RU2323912C2 - Method to manufacture oxide ceramic products with high thermal conductivity - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: manufacturing of oxide ceramic products implies preparation of an acidic solution containing cations of one metal or several metals, incl. those fissionable, precipitation of metal salt or hydroxide from the solution, heat treatment of the precipitate, modelling of products and their sintering. At the precipitation stage, coarse particles of at least 0.1 mcm in size and nanoparticles of not more than 30 nm in size, fraction of the latter being 0.05-2.0 wt.%, are precipitated. Metal hydroxide is ammonia-precipitated in two steps, рН at the first step being less than metal complete precipitation pH at least by 0.5, while рН at the second stage is 9.5-10.5. Metal salt (oxalate) is precipitated with strong oxalic acid solution in at least 20% excess as regards to stoichiometric value.

EFFECT: products with abnormally high thermal conductivity, high plasticity and thermal stability can be produced.

6 cl, 3 ex, 3 tbl, 7 dwg

Description

The invention relates to a technology for the manufacture of oxide ceramic products and can be used in the nuclear, chemical, hydrometallurgical, electronic and other industries.

A known method of producing ceramic products from magnesium oxide, by which magnesium hydroxide or carbonate is precipitated from a solution, calcined the obtained precipitate in an oven at a temperature of 600 ° C [A.M. Cherepanov, S.G. Tresvyatsky. Highly refractory materials and products from oxides. // M .: "Metallurgy", 1964]. The resulting magnesium oxide (MgO) is subjected to additional calcination at high temperatures, then the products are molded from the obtained powder by compression or slip casting and sintered.

The disadvantages of the method are, firstly, a high probability of obtaining defective products due to the instability of the properties of the powders used, obtained by calcining precipitates at an optimal temperature; secondly, sintered products do not have high thermal conductivity.

A known method of producing a ceramic fuel composition of 15 vol.% UO ₂ +85 vol.% MgO with a thermal conductivity of 5.7 W / m · deg. at a temperature of 1000 ° C (~ 1.5 times higher than the calculated thermal conductivity) [ISKurina, VNLopatinsky, NPYermolayev, NNShevchenko. Research and Development of MgO based matrix fuel. // Proceedings of a Technical Committee meeting held in Moscow, 1-4 October 1996. IAEA-TECDOC-970, 1997, p.169-181].

The disadvantage of this method is the significant content in the ceramic fuel composition of UO ₂ + MgO magnesium oxide (85 vol.%, Or 60 wt.%), As a result of which the properties of this composition, which is used as fuel for nuclear power reactors, are deteriorated.

The closest technical solution is a method for producing oxide ceramics [I.S. Kurina. RF patent for the invention No. 2135429. A method of producing ceramic products (1999)], in which precipitation of metal hydroxides, carbonates or oxalates is obtained by precipitation from a solution. Precipitation is subjected to heat treatment at the optimum temperature at which a morphological change in the shape of the particles occurs. If necessary, the resulting oxide is reduced to a lower oxide. The powder is then molded into articles by compression or slip casting and sintered. The method allows to obtain defect-free products (in appearance, cracks, chips, etc.) with an adjustable stable density.

The disadvantage of this method is the lack of conditions allowing to produce sintered products with increased thermal conductivity, which is very important for ceramic materials when operating in significant temperature gradients, as well as during thermal cycling.

The authors were faced with the task of eliminating this drawback, namely: to find and ensure the basic technological parameters responsible for obtaining a microstructure from sintered ceramic products, due to which the products have increased thermal conductivity.

To solve this problem in a method of manufacturing ceramic products, including the deposition of salt or metal hydroxide from a solution, heat treatment of the precipitate at a temperature not lower than the temperature of the morphological change in the shape of the particles, the molding of products and their sintering, it is proposed to carry out the deposition according to a special mode, which allows to obtain large particles of size not less than 0.1 microns and 0.05-2.0 wt.% nanoparticles not more than 30 nm.

The method of obtaining products from oxide ceramics with high thermal conductivity is as follows.

