WO2005009916A1

WO2005009916A1 - Negative thermal expansion glass ceramic

Info

Publication number: WO2005009916A1
Application number: PCT/JP2004/011003
Authority: WO
Inventors: Takayuki Kishi
Original assignee: Kabushiki Kaisha Ohara
Priority date: 2003-07-28
Filing date: 2004-07-26
Publication date: 2005-02-03

Abstract

A glass ceramic having an average line coefficient of more than -25x10-7 to -15x10-7 K-1 in a temperature range of -40°C to 80°C and a spectral transmittance of 85% or more in a wavelength range of 1000 to 1700nm, wherein a predominant crystal phase is one or more crystals selected from β-eucryptite (β-Li2O-Al2O3-2SiO2), β-eucryptite solid solution (β-Li2O-Al2O3-2SiO2 solid solution), β-quartz (β-SiO2), β-quartz solid solution (β-SiO2 solid solution).

Description

DESCRIPTION

NEGATIVE THERMAL EXPANSION GLASS CERAMIC

Technical Field The present invention is related to a crystallized glass having negative average linear thermal expansion coefficient and high transmittance, which is applicable in various fields such as an information communication field, energy related field, electronics field and especially an optical element in an optical communication field, and one of members constituting a multiplexer/demultiplexer.

Background Art Recently, various types of devices for transmitting a large amount of signals in one fiber line simultaneously and quickly are studied and developed in an optical communication field. Among them, wavelength division multiplexing type has become diversified. In the wavelength division multiplexing, wavelengths of adjacent channels are very close in order to enhance a density of sent and received information, i.e. signal density. Accordingly, when a wavelength of a desired signal is shifted due to an environmental condition, the shifted wavelength can not be distinguished with an adjacent wavelength. As a result, the shifted signal thereof can be an error signal and it becomes problematic that signals can not be send and received accurately easily. In view of them, various researches have been developed for sending and receiving signals accurately. When a signal is multiplexing and demultiplexing by a multiplexer/demultiplexer of wavelength division multiplexing type, a wavelength shift occurs according to an environmental temperature because of temperature dependency of the multiplexer/demultiplexer. Accordingly, a temperature compensating member for sending and receiving a signals accurately and a filter which does not show a wavelength shift due to an environmental temperature are required. Furthermore, for various equipments and apparatuses used not only in fields relating to an optical fiber but also in fields relating to energy and information, required is a material being capable of regulating a average linear thermal expansion coefficient of the devices and precision components constituting thereof to a proper value and having a desired dimensional accuracy, dimensional stability, strength, thermal stability and the like. In the earlier development, from a viewpoint of high heat resistance, ceramics, glass ceramics, glasses, metals and the like are used as a material for the various devices because they are suitable in a point of temperature variability. However, these materials have large average linear thermal expansion coefficient. That is, they have a property that they expand with temperature rising. Further, many other materials used in the device together with these materials also have a positive average linear thermal expansion coefficient. Therefore, these materials are far from optimum materials for preventing the thermal effect when considering a whole device. Glass ceramics, ceramics, polymer and the like in earlier development having negative coefficient of expansion easily have a problem of low reliability according to low strength and an environmental condition, since they have pores characteristic to ceramic materials. Thus, these materials have not been unmanufactured. Therefore, a material sustainable to temperature changes is required, where the material has a negative line coefficient of expansion which can negate a large positive average linear thermal expansion coefficient of other materials, and is also sustainable to environmental changes. In earlier development, a solid etalon filter has been used for multiplexing and demultiplexing. Presently a Peltier element is provided to the filter since it requires temperature control. However, a technology where a temperature controller is not required has been researched in order to achieve low cost and weight saving. A filter material having small variability in optical path due to temperature, i.e. having the value represented by the following formula (1) of close to 0, is very useful as a material for a solid etalon filter. nα+dn/dt » 0 ^{• • •} (D where n represents a refractive index in applied wavelength, α represents an average linear thermal expansion coefficient, and dn/dt represents rate of change of the refractive index according to a temperature. However, a material which fulfill the equation of formula (1) = 0 has not found and the researches are ongoing in various fields. An application of a material having a negative average linear thermal expansion coefficient may enable to achieve that the value of the above formula is close to 0. Furthermore, such material enables to design a material in which the value of the above formula is negative. Further, it becomes possible to manufacture a substrate material having a value of the formula (1) of almost 0, when a substrate designed to have negative value of the formula (1) and a substrate having positive value of the formula (1) are adhered by optical contact or with an adhesive having similar refractive index and the like, in which the thickness of the two sheets are calculated and designed to be an optimum value. JP Tokukai 2000-313654A discloses a temperature compensating member consisting of sintered substance of ceramic having negative average linear thermal expansion coefficient thereof is -30 to -85 (10^"7-K^_1), where crystal powder, glass powder and the like are sintered and many micro cracks are generated in grain boundary of the crystal US patent 4507392B discloses a negative thermal expansion transparent glass ceramic. JP Tokukaihei 2-208256A discloses a ZnO-Al₂0₃-Si0₂ system low thermal expansive ceramic in which predominant crystal phase thereof is β-quartz solid solution and/or zinc petalite solid solution. In US patent 5694503B proposes a package in which an optical fiber having refractive index grating is attached to a supporting member having negative average linear thermal expansion coefficient. As a negative thermal expansion material, a composition based on zirconium- tungstate (for example, ZrW₂0s) or hafnium-tungstate (for example, HfW₂0e) is used. For example, and a material having extremely large negative average linear thermal expansion coefficient is obtained by using ZrW₂0s crystal having average linear thermal expansion coefficient of -47 to -94 (10^-7,K) and regulating a manufacturing condition. It is explained that a wavelength variation due to temperature variation can be eliminated by fixing an optical fiber onto the support member of the above material with loading proper stress. W097/14983A and JP Tokukaihei 10-90555A disclose a liquid crystal polymer which is one of other materials having negative average linear thermal expansion coefficient .

