- Email: [email protected]
Improvement in hydriding property of LaNi4.8Al0.2 alloy encapsulated by SiO2 sol
Fusion Engineering and Design 89 (2014) 2975–2980
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Improvement in hydriding property of LaNi4.8 Al0.2 alloy encapsulated by SiO2 sol Xiaojing Qian ∗ , Guoqiang Huang China Academy of Engineering Physics, P.O. Box 919-71, Mianyang 621900, SiChuan, People’s Republic of China
h i g h l i g h t s • • • •
LaNi4.8 Al0.2 powder can be enwrapped in the SiO2 network effectively. Fine pores can restrict impurities entering into matrix avoiding metal poisoning. Anti-pulverization property of LaNi4.8 Al0.2 is enhanced after being encapsulated. Anti-poisoning property of LaNi4.8 Al0.2 is enhanced after being encapsulated.
a r t i c l e
i n f o
Article history: Received 16 August 2012 Received in revised form 22 June 2014 Accepted 22 August 2014 Available online 23 September 2014 Keywords: Metal hydride LaNi4.8 Al0.2 alloy Sol–gel method Pulverization Poisoning
a b s t r a c t LaNi4.8 Al0.2 alloy particles encapsulated by SiO2 matrix were prepared by the sol gel method. Scanning electron microscope (SEM) imaging was applied to determine the silica network outside the encapsulated alloy. The hydriding kinetics, pulverization and poisoning characteristics of LaNi4.8 Al0.2 alloy were investigated before and after being encapsulated by silica. The results reveal that the hydriding properties of encapsulated alloy are excellent. The hydrogenation rate of encapsulated alloy is faster than that of the original alloy. The quantities of hydrogen stored by the encapsulated and original alloy are 169.3 Nml/g and 147.1 Nml/g, respectively. The LaNi4.8 Al0.2 alloy particles are broken up into powder after 10 times hydrogen absorption/desorption cycles, while the encapsulated alloy do not show any breakdown after 30 times hydrogen adsorption/desorption cycles. The quantities of hydrogen absorbed by original alloy particles are less than 8.2 Nml/g in H2 -14.4% CO and 18.6 Nml/g in H2 -12.8% CO2 , while the quantities of hydrogen absorbed by encapsulated alloy agglomerations are 84.5 Nml/g in H2 -14.4% CO and 168.9 Nml/g in H2 -12.8% CO2 . These results clearly indicate that the pulverization and poisoning resistance properties of LaNi4.8 Al0.2 alloy are evidently enhanced after being encapsulated by silica network. © 2014 Published by Elsevier B.V.
1. Introduction Metal hydrides are known to absorb hydrogen isotopes reversibly at moderate conditions. They have been proposed as safe hydrogen isotopes storage and separation materials except for some practical problems. Metal hydride particles will expand upon hydrogen absorption and contract upon hydrogen desorption. As a result, the volume changes during each hydrogen absorption/desorption cycle and the metal hydride particles are broken up into fines after repeated reaction with hydrogen [1]. The fine particles are not suitable for usage in storage and separation columns, since they can plug up columns and restrict gas flow. Furthermore,
∗ Corresponding author. Tel.: +86 0816 3626483; fax: +86 0816 3625900. E-mail address: [email protected] (X. Qian). http://dx.doi.org/10.1016/j.fusengdes.2014.08.011 0920-3796/© 2014 Published by Elsevier B.V.
hydrogen storage alloys can be poisoned by certain impurities such as CO, CO2 and hydrocarbons. If exposure to such reactive impurities, the hydriding reaction rates of the hydrogen storage alloys are significantly slowed down and the hydrogen absorption capacities of them are markedly decreased because of surface poisoning by absorbed impurities and formation of oxides and/or carbides in subsurface layers [2–4]. Therefore, before hydrogen storage alloys are successfully applied in large hydrogen storage or separation processes, they must be converted into a stable form that will not be broken down to fines and they also must be resistant to oxygen and other impurities. Attempts have been made to convert hydrogen storage alloy into stable packing materials in the form of pellets or granules. Heung et al. [5–7] had invented a new kind of method, which could convert fine particles of hydrogen storage alloy into big stable granules. In that method, the hydrogen storage alloy particles are encapsulated in a porous silica matrix by using several kinds of liquids, such as
2976
X. Qian, G. Huang / Fusion Engineering and Design 89 (2014) 2975–2980
ethanol. That method could successfully convert the small particles into large granules, which are suitable for hydrogen isotopes storage, but the large granules are not applied to the pure tritium storage. The pores of the silica matrix were large enough to allow hydrogen to easily transfer within the matrix and react with the metal particles, and small enough to act as filters to prevent passage of larger molecules including harmful impurities. Accordingly, the granules could absorb hydrogen readily and withstand many times hydrogen absorption/desorption cycles without producing fines. Although the anti-poisoning and anti-pulverization characteristics of the granules are enhanced, that kind of encapsulation process is too complex. That method had to use several kinds of liquids. The ethanol was generally added to the water while stirring and other acids were also needed to adjust the acidity of the solution. In addition, the sizes of alloy particles expected in that method were very small, usually smaller than 45 m in diameter. In the present work, the LaNi4.8 Al0.2 alloy particles were modified with a very simple encapsulation process. Sizes of the alloy particles are between 58 and 75 m, which are bigger than the one desired in L.K. Heung’s. Moreover, the present encapsulation process only needs one liquid that is deionized water. LaNi4.8 Al0.2 alloy particles were encapsulated in a porous silica matrix by sol–gel method and the encapsulated alloy are also not applied to the pure tritium storage. The pulverization and poisoning resistance properties of LaNi4.8 Al0.2 alloy before and after being encapsulated were studied.
