NANOSTRUCTURED AMORPHOUS BORON MATERIAL

Abstract

A nanostructured material consisting essentially of boron. The material is in amorphous form and comprising aggregates of boron nanoparticles. A method of preparation thereof and the uses thereof.

Claims

1. Nanostructured material consisting essentially of boron, wherein said material is in amorphous form and that it comprises aggregates of boron nanoparticles with a size less than or equal to 25 nm.

2. Material according to claim 1, wherein said material consists of at least 85 mol % of boron, the remainder being inevitable impurities resulting from the method for producing said material and/or from its oxidation and/or from the equipment used in the production process.

3. Material according to claim 2, wherein the inevitable impurities resulting from the method for producing said material are one or more of the following elements: Li, Na, K, Rb, Cs, I, Cl, Br, F.

4. Material according to claim 2, wherein the inevitable impurities comprise at most 5 mol % of oxygen.

5. Material according to claim 1, wherein said material has a specific surface area S.sub.BET of at least 500 m.sup.2/g, said specific surface area S.sub.BET being calculated by the BET method.

6. Material according to claim 1, wherein said material has a density in the range from 1.1 to 2.3, said density being measured with a helium pycnometer.

7. Method for preparing a nanostructured material consisting essentially of boron as per claim 1, wherein said method comprises at least one step i) of heating, under an inert atmosphere, of a mixture comprising at least one boron hydride and at least one inorganic salt, step i) being carried out at a temperature T.sub.s sufficient to decompose said boron hydride and for said salt to be at least partially in the molten state, and in that it further comprises: a step ii) of cooling the mixture obtained at the end of step i), and a step iii) of purification of the mixture obtained at the end of step i) or step ii).

8. Method according to claim 7, wherein the boron hydride is selected from boranes and alkali metal borohydrides.

9. Method according to claim 7, wherein the temperature T.sub.s of step i) is from 600 to 1000 C.

10. Method according to claim 7, wherein the inorganic salt is an alkali metal halide.

11. Method according to claim 7, wherein the mixture comprises from 0.1 to 20 wt % of the boron hydride and from 80 to 99.9 wt % of inorganic salts.

12. Method according to claim 7, wherein said method further comprises a step iv) of drying the material from step iii).

13. A nanostructured material consisting essentially of boron as defined in claim 1, wherein said material is configured for the storage of dihydrogen, as a precursor for the synthesis of boron compound, as a catalyst, or as an additive with high energy density in hypergolic liquids or solids, explosives or fuels.

14-16. (canceled)

Description

EXAMPLES

[0089] The raw materials used in the examples are listed below: [0090] Lithium iodide, Alfa Aesar, of 99% purity, [0091] Sodium iodide, Alfa Aesar, of 99.9% purity, [0092] Potassium iodide, Alfa Aesar, of 99% purity, [0093] Lithium chloride, Alfa Aesar, of 99.995% purity, [0094] Sodium borohydride, Alfa Aesar, of 98% purity, [0095] Lithium borohydride, Alfa Aesar, of 95% purity, [0096] Magnesium, Sigma Aldrich, of 99% purity, [0097] Commercial boron oxide LiBO.sub.2 (M.sub.01), 2H.sub.2O, Sigma Aldrich, purity 98.0%, and [0098] Commercial amorphous boron (M.sub.02), ABCR, of 95-97% purity.

[0099] Unless stated otherwise, all the materials were used as received from the manufacturers.

[0100] The materials, prepared or commercial, were characterized by: [0101] X-ray diffraction (XRD) to verify the amorphous nature and check that no crystalline impurity is present, [0102] nuclear magnetic resonance (NMR) to determine the presence of oxygen and the quantitative nature of the synthesis, [0103] scanning electron microscopy coupled to energy-dispersed X-ray analysis (SEM-EDX) (quantitative elemental analysis), [0104] adsorption-desorption of nitrogen to determine their specific surface area by the BET method, and [0105] transmission electron microscopy (TEM) to evaluate the morphology of the nanoparticles and/or aggregates.

