Ceramic Window Assembly

Abstract

A ceramic window assembly comprising a ceramic window, a mount that adheres to the ceramic window and creates an asymmetry, and a mechanically stressed superstructure.

Description

[Technical Field]

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

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

[0003] Synthetic diamond material plates are currently available in a variety of grades and can be used for multiple applications. Examples include optical-grade synthetic diamond material for optical applications, thermal-grade synthetic diamond material for thermal management in semiconductor products, and conductive boron-doped diamond grades for electrodes in electrochemical products. Synthetic diamond material has many advantageous characteristics for such applications, including extreme hardness, high optical transparency over a wide frequency range and field of view, high thermal conductivity, and chemical inertness.

[0004] Synthetic diamond plates present several challenges for certain applications. The primary limitation for applications requiring large-area plates is that synthetic diamond material plates are only available up to a certain size. This plate size limitation results from the difficulty of generating and maintaining the extreme conditions required 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 manufactured as circular wafers up to approximately 120 mm in diameter. The circular symmetry of these large-area wafers derives from the circular symmetry of the microwave plasma-activated chemical vapor deposition equipment used in the synthesis process, although other shapes can also be produced.

[0005] While synthetic diamond plates are extremely hard and scratch-resistant, the diamond material is brittle and prone to breakage if not properly mounted and handled. Furthermore, the combination of high hardness and low toughness makes it difficult to machine the diamond material into precise shapes without breaking it or causing significant surface or subsurface damage. Furthermore, while diamond's chemical inertness can be an advantage in many applications, it also means that it can be difficult to adhere diamond components to mounting structures using standard adhesives and mounting structures. Furthermore, while the low thermal expansion coefficient of diamond material is advantageous for avoiding thermal lensing, for example, the stiffness of diamond material, combined with the thermal expansion mismatch with the mounting material, can create thermally induced stresses that can lead to separation or breakage of the diamond component.

[0006] Ceramic materials, such as diamond, typically have high compressive strength but relatively low tensile strength. Because these materials are brittle, their mechanical failure threshold is determined by the largest defect in the area subjected to tensile stress. The distribution of these defects results in a statistical distribution of threshold stress, which depends on the critical defect size. When these materials fail under stress, the mechanism is usually 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 primarily for compression while avoiding tensile forces. If tensile forces cannot be avoided, a large margin of safety is required to ensure the component does not fail during use.

[0007] Most ceramic materials have a lower coefficient of thermal expansion (CTE) than metals. When bonding a ceramic window to a metal mount, the bonding process is typically performed at elevated temperatures. Both components, the ceramic window and the metal mount, are typically under low stress during the bonding process or immediately after bonding while the temperature is still high. However, when the window and attached metal mount cool, the metal mount contracts more than the ceramic material, creating significant stresses in the mount and window. Summary of the Invention

[0008] According to a first aspect of the present invention, there is provided a ceramic window assembly including a ceramic window, a mount bonded to the ceramic window and having an asymmetry perpendicular to the plane of the ceramic window, such as by a mount bonding to only one planar surface of the ceramic window and not to the opposite planar surface of the ceramic window.

[0009] The superstructure may be adjusted to reduce tensile stress in the window and / or may be adjusted to change tensile stress to compressive stress. The ceramic window material may be synthetic diamond, and as a more specific example, the material may be polycrystalline chemical vapor deposition diamond.

[0010] The superstructure can be mechanically coupled to the mount during attachment of the mount to the ceramic window and can be removed from the mount after attachment of the mount to the ceramic window. An inner diameter of the superstructure can match an outer diameter of the mount. The superstructure may have a lower coefficient of thermal expansion than the mount. The superstructure can be a second ceramic window coupled to the mount on a side of the mount opposite the side on which the ceramic window is attached, and the dimensions of the second ceramic window can be substantially the same as the dimensions of the ceramic window.

[0011] The superstructure may extend in a major plane of the window assembly and may have a shape that substantially matches the shape of the mount in the major plane.

