VACUUM GETTER PUMP WITH THERMO-SEDIMENTATIONAL ACTIVATION

Abstract

The new ultra- and extreme high vacuum getter pump with an ultimate pressure of 10−14 mbar and the absence of free solid particles is also distinguished by its simplicity of design and low-cost getter material. Unlike known getter pumps, a new, dust-free solution to the problem of capturing gases with getters reactants is achieved through the use of two types of sorbing boundaries, gas/solid and gas/melt, as well as due to thermo-sedimentational activation of the getter material.

Description

FIELD OF THE INVENTION

The invention relates to the field of vacuum pumps, in particular to getter pumps, in which the consumable material is getters reactants in both solid and molten states.

BACKGROUND OF THE INVENTION

Vacuum getter pumps are capable of being integrated into pumping systems with ultimate pressures up to 10⁻¹² mbar, becoming an important component of these systems. These pumps such as lon Pumps (e.g., Vaclon Plus 40 Pump from Agilent), Titanium Sublimation Pumps (e.g., Titanium Sublimation Pumps from Edwards), NEG Pumps (e.g., Capacitor NEG Pump from SAES Getters), or some kind of a combined variant (e.g., lon Pump from Demaco Holland bv) with all their difference pump out gases using a similar mechanism, by capturing them by solid getter material forming non-volatile solid compounds. However, this also became the reason for the general disadvantage of these pumps, which over time turn into a source of free solid particles as the getter degrades with an increase in the amount of sorbed gas.

In the mentioned pumps, the material basis of the getter is traditionally transition refractory metals and their alloys, which are activated by electromagnetic and/or thermal effects. But in any case gas sorption takes place here at the gas/solid interface.

Another direction of getter technologies is based on IA and IIA metals, as well as their alloys. These metals and alloys capture gases without thermal activation, reacting with them at room temperature to completion (C. A. Hart, C. H. Skinner, A. M. Capece, and B. E. Koel. Sorption of Atmospheric Gases by Bulk Lithium Metal. J. Nucl. Mater. 468 (2016) 71-77; K. Chuntonov, A. O. Ivanov, V. L. Kozhevnikov. Reactive Alloys of IIA Metals: Gas Sorption and Corrosion as One Process. Journal of Material Science and Chemical Engineering, 2021, 9, 39-69.). But here, too, the same problem of free particles remains unsolved.

A new solution in the field of getter pumps taking into account, additionally to above said, the research results connected with using liquid lithium on the walls of ultra-high vacuum chambers to improve their performance (A. de Castro, C. Moynihan, S. Stemmley, M. Szott, D. N. Ruzic; Lithium, a path to make fusion energy affordable. Phys. Plasmas 1 May 2021; 28 (5): 050901, www.doi.org/10.1063/5.0042437) is given below.

SUMMARY OF THE INVENTION

The following summary of the invention is provided to exhibit the basic understanding of some principles, underlying various aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not necessarily intended to particularly identify all key or critical elements of the invention and is not to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the following more detailed.

The invention was made in view of the deficiencies of the prior art and provides systems, methods and processes for overcoming these deficiencies. According to some embodiments and aspects of the present invention, there is provided a new ultra- and extreme high vacuum getter pump with an ultimate pressure of 10-14 mbar and the absence of free solid particles is also distinguished by its simplicity of design and low-cost getter material. Unlike known getter pumps, a new, dust-free solution to the problem of capturing gases with getters reactants is achieved through the use of two types of sorbing boundaries, gas/solid and gas/melt, as well as due to thermo-sedimentational activation of the getter material.

The new type of getter pump represents by itself a very simple device: a cylindrical stainless steel container with a flange, with a getter material inside and an external heater. In the present invention, the getter are preferably lithium alloys with Ca, Sr and Ba, provided that their liquidus temperature does not exceed 200° C. These are alloys with a high Li content, such as Li_x (Sr_yBa_1-y)_1-x, where 0.8≤x≤0.9 and 0≤y≤1, or alloys corresponding to the formula Li_xCa_1-x, where 0.8≤x≤0.9 and where Ca contains significant additions of Sr and/or Ba.

In the first case, the getter is maintained in a molten state at 180° C.≤T≤200° C., in the second—at 180° C.≤T≤240° C., which keeps the ultimate pressure in the range of 10⁻¹⁰-10⁻¹¹ mbar. In the operating mode of the pump, i.e., at the stage of pumping out gases, the liquid state of the getter material radically changes the mechanism of interaction of the reactive alloy with gases. While earlier the diffusion of gas into the volume of the crystalline body limited the rate of gas sorption, now the continuous reaction between the gas and the melt and the continuous removal of reaction products from the surface of the melt according to the sedimentation law, are responsible for the rate of pumping out gases.