[0106] The specific surface area was evaluated using apparatus sold under the trade name Belsorp max, by the company Bel Japan.

[0107] Analysis by transmission electron microscopy (TEM) was performed using apparatus sold under the trade name Tecnai spirit G2, by the company FEI.

[0108] The nuclear magnetic resonance analyses of boron were performed using apparatus sold under the trade name AV700, by the company Bruker, with the following parameters: magnet 16.4 T, rotation of the sample at 20 kHz about the magic angle.

[0109] The X-ray diffraction analyses were performed using apparatus sold under the trade name D8p, by the company Bruker.

[0110] The energy-dispersed X-ray analyses for elemental assay of the samples were performed using a scanning electron microscope sold under the trade name (S-3400-N), by the company (Hitachi), equipped with an EDX detector sold under the name X-max by the company Oxford Instruments.

Example 1: Preparation of a Nanostructured Material Consisting Essentially of Boron M.SUB.1 .According to the First Object of the Invention and Comparison with Materials not According to the Invention M.SUB.1., M.SUB.2., M.SUB.3 .and M.SUB.4

[0111] 606 mg of sodium borohydride (NaBH.sub.4) was mixed with 5.8 g of lithium iodide (LiI) and 4.2 g of potassium iodide (KI) for about 2 minutes in a ball mill marketed under the reference MM400 by the company Retsch, to obtain a homogeneous fine powder.

[0112] The melting point of the LiI/KI eutectic mixture (LiI/KI weight ratio of 58/42) as used is about 286 C.

[0113] The resultant mixture was transferred under an inert atmosphere (argon) to a vessel made of vitreous carbon, it was heated under an inert atmosphere (argon) at a temperature of about 800 C. for about 1 hour, and was then cooled to room temperature to solidify.

[0114] The resultant mixture was washed several times with methanol by successive centrifugations in order to dissolve the salts that remained or had formed, and remove them. The moist powder obtained was dried under vacuum at about 40 C. for about 2 hours to give the material M.sub.1. The material M.sub.1 was then stored under an argon atmosphere.

[0115] The same method was repeated, but changing the heating temperature of 800 C. Temperatures of about 400, 500, 550 and 600 C. were used, giving respectively the materials M.sub.1, M.sub.2, M.sub.3 and M.sub.4.

[0116] Table 1 below shows the characteristics of the materials M.sub.1, M.sub.1, M.sub.2, M.sub.3 and M.sub.4 obtained depending on the heating conditions, namely the specific surface area S.sub.BET (in m.sup.2.Math.g.sup.1), the composition of starting products and/or reaction intermediates and/or contaminants (by .sup.11B NMR analysis) and the molar proportions of the different elements (by SEM-EDX analysis).

TABLE-US-00001 TABLE 1 Presence of starting products, Analysis by SEM-EDX T S.sub.BET intermediates or mol % of the elements Material ( C.) (m.sup.2 .Math. g.sup.1) contaminants % B % O % K % Na % I M.sub.1 800 860 Traces of BO.sub.3 and >94.9 0.0 4.2 <0.7 <0.2 BO.sub.4 M.sub.1 (*) 400 NM BO.sub.3, BO.sub.4, NM M.sub.2B.sub.12H.sub.12 and MBH.sub.4 (M = Li, Na or K) M.sub.2 (*) 500 NM BO.sub.3, BO.sub.4, NM M.sub.2B.sub.12H.sub.12 and MBH.sub.4 (M = Li, Na or K) M.sub.3 (*) 550 NM Traces of BO.sub.3 >82.4 7.6 9.5 <0.4 <0.1 BO.sub.4, M.sub.2B.sub.12H.sub.12 (M = Li, Na or K) M.sub.4 (*) 600 25 Traces of BO.sub.3 >89.4 0.5 9.7 <0.4 0.0 BO.sub.4, M.sub.2B.sub.12H.sub.12 (M = Li, Na or K) (*) Materials not forming part of the invention NM: parameter not measured

[0117] Table 1 shows firstly that the material M.sub.1 has a specific surface area S.sub.BET of about 860 m.sup.2.Math.g.sup.1, whereas the material M.sub.4 has a specific surface area of only about 25 m.sup.2.Math.g.sup.1.

