none

none

In part I, MgO and Co1-xO powders in 9:1 and 1:9 molar ratio (denoted as M9C1 and M1C9 respectively) were sintered and homogenized at 1600oC followed by annealing at 850 and 800oC, respectively to form defect clusters and precipitates. Analytical electron microscopic (AEM) observations indicated the protoxide remained as rock salt structure with complicated planar diffraction contrast for M9C1 sample, however with spinel paracrystal precipitated from the M1C9 sample due to the assembly of charge- and volume-compensating defects of the 4:1 type, i.e. four octahedral vacant sites surrounding one Co3+-filled tetrahedral interstitial site. The spacing of such defect clusters is 4.5 times the lattice spacing of the average spinel structure of Mg-doped Co3-dO4, indicating a higher defect cluster concentration than undoped Co3-dO4. The {111} faulting of Mg-doped Co3-dO4/Co1-xO in the annealed M1C9 sample implies the possible presence of zinc blend-type defect clusters with cation vacancies assembled along oxygen close packed (111) plane. In part II, the Mg2TiO4/MgO composites prepared by reactive sintering MgO and TiO2 powders (9:1 molar ratio) at 1600oC and then air-cooled or further aged at 900oC were studied by X-ray diffraction and (AEM) in order to characterize the microstructures and formation mechanism of nanosized Mg2TiO4 spinel precipitated from Ti-doped MgO. Expulsion of Ti4+ during cooling caused the formation of (001)-specific G.P. zone under the influence of thermal/sintering stress and then the spinel precipitates, which were about 30 nm in size and nearly spherical with {111} and {100} facets to minimize coherency strain energy and surface energy. Secondary nano-size spinel was precipitated and became site saturated during aging at 900oC, leaving a precipitate free zone at the grain boundaries of Ti-doped MgO. The intergranular spinel became progressively Ti-richer upon aging 900oC and showed <110>-specific diffuse scatter intensity likely due to short range ordering and/or onset decomposition. In part III, the Co1-xO/Co2TiO4 composite prepared by reactive sintering CoO and TiO2 powders (9:1 molar ratio) at 1450oC and then air-cooled were studied by X-ray diffraction and AEM in order to characterize the microstructures and formation mechanism of nanosized Co2TiO4 spinel precipitated from Ti-doped Co1-xO. Slight expulsion of Ti4+ during cooling caused the precipitation of nanosize Co2TiO4 spinel. Bulk site saturation also caused impingement of the Co2TiO4 precipitates upon growth. The Co3-dO4 spinel, as an oxidatin product of Co1-xO, was found to form at free surface and the Co1-xO/Co2TiO4 interface. The Co2TiO4 spinel particles formed by reactive sintering rather than precipitation were able to detach from the Co1-xO grain boundaries to reach parallel epitaxial orientation with respect to the host Co1-xO grains via Brownian-type rotation of the embedded particles. In part IV, AEM was used to study the defect microstructures of Zr-dissolved TiO2 prepared via reactive sintering the ZrO2 and TiO2 powders (8:92 in molar ratio, designated as Z8T92) at 1600oC for 24 h and then aged at 900oC for 2-200 h in air. The Zr-dissolved TiO2 with rutile structure showed dislocation arrays, defect clusters, G.P. zone, superlattice, nanometer-size domains incommensurate and commensurate superstructure, may be the precursor of ZrTi2O6 precipitates at 900oC. The rutile showed diffuse diffractions along [001] direction as a result of Zr4+ substitution for Ti4+ with volume compensating defect clusters. Incommensurate and commensurate structures, as indicated by diffraction splitting and extra diffraction along <100> and <010> directions may be attributed to the ordering and clustering process of Zr and Ti atoms in these directions. Part V, deals with the reactive sintering of ZrO2 and TiO2 powders (1:4 molar ratio) at 1400 to 1600oC in air to form orthorhombic ZrTiO4 (a-PbO2-type structure, denoted as a) and to study its epitaxial reorientation in the matrix of tetragonal TiO2 (rutile) grains with Zr4+ (15 mol %) dissolution. The epitaxial relationship of intragranular ZrTiO4 and Zr-dissolved rutile (denoted as r) was determined by electron diffraction as [010]a//[011]r; (001)a // (011)r (i.e. [100]a // [100]r; (001)a // (011)r). The reorientation of the intragranular particles in the composites can be reasonably explained by rotation of the nonepitaxial particles above a critical temperature (T/Tm > 0.8) and below a critical particle size for anchorage release at interface with respect to the host grain. Reactive sintering facilitated the reoreientation process for the particles about to detach from the grain boundaries. The Brownian rotation of the confined ZrTiO4 particles in rutile grains was activated by a beneficial lower interfacial energy for the epitaxial relationship, typically forming lath-like ZrTiO4 with (101)a/(211)r habit plane having fair match of oxygen atoms at the interface. Further aging at 900oC for 50 h in air caused modulated and periodic antiphase domains in ZrTiO4 matrix, as likely precursor of equilibrium ZrTi2O6.

