No abstract

Thermal transformation of illite and smectite group clay minerals was studied using X‐ray diffraction and transmission electron microscopy, and the results were compared with those of kaolinite. Differences were observed in the types and sizes of crystals formed at temperatures between 800° and 1400°C. Spinel‐type phases were formed after 3 h at 1000°C in all three clays, but the morphology and composition of the spinel crystals varied. Mullite crystals reached greater sizes in the illite (>10 μm needles) and smectite (>1 μm) materials than in the kaolinite (0.5 μm). Iron‐rich crystals formed in the illite and smectite materials, and feldspars present as impurities in these clays, along with interlayer cations, resulted in large quantities of liquid being formed above 1000°C. Similar thermal breakdown and phase development schemes are proposed for all three materials, with the major differences observed ascribed to the presence or absence of interlayer cations, substitutional impurities, and accessory minerals. These results provide a basis for predicting phase development on firing a given clay mineral, if its characteristics and impurity levels are known.

Strontium-doped lanthanum ferrites (LSF) were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), 4-point D.C. electrical conductivity and bulk property measurements. The results were compared to those of previous studies as well as selected processing conditions. The investigation focused on effects of sintering temperature, time, atmosphere (air, O 2 and N 2 ) and composition of La 1−x Sr x FeO 3−δ (x = 0.2-0.9), on the sintering behavior, microstructural development and electrical conductivity. An oxalate precipitation method was used to prepare lanthanum ferrite powders. Simultaneous thermogravimetric and differential thermal analysis (TGA/DTA) studies found calcination temperatures of 800 and 850 • C were necessary to form single-phase crystalline powders, as determined by XRD. Specimens were sintered from 1300 to 1400 • C with dwell times from 1 2 to 2 hrs. Results from SEM/EDS analysis showed the presence of a second phase in the samples fired in air or oxygen. The second phase was not detected by x-ray diffraction due to the small amount of material present. Samples fired in nitrogen had the lowest conductivity while those fired in oxygen had the highest. A composition of x = 0.5 resulted in the highest conductivity, 352 S/cm, at an operating temperature of 550 • C in air. High strontium additions (x = 0.9) lowered the linear shrinkage of LSF.

X7R‐type BaTiO3 materials were analyzed using X‐ray diffraction (XRD) and transmission electron microscopy (TEM). Powder XRD indicated that the materials had pseudocubic lattices, but core–shell grain structures predominated in bright‐field (BF) TEM images. Electron diffraction patterns across the core–shell boundaries and convergent beam electron diffraction patterns of cores and shells indicated that coherent grain boundaries existed between cores and shells. The flat dielectric constant–temperature curves obtained from these materials can be interpreted in terms of the internal stress states in individual grains. The stress states were observed using weak‐beam dark‐field (WBDF) microscopy, and strain contours formed by distorted crystal planes were visible in the WBDF images. The contours observed were dependent on the stress state of the crystal instead of crystal symmetry and the stress distribution in individual grains was determined by both the thickness ratio of shell and core, and the geometrical relationship of the core and the shell. Twins observed in this material were determined to be growth rather than mechanical twins, through observation of the strain contour distribution.