5048 J Mater Sci(2007)42:5046-5056 Results and discussion The very sharp diffraction peaks of p-SiC in as- received HNLS and TySA fibers indicated that these XRD characterization ave een high-crystallite structure Figure la-c showed X-ray diffraction patterns of three (Fig 1b, c), because of their very high fabrication temperature(about 1,600C and 1, 800C for HNLS types of fibers annealed at elevated temperatures in Ar and TySA, respectively). The annealing at tempera- for 1 h. The XRD patterns of the as-received SiC fibers tures beyond 1, 600.C caused gradual crystallization of (111)(20=357°;d=0.251nm),(220)(20=60 B-SiC in HNLS fiber(Fig 1b) USing the Scherrer's formula, the apparent crystal d=0.154 nm)and(311)(20=7200. d=0.131 nm). lite size of B-SiC, Dsic, was calculated from the half- The phases present in the hNL fibers were B-SiC and value width of the (111) peak. The plot of the p-Sic XRD-amorphous carbon. After annealing at temper- crystallite size as a function of annealing temperature ature over 1,400C, two other peaks are also observed was shown in Fig. 2. Following Teatures we len and peak height increased with increasing the anneal- (i The grain coarsening of HNL fiber started at ing temperature which are indexed as the(200) and 1,400C. ( i)the crystallite size of B-Sic in HNLS and (222)crystal planes and more obvious in HNLS fibers TySA fiber remained almost constant as annealing annealed at temperatures over 1, 600C(ig 1b) temperature 1,600C, while higher temperature The diffraction peaks in HNL fiber become sharp annealing caused an continuous coarsening in crystal and narrow when temperature is higher than 1, 300C lite size of SiC in HNLS. The crystallite size of B-SiC in (Fig. la), while they are not so obvious for near-sto- TySA fibers appears to be little dependent on the ichiometeric fibers(Fig. 1b, c). Such changes in the annealing temperature. The crystallite sizes for as diffraction peaks of hNL fibers are due to the coales- ceived HNL, HNLS and TySA fibers are 4.0, 11. 4, cence of p-SiC nano-crystals caused by the decompo- and 22.7 nm, while they are 15.5, 30.3, and 25.5 nm for sition of the intergranular amorphous phase and fibers annealed at 1, 900 oC for 1 h, respectively thermally activated diffusion. The decomposition the intergranular amorphous SiC,Oy phase occurred at coalescence of B-Sic nanocrystals due to either about 1,300C in HNL fiber resulted in progressive decomposition of amorphous phase or diffusion of crystallization of B-SiC according to the reaction [16]: and C atoms at grain boundaries during exposure at high temperatures. For the bulk materials with clean SiCrOy-SiC(s)+ Sio(g)+Co(g) (2) grain boundaries, the grain growth proceeds through Fig. 1 X-ray diffractic patterns for SiC fibe (c)"1 annealed at elevated tem itures:(a)HNL fib (220(1 b)HNLS fiber;(c) TySA 1020304050 304050607080 28/degree 28/degree 2 SpringerResults and discussion XRD characterization Figure 1a–c showed X-ray diffraction patterns of three types of fibers annealed at elevated temperatures in Ar for 1 h. The XRD patterns of the as-received SiC fibers show three main peaks which were assigned to the (111) (2h = 35.7; d = 0.251 nm), (220) (2h = 60.0; d = 0.154 nm) and (311) (2h = 72.0; d = 0.131 nm). The phases present in the HNL fibers were b-SiC and XRD-amorphous carbon. After annealing at temper￾ature over 1,400 C, two other peaks are also observed and peak height increased with increasing the anneal￾ing temperature, which are indexed as the (200) and (222) crystal planes and more obvious in HNLS fibers annealed at temperatures over 1,600 C (Fig. 1b). The diffraction peaks in HNL fiber become sharp and narrow when temperature is higher than 1,300 C (Fig. 1a), while they are not so obvious for near-sto￾ichiometeric fibers (Fig. 1b, c). Such changes in the diffraction peaks of HNL fibers are due to the coales￾cence of b-SiC nano-crystals caused by the decompo￾sition of the intergranular amorphous phase and thermally activated diffusion. The decomposition of the intergranular amorphous SiCxOy phase occurred at about 1,300 C in HNL fiber resulted in progressive crystallization of b-SiC according to the reaction [16]: SiCxOy ! SiC(s) + SiO(g) + CO(g) ð2Þ The very sharp diffraction peaks of b-SiC in as￾received HNLS and TySA fibers indicated that these fibers have already been high-crystallite structure (Fig. 1b, c), because of their very high fabrication temperature (about 1,600 C and 1,800 C for HNLS and TySA, respectively). The annealing at tempera￾tures beyond 1,600 C caused gradual crystallization of b-SiC in HNLS fiber (Fig. 1b). Using the Scherrer’s formula, the apparent crystal￾lite size of b-SiC, DSiC, was calculated from the half￾value width of the (111) peak. The plot of the b-SiC crystallite size as a function of annealing temperature was shown in Fig. 2. Following features were observed: (i) The grain coarsening of HNL fiber started at 1,400 C. (ii) the crystallite size of b-SiC in HNLS and TySA fiber remained almost constant as annealing temperature < 1,600 C, while higher temperature annealing caused an continuous coarsening in crystal￾lite size of SiC in HNLS. The crystallite size of b-SiC in TySA fibers appears to be little dependent on the annealing temperature. The crystallite sizes for as￾received HNL, HNLS and TySA fibers are 4.0, 11.4, and 22.7 nm, while they are 15.5, 30.3, and 25.5 nm for fibers annealed at 1,900 C for 1 h, respectively. The grain coarsening could be attributed to the coalescence of b-SiC nanocrystals due to either decomposition of amorphous phase or diffusion of Si and C atoms at grain boundaries during exposure at high temperatures. For the bulk materials with clean grain boundaries, the grain growth proceeds through 10 20 30 40 50 60 70 80 2θ/degree 2θ/degree 2θ/degree (111) (200) (220) (311) 1900°C (222) 1780°C 1400°C As recived 1600°C 10 20 30 40 50 60 70 80 (111) (200) (220) (311) (222) 1900°C 1780°C 1400°C As recived 1600°C (a) (b) (c) 10 20 30 40 50 60 70 80 (1 (200) (220) (311) 1900 (222) °C 1600°C 1400°C As recived 1780°C (111) 1600°C 1400°C 1780°C Fig. 1 X-ray diffraction patterns for SiC fibers annealed at elevated temperatures: (a) HNL fiber; (b) HNLS fiber; (c) TySA fiber : b-SiC 5048 J Mater Sci (2007) 42:5046–5056 123