Mechanochemical process for the production of BP, B12P2 and mixtures thereof, in particular as nanopowders

Abstract

The present invention relates to a process for the preparation of a boron phosphide, more specifically BP and/or B.sub.12P.sub.2, comprising the mechanochemical reaction of boron phosphate (BPO.sub.4) with at least one alkaline earth metal (EA). In particular, the process of the invention allows for the selective preparation of either BP or B.sub.12P.sub.2 with more than 95% purity, through the reduction of boron phosphate (BPO.sub.4) with at least one alkaline earth metal (EA) according to reaction (1) for BP and according to reaction (2) for B.sub.12P.sub.2: BPO.sub.4+4 EA.fwdarw.BP+4 EA(O) (1) 2BPO.sub.4+5 (EA)B.sub.2+3 EA.fwdarw.B.sub.12P.sub.2+8 EA(O) (2). The present invention further relates to boron phosphide powders, in particular BP or B.sub.12P.sub.2 powders, of nanometric or micrometric size.

Claims

1. A process for the preparation of a boron phosphide, wherein the process is a mechanochemical reaction of a powder of boron phosphate (BPO.sub.4) mixed with a powder of at least one alkaline earth metal (EA) by a mechanical device providing mechanical energy which allows to initiate and carry out the mechanochemical reaction between the powder of boron phosphate (BPO4) and the powder of at least one alkaline earth metal (EA), for directly obtaining said boron phosphide without any external thermal heating, said boron phosphide being BP and/or B.sub.12P.sub.2, and the mechanochemical reaction is carried out in a mill.

2. The process of claim 1, wherein the mechanochemical reaction is carried out thanks to mechanical stresses between the powder of boron phosphate (BPO4) and the powder of at least one alkaline earth metal (EA).

3. The process of claim 1, wherein the temperature of the reaction is below 80 C.

4. The process of claim 1, wherein the particle size of the boron phosphide obtained by the mechanochemical reaction is a function of the time of the mechanochemical reaction between, which is between 30 s and 20 min.

5. The process of claim 1, wherein the mechanical device is a rotative device and the rotation speed of the rotative device is between 50 and 1100 rpm.

6. The process of claim 5, wherein the mechanochemical reaction comprises a ball-milling step of a powder mixture comprising boron phosphate and the at least one alkaline earth metal.

7. The process of claim 1, wherein the mechanochemical reaction is carried out in air.

8. The process of claim 1, wherein the boron phosphide is BP, and the process comprises the reduction of boron phosphate (BPO.sub.4) with at least one alkaline earth metal (EA) according to mechanochemical reaction (1):
BPO.sub.4+4 EA.fwdarw.BP+4 EA(O)(1).

9. The process of claim 1, wherein the boron phosphide is B.sub.12P.sub.2, and the process comprises the reduction of boron phosphate (BPO.sub.4) with at least one alkaline earth metal (EA) according to mechanochemical reaction (2):
2BPO.sub.4+5(EA)B.sub.2+3EA.fwdarw.B.sub.12P.sub.2+8EA(O)(2).

10. The process of claim 1, wherein the at least one alkaline earth metal is magnesium and/or calcium.

11. The process of claim 1, wherein the mixture comprising boron phosphate and the at least one alkaline earth metal further comprises a chemically inert diluent.

12. The process of claim 6, wherein it comprises: (a) mixing boron phosphate, the at least one alkaline earth metal, optionally alkaline earth metal boride, and optionally a chemically inert diluent, provided as powders, so as to obtain a mixed powder, (b) pre-milling said mixed powder with at least one ball at a first rotation speed of the ball-mill of between 50 and 200 rpm allowing for grinding the reagents, (c) milling said pre-milled powder with at least one ball at a second rotation speed of the ball mill of between 400 and 1100 rpm enabling the reaction to occur, so as to obtain a milled powder which comprises the boron phosphide, the second rotation speed being superior to the first rotation speed, (d) recovering boron phosphide from the milled powder.

13. The process of claim 10, wherein the at least one alkaline earth metal is magnesium.

14. The process of claim 11, wherein the chemically inert diluent is sodium chloride.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1. Pressure and temperature variation in the vial during the milling step of the process of the invention, using (a) BPO.sub.4 and Mg (60 sec real time milling) and (b) BPO.sub.4, Mg and MgB.sub.2 (120 sec real time milling). The left Y-axis represents the pressure in bar, the right Y-axis represents the temperature in K, and the X-axis represents the time in seconds.

(2) FIG. 2. Diffraction pattern (CuK) of the washed sample of (a) BP (cubic) and (b) B.sub.12P.sub.2 (icosahedral) produced by the process of examples. The arrow shows the location of 104 and 021 diffraction lines of B.sub.12P.sub.2. The X-axis represents the value 20 in degrees.

