Note: Descriptions are shown in the official language in which they were submitted.
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DIAMOND COATED ELECTRODES FOR ELECTROCHEMICAL PROCESSING AND
APPLICATIONS THEREOF
TECHNICAL FIELD
[0001] In at least one aspect, the present invention relates to electrodes
for water
electrolyzers and ozonizers.
BACKGROUND
[0002] Ozone is a strong oxidant that is used for water treatment and
disinfection. In many
applications, ozone replaces chlorine because of unwanted by-product formation
connected with the
latter. Ozone dissolved in water is used for disinfection of microbes and
organic pollutants,
wastewater treatment, and the like. The electrochemical production of ozone
has advantages over the
conventional technologies such as corona discharge. Ozone from electrochemical
production is
directly dissolved in water; thereby minimizing technical problems associated
with handling ozone
gas which is toxic at high concentrations.
[0003] During water electrolysis, oxygen evolution is the main rival
reaction to ozone
production. Thermodynamically, oxygen evolution is strongly favored versus
ozone production.
Therefore, high current efficiencies for electrochemical ozone production are
only possible for anode
materials with a high overpotential for oxygen evolution. In the recent years,
doped diamond
electrodes have been developed and investigated for generation of dissolved
ozone. Besides other
interesting properties, doped diamond is distinguished by an exceptionally
high overvoltage for
oxygen evolution in aqueous electrolytes which makes even highly efficient OH
radical production
possible. In addition, diamond and related materials are stable in aqueous
electrolytic processes.
[0004] Figure 1 provides a schematic of a typical scheme of the
electrolytic cell for ozone
production. Ozonizer 10 includes solid polymer electrolyte (SPE) membrane 12,
which functions as
both electrolyte and separator between anode 14 and cathode 16 contacting the
activated electrodes
on both sides. Water fed to the anode side of the cell is dissociated at the
interface 20 between the
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anode and membrane 12 as a result of an applied DC current from power supply
22. To ensure that
as much ozone as possible is produced, anode 14 typically has an over-
potential that is above the
decomposition of water and the ozone reaction potential. The electrodes
usually have holes, grooves
or porosity to provide both a path for water to reach the active inside
surface and an escape route for
gas produced by the electrolytic reactions; e.g., hydrogen on cathode side,
oxygen and ozone on
anode side. Moreover, membrane 12 is impermeable for ozone molecules hence
limiting
recombination between ozone and hydrogen. Typically, electrolytic cells for
production of ozone
utilize anodes made of boron-doped diamond (BDD). Typical forms for the
diamond electrode are
either free standing perforated plates or thick coatings over perforated metal
substrates. Typically,
the BDD is produced by chemical vapor deposition, which is a high temperature
process.
[0005]
Another prior art SPE ozonizer design is described in detail in Sang-Do Han
et. al.
"Electro-chemical production of ozone using water electrolysis cell of solid
polymer electrolyte
(SPE)", Indian Journal of Chemical Technology Vol.13, March 2006, pp.156-161.
In this design, the
NAFION SPE membrane is sandwiched between two porous titanium electrodes
having
electroca' talytic oxi-ceramic coatings on the sides facing the membrane. The
electrode-SPE assembly
is pressed together by two current collector flow plates also made of titanium
positioned on opposite
sides of the assembly. The current collector flow plates have grooves serving
as channels for
conducting a flow of gas and/or liquid. These grooves ensure sufficient and
uniform supply of water
during electrolysis and also an escape route for oxygen and ozone gases in the
anodic compartment
and hydrogen and water molecules in a cathodic compartment. On the anode side,
the water flow
provides a supply of water for the electrolysis. The water flow on the anode
side also serves as a
carrier to transport oxygen and ozone as products of the electrochemical
reaction taking place at the
electrode-SPE interface. The hydrogen ions first diffuse through the NAFION
SPE membrane;
then, react at the cathode electrode creating hydrogen gas. On the cathode
side of the cell, the
hydrogen is removed either by gas transport or water carrier transport along
the channels in the
cathode current collector. In this SPE-based design, a porous anode electrode
can include a doped
diamond electrocatalytic layer on the side facing the NAFION membrane. The
boron-doped
diamond features both a large overpotential for evolution of oxygen and a
sufficient electrical
conductivity ¨ both of which are necessary for high efficiency production of
ozone by water
electrolysis. The active part of the diamond electrode is the area in close
vicinity to the boundary
where the diamond surface, polymer electrolyte and water meet.
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[0006] A SPE-based electrolytic cell utilizing boron-doped diamond electro-
catalytic layer
deposited by CVD process is also described in U.S. Pat. Appl. No.
2010/0320082. A refinement of
this design utililizes a boron-doped diamond coating on the surface of a
porous anode interfacing the
SPE membrane (NAFION ). This approach is described in Alexander Kraft et.al.
"Electrochemical
ozone production using diamond anodes and a solid polymer electrolyte,"
Electrochemistry
Communications 8 (2006) 883-886. In this work, the anode electrode is porous
niobium coated with
boron¨doped diamond, a few p.m thick, on the side facing a NAFION SPE
membrane.
Alternatively, amorphous tetrahedral nitrogen-doped films can be deposited by
condensation of a
highly-ionized carbon ion beam or carbon plasma using either ion beams or
filtered cathodic arc
deposition or another suitable PVD coating processes as described in U.S. Pat.
No. 6,423,193.
