**5.2. Chemisorption by non-evaporable getter materials**

Getters are solid materials, usually metallic alloys, which can chemisorb gas molecules in its surface; they can be considered as chemical pumps. They are widely used for a variety of applications such as vacuum systems, electronic devices, sensors and MEMS, energy devices, gas purification, and so on. [36]

For a proper absorption of gas molecules, the surface of the getter material must be clean. The surface cleaning process, also called getter activation, is done in two different ways, depending on the type of getter:


For gas purification systems, NEGs are generally used, and from now on, we focus on them.

NEGs are typically based on zirconium alloys. Examples of these alloys are Zr(84%)-Al(16%) and Zr(70%)-V(24.6%)-Fe(5,4%). Zirconium-based systems are very reactive for a wide variety of gas molecules such as H2 , H2 O, O2 , N2 , CO, CO2 , and so on.

For active gases such as N2 , O2 , CO, CO2 , and so on, the reactions proceed by dissociative chemisorption followed by a reaction to form oxides, carbides or nitrides [37]. If the concentration of these gases is high, the getter surface is quickly passivated. To maintain active state of the getter surface, the material can be maintained at high temperature (e.g., 400°C), thus avoiding the formation of a passivation layer. In this way, the surface contaminants diffuse into the bulk of the NEG material.

Hydrogen sorption is governed by a different reaction. Hydrogen easily diffuses into a getter because it dissociates on the getter surface into atomic hydrogen. The hydrogen atoms easily slip into the atomic lattice of the metal grains [37]. As Rameshan explains, hydrogen in the interior of a NEG forms a solid solution that exhibits an equilibrium pressure, which depends on the concentration of the hydrogen and the temperature of the material. Sieverts' law describes this relationship:

$$
\log P = A + 2\log Q - B/T \tag{4}
$$

where P is the H2 equilibrium pressure in torr, Q is the H2 concentration in the NEG alloy in torr∙L/g, T is the temperature of the getter in K and A and B are constants for different NEG alloys (e.g., A = 4.8, B = 6116 for Zr(70%)-V(24.6%)-Fe(5,4%), commercialized under the name St707 [38]). When the hydrogen concentration exceeds 20 torr∙L/g, a phenomenon called "hydrogen embrittlement" occurs due to the change of the lattice parameters [37]. With enough time under these conditions, the getter alloy becomes a fine powder that can cause problems in the getter application.

An NEG material working at ambient temperature is an ideal candidate for the elimination of the remaining molecular hydrogen in helium that has been purified by cryosorption in the ATP. **Figure 5** shows that the hydrogen concentration in helium after passing through the getter will be better than grade 14 (*yH*<sup>2</sup> <<10−14), that is, several orders of magnitude lower than at 400°C. Even more, the hydrogen capacity of the getter is higher, and the sorption speed is still reasonably high [38].

#### **5.3. Clean helium recovery plant configuration**

This system can purify gas flows up to 30 sL/min with 10,000 ppm of impurities. The output flow quality is about six orders of magnitude better for the main contaminants (i.e., air in the

The purifier can operate without interruptions during, at least, 1 month, and can purify more than 1 million sL of recovered helium (with a typical average impurity volume concentration of 300 ppms in total). The regeneration procedure is totally automated and it takes 7 h. Thus,

Once the main contaminants have been removed, the second purification stage needs only to eliminate the remaining hydrogen via chemisorption by the non-evaporable getter (NEG)

Getters are solid materials, usually metallic alloys, which can chemisorb gas molecules in its surface; they can be considered as chemical pumps. They are widely used for a variety of applications such as vacuum systems, electronic devices, sensors and MEMS, energy devices,

For a proper absorption of gas molecules, the surface of the getter material must be clean. The surface cleaning process, also called getter activation, is done in two different ways, depend-

• For evaporable getters, the active surface is obtained by sublimation under vacuum of a

• For non-evaporable getters (NEGs), the active surface is produced by thermal diffusion of the surface contaminants into the bulk of the NEG material itself. After air exposure, the

For gas purification systems, NEGs are generally used, and from now on, we focus on them. NEGs are typically based on zirconium alloys. Examples of these alloys are Zr(84%)-Al(16%) and Zr(70%)-V(24.6%)-Fe(5,4%). Zirconium-based systems are very reactive for a wide vari-

