WO2021240074A1

WO2021240074A1 - Preparation of alpha-rhombohedral boron in the presence of a carbon source

Info

Publication number: WO2021240074A1
Application number: PCT/FR2020/050872
Authority: WO
Inventors: Yann Le Godec; Amrita CHAKRABORTI; Nathalie Madeleine Marquerite VAST
Original assignee: Centre National De La Recherche Scientifique; Commissariat à l'Energie Atomique et aux Energies Alternatives; Ecole Polytechnique
Priority date: 2020-05-25
Filing date: 2020-05-25
Publication date: 2021-12-02

Abstract

The present invention relates to obtaining alpha-rhombohedral boron from a reagent comprising essentially boron, but in a form other than the alpha-rhombohedral form, such as boron in the amorphous state and/or in a crystalline state which may be beta-rhombohedral, tetragonal or even orthorhombic, taken, for example, in one of the beta and/or gamma phases of the boron.

Description

Title of the invention: Preparation of rhombohedral alpha boron in the presence of a carbon source

Field of the invention

The present invention is in the field of the synthesis of alpha boron, and relates in particular to the preparation of alpha boron in the qualified form of alpha rhombohedral boron. More particularly, the present invention relates to obtaining alpha rhombohedral boron from a reagent comprising essentially boron, but in another form than the alpha rhombohedral form, such as boron in the amorphous state and / or in a crystalline state which may be either rhombohedral beta, or tetragonal, or even orthorhombic, taken for example in one of the beta and / or gamma phases of boron.

Technological background

The thermodynamically stable phase of boron at ambient pressure and temperature conditions is the beta rhombohedral phase when it is in crystalline form, but boron can also exist in other forms, for example in amorphous form. Rhombohedral boron beta is generally obtained from boric acid B ₂ O3 in the presence of a reducing metal, and is commercially available. The commercial forms of boron are beta boron and amorphous boron. There is no commercial source where alpha boron is available in large quantities and at a cost comparable to that of beta boron. Boron in its alpha phase is indeed much more difficult to access, and the methods developed to date use synthetic routes incompatible with the hygiene and safety standards recommended for manufacture on an industrial scale. . Thus, a synthesis of alpha boron is described in an article published in 1958 in J. Am. Chem. Soc., 80, 2592, from the toxic and corrosive reagent BI3, on a surface heated between 800 and 1000 ° C. The use of BBr3 OR BCI3 reagents is described in application FR 2 178799 for obtaining alpha boron, but this synthetic route suffers from the same drawbacks linked to the dangers of using boron halides.

[0004] Other known routes for the synthesis of alpha boron use noble metals and / or heavy metals such as platinum (Pt) or palladium (Pd), with often low yields, as described in the article J. Phys. : Conf. Ser., 2008, 121, 042017.

[0005] More recently, the synthesis of alpha boron from rhombohedral beta boron, published in Journal of Crystat Growth, 2011, 321, 162-166, is described. This preparation of alpha boron must be done in the presence of platinum (Pt ) finely divided, and in an enclosure in Pt, or even in gold (Au); also the implementation on an industrial production site of a reactor consisting of these noble metals is not possible, this synthetic route is therefore incompatible with the manufacture of large parts comprising alpha boron and / or with production rates requiring the handling of large volumes of alpha boron.

[0006] Commercial boron, in beta or amorphous form, is useful in various fields and is used in particular as a neutron absorber in nuclear reactors. For example, a layer of boron covering reactor parts act as a shield against neutron radiation.

[0007] In fact, the damage under irradiation, as described in IEEE Transactions on Nudear Science, 2013, 60, 1740-1766, of crystals containing B12 icosahedra, such as beta boron or alpha boron, is much less. severe than in other materials with a cubic crystal structure such as silicon - or with a hexagonal crystal structure - such as gallium nitride or boron nitride. The atoms of the icosahedra, displaced by irradiation, return to their original position by a self-healing mechanism (seif-heaiing), as reported in Journal of So / id State Chemistry 2006, 179, 2791-2798; ibid, 2004, 177, 1619-1623; Journal of Physics and Chemistry of Soi id s, 2014, 75, 1054-1074.

