1138 Journal of the American Ceramic Society--Gouadec et al. Vol 84. No 5 increasing powers, for a variety of t A values. This will be Table L. Ordinate at Origin vi and Slope s: of Linear aracterized by negative S; parameters, in units of cm/mw Regressions calculated after the fitted wavenumbers of Modes i, Plotted as a Function of the Laser Power P(in T= T+ S-P mw, Measured on the Sample) Mode i/fiber/matrix/(nm)t Fo(cm-) Examples of S: measurement for different test conditions(fiber Si(cm/mw) setting, wavelength, ... )are shown in Fig. 2 and Table I Only for Carbon D band/Hi-S 1326.11 SiC (TO mode) was a"plateau clearly identified, up to 17 mW on//647 SiC bonds being nonresonant in the visible range there is there is"D"Hi-Nicalon//514.5 1353.96 therefore no direct conversion of the photonic energy into heat. "D/Hi-Nicalon/ /514.5 1357 Heating is postponed, and measurement confidence would proba- 1354.18 bly be better using SiC as the stress probe when new generations D/SCS6/Ti alloy/514.5 "D"/Hi-Nicalon/celsian/514.5 of nearly stoichiometric SiC fibers are studied (instead of carbon"D"/Hi-Nicalon/celsian/457.9 phase, which becomes so dispersed it no longer probes the actual "D"/Hi-Nicalon/celsian/6471I stress). As for carbon, regressions were extrapolated down to P=Omw although it looks as though a very short "plateau"exists Sic to modet/SCS6 791.49 below 0.5 mw(we cannot rule out heating, but noise contribution (SiC)Ti alloy/514.5 tops being negligible at such low powers and might be disturbing mahm①mm Textron, U.S.A. )is a 33 D One expected result that Table I confirms is the poor fiber edge. #so. 0.07 cm-I/mw for dissipation ability of free-standing fibers, due to the low thermal conductivity of air. In contrast, the matrix surrounding a fiber in a omposite acts as a heat sink, the dissipating effect being greater for a metallic matrix(Ti6242 alloy) than for ceramics(celsian). observed with the A= 514.5 nm line(Table 2 caption wrong- (iii) The effect, if there is one, on changing A on the surface fully mentioned air annealing ). Note that changing the power from of free-standing fibers from I um"(spot size)to 100 um(spot with the 9.2 cm-I shift predicted based on Table 1(6X 1.54).In scanning by a mobile mirror) when (A, P, 1) are set at given values seems to be negligible addition, Sp measurement showed a better reproducibility for v) If there is any"time effect fibers analyzed under equivalent conditions than SG. Figure 3(a) equilibrium state is reached very quickly since wavenumbers did shows that for a pure carbon fiber(FT700 grade ), both vp and Sp change linearly with the exciting laser line energy. A similar linea not systematically decrease(at least for power below 10 mw) behavior is found for n in Hi-Nicalon fibers(Fig. 3(b)and can Power clearly is the most important parameter and data in Table consequently be expected also for their Se when I increased for a given set of (A, M, A) I will help us compensate the thermally induced Raman shift as a (B) In Situ Results, Comparison with Stress-Free References function of the sample and the waveler Once S: is known, it becomes possible to interpret wavenumber shifts from one place to another in terms of stress difference. The actual loading at each point requires a vistress-free"reference measured at room temperature. vi might unfortunately differ from Result ne sample to another and a value found in the sample itself is (I Fiber Analysis expected to be more reliable. Besides, measuring the stress in the (A SD Calibration under Axial Tension: A-2.7(+0.4) ame sample as the reference would avoid compensating for the cm / value has been obtained by gouadec et al. for Sp in annealed Hi-Nicalon fibers (1000.C in reducing atmosphere) Sic mw 5 mw 17 mw 15--920s8 10 786 1320 11 802002202402602.80 1340 776 ▲Free- standing H 13310occs6m6242A 1325 1340 1330 o Hi-Nicalon 20 Fig. 2. Wavenumber vs laser power(A 514.5 nm) for the D band 802002202.40260280 free-standing in air or embedded in composite 1; left-hand scale)and for SiC TO, mode in an SCS-6 fiber(right-hand scale). The labels indicate the Fig 3.