METHOD AND DEVICE FOR MANUFACTURING A GLASS COMPONENT

Description

Technical background

[0001] The invention relates to a method for producing a glass component which consists at least partially of doped, in particular rare earth metal-doped, SiO 2

[0002] The invention also relates to a device for producing a glass component.

[0003] To create special properties, quartz glass is doped with other substances. Components made of doped quartz glass are used, for example, in the manufacture of fittings and components for semiconductor manufacturing, fiber amplifiers, fiber lasers, microstructured optical fibers, passive fibers with customized core refractive index or attenuation properties, for example, for telecommunications, filter glasses, or sensor materials. State of the art

[0004] In known processes for producing optical preforms, starting components are placed in a cladding tube made of quartz glass and fused together with it to form a preform.

[0005] Such a "cladding tube vitrification" for producing a preform for active optical fibers is described, for example, in US Pat. No. 8,720,230 B2. SiO2 powder and a powder of a rare earth metal compound, which can also be present in soluble form, for example as a water-soluble chloride, are mixed with water or alcohol to form a slurry. To adjust the desired refractive index, dopants such as GeO2, P2O5, Al2O3, and / or B2O3 can be added to the slurry. The slurry is dried and ground to a mixed powder of SiO2, rare earth metal, and dopant. The mixed powder is filled into a quartz glass tube with a vertically oriented longitudinal axis, the lower end of which is sealed with a porous glass stopper. For cleaning and calcination, the mixed powder is heated to a temperature below the vitrification level (for example, at approx.The chamber is pressurized with a gas stream containing oxygen, helium, chlorine, or fluorine, which is introduced into the chamber through the porous plug. Finally, the gas is pumped out of the chamber, and the mixed powder and the quartz glass tube are melted zone by zone at a temperature above the vitrification level (preferably 2100°C) and simultaneously elongated into a first preform. The vitrified mixed powder forms the core material of the preform, and the quartz glass tube forms the cladding material.

[0006] The first preform thus produced is mounted in another quartz glass cladding tube, optionally with additional rods, tubes, or preforms, and the remaining space is filled with a second SiO 2 powder, which may also be doped. Subsequently, the interior of the quartz glass cladding tube is placed under vacuum, and the entire package is sintered and vitrified to form a second preform, or directly drawn into an optical fiber.

[0007] US 2004 / 0005127 A1 describes a method for manufacturing a preform for rare-earth metal-doped fibers. The preform is created from a bundle of glass rods inserted into a thin-walled quartz glass tube, which serves as the preform's second cladding tube. Both ends of the cladding tube are sealed with porous glass wool plugs. The glass wool plugs rest against the end faces of the glass rod bundle and prevent lateral displacement. The glass rod bundle undergoes cleaning and drying in the cladding tube, and the cladding tube is then collapsed. Technical task

[0008] During pretreatment of the intermediate product in the cladding tube with inert or reactive treatment gas, the cladding tube is flooded with the treatment gas and the intermediate product is exposed to the treatment atmosphere.

[0009] It can happen that the treatment gas finds preferential flow paths, resulting in "dead zones" in the intermediate product that are not exposed to the treatment gas or are exposed to comparatively little. Therefore, homogeneous pretreatment, such as complete cleaning, is not guaranteed. This is especially true when there is empty space in the cladding tube's inner bore and for intermediate products with large pore volumes.

[0010] When sintering or vitrifying the intermediate product in cladding tube vitrification, it is advisable to apply a vacuum to prevent bubbles from being trapped in the melting glass. Furthermore, the porous intermediate product shrinks significantly during sintering or vitrification, which can lead to the formation of voids.

[0011] It has been shown that the cladding tube can collapse in an uncontrolled manner, causing the intermediate product to be unintentionally deformed and gases to become trapped, resulting in the resulting preform or fiber exhibiting non-reproducible properties. This problem increases with increasing cladding tube diameter.

[0012] The invention is therefore based on the object of specifying a process by which an intermediate product containing cavities can be subjected to a thermal and / or reactive treatment which is reproducible and uniform in its effect.

[0013] Furthermore, the invention is based on the object of providing a device by means of which a thermal and / or reactive treatment of an intermediate product containing cavities in a cladding tube can be carried out reliably and with reproducible results. Summary of the invention

[0014] This object is achieved with regard to the method by a method having the features of claim 1.

[0015] The glass component produced by the process according to the invention consists at least partially of doped, in particular rare earth metal-doped, SiO 2 (hereinafter also referred to as "doped quartz glass"). The doped quartz glass contains at least one dopant. The glass component is produced using an intermediate product from naturally occurring, but preferably synthetically produced, SiO 2 raw material.

[0016] In the process according to the invention, either an intermediate product containing pre-doped SiO 2 raw material is further processed. In this case, the thermal treatment to be carried out results in at least sintering or vitrification of the intermediate product; however, it can also serve to introduce an additional dopant.

[0017] Or, in the process according to the invention, an intermediate product containing a non-doped SiO 2 raw material is further processed. In this case, the thermal treatment to be carried out serves to introduce at least one dopant into the intermediate product via the gas phase, and it serves for the subsequent sintering or vitrification of the thus doped intermediate product.

[0018] At least one component, referred to here as a "gas diffuser," is located in the cladding tube's inner bore in direct or indirect contact (contact via an intermediate element) with the intermediate product. It is porous, allowing gas transport through the gas diffuser along the cladding tube's longitudinal axis. It consists, for example, of glass with open porosity or of pressed and / or sintered particles, such as a glass frit. Advantageously, the gas diffuser has the same or similar porosity as the intermediate product, but exhibits lower shrinkage with increasing temperature.

