Study of an industrially oriented Mg content control technology during annealing process for the LaMg(NiAl)3.5 hydrogen storage alloy
Introduction
Although facing with harsh challenges from novel batteries such as lithium-ion batteries, the Ni-MH batteries still play an important role in the application of medical devices, hybrid car, emergency rescue and other devices due to its outstanding properties in safety, durability et al. [1]. Since facilities mentioned above own remarkable market share, the further development of the Ni-MH batteries still has an attractive potential future prospects.
However, the low capacity [2] and the relative high cost, which dues to the application of expensive metals such as Co [3], [4], of the traditional anode material based on AB5 type hydrogen storage alloy no longer fits the requirement and limit of the application of Ni-MH batteries for future development. Hence, the development of the LaMgNi based A2B7 type anode material, which has higher capacity, lower self-discharge rate and better comprehensive performance [5], is necessary and attractive for future prospects.
In the last decades, researchers had developed various kinds of A2B7 LaMgNi based hydrogen storage alloys [6], [7], [8] and benefits the industrial scale production. Several methods have been adopted to improve the dehydrogenation kinetics and lowered the thermodynamic stability [9], [10], [11], [12], [13]. However, the production of the Mg-based alloys is suffering from two major technical problems. Firstly, the volatilization of the raw material during the melting and casting process causes an obvious loss of Mg in the final alloy and serious safety issues. Although people adopt special kinds of protecting atmosphere and casting processes during the melting, the control of the process depends mainly on the experience of the operator. Secondly, in order to increase the cycle life and the capacity of the material, an annealing process is needed to homogenize the composition and to reduce component segregation. However, the Mg volatilization during the annealing process complicates the conditions for optimization of the as-prepared alloys performance [14], [15]. It is difficult to control and prevent the Mg volatilization from the alloy during the batch production process at high temperatures and to maintain a stable Mg concentration in the alloy after the annealing procedure. Typically, the produced alloy possesses a multi-phase structures, which contains a CaCu5 type AB5 phase, a PuNi3 type AB3 phases and a Ce2Ni7 type A2B7 phase [16], [17]. The types and the ratio of these different phases determine the performance of the alloy and is the key element for optimizing the hy-/dehydrogenation capacity and the cycle life of the alloy. Without constant Mg concentrations of the alloys a precise design of phase abundance and the ratio of the hydrogen storage material is impossible Therefore, an effective way to control or to reduce the Mg volatilization during the annealing process is required for a reliable mass production. In this paper, we focus on developing and studying of a novel possibly industrial-scaled feasible method to prevent Mg volatilization from the LaMg(NiAl)3.5 alloy during the annealing process.
Several researches [18], [19], [20], [21] have shown that Mg-based hydrogen storage materials are not only useful as anode material for Ni-MH batteries, but also appropriable as hydrogen carrier in hydrogen compressing applications or heat storage management system. By solving the problem of uncontrollable Mg volatilization during the production process, the production of alloys with a high degree of consistency and stability in the composition and performance becomes feasible.
Although people came up with several ideas reducing the volatilization rate of Mg during melting and casting processes [22], [23], [24], few efforts has been made in impeding the Mg loss during annealing processes. Hayakawa et al. [25] tried to control the Mg vapor pressure during annealing to obtain La4MgNi19 with homogeneous composition. Li et al. [26] investigated the effect of annealing temperature on microstructures and electrochemical performances of La0.75Mg0.25Ni3.05Co0.2Al0.1Mo0.15 hydrogen storage alloy. The comprehensive electrochemical properties of the annealed alloy at 1173 K presents a good balance between superior discharge capacity, kinetic property and remarkably improved cyclic stability. Young et al. [27] pointed out that extending the annealing period from 5 hours to 16 hours further increased the A2B7 phase abundance and improved discharge electrochemical capacity, charge retention rate, and cycle life, but deteriorated the activation and high-rate discharge ability due to the reduction of AB5 catalytic phase. However, the hydrogen storage alloy manufacturers are still following the traditional annealing way, which cannot control the volatilization of Mg in the heat-treatment process of Mg-containing alloys by far. This is one important reason why only few manufacturers can produce batch-qualify A2B7 hydrogen storage alloys.
