal member 48-in accordance with the present-inventiorr can-
advantageously be impregnated with a material selected from the group
consisting of
zirconia, alumina, yttrium aluminum garnate, alumino-silicate and magnesium
silicate
ceramics, and similar oxides, and combinations thereof, and it is preferred
that seal
member 48 be provided so as to reduce permeability to gas.
[0070] Seal member 48 can advantageously be provided having a fiber
architecture
such as tows, yarns, fiber weave architecture and the like. Such architectures
can be
loaded with secondary particles within the fibers as discussed above so as to
provide
desired seal properties. Further, rail/spacer 44 and compression stop 50 is
provided
having a height and groove depth which are selected to provide for additional
decoupling
of various parameters which are conventionally required to be related.
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[0071] It should be noted that a significant parameter is the response of the
interconnect
and seal to the clamping compressive load which must be applied to the fuel
cell stack as
schematically illustrated in Figure 1.
[0072] Figure 1 shows a compressive load applied to the top and bottom of
assembly 10
which compressive load is advantageously selected to provide for sufficient
interconnect
bonding and sufficiently reduced leakage in the seals while nevertheless
allowing micro-
sliding in the seal area to relieve thermal mismatch stresses and to minimize
compressive
creep of the interconnects.
[0073] From a manufacturing standpoint, the system of the present invention
provides
for cells and interconnects having less stringent dimensional tolerances since
the
interconnect provides out-of plane compliance and, therefore, increased
dimensional
freedom. Further, the provision of fixed thickness rail/spacers 44 and
compression stops
50 ensures decoupling of the sealing and interconnection requirements and
therefore
provides substantial flexibility for building stacks that are based upon
stable and
compatible materials.
[0074) It should of course be appreciated that in accordance with the present
invention,
an interconnect superstructure and compliant seal assembly have been provided
which
advantageously allow for reduced stringency in tolerances in manufacture and
assembly
of solid oxide fuel cell stacks, and further which reduce the stresses
conveyed between
various components of the stack, thereby advantageously decoupling different
design
concerns of the stack and allowing selection of materials to provide
long~stack life:
[0075] It is to be understood that the invention is not limited to the
illustrations
described and shown herein, which are deemed to be merely illustrative of the
best modes
of carrying out the invention, and which are susceptible of modification of
foam, size,
arrangement of parts and details of operation. The invention rather is
intended to
encompass all such modifications which are within its spirit and scope.
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