WO2007039426A1

WO2007039426A1 - Welding method for joining components consisting of a high-silica material, and apparatus for performing the method

Info

Publication number: WO2007039426A1
Application number: PCT/EP2006/066354
Authority: WO
Inventors: Oliver Ganz; Thomas Bogdahn
Original assignee: Heraeus Quarzglas Gmbh & Co. Kg; Shin-Etsu Quartz Products Co., Ltd.
Priority date: 2005-09-20
Filing date: 2006-09-14
Publication date: 2007-04-12
Also published as: JP5114409B2; DE102005044947A1; JP2009508789A; DE102005044947B4

Abstract

In a known welding method for joining a first and at least one second component consisting of a high-silica material, an integral bond is formed between connection surfaces of the components, comprising the steps of clamping the components in a rotation and displacement means, moving the components towards each other by means of the rotation and displacement means in the direction of a central axis, simultaneously heating and softening the components in the area of the mutual connection surfaces by means of at least one heating burner, pressing the connection surfaces against each other to form a component assembly including a weld seam, and cooling the component assembly. Starting therefrom, to provide a method by means of which a solid weld seam can be produced in an exact and reproducible manner between quartz glass components to be joined, with impurities being largely avoided, it is suggested according to the invention that the components should be heated and softened in the area of the mutual connection surfaces within an enclosure enclosing an interior which is open at both sides.

Description

Welding method for joining components consisting of a high-silica material, and apparatus for performing the method

The present invention relates to a welding method for joining a first and at least one second component consisting of a high-silica material by forming an integral bond between connection surfaces of the components, comprising the steps of clamping the components in a rotation and displacement means, moving the components towards each other by means of the rotation and displacement means in the direction of a central axis, simultaneously heating and softening the components in the area of the mutual connection surfaces by means of at least one heating burner, pressing the connection surfaces against each other to form a component assembly including a weld seam, and cooling the component assembly.

Furthermore, the present invention relates to an apparatus for producing a welded joint between a first and at least one second component consisting of a high-silica material, comprising a rotation and displacement means for clamping and moving the components in the direction of a central axis, and at least one heating burner for heating and softening the components in the area of connection surfaces of the components.

A high-silica material is here understood to be a doped or undoped quartz glass having an SiO₂ content of at least 85%. This material shall also be briefly called "quartz glass" in the following. Quartz glass is distinguished by a low coefficient of thermal expansion, by optical transparence over a wide wavelength range and by high chemical and thermal resistance.

Quartz glass components are used for many applications, e.g. as semifinished products in the making of optical fibers in the form of tubes or solid cylinders, in lamp manufacture as sleeve tubes, bulbs, cover plates or reflector carriers for lamps and radiators in the ultraviolet, infrared and visible spectral range, in chemical apparatus construction or in semiconductor manufacture in the form of reactors and apparatus of quartz glass for the treatment of semiconductor components, jigs, bell jars, crucibles, protective shields or simple quartz glass components, such as tubes, rods, plates, flanges, rings or blocks. For producing special properties quartz glass is doped with other substances, such as titanium, aluminum, boron, germanium.

The task often arises to join individual quartz glass components to one another, for example for making quartz glass bodies having a complex shape or for connection to a quartz glass element fulfilling a special function, for example for mounting or shaping. Reference should be made by way of example to a component assembly of quartz glass cylinders as is used in the making of optical fibers or optical preforms in an elongation process. In the elongation process, special emphasis is laid on the pulling phase during which at the softened end of the preliminary product a drawing bulb is formed, from which the component can be drawn off with a predetermined geometry and desired dimension. The formation of the drawing bulb requires some time, which is at the expense of the productivity of the drawing furnace and, since the drawing furnace is thermally very stressed during the pulling phase, also at the expense of the service life of the drawing furnace. With an increasing outer diameter of the preliminary product the time spent on the pulling phase is also increasing.

The unproductive pulling phase can be reduced considerably in that the lower end of the preliminary product is first provided with an outer shape resembling a drawing bulb. To this end it is suggested in JP 10-182179 A (1998) that the lower end of a quartz glass cylinder to be elongated should have welded thereto an attachment piece in the form of a dummy tube with a smaller diameter. This shortens the pulling process and reduces the loss of material at the same time.

