U.S. patent application number 15/901426 was filed with the patent office on 2018-06-28 for electrochemical system and method for electropolishing superconductive radio frequency cavities.
This patent application is currently assigned to Faraday Technology, Inc.. The applicant listed for this patent is Timothy Hall, Maria E. Inman, E. Jennings Taylor. Invention is credited to Timothy Hall, Maria E. Inman, E. Jennings Taylor.
Application Number | 20180178302 15/901426 |
Document ID | / |
Family ID | 49914471 |
Filed Date | 2018-06-28 |
United States Patent
Application |
20180178302 |
Kind Code |
A1 |
Taylor; E. Jennings ; et
al. |
June 28, 2018 |
ELECTROCHEMICAL SYSTEM AND METHOD FOR ELECTROPOLISHING
SUPERCONDUCTIVE RADIO FREQUENCY CAVITIES
Abstract
A method and system for electrochemically machining a hollow
body of a metal or a metal alloy. An electrode is positioned within
a hollow body including a metal or metal alloy, where the hollow
body has a variable internal diameter. The hollow body is oriented
vertically, with the electrode oriented vertically therein. The
hollow body is at least partially filled with an aqueous, acidic
electrolyte solution, the electrolyte solution being devoid of
hydrofluoric acid and having a viscosity less than 15 cP. An
electric current is passed between the hollow body and the
electrode, where the electric current includes a plurality of
anodic pulses and a plurality of cathodic pulses, and where the
cathodic pulses are interposed between at least some of the anodic
pulses.
Inventors: |
Taylor; E. Jennings; (Troy,
OH) ; Inman; Maria E.; (Yellow Springs, OH) ;
Hall; Timothy; (Englewood, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Taylor; E. Jennings
Inman; Maria E.
Hall; Timothy |
Troy
Yellow Springs
Englewood |
OH
OH
OH |
US
US
US |
|
|
Assignee: |
Faraday Technology, Inc.
Clayton
OH
|
Family ID: |
49914471 |
Appl. No.: |
15/901426 |
Filed: |
February 21, 2018 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14585897 |
Dec 30, 2014 |
|
|
|
15901426 |
|
|
|
|
13546072 |
Jul 11, 2012 |
9006147 |
|
|
14585897 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 39/2406 20130101;
B23H 3/00 20130101; H05H 7/20 20130101; C25F 3/26 20130101 |
International
Class: |
B23H 3/00 20060101
B23H003/00; H05H 7/20 20060101 H05H007/20; C25F 3/26 20060101
C25F003/26; H01L 39/24 20060101 H01L039/24 |
Goverment Interests
GOVERNMENT RIGHTS
[0002] This Disclosure was developed under DOE Fermi National
Accelerator Laboratory Purchase Order Number 594128.
Claims
1-20. (canceled)
21. A method for electrochemically machining a hollow body of a
metal or metal alloy, the method comprising: positioning an
electrode within a hollow body comprising a metal or metal alloy,
wherein the hollow body has a variable internal diameter; orienting
the hollow body vertically, with the electrode oriented vertically
therein; at least partially filling the hollow body with a
hydrofluoric acid free electrolyte solution having a viscosity less
than 15 cP; and passing an electric current between the hollow body
and the electrode, wherein the electric current is comprised of a
plurality of anodic pulses and a plurality of cathodic pulses, and
wherein the cathodic pulses are interposed between at least some of
the anodic pulses.
22. The method of claim 21, wherein the electrolyte solution has a
viscosity less than about 4 cP.
23. The method of claim 22, wherein the electrolyte solution has a
conductivity greater than about 200 mS/cm.
24. The method of claim 23, wherein the voltage and on time of the
anodic pulses are adjusted to polish the hollow body while limiting
the formation of passivating metal oxide to a thickness that can be
removed effectively by the cathodic pulse.
25. The method of claim 24, wherein the cathodic pulse voltage is
greater than 4 V.
26. The method of claim 21, wherein the metal or metal alloy forms
a strongly-bonded passivation layer during the anodic pulses of the
passing an electric current step.
27. The method of claim 26, wherein the metal or metal alloy of the
hollow body is selected from the group consisting of niobium and
niobium alloys, titanium and titanium alloys, zirconium and
zirconium alloys, hafnium and hafnium alloys, tantalum and tantalum
alloys, molybdenum and molybdenum alloys, tungsten and tungsten
alloys, and chromium cobalt alloys.
28. The method of claim 27, wherein said hollow body comprises
niobium or niobium alloy.
29. The method of claim 21, wherein the electrolyte contains at
least about 10% water.
30. The method of claim 21, wherein the hollow body does not rotate
during the passing an electric current step.
31. A method for electrochemically machining a hollow body of a
metal or metal alloy, the method comprising: positioning an
electrode within a hollow body comprising a metal or metal alloy,
wherein the hollow body has a variable internal diameter; orienting
the hollow body vertically, with the electrode oriented vertically
therein; at least partially filling the hollow body with an aqueous
electrolyte solution having a viscosity less than 15 cP; and
passing an electric current between the hollow body and the
electrode, wherein the electric current is comprised of a plurality
of anodic pulses and a plurality of cathodic pulses, and wherein
the cathodic pulses are interposed between at least some of the
anodic pulses.
32. The method of claim 31, wherein the electrolyte solution has a
viscosity less than about 4 cP.
