Method for producing zirconium-based ceramic composites

Abstract

FIELD: composites.SUBSTANCE: invention relates to a method for producing ceramic composites based on the zircon-hafnium oxide system (ZrSiO4 -HfO2), which can be used as matrices for hardening and isolating radionuclides from the biosphere or high-level waste from the reprocessing of spent nuclear fuel comprising isotopes of rare earth and transplutonium elements. (1-x)(H2SiO3-ZrO(OH)2)-xHf(OH)4is synthesized by intensive mixing of solutions in a two-stage microreactor with intensively swirling flows. A solution of tetraethoxysilane with a concentration of 0.0325-0.092 mol / l in ethyl alcohol with the addition of distilled water is fed through an axial pipe, an aqueous solution of zirconium oxychloride with a concentration of 0.0325-0.092 mol / l is fed through a tangential pipe of the first stage of the microreactor, an aqueous solution of hafnium oxychloride with a concentration of 0.0325-0.092 mol / l is fed through a tangential pipe of the second stage of the microreactor, and an aqueous solution of ammonia is fed through the tangential pipes of the first and second stages of the microreactor. The synthesis product in the form of a precursor gel is collected at the outlet of the microreactor in containers, after which the particles are separated from the liquid. The resulting precursor gel is left to mature, then filtered and dried, after which it is ground, pressed into tablets under a pressure of 8-10 MPa and fired in air at 850 °C for 12 hours to form a mixture of ZrSiO4 and HfO2 oxides, then sintered in air at a temperature of 1000-1300 °C.EFFECT: physical and chemical characteristics of ceramic composites, high completeness of the reaction at a high rate of interaction of the reagents.1 cl, 9 dwg

Description

[0001] The invention relates to a method for producing ceramic composites based on the zircon-hafnium oxide (ZrSiO4-HfO2) system from nanosized precursor powders and can be used as matrices for curing and isolating radionuclides from the biosphere or high-level waste (HLW) from the reprocessing of spent nuclear fuel (SNF) containing isotopes of rare earth and transplutonium elements.

[0002] To assess the novelty of the claimed solution, let us consider a number of well-known technical means of similar purpose.

[0003] No data on the ZrSiO4-HfO2 system were found in the scientific literature.

[0004] The authors of the work (Ugolkov VL et al. "Sol-gel synthesis of nano-sized powders and fabrication of ceramic composites based on zircon and hafnium oxide", Glass Physics and Chemistry, 2024. V. 50. No. 3. P. 277-285) developed a sol-gel synthesis technique based on the separate deposition of components (ZrSiO4 and Hf(OH4). According to this technique, a nanosized precursor powder ((lx)(H2SiO3-ZrO(OH)2)-xHf(OH)4, where x = 0.0, 0.5, 0.7, 0.8 and 1.0, was synthesized to obtain a ceramic composite (lx)ZrSiO4-xHfO2 by sequential sintering in air in the temperature range 1000 1300°С (24 hours at each stage).

[0005] The starting materials for the synthesis were TEOS, ZrOCl2-8H2O, HfOCl2-8H2O, NH4OH and ethyl alcohol (C2H5OH).

[0006] The synthesis of the H2SiO3-ZrO(OH)2 precursor powder (for obtaining zircon, ZrSiO4) was carried out separately: tetraethoxysilane (TEOS, Si(OC2H5)4) was dissolved in ethyl alcohol (C2H5OH), then distilled water was added (pH≈7). During the hydrolysis of TEOS, silicic acid (H2SiO3) is formed. Next, an aqueous solution of ZrOCl2⋅8H2O was added to the resulting alcoholic TEOS solution and thoroughly mixed using a magnetic stirrer at room temperature. Then, a precipitating agent - an aqueous solution of ammonia (NH4OH) - was added to the solution to pH≈8. The result was a mixture of H2SiO3 and ZrO(OH)2 in the form of a colloid. A colloidal solution of Hf(OH)4 was prepared separately by dissolving HfOCl2⋅8H2O in water and adding NH4OH to it until pH ≈ 8. Then, all the resulting solutions (colloidal solutions of H2SiO3 and ZrO(OH)2, as well as Hf(OH)4) were combined, and NH4OH was added until pH 8-9 for complete coagulation of the colloid with thorough stirring using a magnetic stirrer at room temperature.The resulting precipitate (gel) was left to mature for 24 hours. The gel was then washed, filtered, and dried in a convection oven at 110°C for 12 hours.

