Properties of tritium and 3He in metals

Journal of the Less-Common

PROPERTIES

Metals, 131 (1987)

OF TRITIUM

263 - 273

263

AND 3He IN METALS*

R. L#SSER lnstitut fiir Festkiirperforschung,

Rernforschungsanlage

Jiilich, D-51 70 Jiilich (F.R.G.)

(Received May 5, 1986)

Summary Several aspects of the behaviour of tritium in metals such as the localized vibrations of tritium in niobium and the solubility, phase boundaries and superconducting temperatures of tritium in palladium are compared with the corresponding behaviour of the stable hydrogen isotopes. Properties of 3He produced by the “tritium trick” in metal tritides are presented: precipitation of 3He in bubbles, swelling of metal-tritide samples and changes in lattice parameter and in the width of the rocking curve as a function of aging time.

1. Introduction Because of the unfilled electron shell the hydrogen atom reacts with almost all element-s of the periodic table resulting in metallic, ionic or covalent hydrides. In contrast, helium with its filled electron shell may form only a limited number of compounds with mainly van der Waals binding character and shows an extremely low solubility in metals. The loading of metals (M) with helium from the gas phase is difficult because of the very high helium equihbrium pressures [ 11. Most studies of the behaviour of helium in metals have been performed using metals charged with helium by implantation [ 21. Another technique (the so-called tritium trick) uses tritiurn decay: the superheavy hydrogen isotope tritium decays with a half-life of Tli2 = 12.361 years according to T --+

3He++ fip + G, + 18.6 keV

(1)

into a 3He+ ion, an electron fl- and an antineutrino i;,. Charging metals with tritium yields ternary systems in which tritium decay causes an increase in the atomic helium COnCentratiOn CHe

*Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides V, Maubuisson, France, May 25 - 30, 1986. 0022-5088/871$3.50

0 Elsevier SequoialPrinted in The Netherlands

264

and a decrease in the atomic tritium xT

= XT, 0 exp?-t

ln(2)~~~,~~

concentration

XT (3)

where XT, o is the initial atomic tritium concentration and t is the time. Metals are loaded with tritium from the gas phase in a similar way as for the stable hydrogen isotopes with only the modifications necessary to ensure safe handling of the radioactive hydrogen isotope [3,4]. Changes in the physical properties owing to the presence of helium can be studied either in the ternary alloys or after removal of the tritium. The latter is achieved by degassing the metal Wide sample in an ultrahigh vacuum system at temperatures as low as possible or by welding it to other metals with a higher affinity for tritium. In this paper selected results obtained for tritium and 3He in metals are given in Sections 2 and 3 respectively. No attempt has been made to present all the new information about tritium and 3He in metals; the choice given here is arbitrary. Some concluding remarks are given in Section 4.

2. Properties

of tritium

in metals

In this section a few of the physical properties of tritium in metals determined in the last years for the first time are discussed. Most of these properties have been known for the stable hydrogen isotopes for more than 10 years and have been measured by various groups (see review books [ 51). This lack of knowledge in the case of tritium is mainly owing to difficulties in handling large amounts of tritium, in conserving a high purity of the T2 gas and in overcoming the disturbances of 3He in the metal tritide sample under study. 2.1. Tritium vibmtions in niobium In the last decade neutron scattering has been a very impo~~t tool to determine the structure, the occupied interstitial positions and the vibrational energies of protium and deuterium in metals. The vibration spectrum of tritium in a metal was first published in a paper a few years ago [6] and the metal chosen was niobium. The vibration spectra of hydrogen and deuterium are well known [ 7) and oxide layers at the surface hinder the loss of hydrogen atoms from the metal 133. Such measurements are difficult because the cross-section of tritium (1.5 barn) is about two orders of magnitude smaller than that of protium and samples with an activity of about 37 TBq must be handled. This requires that the amount of protium in the T, gas should be as low as possible. Figure 1 shows the neutron spectra for NbToez, NbDo.72 and NbHo.,, at 295 K [6]. The NbDoeT2 and NbHo.32 spectra agree with published data [ 71 demonstrating that the existence of less than 15% ty phase does not significantly affect the spectra attributed to the p phase. In the spectra of NbHo.s,, and NbDo,,2 two peaks are observed which can be assigned to the optical vibrations of the hydrogen isotopes in the slightly

