Ultrahigh Vacuum Applications Research Articles

Utilizing frustration in Gd- and Yb-based oxides for milli-Kelvin adiabatic demagnetization refrigeration

Abstract The manifold of energetically degenerate configurations arising from competing interactions in frustrated magnets gives rise to an enhanced entropy at lowest temperatures, which can be utilized for adiabatic demagnetization refrigeration (ADR). We review structural and magnetic properties of various Yb- and Gd-based oxides featuring frustration related to different triangular moment configurations and (for some cases) structural randomness. In comparison to paramagnetic hydrated salts, which have traditionally been employed for mK-ADR, these novel ADR materials enable cooling to temperatures several times lower than the magnetic interaction strength, significantly enhancing the entropy density and cooling power at a given target temperature. A further advantage is their chemical stability, allowing for a much simpler ADR pill design and ultra-high vacuum applications. For the temperature range between 0.02 and 2~K, a systematic comparison of the field-induced entropy density change is provided, that illustrates the advantages of frustrated magnets for low-temperature ADR.

Design and performance of indium seals for size-constrained tunable laser spectrometers.

Indium seals have been used extensively in ultra-high vacuum and cryogenic applications. Typically, these seals use indium alongside or in place of other metal gaskets in stainless-steel vacuum flanges, with some custom applications for flanges sealing directly with glass (optics or tubes). Here, we present the design and performance of three pressed indium seals (99.99% In) between aluminum and 0.5 in. diameter sapphire optics and aluminum and gold coated Kovar semiconductor packages (TO-66 and TO-39). Test fixtures were designed to mimic those of future tunable diode laser spectrometers for Earth, planetary, and manned spaceflight environmental monitoring applications. Successful high-hermeticity seals [<10-10 atm cc/s (He)] were achieved for all seals formed with sufficient pressure applied to allow indium to flow between mating surfaces. The hermeticity of the seals was maintained after temperature cycling (-10 to +80 °C, 20 cycles), with the optical seals surviving extended duration tests (-55 to +85 °C, per MIL-STD-883). Semiconductor packages (TO-39) subjected to these extended tests saw a moderate increase in leak rate [∼5 × 10-9 atm cc/s (He)]; however, further testing showed that either the glass-metal package seals or the indium were affected (the sample size was too small to draw firm conclusions for future applications). Overall, these results suggest long-term survivability of indium seals for Kovar-aluminum and sapphire-aluminum interfaces [>10 years at 10-10 atm cc/s (He)], where the coefficient of thermal expansion differs by approximately four times.

Molecular dynamics simulation to investigate titanium metallization on sapphire for fiber optic-based pressure sensing application

Molecular dynamics simulation to investigate titanium metallization on sapphire for fiber optic-based pressure sensing application

Systematic investigation of wear-induced cold welding in ultrahigh vacuum piezoelectric motors with non-metallic coatings.

Piezoelectric motors are widely used in various applications where both precision positioning and miniaturization are required. Inertial or quasi-static motors are commonly employed because of their high accuracy, which demands consistent sliding friction between moving sliders and their static counterparts for reliable operation. In general, slider wear is unavoidable after long-term use. This wear can often lead to more serious cold welding in vacuum, which is also referred to as friction welding induced by direct contact between similar metal surfaces. Non-metallic coatings can prevent such unwanted cold welding in ultrahigh vacuum (UHV) applications. However, the practical reliability of available coatings under UHV conditions still remains to be elucidated. Here, we systematically investigate the practical reliability of commonly used, UHV-compatible lubricant coatings for piezoelectric motors in vacuum. We demonstrate that polytetrafluoroethylene (PTFE) shows the most reliable long-term operation in vacuum, while other coatings eventually lead to wear-induced cold welding and motor failure. Our findings provide a simple and effective method to improve the long-term performance of UHV piezoelectric motors by coating the slider surface with PTFE.

Transient liquid phase bonding technique for ultrahigh-vacuum compatible bimorph mirrors used in soft x-ray beamlines.

