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conductivity of doped graphites
ELSEVIER
Journal of Nuclear Materials 224 (1995) 245-253
jouroalof smear materials
Thermal diffusivity/conductivity of doped graphites B.N. Enweani, J.W. Davis, A.A. Haasz Fusion Research Group, University of Toronto Institute for Aerospace Studies, 4925 Dufferin Street, North York, Ontario, Canada M3H 5T6
Received 26 January 1995; accepted 5 April 1995
Abstract The thermal properties of doped graphite materials were measured using a pulsed laser diffusivity/calorimetry technique. Dopants included: boron, silicon, titanium, nickel and tungsten. Measurements were made in the 300-1000 K temperature range. Differential scanning calorimetry (DSC) was also employed for some specimens to calibrate the pulsed laser method of heat capacity measurement. The specimens were found to be highly anisotropic visa vis thermal diffusivity/conductivity. For the high thermal conductivity direction, all specimens showed a general trend to lower conductivities with increasing dopant concentration - more so at lower temperature, resulting in a reduced temperature dependence. Typically, the high direction thermal conductivities of specimens with dopant concentrations > 7 at% were about a factor of two lower than the values obtained for an undoped reference specimen. For the low conductivity direction, the diffusivity/conductivity was relatively independent of temperature and dopant species and concentration.
1. Introduction Carbon-based materials are important candidates for plasma-facing components in magnetic fusion reactors. In addition to heing a Iow-Z material, graphite's other positive characteristics include excellent resistance to transient thermal shock, high operating temperatures, low nuclear activation and high thermal conductivity. Graphite's major drawback, however is its erosion behaviour due to plasma exposure. Attempts are being made to improve graphite's erosion resistance via the addition of non-carbon elements into the carbon matrix. Unfortunately, the addition of "dopants" might have an effect on the materials's thermal conductivity. Behrisch and Venus [1] have compiled results on the thermal conductivity of various materials relevant to fusion and evaluated their ability to remove heat. Few data are available for doped graphites, but the data that are available generally indicate a reduction in thermal conductivity due to the addition of dopants, or the change in structure associated with the addition of dopants. The objective of the present experiment was to study the effect of a wide range of low-Z to high-Z
dopant elements, in different concentrations, on the thermal diffusivity and conductivity of high density graphite. Test specimens doped with B, Si, Ti, Ni and W were fabricated by Ceramics Kingston Ceramique Inc. (CKC). The specific properties investigated were the temperature dependence of thermal diffusivity and heat capacity; thermal conductivity was then calculated. Comparisons are made with EK98 (isotropic fine grain graphite manufactured by Ringsdorf, Germany) and a CKC reference graphite specimen manufactured by the same method as the doped specimens, but containing no dopants. This work was part of a larger study which also examined the effect of dopants on chemical reactivity [2], radiation-enhanced sublimation [3] and hydrogen retention and recycling [4].
2. Experiment 2.1. Specimens
The doped specimens were manufactured by CKC from finely ground pyrolytic graphite (10-45 txm) mixed
0022-3115/95/$09.50 ~ 1995 Elsevier Science B.V. All rights reserved SSDI 0022-3115(95)00068-2
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B.N. Enweani et al. /Journal of Nuclear Materials 224 (1995) 245-253
Table 1 Properties of the CKC doped graphite specimens Specimen designation a
Ringsdorf EK98 no dopants CKC reference no dopants CKC-B2 -B10 -B20 CKC-Si3 -Si8 -Sil4 CKC-Ti2 -Ti8 -Til6 CKC-Ni8 CKC-W10
Dopant Form/size of concentration dopants in bulk (at%) [3]
Density (kg/m 3)
-
-
1860
-
-
2060
metal/few I~m metal/few Ixm metal/few Ixm metal/few Ixm metal/few Ixm metal/few Ixm T i C / < ~,m metal/few Ixm T i C / < o,m metal/few Ixm metal/few Ixm
2060 2020 1970 2060 1870 1750 1990 2250 2530 2370 4890
2.0+0.9 9.4+ 1.6 20.1+3.6 3.0+0.5 7.5_+0.5 14 +0.5 2.0 + 0.5 8.5_+0.5 16 _+0.5 7.5_+0.5 10 b
a The number following the symbol of the dopant element represents the nominal bulk concentration in at%. b The nominal concentration for W was estimated from the measured density of the W-doped specimen, and the porosity of the reference specimen [2,3].
diffusivity as the "high direction" (hd) and the direction of low thermal diffusivity as the "low direction" (ld). From SEM photographs of the specimens, the graphite particulates are seen to be in the form of flakes of dimensions ~ 10 ~m; see Fig. la. A planar orientation of these flakes and layering are evident. In some of the specimens, most noticeably the Si-doped specimens, dopant particulates of ~ few I~m dimensions were observable as separate features, see Fig. lb. Experiments involving the combustion of Si-doped specimens in air at ~ 1000 K [2] revealed [using energy dispersive x-ray microanalysis (EDS)] that the graphite portion was burned off leaving only pure silicon particulates (Fig. lc). This macroscopic mixture of graphite and silicon particulates may be the reason for the peculiar heat capacity values for these specimens; see below. The specimens were cut (with a precision diamond grinding wheel) in the form of 10 mm diameter disks with thicknesses ranging from 3 to 5 mm. Thicknesses were chosen to provide acceptable temperature rise times for the analysis. Specimen densities were determined on the basis of the mass and physical dimensions. Generally, density measurements were within a few per cent of the manufacturer's quoted values. 2.2. Facility and procedure
with an organic binder. Dopants were added in particulates of pure metals or carbides. Specimen blocks were prepared with a final heating stage at 2270 K. A summary of the properties of the specimens studied is given in Table 1. Dopant concentrations were determined via ion beam analysis - nuclear reaction analysis (NRA) for the B-doped specimens, and Rutherford backscattering (RBS) for the remaining specimens [3]. The depth profiles showed gradually increasing dopant concentrations with increasing depth. With the exception of W, the profiles levelled off beyond a depth of ~ 0.5-1 txm. The concentrations corresponding to the level portions of the profiles, i.e., bulk concentrations, are used to identify the specimens; e.g., B2 and Til6 designate specimens with 2 at% B and 16 at% Ti concentrations, respectively. The W concentration, however, was still increasing as the range of the primary ion beam was reached ( ~ 2.5 Ixm). The nominal 10 at% value for W was based on the measured density of the W-doped specimen, and the porosity of the reference specimen. As the porosity of some of the doped specimens was greater than that of the reference [2,3], this may be a lower bound on the W concentration. Based on the thermal transport results presented here, the specimens were found to be highly anisotropic. Anisotropy was also observed in chemical activity [2] and radiation-enhanced sublimation [3]. In this paper, we refer to the direction of high thermal
