Is Resonance essential for induction heating? - Page 1
Forum section: electronics / projects

author: BennVenn
author_rank: Regular Contributor
timestamp: June 05, 2015, 12:45:43 am
content: I'm about to begin designing a low power (2kw) induction heater for flashing getters in homemade tubes. I’ve built a few smaller ZVS based heaters though getting 2kw from the lower voltage supply and the lower working frequencies make these unsuitable.Is it necessary to use a resonant circuit in the work coil to get the power into the work piece? If it’s only a matter of pushing 100A though a 4 turn coil, would a full bridge converter work just as well? Or is it more the high voltage/current oscillations in the work coil that make the transfer more efficient?I’d much prefer to not have to tune the oscillator, or rely on PLL to prevent the output stage destroying itself. If resonance is the key to induction heating, then I’ll design around that. Just not sure which way to go!

author: coppice
author_rank: Super Contributor
timestamp: June 05, 2015, 01:17:35 am
content: Quote from: BennVenn on June 05, 2015, 12:45:43 amWhy go to all that effort when an induction hob is so cheap to buy?

author: BennVenn
author_rank: Regular Contributor
timestamp: June 05, 2015, 01:22:13 am
content: Not sure how happy a hob would be driving the impedance of a 4 turn coil - it would require consderable modification to the LC network, plus the work coil is live... And I'd assume the frequency would be far too low (need 400khz +)

author: moffy
author_rank: Super Contributor
timestamp: June 05, 2015, 01:58:21 am
content: I guess you know that you use the capacitance to cancel out the inductance, so it reduces your voltage demands, it also reduces your overall power because the cap is effectively power factor correcting your inductance. You can do it brute force but your power and voltage demands will be ridiculous in my opinion compared to the resonant circuit.

author: Marco
author_rank: Super Contributor
timestamp: June 05, 2015, 02:44:57 am
content: Quote from: moffy on June 05, 2015, 01:58:21 amThis doesn't really matter, when you switch the bridge the energy in the inductor goes back into your power supply (until the current reaches zero again). Assuming it's properly designed most of that energy is recovered by the power supply caps.

author: moffy
author_rank: Super Contributor
timestamp: June 05, 2015, 02:49:21 am
content: Quote from: Marco on June 05, 2015, 02:44:57 amBut you have all your extra switching losses, resistance etc. because of the higher voltage supply.

author: Dago
author_rank: Frequent Contributor
timestamp: June 05, 2015, 05:01:34 am
content: Well I guess resonance is not "mandatory" in a sense. You just need to be able to push large amounts of power to the work coil at the desired frequency... Which is easiest at the resonance frequency because the impedance of the tank is at a minimum then. So in practice it probably has to be resonant or otherwise you'd just be cycling power in between your tank and power supply (due to bad power factor).

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: June 05, 2015, 05:42:57 am
content: I'm no expert at induction heating, but my RF sense tells me that no resonance = little energy transfer.

author: Richard Head
author_rank: Frequent Contributor
timestamp: June 05, 2015, 06:10:56 am
content: You just have to push the material from one end of the B-H curve to the other and back again. The more often you do it (ie the higher the frequency) the greater the heating. Resonance is not required, but it does make it easier.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 05, 2015, 07:32:48 am
content: Seems like resonance is the way to go then. I don't think heating due to saturation/hysteresis/B-H is dominant in induction heating, more eddy current heating- at least up to the curie point. I'd like line isolation on the work coil so I'll have to use some ferrite to couple the energy before the tank which is going to limit the output power unless I can source some bulky cores. Decisions...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2015, 08:04:54 am
content: Unless you're doing special purpose (I've done industrial direct drive before, and yes, it's as expensive as it sounds), yes, resonance is important.BTW, why do you need 400kHz? I've lit up getters inside common tubes at much lower frequencies. If you're just going on reference material, consider the origin: they would've used oscillators themselves, because until very recently, it was the easiest way to get controlled power into a coil. And they were always in the 400kHz range. No sense in forcing it lower -- the big capacitors get expensive, and the lower matched impedance just requires more tuning to match to the oscillator's plate load. Higher, I think, is limited by nearby LW/MW radio services, marine emergency or something.Irony being, since tubes are no longer mainstream, and semiconductors are, the load impedances are much lower (~ohms, not ~kohms!!), so the frequencies can be much lower (which helps on switching losses and EMI). So that old application (gettering) could never benefit; until the 70s or so, they only had two choices, LF gensets (~600Hz, with some alternators up to 10kHz) or vacuum tube oscillators (~400kHz, or ISM band).Are you familiar withhttp://www.fluxeon.com/?Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2015, 08:10:45 am
content: Quote from: BennVenn on June 05, 2015, 07:32:48 amGuessing your getter will be, at best, a nickel wire or frame, like the old ones; or at worst, a blob of alkaline earth metal that you want to obliterate. Either way, once it's red hot (or not), it's only eddy currents. Which is fine, those materials heat quite easily that way.Coupling depends on distance from coil to work. For the typical distances of electrodes inside a glass envelope, you'd probably be looking at a Q-factor of 10 to 50. Some of which is due to the coil itself (losses dissipated by cooling water), the rest goes into the work. Efficiency will probably be worse for side-fire getters (which were commonly seen on sweep tubes I'd say), since the getter is partially shielded by the plate behind it. I guess if you're making customs right now, you can put the getter wherever you please, and make the envelope taller if need be.So, if you want to get 2kW into a coil with a Q-factor of 10, you need to deliver 20kVA, almost entirely reactive. That's a lot of transistors, so yes, capacitors are a big win.Tim

author: BennVenn
author_rank: Regular Contributor
timestamp: June 05, 2015, 11:06:46 am
content: I'd settled on 400khz after reading tech notes from the big tube manufacturers from the 50's as well as limited success at 80khz with my current zvs setup. It has no problem bringing an iron rod to incandescence yet can barely push 80 watts into an inch diameter nickel ring getter. A hobby tube builder (Ron soyer) suggests frequencies ranging 400-600khz.Efficiency is of no great concern (excess heating at least), duty cycle would be under 10 seconds with tens of minutes rest between.I do have some very high power triodes I could use for the task. Maybe a tube based solution would be more forgiving and high power/voltage smps's are well within my capability.So resonant it is! Thanks everyone for their input!

author: LaserSteve
author_rank: Super Contributor
timestamp: June 05, 2015, 03:50:55 pm
content: Having processed tubes, and working with ultra high vacuum for years, I can say this: Remember your elements are in a highly insulating vacuum and it does not take much energy to get something to red or white heat unless it contacts the atmosphere or something grossly conducting heat. I design my tubes for oven bakeout, only my neon tube electrodes are inductive heated, and that only needs a few tens of watts.Usually one or two resonant structures in the tube are enough to get heating started and it can rapidly transfer heat by radiation to other electrodes.I find it more important to be able to dial energy down with fine control then have large amounts of it. You need to bring heat up very slowly to avoid cracking the seals, anyways.TV picture tubes were outgassed by resonanting the coil around the tube and having a low Q resonant circuit as one part of the structure.2 KW sounds like a huge amount of energy for a small tube.Steve