[0008] Alpha boron combines the described radiation resistance and electronic properties useful for use in semiconductors. This combination of alpha boron properties can be used in many applications. One such application is the solid state materials neutron detector shown in US Patent US2015 / 0060680. Accessibility to the production of alpha boron in larger quantities should thus make it possible to make optimized neutron detectors, and potentially facilitate the production of individual dosimeters.

The present invention aims to synthesize alpha boron in a simpler manner, without necessarily resorting to the use of noble metals, and not to use harmful or dangerous reagents. It is therefore compatible with large-scale production in an industrial environment. Summary of the invention

To this end, the present invention relates to a process for obtaining alpha rhombohedral boron from a carbon source and at least one source of boron other than rhombohedral alpha boron selected from at least one of the selected sources. from amorphous boron, crystalline boron itself selected from beta boron, tetragonal boron and gamma boron, under a pressure of 1 GPa to 5.5 GPa; and at a temperature of 1000 ° C to 2400 ° C.

The inventors have demonstrated, using the process according to the invention, that different phases of boron, apart from those of alpha rhombohedral boron itself, could lead to the preparation of alpha rhombohedral boron - in the presence of a source of carbon - with an excellent degree of purity not attainable by the routes known in the state of the art, and conversion yields close to 80-99%. The process according to the invention is all the more advantageous as it makes it possible to dispense with the presence of metals such as transition metals, in particular those from the nickel column such as platinum (Pt), or alkali or alkaline earth metals such as magnesium (Mg). Advantageously, the method according to the invention does not use Pt or Mg.

In the method according to the invention making it possible to obtain rhombohedral alpha boron, the carbon source is advantageously selected from at least one of the elements chosen from amorphous carbon, diamond and graphite. Preferably, the carbon source is amorphous carbon. The amorphous carbon makes it possible to work - having regard to the reaction temperature in the context of the invention - at slightly more low temperature (100 ° C to 200 ° C cooler), compared to graphite, or diamond which is itself transformed into graphite at very high temperature.

In the method according to the invention making it possible to obtain rhombohedral alpha boron, the boron source is advantageously in a crystalline state comprising at least one of the crystal systems selected from the beta rhombohedral reticular system, the tetragonal reticular system or the orthorhombic reticular system.

In the process according to the invention, the preferred source of boron is that of rhombohedral beta boron. Thus, the source of boron for obtaining rhombohedral boron alpha is free of the latter and is composed of rhombohedral boron beta.

The rhombohedral alpha boron obtained has the same isotopic composition as the initial boron source. At its natural isotopic composition, the boron source contains mainly two isotopes: ¹¹ B nuclei, at a molar concentration of 80.1%, and ¹⁰ B nuclei, at a molar concentration of 19.9%. The source of boron, in particular beta boron, can be enriched up to at least 99% in isotope ¹⁰ B, and up to 96% as standard in commercial products. Alternatively, the source of boron, in particular beta boron, can be enriched in isotope ¹¹ B up to at least 99%. The final product (rhombohedral alpha boron) may contain any proportion of the two isotopes, between more than 99% of ¹⁰ B isotopes and less than 1% of ¹¹ B, up to less than 1% of ¹⁰ B nuclei and more 99% of isotopes ¹¹ B. In order to produce rhombohedral alpha boron in the presence of amorphous and crystalline boron other than rhombohedral alpha boron, the reagents used for this purpose are advantageously composed:

- at least 75% of the boron source expressed as a molar percentage taken relative to the total number of moles of all of said reagents; and

- not more than 25% of the carbon source expressed as a molar percentage taken relative to the total number of moles of all of said reagents.

The inventors have demonstrated that mixing the ingredients in these relative proportions makes it possible to obtain the best conversion yields of the boron source used into targeted rhombohedral alpha boron.

[0017] In a preferred embodiment of the invention the source of boron is taken in a content of 75 to 85% by mole percentage and the source of carbon is taken in a content of 15 to 25% by mole percentage. Advantageously, the source of boron is 78 to 80% in molar percentage and the source of carbon is 20 to 22% in molar percentage.

