METHOD FOR COLLOIDAL PREPARATION OF A METAL CARBIDE, SAID METAL CARBIDE THUS PREPARED AND USES THEREOF

Abstract

The present invention relates to a method for preparation of a powder comprising at least one carbide of at least one metal, comprising the steps consisting of: (a) preparing a solution comprising at least one organic gelling agent and at least one inorganic salt of at least one metal in a solvent; (b) modifying the pH of the solution prepared in step (a) in such a way as to precipitate said at least one metal and to obtain a colloidal suspension comprising nanoparticles of oxyhydroxides of said at least one metal; (c) removing the solvent from the colloidal suspension obtained in step (b) by which means a precursor of at least one carbide of at least one metal is obtained; and (d) subjecting the precursor obtained in step (c) to a thermal treatment in order to transform same into a powder comprising at least one carbide of at least one metal. The present invention also relates to the powder thus prepared and the various uses thereof.

Claims

1. A method for preparation of a powder comprising at least one carbide of at least one metal comprising the steps consisting of: a) preparing a solution comprising at least one organic gelling agent and at least one inorganic salt of at least one metal in a solvent; b) modifying the pH of the solution prepared in step (a) in such a way as to precipitate said at least one metal and to obtain a colloidal suspension comprising nanoparticles of oxyhydroxides of said at least one metal; c) removing the solvent from the colloidal suspension obtained in step (b) to obtain a precursor of at least one carbide of at least one metal; and d) subjecting the precursor obtained in step (c) to a thermal treatment in order to transform same into a powder comprising at least one carbide of at least one metal, said at least one organic gelling agent being the only carbon source used for the production of carbide.

2. The method according to claim 1, wherein said organic gelling agent is selected from the group consisting of a polyvinyl alcohol constituted of the repetition of the —CH.sub.2—CH(OH)— monomer unit, a polymer of acrylic acid constituted of the repetition of the —CH.sub.2—CH(CO.sub.2H)— monomer unit, a copolymer of acrylic acid-acrylamide comprising —CH.sub.2—CH(CO.sub.2H)— and —CH.sub.2—CH(CONH.sub.2)— monomer units, and a polyvinylpyrrolidone constituted of the repetition of the —CH.sub.2—CH(NCOC.sub.3H.sub.6)— monomer unit.

3. The method according to claim 1, wherein said organic gelling agent is selected from the group consisting of methyl cellulose, hydroxyl ethyl methyl cellulose, hydroxyl ethyl cellulose, carboxy methyl cellulose, hydroxyl propyl methyl cellulose, starch, methyl starch, hydroxyl ethyl methyl starch, hydroxyl ethyl starch, carboxy methyl starch, hydroxyl propyl methyl starch, carrageenan, alginate, furcellaran, agar-agar, glucomannan, galactomannan, xanthan gum, fenugreek gum, guar gum, tara gum, carob gum, cassia gum and gellan gum.

4. The method according to claim 1, wherein said metal is selected from the group consisting of cerium, titanium, zirconium, hafnium, thorium, uranium, neptunium, plutonium, americium and curium.

5. The method according to claim 1, wherein said inorganic salt of at least one metal is in the form of a halide of at least one metal or of a nitrate of at least one metal.

6. The method according to claim 1, wherein the solvent of the solution comprising at least one organic gelling agent and at least one inorganic salt of at least one metal is selected from the group consisting of water, deionised water, distilled water, hydroxylated solvents low molecular weight liquid glycols, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), acetonitrile, acetone, tetrahydrofuran (THF) and one of the mixtures thereof.

7. The method according to claim 1, wherein the removal of the solvent during said step (c) is carried out by at least one technique selected from the group consisting of decantation, air drying, vacuum drying, microwave drying, high frequency drying, sublimation, lyophilisation and any combination thereof.

