Alloys of Li with IIA metals named getter reactants [1][2] possess excellent sorption properties, however, very high chemical activity of these alloys in the form of films, powders or bodies with a porous structure limits their widespread use. Mechanochemistry was also bet on, for example, in flow-type sorption reactors [3]-[5], where the starting material is simply ingots, which are introduced into the working chamber in air and then ground in the process medium at the required rate. However, this solution also has drawbacks. Among them, let us point out pressure drop and problems with the separation of the final products of the gas-metal reaction.

This critical view of mechanochemical methods has become more acute with the growing demand for pure materials and ultra-clean vacuum level production and laboratory conditions. New technological frontiers place more stringent demands on sorbents, requiring them not only to have increased productivity but also higher purity levels of the final product, with an emphasis on the complete absence of dust particles.

And here, a real surprise from Li alloys with IIA metals is the new knowledge about how they interact with gases [6]-[10], suggesting that many clean‑technology problems could be solved using these reactive alloys. Their sorption behavior in gas environments stands out sharply from that of traditional sorbents such as MOFs, NEGs, and zeolites. Let us show the special properties of the given alloys and their advantages in solving clean technology problems.

What are the main peculiarities of getter reactants as a sorbent material? At the level of empirical approach, we can talk about three features that elevate these getters over existing sorbents in solving the urgent technological problems. These are:

Figure 1shows the sorption curve c ( t ) of Li–IIA metal alloys in a usual atmosphere at T r o o m , where c ( t ) = Δ m ( t ) / m 0 , m 0 is the initial mass of the sample in the form of a macrobody with a monolithic structure, and Δ m ( t ) is the mass increment over time t .

Figure 1.General view of the sorption curve of intermetallic phases in Li-IIA metal systems. I: Sorption pause; II: Stage of corrosive decomposition of the cast body, β : average slope of the c ( t ) curve; III: Stage of attenuation of the sorption process, and c max : limit of sorption capacity.

Let us emphasize that here the samples are understood to be alloys with a large fraction of the intermetallic phase or consisting entirely of it. This choice is based on experience [8], which shows that the sorption of gases by intermetallic compounds of Li-IIA metal systems occurs many times faster than sorption by elemental metals [15][16].

This behavior of reactive intermetallics is explained by their mechanical weakness and brittleness, which accelerate the corrosive decomposition of the alloy, and with it, the sorption kinetics. The new sorption mechanism of these alloys changes the usual image of the monotonic curve c ( t ) , giving it a stepped shape (Figure 1). In general, it can be seen that there is a certain similarity with the parabolic breakaway curves [17], however, the latter describe the destruction processes only in the layer of products on the surface, and not the disintegration of the entire getter body, as in Figure 1.

Let us divide according to [9] the c ( t ) curve into three parts (Figure 1), where I is the initial stage, called the sorption pause, II is a series of steps indicating the corrosive decomposition of the ingot and intensive gas capture, and III is the stage of attenuation of the sorption process with the exhaustion of the getter mass at the c max level. This approach to analyzing the sorption process is pragmatic, highlighting those of its features that are of interest for applications.

Sorption pause I on the graph looks like a vertical segment with ends ( 0 , 0 ) and ( 0 , c p ) , which then continues as a horizontal segment with ends ( 0 , c p ) and ( t p , c p ) . This pause provides applications with an ideal chance for open installation of the getter reactant at its working place, since, according to available data, the point t p falls within the range of 10 - 15 minutes to several hours [8], which satisfies any time requirements. Moreover, at stage I the thickness of the sacrificial layer is small and therefore, c p is also small. The share of getter material lost during assembly in air can be reduced by increasing the average size of the getter body [5][7].

Stage II, otherwise known as self-grinding of reactive intermetallic compounds, makes them unattainable for other sorbents in terms of specific sorption capacity c max , as shown in [8]. In its turn, sorption kinetics, determined by the slope β (Figure 1) can be preset. Factors regulating sorption kinetics include the getter composition (Figure 2), its microstructure (Figure 3), and the value of the initial specific surface area of the getter (Figure 4).

