Method for producing rare earth element-doped glass by sol-gel process

Abstract: A rare earth element-doped glass in which the rare earth element is homogeneously doped is produced by a method comprises hydrolyzing a metal alkoxide of the formula: M(OR)₄ wherein M is a metal and R is an alkyl group in a liquid medium selected from the group consisting of alcohols and water in the presence of a compound of a rare earth element and condensing the hydrolyzed material to form a rare earth element-doped glass.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for producing a rare earth element-doped glass which exhibits functions of the rare earth by a sol-gel process. Description of the Related Art In the production of a glass optical fiber such as quartz glass optical fiber, techniques for producing a high pure material and controlling an optical structure have been almost completed, and the glass optical fiber is practically and widely used. Since it was found that a glass added with, for example, Nd³⁺ ions as active ions could generate a laser beam in 1960s, it has been confirmed that a glass added with various rare earth elements such as Er³⁺, Yb³⁺, Ho³⁺ or Tm³⁺ can generate laser beam. This is because the laser transition in the trivalent rare earth element ions is the f-f transition so that it gives a narrow spectrum width in the glass. The glass has many advantages that it can provide a large size mass in comparison with a crystalline material, and it is homogeneous and easily processed. In addition, the glass can be easily processed in a fiber form and used to assemble a telecommunication optical fiber system. To add the rare earth element to the glass optical fiber such as the quartz glass optical fiber, a vapor phase method and a solution impregnating method are known. The vapor phase method includes a method comprising heating a chloride of the rare earth element to vaporize it and introducing the vaporized chloride in a reaction tube together with a quartz glass-forming raw material such as silicon tetrachloride to form a glass while adding the rare earth element to the glass and a method comprising heating a mass of silica particles to vitrify it in an atmosphere comprising a vapor of the rare earth element to add the rare earth element to the glass. Typically, the solution impregnating method comprises dipping a mass of silica particles in a solution of the rare earth element in an alcohol or water, evaporating the alcohol or water and sintering the mass of silica particles to produce a glass containing the rare earth element. The above conventional methods for producing the rare earth element-doped glass are based on the technique for producing the quartz glass optical fiber and have various merits in quality and productivity. However, by the vapor phase method, the rare earth element is added to the glass in an amount of several ppm to several ten ppm. On the other hand, by the solution impregnating method, the rare earth element can be added in an amount up to several thousand ppm, but amounts of the impregnated rare earth element vary due to a bulk density profile in the mass of silica particles so that it is difficult to produce the glass in which the rare earth element is uniformly added. SUMMARY OF THE INVENTION One object of the present invention is to provide a method for producing a rare earth element-doped glass in which the rare earth element is homogeneously doped. Another object of the present invention is to provide a method for producing a rare earth element-doped glass by which a doped amount of the rare earth element can be controlled. These and other objects of the present invention can be achieved by a method for producing a rare earth element-doped glass comprising the steps of: hydrolyzing at least one metal alkoxide selected from Si(OR)₄, Ge(OR)₄, Al(OR)₃, Ti(OR)₄ and B(OR)₃ wherein R is an alkyl group in an aqueous or alcoholic liquid medium having a pH of 6-9 in the presence of a compound of a laser-generating rare earth element to form a gel, drying said gel at a temperature of 60-200°C, dehydrating the dried gel in an atmosphere containing chlorine, and vitrifying the dehydrated gel at a temperature of 1200-1300°C in a helium atmosphere or under vacuum to form the rare earth element-doped glass. The doped glass produced by this method is suitable for making into an optical device such as a fiber laser, a glass laser or an optical switch. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing a relationship between pH values and the bulk density of the gel, Fig. 2 is a Raman spectra for erbium-doped glass produced in Example 3 and the pure SiO₂ glass, and Fig. 3 is a graph showing a relationship between the charged amount of erbium and the doped amount erbium in Example 5. DETAILED DESCRIPTION OF THE INVENTION In the metal alkoxides of formulae Si(OR)₄, Ge(OR)₄, Al(OR)₃, Ti(OR)₄ and B(OR)₃, the alkyl group R is preferably a straight or branched alkyl group having 1 to 8 carbon atoms. Specific examples of the alkyl group are a methyl group, an ethyl group, a n-propyl group, an isopropyl group, a n-butyl group and a tert.-butyl group. When Si(OR)₄ is used as the metal alkoxide, a quartz glass is formed. The compound of rare earth element to be used in the present invention is at least one compound of a lanthanoid element. Examples of the lanthanoid elements are La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Among these, the lanthanoid elements which generate the laser such as Er, Yb, Ho, Tm, Nd, Eu, Tb and Ce are used. Examples of the compound of the rare earth element are chloride, nitrate, sulfate, acetate, etc.. In the method of the present invention, the metal alkoxide is added to the alcohol or water and hydrolyzed in the presence of at least one compound of the rare earth element. Preferred examples of the alcohol are methanol, ethanol, n-propanol, isopropanol and butanol. The hydrolysis conditions may be substantially the same as those employed in the conventional hydrolysis of the metal alkoxide. The pH of the mixture is adjusted in the range 6 and 9 with an alkali solution such as aqueous ammonia. When the gel is formed through hydrolysis of Si(OR)₄ in the pH range between 7 and 9, it is not cracked during drying so that a large size bulk can be produced. The reason for this may be explained from a relationship between the pH of the mixture for hydrolysis and a bulk density of the synthesized gel. As shown in Fig. 1, the bulk density is minimized in the pH range between 7 and 9. The gel having the small bulk density has large pores therein so that water or other liquids are easily evaporated off and the gel is hardly cracked during drying. The amount of the added rare earth element depends on the solubility of the rare earth element compound. In general, the rare earth element chloride such as NdCl₃, YbCl₃, HoCl₃ or TmCl₃ is soluble in water and has large solubility in the alcohol. Then, the rare earth element chloride can be practically used. An amount of the rare earth element to be added to the glass depends on the solubility of rare earth element compound in the alcohol or water and is usually from 100 to 100,000 ppm. For example, 50 grams of NdCl₃ can be dissolved in 100 grams of water. Such solubility is practically satisfactory. Since the rare earth element is added in a liquid phase, it can be homogeneously added to the glass. The amount of the rare earth element added to the glass can be adjusted by changing the amount of the rare earth element compound to be added to the starting mixture. This is one of the advantages of the present invention. Since, in the conventional methods, the rare earth element is added to the synthesized glass, the rare earth element and the glass-forming elements are not sufficiently bonded. In such case, the rare earth element forms clusters or crystals, which adversely affect the generation of laser or optical properties of the doped glass. However, in the present invention, the rare earth element is trapped in the network of the glass during formation of the glass from the metal alkoxide, so that the glass with good quality is produced. If the amount of the rare earth element compound is increased to increase the doped amount of the rare earth element, atoms of the rare earth element associate together so that the functions of the rare earth element cannot be effectively activated. This may be because, in the quartz glass, the rare earth atoms which are independently doped in the SiO₂ network cannot coordinate the oxygen ions sufficiently so that the network is unstabilized by strain generated in the network. Then, the rare earth atoms associate together to restabilize