[0007] One disadvantage of all of the approaches is the precipitation of
calcium salt on and
near the surface of the cathode during operation in tap water. To minimize
such precipitation, the
polarity of the potential on the anode and cathode can be switched
periodically. However, materials
that are resistant to both oxidation during the anodic polarity and hydrogen
embrittlement during the
cathodic polarity are limited and can be both costly (e.g. standalone diamond)
and difficult to
process (e.g. silicon). Having different materials for the anode and cathode
can be more cost-
effective and allow a larger flexibility in design of the cell. Another
disadvantage of some of these
approaches is insufficient flow of water near the surface in which ozone is
formed. Insufficient flow
of water promotes degradation of the material at the surface of the anode
through prolonged
exposure to the generated chemical species and limits efficient transfer of
the ozone into solution.
[0008] Deposition of polycrystalline diamond coatings by CVD or plasma
assisted CVD
(PACVD) processes is a well-known technology. Examples of process for forming
diamond coating
include hot filament CVD (HFCVD); combustion flame CVD CFCVD); arc jet plasma-
assisted
CVD (AJCVD); laser-assisted CVD (LCVD); and RF or microwave plasma-assisted
CVD (RF or
MW CVD). Typically, a polycrystalline diamond CVD coating deposition process
includes the
following steps:
(1) Cleaning in an ultrasonic bath with acetone;
(2) Seeding in submicron diamond slurry in ultrasonic bath to increase the
density of
diamond nucleation sites;
(3) Cleaning in acetone;
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(4) Drying by isopropyl alcohol;
(5) Subjecting to polycrystalline diamond coating deposition process in CVD
reactor.
The gas composition coatings in hot filament reactors, DC arc plasma reactors
and MW reactors is
typically consists of less than 1% CH4 in H2 which can be mixed with argon as
a buffer gas. For
deposition of boron-doped diamond coating which is necessary for ozone
generating electrodes,
trimethylborane can be added to reactive gas atmosphere in proportion ranging
from 0.1 to 1% of
hydrocarbon (HC) gas. The pressure during deposition of diamond CVD coating
can range from 1
mtorr to atmospheric pressure and typically ranging from 1 to 100 torr. The
substrate temperature
during deposition of CVD diamond coatings is typically ranging from 600 to 950
deg C.
[0009] CVD
diamond coatings can be deposited on many different substrates, but substrates
materials which can form carbides in HC contained atmospheres at high
temperatures are preferable
for achieving better coating properties like a continuous film, no voids or
holes and a density near
the theoretical value of 3.5 g/cm3. The electrodes in an electrolytic ozone
generator can be subjected
to periodic switching of the polarity which shifts them from anode to cathode
during operation.
During exposure as a cathode, the electrodes are subjected to reducing
conditions with a high
concentration of nascent hydrogen. During exposure as an anode, they are
subjected to intense
oxidizing conditions. Because of these operating conditions the electrodes
must be stable against
degradation both as anode and as a cathode in electrochemical process.
Moreover, any exposed
metal in contact with water as a part of anode in electrochemical process, has
a much lower
overpotential than that of diamond coating. In this case, oxygen will be
generated on the metal
surface competing with ozone generated on surface of diamond, which reduces
the effectiveness of
the ozone generating process. To overcome this drawback, the metal substrate
must have the ability
to passivate by forming a stable and dense oxide film when exposed as an anode
in electrochemical
process. Metals such as Nb, Ta, W, Mo and Si are known as favorable substrates
for growing
polycrystalline diamond coatings by CVD and PACVD processes. Nb and Ta form a
stable oxide in
an oxidizing (anodic) environment, but are prone to hydrogen embrittlement and
subsequent
cracking and degradation as cathodes. W and Mo are stable as cathodes, but as
anodes they form
unstable oxides transforming their surface into powder. Conductive boron- or
phosphorus-doped
silicon is stable both as anode and as a cathode, but it is brittle and
requires expensive
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micromachining to form electrodes for electrochemical processes. As a result,
a need exists for
improved electrodes for the electrochemical production of ozone to treat
water.
[0010] Accordingly, there is a need for improved design for the
electrochemical production
of ozone to treat water.
SUMMARY
[0011] The present invention solves one or more problems of the prior art
by providing in at
least one embodiment an electrode for an ozone generator or a chlorine
generator. The electrode
includes an electrically conductive substrate, a doped-Si layer disposed over
the electrically
conductive substrate, and a boron-doped diamond (BDD) layer disposed over the
doped-silicon
layer. The doped-silicon layer defines a discrete architecture that maintains
adhesion throughout a
high temperature CVD boron-doped diamond process.
[0012] In another embodiment, an electrode for an ozone generator or a
chlorine generator is
provided. The electrode includes an electrically conductive substrate and a
PVD nitrogen-doped
diamond (ta-C:N) layer disposed over the electrically conductive substrate.
Typically, the substrate
is a doped-Si substrate or doped-Si coating on an electrically conductive
substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIGURE 1 provides a schematic of a typical scheme of the
electrolytic cell for ozone
production;
[0014] FIGURE 2A provides a schematic cross section of an electrode for an
ozone
generator or chlorine generator;
[0015] FIGURE 2B provides a schematic cross section for another electrode
for an ozone
generator or chlorine generator;
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[0016] FIGURE 2C provides a schematic cross section of another electrode
with a nitride
barrier layer.
