Microwave Dielectric Properties of Ca4(La4Pr2)(SiO4)4(PO4)2O2 Ceramics Doped with Isovalent and Aliovalent Ions
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Isovalent and aliovalent substitutions in the cationic and anionic sites of Ca4(La4Pr2)(SiO4)4(PO4)2O2 and Ba4(La4Pr2)(SiO4)4(PO4)2O2 ceramics were explored in this study. Substitutions of (SiO4)4− ions with (GeO4)4− in Ca4(La4Pr2)(SiO4)4(PO4)2O2 ceramic increased its densification temperature and thus led to a significant grain growth; however, replacement of Ca2+ and (PO4)3− ions by Mg2+ ions and (VO4)3− ions, respectively, decreased its densification temperature and resulted in grain refinement. Various rare-earth-metal and (WO4)2− ion substitutions on La3+ and Pr3+ and (PO4)3− sites, respectively, of Ca4(La4Pr2)(SiO4)4(PO4)2O2 ceramic had no impact on the sintering temperature. In the case of Ca4(La4Pr2)(SiO4)4(PO4)2O2 apatite with Mg2+ and (WO4)2− ion substitutions, second phases were observed in the x-ray diffraction patterns; however, only the pure hexagonal apatite phase with space group P63/m was visible in all the other systems. Overall, Ca4(La2Nd2Pr2)(SiO4)4(PO4)2O2 ceramic has the best microwave dielectric properties including εr = 14.2, Q × f = 28,745 GHz and τf = 0.9 ppm/°C. The densification temperatures of Ba4(La4Pr2)(SiO4)4(PO4)2O2 ceramics decreased as the SiO44− and PO43− ions were substituted by WO42− ions. The formation of the secondary phases BaWO4 and Ba2SiO4 decreased their dielectric constant and increased their Q × f values.
KeywordsMicrowave dielectric ceramics apatite microstructure XRD
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