Method of producing amorphous carbon-coated nanoparticles and method of producing transition metal carbide in nanocrystallite form. RU patent 2485052.

FIELD: chemistry.

SUBSTANCE: invention relates to a method of producing nanoparticles of a transition metal oxide coated with amorphous carbon. A liquid mixture containing as precursors at least one alkoxide of a transition metal selected from Ti, Zr, Hf, V, Nb and Ta, an alcohol and excess acetic acid with respect to the transition metal is diluted with water to obtain an aqueous solution. Precursors are contained in the solution in a molar ratio which is sufficient to prevent or significantly limit sol formation. The transition metal, carbon and oxygen are contained in said solution in a stoichiometric ratio which corresponds to the composition of nanoparticles. The aqueous solution is freeze-dried and the freeze-dried product undergoes pyrolysis in a vacuum or an inert atmosphere to obtain nanoparticles. The obtained nanoparticles can be subjected to carbothermal reduction to obtain carbide nanoparticles.

EFFECT: obtaining nanoparticles with a high degree of purity and smaller average size.

15 cl, 4 ex

The technical field to which the invention relates.

The present invention relates to the field of nanomaterials on the basis of transition metals that can be included in the composition of the elements for nuclear reactors. First of all, the present invention relates to a method for producing coated with carbon nanoparticles of at least one oxide of the transition metal.

The level of technology

Carbides of transition metals are materials suitable for certain elements for nuclear reactors of the next generation (primarily the so-called fourth generation reactors) because of its extremely high heat-resistant properties, high thermal conductivity, low absorption of neutrons, low absorption cross-section and high resistance to radiation.

However, they are characterized by fragility, which limits the use in these industries.

Described attempts to reduce the specified fragility due to the decrease of the crystallite size of such carbides to medium size, usually in the range from several nanometers to several hundred nanometers, while received nanocrystallites.

These nanocrystallites can be obtained by a carbothermic recovery of particles of oxides of transition metals coated with amorphous carbon, with srednerazmernogo in the range from several nanometers to several hundred nanometers (in the present description - "nanoparticles of oxides"). The specified response and recovery should be possible to complete that the nanocrystallites of carbides of transition metals contained the minimum amount of impurities.

To this end carbothermic recovery is usually carried out at high temperature, which should be even higher, if the crystallites in nanoparticles of oxides are characterized by a higher average size and/or source material contains more impurities.

However, although the use of very high temperatures allows to obtain crystals of carbides of transition metals of sufficient purity, the disadvantage of this method is the significant increase in the average size of the particles or the formation of coarse powder.

Consequently, often at the last stage you want to grind the resulting powder to decrease the average size of the crystallites contained in the powder. This grinding should be carried out in an inert atmosphere (usually in a protective camera with gloves) to prevent oxidation of the carbides. However, the disadvantage of this method lies in making the dirt together with the material for grinding and in the best case, in obtaining crystals of carbides of transition metals with an average size of about 1 micron.

For Carbo is termicheskogo recovery at lower temperatures compared to the standard terms and conditions article Dolle and others, Journal of the European Ceramic Society, v.27, No.4, SS-2067 (2007) described a new approach to the synthesis of nanoparticles of zirconium oxide. In the first stage of the specified synthesis of conducting the reaction of the Sol-gel, in which the sucrose is dissolved in acetic acid, and then add n-propoxide Zirconia, you get a viscous gel. After drying and pyrolysis of the resulting gel receive oxide nanoparticles with an average particle size of 15 nm, which aggregate and form agglomerates with an average size of from 2 to 3 μm. After carbothermic reduction of these oxide particles at 1400°C have the nanocrystallites carbide of zirconium, which, although they have relatively small average size (about 93 nm), but contain impurities.

As shown below, we can assume that these impurities include first of all the free carbon, dissolved oxygen and oxycarbide.

To increase the purity of these nanocrystallites carbide of zirconium, in turn, is heated at a high temperature of 1600°C, which leads to undesirable consequences, namely the increase in the average particle size of 150 nm, and still unable to completely remove impurities.

