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Experimental investigation on hydrogen adsorption performance of composite adsorbent in the tank with high vacuum multilayer insulation
Vacuum 83 (2009) 1184–1190
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Experimental investigation on hydrogen adsorption performance of composite adsorbent in the tank with high vacuum multilayer insulation Shujun Chen, Xiangdong Li, Rongshun Wang*, Gaofeng Xie, Yuwu Zeng Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 October 2008 Received in revised form 19 March 2009 Accepted 21 March 2009
Hydrogen is one of the main residual gases in the high vacuum multilayer insulated tank. H2 adsorption performance of the composite adsorbent was investigated by using a test bench, an X-ray diffraction and a scanning electron microscopy. The composite adsorbent is composed of molecular sieve 5A and getter that includes the different proportion of PdO and Ag2O. The getter shall is laid as flat as possible when fed into the tank. Leakage and outgassing rate can be decreased by 64% after placing getter. The crystal phase structure of PdO and Ag2O in getter is unchanged by adsorption performance. Experimental results showed that the H2 adsorption rate is high at the initial stage, and then it starts to become slow during a relatively long period. The type IV isotherms were obtained with these samples, and the H2 adsorption principle is also discussed. The optimum percentage content of Ag2O in the getter is 22%. Under the allowed highest pressure, the composite adsorbent that includes 15% Ag2O, 85% PdO and molecular sieve 5A is preferentially used in the tank interlayer. The experimental results and performance analyses can be used in the design of the high vacuum multilayer insulated tank. Ó 2009 Published by Elsevier Ltd.
Keywords: Vacuum Composite adsorbent Getter Hydrogen adsorption Adsorption capacity
1. Introduction It has been a haunting concern for designers as to how to obtain and keep the required interlayer pressure in the high vacuum multilayer insulated tank. The adsorbents in the interlayer play a very important role in the process. The interlayer pressure depends largely on the characteristics of the adsorbents. In addition, how to achieve its adsorption adequately is also important. The adsorption performance of adsorbents can be illustrated effectively by the adsorption isotherm. Under the vacuum and thermal environment conditions, the metal materials and multilayer insulated materials will deflate. Based on much research on the compositions of the gases generated by leakage and outgassing, the results show that the amount of H2 exceeds 70% [1–3]. At temperature of liquid oxygen and liquid nitrogen, H2 adsorption capacity of molecular sieve 5A and activated carbon is very small. Therefore, the major problem is the residual hydrogen in the tank. An effective H2 getter is necessary in vacuum technology. Zr-based non-evaporable getters are widely used in high vacuum applications. However, these getters cannot be used in
* Corresponding author. Tel./fax: þ86 21 3420 6055. E-mail address: [email protected] (R. Wang). 0042-207X/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.vacuum.2009.03.007
the high vacuum multilayer insulated tank due to their limited adsorption capacity and the need to be activated at relatively high temperatures (>300 C) [4]. The use of transition metal oxides is promising for these purposes. It has been reported that PdO is an outstanding hydrogen getter for a vacuum tank [5–7]. So far, little work has been performed to study the effect of H2 adsorption performance of getters on the interlayer pressure. The experiential and semiempirical design methods are used to select the related adsorbents, amounts of PdO, etc. For instance, it is found in the field survey that Zhangjiagang CIMC Sanctum Cryogenic Equipment Co. Ltd and Chart Cryogenic Engineering Systems (Changzhou) Co. Ltd used, respectively, 1 g and 0.75 g getters in the 175 l cylinder. Although the amount is different, the same effect is achieved. Expenditure is greatly reduced in the latter case. Therefore, it is urgent to grasp accurately H2 adsorption performance of adsorbents. The main objective of this work is to study H2 adsorption performance of the composite adsorbent in the tank. Firstly, leakage and outgassing rate of the tank before placing the getter is compared with the rate after placing the getter. Secondly, H2 adsorption of composite adsorbent is studied. The quantitative analysis and computation are carried out by using H2 adsorption isotherm, which is obtained at room temperature. Thirdly, morphologies and crystalline phases of the getter are analyzed by
S. Chen et al. / Vacuum 83 (2009) 1184–1190
(2) H2 cylinder: Hydrogen used in H2 adsorption is a highly pure gas (99.999%). The quality of H2 is in accordance with the relevant regulations of GB/T7455-1995. (3) Calibration container: This has three ports that are connected with the vacuum unit, H2 cylinder and the tank respectively. The decreased amount of H2 can be measured by the calibration container. (4) Vacuum unit: This includes a mechanical pump and a molecular pump. The tank and the calibration container are evacuated by vacuum unit. In addition, the pressure of the calibration container is also controlled by it. (5) Measurement system: This composed of an intelligent manometer and two sets of compound vacuum gauges (ZDF-5227).
Nomenclature 2q
a a, b, c g H H2,(p) P DP S STP
Dt V xamax xbmin
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diffraction angle, H2 adsorption capacity, ml (STP)/g lattice constant, nm leakage and outgassing rate, Pa$m3/s H atom of surface adsorption H2 molecule of physical adsorption equilibrium pressure, Pa variation of the interlayer pressure during Dt, Pa adsorption site of Pd surface standard temperature and pressure, 273 K and 1.013 105 Pa time interval (¼ 8.64 104), s interlayer volume (¼ 41.07 103), m3 maximum amount of hydrogen adsorbed in Pd, forming a dilute solid solution, aH/Pd minimum amount of hydrogen adsorbed in Pd, forming b-hydride phase, H/Pd
2.3. Experiments on H2 adsorption
using scanning electron microscopy (SEM) and X-ray diffractometry (XRD) respectively. The intention of the research is to obtain the principle for H2 adsorption.
2. Experimental procedure 2.1. Samples The composite adsorbent is composed of molecular sieve 5A and getter. Molecular sieve 5A is selected due to its unique adsorption characteristics and ability to trap contaminated molecules. The getter includes different proportions of PdO and Ag2O. More details about the composition and condition of samples are shown in Table 1.
2.2. Experimental setup The experimental apparatus is shown in Fig. 1. H2 adsorption is studied by means of the pressure drop in a vacuum tank, whose volume is known. H2 consumption is determined by measuring the decreased H2 pressure. The apparatus is mainly composed of the following parts. (1) The high vacuum multilayer insulated tank: The tank is a cylindrical stainless-steel column, with internal and external diameters of 200 and 300 mm respectively, and internal length of 750 mm. Not only the experiment of the leakage and outgassing rate can be carried out in the vacuum tank, but also H2 adsorption kinetics and isotherm can be measured.
