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ScienceDirect Materials Today: Proceedings 5 (2018) 19614–19627
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ICMPC_2018
Design and Analysis of Experimental Setup for Hydrogen PPM Measurement Manoj Kumar Gupta a , Mihir Parekh b,*, Nirvesh Mehta b. A
institute For Plasma Research,Bhat,Gandhinagar 382428,India B l.D.R.P-Itr,Sector-15,Gandhinagar 382015,India
Abstract Vacuum Chambers are used to achieve Ultra High Vacuum Pressures (10-7 to 10-16 Torr.) for the processes like Plasma Generation, X-ray Photoelectron Spectroscopy, Surface analysis etc. The reason behind the application of UHV pressures in above fields is to enable readily clean and contamination free surfaces during the experiment. These vacuum chambers are constructed by materials like Aluminum, Glass, Brass, Inconel, Monel Metal and Stainless Steels. Among all, Austenitic Stainless Steel (ASST 304 L) are the most reliable choice for high and ultra-high vacuum Systems because they are cost effective, corrosion resistant, effectively outgassed, good machinability and weldability. Therefore, vacuum chambers of ASST 304 L for use in lower 10-9-10-10 Pa region are often pretreated by baking in order to decrease the hydrogen outgassing, which is the main residual gas in that pressure range. The material for the construction of ultra-high vacuum chambers has to process at different time for different temperatures. Baking in ambient air is easy and cost effective. Therefore, design and analyze of the experimental setup to investigate the hydrogen ppm in different sample, can be carried out for different bake out. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords: Outgassing; Ultra-High Vacuum; Baking; Hydrogen PPM; Austenitic stainless steel (ASST 304 L).
* Corresponding author. Tel.:+91-846-061-2058; E-mail address:
[email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.
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1. Introduction Ultra-High Vacuum pressures [1] (10-7 to 10-16 Torr) finds a unique application in the areas of Surface analysis experiments such as material surface characterization, fundamental particle studies, plasma generation, X-ray Photoelectron Spectroscopy. Such experiments require a dirt free surface with negligible contaminations and trace of adsorbents. Major difficulties associated with UHV systems is Outgassing (Desorption). Outgassing refers to the gas originating at the surfaces or spontaneous removal of gases from the walls of a vacuum chamber as shown in below Fig 1. Mechanisms contributing Outgassing are as follows [2]: Adsorption Desorption Permeation Bulk Diffusion Recombination Surface Diffusion
DESORPTION DIFFUSION
RECOMBINATION
ADSORPTION SURFACE DIFFUSION
PERMEATION
Fig. 1. Mechanisms Contributing Outgassing.
Outgassing equation is depicted as
Q=V Where, V=Chamber Volume,
(1) is rate of pressure change in a closed chamber at constant temperature.
Outgassing is measured by the unit Torr. L s-1 cm-2. One of the major gas constituent in outgassing at UHV is hydrogen which results to inaccuracy and limited performance of a vacuum System.
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2. Hydrogen Traps in Metals After manufacturing, the atmospheric gases adhere on the surface of vacuum components which contaminates the surface [3]. This process always happens on all the vacuum components. Additionally, the molecules of gases are originating inside bulk of material during the production. These molecules will form as source of gas in ultrahigh vacuum components. Hence the vacuum components need to be cleaned thoroughly. The travelling time for the H2 and O2 atoms is approximately one day and 1000 years respectively, to move by distance of 4μm in ASST 304 L. Several kind of atoms are dissolved in bulk but only hydrogen atoms release out at room temperatures [3]. Normally hydrogen has less solubility in most of metals like 0.0001 ppm in pure iron at 1.0132 bar pressure and room temperature [4]. But hydrogen gets trapped inside dislocation and vacancies in molecular structure during the processes like welding, surface electrochemical processes. Hydrogen trapped on the surface is in the amounts of ppm (parts per million). 1.