Thermal diffusivity was measured using the laser flash technique. The technique involves the deposition of heat via a laser pulse on one face of a thin disk specimen and monitoring the temperature rise on the other face; thermal diffusivity is derived from the time dependence of the measured temperature rise. The apparatus consists of a pulsed laser energy source, a vacuum oven for heating the test specimens, and a data acquisition system; see Fig. 2. Further details are provided in Ref. [5]. A 1064 nm, 40 J neodymium-glass laser (Lumonics Inc. FQ class) with a ~ 0.5 ms pulse is used to impact on the front surface of a test specimen which is installed in the vacuum oven. The laser pulse is observed by two instruments along its trajectory: a cleaved fibre-optic pigtail at the rear cavity mirror transmits some of the laser light to a reverse-biased photon detection diode and an energy meter (Scientech 365) which samples some of the laser light deflected by a glass beam splitter. The diode signal is used to indicate the start of the laser pulse for data analysis and also allows the laser pulse shape to be digitized. The temperature rise on the back face of the specimen is measured by either infrared pyrometers ( ~ 450 to > 1000 K), or a chromel alumel thermocoupie ( ~ 300 to 750 K). An infrared filter is used to block any stray laser light from entering the pyrometers. For the heat capacity experiments, a 6 mm diameter aperture is used to restrict the diameter of the laser beam
B.N. Enweani et al. /Journal of Nuclear Materials 224 (1995) 245-253
~iii!ilij¸ /i~
Fig. 1. SEM photographs of characteristic specimen surfaces: (a) undoped CKC reference material, (b) 8 at% Si-doped CKC material, and (c) 8 at% Si-doped material which has had much of the graphite c~omponentremoved through oxidation [2]. In the first two photographs, the surface resulted from the cutting operation using a precision diamond grinding wheel. For all photographs, the black bar at the top of the number 10 in the legend represents a length of 10 Ixm.
247
on the front face of the specimen to slightly less than the specimen diameter ( ~ 10 mm). Thus the energy measured by the energy meter is always a known fraction of that striking the specimen. For the low direction diffusivity and heat capacity measurements, a 25 i~m chromel-alumel thermocouple was attached to the back of the specimen with a graphite-based epoxy. The mass of the epoxy was << 1% of the total specimen mass and thus was not expected to interfere with the measurements. The vacuum oven is housed in an ultrahigh vacuum (uhv) chamber, pumped by a turbomolecular pump. System pressure is typically in the 10 -5 Pa range with the specimen at room temperature, and below ~ 10 -3 Pa at a specimen temperature of ~ 1000 K. Operating in vacuum reduces convective heat losses to essentially zero, thereby improving the approximation of an adiabatic experimental state. The laser beam enters the vacuum system through a sapphire window. Test specimens are held by three electrically and thermally isolated tungsten pins. Specimen heating to temperatures of up to 750 K is achieved via direct radiation from hot tungsten filaments (thin strips in the form of rings) near the specimen. By biasing the specimen to > 1000 V, electrons emitted from the W filament may be used to further heat the specimen to > 1000 K. Electron bombardment is not possible when a thermocouple is attached to the specimen. Data from the temperature-measuring devices and the photon detection diode were collected with a PCbased data acquisition system, at a rate of 2048 Hz for 2 s, manually triggered just prior to firing the laser. The sampling rate could be increased or decreased depending on the time resolution desired. The specimen thicknesses were selected such that the response time of the data acquisition system and the measuring devices were much less than the rise time of the temperature on the back face of the specimen. Experiments were performed starting with the specimen at room temperature, and gradually increasing the oven temperature. After a change in oven temperature, the specimen was allowed to come to equilibrium for ~ 10 min before a measurement was made. The pyrometer measurements were corrected for reflected light originating from the W filaments by monitoring the specimen temperature during cooling (with the W filaments turned off) and extrapolating the cooling curve back to the time when the filaments were turned off. Temperature corrections were on the order of 20 K at low temperatures, decreasing to < 1 K at 1000 K. The heat capacities of some of the specimens were also measured using a Du-Pont 910 digital differential scanning calorimeter (DSC) in the Department of Chemical Engineering and Applied Chemistry at the University of Toronto. This device compares the speci-
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B.N. Enweani et al. /Journal of Nuclear Materials 224 (1995) 245-253
men temperature response to a constant heating or cooling rate, relative to that of an inert reference. The device was calibrated using a National Bureau of Standards sapphire reference; errors are expected to be less than 3%. The DSC data were used to calibrate the laser flash technique for heat capacity measurements via the determination of an "energy absorption efficiency" for a particular specimen; see below.
of the temperature transients may be used to determine the specimen cooling rate or the presence of undesired radial heat transport. The laser energy was chosen to produce a temperature rise on the order of 5 K to provide good signal resolution, while minimizing surface effects, such as ablation. The heat capacity (Cp) of a specimen may also be measured by the pulsed laser technique, according to r/Q
3. Derivation of thermal properties from experimental measurements
The thermal diffusivity ( a ) is obtained from the time required to reach 1 / 2 of the total temperature rise on the back face of the specimen due to a laser pulse impacting on the front face [6]: 1.38 L 2
ce
1T2/1/2
,
(1)
where L is the specimen thickness and tl/2 is the time to reach 1 / 2 of the total temperature rise. The effective specimen temperature for the thermal diffusivity measurement is [6] Zeff = ZinitiaI + 1.6AT,
Co = pLAT'
(3)
where r/ is the energy absorption efficiency, Q is the laser beam energy density, and p is the specimen density. The effective temperature used in the specific heat calculation is (4)
Zeff = ZinitiaI + 0 . 5 A T .
The total temperature rise, AT, is available as part of the diffusivity measurement as discussed above. The only unknown quantity remaining in Eq. (3) is 77, the fractional amount of incident laser energy absorbed by the specimen. In the present experiment, 7/ was obtained by comparing pulsed laser results with DSC results available for some of the specimens.
