Interacllon of Energetlci Trltlum wlth SIC
The Journal of PhY8lCal Chemlstry, Vol, 83, No, 4, 1070 4117
Interaction of Energetlc Tritium wlth Slllcon Carblde Thomas E. Boothe and Hans J, Ache* Department of Chemlstry, Vlrglnla Polytechnic InStltUte and State Universlty, Blacksburg, Vlrglnla 2406 1 (Received June 23, 1978; Revised Manuscrlpt Recelved September 11, 1978)
In order to investigate the physical and chemical interactions of energetic hydrogen isotope species with silicon carbide, recoil tritium from the SHe(n,p)Treaction has been allowed to react with K-T silicon carbide and silicon carbide powder. The results show that if the silicon carbide has been degassed and annealed at 1400 OC prior to tritium bombardment, a considerable fraction of the tritium (ca. 40%) is released as HTO from the Sic upon heating to 1350 O C under vacuum conditions, Most of the remaining tritium is retained in Sic, e.g., the retention of the tritium in the K-T SIC was found to be 62 and 22% upon heating to 600 and 1350 O C , respectively. This is in direct contrast to graphite samples in which the tritium is not released to any significant extent even when heated to 1350 OC. Samples which were exposed to H2O and H2 prior to tritium bombardment were heated to 600 O C after the irradiation, The results obtained indicate that a total of 38.7 and 2.49% of the tritium is released in the form of HT and CH3T in the case of H2or H20exposure, respectively. Treatment of degassed samples after tritium bombardment with H 2 0 and H2 at temperatures up to 1000 O C leads to the relealse of up to 44.9% of the tritium as HT and CHBT.
a stated purity of 99.8 mol % and was used as such, Introduction Hydrogen (99.9995% ) was purchased from the Matheson The interaction of energetic plasma particles with the Co. SM4, which was used as a gas chromatographic metallic vacuum wall (first wall) of a controlled thermostandard, was prepared according to the literature."6 nuclear reactor (CTR,) will produce impurities by processes B. Sample Preparation and Irradiation. K-T silicon such as sputtering, blistering, and evaporation.2 Since it carbide chips, varying in size from 1 mm X 1 mm X: 0,2 has been found that high-Z impurities have the most mm to 3 mm X 3 mm X 0.5 mm were cut from a block. significant effect 0x1 plasma performance, it has been Enough chips were used to give a sample of 0.75 g with a suggested to protect; the first wall of the CTR by using total surface area of approximately 14 cm2. The silicon different types of grriphite or low-Z refractory carbides as carbide powder, 0.75 g, was used as such and had a surface nonstructural complonents such as limiters in tokamak area of 46 cm2. device~i.~-~ The S i c samples were placed into cylindrical quartz Numerous investigations have been carried out to study torr ampoules (volume 0'75-1.0 cm3) and evacuated to the reactionfi of low energetic hydrogen species, which result in mostly surface reactions6-16and s p ~ t t e r i n g , l ~ - ~ ~for various lengths of time with or without heating, Heating of the samples was accomplished by means of a with different forms of graphite. Several investigations furnace, Some samples were evacuated a t 1350 O C for 3 have also extended the study to various carbides, parh and subsequently exposed to hydrogen gas (52 torr) and ticularly silicon carbide.16-23 Sic has been found to be water vapor (18 torr) for 5 min. After evacuation of the somewhat more resistant to attack by thermal hydrogen gases for 5 min, helium-3 was added. produced in a hydrogen plasma,22however, when more In all cases the appropriate amounts of helium-3 were energetic hydirogen species are used some surface swellingz1 added, usually between 130 and 200 torr, and the ampoule and sputteringlgwas observed. In comparison to carbon was sealed. The sample was subsequently exposed in the samples, S i c was found to exhibit lower erosion yields.17 VPI 4- SU nuclear reactor to a neutron flux of 0.8-1.3 X Attack by hydrogen ions of energies up to 15 keV has been 10l2 n cm-2 s-l for 4 h a t a temperature of 65 OC. shown to produce stable Si-H and C-H bonds.23 C. Post-Irradiation Treatment and Sample Analysis. As of yet, very little information is available about the The postirradiation treatment and sample analysis was fate of the higher energetic hydrogen, deuterium, or tritium carried out via two different techniques, species that become incorporated into silicon carbide or (1) The first one was applied to the determination of their subsequent interactions and their potential effect on HT, CHaT and other tritiated hydrocarbon gases, H'I'O, plasma contamination.16 and the retained tritium activity in Sic samples, which Most recently it has been shown that the use of recoil either (a) were not degassed prior to the tritium bomtritium produced in the 3He(n,p)T reaction furnishes an bardment, Le., they contained some residual gases absorbed excellent method for studying the fate of individual enin the bulk Sic, or (b) which were degassed prior to the ergetic tritium species in carbon without any interference tritium bombardment and were heated after the tritium from other reaction products forrned in the tritium bombardment in the presence of PI2 or H20 additives. b ~ m b a r d m e n t This . ~ ~ ~present ~ investigation extends this After irradiation the sealed (irradiated) ampoules were technique to the study of the interaction of energetic heated under conditions varying from room temperature tritium with silicon carbide. to 1000 " C for 4 h. Several samples which were degassed Experimental Section before and after tritium bombardment were subsequently A. Materials. K-T' silicon carbide, a silicon rich heated €or 16 h a t 600 "C for 6 h a t 1000 OC with 18 torr was donated by the lCarborundum Co. Silicon carbide of water vapor or 50 torr of hydrogen gas present. H20 powder, 120-150 mesh, was from Matheson Coleman and or H2was added by vacuum line techniques without exBell. The graphite, H-451nuclear grade, waB donated by posing the samples to air. Great Lakes Carbon Gorp, After the post-irradiation treatment the ampoule wa8 Helium-3 was obtained from Mound Laboratory with placed in a heated (sa "C)ampoule crusher which was 0022-3654/78/2083-0457$01 .OO/O 63 1878 American Chemlcal Soclety
