- Email: [email protected]
Development of doped graphites for plasma-facing components
Fusion Engineering and Design 56 – 57 (2001) 325– 330 www.elsevier.com/locate/fusengdes
Development of doped graphites for plasma-facing components P. Paz a, C. Garcı´a-Rosales a,*, J. Echeberria a, M. Balden b, J. Roth b, R. Behrisch b a
Centro de Estudios e In6estigaciones Te´cnicas de Gipuzkoa and Escuela Superior de Ingenieros, Uni6ersidad de Na6arra, P.O. Box 1555, 20009, San Sebastia´n, Spain b Max-Planck-Institut fu¨r Plasmaphysik, EURATOM Association, Boltzmannstr. 2, D-85748, Garching, Germany
Abstract Doping of carbon leads to a reduction of chemical erosion. This reduction is more effective if dopants are in the form of sub-mm precipitates with a very homogeneous distribution. However, dopants should not degrade but rather improve the thermal conductivity of graphite. Taking into account the catalytic effect of some metals and carbides on the graphitization, the potential for a development of improved doped carbon materials with reduced chemical erosion and optimized thermomechanical properties becomes evident. First results on the development of doped carbon materials starting from mesophase carbon powder and with different fine grain carbides as dopants show that VC acts as an effective catalyst for the graphitization. The materials obtained show high densities, a relatively low open porosity and good mechanical properties. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Doped graphites; Chemical erosion; Graphitization; Thermomechanical properties
1. Introduction In today’s fusion devices carbon materials are preferred for those areas receiving the highest power densities, i.e. limiters and divertor plates, due to their unequalled thermomechanical properties. A critical point, however, is the relative large erosion of carbon during hydrogen bombardment from the fusion plasma. Apart from physical sputtering, enhanced erosion by chemical sputtering,
* Corresponding author. Tel.: + 34-943-21-2800; fax: + 34943-21-3076. E-mail address: [email protected] (C. Garcı´a-Rosales).
i.e. the formation and release of hydrocarbons, is observed [1–3]. Besides the corresponding component lifetime limitation, this enhanced erosion contributes to a high tritium inventory trapped in redeposited carbon layers at other vessel wall areas [4]. The addition of small amounts (several at.%) of B, Si, Ti and W to carbon is known to influence both chemical erosion processes, i.e. the thermal activated process at elevated temperatures, and the surface process at low temperatures and low impact energies [3,5,6]. In addition, the dopant enrichment at the surface due to preferential sputtering of the carbon reduces the erosion yield. Recent investigations suggest that, besides the dopant element, also microstructure effects,
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such as dopant distribution, dopant particle size and porosity, play also an important role [5,6]. On the other hand, dopants should not degrade but rather improve the thermal conductivity and thermal shock resistance of graphite. As reported in [6], the keys for developing improved doped-carbon materials are the proper selection of dopants as well as the improvement of the microstructure. Dopants should reduce, on the one hand, both processes of chemical erosion and, as far as possible, the hydrogen retention. This requires a graphite material with a very fine and homogeneous dopant distribution and with low porosity. On the other hand, dopants should act at the same time as catalyst for graphitization [7] to obtain a carbon material with improved thermal conductivity. In this paper first results on the development and characterization of graphites doped with different carbides and with an optimised microstructure in view of a reduction of chemical erosion while improving their thermomechanical properties are presented.
2. Experimental As starting material a self-sintering powder of microspheres of carbonaceous mesophases (MMC) based on petroleum residues was used. This MMC powder is an excellent starting material for the production of isotropic, extremely fine-grained binderless carbon and represents a novel path for the production of isotropic graphite with significantly improved strength [8].
