1138 G E. Youngblood et al. / Composites Science and Technology 62(2002)1127-1139 Tyranno SA/CVI-SiC exhibit continuously positive thermal conductivity degradation induced in these temperature dependence. Even with near-perfect thermal woven composites by neutron radiation or by other coupling, Ke <5W/(m K) at 27C and <14 W/(m K) mechanical or environmental treatments. If a 2D-SiCd at 1000oC, primarily because of the degradation in the SiC composite with initially high Kr and h-values were irradiated matrix K-values. irradiated, Ker could easily be reduced by a factor of or an irradiated SiCr/SiC composite with a PyC fiber five or six due to the degradation of the interface con- coating, the thermal conductivity of the coating will be ductance and the matrix conductivity degraded and some debonding certainly is expected due To further examine this issue, the effects of tempera to fiber/matrix differential swelling-shrinkage. There- ture and irradiation on Keff were predicted for a hypo fore, KedT) likely will fall somewhere between the thetical 2D-SiCe/Sic composite made with a CVI-SiC h=50 and 500 W/(cmK)curves in Fig 10(b), i.e. in the matrix and reinforced with high conductivity Tyranno range 3-10 W/(m K). Although these predicted Kefr SA fiber. For this example, it was predicted that Keff at values are much lower than the predicted unirradiated 200C would decrease from 34 W/(m K) before irra- values, they are somewhat greater than the Keft-values diation to <6 W/(m K)after irradiation at 200C. measured for an irradiated 2D-SiCe/SiC composite Similarly, at 1000C Keff would decrease from 26 W/ made with less crystalline Nicalon CG fiber(Kr N2 W/(m K) before irradiation to <10 W/(m K) after irra- (m K) with a similar PyC coating. For that composite, diation unirradiated Kerr-values ranged from 9 down to 6.5 W/ (m K)(RT-1000C). When irradiated at 500 or 1200C. the respective Kefr-values were degraded to 2 and 5 W/ Acknowledgements (m K), respectively [37]. Note that the Kefrvalues for the irradiated Nicalon CG composite exhibited higher Work supported by the N gy values for higher irradiation temperatures, a trend also Initiative (NERD, US Department of Energy under predicted for the irradiated hypothetical Tyranno SA/ Contract DE-ACO6-76RLO 1830 CVI-SiC composite 5. Summary and conclusions References For fiber-to-matrix conductivity ratios less than 10 carbide. J (r<10)and for f <0.5, the H-J model predictions of the [2] Hopkins GI i NIc ma J SiC matrix/ SiC fiber composite: a high-heat effective transverse thermal conductivity, Keff, given by flux, low activation, structural material. J Nucl Mater 19 Eq (1)deviate from numerical FEM results by less than 5%. For dispersed fibers in a matrix, Kefr is primarily 3 Sharafat S, Najmabadi F, Wong CPC, the ARIES Design Team. ARIES-I Fusion- Power-Core engineering. Fusion Eng Design controlled by the thermal conductivity of the con 1991;18:21 tinuous matrix phase, Km, and the interfacial con- [4] Ueda S, Nishio S, Seki Y, Kurihara R, Adochi J, Yamazaki S, uctance, h. a simple thermal barrier model, Eq the drEAM Design Team. A fusion power reactor concept using was introduced to describe h in terms of the gaseous and SiC/SiC composites. J Nucl Mat 1998; 1589: 258-63. direct contact components (ghg and fahd, respectively) 5 Perez Ramirez S, Caso A, Giancarli L, Le Bars N, Chaumat G, Salavy JF, et al. TAURO: a ceramic composite structural mate Experimental values of h, determined for a uniaxial Hi- Nicalon/amorphous PIP-SiC composite in vacuum, rial self-cooled Pb-17Li breeder blanket concept. J Nucl Mater 1996:1257:233-7 argon and helium, compared favorably with values [6] estimated by the simple thermal barrier model. Reason tures of isotropic materials. Annal Physik 1935: 24: 636 able agreement between numerical FEM and experi Nomura S, Chou Tw. Bounds of effective thermal conductivities of short-fiber composites. J Comp Mater 1980: 14: 120 mental results with H- model predictions suggest that [8 Brennan JJ, Bentsen LD, Hasselman DPH. Determination of the Ke for a SiCt/Sic composite with fiber volume frac- hermal conductivity and diffusivity of thin fibers by the com- tions f<0.5 and with simple unidirectional or cross-ply osite method. J Mater Sci 1982: 17: 2337. fiber architecture are well described by eq (1) 9 Tawil H, Bentsen LD, Baskaran S, Hasselman DPH. Thermal For a woven 2D-SiCrSiC composite, the localized conductivity of chemically vapor deposited silicon carbide rein- forced with silicon carbide or carbon fibers. J Mater Sci 1985: 20: effects of dense fiber packing within individual tows f usually >0.6 within tows) and the occurrence of [10] Hasselman DPH, Johnson LF, Syed R, Taylor MP, Chyung K many direct fiber-fiber contacts, especially near the Heat conduction characteristics of a carbon fiber-reinforced numerous fiber bundle crossover points, will introduce lithia-aluminosilicate ceramic. J Mater Sci 1987: 22: 701 positive or negative deviations from Eq (1)when r>or [1 Hasselman DPH, Venkateswaran A, Yu M, Tawil H. Role of espectively. However, the analytic solution diffusivity of SiC fiber-reinforced chemically vapor deposited Sic expressed by Eq (1)should be appropriate to examine matrix composite. J Mater Sci Lett 1991: 10: 1037.Tyranno SA/CVI-SiC exhibit continuously positive temperature dependence. Even with near-perfect thermal coupling, Keff<5 W/(m K) at 27
