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SYNTHESIS AND PROCESSING: GENERAL METHODS 31 Mechanical alloying is another approach to the fabrication of inert-particle dispersion-strengthened materials. Typically, ceramic particles are mechanically mixed with metal powders in a high-energy ball mill to produce an intimate mixture of the two phases. The attrition process, by a repetitive particle fragmentation mechanism, gives rise to a fine dispersion (less than 0.1 µm) of ceramic particles in a metal matrix. After consolidation by hot extrusion, the resulting grain size of the metal matrix is typically 1 to 5 µm in diameter, with the dispersed phase in the 10-to 30-nm size range, with comparable interparticle spacings. In optimized systems, the volume fraction of the dispersion ranges from 1 to 5 percent. Such materials have excellent resistance to high-temperature creep and are capable of being used effectively in structural applications up to about 0.8 Tm. The most recent innovation (M. J. Luton, private communication, 1989) in this field dispenses with the need to add the dispersed phase and relies on gas-solid reactions to produce the desired dispersion. In one preferred methodology, the metal particles (e.g., Al) are milled in liquid N2. Such cryomilling takes advantage of sluggish reaction kinetics between the clean Al-particle surfaces and the N2 molecules to produce small AlN clusters, which become embedded within the Al particles during the further mechanical deformation in the ball mill. The resulting microstructure has a grain size less than 50 nm, with a particle size of the disperse phase of approximately 20 nm, which is the smallest uniform-scale microstructure yet achieved by mechanical working processes. This microstructure has been found to be remarkably resistant to coarsening, even at temperatures approaching the melting point of the material. Obvious extensions of this technology are being considered, such as the synthesis of metal-polymer, polymer-ceramic, and other mixtures with nanoscale dimensions. The synthesis of metastable phases, such as amorphous and quasicrystalline materials, can also be produced by mechanical mixing of crystalline precursors (that are ready glass formers) followed by a low-temperature interdiffusion annealing treatment. GAS-CONDENSATION SYNTHESIS The gas-condensation method (Granqvist and Buhrman, 1976; Kimoto et al., 1963; Thölén, 1979) for the production of ultrafine metal particles in an inert-gas atmosphere has been extensively studied during the past 25 years. Using this technique, powders with rather narrow size distributions can be produced in technologically significant amounts. The particle sizes can be varied over a wide range by changing the gas pressure, the partial pressure of the evaporating material (i.e., its temperature or evaporation rate), or the inert gas itself. All of these parameters are in principle easily controlled to yield particles in the size range of 5 to 100 nm. Nanophase materials are synthesized in a two-step process that consists of the production of small powder particles by the gas-condensation method and their subsequent in situ compaction and sintering into a solid material
SYNTHESIS AND PROCESSING: GENERAL METHODS 32 without exposure to air. The resulting new class of nanophase materials may contain crystalline, quasicrystalline, or amorphous phases; they can be metals, ceramics, or composites with rather different and improved properties than normal coarser-grained polycrystalline materials (Gleiter, 1981; Siegel and Hahn, 1987). An alternative single-step process (P. R. Strutt, University of Connecticut, private communication, 1989) for the production of thin-film nanophase materials involves laser-induced evaporation of metallic or ceramic species and condensation as a thin film on a cold substrate. When carried out in a highly reducing environment, it is possible to convert a ceramic into primarily metallic material. This transformation is facilitated by the creation of an intense plasma flame, which is typical of the interaction of a high-power-density laser beam with a material surface. Critical to the success of this single-step process is the condensation from a highly excited, fully ionized state of the material. A specific advantage of the gas-condensation method is the exceptional physical and chemical control available, which permits the particle surfaces to be maintained clean, or to be reacted or coated, allowing subsequent high grain-boundary purity or selective interfacial doping and phase formation. In addition, exceptionally high (rather surface-like) atomic diffusivities along the dense grain-boundary network in nanophase materials allow for efficient doping, and property modification, subsequent to consolidation. The major components of a system for the synthesis of nanophase materials using the gas-condensation method are shown in Figure 9. The system ideally consists of an ultrahigh-vacuum (UHV) chamber equipped with a turbomolecular pump capable of a base pressure of better than 1 Ã 10-6 Pa. The starting material or materials from which the nanophase compacts are to be made are evaporated using conventional methods but in a high-purity gas atmosphere introduced after the production chamber has been pumped to better than 1 Ã 10-6 Pa. During the evaporation in the gas atmosphere (usually an inert gas such as helium), convective gas flow transports the particles, which are formed by homogeneous condensation in close proximity (within a few mm) to the resistance-heated evaporation source, to a liquid-nitrogen-filled cold finger, where the particles are collected. Binary mixtures of powder particles or nanocrystals of different materials can be prepared in the same way by using two evaporation sources simultaneously. To achieve a better mixture in such a case, the cold finger can be rotated during the deposition process. Although resistance-heated evaporation sources have been commonly used in the gas-condensation method, radio-frequency sources, electron-beam, laser, or plasma-torch-heated sources, and ion-sputtering sources are now being used or investigated to allow for greater control of the evaporation and powder-production conditions for a wider range of starting materials. The synthesis of more complex nanophase materials will also be facilitated.
SYNTHESIS AND PROCESSING: GENERAL METHODS 33 Figure 9 Schematic drawing of a gas-condensation chamber for the synthesis of nanophase materials. The material evaporated from source A and/or B condenses in the gas and is transported via convection to the liquid-N2-filled cold finger. The powders are then scraped from the cold finger, collected, and compacted in situ (Siegel and Eastman, 1989).