So, two phenomena, the sorption pause I and the corrosion self-grinding II, contribute new insights into sorption processes, correcting common understanding in this area and confirming the practical value of reactive alloys. Thus, a loose cover layer, typically perceived negatively, proves beneficial here, minimizing getter loss during installation in air but allowing gases to pass to the boundary with the alloy, creating conditions for its subsequent disintegration [8]. Corrosive decay, a clear scourge for most metal products and constructions, serves as the main driving force of the sorption process for getter reactants [6][8].

Let’s move on to the potential applications of solid-phase getter reactants and demonstrate their strengths in solving pressing problems in resource efficient and clean technologies.

Figure 2.Sorption curves of reactive intermetallic compounds under normal atmospheric conditions at T r o o m [7]. Here, curve 1 describes the phase of composition Li 23 Sr 6 , curve 2—the phase of composition CaLi 2 , and curve 3—the phase of composition CaMg 2 .

Figure 3.Decomposition of two samples of the composition Ca 0 .35 Li 0 .45 Mg 0 .20 in standard conditions [9]. (a) Initial state of the samples and a ruler for estimating their size; (b) Decay process over time (hours). The sample on the left was taken from the bottom of the growth crucible; the sample on the right, from its central part.

Figure 4.Sorption curves of the Laves phase Ca 0 .33 Li 0 .48 Mg 0 .19 . Here, curves 1, 2, and 4 [8] are supplemented by curve 3, creating a convincing picture of the influence of the initial values of the specific surface area of the getter material on its sorption kinetics in air. These values in the given case are 71 cm²/g (curve 1), 39 cm²/g (curve 2), 30 cm²/g (curve 3), and 22 cm²/g (curve 4).

A true breakthrough in the field of ultraclean vacuum and ultrapure inert gas environments is becoming possible with the simple step of converting getter reactants into a liquid state (see Chapter 2.3). Changing the phase state of the getter material results in the emergence of new mechanisms for influencing the sorption process in Li-IIA metal alloys [10].

The liquid state of the alloy provides a simple and reliable way to control the reactant surface area, and therefore the sorption kinetics. Furthermore, melting the getter initiates its self-cleaning mechanism, where solid reaction products are released from the melt surface into its volume under the influence of gravity. Thus, the melt surface is cleaned, ridding the environment from free solid particles and maintaining a high specific rate of gas pumping.

This behavior of reactive melts should be of interest in cases where traditional sorbents are used, which become sources of dust for the environment as they wear out. Furthermore, the liquid reactant helps achieve a maximum vacuum level of ~ 10 − 14 mbar [10][14], which is two orders of magnitude higher than currently available level.

Like solid-phase getter reactants, melts have their own criteria for selecting the optimal composition. While the advantages of the former are based on the brittleness of intermetallic phases in Li-IIA metals systems, the latter are the more successful in cleaning the medium the lower their liquidus temperature. Low operating temperatures of the melt keep its components in the condensed state, which is in the interest of a clean environment.

Eutectic alloys based on Li in the Li-Ca , Li-Sr , Li-Ba , Li-Ca-Sr , Li-Ca-Ba , Li-Sr-Ba , Li-Ca-Sr-Ba systems, and the eutectic Li-10   at%   Ce [21] meet these conditions. If we limit the operating range of the melt to 150 ∘ C   -   200 ∘ C , we come to a family of reactive alloys with Li with concentrations ranging from 90   to   80   at% and below, for which vacuum of 10⁻¹⁰ - 10⁻¹¹ mbar is achievable in accordance with the data of [22].

Reactive melts are easily adapted to any sorption application, as they require only one thing for success: to be in contact with the process medium. Applications for this product are seen not only in nuclear energy or particle accelerators, but also in many other areas requiring pure materials and a clean operating environment. Let us consider two such examples, one from the field of scientific instruments and the other from the field of production.

