GB2623540A

GB2623540A - Window assembly and method of manufacture thereof

Info

Publication number: GB2623540A
Application number: GB2215431.4A
Authority: GB
Inventors: Ian Pearson Michael; Graziosi Teodoro; Mark Reininger Francis
Original assignee: Element Six Technologies Ltd
Current assignee: Element Six Technologies Ltd
Priority date: 2022-10-19
Filing date: 2022-10-19
Publication date: 2024-04-24
Also published as: GB202215431D0

Abstract

A window assembly 300 includes a mount 302 defining an aperture 304 and a window 306 attached to the mount via a first affixing member 308. A second affixing member 310 is attached to the window and is for attaching to the mount or a structure peripheral to the window assembly. The first affixing member withstands temperatures up to a first temperature and the second affixing member withstands temperatures up to a second, higher temperature. The first temperature may be 600-950K. The second temperature may be higher than 950K or higher than 1300K. The first affixing member may be applied at a temperature less than 800K or less than 600K and may include a braze or one or more metallic pads affixed to the window with the braze. The one or more metallic pads may be independently affixed to the window and a bonding area of the braze between each of the one or more metallic pads and the window may be less than 10% or less than 5% of the surface area of the window. An optical imaging device includes the window assembly and a detector where both are in the transmission path of the imaging device.

Description

I

WINDOW ASSEMBLY AND METHOD OF MANUFACTURE THEREOF

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

Field of the Invention

The present invention relates to a window assembly, and a method of manufacture thereof, and more specifically to a synthetic diamond window assembly.

Background of the Invention

Ceramics, such as diamond, have high transmissivity across a wide range of frequencies, high scratch resistance, and a low thermal expansion coefficient, amongst other properties, which make them a suitable material for an optical window. On the other hand, ceramics are inherently prone to brittle fracture, particularly under tension. For this reason, it is desirable that tensile stresses in a ceramic window are reduced, minimised and/or removed during storage and/or operation. One known source of residual tension within the ceramic window arises due to the mismatch in the coefficient of thermal expansion (CIE) between the window and a mount.

Summary

According to a first aspect of the invention, there is provided a window assembly, comprising: a mount defining an aperture; a window attached to the mount via a first affixing member; a second affixing member, attached to the window and configured to be attached to the mount or a structure peripheral to the window assembly, wherein the first affixing member is able to withstand temperatures up to a first temperature and wherein the second affixing member is able to withstand temperatures up to a second temperature, and wherein the second temperature is higher than the first temperature.

The first temperature may be between 600K and 950K, and/or, the second temperature may be higher than 950K, or higher than 1300K.

The first affixing member may be applied at a temperature less than 800K, more preferably less than 600K.

The second affixing member may comprise a braze, and, optionally, one or more metallic pads affixed to the window with the braze. Each of the one or more metallic pads are independently affixed to the window. A bonding area of the braze between each of the one or more metallic pad and the window may be less than 10%, preferably less than 5%, of the surface area of the window.

The second affixing member may further comprise a coupling member configured to respectively affix each of said one or more metallic pads to the mount or a peripheral surrounding structure. The coupling member may be moveable relative to the one or more metallic pads, under loading. The coupling member may be resiliently deformable, and is for example a spring.

The braze may comprise an Au-Ta alloy. The first affixing member may be a diffusion bond comprising one or more metallic elements, and in a specific example the one or more metallic elements include aluminium.

The mount and/or the one or more metallic pads comprise molybdenum or an alloy thereof. The second affixing member may be affixed to the mount or surrounding structure. The window may comprise synthetic diamond.

As an option, the ceramic window has a maximum deflection, measured perpendicular to a main plane of the window, of no more than 4.5 xl a5 times a longest linear dimension of the window, and preferably no more than 2.0 x 1 05 times the longest linear dimension of the window. It is beneficial to reduce deflection to ensure that lensing of light or other radiation passing through the ceramic window is minimised.

As an option, the ceramic window has a largest linear dimension selected from any of between 10 mm and 130 mm, between 20 mm and 60 mm, and between 25 mm and 50 mm.

The ceramic window optionally has an average thickness selected from any of between 200 pm and 1500 pm, between 300 pm and 1000 pm, and between 400 pm and 800pm.

