U.S. patent application number 10/975362 was filed with the patent office on 2005-03-17 for minimum volume oven for producing uniform pyrolytic oxide coatings on capacitor anodes.
This patent application is currently assigned to Kemet Electronics Corporation. Invention is credited to Hahn, Randolph S., Henley, John D., Kinard, John T., Melody, Brian J..
Application Number | 20050056221 10/975362 |
Document ID | / |
Family ID | 25488179 |
Filed Date | 2005-03-17 |
United States Patent
Application |
20050056221 |
Kind Code |
A1 |
Henley, John D. ; et
al. |
March 17, 2005 |
Minimum volume oven for producing uniform pyrolytic oxide coatings
on capacitor anodes
Abstract
A pyrolysis oven provides uniform pyrolytic coatings on
capacitor anodes. An oven chamber contains cross-flow blowers
situated to provide uniform laminar flow of oven atmosphere over
the objects to be treated. The top and side walls of the chamber
meet in an inverted V such that when the blower operate, a vortex
is created in the inverted V in the chamber.
Inventors: |
Henley, John D.; (Six Mile,
SC) ; Melody, Brian J.; (Greer, SC) ; Kinard,
John T.; (Greer, SC) ; Hahn, Randolph S.;
(Simpsonville, SC) |
Correspondence
Address: |
BANNER & WITCOFF
1001 G STREET N W
SUITE 1100
WASHINGTON
DC
20001
US
|
Assignee: |
Kemet Electronics
Corporation
Greenville
SC
|
Family ID: |
25488179 |
Appl. No.: |
10/975362 |
Filed: |
October 29, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10975362 |
Oct 29, 2004 |
|
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|
09948717 |
Sep 10, 2001 |
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6849134 |
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Current U.S.
Class: |
118/722 |
Current CPC
Class: |
F27B 9/3005 20130101;
F27B 9/20 20130101; F27B 9/10 20130101 |
Class at
Publication: |
118/722 |
International
Class: |
C23C 016/00 |
Claims
1-14. (Canceled)
15. A process of producing uniform, laminar oven atmosphere flow
over objects to be treated in a pyrolysis oven, said pyrolysis oven
comprising a chamber formed by a top, a bottom, an entrance wall,
an exit wall, a first side wall and a second side wall, wherein the
top and at least the first side wall meet in a first inverted V
shape, wherein the oven further comprises at least one cross-flow
blower situated at the bottom of the chamber adjacent the first
side wall, wherein the process comprises forming a first flow of
oven atmosphere from the cross-flow blower up the first side wall
of the chamber, the first flow of oven atmosphere creating a first
vortex in the first inverted V, the first vortex forcing the first
flow of oven atmosphere over the objects to be treated.
16. The process of claim 15 wherein the top and the first side wall
meet in a first inverted V shape and the top and the second side
wall meet in a second inverted V shape, wherein the chamber
comprises first and second cross-flow blowers, the first cross-flow
blower located in the bottom of the chamber adjacent the first side
wall and the second cross-flow blower located in the bottom of the
chamber adjacent the second side wall, wherein the process
comprises forming a first flow of oven atmosphere from the first
cross-flow blower up the first side wall of the chamber, said first
flow of oven atmosphere creating a first vortex in the inverted V,
said first vortex forcing the first flow of oven atmosphere over
the objects to be treated, and the process further comprises
forming a second flow of oven atmosphere from the second cross-flow
blower up the second side wall of the chamber, the second flow of
oven atmosphere creating a second vortex in the inverted V, the
second vortex forcing the second flow of oven atmosphere over the
objects to be treated.
17. The process of claim 15 further comprising heating the first
flow of oven atmosphere.
18. The process of claim 16 further comprising heating the first
flow of oven atmosphere and second flow of oven atmosphere.
19. The process of claim 15 wherein the objects are continuously
moved through the oven chamber.
20. The process of claim 16 wherein the objects are continuously
moved through the oven chamber.
21. The process of claim 15 further comprising adding a gas to the
oven chamber.
22. The process of claim 16 further comprising adding a gas to the
oven chamber
Description
FIELD OF THE INVENTION
[0001] The invention relates to a minimum volume oven having a
heating zone with uniform laminar oven atmosphere flow for
economically thermally treating objects.
