Hello everyone:
I'm trying to solve the following problem:
I have a cylindrical reactor, made from a Hot Working tool steel, and I applied a graphite spray coating on the interior walls of the reactor.
I use it to try to process the Thermoelectric material Bi2Te3 in powder ,
So I'm trying to use the Hot Press method to try to synthesize my material into a 1cm X 3.5 mm compacted capsule.
So I make the mix of, (Bi2Te3 in powder + Graphite) then I put the powder into this small cylindrical reactor, , the conditions of the Hot Press process are:
T = 600 °C
t = 40 min.
P = 100 bar
but, the problem I'm having is :
My material (Bi2Te3) difusses into the reactor walls, so this makes my reactors to get consumed very fast. Also, when I want to use it for the following processing; these regions in the walls (which now are formed of an alloy of Bi2Te3 + the steel) melts before reaching the Bi2Te3 melting point and the material inside the cavity of the reactor flows through the walls and spill out from the bottom of the reactor.
,
So I think I'm not using the right material to make the reactors, and also I need a better coating which protects them and prevent the material inside to diffuse through the walls
Does someone have any experience on this ?? Or have seen this problem,
Best Regards !
When in contact with iron your Bi2Te3 is reduced to metallic bismuth:
Bi2Te3 + 3Fe = 2Bi + 3FeTe
Metallic bismuth has a melting point of 271°C and at 600°C is, of course, liquid. It is no wonder that bismuth melt quickly destroys steel at 600°C.
But this 600°C is far too high anyway, because bismuth telluride has a melting point of 580°C (during sintering or hot pressing, the temperature of the material must always be lower than its melting temperature).
So, reduce the temperature below 580°C and build the mold from the material that is inert to bismuth telluride. It can be, for example, Al2O3 ceramic (an Al2O3 husk in a steel mold). You can also try using a graphite mold for it.
Bismuth has always such problems. Try to reduce not only temperature but also processing time and follow what suggested by Dr Verlotski.
Dear Vadim:
Thank you for your comments, is very useful information!
The material I'm trying to process with the Hot Press method is n-type Bi2Te3 , which has Sb and Se impurities within. So its melting point is a little higher than pure Bi2Te3(580 °C)
I have found in the literature that the melting point for this material is 600 °C.
Anyways, now I'm trying to reduce the processing temperature to 580 °C or bit lower, lets see if I get better results.
About the options for coatings, I was thinking earlier of a graphite or a quartz coating, since I have seen these options for the crucibles used in the e-beam evaporators.
But i wonder whether as you say, a ceramic coating as Al2O3 would work too, what do you think of TiO2 ?
I'm trying to find more references of these two materials as barriers of diffusion for Bi2Te3 or materials alike, at operating temperatures of 550°C - 600°C.
I will appreciate if you have more information that can help me
Best Regards !
Dear Imad:
Thank you for the article,
It did help me.
So according to the article, a bi-layer of Ti/Au is a promising option...
What I can try with faster than that is TiO2 . , deposited by ALD .
What I need to see is wheter I can find a way to deposit the thin layer of Au needed for the adhesion of the Ti (TiO2 in my case) , or maybe I can try first without this layer.
Regards ! :)
Dear Franklin,
Any ceramic material that does not react with Bi2Te3 is suitable for the mold. Apart from graphite, Al2O3, SIO2 (quartz glass) and TiO2, these are also ZrO2, Cr2O3, Si3N4, SiC, AlN and many others. Even simple porcelain would be suitable. Also, some metals, such as precious metals (silver, gold, platinum, palladium and the like) and, possibly, tantalum, niobium, molybdenum, tungsten, zirconium, chromium (nickel chromium) and titanium alloys are useful. The prerequisite for the selection of the metal is its reaction with tellurium: if the metal does not form tellurides it is suitable.
Regards
Vadim
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