Particles of chemical compounds of gas with metal that originate on the surface of the melt immediately go under the influence of gravity into the volume of the melt, accumulating in the bottom of the container. This sedimentational “cleaning” of the melt surface sets the resulted rate of gas pumping.

The sedimentation of solid particles and the accompanying renewal of the melt surface are also useful for traditional getter technologies based on gas capture by a boundary gas/solid. Traditional methods have their advantages, and the task is to preserve them by building a common sorption platform for vacuum technologies.

Since getters reactants intensively sorb gases at room temperature, this makes it possible to reduce the ultimate pressure of the getter pump to the level of 10-14 mbar without generating free solid particles. This is implemented in one of the embodiments of this invention, where two pumps are combined into a common pumping system. In this system, each of the pumps alternately performs two functions, either restoring its functionality through sedimentation when the getter is in the melt state or pumping gases from the vacuum chamber when the getter is transferred to the solid state.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more comprehensively from the following detailed description taken in conjunction with the appended drawings in which:

FIG. 1. Pump design.

FIG. 2. Mechanism of thermo-sedimentational activation.

FIG. 3. General scheme of the pumping system.

FIG. 4. Operating characteristics of the pump.

FIG. 5. Additional illustration to FIG. 4.

FIG. 6. Purification of noble gases.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown merely by way of example in the drawings. The drawings are not necessarily complete and components are not essentially to scale; emphasis instead being placed upon clearly illustrating the principles underlying the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The new getter pump with molten getters reactants and an ultimate pressure of 10⁻¹⁰-10⁻¹¹ mbar pumps out all gases except noble gases. The consumable getter material here are alloys of the family Li_x(Sr_yBa_1-y)_1-x, where 0.8≤x≤0.90≤y≤1, or alloys Li_x Ca_1-x, where 0.8≤x≤0.9 and where calcium is highly enriched with strontium and barium. The choice of these alloys is due to the fact that they are getters reactants and have low liquidus temperatures. This choice guarantees high sorption efficiency and low vapor pressure of the getter melt near the melting point.

The design of this pump is extremely simple; it is a cylindrical container made of stainless steel with a flange for connection to a vacuum chamber and with an external heater (FIG. 1). The pump is loaded with the reactive alloy and thoroughly degassed in the molten state, after which the pump is ready for operation.

The essence of the proposed sorption solution comes down to a new, thermo-sedimentational method for activating getters reactants. The main innovative steps of this invention are disclosed below.

1. The pump is started and the pumping mode is established when the getter is heated to a temperature approximately 40° C. higher than the liquidus temperature of the corresponding alloy.

2. The stable gas pumping mode lasts exactly as long as the alloy temperature specified in paragraph 1 is maintained.

3. Pumping out gases according to paragraphs 1 and 2 proceeds with high sorption kinetics due to a new phenomenon for getter practice, i.e. due to the release of the gas/melt boundary from the solid products formed on it by their sedimentational entrainment into the volume of the melt.

4. The initial height of the getter melt in the interests of a long operating cycle of the pump must be significant and measured not in fractions of a millimeter but in centimeters, and preferably, in tens of centimeters.

Let us explain points 3 and 4, starting with the latter: if the initial thickness (or height) of the getter mass after melting is small, for example, less than 1 mm, then for a long operating cycle a mobile replenishment of the pump with the melt would be required, which would complicate the pump design and increase costs. On the contrary, a liquid column of melt of large height under ultra-high vacuum conditions could be called an “eternal” pump.

As for point 3, it refers us to sedimentational processes in a gravitational field. This is shown schematically in FIG. 2: when gas comes into contact with the reactive melt, islands of a solid product appear on its surface, consisting of particles of chemical compounds of gas and metal. Compounds of Li, Ca, Sr and Ba with gases are characterized by high volumetric compression and their density is much higher than the density of the melt. This starts a mass transfer mechanism that is unusual for getter applications—the removal of solid particles containing a gas component from the surface of the melt into the volume of the melt under the influence of gravity.