[0017] FIGURE 3 illustrates a discrete ceramic coating that can be
deposited on metal
substrate by a cathodic arc deposition pocess with the substrate masked by a
mesh metal screen;
[0018] FIGURE 4 provides a schematic perspective view of coating system
using a remote
anode arc discharge (RAAD);
[0019] FIGURE 5 provides an image of a ta-C:N coating on a Si wafer along
with various
grids for forming a discrete architecture;
[0020] FIGURE 6 is an schematic illustration of a deposition system using a
mask;
[0021] FIGURE 7 is an schematic illustration of a deposition system using a
mask;
[0022] FIGURE 8 is a micrograph showing dimples in a silicon layer;
[0023] FIGURE 9 is an illustration of a randomly roughened surface;
[0024] FIGURE 10 provides a micrograph of a tungsten substrate that has
been randomly
roughened and coated with a diamond powder;
[0025] FIGURE 11 provides a high resolution scanning electron microscope
(SEM) image
of a Ti substrate after etching;
[0026] FIGURE 12 is a micrograph showing a portion of Figure 11 coated with
doped
silicon;
[0027] FIGURE 13 is an SEM of a 0.032" thick Ti sheet that was grit
blasted;
[0028] FIGURE 14 is an SEM of a Ti sheet that was grit blasted followed by
coating with a
doped silicon layer;
[0029] FIGURE 15 is a schematic illustration of the coating architecture of
an
electrochemical electrode;
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[0030] FIGURE 16 provides a schematic illustration of an electrolytic cell
for an ozone
generator;
[0031] FIGURE 17 is cross sectional schematic view of the electrolytic
cell;
[0032] FIGURE 18 is a schematic showing the water distribution on the anode
side of the
electrolytic cell of Figure 17;
[0033] FIGURE 19 is cross sectional schematic view of the electrolytic
cell; and
[0034] FIGURE 20 is a schematic showing the water distribution on the anode
side of the
electrolytic cell of Figure 19.
DETAILED DESCRIPTION
[0035] As required, detailed embodiments of the present invention are
disclosed herein;
however, it is to be understood that the disclosed embodiments are merely
exemplary of the
invention that may be embodied in various and alternative forms. The figures
are not necessarily to
scale; some features may be exaggerated or minimized to show details of
particular components.
Therefore, specific structural and functional details disclosed herein are not
to be interpreted as
limiting, but merely as a representative basis for teaching one skilled in the
art to variously employ
the present invention.
[0036] In general, the present invention provides coating architecture that
includes a doped-
Si sublayer that can be continuous, patterned, modulated or discrete. The
boron-doped diamond
coated electrodes overcome many of the disadvantages of the prior art thin
polycrystalline boron-
doped diamond films over metallic substrates. In particular, the present
embodiment provides an
electrically conductive doped-silicon barrier interlayer which effectively
protects metal substrates
against degradation during exposure at both the anode and the cathode in an
electrochemical process.
Moreover, the present embodiment advantageously retains a surface composition
favorable for
growing polycrystalline diamond chemical vapor deposition (CVD) films. A
silicon sublayer, barrier
coating positioned between the metal substrate and top diamond CVD layer
allows the use of
virtually any refractory metal as a substrate for boron-doped diamond coated
electrodes in
electrochemical water treatment. Alternatively, the ferrous metals, even when
coated with a thin film
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of silicon, are still unfavorable as a substrate material due to high inward
carbon diffusion rate
through thin silicon interlayer into bulk ferrous metal substrate. Examples of
the metal substrates
which can be used for diamond coated electrodes with silicon barrier sublayer
include Ti, W, Mo,
Nb, Ta, Cr, Zr, V. Among these metals, Ti is economical and forms a
passivating oxide film in
anodic mode.
[0037] With reference to Figurb 2A, a schematic cross section of an
electrode for an ozone
generator or chlorine generator is provided. The electrode 30 includes an
electrically conductive
substrate 32, a doped-silicon layer 34 disposed over the conductive substrate,
and a CVD boron-
doped diamond (BDD) layer 36 disposed over the doped-silicon layer 34. Doped-
silicon layer 34 is
also referred to as silicon interlayer 34. The doped-silicon layer 34 defines
a discrete architecture
(e.g., a pattern) that maintains adhesion throughout a high temperature CVD
boron-doped diamond
process. In a refinement, doped-silicon layer 34 has a pattern with features
less than 100 nm.
[0038] With reference to Figure 2B, a schematic cross section for another
electrode for an
ozone generator or chlorine generator is provided. The electrode 40 includes a
conductive substrate
42 and a PVD nitrogen-doped diamond (ta-C:N) layer 44 disposed over the
conductive substrate 42.
In a refinement, the PVD nitrogen-doped diamond (ta-C:N) layer 44 has a
thickness from 10 nm to
microns. Typically, the substrate is a doped-silicon substrate or doped-Si
coating on a conductive
substrate.
[0039] Coating adhesion and stability of boron-doped diamond (BDD) layer 36
(Figure 2A)
or PVD nitrogen-doped diamond (ta-C:N) layer 44 (Figure 2B) are improved by
reduction of coating
stresses. In this regard, the morphology of the silicon interlayer coating 34
can possess a discrete
morphology that minimizes internal coating stresses and stresses due to the
thermal expansion
coefficient mismatch between the substrate and silicon sublayer. This
modulated surface
morphology survives after deposition bf the boron-doped diamond CVD top layer.
For continuous
films that experience significant residual compressive stresses, the bulging
and subsequent
delamination of the coating can be avoided through the formation of discrete
or modulated patterns
in the coating. Figure 3 illustrates a discrete ceramic coating that can be
deposited on metal substrate
by a cathodic arc deposition process with the substrate masked by a mesh metal
screen. The discrete
topography can improve both self-organization of the substrate¨coating system
and coating stability
and durability under strain. In addition, the modulated morphology of the
diamond-coated electrodes
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can develop a micron-sized microchannel at the electrode-polymeric electrolyte
membrane interface
promoting the formation of ozone through increased microfluidic circulation.
