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. The invention relates also to a light filter using such glass-ceramics.
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.
For example, the glass-ceramics of a SiO2xe2x80x94Li2Oxe2x80x94MgOxe2x80x94P2O5 system disclosed in U.S. Pat. No. 5,626,935 containing lithium disilicate (Li2O.2SiO2) and xcex1-quartz (xcex1-SiO2) as main crystal phases and the size of the xcex1-quartz globular grains is controlled. No discussion, however, is made in this patent about optimum values of crystal grain diameter, degree of crystallization and mechanical strength including Young""s modulus and specific gravity which are suitable for use as a light filter.
Japanese Patent Application Laid-open Publication No. Hei 10-45426 discloses a SiO2xe2x80x94Li2Oxe2x80x94K2Oxe2x80x94MgOxe2x80x94ZnOxe2x80x94P2O5xe2x80x94Al2O3 system glass-ceramic or a SiO2xe2x80x94Li2Oxe2x80x94K2Oxe2x80x94MgOxe2x80x94ZnOxe2x80x94P2O5xe2x80x94Al2O3xe2x80x94ZrO2 system glass-ceramic suitable for laser texturing which contains, as its predominant crystal phase or phases, at least one of lithium disilicate (Li2O.2SiO2), a mixed crystal of lithium disilicate and xcex1-quartz (xcex1-SiO2), and a mixed crystal of lithium disilicate and xcex1-cristobalite (xcex1-SiO2). However, no discussion has been made in the prior art publication about optimum values of crystal grain diameter, degree of crystallization, and mechanical strength including Young""s modulus and specific gravity which are suitable for use as a light filter.
Japanese Patent Application Laid-open Publication No. Hei 9-35234 discloses a magnetic disk substrate made of a glass-ceramic of a SiO2xe2x80x94Al2O3xe2x80x94Li2O system having predominant crystal phases of lithium disilicate (Li2O.2SiO2) and xcex2-spodumene (Li2O.Al2O3.4SiO2). This glass-ceramic, however, contains, as its predominant crystal phase, xcex2-spodumene which has a negative thermal expansion characteristic which causes the substrate to have a negative thermal expansion characteristic and, in this glass-ceramic, growth of SiO2 crystals such as xcex1-quartz (xcex1-SiO2) and xcex1-cristobalite (xcex1-SiO2) which have a positive thermal expansion characteristic and thereby cause the substrate to have a positive thermal expansion characteristic is extremely restricted. It is difficult in this prior art glass-ceramic to obtain a coefficient of thermal expansion required in the present invention. Besides, since this glass-ceramic is so hard that excellent processability cannot be obtained. Further, since this glass-ceramic requires a high temperature of 820xc2x0 C. to 920xc2x0 C. for crystallization which prevents a large scale production of the product at a competitive cost. Furthermore, no discussion has been made in the publication as to crystal grain diameter, degree of crystallization and mechanical strength.
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 temperature for crystallization is reduced (680xc2x0 C.-770xc2x0 C.). A sufficient improvement however cannot be achieved in this substrate. Besides, crystals grown in all examples disclosed are xcex2-eucryptite (Li2O.Al2O3.2SiO2) which has a negative thermal expansion characteristic. Further, no discussion has been made in this publication as to crystal grain diameter, degree of crystallization and mechanical strength so that this glass-ceramic has the same disadvantages as the aforementioned prior art glass-ceramics.
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 multilayer 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.
Accumulated studies and experiments made by the inventors of the present invention for achieving the above described objects of the invention have resulted in the finding, which has led to the present invention, that a glass-ceramic which is obtained by heat treating within a limited range a SiO2xe2x80x94Li2Oxe2x80x94K2Oxe2x80x94MgOxe2x80x94ZnOxe2x80x94P2O5xe2x80x94Al2O3xe2x80x94ZrO2 system glass contains, as its predominant crystal phase or phases, lithium disilicate (Li2O.2SiO2) or a combination of lithium disilicate and at least one crystal phase selected from the group consisting of xcex1-quartz (xcex1-SiO2), xcex1-quartz solid solution (xcex1-SiO, solid solution), xcex1-cristobalite (xcex1-SiO2) and xcex1-cristobalite solid solution (xcex1-SiO2 solid solution), has crystal grains of the predominant crystal phase or phases having a specific range of grain diameter, and has specific mechanical properties including a coefficient of thermal expansion, Young""s modulus and specific gravity which are suitable for use as a light filter, more particularly, as a band-pass filter or a gain flattening filter.
For achieving the above described object of the invention, there is provided a glass-ceramic comprising, as a predominant crystal phase, lithium disilicate (Li2O.2SiO2) having a crystal grain diameter (average) of 0.05 xcexcm or below.
In another aspect of the invention, degree of crystallization of the lithium disilicate crystal phase is within a range from 3% to 20%.
In one aspect of the invention, the glass-ceramic has Young""s modulus (GPa)/specific gravity of 37 or over.
In another aspect of the invention, the glass-ceramic comprises further at least one crystal phase selected from the group consisting of xcex1-quartz (xcex1-SiO2), and xcex1-quartz solid solution (xcex1-SiO2 solid solution), degree of crystallization of the xcex1-quartz (xcex1-SiO2) and the xcex1-quartz solid solution (xcex1-SiO2 solid solution) being within a range from 5% to 25% and a crystal grain diameter thereof (average) being 0.10 xcexcm or below.
In another aspect of the invention, the glass-ceramic comprises further at least one crystal phase selected from the group consisting of xcex1-cristobalite (xcex1-SiO2) and xcex1-cristobalite solid solution (xcex1-SiO2 solid solution), degree of crystallization of the xcex1-cristobalite (xcex1-SiO2) and the xcex1-cristobalite solid solution (xcex1-SiO2 solid solution) being within a range from 2% to 10% and a crystal grain diameter thereof (average) being 0.10 xcexcm or below.
In another aspect of the invention, the glass-ceramic comprises, as predominant crystal phases, lithium disilicate (Li2O.2SiO2) and at least one crystal phase selected from the group consisting of xcex1-quartz (xcex1-SiO2), xcex1-quartz solid solution (xcex1-SiO2 solid solution), xcex1-cristobalite (xcex1-SiO2) and xcex1-cristobalite solid solution (xcex1-SiO2 solid solution).
In another aspect of the invention, crystal grains of the predominant crystal phase or phases are fine and substantially of a globular shape.
In another aspect of the invention, the glass-ceramic is substantially free of Na2O and PbO.
In another aspect of the invention, the glass-ceramic has a surface roughness Ra (arithmetic mean roughness) after polishing of 5.0 xc3x85 or below.
In another aspect of the invention, the glass-ceramic has a coefficient of thermal expansion within a range from +65xc3x9710xe2x88x927/xc2x0 C. to +130xc3x9710xe2x88x927/xc2x0 C. in a temperature range from xe2x88x9250xc2x0 C. to +70xc2x0 C.
In another aspect of the invention, the glass-ceramic substrate has bending strength of 400 MPa or over.
In another aspect of the invention, the glass-ceramic has a composition which comprises in weight percent:
In another aspect of the invention, the glass-ceramic is obtained by heat treating a base glass for nucleation under a temperature within a range from 500xc2x0 C. to 600xc2x0 C. for one to seven hours and further heat treating the glass for crystallization under a temperature within a range from 700xc2x0 C. to 780xc2x0 C. for one to twelve hours.
In another aspect of the invention, there is provided a glass-ceramic substrate for a light filter using the glass-ceramic described above.
In still another aspect of the invention, there is provided a light filter provided by forming a multi-layer film on the glass-ceramic described above.