(2)
where AT is the total temperature rise. The method of Heckman [7] was used to make corrections for the finite laser pulse time effect. For the range of temperatures of the present experiments, corrections for radiation losses from the heated face of the specimen were not required [5]. The diffusivity measurement, therefore involves primarily the knowledge of the laser firing time and the time at which the back face temperature has reached 1 / 2 of its maximum value. The shape of the initial temperature rise, as well as the longer cooling curves
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The thermal diffusivity, heat capacity and thermal conductivity of the specimens studied are presented in Figs. 3-7. A comparison of the present diffusivity results for EK98 (Fig. 3a) which were obtained using a thermocouple, show very good agreement with results by Roth [9]. The measured values were typically ~ 10% lower in the present case, although there is some overlap in the scatter. Sample to sample variations are thought to account for the difference. The diffusivity
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4. Results and discussion
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Fig. 2. Schematic diagram of the experimental apparatus used for the laser flash method.
B.N. Enweani et al. /Journal of Nuclear Materials 224 (1995) 245-253 400
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ing. The nickel coating on the laser entry window led to a degradation of system performance. There was no evidence of vaporization for any of the other specimens. The heat capacity of the CKC reference (Fig. 3b), as measured by the DSC technique, is very similar to published values for graphite [8]. Comparison of the DSC and laser pulse (LP) measurements of Cp for the CKC reference and the CKC-Si8 specimens allowed the determination of 7/as required in Eq. (3) above. As
CKC Reference EK98 EK98 (Roth et el. [g]) Rt to all EK98 data
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curve for t h e C K C r e f e r e n c e h d s p e c i m e n is generally h i g h e r t h a n t h e EK98 curve, while t h e ld diffusivity curve is significantly lower. T h e a d d i t i o n of d o p a n t s results in a d e c r e a s e in diffusivity relative to t h e und o p e d C K C r e f e r e n c e in almost all cases, with t h e d e c r e a s e b e i n g larger for h i g h e r d o p a n t c o n c e n t r a tions. Results for t h e N i - d o p e d s p e c i m e n are limited to t h e single diffusivity r u n (Fig. 7a) d u e to t h e high r a t e of Ni e v a p o r a t i o n from t h e s p e c i m e n d u r i n g laser h e a t -
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Fig. 3. (a) Thermal diffusivity of the CKC reference specimen and EK98. Shown for comparison are published data for EK98 [9]. (b) Heat capacity for the CKC reference specimen as measured by the DSC technique, as well as published data for graphite [8]. (c) Thermal conductivity of the CKC reference specimen and EK9$ derived from the data in (a) and (b).
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250
B.N. Enweani et al. /Journal of Nuclear Materials 224 (1995) 245-253
seen in Fig. 8, the amount of energy absorbed (*7) is clearly independent of temperature, and is very similar for the two CKC specimens, 0.58 for the CKC reference and 0.55 for CKC-Si8. This indicates that the presence of the Si dopant has a negligible effect on *7. This observation is not unexpected since dopant concentrations on the outer surface of these CKC specimens were found to be considerably less than the corresponding bulk concentrations [3]. Therefore, for the derivation of the LP values of heat capacity (Figs. 5b, 6b, 7b), *7 was assumed to be the CKC reference
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value (*7 = 0.58) for all of the doped specimens. The derived values of .7 do not include losses occurring at the beam splitter or the vacuum window; these losses were determined empirically and were calculated separately. For EK98, 7/ was evaluated by comparing the measured LP values of Cp with published values [8]. The higher *7 for EK98 (0.69) is possibly due to its blacker appearance. For all LP measurements of C o, the laser energy was kept constant, at ~ 50 k J / m 2 ( ~ 4 J on an area of 9 - 1 0 mm diameter), although *7 was not found to be dependent on laser energy in this
251
B.N. Enweani et aL /Journal of Nuclear Materials 224 (1995) 245-253
energy range. The fact that the measured absorption efficiency is close to the expected emissivity for the specimens (based on carbon) is an indication that processes leading to lower energy absorption (such as surface melting and vapor shielding [10]) do not play a significant role in these experiments. With the exception of boron, the dopants lead to a reduction in heat capacity and the extent of the reduction increases with temperature and dopant concentration; the CKC-Si8 and -Sil4 cases do not follow the
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noted concentration trend. The measured heat capacities for the B-doped specimens are only marginally ( < 10%) lower than that for the CKC reference material; within the scatter of the data ( < 10%) the effect of dopant concentration is also negligible. For the Si-doped specimens, where the DSC technique was used, a strange drop in Cp occurred at 700-800 K; see Fig. 4b. This behaviour is contrary to that observed for graphite and both the et and 13 phases of silicon carbide [8]. The 3 at% Si specimen was run again, and produced the second trace shown in Fig. 4b. A general shift to higher C o values is observed, however, the drop at ~ 725 K remains. It is suspected that the behaviour is related to the large-scale grain structure observed on the specimens (see Section 2.1) or due to exothermic reactions occurring during DSC heating; more detailed explanation of this behaviour will require further work. Thermal conductivity values were calculated by fitting a second order polynomial to the heat capacity values and using this Cp function and the measured density and thermal diffusivity in the relation k = paCp,
(5)
where k is the thermal conductivity. As the heat capacity was not measured above ~ 750 K for most of the specimens, the polynomial fits were extrapolated to ~ 900 K as shown on the (b) parts of Figs. 5 to 7. We note that this assumes that the dip in the DSC Cp curves for Si-doped specimens (Fig. 4b) is an artifact of the technique and the shape of the Cp curve obtained for the reference specimen also characterizes the doped specimens. The trends observed in the thermal diffusivities are generally also observed in the thermal conductivity, as the product pCp is similar for most specimens. The
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B.N. Enweani et al. /Journal of Nuclear Materials 224 (1995) 245-253
thermal conductivity for EK98 is nearly independent of temperature; ~ 90 W / m K over the temperature range 300-500 K, gradually decreasing to ~ 70 W / m K at 900 K. In the high conductivity direction, the CKC undoped specimen has a higher thermal conductivity than EK98 for most of the temperature range studied. The CKC reference values are about a factor of 1.5 higher at 500 K ( ~ 140 W / m K ) and the ratio gradually falls to unity as the temperature increases to ~ 900 K (70 W / m K ) . The low direction thermal conductivity curve for the CKC reference specimen is significantly lower than the hd curve, and it is nearly temperature independent ( ~ 30 W / m K at 300 K, gradually falling to ~ 20 W / m K at 500 K, then remaining at this value to 900 K). It is apparent from the high direction results for all doped specimens that the dopants have the effect of reducing the hd thermal conductivity. The larger the dopant concentration, the lower the thermal conductivity. In general, specimens with 2-3 at% dopant concentrations have hd thermal conductivities between that of the undoped CKC reference and EK98. Thermal conductivities for specimens with dopant concentrations > 7 at% are generally lower than the EK98 values. In the low direction, the thermal conductivity is nearly independent of temperature and dopant type and concentration. Generally, the ld conductivity is similar to that obtained for the undoped reference specimen. Exceptions are the CKC-Ti8 and -B2 specimens with ld conductivities about 50% higher than the CKC reference case. In Fig. 9, a compilation of thermal conductivity results at 700 K are shown as a function of dopant type
and concentration. N o trend as a function of Z is observed. Also shown for comparison are results from [1,11-13]. The B-doped materials from Ref. [1] have thermal conductivities which are intermediate between the low and high direction specimens of the present study, while the Ti-doped specimens, which are similar in nature to pyrolytic graphite [11], have thermal conductivities much higher than the ones in the present study, but lower than pyrolytic graphite [1].