8. The method according to claim 1, wherein said step of thermal treatment (d) comprises the following operations: an operation of pyrolysis of the precursor obtained following step (c) carried out at a temperature between 600° C. and 1200° C. maintained for a duration between 1 h and 8 h whereby said precursor decomposes; an operation of carboreduction of the pyrolysed precursor obtained at the end of the pyrolysis operation carried out at a temperature above 1050° C. maintained for a duration between 15 min and 8 h whereby a powder comprising at least one carbide of at least one metal is obtained.

9. The method according to claim 8, wherein said pyrolysis operation is carried out at a temperature of 1000° C.±100° C.

10. The method according to claim 8, wherein said carboreduction operation is carried out at a temperature between 1050° C. and 1700° C. under argon or at a temperature of 1200° C.±100° C. under rough vacuum.

11. The method according to claim 1, comprising the steps consisting of: preparing a 1.sup.st colloidal suspension comprising nanoparticles of oxyhydroxides of a 1.sup.st metal according to steps (a) and (b), preparing a 2.sup.nd colloidal suspension comprising nanoparticles of oxyhydroxides of a 2.sup.nd metal, different to said 1.sup.st metal according to steps (a) and (b) as defined in claim 1, mixing the two colloidal suspensions thus prepared, then subjecting them to steps (c) and (d), to obtain a mixed carbide of these two metals.

12. A powder of at least one carbide of at least one metal capable of being prepared according to a method as defined in claim 1.

13. A method for preparation of a dense material of at least one carbide of at least one metal, said method comprising the steps consisting of: preparing a powder of at least one carbide of at least one metal according to a method as defined in claim 1, then subjecting said powder to a sintering step.

14. The method according to claim 6, wherein the solvent is methanol, ethanol or isopropanol.

15. The method according to claim 6, wherein the solvent is ethylene glycol.

16. The method according to claim 10, where the carboreduction operation is carried out at a temperature of 1400° C. under argon.

17. The method according to claim 9, wherein said carboreduction operation is carried out at a temperature between 1050° C. and 1700° C. under argon or at a temperature of 1200° C.±100° C. under rough vacuum.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0108] FIG. 1 shows a scanning electron microscope (SEM) micrograph of a powder of uranium oxide (UO.sub.2) used for the synthesis of carbide by carboreduction according to a method of the prior art.

[0109] FIG. 2 proposes a schematic representation of the synthesis of dense sintered pellets of uranium carbide according to a method of the prior art.

[0110] FIG. 3 shows a SEM micrograph of a fragment of precursor mixture of carboreduction implemented in a method of the prior art.

[0111] FIG. 4 shows the behaviour during carboreduction treatment of a mixture of powders of uranium oxide and carbon according to a method of the prior art.

[0112] FIG. 5 proposes a schematic representation of the synthesis of uranium carbide according to the method of the invention in the presence of methyl cellulose.

[0113] FIG. 6 shows the stability domain of the suspensions of nanoparticles of uranium oxide as a function of the methyl cellulose concentration.

[0114] FIG. 7 shows a transmission electron microscopy and wet-STEM mode scanning electron microscopy image of the colloidal suspension of nanoparticles of UO.sub.2, 2 H.sub.2O (amorphous) in methyl cellulose solution.

[0115] FIG. 8 shows the X-ray diffractogram of the powder obtained after the thermal pyrolysis treatment at 1000° C.

[0116] FIG. 9 shows a transmission electron microscopy (TEM) image of the nanoparticles of UO.sub.2 in a carbon coating.

[0117] FIG. 10 shows scanning electron microscopy images of a fragment of the sample produced by mixing of powders according to the prior art (R=3; FIGS. 10A and 10C) and of a sample prepared by pyrolysis of a suspension of nanoparticles stabilised by methyl cellulose according to the present invention (R=3.4; FIGS. 10B and 10D) and this, at two enlargements (×10000: FIGS. 10A and 10B and ×1000: FIGS. 10C and 10D).

[0118] FIG. 11 shows the carboreduction under vacuum of a mixture of powders according to the prior art (REF R.sub.C/U=3) and of a sample obtained by the synthesis route according to the present invention (R.sub.C/U=3.4).

