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B.P.Tarasov
Institute of Problems of Chemical Physics ofRussian Academy of Sciences, Chernogolovka 142432, Moscow Region, Russia
Ultradispersed hydrogen-
sorbing metals and intermetallic compounds: preparation and properties
Abstract
The generalized results of works on synthesis and research of properties of ultradispersed (including nanocrystalline and amorphous) hydrogen-sorbing metals and intermetallic compounds and their hydrides are submitted. Specifically the following methods of synthesis are examined: (1) the hydride dispergation with subsequent re-milling on ball mills; (2) the interaction of compact metallic phases with ammonia; (3) the reduction of mixed basic carbonates of metals by the calcium hydride; (4) the mechanochemical synthesis from mixtures of metals or their hydrides; (5) the realization of a "hydrogenation-disproportionation-dehydrogenation-recombination" cycle.
Introduction
The powders of the intermetallic compounds and their hydrides, various on the dispersed composition and the crystalline state, are necessary for a manufacture of the constant magnets, the materials for a storage of hydrogen and the metallohydride electrodes. For preparation of such powders the various methods of hydrogen technology, which allow to obtain more homogeneous powders and to avoid an oxidation of their surface, can be used. Besides the application of hydrogen results in a reduction of a number of the surface oxides, allows to obtain the powders of the compounds of the high-plastic and sensitive to oxygen metals, to avoid a dust content and a loss of an initial material, as it occurs by mechanical comminution of cast alloys.
In the given paper the results of works on a development of the methods of synthesis and of dispergation of a number of the intermetallides and their hydrides are generalized. In more details they are submitted in works [1-9]. The powders were obtained in a microcrystalline or amorphous state by the following methods: (1) the hydride dispergation with subsequent re-milling on ball mills (on an example of LaNi5, CeCo3,
TiFe, SmCo5); (2) the interaction of compact metallic phases with the ammonia (on an example of Ti, V, Zr, ScFe2); (3) the reduction of mixed basic carbonates of metals by the calcium hydride in hydrogen environment (on an example of LaNi5); (4) the mechanochemical synthesis from mixtures of metals or of their hydrides (on an example of TiFe); (5) the realization of "hydrogenation-disproportionation-dehydrogenation-recombination" cycle (on an example of CeNi3 and Nd2Fei4B).
1. Hydride dispergation
The most wide-spread method of obtaining powders of the hydrogen-sorbing intermetallic compounds in the crystalline and amorphous states is making an alloy of metallic components in the appropriate proportions and the subsequent dispergation of an alloy under an action of hydrogen [1, 5, 10, 11] or under an action of external mechanical influences in the ball mills [2, 12-14].
The reversible interaction of the intermetallides with hydrogen, for example, on the reactions: LaNi5 + 3 H2 <=====> LaNi5H6, SmCo5 + 2.5 H2 <=====> SmCo5H5, CeCo3 + 2 H2 <=====> CeCo3H4, TiFe + H2 <=====> TiFeH2, results in an embrittlement and a comminution of the compact materials. With an increase of the number of "absorption I2 <==> desorption I2" cycles the size of the particles decreases in all cases [1]. The least average size is reached after realization of 15 cycles and corresponds to 2-5 mcm for LaNi5 and NâNî3 and 7-10 mcm for TiFe and SmCo5. With an increase of the number of cycles the kinetics of hydrogen sorption is improved, and the absorbing characteristics essentially do not vary [1, 2]. The density of an exchange current in reaction of the hydrogen evolution on the electrodes from the received thus samples of LaNi5 considerably exceeds the similar magnitude for a Ni powder [2, 3].
2. Mechanical comminution of hydrides of metals and intermetallides
With a treatment of initial alloys on the vibromill the alloy button badly reduce in size, and the samples after the hydrogen embrittlement disperse to the particles of the size of 0.8-1.5 mcm without appreciable change of chemical and phase composition. The hydrogen-sorbing and electrocatalytic property of the samples received thus essentially do not differ from the characteristics of the initial intermetallide [2, 3].
With a high-energy influence on initial alloys in a planetary mill the powder will be formed nonuniform on the size. Such treatment of the samples, previously embrittled by hydrogen, results in a formation of homogeneous fine-dispersed powders, and a size of particles and the properties practically do not depend on the number of "absorption <==> desorption I2" cycles, but essentially depend on a nature of the intermetallic compound, a magnitude of the ball load and milling time. Thus, with a small load of the milling balls (from 1:1 up to 5:1) the particles of LaNi5 and CeCo3 in addition reduce in size up to 0.4-0.8 mcm, and TiFe and SmCo5 -up to 1-3 mcm. With a greater load (from 10:1 up to 50:1) and milling time of 2 h there is an additional
comminution of the samples till 0.1-0.2 mcm. With an increase of time of the high-energy treatment up to 16-24 h the samples become X-ray amorphous, and on a surface of powders the oxides or hydroxides of metals are found. The measurements of the magnetic characteristics, the specific resistance and the electronic work function in the amorphous samples testify about decomposition of the intermetallide matrix, as a result of high-energy treatment, with a formation of the 3d-metal clusters. With a heat treatment of such amorphous powders above 800 K the crystallization of 3d-metals is observed, and besides with increase of the temperature of heating and the hydrogen influence this effect intensifies [2, 3].