(3) FIG. 3. Representative TEM micrographs of the washed sample of (a) BP (cubic) and (b) B.sub.12P.sub.2 (icosahedral) produced by mechanochemical process of the invention.

(4) FIG. 4. Representative SEM micrographs (10000) of the washed sample of (a) BP (cubic) and (b) B.sub.12P.sub.2 (icosahedral) produced by mechanochemical process of the invention.

(5) FIG. 5 Raman spectra of the washed sample of (a) BP (cubic) and (b) B.sub.12P.sub.2 (icosahedral) produced by mechanochemical process of the invention. The arrow points to the characteristic band of B.sub.12P.sub.2 (476 cm.sup.1). Insets show optical images (100) of the sample surfaces. The X-axis represents the Raman shift in cm.sup.1.

EXAMPLES

(6) The following examples aim at illustrating particular embodiments of the present invention, and thus should not be construed as limiting in any way the scope of the present invention.

(7) Chemicals

(8) Boron phosphate (BPO.sub.4) was produced according to the procedure described in Handbuch der Praparativen Anorganischen Chemie (von G. Brauer, Ed.) vol. 2, pp. 811, Stuttgart: Ferdinand Enke Verlag, 1975:

(9) Reaction of boric acid (Alfa Aesar, 99.8%) with orthophosphoric acid (Alfa Aesar, 85% aq. sol.) in alumina crucible at 520 K with subsequent annealing of the obtained product in a muffle furnace at 770 K yields BPO.sub.4 in 98-99% yield, as a powder with a particle size of less than 200 m.

(10) Magnesium metal (Alfa Aesar, 99.8%; 325 mesh) and magnesium diboride (Alfa Aesar, 99%, 100 mesh) were used as starting materials.

(11) Protocol

(12) The stoichiometric amounts of BPO.sub.4 (powder with a particle size of less than 200 m.), Mg and MgB.sub.2 were thoroughly mixed in an agate mortar, according to equation (1) or (2):
BPO.sub.4+4Mg.fwdarw.BP+4MgO(1)
2BPO.sub.4+5MgB.sub.2+3Mg.fwdarw.B.sub.12P.sub.2+8MgO(2).

(13) Therefore, when production of BP is sought for, the molar ratio BPO.sub.4/Mg is 1:4. When production of B.sub.12P.sub.2 is sought for, the molar ratio BPO.sub.4/MgB.sub.2/Mg is 2/5/3.

(14) Mechanochemical syntheses were carried out in a commercial high-energy planetary ball mill (Fritsch, Pulverisette 7). The milling container was a hardened steel vial with a capacity of 80 ml, and 25 tungsten carbide balls with a diameter of 10 mm (for a total weight of 198 g) were used. Ball-to-powder weight ratio was varied from 100:1 to 20:1. Of note, the ball-to-powder ratio did not have significant influence on the outcome of the process, in particular in terms of yield, particle size and purity of the obtained powders.

(15) Loading/unloading operations and milling were performed in air. Typically, the raw mixture of starting materials was pre-milled with a rotation speed of 100 rpm during 1 min, then the rotation speed was increased up to 700 rpm (see Tables 1 and 2) for 1, 2, 5 or 7 minutes. Milling under high speed (700 rpm) was continually conducted for 1 to 5 minutes. After milling, the resulting powders were extracted, subsequently treated with diluted hydrochloric acid (Alfa Aesar, 10% aqueous solution), washed several times with deionized water, and dried in air at 323 K. In some experiments sodium chloride (Prolabo, 99.5%, 100 m) was used as chemically inert diluent (see Tables 1 and 2).

(16) Characterization

(17) In situ monitoring of pressure and temperature during milling was achieved by incorporating a gas-temperature measurement system based on pressure and temperature sensors as well as a transmitter into the vial lid and a receiver, external to the mill, sending the data to a computer.

(18) The washed samples were studied by X-ray powder diffraction (Equinox 1000, Inel powder diffractometers with CuK1 =1.540598 and CoK1 radiation =1.789007 ). The particle size of the produced powders was calculated from the width of X-ray diffraction lines using Williamson-Hall method (see Williamson et al. Acta Metall., 1953, vol. 1, pp. 22-31).

(19) Raman spectra were excited by 632.8 nm HeNe laser (beam size 10 m) and recorded on a Horiba Jobin Yvon HR800 micro-Raman spectrometer.

(20) Particles sizes and microstructural morphology of boron phosphides powders were examined by scanning electron microscopy (SEM) at a Leo Supra 40 and 50VP instruments, and by transmission electron microscopy (TEM) at a Leo 912AB Omega.