As a rule, a component of quartz glass that serves as a holder is also welded to the upper end of the quartz glass cylinder to be elongated. A method suited for producing such an assembly is described in EP 1 042 241 A1. To butt weld a quartz glass tube, which serves as a preform for an optical fiber, to a dummy tube, it is suggested there that the front surfaces to be connected to each other should be chamfered before the formation of the melt connection and the connection surfaces should thereafter be molten by means of a propane/oxygen burner and the connection surfaces should then be pressed against each other. As an alternative to the propane/oxygen burner, it is also possible to use an electrically heated furnace for softening the connection surfaces.

The welding process includes the steps of clamping both quartz glass components in a lathe, moving the quartz glass components towards each other by means of the lathe, the joint and simultaneous heating and softening of the faces of both quartz glass components and the subsequent pressing of the softened faces against each other to form a component assembly having a weld seam, and cooling the component assembly to ambient temperature.

If necessary, a graphite template is pressed against the softened outer surface and the surface is shaped in this process.

When quartz glass components are welded, impurities may be formed or released from the ambient atmosphere, the heating burner or from boundary walls. The particles may here deposit on the quartz glass components to be joined and particularly on the softened connection surfaces, which particles represent harmful impurities and may lead to bubbles or other flaws on boundary surfaces or even to breakage during further processing of the assembly.

Moreover, during welding an undesired plastic deformation is likely to occur in the area of the melting zone. Although the deformation can again be eliminated by troublesome mechanical reworking, dimensional deviations are normally found. Such plastic deformations are promoted by irregular and undefined heating conditions during the welding process.

It is the object of the present invention to indicate a method by means of which a solid weld seam can be produced in an exact and reproducible manner between quartz glass components to be joined, with impurities being largely avoided.

It is further the object of the present invention to provide a simple, operationally reliable and inexpensive apparatus for producing a welded joint between quartz - A -

glass components, which apparatus is particularly suited for carrying out the method of the invention.

As for the method, this object, starting from the above-indicated welding method, is achieved according to the invention in that the components are heated and softened in the area of the mutual connection surfaces within an enclosure enclosing an interior, which comprises openings which are opposite each other in the direction of the central axis and through which the components to be joined project into the interior, and which comprises a wall of quartz glass which is provided with a free surface in the form of an inner layer of high-purity quartz glass.

According to the invention one or several heating burners are used for heating and softening the components in the area of the connection surfaces. In comparison with the use of an electric furnace employed for this purpose, said measure entails a better energy balance and less capital spending on the system.

In contrast to the prior art the components are heated and softened by means of the heating burner inside an enclosure. The enclosure serves thermal insulation and acts as a heat reservoir at the same time. This yields a locally homogeneous heating profile that is uniform in time and has an advantageous effect on the quality of the welded joint and facilitates the reproducible manufacture thereof. The enclosure reduces the heat loss of the heating burner and also facilitates a defined slow cooling of the components that are welded together, so that thermal stresses that might lead to cracks and breakage of the weld seam are minimized.

Moreover, the enclosure substantially shields the interior from the external environment, so that stray particles or other impurities are largely kept away from the heating zone.

The enclosure substantially surrounds only that area in which the components are softened and welded together. At both sides with respect thereto, it is provided with two openings that are opposite each other in the central axis and through which the components to be joined are introduced into the interior. Moreover, flushing gas which discharges impurities can flow through the interior. The enclosure comprises a wall of quartz glass. Quartz glass is a specific material with respect to the material of the components to be joined.

Moreover, the wall is provided with a free surface in the form of an inner layer of high-purity quartz glass. The wall itself may here consist of inexpensive quartz glass of minor quality. The inner layer of high-purity quartz glass prevents or reduces the exit of impurities out of the wall towards the heating zone.

High-purity quartz glass is here understood to be a quartz glass whose total content of the oxides of Li, Na, K, Mg, Ca, Fe, Cu, Ni, Cr and Mn is less than 10 wt ppm.

The high-purity quartz glass may be synthetically produced quartz glass or quartz glass of naturally occurring raw material, which has been subjected to hot chlohnation in a chlorine-containing atmosphere.

A particularly preferred variant of the method is distinguished in that the interior, viewed in the direction of the central axis, has a round cross-section.

