Asymmetrical 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 government support under contract number FA8651-21-C-0003 awarded by the United States Air Force. The United States government has certain rights in this invention.

[0002] This invention relates to ceramic window assemblies, particularly synthetic diamond windows and mounting configurations for such windows.

Background Art

[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 materials have 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] When using synthetic diamond plates for certain applications, several problems remain. One of the main limitations for applications that require large-area plates 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 a large area. The largest high-quality synthetic diamond plates currently available are polycrystalline chemical vapor deposition (CVD) diamond plates that can be fabricated as circular wafers up to about 120 mm in diameter. The circular symmetry of such large-area wafers is due to the circular symmetry of the microwave plasma-activated chemical vapor deposition apparatus used in the synthesis process.

[0005] While synthetic diamond plates are extremely hard and scratch-resistant, diamond materials are brittle and prone to fracture if not properly mounted and handled. Furthermore, the combination of high hardness and low toughness can make it difficult to machine diamond materials into precise shapes without fracturing the material or introducing significant surface and subsurface damage. Additionally, while the chemical inertness of diamond can be an advantage in many applications, it also means that bonding diamond components to mounting structures using standard adhesives and mounting structures can be challenging. Moreover, while the low coefficient of thermal expansion of diamond materials can be advantageous, for example, in avoiding thermal lensing effects, the rigidity of diamond materials, combined with thermal expansion mismatch with the mounting material, can lead to thermally induced stress and potential delamination or fracture of the diamond components.

[0006] Ceramic materials like diamond typically possess high compressive strength, but their tensile strength is relatively low. These materials are brittle, and therefore, their mechanical failure threshold is determined by the maximum defect in the region under tensile stress. The defect distribution, depending on the critical defect size, results in a statistical distribution of threshold stress. When these materials fail under stress, this mechanism is typically brittle fracture, leading to catastrophic failure of the component. Due to the low tensile strength compared to compressive strength, the statistical distribution of strength, and the brittle fracture mechanism, it is desirable to design ceramic components so that they are primarily under compression while avoiding tensile forces. If avoiding tensile forces is not possible, a large safety margin is necessary to ensure that the component does not fail during use. [Overview of the project] [Problems that the invention aims to solve]

[0007] Most ceramic materials have a lower coefficient of thermal expansion (CTE) than 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, namely 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, as the window and the mounted mount subsequently cool, the mount will shrink more than the ceramic material, causing significant stress on both the mount and the window. [Means for solving the problem]

[0008] A first aspect of the present invention provides a window assembly comprising a frame defining an opening and a ceramic window, wherein the frame is bonded to the ceramic window so as to substantially cover the opening, and the frame comprises two parts, the thickness of the first part of the two parts in a direction perpendicular to the main plane of the opening is less than the thickness of the second part of the two parts in a direction perpendicular to that plane of the opening.

[0009] The first part may be a U-shaped section, and the second part may bridge the legs of the U-shaped section. The U-shape may include three nearly straight parts, so that each part is connected perpendicularly to the adjacent part. Alternatively, the connection may include rounded corners between adjacent parts, 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 parts may form a rectangular frame with respect to each other.

[0010] The first part may have a cross-section perpendicular to the main plane of the opening, the cross-section being wedge-shaped, with the narrower part of the wedge facing the opening and the wider part of the wedge facing the perimeter of the frame.

[0011] Alternatively, the first portion may have a cross-section perpendicular to the main plane of the opening, and the cross-section may be substantially rectangular.

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

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

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

[0015] The first part may contain a different coefficient of thermal expansion than the second part.

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

[0017] Window assemblies may be able to operate at temperatures up to 800°C without breaking or delamination of the joint between the window and the frame.

[0018] The window assembly may further include a detector positioned within the optical path defined by the frame and the window, where the optical path is at a non-zero angle with respect to the normal to the principal plane.

[0019] Ceramic windows can optionally have a longest straight-line dimension of 4.5 × 10 -5 Not larger than twice the length, preferably 2.0 × 10 of the longest straight dimension of the window. -5 The maximum deflection, measured perpendicular to the main plane of the window, is no greater than twice the original deflection. Reducing the deflection is beneficial to ensure that the lensing effect of light or other radiation passing through the ceramic window is minimized.

[0020] As an option, the ceramic window has a maximum straight-line dimension selected from between 10mm and 130mm, between 20mm and 60mm, and between 25mm and 50mm.

[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, a thicker ceramic window is less flexible but subject to higher stress, while a thinner ceramic window has lower stress but is more flexible.