Hydrogen Trapping in ASST 304 L
The phenomena of hydrogen embrittlement causes failure, loss of ductility and decrease in toughness due to induction of hydrogen into metals. Hydrogen is also induced inside the material during the forming and finishing operations. [5] Steels exposed to hydrogen environment or humid environment at higher temperatures, hydrogen diffuses into alloy and combines with carbon forming very small pressure pockets of methane and voids at grain boundaries. As methane cannot come out of the alloy, it increases the pressure inside the voids resulting to high pressure pocket inside the voids, creating crack on stainless steel surface. The above process is so called hydrogen attack in stainless steel. At lower temperatures the hydrogen can diffuse provided the amount of hydrogen in atmosphere is more than the concentration inside the metal, by the cause of concentration gradient. 2.
Hydrogen trapping during Welding Process.
During the fusion welding process, the hydrogen gets dissociated from the water present on filler materials, welding consumables and indulge in weld pool which is in molten state and gets trapped on solidification. As the weld area remains hot, the hydrogen migrates from weld fusion zone to neighboring parent material also called as Heat Affected Zone (Welding heat is transferred to this zone). 3.
Hydrogen trapping during Electrochemical Process.
During electroplating the hydrogen is found in mono-atomic form in cathode reactions, which causes postprocessing failure by hydrogen embrittlement. A is a trap site while B is lattice site as shown on below Fig 2.
Ea = De-trapping Activation Energy Free Energy
Ed
Ea B Eb
Eb = Binding Energy Ed = Activation Energy for Diffusion
A Fig. 2. Energy Levels of Hydrogen Traps [5]
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3. Methods to decrease hydrogen outgassing in UHV systems Methods to decrease hydrogen outgassing are listed as below 1. 2. 3.
Metals having low solubility of hydrogen (copper) can be used. Baking the ASST 304 L at 4000C in vacuum or in air to remove the traces of water. [6] Pre-heating and Post-heating is done on metals so that the hydrogen is removed before it causes damage to parent material, in case of welding. TIN coating decreases the hydrogen diffusion rate. [7] Chemical cleaning and deposition of thin films does not allow hydrogen to permeate out, on inner surface. Reduced hydrogen desorption can be obtained by the process of Laser Surface Melting. [8] Post Baking the metals at temperatures 1500-2000 C helps to preserve the decreased outgassing rates for longer time. Thinner parts desorb less hydrogen than thicker parts, almost lower by factor of 5. [9]
4. 5. 6. 7. 8.
Stainless Steel (SS 304 L or SS 316 L) find a unique application in designing the UHV systems due to following reasons: [10] 1. 2. 3. 4. 5. 6.
SS is corrosion resistant, non-magnetic and chemically inert. SS is ferrous, ductile, good drawing and forming properties. SS restricts atmospheric gases to enter in the chamber, at room temperature. Low carbide precipitation in the heat affected zone during welding due to small carbon content (0.03%). Less outgassing rates. Cheap and easily available.
The Methodology of the defined work is 1. 2. 3. 4. 5.
Heating chamber layout and design for baking of SS Pieces. Creating a model of Heating Chamber. Transient Analysis of Heating Chamber and calculate the temperature required for heating chamber to bake the ASST 304 L is calculated by modelling software ANSYS. Benchmarking examples on radiation heat transfer are calculated to analyze the radiation transfer between heating chamber and SS pieces. Calculating the time needed for baking the SS pieces to reach bake out temperature of 4000C for each piece and evaluating the time required for the SS piece to reach the room temperature after baking.
The temperature and processing time required to bake the SS pieces, can be denoted by the number called Fourier Number [11] F0. Dimensionless Time (F0)
F =
D
(2) D = constant of diffusion for hydrogen at temperature T, t = processing time, d = wall thickness. D = D0 exp (−Ed /RT). D0 = 0.012 cm2/s was used, obtained from permeation calculations when bulk diffusion is the rate limiting mechanism. The energy of activation Ed =14.5 kcal/mol.
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4. Experimental Layout Layout for the Baking Chamber is shown in figure 3 INDUCTION HEATERS VALVE SSS PIECES
P.S
TRAYS
BAKED GASES OUT
Fig. 3 Layout of Baking Chamber.