The round cross-section of the interior facilitates the setting of a heating profile that is coaxial to the central axis, and thus a uniform heating of components having a round cross-section. Moreover, this simplifies the adjustment of a more or less laminar flushing gas flow in the direction of the central axis, and dead corners are avoided where impurities or particles may accumulate in the course of time.

It has turned out to be particularly useful when the wall of the enclosure consists essentially of opaque quartz glass that is molten from naturally occurring raw material.

The opacity of the quartz glass improves the heat-insulating effect of the enclosure and thus facilitates the setting of a homogeneous temperature profile that is constant in time, in the area of the heating zone. Preference should here be given to the use of quartz glass of naturally occurring raw materials over synthetic quartz glass for reasons of costs. Opaque quartz glass tubes are particularly suited for this purpose and are e.g. commercially available at low costs under the trade name "Rotosil" of Heraeus Quarzglas GmbH & Co., KG, Hanau. As a rule, when opaque tubes of quartz glass are produced, a transparent dense surface layer is formed on the surface closest to the heating element during vitrification. This surface layer has a maximum thickness of 3 mm and does not impair the heat- insulating effect of the otherwise opaque wall.

For reasons of costs a procedure is preferred in which the high-purity quartz glass of the inner layer is obtained from naturally occurring raw material that has been subjected to hot chlorination in a chlorine-containing atmosphere.

It has also turned out to be useful when the high-purity quartz glass consists of opaque quartz glass having a density ranging from 2.15 to 2.18 g/cm³ and is made from raw material which has been obtained by wet milling amorphous SiO₂ grain. Such a quartz glass is e.g. commercially available under the trade name OM 100" of Heraeus Quarzglas GmbH & Co. KG, Hanau. The quartz glass is sintered from SiO₂ particles having a particle size in the range of not more than 500 μm, preferably not more than 100 μm, SiO₂ particles with particles sizes in the range between 1 μm and 100 μm accounting for the largest volume portion. The particles are produced in a slip method by wet milling coarse amorphous particles, wherein due to interactions the SiO₂ particles are already interacting in the aqueous slip of high density, said particles improving the stability of the slip and enhancing the density of the resulting opaque quartz glass.

It has turned out to be advantageous when the thickness of the inner layer is in the range of 10 mm to 12 mm.

The thicker the inner layer, the more efficient is its action regarding the prevention of contamination of the components to be joined. On the other hand, it also gets the more expensive because of the use of high-quality raw material for making the inner layer. The said thickness range constitutes a suitable compromise between these requirements.

The heating burner may be arranged inside the enclosure. Preferred is however a variant of the method wherein the at least one heating burner projects through a lateral opening of the enclosure into the interior. The heating burner is thus substantially arranged outside the enclosure. The heat- insulating wall of the enclosure shields the heating burners and the media supply lines from the heat of the heating zone. Moreover, as a consequence, the heating burner does not interfere with, or only interferes little, with a possible flushing gas flow through the enclosure.

It has turned out to be particularly advantageous when purified air is supplied to the heating burner.

This ensures that the at least one heating burner sucks pure air, so that contamination of the components to be joined by particles or the like is substantially avoided.

Advantageously, the enclosure is a multi-part structure.

The multi-part configuration facilitates the mounting of the enclosure. Moreover, this makes maintenance easier and less expensive because only worn parts of the enclosure have to be replaced.

Moreover, it has turned out to be advantageous when the component assembly is cooled within the enclosure.

The enclosure ensures a comparatively slow cooling of the component assembly heated up to the process temperature. In this respect the enclosure can assume the function of an annealing furnace.

Preferably, the components are heated and softened at a process temperature above 2200 °C, cooling from the process temperature to a temperature of 1000°C requiring a time interval of at least 5 minutes.

Thermal stresses are minimized by way of a comparatively slow cooling of the component assembly from the process temperature.

In one embodiment of the enclosure with a round cross-section, a procedure is preferred in which the components to be welded to each other have a cylindrical shape, with the ratio between the maximum outer diameter of the cylinder and the inner diameter of the interior being set in the range between 1.5 and 3, preferably in the range between 2 and 2.5.

It has been found that the annular gap remaining in this process between the enclosure and the outer surface of the component is optimum with respect to heat transfer and a possible flushing gas flow.