An acid (nitric acid, hydrochloric acid or sulfate) solution is prepared containing at least one metal cation (aluminum, magnesium, gadolinium, zirconium, uranium, etc.). Precipitation is carried out by the method of simultaneously draining the acid solution and oxalic acid with its excess from stoichiometry of at least 20% or by simultaneously draining the acid solution and ammonia in two stages, the pH in the first stage being lower than the pH of the complete metal deposition by at least 0.5, and the pH in the second stage is 9.5-10.5. During the deposition, large particles with a size of at least 0.1 μm are obtained, as well as nanoparticles with a size of not more than 30 nm, the proportion of which is 0.05-2.0 wt.%. Then the precipitate is filtered and calcined at the optimum temperature. The choice of the optimal temperature is carried out according to [I.S. Kurina. RF patent for the invention No. 2135429. A method of obtaining products from ceramics (1999)]. If necessary, the resulting oxide is reduced to a lower oxide (for example, U ₃ O ₈ to UO ₂ ). Then the powder is molded into articles by pressing or slip casting and sintered in vacuum, in a hydrogen or hydrogen-nitrogen medium.

The obtained effect of increased thermal conductivity in sintered products is based on the fact that the simultaneous presence of large particles and an insignificant fraction of nanoparticles (no more than 30 nm in size) in the precipitate strongly influences the processes occurring inside the crystals during calcination of the precipitate and sintering of the products. When annealing at the optimum temperature of the precipitate with the main fraction of particles with a size of at least 0.1 μm, as well as 0.05-2.0 wt.% Particles with a size of no more than 30 nm, significant stresses arise in the oxide powders created by the new phase. Particles of nanoscale have high surface energy and, accordingly, increased reactivity. As a result, defects and dislocations are formed in the powders, which can lead to an increase in the concentration of vacancies in the crystals of sintered objects. This is confirmed by the method of X-ray photoelectron spectroscopy, with the help of which it was established that the phenomenon of increased thermal conductivity of oxides is explained by the presence of metal cations with different valencies, and therefore the structure of the crystal lattice changes.

The obtained effect of increased thermal conductivity was revealed for all materials studied by the applicants. For all ceramic oxide materials obtained by the developed method, the temperature dependences of thermal conductivity are of the same type: with increasing temperature from 100 to 700-800 ° C, the thermal conductivity decreases, and then with a further increase in temperature above 1000 ° C, the thermal conductivity also increases at a temperature of 900- 1000 ° C becomes several times higher than the known data.

In addition to increased thermal conductivity, ceramic oxide products obtained by the developed method have increased ductility and heat resistance. For all ceramic oxide materials, anisotropy of microhardness is established: the material has a microhardness that matches the known data, and at the same time 2-3 times less. This indirectly indicates an increase in the ductility of the material. The thermal stability of ceramic oxide materials manufactured by the developed method is 2-3 times higher than similar materials manufactured by standard technologies.

As evidence of the practical feasibility of the solution, the following are examples of the implementation of the method for producing products (tablets, sleeves) from oxide ceramic materials.

EXAMPLE 1

An initial nitric acid solution was prepared containing two components, aluminum and magnesium, in the ratio necessary to obtain stoichiometric MgAl ₂ O ₄ spinel. Conducted a simultaneous precipitation of aluminum and magnesium hydroxides with ammonia by the method of simultaneous draining at a pH in the first stage of 9.9 ± 0.1 (the pH of the complete precipitation of magnesium oxide with ammonia corresponds to 10.5 [1]) and the pH of the second stage 10.6 ± 0.1. The filtered precipitate was calcined at temperatures from 700 to 1200 ° C for 4 hours. The optimum temperature was 1000 ° C. The powder calcined at this temperature had a total specific surface area (S _BET ) value of 37-40 m ² / g, and the phase composition was MgAl ₂ O ₄ . The powder particles were clustered conglomerates consisting of loosely sintered individual particles of different sizes (from 20 nm to more than 0.1 μm). Using an electron microscope, it was determined that the number of particles with sizes from 20 to 30 nm was 1.7-2 wt.% (Figure 1). From the obtained powder, tablets were compressed in various versions:

- with preliminary grinding of the powder (using grinding bodies made of spinel obtained by synthesis of MgO and Al ₂ O ₃ powders from a mechanical mixture) and with a plasticizer;

- without grinding and without plasticizer;

- with plasticizer and the addition of 0.5% Y ₂ O ₃ to the crushed powder.

In addition, for comparison, tablets were prepared from spinel powder obtained by synthesis from a mechanical mixture ("MS") of MgO and Al ₂ O ₃ powders. After grinding, the “MS” spinel powder had S _BET = 3.94 m ² / g and a particle size of 0.3-3 μm (FIG. 2).