[0019] It can be positioned upstream of the intermediate product in the direction of flow of an incoming gas, thus contributing to a more uniform gas distribution on and within the intermediate product, both spatially and temporally, and thus to a more homogeneous effect of the gas on the intermediate product and a more reproducible quality of the intermediate product. Furthermore, the gas diffuser stabilizes the spatial position and shape of the intermediate product by preventing or reducing the possibility of particles of the SiO2 raw material being entrained or displaced by gas flows or during evacuation during thermal treatment, or by undesirable plastic deformation of the softened intermediate product.

[0020] This is also due to the fact that the gas diffuser is movable along the longitudinal axis of the cladding tube and is pressed against one end of the intermediate product during thermal treatment. The gas diffuser thus enables the transfer of the pressing force to the intermediate product, while its gas permeability allows both degassing and gassing of the intermediate product for doping or purification purposes.

[0021] The intermediate product rests against an abutment. The abutment is usually a fixed bearing; however, it can also be a variable bearing. During thermal treatment, the porous intermediate product typically experiences a shortening in the direction of the cladding tube's longitudinal axis, for example, due to a decrease in volume (shrinkage) or deformation. This shortening dimension is also referred to below as the "length" of the intermediate product. This can cause gaps or larger cavities in the structure, which can lead to an uncontrolled collapse of the cladding tube. This can cause cavities with trapped gases to form, resulting in unintentional deformation of the intermediate product. The gas diffuser is adjusted according to the shortening of the intermediate product in the direction of the cladding tube's longitudinal axis.In a first advantageous process variant, this tracking can be carried out by external regulation or control, and in another advantageous process variant, it is carried out automatically by the length shortening during the thermal treatment itself, in that the tracking of the gas diffuser is effected by weight and / or by a mechanical pre-tension.

[0022] A pressing force is generated by means of a clamping device, which acts directly or indirectly - via a movable intermediate element, such as a fixing element - on the first gas diffuser and the intermediate product.

[0023] In a first process variant, the clamping device comprises an elastically deformable spring element inserted into the inner bore of the cladding tube, for example a bellows or elastically deformable springs, each made of a temperature-resistant plastic or metal such as Hastelloy, ceramic, or quartz glass. Depending on its current spring force, the spring element exerts the pressing force on the axially movable gas diffuser directly or indirectly – via a longitudinally movable intermediate element, such as a fixing element – ​​so that the latter is pushed towards the intermediate product whenever the intermediate product shortens in length, thereby stabilizing it. The pushing of the gas diffuser by means of the spring element is controlled by the shrinkage of the components in the cladding tube as a result of the thermal treatment and requires no external intervention.

[0024] In another method variant, the clamping device comprises a slider which can be moved in the inner bore of the cladding tube and has a first magnetic element made of a magnetic material, and a second magnetic element which can be moved in the longitudinal axis direction outside the inner bore of the cladding tube and acts on the magnetic element.

[0025] The second magnetic element is, for example, a permanent magnet or a current-carrying coil. The slider, driven by the movement of the second magnetic element, exerts a pressing force on the axially movable gas diffuser directly or indirectly—via a longitudinally movable intermediate element, such as a fixing element. The magnitude of the pressing force can be adjusted by means of a feed control or regulation of the outer coil; it can, for example, be constant or variable as needed. In this process variant, particularly high and variable-magnitude compressive forces can be exerted on the intermediate product by means of the controllable or adjustable feed of the second magnetic element and slider. The first magnetic element is preferably enclosed in a capsule made of dielectric material to prevent contamination. The slider must be designed so that it does not impede the axial gas flow in the cladding tube.During the thermal treatment, the pressing force preferably acts continuously (permanently) on the intermediate product in the direction of the cladding tube's longitudinal axis. The magnitude of the pressing force can be variable or constant.

[0026] By using the clamping device, high pressing forces can be applied even when the cladding tube's longitudinal axis is horizontally oriented, which can reduce the formation of bubbles in the intermediate product or even remove them.

[0027] To ensure the gas diffuser retains its shape during thermal treatment, it is preferably made of a porous glass that has a higher viscosity than the SiO2 raw material, even at high treatment temperatures, such as between 900°C and 1400°C. Quartz glass generally meets this requirement and, moreover, it contains no harmful impurities. The quartz glass is preferably undoped or contains dopants in lower concentrations than the doped or undoped SiO2.

[0028] In order to prevent the formation of preferential gas flow paths around the intermediate product, to minimize the loss of intermediate product material due to gas flow and evacuation, and to counteract undesirable plastic deformations, the gas diffuser initially fills the inner cross-sectional shape of the cladding tube as completely as possible.

[0029] On the other hand, with a view to minimizing material loss, it is advantageous if, after completion of the thermal treatment, the outer contours of the treated intermediate product and the gas diffuser differ as little as possible. Since the intermediate product shrinks as a result of the thermal treatment and assumes a final product outer contour, and the gas diffuser also shrinks as a result of the thermal treatment and assumes a final gas diffuser outer contour, a gas diffuser is preferably used that has an outer cross-sectional shape prior to the thermal treatment. Taking into account the shrinkage of the intermediate product and the gas diffuser, it is designed such that the final product outer contour and the final gas diffuser outer contour essentially match, for example, within + / - 10% of the largest radial outer contour dimension.In the simplest case, the respective initial outer contours as well as the total shrinkages of the intermediate product and the gas diffuser are the same.

