Abstract :
This article reviews our approach to render
12CaO 7Al2O3 (C12A7) electronically active using a
new concept of ‘active anion manipulation’, where
nanostructures embedded within the C12A7 crystal
lattice are intentionally utilized to generate chemically
unstable (‘water-free active’) anions. Anionic active
oxygen radicals, O– and O2 – , are formed efficiently in
C12A7 cages under high oxygen activity conditions.
The configuration and dynamics of O2 – in cages are
revealed by a combination of continuous-wave and
pulsed electron paramagnetic resonance (EPR). It is
demonstrated that metal-loaded C12A7 is a promising
oxidation catalyst for syngas (CO + H2) formation
from methane. Furthermore, the O– ion, the strongest
oxidant among active oxygen species, can be extracted
from the cage into an external vacuum by applying an
electric field with thermal assistance, generating a highdensity
O– beam in the order of lA cm–2. In contrast,
heat treatment of C12A7 in a hydrogen atmosphere
forms H– ions in the cages. The resultant C12A7:H–
exhibits a persistent insulator-conductor conversion
upon ultraviolet-light or electron-beam irradiation.
The irradiation-induced conversion mechanism is
examined by first-principle theoretical calculations.
Furthermore, the presence of a severely reducing
environment causes the complete substitution of electrons
for anions in the cages. The resulting C12A7:e–,
which exhibits excellent stability and an electrical
conductivity greater than 100 S cm–1, is regarded as an
‘electride’, an ionic compound in which electrons serve
as anions. The C12A7 electride exhibits a high
potential for applications involving cold cathode and
thermal field electron emissions due to its small work
function. Electride fabrication methods suitable for
large-scale production via melt processing are described.
It is also demonstrated that proton or inert gas
ion implantations into C12A7 thin films at elevated
temperatures are effective for both H– and electron
doping.
