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The structure, microstructure, coefficient of thermal expansion (CTE), and mechanical properties of Na1+2xZr2−xCox(PO4)3 ceramics (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were studied. Na1+2xZr2−xCox(PO4)3 submicron powders with the NaZr2(PO4)3 structure (NZP, kosnarite type) were obtained by the solid-phase method. The starting reagents (NaNO3, ZrOCl2·8H2O, NH4H2PO4, CoCl2·6H2O, ethanol) were mixed with the addition of ethyl alcohol. The resulting mixtures were annealed at 600 °C (20 h) and 700 °C (20 h). The obtained phosphates crystallized in the expected structure of the NaZr2(PO4)3 type (trigonal system, space group R 3 ¯ c). Thermal expansion of the powders was studied with high-temperature X-ray diffraction at temperatures ranging from 25 to 700 °C. CTEs were calculated, and their dependence on the cobalt content was analyzed. Na1+2xZr2−xCox(PO4)3 ceramics with high relative density (93.67–99.70%) were obtained by Spark Plasma Sintering (SPS). Ceramics poor in cobalt (x = 0.1) were found to have a high relative density (98.87%) and a uniform fine-grained microstructure with a grain size of 0.5–1 µm. Bigger cobalt content leads to a smaller relative density of ceramics. During the sintering of ceramics with high cobalt content, anomalous grain growth was observed. The powder compaction rate was shown to be determined by creep and diffusion intensity in the Na1+2xZr2−xCox(PO4)3 crystal lattice. SPS activation energy in ceramics increased as the cobalt content grew. The microhardness and fracture toughness of ceramics did not depend on their cobalt content. [ABSTRACT FROM AUTHOR]

Submicron-grade powders of Na1-xZr2(PO4)3-x(XO4)x compounds (hereafter referred to as NZP) and Ca1-xZr2(PO4)3-x(XO4)x compounds (hereafter, CZP), X = Mo, W (0 ≤ x ≤ 0.5) were obtained by sol-gel synthesis. The compounds obtained were studied by X-ray diffraction phase analysis and electron microscopy. An increase in the W or Mo contents was shown to result in an increase in the unit cell volume of the NZP and CZP crystal lattices and in a decrease in the coherent scattering region sizes. Thermal expansion behavior at high temperatures of synthesized NZP and CZP compounds has been investigated. The dependencies of the parameters a and c on the heating temperature, as well as the temperature dependence of the crystal lattice unit cell volume V in the range from the room temperature up to 800 °C, were obtained. The dependencies of the average thermal expansion coefficient (αav) and of the volume coefficient (β) on the W and Mo contents in the compositions of NZP and CZP compounds were studied. Ceramics Na1-xZr2(PO4)3-x(XO4)x with relatively high density (more than 97.5%) were produced by spark plasma sintering (SPS). The increase in the W or Mo contents in the ceramics leads to an increase in the relative density of NZP and to a decrease of the optimum sintering temperature. The mean grain size in the NZP ceramics decreases with increasing W or Mo contents. The study of strength characteristics has revealed that the hardness of the NZP ceramics is greater than 5 GPa, and that the minimum fracture toughness factor was 1 MPa·m1/2. [ABSTRACT FROM AUTHOR]

This paper introduces the results of hydrolytic stability tests and radiation resistance tests of phosphate molybdates and phosphate tungstates Na1−xZr2(PO4)3−x(XO4)x, X = Mo, W (0 ≤ x ≤ 0.5). The ceramics characterized by relatively high density (more than 97.5%) were produced by spark plasma sintering (SPS) of submicron powders obtained by sol–gel synthesis. The study focused on hydrolytic resistance of the ceramics in static mode at room temperature. After 28 days of testing in distilled water, the normalized leaching rate was determined. It was found that the ceramics demonstrated high hydrolytic resistance in static mode: the normalized leaching rates for Mo- and W-containing ceramics were 31·10−6 and 3.36·10−6 g·cm−2·day−1, respectively. The ceramics demonstrated high resistance to irradiation with 167 MeV Xe+26 multiple-charged ions at fluences ranging from 1·1012 to 6·1013 cm−2. The Mo-containing Na0.5Zr2(PO4)2.5(XO4)0.5 ceramics were shown to have higher radiation resistance than phosphate tungstates. Radiation was shown to trigger an increase in leaching rates for W and Mo in the crystal structure of NZP ceramics. [ABSTRACT FROM AUTHOR]

