Sorption properties of novel-fashioned low-cost hydrogen getters in a high-vacuum-multilayer insulation structure
Introduction
Getters are a vital tool for maintaining high vacuum in a high-vacuum-multilayer-insulation structure (HVMLIS) [1], vacuum tubes, fluorescent lamps, particle accelerators and other high vacuum equipment [2]. Electronic industry practitioners generally use non-evaporable getters (NEGs) to maintain ultra-high vacuum in sealed cavities [3]. These NEGs are Zr-based, which have previously been applied to the inner wall of the external container of HVMLIS [4]. However, these getters generally require to be activated [5,6]. If high-storage HVMLIS requires a large area to be coated to attain the desired sorption capacity, then this large area must be subjected to a large amount of heat. The activation temperature may exceed the temperature that some accessories of the cryogenic tank can withstand. Hence, to substantially reduce the activation temperature of NEG, Miyazawa et al. developed a new type of NEG (oxygen-free Pd/Ti) [7], which is activated at 133 °C. However, this NEG required degassing during installation. Moreover, mixing the getter with Pd is expensive [8]. Otherwise, if NEG pills are used in tanks containing flammable gases, which may cause safety concerns. This work is mainly aimed at developing and optimising H2 getters that are inexpensive and easy to utilise in HVMLIS; in addition, this work provides some references for the application of these getters in other industrial products (e.g., vacuum tubes, heavy ion accelerator).
HVMLIS is mainly divided into the following compartments: the inner container, the vacuum jacket, and the outer shell. The vacuum jacket contains multilayer perforated insulation materials (MLPIMs) and high vacuum. MLPIMs are largely composed of Al foils and glass fibre papers in a 1:1 ratio in which the effect of the Al foils minimises the radiation between the internal and external containers while the fibre papers weaken the heat conduction between the reflective shields. Furthermore, the high vacuum can prevent the heat conduction and convection heat transfer of gas molecules. Moreover, high vacuum acts as the only factor that affects the insulation life of equipment after the equipment leaves the factory. Notably, owing to the air leakage from the shell of the equipment and the outgassing of the manufactured materials, the production of the original high vacuum is gradually lost, which considerably shortens the service life of HVMLIS. According to previous studies, the pressure in the vacuum jacket was mainly increased by the gases emitted from the Al foils, the metallic materials and the glass fibre-reinforced plastic support structures of HVMLIS [1,9,10]. Additionally, Ninety-nine percent of the gases, which were emitted from the stainless steel, had H2 solved in it; this H2 constituted over 70% of the total residual gases [11]. Furthermore, the thermal conductivity (K) of H2 was larger than those of the other gases. Apart from H2, water vapour was the main gas emitted from the glass fibre papers. This water vapour can be effectively adsorbed by 5A zeolite or activated carbon at low temperature [12].
The removal of H2 at low temperature and pressure is a significant challenge in the industry and has hence become the focus of recent research. Philippe et al. [13] combined MnO2 and Ag2O as the getter for the sorption of H2 produced by nuclear waste. That study revealed that MnO2 effectively absorbed H2 under nuclear radiation, although the absorptivity of MnO2 was low at low pressure. Belousov et al. [14] tested the H2 sorption performances of MnO2, WO3, Co3O4, Ag2O and other metal oxides at low temperatures and pressures, thereby exploring the influences of these metal oxides, which were mixed with different amounts of Pd during sorption. The results indicated that adding 0.5% Pd could increase the H2 sorption capacities of Co3O4, WO3, MnO2 and NiO by 15–100 times. Londer et al. [15] developed a special sorption device that could be applied to liquid hydrogen (LH2) storage tanks. The inner surface of the device could be coated with more than 1000 Ba films, which were utilised to adsorb residual gases (e.g., H2, N2 and O2) in the vacuum, to produce BaH2, BaN2 and BaO2, respectively, although the adsorptivity was relatively limited. Wang et al. [16,17] studied the application of a CuO composite-based H2 getter in HVMLIS. The result indicated that CuO is an inexpensive H2 getter with good performance, which required a certain temperature to react with H2. Chen et al. [18,19] tested the H2 absorptivities of different proportions of PdO and Ag2O in vacuum and concluded that they exhibited the best H2 sorption performance in a 7:3 ratio [20,21]. However, PdO is relatively expensive and cannot be applied to LH2 storage tanks. Once LH2 leaks out, PdO initiates a strong redox reaction with LH2, thereby generating sparks. Only few studies have investigated H2 getters, such as Ag-Z getter (Ag–Z), a silver molecular sieve (SMS) and an active material getter (AMG), which are inexpensive, exhibit stable performance and are remarkably safe. In this study, the absorptivities of the three H2 getters were compared with that of traditional PdO, thus presenting these inexpensive and excellent H2 getters as substitutes to the relatively expensive PdO. In addition, the sorption capacities (Qn), saturation sorption capacities (Qs) and sorption coefficients (S) of the three H2 getters were calculated using the sorption equation. Finally, the microstructures of the several getters were compared before and after H2 absorption, thus providing a basis for subsequent optimisation of the four getters to obtain improved H2 getters.