The first step is the preliminary stage of H2 adsorption. Approximately 1 g of getter was first fed into the tray in the tank interlayer. The volume of the interlayer was carefully calibrated. Molecular sieve 5A of 800 g was then divided evenly into two groups, which were placed on the upper and lower heads of inner cylinder in the tank interlayer respectively. The tank was evacuated by vacuum unit to 1 102 Pa at room temperature. The pretreatment of composite adsorbent was conducted, because there might be some contamination on their. At the same time, molecular sieve 5A was activated. So the sample was preheated continuously under vacuum for 1 day at a higher maximum temperature of no more than 150 C. Afterwards, the tank was cooled down to room temperature. The second step is the measurement of the leakage and outgassing rate. Firstly, leakage detection was carried out on the tank with a helium mass spectrometer. Then the experiment of the leakage and outgassing rate couldbe performed in the tank within permissive leakage. At the third step, H2 adsorption was carried out in the vacuum interlayer. The detailed procedure of H2 adsorption is as follows: Firstly, the room temperature and atmospheric pressure were recorded by using the intelligent manometer. Secondly, the calibration container was filled with H2, when it was evacuated to a suitable pressure. After the pressure of calibration container attained a certain value, valve 14 was closed. Meanwhile, the value was recorded. Thirdly, valve 11 was opened. The background pressure of the tank was recorded before valve 11 was opened. Fourthly, when the pressure of the tank reached a certain value, valve 11 was
15
4 6 7
14
10 9
Composite adsorbent Getter
#
1 A 1#B 2# 3
#
Adsorbent
100% PdO Molecular sieve 5A 100% PdO Molecular sieve 5A 85% PdO, Molecular 15% Ag2O sieve 5A 70% PdO, Molecular 30% Ag2O sieve 5A
Mass of getter (g)
Placement method of getter
Manufacturer
0.9813
Pack
0.9984
Flat
Shanghai Elegant Molecular Sieve Co. Shanghai Elegant Molecular Sieve Co. Shanghai Elegant Molecular Sieve Co. Shanghai Elegant Molecular Sieve Co.
1.0281 1.0375
Flat Flat
12 Ltd
5
8 9 10
13 Sample
1 2 3
17 16
Table 1 Composition and condition of samples.
19
5
18
11
1. Filling pipe 2.Getter 3.Tray 4. Molecular sieve 5A 5.Compound vacuum gauge 6.Multilayer insulation material 7.Vacuum interlayer 8.Tank 9.Resistance gauge 10.Ionozation gauge
Ltd Ltd Ltd
11,12,14.Vacuum valve 13.Calibration container 15.Vacuum unit 16.H2 cylinder 17.Pressure reducing valve 18.Computer 19.Outgassing pipe Fig.1. Experimental apparatus of H2 adsorption performance of composite adsorbent
Fig. 1. Experimental apparatus of H2 adsorption performance of composite adsorbent.
S. Chen et al. / Vacuum 83 (2009) 1184–1190
closed, and the value was recorded. Fifthly, the pressure of the tank interlayer was recorded automatically by using a personal computer with an RS232 interface. It was not until the pressure became constant within 2 h that adsorption equilibrium was realized. After the equilibrium was established, the whole system was evacuated again. As described above, the same procedure was repeated until the H2 adsorption isotherm was fully obtained.
2.4. X-ray diffraction
0.60 0.55
0.45
XRD was used for the qualitative determination of the crystalline phases of the getter. The XRD pattern of the getter packed in a glass holder was obtained at room temperature with CuKa radiation in a Bruker Advanced D-8 diffractometer having a q–q configuration and a graphite secondary beam monochromator. Diffraction intensities were measured between 30 and 70 , with a step of 0.01 for 10 s per point. Crystalline structures were refined with the Rietveld technique by using FULLPROF98 code [8]. The X-ray intensities were recorded by using a computer system and commercial software. Crystalline phases were identified by comparison with standard reference patterns from the Powder Diffraction File PDF-2 database sets 1–45, maintained by the International Centre for Diffraction Data (ICDD).
The surface morphology of getter was analyzed by using a JEOL JSM-6460LV SEM equipped with energy dispersive spectroscopy microanalyzer LINK AN 1000 EDS (Oxford Instruments). EDS elemental analysis was performed at several different points on the surface in order to minimize any possible anomalies arising from the heterogeneous nature of the sample.
0.40 0.35 0.30 0.25 0.20 0.15 (1) (2) (3)
0.10 0.05 0.00 0
2
4
6
8
10
12
14
16
18
20
22
24
26
Time (h) 0.22 0.20
b
0.18 0.16
Pressure (Pa)
2.5. Scanning electron microscopy
a
0.50
Pressure (Pa)
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0.14 0.12 0.10 0.08 0.06
(1) (2) (3)
0.04 0.02 0
2
4
6
8
10
3. Results and discussion
12
14
16
18
20
22
24
26
Time (h) Fig. 2. Variation of the interlayer pressure in the tank before placing getter (a) and after placing getter (b).
3.1. Leakage and outgassing rate The leakage and outgassing rate is an important parameter to evaluate the performance of the high vacuum multilayer insulated tank. It was measured on grounds of static pressurizing method. Fig. 2 represents the variation of the interlayer pressure in the tank before placing getter and after placing getter. In Fig. 2a, the pressure rises comparatively quickly. The variation of the curves (1) is maximal within 24 h, and the variation of the curves (3) is minimal. Their variations are 5.27 101 Pa and 4.18 101 Pa respectively. The experimental results showed that the average variation is 4.59 101 Pa. The pressure in Fig. 2b rises slowly and the increased degree is smaller than that of Fig. 2a. The maximum variation is just 43% of the minimum variation of Fig. 2a. The average variation of pressure is 1.66 101 Pa in Fig. 2b. Leakage and outgassing rate can be calculated by the formula (1) for the vacuum tank. The calculated values are given in Table 2. The leakage and outgassing rate can be decreased by 64% after placing getter. Although the adsorption capacity of composite adsorbent is very small at room temperature, redox reaction can consume some hydrogen. So the leakage and outgassing rate is reduced.
3.2. Effect of placement method on adsorption capacity Both of samples 1#A and 1#B are composed of getter PdO and molecular sieve 5A. The getter in 1#A was packed by dry formed paper, while the getter in 1#B was fed flatly into the tray. Fig. 3 shows room temperature H2 adsorption isotherms of 1# sample, which are obtained at the different placement methods. It can be seen in Fig. 3 that the changing trend of isotherms is the same, although H2 adsorption capacities of 1#B are obviously larger than those of 1#A. This can be ascribed to three reasons. Firstly, the contact area between 1#B and H2 is larger. Secondly, there is a chemical reaction during H2 adsorption. PdO can be deoxidized to Pd with the increase of H2. Because H2 adsorption is an exothermic reaction, the heat produced in 1#A cannot be removed timely. When the temperature suddenly rises, metal particles of Pd will Table 2 Leakage and outgassing rate. Leakage and outgassing rate (Pa m3/s)
DP V g ¼ Pa m3 =s Dt
(1)
Where g is the leakage and outgassing rate; V is the interlayer volume, and DP is the variation of the interlayer pressure during Dt, Dt is time interval.
