RU2467983C1 - Method of producing nanocrystalline powder and ceramic materials based on mixed oxides of rare-earth elements and subgroup ivb metals - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to production of nanocrystalline powder of mixed oxides of rare-earth elements and subgroup IVB metals and can be used in producing neutron-absorbing and heat-insulating materials and solid electrolytes for high-temperature solid oxide fuel cells. The disclosed method involves preparation of a mixed hydroxide via coprecipitation of salts, filtering and washing the obtained precipitate, drying followed by calcination until a mixed oxide is obtained, grinding said oxide, pressing and annealing the obtained compact product. The step for calcining the mixed hydroxide is carried out in the temperature range of 800-1200°C, and the powder of mixed oxides is ground through mechanical activation in a planetary mill for 18-36 minutes. The rare-earth element used is dysprosium and the subgroup IVB metals are titanium, zirconium and hafnium.

EFFECT: increase in density of ceramic tablets obtained from nanocrystalline powder to 7.5 g/cm³ or higher.

3 cl, 13 ex, 2 tbl

Description

The present invention relates to the technology of inorganic materials, in particular to methods for producing nanocrystalline powders and ceramics based on substances with the general formula Ln _{2 + x} M _2-x O _{7-x / 2} , where Ln is a rare-earth element (REE), M is a metal subgroup IVB (Ti, Zr, Hf), and can be used for the manufacture of neutron-absorbing [1] and heat-insulating [2] materials, solid electrolytes for high-temperature solid oxide fuel cells [3].

A known method of producing a polycrystalline neutron-absorbing material based on dysprosium hafnate [4], in which dysprosium oxide (65-85 wt.%) Is mixed with hafnium dioxide and then the resulting mixture in the form of a compacted sample is sintered in the temperature range 1500-2000 ° C. In connection with the use of the solid-phase synthesis process, the disadvantage of this method is the multiphase nature of the obtained material due to the possibility of the presence of residues of unreacted starting materials (mainly dysprosium oxide) in it.

There is also a method of producing a neutron absorber based on dysprosium hafnate [5], according to which a mixture of dysprosium oxides (12-85 wt.%), Hafnium (0.5-87 wt.%) And niobium (0.5-20 wt.% ) are melted by high-frequency induction melting in a cold crucible. The rapid cooling of the melt, after the synthesis is completed, leads to the production of a single-phase dysprosium hafnate having a face-centered cubic (fcc) fluorite structure. The disadvantage of this method is the high synthesis temperature (over 2300 ° C), which leads to an increase in operating costs due to the use of a special complex of equipment (Crystal-401 installation) and an additional reagent (niobium oxide, which positively affects the resulting crystal structure), as well as the limited operational capabilities of the resulting dysprosium hafnate, namely its use only in powder form. This is because it is impossible to obtain strong and dense products (tablets) from a material with a cubic structure such as fluorite, which was synthesized in the melt at a temperature significantly higher than the sintering temperature of the tablets.

The most effective way to obtain nanocrystalline powders of mixed oxides containing several cations is a chemical method based on the coprecipitation of a mixture of salts of these metals by neutralizing with an alkaline agent, filtering and washing the resulting precipitate of mixed hydroxide, drying and further calcining to the corresponding oxide [6]. This method can significantly reduce the annealing temperature, while obtaining single-phase nanocrystalline powders in a wide range of crystallite sizes (from 3-5 to 100 nm).

Closest to the proposed invention and adopted as a prototype is a method for producing powders of the composition Ln ₂ Zr ₂ O ₇ (where Ln = Gd, Nd, Sm), described in [7], according to which the solution obtained by dissolving salts of Ln ( NO ₃ ) ₃ · 6H ₂ O and ZrOCl ₂ · 8H ₂ O, are slowly added to the ammonia solution (NH ₄ OH; pH 12.5). The resulting precipitate was filtered, washed, dried at 120 ° C for 12 hours and then heat treated at 500 ° C for 10 hours. The powder thus prepared was calcined at 1600 ° C for 24 hours. The resulting powder was further ground in a ball mill in ethanol within 24 hours. Pressing of the tablets is carried out by hot pressing at a pressure of 50 MPa in an argon flow at 1500 ° C for 0.5 hours. As a result, tablets with a density of 6.0 to 6.7 g / cm ³ were obtained. The main disadvantage of the described method is the formation of coarse-grained powders with a coherent scattering region (CSR) size of more than 100 nm and a high degree of aggregation (aggregate size up to 10-15 microns), which does not allow obtaining dense ceramic products.

