Tritium migration in vapor-deposited β-silicon carbide

Journal of Nuclear Materials 203 (1993) 196-205 North-Holland

Tritium migration in vapor-deposited

f%silicon carbide *

R.A. Causey Sundia National ~~ratorigs,

Livermore, CA 94550, USA

W.R. Wampler Sandia National Laboratories, Albuquerque, NM 87185, USA

J.R. Retelle US Deparfment

of Energy, Albuquerque, NM 871&+%54iW,USA

J.L. Kaae General Atomics, San Diego, CA 92121, USA

Received 2 March 1993; accepted 30 April 1993

Tritium diffusivities and solubilities have been measured for vapor-deposited p-silicon carbide. The solubility measurements were performed over the temperature range of 1000 Lo 1600°C and the pressure range 0.01 to 1.0 atm. Diffusivities were determined for the temperature range of 1100 to 1500°C. The magnitude of the diffusivi~ was much lower than that for metals and the activation energy was much higher. The measured ~Iubili~ had a negative heat of solution and, when corrected for surface absorption, varied linearly with the square-root of pressure. The low diffusivity along with the apparent negative heat of solution are indicative of trap-controlled migration of tritium in the silicon carbide.

1. Introduction The New Production Modular High-Temperature Gas-Cooled Reactor (NP-MHTGR) will use multilayered microspheres with lithium aluminate cores for the production and containment of tritium. The lithium aluminate cores, enriched in the 6Li isotope, will be surrounded by layers of low and high density pyrolytic carbon and a layer of silicon carbide. For successful operation of this reactor, it is necessary that the tritium be retained in the particles during normal operation of the reactor. It is also necessary that the behavior of the tritium in these particles during off-normal operation be understood so that an accurate safety assessment can be performed. To understand both the normal and

* Work supported by US Department duction Reactor (DOE-NP). ~22-3115/93/$~.~

of Energy New Pro-

off-normal behavior of tritium in this reactor, it is required that the tritium migration parameters for both silicon carbide and pyrolytic carbon be known. This report describes the work performed to determine those parameters for silicon carbide. All experiments were performed using samples produced in a laboratory-sized coater. The small silicon carbide samples were exposed to tritium and deuterium gas at pressures varying between 0.01 and 1.0 atm at temperatures from 1000 to 1600°C for times from 0.5 to 8 h. In experiments performed at Sandia National Laboratories in Albuquerque, the deuterium concentrations in silicon carbide samples subsequent to the gas exposures were determined directly using a nuclear reaction profiling technique. In the experiments performed at Sandia National Laboratories in Livermore, the tritium was driven back out of the samples after exposure by heating to very high temperatures. The amount of desorbed tritium was deter-

0 1993 - Eisevier Science Publishers B.V. All rights

reserved

RA. Causey et al. / Tiitium migration in @silicon carbide

mined by flowing the released gas through an ionization chamber. The diffusivity was determined by measuring the rate of release of the hydrogen isotopes from the silicon carbide samples at different temperatures.

2. Background While the data base on the migration of hydrogen isotopes in metals is rather extensive, that existing for nonmetaIs such as silicon carbide is very limited. One of the most extensive studies on the behavior of hydrogen isotopes in silicon carbide was performed by Causey et al. [l] in 1978. In this study, recoil injection of tritium into silicon carbide from the ‘jLi(n, u)~H reaction was used to load the samples with tritium. The tritium was driven back out during isothermal anneals to determine the diffusivity as a function of temperature. While the measured diffusivities differed significantly for the several types of silicon carbide tested, all were significantly lower than those reported for metals. Also, the activation energies were significantly higher than those for metals, suggesting the possibility of chemical bonding of the tritium to the silicon or carbon atoms. However, small amounts of impurities were seen to largely affect the diffusivity. Deuterium solubility measurements were performed on two different materials, vapor-deposited p-silicon carbide and (Ysilicon carbide powder. In the o&con carbide measurements, the powder was exposed to deuterium gas for times sufficient to allow equilibrium saturation. Equilibration was not possible for the vapor deposited p-silicon carbide as thicker samples were used. It was necessary in those experiments to use the measured diffusion coefficient to back out an apparent solubility. The differences between the solubilities determined for the two materials were not large, and both were seen to fall with increasing temperature (to have a negative heat of solution). Verghese et al. [2] directly measured the permeation rate for hydrogen through KT silicon carbide. KT silicon carbide is a sintered material. Their measured permeation rate was about two orders of magnitude higher than would be caleulated using the product of the diffusivity times the solubility when Causey et al. [l] values are used. Verghese et al. [2] also measured the tritium diffusion coefficient for his KT silicon carbide and found it to be about two orders of magnitude higher than that reported for vapor deposited p-silicon carbide. This higher diffusivity was thought to be due to excessive silicon in the material. Similarly high permeation rates