[0012] One example of a material for the mount is a metallic material, and an example of a suitable metal is molybdenum.

[0013] The material of the superstructure may have a thermal expansion coefficient lower than that of the mount. Examples of materials for the superstructure are ceramic, synthetic diamond, tungsten, or fused silica.

[0014] Optionally, the ceramic window has a maximum deflection measured perpendicular to a major plane of the window of less than 4.5×10 of the longest linear dimension of the window. -5 less than 2.0 x 10 times the longest linear dimension of the window, preferably -5 The reduced deflection is beneficial to minimize the lensing effect of light or other radiation passing through the ceramic window.

[0015] The maximum linear dimension of the ceramic window may optionally be between 10 mm and 130 mm, between 20 mm and 60 mm, between 25 mm and 50 mm.

[0016] The average thickness of the ceramic window may optionally be between 200 μm and 1500 μm, between 300 μm and 1000 μm, or between 400 μm and 800 μm. In practice, a thicker ceramic window will be less flexible but will have higher stresses, while a thinner ceramic window will have lower stresses but will be more flexible.

[0017] Further optionally, the ceramic window has a peak-to-valley flatness selected from less than 100×λ / 2 fringes, less than 80×λ / 2 fringes, or less than 40×λ / 2 fringes across the maximum linear length of the ceramic window. Flatness can be measured using an interferometer with 633 nm light. Light interference produces fringes, each fringe corresponding to a λ / 2 change in flatness. Thus, the value of the λ / 2 fringes is an indication of the flatness of the ceramic window.

[0018] According to a second aspect of the present invention, there is provided a method of manufacturing the ceramic window assembly of the first aspect, the method comprising providing the ceramic window, providing the mount, providing the superstructure, attaching the superstructure to the mount, bonding the mount to the window, and inducing stress in the superstructure.

[0019] The manufacturing method may further include removing the superstructure from the ceramic assembly after adhering the mount to the window.

[0020] Stress creating steps can occur when the ceramic window assembly is heated to bond the mount to the ceramic window, or when the ceramic window assembly is cooled from the elevated temperatures used to bond the mount to the ceramic window.

[0021] 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.

[0022] Some embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 is a top view of two schematic window assemblies with asymmetric mounts. [Figure 2] 1A-1C are top views of two schematic window assemblies including a restraining superstructure. [Figure 3] FIG. 1 is a side view of two schematic window assemblies including a superstructure. [Figure 4] 1 shows a window assembly with a bridge portion as the superstructure. [Figure 5] 1 shows a top view and a cross section of the window assembly and superstructure. [Figure 6] 1 shows stress modeling of an asymmetric window assembly without a restraining superstructure. [Figure 7] 1 shows stress modeling of an asymmetric window assembly with a restrained superstructure. [Figure 8] 1 is a diagram of the method. DETAILED DESCRIPTION OF THE INVENTION

[0024] (Specific description) The inventors have discovered that stress in a ceramic window and attached mount can be controlled by controlling the distribution of stress throughout the window, particularly by alleviating tensile stress in the window. The use of one or more additional components, also referred to herein as superstructures, can convert tensile stress to compressive stress. Ceramic windows have a much higher failure threshold under compressive stress than under tensile stress.

[0025] As mentioned above, ceramic windows bond to metal mounts at high temperatures. One reason for the high temperatures may be the need to use the window assembly, including the adhesive, at operating temperatures up to 800°C. The adhesive between the mount and the window must maintain its integrity at least up to those operating temperatures. To ensure the adhesive can withstand high temperatures, it can be manufactured at high temperatures. For example, a gold-based braze, with a melting temperature of approximately 1100°C, can be used. Another example of a high-temperature adhesive is Ag-Ti braze. However, low-temperature diffusion adhesives are also possible. Both components, the ceramic window and the metal mount, are typically not stressed during the bonding process or immediately after bonding while the temperatures are high. However, as the window and attached metal mount cool, the metal mount shrinks more than the ceramic material, creating stress in both the mount and the window. The inventors have found that if the mount's shape is symmetrical relative to the plane of the ceramic window, the window is primarily under compressive stress after cooling, and the risk of window breakage is often below a critical breakage threshold. The risk of window failure being below a critical failure threshold is due to the high compressive strength of the ceramic material compared to the compressive strength of other materials or compared to the tensile strength of the ceramic material.

