Specific Outgassing Research Articles

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.

Evolution of hydrogen from AISI 200 stainless steel: A study to determine whether it is a diffusion or recombination limited process and experimental evidence for strongly bound hydrogen

The hydrogen outgassing rate from three equivalent UHV chambers made of 0.5-mm-thick AISI 204 stainless steel was measured in situ over several 10-h cycles at 150 °C, 200 °C and 250 °C. After each cycle, the specific outgassing rate at room temperature was determined using a spinning rotor gauge. A parallel analysis based on thermal desorption spectroscopy, extraction melting and numerical simulation of the recombination limited process reveals a more consistent picture of the outgassing process compared with models given in old vacuum textbooks. At the applied moderate temperatures, the majority of the diffusible hydrogen is released within less than 100 h. Consequently, the specific outgassing rate at room temperature was reduced to the level of q ∼ 1 × 10−13 mbar L/cm2s. At temperatures above 400 °C, the residual hydrogen is activated and is depleted very slowly. It seems that it represents an inner source of hydrogen that is manifested by a stable outgassing rate at moderate temperatures and on the long-term scale at room temperature.

Study on Vacuum Materials Outgassing Rate Using the Method of Switching Between Two Pumping Paths

A new measurement apparatus for vacuum materials outgassing rate was developed, which is mainly composed of three systems, including ultra-high vacuum system, high vacuum system, switching between two pumping paths (SPP) measurement system. The test principle of the modified SPP method is described in details, and the newly-developed technique has two main advantages over the conventional one. Firstly, the errors due to the use of two gauges with different physical properties are avoided by using only one gauge during data processing. Secondly, the outgassing rate of sample chamber, the test chamber and the outgassing or pumping effect of the gauge head can be eliminated enormously. Based on the SPP method, this paper gives the details of the experiments and the results on specific outgassing rate of the 7075 aluminum alloy sample in different conditions, namely, at different times and at different temperatures generated by infrared light irradiation heating.

Plasma–silicone interaction during a-Si:H deposition on solar cell wafers bonded to glass

A scenario for future silicon photovoltaics is to use wafer-based solar cells with a thickness below 100μm. A way to realize this scenario is the merging of cell and module processing, with thin wafers processed while attached to the superstrate glass. One of the challenges for this type of processing is the achievement of high performing surface passivation, i.e., with surface recombination velocities below 10cm/s. In this paper, a detailed explanation for lifetime degradation on wafers bonded to glass by means of silicone is proposed. The degradation is due to cyclic silicone molecules outgassing during a-Si:H deposition from the adhesive used to bond the wafers. The cyclic molecules are incorporated in the a-Si:H layer and modify the amorphous silicon network. By the application of specific outgassing conditions before a-Si:H deposition, a significant amount of cyclic molecules is removed from the adhesive. Non-degraded a-Si:H layers and surface passivation comparable to standalone wafers are obtained, as shown with measures of lifetime and solar cell open circuit voltage.

A Review of Vacuum Degradation Research and the Experimental Outgassing Research of the Core Material- Pu foam on Vacuum Insulation Panels

Abstract Vacuum Insulation Panels(VIPs) have been regarded as a super thermal insulation material with a thermal resistance of about 5-8 times higher than that of equally thick conventional polyurethane boards. In this paper, the researches on factors influencing interior pressure in VIPs, including gas and water vapor permeation through the barrier and outgassing of the core materials, were reviewed respectively. Following this, aiming at the outgassing from open cell PU foam, the specific outgassing rate of the core material is tested not only at room temperature but also at low and high temperatures by an orifice known-conductance method.

Outgassing characteristics of a polycarbonate core material for vacuum insulation panels

Performance deterioration with time is one of the most important issues in a vacuum insulation panel (VIP), which is mostly due to the inner gas pressure rise. Outgassing from the interior of core materials is the major gas source when the core material is a polymer. Outgassing characteristics of a polycarbonate as the VIP’s core material are examined theoretically and experimentally. To measure the outgassing rate, specific outgassing tests are carried out using a pressure rise method. Diffusive outgassing mechanism is discussed based on the Fick’s law. As the result, the total amount of dissolved gas and the diffusion coefficients of various gases in the polycarbonate are obtained by using the measured outgassing rate. Temperature dependence of the diffusion coefficient of nitrogen is also examined. It is shown that the outgassing rate of polymer core materials can be significantly reduced to a negligible level by a baking pre-treatment in vacuum and/or by a metal coating on the polymer surface.

Outgassing rate measurement of copper plated stainless steel

Stainless steel (SS) and Copper are very suitable structural metals for fabrication of UHV vessels. INDUS-1 is an electron storage ring, with 450 MeV electron energy. A new Radio Frequency (RF) cavity was fabricated for INDUS-1, with copper plating on SS. The main factor deciding the vacuum compatibility of the surface is the specific outgassing rate at room temperature as well as at the operating temperature. Surface properties are extremely important in this case. i.e., surface finish should be maintained without any blisters and micro cracks, after baking. In order to evaluate these factors, one sample was prepared (SS pipe plated with copper) and tested with simulated conditions in UHV environment. This paper gives the details of the experiments and the results on specific outgassing rate of the copper plated SS sample in different conditions, namely, at room temperature and after baking at different temperatures.

