Abstract:
The present invention provides a method and apparatus for manufacturing high-purity hydrogen storage alloy Mg 2 Ni applicable to industry and capable of manufacturing continuously. First, raw materials of magnesium-nickel with weight percentage of nickel between 23.5 and 50.2 are heated, melt, and mixed uniformly. Cool the magnesium-nickel liquid and control the temperature to be above the solidification temperature and below the liquification temperature in the phase diagram of magnesium-nickel. By making advantage of segregation principle in phase diagrams, solid-state high-purity γ-phase Mg 2 Ni hydrogen storage alloy is given. The residual waste magnesium-rich liquid in the crucible is poured to another independent crucible, and switch with the position of the crucible originally containing the γ-phase Mg 2 Ni hydrogen storage alloy. Then, new raw materials of magnesium and nickel are added and heated. Repeat the smelt steps described above continuously, and a continuous manufacturing method is introduced. After the original crucible is cooled, the solid substances at the bottom of the crucible can be tapped down without further special treatments. Then high-purity γ-phase Mg 2 Ni hydrogen storage alloy with atomic ratio of 2:1, no other phases, and with excellent hydrogen absorption-desorption dynamics is given.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to a hydrogen storage technology for new energies, and particularly to a method and apparatus for manufacturing high-purity hydrogen storage alloy Mg 2 Ni. 
       BACKGROUND OF THE INVENTION 
       [0002]    Owing to substantial growth of usage in fossil energy while which energy is drying up gradually, to pernicious substances harmful to human bodies produced by extensive application of fossil energy, such as SO 2 —CO—NO x , and to global climate changes caused by the greenhouse effect due to considerable quantity of exhausted CO 2 , the world is devoted to the development of new energy technologies. In particular, hydrogen energy is planned to be one of the major energies in the future by the International Energy Agency (IEA), because the byproduct thereof is water only, without CO 2 , which completely prevents pollution and the greenhouse effect. However, in practical applications, due to the light molecular weight of hydrogen, the storage volume will be immensely huge. Though super-high pressure can be adopted for storage, safety will be another issue. 
         [0003]    The problems of storage density and safety of hydrogen are not solved until 1980 when the hydrogen storage alloys that can stores hydrogen in solid state is introduced. Nevertheless, the hydrogen storage density of current commercial hydrogen storage alloys, including transition-metal-based hydrogen storage alloys AB 2  or rare-earth-metal-based hydrogen storage alloys AB 5 , is still too low, less than 2.0% in weight. Thereby, the research and development of high-capacity hydrogen storage alloys is the current international trend. Particularly, magnesium-based hydrogen storage alloys are regarded as potential hydrogen storage alloys due to their low costs in raw materials. However, because pure magnesium is very active, the surface thereof tends to form an oxidation layer that can block absorption of hydrogen molecules, and hence affect diffusion rate of hydrogen atoms on the surface of alloys. As a result, pure magnesium is difficult to be activated and has bad hydrogen absorption-desorption dynamics. In addition, the temperatures of hydrogen absorption and desorption are too high. Accordingly, it cannot be developed to be a practical hydrogen storage alloy. 
         [0004]    Regarding to the issue of bad hydrogen absorption-desorption dynamics of pure magnesium, by many researches, it is discovered that by adding nickel with catalyzing effect, the reaction rate of hydrogen absorption-desorption in the hydrogen storage alloy Mg—Ni can be improved, and the initial activation properties is catalyzed as well. In the Mg—Ni-based hydrogen storage alloys, Mg 2 Ni in the γ-phase has the fastest activation reaction rate and the best hydrogen absorption-desorption property. 
