Asymmetric mounting device

Abstract

A window assembly comprising a frame defining an opening and a ceramic window, wherein the frame is joined to the ceramic window so as to substantially cover the opening, and the frame comprises two portions, the thickness of the first of the two portions in a direction perpendicular to a major plane of the opening being less than the thickness of the second of the two portions in a direction perpendicular to the plane of the opening.

Description

[Technical Field]

[0001] This invention was made with United States Government support under Contract No. FA8651-21-C-0003 awarded by the United States Air Force. The United States Government has certain rights in this invention.

[0002] The present invention relates to ceramic window assemblies, and in particular to synthetic diamond windows and mounting arrangements for such windows. [Background technology]

[0003] Plates of synthetic diamond material are currently available in a variety of different grades and for a range of applications. Examples include optical grades of synthetic diamond material for optical applications, thermal grades of synthetic diamond material for thermal management in semiconductor applications, and conductive boron-doped diamond grades for electrodes in electrochemical applications. Synthetic diamond material has several advantageous characteristics for applications, including extremely high hardness, high optical transparency over a wide frequency range, high thermal conductivity, chemical inertness, and a wide potential window.

[0004] Several challenges remain in utilizing synthetic diamond plates for certain applications. For applications requiring large-area plates, one of the main limitations is that plates of synthetic diamond material are only available up to a certain size. This size limitation is a result of the difficulty in generating and maintaining the extreme conditions necessary to grow diamond material over large areas. The largest high-quality synthetic diamond plates currently available are polycrystalline chemical vapor deposition (CVD) diamond plates, which can be fabricated as circular wafers up to approximately 120 mm in diameter. The circular symmetry of such large-area wafers results from the circular symmetry of the microwave-plasma-activated chemical vapor deposition equipment used in the synthesis process.

[0005] Although synthetic diamond plates are extremely hard and scratch-resistant, diamond material is brittle and can easily break if not properly mounted and handled. Furthermore, the combination of high hardness and low toughness can make it difficult to machine diamond material into precise shapes without breaking the material or introducing significant surface and subsurface damage. Furthermore, while the chemical inertness of diamond can be an advantage in many applications, it does mean that it can be difficult to bond diamond components to mounting structures using standard adhesives and mounting structures. Furthermore, the low thermal expansion coefficient of diamond material can be advantageous, for example, in avoiding thermal lensing, but the rigidity of diamond material, combined with the thermal expansion mismatch with the mounting material, can lead to thermally induced stress and potential delamination or fracture of the diamond component.

[0006] Ceramic materials, such as diamond, typically have high compressive strength, but relatively low tensile strength. These materials are brittle, and therefore their mechanical failure threshold is determined by the largest defect in the region under tensile stress. The distribution of defects results in a statistical distribution of threshold stresses depending on the critical defect size. When these materials fail under stress, the mechanism is typically brittle fracture, leading to catastrophic failure of the component. Due to the low tensile strength compared to the compressive strength, the statistical distribution of strength, and the brittle fracture mechanism, it is desirable to design ceramic components so that they are primarily in compression while avoiding tensile forces. If it is not possible to avoid tensile forces, a large safety margin is needed to ensure that the component will not fail in service. Summary of the Invention [Problem to be solved by the invention]

[0007] Most ceramic materials have a low coefficient of thermal expansion (CTE) compared to metals and / or alloys. When a ceramic window is bonded to a metal-containing mount, the bonding process is typically performed at high temperatures. Both components, i.e., the ceramic window and the mount, are typically under low stress while the temperature is still high during or immediately after the bonding process. However, when the window and attached mount subsequently cool, the mount will shrink more than the ceramic material, causing significant stress in the mount and window. [Means for solving the problem]

[0008] According to a first aspect of the present invention, there is provided a window assembly including a frame defining an opening and a ceramic window, the frame being joined to the ceramic window so as to substantially cover the opening, the frame including two portions, the thickness of a first of the two portions in a direction perpendicular to a major plane of the opening being less than the thickness of a second of the two portions in a direction perpendicular to the plane of the opening.