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2015, 12:30:53 pm
content: If you've been having trouble, your problem is almost certainly matching, regardless of frequency. The classic ZVS circuit will deliver limited voltage, which is a problem when you need all that extra coil voltage to "reach out and touch" something (which also carries more coil current, hence more VARs, because the Q is high). The difference in supply current for unloaded vs. loaded will be small, because you'll mostly have coil losses.Related pic: "ZVS" circuit driving a fluorescent tube, CCFL style. ~500kHz MOSFET.http://seventransistorlabs.com/Images/Fluo1.jpgThe transformer lets me match to pretty much whatever I want, as long as I can get the turns ratio I need. But, it's not really practical to put capacitors after the transformer, because this circuit needs a low impedance everywhere. Meaning, loading capacitors everywhere, including those directly at the transistors. If capacitors are used in multiple locations, they will interact and resonate in unintended modes, that often don't result in power delivered to the coil. If you stick with primary side capacitors, the transformer has to be quite large, because it has to handle full coil VAs, not just W.And unlike a proper PLL controlled power supply, there's no way to adjust frequency to make it behave, it simply resonates wherever it does. So, maybe it even starts up fine, but then gets stuck on a low power resonant mode, and that's that.A tube oscillator will have the same problem, but fortunately, you have the option of wasting plate efficiency in exchange for relatively easy tuning. You still need matching to get power, of course.Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 06, 2015, 12:40:30 pm
content: Quote from: LaserSteve on June 05, 2015, 03:50:55 pmPower.

author: calexanian
author_rank: Super Contributor
timestamp: June 06, 2015, 08:40:51 pm
content: Without all the back story I work occasionally for M.U. inc who is a tube company. I fix the induction heaters and make my own tubes at home as well. As for your original question, do you need resonance. No. You do not. So long as you can induce enough current to heat. Remember that commercial getters flash at near yellow heat.A few questions.Are you using commercial getters, if so which type.Can you get your coil concentric around the getter, which I am assuming is a loop or ring type.How close can you get it.If not concentric how close can you get the coil. Let's start there before I really dive into it from a practical application perspective.Next bravo for making tubes at home. I do it, and there are some others out there who do too. It's a fun hobby. Sure makes you appreciate the cheapImport tubes for sub $20 really fast! Even though you need to buy two just in case!

author: BennVenn
author_rank: Regular Contributor
timestamp: June 07, 2015, 01:32:51 am
content: Thank you all for you input.I'll design around a high voltage, voltage fed resonant circuit. I'll aim for a few hundred khz operating frequency unless someone can confirm this is not required?Calexanian, I'll be using recycled getters from a large box of milspec dual triodes I got for a few cents each. I've also been told by other hobby tube builders that getter pellets have been homemade from barium and aluminium oxide, pressed with a great deal of force and heated in a vacuum in a tungsten/graphite bath. The high temperature reduces the oxides to elemental metals where they are free to react with limited unwanted elements.I blow the glass myself so I'm free to orientate the getter however I choose. Ultimately I'll be making the pellets and adding them to the tube so they may not be in a nickel carrier, just floating until they are fired and evaporated.I'm using borosilicate and while fairly resistant to high temperatures, I'll fix the pellets a few mm from the glass wall to be safe. So to answer the question, I'd say 5mm between work coil and pellet whose size is will be about that of a tic-tac.I've evaporated tungsten in a tube via a hv discharge and that dramatically decreased the tubes pressure below that which an arc can strike (30kv over a 10mm arc gap). This was just with a single stage rotary pump. I've since got hold of a small diffusion pump but can't fire it up till I find a better roughing pump. Diffusion oil is not cheap!

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: June 07, 2015, 01:36:57 am
content: I sense a need for some YouTube videos.

author: Marco
author_rank: Super Contributor
timestamp: June 07, 2015, 04:23:48 am
content: Quote from: BennVenn on June 07, 2015, 01:32:51 amSo you want to use a transformer?

