Helium Leak Test Research Articles

Long-term evaluation of a liquid crystal polymer (LCP)-based retinal prosthesis

Objective. The aim of this study is to evaluate the long-term reliability of a recently presented liquid crystal polymer (LCP) -based retinal prosthesis in vitro as well as in vivo. Because an all-polymer implant introduces another intrinsic leak type due to gas permeation, for which the traditional helium leak test for metallic packages was not designed to quantify, a new method to investigate its durability is required. Approach. We designed and carried out a series of reliability tests specifically for all-polymer implants by quantitatively investigating moisture ingress through various pathways of the polymer surface, and the polymer–polymer and polymer–metal adhesions. Moisture permeation through the bulk material was estimated by analytic calculation, while water ingress through the adhesively sealed LCP–LCP and LCP–metal interfaces was investigated using the separate parts of an electrode array and a package in an accelerated aging condition. In vivo tests were done in rabbits to examine the long-term biocompatibility and implantation stability by fundus observation and optical coherence tomography (OCT) imaging. Main results. The analytic calculation estimated good barrier properties of the LCP. Samples of the LCP-based electrode array failed after 114 days in 87 °C saline as a result of water penetration through the LCP–metal interface. An eye-conformable LCP package survived for 87 days in an accelerated condition at 87 °C. The in vivo results confirmed that no adverse effects were observed around the retina 2.5 years after the implantation of the device. Significance. These long-term evaluation results show the potential for the chronic use of LCP-based biomedical implants to provide an alternative to traditional metallic packages.

Verification of dimensional stability on ITER blanket shield block after stress relieving

The tight tolerance requirement is one of key issue to manufacture the ITER blanket shield blocks (SBs) which have many interfaces with the First Wall (FW) and Vacuum Vessel (VV). Manufactured SB shall be satisfied with general tolerances (Class “C” of ISO 2768-1 and “L” of ISO 2768-2) and specific tolerance in 2D general assembly drawings. In order to fulfill the tight tolerance requirements in the final stage of SB, stress relieving after welding operations in the manufacturing process shall be performed. Hot helium leak test, Post Welding Heat Treatment (PWHT) and three-dimensional inspection before and after heat treatment were implemented by using the Full Scale Prototype (FSP) of SB in the framework of domestic R&D activities. The hot He leak test was performed at 250°C for 30min, and the result was satisfied the requirements. PWHT was carried out at 400°C for 24h by brazing furnace with test chamber. The deformation value before and after was measured by contact type coordinate measuring machine. The objective of this study is to verify dimensional stability of SB after stress relieving. The results will support to determine the machining allowance prior to welding process.

Gas-tight Thermally Joined Metal-thermoplastic Connections by Pulsed Laser Surface Pre-treatment

Thermal joining of laser-structured metals and thermoplastics has been proven to deliver a high joint strength. A novel idea is to apply this process onto metal-thermoplastic joints in order to produce gas-tight hybrid connections which can be used in several technical applications.The surface pre-treatment of aluminum by infrared pulsed laser radiation was focused in this study. Depending on different cumulative energies two main surface topologies, stochastic and deterministic groove structures, were manufactured. Consequently, heat conduction joining by infrared laser radiation was used to join the metal with polypropylene. The influence of laser structuring on surface roughness and groove geometry was analyzed by scanning electron and laser scanning microscopy. The wetting of the thermoplastic was investigated by optical microscopy. The gas-tightness was identified by a helium leak test and climate testing was performed to analyze the long-term durability. The results indicate that depending on the surface morphology of the metal, durable gas-tight connections can be created by thermal joining.

Fabrication of Reliable Joints of Alumina Ceramics by Microwave-Assisted Reactive Brazing Technique

Microwave-assisted reactive brazing technique was utilized for joining of alumina ceramics at 950 degrees C and 1050 degrees C for 20 min in argon atmosphere using TICUSIL (68.8Ag-26.7Cu-4.5Ti in mass%) paste as the braze alloy. Conventional heating technique was also employed for comparison purpose only. The microwave and conventionally brazed joints were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis and Vickers microhardness measurement. X-ray diffraction data showed that the Ti-based compounds were formed at the substrate-filler alloy interfaces of the microwave and conventionally brazed joints. Scanning electron microscopy exhibited the formation of thicker reaction region in the case of joints microwave brazed at higher temperature. Energy dispersive X-ray analysis determined the elemental compositions across the joint cross-section. Vickers microhardness measurements indicated more reliable performance of the joints microwave brazed at lower temperature. Hermiticity of the microwave and conventionally brazed joints was evaluated by Helium leak test and found to be acceptable for actual applications.