I97

for hydrogen in CVD silicon carbide were reported by Sinharoy and Lange [3]. In their study, the silicon carbide was deposited on a tungsten tube. Tritium release characteristics of TRISO-coated lithium target particles (microspheres of lithium aluminate surrounded by layers of pyrolytic carbon and silicon carbide) were measured in an experimental program conducted at General Atomics [4]. In the analysis of these experiments, it was assumed that the pyrolytic carbon layers did not strongly affect the tritium release. This assumption allowed the results to be applied to the determination of the parameters for tritium in silicon carbide alone. The experiments determined the tritium breakthrough times as a function of temperature for particles with different ‘Li burnups. The breakthrough times were not a function of burnup. According to classical diffusion theory, breakthrough times can be directly related to the diffusion coefficient. The measured breakthrough times were 5 to 10 times longer than those predicted by Causey et al. [l]. Applying the assumption that the effect of the pyrolytic carbon is negligible, these results suggest a diffusion coefficient that is 5 to 10 times lower than the previously reported values. With other studies reporting widely varying results for the tritium migration parameters for silicon carbide, the need for careful measurements is obvious. In the study by Causey et al. [l], the diffusivities for the different types of silicon carbide vary by several orders of magnitudes at some of the temperature extremes. These differences also point out the need for very thorough characterization of the material used in the experiments.

3. Experimental procedures 3. I. Materials The samples used in this experimental program were produced by General Atomics in a laboratory sized coater at a temperature of 1550°C with a deposition rate of 0.3 Fm/min. The silicon carbide was deposited on thin graphite squares. At the end of the deposition process, the edges of the silicon carbide were cut off, and the specimen was cleaved into two slabs. The graphite was burned back by heating to 750°C in air. The thin SiO, layer that formed during the carbon removal was etched away using hydrofluoric acid. The dimensions of the resulting samples were 5 by 5 mm with a thickness varying between 50 and 90 hrn. Sample characterization showed the samples to

R.A. Causeyet ai. / Tritiummi~rat~an in p-siliconcwbide

198

TO Vacuum Eftluent Recovery System

Fig. 1. Tritium exposure system. Maximum exposure temperature is 1600°C.

have a measured density of 3.21 to 3.22 gm/cm3, as compared to a theoretical density of 3.217 gm/cm3. According to X-ray diffraction, the material was predominantly @(cubic) silicon carbide with an indication of a very faint graphite peak in the data. Moreover, there was a very strong preferred orientation of the (111) planes parallel to the deposition plane. Micrographs showed the grain size to vary through the thickness with the finest grain size occurring near the substrate. The grains were elongated and columnar shaped.

consisting of approximately 99% deuterium and 1% tritium at varying gas pressures, varying temperatures, and varying times. The gas pressure in the system was controlled by an Edwards flow control system. A small gas flow of several cc/min was maintained during the experiments to minimize the buildup of impurities. The furnace temperature was measured using a molybdenum sheathed tungsten-rhenium thermocouple. After several samples had been loaded with tritium, the system was reconfigured into a form represented by fig. 2. The same furnace with different furnace tubes was used for both the loading and outgassing of the tritium. For the outgassing, the temperature of the furnace was rapidly elevated to 1600°C and held there until the response of the ionization chamber dropped to background. During this time, a mixture of 99% helium and 1% hydrogen was sent through the furnace tube and ionization chamber at a rate of 250 cc/min. The ionization chamber had a volume of 250 ml and used a charge collection voltage of 100 V. The ionization chamber was connected to a Cary 401 electrometer for charge collection. Calibration of the ionization chamber revealed almost 100% efficiency of ionization collection over the range of concentrations used in these experiments. The outgassed tritium was released from the ionization chamber to the building stack. 3.3. Deuterium experiments

3.2. Tritium experiments The tritium retention experiments were performed in the systems shown schematically in figs. 1 and 2. The thin silicon carbide specimens were exposed to gas

I

I

Fig. 2. Tritium thermal desorption system. Tritium was released directly to the stack due to very limited amounts of tritium in each sample.

