microwave kiln heating elements? (new susceptor questions) - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: February 06, 2022, 03:28:16 am
content: So I have a fire brick (the insulating crumbly one) and some A1 wire to make a little heater with for sintering glass/silver mix on ceramic for small PCB, but I was kind of thinking if I can do this easier. That wire is real tough and I don't want to wind springs with it unless I have to... looks like a pain in the ass.I have seen microwave kilns of various kinds used to melt small pots of metal for casting.I thought to take a fire brick, cut some notches in there parallel to each other and stuff old carbide endmills/drills in there. I think if I put this in a microwave with another brick on the bottom, it might melt a small amount of silver paste on the ceramic PCB to make traces?This has to do with suceptor choice. Will this idea work? I was hoping each of the carbide end mills would get real hot and heat up the ceramic sheet under neath it. Everything I find is related to like casting metal.. but I really just want a very powerful 'broiler'.

author: coppercone2
author_rank: Super Contributor
timestamp: February 07, 2022, 02:02:48 pm
content: having studied it more, it looks like people use old grinding stones that are cut as suceptors.. seem to hold up quite nice.if anyone is trying their own silver PCB that might be a option thats cheap, the big ass kiln in garage game is something I do not want to get into.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 07, 2022, 10:33:31 pm
content: Cool! I wonder about arcing and sparking on the microwave pcb on account of, you know, the microwaves -- but it feels like shielding it enough to not spark might be possible. I dunno if that means just a few absorptive elements, or a lot of absorptive elements, or a cage of absorptive elements, but in any case sounds like good fun! Let us know how it goes!

author: coppercone2
author_rank: Super Contributor
timestamp: February 11, 2022, 03:12:57 pm
content: I got a SiC crucible and also a SiC stone to try this with. I can put the crucible over the sample like a hat or try SiC vitrified block cuttings around the perimeter of the interior to see if any of them work. I also got HT100 infrared reflective compound to paint the interior of the kiln withI do wonder whats gonna happen to the traces, I was thinking maybe the SiC would lower the field strength near the PCB to non sparking levels. It may also be possible to add silicon carbide powder to the traces themselvesvalve grinding compound looks to be 40% SiC too... so you can do something with that maybeKinda wonder if it will even regulate properly. I think it would need a slot for a glass fiber optic line and a optical pyrometer to be really goodI kinda wonder too what will happen if you just paint the brick with valve grinding compound and microwave it, maybe that would make a suceptor paint, or to mix SiC with SiO2 and run microwave at low power levels to make a sintered heater

author: coppercone2
author_rank: Super Contributor
timestamp: February 11, 2022, 11:41:52 pm
content: So I have a question about susceptors in general:How are dimensions of a susceptor relevant to suceptance of fields in cavities?Is there anything different about a susceptor inside of a wave guide, with different propagation modes (round waveguide, oversized waveguide, etc)?If you had spaced out chunks (i.e. like not touching electrically), what would happen?What is the effect of density?.. i.e. if I fill a trench with compacted vs non-compacted SiC powder.What is the effect of microwaves on something graphene-like, where it practically exists in only two dimensions?I see that some microwave kilns draw a 'band' of silicon carbide paste around the interior walls of the kiln, that kind of makes me think of a shorted loop. is this relevant, like a full band vs a strong open C shape that does not form a 'shorted turn'?I put a bit of valve grinding compound (40% SiC + ethylene glycol) on a fragment of ceramic and its not doing much in the microwave, but its just a little patch maybe 4x4mm. I figured it would be glowing after a few minutes.So the over is 2.4GHz, which is approximately 12.5 cm. When you have susceptor structures, do they also follow antenna/electric size rules? Like if I were to draw a 12.5cm long structure, would it heat better?I did not want to do larger scale experiments with the grinding paste because I don't want to screw up that microwave with odors.I want a method of thinking about this heating system.. I am not sure what direction to take my thoughts. Initially I thought that it might have nothing to do with size because the microwave seems to dehydrate stuff very well but that is usually with water trapped in a material so it seems that the water molecules are connected to each other because of evapotranspiration/capillary action.I suppose I can spritz a sheet of glass coated with a bead forming chemical (ceramic coat or slightly greased) and see how isolated droplets dehydrate vs a moist cloth or a really clean sheet forming surface.. would this experiment be relevant?

