IL285365B2 - Vacuum windows with getters reactants - Google Patents

Description

285365 / VACUUM WINDOWS WITH GETTERS REACTANTS FIELD OF THE INVENTION The present invention relates to the field of vacuum insulating glasses, and in particular, to the field of activationless getters reactants. [0001] BACKGROUND The mass production of vacuum insulating glasses (VIGs) is still being held back by the unsolved getter problem. The existing getter products such as NEGs and Ba-EGs [B.

Ferrario, Getters and getter pumps, in: J.M. Lafferty (Ed.), Foundation of Vacuum Science and Technology, Wiley, New York, 1998, pp. 261–290; M. Wutz, H. Adam, W. Walcher, K. [0001] Jousten, Handbuch Vakuumtechnik, Theorie und Praxis. 7. Aufl. Braunschweig: Vieweg; 2000] turned out to be unsuitable here, the first ones due to their low specific sorption capacity, and the second ones due to their incompatibility with the geometric parameters of the vacuum gap of the window.

In this situation, the way out is seen in replacing traditional getters, which besides required [0001] thermal activation, with activationless getters of the class of reactants [Chuntonov, K., Atlas, A., Setina, J. and Douglass, G. (2016) Getters: From Classification to Materials Design.

Journal of Materials Science and Chemical Engineering, 4, 23-34]. The first attempt of this kind came down to the introduction of reactive particles of getter material inside the window through a vacuum pumpout tube, which was then sealed-off [Chuntonov, K., Ivanov, A.O., [0001] Verbitsky, B. and Setina, J. (2018) Getters for Vacuum Insulated Glazing. Vacuum, 155, 300-306]. However, such a procedure was estimated as technically risky and costly.

A new solution was required for the installation of the getter reactant into the window during its assembly in air. This solution is disclosed below. id="p-1"

[0001] 25 285365 / SUMMARY OF THE INVENTION The present invention is built up on the positive view on the corrosive decomposition of metallic solids as a phenomenon that increases the surface area in contact with gases and thereby accelerates the sorption process.

Basing on the new knowledge about reactions in systems air/getter reactant, the authors [0001] solved the getter problem of vacuum windows with the help of a particular family of reactants designed specifically for this application.

The material basis of these getters is formed by the alloys, the composition of which ranges from the binary alloy CaxMg1-x, where 0.75 ≥ ? ≥ 0.60, to ternary alloys obtained by additional alloying of the binary base alloy with lithium. The end getter product, produced by [0001] vacuum casting, or by casting with subsequent mechanical processing, for example, rolling, forging, etc., has the form of tablets, beads, ribbons, wires, etc., with a monolithic structure and with dimensions that are consistent with geometric parameters of the window.

However, within the framework of this topic, there is a demand for getters reactants of a different composition, namely, for the eutectic alloy Sr0.7Mg0.3, or for the whole range of [0001] alloys of the composition SrxMg1-x, where 0.75 ≥ ? ≥ 0.60. These alloys can also be alloyed with lithium to adjust their properties to specific practical needs.

The monolithic structure and the small specific surface area of the described getters reactants explain their stability to the normal atmosphere for the period from tens of minutes to many hours, which makes it possible to install such a getter into a window under air [0001] conditions. On the other hand, the eutectic structure of the base alloy, its additional alloying with Li as well as the mechanical deformation at the shaping stage, all these separately and taken together, lay the material basis for the subsequent corrosion decay of the getter body during the operation of the vacuum window.

This "chemical" decay of the cast bodies of the reactive alloy serves as a substitute for [0001] mechanical milling and is the driving force of the sorption process in this new solution to the 285365 / VIGs getter problem.

BRIEF DESCRIPTION OF THE DRAWINGS Fig.1. General view of sorption curve ∆? (? )/? for reactants with monolithic structure.

Fig.2. Corrosion decay of alloy CaMg2 in the air.

Fig.3. Corrosion decay of alloy Ca0.7Mg0.3 in the air. [0001] Fig.4. Sorption curves of eutectic Ca0.7Mg0.3 and intermetallic phase CaMg2.

Fig.5. Sorption curve of intermetallic phase Li2Sr3.

Fig.6. Corrosion decay of alloy Ca0.35Li0.45Mg0.20 in the air.

Fig.7. Sorption curves of alloys Ca0.30Li0.10Mg0.60 and CaMg2.