Abstract This research is designed to investigate the occurrence of interdiffusion-induced mesopores, Brownian motion, cleaving and healing and defect clusters in three binary composites, i.e. Ni1-xO/Co1-xO, Ni1-xO/ZrO2 and Co1-xO/ZrO2 of the Ni1-xO-Co1-xO-ZrO2 system. Firstly, the (NimCo1-m)1-£_O/Ni-doped Co3-dO4 composites prepared by reactive sintering Ni1-xO and Co1-xO powders (1:2 molar ratio, denoted as N1C2) at 1000oC with or without further annealing at 720oC in air were studied by X-ray diffraction and electron microscopy to clarify the formation mechanism of mesoporous spinel precipitates. Submicron-sized inter- and intragranular pores, due to incomplete sintering and grain boundary detachment, prevails in (Ni0.33Co0.67)1-£_O protoxide with rock salt structure; whereas nanosize pores due to Kirkendall effect were restricted to the spinel precipitates having Ni component progressively expelled upon annealing. A rapid net vacancy flux and a tensile misfit stress perpendicular to the protoxide/spinel interface caused the formation of elongated and aligned {100}-faceted mesopores in the spinel precipitates with a relatively low equilibrium vacancy concentration. Aligned mesopores in diffusion zone of nonstoichiometric metal oxides have potential applications on thermal barrier bond coating and mass-transport limited heterogeneous catalysis. Also, this thesis deals with the reorientation and shape change of low-crystal-symmetry (non-cubic) ZrO2 within the high-crystal-symmetry grains of Co1-xO/Ni1-xO cubic rock salt-type structure. ZrO2/Co1-xO composites 1:99 and ZrO2/Ni1-xO composites 1:9 in molar ratio were sintered and then annealed at 1650oC for 24 and 100 h in air to induce reorientation of the embedded particles. Transmission electron microscopic observations in both systems indicated that the submicron tetragonal/monoclinic (t/m) ZrO2 particles fell into three topotaxial relationships with respect to the host Co1-xO/Ni1-xO grain: (1) parallel topotaxy, (2) ¡§eutectic¡¨ topotaxy i.e. [100]Z//[111]C,N, [010]Z//[0 1]C,N and (3) ¡§occasional¡¨ topotaxy [100]Z//[111]C,N, [01 ]Z//[0 1]C,N. The parallel topotaxy has a beneficial low energy for the family of {100}Z/C,N and {111}Z/C,N interfaces. The change from the occasional topotaxy to an energetically more favorable eutectic topotaxy was likely achieved by a rotation of the ZrO2 particles over a specific (100)Z/(111)C,N interface. Brownian-type rotation is probable for the embedded t-ZrO2 particles in terms of anchorage release at the interphase interface with the Co1-xO/Ni1-xO host. Detachment or bypassing of rock salt type grain boundaries could also cause orientation as well as shape changes of intergranular ZrO2 particles. Zirconia-polymorphism-induced cleaving and spontaneous healing by precipitation was studied in Co1-xO polycrystals containing a dispersion of ZrO2 particles. Conventional, analytical, and high-resolution transmission electron microscopy indicated that the Co1-xO matrix cleaves parallel to {100} and {110} planes and heals itself by co-precipitation of parallel-topotaxial ZrO2/Co3-£_O4 particles upon cooling. Due to size effect and matrix constraint, nanometer-size ZrO2 precipitates at cleavages were able to retain tetragonality upon further cooling to room temperature. Paracrystalline array of defect cluster was shown to form in Zr-doped Ni1-xO and Co1-xO polycrystals while prepared by sintering at relative high temperature, i.e., 1650oC to increase the defect concentration. Paracrystalline array of defect clusters in Co3-£_O4 spinel structure also occurred when doped with Zr4+ at high temperature or cooled below 900oC to activate oxy-precipitation of Co3-dO4 at dislocations. transmission electron microscopic observations indicated the spinel precipitate and its paracrystal predominantly formed at the ZrO2/Co1-xO interface and the cleavages/dislocations of the Co1-xO host. Defect chemistry consideration suggests the paracrystal is due to the assembly of charge- and volume-compensating defects of the 4:1 type with four octahedral vacant sites surrounding one Co3+-filled tetrahedral interstitial site. The spacing of paracrystalline distribution is 3.3, 2.9 and 4.9 times the lattice parameter for Zr-doped Ni1-xO, Zr-doped Co1-xO and Zr-doped Co3-dO4. This spacing between defect clusters is about 0.98 times that of the previously studied undoped Co3-dO4. There is much larger (3.4 times difference) paracrystalline spacing for Zr-doped Co3-£_O4 than its parent phase of Zr-doped Co1-xO.