Disclosure of Invention Generally, known as materials having negative average linear thermal expansion coefficient are inorganic materials such as β-eucryptite, β-eucryptite solid solution, β-quartz, and β-quartz solid solution and Li₂0-Al₂0₃-Si0₂ system ceramics containing two or more crystals selected from the above, and Zn0-Al₂0₃-Si0₂ system ceramics, and titanate, hafnium titanate, zirconium tangstate and tantalum tangstate. The general sintered ceramic consisting of the above crystals is porous compared with a glass ceramic. When a substance having low viscosity and positive average linear thermal expansion coefficient is impregnated into this ceramic material, the expansion property of the material is changed from that of the sintered ceramics itself due to expansion of the impregnated materials caused by temperature variation and humidity variation. Further, the expansion property of the material changes over time since the impregnated material changes over time. Thus, the material easily has problems in a point of long-term stability. On the other hand, the sintered ceramic disclosed in JP Tokukai 2000-313654A does not have optical transparency since it has comparatively large grain size. Further, it has many micro cracks. When grinding fluid is penetrated or an adhesive and the like are impregnated to it, the average linear thermal expansion coefficient thereof is largely changed. Thus, it is difficult to put the ceramic in practical use. The glass ceramic disclosed in US patent 4507392B contains a large amount of Ti0₂ and Zr0₂ as a crystal nucleation agent. When a large amount of Ti0₂ and Zr0₂ as a nucleation agent are contained, a original glass requires high temperature to be melt, and it is impossible to obtain a homogeneous glass. Further, devitrification easily occur in a course of a glass molding. Thus, it has problems in a point of productivity and practical use. The ceramic disclosed in JP Tokukaihei 2-208256A contains a large amount of ZnO component which easily sublimes in high temperature. Therefore it also discloses that long-term melting in a formation of a original glass (parent glass) thereof is not preferable. In the embodiment, the melting time thereof is 10 minutes and it is extremely short. However, Si0₂ and A1₂0₃ components do not melt sufficiently with such a short time, even if the temperature is high. Therefore, it is impossible to obtain a homogeneous original glass. As a result, it is impossible to obtain a homogeneous ceramic by crystallizing such original glass with uneven quality. If the glass is melt for hours as same as a general procedure, an unmelted residue are not left. However, ZnO component sublimes and a composition of the original glass is changed. Thus, it is impossible to obtain a homogeneous ceramic stably. Further, the melting temperature in the embodiment is as high as 1620°C and it makes a high production cost. The negative thermal expansion material used in the package disclosed in US patent 5694503 is not suitable for mass production, since the procedure thereof includes troublesome steps of adding powdered materials having positive average linear thermal expansion coefficient such as A1₂0₃, Si0₂, Zr0₂, MgO, CaO, Y₂0₃ and the like in order to regulate the average linear thermal expansion coefficient and sintering the mixture thereof. Further commensurate skill and equipments are required, and it is difficult to obtain a homogeneous material, since variant materials is needed to be mixed. Additionally, ZrW₂Os and HfW₂Os have a phase transition point at around 157°C, the average linear thermal expansion curvature has an inflection. Thus it can not be said that the material is thermally stable in a wide range of temperature. The liquid crystal polymers disclosed in W097/14983A and JP Tokukaihei 10-90555A shows fine orientation of the crystal, and injection molded product thereof has a problem of curvature. When the material has extremely large negative average linear thermal expansion coefficient in an orientation direction of about -100 (10^~7-K^~1), it has a defect that average linear thermal expansion coefficient thereof in a direction other than the orientation shows large positive value. Further, material properties thereof such as bending strength and elastic modulus are variable due to a direction. Thus it is difficult to put the material in practical use as a certain device. As described above, the materials having negative average linear thermal expansion coefficient in earlier development have several problems. Therefore, as the case now stands, they are not applied well to various fields such as optical communication field, energy related field, information field and others. In view of the above situation, the object of the present invention is to provide a glass ceramic having desired negative average linear thermal expansion coefficient and high optical transmittance at temperature range of -40 to 80 °C, which is a general environmental temperature range in an optical communication field, energy related field, information field and the like, where grinding fluid, adhesive and the like are not impregnated into the glass ceramics because they are highly compact, that is, they do not have pores, voids and cracks, and the glass ceramics can be obtained in a low cost and manufactured stably in a point of composition and material property. The present inventors have performed various researches and tests in order to achieve the above- described object, and finally have found a Li₂0-Al₂0₃-Si0₂ system glass ceramics having a composition in certain range, which are heat-treated to deposit micro crystalline particles therein so that the glass ceramics have high transmittance and do not show anisotropy. Thus, the present invention is accomplished. According to the first aspect of the present invention, a glass ceramic having an average linear thermal expansion coefficient of more than -25xl0^~7 to -15xl0^~7 K^"1 in a temperature range of -40°C to 80°C and a spectral transmittance of 85% or more in a wavelength range of 1000 to 1700nm, wherein a predominant crystal phase is one or more crystals selected from β-eucryptite (β- Li₂0 •A1₂0₃- 2Si0₂) , β-eucryptite solid solution (β- Li₂0-Al₂0₃-2Si0₂ solid solution), β-quartz (β-Si0₂) , β- quartz solid solution (β-Si0₂ solid solution) , . The glass ceramics may comprise by mass % Si0₂ 40 to 59% and/or A1₂0₃ 10 to 30% and/or Li₂0 1 to 5.4%, and wherein the glass ceramic fulfills a formula of Si0₂/ (Al₂0₃+Li₂0) >1.5. The glass ceramic may comprise 12.0% or less of Li₂0 by mol%. The glass ceramic may comprise by mass % ZnO 3 to 10% and/or BaO+SrO 0.5 to 6% and/or Ti0₂+Zr0₂ 1.0 to 5.0%. The glass ceramic may comprise by mass % B₂0₃ 0 to 5% and/or BaO 0 to 4% and/or SrO 0 to 4% and/or CaO 0 to 2% and/or Zr0₂ 0.5 to 3% and/or Ti0₂ 0.5 to 3% and/or Hf0₂ 0 to 3% and/or As₂0₃+Sb₂0₃ 0 to 2%. The glass ceramic may comprise less than 4% of P₂0s by mass %. The glass ceramic may be substantially free from MgO. The glass ceramic may be substantially free from PbO, Na₂0 and K₂0. The glass ceramic may have a Young's modulus of 60GPa or more. The glass ceramic may have a hysteresis of thermal expansion curve of 15ppm or less. The crystal phase of the glass ceramic may be free from Al₂Tiθ5 crystal. The glass ceramics may be obtained by melting, forming and annealing an original glass, performing the first thermal treatment at 650 to 750°C for 0.5 to 50 hours, subsequently performing the second thermal treatment at 700 to 850°C for 0.5 to 100 hours.