33. The method of claim 32, wherein the electrolyte solution has a
conductivity greater than about 200 mS/cm.
34. The method of claim 33, wherein the voltage and on time of the
anodic pulses are adjusted to polish the hollow body while limiting
the formation of passivating metal oxide to a thickness that can be
removed effectively by the cathodic pulse.
35. The method of claim 34, wherein the cathodic pulse voltage is
greater than 4 V.
36. The method of claim 31, wherein the metal or metal alloy forms
a strongly-bonded passivation layer during the anodic pulses of the
passing an electric current step.
37. The method of claim 36, wherein the metal or metal alloy of the
hollow body is selected from the group consisting of niobium and
niobium alloys, titanium and titanium alloys, zirconium and
zirconium alloys, hafnium and hafnium alloys, tantalum and tantalum
alloys, molybdenum and molybdenum alloys, tungsten and tungsten
alloys, and chromium cobalt alloys.
38. The method of claim 37, wherein said hollow body comprises
niobium or niobium alloy.
39. The method of claim 31, wherein the electrolyte contains at
least about 10% water.
40. The method of claim 31, wherein the hollow body does not rotate
during the passing an electric current step.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of U.S.
patent application Ser. No. 13/546,072, filed Jul. 11, 2012, the
entirety of which is incorporated by reference herein.
FIELD
[0003] This application relates to electrochemical machining or
polishing of hollow bodies of metals and metal alloys and, more
particularly, to surface finishing superconductive radio frequency
(SCRF) cavities such as cavities of the type used in
supercolliders.
BACKGROUND
[0004] The most common fabrication technology for SCRF cavities is
to form thin walled (e.g., about 1 to 3 mm) shell components from
high purity niobium sheets by stamping. These shell components are
welded together to form hollow cavities. Horizontal processing of
horizontally situated cavities as described in U.S. Pat. No.
4,014,765 (Siemens Corporation) was developed to avoid the adverse
effects of gas pockets and bubble entrainment that lead to
nonuniform electropolishing. A schematic illustration of an
apparatus for conducting horizontal processing of SCRF cavities is
shown in FIG. 10. The SCRF cavity 100 is oriented horizontally and
mounted on a pair of rotatable end caps 120. One of the end caps
includes a circumferential electrically conductive surface 140. A
cathode 160 passes through the cavity 100. In the illustrated
embodiment, the SCRF cavity includes a single cell which is
schematically represented by the large diameter portion in the
middle of the body shown in the figure. The cathode 160 is
electrically connected to a rectifier 400 by the cathode lead 440.
The anode lead 420 of the rectifier 400 is connected to the
rotating conductive surface 140 which is electrically connected to
the SCRF cavity 100. The cavity 100 is partially filled with a
viscous electrolyte 320. The electrolyte 320 is supplied from a
tank 300 via the electrolyte feed tube 340 which dispenses the
electrolyte to the cavity 100. The electrolyte is continuously
circulated through the cell 100. It leaves the cell through a
return tube 360. The volume above the electrolyte 320 in the cell
100 contains gas generated during the electropolishing process.
This gas is purged from this space by means of a vent shown
schematically at 220. The gas purge 200 is introduced at the end
cap 120A on the opposite end of the cell 100. The cavity is rotated
on the end blocks 120 as shown by the directional arrow A in the
figure.
[0005] One of the vehicles that is often used in electropolishing
passivating metals like niobium is hydrofluoric acid. As explained
herein, the electrolytes used with these passivating metals tend to
be highly viscous and this can leading to the gas entrainment
difficulties that have required the use of the horizontal
processing design discussed above. Accordingly, there is a need for
a method for polishing niobium and other strongly passivating
metals, particularly for use in surface finishing SCRF cavities,
that does not require the use of highly viscous electrolytes.
[0006] As explained in detail in U.S. Published Application No.
2011/0303553 to Inman electrochemical polishing or electrolytic
polishing or electropolishing is a process whereby metal) (M.sup.0)
is selectivity removed from a surface by an electrochemical
reaction, generally of the form
M.sup.0.fwdarw.M.sup.n++ne.sup.- Eq. 1
[0007] As illustrated in FIG. 1, during electropolishing, the
current distribution is controlled so that the peaks or asperities
of the surface are preferentially removed relative to the recesses
or valleys in the subject surface. In the case of primary or
geometric current distribution as depicted in FIG. 2, the resistive
path length from the cathode to the surface asperity (.OMEGA.p) is
shorter than the distance from the cathode to the recess
(.OMEGA.r). Consequently, the peaks are preferentially dissolved.
The difference in the current distribution between the peak and
recess is greater as the electrolyte resistance increases. Highly
resistive electrolytes and low electrolyte temperatures are
desirous to increase the differential between the current at the
peak and the recess. Decreasing temperature increases
resistivity.