Baking chamber consist of 1. Power Supply 2. Induction Heater 3. P.I.D Controllers 4. ASST 304 L mounted on Trays 5. Vacuum Pump 6. Hydrogen Measuring Device Baking of ASST 304 L Pre-treating ASST 304 L pieces by baking at elevated temperatures, removes out hydrogen gas and thus reduces hydrogen outgassing during operation in UHV systems. Extraction methods such as Solid Extraction (Without Melting) Method and Fusion (Melting the Material) Methods are used to remove out hydrogen in ppm [12]. Baking is also referred as de-embrittlement [6] process. Baking the ASST 304 L above 4000C can be done in presence of air or in absence of air that is either in the vacuum or in air. Baking in air have lower desorption rate than vacuum baked samples also air bake out at higher temperatures causes oxidation on surfaces and forms a thick iron-rich oxide layers. But the main comeback is difficulty in creating the vacuum inside the chamber also the costs for producing and maintaining the vacuum may be very high. Hence Air Baking is entertained as it is easy and cheap method. Steps in Baking the ASST 304 L are as follows 1. ASST 304 L are kept at certain arrangements on trays, which are mounted inside the heating chamber. 2. Induction coil heater is used so that a homogenous temperature distribution so that uniform heating of chamber and ASST 304 L can be governed. 3. A 3-Phase power supply is given to induction coil and P.I.D controllers to regulate the temperature inside heating Chamber.
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4.
Heat Treatment is given at fixed temperatures for a fixed intervals of time. For example, the ASST 304 L are heat treated for 2000C for 24 hours, 3000C for 24 hours, 4000C for 36 hours etc. 5. Some of the ASST 304 L pieces are removed after heat treatments at certain temperatures and time. For instance, out of 36 SS pieces, three pieces of SS from each row are removed after the baking at 2000C at 24 hours and the remaining ASST 304 L are left in the chamber for baking at higher temperatures. 6. ASST 304 L are removed by removing out the trays from the chamber. 7. Removed ASST 304 L are then treated in UHV pressures to measure the decrease in hydrogen outgassing after baking. 8. Hydrogen outgassing of baked and unbaked ASST 304 L can be calculated. 9. Gases removed during baking process are continuously removed out from the open valve and if vacuum baking is entertained the valve is connected to vacuum pump. 10. Post Bake out at lower temperature (1500C) can be carried out in the same heating chamber so as to retain the lower outgassing rates. 5. Benchmarking Problems Radiation problems of open and perfect enclosures were analyzed in ANSYS. The difference between both the open and perfect enclosures is also enlisted.
Open Enclosure means, radiation heat transfer in presence room temperature conditions or ambient conditions.
Perfect Enclosure means, radiation heat transfer takes place only between the surfaces and no radiation is lost surroundings. The difference between both the enclosures is shown in following table.
Example: Two parallel plates of same area 0.5 m2 and 0.5 m apart, at temperatures 10000C and 5000C and emissivity 0.2 and 0.5 respectively, arranged parallel to each other. Radiation heat transfer from both the plates is calculated for open and closed enclosure. Subscript H & C represents Hot & Cold respectively. Here IRH, IRC = Incident Radiation ARH, ARC = Absorbed Radiation RRH, RRC = Reflected Radiation ERH, ERC = Emitted Radiation NRH, NRC = Net Radiation. LRH, LRC = Radiation Lost to Atmosphere by Hot and Cold Plate, respectively. Boundary conditions, Interpretation of Radiation Heat Transfer Results in ANSYS and Comparison between both open and prefect enclosures, respectively is shown in figure 4 and 5 respectively and as shown in below Table 1 .
Fig. 4 Boundary Conditions for parallel plates.