The hydrogen-sorbing property of the intermetallide powders, obtained by milling with average loads, are identical to the properties of initial intermetallides with small reduction of a capacity on hydrogen. The high-energy treatment in severe conditions, resulting in an amorphization, reduces a hydrogen capacity by 20-30% and increases a slope of a plateau of phase transition.
The measurement of the density of a exchange current in a reaction of hydrogen evolution on the electrodes, prepared from LaNi5, subjected to high-energy treatment, has shown an unmonotonous influence of milling duration on electrocatalytic activity of the samples. The highest size of current density (0.83 mA/mg) corresponds to milling time of 1.5-2 h and is four times as much as that of initial LaNi5. The further increase of an influence time results in sharp decrease of electrocatalytic activity of samples and after 8 h the density of a exchange current becomes even less, than at initial LaNi5 [2,3].
The additional milling of alloys SmCo5 and Nd2Fe14B, embrittled by hydrogen, essentially reduces the sizes of powder particles, that results in some increase of the magnetic characteristics.
3. Preparation of intermetallide powders by reduction of mixed basic carbonates by calcium hydride in hydrogen
The chemical methods of a synthesis of the intermetallic compounds, based on a reduction of the mixtures of the oxides, hydroxides or chlorides of metals by calcium, calcium hydride etc. in hydrogen or inert environments, are known [4, 15, 16].
In the given work the powders of the intermetallic compound LaNi5 with the average particle size of ~ 220 nm are synthesized from mixed basic carbonate of gross-composition La2Ni10(CO3)8(OH)10(H2O)x, which was received by dissolution of Ni(OH)2 in a solution of LaCl3 in chlorohydric acid (in a ratio La:Ni = 1:5) and by precipitation by the sodium carbonate
(La3+: CO32- = 1:4) and by solution of NaOH (La3+: OH- = 1:5) according to reaction scheme:
300 K
LaCl3 + Ni(OH)2 + Na2CO3 + NaOH + H2O ..............>
.............> La2Ni10(CO3)8 (OH)10- (H2O)x
The mixed basic carbonate was reduced in a current of hydrogen at temperature of 1023 K, obtained mixture was mixed with CaH2 and was maintained in a current of hydrogen at 1273 K:
1023-1273 K
La2Ni10(CO3)8(OH)10.(H2O)x + CaH2 + H2 .......................>
...............> LaNi5 + CaO + CO2
After a wash by water and ether the homogeneous on chemical and phase composition powder of the intermetallic compound of the composition LaNi5 with the content of oxygen no more ~ 0.2% was obtained, and oxygen resided in basic in superficial layers of the particles as a film from lanthanum oxide. The magnetic measurements testify about the content in a sample no more than 0.8 mass % of free nickel. The hydrogen-sorbing properties of the obtained powders of LaNi5 essentially do not differ from the properties of intermetallide, prepared by making an alloy. The dependences of an equilibrium pressure of hydrogen above two-phase region and the appropriate thermodynamic characteristics practically agree, though a maximal hydrogen-sorbing capacity is slightly lesser (about 10 %) [4].
4. Ammoniacal method of obtaining powders of metallic phases
The method is based on the reaction of an interaction of powders of the metals (Ti, Zr, V) or the intermetallides (ScFe2) with various dispersity with ammonia at the temperatures of 520-770 K and pressure of 0.8 MPa in a presence of N^Cl [6,7]:
520-770 K, 0.8 MPa M + NH3 (NH4Cl) ................................> MH2
In the case of Ti such treatment results in a formation of nanocrystalline (the average particle size is 1540 nm), tetragonal (a = 0.4462-0.4476 nm, c = 0.4396-0.4384 nm) [6] titanium dihydride, stabilized by insignificant quantity of nitrogen atoms:
NH3 (NH4CO Ti ..........................> TiH1.92
520 K
At heating up to 770 K the cubic modification of the titanium dihydride (a = 0.4449 nm) forms. At heating up to 900 K the titanium dihydride decomposes, that allows to receive the partially caked powder of titanium:
> 850 K
TiH~ 2 ....................> Ti
The treatment of titanium by ammonia above 700 K results, basically, in a formation of the nanocrystalline (the average size of particles of 15-50 nm) cubic titanium nitride (a = 0.4232-0.4234 nm) [6]:
NH3 (NH4Cl) Ti -------------------------- > TiN
> 620 K
5. Mechanochemical synthesis of intermetallide powders
The method of obtaining the various intermetallides by mechanochemical synthesis from the initial components by a treatment on high-energy ball mills is known [18-20], and we consider the features of synthesis of powders of TiFe from the mixtures of iron and titanium or the titanium hydride [8, 9].
