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Author: Subject: Boron + different oxides
Bender84
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[*] posted on 21-4-2021 at 21:57
Boron + different oxides


Hello World!

I made 3 pyro compositions with boron as fuel. The oxidizer in the first comp was Fe2O3. In the second it was Mn2O3 and in the third: Ni2O3. The fuel to oxidizer mass ratio was 1:7 in all cases (since all oxidizers have similar molar weigth and the assumed products were B2O3 and pure metal) . Fe2O3 melts at approx. 1565*C. Mn2O3 melts at about 940*C and Ni2O3 melts at 600*C.

From these compositions only the one with Mn2O3 burned in air (pressed in a tablet presser). It didn't burn pressed in metal tube, though.

Nonetheless, I can't figure out why the B/Ni2O3 didn't burn since that oxidizer has the lowest melting point, so it should be the easiest to ignite. Afterwards I made an additional one with Co2O3 and it also burned well in air. The Co2O3 melts at 895*C.

What am I missing? What's in the nature of Mn2O3 and Co2O3 that renders them more prone to react even though they have a higher melting point? All these reactions are - in theory - exothermic with B+Mn2O3 being the least energetic (delta HF = -296 kJ/mol) and yet it burns the best.
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papaya
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[*] posted on 21-4-2021 at 23:23


Maybe not strong enough initial impulse. I will suggest to prepare to identical tablets - one composition that burns and other that didn't and put the first on top of the second. One should ignite the other exploiting the whole contact surface. If it still doesn't work then try to increase tablet diameter/ compression force.
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Bender84
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[*] posted on 22-4-2021 at 01:01


Thanks, papaya.

The B/Mn2O3 tablet did not ignite other compositions pressed on top of it. Also, I used a plasma torch to ignite the tablets in the first run. I'll try to increase the compression force. I forgot to mention that all substrates were fine dust/ powder.
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njl
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[*] posted on 22-4-2021 at 07:05


I'm not sure if this is what papaya meant, but maybe pressing a working ignition mix on top of one of the failures (so that both compositions are in one tablet, the top being 100 percent starter like B/Mn2O3 and the bottom being 100 percent B/Ni2O3. In the middle the compositions could be mixed to some degree for better contact).



Reflux condenser?? I barely know her!
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Bender84
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[*] posted on 22-4-2021 at 07:29


Hi njl,

Yes. This is exactly how I tried to make it work. I used B/Mn2O3 as a starter/ignition mix, trying to ignite other compositions pressed on top of the starter. Actually I tried to ignite the other comps from the top and from the bottom as well. None of these worked. I also tried to ignite them with my standard ignition formula which is Si/Pb3O4 with the same result. The B/Ni2O3 behaves like it would start, but the reaction is very "sluggish"/ cold (if I may say so). There are few embers which do not last long.
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Hey Buddy
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[*] posted on 7-5-2021 at 03:31


Without deep examination, it may be simply a stall. I would add a bridge of thermal inertia via graphite or some other bridging material mod to your comp so that its minimum energy level is maintained through the propagation front. Although, if I understand correctly, it seems initiation threshold is not breached, from the description of 'spark' but no initial reaction? In that case, I'd judge it two separate issues:
1) initiation threshold is not met, even using high energy for means of initiation. That would suggest there is a sub optimal ratio of reactants, or the material composition of reactants is different than what it's assumed to be. Or, initiation energy is below threshold of activation.
2) Once initiation threshold is breached, then the next priority is propagation. Occasionally reactants can be too sluggish in their stoichiometric ratios for reliable propagation, or the external factors like heat loss can hamper propagation. Maintaining heat can sometimes help by adding a material like graphite or by using a different liner material which assists in maintaining thermal energy for happy propagation front.

Other thought: ball mixing two reactants together typically lowers initiation threshold and propagation threshold, obviously unintentional ignition during mixing has to be accounted for. In pyrotechnics industry, I believe they avoid co-milling reactants as a general rule, as they are using equipment for extended continuous cycles. In detonator/initiator pyrotechnics it is common to co-mill fuel oxidizer for nano processing, both fluoropolymer and boron/nitrate mixes. Usually inert atmosphere or reduced temp or solvent. Typically equipment is set up to accommodate detonation at any time, which can be as complex as the imagination or as simple as interlocked sand bags or a pvc ball mill defeladed within a hole in the dirt ground, with an extension cord.

[Edited on 7-5-2021 by Hey Buddy]

[Edited on 7-5-2021 by Hey Buddy]

After second consideration, if using the Ni at a 7:1 ratio, assuming products are B2O3, it may be entirely too rich. You may try a Ni2O3:B ratio of 20:1, possibly with greater activity.

[Edited on 7-5-2021 by Hey Buddy]
Scratch that last. I will play around with it a little bit.

[Edited on 8-5-2021 by Hey Buddy]
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Bedlasky
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[*] posted on 8-5-2021 at 01:11


Ni2O3 easily decompose in to NiO and O2 (I doubt that it even melt). Wiki says that 600°C is decomposition temperature, but I wouldn't be surprised if it decompose at lower temperature. This is reason why reactions didn't proceed.

Mn2O3 doesn't melt. Mn2O3 start to decompose in to Mn3O4 and O2 at 900°C. Mn3O4 melts at 1567°C.

Co2O3 decompose in to Co3O4. Co3O4 doesn't melt but decompose at 900°C in to CoO. But apparently Co2O3 or Co3O4 start to react with boron before decomposition.




If you are interested in aqueous inorganic chemistry look at https://colourchem.wordpress.com/main-page/

I can offer GC analysis of samples. Just U2U to me for more info.

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