As for the apparatus, the above-indicated object, starting from an apparatus of the above-mentioned type, is achieved according to the invention in that an enclosure is provided which encloses an interior and within which the components are heated and softened in the area of the mutual connection surfaces, and which comprises openings which are opposite each other in the direction of a central axis and through which the components to be joined project into the interior, and which comprises a wall of quartz glass which is provided with a free surface in the form of an inner layer of high-purity quartz glass.

With the help of one or several heating burners, a heating zone is produced within which the components are heated and softened in the area of their connection surfaces. In contrast to the prior art, the heating zone is provided inside an enclosure. The enclosure serves the purpose of thermal insulation and simultaneously acts as a heat reservoir. This produces a locally homogeneous heating profile that is uniform in time and has an advantageous effect on the quality of the welded joint and facilitates the reproducible manufacture thereof. The enclosure reduces the heat loss of the heating burner and additionally facilitates a defined slow cooling of the components that are welded together, so that thermal stresses that might lead to cracks and breakage of the weld seam are minimized.

The enclosure substantially shields the interior from the external environment, so that stray particles or other impurities are substantially kept away from the heating zone.

The interior substantially surrounds only the heating area where the components to be joined are heated and welded.

Hence, the enclosure comprises openings that are opposite each other in the direction of the central axis and through which the components to be joined extend into the interior. Moreover, flushing gas that discharges impurities can flow through the enclosure, namely through the openings.

Advantageous developments of the apparatus according to the invention become apparent from the subclaims. Insofar as configurations of the apparatus indicated in the subclaims imitate the procedures outlined in subclaims with respect to the method of the invention, reference is made to the corresponding method claims for a supplementary explanation with respect to the above statements.

The invention shall now be explained in more detail with reference to embodiments and a drawing. What is shown in detail in a schematic illustration is in

Fig. 1 an embodiment of the apparatus of the invention with an enclosure in the form of a muffle tube in a view on the jacket surface thereof; and

Fig. 2 the muffle tube according to Fig. 1 in a view on the front side thereof.

The apparatus according to Figs. 1 and 2 serves the front-sided welding of a holder in the form of a quartz glass tube 1 to a hollow cylinder 2. The assembly to be produced, which consists of quartz glass tube 1 and hollow cylinder 2, is to be elongated into a preform for optical fibers or directly into the optical fiber in combination with a so-called core rod, which is inserted into the inner bore of the hollow cylinder 2.

The quartz glass tube 1 consists of quartz glass of minor quality which may for example contain large amounts of impurities, bubbles, etc. At the same inner diameter the quartz glass tube 1 has a slightly thinner wall thickness than the quartz glass cylinder 2. During the elongation process the hollow cylinder 2 is held in the drawing furnace by means of the quartz glass tube 1 , and/or the quartz glass tube 1 serves pulling purposes during elongation. To this end the hollow cylinder 2 of quartz glass is provided at one end or at both ends with such a quartz glass tube.

Furthermore, the apparatus comprises a lathe in the chucks 3 of which the quartz glass tube 1 is clamped on the one hand and the hollow cylinder 2 on the other hand in such a manner that their central axes 4 are coaxial to each other and the front sides to be welded are opposite each other. The opposite areas of quartz glass tube 1 and hollow cylinder 2 are heated and softened inside a muffle tube 5 of opaque quartz glass having an inner diameter "D" of 400 mm. The muffle tube 5 is configured as a three-part member and it is open at both sides. The central part is provided in its lateral wall with an opening 6 through which the two heating burners 7, 8 project into the interior 9.

The muffle tube 5 consists of a tubular basic body 10 of quartz glass which is molten from naturally occurring quartz and which is commercially available under the name "Rotosil". The muffle tube 5 is provided over its whole length with a longitudinal slit which serves as an expansion joint. The inner side of the basic body 10 which faces the interior 9 is lined with an inner layer 11 having a layer thickness of 10 mm to 12 mm, which is molten from high-quality quartz grain. This is grain of naturally occurring quartz sand which is commercially available under the name "IOTA Standard" in an already pre-cleaned form (supplier: Unimin Corp., USA). The already pre-cleaned quartz sand is subjected to a further cleaning process in a chlorine-containing atmosphere at a high temperature (about 900 °C) prior to its use for melting the inner layer. Table 1 reveals the typical impurities of IOTA standard before and after hot chlorination.

Table 1

The concentration data given in table 1 refer to wt ppb. The impurity contents were measured by means of ICP-OES.