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attached to a gas-chromatographic system. After removing the air by sweeping the system with helium gas the ampoule was opened and the contents were allowed to equilibriate for 3 min. Subsequently the released gases were directly injected onto a gas-chromatographic column (60-80 mesh silica gel; 4.5 m long, 0.6 cm i.d.). The helium flow rate was 40 mL/min. The initial column temperature was 40 "C for the separation of H T and CH3T. After passage of CH3T the temperature was raised to 130 "C at a rate of 15 "C/min for the separation of the higher molecular weight hydrocarbons. Hydrocarbons carriers were simultaneously injected bypassing the S i c in the ampoule crusher. No exchange occurred on the column during separation. After passing the mass detector the effluent gases were oxidized on line in a 30 cm X 0.5 cm quartz tube containing 5.5 g of Co304heated to 650 0C.26 The water formed was discontinuously collected a t 1-min intervals in vials containing 10 mL of a 2:l toluene-Triton-X 100 scintillation solution and the tritium activity counted in a Beckmann LS-100 liquid scintillation counter. The counting efficiency was 29%. In order to determine the amount of CHI in helium-3 and the amount of tritiated gases formed with gases absorbed on or released from the surface of the quartz ampoules control experiments were performed by irradiating quartz ampoules filled with helium-3 and using the same analytical procedure. Samples were also heated at 600 and 1000 "C. The amount of tritiated gases formed under these conditions was found to be insignificant as compared with those detected with S i c samples. Only trace amounts of HTO were observed at 1000 "C. In order to determine the amount of tritium remaining in the Sic, after being subjected to the treatment described above, several portions of the S i c were mixed with 3 g of Co304in the presence of NaOH27and heated at 650 "C in a helium stream (20 mL/min) for 6 h. The water formed was collected and its tritium activity determined as described above. The total activities, which varied from 3.8 to 6.7 X lo6 dpm, were derived from the sum of the tritium activities measured in the gas phase and in Sic. Based on the actually calculated tritium activities, in each case these total activities correspond to approximately 30% of the total tritium produced. The missing activity is probably lost to the quartz wall. (2) The second method of post-irradiation treatment and sample analysis was applied to S i c samples and H-451 graphite, which were degassed at 1350 "C for 3 h and subsequently tritium bombarded, in order to determine the tritium retention in these samples under vacuum conditions. The irradiated ampoule was opened to the air and some of the Sic (0.15 g) was removed to determine the total tritium activity. The remainder of the sample was divided into several portions, and each portion was placed in a quartz ampoule and heated to temperatures varying from 300 to 1350 "C for 0.5 h under vacuum torr) with continuous pumping. The tritium activity remaining in the sample was determined by the combustion technique as described above. The H-451 graphite samples were degassed at 1100 "C for 3 h and were treated in a similar manner. In several cases the neutron irradiated ampoule containing the tritiated S i c powder was placed into a special transfer vessel. The entire apparatus was evacuated and flame dried. The ampoule was broken and a portion of the Sic (0.15 g) was transferred to another quartz ampoule
PERCENT RETENTION OF TRITIUM IN SAMPLES HEATED UNDER VACUUM 100
90
30 20 lo
t
01
0
'
1
'
1
'
1
'
200 400 600 800 loo0 1200 1400
TEMPERATURE PC) Figure 1. Percent retention of the tritium in H-451 graphite, K-T SIC, and Sic powder as a function of heating under vacuum at various temperatures for 30 min.
without exposing the sample to air and heated to 1000 "C for 0.5 h. The tritium activity remaining in the sample was then measured as before. To determine the nature of the tritiated product or products released from the S i c upon heating, other aliquots of the above samples, which had been temporarily exposed to air, were placed in a small quartz ampoule (volume 1cm3) which was connected to a trap (volume 5 cm3). The system was evacuated at room temperature and sealed off under vacuum. The trap was cooled to -196 "C and the sample in the quartz ampoule heated at 1000 "C for 0.5 h. The volatile products were allowed to collect in the trap a t -196 "C. The total contents including noncondensable gases of the trap was subsequently subjected to radio gas-chromatographic analysis on the silica gel column as previously described or on a Poropak Q column (80-100 mesh, 2.5 m long, 0.5 cm id.) at room temperature; the helium flow rate was 40 mL/min.