Nuclear reaction analysis was used to measure depth profiles of deuterium in the silicon carbide samples after exposure to deuterium gas. This technique was used to determine deuterium surface and volumetric concentrations in experiments where the temperature was not high enough to obtain equitibrium within reasonable exposure times. The thin silicon carbide samples were exposed to deuterium gas at temperatures up to 1000°C and pressures up to one atmosphere for various lengths of time. Exposures were done inside a quartz tube which was evacuated with an ion pump prior to backfilling with deuterium gas to the desired pressure. The temperature and gas pressure during the exposure were monitored with a chromel-alumel thermocouple and a capacitance manometer. Initial cooling rates at the end of the exposures were - 2”C/s, resulting in short cooldown times in comparison to diffusion times. Nuclear reaction analysis was used to measure the concentration of deuterium versus depth in the silicon carbide. An analysis beam of 3He+ ions from a Van de Graaff ion accelerator was directed onto the sample. Some of these ions undergo the Dc3He, pj4He nuclear

R.A. Causey et al. / Tritium migration in p-silicon carbide

reaction with deuterium atoms in the target giving energetic protons which were counted using a silicon charged particle detector. The yield of protons was measured for various incident 3He energies between 0.3 and 2 MeV. Higher energy analysis beams probe deuterium to greater depths into the target. Since the reaction cross section and stopping power are known [5,6], the data can be numerically unfolded to give the concentration of deuterium versus depth [7]. Deuterium concentrations as small as one atomic part per million can be measured to a maximum depth of about 5 km with a resolution of few tenths of a micron. Surface coverages as small as lOI D/cm* can be measured.

4. Experimental

199

1.0

0.6

results

4.1. Diffusiioity measurements

One series of experiments was performed solely for the purpose of determining the diffusivity. In these experiments, the samples were exposed to gas consisting of 99% deuterium and 1% tritium at a pressure of 1 atm, a temperature of 13OO”C,and a time of 1 h. It was initially believed that these conditions were sufficient to achieve equilibrium loading of the samples. During the outgassing phase of the experiments at 1100 through 15OO”C, the samples were individually lowered into the furnace with the furnace already at temperature. The sweep gas (99% helium and 1% hydrogen) was sent through the molybdenum furnace tube and then through the ionization chamber. The tritium release rate from the sample was determined from the response of the ionization chamber. After long isothermal release measurements were performed, the sample temperatures were elevated to 1600°C to drive out the remaining tritium. As examples of the experimental results and the analysis technique used, the results from the outgassing experiments performed at 1200 and 1300°C are shown in fig. 3. These release rate curves were compared to those calculated by the DIFFUSE computer code [8] where it was assumed the samples were uniformly loaded with tritium. While the short-term release rates did not agree well with computer results, it was still possible to obtain estimates for the diffusion coefficient of tritium in silicon carbide using this technique. Once these estimates were available, it was determined that the 1 h, 1300°C exposure was not sufficient to achieve uniform loading of the samples. The newly estimated diffusion coefficient was then used in the DIFFUSE

0.2

0.0 0

1000

2000 Time

3000

4000

(set)

Fig. 3. Experimental values for the fractional release of tritium from silicon carbide as a function of time at 1200°C.

computer code to duplicate the tritium concentration profile that existed in the samples at the end of the 1300°C exposure. The release rates curves as calculated by the computer assuming the nonequilibrium loading are shown as fig. 4. The results are shown as fractional release versus time for different diffusivities. The diffusion coefficients were again estimated by overlaying the curves and determining the diffusion coefficient that gave the best fit to the experimental data. Greater consideration was given to the longer term response where effects such as surface release would not be a consideration. For the 1200 and 1300°C cases, the curves corresponding to 3 X lo-” and 1 X 10e9 cm*/s gave the best comparisons. Interestingly, the values determined using the corrected tritium profiles were exactly the same as those determined when equilibrium was assumed. The primary difference was the better agreement at shorter times. The diffusivities determined from the release rate curves are shown in fig. 5 along with one data point obtained from tritium retention measurements at 1200°C as a function of time. In those experiments, the silicon carbide samples were exposed to tritium gas at 1200°C for times varying