Bimorph mirrors place stringent requirements on the welding technology of silicon substrates and piezoelectric ceramics to ensure their ultrahigh-vacuum compatibility. Conventional welding techniques usually require high temperature and pressure, which have a high impact on the welding substrate, while the use of organic adhesives for bonding does not guarantee their stability in ultrahigh vacuum. Here, the transient liquid phase bonding technology based on an Au-In metal system was studied to meet the requirement for ultrahigh-vacuum application. The microstructure, chemical composition, and related mechanical properties of the bonding at different welding conditions were investigated. Meanwhile, the piezo ceramics and the bond were baked at 150 °C to test the stability. The results show that a stable bonding was achieved between centimeter-scaled single crystal silicon and lead zirconate titanate ceramics at 200 °C temperature and 2.5 MPa pressure, and the piezo ceramics and the bond are not damaged by baking to 150 °C for 48 h.

A multi-technique tomography-based approach for non-invasive characterization of additive manufacturing components in view of vacuum/UHV applications: preliminary results

In this paper, we have studied an additively manufactured metallic component, intended for ultra-high vacuum application, the exit-snout of the MACHINA transportable proton accelerator beam-line. Metal additive manufacturing components can exhibit heterogeneous and anisotropic microstructures. Two non-destructive imaging techniques, X-ray computed tomography and Neutron Tomography, were employed to examine its microstructure. They unveiled the presence of porosity and channels, the size and composition of grains and intergranular precipitates, and the general behavior of the spatial distribution of the solidification lines. While X-ray computed tomography evidenced qualitative details about the surface roughness and internal defects, neutron tomography showed excellent ability in imaging the spatial density distribution within the component. The anisotropy of the density was attributed to the material building orientation during the 3D printing process. Density variations suggest the possibility of defect pathways, which could affect high vacuum performances. In addition, these results highlight the importance of considering building orientation in the design for additive manufacturing for UHV applications.Graphical

Systematic study of the outgassing behavior of different ceramic materials

The outgassing rates of various ceramic materials were systematically investigated. This paper intercompares the most relevant ceramic material classes used in vacuum technology, namely, oxide (five different types investigated) and non-oxide based ceramics, including nitrides (four different types) and carbides (three different types). For this purpose, the Outgassing Measurement Apparatus, which uses the difference method, was applied. Besides time, also, temperature dependence of outgassing was studied to check whether a previous heat treatment of the ceramics is necessary. Additionally, the outgassing species were qualitatively determined by a quadrupole mass spectrometer. In total, relatively large differences were found between the investigated ceramic materials for their initial outgassing rates as well as for their behavior at elevated temperatures. All investigated types of carbide ceramics showed very low outgassing rates of less than 3 × 10−8 Pa m3/(s m2) after 100 h at room temperature, whereas rather high outgassing rates of about 10−5 Pa m3/(s m2) were found for magnesia as well as the tested nitride ceramic types. Thus, depending on the ceramic material, a heat treatment is strongly recommended before use in ultrahigh vacuum applications.

Outgassing rate comparison of seven geometrically similar vacuum chambers of different materials and heat treatments

We have measured the water and hydrogen outgassing rates of seven vacuum chambers of identical geometry but constructed of different materials and heat treatments. Chambers of five different materials were tested: 304L, 316L, and 316LN stainless steels; titanium (ASTM grade 2); and 6061 aluminum. In addition, chambers constructed of 316L and 316LN stainless steel were subjected to a vacuum-fire process, where they were heated to approximately 950 °C for 24 h while under vacuum. These latter two chambers are designated as 316L-XHV and 316LN-XHV. Because all the chambers were of identical geometry and made by the same manufacturer, a relative comparison of the outgassing rates among these chambers can be made. Water outgassing rates were measured as a function of time using the throughput technique. The water outgassing results for 316L, 316LN, 316L-XHV, and 316LN-XHV were all similar but lower than those for 304L by a factor of 3–5 lower at 104 s. The water outgassing results for the Ti and Al chambers were closer to that of 304L, Ti being slightly lower. Hydrogen outgassing rates were measured using the rate-of-rise method and performed after a low-temperature bake of 125–150 °C for a minimum of 72 h. The Ti, Al, 316L-XHV, and 316LN-XHV chambers all have ultralow specific outgassing rates below 1 × 10−11 Pa l s−1 cm−2 and are a factor of 100 or better than the 304L chamber. The 304L, 316L, and 316LN chambers with no vacuum-fire heat treatment have larger hydrogen outgassing rates than the other chambers, with determined specific outgassing rates ranging between 4.0 and 8.0 × 10−11 Pa l s−1 cm−2. We conclude that Ti, Al, 316L-XHV, and 316LN-XHV have hydrogen outgassing rates that make them excellent choices for ultrahigh vacuum and extreme-high vacuum applications, the choice depending on cost and other material properties.

