U.S. patent application number 12/919604 was filed with the patent office on 2010-12-30 for method for the electrochemical deposition of carbon nanotubes.
This patent application is currently assigned to E. I. Du Pont De nemours and Company. Invention is credited to Gillian Althea Maria Reynolds, Ming Zheng.
Application Number | 20100326834 12/919604 |
Document ID | / |
Family ID | 41056554 |
Filed Date | 2010-12-30 |
United States Patent
Application |
20100326834 |
Kind Code |
A1 |
Reynolds; Gillian Althea Maria ;
et al. |
December 30, 2010 |
METHOD FOR THE ELECTROCHEMICAL DEPOSITION OF CARBON NANOTUBES
Abstract
This invention relates to the electrochemical deposition of
carbon nanotubes ("CNTs") on a substrate using an electrochemical
cell. A dispersion of a complex of CNTs and an anionic polymer is
neutralized and thereby caused to deposit on the anode plate of the
cell.
Inventors: |
Reynolds; Gillian Althea Maria;
(Wilmington, DE) ; Zheng; Ming; (Wilmington,
DE) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1122B, 4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Assignee: |
E. I. Du Pont De nemours and
Company
|
Family ID: |
41056554 |
Appl. No.: |
12/919604 |
Filed: |
February 27, 2009 |
PCT Filed: |
February 27, 2009 |
PCT NO: |
PCT/US09/35408 |
371 Date: |
August 26, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61032505 |
Feb 29, 2008 |
|
|
|
Current U.S.
Class: |
205/50 ; 205/316;
205/317; 977/742 |
Current CPC
Class: |
C25D 13/04 20130101;
C25D 15/02 20130101; B82Y 10/00 20130101; H01L 51/0006 20130101;
C09D 5/4407 20130101; H01L 51/0093 20130101; C09D 5/448 20130101;
H01L 51/105 20130101; C09D 5/4484 20130101; H01J 9/025 20130101;
C25D 13/02 20130101; H01L 51/0048 20130101 |
Class at
Publication: |
205/50 ; 205/316;
205/317; 977/742 |
International
Class: |
C25D 9/04 20060101
C25D009/04; C25D 7/00 20060101 C25D007/00 |
Claims
1. A method for the deposition of carbon nanotubes, comprising: (a)
providing an electrochemical cell that comprises a cathode, an
anode plate, a first electrically conducting pathway connecting the
cathode to an electrical power supply, and a second electrically
conducting pathway connecting the electrical power supply to the
anode plate; (b) providing as an aqueous electrolyte disposed
between the cathode and the anode a dispersion of a complex formed
from carbon nanotubes and a first anionic polymer; (c) applying a
voltage to the electrochemical cell to deposit the complex on the
anode; and (d) removing the anode plate from the electrochemical
cell and firing the plate in air.
2. A method according to claim 1 wherein the aqueous electrolyte
further comprises a coagulant.
3. A method according to claim 2 wherein coagulant residue is
deposited on the anode together with the complex.
4. A method according to claim 1 wherein the first polymer
comprises a nucleic acid molecule.
5. A method according to claim 1 wherein the first polymer
comprises RNA.
6. A method according to claim 1 wherein the electrolyte further
comprises a second anionic polymer.
7. A method according to claim 6 wherein the second ionic polymer
comprises a styrenic ionomer or an ionized ethylene/(meth)acrylic
acid
8. A method according to claim 6 wherein the complex, as deposited
on the anode, is deposited in a matrix of the second anionic
polymer.
9. A method according to claim 7 wherein the first polymer
comprises a nucleic acid molecule.
10. A method according to claim 1 further comprising a step of
removing the anode plate from the cell, and installing it in a
field emission device.
11. A method for the deposition of carbon nanotubes, comprising:
(a) providing an electrochemical cell that comprises a cathode, an
anode plate, a first electrically conducting pathway connecting the
cathode to an electrical power supply, and a second electrically
conducting pathway connecting the electrical power supply to the
anode plate; (b) providing an aqueous electrolyte disposed between
the cathode and the anode, wherein the electrolyte comprises boric
acid and/or a borate compound, and a dispersion of a complex formed
from carbon nanotubes and a first anionic polymer; and (c) applying
a voltage to the electrochemical cell to deposit the complex on the
anode.
12. A method according to claim 11 wherein boric acid and/or a
borate compound is deposited on the anode together with the
complex.
13. A method according to claim 11 wherein the aqueous electrolyte
further comprises a coagulant.
14. A method according to claim 13 wherein coagulant residue is
deposited on the anode together with the complex.
15. A method according to claim 11 wherein the first polymer
comprises a nucleic acid molecule.
16. A method according to claim 11 wherein the first polymer
comprises RNA.
17. A method according to claim 11 wherein the electrolyte further
comprises a second anionic polymer.
18. A method according to claim 17 wherein the second ionic polymer
comprises a styrenic ionomer or an ionized ethylene/(meth)acrylic
acid copolymer.
19. A method according to claim 17 wherein the first polymer
comprises a nucleic acid molecule.
20. A method according to claim 11 further comprising a step of
removing the anode plate from the cell, and installing it in a
field emission device.
21. A film comprising a substrate and, disposed on the substrate,
(a) boric acid and/or a borate compound, and (b) a complex formed
from carbon nanotubes and a first anionic polymer.
22-28. (canceled)
29. A cathode assembly for a field emission device comprising a
film according to claim 21.
30. A field emission device comprising a cathode assembly according
to claim 29.
31-35. (canceled)
Description
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e) from, and claims the benefit of, U.S. Provisional
Application No. 61/032,505, filed Feb. 29, 2008, which is by this
reference incorporated in its entirety as a part hereof for all
purposes.
TECHNICAL FIELD
[0002] This invention relates to the electrochemical deposition of
carbon nanotubes ("CNTs") on a substrate.
BACKGROUND
[0003] Carbon nanotubes are well known to have unique and useful
electrical properties, and are frequently used in the fabrication
of the cathode of a field emission device ("FED"). However,
adoption of these materials is constrained by their high cost
[0004] US 2006/0063464 describes the deposition of carbon nanotubes
by electrochemical methods. A need remains, however, for methods
for the electrodeposition of carbon nanotubes that produce electron
field emitters with good uniformity and low material consumption,
and in which a relatively high emission current is consistently
obtained from a relatively low voltage input.
SUMMARY
[0005] One objective of this invention is thus to provide a method
for making a uniform CNT film on a substrate such as a conducting
substrate with good uniformity and low material consumption.