[0119] FIG. 12 shows the X-ray diffractogram of the sample obtained after carboreduction under vacuum of the precursor mixture according to the present invention (R.sub.C/U=3.4).

[0120] FIG. 13 shows scanning electron microscopy images in such a way as to compare the morphology of the compounds obtained after carboreduction of the mixture of powders according to the prior art (REF R.sub.C/U=3; FIG. 13A) and the precursor mixture according to the present invention (R.sub.C/U=4.2; FIG. 13B).

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0121] I. Synthesis of Uranium Monocarbide by Carboreduction (Prior Art).

[0122] This elaboration route is that which is generally retained for the synthesis of uranium carbides at an industrial scale.

[0123] The uranium oxide used may be constituted, for example, of spherical agglomerates the size of which is between 10 μm and 60 μm (FIG. 1).

[0124] The carbon used may be, for example, a powder of ground natural graphite constituted of particles having diameters between 10 μm and 110 μm.

[0125] With the aim of producing a monophase uranium carbide of overall formula UC, with a molar ratio n.sub.C/n.sub.U=1, the quantities of uranium and carbon introduced into the mixture have to be precise because the stoichiometry domain is very narrow. These quantities are thus weighed out accurately. The different uranium carbide synthesis steps are shown in FIG. 2.

[0126] The mixture of powders (oxide+carbon) observed by SEM micrography shows a heterogeneous mixture at the hundreds of micron scale (BSE mode, FIG. 3). The SEM micrograph reveals the heterogeneity of the mixture of precursors obtained by this conventional route. Zones rich in UO.sub.2 (in white) and zones rich in carbon (in black) of several tens to hundreds of microns exist.

[0127] A thermal cycle called carboreduction, at a temperature above 1600° C. under rough vacuum is carried out to achieve the synthesis of the carbide. Monitoring the pressure in the furnace makes it possible to identify the synthesis temperatures (FIG. 4). The rise in pressure observed between 300 and 600 min corresponds to gaseous release during the carboreduction reaction. This increase in pressure (around 300 min) occurs at 1300° C., considered as the temperature of start of carboreduction in these conditions.

[0128] The powders obtained cannot directly lead to a dense solid by sintering. Steps of grinding and compacting of the powders are necessary before the thermal sintering treatment to lead to dense products.

[0129] The synthesis of mixed carbides (U,Pu)C may be carried out in similar conditions by introducing the two types of oxide at the start.

[0130] II. Preparation of Nanoparticles of Uranium Oxide According to the Method of the Invention.

[0131] The different steps of the method according to the invention described hereafter are shown schematically in FIG. 5.

[0132] II.1. Preparation of the Suspension.

[0133] This novel route has been implemented for the synthesis of uranium mono- and dicarbides. The physical properties (specific surface, particle size, oxygen content) of the uranium carbides obtained from this novel synthesis route are improved with a view to the later step of sintering compared to compounds obtained by current methods. The carboreduction temperature is reduced by 200 to 400° C. (under rough vacuum) and the method is simplified regarding the grinding and pelletizing steps compared to the conventional route. This gain will make it possible to reduce both losses in actinides during the carboreduction and the oxygen content of the final product.

[0134] A method for preparation of a stable colloidal suspension of nanoparticles has been developed for this novel route. Particular conditions are required to obtain the expected precursor.

[0135] The solubility of uranium (IV) (or U(IV)) in basic medium is low (of the order of 2.10.sup.−10 mol/L at pH 8). The nanoparticles of precursors of UO.sub.2 are formed by raising the pH of a solution of UCl.sub.4 initially in 7 M HCl. The precipitation of U(IV) in uranium hydroxide form takes place as soon as the pH exceeds 8. Uranium hydroxide is not stable and is transformed in several hours into amorphous dihydrated uranium oxide.