[0118] .sup.11B NMR analysis of the materials M.sub.1, M.sub.2, M.sub.3 and M.sub.4 as prepared above was performed (cf. FIG. 1a) as well as of the material M.sub.1 and by comparison with the commercial boron oxide M.sub.01 and the commercial amorphous boron M.sub.02 (cf. FIG. 1b).

[0119] More particularly, .sup.11B NMR analysis of the material M.sub.1 showed the presence of a broad single peak, as well as very weak signals corresponding to the oxidized boron derivatives BO.sub.4 and BO.sub.3. The product obtained directly from the process (i.e. before NMR analysis) comprises even less oxygen than is seen by NMR since characterization of the samples for the NMR analyses involves confinement in a rotor that is not perfectly hermetic. In these conditions, slight exposure to the ambient air for about 15 minutes, and therefore possible oxidation of the boron, is probable. According to the NMR analysis, the material M.sub.1 obtained does not comprise starting product and/or intermediates of the reaction of decomposition of the alkaline borohydrides that may possibly form during heating, such as M.sub.2B.sub.12H.sub.12 and MBH.sub.4 (with M=Li, Na or K).

[0120] .sup.11B NMR analysis of the material M.sub.01 is a reference for positioning the signature of the oxidized groups BO.sub.3 and BO.sub.4 in an .sup.11B NMR spectrum.

[0121] .sup.11B NMR analysis of the material M.sub.02 (commercial amorphous boron) showed the presence of BO.sub.3 groups, as well as a distribution of chemical shifts different from that of M.sub.1, reflecting different B-B environments in M.sub.02 and M.sub.1, and therefore differences in short-range order.

[0122] However, when the heating temperature is 400 C., 500 C. or 550 C., NaBH.sub.4 did not decompose completely to amorphous boron, as demonstrated by the presence, according to boron .sup.11B NMR, of MBH.sub.4 and/or M.sub.2B.sub.12H.sub.12 (with M=Li, Na or K) in the materials M.sub.1, M.sub.2 and M.sub.3.

[0123] At 600 C., the .sup.11B NMR signal is made up of multiple broad peaks. The NMR signature of M.sub.2B.sub.12H.sub.12 is still present in the material M.sub.4, although faintly discernible in the form of a shoulder of the broadest peak corresponding to amorphous boron.

[0124] Furthermore, analysis by SEM-EDX showed that potassium is still well represented in the materials M.sub.3 and M.sub.4 (about 9.5 mol % and about 9.7 mol %, respectively), notably owing to the presence of K.sub.2B.sub.12H.sub.12.

[0125] TEM analysis of the material M.sub.4 (cf. FIG. 2a: scale 20 nm, FIG. 2b: scale 10 nm) showed the presence of aggregates of amorphous boron with smoothed contours. It is not possible to discern the nanoparticles making up the clusters or aggregates. This TEM observation is consistent with the low value of the specific surface area of said material M.sub.4, itself evidence of the absence of intergranular porosity in said material M.sub.4.

[0126] For comparison, TEM analysis of the material M.sub.1 (FIG. 3a: scale 20 nm, FIG. 3b: scale 10 nm, FIG. 3c: scale 5 nm) clearly showed the presence of aggregates of nanoparticles smaller than about 20 nm.

[0127] FIG. 4 shows the distribution of the size of the nanoparticles in the material M.sub.1, the calculation having been based on 150 measurements of size obtained from TEM images on several regions of different aggregates. Thus, the material M.sub.1 according to the invention comprises aggregates of nanoparticles ranging in size from about 2 to 10 nm, and notably with an average size of about 5.6 nm.