All tablets were sintered at a temperature of 1800 ° C in a hydrogen medium.

Measurements were made of the thermal conductivity coefficient (λ) of sintered MgAl ₂ O ₄ tablets depending on the temperature in the range from 100 to 1000 ° C (Fig. 3) by the axial heat flux method. The thermal conductivity coefficient of spinel from [2] varies from 17.5 to 6 W / m · deg. The thermal conductivity of the "MS" spinel with a density of 95% theoretical (TP) varies from 15 to 9 W / m · deg (curve 2 in figure 3). The thermal conductivity of the spinel manufactured by the developed method, without grinding and plasticizer, had high values (from 43.0 to 20.8 W / m · deg.), Although the density of the samples was only 89.7% TP. During grinding, spinel powder was milled from grinding media. The thermal conductivity of the spinel immediately decreased (curve 3 in figure 3). With the addition of yttrium oxide with low thermal conductivity, the thermal conductivity of the spinel obtained by the developed method significantly decreased and amounted to 13 to 6 W / m · deg. when the temperature changes, respectively, from 100 to 1000 ° C (curve 5 in figure 3). The introduction of impurities into powders leads to the scattering of phonons and the deterioration of thermal conductivity. For the studied spinel samples obtained by the developed method, the temperature dependence of λ has a special character: up to 700-800 ° C, the λ values decrease and then increase (except for spinel with the addition of Y ₂ O ₃ ).

Measurements were made of the heat resistance of spinel products. To do this, we studied the tensile strength of the obtained sleeves during heating. At a heating rate of 10 ° C / s, the destruction of the spinel and “MS” spinel bushings occurred at an external surface temperature of ~ 550 ° C and ~ 465 ° C, respectively. The values of the breaking stress for the bushings of the modified and "MS" spinel were respectively ~ 21 and ~ 7 kg / mm ² . Thus, the heat resistance of sintered products from modified spinel is about three times higher than for products from “MS” spinel.

Young's modulus (elasticity characteristic), measured by an ultrasonic pulsed method, for samples of spinel made of powder without grinding and plasticizer, amounted to 294-296 GPa. Young's modulus for MgAl ₂ O ₄ according to published data [3] is 220-250 GPa. The increased Young's modulus MgAl ₂ O ₄ , manufactured by the developed method, indicates the increased elasticity of these products.

The temperature coefficient of linear expansion (TEC) of the spinel was measured on a high-temperature dilatometer using the relative method. The tests were carried out in a vacuum of 5 · 10 ^-4 mm Hg at temperatures from 600 to 1300 ° C. The average values from three to four measurements (at each temperature) are shown in table 1 and figure 4. The TECL values of the MS spinel samples monotonically increase from 7.25 · 10 ⁶ deg. ^-1 to 8.8 · 10 ⁶ deg. ^-1 when the temperature increases from 600 to 1300 ° C, they are in satisfactory agreement with the data of [4]. For the studied samples of modified MgAl ₂ O ₄ spinel obtained without preliminary grinding of the powder, without introducing any impurities, the LTEC is practically independent of the temperature in the range from 600 to 1300 ° С, and a slight increase in the LTEC is observed to a temperature of 1000 ° С and then decline again. This effect is very important when thermal cycling products from such spinel. In addition, in [5] a close relationship between the thermal expansion coefficient and thermal conductivity is described: if the thermal expansion coefficient decreases with increasing temperature, then, accordingly, the thermal conductivity increases.

The microstructure of the modified spinel (Fig. 5) with a density of 97.2% TP differs from the "MS" spinel (Fig. 6) with a density of 95% TP. Inside the grains, the pores (size from 0.2 to 2 μm) are located in the form of small clusters (Fig. 5). They are very few, or the pores inside the grains are completely absent. Mostly the pores are located along the grain boundaries. Some pores have a pronounced polyhedral cut. The spinel material is similar to a single crystal: no grain boundaries are observed when the grain boundaries are broken. The porosity in the spinel "MS" (Fig.6) is distributed throughout the volume, more fragile fractures are visible, the grain boundaries at break are visible.

The crystal lattice parameter for the new spinel (without impurities) is 8.064 Å, i.e. below reference data for stoichiometric spinel. For the "MS" spinel, it is 8.081 Å, which coincides with the known data.