, CO, CO2

chemisorption followed by a reaction to form oxides, carbides or nitrides [37]. If the concentration of these gases is high, the getter surface is quickly passivated. To maintain active state of the getter surface, the material can be maintained at high temperature (e.g., 400°C), thus avoiding the formation of a passivation layer. In this way, the surface contaminants diffuse

Hydrogen sorption is governed by a different reaction. Hydrogen easily diffuses into a getter because it dissociates on the getter surface into atomic hydrogen. The hydrogen atoms easily slip into the atomic lattice of the metal grains [37]. As Rameshan explains, hydrogen in the interior of a NEG forms a solid solution that exhibits an equilibrium pressure, which

, and so on.

, and so on, the reactions proceed by dissociative

main contaminant is oxygen present in the passivating oxide layer.

, H2 O, O2 , N2

, CO, CO2

, O2

case of recovered helium).

78 Superfluids and Superconductors

gas purification, and so on. [36]

ing on the type of getter:

fresh metallic film.

ety of gas molecules such as H2

into the bulk of the NEG material.

For active gases such as N2

material.

the operational down-time ratio is only 1.25%.

**5.2. Chemisorption by non-evaporable getter materials**

Our "Clean helium" (extreme pure helium free of molecular H2 ) low-pressure (P < Pc) SS-HRP concept is depicted in **Figure 6**. The plant is initially fed with commercial grade 5 (99.999% pure) helium gas that may contain up to a H2 molar fraction of 10−6 [25]. The gas is further purified by cryocondensation by one or more cryo-refrigerator-based purifiers (ATPs), each with a total effective volume to store solid impurities of several liters and a maximum purification flow rate of around 30 sL/min at 20 K.

**Figure 5.** Hydrogen molar fraction, *yH*<sup>2</sup> , calculated from equilibrium isotherms of the St 707 getter alloy obtained from Sievert's law. *yH*<sup>2</sup> increases accordingly when the material captures hydrogen molecules until it reaches the embrittlement area.

bag and compressed in the recovery bottles at 2 × 10<sup>4</sup>

input gas contains other impurities (e.g., some ppms of O2

the accumulation of solid impurities (H2

until the saturation is reduced significantly.

, O2

concentration of molecular content (*yj*

, CO, CO2

, etc).

different impurities (N2

the main source of H2

kPa (200 bar). A H2

, N2 , H2

< 10−14) of all impurity constituents (i.e., N2

), an ATP regeneration process is automati-

http://dx.doi.org/10.5772/intechopen.74907

the series after the compressor, not shown in the scheme of **Figure 6**, should always be used.

When a pressure drop develops between the input and the output of one of the ATPs, due to

cally initiated. The input and the output gas ports of the given ATP are closed, so that this ATP is now isolated and the entire ATP Dewar volume is heated up to around 130 K so that

are sublimated and released to the atmosphere through a vent valve. Before restarting a new purification cycle, the ATP cools down again to the temperature of normal operation at 10 K.

Nevertheless, as we have seen in Section 5.2, some getter materials are capable of eliminating other impurities besides hydrogen; the chemical reactions are competitive. Therefore, if the

In the first version of the "Clean Helium" plant, we used a getter placed after the commercial pure helium (99.999%, less than yj < 10−5 in total) bottles (**Figure 7**). With this configuration, we were able to produce hydrogen-free liquid helium in 3 months. From that moment, impedance blockages start to appear due to the saturation of the NEG produced by the presence of

The purified helium at the ATP output, with a working temperature < 20 K, has a negligible

**Figure 7.** Schematic configuration of a first-generation small-scale helium recovery plant . The commercial He bottles are

contamination (red), and it is purified with a heated getter before helium enters in the recovery plant.

, N2 , O2

all the low-vapor pressure impurities, collected in solid form, for example, H2

O dryer, plumbed in

"Clean" Liquid Helium

81

, N2

O, etc.), the getter duration

, O2 , H2 O,

 and O2 ,

**Figure 6.** Schematic configuration of a small-scale "Clean Helium" recovery plant (free of hydrogen). Gas bag, compressor and recovery helium bottles are not completely free of H2 (orange). The commercial He bottles are the main source of contamination (red). The bypass is closed when the ATP operation temperature is T > 3 K.