2. Experimental 2.1. Preparation of encapsulated alloy LaNi4.8 Al0.2 alloy powder was in-house fabricated and prepared by mechanical milling for 5 min at 300 r.p.m under high purity argon (99.999% purity). Most of the metal particles were broken up into powder, which was small enough to go through a 75 m sieve. The fresh powder must not be exposed to air immediately, because the fresh surfaces of metal powder have high activity. If the powder is exposed to air immediately after ball-milling, the fresh surfaces of powder will be oxidated acutely and the oxide layers will become deep. Powder was firstly kept in a glove box under argon atmosphere, and then was exposed to air gradually over several days by adjusting the amount of air in the glove box gradually. After being sieved through 58–75 m sieves, the alloy powder was stored for later use. The porous silica matrix was prepared using amorphous fumed silica particles via a sol–gel process. Commercial powder of fumed silica is produced by burning silicon tetrachloride vapor in a flame of hydrogen and oxygen [6]. The fumed silica used in this paper was purchased from Degussa Corporation with the model of AEROSIL® 200. The weight ratio of the deionized water, fumed silica and LaNi4.8 Al0.2 alloy was 8:1:0.2. Firstly, the fumed silica particles were separated into several portions and one portion was blended into the deionized water at a time. After being stirred continuously for more than 5 min at room temperature, a waterlike sol called silica sol was formed. Secondly, the alloy powder portions were added and blended into the silica sol in steps. The mixture was continuously stirred during the process for more than 10 min at room temperature. In the end, a thick mixture was formed. Finally, the mixture was set in the vacuum dryer and dried at 423 K under argon atmosphere to form a solid. Sizes of the dried encapsulated alloy agglomerations were between 5 and 20 mm. The specific surface area of the dried encapsulated alloy was about 165 m2 /g after being analyzed by the adsorption and desorption instrument with the model of AUTOSORB-1 using nitrogen.
Fig. 1. The sketch map of the hydriding test system.
2.2. Test procedure Surface topography of the encapsulated alloy was observed by scanning electron microscope (SEM). The sketch map of the hydriding test apparatus is shown in Fig. 1. The hydriding kinetics, pulverization and poisoning characteristics of the encapsulated alloy were tested and compared with original alloy in the same hydriding test apparatus. The initial sizes of the encapsulated and original alloy were both more than 10 mm in the experiments. Each test involved several steps as follows: the encapsulated and original LaNi4.8 Al0.2 alloy particles were firstly put into the same sample containers, and then the sample containers were evacuated at room temperature and at 523 K, and subsequently the encapsulated and original alloy particles were activated by hydrogen absorption at room temperature and hydrogen desorption at 523 K for three times. During the evacuation especially the heating process, the water vapor and other gases adsorbed by the piping and valves will be mostly released. During the hydrogen absorption process, the sample container was put into the isothermal water bath to maintain isothermal condition, and before the hydrogen desorption process, the sample container was take out from the isothermal water bath for heating by electric stove. Hydrogen absorption capacities of the LaNi4.8 Al0.2 alloy before and after being encapsulated were measured at room temperature with a hydrogen pressure of 0.2 MPa. To determine the pulverization resistance property of the LaNi4.8 Al0.2 alloy after being encapsulated, the encapsulated and original alloy particles were tested for 30 and 10 times hydrogen absorption/desorption cycles with equal amounts of hydrogen, respectively. Before test, the reservoir was filled with hydrogen under 0.193 MPa. For each cycle, the alloy particles firstly absorbed hydrogen at room temperature till equilibrium, and then desorbed hydrogen at 473 K till equilibrium. The amount of hydrogen was maintained constant during the study, that is, the hydrogen released from the alloy was kept in the reservoir and not evacuated during the hydrogen desorption process. The variations of pressure in the reservoir were measured during the hydrogen absorption/desorption cycles. After 30 or 10 times absorption/desorption cycles, the encapsulated and original alloy particles were taken out for optical examination, respectively. The hydrogen absorption capacities of the encapsulated and original alloys were also tested with hydrogen mixed with harmful gases
X. Qian, G. Huang / Fusion Engineering and Design 89 (2014) 2975–2980
Fig. 2. SEM image of the surface of the LaNi4.8 Al0.2 alloy just after being encapsulated.
as H2 -14.4% CO and H2 -12.8% CO2 at 0.2 MPa, respectively, in order to evaluate the anti-poison property of the encapsulated alloy. 3. Results and discussion Fig. 2 shows the SEM image of the surface of the LaNi4.8 Al0.2 alloy just after being encapsulated. There is a porous network on the surface of the LaNi4.8 Al0.2 alloy after encapsulation, which is formed by the silica particles. The sizes of the pores in the network are about less than 0.2 m by cursory inspection of the SEM image. Hence, the pores are large enough to allow hydrogen to easily transfer within the matrix and react with the metal particles. On the other hand, the sizes of the LaNi4.8 Al0.2 particles are between 58 and 75 m. The sizes of metal powder are larger than that of the pores, so the LaNi4.8 Al0.2 alloy powder can be encapsulated effectively by the silica network and the encapsulated metal powder cannot drop from the matrix even if the alloy particles are broken down during hydrogen absorption/desorption cycles. In addition, the fine pores are small enough to act as filters, so it can restrict larger molecules including harmful impurities entering into the matrix to avoid the poisoning of the metal particles. Fig. 3 presents the hydriding kinetics of the encapsulated and original LaNi4.8 Al0.2 alloys at 0.2 MPa. The plot shows that the hydrogen absorption property of encapsulated alloy is excellent.
Fig. 3. The evolution of the hydrogen adsorption capacities of the encapsulated and original LaNi4.8 Al0.2 alloy with time.