In order to reach an appropriate particle size distribution, the MMC powder was first micronized by jet-milling, after which it shows a bimodal distribution, with the first peak at 0.4 mm and the second one at 3.5 mm. The mean particle size is 0.6 mm. As dopants, the following carbides were selected up to now: B4C, a-SiC, TiC, V8C7, ZrC, WC. All these carbides except SiC are known to show a catalytic effect on graphitization [7]. As can be seen in Table 1, they have a high melting point and a relatively low vapor pressure. The particle size of the carbide powders was around 1 mm (see Table 1). The metals were selected so that they cover a wide range in atomic mass. The carbides were added to the carbon powder up to a metal concentration of about 5 at.%. For the case of TiC, a series ranging from 0.7 to 5 at.% was prepared to see the influence of the dopant concentration on chemical erosion. The fabrication procedure starting from mesophase carbon powder was as follows: after jet-milling and mixing, the powders were molded by uni-axial compaction (pressure between 100 and 225 MPa) to cylindrical bodies of ¥ 16× 5 mm and rods of 55× 18× 5 mm. The cylinders were used for density and porosity measurements and for the determination of the degree of graphitization, while the rods were used for the measurement of the mechanical properties. Sintering was performed at 1000 °C in a nitrogen atmosphere (heating rate 0.5 °C/min for 25– 600 °C, 2 °C/min for 600–1000 °C and dwell of 1 h at this temperature). The mass loss of the material without carbides was 13%. Graphitization occurred in a graphite resistance furnace
Table 1 Physical properties of the carbides selected as dopants [9–11] Carbide
Melting point (°C)
Vapor pressure at 2000 Particle size (mm) K (bar)
B4C a-SiC TiC V8C7 ZrC WC
2.470 2.540 3.150 2.850 3.500 2.870
4.5×10−8 2.7×10−6 7.6×10−10 9.4×10−9 3.0×10−11 4.0×10−11
1 1 1 0.9–1.3 1 0.7
Density (g/cm3)
Thermal conductivity at RT (W/mK)
2.5 3.2 4.9 5.7 6.7 15.6
30–90 125 30 29 100
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30 eV and 1 keV D+, as well as the energy dependence for 810 K was determined, in order to see the effect of dopants on the thermal chemical yield. These measurements are described in detail in [13].
3. Results and discussion
Fig. 1. Dependence of the flexural strength on the molding pressure for two mesophase powders of the same quality (without carbides) but different particle size.
in He atmosphere at temperatures between 2000 and 2350 °C (heating rate 20 °C/min from room temperature to 850 °C, 10 °C/min to the maximum temperature, dwell of 1 h at this temperature). The flexural strength, Young’s modulus and strain-to-failure was measured after sintering and after graphitization by a three-point bending test according to ISO 4492. The bulk densities were determined geometrically (and in mercury for comparison). The powder density and the open porosity were determined with a He pycnometer. After sintering and before graphitization the carbon material shows a turbostratic structure with very low crystalline order. During graphitization the unordered stacked carbon layer planes becomes more crystalline towards the perfect graphite structure while the crystallite size increases. The crystallite height Lc can be easily determined from X-ray diffraction by measuring the width of the (002) basal plane reflection. Hence, the value of Lc is generally used for the determination of the degree of graphitization, indicating the catalytic effect of the dopants. The Lc parameter was measured with a Philips diffractometer using the Cu-Ka1 radiation. First chemical erosion measurements were performed at the Garching high current ion source [12] by bombarding the samples during a long period with 30 eV D+ ions at room temperature, in order to see the evolution of the surface chemical erosion yield with fluence. Furthermore, the temperature dependence of the chemical yield for