(1) (2) (3) Average value
Before placing getter
After placing getter
2.51 107 2.05 107 1.99 107 2.18 107
7.84 108 8.51 108 7.37 108 7.91 108
S. Chen et al. / Vacuum 83 (2009) 1184–1190
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0.4 1#A
α (ml (STP) / g )
1#B
0.1
0.01
5E-3 5E-4 1E-3
0.01
0.1
1
8
Equilibrium pressure (Pa) Fig. 3. H2 adsorption isotherms of 1# sample.
agglomerate or move out, which can weaken the chemical adsorption of Pd and decrease the adsorbed amount of H2 gas. Though there is a similar reaction in 1#B, the heat can be removed timely. Getter is the well-proportioned solid powder after H2 adsorption. Thirdly, a skin of Pd on PdO particles is formed due to the agglomeration and migration of Pd in 1#A, which can reduce the likelihood of the further contact between H2 and PdO. Meanwhile, catalytic activity of Pd is also decreased in the skin. This is consistent with Fig. 4 which shows the SEM images of getter in both 1#A (a) and 1#B (b). It is shown in Fig. 4 that the particles are massive and rough in shape, and the particles in 1#A look larger than those in 1#B at the same amplification. It is also observed that a skin of Pd on PdO particles is formed, as indicated by the arrow in Fig. 4a. 3.3. Phase analysis The XRD patterns of getter in both 1#B (a) and 2# (b) are shown in Fig. 5. The dotted line represents the XRD pattern of getter before H2 adsorption, and the solid line shows their XRD pattern of getter after H2 adsorption. PdO crystallizes in a tetragonal structure with lattice constants a ¼ b ¼ 0.304 nm and c ¼ 0.534 nm at room temperature. The detailed experimental XRD data are shown in Table 3. It can be seen in Fig. 5 and Table 3 that the characteristic peaks of PdO and Ag2O are the same before and after H2 adsorption in the getter. XRD results showed that their crystal phase structure is unchanged by adsorption performance, and new substances are observed. This is consistent with the results of Section 3.2. The diffraction peaks of the getter in 1#B appear at 34 , 42 , 55 and 61 before H2 adsorption. 2q of the biggest diffraction intensity is 34 . Compared with standard reference patterns from the Powder Diffraction File PDF-2 database sets 1–45, crystalline phases in 1#B before H2 adsorption are identified. The substance is PdO. The diffraction peaks of the getter in 1#B appear at 34 , 39 ,42 ,45 ,55 ,61 and 66 after H2 adsorption. 2q of the biggest diffraction intensity shifts to the right side compared with that before H2 adsorption, and the value is 39 . The mixture of PdO and PdH0.706 is observed by XRD. It can be seen from diffraction intensities that the content of PdO in the getter decreases obviously after H2 adsorption. The formation process of PdH0.706 is described as follows: Firstly, PdO is deoxidized by hydrogen. The getter becomes the mixture of PdO and Pd. Pd has not only catalytic ability but also adsorption ability for H2. Secondly, the physical adsorption occurs in the Pd/H2 system. The intermolecular interaction between H2 molecules and
Fig. 4. SEM images of getter in both 1#A (a) and 1#B (b).
Pd surfaces leads easily to dissociation of H2 molecules. So a fraction of molecules impinging on the surface will dissociate into two isolated atoms and become the chemical adsorption. At room temperature, and for H concentrations less than xamax z0:01 (hydrogen concentrations, x, are in atom fractions, H/Pd), the H atoms remain at random in the octahedral-symmetry interstitial positions of type (0,1/2, 0) in the fcc Pd lattice, and form a dilute solid solution, a. As the H concentration increases above xamax , some of the H atoms coalesce into an ordered palladium–hydride phase, b, having the composition xbmin z0:62. In the b-hydride phase, the Pd atoms retain the fcc lattice of the a-phase, and the volume of the lattice expands by approximately 12% [9]. The hydrogen atoms remain in (0, 1/2, 0) type positions, occupying only 62% of the available interstitial sites. The position model of H atoms in Pd lattice is shown in Fig. 6. For xamax < x < xbmin, Pd is a two-phase mixture of a and b phases. These steps can be expressed by Eqs. (2)–(5): PdO deoxidation by hydrogen:
PdO þ H2 ¼ Pd þ H2 O
(2)
Physical adsorption between H2 and Pd surface:
physisorption
H2 þ 2S ! H2;ðpÞ
(3)
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S. Chen et al. / Vacuum 83 (2009) 1184–1190
0.7
0.7
a
0.6
PdO PdH0.706
0.6 0.5
After H2 adsorption Before H2 adsorption
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
Counts
Counts
0.5
0.0
0.0 30
35
40
45
50
55
60
65
70
2 (°)
b
PdO PdH0.706 Ag2O Ag
0.6
The kinetic curves of 1#B were obtained at constant temperature and volume. Fig. 7 shows H2 adsorption kinetic curves of 1#B at room temperature and different equilibrium pressures. H2 adsorption processes of 1#B can be divided into two stages: Firstly the reaction rate is high at the initial stage, and then it starts to become slow during a relatively long period. At the initial stage, the variation of pressure–time accords with the exponential decay relation. The coefficient of the exponential term is a variable and depends on the equilibrium pressure. The degree of the pressure decreasing becomes smaller as the equilibrium pressure increases. H2 adsorption processes of other samples are similar with those of 1#B. In the system of the composite adsorbent and H2, H2O produced in the reaction is adsorbed by molecular sieve 5A. Because H2O adsorption ability of molecular sieves 5A is stronger than O2 adsorption ability, the adsorbed H2O can displace O2 that has been adsorbed. There is a physical adsorption between O2 with Pd surface. The adsorbed O2 molecules can be dissociated under the temptation of hydrogen ions. The synthetic reaction between hydrogen ions and oxygen ions can easily take place due to the catalytic activity of Pd. So the interlayer pressure can be improved.
0.5
After H2 adsorption Before H2 adsorption
0.2
3.4. H2 adsorption kinetic analysis
0.4 0.3
Counts
Counts
0.3
0.7
0.2
0.1
0.1 0.0
0.0 30
35
40
45
50
55
60
65
70
2 (°) Fig. 5. XRD patterns of getter in both 1#B (a) and 2# (b).
H2 molecules dissociation on Pd surface: H,,,,,H
H2;ðpÞ ! 2H
(4) 3.5. H2 adsorption isotherms
Pd–hydride phase formation:
xH þ Pd ¼ b-PdHx
(5)
Where S is the adsorbing site of Pd surface; H2,(p) is H2 molecule of physical adsorption, and H is H atom of surface adsorption. In Fig. 5b, besides the above-mentioned reactions, there is a decomposition reaction because one of Ag2O’s characteristics is unstable. So the elementary substance of Ag is observed.
Table 3 XRD data of getter in both 1#B and 2# Sample
Composition
Getter in 1#B
B A
Getter in 2#
B A
PdO PdO PdH0.706 PdO Ag2O PdH0.706 PdO Ag2O Ag
Structure
Tetragonal Tetragonal fcc Tetragonal Cubic fcc Tetragonal Cubic fcc
2q of diffraction peaks ( )
Lattice constant a (nm)
c (nm)
0.304 0.304 0.402 0.304 0.473 0.402 0.304 0.473 0.409
0.534 0.534 0.402 0.534 0.473 0.402 0.534 0.473 0.409
Notes: a ¼ b; B ¼ before H2 adsorption; A ¼ after H2 adsorption.
34, 34, 39, 34, 33, 39, 34, 33, 38,
42, 55, 61 42, 55, 61 45, 66 42, 55, 61 38, 47, 55, 65.5 45, 66 42, 55, 61 38, 47, 55, 65.5 44.5, 64.5
Fig. 8 represents H2 adsorption isotherms of the different samples. These isotherms are obtained at room temperature. The 0.1
(7)
Pressure (Pa)
0.4
Fig. 6. Position model of H atoms in Pd lattice for b-PdH.