The implementation of the method according to the invention advantageously comprises at least two steps. In a first step i) the reactants comprising the source of boron and the source of carbon are mixed in the proportions described above. In a second step ii) the mixture obtained in the first step is subjected to a pressure of between 1 GPa and 5.5 GPa and at a temperature of between 1000 ° C and 2400 ° C. Without being bound by theory to higher pressures, there seems to be a kinetic competition between the formation of rhombohedral alpha boron and the formation of boron carbides, which would decrease the purity and the conversion efficiency of alpha boron accordingly. rhombohedral likely to be obtained. In a preferred embodiment of the invention the source of boron is beta boron and the carbon source is amorphous carbon, the pressure used is between 4 GPa and 5 GPa and the temperature is 1200 ° C. at 2200 ° C.

During the implementation of the second step, the rise in pressure is advantageously done initially in at least 10 minutes at room temperature and then it is followed by a rise in temperature until the temperature range is reached. reaction defined in step ii) above, and maintaining the latter at 1000 ° C to 2400 ° C, preferably from 1200 ° C to 2200 ° C, more preferably from 1400 to 2000 ° C, or even from 1600 to 1800 ° C, for a period of at least 2 hours and not exceeding 4 hours.

In a preferred embodiment, the mixture is maintained for at least 2 hours at a pressure of between 2 to 5 GPa. The temperature for its part is advantageously maintained between 1000 and 1600 ° C, more preferably between 1200 and 1400 ° C.

In order to separate at the end of the reaction the boron, the excess carbon source, and the rhombohedral alpha boron obtained, it is advantageously implemented a treatment step iii), subsequent to step ii ), consisting in sieving the products obtained. Which treatment step advantageously comprises a first sieving step. The first screen used advantageously comprises holes with a diameter less than or equal to 5 microns. At the end of this first sieving, a second sieving step will advantageously be carried out again on a second sieve the pores of which are between 50 and 250 microns. Advantageously, the method according to the invention comprises a final step iv) of deposition by sintering of the sieved products as described above in the context of the invention, on a support of the neutron detector cell type. Thus, the alpha rhombohedral boron obtained by implementing the method according to the invention can be subjected to a sintering process consisting in heating said alpha rhombohedral boron in powder form without leading it to melting in order to obtain a part mainly comprising rhombohedral alpha boron. The product resulting from this sintering can be the subject of a detector cell, for example a neutron detector or a personal dosimeter.

The pressure implemented during the sintering process being carried out under a pressure between 1 GPa and 5 GPa and at a temperature between 1100 ° C and 1300 ° C.

Another object of the invention corresponds to rhombohedral alpha boron having a degree of purity of at least 95%. Said rhombohedral alpha boron obtained by the process of the invention comprising an aggregate of particles consisting of rhombohedral alpha boron, the diameter of which is between 50 and 250 microns.

Another object of the invention consists of electronic devices comprising rhombohedral alpha boron giving said device the character of so-called hardened electronic devices.

More particularly, the present invention also relates to the use of rhombohedral alpha boron obtained according to the method described above in the context of the invention in solar cells for photovoltaics. as a replacement for the polycrystalline semiconductor, or in hardened electronics used in space, nuclear or defense. In fact, semiconductors of cubic or hexagonal structure subjected to irradiation exhibit energy levels in the forbidden band due to damage by displacement of atoms, which modifies their electronic properties. The atomic and electronic structures of rhombohedral alpha boron are less altered by irradiation: the production of rhombohedral alpha boron in greater quantity thus opens the way both towards the production of solar cells for space or defense, and towards the production of a hardened electronics, based on rhombohedral alpha boron.

[0026] The rhombohedral alpha boron obtained according to the process of the invention has a degree of purity close to that of the initial boron source, and in a standard manner greater than 95%. Depending on the degree of purity of the initial boron source, it can therefore have different shades such as those found for materials such as silicon carbide, namely metallurgical grade (metallurgical grade), solar type ( solar grade) for solar cell type applications for photovoltaics in a hostile environment, or an electronic grade (electronic grade) for microelectronic applications suitable for use in a hostile environment.

