Alternate Electrolyte Composition for Electropolishing of Niobium Surfaces Page: 2 of 9
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Ceti ls o Sampl
Fig.1:ScimWti oss- ecdinofthe el pisi syst
Several systems have been designed and set up to study
the polishing process. One typical simple and useful
system is schematically illustrated in Fig l- Samples with
dimension of 40x110 mm3 were sheared from a 3 mm
niobium sheet. The sample holder had nine slots. The
separation between each slot was 25 mm. Cathode of the
same dimension was placed at one end. Therefore the
longest distance between cathode and anode was 200 mm
corresponding to the largest radius of a practical cavity.
Cooling of the electrolyte turned out to be necessary
because of the heat generated during polishing, especially
when a large current density was required. This was done
by filling the outer container with cooling fluid, and kept
the cooling fluid flowing all the time during the polishing.
In this way, we were able to keep temperature of the
electrolyte below about 30 0C. The length of sample or
cathode immersed inside the electrolyte was 90 mm.
Before electropolishing, all samples were first cleaned
using acetone and then isopropyl alcohol and
subsequently by ultrasonic cleaning using de-ionized
water. Finally dry nitrogen gas was used to blow away
the remaining water on sample surfaces.
3 RESULTS AND DISCUSSION
Generally speaking, any electropolishing process can be
characterized by plotting its anode current density (Id)
against its voltage (V) drop between cathode and anode.
The detailed shape of each Id vs. V curve depends on the
configuration of each polishing set-up and is largely
influenced by the distance between cathode and anode-
We felt that it was less economic to study the polishing
process by going though Id vs. V curves for all different
distances between cathode and anode in our set-up-
Instead, we chose the middle distance as a typical one to
be studied in detail, then we used the established limiting
current density to study the effect of the distance between
cathode and anode on the surface finish of a Nb sample-
3.1 Limiting Current Density
Normally, a typical Id vs. V curve contains two peaks
followed by a plateau and then a rapid increase of Id with
V. A sample may experience: etching, periodic oscillation
of anode current density, polishing, gas evolution on the
anode surface, and finally the occurrence of a comparable
amount of etching and polishing. The optimum polishing
conditions are located right at the transition point between
the plateau area and the rapid increase of Id with V.
Fig.2 shows a measured Id vs. V curve for the samples
being placed at the middle location of the sample holder.
We did not see the expected two peaks, but only one peak.
In the vicinity of the peak, a periodic oscillation of anode
current was observed. This Id vs. V curve was reproduced
more than one time, indicating therefore it was an
intrinsic property of the electropolishing process for the
Cell Voltage (V)
Fg.2: Aw ols ni ty lms d ma fiuc.i of applied
vdlA fr the pohzing system dadWninFig.l. Fur aras e
i Qed(s 00 om.r detk.
Fig.2 can also be divided into four regions
corresponding to etching, periodic oscillation of anode
current density, polishing, and gas evolution on the anode
surface as indicated. The comparable amount of etching
and polishing process was not observed up to the
maximum applied voltage of 40 V. The optimum
polishing conditions were, therefore, identified as a cell
voltage of 23 V and an anode current density of 50
mA/cm2. Since we are going to use current density as the
control parameter for polishing, we can discard the
voltage. This optimum current density is almost the same
as that for the acid mixture used by KEK . The
difference is that we find the occurrence of current
oscillation before polishing which is opposite to that
found  by KEK. Sometimes, we did find a slow
oscillation of current taking place after the polishing
region. The oscillation region was confined to a narrow
voltage region from about 24 to 26 V. However, this was
not reproducible and seemed to occur only when the
electrolyte temperature was allowed to rise to 55 "C.
Fig.3 shows how surfaces look under an optical
microscope for samples from each region after
electropolishing for half an hour. In the etching region,
the surface morphology is almost identical to that of a
sample treated by Buffered Chemical Polishing (BCP)
(see Fig.4), except that in Fig.4 various surface
contaminants are present, which are not seen in Fig.3a.
Nb surfaces in this region appear to be very rough. The
roughness is due to the enhanced etching near grain
boundaries, which is, in fact, the major drawback  of
BCP. Surfaces tend to become brighter and smoother in
the oscillation region as shown in Fig.3b. Although grain
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Delayen, Jean R.; Mammosser, John; Phillips, Larry & Wu, Andy T. Alternate Electrolyte Composition for Electropolishing of Niobium Surfaces, article, September 1, 2001; Newport News, Virginia. (digital.library.unt.edu/ark:/67531/metadc739035/m1/2/: accessed December 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.