Patent Application
20250114878
This invention relates to braze alloys for high temperature hermetic envelopes and assemblies comprising thereof; brazed joints; and the process of producing braze joints using said braze alloys.
High voltage vacuum tubes comprise an anode and a cathode, which are disposed opposite one another in a vacuumized inner space. The vacuumized inner space is typically enclosed by a cylindrical metal housing, with the anode and/or cathode being electrically insulated by means of an annular insulator.
The high voltage vacuum tube is a device which controls electric current flow in a high vacuum between electrodes, with a considerable amount of heat produced from both the filament (heater) and the electron bombardment onto the anode. High voltage applications of vacuum tubes include x-ray tubes, magnetrons, traveling-wave tubes, carcinotrons and klystrons.
High voltage vacuum tubes typically use ceramic insulators to offset a high voltage from a lower voltage. For example, an anode at a high voltage may be offset from a body of the vacuum tube by the ceramic insulator. The body of the vacuum tube is typically attached to a ceramic insulator. A metallic seal ring may be brazed onto an outer surface of the ceramic insulator to attach the body to the ceramic insulator. This attachment creates a triple junction between the ceramic insulator, the seal ring, and a surrounding media. An electric field at this triple junction may be relatively high, resulting in electrons that may become the source of arcing and/or punctures. High voltage structures with an insulator forming part of a vacuum chamber are known from U.S. Pat. No. 4,126,803.
X-ray tubes for various applications are known to be operated with high direct or alternating voltages applied between the anode and the cathode, and depending on the desired radiation intensity, the voltages applied reach up to several 100 kV. In such x-ray tubes, the necessary insulating paths are arranged predominantly in the axial direction.
As highlighted in U.S. 20210134553, the bonding of iron-nickel cobalt or iron-nickel alloy to niobium may demand a braze alloy with a liquidus greater than approximately 900° C. to achieve the required wetting and braze flow. The braze material may be selected to achieve the desired wetting and braze flow, such as a 50/50 mix of Au/Cu, 81.5/16.5 mix of Au/Cu (Nicoro™-80), and 82/18 mix of Au/Cu (Nioro™).
Gold copper braze materials are part of a broader family of high temperature braze materials based on precious metals (palladium, platinum, gold and silver) with nickel and copper additions. They possess good mechanical properties at elevated temperature and good oxidation resistance. Cu—Ge based alloys are often used as an alternative to precious metal braze alloys (e.g. Au—Cu and Cu—Ag based alloys) for vacuum brazing applications.
Non-precious metal alloys, such as Cu—Ge alloys (tradename Gemco™ with a nominal composition of 87.75% Cu, 12% Ge and 0.25% Ni), have been used for vacuum brazing of copper, steel, and nickel-based metals. However, such an alloy exhibits a wide range between their solidus and liquidus temperatures known as the solidification temperature range (STR), which create a liquation problem in brazing applications. Liquation in brazing is defined as the tendency of the lower-melting constituents of a braze alloy to separate out and flow away from the higher-melting constituents of the braze alloy during heating. It occurs when the alloy is heated slowly through that melting range such as when furnace brazing and it manifests as a non-melted skull of alloy that remains at the point where the braze alloy was applied. This often leads to poor joint strength due to the presence of a brittle intermetallic phase in the brazed joint. Liquation is usually apparent in alloys having a large STR. Brazing operations conducted within a furnace which has temperature variations are also prone to liquation, particularly if the braze alloy comprises a large STR.
Another problem with high STR braze alloys is their inability to be used in step-brazing when a lower temperature alloy is used in step to braze complex joints. During step brazing, the liquidus temperature of the low temperature braze is close to the solidus temperature of the high temperature braze (due to the high STR of the latter), which causes a dimensional shift of the components.
SU564128 discloses a braze alloy composition with good wettability properties at 1000° C. with a composition range of 9.5-11 wt % Ge; 0.8-1.3 wt % Sn; 0.03 to 0.15 wt % of elements from the group of B, Co and Fe; and the balance Cu. Through increasing the amount of Ge and reducing the amount of B, the composition overcame the deficiencies of the braze alloy composition of SU255015 which while suitable for brazing at temperatures between 1000-1040° C., had poor formability with cracks first appearing after rolling of the starting ingots to a reduction value of only 14% for compositions in the range of 6.5 to 9 wt % Ge; 0.1 to 2.5 wt % Ni; 0.5 to 1.5 wt % Sn; 0.05 to 0.25 wt % Co; 0.05 to 0.25 wt % B; and the balance Cu.
While these braze alloy compositions address some of the needs for alternatives for braze alloys with a significant precious metal content, there is still a demand for braze alloys braze assemblies and devices comprising thereof which comprise components brazed together using alternative braze materials which are sufficiently malleable to form into wires, foils and preforms and which are not reliant upon large portions of precious metals. Additionally, the braze alloys should have a sufficiently small STR to avoid or reduce the risk of liquation compared to conventional non-precious metal braze alloys.
In a first aspect of the present invention, there is provided a braze alloy composition comprising in weight %:
where the braze alloy composition comprises no more than 0.4 wt % Sn.
The copper content may be greater than 85 wt % or greater than 87 wt % or greater than 89 wt % or greater than 91 wt % or greater than 93 wt % of the total weight of the braze alloy composition. The copper content may be no more than 96 wt % or no more than 94 wt % or no more than 92 wt % of the total weight of the braze alloy composition. In some embodiment, the braze alloy composition may further comprising a balance of additives, excluding Cu, Ge, and B. In another embodiment, the braze alloy composition may further comprise 0 to 10 wt % additives (or 0 to 5 wt % additives), not including Cu, Ge and B. In some embodiments, the additives are selected from one or more elements from the group consisting of rare earth metals and transition metals, with the exception of Cu. In some embodiments the Sn content is no more than 0.2 wt % or is present as an incidental impurity. In other embodiments, Sn is present as an optional additive.
In some embodiments, the additives make up the balance of the braze alloy composition in addition to the Cu, Ge, B and Sn. The additives may comprise greater than 0.1 wt % of the total weight of the braze alloy composition.
The braze alloy composition is typically crystalline. However, in some embodiments, it may be at least partially amorphous.
In an alternative aspect, the braze alloy composition comprises in weight %:
wherein the additives and impurities do not comprise Sn in an amount exceeding 0.4 wt % of the total weight of the braze alloy composition.
The additives may comprise or consist of one or more additives (i.e. elements) selected from the group consisting of transition and rare earth metals.
The braze alloy composition is preferably suited for use in a vacuum tube and as such as the required properties, including low vapour pressure and low magnetism. The need for low magnetism precludes the use of excess amounts of Ni, Co and Fe. In some embodiments, the combined amounts of Ni, Co and Fe is no more the 4.0 wt % or no more than 3.0 wt % or no more than 2.0 wt % or no more than 1.0 wt %.
The braze alloy compositions of the present invention provides a non-precious metal, or low precious metal content, alternative for high temperature brazing, particularly in a vacuum environment. The applicants have found a small compositional window for a braze alloy with desirable liquidus temperature; solidification temperature range (STR), with good formability, which is able to form hermetic seals within low pressure environments.
For the purposes of the present invention, good formability means that the braze alloy has a formability of at least 38% as determined by the formability test, described herein, and/or the braze alloy is able to be drawn into a wire with a diameter of down to 0.030″ (0.76 mm) and preferably down to at least 0.015″ (0.38 mm). Typically, wire diameters of up to 0.10″ may be used, although larger diameter wires may be manufactured as required.