[0008] In the case of tertiary or mass transport controlled current
distribution as depicted in FIG. 3, the diffusion distance from the
peak to the bulk solution (Dp) is less than the diffusion distance
from the recess to the bulk solution (Dr). Since one skilled in the
art would understand that the diffusion limited current is based on
either dissolved metal ions diffusing away from the peaks or
acceptor ions diffusing to the peaks, the diffusion limiting
current for metal dissolution at the peaks is higher than the
diffusion limiting current at the recesses. Consequently the peaks
are preferentially dissolved. The difference in the diffusion
limited current distribution between the peaks and recesses is
higher for viscous solutions. Viscous solutions have the effect of
slowing down the diffusion process. Consequently, highly viscous
electrolytes (e.g., about 15 to 30 cP) and low temperatures (e.g.,
10.degree. C. to 30.degree. C.) leading to higher viscosity are
able to increase the differential between the current at the peak
and the recess. Consequently, electropolishing solutions used in
the systems discussed above are generally highly resistive (e.g.,
10 mS/cm to 200mS/cm) and high viscosity (e.g., about 15 cP to 30
cP) solutions, in some cases operating at low temperatures as
disclosed by D. Ward "Electropolishing" in Electroplating Handbook
ed. L. Durney 4.sup.th edition pg. 108, Van Nostrand Reinhold, New
York (1984).
[0009] Despite the obstacles presented by strongly-bonded
passivation layers, various techniques have been developed for
electrochemically processing such metals as niobium and niobium
alloys. In addition to highly resistive and high viscosity
electrolytes, these techniques typically require high voltages
and/or hydrofluoric acid in the electrolyte solution. The
electrochemical conditions which drive the reaction shown in Eq. 1
above also drive the following reaction which results in the
formation of passivating oxides.
M.sup.0+xH.sub.2O.fwdarw.MOx+2xH.sup.++2xe.sup.- Eq. 2
By electropolishing in non-aqueous or minimally aqueous
electrolytes, the source of the oxygen that forms these passivating
oxides is eliminated. However, maintaining low water content
presents an additional set of control challenges. Using reverse
current pulse conditions in accordance with this disclosure
provides the means to manage the formation of this layer of
passivating oxides, even in the presence of substantial water, so
that the oxides do not interfere with electropolishing.
SUMMARY
[0010] One manifestation of this disclosure is a method for
electropolishing SCRF cavities using relatively low viscosity
hydrofluoric acid free electrolytes that enable one to process the
cavities without gas entrainment. In accordance with another
manifestation, the SCRF cavities can be processed in a vertical
orientation. Another manifestation of the disclosure is a process
that does not require cavity rotation or the need to purge gases
and that may overcome other disadvantages of the horizontal
processing system disclosed in U.S. Pat. No. 4,014,765. Another
manifestation is a process for polishing hollow niobium bodies that
may lead to a cost effective, scalable, high yield process to meet
the demand for SCRF cavities.
[0011] According to this disclosure, an electrically mediated
approach is used to eliminate the need for hydrofluoric acid and/or
fluoride salts and to reduce the effect of hydrogen in finishing
the highly passive metal surfaces of SCRF cavities. The
electrically mediated process is environmentally benign and retains
the advantages of electrochemical processes in terms of speed and
investment. Depending on the flow rate of the electrolyte across
the work piece, a waveform is selected to polish the surface. As
the surface roughness is reduced and macroroughness is reduced to
microroughness, the waveform may be changed as required. These
distinct waveforms can be preprogrammed into the rectifier.
[0012] In another aspect, the disclosed method may include the
steps of positioning an hydrofluoric acid free electrolyte solution
that may optionally be an aqueous electrolyte solution between a
workpiece and an electrode, and passing an electric current between
the workpiece and the electrode, wherein the electric current is
comprised of anodic pulses and cathodic pulses, and wherein the
cathodic pulses are interposed between at least some of the anodic
pulses.
[0013] In another aspect the electrolyte solution contains a
surfactant such as Triton-X to facilitate the release of oxygen
bubbles generated in the electrolytic process.
[0014] Other aspects of the disclosed electrochemical machining
system and method will become apparent from the following
description, the accompanying drawings and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic representation of
electropolishing.
[0016] FIG. 2 is a schematic representation of electropolishing
with resistive electrolytes.
[0017] FIG. 3 is a schematic representation of electropolishing
with viscous electrolytes.
[0018] FIG. 4 is a schematic illustration of one particular
implementation of the disclosed electrochemical machining
system.
[0019] FIGS. 5A and 5B are graphical illustrations of anodic
pulse-cathodic pulse waveforms used in connection with the
electrochemical machining system of FIG. 4.
[0020] FIG. 6 is an example of polarization curves for metal in
different electrolytes.
[0021] FIG. 7 is a two electrode polarization curve for Niobium in
different electrolytes.
[0022] FIG. 8 is a graph showing the effect of Vf on the average
surface roughness of Nb after electropolishing in 31% by weight
H.sub.2SO.sub.4 electrolytes at ambient temperature
(.about.20.degree. C.) using (a) Vf=20 V; (b) Vf=30 V; and (c)
Vf=40 V.
[0023] FIG. 9 is a graph showing the effect of time on the average
surface roughness of Nb after electropolishing in 31% by weight
H.sub.2SO.sub.4 electrolytes at room temperature (.about.20.degree.
C.) using Vf=30 V, Vr=8 V, frequency=1000 Hz, Df/Dr=10%/90% and
different Rao.
[0024] FIG. 10 is a schematic of an electropolishing apparatus for
horizontal processing as disclosed in the prior art.
[0025] FIG. 11 is a schematic of an electropolishing apparatus for
vertical processing a SCRFC in accordance with one embodiment.