Disclosure of inventions

One object of the present invention proposes a method of producing nanoparticles of oxides with the smallest possible average size, with the Le holding the carbothermic reduction at moderate temperatures such nanoparticles are formed nanocrystallites of carbides of transition metals higher degree of purity and/or nanoparticles are formed with a smaller the average size compared to nanoparticles, obtained by the most efficient standard method, primarily by way of a Sol-gel.

Therefore, the object of the present invention proposes a method of producing nanoparticles of at least one oxide of a transition metal selected from the group comprising Ti, Zr, Hf, V, Nb and TA, covered with amorphous carbon, and the method includes the following successive stages:

(1) obtaining a liquid mixture containing as precursors of at least one transition metal alkoxide, alcohol, acetic acid in excess relative to the transition metal, and then diluting the mixture with water, you get water solution, and the predecessors are contained in the aqueous solution in a molar ratio that prevents or adequately restrict the formation of Zola and liofilizirovanny aqueous solution, and, in addition, a transition metal, carbon, and oxygen must be contained in a stoichiometric ratio, as nanoparticles,

(2) freeze-drying the resulting aqueous solution,

(3) pyrolysis of the resulting lyophilisate obtained at the previous stage, in a vacuum or in an inert atmosphere, thus receive nanoparticles.

According to the present invention, the nanoparticles of transition metal oxides "covered" and artnum carbon so their surfaces are partially or completely covered with amorphous carbon. Carbon, in turn, referred to as "amorphous", as it mostly is in the form of crystallites, however, in some parts of its structure may be a short-range order in the arrangement of the atoms.

Preferably freeze-drying involves spraying an aqueous solution in liquid nitrogen, you get a frozen particles, characterized by a homogeneous composition of a specified solution, and then the particles are dried in vacuum to remove water by sublimation, while the powder can be received and after further drying receive a lyophilisate. According to the present invention, the term "homogeneous composition" means a composition containing particles of the same or almost the same micrometer size, preferably of nanometric size.

Spraying is carried out with the use of different nozzles, such as nozzle with a nozzle or an ultrasonic atomizer.

For nanoparticles of oxides, characterized by the highest possible degree of purity, preferably lyophilized should not contain any other elements, in addition to the transition metal, carbon, hydrogen or oxygen. To this end, the alkoxides are preferably chosen from the group including isopropoxide and n-propoxide.

The alkoxides, the content is the following various transition metals, you can also mix and obtain nanoparticles containing a mixture of the corresponding oxides, for example a mixture of oxides of Ti and Zr.

Alcohol is used primarily as a solvent for the alkoxide. Alcohol is selected from the group comprising isopropanol or 2-propanol and 1-propanol, since these alcohols contain hydrocarbon radicals belonging to the same class as the radicals of the above-mentioned preferred alkoxides.

Acetic acid, in turn, is a chemical modifying agent, providing for the substitution of alkoxygroup in the alkoxide Matala on the acetate groups. Thus preferably receive the modified alkoxide, which is compared with the original alkoxide is characterized by low reactivity with water, thus, is prevented or limited by spontaneous condensation reaction (the reaction of the Sol-gel) alkoxide, which can lead to the formation of sludge. If this reaction occurs with a limited speed, you can begin the education Zola due to the initiation of the reaction, Sol-gel and Sol obtained according to the present invention contains oligomers and/or colloidal particles suspended in water. Moreover, in order to prevent or limit the formation of Zola, which may lead primarily to education is too viscous aqueous solution, that doesn't freeze drying, and/or to the formation of an aqueous solution with a low degree of homogeneity, acetic acid, which, in addition, can be used to reduce the viscosity of the solution, is added in excess in relation to the alkoxide and alcohol. For the specialist in this area seems clear, for example, that an aqueous solution of the present invention, satisfying the specified criteria, represents a clear solution. This property is one of the most important features of the process of the present invention, since the fact that the precursors of oxides nanoparticles (namely, the alkoxides of transition metals, acetic acid, alcohol, and optionally carbon compound) are, for example, in the form of a clear aqueous solution, provides a homogeneous distribution of these predecessors at the molecular level and, therefore, contributes to the formation of a homogeneous composition of nanoparticles of oxides.

As a rule, it is convenient to use aqueous solutions with the lowest possible concentration, while under other equal conditions, a decrease in the solution concentration leads to a decrease in the average size of nanoparticles of oxides obtained according to the method of the present invention.