Outgassing of Rigid Open-celled Polyurethane Foam Used in Vacuum Insulation Panels Under Vacuum Condition

Rigid open-celled polyurethane foam is the main type of core material in vacuum insulation panels (VIPs). The formulation and manufacturing process of the foam are experimentally analyzed, and the factors that affect cell structure and heat insulation performance are discussed. Vacuum level in VIPs is one of the most important factors contributing to the heat insulation performance of VIPs. By the throughput method, outgassing properties of the core material of open-celled PU foam are investigated. Outgassing has a great negative influence on vacuum level in VIPs. The specific outgassing rate of the foam is tested not only at room temperature but also at low and high temperatures. It has been found that the specific outgassing rate is large, and has a decreasing trend with decreasing temperature. In a log–log plot of specific outgassing rate versus time, the decay exponent ranges from 0.62 to 0.8 with temperature changes from 20 to 70 C. At room temperature, pressure in VIPs used in refrigerators will reach more than 300 Pa due to outgassing in its service life, if there are no getters adopted. After the foam is baked at 120 C for 15 min, the specific outgassing rate is greatly reduced.

Outgassing from stainless steel and the effects of the gauges

The true outgassing from surfaces is often masked by the production of gases and pumping within the measuring gauge. Quadrupole mass analysers have been used to measure the outgassing from carefully prepared samples of stainless steel. By using an isolation pressure rise technique coupled to non-evaporable getter pumping, the problem of production of methane by the gauge is overcome and it has been possible to measure the outgassing of methane to very low levels. The specific outgassing rate of methane from 316 L stainless steel is found to be ⩽5×10 −22 mbar l s −1 cm −2. The outgassing of the other gases has been measured using several gauges. The specific outgassing of hydrogen is found to be 4.5×10 −15 mbar l s −1 cm −2 and the total of all other gasses amounted to no more than 10% of this figure. Although the measurements of the other gases are more ambiguous than those for methane, it is concluded that, in all probability, only hydrogen is outgassed from stainless steel.

Vacuum and beam transport lines: main principles

Some details of vacuum system designs of beam transport lines are presented. The effect of gas desorption on vacuum equilibrium and stability and ion-induced pressure instability in the beam transport lines are discussed. Main vacuum relationships are also given as a basis for understanding the vacuum system configuration of beam transport lines. Residual gas spectra together with specific outgassing rates of main materials used for construction of beam transport lines are shown.

Measurement system for low outgassing materials by switching between two pumping paths

A method of switching between two pumping paths (SPP) has been developed to measure a low outgassing rate accurately by subtracting the outgassing of all the elemental parts of a measurement system. The mass spectra of only the sample can also be measured by the SPP method. To prove the capability of the SPP method, outgassing properties of two stainless steel chambers with different treatments were examined. One chamber was electrolytically polished and subsequently 450 °C vacuum baked, and the other was glass beads blasted. The specific outgassing rate of the electrolytically polished and vacuum baked sample was less than 1 × 10 −11 Pa · m · s −1 at 28 °C after in situ 150 °C bakeout. The outgassing species was only hydrogen which was mainly caused by diffusion from the bulk stainless steel chamber wall.

A molecular beam detector to determine the specific outgassing rate of small specimens

A molecular beam detector was designed to measure the specific outgassing rate of metal samples in the UHV region at reduced influence of the gas desorbed from the walls of the chamber. The detector consists of a nude Bayard-Alpert gauge surrounded by a liquid nitrogen cooled stainless steel tube. The orifice in direction to the sample was designed as a multicapillary system (MCS) containing 85 capillaries of 1 mm diameter. Molecules desorbed from the sample can reach the interior of the detector directional resolved by the MCS and can be detected. A shutter between the MCS and the sample is used for the determination of the background of the equipment. The derivation of the formulae for the determination of the specific outgassing rates from the pressure measured in the detector chamber is explained. First experimental results are discussed.

Influence of vacuum gauges on outgassing rate measurements

When the outgassing rates from eighteen chamber samples with different surface treatments for ultrahigh vacuum were measured by the throughput method, the minimum specific outgassing rate measured was limited to about 10−13 Torr ℓ/s/cm2 due to the outgassing of a Bayard–Alpert gauge (BAG), about 10−9 Torr ℓ/s. A measurement of outgassing rates with a BAG and an extractor gauge (EG) showed that the outgassing rate of the EG is lower than that of the BAG and that outgassing due to thermal desorption from the wall surrounding of the gauges cannot be neglected in seeking to obtain correct outgassing rates for the gauges, especially for the case after baking. A new type of quadrupole mass spectrometer (QMS) and an extractor gauge (EG) showed an outgassing rate of 6–7×10−11 Torr ℓ/s. Using the QMS and EG, a minimum specific outgassing rate from the sample chambers was measured on the order of 10−14 Torr ℓ/s/cm2 from the total pressure. The measurement showed that the main residual gas species are H2O, before baking, and H2, after baking. Therefore, the outgassing rate (q) must be described using the partial pressures of the residual gases and was 2.0×10−10 Torr ℓ/s/cm2 for water (qH2O) and 1.4×10−13 Torr ℓ/s/cm2 for hydrogen (qH2) from an A6063 EX mirror-finished sample.