EXAMPLE 2

A nitric acid solution containing gadolinium was prepared. The gadolinium oxalate was precipitated with a solution of oxalic acid (with an excess of 20% to stoichiometry) by the method of simultaneous draining. The filtered precipitate was calcined at temperatures from 500 to 800 ° C for 4 hours. The optimum calcination temperature was 600 ° C. The powder calcined at this temperature had an S _{BET of} 6.2 m ² / g, and the phase composition was Gd ₂ O ₃ with a cubic crystal lattice having the parameter a = 10.816 ± 0.002 Å. Using an electron microscope, it was found that the powder was a mixture of randomly broken particles in the form of rectangular or square plates (parallelepipeds). The characteristic overall particle sizes varied within the following limits: length - 5-10 microns; width - 3-5 microns; thickness - 0.01-1 microns. At the same time, from 0.05 to 0.1 wt.% Nanoparticles with a size of from 5 to 6 microns was present in the powder; width - 3 microns; thickness - 10-30 nm.

From the obtained powder (with the addition of a plasticizer), tablets were pressed, which were sintered at 1800 ° C in a hydrogen medium. Sintered tablets (batch No. 1) had a density of 8.02 g / cm ³ (97.6% TP), the phase composition was Gd ₂ O ₃ with a monoclinic crystal lattice having parameters, Å: a = 14.09 ± 0.01 , b = 3.570 ± 0.003, c = 8.760 ± 0.005, β = 100.06 ± 0.05. For comparison, Gd ₂ O ₃ tablets were studied with a calcining temperature of the precipitate of 800 ° C (batch No. 2); their density after sintering at 1800 ° C in a hydrogen medium was 7.96 g / cm ³ (96.8% TP), the phase composition was Gd ₂ O ₃ with a monoclinic crystal lattice having the parameters: a = 14.08 ± 0, 01, b = 3.571 ± 0.003, s = 8.761 ± 0.005, β = 100.05 ± 0.05 Å.

The axial heat flux method was used to measure the thermal conductivity of Gd ₂ O ₃ tablets _of batch No. 1 (3 samples) and batch No. 2 (3 samples). Table 2 shows the averaged experimental values of the thermal conductivity Gd ₂ O ₃ depending on temperature. The thermal conductivity coefficient Gd ₂ O _{3 of} batch No. 1 has high values in the entire studied temperature range: with increasing temperature from 100 to 1000 ° C, λ Gd ₂ O ₃ changes from 29 to 8.9 W / m · deg, respectively. At 1000 ° C, λ Gd ₂ O ₃ for tablets manufactured by the developed technology is more than 4 times higher than the value of λ Gd ₂ O ₃ (2.1 W / m · deg.), Taken from [6], and almost 3 times higher than λ Gd ₂ O ₃ party number 2.

In the study of the microstructure of samples of Gd ₂ O ₃ party No. 1, deformation twins (Fig. 7a) and strips of the type of fault formation (Fig. 7b) were detected. In this case, conditions are created (due to the formation of dislocations) for the local stress concentration necessary for nucleation of twins. It is known that twinning processes play a major role in the hardening of the material. When the indenter was pressed, a microhardness meter did not form cracks in the grains.

On the PMT-3 was determined by Gd ₂ O _3, the microhardness of the samples at a load of 100 g Microhardness beans Gd ₂ O ₃ batch №2 was 667-744 kg / ^mm2. When an indenter of a microhardness meter is pressed in, Gd ₂ O ₃ grains _of batch No. 2 cracked (Figs. 7c, d). The microhardness anisotropy was established on Gd ₂ O ₃ samples obtained by the developed technology (batch No. 1): 346-372 kg / mm ² and 555-693 kg / mm ² (10 measurements in total). A decrease in microhardness indirectly indicates an increase in the ductility of the material.

In the microstructure of the samples of Gd ₂ O _3, batch No. 2, the grains are predominantly smooth, with no visible substructure (Figs. 7c, d), deformation twins and bands of the type of fault formation were not found. In addition, cracks appeared in some grains during indentation of the microhardness tester (Fig. 7d), which indicates the fragility of the material.

Young's modulus, measured by an ultrasonic pulsed method, for samples of batches No. 1 and No. 2 was 135 and 109 GPa, respectively. Young's modulus for Gd ₂ O ₃ according to [6] is from 120 to 124 GPa. The increased Young's modulus of the products Gd ₂ O ₃ manufactured by the developed method indicates an increased elasticity of these products.