5. Conclusions
Thermal diffusivity/conductivity data for the CKC specimens show that these materials are highly anisotropic. For an undoped reference specimen the hd conductivity over the 500-900 K temperature range decreases with increasing temperature from ~ 140 W / m K to ~ 70 W / m K . The ld conductivity is nearly temperature independent over this temperature range with a value of ~ 20 W / m K . For comparison, for the isotropic fine grain EK98, the thermal conductivity changes only slightly with temperature; as the temperature increases from 500-900 K, the conductivity decreases from ~ 90 to ~ 70 W / m K . The effect of dopant species and concentration on the ld conductivity is negligible. In the high conductivity direction, however, the conductivity generally decreases with increasing dopant concentration - more so at low temperatures, resulting in a reduced temperature dependence. No clear trend is observed for the conductivity as a function of dopant mass.
Acknowledgements Present Results
o r~ >' >
2-3 aL% dopant 7-1 0 at.% dopant 14-20 at.% dopant
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$2508 [1 ] GB120y [1 ] CFC 2.5 wL% SIC (//) (1 2] CFC 8 wL% SIC (//) [1 2] B4O [121, SIC [1,1 3], TIC (RT) [12], WC (RT) [1 s] RG-TI-91 [11 ] RG-TI-90 [11 ]
This work was supported by the Natural Sciences and Engineering Research Council of Canada, the Ontario Centre for Materials research and the Canadian Fusion Fuels Technology Project. Test specimens were provided by Ceramics Kingston Ceramique Inc., Kingston, Ontario. The DSC measurements were performed with the help of Dr. J. W. Graydon in the Department of Chemical Engineering and Applied Chemistry at the University of Toronto. We gratefully acknowledge the contribution of Mr. Charles Perez to the preparation of the experimental facility and the cutting of the specimens.
Dopant
Fig. 9. Thermal conductivities at 700 K for the CKC specimens compared with those from Refs. [1,11,12]. The RG-Ti specimens have 6 wt% Ti dopant [11]; GB120y (Toyo Tanso, Japan) has 22 wt% B dopant, and the B concentration of the $2508 material (Carbonne Lorraine, France) is 3.5%. Carbide thermal conductivities are also shown [1,13], however, we note that the TiC and WC values are for room temperature.
References
[1] R. Behrisch and G. Venus, J. Nucl. Mater. 202 (1994) 1. [2] A.Y.K. Chen, A.A. Haasz and J.W. Davis, Hydrocarbon formation during H + irradiation of doped graphites, to be published.
B.N. Enweani et al. /Journal of Nuclear Materials 224 (1995) 245-253 [3] P. Franzen, A.A. Haasz and J.W. Davis, Radiation-enhanced sublimation of doped graphites, to be published. [4] A.A. Haasz and J.W. Davis, Deuterium retention in and reemission from doped graphites, J. Nucl. Mater., in press. [5] B.N. Enweani, Thermal Properties of Advanced Doped Carbon Materials Determined on a New UTIAS Laser Flash Facility, University of Toronto Institute for Aerospace Studies, M. A. Sc. Thesis (1994). [6] W.J. Parker, R.J. Jenkens, C.P. Buttler and G.L. Abbott, J. Appl. Phys. 32 (1961) 1679. [7] R.C. Heckman, J. Appl. Phys. 44 (1973) 1455. [8] JANAF Thermochemical Tables, 2nd ed. (1971). [9] E.P. Roth, R.D. Watson, M. Moss, and W.D. Drotning, Thermophysical Properties of Advanced Carbon Materi-
[10] [11]
[12] [13]
253
als for Tokamak Limiters, Sandia National Laboratories Report No. SAND88-2057 (1989). J.F. Ready, Effects of High Power Laser Radiation (Academic Press, New York, 1971). T.A. Burtseva, O.K. Chungunov, E.F. Dovguchits, V.L. Komarov, I.V. Mazul, A.A. Mitrofansky, M.I. Persin, Yu. G. Prokofiev, V.A. Sokolov, E.I. Trofimchuk and L.P. Zav'jalsky, J. Nucl. Mater. 191-194 (1992) 309. P. Magaud, Fusion Technology, Annual Report of the Association CEA/Euratom (1993). CRC Handbook of Chemistry and Physics, ed. R.C. Weast, 69th ed. (1988), and CRC Handbook of Tables for Applied Engineering Science, eds. R.E. Bolz and G.L. Tuve, 2nd ed. (1973).