It should be appreciated that in embodiments, the braze alloy composition may consist of the components in weight % defined above. Thus, the first aspect of the present invention can also relate to a braze alloy composition consisting of in weight %:
wherein the additives and impurities do not comprise Sn in an amount exceeding 0.4 wt % of the total weight of the braze alloy composition.
In one embodiment, the braze alloy composition comprises in weight %:
wherein the additives and impurities do not comprise Sn in an amount exceeding 0.4 wt % of the total weight of the braze alloy composition.
In another embodiment, the braze alloy composition comprises in weight %:
wherein the additives and impurities do not comprise Sn in an amount exceeding 0.4 wt % of the total weight of the braze alloy composition.
In a further embodiment, the braze alloy composition comprises in weight %:
wherein the additives and impurities do not comprise Sn in an amount exceeding 0.4 wt % of the total weight of the braze alloy composition.
In a further embodiment, the braze alloy composition comprises in weight %:
wherein the additives and impurities do not comprise Sn in an amount exceeding 0.4 wt % of the total weight of the braze alloy composition.
It should be appreciated that in embodiments, the braze alloy composition may consist of the components in weight % defined within the specification.
Braze alloys within this compositional range have a combination of low STR values and high liquidus temperatures which are suitable for joining together substrates of which at least one comprises copper.
In some embodiment, the sum of Ge+B+Cu is greater than 85.0 wt % or greater than 90.0 wt % or greater than 95.0 wt %, or greater than 96.0 wt %, or greater than 97.0 wt %, or greater than 98.0 wt %, or greater than 99.0 wt %, or greater than 99.5 wt %, or greater than 99.7 wt % of the total weight of the braze alloy composition.
It is noted that where an element or compound or other constituent is stated to have a content in a numerical range including 0 or a numerical range without a lower limit, the content of this element or compound or other constituent may be zero. In other words, this element or compound or other constituent may be absent, and is therefore optional.
Elemental Germanium levels below 2.0 wt % result in the braze alloy having a too high a liquidus temperature to braze copper or copper-based alloys. In some embodiments, the braze alloy comprises at least 2.2 wt %, or at least 2.5 wt %, or at least 2.7 wt %, or at least 2.9 wt %, or at least 3.1wt %, or at least 3.3 wt %, or at least 3.5 wt %, or at least 3.7 wt %, or at least 3.9 wt %, or at least 4.1 wt %, or at least 4.3 wt %, or at least 4.5 wt %, or at least 4.7 wt %. Higher levels of Germanium above 10 wt % correspond to a braze alloy composition comprising a lower liquidus temperature.
Germanium levels above 9.5 wt % Ge comprise a higher solidification temperature range (STR) which is less desirable for brazing applications, particularly involving step brazing. It has been found that Ge contents below 10 wt % possess an acceptable STR value for many applications, whilst the liquidus temperatures increases, but remains in the target liquidus temperature range for brazing, for example copper (or a copper alloy) to a substrate, such as stainless steel (e.g. 900 to 1050° C.).
In some embodiments, the Ge content is no more than 9.3 wt % or no more than 9.0 wt % or no more than 8.5 wt %, or no more than 8.0 wt %, or no more than 7.5 wt %, or no more than 7.2 wt %, or no more than 7.0 wt %, or no more than 6.5 wt %, or no more than 6.4 wt %, or no more than 6.3 wt %, or no more than 6.2 wt %, or no more than 6.1 wt %, or no more than 6.0 wt %, or no more than 5.8 wt %, or no more than 5.5 wt %, or no more than 5.2 wt %, or no more than 5.0 wt %.
In one embodiment, the Germanium content is in the range of 2.5 wt % to 7.4 wt %, or in the range of 3.0 wt % to 6.5 wt %, or in the range 3.5 wt % to 6.3 wt %, or in the range 3.8 wt % to 6.0 wt %, or in the range 4.0 wt % to 5.8 wt %. Germanium contents within these compositional windows have been found to have a combination of low STR values and relatively good formability at a given boron content.
Optional liquidus temperature suppressant amounts of aluminum indium and silicon, as defined in the first aspects of the present invention, contribute to reducing the liquidus temperature of the braze alloy, whilst maintaining a relatively small solidification temperature range (STR).
It has been found that Al, In and Si may decrease the liquidus temperature of the braze alloy composition within relatively smaller amounts than Ge. As a general rule, it is thought that more Ge is required to reduce the liquidus temperature than any combination of Al, In and Si on a wt % basis (e.g. 2.5 wt % Ge has the equivalent liquidus temperature lowering power as 1.0 wt % of Al, In and Si). For example, rather than using 7.25 wt % Ge, an alternative starting point may include 3.0 wt % Ge and 1.7 wt % In or 5.5 wt % Ge and 0.7 In.
As the skilled artisan would appreciate, the exact equivalent liquidus temperature lower amounts may vary depending upon the specific liquidus temperature suppressant used and the overall composition of the braze alloy. It has also been found that relatively small amounts (compared to Ge) of combinations of Al, In and Si may adversely affect the STR or liquidus temperature of the braze alloy composition. As such, Ge should preferably form at least 50 wt % or at least 60 wt % or at least 70 wt % or at least 80 wt % or at least 90 wt % or at least 95 wt % of the total amount of liquidus temperature suppressants (Ge, In, Si and Al).
The addition of elemental boron (e.g. boron>0.02 wt % or >0.05 wt % or >0.10 wt %) has been shown to result in decreased STR values, whilst boron levels above 1.25 wt % result in braze alloys with insufficient formability, thereby preventing the fabrication of the desired forms of the braze alloys (e.g. in wire or sheet/foil form).
In some embodiments, the boron level is greater than 0.05 wt % or greater than 0.10 wt % or greater than 0.12 wt %, or greater than 0.15 wt % or greater than 0.20 wt % or greater than 0.23 wt %, or greater than 0.25 wt %, or greater than 0.27 wt %, or greater than 0.28 wt %, or greater than 0.29 wt %, or greater than 0.30 wt %, or greater than 0.31 wt %, or greater than 0.32 wt %, or greater than 0.33 wt %, or greater than 0.34 wt %, or greater than 0.35 wt %, or greater than 0.37 wt %, or greater than 0.39 wt %. In some embodiments, the boron level is no more than 1.2 wt %, or no more than 1.1 wt %, or no more than 1.0 wt %, or no more than 0.90 wt %, or no more than 0.80 wt %, or no more than 0.75 wt %, or no more than 0.70 wt %, or no more than 0.65 wt %, or no more than 0.60 wt %, or no more than 0.55 wt %, or no more than 0.50 wt %.
In a preferred embodiment, the boron content is in the range of greater than 0.27 wt % and no more than 0.80 wt %. Within this range, the braze alloys possess both good formability and low STR values.
A range of elemental additives (preferably metallic) may be added to the alloy composition to assist the wettability, flowability during the formation of the braze joint and/or the mechanical strength of the resultant braze joint. The additives should be selected as to not significantly adversely affect the STR, liquidus temperature; formability and/or vapour pressure of the braze alloy; mechanical integrity or hermeticity of the resultant joint.
As would be apparent to the skilled addressee, small amounts of additives may be added to the braze alloy compositions which may enhance or at least not be detrimental to the functionality of the braze alloy composition within a given system or application. The determination of the type and amounts of additives would be within the competency of the skilled addressee, without the need for undue experimentation. The scope of this disclosure covers such additive additions.
In some embodiments the additives comprise one or both of transition metals and rare earth metals. The additives may comprise one or more elements from the group consisting of transition metals. The additives may comprise one or more elements from the group consisting of rare earth metals.