(21) The chemical composition and purity of the samples were revealed by energy dispersive X-ray (EDX) microanalysis (performed in Leica S440 electron microscope with a Princeton Gamma-Tech energy-dispersive spectrometer and LEO Supra 50VP electron microscope with Oxford Instruments INCA Energy+ microanalytical system). According to the quantitative EDX microanalysis, the samples of boron phosphides did not show any detectable non-stoichiometry.

(22) Specific surface areas (SSA) were measured using the Brunauer-Emmett-Teller (BET) method (see Brunauer, Emmett, and Teller, J. Am. Chem. Soc., 1938, vol. 60, pp. 309-319) by nitrogen adsorption on a Coulter SA 3100.

(23) Particles size distributions (for a few samples) were calculated from static light scattering (SLS) experiments carried out on a Fritsch Analysette 22 NanoTec plus.

(24) Results

(25) The obtained results are summarized in tables 1 (for BP) and 2 (for B.sub.12P.sub.2) below.

(26) TABLE-US-00001 TABLE 1 Milling conditions of key experiments for cubic BP producing and results of X-ray analysis Starting XRD phases after composition milling and washing Crystalline BPO.sub.4/Mg/NaCl (in % by weight of size.sup.1 SSA (g) Pre-milling step Milling step the solid) Yield (nm) (m.sup.2/g) 1.06/0.96/0.00 100 rpm 700 rpm BP (90%) B.sub.12P.sub.2 (10%) 72% 80(7) 1 min 1 min 2.12/2.00/0.00 100 rpm 700 rpm BP (95%) B.sub.12P.sub.2 (5%) 76% 78(8) 1 min 1 min 1.59/1.50/1.55 100 rpm 700 rpm BP (97%) B.sub.12P.sub.2 (3%) 65% 65(9) 18.7 1 min 1 min 1.06/0.98/2.54 100 rpm 700 rpm BP (97%) B.sub.12P.sub.2 (3%) 75% 59(7) 33.6 1 min 5 min 2.12/1.92/4.04 100 rpm 700 rpm BP (98%) B.sub.12P.sub.2 (2%) 75% 67(5) 33.7 1 min 5 min 3.18/2.88/4.03 100 rpm 700 rpm BP (97%) B.sub.12P.sub.2 (3%) 75% 53(7) 10.1 1 min 5 min .sup.1The crystalline size corresponds to the particle size of the obtained powder. The first number corresponds to the measure particle size, while the number in parenthesis indicates the uncertainty. The uncertainty applies to the least significant figure(s) of the number prior to the parenthesized value.

(27) TABLE-US-00002 TABLE 2 Milling conditions of key experiments for B.sub.12P.sub.2 producing and results of X-ray analysis Starting XRD phases after composition milling and washing Crystalline BPO.sub.4/Mg/MgB.sub.2 (in % by weight of size1 SSA (g) Pre-milling step Milling step the solid) Yield (nm) (m.sup.2/g) 1.06/0.00/1.32 100 rpm 700 rpm initial substance only 1 min 7 min 1.06/0.37/1.15 100 rpm 700 rpm BP (20%) B.sub.12P.sub.2 (80%) 67% 32(6) 1 min 1 min 2.12/0.73/2.76 100 rpm 700 rpm BP (3%) B.sub.12P.sub.2 (97%) 84% 32(6) 27.3 1 min 2 min 1The crystalline size corresponds to the particle size of the obtained powder. The first number corresponds to the measure particle size, while the number in parenthesis indicates the uncertainty. The uncertainty applies to the least significant figure(s) of the number prior to the parenthesized value.

DISCUSSION

(28) FIGS. 1a (production of BP) and 1b (production of B.sub.12P.sub.2) show a typical variation of the gas pressure and the integrated temperature in the reaction tank after setting the rotation speed to 700 rpm with the corresponding holding time (see tables 1 and 2) for the milling step. The small increase of the temperature in the reaction tank gave a corresponding signal of the sensitive gas pressure sensor. In the few first seconds, the temperature and pressure are stable. A sharp pressure peak of about 4 (FIG. 1a)-9 (FIG. 1b) bars is observed, reflecting the start of an exothermal reaction in the powder mixture. The incubation period, i.e. the milling time under high speed up to this peak, is 6-20 seconds in the case of the reaction according to equation (1), and without any addition of chemically inert diluent (i.e. with pure BPO.sub.4 and Mg, FIG. 1a). This incubation time is increased up to 160-220 seconds upon addition of a chemically inert diluent (sodium chloride). The incubation time is also increased in the case of production of B.sub.12P.sub.2 (see FIG. 1b). The pressure peak could be attributed to the presence of water (not more than 0.4 wt %) in initial BPO.sub.4. Alternatively, the pressure peak could correspond to the formation of small amounts of gaseous products as a result of side reactions in the experimental mixture (perhaps elemental phosphorus). This second hypothesis seems more accurate, as it would explain the light phosphorus smell identified after unloading.