2977
The absorption rate of encapsulated alloy is also faster, that may be due to the smaller sizes of the alloy particles in the encapsulated alloy. The smaller the alloy particles are, the more contact surfaces are. As a result, the hydrogen can react with the alloy particles faster. After one minute, the hydrogen absorption amounts reach 83.4 Nml/g for the encapsulated alloy and 19.1 Nml/g for the original alloy. The amount of hydrogen absorbed by the original alloy adds gradually with the absorption time, while the absorption amount of the encapsulated alloy increases quickly with the absorption time. The hydrogen absorption of the encapsulated alloy is saturated after 22 min and the amount reaches 169.3 Nml/g. However, the original alloy reaches the absorption equilibrium after about 120 min and the absorption capacity is 147.1 Nml/g. In this experiment, the weight of encapsulated alloy is about 4.79 g. The weight ratio of silica and LaNi4.8 Al0.2 is 1:0.2. That is to say, the weight of LaNi4.8 Al0.2 is about 0.8 g and the remaining weight about 3.99 g is of silica. The capacities of hydrogen stored by the encapsulated alloy are calculated only considering the weight of alloy in this experiment. The absorption capacity of the original LaNi4.8 Al0.2 alloy is 147.1 Nml/g when reaching absorption equilibrium whereas the adsorption capacity of the encapsulated alloy is about 169.3 Nml/g when only considering the weight of alloy. If we assume that the quantity of hydrogen absorbed by LaNi4.8 Al0.2 alloy is the same in both experiments. There is an excess in the experiment with encapsulated alloy. How to explain the excess? If we suppose that the excess of hydrogen is stored by the encapsulated alloy mostly because of the adsorption of silica. The hypothetical amount of hydrogen stored by silica can be estimated as follows: qsilica =
(qencapsulated − qoriginal ) × malloy msilica
(169.3 − 147.1) × 0.8 ≈ 4.45 3.99
(1)
(2)
The hypothetical quantity of hydrogen stored by silica is about 4.45 Nml/g in this experiment. The specific surface area of the encapsulated alloy used in this experiment is about 165 m2 /g. There are a great number of micropores formed by the silica on the surface of the encapsulated alloy. It is possible for the surface formed by the silica to adsorb few hydrogen. The hypothetical amount of hydrogen adsorbed by silica is very small compared with the amount absorbed by LaNi4.8 Al0.2 alloy. Hence, the capacities of hydrogen stored by encapsulated alloy were calculated only considering the weight of alloy. That is why the adsorption quantities of encapsulated alloy are larger than that of the original alloy in Fig. 3. Figs. 4 and 5 show the photographs of the original and encapsulated alloy before and after 10 and 30 times hydrogen absorption/desorption cycles, respectively. The metal volume will expand after hydrogen absorption and shrink after hydrogen desorption. After several times hydrogen absorption/desorption cycles, the metal undergoes repeated expansion and shrinkage, resulting in the breaking up of the metal particles. The results show that the original LaNi4.8 Al0.2 alloy blocks are broken down after 10 times hydrogen absorption/desorption cycles. After being sieved through fine mesh sieves, the sizes of metal powder are smaller than 150 m. Meanwhile the encapsulated alloy do not show obviously breakdown after 30 times hydrogen absorption/desorption cycles except for the slight change of color. The pulverization resistance property of the LaNi4.8 Al0.2 alloy after being encapsulated in the silica network is evidently enhanced. The variations of hydrogen pressure during hydrogen absorption and desorption process are shown in Fig. 6. For both cases the absorption times are about 5 times longer than desorption times in each cycle, although the absorption times of the encapsulated alloy are delayed gradually after each cycle.
2978
X. Qian, G. Huang / Fusion Engineering and Design 89 (2014) 2975–2980
Fig. 4. (a) The photographs of LaNi4.8 Al0.2 alloy before 10 times hydrogen cycles. (b) The photographs of LaNi4.8 Al0.2 alloy after 10 times hydrogen cycles.
Fig. 5. (a) The photographs of the encapsulated LaNi4.8 Al0.2 alloy agglomerations before 30 times hydrogen cycles. (b) The photographs of the encapsulated LaNi4.8 Al0.2 alloy agglomerations after 30 times hydrogen cycles.
The fresh surface of the metal has high activity and hydrogen molecules can be absorbed and desorbed easily. The reaction rates and absorption quantities, however, are significantly reduced in the presence of impurities such as CO, CO2 because the surface is poisoned by the absorbed impurities [2]. Fig. 7 displays the evolutions of the hydrogen absorption quantities of the encapsulated and original alloy with time during one absorption process in H2 -14.4% CO. The results indicate that the CO exposure causes serious reduction in hydrogen absorption capacity of the encapsulated and original alloy. For example, the largest absorption amounts reduce from 147.1 Nml/g to 8.2 Nml/g for the original alloy and from 169.6 Nml/g to 84.5 Nml/g for the encapsulated alloy in H2 -14.4% CO. As we know, the metal hydride can be poisoned by CO. When H2 mixes with CO, CO is also adsorbed by the surface of the alloy, and it can react with alloy particles to form a compound layer [8]. The layer can prevent hydrogen from diffusing inside the alloy particles. So the quantity of H2 diffusing inside the alloy particles will become small and the amount of H2 absorbed by the metal powder will decrease. The absorption capacity of the encapsulated alloy is approximately 10 times larger than that of the original one. The
absorption speed of the metal powder with silica is also faster than the one without silica. When the alloy particles are encapsulated by the SiO2 sol, there is a porous network outside the encapsulated alloy, and the sizes of most pores are smaller than the diameter of the CO molecule. So CO cannot transfer into most of the network and cannot be absorbed by the metal powder. As a result, most of the alloy particles are not poisoned. While H2 can also transfer into the network and can be absorbed by the metal powder. Hence, the quantity of H2 diffusing inside the metal powder with silica is larger and the amount of H2 absorbed by encapsulated alloy will increase. The absorption amounts are 6.2 Nml/g for the original alloy and 67.6 Nml/g for the encapsulated alloy at the beginning. The two amounts hardly change before 18 min and 23 min, respectively. When H2 mixes with CO, the alloy particles can absorb both of them. H2 competes with CO for reacting with alloy particles, once the valves between reservoir and sample container are open. With the absorption of CO, the poisoning of the alloy becomes more and more serious. The poisoned alloy particles rarely go on absorption, so the absorption amounts of the two alloys increase few. There are several increases during the absorption process
X. Qian, G. Huang / Fusion Engineering and Design 89 (2014) 2975–2980
2979
Fig. 8. The evolution of the hydrogen adsorption capacities of the encapsulated and original LaNi4.8 Al0.2 alloy with time in H2 -12.8%CO2 at approximately 0.2 MPa.