Journal of Nuclear Materials 224 (1995) 245-253
jouroalof smear materials
Thermal diffusivity/conductivity of doped graphites B.N. Enweani, J.W. Davis, A.A. Haasz Fusion Research Group, University of Toronto Institute for Aerospace Studies, 4925 Dufferin Street, North York, Ontario, Canada M3H 5T6
Received 26 January 1995; accepted 5 April 1995
Abstract The thermal properties of doped graphite materials were measured using a pulsed laser diffusivity/calorimetry technique. Dopants included: boron, silicon, titanium, nickel and tungsten. Measurements were made in the 300-1000 K temperature range. Differential scanning calorimetry (DSC) was also employed for some specimens to calibrate the pulsed laser method of heat capacity measurement. The specimens were found to be highly anisotropic visa vis thermal diffusivity/conductivity. For the high thermal conductivity direction, all specimens showed a general trend to lower conductivities with increasing dopant concentration - more so at lower temperature, resulting in a reduced temperature dependence. Typically, the high direction thermal conductivities of specimens with dopant concentrations > 7 at% were about a factor of two lower than the values obtained for an undoped reference specimen. For the low conductivity direction, the diffusivity/conductivity was relatively independent of temperature and dopant species and concentration.
1. Introduction Carbon-based materials are important candidates for plasma-facing components in magnetic fusion reactors. In addition to heing a Iow-Z material, graphite's other positive characteristics include excellent resistance to transient thermal shock, high operating temperatures, low nuclear activation and high thermal conductivity. Graphite's major drawback, however is its erosion behaviour due to plasma exposure. Attempts are being made to improve graphite's erosion resistance via the addition of non-carbon elements into the carbon matrix. Unfortunately, the addition of "dopants" might have an effect on the materials's thermal conductivity. Behrisch and Venus [1] have compiled results on the thermal conductivity of various materials relevant to fusion and evaluated their ability to remove heat. Few data are available for doped graphites, but the data that are available generally indicate a reduction in thermal conductivity due to the addition of dopants, or the change in structure associated with the addition of dopants. The objective of the present experiment was to study the effect of a wide range of low-Z to high-Z
dopant elements, in different concentrations, on the thermal diffusivity and conductivity of high density graphite. Test specimens doped with B, Si, Ti, Ni and W were fabricated by Ceramics Kingston Ceramique Inc. (CKC). The specific properties investigated were the temperature dependence of thermal diffusivity and heat capacity; thermal conductivity was then calculated. Comparisons are made with EK98 (isotropic fine grain graphite manufactured by Ringsdorf, Germany) and a CKC reference graphite specimen manufactured by the same method as the doped specimens, but containing no dopants. This work was part of a larger study which also examined the effect of dopants on chemical reactivity [2], radiation-enhanced sublimation [3] and hydrogen retention and recycling [4].
2. Experiment 2.1. Specimens
The doped specimens were manufactured by CKC from finely ground pyrolytic graphite (10-45 txm) mixed
0022-3115/95/$09.50 ~ 1995 Elsevier Science B.V. All rights reserved SSDI 0022-3115(95)00068-2
246
B.N. Enweani et al. /Journal of Nuclear Materials 224 (1995) 245-253
Table 1 Properties of the CKC doped graphite specimens Specimen designation a
Ringsdorf EK98 no dopants CKC reference no dopants CKC-B2 -B10 -B20 CKC-Si3 -Si8 -Sil4 CKC-Ti2 -Ti8 -Til6 CKC-Ni8 CKC-W10
Dopant Form/size of concentration dopants in bulk (at%) [3]
Density (kg/m 3)
-
-
1860
-
-
2060
metal/few I~m metal/few Ixm metal/few Ixm metal/few Ixm metal/few Ixm metal/few Ixm T i C / < ~,m metal/few Ixm T i C / < o,m metal/few Ixm metal/few Ixm
2060 2020 1970 2060 1870 1750 1990 2250 2530 2370 4890
2.0+0.9 9.4+ 1.6 20.1+3.6 3.0+0.5 7.5_+0.5 14 +0.5 2.0 + 0.5 8.5_+0.5 16 _+0.5 7.5_+0.5 10 b
a The number following the symbol of the dopant element represents the nominal bulk concentration in at%. b The nominal concentration for W was estimated from the measured density of the W-doped specimen, and the porosity of the reference specimen [2,3].
diffusivity as the "high direction" (hd) and the direction of low thermal diffusivity as the "low direction" (ld). From SEM photographs of the specimens, the graphite particulates are seen to be in the form of flakes of dimensions ~ 10 ~m; see Fig. la. A planar orientation of these flakes and layering are evident. In some of the specimens, most noticeably the Si-doped specimens, dopant particulates of ~ few I~m dimensions were observable as separate features, see Fig. lb. Experiments involving the combustion of Si-doped specimens in air at ~ 1000 K [2] revealed [using energy dispersive x-ray microanalysis (EDS)] that the graphite portion was burned off leaving only pure silicon particulates (Fig. lc). This macroscopic mixture of graphite and silicon particulates may be the reason for the peculiar heat capacity values for these specimens; see below. The specimens were cut (with a precision diamond grinding wheel) in the form of 10 mm diameter disks with thicknesses ranging from 3 to 5 mm. Thicknesses were chosen to provide acceptable temperature rise times for the analysis. Specimen densities were determined on the basis of the mass and physical dimensions. Generally, density measurements were within a few per cent of the manufacturer's quoted values. 2.2. Facility and procedure
with an organic binder. Dopants were added in particulates of pure metals or carbides. Specimen blocks were prepared with a final heating stage at 2270 K. A summary of the properties of the specimens studied is given in Table 1. Dopant concentrations were determined via ion beam analysis - nuclear reaction analysis (NRA) for the B-doped specimens, and Rutherford backscattering (RBS) for the remaining specimens [3]. The depth profiles showed gradually increasing dopant concentrations with increasing depth. With the exception of W, the profiles levelled off beyond a depth of ~ 0.5-1 txm. The concentrations corresponding to the level portions of the profiles, i.e., bulk concentrations, are used to identify the specimens; e.g., B2 and Til6 designate specimens with 2 at% B and 16 at% Ti concentrations, respectively. The W concentration, however, was still increasing as the range of the primary ion beam was reached ( ~ 2.5 Ixm). The nominal 10 at% value for W was based on the measured density of the W-doped specimen, and the porosity of the reference specimen. As the porosity of some of the doped specimens was greater than that of the reference [2,3], this may be a lower bound on the W concentration. Based on the thermal transport results presented here, the specimens were found to be highly anisotropic. Anisotropy was also observed in chemical activity [2] and radiation-enhanced sublimation [3]. In this paper, we refer to the direction of high thermal
Thermal diffusivity was measured using the laser flash technique. The technique involves the deposition of heat via a laser pulse on one face of a thin disk specimen and monitoring the temperature rise on the other face; thermal diffusivity is derived from the time dependence of the measured temperature rise. The apparatus consists of a pulsed laser energy source, a vacuum oven for heating the test specimens, and a data acquisition system; see Fig. 2. Further details are provided in Ref. [5]. A 1064 nm, 40 J neodymium-glass laser (Lumonics Inc. FQ class) with a ~ 0.5 ms pulse is used to impact on the front surface of a test specimen which is installed in the vacuum oven. The laser pulse is observed by two instruments along its trajectory: a cleaved fibre-optic pigtail at the rear cavity mirror transmits some of the laser light to a reverse-biased photon detection diode and an energy meter (Scientech 365) which samples some of the laser light deflected by a glass beam splitter. The diode signal is used to indicate the start of the laser pulse for data analysis and also allows the laser pulse shape to be digitized. The temperature rise on the back face of the specimen is measured by either infrared pyrometers ( ~ 450 to > 1000 K), or a chromel alumel thermocoupie ( ~ 300 to 750 K). An infrared filter is used to block any stray laser light from entering the pyrometers. For the heat capacity experiments, a 6 mm diameter aperture is used to restrict the diameter of the laser beam
B.N. Enweani et al. /Journal of Nuclear Materials 224 (1995) 245-253
~iii!ilij¸ /i~
Fig. 1. SEM photographs of characteristic specimen surfaces: (a) undoped CKC reference material, (b) 8 at% Si-doped CKC material, and (c) 8 at% Si-doped material which has had much of the graphite c~omponentremoved through oxidation [2]. In the first two photographs, the surface resulted from the cutting operation using a precision diamond grinding wheel. For all photographs, the black bar at the top of the number 10 in the legend represents a length of 10 Ixm.