In order to see the influence of the particle size and the molding pressure on the mechanical properties, rods of the same mesophase powder (without carbides) but with different particle size distribution were molded at different pressures between 100 and 225 MPa. One of the powder was jet-milled to the particle size distribution described before. The other one was only screened by a 100 mm sieve. In Fig. 1 the flexural strength |f for the two powders is shown as a function of the molding pressure. It can be observed that the powder with the small particle size results in considerably higher strength values than the rough powder. The optimum pressure for which the maximum values of |f are reached lies between 150 and 200 MPa. At 225 MPa the rods molded with the fine powder present cracks likely originated during the molding procedure and/or by the exit of volatile compounds during the sintering process. Thus, all further samples were molded at 150 MPa. The dopant distribution have been investigated for polished surfaces by scanning electron microcopy. One observed in all cases a fine and homogeneous carbide distribution. The bulk density of the specimen without carbides was determined in various stages of the graphitization treatment. It increases almost linearly from 1.62 g/cm3 after sintering to a value of 1.92 g/cm3, which is reached at 1700 °C and remains nearly constant up to the maximum temperature. In Table 2 the bulk density as well as the powder density is shown for all manufactured samples after sintering and after graphitization at 2350 °C. The Lc value, which is a mass of the degree of graphitization, was measured for all manufactured samples after sintering and after graphitization. After sintering all samples showed Lc values
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bon in a metal carbide particle followed by the precipitation as ordered graphite is proposed as possible mechanism for catalytic graphitization. As reported in [14], V8C7 experiences an orderdisorder transition at a temperature of 1360–1380 K, i.e. just at the beginning of the graphitization process. In a disordered phase the amount of vacancies and thus the carbon diffusion should increase, which would explain the larger catalytic effect of this carbide at lower temperatures, if the mentioned mechanism applies. For comparison, the Lc values of a commercial graphite (POCO) and of the 2 at.% Ti doped graphite RG-Ti 91 (Efremov Institute, St. Petersburg [15]) are also shown. The RG-Ti material, which shows an extremely high thermal conductivity in one direction [15], exhibits also a high Lc value, indicating a
of the order of 1– 2 nm. The value of Lc after graphitization is shown in Fig. 2 for samples with different dopants but the same at.% metal concentration for the two graphitization temperatures (Fig. 2a), and with Ti as dopant in different concentrations (Fig. 2b). As can be noticed from Fig. 2a, all carbides except SiC lead to an increase in Lc as compared with the Lc value of graphite without carbides. This is a clear indication of the existence of a catalytic effect on graphitization. At 2100 °C V8C7 shows the largest catalytic effect of all investigated carbides. For higher temperatures, however, the Lc value of this sample does not increase but rather decreases. In contrast, the Lc value of the W- and Ti-doped samples increases considerably with increasing graphitization temperature. In [7] the dissolution of unordered car-
Table 2 Properties of doped carbon materials after sintering at 1000 °C and after graphitization at 2350°C Dopant
Without B4C TiC V8C7 WC
1000 2350 1000 2350 1000 2350 1000 2350 1000 2350
°C °C °C °C °C °C °C °C °C °C
Bulk density (g/cm3)
Powder density (g/cm3)
Flexural strength (MPa)
Young’s modulus (GPa)
Strain-to-failure (%)
1.62 1.94 1.52 1.97 1.81 2.04 1.84 1.96 2.46 2.91
1.99 2.15 2.01 2.16 2.23 2.39 2.28 2.45 3.12 3.35
144 69 111 64 99
20.0 6.9 15.6 7.0 12.4 6.9 13.8 5.0 14.0 6.7
2.2 0.9 0.93 0.93 1.12 0.73 0.82 1.02 0.93 0.91
88 57 97 72
Fig. 2. Mean crystallite height Lc: for samples with different dopants in the same at.% metal concentration graphitized at 2100 °C (left), and for Ti-doped samples graphitized at 2025 °C as a function of the Ti concentrations (right). In the left figure the Lc values for the commercial fine grain graphite POCO and for the Ti-doped graphite RG-Ti 91 are also shown.
P. Paz et al. / Fusion Engineering and Design 56–57 (2001) 325–330
Fig. 3. Open and close porosity after calcination (1000 °C) and after graphitization (2100 °C) for the manufactured specimens: pure graphite (MMC) and graphite doped with different carbides.