Fig. 6. (a) The variations of pressure in first and last hydrogen cycle of original LaNi4.8 Al0.2 alloy particles. (b) The variations of pressure in first and last hydrogen cycle of encapsulated LaNi4.8 Al0.2 alloy agglomerations.
of the two alloys. The absorption amount of the original alloy is firstly increased from 6.2 Nml/g to 6.5 Nml/g at 18 min, after about 5 min slow increase, the amount reaches the maximum, which is 8.2 Nml/g. Subsequently, the absorption amount of the encapsulated alloy is increased from 67.6 Nml/g to 69.9 Nml/g at 23 min,
Fig. 7. The evolution of the hydrogen adsorption capacities of the encapsulated and original LaNi4.8 Al0.2 alloy with time in H2 -14.4%CO at 0.2 MPa.
after about 6 min slow increase, the absorption value reaches the maximum that is 84.5 Nml/g. And then the absorption amounts of the two alloys do not change till the experiment is over. The increase may be attributed to the absorption of the new-formed surface on the alloy particles. Maybe a few alloy particles break after absorption and reaction with H2 , it can produce some new surface. It is possible for the fresh surface of the metal to absorb the remainder hydrogen in the system. Fig. 8 presents the evolutions of the hydrogen absorption quantities of the encapsulated and original alloy with time during one absorption process in H2 -12.8% CO2 at 0.2 MPa. The result shows that the absorption capacity of the original LaNi4.8 Al0.2 alloy reduces remarkably because of the CO2 exposure, but the absorption amount of the encapsulated alloy is almost invariable after reaching equilibrium. For example, the largest absorption amount of the original alloy reduces from 147.1 Nml/g to 18.6 Nml/g, while the absorption capacity of the encapsulated alloy is 168.9 Nml/g, which is very close to 169.3 Nml/g in pure H2 . The absorption amount of the encapsulated alloy is almost 10 times larger than that of the original alloy. There are also some fluctuations during the hydrogen absorption process of the two alloys, which is the same with the status of the CO exposure. The LaNi4.8 Al0.2 alloy can be poisoned by absorption of CO and CO2 , but CO is considered as a strong poisonous gas, because the poisoning effect by CO absorption is more serious than that by CO2 absorption. For example, the absorption quantities of the original LaNi4.8 Al0.2 alloy are 18.6 Nml/g in H2 -12.8% CO2 and 8.2 Nml/g in H2 -14.4% CO. If CO is introduced into H2 , when the hydrogen storage alloy particles absorb hydrogen, CO is absorbed by the alloy surface and reacts with the alloy particles to form a compound layer [8]. The compound layer can prevent hydrogen from diffusing inside the alloy particles. Sang et al. [9] also reported that the formed compound film on the alloy surface can inhibit further hydrogen absorption, as a result, the absorption rate will be reduced and the hydrogen absorption amount will decrease. In present experiments, there is a porous network outside the encapsulated alloy, and all the sizes of the pores in the network are almost smaller than the diameter of the CO2 molecule. So CO2 cannot transfer into the network and cannot be absorbed by the LaNi4.8 Al0.2 alloy, while the hydrogen can be absorbed by the encapsulated alloy continuously. Hence the absorption amount in the presence of CO2 is larger than that in CO. CO can also affect the hydrogen absorption capacity of the encapsulated alloy. In the presence of CO, the largest absorption
2980
X. Qian, G. Huang / Fusion Engineering and Design 89 (2014) 2975–2980
value of the encapsulated alloy decreases from 169.6 Nml/g to 84.5 Nml/g, but it is still larger than that of the original alloy. There is almost no influence on the absorption capacity of the encapsulated alloy in the presence of CO2 , because the absorption capacity of the encapsulated alloy is hardly change with or without CO2 . The poisoning resistance property of the LaNi4.8 Al0.2 alloy improves after being encapsulated by the silica matrix. Once the alloy particles are encapsulated by SiO2 sol, there is a porous network of the silica matrix formed outside the alloy. The pores in the network are very small and can restrict the transfer of large molecules, such as CO, CO2 , and other harmful impurities. Therefore it can protect the metal hydrides from such impurities.
properties of the LaNi4.8 Al0.2 alloy are noticeably enhanced. Hence, this process is suitable for the surface modification of the hydrogen storage alloy with cost-saving and energy-saving advantages.
4. Conclusions
[1] I.A. Fisher, Structural stability of 1100[degree]C heated Pd/k during absorption cycling in protium, WSRC-RP-93-401. [2] R.D. Penzhorn, M. Devillers, M. Sirch, Storage of tritium in ZrCo alloy: effect of preexposure to impurities relevant to the fusion fuel cycle, J. Nucl. Mater. 179–181 (1991) 863–866. [3] C. Boffito, B. Ferrario, D. Martelli, Equilibrium pressures of H2 and D2 for different getter materials and the effect of Co impurities, J. Vac. Technol. A1 (1983) 1279–1282. [4] K. Ashida, K. Watanabe, S. Ikeno, K. Mori, Surface modification of Zr-based alloys applicable for tritium processing, Fusion Eng. Des. 39–40 (1998) 1049–1054. [5] L.K. Heung, G.G. Wicks, G.L. Enz, N. Augusta, Composition for absorbing hydrogen, US pat. 5411928, 1995. [6] L.K. Heung, Method and composition in which metal hydride particles are embedded in a silica network, US pat. 5958098, 1999. [7] L.K. Heung, G.G. Wicks, Silica embedded metal hydrides, J. Alloys Compd. 293 (1999) 446–451. [8] J.I. Han, J.-Y. Lee, The effect of CO impurity on the hydrogenation properties of LaNi5 , LaNi4.7 Al0.3 and MmNi4.5 Al0.5 during hydriding–dehydriding cycling, J. Less-Common Met. 152 (1989) 319–327. [9] G. Sang, M.J. Tu, Q.A. Li, K.P. Yan, Y.G. Ning, Y.G. Chen, et al., Sims analysis of CO stabilization of LaNi4.7 Al0.3 alloy, Acta Metall. Sin. 36 (2000) 251–253.
There is a porous network of the silica matrix formed outside the LaNi4.8 Al0.2 alloy powder after being encapsulated by SiO2 sol. The pores in the network are so fine that the alloy particles can be encapsulated in the network effectively, even the smaller broken alloy particles. The fine pores can also restrict large molecules entering into the matrix to prevent the poisoning of alloy particles. After being encapsulated by the silica matrix, the hydrogen absorption capacity of the LaNi4.8 Al0.2 alloy does not decrease. Sizes of the encapsulated alloy agglomerations hardly change after 30 times absorption/desorption cycles. When hydrogen is mixed with CO or CO2 , the absorption capacity of the encapsulated alloy is reduced to half of the case without CO. But there is little effect on the absorption capacity of the encapsulated alloy in the presence of CO2 . After surface modification, the pulverization and poisoning resistance
Acknowledgements We are grateful to Doctor Huang HG from National Key Laboratory of Surface Physics and Chemistry, China, for alloy surface analysis by SEM. We also would like to thank Tang T and Huang ZY for insightful discussion. Thanks to Xiang X and Chen M for providing language help. References
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Improvement in hydriding property of LaNi4.8 Al0.2 alloy encapsulated by SiO2 sol Xiaojing Qian ∗ , Guoqiang Huang China Academy of Engineering Physics, P.O. Box 919-71, Mianyang 621900, SiChuan, People’s Republic of China
h i g h l i g h t s • • • •
LaNi4.8 Al0.2 powder can be enwrapped in the SiO2 network effectively. Fine pores can restrict impurities entering into matrix avoiding metal poisoning. Anti-pulverization property of LaNi4.8 Al0.2 is enhanced after being encapsulated. Anti-poisoning property of LaNi4.8 Al0.2 is enhanced after being encapsulated.