Theoretically, although the lower limit of the concentration of the solution and there is, preferably, PR is continued only for economic reasons, do not use too low a concentration of the solution to reduce the cost of implementing method.

Therefore, preferably the concentration of the transition metal in the aqueous solution is 0.1 mol/l or less, more preferably is in the range from 0.001 to 0.1 mol/l, more preferably in the range from 0.01 to 0.1 mol/L. These concentrations primarily preferred to prevent or limit any aggregation of particles present in the form of Zola. In addition, aqueous solutions with the indicated concentration can be liofilizirovanny on standard setting for freeze-drying, as the triple point solution is almost not different from the value for pure water.

Moreover, to decrease the average size of the oxide nanoparticles to the maximum possible value you can get frozen particles formed during lyophilization, which are characterized by an average size of from 0.1 μm to 10 μm, preferably less than 2 μm, even more preferably from 0.5 μm to 1 μm.

In the present description, the term "average size" means the average diameter of the analyzed particles (oxide nanoparticles, nanocrystallites transition metal carbides and the like), if the particles are mainly characterized by a spherical shape or an average value of the main dimensions of the particles are not spheres is practical form.

The above objective is achieved preferably by spraying an aqueous solution in liquid nitrogen contained in a Dewar vessel, and/or when sprayed through a nozzle with a calibrated output hole, such as hole size of 0.51 mm, through which the aqueous solution is fed under pressure from 0.03 to 0.4 MPa, preferably 0.3 MPa, usually in the atmosphere carrier gas, for example compressed air or neutral industrial gas, preferably filtered, for example argon or nitrogen.

In a preferred embodiment of the present invention, the flow of the aqueous solution is twisted into the spray nozzle through the corrugated conical nozzle. In the specified conical nozzle due to the centrifugal effect of the aqueous solution hits the inner wall of the nozzle before the liquor enters the hole. The result is usually formed of a liquid jet in the form of a hollow cone with a turbulent effect.

Freeze drying can be carried out in any suitable installation. At this stage, the preferred conditions are not critical, but preferably the particles should be maintained in a frozen condition until removal of water first to avoid the agglomeration of particles.

It is most preferable at this stage to provide a virtually complete removal of water prid is just, to avoid the formation of pores in the oxide nanoparticles during pyrolysis of lyophilisate. To this end lyophilic drying is preferably carried out at a temperature from -200°C to +50°C, more preferably from -20°C to +30°C, and at pressures of from 0.1 PA to 100 PA, most preferably at 10 PA or less. Thus, to improve the efficiency and speed lyophilization drying is carried out at a temperature of approximately -20°C and a pressure of approximately 0.1 PA.

Stage lyophilization, moreover, preferably includes a step of removal of adsorbed water, which is contained in the lyophilized, and the pressure is preferably 0.1 PA and the stage of increasing the temperature is preferably in the range from 30°C to 100°C, more preferably at 30°C.

The lyophilisate obtained from the aqueous solution contains precursors that have the following characteristics:

the lyophilisate is characterized by a homogeneous composition throughout the volume primarily due to the fact that in the process of freeze-drying removes water in the absence of a concentration gradient in the solution,

the lyophilisate is formed in the form of finely ground material, which increases its reactivity, for example, by heat treatment, moreover, the advantage is that it can be processed in an open atmosphere and therefore clicks the zoom to get the oxide nanoparticles with a smaller average size.

Thus, the average crystallite size of the oxide of the transition metal (which is regarded as equivalent to the average size of the oxide nanoparticles) is usually from 10 to 100 nm, preferably from 10 to 50 nm, even more preferably from 10 to 20 nm.

Characteristics of the lyophilisate allow the pyrolysis and the result is the formation of the oxide nanoparticles, which can be subjected to a carbothermic recovery to almost full completion of the reaction, while having nanocrystallites of carbides of transition metals with reduced average size and high degree of purity, it does not require processing at high temperatures.

Moreover, it is important to note that phase pyrolysis of lyophilisate hold 1) in vacuum or in an inert atmosphere to prevent the formation of side products such as oxycarbide, 2) when the temperature at which the crystallization of the oxide nanoparticles, and at the same time at this stage of obtaining nanoparticles of oxide during carbothermic reduction do not form undesirable carbide nanocrystallites. This temperature is usually in the range from 400°C to 900°C, preferably from 400°C to 600°C, most preferably from 400°C to 450°C.