265

Nb To.2 3H

SO

3H

100

ENERGY

I50 (meV)

Fig. 1. Inelastic neutron spectra of NbT 0.2, NbD0.725 and NbHc.32 at 295 K 161.

distorted tetrahedral sites in the orthorhombic P-phase structure. Because of the double degeneracy of the higher vibrational modes the intensity of the higher peak is about twice that at the lower energy. Four peaks are seen in the NbTOe2spectrum. The two dominant peaks in the middle are caused by the optical vibrations of tritium atoms in the j3 phase of the Nb-T system and are shifted to lower energies compared with the peaks of the deuterium and protium spectra owing to the larger mass of the tritium atoms. The small peak on the right is attribu~d to protium and occurs at an energy about 5 meV higher than the corresponding position of the protium peak in the /3

266

phase of isotopically pure NbH,. This peak is due to hydrogen impurities in the T, gas employed to load the niobium sample. A similar shift in energy of protium impurities in PdD, has been recently reported 181. The small peak at the left side is attributed to vibrations of about 1 at.% 0 atoms present in the niobium. The energies of the various isotopes are not proportional to the inverse of the square root of the masses indicating that the simple harmonic model is not appropriate to describe the system and that anharmonic contributions to a parabolic potential have to be taken in account. 2.2. Tritium in palladium ~though the Pd-H and Pd-D systems have been studied for 120 and 50 years respectively, reliable ~fo~ation on the Pd-T system has been obtained only in the last few years. In the following we will discuss the solubility, phase boundaries and superconductivity in the Pd-T system. The diffusion of tritium in metals has recently been reviewed [ 91. 2.2.1. Solubility and phase boundaries of the Pd-T system The solubility in the (x phase of the Pd-X (X z H, D, T) system is best described by the Sievert constant which in this paper is defined as the square of the concentration divided by the equilibrium gas pressure at infinite dilution. The values of the Sievert constant determined by LHsser and Powell [lo] are shown as a function of the reciprocal of the temperature in Fig. 2 and are compared with the results for protium and deu~~um obtained by other groups [11,12]. It can be seen that the solubility decreases with the mass of the hydrogen isotope at constant temperature. Desorption isotherms taken at 70, 80 and 90 “C for protium, deuterium and tritium in palladium are plotted in Fig. 3 (13, 141. Again a large isotopic effect is observed. In all three phases the decomposition pressure of tritium is higher than those of D, or H, for the same concentration and temperature. The isotopic dependence of part of the phase boundaries of the Pd-X (X E H, D, T) system is presented in ref. 15. The CY-+ (x + p phase boundary moves to higher concentrations with increasing hydrogen isotope mass whereas the fi + (Y+ /3 phase limit is shifted to lower concentrations in the sequence protiurn, deuterium, tritium. As a consequence the miscibility gap shrinks and the critical temperature decreases in the sequence protium, deuterium, tritium. In a recent paper by Oates et al. [ 161 the behaviour of protium, deuterium and tritium in paliadium discussed above and the critical temperatures for protium and deuterium in palladium were correctly modelled. The Sievert constant determined by L&ser and Powell [lo] and the excess chemical potential for protium measured by Kuji et al. [17] are used in the model and it is assumed that the Einstein temperature 19,(x) varies linearly with concentration according to 0,(x) = 19; + @x where 13; and $I were determined from inelastic neutron scattering experiments performed for various concentrations of protium and deuterium in palladium [ 18, 191. Using now only the isotopic dependence of the optic modes in an harmonic approximation this model gives excellent agreement with the expe~en~ data for the o-, (a + p)- and P-phase regions of the Pd-D and Pd-T systems.

267

Fig. 2. Sievert’s constant Kx (X 2 H, D,T) defined as the square of the hydrogen concentration divided by the equilibrium pressure at infinite dilution as a function of the reciprocal of the temperature in palladium [lo]: 0, data reported by Powelf; n, data reported by LBisser; 0, data reported by Wicke and Nernst [ll]; w, data reported by Clewley et al. [12]. Fig. 3. Desorption isotherms for protium, deuterium and tritium in palladium taken at 70,&J and 90 “C [13,14].