Fig. 4. Flexural strength of pure graphite (MMC) and the doped graphites after sintering (1000 °C) and after graphitization at 2350 °C.
high degree of crystallinity. The POCO material, on the other hand, shows about the same crystallinity as our graphite without dopants. Fig. 2b shows that the catalytic effect of TiC seems to saturate at about 3 to 4 at.% Ti. In Fig. 3 the open and closed porosity is shown for pure graphite (MMC) and for the doped graphites after sintering (1000 °C) and after graphitization at 2100 °C. While pure carbon experiences a reduction of the total porosity of about 33% during graphitization, the reduction observed for the graphites doped with SiC and VC, and in a lower extent for ZrC, is surprisingly low. Furthermore, in the pure carbon sample the closed porosity remains nearly constant during the graphitization process, and the open porosity experiences a small decrease. In contrast, in all doped graphites the open porosity decreases significantly while the closed porosity becomes even
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larger than before graphitization. This effect is especially pronounced for the VC-doped graphite. This carbide shows, on the other hand, the highest catalytic effect on graphitization, as mentioned above. These observations could be related to the mechanisms taking place during catalytic graphitization. Fig. 4 shows the flexural strength for pure graphite (MMC) and for the doped graphites after sintering (1000 °C) and after graphitization at 2350 °C. After graphitization an inevitable decrease of strength is observed for all samples due to the recrystallization process taking place. However, the decrease in strength for the doped samples is not as pronounced as for the undoped sample. Hence the carbides seem to play a beneficial role during graphitization regarding the mechanical properties. In Table 2 the Young’s modulus and the strain-to-failure after sintering and after graphitization at the highest temperature are compiled. While after sintering there is a strong difference between the strain-to-failure of pure graphite and that of the doped graphites, after graphitization all samples show about the same strain-to-failure of 1%. The same tendency is observed for the Young’s modulus. Concerning chemical erosion, first results show that carbide addition leads to a reduction of both the thermal and the surface processes, which is only partly attributed to surface enrichment of the carbides but is also the result of a chemical influence in both cases [13].
4. Summary First results on the development of new doped carbon materials show that VC acts as an effective catalyst for the graphitization at relatively low graphitization temperatures. TiC, B4C and WC are effective catalysts at higher temperatures. For B4C-doping, however, a strong reduction of the thermal conductivity is expected due to solid solution in the graphite lattice. The materials obtained show a high density, low open porosity and mechanical properties comparable or better than most commercial fine grain graphites.
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Acknowledgements We would like to thank the company REPSOLYPF for providing the mesophase carbon powder for sample manufacturing, as well as T. Go´ mezAcebo for his kind assistance by using the Thermo-Calc software for the calculation of the carbide vapor pressures. Thanks are also due to I. Gil and E. Oyarzabal for their efficient experimental assistance.