[0040] With reference to Figure 2C, a schematic cross section for another
electrode with a
nitride barrier layer for an ozone generator or chlorine generator is
provided. Electrode 40 includes
a nitride barrier layer 46 (e.g. TiN or ZrN) deposited between Si interlayer
34 and metal substrate 32
is provided. The nitride-based barrier interlayer 46 inhibits silicon
diffusion into the bulk metal
substrate thereby reducing the reactitie interaction between the silicon
coating and bulk metal
substrate. This inhibition of silicon diffusion allows for subsequent diamond
nucleation pretreatment
of the Si layer under high temperature conditions during exposure of the
coated and pre-treated
substrate in a polycrystalline diamond CVD reactor. In a refinement, the
thickness of the nitride
barrier layer is from 0.1 inn to 10 Itm. A nitride barrier layer thickness
less than 0.1 p.m is not
sufficient to protect against reaction of the bulk metal substrate with
silicon and silicon diffusion into
substrate. A nitride barrier thickness greater than 10 pm is impractical and
not necessary to provide
the necessary barrier properties. In a refinement, prior to exposure to micro-
scratch ultrasonic
treatment in a diamond powder suspension, the silicon-coated metal substrate
with or without a
nitride barrier interlayer can be subjected to one or more heat cycles under
an inert atmosphere or in
vacuum up to maximum temperature ranging from 700 to 1000 C.
[0041] The discrete coating approach set forth above is also beneficial for
deposition of
nitrogen-doped amorphous diamond coating or tetrahedral amorphous nitrogen
doped carbon (ta-
C:N) which can be used for electrodes in chlorine generators. Typically, ta-
C:N coatings can be
deposited on different substrates such as W, Mo, Nb, Ta, Cr, Zr, V, Pt, Au and
Si in addition to
metal substrates with a Si, Pt or Au barrier coating. For example, ta-C:N
coatings can be deposited
by physical vapor deposition (PVD) processes such as filtered cathodic arc
deposition with
magnetically steered cathodic arc spots. In this process, a carbon plasma is
generated by vacuum
cathodic arc spots on the surface of a graphite cathode target. Unwanted
macroparticles and neutrals
are filtered, either mechanically or magnetically, by bending the carbon vapor
plasma stream in the
curvilinear magnetic field while neutrals, which are not affected by
electromagnetic field, are
trapped by the baffles. A schematic illustration of such a coating deposition
process is provided by
Figure 4 which incorporates features from US Pat. Application No.
2014/0076715. Referring to
Figure 4, deposition system 48 uses the magnetic field generated by the duct
coil 60 (i.e., 60a and
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60b) to steer the cathodic arc spots along the sides of the cathode target bar
62 parallel to the
magnetic field force lines as set forth above. The direction of the movement
of the cathodic arc spots
is shown by the arrows AD. The ends of the plasma duct 64 are opened which
allows the cathodic
metal vapor plasma to flow along magnetic force lines toward substrates 52
installed on substrate
holder 50 in the coating chamber. The neutrals and macroparticles are trapped
within the cathode
chamber on the inner walls of the duct 68 yielding near 100% ionized metal
vapor plasma to enter in
the coating area outside of the plasma duct 68. This design of the cathode
chamber is essentially that
of a filtered cathodic arc metal vapor plasma source capable of getting rid of
macroparticles and
neutrals in the outcoming metal vapor plasma and yielding nearly 100%
atomically clean ionized
metal vapor for deposition of advanced coatings. The RAAD plasma established
between the
cathode 62 and the remote anodes 72, 74 enhances ionization and activation of
the plasma
environment in the RAAMS coating deposition process, resulting in improved
coating properties. In
this design, the hybrid coating deposition processes can be conducted as a
single cathodic arc or
magnetron coating deposition or as a hybrid process combining cathodic arc
metal vapor plasma
with magnetron metal sputtering flow immersed in a highly ionized remote arc
plasma environment.
[0042] Still
referring to Figure 4, the issue of arc plasma enhancement of large area
magnetron sputtering coating deposition process and hybrid processes is
addressed by positioning at
least one remote arc anode off line-in-sight with the cathode target bar 62.
In this variation, at least
one substrate 52 held by substrate holder 50 and magnetron sputtering sources
76-82 are positioned
in a coating chamber region outside of the plasma duct 68. The present RAAMS
process effectively
immerses the metal sputtering flow generated by conventional magnetron sources
in the dense and
highly ionized remote anode arc discharge (RAAD) gaseous plasma. The remote
arc power supply
(not shown) which powers the RAAD plasma is installed between the arc cathode
target 62 and the
at least one remote anode 72. Remote anodes 72 and 74 have a linear remote
anode dimension Da.
Magnetron sputtering sources 76-82 have linear source dimension D. Cathode
target 62 has a linear
cathode target dimension D. Substrate holder 50 has a linear holder dimension
Dh. In a refinement,
the linear remote anode dimension Da, the linear cathode target dimension D.
and the linear holder
dimension ph are parallel to each other. The remote anodes 72, 74 provide at
least 20% higher open
circuit voltage than the power supply which powers the primary arc discharge
in a cathode chamber
which is ignited between the arc cathode 62 and the proximate anode. The
proximate anode can be
an inner wall of the plasma duct enclosures 86a, 86b or, optionally, an
independent anode electrode
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within plasma duct 68. In another refinement, several additional remote
anodes, each of them
associated with at least one arc cathode positioned within plasma duct 68, may
be utilized. The
remote anodes are positioned at strategic positions within the coating chamber
between the end-
openings of the plasma duct 68 off line-in-sight from cathode 62. This process
is capable of
depositing by remote arc assisted magnetron sputtering (RAAMS) process a
silicon passivation
promoting layer and, optionally, the nitride diffusion barrier interlayer
between silicon layer and
metal substrate prior to deposition of nitrogen-doped diamond-like carbon
coating ta-C:N layer by
filtered cathodic arc deposition.