Brief Description of Drawings Fig. 1 is a TEM photograph of the example 1 according to the present invention. Fig. 2 is a TEM photograph of the example 3 according to the present invention. Fig. 3 is a TEM photograph of the example 4 according to the present invention.

Best Mode for Carrying Out the Invention Hereinafter a negative thermal expansive glass ceramic of the present invention will be explained in detail. In the present invention, a glass ceramic designates a material where a glass is heat-treated to deposit a crystal phase in a glass phase, and it includes not only a material consisting of a glass phase and a crystal phase but also a material where a glass phase is completely phase-transferred to a crystal phase, i.e. proportion of a crystal in the material (crystallinity) is 100 mass%.

Compositions of every component are described by mass%, unless otherwise stated. In this specification, "be substantially free" designates at least "can contain a certain amount within a range where a feature of the invention is not changed in nature". However, when stated as such, it preferably designates "can contain an amount considerable as impurities, but should not contain an amount which is assumed to be intentionally". In this specification, a predominant crystal phase designates all crystal phases whose deposited ratio is comparatively large. That is, in X-ray chart (where the longitudinal axis is X-ray diffraction intensity and a horizontal axis is an angle of diffraction) of X-ray diffraction, when a main peak of a certain deposited crystal phase has a ratio of X-ray diffraction intensity (hereinafter, also referred to as X-ray intensity ratio) of 30 or more, where a main peak of a crystal phase which has deposited the most (the highest peak) is rendered 100, such crystal phase is totally included in a predominant crystal phase. Here, Crystals other than a predominant crystal phase preferably have X-ray intensity ratio of less than 20, more preferably less than 10, and the most preferably less than 5. In this specification, hysteresis of a thermal expansion curve designates the largest difference of ΔL/L value between ΔL/L curve in temperature rising and that in temperature falling (i.e. the largest one among differences of thermal expansion rate between temperature rising and temperature falling at individual temperatures) , where ΔL/L curve is drawn by measuring average linear expnasion coefficient in courses of low to high temperature and high to low temperature. A predominant crystal phase of a negative thermal expansive glass ceramic according to the present invention consists of at least one or more crystals selected from β- eucryptite (β-Li₂0 •A1₂0₃- 2Si0₂) , β-eucryptite solid solution (β-Li₂0-Al₂0₃-2Si0₂ solid solution), β-quartz (β-Si0₂) , β- quartz solid solution (β-Si0₂ solid solution) . It is to be noted here that a solid solution designates one where a part of a crystal of β-eucryptite or β-quartz is substituted with a element which does not constitute the crystal thereof, or an atom is penetrated between the crystals . The predominant crystal phase is an important factor which contributes to negative average linear thermal expansion coefficient of a glass ceramic according to the present invention. A original glass having a certain composition is heat-treated in a predetermined condition so that the above predominant crystal phase having negative average linear thermal expansion coefficient is deposited in a glass phase having positive average linear thermal expansion coefficient, or that all the glass phase is phase-transferred to crystal phase including the above predominant crystal phase. Thus, it becomes possible to control average linear thermal expansion coefficient of the total glass ceramic to a negative value within a desired range. In the present invention, composition, selection of type of deposited crystal phase, deposition rate of a deposited crystal phase, i.e. crystallinity, and grain size of deposited crystals and the like of the glass ceramic are optimized. As a result, it becomes possible to obtain a glass ceramic having desired average linear thermal expansion coefficient which is lower than that in earlier development, by using deposited crystals by which a material having only large negative average linear thermal expansion coefficient is obtained in earlier development. It is to be noted that the type of a predominant crystal phase and crystallinity based on a whole glass ceramic are determined by contents of Li₂0, A1₂0₃ and Si0₂ within a specific range of composition, and temperatures of all heat treatments for crystallization which is to be described. In the present invention, it is important that a glass ceramics has negative average linear thermal expansion coefficient within a specific range. In order to achieve this, it is preferable for the glass ceramic not to contain a crystal phase having positive average linear thermal expansion coefficient, i.e. lithium disilicate, lithium silicate, α-quartz, α-cristobalite, α-tridymite, petalites such as Zn-petalite, Al₂TiOs, β-supodumene, cordierite, spinel system crystals such as gahnite, wollastonite, forsterite, diopside, nepheline, clinoenstatite, anorthite, celsian, gehlenite, feldspar, willemite, mullite, corundum, rankinite, larnite, and the solid solutions thereof. In addition to the above, it is also preferable for the glass ceramic not to contain tungstates such as Hf tungstate and Zr tungstate, titanates such as aluminum titanate, barium titanate, manganese titanate and lead titanate, and mullite, dibarium trisilicate, Al₂0₃-5Si0₂ and the solid solutions thereof. As for the average linear thermal expansion coefficient, a main object is to make the formula (1) being approximately 0 in the above-described optical devices. In order to achieve this, it is required that variation of a light path caused by a material having positive average linear thermal expansion coefficient is reduced. Accordingly, it is required that a material having negative average linear thermal expansion coefficient will be combined for temperature compensating. Furthermore, average linear thermal expansion^' coefficient is depended on various factors such as type of a deposited crystals, crystallinity, grain size, average linear thermal expansion coefficient of a glass matrix part and the like. These factors also closely affect mechanical strength (for example, Young's modulus, modulus of rigidity, hardness and the like) . As for the glass ceramic used as a member of an optical device, not only property to temperature compensating but also mechanical strength is important in a point of durability. In a manufacture of devices, hardness, which is a factor of workability, is also important. As a result of various investigations for optimization in order to fulfill the above in a balanced manner, it has been found that a glass ceramic of the present invention suitably has negative average coefficient of expansion of more than -25xl0^~7 to -15xl0^"7 K^"1. It is more preferable that the upper limit is -16xl0^~7 K^"1 and/or the lower limit is -24xl0^~7 K^"1, and the most preferable that the upper limit is -17xl0^~7 K^-1 and/or the lower limit is -23X10^"7 K^"1. In the present invention, transmittance or light transmittance designates spectral transmittance of a sample having 10mm thickness at light wavelength of 1000 to 1700nm. In an optical communication field, the higher a material has transmittance at wavelength range generally used, the lower a loss is, when it is used as a filter material. As a result of various investigations for transmittance, it has been found that a material having transmittance of 85% or more is easily applicable to a filter material.

Furthermore, the transmittance of 88% or more is more preferable, and that of 90% or more is the most preferable. As for the hysteresis (ΔL/L) of thermal expansion curve, since the glass ceramic of the present invention is designed to provide a temperature compensating effect to an optical communication device by being combined with a positive expansive material. When a material has a large hysteresis of thermal expansion curve, a temperature compensating effect is decreased. Thus, it is preferable that a hysteresis of thermal expansion curve is low, and the value is preferably 15 ppm or less. Furthermore, that of 10 ppm or less is more preferable, and that of 8 ppm or less is the most preferable. As for Young's modulus, when a material does not have a certain value or more, durability of a device where the material is used is decreased. Thus, it is preferable that the material has a Young's modulus as high as possible in a point of an intended purpose for a optical communication device. However, when it is too high, hardness of the glass ceramic also becomes too high, so that workability thereof is easily decreased so much. Further, a glass ceramic having too high Young's modulus easily has a tendency that the original glass thereof has poor meltability, which becomes an impediment to obtain a homogeneous glass ceramic. Taking the above into account, the lower limit of Young's modulus is preferably 60 GPa, more preferably 70 GPa, and particularly preferably 80 GPa, and the upper limit thereof is preferably 140 GPa, more preferably 125 GPa, and particularly preferably 110 GPa. Rigidity is positively proportional to Young's modulus. As described above, it is preferable that the material has rigidity as high as possible for a member of a device in optical communication. However, when it is intended to obtain a glass ceramic having favorable workability and homogeneous quality easily, there exists a preferable range of rigidity as same as Young's modulus.

