**2.2 High-temperature electromagnetic measurements**

In order to evaluate the high-temperature permittivity, specimens with the size of 22.86 × 10.16 × 1.5 mm3 were polished and determined in X-band through the wave-guide method with a vector network analyzer (Agilent N5230A, USA). As shown in **Figure 2**, the as-prepared Si3N4 ceramic sample was heated by an inner heater with a ramp rate of 10°C/min up to 800°C in air. For accuracy of measurement, the device was carefully calibrated with the through-reflect-line (TRL) approach, and a period of 10 min was applied to guarantee system stability at each evaluated temperature.

## **Figure 1.**

*The gelcasting process for preparation of multilayer Cf/Si3N4 composites (reprinted with permission from Ref. [35]).*

**Figure 2.**

*Schematic diagram of complex permittivity test apparatus (reprinted with permission from Ref. [48]).*
