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 £^-type related
structures, i.e. £^- and its distortion derivatives £_- and/or £c-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 £\-type.
The apparent activation energy for such a rapid coarsening-coalescence event was
estimated as 241 ¡Ó 18 and 119 ¡Ó 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
£^−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 £\-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 £\-Al2O3 was alternatively
formed from the minor £^-Al2O3 nanocondensates upon electron irradiation. In such a
case, the polymorphs follow a special crystallographic relationship [110]£^//[2110]£\;
(111) £^//(0114)£\ with a mixed mismatch strain yet nonparallel close packed planes
indicating a reconstructive type transformation. The formation of metastable
£\-Al2O3 in the dynamic processes can be rationalized by the kinetic phase change
from the amorphous lamellar and/or £^-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
¡Ñ 1011 W/cm2 using Q-switch mode and 1064 nm excitation was used to fabricate
(H+,Al2+)-codoped Al2O3 nanocondensates having £^- and its derivative £c-type
structure as characterized by electron microscopy and spectroscopy. The as-formed £^-
and £c-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 £^-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 £^¡÷£c transformation
following the crystallographic relationship (3 11 )£c //(02 2)£^; (0 2 4 )£c//(3 11)£^. The
formation of dense and (H+,Al2+)-codoped £^/£c-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 £\-Al2O3 for potential optocatalytic applications.
Moreover, the Al2O3 nanocondensates of spinel-type related structures, i.e. £^- and
£c- 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 £^-Al2O3. Transmission electron microscopic
observations indicated the £^-Al2O3 follows the crystallographic relationship
(100)b//(011)£^; [001]b//[111]£^ with relic bayerite (denoted as b). The £^-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 £^-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 £^-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 £^-Al2O3 following a specific crystallographic relationship [211]£^//[110]N;
( 2 22) £^//(002)N and (0 2 2) £^//(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.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-0715110-235228 |
| Date | 15 July 2010 |
| Creators | Liu, I-Lung |
| Contributors | Gan Der-Shin, Chen Shui-Yuan, Hsieh Ke-Chang, Ho New-Jin, Shen Pouyan, Kao Po-We |
| Publisher | NSYSU |
| Source Sets | NSYSU Electronic Thesis and Dissertation Archive |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lib.nsysu.edu.tw/ETD-db/ETD-search/view_etd?URN=etd-0715110-235228 |
| Rights | not_available, Copyright information available at source archive |
Page generated in 0.0021 seconds