Annotation of the results obtained in 2018
In 2018, the studies, carried out under this project in 2016-2017, were continued. The problem includes the development and improvement of the properties of inorganic materials (powders and ceramics) for the consolidation of nuclear waste, for structural applications (for products of atomic and space technology). Improving the properties of the materials was carried out by choosing the structure, changing the composition of compounds, choosing a method of synthesis. The objects of the research are compounds with the structures of natural minerals and composites based on them. The basic compounds had the structure of natural minerals: fluorite, garnet, monazite, kosnarite (NZP), scheelite. Oxide with the fluorite structure CeO2 was obtained in the form of powders by the decomposition of cerium nitrate. Composites based on it, CeO2-x (vol.%) W, Mo, SiC, x = 0, 10, 20, were obtained by combining aqueous solutions of tungstate or ammonium molybdate salts and the finished CeO2 powder (composites with W, Mo) and dispersing a mixture of powders using a laboratory planetary ball mill (composites with SiC). Ceramics were obtained by the SPS method. Powder and ceramic samples were characterized by XRD and SEM. In ceramic samples, inclusions of metal/SiC microparticles are visible on the surfaces of cerium oxide grains. Experiments on refining the sintering mode of СeO2-based ceramics were carried out. The optimal parameters of the sintering process were established: Tsintering = 1025 °C, P = 70 MPa, t = 5 min. The values of relative density were 91.1-95.9%, microhardness of composites with Mo is 3.5 and 2.9 GPa, fracture toughness is 0.68 and 0.75 MPa·m^ 1/2 in the series x = 10, 20 vol. %, microhardness of composites with W is 4.0 and 3.8 GPa, fracture toughness is 0.62 and 0.69 MPa·m ^ 1/2 in the series x = 10, 20 vol. %, microhardness of composites with SiC is 5.1 and 5.18 GPa, fracture toughness is 0.72 and 0.94 MPa·m ^ 1/2 in the series x = 10, 20 vol. % The thermal conductivity coefficient was calculated for CeO2-xSiC composites. It was found that an increase in the SiC concentration in the composite led to an increase in thermal conductivity: from 6.1 W/(m·K) for CeO2 (25 °C) to 9.1 and 10.9 W/(m·K) for CeO2-10% SiC and CeO2-20 % SiC (25 °C), respectively. ZrO2-0.25 (mol) SmO1.5, ZrO2-0.25 (mol) YbO1.5 (fluorite structure) Studies were performed in a static mode (T = 90 °C, P = 1 atm) in distilled water for 56 days with sampling at 1-, 4-, 7-, 11-, 14-, 21-, 56th day . According to the results of the experiment, the mass of the tested samples did not change, indicating a high chemical resistance of the studied ceramics. According to XRD data, no phase changes in the studied ceramics occurred after the test under hydrolytic conditions. The achieved leaching rates were ~10^(-10) g/(cm^2·day). Phosphates Na(1+2x)Zr(2-x)Cux(PO4)3, Ca0.5(1+x)Zr(2-x)Cux(PO4)3, Ca(0.5+x)Zr(2-x)Cox(PO4)3, Na(1+2x)Zr(2-x)Cox(PO4)3 and phosphate-silicates (Ca0.75+0.5x)Zr1.5Fe0.5(PO4)(3-x)(SiO4)x (0≤x≤0.5) with NaZr2(PO4)3 (NZP) structure are proposed based on the simulation of compositions with the expected small and adjustable thermal expansion with the application of crystal chemical principles. Phosphate powders were obtained by the solid phase method, phosphate-silicates were obtained by the sol-gel method. Using the method of high-temperature X-ray diffraction, the parameters of thermal expansion for the temperature range from 25 to 800 °C were determined (step 100 °C). The values of axial (αa and αс), average (αav) and volume (β) coefficients of thermal expansion, as well as anisotropy of thermal expansion (Δα) of the studied phosphates and phosphate-silicates were calculated. In Cu samples with sodium Na(1+2x)Zr(2-x)Cux(PO4)3, the introduction of copper and the increase in the population of M2 positions with Na cations did not significantly affect the behavior of the compounds when heated: αa, αs and Δα increased slightly, αav and β decreased slightly. In Cu and Co samples with calcium Ca0.5(1+x)Zr(2-x)Cux(PO4)3, Ca(0.5+x)Zr(2-x)Cox(PO4)3 with the introduction of a copper and cobalt (instead of zirconium) and increase in the population of M1 positions with Ca cations, the absolute values of αa, αs and Δα decreased, while the values of αav and β increased. In Co-containing compounds with sodium Na(1+2x)Zr(2-x)Cox(PO4)3, with the introduction of cobalt and a corresponding increase in the population of M2 positions with Na cations, all the parameters of thermal expansion (αa, αс, αav, β, and Δα) decreased. In phosphate-silicates Ca(0.75+0.5x)Zr1.5Fe0.5(PO4)(3-x)(SiO4)x, the