As for the range, the lower limit thereof is preferably 20 GPa, more preferably 25 GPa and particularly preferably 30 GPa, and the upper limit thereof is preferably 50 GPa, more preferably 45 GPa, and particularly preferably 40 GPa. Si0₂ component is a main component of the above- described main crystal having negative average linear thermal expansion coefficient. When the content is less than 40%, the desired predominant crystal phase is not deposited sufficiently, and when the content is more than 59%, melting and clarity of a glass become difficult and a crystal phase other than a desired predominant crystal phase easily deposits. Thus, as for the preferable range of Si0₂ content, the lower limit thereof is preferably 40 %, more preferably 45 % and particularly preferably 50 %, and the upper limit is preferably 59%, more preferably 58.9%, and particularly preferably 58.8%. A1₂0₃ component is a important component constituting β-eucryptite and the solid solution thereof, and β-quartz solid solution where Al component is substituted and/or impregnated, which are included in the above described main crystal. When the content is less than 10 %, a homogeneous quality of an original glass is deteriorated, desired amount of a predominant crystal phase is not deposited, and chemical durability is also deteriorated. When the content is more than 30 %, a melting point of an original glass is too high and melting and clarification of the glass becomes difficult. Thus, as for the preferable range of A1₂0₃ component, the lower limit is preferably 10 %, more preferably 13 % and particularly preferably 15 %, and the upper limit is preferably 30 %, more preferably 28.5 % and particularly preferably 27 %. Li₂0 component is an important component constituting β-eucryptite and the solid solution thereof, and β-quartz solid solution where Li component is substituted and/or impregnated, which are included in the above described main crystal. When the content is less than 1%, it is difficult to melt an original glass and obtain required amount of desired predominant crystal phase. When the content is more than 5.4%, vitrification is difficult so that strength and transmittance of a glass ceramic after heat treatment decrease. Thus, a desired range of Li₂0 component is 1 to 5 %. It is more preferable that the lower limit is 2 %, and the most preferably it is 3 %. It is to be noted that the content of Li₂0 component in multicomponent system becomes extremely large proportion when it is denoted in terms of the molar ratio thereof, since molecular weight of Li₂0 component is small. Therefore, it is preferable that the molar ratio thereof based on a whole glass is calculated. When Li₂0 content is more than 12 mol%, a platinum crucible is damaged and desired thermal expansion coefficient can not be obtained. Thus, Li₂0 component is preferably 12 mol% or less, 11.5 mol% or less is more preferable, and 11 mol% or less is the most preferable. When Si0₂/ (Al₂0₃+Li₂0) is less than 1.5, fracture, crack of a glass ceramic and deterioration of transmittance become problematic due to extraordinary crystal growth. Thus Si0₂/ (Al₂0₃+Li₂0) is desirably 1.5 or more, and more preferably 1.6 or more and the most preferably 1.7 or more. ZnO component is an important component constituting β-eucryptite solid solution (β-Li₂0 •A1₂0₃- 2Si0₂ solid solution) and β-quartz solid solution (β-Si0₂ solid solution) , where Zn component is substituted and/or impregnated. When the content is less than 3 %, it is difficult to obtain desired average linear thermal expansion coefficient after crystallization. When the content is more than 10 %, possibility to generate devitrification is high, so that productivity is deteriorated. Thus, as for the range of ZnO component content, the lower limit is preferably 3 %, more preferably 3.5 % and the most preferably 4 %, and the upper limit is 10 %, more preferably 8 % and the most preferably 7 %. B₂0₃ component can be added optionally to a glass ceramic for improving a meltability of a original glass thereof. When B₂0₃ component is added, it constitutes a glass phase part of a negative thermal expansive glass ceramic of the present invention. When the content is more than 5 %, it causes a trouble in generation of a desired predominant crystal phase, and property to heat resistance of a glass ceramics is deteriorated. Thus, the content is preferably 3.5 % or less, more preferably 2 % or less. Further it is the most preferable that a glass ceramic is substantially free from B₂0₃ component. BaO component can be optionally added to a glass ceramic, where it constitutes β-eucryptite solid solution (β-Li₂OΑl₂0₃-2Siθ2 solid solution) and β-quartz solid solution (β-Si0₂ solid solution) in which Ba component is substituted and/or impregnated. BaO component is effective in preventing other metal elements in an original glass from being alloyed with platinum of a crucible in a course of melting a original glass, and maintaining resistance to devitrification property of a original glass. Thus, it is preferable for a glass ceramic to contain BaO component depending on a