[0043] In general, ta-C:N coatings usually have a high level of stress that
makes it difficult to
deposit thick ta-C:N coatings on different metal substrates. As illustrated by
Figure 5, discrete
coatings of different patterns obtained by deposition of metal vapor on masked
substrate surfaces can
be used to deposit thick ta-C:N coatings which will extend the service life of
the electrodes operating
in salted water in a chlorine generator. Figure 5 provides an image of a ta-
C:N coating on a Si
wafer. During deposition, areas of the wafer can be masked with different
grids. Examples of such
grids include, but are not limited to, Ti expanded metal grids, perforated
sheet of highly oriented
pyrolytic graphite, expanded Nb coated with Pt and a stainless steel screen
with a fine mesh. Figure
shows different mesh metal masks adjacent to masked coated areas on the
surface of a silicon
substrate. Advantageously, coatings are modulated in the pattern of the mask.
It should be noted that
ta-C:N coatings delaminate in unmasked areas while adhere in masked areas. In
these unmasked
areas, the stress of the coatings overcomes the strength of adhesion of the
coating while in the
masked areas stress was lower due to the discrete architecture.
[0044] Figures 6 and 7 provide illustrations of a deposition system using a
metal mask. An
advantage of using metal masks is the ability of accelerating carbon ions
during coating deposition
on substrates made of dielectric materials such as glass, alumina or silica or
semiconductive or low
conductivity materials such as silicon or germanium. Figure 7 shows deposition
system 90 and mesh
metal mask 92 positioned above the surface of the substrate 94 to be coated
and spaced from the
substrate by the distance d. When the ilegative bias voltage is applied to the
substrate 94, carbon ions
are extracted from the incoming carbon plasma flow 96 generated by the
filtered cathodic arc source
98 or other type of plasma source capable of generating carbon vapor plasma.
An accelerated ion
flow 100 is produced within the gap between the mesh metal screen 92 toward
the surface of the
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substrate 94 to be coated by the potential difference V; = Vs-Vp, which is the
plasma potential in the
area of plasma flow 96 near the metal mesh 92 and Vb is the bias potential
applied to the substrate 94
to be coated with attached metal mesh 92. To achieve adequate plasma
acceleration by the metal
mesh screen, the openings in the mesh typically have a largest spatial
dimension from 10 gm to
lmm. When openings are greater than 1 mm, the screen loses the ability to
extract ions from the
plasma flow with the plasma leaking throughout the metal mesh screen 92.
Openings less than 10
gm are impractical and also unable to produce a discrete pattern of the DLC
coating.
[0045] Conductive doped Si layers can be deposited on different metal
substrates by a
number of methods known to those skilled in the art. For example, doped Si
layers can be deposited
by CVD or plasma assisted CVD (PACVD) processes in a reactive atmosphere
containing disilane
(Si2H6) as a source of silicon and phosphine (PH3) and /or diborane (B2H6) as
a source of phosphorus
and boron. The conductive doped Si layers can also be deposited by a
chemically enhanced physical
vapor deposition (CAPVD) method using either e-beam evaporation of silicon or
magnetron
sputtering of silicon target as a source of silicon with phosphorus and/or
boron doping achieved by
adding phosphine and/or diborane to the reactive gas atmosphere or by using a
silicon target that is
already doped. Boron doped silicon has p-type conductivity while phosphorus-
doping silicon has n-
type conductivity. Such electrical conductivity is necessary for sub-layers in
water treatment
diamond coated electrodes.
[0046] A number of techniques can be employed to roughen the substrate
surface prior to
deposition of the doped silicon interlayer. For example, texturing can be
created by wet or dry grit or
bead blasting, vibratory tumbling or etching. Moreover, patterns on the
surface can be produced by a
programmed laser ablation technique, which can provide a precisely controlled
surface profile.
Figure 8 provides on the left side a micrograph of an array of dimples with
different diameters,
depths, and spacing produced by a pre-programmed laser ablation texturing
technique. A micrograph
of a single laser dimple is shown on the right side of the Figure 8. The
density and dimensions of
these micro-holes can be used as variables for optimizing the laser patterning
of the substrate surface
of electrochemical electrodes. A less expensive technique for achieving
surface texturing is by
surface graining of the metal substrat with randomly moving vacuum arc spots
as set forth in US
Patents Nos. 5012062, 5462609, and 5508492. These techniques produce a
randomly roughened
surface with large differences between ridges and valleys. An example of a
randomly roughened
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surface is shown in Figure 9. A desirable pattern including microchannels can
also be produced on
the surface of metal substrate by micromachining techniques involving
photolithography followed
by chemical (wet) etching or plasma (dry) etching. Thin polycrystalline
diamond CVD coatings
deposited on the roughened substrate advantageously retain the pre-deposition
surface profile.
Figure 10 provides a micrograph of a tungsten substrate that has been randomly
roughened and
coated with diamond. In another refinement, a pattern of channels can be
directly stamped into a
metal surface.