[0026] However, if the use of the mounted window requires an asymmetric mount due to technical design constraints, asymmetric contraction of the metal mount will cause a bending moment in the window, inducing tensile stress in the window. Deformation of the window perpendicular to its major planes may occur. Some degree of "strain" of the ceramic window may also occur. In the case of a diamond window, the strain acts as a force that separates atoms in the lattice structure, as opposed to the favorable compressive force that brings atoms closer together. In practice, there may be a combination of these different undesirable effects, and it is not always possible to determine the individual contributions of deformation, strain, and other effects that cause stress in ceramic materials. Therefore, the proposed superstructure mount was arrived at not only through theoretical considerations, but also through a partially experimental inventive process.

[0027] Figure 1 shows two examples of asymmetric mounts. In Figure 1, a window 10 is supported by a U-shaped mount 11. The mount has an axis of symmetry about line L1. If the mount supported the lower portion of the window, and the U-shape were an O-shape, the mount would also be symmetric about line L2, reducing the tensile stress. The fact that the mount is only on one side of the window also causes tensile stresses, which are typically greater than those caused by asymmetry about line L2.

[0028] An example of an application requiring a U-shaped mount for certain operating conditions is an optical application where light must pass through the lower portion of the window. Upon cooling, the upper portion of the mount contracts, but the lower portion of the window lacks a corresponding mount portion to reflect the mount's contraction, resulting in an asymmetric contraction force and tensile stress in the window. Figure 1B shows another asymmetric mount, including a first component 12 attached to the left side of window 10 and a second component 13 attached to the right side of the window. The first component 12 is larger than the second component 13, causing an asymmetry and corresponding tensile stress during temperature changes. The contraction of the mount upon cooling and the corresponding stress, indicated by the double-headed arrow E1 at the top, will not be the same as the double-headed arrow E2. While stresses E1 and E2 lie in the major plane of the window, stresses can also occur perpendicular to the major plane of the window, perpendicular to E1 and E2, or even in the plane of the window.

[0029] A further example of an asymmetric mount is an elliptical mount, as opposed to a circular mount. While an elliptical shape has only two axes of symmetry in the major plane of its shape, a circular shape is symmetrical around any line passing through the center of the circle in the major plane of the mount. While a circular shape is preferred for reducing tensile stress, this is not always desirable depending on the use scenario. For example, the field of view of a circular mount may be too small for certain applications. The presence of a mount on one side of the window, rather than both sides, is a significant source of tensile stress for both circular and elliptical windows.

[0030] The inventors have devised a superstructure to attenuate or avoid tensile stresses by attenuating some or all of the asymmetric forces. The superstructure may be arranged to attenuate the total amount of stress on the window, or to symmetrize the stresses and reduce tensile stresses without attenuating all of them, or to perform a combination of stress attenuation and symmetrization. As explained in more detail in the examples below, the superstructure may be thought of as "transforming" tensile stresses into compressive stresses. Bending moments are also avoided or attenuated by the superstructure.

[0031] Thus, a ceramic window assembly is provided that includes a ceramic window and a mount bonded to the window, the mount having an asymmetrical configuration, and the assembly further includes a superstructure, the superstructure being mechanically stressed, the stress in the superstructure relieving the tensile stress on the window.

[0032] The characteristics of the superstructure that are subjected to stress are considered primarily structural rather than functional characteristics because the stresses are objectively observable and measurable and are inherent to the particular configuration. The stresses are characteristics of the materials of the assembly rather than of the way they are performed by an external process.

[0033] The superstructure keeps the tensile stress well below the failure threshold, preventing damage to the window even when compressive forces are increased by the superstructure.