References [1] J. Roth, C. Garcı´a-Rosales, Nucl. Fusion 36 (1996) 1647 with corrigendum Nucl. Fusion 37 (1997) 897. [2] B.V. Mech, A.A. Haasz, J.W. Davis, J. Appl. Phys. 84 (3) (1998) 1655. [3] J. Roth, J. Nucl. Mater. 266 –269 (1999) 51. [4] G. Federici, R.A. Anderl, P. Andrew, M.A. Pick, J.N. Brooks, R.A. Causey, J.P. Coad, D. Cowgill, R.P. Do-
[5] [6] [7] [8] [9] [10] [11] [12]
[13] [14] [15]
erner, A.A. Haasz, G. Janeschitz, W. Jacob, G.R. Longhurst, R. Nygren, A. Peacock, V. Philipps, J. Roth, C.H. Skinner, W.R. Wampler, J. Nucl. Mater. 266 –269 (1999) 14. M. Balden, Physica Scripta T81 (1999) 64. C. Garcı´a-Rosales, M. Balden, J. Nucl. Mater. 290 –293 (2001) 173. A. O( ya, H. Marsh, J. Mater. Sci. 17 (1982) 309. K. Hu¨ ttinger, Adv. Mater. 2 (8) (1990) 349. O. Kubaschewski, E.L.L. Evans, C.B. Alcock, Metallurgical Thermochemistry, 4th, Pergamon Press, Oxford, 1967. Handbook of Chemistry and Physics, 79th Ed. R.J. Kotelinikof, et al., Handbook on Super Refractory Materials, Metallurgy publisher, Moscow, 1976. W. Eckstein, C. Garcı´a-Rosales, J. Roth, W. Ottenberger, Sputtering Data, Techn. Rep. IPP 9/82, Max-Planck-Institut fu¨ r Plasmaphysik, Garching, 1993. M. Balden, C. Garcı´a-Rosales, R. Behrisch, J. Roth, P. Paz, J. Echeberria, J. Nucl. Mater. 290 – 293 (2001) 52. V.N. Lipatnikov, A.I. Gusev, P. Ettmayer, W. Lengauer, J. Phys.: Condens. Matter 11 (1999) 163. T.A. Burtseva, O.K. Chugunov, E.F. Dovguchits, et al., J. Nucl. Mat. 191 – 194 (1992) 309.
Development of doped graphites for plasma-facing components P. Paz a, C. Garcı´a-Rosales a,*, J. Echeberria a, M. Balden b, J. Roth b, R. Behrisch b a
Centro de Estudios e In6estigaciones Te´cnicas de Gipuzkoa and Escuela Superior de Ingenieros, Uni6ersidad de Na6arra, P.O. Box 1555, 20009, San Sebastia´n, Spain b Max-Planck-Institut fu¨r Plasmaphysik, EURATOM Association, Boltzmannstr. 2, D-85748, Garching, Germany
Abstract Doping of carbon leads to a reduction of chemical erosion. This reduction is more effective if dopants are in the form of sub-mm precipitates with a very homogeneous distribution. However, dopants should not degrade but rather improve the thermal conductivity of graphite. Taking into account the catalytic effect of some metals and carbides on the graphitization, the potential for a development of improved doped carbon materials with reduced chemical erosion and optimized thermomechanical properties becomes evident. First results on the development of doped carbon materials starting from mesophase carbon powder and with different fine grain carbides as dopants show that VC acts as an effective catalyst for the graphitization. The materials obtained show high densities, a relatively low open porosity and good mechanical properties. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Doped graphites; Chemical erosion; Graphitization; Thermomechanical properties
1. Introduction In today’s fusion devices carbon materials are preferred for those areas receiving the highest power densities, i.e. limiters and divertor plates, due to their unequalled thermomechanical properties. A critical point, however, is the relative large erosion of carbon during hydrogen bombardment from the fusion plasma. Apart from physical sputtering, enhanced erosion by chemical sputtering,
* Corresponding author. Tel.: + 34-943-21-2800; fax: + 34943-21-3076. E-mail address: [email protected] (C. Garcı´a-Rosales).
i.e. the formation and release of hydrocarbons, is observed [1–3]. Besides the corresponding component lifetime limitation, this enhanced erosion contributes to a high tritium inventory trapped in redeposited carbon layers at other vessel wall areas [4]. The addition of small amounts (several at.%) of B, Si, Ti and W to carbon is known to influence both chemical erosion processes, i.e. the thermal activated process at elevated temperatures, and the surface process at low temperatures and low impact energies [3,5,6]. In addition, the dopant enrichment at the surface due to preferential sputtering of the carbon reduces the erosion yield. Recent investigations suggest that, besides the dopant element, also microstructure effects,
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such as dopant distribution, dopant particle size and porosity, play also an important role [5,6]. On the other hand, dopants should not degrade but rather improve the thermal conductivity and thermal shock resistance of graphite. As reported in [6], the keys for developing improved doped-carbon materials are the proper selection of dopants as well as the improvement of the microstructure. Dopants should reduce, on the one hand, both processes of chemical erosion and, as far as possible, the hydrogen retention. This requires a graphite material with a very fine and homogeneous dopant distribution and with low porosity. On the other hand, dopants should act at the same time as catalyst for graphitization [7] to obtain a carbon material with improved thermal conductivity. In this paper first results on the development and characterization of graphites doped with different carbides and with an optimised microstructure in view of a reduction of chemical erosion while improving their thermomechanical properties are presented.
