The microstructure of SiCN ceramics and their excellent electromagnetic wave absorbing properties

doi:10.1016/j.ceramint.2015.05.097

Ceramics International

Volume 41, Issue 9, Part A, November 2015, Pages 11372-11378

https://doi.org/10.1016/j.ceramint.2015.05.097 Get rights and content

Abstract

In order to increase the controllability of the deposition process of SiCN ceramics, SiCN ceramics were introduced into porous Si₃N₄ substrates by low pressure chemical vapor infiltration (LPCVI) using a simpler gaseous mixture of SiCl₃CH₃/NH₃/H₂ to substitute the gaseous mixture of SiCl₄/C₃H₆/NH₃/H₂. The as-prepared SiCN ceramics attained the microstructure of nano-sized graphite crystals distributing in amorphous SiCN phase. The SiCN ceramics with this unique microstructure had suitable relative complex permittivity, leading to excellent electromagnetic (EM) wave absorbing performance. The minimum reflection coefficient of SiCN–Si₃N₄ ceramics was −40 dB (>99.99% absorption) at 11.1 GHz with the thickness of 3 mm. When the thickness was 3.25 mm, reflection coefficient of SiCN–Si₃N₄ ceramics was lower than −10 dB (>90% absorption) at the whole X-band.

Introduction

Silicon carbonitride (SiCN) ceramic is a kind of promising material due to the good corrosion resistance, high temperature piezoresistivity, good photoelectronic and electromagnetic (EM) properties [1], [2], [3], [4], [5], [6]. Recently, with the rapid development of advanced electronic devices and telecommunications applications, more and more attentions are paid on exploring the EM wave absorbing properties of SiCN ceramics to prevent EM wave interference [7], [8], [9], [10], [11]. SiCN ceramics fabricated by polymer derived ceramics (PDCs) route possessed good EM wave absorbing property only when they were annealed at above 1400 °C owing to the formation of SiC nanocrystals and free carbon nanodomains [9]. However, such high heat-treatment temperature influenced the application of SiCN ceramics in ceramic matrix composites (CMC) considering the performance degradation of fibers [12]. Therefore, for preparing fiber reinforced SiCN ceramic matrix composites, it is necessary to develop a low temperature fabrication process.

Chemical vapor infiltration (CVI) has the advantages of the relatively low preparation temperature and controllable deposition rate [13], [14] and has been widely used in fabricating the interphase, matrix and coating of CMC [15], [16]. Our previous work revealed that SiCN ceramics prepared by CVI achieved good EM wave absorbing properties [8] with the minimum reflection coefficient (RC) of −42.60 dB. The gaseous mixture of SiCl₄/C₃H₆/NH₃/H₂ used to prepare SiCN ceramics increased the complexity of chemical reaction during the deposition process, thus gaseous mixture of SiCl₃CH₃/NH₃/H₂ is explored. Methyltrichlorosilane (SiCl₃CH₃) was chosen as silicon and carbon source, and ammonia (NH₃) was used as nitrogen source. The as-prepared SiCN ceramics are expected to consist of graphite and Si₃N₄ phases or nano-sized crystals and amorphous SiCN phase through controlling the deposition parameters according to the deposition process of CVD SiC from SiCl₃CH₃/H₂ [17], [18], [19]. As is known, graphite has high electrical conductivity and Si₃N₄ has low dielectric constant and dielectric loss. SiCN ceramics composed of graphite and Si₃N₄ may have excellent EM absorbing capability due to the special A+B type structure (A represents the electrically insulating Si₃N₄ or amorphous SiCN phase, B represents the conductive graphite) [20], [21]. Meanwhile, aggregation problems of absorbent could be solved at the same time.

In this work, SiCN ceramics were prepared by LPCVI using the gaseous mixture of SiCl₃CH₃/NH₃/H₂. The microstructure and deposition mechanism of SiCN ceramics were studied in details. The dielectric and EM wave absorbing properties were measured. The aim of this work is to develop another simpler gaseous mixture to prepare SiCN ceramics with excellent EM wave absorbing properties.

Section snippets

Experimental procedures

Si₃N₄ ceramics with high porosity of about 40% fabricated using the previous method [22] were machined into specimens with dimensions of 22.86×10.16×2.16 mm³ for dielectric properties measurement. The Archimedes method was employed to measure the open porosity and density of as-obtained ceramics. Si₃N₄ ceramics were then placed into a vertical LPCVI furnace to infiltrate SiCN ceramics from gaseous mixture of CH₃SiCl₃ (≥99.99 wt%), NH₃ (≥99.99%), H₂ (≥99.99%) and Ar (≥99.9%). H₂ was used as the

The microstructure of SiCN ceramics and deposition mechanism

The morphologies of SiCN–Si₃N₄ ceramics are shown in Fig. 1. Porous Si₃N₄ ceramics were composed of rod-like β-Si₃N₄ [8] intercrossing with each other and SiCN ceramics were infiltrated into the porous preforms (Fig. 1). The porosity of SiCN–Si₃N₄ ceramics gradually decreased (Table 1). The elemental compositions of LPCVI SiCN ceramics were measured by XPS, and the results are shown in Table 1. With the increasing N/Si ratio, the content of carbon decreased distinctly from 90.9% to 42.4%, while

Conclusion

In this paper, gaseous mixture of SiCl₃CH₃/NH₃/H₂ was used to prepare SiCN ceramics by LPCVI. The as-received SiCN ceramics obtained the microstructure of nano-sized graphite crystals distributing in amorphous SiCN phase. Nanometer effect and interfacial effect of the unique microstructure made the obtained SiCN ceramics possess suitable relative complex permittivity and excellent EM wave absorbing properties. The minimum RC of SiCN–Si₃N₄ ceramics reached −40 dB (>99.99% absorption) at 11.1 GHz

Acknowledgment

This work is supported by the National Natural Science Foundation of China (Grants: 51332004 and 51221001), the Fundamental Research Funds for the Central Universities (No. 3102014JC02010403), and the 111 Project (B08040).

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The microstructure of SiCN ceramics and their excellent electromagnetic wave absorbing properties

Abstract

Introduction

Section snippets

Experimental procedures

The microstructure of SiCN ceramics and deposition mechanism

Conclusion

Acknowledgment

J. Alloy. Compd.

J. Alloy. Compd.

Scr. Mater.

Thin Solid Films

J. Eur. Ceram. Soc.

Physica E

Mater. Lett.

J. Eur. Ceram. Soc.

Carbon

J. Eur. Ceram. Soc.

Carbon

J. Alloy. Compd.

J. Cryst. Growth

J. Alloy. Compd.

Compos. Sci. Technol.

Thin Solid Films

J. Nucl. Mater.

J. Alloy. Compd.

J. Eur. Ceram. Soc.

J. Magn. Mater.