L. Cheng et al./ Carbon 41(2003)707-711 on the longitudinal and transverse thermal diffusivity of 3. Results and discussion the C/SiC composite with and without a CVD SiC coating were investigated 3. I. Thermal difusivity of CVD Sic The thermal diffusivity as of the CVd Sic prepared in this paper was compared with that a s of the CVD SiC 2. Experimental procedure prepared by rohm and Haas Company and was shown in Fig. 1. Although the latter was much larger than the 2.I. Fabrication of the specimens ormer, they could be well fitted by a simple function over the full temperature range Two kinds of specimens were machined from a 3I C/SiC composite prepared by low-pressure chemical vapor 994 deposition method (LPCVD), one being along the fiber 1.35 +00285 (3) axial and the other cross the axial.a bulk cvd sic specimen prepared by usual pressure chemical vapor 48800 deposition method (UPCVD) was used to investigate the a p1.8+00700 thermal diffusivity of C/SiC composites The deposition conditions of bulk SiC material were as where T is the Kelvin temperature. It could be seen that a follow. temperature 1 100C, time 200 h, H, flow 400 ml decreased much rapidly than as with increasing tempera min. Ar flow 400 ml min and the molar ratio of h ture. The higher the temperature, the smaller the difference and MTS was 3-5. The 3D preforms were deposited with between as and a s pyrolytic carbon (PyC) and Sic using butane and The bulk CVD SiC material used in this paper was very methyltrichlorosilane(MTS). The deposition conditions of different from that prepared by rohm and Haas Company PyC interlayer were as follow: temperature 960C, pre in microstructure and composition due to different depo sure 5 KPa, time 20 h, Ar flow 200 ml min, butane flow tion conditions [5, 6]. When the molar ratio of H, and MTs 15 ml min. The deposition conditions of Sic matrix is about 10, stoichiometric silicon carbide can be eventual- were as follow: temperature 1000C, pressure 5 KPa, time ly obtained. Because this ratio under the UPCVd was only 120 h, H, flow 350 ml min Ar flow 350 ml min and 3-5, the Sic deposited was rich in carbon. The XRD the molar ratio of H, and MTs was 10. A CVD Sic analysis confirmed that the free carbon existed in graphit coating was prepared on the C/SiC specimens after (Fig. 2). The composition of the Sic was calculated to be machining under the same conditions as the sic matrix to 88% B-SiC, 4% a-SiC and 7% graphite. Some defects investigate its effect on the thermal diffusivity. Although where the Sic was loosely packed could be found owing to the deposition conditions of the bulk SiC material were the rapid deposition under UPCVD(Fig. 3). The defects different from those of the SiC matrix and the Sic coating, and the graphite made as much lower than a s. Of course, thermal diffusivity measurement of the bulk material will it would take very short time to prepare a bulk si be helpful to investigate that of the c/Sic composit material by UPCVD 2.2. Diffusivity measurements Laser Flash Apparatus LFA 427 made by NETZSCH company was employed for measurements of the 1.4 o CVD SiC Ref diffusivity. The required specimen size was 12.5X2 mm. All measurements were conducted in Argon atmos- 1400C. In order to investigate the effect of the heat treatment on the thermal diffusivity, all specimens were measured two or three times. The thermal diffusivity at each appointed temperature was measured three times Below 1000C. the relative error Aala is less than 0.5% and above 1000C it is less than 0. 1%. So the measuring 500 00 1500 error have not effect on the relations of thermal diffusivity to temperature. Because the uncertainty of sample thick- ness is less than 1%, it leads to about 2% uncertainty of Fig. 1. Thermal diffusivity of CVD SiC as a function of tempera- ture708 L. Cheng et al. / Carbon 41 (2003) 707–711 on the longitudinal and transverse thermal diffusivity of 3. Results and discussion the C/SiC composite with and without a CVD SiC coating were investigated. 3 .1. Thermal diffusivity of CVD SiC The thermal diffusivity aS of the CVD SiC prepared in this paper was compared with that a9 S of the CVD SiC 2. Experimental procedure prepared by Rohm and Haas Company and was shown in Fig. 1. Although the latter was much larger than the 2 .1. Fabrication of the specimens former, they could be well fitted by a simple function over the full temperature range Two kinds of specimens were machined from a 3D C/SiC composite prepared by low-pressure chemical vapor 994 a 5 1 ]] 0.0285 (3) S 1.35 deposition method (LPCVI), one being along the fiber T axial and the other cross the axial. A bulk CVD SiC specimen prepared by usual pressure chemical vapor 48800 a9 5 1 ]] 0.0700 (4) S 1.8 deposition method (UPCVD) was used to investigate the T thermal diffusivity of C/SiC composites. The deposition conditions of bulk SiC material were as where T is the Kelvin temperature. It could be seen that a9 S follow: temperature 1100 8C, time 200 h, H flow 400 ml decreased much rapidly than a with increasing tempera- 2 S 21 21 min , Ar flow 400 ml min and the molar ratio of H ture. The higher the temperature, the smaller the difference 2 and MTS was 3–5. The 3D preforms were deposited with between a and a9. S S pyrolytic carbon (PyC) and SiC using butane and The bulk CVD SiC material used in this paper was very methyltrichlorosilane (MTS). The deposition conditions of different from that prepared by Rohm and Haas Company PyC interlayer were as follow: temperature 960 8C, pres- in microstructure and composition due to different deposi- 21 sure 5 KPa, time 20 h, Ar flow 200 ml min , butane flow tion conditions [5,6]. When the molar ratio of H and MTS 2 21 15 ml min . The deposition conditions of SiC matrix is about 10, stoichiometric silicon carbide can be eventual￾were as follow: temperature 1000 8C, pressure 5 KPa, time ly obtained. Because this ratio under the UPCVD was only 21 21 120 h, H flow 350 ml min , Ar flow 350 ml min and 3–5, the SiC deposited was rich in carbon. The XRD 2 the molar ratio of H and MTS was 10. A CVD SiC analysis confirmed that the free carbon existed in graphite 2 coating was prepared on the C/SiC specimens after (Fig. 2). The composition of the SiC was calculated to be machining under the same conditions as the SiC matrix to 88% b-SiC, 4% a-SiC and 7% graphite. Some defects investigate its effect on the thermal diffusivity. Although where the SiC was loosely packed could be found owing to the deposition conditions of the bulk SiC material were the rapid deposition under UPCVD (Fig. 3). The defects different from those of the SiC matrix and the SiC coating, and the graphite made a much lower than a9. Of course, S S thermal diffusivity measurement of the bulk material will it would take very short time to prepare a bulk SiC be helpful to investigate that of the C/SiC composite. material by UPCVD. 2 .2. Diffusivity measurements Laser Flash Apparatus LFA 427 made by NETZSCH company was employed for measurements of the thermal diffusivity. The required specimen size was f12.532 2 mm . All measurements were conducted in Argon atmos￾phere at different temperatures from room temperature to 1400 8C. In order to investigate the effect of the heat treatment on the thermal diffusivity, all specimens were measured two or three times. The thermal diffusivity at each appointed temperature was measured three times. Below 1000 8C, the relative error Da/a is less than 0.5%, and above 1000 8C it is less than 0.1%. So the measuring error have not effect on the relations of thermal diffusivity to temperature. Because the uncertainty of sample thick￾ness is less than 1%, it leads to about 2% uncertainty of Fig. 1. Thermal diffusivity of CVD SiC as a function of tempera￾diffusivity. ture