Coarsening/coalescence and phase change of Al2O3 nanoparticles by PLA in air, vacuum and aqueous solutions with/without NaOH

• Main Authors: I-Lung Liu, 劉宜隴
• Other Authors: Shen Pouyan
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/73380419812003060819

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.