introduction of silicate groups led to a sharp decrease in αa values and a small increase in αs values, as well as a sharp increase in β values, as a result of Δα varied slightly, αav values increased. According to the values of thermal expansion coefficients, the obtained samples can be classified as low- and medium-expansion materials when heated. Ceramics based on phosphate-silicate Ca0.875Zr1.5Fe0.5(PO4)2.75(SiO4)0.25 was obtained by SPS. The relative density of ceramics is ~ 99.7%. The phase composition of ceramics after sintering did not change. On micrographs, ceramics had a high-density fine-grained structure - the grain size was 1-3 μm, residual microporosity was observed, the pore size did not exceed 0.5 μm. Microhardness of ceramics was 6.2 GPa, fracture toughness was 1.2 MPa·m^1/2. These are typical values of microhardness and fracture toughness for ceramics of a given structural type. The synthesis of compounds Nd0.33Zr2(PO4)3, Ba0.5Zr2(PO4)3, K2YZr(PO4)3 was performed using various methods with varying synthesis conditions. Ceramic samples were synthesized from the powder sample Nd0.33Zr2(PO4)3 (structural type NZP) using the SPS method. The powder was placed in a graphite mold. The sintering mode of the powder in ceramics of various densities using the SPS method was worked out. The duration of sintering processes did not exceed 15 minutes. Sintering of the powder occurred at sufficiently high temperatures: T = 1050-1300 °C. The relative density of the samples had values ~ 69-96%. Helium permeability was studied on the example of ceramics of the composition Nd0.33Zr2(PO4)3 (structure NZP), Na0.1Ca0.8Nd0.1WO4, Na0.5Nd0.5WO4 (scheelite structure). As expected the helium permeability increased with increasing porosity of NZP ceramics and was significant even at a relative density of the sample 96% (1.5·10^(-5) Pa·m^3/s). For ceramics with scheelite structure with a relative density of 98%, helium permeability was at the level of the instrument sensitivity (10^(-6) - 10^(-8) Pa·m^3/s). For ceramics Na0.5Nd0.5WO4 with a relative density of 95%, helium permeability was not recorded. Phosphate with monazite structure Hydrolytic studies were carried our in a static mode at T = 90 °C in distilled water for 56 days. According to XRD data, no phase changes in the studied ceramics occurred after the test. The leaching rate of Pr from phosphate PrPO4 was ~10^(-9)-10^(-10) g/(cm^2·day) and did not depend on the sintering mode of the ceramics. Molybdates and tungstates with scheelite structure. NaxCa/Sr(1-2x)NdxMo/WO4 molybdates and tungstates (x = 0–0.5, Δx = 0.1) were obtained by precipitation from aqueous solutions. High-temperature studies of molybdates were carried out at T from 25 to 1000 °C (∆T = 75-100 °C). With increasing temperature, parameters a, c, V increased. Values of axial (αa and αс), average (αav) and volume (β) coefficients of thermal expansion of the studied molybdates were calculated. Thermal expansion anisotropy was visualized using thermal expansion patterns. The content of lanthanide in the composition of the samples had little effect on the change in the parameters of thermal expansion. The studied molybdates were classified as highly expanding substances. The SPS method was used to compact powder samples of molybdates of the NaxSr(1-2x)NdxMoO4 series (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) in ceramics. The duration of the sintering process did not exceed 2 minutes. Sintering temperatures were rather low (up to 1000 °C). The relative density of ceramic samples was ≈ 97-99%. The phase composition of ceramics after sintering has not changed (according to XRD data). NaxSr(1-2x)NdxMoO4 molybdate ceramics (x = 0, 0.1, 0.2, 0.3, 0.4) were tested in distilled water in a static mode, T = 25 °C, t = 28 days. Water samples were analyzed for Sr. The minimum leaching rates of the Sr2+ cation were in the range of 6.3∙10^(-6)-1.3∙10^(-5) g/(cm^2·day). With increasing x, the leaching rate of strontium increased significantly. After hydrolytic testing, the ceramic samples retained their initial phase composition (according to XRD data). For the compounds NaxCa/Sr(1-2x)NdxWO4 (x = 0, 0.1, 0.2, 0.5) with the scheelite structure, the dominant mechanisms arising at different stages of sintering were established. The sintering kinetics of monophasic fine-grained tungsten NaxCa/Sr(1-2x)NdxWO4 ceramics was determined by the intensity of the grain boundary diffusion process, while the concentration of the x component had a significant effect on the activation energy of the sintering process associated with grain boundary diffusion during active grain growth.