[0026] FIG. 12 is a schematic of an electropolishing apparatus for
vertical processing a multi-cavity SCRFC in accordance with another
embodiment.
DETAILED DESCRIPTION
[0027] U.S. Published Application 2011/0303553 to Inman is
incorporated herein by reference.
[0028] As used herein, "electrochemical machining" broadly refers
to any electrochemical process that involves the removal of
material from a workpiece, regardless of the extent of removal. For
example, electrochemical machining includes electrochemical
polishing, electrochemical etching, electrochemical through-mask
etching, electrochemical shaping and electrochemical deburring,
among other processes.
[0029] As used herein the term "hydrofluoric acid free" includes
electrolytes that may be formulated to be substantially free of
fluoride acids and salts as well as hydrofluoric acid.
[0030] FIGS. 11 and 12 illustrate schematically two embodiments of
the invention wherein the superconducting radio frequency cavity is
vertically oriented and electropolished. In FIG. 11, the cavity 100
includes a single cell (similar to FIG. 10) whereas in FIG. 12, the
cavity 500 includes multiple (nine) cells. A cathode 160 is
vertically oriented in the cells 100 and 500 and is retained by the
end blocks 120. The end blocks 120, unlike the end blocks shown in
FIG. 10, are not designed to rotate nor do they include means for
purging gas generated during the electropolishing process. This is
unnecessary because at the viscosities used, the oxygen gas
generated during processing is readily dispersed in the electrolyte
such that it does not interfere with the surface finishing process.
The cathode 160 is electrically connected to the cathode 440 of the
rectifier 400. The cell 100/500 is electrically connected to the
anode 420 of the rectifier 400 by the end plate 120. The
electrolyte 300 is introduced to the bottom of the cavity 100/500
by a feed tube 340 from the electrolyte supply tank 300. At the top
of the cavity 100/500, electrolyte is expelled from the cavity
through the end cap 120 through the electrolyte return tube
360.
[0031] The flow of the electrolyte through the cavity is adjusted
such that the electrolyte is refreshed during the electropolishing
process.
[0032] The power source or rectifier is configured to pass an
electric current between the electrode and the workpiece, wherein
the electric current is comprised of anodic pulses and cathodic
pulses, and wherein the cathodic pulses are interposed between at
least some of the anodic pulses. The electrolyte is characterized
in that it is hydrofluoric acid and/or fluoride salt free. In one
embodiment it is an aqueous solution of sulfuric acid at
concentrations of 70% by weight and less. In certain embodiments,
the amount of water in the electrolyte may be greater than 5% by
weight, or greater than 10%, or greater than 20%, or greater than
70%.
[0033] The electrolyte solution disclosed herein may be used with
an anodic pulse-cathodic pulse waveform to electrochemically
machine niobium and alloys thereof, as well as other metals and
metal alloys, including other metals and metal alloys that tend to
have strongly bonded passivation layers. For example, the disclosed
electrolyte solution may be used with an anodic pulse-cathodic
pulse waveform to electrochemically machine niobium and niobium
alloys, titanium and titanium alloys (such as titanium and
molybdenum alloys, and titanium and nickel alloys also know as
nitinol), zirconium and zirconium alloys, hafnium and hafnium
alloys, tantalum and tantalum alloys, molybdenum and molybdenum
alloys, and tungsten and tungsten alloy. In another embodiment the
process may be used to polish cobalt chrome alloys of a type used
in medical applications such as vascular and other stents.
[0034] The disclosed hydrofluoric acid free electrolyte solution in
one embodiment may be an electrolyte having a low viscosity such as
a viscosity of about 1 cP to 15cP or about 1 cP to 8 cP, or about 1
cP to 4 cP. In a particular embodiment it may be an aqueous
solution comprised of low concentrations of sulfuric acid, for
example, concentrations of about 1% by weight to 70% by weight,
more particularly about 15% by weight to 40% by weight, still more
particularly about 20% by weight to 40% by weight. In other
embodiments of the invention, other acidic, hydrofluoric acid and
fluoride acid and salt free electrolytes may be used such as
combinations of sulfuric/chromic/phosphoric acids,
phosphoric/chromic acids, phosphoric/sulfuric acids, phosphoric
acid, phosphoric/sulfuric/chromic acids,
phosphoric/sulfuric/hydrochloric acids, sulfuric/glycolic acids,
phosphoric/sulfuric acids, sulfuric/chromic acids, sulfuric/citric,
and others. Generally, it will be desirable to select electrolytes
having a high conductivity such as greater than 200 mS/cm, or
greater than 400 mS/cm, or greater than 600 mS/cm or greater than
800 mS/cm.
[0035] In one implementation of the invention, aqueous electrolytes
containing substantial water as disclosed above may be use.
However, another implementation may employ non-aqueous or minimally
aqueous electrolytes containing less than 15%, less than 10% or
less than 5% water. When water-containing electrolytes are used,
oxygen is generated according to the equation:
H.sub.2O.fwdarw.2H.sup.++O.sub.2+2e.sup.- Eq. 3
It has been found that the addition of a surfactant facilitates
electropolishing. One possible reason for this that the surfactant
promotes the formation of small bubbles that do not interfere with
the diffusion process by stirring the electrolyte. Conventional
surfactants may be used for this purpose such as Triton X
(polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether), a
product of Rohm and Haas, in conventional amounts.