EXAMPLE 3

Three nitric acid solutions were prepared:

- The first solution contained magnesium cations;

- the second solution contained magnesium cations, as well as catalyst cations - tin (Sn ²⁺ ) in an amount of 0.5 wt.%;

- the 3rd solution contained magnesium cations, as well as catalyst cations - aluminum (Al ³⁺ ) in an amount of 0.5 wt.%.

Magnesium hydroxide was precipitated from the first solution, magnesium and tin hydroxides from the second solution, and magnesium and aluminum hydroxides from the third solution. Precipitation was carried out by ammonia by the method of simultaneous draining of nitric acid solutions and ammonia at a pH of 9.9 ± 0.1 in the first stage (the pH of complete precipitation of magnesium oxide with ammonia corresponds to 10.5 [1]) and the pH of the second stage was 10.6 ± 0.1. The filtered precipitates were calcined at an optimum temperature for magnesium oxide, namely, at 900 ° C for 4 hours. The powders calcined at this temperature had a total specific surface area (S _BET ) of 5.0 m ² / g, phase composition of powder No. 1 — MgO powder No. 2 - MgO + SnO ₂ and powder No. 3 - MgO + MgAl ₂ O ₄ . After calcination, the MgO powder particles underwent morphological changes and took the form of plates and flakes of irregular shape with rounded edges, the longitudinal size of which varied from 2 to 10 μm, and the transverse size from 2 to 5 μm with a thickness of about 0.2 μm. At the same time, from powders No. 1, No. 2 and No. 3 were present from 0.5 to 1.0 wt.% Nanoparticles in the form of flakes 5-30 nm thick. In addition, powder No. 2 is not more than 0.5 wt.% SnO ₂ particles _of indefinite shape with a size of less than 10 nm.

From the obtained powders (with the addition of a plasticizer), tablets were compressed, which were sintered at 1800 ° С, and tablets No. 1 (MgO) and No. 2 (MgO + SnO ₂ ) were sintered in a hydrogen medium, and tablets No. 3 (MgO + MgAl ₂ O ₄ ) - in a vacuum. Sintered tablets No. 1 (MgO), No. 2 (MgO + SnO ₂ ) and No. 3 (MgO + MgAl ₂ O ₄ ) respectively had a density of 3.52 g / cm ³ (97.8% TP), 3.50 g / cm ³ (97.2% TP) and 3.35 g / cm ³ (93.1% TP).

The axial heat flux method was used to measure the thermal conductivity of tablets No. 1 (MgO), No. 2 (MgO + SnO ₂ ) and No. 3 (MgO + MgAl ₂ O ₄ ). Table 3 shows the average experimental values of the thermal conductivity of products (recalculated for a density of 95% TP) depending on temperature. The thermal conductivity coefficients of MgO batches No. 2 and No. 3 with the addition of catalysts SnO ₂ and Al ₂ O _3, respectively, have increased values in the entire studied temperature range. At 800 ° С, λ for MgO + SnO ₂ and MgO + MgAl ₂ O ₄ pellets manufactured using the developed technology is approximately 2 times higher than the value of Mg MgO (8.1 W / m · deg.), Taken from [7] . Also, at 800 ° С, λ for MgO pellets made according to the developed technology is approximately 1.5 times higher than the λ MgO value taken from [7].

Similar phenomenal phenomena were observed on many oxide materials manufactured by the developed technology. Examples can be given using materials such as UO ₂ , (U, Th) O ₂ , PuO ₂ + MgO, PuO ₂ + Fe + MgO, BaPuO ₃ , (Pu, Zr, Y) O _2-x , (Pu, Th ) O ₂ and others.

Using the invention will allow to obtain products with abnormally high thermal conductivity, as well as with increased ductility and heat resistance. The technology is simple to implement, does not require significant investment, can be introduced into the production of ceramic products with a wet manufacturing process scheme and standard equipment. Ceramic oxide materials with high thermal conductivity and ductility can be used under conditions of high mechanical stresses, high temperature gradients and in thermal cycling modes, as well as in electronics and electrical engineering. An increase in the thermal conductivity of ceramic products leads to a decrease in the temperature difference from the center to the periphery of the products and, consequently, to a decrease in mechanical stresses. This will increase the term of use of the product, which, in particular, will lead to an improvement in the ecological situation of the environment due to a decrease in the volume of waste, as well as to an increase in the efficiency of the production of products.

The use of the invention in nuclear energy will significantly improve the safety of reactors and will give a significant economic effect.