Qualification of electron-beam welded joints between copper and stainless steel for cryogenic application

Joints between copper and stainless steel are commonly applied in cryogenic systems. A relatively new and increasingly important method to combine these materials is electron-beam (EB) welding. Typically, welds in cryogenic applications need to withstand a temperature range from 300K down to 4K, and pressures of several MPa. However, few data are available for classifying EB welds between OFHC copper and 316L stainless steel. A broad test program was conducted in order to qualify this kind of weld. The experiments started with the measurement of the hardness in the weld area. To verify the leak-tightness of the joints, integral helium leak tests at operating pressures of 16 MPa were carried out at room- and at liquid nitrogen temperature. The tests were followed by destructive tensile tests at room temperature, at liquid nitrogen and at liquid helium temperatures, yielding information on the yield strength and the ultimate tensile strength of the welds at these temperatures. Moreover, nondestructive tensile tests up to the yield strength, i.e. the range in which the weld can be stressed during operation, were performed. Also, the behavior of the weld upon temperature fluctuations between room- and liquid nitrogen temperature was tested. The results of the qualification indicate that EB welded joints between OFHC copper and 316L stainless steel are reliable and present an interesting alternative to other technologies such as vacuum brazing or friction welding.

Contact Stress and Contact Width Analysis of Corrugated Metal Gasket

Previous studies on corrugated metal gaskets have established that the contact width, contact stress, and surface roughness are important design parameters for optimizing gasket performance. However, the contact stress and contact width considering the surface roughness when the leakage occur did not defined yet. In this study, we determined the real contact stress and contact width when leakage started on 25A-size metal gasket. The contact width determined through a comparison between simulation and experimental results. The contact stress determined through a simulation analysis. The experiment involves a helium leakage test using new metal gaskets having different surface roughness levels. The result justified that the real contact width and average contact stress when the leakage started occur were 0.195mm and 800MPa, respectively. There is a good agreement for contact width both by simulation and experiment result. The contact width for flange having surface roughness 1.5μm is longest than the others.

Novel Electroless Plating of Ruthenium for Fabrication of Palladium-Ruthenium Composite Membrane on PSS Substrate and Its Characterization

Article history: This paper focused on a novel method of electroless plating ruthenium (Ru) on solid or porous substrates like porous stainless steel (PSS) discs or ceramic tubes. A novel complexing plating bath of Ru was developed. It is proven that Ru can be deposited directly on these substrates by the bath at a temperature of 328 K and strong alkaline environment. TGA, SEM, EDX and XRD confirmed the successful deposition of Ru. Relationships among plating rate, plating time, plating bath temperature and concentrations were also investigated. The Ru plating procedure was optimized. Based on the optimized procedure, a Ru layer was deposited by electroless plating of Ru onto the surface of pretreated aluminum-oxide modified PSS substrate. A 7.6 µm thick Pd-Ru membrane could be fabricated by electroless plating with a palladium (Pd) layer overlying the PSS substrate and a Ru layer overlying the Pd layer. Helium leakage and hydrogen permeation tests confirmed that the membrane was free of defects. Hydrogen diffusion through the Pd-Ru membrane was found to be the rate-controlling step in permeation. A new Pd-Ru composite membrane had the same features and permeation behavior as a previous one, indicating that electroless plating of both Ru and Pd is reliable and repeatable.

End Closure Joining of Ferritic-Martensitic and Oxide-Dispersion Strengthened Steel Cladding Tubes by Magnetic Pulse Welding

The magnetic pulse welding (MPW) technique was employed for the end closure joining of fuel pin cladding tubes made of ferritic-martensitic (FM) steel and oxide-dispersion strengthened (ODS) steel. The technique is a solid-state impact joining process based on the electromagnetic force, similar to explosive welding. For a given set of optimal process parameters, e.g., the end-plug geometry, the rigid metallurgical bonding between the tube and end plug was obtained by high-velocity impact collision accompanied with surface jetting. The joint region showed a typical wavy morphology with a narrow grain boundary-like bonding interface. There was no evidence of even local melting, and only the limited grain refinement was observed in the vicinity of the bonding interface without destructing the original reinforcement microstructure of the FM-ODS steel, i.e., a fine grain structure with oxide dispersion. No leaks were detected during helium leakage test, and moreover, the rupture occurred in the cladding tube section without leaving any joint damage during internal pressure burst test. All of the results proved the integrity and durability of the MPWed joints and signified the great potential of this method of end closure joining for advanced fast reactor fuel pin fabrication.

