R.W. Siegel

Mechanical behavior of nanocrystalline Cu and Pd

This report gives results of a study of the bulk mechanical properties of samples of nanocrystalline Cu and Pd consolidated from powders prepared by inert gas condensation. Fourier analysis x-ray diffraction techniques, used to determine average grain size and mean lattice strains of the as-consolidated samples, show grain sizes in the range of 3--50 nm and lattice strains ranging from 0.02--3%. Sample densities range from 97--72% of the density of a coarse-grained standard. Microhardness of the nanocrystalline samples exceeds that of annealed, coarse-grained samples by a factor of 2--5, despite indications that sample porosity reduces hardness values below the ultimate value. Uniaxial tensile strength of the nanocrystalline samples is similarly elevated above the value of the coarse-grained standard samples. Restrictions on dislocation generation and mobility imposed by ultrafine grain size are believed to be the dominant factor in raising strength. Residual stress may also play a role. Room temperature diffusional creep, predicted to be appreciable in nanocrystalline samples, was not found. Instead, samples appear to show logarithmic creep that is much smaller than the predicted Coble creep.

Research opportunities on clusters and cluster-assembled materials —A Department of Energy, Council on Materials Science Panel Report

The Panel was charged with assessing the present scientific understanding of the size-dependent physical and chemical properties of clusters, the methods of synthesis of macroscopic amounts of size-selected clusters with desired properties, and most importantly, the possibility of their controlled assembly into new materials with novel properties. The Panel was composed of both academic and industrial scientists from the physics, chemistry, and materials science communities, and met in January 1988. In materials (insulators, semiconductors, and metals) with strong chemical bonding, there is extensive spatial delocalization of valence electrons, and therefore the bulk physical properties which depend upon these electrons develop only gradually with cluster size. Recent research using supersonic-jet, gas-aggregation, colloidal, and chemical-synthetic methods indeed clearly establishes that intermediate size clusters have novel and hybrid properties, between the molecular and bulk solid-state limits. A scientific understanding of these transitions in properties has only been partially achieved, and the Panel believes that this interdisciplinary area of science is at the very heart of the basic nature of materials. In Sec. V (Future Challenges and Opportunities), a series of basic questions for future research are detailed. Each question has an obvious impact on our potential ability to create new materials. Present methods for the synthesis of useful amounts of size-selected clusters, with surface chemical properties purposefully controlled and/or modified, are almost nonexistent, and these fundamentally limit our ability to explore the assembly of clusters into potentially novel materials. While elegant spectroscopic and chemisorption studies of size-selected clusters have been carried out using molecular-beam technologies, there are no demonstrated methods for recovery and accumulation of such samples. Within the past year, the first reports of the chemical synthesis of clusters with surfaces chemically modified have been reported for limited classes of materials. Apparatus for the accumulation and consolidation of nanophase materials have been developed, and the first promising studies of their physical properties are appearing. In both the chemical and nanophase synthesis areas, clusters with a distribution of sizes and shapes are being studied. Progress on macroscopic synthetic methods for size-selected clusters of controlled surface properties is the most important immediate goal recognized by the Panel. Simultaneous improvement in physical characterization will be necessary to guide synthesis research. Assuming such progress will occur, the Panel suggests that self-assembly of clusters into new elemental polymorphs and new types of nanoscale heterogeneous materials offers an area of intriguing technological promise. The electrical and optical properties of such heterogeneous materials could be tailored in very specific ways. Such ideas are quite speculative at this time; their implementation critically depends upon controlled modification of cluster surfaces, and upon development of characterization and theoretical tools to guide experiments. The Panel concluded that a number of genuinely novel ideas had been enunciated, and that in its opinion some would surely lead to exciting new science and important new materials.

Synthesis, characterization, and properties of nanophase TiO 2

Ultrafine-grained, nanophase samples of TiO/sub 2/ (rutile) were synthesized by the gas-condensation method and subsequent in situ compaction. The samples were studied by a number of techniques, including transmission electron microscopy, Vickers microharness measurements, and positron annihilation spectroscopy, as a function of sintering temperature. The nanophase compacts with average initial grain sizes of 12 nm were found to densify rapidly above 500 /sup 0/C, with only a small increase in grain size. The hardness values obtained by this method are comparable to or greater than those for coarser-grained compacts, but are achieved at temperatures 400 to 600 /sup 0/C lower than conventional sintering temperatures and without the need for sintering aids.

Mechanical properties of nanophase metals

Abstract Nanophase metals have grain-size dependent mechanical properties that are significantly different than those of their coarse-grained counterparts. Pure metals are much stronger and apparently less ductile than conventional ones; intermetallics are also strengthened, but they tend toward increased ductility at the smallest grain sizes. These property changes are primarily related to grain size limitations, but they are also affected by the large percentage of atoms in grain boundaries and other microstructural features. Strengthening appears to result from a limitation of dislocation activity, while increased ductility probably relates to grain boundary sliding. A brief overview of our present understanding of the mechanical properties of nanophase metals is presented.