Anomalies in the measurement of the residual gases in a large uhv system using a quadrupole mass analyser

A small quadrupole mass analyser was used to measure the residual gases in a stainless steel ultra-high vacuum system (vacuum baked at 150°C for 10 days) of 7500 l volume, pumped by a NEG pump (nominal 100 l s −1). A series of experiments is described which shows that the indicated partial pressures of all of the residual gases, except hydrogen, are increased due to their production in the analyser. It has not been possible to accurately quantify the effect, but upper limits for the specific outgassing rates (mbar l s −1 cm −2) are : 2 × 10 −13 for hydrogen, ⩽1 × 10 −18 for methane, ⩽3 × 10 −17 for water, ⩽4 × 10 −16 for carbon monoxide and ⩽2 × 10 −17 for carbon dioxide.

The outgassing rate of titanium-aluminium alloys

The outgassing rates of the Soviet titanium alloys WT 20 and OT 4-1 were measured with a throughput method. The construction of the all-glass uhv apparatus with mercury diffusion pumps is described in detail. After 1 h of pumping at a temperature of 573 K the specific outgassing rate of the alloy WT 20 is of the order of magnitude of 10 −6 Pa l s cm −2. The exact value depends essentially on the pretreatment and on the temperature. The alloy OT 4-1 has at 50°C a somewhat higher specific outgassing rate of about 4 × 10 −6 Pa l s cm −2. Both materials can be used for the construction of uhv devices.

Convoluted vacuum tubes for long baseline interferometric gravitational wave detectors

A discussion is given of the advantages of thin-walled convoluted vacuum tubes for the long arms of interferometric gravitational wave detectors currently being proposed by a number of institutions around the world. Typically the tubes will be 3 km long and 1.5 m in diameter. Pressures of ⩽ 10 −7 mbar are required for hydrogen and at least an order of magnitude lower for the sum of all the other gases. The results of tests of some of the mechanical and vacuum properties of a prototype section of convoluted tube are presented. The tube is 5 m long, 1.4 m internal diameter and is constructed from 0.91 mm thick 316L stainless steel sheet. The convolutions have a semicircular form of 20 mm radius, giving the tube a strength equivalent to a plain tube of the same diameter but 13 mm wall thickness. A specific outgassing rate of ∼3x10 −13 mbar I s −1 cm −2 is achieved by a vaccum bake at 150°C. Examination of the residual mass spectra reveals that > 99% of the gas is hydrogen.

Industrial fabrication of uhv quadrupole chambers for LEP

CERN has constructed a Large Electron Positron Storage Ring (LEP) for colliding beam experiments. Particles storage requires an evacuated (uhv) beam pipe at p < 3 × 10 −7 Pa. The quadrupole vacuum chambers (VCQ) are made of aluminium without their own pumping system, hence they must have a very low outgassing rate. For the manufacture of 2635 m of VCQ, the Plasma arc welding process was chosen. After vacuum commissioning at 150°C, all VCQ had a vacuum limit corresponding to a specific outgassing rate lower than 2 × 10 −13 Pa m 3 s −1 cm −2.

Ultimate pressure of the order of 10−13 Torr in an aluminum alloy vacuum chamber

An extremely high vacuum chamber for the calibration of ultrahigh vacuum gauges was fabricated and tested. The chamber has an inner diameter of 250 mm and is 1800 mm long. It has 20 gauge ports and a liquid-nitrogen shroud inside to suppress interference between gauges and to avoid pressure rise due to thermal radiation from the gauges. The vacuum chamber was made of specially extruded aluminum alloy A6063-T6-EX in an oxygen and argon atmosphere. The chamber was assembled using tungsten inert gas welding in an argon atmosphere. After a bakeout at 150 °C for 24 h, a specific outgassing rate of the order of 10−13 Torr l/s cm2 was obtained. The system was evacuated with a turbobacked 300 l/s turbomolecular pump. The extremely high vacuum is maintained by two 300 l/s sputter ion pumps and a titanium sublimation pump with a liquid-nitrogen shroud. These pumps are also made of aluminum alloys. An ultimate pressure of 3×10−13 Torr was measured with a point collector gauge with spherical anode. Residual gas analysis in the 3×10−12 Torr was performed by a newly developed Q-mass filter.