[0030] In a process in which the gas diffuser experiences less shrinkage during vitrification than the intermediate product, a gas diffuser is preferably used which, prior to thermal treatment, has an outer contour that essentially matches the initial cladding tube inner contour, for example, leaving a gap of less than 2 mm. This process has proven successful, for example, when thermal treatment is accompanied by doping of the intermediate product by treatment with a gaseous dopant, such as fluorine or chlorine. In order to achieve the narrowest possible gap and to avoid uncontrollable flow paths and thus also an inhomogeneous dopant distribution, it is accepted that the final product outer contour and the final gas diffuser outer contour differ; more precisely, that the final product outer contour has a larger circumference than the final gas diffuser outer contour.

[0031] With a view to ensuring that the flowing gas acts as evenly as possible on the intermediate product and to avoid preferential gas flow paths around the outer circumference of the intermediate product, it has also proven advantageous if the intermediate product is designed as a soot body, sintered body or compact and has an outer cross-sectional shape that fills an inner cross-sectional shape of the cladding tube while leaving a gap with a gap size of 1 mm or less.

[0032] In the case of an inner bore and a cylindrical intermediate product with a circular cross-section, the nominal inner diameter of the inner bore is slightly larger and, in the limiting case, the same as the nominal outer diameter of the intermediate product.

[0033] In the case of an intermediate product in the form of a bed of particles, the cladding tube is filled with the bed in such a way that as few voids as possible remain, with the bed being pressed as tightly as possible into the cladding tube.

[0034] The effects explained relate to the use of a first gas diffuser, which is arranged upstream of the intermediate product in the flow direction of a treatment gas. They are intensified when a process uses a second gas-permeable gas diffuser, which is arranged downstream of the intermediate product in the flow direction of the treatment gas. The intermediate product is optionally arranged between the first gas diffuser and the second gas-permeable gas diffuser. During thermal treatment, the first gas diffuser presses against one end of the intermediate product and the second gas diffuser against the opposite, other end. To apply the pressing force, it is not necessary for the second gas diffuser to be moved in the direction of the cladding tube's longitudinal axis; it forms an abutment against which the intermediate product rests.Otherwise, the same applies to the second gas diffuser regarding geometric shape, chemical composition, function, and shrinkage behavior as for the first gas diffuser. In the simplest case, the first and second gas diffusers are manufactured using the same manufacturing process and are identical in terms of shape, porosity, and material.

[0035] The gas diffusers located on both sides of the intermediate product stabilize the intermediate product and prevent, for example, loose or molten, low-viscosity material of the intermediate product from being drawn uncontrollably through the cladding tube during thermal treatment under vacuum, which could clog existing gas channels so that gases generated during the thermal treatment can no longer be extracted.

[0036] Advantageously, the first gas diffuser and the second gas diffuser are cylindrical and each have a flat end face adjacent to the intermediate product.

[0037] In a further advantageous embodiment of the method, a first fixing element is inserted into the cladding tube inner bore, in or on which at least one continuous gas channel is formed and which is displaced along the cladding tube longitudinal axis and pressed against the gas diffuser.

[0038] The fixing element inserted into the cladding tube's inner bore rests directly or indirectly (via another intermediate element) against the gas diffuser and transmits a pressing force to it. It has at least one gas channel that runs axially through it, so that it does not prevent gas flow through the cladding tube's inner bore. In contrast to the gas diffuser, however, it is preferably made of a completely impermeable material, such as glass; and preferably of quartz glass made from synthetically produced raw materials. The fixing element partially seals off the porous gas diffuser from the outside, stabilizes it, and thus also contributes to holding the intermediate product in its predetermined axial position. Furthermore, even when the pressing force is applied to a small contact surface, it is not as easily damaged as the porous gas diffuser material, such as a glass frit.On the other hand, the gas diffuser protects the fixing element from viscous material or material moved by gas flow or evacuation from the intermediate product entering the gas channel or channels of the fixing element during sintering or vitrification and thereby clogging them.

[0039] The fixing element also exerts a stabilizing effect on the cladding tube; in particular, it counteracts premature collapse of the cladding tube during thermal treatment at high temperature and under vacuum.

[0040] The at least one gas channel runs completely within the fixing element, for example as a through-bore, or it is designed as a continuous intermediate space between the fixing element outer wall and the cladding tube inner wall, for example as a longitudinal groove in or chamfer on the outer wall.

[0041] The outer cross-sectional shape of the fixing element is preferably adapted to the inner cross-sectional shape of the cladding tube in such a way that a gap with the smallest possible width remains, for example, with a gap dimension of less than 2 mm. The fixing element, for example, has a circumference that is adapted to the inscribed circle of the cladding tube. The inscribed circle is understood here to be the largest circle lying entirely within the cladding tube's inner bore. The outer cross-sectional shapes of the gas diffuser and the fixing element are, for example, identical.

[0042] As explained for the outer cross-sectional shape of the gas diffuser, it is also advantageous for the fixation element, with a view to minimizing material loss, if the outer contours of the treated intermediate product and the fixation element differ as little as possible after completion of the thermal treatment. It should be noted that at least one gas channel can collapse completely or partially during the thermal treatment. Thus, not only does the intermediate product take on a smaller final product outer contour as a result of the thermal treatment, but the fixation element also shrinks as a result of the thermal treatment and takes on a smaller final fixation element outer contour.Therefore, it is preferable to use a fixing element which, prior to thermal treatment, has an external cross-sectional shape which, taking into account the shrinkage of the intermediate product and the fixing element, is designed so that the final product outer contour and the final fixing element outer contour match as closely as possible; for example, + / - 10% based on the largest radial dimension.