The present invention also relates to the application of the method for producing nanoparticles of oxides is La obtain carbides of transition metals in the form of nanocrystallites when conducting carbothermic reduction of the nanoparticles in the next stage or simultaneously with the specified process.

Carbothermic recovery can be performed in the process of obtaining nanoparticles of oxides, i.e., the lyophilisate is subjected to only one stage of heat treatment, including pyrolysis (for the formation of nanoparticles of oxides), and directly following it carbothermic recovery. The lyophilisate can first be subjected to heat treatment in an inert atmosphere, for example, at the stage of pyrolysis, and the thus obtained oxide nanoparticles then subjected to a second stage heat treatment, for example, at the stage of carbothermic reduction.

Carbon, oxygen and transitional elements necessary for the formation of the oxide nanoparticles, preferably added only in the form of alkoxide, acetic acid and alcohol. The number of components calculated in advance with respect to chemical formula predecessors and/or carbon and oxygen, which is defined according to thermogravimetric analysis (TGA) predecessors or oxide nanoparticles.

However, in the preferred embodiment, the carbon and/or oxygen can be added in the form of additional connections, for example in the form of a precursor comprising at least one carbon-containing compound that is added in aqueous solution. The specified connection is chemically inert in aqueous solution in which the compared alkoxide and above all IT does not contain group(s), which can cause hydrolysis of the alkoxides, and the above compounds selected from the group comprising cellulose derivatives that meet the above criteria, for example, preferably methylcellulose.

The method according to the present invention is quite versatile, since it allows to obtain the oxide nanoparticles in a wide range of molar relationship amorphous carbon/oxide of the transition metal, thus, it is possible to obtain the oxide nanoparticles, in which the oxide of the transition metal is coated with amorphous carbon and the ratio of the coating to the oxide is a wide range, preferably from 1 to 4, more preferably from 2 to 3.

In a preferred embodiment, the mixture was added an excess of acetic acid and the molar ratio between the amounts of acetic acid, alcohol and alkoxide is in the range from 20:6:1 to 3:1:1, more preferably is 16:4:1. In addition, it was found that the specified molar ratio allows you to limit the increase in viscosity after adding, if necessary, carbon compounds according to the present invention.

Preferably the pH of an aqueous solution of the present invention is from 3 to 10, more preferably from 3 to 5, it is possible to exclude a significant decrease in the freezing temperature of the specified solution, which allows you to accelerate the th lyophilization.

The implementation of the invention

Other objects, features and advantages of the present invention are illustrated in the following description, which does not restrict the scope of the present invention and shown for illustration purposes only.

In the examples described method of producing nanoparticles of dioxins various transition metals according to the present invention containing different amounts of coverage and application of these nanoparticles to obtain the corresponding carbides.

Example 1. Obtaining nanoparticles of titanium dioxide (TiO₂with the floor, the molar ratio of carbon/TiO₂which is approximately 3

of 2.27 ml isopropoxide titanium (IsopTi), which corresponds 0,59 g TiO₂added 2.27 ml isopropanol (2-propanol) and for 6.81 ml of glacial acetic acid (100%), while the molar ratio of acid/alcohol/isopropoxide was 16:4:1, and the volumetric ratio of acid/alcohol/isopropoxide was 3:1:1. In the obtained liquid mixture was added 200 ml of an aqueous solution, which was pre-dissolved 1,0790 g of methylcellulose (MC), which corresponds to 0,1802 g of carbon (according to preliminary thermogravimetric analysis).

It was obtained a clear solution, which indicated the lack of education Zola and the homogeneity of the solution. Then the solution was diluted with water and received 600 ml of water is of astora, the concentration of Ti in which amounted to 0.03 mol/L.

The resulting aqueous solution was sprayed (used sprayer Spraying Systems company Emani Co., the diameter of the nozzle 0.51 mm) and received droplets with an average size of 1 μm, which when injected into liquid nitrogen to form frozen particles.

