Y Liu et al. Materials Science and Engineering A 475(2008)217-223 Table 3 Flexural strength and dielectric constant of CVI Si3 Nap/Si3 N4 and other Si Na materials lexural strength(MPa) Density (gcm-3) Dielectric constant 113.4 4.32(30MHz) CVI 104.5 4.27(30MHz) 104.3 4.13(30MHz) 4.34(30MHz) Reaction sintered Si3 N4 [31] 5.6(810GHz) SioN nano-composite [331 747(8.5GHz),7.14(35GHz) Electromagnetic window that of SiC(p/Sic composites, the flexural strength of CVI Acknowledgments NA(P/Si3N4 composite is low, only 100 MPa around. The low strength of Si3 N4(p/Si3 N4 composite has resulted from This work was supported by the Key Foundation of National the low density, 2.03-2.10g/cm-3, and high open pore ratio, Science in China(90405015)and National Elitist Youth Founda- about 12% of the composite. The microstructure of the tion in China(50425208) This work was also supported by the two types of composites also verified that the SiC(py/sic Doctorate Foundation of Northwestern Polytechnical University has more high strength due to the denser cross-section ( CX200505) Compared with other reports, CVI Si3N4(P/Si3 N4 compos- References Although reaction sintered Si3N4 composite has a high strength, [1) Mitomo Mamoru, Petzow Gunter, MRS Bull. 20(1995)I 500 MPa 31], the composites also has a high dielectric con- [23J. Barta, M Manela, R. Fischer, Mater. Sci. Eng. 71(1984)265. tant, which is disadvantageous for radome applications On the [3] J.H. She, D L Jiang, S.H. Tan, J.K.Guo, Key Eng Mater. 108-110(1995) other hand, CVI Si3 N4(p)/Si3 N4 composite has a higher strength and almost similar dielectric constant, compared with sintered 4 Y Hua, L. Zhang, L. Cheng, J. Wang, Mater. Sci. Eng. A 428(2006) nano Si3N4 [33]. Therefore, the CVI Si3N4(p)/Si3N4 compos- (5)R.Naslain, Phys IV France 123(2005)3-17 ite is a good candidate materials for high temperature radome (6)X D Wang, G.J. Qiao, Z.H. Jin, J.Am. Ceram Soc. 87(2004)565 [8] G.C. Dodds, R.A. Tanzilli. United States Patent, Appl. 5891815, 1999 4. Conclusions 9] I.G. Talmy, C.A. Martin, H.A. Deborah, et al. United States Patent, Appl 5573986,1996 [10] K. Strecker, S Ribeiro, R. Oberacker, M.J. Hoffmann, Int J Refract Met. (1) The designed particle preform exist with two kinds of pores H Mater.22(2004)169 with size of 500-800 um inter-agglomerations and 5-10 um [11] U Paik, H C. Park, S.C. Choi, C.G. Ha, J.W. Kim, Y.G. Jung, Mater. Sci. intra-agglomerations, respectively. The agglomerate parti EngA334(2002)267 cle preform is uniform and porous, with density 1.29 of [12] H.J. Choi, Yw.Kim, M.Mitomo,TNishimura,JHLee,DY.Kim,Scripta g/cm and pore ratio 59.6%. The gas diffusion mechanism [13] M.J. Hoffmann, A. Geyer, R. Oberacker, J. Eur. Ceram Soc. 19(13-14) through inter-agglomerations pores belongs to transi- (1999)2359 tion diffusion and Knudsen diffusion intra-agglomerations [14] J.S. Park, Y. Katoh, A Kohyama, J K. Lee, J.J. Sha, H.K. Yoon, J Nucl pores Mater.329-333(2004)558 (2)SiC()/SiC and Sis N4(p)/Si3 N4 composites were fabricated [15K. Biswas, G Rixecker, EAldinger,J.Eur.CeramSoc.23(2003)1099 using the designed particle preform and chemical vapor 17]K. Strecker, M.J. Hoffmann, J. Eur. Ceram Soc. 25 (2005)80 infiltration technique. There existed large amounts of [18]QWHuang, L.H. Zhu, Mater. Lett. 59(2005)1732. silicon carbide and silicon nitride in the inter-and intra- [19] T. Tani, Compos. Part A-Appl S 30(1999)419. agglomerations pores, although there are some residual [20] P. Bhandhubanyong, T. Akhadejdamrong, JMater. Process. Technol. 63 pores ()The flexural strength of SiC(p /SiC composite changes with [21 R. Naslain, R Paller, X. Bourrat, G. Vignoles, Key Eng Mater. 