The microstructure of SnO& prepared by the sol-gel method, was studied by scanning electron microscopy, transmission electron microscopy, high-resolution electron microscopy, and x-ray difFraction. A nanosponge structure was observed. There is much surface and interface structure. The interfaces vary in type from amorphous to crystalline. The high-density of defects greatly influences the physical and chemical properties of this material.

G. VAR TENDELOO et al. : Direct Observation of "Discommensurations" 483 phys. stat. sol. (a) 91, 483 (1985) Subject classification: 1.2 and 1.4; 22.8.1 R i~k s u n~v e r s~t a i r Centrum Antwerpenl) (u),

The purpose of this investigation is to reveal the correlation between microstructures and curitical current density (Jc) in YBCO ceramic superconductors by TEH. Further analyses have shown that these microstructural features are denpendent on Cu-O planes behaviour of two dimensions and key to explain the mechanism of the relation between microstructure and Jc. Three YBCO samples which have the same composion but different processing were measured and compared. The Tc values of three samples were approximately the same (90K) but the Jc values were quite different:30A/cm2 for sample S1 300A/cm2 for sample S2 and 2000A/cm2 for sample S3. Hundreds pictures for three samples were randomly taken in low magnification by TEM. Using the statistical method, the microstructural information were measured from TEM pictures.A number of observations by TEM have found that three kinds of microstructural features, including the grain of c/a≥3 (grain length c, width a),the grain boundary between parallel-straight grains and the grain with twin domains play a major role in YBCO ceramics superconductors as shown in Fig. 1. The results of investigation have been shown that their content increases as Jc value are increased and as a function of Jc as shown in Fig. 2.

The structure defects were observed by HREM in α-Si3N4, which was made by hot-pressing method with MgO and LiF as the additives. The observations indicate that the phenomenon of phase seperation occured in the grain boundaries of three grains junction, showing inhomogeneous chemical composition in glassy phase of grain boundaries. In the grains, the stress areas caused by lattice distortion and displacement of (100) planes were found. The distortions of the lattices are so serious in some regions that the hexagonal symmetry in the unit-cell gets lost. The distortion of the lattices could be measured by the deviation of the spacings of (100) planes from the normal value, 6.771?. With HREM we have found electron radiation damage of α-Si3N4 grains at lattice level caused probably by the distortion of the unit-cells of α-Si3N4. As compared with β-Si3N4, α-Si3N4 is less stable thermodynamically. According to our observations of both α-Si3N4 and β-Si3N4 made by hot-pressing method, the defects of the structure in α-Si3N4 are much more than that in β-Si3N4. This indicates that the phenomenon probably arises from easier distortion of the α-Si3N4 lattices than that in β-Si3N4.

The defects in Hg1-xCdxTe crystals are observed using HREM. Two kinds of 60° dislocation as well as the twin dislocation in the twin-plane boundary are obserred directly. It is showed that twin dislocation is the activated step of the migration of twin-plane boundary during recrystallization.