BACKGROUND OF THE INVENTION
[0002] Solid tantalum capacitors were introduced in the 1950's, and
since that time such devices have replaced many of the liquid
electrolyte-containing aluminum electrolytic capacitors of similar
rating used in the fabrication of electronic circuits. Solid
tantalum capacitors have higher capacitance per unit volume, lower
equivalent series resistance, lower temperature dependence of
capacitance and equivalent series resistance, and higher
reliability than the liquid electrolyte aluminum capacitors.
[0003] The high capacitance per unit volume of solid tantalum
capacitors is a function of the high surface area tantalum powder
used to fabricate the powder metallurgy compacts making up the
anodes of electronic devices and is also a function of the high
dielectric constant of the anodic oxide dielectric film. The high
reliability of solid tantalum capacitors is a function of the high
stability of the anodic tantalum oxide dielectric layer applied to
each sintered powder metallurgy tantalum compact via an anodizing
process step. The low equivalent series resistance and small
temperature dependence of capacitance and equivalent series
resistance are largely a function of the manganese dioxide cathode
material used in the fabrication of these devices.
[0004] The manganese dioxide cathode material in solid tantalum
capacitors is produced in situ via pyrolysis of manganese nitrate
solution introduced into the powder metallurgy anode bodies by a
dipping step prior to the pyrolysis step. The manganese nitrate
dipping and pyrolysis sequence is repeated until the pore structure
is sufficiently coated with manganese dioxide.
[0005] After the application of the manganese dioxide cathode
material to the sintered and anodized powder metallurgy tantalum
anode compacts is complete, the compacts are coated with carbon and
silver paint, then assembled into finished devices. The finished
devices may be on the leaded (hermetically-sealed metal can,
molded, or fluidized bed epoxy coated) or surface mount (molded or
conformally resin coated) configuration.
[0006] Manganese dioxide is a complex substance having many crystal
forms, hydration states, and crystal densities. In addition to the
above variables, manganese dioxide produced via pyrolysis of
manganese nitrate solutions is of varying porosity and surface
smoothness depending upon pyrolysis conditions. The focus of a good
deal of the development work conducted in the field of solid
tantalum capacitors has been the production of manganese dioxide
coatings which are dense, adherent, and highly electrically
conductive.
[0007] Early in the development of solid tantalum capacitors, it
was recognized that carrying out the pyrolysis process in the
presence of steam gives rise to smoother, denser, and more
electrically conductive manganese dioxide than when the pyrolysis
is carried out in air. A denser, smoother, and more conformal
manganese dioxide coating can be obtained with pyrolysis carried
out in an essentially steam atmosphere. Prior to the development of
steam atmosphere pyrolysis, the manganese dioxide pyrolytic
coatings on tantalum capacitors produced in air were sufficiently
non-uniform to require mechanical sizing, such as by external
grinding, prior to fabrication of the finished devices.
[0008] It was discovered that confining the pyrolysis reaction
gases in close proximity to the manganese nitrate coated substrate
gives rise to the production of manganese dioxide having higher
density and conductivity than manganese dioxide produced in an
atmosphere of air or steam alone ("Electrical Properties of
Manganese Dioxide and Manganese Sesquioxide", by Peter Klose,
Journal of the Electrochemical Society, Vol. 117, No. 7, pages
854-858). Others made use of this effect, i.e., the improvement in
manganese dioxide density and conductivity when the pyrolysis gases
are confined in close proximity to the reaction mass, to produce
tantalum capacitors having improved electrical parameters (lower
leakage current and dissipation factor, higher capacitance) by
confining the manganese nitrate solution-dipped anodes within small
radiant ovens having a small degree of positive pressure, with or
without horizontal circulation of the oven atmosphere. See U.S.
Patents, 4,038,159, 4,042,420, 4,105,513, and 4,148,131; also
described by Nishino, et. al., at the Manganese Dioxide Symposium,
1980, Tokyo, published by The Electrochemical Society, 1981,
Symposium Proceedings, pages 305-320.
