I'm trying to synthesize bulk hBN powder from boric acid and urea precursors in a tube furnace. My goal is to get 100s (or at least 10s) of milligrams of hBN from each synthesis that I can use in catalytic reactions. I have a tube furnace that can go up to 1200C. Based off many references, I know that I need a high molar ratio of urea: boric acid, so I've been doing 30:1 (though I've also tried lower). I add ~150 mg boric acid and ~4.5g urea to a beaker with 20 mL DI H2O + 20 mL methanol (based on: Article A Template-free Solvent-mediated Synthesis of High Surface A...
), then leave it in a 65C oven overnight to evaporate and recrystallize. In the morning, I grind it to a fine powder using a mortar and pestle, then load the powder into a ceramic boat with a lid (see photo), and load that into the quartz tube. I attach the gas line (I've used both Argon and N2) and I run the gas for 30 min, then turn on the instrument. I heat up at 5C/min until 950C or 1100C. The tube turns white (and sometimes a bit yellow) with urea (decomposition). Both have yielded charred, burnt material, not white hBN powder, or only a tiny amount of white, flaky powder amidst the char (see photo). I've talked to a couple of collaborators and read a number of papers and can't find anyone who has experienced this problem, though no one has tried this synthesis on my lab's exact tube furnace.Does anyone have advice on how to eliminate these problems and move forward? Any advice would be much appreciated!
Some potential questions
-is oxygen getting into the reaction and causing it to char?
-should the ceramic boat's lid be removed? Is that a problem?
-should I switch to a rounded ceramic crucible, with or without a lid? (though that's harder to put in a round tube without spilling)
-should I change my temperature or ramp rate?
Not absolutely sure, but too much of Urea might have polycondensed to some aromatics (probably catalyzed by Boric acid, a good electrophile), that might have condensed/pyrolyzed further to make Char. Try going from Borazine to hBN, the former can be synthesized in various ways (Ammonia+Borane, Sodium borohydride+ Ammonium sulfate...) , allow some polymerization and then that polymer to BN (note sure though whether leads to cBN or hBN) as per the paper
Wideman, Thomas; Fazen, Paul J.; Lynch, Anne T.; Su, Kai; Remsen, Edward E.; Sneddon, Larry G. (1998). "Borazine, Polyborazylene, β‐Vinylborazine, and Poly(β‐Vinylborazine)". Inorganic Syntheses. 32. doi:10.1002/9780470132630.ch39
Remember- no carbon, no char. Why do not you reduce Boric acid to Borane or use borohydride instead to drive C and O out of Urea?
Best of luck.
Thanks for the tips! I haven't yet tried borazine, but when I changed a couple of parameters, including fixing a problem with the gas line and moving from a ceramic boat to a quartz boat, I've consistently gotten no char, just white powder (see the image). I'm not sure what the exact issue was, but I'll stick with the quartz. boat without a lid, as it seems to be working.
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