a r t i c l e
i n f o
Article history: Received 16 August 2012 Received in revised form 22 June 2014 Accepted 22 August 2014 Available online 23 September 2014 Keywords: Metal hydride LaNi4.8 Al0.2 alloy Sol–gel method Pulverization Poisoning
a b s t r a c t LaNi4.8 Al0.2 alloy particles encapsulated by SiO2 matrix were prepared by the sol gel method. Scanning electron microscope (SEM) imaging was applied to determine the silica network outside the encapsulated alloy. The hydriding kinetics, pulverization and poisoning characteristics of LaNi4.8 Al0.2 alloy were investigated before and after being encapsulated by silica. The results reveal that the hydriding properties of encapsulated alloy are excellent. The hydrogenation rate of encapsulated alloy is faster than that of the original alloy. The quantities of hydrogen stored by the encapsulated and original alloy are 169.3 Nml/g and 147.1 Nml/g, respectively. The LaNi4.8 Al0.2 alloy particles are broken up into powder after 10 times hydrogen absorption/desorption cycles, while the encapsulated alloy do not show any breakdown after 30 times hydrogen adsorption/desorption cycles. The quantities of hydrogen absorbed by original alloy particles are less than 8.2 Nml/g in H2 -14.4% CO and 18.6 Nml/g in H2 -12.8% CO2 , while the quantities of hydrogen absorbed by encapsulated alloy agglomerations are 84.5 Nml/g in H2 -14.4% CO and 168.9 Nml/g in H2 -12.8% CO2 . These results clearly indicate that the pulverization and poisoning resistance properties of LaNi4.8 Al0.2 alloy are evidently enhanced after being encapsulated by silica network. © 2014 Published by Elsevier B.V.
1. Introduction Metal hydrides are known to absorb hydrogen isotopes reversibly at moderate conditions. They have been proposed as safe hydrogen isotopes storage and separation materials except for some practical problems. Metal hydride particles will expand upon hydrogen absorption and contract upon hydrogen desorption. As a result, the volume changes during each hydrogen absorption/desorption cycle and the metal hydride particles are broken up into fines after repeated reaction with hydrogen [1]. The fine particles are not suitable for usage in storage and separation columns, since they can plug up columns and restrict gas flow. Furthermore,
∗ Corresponding author. Tel.: +86 0816 3626483; fax: +86 0816 3625900. E-mail address: [email protected] (X. Qian). http://dx.doi.org/10.1016/j.fusengdes.2014.08.011 0920-3796/© 2014 Published by Elsevier B.V.
hydrogen storage alloys can be poisoned by certain impurities such as CO, CO2 and hydrocarbons. If exposure to such reactive impurities, the hydriding reaction rates of the hydrogen storage alloys are significantly slowed down and the hydrogen absorption capacities of them are markedly decreased because of surface poisoning by absorbed impurities and formation of oxides and/or carbides in subsurface layers [2–4]. Therefore, before hydrogen storage alloys are successfully applied in large hydrogen storage or separation processes, they must be converted into a stable form that will not be broken down to fines and they also must be resistant to oxygen and other impurities. Attempts have been made to convert hydrogen storage alloy into stable packing materials in the form of pellets or granules. Heung et al. [5–7] had invented a new kind of method, which could convert fine particles of hydrogen storage alloy into big stable granules. In that method, the hydrogen storage alloy particles are encapsulated in a porous silica matrix by using several kinds of liquids, such as
2976
X. Qian, G. Huang / Fusion Engineering and Design 89 (2014) 2975–2980
ethanol. That method could successfully convert the small particles into large granules, which are suitable for hydrogen isotopes storage, but the large granules are not applied to the pure tritium storage. The pores of the silica matrix were large enough to allow hydrogen to easily transfer within the matrix and react with the metal particles, and small enough to act as filters to prevent passage of larger molecules including harmful impurities. Accordingly, the granules could absorb hydrogen readily and withstand many times hydrogen absorption/desorption cycles without producing fines. Although the anti-poisoning and anti-pulverization characteristics of the granules are enhanced, that kind of encapsulation process is too complex. That method had to use several kinds of liquids. The ethanol was generally added to the water while stirring and other acids were also needed to adjust the acidity of the solution. In addition, the sizes of alloy particles expected in that method were very small, usually smaller than 45 m in diameter. In the present work, the LaNi4.8 Al0.2 alloy particles were modified with a very simple encapsulation process. Sizes of the alloy particles are between 58 and 75 m, which are bigger than the one desired in L.K. Heung’s. Moreover, the present encapsulation process only needs one liquid that is deionized water. LaNi4.8 Al0.2 alloy particles were encapsulated in a porous silica matrix by sol–gel method and the encapsulated alloy are also not applied to the pure tritium storage. The pulverization and poisoning resistance properties of LaNi4.8 Al0.2 alloy before and after being encapsulated were studied.