author: calexanian
author_rank: Super Contributor
timestamp: June 07, 2015, 06:04:36 am
content: A couple of things to be aware of, but first a few words about high vacuum and electron tube practices. Life for a receiving tube during assembly and exhaust is as follows.all internal parts are hydrogen fired for a number of reasons. First it reduces a great many things to pretty much gas that is easy to remove. all parts are extremely clean. They are assembled by people in the equivalent of clean rooms. The cage is mounted to the stem then put on a carousel type sealing machine. It rides around and the bulb seals and the excess bulb glass falls off by gravity and the flames make the seal good. Before the glass goes below its annealing point its placed on the exhaust machine, typically right next to the sealing machine.It will spend about 2 minutes going around and will step through about a dozen positions. the first stages are just rough vacuum. As soon as that is established most of the successive positions have the tube under heavy RF bombardment. The anode will glow bright red. The getters at this point are also heated up to somewhat below the flash point to drive off gas within it. Part of the reason they use 400khz is that it penetrates and also helps heat internal grids. Also the high frequencies help heat non cylindrical things. Side rods, posts, flat sheets and shields. The filament is then lit and breakdown begins. The idea is to convert the carbonates as quickly as possible so carbon gas, carbon monoxide, and carbon dioxide don't begin condensing on parts. While all of this is going on the glass is still screaming hot around its annealing point. The actual pressure inside the tube at this point is rarely below the 10 micron range because so much gas is being driven off from all of the surfaces including the glass. Also gas is evolving from within the metals. typically the last induction bombing station will have a coil that flashes the getter. Sometimes it is not completely flashed, but most of the time it is. You had to be sure during the bombardment that you drove off enough gas from the getter ring that by now even getting it even hotter to flash you wont liberate so much more gas that the flash will be consumed by it. After that the filament is shut off and he tube is tipped off, still screaming hot. The tube is then off to annealing. that was all done in about 2 minutes. When the tube is cool the internal pressure will be around 5X10-5 torr. This is limited by the remaining gases that the getter cannot sorption. Hydrogen and the noble gases primarily. The tube is considered to be at the end of its life when it reaches about the middle of the -4 scale. Any more and glow discharge will occur by electron emission enhancement, then eventually bang. Arc. You can take a perfectly good receiving tube and drive it to being gassy by just simply letting it run red hot for a long time. This system only works because they are continuously running parts through and nothing sits still long enough to get dirty or gassy. Material sciences are perfect. In particular the nickel cathodes have lots of reducing agents to help deal with the relatively high gas pressures and not be poisoned.Now the life of a transmitting or large military tube, or anything with a tungsten filament.Parts are individually cleaned and bright dipped by very exacting standards. Procedures are followed exactly the same each time. Typically these types of tubes have hard glass bulbs which makes sealing and exhaust timings far less critical. Mounting and spot welding is done by hand. Coatings such as zirconium, chrome or nickel plating, graphite, thoria in the case of thoria bonded filaments (such as found in true 572b tubes) are performed. Everything is hydrogen fired as well, but for much longer periods of time. Some parts are also vacuum fired. Special treatment is required for solid graphite anodes and zirconium power coatings on both graphite and metal anodes. The cages and mounts are assembled by hand using fixtures. The mounts and bulbs are typically sealed by hand on a lathe. They are then exhausted a small number at a time on a station specifically designed for that purpose. Think of a helium leak detector with a bake out oven/hood over it. A manifold with compression ports serves as the connections to the tubes. Most i have seen and the ones we use can typically handle anywhere from 2 to 6 tubes at a time. With a warmed up and ready to go system the tubes are sealed to the manifold and and rough pumped. *a word on pumping systems* Most of these systems were pretty standard. A welch 1402 or 1405 pump and a diffusion pump. the NRC H-2-P type pump using dow corning 704 fluid was by far the most popular for receiving tube use and still can serve as a excellent system. Cold traps are not required with this setup so long as there is no direct optical path to the tube port. Back to the tubes on the pump.... During rough pumpdown you close off the foreline to the diffusion pump. Even though the main inlet valve is closed you don't want to run the diffusion pump with the foreline valve closed for too long or bad things happen. During roughing a hand held tesla spark generator is run over the tube and any leaks in the glass will cause a distinct arc to jump to said leak. Quickly the glow discharge from the tesla coil will reduce to a pale blue glow as the pressure drops below 30 micron or so. At this point the roughing valve is closed, the foreline reopened. When the pressure re stabilize and the inlet to the diffusion pump is back to the base pressure of the system, typically below 10-6 torr scale, the main valve is opened. After a few minutes and pressure is stable the tube will be RF bombarded. The pressure is watched as it will jump way up each time. It is never allowed to go above 10-4 each time it is heated. Eventually after a few go arounds of this the pressure will not spike as much and the first round of bombardment is over. The bake out hood is then closed or lowered and the tubes are backed out. Typically this is done up to just about the annealing point of the glass and held there for a while. A proper bakeout cycle is an hour minimum. Some types require many hours. The tubes are cooled slowly down to about 300 Celsius before they are allowed to cool more rapidly. The filaments are then connected and a careful breakdown schedule of voltages are applied. These are very critical to make the filament characteristics right for the particular design. Some tube filaments require previous carburization. After that another round of rf bombardment with the filament still lit. then the tube is allowed to cool down to about 100 C and the pressure is read. Typically the tube will be down at the base pressure of the system. 5X10 -7 to 6X10 -8 are typical for us depending of station and tube. The tube is then sealed off and tubes with getters are flashed after tip off while the tube is still hot. Some tubes do not require gettering depending on design. Some use zirconium as the gettering agent.The hard to swallow stuffThe idea of reusing getters is whats called a soft flash. This used to be done in experimental tubes where they new they were going to have an evolution of gas down the road, or they had getters of a very large yield where a actual internal short may occur if they fully flash the getter. For all intents and purposes as soon the breakdown of the aluminum begins and the barium starts reducing out that material changes in a manner where if exposed to air, when re sealed and heated it will liberate more gas than the barium can recapture.Everything has gas in it. Heat is what gets gas to move.. Try to get as much of it as you can out first.As for commercial getters themselves they are typically made of nickel plated steel which heats by induction many times easier than pure nickel. Most receiving tube anodes are actually steel that is nickel plated, then blackened. Or a multi ply which is typically steel then copper, then nickel, then aluminum of a very special surface preparation. This was the gray anode material popular from the 60's on. It had excellent heat conduction properties. The current import tubes are aluminum on steel typically. The aluminum surface is specially treated so it does not evaporate at red heat. 2KW is a minimum of power for tube use. I have a 2.5 KVA lepel in my home shop and its barley enough for small tubes about the size of a T12 bulb.Its after my bed time and I am running out of gas. Long story short if you need a few getters I can send you a dozen. there are a lot of things you can do first though before you get to that point. Any more questions feel free to ask.

author: Arjan Emm
author_rank: Regular Contributor
timestamp: June 07, 2015, 07:36:16 am
content: I quickly latched a induction heater together a few years ago for fun. It could heat up a m10 nut to curie temperature in 30 seconds or so in open air.I found this topology very interesting:http://www.richieburnett.co.uk/indheat.htmlThis is how i implemented it:http://www.xs4all.nl/~vam/pics/indschem.JPGhttp://www.xs4all.nl/~vam/pics/indheat.jpgAs a drive i simply used a function generator and tune it for resonance. The ferrite core is from an atx supply.At 50V it would draw 4A so about 200 watt.The work coil measured a whopping 86A at 212V at 113 KHz! That's the benefit of using resonance. The cooling elements on the igbt's were not needed at all. They remained completely cold. The nice part is that igbt's are switching when the current through them is 0.I guess you have to beef this up a bit. I never pussed the circuit any further because my workcoil got very hot and i stopped experimenting with it.Make sure you use capacitors that are up to the job. polypropylene high current ones. I used cornell dubilier 942C20P15K-F, the favourite choice in teslacoils..Edit: I'm NOT magically creating 18KW out of 200W. Current and voltage in the tank circuit are almost perfectly out of fase. Cos phi is almost 0. The 200 watts is what is lost in the heating of the workcoil and the heating of the work piece.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 08, 2015, 12:20:10 am
content: Thank you very much for the information, this kind of stuff is very hard to find and seems most knowledge is dying with the original workers/engineers of the time.My setup so far - I'm using a chinese HHO 'water welder' as a flame source. This seemed the most cost effective way to produce a flame hot enough to work boro. Infact the flame is so hot it can actually boil the boro and reduce it to its component elements. Adding acetone to the boiler reduces the flame temp and boosts the BTU output. This means my flame is slightly oxidising when run pure and hydrogen/carbon rich when passed through the acetone.I've been working on making reliable glass-metal seals. I've purchased some Kovar but cant get a reliable seal with the glass. It could be that the boro I'm using is miss-matched to the kovar. I've had good results using Tungsten. Getting the oxide layer right before pinching still needs some more experimentation. It is very hard to source soda glass here in pre-formed tubes so I'm stuck with boro and all the problems it comes with.I'll keep an eye out for a reasonably priced Welch Duo-Seal. A unit that heavy attracts hefty shipping fees to get it here (Australia). I was thinking a 2/3 stage rotary (A/C evac pump) might be more suitable with some high quality oil. Locally available and relatively cheap. I've got DC-704 fluid for the diffusion pump, I've read it is more immune to oxidation when something goes wrong.As for the induction heater, I'll now be looking at a 2.4kw unit (as much power as a single phase outlet will allow here) I was considering a bake-out oven but induction heating the plate/structure seems like a more efficient way to go about it. This also changes my design to a 100% duty cycle.Thanks Arjan Emm for the links, I was looking at those designs a year or so ago. I threw together something last night as a kind of prototype. A 50% duty push pull mosfet setup into a toroid pushing out 100v p-p square out the secondary. 3.3uH made of 80 strands of 0.1mm litz wound 8 turns around an old tube. 0.33uF resonant cap found in an induction cooktop (I assume these would be rated to the high current?) I was capacitively coupling the toroid to the tank via a cap which was a bad idea (Didn't have that LLC schematic in front of me - just experimenting) . Ended up cooking my fets. I'll wind the matching inductor today and try again. At resonance the Coil got a little warm but the cap remained cool. ~150khz when loaded with no noticeable heating of the tube when pulling ~120watts. A stable RC oscillator at 400khz seems like a major achievement in its own.Calexanian, what particular area should I work on first? It seems an induction heater is a little way down on my list of things to do. I've got a few discharge tubes I've made that are still gas-tight after 6 month's sitting on my desk. Gas tight at 30 microns is probably a little different to 10^-6...Thanks again everyone!

