Effect of Zn and Ca modifications on crystallization and microwave dielectric properties of lanthanum borates

▶ Low loss dielectrics based on two borate glass systems, (13–19)CaO·(13–19)La 2O 3·68B 2O 3 and (13–19)ZnO·(13–19)La 2O 3·68B 2O 3, are investigated by focusing on effects of the CaO/La 2O 3 and ZnO/La 2O 3 ratios on the physical and dielectric properties. Identical compositions have not been reported so far. ▶ The effects of increasing the relative content of Ca and Zn were found to be opposite specifically in microstructure evolution and crystallization behavior, and, thus, dielectric properties. It suggests that the glass modifiers can be very critical in controlling the densification and crystallization behavior. ▶ In the CaO·La 2O 3·B 2O 3 samples, the lower content of Ca is preferred since densification is enhanced and more LaBO 3 phase is produced. ▶ In the ZnO·La 2O 3·B 2O 3 samples, on the contrary, the higher content of Zn seems to be more effective in generating a larger difference between the softening and crystallization temperatures, and, hence, better densification with least porosity. ▶ As the best glass composition, 19ZnO·13La 2O 3·68B 2O 3 having the increased content of the LaAl 2.03(B 4O 10)O 0.54 phase exhibited promising dielectric properties of k ∼ 7.4 and Q × f ∼ 18,100. Low loss dielectrics based on two borate glasses, (13–19)CaO·(13–19)La 2O 3·68B 2O 3 and (13–19)ZnO·(13–19)La 2O 3·68B 2O 3, have been investigated with a focus on effects of the CaO/La 2O 3 and ZnO/La 2O 3 ratios on physical and dielectric properties. All samples were prepared by mixing glass with 40 wt% Al 2O 3 filler and by firing at 850 °C for 30 min. The effects of increasing relative contents of Ca and Zn were found to be opposite specifically in microstructure evolution and crystallization behavior, and thus, dielectric properties. For example, the 13CaO·19La 2O 3·B 2O 3 sample having the lowest Ca content showed the highest Q × f of ∼8100, while the 19ZnO·13La 2O 3·B 2O 3 sample having the highest Zn content showed the highest Q × f of ∼18,100. The extensive crystallization of low loss phases, i.e., LaBO 3 and LaAl 2.03(B 4O 10)O 0.54, as well as the improved level of densification are assumed to be responsible for better dielectric performance.