Effect of niobium pentoxide (Nb2O5) on the microstructure and properties of the diopside glass-ceramics produced from Bayan Obo mine tailing

Diopside (CaMgSi2O6) glass-ceramic, a typical derivative of the pyroxene system, has attracted considerable attentions in the fields of anti-erosion, anti-abrasion, light-emitting diode (LED) packaging, or electric circuit substrate materials because of its high mechanical strength, good physical and chemical stability, as well as remarkable low dielectric coefficient and loss. However, the effect of niobium pentoxide (Nb2O5) on the microstructure and property of this technically important glass-ceramic is still not fully understood. Therefore, diopside glass-ceramics with 0 wt%, 0.1 wt%, 0.025 wt%, 0.5 wt%, 1 wt%, 2 wt%, and 4 wt% of Nb2O5 were synthesized respectively by the traditional melting method using Bayan Obo mine tailing and fly ash as the main starting materials. The obtained materials were studied by differential scanning calorimetry, X-ray diffractometry, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, mechanical property, and dielectric property measurements to reveal the effect of Nb2O5 on its microstructure and property. The result shows the addition of Nb2O5 has a clear effect on increasing the formation of diopside crystals and the crystallization of calcium niobate (Ca2Nb2O7) secondary phase at the intergranular areas between diopside crystals. And this latter microstructural change relates closely with the decrease in the average size of diopside crystals and their morphology changing from the ones resembling chrysanthemum flowers to clusters of roughly round islands. Addition of Nb2O5 up to 2 wt% exhibits a near monotonous effect on improving the bending strength. The maximum value of it can reach as high as 236 MPa. The dielectric constant rises from 7 to 9.7 with the increase of Nb2O5 addition from 0 to 4 wt%, while the dielectric loss fluctuates within the range of 0.015 to 0.035. The comparison of the results of this study with those of peers showed the crystallization of diopside crystals was more likely enhanced through the mechanism related to the Nb2O5-induced phase separation in the base glass rather than the normally reported heterogeneous nuclei mechanism reported for Ferric oxide (Fe2O3), chromium trioxide (Cr2O3), and titania (TiO2).