author: calexanian
author_rank: Super Contributor
timestamp: June 08, 2015, 03:27:16 am
content: Just read your last post. I just got back from a pretty strenuous day hike in Yosemite. Give me some time to begin stringing coherent thoughts together.A side note... If you have access to somebody with a helium leak detector you can use them to evacuate tubes pretty well. Also a national hand torch is pretty handy as an all around torch. They run well with oxygen and propane or natural gas. good for tipping off and making seals. As for the metal to glass seal kovar and pyrex are not the greatest seal. It can be done but extreme care must be taken in forming a proper oxide layer on the kovar. Small diameter tungsten works fat better. Copper can be used as well but its hard not to melt it because it melts or oxidizes through right around the working point of the boro glass. It is used extensively through the "Housekeeper" method with good success. Re using old receiving tube pins works reasonable well so long as there is still glass attached all the way around the dumet section. That in turn seals well to other lime or lead glass such as neon tubing. Neon electrodes also can serve as a good source of parts.

author: LaserSteve
author_rank: Super Contributor
timestamp: June 08, 2015, 02:44:49 pm
content: Common 7740 Pyrex does not seal well directly to Kovar, despite what you have read, the COEs mismatch by 14 units, and you want no more then 7 COE of error. Your best bet is Schott 8250 patch clamp glass to Kovar, Followed by 3320 Uranium Glass, then Pyrex. Corning 7056 Patch Clamp Glass is expensive but also available.A lifetime pile of patch clamp tubes will set you back about 75$. Real Uranium Glass rod from the last melt(1959?) shows up from one seller on Ebay...You can also go 8250 to 7740 but you must anneal properly.304 SS can seal direct to Pyrex, but the required machining to make the Housekeeper seal is beyond most home users. It must also go thru a dry hydrogen baking step.Kovar usually needs a wet or dry Hydrogen furnace bake to form the blank surface for the oxide to form on.I have a huge spool of very fine Kovar wire. When I seal it direct to the patch clamp tubing, I find if I do not bake the wire, it badly bubbles, and then you get a leak along the axis of the wire. ... For example, fingerprints on the wire bubble, they do not boil off.You really need a copy of Fred Rosebury's book: "Handbook of Electron Tube and Vacuum Techniques" or one of RCA's private tomes on Vacuum Tube Construction.... Those show up online with careful searching...With care, one can order pre-beaded seal pins,... Your far better off if you do.There is one vendor in the US who will sell the exotic old Corning sealing glasses to hobbyists, but he is a lawyer who bought a warehouse of glass. His idea is a 300$ minimum order of any one type of glass... Yet he wonders why he has few sales...Corning and Schott do one single melt every two or three years, making sealing glasses rare and expensive.I did find certain metal halide lamps have a Kovar compatible outer envelope made of hard glass, but it takes considerable experimentation to find the right ones. Far better just to buy the 8250 patch clamp glass.For soft glass:Good Borated Dumet is readily available from India (best) and China (ok but not great). This seals nicely to Technolux glass, which is available in the US, and far easier to use then 088 Neon Tubing, which leaches lead and turns brown/black if the oxidation state of the flame is not perfect.Steve

author: calexanian
author_rank: Super Contributor
timestamp: June 08, 2015, 06:29:16 pm
content: The AVS (American vacuum society) has reprinted many of the classic books covering these subjects. Vacuum Sealing techniques, or something like that. i have the book at home and i am at work now. Anyways thats a good one. Goes over this stuff quite well. as said sealing glasses are hard to come by. I like the Technolux and dumet idea. I intend on buying a few cases and giving it a try. Boro glasses are so expensive. They are easier to work from an annealing perspective, but at a cost. Fortunately I have a rather large supply of old nonex, but that stuff does not last forever.

author: LaserSteve
author_rank: Super Contributor
timestamp: June 08, 2015, 07:13:50 pm
content: Next up is finding a good Capacitive Discharge Welder for bonding the electrode structure to the feed through leads. Its harder then it looks. I'd probably spend far more time on that then the inductive heater.There is a good tube making video on Youtube of the Mullard Factory, including how they spin mold the base stems with the pins in place. The pins are placed in a spinning block of carbon, powered glass is added, and the mold is spun while being heated with a torch. Goodfellow sells rods of the correct "42" alloy, but they are NOT cheap.Steve

author: calexanian
author_rank: Super Contributor
timestamp: June 08, 2015, 08:32:52 pm
content: Good results can be had with a 120 volt spot welder from harbor freight and a 10 amp variac. I use a foot pedal to apply power. You can drill a hole through the large jaws and use smaller spot welding electrodes. I use 1/8" inch electrodes for most things. With carefull adjustment of the voltage you can get very good results. Capacitive discharge is good, but with the availability of those cheap welders you may as well use one.

author: LaserSteve
author_rank: Super Contributor
timestamp: June 08, 2015, 10:20:27 pm
content: I'm spoiled. We have two different forms of CD Welding at the university where I work. Both single and dual pulse, plus the TIG like plasma transfer welding option. I so want one...But I'll borrow an HF unit and try it.Steve

author: BennVenn
author_rank: Regular Contributor
timestamp: June 08, 2015, 10:49:55 pm
content: I have the spot welder sorted, I've re-wound a MOT with 6 gauge, around 18volts into copper spot welder electrodes. I'm switching the primary with a micro set for two different pulse widths, one short pulse (1/2 cycle @ 50hz) followed by a longer pulse 100ms. Foot switch activated. A lot of experimenting has ended up with these numbers. It will fuse just about anything to anything. The tungsten becomes very brittle after heating though.I've contacted a few Neon suppliers in my country, hopefully get a reply from them today. They stock both lead and soda in 1.2m lenghts from 6mm up to 28mm. MOQ is quoted at 50lbs but if they are happy to mix and match I'm happy with 50lbs. I'll probably grab some dumet from the electron tube store online and fingers crossed they are matched to the soda.I'm a bit hesitant working with lead glass as my HHO flame isn't oxidising enough and always end up with a blackened glass. Not sure if this compromises the seal but it doesn't look too great. I'll start looking for a oxygen concentrator.I have a copy of that old RCA tube design book. So much usefull information in there! Also found a scientific lampwork book from the 40's/50's with lots of practice examples in joining, annealing, blowing complex shapes etc.I'll see if I can source a digital copy of "Handbook of Electron Tube and Vacuum Techniques". The more reference material the better.Any idea on the material used in neon seals? They seem very cheap, MOQ 1000pcs. If i could butt weld the material to nickel wire that would be ideal.I've been looking at the construction of the newer mass produced tubes and appears the base is stamped onto the pins much like injection molding. I'd like to replicate this, Graphite mold + hot glass? Maybe a re-purposed microwave to heat the glass to a liquid then pour and press?As for the induction heater, I've rebuilt my test rig to compare 'brute force' heating to resonance. Brute force does work with significant coil heating and low heating 'distance'. Resonance is the way to go though it seems to stress my mosfets. 200v 30a fets running on 12votls and still a lot of heat is being produced. Likely from excessive drain peak voltages. I'll confirm with the scope.

