WO2003043936A1

WO2003043936A1 - A METHOD FOR SYNTHESIZING MAGNESIUM DIBORIDE (MgB2) IN BULK FORM

Info

Publication number: WO2003043936A1
Application number: PCT/IB2002/005158
Authority: WO
Inventors: Andrea Palenzona; Pietro Manfrinetti; Valeria Braccini
Original assignee: Infm Istituto Nazionale Per La Fisica Della Materia
Priority date: 2001-11-23
Filing date: 2002-11-21
Publication date: 2003-05-30
Also published as: AU2002351128A8; WO2003043936A8; AU2002351128A1; ITTO20011098A1

Abstract

A method for producing magnesium diboride in bulk form by means of a reaction between boron and magnesium in their elementary states, characterised in that it includes the step of causing stoichiometrically measured quantities of boron in the form of crystalline powder and magnesium in solid, compact form to react in a sealed environment, in an atmosphere of inert gas and at a temperature of between 750 and 850°C.

Description

A method for synthesizing magnesium diboride (MgB₂) in bulk form

The present invention relates to a synthesizing method for producing magnesium diboride in bulk form, from boron and magnesium in their elementary states .

The discovery of the superconductor properties of the compound MgB₂ with T_c = 39 K is extremely recent [1] . The find immediately gave rise to a vast quantity of research across the scientific world, with the metallic properties of this compound causing scientists to turn their attention once again to developing a new "conventional" superconductor. The high number of papers which have already been published, or are due to be published [2] in the first half of this year, bear witness to the effort put into this and to the results achieved. Research has centred mainly on understanding the relationship between methods of synthesis, the purity, morphology and dimensions of the granules, the presence of and constitution of grain boundaries and the physical criteria which chiefly characterize it (critical temperature and current T_c and J_c, coherence length ξ₀) and thus on the possibility of using this material industrially.

Up until now, however, all the methods of synthesis have produced only products in the form of powders, film or single crystals; all the data submitted in the literature has thus referred to samples of MgB₂ in the form of powder or pellets obtained by means of successive sintering processes.

The method currently used consists in directly synthesizing the two elements, which are both available commercially in very pure forms (> 99.9 and 99.6% by weight for g and B respectively) . However, owing to the thermodynamics of its formation (peritectic reaction) , the MgB₂ compound cannot be prepared from a liquid by fusion and solidification; while the high volatility of the Mg (Tf_USion = 649°C, T_b0iii_ng = 1,090°C) means that it must be prepared in a sealed container. The Mg and the B, in stoichiometric quantities, with the first in the form of shavings or powder and the second in the form of powder, are mixed together well and placed in crucibles (Nb, Mo, Ta, Fe) which are then welded shut in a flow of inert gas (Ar) . The crucible is vacuum sealed in turn in a quartz tube and heated in an oven (usually of an electrical resistance type) to a temperature ranging between 900 and 950°C, for a duration of 30 minutes to several hours: under these conditions, the Mg which reacts is mostly in a gaseous state. The compound thus obtained is in the form of a powder, with single granules of various shapes and dimensions (10-20 to 400-500nm) and agglomerations of granules which can reach a diameter of several millimetres, in dependence on the various synthesizing conditions [SEM photographs: Figures 1.1 and 1.2] .

The powder is then compressed into pellets and sintered at a temperature of 1000-1100 °C, still in a closed container with an inert atmosphere, for a duration of up to several dozen hours .

Whatever the method used, the series of operations to be carried out (together with the aforesaid high-temperature heat treatments) leads inevitably to the formation of impurities and of unwelcome secondary phases (MgO, MgO_x, B₂0₃, BO_y MgB₄) , often accompanied by Mg which has not reacted, for a total of several percentage units by weight. These spurious phases themselves form the boundary between the granules (grain boundary phases) , having a deleterious effect on the technological properties of this new superconductor in use and making it difficult to interpret the results of chemical-physical characterization measurements [3] . These impurities also constitute targets for atmospheric agents, especially humidity, which attack and damage the samples even if the compound is highly stable.

One object of the present invention is therefore to provide a new synthesizing method for the production of magnesium diboride in bulk form, as defined in the Claims which follow.

A further object of the invention is to provide magnesium diboride in bulk form, as obtainable according to the aforementioned method.

In the appended drawings :

Figures 1.1-1.6 are electronic-scanner microscope photographs, enlarged several times, which show the morphology of magnesium diboride in powder form (Figures 1.1 and 1.2) and in bulk form, as obtained by the method of the invention (Figures 1.3-1.6) ;

Figure 2 is a graph showing the curve illustrating the variation in magnetization as a function of temperature, in a constant magnetic field (10 Gauss) , of magnesium diboride obtained by the method of the invention as compared to that of a sample of magnesium diboride sintered from powder; and

Figure 3 is a graph illustrating variation in critical current density J_c as a function of the magnetic field applied, resulting from measurement of hysteresis cycles where T= 5K for the same samples as Figure 2. The new method still employs direct synthesis of the two elements, but in one manufacturing process involving a single step.