Section snippets
Experimental platform
Fig. 1 shows that the experimental system mainly afforded a vacuum similar to the engineering of getters. A buffer system was used to facilitate transition when introducing H2 into the vacuum jacket to prevent the excessive introduction of H2. A pressure-measurement system was used to constantly monitor the change in the pressure of the vacuum jacket and the pressure changes before and after the introduction of H2 to the buffer system. The pump system afforded high vacuum for the vacuum jacket
Influence of the vacuum degree on the thermal insulation performance
The thermal insulation performance of the experimental equipment, which was wrapped with 30 MLPIMs, was tested at a pressure of ≤10−3 Pa (the experimental vessel stood for 24 h prior to measurement and was filled with LN2). Fig. 4 shows that the static evaporation rate was 0.22 L/min; the both peaks were produced by accelerating the evaporation when filling the inner container with LN2. After the experimental equipment was restored to thermal equilibrium, the evaporation rate was stabilised to
Conclusion
The goal of this study, which was the comparison of three getters with traditional PdO, was achieved, and their S and Qs values were obtained. The results indicated that the inexpensiveness and safety of SMS and Ag–Z in the ratios of 2:1 and 3:1 makes them adequate substitutes for PdO, respectively, to save the application cost of getters in HVMLIS by >70%. Regarding AMG, performance improvement will be comprehensively studied in subsequent works. Furthermore, the two new substances (Pd1.5H2
Funding
This work was financially supported by the National Key R&D (Grant numbers: 2017YFC0805601).
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the
References (30)
Gas problem and gettering in sealed-off vacuum devices
Vacuum
(1996)- et al.
Getters for vacuum insulated glazing
Vacuum
(2018) - et al.
Ultimate pressures achieved in TiZrV sputter-coated vacuum chambers
Vacuum
(2001) - et al.
Decreasing surface outgassing by thin film getter coatings
Vacuum
(1998) - et al.
Gas desorption from a stainless-steel surface in ultrahigh vacuum devices
Vacuum
(2003) - et al.
A new costly effective composite getter for application in high vacuum multilayer insulation tank
Vacuum
(2016) - et al.
Experimental investigation and theoretical analysis on measurement of H2 adsorption in vacuum system
Int. J. H2 Energy
(2010) - et al.
Surface area, pore size distribution and microstructure of vacuum getter
Vacuum
(2011) - et al.
Experimental investigation on H2 adsorption performance of composite adsorbent in the tank with high vacuum multilayer insulation
Vacuum
(2009) - et al.
Optimization and performance of highly efficient H2 getter applied in high vacuum multilayer insulation cryogenic tank
Vacuum
(2018)
Research progress of getter in cryogenic container
Chin. J. Vac. Sci. Technol
Cropper single metal zirconium non-evaporable getter coating
Vacuum
H2 incorporation and release from nonevaporable getter coatings based on oxygen-free Pd/Ti thin films
J. Vac. Sci. Technol.
Oxygen-free palladium/titanium coating, a novel nonevaporable getter coating with an activation temperature of 133 °C
J. Vac. Sci. Technol.
Adsorption on carbon-filled paper in high-vacuum multi-layered thermal insulation at liquid nitrogen temperature, Chin
J. Vac. Sci. Technol.
Cited by (10)
Study on unsteady evacuation characteristics of multi-layer insulation during launch/in orbit of the cryogenic propellant tanks
2024, International Journal of Thermal SciencesExperimental study on the sorption performance of New–Fashioned silver getters compared with the traditional PdO in High–Vacuum multi–layer insulation vessels
2023, VacuumCitation Excerpt :6–8 was marked as the slow sorption stage, in which the Qn only increased by 31.4%, whereas the Pes increased by a factor of 192.3. There are three reasons for this:(1) the sorption of PdO was about to reach its saturation sorption capacity; (2) Pd was generated after PdO sorbed H2, and then Pd sorbed H2 to produce PdxHy [26], which led to the decline in overall sorption performance; (3) The Pd have a large cohesive force, which caused the aggregation of Pd and hindered the further reaction of PdO, Pd and H2, thus leading to the reduction of H2 sorption performance. Finally, compared with the BDDT model, it is concluded that the sorption model of PdO was consistent with that of the Langmuir model.