RogueRose - 9-1-2019 at 21:31
I remembered seeing someone make a shaped heating mantle out of, IIRC, CaSO4 and it was a perfectly shaped heating mantle for their RBF. I don't
remember if it was on this board or not or if it was on Instructables or not but I'm wondering if anyone remembers this and if the mantle held up.
I have a few RBF's that have a broken neck, so I though I might try to make one using these, so if something goes wrong, I don't care if it breaks.
I plan to use some nichrome or similar wire and implant a thermocouple or 2 and control it with either a PWM or a variac.
I thought about doing the same making a flat hotplate top and use either cartridge heaters or resistance wire (maybe leaving space in center for
stirring). I could cast the heating block on top of a stainless steel, aluminum or ceramic top, so when it dries, the top is firmly attached and it
has good contact. IDK if it would be better to use something like thermal paste for contact or not.
On another note, I've found that I can find old hot plates that don't work (for whatever reason, often no heating but stirring and power supply is
fine) and the replacement elements are very expensive, compared to the $5-10 for a broken unit or $100 for a used one, and thought making a
replacement element like I described above would be adequate and I can't see how it would be "vulnerable" while it was under the heating top (Al, SS,
ceramic, etc). Any thoughts on this?
I was thinking of trying a different mix than just CaSO4 such as a mix of dry clay (kaolin), CaSO4 and maybe Na2SiO3 (or sodium silicate)
I also have SiO2 and Al2O3 (which is the main composition of kaolin clay) but IDK if this has to go through a very high heating process (with water)
to make it "clay". Could these be added separately to the CaSO4 and get a similar result, or mixed with the sodium silicate?
It'd be nice to come up with a workable formula for high temp refractory, I know this doesn't need to be very high temp, but I have use for the very
high temp refractory in other apps, and this seems like a good project to try it out.
happyfooddance - 9-1-2019 at 22:15
I would imagine that CaSO4 would effloresce a lot, avoiding that may be a challenge.
Ubya - 10-1-2019 at 00:12
https://youtu.be/QKFC0ke_DOU
is this what you are referring to?
wg48temp9 - 10-1-2019 at 02:45
I don't understand why anyone who knows that that a hydrated compound that contains water of crystallization and can be dehydrated at temperatures
just over 100C, would think it could be used as insulation significantly above 100C ???
Most compounds that contain water of crystallization when dehydrated fall apart and the resultant dehydrated material usually has a smaller volume
(cracking and shrinkage)
The setting (hardening) of new plaster (dehydrated calcium sulfate) is caused by hydrated crystals growing from the mix of water and anhydrous calcium
sulfate. The growing crystals form an interlocking matrix ie set plaster. That matrix will be destroyed when its is heated past its dehydration
temperature.
Anhydrous copper sulfate would behave similarly.
In addition after dehydration and the when cooled down it will absorb water from the atmosphere which will have to be driven off next time its heated
even it remains sufficiently coherent to use it.
If the insulation is contained in a earthed metal can there will be a good chance the earth leakage trip will be triggered as the driven off moisture
condenses on the cooler parts form a leakage path between the can and the heating element or connections to it. It could also be a shock hazard if the
damp outer insulation is touched.
The only legitimate reason I can think of for using copper sulfate would be in small amounts (<1%) as a sintering aid in a real (not a hydrated
salt) ceramic. I guess that small amounts of plaster could also be used as a sintering aid also. But how are you going to heat the whole mantel to
about 1,000C for the aid to help sinter the ceramic precursors.
One more point copper connections to the heating element that are heating much above 300C will gradually oxidize over time and eventually fail. The
usual method of connection is to use a nickle wire welded to the heating wire or double up (preferably triple up) the element wire (fold back a
several inches at the ends and twist together which is then connected to the copper wires outside of the insulation in a cool location.
From https://www.escholar.manchester.ac.uk/api/datastream?publica...
"shown in Figure 2.7, the results suggest that strength and
stiffness of gypsum reduce to zero by 120°C."
On a different point aluminium oxide and silicon dioxide do chemically combine (no water required)to form a high temperature ceramic but > 1,000c
is needed even with a sintering aide.
If you want your ceramic insulation to last do not make it from a hydrated anything that includes cement, plaster or any sulphate, wood, plastic or
dry road kill LOL