composition thereof. When the above effect is required, the content is preferably 0.5 % or more. However, when it is more than 4 %, average linear thermal expansion coefficient of a glass is too high. As a result, average linear thermal expansion coefficient of a glass ceramic easily becomes large, so that it becomes difficult to obtain desired average linear thermal expansion coefficient. Thus, the upper limit is more preferably 2.5 % and the most preferably 2.0 %. SrO component can be optionally added to a glass ceramic, where it constitutes β-eucryptite solid solution (β-Li₂0-Al₂0₃-2Si0₂ solid solution) and β-quartz solid solution (β-Si0₂ solid solution) in which Sr component is substituted and/or impregnated. SrO component is effective in making a hysteresis of average linear thermal expansion coefficient smaller by being combined with other RO (metal oxide) components. Thus, it is preferable for a glass ceramic to contain SrO component depending on the composition thereof. When the above effect is expected, the content is preferably 0.5 % or more. However, when it is more than 4 %, SrO component not only constitutes predominant crystal phase but also is largely included in a glass matrix. Therefore average linear thermal expansion coefficient of a glass matrix becomes large. As a result, average linear thermal expansion coefficient of a glass ceramic easily becomes large, so that it becomes difficult to obtain desired average linear thermal expansion coefficient. Thus the upper limit is preferably 3.0%, more preferably 2.5 %, and the most preferably 2.0 %. As described above, since BaO and SrO components have the above-described preferable effects for a glass ceramic, the total content of them is preferably 0.5 % or more. However, when a glass ceramic contains a large amount of them, the above described problem that average linear thermal expansion coefficient thereof deviates from the desired range easily occur. Thus the total content is preferably 6 % or less. It is more preferable that the upper limit is 5.5 % and the most preferable is 4.5 %. CaO component is effective in improving melting and clarification of a glass. However, when the content is more than 2 %, sufficient negative average linear thermal expansion coefficient is not obtained. Thus, the content is preferably 2 % or less. It is more preferable that the upper limit is 1.8 % and the most preferable is 1.5 %. Zr0₂ component acts as a crystal nucleation agent. Thus, 0.5 % or more of Zr0₂ component is preferably added. However, when the content is more than 3 %, melting and clarification of an original glass easily become difficult, so that unmelted residue is easily left. Thus, the content is preferably 3 % or less. It is more preferable that the upper limit is 2.8 % and the most preferable is 2.5 %. Ti0₂ component also acts as a crystal nucleation agent. Thus, 0.5 % or more of Ti0₂ component is preferably added. However, when the content is more than 3 %, devitrification easily arises in a course of glass molding. Thus the content is preferably 3 % or less. It is more preferable that the upper limit is 2.8 % and 2.5 % is the most preferable. As for the total content of Ti0₂ and Zr0₂ components, they improve Young's modulus and make it easier to control grain size of a predominant crystal phase to desired particle size, so that it become easy to achieve high light transmittance. Thus, the total content of Ti0₂ and Zr0₂ components is preferably 1.0 % or more. However, when the total content is more than 5.0 %, it becomes difficult to obtain desired average linear thermal expansion coefficient, Thus, the total content is preferably 5.0 % or less. It is more preferable that the lower limit is 1.5 %, and the most preferable is 2.0 %. Hf0₂ component decreases average linear thermal expansion coefficient of a original glass. However, when the content is more than 3 %, meltability is easily deteriorated. Thus the content is preferably 3 % or less. It is more preferable that the upper limit is 2 %, and it is the most preferable that the glass ceramic is substantially free from Hf0₂ component. As₂0₃ and Sb₂0₃ components can be added as a clarification agent in melting a glass for obtaining a homogeneous product. It is sufficient that the total content is less than 2%. Other than the above described components, a glass ceramic of the present invention can include respectively 3 % or less of F₂, La₂0₃, Ta₂0₅, Ge0₂, Bi₂0₃, W0₃, Y₂0₃, Gd₂0₃, Sn0₂, Te0₂, CoO, NiO, CuO, AgO, MoO, Mn0₂, Fe₂0₃, Cr₂0₃, Nb₂0₅, V₂θ5, Yb₂0₃, Ce0₂, Cs₂0, other rare earth elements and the like. However it is preferable that the glass ceramic is substantially free from them since they tend to affect a promotion of crystallization and decreasing a transmittance. P₂0₅ component has a tendency to increase average linear thermal expansion coefficient of a crystallized glass ceramic and to deteriorate meltability. Thus, the content is preferably less than 4 %, more preferably 3 % or less. It is the most preferable that the glass ceramic is substantially free from P2O5 component. MgO component easily has a tendency to make crystals