In 1972 Skoskiewicz [20] discovered superconductivity in PdH,. Only one year later the reverse isotope effect in the superconducting transition temperature T, was reported by Buckel and Stritzker [21] who observed a higher T, for PdD, than for PdH,. In 1984 Schirber et al. 1223 published preliminary results for the superconducting transition temperature for PdT,. Their palladium powder sample was loaded with Tz gas up to pressures of 700 bar at room temperature, One-third of the sample was mixed with tin powder to avoid heating of the PdT, sample owing to the p- decay of tritium, because the temperature was measured outside the pressure vessel. The tin thermometer indicated

268

that the temperature inside the vessel was about 0.2 K higher than at the outer surface. Owing to the expectation that the PdT, powder not in contact with tin is probably warmer than the PdT, mixed with tin the values given in Fig. 4 represent a lower bound of T, for PdT,. In addition, the T, values for PdH, and PdD, reported by Schirber and Northrup [23] are also shown. In spite of these uncertainties in the absolute T, values the determined !I’, values for PdT, follow the trends observed earlier in the T, values for PdH, and PdD,. The large isotope effect is described by the following inequality for a given concentration x: TcH(x) < TcD(x) < TeT(x)

(4)

Electron-phonon effects associated with optic modes [ 241 are considered to be the main origins of the reverse isotope effect in the superconducting temperature, whereas zero point motion effects [ 251 play a minor role. 2.3. Phase diagram for the V-T system The phase boundaries of the Pd-X (X = H, D, T) system [ 14,151 show only a weak dependence on the hydrogen isotope. In contrast, the V-H and V-D phase diagrams are known to show a large isotopic dependence [26], and a large additional shift of the solvus line to higher concentrations in the case of VT, has been reported [27] recently. In the meantime we also determined the phase boundaries at higher concentrations [ 281. Figure 5 shows the first presentation of the VT, phase diagram together with the wellknown phase boundaries of the V-H and V-D systems. The full lines correspond to well-characterized phase limits, whereas the broken lines were obtained by ex~apolation of the full lines using the known behaviour of the VD, phase diagram. This is justified because it can be concluded from the full lines in Fig. 5 which represent the main part of the VT, phase diagram

9, 8.

l

Pd

n,

- Pd D, .

Pd T,

7.

st -

5.

c’ 4. 32.

-.-

1.

,970 i

0.75

0.80

085

0.90

0.95

f

1.00

0

02

04

06

08

domtcrotio ~T/v~,lD/Vl,~H/vl Fig. 4. Superconducting transition temperatures for PdT, [ 22 ] in comparison with the data for PdH, and PdD, [ 23 ] as a function of hydrogen isotope concentration. X

Fig. 5. Phase diagram for the V-T system (-, V-D(+.=)andV-H(-*-)systems [26].

x

- - -) (28 J compared with those for the

269

that the phase diagram of the V-T and V-D systems have the same topology, whereas the phase diagrams of VT, and VH, are very different for concentrations x > 0.33. A comparison of the V-T and V-D phase diagrams shows that the (x -+ cy + /3 and cy-+ /3 phase transitions are shifted to lower temperatures for tritium compared with those for deuterium in vanadium resulting in a larger a-phase region for VT, than for VD,, whereas the fl + 5 phase boundaries are about equal for deuterium and tritium in vanadium.