Explosive welding method for manufacturing ITER-grade 316L(N)/CuCrZr hollow structural member

In this study, a new explosive welding method provided an effective way for manufacturing ITER-grade 316L(N)/CuCrZr hollow structural member. The welding parameters (stand-off distance and explosion rate) were calculated respectively using equivalent frontal collision wave model and effective energy model. The welded samples were subject to two step heat treatment cycles (solution annealing and aging). Optical microscopy (OM) and scanning electron microscopy (SEM) were utilized to analyze the microstructure of bonding interface. The mechanical properties of the welded samples were evaluated through microhardness test and tensile test. Moreover, the sealing property of the welded specimens was measured through helium leak test.Microstructural analysis showed that the welded sample using effective energy model had an ideal wavy interface. The results of microhardness test revealed an increase in hardness for both sides near to the bonding interface. And the hardening phenomenon of interface region disappeared after the solution annealing. SEM observation indicated that the samples with the post heat treatments exhibited a ductile fracture with dimple features after tensile test. After the specimens undergo aging strengthening, there was an obvious increase in the strength for all specimens. The helium leak test results have proven that the welded specimens are soundness.

Experimental study on hollow structural component by explosive welding

A large thin-walled hollow structural component with sealed channels is required for the vacuum chamber of a thermonuclear experimental reactor, with inconel625 as its fabrication material. This hollow structural component is rarely manufactured by normal machining method, and its manufacture is also problematic in the field of explosive welding. With this in mind, we developed an adjusted explosive welding technology which involves a two-step design, setting and annealing technology. The joints were evaluated using optical microscope and scanning electron microscope, and a mechanical experiment was conducted, involving micro-hardness test, cold helium leak test and hydraulic pressure test. The results showed that a metallurgical bonding was realized by the ribs and slabs, and the shearing strength of the bonding interface exceeded that of the parent metal. Hence, the hollow structural component has a good comprehensive mechanical performance and sealing property.

Thermal cycling behavior of alumina-graphite brazed joints in electron tube applications

Alumina was joined with graphite by active metal brazing technique at 895, 900, 905, and 910 °C for 10 min in vacuum of 0.67 mPa using Ti-Cu-Ag (68.8Ag-26.7Cu-4.5Ti; mass fraction, %) as filler material. The brazed samples were thermal cycled between 30 and 600 °C and characterized. X-ray diffraction results show strong reaction between titanium and carbon as well as titanium and alumina. Scanning electron microscopy and helium leak tests show that the initial and thermal cycled brazed samples are devoid of cracks or any other defects and hermeticity in nature. Brazing strength of the joints is found to be satisfactory.

Stiction-Induced Sealing of Surface Micromachined Channels

We established a technique to achieve low temperature hermetic sealing of surface micromachined channels by exploiting stiction. Using a wing-shaped structural layer, microchannels automatically seal during the drying step that follows the sacrificial etch. This avoids the need of plugging layers to close the apertures of the sacrificial etch. To demonstrate the technique, we designed a surface micromachined channel with a structural layer made of polycrystalline silicon carbide (poly-SiC). This layer integrates an array of anchored pillars to achieve long and wide microchannels (5.4 mm × 0.43 mm × 0.001 mm). To dimension the sealing-wing, we estimate the minimum adhesion energy between the poly-SiC and silicon. This is done by analytical modeling and experimental characterization of test structures in the shape of centrally-supported circular plates. The bending and adhesion of the sealing-wing is followed in situ by optical microscopy. The closed microchannels are annealed at 100 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. Some structures are exposed to thermal stress at 760 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. The results are inspected by white light interferometry, scanning electron microscopy, and helium leak testing. Microchannels result hermetically sealed showing leak rates below the detection limit (4×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> Pa·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> /s). The seal is effective to at least 600 kPa.

Durability and field condition study of seal of SAVY-4000 storage container

The conditions of use for rubber O-rings are at least as important as their physical properties in their effect on the quality of a seal. Under normal use conditions, O-rings may be subject to wear and environmental contaminants such as hair and dirt. This study examines how these factors impact the leak tightness of a nuclear material storage container and the likelihood of the inadvertent release of radioactive material. The durability lifetime of an O-ring was explored by opening and closing four SAVY-4000 1 quart containers 100 times and periodically performing helium leak testing, though no significant change in leak rate was observed. This study also explored how the accumulation of dust or hair on the O-ring surface would affect the leak rate of the containers. A single hair crossing the seal, or a sufficient amount of particulate matter would compromise the seal, but after cleaning, the seal was re-established.