Mechanical properties of nanophase TiO 2 as determined by nanoindentation

Nanoindenter techniques have been used to determine the hardness, Youngs modulus, and strain rate sensitivity of nanophase TiO{sub 2}, which is currently available only in very small quantities and which cannot be tested by most conventional techniques. Hardness and Youngs modulus both increase linearly with sintering temperature over the range 25--900 {degree}C but come to within only 50--70% of the single crystal values. Strain rate sensitivity, on the other hand, is measurably greater for this material than for single crystal rutile, and the value of strain rate sensitivity increases as the grain size and the sintering temperature are decreased. In its as-compacted form, the strain rate sensitivity of nanophase TiO{sub 2} is approximately a quarter that of lead at room temperature, indicating a potential for significant ductility in these ceramic materials. Finally, a significant scatter in hardness values has been detected within individual nanophase samples. This is interpreted as arising from microstructural inhomogeneity in these materials.

Raman microprobe study of nanophase TiO sub 2 and oxidation-induced spectral changes

A Raman microprobe study of as-compacted nanophase TiO{sub 2} was carried out to investigate the spatial inhomogeneity of its anatase and rutile phases. Also, changes in the observed Raman spectra (line shifts and broadening) were investigated as a function of annealing at temperatures up to 600 {degree}C in argon or air. Microscopic phase inhomogeneity is observed and Raman spectral changes are shown to result from inhomogeneous oxygen deficiency in the nanophase TiO{sub 2}. The line positions corresponding to the Raman active {ital E}{sub {ital g}} modes in both anatase and rutile are found to be sensitive to this oxygen deficiency and are potential quantitative indicators of such deviations from stoichiometry.

Nanostructured materials -mind over matter-

Abstract Considerable interest is being exhibited in the novel and enhanced properties of nanostructured materials. These materials, with their constituent phase or grain structures modulated on a length scale less than 100nm, are now artificially synthesized by a wide variety of physical, chemical, and mechanical methods. Nanostructured materials with modulation dimensionalities of zero (clusters), one (multilayers), two (ultrafine-grained overlayers), and three (nanophase materials) are considered. The basic principles involved in the synthesis of these new materials are discussed in terms of the special properties sought using selected examples from particular synthesis and processing methodologies. Some examples of the property changes that can result from one of these methods, cluster assembly of nanophase materials, are presented.

Microhardness of nanocrystalline palladium and copper produced by inert-gas condensation

This report describes the major results of a Vickers microhardness study of nanocrystalline palladium and copper produced by the inert-gas condensation method. Small grain size is known to have a strong influence on mechanical properties at low and intermediate temperatures, as observed in the Hall--Petch effect, Coble creep, and superplasticity. Until recently, the mechanical properties of nanocrystalline materials with mean grain diameters of 5 to 50 nm have not been explored. A microhardness study on conventional compacted tungsten powder indicated that the Hall--Petch effect extends to the 150 nm range; and effects of nanocrystalline grain size on mechanical properties of TiO{sub 2} have been investigated.

Raman spectroscopy of nanophase TiO 2

Raman spectra are reported for consolidated nanophase TiO{sub 2} particles in their as-compacted state and after annealing at a variety of temperatures up to 1273 K. The Raman-active bands normally observed for the rutile form of TiO{sub 2} were present in as-compacted samples having average grain sizes in the range from about 10 to 100 nm. However, significant broadening of these bands was found, which was uncorrelated with initial grain size, but not necessarily with other synthesis-related factors. This broadening decreased upon isochronal annealing at elevated temperatures in air. Based upon these observation, it is concluded that nanophase TiO{sub 2} in the as-consolidated state contains significant defect concentrations within the rutile grains and that these intragrain defects and the grain-boundary regions as well have local atomic structures with the rutile symmetry, albeit with some short-range displacements. Some sporadic sample regions containing small amounts ({lt}5) of the anatase form of TiO{sub 2} were also found; these traces of anatase transformed to rutile upon annealing in air at temperatures above 883 K.

Mechanical behavior of nanocrystalline metals

The influence of grain size or cell size on the mechanical behavior of metals is well known, although debate continues about details of defect control mechanisms. In the past, difficulty in producing bulk samples with grain sizes smaller than about 1 {mu}m limited mechanical behavior studies in finer-grained materials. Expressions relating strength to grain size, determined from studies of coarse-grained materials, suggest that reducing grain size into the sub-micrometer range results in increased mechanical strength at low homologous temperatures. At high temperatures, diffusional creep effects may lead to increased ductility. This paper reports the major results to date of an ongoing study of the mechanical behavior of nanocrystalline metals produced by the inert-gas condensation method; some results have been reported elsewhere. Studies of the tensile strength, low-temperature creep and Vickers microhardness of Cu, Pd and Ag reported here are complemented in this broader study by processing studies, x-ray grain-size and strain analyses, and high resolution microscopy studies of nanostructure and microstructure. The work provides a basis for predicting low-temperature mechanical behavior of ultrafine-grained metals, subject to some significant constraints imposed by the processing conditions. 16 refs., 2 figs., 3 tabs.