On the Mechanism of Niobium Electropolishing: Some effects of Gravity and Geometry Ashwini Chandra M.D. Sumption, E.W. Collings G.S. Frankel This work supported by United States Department of Energy Grant DE-SC0004217 Nb coupons L Cooley, FNAL
Motivation Surface defects like pits, protrusions, and grain boundaries are detrimental to the performance of SRF cavities Electropolishing is so far the best technique to get a high quality surface finish The electropolishing mechanism for niobium is not very well understood, specially the association of pits with the EBW region Interesting to look at the interface between pure electrochemistry, and its realization in cavity work Will look at effects of gravity on film present during EP, and infleunce on EP rate Will look at EP on BCP and no BCP Nb coupon in HAZ
Our Schematic Bath for Nb EP Working Electrode: High purity polycrystalline Nb (99.999%) Counter Electrode: High purity Al (99.99%) Reference Electrode: mercurous sulfate electrode (MSE) Electrolyte: HF(48%)+ H2SO4(96%) in 1:9 vol ratio Fig: Schematic of the experimental setup.
Experimental Setup
Replicating Acid Ratio/Temperature, and stirring results in our set-up By know well known effect of acid ratios, T, and agitation Effect of bath agitation Note: Effect of [F-] supports acceptor limited model
Verification of Evolution of surface roughness with EP time V= 15V, T= 26C, HF:H2SO41:9 Verification of Evolution of surface roughness with EP time EP time: 2 h EP time: 4 h EP time: 6 h Surface changes from scalloped appearance after 2 h of electropolishing to a relatively smoother surface after 6 h of polishing. economic consideration for optimizing time. 6
Evidence of the presence of surface film while Niobium electropolishing A mass transport limited current plateau is necessary for electropolishing to take place There are evidences of some form of film formation on the surface through which there is diffusion limited transport of ions involved in polishing mechanism The surface film formed is soluble and dissolves in the electrolyte on switching off the current Surface Film formed during electropolishing
There is an effect of gravity on the film Evidence of the presence of surface film while niobium electropolishing There is an effect of gravity on the film Thinner film at the top of the sample because of hydrodynamics Higher dissolution rate at the top of the sample TOP 0.96mm by 15 mm BOTTOM
Evidence of Possible Gravity-Convective Effects Jerry suggested that there is not much difference in film thickness, it is the diffusion layer which is thinner at the top and thicker at the bottom because of natural convection. Natural, gravity driven convection results in thinner diffusion layer at top of sample, hence higher dissolution rate 9
Evidence of Possible Convective Effects Bottom edge of sample Pits at surface of flag electrode that was facing down. Possibly: Gravity + convection of film prevented the formation of a stable film and hence pitting instead of electropolishing Selective dissolution resulted in pitting Ridges close to bottom edge of sample due to convection caused by dissolution products (sliding of the film under gravity) 10
I vs t for EP done at 15V for 2 hours at 26oC Surface film, gravity, convective effects and EP rate for EP “face up” and vertical Flag EP rate should depend on film thickness Film is weakly held on and may be affected by gravity SO –current density vs time was measured for a films only exposed to the EP bath on top and bottom surfaces, respectively The face up configuration (blue) had a very low current density and polishing rate (thick film) The vertical configuration (red) had a relatively high current density and polishing rate I vs t for EP done at 15V for 2 hours at 26oC
Surface film and polarization curve instability (“noise”) during niobium electropolishing of VERTICAL flag Film thickness increases with the applied potential till it falls under its own weight Formation and re-dissolution of the film causes current oscillation – may well be chaotic (in the physics sense) Stirring causes the film to remain thin, weight could be supported till relatively higher potential No oscillations in case of electropolishing of masked sample due to controlled hydrodynamics at the surface
Surface (viscous) film, gravity, and EP rate for EP “face up” and “face down” Assume weakly adhering surface film Gravity assist in “face up” mode leads to thick films Gravity “attack” in “face down” mode leads to thin films Un-masked vertical films may show preferential attack at bottom edges Masked vertical films extra support for film at bottom edge Face-up Face down Erosion at bottom edge Critical angle of the “sand-pile “mask”
How will this play out in a cavity as it is polished? Will topology of the surface be an issue in any gravity and convection moderated effects?
Opticals of EP of HAZ I (Nb with no BCP) Top two micros-- the HAZ near the weld before EP Bottom two micros -- pits formed on the surface of Nb electropolished for 15 min. Possible the pits initiated at surface defects such as impurities, structural defects, etc. that formed during the welding process.
Optical of EP in HAZ II (Nb with no BCP) HAZ after 30 min eliminates the pits on the surface Conditions: applied voltage of 15 V, a working temperature of 25 °C and a HF:H2SO4 acid volume ratio of 1:9
Optical of EP in HAZ III (Nb with no BCP) Top two Micros -- Further EP (60 min) further smoothens the surface of the Nb. Lower two micros -- After EP for 120 min, a slightly enhanced dissolution at grain boundaries provides a contrast that allows grains over the entire surface to be imaged,
Optical of EP in HAZ IV (Nb with no BCP) EP for 240 min re-roughens the surface -- and a fine pit structure is seen. In addition, preferential dissolution of certain grains is found in some places, which leaves large holes on the surface of the electropolished Nb Conditions: applied voltage of 15 V, a working temperature of 25 °C and a HF:H2SO4 acid volume ratio of 1:9 for different electropolishing durations: (a and b) 60, (c and d) 120 and (e and f) 240 min.
Optical of EP in HAZ I (Nb with BCP) Top two figs – BCP Nb no EP -- grain morphology is readily evident by optical microscopy. Some grains darker from surface morphology. Bottom two micros – EP for 15 min. More dissolution of the dark grains than that of the white grains with the formation of pits on both grains.
Optical of EP in HAZ II (Nb with BCP) EP for 30 min. More dissolution of the dark grains than that of the white grains with the formation of pits on both grains. Conditions: applied voltage of 15 V, a working temperature of 25 °C and a HF:H2SO4 acid volume ratio of 1:9 for different electropolishing durations: (a and b) 60, (c and d) 120 and (e and f) 240 min.
Optical of EP in HAZ III (Nb with BCP) Top two micros – EP for 60 min. The smaller grains almost disappear, the pits are more apparent on the dark grains. Bottom two micros – EP for 120 min. Formation of pits on the dark grains while the light grains do not have the significant pitting -- indicates that the light grains have much higher resistance to pitting
Optical of EP in HAZ IV (Nb with BCP) EP for 240 min. Severe pitting of the treated surfaces, on both dark and light grains. It is clear that the electropolishing for 240 min cannot smoothen the surface of the treated Nb by levelling the protruded grains above the surface. Conditions: applied voltage of 15 V, a working temperature of 25 °C and a HF:H2SO4 acid volume ratio of 1:9 for different electropolishing durations: (a and b) 60, (c and d) 120 and (e and f) 240 min.
Conclusions Film forms during EP, thickness influenced by topilogy and gravity, in turns affects etch rate BCP and no BCP Nb were electropolished for different durations, the HAZ was examined EP of the no BCP Nb after led to pits after 15 min, but further EP (30-60-120). EP up to 120 min continued to smoothen the surface, but EP at 240 minutes cause severe pitting. BCP significantly changed the Nb surface morphology which was covered by grains of different size, appearing light or dark in the optical microscope. BCP Nb was susceptible to pitting during the EP sequence Dark grains (more texture) had more susceptibility to pitting than the light grains. EP for 240 min again resulted in severe pit formation.