FORMATION OF BORON CARBIDE NANOPARTICLES FROM A BORON ALKOXIDE AND A POLYVINYL ALCOHOL

Abstract

The present invention relates to a process for the preparation of boron carbide nanoparticles, characterized in that it comprises at least the stages consisting in: (i) interacting boric acid, boron oxide B.sub.2O.sub.3 or a boric acid ester of B(OR).sub.3 type, with R, which are identical or different, representing C.sub.1-4-alkyl groups, with 1 to 2 molar equivalents of at least one C.sub.2 to C.sub.4 polyol, under conditions favorable to the formation of a boron alkoxide powder; (ii) interacting, in an aqueous medium, the boron alkoxide powder obtained on conclusion of stage (i) with an effective amount of one or more completely hydrolyzed polyvinyl alcohols, with a molar mass of between 10 000 and 80 000 g.mol.sup.1, under conditions favorable to the formation of a crosslinked PVA gel, and (iii) carrying out an oxidizing pyrolysis of the crosslinked gel formed on conclusion of the preceding stage (ii), under conditions favorable to the formation of the CB.sub.4 nanoparticles.

Claims

1. Process for the preparation of boron carbide nanoparticles, comprising at least the stages consisting in: (i) interacting boric acid, boron oxide B.sub.2O.sub.3 or a boric acid ester of B(OR).sub.3 type, with R, which are identical or different, representing C.sub.1-4-alkyl groups, with 1 to 2 molar equivalents of at least one C.sub.2 to C.sub.4 polyol, under conditions favorable to the formation of a boron alkoxide powder; (ii) interacting, in an aqueous medium, the boron alkoxide powder obtained on conclusion of stage (i) with an effective amount of one or more completely hydrolyzed polyvinyl alcohols, with a molar mass of between 10 000 and 80 000 g.mol.sup.1, under conditions favorable to the formation of a crosslinked PVA gel, and (iii) carrying out an oxidizing pyrolysis of the crosslinked gel formed on conclusion of the preceding stage (ii), under conditions favorable to the formation of the CB.sub.4 nanoparticles.

2. Process according to claim 1, said CB.sub.4 nanoparticles having a mean size of less than or equal to 100 nm.

3. Process according to claim 1, in which stage (i) is carried out starting from boric acid, trimethyl borate or triethyl borate.

4. Process according to claim 1, in which the polyol in stage (i) is chosen from ethylene glycol, propylene glycol, diethylene glycol, propane-1,3-diol, butane-2,3-diol, butane-1,2-diol, butane-1,2,4-triol, glycerol and their mixtures.

5. Process according to claim 1, in which the polyol in stage (i) is chosen from ethylene glycol, propylene glycol, glycerol and their mixtures.

6. Process according to claim 1, in which stage (i) is carried out via the bringing together of boric acid or one of its esters B(OR).sub.3 or boron oxide B.sub.2O.sub.3 and of said polyol(s), followed by the heating of the reaction medium.

7. Process according to claim 6, in which the heating is carried out at a temperature of between 50 C. and 150 C.

8. Process according to claim 7, in which the heating is carried out under an oxidizing atmosphere.

9. Process according to claim 1, in which stage (ii) is carried out by addition of the boron alkoxide powder to an aqueous solution of polyvinyl alcohol(s), followed by the heating of the reaction medium.

10. Process according to claim 9, in which the heating is carried out at a temperature of between 5 C. and 100 C.

11. Process according to claim 9, in which the duration of the heating is between 1 hour and 5 hours.

12. Process according to claim 1, in which the pyrolysis in stage (iii) is carried out by heating at a temperature of between 500 C. and 1200 C.

13. Process according to claim 1, in which the pyrolysis in stage (iii) is carried out under flushing with air.

Description

EXAMPLES

Synthesis of Boron Carbide Nanoparticles

Synthesis Protocol

[0101] 5 g of B(OH).sub.3 (0.083 mol) are mixed with 7.75 g of ethylene glycol. The mixture obtained is heated under air at 120 C. for two hours, then crystallizes from return to ambient temperature.

[0102] The powder obtained, which is transparent and slightly yellow, is ground. 4 g of this powder are added to 100 g of a 4% aqueous solution of hydrolyzed PVA (Mowiol 4-98 MW 27000, Mowiol 6-98 MW 47000 or PVA Aldrich MW 77000-79000, 98% hydrolyzed).

[0103] The reaction medium is heated at 80 C. for 2 hours. Complete dissolution of the boron alkoxide is observed, followed by an increase in the viscosity with formation of a solid homogeneous gel which is transparent or slightly white.

[0104] The gel is dried and then ground. It is subsequently pyrolyzed at 800 C. in a porcelain boat in a quartz tubular furnace under air (50 liters of air per hour; rise of 160 C. per hour).

[0105] The gray powder obtained on conclusion of the oxidizing pyrolysis is washed with water, in order to remove the traces of B.sub.2O.sub.3, and then dried at 300 C. in an oven.

[0106] Similar syntheses were carried out by employing propylene glycol or glycerol in place of ethylene glycol and/or trimethyl borate (B(OMe).sub.3) or triethyl borate (B(OEt).sub.3) in place of boric acid.

Results

Characterization of the Boron Carbide Powders

[0107] The analysis by infrared absorption spectroscopy of the powders obtained under the abovementioned conditions confirms the formation of boron carbide with a peak, attributable to the CB bond, at 1170 cm.sup.1.

[0108] The boron carbide powders may also be observed by scanning electron microscopy (SEM). The photographs obtained by SEM testify to a population of homogeneous spherical crystals, with a size of less than 90 nm.

Synthesis Yield

[0109] The yield of boron carbide obtained is measured with respect to the initial weight of B(OH).sub.3 (or of boric acid ester) introduced at the start of the synthesis.

[0110] Under the conditions described above, using boric acid and, as polyol of low molecular weight, ethylene glycol, propylene glycol or glycerol, a yield of boron carbide of approximately 40% by weight is obtained on conclusion of the oxidizing pyrolysis.

[0111] In the same way, the use, as starting material, of B(OMe).sub.3 and of B(OEt).sub.3 instead of boric acid with 1 to 2 molar equivalents of a polyol of low molecular weight (ethylene glycol, propylene glycol or glycerol) makes it possible to access, on conclusion of the oxidizing pyrolysis, a synthesis yield of boron carbide of 35 to 40% by weight.

[0112] On the other hand, the synthesis of boron carbide by employing the protocol of Fathi et al. [1], with a hydrolyzed PVA of Mowiol 4-98 type, results, after washing the pyrolyzed gray powder with water, in a yield of boron carbide of 9 to 10% by weight, with respect to the boric acid charged.

[0113] A similar synthesis, according to the protocol of Fathi et al [1], with a grade of PVA sold by Aldrich, 98% hydrolyzed and with a molecular weight of 3000 g.mol.sup.1, still results in a synthesis yield of boron carbide of approximately 10% by weight.

REFERENCES

[0114] [1] Fathi et al., Synthesis of boron carbide nano particles using polyvinyl alcohol and boric acid, CeramicsSilikaty, 56(1), 32-35 (2012);

[0115] [2] Kakiage et al., Low-temperature synthesis of boron carbide powder from condensed boric acid-glycerin product, Materials Letters, 65 (2011), 1839-1841;

[0116] [3] Murray, Low temperature Synthesis of Boron Carbide Using a Polymer Precursor Powder Route, School of Metallurgy and Materials, University of Birmingham, Sept 2010-Sept 2011.