## 3 Results and Discussion
Subsequent analysis via EDS (Figure S3 of the Supplementary Materials) reveals that the precipitated grains (marked as point B in Figure 7b) are mainly composed of Cu-rich phase, which is consistent with the analysis of XRD refinement. The composition of the matrix (marked as point A in Figure 7b) exhibits a Cu-deficient Cu\({}_{0.3}\)Zn\({}_{0.7}\)Ti\({}_{0.2}\)Zr\({}_{0.8}\)Nb\({}_{2}\)O\({}_{8}\) phase, which is probably due to some of the Cu diffusing into the precipitated grains. When \(y=\) 40 (Figure 7c), the average grain size decreased even further to approximately (4.6 \(\pm\) 0.7) \(\upmu\)m, with the grain morphology becoming more uniform and the grain boundary clarity increasing, and the composition of the precipitated phase is similar to that of \(y=\) 20. By \(y=\) 50 (Figure 7d), the grain size significantly decreased to approximately (3.0 \(\pm\) 0.4) \(\upmu\)m. After doping with LiF, grain refinement was observed, indicating that LiF plays a regulatory role in the grain growth process of Cu\({}_{0.3}\)Zn\({}_{0.7}\)Ti\({}_{0.2}\)Zr\({}_{0.8}\)Nb\({}_{2}\)O\({}_{8}\) ceramics. The reduction in average grain size may be attributed to the potential decrease in the optimal sintering temperature and the inhibition of grain growth by the higher surface energy associated with increasing sintering aid (i.e., LiF) doping levels [32]. Figure 8 shows the relative densities, \(\varepsilon_{r}\), \(Q\times f\), and \(\tau_{f}\) values of the \(y\) mol% LiF (\(10\leq y\leq 50\))-added Cu\({}_{0.3}\)Zn\({}_{0.7}\)Ti\({}_{0.2}\)Zr\({}_{0.8}\)Nb\({}_{2}\)O\({}_{8}\) ceramics sintered at 950 \({}^{\circ}\)C for 4 h. The relative densities of the specimens with \(10\leq y\leq 40\) exhibited high values (\(\geq\)95%), indicating that they were well sintered under the current conditions. However, when \(y=50\), its relative density decreased significantly to around 90%, probably due to the generation of an excessive liquid phase caused by the LiF. The trend of \(\varepsilon_{r}\) is similar to that of relative density, both of which first increased and then decreased with the addition of LiF, followed by another rise and subsequent fall. As the quantity of LiF increased, the \(Q\times f\) values discernibly followed an upward trend, culminating in a peak value of approximately 45,543 GHz at \(y=40\). However, when \(y\) exceeded 40, the \(Q\times f\) value decreased to 34,536 GHz at \(y=50\), probably because of the sharply decreased relative density. For mixtures, \(\tau_{f}\) is mainly related to the phase composition. For the specimens with low LiF addition, the variation in \(\tau_{f}\) values is nearly identical to \(\varepsilon_{r}\). However, when \(y\geq 30\), the deeply negative \(\tau_{f}\) of LiF (\(-117.7\) ppm/\({}^{\circ}\)C) [33] deteriorates the \(\tau_{f}\) of the specimen with \(y=50\) to around \(-57.7\) ppm/\({}^{\circ}\)C. Overall, the 40 mol% LiF-added Cu\({}_{0.3}\)Zn\({}_{0.7}\)Ti\({}_{0.2}\)Zr\({}_{0.8}\)Nb\({}_{2}\)O\({}_{8}\) ceramics sintered at 950 \({}^{\circ}\)C for 4 h showed good microwave dielectric properties: \(\varepsilon_{r}\) of 32, \(Q\times f\) of 45,543 GHz, and \(\tau_{f}\) of \(-43.5\) ppm/\({}^{\circ}\)C. Furthermore, when compared to the \(M^{2+}N^{4+}\)Nb\({}_{2}\)O\({}_{8}\)-type ceramics listed in Table 1, which have been reported in recent years, the Cu\({}_{0.3}\)Zn\({}_{0.7}\)Ti\({}_{0.2}\)Zr\({}_{0.8}\)Nb\({}_{2}\)O\({}_{8}\) ceramics doped with 40 mol% LiF exhibit an exceptionally outstanding performance in terms of both sintering temperature and microwave dielectric properties. Figure 7: SEM images of Cu\({}_{0.3}\)Zn\({}_{0.7}\)Ti\({}_{0.2}\)Zr\({}_{0.8}\)Nb\({}_{2}\)O\({}_{8}\) supplemented with \(y\) mol% LiF (\(0\leq y\leq 50\)): (**a**) \(y=10\); (**b**) \(y=20\) (Point A was taken from the matrix, while B was taken from precipitated grain); (**c**) \(y=40\); (**d**) \(y=50\). Silver, owing to its exceptional conductivity coupled with a comparatively economical cost, has been ubiquitously employed as an electrode in LTCC devices.