Another objective is to provide a method for making a CNT film
that, when used as electron field emitter, consistently produces a
relatively high emission current from a relatively low voltage
input A further objective is to provide from this method a CNT film
that may be easily patterned for use in electronic applications.
The CNT film so patterned may be used, for example, in a cathode
assembly that is installed in a field emission device.
[0006] One embodiment of this invention thus provides a method for
the deposition of carbon nanotubes by (a) providing an
electrochemical cell that comprises a cathode, an anode plate, a
first electrically conducting pathway connecting the cathode to an
electrical power supply, and a second electrically conducting
pathway connecting the electrical power supply to the anode plate;
(b) providing as an aqueous electrolyte disposed between the
cathode and the anode a dispersion of a complex formed from carbon
nanotubes and a first anionic polymer; (c) applying a voltage to
the electrochemical cell to deposit the complex on the anode; and
(d) removing the anode plate from the electrochemical cell and
firing the plate in air.
[0007] In another embodiment, this invention provides a method for
the deposition of an electron emitting material on a substrate by
(a) providing an electrochemical cell that comprises a cathode, an
anode plate, a first electrically conducting pathway connecting the
cathode to an electrical power supply, and a second electrically
conducting pathway connecting the electrical power supply to the
anode plate; (b) providing an aqueous electrolyte disposed between
the cathode and the anode, wherein the electrolyte comprises boric
acid and/or a borate compound, and a dispersion of a complex formed
from carbon nanotubes and a first anionic polymer; and (c) applying
a voltage to the electrochemical cell to deposit the complex on the
anode.
[0008] In a further embodiment, this invention provides a film that
includes a substrate and, disposed on the substrate, (a) boric acid
and/or a borate compound, and (b) a complex formed from carbon
nanotubes and a first anionic polymer. Alternatively, in this
embodiment, there may further be disposed on the substrate
coagulant residue.
[0009] In yet another embodiment, this invention provides a method
for the deposition of an electron emitting material on a substrate,
by (a) depositing an electron emitting material on a substrate to
prepare an electron field emitter; (b) installing the electron
field emitter as the anode plate in an electrochemical cell,
wherein the electrochemical cell further comprise a cathode, a
first electrically conducting pathway connecting the cathode to an
electrical power supply, and a second electrically conducting
pathway connecting the electrical power supply to the anode plate;
(c) providing an electrolyte disposed between the cathode and the
anode plate that comprises boric acid and/or a borate compound; and
(d) applying a voltage to the electrochemical cell.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 shows a schematic representation of the mechanism of
deposit in one embodiment of the methods of this invention.
[0011] FIG. 2 shows the configuration of an electrochemical cell as
used in the examples.
[0012] FIG. 3 shows a plot of the results of Example 1.
[0013] FIG. 4 shows a plot of the results of Example 2.
DETAILED DESCRIPTION
[0014] A CNT film is made by the method of this invention by the
deposition of CNTs on a substrate by electrochemical means, and for
such purpose the method hereof involves the use of an
electrochemical cell. The cell contains a cathode, an anode plate,
a first electrically conducting pathway connecting the cathode to
an electrical power supply, and a second electrically conducting
pathway connecting the electrical power supply to the anode plate.
An aqueous electrolyte is provided to the cell and is disposed
between the anode plate and the cathode. Contained in the
electrolyte is a dispersion of a complex formed from CNTs and a
first anionic polymer, and optionally a coagulant.
[0015] CNTs as used herein are generally about 0.5-2 nm in diameter
where the ratio of the length dimension to the narrow dimension,
i.e. the aspect ratio, is at least 5. In general, the aspect ratio
is between 10 and 2000. CNTs are comprised primarily of carbon
atoms, however may be doped with other elements, e.g. metals. The
carbon-based nanotubes of the invention can be either multi-walled
nanotubes (MWNTs) or single-walled nanotubes (SWNTs). A MWNT, for
example, includes several concentric nanotubes each having a
different diameter. Thus, the smallest diameter tube is
encapsulated by a larger diameter tube, which in turn, is
encapsulated by another larger diameter nanotube. A SWNT, on the
other hand, includes only one nanotube.
[0016] CNTs may be produced by a variety of methods, and are
additionally commercially available. Methods of CNT synthesis
include laser vaporization of graphite [A. Thess et al, Science
273, 483 (1996)], arc discharge [C. Journet et al, Nature 388, 756
(1997)] and HiPCo (high pressure carbon monoxide) process [P.
Nikolaev et al, Chem. Phys. Lett. 313, 91-97 (1999)]. Chemical
vapor deposition (CVD) can also be used in producing carbon
nanotubes [J. Kong et al, Chem. Phys. Lett. 292, 567-574 (1998); J.
Kong et al, Nature 395, 878-879 (1998); A. Cassell et al, J. Phys.
Chem. 103, 6484-6492 (1999); H. Dai et al, J. Phys. Chem. 103,
11246-11255 (1999)]. Additionally CNTs may be grown via catalytic
processes both in solution and on solid substrates [Yan Li et al,
Chem. Mater.; 2001; 13(3); 1008-1014); (N. Franklin and H. Dai,
Adv. Mater. 12, 890 (2000); A. Cassell et al, J. Am. Chem. Soc.
121, 7975-7976 (1999)].
[0017] A major obstacle to the use of CNTs has been the diversity
of tube diameters, chiral angles, and aggregation states in
nanotube samples obtained from the various preparation methods.
Aggregation is particularly problematic because the highly
polarizable, smooth-sided fullerene tubes readily form parallel
bundles or ropes with a large van der Waals binding energy. This
bundling perturbs the electronic structure of the tubes, and it
confounds almost all attempts to separate the tubes by size or type
or to use them as individual macromolecular species.
[0018] There is thus provided by this invention a method for
dispersing a population of bundled carbon nanotubes by contacting
the bundled nanotubes with an aqueous solution of an anionic
polymer. A complex containing the anionic polymer and the CNTs is
thereby formed, but the association between the anionic polymer and
the CNTs in the complex is a loose association, is formed
essentially by van der Waals bonds or some other non-covalent
means, and is not formed through the interaction of specific
functionalized groups. The structural integrity of the CNTs is
therefore retained, but the complexes they form with the anionic
polymers become, when present in the electrolyte, suspended in a
dispersion in the electrolyte.