[0036] Referring to FIG. 4, one particular implementation of the
disclosed electrochemical machining system, generally designated
200, may include a working chamber 202 defined by a tank 204 and a
cover 208, an electrolyte holding tank 222, a conduit 218, a pump
220, an electrode 304, a workpiece 302, a power source 228 and the
disclosed electrolyte solution. The working chamber 202 may be in
fluid communication with the electrolyte holding tank 222 by way of
a gravity drain 206. A filter 224 may be associated with the drain
206 to filter the electrolyte solution flowing from the working
chamber 202 to the electrolyte holding tank 222. The working
chamber 202 may also be in fluid communication with the electrolyte
holding tank 222 by way of the conduit 218, wherein the pump 220
may pump the electrolyte solution from the electrolyte holding tank
222 to the working chamber 202, as shown by arrow 234.
[0037] Within the working chamber 202, a workpiece holder 210 may
be mounted near the bottom of the tank 204 with adequate spacing
from the walls and bottom of the tank 204 to allow for drainage of
the electrolyte solution into the electrolyte holding tank 222. The
workpiece 302 may be supported on the workpiece holder 210 and may
be connected to a first electrical lead 302 of the power source
228.
[0038] The workpiece 302 may be any apparatus or thing capable of
being electrochemically machined. In one embodiment the work piece
can be a medical stent. In another embodiment it may be a dental
implant. In another embodiment it may be RF superconducting
cavities that are components of linear particle accelerators. In a
first expression, the workpiece 26 may be any apparatus or thing
that is formed from or includes a metal or a metal alloy. In a
second expression, the workpiece may be any apparatus or thing that
is formed from or includes a metal or a metal alloy that forms a
strongly-bonded passivation layer. Examples of metals that form
strongly-bonded passivation layers include niobium, titanium and
tantalum. In a third expression, the workpiece may be any device
that is formed from or includes niobium or a niobium alloy. For
example, the workpiece may be a niobium structure or a portion of a
niobium structure that defines RF superconducting cavities in a
particle accelerator. In a fourth expression, the workpiece may be
any apparatus or thing that is formed from or includes titanium or
a titanium alloy. For example, the workpiece may be a nitinol tube
coated with a resist material, wherein the resist material is
patterned to define a surgical stent after electrochemical
machining. In another expression the workpiece may be any device
that is formed from or includes cobalt chromium alloy.
[0039] An electrode holder 214 may be positioned in the working
chamber 202 above the workpiece holder 210. The electrode holder
214 may be supplied with the electrolyte solution by way of the
conduit 218. The electrode 304 may be connected to a second
electrical lead 232 (opposite polarity than the first electrical
lead 302) of the power source 228 and may be supported by the
electrode holder 214 such that the tool holder 214 may move the
electrode 304 in a vertical axis under control of an electrode feed
controller 226.
[0040] In one particular aspect, the electrode 304 may include a
central bore and the electrode 304 may be connected to the
electrode holder 214 such that the central bore of the electrode
304 is directed at the workpiece 302. During electrochemical
processing, the electrolyte solution may be pumped by pump 220 from
the electrolyte holding tank 222 to the electrode holder 214 and,
ultimately, to the electrode 304 by way of the conduit 218. The
rate of the electrolyte solution flow is herein referred to as
E.sub.v. The electrolyte solution may flow through the central bore
of the electrode 304 and may exit between the electrode 304 and the
workpiece 302 before returning to the electrolyte holding tank 222
by way of the drain 206. The power source 228 may supply electric
current to the workpiece 302 and the electrode 304 by way of the
first and second electrical leads 230, 232 in accordance with the
disclosed anodic pulse-cathodic pulse waveform.
[0041] The spacing between the electrode and workpiece 302 during
processing may be considered an optimizeable parameter and may
depend on the composition of the electrolyte solution and the type
of electrochemical machining process being performed, among other
factors. For example, the spacing between the electrode and
workpiece may range from about 0.5 to 20 millimeters or more
particularly 0.5 to 10 millimeters for an electrochemical shaping
process, about 5 to about 12 millimeter for an electrochemical
polishing process and about 5 to about 50 millimeters for an
electrochemical deburring process.
[0042] As shown in FIG. 5A, an anodic pulse-cathodic pulse
waveform, generally designated 50, may include a plurality of
anodic pulses 52 and a plurality of cathodic pulses 54. One
particular implementation is shown in FIG. 5B.
[0043] The period T of the waveform is the sum
(T=t.sub.1+t.sub.2+t.sub.0+t.sub.1) of the anodic on-time t.sub.1,
cathodic on-time t.sub.2, relaxation period t.sub.0 and
intermediate period t.sub.i. The inverse (1/T) of the period T of
the waveform is the frequency f of the waveform. The ratio
(t.sub.1/T) of the anodic on-time t.sub.1 to the period T is the
anodic duty cycle D.sub.1 and the ratio (t.sub.2/T) of the cathodic
on-time t.sub.2 to the period T is the cathodic duty cycle D.sub.2.