[0047] In another variation, a random pattern with both micron and
nanometer-sized features
can be generated on a surface by chemical and/or electrochemically etching
and/or grit blasting.
Examples of chemical etchants for titanium are ammonium bifluoride, sulfuric
acid with or without
an oxidizer (e.g., hydrogen perioxide) and other strong acids (e.g.,
trifluoromethanesulfonic acid,
trifluoroacetic acid) and ammonium hydroxide and combinations thereof. The
article Fiorenzo
Vetrone et. al. "Nanoscale Oxidative Patterning of Metallic Surfaces to
Modulate Cell Activity and
Fate", Nano Letters Vol.09, No. 2, 2009, pp.659-665 provides examples of
suitable etchants. Figure
11 provides a high resolution scanning electron microscope (SEM) image of a Ti
substrate that was
etched in a 50% solution of H2SO4 for four hours at room temperature under
agitation. This etching
produces a random pattern of both micron-sized features between crystalline
facets and nanometer-
sized feature from etching within the facets. Subsequently, the surface was
coated with Ti; then,
TiN; then, doped-Si using magnetron sputtering. Figure 12 shows the etched
part from Figure 11
after this coating process. This same sample was placed in a sealed quartz
tube furnace which was
flushed for 20 minutes with Ar gas at 3 1/min and continually flushed while
ramping the temperature
from room temperature to 800 C over 30 minutes followed by cooling over 4 hrs
to room
temperature. It is observed that the Si coating includes discrete islands that
survive the heating
process. Moreover, grit blasting can be incorporated into the texturing
process. Figure 13 is an
SEM of a 0.032" thick Ti sheet that was grit blasted at 80 psi with size 50
SiC grit prior to etching
and then coated with Ti, TiN and Si using physical vapor deposition. Figure 14
is an SEM of a Ti
sheet that was grit blasted following etching then cleaned with an aqueous
automated cleaning line
with ultrasonic agitation prior to coating with Ti, TiN and Si using physical
vapor deposition.
[0048] To determine if a Ti-coated substrate with a doped-Si coating can
survive the high
temperature cycling of the CVD BDD process, samples were exposed to a heating
cycle in an argon
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atmosphere in a quartz tube furnace from MTI Corporation (model number GSL-
1100X). In these
experiments, samples were placed in the quartz tube which was sealed and then
flushed for 20
minutes with Ar gas at 3 1/min while maintaining a slight overpressure of 0.08
MPa. Flushing and
overpressure continued while ramping the temperature from room temperature to
800 C over a 30
minute period followed by a slow cooling over 4 hrs to room temperature.
Subsequently,
SCOTCH tape was pressed on the coating and then pulled off to test the
adhesion of the doped-Si
coating. The Table below shows the results of coating adhesion after the
heating cycle as related to
the texturing operation. The Si coating deposited on the etched-only surface
fully survived the
heating process.
% of coating that survived after tape
Texturing operation
application/removal from the surface
No surface treatment 2%
Grit blasting prior to etching in sulfuric acid 90%
Grit blasting after etching in sulfuric acid 90%
Etching in sulfuric acid >99%
[0049] As shown illustratively in Figure 6, a predefined pattern of a doped
silicon layer can
also be produced by deposition of silicon by PVD techniques such as e-beam
evaporation or
magnetron sputtering through a mesh mask disposed above the metal substrate
and spaced from the
metal substrate at the distance from 10 gm to 3 mm. In this figure, the
silicon sputtering flow 96
generated by the magnetron sputtering source 98 is deposited on the surface of
the metal electrode
substrate 94 through mask 92 made of mesh metal screen. In the mask-substrate
assembly 35, the
distance between the mask 92 and the substrate 94 is fixed by spacers (not
shown). When the
distance of the mask to the substrate surface is less than 10 gm, it can leave
the exposed bare metal
substrate unprotected with the barrier silicon coating, while a distance above
3 mm may not be
sufficient to produce a modulated discrete surface profile of the silicon
coated metal substrate. The
optical photograph of the surface of metal plate with modulated TiN coating
deposited through the
mask is shown in Figure 3. Alternatively, the substrate itself can be
fabricated in the shape of a
defined pattern; for example, a grid, screen or expanded metal piece.
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[00501 Doped silicon deposited over a modulated surface profile of a metal
substrate
includes elevated islands separated from each other by small gaps. Modulating
the topography of the
coating limits both stress in the coating and mismatch with the thermal
expansion of the substrate.
Depositing a silicon sublayer directly on a metal substrate is possible.
Alternatively, the doped
silicon sublayer, which is conductive, is deposited on top of a passive
sublayer composed of
semiconductor oxide, nitride or oxinitride. For example, titanium substrate
can be coated initially
with semiconducting titanium oxide and/or titanium nitride having a thickness
ranging from 1 to
1000 nm followed by a boron-doped silicon coating having a thickness ranging
from 1 to 10 p.m.
Figure 15 illustrates the coating architecture of an electrochemical
electrode. In this figure, the metal
substrate 102 is coated by a semiconductive oxide, nitride or oxinitride
coating sublayer 104.
Subsequently, sublayer 104 is at least partially covered with the boron-doped
silicon interlayer 106
with an orderly pattern of elevated islands 108 separated by the gaps 110,
which is deposited through
a mask by a DC magnetron sputtering process using a conductive boron-doped
silicon target.
Deposited on top of a silicon interlayer 106 is a catalyst layer 112 for the
electrolytic generation of
ozone. Appropriate materials for catalyst layer 112 include conductive CVD
diamond, amorphous
carbon, graphite, lead dioxide, noble metals and noble metal oxides.