[0007] The dried precipitate was ground in a vibrating mill and the resulting precursor powder ((lx)(H2SiO3-ZrO(OH)2)-xHf(OH)4 was pressed into tablets under a pressure of 8-10 MPa and calcined at 850°C for 12 h to dehydrate the synthesis product and decompose H2SiO3, ZrO(OH)2 and Hf(OH)4 to the corresponding oxides SiO2 and ZrO2 (with the simultaneous formation of ZrSiO4) and HfO2; heating was carried out at a high rate (~ 20 deg / min) to avoid grain coarsening due to mass transfer (to maintain high dispersion of the powder in the press). After re-grinding the (l-x)ZrSiO4-xHfO2 powder in a vibrating mill, it was again pressed into tablets under pressure of 8-10 MPa for sequential sintering in the temperature range of 1000-1300°C in air for 24 hours at each stage with intermediate grinding to obtain a ceramic composite (1-x)ZrSiO4-xHfO2.

[0008] The resulting ceramic composite exhibits high Vickers microhardness (15 to 18 GPa depending on x), low thermal conductivity and high thermal and chemical stability in distilled water.

[0009] This technical solution, as the closest to the declared one in terms of technical essence and the achieved result, is accepted as its prototype.

[0010] This method allows to obtain a nano-sized precursor powder, which improves the sinterability of the powder and further improves the physicochemical and physicomechanical properties of the ceramic composite.

[0011] The disadvantages of the described method are, firstly, the synthesis of the precursor powder in a small volume (a 1000 ml beaker); secondly, the use of a magnetic stirrer, which does not allow for high-quality mixing at the micro-level; thirdly, the mixing process itself is quite long (at least 6 hours) when using a magnetic stirrer. This significantly limits the industrial applicability of this method. Furthermore, due to insufficient micro-mixing, incomplete consumption of silicic acid in the reaction is possible.

[0012] It should be noted that the result of sintering the precursor powder (1-x)ZrSiO4-xHfO2 at a temperature of 1200 °C is the formation of a monoclinic solid solution of Hf x Zr 1-x O2 and X-ray amorphous SiCO2. That is, a composite with a mole fraction of HfO2 of 0.5, 0.7, and 0.8 is a mixture of zircon, a monoclinic solid solution of Hf x Zr 1-x O2 and X-ray amorphous SiO2.

[0013] The ceramic composite (1-x) ZrSiO4-xHfO2 after sintering at 1300°C exhibits Vickers microhardness values ​​(from 23.2 to 26.7 GPa), shows low thermal conductivity (2.1-1.5 W / mK) in the temperature range from room temperature to 250°C and a low linear temperature expansion coefficient (LTEC) (25.7 23.8)×10 -6 ⋅K -1 ) in the temperature range of 200-1100°C.

[0014] Modern literature provides data indicating that the synthesis method can influence the morphology, dispersion and properties of the obtained precursor powder, and, accordingly, the properties of the sintering product.

[0015] The objective of the claimed invention is to obtain composite ceramics based on the ZrSiO4-HfO2 system using a microreactor method specially developed for the specified materials, which allows, due to the speed and quality of mixing of the reagents, to increase the completeness of the reaction due to the complete consumption of silicic acid, to increase the microhardness of ceramic composites, to reduce their porosity, which in general will reduce the overconsumption of the TEOS reagent, to scale up the experiment and to ensure a reduction in the rate of extraction of Zr, Hf and Si, as well as radionuclides ( 137 Cs, 90 Sr, 152 Eu), which will allow the use of this composite ceramic as a matrix for the solidification and isolation of radionuclides from the biosphere or high-level waste (HLW) from the reprocessing of spent nuclear fuel (SNF), containing isotopes of rare earth and transplutonium elements. Furthermore, according to literature data, a material based on solid solutions of Hf x Zr 1-xO2 is recognized as a new generation Si-integrable material due to the discovery of ferroelectricity and antiferroelectricity in their Si-compatible thin films (Eliseev E.A. et al., “Phase diagrams and polarization reversal in nanosized Hf x Zr 1-x O 2-y " published in the journal AIP Advances. 2024. V. 14. N 5. Article number 055224). The method presented in the application opens up new possibilities for scaling up the synthesis of solid solutions of Hf x Zr 1-x O2 to obtain thin films compatible with Si.