In practice, a thicker ceramic window is less prone to deflection but is more highly stressed, whereas a thinner ceramic window has lower stress but is more prone to deflection.

As a further option, the ceramic window has a peak to valley flatness selected from any of less than 100, less than 80 and less than 40 x A/2 interference fringes over a largest linear length of the ceramic window. Flatness can be measured using a 633 nm light interferometer. Optical interference creates a fringe pattern, and each fringe corresponds to a A/2 variation in flatness. The number of A/2 interference fringes is therefore a measure of the flatness of the ceramic window.

According to a second aspect of the invention, there is provided a method of preparing the window assembly according to the first aspect, the method comprising: applying a second affixing member to the window; attaching the window to the mount using a first affixing member to substantially cover an aperture defined in the mount; wherein, the first affixing member is able to withstand temperatures up to a first temperature, and wherein the second affixing member is able to withstand temperatures up to a second temperature, the second temperature being higher than the first temperature.

The step of applying the second affixing member to the window may comprise brazing one or more metallic pads to the window; and attaching a coupling member to said one or more metallic pads.

The step of attaching the window to a mount using a first affixing member may comprise: arranging the window and a bonding material relative to the mount, the window substantially covering the aperture in the mount and the bonding material separating the window from the mount; and applying heat and pressure to establish a diffusion bond between the window and mount, said diffusion bond comprising the bonding material, wherein during said heating the temperature remains no higher than 800K, preferably no higher than 600K.

The method may further comprise attaching the coupling member to the mount or a structure peripheral to the window assembly.

According to a third aspect, there is provided an optical imaging device, comprising: the window assembly according to the first aspect; a detector, wherein the window and detector of said window assembly are arranged in the transmission path of the imaging device. The optical imaging device may further comprise: a source configured to produce electromagnetic radiation along said transmission path.

Brief Description of the Drawings

Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1A and 18 show a top-view and side-view of a window assembly; Figure 2 shows the fractional change in length of a typical ceramic and metal with temperature; Figure 3A and Figure 3B show a top-view and side-view of a window assembly; Figure 4 shows a side-view of a second affixing member; Figure 5A and Figure 5B show a top-view and side-view of a window assembly; and Figure 6 is a flow diagram illustrating a method of preparing the window assembly.

Detailed Description

FIGs. 1A and 1B show a top-view and a side-view of a window assembly 100. The window assembly 100 comprises: a mount 102, defining an aperture 104; and a window 106 attached to the mount 102, via a bond 108, around the periphery of the aperture 104. The window is substantially planar (e.g., it may be a plate), having planar dimensions greater than the dimensions of the aperture 104 in the mount 102, such that, when the window 106 is attached to the mount 102, the window 106 spans across the aperture 104 to cover the aperture 104. More specifically, the bond 108 is arranged on at least part of the overlapping region between the window 106 and aperture 102. The mount of the window assembly 100 may include one or more through holes for securing the mount to a surrounding structure, for example using screws, bolts and nuts, etc to mount the assembly to a plate.

As the skilled reader would appreciate, the shape of the aperture 104 and window 106 is not to be considered limiting. The shape of the aperture 104 and window 106 in section may be circular, rectangular, square, hexagonal or the like. The aperture 104 and window 106 may be similar or dissimilar in shape. When used as a window for detecting light, the field of view and therefore the shape of the aperture are preferably as large as possible within the design constraints, and a rectangular window may be preferred for this reason.

Although the word 'mount is used for element 102, and one of the functions is that of mounting the assembly, the element may also be used for a different purpose than mounting. For example, element 102 may define a cooling channel and be used to control the temperature of the assembly by guiding a cooling fluid through the cooling channel. The word 'frame' is therefore also used herein instead of 'mount.

In an example, the mount 102 comprises a metal, in particular molybdenum or aluminium. The window 106 comprises a ceramic, such as diamond. The bond 108 comprises one or more metallic elements, such as aluminium. The bond may be a diffusion bond, or a different type of bond. One or more base layers may be deposited onto the diamond surface before diffusion bonding to the mount. The one or more base layers chemically bond to the diamond, and provide good adhesion to a primary bonding layer provided by the diffusion bonding.