2. Experimental As starting material a self-sintering powder of microspheres of carbonaceous mesophases (MMC) based on petroleum residues was used. This MMC powder is an excellent starting material for the production of isotropic, extremely fine-grained binderless carbon and represents a novel path for the production of isotropic graphite with significantly improved strength [8].
In order to reach an appropriate particle size distribution, the MMC powder was first micronized by jet-milling, after which it shows a bimodal distribution, with the first peak at 0.4 mm and the second one at 3.5 mm. The mean particle size is 0.6 mm. As dopants, the following carbides were selected up to now: B4C, a-SiC, TiC, V8C7, ZrC, WC. All these carbides except SiC are known to show a catalytic effect on graphitization [7]. As can be seen in Table 1, they have a high melting point and a relatively low vapor pressure. The particle size of the carbide powders was around 1 mm (see Table 1). The metals were selected so that they cover a wide range in atomic mass. The carbides were added to the carbon powder up to a metal concentration of about 5 at.%. For the case of TiC, a series ranging from 0.7 to 5 at.% was prepared to see the influence of the dopant concentration on chemical erosion. The fabrication procedure starting from mesophase carbon powder was as follows: after jet-milling and mixing, the powders were molded by uni-axial compaction (pressure between 100 and 225 MPa) to cylindrical bodies of ¥ 16× 5 mm and rods of 55× 18× 5 mm. The cylinders were used for density and porosity measurements and for the determination of the degree of graphitization, while the rods were used for the measurement of the mechanical properties. Sintering was performed at 1000 °C in a nitrogen atmosphere (heating rate 0.5 °C/min for 25– 600 °C, 2 °C/min for 600–1000 °C and dwell of 1 h at this temperature). The mass loss of the material without carbides was 13%. Graphitization occurred in a graphite resistance furnace
Table 1 Physical properties of the carbides selected as dopants [9–11] Carbide
Melting point (°C)
Vapor pressure at 2000 Particle size (mm) K (bar)
B4C a-SiC TiC V8C7 ZrC WC
2.470 2.540 3.150 2.850 3.500 2.870
4.5×10−8 2.7×10−6 7.6×10−10 9.4×10−9 3.0×10−11 4.0×10−11
1 1 1 0.9–1.3 1 0.7
Density (g/cm3)
Thermal conductivity at RT (W/mK)
2.5 3.2 4.9 5.7 6.7 15.6
30–90 125 30 29 100
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30 eV and 1 keV D+, as well as the energy dependence for 810 K was determined, in order to see the effect of dopants on the thermal chemical yield. These measurements are described in detail in [13].
3. Results and discussion
Fig. 1. Dependence of the flexural strength on the molding pressure for two mesophase powders of the same quality (without carbides) but different particle size.
