Effects of Elemental Additives on Densification, Microstructure, Strength, and Thermal Conductivity of Silicon Carbide Ceramics

Abstract

Effects of elemental additives on the densification, microstructure, strength, and thermal conductivity of hot-pressed Sic ceramics were investigated. The addition of Be, B, and A1 gave up to 98% of theoretical density, whereas other additives had only a slight effect on densification. The microstructure, flexural strength, and thermal conductivity of dense Sic ceramics varied considerably depending on the additive used. For example, Sic ceramics with added A1 had fine grains -2.3 pn in size and high flexural strength, =940 MNlm'. On the other hand, those with added Be or B had larger grains -5.1 and -4.2 p n in size and lower flexural strengths -400 and =290 MN/m2, respectively. Moreover, with added Be an extremely high thermal conductivity, =260 W/m K, was obtained; this value is much larger than those for Sic ceramics with B or A1 additions.

Microstructural characterization of high-thermal-conductivity SiC ceramics

“…The mixed powder was pressed with a carbon-steel die (50 mm diameter) and followed by hot pressing at 2313 K for 1 h under 20 MPa. 3,4 The thermal conductivity values between 5 and 300 K and between 300 and 1300 K were measured using a steady-state longitudinal heat flow technique and laser-flash technique, respectively. 6 Phase composition of the sample was identified by X-ray diffractometry (XRD).…”

“…The secondary phase was composed of O, Fe, Si, Ti, and Ni, and these elements, except for Si, were impurities present in the SiC ceramics ( Table 1) that were derived from the ␣-SiC source powder. 3 The impurity-crystalline phase seemed to be crystallized along the SiC grain surfaces; therefore, such precipitates should be crystallized from the liquid phase during the cooling process, as seen in AlN ceramics. 8,9 In Fig.…”

“…1 Furthermore, SiC ceramics with high-thermal conductivities (>200 W m −1 K −1 ) have been employed as electronic substrates for highly integrated circuits and laser-reading devices. 2 The maximum experimental value in SiC ceramics at room temperature is reported to be approximately 270 W m −1 K −1 , 3,4 representing roughly 50% of the intrinsic crystal value (490 W m −1 K −1 . 5 The ceramic was obtained by hot-pressing with a Be dopant.…”

“…5 The ceramic was obtained by hot-pressing with a Be dopant. 3,4 In previous work, 6 we measured the thermal conductivity of the ceramic at temperatures between 5 and 1300 K. Moreover, the thermal conductivity mechanism of SiC ceramics has been discussed through calculation of the phonon mean free path and the significance of its temperature variation for the thermal conductivity. We concluded that at temperatures less than 30 K, the conductivities are controlled by the grain boundaries and grain junctions.…”

Evaluation of thermal conductivity of grains and fillers by using thermoreflectance technique—Si₃N₄, AlN, SiC—

“…High-thermal-conductivity SiC ceramic with small amount of BeO additive, having a value of 270 Wm ¹1°C¹1 , corresponding to roughly 0.5 of its intrinsic conductivity, was successfully obtained by Takeda et al 19) Microstructure characterization of the ceramic has been reported by Nakano et al recently. They mentioned that there exists large amount of stacking fault and BeO particles inside the SiC grains.…”