Thermite

[YT2095]

Not nesscary that only.The oxidation of sulphur also acts as an energy source,thereby reducing the external heat needed to activate thermite compound indirectly.(the activation energy remains the same)

that`s just nit picking / splitting hairs!

 

the point is perfectly valid as intended

[anerce]

hi everyone, im new here. this thread seems to have been idle for quite some time. but it would be great if you guys could help me out somehow. I'm a summer student at McGill University in Canada and my summer research project primarily involves thermites. I place stoichiometric thermite powder mixtures in a Setaram DTA/TGA machine to plot a thermogram of heat flow of the mixture during heating and cooling. we are interested in the temperature where it starts reacting and if the reactions occur before or after the melting point of the metal. the heating is held in an inert argon atmosphere. i was told that most people beleive that the melting of aluminum coincides with the onset of a reaction (with Fe2O3 for example..). but our thermograms show that the bulk of the reactions (characterized by large exotherms on the thermogram) occur much later than 660C (m.p. Al). does this make sense? someone in our group suggested that this happens because the aluminum powder melts inside its own aluminum oxide shell (All aluminum quickly develops an oxide shell when in contact with atmosphere). so even though it is melted, it does not come in contact with the metal oxide powder because the shell (very high m.p.) prevents it.. maybe eventually, something happens and the aluminum leaks out and starts reacting at around 1200C (according to our graphs). which of these theories do you think is more valid? does the reaction genuinely occur much later than the melting point, or is it just because of the oxide shell which delays the reaction. also, i tried melting pure aluminum powder 99.9% in the DTA machine. it clearly melted (based on the thermogram) and then resolidified upon cooling. but it came back out in powder form, as if it never even melted in bulk form... very strange. i tried the same with pure aluminum wire.. and it came back out in wire form.. no bulk melting.. this is why the oxide shell theory was raised by one of my friends.. please give me some feedback into this.. thanks a lot.. I trust everyone's experience here...

[evilwalnut]

I was also wondering the initiation temperature for thermite, and if any catalysts could lower the activation energy. I dont think that the melting of Al is what begins the reaction, rather the Al begins to melt at a lower energy level that is needed to begin it's reaction with Fe2O3, magnetite or what have you.

 

If this is right, you could very well have melted Al without the reaction begining. As a fan of Occam's Razor, I choose to accept this simpler explanation rather than that of you friend's. Any criticism or feedback is, of course, appreciated.

[bob000555]

MgO and Al make a grate termite because both the reactants are flammable so you get

3MgO + 2 Al -> Al2O3

3Mg+Al2O3-> 3MgO+2Al

And then back to the first reaction and this keeps happening until the heat is to low for another reaction

Also good is lime (CaCO3) plus aluminum.

[woelen]

So, you have a perpetuum mobile???? Bob000555, please think twice before you post. This is nonsense. MgO and Al make no good thermite. Lime plus aluminium also is crap thermite. Did you ever try it yourself???

 

For a thermite reaction you need a free very electropositive metal, mixed with the oxide (or halogenide) of a much lesser electropositive metal. Some examples:

 

Al and Fe2O3

Al and CuO

Al and Cr2O3

Mg and V2O5

Mg and MnO2

Al and CuCl2

Mg and Ag2O

 

Also, the metals and the oxides must be very finely powdered. If you ever try these reactions, also keep in mind that some oxides and chlorides are volatile (e.g CuCl2, to some extent V2O5).

[bob000555]

I ment MgO2 and not perpetual just until to much heat has dissipated to initiate the next reaction. Also I tryed the Al CaCO3 sometimes to produce lots of CO2 but some of the CO2 that reacts with the Al. Actuly the Al CaCO3may not be thermite...but it burns well.

[woelen]

I doubt whether MgO2 does exist. It might be a peroxide, but I wonder if you ever can get your hands on that (if it exists at all). Probably you mean MnO2. That indeed can make a good thermite. I mentioned it in my list of examples in the previous post.

 

The reaction never goes circular. What you describe is going circular from Al + metal oxide to Al-oxide + metal and then vice versa. That does not happen. You only have one reaction and when one of the reactants is used up, then the reaction stops. This has nothing to do with heat being dissipated. Heat is not the driving force of the reaction (although it may help initiating the reaction). The driving force is the potential energy, stored in the reactants.

 

Does CaCO3 / Al really burn well? How easy is it to ignite this mix?

[[w00t]]

hold on, how can CaCO3 / Al work?

 

how will that oxidize the Al to get a reaction happening

[woelen]

I also have serious doubts on this one, but if bob000555 really has tried this and it really burns well, then of course we have an interesting thing.