The rhombohedral alpha boron obtained according to the method of the invention can also be used as a replacement for silicon in SOI devices "Silicon on insuiator" also called silicon on insulator which is a structure consisting of a stack of a layer silicon (50 nm to a few µm thick) on an insulator layer. This insulator can be sapphire (siiicon-on-sapphire), air (si / icon-on-nothing) or carbon dioxide. silicon (S1O2). This technology is a promising alternative to raw silicon in the production of transistors operating at high frequencies. The rhombohedral alpha boron obtained according to the process of the invention can also be used instead of the insulator, whether in the form of S1O2 or of silicon on sapphire. The presence of rhombohedral alpha boron reinforcing the protection of said transistors.

Detailed description of the invention

Rhombohedral alpha boron is generally in the form of clear red crystals. Rhombohedral alpha red boron is an elemental semiconductor. It has been discovered that this boron allotrope, unlike other materials used in neutron detectors, has a high unit-volume sensitivity which cannot be achieved by other materials used in neutron detectors. Potentially, alpha rhombohedral boron solid state neutron detectors have a neutron detection sensitivity that can be over 500 times that of detectors made from semiconductor or boron trifluoride neutron detectors or lithium scintillation 6.

Their efficiency as a neutron detector comes from the fact that the neutrons absorbed by the ¹⁰ B atoms of rhombohedral alpha boron cause the generation of millions of electron-hole pairs which can be scanned through the body of the device by a field applied, then collected to produce an electronic pulse that can be quantitatively measured with high sensitivity with standard electronic instruments. Alpha boron neutron detectors rhombohedral can operate at high temperatures while maintaining high wearer mobility and high service life.

The phases of boron other than the rhombohedral alpha form, are bad semiconductors because they have, in the forbidden band, many deep levels of defects which trap electric charges, or because their absorption spectrum optic presents an Urbach tail before the absorption threshold. Rhombohedral boron alpha, however, does not suffer from this disadvantage and therefore exhibits the electronic properties of an elemental semiconductor whose electronic bandgap does not contain point defects.

Thus easily having large quantities of rhombohedral alpha boron capable of being obtained by non-polluting techniques, easily applicable industrially or having a low cost, constitutes an advantage of the invention.

The invention is also illustrated by figures:

Figure 1 illustrates three Raman scattering spectra using a 514.53 nm laser source of rhombohedral alpha boron obtained by the process according to the invention at 5 GPa at the respective temperatures of 1800 ° C, 1400 ° C and 1200 ° C.

Figure 2 illustrates the X-ray diffraction spectrum using a copper source, of a rhombohedral alpha boron crystal obtained by the method of the invention at 5 GPa and 1200 ° C.

Figure 3 illustrates the X-ray diffraction spectrum, using a source of copper, of the initial reagent, beta boron. The inset in Figure 3 illustrates the superposition of this X-ray diffraction spectrum of the initial reagent (beta boron) comprising at least 5 peaks of high relative intensity, between 2000 a.u and 4000 a.u. (arbitray unit) with the rhombohedral alpha boron spectrum obtained according to the process of the invention, at 5 GPa and 1200 ° C, said final product comprising two peaks of high relative intensity.

Figures 4 and 5 show the dimensions of capsules used, as well as their sealing lids, in Paris-Edinburgh type presses.

Figure 6 shows a sectional diagram of said Paris-Edinburgh press. Said press comprising the sample to be treated included between the anvils (6) supported by the upper and lower seats (3). The press operating thanks to the hydraulic pressure exerted by an oil pump device connected to the pressure inlet (4), the piston (5) allowing when it moves along the column (2) to close the device which comes into abutment on the plate (1) held stationary by the nuts (8). EXPERIMENTAL PART

Reagents: they are used in powder form. For boron, the source used is beta boron sold by the company Prolabo® under a reference exhibiting a degree of purity of 99.9%, and amorphous carbon sold by the company Sigma-Aldrich® under a reference exhibiting a degree of purity. 99.95% purity.

Alternatively, diamond or graphite can be used, the diamond being converted into graphite according to the high temperature process detailed below.

