NaZr 2-x B x (PO 4) 3-2x (SO 4) 2x (0 ≤ x ≤ 1.25, B = Mg, Co, Ni, Cu, Zn), and NaZr 2x R x (PO 4) 3-x (SO 4) x (0 ≤ x ≤ 1.25, R = Al, Fe) phosphate-sulfates series have been prepared by a sol-gel process. These compounds belong to the NaZr 2 (PO 4) 3 (NZP) structure family and crystallize in hexagonal crystal system, space group R3 c. Limited solid solution series were found to exist; their formation temperatures and thermal stability limits were determined. Particle sizes as determined by microstructure observation were 50-200 nm, and for Cu-and Zn-containing samples, 200-500 nm. The thermal expansion of phosphate-sulfate NaZr 1.25 Cu 0.75 (PO 4) 1.5 (SO 4) 1.5 was studied in the range 25-700°C. Thermal expansion coefficients and thermal expansion anisotropy were found to be α a =
Ca 0.5(1+x) Zr 2-x Fe x (PO 4) 3 phosphates have been synthesized by a sol-gel process. The individual compounds and solid solutions obtained crystallize in the NaZr 2 (PO 4) 3 structure (trigonal symmetry, sp. gr. R3). Using high-temperature X-ray diffraction, we have determined their thermal expansion parameters in the temperature range from 25 to 800°C. With increasing x, the magnitudes of their linear thermal expansion coefficients and thermal expansion anisotropy decrease. Most of the synthesized phosphates can be rated as low-thermal-expansion compounds and can be regarded as materials capable of withstanding thermal "stress."
Ca 0.75+0.5x Zr 1.5 Fe 0.5 (PO 4 ) 3-x (SiO 4 ) x (x = 0-0.5) solid solutions have been synthesized by a sol-gel process and characterized by X-ray diffraction, IR spectroscopy, and differential scanning calorimetry. As expected, the synthesized phosphatosilicates crystallize in a NaZr 2 (PO 4 ) 3 -type structure (trigonal symmetry, sp. gr. R3 c). The thermal expansion of the solid solutions has been studied by high-temperature Xray diffraction in the temperature range from 25 to 800°C. Their thermal expansion parameters have been calculated and analyzed as functions of composition. Highdensity ceramics based on the Ca 0.875 Zr 1.5 Fe 0.5 (PO 4 ) 2.75 (SiO 4 ) 0.25 phosphatosilicate have been produced by spark plasma sintering and their structure and properties have been studied in detail.
CONCLUSIONSCa 0.75+0.5x Zr 1.5 Fe 0.5 (PO 4 ) 3-x (SiO 4 ) x (x = 0-0.5) solid solutions isostructural with NaZr 2 (PO 4 ) 3 have been synthesized and characterized. According to the present high-temperature X-ray diffraction data obtained in the range 25-800°C, their lattice parameters a and c are linear functions of temperature: heating reduces the a cell parameter and increases c. The incorporation of silicate groups has been shown to lead to a sharp decrease in the absolute value of a-axis thermal expansion coefficient and a slight increase in the c-axis coefficient. The Ca 0.875 Zr 1.5 Fe 0.5 (PO 4 ) 2.75 (SiO 4 ) 0.25 (x = 0.25) and CaZr 1.5 Fe 0.5 (PO 4 ) 2.5 (SiO 4 ) 0.5 (x = 0.5) phosphatosilicates have intermediate thermal expansion coefficients: 2 × 10 -6 ≤ α av ≤ 8 × 10 -6°C-1 .The observed general trends in the effect of anion substitutions in the framework on the thermal expansion of NaZr 2 (PO 4 ) 3 -type compounds can be helpful in engineering materials with tailored thermal expansion parameters.Using one composition as an example, we have demonstrated the possibility of producing ceramic samples with high relative density (~99.7%) and high mechanical strength by SPS.
ACKNOWLEDGMENTSThis work was supported by the Russian Science Foundation, project no. 16-13-10464: Advanced ceramic like mineral materials with improved and adjustable service characteristics: design, synthesis, study.
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