author: coppercone2
author_rank: Super Contributor
timestamp: February 13, 2022, 03:38:42 am
content: Well the silicon carbide rectangular 0.5x0.5x1.5 '' stones appear to work well, they heated to red hot in 1 minute and only seemed to produce a little odor on first heating... snapped off from a norton vitrified SiC stone after a incision with diamond wheel in dremel.If you do this and wet cut the brick, be sure to dehydrate it well before first use (2 hours in oven at 350 and then a few 4 minute cycles in microwaves till its fully dry, possibly reinsert into oven again for 1 hr).. it seemed hard to get all the water out... but on low power it should be possible to just slowly microwave the water out (inverter microwave preferred), just beware that you will get alot of condensation in the microwave so I recommend short heating/cooling/wipe off cycles.Can't wait to try it with the silver mix, just need to precisely weigh out the powders with good old metler.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 13, 2022, 01:08:36 pm
content: Thanks for the update! I look forward to hearing how the next part goesHere's hoping that SiC Stonehenge can keep the fields down enough to prevent sparks from disturbing the traces.

author: coppercone2
author_rank: Super Contributor
timestamp: February 14, 2022, 03:20:39 am
content: Ok, so I think the problem is I used pure SiO2 which has too high a melting point for my little kiln, its gotten plenty hot but its just not fusing. I need to try to get a lower melting point binder.

author: coppercone2
author_rank: Super Contributor
timestamp: February 14, 2022, 03:57:07 am
content: actually analyzing the sample I prepared it does look like its somewhat solid and conductive but it does not look like anything was molten.I took a small segment of the hard fire brick (the liner kind) to act as a rough substrate for the SiO2/Ag powder. When I rub it gently with a multimeter probe it does seem to hold together despite still looking like a powder. Contact testing with a multimeter reads continuity that is quite low resistance.The material tins readily with eutectic lead solder. The material reads close to a dead short on a kelvin measurement with 1mm of separation (2 miliohms or so). Unfortunatly it came off the substrate so I had to do a mid air measurement before it broke in half.The sintered bit lifted off pretty easily (poor adhesion), but it is just a shitty cheap firebrick.. not a nicely prepared engineering ceramicI would say this looks somewhat promising, given that my binder has twice the melting point required. I did not have to do any cleaning, the stuff adhered to the 'solid powder' very easily with regular flux and soldering technique. I am surprised I had any kind of result.The problem right now is that my substrate is as thick as the heaters, so essentially the heaters are only sticking out about 0.5mm around the perimeter of the kiln, so the thick porous substrate is being blasted hard but there is no light falling on the substrate. it should shine like the shadow of a obelisk.. I think maybe if I put carbide overhead and use a thin substrate it will work. I believe that I will attempt this on a ceramic razor blade since its not ready for my better substratethe next structure will be a jenga typeI really want a carbide parabola to put over the substrate. I am starting to think standoffs + a sheet of silicon carbide would be the best solution without anything custom.

author: coppercone2
author_rank: Super Contributor
timestamp: February 14, 2022, 07:05:54 am
content: Ok, so I tried a bigger pile of braze powder on a rather well finished or possibly glazed razor exacto blade surrounded by carbide elements from all sides including the top (stonehenge with cross members).. the silver is really stuck on their hard but the pile was like 20x thicker then it needed to be for the trace (mountain) and while its totally conductive and super well attached, its really really bumpy with a few mountains.Anyway, that goes to say it might be possible to use SiO2 instead of glass. FYI SiO2 is used in stainless steel welding flux (for gas welding).. so it has a flux reaction with the substrate maybe.I need to make some kinda stencil and maybe try to gel-up the powder to see if you can make a neat trace.. but it seems promising.When I mixed the powder I rolled it around in a crucible with a ball bearing.

author: coppercone2
author_rank: Super Contributor
timestamp: February 18, 2022, 07:50:06 pm
content: I tried the HT100 ceramic reflective coating on the microwave kiln. It made a huge improvement, much less energy is radiated from the brick when its in the microwave and it heats up faster.. so the platter does not feel like its going to explodeI could not find any information on HT100 being microwave transparent, but it appears that it is.Also, if you need a sample substrate, a ceramic razor blade seems to work. You can clean it off using a silicon carbide grinding stone in a dremel as well to reuse it, since spending money on substrates sucks when you are already spending money on silver. It also polishes nicely, so if you make it thick, it will most likely be possible to surface grind it nicely.. even when molten. After a bit of grinding I got a shiny silver surface.If you do a ceramic razor, be sure to use the carbide dremel stone to dull the blade first, it does a fine job with some white sparks.

author: coppercone2
author_rank: Super Contributor
timestamp: February 19, 2022, 08:34:58 pm
content: I was experimenting for a while.. I really think SiO2 is not the answer.Every time I get strong adhesion, there is molten silver in the mix. When I adjust the power level on multiple samples (using a inverter controlled microwave), I can either get it looking like its partially sintered, somewhat conductive but weak, or get a very strong bond to the substrate that has molten droplets of silver and the trace looks screwed up. The sintering/melting temperature of the SiO2/Ag are too different.. I think the key is in using a SiO2 alloy?mixture? that has a lower melting point (glass glaze extract), as in the youtube videoMaybe there is a different approach that you can take i.e. sprinkle the silk screen with SiO2, then sinter it at extreme temp, then add a silver SiO2 mixture.. maybe it would help if there was already some roughness attached to the PCB but it seems too complicated compared to buying some reagents