[0009] Confining decomposition gases from manganous nitrate
pyrolysis (mainly nitrogen dioxide and steam) in close proximity to
manganese nitrate solution-dipped anodes in order to obtain
improved pyrolytic manganese dioxide properties has several
drawbacks under manufacturing conditions. In order to obtain
uniform results, the pyrolysis oven must be loaded with the same
number of anodes of the same size containing the same amount of the
same concentration of manganous nitrate. However, it is very
desirable to be able to vary the number and size of the anodes
undergoing pyrolysis in order to meet manufacturing demands.
[0010] Aronson, et. al., U.S. Pat. No. 4,164,455, reasoned, because
the major nitrogen-containing species evolved during manganese
nitrate pyrolysis is nitrogen dioxide, that this is the material
responsible for the results obtained in Klose's experiments and
Nishino's pyrolysis process. Aronson found similar results could be
obtained by employing a small-volume oven into which is introduced
a stream of nitrogen dioxide as well as steam. The introduction of
nitrogen dioxide as well as into the oven would seem to free the
process from a dependence upon loading uniformity from pyrolysis
run to pyrolysis run in order to obtain uniform pyrolytic manganese
dioxide properties.
[0011] A series of experiments indicated that gaseous oxidizing
agents more oxidizing than nitrogen dioxide, such as nitric acid,
hydrogen peroxide/nitric acid mixtures, and ozone, are
significantly more effective than nitrogen dioxide in facilitating
the production of the higher density, higher electrical
conductivity beta crystal form of manganese dioxide associated with
superior electrical performance in the finished solid capacitors
(U.S. Pat. No. 5,622,746, and "A Process For Producing Low ESR
Solid Tantalum Capacitors", by Randy Hahn and Brian Melody,
presented at The 15th Annual Capacitor and Resistor Technology
Symposium, Mar. 11, 1998, Symposium Proceedings, pages 129-133).
The oxidizing agent(s) may be present at relatively low
concentrations, e.g. 1-2% of ozone, to 50% or more of the oven
atmosphere.
[0012] The oxidizing agents employed by Hahn tend to be expensive
and corrosive (nitric acid) as well as unstable at pyrolysis
temperatures (hydrogen peroxide, ozone). The instability of these
reagents makes frequent oven atmosphere turnover necessary in order
to maintain the most favorable conditions for high density/high
conductivity manganese dioxide production, while the expense of
these materials mandates minimal oven size for economic process
operation, i.e., a 50% reduction in oven volume for the same oven
capacity, in terms of anodes processed in a batch, results in a 50%
savings of oxidizing agent and steam consumed per part
processed.
[0013] Oven size (volume versus anode capacity) is not the only
consideration in oven design. Circulating air ovens have been found
to offer several advantages over non-circulating radiant ovens for
the processing of tantalum anodes through the manganese nitrate
pyrolysis process. Circulating air (circulating atmosphere) ovens
are more readily maintained at uniform temperature than
non-circulating ovens. Circulating air ovens heat the anodes more
rapidly than radiant ovens maintained at the same temperature.
Atmospheric doping and composition control is more easily
accomplished with a circulating atmosphere oven than with a radiant
oven.
[0014] Applying manganese dioxide to tantalum powder metallurgy
anodes provided a decided advantage for pyrolysis ovens having
top-down air flow. Top-down air flow dries the tops of the anodes,
which are suspended from bars held in a horizontal rack (process
lid) faster than the lower portions of the anodes, resulting in
liquid phase material being transported to the tops of the anodes
by capillary action, counterbalancing the tendency for the liquid
manganese nitrate solution to migrate to the lower portions of the
anodes due to the action of gravity. The overall result is the
production of more uniform manganese dioxide coatings in top-down
circulating air pyrolysis ovens.
[0015] In order to direct the airflow inside of circulating air
process ovens, conventional ovens contain ducts, baffles, and
plenums through which the oven atmosphere flows under the impetus
of a motorized fan or fans contained within the ductwork. One of
the most difficult goals to accomplish in circulating atmosphere
oven design is the production of uniform and laminar flow of the
oven atmosphere past the objects to be heated, which are contained
within the main chamber of the oven during use. In order to render
the atmospheric flow uniform across the entire load within an oven,
oven manufacturers employ expensive plenums, stacked diffusion
screens, and multiple blowers in oven construction. One consequence
of using extensive plenums and diffusion screen stacks, etc., is
that the volume of the oven atmosphere is many times larger than
the volume of the parts being processed. The resulting large size
and cost of circulating atmosphere ovens are disadvantageous for
the user of these devices. The large volume, associated with the
ducting and plenums employed in conventionally designed ovens, also
necessitates the use of a relatively large amount of atmospheric
doping chemicals for applications such as the manufacture of
tantalum capacitors.