A / The reagents (source of boron and source of carbon) are mixed beforehand with the mortar for 5 minutes with the following proportions:

- 80% of the boron source expressed as a molar percentage; and

- 20% of the carbon source expressed as a molar percentage.

The powders are mixed all at once and are mixed "by hand" with the mortar without pressing strongly on the pestle, so as to obtain a homogeneous mixture of the powders.

B / The mixture obtained in A / is placed in a hexagonal boron nitride capsule and the resulting assembly is inserted into a large volume hydraulic press of the Paris-Edinburgh (PE) type, described in the article High Pressure Research, 2003 , 23, 281-287; 10mm joint anvils are used with a hexagonal boron nitride (h-BN) capsule having the dimensions shown in figure 4. A cap is added, when the capsule is filled with reagents (sealing is then done by pressure, see figure 5 for dimensions). It is also possible to use PE anvils for 16mm joints, with capsule 4.5mm high, 7mm outer diameter, 6.6mm inner diameter. The final sample has a volume of approximately 130 mm ³ .

Hexagonal boron nitride (h-BN) has the advantage of being chemically inert towards carbon and boron, and said h-BN transmits force well because it is very ductile; h-BN is also a material exhibiting excellent thermal conduction properties and ideally as a container enables heat transfer to the contents to be effected efficiently. Developed in the late 1980s, the Paris-Edinburgh type presses were designed specifically for high pressure experiments in neutron sources, such as the ISIS spallation source in England. The design idea was to generate a maximum of force in a minimum of footprint in order to reduce the storage space of the press. Depending on the version, the Paris-Edinburgh press can generate a force of 500 kN to 4500 kN for a mass ranging from 10 kg to 90 kg, it is also easily removable, with parts not exceeding 20 kg, which facilitates its handling and transport. Associated with optimized design anvils, it can carry a sample of several tens of mm ³ at a pressure of 15 GPa and at temperatures ranging from 2 K to 3000 K with a cryostat or resistive tube furnaces in the joint where is located sample. FIG. 6 represents a sectional view of a Paris-Edinburgh press of the V3 type and makes it possible to simply illustrate its operating principle: a hydraulic fluid pressure is applied to the base of the piston (5) via a high pressure generator ( 4) (compressor, hand pump, etc.) connected to the press pot by a high pressure flexible tube. The piston, of different size depending on the type of press Paris Edinburgh (100 cm ² for a V3), then pushes a lower seat (3) in tungsten carbide (CW) towards an upper seat also in carbide of tungsten, wrapped in a steel part (with a radial stress of 1 GPa), which transmits the force to 2 opposing anvils (6), the high pressure part is located between the anvils. The force applied to the back faces of the anvils over a relatively large area is transmitted between the anvils over a much smaller area, causing the pressure to multiply.

Another type of press is also used, of the multi-anvil type, described in the article Zeitschrift für Kristaiiographie, 2004, 219, 330-338. The capsules used with this type of device can have dimensions of the order of a centimeter in diameter.

In all cases, the mixture is then subjected to a High Pressure and High Temperature treatment, also called HPHT treatment.

The pressure build-up step reached a value of 2 GPa or 5 GPa. The pressure value is determined by the value of the primary pressure on the press pot, determined by a high pressure sensor (wika type HP-2-S sensor, 0.25 GPa bars) connected to the compressor, brought to values between 0.1 and 0.25 GPa.

Once the sample is at the target pressure of 5 GPa, as shown in Figure 2, the resistive heater is activated by the controlled passage of a direct current through the anvils. This current is generated by an Elektronika 0-15 V and 0-200 A DC current generator, and the temperature is determined by the transmitted electric power as measured by the current generator, previously calibrated in preliminary experiments according to the temperature of the sample (with thermocouple in the center of the sample). The temperature ramp is 125 ° C / minute. Once the target temperature has been reached, of 1200 ° C as shown in Figure 2, the temperature is kept constant for a period of between 1 to 4 hours.

The heating is then suddenly stopped (i.e. without a ramp), and the decompression takes place between 1 and 4 hours. The final sample is thus recovered from the press and, after sonication of the sample obtained in a glass beaker under acetone in an ultrasonic bath marketed by the company Labo-Moderne ™ under the reference SONOCLEAN SHE1501 for 6 minutes. The sample produced as a result of the reaction of the powdered reaction mixture presented above is then separated from the boron nitride capsule which is, in turn, destroyed.