[0019] A variety of anionic polymers may thus be used as
dispersants for the purpose of dispersing CNTs in an aqueous
solution by facilitating the formation of the polymer/CNT complex,
but a preferred polymer for use for such purpose is a nucleic acid,
particularly a stabilized solution of nucleic acid molecules.
Nucleic acids are very effective in dispersing CNTs by the
formation of nanotube-nucleic acid complexes based on non-covalent
interactions between the nanotube and the nucleic acid molecule.
The method of this invention therefore includes a method for the
dispersion of bundled CNTs by contacting the nanotubes with a
solution of anionic polymers such as nucleic acid molecules.
[0020] In the following discussion of the use of nucleic acid
molecules to form complexes with and thereby disperse CNTs, the
following terms and abbreviations are used:
[0021] "cDNA" means complementary DNA
[0022] "PNA" means peptide nucleic acid
[0023] "SEM" means scanning electron microscopy
[0024] "ssDNA" means single stranded DNA
[0025] "tRNA" means transfer RNA
[0026] "CNT" means carbon nanotube
[0027] "MWNT" means multi-walled nanotube
[0028] "SWNT" means single walled nanotube
[0029] "TEM" means transmission electron microscopy
[0030] A "nucleic acid molecule" is defined as a polymer of RNA,
DNA, or peptide nucleic acid (PNA) that is single- or
double-stranded, optionally containing synthetic, non-natural or
altered nucleotide bases. A nucleic acid molecule in the form of a
polymer of DNA may be comprised of one or more segments of cDNA,
genomic DNA or synthetic DNA.
[0031] The letters "A", "G", "T", "C" when referred to in the
context of nucleic acids will mean the purine bases adenine
(C.sub.5H.sub.5N.sub.5) and guanine (C.sub.5H.sub.5N.sub.5O) and
the pyrimidine bases thymine (C.sub.5H.sub.6N.sub.2O.sub.2) and
cytosine (C.sub.4H.sub.5N.sub.3O), respectively.
[0032] The term "peptide nucleic acids" refers to a material having
stretches of nucleic acid polymers linked together by peptide
linkers.
[0033] A "stabilized solution of nucleic acid molecules" refers to
a solution of nucleic acid molecules that are solubilized and in a
relaxed secondary conformation.
[0034] A "nanotube-nucleic acid complex" means a composition
comprising a carbon nanotube loosely associated with at least one
nucleic acid molecule. Typically the association between the
nucleic acid and the nanotube is by van der Waals bonds or some
other non-covalent means.
[0035] The term "agitation means" refers to a devices that
facilitate the dispersion of nanotubes and nucleic acids. A typical
agitation means is sonication.
[0036] The term "denaturant" refers to substances effective in the
denaturation of DNA and other nucleic acid molecules.
[0037] Standard recombinant DNA and molecular biology techniques
used here are well known in the art and are described, for example,
by Sambrook, Fritsch and Maniatis, Molecular Cloning: A Laboratory
Manual, Second Edition, Cold Spring Harbor Laboratory Press, Cold
Spring Harbor, N.Y. (1989) (hereinafter "Maniatis"); by Silhavy,
Bennan and Enquist, Experiments with Gene Fusions, Cold Spring
Harbor Laboratory Cold Press Spring Harbor, N.Y. (1984); and by
Ausubel et al, Current Protocols in Molecular Biology, published by
Greene Publishing Assoc. and Wiley-Interscience (1987).
[0038] Nucleic acid molecules as used in a method of this invention
may be of any type and from any suitable source, and include
without limitation DNA, RNA and peptide nucleic acids. The nucleic
acid molecules used herein may be generated by synthetic means or
may be isolated from nature by protocols known in the art (see,
e.g., Sambrook supra). The nucleic acid molecules may be either
single stranded or double stranded and may optionally be
functionalized at any point with a variety of reactive groups,
ligands or agents. Functionalization of nucleic acids is not,
however, required for their association with CNTs for the purpose
of dispersion, and most of the nucleic acids used herein for
dispersion do lack functional groups and are therefore referred to
herein as "unfunctionalized".
[0039] Peptide nucleic acids (PNA) are particularly useful herein
for dispersion as they possess the double functionality of both
nucleic acids and peptides. Methods for the synthesis and use of
PNA's are known in the art as discussed, for example, in
Antsypovitch, Peptide nucleic acids: Structure, Russian Chemical
Reviews (2002), 71(1), 71-83.
[0040] Nucleic acid molecules as used herein may have any
composition of bases and may even consist of stretches of the same
base (poly A or poly T for example) without impairing the ability
of the nucleic acid molecule to disperse the bundled CNTs.
Preferably the nucleic acid molecules will be less than about 2000
bases where less than 1000 bases is preferred and where from about
5 bases to about 1000 bases is most preferred. Generally the
ability of nucleic acids to disperse CNTs appears to be independent
of sequence or base composition, however there is some evidence to
suggest that the less G-C and T-A base-pairing interactions in a
sequence, the higher the dispersion efficiency, and that RNA and
varieties thereof is particularly effective in dispersion and is
thus preferred herein. Nucleic acid molecules suitable for use
herein include without limitation those having the general formula:
[0041] 1. An wherein n=1-2000; [0042] 2. Tn wherein n=1-2000;
[0043] 3. Cn wherein n=1-2000; [0044] 4. Gn wherein n=1-2000;
[0045] 5. Rn wherein n=1-2000, and wherein R may be either A or G;
[0046] 6. Yn wherein n=1-2000, and wherein Y may be either C or T;
[0047] 7. Mn wherein n=1-2000, and wherein M may be either A or C;
[0048] 8. Kn wherein n=1-2000, and wherein K may be either G or T;
[0049] 9. Sn wherein n=1-2000, and wherein S may be either C or G;
[0050] 10. Wn wherein n=1-2000, and wherein W may be either A or T;
[0051] 11. Hn wherein n=1-2000, and wherein H may be either A or C
or T; [0052] 12. Bn wherein n=1-2000, and wherein B may be either C
or G or T; [0053] 13. Vn wherein n=1-2000, and wherein V may be
either A or C or G; [0054] 14. Dn wherein n=1-2000, and wherein D
may be either A or G or T; and [0055] 15. Nn wherein n=1-2000, and
wherein N may be either A or C or T or G.
[0056] In addition to the combinations listed above, any of these
sequences may have one or more deoxyribonucleotides replaced by
ribonucleotides (i.e. RNA or RNA/DNA hybrid) or one or more
sugar-phosphate linkages replaced by peptide bonds (i.e. PNA or
PNA/RNA/DNA hybrid).