2. Experimental 2.1. Preparation of encapsulated alloy LaNi4.8 Al0.2 alloy powder was in-house fabricated and prepared by mechanical milling for 5 min at 300 r.p.m under high purity argon (99.999% purity). Most of the metal particles were broken up into powder, which was small enough to go through a 75 m sieve. The fresh powder must not be exposed to air immediately, because the fresh surfaces of metal powder have high activity. If the powder is exposed to air immediately after ball-milling, the fresh surfaces of powder will be oxidated acutely and the oxide layers will become deep. Powder was firstly kept in a glove box under argon atmosphere, and then was exposed to air gradually over several days by adjusting the amount of air in the glove box gradually. After being sieved through 58–75 m sieves, the alloy powder was stored for later use. The porous silica matrix was prepared using amorphous fumed silica particles via a sol–gel process. Commercial powder of fumed silica is produced by burning silicon tetrachloride vapor in a flame of hydrogen and oxygen [6]. The fumed silica used in this paper was purchased from Degussa Corporation with the model of AEROSIL® 200. The weight ratio of the deionized water, fumed silica and LaNi4.8 Al0.2 alloy was 8:1:0.2. Firstly, the fumed silica particles were separated into several portions and one portion was blended into the deionized water at a time. After being stirred continuously for more than 5 min at room temperature, a waterlike sol called silica sol was formed. Secondly, the alloy powder portions were added and blended into the silica sol in steps. The mixture was continuously stirred during the process for more than 10 min at room temperature. In the end, a thick mixture was formed. Finally, the mixture was set in the vacuum dryer and dried at 423 K under argon atmosphere to form a solid. Sizes of the dried encapsulated alloy agglomerations were between 5 and 20 mm. The specific surface area of the dried encapsulated alloy was about 165 m2 /g after being analyzed by the adsorption and desorption instrument with the model of AUTOSORB-1 using nitrogen.
Fig. 1. The sketch map of the hydriding test system.
2.2. Test procedure Surface topography of the encapsulated alloy was observed by scanning electron microscope (SEM). The sketch map of the hydriding test apparatus is shown in Fig. 1. The hydriding kinetics, pulverization and poisoning characteristics of the encapsulated alloy were tested and compared with original alloy in the same hydriding test apparatus. The initial sizes of the encapsulated and original alloy were both more than 10 mm in the experiments. Each test involved several steps as follows: the encapsulated and original LaNi4.8 Al0.2 alloy particles were firstly put into the same sample containers, and then the sample containers were evacuated at room temperature and at 523 K, and subsequently the encapsulated and original alloy particles were activated by hydrogen absorption at room temperature and hydrogen desorption at 523 K for three times. During the evacuation especially the heating process, the water vapor and other gases adsorbed by the piping and valves will be mostly released. During the hydrogen absorption process, the sample container was put into the isothermal water bath to maintain isothermal condition, and before the hydrogen desorption process, the sample container was take out from the isothermal water bath for heating by electric stove. Hydrogen absorption capacities of the LaNi4.8 Al0.2 alloy before and after being encapsulated were measured at room temperature with a hydrogen pressure of 0.2 MPa. To determine the pulverization resistance property of the LaNi4.8 Al0.2 alloy after being encapsulated, the encapsulated and original alloy particles were tested for 30 and 10 times hydrogen absorption/desorption cycles with equal amounts of hydrogen, respectively. Before test, the reservoir was filled with hydrogen under 0.193 MPa. For each cycle, the alloy particles firstly absorbed hydrogen at room temperature till equilibrium, and then desorbed hydrogen at 473 K till equilibrium. The amount of hydrogen was maintained constant during the study, that is, the hydrogen released from the alloy was kept in the reservoir and not evacuated during the hydrogen desorption process. The variations of pressure in the reservoir were measured during the hydrogen absorption/desorption cycles. After 30 or 10 times absorption/desorption cycles, the encapsulated and original alloy particles were taken out for optical examination, respectively. The hydrogen absorption capacities of the encapsulated and original alloys were also tested with hydrogen mixed with harmful gases
X. Qian, G. Huang / Fusion Engineering and Design 89 (2014) 2975–2980
Fig. 2. SEM image of the surface of the LaNi4.8 Al0.2 alloy just after being encapsulated.
as H2 -14.4% CO and H2 -12.8% CO2 at 0.2 MPa, respectively, in order to evaluate the anti-poison property of the encapsulated alloy. 3. Results and discussion Fig. 2 shows the SEM image of the surface of the LaNi4.8 Al0.2 alloy just after being encapsulated. There is a porous network on the surface of the LaNi4.8 Al0.2 alloy after encapsulation, which is formed by the silica particles. The sizes of the pores in the network are about less than 0.2 m by cursory inspection of the SEM image. Hence, the pores are large enough to allow hydrogen to easily transfer within the matrix and react with the metal particles. On the other hand, the sizes of the LaNi4.8 Al0.2 particles are between 58 and 75 m. The sizes of metal powder are larger than that of the pores, so the LaNi4.8 Al0.2 alloy powder can be encapsulated effectively by the silica network and the encapsulated metal powder cannot drop from the matrix even if the alloy particles are broken down during hydrogen absorption/desorption cycles. In addition, the fine pores are small enough to act as filters, so it can restrict larger molecules including harmful impurities entering into the matrix to avoid the poisoning of the metal particles. Fig. 3 presents the hydriding kinetics of the encapsulated and original LaNi4.8 Al0.2 alloys at 0.2 MPa. The plot shows that the hydrogen absorption property of encapsulated alloy is excellent.
Fig. 3. The evolution of the hydrogen adsorption capacities of the encapsulated and original LaNi4.8 Al0.2 alloy with time.