[0016] What is desired, then, is a circulating process oven
designed and fabricated so as to facilitate laminar and uniform
atmospheric flow within the oven without the need for the large
volume of ducting, plenums, and diffusion screens required to
produce uniform oven atmosphere circulation in ovens atmosphere
circulation in ovens of conventional design in order to minimize
the parasitic oven volume such as ducting, plenums, diffusion
screens, etc. versus the useful oven volume in which the load
resides during processing.
BRIEF SUMMARY OF THE INVENTION
[0017] In a first embodiment, a pyrolysis oven comprises a chamber
formed by a top, a bottom, a first side wall, a second side wall,
an entrance wall and an exit wall, wherein the top and at least the
first side wall meet in an inverted V shape, wherein the oven
further comprises at least a first cross-flow blower situated at
the bottom of the chamber adjacent the first side wall such that
when the cross-flow blower operates, at least a first flow of air
flows up the first side wall of the chamber and meets a first
vortex created in the first inverted V which first vortex forces
the first flow of air over the objects to be treated.
[0018] In a preferred embodiment, the top and first side wall meet
in a first inverted V shape and the top and second side wall meet
in a second inverted V shape, wherein the chamber comprises first
and second cross-flow blowers, the first blower located in the
bottom of the chamber adjacent the first side wall and the second
blower located in the bottom of the chamber adjacent the second
side wall such that when the cross-flow blowers operate, a first
flow of air flows up the inside of the first side wall and meets a
first vortex created in the first inverted V which first vortex
forces the first flow of air over the objects to be treated and a
second flow of air flows up the inside of the second side wall of
the chamber and meets a second vortex created in the second
inverted V which second vortex forces the second flow of air over
the objects to be treated.
[0019] The invention is also directed to a method for treating
objects in the pyrolysis oven described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic sectional view showing the components
of the pyrolysis oven of the invention with the entrance wall
removed taken along line 1-1 of FIG. 2.
[0021] FIG. 2 shows a schematic view of the components of the
pyrolysis oven of the invention when viewed with a side wall
removed.
DETAILED DESCRIPTION OF THE INVENTION
[0022] It was discovered that a pyrolysis oven having a cross-flow
blower in the oven chamber produces uniform, laminar oven
atmosphere flow over the objects to be treated in the oven. The
oven provides uniform atmospheric flow past a high aspect ratio
(near planar) load with a minimum of parasitic oven volume compared
with the active or load oven volume.
[0023] The pyrolysis oven has a chamber formed by a top, a bottom,
a first side wall, a second side wall, an entrance wall and an exit
wall. A conveyor transports objects for pyrolysis treatment through
an opening in the entrance wall of the oven chamber in the
direction of the arrow in FIG. 2. The top and at least one of the
side walls meet in an inverted V shape. At the bottom of chamber,
adjacent the side wall, below the inverted V shape, at least one
cross-flow blower is present. The cross-flow blower rotating
clockwise as seen in FIG. 1 creates a flow of oven atmosphere that
flows up the inside of the side wall, through means for heating the
flow of oven atmosphere. The flow of oven atmosphere meets a vortex
of oven atmosphere that is created in the inverted V-shape, which
forces the flow of oven atmosphere down onto the objects to be
treated. Preferably the side wall and bottom meet in a curve to
accommodate the cross-flow blower and to allow a smooth flow of
atmosphere up the side wall.
[0024] The means for heating may be any suitable means such as, but
not limited to, electrically heated coils or steam heated coils,
which may be contained in or passed through the walls of the
entrance or exit sides. Such heating means are known in the art and
any suitable heating means may be used.
[0025] A preferred embodiment is illustrated in FIG. 1, which shows
the generally designated oven 1 in accordance with the invention.