in He atmosphere at temperatures between 2000 and 2350 °C (heating rate 20 °C/min from room temperature to 850 °C, 10 °C/min to the maximum temperature, dwell of 1 h at this temperature). The flexural strength, Young’s modulus and strain-to-failure was measured after sintering and after graphitization by a three-point bending test according to ISO 4492. The bulk densities were determined geometrically (and in mercury for comparison). The powder density and the open porosity were determined with a He pycnometer. After sintering and before graphitization the carbon material shows a turbostratic structure with very low crystalline order. During graphitization the unordered stacked carbon layer planes becomes more crystalline towards the perfect graphite structure while the crystallite size increases. The crystallite height Lc can be easily determined from X-ray diffraction by measuring the width of the (002) basal plane reflection. Hence, the value of Lc is generally used for the determination of the degree of graphitization, indicating the catalytic effect of the dopants. The Lc parameter was measured with a Philips diffractometer using the Cu-Ka1 radiation. First chemical erosion measurements were performed at the Garching high current ion source [12] by bombarding the samples during a long period with 30 eV D+ ions at room temperature, in order to see the evolution of the surface chemical erosion yield with fluence. Furthermore, the temperature dependence of the chemical yield for
In order to see the influence of the particle size and the molding pressure on the mechanical properties, rods of the same mesophase powder (without carbides) but with different particle size distribution were molded at different pressures between 100 and 225 MPa. One of the powder was jet-milled to the particle size distribution described before. The other one was only screened by a 100 mm sieve. In Fig. 1 the flexural strength |f for the two powders is shown as a function of the molding pressure. It can be observed that the powder with the small particle size results in considerably higher strength values than the rough powder. The optimum pressure for which the maximum values of |f are reached lies between 150 and 200 MPa. At 225 MPa the rods molded with the fine powder present cracks likely originated during the molding procedure and/or by the exit of volatile compounds during the sintering process. Thus, all further samples were molded at 150 MPa. The dopant distribution have been investigated for polished surfaces by scanning electron microcopy. One observed in all cases a fine and homogeneous carbide distribution. The bulk density of the specimen without carbides was determined in various stages of the graphitization treatment. It increases almost linearly from 1.62 g/cm3 after sintering to a value of 1.92 g/cm3, which is reached at 1700 °C and remains nearly constant up to the maximum temperature. In Table 2 the bulk density as well as the powder density is shown for all manufactured samples after sintering and after graphitization at 2350 °C. The Lc value, which is a mass of the degree of graphitization, was measured for all manufactured samples after sintering and after graphitization. After sintering all samples showed Lc values
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bon in a metal carbide particle followed by the precipitation as ordered graphite is proposed as possible mechanism for catalytic graphitization. As reported in [14], V8C7 experiences an orderdisorder transition at a temperature of 1360–1380 K, i.e. just at the beginning of the graphitization process. In a disordered phase the amount of vacancies and thus the carbon diffusion should increase, which would explain the larger catalytic effect of this carbide at lower temperatures, if the mentioned mechanism applies. For comparison, the Lc values of a commercial graphite (POCO) and of the 2 at.% Ti doped graphite RG-Ti 91 (Efremov Institute, St. Petersburg [15]) are also shown. The RG-Ti material, which shows an extremely high thermal conductivity in one direction [15], exhibits also a high Lc value, indicating a
of the order of 1– 2 nm. The value of Lc after graphitization is shown in Fig. 2 for samples with different dopants but the same at.% metal concentration for the two graphitization temperatures (Fig. 2a), and with Ti as dopant in different concentrations (Fig. 2b). As can be noticed from Fig. 2a, all carbides except SiC lead to an increase in Lc as compared with the Lc value of graphite without carbides. This is a clear indication of the existence of a catalytic effect on graphitization. At 2100 °C V8C7 shows the largest catalytic effect of all investigated carbides. For higher temperatures, however, the Lc value of this sample does not increase but rather decreases. In contrast, the Lc value of the W- and Ti-doped samples increases considerably with increasing graphitization temperature. In [7] the dissolution of unordered car-
Table 2 Properties of doped carbon materials after sintering at 1000 °C and after graphitization at 2350°C Dopant
Without B4C TiC V8C7 WC
1000 2350 1000 2350 1000 2350 1000 2350 1000 2350
°C °C °C °C °C °C °C °C °C °C
Bulk density (g/cm3)
Powder density (g/cm3)
Flexural strength (MPa)
Young’s modulus (GPa)
Strain-to-failure (%)
1.62 1.94 1.52 1.97 1.81 2.04 1.84 1.96 2.46 2.91
1.99 2.15 2.01 2.16 2.23 2.39 2.28 2.45 3.12 3.35
144 69 111 64 99
20.0 6.9 15.6 7.0 12.4 6.9 13.8 5.0 14.0 6.7
2.2 0.9 0.93 0.93 1.12 0.73 0.82 1.02 0.93 0.91
88 57 97 72
Fig. 2. Mean crystallite height Lc: for samples with different dopants in the same at.% metal concentration graphitized at 2100 °C (left), and for Ti-doped samples graphitized at 2025 °C as a function of the Ti concentrations (right). In the left figure the Lc values for the commercial fine grain graphite POCO and for the Ti-doped graphite RG-Ti 91 are also shown.