[0043] In the simplest case, the initial outer contours and the total shrinkage of the intermediate product and the fixing element are the same.

[0044] It has also proven advantageous if the fixing element has a through-bore and a flat first end face facing the gas diffuser, in which gas distribution grooves are embedded starting from the through-bore in order to influence the gas flow conditions in the cladding tube.

[0045] In a further preferred embodiment of the method, a second fixing element is also provided in conjunction with a second gas diffuser. The first fixing element is preferably arranged upstream of the first gas diffuser in the flow direction of a treatment gas, and the second fixing element is arranged downstream of the second gas diffuser in the flow direction and rests against it.

[0046] If necessary, the first fixing element, the first gas diffuser, the intermediate product to be treated, the second gas diffuser and the second fixing element are arranged in abutting relation in this order.

[0047] The intermittent arrangement over plane-parallel contact surfaces results in better temperature homogeneity, especially across the contact surfaces.

[0048] Advantageously, the first fixing element and the second fixing element are cylindrical and have flat end faces with which they rest against a flat end face of the respective associated gas diffuser.

[0049] What has been said for the first fixation element regarding geometric shape, chemical composition, function and collapse behavior also applies to the second fixation element.

[0050] In an advantageous procedure, it is provided that a negative pressure is applied in the cladding tube inner bore at one cladding tube end and / or a treatment gas is introduced into the cladding tube at an opposite cladding tube end.

[0051] For example, the intermediate product can be exposed to the treatment gas introduced from one end of the cladding tube, while the used or contaminant-laden treatment gas can be simultaneously drawn from the intermediate product through the other end of the cladding tube. During sintering or vitrification of the intermediate product, a negative pressure is created and maintained in the cladding tube, but no treatment gas is introduced. The negative pressure, combined with the compression force, allows bubbles to be removed from the porous intermediate product even when the cladding tube's longitudinal axis is oriented horizontally.

[0052] Advantageously, the intermediate product is heated in an electrically heated ring furnace for thermal treatment.

[0053] Compared to heating with gas-fired burners, an electrically heated ring furnace easily creates a long heating zone within which the internal components of the cladding tube are exposed to high temperatures. This allows for more even heat radiation and better heat distribution. This allows for the thermal treatment of larger cladding tubes and thus increases the yield.

[0054] The above-mentioned technical problem is solved according to the invention with regard to the device by a device having the features of claim 12.

[0055] The device is particularly suitable for carrying out the method according to the invention. The statements regarding the method apply equally to the device itself. Advantageous embodiments of the device are set forth in the subclaims. To the extent that embodiments of the device specified in the subclaims are modeled on the procedures mentioned in the subclaims for the method according to the invention, reference is made to the above statements regarding the corresponding method claims for additional explanation. Definitions

[0056] Individual process steps and terms in this description are defined below for additional information. These definitions are part of the description of the invention. In the event of a substantive contradiction between one of the following definitions and the rest of the description, the provisions of the description shall prevail. Doped or undoped SiO 2 - quartz glass

[0057] Undoped SiO 2 is mixed with at least one dopant as a result of the thermal treatment. The SiO 2 already initially mixed with at least one dopant consists of SiO 2 containing the dopant and / or it contains SiO 2 particles mixed with powder of the dopant(s); it can be mixed with at least one additional dopant during the thermal treatment. Quartz glass is understood here to be high-silica glass with an SiO 2 content of at least 90 mol%. Endowment

[0058] The dopant consists of one or more dopants. The "dopant" comprises at least one substance intentionally added to the glass to achieve desired properties, such as a rare earth metal and / or other dopants. Rare earth metals include the elements scandium, yttrium, and lanthanum, as well as the lanthanides. Other dopants include, for example, fluorine, chlorine, P, Al, Ge, Ga, Cs, K, B, Li, F, Cl, Fe, Ni, Co, Cr, Mn, Ti, and V. Thermal treatment

[0059] It includes process steps at elevated temperatures for the purpose of sintering (densification), vitrification (melting), purification or doping of the intermediate product.

[0060] "Sintering" or "densification" refers here to a treatment at an elevated temperature of more than 1100 °C, which results in a glass component made of opaque, non-transparent glass, in particular quartz glass.

[0061] By "vitrification" or "melting" a glass component is obtained from dense, transparent glass, especially quartz glass.

[0062] Purification involves reducing the impurity content of the intermediate product. The main impurities are OH groups, carbon-containing compounds, transition metals, alkali metals, and alkaline earth metals, which originate from the feedstock or are introduced during processing. Purification involves treatment at high temperatures (> 800°C) in a chlorine-, fluorine-, and / or oxygen-containing atmosphere.

[0063] Doping of the intermediate product is performed to adjust the refractive index or to influence other material properties such as stress birefringence, photosensitivity, and photodegradation. Doping during thermal treatment involves treatment at a temperature in the range of 400 to 1100 °C, particularly in an atmosphere containing chlorine, fluorine, boron, phosphorus, and / or germanium. Granules

[0064] A distinction can be made between built-up granulation and press granulation, and from a process engineering perspective, between wet and dry granulation processes. Common methods include roller granulation in a granulating pan, spray granulation, freeze granulation, centrifugal atomization, fluidized bed granulation, granulation using a granulating mill, compaction, roller pressing, briquetting, slurry production, or extrusion.

