Outgassing Rate Measurements Research Articles

Vacuum metrology and its impact on research and industry

SummaryTo mark our anniversary, we are presenting articles that have been particularly appreciated by readers of our online edition once again in print. Here:By accurate vacuum measurement, it is possible to control processes in industry, to ensure the necessary environmental conditions for many experiments in science and research and to support the commercial relationship between manufacturers and users of vacuum equipment. The main task of vacuum metrology, the science of vacuum measurement, is to provide traceability of vacuum measurements to the SI. In recent years, also the characterization of vacuum by partial pressure analysis and outgassing rate measurement, and the dynamics of vacuum measurement became an issue for vacuum metrology.

Research on vacuum properties of CuCrZr UHV flanges for the Hefei Advanced Light Facility

Hefei Advanced Light Facility (HALF), one of the Diffraction-Limited Storage Rings (DLSR), requires low-impedance flanges to be welded with Oxygen-free with Ag (OFS) vacuum chamber. In this paper, CuCrZr alloy as flange material and spring energized metal C-ring sealing structure are proposed. Two types of CuCrZr flanges have been designed, which are chain clamp and bolted flange, respectively. Chain clamp flanges are planed for compact areas, due to its small axial dimension. Bolted flanges with better positioning ability are designed to place in sections without space limitation. A series of leak rate and outgassing rate measurements were performed, and force and finite element analyses were carried out. Leak rates of both two flanges are lower than 5×10−13Pam3s−1 after 10 times seal-changing tests and 15 times same-seal bakeout cycles. Required fastening torques of chain clamp and bolted flange are 5–7 N m and 3–5 N m, respectively, in order to achieve sealing. The outgassing rate of CuCrZr is about an order of magnitude less than that of stainless steel after bakeout to 250 °C. The simulation results indicate that stress on the CuCrZr flange is far lower than metal C-ring due to the large deformation of the seal.

Vacuum metrology and its impact on research and industry

SummaryBy accurate vacuum measurement, it is possible to control processes in industry, to ensure the necessary environmental conditions for many experiments in science and research and to support the commercial relationship between manufacturers and users of vacuum equipment. The main task of vacuum metrology, the science of vacuum measurement, is to provide traceability of vacuum measurements to the SI. In recent years, also the characterization of vacuum by partial pressure analysis and outgassing rate measurement, and the dynamics of vacuum measurement became an issue for vacuum metrology.

Outgassing characteristics analysis of mechanical cryocooler materials

Three types of outgassing rate measurements were conducted for the constituent materials of a single-stage Stirling (1ST) cooler: outgas measurement by total mass loss in a vacuum environment; outgassing rate measurement using the Quartz Crystal Microbalance (QCM) in a vacuum environment; and outgassing rate measurement using the Atmospheric Pressure Ionization Mass Spectrometer (API-MS) in an argon gas flowing environment. Relative comparisons of the adsorption energy and outgassing rate were made based on these measurement results. It was confirmed that the outgassing rate of the cold head polyimide part and compressor voice coil was dominant.

Outgassing measurement of various activated carbon sorbents for application in prototype cryopump

The ultimate pressure in any Ultra High Vacuum (UHV) system is related to the magnitude of the gas load; therefore, the measures taken to reduce outgassing are critical for the production of UHV. An OutGassing Measurement System (OGMS) has been established to measure the outgassing rate of different materials to be used in the cryopump in vacuum condition [1]. The outgassing rate measurement in OGMS is based on throughput method and has the base outgassing rate of < 5 x 10-12 mbar-liter/sec-cm2.For standard materials, Outgassing data are available in literature. It is not available for the specific materials in use of particular application. Micro porous activated carbonaceous compounds are the best sorbents and suitable for adsorbing gases to cater the cryopumping [2]. Coconut shell charcoal (of various granular size), activated carbon spheres (ACS), carbon cloth (woven and nonwoven fabric) are some of the examples of sorbents serving the said purpose.This paper shows the results of the Outgassing measurements of various activated charcoal sorbents coated on stainless steel samples having physical dimensions of 10 cm (L) x 5 cm (W) x 0.15 cm (thick) for each sample and the surface area is about 104 cm2.

表面処理したアルミニウム合金のガス放出特性—開発した測定装置の性能評価とガス放出速度測定—

表面処理したアルミニウム合金のガス放出特性—開発した測定装置の性能評価とガス放出速度測定—

Outgassing rate measurements with the difference method in the framework of EMRP IND12

A new outgassing rate facility applying the difference method was established at KIT. This system was validated within the framework of Joint Research Project IND 12 of the European metrology Research programme by several outgassing reference probes. These served as transfer standards for comparison with three other outgassing rate systems available in the project. Two of the probes were newly developed reference devices and exhibit a known, constant outgassing rate of water. Further two probes were also newly developed outgassing reference devices which emit mixtures of gases. Additionally another probe was an elastomer which was loaded with a known gas mixture. The last probe was a commercially available plastic material whose outgassing shows a large spectrum of gases. The other outgassing rate facilities applied the in-situ calibration of quadrupole mass spectrometers by a primary method, the continuous expansion method (Physikalisch-Technische Bundesanstalt) and the known gas quantity method (Institute of Metals and Technology) or the throughput method (VACOM company).The comparison of the results shows a satisfactory agreement between the measurements.

