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

Microwave dielectric ceramics with permittivity (ε) ∼ 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 V ions ( = 0.355 Å, with coordination number (CN) = 4) were substituted for Nb ( = 0.48 Å with CN = 4) in the Nd(NbV)O ceramics, which, according to X-ray diffraction data, lowered the fergusonite-to-scheelite phase transition () to 400 °C for = 0.2. The thermal expansion coefficient (α) of the high-temperature scheelite phase was +11 ppm/°C, whereas for the low-temperature fergusonite phase, it was + 14 < α < + 15 ppm/°C. The abrupt change in α, the associated negative temperature coefficient of permittivity (τ), and the minimum value of ε at resulted in a near-zero TCF ∼ (+7.8 ppm/°C) for Nd(NbV)O (ε ∼ 18.6 and Qf ∼ 70,100 GHz). A method to design near-zero TCF compositions based on modulation of τ and α at is thus demonstrated that may also be extended to other fergusonite systems.