[0016] The essence of the invention as a technical solution is expressed in the following set of essential features, sufficient to achieve the above-mentioned technical result provided by the invention.

[0017] A method for producing zircon-based ceramic composites comprising the stages of solution synthesis, drying, grinding the dried powder and pressing into tablets, as well as high-temperature heat treatment, characterized in that the synthesis of the compositions ((lx)(H2SiO3-ZrO(OH)2)-xHf(OH)4 is carried out with intensive mixing of solutions in a two-stage microreactor with intensively swirling flows, the first stage of which is equipped with one axial and two tangential pipes, and the second stage is equipped with two tangential pipes, and the supply of a tetraethoxysilane solution with a concentration of 0.0325-0.092 mol / l in ethyl alcohol with the addition of distilled water is carried out through an axial pipe, an aqueous solution of zirconium oxychloride with a concentration of 0.0325-0.092 mol / l is fed through the tangential branch of the first stage of the microreactor, an aqueous solution of ammonia is also fed through the tangential branch of the first stage of the microreactor, an aqueous solution of hafnium oxychloride with a concentration of 0.0325-0.092 mol / l is fed through the tangential branch pipe of the second stage of the microreactor, and an aqueous solution of ammonia is also fed through the tangential branch pipe of the second stage of the microreactor, with the pH in both stages of the microreactor being maintained in the range of 8-9, the collection of the synthesis product in the form of a precursor gel is carried out at the outlet of the microreactor in a container, in which the separation of particles from the liquid is then carried out; The resulting precursor gel is left to mature for 24 hours, then filtered and dried in a convection oven at 110°C for 12 hours, ground, pressed into tablets under a pressure of 8-10 MPa and fired in air at 850°C for 12 hours to dehydrate and decompose the synthesis products to form a mixture of ZrSiCO4 and HfO2, and then sintered in air at a temperature of 1000-1300°C for 24 hours to obtain a ceramic composite.

[0018] The technical result achieved by using the invention consists in ensuring the production of composite ceramics, as well as in the fact that the nano-size of the precipitate (gel precursor) is ensured due to the high feed rate of reagent solutions and due to preliminary calcination at a high heating rate of compacted powders for dehydration and decomposition of synthesis products.The specified technical result, obtained by implementing the claimed set of essential features of the invention, ensures the production of composite ceramics based on the ZrSiO4-HfO2 system with increased microhardness, low porosity, low thermal conductivity and high chemical resistance, which allows it to be used as matrices for the solidification and isolation of radionuclides from the biosphere or high-level waste (HLW) from the reprocessing of spent nuclear fuel (SNF) containing isotopes of rare earth and transplutonium elements, as well as ferroelectrics and antiferroelectrics in the form of thin films compatible with Si.

[0019] The essence of the invention is explained by illustrations, where Fig. 1 shows a diagram of the microreactor synthesis of the precursor gel (lx)ZrSiO4-xHfO2, where x = 0.0, 0.5, 0.7, 0.8, 1.0.

[0020] Fig. 2 shows the X-ray diffraction patterns of the (1 x)ZrSiO4-xHf(OH)4 precursor powder: (a) obtained by reverse precipitation using a magnetic stirrer; (b) obtained by coprecipitation using a microreactor; (for powder with a mole fraction x = 0.0(1), 0.5(2), 0.7(3), 0.8(4), 1.0(5)).

[0021] Fig. 3 shows the X-ray diffraction patterns of (1-x) ZrSiO4-xHfO2 powder after heat treatment at 850°C: (a) obtained by reverse precipitation using a magnetic stirrer; (b) obtained by coprecipitation using a microreactor; (for powder with a mole fraction x = 0.0(1), 0.5(2), 0.7(3), 0.8(4), 1.0(5)).

[0022] Fig. 4 shows the X-ray diffraction patterns of the ceramic composite (1-x) ZrSiO4-xHfO2 after sintering the powder at 1000°C for 24 hours, depending on the powder synthesis method: (a) obtained by reverse precipitation using a magnetic stirrer; (b) obtained by coprecipitation using a microreactor; (for a composite with a mole fraction x = 0.0(1), 0.5(2), 0.7(3), 0.8(4), 1.0(5)).

