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,...
Reexamination Certificate
1999-11-19
2002-05-28
Fiorilla, Christopher A. (Department: 1731)
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,...
C264S125000, C264S667000
Reexamination Certificate
active
06395214
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for manufacturing dense nanocrystalline ceramic articles, and in particular, to a hot pressing method that simultaneously uses high compaction pressures in the range of about 1.5-8 GPa and low sintering temperatures in the range of about 0.2-0.6 T
m
where T
m
is the absolute melting temperature to consolidate amorphous or nanocrystalline powder compacts into fully dense nanocrystalline ceramic articles.
BACKGROUND OF THE INVENTION
Methods have been developed for the production of nanostructured oxide and non-oxide ceramic powders. Nanopowders have been formed by rapid condensation of precursor species from the vapor state. However, processing these powders into bulk nanocrystalline forms has proven to be difficult, due to complications arising from the presence of high levels of chemisorbed species on the nanoparticle surfaces, as well as the occurrence of severe interparticle friction effects during powder compaction. Even if these difficulties are overcome, high temperature sintering of green state compacts often results in rapid grain growth, thus losing the opportunity to realize a bulk nanocrystalline structure. This effect can be mitigated to some extent by adding grain growth inhibitors, but usually at the expense of degrading mechanical and/or physical properties.
Various approaches to the consolidation of ceramic powders have been investigated, with varying degrees of success. Microwave-assisted sintering of nanopowder compacts of &ggr;-Al
2
O
3
is examined in J. Freim et al., “Microwave Sintering of Nanocrystalline &ggr;-Al
2
O
3
”, Nanostructure Mater
., 4, 371-385 (1994). It was observed that the occurrence of a phase transformation (&ggr;-phase to &agr;-phase) during sintering without an applied pressure resulted in rapid grain growth, which hindered further densification. Hot pressing of &ggr;-Al
2
O
3
and rapid sintering at 1125-1175° C. is examined in S. J. Wu et al., “Sintering of Nanophase &ggr;-Al
2
O
3
Powder”,
J. Am. Ceram. Soc
., 79, 2207-11 (1996) which described much reduced sintering rate at higher temperatures. At 1350° C., a relative density of 0.82 was realized. In another approach described in R. S. Mishra et al., “High-Pressure Sintering of Nanocrystalline &ggr;-Al
2
O
3
”, J. Am. Ceram. Soc
., 79, 2989-92 (1996), &ggr;-Al
2
O
3
was sintered at 650-1100° C. under high pressure (~1 GPa). The &ggr; to &agr; transformation temperature was reduced from 1200° C. at ambient pressure to about 750° C. at 1 GPa. Fully dense compacts with a grain size of about 142 nm were obtained at 1000-1100° C. and 1 GPa in 10 minutes. The transformation and densification of nanocrystalline &thgr;-alumina during sintering forging is described in C. S. Nordahl et al., “Transformation and Densification of Nanocrystalline &thgr;-Alumina during Sinter Forging”,
J. Am. Ceram. Soc
., 79, 3149-54 (1996). It was demonstrated that by using seeded nanocrystalline &thgr;-alumina as the starting material, dense &agr;-Al
2
O
3
with a grain size of 230 nm could be obtained at 235 MPa/1060° C. for 30 minutes. The compaction and heat treatment behavior of nanocrystalline (~20 nm) &ggr;-Al
2
O
3
at high pressure is described in M. R. Gallas et al. “Fabrication of Transparent &ggr;-Al
2
O
3
form Nanosize Particles”,
J. Am. Ceram. Soc
., 77, 2107-12 (1994). Pressures up to 3 GPa and liquid nitrogen as a lubricant were utilized to form transparent green-state compacts, followed by pressure-less sintering at 800° C. for a few hours. Transmission Electron Microscope (TEM) examination of the sintered sample revealed a random dense-packed particle structure and interconnected porosity. Interstitial void dimensions, however, were always less than the average particle diameter.