[0051] Figure 16 provides a schematic illustration of an electrolytic cell
for an ozone
generator. Electrolytic cell 120 depicted in this figure incorporates the
design of the electrochemical
electrodes of Figure 15. In Figure 16, the NAFION solid polymer membrane 122
separates the
cathode 124 and the anode 126 of the electrolytic cell 120. Cathode 124 has
holes 130 and anode
126 has holes 132 for removing the reactive species generated along
membrane/electrode interface
toward the surrounding water environment. The electrode's surface adjacent to
the membrane is
coated by rough boron-doped diamond coating 133 with a silicon interlayer.
Microchannels can form
between the electrode and membrane by intentional patterning of the surface of
the diamond. The
topography of the surface of the diamond includes the texture of the diamond
coating and the
topography of both the doped-Si interlayer and the substrate. In operation,
microchannels will
induce a microfluidic mass transfer along the electrode-membrane interface
which includes transport
of reactive species, ozone on anode side and hydrogen on cathode side toward
the holes 130, 132 in
the electrochemical electrodes and into the surrounding water environment. To
avoid accumulation
of mineral deposits on the cathode, the polarity of the two electrodes is
swapped periodically using
switches Si and s2 and power supplies P1 and P2.
CA 02894591 2015-06-18
Example 1. Fabrication of BDD coating for ozone generator
[0052] A
titanium plate 3 cm x 3 cm x 0.1 cm with a 10 x 10 array of holes, each hole
having
a diameter of 0.5 mm with a spacing of 1.5 mm between, is used as a substrate.
First, the substrate is
etched. For this, 50% sulfuric acid is poured into a 500 ml beaker. A TEFLON -
coated stirring bar
is dropped into the beaker. The beaker is stirred using a magnetic stirring
device. The titanium
plates are submerged into the sulfuric acid and held under for 4 hrs to create
a textured surface with
features on the scale of nanometers. The titanium is then removed and rinsed
in de-ionized water.
The substrates are then installed on a rotatable holder in a vacuum coating
processing chamber with
a DC magnetron sputtering source equipped with a boron-doped silicon target.
After evacuation, the
chamber is filled with argon to a pressure of 30 mT. Subsequently, a glow
discharge is ignited on the
substrate by applying 500 volts bias to the holder. The holder is rotated at
10 rpm. After 30 min of
ion cleaning in glow discharge in argon, oxygen is introduced at a 30% partial
pressure in relation to
argon. Substrate treatment continues in the Ar/02 mixture for 1 hr to produce
a titanium oxide
interlayer. Optionally, a TiN sublayer can be deposited by magnetron
sputtering and/or cathodic arc
evaporation prior to silicon coating. The TiN sublayer serves as a barrier
against silicon diffusion
into bulk metal substrate. After this stage, the oxygen is evacuated and the
DC magnetron sputtering
of boron-doped silicon is started to produce a modulated boron-doped
conductive silicon interlayer
coating deposited on a titanium substrate. The silicon coating process lasts 2
hrs to produce a 1.5 urn
thick modulated boron-doped silicon interlayer. After this stage, the coating
process is interrupted
and the substrate is removed from the chamber. For the next stage, the
substrate with the modulated
conductive boron-doped silicon coating is subjected to diamond seeding by
exposure in an ultrasonic
bath with a nano-diamond suspension for 1 hr. After the seeding stage, the
substrate is cleaned in
acetone and dried in isopropyl alcohol before loading in a hot-filament CVD
reactor for deposition
of a boron-doped polycrystalline diamond coating that is 2-3 um thick. This
process lasts for 5 hrs at
a substrate temperature of 950 C in a mixture of hydrogen/0.3% methane/0.001%
triethylborane
mixture at 100 torr. After this step, the substrate is removed from the
chamber and assembled with a
NAFION SPM and a counter electrode made of tungsten to form an electrolytic
cell for water
treatment. The diamond coated electrode is connected to the positive pole of
the power supply to
serve as an anode and the counter electrode is connected to the negative pole
to serve as a cathode in
water treatment electrochemical process. The polarity of the electrodes is
switched periodically to
minimize build-up of mineral deposits on the cathode. The NAFION can be
either a solid piece or
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have holes aligned and perhaps concentric with the holes of the electrodes. If
solid, the NAFION
would separate the water and reactive species in the cathodic and anodic
regions. If perforated with
holes, the anodic and cathodic regions would not be separated and water could
flow through the cell.
[0053] In a preferred embodiment of the invention, the water flow can be
forced through the
channels between the electrodes and the NAFION SPE membrane. Figure 17 is
cross sectional
schematic view of the electrolytic cell 130 with electrodes 132, 134 featuring
forced flow of water.
In this figure, the anode 132 and cathode 134 are separated by the NAFION SPE
membrane 136.
The water is supplied through the pipes 138 and 140 in the water collectors
positioned on a top of
each electrode 132 and 134. The water distributing channels 140 are provided
at the surface of the
electrodes 132 and 134 facing the SPE membrane 136. The inlet holes 144 in the
electrodes connect
the water collector 142 to the channels 150. The outlet holes 146, generally
having greater diameter
than inlet holes 144, direct the water flow outside of the electrolytic cell
130. The direction of the
water flow on each electrode side is shown by the arrows. The power supplies
152 and 154 and
switches 156 and 158 allow switching the polarity of the voltage on the
electrodes during operation.