[0023] Fig. 5 shows the X-ray diffraction patterns of the ceramic composite (1-x) ZrSiO4-xHfO2 after sintering the powder at 1300°C for 24 hours, depending on the powder synthesis method: (a) obtained by reverse precipitation using a magnetic stirrer; (b) obtained by coprecipitation using a microreactor; (for a composite with a mole fraction x = 0.0(1), 0.5(2), 0.7(3), 0.8(4), 1.0(5)).

[0024] Fig. 6 shows the temperature-concentration dependences of the Vickers microhardness values ​​of the ceramic composite (1-x) ZrSiO4-xHfO2 for various methods of synthesis of the initial powders: (a) obtained by reverse precipitation using a magnetic stirrer; (b) obtained by coprecipitation using a microreactor.

[0025] Fig. 7 shows the temperature-concentration dependences of the thermal conductivity of the ceramic composite (1-x)ZrSiO4-xHfO2, sintered at 1300°C, for different methods of synthesis of the initial powder: (a) obtained by reverse precipitation using a magnetic stirrer; (b) obtained by coprecipitation using a microreactor.

[0026] Figure 8 shows the dependence of the leaching rate (R) of silicon, zirconium, and hafnium on time (t) in distilled water from a ceramic matrix (1-x) ZrSiO4-xHfO2 obtained from powder by reverse precipitation using a magnetic stirrer. Sintering temperature is 1300°C.

[0027] Fig. 9 shows the dependence of the leaching rate (K) of introduced radionuclides (Cs-137, Sr-90, Eu-152) from the 0.8ZrSiO4-0.2HfO2 ceramic matrix obtained using a microreactor on the time (f) of exposure to distilled water. Sintering temperature is 1100°C.

[0028] The claimed method is implemented as follows.

[0029] To obtain a ceramic composite based on the ZrSiO4-HfO2 system, a nanosized precursor powder ((lx)(H2SiO3-ZrO(OH)2)-xHf(OH) is used 4,Synthesized by a sol-gel technique using microreactor synthesis. The HfO2 oxide content varied from 0.0 to 100 mol.% (i.e., mole fractions = 0.0, 0.5, 0.7, 0.8, and 1.0). The synthesis was carried out in a two-stage microreactor with intensely swirling reactant flows, the design of which is described in detail in Russian Patent No. 2748486. This microreactor ensures a high level of macro- and micro-mixing and enables the introduction of solutions distributed along the length of the apparatus, which ultimately allows for multi-stage, fast reactions, including in multiphase environments, and the production of nanoscale core-shell particles and other composite materials requiring the sequential introduction of components.

[0030] The solutions were pre-prepared as follows: TEOS was dissolved in ethyl alcohol (C2H5OH); zirconium oxychloride and hafnium oxychloride were separately dissolved in water at room temperature. To synthesize zirconium hydroxide, an aqueous solution of zirconium oxychloride was fed through port A, and an aqueous solution of ammonia through port B. Intensive mixing of these solutions took place in the upper neck (intensive mixing 1, Fig. 1).

[0031] To synthesize hafnium hydroxide, an aqueous solution of hafnium oxychloride was fed through pipe G, and an aqueous solution of ammonia was fed through pipe D. Intensive mixing of these solutions took place in the lower neck (intensive mixing 2, Fig. 1).

[0032] For the synthesis of zircon, a solution of TEOS in ethyl alcohol was fed through the axial pipe (B), an aqueous solution of zirconium oxychloride was fed through the tangential pipe A, and an aqueous solution of ammonia was fed through the tangential pipe B. Intensive mixing of the solutions also took place in the neck (intensive mixing zone 1, Fig. 1).

[0033] The synthesis of the compositions ((lx)(H2SiO3-ZrO(OH)2)-xHf(OH)4 required a two-stage process - a solution of TEOS in ethyl alcohol was fed through the axial pipe (B), an aqueous solution of zirconium oxychloride or hafnium oxychloride in the appropriate concentration was fed through tangential pipes A and G, an aqueous solution of ammonia - through tangential pipes B and D. Intensive mixing of the solutions in this case took place sequentially in two necks, the upper and lower (intensive mixing zones 1 and intensive mixing 2 in Fig. 1). The flow rates of the solutions were: through pipe A: Q A = 1.410±0.023 l / min, through pipe B: Q Б=± 1.382±0.037 l / min, through port B: Q В = 1.352±0.032 l / min, through pipe G: Q Г = 1.419 ± 0.035 l / min, through pipe D: Q Д = 1.4 ± 0.05 l / min.