[0057] Nucleic acid molecules as used herein may be stabilized in a
suitable solution. It is preferred that the nucleic acid molecules
be in a relaxed secondary conformation and only loosely associated
with each other to allow for the greatest contact by individual
strands with the CNTs. Stabilized solutions of nucleic acids are
common and well known in the art (see Sambrook, supra) and
typically include salts and buffers such as sodium and potassium
salts, and TRIS (Tris(2-aminoethyl)amine), HEPES
(N-(2-hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid), and MES
(2-(N-Morpholino)ethanesulfonic acid. Preferred solvents for
stabilized nucleic acid solutions are those that are water miscible
where water is most preferred. The process of dispersion may be
improved with the optional addition of nucleic acid denaturing
substances to the solution. Common denaturants include but are not
limited to formamide, urea and guanidine. A non-limiting list of
suitable denaturants may be found in Sambrook, supra.
[0058] To prepare a dispersion according to one embodiment of the
method hereof, an anionic polymer such as one or more nucleic acid
molecules may be contacted with a population of bundled carbon
nanotubes. It is preferred, although not required, that contact be
made in the presence of an agitation means of some sort. Typically
the agitation means employs sonication, but may also include
devices that produce high shear mixing of the nucleic acids and
CNTs (i.e. homogenization), or any combination thereof. Upon
agitation, the CNTs will become dispersed and will form
nanotube-nucleic acid complexes comprising at least one nucleic
acid molecule loosely associated with the CNT by hydrogen bonding
or some other non-covalent means.
[0059] Temperature during the process of contacting CNTs with a
nucleic acid may have an effect on the efficacy of the dispersion.
Mixing at room temperature or higher has been seen to give longer
dispersion times whereas mixing at temperatures below room
temperature (23.degree. C.) has been seen to give more rapid
dispersion times, and temperatures of about 4.degree. C. are
preferred. The dispersion of CNTs by contact with nucleic acid
molecules is also described in US 2004/0132072 and US 2004/0146904,
each of which is by this reference incorporated in its entirety as
a part hereof for all purposes.
[0060] In addition to the nucleic acid molecules described above,
one or more other anionic polymers may be used for the purpose of
preparing an aqueous dispersion of CNTs. Examples of other anionic
polymers that are suitable for use in the preparation of a
dispersion of CNTs include without limitation ionized poly(acrylic
acid) ("PAA") or ionized ethylene/(meth)acrylic acid copolymer
("EAA" or "EMAA"), either of which may be neutralized with cations
such as Na.sup.+, K.sup.+, NH.sub.4.sup.+ or Cr.sup.+; styrenic
ionomers such as styrene/sodium styrene sulfonate copolymer (PSS)
or styrene/sodium styrene methacrylate copolymer; and ionized
tetrafluoroethylene/sulfonic acid copolymers such as Nafion.TM.
copolymer (from DuPont) in which the sulfonic acid group in a
tetrafluoroethylene/perfluorovinyl ether copolymer may be sodium
neutralized. As indicated above with respect to nucleic acid
molecules, ultrasonication or other mixing means may be applied to
facilitate the dispersion of CNTs in an aqueous solution of one or
more of the anionic polymers discussed above.
[0061] In one embodiment, deposition on the anode plate of the cell
of the complexes formed from CNTs and molecules of an anionic
polymer, as dispersed in the electrolyte solution contained in the
cell, will be facilitated by the presence therein of the optional
coagulant. The coagulant will neutralize the negative charge on the
anionic polymer in the complex. As the population of anionic
polymer/CNT complexes has been maintained in dispersion primarily
by the repulsion of one negatively charged complex from another (or
by the repulsion of positively charged double layers surrounding
the complexes), neutralization of those negative charges (or
compression of the double layer) by the coagulant will remove the
force enabling the population of complexes to remain in dispersion
in the electrolyte solution. As the action of the coagulant to
neutralize the complexes occurs in close proximity to the anode
plate, the complexes (as no longer dispersed) will in varying
degrees undergo a transition from solution phase to solid phase,
become progressively aggregated and agglomerated (similar to the
formation of floccules and flocs), and then be collected and
deposited on the surface of the anode plate. In addition to the CNT
complexes, the material as deposited on the plate may include
coagulant residue.
[0062] When first and second anionic polymers are present in the
electrolyte solution, they may be, for example, a first polymer
that forms a complex with CNTs, and a second polymer that does not
form a complex, or that is more loosely associated with CNTs than
the first polymer. The first and second polymers may become
deposited on the surface of the anode at the same time, and the
first polymer may, for example, be deposited in a matrix of the
second polymer. If additional materials, such as conductive or
functionalized particles, are needed to enhance the usefulness and
performance of the cell anode plate as a component in a field
emission device are present in the electrolyte solution, those
materials may be deposited on the anode plate at the same time as
the anionic polymer/CNT complexes. FIG. 2 shows a typical example
of the type of film formed by such deposition on the anode plate,
which film has good uniformity of evenly deposited, well-adhered
material all across its surface.
[0063] Coagulants suitable for use herein for the purpose of
neutralizing an anionic polymer/CNT complex include inorganic
coagulants such as trivalent cations formed from metals including
Group VIII/VIIIA metals such as iron, cobalt, ruthenium or osmium.
As a trivalent cation can be up to as much as ten times more
effective in neutralizing the complex than a divalent cation, a
convenient way to provide the coagulant is to supply a divalent
cation such as tris(2,2'-bipyridyl)dichloro-ruthenium (II) to the
electrolyte solution wherein the 2.sup.+ cation is oxidized to a
3.sup.+ valence by the loss of electrons to the anode plate. A
schematic representative of this mechanism is shown in FIG. 1. In
the case, for example, of the use of a metal cation as the
coagulant, coagulant residue will thus be the cation as oxidized by
interaction with an anionic polymer/CNT complex.
[0064] In an alternative embodiment, however, a coagulant is not
used where the anode plate is formed from a metal such as silver or
nickel. In such case, the metal on the plate dissolves in the
electrolyte solution, and the charge on an anionic polymer/CNT
complex is neutralized by cations formed from metal atoms that have
gone into solution from the solid metal from which the plate if
formed.
[0065] In a further alternative embodiment, the electrolyte
solution may contain, in addition to one or more anionic polymers
and the optional coagulant, boric acid and/or a borate compound.