The window assembly 100 may form part of an imaging device, comprising a detector and optionally an electromagnetic radiation source such as an infrared or other light source. The window assembly 100 in the imaging device is then arranged in the transmission path between the source and the detector, such that the transmission path passes through the window 106. In an example, the window 106 is co-aligned with the transmission path. In some examples, the source emits a particle such as a neutron and the source is not part of the imaging device. For example, the source could be a neutron source and the imaging device, comprising the window assembly 100, is arranged in the transmission path of the produced neutrons, but is a separate part to the source.

In some applications, the window assembly 100 is subject to thermal loading. To avoid excessive heating, the window assembly 100 optionally includes a cooling arrangement. Excessive heating is preferably avoided because it leads to residual stress/strain in the window 106, as explained below. Cooling arrangements, such as cooling channels within the mount 102 close to the window 106 and bond 108, can be included as mentioned before.

FIG. 2 shows a plot of the fractional change in length 202 of a typical ceramic, denoted by a circle, and a typical metal, denoted by a square, with temperature 204, at room temperature 206, an operating temperature 208 (Ti) and a bonding temperature 210 (T2).

As FIG. 2 shows, the CTE of a ceramic is generally lower than a metal, although this is not always the case. For example, diamond has a CIE of 1.07x10-6K-1 at 300K, whereas molybdenum has a CTE of 4.8x10-6K-1 at 300K and aluminium has a CTE of 2.4x10-5K-1 at 300K.

Room temperature refers to 300K. The operating temperature 208, also denoted Ti, refers to the temperature of the window 106 during operation at steady state. If steady state is not reached during operation, the temperature refers to the temporal average temperature of the window 106 during operation. The bonding temperature 210, also denoted as 12, refers to the temperature at which the window 106 is bonded to the mount 102.

Referring back to FIG. 1, after the window assembly 100 is allowed to cool following bonding of the window 106 to the mount 102, a residual stress will develop in the window 106 and within the mount 102 as the ceramic window 106 shrinks to a lesser extent than the metallic mount 102, and the ceramic window 106 is constrained in position by the bond 108.

The magnitude of the residual stress depends on: * the relative difference between the CTEs of the material, e.g., diamond, comprising the window, the material, e.g., molybdenum, comprising the mount and the material, e.g., aluminium, comprising the bond between the mount and window * the difference in magnitude between the current temperature, e.g., operating temperature, Ii, and the bonding temperature, 12.

The directionality of the residual stress depends on: * the shape and relative placement of the window 106 with respect to the mount 102, for example whether the window 106 is mounted in the space completely within the mount 102 defined by the aperture 104, or, against the mount 102 to cover the aperture 104, as shown in FIG. 1; and * the shape and relative placement of the bond 108 with respect to the window 106 and mount 102. For example, whether the bond between the window 106 and mount 102 is around the entire peripheral edge of the aperture 104, or only along certain sections of the peripheral edge, the relative placement of those sections etc. The magnitude and directionality of the stress and strain in the window 106 and mount 102 can be calculated using commercial software, such as ABAQUSTM.

Ideally, the arrangement of the mount 102, window 106 and bond 108 therebetween are designed such that the residual tensile stresses within the window 106 are minimised at the operating temperature, Ti. As an example of an assembly that avoids the development of residual tensile stress in the window following cooling after bonding of the window to the mount is an assembly comprising a mount having a aperture of circular section (or other shape with at least one rotational axis of symmetry); and a window of matching section, which is located completely within the space defined by the aperture and bonded to the mount around the periphery of the aperture. In such an assembly, cooling after bonding the window to the mount would lead to the development of compressive residual stresses in the window because the symmetry of the assembly prevents a bending moment about any axis contained within the plane including the bond from developing.

In practice, however, design constraints may require window assemblies 100 with a degree of asymmetry, as for example shown in FIG. 1. One source of asymmetry in the window assembly 100 of FIG. 1 is the presence of the window 106 on one side of the bond between the mount 102 and the window 106, but not the other. This asymmetry results in a bending moment after the window assembly 100 is cooled following bonding of the window 106 to the mount 102. Specifically, as the ceramic window 106 contracts to a lesser extent during cooling than the metallic mount 102, the window 106 of FIG. 1 would bend as it cools with one side being in compression and the opposing side being in tension. Bending of the ceramic window 106 also distorts radiation passing therethrough and could possibly lead to fracture. Bending is therefore undesirable.