[0136] To obtain a stable suspension of nanoparticles, it is necessary to control precipitation and avoid agglomeration of the particles. It is thus necessary to use a water soluble polymer capable both of confining precipitation in the nano-domains and avoiding agglomeration of the nanoparticles. Methyl cellulose has been chosen as stabiliser of the nanoparticles. Adjustment of the methyl cellulose concentration is necessary to stabilise the nanoparticles formed (FIG. 6).

[0137] In order to avoid oxidation of the suspensions, it is advisable that the latter are prepared in an inert glove box to avoid oxidation of the uranium dioxide into higher oxides.

[0138] From a certain methyl cellulose concentration, the absence of sedimentation for a period of 24 hours is noted. This stability seems practically independent of the uranium content in the solution and thus of the concentration of nanoparticles.

[0139] This particular property therefore enables the preparation of suspensions of variable and controllable concentrations of uranium leading to variable ratios R.sub.C/U (number of moles of carbon:number of moles of uranium R.sub.C/U=n.sub.C/n.sub.U), in particular, close to the stoichiometry (R.sub.C/U=3) to carry out the synthesis of monocarbide according to the following carboreduction reaction:

UO.sub.2+3 C⇄UC+2 CO (gas)  (equation 2)

[0140] In an identical manner, by changing the ratio R, it is possible to elaborate higher carbides such as a dicarbide.

[0141] II.2. Example of Preparation.

[0142] For the preparation of suspensions of nanoparticles, a 30 g.Math.L.sup.−1 methyl cellulose solution is prepared beforehand. To do so, 100 mL of deionised water are heated to boiling, 3 g of Methocel® (water soluble methyl cellulose, Sigma-Aldrich, CAS No.: 9004-67-5) are added to the boiling water maintained under vigorous agitation for 10 min. The solution is cooled in an ice batch while maintaining agitation. After cooling, the solution is decanted into a 100 mL flask, the volume is made up to 100 mL with deionised water, the missing volume having been lost during boiling and from volume contraction due to the addition of methyl cellulose. The solution is agitated for 24 hours before use.

[0143] The pH of the solution is monitored using a pH-meter throughout the experiment. 17 mL of the 30 g.Math.L.sup.−1 methyl cellulose solution are introduced into a 50 mL beaker, then 3 mL of a pure ammonia solution (28.0%-30.0%) are added under mechanical agitation using a magnetic stirring bar. The agitation is maintained for 10 min. Using a micropipette, 6 mL of a solution of UCl.sub.4 ([U]=0.47 M in 7 M HCl medium) are collected and added to the solution under agitation. The pH is immediately adjusted to attain a value above 8 using pure ammonia solution (28.0%-30.0%). Stirring is maintained up to stabilisation of the pH at a value above or equal to 8.

[0144] FIG. 7 shows the presence of non-agglomerated nanoparticles of several nanometres. The nanoparticles obtained by the proposed synthesis route have a homogeneous size distribution.

[0145] II.3. Removal of the Solvent.

[0146] The solvent, here water, may be removed by lyophilisation. It may also be removed by evaporation by heating the colloidal suspension to 100° C. or any other means of drying. At this stage, a new precursor mixture is obtained.

[0147] II.4. Thermal Pyrolysis Treatment.

[0148] Thermal pyrolysis treatment makes it possible to transform the organic molecules into solid amorphous carbon and also makes it possible to improve the crystallisation of UO.sub.2. This treatment may, for example, be carried out with a heating rate of 200° C..Math.h.sup.−1 and an isothermal stage at 1000° C. of 4 hours, under argon with a flow rate of 30 L.Math.h.sup.−1. During this step, a strong release of decomposition gas takes place (CO.sub.2, CO, H.sub.2O) and compounds rich in nitrogen and chlorine from the synthesis are also removed.

[0149] A control of the pyrolysis conditions, in particular the argon flow rate, is necessary in order to obtain a ratio R.sub.C/U=n.sub.C/n.sub.U reproducible at the end of the treatment.