[0128] Measurement of density of the material M.sub.1 with a helium pycnometer showed that the material has an apparent density of 1.330.03 g.Math.cm.sup.3.

[0129] The calculated specific surface area S.sub.calculated for a perfectly smooth spherical particle with a diameter of 5.6 nm and a density of 1.33 g.Math.cm.sup.3 is 810 m.sup.2.Math.g.sup.1. This value is consistent with the specific surface area S.sub.BET measured on the material M.sub.1.

[0130] The nitrogen adsorption and desorption isotherms of the material M.sub.1 (cf. FIG. 5) showed the presence of mesopores formed by the intergranular spaces between the nanoparticles.

[0131] X-ray diffraction analysis of the material M.sub.1 (FIG. 6) showed the amorphous phase of boron, since no peak is present (i.e. phase without long-range order). The broad peaks at low angles are due to scattering of the X-rays when they pass through the dome under which the powder is stored, away from the ambient air.

[0132] In conclusion, in all the tests in this example 1, the heating temperature used was above the melting point of the eutectic (i.e. 286 C.), thus allowing the mixture of inorganic salts to be in the molten state. However, use of a temperature that is too low, for example less than or equal to 600 C., does not allow the boron hydride (i.e. NaBH.sub.4) to be decomposed completely, and therefore cannot give a material according to the invention, i.e. comprising aggregates of boron nanoparticles with a size of at most about 25 nm, the boron being in amorphous form.

[0133] It should be noted that in this example 1, a temperature of 600 C. is not sufficient to decompose NaBH.sub.4 completely when it is reacted with lithium iodide and potassium iodide in proportions as described above. However, this sufficient temperature T.sub.s depends both on the inorganic salt used (i.e. its melting point or softening point), whether it is mixed with other inorganic salts, the decomposition temperature of the boron hydride used, etc.

[0134] Consequently, a temperature of 600 C. might be sufficient if the reaction is carried out with some other boron hydride mixed with identical inorganic salts, or with an identical boron hydride mixed with one or more different inorganic salts, or with some other boron hydride mixed with one or more different inorganic salts.

Example 2: Preparation of a Nanostructured Material Consisting Essentially of Boron M.SUB.2 .According to the First Object of the Invention and Comparison with a Material not According to the Invention M's

[0135] 174 mg of lithium borohydride (LiBH.sub.4) was mixed with 0.76 g of lithium chloride (LiCl) and 4.24 g of lithium iodide (LiI) for about 2 minutes in a ball mill such as that used in example 1, to obtain a homogeneous fine powder.

[0136] The melting point of the LiCl/LiI eutectic mixture as used is about 371 C.

[0137] The resultant mixture was transferred under an inert atmosphere (argon) to a vessel made of vitreous carbon, it was heated under an inert atmosphere (argon) at a temperature of about 800 C. for 1 hour, and was then cooled to room temperature to solidify.

[0138] The resultant mixture was washed several times with methanol by successive centrifugations in order to dissolve the salts that remained and had formed, and remove them. The moist powder obtained was dried under vacuum at about 40 C. for about 2 hours to give the material M.sub.2. The material M.sub.2 was then stored under an argon atmosphere.

[0139] The same method was repeated, but changing the heating temperature of 800 C. to a temperature of about 550 C., giving the material M.sub.5.

[0140] Table 2 below shows the characteristics of the materials M.sub.2 and M.sub.5 obtained according to the heating conditions, namely the specific surface area S.sub.BET (in m.sup.2.Math.g.sup.1), the composition of starting products and/or reaction intermediates and/or contaminants (by .sup.11B NMR analysis) and the molar proportions of the different elements (by SEM-EDX analysis).

