An onset coarsening-coalescence event based on the incubation time of cylindrical mesopore formation and a significant decrease of specific surface area by a certain fraction relative to the dry pressed samples was determined by N2 adsorption-desorption hysteresis isotherm for amorphous SiO2 nanoparticles (ca. 40 nm in size). In the temperature range of 1150-1300oC, the nanoparticles with binder (PVA) additive underwent onset sintering coupled with coarsening-coalescence without appreciable crystallization. The apparent activation energy of such a rapid process for amorphous SiO2 nanoparticles was estimated as 177 ¡Ó 31.5 kJ/mol, based on 30% change of specific surface area. As a comparison, in much lower temperature range of 600-900oC, the amorphous Zn2SiO4 nanoparticles underwent onset sintering coupled with coarsening-coalescence accompanied more or less with the formation of ZnO The apparent activation energy of such a rapid process for a amorphous Zn2SiO4 was estimated as 105 ¡Ó 3.8 kJ/mol based on 50% change of specific surface area. The minimum temperatures for sintering/coarsening/coalescence of the amorphous SiO2 and Zn2SiO4 are 1120¢J and 635oC, respectively based on the extrapolation of steady specific surface area reduction rates to null.
PLA fragmentation of amorphous and nearly spherical SiO2 nanoparticles (40 nm in size) in water (i.e. PLAL process) with optional NaCl addition was conducted under Q-switch mode (532 nm, 400 mJ per pulse) having laser focal point fixed at ca. 10 mm beneath the water level for an accumulation time of 20 and 30 min at 10 Hz. The 532 nm laser incidence suffered little water absorption and was effective to produce irregular shaped amorphous nanocondensates as small as 10nm~20nm in diameter with accompanied change of medium range order (MRO) as indicated by single rather than two broad x-ray diffractions at low 2theta angle. Whereas the Na+ uptake in the amorphous silica from the salty water account for a lower wave number of FTIR bands. The combined effects of nanosize, MRO change and H+ -signature may cause a lower minimum band gap of the amorphous products (analogous to opal-A) which become partially crystallized as £]-cristobalite (analogous to opal-CT) with additional £\-tridymite when Na+ is present.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-0622111-172051 |
| Date | 22 June 2011 |
| Creators | Chen, Zih-ling |
| Contributors | none, none, none |
| Publisher | NSYSU |
| Source Sets | NSYSU Electronic Thesis and Dissertation Archive |
| Language | Cholon |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lib.nsysu.edu.tw/ETD-db/ETD-search/view_etd?URN=etd-0622111-172051 |
| Rights | not_available, Copyright information available at source archive |
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