the network. To prevent the association of the rare earth atoms, co-doping of the rare earth element with other metal such as Al or P is effective. Accordingly, in an embodiment of the present invention in which Si(OR)₄ is used as the metal alkoxide, preferably the rare earth element compound and other metal alkoxide are previously hydrolyzed to prepare a solution containing the rare earth element, and this solution is added to the mixture of Si(OR)₄ and the alcohol or water. This previous hydrolysis may be carried out at a reaction temperature of from 20 to 40°C for 1 to 3 hours in the presence of a catalyst such as hydrochloric acid or aqueous ammonia. Preferred examples of the other metal alkoxide are alkoxides of Ge, Al, P and B. Further, a glass-forming element such as Wo and Te may be used. Among them, Al and P are most preferred. Usually, 5 to 6 moles of the other metal is used per one mole of the rare earth element. As hydrolysis of the metal alkoxide proceeds in the alcohol or water, a sol is formed and then the sol is changed to a gel. The gel is heated and dried. The drying is carried out at a temperature of at least 60°C, preferably from 60°C to 200°C, for example from 120°C to 200°C. When the drying temperature is lower than 60°C, residual water cannot be sufficiently removed. The dried gel is then dehydrated preferably in a chlorine-containing atmosphere and vitrified at a sufficiently high temperature, for example, higher than 200°C in a helium atmosphere or under vacuum to give a glass containing the rare earth element in the form of an oxide. PREFERRED EMBODIMENTS OF THE INVENTION The present invention will be illustrated by the following Examples. Example 1 Silicon tetramethoxide (500 ml), water (250 ml), 0.1N HCl (20 ml) and erbium chloride (3.3 g) dissolved in methanol (50 ml) were charged in a beaker and mixed with a magnetic stirrer for about two hours. Then, the mixture was poured in a messcylinder made of polytetrafluoroethylene (Teflon, a trade mark), and the messcylinder was sealed with an aluminum adhesive tape and kept standing at 60°C for one day. After making several holes in the aluminum tape, the mixture in the messcylinder was kept standing at 60°C for ten days to obtain a dried gel. The dried gel was dehydrated in an atmosphere containing 7 % of chlorine at 800°C for about two hours followed by heating in a helium atmosphere at 1300°C for two hours to obtain a transparent pink glass. Elemental analysis of the glass revealed that it contained about 1 % by weight of erbium. Example 2 In a mixture of aluminum methoxide (5 ml), ethanol (10 ml) and 0.1N aqueous ammonia (0.1 ml), erbium chloride (3.3 g) was dissolved and stirred for two hours. Then, to the mixture, silicon tetramethoxide (500 ml), water (500 ml), ethanol (750 ml) and 0.1N aqueous ammonia (20 ml) were added and further stirred for two hours. The resulting mixture was poured in a messcylinder made of polytetrafluoroethylene (Teflon, a trade mark), and the messcylinder was sealed with an aluminum adhesive tape and kept standing at 35°C for one day to form a gel. Then, the mixture was heated to 80°C over two days. After making small holes in the aluminum tape, the mixture in the messcylinder was kept standing at 80°C for five days, heated to 150°C over four days, kept standing at 100°C for one day and then evacuated under vacuum at 200°C to obtain a dried gel. The dried gel was heated to 1200°C at a heating rate of 1°C/min. and kept at that temperature for two hours to obtain a transparent glass. Elemental analysis of the glass revealed that it contained 1 % by weight of erbium. Example 3 In the same manner as in Example 2, a SiO₂ glass containing 0.7 % by weight or 2.1 % by weight of erbium was produced and its Raman spectrum was recorded and compared with the Raman spectrum of pure SiO₂. The spectra are shown in Fig. 2. The peaks indicated by the arrows are assigned to SiO₂ and are not affected by the addition of erbium. This result means that erbium is homogeneously dispersed in the erbium-doped glass. Example 4 To a mixture of phosphorus methoxide (6 ml), ethanol (12 ml) and water (6 ml), 0.1N hydrochloric acid (0.5 ml) and then neodymium chloride (3.4 g) were added and stirred at 30°C for two hours to hydrolyze phosphorus methoxide. To the mixture, silicon methoxide (500 ml), water (500 ml), ethanol (750 ml) and 0.1N aqueous ammonia (20 ml) were stirred for one hour. The resulting mixture was charged in a messcylinder made of polytetrafluoroethylene (Teflon, a trade mark), and the messcylinder was sealed with an aluminum adhesive tape and kept standing at 35°C for one day to form a gel. Then, the mixture was heated to 80°C over two days. After making small holes in the aluminum tape, the mixture in the messcylinder was kept standing at 80°C for five days, heated to 150°C over four days and kept standing at that temperature for one day followed by evacuation under vacuum at 200°C for eight hours to obtain a dried gel. The dried gel was heated in the air to 350°C at a heating rate of 1°C/min. to remove the residual carbon. Thereafter, the gel was heated to 800°C at a heating rate of 1°C/min. and a chlorine gas was introduced in the messcylinder to dehydrate the gel. The dehydrated gel was heated to 1200°C at a heating rate of 1°C/min. and kept at that temperature for two hours to obtain a transparent glass. Elemental analysis of the glass revealed that it contained 1 % by weight of neodymium. Example 5 Silicon tetramethoxide (100 ml), ethanol (220 ml), water (100 ml), 0.1N aqueous ammonia 720 ml) and erbium chloride.hexahydrate (ErCl₃.6H₂O) (0.93 g) were mixed and stirred with a magnetic stirrer for about 15 minutes. Then, the mixture was poured in a messcylinder made of polytetrafluoroethylene (Teflon, a trade mark), and the messcylinder was sealed with an aluminum adhesive tape and kept standing at 35°C for two days. After making several holes with a diameter of 0.1 mm in the aluminum tape, the mixture in the messcylinder was kept standing at 80°C for five days, at 100°C for two days, at 130°C for two days and at 150°C for two days to obtain a dried gel. The dried gel was heated with an electric furnace in an oxygen atmosphere at 350°C for two days to burn out the residual carbon followed by dehydration in a chlorine atmosphere at 800°C for two hours. Then, the dehydrated gel was heated in a helium atmosphere at 1200°C to obtain a transparent pink glass. Elemental analysis of the glass revealed that it contained about 1 % by weight of erbium. The glass contained no scattering materials, which indicated that Er atoms did not form clusters and were homogeneously dispersed in the glass. With changing the charged amount of erbium methoxide, erbium-doped glass was produced. The relationship between the charged amount of erbium methoxide and the amount of dope erbium is shown in Fig. 3. The doped amount of erbium was measured by atomic absorption analysis. The results indicate that erbium was added to the glass at a yield of 70 %. Example 6 In the same manner as in Example 5 except that silicon tetramethoxide (100 ml), ethanol (270 ml), water (100 ml), 0.1N aqueous ammonia (70 ml) and uranium chloride (UCl₃) (5.8 g) were used, a glass doped with 1 % of uranium was produced.

Claims: A method for producing a rare earth element-doped glass comprising the steps of: hydrolyzing at least one metal alkoxide selected from Si(OR)₄, Ge(OR)₄, Al(OR)₃, Ti(OR)₄ and B(OR)₃ wherein R is an alkyl group in an aqueous or alcoholic liquid medium having a pH of 7-9 in the presence of a compound of a laser-generating rare earth element to form a gel, drying said gel at a temperature of 60-200°C, dehydrating the dried gel in an atmosphere containing chlorine, and vitrifying the dehydrated gel at a temperature of 1200-1300°C in a helium atmosphere or under vacuum to form the rare earth element-doped glass. A method according to claim 1, wherein said metal alkoxide is Si(OR)₄, and at least one other metal alkoxide is previously mixed with said compound of rare earth element and hydrolyzed and then added to a reaction mixture for hydrolyzing Si(OR)₄. A method according to claim 2, wherein said other metal alkoxide is Ge(OR)₄, Al(OR)₃, P(OR)₅ or B(OR)₃. A method according to claim 1, comprising the step of adjusting the pH of the liquid medium using aqueous ammonia.

Inventors: ITO MASUMI, SAITOH TATSUHIKO, YOKOTA HIROSHI, ITO, MASUMI, SAITOH, TATSUHIKO, YOKOTA, HIROSHI, ITO, MASUMI, C/O YOKOHAMA WORKS OF, SAITOH, TATSUHIKO, C/O YOKOHAMA WORKS OF, YOKOTA, HIROSHI, C/O YOKOHAMA WORKS OF
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: C03B8
IPC codes: C03B37, C03C1, C03C3, C03C4, C03B8