Fig.8. Sorption curves of alloys Li2Sr3, Ca0.7Mg0.3 and CaMg2. [0001] Fig.9. Sorption curves of alloys Ca0.56Li0.20Mg0.24 and Ca0.7Mg0.3.

Fig.10. Incubation stage of the sorption processes for alloys Ca0.56Li0.20Mg0.24 and Ca0.7Mg0.3.

Fig.11. Assembly of the vacuum window.

DETAILED DESCRIPTION OF THE INVENTION [0001] From the standpoint of Prior Art, installing getter material into a vacuum chamber in air without subsequent thermal activation is an act, which does not make sense. However, the situation with getter reactants is fundamentally different from the usual practices with the participation of getters adsorbents.

Experiments with getters reactants in air conditions show that the slowing layer, which is [0001] formed on their surface [Chuntonov, K., Ivanov, A.O. and Kozhevnikov, V.L. (2020) Tribochemical Purification of Gases. I. The Process Model. Journal of Materials Science and Chemical Engineering, 8, 37-54], protects the getter material from damage only for a certain limited time. This time, shown in Fig. 1 by the segment (0 – t? ), can be called the 285365 / incubation stage of the general sorption process, in which the intensive stage of the interaction of the gas with the getter occurs later (region II in Fig. 1), when the corrosion destruction of the initial material starts.

In Figures 2 and 3, it can be seen how getter material, which was left at rest, without external forces acting on it, was decaying. Comparing this picture of decay with the sorption curves [0001] of the same reactants (Fig. 4), it is easy to make sure that the faster the material is destructed, the higher its sorption kinetics. This kinetics is defined here by the expression (1/? )? [∆? (? )]/?? .

The process of structural decay attenuates in the vicinity of the point ? = ? ? , which is the boundary between sections II and III (Fig. 1). By this time, the reactant turns into a powder [0001] mixture consisting of solid reaction products and small alloy residues, which continue reacting with gases with decreasing kinetics till the final at cmax . As an illustrative example of this behavior, we can refer to the sorption curve of reactant Li2Sr3 (Fig. 5).

Two factors have the greatest influence on the corrosion destruction of reactants and accordingly on the shape of the sorption curve ∆? (? )/? . These factors are the structure [0001] of the material and its composition. The influence of the structure is shown in Fig. 6: a single- crystal sample of Ca0.35Li0.45Mg0.20 alloy did not visually change after 20 hours of exposure to air, while a polycrystalline sample of the same alloy during the same period of time disintegrated into a bunch of crystal fragments.

The factor of the chemical composition, which gives one more opportunity to control the [0001] process of corrosive decomposition of reactive alloys in gases, is no less effective. Thus, adding only 10 at% of lithium to the alloy of CaMg2 increases the sorption capacity of the material by approximately 30% (Fig.7). The replacement of Ca – Mg alloys by Li – Sr alloys looks more impressive (Fig. 8), but these active metals are less common in nature, so it is more reasonable to use them not as a basis, but as an additional element for alloying. [0001] The presented here data allow revealing two characteristic differences between getter 285365 / reactants and getters adsorbents. The first one is the slowing layer formed on the surface of the reactants upon contact with air, and the second one is the corrosive decomposition of reactants in a gaseous medium. 1. Slowing layer. While a passivated film is rapidly formed on the surface of getters adsorbents in air conditions and completely stops the sorption process, getters reactants in [0001] air get covered with a layer of products with porous structure. On the one hand, such a layer makes it difficult for gases to access the surface of the reactant, which manifests itself on the ∆? (? )/? curve (Fig. 1) in the form of a zero segment (0 – t? ). This zero segment gives a chance for installing reactants in a window in air conditions.

On the other hand, gases partially penetrate through the defects of the slowing layer to the [0001] surface of the reactant, filling microcracks and with time the grain boundaries as well. As the products of the reaction of gases with metals grow in the places, where the active gases penetrate and where they are concentrated, forces of destruction of the crystalline body begin to appear. These forces, directed from the inside along the normal to the wall of the microcrack or grain boundaries, become noticeable when approaching the end of the [0001] incubation stage of the sorption process, that is, somewhere in the vicinity of the point t? (Fig. 1). 2. Corrosive decomposition of reactants. Unlike getters adsorbents of the NEGs type, representing by themselves sintered powder materials with high surface area, which require thermal activation after the contact with air, getters reactants are simple macrobodies with [0001] monolithic structure. After exposure to the air for a period of time (0 – t? ) these macrobodies come to a state of readiness for structural decomposition.