247
on the front face of the specimen to slightly less than the specimen diameter ( ~ 10 mm). Thus the energy measured by the energy meter is always a known fraction of that striking the specimen. For the low direction diffusivity and heat capacity measurements, a 25 i~m chromel-alumel thermocouple was attached to the back of the specimen with a graphite-based epoxy. The mass of the epoxy was << 1% of the total specimen mass and thus was not expected to interfere with the measurements. The vacuum oven is housed in an ultrahigh vacuum (uhv) chamber, pumped by a turbomolecular pump. System pressure is typically in the 10 -5 Pa range with the specimen at room temperature, and below ~ 10 -3 Pa at a specimen temperature of ~ 1000 K. Operating in vacuum reduces convective heat losses to essentially zero, thereby improving the approximation of an adiabatic experimental state. The laser beam enters the vacuum system through a sapphire window. Test specimens are held by three electrically and thermally isolated tungsten pins. Specimen heating to temperatures of up to 750 K is achieved via direct radiation from hot tungsten filaments (thin strips in the form of rings) near the specimen. By biasing the specimen to > 1000 V, electrons emitted from the W filament may be used to further heat the specimen to > 1000 K. Electron bombardment is not possible when a thermocouple is attached to the specimen. Data from the temperature-measuring devices and the photon detection diode were collected with a PCbased data acquisition system, at a rate of 2048 Hz for 2 s, manually triggered just prior to firing the laser. The sampling rate could be increased or decreased depending on the time resolution desired. The specimen thicknesses were selected such that the response time of the data acquisition system and the measuring devices were much less than the rise time of the temperature on the back face of the specimen. Experiments were performed starting with the specimen at room temperature, and gradually increasing the oven temperature. After a change in oven temperature, the specimen was allowed to come to equilibrium for ~ 10 min before a measurement was made. The pyrometer measurements were corrected for reflected light originating from the W filaments by monitoring the specimen temperature during cooling (with the W filaments turned off) and extrapolating the cooling curve back to the time when the filaments were turned off. Temperature corrections were on the order of 20 K at low temperatures, decreasing to < 1 K at 1000 K. The heat capacities of some of the specimens were also measured using a Du-Pont 910 digital differential scanning calorimeter (DSC) in the Department of Chemical Engineering and Applied Chemistry at the University of Toronto. This device compares the speci-
248
B.N. Enweani et al. /Journal of Nuclear Materials 224 (1995) 245-253
men temperature response to a constant heating or cooling rate, relative to that of an inert reference. The device was calibrated using a National Bureau of Standards sapphire reference; errors are expected to be less than 3%. The DSC data were used to calibrate the laser flash technique for heat capacity measurements via the determination of an "energy absorption efficiency" for a particular specimen; see below.
of the temperature transients may be used to determine the specimen cooling rate or the presence of undesired radial heat transport. The laser energy was chosen to produce a temperature rise on the order of 5 K to provide good signal resolution, while minimizing surface effects, such as ablation. The heat capacity (Cp) of a specimen may also be measured by the pulsed laser technique, according to r/Q
3. Derivation of thermal properties from experimental measurements
The thermal diffusivity ( a ) is obtained from the time required to reach 1 / 2 of the total temperature rise on the back face of the specimen due to a laser pulse impacting on the front face [6]: 1.38 L 2
ce
1T2/1/2
,
(1)
where L is the specimen thickness and tl/2 is the time to reach 1 / 2 of the total temperature rise. The effective specimen temperature for the thermal diffusivity measurement is [6] Zeff = ZinitiaI + 1.6AT,
Co = pLAT'
(3)
where r/ is the energy absorption efficiency, Q is the laser beam energy density, and p is the specimen density. The effective temperature used in the specific heat calculation is (4)
Zeff = ZinitiaI + 0 . 5 A T .
The total temperature rise, AT, is available as part of the diffusivity measurement as discussed above. The only unknown quantity remaining in Eq. (3) is 77, the fractional amount of incident laser energy absorbed by the specimen. In the present experiment, 7/ was obtained by comparing pulsed laser results with DSC results available for some of the specimens.