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Table 1. Comparison between Open and Perfect Enclosure. PARAMETERS
PERFECT ENCLOSURE
OPEN ENCLOSURE
ENCLOSURES SHAPE FACTORS
2-BODY ENCLOSURE F12=F21=1
RESISTANCES
1- ɛ1/ ɛ1A1=1-0.2/(0.2)(0.5)=8 1- ɛ2/ ɛ2A2=1-0.5/(0.5)(0.5)=2
3-BODY ENCLOSURE F12=0.308 F21=0.30037 F13=1-F12=0.695 F23=1-F21=0.705 1- ɛ1/ ɛ1A1=1-0.2/(0.2)(0.5)=8 1- ɛ2/ ɛ2A2=1-0.5/(0.5)(0.5)=2
HOT PLATE (kW)
CALCULATIONS
COLD PLATE CALCULATIONS (kW)
RADIATION LOST TO ATMOSPHERE FROM BOTH PLATES. (kW)
1/A1F12=1/(0.5)(1)=2
1/A1F12=1/(0.5)(0.285)=7.018 1/A1F13=1/(0.5)(0.715)=2.797 1/A2F23=1/(0.5)(0.715)=2.797
IRH = 20.852 ARH = 4.1702 RRH = 16.681 ERH = 14.894 NRH = 10.724 IRC = 31.576 ARC = 15.787 RRC = 15.787 ERC = 5.0641 NRC = -10.724 HOT PLATE= 0 COLD PLATE=0 TOTAL = 0
IRH = 2.3245 ARH = 0.4649 RRH = 1.8596 ERH = 14.887 NRH = 14.422 IRC = 4.949 ARC = 2.4745 RRC = 2.4745 ERC = 5.060 NRC = 2.5856 HOT PLATE=11.7975 COLD PLATE=5.2102 TOTAL=17.008
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6. Experimental Setup of Heating Chamber Baking chamber for heating the ASST 304 L pieces and the arrangement of ASST 304 L pieces in the chamber is depicted in fig 6 (a-b).
Fig 6. (a) SS Heating Chamber, (b) SS 304L Pieces arrangement in Heating Chamber.
The above configuration depicts the 3-D configuration of ASST 304 L pieces inside the baking chamber. For the experimental purpose 36 pieces of 5 mm × 5 mm are arranged on the steel rods enclosed by the SS chamber. For supporting each SS piece two steels rods are provided so that ASST 304 L are baked from all the faces. We can evaluate the hydrogen outgassing properties of the same material (ASST 304 L) provided by different vendors. Suppose vendors from three different locations are providing ASST 304 L materials, 12 pieces of SS by first vendor can be kept on first row and accordingly for others. After baking for 4000C, 3 pieces from each rows are removed and decrease in hydrogen outgassing is measured. Similar process can be repeated at 4000C, 4500C, 5000C. The optimum temperature for baking can be found by the above process. Time required to bake the ASST 304 L and temperature of heating chamber required for baking the ASST 304 L pieces can be known by applying thermal boundary conditions on heating chamber and SS Pieces. The SS pieces are heated due to radiation heat of heating chamber. Time v/s Temperature graphs for baking ASST 304 L are plotted in the results section.
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6.1 Thermal Boundary Conditions Transient thermal analysis is carried out along with radiation boundary conditions as shown in figure 7.
Fig. 7 Boundary Conditions of Heating Chamber
Boundary Conditions are enlisted for ASST 304 pieces and the heating chamber in Table 2. Here power to the induction heater is supplied such that the heating chamber is heated to 5250C in 1800 seconds. After that the temperature is held constant to 5250C for 1800 seconds, so that all the pieces are baked to 4000C. Table 2: Boundary Conditions for Radiation between ASST 304 L pieces and Cylinder.
BOUNDARY CONDITIONS
VALUE
Cylinder Temperature
5250 C
ASST 304 L Pieces Temperature
220 C
Ambient Temperature
220 C
Emissivity of SS 304 L Cylinder
0.3
Emissivity of ASST 304 L Pieces[15]
0.3
Step End Time
3600
If ASST 316 L (Austenitic Stainless Steel 316 L) pieces are baked then emissivity of the pieces will change whereas, the material for the cylinder (ASST 304 L) will remain same.
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6.2 Temperature and Time Calculations Results are obtained in the form of radiation heat transfer between ASST 304 L and the chamber. The time required to bake the steel pieces to 4000C is almost same for all the ASST 304 L for heating chamber heated to 5250C. ASST 304 L pieces baked to 4000C are the left to room temperature for ambient cooling. Therefore, the time required for the ASST 304 L to reach the ambient temperature after baking can be evaluated. Results show that time required to reach the temperature to 4000C of ASST 304 L is 3600 seconds that implies 1 hour, for the temperature of heating chamber held constant to 5250C As shown in below Fig 8 and Fig 9
Fig 8. Baking of ASST 304 L to 4000C
Fig 9. Temperature v/s Time Graph of Baking ASST 304 L Pieces.