rough and large. Thus the glass ceramic is substantially free from MgO component . PbO component is environmentally unfavorable component. As for Na₂0 and K₂0 components, the ions thereof are diffused in a course of after-treatment such as film coating and cleaning, so that material property of a negative thermal expansion glass ceramic of the present invention is labile to variation. Thus, it is preferable that the glass ceramic is substantially free from PbO, Na₂0 and K₂0 components. As for the deposited grain size of a glass ceramic, when difference of refractive index between glass matrix part and deposited crystal phase and/or deposited grain size are large, light transmittance of the glass ceramic decreases. Hereof, all the glass ceramics of the invention have an extremely micro grain size whose average is 0.3 μm or less. Thus light transmittance thereof is fine. The average grain size is preferably 0.2 μm or less, more preferably 0.15 μm or less and the most preferably 0.1 μm or less. A glass ceramic of the present invention is homogenous. For example, various alkaline components (ions) such as Li, Na, K and the like show no difference of concentration between surface phase and internal part. Furthermore, generation of cracks are extremely few, and there is substantially no cracks of 5 μm or more. Also, there is substantially no cracks of 1 μm or more. It is to be noted that the most preferable glass ceramic has no cracks . A glass ceramic of the present invention having the above-described composition is manufactured as follows. Firstly, materials of glass such as oxides, carbonates, hydrates, nitrates and the like are weighed and mixed, being put into a crucible and the like, and melted with stirring at about 1300 °C to 1550 °C for about 6 to 8 hours Thus a clarified original glass is obtained. As described above, after the original glass is melted, it is molded to be tabular shape and annealed by casting it to a metal mold and the like. Next, heat treatment to be a glass ceramic is performed. Generally in a glass ceramic having negative thermal expansion property, cracks are easily generated at a boundary of a residual matrix glass and deposited crystal phase. This is due to a stress caused by large difference of thermal expansion between a residual glass matrix having positive thermal expansion property and deposited crystal phase having negative thermal expansion property. It has been problematic in earlier development that it is impossible to manufacture a product having large size due to the above problem. However, the present invention is successful in obtaining a material which can be manufactured stably by controlling a grain size according to a crystallization condition. First, the original glass is kept at 650 °C to 750 °C to promote nucleation (the first thermal treatment) . When the nucleation temperature is lower than 650 °C, it is impossible to generate crystal nucleation. Oppositely, when it is higher than 750 °C, nuclei are generated earlier and grow. As a result, crystals grow larger in the second thermal treatment step, which cause a generation of cracks in the material. Thermal treatment time is desirably 0.5 to 50 hours in order to obtain desired property. It is more preferable in a point of more preferable property, productivity and cost, the lower limit is 1 hour and/or the upper limit is 30 hours. After the nucleation, grain size is controlled by an optimization of temperature rising process toward nuclei growing temperature. As for a product of large size, when a temperature rising rate is high, the product may break due to distortion caused by difference of temperature between internal and external of the product, because the product at this point has large glass matrix part having positive average linear thermal expansion coefficient and proportion of deposited crystal phase is small. Therefore, it becomes important that temperature rising rate is controlled as slow as possible. Temperature rising rate is preferably 10 °C /hour or less in order to obtain desired property. Subsequently, As for crystallization at 700 to 850 °C (the second heat treatment) , when a crystallization temperature is lower than 700 °C, a predominant crystal phase does not sufficiently grow, and when it is higher than 850 °C, an original glass can be subject to softening to deform and remelting. Thus it is preferable that the lower limit is 750 °C and/or the upper limit is 850 °C. After crystallization, annealing rate is desirably 50 °C/hour or less, more preferably 25 °C/hour or less. Heat treatment time of nuclei growing is also desirably 0.5 to 100 hours. For the same reasons as the first heat treatment, it is more preferable that the lower limit is 1 hour and/or upper limit is 30 hours. However, when the above temperature rising rate is slow, heat treatment time of nuclei growing can be shorter than the above from a productive view. Next, examples of a negative thermal expansion glass ceramics according to the present invention will be explained. However, it is to be noted here that the present invention is not limited to the examples.