It should be understood that, although a ceramic window 106 generally has a lower CTE than the mount 102 comprising a metal, this is not necessarily the case. If the mount contracted to a lesser extent than the window, a bending moment would still develop, albeit of opposite sign.

In order to reduce the magnitude of the residual stress within the window 102 following bonding with the mount 102, the inventors propose selecting a suitable bond material and method of bonding that allows for relatively low temperature bonding between the window 106 and mount 102. An example of a suitably low temperature is a temperature less than 1000K, more preferably less than 800K, even more preferably less than 600K.

A lower bonding temperature means the temperature difference between 12 and, the operating temperature Ti, or room temperature, respectively, is reduced. A corollary is that the induced strain following cooling due to the CTE mismatch, and hence the residual stress in the window 106, is reduced.

However, the inventors appreciate that a problem with this approach is that in order to produce such a bond at the above-mentioned "relatively low" temperatures, the material comprising the bond tends to have a low melting point accordingly. An example of a melting point corresponding to a low temperature bond is a temperature less than 1000K. A bond using such a low melting point material is vulnerable to softening or melting if the operating temperature Ti becomes greater than a first (Ti) or second critical temperature (1.2) respectively. An excess temperature may occur because of a loss of coolant.

Li refers to the temperature at which the yield stress of the bond 108 is less than 25% of the yield stress measured at room temperature using known techniques. T.2 refers to the melting point of the bond 108. As the bond may comprise a diffusion bond with varying melting point, T.2 may be interpreted as the minimum melting point of the bonding 108.

If the bond partially or completely melts, the window 106 is no longer mounted securely against the mount 102 and it may fall, be damaged or be misaligned with the aperture 104. If the bond softens, the bond may become unable to resist small loads (present during normal operation) and similarly be damaged and/or misaligned.

In a specific example, the bond 108 comprises aluminium or an alloy thereof, when the bond is an Al diffusion bond. The mount is molybdenum or an alloy thereof. The window comprises diamond. In a preferred example, the diamond is a polycrystalline chemical vapour deposited (CVD) diamond plate. With such a bond 108, Tel is approximately 600K, more specifically 570K, and T02 is approximately 950K, more specifically 930K.

In order to address this problem, the inventors propose an improved window assembly 300, as shown in FIG. 3A (top-view) and FIG.3B (side-view).

The window assembly 300 comprises: a mount 302, having an aperture 304; and a window 306 attached to the mount 302 via a first affixing member 308, and a plurality of second affixing members 310, which attach the window 306 to a surrounding structure, such as a plate which the mount 302 is secured against, or mount 302.

The first affixing member 308 is adapted to withstand temperatures up to a first temperature, TA. The lower bound for the first temperature (TA) is the softening temperature (T01) of the material comprising the first affixing member, as defined above. The upper bound for the first temperature (TA) is the melting point (fez) of the material comprising the first affixing member 308 (Te2), as defined above. The upper bound for the first temperature (TA) may be greater than the normal operating temperature Ti but is less than the temperature during operation in the event of loss of coolant. The second affixing member 310 is adapted to withstand temperatures up to a second temperature, TB. The lower bound for the second temperature is the softening temperature (Ti) of the material comprising the second affixing member, as defined. Similarly, the upper bound for the second temperature (TB) is the melting point of the material comprising the second affixing member 310 (Te2). The lower bound (and hence the upper bound) for the second temperature (TB) is greater than the maximum expected temperature during operation, even, for example, in the event of a loss of coolant. The second temperature is therefore greater than the first temperature and the maximum operating environment temperature, preferably with an addition of a safety margin. In an example, the operating environment temperature may be 1000K, and the second temperature is at least 1300 K. The first affixing member 308 may comprise a bond between the window 306 and mount 302, as for example shown in FIG. 1A and 1B and described in detail above. The window assembly 300 of FIG. 3A and 3B differs from the window assembly 100 of FIG. 1A and FIG.1B in that window assembly 300 further comprises the second affixing member 310, and the planar extent of the window 306 is larger than the mount 302 in order to provide sufficient space for the second affixing member 310. However, it should be understood that this is not essential because the second affixing member 310 could be affixed on the opposing side of the window to the first affixing member 308 and/or fixed to the mount 302, rather than the surrounding structure. Such an example is shown in FIG. 4.

