Neutron monitoring during evolution of deuteride precipitation in Nb, Ta and Ti

Solid State Communications, Vol. 76, No. 6, pp. 815-819, 1990. Printed in Great Britain.

0038-1098/90 $3.00 + .00 Pergamon Press plc

NEUTRON MONITORING DURING EVOLUTION OF DEUTERIDE PRECIPITATION IN Nb, Ta AND Ti R. Bernabei*, G. Gannelli?, R. Cantelli*, F. Cordero~, S. d'Angelo*, N. Iucci§, P.G. Picozza* and G. Villoresi¶ * II Universifft di Roma, Dipartimento di Fisica, Via E. Carnevale, 1-00173 Roma, Italy ?Universit~ di Perugia, Dipartimento di Fisica, Perugia, Italy C.N.R., Instituto di Acustica "O.M. Corbino", 1-00189 Roma, Italy §Universith di Roma "La Sapienza", Piazzale A. Moro 2, 1-00184 Roma, Italy ¶C.N.R., Instituto di Fisica dello Spazio Interplanetario, Frascati, Italy

(Received 17 March 1990 by E. Tosatti) The neutron emission has been monitored during deuteride precipitation in Nb, Ta, and Ti and during the plastic deformation and crack nucleation and propagation phenomena accompanying this phase transformation. No evidence is obtained that the counts recorded can be attributed to a real physical mechanism. EMISSION of neutrons from Ti and Pd charged with deuterium either electrolytically or from the gas phase has been recently reported [1-6] and attributed to the possible fusion of pairs of interstitial deuterons. Much controversy exists at present about those results and their respective interpretation, and because the emission has been observed only sporadically, a definite attribution about the real nature of the event has not been possible yet; in addition, also contrary evidence has been reported [7-10]. The detected neutron emission does not seem to be due to a particular metal-deuterium system but rather to non-equilibrium conditions, as often suggested. Non-equilibrium conditions due to the evolution of deuteride precipitation were certainly occurring during most of those experiments. If those are the relevant ones, the efficiency of the effect should be remarkably enhanced when precipitation is stimulated by varying the temperature of the doped samples, compared with the case of doping at a fixed temperature. Indeed, in the former case larger quantities of deuterium in solid solution a-phase can be transformed into the precipitate fl-phase in shorter times. In the present paper we report a study of temperature-stimulated deuteride precipitation in Nb, Ta, and Ti while counting neutrons. The two transition metals of group Va, Nb and Ta, have been chosen because of the detailed knowledge of the behaviour of hydride [11] and deuteride [12] precipitation in them. Particular attention was devoted to the physical mechanisms stimulated in the samples, because up to now this aspect was neglected.

Deuterium doping was conducted by thermal treatments at 550 or 400°C for 1-2 h in 99.96% pure deuterium atmospheres and the concentrations ranged from 7 to 43 at %. The cooling and heating runs were performed between room temperature and 77 K at various rates; temperature was measured by a copper-constantan thermocouple. Samples were Nb, Ta, and Ti discs or bars whose characteristics are reported in Table 1. Neutron flux measurements were carried out using a cylindrical FB3 proportional counter (15.3 cm in diameter, 191 cm of active length, 33 1 of effective volume and 765 (cps/n)s -1 cm2 of nominal thermal neutron sensitivity), which was completely shielded by a 2 cm thick polyethylene sheet whole role was also that of moderating the expected 2.45 MeV neutrons. During the experiments the cylinder was vertically installed at 3 cm from the samples. Under the above conditions the total efficiency, e, to detect 2.45 MeV neutrons was estimated to be about 2% by means of the MNCP programme [13]. The output pulses from the BF3 counter was CR-RC shaped to obtain the best signal-to-noise ratio and to keep the dead time below 10/~s. The counting rate was integrated over 1 or 2 min and was recorded continuously. The background neutron emission was recorded for several days, before and between the experiments, and a sample of it is reported in Fig. 1 in count/s (2min integration time). The neutron count rate during the I st cooling after doping and the subsequent heating of PTaHr30 containing 7.8 at %D is reported in the same Fig. 1 together with the temperature curve of the sample vs time. On cooling, deuteride precipi-

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NEUTRON MONITORING IN Nb, Ta AND Ti

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Table 1.

