This thesis is about the synthesis and characterization (electron microscopy and spectroscopy) of chromium oxide condensates prepared by a dynamic process of pulsed laser ablation (PLA) or pulsed laser ablation in liquid (PLAL) regarding the composition, size, shape, structure and internal stress of the condensates under the influence of laser parameters and dopant.
Firstly, dense chromium oxide nanocondensates dissolved with ca. 4 atomic % Si, according to energy dispersive X-ray analysis, were fabricated by PLA on a clamped Cr/Si target in air purged with oxygen for a very rapid heating/cooling and hence pressure effect. Transmission electron microscopic observations indicated the predominant corundum-type Si4+:£\-Cr2O3 nanocondensates are hexagonal in shape with significant internal compressive stress, and the minor spinel-like Si4+:Cr3O4 nanocondensates are octahedral in shape with considerable tetragonal distortion. The predominant Si4+:£\-Cr2O3 condensates tended to coalesce over stepwise (0001) or lateral (1-102) surface to generate dislocations until parallel epitaxial relationship was exactly reached via a Brownian rotation process of the particles. X-ray diffraction indicated that the internal compressive stress was quite released for the coarsened/coalesced condensates. The laser ablation results in this part shed light on the condensation effect, as an alternative to a solidification process, on the formation of Cr-rich oxide particles on the surface of Cr4+ doped YAG fiber.
In addition, the £\-Cr2O3 single crystal nanocondensates were fabricated by pulsed laser ablation in air purged with oxygen and characterized by analytical electron microscopy regarding shape dependent local internal stress of the anisotropic crystal. The nanocondensates formed predominantly as rhombohedra with well-developed {01-12} surfaces and occasionally hexagonal plate with thin {11-20} edges and blunt corners. Such nanocondensates showed Raman shift for the CrO6 polyhedra, indicating a local compressive stress up to 5 GPa on the average. Careful analysis of the lattice fringes revealed a local compressive stress (0.5% strain) at the thin edge of the hexagonal plates and a local tensile stress (0.3-1.0 % strain) near the relaxed {-1012}, {10-11} and (0001) surfaces of truncated rhombohedra. The combined effects of nanosize, capillarity force at sharp edge, and specific surface relaxation account for the retention of a local internal compressive stress built up in an anisotropic crystal during a very rapid heating-cooling process.
Furthermore, amorphous chromium oxide nanocondensates were fabricated by energetic PLA on Cr target in vacuum for a fine particle size and a pronounced quenching effect. Analytical electron microscopy indicated the amorphous phase thus quenched has corrugated lamellar layers with a bimodal interspacing 0.259-0.266 and 0.355-0.371 nm which are close to that of specific lattice planes of the stable £\-type structure, i.e. (-1104) and (01-12) having the Cr-filled octahedral sites assembled as 0 and 1 periodic bond chains (PBC), respectively. Such amorphous nanocondensates were observed in-situ to became more polymerized by forming (01-12)-like layers and then fully crystallized as £\-Cr2O3 for further (01-12)-specific coalescence when irradiated by electron beam. The partially crystallized lamellae showed Raman shifts similar to that of the ambient £\-Cr2O3 yet at higher frequencies due to an internal compressive stress up to ca. 4 GPa. This implies a rather tight 6-coordination of Cr3+ in the corundum-like structure for the rapidly quenched amorphous phase.
Moreover, the chromium oxide condensates nearly spherical in shape ranging from 0.1 to 0.2 micron in diameter were fabricated by laser ablation on Cr target at a very high power density of 1.8¡Ñ1012 W/cm2 for a very rapid heating and cooling effect. Analytical electron microscopic observations of such spherical particulates revealed three types: (1) £\-Cr2O3 single crystal with (-1101), (-1012) and (1-210) facets, (2) spinel-like Cr3O4 polycrystals with spherulitic texture, i.e. a rather corrugated solidification front, (3) recrystallized Cr3O4 polycrystals derived from type 2 by radiant heating. The microstructure and phase difference among the particulates can be attributed to varied extent of supercooling under the influence of rather complicated Cr2+ solute trapping of the molten and solid phases in the Cr3O4-O pseudo-binary in vacuum. The chromium oxide condensates being spherical yet full of facets, with significant internal compressive stress up to ca. 3.4 GPa according to Raman shift, and with UV-absorbance close to violet light due to the presence of internal stress and/or Cr2+, may have potential optoelectronic and catalytic applications.
Finally, analytical electron microscopic observations indicated that the chromium oxide nanocondensates fabricated by PLAL (in water) are predominantly dodecahedral Cr3O4 with varied extent of tetragonal (t-) distortion from the spinel (sp)-type following the Bain relationship [21-1]sp//[01-1]t; (011)sp//(100)t. The t-Cr3O4 nanocondensates have {101}-twining due to tetragonal distortion and/or a coalescence event. The additional x(200) and 2x(211) commensurate superstructures can be attributed to periodic presence of Cr2+, Cr3+ and/or H+ in the 4- and/or 6-coordinated lattice sites with an internal compressive stress up to ~ 5 GPa according to X-ray photoelectron and vibrational spectroscopic evidences. The presence of internal stress and Cr2+ ion caused red shift of the UV absorbance, thus shedding light on potential optoelectronic applications of the Cr3O4 nanocondensates.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-0722109-152303 |
| Date | 22 July 2009 |
| Creators | Lin, Chun-hung |
| Contributors | none, none, none, Pouyan Shen, none, none, none, none, none |
| 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-0722109-152303 |
| Rights | not_available, Copyright information available at source archive |
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