In some embodiments, there are greater than 0.0 wt % additives (>0.0 wt %) additives, requiring the alloy composition to include at least some additive content. For example, in some embodiments, the composition includes >0 to 5.0 wt % additives selected from the group consisting of transition metal and rare earth metals, with the exception of copper. The upper limit of the total and individual additive components will be limited by their ability to maintain the functional performance of the braze alloy, whilst the lower limit will be limited by the amount required to provide a functional benefit to the braze alloy.
In some embodiments the additives comprise 0 or >0 to 5 wt % transition metals.
The transition metals and rare earth metals may include scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, zinc, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, silver, cadmium, lanthanum, hafnium, tantalum, tungsten, rhenium, osmium, iridium, mercury, actinium, rutherfordium, dubnium, seaborgium, bohrium, hassium, meitnerium, darmstadtium, roentgenium, copernicium, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.
In some embodiments, the additives may comprise wetting additives selected from the group consisting of Nb, Ni, Mo, W, Co, and Fe.
In some embodiments, the braze alloy composition may comprise one or more liquidus temperature suppressants in the concentration ranges as follows:
Greater amounts of these elements may result in a deterioration in ductility and/or the STR of the braze alloy composition.
In some embodiments, the additives may comprise Ag or Zn to further enhance workability of the braze alloys.
Ti, V and/or Zr may also be added to assist in the bonding of the braze alloy to ceramic surfaces.
Other transition metals, including noble and/or precious metals, may also be added to assist functional performance. In some embodiments, the proportion of precious metals (combined or as any one elemental additive) is less than 4.0 wt % or less than 3.0 wt % or less than 2.0 wt % or less than 1.0 wt % or less than 0.5 wt %. While a small proportion of precious metals may provide some functional benefits, their addition is optional.
In one embodiment, the additives comprise one or more of Nb, Ni, Mo, W, Co, Cr, Fe, Ti, V, Zr, Au, Ag, Zn, Pt & Pd.
In some embodiments, the additives comprise 0 or >0 to 3 wt % rare earth metals. The amount of rare earth metals may be no more than 2.0 wt % or no more than 1.0 wt % or no more than 0.5 wt % or no more than 0.3 wt % or no more than 0.1 wt %.
Rare earth metals, such as Nd, Y, Yb and Ce may be added to further improve strength and/or hermeticity through grain refinement.
In one embodiment, the additives comprise one or more of Nb, Ni, Mo, W, Co, Cr, Fe, Ti, V, Zr, Au, Ag, Zn, Pt, Pd, Y, Yb, Nd & Ce.
In another embodiment, the additives comprise one or more of Ni, Co, Cr, Fe, Zr, Au, Ag, Zn, Y and Nd.
In a preferred embodiment, the additives comprise Ni. In a preferred embodiment, the additives are selected from the group consisting of Ni, Co, Fe, Au and Ag. In a preferred embodiment, the braze alloy composition comprises at most 4.8 wt % of said additives. In a preferred embodiment, the braze alloy composition comprises at most 1.0 wt % of said additives. In a preferred embodiment, the braze alloy composition comprise at least 0.2 wt % of said additives.
The addition of Sn was found to detrimentally affect both the STR and the formability of the braze alloy. As such, the addition of Sn should be avoided and, if present, should be in quantities (additives+incidental impurities) that do not exceed 0.4 wt % or do not exceed 0.3 wt % or do not exceed 0.25 wt % or do not exceed 0.2 wt % or do not exceed 0.15 wt % or do not exceed 0.1 wt % of the total weight of the braze alloy composition. In some embodiments, the Sn content in the braze alloy composition is 0.05 wt % or less, and more preferably 0.01 wt % or less.
The additives preferably comprise metals with a liquidus temperature greater than at least 500° C. or at least 800° C. or at least 900° C. or at least 1000° C. Due to the need for low vapour pressure and high temperature performance, high liquidus temperature additives are preferred.
In some embodiments there is ≤4.0 wt % or ≤3.0 wt % or ≤2.0 wt % or ≤1.0 wt % or ≤0.5 wt % additives. In other embodiments, when present, the additive level may be ≥0.05 wt % or ≤0.10 wt % or >0.15 wt % or >0.20 wt % additives. Additives levels below this amount may not be sufficient to provide the desired functional effect, such as improved wettability or improved braze joint strength. In some embodiments, the additives comprise or consist of metallic wetting additive(s), which when present, may be selected to improve wettability of the braze alloy on the substrate surfaces being joined (e.g. Nb, Ni, Co and/or Fe). Wettability of the braze alloy to the substrate is important to ensure a strong mechanical and hermetic joint. Due to the requirements of the braze alloy to have a low vapour pressure in some applications, additives such as Cd and Zn, are preferably not used.
In one embodiment, each additive has a vapour pressure of no more than1.0×10−7 mm Hg (1.33×10−5 Pa) at 700° C. and preferably no more than 5×10−8 mm Hg (6.65×10−6 Pa) at 700° C. or no more than 1×10−8 mm Hg (1.33×106 Pa) at 700° C. In another embodiment, the addition of the additives (including wetting additives) does not result in the vapour pressure increasing to more than 1.0×10−7 mm Hg (1.33×10−5 Pa) at 700° C. or more than 5×10−8 mm Hg (6.65×10−6 Pa) at 700° C. or more than 1×10−8 mm Hg (1.33×10−6 Pa). These undesirable resultant vapour pressures would typically be a higher vapour pressure than the braze alloy without the addition of the additives. The braze alloy preferable comprises no more than 0.5 wt % or no more than 0.4 wt % or no more than 0.3 wt % or no more than 0.2wt % or no more than 0.1 wt % or no more than 0.05 wt % of additives that do not meet this requirement.
It will be appreciated that the braze joint composition may be derived from the composition of the braze alloy and the composition of the substrates being joined together, including coatings thereof. At least part, if not all, of the additives in the braze joint composition may be derived through diffusion of components in the substrates being joined into the braze joint.
Incidental impurities as used herein refer to unavoidable traces of elements (including oxidised or reduced forms thereof) which occur during the production process of braze alloys.
Unless otherwise specified, incidental impurities may include any element or compound which is not already specified within the braze alloy composition (e.g. excluding Ge, Cu, B, Al, In, Si, Sn, and the additives). Incidental impurities may include elements (and compounds derived therefrom) from Group 1, 2, 3A (except B), 4A (except Sn and Ge) 5A, 6A, 7A, 8A of the periodic table.
Group 1 elements comprise H, Li, Na, K, Rb, Cs and Fr.
Group 2 elements comprise Be, Mg, Ca, Sr, Ba and Ra.
The Group 3A elements comprise Ga and TI.
The Group 4A elements comprise C and Pb.
The Group 5A elements comprise N, P, As, Sb and Bi.
The Group 6A elements comprise O, S, Se, Te and Po
The Group 7A elements comprise F, Cl, Br, I and At
The Group 8A elements comprise He, Ne, Ar, Kr, Xe and Rd.
In a preferred embodiment, the braze alloy composition comprises less than 1.0 wt % incidental impurities or at most 0.5 wt % of incidental impurities, preferably at most 0.3 wt % of incidental impurities, and most preferably at most 0.15 wt % of incidental impurities. These small quantities of elements typically do not contribute to or modify the actual purpose and/or performance of the braze alloy.
In one embodiment, the braze alloy composition, preferably with the exception of oxygen, comprises no more than 0.2 wt % or no more than 0.15 wt % or no more than 0.1 wt % or no more than 0.05 wt % of any one individual incidental impurity element.