P. Paz et al. / Fusion Engineering and Design 56–57 (2001) 325–330
Fig. 3. Open and close porosity after calcination (1000 °C) and after graphitization (2100 °C) for the manufactured specimens: pure graphite (MMC) and graphite doped with different carbides.
Fig. 4. Flexural strength of pure graphite (MMC) and the doped graphites after sintering (1000 °C) and after graphitization at 2350 °C.
high degree of crystallinity. The POCO material, on the other hand, shows about the same crystallinity as our graphite without dopants. Fig. 2b shows that the catalytic effect of TiC seems to saturate at about 3 to 4 at.% Ti. In Fig. 3 the open and closed porosity is shown for pure graphite (MMC) and for the doped graphites after sintering (1000 °C) and after graphitization at 2100 °C. While pure carbon experiences a reduction of the total porosity of about 33% during graphitization, the reduction observed for the graphites doped with SiC and VC, and in a lower extent for ZrC, is surprisingly low. Furthermore, in the pure carbon sample the closed porosity remains nearly constant during the graphitization process, and the open porosity experiences a small decrease. In contrast, in all doped graphites the open porosity decreases significantly while the closed porosity becomes even
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larger than before graphitization. This effect is especially pronounced for the VC-doped graphite. This carbide shows, on the other hand, the highest catalytic effect on graphitization, as mentioned above. These observations could be related to the mechanisms taking place during catalytic graphitization. Fig. 4 shows the flexural strength for pure graphite (MMC) and for the doped graphites after sintering (1000 °C) and after graphitization at 2350 °C. After graphitization an inevitable decrease of strength is observed for all samples due to the recrystallization process taking place. However, the decrease in strength for the doped samples is not as pronounced as for the undoped sample. Hence the carbides seem to play a beneficial role during graphitization regarding the mechanical properties. In Table 2 the Young’s modulus and the strain-to-failure after sintering and after graphitization at the highest temperature are compiled. While after sintering there is a strong difference between the strain-to-failure of pure graphite and that of the doped graphites, after graphitization all samples show about the same strain-to-failure of 1%. The same tendency is observed for the Young’s modulus. Concerning chemical erosion, first results show that carbide addition leads to a reduction of both the thermal and the surface processes, which is only partly attributed to surface enrichment of the carbides but is also the result of a chemical influence in both cases [13].
4. Summary First results on the development of new doped carbon materials show that VC acts as an effective catalyst for the graphitization at relatively low graphitization temperatures. TiC, B4C and WC are effective catalysts at higher temperatures. For B4C-doping, however, a strong reduction of the thermal conductivity is expected due to solid solution in the graphite lattice. The materials obtained show a high density, low open porosity and mechanical properties comparable or better than most commercial fine grain graphites.
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Acknowledgements We would like to thank the company REPSOLYPF for providing the mesophase carbon powder for sample manufacturing, as well as T. Go´ mezAcebo for his kind assistance by using the Thermo-Calc software for the calculation of the carbide vapor pressures. Thanks are also due to I. Gil and E. Oyarzabal for their efficient experimental assistance.
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