Results The interaction of tritium produced by the 3He(n,p)T reaction with Sic was investigated by carrying out several series of experiments. Tritium Retention. In the first series of experiments the amount of tritium remaining incorporated in the S i c samples was studied as a function of the temperature to which the samples were subjected to under vacuum with continuous pumping after the tritium bombardment. In order to minimize any interaction between the implanted tritium and impurities such as adsorbed or absorbed residual gases in the Sic, with the recoiling tritium species the samples were heated at 1350 "C for 3 h under vacuum in order to remove the major function of these residual gases, prior to the tritium bombardment. After tritium bombardment the samples, which consisted of K-T S i c or S i c powder, were heated to temperatures varying from 300 to 1350 "C for 0.5 h after briefly being exposed to the air during the necessary transfer process. A comparison was made with H-451 graphite which had been degassed at 1100 "C for 3 h. As can be seen from Figure 1, which shows the percent retention of tritium as a function of temperature, the S i c samples begin to lose their tritium activity at temperatures as low as 300 "C, and at 1350 "C only %-3o% of the total tritium remains incorporated. The S i c powder seems to
Interaction of Energetic Tritium with Sic
The Journal of Physical Chemistry, Vol. 83, No. 4, 1979 459
TABLE I: Tritiated Products in % of the Total Tritium Activity Obtained from Tritiated Sic Samples upon Heating in the Presence of Water and Hydrogen Gas type of sample
additive (torr)
% of total tritium activity
treatmenta HT CH,T HTO 600 ' C , 1 6 h 0.98 f 0.15 0.14 * 0.02 0.13 c 0.02 K-T SilC H*Q(18) 19.7 3.0 3.27 f 0.19b 0.38 c 0.06 1000 "C, 6 h K-T SiC H*0(18) 0.053 f 0.003 0.58 * 0.09 600 'C, 16 h K-T SiC H2t50) 3.24 * 0.49 1000 'C, 6 h 28.9 f 4.3 K-T S i c HA50) 5.55 f 0.67 1000 ' C , 6 h 39.3 * 4.7 Sic powder H2(50) a All samples were degassed at 1350 "C for 3 h and 1000 "C for 1 0 min before and after irradiation, respectively. They were subseauentlv heated in the Presence of the indicated amount of additive at the conditions stated, without exposing the 0.085% of C,H,T obtained. samples t o the air.
show a slightly higher retention than the K-T Sic. To compare these results, which were obtained from samples briefly exposed to air, to the retention in samples which were transferred without being exposed to air, a Sic powder sample which had been degassed a t 1350 "C for 3 h and irradiated was heated at 1000 "C for 0.5 h. The retention of the tritium in the sample was 58.6% which is only slightly higher than in the samples which have been handled in the laboratory atmosphere and which show a tritium retention of about 49%. A prolonged heating time after the tritium bombardment results in a further reductio11 of the amount of tritium retained in the Sic, e.g., if the heating time of the K-T SiC sample is extended from 0.5 to 3 h under these conditions the tritium contents in the S i c is reduced from about 20 to 10.9%. On the other hand, the effect of a prolonged degassing time in excess of 3 h prior to the tritium bombardment is relatively small, e.g., a S i c powder sample which has been degassed under vacuum at 1350 "C for 10 h retains (24%) approximately the same amount of tritium as a sample which has been degassed under the same conditions for only 3 h (32%). The percentages quoted refer in both cases to the percentages of tritium retained after a 0.5-h post-bombardment heat treatment at 1350 "C. The latter result seems to indicate that a 3-h heating period (1400 "C) is sufficient to remove most of the ad- or absorbed gases from the sample. Also sihown in Figure 1 are the results obtained from H-451 graphite. In contrast to the Sic only minor imounts of the tritium are removed from the graphite even if the sample is heated to 1350 "C at which temperature 84% of the tritium still are retained in the carbon samples after 0.5-h heating time. In order to determine the nature of the tritiated products released from the S i c upon heating, samples of tritium bombarded Si(: powder, which were degassed prior to the tritium bombardment as described above, were heated to 1000 "C for 0.5 h under vacuum, the released products collected by vacuum line techniques, and radio gas-chromatographically analyzed. It was found that 40.0% of the tritium was released as HTO while 49.2% was retained in the Sic. The remainder of the activity could not be assigned to defined compounds, but an increase of the radioactive background was observed during the radio gas-chromatographic analysis. Furthermore, the radio gas-chromatographic analysis revealed only traces of tritium activity in form of HT or in tritiated hydrocarbons and none was observed in SiH3T. Reactions of Tritium Incorporated in Sic with Various Additives. In order to study the fate of the tritium in the S i c samples when various hydrogen sources were provided, in a second seiries of experiments, the K-T S i c was degassed at 1350 "C for 3 h before and 1000 "C for 10 min after the tritiuin bombardment. Water vapor (18 torr) or hydrogen gas (50 torr) were added to the samples, which
were subsequently heated a t 600 "C for 16 h and at 1000 "C for 6 h. Also, for comparison, hydrogen gas was added to S i c powder, which had been pretreated under similar conditions and heated at 1000 "C for 6 h. The results, as shown in Table I, indicate that only small amounts of tritiated products were produced at 600 "C with both the water and hydrogen gas additives. If, however, the sample was heated at 1000 "C, significant amounts of H T and CH,T were produced with H T being the predominant tritiated product. Exposure to hydrogen gas gave approximately twice the amount of CH8T as exposure to H 2 0 (Table I). The exposure of the Sic powder to hydrogen gas at lo00 "C gave very similar results to the results obtained from the K-T samples. It was also of interest to investigate the effect which impurities initially present in the S i c might have on the fate of the implanted tritium. In this third series of experiments, the impurities in these