[0040] The sample obtained is then sieved, allowing the elimination of carbon. At the end of the reaction, the obtained alpha rhombohedral boron comes in two forms. Small aggregates of about 3 to 5 microns and larger aggregates with an average diameter of 50 to 250 microns. Aggregates with a diameter of about 3 to 5 microns may be tainted with many carbon impurities present in excess and unreacted. These 3 to 5 micron aggregates are removed by a first sieving with holes no larger than 5 microns. Aggregates larger than 50 microns having a degree of purity of at least 95% are subjected to a second pass through a sieve of the appropriate diameter for their extraction.

From 2.46 g of beta boron, nearly 1 cm ³ of rhombohedral alpha boron is obtained on average, which corresponds to approximately 2.46g of rhombohedral alpha boron. This is an advantage of the invention, since obtaining such a quantity of rhombohedral alpha boron by conventional chemical routes would be more complicated and would require successive purifications to remove metals allowing the synthesis of said boron, which would correspondingly decrease the yield of obtaining the latter.

The powders thus recovered were characterized by Raman spectroscopy, the results are shown in FIG. 1. The Raman spectra were collected for 3 minutes each, using the spectrometer marketed under the reference T64000 by the company Jobin-Yvon® ( Horiba Scientific). Monochromatic light of wavelength 514.53 nm is generated by ionized argon used by a laser device marketed by the company Spectra Physics®. An optical device makes it possible to route the beam of said laser to the sample composed of the sieved powder described above. The mirrors are spaced on the editing table to minimize the angle of deflection.

Principle of Raman - Laser spectrometry: collimators are placed between the mirrors to ensure that the light is reflected in the right direction. Before reaching the microscope, an opaque glass filter is placed in the beam path to reduce the intensity of the radiation. Two other filters are used: a plasma filter to avoid the parasitic radiation of the ionized gas, and a "pinhole" which focuses the light again. A semi-reflecting mirror is used to make our backscattering measurements: part of the radiation is reflected and strikes the sample, then follows the reverse path before entering, once transmitted, into the spectrometer for analysis.

The result reported in Figure 1 demonstrates that only the rhombohedral alpha boron is present in the reaction mixture at the end of the reaction, and that the spectrum conforms to the description of the Raman spectrum of boron rhombohedral alpha given in the literature, in particular in the article Phys. Rev. Lett. 1997, 78, 693-696.

The powders recovered at the end of the reaction, after sieving, were also characterized by X-ray diffraction using an X'Pert laboratory diffractometer marketed by the company PANalytical equipped with a copper source and a detector marketed under the reference XCelerator®, comprising “Bragg-Brentano” type optics in transmission. The acquisition parameters are as follows: 0.04 ° Soller slits; 0.5 ° programmable s / it divergence, 1st incident antiscatter s Ht, 0.5 ° diffracted antiscatter s / it, fiiter iron.

The result reported in Figure 2 shows that the diffraction peaks are all characteristic of rhombohedral alpha boron in comparison with the crystallographic data in the literature, for example those reported by Wyckoff, RWG, in the book Crystai structures, 1963, 1 , 7-83 (second edition, Interscience Publishers, New York). It is thus proved that the overwhelming majority of rhombohedral alpha boron is synthesized by implementing the process according to the invention.

As for Figure 3, the diffraction diagram of the initial reagent is shown (commercial beta boron from ProLabo®, purity 99.9%), and its superposition with the final product of Figure 2. It is thus established that the transformation of beta boron into rhombohedral alpha boron has indeed taken place.

Claims

Claim 1. Process for obtaining alpha rhombohedral boron from a carbon source and at least one source of boron selected from at least one of the sources selected from amorphous boron, crystalline boron including beta boron, gamma boron, tetragonal boron under a pressure of 1 GPa to 5.5 GPa and at a temperature of 1000 to 2400 ° C.