The sides of the electrodes facing the SPE membrane, including the channels
are coated with
catalytic coating consisting of boron-doped diamond, metal oxide, noble metal
or noble metal oxides
having high overpotential and good corrosion resistance both in anodic and
cathodic electrolytic
environments. The ozone and oxygen are produced along the channels 150 mostly
at the 3-phase
anode-to-SPE junction on anode side while the hydrogen is produced along the
channels 150 mostly
at the 3-phase cathode-to-SPE junction on cathode side. The water distribution
on the anode side 132
of Figure 17 is shown in Figure 18. As depicted in Figure 18, the water is
supplied through water
inlet holes 115 from the water collector (not shown) toward water channels 150
positioned at the
side of the electrodes facing SPE membrane 136 (not shown). The water flows
along the channels
150, where the ozone and oxygen are produced on anode side and hydrogen on
cathode side, toward
outlet holes 146. Water flows contain high concentrations of ozone and oxygen
on the anode side,
and hydrogen on the cathode side. Water leaves the electrolytic cell 130
through the outlet holes
146.
[0054] In a refinement of the forced flow design, the electrolytic cell
design features flow
through a perforated SPE membrane as shown schematically in a cross-sectional
view in Figure 19.
In this embodiment of the invention, the water flow is supplied into
collectors 142 through inlet
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tubes 138 and 140 into collectors on the side of anode 132 and cathode 134,
respectively. From
collectors 142, the water flow is directed through the inlet holes 144 toward
channels 150, which
connect the inlet holes 144 to the outlet holes 146 in SPE membrane 136 on the
side of anode 132
and to the outlet holes 146 in SPE membrane 136 on the side of cathode 134. As
shown in Figure 17,
the ozone and oxygen in this flow-through design are produced along the
channels 150 on the side of
electrodes facing the SPE membrane where the catalytic coating with high
overpotential and high
resistance against corrosion is provided on the anode side. Accordingly,
hydrogen is produced along
the channels 150 mostly at the 3-phase cathode-to-SPE junction on the cathode
side. The ozonized
water is directed outside of the electrolytic cell 10 through the cathodic
side of the electrolytic cell
130 through outlet holes 146 in the SPE membrane 136, while hydrogen is
directed in the opposite
direction through the anodic side of the electrolytic cell 10 through outlet
holes 146 in the SPE
membrane 136. Water flows contain high concentrations of ozone and oxygen on
the anode side, and
hydrogen on the cathode side. Water leaves the electrolytic cell 130 through
the outlet holes 146 and
147 in SPE membrane, respectively. The water distribution diagram on the anode
132 side of the
flow-through water circulation electrolytic cell design of Figure 19 is shown
in Figure 20. In this
figure, the water is supplied through water inlet holes 134 from the water
collector (not shown)
towards water channels 150 positioned at the side of the electrodes facing SPE
membrane 150 (not
shown). The water flows along the channels 150, where the ozone and oxygen are
produced on the
anode side and hydrogen on the cathode side, toward outlet holes 147. Water
flows contain high
concentrations of ozone and oxygen on the anode side, and hydrogen on the
cathode side. Water
leaves the electrolytic cell 10 through the outlet holes 147 and 146,
respectively.
Example 2. Fabrication of metal electrodes with discreet ta-C:N coating for
chlorine generator.
[0055] A
solid titanium plate 3 cm x 12 cm x 0.1 cm is used as a substrate. First, the
substrate is cleaned in acetone and dried in isopropyl alcohol. After, a
stainless steel mask screen
with 0.15 mm opening and 0.05 mm diameter wire is attached to the front side
of the substrate with a
spacing of 0.1 mm from the substrate. The substrate¨mask assembly is then
installed on rotatable
holder in a vacuum coating processing chamber with a DC magnetron sputtering
source equipped
with a boron-doped silicon target. After evacuation, the chamber is filled
with argon to a pressure of
30 mT. Subsequently, a glow discharge is ignited on the substrate by applying
500 volts bias to the
holder. The holder is rotated at 10 rpm. Substrate treatment continues in the
Ar plasma for 1 hr to
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clean the surface of the titanium. After this stage, the DC magnetron
sputtering of boron-doped
silicon is started to produce a modulated boron-doped conductive silicon
interlayer coating deposited
on a titanium substrate through the mask. The silicon coating process lasts
for 2 hrs to produce a 1.5
urn thick modulated boron-doped silicon interlayer. After this stage, the
argon flow is replaced by a
flow of nitrogen gas and the background pressure is held at 0.05 mT to prepare
for a deposition of
nitrogen-doped tetrahedral amorphous carbon (ta-C:N). For this, a current of
140 amperes is ignited
on a graphite plate. Carbon ions are evaporated from the graphite plate and
steered using an
electromagnetic field onto the substrates, which are held at a 50 V bias. The
samples are rotated
around the graphite plate and exposed intermittently to the carbon ions for a
period of 4 hrs to build
the thickness of ta-C:N to about 1.5 microns. After this stage, the substrate
is removed from the
chamber and assembled with a counter electrode made of tungsten to form an
electrolytic cell for
generation of chlorine in saltwater. The diamond coated electrode is connected
to the positive pole of
the power supply to serve as an anode, and the counter electrode is connected
to the negative pole to
serve as a cathode in saltwater treatment electrochemical process. The
polarity of the electrodes is
switched periodically to minimize build-up of mineral deposits on the cathode.
[0056] While
exemplary embodiments are described above, it is not intended that these
embodiments describe all possible forms of the invention. Rather, the words
used in the
specification are words of description rather than limitation, and it is
understood that various
changes may be made without departing from the scope of the invention.
Additionally, the features
of various implementing embodiments may be combined to form further
embodiments of the
invention.
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