C and <14 W/(m K) at 1000
C,primarily because of the degradation in the irradiated matrix Km-values. For an irradiated SiCf/SiC composite with a PyC fiber coating,the thermal conductivity of the coating will be degraded and some debonding certainly is expected due to fiber/matrix differential swelling-shrinkage. There￾fore, Keff(T) likely will fall somewhere between the h=50 and 500 W/(cm2 K) curves in Fig. 10(b),i.e. in the range 3–10 W/(m K). Although these predicted Keff- values are much lower than the predicted unirradiated values,they are somewhat greater than the Keff-values measured for an irradiated 2D-SiCf/SiC composite made with less crystalline Nicalon CG fiber (Kf 2 W/ (m K) with a similar PyC coating. For that composite, unirradiated Keff-values ranged from 9 down to 6.5 W/ (m K) (RT-1000
C). When irradiated at 500 or 1200
C, the respective Keff-values were degraded to 2 and 5 W/ (m K),respectively [37]. Note that the Keff-values for the irradiated Nicalon CG composite exhibited higher values for higher irradiation temperatures,a trend also predicted for the irradiated hypothetical Tyranno SA/ CVI-SiC composite. 5. Summary and conclusions For fiber-to-matrix conductivity ratios less than 10 (r<10) and for f40.5,the H–J model predictions of the effective transverse thermal conductivity, Keff,given by Eq. (1) deviate from numerical FEM results by less than 5%. For dispersed fibers in a matrix, Keff is primarily controlled by the thermal conductivity of the con￾tinuous matrix phase, Km,and the interfacial con￾ductance, h. A simple thermal barrier model,Eq. (4), was introduced to describe h in terms of the gaseous and direct contact components ( fghg and fdhd,respectively). Experimental values of h,determined for a uniaxial Hi￾Nicalon/amorphous PIP-SiC composite in vacuum, argon and helium,compared favorably with values estimated by the simple thermal barrier model. Reason￾able agreement between numerical FEM and experi￾mental results with H–J model predictions suggest that Keff for a SiCf/SiC composite with fiber volume frac￾tions f40.5 and with simple unidirectional or cross-ply fiber architecture are well described by Eq. (1). For a woven 2D-SiCf/SiC composite,the localized effects of dense fiber packing within individual tows ( f usually50.6 within tows) and the occurrence of many direct fiber–fiber contacts,especially near the numerous fiber bundle crossover points,will introduce positive or negative deviations from Eq. (1) when r> or <1,respectively. However,the analytic solution expressed by Eq. (1) should be appropriate to examine thermal conductivity degradation induced in these woven composites by neutron radiation or by other mechanical or environmental treatments. If a 2D-SiCf/ SiC composite with initially high Kf- and h-values were irradiated, Keff could easily be reduced by a factor of five or six due to the degradation of the interface con￾ductance and the matrix conductivity. To further examine this issue,the effects of tempera￾ture and irradiation on Keff were predicted for a hypo￾thetical 2D-SiCf/SiC composite made with a CVI-SiC matrix and reinforced with high conductivity Tyranno SA fiber. For this example,it was predicted that Keff at 200
C would decrease from 34 W/(m K) before irra￾diation to <6 W/(m K) after irradiation at 200
C. Similarly,at 1000
C Keff would decrease from 26 W/ (m K) before irradiation to <10 W/(m K) after irra￾diation. Acknowledgements Work supported by the Nuclear Energy Research Initiative (NERI),US Department of Energy under Contract DE-AC06–76RLO 1830. References [1] Price RJ. Effects of fast neutron irradiation on pyrolytic silicon carbide. J Nucl Mater 1969;33:17. [2] Hopkins GR,Chin J. SiC matrix/SiC fiber composite: a high-heat flux,low activation,structural material. J Nucl Mater 1986;148: 141–3. [3] Sharafat S,Najmabadi F,Wong CPC,the ARIES Design Team. ARIES-I Fusion-Power-Core engineering. Fusion Eng Design 1991;18:21. [4] Ueda S,Nishio S,Seki Y,Kurihara R,Adochi J,Yamazaki S, the DREAM Design Team. A fusion power reactor concept using SiC/SiC composites. J Nucl Mat 1998;1589:258–63. [5] Perez Ramirez S,Caso A,Giancarli L,Le Bars N,Chaumat G, Salavy JF,et al. TAURO: a ceramic composite structural mate￾rial self-cooled Pb-17Li breeder blanket concept. J Nucl Mater 1996;1257:233–7. [6] Bruggeman DAG. Dielectric constant and conductivity of mix￾tures of isotropic materials. Annal Physik 1935;24:636. [7] Nomura S,Chou TW. Bounds of effective thermal conductivities of short-fiber composites. J Comp Mater 1980;14:120. [8] Brennan JJ,Bentsen LD,Hasselman DPH. Determination of the thermal conductivity and diffusivity of thin fibers by the com￾posite method. J Mater Sci 1982;17:2337. [9] Tawil H,Bentsen LD,Baskaran S,Hasselman DPH. Thermal conductivity of chemically vapor deposited silicon carbide rein￾forced with silicon carbide or carbon fibers. J Mater Sci 1985;20: 3201. [10] Hasselman DPH,Johnson LF,Syed R,Taylor MP,Chyung K. Heat conduction characteristics of a carbon fiber-reinforced lithia-aluminosilicate ceramic. J Mater Sci 1987;22:701. [11] Hasselman DPH,Venkateswaran A,Yu M,Tawil H. Role of interfacial debonding and matrix cracking in the effective thermal diffusivity of SiC fiber-reinforced chemically vapor deposited SiC matrix composite. J Mater Sci Lett 1991;10:1037. 1138 G.E. Youngbloodet al. / Composites Science andTechnology 62 (2002) 1127–1139