Additively manufactured ultra-high vacuum chamber for portable quantum technologies

Additive manufacturing is having a dramatic impact on research and industry across multiple sectors, but the production of additively manufactured systems for ultra-high vacuum applications has so far proved elusive and widely been considered impossible. We demonstrate the first additively manufactured vacuum chamber operating at a pressure below 10−10 mbar, measured via an ion pump current reading, and show that the corresponding upper limit on the total gas output of the additively manufactured material is 3.6 × 10−13 mbar l/(s mm2). The chamber is produced from AlSi10Mg by laser powder bed fusion. Detailed surface analysis reveals that an oxidised, Mg-rich surface layer forms on the additively manufactured material and plays a key role in enabling vacuum compatibility. Our results not only enable lightweight, compact versions of existing systems, but also facilitate rapid prototyping and unlock hitherto inaccessible options in experimental science by removing the constraints that traditional manufacturing considerations impose on component design. This is particularly relevant to the burgeoning field of portable quantum sensors — a point that we illustrate by using the chamber to create a magneto-optical trap for cold 85Rb atoms — and will impact significantly on all application areas of high and ultra-high vacuum.

Hydrogen outgassing and permeation in stainless steel and its reduction for UHV applications

Stainless steel, grade 304L and 316L are widely used structural steel in most of the Ultra High Vacuum (UHV) systems in present-day research. Large size vacuum chambers and beam lines are used in much scientific applications, few examples are CERN accelerators, ITER, LIGO, Neutrino observatory, FAIR, Space research, etc. The selection and operation of the large size vacuum pumping system and the material to be used for UHV application is one of the crucial tasks for the vacuum system engineers as it relates the capital and operational cost linked to it. Any failure in operation has a direct impact on the project execution timeline and the huge refurbishment cost. Hydrogen in structural steel and its reduction for ultra-high vacuum systems is one of the major challenges for the present day. Therefore, prior understanding is required for knowing hydrogen content and its permeation and outgassing behaviour in vacuum system establishment and operation. Based on the available literature, a theoretical model for estimating permeation and outgassing rate is discussed. Relating the permeation and outgassing, the theoretical model contains first and second-order Fick's law and their solutions. Permeation and outgassing rate of 3 mm thick stainless steel is calculated for different thermal bake-out conditions and are discussed. A brief discussion (after going through various references) is mentioned for lowering the outgassing rate for hydrogen in steel.

Versatile optical fiber feedthroughs for ultra-high vacuum applications

In this article we present an investigation of three different fiber optic vacuum feedthroughs. Using Swagelok-type tube fittings, metal tubings containing the optical fibers are sealed into a vacuum flange. This allows for easy replacement of the feedthrough without replacing the entire flange. Employing only epoxy resin for sealing the optical fiber into the feedthough we have measured a helium diffusion rate of 2.5⋅10−10 mbar ⋅ l/s, whereas with a combination of a solderglass and epoxy resin seal we have obtained a diffusion rate below 1⋅10−12 mbar ⋅ l/s. In a third approach, using small tolerance fiber optic ferrules in our feedthroughs, we have obtained helium diffusion rates of below 1⋅10−12 mbar ⋅ l/s, making these feedthroughs well suited for ultra-high vacuum applications.