author: calexanian
author_rank: Super Contributor
timestamp: June 09, 2015, 03:53:50 am
content: Typically the molded wafer stems are made with two concentric rings of glass. they use a graphite mold, hand load the pins, which sometimes have already been beaded with glass. Typically for octal types there is a internal and an external ring along with the evacuation tubeulation. For noval 9 pin bases there is typically only one ring. that is then heated to the working point of the particular glass and the top half of the mold smashes down.What country are you in?

author: BennVenn
author_rank: Regular Contributor
timestamp: June 09, 2015, 04:11:33 am
content: I'm in Australia.I think I'm going to attempt the noval approach. I have a small cnc mill and Graphite blocks seem cheap enough. A measured amount of glass powder, heat and pressure sounds like it will give good results and bringing the temperature of the graphite down slowly should anneal the base. A reliable gas tight seal seems to be the point of failure in most homemade tubes. If I can get this perfected I'll have the groundwork for all my tube designs I want to bring to life. Plus I'll be able to use standard tube sockets.I've read dumet wire is available new from india in 3kg spools. Not sure what that will cost me just yet but it is available in sizes up to 2mm. Electron Tube Store has none in stock.

author: calexanian
author_rank: Super Contributor
timestamp: June 09, 2015, 05:04:49 am
content: Powdered glass, or Frit as it is called is nasty and hard to work with stuff. It has to be melted under vacuum conditions or it will be terribly gassy and leaky. It can be done, but the pinch method is much easier to do. I suggest reviewing the videos of Roy Soyland. Don't worry about the lead in neon tubing. Just don't lick any oxides if they precipitate out! hahaha. Anyways the lead is so tightly chemically bonded you have nothing to worry about. Otherwise we would have all been poisoned by crystal stemware from our new years ever champagne! That stuff is about as high in lead content as they could manage before it appears metallic!

author: BennVenn
author_rank: Regular Contributor
timestamp: June 09, 2015, 05:22:41 am
content: It was more the discoloration of the lead when heating in a oxygen deficient flame. How about these for lead in wires? Already coated with soda glass.http://en.wikipedia.org/wiki/File:Germanium_Diode_1N60.jpg

author: LaserSteve
author_rank: Super Contributor
timestamp: June 09, 2015, 03:07:58 pm
content: Quote from: Alex Eisenhut on June 06, 2015, 12:40:30 pmYour being Pedantic, and its the delivered energy that matters...Steve

author: calexanian
author_rank: Super Contributor
timestamp: June 09, 2015, 03:18:56 pm
content: You are better off using old broken vacuum tubes. You will find that your filaments will draw lots of amperage and you will need feed throughs that can handle several amps without getting hot and cracking. Are you in Uni still? you may be able to buddy up with somebody who has a vacuum system with feed thoughts and view ports and such and begin playing with filament/electrode dynamics and get your chemistry and cleaning practices down utilizing demountable apparatus. No need for glass or getters when the system is pumping at a nice appropriate level. It will make data collection way easier. Need to change anode spacing, no prob. Open her up, bend the anode over, and suck her out again, pick up where you left off.

author: LaserSteve
author_rank: Super Contributor
timestamp: June 09, 2015, 03:29:44 pm
content: I reprocess Argon and Krypton Ion laser tubes at home, as well as do some Neon for fun. As my laser tubes have up to 40 amp arcs, I use a bit different processing scheme. My tubes have to reach a minimum of 2 x 10-7 Torr for cathode activation, which takes about two hours and is done in step, by ramping the current to the cathode. The factory cathodes are tungsten matrix dispenser cathodes, requiring a far different activation /reactivation process then a small vacuum tube. A cathode consumes 70-120 watts alone. Also I have optical windows on the tubes that need to stay clean, so processing is far more stringent.Final processing is done using plasma flushes while the tube is actually running an arc.Any part other then the fill stem or windows needs a hydrogen furnace bake prior to welding. Any hydrogen left in the tube from a accidental outgas stops lasing, hence the baking step. Windows are either frit sealed or are optically contacted.So my home system is a very large two stage roughing pump, a TMP150 turbo pump, and a 250 L/Sec ion pump. An oven is used to cook the tube as long as needed, which can often be two days if a tube is fully up to air. Usually I can get in via a gas fill system on the tube, so air exposure is rare.While a few laser tubes use a 7052 glass seal, most have a cold welded copper pinch like a high end ceramic RF tube. The pinch tool alone was 3k$. It was kindly donated to the cause by a customer who needed to learn the reprocessing technique. Otherwise I could never afford it.I have a 100 watt inductive system for the neon electrodes. It only takes a few seconds to get that electrode red hot, however they have a large shell that is very susceptible to low frequency induction.At work, which is a University, I often have to hit 1x10-8 Torr, and we have both resistive and inductive bake out depending on the instrument in use.While you can get away with far less hardware for a home system, long life tube processing is expensive. Its an art.

author: LaserSteve
author_rank: Super Contributor
timestamp: June 09, 2015, 03:40:45 pm
content: BTW, Calexanian has a serious point...When serious, college age, hobbyists show up to a lab, we technicians often hand them off to Professors for an unscheduled interview. This has often lead to scholarships and grad school for the individual with the right skill set.When middle school and high school students show up, with a teacher or serious parent in tow, they often find themselves sponsored for Intel-ISEF or a summer internship. At a minimum here, they get a serious lab tour. This is NOT going to happen to everyone who shows up, but it does happen. The US NSF funding agency often awards professors additional funding for STEM Education participation.Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: June 09, 2015, 03:46:57 pm
content: I bought Borated Dumet from the fellow on Ebay. He gets it from JLC Electromet...Works fine, but must be kept in a sealed, desiccated chamber when not in use. Which is no problem with modern Silica Gel.I have also purchased small bags of pre-welded lead thru wires. These are typically Nickel, Dumet, Copper in series, and machine welded. There are a few manufacturers of these. They are nice for making pinches, as shown in the usual hobby videos.There is a video on line some place of the home "butt" welding technique for small gauge sealing wires. A bank of lytic caps is charged to a low voltage. One lead to be welded is shoved down a long glass capillary tube and connected to one side of the cap bank. The other wire is connected to the cap bank via a flex cable. Its rapidly shoved into the tube with a pair of Pliars. The resulting contact flash welds the Dumet to the Nickel. The capillary is a tight constraint, and self aligns the wires.Steve

author: calexanian
author_rank: Super Contributor
timestamp: June 09, 2015, 07:36:05 pm
content: Steve, do you have a handy source for the pre butt welded lead in wires? I am using up my old stocks and will need more soon.