samples were removed in a stepwise fashion by heating the samples under vacuum (10" torr) at temperatures varying from room temperature to 1400 "C for various time periods of 1-10 h before tritium bombardment. No analysis of the removed gases was attempted. It was observed that the evacuation of the Sic a t 1350 "C for 3 h produced a gray deposit on the walls of the quartz tube where the tube passed from the furnace. Infrared analysis of the deposit indicated frequencies corresponding to the 0-H, C-H, C=O, Si-C, and Si-0 functional groups. No evidence for Si--H bonds wlere observed. After the neutron irradiation and concurrent tritium bombardment, the samples were heated at 1000 "C for 4 h, and the tritium activity in the released gases was measured. The observed tritium activities in the form of HT, CH3T, and minor amounts of the higher hydrocarbons, C2H5Tand C2H3T,in percent of the total tritium activity, are given in Table 11. It can be seen that the degassing of the sample prior to the tritium bombardment reduces the total amount of these gaseous products released in the post-irradiation treatment. Typical total activities obtained in form of' H T and tritiated hydrocarbons upon heating to 1000 "C for 4 h are 47.4 and 69.6% of the total tritium for the K-T Sic and S i c powder, respectively, if the samples were not subjected to any preirradiation heating. This compares to 0.72 and 0.39% if the samples were degassed for 3 h at 1350 "C prior to the irradiation and tritium bombardment. For other products formed in the latter case, e.g., HTO and T retention, see the previous section and Table I. If a sample has been degassed at 1400 "C for 10 h the total activity of these gases is further reduced to 0.11%. In samples which were heated prior to neutron irradiation, the relative as well as the total amount of tritiated hydrocarbons is generally lower than in samples which were irradiated without pretreatment (Table 11). Also of interest was the dependence of the compositlon of the various tritiated gases released from samples which
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T. E. Boothe and H. J. Ache
TABLE 11: Tritiated Products in % of the Total Tritium Activity as a Function of the Temperature of the Preirradiation Degassing Pr aced degassing conditions before irradiation: % of total tritium activity type of temp, "C; sample time, h HT CH3T C2HST C2H3T K-T Sic 25, 4 46.8 i 6.1 0.60 i 0.08 Sic powder 25, 4 68.6 i: 8.9 6.70 i: 0.87 0.082 i 0.001 K-T Sic 1100, 1 7.77 i: 1.01 0.102 i ,013 0.0095 i 0.0012 0.00020 i 0.00003 K-T Sic 1100,3 3.45 i: 0.48 0.063 i 0,009 K-T Sic 1350, 3 0.72 i 0.11 Sic powder 1350, 3 0.32 f 0.04 0.069 i: ,009 SIC powder 1400, 10 0.07 ?: 0.02 0.04 i: 0.01 a Post-irradiation treatment in each case consisted of heating the samples to 1000 "C For 4 h. No hydrogen source was added, TABLE 1x1: Tritiated Products in % of the Total Tritium Activity Observed when Degassed Sic Samples Were Exposed t o Hydrogen Gas and H,O Vapor Exposure Prior to Irradiation % of total tritium activity type of additive post-irradiation sample (torr) treatmenta HT CH 3T CAT HTO K-T Sic H,0(18) none 0.034 i 0.004 0.012 i: 0.002 Sic powder H,O(18) none 0.21 i 0.02 0.035 f 0.005 0.003 f 0.001 K-T Sic H,0(18) 600 O C , 4 h 34.2 .i. 3.5 4.52 i: 0.46 0.96 f 0.12 1.68 ?: 0.17 Graphite (H-451)b H,0(30) none 0.047 i: 0.007 Sic powder H,(52) 600 OC, 4 h 2.20 ?- 0.31 0.59 ?: 0.08 0.089 f 0.013 a Samples were degassed at 1350 "C for 3 h prior t o irradiation, exposed t o the indicated amount of substrate for 5 min, degassed for 5 min before addition of He-3 and neutron irradiation. H-451 graphite degassed at 1100 "C for 3 h and exposed t o H,O for 30 rnin at 600 'C, cooled to room temperature and evacuated for 5 rnin before addition of He-3 and neutron irradiation,
received no preirradiation heating on the temperature which appeared in the post-irradiation heat treatment without adding any hydrogen source. In Figure 2 the amount of tritium found in H T and CH3T in percent of the total tritium is plotted as a function of the temperature to which the samples, without any preirradiation treatment, were subjected to for 4 h after tritium bombardment. From the figure it can be seen that at 20 "C the relative amounts of HT and CH3T are about equal for both types of Sic; the H T begins to predominate as the temperature increases. At the higher temperatures (600-1000 "C) the amount of H T produced from the K-T Sic begins to level off, whereas the H T released from the Sic powder still shows a slight increase in this temperature range. Heating of the samples to 1000 "C definitely reduces the relative amounts of tritium found in hydrocarbons. A t this temperature H T is the predominant labeled product obtained. To gain further insight into the type of impurities which might be responsible for the tritiated products obtained, in the fourth series of experiments, samples of the K-T Sic and the S i c powder were degassed at 1350 "C for 3 h before irradiation and subsequently exposed to a hydrogen gas atmosphere (52 torr) and water vapor (18 torr) for 5 min at room temperature. After removal of the hydrogen gas or water vapor by vacuum, at room temperature, helium-3 was added and the sample neutron irradiated. As shown in Table 111the simple exposure of a sample to water vapor before tritium bombardment results in only minor amounts of HT, CH3T, or HTO if no post-irradiation heating is applied. Similar results are obtained using an H-451 graphite sample as shown in Table 111. If, however, the S i c sample is heated at 600 "C for 4 h following the tritium bombardment, significant amounts of both H T (34.2%) and CH3T (4.52%) are observed with minor amounts of C2H5Tand HTO. Exposure of S i c powder to hydrogen gas before irradiation also resulted in the production of H T and CH3T when the sample was
100
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I
1
1
I
I
I
200
400
€00
800
1000
1200
TEMPERATURE,
(OC)
Figure 2. Tritiated hydrogen and methane in % of the total tritium activity as a function of the post-irradiation temperature (4-h heating time) of samples which have not undergone any preirradiation heat treatment.