 

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Annotation of the results obtained in 2016
State of works in the world in investigated subjects was described.Modelling of compositions of the compounds, which are basic for ceramic materials for nuclear technology and construction materials, was carried out. Ceramics and powders were obtained, their properties were studied. Oxides. Oxide Y2.5Nd0.5Al5O12 with garnet structure was obtained by the coprecipitation method with ammonium hydroxide NH4OH (5 mol. % in water) with subsequent thermal processing steps up to 1000°C. For forming of the composites Y2.5Nd0.5Al5O12 – x Ni, W, Mo (where х = 10, 20%) powders were withstood in aqueous solutions of NiCl2, (NH4)4W5O17•2.5H2O, (NH4)6Mo7O24•4H2O, then heated at 700°C (for Ni, W) and 1000°C (for Mo) in a H2-atmosphere for 2 h. Composites were two-phase systems: ceramic and metal. Inclusions of metal microparticles are visible on the surfaces of the garnet grains (microstructural analysis). For ceramics obtained by SPS, the optimal process conditions are found. Sintering time of the composites is not exceeded 9 min, values of relative density ranged from 94 to 99%. Microhardness of the composites with Ni in row x = 0, 10, 20 wt. % ranged from 13.2 to 11.0GPa, fracture toughness – 0.7 to 0.9 MPa•m-1. Microhardness of the composites with Mo in row x = 10, 20 vol. % ranged from 10.8 to 9.6GPa, fracture toughness – 1.0 to 1.1 MPa•m-1. Microhardness of the composites with W in row x = 10, 20 vol. % ranged from 13.3 to 11.9GPa, fracture toughness – 0.9 to 1.0 MPa•m-1. Phosphates. Phosphate NdPO4 with monazite structure (Sp. gr. P21/n, а = 6.7102 Å, b = 6.9216 Å, c = 6.3756 Å; α = 90o, β = 103.7084o, γ = 90o) was obtained by precipitation method with subsequent heat treatment up to 1000°C and dispersing in a planetary mill. For obtaining of the composit NdPO4 + xMgO (where х = 5, 10, 20 wt.%) mixture of powders NdPO4 and MgO as a suspension in water was stirred on a magnetic stirrer for 1 d, and then dried at 135°C and heated at 900°C for 3 h. For the first time SPS method was used to produce ceramics. Based on the scanning electron microscopy (SEM) data was established inhomogeneous microstructure formation (small and large particles) for the sample without MgO, while compared with the powder, there was separation of individual particles in the agglomerates, the same is observed for the composites. Phosphate CsCoPO4. Structure of tridymite and other polymorphs. The single-crystal samples synthesized by crystallization of molten powders previously were obtained by sol-gel method at T = 600, 800 ° C (5, 15 h) according to the classical technology. New structural data for all 4 polymorphs with anisotropically refined displacement parameters were obtained and compared with those known for powder samples. Temperature regions of phase transitions must be considered when handling and transport of sources of ionizing radiation with composition CsCoPO4 with radioactive isotopes of cesium and cobalt, including cesium and cobalt together. Phosphate-sulfates with NaZr2(PO4.)3 (NZP) structure. A2xZr2-xCux(PO4)2(SO4), A = Na, K. Synthesis was carried out by the sol-gel process using ethanol as a salting-out agent.The gel was thermostated in stages from 90°C for 1 d to T = 500, 600, 700, 800°C for 20 h at each stage. Monophasic products were obtained for x between 0 and 0.75, NZP structure was implemented in them, hex. sing., sp. gr. R c. For the compound NaZr1.5Cu0.5(PO4)1.5(SO4)1.5 values of the parameters of thermal expansion at temperatures range from 25 to 700°C amounted: αa= -5.40•10-6 °C-1, αс= 18.88•10-6 °C-1, αav= 2.69•10-6 °C-1, Δα = 24.28•10-6 °C-1 (high-temperature X-ray diffraction method). Substitution of phosphorus by sulfur led to reduction of characteristics of thermal expansion in accordance with the reduction of the size of this cation compared to phosphate NaZr2(PO4)3: αa on 16 %, αс on 26 %, Δα on 24 %. Thus, a crystallochemical principle of reducing