While incidental impurities (also known as unavoidable impurities) may vary depending upon the purity of the raw material used, typical levels of incidental impurities are less than 0.8 wt % or less than 0.5 wt % or less than 0.2 wt % or less than 0.1 wt % or less than 0.05 wt % of the total weight of the braze alloy composition. Due to oxidation reactions, oxygen (O) may be present as an incidental impurity up to 1.0 wt % or up to 0.8 wt % of up to 0.5 wt % of the total weight of the braze alloy composition. Some applications required even stricter limits. For example, limits for each of Zn, Cd, Pb, C may be less than 0.1 wt % or less than 0.05 wt % or less than 0.01 wt % or less than 0.005 wt % or less than 0.01 wt % of the total weight of the braze alloy composition. In one embodiment, Zn and Cd have limits of less than 0.002 wt % or less than 0.001 wt %. Pb and P may have a limit of less than 0.01 wt % or less than 0.002 wt %. C may have a limit of less than 0.05 wt % or less than 0.01 wt % of the total weight of the braze alloy composition. All other elemental impurities (including metallic impurities) having a vapour pressure higher than 10−7 mm Hg (1.33×10−5 Pa) at 500° C. are preferably limited to 0.1 wt % or less than or 0.01 wt % or less; or 0.005 wt % or less or 0.002 wt % or less each. Elemental impurities having a vapour pressure lower than 10−7 mm Hg at 500° C. are preferably limited to a total no more than 0.2 wt % or no more than 0.1 wt % or no more than of 0.075 wt % of the total weight of the braze alloy composition.
A list of incidental impurities which are typically tested for includes Al, P, Pb, Cd and Zn. For a duplicate analysis of Example 8 (Table 1), all of the incidental impurities were measured to be less than the detection limit (% wt) being Al<0.001; P<0.002; Pb<0.001; Cd<0.001; and Zn<0.001. Whilst Cd and Zn are transition metals, for the application the braze alloy is to be used for (e.g. x-ray tubes), Cd and Zn are each considered incidental impurities for this specific end use application. Any use of ultrapure raw materials such as used in Example 8 is not reflective of the typical incidental impurity levels that may be used.
The braze alloy is preferably malleable enough to be processed as a standard alloy. It has sufficient workability, i.e., easy to deform into required sizes and shapes via standard metal forming processes such as rolling, wire forming, wire drawing, and stamping. In one embodiment, the braze alloy may be manufactured into a wire with a diameter of down to 0.030″ (760 μm) or down to 0.015″ (380 μm) or down to 200 μm. Wire diameters typically are no more than 5.0 mm or 2.54 mm in diameter. In another embodiment, the braze alloy may to manufactured into a foil with a thickness of down to at least 0.002″ (50 μm) or lower. Generally, foils may be produced with a thickness in the range of 0.001″ (25 μm) to 0.010″ (250 μm) or 0.020″ (500 μm) or higher.
The braze alloys may be used to form braze assemblies in aeroengines (OEM and repairs), aerospace fuel-line assemblies, semiconductor process chamber components, vacuum tubes (including high voltage vacuum tubes, such as x-ray tubes), wave guides and Klystron assemblies, power supply surge arrestors and automotive components.
In a second aspect of the present invention there is provided an envelope comprising a first component and a second component, wherein a brazed joint (preferably hermetic) joins the first component and the second component together, said envelope comprising a braze alloy composition of the first aspect of the present invention or a braze assembly of the fourth aspect of the present invention.
It should be appreciated that in other embodiments, the braze alloy composition may consist of the components in weight % defined above.
It is to be understood that envelope in the context of the present specification and claims means a high temperature hermetic envelope that is capable of being operated at least 500° C. or at least 800° C. and that is capable of operating under vacuum or containing gases such that the brazed joints therein prevent gases leaking into or out of the envelope. Preferably, the hermeticity is such that the braze joint passes a leak test with a gas tightness of 1×10−6 atm.cc/s or less (ASTM F2391 using helium gas).
“Envelope” refers to a vessel, tube or enclosure which defines an enclosed space in which enclosed components, fluid, vacuum or gases may reside. The envelope separates or isolates the enclosed spaced from the space external to the envelope.
“Hermetic” and its variations herein refer to a sealed, gas-tight, and fluid-tight braze joint, vessel, tube, or enclosure relative to the environmental conditions under which a housing or enclosure relative to the environmental conditions under which an envelope described herein would normally be subjected. Hermetic may generally mean that a braze joint is capable of isolating an environment on the outside of a device from the inside of the same device. The envelope may be part of an assembly such as all or part of a high voltage vacuum tube or a semiconductor process chamber component.
Envelopes by their nature typically comprise at least one braze joint one of which necessarily has to be formed as a blind joint. That is, a joint which is formed through the molten braze alloy being drawn into a gap (e.g. 5 μm to 500 μm or to 200 μm) between the two components to be joined (e.g. via capillary action), often after the sub-subassembly components within the envelope are in place. Thus, the braze alloy properties are critical to firstly form the braze of the desired dimensions to be placed immediate adjacent the components to be joined. Secondly, the braze joint should have sufficient wettability and flowability for form a high-quality connection (e.g. without liquation) between the components through capillary action. Thirdly, the brazed joint requires sufficient hermeticity, corrosion resistance, high temperature resistance and strength to provide the envelope with long term functional integrity. At least some of the braze joints within the envelope (or other brazed assembly) should be accomplished blindly.
Additionally, the joints of the many designs cannot be inspected after fabrication. This is particularly important because brazed joints require finite gaps for proper brazing. If the manufacturing tolerances vary, a required gap appears only problematical because the final brazing gap is unknown and too large. Consequently, there is a need to maintain the stress carrying capability of the brazed joint and its capacity to be inspected and produced with consistency.
The average thickness of the braze joint is typically in the range of 5 μm to 500 μm or 8 μm to 200 μm or 8 μm to 100 μm. The depth of the braze joint may be in the range of 5 μm to 50 mm or 30 μm to 10 mm or 40 μm to 5.0 mm. In the formation of blind joints, the flowability of the braze is particularly important the greater distance the molten braze alloy is required to flow to cover the require depth of the braze joint.
Such assemblies are typically required to operate at high temperatures and/or with high precision with highly conductive components, such as copper, being integral to their effective thermal management system. In the vacuum tube, the envelope may further comprises a heat source enclosed therein, such as a cathode filament of the x-ray vacuum tube or a lamphead in a rapid thermal processing assembly in a semiconductor processing chamber. The heat source is preferably capable of heating at least a portion of the contents of the envelope to at least 800° C. or at least 900° C. or at least 1000° C. The maximum operating temperatures of the envelopes is governed by the softening and liquidus temperatures of the materials used. At least part of the brazed joint is exposed to the internal side of the envelope (e.g. internal side of the vacuum tube), which may be subjected to high temperatures and high vacuum.
In one embodiment, there is provided an x-ray tube comprising:
wherein said x-ray tube comprises a braze assembly, said braze assembly comprising a first component and a second component joined together by a first braze joint, said first braze joint comprising a composition configured to comprise a solidification temperature range of no more than 90° C. and a liquidus temperature in the range of 950° C. to 1060° C.; said braze joint comprising a composition as defined in the first aspect of the present disclosure, wherein at least a portion of the braze joint is exposed to the interior of the x-ray tube envelope and wherein at least one of the first and second component form part of one or more of the x-ray tube envelope, the anode assembly and the cathode assembly. In one embodiment, there is provided an apparatus comprising a heat source and an envelope, said envelope comprises a first component and a second component, wherein a brazed joint joins the first component and the second component together, said brazed joint comprising a braze alloy composition comprising in weight %:
wherein the additives and impurities do not comprise Sn in an amount exceeding 0.4 wt % of the total weight of the braze alloy composition; and wherein the envelop is hermetic. In another embodiment, the braze joint comprises:
In another embodiment, there is provided an apparatus comprising an envelope and a heat source, said envelope comprises a braze assembly, said braze assembly comprising a first component and a second component joined together by a first braze joint, wherein the braze joint comprises a solidification temperature range of no more than 90° C., said braze joint comprising a liquidus temperature in the range of 950° C. to 1060° C. and a precious metal content of less than 4.0 wt % relative to the total weight of the braze joint.
