DE102020124916A1 - Attachment of an optically transparent window to a metal work piece - Google Patents

Abstract

The invention relates to a method for closing an opening (2) in a metallic workpiece (1) with an optically transparent window (3), with the following method steps: - producing a stacked arrangement (4) over the opening (2) in the metallic workpiece (1 ) with the following components: a first soldering ring (5) on the metallic workpiece (1), a balancing ring (6) on the first soldering ring (5), a second soldering ring (7) on the balancing ring (6), a retaining ring (8) on the second solder ring (7), a third solder ring (9) on the retaining ring (8) and the optically transparent window (3) on the third solder ring (9), - heating the metallic workpiece (1) together with the stacked assembly (4 ) so that the solder rings (5, 7, 9) melt, and- cooling the metallic workpiece (1) together with the stacked assembly (4) to room temperature, the retaining ring (8) being made of a material whose maximum thermal expansion coefficient 15% from that of the material of the optically transparent window (3), and the compensating ring (6) is made of a material whose modulus of elasticity is at most 80% of the modulus of elasticity of the retaining ring (8) and at most 80% of the modulus of elasticity of the metallic workpiece (1) amounts to. This provides a possibility for stable sealing of an opening (2) in a metal workpiece (1) with an optically transparent window (3), which is inexpensive and reliable over a wide temperature range.

Description

The invention relates to a method for closing an opening in a metal workpiece with an optically transparent window and an arrangement of an optically transparent window on a metal workpiece for closing an opening in the metal workpiece.

Optical methods are used in many areas of research and industry. In many cases, the system or material to be examined must be enclosed in a container with a limited volume. Accordingly, these sealed enclosures require optical access which must be sealed to the rest of the enclosure. An example are sight glasses for process control in the chemical industry. The seal required between the actual container material and an optical access point, also referred to below as a window, is in many cases exposed to high mechanical stress. In this case, the seal must meet the following criteria: It must create a connection between a container made of temperature-resistant stainless steel, for example, and an optically transparent window (e.g. made of sapphire). The seal must be stable, have a very low leakage rate and be chemically inert even at high pressures. Pressures of up to several hundred bar can prevail in the container. In many cases, the seal must also be stable at a negative pressure of up to 1 bar, namely when the container is evacuated. These requirements often apply over a very wide temperature range, both below and above room temperature. This can be used in particular for cleaning vacuum apertures in the baking process at comparatively very high temperatures, or also at high process temperatures in the chemical industry.

In principle, there are various options for a temperature-stable seal between the window material and the container. It is always important that there are as few gaps as possible between the materials, as the enclosed medium can escape through these. The simplest option is to process both materials as precisely as possible and then join them together mechanically and, if necessary, press them. This method can be extended by inserting a sealing material that is as flexible as possible between the two materials, which is beneficial due to its mechanical properties, since it can compensate for unevenness. In principle, such a procedure is also known from everyday life, e.g. in the form of using a rubber seal. Mica materials, for example, are known from the prior art for temperature-resistant seals. Finally, the optically transparent material can also be soldered into the container material. The two materials are joined with molten metal. However, there are certain technical obstacles to this approach, as explained below.

The aforementioned method, in which both materials are processed as accurately as possible and then mechanically joined together and possibly pressed together, has the disadvantage that high demands are placed on the surface quality of the materials, since unevenness can result in leaks. Furthermore, the materials must also be mounted with sufficient force so that there are no gaps even when there is internal pressure. This can lead to problems with the mechanical stability of the materials used. This is intensified in particular when the materials expand at different rates at extreme temperatures, creating stresses in the material that can lead to fracture.

As mentioned above, these problems can be alleviated by using a suitable sealing material, since the sealing material can even out any unevenness. Furthermore, if the sealing material is flexible enough, it can compensate for slight displacements of the materials relative to one another. Nevertheless, fundamental problems remain even when using a sealing material. In particular, the sealing materials available at extreme temperatures are very limited in terms of their flexibility. Depending on the requirements, there is also the problem that the sealing materials are not always chemically inert, especially at high temperatures.

Finally, the previously mentioned soldering has the disadvantage that both when using the windows at extreme temperatures and even during the soldering process, stresses can arise due to the different expansion coefficients of the materials. In the case of crystals as window material, this leads to problems in particular because these, like most glasses that can be used as window materials, are quite brittle and tend to break when force is applied. To prevent this, there are several options.