(6) 0.01
(5) (4) (3) (1) (2) (3) (4) (5) (6) (7)
1E-3 (2) (1) 0
2
4
6
8
10
12
14
16
18
20
0. 00054Pa 0. 00079Pa 0. 0068Pa 0. 0075Pa 0. 01Pa 0. 013Pa 0. 025Pa 22
24
Time (h) Fig. 7. H2 adsorption kinetic curves of 1#B at room temperature.
26
28
S. Chen et al. / Vacuum 83 (2009) 1184–1190
0.7
α ( ml (STP) / g )
(c)
(b)
0.1
(a) 1#B 2#
0.01
3# 1E-3
0.01
0.1
1
10
Equilibrium pressure (Pa) Fig. 8. Room-temperature H2 adsorption isotherms of different samples.
isotherms of these samples exhibit type IV isotherms according to the BDDT theory [10]. The changing process of H2 adsorption isotherms generally undergoes three stages, which are shown as follows: (1) The first stage (a) represents type Langmuir curves of the upper convex. It belongs to the favorable H2 adsorption isotherm. Although the concentration of H2 is very low, the composite adsorbent has rather high equilibrium adsorption capacity. (2) The second stage (b) is a type whose changing trend is inverse to that of type Langmuir curve. It belongs to the unfavorable H2 adsorption isotherm. The curve shape is concave because of the weak intermolecular interaction between H2 and the getter. The adsorbed amount is very low in spite of H2 concentration. (3) The third stage (c) is the repetition of the first stage. The upper convex degree is smaller than that of the first stage. The experimental data is 4–5 orders of magnitude for pressure. H2 adsorption isotherms are in agreement with Temkin’s empirical equation in the first and third stages. In this instance, the error is very small. However, the curves fitting deviate from the experimental data in the second stage. Freundlich’s empirical equation is consistent with the experimental data under this condition. The formula (6) is the fitting equation of 1#B. The H2 adsorption capacity is 0.69 ml (STP)/g (STP represents the standard state) at 700 Pa. This is consistent with the experimental result of Belousov [6]:
8 < a ¼ 0:02599 þ 0:00122lnðP 0:00057Þ p < 0:0047 Pa a ¼ 1:24396P 0:78359 0:0047 Pa P 0:025 Pa : a ¼ 0:19433 þ 0:07544lnðP þ 0:15168Þ P > 0:025 Pa (6) Where a is H2 adsorption capacity and P is the equilibrium pressure. It can be seen in Fig. 8 that the general order of H2 adsorption capacities is 2# > 1#B > 3# at a pressure higher than 102 Pa. H2 adsorption capacity of 2# is 0.6 ml (STP)/g at 5 Pa. The adsorbed amount of H2 gas is 0.3 ml (STP)/g at 5.8 Pa for 1#B, which just accounts for 50% of H2 adsorption capacity of 2# under the same conditions. The highest measured adsorption capacity of H2 for 2#, compared to 1#B and 3#, can be explained by its compositions. It is composed of 15% PdO, 85% Ag2O and molecular sieve 5A. The roles of Ag2O are as follows: It is a catalyst that can accelerate H2 adsorption. It can also provide oxygen for H2 adsorption due to
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the unstable characteristic of Ag2O, and simultaneously, the elementary substance of Ag is obtained. The heat transfer property of Ag is the highest in all the metals. The heat produced can be removed timely, which makes the temperature of the getter steady. The isotherm curve shifts to the left side with the increase of Ag. This is consistent with the observation reported by Brodowaky [11]. Pd and Ag form a continuous solid solution with an fcc structure. Due to the only slightly negative enthalpy of formation of Pd–Ag alloys, a random distribution of Pd and Ag in the alloy can be expected. In a substitutional coarse-grained Pd– Ag alloy whose unit cell is larger than that of Pd, more hydrogen dissolves at a given low pressure than in Pd, and its plateau pressure is lower than that of pure Pd. In addition, the (a þ b) plateau becomes narrower as the amount of Ag increases. The region of coexistence of a and b disappears as the composition approaches a critical value near Pd77Ag23 [12]. With the increase of Ag to Pd up to 23 at.%, hydrogen permeability increases linearly [11,13]. But the ratio of PdO and Ag2O reaches an optimum value. The optimum percentage content of Ag2O is 22% in the getter according to the study of Pd–Ag alloys. Although there is also Ag2O in 3# sample, the adsorbed capacities of H2 gas are the smallest at the same pressure, because it contains too much Ag2O. In order to achieve the optimal effect of the multilayer insulation, it is desirable to keep the interlayer pressure lower than 102 Pa. H2 adsorption capacity of 1#B, 2# and 3# is 0.0320 ml (STP)/ g, 0.0312 ml (STP)/g and 0.0276 ml (STP)/g at 1 102 Pa respectively. H2 adsorption capacity of 2# is slightly lower than that of 1#B. But the order of the price per unit mass is 1#B > 2# > 3#. So 2# sample is preferentially used in the tank interlayer. 4. Conclusions The interlayer pressure depends largely on H2 adsorption performance of composite adsorbent, which can be illustrated effectively by the adsorption isotherm. The getter should be laid as flat as possible when fed into the tank. Leakage and outgassing rate can be decreased by 64% after placing the getter. XRD results showed that the crystal phase structure of PdO and Ag2O is unchanged by adsorption performance, and new substances such as PdH0.706 and Ag are observed. The H2 adsorption rate is high at the initial stage, then it starts to become slow during a relatively long period. The isotherms of these samples exhibit type IV isotherms. The general order of H2 adsorption capacities is 2# > 1#B > 3# at a pressure higher than 102 Pa. The optimum percentage content of Ag2O in the getter is 22%. Under the allowed highest pressure, 2# sample is preferentially used in the tank with high vacuum multilayer insulation. References [1] Yang Y, Saitoh K, Tsukahara S. An improved throughput method for the measurement of outgassing rates of materials. Vacuum 1995;46:1371–6. [2] Xi GK, Shao SM, Li SL, Wang JR. Studies on magnesium rich rare-earth alloys for hydrogen absorption in vacuum. Vacuum 1998;39:531–5. [3] Nemanic V, Zumer M, Zajec B. The influence of a hot cathode vacuum gauge on the residual gas composition. Vacuum 2003;70:523–30. [4] Della Porta P. Gas problem and gettering in sealed-off vacuum devices. Vacuum 1996;47:771–7. [5] Boffito C, Schiabel A, Gallitognotta A. Thermally insulating jacket and related process. U.S. Patent. Patent Number 1995;5408832. [6] Belousov VM, Vasylyev MA, Lyashenko LV, NYu Vilkova, Nieuwenhuys BE. The low-temperature reduction of Pd-doped transition metal oxide surface with hydrogen. Chem Eng J 2003;91:143–50. [7] Boffito C, Ferrario B. A device for the removal of hydrogen from a vacuum enclosure at cryogenic temperatures and especially high energy particle accelerators. European Patent; Application number: EP19920830028 1995. [8] Chen LF, Wang JA, Valenzuela MA, Bokhimi X, Acosta DR, Novaro O. Hydrogen spillover and structural defects in a PdO/zirconia nanophase synthesized through a surfactant-templated route. J Alloys Compd 2006;417:220–3.
