**1. Introduction**

Wireless electronic devices and communication instruments have found wide application in our daily life. Their efficient operation depends strongly on transmission behavior of alternating electromagnetic wave with frequency ranging from kilohertz (KHz) to gigahertz (GHz), and vice versa, are very sensitive to interference from external electromagnetic wave. Driven by the demand for both adequate interference rejection and controlled radiation, more and more efforts have been devoted to high-performance electromagnetic compatibility/interference (EMC/ EMI) materials. As we all know, the propagation behavior of electromagnetic wave when encountering a material could be divided into three types in principle: reflection, absorption, and transmission. As typical EMI materials, metals or materials with high electrical conductivity could prevent external electromagnetic wave from penetration due to the large amount of free electrons. Last decades have witnessed intensive efforts toward exploring lightweight and cost-effective electromagnetic interference (EMI) materials with adequate shielding effectiveness [1–5], involving carbonaceous fillers-enabled polymers, novel lightweight metal composites, etc.

However, the primary function of EMI shielding is to reflect radiation using charge carriers that interact directly with incident electromagnetic field, and the back-radiation would in turn affect the surrounding environment and devices. What is more, the reflected radiation may also be caught by radar observation systems and lead to exposure of moving trace, which is extremely undesirable from the defense-oriented point of view. As a result, electromagnetic wave-absorbing materials with reduced reflection on the surface as well as enhanced internal attenuation are more favorable candidates for EMI shielding, especially in the GHz range. Polymers modified by carbon nanomaterials (e.g., carbon nanotubes [6–8], carbon nanofibers [9, 10], graphene [11, 12], etc.), metal powders [13, 14], and ferrite [15] have been demonstrated to be excellent microwave-absorbing/ shielding materials especially in the X-band (8.2–12.4 GHz) [16, 17], and have achieved successful application [18–20]. However, due to their inferior temperature stability and mechanical properties, their application is limited toward application under high temperature. For example, the temperature on the windward side of high-speed aircraft (>3 Ma) could reach up to 1000°C due to the aerodynamic heating effect. As a result, ceramics and their derivative architecture (r-GOs/SiO2, CNT/SiO2, ZnO/ZrSiO4, SiC*f*/SiC, etc.) [21–30] with the integration of desirable dielectric responses, high strength, oxidation resistance, thermo-stability, and low density have attracted growing attention for high-temperature-absorbing materials. Besides, Si3N4 ceramics are one of the most intensively studied ceramics in high-temperature applications due to their superior antioxidation (>1200°C) and mechanical and chemical stabilization properties [31–38]. More importantly, owing to the excellent electrical insulation property and low dielectric constant, Si3N4 ceramics are expected to be a promising candidate matrix as high-temperature microwave-absorbing materials [25, 29, 30, 39–42]. However, previous work mainly focused on experimental evolution of complex dielectric responses with temperature and qualitative analyses according to the Debye theory. Still, modeling for high-temperature dielectric behaviors is relatively limited and remains a great challenge due to the complexity of the components and microstructures for high-temperature microwave-absorbing materials, as well as high-temperature measurement system. It also should be noted that microwave dissipation capacity of a composite is strongly dependent on the structural design. Many investigations have shown that incorporation of reasonable structural design, involving multilayer structure [43–45] and periodic structure in metamaterials [46, 47], is an effective way to regulate dielectric response and guarantee desirable attenuation performance. Moreover, taking full advantage of tunable electromagnetic parameters in each layer, optimal microwave impedance matching as well as absorbing capability could be achieved. This fact means that it is essential to explore the mechanism of dielectric behavior of laminate-structure materials from new viewpoints.

In this chapter, we mainly focus on the microwave dielectric responses in laminate-structure or multilayer-structure C*f*/Si3N4 composites from both experimental and theoretical points of view. Furthermore, a general model with respect to permittivity as a function of temperature and frequency would be established to reveal mechanisms of temperature-dependent dielectric responses for C*f*/Si3N4 composites. These findings point to important guidelines to reveal the mechanism of dielectric behavior for carbon fiber functionalized composites including but not limited to C*f*/Si3N4 composites at high temperatures, and pave the way for the development of high-temperature radar absorbing materials.

**31**

**Figure 1.**

*Ref. [35]).*

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

Commercially available carbon fibers (T700, 12 K, TohoTenax Inc., Japan) were used as starting materials in this work. In order to avoid damage at hightemperature sintering, pyrolytic carbon(PyC)/SiC dual-coating on carbon fibers was prepared by chemical vapor deposition based on Methyltrichlorosilane (MTS)-H2-Ar system at 1150°C. The powder mixture of 85 wt% α-Si3N4 (purity > 93%, d50 = 0.5 μm, Beijing Unisplendor Founder High Technology Ceramics Co. Ltd., China), 5 wt% Al2O3, and 10 wt% Y2O3 was mixed with solvent-based acrylamide-N,N′-methylenebisacrylamide (AM-MBAM) system, and consolidated via gelcasting and pressureless-sintering route (as illustrated in **Figure 1**). Details of the multilayer C*f*/Si3N4 samples' preparation are given in our previous work [35]. Note that each layer of carbon fiber plays dominant role to attenuate microwave energy, which could be adjudged the surface density of short carbon fiber.

In order to evaluate the high-temperature permittivity, specimens with the size

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

*The gelcasting process for preparation of multilayer Cf/Si3N4 composites (reprinted with permission from* 

were polished and determined in X-band through the

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

**2.1 Preparation of multilayer C***f***/Si3N4 composites**

**2.2 High-temperature electromagnetic measurements**

**2. Experiments**

of 22.86 × 10.16 × 1.5 mm3

evaluated temperature.