The technical result, which consists in obtaining nanocrystalline powders of mixed REE oxides and a metal of subgroup IVB with an increase in the density of ceramic tablets obtained on their basis, to 7.5 g / cm ³ and above, is achieved by the fact that in the known method, comprising manufacturing a mixed hydroxide by coprecipitation of salts, filtration and washing of the precipitate obtained, drying followed by calcination to obtain a mixed oxide, grinding, pressing and annealing of the obtained compacts, the calcination stage of the mixed hydroxide is carried out in the temperature range of 800-1200 ° C, and the grinding of powders of mixed oxides is carried out by mechanical activation in a planetary mill for 18-36 minutes In a particular case, it is proposed to use dysprosium as REE, and titanium, zirconium, hafnium as metals of subgroup IVB.

The use of calcination of an X-ray amorphous mixed REE hydroxide and a metal of the IVB subgroup in the temperature range 800-1200 ° C leads to the formation of nanocrystalline powders Ln _{2 + x} M _2-x O _{7-x / 2} . Using a temperature of less than 800 ° C leads to the preservation of the X-ray amorphous state of the powder, which contains a significant amount of residual structurally bound water (in the form of hydroxyl groups). Using a temperature of more than 1200 ° C leads to the production of coarse-grained powders of mixed REE oxides and a metal of subgroup IVB with a crystallite size of more than 100 nm.

The use of mechanical activation of powders in a planetary mill leads not only to the grinding of aggregates and a decrease in their size, but also to a decrease in the size of crystallites and an increase in the microstresses of the crystal lattice and, accordingly, to a significant increase in the density of the obtained ceramic tablets after pressing and annealing. Used mechanical activation in a planetary mill (centrifugal acceleration of grinding media of the order of 1000 m ² / s) for 18-36 minutes The use of mechanical activation for less than 18 minutes does not allow to increase the density of tablets to 7.5 g / cm ³ and above. Exposure for more than 36 minutes practically does not lead to an increase in tablet density compared with samples obtained at shorter times.

This production method was implemented using a high-temperature furnace “Nabertherm NT 08/18” and a planetary mill “Pulverisette 7 premium line. The starting materials used were: tetrabutoxy titanium Ti (OC ₄ H ₉ ) ₄ , zirconium oxychlorides ZrOCl ₂ · 8H ₂ O and hafnium HfOCl ₂ · 8H ₂ O, dysprosium nitrate Dy (NO ₃ ) ₃ · 5H ₂ O, 25% aqueous ammonia solution NH ₄ OH, distilled water.

Example 1. 20.9 g of Dy (NO ₃ ) ₃ · 5H ₂ O was dissolved in 200 ml of distilled water. The resulting solution was filtered to remove insoluble suspended particles. 27 ml of 25% NH ₄ OH was adjusted to 200 ml with distilled water. With vigorous stirring, the resulting solution of Dy (NO ₃ ) ₃ and 8.1 g of Ti (OC ₄ H ₉ ) _{4 were} metered into an ammonia solution to obtain a viscous white suspension with a pH of 10.42. The resulting suspension was filtered and then the precipitate of mixed dysprosium hydroxide and titanium was washed with distilled water until there were no soluble anions in the washings. The washed precipitate was dried at a temperature of 90 ° C for 12 hours. The dried powder was calcined in a muffle furnace in air at 800 ° C for 3 hours. The obtained powder was a single-phase nanocrystalline powder Dy ₂ TiO ₅ with a crystallite size of 20 nm having an fcc structure .

Dy ₂ TiO ₅ powder was mechanically activated for 24 min and then pressed (at a pressure of 180 MPa) into a tablet with a diameter of 12.1 mm and a height of 15.5 mm, having a density of 4.28 g / cm ³ . The tablet was calcined at a temperature of 1550 ° C for 4 hours. The tablet obtained after calcination was light cream in color, uniform, without cracks, had a diameter of 10.0 mm, a height of 13.6 mm and a density of 7.50 g / cm ³ . X-ray analysis showed that the tablet consisted of single-phase nanocrystalline Dy ₂ TiO ₅ .