3. Properties

of 3He in metals

Owing to the tritium decay described in the introduction, metals which absorb large amounts of tritium can easily be loaded with 3He. In contrast with most helium ion implantation techniques (the exception is subthreshold implantation) and (n, a) reactions in fission reactors the “tritium trick” method introduces no lattice damage in the metal because the averaged recoil energy of about 1 eV transferred to the 3He nucleus is far too low to generate Frenkel defects. In addition, in contrast with the implantation techniques, large samples can be charged homogeneously with tritium and therefore with 3He. Four properties of 3He generated by the “tritium trick” are discussed: bubble structure, swelling of MT, samples, lattice parameter changes and broadening of the rocking curve as a function of 3He concentration. All experiments on these properties were performed at room temperature using metals with oxide layers at the surface to prevent tritium loss [3,14, 271. It is assumed that no leakage of helium from the MT, samples occurs during the early stages of aging. 3.1. Helium bubbles in VT, alloys Using transmission electron microscopic techniques we studied [29,30] the aging behaviour of VTX samples with concentrations x of the two-phase region (x + /3 (see Fig. 5). A rather dense dist~bution of small in~~titi~ dislocation loops was observed in the @-phase domains after 1 - 2 weeks aging at room temperature. Bubbles with a mean diameter of about 1.2 nm were detected after about 13 weeks of aging. The bubble density was about 5 X 1O23 me3. Isolated bubbles were observed in the a-phase regions of the VT, sample. Owing to the high inter4 helium pressure these bubbles can eject dislocation loops. Low angle grain boundaries and dislocations in the LYphase were found to be preferential sites for helium bubble nucleation. These results show that 3He generated in the metal forms bubbles, even without radiation-induced traps such as vacancies, owing to the negligible helium solubility. A mechanism resulting in the generation of helium-filled cavities is the so-&led self-trapping process [ 31 J. The first step is mobile clustering of interstitial 3He atoms. If the cluster size exceeds certain values the stored energy is high enough to generate spontaneously a Frenkel pair. Additional helium atoms may be absorbed by this resulting defect leading to further emission of self-interstitial atoms (SIAs). When the helium bubbles

270

reach a certain size emission of SIA clusters, in particular dislocation loops [32, 331, may be energetically more favorable than single SIA emission. In the vicinity of these defects a change in the averaged tritium concentration may occur. 3.2. Swelling of TaT, samples Using dilatometric measurements Schober ef al. [34] determined the relative length change of two different TaT, samples with XT, e = 0.103 and xT, 0 = 0.42 as a function of time or 3He concentration. The relative length change AL/L or volume change AV/V can be described by the following equation: (5) where the first ‘and second terms on the right-hand side are due to the volume change associated with the generation of helium Frenkel pairs and the decay of tritium respectively. (Au/G!)~ + JHe is the average volume change per decay event divided by the host atom volume Sz. cHe is the concen~ation of helium which can easily be calculated using eqn. (2). Une is the volume occupied by 3He in the high pressure bubble and Aii;r is the volume change caused by the emission of SIAs into the lattice, loops, SIA clusters, dislocations and internal surfaces. If the emitted atoms exist as isolated SIAs, Ai?& = 1.1 [34], whereas in the case where the SIAs are incorporated in interstitial dislocation loops, Aii,/G! = 1.0. The volume change, -A+, when a tritium atom is taken out of the MT, sample. Furthermore, to derive eqn. (5) it was assumed that almost all helium atoms are contained in bubbles (see ref. 2). Using the knowledge of Au&2 and the experimental values of AV/V (see ref. 34) and assuming that A&/G? = 1.0 the volume Une can be calculated. From this value the density and pressure of 3He in the bubbles could be determined and were in good agreement with those determined using electron loss spectroscopy [35] after 4He implantation. A list of the obtained results is given in Table 1. TABLE 1 Results of the swelling experiments [ 341 on TaTX samples at room temperature Sample

(Av/Q)~“~~

vHela

uHe ( x 1oe3’

TaTo. 103 TaTo.42

0.37 0.37

0.53 0.52

9.64 9.98

m3)

P @bar) 53 47

3.3. Lattice parameter and width of the rocking curve of a TaT,. 164sample The lattice parameter and the width of the rocking curve of a TaT, sample with an initial tritium concentration %r, 0 = 0.164 were measured for

273

Fig. 6. Rocking curves for (a) a single crystal of tantalum before loading with tritium and (b) a TaT,He,, sample 6 days (0) and 75 days (m) after loading with tritium at an initial concentration q-e = 0.164 as a function of the scattering angle 8 [36]. Cu K@ rays were used to measure the (222) reflexion of the tantalum b.c.c. structure.