Material

Sample

Shape

Mass

Manufacturer

Purity

Heraeus Haynes Heraeus Kawecki MRC MRC MRC

Reactor 99.9% Reactor 99.9% Marz Marz Marz

(g) Ta Ta Ta Nb Ti Ti Ti

PTaHr30 PTaHy30 PTa3 SNbKal STi 1 STi 2 STi 3

disc disc disc bar bar bar bar

32 70 43 13 6 36 5

accommodation of the misfitting deuteride particles during their growth. It is seen that during evolution of the plastic deformation processes of the experiment of Fig. 1, the neutron counts did not exceed the background fluctuation. Subsequently, the same sample has undergone three cooling-heating runs between room and liquid

tation started at the transformation temperature Tt = 242.4 K, as indicated by the temperature arrest at constant cooling power (due to the exothermic character of the 2nd phase formation; see sketch in Fig. 1) and as predicted by the Ta-D phase diagram [12]. T, is also the temperature at which the dislocation nucleation and multiplication starts, due to the

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N E U T R O N MONITORING IN Nb, Ta AND Ti

Vol. 76, No. 6

817

nitrogen temperature at a rate of about 1.5 K min -| . formation (e.g. 12% volume increase in N b - H ) Afterwards, a thermal treatment at 150°C for 2h produces stress fields at the ends of the precipitate in vacuum followed to let oxygen migrate onto dislo- particles which tend to attract hydrogen and nucleate cations, in order to recover the effects of precipitation new hydride, with consequent occurrence of dislo[12], and the cooling-heating procedure was repeated. cation multiplication processes [15]. Dislocation The concomitant neutron monitoring always gave pile up at the hydride boundary can produce crack signals which did not emerge from the background. nucleation, which then rapidly progresses through or Samples doped at higher D concentrations along the interface of the second phase. Crack propa(PTaHr30:11.6 at %D; PTaHy30:11.7 at %D; PTa3: gation proceeds with the particle growth, which in 14.0 at %; SNbKal: 10.2 and 9.3 at %), were submitted turn is driven by the H(D) flux generated by the stress to thermal cycles at various rates, what produced concentration. many severe precipitations; even in this case no It has been found [16-18] that the nucleation and neutron emission above the background was detected. propagation of microcracks during hydride precipiIn transition metals, hydride (deuteride) precipi- tation is accompanied by intense ultrasound emission tation is a non-equilibrium and diffusion-controlled which is originated by sudden releases of elastic energy process characterized by a rapid formation of mis- in the lattice and whose intensity can reach particularly fitting precipitate particles due to the high H(D) high values. However, this irreversible lattice damagmobility [14]. Hydrogen (deuterium) preferably ing, and the concomitant energy release, does not segregates at the stress fields such as those associated manifest itself during every precipitation, and even a with dislocations, and below the solvus temperature it precipitation from high H(D) concentrations can forms "embrios" or nucleation sites for hydride occur without progression of fracture processes. (deuteride) particles. The local increase in volume and Indeed, for crack formation and growth to occur, change in symmetry accompanying the fl-phase particular conditions must be fulfilled [16, 19]: it was I

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818

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NEUTRON MONITORING IN Nb, Ta AND Ti

shown that the multiple thermal cycling of hydrogenated samples or the severe precipitations can trigger the crack formation and growth, which, however, is progressively stopped during the subsequent precipitations when most of the accommodation has already occurred; therefore, annealing at high temperature and recharging is required for crack propagation to proceed. Thus, in spite of its non-equilibrium character, the crack formation is an event which can be stimulated reproducibly. It has been reported that, in Li, intense electrical potential differences ( ,~ 100 kV) are produced between the two separating surfaces of propagating cracks [20] and if the material is deuterated, gas discharge processes may occur with consequent d-d nuclear reactions and neutron emission. Indications of the occurrence of charge separation across the crack tip during fracture have recently been produced also for a metal (Ti) [21, 22]. Therefore, if cracks are directly involved, the reproducibility for the reported neutron emission must be searched among the conditions necessary to produce fracture processes [16]. In the experiments illustrated below, the above procedure for the crack formation and growth has been followed and neutron emission has been simultaneously monitored. On samples PTa3 and PTa Hy30,

which showed visible cracks from the previous precipitations, an outgassing thermal treatment was carried out at 800° C in vacuum for 1 h, which was followed by a furnace doping (18.7 and 16.2 at %D for the two samples, respectively). During their 1st precipitation, when progression of crack formation and growth occurred [16], neutron monitoring was performed and the results are reported vs time in Fig. 2, together with the specimen temperature and a sample of background emission; no neutron emission was detected. For the Ti specimens a similar procedure was followed. Neutron counting during cooling at different rates of sample STi 2 charged in the "as received" state with 36.4 at %D is reported in Fig. 3. After the outgassing ofSTi 2 and its recharging at 43 at %D, the neutron counts gave similar results. Also thermal cycling of samples STi 1 and STi 3 charged at lower concentrations (5.4 and 5.2 at %D, respectively), gave counts comparable with those from STi2. The non-reproducibility character of the neutron emission from metal-D systems recently reported was generically attributed to the occurrence of nonequilibrium processes, which however were not identified by the various authors. Within the sensitivity limit of our detector, we can affirm that such unknown processes are hardly identifiable with the dislocation multiplication or the crack formation caused by the I