Measurement and Modeling of Water-Vapor Diffusion in Elastomers with Impact in Humidity and Vacuum Measurements

The dynamics of water-vapor dissolution in Viton O-rings is measured with a gravimetric method using a precise mass comparator. A sample gasket was degassed in high vacuum for a sufficiently long period to remove more than 99 % of the dissolved water vapor. After that, it was exposed to the ambient atmosphere with a controlled temperature, and relative humidity and water-vapor uptake curves were measured gravimetrically with a precise balance. The dynamics of a water-vapor release into vacuum from another sample that was previously saturated with water vapor at room temperature was determined. The sample was placed in a vacuum outgassing rate measurement apparatus. The time dependence of the evolved water vapor was calculated by integrating the measured outgassing rate. The physical process of water absorption can be described by the diffusion equation. The geometry of the samples required solving the diffusion equation in cylindrical coordinates. This was done numerically using a finite-difference method. As a result of the modeling, room temperature values of the diffusion constant $$D$$ , the solubility $$s$$ , and the permeability $$K = D\times s$$ of water vapor in the sample material (Viton A-401C) were obtained. For sample 1, we obtained $$D = 8.0 \times 10 ^{-8}$$ cm $$^{2}\,\,{\cdot }\,\,$$ s $$^{-1}$$ and $$s = 6.5 \times 10^{-7}$$ g $$\,{\cdot }\,$$ cm $$^{-3}\,{\cdot }\,$$ Pa $$^{-1}$$ , while for sample 2, $$D = 3.0 \times 10^{-7}$$ cm $$^{2}\,{\cdot }\,$$ s $$^{-1}$$ and $$s = 3.5 \times 10^{-7}$$ g $$\,{\cdot }\,$$ cm $$^{-3}\,{\cdot }\,$$ Pa $$^{-1}$$ .

合成樹脂を用いた材料の気体放出速度測定

合成樹脂を用いた材料の気体放出速度測定

Outgassing measurements for the ITER EC H&CD Upper Launcher

In most of applications involving both vacuum and high temperatures, outgassing of structural materials is a critical issue. For instance, this is the case of fusion test devices, where the gas released from the vessel walls contaminates the plasma. Four upper ports in the ITER vacuum vessel are reserved for Electron Cyclotron Heating and Current Drive (EC H&CD) Upper Launchers (UL), which have to provide plasma MHD stabilization by localized deposition of high power microwave beams. The structural material foreseen for the UL is the 316L(N)-IG stainless steel. It has to withstand temperatures in the range 120–150 °C during normal operation and 240 °C during the baking process. One of the preferred manufacturing routes for the UL is the Hot Isostatic Pressing (HIP) which is a very sophisticated method to manufacture structural components of complex geometry with good mechanical properties. The materials for use in the ITER vacuum systems have to comply with the outgassing limits given in the ITER vacuum handbook, but no outgassing data for HIPed stainless steel are available in literature, thus they must be obtained by experimental measurements. In this paper, measurements of partial outgassing rates are shown and discussed for stainless steel prototype samples AISI 316LN (on which the 316L(N)-IG is based) and AISI 317LMN, obtained by powder and solid HIPing method. A variant of the gas throughput method in vacuum systems was used for the measurements which were carried out over periods larger than 8 h and at different temperatures.

微小ギャップの排気 電磁石の鉄心用積層鋼板の真空特性

A large amount of outgassing from the laminated cores in the pulse-operating magnets is one of the causes of pressure rise in an accelerator vacuum chamber. Outgassing rate measurements were carried out for a thousand of 0.1 mm thickness steel laminations, being stacked, in the compressed and uncompressed conditions. The measured rates of these samples did not show a significant difference, and indicated a t-1/2 time dependence. A pre-baking treatment for the laminated core samples was also examined.

Pit-free electropolishing of aluminum and its application for process chamber

Aluminum is one of the candidate materials of the vacuum chamber and components in the ultra-large scale integration device production facilities, because of high thermal conductivity and minimizing weight. For that purpose, their surface must be smooth and passivated so as to have a low outgassing rate. An electropolishing method to obtain a mirror-finish and pit-free smooth surface has been developed by controlling the flow of the electrolytic fluid during polishing. The polished surface, which shows an average roughness of 0.03 μm was found to be covered with an oxide film of 200 nm thick as determined by Auger electron spectroscopy, indicating a chemically stable surface. The outgassing rate measurement using a conduction-modulation method shows the value of 10−10 Pa m3 s−1 m−2, after baking treatment at 150 °C.