subsequently heated to 600 "C (Table 111). Discussion The reaction of recoil tritium from the 3He(n,p)T reaction furnishes an excellent method of studying the fate of individual energetic tritium atoms deeply implanted in various materials to be used in the CTR without any interference from reactions between two or more reaction products formed in the initial tritium attack. The average kinetic energy of the tritium recoil species generated in the
Interaction of Energetic Tritium with SIC 3He(n,p)Tnuclear process is 0.195 MeV. The arrangement of the Sic in the quartz ampoule filled with 175 torr of 3He gas assured that the average distance which the hot tritium species has to travel before it strikes the S i c was less than 1 mm. A calculation28based on the available data on the energy-range irelationships shows that the tritium species loses on the average less than 3% of its initial energy before it impinges on the Sic: surface. If a residual kinetic energy of 0.9 MeV is assumed, the average penetration depth of the tritium in the S i c would be on the order of 0.25 mg cm-2, Therefore, the tritium should have sufficient energy to penetrate into the solid sample so that primary reactions on the surface contribute only to a small fraction' of the overall results. One of the most interesting results of this investigation seems to be that even if the S i c is evacuated at 1400 "C prior to tritium bombardment, a majority of the tritium that has penetrated into the S i c sample can be removed as HTO upon heating of the sample under vacuum. This however occurs only if no apparent hydrogen source such as Hz or HzO additive is available and if the amount of residual gas impurities in the samples is greatly reduced by prior degassing at high temperatures. If a hydrogen source is furnished either in the form of HzO or Hz, or in form of residual gases still present in the sample a fraction of the tritium can be removed from the Sic at elevated temperatures in the form of H T and CH3T. While at lower temperatures (600 "C), only small amounts of tritiated products are produced regardless of the additive used, at 1000 "C a significant amount of the tritium can be removed in form of these gaseous products (Table 1). The exposure of thoroughly degassed samples of S i c to HzO and H2 environments before the addition of He-3 and irradiation yielded pertinent information regarding the initial attack of the recoil tritium on the S i c and the influence of impurities which might be present in the Sic. The results that are shown in Table I11 indicate that in the case of HzO exposure little direct interaction occurs to form H T or CH3T. If, however, such a sample is heated to 600 "C for 4 h, significant amounts of both H T and CH3T are produced. The amount of H T and CH3T formed under these conditions are very similar to the results obtained from heating samples at 600 "C which had received nto preirradiation treatment (Figure 2). This would indicate that HzO which is adsorbed on or in the nondegassecl S i c samples may be the predominant factor in the formation of the tritiated gases. In the case of Hz gas exposure before tritium bombardment, the production of small amounts of H T and CH3T when the sample was heated at 600 "C after irradiation indicates that H2adsorbs on the S i c only to a limited extent. It would appear then in this investigation, that if the recoil tritium has penetrated into the S i c matrix and become incorporated in the interior, some mechanism exists for its facile removal. Past experience with the reactions of hot atoms such as carbon-11 or carbon-14 atoms in seems to indicate that even in such rigid lattice structures as magnesium nitride or sodium chloride the energetic (hot) atom causes momentarily the break up of existing formations and new bonds are being formed between the implanted atom and the matrix components. In the present case one might expect to observe C-T or Si-T bonds formed as a result of the tritium bombardment. This assumption is supported by the result of previous experiments, where in the bombardment of S i c with 15-keV hydrogen ions evidence for the formation of Si-H
The Journal of Physical Chemistry, Vol. 83, No. 4, 1979 461
and C-H bondsz3has been found. The formation of Si-H bonds has been observed following the bombardment of Si with H+ and D+ of energies of 15-38 keVa31However, also of significance are the results obtained from the bombardment of 6H and 4H S i c with 138-keV hydrogen ions,32i33which demonstrate the formation of C-H bonds a t a bombardment-induced Si vacancy. In this case, however, no evidence for the formation of Si-H bonds was obtained. Assuming that tritium indeed forms such bonds with silicon or carbon, two mechanisms, which could be operative in the removal of the tritium from the Sic, would have to be considered. The first mechanisms involves the interaction of the S i c bound tritium with some reactant gas (H-R), which may be present or have been diffused into the bulk Sic, in the interior of the S i c sample as shown in eq 1and 2. (It is +C-T H-R HT, CH3T (1) +Si-T H-R HT, SiH3T (2) unlikely, however, that SiH3T would be observed because SiH4 is known to decompose to silicon and Hz a t temperatures greater than 380 0C.34) This mechanism would be controlled by the diffusion characteristics of (1)the reactant gases, Hz or HzO, which have to diffuse to the S i c bound tritium to react and ( 2 ) the compounds formed in these reactions. Very few such data are available for the systems under consideratiion. One could, however, reasonably expect, that HzO is less capable of diffusing into S i c than Hz, which would be consistent with the lower yields of gaseous products released upon heating (6 h at 1000 "C), if HzO is the additive