author: LaserSteve
author_rank: Super Contributor
timestamp: June 09, 2015, 07:57:51 pm
content: My last batch came from a surplus shop that used to be next to the famous Nela Park, which is an hour from my home.So far I've found when searching..India:http://www.gemelectro.com/leadwire.aspIreland:http://www.cit-ireland.com/index.php/welded-leadsChina:http://www.titan-lighting.com/cp/class/9.htmlI had a US supplier, once, , I'm trying to find them via Google.I'll go in with you on a buy if needed.Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: June 09, 2015, 08:08:59 pm
content: Calexanian, Check your PMs.Steve

author: BennVenn
author_rank: Regular Contributor
timestamp: June 09, 2015, 08:43:27 pm
content: Thats a good point, I'll start with setting up a nice High Vac chamber to prototype everything and experiment around with cathode coatings etc... No point assembling and spending hours baking a failed tube by design.I'm 30 this year (Yikes!) so my opportunity for uni has passed. 11yrs in the Navy as a Elec/Radar tech but most of what I learn is from building things at home (www.bennvenn.com) and I've wanted to build my own tubes since I built my first tube regen radio 20yrs ago. Slowly collecting/building the equipment to do so.I would love to get in on this group buy for the lead in wires! I will need them eventually and they'll only get harder to source with LED lighting taking over CFL, CRT's are just about dead and modern lamps are all using moly/tungsten in a pyrex tube.

author: calexanian
author_rank: Super Contributor
timestamp: June 09, 2015, 08:48:25 pm
content: Its typically not pyrex. Its typically a corning or Schott product for tungsten sealing. Then for the HID stuff its vycor with moly ribbon. Pyrex can seal directly to tungsten of about .010 or so, but its not a great seal and cannot pass much current. Not enough to light a filament anyways. Mostly used for discharge tubes and such.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 09, 2015, 08:54:07 pm
content: Ahh right, sorry. Pyrex is a brand name. Boro is the glass category.So, feed through insulators! I've read spark plugs can be used - would the ceramic to metal seal be that good for high vacuum work?Or should I try source commercial feedthroughs?

author: LaserSteve
author_rank: Super Contributor
timestamp: June 09, 2015, 09:22:39 pm
content: BennVenn, Since your way down under where vacuum parts do not grow on trees, pay the packing and shipping and I'll send you two 8 lead header feeds with solder tabs for your experiment. These have a one inch diameter base with a solderable flange on them to contain the vacuum. They are basically a big chunk of precision machined Kovar, with a pyrex like glass beads in them for the leads. They are then nickel plated and tinned for solderingThese were used on Mil Spec relays and aircraft radios etc for hermetic sealing.While 60/40 solder is a vacuum no-no, (lead actually has a vapor pressure and poisons cathodes) these will get you started.Send me a PM with an address. Weight will be about a pound counting the box.Looks like one of these:http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=10&cad=rja&uact=8&ved=0CEIQFjAJ&url=http%3A%2F%2Fwww.ametekinterconnect.com%2Fdownload%2FHCC%2520Connector%2520Terminals%2520and%2520Headers%2520Design%2520Guide.pdf&ei=Dlh3VdnWIJH1oASa8oPACg&usg=AFQjCNHmx9fjPGboq7d969BbxJVwy_L9Lw&sig2=oI8yDxM-qTSgfSxwnTmw0ASteve

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 09, 2015, 09:24:47 pm
content: Quote from: LaserSteve on June 09, 2015, 03:07:58 pmYou're*To be perfectly pedantic, it's neither; not all of the energy delivered is deposited in the electrode structure, as there is plenty of time for it to radiate away, heating the glass (and ultimately dissipating it in air). The delivered energy will be greater than that required by heat capacity alone. The delivered power will also be greater than the steady state dissipation at the desired temperature, but for a shorter period of time.So you're both wrong.</pedant>

author: LaserSteve
author_rank: Super Contributor
timestamp: June 09, 2015, 09:31:48 pm
content: You forgot the energy carried away by the liberated gas. As well as the temporary cooling effect caused by the electron emission while the metal is glowing red. Yes, removing electrons to form the space charge cloud cools a metal slightly, proportional to its work function.Steve

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 09, 2015, 09:37:32 pm
content: Included in "dissipation". I didn't specify what type of process(es) were at work, only concerned that heat was going away.Tim

author: BennVenn
author_rank: Regular Contributor
timestamp: June 09, 2015, 10:37:37 pm
content: Steve! That would be fantastic! I was looking at some hermetic relays a while back but wasn't sure if they would be leaky.As for the tin/lead solder, would silver solder outgass too? And would the glass-metal seal be compromised with the excess heat required for silver soldering? The diffusion pump seems to have a bit of silver solder on its output port/lines so it must be OK.So I should solder the feed through in? Or a gasket type system? Gasket will allow me to use aluminium blocks which I already have or I can source a block of copper if it would be more suitable. Cu-Be would be ideal but I'm not sure if anyone would machine it for me.PM sent.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 10, 2015, 12:58:17 am
content: As far as I know, ceramic-metal seals were (usually?) silver soldered (ceramic transmitting tubes, microwave parts, nuvistors..). But as far as I know, it was done completely under vacuum...I recall that cadmium and zinc bearing solders are utterly useless for vacuum duty, so make sure to avoid them. On the upside, everything under vacuum means no need for fluxes!I don't remember if soft solder is suitable for vacuum. It would be kind of hard to use for power tubes though, or during degassing...Tim

author: BennVenn
author_rank: Regular Contributor
timestamp: June 10, 2015, 01:20:40 am
content: Silver solder it is then. Need to find a suitable copper plate and a bell jar to suit. I've seen a few thick walled small bell jars at a local bargain shop. They would need to be polished and ground and hopefully wont implode. Smaller is better for pump down times and implosion protection?And now my induction heater needs to fit around a bell jar! Or is RF bomardment generally not required while a diffusion pump is running?First project should be an ion vac gauge...

author: calexanian
author_rank: Super Contributor
timestamp: June 10, 2015, 01:48:04 am
content: Bombardment is not really required while on the pump so long as you are in the -6 range. That requires a very very good system though. Here it why..Getting back to what you were saying about after lighting your tungsten filaments your pressure drops. This is correct. Unfortunately what is happening is the nitrogen and oxygen in the residual gas is combining with the tungsten to form either Tungsten Nitride, or oxidized tungsten, both of which will reduce emission greatly. Essentially the tungsten becomes a getter. Flashing the filament even hotter does not really help at a certain point. This in part can be mitigated in a thoriated tungsten filament my forming a tungsten carbide layer by decomposition of hydrocarbon gas at elevated temperature after heating in a reducing atmosphere in a partial vacuum. This carbide layer will still allow the thoria to diffuse to the surface and emit, and to a degree dissuade the remaining gas molecules from reacting with the tungsten core. In very very old tubes before getters were sorted out they would create sublimation pumps with a sacrificial tungsten filament in the bulb, sometimes wrapped with an aluminum wire. When the aluminum is out gassed, then subsequently evaporated (Titanium is used today) it will adsorb the remaining oxygen and hydrogen vigorously. Then in turn the tungsten will adsorb any nitrogen or hydrocarbon gas into nitrate or carbide deposits therefore reducing the final pressure.Long story short.. Even though the pumping system may be maintaining a very clean vacuum. There is still a continuous gas load and a hot filament is picking things up as they pass through. For short tests though demountable systems are great. It is also why a tube can be sealed off at a relative high pressure and be ok, but a surface physics experiment must be kept at extreme levels of vacuum and cleanliness. Once you seal that glass its deterministic whats in there, less leaks which are very quickly known. Chemical whats in there, is whats in there. Nothing can be added, or taken out.As far as the spark plug idea. I was told by an old timer friend of mine that they work very well. Just make sure you get bog standard plugs and not the platinum tip or any other fancy ones with the tiny conductor in the middle. Some plugs have an intrinsic impedance that does you no favors. Head taps for the spark plug holes are easy to find. Also you can get replacement spark plug bushings that are already threaded, or you can silver solder them.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 10, 2015, 03:30:58 am
content: Vacuum thru fittings for the coil (water included), will be highly desirable. There's just no way you're going to induction heat something through a bell jar.Tim