[0009] The first may be a U-shaped section, and the second section may bridge the legs of the U-shaped section. The U-shape may include three generally straight sections, whereby each section is connected perpendicularly to the adjacent section. Alternatively, the connection may include rounded corners between adjacent sections, and / or at least a portion of the U-shaped section may be curved. The bridging section improves the symmetry of the frame and reduces tensile stress. The first and second sections may together form a rectangular frame.

[0010] The first portion can have a cross-section perpendicular to a major plane of the opening, the cross-section having a wedge shape, with the narrow part of the wedge being on the opening side and the wide part of the wedge being on the periphery side of the frame.

[0011] Alternatively, the first portion may have a cross-section in a direction perpendicular to a major plane of the opening, the cross-section being substantially rectangular.

[0012] The window assembly may include a gradual transition from the thickness of the first portion to the thickness of the second portion.

[0013] The perimeter of the opening may include an oval, a rectangular shape, or a rectangular shape with rounded corners.

[0014] The periphery of the frame may include an oval, a rectangular shape, or a rectangular shape with rounded corners.

[0015] The first portion may include a coefficient of thermal expansion that is different from the coefficient of thermal expansion of the second portion.

[0016] The frame may be molybdenum and / or the window material may be synthetic diamond. The material of the first portion may include a ceramic, synthetic diamond, tungsten, or fused silica material.

[0017] The window assembly may be capable of operating at temperatures up to 800°C without breaking or the bond between the window and the frame failing.

[0018] The window assembly may further include a detector disposed in an optical path defined by the frame and the window, the optical path being at a non-zero angle relative to the normal to the major plane.

[0019] The ceramic window is optionally 4.5 x 10 mm thick, measured from the longest linear dimension of the window. -5 Not more than 10 times the longest linear dimension of the window, preferably 2.0 x 10 -5 The ceramic window has a maximum deflection, measured perpendicular to the major plane of the window, of no more than 1 / 2. It is beneficial to reduce the deflection to ensure that the lensing effect of light or other radiation passing through the ceramic window is minimized.

[0020] Optionally, the ceramic window has a maximum linear dimension selected from any of the following: between 10 mm and 130 mm, between 20 mm and 60 mm, and between 25 mm and 50 mm.

[0021] The ceramic window optionally has an average thickness selected from any of between 200 μm and 1500 μm, between 300 μm and 1000 μm, and between 400 μm and 800 μm. In practice, thicker ceramic windows are less flexible but are subject to higher stresses, while thinner ceramic windows have lower stresses but are more flexible.

[0022] As yet another option, the ceramic window has a peak-to-valley flatness selected from any of the following: less than 100, less than 80, and less than 40 times the λ / 2 fringes over the maximum linear length of the ceramic window. The flatness can be measured using a 633 nm optical interferometer. Optical interference produces interference fringes, each corresponding to a λ / 2 variation in flatness. The number of λ / 2 fringes is therefore a measure of the flatness of the ceramic window.

[0023] Optionally, the frame is chemically bonded to the ceramic window so as to substantially cover the opening.

[0024] According to a second aspect of the present invention there is provided a method of manufacturing a window assembly according to any one of the preceding claims, the method comprising providing a ceramic window, providing a frame and bonding the ceramic window to the frame, the frame comprising two parts, wherein a thickness of a first of the two parts in a direction perpendicular to a main plane of the opening is smaller than a thickness of a second of the two parts in a direction perpendicular to the main plane of the opening.

[0025] The method may further comprise, prior to the step of providing a frame, producing a frame using the first part comprising a different material than the second part.

[0026] The method optionally further includes machining the ceramic window after bonding the ceramic window to the frame.

[0027] According to a third aspect of the present invention there is provided an optical device comprising a ceramic window assembly according to the first aspect.