[0034] A first example of a superstructure is a constraining superstructure positioned to limit deformation of the mount during heating. The superstructure limits deformation of the mount during the bonding process, thereby creating stresses in the mount and superstructure but not in the window or in the mount after subsequent cooling. The superstructure is constructed of a material strong enough to withstand the stresses created by the mount's expansion and with a lower coefficient of thermal expansion than the metal mount. An example material is a ceramic material, although other materials may be used. Figure 2A shows a schematic diagram of the assembly of Figure 1A, including an additional superstructure 21, window 10, and mount 11. The superstructure 21 has internal dimensions that match the external dimensions of the mount 11 when the mount is at a low temperature, such as room temperature, and may comprise a strong ceramic material or other material with a low coefficient of thermal expansion. This matching is intended to prevent expansion of the mount, but the superstructure may be slightly larger in size depending on the relative thermal expansion coefficients and bonding temperature. The superstructure is placed around the mount before the assembly is heated for the bonding process. When the assembly including the superstructure is heated, the contraction caused by the superstructure places stress on the mount and superstructure. After the bonding process is completed, the assembly is cooled and the stress disappears. In this embodiment, the technical effect of the superstructure after cooling is to remove stress rather than convert tensile stress to compressive stress.

[0035] If the mounted window is to be used in high-temperature applications, the superstructure may need to remain in place, but if the mounted window is to be used only in low-temperature applications, the superstructure may be removed after the high-temperature bonding process is complete. The reason the superstructure optionally remains in place is because tensile stresses can be generated by the opposite scenario to that described in connection with FIG. 1A , whereby there is little or no tensile stress at room temperature (or other lower operating temperatures), but excessive tensile stresses at elevated temperatures. This is the opposite of the scenario described in connection with FIG. 1A , because without the superstructure, there would be little or no tensile stress at elevated temperatures during the bonding process, but excessive tensile stresses can be generated after cooling.

[0036] Figure 2B shows a different example of a constraining superstructure. The superstructure is oval or circular. A disadvantage compared to the shape of Figure 2A is that there are fewer contact points that could induce deformation, such as buckling, between the mount and the window. There are four compressive contact points between the superstructure and the mount, located at the four corners of the mount. Therefore, this structure may be less desirable because a mismatch between the shape of the mount and the shape of the superstructure could result in buckling of the mount and / or window. However, when the superstructure of Figure 2B is used with an oval-shaped mount that matches the shape of the superstructure, it is again suitable for preventing tensile stresses.

[0037] The asymmetry of having the mount on one side of the window and not on the other side also causes tensile stress, as discussed above. Figure 3 shows an assembly in which the superstructure is a second ceramic window on the opposite side of the mount. Figure 3A is a schematic side view, with window 10 and mount 11 corresponding to the same numbers as in Figures 1 and 2. A second window 31 is provided. Preferably, the dimensions and material properties of the second window are the same as or very similar to those of the first window. Preferably, the orientation of the first and second windows is symmetrical about the major plane of the mount. The technical effect of this symmetry, identical dimensions, and material properties is a reduction in the tensile stress. However, some of the dimensional, material, and orientation parameters can be varied while still achieving the same effect. For example, if the material of the first window is diamond and a different, less expensive material with a slightly different thermal expansion coefficient is selected for the second window, the dimensions of the windows can be selected to be different to achieve the same overall effect of canceling bending moments.

[0038] FIG. 3B is a schematic side view, showing window 10 and mount 11 corresponding to those previously described. Instead of the additional window superstructure, an additional mount 32 mirroring mount 11 restores or improves symmetry perpendicular to the major plane and eliminates the bending moment. An advantage over the embodiment of FIG. 3A is reduced cost, since metal mounts typically cost less than diamond windows. However, this approach may not be suitable depending on design constraints.