of the parameters of thermal expansion due to decrease of cation radiuses in tetrahedron XO4 (when phosphorus is replaced by sulfur) used in the present the work is performed. For phosphate-sulfates A2xZr2-xCux(PO4)2(SO4) the parameters of thermal expansion at temperature range from -100 to 100°C are in table in Attachment. Test materials are characterized by a negative thermal expansion along all crystallographic directions in this temperature range. This unusual behavior during heating opens prospects for the application of these compounds in technology of reception of composite materials as compensators of thermal expansion. Through a combination in composite materials of the test phosphate-sulfates with “usual”, expanding during heating compounds it becomes possiblecontrollably change the overall values of the parameters of thermal expansion. Tungstates with scheelite structure. NaxSr1-2xNdxWO4, where x = 0.1, 0.2, 0.3, 0.4, 0.5. Powders were obtained by sol-gel method with subsequent heat treatment up to 1100°C for 10 h at each stage.Obtained powders were monophasic, scheelite structure was implemented (tetragonal. sing., sp. gr. I41/a).The calculated parameters of elementary cells modified in a series of compounds with x from 0.1 to 0.5: a = b from 5.4293 to 5.3463 Å, c from 11.9781 to 11.7078 Å, V from 353.0820 to 334.6432 Å3 , α = β = γ = 90° and were in accordance with changes in the ionic radii of the cations involved in isomorphic substitution. Ceramics were obtained bymethods SPS and CPS. Relative density of ceramics obtained by SPS were 92.68 - 99.9%, CPS - 70 - 80%. Despite the fact that the processes of sintering of ceramic samples occur in such a short time, the relative density of the ceramic is very high. We can say that formed samples are almost non-porous, which is important for a material resistant to a variety of environments. SEM was used for studying microstructure of powders and ceramics. When comparing the particle size of the starting powder and the ceramics obtained from that can be seen that the value of the individual particles is not substantially changed, agglomeration of individual grains of neodymium compounds to the big particles not occurred. Hydrolytic stability under static conditionsfor ceramic samples NaxSr1-2xNdxWO4, x = 0.1 - 0.3, obtained by CPS were studied. Achieved leaching rate is R, g/(cm2•d) ~ 10-5. Studied ceramics are offered as a matrix for the consolidation AEE and REE (fission products), including disposal them from the alkaline-chloride melts used in pyroelectrochemical regeneration technology of irradiated nuclear fuel. Composites with NZP–leucite structure KZr2(PO4)3-KAlSi2O6and NZP–pollucite structure NaZr2(PO4)3-Cs[MgAl0.5P1.5O6]. The principle implemented in the natural mineral itakolumite, containing compounds with very different the parameters of thermal expansion, is the basis for modeling of the composites with improved ductility. Synthesis of phosphates powders with NZP and complex oxides with leucite and pollucite structures was performed according to previously developed sol-gel techniques. Complexing agents and alcohols, calcium fluoride were added. Also was used the technique of cationic substitution: with bentonite clay and industrially manufactured Zeolite A. To determine the microstructure and phase composition of powders and ceramics, including in composites, were used XRD and SEM. Optimal methods were selected from used technologies that lead to the formation of single-phase crystalline products. Optimum temperatures were in the range 700-1000 ° C, duration 5-15 h. According SEM data the particle sizes of KZr2(PO4)3 and NaZr2(PO4)3 were in range 100-300 nm, and the particle sizes of KAlSi2O6 and CsMgAl0.5P1.5O6 is 30-60 and 5-10 µm respectively. The composites were prepared by mechanical mixing of the stoichiometric calculated batches of powders, followed by brief dispersing. Then, ceramic based on them was sintered by SPS method. Ceramics obtained by sintering SPS of early