[0065] During granulation, the SiO2 primary particles aggregate to form discrete, larger agglomerates, referred to here as "SiO2 granulate particles" or "granule particles" for short. Collectively, the granulate particles form "SiO2 granules." Vacuum / negative pressure

[0066] The intermediate product can be heated under vacuum. The negative pressure is expressed as absolute gas pressure. A vacuum is defined as an absolute gas pressure of less than 50 mbar. viscosity

[0067] The viscosity of quartz glass is measured using a beam bending viscometer. Beam bending viscometry covers a viscosity range from 10 8 < to 10 15 < dPa s. The measurement setup includes a heatable three-point bending device with a measuring beam made of the quartz glass to be measured (beam / strip: 50 mm long, 3 mm high, 5 mm wide). The measured variable is the bending rate at the respective temperature. For undoped quartz glass, typical viscosity values ​​are around 10 12.1 < dPa s at a temperature of 1280°C and around 10 11.3 < dPa s at 1133°C. Softening temperature

[0068] As is well known, glasses cannot be assigned a specific softening temperature, but rather a softening temperature range. For the purpose of determining a temperature value, reference is made here to the definition according to DIN ISO 7884 (1998), which defines the softening temperature as the temperature at which the glass has a viscosity of 10 7.6 < dPas. Temperature values ​​in the range of 1600°C to 1730°C are cited in the literature for the softening temperature of undoped quartz glass. Intermediate product

[0069] The intermediate product can be in the form of a bed of particles or granules of a raw material containing SiO2. Alternatively, it can be a porous body made of SiO2 soot (also known as an SiO2 soot body) or a mechanically or thermally pre-compacted pressed body (pressed part), sintered body, or green body. The physical intermediate product consists of one part or is composed of several parts, which are arranged in stacks, for example. The intermediate product contains cavities, is gas-permeable, and can be modified and further compacted by thermal treatment, in particular by sintering or vitrification. Sintering or vitrification of the intermediate product produces the glass component, which consists at least partially of the doped SiO2. Incircle / Circumference

[0070] The "incircle" is the largest circle that lies entirely within a given inner contour. The "circumference" is the smallest circle that lies entirely outside a given outer contour. Example

[0071] The invention is described in more detail below with reference to an embodiment and a drawing. The drawing shows a schematic representation Figure 1 an embodiment of a cladding tube structure in a side view, Figure 2 a fixing tube for the cladding tube assembly of Figure 1 in an enlarged view and in a view of the radial cross-section, Figure 3 another embodiment of a fixing tube in radial cross-section, Figure 4 another embodiment of a fixing tube in radial cross-section, Figure 5 another embodiment of a fixing tube in radial cross-section, and Figure 6another embodiment of a cladding tube structure in a side view with an alternative clamping device. Production of a porous intermediate from doped quartz glass

[0072] In a conventional manner, a slurry of discrete, synthetically produced SiO2 particles with an average particle size of around 10 µm is prepared in ultrapure water. A quantity of 285.7 g of the slurry with a residual moisture content of 37.4% is diluted with 1000 ml of ultrapure water. A pH of 10 is adjusted by adding 75 ml of concentrated ammonia solution.

[0073] To produce quartz glass doped with Yb 2 O 3 and Al 2 O 3, an aqueous dopant solution of AlCl 3 and YbCl 3 (molar ratio 4:1) is prepared in parallel in 400 ml of ultrapure water and filtered through a 25 µm membrane filter. Instead of chlorides, other starting substances can also be used, such as organic compounds, nitrides, or fluorides.

[0074] The dopant solution is added to the agitated suspension in the form of a spray over a period of 65 minutes. To generate the spray, the dopant solution is atomized using a spray nozzle, with an operating pressure of 2 bar and a flow rate of 0.8 l / h. The spray thus generated contains droplets with an average diameter between 10 µm and 40 µm. Due to the high pH of the suspension, a coprecipitation of hydroxides and oxides of the two dopants in the form of Al(OH) 3 and Yb(OH) 3 occurs immediately. The solid particles thus formed adsorb to the existing surfaces of the SiO 2 particles and are thereby immobilized, preventing coagulation of the solid particles or sedimentation. In this way, a dopant concentration of 2 mol-% Al and 0.5 mol-% Yb (relative to the Si content of the suspension) is set in the slurry.The slurry containing the dopants is then homogenized by stirring for a further 2 hours. This procedure ensures that an optimally homogeneously doped SiO2 slurry is obtained.

[0075] The doped SiO2 slurry is frozen and further processed into granules through a process known as freeze granulation. The resulting granulate slurry is washed several times with ultrapure water, and the excess water is decanted off each time.

[0076] The ammonia-free and purified granulate sludge is then dried at a temperature of around 400°C for 6 hours. The dried granulate is sealed in a plastic mold and isostatically pressed at 400 bar. A compact is obtained (hereinafter also referred to as the "granulate compact"). Turning the pellet

[0077] The length of a pellet is typically between 70 and 150 mm. The outer surface of the pellet is machined with a turning tool until a specified outer diameter is achieved. This diameter is adjusted to the inner diameter of the cladding tube with a gap of 0.3 to 0.5 mm. In addition, the end faces of the pellet are machined to ensure they are plane-parallel.

[0078] The mechanically processed compact has a cylindrical shape with a diameter of 29.5 mm and a length of 100 mm. Its average density is approximately 40-50% of the nominal density of the fully densified doped quartz glass. It is thermally treated as an intermediate product in a modified treatment and vitrification process in the cladding tube and melted into a component made of doped, transparent quartz glass. Installation in a duct

[0079] The cladding tube structure is explained below using an example and Figure 1explained in more detail. The schematically shown cladding tube structure comprises a cladding tube 1 made of quartz glass with a circular inner cross-section, an inner diameter of 30.0 mm, and a wall thickness of 2 mm. A first holding tube 2a and a second holding tube 2b are fused to its two front ends. Both holding tubes 2a, 2b have a smaller inner diameter than the cladding tube 1, so that the respective transitions from the cladding tube 1 to the holding tubes 2a; 2b form a constriction of the inner diameter 2c.