The purification temperature in the coldest zone of the ATP Dewar will be in the range between 10 and 30 K, and this does not guarantee a negligible vapor pressure of solid H2 nor a negligible solubility in liquid He. Thus, the purified gas will contain H2 molecules that need to be eliminated before liquefaction. A solution tested in our plant consists of the chemisorption of the remaining H2 molecules in the ATP output gas by a getter material at room temperature (**Figure 6**). The non-evaporable getter (NEG) materials used in this study are:


This solution is extremely efficient since there are no helium losses at all. On the other hand, in this configuration, the St707 getter only traps hydrogen, and it does in a reversible way. Therefore, once it is near saturation, typically, every two years, it can be regenerated by heating it up to a specific H2 desorption temperature (typically >500°C).

The H2 -free He from the double purification stage (cryocondensation + chemisorption) is then fed a parallel network of advanced technology liquefiers (ATLs) [31] that produce H2 -free ultra-pure liquid helium (named by us as "Clean Helium"). The instruments are always filled with ATL "Clean Liquid Helium." Obviously, commercial liquid helium should never be transferred to hydrogen-sensitive instruments because the absence of H2 is not guaranteed. In this small-scale HP-HRP, helium boil-off from the cryogenic instruments is collected in a gas bag and compressed in the recovery bottles at 2 × 10<sup>4</sup> kPa (200 bar). A H2 O dryer, plumbed in the series after the compressor, not shown in the scheme of **Figure 6**, should always be used.

When a pressure drop develops between the input and the output of one of the ATPs, due to the accumulation of solid impurities (H2 , N2 , O2 ), an ATP regeneration process is automatically initiated. The input and the output gas ports of the given ATP are closed, so that this ATP is now isolated and the entire ATP Dewar volume is heated up to around 130 K so that all the low-vapor pressure impurities, collected in solid form, for example, H2 , N2 and O2 , are sublimated and released to the atmosphere through a vent valve. Before restarting a new purification cycle, the ATP cools down again to the temperature of normal operation at 10 K.

Nevertheless, as we have seen in Section 5.2, some getter materials are capable of eliminating other impurities besides hydrogen; the chemical reactions are competitive. Therefore, if the input gas contains other impurities (e.g., some ppms of O2 , N2 , H2 O, etc.), the getter duration until the saturation is reduced significantly.

In the first version of the "Clean Helium" plant, we used a getter placed after the commercial pure helium (99.999%, less than yj < 10−5 in total) bottles (**Figure 7**). With this configuration, we were able to produce hydrogen-free liquid helium in 3 months. From that moment, impedance blockages start to appear due to the saturation of the NEG produced by the presence of different impurities (N2 , O2 , CO, CO2 , etc).

The purified helium at the ATP output, with a working temperature < 20 K, has a negligible concentration of molecular content (*yj* < 10−14) of all impurity constituents (i.e., N2 , O2 , H2 O,

The purification temperature in the coldest zone of the ATP Dewar will be in the range between 10 and 30 K, and this does not guarantee a negligible vapor pressure of solid H2

**Figure 6.** Schematic configuration of a small-scale "Clean Helium" recovery plant (free of hydrogen). Gas bag,

be eliminated before liquefaction. A solution tested in our plant consists of the chemisorption

This solution is extremely efficient since there are no helium losses at all. On the other hand, in this configuration, the St707 getter only traps hydrogen, and it does in a reversible way. Therefore, once it is near saturation, typically, every two years, it can be regenerated by heat-

desorption temperature (typically >500°C).

fed a parallel network of advanced technology liquefiers (ATLs) [31] that produce H2

transferred to hydrogen-sensitive instruments because the absence of H2

ultra-pure liquid helium (named by us as "Clean Helium"). The instruments are always filled with ATL "Clean Liquid Helium." Obviously, commercial liquid helium should never be

this small-scale HP-HRP, helium boil-off from the cryogenic instruments is collected in a gas


molecules in the ATP output gas by a getter material at room temperature

(24%)-MgO(13%)-based oxides working at room temperature.

negligible solubility in liquid He. Thus, the purified gas will contain H2

compressor and recovery helium bottles are not completely free of H2

(**Figure 6**). The non-evaporable getter (NEG) materials used in this study are:

source of contamination (red). The bypass is closed when the ATP operation temperature is T > 3 K.