The destruction of reactants in the environment of residual gases and the transformation of their monolithic structure into a pile of fine-grained fragments of the original body is accompanied by a multiple increase in the sorption surface area and, and consequently, by [0001] the same increase in the amount of sorbed gas. It is the target of the present invention to 285365 / create all conditions for the "chemical" grinding of the reactant in the getter cell of the vacuum window.

So, returning to the initial problem, let us formulate it as a search for a family of reactants capable of satisfying the two main requirements: their stability under air conditions at the stage of the window assembly and their high sorption activity at the stage of operation of [0001] the vacuum window. In view of economic and technical considerations, binary alloys of the composition CaxMg1-x or SrxMg1-x, where 0.75 ≥ ? ≥ 0.60, or the mentioned CaxMg1-x or SrxMg1-x alloys doped with lithium, were chosen as the material basis for getters reactants for vacuum windows The end getter product in the form of tablets, plates, beads, ribbons or wire is made by [0001] vacuum casting of the melt into molds of the given size at an argon pressure of 5 - 25mbar with the final mechanical adjustment of the material to the parameters of the getter cell of the window. The mentioned treatment of ingots by rolling, cutting, forging, etc. is useful for another reason as well. Indeed, it is accompanied by the saturation of the getter material with various structural defects, point ones, in the form of surface microcracks, shattered [0001] grains and their deformation up to the appearance of a special texture, i.e., everything that increases the rate of corrosion decomposition and the reactivity of the getter material with respect to gases.

Additional alloying the binary base alloy with active metals provides another way to accelerate the process of destruction of the getter material and to increase its sorption [0001] capacity. Figure 9 shows the results of sorption tests for the eutectic alloy Ca0.7Mg0.3 and for the same alloy with the addition of 20 at% of Li. The gas sorption rate of the Li-containing alloy is noticeably higher than in the case of the eutectic. A similar picture has already been demonstrated above for the example of the intermetallic phase of CaMg2 (Fig. 7), however, the Ca0.7Mg0.3 eutectic is much more promising in terms of sorption. [0001] For a better understanding, let us explain once again according to what mechanism the 285365 / chemical capturing of gases by IIA metal alloys takes place. This capturing is the total result of the following processes: growth of the layer of products on the surface of the reactant according to a linear or a parabolic law; diffusion of gases along the grain boundaries and the appearance of the products of reactions in them, weakening the forces of cohesion between the grains; capillary condensation of gases (for example, moisture, carbon dioxide) [0001] in the microcracks and formation there the products of reactions, which grow and move these cracks apart; the decay of the crystalline body of the reactant under the influence of such internal forces into separate fragments, which is accompanied by a multiple, reaching several orders of magnitude, increase of the area of the sorbing surface. By the end of this total process, the theoretical limit of the sorption capacity of the reactant is reached. [0001] For such reactants, which react with gases to the end, it is possible to determine in advance the amount of getter material that is sufficient to guarantee the operability of the window for the required time [Chuntonov, K., Ivanov, A.O., Verbitsky, B. and Setina, J. (2018) Getters for Vacuum Insulated Glazing. Vacuum, 155, 300-306]. Using the mentioned calculation method, we obtain, for example, in the case of a reactant with the composition [0001] Ca0.56Li0.20Mg0.24, that in order to maintain high thermal insulation of a vacuum window with a 0.2 mm gap for 20 years, it is necessary that for each 1m of such a window there should be an amount of the reactant contained in 0.2 cm of cast alloy. This requirement can be realized, for example, in the form of three tablets with a diameter of 1 cm and thickness of 1 mm, or in the form of one plate with a cross section of 1 mm x 4 mm and 5.5 cm long, or [0001] a square wire of 1 mm x 1 mm and 22 cm long. The production cost of such a getter product is negligible compared to the production cost of the window itself.

Another requirement for reactants put forward by VIGs manufacturers is that the coordinates of the point t? (Fig. 1), which determines the duration of the incubation stage of the sorption process, should be as close as possible in time to the end of the assembly process. The [0001] position of the point t? on the ? axis can be changed using the same factors that affect the 285365 / sorption rate, namely, varying the composition and structure of the reactant.