Erratum: Effects of readsorption on outgassing rate measurements [J. Vac. Sci. Technol. A 14, 2599 (1996)

Erratum: Effects of readsorption on outgassing rate measurements [J. Vac. Sci. Technol. A <b>14</b>, 2599 (1996)

Effects of readsorption on outgassing rate measurements

Outgassing rates from the walls of a vacuum chamber are usually measured by one of two methods, (1) the throughput method, where the outgassing rate is derived from the pressure drop across a known conductance during evacuation, or (2) the pressure rise method (aka the gas accumulation method) where the chamber is sealed off at a low pressure and the rise in pressure with time measured. A model of the throughput method is analyzed to show that readsorption of desorbed gas results in the measured outgassing rate Qm being considerably less than the intrinsic rate Qi (the outgassing rate in the absence of readsorption). The ratio of outgassing rates Qm/Qi varies with the pumping parameter S/A (S is the pumping speed and A the adsorbing area) and the sticking probability s. Qm is found to vary with the pumping parameter and the sticking probability at zero coverage s0, except when a/As0≪kt−1, where a is the effective area of the orifice, A the adsorbing area, k is a constant involving the heats of adsorption, and t the pumping time. Measurements by the pressure-rise method on stainless steel systems show that the gas evolved is hydrogen, and that the pressure rise is linear for as long as 103 h with the pressure increasing by a factor as large as 106, indicating a constant outgassing rate. It is shown that if readsorption occurs then the p(t) curve will be nonlinear. A model is proposed to explain the observed constancy of outgassing rate and the apparent absence of readsorption which assumes that (a) the outgassing rate is desorption limited and (b) the rate of diffusion of H atoms to the surface is much greater than the rate of desorption of H2 or the rate of arrival of molecules from the gas phase at empty sites.

Outgassing rate characteristic of a stainless-steel extreme high vacuum system

Using an extremely dry nitrogen venting system, fast return to ultrahigh vacuum condition was achieved. The apparatus is similar to that of our earlier extreme high vacuum publication. The system pumps to 1.1×1−10 and 1.4×10−11 Torr in 5 and 50 h, respectively, after the start of pumpdown and without bakeout. The pressure-time curve for pumpdown shows that the system pressure is determined by surface outgassing which decays algebraically as t−α, where the slope α changes from 2.13 for p≥10−9 Torr, 1.24 for 10−10≤p≤10−9, and 0.82 for p≤10−10 Torr. However, α=0.5, which would be expected for bulk diffusion, was not observed. Curve fitting yields desorption energies of 19, 20, 21, 22, and 23 kcal/mol for five types of adsorption sites on the surface for water. Analysis suggests that the elimination of water with desorption energies of 20 and 21 kcal/mol is essential for rapid pumpdown to the 10−10 Torr decade. A desorption energy of 10.4 kcal/mol for hydrogen was obtained by outgassing rate measurement. This relatively small value is suggestive of a smooth oxide layer formed on the chamber wall.

Effects of re-adsorption on outgassing rate measurements

Effects of re-adsorption on outgassing rate measurements

Measurements of partial outgassing rates

To investigate the surface composition and outgassing rates of a series of samples, the size of these should be small and the measuring time variable. Because the species of desorbed gases vary with dependence on time and temperature, the outgassing measurements using total pressure gauges include some errors caused by the conductance and the gas correction factor for the gauges. To reduce these errors we developed an outgassing measurement system on the basis of a modified throughput method with quadrupole mass spectrometers. By this method it is possible to measure partial outgassing rates. The change of major gas components is discussed. Some examples of the outgassing behaviour of stain less steel, nickel and chromium coated brass are given.

Assessment of Tritium Inventory in Waste by Outgassing Measurements

The production of large volumes of low specific activity waste is a byproduct of working with tritium. Reliable, labour minimized techniques for assessing the tritium bound in the waste is essential for an effective waste management program. This work examines the use of tritium outgassing rates from waste as a means of determining the bound inventory. A simple and inexpensive technique has been developed which, when combined with a sampling methodology, leads to accurate measurements of tritium outgassing rates. These outgassing rates when used in conjuction with the appropriate database will yield the inventory bound in the waste. The results of this study support the viability of the proposed approach.

Outgassing rate of preconditioned vacuum systems after short exposure to the atmosphere: Outgassing rate measurements on Viton-A and copper

When the sorption and desorption rates depend primarily on the diffusion of water vapor and atmospheric gases through the bulk of the sorbent (which may be an elastomer or plastic) and the process is reversible, then by an application of Fick’s law and Duhamel’s theorem equations can be derived for the outgassing rate as a function of time during subsequent pumpdowns in terms of the time of exposure to atmosphere with known relative humidity and the outgassing rate as a function of time during the initial conditioning cycle. Good agreement with the predictions of these equations has been obtained from measurements of repeated pumpdowns of a large-area sample of a Viton-A elastomer. However, similar experiments on copper plates showed relatively poor agreement with the theoretical equations indicating that the sorption–desorption process for water vapor in the oxide layer on metal surfaces may not be exactly reversible or that the process is not strictly controlled by diffusion.

Study of wall coatings for vapor-cell laser traps

Efficient collection of atoms into a vapor-cell laser trap requires a special wall material for the cell that minimizes the interactions between the vapor and the wall. Tests of several different wall coatings and materials are reported, and measurements of adsorption energies, outgassing, and chemical reaction rates between the alkali vapor and the walls are described. It is demonstrated that each of these parameters affects the collection efficiency.