**3.1 Structure of multilayer C***f***/Si3N4 composites**

The optical image of cross-section of multilayer C*f*/Si3N4 composites is shown in **Figure 3(a)**. As expected, three layers filled with short carbon fibers are uniformly embedded in the Si3N4 matrix. The microstructure of Si3N4 ceramic was formed by rod-like particles, which are evenly distributed and intercross with each other to form the main pores. Energy dispersive spectroscopy (EDS) analysis at spots A and B in **Figure 3(c)** demonstrates that the PyC/SiC interphase could effectively promote the chemical compatibility between carbon fibers and Si3N4 ceramic at high-temperature circumstance, which could be further proved by the XRD investigations (see **Figure 4**). As seen in **Figure 4**, in addition to the main *β*-Si3N4 peaks

#### **Figure 3.**

*(a) Cross-section of multilayer Cf/Si3N4 composites, facture surface located at (b) Si3N4 matrix and (c) carbon fibers (reprinted with permission from Ref. [39]).*

**33**

**Figure 4.**

*Dielectric Responses in Multilayer Cf/Si3N4 as High-Temperature Microwave-Absorbing Materials*

and Y-Al oxide peaks, additional C and *β*-SiC peaks, corresponding to the carbon

**3.2 Room-temperature dielectric properties of multilayer C***f***/Si3N4 composites**

constant is considered to be helpful for microwave impedance matching with free space, which tends to reduce reflection of electromagnetic wave from the surface of

However, both the real permittivity and imaginary permittivity of C*f*/Si3N4 sandwich composites decrease markedly as frequency increases at X-band, varying from 12.3 and 5.1 to 7.9 and 1.2, respectively. This phenomenon is usually called frequency dispersion characteristic, which is acknowledged to be beneficial to broaden the microwave absorption bandwidth. The reflection loss (*R*) of Si3N4 and C*f*/Si3N4

sandwich composites was calculated according to the formula as follows:

\_\_ \_\_ *μr*

*<sup>ε</sup><sup>r</sup>* tanh[*j*(\_\_\_ <sup>2</sup>*<sup>π</sup>*

where *Zin* refers to input impedance, *j* is the imaginary unit (i.e., equals to √

is the velocity of electromagnetic waves in free space, *f* is the microwave frequency,

*XRD patterns of as-prepared Si3N4 ceramics and Cf/Si3N4 composites (reprinted with permission from Ref. [39]).*

**Figure 5(a)** shows the real and imaginary permittivity of multilayer C*f*/Si3N4 composites at X-band, as well as as-prepared Si3N4 ceramics. Clearly, the dielectric constant of Si3N4 ceramics presents frequency-independent behavior. The mean

real and imaginary parts of permittivity and dielectric loss (tan*<sup>δ</sup>* <sup>=</sup> *<sup>ε</sup>*″

material and enhance energy propagating in the material.

*R*(*dB*) = 20 log|

*Zin* <sup>=</sup> <sup>√</sup>

According to the classical transmission line theory, microwave complex permit-

) is an important parameter to determine the absorbing performance.

*Zin* − 1 \_\_\_\_\_

\_\_\_\_

*c* )√

/*ε*′

*Zin* <sup>+</sup> <sup>1</sup><sup>|</sup> (1)

*μr ε<sup>r</sup> fd*] (2)

, respectively. The relatively low dielectric

) of pure Si3N4

\_\_\_ −1), *c*

fiber and modification coating, were detected for C*f*/Si3N4 composites.

*DOI: http://dx.doi.org/10.5772/intechopen.82389*

ceramic were 7.7, 0.04, and 5.3 × 10<sup>−</sup><sup>3</sup>

tivity (*<sup>ε</sup>* <sup>=</sup> *<sup>ε</sup>*′ <sup>−</sup> *<sup>j</sup> <sup>ε</sup>*″

and

and Y-Al oxide peaks, additional C and *β*-SiC peaks, corresponding to the carbon fiber and modification coating, were detected for C*f*/Si3N4 composites.