compared with those observed with H2 additives (Table 111). Furthermore, the interaction of HzO with Sic bound tritium might involve the prior conversion of HzO to Hz (eq 3) on the S i c surface, a reaction which occurs a t a S i c + 3H20 Si02 + CO + 3Hz (3) significant rate only at temperatures greater than 1000 0C,35-37The formation of CH4 has also been postulated to result from the interaction of HzO on Sic, although tbe experimental evidence is lacking.38 If Hz gas is the reactant involved, the tritiated products could result from a simple, possibly catalyzed, hydrogen-tritium exchange reaction between the S i c bound tritium and H2 or CH4. There is evidence that CHI can be produced from the interaction of H2 with The second mechanism would postulate that the tritium atoms are re-emitted upon heating of the sample, i.e., that they diffuse to the S i c surface, where the reaction wiith the additives or surface impurities take place. The yields and product distribution is then determined by the kinetics and thermodynamics of the various possible reactions. This mechanism is similar to that postulated by Braganza et a1.16for the methane formation during irradiations of S i c with 20-keV D+ ions. We would have to assume that tritium atoms which have become incorporated in the interior of the S i c and have possibly formed C-T bonds will become mobile (thermally induced bond scission) and diffuse to the surface of the Sic. Such a thermally induced bond scission of C-halogen in crystals was recently observed in our l a b o r a t ~ r y In .~~~~~ the presence of hydrogen sources these tritium atoms will combine with available H atoms on the surface or form if thermodynamically possible tritiated methane. The observed temperature dependence of the products which show a falloff in methane at higher temperatures (Figure 2) is consistent with the thermodynamic instability of methane a t these temperatures.
+ +
-+
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T. E. Boothe and H. J. Ache
The Journal of Physical Chemistry, Vol. 83, No. 4, 1979
Another argument in favor of this second mechanism appears to be the fact that a degassed sample of tritium bombarded S i c when heated at high temperatures under vacuum conditions after irradiation in the absence of a hydrogen source results in the formation of HTO only. According to the second mechanism the HTQ could be produced from the interaction of the tritium with some oxygen source such as SiOz or &(OH), on the surface of the sample. SiOz or Si(OH), could result from the reaction of adsorbed oxygen with the Sic under the initial degassing temperature of 1350 0C.35338,40 Oxygen and small amounts of HzO leading to a surface oxidation of the S i c samples could also be introduced from the walls of the quartz ampoules. Once the HTO is formed it is unlikely that complete reduction would occur as shown in eq 3, because a sufficient amount of HTO would not be available, due to its rapid removal under the high vacuum conditions used in these experiments. Comparison of S i c to Graphite. The fact that the tritium is easily removed from both the K-T S i c and the S i c powder when the samples are heated under vacuum is in direct contrast to the results obtained using graphite samples as shown in Figure 1. However, the retention of the tritium in the graphite is consistent with previous result^,^^^^ which demonstrated that if a graphite sample is thoroughly degassed by heating it to 1100 "C under vacuum prior to tritium bombardment, no labeled gaseous products were released upon subsequent heating of the graphite to 1000 "C. Indeed, as shown in Figure 1, the retention of the tritium a t this temperature in the H-451 graphite is essentially 100%. In terms of the second mechanism the basic difference in the behavior between the graphite and silicon carbide samples can be explained by the availability of an oxygen source in form of SiOz or &(OH), for the out-diffusing tritium atoms to form HTO in the case of Sic, which can be removed, while no such source is available in the case of graphite. Surface oxidation of graphite results in the formation of CO or COz, gaseous products, which will not remain adsorbed to a significant degree on the graphite surface under these experimental conditions. Thus tritium diffusing to the graphite surface will remain chemisorbed for lack of a suitable reaction partner, with which it can react to form a gaseous product. Also, in relation to the previously studied graphite samples,24one significant difference between the behavior of graphite and Sic appears to be that the overall reactivity of the S i c bound tritium toward HzO and H2is somewhat less, especially at 600 "C. It also appears that in the reactions of the additives with the tritium in Sic, much more CH3T relative to H T is formed; for example, the HT/CH3T ratio obtained with the graphite samples was 39.8 and 31.3 for reactions with H20 and Hz, respectively, whereas the ratio produced with Sic is 8.9 and 7.1 for HzO and Hz at 600 "C. This is due not only to a decrease in the amount of HT in the S i c samples but mainly due to a large increase in the CH3T produced. These results seem to emphasize the effect of the nature of the matrix on the relative product distribution, which shows that the formation of the compounds is not entirely thermodynamically but also kinetically controlled. A conclusive answer, however, as to which one of these two mechanisms or whether a combination of both is operative, will have to wait for the results of a systematic study in which the tritium recoil species have been moderated to lower kinetic energies and thus reduced penetration depth, which would allow a more precise evaluation of the diffusive behavior of the species involved.