[0028] Some embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0029] [Figure 1A] FIG. 1 is a cross-sectional view of a schematic window assembly. [Figure 1B] FIG. 1 is a top view of a schematic window assembly. [Figure 2A] FIG. 1 is a cross-sectional view of a schematic window assembly. [Figure 2B] FIG. 1 is a top view of a schematic window assembly. [Figure 3A] 1 is a cross-sectional view of a schematic window assembly. [Figure 3B] FIG. 1 is a top view of a schematic window assembly. [Figure 4A] FIG. 1 is a side view of two schematic window assemblies. [Figure 4B] FIG. 1 is a side view of two schematic window assemblies. [Figure 5A] FIG. 1 is a top view of a schematic window assembly. [Figure 5B] FIG. 1 is a top view of a schematic window assembly. [Figure 6] FIG. 1 illustrates stress modeling of a window assembly. [Figure 7] FIG. 3 illustrates stress modeling of the window assembly shown in FIG. 2. [Figure 8] FIG. 4 illustrates stress modeling of a window assembly such as that shown in FIG. 3. [Figure 9] 1 is a flow chart illustrating a method of manufacturing a window assembly. DETAILED DESCRIPTION OF THE INVENTION

[0030] The inventors have realized that it is possible to enable light transmission at low angles of incidence through a ceramic window, such as a synthetic diamond window, while simultaneously controlling the distribution of stress across the window. In particular, the distribution of stress can be controlled to reduce tensile stresses acting on the window. Figures 1A and 1B show a window assembly having a frame 11 defining an opening and a ceramic window 12 disposed across the opening. The frame comprises a metal or alloy (e.g., molybdenum or a molybdenum alloy). The frame is chemically bonded to the window. This window assembly is intended for use at temperatures up to 800°C. Chemical bonds 13 between the frame and window that can withstand such operating temperatures are typically created at temperatures above 800°C. For example, gold-based brazing, with a melting point of approximately 1100°C, can be used. Another example of a high-temperature bond is Ag-Ti brazing. However, lower-temperature diffusion bonding is also a viable option. Large temperature fluctuations during the bonding process and during operation can create compressive stresses within the window due to the different expansion rates of the ceramic window and the frame.

[0031] FIG. 1A also shows a photodetector 15 that is sensitive to light propagating through window 12, but which does not reach the detector because it is blocked by the frame.

[0032] The words "frame" and "mount" are used interchangeably herein. A frame can be used to mount a window assembly, but that is not necessarily its purpose. An alternative use for the frame is as a cooling channel. In such a configuration, the frame defines hollow channels for directing cooling fluid through the frame, and a separate fixture can be provided for mounting the assembly.

[0033] Both components, i.e., the ceramic window and the mount, are generally not under stress while temperatures are still high during the bonding process or immediately after bonding. However, as the window and attached mount cool, the metal mount shrinks more than the ceramic material, causing stress in both the mount and the window. The inventors have realized that if the mount shape is symmetrical with respect to the plane of the ceramic window, the window will be primarily under compressive stress after cooling, and the risk of fracture will often be lower than the critical fracture threshold. Due to the high compressive strength of the ceramic material compared to other materials or the high tensile strength of the ceramic material, the risk of fracture is lower than the critical fracture threshold.

[0034] The coefficient of thermal expansion (CTE) of ceramics is generally (but not always) lower than that of metals. For example, diamond has a CTE of 1.07×10-6 K at 300 K (room temperature). -1 At 300 K, molybdenum has a CTE of 4.8 x 10 -6 K -1 and aluminum has a CTE of 2.4 x 10 at 300 K. -5 K -1 It has a CTE of

[0035] The directionality of residual stresses (i.e., tension, compression, shear, etc.) depends on the shape and relative orientation of the window with respect to the frame, for example, whether the window is fully mounted in the space within the frame defined by the opening or mounted over the opening relative to the frame as shown in FIG. 1B. The directionality also depends on the shape and relative orientation of the joint with respect to the window and frame. For example, whether the joint between the window and frame runs along the entire periphery of the window or only along a certain section of the window, and the relative orientation of those sections. The shape of the frame also determines the stress. The magnitude and directionality of stresses and strains in the window 12 and mount 11 can be calculated using commercially available software such as ABAQUS®.

[0036] The frame's generally rectangular shape provides symmetry along at least two axes passing through the center of the assembly. The frame's symmetry ensures that the window experiences primarily compressive stresses, while tensile stresses due to window deformation perpendicular to the window's main frame are lower than if one of the frame's edges were omitted. The window's symmetry reduces tensile stresses, resulting in primarily compressive stresses. However, a drawback of this frame configuration is that the frame blocks light propagating through the window at shallow angles. As shown in Figure 1A, light propagating through the window in the direction of arrow A is blocked by the lower frame part 14.