         [0005]    Because the melting points of magnesium (649° C.) and nickel (1455° C.) differ greatly, melting tends to be ununiform, which would result in ununiformity in composition of the hydrogen storage alloy. In addition, the vapor pressure of magnesium is high, thereby magnesium is easy to vaporize during melting, which causes severe deviation in initial composition, and excess eutectic structure and formation of the β-phase MgNi 2 , which is incapable of absorbing hydrogen. In order to solve the problem the severe deviation in composition during melting as described above, next-generation vacuum induction furnaces are introduced. However, although the vacuum induction furnaces are equipped with in-situ inspection, for the hydrogen storage alloy Mg—Ni, owing to its natural characteristic in the phase diagram, the melt liquid of Mg—Ni still cannot give 100%-pure γ-phase Mg 2 Ni after solidification, even the composition of magnesium and nickel are controlled to be accurately 2:1 via the most precise in-situ inspection function. This is because according to the binary equilibrium phase diagram of magnesium and nickel, in such a composition, far above the melting point 761° C. of the γ-phase Mg 2 Ni, the β-phase MgNi 2 , which has a meting point of 1147° C. and is incapable of absorbing hydrogen, has solidified and precipitated first. Besides, because the composition of the β-phase MgNi 2  has much more nickel than the γ-phase Mg 2 Ni, the residual Mg—Ni melt liquid yet solidified deviates from the original composition of the γ-phase Mg 2 Ni with a magnesium-to-nickel atomic ratio of 2:1, and becomes a magnesium-rich state. The Mg—Ni melt liquid in the magnesium-rich state, according to the binary equilibrium phase diagram of magnesium and nickel, not only will form the γ-phase Mg 2 Ni if the temperature is lower than 761° C. in the present composition, but also will give an eutectic structure including the pure-magnesium phase at the eutectic temperature of 507° C. That is to say, even the macroscopic composition complies with the proportion of the γ phase, the microscopic structure thereof includes the β-phase MgNi 2  and the solid solution phase of pure-magnesium in the γ-phase Mg 2 Ni. Thereby, the smelt method according to the prior art cannot be used for preparing high-purity hydrogen storage alloy Mg 2 Ni with fast activation reaction rate and with excellent hydrogen absorption and desorption properties. 
         [0006]    Accordingly, the authors of the present invention make advantage of the segregation principle in physical metallurgy, in a broad range of composition and in low temperatures (far lower than the melting point of pure nickel), and propose a simple apparatus for continuously manufacturing high-purity hydrogen storage alloy Mg 2 Ni. 
       SUMMARY 
       [0007]    An objective of the present invention is to provide a method and apparatus for manufacturing high-purity magnesium-nickel hydrogen storage alloy without the need of precisely controlling the composition of magnesium and nickel in the magnesium-nickel alloy. 
         [0008]    Another objective of the present invention is to provide a method and apparatus for manufacturing high-purity magnesium-nickel hydrogen storage alloy, which can recycle the residual magnesium-rich liquid after the precipitation reaction and continuously manufacture high-purity magnesium-nickel hydrogen storage alloy according to the method provided by the present invention. 
         [0009]    In order to achieve the objectives described above, the present invention provides a method and apparatus for manufacturing high-purity magnesium-nickel hydrogen storage alloy. The apparatus comprises a vacuum chamber with a material feeding tube, a first crucible, a heating device, a stirring device, and a second crucible. First, put the raw material of pure magnesium into the first crucible, and place the first crucible into the vacuum chamber gassed with an inert gas. Then, use the heating device to heat the magnesium raw material until it melts completely into a magnesium liquid. Next, use the material feeding tube to add slowly pure nickel powders to the first crucible with the magnesium liquid, and use the stirring device to stir unceasingly while using the heating device to heat up, so that the nickel powders are melt completely and mixed with the magnesium liquid to become a uniform magnesium-nickel liquid. It is not necessary for the apparatus and method according to the present invention to install delicate in-situ inspection, nor to control precisely the composition of the magnesium-nickel liquid. It is only required that the weight percentage of the amount of the added nickel to the whole magnesium-nickel melt is between 23.5 and 50.2, then it is guaranteed to give pure γ-phase Mg 2 Ni hydrogen storage alloy with composition of Mg-54.6 wt % Ni (that is, the atomic ratio between magnesium and nickel is 2:1) without other phases. 