[0022] As yet another option, the ceramic window has a valley-to-valley flatness selected from any of less than 100 times, less than 80 times, and less than 40 times the λ / 2 interference fringes across the maximum straight-line length of the ceramic window. The flatness can be measured using a 633 nm optical interferometer. Optical interference generates interference fringes, and each fringe corresponds to a λ / 2 variation in flatness. The number of λ / 2 interference fringes is thus a measure of the flatness of the ceramic window.

[0023] As an option, 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 including two parts, the thickness of the first part of the two parts in a direction perpendicular to the main plane of the opening being smaller than the thickness of the second of the two parts in a direction perpendicular to the main plane of the opening.

[0025] The method may further include generating the frame using the first part including a material different from the second part before the step of providing the frame.

[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 including a ceramic window assembly according to the first aspect.

[0028] Some embodiments of the present invention will be described below simply by illustrative reference to the accompanying drawings. [Brief explanation of the drawing]

[0029] [Figure 1A] This is a schematic cross-sectional view of a window assembly. [Figure 1B] This is a schematic top view of a window assembly. [Figure 2A] This is a schematic cross-sectional view of a window assembly. [Figure 2B] This is a schematic top view of a window assembly. [Figure 3A] This is a schematic cross-sectional view of a window assembly. [Figure 3B] This is a schematic top view of the window assembly. [Figure 4A] These are side views of two schematic window assemblies. [Figure 4B] These are side views of two schematic window assemblies. [Figure 5A] This is a schematic top view of a window assembly. [Figure 5B] This is a schematic top view of a window assembly. [Figure 6] This diagram illustrates the stress modeling of a window assembly. [Figure 7] Figure 2 illustrates the stress modeling of the window assembly shown in the diagram. [Figure 8] Figure 3 illustrates an example of stress modeling for a window assembly. [Figure 9] This is a flowchart illustrating a method for manufacturing a window assembly. [Modes for carrying out the invention]

[0030] The inventors have found that it is possible to enable light transmission at low angles of incidence to ceramic windows, such as synthetic diamond windows, while simultaneously controlling the stress distribution across the window. In particular, it is possible to control the stress distribution to reduce the tensile stress acting on the window. Figures 1A and 1B show a window assembly having a frame 11 that defines an opening and a ceramic window 12 positioned across the opening. This frame includes 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 generally produced at temperatures above 800°C. For example, gold-based brazing with a melting point of about 1100°C can be used. Another example of high-temperature bonding is Ag-Ti brazing. However, lower-temperature diffusion bonding is also a possible option. Large temperature fluctuations during the bonding process and during operation create compressive stress within the window due to the different expansion rates of the ceramic window and the frame.

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

[0032] In this specification, the words “frame” and “mount” are used interchangeably. A frame can be used to mount a window assembly, but that is not necessarily its intended purpose. An alternative use for a frame is as a cooling channel. In such a configuration, the frame may define a hollow channel for guiding a cooling fluid through to the frame and provide separate mounting fixtures for mounting the assembly.

[0033] Both components, namely the ceramic window and the mount, are generally not under stress during or immediately after the joining process while their temperatures are still high. However, as the window and the mounted mount cool, the metal mount shrinks more than the ceramic material, causing stress in both the mount and the window. The inventors have found that when the mount shape is symmetrical with respect to the plane of the ceramic window, the window is under primarily compressive stress after cooling, and its risk of fracture is often below the critical fracture threshold. The risk of fracture is below the critical fracture threshold due to the fact that the compressive strength of the ceramic material is higher than that of other materials or higher than that of the ceramic material's tensile strength.

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

[0035] The direction of residual stress (i.e., tension, compression, shear, etc.) depends on the shape and relative position of the window relative to the frame, for example, whether the window is fully fitted into the space within the frame defined by the opening, or fitted to cover the opening relative to the frame as shown in Figure 1B. The direction also depends on the shape and relative position of the joints between the window and the frame. For example, whether the joint between the window and the frame extends along the entire periphery of the window, or along only certain sections of the window, and the relative position of those sections. The shape of the frame also determines the stress. The magnitude and direction of stress and strain in the window 12 and mount 11 can be calculated using commercially available software such as ABAQUS®.

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

[0037] As described above, the symmetry of the rectangular frame reduces or avoids tensile stress within the window. Therefore, it is undesirable to remove the lower part 14 of the frame to provide an unobstructed optical path. If the lower part 14 of the window is removed and the remaining frame has an inverted U-shape, the tensile stress within the window will increase or the window will deform in a direction perpendicular to the main plane compared to a quadrilateral frame. The tensile stress may cause the window to break or limit its operating range to avoid breakage.