On the one hand, a construction that is as flexible as possible can be used. Here the wall thickness of the metallic workpiece that accommodates the window in the form of the optically transparent material should be so thin that the transmitted force it is not damaged. It is also ensured that the contact surface between the workpiece and the window is as small as possible in order to reduce stresses that occur there. This option is widely used in vacuum technology, but due to the design it is not suitable for high pressures and the high mechanical forces associated with them.

Furthermore, particularly suitable metal alloys can be used. These have an expansion coefficient that is close to that of the window. These materials entail other disadvantages, such as difficult processing in particular. In addition, the overall construction can be strengthened. However, this is often not practicable due to other design aspects to be considered. In addition, cracks can occur at the connection points between the window and the workpiece, since the basic problem of the different expansion of the materials used remains.

From the U.S. 6,212,989 B1 discloses a window assembly for transmitting light in and out of a container containing a high pressure, high temperature, highly reactive environment. This window assembly includes a frusto-conical window, a frusto-conical metal gasket that fits circumferentially against the window, and a housing to which the window and gasket are attached. To facilitate automatic cleaning of the window, the surface of the window exposed to the high pressure environment is substantially flush with the high pressure surface of the housing. During its assembly, the window assembly is first set by mechanical pressing and then subjected to a series of successively higher combustion pressures. the U.S. 6,065,630 A describes a pressure vessel in which PTFE is used as the sealing material. Furthermore, in the U.S. 8,564,768 B2 called the soldering of sapphire windows. Finally in the U.S. 4,614,428 A describes a pressure vessel that is made entirely of glass and can withstand pressures of up to 1000 bar.

Proceeding from this, it is the object of the invention to specify a possibility for the stable closing of an opening of a metal workpiece with an optically transparent window, which is cost-effective and reliable over a wide temperature range.

This object is solved by the subject matter of the independent patent claims. Preferred developments can be found in the dependent claims.

• - Herstellen einer gestapelten Anordnung über der Öffnung des metallischen Werkstücks mit folgenden Komponenten:
  • einem ersten Lotring auf dem metallischen Werkstück,
  • einem Ausgleichsring auf dem ersten Lotring,
  • einem zweiten Lotring auf dem Ausgleichsring,
  • einem Haltering auf dem zweiten Lotring,
  • einem dritten Lotring auf dem Haltering und
  • dem optisch transparenten Fenster auf dem dritten Lotring,
• - Erhitzen des metallischen Werkstücks zusammen mit der gestapelten Anordnung, so dass es zum Schmelzen der Lotringe kommt, und
• - Abkühlen des metallischen Werkstücks zusammen mit der gestapelten Anordnung auf Zimmertemperatur, wobei

der Haltering aus einem Material hergestellt ist, dessen thermischer Ausdehnungskoeffizient maximal 15 % von dem des Materials des optisch transparenten Fensters abweicht, bevorzugt maximal 10 %, ganz besonders besonders bevorzugt maximal 5 %, und
der Ausgleichsring aus einem Material hergestellt ist, dessen Elastizitätsmodul höchstens 80 %, bevorzugt maximal 70 %, ganz besonders besonders bevorzugt höchstens 50 % des Elastizitätsmoduls des Haltrings sowie höchstens 80 %, bevorzugt maximal 70 %, ganz besonders besonders bevorzugt höchstens 50 % des Elastizitätsmoduls des metallischen Werkstücks beträgt.According to the invention, a method for closing an opening in a metallic workpiece with an optically transparent window is thus provided, which has the following method steps:

• - Making a stacked assembly over the opening of the metal work piece with the following components:
  • a first solder ring on the metallic workpiece,
  • a balancing ring on the first solder ring,
  • a second solder ring on the balancing ring,
  • a retaining ring on the second solder ring,
  • a third solder ring on the retaining ring and
  • the optically transparent window on the third solder ring,
• - heating the metallic workpiece together with the stacked assembly so that the solder rings melt, and
• - Cooling the metallic workpiece together with the stacked assembly to room temperature, wherein

the retaining ring is made of a material whose coefficient of thermal expansion differs by a maximum of 15% from that of the material of the optically transparent window, preferably by a maximum of 10%, very particularly preferably by a maximum of 5%, and
the compensating ring is made of a material whose modulus of elasticity is at most 80%, preferably at most 70%, very particularly preferably at most 50% of the modulus of elasticity of the retaining ring and at most 80%, preferably at most 70%, very particularly particularly preferably at most 50% of the modulus of elasticity of the metallic workpiece is.