The current density (i.e., current per unit area of the electrode)
during the anodic on-time t.sub.1 and cathodic on-time t.sub.2 may
be referred to as the anodic peak pulse current density and
cathodic peak pulse current density, respectively. The anodic
charge transfer density Q.sub.1 is the product (I.sub.1t.sub.1) of
the anodic current density I.sub.1 and the anodic on-time t.sub.1,
while the cathodic charge transfer density Q.sub.2 is the product
(I.sub.2t.sub.2) of the cathodic current density I.sub.2 and the
cathodic on-time t.sub.2.
[0044] In a first expression of the anodic pulse-cathodic pulse
waveform, the anodic peak current I.sub.1 may range from about 2 to
6 A/cm.sup.2, the cathodic peak current I.sub.2 may range from
about 8 to 15 A/cm.sup.2.
[0045] In one embodiment, the voltage and on-time of the anodic
pulse is adjusted to remove metal from the micropeaks in the
surface via Eq. 1 above without accumulating a passivating layer
thickness via Eq. 2 above that cannot be effectively removed during
the cathodic duty cycle. Accordingly, the anodic voltage and
on-time are adjusted to oxidize the metal on the micropeaks while
generating only that amount of passivating oxides that can be
removed by the cathodic pulse. If the passivating oxide layer
cannot be removed, it prevents or terminates polishing. The
appropriate conditions will vary depending on the nature of the
metal. In one embodiment a relatively short anodic pulse t.sub.1,
typically from about 0.01 ms to about 100 ms, preferably from about
0.05 ms to about 10 ms, and an anodic duty cycle from about 1 to
60% or from about 5% to about 60%, preferably from about 1 to 40%
or from about 10% to about 40%. The cathodic pulse t.sub.2 may have
a pulse width from about 0.01 ms to about 900 ms, preferably from
about 0.1 ms or from about 0.5 ms to about 90 ms, and a duty cycle
from about 40% to about 99% or about 95%, preferably from about 60%
to about 99% or about 90%. The relaxation period t.sub.o may range
from about 0 to about 600 s and the intermediate off period t.sub.i
may range from about 0 to about 1000 ms. The frequency f of the
waveform 50 may range from about 1 Hertz to about 5000 Hertz,
preferably from about 10 Hz to about 2000 Hz and more preferably
from about 100 Hz to about 2000 Hz or about 100 Hz to 1000 Hz.
[0046] At this point, those skilled in the art will appreciate that
the parameters of the pulse waveform 50 can be selected to provide
uniform metal removal from the workpiece and thereby provide more
accurate conformity of the workpiece. Additionally, the field can
be implemented to reduce or anodically consume some of the hydrogen
generated at the workpiece surface and reduce or eliminate the
effects of a nonuniform oxide film. The anodic peak current
I.sub.1, the anodic on-time t.sub.i, the cathodic peak current
I.sub.2, the anodic on-time t.sub.2, the relaxation period t.sub.0
and the intermediate off period t.sub.1, may be varied depending on
the composition of the workpiece 26, the composition of the
electrode, the composition of the electrolyte solution and the type
of electrochemical machining process being performed, among other
factors to achieve these objectives. Furthermore, those skilled in
the art will appreciate that the voltage and current may be
proportional under the circumstances of the disclosed system and
method and, therefore, the ordinate in FIG. 2 could represent
either current or voltage, although it is generally more convenient
in practice to control the voltage. Furthermore, the waveform 50
need not be rectangular as illustrated. The anodic pulses and the
cathodic pulses may have any voltage-time (or current-time)
profile. Rectangular pulses are assumed merely for simplicity.
Again, one skilled in the art will recognize that the point in time
chosen as the initial point of the pulse train is entirely
arbitrary. Either the anodic pulse or the cathodic pulse (or any
point in the pulse train) could be considered as the initial point.
The representation with the anodic initial pulse is introduced for
simplicity in discussion. In accordance with one embodiment, the
cathodic voltage is about 4 to 40 volts or about 4 to 15 volts or
about 8 to 35 volts or about 6 to 12 volts, and in one embodiment
about 35 volts. This is in contrast to processes in which the
workpiece is not made from a strongly passivating metal in which
case a cathodic voltage of 4 volts or less may be satisfactory. The
cathodic voltage is used to depassivate the surface, and for
strongly passivating materials the cathodic voltage needs to be
greater than 4 volts, one skilled in the art can determine the
anodic voltage required for the desired electrochemical
dissolution, i.e. etching and/or polishing.
[0047] Without being limited to any particular theory, it is
believed that the introduction of cathodic pulses between the
anodic pulses has the effect of cathodically consuming the nascent
oxygen or cathodically reducing the oxide film, thereby reducing or
eliminating the adverse effects due to the formation of a
non-uniform oxide film. Consequently, when the next anodic pulse is
applied, any passive layer that may have formed will be more easily
broken down, and therefore less capable of forming local islands of
passivity that tend to resist erosion of the underlying metal.
EXAMPLES
[0048] Niobium foil, 99.9% pure, was purchased from GoodFellow
(GoodFellow, Oakdale, Pa.) (FIG. 131) and cut into two different
coupon sizes to use for the electropolishing studies. Final coupon
sizes had the following dimensions: 1) 25.4 mm.times.25.4
mm.times.3 mm, and 2) 30 mm.times.10 mm.times.3 mm.