[0037] As mentioned above, the symmetry of the rectangular frame reduces or avoids tensile stresses within the window. Therefore, it is not desirable to remove the lower part 14 of the frame to provide an unobstructed light path. If the lower part 14 of the window were removed and the remaining frame had an inverted U-shape, the tensile stresses within the window would increase or the window would deform in a direction perpendicular to its major planes compared to a quadrilateral frame. The tensile stresses could cause the window to fracture or limit its operating range to avoid fracture.

[0038] A first embodiment of a window assembly that addresses these challenges is shown in Figures 2A and 2B. Parts of the assembly that are the same as corresponding parts in Figures 1A and 1B are similarly numbered and will not be described again. The top views shown in Figures 1B and 2B appear identical because the frame shape in the major plane of the window assembly is substantially the same. However, the cross-sectional views shown in Figures 1A and 2A are different. Compared to the configuration of Figure 1A, the reduced thickness of the lower portion 21 of the frame in a direction perpendicular to the major plane provides an unobstructed optical path. At the same time, the quadrilateral frame avoids or reduces tensile stresses due to uneven expansion across the frame. Although the asymmetrical thickness of the frame breaks symmetry along a horizontal axis passing through the center of the assembly shown in Figure 2B, symmetry is restored along a vertical axis passing through the center of the assembly shown in Figure 2B.

[0039] The introduction of a thin frame section reduces the maximum tensile stress without significantly compromising field of view. Because this small rectangular cross section is less stiff than the other three bonded sides, the tensile stress still increases when compared to a full four-sided bonded mount as shown in Figures 1A and 1B. The tensile stress may be tolerable if it is kept below the fracture threshold during use so that the window does not fracture.

[0040] As a further optional feature, the material of the lower portion 21 can be different from the material of the remainder of the frame to further reduce tensile stresses. The stiffness of the material of the lower portion 21 can be greater than the stiffness of the material of the remainder of the frame to improve the symmetry of the stress distribution. The material of the frame can be molybdenum, and the stiffness of the lower portion can be increased by selecting a different material or by creating a molybdenum alloy with higher stiffness. To significantly change the stiffness, the other alloy material must be in a relatively high concentration.

[0041] The material can be selected to achieve the technical effect of compensating for a thinner structure with a lower CTE. The material of the lower portion 21 can be one of ceramic, synthetic diamond, tungsten, or fused silica materials. It is contemplated that each of these options can be combined with the mounting material being molybdenum.

[0042] The material of the upper part of the frame can also be selected to reduce the coefficient of thermal expansion. As the overall coefficient of thermal expansion of the entire frame is reduced, the effect of asymmetry in the frame thickness on tensile stresses is also reduced.

[0043] A second example of a window assembly is shown in Figures 3A and 3B. By replacing the thin frame section 21 with a frame section having a wedge-shaped cross-section 31, the maximum tensile stress can be further reduced compared to the embodiment of Figures 2A and 2B. The wedge is oriented so that the narrow part of the wedge is at the opening, while the wider part is on the perimeter of the frame. Advantageously, the angle of the wedge is such that light is not blocked by the wedge-shaped frame section when imaging at shallow angles. Due to the greater thickness of the wedge-shaped mounting section compared to the thin frame portion, the maximum tensile stress is reduced because the overall thickness mismatch of the frame is reduced.

[0044] Both Figures 2 and 3 provide a schematic illustration of a ray A propagating towards a detector 15 with no or little obstruction by a first portion of the frame.

[0045] To further reduce the maximum tensile stress of the window, the cross-section can be gradually transitioned between the lower part of the mount and the remainder of the mount. A gradual transition avoids abrupt changes in the mount's thickness and the associated expansion discontinuities. Figure 4A shows a tapered thickness from the wide portion at the top of the mount to the narrow portion of the lower part of the mount. Figure 4B shows a mount that tapers to a narrow portion, like the narrow portion of the embodiment in Figure 3, before widening slightly again.