Example 2. 21.7 g of Dy (NO ₃ ) ₃ · 5H ₂ O and 16.0 g of ZrOCl ₂ · 8H ₂ O were dissolved in 200 ml of distilled water. The resulting solution was filtered to remove insoluble suspended particles. 36 ml of 25% NH ₄ OH was adjusted to 200 ml with distilled water. With vigorous stirring, a solution of a mixture of salts of Gd and Zr was dosed into an ammonia solution, obtaining a viscous white suspension with a pH of 10.08. The resulting suspension was filtered and then the precipitate of the mixed dysprosium and zirconium hydroxide was washed with distilled water until there were no soluble anions in the washings. The washed precipitate was dried at a temperature of 90 ° C for 12 hours. The dried powder was calcined in a muffle furnace in air at 800 ° C for 3 hours. The obtained powder was a single-phase nanocrystalline powder Dy ₂ Zr ₂ O ₇ (crystallite size 11 nm), having an fcc structure of fluorite.

Dy ₂ Zr ₂ O ₇ powder was mechanically activated for 18 min and then pressed (at a pressure of 180 MPa) into a tablet with a diameter of 12.1 mm and a height of 15.5 mm, having a density of 4.68 g / cm ³ . The tablet was calcined at a temperature of 1550 ° C for 4 hours. The obtained tablet after calcination was light cream in color, uniform, without cracks, had a diameter of 10.0 mm, a height of 13.6 mm and a density of 7.70 g / cm ³ .

X-ray diffraction analysis showed that the tablet consisted of a single-phase nanocrystalline Dy ₂ Zr ₂ O ₇ (crystallite size 100 nm) having an fcc structure.

Example 3. 21.8 g of Dy (NO ₃ ) ₃ · 5H ₂ O and 10.3 g of HfOCl ₂ · 8H ₂ O were dissolved in 200 ml of distilled water. The resulting solution was filtered to remove insoluble suspended particles. 35 ml of 25% NH ₄ OH was adjusted to 150 ml with distilled water. With vigorous stirring, a solution of a mixture of salts of Dy and Hf was dosed into an ammonia solution to obtain a viscous white suspension with a pH of 10.36. The resulting suspension was filtered and then the precipitate of mixed dysprosium and hafnium hydroxide was washed with distilled water until there were no soluble anions in the washings. The washed precipitate was dried at a temperature of 90 ° C for 12 hours. The dried powder was calcined in a muffle furnace in air at 800 ° C for 3 hours. The obtained powder was a single-phase nanocrystalline powder Dy ₂ HfO ₅ (crystallite size 8 nm) having fcc fluorite structure.

Dy ₂ HfO ₅ powder was mechanically activated for 33 min and then pressed (at a pressure of 180 MPa) into a tablet with a diameter of 12.1 mm and a height of 12.0 mm, having a density of 4.71 g / cm ³ . The tablet was calcined at a temperature of 1550 ° C for 4 hours. The tablet obtained after calcination was light cream in color, uniform, without cracks, had a diameter of 9.6 mm, a height of 9.4 mm and a density of 8.00 g / cm ³ . X-ray diffraction analysis showed that the tablet consisted of a single-phase nanocrystalline Dy ₂ HfO ₅ (crystallite size 85 nm) having an fcc structure.

Example 4. Nanocrystalline powder Dy ₂ HfO ₅ (crystallite size 8 nm) obtained in example 3, was pressed (at a pressure of 180 MPa) without preliminary mechanical activation. The original tablet had a diameter of 12.2 mm, a height of 11.6 mm and a density of 5.10 g / cm ³ . As in the case of example 1, the tablet was calcined at a temperature of 1550 ° C for 4 hours

The tablet obtained after calcination was light cream in color, uniform, without cracks, had a diameter of 11.1 mm, a height of 10.5 mm and a density of 6.57 g / cm ³ . X-ray diffraction analysis showed that the tablet consisted of a single-phase nanocrystalline Dy ₂ HfO ₅ (crystallite size 113 nm) having an fcc structure.