R.A. Cuusey et al. / Tritium migration in p-silicon carbide

temperature and time to be sufficient to achieve equilibrium loading of the samples. The amount of retained tritium as determined by the subsequent outgassing of the samples is shown in fig. 6. The data is shown on a log-log plot under the assumption that the solubility is proportional to the pressure raised to some power. With hydrogen being a diatomic gas, the solubility was anticipated to vary with the square root for pressures in the range where measurements were performed. As can be seen in the figure, the power is closer to l/3 than it is to l/2. A possible explanation for this unexpected behavior is a constant surface loading that is not part of the volumetric retention. Evidence for this came from experiments using nuclear reaction profiling of deuterium exposed samples. This technique is able to determine both the surface retention as well as the concentration as a function of depth in the near surface region. Samples exposed to deuterium gas at 1000°C and 1200°C showed surface retentions of 2.1 X lOI and 1.4 X 10’” atoms/cm2, respectively (each side of the sample only has one half of this concentra-

0.6

0.4

0.2

0.0 1000

3000

2000 Time

4000

(see)

llooOc I

1500°c I

Fig. 4. Computer generated fractional releases of tritium as a function of time for different diffusivities. Curves are based on actual sample thicknesses of 80 Km.

between 30 min and 8 h. The DIFFUSE computer code was used to calculate a series of curves for tritium retention assuming different diffusivities; A value of 5 X lo-” cm2/s for the diffusivity at 1200°C gave the best fit to the experimental data. The least squares fit of all of the data in fig. 5 to the Arrhenius expression yields D = 9.8 X 10m4 exp( - 1.89 eV/kT) {R’=0.94}.

/;I

\ Yerghese KT-SIC

et

al.

(1979)

cm2/s (1)

Also shown in fig. 5 are the earlier

results given by Causey et al. [l] and those of Verghese et ai. [2]. This data comparison is discussed in section 5.

4.2. Solubility measurements Both the pressure and temperature dependences of the solubility were measured. The pressure dependence of the solubility was measured by exposing samples to the tritium/deuterium gas at pressures varying from 0.01 up to 1.0 atm with a temperature of 1400°C for 1 h. Prelimina~ measurements had shown this

W Release Rate Data 0

Absorption

Rate Data

~1

~.

0.6

~~,

.

0.7

C 3

1000/T(K)

Fig. 5. Tritium diffusivity for vapor-deposited p-siiicon carbide. Data was obtained from both release rates and absorption rates results.

R.A. Causey et al. / Tn’tium migration in p-silicon carbide

experiment at the higher temperature was based on a signal-to-noise ratio problem. At the lower temperatures, the ratio of the surface coverage to the internal concentration was high. This ratio drops significantly at 14Oo”C, rendering the direct measurement impossible. Inherent in this explanation is the assumption that the surface retention is not also pressure dependent. It is anticipated that the surface sites became saturated during the furnace cool down cycle. The deuterium and tritium gas had been removed, but there was sufficient trace amounts remaining to fill these easily accessible surface sites which readily retain the hydrogen isotope atoms at the lower temperatures. The temperature dependence of the solubility was measured over the temperature range of 1000 through 1600°C using a gas pressure of 1 atm. The results for these experiments are shown in fig. 8. The data for 1200°C and above were determined by the tritium experiments where equilibrium loading of the samples was used. For the 1200°C data, it was necessary to

.37

.l

iquilibrium

Retention

Exposure

Temperature

Proportional

1

I

P

14OO’C

10 Pressure

to

201

100

1000

(tow)

Fig. 6. Uncorrected pressure dependence of the tritium bility in vapor-deposited l3-silicon carbide.