         [0010]    The next step is to control the heating temperature of the heating device to be within a temperature range, which is between 507° C. and 761° C. According to the segregation principle of physical metallurgy and to the Mg—Ni phase diagram, high-purity magnesium-nickel hydrogen storage alloy will be formed and precipitated automatically, and the purity thereof is independent of the precipitation temperature within said temperature range. Thereby, according to the present invention, it is not necessary to adopt accurate and costly temperature control systems. In addition, the precipitated quantity (weight) of the hydrogen storage alloy Mg 2 Ni depends on the composition of the magnesium-nickel liquid and the precipitation temperature. In general, within the broad ranges of composition and temperature conditions according to the present invention, the higher the proportion of nickel and the lower the precipitation temperature, the more the precipitated quantity of high-purity γ-phase Mg 2 Ni. The exact precipitated quantity (weight) can be calculated according to the level rule of phase diagram in physical metallurgy. 
         [0011]    Because the nickel composition (54.6 wt %) of the precipitated high-purity γ-phase Mg 2 Ni according to the present invention is higher than that of the original magnesium-nickel composition (that is, the weight percentage of nickel is between 23.5 and 50.2), with the progress of precipitation reaction, according to the law of conservation of mass, the composition of the residual magnesium-nickel liquid will become more and more magnesium-rich. The density of nickel (8.9 g/cm 3 ) is much greater than that of magnesium (1.74 g/cm 3 ), therefore, the precipitated high-purity magnesium-nickel hydrogen storage alloy will sink at the bottom of the crucible given that the density of solid-state magnesium-nickel hydrogen storage alloy is much greater than the specific weight of the magnesium-nickel liquid. Thereby, pour the residual liquid in the first crucible after the precipitation reaction into the second crucible, draw out the first crucible loaded with the precipitated magnesium-nickel hydrogen storage alloy from the heating device, and cool the first crucible. After cooling, pick out the magnesium-nickel hydrogen storage alloy from the first crucible, and repeat the procedure described above for the second crucible loaded with the residual liquid. Then high-purity magnesium-nickel hydrogen storage alloy is given continuously. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows a flowchart according to a preferred embodiment of the present invention; 
           [0013]      FIG. 2A  shows a schematic diagram of the apparatus in the steps S 10  and S 11  according to a preferred embodiment of the present invention; 
           [0014]      FIG. 2B  shows a schematic diagram of the apparatus in the step S 12  according to a preferred embodiment of the present invention; 
           [0015]      FIG. 2C  shows a schematic diagram of the apparatus in the step S 13  according to a preferred embodiment of the present invention; 
           [0016]      FIG. 2D  shows a schematic diagram of the apparatus in the step S 14  according to a preferred embodiment of the present invention; 
           [0017]      FIG. 2E  shows a schematic diagram of the apparatus in the step S 15  according to a preferred embodiment of the present invention; 
           [0018]      FIG. 2F  shows a schematic diagram of the apparatus in the step S 16  according to a preferred embodiment of the present invention; and 
           [0019]      FIG. 3  shows a schematic diagram of the apparatus according to another preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    In order to make the structure and characteristics as well as the effectiveness of the present invention to be further understood and recognized, the detailed description of the present invention is provided as follows along with preferred embodiments and accompanying figures. 
         [0021]      FIG. 1  and  FIG. 2A  show a flowchart and a schematic diagram of the apparatus in the steps S 10  and S 11  according to a preferred embodiment of the present invention. As shown in the figure, the present invention provides a method and apparatus for manufacturing high-purity magnesium-nickel hydrogen storage alloy. The apparatus comprises a vacuum chamber  10  with a material feeding tube  104 , a first crucible  12 , a heating device  14 , a stirring device  16 , and a second crucible  18 . By using the apparatus, the step S 10  is executed for putting a raw material of magnesium  11  into the first crucible  12 , where the raw material of magnesium  11  is a magnesium metal bulk, and the material of the first crucible  12  is a metal material with melting point greater than that of the magnesium metal. Then, gas an inert gas  13  into the vacuum chamber  10 , and put the first crucible  12  with the raw material of magnesium  11  into the vacuum chamber  10 . Before gassing the inert gas  13  into the vacuum chamber  10 , the inert gas  13  is first used to purge the vacuum chamber  10 . Finally, seal the vacuum chamber  10 , and let the inert gas  13  be maintained in the vacuum chamber  10 . After the first crucible  11  loaded with the raw material of magnesium  11  is put into the vacuum chamber  10 , the step S 11  is executed for setting the first crucible  12  in the heating device  14 , which is used for heating the raw material of magnesium  11  in the first crucible  12  to be totally melt and become a magnesium liquid  110 . The heating device  14  is a resistive heater with a temperature adjustment function. 