author: calexanian
author_rank: Super Contributor
timestamp: June 10, 2015, 03:49:02 am
content: generally for just making test measurements induction heating will not be required in a high quality vacuum chamber. The volume of the system and high pumping rates will overcome the outgassing along with radiant heating from the filament.Generally I would agree with you about induction heating through bell jars. However.. It has been done and continues to be done very successfully. We fire 2X3 inch cylindrical graphite anodes stacked several high suspended from a top hook in a bell jar with a 7" wide 4" tall coil of about 8 turns to red heat easily. It just takes 10KW... We have a little gear driven rack that raises and lowers the coil along the jar and the anodes turn red as the coil moves over. Kinda fun actually. In this instance however physics is working with us due to the size and cylindrical nature of the part. Generally an internal coil or other heating method is preferred.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 10, 2015, 04:06:45 am
content: Interesting. Given the majority of gas in my tubes would be hydrogen or oxygen (I could even flush continuously with these gasses) would sacrificial tungsten as a getter work? I can get a coil of tungsten incandescent with my current induction heater. This should evaporate within the tube. Just thinking of things I can try while I get together my High vac system. I'll see if titanium wire is available here.Found a 150x250mm boro bell jar from a scientific equipment supplier here in Aus so no more implosion worries. They don't stock the base though. It'll have to be custom anyway being copper. Or Cu electroplated aluminium?

author: LaserSteve
author_rank: Super Contributor
timestamp: June 10, 2015, 03:36:16 pm
content: FYICeramic tubes are furnace metalized using typically a Moly-Manganese or Indium-Gold alloy paste, usually activated by including titanium hydride powder. A copper or gold plating is often then used over the metallization They are then active brazed at a very high temperature, again using a hydride or metal activator instead of a flux. Rarely are they soldered. Solder is frail and micro cracks when temperature cycled. A typical braze might be Titanium-Incusil or a Wesgo alloy.This requires a very hot hydrogen or vacuum furnace.Soldering would be a lab option for one offs.Steve

author: BennVenn
author_rank: Regular Contributor
timestamp: June 10, 2015, 10:47:55 pm
content: I suppose the ceramics are of a unique composition too? Standard clay based ceramics would be too porous?I've got a reply from both the local glass distributor and the dumet supplier over in China. Duran brand soda glass at AU$15/kg so I'll be stocking up on that - he doesn't have a large range but can order it in at a 12week lead time. And the Dumet was quoted at US$39.50/kg MOQ 1kg and shiping at US$64.50/KG. All up close to AUD$200/kg. I suppose a KG would last a very long time.Next up is building an annealing oven for the glass. Kiln heater coil in a refractory tile box should do the trick, with a quality thermocouple and controller.Did you get a quote on those pre-made lead in wires? They would save me quite a bit of time spot welding Nickel-Dumet-Nickel.For an ion gauge - is tungsten ok for a fillament or will I need thoriated tungsten?And an Ion pump, as simple as it sounds? Positive on the tube, Negative on the pump and let the ions work their way down?

author: calexanian
author_rank: Super Contributor
timestamp: June 10, 2015, 10:57:28 pm
content: Quote from: BennVenn on June 10, 2015, 10:47:55 pmAn ion pump will not have the volume you will need. They are only good for systems that are already extremely clean. Use either a diffusion pump or a turbomolecular pump. Diffusion pumps are the lowest cost and are silent.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 10, 2015, 11:07:14 pm
content: I've got a diffusion pump and enough oil for 2 charges incase I make a mistake, just waiting on a nice rouging pump to become available.The traveling wave tubes and kylstrons at work use ion pumps to keep a hard vacuum and also give a good indication of a leaky tube, I've read a HV discharge can move a large amount of gas through a tube when already pumped down.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 11, 2015, 12:49:43 am
content: AFAIK, BeO and Al2O3 ceramics are the most popular. I recall Tektronix used a forsterite (magnesium silicate) blend for their press molded ceramic CRT parts. I haven't heard anything about others, really.Al2O3 wouldn't be very easy to do by hand, as you'd want either HIP (hot isostatic pressing) or a very high firing temperature to get the porosity down. Liquid phase sintering (i.e., adding a flux to goop it together with a transient or permanent glassy phase) might be okay, but beware of trapped gas pockets I suppose, and the influence on expansion rate (e.g., adding a sodium silicate phase might make firing easier, but also worsen the expansion beyond what you were hoping for). Different fluxes have different TCEs, too (I recall lithium is better in that regard, usually sourced from lithium carbonate or spodumene).Just offhand, I wouldn't think any pottery style ceramic would be a problem, as long as it meets requirements for porosity*, TCE, service and processing temperature range, and chemical compatibility with your process (e.g., the leaded glass not behaving well under HHO).*Comes to mind, glazes could be used -- but -- they might trap gas in the internal pores, too. So going for a solid unglazed body is probably a better bet.Beware that most pottery has a large quantity of SiO2, as quartz or cristobalite, both of which go through an expansion phase transformation at rather modest temperatures. Whatever you use, it will have to be formulated for low silica content (mullite, forsterite, cordierite, etc.?), or high glassy content (porcelain). And ensuring homogeneity may demand technical processing methods (chemical mix --> ball mill --> first firing --> pulverize and ball mill --> add organic binders --> mold and fire), at which point you might be better off doing something else.Hope I'm not too far off base. Armchair physics and all that.Oh, Macor and other machinable ceramics come to mind. Think I've heard of them being used for vacuum, before. No idea about direct bonding (glass seals or metallization).Tim

author: calexanian
author_rank: Super Contributor
timestamp: June 11, 2015, 01:02:50 am
content: Most of the ceramics are completely vitrified. porosity is typically not a problem. Maccor is very expensive. Using silicone oil for diffusion pumps is recommended. Hundreds of cycles to air before any degradation occurs. Octoil and other non silicone oils will not survive exposure to air at operating temperatures. they make a terrible mess as a matter of fact. Hard asphalt like gunk forms.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 11, 2015, 01:17:29 am
content: Yes, I have some Dow 704 silicone oil. Expensive but worth it from what I've read.There is a local ceramics supply store in my area (https://cdn.shopify.com/s/files/1/0572/0029/files/Summer_2015_Catalogue_w.pdf?2707) that sell allkidskinds of raw materials - which is where I get my barium/strontium carbonate that I plan on making into emission materials. Most of which you've mentioned is available. Beryllium is one of those things that is hard to get hold of, similar to mercury and has the same stigma as asbestos. A high grade porcelain should accessible.Would ceramics have the same 'bubbling' problems as glass melted from fritt? I think making a noval style base from glass fritt in a vacuum would be achievable. The vacuum to degass the glass would be easy enough to maintain with a single stage pump which I have now.I'll put that order in for the Dumet (0.6mm) which should carry a fair bit of current and bond well to 1mm nickel wire.There is also an almost endless supply of magnetrons at my local scrap yard from gutted microwaves. Could the ceramic/metal seals be re-used? Can ceramics be brazed to other ceramics after firing? My toch is more than hot enough to melt porcelain, not sure about anenaling it though.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 11, 2015, 01:35:48 am
content: An update on the Dumet, another supplier just offered 0.35mm at US$12/kg with $40 shipping/kg. Much more reasonable. Seems alibaba does the negotiating for you! Checking if they also have pre-made lead in wire.Update: Ordered 1kg of 0.6mm at $20/Kg. Should be thick enough to butt weld to 1mm nickel.