The first side wall 2 and second side wall 3 of the oven are
preferably symmetrical. The top 4 of the oven has a V-shape with
the connection of the legs of the V running from the front to the
back of the oven. The top and first side and top and second side
each meet in an inverted V shape. Preferably, the bottom 5 also has
a V shape with the connection of the legs of the V running parallel
with the connection of the legs of the V of the top of the oven of
the top V.
[0026] The angles "A" of the V shape between the first side wall 2
and the top 4 and second side wall 3 and bottom 5 are preferably in
the range of about 67.degree. to about 77.degree., most preferably
in the order of about 72.degree.. The sidewall preferably is about
vertical on the lower part of the oven and angles inward on the
upper part of the oven. The angle "C" inward from a line extending
the vertical side wall part of the oven and the upper side wall is
about 28.degree. to about 32.degree., preferably about
30.degree..
[0027] The angle "B" between the connecting legs of the V in the
top are in the range of about 115.degree. to about 135.degree.,
preferably about 125.degree.. The angle "D" between the connecting
legs of the V in the bottom are in the range of about 120.degree.
to about 140.degree., preferably about 130.degree..
[0028] Cross-flow blower fans, 6 and 7, are located in the lower
sections on either side of the oven. The cross-flow blowers
resemble "squirrel cage" blowers used almost universally in air
conditioning and heating systems due to their quiet operation. Both
types of blowers have a series of blades arranged around the
perimeter of a circle and running parallel to the axis of rotation.
(See the Fan Handbook, by Frank R. Bleier, 1998, McGraw-Hill,
Boston.) As shown in FIG. 1, fan 6 is driven clockwise, while fan 7
is driven counterclockwise by motors not shown.
[0029] The operating principles of the two types of blowers are
quite different however. The air or other gas passing through a
squirrel cage blower enters the fan of the blower axially and
passes through the fan blades once, exiting the fan radially. Gases
passing through a cross-flow blower, in contrast, pass through the
fan blades of the blower twice, both when entering and when exiting
the blower radially. See, for example chapter 13, page 13.1, of the
Fan Handbook, supra.
[0030] An advantage of the cross-flow blower design versus other
blower designs is the reduction in volume of ducting. The
cross-flow blower may be located within rectangular ductwork
without the need for auxiliary or oversized ductwork to convey the
air or other gases to the axial input of squirrel cage blowers. See
FIGS. 7.7, 7.8, and 7.9 in the Fan Handbook, supra. Cross-flow
blowers can be enclosed in a housing not or slightly larger than
the ductwork into which they exhaust. This is due to the unique
double flow of air or other gas through the fan as depicted
schematically in FIG. 13. 1, Fan Handbook. The modest size of the
fan housing versus the ductwork size may be seen in FIG. 13.2 of
the Fan Handbook.
[0031] As previously stated, the two cross-flow blowers 6 and 7
rotate in opposite directions, preferably on graphite bearings to
produce twin flow streams. Each blower rotates in a direction to
circulate the atmosphere of the oven in a flow stream, which passes
up along the adjacent side wall. The twin flow streams pass through
heating coils, 8 and 9. As the atmospheric flow on both sides of
the oven continue past the heating coils, vortexes of rotating oven
atmosphere 11 and 12 positioned below the inverted "V" on either
side of the interior surface of the top 4 is encountered by the
circulating oven atmosphere which serves to direct the flows 13 and
14 of the oven atmosphere downward at a uniform rate of flow past
the load zone 21 of the oven which contains the objects to be
treated by the pyrolysis process.
[0032] FIG. 2 depicts a schematic side view of the oven 1 of the
invention with only the cross-flow blower 6 on one side wall and a
conveyor 18. The conveyor transports objects to be treated through
an opening on the entrance wall and then through the chamber of the
oven.
[0033] The objects are placed in a process lid, for example. FIG. 2
shows three process lids. Process lid 15 is on the conveyor outside
the front 19 of the oven. Process lid 16 is within the oven.
Process lid 17 is on the conveyor outside the back 20 of the
oven.
[0034] The objects to be treated may be placed on a single conveyor
or a series of conveyers as is within the skill of the art.