         [0022]      FIG. 2B  shows a schematic diagram of the apparatus in the step S 12  according to a preferred embodiment of the present invention. As shown in the figure, after the raw material of magnesium  11  in the first crucible  12  is melt into the magnesium liquid  110 , the step S 12  is executed for adding nickel powders  15  slowly to the magnesium liquid  110  in the first crucible  12  by using the material feeding tube  104 , and stirring the magnesium liquid  110  and the nickel powders  15  loaded in the first crucible  12  by using the stirring device  16 . Besides, the heating device  14  is used for heating the first crucible  12  with the magnesium liquid  110  and the nickel powders  15  so that the temperature of the magnesium liquid  110  is heated above 770° C. Thereby, the nickel powders  155  are melt completely in the magnesium liquid  110  and a uniformly mixed magnesium-nickel liquid  112  is produced. The stirring device  16  includes a motor  161  and a paddle  163 . In addition, the stirring device  16  can be elevated. When the stirring device  16  is used for stirring, the paddle  163  can elevated to a proper position, and the motor  161  will drive the paddle  163  for stirring. Furthermore, an oar-shaped blade  165  is adapted on one end of the paddle  163  for increasing stirring area and speed. When stirring is performed, the paddle  163  of the stirring device  16  is retracted. The weight percentage of the nickel element in the magnesium-nickel liquid  112  is between 23.5% and 50.2%, which represents the composition of the added nickel powders. Thereby, the composition ratio of the magnesium and nickel elements in the final precipitated high-purity solid-state magnesium-nickel hydrogen storage alloy is 2:1 without other phases. 
         [0023]      FIG. 2C  shows a schematic diagram of the apparatus in the step S 13  according to a preferred embodiment of the present invention. As shown in the figure, when the magnesium-nickel liquid  112  is produced, the step S 13  is executed for controlling the temperature of the heating device  14  to fall within a temperature range. Thereby, the temperature of the magnesium-nickel liquid  112  will be within the temperature range, which is above the solidification temperature and below the liquification temperature of the magnesium-nickel liquid  112 . That is, between 507° C. and 761° C. According to the segregation principle of physical metallurgy and to the Mg—Ni phase diagram, high-purity magnesium-nickel hydrogen storage alloy  114  will be formed and precipitated from the magnesium-nickel liquid  112  automatically, and the purity thereof is independent of the precipitation temperature within said temperature range. Thereby, according to the present invention, it is not necessary to adopt accurate and costly temperature control systems. In addition, the precipitated quantity (weight) of the hydrogen storage alloy  114  depends on the composition of the magnesium-nickel liquid and the precipitation temperature. In general, within the broad ranges of composition and temperature conditions according to the present invention, the higher the proportion of nickel and the lower the precipitation temperature, the more the precipitated quantity of high-purity magnesium-nickel hydrogen storage alloy  114 . The exact precipitated quantity (weight) can be calculated according to the level rule of phase diagram in physical metallurgy. 
         [0024]      FIG. 2D  shows a schematic diagram of the apparatus in the step S 14  according to a preferred embodiment of the present invention. As shown in the figure, the solid-state magnesium-nickel hydrogen storage alloy  114  is precipitated from the magnesium-nickel liquid  112 . The nickel composition of the magnesium-nickel hydrogen storage alloy  114  is greater than that in the magnesium-nickel liquid  112 . With the progress of precipitation reaction, according to the law of conservation of mass, the composition of the residual magnesium-nickel liquid  116  will become magnesium-rich. The density of nickel (8.9 g/cm 3 ) is much greater than that of magnesium (1.74 g/cm 3 ), therefore, the solid-state magnesium-nickel hydrogen storage alloy  114  will sink at the bottom of the first crucible  12 . After the magnesium-nickel liquid  112  precipitated the solid-state magnesium-nickel hydrogen storage alloy  114 , the step S 14  is executed for separating the residual liquid  116  in the first crucible  12  from the solid-state magnesium-nickel hydrogen storage alloy  114  suck at the bottom of the first crucible  12  by pouring the residual liquid  116  in the first crucible  12  into the second crucible  18 . In order to pour the residual liquid  116  in the first crucible  12  into the second crucible  18  easily, an inclinable base  19  is adapted in the vacuum chamber  10  with the first crucible  12  and the heating device  14  set thereon. When the base  19  inclines, the first crucible  12  and the heating device  14  incline with the base  19 , and the residual liquid  116  will be poured into the second crucible  18 . Finally, the solid-state magnesium-nickel hydrogen storage alloy  114  will be left at the bottom of the first crucible  12 . 