Approaches described in S. C. Liao et al., “High Pressure and Low Temperature Sintering of Bulk Nanocrystalline TiO
2
”, Materials Science and Engineering A
, 204, pp. 152-159 (1995); “Theory of High Pressure/Low Temperature Sintering of Bulk Nanocrystalline TiO
2
”, Acta Materialia
, 45 [10], pp. 4027-4040 (1997); “Retention of Nanoscale Grain Size in Bulk Sintered Materials via a Pressure-Induced Phase Transformation”,
Nanostructured Materials
, 8[6], pp. 645-656 (1997); and “The Effect of High Pressure on Phase Transformation of Nanocrystalline TiO
2
during Hot-Pressing”,
Nanostructured Materials
, 5[3], pp. 319-325 (1995) found that bulk TiO
2
with nanoscale grain structure could be produced by high pressure/low temperature sintering of a metastable TiO
2
polymorph (anatase). The high pressure was described as not exceeding 1.5 GPa in these works and the low temperature was described in the range of 400-445° C. The combined high pressure and low temperature sintering was found to reduce the diffusion rate while increasing the nucleation rate of the stable phase (rutile). The net result was the production of nanocrystalline (~36 nm grain size) TiO
2
(rutile) with high density (>98%), high hardness (800 VHN, and about 6 times improvement in wear resistance. See Y. Iwai et al, “Tribological Properties of Nanocrystalline TiO
2
”, Proceedings of JAST Tribology Conference
, Osaka, November, 1997, Japanese Society of Tribologists, Osaka, Japan, 1997, 1997, pp. 209-212.
It is desirable to provide nanocrystalline ceramics with improved hardness and wear resistance, combined with good fracture toughness, compared with their microcrystalline counterparts.
SUMMARY OF THE INVENTION
The present invention relates to a method for fabricating nanocrystalline ceramic articles in which a loosely agglomerated ceramic nanopowder is synthesized. For example, the nanopowder can be synthesized by chemical vapor condensation (CVC) using a hot-wall reactor for the production of non-oxide ceramics and a combustion-flame reactor for the production of oxide ceramic powders. The nanopowder is formed into a compact and consolidated at pressures of at least 1.5 GPa and temperatures of no greater than 0.6 times the absolute melting temperature of the ceramic nanopowder (T
m
). For example, the nanopowder can be compacted at pressures in the range of 1.5-8 GPa and temperatures in the range of 0.2-0.6 T
m
.
It has been found that high compaction pressure causes deformation, such that the green density increases with pressure up to a maximum at about 8 GPa. Low sintering temperature mitigates grain growth during consolidation. The simultaneous application of high pressure and low temperature to a nanocrystalline powder compact under near isostatic conditions produces a sintered nanophase ceramic article with high density and a grain size comparable with the original powder particle size. The article can be produced with superior properties and performance for specific applications. For example, higher strength and toughness is advantageous for components in combustion and gas turbine engines, higher hardness and wear resistance is advantageous for protective coatings, and enhanced optical transparency is advantageous for infra-red windows, aircraft canopies, and high intensity lamps.
In one aspect of the invention nanophase powder with a metastable structure can be used as a starting material. Under high pressure, the metastable phase transforms to a more stable phase, which effect promotes the consolidation process. This transformation-assisted consolidation has been successfully applied to produce sintered oxide and non-oxide bulk nanocrystalline ceramics having a grain size of less than 100 nm, starting with even finer-scale ceramic nanopowders in the range of 5 to 50 nms. Also, under appropriate conditions for example sintering pressures in the range of 3 GPa to 5.5 GPa, the grain size of the nanocrystalline sintered product can be smaller than the original nanopowder particle size. These effects have not previously been reported, since prior work on sintering of ceramic powder compacts has been limited to pressures less than 1.5 GPa.
Although the method of the present invention is especially useful for processing nanophase Al
2
O
3
, TiO
2
,
Kear Bernard H.
Liao Shih-Chieh
Mayo William E.
Fiorilla Christopher A.
Mathew, Collins, Shepherd & McKay, P.A.
Rutgers The State University of New Jersey
LandOfFree
High pressure and low temperature sintering of nanophase... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High pressure and low temperature sintering of nanophase..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High pressure and low temperature sintering of nanophase... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2862545