159-160 molding-agglomerations pressure and molding-preforms [22]R. Naslain, Compos. Sci. Technol. 64(2004) pressure. The maximum of the strength was 284 MPa, and [23]N. Igawaa, T. Taguchi, T Nozawa, L L. Snead, T Hinoki, J C. McLaughlin, the ratio of the retained strength was 95.4% at 1600C, and Y. Katoh, S Jitsukawa, A. Kohyama, J. Phys. Chem. Solids 66(2005) fracture roughness was 7. 11 MPam. The Si3N4(p/Si3 N4 composite had an acceptable strength, 113. 4 MPa and low 4] Y Liu, L Cheng, L Zhang, et al., J Inorg. Mater. 20(5)(2005)979(in dielectric constant. about 4.2-4.3 [25] Y. Liu, L Cheng, L. Zhang, et al., J. Univ Sci. Technol. B, 2007, in press222 Y. Liu et al. / Materials Science and Engineering A 475 (2008) 217–223 Table 3 Flexural strength and dielectric constant of CVI Si3N4p/Si3N4 and other Si3N4 materials Materials Flexural strength (MPa) Density (g cm−3) Dielectric constant CVI Si3N4p/Si3N4 113.4 2.08 4.32 (30 MHz) 104.5 2.03 4.27 (30 MHz) 104.3 2.09 4.13 (30 MHz) 104.3 2.15 4.34 (30 MHz) Reaction sintered Si3N4 [31] 500 – 5.6 (8–10 GHz) Sintered nano-Si3N4 [32] 89 2.29 4.8–5.7 SiON nano-composite [33] 190 – 4.78–5.00 SiBAlON composite [8] – – 7.47 (8.5 GHz), 7.14 (35 GHz) Electromagnetic window [9] 85 – 4.03 that of SiC(p)/SiC composites, the flexural strength of CVI Si3N4(p)/Si3N4 composite is low, only 100 MPa around. The low strength of Si3N4(p)/Si3N4 composite has resulted from the low density, 2.03–2.10 g/cm−3, and high open pore ratio, about 12% of the composite. The microstructure of the two types of composites also verified that the SiC(p)/SiC has more high strength due to the denser cross-section morphology. Compared with other reports, CVI Si3N4(p)/Si3N4 compos￾ite has an acceptable strength and low dielectric constant. Although reaction sintered Si3N4 composite has a high strength, 500 MPa [31], the composites also has a high dielectric con￾stant, which is disadvantageous for radome applications. On the other hand, CVI Si3N4(p)/Si3N4 composite has a higher strength and almost similar dielectric constant, compared with sintered nano Si3N4 [33]. Therefore, the CVI Si3N4(p)/Si3N4 compos￾ite is a good candidate materials for high temperature radome applications. 4. Conclusions (1) The designed particle preform exist with two kinds of pores with size of 500–800m inter-agglomerations and 5–10 m intra-agglomerations, respectively. The agglomerate parti￾cle preform is uniform and porous, with density 1.29 of g/cm3 and pore ratio 59.6%. The gas diffusion mechanism through inter-agglomerations pores belongs to transi￾tion diffusion and Knudsen diffusion intra-agglomerations pores. (2) SiC(p)/SiC and Si3N4(p)/Si3N4 composites were fabricated using the designed particle preform and chemical vapor infiltration technique. There existed large amounts of silicon carbide and silicon nitride in the inter- and intra￾agglomerations pores, although there are some residual pores. (3) The flexural strength of SiC(p)/SiC composite changes with molding-agglomerations pressure and molding-preforms pressure. The maximum of the strength was 284 MPa, and the ratio of the retained strength was 95.4% at 1600 ◦C, and fracture roughness was 7.11 MPa m1/2. The Si3N4(p)/Si3N4 composite had an acceptable strength, 113.4 MPa and low dielectric constant, about 4.2–4.3. Acknowledgments This work was supported by the Key Foundation of National Science in China (90405015) and National Elitist Youth Founda￾tion in China (50425208). This work was also supported by the Doctorate Foundation of Northwestern Polytechnical University (CX200505). 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[25] Y. Liu, L. Cheng, L. Zhang, et al., J. Univ. Sci. Technol. B, 2007, in press.