A rapid densification technology uses nanostructured powders to produce ceramic devices and components. This technology provides ceramic monoliths and composites that can be used in the automobile, energy, electrochemical, magnetic, structural, biomedical, computing, information-transfer, and pollution-prevention-and-control industries.

Nanoscale powders of ceramic, measuring less than 100 nm, were mixed with less than 5 weight percent of sintering aid, cold pressed into pellets, and pressureless sintered between 1,400 and 1,600 °C. The sintering environment was evacuated to remove oxygen and maintained in evacuated reducing state.

This image shows Nano-Sized SiC Powders (magnified 120,000 times).

Micron-scale powders of ceramics with the same composition were also processed through the same steps under similar environments.

The densification of a ceramic compact, or sintering, is the process of removing the pores between the starting particles combined with growth and strong bonding between adjacent particles. The driving force for densification is the decrease in surface area and lowering of the surface free energy by the elimination of the solid-vapor interface

Nanostructured powders are a novel class of materials whose distinguishing feature is that their average grain or other structural domain sizes are below 100 nm. Within this size range, a variety of confinement effects significantly change the properties of the material. The confinement effects lead to several commercially useful characteristics. From a processing viewpoint, nanostructured powders offer the potential for very high sintering rates at lower temperatures.

In order to reduce the concept of this new technology to practice, the innovators focused on β-SiC and continuous SiC fiber-reinforced SiC composites. Nanoceramic powders were produced and then formed into monoliths. These monoliths were dispersed into the fibers by several alternative methods, isostatically compacted, and hot pressed to achieve high densities.

Nano-sized SiC powders synthesized were characterized using x-ray diffraction and transmission electron microscopy. The nano-sized SiC powders were further characterized using the B.E.T. method, and surface area was found to be 100 m2/g. Using 3.2 g/cm3as the density of SiC, the average powder size was calculated as 9.4 nm.

The pressureless sintering was carried out in a high-temperature graphite furnace. The graphite heating element is insulated from the water-cooled stainless steel chamber. A EUROCUBE 425 Thyristor unit was used to precisely control the furnace temperature and heating/cooling cycle. The chamber was first pumped down to ~10 torr using a mechanical pump, then flashed with argon three times. Either an argon or argon-reducing atmosphere was used during the sintering process.

Rapid densification technology will be of great value in the development of new ceramic-matrix-composite technologies of the future.

With 10-nm-sized SiC nanopowders, monolithic SiC and SiC matrix composite samples were pressureless sintered to over 90 percent of the theoretical density in 240 minutes at 1,450 and 1,500 °C respectively. At temperatures such as those presently used in conventional SiC and SiC/SiC densification practices (>2,000 °C), the densification of nano-sized SiC and SiC/SiC is expected to be more than 30 times faster. Beyond processing benefits, the invention offers performance benefits as well. The densified composite samples prepared with nano-sized SiC, for example, offer much higher fracture strength (400 percent) than those prepared with micron-sized SiC as starting powders. This technology breakthrough is applicable to other commercially important carbide, nitride, boride, silicide, and oxide ceramic compound compositions as well.

This work was done by Mark Liu Yang and Tapesh Yadav, Nano-materials Research Corporation, for the Marshall Space Flight Center. Inquiries concerning rights for the commercial use of this invention should be addressed to

the Patent Counsel, Marshall Space Flight Center; (205) 544-0021

Refer to MFS-26458.

NASA Tech Briefs Magazine

This article first appeared in the October, 1998 issue of NASA Tech Briefs Magazine.

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