In the first part of this thesis, CaCO3 (calcite) powder ball-milled to micro/nano scale were subjected to isothermal firing in the temperature range of 450-600 ¢XC in an open air furnace in order to study specific-surface area reduction as a result of early-stage sintering/coarsening/coalescence/repacking (denoted as SCCR process) of the fine particles. The surface area and pore size distributions were obtained from the BET and BJH methods, respectively. The H1 type adsorption/desorption hysteresis loop of the type IV isotherm was used as an indicator of cylindrical pore formation upon dry pressing and firing of the powder. The apparent activation energy for the onset SCCR of the dry-pressed calcite powder turned out to be 57.5¡Ó1.0 kJ/mol based on t0.5, i.e. time for 50 % specific surface area reduction. The minimum temperature for such an incipient SCCR process was estimated to be 590K (317¢J) by extrapolating the specific-surface-area reduction rate to null. The mechanism of specific surface area change includes the Brownian motion, coarsening and coalescence/repacking of calcite particles besides sintering via synchronizing diffusion of calcium ion and carbonic acid ion along grain surface and boundary. In the second part of this thesis, pulsed laser ablation on micrometer-sized calcite (type I) powder in liquid H2O (PLAL) was conducted to study the structure and optical property change of calcium carbonate under a dynamic high-temperature. high-pressure aqueous condition. X-ray diffraction (XRD) indicated the fragmented calcite I powder via such a PLAL process change predominantly into a metastable CaCO3 II phase presumably by a displacive type transformation from calcite I and/or nucleated from atom clusters. The refined XRD lattice parameters indicate a significant internal compressive stress (up to 1.5 GPa) was retained for the predominant CaCO3 II nanoparticles having well-developed (013), (010) and (013) faces as revealed by transmission electron microscopy (TEM). Minor calcium carbonate nanoparticles were also identified by TEM to be other high-pressure polymorphs (type III and aragonite), hydrated (monohydrocalcite, ikaite), amorphized (amorphous calcium carbonate), and even decomposed as cubic lime (CaO). Monohydrocalcite occasionally occurred as epitaxial intergrowths within the predominant CaCO3 II matrix. Vibrational spectroscopy (Raman and FTIR) indicated the structure units of the overall nanoparticles by the PLAL process were considerably modified as a combined results of size miniature, protonation and internal compressive stress. The UV-visible absorption results further indicate that the minimum band gap of the colloidal solution was narrowed down to ca. 5 eV and 3 eV for the predominant CaCO3 II and minor accessory phases, respectively, thus shedding light on their potential opto-catalytic applications.

The specific surface area reduction and pore size distribution coupled with N2 adsorption-desorption hysteresis isotherm were studied in the temperature range of 1400-1550¢J for periclase MgO powder having 0.1£gm in size and with face-centered cubic structure. The apparent activation energy of such a rapid coarsening-coalescence process for MgO powder was estimated as 181¡Ó3kJ/mol. The minimum temperature for sintering/coarsening/coalescence of submicron MgO particles was estimated to be near 1300¢J based on the extrapolation of steady specific surface area reduction rates to zero. Pulsed laser ablation (PLA) of periclase MgO powders in water was conducted under Q-switch mode and specified water height and water depth (10 mm) for an accumulation time of 5 and 20 minutes at 10 Hz. Such a PLA process has successfully synthesized nanosized and protonated MgO particles from Mg(OH)2 and lamellar precusors, implying the three phases may co-exist at high pressure and temperature conditions upon dynamic shock loading. A significant internal compressive stress up to 10 GPa was built up for the MgO but not the readily relaxed Mg(OH)2 nanocondensates. The lamellae-derived Mg(OH)2 tended to undergo a dehydroxylation process to become MgO following a specific crystallographic relationship, i.e. lamellar basal layer parallel to Mg(OH)2(0001) and MgO(111). The minimum band gap of the colloidal solution of MgO/Mg(OH)2/lamellae was lowered to ca. 5.2eV after the PLA process.