Figure 10 shows the initial segment of the sorption curves for the eutectic Ca0.7Mg0.3 and the same alloy after its additional alloying with lithium. It can be seen that, if necessary, using the addition of an active metal, it is possible to reduce the duration of the incubation stage of the sorption process. If the assembly process is time-consuming and the interval [0001] (0 − t? ) has to be prolonged, then a simple way to this is to increase the share of Mg in the alloys.

So, the new reactant, considered as a family of related alloys, not only satisfies the basic requirements for it, i.e., it is stable in air conditions with preservation of getter properties, but also provides the possibility of adaption to the changing conditions of VIGs production by [0001] adjusting the composition and structure of the alloy. Moreover, this reactant, due to its extremely high sorption capacity, makes it possible to simplify the production process by not using the services of vacuum technology, i.e. pumps. In this case, all the work on creating and maintaining a vacuum inside the window is performed by the getter material.

There are two variants of this technology. [0001] According to one variant, the reactant is introduced into the window, and then the window is sealed together with the air in it. The composition of the reactant in this case corresponds to the formula (Ca, Sr)xMg1-x , where 0.75 ≥ ? ≥ 0.60 and Ca and Sr are approximately in an equiatomic ratio. The volume of the reactant here exceeds the norm that was given earlier in the example, which assumes the evacuation of gas from the gap by a vacuum [0001] pump. That is, now we are talking about a new norm, when for each square meter of the window there is already not 0.2 cm, but 0.3 cm of a monolithic getter body. Under the described conditions, after sealing the window the residual argon atmosphere in the gap between the glass plates is stabilized at a pressure of about 10 mbar.

According to another variant, high vacuum is created inside the window, although not with [0001] the help of a vacuum pump, but due to the chemical capturing of gases by a getter reactant. 285365 / For this purpose, the volume of the reactant having the composition CaxMg1-x , where 0.75 ≥ ? ≥ 0.60, is brought to a new norm equal to 0.5 cm of a monolithic getter body per one square meter of the window. After installing the getter in air conditions into the window, the air is removed by blowing the window with carbon dioxide, and then the window is sealed with CO2 inside it. This gas is completely captured by the reactant, so that the pressure of [0001] the residual gases in the vacuum gap here is determined by the content of inert gases in the purging CO2. Fig.11 explains the above said.

DETAILED DESCRIPTION OF THE DRAWINGS Fig.1. General view of sorption curve ∆? (? )/? for reactants with monolithic structure. [0001] The presented curve shows the dependence of the sorption capacity of the reactant on time t in relative units. Here ? is the initial weight of the reactant, ∆? (? ) = ? (? ) − ? , where ? (? ) − is the current weight of the reactant, and ∆? (? ) is its weight gain, equal to the amount of gas sorbed by the time ? , ? ??? – the saturation state.

The sorption curve is divided into three zones: I - the initial curve section, practically merging [0001] with the axis ? in the interval (0 − ? ? ), II – the central section corresponding to the stage of intensive gas sorption, and III - the final section with the process termination at ?? (? ) / ? = ? ???. The scheme of such a division is clear from the graph itself, where the coordinate ? ? is the point of intersection of the axis ? and the tangent to the sorption curve in section II, the coordinate ? ? is the projection of the point ? onto the axis ? , and the point ? [0001] itself appears as a result of the intersection of the mentioned tangent with the horizontal line ?? (? ) / ? = ? ???.

Fig.2. Corrosion decay of alloy CaMg2 in the air.

Microimages of the sample with the initial mass of 0.12g. Under each image the time of the exposure in the air is indicated (hours). [0001] 25 285365 / Fig.3. Corrosion decay of alloy Ca0.7Mg0.3 in the air.

Microimages of the sample with the initial mass of 0.416g. The time of the exposure is given in hours.

Fig.4. Sorption curves of eutectic Ca0.7Mg0.3 and intermetallic phase CaMg2.

Fig.5. Sorption curve of intermetallic phase Li2Sr3. [0001] The shape of the curve and the level of the plateau give the ground to assume that the sample reached the limit of the sorption capacity.

Fig.6. Corrosion decay of alloy Ca0.35Li0.45Mg0.20 in the air.

Photos of two samples of a ternary alloy. The left sample was taken from the bottom part of the growth crucible; the right one – from the central part. [0001] Fig.7. Sorption curves of alloys Ca0.30Li0.10Mg0.60 and CaMg2.

Fig.8. Sorption curves of alloys Li2Sr3, Ca0.7Mg0.3 and CaMg2.