Such an investigation has been recently initiated in our laboratory. In relation to the CTR operation using S i c as a protective component the results of this work would indicate that the highly energetic (hot) fraction of the hydrogen species escaping the plasma of the CTR would initially become strongly incorporated into the Sic. However, these species can be subsequently released back into the plasma at elevated temperatures as HTO if no hydrogen source is introduced into the system. Under actual CTR operation conditions, however, where the flux of D+ and T" which reaches the first wall has been estimated to vary from lOI3 to 10l6cm-z/s-l depending on the type of reactor with kinetic energies between thermal to 580-keV,2,41~42 an accumulation of hydrogen species on or near the surface can be expected, in which case the implanted highly energetic tritium which represents only a relatively small fraction of the tritium escaping the plasma would be predominantly released in form of HT and GH3T if the wall is kept at temperatures of greater than 600 "C. At lower temperatures S i c would act as a sink for highly energetic tritium species in the CTR reactor. References and Notes This work was supported by the U S . Energy Research and Development Administration. For general reviews see, e.g., (a) F. L. Vook, H. K. Birnbaum, T, H. Blewitt, W. L. Brown, J. W. Corbett, J. H. Crawford, A. N. Goland, G. L. Kulcinski, M. T. Robinson, D. N. Seidman, and F. W. Young, Jr., Rev. Mod. Phys., Suppl. 3 (1975); (b) F. L. Vook, Phys. Today, 28, 34 (1975); (c) G.L. Kulcinski and G. A. Emmert, J . Nucl. Mater., 53,3 1 (1974); (d) S. 0. Dean et al., "Status and Objectives of Tokamak Systems for Fusion Research", WASH-1295, 1974; (e) R. Behrisch, J . Nucl. Fusion, 12, 695 (1972). 0. L. Kulcinski, R. W. Conn, G. Lang, L. Wittenberg, J. Kesner, and D. C. Kummer, UWFDM-108, University of Wisconsin, Aug 1974. G. L. Kulcinski, R. W. Conn, and G. Lang, J . Nucl. Fusion, 15, 327 (1975). R. W. Conn and J. Kesner, J. Nucl. Fusion, 15, 775 (1975). G. P. Lang and V. L. Holmes, J . Nucl. Fusion, 18, 162 (1976). G. A. Beitel, J . Vac. Sci. Techno/., 6, 224 (1969). K. Flaskamp, G. Stocklin, E. Vietzke, and K. Vogelbruch, "Sticking Coefficient of Atomic Hydrogen on Graphite", paper presented at the V I International Symposium on Molecular Beams, Nordwijkerhout, April 18-22, 1977. K. J. Dietz, E. Geissler, F. Waelbroeck, J. Kirschner, E. A. Niekisch, K. G. Tscherisch, G. Stocklin, E. Vietzke, and K. Vogelbruch, J. Nucl. Mater. 63, 167 (1976). M. Balooch and D. R. Olander, J . Chem. Phys., 63,4772 (1975). 8.J. Wood and Henry Wise, J . Phys. Chem., 73, 1348 (1963). R. K. Gould, J . Chem. Phys., 63, 1825 (1975). P. S. Gill, R. E. Tomey, and H. C. Moser, Carbon, 5, 43 (1967). P. L. Walker, F. Rusinko, and L. G. Austin, A&. Caul., 11, 133 (1959)). S.K. Erents, C. M. Braganza, and G. M. McCracken, J. Nucl. Mater., 63,399 (1976). C. Braganza, G. M. McCracken, and S. K. Erents, Proceedings of the International Symposium on Plasma Wall Interaction, Julich, Oct 1976, p 257. R. Behrisch, J. Bohdansky, G,H. Oetjen, J. Roth. G. Schilling, and H. Verbeek, J . Nucl. Mater., 60,321 (1976). N. P. Busharov, E. A. Gorvatov, V. M. Gusev, M. I. Guseva, and Yu. V. Martynenko, J . Nucl. Mater., 63,230 (1976). J. Roth, J. Bohdansky, W. Poschenrieder, and M. K. Sinha, J . Nucl. Mater., 63,222 (1976). J. Bohdansky, J. Roth, and M. K. Sinha, Proceedings of the Ninth Symposium on Fusion Technology, July 14-18, 1976, I(. H. Schmitter, Chairman. J. N. Smith, Jr., C. H. Meyer, Jr., J. K. Layton, G. H. Hopkins, and L. H. Rovner, J . Nucl. Mater., 63,392 (1976). S.Veprek, M. R. Hague, and H. R. Oswald, J . Nucl. Mater., 63,405 (1976). R. B. Wright, R. Varma, and D. M. Gruen, J . Nucl. Mater., 63,415 (1976). T. E. Boothe and H. J. Ache, J. Phys. Chem.. 82, 1362 (1978). A. E. Finholt, A. C. Bond, Jr., K. E. Walzbach, and H. I. Schlesinger, J . Am. Chem. Soc., 69, 2692 (1947). 5. Diehn, A. P. Wolf, and F. S. Rowland, 2. Anal. Chem., 204, 112 (1966). E. G, Rachow in "Comprehensive Inorganic Chemistry", Voi. I, Pergamon Press, Oxford, 1973, p 1421. See, e.g., G. Friedlander, J. W. Kennedy, and J. M. Miller, in "Nuclear and Radiochemistry", 2nd ed., Wiley, New York, 1966, p 95 f f . H. J. Ache and A. P. Wolf, J . Phys. Chem., 73, 3499 (1969).