Sodium zirconium phosphate (NaZr 2 (PO 4) 3 , NZP) type phosphate compounds have been considered as a candidate material for the immobilization of radionuclides. In this work, the highly densified NZP-type ceramic waste forms for immobilizing simulated radionuclides Sr and Cs, which were designed as the formula of Cs 1-2 x Sr x Zr 2 (PO 4) 3 (x = 0, 1/12, 2/12, 3/12, 4/12, 5/12, and 6/12), were prepared by microwave-assisted solid-state sintering method. The effects of Sr and Cs incorporation on the phase composition, microstructure, densification, and chemical durability of Cs 1-2x Sr x Zr 2 (PO 4) 3 ceramic waste forms were systematically discussed. It was shown that the single CsZr 2 (PO 4) 3 (CsZP) phase was generated in the samples when x ≤ 2/12, while two phases of CsZP and Sr 0.5 Zr 2 (PO 4) 3 (SrZP) were formed when 3/12 ≤ x ≤ 5/12. The Rietveld refinement results revealed that Sr/Cs could be incorporated in the NZP crystal structure. The as-prepared samples all presented a well dense microstructure, whose relative density reached up to approximately 98% with Sr incorporation. In addition, the Product Consistency Test (PCT) leaching results demonstrated that the ceramics waste forms simultaneously immobilizing Sr and Cs exhibited superior leaching resistance, and the leaching rates of Sr and Cs elements were in the order of 10−3-10−4 g m−2 d−1. The increase of Sr incorporation brought about the decreased leaching rates of ceramic samples. [ABSTRACT FROM AUTHOR]

This paper introduces the results of hydrolytic stability tests and radiation resistance tests of phosphate molybdates and phosphate tungstates Na1−xZr2(PO4)3−x(XO4)x, X = Mo, W (0 ≤ x ≤ 0.5). The ceramics characterized by relatively high density (more than 97.5%) were produced by spark plasma sintering (SPS) of submicron powders obtained by sol–gel synthesis. The study focused on hydrolytic resistance of the ceramics in static mode at room temperature. After 28 days of testing in distilled water, the normalized leaching rate was determined. It was found that the ceramics demonstrated high hydrolytic resistance in static mode: the normalized leaching rates for Mo- and W-containing ceramics were 31·10−6 and 3.36·10−6 g·cm−2·day−1, respectively. The ceramics demonstrated high resistance to irradiation with 167 MeV Xe+26 multiple-charged ions at fluences ranging from 1·1012 to 6·1013 cm−2. The Mo-containing Na0.5Zr2(PO4)2.5(XO4)0.5 ceramics were shown to have higher radiation resistance than phosphate tungstates. Radiation was shown to trigger an increase in leaching rates for W and Mo in the crystal structure of NZP ceramics.

Submicron-grade powders of Na1-xZr2(PO4)3-x(XO4)x compounds (hereafter referred to as NZP) and Ca1-xZr2(PO4)3-x(XO4)x compounds (hereafter, CZP), X = Mo, W (0 ≤ x ≤ 0.5) were obtained by sol-gel synthesis. The compounds obtained were studied by X-ray diffraction phase analysis and electron microscopy. An increase in the W or Mo contents was shown to result in an increase in the unit cell volume of the NZP and CZP crystal lattices and in a decrease in the coherent scattering region sizes. Thermal expansion behavior at high temperatures of synthesized NZP and CZP compounds has been investigated. The dependencies of the parameters a and c on the heating temperature, as well as the temperature dependence of the crystal lattice unit cell volume V in the range from the room temperature up to 800 °C, were obtained. The dependencies of the average thermal expansion coefficient (αav) and of the volume coefficient (β) on the W and Mo contents in the compositions of NZP and CZP compounds were studied. Ceramics Na1-xZr2(PO4)3-x(XO4)x with relatively high density (more than 97.5%) were produced by spark plasma sintering (SPS). The increase in the W or Mo contents in the ceramics leads to an increase in the relative density of NZP and to a decrease of the optimum sintering temperature. The mean grain size in the NZP ceramics decreases with increasing W or Mo contents. The study of strength characteristics has revealed that the hardness of the NZP ceramics is greater than 5 GPa, and that the minimum fracture toughness factor was 1 MPa·m1/2.

New NZP-type solid solutions, Na1+2xZr2–xNix(PO4)3 and Ca0.5+xZr2–xNix(PO4)3 with x = 0.10, 0.25, and 0.50, have been prepared by solid-state reactions. The synthesized samples have been characterized by X-ray diffraction and IR spectroscopy. Crystal structure of the compound CaZr1.5Ni0.5(PO4)3 has been refined by full-profile X-ray diffraction analysis method, reliability factors is Rp = 4.78% and Rwp = 6.45%, respectively. Thermal expansion of the phosphates has been studied by high-temperature X-ray diffraction in the temperature range 25–700°C. The synthesized compounds have been shown to be intermediate- and low-thermal-expansion materials. Using SPS method we have produced ceramic samples with a high relative density (ρrel > 97%), based on phosphate powders. Strength characteristics (microhardness and fracture toughness) of the ceramics have been determined. [ABSTRACT FROM AUTHOR]