Low loss and good chemical compatibility with silver electrodes of Cu2+-substituted Ba3Ti4Nb4O21 ceramics realizing low temperature co-fired ceramics technology: Crystal structure, vibration modes and chemical bonds studies

New low loss and low-sintering temperature co-fired Ba 3- x Cu x Ti 4 Nb 4 O 21 (BCTN, 0 ≤  x  ≤ 0.12) ceramics with 0.60  wt % Li 2 O-B 2 O 3 -SiO 2 -CaO-Al 2 O 3 (LBSCA) glass were prepared by solid-state reaction methodology. This work showed that CuO and LBSCA were effective sintering aid, which improved the densification and decreased sintering temperature . Thus, the excellent microwave dielectric properties of BCTN ceramics ( x  = 0.08) were obtained after sintering at 925 ℃ with ε r ~ 44.18, Q× f ~ 17,860 GHz (@ 5.6 GHz) and τ f ~ 94.76 ppm/℃. Q× f value was increased nearly 3-fold compared to pure BTN ceramics (~ 6090 GHz). Based on the P-V-L bond theory, the Ti-O and Nb-O bonds together contributed greatly to ε r . The Nb-O bonds was the main factor affecting the internal loss on Q× f . The τ f closely related to the oxygen octahedron [Ti1/Nb1O 6 ]. The BCTN ceramics would not react with Ag electrodes, and had great potential to be used in LTCC microwave devices.