2977
The absorption rate of encapsulated alloy is also faster, that may be due to the smaller sizes of the alloy particles in the encapsulated alloy. The smaller the alloy particles are, the more contact surfaces are. As a result, the hydrogen can react with the alloy particles faster. After one minute, the hydrogen absorption amounts reach 83.4 Nml/g for the encapsulated alloy and 19.1 Nml/g for the original alloy. The amount of hydrogen absorbed by the original alloy adds gradually with the absorption time, while the absorption amount of the encapsulated alloy increases quickly with the absorption time. The hydrogen absorption of the encapsulated alloy is saturated after 22 min and the amount reaches 169.3 Nml/g. However, the original alloy reaches the absorption equilibrium after about 120 min and the absorption capacity is 147.1 Nml/g. In this experiment, the weight of encapsulated alloy is about 4.79 g. The weight ratio of silica and LaNi4.8 Al0.2 is 1:0.2. That is to say, the weight of LaNi4.8 Al0.2 is about 0.8 g and the remaining weight about 3.99 g is of silica. The capacities of hydrogen stored by the encapsulated alloy are calculated only considering the weight of alloy in this experiment. The absorption capacity of the original LaNi4.8 Al0.2 alloy is 147.1 Nml/g when reaching absorption equilibrium whereas the adsorption capacity of the encapsulated alloy is about 169.3 Nml/g when only considering the weight of alloy. If we assume that the quantity of hydrogen absorbed by LaNi4.8 Al0.2 alloy is the same in both experiments. There is an excess in the experiment with encapsulated alloy. How to explain the excess? If we suppose that the excess of hydrogen is stored by the encapsulated alloy mostly because of the adsorption of silica. The hypothetical amount of hydrogen stored by silica can be estimated as follows: qsilica =
(qencapsulated − qoriginal ) × malloy msilica
(169.3 − 147.1) × 0.8 ≈ 4.45 3.99
(1)
(2)
The hypothetical quantity of hydrogen stored by silica is about 4.45 Nml/g in this experiment. The specific surface area of the encapsulated alloy used in this experiment is about 165 m2 /g. There are a great number of micropores formed by the silica on the surface of the encapsulated alloy. It is possible for the surface formed by the silica to adsorb few hydrogen. The hypothetical amount of hydrogen adsorbed by silica is very small compared with the amount absorbed by LaNi4.8 Al0.2 alloy. Hence, the capacities of hydrogen stored by encapsulated alloy were calculated only considering the weight of alloy. That is why the adsorption quantities of encapsulated alloy are larger than that of the original alloy in Fig. 3. Figs. 4 and 5 show the photographs of the original and encapsulated alloy before and after 10 and 30 times hydrogen absorption/desorption cycles, respectively. The metal volume will expand after hydrogen absorption and shrink after hydrogen desorption. After several times hydrogen absorption/desorption cycles, the metal undergoes repeated expansion and shrinkage, resulting in the breaking up of the metal particles. The results show that the original LaNi4.8 Al0.2 alloy blocks are broken down after 10 times hydrogen absorption/desorption cycles. After being sieved through fine mesh sieves, the sizes of metal powder are smaller than 150 m. Meanwhile the encapsulated alloy do not show obviously breakdown after 30 times hydrogen absorption/desorption cycles except for the slight change of color. The pulverization resistance property of the LaNi4.8 Al0.2 alloy after being encapsulated in the silica network is evidently enhanced. The variations of hydrogen pressure during hydrogen absorption and desorption process are shown in Fig. 6. For both cases the absorption times are about 5 times longer than desorption times in each cycle, although the absorption times of the encapsulated alloy are delayed gradually after each cycle.
2978
X. Qian, G. Huang / Fusion Engineering and Design 89 (2014) 2975–2980
Fig. 4. (a) The photographs of LaNi4.8 Al0.2 alloy before 10 times hydrogen cycles. (b) The photographs of LaNi4.8 Al0.2 alloy after 10 times hydrogen cycles.
Fig. 5. (a) The photographs of the encapsulated LaNi4.8 Al0.2 alloy agglomerations before 30 times hydrogen cycles. (b) The photographs of the encapsulated LaNi4.8 Al0.2 alloy agglomerations after 30 times hydrogen cycles.
The fresh surface of the metal has high activity and hydrogen molecules can be absorbed and desorbed easily. The reaction rates and absorption quantities, however, are significantly reduced in the presence of impurities such as CO, CO2 because the surface is poisoned by the absorbed impurities [2]. Fig. 7 displays the evolutions of the hydrogen absorption quantities of the encapsulated and original alloy with time during one absorption process in H2 -14.4% CO. The results indicate that the CO exposure causes serious reduction in hydrogen absorption capacity of the encapsulated and original alloy. For example, the largest absorption amounts reduce from 147.1 Nml/g to 8.2 Nml/g for the original alloy and from 169.6 Nml/g to 84.5 Nml/g for the encapsulated alloy in H2 -14.4% CO. As we know, the metal hydride can be poisoned by CO. When H2 mixes with CO, CO is also adsorbed by the surface of the alloy, and it can react with alloy particles to form a compound layer [8]. The layer can prevent hydrogen from diffusing inside the alloy particles. So the quantity of H2 diffusing inside the alloy particles will become small and the amount of H2 absorbed by the metal powder will decrease. The absorption capacity of the encapsulated alloy is approximately 10 times larger than that of the original one. The
absorption speed of the metal powder with silica is also faster than the one without silica. When the alloy particles are encapsulated by the SiO2 sol, there is a porous network outside the encapsulated alloy, and the sizes of most pores are smaller than the diameter of the CO molecule. So CO cannot transfer into most of the network and cannot be absorbed by the metal powder. As a result, most of the alloy particles are not poisoned. While H2 can also transfer into the network and can be absorbed by the metal powder. Hence, the quantity of H2 diffusing inside the metal powder with silica is larger and the amount of H2 absorbed by encapsulated alloy will increase. The absorption amounts are 6.2 Nml/g for the original alloy and 67.6 Nml/g for the encapsulated alloy at the beginning. The two amounts hardly change before 18 min and 23 min, respectively. When H2 mixes with CO, the alloy particles can absorb both of them. H2 competes with CO for reacting with alloy particles, once the valves between reservoir and sample container are open. With the absorption of CO, the poisoning of the alloy becomes more and more serious. The poisoned alloy particles rarely go on absorption, so the absorption amounts of the two alloys increase few. There are several increases during the absorption process
X. Qian, G. Huang / Fusion Engineering and Design 89 (2014) 2975–2980
2979
Fig. 8. The evolution of the hydrogen adsorption capacities of the encapsulated and original LaNi4.8 Al0.2 alloy with time in H2 -12.8%CO2 at approximately 0.2 MPa.
