One-step synthesis and consolidation of nanophase materials

Details One-step synthesis and consolidation of nanophase materials One-step synthesis and consolidation of nanophase materials

1999-08-13

2001-03-13

Theisen, Mary Lynn (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Direct application of electrical or wave energy to work

Producing or treating inorganic material, not as pigments,...

C264S449000, C419S045000, C419S048000, C419S051000, C419S052000

Reexamination Certificate

active

06200515

ABSTRACT:

This invention relates to materials science, and particularly to the preparation of solid crystalline ceramic, intermetallic, and composite materials with grain sizes in the nanometer range.
BACKGROUND OF THE INVENTION
Synthesized solid materials such as ceramics, composites and intermetallic materials in which the grain size is in the nanometer (10
−9
m) range are the subject of active development due to their unique properties. For example, nanometer-scale crystals have the potential of improving the processing and performance characteristics of ceramics, composite polymers, catalysts, filtration systems, and transmission media.
Products and materials with nanometer-scale crystallites are formed from nanometer-scale particles in processes that entail first forming the particles of the desired chemistry and size scale, combining the particles into a green body, and then densifying the particles. Traditional metallurgical techniques such as casting, hot rolling and powder metallurgy have been used in combining the particles, and a reactive process known as combustion synthesis, reactive sintering, or self-propagating high-temperature synthesis has been used in some cases.
A group of processes that have not heretofore been used with nanoparticles but otherwise form the background of this invention are field-assisted combustion synthesis and field-activated pressure-assisted synthesis. A description of field-assisted combustion synthesis is found in U.S. Pat. No. 5,380,409, issued Jan. 10, 1995, to Munir et al, and a description of field-activated pressure-assisted synthesis is found in U.S. Pat. No. 5,794,113, issued Aug. 11, 1998, to Munir et al. The entire contents of both of these patents are incorporated herein by reference. In field-assisted combustion synthesis, a precursor material consisting of the starting materials that will react or combine to form a desired product is exposed to an electric field that energizes the material by propagating a current through the material that energizes the material but is not high enough to ignite the reaction. The reaction is then ignited in a subsequent step by radiative energy while the energizing wave sustains the propagation of the reaction through the material. Field-activated pressure-assisted synthesis, by contrast is the simultaneous application of a high current and pressure to effect both the synthesis reaction and densification of the product. Unlike field-assisted combustion synthesis, the current used is high enough to cause Joule heating of the material to the ignition temperature.
The utility and success of both field-assisted combustion synthesis and field-activated pressure-assisted synthesis have only been demonstrated with particles in the micron (10
−6
m) size range. Accordingly, neither process carries an expectation that it can be applied to nano-scale particles to result in a product that will successfully retain the nanocrystalline structure of the starting particles. Due to the delicate nature of the nano-scale particles and the extreme conditions imposed during these two processes, the risk that the nano-phase will be lost or substantially reduced is great enough to prevent one from predicting that a nanocrystalline product will be formed.
SUMMARY OF THE INVENTION
It has now been discovered that field-activated pressure-assisted synthesis can be performed on nano-scale particles of reactant materials to achieve a dense reaction product that substantially retains the nanocrystalline structure of the starting materials. Accordingly, solid particulate reactants in accordance with this invention are milled to particle sizes in the nanometer range (a step referred to herein as “mechanical milling”), then compacted into a green body in a stoichiometric ratio corresponding to that of the desired product, and the green body is then simultaneously exposed to an electric current (AC or pulsed DC) sufficient to cause Joule heating of the body to a temperature at which initiation of the reaction occurs and to a pressure sufficiently high to density the product. The reaction is completed within a very short period of time and the product is consolidated to a high density as the reaction is occurring. Grain growth and hence the loss of the nanocrytalline structure, which would normally be expected to occur subsequent to product formation, are either minimized or avoided entirely.
Further objects, features, and advantages of the invention will become apparent from the description that follows.


REFERENCES:
patent: 5380409 (1995-01-01), Munir et al.
patent: 5794113 (1998-08-01), Munir et al.
Abdellaoui and Gaffet,Acta. Mater.(1996) 44(2): 725-734.
Charlot et al.,Acta. Mater.(1999) 47(2): 619-629.
Munir,Journal of Materials synthesis and Processing(1993) 1(6): 387-394.

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