The velocity V of sedimentation in a gravitational field is expressed by the formula V=Δρd²g/18η, where Δρ is the difference in the density of the particle and the melt, d is the size of the solid particle, g—gravitational acceleration and η—melt viscosity. In the case under consideration, η is small, because the basis of the melt is Li (Takamichi lida and Roderick I. L. Guthrie. The Physical Properties of Liquid Metals. Oxford Science Publications, 1988.), d is large, even on the solid surface of these alloys at T_room micron-scale coatings grow quickly (K. Chuntonov. Intermetallic Getters Reactants for Vacuum Applications. Materials Sciences and Applications, Vol. 14 (2023) 222-239.), and Δρ has already been mentioned above. These data are in good agreement with the experimentally observed behavior of getters reactants, which already at high vacuum demonstrate a mirror-clean surface of their melt and stable sorption kinetics. It is also possible to refer to other results obtained in the research of sedimentation in metal melts (K. A. , B. Γ. , E. Π. . . , No3, 1991, c. 40-50. Hayka [K. A. Chuntonov, V. G. Postovalov, E. P. Romanov E. P. Rate characteristics of solidification of alloys under sedimentation conditions. Melts. 1991,V. 3. p. 40-50. Science.] and K. A. , B. Γ. , E. Π. . . , No4, 1993, c. 31-36. Hayka. [K. A. Chuntonov, V. G. Postovalov, E. P. Romanov. Cleaning the melt from suspended particles using centrifugation. Melts, V. 4, 1993, p. 31-36. Science.]).

As can be seen from FIG. 2, stabilization of sorption kinetics occurs almost immediately after the start of the pumping process. On the surface of the melt a dynamic equilibrium is established between the amount of gases entering it and the amount of gas removed into the volume of the melt through sedimentation of reaction products. The dependence j (t), where j is the specific pumping speed (i.e. the speed per unit area of the melt surface), and t is time, has the form of an extended horizontal section, which ends with a sharp decline in the curve j (t) at t=τ (FIG. 2, b), when a column of solid particles growing from the bottom of the container reaches the surface of the melt.

However, by the time the pumping process is completed, 15-20% of unused liquid alloy remains in the pump. This residue can be used when depositing getter films by heating the pump to 500-600° C., or it can be evaporated into a special container for subsequent consumption, for example, in the same getter pumps.

The described pump, like all getter pumps, is included in the process of pumping out the vacuum chamber after a preliminary vacuum of the order of 10⁻² mbar has already been created in it. Then the getter pump gradually reduces the pressure in the chamber to 10-11 mbar with the support of the cryo pump, which removes noble gases. Let us also mention, that the pressure of 10⁻¹¹ for the pump we are describing is not the limit, and it can be reduced to ˜10⁻¹⁴ mbar if not one, but two such pumps are used (FIG. 3).

The solution comes down to creating a system of two alternately operating pumps, pump No. 1 and pump No. 2 (FIG. 3). Let both pumps operate with half-open valves at the start of the pumping process, and let the pressure in the vacuum chamber stabilize at 10⁻¹¹ mbar. Then the following steps are taken: fully open the valve of pump No. 1 and close the valve of pump No. 2, also turning off its heating. After the temperature of pump No. 2 drops to T_room, simultaneously open the valve of pump No. 2 and close the valve of pump No. 1.

From this moment the pumping process changes essentially: now the solid surface of the reactive alloy is responsible for the sorption of gases and the total vapor pressure of Li, Ca, Sr and Ba decreases to ˜10⁻¹⁴ mbar. Over time the surface of the ingot becomes covered with a layer of reaction products, and when the rate of gas capture approaches a critical value (or earlier if other reasons appear), pumps No. 1 and No. 2 must exchange their roles in the sorption process.

To do this, the valve of pump No. 1 with pure hard alloy, which takes over the baton from pump No. 2, is opened and the valve of pump No. 2 is closed to restore its functionality. That is, the getter material of pump No. 2 is melted to clean the surface from solid products by sedimentation method and the alloy is returned to the solid state again so that at the right time it becomes a replacement for pump No. 1. Such alternation of operations of mutual support of pumps No. 1 and No. 2 can continue until the sorption resource of the reactive alloy is exhausted.