As an alternative, the muffle tube 5 of high-purity opaque quartz glass is made from a material which is commercially available under the trade name ,,OM 100" of Heraeus Quarzglas GmbH & Co. KG, Hanau. The opaque quartz glass is sintered from SiO₂ particles having a mean particle size of about 60 μm, which have been produced in a slip method by wet milling coarse amorphous particles. Said opaque quartz glass is distinguished by high purity and a density in the range around 2.16 g/cm³.

At the side opposite to the heating burners 7, 8, the inside of the muffle tube 5 is covered with a simple shell-like insertion part 13 of high-purity quartz glass, which protects the constructionally comparatively complicated central part of the muffle tube 5 from the action of heat and which, in addition, enhances the heat capacity of the total structure and thus makes the temperature distribution inside the muffle tube 5 uniform.

Holders (not shown in the figure) of high-purity graphite serve to position and fix the muffle tube 5.

A suction device 12 is provided above the muffle tube 5. Said suction device extends in part along the front openings of the muffle tube 5 and sucks off the hot exhaust gas.

Furthermore, a supply device (not shown in Fig. 1 ) for ultrapure air is provided around the burner mouth of the heating burners 7, 8 and ensures that the two heating burners 7, 8 only suck pure air so that contamination of the components 1 , 2 to be joined is largely avoided. The heating burners 7, 8 are made from quartz glass.

This embodiment of the apparatus according to the invention using a muffle tube 5 is distinguished by a reduced output of impurities to the environment in comparison with an enclosure composed of individual modules of refractory material, particularly bricks made of quartz glass.

An embodiment of the method according to the invention shall now be explained in more detail with reference to Figs. 1 and 2.

A quartz glass blank is produced with the help of the known OVD method by evaporating high-purity silicon tetrachloride and by flame hydrolysis in an oxyhydrogen gas flame. In this process SiO₂ particles are deposited on a rotating quartz glass rod, resulting in a large-volume porous soot material that is subsequently vitrified at 1600 °C with formation of the quartz glass blank. Both ends of the quartz glass blank are cut off and the outer peripheral surface is ground by means of a cylinder grinding machine. This yields a hollow cylinder of quartz glass in that the quartz glass blank is pierced by using a core drill. The resulting hollow cylinder of quartz glass has an outer diameter of 180 mm and an inner diameter of 50 mm and is subsequently subjected to an etching treatment in hydrofluoric acid and flushed with pure water and dried.

For joining this high-quality hollow cylinder 2 with a quartz glass tube 1 of low- purity quartz glass, hollow cylinder 2 and quartz glass tube 1 are clamped with coaxial central axes 4 into the chucks 3 of the lathe and moved towards each other by means of the lathe drive so that they are opposite each other inside the muffle tube 5 in the operative area of the heating burners 7, 8. The areas facing the weld are heated by means of the heating burners 7, 8 to a temperature of about 2200 °C to 2300 °C for a period of 20 minutes and are softened in this process, the front faces of hollow cylinder 2 and quartz glass tube 1 being simultaneously pressed against each other. An oxygen flow is here passed through the bores of hollow cylinder 2 and quartz glass tube 1.

The muffle tube 5 ensures a homogeneous temperature distribution during the welding process. After the completion thereof, the component assembly (2, 1 ) produced in this way remains inside the muffle tube 5 for about 10 minutes, slowly cooling in this process to a temperature below 1000°C.

Static tensile-strength measurements revealed that no breakage had occurred in the area of the weld even if the maximum test load of 3 tons was applied.

In an alternative procedure, which has turned out to be particularly advantageous for welding large components, the heating burners 7, 8 are not arranged in parallel with each other, but are distributed in the area of the mutual connection surfaces around the wall of the muffle tube 5. This distributes the heating action of the heating burners 7, 8 over a larger area of the muffle tube 5, resulting in a smaller thermal load thereon. The heating burners 7, 8 may e.g. be positioned opposite each other on the muffle tube wall.