Simple O-ring sealed Brewster-angle window for ultrahigh vacuum applications

Simple O-ring sealed Brewster-angle window for ultrahigh vacuum applications

Gas tungsten arc welding of copper to stainless steel for ultra-high vacuum applications

Abstract The GTAW method was employed for the dissimilar joining of OFHC copper and AISI 304 stainless steel. The special weld joint design of a modified socked-weld flange with edge joints made on short-flanged edges was used. The butt joint of the flange edges was designed to have different height and thickness. The welding was done without filler metal using a DC current of 80 A with pre-heating of the joint at 200 °C. The cross-sectional image of the specimen clearly indicated the fusion zone as a semicircular-shaped bead with the molten region about 0.7 mm from the top. The micro XRF mapping image revealed the heterogeneous elemental distributions of Cu, Fe, Cr, Ni and Mn in the fusion zone. The highest Mn concentration appeared in the Cu-rich area. At the interface, SEM images show the formation of small granules of Fe on the Cu side while the dendrites form near stainless steel. The structures of Fe and Cu were not changed during the welding process as determined by the XAS.

Particle radiation effects on shape memory alloy couplers for ultra-high vacuum sealing: a preliminary study

A new shape memory alloy (SMA)-based coupling system for ultra-high vacuum (UHV) applications in particle in accelerators is currently under investigation at the European organization for nuclear research (CERN). The use of such technology in some restricted-access radioactive areas within CERN accelerators could result in noticeable advantages, especially during maintenance operations. Bolt-free SMA couplers, can be activated remotely by temperature changes, resulting in significant reduction of the radiation doses collected by the technical personnel. The functional performance of SMA-based prototype systems, in terms of leak tightness, thermal mounting/dismounting and thermal outgassing properties, has been already verified. Radiation-induced microstructural damages usually cause losses in the mechanical properties, such as embrittlement in conventional engineering metals. The particle radiation effects on the functional characteristics of SMAs, in terms of microstructural transition mechanisms, represent a key issue for their application in critical accelerators areas. To this aim, specific research activities are being carried out at CERN for capturing the evolution of SMA mechanical and functional properties during and/or after particle irradiation. In this preliminary study, a few selected SMA-based prototype UHV chambers, have been exposed to a high-energy mixed particle field (up to ∼140 kGy of absorbed dose) at the CERN high-energy accelerator mixed-field (CHARM) facility. The results, in terms of post-irradiation measurements, have revealed that leak tightness and thermal dismounting are unaffected by irradiation.

The Effect of Pulsed Current and Organic Additives on Hydrogen Incorporation in Electroformed Copper Used in Ultrahigh Vacuum Applications

The presence of hydrogen in electroformed copper from two different acidic copper sulfate solutions was evaluated: an additive-free solution and a solution including a sugar. D-xylose addition is found to inhibit H incorporation and allows the use of higher cathodic pulses before the copper diffusion limited range starts. TDS experiments show that hydrogen is trapped in the copper samples in two different forms. Hydrogen diffused from copper vacancies was found on all samples at an outgassing temperature of around 450°C. For samples with long pulse times, an additional H2 outgassing peak was found at around 600°C. XRD measurements allowed us to determine the preferential orientation of the plated samples and to monitor lattice parameter evolution with increasing temperature.

Vapor-Phase Cleaning and Corrosion Inhibition of Copper Films by Ethanol and Heterocyclic Amines.

Cleaning and passivation of metal surfaces are necessary steps for selective film deposition processes that are attractive for some microelectronic applications (e.g., fully aligned vias or self-aligned contacts). For copper, there is limited knowledge about the mechanisms of the copper oxide reduction process and subsequent passivation layer formation reactions. We have investigated the in situ cleaning (i.e., oxidation and reduction by vapor-phase species) and passivation of chemical-mechanical polishing (CMP)-prepared Cu films in an effort to derive the mechanisms associated with selectively tailoring the surface chemistry. By monitoring the interaction of vapor-phase ethanol with the surface species generated after ozone cleaning at 300 °C, we find that the optimum procedure to remove these species and avoid byproduct redeposition is to use atomic layer deposition (ALD)-like binary cycles of ethanol and N2, with active pumping. We have further explored passivation of clean Cu using benzotriazole and 2,2'-bipyridine in an ALD environment. Both molecules chemisorb on clean Cu in an upright orientation, with respect to the metal surface at temperatures higher than the melting point of the organic inhibitors (100 ≤ T < 300 °C). Both molecules desorb without decomposition from clean Cu above 300 °C but not from Cu2O. Previous studies related to the passivation of Cu surfaces using heterocyclic amines have focused on solution-based or ultrahigh vacuum applications of the passivation molecules onto single crystalline Cu samples. The present work explores more industrially relevant vapor-phase passivation of CMP-cleaned, electroplated Cu samples using ALD-like processing conditions and in situ vapor-phase cleaning.