TABLE-US-00002 TABLE 2 Presence of starting Analysis products, by SEM-EDX T S.sub.BET intermediates or mol % of the elements Material ( C.) (m.sup.2 .Math. g.sup.1) contaminants % B % O % I M.sub.2 800 900 >98.3 1.6 <0.1 M.sub.5 (*) 550 NM traces of BO.sub.3 and NM NM NM BO.sub.4 Li.sub.2B.sub.12H.sub.12 and LiBH.sub.4 (*) Materials not forming part of the invention NM: parameter not measured

[0141] Table 2 shows firstly that the material M.sub.2 has a specific surface area S.sub.BET of about 900 m.sup.2.Math.g.sup.1.

[0142] .sup.11B NMR analysis of the materials M.sub.2 and M.sub.5 was undertaken (cf. FIG. 7).

[0143] In particular, .sup.11B NMR analysis of the material M.sub.2 showed the presence of a broad single peak. No signal corresponding to oxidation of the boron to oxidized boron derivatives BO.sub.4 and BO.sub.3 was present. The material M.sub.2 therefore does not comprise the starting product and/or the intermediate of the reaction of decomposition of the alkaline borohydrides that may possibly form during heating, such as Li.sub.2B.sub.12H.sub.12 and LiBH.sub.4.

[0144] However, when the heating temperature was about 550 C., LiBH.sub.4 was not decomposed to amorphous boron completely, as demonstrated by the presence, according to .sup.11B boron NMR, of LiBH.sub.4 and Li.sub.2B.sub.12H.sub.12 in the material M's.

[0145] TEM analysis of the material M.sub.2 (FIG. 8a: scale 20 nm, FIG. 8b: scale 10 nm) showed morphology of the material M.sub.2 very similar to that observed for the material M.sub.1, with very similar sizes of nanoparticles (smaller than about 20 nm). This TEM observation is consistent with the high value of the specific surface area of said material M.sub.2, which is about 900 m.sup.2.Math.g.sup.1.

[0146] SEM-EDX analysis of the material M.sub.2 showed that amorphous boron was obtained with purity above 98%. Lithium was not detected. The presence of oxygen might be due to oxidation during preparation of the sample for analysis by SEM-EDX (exposure to the air for about 1 minute).

[0147] In conclusion, in the tests in this example 2, the heating temperature used was above the melting point of the eutectic (i.e. 371 C.), thus allowing the mixture of inorganic salts to be in the molten state. However, use of a temperature that is too low, for example less than or equal to 550 C., does not allow the boron hydride (i.e. LiBH.sub.4) to be decomposed completely, and therefore cannot give a material according to the invention, i.e. comprising aggregates of boron nanoparticles with a size of at most about 25 nm, the boron being in amorphous form.

Example 3: Preparation of a Nanostructured Material Consisting Essentially of Boron M.SUB.3 .According to the First Object of the Invention and Comparison with a Material not According to the Invention M.SUB.6

[0148] 303 mg of sodium borohydride (NaBH.sub.4) was mixed with 5.00 g of sodium iodide (NaI) for about 2 minutes in a ball mill such as that used in example 1, to obtain a homogeneous fine powder.

[0149] The melting point of NaI is about 660 C.

[0150] The resultant mixture was transferred under an inert atmosphere (argon) to a vessel made of vitreous carbon, it was heated under an inert atmosphere (argon) at a temperature of about 800 C. for about 1 hour, and was then cooled to room temperature to solidify.

[0151] The resultant mixture was washed several times with methanol by successive centrifugations in order to dissolve the salts that remained and had formed, and remove them. The moist powder obtained was dried under vacuum at about 40 C. for about 2 hours to give the material M.sub.3. The material M.sub.3 was then stored under an argon atmosphere.

[0152] The same method was repeated, but changing the heating temperature of 800 C. to a temperature of about 600 C., to give the material M.sub.6.