An all-diamond, hermetic electrical feedthrough array for a retinal prosthesis

The interface between medical implants and the human nervous system is rapidly becoming more and more complex. This rise in complexity is driving the need for increasing numbers of densely packed electrical feedthrough to carry signals to and from implanted devices. This is particularly crucial in the field of neural prosthesis where high resolution stimulating or recording arrays near peripheral nerves or in the brain could dramatically improve the performance of these devices. Here we describe a flexible strategy for implementing high density, high count arrays of hermetic electrical feedthroughs by forming conducting nitrogen doped nanocrystalline diamond channels within an insulating polycrystalline diamond substrate. A unique feature of these arrays is that the feedthroughs can themselves be used as stimulating electrodes for neural tissue. Our particular application is such a feedthrough, designed as a component of a retinal implant to restore vision to the blind. The hermeticity of the feedthroughs means that the array can also form part of an implantable capsule which can interface directly with internal electronic chips. The hermeticity of the array is demonstrated by helium leak tests and electrical and electrochemical characterisation of the feedthroughs is described. The nitrogen doped nanocrystalline diamond forming the electrical feedthroughs is shown to be non-cyctotoxic. New fabrication strategies, such as the one described here, combined with the exceptional biostability of diamond can be exploited to generate a range of biomedical implants that last for the lifetime of the user without fear of degradation.

Preparation of a novel Pd/layered double hydroxide composite membrane for hydrogen filtration and characterization by thermal cycling

A layered double hydroxide (LDH) layer was grown directly on a porous stainless steel (PSS) surface to reduce the pore opening of the PSS and to be a middle layer retarding Pd/Fe interdiffusion. A thin Pd film (∼7.85 μm) was plated on the modified PSS tube by an electroless plating method. A helium leak test proved that the thin Pd on the LDH-modified PSS substrate was free of defects. The membrane had a H2 flux of 28–36 m3/(m2 h) and H2/He selectivity larger than 2000 at a pressure difference of 1 bar. Thermal cycling between room temperature and 673 K was performed and showed that the membrane exhibited good permeance and selectivity. Long-term evaluation (1500 h) of the membrane at 673 K showed static results of H2 flux (∼30 m3/(m2 h)) and H2/He selectivity (∼2000) over the 1500 h test period.

Quality Management during Manufacturing of High Tempertaure Thin Walled Austenitic Stainless Steel Sodium Tanks of Prototype Fast Breeder Reactor

Austenitic stainless steels are the major material of construction for the fast breeder reactors in view of their adequate high temperature mechanical properties, compatibility with liquid sodium coolant, good weldability, availability of design data and above all the fairly vast and satisfactory experience in the use of these steels for high temperature service. All the Nuclear Steam Supply System (NSSS) components of FBR are thin walled structure and require manufacture to very close tolerances under nuclear clean conditions. As a result of high temperature operation and thin wall construction, the acceptance criteria are stringent as compared to ASME Section III. The material of construction is Austenitic stainless steel 316 LN and 304 LN with controlled Chemistry and calls for additional tests and requirements as compared to ASTM standards. Prototype Fast Breeder Reactor (PFBR) is sodium cooled, pool type, 500 MWe reactor which is at advanced stage of construction at Kalpakkam, Tamilnadu, India. In PFBR, the normal heat transport is mainly through two secondary loops and in their absence; the decay heat removal is through four passive and independent safety grade decay heat removal loops (SGDHR). The secondary sodium circuit and the SGHDR circuit consist of sodium tanks for various applications such as storage, transfer, pressure mitigation and to take care of volumetric expansion. The sodium tanks are thin walled cylindrical vertical vessels with predominantly torispherical dished heads at the top and bottom. These tanks are provided with pull-out nozzles which were successfully made by cold forming. Surface thermocouples and heaters, wire type leak detectors are provided on these tanks. These tanks are insulated with bonded mineral wool and with aluminum cladding. All the butt welds in pressure parts were subjected to 100% Radiographic examination. These tanks were subjected to hydrotest, pneumatic test and helium leak test under vacuum. The principal material of construction being stainless steel for the sodium tanks shall be handled with care following best engineering practices coupled with stringent QA requirements to avoid stress corrosion cracking in the highly brackish environment. Intergranular stress corrosion cracking and hot cracking are additional factors to be addressed for the welding of stainless steel components. Pickling and passivation, Testing with chemistry controlled demineralised water are salient steps in manufacturing. Corrosion protection and preservation during fabrication, erection and post erection is a mandatory stipulation in the QA programme. Enhanced reliability of welded components can be achieved mainly through quality control and quality assurance procedures in addition to design and metallurgy. The diverse and redundant inspections in terms of both operator and technique are required for components where zero failure is desired &amp; claimed. This paper highlights the step by step quality management methodologies adopted during the manufacturing of high temperature thin walled austenitic stainless steel sodium tanks of PFBR.