(a)vo (left-hand scale, in cm)and SD(right-hand scale, in umber of measured values( when more than one)for the corresponding ser line energy (in electronvolts) points. The displayed regression lines would be straight lines for a linear for FT700 carbon fibers. (b) Comparison of vo dependency to the lin ower scale energy in Hi-Nicalon and FT700 fibersincreasing powers, for a variety of {t;A} values. This will be characterized by negative Si P parameters, in units of cm21 /mW: n#i 5 n#i 0 1 Si P zP (5) Examples of Si P measurement for different test conditions (fiber, setting, wavelength, . . . ) are shown in Fig. 2 and Table I. Only for SiC (TO mode) was a “plateau” clearly identified, up to 17 mW. Si–C bonds being nonresonant in the visible range, there is therefore no direct conversion of the photonic energy into heat. Heating is postponed, and measurement confidence would proba￾bly be better using SiC as the stress probe when new generations of nearly stoichiometric SiC fibers are studied (instead of carbon phase, which becomes so dispersed it no longer probes the actual stress12). As for carbon, regressions were extrapolated down to P 5 0 mW although it looks as though a very short “plateau” exists below 0.5 mW (we cannot rule out heating, but noise contribution stops being negligible at such low powers and might be disturbing the fitting). (ii) One expected result that Table I confirms is the poor dissipation ability of free-standing fibers, due to the low thermal conductivity of air. In contrast, the matrix surrounding a fiber in a composite acts as a heat sink, the dissipating effect being greater for a metallic matrix (Ti6242 alloy) than for ceramics (celsian). (iii) The effect, if there is one, on changing A on the surface of free-standing fibers from 1 mm2 (spot size) to 100 mm2 (spot scanning by a mobile mirror) when {l,P,t} are set at given values seems to be negligible. (iv) If there is any “time effect” on heat accumulation, the equilibrium state is reached very quickly since wavenumbers did not systematically decrease (at least for power below 10 mW) when t increased for a given set of {l,M,A}. Power clearly is the most important parameter and data in Table I will help us compensate the thermally induced Raman shift as a function of the sample and the wavelength used. IV. Results (1) Fiber Analysis (A) SD ε Calibration under Axial Tension: A 22.7 (60.4) cm21 /% value has been obtained by Gouadec et al. for SD ε in annealed Hi-Nicalon fibers (1000°C in reducing atmosphere) observed with the l 5 514.5 nm line11 (Table 2 caption wrong￾fully mentioned air annealing). Note that changing the power from 2 to 8 mW provoked a 9.1 cm21 shift of n#D, which agrees well with the 9.2 cm21 shift predicted based on Table I (6 3 1.54). In addition, SD ε measurement showed a better reproducibility for fibers analyzed under equivalent conditions than SG ε . Figure 3(a) shows that for a pure carbon fiber (FT700 grade), both n#D and SD ε change linearly with the exciting laser line energy. A similar linear behavior is found for n#D in Hi-Nicalon fibers (Fig. 3(b)) and can consequently be expected also for their SD ε . (B) In Situ Results; Comparison with Stress-Free References: Once Si ε is known, it becomes possible to interpret wavenumber shifts from one place to another in terms of stress difference. The actual loading at each point requires a n#i 0 “stress-free” reference measured at room temperature. n#i 0 might unfortunately differ from one sample to another and a value found in the sample itself is expected to be more reliable. Besides, measuring the stress in the same sample as the reference would avoid compensating for the Fig. 2. Wavenumber vs laser power (l 5 514.5 nm) for the D band (“sp3 –sp2 /sp3 ”-hybridized carbon atoms) of a Hi-Nicalon fiber (either free-standing in air or embedded in composite 1; left-hand scale) and for SiC TO1 mode in an SCS-6 fiber (right-hand scale). The labels indicate the number of measured values (when more than one) for the corresponding points. The displayed regression lines would be straight lines for a linear power scale. Table I. Ordinate at Origin n#i 0 and Slope Si P of Linear Regressions Calculated after the Fitted Wavenumbers of Modes i, Plotted as a Function of the Laser Power P (in mW, Measured on the Sample) Mode i/fiber/matrix/l (nm)† n#i 0 (cm21 ) Si P (cm21 /mW) Carbon D band/Hi-S Nicalon/‡ /647.1 1326.11 21.30 “D”/Hi-Nicalon/‡ /514.5 1353.96 21.54 “D”/Hi-Nicalon§ / ‡ /514.5 1357.76 21.60 “D”/FT700/2/514.5 1354.18 20.20 “D”¶ /SCS6/Ti alloy/514.5 1354.26 20.47 “D”/Hi-Nicalon/celsian/514.5 1355.06 20.55 “D”/Hi-Nicalon/celsian/457.9 1368.90 20.70 “D”/Hi-Nicalon/celsian/647.1 1328.88 20.57 SiC TO mode††/SCS6 (SiC)/Ti alloy/514.5 791.49 0‡‡ † FT700 is a pure carbon fiber (Tonen, Japan); SCS6 fiber (Textron, U.S.A.) is a 33 mm diameter carbon filament surrounded by a 50 mm thick SiC layer. ‡ Free-standing fiber. § Scanning over 100 mm. ¶ In the center of carbon filament. ††12.5 mm from the fiber edge. ‡‡STO-SiC P 5 20.07 cm21 /mW for P . 17 mW. Fig. 3. (a) n#D (left-hand scale, in cm21 ) and SD ε (right-hand scale, in cm21 /%) as a function of the exciting laser line energy (in electronvolts) for FT700 carbon fibers. (b) Comparison of n#D dependency to the line energy in Hi-Nicalon and FT700 fibers. 1138 Journal of the American Ceramic Society—Gouadec et al. Vol. 84, No. 5