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 20, 2022, 04:12:03 pm
content: The melting temperature difference theory sounds plausible, but could it also be a situation where once the silver starts sintering it turns into an antenna and you get arcing / splattering? Or do you think the setup as it stands is pretty good at keeping the fields down in the region with the traces?

author: coppercone2
author_rank: Super Contributor
timestamp: February 20, 2022, 07:52:34 pm
content: no way to tell without quartz inserts to look through or something like that.. I am not modding this microwave either (that is a huge project if I am gonna be doing things to a microwave oven cavity to make it observable. My kiln is too big to look into while its in thereI got the glass powder gloss thing I will try it with so if that looks damaged it means the field strength is too high so maybe reducing power and increasing time would help or just going to a normal kiln. I can also try to put more carbide around it, but I ran out of sharpening stones.I thought about the arcing too, hopefully soda lime or whatever glass has a low enough melting point that I can throw the field down to setting 5.. but if its forming a good antenna then I dunno. All I can think of is to put the PCB in like a sarcogphagus of SiC with no field inside (i.e. in a crucible with a lid).These sharpening stones are also machinable I guess so its probobly possible to make like mortises out of them and join a bunch of stones together to make a box out of sharpening stones lol

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2022, 10:21:55 pm
content: SiO2 powder? That's not glass... not until obscene temperatures at least.Mixed with a flux (alkaline or alk-earth oxides) it'll melt at a lower temperature, but better to just use frit (powdered glass), better still a properly formulated metal enamel that's, y'know, manufactured specifically to fuse into a conductive deposit.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: February 20, 2022, 10:25:35 pm
content: What are the commercial products available?The silica was a bad choice, I thought maybe some kinda chemistry would happen if the two powders were ball milled together and really fine that changed the properties (the silver is near nano powder size). I have been waiting on a shipment of the kiln gloss to get fine glass out of.How does the flux chemistry work exactly in this situation? It gets rid of the oxygen making a portion of elemental silicone on the crystal of SiO2 that forms a (slow sintering) bond with the silver? Is it a kind of thermite reaction ? Kinda wonder what would happen if you rinse it with a NaOH solution and let it dry and repowder, or KOH. That would be a alkaline flux ..?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2022, 10:32:17 pm
content: It's just acid base reaction... mildly so at that. Just combining oxides. Nothing crystal about it, I mean it's making glass after all. So, Na2O.SiO2 has lower MP than SiO2, further still with some CaO mixed in, etc. Or B2O3 or P2O5 based glasses can have even lower melting points (I think relevant for semiconductor manufacture, i.e. frit-brazed ceramic packages).Tim

author: coppercone2
author_rank: Super Contributor
timestamp: February 21, 2022, 01:14:52 am
content: would that work though? I think I read about a thermite that makes silicon metal, using magnesium. I wonder if you can mix Mg/Ag/SiO2 to make something that would fuse into what I imagined, silicon on SiO2 crystals. But I guess that would be alot of diodes there

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 21, 2022, 02:20:18 am
content: Man, I'm just talking about pottery. No idea what kind of pot you're smokin.(Apologies. But too good of an opportunity to pass up.)Tim

author: coppercone2
author_rank: Super Contributor
timestamp: February 21, 2022, 02:54:01 am
content: what is the commercial conductive glaze?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 21, 2022, 08:20:28 am
content: What was it bkraz used again? Or wait, did he mix that too? I forget.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: February 21, 2022, 09:39:48 pm
content: I found a old tektronix film about making ceramic CRT's, they say the silver traces are made with a mixture of silver powder and 'glass frit'. So I assume its the same type of glass they use to make filter elements in lab glassware. It's kind of expensive though, i hope I can get good results from the gloss ceramic stuff

author: coppercone2
author_rank: Super Contributor
timestamp: February 23, 2022, 05:53:40 am
content: the ITC-296A kiln coating is also microwave transparent, and it seems to make a difference in heat up time.

author: twospoons
author_rank: Frequent Contributor
timestamp: February 24, 2022, 12:23:19 am
content: I would have thought you'd want better temperature regulation than you would get with just microwaving broken grinding wheels.Surely you want a controlled sinter, rather than just melting at whatever temp you can achieve?

author: coppercone2
author_rank: Super Contributor
timestamp: February 24, 2022, 01:49:55 am
content: Its like a thousand percent cost and size savings lol, I will try it with the frit powder and see what I can get. Hardly going to feel bad about the materials I used. If it does not work I will just cut it to thermal blocks to have materials for arranging around gas welds and soldering at no loss.