[0034] In all cases, the final synthesis product, in the form of a gel, was collected at the bottom of a common container (particle-liquid separation, Fig. 1). The pH was maintained at ≥8. The resulting gel precursor was allowed to mature for 24 h, then filtered and dried in a convection oven at 110°C for 12 h.

[0035] The finely dispersed (nanosized) and ground powder of the precursor gel is pressed into tablets under a pressure of 8-10 MPa and heat-treated at 850°C for 12 hours to further dehydrate and decompose the synthesis products, forming a mixture of zircon and HfO2. The tablets are then crushed in a vibratory mill, pressed again into tablets under a pressure of 8-10 MPa, and sequentially sintered in air at a temperature range of 1000-1300°C for 24 hours at each stage, with intermediate grinding, to obtain a ceramic composite (1-x) ZrSiO4-xHfO2.

[0036] Comparing the X-ray diffraction patterns in Fig. 2, it can be noted that in both cases the synthesis is effective, forming X-ray amorphous mixtures that no longer contain crystallized silicic acid, i.e., at the moment of mixing the solutions, the components interact with the formation of highly dispersed (X-ray amorphous) products. The precursor powder (lx)ZrSiO4-xHf(OH)4 (Fig. 2a, b) is apparently a mixture of X-ray amorphous zircon (in diffraction patterns 1-4, a halo in the region of 20 = 31° indicates the onset of zircon crystallization) and hafnium oxyhydroxide (diffraction patterns 5).

[0037] The synthesized (1-x)ZrSiO4-xHfO2 powder after calcination at 850°C retains its high dispersion both in the case of reverse precipitation using a magnetic stirrer and in the microreactor synthesis (Fig. 3a, b). However, in the latter case, zircon is better crystallized (Fig. 3b, diffraction pattern I) and it is more stable, i.e. the addition of HfO2 (x = 0.5) does not destroy its structure, i.e. the presence of ZrO2 reflections is not observed (Fig. 3b, diffraction pattern 2).

[0038] After sintering at 1000°C, in the X-ray diffraction patterns of the (1-x) ZrSiO4-xHfO2 composite, both zircon and HfO2 reflections are observed in both cases (Fig. 4a, b, diffraction patterns 2-4).

[0039] The composite (lx)ZrSiO4-xHfO2 after sintering at 1300°C is a mixture of monoclinic solid solutions of Hf x Zr 1-xO2 (designated as HfO2) and hexagonal SiO2 (Fig. 5a, b, diffraction patterns 2-4), with the exception of the composition x = 0.0, where partial decomposition of zircon is observed (Fig. 5a, b, diffraction patterns I). Moreover, in the case of microreactor synthesis, a significantly larger number of reflections belonging to zircon itself are observed in the diffraction pattern of zircon (Fig. 5b, diffraction pattern I), compared to the synthesis by inverse precipitation using a magnetic stirrer (Fig. 5a, diffraction pattern I).

[0040] From the temperature-concentration dependences of the Vickers microhardness values ​​of the ceramic composite (1-x)ZrSiO4-xHfO2 shown in Fig. 6, it is evident that they are identical.

[0041] In general, the microhardness values ​​for the composite increase with increasing temperature. The values ​​themselves significantly exceed the few data available in the literature. Thus, according to the work (Suarez G. et al. "Colloidal processing, sintering and mechanical properties of zircon (ZrSiO4)" in the journal Ceramics International, 2015, Vol. 41, No. 1, Pt. B, pp. 1015-1021.), the microhardness value of a ceramic zircon sample obtained from commercial ZrSiO4 powder after sintering at 1680°C corresponded to 8.5 GPa. The value of 7.39 GPa was obtained for a ceramic sample of zircon, also prepared from commercial ZrSiO4 powder, but by hot pressing at 1350°C under a pressure of 30 MPa (Rendtorff NM et al. "Dense zircon (ZrSiO4) ceramics by high energy ball milling and spark plasma sintering" in the journal Ceramics International, 2012. V. 38. N 3. P. 1793-1799.). A higher value of Vickers microhardness (13.67 GPa) was obtained for a ceramic sample of zircon prepared from commercial ZrSiO4 powder after dispersion in a planetary mill and sintered at 1400°C (Musyarofah et al. “Phase study of SiO2-ZrO2 composites prepared from polymorphic combination of starting powders via a ball-milling followed by calcination” in Journal of Physics: Conference Series. 2017, Vol. 817, No. 1, Article No. 012033).