These particles were placed in a unit for freeze drying (Alpha 2-4 Christ LSC) at the temperature of liquid nitrogen. The pressure in the freeze drying was reduced to 0.1 PA and was supported by the specified low pressure and temperature -20°C for 48 hours Then the space in the installation was heated at +30°C for 3 h under a pressure of 0.1 PA.

In the drying process at the specified low pressure and a temperature of 20°C for 48 h and +30°C for 3 h, water is removed by sublimation, and then desorption, with the specified processing received 16 g of the particles in the form of a dry powder.

Dry powder (lyophilized)obtained in the previous phase was poured into the graphite boat and were subjected to pyrolysis in a tubular furnace, made of aluminum (Adamel) in an argon flow U (Arcal, flow rate 1.2 l/min) with increasing temperature at a speed of 5°C./min until reaching 450°C, then the specified temperature is maintained for 0.1 h and reduced at a speed of 5°C/min up to CT. After completion of the pyrolysis got black powder. Data analysis the obtained powder is Ecodom x-ray diffraction and scanning electron microscopy suggests, what it consists of nanoparticles of titanium dioxide TiO₂the tetragonal structure (anatase) in the form of nanocrystallites with an average size of 16 nm. The presence of carbon coatings on these nanoparticles was estimated using TGA in air. By using these ratios IsopTi, alcohol, acetic acid and MC molar ratio of carbon/TiO₂was 3.04 from, provided that in the ideal case, the reaction is completed recovery dioxide molar ratio of carbon/TiO₂should be 3 in accordance with the following equation carbothermic reduction:

TiO₂(TV.)+3C(TV.)→TiC(TV.)+2SD(gas)

Example 2. Getting nanocrystallites titanium carbide TiC

Obtained at the stage of freeze drying the freeze-dried, placed in a graphite boat, which was subjected to heat treatment in the above aluminum tube furnace (Adamel) in an argon flow temperature increase at 5°C/min up to 1300°C, the specified temperature is maintained for 2 h, and then the temperature was lowered at a speed of 5°C/min up to CT. In this case carbothermic restoration carried out simultaneously with the pyrolysis, thus obtained oxide nanoparticles, thus the lyophilisate was subjected to heat treatment only on one stage. To receive particles of titanium carbide TiC nanometer size Gras is centrirovannomu cubic structure of the crystal lattice, the lattice parameter was 4,326 Å (almost equal to theoretical value, which is 4,327 Å), the average crystallite size according to the methods of x-ray diffraction and scanning electron microscopy was 65 nm.

Thermogravimetric analysis allowed to determine the stoichiometric composition of the TiC and it was found that the residual oxygen content is less than 1 wt.% and that the compound contains an excess of carbon (weight gain on 13,40% compared with theoretical value 33,40%). Given this excess carbon, which is an impurity, it is possible to change the amount of carbon which is added in the form of methylcellulose, in repeated experiments to reduce or even exclude the carbon content of the carbide of the transition metal.

Example 3. Obtaining nanoparticles of titanium dioxide (TiO₂with the floor, the molar ratio of carbon/TiO₂which is 0.05

Nanoparticles of titanium dioxide (TiO₂with coating, in which the molar ratio of carbon/TiO₂0.05, obtained similarly as described in example 1, but changed the number of added elemental carbon. The obtained nanoparticles primarily can be used as a component material for electrodes in lithium batteries. The degree of coverage of such particles Oba is also determined by the molar ratio of carbon/TiO ₂that is from 0.01 to 0.06, preferably from 0.02 to 0.05.

Example 4. Obtaining nanoparticles of zirconium dioxide (ZrO₂) or hafnium dioxide (HfO₂) coated and carbides of zirconium and hafnium

Nanoparticles of zirconium dioxide (ZrO₂) and hafnium dioxide (HfO₂)coated with amorphous carbon was obtained in the same way as described in the previous examples.

A similar technique was used for the carbothermic reduction: the lyophilisate obtained in the same way as described in the previous examples was subjected to heat treatment at 1400°C for 3 h and 5 h, respectively, to receive the crystallites ZrC and HfC, the average size of which was 40 and 30 nm, respectively.