[0153] Table 3 below shows the characteristics of the materials M.sub.3 and M.sub.6 obtained according to the heating conditions, namely the specific surface area S.sub.BET (in m.sup.2.Math.g.sup.1), the composition of starting products and/or reaction intermediates and/or contaminants (by .sup.11B NMR analysis) and the molar proportions of the different elements (by SEM-EDX analysis).

TABLE-US-00003 TABLE 3 Presence of starting products, intermediates Analysis by SEM-EDX T S.sub.BET or mol % of the elements Material ( C.) (m.sup.2 .Math. g.sup.1) contaminants % B % O % Na % I M.sub.3 800 700 traces of BO.sub.3 >90.1 4.3 5.4 <0.2 and BO.sub.4 M.sub.6 (*) 600 33 BO.sub.3, BO.sub.4 and >82.2 7.6 9.5 <0.7 Na.sub.2B.sub.12H.sub.12 (*) Materials not forming part of the invention

[0154] Table 3 shows firstly that the material M.sub.3 has a specific surface area S.sub.BET of 700 m.sup.2.Math.g.sup.1, whereas the material M.sub.6 has a far lower specific surface area S.sub.BET of 33 m.sup.2.Math.g.sup.1.

[0155] .sup.11B NMR analysis of the materials M.sub.3 and M.sub.6 was undertaken (cf. FIG. 9).

[0156] In particular, .sup.11B NMR analysis of the material M.sub.3 showed the presence of a broad single peak. The material M.sub.3 therefore does not comprise the starting product and/or intermediates of the reaction of decomposition of the alkaline borohydrides that may possibly form during heating, such as Na.sub.2B.sub.12H.sub.12 and NaBH.sub.4. Traces of chemical groups BO.sub.3 and BO.sub.4 were detected in the form of a shoulder of the main broad peak corresponding to amorphous boron, at the corresponding values of chemical shifts. These groups indicate slight oxidation, probably due to transfer of the powder to a rotor that is not hermetic, for NMR characterization, causing slight exposure to the ambient air for about 15 minutes.

[0157] However, when the heating temperature was about 600 C., NaBH.sub.4 did not decompose completely to amorphous boron, as demonstrated by the presence, according to .sup.11B boron NMR, of Na.sub.2B.sub.12H.sub.12 in the material M.sub.6. The material M.sub.6 also comprises oxidized boron derivatives such as BO.sub.3 and BO.sub.4.

[0158] TEM analysis of the material M.sub.3 (FIG. 10a: scale 20 nm, FIG. 10b: scale 10 nm) showed morphology of the material M.sub.3 very similar to that observed for the materials M.sub.1 and M.sub.2, with very similar sizes of nanoparticles (smaller than about 20 nm). This TEM observation is consistent with the high value of the specific surface area of said material M.sub.3, which is about 700 m.sup.2.Math.g.sup.1.

[0159] SEM-EDX analysis of the material M.sub.3 showed that amorphous boron with a purity of 90.1% was obtained. The presence of oxygen may be due to oxidation during preparation of the sample for SEM-EDX analysis (exposure to the air for about 15 minutes).

[0160] For comparison, TEM analysis of M.sub.6 (FIG. 11a: scale 50 nm, FIG. 11b: scale 20 nm) showed smooth spherical particles, at least an order of magnitude larger than in molten medium. This observation explains the lower specific surface area obtained, of about 33 m.sup.2.Math.g.sup.1.

[0161] This example 3 provides evidence of the need to carry out the method of the invention (cf. step i)) in a medium that is at least partially molten, i.e. at a temperature T.sub.s at least greater than or equal to the melting point of the salt or of the eutectic, or the start of fusion of the mixture if several salts are used.

[0162] Furthermore, as was demonstrated in examples 1 and 2, the method of the invention must take place at a temperature T.sub.s sufficient to ensure complete decomposition of the borohydride to amorphous boron, which is not the case at 600 C., regardless of the medium envisaged. As a result, the properties of specific surface area and purity with respect to alkali metals are greatly decreased at a temperature T that is not sufficient to decompose said boron hydride and for said salt or mixture of salts to be at least partially in the molten state.