The heat source may be located within the envelope or form part of the envelope.
The first or the second component may comprise copper or a copper alloy, which has excellent conductivity. The first or the second component may also comprise other metals including, but not limited to, stainless steel, copper alloys or other metal or metal alloys with a melting temperature greater than 1050° C. or greater than 1080° C. The envelope may further comprise a cooling system to remove heat from the system. The envelope may be under vacuum as in a vacuum tube or may comprise a process gas (e.g. in a semiconductor processing chamber).
The first or the second component may also comprise ceramic or a metallised ceramic (i.e. a ceramic comprising a metallised coating).
In embodiments, in which the envelope forms part of a vacuum tube, the envelope may comprise an anode and a cathode, which are disposed opposite one another in a vacuumized inner space. The vacuumized inner space may be enclosed by a cylindrical shaped metal envelope, with the anode and/or cathode being electrically insulated by means of an annular insulator (e.g. a ceramic or metallised ceramic component). The braze joint may form a seal between the cylindrical metal envelope and the annular insulator.
The braze joint may comprise a ceramic (or metallised coated ceramic) component and a metallic component, wherein the brazed joint joins the ceramic/metallised coated ceramic and the metallic components together. In one embodiment, the braze joint comprises a braze alloy composition comprising in weight %:
wherein the additives and impurities do not comprise Sn in an amount exceeding 0.4 wt % of the total weight of the braze alloy composition.
It should be appreciated that in embodiments, the braze alloy composition may of the components in weight % defined above.
The braze joint may also comprise compositional variations of the braze alloy according to any of the embodiments of the first aspect of the present invention.
The ceramic component may comprise a metallised coating such as a molybdenum-manganese coating or nickel plating.
In a third aspect of the present invention there is provided a vacuum tube assembly or a semiconductor process chamber such as a rapid thermal processing assembly comprising an envelope of the second aspect of the present invention. In some embodiments, the vacuum tube assembly or the rapid thermal processing assembly are operated at temperatures of up to 800° C. or greater, or 900° C. or greater, or 1000° C. or greater.
The wetting/bonding additive(s), when present, may be selected to improve wettability and/or bonding of the alloy on the first and or second component.
The vacuum tubes of the present invention may comprise a braze joint formed from a high copper content braze alloy with specific amounts of minor components which may be varied to adjust the desired solidus temperature, whilst maintaining a relatively narrow liquidus solidus temperature range. Further, the braze joints typically have excellent wettability and flowability over a range of base materials including copper, nickel and alloys thereof, stainless steel, nickel cobalt iron alloys and molybdenum-manganese metallised substrates. These braze alloy properties enable a diverse variety of vacuum tubes to be formed therefrom using a variety of different brazing techniques and conditions.
The braze alloy generally has excellent wettability and flow properties which enable it to make reliable hermetic seals at high temperatures. The braze alloys are particularly suitable for applications in high temperature environments including vacuum tubes (such as x-ray tubes, wave guides and klystrons assemblies); in aerospace applications (such as engine components and repairs, fuel-line assemblies); gas turbine assemblies;
semiconductor process chamber components such as a power supply of a rapid thermal processing apparatus; power supply surge arrestors and automotive components. A braze alloy needs to melt to operate. The melting behaviour is specified by the solidus and liquidus temperature, with the melting onset temperature (the solidus) and the melting range (the difference between the two points) being most significant for brazing. A braze alloy should have a solidus temperature above the maximum temperature it will experience in service, but below the solidus of the lowest melting parent material.
The liquidus temperature of the braze alloy composition is typically below about 1060° C. or typically below 1055° C. or below 1050° C. or below 1045° C. The liquidus temperature is typically at least 890° C., or at least 900° C., or at least 910° C., or at least 920° C., or at least 925° C., or at least 950° C. Therefore, the braze alloys are particularly suited for brazing copper or copper alloy substrates. In one embodiment, the braze alloy comprises a liquidus temperature in the range of 950° C. to 1060° C., or in the range of 970° C. to 1050° C. The braze alloy composition is preferably configured to obtain the abovementioned liquidus temperatures or ranges thereof.
Some braze alloys have a narrow melting range and some a wide one. Melting range is often linked to flow and this may drive selection, as may the required heating rate. A braze alloy with a narrow melting range (a small temperature range between the solidus and liquidus) can be used with fast (e.g. greater than 30° C./minute or greater than 40° C./minute from the solidus to the liquidus temperature) or slow heating rates (e.g. in the range of 5° C./minute or 10° C./minute to 30° C./minute from the solidus to the liquidus temperature). A slow heating rate, such as in furnace brazing, for a braze alloy with a wide melting range can result in extensive time where solid and liquid phases are in equilibrium and coexist. This leads to liquation, where the liquid first formed (of a particular composition distinct from the bulk) flows into the joint gap, becoming physically separated from the solid residue. The resulting chemical inhomogeneity can be detrimental to the strength of the joint, and is often aesthetically displeasing.
In some brazing applications the filler metal (i.e. braze alloy) may need to flow to enter the joint gap, but even when preplaced, flow characteristics can still be important in making sure that all of the joint gap is filled. Better flowing alloys can penetrate smaller capillary gaps, but if an alloy is too free-flowing in larger gaps it may fail to be retained in the joint, leading to voids and lower strength. The flow of an alloy is primarily dictated by the relative amounts of solid and liquid present at the brazing temperature. If the alloy melts at a single point (e.g. an eutectic composition or a pure metal) then it will be fully liquid at the brazing temperature and will flow easily. An alloy brazed within its melting range will have some quantity of solid and liquid present; if it is largely molten, it will flow well; if there is a significant solid fraction, the flow will be more sluggish.
The braze alloys of the present invention are suitable for a range of brazing including, but not limited to assemblies having substrates comprising copper or copper alloys, Kovar™ (Ni—Co—Fe alloy), ceramics components that have been metallised (e.g. molybdenum-manganese metallised or nickel or copper plating); steel, including stainless steel; nickel and nickel alloys including Ni-super alloys, and other refractory metals (e.g. molybdenum and alloys thereof) comprising a melting temperature preferably at least 20° C. or at least 50° C. above the liquidus temperature of the braze alloy.
To avoid liquation, while promoting good braze coverage over the joint, the braze alloy preferably possesses a narrow temperature difference between the solidus temperature and the liquidus temperature (i.e. low STR values). In some embodiments, the boron and/or the Germanium content of the braze composition is configured to obtain the temperature difference between the solidus temperature and the liquidus temperature of the braze alloy of no more than 90° C., or no more than 88° C., or no more than 85° C., or no more than 82° C., or no more than 80° C., or no more than 75° C., or no more than 70° C., or no more than 65° C., or no more than 60° C., or no more than 55° C., or no more than 50° C., or no more than 45° C., or no more than 40° C., or no more than 35° C., or no more than 30° C., or no more than 25° C., or no more than 20° C. The abovementioned STR ranges are considered to be low STR values.
In a fourth aspect of the present invention there is provided a braze assembly comprising a first joint and a second joint wherein at least one of the joints comprises a braze alloy composition according to the first aspect of the present invention. Alternatively, the braze assembly be part of an apparatus, the apparatus comprising an envelope and a heat source, said envelope comprises a braze assembly, said braze assembly comprising a first component and a second component joined together by a first braze joint, wherein the first braze joint comprises a solidification temperature range of no more than 90° C., said first braze joint comprising a liquidus temperature in the range of 950° C. to 1060° C. and a precious metal content of less than 4.0 wt % relative to the total weight of the first braze joint.