The conditions in which you can get the carbides of transition metals of nanoparticles of oxides, which are formed at approximately 450°C. while increasing the temperature at which there is a carbothermic recovery may vary slightly depending on the transition metal. These conditions typically include a temperature rising speed of 5°C./min to 10°C/min, preferably 5°C./min, to a temperature in the range from 1000°C to 1600°C, preferably from 1300°C to 1400°C, the specified temperature is maintained for 2-6 hours, preferably within 2 hours for a TiC, 3 h for ZrC and 5 h for HfC.

JV is the expert in the art can find conditions when conducting repeated experiments, to ensure the full completion of the carbothermic reduction reaction and to obtain particles of nanocrystallites of the present invention, the smallest average size, which can be in the range from 30 to 100 nm, preferably from 30 to 70 nm, even more preferably from 30 to 40 nm. Moreover, carbothermic carried out restoration in the stream of carrier gas, for example argon, more preferably argon U or Arcal. The above examples relate to obtaining nanoparticles of oxides and carbides, including titanium, zirconium and hafnium. Specialist in the art may find other transition metals such as vanadium, niobium and tantalum.

According to the present description should be understood that the method according to the present invention allows to obtain nanoparticles of oxides with reduced average size, which in turn allows after the carbothermic reduction at moderate temperatures to get the nanocrystallites of carbides of transition metals higher degree of purity and/or with reduced average size compared to nanoparticles, which receive the standard method the Sol-gel. This method, moreover, is characterized by simplicity when scaling process and allows a simple way to obtain an oxide nanoparticles to the e are characterized by a wide interval of the ratios of the coating of amorphous carbon/oxide.

1. The method of obtaining covered with amorphous carbon nanoparticles of at least one oxide of a transition metal chosen from the group comprising Ti, Zr, Hf, V, Nb and TA, including first obtaining a liquid mixture containing as precursors of at least one alkoxide of the mentioned transition metal, alcohol and excess acetic acid in relation to the specified transition metal, then dilute the mixture with water to obtain an aqueous solution in which the precursor is contained in a molar ratio sufficient to prevent or substantially limit the formation of Zola, and obtain thereby the possibility of liofilizirovanny specified aqueous solution and in which the transition metal, carbon, and oxygen contained in the stoichiometric ratio corresponding to the composition of these nanoparticles, and subsequent lyophilization of the specified aqueous solution and then the pyrolysis of the lyophilisate obtained at the previous stage, in a vacuum or in an inert atmosphere with obtaining these nanoparticles.

2. The method according to claim 1, in which the specified molar ratio change when added to an aqueous solution of at least one carbon compounds, chemically inert in relation to the alkoxide.

3. The method according to claim 2, in which the specified hydrocarbon compound selected from the derivatives of adnych cellulose, such as methylcellulose.

4. The method according to claim 1, wherein said alkoxide is selected among isopropoxide and n-propoxide.

5. The method according to claim 1, wherein said alcohol is selected among 1-propanolol and 2-propanolol.

6. The method according to claim 1, wherein the concentration of transition metal in the aqueous solution is at most 0.1 mol/L.

7. The method according to claim 1, in which the molar ratio between the amounts of acetic acid, alcohol and alkoxide take in the range of 20:6:1 to 3:1:1.

8. The method according to claim 1, in which the specified lyophilization is carried out at temperatures from -200°C to 50°C and at a pressure ranging from 0.1 PA to 100 PA.

9. The method according to claim 8, in which the specified lyophilization involves removal of adsorbed water from freeze-dried at a pressure of freeze-drying and the temperature increase to values in the range from 30°C to 100°C.

10. The method of claim 1, wherein the pyrolysis of the lyophilisate is carried out at a temperature in the range from 400°C to 900°C.

11. The method according to claim 1, in which the average size of these nanoparticles is from 10 to 100 nm.

12. A method of producing a carbide of the transition metal in the form of nanocrystallites, including carbothermic recovery of the nanoparticles obtained by the method according to any one of claims 1 to 11.

13. The method according to item 12, which receive the nanocrystallites with an average size in the range from 30 to 100 nm.

14. The method according to item 12, in which a carbothermic recovery on the denotes the temperature rise speed of 5°C./min to 10°C/min to a temperature in the range from 1000°C to 1600°C and maintaining this temperature for from 2 to 6 o'clock

15. The method of item 12, in which a carbothermic restoration carried out in the atmosphere carrier gas containing argon.