prepared and characterized powders of composites with different ratios of constituent phases: KZr2(PO4)3:KAlSi2O6 and NaZr2(PO4)3:Cs[MgAl0.5P1.5O6] 2:8, 4:6, had a relative density 95.2 – 97.6 %. Optimal Т(t) of sintering process were 960 °С (1 – 4 min). The values of microhardness, fracture toughness are established and amounted 3.6 – 6.6 GPa and 0.33 – 0.46 MPa•m1/2. The imprint of indentor has a square shape with concave sides inward. This suggests a fairly significant elastic recovery after unloading the print, which is characteristic of amorphous materials (eg fused silica). An unusual phenomenon was observed for composites 2:8 and 4:6 with compositions KZr2(PO4)3:KAlSi2O6. During the sintering process under certain conditions, there was a strong softening of the sample to the consistency of plasticine and its outflow from the mold under an applied pressure. According to XRD, the phase composition of the composite did not undergo changes.

 

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Annotation of the results obtained in 2017
In 2017, the research carried out under this project in 2016 was continued. The results of the work of 2016 were presented in the Report for 2016. The problem includes developing and perfection of properties of inorganic material (powders and ceramics) for consolidation of nuclear wastes, construction applications – for products of nuclear and space technology. Perfection of material properties was carried out due to the choice of structure, change of composition of the compound, choice of synthesis method. Investigations of composite materials contained additional phases in the form of metals, metal oxides, silicon carbide SiC along with the base compound with selected structure deserved special attention. Thus, a solution was proposed for the controlled change in plasticity - an important operating characteristic of a ceramic material. The objects of investigation - compounds with structures of the natural minerals and com-posites based on these compounds as basic ones. Base compounds had the structures of natural minerals: fluorite, garnet, monazite, kosnarite (NZP), scheelite. Oxides with fluorite structure ZrO2-xLnO1.5 (Ln = Sm, x = 0.22, 0.25, 0.27; Ln = Yb, x = 0.22, 0.27, 0.30) were synthesized by precipitation and sol-gel technology in four ways. The synthesis conditions were changed. The synthesis procedure was optimized according to analyzis of the microstructure of the obtained samples. The lots of powdered target products for following investigations were obtained using this technique. The behavior of the resulting powders when heated was investigated by the DSC method. The J (T) dependences were identical for the powders ZrO2 with Sm and ZrO2 with Yb. Peaks of heat extraction/absorption at the heating temperatures typical for phase transitions of the individual oxide ZrO2, were absent. The obtained data indicated a high thermal stability of a stabilized cubic phase with a fluorite-type structure (sp.gr. Fm3m) in zirconium oxide powders with samarium and ytterbium. Ceramics using the SPS method were obtained by changing the sintering regime: or with a constant grain size (0.4 and 1.2 μm) or with a constant density (ρrelative ~ 98%), affecting the microstructure of the samples. The sintering time of the samples of solid solutions was changed from 1 to 30 minutes at a constant pressure of P = 70 MPa. Microhardness was measured: Hv = 7.4-13.1 GPa (average value – 12.4 GPa). The activation energy of sintering Qs of the obtained ceramics was determined. The phase composition of the ceramics did not change. The influence of the microstructure (SEM method) on chemical stability was established. The studies of ceramics with different grain sizes and different density values were carried out in a static mode at T = 90 °C, P = 1 atm in distilled water for 56 days. According to XRD data phase compositions of the ceramics did not change after tests, that indicates their stability under the selected conditions. Garnet-type oxide Y2.5Nd0.5Al5O12 and composites based on Y2.5Nd0.5Al5O12 – W, Mo, MgO, SiC, х = 10, 20 vol.