In one embodiment, the assembly comprises two joints with each joint comprising a braze alloy composition according to the first aspect of the present invention. Each of the braze alloys may be different. The first joint may comprise a braze alloy comprising a liquidus temperature below the solidus temperature of the second braze alloy.
In one embodiment, the first and second braze alloys comprise a braze alloy composition according to the first aspect of the present invention. Preferably the difference between the solidus temperature of the first joint and the liquidus temperature of the second joint is at least +15° C., or at least +20° C. The first joint may comprise a braze alloy composition with a solidus temperature of at least 950° C., or at least 990° C., and the second joint comprises a liquidus temperature of no more than 1017° C. or no more than 1000° C. or no more than 980° C.
This type of assembly is ideally suited to a step brazing process in which the higher temperature braze joint is first assembled and cooled before the lower temperature second braze joint is assembled. As the solidus temperature of the first braze joint is higher than the liquidus temperature of the second braze joint, the integrity of the first braze joint should not be compromised if the brazing temperature of the second braze joint is kept below the solidus temperature of the braze alloy of the first braze joint.
Whilst the present invention encompasses step brazing using two braze alloy compositions, the present invention also encompasses step brazing wherein only one of the braze joints comprise a braze alloy according to the first aspect of the present invention.
In some embodiments, the brazing process may result in the migration of boron from the braze joint. The reduction of the boron content in this first braze joint may result in an increase of the solidus & liquidus temperature compared to original braze alloy composition, thereby enabling the same braze alloy composition to be used in a step brazing operation. This rise in liquidus temperature may enable the same original braze alloy to be used to subsequently braze an adjacent component (second braze joint), with the brazing temperature being below the solidus temperature of the first braze joint.
In some embodiments, the braze joints are derivable from the braze alloy composition of the first aspect of the present invention. The derivable braze joints may have a lower boron content compared to the braze alloy composition from which it is derived. The adjacent materials/components to the braze joint may also have an evaluated level of boron relative to the materials/components prior to the braze joint being formed.
Brazing, as used herein, refers to a joining process of two (or more) materials to be joined using a braze alloy which blends with the materials to be joined upon melting. The melting temperature of the braze alloy is lower than the melting temperature of the materials to be joined. The liquefied/molten braze alloy interacts with the materials to be joined and forms the braze joint during cooling. The interaction of the braze alloy and the materials to be joined can be described by diffusion processes and formation processes of intermetallic phases and other compounds. The brazing may be performed in a vacuum, reducing or protective atmosphere (e.g. mixtures of hydrogen and nitrogen gases). A flux may be used during brazing in order to remove oxides from the brazing surfaces of the materials to be joined and to prevent the formation of oxides during brazing, thereby allowing a thorough wetting of the surfaces of the materials to be joined by the liquefied/molten braze alloy. However, a fluxless braze is preferred.
High voltage vacuum tubes comprise an anode and a cathode, which are disposed opposite one another in a vacuumized inner space. The vacuumized inner space is typically enclosed by a cylindrical metal housing, with the anode and/or cathode being electrically insulated by means of an annular insulator. The high voltage vacuum tubes may have an operating temperature above 800° C., or above 900° C., or above 950° C., or above 1000° C.
In some embodiments, the anode is a rotating anode. Rotating anodes place additional stresses of the braze joints contained therein and, as such, the strength of these braze joints are of particular importance compared to brazed joints within vacuum tubes with static anodes.
In a fifth aspect of the present invention there is provided a process of producing a braze joint between a first component and a second component using the braze alloy composition according to the first aspect of the present invention.
The brazing process may comprise:
The process may include ramping the temperature between the solidus and the liquidus temperature at a rate of 1° C./min to 30° C./min between the solidus and the liquidus temperature of said braze alloy. The temperature ramp rate may be less than 28° C./min, or less than 26° C./min, or less than 24° C./min, or less than 22° C./min, or less than 20° C./min, or less than 18° C./min, or less than 16° C./min, or less than 14° C./min, or less than 12° C./min. In contrast to braze alloys with a high STR, the braze alloys of the present invention are able to be brazed at a lower rate of heating without the same risk of liquidation resulting in poor joint performance. The use of lower heating rates also avoids other disadvantages associated with faster heating rates such as component distortion, spalling, and excessive outgassing. This enables the braze alloys to be effectively used in a greater array of brazing environments, including the brazing of components in which one component has a low conductivity (e.g. a ceramic) and/or a large thermal mass such that fast heating rates are difficult to achieve.
In some embodiments, the braze cycle also includes holding the braze joint and associated substrates at a temperature of typically between 10° C. and 400° C. below the solidus temperature for between 10 and 30 minutes prior to brazing the braze joint at the brazing temperature, typically between 15° C. and 60°° C. above the liquidus temperature of the braze alloy.
In other embodiments, the double braze is used, in which the braze joint is cooled approximately 100° C. (to below the solidus temperature) between braze cycles.
The brazed joint may be formed in a brazing furnace including a vacuum, reducing (e.g. H2) or protective (e.g. N2 or Argon) furnace. The brazing of blind joints is particularly suited to being formed in a brazing furnace or oven, as the temperature and atmospheric conditions of brazing can be reliably controlled. The vacuum furnace may have an evacuated vacuum of less than 8×10−4 mmHg (1.17×10−3 Pa) and preferably less than 5 ×10−4 mmHg (6.65×10−2 Pa). The use of laser brazing in a controlled atmosphere may also be able to achieve the required levels of temperature and atmospheric control.
In some embodiments, a two-step brazing process is employed comprising heating a first braze alloy composition to a first brazing temperature and allowing to cool to form a first braze joint and then heating a second braze alloy composition to a second brazing temperature and allowing to cool to form a second braze joint, wherein the solidus temperature of the first braze joint is higher than the liquidus temperature of the second braze joint, wherein the second brazing temperature is kept below the solidus temperature of the first braze joint.
In one embodiment, the braze joint is form through placing the braze alloy composition in the form of a wire, powder, paste or foil adjacent two components to be joined and heating the braze composition above the liquidus temperature of the braze alloy composition and allowing a molten braze alloy to flow between the two components via capillary action to form the braze joint.
In a sixth aspect of the present invention, there is provided braze joint produced by or obtainable by the process according to fifth aspect of the present invention. The process of preparing the braze joint enables a braze alloy composition to have a sufficiently low liquidus temperature and sufficient flow and wettability properties to form a blind joint through capillary action drawing the braze alloy between the gap between the substrates to be joined. The cooled braze joint may have a decreased amount of boron compared to the original braze alloy composition. Thus, without the brazing process of the present invention, the braze joint composition could not have been obtained, as the liquidus temperature of the braze joint composition would have been higher.
The braze alloys and derived joints of the present invention are preferably hermetic, have good mechanical strength and have a low vapour pressure.
The braze joints of the present invention preferably have a hermeticity with a maximum permissible leakage rate of closed vacuum assemblies of 1×10−6 atm.cc/s or less, 1×10−7 atm.cc/s or less, or 1×10−8 atm.cc/s or less (ASTM F2391 using helium gas). In some applications, such as RTP assemblies, lower seal integrity may be sufficient, although braze joint integrity should be such that the process gases are contained within the process chamber and do not leak through the braze joint.
