Compositions: ceramic – Ceramic compositions – Devitrified glass-ceramics
Reexamination Certificate
2000-11-29
2002-12-10
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Devitrified glass-ceramics
C501S007000
Reexamination Certificate
active
06492288
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a glass ceramic and a temperature compensating member using the glass ceramic, which can be used in many fields, e.g., information communication field, energy related field, electronics field, and the like. In particular, in the optical communication field, they can be used as a part of a device including an optical fiber, e.g., an optical fiber grating, connector or the like, and have a negative coefficient of thermal expansion and can provide a temperature compensation to the device.
2. Description of the Related Art
Recently, an optical fiber is frequently used in the optical communication field and the like. The optical fiber related device is required not to be adversely affected by temperature, on the characteristic of the optical fiber itself.
For example, the connector for the optical fiber is used as an input/output terminal for an optical transmission device or an optical measurement apparatus, or a connector for connecting a plurality of optical cables to each other in an optical transmission path. Such a device having a purpose of fixing, connecting or protecting the optical fibers is required not to be adversely affected on the optical fibers, by the strain caused by expansion or shrinkage of the device according to a temperature change, for example, to have a structure of combination of materials having desired coefficients of thermal expansion, or the like.
Applications of fiber grating are being expanded in wavelength division multiplexing system, for example, as a device for performing dispersion compensation, wavelength stabilization for a semiconductor laser, or the like, which uses a narrow wavelength band selection property thereof. However, it is known that the fiber grating has a temperature dependency of the center wavelength of a reflected light because the effective refractive index of the core portion changes with temperature. Therefore, it is also required that the fiber grating reduces an adverse influence from temperature change as much as possible.
For various types of equipment, apparatus and the like, used not only in the optical fiber related field but also in the energy related field, the information related field, or the like, a material which can adjust the coefficient of thermal expansion of a device or a precision component included in the equipment, apparatus and the like, to an appropriate value, in order to prevent generation of strain or internal stress, caused by a temperature difference, and which is qualified for giving a good dimensional accuracy, a good dimensional stability, a high strength, a good thermal stability or the like, is required.
Conventionally, as materials suitable for various types of devices on the above-described point of temperature change, ceramics, glass ceramics, glass, metal and the like have been used because of having a high heat resistance, a small coefficient value of thermal expansion and the like.
However, these materials have a positive coefficient of thermal expansion, that is, a characteristics of expanding in volume as the temperature increases. Many of other materials used for the devices together with the materials having the positive coefficient have also a positive coefficient of thermal expansion. Therefore, these materials are not necessarily the optimum ones for preventing the adverse influence from the temperature change of the whole device. For this reason, a material having a negative coefficient of thermal expansion, to cancel the positive coefficient of thermal expansion, that is, characteristics of shrinking in volume as the temperature increases, is desired as a material which opposes temperature changes.
As a material having a negative coefficient of thermal expansion, inorganics, e.g., &bgr;-eucryptite crystal, Li
2
O—Al
2
O
3
—SiO
2
system ceramics including the &bgr;-eucryptite crystal, Li
2
O—Al
2
O
3
—SiO
2
system glass ceramics, ZnO—Al
2
O
3
—SiO
2
system glass ceramics, lead titanate, hafnium titanate, zirconium tungstate, tantalum tungstate, and the like are known.
U.S. Pat. No. 6,087,280 discloses athermal optical device and a method for producing the device. The device comprises a negative thermal expansion substrate and an optical fiber mounted on the substrate surface and having a grating. In the optical fiber reflective grating device, although the change of the grating center wavelength is approximately 1.9 nm when not attached to the substrate, it comes to be only 0.2 nm when attached to the substrate. The negative thermal expansion substrate comprises a glass-ceramic having &bgr;-eucryptite crystal, wherein the substrate has a coefficient of thermal expansion in the range from −20×10
−7
/° C. to −100×10
−7
/° C. in the temperature range of −40° C. to 85° C. and the hysteresis of the coefficient of thermal expansion is restrained to not more than 20 ppm.
However, the glass-ceramic used in the technique includes many microcracks because of its negative coefficient of thermal expansion and the crystal size to form the microcracks has a diameter larger than 5 &mgr;m. There are problems that such a material cannot obtain an enough mechanical strength and impregnates chemical agents or the like easily during a treatment. Impregnation of a chemical agent having a positive coefficient of thermal expansion cancels the inherent negative thermal expansion property of the ceramic. As a result, it is not possible to obtain a desired coefficient of thermal expansion at all.
The crystal phase thereof includes Al
2
TiO
5
. It is generally known that because Al
2
TiO
5
has a large anisotropic expansion, the thermal expansion is highly anisotropic for a sintered body and therefore the results of repeated measurements do not coincide with one another in general, and that it is difficult to make the strength large because of the existence of microcracks. There is also another problem that such a material makes the production costs large because a heat-treatment for crystallizing the glass at a temperature of not less than 1,300° C. for 3 or more hours is required, in order to obtain an enough negative coefficient of thermal expansion.
U.S. Pat. No. 4,209,229 discloses a glass ceramic comprising a main crystal phase of &bgr;-eucryptite or &bgr;-quartz solid solution, and the reference says that the material is particularly useful for a protective outer layer for the molten silica or useful for a cladding layer for another optical fiber waveguide member.
However, because the glass ceramic includes much TiO
2
to make the crystals fine and therefore lacks for stability of glass, only a thin material thereof can be obtained. Further, there are also problems that a very high temperature, preferably, 1,000° C. to 1,300° C., is required for crystallizing the glass and that the negative coefficient of thermal expansion of the glass ceramic is not enough, i.e., about −2×10
−7
/° C.
U.S. Pat. No. 4,507,392 discloses a transparent glass ceramic material containing &bgr;-quartz solid solution as the predominant crystal phase especially suitable for application as a decorative glaze to glass, glass ceramic, and ceramic bodies.
However, because the glass ceramic includes a large quantity of nucleation agents, it is difficult to obtain a material having a large negative coefficient of thermal expansion. The minimum negative coefficient of thermal expansion of the material obtained by the technique of the reference is only −29.4×10
−7
/° C. which is not enough.
Japanese Patent Application Publication (Laid-open) No. Tokukai-sho 63-201034 discloses a method for producing a crystallized glass (glass ceramic) having a negative coefficient of thermal expansion. The method comprises the steps of: mixing volcanic vitreous sediment powder with Al
2
O
3
powder and Li
2
O powder, heating to melt the mixture, thereafter performing a treatment for removing strain, reheating the performed one at a temperature in a specific range for 12-24 hours,
Group Karl
Kabushiki Kaisha Ohara
Oliff & Berridg,e PLC
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