author: calexanian
author_rank: Super Contributor
timestamp: June 11, 2015, 05:55:33 am
content: I would not really even consider ceramic. Its hard even for the specialist companies who use it.

author: LaserSteve
author_rank: Super Contributor
timestamp: June 11, 2015, 02:26:10 pm
content: I'll call the company that makes the pre welded leads today.Ceramic is not for home use, the vacuum metallization and brazing furnaces run well above the 1100' C limit for typical nichrome based lab furnaces.I mentioned it for completeness.Steve

author: BennVenn
author_rank: Regular Contributor
timestamp: June 11, 2015, 11:13:31 pm
content: Yes, baby steps!Dumet has been ordered, I'll pick up the tube next week. I'll machine a flange to bolt onto the diffusion pump and interface to 6mm dia glass tube. Compression seal should be ok for the evac tube?I'll also need to strip down the diffusion pump to clean out the packing peanuts that found their way inside during transit.No need for a getter while its on the pump, I'll add a loop of tungsten to evaporate in the tube if I plan on sealing.I have a few meters of 0.15mm tungsten and molly wire to experiment with as emitter sources. I'll mix up some barrium&strontium carbonate and see if I can activate it in the tube.First project will be a simple diode with different emission materials. How are the oxides adhered to the fillament? Sodium sillicate as a binder perhaps? Or does the tungsten alloy with the barrium+strontium?

author: BennVenn
author_rank: Regular Contributor
timestamp: June 12, 2015, 12:57:32 am
content: Supplier has asked do I want "oxydic or boraxing". I am after borated, yes?

author: calexanian
author_rank: Super Contributor
timestamp: June 12, 2015, 01:03:06 am
content: The oxides are typically mixed with a nitrocellulose or Methacralate binder then applied like paint. Everything must be chemicaly cleaned. You may not have success because oxide cathode bases must be cleaned of their oxides in a hydrogen reduction environment. Tungsten is more forgiving. Thoriated tungsten requires the thorium to be diffused to the surface and requires the curface to be previously converted to a carbide. Pure tungsten need only be de oxidized and cleaned. Sometimes you can just polish it to remove oxides. It will emit consistently unless it is oxidized or becomes too hot and begins to evaporate.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 12, 2015, 01:16:36 am
content: So to convert the barium carbonate to an electron emitter simply heating in a vacuum is not enough? I should backfill with hydrogen then heat the filament?

author: calexanian
author_rank: Super Contributor
timestamp: June 12, 2015, 01:53:20 am
content: I think we jumped the tracks somewhere. The base material must be completely chemically clean and oxide free for the disassociation of the alkali metals to occur. Somply being physicaly clean is not good enough. The activation of the oxide cathode requires the reduction to metallic barium, etc. and it needs a clean base metal to sublimate to. certain reducing metals can also be present such as aluminum and titanium, in very small quantities and help speed activation. But cleanliness is essential. very hard to do in a non laboratory environment. I have had some success with nickel ribbon electro cleaned, but the results are not consistent attempt to attempt.

author: LaserSteve
author_rank: Super Contributor
timestamp: June 12, 2015, 01:36:20 pm
content: Most commercial cathode materials have a "activator" reactive metal, such as aluminum, nickel, titanium or zirconium, usually contained as titanium powder or titanium hydride or zirconium metal powder. The reactive metal acts as a flux to lower the reaction temperature of the reduction to base metal. The exception being plasma processed cold cathodes for neon tubing. But neon cathodes have such a huge area that they can afford to be in-efficient. The mixture has a aluminum oxide backing that is porous and has a huge surface area, yet is insulating.This page gives some mixtures:http://transene.com/cathode/A directly heated pure tungsten cathode will get you started. After all, that worked quite well in Thomas Edison's somewhat accidental discovery of the diode(1). Production tubes usually have indirectly heated cathodes that are coated with emissive mixes.This gets complex:http://www.cathode.com/pdf/tb-135.pdfRosebury's book goes into detail on making reliable cathode mixes that are dipped, wet electro-deposited, or airbrushed on cathodes. Almost all are based on having a very fine Aluminum Oxide powder as a base. It also goes into detail on cleaning the metals. Most electrodes are hydrogen baked, cleaned in a chemical bath or electrolytically cleaned.As for using the tungsten wire as a getter, simply place a small amount of cleaned aluminum foil or fine aluminum wire on the tungsten and use that as the getter. Aluminum evaporation wires are usually premade by placing the tungsten in a vacuum and heating the wire using a transformer on a variac. The idea being to premelt the aluminum, which naturally wets and coats the tungsten, before welding it in the tube. Magnesium has also been used by hobbyists in the same way. AL-MG would probably work better.I used to pre-wet my wires for evaporating Al on glass slides in the lab. I'd describe it as easy if you have some form of demountable vacuum jar or quick-flange. I had a window where I could watch the process as I outgassed the wire and then took it to just above melting. There is quite a difference in the current between wetting and full scale evaporating.But keep in mind a getter without a reactive metal is not efficient. So a AL, MG, or AL-MG getter without the Barium or other active metal is just sweeping the residual gas out of the tube and burying it in a metal film on the tube wall. . Then design the tube so the deposited aluminum is shadowed only where you want it, otherwise it could short out things.Any gettering you have is a a good start.This is like chess. Its lots of tiny steps.1. Mr. Edison hired a lot of bright people, however it can be argued he went out of his way to claim all their inventions as his. I think he's more or less the actual inventor of Self Promotion in the technical fields.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 23, 2015, 03:44:12 am
content: My dumet has arrived! For a while there I thought they might have done the dodgey with my money but no! A 1kg parcel has arrived at my post office.I changed my order from 0.6mm to 0.35mm as it will be more tollerant to an expansion mismatch (I hope) although handle a lot less current.I'll grab some refractory tiles this week and build an anneal/bake-out oven and try a few glass-metal seals. Might even get a few tube blanks blown. Excited!After reading that cathode.com pdf, I now understand what you mean about the reduction to metalic barium. I'll be using tungsten for the filament, at least until I get a few working tubes under my belt. I've watched a few videos on home made plasma cleaning materials, that should prepare a nickel cathode for an emissive coating?