[0150] During this thermal treatment, the amorphous uranium oxide becomes crystalline, this phase is the only one identified on the powder X-ray diffractograms (FIG. 8).

[0151] After pyrolysis, the morphology of the samples may be observed by transmission electronic microscopy (FIG. 9). The latter shows nanoparticles of UO.sub.2 between 10 and 17 nm diameter, encapsulated in a carbon coating.

[0152] Observations by scanning electron microscopy also make it possible to compare the morphology of samples obtained by this novel route with precursors obtained by the conventional route (FIG. 10).

[0153] The homogeneity of the precursor mixture obtained after pyrolysis is very considerably improved compared to the reference sample. For samples prepared from colloidal suspensions, the morphology of the colloidal precursor is conserved after the pyrolysis step and nanoparticles of UO.sub.2 enveloped in a carbon coating are observed.

[0154] II.5. Thermal Carboreduction Treatment.

[0155] The carboreduction of the precursor is carried out under rough vacuum. In the example that follows, the rate of heating used is 240° C..Math.h.sup.−1, followed by an isothermal stage for 2 hours at 1450° C. The pressure in the furnace is measured using a Pirani gauge, which makes it possible to record with accuracy small pressure variations, notably due to the release of carbon monoxide during carboreduction. A sample obtained by mixing of powders (REF R.sub.C/U=3) has undergone the same thermal treatment in order to demonstrate the gain obtained using a new precursor mixture (R.sub.C/U=3.4) (FIG. 11).

[0156] Under rough vacuum, the lowering of the temperature of start of carboreduction thanks to the use of the novel synthesis route is around 200° C. The difference between the samples is also notable for the end of carboreduction temperature.

[0157] The carboreduction of the precursor mixture according to the invention is complete after 1 hour at 1450° C., whereas this temperature maintained for 2 hours does not seem sufficient to complete the carboreduction of the sample obtained by simple mixing of powders. The temperature necessary for the total carboreduction of the reference sample (REF R.sub.C/U=3) under rough vacuum is 1750° C. (FIG. 4).

[0158] The difference observed between the temperatures of start of carboreduction is explained by the fact that uranium and carbon are intimately mixed at the nanometric scale in the precursor mixture according to the invention. The reaction kinetic increases thanks to the size of the UO.sub.2 domains and their coating in a carbon coating. This reduction in the distances between the reagents accelerates the steps of diffusional transport, which are generally limiting.

[0159] The samples obtained after carboreduction under vacuum have been characterised by powder XRD. The diffractogram of the product obtained by the novel synthesis route after carboreduction is shown in FIG. 12.

[0160] The sample derived from carboreduction under vacuum of the precursor according to the present invention (R.sub.C/U=3.4) is well crystallised. A mixture of phases is identified as containing 59% by weight of UC and 41% by weight of UC.sub.2 after the Rietveld refinement of the diffractogram shown in FIG. 12. Within the precision boundaries of this analysis, the reaction may be considered as total.

[0161] In order to complete the characterisation of the samples after carboreduction, the morphology of the carbides obtained is compared to the conventional route by scanning electron microscopy (FIG. 13).

[0162] The morphology of the compounds obtained is extremely different. The sample REF R.sub.C/U=3 is in the form of grains of several tens of microns, seemingly homogenous (FIG. 13A). The sample obtained by carboreduction of the precursor mixture according to the invention (R.sub.C/U=4.2) is in the form of sub-micrometric grains of carbide (FIG. 13B). The morphology of the carbides depends on the stoichiometry of the samples before carboreduction (ratio R.sub.C/U). In the case of the synthesis route proposed in the invention, the residual presence of carbon makes it possible to conserve the carbide in fine particle form. This property is favourable to the later sintering required for the shaping of the fuel.

[0163] II.6. Shaping and Sintering.

[0164] The carbide powders thus synthesised may then, without any prior step of grinding, be shaped and undergo a conventional sintering cycle.

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