Novel Method to Achieve Temperature-Stable Microwave Dielectric Ceramics: A Case in the Fergusonite-Structured NdNbO4 System

Microwave dielectric ceramics with permittivity (epsilon r) similar to 20 play an important role in massive multiple-input multiple-output (MIMO) technology in 5G. Although fergusonite-structured materials with low dielectric loss are good candidates for 5G application, tuning the temperature coefficient of resonant frequency (TCF) remains a problem. In the present work, smaller V5+ ions (rV = 0.355 angstrom, with coordination number (CN) = 4) were substituted for Nb5+ (rNb = 0.48 angstrom with CN = 4) in the Nd(Nb1-xVx)O4 ceramics, which, according to in situ X-ray diffraction data, lowered the fergusonite-to-scheelite phase transition (TF-S) to 400 degrees C for x = 0.2. The thermal expansion coefficient (alpha L) of the high-temperature scheelite phase was +11 ppm/degrees C, whereas for the low-temperature fergusonite phase, it was + 14 < alpha L < + 15 ppm/degrees C. The abrupt change in alpha L, the associated negative temperature coefficient of permittivity (tau epsilon), and the minimum value of epsilon r at TF-S resulted in a near-zero TCF similar to (+7.8 ppm/degrees C) for Nd(Nb0.8V0.2)O4 (epsilon r similar to 18.6 and Qf similar to 70,100 GHz). A method to design near-zero TCF compositions based on modulation of tau epsilon and alpha L at TF-S is thus demonstrated that may also be extended to other fergusonite systems.