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Experimental investigation on hydrogen adsorption performance of composite adsorbent in the tank with high vacuum multilayer insulation Shujun Chen, Xiangdong Li, Rongshun Wang*, Gaofeng Xie, Yuwu Zeng Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 October 2008 Received in revised form 19 March 2009 Accepted 21 March 2009
Hydrogen is one of the main residual gases in the high vacuum multilayer insulated tank. H2 adsorption performance of the composite adsorbent was investigated by using a test bench, an X-ray diffraction and a scanning electron microscopy. The composite adsorbent is composed of molecular sieve 5A and getter that includes the different proportion of PdO and Ag2O. The getter shall is laid as flat as possible when fed into the tank. Leakage and outgassing rate can be decreased by 64% after placing getter. The crystal phase structure of PdO and Ag2O in getter is unchanged by adsorption performance. Experimental results showed that the H2 adsorption rate is high at the initial stage, and then it starts to become slow during a relatively long period. The type IV isotherms were obtained with these samples, and the H2 adsorption principle is also discussed. The optimum percentage content of Ag2O in the getter is 22%. Under the allowed highest pressure, the composite adsorbent that includes 15% Ag2O, 85% PdO and molecular sieve 5A is preferentially used in the tank interlayer. The experimental results and performance analyses can be used in the design of the high vacuum multilayer insulated tank. Ó 2009 Published by Elsevier Ltd.
Keywords: Vacuum Composite adsorbent Getter Hydrogen adsorption Adsorption capacity
1. Introduction It has been a haunting concern for designers as to how to obtain and keep the required interlayer pressure in the high vacuum multilayer insulated tank. The adsorbents in the interlayer play a very important role in the process. The interlayer pressure depends largely on the characteristics of the adsorbents. In addition, how to achieve its adsorption adequately is also important. The adsorption performance of adsorbents can be illustrated effectively by the adsorption isotherm. Under the vacuum and thermal environment conditions, the metal materials and multilayer insulated materials will deflate. Based on much research on the compositions of the gases generated by leakage and outgassing, the results show that the amount of H2 exceeds 70% [1–3]. At temperature of liquid oxygen and liquid nitrogen, H2 adsorption capacity of molecular sieve 5A and activated carbon is very small. Therefore, the major problem is the residual hydrogen in the tank. An effective H2 getter is necessary in vacuum technology. Zr-based non-evaporable getters are widely used in high vacuum applications. However, these getters cannot be used in
* Corresponding author. Tel./fax: þ86 21 3420 6055. E-mail address: [email protected] (R. Wang). 0042-207X/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.vacuum.2009.03.007
the high vacuum multilayer insulated tank due to their limited adsorption capacity and the need to be activated at relatively high temperatures (>300 C) [4]. The use of transition metal oxides is promising for these purposes. It has been reported that PdO is an outstanding hydrogen getter for a vacuum tank [5–7]. So far, little work has been performed to study the effect of H2 adsorption performance of getters on the interlayer pressure. The experiential and semiempirical design methods are used to select the related adsorbents, amounts of PdO, etc. For instance, it is found in the field survey that Zhangjiagang CIMC Sanctum Cryogenic Equipment Co. Ltd and Chart Cryogenic Engineering Systems (Changzhou) Co. Ltd used, respectively, 1 g and 0.75 g getters in the 175 l cylinder. Although the amount is different, the same effect is achieved. Expenditure is greatly reduced in the latter case. Therefore, it is urgent to grasp accurately H2 adsorption performance of adsorbents. The main objective of this work is to study H2 adsorption performance of the composite adsorbent in the tank. Firstly, leakage and outgassing rate of the tank before placing the getter is compared with the rate after placing the getter. Secondly, H2 adsorption of composite adsorbent is studied. The quantitative analysis and computation are carried out by using H2 adsorption isotherm, which is obtained at room temperature. Thirdly, morphologies and crystalline phases of the getter are analyzed by
S. Chen et al. / Vacuum 83 (2009) 1184–1190
(2) H2 cylinder: Hydrogen used in H2 adsorption is a highly pure gas (99.999%). The quality of H2 is in accordance with the relevant regulations of GB/T7455-1995. (3) Calibration container: This has three ports that are connected with the vacuum unit, H2 cylinder and the tank respectively. The decreased amount of H2 can be measured by the calibration container. (4) Vacuum unit: This includes a mechanical pump and a molecular pump. The tank and the calibration container are evacuated by vacuum unit. In addition, the pressure of the calibration container is also controlled by it. (5) Measurement system: This composed of an intelligent manometer and two sets of compound vacuum gauges (ZDF-5227).
Nomenclature 2q
a a, b, c g H H2,(p) P DP S STP
Dt V xamax xbmin
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diffraction angle, H2 adsorption capacity, ml (STP)/g lattice constant, nm leakage and outgassing rate, Pa$m3/s H atom of surface adsorption H2 molecule of physical adsorption equilibrium pressure, Pa variation of the interlayer pressure during Dt, Pa adsorption site of Pd surface standard temperature and pressure, 273 K and 1.013 105 Pa time interval (¼ 8.64 104), s interlayer volume (¼ 41.07 103), m3 maximum amount of hydrogen adsorbed in Pd, forming a dilute solid solution, aH/Pd minimum amount of hydrogen adsorbed in Pd, forming b-hydride phase, H/Pd
2.3. Experiments on H2 adsorption
using scanning electron microscopy (SEM) and X-ray diffractometry (XRD) respectively. The intention of the research is to obtain the principle for H2 adsorption.
2. Experimental procedure 2.1. Samples The composite adsorbent is composed of molecular sieve 5A and getter. Molecular sieve 5A is selected due to its unique adsorption characteristics and ability to trap contaminated molecules. The getter includes different proportions of PdO and Ag2O. More details about the composition and condition of samples are shown in Table 1.
2.2. Experimental setup The experimental apparatus is shown in Fig. 1. H2 adsorption is studied by means of the pressure drop in a vacuum tank, whose volume is known. H2 consumption is determined by measuring the decreased H2 pressure. The apparatus is mainly composed of the following parts. (1) The high vacuum multilayer insulated tank: The tank is a cylindrical stainless-steel column, with internal and external diameters of 200 and 300 mm respectively, and internal length of 750 mm. Not only the experiment of the leakage and outgassing rate can be carried out in the vacuum tank, but also H2 adsorption kinetics and isotherm can be measured.
