| Summary: | 博士 === 國立中山大學 === 材料與光電科學學系研究所 === 98 === This research is focused on the synthesis and characterization (BET, transmission
electron microscopy and optical spectroscopy) of aluminum oxide condensates via a
static sintering process and dynamic process of pulse laser ablation
(PLA) and pulse laser ablation in liquid (PLAL).
For a start, the static route of an onset coarsening-coalescence event based on the
incubation time of cylindrical mesopore formation and a significant decrease of
specific surface area by 50% and 70% relative to the dry pressed samples was
determined by N2 adsorption-desorption hysteresis isotherm for two Al2O3 powders
having 50 and 10 nm in diameter respectively on an average and with γ-type related
structures, i.e. γ- and its distortion derivatives δ- and/or θ-types with {100}/{111}
facets and twinning according to transmission electron microscopy. In the
temperature range of 1100 to 1400oC, both powders underwent onset
coarsening-coalescence before reconstructive transformation to form the stable α-type.
The apparent activation energy for such a rapid coarsening-coalescence event was
estimated as 241 ± 18 and 119 ± 19 kJ/mol, for 50 and 10 nm-sized particles,
respectively indicating easier surface diffusion and particle movement for the latter.
The size dependence of surface relaxation and onset coarsening-coalescence of the
γ−type related Al2O3 nanoparticles agrees with their recrystallization-repacking upon
electron irradiation and accounts for their assembly into nano chain aggregates or a
close packed manner under the radiant heating effect in a dynamic laser ablation
process.
In addition, ultrafine (5 nm) Al2O3 nanoparticles having a predominant α-type
structure and with an internal compressive stress up to ca. 15 GPa were synthesized by pulsed laser ablation on Al target under a very high peak power density (1.8x1011
W/cm2) with oxygen flow in vacuum. The ultrafine α-Al2O3 was alternatively
formed from the minor γ-Al2O3 nanocondensates upon electron irradiation. In such a
case, the polymorphs follow a special crystallographic relationship [110]γ//[2110]α;
(111) γ//(0114)α with a mixed mismatch strain yet nonparallel close packed planes
indicating a reconstructive type transformation. The formation of metastable
α-Al2O3 in the dynamic processes can be rationalized by the kinetic phase change
from the amorphous lamellar and/or γ-Al2O3 depending on their free energy versus
cell volume curves. The dense and ultrafine sized Al2O3 polymorphs with a rather
low minimum band gap of 3.7 eV shed light on their natural occurrence in dynamic
settings and abrasive as well as catalytic/optoelectronic applications.
Furthmore, pulsed laser ablation in water under a high peak power density of 1.8
× 1011 W/cm2 using Q-switch mode and 1064 nm excitation was used to fabricate
(H+,Al2+)-codoped Al2O3 nanocondensates having γ- and its derivative θ-type
structure as characterized by electron microscopy and spectroscopy. The as-formed γ-
and θ-Al2O3 nanocondensates are mainly 10 to 100 nm in size and have a significant
internal compressive stress (> 10 GPa) according to cell parameters and vibrational
spectroscopy, due to a significant shock loading effect in water. The γ-Al2O3
nanocondensates are nearly spherical in shape but became cubo-octahedra when grew
up to ca. 100 nm to exhibit more facets as a result of martensitic γ→θ transformation
following the crystallographic relationship (3 11 )θ //(02 2)γ; (0 2 4 )θ//(3 11)γ. The
formation of dense and (H+,Al2+)-codoped γ/θ-Al2O3 rather than aluminum hydrates
sheds light on the favored phases of the Al2O3-H2O binary at high temperature and
pressure conditions in natural dynamic settings. The nanocondensates thus formed
have a much lower minimum band gap (5.2 eV) than bulk α-Al2O3 for potential optocatalytic applications.
Moreover, the Al2O3 nanocondensates of spinel-type related structures, i.e. γ- and
θ- type with a significant internal compressive stress via pulsed laser ablation in water
were subjected to prolonged dwelling in water to form columnar bayerite plates for
further transformation as platy γ-Al2O3. Transmission electron microscopic
observations indicated the γ-Al2O3 follows the crystallographic relationship
(100)b//(011)γ; [001]b//[111]γ with relic bayerite (denoted as b). The γ-Al2O3 also
shows {111} twin/faults and rock salt-type domains due to dehydroxylation of
bayerite which involves {111} shuffling and disordering of the Al ions in the
octahedral and tetrahedral sites. The combined evidences of X-ray photoelectron
spectroscopy, vibrational spectroscopy and UV-visible absorbance indicated that the
H+, Al+ and Al2+ co-doped bayerite and γ-Al2O3 composite plates have a minimum
band gap as low as ~ 5 eV for potential catalytic and electro-optical applications in
water environment.
Finally, pulsed laser ablation in aqueous solution of NaOH up to 1 M was
employed to fabricate epitaxial NaAlO2 and γ-Al2O3 nanopartricles for electron
microscopic and spectroscopic characterizations. The NaAlO2 phase (denoted as N),
presumably derived from NaAlO2
.5/4H2O, was found to form intimate intergrowth
with the γ-Al2O3 following a specific crystallographic relationship [211]γ//[110]N;
( 2 22) γ//(002)N and (0 2 2) γ//(110)N for a parallel close packed planes in terms of
corner linked AlO4 tetrahedra and a beneficial lower interfacial energy and/or strain
energy. The composite phases have significant internal compressive stress up to 7
and 40 GPa according to cell volume and IR shift results and a low minimum band
gap of 5.9 eV for potential applications in UV region.
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