[0039] Another example of a superstructure is shown in FIG. 4A. As a non-limiting example of an asymmetric window, a window 10 and a mount 11 are provided as described above. A superstructure 41 is provided to close the gap between the two legs of the U-shaped mount 11. The superstructure has a smaller thickness than the mount 11 to prevent it from obstructing the beam path. This is an example of why a mount on the window side may be omitted. FIG. 4B is a vertical cross section taken along line L3 in FIG. 4A, illustrating the window's thin thickness perpendicular to its major plane. The material of the superstructure is selected so that its thermal expansion matches that of the rest of the window, avoiding tensile stress in the window. A relatively thin layer of material with a high Young's modulus and a high thermal expansion coefficient can restore a symmetric stress state even if the structure shape is not symmetric.

[0040] The material can be selected to achieve the technical effect of a thin structure complemented by a low CTE. The material of the superstructure can be one of ceramic, synthetic diamond, tungsten, or fused silica materials. Each of these options can be combined with a mounting material of molybdenum.

[0041] 4C is a schematic diagram of a variation of the superstructure 41, which includes a bridge portion 42. The bridge portion 42 is connected to both legs of the U-mount, but not to the window. As with the superstructure 41, the bridge portion 42 creates a symmetrical stress distribution, thereby avoiding tensile stresses and providing only compressive stresses.

[0042] Further examples are shown in Figures 5A and 5B. Figure 5A is a top view, and Figure 5B is a cross-sectional view taken along line L5 in Figure 5A. Window 51 is attached to mount 52. The shape of the mount can be as previously described, i.e., U-shaped, O-shaped, oval, or circular, or any other shape dictated by design specifications. A generally rectangular shape with rounded corners is shown. Rounded corners provide a smooth transition between sides and reduce potential stresses. Superstructure 53 is attached to the side of the mount opposite the side on which the window is attached. Instead of bonding the entire window to the back of the mount, as shown in the embodiment of Figure 3A, a superstructure with a low coefficient of thermal expansion, such as diamond or fused silica, is bonded to the back of the mount. The superstructure can be the same or similar shape as the mount, e.g., U-shaped, O-shaped, oval, or circular, or any other shape of the mount. However, the shapes do not need to match perfectly, and the superstructure can be slightly larger to achieve the same technical effect of bearing some stress and reducing tensile stress in the window, while avoiding the cost of an entire window. Because the superstructure in Figure 5A matches the shape of the mount, you cannot see the mount underneath the superstructure in a top view.

[0043] Optionally, the mount may define cooling channels for conducting a cooling fluid during use. A reduction in the temperature of the mount reduces expansion and some of the stresses associated therewith.

[0044] Figure 6 shows the stress modeling of an asymmetric window assembly without a constraining superstructure. The modeling was performed using Abaqus and assumes a polycrystalline diamond window with dimensions of 40 mm x 25 mm x 600 μm and a molybdenum mount. The diamond-molybdenum bond was modeled as a 100 μm thick high-temperature braze and assumed to be fully plastic.

[0045] Figure 6A shows an isometric view of one quarter of the window assembly, with the ceramic window 61 shown bonded to the mount 62. As can be seen from Figure 6B, which shows a plot of modeled principal stresses, high stresses occur away from the mount area of ​​the diamond window, with a modeled maximum tensile stress of 96.73 MPa.

[0046] In comparison, Figure 7 shows stress modeling of an asymmetric window assembly with a mechanically stressed constraining superstructure, using the same assumptions as Figure 6 listed above. Figure 7A shows a mesh isometric view of a quarter of the window assembly with the mechanically stressed constraining superstructure 63. As can be seen from Figure 7B, which shows a plot of the modeled principal stresses, the modeled stresses generated in the ceramic window are much lower than those shown in Figure 6B, with a maximum modeled tensile stress of 63.70 MPa.

[0047] FIG. 8 illustrates a method for fabricating a ceramic window assembly according to the following steps: S1 prepare the ceramic window; S2 prepare the mount; S3 prepare the superstructure; S3 attach the superstructure to the mount or the ceramic window; S4 bond the mount to the window; and S5 create stress in the superstructure. The order of the steps may be varied. For example, the step of attaching the superstructure to the mount or window may be performed before or after attaching the mount to the window. Alternatively, the steps may be performed substantially simultaneously. The final step of creating stress may be performed by increasing or decreasing the temperature of the assembly, as previously described.