[0038] A first embodiment of a window assembly that addresses these challenges is shown in Figures 2A and 2B. The parts of the same assembly as the 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 main 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 in Figure 1A, the reduced thickness of the lower portion 21 of the frame in the direction perpendicular to the main plane provides an unobstructed optical path. At the same time, the quadrilateral frame avoids or reduces tensile stress due to uneven expansion across the frame. Although the symmetry along the horizontal axis passing through the center of the assembly shown in Figure 2B is broken due to the asymmetric thickness of the frame, the symmetry along the vertical axis passing through the center of the assembly shown in Figure 2B is restored.

[0039] The introduction of a thin frame reduces the maximum tensile stress with minimal loss of field of view. Because this small rectangular cross-section is less rigid than the other three joint sides, the tensile stress is still higher compared to a fully four-sided jointed mount as shown in Figures 1A and 1B. The tensile stress may be acceptable if it is kept below the failure threshold during use to prevent window breakage.

[0040] As an optional further feature, the material of the lower section 21 can be different from the material of the rest of the frame to further reduce tensile stress. The stiffness of the material of the lower section 21 can be made greater than that of the rest 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 section can be increased by selecting a different material or by creating a molybdenum alloy with high stiffness. To significantly change the stiffness, the other alloy material must be present in a relatively high concentration.

[0041] The materials 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 the following: ceramic, synthetic diamond, tungsten, or fused silica. Each of these options can be considered to be combined with a mounting material which is molybdenum.

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

[0043] A second embodiment of the 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 embodiments in Figures 2A and 2B. The wedge is oriented such 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 formed by the wedge is such that light is not obstructed by the wedge-shaped frame section when imaging at a shallow angle. 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 mismatch in the overall thickness of the frame is reduced.

[0044] Both Figures 2 and 3 provide schematic diagrams of the light ray A propagating toward the detector 15 with little to no obstruction by the first portion of the frame.

[0045] To further reduce the maximum tensile stress on the window, the cross-section can be gradually transitioned between the lower part of the mount and the rest of the mount. This gradual transition avoids abrupt, stepwise changes in the mount's thickness and the resulting discontinuities in expansion. Figure 4A shows the tapering thickness from the wider upper part of the mount to the narrower lower part of the mount. Figure 4B shows a mount that tapers towards a narrow section, similar to the narrow section in the embodiment of Figure 3, and then widens slightly again.

[0046] Figures 5A and 5B show yet another embodiment of the window assembly. The outer perimeter of the frame is approximately rectangular. However, in the opening defined by the frame in Figure 5A, the sides are perpendicular to each other but have rounded corners. This further reduces the maximum tensile stress applied to the ceramic window. Figure 5B shows an alternative embodiment in which both the outer perimeter and the opening are oval. We also consider the case where the outer perimeter of the frame is approximately rectangular and the opening is oval. All of these external shapes in the main plane of the window assembly are combined with a lower portion of the frame with a reduced diameter, as described above. The technical effect of these shapes is a more uniform distribution of stress and a reduction in 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 depends on the specific application, and a person skilled in the art will be able to select the optimal parameters. For example, an oval shape like that shown in Figure 5B may be required for use in a window assembly at high temperatures where a narrow laser beam propagates through the window, while a frame like that shown in Figure 2 may be required for use in a window at low temperatures to detect a wide beam of scattered light.

[0048] By combining the front view in Figure 5 with the side view in Figure 4, the same technical effect as the above-described embodiment, in which the field of view is improved, can be achieved.

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

[0050] Figure 6A shows a mesh-like isometric projection of a quarter of the window assembly. The quarter of the illustrated ceramic window 61 is joined to the mount 62. As can be seen from Figure 6B, this shows a plot of the modeled principal stresses in the half of the window, with high stress occurring away from the mounting area of ​​the diamond window, and the modeled maximum tensile stress is 96.73 MPa.

[0051] For comparison, Figure 7 illustrates the stress modeling of a window assembly using the same assumptions as those mentioned above for Figure 6, but with the thickness of the lower frame portion 21 reduced to provide an unobstructed optical path as shown in Figure 2. Figure 7A is a mesh-like isometric projection of half of the window assembly. As can be seen from Figure 7B, this shows a plot of the modeled principal stresses, and the modeled stresses occurring in the ceramic window are higher than those shown in Figure 6B, but still acceptable, with the maximum modeled tensile stress being 248.0 MPa. Unsurprisingly, the stresses are highest around the area joined to the thickness reduction portion of the lower frame portion 21.