(2)
where AT is the total temperature rise. The method of Heckman [7] was used to make corrections for the finite laser pulse time effect. For the range of temperatures of the present experiments, corrections for radiation losses from the heated face of the specimen were not required [5]. The diffusivity measurement, therefore involves primarily the knowledge of the laser firing time and the time at which the back face temperature has reached 1 / 2 of its maximum value. The shape of the initial temperature rise, as well as the longer cooling curves
Ore n \ Vacuum \ chamber Sample \ \ ~ X Pyrometer ~ , ~
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The thermal diffusivity, heat capacity and thermal conductivity of the specimens studied are presented in Figs. 3-7. A comparison of the present diffusivity results for EK98 (Fig. 3a) which were obtained using a thermocouple, show very good agreement with results by Roth [9]. The measured values were typically ~ 10% lower in the present case, although there is some overlap in the scatter. Sample to sample variations are thought to account for the difference. The diffusivity
6mm diameter B e a m aperture splitter t k
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4. Results and discussion
40J 0.5 ms Nd-glasslaser
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Fig. 2. Schematic diagram of the experimental apparatus used for the laser flash method.
B.N. Enweani et al. /Journal of Nuclear Materials 224 (1995) 245-253 400
200
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ing. The nickel coating on the laser entry window led to a degradation of system performance. There was no evidence of vaporization for any of the other specimens. The heat capacity of the CKC reference (Fig. 3b), as measured by the DSC technique, is very similar to published values for graphite [8]. Comparison of the DSC and laser pulse (LP) measurements of Cp for the CKC reference and the CKC-Si8 specimens allowed the determination of 7/as required in Eq. (3) above. As
CKC Reference EK98 EK98 (Roth et el. [g]) Rt to all EK98 data
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curve for t h e C K C r e f e r e n c e h d s p e c i m e n is generally h i g h e r t h a n t h e EK98 curve, while t h e ld diffusivity curve is significantly lower. T h e a d d i t i o n of d o p a n t s results in a d e c r e a s e in diffusivity relative to t h e und o p e d C K C r e f e r e n c e in almost all cases, with t h e d e c r e a s e b e i n g larger for h i g h e r d o p a n t c o n c e n t r a tions. Results for t h e N i - d o p e d s p e c i m e n are limited to t h e single diffusivity r u n (Fig. 7a) d u e to t h e high r a t e of Ni e v a p o r a t i o n from t h e s p e c i m e n d u r i n g laser h e a t -
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0.6
T e m p e r a t u r e / (K)
Fig. 3. (a) Thermal diffusivity of the CKC reference specimen and EK98. Shown for comparison are published data for EK98 [9]. (b) Heat capacity for the CKC reference specimen as measured by the DSC technique, as well as published data for graphite [8]. (c) Thermal conductivity of the CKC reference specimen and EK9$ derived from the data in (a) and (b).
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Fig. 4. Si-doped CKC specimens: (a) thermal diffusivity, (b) DSC measurements of heat capacity, and (c) thermal conductivity derived from data in (a) and (b). Curves from Fig. 3 for the CKC reference and EK98 are shown for comparison.
250
B.N. Enweani et al. /Journal of Nuclear Materials 224 (1995) 245-253
seen in Fig. 8, the amount of energy absorbed (*7) is clearly independent of temperature, and is very similar for the two CKC specimens, 0.58 for the CKC reference and 0.55 for CKC-Si8. This indicates that the presence of the Si dopant has a negligible effect on *7. This observation is not unexpected since dopant concentrations on the outer surface of these CKC specimens were found to be considerably less than the corresponding bulk concentrations [3]. Therefore, for the derivation of the LP values of heat capacity (Figs. 5b, 6b, 7b), *7 was assumed to be the CKC reference
200 ~E
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Fig. 6. Ti-doped CKC specimens: (a) thermal diffusivity, (b) heat capacity measured by laser pulse method, and (c) thermal conductivity derived from data in (a) and (b). Curves from Fig. 3 for the CKC reference and EK98 are also shown.
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Fig. 5. B-doped CKC specimens: (a) thermal diffusivity, (b) heat capacity measured by laser pulse method, and (c) thermal conductivity derived from data in (a) and (b). Curves from Fig. 3 for the CKC reference and EK98 are also shown.
value (*7 = 0.58) for all of the doped specimens. The derived values of .7 do not include losses occurring at the beam splitter or the vacuum window; these losses were determined empirically and were calculated separately. For EK98, 7/ was evaluated by comparing the measured LP values of Cp with published values [8]. The higher *7 for EK98 (0.69) is possibly due to its blacker appearance. For all LP measurements of C o, the laser energy was kept constant, at ~ 50 k J / m 2 ( ~ 4 J on an area of 9 - 1 0 mm diameter), although *7 was not found to be dependent on laser energy in this
251
B.N. Enweani et aL /Journal of Nuclear Materials 224 (1995) 245-253
energy range. The fact that the measured absorption efficiency is close to the expected emissivity for the specimens (based on carbon) is an indication that processes leading to lower energy absorption (such as surface melting and vapor shielding [10]) do not play a significant role in these experiments. With the exception of boron, the dopants lead to a reduction in heat capacity and the extent of the reduction increases with temperature and dopant concentration; the CKC-Si8 and -Sil4 cases do not follow the
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noted concentration trend. The measured heat capacities for the B-doped specimens are only marginally ( < 10%) lower than that for the CKC reference material; within the scatter of the data ( < 10%) the effect of dopant concentration is also negligible. For the Si-doped specimens, where the DSC technique was used, a strange drop in Cp occurred at 700-800 K; see Fig. 4b. This behaviour is contrary to that observed for graphite and both the et and 13 phases of silicon carbide [8]. The 3 at% Si specimen was run again, and produced the second trace shown in Fig. 4b. A general shift to higher C o values is observed, however, the drop at ~ 725 K remains. It is suspected that the behaviour is related to the large-scale grain structure observed on the specimens (see Section 2.1) or due to exothermic reactions occurring during DSC heating; more detailed explanation of this behaviour will require further work. Thermal conductivity values were calculated by fitting a second order polynomial to the heat capacity values and using this Cp function and the measured density and thermal diffusivity in the relation k = paCp,
