wiyosaya - 7-1-2013 at 09:48
I am new to the board and, to me, it seems like this is the right place to be posting this. My apologies if it is not the right place. I am also very
likely to show lack of knowledge, however, I consider myself very capable of understanding at least some of the answers.
I am looking to melt very small quantities of ceramic nanopowders without the use of a lab furnace - as I am unable to afford one. I understand the
sintering process, however, I would really like to achieve actual melt temperatures - in the neighborhood of 2,400C give or take a few hundred degrees
C.
I figure that I should be able to achieve these temperatures with an oxy-acetylene (which I have) or oxy-hydrogen torch (which I could convert to). I
also figure that I will need some sort of crucible or boat to hold the melt and stand up to these temperatures in air.
Does anyone know of commercially available boats/crucibles made of a material that would operate in air for brief periods of time at temperatures
approaching 2,400C without destroying the crucible? I am thinking that perhaps a boat/crucible made of high-purity ZrO2 would get me close.
Also, what torch gas combination would be the best to use - if it makes a difference with a properly adjusted flame? I would like to minimize any
secondary reactions due to the gases used for the torch and, if possible, the "air" operating environment.
Thanks in advance.
elementcollector1 - 7-1-2013 at 09:58
Crucible: Graphite.
Heat source: I would not recommend a torch with nanopowders, they're going to blow everywhere! Use a furnace, such as a tower of bricks full of coal
(with your graphite crucible at the top, or in the middle, or wherever), and a source for introduction of air at the bottom (this allows temperatures
to rise to the level you need). I would suggest you get a crucible with a lid, as it'll be hotter if it's submerged in the coals than if it's in the
open air.
I have no experience with an acetylene/hydrogen torch, but I would recommend a setup to prevent the nanoparticles from escaping the crucible by being
blown about from the air movements of the torch.
What ceramic nanoparticles are these, and for what purpose do you want to melt them?
daragh8008 - 7-1-2013 at 11:37
A small point, but nano powdered materials typically have melting temperatures significantly lower than the bulk melting temperature. If the particle
size is small enough you can actually melt them without reaching anywhere near the bulk melting temperature. Of course they don't stay molten for very
long as the particle agglomerate into larger particles. Another point to consider is if the ceramic material that you are interested in will melt in
the conventional way. That is for many bulk melting temperature that are quoted the materials will decompose at these temperature if specific
conditions of surrounding gas composition and pressures are not met. But again its very hard to suggest and alternative if we have no idea what the
material is.
wiyosaya - 7-1-2013 at 18:31
Thanks to you both for your replies.
Since one of the materials is Cr2O3, that would seem to preclude the use of graphite as a crucible material as the info on
Wikipedia states that Cr2O3 will will turn into chromium metal and CO2 when Cr2O3 is heated in the presence of
finely divided carbon. Will Cr2O3 react with zirconia or other crucible materials when the two are heated together?
The other material is TiO2. I understand its bulk melting temperature is significantly less than that for Cr2O3. In a
quick search, I did not find anything that would indicate that it would decompose at elevated temperatures.
Thanks for the tip on using a torch with nano particles; I had not thought of the possibility of blowing the particles away with the torch. A covered
crucible sounds like a good idea whether or not I end up using a torch.
What temperatures might I expect when using the tower of coal approach?
My aim is to make small, ~1mm dia., pieces of both.
elementcollector1 - 7-1-2013 at 18:42
"Finely divided" is the key word here, I think. Graphite will probably still work.
Chromium (III) oxide shouldn't react with zirconia, and I can't think of any products they might produce.
You would expect enough temperature to at least partially melt iron (as this is the basic forge setup they use at my Scout camp), or 1538 degrees C.
Not high enough, I know, but the bulk melting thing might help out here.
wiyosaya - 8-1-2013 at 11:49
I was wondering about the words "finely divided" myself. I would not think that a graphite crucible would fall into that category, however, I may go
with a zirconia crucible anyway just to be safe.
How long do the crucibles that are used at the scout camp survive? I am asking because specs on graphite crucibles such as these indicate that they are good in an oxidizing atmosphere up to only 427C. It sounds like the temperatures achieved in the "coal pile"
are higher than that. To me, unless the graphite crucibles you have used have different operating characteristics, it sounds like within the coal pile
the "atmosphere" is not oxidizing.
On the chance that the crucibles you use are somehow different, where do you get the ones you have used?
Thanks again.
elementcollector1 - 8-1-2013 at 15:29
Sorry, I really should clarify. That "coal tower" setup is what we use at camp, and we just stick the pieces of iron in to soften. They can melt if
you leave them in long enough. We don't use crucibles at the camp because for some reason, no one teaches lost wax casting and foundry processes
anymore (even though it's *in* the Metalworking merit badge).