more than a year using a high precision X-ray diffractometer [ 361 at 30 “C. The Bragg peaks of the tantalum crystal before and 6 and 75 days after charging with tritium are presented in Fig. 6. The peaks in the lower part of Fig. 6 are shifted to smaller Bragg angles than those in the pure tantalum crystal owing to the expansion of the lattice caused by the absorbed tritium atoms. The full width at half maximum (FWHM) of the Bragg peaks determined for pure tantalum and for samples loaded with tritium 6 days after charging are equal, whereas the peak measured 75 days after tritium loading shows a considerable increase in the FWHM but appears at almost the same Bragg angle. This means that the lattice parameter changes on aging or on increasing the amount of helium are very small. The following values for the relative lattice parameter change Aala, and for the relative increases in the FWHM AG/G with atomic helium concentration cne were obtained:

WWad

WG/G)

= 0.21 + 0.06 dc Hi? dcue where a, is the lattice parameter of the pure tantalum crystal and AG/G the relative deconvoluted FWHM resulting from helium-induced effects alone. In Section 3.1 the emission of SIAs and the punching of dislocation loops by helium clusters or helium bubbles was described. As long as the emitted metal atoms exist as SIAs or are in~o~ora~d in very small isolated SIA clusters they will contribute to the shift in the Bragg peaks and generate = (0.6 & 1.7) X 10e2

and

272

diffuse scattering around the Bragg peaks. In contrast, they will not contribute to the peak shifts but broaden the Bragg peaks if they are incorporated in a dislocation network. A comparison of these two statements with our experimental results [ 361 shows that the main part of the SIAs are incorporated in the dislocation network and contribute to the dislocation density surprisingly early (for t 2 6 days). An analysis according to Simmons and Balluffi [37] using the experimentally determined relative volume change (see Section 3.2) and lattice parameter change (see eqn. (6)) determined the total concentration of SIAs which contributes to the dislocation network. It was found that about 70% of the emitted SIAs were transferred to the dislocation network. This was in excellent agreement with the analysis of the peak broadening. Therefore we could explain both the peak broadening and the relative change in the volume and in the lattice in a consistent way combining different experiments [ 34, 361. The very small changes in the lattice parameter are fortuitous. This is because of compensating effects: tritium decay would be expected to cause shrinkage of the lattice and this is compensated by the expansion of the lattice owing to the about 30% emitted metal atoms which exist as SIAs or isolated SIA clusters. 4. Conclusions In this paper the isotopic dependence of various physical quantities such as phase boundaries, solubilities, localized vibrations and superconductivity are presented: in all cases the observed shift for tritium followed the trends expected from the known behaviour of protium and deuterium. We also studied the aging behaviour of metal tritide samples using transmission electron microscopy, dilatometric and diffractometric techniques. All data can be consistently explained in terms of the self-trapping process of helium atoms and the emission of SIAs. In addition to the above mentioned changes it may also be possible that the physical properties of tritides change owing to the various defect structures induced by the increasing helium concentration. This may even be an interesting research field in the future with technological importance because metal tritide beds are considered to be promising getters for long-term tritiurn storage. Acknowledgments We thank T. B. Flanagan, W. Jagger, W. A. Oates, T. Schober, G. Thomas, H. Trinkaus and H. Wenzl for interesting discussions and support. In addition, the critical reading of the manuscript by T. Schober is acknowledged.

References 1 H. J. von den Driesch and P. Jung, High Temp. High Pressures, 12 (1980) 635;H. von den Driesch, Rep. 1663, 1980, (Kernforschungsanlage Jiilich, D-5170 (Jiilich).

J.