The first step is the preliminary stage of H2 adsorption. Approximately 1 g of getter was first fed into the tray in the tank interlayer. The volume of the interlayer was carefully calibrated. Molecular sieve 5A of 800 g was then divided evenly into two groups, which were placed on the upper and lower heads of inner cylinder in the tank interlayer respectively. The tank was evacuated by vacuum unit to 1 102 Pa at room temperature. The pretreatment of composite adsorbent was conducted, because there might be some contamination on their. At the same time, molecular sieve 5A was activated. So the sample was preheated continuously under vacuum for 1 day at a higher maximum temperature of no more than 150 C. Afterwards, the tank was cooled down to room temperature. The second step is the measurement of the leakage and outgassing rate. Firstly, leakage detection was carried out on the tank with a helium mass spectrometer. Then the experiment of the leakage and outgassing rate couldbe performed in the tank within permissive leakage. At the third step, H2 adsorption was carried out in the vacuum interlayer. The detailed procedure of H2 adsorption is as follows: Firstly, the room temperature and atmospheric pressure were recorded by using the intelligent manometer. Secondly, the calibration container was filled with H2, when it was evacuated to a suitable pressure. After the pressure of calibration container attained a certain value, valve 14 was closed. Meanwhile, the value was recorded. Thirdly, valve 11 was opened. The background pressure of the tank was recorded before valve 11 was opened. Fourthly, when the pressure of the tank reached a certain value, valve 11 was
15
4 6 7
14
10 9
Composite adsorbent Getter
#
1 A 1#B 2# 3
#
Adsorbent
100% PdO Molecular sieve 5A 100% PdO Molecular sieve 5A 85% PdO, Molecular 15% Ag2O sieve 5A 70% PdO, Molecular 30% Ag2O sieve 5A
Mass of getter (g)
Placement method of getter
Manufacturer
0.9813
Pack
0.9984
Flat
Shanghai Elegant Molecular Sieve Co. Shanghai Elegant Molecular Sieve Co. Shanghai Elegant Molecular Sieve Co. Shanghai Elegant Molecular Sieve Co.
1.0281 1.0375
Flat Flat
12 Ltd
5
8 9 10
13 Sample
1 2 3
17 16
Table 1 Composition and condition of samples.
19
5
18
11
1. Filling pipe 2.Getter 3.Tray 4. Molecular sieve 5A 5.Compound vacuum gauge 6.Multilayer insulation material 7.Vacuum interlayer 8.Tank 9.Resistance gauge 10.Ionozation gauge
Ltd Ltd Ltd
11,12,14.Vacuum valve 13.Calibration container 15.Vacuum unit 16.H2 cylinder 17.Pressure reducing valve 18.Computer 19.Outgassing pipe Fig.1. Experimental apparatus of H2 adsorption performance of composite adsorbent
Fig. 1. Experimental apparatus of H2 adsorption performance of composite adsorbent.
S. Chen et al. / Vacuum 83 (2009) 1184–1190
closed, and the value was recorded. Fifthly, the pressure of the tank interlayer was recorded automatically by using a personal computer with an RS232 interface. It was not until the pressure became constant within 2 h that adsorption equilibrium was realized. After the equilibrium was established, the whole system was evacuated again. As described above, the same procedure was repeated until the H2 adsorption isotherm was fully obtained.
2.4. X-ray diffraction
0.60 0.55
0.45
XRD was used for the qualitative determination of the crystalline phases of the getter. The XRD pattern of the getter packed in a glass holder was obtained at room temperature with CuKa radiation in a Bruker Advanced D-8 diffractometer having a q–q configuration and a graphite secondary beam monochromator. Diffraction intensities were measured between 30 and 70 , with a step of 0.01 for 10 s per point. Crystalline structures were refined with the Rietveld technique by using FULLPROF98 code [8]. The X-ray intensities were recorded by using a computer system and commercial software. Crystalline phases were identified by comparison with standard reference patterns from the Powder Diffraction File PDF-2 database sets 1–45, maintained by the International Centre for Diffraction Data (ICDD).
The surface morphology of getter was analyzed by using a JEOL JSM-6460LV SEM equipped with energy dispersive spectroscopy microanalyzer LINK AN 1000 EDS (Oxford Instruments). EDS elemental analysis was performed at several different points on the surface in order to minimize any possible anomalies arising from the heterogeneous nature of the sample.
0.40 0.35 0.30 0.25 0.20 0.15 (1) (2) (3)
0.10 0.05 0.00 0
2
4
6
8
10
12
14
16
18
20
22
24
26
Time (h) 0.22 0.20
b
0.18 0.16
Pressure (Pa)
2.5. Scanning electron microscopy
a
0.50
Pressure (Pa)
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0.14 0.12 0.10 0.08 0.06
(1) (2) (3)
0.04 0.02 0
2
4
6
8
10
3. Results and discussion
12
14
16
18
20
22
24
26
Time (h) Fig. 2. Variation of the interlayer pressure in the tank before placing getter (a) and after placing getter (b).
3.1. Leakage and outgassing rate The leakage and outgassing rate is an important parameter to evaluate the performance of the high vacuum multilayer insulated tank. It was measured on grounds of static pressurizing method. Fig. 2 represents the variation of the interlayer pressure in the tank before placing getter and after placing getter. In Fig. 2a, the pressure rises comparatively quickly. The variation of the curves (1) is maximal within 24 h, and the variation of the curves (3) is minimal. Their variations are 5.27 101 Pa and 4.18 101 Pa respectively. The experimental results showed that the average variation is 4.59 101 Pa. The pressure in Fig. 2b rises slowly and the increased degree is smaller than that of Fig. 2a. The maximum variation is just 43% of the minimum variation of Fig. 2a. The average variation of pressure is 1.66 101 Pa in Fig. 2b. Leakage and outgassing rate can be calculated by the formula (1) for the vacuum tank. The calculated values are given in Table 2. The leakage and outgassing rate can be decreased by 64% after placing getter. Although the adsorption capacity of composite adsorbent is very small at room temperature, redox reaction can consume some hydrogen. So the leakage and outgassing rate is reduced.
3.2. Effect of placement method on adsorption capacity Both of samples 1#A and 1#B are composed of getter PdO and molecular sieve 5A. The getter in 1#A was packed by dry formed paper, while the getter in 1#B was fed flatly into the tray. Fig. 3 shows room temperature H2 adsorption isotherms of 1# sample, which are obtained at the different placement methods. It can be seen in Fig. 3 that the changing trend of isotherms is the same, although H2 adsorption capacities of 1#B are obviously larger than those of 1#A. This can be ascribed to three reasons. Firstly, the contact area between 1#B and H2 is larger. Secondly, there is a chemical reaction during H2 adsorption. PdO can be deoxidized to Pd with the increase of H2. Because H2 adsorption is an exothermic reaction, the heat produced in 1#A cannot be removed timely. When the temperature suddenly rises, metal particles of Pd will Table 2 Leakage and outgassing rate. Leakage and outgassing rate (Pa m3/s)
DP V g ¼ Pa m3 =s Dt
(1)
Where g is the leakage and outgassing rate; V is the interlayer volume, and DP is the variation of the interlayer pressure during Dt, Dt is time interval.