The second affixing member 310 may comprise: * one or more metallic pads, each having a first portion brazed to the window 306 using a braze, and a second portion mechanically coupled to a surrounding structure or mount 302, e.g., a plate to which the mount 302 is secured. As detailed above, the mount 302 may include one or more through holes for securing the mount to the surrounding structure; * the braze; and * one or more coupling members (312), which respectively affix the second portion of the metallic pad to the surrounding structure or mount 302.

The bonding area between each of the metallic pads and the window is preferably small, to avoid restricting the field of view, and to reduce perturbations due to expansion or contraction. The bonding area may be less than for example 10%, more preferably less than 5%, even more preferably less than 1% of the total surface area of the window 306. A technical effect of this difference in surface area is that even if the metallic pads expand or contract at a significantly different rate than the window 306, they do not cause any significant stress (compressive or tensile) because the change in size of the affixing member is small compared to the size of the window. Although even small metallic pads can cause high stress, it will be easier to reduce perturbations with a smaller pad than with a larger pad.

Given the small relative size of the second affixing member, a plurality of affixing members 310 are provided to allow sufficiently secure handling. In the example, as illustrated in Fig. 3A, four affixing members 310 are illustrated: one for each side of a generally rectangular window assembly. Fewer second affixing members than four is, however, possible. The plurality of second affixing members 310 are indirectly connected to each other via the window 306, but are otherwise independent from one another. A technical advantage of using a plurality of independent second affixing members 310 is a reduction in tensile stress when compared to a larger mount securing the window along more connected portions, which extend in different directions.

Prior to attaching the window 306 to the mount 302 and the window assembly 300 to the surrounding structure, one end of the metallic pad is "free", or not mechanically coupled to anything else. The coupling member attached to the surrounding structure may be resiliently deformable to ensure that any expansion or contraction of the plurality of second affixing members 310 does not result in any significant stress (in particular tensile stress) in the window 306, but rather reversible deformation of the coupling member.

As shown in FIG. 3A, the one or more metallic pads are disposed around the window (e.g., a metallic pad for each edge of the window). The skilled reader should appreciate that the number, spacing and relative placement of these metallic pads can be varied and this example is not to be considered limiting.

As discussed above, the coupling member is preferably able to reversibly deform (i.e., elastically) under stress/strain such that it is movable relative to the metallic pads. An advantage of such a coupling member is that any relative expansion/contraction between any of the metallic pads, the mount 302, the braze therebetween and/or the window 306, due to thermal expansion mismatch can be, at least partially, accommodated by movement/strain of the coupling member relative to any of the above. The residual stress induced in the assembly 300 (e.g., the window) due to the thermal expansion mismatch is therefore reduced.

As shown in FIG. 4, an exemplary coupling member comprises one or more springs 412 (optionally, steel), having a stiffness sufficiently low to enable elastic deformation at the stresses expected during operation. The one or more springs 412 can be arranged in series and/or parallel. The springs 412 in FIG. 4 are affixed by welding, brazing, or the like, to the mount 302 or surrounding structure at one end and the metallic pads 414 at the other end, to thereby establish the mechanical coupling between the metallic pads 414 and mount 302 or surrounding structure, as the case may be. In another example, the coupling member comprises another compliant metal connection, such as a thin copper sheet folded to form a U-shape. For the avoidance of doubt, the metallic pads 414, mount 302, the coupling member (e.g., springs 412) and any brazing/welding therebetween comprise suitable materials that can withstand temperatures at least up to the second temperature (TB), referred to above.

Note that the metallic pads 414 in FIG. 4 are brazed to the window 306 on the opposing side to the first affixing member 308, in particular the diffusion bond. In this configuration, there is space for the second affixing members 310 to secure against the mount 302 (although it is still possible to affix them to a surrounding structure) and the planar extent of the window need not be greater than the mount 302.

In the event that the first affixing member 308 softens or melts at an excess temperature T, the second affixing member 310, which is able to withstand such excess temperature, ensures the window 306 maintains mechanical coupled and secured to the mount 302 or surrounding structure. Using the earlier notation: (Tcvc2 of the first affixing member 308) < T < (T01102 of the second affixing member 310). Hence, damage, loss and/or misalignment of the window 306 are avoided. The key failure which is avoided by the second affixing member is a failure of the window separating from a surrounding structure.