Borate compounds suitable for use in the electrolyte solution
include, for example, those represented by the structural formula
B--(--R.sup.3)(--R.sup.4)(--R.sup.5), wherein R.sup.3, R.sup.4 and
R.sup.5 may be the same or different, and each independently
represents an alkyloxy group, an alkenyloxy group, an aryloxy
group, an aralkyloxy group or a halogen atom; and when R.sup.4 and
R.sup.5 are an alkyloxy group, an alkenyloxy group, an aryloxy
group or an aralkyloxy group, R.sup.4 and R.sup.5 may be combined
to each other to form a cyclic structure together with the boron
atom.
[0066] The alkyloxy group represented by R.sup.3, R.sup.4 or
R.sup.5 may have a substituent and specifically, the alkyloxy group
is preferably a substituted or unsubstituted, linear or branched
alkyloxy group having from 1 to 10 carbon atoms. Examples thereof
include methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy,
sec-butoxy, pentyloxy, hexyloxy, heptyloxy, octyloxy,
3-methoxypropoxy, 4-chlorobutoxy and 2-diethylaminoethoxy.
[0067] The alkenyloxy group represented by R.sup.3, R.sup.4 or
R.sup.5 may have a substituent and specifically, the alkenyloxy
group is preferably a substituted or unsubstituted, linear or
branched alkenyloxy group having from 3 to 12 carbon atoms.
Examples thereof include a propenyloxy group, a butenyloxy group, a
pentenyloxy group, a hexenyloxy group, a heptenyloxy group, an
octenyloxy group, a dodecenyloxy group and prenyloxy group.
[0068] The aryloxy group represented by R3, R.sup.4 or R.sup.5 may
have a substituent and specifically, the aryloxy group is a
substituted or unsubstituted aryloxy group. Examples thereof
include phenoxy, tolyloxy, xylyloxy, 4-ethylphenoxy,
4-butylphenoxy, 4-tert-butylphenoxy, 4-methoxyphenoxy,
4-diethylaminophenoxy, 2-methylphenoxy, 2-methoxyphenoxy,
1-naphthoxy, 2-naphthoxy and 4-methylnaphthoxy.
[0069] The aralkyloxy group represented by R.sup.3, R.sup.4 or
R.sup.5 may have a substituent and specifically, the aralkyloxy
group is a substituted or unsubstituted aralkyloxy group. Examples
thereof include a benzyloxy group, a phenethyloxy group, a
phenylpropyloxy group, a 1-naphthylmethyloxy group, a
2-naphthylmethyloxy group and a 4-methoxybenzyloxy group.
[0070] Specific examples of a borate compound suitable for use
herein include a trimethyl borate, a triethyl borate, a
tri-n-propyl borate, a truisopropyl borate, a tri-n-butyl borate, a
truisobutyl borate, a tri-n-octyl borate, a butyldiethyl borate, an
ethyldi(2-phenethyl)borate, a triphenyl borate, a
diethyl-4-methoxyphenyl borate, a diethylcyclohexyl borate,
trichloroborane, trifluoroborane, diethoxychloroborane,
n-butoxydichloroborane, and tris borate(ethylenediaminetetraacetic
acid).
[0071] Specific examples of a compound having a cyclic structure
containing a boron atom and two oxygen atoms within the ring formed
by combining R.sup.4 and R.sup.5 to each other include
2-methoxy-1,3,2-dioxaborinane, 2-ethoxy-1,3,2-dioxaborolane,
2-butoxy-1,3,2-dioxaborinane, 2phenoxy-1,3,2-dioxaborinane,
2-phenoxy-4,4,6-trimethyl1,3,2-dioxaborinane,
2-naphthoxy-1,3,2-dioxaborinane, 2-methoxy-1,3,2-benzodioxaborole
and 2-ethoxy-1,3,2-benzodioxaborin.
[0072] In this embodiment, boric acid and/or a borate compound may
be used in the electrolyte solution at a concentration therein in
the range of about 0.1 wt % or more, or about 0.5 wt % or more, and
yet about 10 wt % or less, or about 5 wt % or less.
[0073] In this embodiment of the method hereof, where boric acid
and/or a borate compound is present in the electrolyte solution,
the material as deposited on the cell anode plate may thus include,
in addition to the CNT complexes, residue of the optional coagulant
and/or some of the boric acid and/or borate compound. In such
event, a further embodiment of this invention includes a film that
is composed of a substrate and, disposed or deposited on the
substrate, a complex formed from carbon nanotubes and one or more
anionic polymer(s), coagulant residue and/or boric acid and/or a
borate compound.
[0074] As the plate that is used an the anode in the electrolytic
cell will ultimately be used in the cathode assembly of a field
emission device, it is desirable that the plate as used in the cell
already be provided with conductive means onto which the CNTs may
be deposited. One example of a suitable plate to use for such
purpose is a glass plate, such as a soda lime glass plate, that is
coated with a conductive material such as indium tin oxide ("ITO").
Alternatively, however, the plate used for such purpose could be a
substrate on which conductive materials have first been deposited
by thick film paste methods such as described below.
[0075] The method hereof may be used to produce a film in which the
deposited material is deposited in a pre-determined pattern. This
may be accomplished by patterning the surface of the plate used as
the cell anode using conventional photoimaging techniques. Thus a
photoresist may be activated through a mask and then developed to
provide on the surface of the cell anode a pattern such as an array
of circular wells. As the anionic polymer/CNT complexes are
aggregated and settle out of solution, they are deposited only in
the circular wells, and the photoresist may be removed. This
provides a patterned CNT film, with the anode plate serving as a
substrate for the film, for use by installation in a field emission
device.
[0076] The method hereof is generally performed by operation of the
electrochemical cell at lower potential such as less than about 5
volts, or from about 2 to less than about 5 volts, or from about 2
volts to about 3 volts. Thickness of the deposited film is to a
large extent directly related to length of deposition time. A
deposition time in the range of about 1 to about 10 minutes, or in
the range of about 1 to about 2 minutes, may be used. A positive
potential is maintained at the cell anode plate relative to the
cathode of the cell.
[0077] After completion of the deposition of CNT complex material
on the anode plate in the cell, the plate may be removed from the
cell, rinsed, dried and installed in such condition in a field
emission device for use as part of the cathode assembly therein to
provide electron emission. Alternatively, however, before
installation in a field emission device, the plate may be baked
and/or fired to melt the deposited polymer(s) and utilize them in
that form as an adhesive to more securely anchor the CNTs to the
surface of the plate, resulting in a CNT-containing film with
excellent abrasion resistance. Firing may be performed at a
temperature in the range of about 250.degree. C. to about
650.degree. C., or about 350.degree. C. to about 550.degree. C., or
about 450.degree. C. to about 525.degree. C., for a period of time
in the range of about 5 to about 30 minutes, or about 10 to about
25 minutes, or about 10 to about 20 minutes, in an inert gas such
as nitrogen or in air.