[0080] Within the cladding tube inner bore 5, the following internal components are arranged coaxially to the horizontally oriented cladding tube longitudinal axis 11: A gas-permeable compact 3 made of doped SiO 2 raw material, which is to be thermally treated. Compact 3 is cylindrical with flat end faces and an outer diameter of 29.5 mm, slightly smaller than the inner diameter of the cladding tube. On either side of compact 3, a gas-permeable dummy compact 4a, 4b made of undoped SiO 2 is arranged, acting as a gas diffuser. The dummy compacts 4a, 4b are obtained in a similar manner to the compact by isostatically pressing granules of undoped SiO 2 in a plastic mold. Their porosity corresponds to that of the granulate compact; the average density is between 0.9 and 1.0 g / cm 3 (i.e., approximately 40-45% of the nominal density of quartz glass at 2.21 g / cm 3 ). The dummy compacts 4a, 4b are also cylindrical and have flat faces on both sides. They sandwich the compact 3 to be treated (sandwich grouping 7a). Their initial outer diameter is also 29.5 mm.This outer diameter is designed to ensure the most effective and even gas distribution possible and to protect against material loss from the compact. The porous, undoped SiO2 of the dummy compacts 4a, 4b is further densified during thermal treatment and particularly during vitrification; however, shrinkage is somewhat less than for compact 3, so that the outer diameter after vitrification is somewhat larger than the outer diameter of the blank to be produced. In an alternative procedure, the initial outer diameter of the two dummy compacts 4a, 4b is designed so that after vitrification it approximately corresponds to the outer diameter of the blank to be produced. The initial outer diameter of the two dummy compacts 4a, 4b is approximately 25 mm. One fixing tube 6a, 6b is located on either side of the sandwich grouping 7a consisting of compact 3 and dummy compacts 4a, 4b.The fixing tubes 6a, 6b are made of non-doped and non-porous quartz glass (with a nominal density of around 2.21 g / cm 3 ). They have a central through-bore with an inner diameter of 14 mm and an outer diameter that corresponds to the initial outer diameter of the compact 3, i.e. 29.5 mm s in the exemplary embodiment. Both end faces of the fixing tubes 6a, 6b are also flat. The two fixing tubes 6a, 6b close off the sandwich grouping 7a to the outside and together with this form a butt-joint arrangement 7b of the inner components 3; 4a; 4b; 6a and 6b. The inner bore of the fixing tubes 6a, 6b collapses during vitrification only in the area adjacent to the dummy compact 4a; 4b, resulting in an outer diameter of approximately 25.5 mm. Otherwise, the inner bore and thus also the initial outer diameter of the fixing tubes 6a, 6b remain unchanged.An inner pipe 8 with an outer diameter smaller than the inner diameter of the cladding tube, which is attached at one end to the free end of the fixing tube 6a and whose opposite end is provided with a standard ground joint 9. A bellows 10 made of PTFE, provided with a compression spring, is connected to the inner pipe 8 via the standard ground joint 9 and acts on it.

[0081] For heating during cleaning, doping, sintering or vitrification of the compact 3, an electrically heated ring furnace 14 is provided, which can be moved reversibly along the longitudinal axis 11, as indicated by the directional arrow 15.

[0082] The entire cladding tube internal structure with the internal components 3; 4a; 4b; 6a, 6b; 8; 9 and 10 is axially symmetrical with respect to the longitudinal axis 11 and gas-permeable for an inflowing treatment gas (indicated by the directional arrow 13).

[0083] The cladding tube internal structure is manufactured, for example, through the following steps: Step 1: The cladding tube 1, attached to the holding tube 2a on one side, is clamped in the three-jaw chuck of a glass lathe. The other end of the cladding tube 1 remains freely accessible within the working area of ​​the glass lathe. Step 2: The internal components are inserted into the cladding tube 1 through the inner bore 5 in the following order: the bellows 10, the inner pipe 8 previously connected via the ground joint 9, the first (right) fixing tube 6a, the first (right) dummy compact 4a, the compact 3 to be thermally treated, the second (left) dummy compact 4b, and the left (second) fixing tube 6b. Step 3: The internal components are pushed into the cladding tube 1 so that the bellows 10 rests against the diameter constriction 2c of the cladding tube 1 to the holding tube 2a and is compressed, i.e. tensioned, as much as possible.With the bellows 10 tensioned, the cladding tube 1 behind the last installed fixing tube 6b is heated selectively with a hand torch and pressed in at three points distributed around the circumference to create nubs 12 protruding into the cladding tube's inner bore 4, which serve as abutments for the fixing tube 6b. Step 4: The left-hand holding tube 2b is then attached to the cladding tube using a hand torch while continuously rotating around the longitudinal axis 11. Thermal treatment of the mechanically processed compact in the cladding tube

[0084] The granulate pellet 3 is heated under helium purge and then treated for approximately 8 hours in a chlorine-containing atmosphere at approximately 900 °C. The chlorine-containing treatment gas is introduced into the cladding tube 1 via the holding tube 2a at a gas flow volume of 35 sccm Cl2 and 15 sccm He (in the direction of arrow 13). At the same time, gas is extracted from the opposite side, creating a negative pressure of approximately 200 mbar (absolute pressure) in the cladding tube's inner bore. This draws the treatment gas into the pores of the pellet 3 and improves the purging.