(5)
where k is the thermal conductivity. As the heat capacity was not measured above ~ 750 K for most of the specimens, the polynomial fits were extrapolated to ~ 900 K as shown on the (b) parts of Figs. 5 to 7. We note that this assumes that the dip in the DSC Cp curves for Si-doped specimens (Fig. 4b) is an artifact of the technique and the shape of the Cp curve obtained for the reference specimen also characterizes the doped specimens. The trends observed in the thermal diffusivities are generally also observed in the thermal conductivity, as the product pCp is similar for most specimens. The
252
B.N. Enweani et al. /Journal of Nuclear Materials 224 (1995) 245-253
thermal conductivity for EK98 is nearly independent of temperature; ~ 90 W / m K over the temperature range 300-500 K, gradually decreasing to ~ 70 W / m K at 900 K. In the high conductivity direction, the CKC undoped specimen has a higher thermal conductivity than EK98 for most of the temperature range studied. The CKC reference values are about a factor of 1.5 higher at 500 K ( ~ 140 W / m K ) and the ratio gradually falls to unity as the temperature increases to ~ 900 K (70 W / m K ) . The low direction thermal conductivity curve for the CKC reference specimen is significantly lower than the hd curve, and it is nearly temperature independent ( ~ 30 W / m K at 300 K, gradually falling to ~ 20 W / m K at 500 K, then remaining at this value to 900 K). It is apparent from the high direction results for all doped specimens that the dopants have the effect of reducing the hd thermal conductivity. The larger the dopant concentration, the lower the thermal conductivity. In general, specimens with 2-3 at% dopant concentrations have hd thermal conductivities between that of the undoped CKC reference and EK98. Thermal conductivities for specimens with dopant concentrations > 7 at% are generally lower than the EK98 values. In the low direction, the thermal conductivity is nearly independent of temperature and dopant type and concentration. Generally, the ld conductivity is similar to that obtained for the undoped reference specimen. Exceptions are the CKC-Ti8 and -B2 specimens with ld conductivities about 50% higher than the CKC reference case. In Fig. 9, a compilation of thermal conductivity results at 700 K are shown as a function of dopant type
and concentration. N o trend as a function of Z is observed. Also shown for comparison are results from [1,11-13]. The B-doped materials from Ref. [1] have thermal conductivities which are intermediate between the low and high direction specimens of the present study, while the Ti-doped specimens, which are similar in nature to pyrolytic graphite [11], have thermal conductivities much higher than the ones in the present study, but lower than pyrolytic graphite [1].
5. Conclusions
Thermal diffusivity/conductivity data for the CKC specimens show that these materials are highly anisotropic. For an undoped reference specimen the hd conductivity over the 500-900 K temperature range decreases with increasing temperature from ~ 140 W / m K to ~ 70 W / m K . The ld conductivity is nearly temperature independent over this temperature range with a value of ~ 20 W / m K . For comparison, for the isotropic fine grain EK98, the thermal conductivity changes only slightly with temperature; as the temperature increases from 500-900 K, the conductivity decreases from ~ 90 to ~ 70 W / m K . The effect of dopant species and concentration on the ld conductivity is negligible. In the high conductivity direction, however, the conductivity generally decreases with increasing dopant concentration - more so at low temperatures, resulting in a reduced temperature dependence. No clear trend is observed for the conductivity as a function of dopant mass.
Acknowledgements Present Results
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$2508 [1 ] GB120y [1 ] CFC 2.5 wL% SIC (//) (1 2] CFC 8 wL% SIC (//) [1 2] B4O [121, SIC [1,1 3], TIC (RT) [12], WC (RT) [1 s] RG-TI-91 [11 ] RG-TI-90 [11 ]
This work was supported by the Natural Sciences and Engineering Research Council of Canada, the Ontario Centre for Materials research and the Canadian Fusion Fuels Technology Project. Test specimens were provided by Ceramics Kingston Ceramique Inc., Kingston, Ontario. The DSC measurements were performed with the help of Dr. J. W. Graydon in the Department of Chemical Engineering and Applied Chemistry at the University of Toronto. We gratefully acknowledge the contribution of Mr. Charles Perez to the preparation of the experimental facility and the cutting of the specimens.
Dopant
Fig. 9. Thermal conductivities at 700 K for the CKC specimens compared with those from Refs. [1,11,12]. The RG-Ti specimens have 6 wt% Ti dopant [11]; GB120y (Toyo Tanso, Japan) has 22 wt% B dopant, and the B concentration of the $2508 material (Carbonne Lorraine, France) is 3.5%. Carbide thermal conductivities are also shown [1,13], however, we note that the TiC and WC values are for room temperature.
References
[1] R. Behrisch and G. Venus, J. Nucl. Mater. 202 (1994) 1. [2] A.Y.K. Chen, A.A. Haasz and J.W. Davis, Hydrocarbon formation during H + irradiation of doped graphites, to be published.
B.N. Enweani et al. /Journal of Nuclear Materials 224 (1995) 245-253 [3] P. Franzen, A.A. Haasz and J.W. Davis, Radiation-enhanced sublimation of doped graphites, to be published. [4] A.A. Haasz and J.W. Davis, Deuterium retention in and reemission from doped graphites, J. Nucl. Mater., in press. [5] B.N. Enweani, Thermal Properties of Advanced Doped Carbon Materials Determined on a New UTIAS Laser Flash Facility, University of Toronto Institute for Aerospace Studies, M. A. Sc. Thesis (1994). [6] W.J. Parker, R.J. Jenkens, C.P. Buttler and G.L. Abbott, J. Appl. Phys. 32 (1961) 1679. [7] R.C. Heckman, J. Appl. Phys. 44 (1973) 1455. [8] JANAF Thermochemical Tables, 2nd ed. (1971). [9] E.P. Roth, R.D. Watson, M. Moss, and W.D. Drotning, Thermophysical Properties of Advanced Carbon Materi-
[10] [11]
[12] [13]
253
als for Tokamak Limiters, Sandia National Laboratories Report No. SAND88-2057 (1989). J.F. Ready, Effects of High Power Laser Radiation (Academic Press, New York, 1971). T.A. Burtseva, O.K. Chungunov, E.F. Dovguchits, V.L. Komarov, I.V. Mazul, A.A. Mitrofansky, M.I. Persin, Yu. G. Prokofiev, V.A. Sokolov, E.I. Trofimchuk and L.P. Zav'jalsky, J. Nucl. Mater. 191-194 (1992) 309. P. Magaud, Fusion Technology, Annual Report of the Association CEA/Euratom (1993). CRC Handbook of Chemistry and Physics, ed. R.C. Weast, 69th ed. (1988), and CRC Handbook of Tables for Applied Engineering Science, eds. R.E. Bolz and G.L. Tuve, 2nd ed. (1973).