 

I can imagine that at sufficient high temperature CaCO3 decomposes to CaO and CO2 and that the CO2 supports combustion of Al. The heat of that reaction could decompose more CaCO3 and so the reaction can be sustained.

It is a well known fact that magnesium burns brilliantly in an atmosphere of pure CO2, producing MgO, CO and C. In fact, you get lots of soot when burning Mg in an atmosphere of CO2. So, it is imaginable that the Al/CaCO3 mix works by formation of Al2O3 and C (and CO) from the Al and CO2.

Let bob000555 comment on this and describe how he did the experiment.

[YT2095]

actualy I HAVE heard instances of folks adding Calcium X to regular thermit mix (al fe types), but I don`t know it it was the Oxide, Hyrdoxide or maybe even the carbonate?

Plaster of Paris was mentioned, so it`s probably That stuff, and don`t ask me Why they added it, I never got an answer!?

[[w00t]]

actualy I HAVE heard instances of folks adding Calcium X to regular thermit mix (al fe types)' date=' but I don`t know it it was the Oxide, Hyrdoxide or maybe even the carbonate?

Plaster of Paris was mentioned, so it`s probably That stuff, and don`t ask me Why they added it, I never got an answer!?[/quote']

 

i think its Calcium sulfate ( plasters of paris ) used to make thermite in to a paste or something

[woelen]

Adding calcium oxide or hydroxide does not seem very beneficial to me, adding calcium carbonate or calcium sulfate can be beneficial, because of the reaction, I described in my previous post. Calcium sulfate can do a similar thing, now SO3 being the oxidizer, itself beingng reduced to SO2 and the aluminium being oxidized to Al2O3.

[SilverFox]

I do not know much chemistry, or even if its some reaction happening... But the very coolest thing that I've done with thermite only uses about 100 grams of Fe3O4 + Al mix, poured into a brick hole, topped with pennies throughout the mixture. The pennies cause explosions like eruptions from a volcano! It's pretty damn cool to do and I highly recommend it (I'm not responsible for burned property or personal damages or loss of anything caused by you trying this) lol...

 

I really need a picture to show you, but it's really amazing and I'd love to know what the hell causes this violent eruption!

[UC]

I do not know much chemistry, or even if its some reaction happening... But the very coolest thing that I've done with thermite only uses about 100 grams of Fe3O4 + Al mix, poured into a brick hole, topped with pennies throughout the mixture. The pennies cause explosions like eruptions from a volcano! It's pretty damn cool to do and I highly recommend it (I'm not responsible for burned property or personal damages or loss of anything caused by you trying this) lol...

 

I really need a picture to show you, but it's really amazing and I'd love to know what the hell causes this violent eruption!

 

Modern pennies are zinc, which has a low boiling point. You can probably figure out the rest. Use some pre-1982 pennies. I bet it doesn't work (they're solid copper).

 

This is pretty ridiculously dangerous, what with flying thermite and zinc and zinc oxide fumes, which can cause metal fume fever.

[hermanntrude]

I've done it!

 

I just had my first thermite!

 

I couldn't get it to ignite for ages... I tried various variations on magnesium fuses, bent and folded in different ways but none of them worked. I also tried KMnO4 and glycerol in varying quantities but it never worked. The thing that finally worked for me was a combination of the two... I placed a 3 inch strip of magnesium, folded into thrids into the thermite mixture, then poured some KMnO4 on top until the magnesium was almost covered up, then ignited the mixture by adding about 3 ml of glycerol.

 

IT WAS AWESOME.

 

next time I do it (and there WILL be next times) I will video it.

[Theophrastus]

Yay you!!!

 

Now that I think of it, I ought to try a little thermite some time, if I have the freedom to pop into my local art store (40 min. away) and fetch some fine aluminium powder.

[hermanntrude]

be extremely careful, theo... this one's not forgiving at all... spitting liquid iron around the place and generally being extremely hot and uncontrollable.

[UC]

be extremely careful, theo... this one's not forgiving at all... spitting liquid iron around the place and generally being extremely hot and uncontrollable.

 

You know plain old metal sparklers? The cheap ones? Or do they not sell them up there? Just tie one to the end of a meterstick or a broom handle or similar and touch the thermite with the lit end. So far, I've had no problems lighting it like this.

[hermanntrude]

You know plain old metal sparklers? The cheap ones? Or do they not sell them up there? Just tie one to the end of a meterstick or a broom handle or similar and touch the thermite with the lit end. So far, I've had no problems lighting it like this.

 

I had heard of this method too, but I didn't have one with me that day. I may try it out in future but my thermite is quite coarse-grained so whatever method I use has to be good :0)