Stock material or miscellaneous articles – Composite – Of quartz or glass
Reexamination Certificate
2000-06-14
2003-06-24
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of quartz or glass
C501S004000, C501S068000, C501S069000, C501S072000
Reexamination Certificate
active
06582826
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to novel glass-ceramics and, more particularly, to glass-ceramics suitable for use as a light filter and, more particularly, to glass-ceramics suitable for use as a band-pass filter and a gain flattening filter.
There are light filters which cut or pass light of a specific wavelength and there are also light filters which reduce intensity of light without depending upon wavelength. The former includes a band-pass filter which passes only a specific wavelength, a notch pass filter which cuts a specific wavelength and high-pass and low-pass filters which pass only wavelengths shorter or longer than a specific wavelength. The latter includes an ND filter.
Light filters can be classified also into an absorption type filter and an interference type filter. A representative absorption type filter is the ND filter and a representative interference type filter is the band-pass filter. A substrate made of plastic is used for absorption type filters such as those for photography. Since a substrate for light filters which are subject to a strong laser beam requires durability and heat resistance property, amorphous glass is exclusively employed for such substrate.
The band-pass filters are made by forming, on a substrate made of, e.g., glass, a multi-layer film of dielectric by alternately laminating an H layer of a dielectric thin film having a high refractive index and an L layer of a dielectric thin film having a low refractive index.
In a band-pass filter which is used for the WDM(wavelength division multiplexing) optical communication system, temperature stability of the center wavelength of the band poses a problem when a narrow band width for passing wavelengths is set for applying the band-pass filter to a wavelength of a higher density. More specifically, the band-pass filter is a sensitive element in which the center wavelength of the band varies even with a slight variation in temperature and, therefore, temperature compensation should be made by a temperature controller when the band-pass filter is used. Such temperature controller, however, cannot actually be employed because of limitation in the space where the band-pass filter is located. The temperature stability has become a matter of increasing importance since it is necessary to reduce the band width as the amount of light information increases.
In the past, amorphous glass has been used as a substrate for the band-pass filter. This prior art substrate is not sufficient in its compressive stress to the film and its durability since its thermal expansion property and mechanical strength are not sufficiently high. Further, amorphous glass has low mechanical strength and therefore tends to produce micro-cracks in processing with resulting cracking or chipping off of corner portions of the product which reduces the yield of the product. Moreover, in amorphous glass, a relatively large amount of alkali ingredient must be added if a high thermal expansion property is to be provided and this poses a problem of elution of alkali ingredient during and after forming of the dielectric film on the substrate. Thus, amorphous glass cannot sufficiently satisfy the demands for a substrate for a light filter, particularly a substrate for a band-pass filter.
Known in the art are some glass-ceramics. For example, the glass-ceramics of a SiO
2
—Li
2
O—MgO—P
2
O
5
system disclosed in U.S. Pat. No. 5,626,935 containing lithium disilicate (Li
2
O.2SiO
2
) and &agr;-quartz (&agr;-SiO
2
) as main crystal phases is an excellent material as a material textured over the entire surface in which, by controlling the grain diameter of globular crystal grains of &agr;-quartz, the conventional mechanical texturing or chemical texturing can be omitted and the surface roughness after polishing (Ra) can be controlled within a range from 15 Å to 50 Å. In this glass-ceramic, however, no discussion or suggestion is made about Young's modulus and a coefficient for thermal expansion which are important features of the present invention.
Japanese Patent Application Laid-open Publication No. Hei 9-35234 discloses a magnetic disk substrate made of a glass-ceramic of a SiO
2
—Al
2
O
3
—Li
2
O system having predominant crystal phases of lithium disilicate (Li
2
O.2SiO
2
) and &bgr;-spodumene (Li
2
O.Al
2
O
3
.4SiO
2
) which has a negative coefficient of thermal expansion. This glass-ceramic has a composition which contains a relatively large amount of Al
2
O
3
ingredient and in which growth of SiO
2
crystals such as &agr;-quartz (&agr;-SiO
2
) is extremely restricted and, therefore, it is difficult in this glass-ceramic to obtain a coefficient of thermal expansion required in the present invention and, moreover, since the glass-ceramic is so hard that it has no good processability. Further, since this glass-ceramic requires a high temperature of 820° C. to 920° C. for crystallization which prevents a large scale production of the product at a competitive cost.
International Publication WO97/01164 which includes the above described Japanese Patent Application Laid-open Publication No. Hei 9-35234 discloses a glass-ceramic for a magnetic disk in which the lower limit of the Al
2
O
3
ingredient is lowered and temperature for crystallization is reduced (680° C.-770° C.). A sufficient improvement however cannot be achieved by merely lowering the lower limit of the Al
2
O
3
ingredient. Besides, crystals grown in all examples disclosed are &bgr;-eucriptite (Li
2
O.Al
2
O
3
.2SiO
2
) which has a negative coefficient of thermal expansion and, therefore, has the same disadvantage as the above described prior art glass-ceramic.
It is, therefore, an object of the invention to provide a material suitable for a substrate for a light filter which has eliminated the above described disadvantages of the prior art substrate and has a thermal expansion property which is sufficient for avoiding variation in the refractive index at a temperature at which a filter formed with a mono-layer or multi-layer film is used (i.e., having a high coefficient of thermal expansion and thereby imparting compressive stress to the film to improve temperature stability of the refractive index of the film) and also has a mechanical property which imparts sufficient durability to the filter and further has excellent light transmittance.
SUMMARY OF THE INVENTION
Accumulated studies and experiments made by the inventors of the present invention for achieving the above described object of the invention have resulted in the finding, which has led to the present invention, that, glass-ceramics having, as their predominant crystal phases, lithium disilicate (Li
2
O.2Si
O
2
) and &agr;-quartz (&agr;-SiO
2
) or &agr;-quartz solid solution (&agr;SiO
2
solid solution) and having Young's modulus (GPa) of 95 to 120 have an excellent processability and is suitable for use as a substrate for a light filter and, more particularly, as a substrate for a band-pass filter or a gain flattening filter.
For achieving the object of the invention, there is provided a glass-ceramic having Young's modulus (GPa) within a range from 95 to 120 and comprising 5.3 to less than 10 weight percent (expressed on oxide basis) of Al
2
O
3
.
In one aspect of the invention, the glass-ceramic has specific gravity within a range from 2.4 to 2.6.
In another aspect of the invention, the glass-ceramic has a coefficient of thermal expansion which is within a range from 65×10
−7
/° C. to 130×10
−7
/° C. within a temperature range from −50° C. to +
70° C.
In another aspect of the invention, predominant crystal phases the glass-ceramic are (a) lithium disilicate (Li
2
O.2SiO
2
) and (b) at least one of &agr;-quartz (&agr;-SiO
2
). and &agr;-quartz solid solution (&agr;-SiO
2
solid solution).
In another aspect of the invention, the glass-ceramic is substantially free of Na
2
O and PbO.
In another aspect of the invention, the glass-ceramic comprises 0.3 weight percent or over (expressed on the basis of composition of the oxide) of Mg
Goto Naoyuki
Ishioka Junko
Kawashima Yasuyuki
Hedman & Costigan ,P.C.
Jones Deborah
Kabushiki Kaisha Ohara
McNeil Jennifer
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