Radiant energy – Irradiation of objects or material
Reexamination Certificate
1998-03-31
2001-04-24
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Irradiation of objects or material
Reexamination Certificate
active
06222194
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of materials science, more particularly the production and use of sapphire bodies with increased strength at elevated temperature, particularly compressive strength. The invention further relates to sapphire windows, bearing assemblies, semi-conductor support materials, or fiber optic devices, which are exposed to high temperature compressive conditions either during manufacture or use.
BACKGROUND OF THE INVENTION
Conventional methods have failed to create sapphire bodies which exhibit sufficient compressive strength at high temperatures to be useful in many applications. When exposed to elevated temperature, sapphire is generally believed to lose strength at least partially because of “twinning” wherein twin planes slip along adjoining regions. Twinning results in a reorientation of the planes of atoms in one slice of a crystal relative to the alignment in neighboring regions. Twins appear very sharp in cross-polarized light because of the shift in orientation.
Prior attempts to increase the strength of sapphire at elevated temperature have included both ion implantation utilizing particle accelerators and solid solution strengthening methods. Ion implantation techniques have failed, at least in part due to the fact that they only modify the extreme outer portion of a sapphire body due to the shallow range of heavy ion penetration. Solid solution strengthening methods, on the other hand, are based upon introducing substitutional alloys into sapphire during crystal growth by adding small quantities of an inert material during crystal growth. The impurities in the sapphire structure can impede dislocation movements and result in more stable structures, but the materials have still been found to exhibit poor strength or poor optical properties when the sapphire is placed in an elevated temperature environment.
Harris et al,
Mechanism of Mechanical Failure of Sapphire at High Temperature,
Proc. SPIE, Volume 2286 (1995) discloses that the compressive strength of sapphire (single crystal Al
2
O
3
) at 800° C. was reduced by more than 95% of its room temperature value. Harris et al, Mechanical Strength of Sapphire at Elevated Temperature, Proceedings of the 6th DoD Electromagnetic Windows Symposium, Huntsville, AL, Oct. 16-19, 1995 discloses that sapphire specimens had a compressive strength at 800° C. of only from 29 to 45 MPa and similar low compressive strength at 600° C. Others have reported similar relatively low compressive strengths at high temperature when varying the crystal growth method (edge-defines film-fed growth (EFG) vs. heat exchanger method (HEM)), surface finish (as-grown vs. polished), and test atmosphere (air vs. argon).
Dients et al,
J. Nucl. Mater.,
191-194 (Pt. A), 555-9 (1992) discloses that alumina, aluminum nitride, and silicon carbide exhibit a reduction in bending strength at 400-600° C. after neutron irradiation at 10
24
and 10
26
n/m
2
(10
20
and 10
22
nvt). Unless otherwise specified, “nvt” is used herein refers to a neutron flux in neutrons/cm
2
for neutrons ≧1 MeV. At 10
26
n/m
2
(10
22
nvt), the mean ultimate bending strength was reported to be 50-60% of the original strength of the material. The loss of strength after irradiation was always accompanied by a large decrease of the Weibull modulus. No considerable difference was found at a fluence of about 5×10
24
n/m
2
(5×10
20
nvt).
Pells,
Radiation Damage Effects in Alumina,
Journal of Nuclear Materials 191-194 (1992) 555-559 reports that the a- and c-axes of sapphire increase in length, for 14−meV neutron fluences on the order of 10
20
n/m
2
(10
16
nvt) at 325° K.
Heidinger et al,
The Impact of Neutron Irradiation on the Performance of Cryogenically Cooled Windows for Electron Cyclotron Resonance Heating,
Fusion Engineering and Design 18 (1991) 337-340 discloses dielectric loss tangent and thermal conductivity calculations for a limited set of data. The calculations are based on data of sapphire irradiated at 3.5×10
19
f.n. (fast neutrons)/cm
2
, (1.5-50.2)×10
17
f.n./cm
2
and (0.3-18)×10
19
f.n./cm
2
. Heidinger et al disclose sapphire disks used in gyrotrons and torous windows in devices used for electron resonance heating fusion plasmas.
It has now been unexpectedly discovered that a sapphire body having high strength at elevated temperature can be produced by controlled neutron irradiation processing to introduce point defects within the body.
It is an object of this invention to produce sapphire bodies having increased compressive strength at elevated temperature by irradiating sapphire with a limited amount of fast neutrons.
It is a further object of this invention to develop strengthened sapphire bodies that exhibit desirable transmission characteristics in the midwave region.
These and still further objects will be apparent from the following description of this invention.
SUMMARY OF THE INVENTION
The present invention is directed to a process of irradiating sapphire with fast neutron radiation to increase the strength of the sapphire and a material so produced. The process generally entails placing a sapphire body in or near a radiation source and exposing the body to radiation of an integrated fluence ranging from about 1×10
17
nvt to about 9×10
19
nvt.
Preferably the process entails encasing a sapphire body inside a radiation filter which absorbs or reflects low energy thermal neutrons, placing the encased body in a radiation reactor, passing radiation through the filter to the sapphire body, and controlling the extent of radiation to an integrated fluence ranging from about 1×10
17
nvt to about 9×10
19
nvt. Use of the filter allows the preparation of sapphire bodies which require substantially less storage prior to being safe for use.
The invention is further directed to a sapphire body produced by the method and the use of the body in a sapphire window assembly which is exposed to a temperature of at least about 500° C.
The invention is further directed to a sapphire body which, at temperatures of 400° C. and above, exhibits a c-axis compressive strength which is greater than both the c-axis and a-axis tensile strengths of prior sapphire bodies.
The invention is further directed to a sapphire body which at a temperature of 600° C. exhibits a c-axis compressive strength of at least 550 MPa.
REFERENCES:
patent: 5200370 (1993-04-01), Lennox et al.
patent: 5702654 (1997-12-01), Chen et al.
Aspen Systems Inc.
Jacobs Bruce F.
Nguyen Kiet T.
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