Unburned carbon behavior in sintered coal fly-ash bulk material by spark plasma sintering

Coal fly-ash bulk materials were prepared by spark plasma sintering (SPS). The as-received coal fly ash produced by Misumi Power Station (The Chugoku Flectric Power Co., Inc.), had an average particle size of 19 pm and contained about 2% carbon from unburned coal. The sintering temperature was 1000 degrees C for 10 min. The mass density of the sintered compact was 2.4 x 10(3) kg/m(3). After three-point flexural testing of the compact. the average flexural strength and Young's modulus were 25.6 MPa and 23.0 GPa, respectively. From the flexural strength, the Weibull modulus was found to be m = 6.13, indicating that the compact was a typical ceramics. Fractographic examination indicated that in all specimens the fracture origin was located on the bottom surface and was not an intrinsic flaw. Vickers indentation test showed that the fracture toughness was 0.61 MPa.m(0.5) and the calculated critical flaw size, c(0), was 0.18 mm. This c(0) value was larger than that of the voids and unburned carbon on the fracture surface. It is noteworthy that the mechanical strength of the sintered compact was not affected by the voids and unburned carbon.