Fig. 6. (a) The variations of pressure in first and last hydrogen cycle of original LaNi4.8 Al0.2 alloy particles. (b) The variations of pressure in first and last hydrogen cycle of encapsulated LaNi4.8 Al0.2 alloy agglomerations.
of the two alloys. The absorption amount of the original alloy is firstly increased from 6.2 Nml/g to 6.5 Nml/g at 18 min, after about 5 min slow increase, the amount reaches the maximum, which is 8.2 Nml/g. Subsequently, the absorption amount of the encapsulated alloy is increased from 67.6 Nml/g to 69.9 Nml/g at 23 min,
Fig. 7. The evolution of the hydrogen adsorption capacities of the encapsulated and original LaNi4.8 Al0.2 alloy with time in H2 -14.4%CO at 0.2 MPa.
after about 6 min slow increase, the absorption value reaches the maximum that is 84.5 Nml/g. And then the absorption amounts of the two alloys do not change till the experiment is over. The increase may be attributed to the absorption of the new-formed surface on the alloy particles. Maybe a few alloy particles break after absorption and reaction with H2 , it can produce some new surface. It is possible for the fresh surface of the metal to absorb the remainder hydrogen in the system. Fig. 8 presents the evolutions of the hydrogen absorption quantities of the encapsulated and original alloy with time during one absorption process in H2 -12.8% CO2 at 0.2 MPa. The result shows that the absorption capacity of the original LaNi4.8 Al0.2 alloy reduces remarkably because of the CO2 exposure, but the absorption amount of the encapsulated alloy is almost invariable after reaching equilibrium. For example, the largest absorption amount of the original alloy reduces from 147.1 Nml/g to 18.6 Nml/g, while the absorption capacity of the encapsulated alloy is 168.9 Nml/g, which is very close to 169.3 Nml/g in pure H2 . The absorption amount of the encapsulated alloy is almost 10 times larger than that of the original alloy. There are also some fluctuations during the hydrogen absorption process of the two alloys, which is the same with the status of the CO exposure. The LaNi4.8 Al0.2 alloy can be poisoned by absorption of CO and CO2 , but CO is considered as a strong poisonous gas, because the poisoning effect by CO absorption is more serious than that by CO2 absorption. For example, the absorption quantities of the original LaNi4.8 Al0.2 alloy are 18.6 Nml/g in H2 -12.8% CO2 and 8.2 Nml/g in H2 -14.4% CO. If CO is introduced into H2 , when the hydrogen storage alloy particles absorb hydrogen, CO is absorbed by the alloy surface and reacts with the alloy particles to form a compound layer [8]. The compound layer can prevent hydrogen from diffusing inside the alloy particles. Sang et al. [9] also reported that the formed compound film on the alloy surface can inhibit further hydrogen absorption, as a result, the absorption rate will be reduced and the hydrogen absorption amount will decrease. In present experiments, there is a porous network outside the encapsulated alloy, and all the sizes of the pores in the network are almost smaller than the diameter of the CO2 molecule. So CO2 cannot transfer into the network and cannot be absorbed by the LaNi4.8 Al0.2 alloy, while the hydrogen can be absorbed by the encapsulated alloy continuously. Hence the absorption amount in the presence of CO2 is larger than that in CO. CO can also affect the hydrogen absorption capacity of the encapsulated alloy. In the presence of CO, the largest absorption
2980
X. Qian, G. Huang / Fusion Engineering and Design 89 (2014) 2975–2980
value of the encapsulated alloy decreases from 169.6 Nml/g to 84.5 Nml/g, but it is still larger than that of the original alloy. There is almost no influence on the absorption capacity of the encapsulated alloy in the presence of CO2 , because the absorption capacity of the encapsulated alloy is hardly change with or without CO2 . The poisoning resistance property of the LaNi4.8 Al0.2 alloy improves after being encapsulated by the silica matrix. Once the alloy particles are encapsulated by SiO2 sol, there is a porous network of the silica matrix formed outside the alloy. The pores in the network are very small and can restrict the transfer of large molecules, such as CO, CO2 , and other harmful impurities. Therefore it can protect the metal hydrides from such impurities.
properties of the LaNi4.8 Al0.2 alloy are noticeably enhanced. Hence, this process is suitable for the surface modification of the hydrogen storage alloy with cost-saving and energy-saving advantages.
4. Conclusions
[1] I.A. Fisher, Structural stability of 1100[degree]C heated Pd/k during absorption cycling in protium, WSRC-RP-93-401. [2] R.D. Penzhorn, M. Devillers, M. Sirch, Storage of tritium in ZrCo alloy: effect of preexposure to impurities relevant to the fusion fuel cycle, J. Nucl. Mater. 179–181 (1991) 863–866. [3] C. Boffito, B. Ferrario, D. Martelli, Equilibrium pressures of H2 and D2 for different getter materials and the effect of Co impurities, J. Vac. Technol. A1 (1983) 1279–1282. [4] K. Ashida, K. Watanabe, S. Ikeno, K. Mori, Surface modification of Zr-based alloys applicable for tritium processing, Fusion Eng. Des. 39–40 (1998) 1049–1054. [5] L.K. Heung, G.G. Wicks, G.L. Enz, N. Augusta, Composition for absorbing hydrogen, US pat. 5411928, 1995. [6] L.K. Heung, Method and composition in which metal hydride particles are embedded in a silica network, US pat. 5958098, 1999. [7] L.K. Heung, G.G. Wicks, Silica embedded metal hydrides, J. Alloys Compd. 293 (1999) 446–451. [8] J.I. Han, J.-Y. Lee, The effect of CO impurity on the hydrogenation properties of LaNi5 , LaNi4.7 Al0.3 and MmNi4.5 Al0.5 during hydriding–dehydriding cycling, J. Less-Common Met. 152 (1989) 319–327. [9] G. Sang, M.J. Tu, Q.A. Li, K.P. Yan, Y.G. Ning, Y.G. Chen, et al., Sims analysis of CO stabilization of LaNi4.7 Al0.3 alloy, Acta Metall. Sin. 36 (2000) 251–253.
There is a porous network of the silica matrix formed outside the LaNi4.8 Al0.2 alloy powder after being encapsulated by SiO2 sol. The pores in the network are so fine that the alloy particles can be encapsulated in the network effectively, even the smaller broken alloy particles. The fine pores can also restrict large molecules entering into the matrix to prevent the poisoning of alloy particles. After being encapsulated by the silica matrix, the hydrogen absorption capacity of the LaNi4.8 Al0.2 alloy does not decrease. Sizes of the encapsulated alloy agglomerations hardly change after 30 times absorption/desorption cycles. When hydrogen is mixed with CO or CO2 , the absorption capacity of the encapsulated alloy is reduced to half of the case without CO. But there is little effect on the absorption capacity of the encapsulated alloy in the presence of CO2 . After surface modification, the pulverization and poisoning resistance
Acknowledgements We are grateful to Doctor Huang HG from National Key Laboratory of Surface Physics and Chemistry, China, for alloy surface analysis by SEM. We also would like to thank Tang T and Huang ZY for insightful discussion. Thanks to Xiang X and Chen M for providing language help. References