Surface-strain imaging

Figure 7a and b show typical three-dimensional surface topographic images of the PHB fibers drawn at a draw ratio of 4.0 and 7. The surfaces of the fibers differ considerably. Depending on the draw ratio, spherulitic or fibril-like surface structures were formed. The textile physical properties of the fibers can be explained by these different structures. The fibers, spun at a draw ratio of 4.0, are brittle without a sufficient elongation at break visible in the stress-strain curve (Fig. 5). The fibers spun at a draw ratio of 7 show a completely different stress-strain behavior with a sufficient elongation at break and a sufficient tenacity, as can be seen from the stress-strain curve (Fig. 5).

Summary. Scanned probe methods for imaging electrochemical deposition on surfaces are now well established. For such methods the smface structure at the atomic scale can be measured so that surface strains may be inferred. Here we demonstrate how extremely sensitive and fast stress sensors can be constructed from atomic force microscope (AFM) cantilevers for studies of interfacial processes such as adsorption and reconstruction. The surface stress sensor has submonolayer sensitivity for use in electrochemistry, whereby simultaneous cyclic voltammograms and stress changes can be recorded. This is demonstrated with measurements of the electrocapillary curve of gold, and stress changes associated with the underpotential deposition of silver on gold (111). 

The tensile tests have been accompanied by acoustic emission (AE) registration, fuU-field surface strain-mapping (SM), progressive crack imaging at different load levels and micrographic inspection of the cross-sections of samples. The results of testing have been reported in [108,109]. 

Zinc sulfide, ZnS, has been epitaxially deposited by the dual bath approach on Au(lll) surface and studied by STM and XPS [48]. The first complete ECALE cycle resulted in the formation of nanocrystallites of ZnS randomly distributed across Au(l 11) terraces, on account of lattice mismatch induced strain between ZnS and Au(lll) - although the mismatch is only 0.13% for ZnS/Au(lll). Atomically resolved STM images showed the ZnS/Au(lll) monolayer to be sixfold symmetric. The average diameter of the crystallites was 10 5 nm and the apparent coverage 0.38. 

The essence of the topographic methods is that they map the interrsity of the diffracted beam over the surface of the crystal. Defects affect the diffracted intensity, so give contrast in the image. The methods are quite sertsitive enough to reveal individual dislocatiorrs, precipitates, magnetic domains and other long-range strain fields but cannot reveal point defects except in dense clusters. 

An important feature of the use of soft radiation is that both the extinction distance and the absorption distance are small, and therefore the X-rays penetrate only a veiy small distance into the crystal. We therefore examine only a small slice of the crystal close to the surface. The strain fields of dislocations deeper into the crystal do not contribute significantly to the image and, in a transmission experiment, overlapping of the images leads to an upper limit of about 10" cm on the dislocation density for individual defect imaging. With the Berg-Barrett technique this can be pushed to about 10 cm ... 

The past decade saw introduction of the low-energy electron microscope (LEEM) (12). It was expected potentially to be capable of moderately high resolution (6-mn) video-rate imaging of surfaces and interfaces. This would make it capable of studying dynamic processes at surfaces, thin-fibn growth, strain relief, etching, absorption. 

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