(1) (2) (3) Average value
Before placing getter
After placing getter
2.51 107 2.05 107 1.99 107 2.18 107
7.84 108 8.51 108 7.37 108 7.91 108
S. Chen et al. / Vacuum 83 (2009) 1184–1190
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0.4 1#A
α (ml (STP) / g )
1#B
0.1
0.01
5E-3 5E-4 1E-3
0.01
0.1
1
8
Equilibrium pressure (Pa) Fig. 3. H2 adsorption isotherms of 1# sample.
agglomerate or move out, which can weaken the chemical adsorption of Pd and decrease the adsorbed amount of H2 gas. Though there is a similar reaction in 1#B, the heat can be removed timely. Getter is the well-proportioned solid powder after H2 adsorption. Thirdly, a skin of Pd on PdO particles is formed due to the agglomeration and migration of Pd in 1#A, which can reduce the likelihood of the further contact between H2 and PdO. Meanwhile, catalytic activity of Pd is also decreased in the skin. This is consistent with Fig. 4 which shows the SEM images of getter in both 1#A (a) and 1#B (b). It is shown in Fig. 4 that the particles are massive and rough in shape, and the particles in 1#A look larger than those in 1#B at the same amplification. It is also observed that a skin of Pd on PdO particles is formed, as indicated by the arrow in Fig. 4a. 3.3. Phase analysis The XRD patterns of getter in both 1#B (a) and 2# (b) are shown in Fig. 5. The dotted line represents the XRD pattern of getter before H2 adsorption, and the solid line shows their XRD pattern of getter after H2 adsorption. PdO crystallizes in a tetragonal structure with lattice constants a ¼ b ¼ 0.304 nm and c ¼ 0.534 nm at room temperature. The detailed experimental XRD data are shown in Table 3. It can be seen in Fig. 5 and Table 3 that the characteristic peaks of PdO and Ag2O are the same before and after H2 adsorption in the getter. XRD results showed that their crystal phase structure is unchanged by adsorption performance, and new substances are observed. This is consistent with the results of Section 3.2. The diffraction peaks of the getter in 1#B appear at 34 , 42 , 55 and 61 before H2 adsorption. 2q of the biggest diffraction intensity is 34 . Compared with standard reference patterns from the Powder Diffraction File PDF-2 database sets 1–45, crystalline phases in 1#B before H2 adsorption are identified. The substance is PdO. The diffraction peaks of the getter in 1#B appear at 34 , 39 ,42 ,45 ,55 ,61 and 66 after H2 adsorption. 2q of the biggest diffraction intensity shifts to the right side compared with that before H2 adsorption, and the value is 39 . The mixture of PdO and PdH0.706 is observed by XRD. It can be seen from diffraction intensities that the content of PdO in the getter decreases obviously after H2 adsorption. The formation process of PdH0.706 is described as follows: Firstly, PdO is deoxidized by hydrogen. The getter becomes the mixture of PdO and Pd. Pd has not only catalytic ability but also adsorption ability for H2. Secondly, the physical adsorption occurs in the Pd/H2 system. The intermolecular interaction between H2 molecules and
Fig. 4. SEM images of getter in both 1#A (a) and 1#B (b).
Pd surfaces leads easily to dissociation of H2 molecules. So a fraction of molecules impinging on the surface will dissociate into two isolated atoms and become the chemical adsorption. At room temperature, and for H concentrations less than xamax z0:01 (hydrogen concentrations, x, are in atom fractions, H/Pd), the H atoms remain at random in the octahedral-symmetry interstitial positions of type (0,1/2, 0) in the fcc Pd lattice, and form a dilute solid solution, a. As the H concentration increases above xamax , some of the H atoms coalesce into an ordered palladium–hydride phase, b, having the composition xbmin z0:62. In the b-hydride phase, the Pd atoms retain the fcc lattice of the a-phase, and the volume of the lattice expands by approximately 12% [9]. The hydrogen atoms remain in (0, 1/2, 0) type positions, occupying only 62% of the available interstitial sites. The position model of H atoms in Pd lattice is shown in Fig. 6. For xamax < x < xbmin, Pd is a two-phase mixture of a and b phases. These steps can be expressed by Eqs. (2)–(5): PdO deoxidation by hydrogen:
PdO þ H2 ¼ Pd þ H2 O
(2)
Physical adsorption between H2 and Pd surface:
physisorption
H2 þ 2S ! H2;ðpÞ
(3)
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0.7
0.7
a
0.6
PdO PdH0.706
0.6 0.5
After H2 adsorption Before H2 adsorption
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
Counts
Counts
0.5
0.0
0.0 30
35
40
45
50
55
60
65
70
2 (°)
b
PdO PdH0.706 Ag2O Ag
0.6
The kinetic curves of 1#B were obtained at constant temperature and volume. Fig. 7 shows H2 adsorption kinetic curves of 1#B at room temperature and different equilibrium pressures. H2 adsorption processes of 1#B can be divided into two stages: Firstly the reaction rate is high at the initial stage, and then it starts to become slow during a relatively long period. At the initial stage, the variation of pressure–time accords with the exponential decay relation. The coefficient of the exponential term is a variable and depends on the equilibrium pressure. The degree of the pressure decreasing becomes smaller as the equilibrium pressure increases. H2 adsorption processes of other samples are similar with those of 1#B. In the system of the composite adsorbent and H2, H2O produced in the reaction is adsorbed by molecular sieve 5A. Because H2O adsorption ability of molecular sieves 5A is stronger than O2 adsorption ability, the adsorbed H2O can displace O2 that has been adsorbed. There is a physical adsorption between O2 with Pd surface. The adsorbed O2 molecules can be dissociated under the temptation of hydrogen ions. The synthetic reaction between hydrogen ions and oxygen ions can easily take place due to the catalytic activity of Pd. So the interlayer pressure can be improved.
0.5
After H2 adsorption Before H2 adsorption
0.2
3.4. H2 adsorption kinetic analysis
0.4 0.3
Counts
Counts
0.3
0.7
0.2
0.1
0.1 0.0
0.0 30
35
40
45
50
55
60
65
70
2 (°) Fig. 5. XRD patterns of getter in both 1#B (a) and 2# (b).
H2 molecules dissociation on Pd surface: H,,,,,H
H2;ðpÞ ! 2H
(4) 3.5. H2 adsorption isotherms
Pd–hydride phase formation:
xH þ Pd ¼ b-PdHx
(5)
Where S is the adsorbing site of Pd surface; H2,(p) is H2 molecule of physical adsorption, and H is H atom of surface adsorption. In Fig. 5b, besides the above-mentioned reactions, there is a decomposition reaction because one of Ag2O’s characteristics is unstable. So the elementary substance of Ag is observed.
Table 3 XRD data of getter in both 1#B and 2# Sample
Composition
Getter in 1#B
B A
Getter in 2#
B A
PdO PdO PdH0.706 PdO Ag2O PdH0.706 PdO Ag2O Ag
Structure
Tetragonal Tetragonal fcc Tetragonal Cubic fcc Tetragonal Cubic fcc
2q of diffraction peaks ( )
Lattice constant a (nm)
c (nm)
0.304 0.304 0.402 0.304 0.473 0.402 0.304 0.473 0.409
0.534 0.534 0.402 0.534 0.473 0.402 0.534 0.473 0.409
Notes: a ¼ b; B ¼ before H2 adsorption; A ¼ after H2 adsorption.
34, 34, 39, 34, 33, 39, 34, 33, 38,
42, 55, 61 42, 55, 61 45, 66 42, 55, 61 38, 47, 55, 65.5 45, 66 42, 55, 61 38, 47, 55, 65.5 44.5, 64.5
Fig. 8 represents H2 adsorption isotherms of the different samples. These isotherms are obtained at room temperature. The 0.1
(7)
Pressure (Pa)
0.4
Fig. 6. Position model of H atoms in Pd lattice for b-PdH.