[0163] Although the purities are better with lithium salts (cf. examples 1 and 2), synthesis in a sodium salt medium may correspond to a good compromise between product quality (specific surface area and purity), ease of reuse of the salts, and cost of the chemical species involved.

Example 4: Use of the Nanostructured Material Consisting Essentially of Boron M.SUB.1 .According to the First Object of the Invention and Comparison with the Use of a Commercial Material not According to the Invention for Preparing MgB.SUB.2

[0164] 30 mg of material M.sub.1 as prepared in example 1 was mixed with 37 mg of magnesium (1.1 equivalents) in an autoclave, under an argon atmosphere. The resultant mixture was heated at about 500 C. for about 4 hours, in a sealed environment, under autogenous pressure. A fine black powder was recovered, and was stored under an inert argon atmosphere.

[0165] X-ray diffraction (FIG. 12) was able to show the presence of a predominant crystalline phase MgB.sub.2, and of a minor phase MgO in the material obtained. The width of the reflections of MgB.sub.2 can be interpreted by a particle size effect. The apparent size of the crystal domains, calculated with Scherrer's formula, is about 40 nm. The apparent sizes of the crystal domains for the reflections of MgO are of about 20 nm.

[0166] The powder was observed by scanning electron microscopy (FIG. 13a: scale 500 nm, FIG. 13b: scale 2 m). The particles of MgB.sub.2 assume a morphology in the form of hexagonal platelets with a thickness ranging from about 50 to 100 nm, and with a length from about 500 nm to 1 m. The chemical nature of the platelets was confirmed by high-resolution transmission electron microscopy (FIG. 14), which shows the hexagonal structure of MgB.sub.2, with interplanar distances corresponding to the (100), (010), and (110) planes (cf. Fourier transform in FIG. 14 as insert in the same figure). Observation of a platelet along the slice also made it possible to observe the (001) planes of MgB.sub.2 (data not supplied).

[0167] The same method was then repeated using a commercial amorphous boron material M.sub.02 identical to that used in example 1.

[0168] The X-ray diffraction patterns of the two powders, synthesized starting from materials M.sub.1 and M.sub.02, are shown together in FIG. 15. Whereas the powder prepared from the amorphous boron material of the invention M.sub.1 consists predominantly of MgB.sub.2 as well as an MgO impurity, the powder prepared from the commercial amorphous boron material not according to the invention M.sub.02 contains MgB.sub.2, MgO and Mg. An additional peak (indicated with a star in FIG. 15) could not be identified, and corresponds to an unknown contaminant. The relative intensities between the peaks of MgB.sub.2 and those of MgO show that MgO is present in higher proportions in the powder obtained from M.sub.02 than in that obtained from M.sub.1. A higher proportion of MgO as well as the presence of residual magnesium metal Mg in the powder obtained from M.sub.02 indicates that conversion of the amorphous boron to MgB.sub.2 is not total in the reaction starting from the commercial boron not according to the invention M.sub.02.

[0169] This result confirms the higher reactivity of the material M.sub.1 according to the invention in formation of MgB.sub.2, compared to a commercial material M.sub.02, not according to the invention. Furthermore, the MgB.sub.2 powder is of higher purity when it is obtained from M.sub.1.

[0170] The results obtained with M.sub.02 confirm those described in the publication [Varin et al., J. Alloys and Comp., 2006, 407, 268-273]. In fact, Varin et al. showed that reaction of magnesium with a commercial amorphous boron (particles with size in the range from 100 to 200 nm) requires a temperature of at least 650 C. so as to be able to form the phase MgB.sub.2 quantitatively, and thus avoid formation of a mixture of several phases. In the case of a material according to the invention such as M.sub.1, a temperature of 500 C. is sufficient.