[0042] Apparently, the high values ​​of Vickers microhardness of the samples obtained by the proposed method are associated with the high dispersion of the initial powder, which ensures good sinterability of the ceramic composite due to mass transfer through the developed intergranular boundary.

[0043] The thermal conductivity of the ceramic composite (Fig. 7) decreases with increasing HfO2 content and temperature. These results can be compared with the limited data available in the scientific literature. For example, for individual zircon, a decrease in thermal conductivity was noted from 5.1 W / (m⋅K) at room temperature to 3.5 W / (m K) at 1000°C (Abajo S. et al. “New processing route for ZrSiO4by powder injection molding using an eco-friendly binder system” in Boletin de la Sociedad Espanola de Ceramica y Vidrio, 2015, Vol. 54, No. 3, P. 93–100; and Musyarofah et al. “Phase study of SiO2-ZrO2composites prepared from a polymorphic combination of starting powders via a ball-milling followed by calcination” in Journal of Physics: Conference Series. 2017, Vol. 817, No. 1, Article No. 012033). The authors of the article (Nakamori F. et al. “Mechanical and thermal properties of ZrSiO4”) in the Journal of Nuclear Science and Technology, 2017, Vol. 54, No. 11, P. 1267-1273.) obtained a higher value of thermal conductivity of zircon at 298 K (~ 25°C), corresponding to 14.3 W / (m⋅K).

[0044] In a recently published paper (Xiang X. et al., “The lattice thermal conductivity of hafnia: The influence of high-order scatterings and phonon coherence”) in Journal of Applied Physics, 2024, Vol. 135, Article No. 125102), the thermal conductivity of HfO2 was measured in a wide temperature range from 300 to 2000 K (27-1727°C), in which the thermal conductivity values ​​decreased from 11.95 to 1.72 W / (m⋅K). In the work (Li S. et al. "Effect of Y doping on microstructure and thermophysical properties of yttria-stabilized hafnia ceramics)) in the journal Ceramics International, 2018, Vol. 44, No. 15, pp. 18213-18221.) data on thermal conductivity are presented for a ceramic sample of HfO2 obtained by sintering the finished reagent at 1600°C for 5 h. In the temperature range from room temperature to approximately 250°C, the thermal conductivity decreases from 9 to 5 W / (m⋅K). The values ​​of thermal conductivity of HfO2 closest to our data (~1.37 W / (m⋅K)) are presented in the work (Chaubey GS et al."Microstructural and thermal investigations of HfO2 nanoparticles" in RSC Advances, 2012, Vol. 2, No. 24, pp. 9207-9213.). The data were obtained for HfO2 nanoparticles synthesized by the sol-gel method, then subjected to heat treatment at 850°C in air by hot pressing, and correspond to a value of ~1 W / (m⋅K) in the range of 300-775 K (27-502°C).

[0045] The ceramic composite as a matrix (1-x) ZrSiO4-xHfO2 exhibits high chemical resistance (Fig. 8).

[0046] The chemical stability of the composite was assessed using a leaching experiment in distilled water. For this, (1-x)ZrSiO4-xHfO2 powder was pressed into 1 cm3 cubes after heat treatment at 850°C. 3 under a pressure of 8-10 MPa, after which it is successively sintered at 1000, 1200, and 1300°C (24 hours at each stage). The cubes are placed in plastic containers and filled with 50 cm 3contact solution (distilled water). Test temperature (25±3)°C; the contact solution is changed on the 1st, 3rd, 7th, 10th, 14th, 21st, and 35th (28th) days. The concentration of Zr, Hf, and Si is determined by inductively coupled plasma mass spectrometry (ICP-MS) in a contact solution acidified with nitric acid to pH ~1. ICP-MS measurements were performed at the A.P. Karpinsky All-Russian Geological Research Institute (St. Petersburg, Russia).