Physical Properties of TBA-H,O
Mixtures
(30) R. D. Finn, A. P.Wolf, and H. J. Ache, J . Phys. Chem., 73,3928 (1969). (31) D. M. Gruon, R. Varma, and R. E. Wright, J. Chem. Phys., 84, 5000 (1976). (32) L Patrick and W. J. Choyke, Phys. Rev. E , 8, 1660 (1973). (33) W. J. Choyke and L. Patrick, Phys. Rev. E , 10,3214 (1974). (34) T. R. Hogness, T. L. Wilson, and W. C. Johnson, J. Am. Chem. Soc., 58, 108 (1936). (35) I. A. Yavorski, V. I. Elchin, G. G. Gnesin, and G. S. Oleinik, Porosh. Met., 8, 59 (1968).
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(36) B. E. Deal, J. Necfrochem. SOC.,110, 527 (1963). (37) A. I. N. Frantsevich in "Silicon Carbide", A. I. N. Frantsevich, Ed., Consultants Bureau, New York, 1970,pp 1-6. (38) S.C. Singhal, J . Am. Ceram. SOC.,59, 81 (1976). (39) M. Kumagawa and H. Kuwabara, Jpn. J. Appl. Phys., 8,421(1969). (40) F. A. Guliransen, K. F. Andrew, and F. A, Brassart, J. Nectrochem. SOC., 163, 1311 (1966). (41)M. Kaminsky in Radiation Test Facilities for the CTR Surface and Materials Program, C. J. Persiani, Chairman, 1975,p 16 ff. (42) R. W. Conn and Y. Kesner, J . Nucl. Mater., 63, 1 (1976).
Light-Scattering Study of Clathrate Hydrate Formation in Binary Mixtures of ferf-Butyl Alcohol and Water. 2. Temperature Effect Kenji Iwasaki and Tsunetake Fujiyarna" Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Setagaya-ku, Tokyo 158, Japan (Received September 1, 1978) Publicaflon costs asslsfed by the Ministry of Education
Temperature dependence of the concentration fluctuations, N ( AX)^), was observed for binary mixtures of tert- butyl alcohol (TI3A)--waterand dioxane-water at various concentrations by the use of lighbscattering spectra. In TBA-water mixtures the rapid increase of N ( AX)^) values due to the temperature increase in the range of 17.5-63 "C was observed for XTBA > 0.05, where XTBA is the mole fraction of TBA. This result is reasonably interpreted by considering the breakdown of the clathrate hydrate-like structure and the growth of the water part of the local structure which is composed of water and TBA molecules. In contrast to the result observed for TBA-water mixtures, the N ( AX)^) values observed for dioxane-water mixtures are small and hardly change magnitude in the temperature range 18-49.5 "C. The analysis of this result suggests that the clathrate hydrate-like structure does not exist in dioxane-water mixtures.
Introduction Anomalous physical properties of tert-butyl alcohol (TBA)-water mixtures have been studied by many workers and their results suggest that TBA--water mixtures are in a particular state of mixing. In the previous publication the concentration fluctuation observed by light-scattering spectra for TBA-water mixtures at 24 "C has been reported1 The concentration dependence of the concentration fluctuation, N ( AX)^), observed for TBA-water mixtures at 24 "C is characterized by the following: (i) the N(C AX)^) value is small in the concentration range of 0 I xTBA 1/22 in TBA-water mixtures. These local structures were considered to have clathrate hydrate-like structures because the value of the molecular ratio of TBA to water in these local structures is close to that of the solid hydrate of TBA.2-4 The present study concerns itself with a further study of the local ,structures formed in TBA-water mixtures through the observation of the temperature dependence of the concentration fluctuations. (Pur interests lie in the temperature effect on hydrate-like structure formation and on the mixing state of TBA-water mixtures. As a reference, the Concentration and temperature dependence for dioxane-water mixtures is observed and analyzed, because 0022-365417912083-0463$0 1 .OO/O
the formation of the hydrate-like structure in the dioxane-water mixtures has also been discussedS2The changes in local structures due to a temperature increase are discussed in the last paragraph in connection with the temperature dependence of the mixing enthalpy.
Experimental Section Light-scattering spectra were recorded at a scattering angle of 90" by the use of a spectrometer designed and constructed in our l a b ~ r a t o r y . ~The spectrometer is composed of a He-Ne gas laser source (NEC, GLG108,50 mW) and a pressure scanning Fabry-Perot interferometer. tert-Butyl alcohol and dioxane purchased from Wako Pure Chemical Industries, Ltd. were used without further purification and water was triply distilled. The binary mixtures of TBA-water and dioxane-water were made dust-free with a Nuclepore filter with a pore size of 0.1 Mm, and a Millipore filter FG with a pore size of 0.2 pm,respectively. The dust-free state was certified by observing the L-P ratio of water to be zero. The preparation (filtration and injection of a sample into a cell) was repeated three times and the sample which showed the least L-P ratio was used for further experiments. The sample temperatures were controlled by a high temperature cell which was made by the authors. A temperature constancy of fl "C was obtained with the apparatus. The lightscattering spectra of TBA-water mixtures were observed at temperatures of 17.5, 24, 37, 49.5, and 63 "C. The light-scattering spectra of dioxane-water mixtures were observed at temperatures of 18 and 49.5 "C. The refractive indices of the sample solutions were measured at each temperature by means of an Atago Abbe refractometer. 0 1979 American
Chemical Society