The operating characteristics of the new pump with the same composition of the getter material are determined by the geometric parameters of the reactive melt column, by its height h and by its radius r (FIG. 1). Thus, the duration τ of the working cycle of the pump measured by the length of the horizontal section of the curve j(t) in FIG. 2,b, is proportional to the value of h (FIG. 4,a), and the total pumping speed js, where s is the surface area of the melt, depends parabolically on t (FIG. 4,b). This introduces simplicity and convenience into pump design and operation. FIG. 5 shows how the operating characteristics of the pump change (FIG. 5, b) depending on its parameters h and r for the same volume of melt (FIG. 5, a). Let us note here that for maximum melt consumption its initial height h should be approximately one third lower than the height of the side heater (FIG. 1 and FIG. 2, a).

The pumps, which we have described as double-pump pumping systems, are intended for the same purposes as other getter pumps, i.e., to create and maintain ultra high vacuum (UHV) and extreme high vacuum (XHV). On the other hand, a single pump with a cryo unit is able to take the place of high vacuum pumps, a diffusion pump, and a turbo vacuum pump, while being oil-free and silent. In this second sector of vacuum technologies the material base of getters reactants can be expanded: while in the case of UHV and XHV applications this base was alloys Li and IIA metals with the liquidus temperature not higher than 200° C., now these are alloys, the liquidus temperature of which is T≤250° C. Besides, additions of Mg are possible here, as well as the use of the eutectic alloy Li—10 at % Ce as a getter.

However, there is another application of a single pump, which does not relate to the vacuum field, although it is based on the same reactions of gas with the melt and on the same sedimentation renewal of the melt surface as in the case of pumping gases from a vacuum chamber. This additional application is not difficult to disclose with the help of FIG. 3.

Let us assume that in the scheme in FIG. 3 all pumps except pump No. 1 are removed, and the vacuum chamber contains a noble gas with a mixture of active and low-active gases. Then, if the valve of pump No. 1 is opened, active and low-active gases begin to react with the melt and escape into its volume. The sorption process will continue in this case until the noble gas is completely purified from all impurities. Below there are two examples of the implementation of this method of noble gas purification using a reactive melt (FIG. 6).

One variant is shown in FIG. 6, a: a tube furnace 1 with a closed bottom is fixed in an inclined position at an angle β relative to the vertical, and inside the furnace pump 2 rotates around its longitudinal axis. This latter is a steel cylinder with a getter melt and a noble gas to be purified under high pressure of about several tens of bar. The cylinder is hermetically sealed with a flange 3, which is equipped with a valve 4 and manometer 5.

Another variant is shown in FIG. 6, b: here 1 is a combination of a furnace and a pump, where the pump is mechanically fixed inside the furnace, without rotating around its axis, as in the case of FIG. 6, a. This element 1 together with the counterweight 2 performs oscillatory movements with a swing angle β relatively to the vertical axis y.

These dynamic solutions, one with rotation of the pump (FIG. 6, a) and the other with its swinging (FIG. 6, b), have clear advantages over the static variant of noble gas purification, which is discussed above hypothetically on the basis of FIG. 3. Firstly, with the appearance of an inclination B of the longitudinal axis of the pump relatively to the vertical, the surface area of the melt increases according to the expression s=ηr²/cosβ, and with it the gas sorption rate also increases. In the case under consideration, this increase can be multiple, but becomes very large if the shape of the pump is changed accordingly. Secondly, the sorption process here is time controlled. Thirdly, both the rotation of the pump and its swinging prevent the formation of a hard crust of products on the surface of the melt, which can appear at the stage of filling the pump with a fresh portion of the processed gas, when the concentration of impurities in it is high.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1. Pump design.

1—cylindrical container made of stainless steel SS 304 or SS 316, 2—getter reactant melt with low liquidus temperature, 3—conflate flange, 4—side heater and 5—bottom heater; h is the height of the melt column and r is its radius. Two independent heaters allow, if necessary, to create convective flows of different configurations in the melt.

FIG. 2. Mechanism of thermo-sedimentational activation.

(a) shows what happens inside the pump over time t; 1—melt, 2—solid particle; t₀—corresponds to the starting state of the melt, t₁—to the state by the time τ/4, t₂—to the state by time 2τ/4, t₃—to the state by the time 3τ/4, and t₄=τ to the end of the working cycle, when there is no longer a common flat gas/melt boundary, but there are only melt residues in the voids between solid particles of reaction products.

(b) it can be seen that the gas pumping speed j(t) has the form of a plateau, the length of which τ determines the duration of the pump operating cycle, and which ends with a decline in the vicinity of point t₄, when the mentioned flat surface of the melt disappears. The dashed vertical lines on the graph, drawn in increments of τ/4, only serve to link the structural images (a) to the time scale t.