         [0025]      FIG. 2E  shows a schematic diagram of the apparatus in the step S 15  according to a preferred embodiment of the present invention. As shown in the figure, the step S 15  is executed. Draw out the first crucible  12  from the heating device  14 , and cool the first crucible  12  loaded with the solid-state magnesium-nickel hydrogen storage alloy  114 . In or to draw out the first crucible  12  from the heating device  14  conveniently, a hoist mechanism  17  is further adapted in the vacuum chamber  10 . The hoist mechanism  17  includes a plurality of twisted ropes  171 , which is fixed on the first crucible  12 . Thereby, the hoist mechanism  17  can draw out the first crucible  12  from the heating device  14 . In addition, in order to secure the connection between the hoist mechanism  17  and the first crucible  12 , a plurality of hanging ears (not shown in the figure) is adapted at the periphery of the opening of the first crucible  12 . A hanging hook (not shown in the figure) is adapted on one end of the plurality of twisted ropes  171  of the hoist mechanism  17 , respectively. Thereby, the hanging hooks are hooked on the plurality of hanging ears of the first crucible  12 . Thus, the connection between the hoist mechanism  17  and the first crucible  12  is secured. 
         [0026]    Another significant technological breakthrough of the present invention is to recycle the residual liquid, and thereby a method and apparatus for continuously manufacturing high-purity magnesium-nickel hydrogen storage alloy is developed.  FIG. 2F  shows a schematic diagram of the apparatus in the step S 16  according to a preferred embodiment of the present invention. As shown in the figure, after the first crucible  12  is drawn out from the heating device  14 , the step S 16  is executed for putting the second crucible  18  loaded with the residual liquid  116  into the heating device  14  by using the hoist mechanism  17 . Then, the steps S 10  through S 16  are executed repeatedly for continuously manufacturing high-purity magnesium-nickel hydrogen storage alloy  114 . The first and the second crucibles  12 ,  18  are used alternately owing to continuous manufacturing. 
         [0027]    While manufacturing continuously, the second and thereafter manufacturing cycles differ from the first manufacturing cycle in that, in the second and thereafter manufacturing cycles, in order to increase productivity of high-purity magnesium-nickel hydrogen storage alloy  114 , the amount of added nickel powders can be increased from the preset range of 23.5% and 50.2% up to 54.6%. The condition still gives high-purity magnesium-nickel hydrogen storage alloy  114  without other phases. Because the residual liquid  116  is a magnesium-rich liquid, which is an excellent composition adjuster, the nickel composition of the magnesium-nickel liquid  112  can be maintained within the range of 20 to 55 wt % without precise and accurate control of chemical composition. 
         [0028]      FIG. 3  shows a schematic diagram of the apparatus according to another preferred embodiment of the present invention. As shown in the figure, the present invention provides an apparatus for manufacturing high-purity magnesium-nickel alloy and comprising a vacuum chamber  10 , a first crucible  12 , a heating device  14 , a stirring device  16 , a second crucible  18 , a hoist mechanism  17 , a water-cooled copper base  100  with recycling cooling water, and a material feeding tube  104 . The vacuum chamber  10  according to the present preferred embodiment is divided into a precipitation chamber  101  and a crucible in/out chamber  103 . One or more isolation valves  102  are adapted between the precipitation chamber  101  and the crucible in/out chamber  103 , so that the precipitation chamber  101  can be maintain in vacuum or in the inert gas no matter separation or crucible in/out is undergoing. 