[0052] Figure 8 illustrates the stress modeling of a window assembly having a wedge-shaped cross section 31 component of the frame to provide an unobstructed optical path, as shown in Figure 3, using the same assumptions as previously mentioned for Figure 6. A second embodiment of the window assembly is shown in Figures 3A and 3B. Figure 8A is a mesh-like isometric projection of half of the window assembly. As can be seen from Figure 8B, this shows a plot of the modeled principal stresses, and the modeled stresses occurring in the ceramic window are higher than those shown in Figure 6B, but still acceptable, with the modeled maximum tensile stress of 138.7 MPa being approximately half of that in the embodiment shown in Figure 7. Unsurprisingly, the stresses are highest around the region joined to the wedge-shaped cross section 31 component of the frame.

[0053] Figure 9 is a flowchart of a method for manufacturing the window assembly described above. The method includes the following steps: step S1 of providing the ceramic window, step S2 of providing the frame, and step S3 of chemically bonding the ceramic window to the frame, where the frame includes two parts, the thickness of the first part of the two parts in a direction perpendicular to the main plane of the opening is less than the thickness of the second part of the two parts in a direction perpendicular to the main plane of the opening.

[0054] It will be acknowledged that the different improvements presented herein can be used together in various synergistic combinations. For example, the embodiment in Figure 2 can be used with some or all of the following: (1) corner rounding, (2) modification of the molybdenum alloy to reduce CTE, and (3) gradual transition between thick and thin sections. Again, those skilled in the art will be able to select the parameters best suited to their specific application.

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

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

Claims

1. A window assembly, A frame that defines an opening having a main plane, Ceramic windows and Includes, The frame is joined to the ceramic window so as to substantially cover the opening. The frame comprises two parts, wherein the thickness of the first part of the two parts in a direction perpendicular to the main plane of the opening is less than the thickness of the second part of the two parts in a direction perpendicular to the main plane of the opening. The first part is a U-shaped portion that is curved in a U-shape, The second part is a portion that extends to connect the legs of the U-shaped portion to one another. Window assembly.

2. The window assembly according to claim 1, wherein the first and second parts form a rectangular frame with respect to each other.

3. The first portion has a cross-section perpendicular to the main plane of the opening, The aforementioned cross-section has a wedge shape, The wedge-shaped narrow portion is located on the side of the opening, The wedge-shaped wide portion is located on the side surrounding the frame. The window assembly according to claim 1.

4. The first portion has a cross-section perpendicular to the main plane of the opening, The aforementioned cross-section is substantially rectangular. The window assembly according to claim 1.

5. The window assembly according to claim 1, comprising a gradual transition from the thickness of the first portion to the thickness of the second portion.

6. The window assembly according to claim 1, which can be operated at temperatures up to 800°C without destruction or delamination of the joint between the ceramic window and the frame.

7. The system further includes a detector positioned within the optical path defined by the frame and the ceramic window, The optical path makes a non-zero angle with respect to the normal of the main plane. The window assembly according to claim 1.

8. The ceramic window has a longest straight dimension of 4.5 × 10 -5 The window assembly according to claim 1, having a maximum deflection of the ceramic window measured perpendicular to the main plane of the ceramic window that is not greater than twice the original deflection.

9. The window assembly according to claim 8, wherein the ceramic window has a maximum deflection measured perpendicular to the main plane of the ceramic window that is not greater than 2.0 × 10⁻⁵ times the longest straight dimension of the ceramic window.

10. The ceramic window assembly according to claim 1, wherein the ceramic window has a maximum linear dimension selected from among 10 mm and 130 mm, 20 mm and 60 mm, and 25 mm and 50 mm.

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

12. A method for manufacturing the window assembly described in claim 1, To provide the aforementioned ceramic window, The aforementioned frame is provided, The ceramic window is joined to the frame, Includes, The frame comprises two parts, wherein the thickness of the first part of the two parts in the direction perpendicular to the main plane of the opening is less than the thickness of the second part of the two parts in the direction perpendicular to the main plane of the opening. method.

13. The method according to claim 12, further comprising generating the frame, in which the first portion is made of a different material from the second portion, before providing the frame.

14. The method according to claim 12, further comprising machining the ceramic window after joining it to the frame.

15. Ceramic window assembly according to claim 1, Optical devices including

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