The retaining ring and the compensating ring are preferably made of a metal or an alloy. Particularly preferred materials for the compensating ring are copper, aluminum, silver, gold, indium, platinum, zinc and tin as well as alloys that mainly consist of these metals and/or have similar material properties. In particular, copper or copper alloys come into consideration as material for the compensating ring, for example, at moderate temperatures up to approximately 500° C., and then, at higher temperatures, nickel, for example.

The material used for the retaining ring is “Kovar” alloy, which has a low coefficient of thermal expansion, typically around 5 ppm/K, which is lower than the coefficient of typical metals. Specifically, Kovar is composed in such a way that it essentially chen has the same thermal expansion properties as borosilicate glass (approx. 5 × 10 ^-6 /K between 30 and 200 °C to approx. 10 × 10 ^-6 /K at 800 °C. A typical composition for Kovar is 54% iron, 29% Nickel and 17% cobalt (in mass percent). This gives thermal expansion properties very similar to sapphire. So sapphire is a preferred material for the optically transparent window in this case. Another preferred material for the window is diamond (1st .18 × 10 ^{-6 /K).In this case, Invar (0.6-1.2 × 10 -6 /} K; 64% iron and 36% nickel (by mass percentage)) is preferably used for the retaining ring 0.6 × 10 ^-6 /K Invar can also be used here. Another preferred material for the optically transparent window is magnesium fluoride (MgF ₂ ), especially for use in the UV range; 8, 9-13.7×10 ^-6 /K). In this case, iron (11.8 × 10 ^-6 /K) is preferably used for the retaining ring.

With the method according to the invention, optically transparent windows can thus be soldered into a metallic workpiece, preferably in a vacuum soldering process, in such a way that there are no leaks either under pressure loads of up to several hundred bar or at temperatures in the range both significantly above and below room temperature. The use of several materials in the brazed joint ensures that the stresses caused by the different thermal expansion coefficients of the materials neither exceed the respective range of plastic deformation nor the strength of the brazed joint.

With this method, the necessary flexibility is not only achieved through a thin-walled construction of the intermediate materials, but also through the use of a relatively soft material. Since its expansion coefficient differs from that of the window material and this would also lead to high stresses at high temperatures, a retaining ring with a selected thermal expansion coefficient is used to protect the window from excessive mechanical stress. This ensures that the connection between the window and the metal work piece is flexible enough not to tear when the temperature changes, but at the same time can be made thick enough to withstand high pressures.

In principle, the opening in the metallic workpiece can have different shapes. According to a preferred development of the invention, however, the opening of the metallic workpiece is provided with a peripheral shoulder, so that the stacked arrangement can be arranged on this shoulder and at least partially inserted into the opening. In this way, the stacked arrangement can be fixed in a particularly stable manner in the area of the opening.

Various materials can be considered as the solder material for the solder rings. However, an active solder containing titanium is preferably used as the solder material for the solder rings. Soldering with active solder is particularly suitable for joining highly stressed constructions. Silver, nickel, copper and/or gold are preferably used as soldering materials, with titanium as the active element that enables the soldering process. Active solders wet the materials used here in that the active elements contained form a reaction layer with the surface of the materials. Solders that can be used for the invention are, for example, APA 7 from Lot-Tek (59% Ag, 27.25% Cu, 12.5% In, 1.25% Ti (data in percent by mass); melting point approx. 650° C.). The resulting soldered connection has a very high strength. It is particularly preferred that the material is the same for all solder rings.

In principle, the opening or also, if necessary, several openings lying next to one another in the metallic workpiece and the components of the stacked arrangement can be used with different shapes. In principle, all shapes that are closed on the circumference and thus enclose an inner opening can be considered for the rings. In particular, the rings can therefore be angular, e.g. square or hexagonal. However, it is preferable for the opening of the metallic workpiece, the solder rings, the other rings and the window to be circular. In this way, a particularly stable construction is achieved.

