MMO for the masses!
Entry posted by Swede
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It's been too long since my last blog. Most of the stuff I've been doing hasn't been worthy of note. Even worse, I couldn't find my camera! I've been using a Sony CD Mavica for years, and it has a great lense that does good macro work. Scouring the house for days, I was totally frustrated. Then, for some odd reason, I entered my teenage son's room, despite the mess. Young APC'ers, enlighten me, why does a teen not mind living in filth? It baffles me. How can your soul find peace when surrounded with gross clutter? 
Anyway, there was my camera. Grrr..... 
Some time ago, I began a very rewarding correspondence with another APC member, who helped me locate a distrubutor for bulk MMO coated Ti sheet. For those a bit in the dark, MMO stands for "Mixed Metal Oxide." The Ti mesh is coated with a variety of exotic conducting oxides, and under the influence of voltage, do the magic that converts our innocent chloride ions into powerful oxidizers.
A bit of emailing and phoning, and one credit card later, this mesh beast was on my doorstep:
That, my friends, is a yardstick, not a one-foot ruler. I could use this to make a 500 gallon cell, but a better idea is to cut it into more useful sizes, like 2" X 6". One problem: how to connect a Ti shank to the cut pieces?
Years ago, I had the need to spot-weld a large number of stainless steel sheet pieces for my model turbojets. Commercial spotwelders were either too expensive or simply not adequate, so I made my own... easier than many think. A typical spot welder is nothing more than an AC transformer that turns 115 or 230V into 3 to 5 volts, at a ferocious current. By cannibalizing a big transformer, removing the secondary, and replacing that with a few winds of #4 copper cable, the job is more or less done.
I made it fancy by giving it a solid-state relay. The heart of the welder is a timer, also solid state, equipped with a foot pedal that gives me control over the duration of the weld. With my old turbojet project, I found that having a larger number of tiny welds secured better, and warped less, than fewer heavier welds. The duration (ON time) of some of those tiny spot welds was 10 milliseconds... the timer works very well. For this heavier Ti, longer duration would be necessary. By programming the timer for "one shot" functionality, and setting the timer, a simple foot-pedal tap will deliver the current for the exact time selected.
The solid state relay is protected with a 20 amp fuse, and by extension, the primary windings of the transformer will see no more than 20 amps. For sake of argument, if we assume the primary is at 15 amps, and we measure the secondary voltage at 4.00V, we can determine the current in the secondary with a simple ratio of voltage and current...
115V / 4V = 28.75
Multiply 28.75 by 15A = 431 amps!
I'm not sure if this is correct, but I believe it to be. The #4 copper cable, and the brass clamp arms, cannot sustain 431 amps for any length of time... they heat rapidly, and after a few welds, the system needs to be allowed to cool.
I had my doubts that this thing would weld Titanium. Two problems... it is relatively lightweight, as it was engineered to weld SS sheet no thicker than 0.015", pretty thin stuff. Secondly, it is widely known that spot welding aluminum is very difficult to do correctly. Commercial spot welders for aluminum have complex circuitry that changes the direction of the current, which "blows" oxides off the weld, and allows a successful fusion of the underlying metal. I fully expected Ti to behave like Al in this matter. The rig is 115V AC, and in the back of my mind was the notion of rewiring this for 230V, thus doubling the current. But it was worth a try in its current form.
I took two pieces of CP strip Ti, 0.040" thick, and thoroughly cleaned them with a wire brush and alcohol. Next, I set the timer at two seconds, and placed the strips in the clamp arm. The clamp arms can be held with bare hands, as the voltage is only about 4V AC. With some trepidation, I tapped the pedal. A puff of smoke was released, and this is what I saw:
The weld looked good. I knew from experience that even if a weld visually seems OK, sometimes the penetration is superficial, and the bond is weak. There's only one way to test it, and that is by forcefully separating the pieces. A good spot weld will have one section literally ripping a nugget of metal from the other. With a pair of pliers on either strip, I attempted to pull it apart. I was successful (as expected) but it took a LOT of force. The weld was a good one, with an acceptable nugget of Ti ripped from one of the sections. One spot weld is rarely enough, and it was a simple matter to add four more spots after rewelding the original. The resultant test weld was excellent, both mechanically and electrically. My plan is to strip the MMO coating off of small sections of the mesh, and spot weld the underlying, bare Ti wire directly to the shank, at several locations. If it works properly, it will be a huge relief, as TIG welding this mesh to an appropriate shank, while it would work, would be a real PITA.
Back to the cell: I decided that I wanted to allow for electrode placement in the CC (the big cell), as well as having a separate EC. My reasoning... while I still believe in the premise of having a recirculating system, I wanted, as a backup, to be able to do basic, single-cell operation, just in case the recirculation method didn't work well. Faced with having to mount electrodes yet again, I executed something similar, and mounted a pair of externally-tied Ti cathodes on the lid, and bored a 1.375" hole in the lid between them. The hole is identical to the one in the lid of the EC, so anodes will fit either rig.
On the underside, I created a PVC bracket that would press down on the Ti strips, and was very liberal with the cement. The cathodes are a bit crooked, but overall, it looks OK, and more importantly, it will be functional.
Power supplies: One of the tougher challenges facing the hobbyist. The deals are out there... fixed voltage switching supplies are plentiful and cheap. I have gathered no less than SEVEN since I started this project, all of them well under $100, most of them less than $25. One 5V fixed supply I snagged is rated at 400 amps, and costed a whopping $21. Variable supplies, if you can find one, are best. Here is one I couldn't let pass by at $62... 0-10V, 0-60A, in mint condition...
I am DONE with power supplies, and wil probably flip some of them, but I did want to see if the super-cheap, 5V fixed supplies will work well for a chlorate cell. I suspect they will! AND, you can run a pair of them in series to generate higher voltages if needed.
I am off to cut some of the MMO sheet and try some experimental spot-welding. Two upcoming tests... one of these will be run to absurdly low chloride levels to see how it behaves. If we can break the "10% chloride" barrier with impunity, it will triple the yields of chlorate in a given run. The second, more involved experiment, under Tentacles' expert tutelage, will be to attempt to prep and plate one of these with PbO2 for perchlorate. It's nice to have surplus MMO material! No more babying my lone MMO anode! 
Added later: Spot welding of the MMO mesh to a Ti shank was a total success!! A tiny test anode was cut from the giant sheet, and a section of the MMO coating was scraped free of the MMO material. The Ti strip was cleaned with alcohol, and wire-brushed. The welding itself went very well, with no quirks or problems. The five spot welds are centered over the "thick" areas of the MMO mesh. On the second picture, look closely at the two spots farthest lower right... note how they are "cratered" more than the others. For those, I switched to a more pointed tip on the spot welder, and as the metal softened, the pressure applied created the crater. This also makes a better bond, as it concentrates the current through a smaller cross-section.
I am tickled. This worked far better than I had dared to hope. I was anticipating TIG welding these, and while a good TIG weld would certainly make an excelent bond, it would also potentially overheat the coating, expose more of the mesh to the electrolyte, and probably boil some of the MMO, which would be a health hazard.
I still wore a respirator doing this, but the welds are so quick, so focused, there is little opportunity to boil MMO. This little guy (which is 3" X 2") will be a fine test anode.
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