(29) As can be seen in FIG. 1a, the integrated temperature increased during milling from 306 K to 323 K. Two sections are clearly visible on the graph. In the first section, the slope is of 0.50 K/sec, and in the second section, it is of 0.27 K/sec. Of note, in the control experiment wherein BPO.sub.4 was loaded without Mg, the temperature slowly increases with a rate of about 0.02 K/sec, which may be explained by internal friction and impact phenomena. The temperature curves show a similar trend for all the trials.

(30) Similar results are observed regarding the integrated temperature during milling of the mixture of BPO.sub.4, Mg, MgB.sub.2, (leading to B.sub.12P.sub.2), as can be seen in FIG. 1b, which increased from 306 K to around 350 K.

(31) The product obtained according to reaction (1) (after washing) proved to be solid boron phosphide, comprising up to 10% of boron subphosphide, B.sub.12P.sub.2. More specifically, the BP powder produced according to the invention, and in particular according to the present examples, may contain following impurities: (i) products of side reactions (mainly B.sub.12P.sub.2, below 3 wt %), (ii) elemental amorphous boron and phosphorus, which may however be removed upon treatment with aqueous nitric acid and (iii) very small amounts of impurities from the mill device (milling balls and vial).

(32) To reduce the excess of Gibbs energy caused by mechanical energy transfer, sodium chloride, a chemically inert diluent, was added to the reagents. Indeed, upon addition of NaCl to the reaction mixture, a decrease in the intensity of the B.sub.12P.sub.2 diffraction lines in the final products was observed. In particular, with a NaCl content of 33 wt %, the formation of the almost single-phase (>97%) boron phosphide is observed (FIG. 2a), with a lattice parameter of a=4.545(2) , which is close to the literature value (4.543(1) (Xiam et al. J. Appl. Phys., 1993. vol. 74, pp. 1660-1662)). The obtained final powder exhibited a mean particle size of 100-200 nm (FIGS. 3a and 4a). The Raman spectra (FIG. 5a) of the washed reaction (1) products exhibits two features: a strong asymmetric line at 818 cm.sup.1 and a weak broad line at 804 cm.sup.1 that are characteristic bands for BP. In some BP samples, a weak sharp line at 476 cm.sup.1 (the most intense band of B.sub.12P.sub.2) was also observed.

(33) The product obtained according to reaction (2) (after washing) proved to be solid boron subphosphide (B.sub.12P.sub.2) almost as a single phase rhomboedric B.sub.12P.sub.2 (>97%, FIG. 2b), comprising up to 3 wt % of boron phosphide BP. Of note, under unoptimized conditions, the obtained solid boron subphosphide (B.sub.12P.sub.2) comprises up to 20 wt % of boron phosphide BP. More specifically, the B.sub.12P.sub.2 powder produced according to the invention, and in particular according to the present examples, may contain: (i) products of side reactions (mainly BP, below 3 wt %), (ii) elemental amorphous boron, which may however be removed upon treatment with aqueous nitric acid, (iii) trace of magnesium borides and (iv) very small amounts of impurities from the mill device (milling balls and vial).

(34) B.sub.12P.sub.2 lattice parameters of a=5.992(4) , c=11.861(8) were found, which are a good agreement with the literature values (a=5.986(6) , c=11.848(9) (Slack et al. J. Phys. Chem. Solids, 2014, vol. 75, pp. 1054-1074)). The obtained final powder exhibited a mean particle size of 100-200 nm (FIGS. 3b and 4b). The Raman spectrum (FIG. 5b) of the washed reaction (2) products exhibits all of the expected peaks for B.sub.12P.sub.2, in particular the strong asymmetric line at 476 cm.sup.1 corresponding to the most intense band of B.sub.12P.sub.2.

(35) The total yield of boron phosphides is of about 75%, according to reactions (1) and (2). The lower yields observed in reactions with high contents of chemically inert diluent (NaCl) demonstrate that when the content of chemically inert diluent (NaCl) is high, the reaction is slower, so that in the reaction conditions of the above examples the reaction did not go to completion.

CONCLUSION

(36) In sum, single-phase boron phosphides BP and B.sub.12P.sub.2 powders have been produced by reduction of boron phosphate by metallic magnesium (BP), or metallic magnesium and magnesium diboride (B.sub.12P.sub.2), through a mechanochemical method, i.e. high energy ball milling.

(37) As demonstrated, the method of the invention is characterized by easy implementation, high efficiency, low cost, and good perspectives for large-scale production of BP and/or B.sub.12P.sub.2.