I've never used a graphite crucible before, but I know that they stand up to just about anything (as carbon, according to the Internet, melts at a
whopping 3500 C). Zirconia, by comparison, melts at 2715 degrees C (Wikipedia). Zirconia would be more inert, but graphite would undoubtedly possess
the better temperature range. For your purposes, however, zirconia will work just fine (provided the 'bulk melting' thing does happen). You would have
to make sure whatever setup you're using doesn't go above 2700 C, though.
On a side note, heating with a hydrogen or acetylene torch on the outside of the crucible *may* work better than the 'coal tower' furnace, given the
temperature you want. However, I would watch out for uneven heating - this causes stress on any crucible, and the potential of breakage.
12AX7 - 8-1-2013 at 16:51
There are only two practical ways of achieving 2500C in a lab setup (i.e., without spending kilobucks at a time).
1. Resistance furnace. There's a description, using graphite, in Brauer, actually. Consider this:
- Bell jar
- Graphite tube
- Water cooled cylindrical clamps to carry current to said tube
- High-ampere connections to clamps (ca. 500A, a couple volts -- spot welder territory, but continuous duty); copper pipes can carry current and water
through the vacuum chamber easily
- Transformer to supply said amperage (a modified microwave oven transformer, or several, might serve nicely)
- And of course, a support peg for the sample and its container, which are placed inside the graphite tube. Heat transfer by radiation, which will
work very nicely indeed at that temperature.
2. Arc furnace. You need a high refractory chamber which doesn't react with the sample to be melted, and some electrodes and a filler gas to do the
arc.
- Melting ceramic inside ceramic is, at best, counterproductive; consider a molybdenum, tantalum, rhenium or other metal housing for this purpose.
No, it won't be cheap (I think molybdenum and tungsten are the "cheapest", but tungsten may still react; molybdenum I would think okay if it handles
the heat).
- You should have some insulation for the chamber as well, which... have fun there. A reflective housing (polished stainless steel?) wouldn't be too
bad an idea; this goes for the graphite tube furnace as well.
- The arc rods can be something simple like graphite (gouging rods perhaps), assuming carbon isn't a problem for your housing or ceramic, or tungsten
(usually lanthanated; again, see local welding supply store)
- Pure vacuum doesn't really arc, so you need a gas, but you still want some vacuum. Such furnaces usually use low pressure argon or something like
that. Not terribly expensive or hard to find, just more tedium to set up. IIRC they're somewhere around 10-30 torr, which could be achieved with a
mechanical vacuum pump, purging the system with a slow leak of argon. Low pressures are more insulating (less convection) and sustain arc discharges
easier than STP gasses. Use an AC or DC welder to drive the arc, preferably one with HF start so you don't have to dick with the electrodes to get it
started (which would be... a problem in a vacuum chamber).
Regarding graphite's stability in air, graphite does indeed burn past red heat (yeah, about 400-500C). A hunk of graphite, orange hot, glows with a
faint blue haze, which is carbon monoxide burning. The erosion rate at only yellow heat, in atmosphere, is, oh, something like 1/16" to 1/8" per hour
or so I would guess. It doesn't sustain itself, not enough surface area -- but heck, maybe up at 2500C it will.
Your options on refractory are severely limited by what you want to melt in contact with it. Induction levitation melting is obviously out of the
question. 
Tim
wiyosaya - 9-1-2013 at 13:09
Tim,
Thanks for your reply. It is helpful even if it is not quite what I wanted to hear. 
I've tried downloading Brauer from the library, and the file refuses to open. I get an error from Acrobat which says "There was an error opening this
document. The file is damaged and could not be repaired."
I tried searching the net for this error and came up with a possible solution in the "Foxit" reader; however, Foxit says that this is not a PDF file.
Any ideas?
Method 1 might be doable for me, assuming I could get Brauer. 
Coincidentally, I've
been working on a high-current, low-voltage controller that could potentially control 2V at 100A. I'd have to parallel a few of these. I've already
got a 1 KW toroidal transformer for that project.
wiyosaya - 11-1-2013 at 11:44
Using a download manager, I finally got Brauer. I am going to post to the general forum about this as perhaps others have experienced similar
problems. I tried downloading the file on several different PCs with different operating systems and all failed without the download manager; I cannot
say that I know the reason for the failure.
I am intrigued by the furnace section as it looks like it might be simpler to build one than I had imagined. Then again, I do have some experience
with high/ultra high vacuum equipment I years back, and I still have that system exclusive of a roughing pump and a gauge. I am seriously considering
going forward with an attempt to build a tube furnace - either carbon or tungsten. It is further intriguing to me that such furnaces were often built
as needed even in professional / academic labs.
Now if I could only find an induction levitation unit on e-bay! 