Emissivity of Silicon Carbide Composites as a Function of temperature And Microstructure
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Intelligent process control requires specific information on the product properties such as temperature and density. Given the "hostile" environment in CVI and microwave assisted CVI, remote probes are desirable. Emissivity can be remotely sensed and it has the potential to provide temperature, density and oxidation state information. [1-4] The goal of this project was to investigate emissivity as a function of density and microstructure, temperature, and atmosphere and determine if it was feasible to use emissivity as a remote, non-invasive probe for intelligent control for CVI and microwave assisted CVI of SiC based composites. Specifically, we wished to determine if the emissivity was sufficiently sensitive to porosity under a wide variety of process conditions to allow it to be used as a probe of both densification rate and product density. In addition, we wished to examine the relationship between emissivity and temperature to allow us to make appropriate recommendations with respect to the use of pyrometry for temperature control. 411
Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Society
EXPERIMENT SiC fiber tube preforms were partially densified using chemical vapor Three infiltration to form the SiC matrix at Ceramic Composites, Inc. samples with nominal porosities of 54%, 47% and 35% were prepared for this study. Uncoated, as-received samples were examined in the ESEM (environmental SEM). In addition, pieces were broken from the end of the tubes to allow examination of the fracture surfaces in the ESEM. (Detailed microstructural characterization will be done after the emissivity measurements are complete.) The spectral radiance of the samples was measured using a PbS detector between 600 and 1000 0C in nitrogen and air. Figure 1 shows a schematic of the experimental set-up. The samples were centered next to a 1 cm diameter "view line" in the furnace door. A reference thermocouple was next to the sample. The 1.2 mm diameter fiber optic cable was centered on the opening on the outside of the door. The angle subtended by the radiation impinging on the cable was < 40. The emissivity was calculated using [1]: E?,=(S-Z)/(H-Z)(1)
where S=sample radiance, (assumed to be 5% H).
H=blackbody
radiance and Z=cold reference
The nitrogen measurements were done before the measurements in air to minimize the oxidation effects from exposure to air at high temperatures. The measurement temperatures (600-1000 0 C) reflect those of interest in
Fused silica fiber optic cable
Bias Supply \ I
\ Light shield i\ Controlled atm. box furnace
\ Qf[I
Oscilliscope
,
~~Chopper
/ 1 .
/ •" )
/
Sample holder and thermocouple Spectropro scanni g monochrometer Figure 1:
True RMS voltmeter Oriel PbS detector
Power supply
Schematic of the experimental set-up. 412
the CVI and microwave assisted CVI manufacturing processes. RESULTS AND DISCUSSION ESEM analysis indicated that the samples all contained significant open porosity consistent with the
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