**Acknowledgements**

*Electromagnetic Materials and Devices*

heated from room temperature to 800°C. This gradually increasing trend may be ascribed to the enhancement of scattering effect between electrons for increase both the number and energy of electrons. Furthermore, the relaxation time *<sup>τ</sup>* (216.1–250.2 ps) is almost twice as much as a single cycle for time-harmonic electromagnetic wave in X-band (*<sup>t</sup>*<sup>0</sup> <sup>=</sup> <sup>1</sup> <sup>⁄</sup> *<sup>f</sup>* = 80.65–121.95 ps). As a result, electronic migration could not keep up with the pace of external alternating electronic field, which leads

 *as a function of* ε *''⁄* ω *with an inset of temperature-dependent relaxation time (reprinted with* 

*(a) Argand diagram of Cf/Si3N4 composites at different temperature, (b) detailed view for the region marked* 

*by black box in (a) (reprinted with permission from Ref. [39]).*

In this chapter, microwave dielectric properties of multilayer C*f*/Si3N4 composites fabricated via gelcasting and pressureless sintering were intensively studied in X-band. Firstly, a strong frequency dependence of the real and imaginary parts of permittivity at room temperature was observed at the X-band. Particularly, an

to continuous decrease of permittivity with frequency increase.

**42**

**4. Conclusion**

**Figure 14.** *Plot of ε'*

**Figure 13.**

*permission from Ref. [39]).*

The authors would like to acknowledge the generous funding from the National Key Research and Development Program of China (Grant No. 2017YFA0204600), the State Key Development Program for Basic Research of China (Grant No. 2011CB605804), and the National Natural Science Foundation of China (Grant No. 51802352).
