**3.1. Liquid helium purity**

With a boiling point of 4.2 K at 100 KPa, liquid helium is the coldest fluid that exists in nature. Below its critical temperature (Tc = 5.2 K), any unwanted substance present in the liquid phase, that is, any impurity, will be in solid form, resulting in mist, snow, suspensions or particulates [6]. The vapor pressure of these solid impurities will be, in general, negligibly small (<<10−9 Pa), except for the case of the hydrogen isotopes and their molecular combinations [7] for which this is of the order of 10−2 Pa and 10−5 Pa, at 5.2 and 4.2 K, respectively. The solid impurities are usually charged and can be easily eliminated by electrostatic precipitation using Petryanov filters to obtain "optically clean" liquid, as demonstrated by Abrikosova and Shal'nikov [7]. But, even "optically clean" filtered liquid helium may contain a relevant quantity of non-solid hydrogen, that is, molecular hydrogen traces.

The He-H2 gas mixture has attracted much interest in the scientific community because it is the simplest system for the study of intermolecular potentials [8–10]. The interaction potential of hydrogen and helium has been extensively studied by Silvera [11]. The Lennard-Jones wells for the weakly interacting He-He, He-H2 and H2 -H2 pairs are 10.8, 13.34 and 34.3 K, respectively. According to this study, H2 molecules may have a bound state with He atoms, reside in liquid He surface states and penetrate the liquid helium. Thus, in addition to the possible presence of hydrogen molecules in the helium vapor, due to the non-negligible vapor pressure of solid hydrogen at 4.2 K, there may also exist a non-negligible amount of these hydrogen molecules "dissolved" in the liquid He phase.

If the impedance value (Z) is too large, there will be insufficient refrigeration and no liquid will accumulate in the evaporation vessel. If the impedance is too small, more liquid than required will flow, with the level rising higher at the vessel. This will not prevent the device from working but will result in higher helium consumption and a higher minimum temperature [5].

Historically, the appearance of a blockage in the capillary has been attributed to nitrogen or air impurities, for example, from [3]: "*During cooldown the refrigerator should be connected to a* 

*He gas in order to prevent N2*

*fill capillary. Sometimes problems arise because impurities in the main liquid helium bath (e.g. frozen air) block the fine capillary used for the impedance. One therefore has to put a filter in front of the capil-*

During an initial cooldown of a cryostat, if the liquid helium transfer is not carefully carried out (e.g., forgetting to purge the Dewar with helium gas prior to transferring liquid helium), any residual air inside the cryostat can enter, freeze and block the impedance during the precool process. But, if the system has been cooled very carefully with high-purity liquid helium, and, the correct flow through the impedance has been verified, there is only a substance capable to pass through the filter and to block the impedance. This is molecular hydrogen, as we

Other authors [4] recommended the impedance construction: "*Problems with plugged capillaries sometimes occurred when the impedance was increased using a fine wire, hence, longer capillaries without wires are favored. The filters, which were necessary to prevent plugging of the impedance by frozen air or other particulate matter, were disks of sintered copper felt compression fitted at both ends* 

As we see, the impedance geometry can affect the time necessary to produce the solid that blocks the impedance, but if the helium bath contains molecular hydrogen traces, sooner or

With a boiling point of 4.2 K at 100 KPa, liquid helium is the coldest fluid that exists in nature. Below its critical temperature (Tc = 5.2 K), any unwanted substance present in the liquid phase, that is, any impurity, will be in solid form, resulting in mist, snow, suspensions or particulates [6]. The vapor pressure of these solid impurities will be, in general, negligibly small (<<10−9 Pa), except for the case of the hydrogen isotopes and their molecular combinations [7] for which this is of the order of 10−2 Pa and 10−5 Pa, at 5.2 and 4.2 K, respectively. The solid impurities are usually charged and can be easily eliminated by electrostatic precipitation using Petryanov filters to obtain "optically clean" liquid, as demonstrated by Abrikosova and Shal'nikov [7]. But, even "optically clean" filtered liquid helium may contain a relevant

quantity of non-solid hydrogen, that is, molecular hydrogen traces.

 *or air from entering and blocking the* 

*volume with pressurized very pure 4*

demonstrate in the following section.

*He clean.*"

*lary and keep the main 4*

70 Superfluids and Superconductors

*of the capillary.*".

later, the problem will occur.

