Arzu Altay

Crystallization of CaAl4O7 and CaAl12O19 powders

Calcium is always present in alumina systems as an unintentional (or intentional) dopant, and yet the fundamental effect of its incorporation into the aluminas is not well understood, and is further complicated by the presence of Si. The synthesis of powders of two calcium aluminate phases (CaAl4O7, which is also known as CaO · 2Al2O3 or CA2, and CaAl12O19, which is also known as CaO · 6Al2O3 or CA6) has been investigated using low-temperature chemical-processing techniques. The crystallization of these powders from the amorphous precursor has been examined using various characterization techniques. The precursors for the powders were prepared by mixing stoichiometric proportions of the nitrate salts into a 5 wt% aqueous solution of poly(vinyl alcohol). Conversion of the amorphous precursors to crystalline powders and the subsequent phase transitions were monitored using differential thermal analysis (DTA), thermogravimetric analysis (TGA) and powder X-ray diffractometry (XRD). While powders with CA2 stoichiometry crystallized directly at 883°C, amorphous powders with CA6 stoichiometry first crystallized into an intermediate structure without partitioning and then transformed into CA6 at 1175°C. Fully and partially crystallized powders were analyzed using transmission electron microscopy and electron energy-loss spectroscopy (EELS). Measured near-edge structures (Al–L2,3, Ca–L2,3 and O–K) are presented for the CA2, γ-Al2O3 and CA6 phases. The intermediate phase, identified as γ-Al2O3, was found to accommodate a significant concentration of Ca.

Calcium in alpha-alumina: the myth and some EM observations

Changes in the microstructure of alpha alumina with increasing amounts of calcium concentrations in the temperature range between 1400degreesC to 1600degreesC were studied by using various electron microscopy techniques. Variations in the average grain sizes were related to hypothetical excess calcium amount at the grain boundaries (Gamma(Ca)) assuming negligibly small solubility of calcium in bulk alpha-alumina. analytical and high-resolution transmission electron microscopy studies were done on these Ca-doped alumina ceramics. Under all sintering conditions, the grains were uniform in size and equi-axed for low calcium concentrations (<3 Ca-atoms/nm(2)). The grain morphology became elongated when the calcium concentration at the grain boundaries reached calcium excess of Gamma(Ca) = 3-3.5 Ca-atoms/nm(2) in all samples. For the samples that were sintered at 1500degreesC and 1600degreesC, slab like abnormally grown grains appeared for critical calcium excess concentrations of Gamma(Ca) = 4.5-8 Ca-atoms/nm(2). No amorphous triple point pocket and grain boundary phases and/or Films were observed by high resolution electron microscopy. SEM/EDS chemical analysis showed precipitation of calcium hexa-aluminate precipitates that are as large as the matrix alumina grains after certain calcium excess level.

Monitoring Faceting on Ceramic Surfaces

Faceting is the transformation of a planar surface into two or more surfaces of lower energy. Metal, semiconductor and ceramic surfaces can all undergo faceting. The evolution of facets formed on the m-plane (10 1 0) of alumina has been monitored using atomic-force microscopy (AFM). When heat-treated, the (10 1 0) surface reconstructs into a hill-and-valley morphology. The present study investigates the manner in which facets originally form and grow to cover a surface. A gravity-loaded indenter (load of 25 grams) was used to mark a 25 μm × 25 μm square area on as-received, polished alumina specimens. An initial heat-treatment of 1400°C for 10 minutes is carried out to initiate faceting. With the indents as guides the same area can be identified and imaged after each subsequent heat-treatment. The morphology of the facets can be described as being comprised of a “simple” and “complex” surface. The simple surface corresponds to the (1 1 02) plane which is stable over the course of heat treatments, whereas the complex surface gradually transforms to a lower energy surface after several heat treatments and acts as a nucleation site for new facet growth.

Technique for Monitoring the Etching Rate of Alumina

The effect of chemical and thermal treatments on the grains and grain boundaries of polycrystalline μ-Al 2 O 3 has been examined using a combination of microscopy techniques. Commercially available alumina samples (Lucalox™) were chemically etched in phosphoric acid at 200°C in increments of 15 min. Thermal treatments were carried out at 1650°C before chemical treatments. Using maps obtained by visible-light microscopy (VLM) as a guide, the same regions were re-examined using atomic force microscopy (AFM) after subsequent treatments. Variations in the dissolution rates of different grains and grain boundaries could then be studied using AFM. The geometry of the grain-boundary grooves was compared after thermal and chemical treatments. Electron backscattered diffraction (EBSD) patterns recorded in the scanning electron microscope (SEM) were used to obtain crystallographic orientations of the grains which enabled variations in dissolution rates between grains to be correlated to orientation.