% were obtained in the form of powders by coprecipitation method. A laboratory planetary ball mono mill (for composites with SiC) was used for dispersing of the powder mixture. The ceramics were prepared by the SPS method. Powder and ceramic samples were characterized by XRF and SEM methods. Inclusions of microparticles of metals/MgO (microstructural analysis) were seen on the surfaces of garnet grains. Optimum process conditions were found for ceramics obtained by the SPS method. The sintering time of the composites did not exceed 9 min. Chemical interaction of MgO with base compound occurred during sintering of composites. The studies of the regularities of sintering of ceramics and composites with W, Mo,SiC under conditions of high-speed heating have been carried out. It was established that the activation energies of the sintering processes of garnet - tungsten/molybdenum composites were small and turned out to be much less than the activation energy of the garnet sintering, the reason for these changes was explained. The introduction of SiC particles leads to a change in the sintering mechanism of composites based on Y2.5Nd0.5Al5O12 – the SiC particles contribute to the formation of a finer granular structure in garnet and, as a result, provide the conditions for accelerated sintering of ceramics at low temperatures. Values of relative density were from 87 to 99%. The microhardness of composites with Mo was 10.8 and 9.6 GPa, the fracture toughness 2.1 and 2.3 MПа·м-1/2 in the series x = 10, 20 vol. %. The microhardness of composites with W was 13.3 and 11.9 GPa, the fracture toughness from 1.9 to 2.1 MПа·м-1/2 in the series x = 10, 20 vol. %. The microhardness of composites with SiC was 13.9 GPa, the fracture toughness 2.0 and 2.3 MПа·м-1/2 in the series x = 10, 20 vol. %. Phosphates with monazite structure PrPO4, NdPO4 and composites NdPO4 - x(wt.%)Ni, x = 5, 10, 20 were obtained in the form of powders: PrPO - by solid-phase method, composites - from aqueous suspensions. Ceramics were sintered by the SPS method. The ceramics of PrPO4 were sintered 1) with a constant grain size, 2) with a constant density. Sintering was carried out in vacuum, Tshrinkage process = 1000 - 1200 °C, t = 4 min. The grain growth in ceramics (d = 3.5 - 8.3 μm) at the same ρrel = 97% was achieved by stage of isothermal holding (1 – 30 min). According to the SEM data, ceramics was almost non-pores, but the initial phase PrPO4 was destroyed. The microhardness values for all the obtained PrPO4 ceramics are within the range of 3.2-6.2 GPa. Sintering of NdPO4 - x(wt.%)Ni, x = 5, 10, and 20 samples was carried out in a vacuum, the shrinkage process occurred at T = 900 - 1200 °C in t = 5-6 min. The stage of isothermal holding was absent. Microhardness and fracture toughness of the obtained ceramics was HV = 5.55 - 5.85 GPa (with a decrease in the value with an increase in the amount of Ni in the composition), KIC = 0.86 - 0.87 MPa·m1/2. According to XRD data, after sintering process the phase composition of the powder samples does not saved. Phosphates with NaZr2(PO4)3 structure (NZP) Ca0.5(1+x)Zr2-xFex(PO4)3, x = 0 – 1.25 were chosen on the basis of modeling of compositions with expected low and controlled thermal expansion using crystallochemical principles. The powders was obtained by a sol-gel method. It was found (XRD data), that obtained phases crystallize in the NaZr2(PO4)3-structure type, trigonal system, sp. gr. R-3c. Using high-temperature X-ray diffraction method, parameters of thermal expansion for a temperature range from 25 to 800 °C (step - 100 °C) were determined. As x increases, the absolute values of the linear coefficients of thermal expansion and the anisotropy of thermal expansion decreases (αa from -3.31 to 0.46 ·10-6 °C-1, αс from 9.88 to 8.73 ·10-6 °C-1, Δα from 13.19 to 8.27 ·10-6 °C-1). As is evident from the obtained results, most of the obtained phosphates of the series Ca0.5(1+x)Zr2-xFex(PO4)3 can be characterized as low expanding upon heating and can be recommended as thermal "stresses" resistant materials. Tungstates with scheelite structure and composites based on them Nax(Ca/Sr)1-2xNdxWO4, x = 0 - 0.5 Δx = 0.1; NaxCa1-2xNdxWO4 – y(wt.