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Vacuum Sealing of a Ceramic Muffle at the Ends in a 1100'C Furnace

03/13/2015 9:01 PM

I want to seal a ceramic muffle at the ends which is being heated from outside in a furnace so that I can make vacuum inside the muffle at 1100'C . Can I do it with a plaster of magnesium powder or any other fire cement soaked in sodium sillicate?

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#1

Re: vacuum sealing of a ceramic muffle at the ends in a 1100'C furnace

03/14/2015 2:56 AM

You may want to so so more research about magnesium and the dangers working with it. Wikipedia has this about it;

The combusting magnesium-bodied Honda RA302 at the 1968 French Grand Prix, after the crash that killed driver Jo Schlesser.

Magnesium metal and its alloys are explosive hazards; they are highly flammable in their pure form when molten or in powder or ribbon form. Burning or molten magnesium metal reacts violently with water. When working with powdered magnesium, safety glasses with welding eye protection are employed, because the bright-white light produced by burning magnesium contains ultraviolet light that can permanently damage the retinas of the eyes.[54

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#2
In reply to #1

Re: vacuum sealing of a ceramic muffle at the ends in a 1100'C furnace

03/14/2015 11:36 PM

I don't have an answer to the OP's question, but he's NOT talking about "magnesium" powder; hes talking about "plaster of magnesium" powder. Plaster of Paris is Calcium Sulphate; I'm guessing that he's referring to Magnesium Sulphate.

Sulphates are very different from their base metals.

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#4
In reply to #2

Re: vacuum sealing of a ceramic muffle at the ends in a 1100'C furnace

03/15/2015 12:43 AM

I hope he's talking about magnesium sulphate. We about lost an aircraft from a brake fire that caught the tire rim on fire. Man, I can tell you the sparks were flying and they weren't all from the rim either!

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#5
In reply to #2

Re: vacuum sealing of a ceramic muffle at the ends in a 1100'C furnace

03/15/2015 9:38 AM

Ac tally when I say magnesium powder it is a compund of magnesium used to form a plaster to be applied for vacuum sealing . However I have changed the powder and instead using silica powder mixed with Sodium Sillicate aqueous solution to form a paste while sealing the ends of a glazed silimanite tube with a stainless steel tube where the ceramic tube is terminating .By a heat seal, temperature of stainless steel tube will not be more than 500'c although center of the sillimanite tube will be at 1100'c . I know quartz tube would be ideal as regards porosity of the ceramic tube is concerned but there is some compulsion for which I am using Sillimanite tube . Even with quartz tube the problem of end point sealing with ss tube for vacuum remains . I am trying with my present arrangement as described but would appreciate if anybody can suggest any better method .

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#6
In reply to #5

Re: vacuum sealing of a ceramic muffle at the ends in a 1100'C furnace

03/17/2015 9:03 AM

Differential thermal expansion will lead to cracking of the seal. Make sure the sillimanite is vacuum tight first. Precision grind and polish a seal area at each end for a silicone o-ring seal. Use stainless steel fittings if you can to reduce cost. Place a ceramic plug in each end to block the radiant energy from overheating the seals. Water cool the stainless steel fittings if necessary if the ceramic plugs do not lower the seal temperature sufficiently.

You should expect to achieve 10^-2 torr with a mechanical roughing pump and 10^-5 with a molecular pump.

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#3

Re: Vacuum Sealing of a Ceramic Muffle at the Ends in a 1100'C Furnace

03/15/2015 12:22 AM

I do not know quality of your ceramic- hope it is not porous and withstand vaccum at 1100 Deg C. Do not know your application. Why not try quartz glass instead container. It can withstand vacuum and 1100Deg C?

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#7

Re: Vacuum Sealing of a Ceramic Muffle at the Ends in a 1100'C Furnace

03/24/2015 9:34 AM

Fused quartz would be much better in your 1100 °C application as this is just below the annealing point of fused quartz (tubing). The softening point is given as 1665 °C,

The material has a very low thermal expansion coefficient which makes it more suitable than other ceramic tubes for transition parts to stainless steel (IMHO). The join may be kept within a safe range of temperature using a jet of forced air, not water cooling (IMHO). The vacuum characteristics should also be superior. All of this depends on what the tube may contain during the test. If you put something in the tube that could form a solution of quartz within its liquid, then this will not work, since the tube would thin out, and fail suddenly under external pressure to the vacuum.

If you have other reasons for using the sillimanite ceramic tubing, then by all means try the magnesium silicate material as a sealing material, but I suspect there will be issues with differential thermal expansion overcoming the seal, such that good vacuum levels will not be reached. I also am not keenly aware of the porosity of sillimanite tubes, but I understand there are porous and impervious grades of "mullite", essentially the same aluminosilicate mineral.

If, on the other hand, you have a "boat" crucible containing such material to prevent contact of any melt with the quartz, then the likelihood of success should be considerably higher. I suggest tantalum, tungsten, or other high melting substances, but probably not graphite, as it may begin to react with the silicon dioxide (quartz) at the furnace temperature just slightly. You could still try the graphite in a test run if you wish, and study the effect on the tube. Fused quartz tubing is not hard to fabricate with, and a good glass blower using hydrogen torch can easily mate this up to the transition pieces (that connect to metal tubing), and fused quartz is readily available at a "reasonable" price.

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