[0047] Based on the obtained ion concentrations in the contact solution, the leaching rates of the ceramic matrix are calculated using the formula:

[0048]

[0049] where R is the leaching rate of the component, g / (cm 2 ⋅day); c is the concentration of the component in the contact solution, g / dm 3 ; V - volume of contact solution, dm 3 ; w is the mass fraction of the component in the sample; S is the geometric area of ​​the open surface of the sample, cm 2; t - duration of the leaching period, days.

[0050] Leaching experiments showed a fairly high stability of the ceramic composite (matrix) (1-x)ZrSiO4-xHfO 2. Lower steady-state zirconium leaching rates of approximately 10 were observed. -7 -10 -8 g / (cm 2 ⋅day) from the studied ceramic matrix compared to silicon ions (Fig. 8). Silicon, as shown by the leaching rate curves, left the matrix faster: steady-state values ​​of the leaching rate were in the range of 10 -3 -10 -5 g / (cm 2 ⋅day) (Fig. 8). This is apparently due to the presence of silica, which may or may not be observed due to its high dispersion in the diffraction patterns of the ceramic composite after sintering at 1300°C (Fig. 5a). Hafnium 10 shows the lowest leaching rates. -8 -10 -9 g / (cm 2 ⋅day), Fig. 8.

[0051] In the Russian Federation, the method for determining the chemical stability of solidified radioactive waste and its simulants through the leaching of radionuclides and macrocomponents during prolonged contact with water and aqueous solutions is established by GOST 52126-2003 "Radioactive Waste. Determination of Chemical Stability of Solidified High-Level Waste by the Long-Term Leaching Method" (essentially a static test). This standard is used, in particular, for the development of solidification technologies. The leaching rate determination methodology used in this study was developed in accordance with the requirements of GOST 52126-2003.

[0052] The main indicator of the chemical stability of a matrix is ​​the rate of leaching of radionuclides, i.e. the rate of their transition into the solvent upon its contact with the matrix.

[0053] To measure the leaching rate of each of the introduced radionuclides ( 137 Cs, 90 Sr, 152Eu) two samples of matrix material of each studied composition (ZrSiO4), obtained by microreactor synthesis, were prepared, as well as a sample of the nominal composition 0.8ZrSiO4-0.2HfO2, obtained by reverse precipitation using a magnetic stirrer.

[0054] The samples, pressed into tablets under a pressure of 10 MPa, were impregnated with an aqueous solution containing a radionuclide as follows. Using a microdispenser, a volume of an aqueous solution of the radionuclide with a pH of ≈1 was applied dropwise onto each tablet so that the counting rate of the radionuclide introduced into each tablet was ≈10 7 pulses / sec. The tablets were then dried under an IR lamp for 2 hours and then re-compressed (under a pressure of 10 MPa). The resulting samples were calcined in a muffle furnace at 1100°C for 24 hours.

[0055] The samples were then placed in lidded Teflon cups of suitable volume, into which 10 cm 3distilled water (contact solution). Throughout the experiment, the water temperature was maintained at (25±5)°C.

[0056] The contact solution (distilled water) was changed after 1, 3, 7, 10, 14, 21, 28, 60, and 90 days from the start of the experiment. At the specified time, each sample was removed from the container, the water was drained, and the sample was rinsed with 5 cm 3 distilled water, which was added to the drained water, after which the combined solution was acidified with 0.05 cm 3 concentrated nitric acid. The sample was immediately placed back into the container and filled with a new portion of contact solution (10 cm 3 distilled water).

[0057] The content of radionuclides in the contact solution was determined radiometrically by evaporating an aliquot of 1 cm 3 contact solution on a stainless steel substrate. The sample count rate on the substrate was determined using an RKBA-01 "RADEK" beta radiometer,

[0058] The leaching rate of radionuclides was determined using the same formula (1) given above.