According to the law of thermodynamics, the relationship between the saturated vapor pressure P0 of Mg and the temperature T is as follow:lnP0 = A/T + B*lgT + C*T + D.
A, B, C, D are constants, which can be found in the manual [28] as Table 1:
As shown in Fig. 1, the saturated vapor pressure P0 of Mg increases rapidly with rising temperatures after 1100 K. The higher the temperature, the higher the saturated vapor pressure will be which is the main driving force for the Mg volatilization.
The saturated vapor pressure of the Mg, which was alloyed with other metals, can be calculated with the following equation [28],Pie = xi*ri*Pi0
xi: mole fraction of component i;
ri: activity coefficient of component i in the melting alloy, in this experiment ri ≈ 1;
Pi0: saturated vapor pressure of the metal element;
Pie: saturated vapor pressure of the metal in the alloy.
The saturated vapor pressure of a metal in the alloy depends mainly from the temperature. From all metals of LaMg(NiAl)3.5 hydrogen storage alloys only Mg shows a significant saturated vapor pressure at the annealing temperatures around 1000 K. Therefore, for a reliable production process it is important to reduce the volatilization of Mg from the alloy during the annealing process. Besides, the “Cold Zones”, in which the temperature is obviously lower than the effective heating zone of the furnace, could also cause further volatilization of Mg. Due to the design and manufacture technology limitation in some furnaces, several “Cold Zones” areas are formed by those huge valves and lines that connect to the outside parts of the furnace, such as gas resources or vacuum pumps. Even in the internal of the furnace chamber, some areas could not be effective heated up like others and therefore could also form several “Cold Zones”. The existence of these “Cold Zones” helps building up the temperature difference inside the furnace chamber. The Mg will condense in these areas which have lower temperatures. The condensed Mg reduces the amount of Mg in the atmosphere of the effective heating zone of the furnace and lowers the vapor pressure of the Mg. The reduction of the Mg vapor pressure will in turn enhances the driving force of the volatilization of Mg from the alloy.
Based on the description above, we propose the construction of a closed container, which could separates the alloy from the internal environment of the furnace, inside the effective heating zone of the furnace and in addition use of Mg element metal as source or compensation material for the forming of Mg vapor pressure. This will help preventing Mg from moving to the “cold zones” of the furnace.
In order to prove the idea, we build a closed container porotype by quartz and run the experiments on the production line of the alloy manufacturer.
Section snippets
Experimental
The determination of the content of Mg was performed with a Thermo Electron IRIS Intrepid II XSP Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) spectrometer. Powders were sieved to a particle size below 400 meshes for the measurement. The Input power is 1000 ± 100 kW. Argon was used as protective and carrier gas.
The phase composition of the sample was measured by X-ray diffractometer (XRD, Bruker D8 Advance) equipped with Cu Ka radiation and LynxEye, scanned in the 2θ range
Influence of the annealing temperature
Fig. 2 shows the Mg loss rate of the alloys during three annealing procedures at different temperatures for 2 h. The Mg loss rates of all three groups increase with rising temperature. At temperatures higher than 800 °C, the Mg loss rate becomes greater due to the steep increase of the vapor pressure P0 of Mg (shown in Fig. 1). The Mg loss rate of group I reached almost 12% after 2 h at the temperature of 1000 °C. Meanwhile, the Mg loss rate is only 8.9% for the closed system in group II. In
Conclusion
We propose a novel annealing method for Mg-contained hydrogen storage alloys and prove the effectiveness of this method. The novel annealing method can effectively impedes the volatilization of Mg from the alloy by limiting the free space of the annealing furnace and achieves the saturated Mg vapor pressure as fast as possible with the help of external Mg metal. The sealed container provides a limited and closed environment, which not only reduces the volume for forming of Mg vapor pressure but
Acknowledgement
The authors of this paper would like to express their gratitude to the financial supported by the High-tech Industrial Projects of Guangdong Province (No. 2016A010104015), Guangdong Science and Technology Project (No. 2017A070701022, 2017B090907026), Guangdong Key Laboratory of Rare Earth Development and Application Project (No. 2017B030314081), BRICS International Cooperation Project (BRIC171030276859, BRICS2017-064 Metal hydride materials and systems for the increase of efficiency in
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