solu-

etention Lxposure

tion). Linearly extrapolating these values to that expected at 1400°C predicts a surface loading of 7 X 1014 atoms/cm’. Because the samples thicknesses were not all the same, it was not possible to make this correction with a simple subtraction from the data shown in fig. 6. Each sample was 0.5 by 0.5 cm, making it necessary to subtract (7 X 1014 atoms/cm2) X (0.25 cm21 = 1.75 X 1014 atoms from the measured release for each sample. This total retention was then divided by the sample volume to get the corrected pressure dependence shown in fig. 7. This correction resulted in a pressure dependence of Po.46 which is much closer to the expected square-root of pressure dependence. It is obvious that a direct measurement at 1400°C would have been preferable to the extrapolation of the lower temperature data. In reality, there is little justification for a linear extrapolation. It is also difficult to predict the error caused by this extrapolation. This extrapolation was performed purely to show that surface effects could have distorted the square root of pressure dependence. The failure to perform the surface retention

Corrected Temperature

Equilibrium

1

by Subtracting

Retention

/

Proportional

10 Gas

7x164atomsic6

= 14OO’C

Pressure

100

to P4

1000

(tow)

Fig. 7. Corrected pressure dependence of the tritium solubility in vapor-deposited p-silicon carbide. Data corrected by subtracting surface absorption component.

R.A. Causeyet al. / Tritiummigrationin p-siliconcarbide

202

continue the tritium exposure for 8 h to obtain equilibrium. For the temperatures above 1200°C it was only necessary to use 1 to 4 h exposures. The amount of surface absorption at 1500 and 1600°C is negligible, but the 1200 through 1400°C results were corrected using the values given in the above pressure dependence discussion. For the 1000°C data point, nuclear reaction profiling was used to determine the solubility. For nuclear reaction profiling, it is only necessary that the deuterium come to equilibrium in the first several microns of the sample. This was achieved by a 45 h exposure. The least squares fit of the data to the Arrhenius relationship yields the following formula for the soiubility: S = 8.2 X 10” exp(0.61 eV/kT)

10”

A

Experimental

-

Computer

Data

Data

atoms/cm3 atm”*

{R2 = 0.834) (2) Also shown in fig. 8 is the earlier data of Causey et al. [l]. The differences obtained in the two different studies are discussed in section 5. Exposure Temperature = t200’C Exposure Pressure q 1 Atmosphere Sample Thickness = 60 pm 10” 0

2

4 Exposure

6 Time

6

10

(Hours)

Fig. 9. Comparison of the measured tritium retention in vapor-deposited p-silicon carbide at 12CWC as a function of time to that calcuiated using the soIubili~ and diffusivity determined in this study.

4.3. Tritium retention as a function of temperature, pressure, and time

1o”J 0.5

0.6

0.7

0.6

1000/T(K)

Fig. 8. Temperature dependence of the solubility of tritium in vapor-deposited p-siticon carbide. Earlier results calculated from absorption rate which was affected by trapping.

Experiments were performed at a series of different temperatures and times to allow a comparison of measured tritium retentions to those predicted using the solubilities and diffusivities determined in this experimental program. Fig. 9 shows the tritium retention in silicon carbide samples as a function of time using a gas pressure of 1.0 atm and a temperature of 1200°C. The solid line was calculated by the DIFFUSE computer code [S] using the diffusivity and solubility given above. fig. 10 shows the measured and calculated retentions of tritium in silicon carbide with an exposure pressure of 1.0 atm, a time of 1 h, and temperatures from 1200 to 1600°C. Backgrounds to account for surface absorption were subtracted from the raw experimental data. For most of the experimental conditions, the measured results were greater than the calculated results. This could be due to an underestimation of

203

R.A. Causey et al. / Tritium migration in p-silicon carbide

-

A Ex~rl~n~l Results Computer Results

Exposure Time I 1 Hour Pressure = 1 Atmosphere Sample Thickness = 60 pm 1100

1200

1300

1400

Temperature

1500

1600

1700

(C)

Fig. 10. Comparison of the measured tritium retention in vapor-deposited p-silicon carbide as a function of temperature to that calculated using the sotubili~ and diffusivity determined in this study.

the diffusivity or the solubility. The other possible explanation is that the sample side with the smaller grains has a solubility greater than the sample average, and that this high solubility results in rapid absorption of tritium initially. This is further discussed in the following section.