Advantageously, the window assembly 300 can have the benefits of a first affixing member 308 with a relatively low bonding temperature without having the vulnerabilities to high temperature.

In a specific example, the second affixing member comprises one or more metallic pads comprising molybdenum or an alloy thereof; a braze between the window and metallic pads comprising an active gold braze alloy, such as an Au-Ta alloy; and the one or more coupling members comprise one or more steel springs 412. The braze may also be another high temperature braze alloy containing a carbide forming element such as Ta or Ti. The window 306 comprises diamond. In a preferred example, the diamond is a polycrystalline chemical vapour deposited (CVD) diamond plate. The first affixing member 308 comprises an aluminium diffusion bond between the diamond window and molybdenum mount.

Figs. 5A and 5B illustrate an exemplary window assembly, which includes the coupling members. FIG. 5A is a front view, and FIG.5B is a vertical cross section along dotted line L in Fig.5A. A window 51 is attached to a mount 52 (which may for example comprise a cooling channel, or be a cooling pipe arrangement) but the mount is attached after the second affixing members 53 have been bonded to the window 51. The second affixing members comprise molybdenum, and the bond to the window 51 is a high temperature Au-Ta braze. In the examples shown, four affixing members 53 are bonded to the window with bonds 54. A larger or smaller number of second affixing members 53 may, however, achieve the same effect of providing a secure connection to a surrounding structure (not shown). Coupling members 55, such as springs, are used to attach the window assembly to the surrounding structure (not shown). As stated above, the surrounding structure may be a plate to which the mount is secured. The coupling members 55 avoid stress in the window assembly due to expansion or contraction as a result of temperature changes. The mount 52 preferably, although not necessarily, includes a cooling channel. The mount 52 connects to the window 51 via a diffusion bond 56.

The perimeter of mount 52 is sufficiently small to leave space on window 51 for attachment of bonds 54 without overlap between the bonds and the mount. A possible disadvantage of a reduced perimeter of the mount is a smaller field of view for optical applications. An option for addressing such disadvantage is a reduced perimeter only in the vicinity of the bonds, while the perimeter extends closer to the edge of the window in the remaining areas of the window without bonds.

FIG. 6 is a method flow diagram, which sets out how the window assembly 300, 400 according to FIG. 3A, 3B, FIG. 4 or FIG. 5A, FIG. 5B, is made.

In step 602, the plurality of second affixing members 310 are applied to the window 306.

For example, the one or more metallic pads 310 are brazed to the window 306.

In step 604, the window 306 is attached to the mount 302 via the first affixing member 308 to substantially cover the aperture 304 in the mount 302.

More specifically, the window 306 and a bonding material is arranged relative to the mount 302 such that the window 306 substantially or completely covers the aperture 304 in the mount 302, but the window 306 is separated from the mount 302 by the bonding material, such as an aluminium foil. Heat and pressure are then applied to the assembly 300, including the window 306, mount 302 and metallic foil, for a sufficient period of time for a diffusion bond to form, thereby establishing attachment of the mount 302 to the window 306.

In optional step 606, the plurality of second affixing members 310 are attached to the mount 302 or surrounding structure. For example, the coupling members are affixed between the mount 302 or surrounding structure and the one or more metallic pads, brazed to the window, via any one of welding/brazing/soldering.

The steps described above may be performed in the order specified or in any other working order. As the second affixing members 310 are able to withstand higher temperatures than the first affixing member 308, it will be applied to the window 306 before the window 306 is affixed to the mount 302 using the first affixing member 308. That said, the coupling member may be attached to attach the metallic pads to the mount or surrounding structure after the first affixing member has been applied. It should therefore be noted that, although the first affixing member 308 has been described with reference to bond 108 shown in FIG. 1A and FIG. 13, the bonding area between the window 306 and mount 302 may differ due to the presence of the second affixing member 310.

Although the invention has been described in terms of preferred embodiments, as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Features from different examples may be combined as appropriate to form other working examples.