[0078] After completion of the deposition of CNT complex material
on the anode plate in the cell, the plate may be installed in a
field emission device for use as part of the cathode assembly
therein to provide electron emission. When a voltage is applied to
the CNTs, the anode of the device is bombarded with electrons. The
anode of the field emission device is an electrode coated with an
electrically conductive layer. When the field emission device is
used in a display device where the cathode contains an array of
pixels of the thick film paste deposits described above, the FED
anode may comprise phosphors to convert incident electrons into
light. The substrate of the FED anode would also be selected to be
transparent so that the resulting light could be transmitted. From
the cathode assembly and FED anode, a sealed unit is constructed in
which the cathode assembly and anode are separated by spacers, and
there is an evacuated space between the anode and the cathode. This
evacuated space is under partial vacuum so that the electrons
emitted from the cathode may transit to the anode with only a small
number of collisions with gas molecules. Frequently the evacuated
space is evacuated to a pressure of less than 10.sup.-5 Torr.
[0079] Such a field emission device is useful in a variety of
electronic applications, e.g. vacuum electronic devices, flat panel
computer and television displays, back-light source for LCD
displays, emission gate amplifiers, and klystrons and in lighting
devices. For example, flat panel displays having a cathode using a
field emission electron source, i.e. a field emission material or
field emitter, and a phosphor capable of emitting light upon
bombardment by electrons emitted by the field emitter have been
proposed. Such displays have the potential for providing the visual
display advantages of the conventional cathode ray tube and the
depth, weight and power consumption advantages of the other flat
panel displays. The flat panel displays can be planar or curved.
U.S. Pat. Nos. 4,857,799 and 5,015,912 disclose matrix-addressed
flat panel displays using micro-tip cathodes constructed of
tungsten, molybdenum or silicon. WO 94-15352, WO 94-15350 and WO
94-28571 disclose flat panel displays wherein the cathodes have
relatively flat emission surfaces. These devices are also described
in US 2002/0074932, which is by this reference incorporated in its
entirety as a part hereof for all purposes.
[0080] In an alternative embodiment of this invention, a field
emission device may be made by preparing an electron field emitter
by conventional means. Such an electron field emitter would take
the form of a substrate on which electron emitting material has
been deposited, and would be suitable for use as, or to further
prepare, a cathode assembly for use in an FED. The conventional
means of preparing the electron field emitter would include, for
example, depositing an electron emitting material on a substrate by
screen printing a thick film paste. After the electron field
emitter has been prepared, it is then installed as the anode plate
in an electrolytic cell, as described elsewhere herein. An aqueous
electrolyte is provided to the cell and is disposed therein between
the cell cathode and the cell anode plate, which is the
previously-prepared electron field emitter. Contained in the
electrolyte is boric acid and/or a borate compound as described
above. A voltage is then applied to the cell, and the cell anode
plate (the previously-prepared electron field emitter) is then
removed from the cell.
[0081] In the preparation of an electron field emitter to be used
in this embodiment as the cell anode plate, there may be, for
example, a deposit on a substrate of a thick film paste containing
an electron emitting material. The electron emitting material
contained in the thick film paste may be any acicular emitting
material such as the CNTs described above, other forms of carbon
such as carbon fibers, a semiconductor, a metal or mixtures
thereof. Carbon fibers useful as an acicular emitting material may
be grown from the catalytic decomposition of carbon-containing
gases over small metal particles are also useful as acicular
carbon, and other examples of acicular carbon are
polyacrylonitrile-based (PAN-based) carbon fibers and pitch-based
carbon fibers. As used herein, "acicular" means particles with
aspect ratios of 10 or more. Typically, glass frit, metallic powder
or metallic paint or a mixture thereof is used to attach the
electron emitting material to the substrate in the electron field
emitter to be used as, or in the preparation of, a cathode
assembly.
[0082] In a conventional attachment of an electron emitting
material to a substrate, various screen printing-type processes can
be used. The means of attachment must withstand and maintain its
integrity under the conditions of manufacturing the apparatus into
which the field emitting cathode is placed and under the conditions
surrounding its use, e.g. typically vacuum conditions and
temperatures up to about 450.degree. C. A preferred method is to
screen print a paste comprised of the electron emitting material
and glass frit, metallic powder or metallic paint or a mixture
thereof onto a substrate in the desired pattern and to then fire
the dried patterned paste. For a wider variety of applications,
e.g. those requiring finer resolution, the preferred process
comprises screen printing a paste which further comprises a
photoinitiator and a photohardenable monomer, photopatterning the
dried paste and firing the patterned paste.
[0083] The substrate can be any material to which the paste
composition will adhere. If the paste is non-conducting and a
non-conducting substrate is used, a film of an electrical conductor
to serve as the cathode electrode and provide means to apply a
voltage to the electron emitting material will be needed. Silicon,
a glass, a metal or a refractory material such as alumina can serve
as the substrate. For display applications, the preferable
substrate is glass and soda lime glass is especially preferred. For
optimum conductivity on glass, silver paste can be pre-fired onto
the glass at 500-550.degree. C. in air or in an inert gas such as
nitrogen, but preferably in air, or the substrate may be coated
with a layer of ITO. The conducting layer so-formed can then be
over-printed with the emitter paste.
[0084] The paste used for conventional screen printing typically
contains the electron emitting material, an organic medium,
solvent, surfactant and either low softening point glass frit,
metallic powder or metallic paint or a mixture thereof. The role of
the medium and solvent is to suspend and disperse the particulate
constituents, i.e. the solids, in the paste with a proper rheology
for typical patterning processes such as screen printing. There are
many organic media known for use for such purpose including
cellulosic resins such as ethyl cellulose and alkyd resins of
various molecular weights. Butyl carbitol, butyl carbitol acetate,
dibutyl carbitol, dibutyl phthalate and terpineol are examples of
useful solvents. These and other solvents are formulated to obtain
the desired viscosity and volatility requirements.