The compensating ring can be rectangular in cross section. According to a preferred development of the invention, however, it has a peripheral groove. As a result, it retains a large contact area with the other components and is still flexible. It thus mediates between the coefficients of expansion, so that the entire mechanical stress is not passed on. The same applies to the retaining ring: it can be rectangular in cross section. According to a preferred development of the invention, however, it has a peripheral groove. These grooves can have different dimensions. Preferably they have a depth between 30 and 50% of the thickness of the rings in the radial direction. The grooves can also have a rectangular cross section. Very particularly preferably, however, their width decreases with increasing depth, so that their cross-sectional shape, for example, essentially follows a triangular shape.

In principle, the heating for melting the solder rings can take place in different suitable ways. However, the heating is preferably carried out in a vacuum brazing furnace. Additionally, the solder rings can be provided as solid rings that can be inserted into the stacked assembly as such. Alternatively, however, the solder rings can also be produced from a solder paste or a solder powder, in that the solder paste or the solder powder is introduced at the desired location for the respective solder ring in the stacked arrangement.

The invention also relates to an arrangement of an optically transparent window on a metal workpiece for closing an opening of the metal workpiece, wherein
a compensating ring is arranged above the opening, which is connected to the metallic workpiece by means of a solder joint,
a retaining ring is arranged on the compensating ring, which is connected to the compensating ring by means of a soldered connection, and
the optically transparent window is arranged on the retaining ring, which is connected to the retaining ring by means of a soldered connection, wherein
the retaining ring is made of a material whose coefficient of thermal expansion differs by a maximum of 15% from that of the material of the optically transparent window, preferably by a maximum of 10%, very particularly preferably by a maximum of 5%, and
the compensating ring is made of a material whose modulus of elasticity is at most 80%, preferably at most 70%, very particularly preferably at most 50% of the modulus of elasticity of the retaining ring and at most 80%, preferably at most 70%, very particularly particularly preferably at most 50% of the modulus of elasticity of the metallic workpiece is.

Preferred developments of this arrangement according to the invention result in analogy to the preferred developments of the method according to the invention described above.

The invention is explained in more detail below using a preferred exemplary embodiment with reference to the drawings.

• 1 schematisch in einer perspektivischen Ansicht die Herstellung einer gestapelten Anordnung für die Befestigung eines optisch transparenten Fensters auf einem metallischen Werkstück gemäß einem bevorzugten Ausführungsbeispiel der Erfindung und
• 2 schematisch die Anordnung des optisch transparenten Fensters auf dem metallischen Werkstück gemäß dem bevorzugten Ausführungsbeispiel der Erfindung.

Show in the drawings

• 1 schematically in a perspective view the production of a stacked arrangement for the attachment of an optically transparent window on a metal workpiece according to a preferred embodiment of the invention and
• 2 schematically shows the arrangement of the optically transparent window on the metallic workpiece according to the preferred embodiment of the invention.

the end 1 shows the production of a stacked arrangement 4 for the attachment of an optically transparent window 3 in the form of a sapphire window on a metallic workpiece 1 according to a preferred embodiment of the invention, wherein the metallic workpiece 1 is only in 2 1 is shown showing the entire assembly according to the preferred embodiment of the invention in the completed state.

As in 1 shown, two rings are used between the metallic workpiece 1 and the optically transparent window 3, namely a balancing ring 6 in the form of a copper ring and a retaining ring 8 in the form of a Kovar ring. The compensating ring 6, which is soldered to the metal workpiece 1, has a notched cross-section, namely it is provided with a circumferential groove 11 on the peripheral side. This is schematic 2 evident. As a result, the compensating ring 6 retains a large contact area with the other components and is still flexible. It thus mediates between the coefficients of expansion, so that the entire mechanical stress is not passed on. The retaining ring 8 also has a circumferential groove 12 .

The retaining ring 8 sits between this compensating ring 6 and the optically transparent window 3. The special alloy of the retaining ring 8 has a similar coefficient of thermal expansion as sapphire. As a result, there is almost no tension between this and the sapphire window. At the same time, the retaining ring 8 can absorb forces on the other side, since being metal it is significantly more flexible than the brittle sapphire window. Compared to metal-to-metal bonds, the bond between the metal and the sapphire window is significantly more prone to cracking. Accordingly, the use of the kovar at the critical junction to the sapphire crystal ensures that the solder joint is not weakened by microcracks that can otherwise occur when the coefficients of expansion of the materials being soldered do not substantially match. Between the respective components, i.e. between the compensating ring 6 and the metallic workpiece 1, between the compensating ring 6 and the retaining ring 8 and between the retaining ring 8 and the optically transparent window 3, a titanium-containing active solder is used, which is in the form of a ring, i.e. in the form of Lorringen 5, 7, 9.