• thermally activated media-based [Zr(70%)-V(24.6%)-Fe(5.4%)] St707 [38] and

of the remaining H2

80 Superfluids and Superconductors

• Ni(31%)-NiO(32%)-SiO2

ing it up to a specific H2

The H2

nor a


is not guaranteed. In

molecules that need to

(orange). The commercial He bottles are the main

**Figure 7.** Schematic configuration of a first-generation small-scale helium recovery plant . The commercial He bottles are the main source of H2 contamination (red), and it is purified with a heated getter before helium enters in the recovery plant.

CO2 , etc.) except for the neon and hydrogen case (see **Figure 4**). The neon is not a problematic substance for the impedance clogging issue, since at liquid helium temperature (4.2 K at Patm), the vapor pressure is negligible; besides, if there exists a molecular concentration at higher temperatures, it is not affected by the getter material because it is a noble gas like helium. Therefore, the best place to put the hydrogen grabber is at the ATP output (**Figure 6**), when the helium is extremely pure. In fact *yj* < 10−14 for all the substances except for the H2; thus, the unique function of the getter is to capture H2 . In this way, the process is optimized and the life of the getter material extends.

**Thanks**

information.

**Author details**

Zaragoza, Spain

Zaragoza, Spain

Zaragoza, Spain

**References**

We want to acknowledge the collaboration and support of the technical team of the Servicio General de Apoyo a la Investigación-SAI, specifically to Mrs. M. Castrillo, Mr. D. Finol, Mr. F. Gómez, Dr. A. Arauzo, Mr. E. Guerrero and Mr. P. Tellez for their technical support. We thank Dr. Christoph Haberstroh from TU Dresden for additional discussion and very useful

Miguel Gabal1,2, Javier Sesé1,3, Conrado Rillo1,4\* and Stefano Spagna5

1 Departamento de Física de la Materia Condensada, Universidad de Zaragoza,

2 Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza,

3 Instituto de Nanociencia de Aragón (INA), Universidad de Zaragoza, Zaragoza, Spain

4 Instituto de Ciencia de Materiales de Aragón (ICMA), Universidad de Zaragoza-CSIC,

[2] Gabal M. New Cryocooler-Based Helium Liquefaction and Purification Techniques. From Recovered Gas to Ultra-Pure Liquid [thesis]. Colección de Estudios de Física. Uni-

[3] Pobell F. Matter and Methods at Low Temperatures. Berlin, Heidelberg: Springer Berlin

[4] Engel BN, Ihas GG, Adams ED, Fombarlet C. Insert for rapidly producing temperatures between 300 and 1 K in a helium storage Dewar. The Review of Scientific Instruments.

[5] DeLong LE, Symko OG, Wheatley JC. Continuously operating <sup>4</sup>

erator. The Review of Scientific Instruments. Nov. 1971;**42**(1):147

concentrations in superfluid 4He. Physica

He evaporation refrig-

"Clean" Liquid Helium

83

http://dx.doi.org/10.5772/intechopen.74907

\*Address all correspondence to: crillo@unizar.es

5 Quantum Design Inc., San Diego, California, USA

[1] Marin JM, Boronat J, Casulleras J. Finite H2

B. Jul. 2000;**284-288**:95-96

versity of Zaragoza; 2016

Heidelberg; 2007

Sep. 1984;**55**(9):1489

The "Clean helium" gas produced by the Clean Helium Recovery Plant (**Figure 6**) is ultimately liquefied in a commercial ATL and transferred directly or by intermediate transport Dewars into the application instruments. The evaporated gas from non-H2 sensitive instruments, that could be initially filled with commercial non-"Clean" liquid (e.g., NMRs, MEGs, high field magnet cryostats, etc.), and can have a hydrogen quantity equal or below that corresponding to the vapor pressure of the hydrogen at 4.2 K and 100 kPa (i.e., *yH*<sup>2</sup> = 3.5 ∙ 10−10), is also collected in the gas bag, compressed and injected again in the ATPs for purification and complete elimination of the H2 impurities.

The validity of the "Clean helium" plant concept is demonstrated by the fact that impedance blockages have been completely eliminated for more than 3 years, when the plant configuration was implemented in the Cryogenic Liquids Service at the University of Zaragoza [39]. Furthermore, the efficiency of the double purification method presented in this chapter was verified by extra-sensitive H2 detection techniques presented in [2], for both gas and liquid phases.