[0085] Because the compact sits snugly in the cladding tube 1, gas flow conditions are created in which preferred flow paths around the outer circumference and dead zones with low exposure to the reactive gas are minimized. The cleaning treatment of compact 3 is therefore homogeneous across its volume and reproducible in its effect. This cleaning treatment removes impurities from compact 3 and reduces the hydroxyl group content to less than 10 ppm by weight.

[0086] The cleaned compact 3 is then subjected to a cladding tube vitrification process. The ring furnace 14 is heated to a temperature of 1250 °C and moved back and forth along the compact 3 in several strokes at a transverse speed of 10 mm / min. Simultaneously, a vacuum is applied to the cladding tube 1 by both holding tubes 1a; 2b. The reversing movement ends on both sides approximately at the midpoint of the respective fixing tube 6a; 6b.

[0087] Since the compact 3 has a comparatively low density of approximately 45% of the regular density of quartz glass, it undergoes compaction during vitrification, resulting in a significant reduction in volume. This contraction creates gaps in the internal structure of the cladding tube 1, which can lead to gas inclusions and undefined collapse of the cladding tube 1. These effects are prevented or mitigated by the cladding tube structure explained above. The clamping device causes the internal components 3; 4a; 4b; 6a and 6b, which together form a butt-joint arrangement 7b, to be butted against one another in a butt-joint manner due to longitudinal pressure. This pressure is exerted by the PTFE bellows 10 and transmitted to the internal components via the inner pipe 8 (a quartz glass tube with a standard ground joint 9 for connection to the bellows 10).The bellows is designed for a maximum deflection of approximately 2 cm and with a spring constant of 700 N / m. The inner tube 8 is long enough to keep the bellows 10 away from the ring furnace 14, preventing it from overheating during vitrification. The bellows 10 acts as a spring and is installed under tension, allowing the aforementioned contraction of the compact 3 to push forward, thus allowing the gaps to be equalized by the vitrification process itself. At the same time, a vacuum of 25 mbar (absolute pressure) is applied to the opposite end of the cladding tube's inner bore.

[0088] During vitrification, the compact 3 is compacted into bubble-free rare earth metal-doped quartz glass. At the same time, the cladding tube 1 collapses onto the compacted granulate compact 3 and the immediately surrounding dummy compacts 4a; 4b, as well as partially onto the fixing tubes 6a; 6b. The volume of the dummy compacts 4a; 4b and the fixing tubes 6a; 6b also decreases as a result of vitrification.

[0089] After the cladding tube glazing process is completed, the resulting blank is sawn off on both sides of the former compact 3 in the area of ​​the former dummy compacts 4a; 4b. This produces a preform with an outer diameter of 24 mm, containing a cylindrical core with a diameter of approximately 20 mm made of rare-earth metal-doped quartz glass and a cladding made of the cladding tube material collapsed onto it. The blank is further processed into a preform. This further processing may include, for example, grinding the cladding tube material, thermal-mechanical homogenization of the resulting ground core rod to dissolve defects (such as crystallites), and cladding the homogenized core rod with a cladding tube. A laser fiber preform is obtained, which is drawn into a fiber in the final step.

[0090] The initial outer diameters (D 0 ) of the cladding tube's inner components: granulate compact 1, dummy compacts 4a; 4b, and fixing tubes 6a; 6b, can be designed depending on the shrinkage expected during cladding tube vitrification so that, after completion of the vitrification process, the final outer diameters (D f ) are as similar as possible, thus resulting in the said inner components being arranged as flush as possible. In the present exemplary embodiment, however, the outer diameters of the cladding tube's inner components are designed for the most effective gas distribution possible during the treatment process and thus for an initially extensive filling of the inner diameter of the cladding tube's inner bore. The following Table 1 summarizes the corresponding information on the inner components. Table 1

[0091] Pressing Dummy pellet Fixing tube material Yb 2 O 3 , Al 2 O 3 , SiO 2 SiO2 SiO2 D0 [mm] 29,5 29,5 29,5 D f [mm] 20 21 26

[0092] Figure 2shows schematically an enlarged representation of the fixing tube 6a with the centrally running inner bore 20 in a view of the radial cross section.

[0093] The fixation tube 6a has a flat end face facing the compact, on which several slots are milled, extending radially from the inner bore to the cylinder surface. The slots 21 ensure optimized guidance of the treatment gases.

[0094] The Figure 3 The schematically shown embodiment of a fixing tube 30 is designed as a quartz glass cylinder with a plurality of small through-bores 31.

[0095] The fixation tube 30 acts as a gas distribution element positioned upstream of the dummy compact, allowing the treatment gas to be evenly distributed in front of the dummy compact. The smaller and more numerous the through-holes 31 are, the more effective the gas distribution. In extreme cases, the through-holes are designed as microchannels or capillaries, in which a certain back pressure builds up.

[0096] The design of the fixing tube 46 of Figure 4 has both a centrally running inner bore 40 and a plurality of grooves 41 in the cylinder surface distributed over the outer circumference, which in conjunction with the cladding tube 1 form additional continuous gas channels.

[0097] In the embodiment of the fixing tube 56 of Figure 5A centrally located inner bore 50 is also provided. Furthermore, the cylinder surface is flattened in four areas evenly distributed around the outer circumference. The flattened areas 51, in conjunction with the cladding tube 1, also form additional continuous gas channels.