(6) 0.01
(5) (4) (3) (1) (2) (3) (4) (5) (6) (7)
1E-3 (2) (1) 0
2
4
6
8
10
12
14
16
18
20
0. 00054Pa 0. 00079Pa 0. 0068Pa 0. 0075Pa 0. 01Pa 0. 013Pa 0. 025Pa 22
24
Time (h) Fig. 7. H2 adsorption kinetic curves of 1#B at room temperature.
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0.7
α ( ml (STP) / g )
(c)
(b)
0.1
(a) 1#B 2#
0.01
3# 1E-3
0.01
0.1
1
10
Equilibrium pressure (Pa) Fig. 8. Room-temperature H2 adsorption isotherms of different samples.
isotherms of these samples exhibit type IV isotherms according to the BDDT theory [10]. The changing process of H2 adsorption isotherms generally undergoes three stages, which are shown as follows: (1) The first stage (a) represents type Langmuir curves of the upper convex. It belongs to the favorable H2 adsorption isotherm. Although the concentration of H2 is very low, the composite adsorbent has rather high equilibrium adsorption capacity. (2) The second stage (b) is a type whose changing trend is inverse to that of type Langmuir curve. It belongs to the unfavorable H2 adsorption isotherm. The curve shape is concave because of the weak intermolecular interaction between H2 and the getter. The adsorbed amount is very low in spite of H2 concentration. (3) The third stage (c) is the repetition of the first stage. The upper convex degree is smaller than that of the first stage. The experimental data is 4–5 orders of magnitude for pressure. H2 adsorption isotherms are in agreement with Temkin’s empirical equation in the first and third stages. In this instance, the error is very small. However, the curves fitting deviate from the experimental data in the second stage. Freundlich’s empirical equation is consistent with the experimental data under this condition. The formula (6) is the fitting equation of 1#B. The H2 adsorption capacity is 0.69 ml (STP)/g (STP represents the standard state) at 700 Pa. This is consistent with the experimental result of Belousov [6]:
8 < a ¼ 0:02599 þ 0:00122lnðP 0:00057Þ p < 0:0047 Pa a ¼ 1:24396P 0:78359 0:0047 Pa P 0:025 Pa : a ¼ 0:19433 þ 0:07544lnðP þ 0:15168Þ P > 0:025 Pa (6) Where a is H2 adsorption capacity and P is the equilibrium pressure. It can be seen in Fig. 8 that the general order of H2 adsorption capacities is 2# > 1#B > 3# at a pressure higher than 102 Pa. H2 adsorption capacity of 2# is 0.6 ml (STP)/g at 5 Pa. The adsorbed amount of H2 gas is 0.3 ml (STP)/g at 5.8 Pa for 1#B, which just accounts for 50% of H2 adsorption capacity of 2# under the same conditions. The highest measured adsorption capacity of H2 for 2#, compared to 1#B and 3#, can be explained by its compositions. It is composed of 15% PdO, 85% Ag2O and molecular sieve 5A. The roles of Ag2O are as follows: It is a catalyst that can accelerate H2 adsorption. It can also provide oxygen for H2 adsorption due to
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the unstable characteristic of Ag2O, and simultaneously, the elementary substance of Ag is obtained. The heat transfer property of Ag is the highest in all the metals. The heat produced can be removed timely, which makes the temperature of the getter steady. The isotherm curve shifts to the left side with the increase of Ag. This is consistent with the observation reported by Brodowaky [11]. Pd and Ag form a continuous solid solution with an fcc structure. Due to the only slightly negative enthalpy of formation of Pd–Ag alloys, a random distribution of Pd and Ag in the alloy can be expected. In a substitutional coarse-grained Pd– Ag alloy whose unit cell is larger than that of Pd, more hydrogen dissolves at a given low pressure than in Pd, and its plateau pressure is lower than that of pure Pd. In addition, the (a þ b) plateau becomes narrower as the amount of Ag increases. The region of coexistence of a and b disappears as the composition approaches a critical value near Pd77Ag23 [12]. With the increase of Ag to Pd up to 23 at.%, hydrogen permeability increases linearly [11,13]. But the ratio of PdO and Ag2O reaches an optimum value. The optimum percentage content of Ag2O is 22% in the getter according to the study of Pd–Ag alloys. Although there is also Ag2O in 3# sample, the adsorbed capacities of H2 gas are the smallest at the same pressure, because it contains too much Ag2O. In order to achieve the optimal effect of the multilayer insulation, it is desirable to keep the interlayer pressure lower than 102 Pa. H2 adsorption capacity of 1#B, 2# and 3# is 0.0320 ml (STP)/ g, 0.0312 ml (STP)/g and 0.0276 ml (STP)/g at 1 102 Pa respectively. H2 adsorption capacity of 2# is slightly lower than that of 1#B. But the order of the price per unit mass is 1#B > 2# > 3#. So 2# sample is preferentially used in the tank interlayer. 4. Conclusions The interlayer pressure depends largely on H2 adsorption performance of composite adsorbent, which can be illustrated effectively by the adsorption isotherm. The getter should be laid as flat as possible when fed into the tank. Leakage and outgassing rate can be decreased by 64% after placing the getter. XRD results showed that the crystal phase structure of PdO and Ag2O is unchanged by adsorption performance, and new substances such as PdH0.706 and Ag are observed. The H2 adsorption rate is high at the initial stage, then it starts to become slow during a relatively long period. The isotherms of these samples exhibit type IV isotherms. The general order of H2 adsorption capacities is 2# > 1#B > 3# at a pressure higher than 102 Pa. The optimum percentage content of Ag2O in the getter is 22%. Under the allowed highest pressure, 2# sample is preferentially used in the tank with high vacuum multilayer insulation. References [1] Yang Y, Saitoh K, Tsukahara S. An improved throughput method for the measurement of outgassing rates of materials. Vacuum 1995;46:1371–6. [2] Xi GK, Shao SM, Li SL, Wang JR. Studies on magnesium rich rare-earth alloys for hydrogen absorption in vacuum. Vacuum 1998;39:531–5. [3] Nemanic V, Zumer M, Zajec B. The influence of a hot cathode vacuum gauge on the residual gas composition. Vacuum 2003;70:523–30. [4] Della Porta P. Gas problem and gettering in sealed-off vacuum devices. Vacuum 1996;47:771–7. [5] Boffito C, Schiabel A, Gallitognotta A. Thermally insulating jacket and related process. U.S. Patent. Patent Number 1995;5408832. [6] Belousov VM, Vasylyev MA, Lyashenko LV, NYu Vilkova, Nieuwenhuys BE. The low-temperature reduction of Pd-doped transition metal oxide surface with hydrogen. Chem Eng J 2003;91:143–50. [7] Boffito C, Ferrario B. A device for the removal of hydrogen from a vacuum enclosure at cryogenic temperatures and especially high energy particle accelerators. European Patent; Application number: EP19920830028 1995. [8] Chen LF, Wang JA, Valenzuela MA, Bokhimi X, Acosta DR, Novaro O. Hydrogen spillover and structural defects in a PdO/zirconia nanophase synthesized through a surfactant-templated route. J Alloys Compd 2006;417:220–3.
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