Abstract
Porous Si3N4-Si3N4 composite ceramics were fabricated by 3D printing combined with low-pressure chemical vapor infiltration (CVI). This technique could effectively improve the designability of porous Si3N4 ceramics and optimize the mechanical and dielectric properties. The effects of process parameters including the deposition time and heat treatment on the microstructure and properties of porous Si3N4-Si3N4 composite ceramics were studied. The study highlights following: When CVI processing time was increased from 0 to 12 h, the porosity decreased from 68.65% to 26.07% and the density increased from 0.99 to 2.02 g/cm3. At the same time, the dielectric constant gradually increased from 1.72 to 3.60; however, the dielectric loss always remained less than 0.01, indicating the excellent electromagnetic (EM) wave-transparent performance of porous Si3N4-Si3N4 composite ceramics. The maximum flexural strength of 47±2 MPa was achieved when the deposition time attained 6 h. After heat treatment, the porosity increased from 26.07% to 36.02% and the dielectric constant got a slight increase from 3.60 to 3.70 with the dielectric loss still maintaining lower than 0.01. It has been demonstrated that the porous Si3N4-Si3N4 composite ceramics are a promising structural and EM wave-transparent material suitable for high temperature service.
Highlights
The radome and antenna windows, which are used on carrier rockets, airships, missiles, and return satellites, are made of excellent electromagnetic (EM) wave-J Adv Ceram 2019, 8(3): 399–407 as Al2O3 [3]
L3 × ∆P 4bh3 × ∆f where σf refers to the flexural strength (Mpa); P represents the largest load when specimens were broken; L corresponds to the span of supporting points; b refers to the rectangular width of the pressure surface; h represents the thickness of specimens; Ef refers to the modulus; ∆P corresponds to the change in load during the elastic stage and ∆f corresponds to the deflection increment
The 3D-printed porous Si3N4 ceramics after binder removal still composed of α-Si3N4 with a little amount of Lu2Si2O7, indicating the unchanged crystal structure
Summary
The radome and antenna windows, which are used on carrier rockets, airships, missiles, and return satellites, are made of excellent electromagnetic (EM) wave-. Si3N4 ceramics are a potential EM wave-transparent material for radome applications, which possess excellent mechanical properties but relatively high dielectric constant (5.6 in 8–10 GHz, compared with 3.3 of fused silica) [4,5]. The porous Si3N4 ceramics with 40%–55% of porosity exhibit the low dielectric constant of 2.7–3.3 [3,6]. The porous Si3N4 ceramic preforms can be prepared by a number of shaping methods, such as die pressing process, cold isostatic pressing process, and gel-casting process [3]. A densification technique is needed to adjust and control the microstructure of porous Si3N4 ceramics in order to optimize the mechanical and dielectric properties. Chemical vapor infiltration (CVI) technique is considered as an effective method to fabricate Si3N4 matrix with expected and controlled composition in the porous preforms. The mechanical and dielectric properties of composite ceramics as a function of deposition time were explored, and the effects of heat treatment on the microstructure and properties were investigated
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