This stacked arrangement 4 is placed in the opening 2 of the workpiece 1 in such a way that it comes to rest on a shoulder 10 provided in the opening 2 . In this way, the stacked assembly 4 is at least partially in the Opening 2 used. The entire ensemble, i.e. the metallic workpiece 1 together with the stacked arrangement 4 arranged in the opening 2 of the metallic workpiece 1, is now slowly heated in a vacuum soldering furnace to above the melting temperature of the solder of the solder rings 5, 7, 9, in accordance with the here described exemplary embodiment as follows: increase to a temperature of 750° C. within 90 minutes, hold at this temperature for 10 minutes, down to a temperature of 450° C. within 120 minutes and then allow to cool further to room temperature.

The solder wets the surfaces of the components and a surface alloy is formed on the metallic components. On the sapphire crystal, the active components of the solder, mainly the titanium, form a chemical compound with the oxygen content of the sapphire crystal. As the solder cools, it hardens and a solid connection is formed.

Reference List

1

metallic workpiece

2

opening

3

optically transparent window

4

stacked arrangement

5

first solder ring

6

balancing ring

7

second solder ring

8th

retaining ring

9

third solder ring

10

unit volume

11

circumferential groove of the compensating ring

12

circumferential groove of the retaining ring

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US6212989B1 [0009]
• US6065630A [0009]
• US 8564768 B2 [0009]
• US4614428A [0009]

Claims (9)

Method for closing an opening (2) of a metallic workpiece (1) with an optically transparent window (3), with the following method steps: - Producing a stacked arrangement (4) over the opening (2) of the metallic workpiece (1) with the following components: a first solder ring (5) on the metallic workpiece (1), a balancing ring (6) on the first plumb ring (5), a second plumb ring (7) on the balancing ring (6), a retaining ring (8) on the second plumb ring (7), a third plumb ring (9) on the retaining ring (8) and the optically transparent window (3) on the third solder ring (9), - heating the metallic workpiece (1) together with the stacked assembly (4) so that the solder rings (5, 7, 9) melt, and - Cooling of the metallic workpiece (1) together with the stacked arrangement (4) to room temperature, wherein the retaining ring (8) is made of a material whose coefficient of thermal expansion differs by no more than 15% from that of the material of the optically transparent window (3), and the compensating ring (6) is made of a material whose modulus of elasticity is at most 80% of the modulus of elasticity of the retaining ring (8) and at most 80% of the modulus of elasticity of the metallic workpiece (1). procedure after claim 1 , wherein the opening (2) of the metallic workpiece (1) is provided with a circumferential shoulder (10), so that the stacked arrangement (4) is arranged on this shoulder (10) and thereby at least partially inserted into the opening (2). can. procedure after claim 1 or 2 , wherein a titanium-containing active solder is used as the solder material for the solder rings (5, 7, 9). Method according to one of the preceding claims, in which the material is the same for all solder rings (5, 7, 9). Method according to one of the preceding claims, wherein the opening (2) of the metallic workpiece (1), the solder rings (5, 7, 9), the compensating ring (6), the retaining ring (8) and the optically transparent window (3) are circular are. Method according to one of the preceding claims, in which the compensating ring (6) has a peripheral groove (10). Method according to one of the preceding claims, in which the retaining ring (8) has a peripheral groove (11). Arrangement of an optically transparent window (3) on a metallic workpiece (1) for closing an opening (2) of the metallic workpiece (1), wherein A compensating ring (6) is arranged above the opening (2) and is connected to the metallic workpiece (1) by means of a soldered joint, a retaining ring (8) is arranged on the compensating ring (6) and is connected to the compensating ring (6) by means of a soldered joint, and the optically transparent window (3) is arranged on the retaining ring (8), which is connected to the retaining ring (8) by means of a soldered connection, wherein the retaining ring (8) is made of a material whose coefficient of thermal expansion differs by no more than 15% from that of the material of the optically transparent window (3), and the compensating ring (6) is made of a material whose modulus of elasticity is at most 80% of the modulus of elasticity of the retaining ring (8) and at most 80% of the modulus of elasticity of the metallic workpiece (1). arrangement according to claim 8 , wherein the compensating ring (6) has a peripheral groove (10) and/or the retaining ring (8) has a peripheral groove (11).

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