Information sources

1. V.N. Tikhonov. Analytical chemistry of magnesium // M .: Publishing house "Science", 1973, p.36.

2. V. Espe. Technology of electrovacuum materials // M .: "Energy", 1968.

3. V.L. Balkevich. Technical ceramics // M .: Publishing house of building literature, 1968, p. 178.

4. A.R. Andrievsky, I.I. Spivak. Strength of refractory compounds and materials based on them. Reference // Chelyabinsk: Metallurgy, 1989.

5. A.A. Vorobyov. Mechanical and thermal properties of alkali halide single crystals // M .: Higher School, 1968, p. 187.

6. Physico-chemical properties of oxides. Handbook edited by G.V. Samsonov // M.: Metallurgy, 1978.

7. B.C. Chirkin. Thermophysical properties of materials of nuclear engineering // M. Atomizdat, 1968.

Table 1 The temperature coefficient of linear expansion of the spinel MgAl ₂ O ₄ depending on temperature Spinel Manufacturing Option Temperature ° C LTEC, α · 10 ⁶ , deg. ^-one The method of "MS" (curve 1 in figure 3) 600 7.25 800 8.1 1000 8.6 1200 8.6 1300 8.8 The developed method, the press without grinding and plasticizer (curve 2 in figure 3) 600 8.2 800 8.5 1000 8.7 1200 8.1 1300 8.2 table 2 Temperature dependence of thermal conductivity coefficients of products Gd ₂ O ₃ with 95% TP Calcination temperature Gd ₂ О ₃ , ° С λ Gd ₂ O ₃ , W / m · deg, depending on temperature, ° С: one hundred 200 300 400 500 600 700 800 900 1000 600 29.0 21.0 15.0 11,4 9,4 8.4 8.1 8.1 8.4 8.9 800 13,4 10.0 7.3 6.0 4.7 4.2 3.8 3,5 3.3 3.2 From the work [6] - - - - - - - - - 2.1 Table 3 The temperature dependence of the thermal conductivity of products with 95% TP: MgO, MgO + 0.5 wt.% SnO ₂ and MgO + 0.5 wt.% Al ₂ O ₃ Structure λ, W / m · deg, depending on temperature, ° С: one hundred 200 300 400 500 600 700 800 MgO No. 1 32.6 23.6 19.1 16.7 14.9 13.9 13,2 12.8 MgO + SnO ₂ No. 2 43,4 34,4 27,2 22.7 twenty 18.3 17,2 16 MgO + Al ₂ O ₃ No. 3 42,2 32.8 25.7 21.1 18,4 16.5 15.7 15.1 From the work [7] 38,2 29.5 22.5 17 13,4 10.6 8.9 8.1

Claims (6)

1. A method of manufacturing products from oxide ceramics with high thermal conductivity, including the operation of preparing an acid solution containing at least one metal cation, including fissile, precipitation of a salt or hydroxide of at least one metal from a solution, heat treatment of the precipitate at a temperature not lower than temperature morphological changes in the shape of the particles of the precipitate, the molding of products and their sintering, characterized in that the metal hydroxide is precipitated with ammonia in two stages, and the pH in the first stage is lower than pH precipitation of the metal by no less than 0.5, and the pH in the second stage is 9.5-10.5, the salt in the form of metal oxalate is precipitated with a concentrated solution of oxalic acid with an excess of stoichiometry of at least 20%, and in the sediment they provide the formation of large particles with a size of at least 0.1 microns and 0.05-2.0 wt.% nanoparticles with a size of not more than 30 nm. 2. The method according to claim 1, characterized in that the acid solution is prepared using nitric, hydrochloric or sulfuric acids. 3. The method according to claim 1, characterized in that the precipitation of the salt or hydroxide is carried out by simultaneously draining the acid solution and the precipitating solution. 4. The method according to claim 1, characterized in that the precipitation of the salt is carried out in the presence of at least one cation of a catalyst, for example tin (Sn ²⁺ ), titanium (Ti ⁴⁺ ), aluminum (Al ³⁺ ). 5. The method according to claim 4, characterized in that the amount of cation of the catalyst is from 0.05 to 1 wt.% To the mass of salt. 6. The method according to claim 1, characterized in that the sintering is carried out at a temperature of from 1400 to 2200 ° C in vacuum in a hydrogen or hydrogen-nitrogen medium.

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