273 2 H. UIlmaier fed.), Proc. Int. Symp. on Fundamental Aspects of Helium in Metals, Radiat. Eff., 78 (1983) 1 - 426. 3 R. Lasser, K.-H. Klatt, P. Mecking and H. Wenzl, Rep. 1800, 1982 (Kernforschungsanlage Jiilich, D-5170 Jiilich). 4 R. LPsser and K.-H. Klatt, Phys. Rev. B, 28 (1983) 748. 5 G. Alefeld and J. Volkl, Top. Appl. Phys., 28 (1978) 1 - 396; 29 (1978) 1 _ 330. 6 J. 3. Rush, A. Magerl, J. M. Rowe, J. M. Harris and J. L. Provo, Phys. Reu. B, 24 (1981) 4903. 7 D. Richter and S. H. Shapiro, Phys. Reu. B, 22 (1980) 599. 8 J. J. Rush, J. M. Rowe and D. Richter, Phys. Rev. 3, 31 (1985) 6102. 9 H. Kronmiiller, G. Higelin, P. Vargas and R. L&ser, 2. Phys. Ckem. NF, 143 (1985) 161. 10 R. Liisser and G. L. Powell, Phys. Rev. B, 34 (1986) 578. 11 E. Wicke and G. Nernst, Ber. Bunsenges. Pkys. Ckem., 68 (1964) 224. 12 J. D. Clewley, T. Curran, T. B. Flanagan and W. A. Oates, J. Ckem. Sot., Faraday Trans. 1, 69 (1973) 449. 13 R. Llsser, Pkys. Rev. B, 26 (1982) 3517. 14 R. Lgsser, 2. Phys. Ckem. NF, 143 (1985) 23. 15 R. Lasser, J. Pkys. Chem. Solids, 46 (1985) 33. 16 W. A. Oates, R. Lasser, T. Kuji and T. B. Flanagan, J. Phys. Ckem. Solids, 47 (1986) 429. 17 T. Kuji, W. A. Oates, B. S. Bowerman and T. B. Flanagan, J. Phys. F, I3 (1983) 1785. 18 J. J. Rush, J. M. Rowe and D. Richter, 2. Pkys. B: Condensed Matter, 55 (1984) 283. 19 B. M. Geerken, R. Griessen, L. M. Huisman and E. Walker, Phys. Rev. B, 26 (1982) 1637. 20 T. Skoskiewicz, Pkys. St&us Solidi A, 11 (1972) 5123. 21 W. Buckel and B. Stritzker, Phys. Lett. A, 43 (1973) 403. 22 J. E. Schrber, J. M. Mintz and W. Wall, Solid State ~ornrn~n,~ 52 (1984) 837. 23 J. E. Schirber and C. J. M. Northrup, Jr., Pkys. Ret). B, 10 (1974) 3828. 24 D. A. Papaconstantopoulos, B. M. Klein, E. N. Economou and L. L. Boyer, Phys. Rev. B, 17 (1978) 141. 25 P. Jena, J. Jones and R. M. Nieminen, Pkys. Reu. B, 29 (1984) 4140. 26 1‘. Schober and H. Wenzl, in G. Alefeld and J. VSlkl (eds.), Hydrogen in Metals II, Top. Appl. Pkys., 29 (1978) 11. 27 R. Liisser and K. Bickmann, J. Nucl. Mater., 126 (1984) 234. 28 R. Llisser and T. Schober, to be published. 29 W. Jtiger, R. L%sser, T. Schober and G. J. Thomas, Radiat. Eff., 78 (1983) 165. 30 ‘I’. Sehober, R. Lasser, W. Jager and G. J. Thomas, J. Nucl. Mater,, 122 - 123 (1984) 571. 31 W. D. Wilson, C. L. Bisson and M. I. Baskes, Phys. Rev. B, 24 (1981) 5616. 32 G. W. Greenwood, A. J. E. Foreman and D. E. Rimmer, J. Nucl. Mater., 4 (1959) 305. 33 H. Trinkaus, Radiat. Eff., 78 (1983) 189. 34 T. Schober, R. Lasser, J. Golczewski, C. Dieker and H. Trinkaus, Pkys. Rev. B, 31 (1985) 7109. 35 W. Jgger, R. Manzke, H. Trinkaus, R. Zeller, J. Fink and G. Crucelius, Radiat. Eff., 78 (1983) 315. 36 R. Lasser, K. Bickmann, H. Trinkaus and H. Wenzl, Phys. Rev. B, 34 (1986) 4364. 37 R. 0. Simmons and R. W. Balluffi, Phys. Reu., 117 (1960) 52.