[0049] As a simple, efficient, and cost-effective screening method,
the polarization curve can be used to select candidate
electrolytes. In FIG. 6, curve 1 shows the behavior of an active
metal, and curve 2 shows the behavior of a passivated metal. Before
the electric field is applied, the metal anode immersed in the
electrolyte has a steady-state voltage (E.sub.ss). When the power
is applied, the electrode voltage will shift in the positive
direction from E.sub.ss to E.sub.ab (the breakdown voltage). Above
E.sub.ab, the current density rises abruptly due to the dissolution
reactions occurring on the anode (region AB). The dissolution rate
of the anode metal stops increasing when a limiting current density
I.sub.lim is reached (BC region), where the metal atoms form metal
ions and compounds with the activating anions and pass into the
electrolyte. The limiting current density I.sub.lim and the ratio
of .DELTA.I to .DELTA.E (the slope of AB on the polarization curve)
can be defined as the metal dissolution rate and current efficiency
in the electrolyte, respectively. In region BC, the current density
remains constant (curve 1) or drops to a lower value (curve 2)
indicating mass transport phenomena that limit the rate of metal
ion removal. The products of metal dissolution reach their
solubility limit and form a loose deposit or passive film on the
electrode surface. If the metal dissolution is conducted in a
passive electrolyte, the passive film can grow faster than metal
ions pass into the electrolyte, with the result that the current
density falls to lower values (curve 2). Generally, the limiting
current decreases with increasing electrolyte concentration, due to
the decrease in the solubility of the reaction products. Since the
limiting current is strongly related to diffusion, it can be
increased in the pulse/pulse reverse process by increasing the
electrolyte flow rate. When the anode voltage increases to region
CD of the polarization curve, the higher voltage can breakdown or
remove the passive film and deposits, and increase the ionization
rate of the metal to increase the current density.
[0050] The metal brightness and smoothness in different
electrolytes can be directly observed from polarization tests,
providing information as to the effect of electrolytes on the
etching process. In region AB of the polarization curve metal is
eroded. The metal surface roughness is high due to the different
dissolution rates of the various microscopic areas on the surface.
At high anode voltages (region BC), the metal surface becomes
smoothed or even polished, as in the case of curve 1. If the anode
voltage reaches the CD region, the metal dissolution at higher
voltages will lead to a polished surface with macrodefects (such as
fine lines, striations and pits). The optimal polarization curve
should (1) indicate a low breakdown voltage (E.sub.ab), (2) have a
high ratio of .DELTA.I /.DELTA.E, and (3) provide a smooth and
shiny surface.
[0051] DC polarization studies were carried out in order to select
an electrolyte that would enable the pulse/pulse reverse process
for electropolishing Nb coupons. The 2-electrode DC polarization
studies were performed on 25.4 mm.times.25.4 mm.times.3 mm Nb
coupons to study the electrochemical activity (e.g. total current
density) of Nb in different electrolyte type and concentrations. A
platinum coated Nb mesh was used as the cathode. All polarization
curve experiments were performed at room temperature
(.about.20.degree. C.). A TecNu power supply was used for this
study (Model SPR-300/100/48-3). The cell voltage was raised by
increments of five volts per minute. Total current densities were
read from the oscilloscope trace recorded on a FLUKE 196C
Scopemeter color system.
[0052] FIG. 7 summarizes the electrochemical activity of Nb
substrates in different electrolytes, 200 and 300 g/L sodium
chloride (NaCl), 31% by weight sulfuric acid (H.sub.2SO.sub.4), 200
g/L sodium bromide (NaBr), 50 g/L sodium fluoride (NaF) and 21% by
weight phosphoric acid (H.sub.3PO.sub.4). In all cases breakdown of
the Nb was not observed; any current measured is assumed to be
associated with water oxidation
(2H.sub.2O.fwdarw.O.sub.2+2H.sup.++2e.sup.-) and Nb anodization.
The highest and lowest total current density observed for voltages
up to 70 V was in the 31% by weight H.sub.2SO.sub.4 and 50 g/L NaF
electrolytes, respectively.
[0053] This data demonstrated the tenacity of the Nb oxide film; DC
polarization studies were unable to shed any light on the
conditions that would be required to break down the oxide film
without the use of hydrofluoric acid. Electrochemical cells with
variable flow as shown in FIG. 4 were used to test the efficacy of
pulse/pulse reverse waveforms in electropolishing Nb. As described
in the prior art, variable flow channel cells are used to
successfully perform metal removal of passive metal and alloys.
.sup.1, 2 An advantage of electrolyte flow is the removal of
undesired byproducts from the surface of the substrate being
electropolished, such as Nb ions, heat and bubbles (resulting
mainly due to oxygen and hydrogen generation from water
electrolysis). .sup.1 A. Lozano-Morales, A. Bonifas, M. Inman, P.
Miller and E. J. Taylor, J. Appl. Surf. Finish., 2 (3), 192-197
(2007)..sup.2 J. J. Sun, E. J. Taylor, R Srinivasan, J. Materials
Processing Technology, 108 356-368 (2001).
[0054] As reported in the prior art, a 300 g/L NaCl electrolyte has
been successfully used for pulse/pulse reverse electropolishing
different passive materials such as nickel based alloys and
stainless steel, and therefore it was used to initially study the
electrochemical activity of Nb. An initial design of experiments
set was developed using statistical software called MINITAB.RTM..