(Embodiment) Relating to the examples 1 to 6 of a glass ceramic according to the present invention, Tables 1 and 2 shows their composition, melting temperature, nucleation temperature, nucleation time, nuclei growing temperature, nuclei growing time, average linear thermal expansion coefficient, relative density, average grain size, thermal expansion hysteresis, Young's modulus, rigidity and transmittance .

(Examples 1 to 6) Glass ceramics of the examples 1 to 6 were manufactured as follows. First, materials of a glass such as oxides, carbonates, hydrates, nitrates and the like were weighed and mixed according to the compositions shown in Tables 1 and 2, and were put into platinum crucibles. The materials were melted with stirring at the temperatures shown in Tables 1 and 2 for 6 to 8 hours by using a general melting apparatus. Next, each melted original glass was cast into a metal die and annealed to obtain individual molded glasses. The molded glass was put into a crystallization furnace and heated up to form crystal nuclei at the nucleation temperature for the nucleation time shown in Tables 1 and 2. Successively the original glass is heated up to crystallize thereof at the nuclei growing temperature for the nuclei ^■growing time also shown in Tables 1 and 2, and annealed at a rate of 50 °C/hour or less to obtain a glass ceramic. The glass ceramics of each example obtained as described above were cut into sample blocks having a diameter of 5 mm and length of 20 mm. Average linear thermal expansion coefficient and thermal expansion hysteresis at -40 °C to 80 °C of each sample were measured by TAS200 thermal mechanical analyzer made by Rigaku Corp. Each glass ceramic obtained as described above was polished with Ce0₂ and its relative density, Young's modulus, rigidity and transmittance were measured. Average grain size was measured with TEM photographs. Thin section of several micrometers thickness was prepared by ion milling and observed at an accelerating voltage of 75 kV and magnification rate of 50,000. As a result of the observation, it has found that average grain size of the examples 1 and 4 is less than 0.1 μm, and that of example 3 is less than 0.05 μm.