[0046] Yet another example of a window assembly is shown in Figures 5A and 5B. The frame has a generally rectangular perimeter. However, the opening defined by the frame in Figure 5A has sides that are perpendicular to one another but with rounded corners. This further reduces the maximum tensile stress on the ceramic window. Figure 5B shows an alternative embodiment in which both the perimeter and the opening are oval. A similar case is contemplated in which the frame has a generally rectangular perimeter and an oval opening. All of these shapes in the main plane of the window assembly are combined with the reduced diameter lower portion of the frame, as described above. The technical effect of these shapes is a more uniform distribution of stress and reduced tensile stress. However, a drawback of Figures 5A and 5B is a reduced field of view; the frame in Figure 5B has a more limited field of view than the frame in Figure 5A.

[0047] Whether a particular trade-off is acceptable will depend on the particular application, and one of ordinary skill in the art will be able to select the optimal parameters. For example, a window assembly used at high temperatures with a narrow laser beam propagating through the window may require an oval shape such as that of Figure 5B, while a window used at low temperatures to detect a wide beam of scattered light may require the frame of Figure 2.

[0048] The front view of FIG. 5 can be combined with the side view of FIG. 4 to achieve the same technical effect as the above-described embodiment of improved field of view.

[0049] Figure 6 illustrates the stress modeling of the window assembly. The modeling was performed using Abaqus, assuming a polycrystalline diamond window with dimensions of 45 mm x 25 mm x 600 μm and a molybdenum mount. The diamond-to-molybdenum bond was modeled as a 100 μm thick high-temperature braze that was assumed to be perfectly plastic.

[0050] Figure 6A shows a meshed isometric view of one quarter of a window assembly. The quarter of the illustrated ceramic window 61 is bonded to a mount 62. As can be seen in Figure 6B, this shows a plot of the modeled principal stresses in the window half, with higher stresses occurring away from the diamond window attachment area, with a modeled maximum tensile stress of 96.73 MPa.

[0051] By comparison, Figure 7 illustrates stress modeling for a window assembly using the same assumptions set forth above for Figure 6, but with a reduced thickness in the lower frame portion 21 to provide an unobstructed light path as shown in Figure 2. Figure 7A is a meshed isometric view of one half of the window assembly. As can be seen from Figure 7B, which shows a plot of the modeled principal stresses, the modeled stresses induced in the ceramic window are higher than those shown in Figure 6B, but are still acceptable, with a modeled maximum tensile stress of 248.0 MPa. Not surprisingly, the stresses are highest around the area bonded to the reduced thickness section of the lower frame portion 21.

[0052] FIG. 8 illustrates stress modeling for a window assembly using the same assumptions as those set forth above for FIG. 6 and having a wedge-shaped cross-section 31 piece of frame to provide an unobstructed light path as shown in FIG. 3. A second example window assembly is shown in FIGS. 3A and 3B. FIG. 8A is a meshed isometric view of one half of the window assembly. As can be seen from FIG. 8B, which shows a plot of the modeled principal stresses, the modeled stresses induced in the ceramic window are higher than those shown in FIG. 6B, but still acceptable, and the modeled maximum tensile stress of 138.7 MPa is roughly half that of the embodiment shown in FIG. 7. Not surprisingly, the stresses are highest around the area bonded to the wedge-shaped cross-section 31 piece of frame.

[0053] 9 is a flow chart of a method for manufacturing a window assembly as described above. The method includes the following steps: S1 providing a ceramic window, S2 providing a frame, and S3 chemically bonding the ceramic window to the frame, wherein the frame includes two portions, the thickness of a first of the two portions in a direction perpendicular to a major plane of the opening being less than the thickness of a second of the two portions in a direction perpendicular to the major plane of the opening.

[0054] It will be appreciated that the different improvements presented herein can be used with each other in various synergistic combinations. For example, the embodiment of Figure 2 can be used with some or all of the following: (1) corner rounding, (2) molybdenum alloy modification to reduce CTE, and (3) gradual transition between thick and thin sections. Again, one skilled in the art will be able to select the parameters that are optimal for a particular application.