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NEUTRON MONITORING IN Nb, Ta AND Ti

deuteride precipitation, because these processes have been massively and reproducibly stimulated in our investigation, and the neutron counts were included in the background fluctuation. Indeed, considering the characteristics of our counter illustrated above and the features of the neutron emission reporterd [3, 5] (which would manifest itself in bursts of about 100 #s), we can conclude that such intense signals [3] would not have escaped our observation. The present results indicate that, if neutron emission from deuterated systems exists, it should manifest itself at very low levels. In the affirmative case, it cannot be concluded whether this subtle effect is really ascribable to a "cold" fusion reaction or to processes of different physical origin or even to artifacts. REFERENCES 1. 2. 3. 4. 5.

6.

7.

M. Fleischmann & S. Pons, J. Electroanal. Chem. 261, 301 (1989). S.E. Jones, E.P. Palmer, J.B. Czirr, D.L. Decker, G.L. Jensen, J.M. Thorne, S.F. Taylor & J. Rafelski, Nature 338, 737 (1989). A. De Ninno, A. Frattolillo, G. Lollobattista, L. Martinis, M. Martone, L. Moil, S. Podda & F. Scaramuzzi, Europhys. Lett. 9, 221 (1989). P. Perfetti, F. CiUoco, R. Felici, M. Capozi & A. Ippoliti, Il Nuovo Cimento 11, 921 (1989). F. D'Amato, A. De Ninno, F. Scaramuzzi, P. Zeppa, C. Pontorieri & F. Lanza, 1st Annual Conference on Cold Fusion, Salt Lake City, March 28-31 (1990). H.O. Menlove, M.M. Fowler, E. Garcia, A. Mayer, M.C. Miller, M.A. Paciotti, R.R. Ryan & S.E. Jones, Ist Annual Conference on Cold Fusion, Salt Lake City, March 28-31 (1990). N.S. Lewis, C.A. Barnes, M.J. Heben, A. Kumar, S.R. Lunt, G.E. McManis, G.M. Miskelly,

8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

21. 22.

819

R.P. Penner, M.J. Sailor, P.G. Santangelo, G.A. Shreve, B.J. Tufts, M.G. Youngquist, R.W. Kawanagh, S.E. Kellogg, R.B. Vogelaar, T.R. Wang, R. Kondrat & R. New, Nature 340, 525 (1989). P.B. Price, S.W. Barwich & W.T. Williams, Phys. Rev. Left. 63, 1926 (1989). L. Bruschi, M. Santini, G. Torzo & G. Nardelli, Europhys. Lett. 10, 303 (1989). S.E. Segre, P. Batistoni, L. Bertalot, L. Bettinali, M. Martone & S. Podda, Technical Report, ENEA RT/FUS/89/12. O. Buck, D.O. Thompson & C.A. Wert, J. Phys. Chem. Solids 32, 2331 (1971). G. Cannelli & R. Cantelli, Appl. Phys. 3, 325 (1974). P. Belli, R. Bernabei, S. D'Angelo, M.P. DePascale, L. Paoluzi & R. Santonico, II Nuovo Cimento 101, 959 (1989). G. Alefeld & J. Vtlkl, eds. Hydrogen in Metals, Springer ~(1978). M.S. Rashid & T.E. Scott, J. Less Common Met. 31, 377 (1973). G. Cannelli & R. Cantelli, J. Appl. Phys. 50, 5666 (1979). G. Cannelli & R. Cantelli, J. Appl. Phys. 51, 1955 (1980). G. Cannelli & R. Cantelli, Scripta Met. 14, 731 (1980). G. Cannelli & R. Cantelli, J. Physique 42, C5-947 (1981). V.A. Klyuev, A.G. Lipson, Yu. P. Toporov, B.V. Deryaguin, V.I. Lushchikov, A.V. Strelkov & E.P. Shabalin, Soy. Tech. Phys. Lett. 12, 551 (1986). B.V. Deryaguin, A.G. Lipson, V.A. Kluev, D.M. Sakov & Yu. P. Toporov, Nature 341, 492 (1989). J.T. Dickinson, L.C. Jensen, S.C. Langford, R.R. Ryan & E. Garcia, submitted to J. Mat. Research (1989).