(Comparative examples 1 to 3) Glass ceramics of the comparative examples 1 to 3 were obtained by the similar procedure as that of the examples 1 to 6, except that: materials of a glass such as oxides, carbonates, hydrates, nitrates and the like were weighed mixed according to the compositions shown in Table 3 and were put into platinum crucibles, stirred at 1400 to 1550 °C by using a general melting apparatus, subjected to formation of crystal nuclei at the nucleation temperature and the nucleation time shown in Table 3, subjected to crystallization at the nuclei growing temperature and the nuclei growing time shown in Table 3. The comparative example 1 contains extremely large amount of Li₂0 and A1₂0₃ components. Crystal growth of β- eucryptite solid solution is fast, and the crystals are bound each other to grow further after crystal growth from a crystal nuclei. Since deposited grain size and crystallinity become large and high by repeating this phenomenon, light transmittance at a desired wavelength becomes low. The comparative example 2 contains a larger amount of Li₂0 component compared with the examples of the present invention. Thus, as same as the comparative example 1, it has high crystallinity so that it is difficult to obtain desired average linear thermal expansion coefficient. The comparative example 3 has Si0₂ component whose content is higher than the scope of the present invention. Thus, as same as the comparative example 1, it has high crystallinity so that it is difficult to obtain desired average linear thermal expansion coefficient.

Table 1

Table 2

Table 3

Industrial Applicability A glass ceramic of the present invention is a Li₂0- Al₂0₃-Si0₂ system glass having a composition in a certain range and a negative average linear thermal expansion coefficient of more than -25xl0^"7 to -15xl0^"7 K^"1 in a temperature range of -40°C to 80°C and a spectral transmittance of 85% or more in a wavelength range of 1000 to 1700nm, where the glass ceramic is heat-treated to be crystallized and can be manufactured stably. Furthermore, since the glass ceramic has an excellent mechanical strength, the procedure thereof does not require a step such as chemical strengthening for a improvement of strength. Therefore the glass ceramic has excellent durability and does not have a problem such as elution of alkali ion. As a result, the glass ceramic can be applied to an optical device in an optical communication field, where the glass ceramic is used in combination with a material having positive average linear thermal expansion coefficient. Further, the glass ceramic can be applied widely to various fields such as an energy related field, information communication field, and electronics field. Compared to an earlier development, the glass ceramic of the present invention can be manufactured by melting a material glass at comparatively low temperature, and a temperature of the heat treatment for crystallization thereof is also low. Thus the glass ceramic can be manufactured at a low cost. Furthermore, since the glass ceramic does not contain unstable components in the composition thereof and the proportion of the components can be easily controlled, it can be manufactured stably from a viewpoint of composition and material property.

Claims

1. A glass ceramic having an average linear thermal expansion coefficient of more than -25xl0^~7 to -15xl0^~7 K^_1 in a temperature range of -40 °C to 80 °C and a spectral transmittance of 85% or more in a wavelength range of 1000 to 1700 nm, wherein a predominant crystal phase is one or more crystals selected from β-eucryptite, β-eucryptite solid solution, β-quartz and β-quartz solid solution.

2. The glass ceramic as claimed in claim 1, comprising by mass %; Si0₂ 40 to 59 % and/or A1₂0₃ 10 to 30 % and/or Li₂0 1 to 5.4 %, and wherein the glass ceramic fulfills a formula of Si0₂/ (Al₂0₃+Li₂0) >1.5.

3. The glass ceramic as claimed in claim 1 or 2, further comprising 12.0% or less of Li₂0 by mol %.

4. The glass ceramic as claimed in any one of claims 1 to 3, further comprising by mass %; ZnO 3 to 10 % and/or BaO+SrO 0.5 to 6 % and/or Ti0₂+Zr0₂ 1.0 to 5.0 %.

5. The glass ceramic as claimed in any one of claims 1 to 4, further comprising by mass %; B₂0₃ 0 to 5 % and/or BaO 0 to 4 % and/or SrO 0 to 4 % and/or CaO 0 to 2 % and/or Zr0₂ 0.5 to 3 % and/or Ti0₂ 0.5 to 3 % and/or Hf0₂ 0 to 3 % and/or As₂0₃+Sb₂0₃ 0 to 2 %.

6. The glass ceramic as claimed in any one of claims 1 to 5, further comprising less than 4% of P₂0₅ by mass %.

7. The glass ceramic as claimed in any one of claims 1 to 6, wherein the glass ceramic is substantially free from MgO .

8. The glass ceramic as claimed in any one of claims 1 to 7, wherein the glass ceramic is substantially free from PbO, Na₂0 and K₂0.

9. The glass ceramic as claimed in any one of claims 1 to 8, having a Young's modulus of 60GPa or more.

10. The glass ceramic as claimed in any one of claims 1 to 9, having a hysteresis of thermal expansion curve of 15ppm or less.

11. The glass ceramic as claimed in any one of claims 1 to 10, wherein a crystal phase does not contain a Al₂Ti0₅ crystal.

12. The glass ceramic as claimed in any one of claims 1 to 11, wherein the glass ceramic is obtained by melting, forming and annealing an original glass, performing the first thermal treatment at 650 to 750°C for 0.5 to 50 hours, subsequently performing a second thermal treatment at 700 to 850°C for 0.5 to 100 hours.

13. The glass ceramic as claimed in claim 12, wherein the original glass is heated up to a temperature of the second thermal treatment at a temperature rising rate of 10 °C/hour or less after the first thermal treatment.