Getter reactants are a special class of chemisorbents based on Li-IIA metal alloys. They are versatile, easily adapting to any vacuum or gas application; in both their phase states, solid and liquid, they outperform existing sorbents on performance parameters; they are also a clear pillar of developing clean technologies. So, the essence of this review:

1) Solid-phase getter reactants are arbitrary-shaped macrobodies with a monolithic structure, belonging in composition to the intermetallic phases of the Li-IIA metal systems. Due to their structure, they withstand assembly in air, and at the same time during the operating stage, due to their brittleness and mechanical weakness, they quickly reach the theoretical limit of sorption capacity in a gas environment in the process of corrosive decomposition. Sorption pause and self-grinding in gas environment make these getters leaders in sorption efficiency.

2) Liquid-phase getter reactants are melts of eutectic and hypereutectic lithium alloys with Ca, Sr, and Ba at temperatures of 150˚C - 200˚C, corresponding to concentrations of 90 to 80 at% Li or even less. A getter melt solves the main problem of clean technologies by ridding them of free solid particles. Purity level is increased by sedimentation of solid particles within the melt, and sorption kinetics becomes controllable due to the melt’s fluidity, allowing for easy modification of its surface area.

3) Contribution to clean technologies. Getter reactants win in the fight against dust particles: the melt surface captures not only active gases and the products of reaction with them but also foreign solid particles. All particles with a density higher than the melt density, go into its volume under the influence of gravity forces. Further, two getter pumps are capable of creating an extremely high vacuum of ~ 10 − 14 mbar, while a sorption bath with a getter melt (see Appendix) under appropriate conditions ensures complete removal of all active and low-activity impurity in a fraction of a second.

4) Contribution to productivity. The simple design of the equipment, the single-act mechanical activation in the solid state, and a one-stage controlled sorption process in the liquid state, the unlimited availability of consumable metals, the alloys of which in the form of primary ingots serve as the initial gas sorbent with record-breaking performance characteristics—all these are constituent parts of a new sorption technology capable of replacing a number of traditional practices. Getter reactants mobilize natural phenomena, manifesting themselves in the form of a beneficial sorption pause, self-grinding, and self-cleaning, increasing productivity and reducing costs in sorption technologies.

Let us single out a gas column in the form of a rectangular parallelepiped G of height h and bases of area s above the melt in the bath (Figure 6), where the lower base serves as the boundary with the melt, and the upper base, with the bath lid. The number of impacts of gas molecules on the melt surface s during a time t is denoted by n = v s t , where v = p / 2 π m k T is the number of impacts per second per 1   cm 2 of melt surface, p is the gas pressure above the melt, m is the average mass of gas molecules, k is Boltzmann’s constant, and T is the bath temperature. Let us calculate the time t c at which the number n equals the number of gas molecules in the above-mentioned column G of volume V = s h .

Using the equation of state for gases p V = N k T , where N is the number of gas molecules within a volume V , we find that t c = α h , where α = 2 π m / k T . Assuming the following values: p = 1.1   bar , h = 1   cm , average molecular mass m = 40   g / mol , and T = 450   K , we arrive at t c = 8 × 10 − 5   sec . Collision of any particle, except for inert gas atoms, with the reactive melt leads to its capture, therefore, the value of t c under certain conditions can provide an estimate of the time required for complete purification of the initial noble gas from active and low-active impurities.

Indeed, in terms of impurity level, rare gases can be divided into two sorts: gases, purification of which involves a sedimentation process, and gases subjected to instantaneous purification. The dividing line here is the concentration at which the amount of impurity in the initial gas column above the melt is sufficient to form a monolayer of reaction products on the melt surface. Let’s estimate the value of this concentration.

Elementary calculations performed for the cases h = 0.1 cm, h = 1 cm, and h = 10 cm show that the desired concentration is 99.95%, 99.995%, and 99.9995% of the noble gas, respectively. In each of these three cases, these gases release all their active impurity to the melt within a time of t c ∼ 10 − 4 sec . In terms of both the time scale and the level of purity of the final product, with the complete removal of all active gases and dust particles, this is a remarkable result.