Challenging Experiences in the Usage of Stainless Steels in a Heavy Fabrication Industry

Stainless steels are regularly used as one of the preferred material of construction in the pressure vessels and heat exchangers manufactured by welding for process plants and energy sector at BHEL, Tiruchirappalli. They are considered mainly because of their corrosion resistance and high temperature suitability. But the practising welding engineers have to face innumerable challenges with stainless steel with regard to defects minimization, distortion control and dimensional stability on large and complex assemblies. Use of Nickel based filler wires or development of welding procedures simulating the true configuration of the product and by conducting specific tests and NDE are followed for weld defects control. Sequence welding, development of special fixtures, etc have come as handy options for welding distortion control of SS. Innovative inspection techniques for the certification of dimensions including geometrical tolerances especially on large constructions using SS welding and evolution of special Helium leak testing procedures for certain in-process checks in critical products are inevitable in the SS fabrication industry as a part of Quality Assurance programme. As a pioneer in SS fabrication, some of our challenging experiences pertaining to these three areas are discussed in this paper.

High-temperature stability of laser-joined silicon carbide components

Silicon carbide is recommended for applications in energy technology due to its good high-temperature corrosion resistance, mechanical durability, and abrasion resistance. The prerequisite for use is often the availability of suitable technologies for joining or sealing the components. A laser-induced process using fillers and local heating of the components represents a possible low-cost option.Investigations in which yttrium aluminosilicate glass was used for laser-induced brazing of SiC components of varying geometry are presented. A four-point bending strength of 112MPa was found for these joints. In burst tests, laser-joined components were found to withstand internal pressures of up to 54MPa.Helium leak tests yielded leak rates of less than 10–8mbarls−1, even after 300h at 900°C. In contrast, the assemblies showed an increased leak rate after annealing at 1050°C.The short process time of the laser technique – in the range of a few seconds to a few minutes – results in high temperature gradients and transients. SEM analysis showed that the filler in the seam predominantly solidifies in a glassy state. Crystallization occurred during later thermal loading of the joined components, with chemical equilibrium being established. Differences in seam structures yielded from different cooling rates in the laser process could not be equalized by annealing.The results demonstrated the long-term stability of laser-brazed SiC assemblies to temperatures in the range of glass transformation (900°C) of the yttrium aluminosilicate filler. In technological investigations, the suitability of the laser joining technique for sealing of SiC components with a geometry approximating that of a fuel element sleeve pin (pin) in a gas-cooled fast reactor was proven.

Development of graphite sealing technique for an irradiation test rig of nuclear fuels

To test a newly developed nuclear fuel, a test rig needs to be made and put into the core of a research reactor. Then, high temperature and highly pressurized coolant is circulated through the test rig during the irradiation test to transfer the heat generated by the fissile activity of the nuclear fuel. Because several decades of instrumentation cables are drawn from a test rig, it is difficult to seal out the coolant in the test rig. In this study, a mechanical sealing method which uses flake type graphite powder has been developed to replace the conventional brazing process. The internal design of the test rig is modified to apply compressive stress on the graphite powder to seal out the gap between the sealing plug and instrumentation cables. The sealing performance is then verified by a hydraulic pressure test and Helium leak test according to the ASME code, section III.

Sacrificial microchannel sealing by glass‐frit reflow for chip scale atomic magnetometer

AbstractA novel sealing technique using sacrificial microchannels was proposed for control of an atmosphere in a micromachined alkali metal vapor cell for a chip‐scale atomic magnetometer. The microchannels act as a feedthrough connecting the cell to the outside atmosphere during evacuation and gas filling steps, and eventually they are sealed by glass‐frit reflow. A silicon microchannel was designed and sealing by glass‐frit reflow was successfully demonstrated in reflow experiments. Simulation results clarified the glass‐frit reflow characteristics and their dependence on cross‐sectional shapes of the microchannels. Hermeticity of the proposed sealing technique at a leak rate of less than 10−12 Pa·m3/s was verified by a high‐resolution helium leak test. © 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(5): 58–66, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10432