FIG. 3. General scheme of the pumping system with two new pumps.

It is assumed that if the vacuum chamber is a UHV or XHV chamber, then valves of the Conflat Gate Valve type are used.

FIG. 4. Operating characteristics of the pump.

(a) the total service life τ of the pump linearly depends on the height h of the getter column; (b) the pumping speed sj depends parabolically on the radius r of the getter column.

FIG. 5. Additional illustrations for FIG. 4.

(a) four pumps 1, 2, 3 and 4 different values of the melt surface area πr², but with the same volume πr²h of the getter melt; (b) graph of the dependence of pumping speed js on time for the same four pumps 1, 2, 3 and 4. It can be seen how easy it is to satisfy a variety of practical needs in such a simple way as varying the parameters h and r of a getter column.

FIG. 6. Purification of noble gases.

(a) inclined pump rotation; 1—furnace, 2—pump, 3—flange with valve 4 and pressure gauge 5, 6—getter melt and β—angle of deviation of the pump axis from the vertical y.

(b) pump swing; 1—common body containing a furnace with a pump inside, 2—counterweight, β—swing amplitude.

A new getter pump has been developed based on getters reactants consisting of Li alloys with Ca, Sr, Ba, Mg, and Ce and with low liquidus temperatures. These alloys serve as consumable getter material in these pumps in both liquid and solid states. The high sorption efficiency of these pumps, their simplicity and their dust-free behavior are a consequence of a new method of activation of the given alloys, namely their thermo-sedimentational activation. There are three options for using these pumps. The first option is a single pump with a getter melt and with an ultimate pressure of 10⁻¹⁰-10⁻¹¹ mbar. The second is a combination of two pumps in the form of a single pumping system, in which the sorption process occurs at both the gas/solid and gas/melt boundaries and with the ultimate pressure of 10⁻¹⁴ mbar. The third option is the purification of noble gases from active and low-active gas impurities.

Let us highlight the most noticeable advantages of the new pumps:

• • 1. No generation of free solid particles.
  • 2. Exploitation of two sorption boundaries, the gas/solid boundary and the gas/melt boundary.
  • 3. Reducing the ultimate pressure to 10⁻¹⁴ mbar by forming a pumping system of two pumps.
  • 4. Simplicity of pump design and maintenance.
  • 5. Low-melting alloys of Li with Ca, Sr, Ba, Mg and Ce as an effective getter in the solid, and especially in the liquid state.
  • 6. The new pump is easily adaptable to many applications in the field of high, ultra-high and extreme vacuum, as well as in the field of noble gas getter purification.

Claims

1. A getter vacuum pump or a pumping system in the form of two such pumps, wherein either of said pumps is a steel cylindrical container with a flange, with getter material inside and an external heater, and wherein said pumps create a pressure of the order of 10−10-10−14 mbar in the evacuated vacuum chamber without generating free solid particles.

2. The getter vacuum pump according to claim 1, wherein the getter material is Li alloys with Ca, Sr and Ba in the molten state, which triggers the process of sedimentational cleaning of the gas/melt interface from the solid product formed thereon, thus accelerating the sorption kinetics and preventing contamination of the vacuum chamber with solid particles.

3. The getter vacuum pump according to claim 2, wherein only alloys with a liquidus temperature of 200° C. or less are used, such as Lix(SryBa1-y)1-x, where 0.8≤x≤0.9 and 0≤y≤1, or alloys LixCa1-x, where 0.8≤x≤0.9 and where Ca is doped with Sr and Ba, which reduces the ultimate pressure to 10−11 mbar.

4. The pumping system in the form of two getter pumps according to claim 1, where in turn each of them is in a state of pumping out gases with the getter material at Troom, and the other is at this time disconnected from the vacuum chamber to restore the functionality of the getter material by melting it and sedimentational cleaning of the surface, which, taken together, reduces the ultimate pressure in the system to 10−14 mbar.

5. The getter vacuum pump according to claim 3, where in addition to the alloys specified therein, the eutectic alloy Li—10 at % Ce is used, and the alloys are also allowed to be alloyed with magnesium, provided that the liquidus temperature of the alloys does not exceed 250° C., thereby guaranteeing the ultimate pressure at the level of 10−10 mbar.

6. The getter vacuum pump according to claim 5, which in its vertical position, and more preferably when installed on a “swing”, or when connected to a rotation system with an inclination, is used for sorption purification of noble gas loaded into this pump at the required pressure, up to pressures of tens of bar.