Sciencemadness Discussion Board

preparation of MgB2

Vadim - 20-9-2005 at 06:17

I would like to know conditions (temperature, pressure and et.) of the following reactions. Who can help me?

B2O3 + 6Mg ---> Mg3B2 + 3MgO
B2O3 + 3Mg ---> 2B + 3MgO
B2O3 + 4Mg ---> MgB2 + 3MgO
2B + Mg ---> MgB2
2B + 3Mg ---> Mg3B2

12AX7 - 20-9-2005 at 12:06

As far as I know, the only problem is ignition; otherwise they all proceed exothermically.

At worst, temperature could be in excess of only 1200°C...(I say "only" because this is well within the reach of a gas burner). That seems rather pessimistic, since Mg boils below that. I know matter-of-factly the thermite reactions ignite at a mere red heat in atmosphere.

Tim

BromicAcid - 20-9-2005 at 16:15

B<sub>2</sub>O<sub>3</sub> + 6Mg ---> Mg<sub>3</sub>B<sub>2</sub> + 3MgO

I've done this reaction about 4 times now. Each time with varying degrees of sucess. Also I have noticed magnesium boride written as Mg<sub>3</sub>B<sub>2</sub> and alternatively MgB<sub>2</sub>. I preformed my most recent reaction with 11 grams B<sub>2</sub>O<sub>3</sub> and 15 grams of magneisum shavings. Without access to oxygen the reaction was hard to initiate by heating from the bottom. But adding a pinch of potassium perchlorate to the top and taking the torch directly to the top of the mixture ignited the magnesium exposed and the mixture was covered. When it was all over I ended up with a solid that was like charcoal that could be broken up easily by hand.

The intention was to make magnesium boride to make boron hydrides and I posted my results in the thread "Borohydride Synth?" and also on my website. I thought that adding acid would produce B<sub>2</sub>H<sub>6</sub> however further research showed that to be wrong. Borane decomposes in water, the higher boron hydrides that are produced can survive water and thus I got to smell them in all of their glory. The flask filled with a smoke that was interesting to watch though. I ended up with a good amount of elemental boron though after reacting the impure magnesium boride with acid. To give better yields the texts I have read recomend absolutely dry powdered boric oxide, beyond what I can make. This is reacted with magnesium in a furnace for several hours and this gives a product high in magnesium boride (somewhere around 80% as I remember).

Searched around some, this is from the post I made to the thread mentioned above:
Quote:
Magnesium diboride was prepared in US patent 2942935 by the reaction of elemental magnesium with boron at 950 C under an argon atmosphere (According to another paper if the temp is too high yields are reduced a thin iron or clay crucible should be used, nickel is attacked). The product was ~85% diborane with the remainder magnesium.

Vadim - 21-9-2005 at 13:07

It is very surprising for me that MgB<sub>2</sub> and Mg<sub>3</sub>B<sub>2</sub> is the identical substance. I don’t understand, please, explain!
The molecule of the first agent contain two boron atoms and one magnesium atom the second contains also two B but three Mg. Molecular composition and weight of these substances must be different and these substances must have different crystal structure and various physical and chemical properties. MgB<sub>2</sub> is well known and there are many publications about one but I found Mg<sub>3</sub>B<sub>2</sub> only in manuals of chemistry.
I would be very grateful for the answer.

BromicAcid - 21-9-2005 at 13:36

My thinking on this is that depending on the preparation there is boron dissolved in magnesium or vice versa and older methods of preparation were assumed to make different compounds. Modern litrature gives MgB<sub>2</sub> as the formula and I agree with that one, but the boride was often just a melt of the two back when it was first discovered and its composition was not something agreed upon, I think there are different compositions formed at different temperatures but I know nothing more then that. What is your intended use for the boride if you don't mind me asking?

12AX7 - 21-9-2005 at 18:33

Some reason they can't both exist? Mg3B2 is stoichiometric, you should question MgB2 if anything. ;)

Heck, the system Ca-Si has three intermetallics.

Tim

praseodym - 21-9-2005 at 22:48

I am not so sure if MgB2 and Mg3B2 are the same substance, but if one of them is wrong, it must be Mg3B2, according to this website:

http://osulibrary.oregonstate.edu/specialcollections/coll/pauling/calendar/1953/12-december/17.html

According to this website, magnesium boride was misrepresented as Mg3B2 by Moissan and Ray in 1953 when it is really MgB2. This could probably lead to the confusion here.

Vadim - 22-9-2005 at 01:18

I investigate superconductivity of MgB<sub>2</sub>. In my laboratory the MgB<sub>2</sub> was synthesized from components in He atmosphere [Supercond. Sci. Technol. 17 (2004) S274–S275]. We varied synthesis temperature in the range from 700C to 1000C but obtained only MgB<sub>2</sub>. It is evident from X-ray analysis and samples weighing (with the assumption, that the quantity of boron remains constant during heat treatment i.e. boron does not evaporate practically). I know that at preparation of the single crystals of MgB<sub>2</sub> at the temperature up to 2000C and pressure up to 30kbar (for example [physical review B71, 174506 (2005)]) Mg<sub>3</sub>B<sub>2</sub> also don’t form.
Mg<sub>3</sub>B<sub>2</sub> was mentioned only in old publications but one had vanished in further. I think there are two reasons that can explain this. The first Mg<sub>3</sub>B<sub>2</sub> don’t exist [see above praseodym] and the second (mysterious) Mg<sub>3</sub>B<sub>2</sub> is secret compound with difficult preparation methods. :cool:

The_Davster - 19-8-2006 at 20:57

I was bored in the lab on my last day there, so I was just screwing around and ended up making MgB2 via the usual:
B(OH)3 --->B2O3 +H2O
B2O3+Mg---> MgB2 +MgO route
So I did an X-ray difraction analysis on it, and as expected there was peaks for MgO, MgB2, and a smal ammount of B2O3 and Mg. But there was also something else in there. In a percentage about half that of MgB2.

No idea what that was...

unionised - 20-8-2006 at 10:43

Just a random thought, do you have diffraction data for magnesium borate(s)?

The_Davster - 20-8-2006 at 14:21

Unfortunatly not, the database of difraction data we use does not contain info on such compounds.