Chemical and Optical Identification of Micrometer-Sized 1.9 Billion-Year-Old Fossils by Combining a Miniature Laser Ablation Ionization Mass Spectrometry System with an Optical Microscope.

The recognition of biosignatures on planetary bodies requires the analysis of the putative microfossil with a set of complementary analytical techniques. This includes localized elemental and isotopic analysis of both, the putative microfossil and its surrounding host matrix. If the analysis can be performed with spatial resolution at the micrometer level and ppm detection sensitivities, valuable information on the (bio)chemical and physical processes that influenced the sample material can be gained. Our miniaturized laser ablation ionization mass spectrometry (LIMS)-time-of-flight mass spectrometer instrument is a valid candidate for performing the required chemical analysis in situ. However, up until now it was limited by the spatial accuracy of the sampling. In this contribution, we introduce a newly developed microscope system with micrometer accuracy for Ultra High Vacuum application, which allows a significant increase in the measurement capabilities of our miniature LIMS system. The new enhancement allows identification and efficient and accurate sampling of features of micrometer-sized fossils in a host matrix. The performance of our system is demonstrated by the identification and chemical analysis of signatures of micrometer-sized fossil structures in the 1.9 billion-year-old Gunflint chert.

Study of the suitability of 3D printing for Ultra-High Vacuum applications

In the recent year additive manufacturing (3D printing) has revolutionized mechanical engineering by allowing the quick production of mechanical components with complex shapes. So far most of these components are made in plastic and therefore can not be used in accelerator beam pipes. We have investigated samples printed using a metal 3D printer to study their behavior under vacuum. We report on our first tests showing that such samples are vacuum compatible and comparing pumping time.

Shape-memory alloy rings as tight couplers between ultrahigh-vacuum pipes: Design and experimental assessment

The ultrahigh-vacuum (UHV) coupling performance of NiTiNb shape memory alloy (SMA) rings was investigated by finite element (FE) simulations and experimental measurements. The contact pressure at the interface between the SMA ring and the vacuum pipe was measured and good agreements with the FE results were obtained. Furthermore, a design method was proposed that involves the FE results and a vacuum sealing model. Finally, leak rate measurements were carried out to assess the sealing performance of selected coupling configurations. The main result found was that the leak rate constraints for UHV applications could be easily satisfied (leak rate &amp;lt;10−10 mbar l s−1), which opens the possibility of remotely clamping/unclamping the tight couplers by well-defined changes in their temperature. Possible applications in the field of particle accelerators and, in particular, in restricted access areas were considered.

Measurement of Outgassing Rates of Steels.

Steels are commonly used materials in the fabrication of vacuum systems because of their good mechanical, corrosion, and vacuum properties. A variety of steels meet the criterion of low outgassing required for high or ultrahigh vacuum applications. However, a given material can present different outgassing rates depending on its manufacturing process or the various pretreatment processes involved during the fabrication. Thus, the measurement of outgassing rates is highly desirable for a specific vacuum application. For this reason, the rate-of-pressure rise (RoR) method is often used to measure the outgassing of hydrogen after bakeout. In this article, a detailed description of the design and execution of the experimental protocol involved in the RoR method is provided. The RoR method uses a spinning rotor gauge to minimize errors that stem from outgassing or the pumping action of a vacuum gauge. The outgassing rates of two ordinary steels (stainless steel and mild steel) were measured. The measurements were made before and after the heat pretreatment of the steels. The heat pretreatment of steels was performed to reduce the outgassing. Extremely low rates of outgassing (on the order of 10-11 Pa m3 sec-1 m-2) can be routinely measured using relatively small samples.