         [0029]    The first crucible  12 , the heating device  14 , the stirring device  16 , the hoist mechanism  17 , the water-cooled copper base  100 , and the material feeding tube  104  are set in the precipitation chamber  101  of the vacuum chamber  10 . The first crucible is set on the heating device  14 . The stirring device is set on top of precipitation chamber  101  of the vacuum chamber  10 , and facing the first crucible  12 . The hoist mechanism  17  is also set on top of precipitation chamber  101  of the vacuum chamber  10 . The water-cooled copper base  100  is set on one side of the first crucible  12 . The material feeding tube  104  penetrates the vacuum chamber  10 . 
         [0030]    According to the present invention, place a raw material of magnesium to the first crucible  12  on the crucible in/out chamber  103  of the vacuum chamber  10 , and gas an inert gas to the vacuum chamber  10 . Use the hoist mechanism  17  to put the first crucible  12  loaded with the raw material of magnesium to the precipitation chamber  101  filled with the inert gas and into the heating device  14 . The heating device  14  heats the first crucible  12  loaded with the raw material of magnesium, melts the raw material of magnesium to a magnesium liquid. Then, through the material feeding tube  104  penetrating the vacuum chamber  10 , nickel powders are added into the first crucible  12  loaded with the magnesium liquid. By using the heating device  14 , the first crucible  12  loaded with the nickel powders and the magnesium liquid. Besides, the stirring device  16  is used for stirring, so that the nickel powders are melt in the magnesium liquid to produce a magnesium-nickel liquid. Next, control the temperate of the heating device  14  to fall within a temperature range for the magnesium-nickel liquid to precipitate a solid-state magnesium-nickel hydrogen storage alloy. Finally, separate the residual liquid in the first crucible from the precipitated solid-state magnesium-nickel hydrogen storage alloy. First, place a raw material of magnesium in the second crucible  18  and put it to the precipitation chamber  101  of the vacuum chamber  10 . Use the hoist mechanism  17 , which is capable of inclining, to put the first crucible  12  loaded with residual liquid to the second crucible  18 , and put the first crucible  12  on the water-cooled copper base  100  in the precipitation chamber  101 . The water-cooled copper base  100  cools the solid-state magnesium-nickel hydrogen storage alloy in the first crucible  12 . After cooling, use the hoist mechanism  17  to pick the first crucible  12  out, and take the solid-state magnesium-nickel hydrogen storage alloy from the first crucible  12 . The water-cooled copper base  100  is adapted in the precipitation chamber  101 . Because the activity of magnesium-nickel hydrogen storage alloy is very high, it tends to react with oxygen or even ignite, deteriorating its characteristics and producing dangers, it is necessary to cool sufficiently before drawing out from the precipitation chamber  101  in vacuum or filled with the inert gas. In mass production, for example, smelt above hundreds of kilograms or tons, the cooling rate of nature cooling is insufficient, and thus limiting the production efficiency. Thereby, the water-cooled copper base is equipped in the precipitation chamber  101 . By taking advantage of the excellent heat-sinking characteristic of copper, the first crucible loaded with high-purity solid-state magnesium-nickel hydrogen storage alloy can be quenched rapidly. 
         [0031]    To sum up, the present invention provides a method and apparatus for manufacturing high-purity magnesium-nickel hydrogen storage alloy, which can be used for manufacturing high-purity magnesium-nickel hydrogen storage alloy with superior hydrogen absorption-desorption dynamics without the need of adopting costly and delicate equipments. In addition, the residual liquid after precipitation reaction can be recycled and high-purity magnesium-nickel hydrogen storage alloy with superior hydrogen absorption-desorption dynamics can be manufactured continuously. 
         [0032]    Accordingly, the present invention conforms to the legal requirements owing to its novelty, non-obviousness, and utility. However, the foregoing description is only a preferred embodiment of the present invention, not used to limit the scope and range of the present invention. Those equivalent changes or modifications made according to the shape, structure, feature, or spirit described in the claims of the present invention are included in the appended claims of the present invention.