Claim 2. A method of obtaining alpha rhombohedral boron according to claim 1, wherein the carbon source is selected from at least one of the elements selected from amorphous carbon, diamond and graphite.

Claim 3. A process for obtaining alpha rhombohedral boron according to one of claims 1 to 2, wherein the carbon source is amorphous carbon.

Claim 4. A process for obtaining alpha rhombohedral boron according to one of claims 1 to 3, in which the source of boron is in a crystalline state comprising at least one of the crystal systems selected from the rhombohedral reticular system, the system. Tetragonal reticular and orthorhombic reticular system.

Claim 5. A process for obtaining alpha rhombohedral boron according to one of claims 1 to 4, wherein the source of boron is beta rhombohedral boron.

Claim 6. A method of obtaining alpha rhombohedral boron according to one of claims 1 to 5, wherein the boron source comprises the isotope 1 ⁰ B in a molar content of between 19.9 and 99%. Claim 7. A method of obtaining rhombohedral alpha boron according to one of claims 1 to 5, wherein the boron source comprises the isotope 1 ¹ B in a molar content of 80.1 to 99%.

Claim 8. A process for obtaining rhombohedral alpha boron according to one of claims 1 to 7, in which the reagents used are composed of:

Claim 9. A process for obtaining rhombohedral alpha boron according to one of claims 1 to 8, in which the reagents used are composed of:

- from 75 to 85% of the boron source expressed as a molar percentage taken relative to the total number of moles of all of said reagents; and

- from 15 to 25% of the carbon source expressed as a molar percentage taken relative to the total number of moles of all of said reagents.

Claim 10. A process for obtaining rhombohedral alpha boron according to one of claims 1 to 9, wherein the process comprises at least two steps: i) mixing the reagents comprising the source of boron and the source of carbon; and ii) subjecting the mixture obtained in i) to a pressure of between 1 GPa and 5.5 GPa and at a temperature of 1000 ° C to 2400 ° C.

Claim 11. A process for obtaining rhombohedral alpha boron according to claim 10, in which the process comprises the two steps: i) mixing the reagents in which the source of boron is beta boron and the carbon source is amorphous carbon; and ii) subjecting the mixture obtained in i) to a pressure between 4 and 5 GPa and at a temperature between 1200 ° C and 2200 ° C.

Claim 12. A process for obtaining rhombohedral alpha boron according to claim 10 or 11, in which step ii) comprises a sub-step ii.1) of increasing the pressure for at least 10 min at room temperature; followed by a second step ii.2) of raising the temperature and maintaining said reaction temperature for at least 2 h.

Claim 13. A process for obtaining rhombohedral alpha boron according to one of claims 1 to 12, in which the mixture is maintained for at least 2 hours at a pressure of between 2 and 5 GPa.

Claim 14. A process for obtaining rhombohedral alpha boron according to one of claims 1 to 13, wherein the mixture is maintained for at least 2 hours at a temperature of 1400 ° C to 2000 ° C.

Claim 15. A process for obtaining rhombohedral alpha boron according to one of claims 7 to 14, in which the process comprises a treatment step iii), subsequent to step ii), consisting in sieving the products obtained.

Claim 16. A method for obtaining rhombohedral alpha boron according to claim 15, wherein the screening uses a first screen comprising holes of a diameter less than or equal to 5 microns.

Claim 17. A process for obtaining rhombohedral alpha boron according to claim 16, in which the product remaining on said first screen is passed again through a second screen, the pores of which are between 50 and 250 microns. Claim 18. A process for obtaining rhombohedral alpha boron according to one of claims 1 to 17, in which the process comprises a final step iv) of deposition by sintering of the screened products according to claim 17, on a cell-type support. neutron detector or personal dosimeter type.

Claim 19. A process for obtaining rhombohedral alpha boron according to claim 18, wherein step iv) is carried out under a pressure of between 1 GPa and at most 5 GPa at a temperature of between 1100 ° C and 1300 ° C.

Claim 20. Rhombohedral alpha boron obtained according to any one of claims 1 to 19, exhibiting a degree of purity of at least 95% comprising alpha rhombohedral boron present in the form of aggregates of particles with a diameter of between 50. and 250 microns.