The braze joints of the present invention preferably have a tensile strength of at least 900MPa or at least 950 MPa or at least 1000 MPa. The braze joints of the present invention preferably have a shear strength of at least 5.0 or at least 7.0 MPa. The tensile and shear strength is measured in accordance with AWS C.3.2M/C3.2:2019 Standard method for evaluating the strength of braze joints. The reference substrates for testing was joining 304 stainless steel to 100% copper.
The braze alloy of the present invention preferably has a vapour pressure less than 1×10 11 mm Hg at 500° C. (1.33×10−9 Pa), or less than 1×10−12 mm Hg (1.33×10−10 Pa) at 500° C., or less than 1×10−13 mm Hg (1.33×10−11 Pa) at 500° C., or less than 5×10−14 mm Hg (1.33×10−12 Pa) at 500° C., or less than 1×10−15 mm Hg (1.33×10−13 Pa) at 500° C. At 700° C., the vapour pressure of the braze alloy is preferably less than 1×10−8 mm Hg (1.33 ×10−6 Pa), or less than 1×10−9 mm Hg (1.33×10−7 Pa), or less than 5×10−10 mm Hg (1.33 ×10−8 Pa), or less than 1×10−11 mm Hg (1.33×10−9 Pa).
For the purpose of this invention, a vacuum tube includes power tubes, x-ray tubes, magnetrons, traveling-wave tubes, carcinotrons and klystrons.
A solidus temperature is the highest temperature at which a metal; or alloy is completely solid. A liquidus temperature is the lowest temperature at which a metal or alloy is completely liquid.
High voltage, for the purposes of the present invention means a voltage of at least 1 kV or at least 10 kV or at least 100 kV. The benefits of the braze alloy of the present invention may also be governed by applications where the Voltage/distance (V/d) ratio is sufficiently high, for example at least 0.5 kV/mm or at least 1 kV/mm or at least 10 kV/mm.
Vacuum brazing is typically performed at about 1×10−5 mm Hg (1.33×10−3 Pa).
The notation of “Balance Cu” means that the copper makes up the remaining portion of the braze alloy composition up to 100.00 wt %. (i.e. % wt copper=100.00 wt %—amount of all the other components (wt %) in the braze alloy).
The sum of all of the components of the braze alloy composition shall not exceed 100 wt %. Theoretical sums of combinations of components exceeding 100 wt % should be disregarded.
Unless otherwise indicated references to % wt amounts are on the basis of the total weight of the braze alloy composition.
Additives are exclusive of elements already defined within the scope of the braze alloy composition (e.g. Cu, Ge, Sn, Al, Si, In, and B).
Precious metals for the purposes of the present invention means gold, silver, and palladium and platinum.
Reference to boron, copper, Germanium and additives are reference to these components in their elemental form (i.e. oxidation number=0). Incidental impurities may be in any permitted oxidation state, but preferably have an oxidation number of zero.
For the purposes of this invention, a braze joint composition which is derivable from a braze alloy composition means that the braze joint whilst being formed from a braze alloy composition may also comprise components from the associated substrates forming the joint which may have diffused into the joint during the brazing process. Similarly, at least a portion of some components of the original braze alloy composition may have diffused into the adjacent substrates (e.g. boron from the braze alloy composition may have at least partially diffused into the adjacent substrates).
The present invention will now be described with reference to the figures of the accompanying drawings, which illustrate particularly preferred embodiments of the present invention, wherein:
Braze alloys need to exhibit deformability (the ability of the alloy to undergo plastic deformation without breaking), in particular, ductility (under tensile stress), to fabricate in various forms, such as wires and foils. The production of a braze alloy begins with mixing all elements in the alloy in appropriate amounts and melting the mixture to a sufficiently high temperature, followed by casting to produce an ingot or a billet in a solid form. The condition of the alloy can be changed from the as-cast state to the wrought form by cold or hot working such as rolling, drawing, and stamping.
Rapid solidification methods (i.e., under extremely high cooling rates), such as melt-spinning, can produce braze alloys in thin foils. Grinding of braze alloys in cast, ingot, or any other form or atomization technique can be used to make the alloy in powder form. Braze alloys in preforms shapes are fabricated by precisely cutting formed and cold-headed wire to ring form or by stamping the alloy strip or foil. Preforms provide an exact volume for the specific region of a particular braze joint. Preforms are placed in a joint region and melt during brazing to join the base materials during cooling into a solid state. In some embodiments, brazing is performed under vacuum or under hydrogen or an inert gas, with the braze temperature typically about at least 20° C. above the liquidus temperature of the braze alloy.
With reference to
Electrons are generated by heating the cathode filament 50, and under the action of the accelerated electric field between the cathode 30 and anode, the electrons hit the target surface 60 at high speed, thereby generating X-rays. The target surface is a rotating disc made from tungsten which is able to withstand the high temperatures generated by the impacted electrons. The induction motor 70 comprises bearings 80 and a rotor stem 90, which is typically made from molybdenum with its relatively lower conductivity providing the motor a degree of thermal isolation from the target surface 60. The stator (not shown) comprising a series of magnets which are able to drive the rotation of the rotor 90 from outside the envelope 20.
The bombardment of electrons on the target surface release a stream of x-rays, which may be selectively transmitted outside the envelope via an emission window 100. High voltage wires 110, 120 are supplied to the cathode 30 and anode 40, via feedthroughs 130, 140 respectively. The feedthrough comprises conductors 110, 120 which are insulated from the envelope by metallised ceramic seals 150, 160. Within this structure there is a need for braze alloys which are able to join various components of metal and/or ceramics components. For example, braze joints B1 are required to hermetically seal the envelope with the anode feedthrough 140 as well as a conductor 120 within the ceramic seal 160. A similar set-up is required at the cathode feedthrough with braze joints B2. Further braze joints B3 are required to hermetically seal the x-ray emission window 100 with the envelope 20. Braze joints B4 may be also required to joint components of the rotating anode 40 or cathode (30).
As only 1% of the total energy is used to generate x-rays, with the remaining 99% of the energy converted into heat energy the x-ray vacuum tube is required to comprise of hermetically sealed components with varying degrees of conductivity and mechanical strength, which must maintain good dimensional stability under extreme temperature variations upon start-up.
The housing assembly (not shown) may further comprise a cooling fluid (e.g. oil) which assists in removing the heat from the envelope and maintaining the temperatures to within target operating zone, which are typically up to 800° C., or up to 900° C., or up to 1000° C., or more. With many of the metal components of the vacuum tube and external housing comprising copper due to its high conductivity, the use of copper based braze alloys have the advantages of having similar coefficients of thermal expansions and being able to operate at temperatures required within the vacuum tube. Thus, the braze alloys of the present invention may be advantageous used in the braze joints of B1, B2, B3 and B4 to joint metal to metal, metal to metallised ceramics and/or metal to ceramics.
As is apparent from the sophistication of the apparatus, it is also critical that the braze alloys can be positioned and flow into tightly dimensioned spaces to produce the required mechanical and hermetic integrity of the braze joint. In contrast to braze alloys with high STR values, the brazing process can employ a relatively low temperature ramp up as the braze joints of the present invention are less prone to liquation.
While the braze alloys and joints thereof of the present invention are particularly suited to high temperature vacuum tube applications, the braze alloys are not limited to these applications.