In one example of synthesis, the B and the Mg in stoichiometric quantities, are used in the form of a crystalline powder (325 mesh) and of a single ingot respectively. The container where the synthesis is carried out (preferably constituted by a cylindrical crucible made of Ta) is kept in a vertical position; the entire quantity of B is compacted and pressed into the bottom of the container, the ingot of Mg is then placed on top of the B and the crucible is closed in an inert atmosphere (flow of Ar) and the lid thereof sealed by arc welding.

The two materials for synthesis are therefore in the form of an upright cylindrical tablet made up of two separate layers: the lower consisting of compressed boron and the upper of a single portion of Mg.

In order to protect it from superficial oxidization, the crucible is either vacuum sealed in a quartz tube or simply placed in a quartz tube where a dynamic vacuum is maintained, it is then placed in a vertical position in an oven which is already at the correct reaction temperature which must be of between 700 and 850 °C, and preferably of between 750 and 800°C. At this temperature a highly exothermic reaction occurs between the Mg in a liquid state and the solid B. A reaction time of 20-30 minutes is sufficient for synthesis to occur; in order, however, to produce optimum characteristics in the compound, it is useful to follow up by annealing it (in the same oven) at 950-1000°C for a duration of at least 12 hours and preferably 2-3 days. The material thus obtained is compact, in bulk form and of a golden colour.

The method described above makes it possible to: a) prepare the MgB₂ in bulk form [Figures 1.3 and 1.4] .

The bulk thus obtained is tougher and has better mechanical properties than bulk material produced by sintering powder, thanks to the almost total absence of air pockets and/or microfractures . Density is of 2.4 g/cm³, compared to a theoretical density (calculated from crystallographic data) of 2.6 g/cm³. b) prepare this material in a single operation instead of the two successive steps of synthesizing+sintering. In addition to a considerable saving of time and materials (by reducing the number of operations, of crucibles made of expensive refractive metals, of highly pure inert gas and of power for sintering in high-temperature ovens) , the compound thus obtained is found to be significantly purer: being virtually free of secondary phases and with a negligible content of impure phases and/or grain boundary phases [Figures 1.5 and 1.6]. c) achieve better physical properties.

The characteristics of samples of MgB₂ prepared according to this new synthesizing method have been studied from a magnetic point of view by a SQUID magnetometer. The curve of Figure 2 shows the variation in magnetization as a function of temperature in a constant magnetic field (10 Gauss) , compared to an equivalent curve typical of a 'standard' sintered sample. It can be seen that the critical temperature T_c increases from 37.3 K to 38.3 K (T_{c onset} from 38.2K to 38.8K) while transition becomes more restricted (ΔT_C decreases from 0.9 K to 0.25 K) . Figure 3 gives the critical current of the same samples, obtained by measuring hysteresis cycles at T=5K. For samples obtained by the new method of preparation, the critical current increases by a factor of around 7 in an applied field of IT, increasing from around 1.2 10⁴ to 8 10⁴ A/cm² (from 5 10⁴ to 2.2 10⁵ A/cm² in null field) .

BIBLIOGRAPHICAL REFERENCES

[1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature 410 (2001) 63.

[2] C. Buzea, C. Yamashita. cond-mat/0108265 (Review article) and references cited therein.

[3] R.F. Klie, J.C. Idrobo, N.D. Browning, K.A. Regan, N.S. Rogado, R.J. Cava, cond-mat/0107324.

Claims

1. A method for producing magnesium diboride in bulk form by means of a reaction between boron and magnesium in their elementary form, characterised in that it includes a step of causing a reaction between stoichiometrically measured amounts of boron in powder form and of magnesium in the form of one or more solid/compact body or bodies in a closed environment, in an atmosphere of inert gas and at a temperature of between 700 and 850°C.

2. A method according to Claim 1, in which the measured quantity of powdered boron is compressed into the said closed environment in contact with the measured quantity of magnesium in the form of one or more compact bodies placed on top of the compacted boron powder.

3. A method according to Claim 1 or Claim 2 , in which the boron is in the form of particles, the grain size of which is equal to or less than 100 mesh (USBS-ASTM) .

4. A method according to any of Claims 1 to 3 , in which the said closed environment is constituted by a tantalum crucible sealed by means of electric arc welding.

5. A method according to Claim 4, characterised in that the said sealed crucible containing the reactants is vacuum- sealed in a quartz tube, or placed in a quartz tube in which a dynamic vacuum is maintained, and placed in an oven at a reaction temperature of between 700 and 850°C.

6. A method according to any of Claims 1 to 5 , characterised in that the reactants are kept at the reaction temperature for a period of between 20 and 30 minutes.

7. A method according to any of Claims 1 to 5, characterised in that, after the reaction at 700-850°C, it includes an annealing step at a temperature of between 950 and 1000°C and lasting for at least 12 hours.

8. A method according to any preceding Claim, in which the boron is in the form of a crystalline powder.

9. Magnesium diboride in bulk form obtainable by means of a method according to any of Claims 1 to 8.

10. Magnesium diboride in bulk form with a density of 2.4 g/cm³.