Examples 5-10. The dried powder of the mixed dysprosium and hafnium hydroxide obtained in Example 1 was calcined in a muffle furnace in air at various temperatures for 3 hours. The powders obtained at a temperature of 800-1600 ° C were a single-phase powder Dy ₂ HfO ₅ having a fcc fluorite structure, with crystallite size depending on the processing temperature (Table 1). From table 1 it is seen that an increase in the calcination temperature of more than 1200 ° C leads to a sharp increase in crystallite size and the production of coarse-grained Dy ₂ HfO ₅ powders.

Examples 11-13. The nanocrystalline powder Dy ₂ HfO ₅ (crystallite size 8 nm) obtained in Example No. 3 was mechanically activated for various times and then pressed (at a pressure of 180 MPa) into tablets, which were calcined at a temperature of 1550 ° C for 4 h. Obtained after The calcinations of the tablets were light cream in color, uniform, without cracks and had different densities (Table 2). X-ray diffraction analysis showed that the tablets consisted of single-phase Dy ₂ HfO ₅ having an fcc structure.

Thus, the above examples show that calcination of a mixed hydroxide obtained by coprecipitation of dysprosium and hafnium salts in the temperature range of 800-1200 ° C makes it possible to obtain single-phase nanocrystalline powders of REE oxides and metal of subgroup IVB, whose mechanical activation is performed for 18-36 min before pressing and subsequent annealing of compacts can significantly increase the density of the resulting tablets.

BIBLIOGRAPHY

1. Risovany V.D., Zakharov A.V., Muraleva E.M., et al. Dysprosium hafnate as absorbing material for control rods // J. Nucl. Mater., 2006, v. 355, No. 1, p. 163-170.

2. Xu Q., Pan W., Wang J., et al. Rare-earth zirconate ceramics with fluorite structure for thermal barrier coatings // J. Amer. Ceram. Soc, 2006, v. 89, No. 1, p. 340-342.

3. Abrantes JCC, Levchenko AV, Shlyakhtina AV, et al. Ionic and electronic conductivity of Yb _{2 + x} Ti _2-x O _{7-x / 2} materials // Solid State Ionics, 2007, v. 177, No. 19-25, p. 1785-1788.

4. US Patent No. 4992225, cl. F27B 9/04, 1991.

5. RF patent №2124240, cl. G21C 7/24, 1998.

6. Shabanova N.A., Popov V.V., Sarkisov P.D. Chemistry and technology of nanosized oxides. M .: Academic book, 2006, 309 p.

7. Wu J., Wei X., Padture N.P., et al. Low-thermal-conductivity rare-earth zirconates for potential thermal-barrier-coating applications // J. Amer. Ceram. Soc, 2002, v. 85, No. 12, p. 3031-3035.

Table 1 Example No. Calcination temperature, ° C The crystallite size, nm Lattice parameters, Å Microstress,% 5 400 amorphous - - 6 600 amorphous - - 3 800 8 5,258 (2) 1,6 7 1000 eighteen 5.2570 (6) 0.6 8 1200 62 5.2602 (2) 0.3 9 1400 270 5.2604 (1) 0.1 10 1600 330 5.2622 (1) <0.1

table 2 Example No. eleven 12 13 Mechanical activation time, min fifteen 36 48 The density of the tablets, g / cm ³ 7.30 7.96 7.97

Claims (3)

1. A method of producing nanocrystalline powders and ceramic materials based on mixed oxides of rare-earth elements and metals of subgroup IVB, including the manufacture of a mixed hydroxide by coprecipitation of salts, filtering and washing the obtained precipitate, drying, followed by calcination to obtain a mixed oxide, grinding, pressing and annealing of the obtained , compacts, characterized in that the stage of calcination of the mixed hydroxide is carried out in a temperature range of 800-1200 ° C, and the grinding of powders of mixed oxides is carried out pour by mechanical activation in a planetary mill for 18-36 minutes. 2. The method according to claim 1, characterized in that dysprosium is used as a rare-earth element. 3. The method according to claim 1, characterized in that titanium, zirconium or hafnium are used as metals of subgroup IVB.