[0085] A glass frit that softens sufficiently at the firing
temperature to adhere to the substrate and to the electron emitting
material is also used. A lead or bismuth glass frit can be used as
well as other glasses with low softening points such as calcium or
zinc borosilicates. If a screen printable composition with higher
electrical conductivity is desired, the paste may also contain a
metal, for example, silver or gold. The paste typically contains
about 40 wt % to about 80 wt % solids based on the total weight of
the paste. These solids include the electron emitting material and
glass frit and/or metallic components. Variations in the
composition can be used to adjust the viscosity and the final
thickness of the printed material.
[0086] If the screen-printed paste is to be photopatterned, the
paste may also contain a photoinitiator, a developable binder and a
photohardenable monomer comprised, for example, of at least one
addition polymerizable ethylenically unsaturated compound having at
least one polymerizable ethylenic group. Typically, a paste
prepared from an electron emitting material such as CNTs, silver
and glass frit will contain about 0.01-6.0 wt % nanotubes, about
40-75 wt % silver in the form of fine silver particles and about
3-15 wt % glass frit based on the total weight of the paste.
[0087] The emitter paste is typically prepared by three-roll
milling a mixture of the electron emitting material, organic
medium, surfactant, solvent and either low softening point glass
frit, metallic powder or metallic paint or a mixture thereof. The
paste mixture can be screen printed using, for example, a
165-400-mesh stainless steel screen. The paste can be deposited as
a continuous film or in the form of a desired pattern.
[0088] After printing, the conventionally-prepared electron field
emitter is further processed by removing any residual photoresist
material, drying the plate, and then installing it as the anode
plate in an electrochemical cell. The cell is similar in
construction to the cell described above, and the cathode therein
may be stainless steel or any non-oxidizable conductor. The
electrolyte, which is disposed between the cathode and the anode,
contains boric acid and/or a borate compound. This embodiment of
the methods hereof is generally performed by operation of the cell
at a potential of less than about 10 volts, or in the range of from
about 2 to about 6 volts, or in the range of from about 3 volts to
about 5 volts. The cell may be operated for a period of time in the
range of from about 1 to about 10 minutes, or in the range of from
about 2 to about 6 minutes, or in the range of from about 3 to
about 5 minutes.
[0089] After completion of operation of the cell, the plate may be
removed from the cell, rinsed, dried and installed in such
condition in a field emission device for use as part of the cathode
assembly therein to provide electron emission in devices such as
described above. Alternatively, however, before installation in a
field emission device, the plate may first be baked and/or fired to
melt the deposited polymer(s) and utilize them in that form as an
adhesive to more securely anchor the CNTs to the surface of the
plate, resulting in a CNT-containing film with excellent abrasion
resistance. Firing may be performed at a temperature in the range
of about 250.degree. C. to about 650.degree. C., or about
350.degree. C. to about 550.degree. C., or about 450.degree. C. to
about 525.degree. C., for a period of time in the range of about 5
to about 30 minutes, or about 10 to about 25 minutes, or about 10
to about 20 minutes, in nitrogen or air. Higher firing temperatures
can be used with substrates that can endure them provided the
atmosphere is free of oxygen. However, the organic constituents in
the paste are effectively volatilized at 350-450.degree. C.,
leaving a layer of the composite of the electron emitting material
and glass and/or metallic conductor.
[0090] In this embodiment of the methods hereof, where (as above)
boric acid and/or a borate compound is present in the electrolyte
solution, the material as deposited on the cell anode plate may
thus include, in addition to the CNT complexes, some of the boric
acid and/or borate compound. In such event, a further embodiment of
this invention includes a film that is composed of a substrate and,
disposed or deposited on the substrate, boric acid and/or a borate
compound and a complex formed from carbon nanotubes and one or more
anionic polymer(s).
[0091] In this embodiment, boric acid and/or a borate compound may
be used in the electrolyte solution at a concentration therein in
the range of about 0.1 wt % or more, or about 0.5 wt % or more, and
yet about 10 wt % or less, or about 5 wt % or less.
[0092] Materials as used in the process hereof may be made by
processes known in the art, or are available commercially from
suppliers such as Alfa Aesar (Ward Hill, Mass.), City Chemical
(West Haven, Conn.), Fisher Scientific (Fairlawn, N.J.),
Sigma-Aldrich (St. Louis, Mo.) or Stanford Materials (Aliso Viejo,
Calif.).
[0093] The advantageous attributes and effects of this invention
may be seen in a series of examples (Examples 1.about.5), as
described below. The embodiments on which the examples are based
are representative only, and the selection of those embodiments to
illustrate the invention does not indicate that materials,
conditions, specifications, components, reactants, techniques and
protocols not described in these examples are not suitable for
practicing this invention, or that subject matter not described in
these examples is excluded from the scope of the appended claims
and equivalents thereof.
Examples
[0094] 150 mg of laser-ablated CNTs (from CNI, Houston, Tex.) was
mixed with 30 mg yeast RNA (from Sigma Aldrich) in 15 mL of
1.times. TBE [tris borate (ethylenediaminetetraacetic acid)] buffer
(from Sigma Aldrich). The mixture was sonicated with a probe
sonicator at a power level of 20 W for 30 min. The resulting
dispersion ("CNT Dispersion") was mixed with two other components
according to the following table (Table 1) to make up 100 mL of
deposition solution. Ru.sup.2+(bipy).sub.3 as used in the
deposition solution is tris(2,2'-bipyridyl)dichloro-ruthenium (II)
and is obtained from Sigma Aldrich. EMMA is ethylene/methacrylic
acid ionomer obtained from DuPont as Surlyn.TM. ionomer.
TABLE-US-00001 TABLE 1 Composition of Deposition Solution Stock
Volume Final Component concentration added conc. CNT 10 mg/mL 4 mL
0.04% Dispersion EMMA 10 mg/mL 2 mL 0.02% Ru.sup.2+(bipy).sub.3 10
mM 2 mL 0.2 mM water 92 mL
Example 1
[0095] A photoresist (PR) patterned glass substrate coated with
indium tin oxide (ITO) (2''.times.2'') (used as the cell anode) was
prepared. The PR layer defines an array of open circular wells with
20 .mu.m diameter. The open circular wells expose the ITO surface
for CNT deposition. Before electrodeposition, the PR coated ITO
plate was dipped into a solution of 0.01% Triton X-100 for 30
seconds, taken out and dried by blowing N.sub.2 gas. This step is
to coat the hydrophobic PR layer with a thin hydrophilic layer for
better wetting.
