Hf3N4 in nanocrystalline
form is produced
by solution phase reaction of Hf(NEtMe)4 with ammonia followed
by low-temperature pyrolysis in ammonia. Understanding of phase behavior
in these systems is important because early transition-metal nitrides
with the metal in maximum oxidation state are potential visible light
photocatalysts. A combination of synchrotron powder X-ray diffraction
and pair distribution function studies has been used to show this
phase to have a tetragonally distorted fluorite structure with 1/3 vacancies on the anion sites. Laser heating
nanocrystalline Hf3N4 at 12 GPa and 1500 K in
a diamond anvil cell results in its crystallization with the same
structure type, an interesting example of prestructuring of the phase
during preparation of the precursor compound. This metastable pathway
could provide a route to other new polymorphs of metal nitrides and
to nitrogen-rich phases where they do not currently exist. Importantly
it leads to bulk formation of the material rather than surface conversion
as often occurs in elemental combination reactions at high pressure.
Laser heating at 2000 K at a higher pressure of 19 GPa results in
a further new polymorph of Hf3N4 that adopts
an anion deficient cottunite-type (orthorhombic) structure. The orthorhombic
Hf3N4 phase is recoverable to ambient pressure
and the tetragonal phase is at least partially recoverable.
The effects of surface oxidation on the capacitance of titanium nitride electrode surfaces are examined. Electrochemical oxidation was effective in increasing capacitance.
Ordered macroporous films of nanocrystalline TiN and amorphous SiN x on silica substrates have been prepared. This involved infiltration of sacrificial templates of close-packed polystyrene microsphere arrays with precursor materials, followed by pyrolysis under ammonia to remove the template and form the ceramic materials. TiN was prepared from a sol-gel route, whereas SiN x was introduced using a Si(NHMe) 4 precursor solution. Hexane was found to be the solvent most compatible with the divinylbenzene-cross-linked polystyrene template and was used for both materials.
Reactions
of Ta(NMe2)5 and n-propylamine
are shown to be an effective system for sol-gel processing of Ta3N5. Ordered macroporous films of Ta3N5 on silica substrates have been prepared by infiltration
of such a sol into close-packed sacrificial templates of cross-linked
500 nm polystyrene spheres followed by pyrolysis under ammonia to
remove the template and crystallize the Ta3N5. Templates with long-range order were produced by controlled humidity
evaporation. Pyrolysis of a sol-infiltrated template at 600 °C
removes the polystyrene but does not crystallize Ta3N5, and X-ray diffraction shows nanocrystalline TaN plus amorphous
material. Heating at 700 °C crystallizes Ta3N5 while retaining a high degree of pore ordering, whereas at
800 °C porous films with a complete loss of order are obtained.
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