either

5. Discussion Two significant features that emerge from the present study of the migration of tritium in Sic are the small but exothermic heat of solution for the solubility and the very low diffusivity. The fact that the solubility is small and has an exothermic, i.e. negative, heat of solution indicates that the number of sites at which tritium can reside in the SiC lattice is small in comparison to the number of host atoms in the Sic lattice. This

is illustrated by the fact that the tritium solubility prefactor (S, = 8 x lo-* atomic fraction/atm1’2) obtained here for Sic is smaller by a factor of about 4 x 10” than prefactors for hydrogen solution in metals [9-N] where the number of solution sites is comparable to the number of metal atoms. This suggests that the observed solubility of tritium in Sic is due to a relatively small number of sites, which we will call traps. Additional information on the nature of the traps comes from the observed negative heat of solution. The activation energy for populating traps (equivalent to the heat of solution for occupation of solution sites) from the gas phase is the energy to break the H-H bond (2.26 eV/atom) minus the binding energy of the H atom to the trap. The measured value of -0.61 eV/atom for the energy of solution for tritium in Sic implies a binding energy of 2.87 eV. Binding this strong must come from formation of covalent chemical bonds between the tritium and C or Si atoms. For comparison, Robe11 et al. [17] obtained a value of 2.94 eV for the C-H bond energy from the studies of surface diffusion of H on carbon. Myers et al. [18] have recently determined a bond energy of 2.5 eV for Si-H at silicon surfaces. Bond strengths in molecules are somewhat higher, ranging from 2.8 to 3.9 eV for Si-H bonds in various molecular species 1191and 3.5 to 4.8 eV for C-H bonds in various hydr~arbons [ZO]. The large activation energy for diffusion (1.9 eV) is also consistent with trap dominated diffusion where the time spent in the traps by tritium atoms is long compared to the time spent moving between traps. The occupation of the traps is described by q/G

-4)

= C

exp(Q,/W,

(3)

where q is the fraction of the trap sites which are occupied, C is the atomic fraction of tritium in the solution sites and Q, is the energy difference between tritium in a trap and tritium in a solution site. The traps will be saturated (i.e. q = 1) when [C exp(Q,/ kT)] z+ 1. In equilibrium with gas C = P’/*C, exp( - Q,/kT)I, where P is the gas pressure and C, is the true lattice solubility prefactor. Also Q, = Q, - Qd, where Q, is the true heat of solution and Q$ = -0.61 eV/atom is the apparent heat of solution or activation energy for populating the traps from the gas phase. The condition for saturation thus becomes [P’/*CO expt-Q,*/kT)I za 1. C, is not known for hydrogen in Sic, but if we assume a value of C, = 0.002 atomic fraction/atm’/* typical for hydrogen in metals [9-161,

then

we find

that

for P=

1 atm

the

traps

204

R.A. Causey et al. / Tritium migration in p-silicon carbide

should not be saturated when T > 900°C. Therefore, because of the modest energy difference between gas phase tritium and trapped tritium (0.61 eV/atom) the traps should not be saturated for the temperatures and gas pressures used in this investigation. This is consistent with the observation that the tritium retention continues to increase with increasing gas pressure (fig. 7) and with decreasing temperature (fig. 8). For the situation described above where trapping determines the effective solubility and diffusivity and where the traps are not saturated, the effective solubility is larger than the solubility for the trap-free lattice by a factor of N, exp(Q,/kT), where N1 is the atomic fraction of the traps. The effective diffusivity is smaller than the value for the trap-free lattice by the same factor. The steady-state pe~eability, which is the product of the diffusivity and the solubility is therefore not affected by the trapping. In steady-state the amount of tritium in the traps does not change with time and is merely a spectator to the permeation process. However, transient effects, such as breakthrough times and the thermal release measured in this study, are strongly affected by trapping. We now consider what is known about the microstructure of the traps. For the reasons discussed above the traps are believed to be dangling bonds on C or Si atoms at the defects in the lattice. Possible lattice defects include grain boundaries, dislocations, point defects such as vacancies or substitutional impurities, and surfaces (internal and external). The samples used in this study had grains approximately 1 km in diameter on one side, and larger grains on the other side. This indicates a higher density of lattice defects and therefore traps on the side with smaller grains. This agrees with our nuclear reaction analysis measurements of near surface concentrations of D in SIC samples exposed at 1000°C to D, gas which showed that the side with the smaller grains retained 2 to 3 times more D. This difference was consistently seen on several samples. A simple calculation shows that the observed retention could be due to defects at grain boundaries. Grains with a diameter of I urn have 3 X lo4 cm2 of surface area per cm3 of material. Using a surface coverage of lOI5 atoms/cm’ yields a maximum trap density of 3 x 10’9/cm”. This assumption overestimates the trap density since many of the atoms at grain boundaries will not have dangling bonds. Furthermore, the concentration of traps is probably larger than the concentration of retained D for the reasons discussed above. Thus, trapping at grain boundaries could account for the observed retained concentrations of about 10’” D or T/cm3. The possibility remains that