[0035] The process may be carried out either in continuous or batch
processes. If continuous, as shown in FIG. 2, the conveyor may
convey the objects into the opening of the entrance wall of the
oven, and after treatment, remove the objects through openings in
either the entrance wall or exit wall of the oven. The openings may
be conventional isolation ports. For example, the conveyor passes
through conventional isolation ports 22 and 23 in the entrance and
exit walls, respectively. The conveyor may be of any suitable
design such as, but not limited to, one or two chains or a belt.
The process lids may hang from the chains or rest on the belt.
Conveyors, process lids, and other means to convey objects to be
treated through an oven are known to those skilled in the art.
[0036] The objects may be moved into the oven for the pyrolysis
process and back out for dipping in the precursor solutions which
give rise to MnO.sub.2, etc., during pyrolysis. Alternatively,
several ovens may be arranged with a single conveyor chain passing
through them with automatic dipping stations located between the
ovens. In this manner, the objects may be repeatedly dipped in the
precursor solutions and cycled through the pyrolysis steps
serially, with the objects being removed from the conveyor after
exiting the last pyrolysis oven.
[0037] Gas inlet means 24 may be used to inject gases into the
chamber to mix with the oven atmosphere. The gas inlet means may be
in any suitable location but are preferably near the bottom of the
oven near the blowers so that the gases are drawn into the blowers
and mixed with the atmosphere of the oven.
[0038] Modifications to the oven design of the invention may be
made without materially changing the value of the oven. For
example, the oven may be equipped with flap doors or guillotine
doors (having recessed areas for the conveyor or chains to pass
through), with no doors, or with tunnels leading into and out from
the oven to minimize oven atmospheric loss. Separate exhaust(s) may
be included in the oven design or the ports for loading/unloading
the oven may serve to allow gases evolving during pyrolysis (or
other processes) to escape. The outer oven side walls, bottom, top,
entrance wall, and exit wall may be coated with insulation to help
prevent heat loss. The oven may be of separate top and bottom
sections welded together or bolted together (with or without
gaskets) to facilitate easy access for repairs, etc.
[0039] If batch processing is desired, the ports may be sealed and
the objects to be treated held on conventional trays during
processing.
EXAMPLE 1
[0040] In order to demonstrate the efficacy of the oven design of
the invention in producing uniform oven atmospheric flow past the
load zone, a prototype oven was constructed having the shape shown
in FIG. 1 and a volume of 1.3 ft.sup.3/lid and 14.5 inches
deep.
[0041] The stainless steel front of the oven was replaced with a
1/4" thick polycarbonate plastic 8 having a series of {fraction
(5/16)}" diameter holes drilled through, both directly above and
below the load zone, as well as into the load zone.
[0042] The cross flow blower fans were turned on and adjusted
(variable speed motors) to provide an oven atmosphere flow rate
similar to that used commonly in production pyrolysis ovens (i.e.,
approximately 300 feet per minute).
[0043] An electric anemometer having a probe 1/4" in diameter was
used to measure the atmospheric flow within the oven by inserting
the probe into each of the holes in the polycarbonate plastic
sheet. The atmospheric flow through the load zone, above and below
the load zone, and from the front to the back of the oven was found
to be 300 +/-50 feet per minute.
[0044] By comparison, a production oven of conventional design,
having baffles and extensive ductwork, was found to have a nominal
flow rate of 300 feet per minute but with extremes in flow rate
from 110 to over 400 feet per minute.
[0045] The inventive oven provided uniform flow better than ovens
of conventional construction. The inventive oven had an interior
volume of approximately 2.6 cubic feet. The smallest circulating
oven atmosphere oven capable of containing two side-by-side process
lids of the size used in the inventive oven and also constructed
with conventional baffles and circulating fans to give uniform
atmosphere flow, had an estimated volume of at least 12-15 cubic
feet. Thus the inventive oven design represents a reduction in oven
volume, of at least a factor of approximately 5 over prior art
technology. Moreover, the amount of oven atmosphere doping
chemicals and steam were likewise reduced.
[0046] While the invention has been described with respect to
specific examples including presently preferred modes of carrying
out the invention, those skilled in the art will appreciate that
there are numerous variations and permutations of the above
described systems and techniques that fall within the spirit and
scope of the invention as set forth in the appended claims.
* * * * *