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Single ASST 304 L from each row is taken into consideration to confirm that uniform temperature is distributed to all the pieces and are baked to 4000C. Results and Graph are shown below.
SS PIECE (R1 C32)
R1 C32 represents the SS piece situated at First Row Third Column and Second Element. The temperature attained by this element is 401.040C as shown below fig 10 (a-b).
Fig 10. (a) Temperature attained by SS Piece R1 C32 (b) Temperature v/s Time graph for R1 C3
SS PIECE (R2 C33)
R2 C33 represents the SS piece situated at second row, third column and third element. The temperature attained by this element is 401.130 C as shown below Fig 11 (a-b). a b
Fig 11. (a) Temperature attained by SS Piece R2 C33 (b) Temperature v/s Time graph for R2 C33
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SS PIECE (R3 C41)
R3 C41 represents the SS piece situated at third Row, fourth column and first element. The temperature attained by this element is 427.960C as shown below Fig 12 (a-b). a b
Fig 12. (a) Temperature attained by SS Piece R3 C41 (b) Temperature v/s Time graph for R3 C41
6.3 Bake Outs Bake out at 4000C temperature is carried out and from the graph of temperature versus time, estimation of time required to reach the temperature to 2500C, 3000 C,3500C, 4000 C is shown below Table 3. Table 3 Results showing Time and Temperature required to bake ASST 304 L at different temperatures
Temperature of SS 304 L Pieces.(Baking Temperature)
Temperature of Heating Chamber
Baking at 2500C Baking at 3000C
Time Required to bake the pieces. (seconds)
Time required for the SS pieces to reach ambient temperature. (seconds)
1400 5250 C
1600
Baking at 3500C
1850
Baking at 4000C
3600
70000
Similarly, if the samples are taken from SS 316 L then emissivity of the material will change. The emissivity [13] of SS316L is between 0.44 and 0.51. For the analysis purpose, the emissivity of ASST 316 L has been taken as 0.47. Here the temperature of heating chamber required to bake the pieces is 5500C.The temperature of heating chamber is increased to 5500C in 1800 seconds and then held constant for same interval.
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Table 4 Results showing Time and Temperature required to bake ASST 316 L at different temperatures
Temperature of SS 316 L Pieces (Baking Temperature)
Temperature of Heating Chamber
Baking at 2500C Baking at 3000C Baking at 3500C Baking at 4000C
Time Required to bake the pieces.( seconds)
Time required for the SS pieces to reach ambient temperature. (seconds)
1350 5500 C
1530 1727
50000
2010
7. Hydrogen Measurement High thermal conductivity of hydrogen is used for measuring the hydrogen concentration. The conduction of heat by hydrogen is more by factor of 7 than air. The metallic wire gets heated due to electric current, in a measuring cell. Surrounding gas decreases the temperature of the wire and after a short time the equilibrium of temperature is reached. Temperature of the metallic wire determines its resistivity and in measured by wheat-stone bridge. If there is presence of hydrogen in the measuring gas, then the temperature and D.C voltage output on measuring section changes. [14]
Fig.13 Hydrogen Measurement Device [14]
8. Conclusion The experimental set has been designed and analysis for the different temperature conditions. The temperature of the heating chamber has been evaluated as 5250C for ASST 304 L and 5500C for ASST 316 L. The samples (SS304L & SS316L) can be withdrawn at different time intervals of 12 hours, 24 hours etc. as per the requirement for the hydrogen ppm study. The maximum temperature of 4000C can be reached within 3600 sec and 2010 sec for SS304 L and SS316L respectively. The time difference for both the material is due to the variation in emissivity. After baking the material samples is cooled in air (220C), the time taken for cool down from the analysis is 70000 seconds (i.e. ~19.44 hours) and 50000 seconds (i.e. ~13.89 hours) for SS304L and SS316L respectively. Before carrying out the actual analysis by ANSYS, a benchmark problem was done which showed good agreement with analytical results. This experimental set up can be used for air baking and vacuum baking (with minor modification and vacuum pumps).
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