[0098] Figure 6 shows an alternative cladding tube construction in a schematic representation. If the same reference numbers as in Figure 1 are used, these refer to identical or equivalent components as described above in the cladding tube structure using Figure 1 The only difference is the tensioning device, which instead of a bellows (10 in Figure 1 ) comprises a slide 60 which is displaceable in the inner bore of the cladding tube and has a magnetic element 61 made of a magnetic material, and a current-carrying coil 62 which is movable in the longitudinal axis direction 11 and is displaceable over the cladding tube 1 and along its longitudinal axis 11.

[0099] The magnetic element 61 is a ring-shaped permanent magnet welded into a double-walled quartz glass tube 63. The slide 60, together with the ring magnet fixed therein, can be displaced against one end of the pipe 8 by means of the coil 62, as indicated by the directional arrows 64, and presses it against the front end of the fixing tube 6a. For axial guidance of the pipe, several spacers 65 are distributed around the inner circumference of the cladding tube 1. An annular gap of approximately 3 mm remains between the slide 60 and the inner wall of the cladding tube 1, so that the gas flow through the cladding tube is not significantly impaired.

Claims

1. A method for producing a glass component which consists at least in part of doped, in particular rare-earth-doped SiO2, comprising the following method steps: (a) providing a cavity-containing intermediate product (3) that contains doped SiO2, (b) introducing the intermediate product (3) into a sheath (1) made of glass, which sheath comprises a longitudinal axis (11) and an inner bore (5), and (c) thermally treating the intermediate product (3) in the sheath (1), wherein a first gas-permeable gas diffuser (4a) is inserted into the sheath inner bore (5), which gas diffuser is displaceable along the sheath longitudinal axis (11) and is pressed against the intermediate product (3) during thermal treatment, wherein a pressing force is generated by means of a clamping device (8; 9; 10; 60; 61, 62; 63), which exerts a pre-pressure on the first gas diffuser (4a), wherein the clamping device (8; 9; 10; 60; 61; 62; 63) comprises an elastically deformable spring element inserted in the sheath inner bore (5) or a slide (61; 63), which is displaceable in the sheath inner bore (5) and has a first magnet element (61) made of a magnetic material, and a second magnetic element, which is movable in the longitudinal axis direction (11) outside the sheath inner bore (5) and acts on the magnet element (61).

2. The method according to claim 1, characterized in that the first gas diffuser (4a) consists of a porous glass which has the same and particularly preferably a higher viscosity than the doped or non-doped SiOz.

3. The method according to either of the preceding claims, characterized in that the intermediate product (3) is designed as a soot body, sintered body or pressed part and has an outer cross-sectional shape which fills an inner cross-sectional shape of the sheath (1) while leaving a gap with a maximum gap width of 1 mm.

4. The method according to any of the preceding claims, characterized in that the intermediate product (3) is arranged between the first gas diffuser (4a) and a second gas-permeable gas diffuser (4b).

5. The method according to claim 4, characterized in that the first gas diffuser (4a) and the second gas diffuser (4b) are cylindrical and each have a flat end face abutting the intermediate product (3).

6. The method according to any of the preceding claims, characterized in that a first fixing element (6a) is inserted into the sheath inner bore (5), in which or on which fixing element at least one continuous gas channel (20; 41; 51) is formed and which is displaced along the sheath longitudinal axis (11) and pressed against the first gas diffuser (4a).

7. The method according to claim 6, characterized in that the first fixing element (6a) consists of a gas-tight material, and preferably consists of quartz glass made of synthetically produced raw materials.

8. The method according to either of claims 4 and 5 and either of claims 6 or 7, characterized in that the first fixing element (6a) is arranged upstream of the first gas diffuser (4a) in the flow direction (13) of a treatment gas, and in that a second fixing element (6b) is arranged downstream of the second gas diffuser (4b) in the flow direction (13) and abuts said gas diffuser, with preferably the first fixing element (6a), the first gas diffuser (4a), the intermediate product (3) to be treated, the second gas diffuser (4b) and the second fixing element (6b) intermittently abutting one another in this order.

9. The method according to claim 1, characterized in that the elastically deformable spring element comprises a bellows (10).

10. The method according to any of the preceding claims, characterized in that a negative pressure is applied in the sheath inner bore at one sheath end (1b) and / or a treatment gas (13) is introduced into the sheath (1) at an opposite sheath end (1a).

11. The method according to any of the preceding claims, characterized in that the intermediate product (3) is heated in an electrically heated ring furnace (14) for thermal treatment.

12. A device for producing a glass component, comprising a sheath (1) made of glass for receiving a cavity-containing intermediate product (3) and an electrically heatable ring furnace (14) for heating the intermediate product (3), the sheath (1) comprising a longitudinal axis (11) and an inner bore (5), characterized in that a first gas-permeable gas diffuser (4a) is inserted into the sheath inner bore (5), which gas diffuser is displaceable along the sheath longitudinal axis (11) against the intermediate product (3), a clamping device (8; 9; 10; 61; 62; 63) being inserted into the sheath inner bore (5), which clamping device exerts a pre-pressure on the first gas diffuser (4a), the clamping device (8; 9; 10; 60; 61; 62; 63) comprising an elastically deformable spring element inserted in the sheath inner bore (5) or a slide (61; 63), which is displaceable in the sheath inner bore (5) and has a first magnet element (61) made of a magnetic material, and a second magnetic element, which is movable in the longitudinal axis direction (11) outside the sheath inner bore (5) and acts on the magnet element (61).

13. The device according to claim 12, characterized in that a first fixing element (6a) is inserted into the sheath inner bore (5), which fixing element is displaceable along the sheath longitudinal axis (11) and presses against the first gas diffuser (4a), and in which or on which at least one continuous gas channel (20; 41, 51) is formed.