%)MgO, x = 0, 0.1, 0.3, 0.5, y = 5, 10, 20 were synthesized in the form of powders in aqueous solution by coprecipitation method, the ceramics - by the SPS method. Nd-containing powders had a greater homogeneity of the microstructure (SEM data) compared to pure (Ca/Sr)WO4. The particle size distribution was 10 μm for (Ca/Sr)WO4 and 2 - 5 μm for Nd-containing samples. Tthe IR spectras were obtained and analyzed. Noticeable shift of the IR-frequency in the samples with different Nd concentrations was not observed. Using the method of high-temperature XRD, the behavior at T = 25 - 1000 ° C of compounds Nax(Ca/Sr)1-2xNdxWO4, x = 0 - 0.5 was studied. The values of thermal expansion were calculated (10^6, °C^(-1)): the NaxCa1-2xNdxWO4 series, αa: 13.43 - 13.22; αс: 22.05 - 25.79; αmv: 16.30 - 17.41; β: 47.48 - 54.44; Δα: 8.62 - 12.57; the NaxSr1-2xNdxWO4 series, αa: 14.04 - 12.94; αс: 22.43 - 25.75; αmv: 16.84 - 17.20; β: 43.55 - 53.43; Δα: 8.39 - 12.79. The change in composition did not affect them. The studied tungstates are classified as highly expandable. During sintering of the ceramics NaxCa1-2xNdxWO4, x = 0, 0.1, 0.2, 0.5 and CaWO4/Na0.1Ca0.8Nd0.1WO4 - x(wt.%)MgO, x = 5, 10, 20 the temperature of the beginning and the end of the shrinkage varied with the variation of x in the composition of the samples and y of the composites. The chemical composition of the ceramics did not have a significant effect on their hardness and fracture toughness: HV = 4.3 - 4.9 GPa, KIC = 0.7 - 0.9 MPa·m1/2. The resulting ceramics retained the phase composition of the initial powder samples. Samples CaWO4-y(масс.%)MgO of composite ceramics were destroyed under the load of the indenter, while experiments to determine microhardness and fracture toughness were carried out, microhardness for Na0.1Ca0.8Nd0.1WO4 – y(масс.%)MgO ceramics was HV = 4.97 – 5.58 GPa. A sample of Na0.2Ca0.6Nd0.2WO4 ceramic was kept in distilled water for 28 days under static conditions at T = 25 °C. The neodymium concentrations found in each of the samples were less than the limit of detection of the device (0.1 mg/l), which makes it possible to speak of this sample of ceramics as hydrolytically stable. Molybdates with scheelite structure NaxSr1-2xNdxMoO4, x = 0 - 0.5 and composites on their basis NaxSr1-2xNdxMoO4 - y (mass.%) MgO, x = 0.1 - 0.5; y = 5, 10, 20 were obtained in the form of powders by sol-gel technology and ceramics by SPS method. According to the XRD data the powders were monophasic and had a structure of scheelite (tetragonal system, sp. gr. I41/a). Powders of composites with magnesium oxide NaxSr1-2xNdxMoO4 – y%MgO, x = 0.1, 0.3, 0.5; y = 5, 10, 20 were obtained from them using methods of “wet” chemistry with subsequent heating. Samples had two phases: 1) NaxSr1-2xNdxMoO4, 2) MgO. Phase compositions did not change in the sintering process. The values of relative density was 97.1 – 99.2 % (system 1) and 95.1 – 99.9 % (system 2) at the temperatures and times of sintering 872 – 985 °C, 870 – 900 °C, and 140-150 s, 650-800 s, respectively. Ceramics obtained at short time intervals can be practically non-porous, that will reduce their reactivity and increase stability in relation to the destructive factors of natural and technogenic character, respectively. The microstructure of powders and ceramics was investigated by SEM method. Size particles practically did not change, the separation of grains in agglomerates up to the individual particles has not occurred. The microhardness values were HV = 3.44 – 3.89 GPa. The studied ceramics are proposed as a matrix for the consolidation of radioactive isotopes of AEE and REE (fission products).

 

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