[0048] Although the present invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail can be made therein without departing from the scope of the invention as defined by the appended claims.

Claims

1. 1. A ceramic window assembly comprising: Ceramic windows, a mount bonded to the ceramic window and asymmetrical in a direction perpendicular to the plane of the ceramic window; and a mechanically stressed superstructure;

2. The ceramic window assembly of claim 1 , wherein the superstructure is positioned to reduce tensile stress in the window and / or to transform tensile stress into compressive stress.

3. 3. The ceramic window assembly of claim 1 or 2, wherein the ceramic window material is synthetic diamond, optionally polycrystalline chemical vapor deposited diamond.

4. 4. The ceramic window assembly of claim 1, wherein the upper structure is mechanically coupled to the mount while the mount is attached to the ceramic window, and the upper structure is removed from the mount after the mount is attached to the ceramic window.

5. The ceramic window assembly of claim 4 , wherein an inner diameter of the superstructure matches an outer diameter of the mount.

6. 6. The ceramic window assembly of claim 4 or 5, wherein the superstructure has a lower coefficient of thermal expansion than the mount.

7. 4. The ceramic window assembly of claim 1, wherein the superstructure is a second ceramic window bonded to the mount on a side of the mount opposite the bonded side of the ceramic window.

8. The ceramic window assembly of claim 7 , wherein the second ceramic window has dimensions substantially the same as the dimensions of the ceramic window.

9. 4. The ceramic window assembly of claim 1, wherein the superstructure extends within a major plane of the window assembly and has a shape within the major plane that substantially matches the shape of the mount.

10. 10. The ceramic window assembly of any of claims 4 to 6 or 9, wherein the mount material is a metal, which may optionally be molybdenum.

11. 11. The ceramic window assembly of claim 1, wherein the thermal expansion coefficient of the material of the superstructure is lower than the thermal expansion coefficient of the mount.

12. The ceramic window assembly of claim 11 , wherein the material of the superstructure is one of ceramic, synthetic diamond, tungsten, or fused silica.

13. The maximum deflection of the ceramic window measured perpendicular to the plane of the window is 4.5 x 10 of the maximum linear dimension of the window. ―5 times or less, preferably 2.0 × 10 ―5 The ceramic window assembly of any one of claims 1 to 12, wherein the thickness is less than or equal to 100 μm.

14. 14. The ceramic window assembly of claim 1, wherein the maximum linear dimension of the ceramic window is selected from the group consisting of 10 mm to 130 mm, 20 mm to 60 mm, and 25 mm to 50 mm.

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

16. 16. The ceramic window assembly of any one of claims 1 to 15, wherein the ceramic window has a peak to valley flatness measured using 633 nm light across its maximum linear length selected from less than 100 x λ / 2 interference fringes, less than 80 x λ / 2 interference fringes, or less than 40 x λ / 2 interference fringes.

17. 17. A method for manufacturing a ceramic window assembly according to any one of claims 1 to 16, comprising the steps of: preparing the ceramic window; preparing the mount; preparing the superstructure; attaching the superstructure to the mount; bonding the mount to the ceramic window; and creating stress in the superstructure.

18. 18. The method of claim 17, further comprising the step of removing the superstructure from the ceramic assembly after adhering the mount to the window.

19. 20. The method of claim 18, wherein stress is induced when the ceramic window assembly is heated to bond the mount to the ceramic window.

20. 18. The method of claim 17, wherein the stress is induced upon cooling the ceramic window assembly from an elevated temperature used to bond the mount to the ceramic window.

21. The method of any of claims 17 to 20, further comprising mechanically machining the ceramic window after bonding the mount to the ceramic window.

22. An optical device comprising the ceramic window assembly of any one of claims 1 to 16.

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