The formation of “dark holes” and their significant influences on microwave dielectric properties of Ba4.2Nd9.2Ti18O54 ceramics

• The relationship between dielectric loss and the size of ceramic has not been reported before.
• The distribution of "dark hole" in the BNT ceramics with different size were clearly presented.
• Dark holes were observed in the center of the ceramics.
• The Q × f value significantly dropped when the amount of Ti3 + ions was considerable.
• It would give a guidance to those people who have confusion about dielectric loss especially in large size microwave dielectric ceramics.

Ba4.2Nd9.2Ti18O54 (BNT) ceramics have been prepared by solid-state ceramic method. Dark holes were observed in the center of the ceramics. All the samples showed typical columnar grain morphology in SEM images. TEM and XRD analysis indicated that there was only a BaNd2Ti4O12 (JCPDS Card No. 44-0061) phase in the ceramics. In HRTEM images, the grain boundary was clear and no impurities and defects were observed. The XPS results showed the difference in valence of Ti. Ti3 + ions appeared in all the samples. The Q × f value significantly dropped when the amount of Ti3 + ions was considerable. BNT ceramics sintered at high temperature (1300 °C to 1400 °C) usually contain dark holes in the center (caused by Ti3 + ions) which could lead to a significant drop in Q × f value.