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Performance Study of NbTi Alloy Low-Temperature Vacuum Sealing Components, Comparison with Rare Metals such as TA2, Tantalum, Molybdenum, C103, and Nb₃Sn
Ultra-high vacuum low-temperature superconducting equipment, low-temperature vacuum chambers, sealing flanges for superconducting devices, and support structures for irregularly shaped sealing gaskets need to operate for extended periods in high vacuum, ultra-low temperature (4.2K), and temperature-changing environments. This necessitates materials with ultra-low outgassing rates, no deformation at low temperatures, high sealing precision, resistance to vacuum irradiation, and no magnetic interference. Traditional rubber and plastic seals harden and fail at low temperatures; stainless steel and titanium alloy seals have high outgassing rates and large low-temperature deformation; rare metals such as molybdenum and tungsten are brittle and have poor sealing fit; Nb₃Sn, pure tantalum, and C103 each have functional shortcomings and cost drawbacks. NbTi niobium-titanium alloy, with its extremely low vacuum outgassing characteristics, excellent low-temperature plasticity, and stable superconducting properties, has become the exclusive sealing structure material for low-temperature ultra-high vacuum superconducting equipment. In the field of precision vacuum sealing, its performance is comprehensively superior to various conventional metals and rare refractory metals.
Conventional metal vacuum sealing components have significant shortcomings in low-temperature service. 316L stainless steel has a high vacuum outgassing rate, and in long-term high-vacuum environments, trace amounts of gas will continuously be released, compromising the vacuum level of the cavity. Furthermore, its large low-temperature deformation makes it prone to gap leaks at the sealing surface. TA2 pure titanium and Ti6Al4V titanium alloy have lower outgassing rates, but their low-temperature rigidity is relatively high, resulting in insufficient sealing toughness. After repeated temperature changes, the sealing accuracy significantly decreases, and they lack superconducting capabilities. Copper and aluminum, being soft metals, offer good sealing fit, but their extremely low low-temperature strength makes them prone to plastic collapse and poor vacuum stability. Ordinary metal sealing structures generally suffer from problems such as leakage, deformation, and accuracy degradation, failing to meet the requirements of long-term ultra-high vacuum operation in superconducting equipment.
This paper repeatedly compares the low-temperature vacuum sealing adaptability of TA2 pure titanium, high-purity tantalum, molybdenum, C103 niobium alloy, and Nb₃Sn. TA2 and Ti6Al4V offer balanced overall performance, but their vacuum outgassing rate is higher than NbTi, resulting in insufficient long-term sealing performance. Pure tantalum boasts excellent vacuum stability, extremely low outgassing rate, and good low-temperature toughness, but it is expensive and lacks superconductivity, limiting its use to pure sealing structures and preventing functional integration. Molybdenum and tungsten exhibit excellent ultra-high vacuum performance, but their extremely high low-temperature brittleness makes them prone to cracking and deformation during flange tightening, rendering them unsuitable for precision sealing structures. C103 niobium alloy offers low-temperature dimensional stability, but its high outgassing rate and insufficient vacuum cleanliness make it unsuitable for precision sealing. Nb₃Sn exhibits strong superconductivity, but its excessive brittleness prevents the machining of precise sealing surfaces, resulting in zero sealing reliability. Considering all rare metals, NbTi is the only vacuum sealing material that combines ultra-low outgassing, low-temperature plasticity, precision forming, and superconductivity.
Within the niobium-titanium alloy system, standard Nb-47Ti offers the best vacuum sealing performance. High-niobium NbTi alloys offer excellent vacuum performance but lack plasticity, resulting in poor sealing fit. High-titanium NbTi alloys exhibit superior plasticity, but suffer from increased vacuum outgassing and decreased superconductivity. Forged NbTi has coarse grains, leading to insufficient flatness of the sealing surface. Additive NbTi can precisely form micron-level sealing surfaces, far exceeding the precision of traditional processes. SLM-printed NbTi sealing components boast high density and no internal pores, eliminating gas adsorption and precipitation at the source. Under ultra-high vacuum conditions, the outgassing rate is significantly lower than that of standard titanium and tantalum alloy components, exhibiting no deformation or leakage even after repeated thermal cycling.
Currently, NbTi cryogenic vacuum sealing flanges and support gaskets are widely used in superconducting quantum devices and cryogenic vacuum testing chambers, replacing traditional titanium and tantalum sealing components. This increases the chamber vacuum maintenance time by 5 times and reduces gas leakage by 95%. The material's weakness lies in its relatively weak high-temperature vacuum performance; the outgassing rate increases significantly above 200°C, making it only suitable for cryogenic vacuum conditions. In the future, by further reducing the outgassing rate through surface passivation modification and optimizing the precision printing process, it will become the core sealing structure material for low-temperature ultra-high vacuum superconducting equipment.
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