The thermal decomposition of guanidinium tetraborate, pentaborate, and nonaborate has been characterized under nitrogen and ammonia atmospheres, and conditions have been defined for bulk powder and aqueous aerosol transformations of these precursors to boron nitride (BN). The bulk pyrolyses under ammonia (600-1400 °C) produce micrometer-and sub-micrometer-sized platelet morphology particles with oxygen contents of 3-18 wt. %. Subsequent pyrolysis of these powders at 1600 °C under NH 3 reduces the oxygen contents to <1 wt. %. Pyrolysis of aqueous aerosols containing the guanidinium borates gives submicrometer spherical morphology BN particles. The chemistry and processing of guanidinium borates are compared with the behavior of alternative precursors.
The reaction of aerosol droplets of the organoborate reagent (MeO)3B, suspended in nitrogen, with
gaseous ammonia in a flight tube heated to 800−1500 °C was used to produce, in high yield, spherical
morphology precursor powders BN
x
O
y
Cz that contain decreasing amounts of oxygen and carbon as the
process temperature and mole fraction of ammonia increased. Subsequent static nitridation of the precursor
powder under ammonia gave spherical morphology BN powders with low (<1 wt %) oxygen and carbon
contents. The powders were characterized by elemental analysis, DRIFT-IR, XRD, SEM, and TEM
methods. A weak molecular complex (MeO)3B·NH3 has been previously reported to exist, and this species
was isolated and structurally characterized by single-crystal X-ray diffraction methods. Aerosol conversions
using (MeO)3B/MeOH, H3BO3/MeOH, and B2O3/MeOH solutions were also examined, and the results
are briefly described.
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