Claims

Patent Claims

1. A welding method for joining a first and at least one second component consisting of a high-silica material by forming an integral bond between connection surfaces of the components (1 , 2), comprising the steps of clamping the components (1 , 2) in a rotation and displacement means (3), moving the components (1 , 2) towards each other by means of the rotation and displacement means (3) in the direction of a central axis (4), simultaneously heating and softening the components (1 , 2) in the area of the mutual connection surfaces by means of at least one heating burner (7, 8), pressing the connection surfaces against each other to form a component assembly including a weld seam, and cooling the component assembly, characterized in that the components (1 , 2) are heated and softened in the area of the mutual connection surfaces within an enclosure

(5) enclosing an interior (9), which comprises openings which are opposite each other in the direction of a central axis and through which the components to be joined project into the interior, and which comprises a wall (10, 1 1 ) of quartz glass which is provided with a free surface in the form of an inner layer of high-purity quartz glass.

2. The method according to claim 1 , characterized in that the interior (9), viewed in the direction of the central axis (4), has a round cross-section.

3. The method according to claim 1 or 2, characterized in that at least part of the wall (10, 1 1 ) consists of opaque quartz glass which is molten from naturally occurring raw material.

4. The method according to any one of the preceding claims, characterized in that the high-purity quartz glass is obtained from naturally occurring raw material which has been subjected to hot chlorination in a chlorine- containing atmosphere.

5. The method according to any one of the preceding claims, characterized in that the high-purity quartz glass consists of opaque quartz glass having a density ranging from 2.15 to 2.18 g/cm³ and is made from raw material which has been obtained by wet milling amorphous SiO₂ grain.

6. The method according to any one of the preceding claims, characterized in that the thickness of the inner layer (1 1 ) is in the range of 10 mm to 12 mm.

7. The method according to any one of the preceding claims, characterized in that the at least one heating burner (7, 8) projects through a lateral opening (6) of the enclosure (5) into the interior (9).

8. The method according to any one of the preceding claims, characterized in that purified air is supplied to the heating burner.

9. The method according to any one of the preceding claims, characterized in that the enclosure (5) is a multi-part structure.

10. The method according to any one of the preceding claims, characterized in that the component assembly is cooled within the enclosure (5).

1 1. The method according to claim 10, characterized in that the components (1 , 2) are heated and softened at a process temperature above 2200 °C and that cooling from the process temperature to a temperature of 1000°C requires a time interval of at least 5 minutes.

12. The method according to claim 2 and one or several preceding claims, characterized in that the components (1 , 2) to be welded to each other have a cylindrical shape, and that the ratio between the maximum outer diameter of the cylinder and the inner diameter of the interior (9) is set in the range between 1.5 and 3, preferably in the range between 2 and 2.5.

13. An apparatus for producing a welded joint between a first and at least one second component consisting of a high-silica material, comprising a rotation and displacement means (3) for clamping and moving the components (1 , 2) in the direction of a central axis (4), and at least one heating burner (7, 8) for heating and softening the components (1 , 2) in the area of connection surfaces of the components (1 , 2), characterized in that an enclosure (5) is provided which encloses an interior (9) and within which the components (1 , 2) are heated and softened in the area of the mutual connection surfaces, and which comprises openings which are opposite each other in the direction of a central axis and through which the components to be joined project into the interior, and which comprises a wall (10, 1 1 ) of quartz glass which is provided with a free surface in the form of an inner layer (11 ) of high-purity quartz glass.

14. The apparatus according to claim 13, characterized in that the interior (9), viewed in the direction of the central axis (4), has a round cross-section.

15. The apparatus according to any one of claims 13 or 14, characterized in that at least part (10) of the wall (10, 1 1 ) consists of opaque quartz glass which is molten from naturally occurring raw material.

16. The apparatus according to any one of claims 13 to 15, characterized in that the thickness of the inner layer (1 1 ) is in the range of 10 mm to 12 mm.

17. The apparatus according to any one of the preceding apparatus claims, characterized in that the at least one heating burner (7, 8) projects through a lateral opening (6) of the enclosure (5) into the interior (9).

18. The apparatus according to any one of the preceding apparatus claims, characterized in that purified air is supplied to the at least one heating burner

(7, 8).

19. The apparatus according to any one of the preceding apparatus claims, characterized in that the enclosure (5) is a multi-part structure.

20. The apparatus according to claim 14 and one or several preceding apparatus claims, characterized in that the components (1 , 2) to be welded to each other have a cylindrical shape, and that the ratio between the maximum outer diameter of the cylinder and the inner diameter of the interior is set in the range between 1.5 and 3, preferably in the range between 2 and 2.5.