**3.1. Liquid helium purity**

**3. Flow impedance blocking issue**

In general, liquid helium in research laboratories is either delivered by a distributor of specialty gases or produced by liquefaction of both commercial grade and recovered gas. Liquid helium is subsequently stored and transferred to the application's cryostat requiring cryogenic cooling at atmospheric pressure and temperatures around 4.2 K. Since the triple point of H2 is at 13.84 K and 7.04 kPa, the equilibrium vapor pressure of solid H2 at those temperatures (≈ 4.2 K) is very small, of the order of ≈ 10−5 Pa. Therefore, if there is enough H2 in the He gas being liquefied to produce a partial pressure higher than the equilibrium vapor pressure at 4.2 K, the H2 molecules will directly nucleate into solid clusters. At atmospheric pressure (105 Pa), those solid clusters will be in equilibrium with a H2 molar fraction in the vapor phase of the order 10−10 (yH2 = (10−5 Pa/105 Pa) = 10−10).

Even though there are no experimental reports about solubility of H2 in liquid helium, theoretical calculations from classical solubility theory [12] indicate that the limiting solubility of solid hydrogen in liquid helium at 4.2 K would yield to molar fractions in the liquid phase, *xH*<sup>2</sup> , of the order of ≈10−10, that is, the same order of magnitude than the H2 molar fraction in the vapor phase, *yH*<sup>2</sup> .

Furthermore, the solid hydrogen vapor pressure and the theoretical limiting solubility of solid hydrogen in liquid helium decrease exponentially with temperature, both becoming very small (≈10−9 Pa and ≈10−14, respectively) below 3 K. Thus, the maximum concentration of H2 molecules present in liquid helium will be determined by the exact temperature and pressure conditions of the helium bath. For this chapter, the H2 molar fractions in the vapor, *yH*<sup>2</sup> , and in the liquid, *xH*<sup>2</sup> , below 3 K, both being of the order of ≈10−14, will be considered negligible. Furthermore, at temperatures near or below 1 K, hydrogen may be regarded as being totally insoluble in He [12].

Thus, unless H2 impurities are completely eliminated prior to He liquefaction, that is, its molar fraction is reduced from its typical values in the range *yH*<sup>2</sup> = 10−6–10−5 down to ≈ 10−14, the liquid He, as produced, will have traces of H2 , up to a maximum concentration level determined by the temperature (e.g., *xH*<sup>2</sup> ≈10−10 at 4.2 K). If the temperature of liquid helium is further reduced, as it is the case in small capillary impedances, for attaining very low temperatures, T < 3 K, the excess H2 will condense and accumulate at the impedance low-pressure side and, after some time, it will produce a total impedance blockage.

Many applications requiring liquid helium cooling are not sensitive to contaminants of any kind and consequently, do not require special provisions for helium cleanness and precautions to avoid contamination during liquid helium refills. On the other hand, there are a considerable number of low-temperature applications that require achieving temperatures below 4 K [5], which are very sensitive to impurities present in the liquid and, therefore, those applications need extreme pure liquid helium for proper operation [13].

**Figure 2** illustrates molecular H2

regarding impedance blockage.

limiting solubility of H2

100 K [16, 17].

10−10) of H2

H2

lary is reduced below 3 K by evaporation cooling, the H2

similar to the vapor pressure of solid hydrogen at 3 K, *xH*<sup>2</sup>

**Figure 2.** Schematic description of low-temperature impedance blockage by molecular H2

, present in the liquid helium bath, flowing through a sub-

vapor pressure, as well as the

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

=0.0075 ppt (7.5∙10−15). This is

present in liquid He.

<10−14). Therefore, all the

"Clean" Liquid Helium

73

=0.35 ppb (3.5∙

molecules

micron-sized metallic-sintered filter (e.g., 500 nm as average pore size) placed to stop solid impurities entering the fine capillary impedance tube. When the temperature in the capil-

in helium, becomes negligibly small (*xH*<sup>2</sup>

As an example, a typical two-phase He flow of only 1 sL/min, having *xH*<sup>2</sup>

duce the same effect when pumping helium with a lower concentration of H2

 present in the liquid helium heterogeneously nucleates along the walls of the impedance tube. A similar mechanism in a completely different working fluid and temperature range, for the freezing of water molecule impurities in nitrogen gas, has been proposed to explain blocking in micromachined Joule-Thomson coolers operating approximately at

uid helium under typical laboratory conditions (4.2 K and 100 kPa)], pumped through a cylindrical tube impedance of 66-μm effective diameter [e.g., the low temperature impedance of a Quantum Design, Physical Properties Measurement System (PPMS)] [18], may produce a solid hydrogen cylinder block of 66-μm diameter that, in about 24 h, will have 132 μm of height. The exact time for the blocking to occur will depend on the exact solid hydrogen distribution in the impedance. Instead, several years would be necessary to pro-

the reason why we consider the vapor pressure of solid hydrogen at 3 K negligibly small

molecules [i.e., corresponding to the vapor pressure of solid hydrogen in liq-