Fig.9. Sorption curves of alloys Ca0.56Li0.20Mg0.24 and Ca0.7Mg0.3.

Fig.10. Incubation stage of the sorption processes for alloys Ca0.56Li0.20Mg0.24 and Ca0.7Mg0.3. [0001] Fig.11. Assembly of the vacuum window. (a) initial stage of the assembly; (b) final stage of the assembly 1 – glass plate (glass pane) with a cell for a getter, 2 – seal material, 3 – getter, 4 – pillar, – second glass pane, 6 – vacuum gap. (а) seal material 2 and getter reactant 3 are placed onto pane 1 in air. [0001] (b) pane 1 is connected with pane 5 involving material 2.

Three variants of the assembly are possible. All of them start with placing seal material and getter reactant 3 onto plane 1 and at the final stage hermetization of the window by 285365 / edge seal at connecting panes 1 and 5 is included.

The first variant: hermetization of the window along the edges with air in gap 6. The second variant: before joining panes 1 and 5 air medium between them is replaced by carbon dioxide, after which the window is hermetized with CO2 in gap 6. The third variant: a pane with a pump out hole is used as pane 5. After hermetization of the window along the edges [0001] air from gap 6 is evacuated by the vacuum pump and then hole is closed tightly. So, the first two variants do not use the vacuum pump, which simplifies the assembly process.

RESUME The given invention radically changes the principles of design and operation of metallic gas [0001] sorbents used in the vacuum field, in particular in vacuum windows. These gas sorbents, called getters reactants, are IIA metal alloys in the form of simple macrobodies with monolithic structure. They are not afraid of short exposure to air, which simplifies assembly operations. Moreover, this contact with air is a kind of "self-activation" time for reactants: diffusion of atmospheric gases along grain boundaries and filling of microcracks with gases [0001] lay the foundation for future corrosion decay of the reactant at the stage of its operation.

The core of the sorption process with the participation of reactants is the decay of a reactive metallic solid according to the laws of corrosion destruction. The kinetics of this decomposition can be controlled within certain limits by pre-selecting the necessary microstructure and composition of the reactive ingot. In this invention the authors limited [0001] themselves to eutectic structure that decays into micron-level particles, which then react with gases to the end according to the parabolic law.

Since Ca and Sr are infinitely soluble in one another in both liquid and solid states, the chemical composition of the eutectic alloy, which the authors of this invention put as the basis for getters reactants, can be briefly written as (CaxSr1-x)yMg1-y , where 0 ≤ ? ≤ [0001] 1 and 0.60 ≤ ? ≤ 0.75.

Claims (7)

285365 / - 12 - CLAIMS

1. A method of maintaining thermo-insulating properties of vacuum windows, comprising: (a) providing at least one getter reactant comprising a binary alloy of a composition (CaxSr1-x)yMg1-y, where 0 ≤ ? ≤ 1 and 0.60 ≤ ? ≤ 0.75 ; (b) doping said binary alloy with lithium; (c) forming said getter reactant into a monolithic structure; (d) exposing said getter reactant to an ambient air during window assembly; (e) sealing said getter reactant within a vacuum gap of a window; (f) allowing a corrosive decomposition of said getter reactant in said vacuum gap, thereby enhancing gas sorption in said vacuum gap; wherein said decomposition is configured to increase a sorption surface area, thereby maintaining a vacuum level within said window.

2. The method according to claim 1, wherein said monolithic structure is produced by casting said at least one getter reactant at a partial argon pressure of 5-25 mbar.

3. The method according to claim 1, wherein said monolithic structure is selected from the group consisting of: tablets, plates, beads, ribbons, and wire.

4. The method according to claim 1, wherein a time of exposure of said reactant to said ambient air during said assembly of said window ranges from several minutes to several hours.

5. The method according to claim 4, wherein said reactant is installed in said window in said ambient air during an assembly process, further comprises sealing said window along edges without removing said ambient air from said vacuum gap in said window.

6. The method according to claim 4, when said reactant is installed in said window in said 285365 / - 13 - ambient air during an assembly process, further comprises : (a) sealing said window at edges, and (b) evacuating of an air from said vacuum gap by a pump.

7. The method according to claim4, when said reactant is installed in said window in said ambient air during an assembly process, further comprises: (a) replacing an air inside said vacuum gap with a carbon dioxide, and (b) sealing said window at edges.