[0096] After this treatment, a 2''.times.2'' stainless steel plate
(used as the cell cathode) and the PR-coated ITO plate (used as the
cell anode) were inserted in a parallel fashion into a rectangular
cell containing 15 mL of the deposition solution. FIG. 2 shows the
rectangular cell containing deposition solution into which the
stainless steel cathode and PR-coated anode are inserted in a
parallel fashion. The electrochemical cell is designated as number
1, the slot for the cathode is designated as number 2 and the slot
for the anode is designated as number 3.
[0097] A DC potential of 2.5 V (obtained from a Princeton Applied
Research, Model 263A, Oak Ridge, Tenn.) was applied between the two
electrodes. After 2 min, the deposition was stopped, and the ITO
plate was taken out of the cell, rinsed with DI water and dried in
air. The PR layer was stripped off by organic solvents such as
acetone or a NMP:H.sub.2O solution. The cell anode was then rinsed
in DI H.sub.2O and dried under flowing N.sub.2 gas.
[0098] Control samples were made using the same laser ablated
carbon nanotube powder from which the dispersion described above
was derived. The nanotube powder was incorporated into a paste and
screen printed onto a 2''.times.2'' PR patterned ITO substrate.
After imaging under UV exposure, the printed substrate was rinsed
for 65 seconds in a NMP:H.sub.2O solution.
[0099] Both the control and the electrochemically deposited
substrates were fired in air in a 10-zone belt furnace (Lindberg,
810 thick-film conveyor, Watertown, Wis.) to 400.degree. C. peak
for 21 minutes. The substrates were then activated by placing an
adhesive in contact with the patterned surface. Each activated
substrate was then incorporated into a diode device as the cathode,
with a 620 .mu.m spacer between the 2''.times.2'' ITO coated
phosphor glass substrate that served as the anode. The diode thus
formed was placed in a vacuum chamber evacuated to a base pressure
below 1.times.10.sup.-5 Torr.
[0100] A negative voltage pulse with a pulse width of 60 us at 60
Hz was applied to each diode using an IRCO high voltage source
(Model F5k-10-02N, IRCO, Columbia Md.). The pulsing was supplied
from a pulse generator (Stanford Research Systems, Inc., model
DG535, Sunnyvale, Calif.). The resulting emission current was
measured as a function of applied voltage using a Keithley 2000
multimeter (Keithley Instruments, Cleveland, Ohio). The field
required to obtain 20 .mu.A or more was noted. For the control
sample, this field was found to be generally 4.5V/.mu.m or greater.
For the electrochemically deposited sample, the field was generally
on the order of 2.5V/.mu.m. FIG. 3 shows the average emission
fields from the samples that were made from an electrochemical
deposition (ECD) technique (square) and from a screen-printing (non
ECD) technique (circle). Lower operational fields are
preferred.
Example 2
[0101] Carbon nanotube powder made from a laser ablation process
was incorporated into a thick film paste and screen printed onto a
2''.times.2'' photoresist (PR) patterned glass substrate coated
with indium tin oxide (ITO). The PR layer defines an array of open
circular wells with 20 .mu.m diameter. The open wells expose the
ITO surface onto which the CNT-containing paste can be screen
printed. After imaging the printed surface under UV exposure, the
substrate was rinsed for 65 seconds in a NMP:H.sub.2O solution to
reveal the patterned structure.
[0102] A 2''.times.2'' stainless steel plate (used as the cell
cathode) and the 2''.times.2'' screen printed substrate on ITO
(used as the cell anode) were inserted in a parallel fashion into a
rectangular cell (as shown in FIG. 2) containing 15 mL of
electrolyte solution (1.times. TBE or 0.1 M Boric acid, Sigma
Aldrich). A DC potential of 3V (Princeton Applied Research, Model
263A) was applied between the two electrodes. After 4 min, the
treatment was stopped, and the ITO plate was taken out of the cell,
and allowed to dry in air.
[0103] The substrate (cell anode) was then fired in air to
400.degree. C. peak for 21 minutes in a 10-zone belt furnace
(Lindberg, 810 thick-film conveyor, Watertown, Wis.). The substrate
was then activated by placing an adhesive in contact with the
patterned surface containing the carbon nanotube paste. The
substrate was then incorporated into a diode device as the cathode,
separated from the ITO coated phosphor glass anode by a 620 .mu.m
spacer. The diode thus formed was placed in a vacuum chamber
evacuated to a base pressure below 1.times.10.sup.-5 Torr.
[0104] A negative voltage pulse with a pulse width of 60 us at 60
Hz was applied using an IRCO high voltage source (Model F5k-10-02N,
IRCO, Columbia, Md.). The pulsing was supplied from a pulse
generator (Stanford Research Systems, Inc., Model DG535, Sunnyvale,
Calif.). The resulting emission current was measured as a function
of applied voltage using a Keithley 2000 multimeter (Keithley
Instruments, Cleveland, Ohio). The field required to obtain 20
.mu.A or more was noted.
[0105] For a control sample not subjected to electrochemical
treatment, this field was found to be generally greater than
5V/.mu.m. For the electrochemically treated sample, the fields
needed were generally on the order of 2.5V/.mu.m to 3.0V/.mu.m.
FIG. 4 shows the emission curves from screen printed samples that
were either treated in an electrochemical cell (solid lines) or not
treated in an electrochemical cell (dotted lines). Lower
operational fields for any given current are preferred.
[0106] Features of certain of the devices of this invention are
described herein in the context of one or more specific embodiments
that combine various such features together. The scope of the
invention is not, however, limited by the description of only
certain features within any specific embodiment, and the invention
also includes (1) a subcombination of fewer than all of the
features of any described embodiment, which subcombination may be
characterized by the absence of the features omitted to form the
subcombination; (2) each of the features, individually, included
within the combination of any described embodiment; and (3) other
combinations of features formed by grouping only selected features
of two or more described embodiments, optionally together with
other features as disclosed elsewhere herein.
[0107] In this specification, unless explicitly stated otherwise or
indicated to the contrary by the context of usage, where an
embodiment of the subject matter hereof is stated or described as
comprising, including, containing, having, being composed of or
being constituted by or of certain features or elements, one or
more features or elements in addition to those explicitly stated or
described may be present in the embodiment. An alternative
embodiment of the subject matter hereof, however, may be stated or
described as consisting essentially of certain features or
elements, in which embodiment features or elements that would
materially alter the principle of operation or the distinguishing
characteristics of the embodiment are not present therein. A
further alternative embodiment of the subject matter hereof may be
stated or described as consisting of certain features or elements,
in which embodiment, or in insubstantial variations thereof, only
the features or elements specifically stated or described are
present.
* * * * *