[0059] The results of the performed measurements of the leaching rate are shown in Fig. 9. From the presented data it follows that in the case of the ZrSiO4 matrix, the leaching rates 137 Cs and 90 Sr are quite close, whereas 152 Eu - more than an order of magnitude lower; radionuclide leaching rate 137 Cs and 90 By the end of the experiment, Sr had dropped to a level of ~10 -7 g / (cm 2 ⋅day), and 152 Eu - up to ~10 -9 g / (cm 2 ⋅days). This may be an indication that 152 Eu can be incorporated into the zircon crystal lattice (from the publication by Friis H. et al. “Photoluminescence of zircon (ZrSiO4) doped with REE 3+(REE = Pr, Sm, Eu, Gd, Dy, Ho, Er)” in the journal Physics and Chemistry of Minerals, 2010, Vol. 37. P. 333-342: “…rare earth elements (REE = La-Lu) are readily substituted in the eight-coordinated Zr center, which forms a triangular dodecahedron with D2d symmetry”), and 137 Cs and 90 Sr - into the crystal lattice of SiO2, formed as a result of partial decomposition of zircon.

[0060] It should be noted that in the case of both matrices (ZrSiO4 and 0.8ZrSiO4-0.2HfO2) the leaching rates obtained at the beginning of the experiment 137 Cs meet the requirements of GOST 51883-2002 and NP-019-15 (Federal norms and rules in the field of atomic energy use “Collection, processing, storage and conditioning of liquid radioactive waste. Safety requirements”) for cemented waste, while by the end of the experiment (on the 90th day) the leaching rate values 137 Cs and 90Sr does not exceed the maximum levels for vitrified waste established by NP-019-15 and GOST R 50926-96 (Highly active solidified waste. General technical requirements. - Moscow: IPK Publishing House of Standards, 1996. - 8 p.).

[0061] The obtained results show that ceramic composites (matrices) based on the ZrSiO4-HfO2 system can be used as matrices for the solidification and isolation of radionuclides from the biosphere or high-level waste (HLW) from the reprocessing of spent nuclear fuel (SNF) containing isotopes of rare earth and transplutonium elements.

[0062] The use of a microreactor method for synthesizing precursor powders made it possible to obtain physicochemical characteristics of ceramic composites after sintering in the temperature range of 1000-1300°C at the level of or even higher (in general by about 5-7%) than the characteristics of ceramic composites obtained by reverse precipitation using a magnetic stirrer, since, due to the high quality of micromixing achieved in a two-stage microreactor with intensively swirling reagent flows, it is possible to achieve a high completeness of the reaction, saving the consumption of TEOS reagent, at a high rate of reagent interaction, to carry out continuous two-stage synthesis in one microapparatus, and also to scale up the process.

Claims

A method for producing zircon-based ceramic composites comprising the stages of solution synthesis, drying, milling the dried powder and pressing into tablets, as well as high-temperature heat treatment, characterized in that the synthesis of the compositions ((1-x)(H2SiO3-ZrO(OH)2)-xHf(OH)4 is carried out with intensive mixing of solutions in a two-stage microreactor with intensively swirling flows, the first stage of which is equipped with one axial and two tangential pipes, and the second stage is equipped with two tangential pipes, wherein the supply of a solution of tetraethoxysilane with a concentration of 0.0325-0.092 mol / l in ethyl alcohol with the addition of distilled water is carried out through the axial pipe, an aqueous solution of zirconium oxychloride with a concentration of 0.0325-0.092 mol / l is supplied through the tangential the first stage branch pipe of the microreactor, an aqueous solution of ammonia is also fed through the tangential branch pipe of the first stage of the microreactor, an aqueous solution of hafnium oxychloride with a concentration of 0.0325-0,092 mol / l is fed through the tangential branch pipe of the second stage of the microreactor, and an aqueous solution of ammonia is also fed through the tangential branch pipe of the second stage of the microreactor, with the pH in both stages of the microreactor being maintained in the range of 8-9, the collection of the synthesis product in the form of a precursor gel is carried out at the outlet of the microreactor in a container, in which the separation of particles from the liquid is then carried out; The resulting precursor gel is left to mature for 24 hours, then filtered and dried in a convection oven at 110°C for 12 hours, ground, pressed into tablets under a pressure of 8-10 MPa and fired in air at 850°C for 12 hours to dehydrate and decompose the synthesis products to form a mixture of ZrSiO4 and HfO2, and then sintered in air at a temperature of 1000-1300°C for 24 hours to obtain a ceramic composite.

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