At a high-energy treatment of the mixtures of the equimolar quantities of Ti and Fe or TiH2 and Fe for from 20 up to 70 h in an atmosphere of Ar or H2 in severe conditions (a ratio of mass of grinding balls to mass of a sample is from 20:1 up to 50:1, an acceleration of grinding balls up to 700 m/s2) the X-ray amorphous
powders of "TiFe" and "TiFeHx" with the average size of particles of 0.5-2 mcm [8, 9] form:
970 K
Ti + Fe .............> "TiFe" .................> TiFe (+ TiFe2)
970 K
TiH2 + Fe ----------> "TiFeHx" .................> TiFe (+ TiFe2)
Such powders absorb hydrogen up to 0.5-0.6 mass % and completely evolve it at heating up to 600 K without the precisely expressed phase transition. The analysis of thermodesorbing, magnetic and spectral data testifies that in such samples individual Fe and Ti or TiH2 account for less than 3% of the total. At a temperature of about 1000 K a exoeffect without gas evolution is observed. This effect connects with irreversible crystallization of a sample, and the obtained sample represents TiFe with a large impurity (up to 20%) of TiFe2. The hydrogen-sorbing properties of crystalline intermetallide TiFe obtained are less than those of an intermetallide received by making an alloy.
It is necessary to note, that the application of the titanium hydride in comparison with a titanium powder in process of mechanochemical synthesis is preferable because of the greater embrittlement of the hydride and an opportunity of protection from oxidation of initial metals and formed intermetallides.
6. Preparation of powders by realization of "hydrogenation-disproportionation-dehydrogenation-recombination" cycle
The recently developed reversible hydrogenolytic method (quite often named by a HDDR-method) of obtaining the ultradispersed powders, in a development of which basic contribution was brought by I.R.Harris and V.A.Yartys' [21-23], has appeared very perspective for preparation of constant magnets with the improved characteristics. The essence of this method is that the some intermetallic compounds under action of hydrogen are exposed to the hydrogenolysis, i.e. the metallic matrix decomposes with a formation of metal hydride with large heat of formation (mostly this is the hydrides of metals of II-IV periodic groups) and of metal, which is not forming the hydride in usual conditions, or of the hydride of the more stable intermetallide. After a removal of hydrogen from the obtained hydride phases at heating, the further heat treatment can result in a formation of the initial intermetallic compounds:
290 K, 1 MPa H2 570 K, 1 MPa H2 CeNi3 ............................> CeNi3H4 ............................>
1070K, 0.01 Pa H2 1170-1270 K CeH2_3 + Ni .............................> Ce + Ni ......................> CeNi3;
290-470 K, 1-5 MPa H2 ~1020 K, 1 MPa H2
Nd2FeMB.................................>Nd2Fe14BH~3_6 .............................>
~1120 K, 0.01 Pa ~1170-1270 K NdH2 + Fe2B + Fe .........................> Nd + Fe2B + Fe ....................>
----- > Nd2Fe14B
Since during such transformations the embrittlement and the comminution occur, then, in consequence,
the fine-dispersed powders form, in a number of cases having the improved characteristics.
An realization of hydride-formation reaction at simultaneous action of mechanical energy or the treatment of the alloys, embrittled by hydrogen, leads to a formation of the finer particles. Besides the further heat treatment will be carried out similarly, that allows to keep the advantages of a HDDR-method.
The further development of conditions of a realization of the processes, leading to a formation of the finer particles, apparently, can improve the characteristics of compositional materials from them.
Conclusions
The powders, obtained by a method of the hydride dispergation, are very attractive in connection with an opportunity of a production of compositional compact materials on their basis, where as linker it is possible to use various polymers (tetrafluoroethylene, polyethylene etc.) or metals (Al, Cu, Ni etc.), which quite often are used for systems of a storage, compression, clearing and separation of hydrogen, in the electrochemical cells. The method of obtaining the intermetallide powders by the hydride dispergation with subsequent re-milling on ball mills is of interest because of an opportunity of an use of this method for obtaining the electrode and magnetic materials with the improved characteristics.
The chemical method of synthesis of the hydrogen-sorbing intermetallic compounds allows to receive the dispersed powders without essential deterioration of the characteristics, excepting the stages of obtaining the high-pure metals, their alloying, hydride dispergation and mechanical comminution, that can essentially simplify and make cheaper a process of preparation of dispersed powders.
The mechanochemical synthesis of hydrogen-sorbing intermetallides can be convenient for a preparation of hydrogenation catalysts.
The HDDR-method is perspective for preparation of constant magnets on a basis of the rare-earth metals, though requires detailed study of all occurring processes.
Acknowledgements
The author thanks V.N.Fokin, E.E.Fokina, I.I.Korobov, S.P.Shilkin (Institute of Problems of Chemical Physics of RAS, Chernogolovka, Russia) and A.A. Novakova (Moscow State University, Russia) for participation in different stages of the experiments.
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