For example, the braze alloys may be used in braze joints within semiconductor process chamber components such as a rapid thermal processing (RTP) chamber component. RTP may be used in a number of high temperature industrial processes, including a process which heats to temperatures exceeding 1,000° C. for not more than a few seconds. During cooling wafer temperatures must be brought down slowly to prevent dislocations and wafer breakage due to thermal shock. Such rapid heating rates are often attained by high intensity lamps or lasers. As with high temperature vacuum tube components, RTP components are required to withstand high temperatures and include highly conductive metals for thermal management of the system. As such, a copper component is often used, which are typically brazed to other metal components, such as stainless-steel components. The RTP chamber typically comprises a lamphead as a heat source, with the chamber comprising a flow of process gas, e.g. an inert gas such as nitrogen gas to facilitate the heat treatment process. Due to the criticality of purity in wafer processing, the chamber is required to be hermetically seal in operation. Examples of RTP assemblies and processes are provided in U.S. Pat. No. 8,698,049 and U.S. Pat. No. 20,210,348302.
Sample braze alloys comprising copper, germanium, and boron with 0.25 wt. % nickel in various compositions listed in Table 1 were prepared via heating the mixture of elemental components to about 1080° C. to form a homogeneous melt. The molten alloy was then cast into the form of an ingot, followed by cold working and annealing to produce the alloy in a wire and/or foil form.
The Differential Scanning calorimetry (DSC) depicted the melting behaviour of these alloys. The liquidus and solidus temperatures were measured by DSC, using small samples of ˜20 mg mass placed in an alumina crucible with a lid. After loading the sample, the chamber was evacuated and backfilled with argon gas. The analysis was carried out in the temperature range from 298 K to 1373 K at a heating rate of 20 K min−1. The output of the analysis is a curve representing the variation of heat flux with the temperature.
With reference to
The compositions of the alloy were prepared by melting 5±1 grams (0.25 inch in height) on a water-cooled copper hearth using a tungsten electrode in an argon gas atmosphere, producing semi-spherical alloy ingots (buttons). Buttons of different compositions were subjected to cold rolling to test their ability to plastically deform into sheet form using a two-roll mill.
The formability of each composition was determined by measuring the deformation required to cause a fracture (as observed with a naked eye) when passed through the rolls at room temperature. In each step, the roll gap is adjusted to up to the equivalent of up to a 10% reduction in the thickness of the button. The formability of alloys that can withstand cold rolling down to a 0.002 inch (=50 μm) thick sheet is considered near ductile. The formability test defines a reduction from 0.25″ to 0.002″ to correspond to 100% formability, with a reduction to 0.125″ corresponding to 50% formability.
The compositions were considered somewhat brittle in nature if there was an early onset of fracture before reaching the target thickness of about 50 μm. This methodology was used to assess the ability of the various compositions of this alloy to fabricate braze filler metal wire or sheet pre-made forms. The early onset of fracture means the visible appearance of factures by the naked eye in the sample, as observed in
Formability percentages of at least 38% are considered feasible to fabricate into braze alloy pre-forms, although formability of at least 45% or at least 50% or at least 60% or at least 70% or at least 80% or at least 90% or 100% are preferred. Accordingly, the boron content of the braze alloy composition may be configured to obtain the abovementioned formability values or value ranges.
A series of experiments were conducted to assess the characteristics of the braze alloys in terms of their solidus and liquidus temperature as well as their formability.
The vapour pressures of the braze alloys of the present invention in Table 1 were found to be less than 1×10−11 mm Hg (1.33×10−9 Pa). The vapor pressure of each alloy is estimated using each elements individual vapor pressure contribution:
where ai is the activity and Pi is the equilibrium vapor pressure at the Liquidus +50° C. temperature.
The preferential vaporization of different alloying elements is a function of the element's volatility and the element's activity within an alloy. The partial contribution of each alloy element to the total equilibrium vapor pressure is a function of the pure element's equilibrium vapor pressure and its activity in the alloy, as described by Equation (1).
We reference to FIG. 3, the effect of boron on braze alloys with 2.5 wt % Germanium (line A) and 4.8 wt % Germanium (line B) demonstrates that for each there is an optimum boron content which results in a minimum STR value. For Germanium contents of 2.5wt % (samples 1, 2, 3, C-1), 4.2 wt % (sample 4); and 4.8 wt % (samples C-2, C-3, 5, 6, 7), the optimum boron content ranged from about 0.5 to 1.25 wt % boron. While a similar decline in STR was observed at Germanium levels at 7.2 wt % (a minimum STR recorded at 0.55 wt % boron) and 10 wt % (a minimum STR recorded at 0.80 wt % boron), the absolute minimum STR values were not as low compared to the lower Ge content braze alloys.
However, a limitation of low Ge contents is that they have elevated liquidus temperatures, which may be detrimental to applications involving the brazing of copper or copper alloy components. Braze alloys with a Ge content of about 4.2 wt % to 4.8 wt % of the present invention were found to have an excellent combination of low STR values; liquidus temperatures in the target range of brazing copper/copper alloy components; and good formability. The braze alloy compositions of the present invention provide a variety of liquidus temperatures, favourable STR ranges and acceptable formability to meet the functional requirements of a many end-uses applications, all without the use of any substantial amounts of precious metals.
For example, the melting profile of sample 4 is similar to the precious metal braze alloy 35Au-65Cu (WESGO™), with the latter have a liquidus temperature of 1010° C. and a STR of 20° C. (Table 2). Whilst other samples may have higher STRs than precious metal braze alloy alternatives, they still offer improved performance relative to existing non-precious metal alternatives. For example, 50Au-50Cu (WESGO™) has a liquidus temperature of 970 and a STR of 15° C. (Table 2). Samples 15 and 16 have a comparable liquidus temperature, but a STR of 68-69° C. This is a significant improvement on the non-precious metal braze alloy (GEMCO™ by WESGOT) which comprises a liquidus temperature of 975° C., but a STR of 95° C. (Table 2).
Whilst higher Ge content braze alloys (e.g. 7.2 wt % or above) may have relatively high STR levels and lower liquidus temperatures (compared to braze alloys with lower levels of Ge), these braze alloy characteristics are still acceptable for some applications.
As indicated in Table 1, the formability of the braze alloys tended to decrease with an increase in boron content, with formability noticeably decreasing as boron content increases above 1.0 wt %. The Ge content appears to have a lesser impact on the formability, with the formability dropping from 43 to 40 to 38 as the Ge content increased from 2.5 wt % to 4.8 wt % to 7.2 wt % respectively, at a constant boron content of 1.25 wt %.
FIG. 6A to 6F illustrate the foils formed in samples 8 to 12; and C-6, with samples 11,12and C-6 showing visible signs of cracking as formability decreases.
The majority of examples contained 0.25 wt % Ni which was added as a wetting aid to assist the wetting of the braze alloy against a stainless-steel substrate. The braze alloy composition tolerated a level of Ni up to 4.8 wt % whilst still maintaining an acceptable STR value, however it is expected that additive levels above 5.0 wt % may detrimentally affect the STR or liquidus temperature as indicated in comparative example 10 (C-10) which comprised 5.25 wt % additives. Higher levels of Ni and Ag (C-9, C-11) further confirmed this negative trend.
The effect of Sn addition was investigated through the addition of 1.0 wt % Sn to the composition for Example 5, which had a low STR of 12° C. The resultant composition (C-6) saw a detrimental deterioration in the STR value to 173° C. Other Sn containing compositions C-7 and C-8 also comprised high STR values of 116° C. and 185° C. respectively. The effect of Sn also saw a significant deterioration in the formability of Example 5 (100% formability), with the addition of 1.0 wt % Sn resulting in a dramatic fall in formability to 20% (
With reference to
A similar performance, with no observed liquation, was obtained when the braze alloy was used to braze a 304 stainless steel strip to a copper strip in a “T” configuration (not shown).
It will be understood that modifications and variations may be affected without departing from the spirit and scope of the novel concepts of the present invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2024/029673 | 5/16/2024 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63467054 | May 2023 | US |
