Foreword

This special Issue of Ultramicroscopy contains invited and contributed papers from the Workshop entitled "The Structure and Properties of Interfaces" held at Wlckenburg, Arizona during January 3 to 6, 1991 This was the eleventh in the series of Workshops sponsored annually by the NSF Facihty for High-Resolution Electron Microscopy in the Center for Sohd State Science at Arizona State University The Workshops are part of the educational program of the Facility and are intended to enhance Interaction between researchers in electron microscopy and important areas of contemporary materials science The next Workshop In the series, planned for January 8-11, 1992, will be devoted to Mlcrostructure of Magnetic Materials

The general objective of the Workshop was to bring together, for several days of uninter- rupted discussion, researchers with strong interests in interfaces in solids, but from different points of view Electron microscopy specialists illustrated current capabihties for structural analysis of interfaces, and researchers speclahzlng in theoretical and experimental s t ruc tu re / property relationships discussed interface parameters needed for advances in materials science We were especially interested in bridging the semantic gap between the continuum view of s t ruc tu re /p rope r t i e s relations researchers and the atomlstlc view of high-resolution electron microscopists By the end of the Workshop it was evident that considerable progress toward that goal had been made, and many interesting joint research projects between these two groups were being discussed

During the last decade rapid progress in experimental high-resolution imaging has led to descriptions of particular boundaries and interfaces as arrays of resolvable atomic columns arranged in special periodicities, with quite localized deviations from periodicity These observations have most often been made on gram boundaries in metals, paradoxically the smallest unit cell materials and therefore reqmrlng higher resolution than other classes of materials, mainly because metal blcrystal specimens were readily available whereas suitable specimens of other materials are not The observations are leading to increasingly sophisti- cated theoretical efforts to calculate the equihbrlum structure of special grain boundaries in a wide variety of metals and for m e t a l / c e r a m i c interfaces Several of the papers presented at the Workshop illustrated outstanding experimental and theoretical results in this area

Applications of high-resolution imaging to the study of the structure sensitive propert ies of materials was discussed most intensively in terms of flow and fracture of composites, as well as from the viewpoints of diffusion and electronic properties Theoretical t reatments of flow and fracture of composites which can now account for lnterfaclal decoheslon in terms of critical i n t e r f ace /boundary strength and specific work of separation, or in terms of discontin- uous normal and shear displacements at interfaces have appeared recently These t reatments lead to "critical lengths" at interfaces that characterize much of the mechanical behaviour for these materials Experimental results for deformation and fracture of composites were presented that illustrated the difference between frictional sliding and decoheslon at inter- faces Both the theoretical and experimental mechanical propert ies results emphasized the

198 Preface

importance of understanding structural continuity, or the lack of It, along boundaries and interfaces in solids These considerations led naturally to discussions of the chemistry at in ter faces /boundar ies , because exastlng experimental data shows clearly that structural &s- contInmtles are often associated with local variations in chemistry

Measurements of chemical variations along in te r faces /boundar ies or at other defects require definitions of resolution parameters relative to both high-resolution Imaging of atomic columns at interfaces and the characteristic dimensions associated with theoretical analyses of mechanical propert ies The primary techniques for chemical microscopy are electron-energy- loss or X-ray energy dispersive spectroscopy A discussion of those methods established that about 1 nm spatial resolution could be expected for measurement of elemental distributions, using suitably thin specimens and with deconvolutlon of small probe current density from the elemental distributions of interest, with adequate signal-to-noise ratio and energy resolution for fine structure analysis in the case of electron-energy-loss spectroscopy Field emission electron sources are required to achieve these results Spectroscopic data illustrating &scon- tlnuous oxygen dlstrlbutlons along whiske r /ma t r ix interfaces in ceramic matrix composites were discussed The discontinuities in the distributions, and therefore in interface bonding or strength, were variable on a length scale of two to several tens of nanometers depending on the processing history of the composites These new results showed that one cannot Justify the assumption of constant interface bond strength or structure, even in single interfaces Similar results were presented for grain boundaries in several materials

Results from In SltU electron microscopy deformation experiments and post-deformation in t e r face /boundary analysis demonstrated that a hierarchy of defects must be considered for analysis of mechanical properties One could reasonably assume that similar considerations apply to chemical distributions These defect distributions extend over distances of tens of nanometers to microns High-resolution defect and chemical analysis is indlspenslble for analysis of individual defects Conventional imaging methods based on diffraction contrast, especially weak beam methods, are indlspenslble for analyzing the collective Interactions of groups of individual defects One newly optimized technique for local elemental analysis at Interfaces is worthy of special mention high-angle annular dark-field STEM, for which contrast depends strongly on atomic number The technique IS not spectroscopic, but the characteristic strong contrast dependence on atomic number and its high resolution (0 2 nm) indicate that relative elemental resolution equal to atom position resolution in H R E M images can at least be obtained from well ordered interfaces It has so far been applied mainly to interfaces In electronic materials, but extension to other materials is of much interest

Given the levels of experimental resolution now demonstrably attainable for both atomic positions and elemental distributions, the participants Indicated that it is now appropriate to begin larger theoretical research efforts to relate materials thermodynamics, symmetry argu- ments, and quantum mechanical studies of bonding to structure of in ter faces /boundar ies , leading to predictions of structure and propert ies that could be experimentally demonstrated This research will require careful experimental interface specimen preparat ion to ensure reproducible results, given the already demonstrated interface structure sensitivity to local chemical distributions

It was apparent during the concluding session of the Workshop that the participants ' interests in interfaces spanned a very wide range of materials Preparat ions to satisfy this wide range of materials interests in the community are being made by the establishment elsewhere of speclahzed laboratories for interface synthesis under very well controlled conditions High-resolution transmission methods will be used nearly exclusively to determine the structure and chemistry of single defects associated with these interfaces A broad range of

Preface 199

electron microscopy methods wdl be employed to analyze the collectwe interaction of defects with mterfaces and to study s t ructure/propemes relations m these materials The pamo- pants indicated a major reqmrement for real time data collection and quantification during mlcrostructural analysis by TEM, to extract all possible reformation from electron beam experiments, and a further reqmrement for improved chemical resolution under condmons that mmlmlze radmtlon effects These reqmrements presuppose marked advances m the apphcatlons of computers to electron microscopy, and the pamopants urged strongly that these new methods be qmckly taught to students who will be the next generanon of materials technology researchers

The Workshop was followed by a 4½ day school on High-Resolution Electron Microscopy, orgamzed by Peter Rez and held at the HREM Faclhty at ASU We are grateful to the microscope manufacturers (Phdlps and JEOL) for generously providing scholarships for several students to attend the school

Finally, we pamcularly wish to thank Sharon Wdhson for her splendid orgamzatlonal assistance

David J Smith and Ray W Carpenter Tempe, Arizona

August 1991