Claims

CLAIMS: 1. A window assembly, comprising: a mount defining an aperture; a window attached to the mount via a first affixing member; a second affixing member, attached to the window and configured to be attached to the mount or a structure peripheral to the window assembly, wherein the first affixing member is able to withstand temperatures up to a first temperature and wherein the second affixing member is able to withstand temperatures up to a second temperature, and wherein the second temperature is higher than the first temperature.
2. The window assembly according to claim 1, wherein the first temperature is between 600K and 950K.
3. The window assembly according to any of claims 1 or 2, wherein the second temperature is higher than 950K, or higher than 1300K.
4. The window assembly according to any one of claims 1 to 3, wherein the first affixing member is applied at a temperature less than 800K, more preferably less than 600K.
5. The window assembly according to any one of claims 1 to 4, wherein the second affixing member comprises: a braze, and, optionally, one or more metallic pads affixed to the window with the braze.
6. The window assembly according to claim 5, wherein each of the one or more metallic pads are independently affixed to the window.
7. The window assembly according to claims 5 or 6, wherein a bonding area of the braze between each of the one or more metallic pad and the window is less than 10%, preferably less than 5%, of the surface area of the window.
8. The window assembly according to any one of claims 5 to 7, wherein the second affixing member further comprises: a coupling member configured to respectively affix each of said one or more metallic pads to the mount or a peripheral surrounding structure.
9. The window assembly according to claim 7 wherein the coupling member is moveable relative to the one or more metallic pads, under loading.
10. The window assembly according to claim 9, wherein the coupling member is resiliently deformable.
11. The window assembly according to claim 10, wherein the coupling member is a spring.
12. The window assembly according to any one of claims 5 to 11, wherein the braze comprises an Au-Ta alloy.
13. The window assembly according to any one of claims 1 to 12, wherein the first affixing member is a diffusion bond comprising one or more metallic elements.
14. The window assembly according to claim 13, wherein the one or more metallic elements include aluminium.
15. The window assembly according to claim 5, wherein the mount and/or the one or more metallic pads comprise molybdenum or an alloy thereof.
16. The window assembly according to any one of claims 1 to 15, wherein the second affixing member is affixed to the mount or surrounding structure.
17. The window assembly according to any one of claims 1 to 16, wherein the window comprises synthetic diamond.
18. The window assembly according to any one of claims 1 to 17, wherein the ceramic window has a maximum deflection, measured perpendicular to a main plane of the window, of no more than 4.5 x 10-5 times a longest linear dimension of the window, and preferably no more than 2.0 x 10-5 times the longest linear dimension of the window.
19. The ceramic window assembly of any one of claims 1 to 18, wherein the ceramic window has a largest linear dimension selected from any of between 10 mm and 130 mm, between 20 mm and 60 mm, and between 25 mm and 50 mm.
20. The ceramic window assembly of any one of claims 1 to 19, wherein the ceramic window has an average thickness selected from any of between 200 pm and 1500 pm, between 300 pm and 1000 pm, and between 400 pm and 800pm.
21. The ceramic window assembly of any one of claims 1 to 20, wherein the ceramic window has a peak to valley flatness selected from any of less than 100, less than 80 and less than 40 x A/2 interference fringes over a largest linear length of the ceramic window, measured using 633 nm light.
22. A method of preparing the window assembly according to any of claims 1 to 21, the method comprising: applying a second affixing member to the window; attaching the window to the mount using a first affixing member to substantially cover an aperture defined in the mount; wherein, the first affixing member is able to withstand temperatures up to a first temperature, and wherein the second affixing member is able to withstand temperatures up to a second temperature, the second temperature being higher than the first temperature.
23. The method of claim 22, wherein applying the second affixing member to the window comprises: brazing one or more metallic pads to the window; and attaching a coupling member to said one or more metallic pads.
24. The method of any of one of claims 22 to 23, wherein attaching the window to a mount using a first affixing member comprises: arranging the window and a bonding material relative to the mount, the window substantially covering the aperture in the mount and the bonding material separating the window from the mount; and applying heat and pressure to establish a diffusion bond between the window and mount, said diffusion bond comprising the bonding material, wherein during said heating the temperature remains no higher than 800K, preferably no higher than 600K.
25. The method of any of claims 22 to 24, further comprising: attaching the coupling member to the mount or a structure peripheral to the window assembly.
26. An optical imaging device, comprising: the window assembly according to any of claims 1 to 21; a detector, wherein the window and detector of said window assembly are arranged in the transmission path of the imaging device.
27. The optical imaging device according to claim 26, further comprising: a source configured to produce electromagnetic radiation along said transmission path.