[0055] While the present invention has been described with reference to the preferred embodiments listed above, it should be understood that these embodiments are merely exemplary and that the claims are not limited to these embodiments. Those skilled in the art can make modifications and alternatives in light of the present disclosure that are contemplated to fall within the scope of the appended claims. Each feature disclosed or exemplified herein can be incorporated into the present invention alone or in any suitable combination with any other feature disclosed or exemplified herein. [Explanation of symbols]

[0056] 11 frames 12. Ceramic Windows 13 Chemical bonding between frame and window 15 Photodetector A: Light ray propagating towards the detector

Claims

1. 1. A window assembly comprising: a frame defining an opening; Ceramic windows and Including, the frame is bonded to the ceramic window so as to substantially cover the opening; the frame includes two portions, and a thickness of a first of the two portions in a direction perpendicular to a main plane of the opening is smaller than a thickness of a second of the two portions in a direction perpendicular to the plane of the opening; Window assembly.

2. the first portion is a U-shaped portion; the second portion bridging the legs of the U-shaped portion; The window assembly of claim 1 .

3. 3. The window assembly of claim 2, wherein the first and second portions together form a rectangular frame.

4. the first portion has a cross section perpendicular to the major plane of the opening; the cross section has a wedge shape; The narrow portion of the wedge is on the side of the opening, The wide portion of the wedge is on the periphery side of the frame.

3. The window assembly of claim 2.

5. the first portion has a cross section perpendicular to the major plane of the opening; The cross section is substantially rectangular.

3. The window assembly of claim 2.

6. 6. A window assembly according to any one of claims 1 to 5, comprising a gradual transition from the thickness of the first portion to the thickness of the second portion.

7. 7. The window assembly of any one of claims 1 to 6, wherein the perimeter of the opening comprises an oval, a rectangular shape, or a rectangular shape with rounded corners.

8. 8. The window assembly of any one of claims 1 to 7, wherein the periphery of the frame comprises an oval, a rectangular shape, or a rectangular shape with rounded corners.

9. 9. The window assembly of claim 1, wherein the first portion comprises a coefficient of thermal expansion that is different from a coefficient of thermal expansion of the second portion.

10. the frame comprises molybdenum; and / or the window material is synthetic diamond; 10. A window assembly according to any one of claims 1 to 9.

11. 11. The window assembly of claim 1, wherein the material of the first portion comprises a ceramic, synthetic diamond, tungsten, or fused silica material.

12. 12. A window assembly according to any one of claims 1 to 11, capable of operating at temperatures up to 800°C without breaking or the bond between the window and the frame delaminating.

13. a detector disposed in a light path defined by the frame and the window; the optical path makes a non-zero angle with respect to the normal to the principal plane; 13. A window assembly according to any one of claims 1 to 12.

14. The ceramic window is 4.5 x 10 in the longest linear dimension of the window. -5 and preferably no more than 2.0 times 10 times the longest linear dimension of the window. -5 14. A window assembly according to any preceding claim, having a maximum deflection measured perpendicular to the plane of the window of no more than 1 / 2 times the normal width.

15. 15. The ceramic window assembly of claim 1, wherein the ceramic window has a maximum linear dimension selected from the group consisting of between 10 mm and 130 mm, between 20 mm and 60 mm, and between 25 mm and 50 mm.

16. 16. The ceramic window assembly of claim 1, wherein the ceramic window has an average thickness selected from the group consisting of between 200 μm and 1500 μm, between 300 μm and 1000 μm, and between 400 μm and 800 μm.

17. 17. The ceramic window assembly of claim 1, wherein the ceramic window has a peak-to-valley flatness selected from any of the following: less than 100 times, less than 80 times, and less than 40 times the λ / 2 interference fringe over a maximum linear length of the ceramic window measured using 633 nm light.

18. 18. The ceramic window assembly of claim 1, wherein the frame is chemically bonded to the ceramic window so as to substantially cover the opening.

19. 10. A method of manufacturing the window assembly of claim 1, comprising: providing said ceramic window; providing said frame; bonding the ceramic window to the frame; Including, the frame includes two portions, and the thickness of a first of the two portions in the direction perpendicular to the main plane of the opening is less than the thickness of a second of the two portions in the direction perpendicular to the plane of the opening; method.

20. 20. The method of claim 19, further comprising, prior to the step of providing the frame, producing the frame in which the first portion comprises a different material than the second portion.

21. 21. The method of any one of claims 19 to 20, further comprising machining the ceramic window after bonding the ceramic window to the frame.

22. 19. The ceramic window assembly according to any one of claims 1 to 18. An optical device comprising:

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