other defects, such as dislocations or point defects within the grains, may contribute to the trapping. While trapping, possibly at the grain boundaries, appears to control retention and transport of tritium in Sic at temperatures used in this study, normal bulk diffusion should dominate at temperatures high enough such that Nt exp(Q,/kT) +Z 1. The tritium diffusion coefficients determined in this study are approximately one order of magnitude larger than those obtained in the earlier experiments by Causey et al. [l]. Because the Sic used in the two studies was almost identical (earlier experiments used SIC microspheres produced by General Atomics), the question arises why different results were obtained. The reason for this difference probably lies in the different methods used to put tritium into the Sic. In the earlier experiments the tritium was energetically recoiled into the Sic using the 6Li(n, ol)‘H reaction. The recoiling tritium as well as the a-particles most likely caused radiation damage in the Sic which would have increased the density of the trapping sites. The effective diffusivity determined from those measurements of thermal release would then have been reduced by the larger density of traps as discussed above. Experiments to characterize the effects of energetic particle bombardment on trapping of D in Sic are now in progress. Preliminary results indicate that irradiation by He ions greatly increases the concentration of retained D. The values given here for tritium diffusion in silicon carbide are also considerable higher than those determined by Verghese et al. [2] for KT silicon carbide. Their higher values were attributed to excess silicon in the material as well as some open porosity. The solubility of retained D was also determined in the earlier experiments by Causey et al. [l]. In those experiments the Sic was exposed to deuterium gas at elevated temperatures and subsequently outgassed. A calibrated residual gas analyzer was used to measure the amount of gas desorbed. Because the Sic was relatively thick, equilibrium saturation of the samples was not possible. In this case the number of D atoms absorbed per unit area is N= 2Cr(Dt)‘/‘/rr, where r = 0.97 X 1023/cm? is the atomic density of Sic and t is time. Concentrations C and thus solubilities were inferred from the measurements of N using the above expression and values of the diffusivity D from the recoil implantation experiments. Because the diffusivity was determined in experiments where radiation damage was a factor and because the samples used in the solubility measurements were not damaged, the solubilities reported in those earlier experiments were too high.

RA. Causey et al. / Tritium migration in p-siiican carbide

6. Conclusions

121K. Verghese, L.R. Zumwalt, C.P. Feng and T.S. Elleman,

The tritium d~ffusivi~ for vapor-deposited carbide was determined for the temperature 1100 to 1500°C. It is given by

D = 9.8

X low4 exp( - 1.89 eV/kT)

&silicon range of

cm’/s.

(1) The solubility of tritium in vapor deposited silicon carbide was measured over the temperature range of 1000 to 1600°C and the pressure range of 0.01 to 1.0 atm. This solubility is given by

S = 8.2 X lOI exp(0.61

205

eV/kT)

atoms/cm3

atm”‘. (2)

When corrected for surface absorption, the solubility was found to vary linearly with the square-root of pressure. It is postulated that both the diffusivity and solubility are controlled by tritium trapping at lattice defects.

Acknowledgement

The authors would like to acknowledge the support of the US Department of Energy New Production Reactor (DOE-NP).

References [l] R.A. Causey, J.D. Fowler, C. Ravanbakht, T.S. Elleman and K. Verghese, J. Am. Ceram. Sot. 61 (1978) 221.

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