Frequency, duty cycle and reverse (cathodic) voltage were varied
with three levels for each variable. Three different frequencies
were varied at 10, 100 and 1000 Hz at three different forward
(anodic) duty cycles (D.sub.f=10, 50 and 90%), and three different
reverse voltages (V.sub.r=2, 4 and 8 V). A Nb coupon of the same
dimension as the anode was used as the cathode. The electrolyte
velocity was kept constant at 12 m/s and a forward (anodic) voltage
(V.sub.f) of 48 V was used in all the experiments at ambient
temperature (.about.20.degree. C.). The temperature of the
electrolyte was not controlled. The total run time in each case was
10 minutes.
[0055] While uniform etching was not achieved in 300 g/1 NaCl,
there was evidence of Nb breakdown at the coupon edges, where the
electrolyte flow entered and exited the cell. Oxides were also
formed on the surface, indicating electrochemical activity. These
oxides were not tenacious, able to be removed using a scotch-brite
pad, soap and water. However, high electrolyte flows and very close
electrode gaps are not likely to be realistically accommodated.
[0056] In initial experiments in 31% by weight H.sub.2SO.sub.4, the
constant parameters were electrolyte velocity (E.sub.v)=0.4 m/s,
V.sub.f=20 V, V.sub.r=8 V, run time=10 minutes, anode to cathode
distance=5 mm, and ambient temperature (.about.20.degree. C.).
Since the low flow channel cell did not have temperature control
built in, the electrolyte temperature rose from an initial value of
20.degree. C. to around 27.degree. C. by the end of every
experiment. In conventional electropolishing, temperatures above
40.degree. C. must be avoided in order to prevent etching pits on
the Nb substrate..sup.3 A design of experiments set was performed
using MINITAB.RTM.. Two different frequencies were used (10 and
1000 Hz) at two different duty cycles (D.sub.f=10 and 90%), (see
Table 1), for a total number of 3 experiments. .sup.3 L. Lilje, E.
Kako, D. Kostin, A. Matheisen, W.-D Moller, D. Proch, D. Reschke,
K. Saito, P. Schmuser, S. Simrock, T. Suzuki, and K. Twarowski,
Nuclear Instruments and Methods in Physics Research A 524 1-12
(2004).
TABLE-US-00001 TABLE 1 Design of experiments using reverse pulse
waveforms to electropolish Nb in a 31% by weight H.sub.2SO.sub.4
electrolyte for 10 minutes using a V.sub.f = 20 V and anode-cathode
distance of 5 mm. Forward (Anodic) Reverse Voltage Run Order
Frequency (Hz) Duty Cycle (%) (V.sub.r) 1 10 90 8 2 1000 10 8 3
1000 90 8
[0057] Run 1 showed no evidence of etching at all. The different
colors observed represent Nb oxide layers formed on the substrate.
Run 2 showed some degree of etching, which suggested that Nb
substrates could be uniformly electrochemically etched in an
electrolyte free of hydrofluoric acid. Run 3 also showed some
degree of etching, but much lower compared to Run 2.
[0058] Based on these preliminary results, the pulse/pulse reverse
waveform used for Run 2 was further explored. Specifically, the
same waveform parameters as Run 2 were used, but the coupon was
electropolished for 37 minutes instead of 10 minutes. 100 .mu.m of
Nb was successfully removed uniformly from the coupon at an average
removal rate of 2.7 .mu.m/min in an area approximately 161
mm.sup.2.
[0059] The effect of raising V.sub.f from 20 to 30 to 40 V on Nb
electropolishing performance was studied. In all cases, the other
pulse/pulse reverse process parameters were kept constant:
V.sub.r=8 V, frequency=1000 Hz, D.sub.f/D.sub.r=10%/90%. FIG. 8
summarizes the effect of anodic peak voltage on Nb surface finish.
The roughest surface finish was obtained at 20 V. When V.sub.f was
increased to 30 V, the surface finish of Nb dropped from 1.38 .mu.m
to 0.29 .mu.m. At 40 V the Nb surface finish started getting
rougher again and surface discoloration was observed.
[0060] The effect of Nb initial surface roughness, Ra.sub.o on
final surface roughness, Ra.sub.f was also studied by performing
electropolishing time studies at different Ra.sub.o. FIG. 9
compares the effect of time on the average surface roughness of a
Nb coupon after electropolishing in a 31% by weight H.sub.2SO.sub.4
electrolyte for Ra.sub.o=0.56 .mu.m (Run #4) and Ra.sub.o=1.53
.mu.m (Run #5). For the higher initial surface roughness, there was
a significant decrease in Ra after 10 minutes, from 1.53 .mu.m to
.about.0.85 .mu.m. Thereafter, Ra decreased further by increasing
electropolishing time up to 60 minutes down to an Ra.sub.f of 0.33
.mu.m. Further decreases in surface finish did not occur after 50
minutes.
[0061] Accordingly, electrochemical machining systems and methods
employing the disclosed electrolyte solution with the disclosed
anodic pulse-cathodic pulse waveform may be used to process niobium
and niobium alloys, as well as other metals and metal alloys,
without the need for fluoride acids or salts, such hydrofluoric
acid.
[0062] Although various aspects of the disclosed electrochemical
system and method for machining niobium and other metals have been
shown and described, modifications may occur to those skilled in
the art upon reading the specification. The present application
includes such modifications and is limited only by the scope of the
claims.
* * * * *