Absorption of deuterium in titanium plates induced by electric discharges

Vol. 23,No. 10,pp. 8855890,1998 I(: 1998InternationalAssociationfor HydrogenEnergy ElsevierScienceLtd All rightsreserved.Printedin GreatBritain

hr. J. Hydrogen Energy,

Pergamon PII: s036&3199(97)@0157-2

036%3199/98

$19.00+0.00

ABSORPTION OF DEUTERIUM IN TITANIUM PLATES INDUCED BY ELECTRIC DISCHARGES M. G. OLAYO,* G. J. CRUZ, L. BALDERAS, L. MELENDEZ, A. CHAVEZ, R. VALENCIA, A. FLORES and R. L6PEZ

E. CHAVEZ,?

Departamento de Fisica, Instituto National de Investigaciones Nucleares, Apdo. Postal 18-1027,Col. Escandon, CP 11801,Mexico, D.F., Mexico; t Also at ICE-ESIME, Instituto Politecnico National, UPALM, Col. Lindavista, Mexico, D.F., Mexico

Abstract-In this work, the absorption of deuterium in titanium plates induced by electric discharges is studied. The objective was to measure the amount of deuterium that is absorbed in the titanium structure under the influence of an electric discharge. The ionization and the free radicals produced by the electric field act as a promoter to the absorption mechanism. Thus, the absorption can be enhanced by the use of an electric discharge. The results indicated that there was a rapid desorption of deuterium at the beginning of the discharge, followed by an additional absorption to higher levels than those before the discharge. The additional absorption of deuterium was about 20% of the initial absorption. When the titanium was completely saturated with the gas, no additional absorption occurred through the electric discharges. As a result of the absorption conditions of deuterium in the titanium structure, anomalous emission of neutrons was recorded as tracks in a CR39 type plastic solid-state nuclear-track detector. Q 1998 International Association for Hydrogen Energy

1. INTRODUCTION The absorption of hydrogen in metallic structures has been studied under different thermodynamic conditions for several systems. However, the mechanisms involved in the process have been explained by chemical interactions of the hydrogen atoms and the metallic structures. The chemical interactions involve the electronic cloud surrounding the atoms. In this process the last electrons share orbits to construct bonds between the atoms participating in the interaction. The bonds can be strong enough to make the atoms join in chemical compounds, or so weak as to link the atoms in physical sorption. There is evidence that hydrogen atoms move inside the metallic lattice to fill part of the structure, and that not all the possible sites are occupied. Consequently, there are always vacanciesin the crystal cells. Hydrogen atoms, thus, have the freedom to move from one site to another depending on the energy incoming to the system. Another interaction that has not beenconsidered is the size of the nucleus of hydrogen atoms. Deuterium moves inside the metallic structure, as hydrogen does in similar thermodynamic conditions, with one exception, namely that evidence exists of anomalous emission of sub-atomic particles in the process. This hydrogen isotope has one * Author to whom correspondence should be addressed.

proton and one neutron in the nucleus, hydrogen has only one proton in the nucleus, and both are stable atoms. The mass of the nucleus influences the size of the entire atom and this change modifies the interaction of deuterium and titanium with respect to similar hydrogentitanium systems. Understanding this phenomenon requires considering the role of the additional neutron in the hydrogen atom. The additional neutron produces some kind of instability among the surrounding nuclei. Some studies have reported that when both elements, titanium and deuterium, are put together under specific thermodynamic conditions, deuterium penetrates in the metallic structure with a releaseof heat and neutrons in the process[l-l 31.The releasedheat can be explained by physical sorption. However, no adequate explanation has yet been given about the neutron phenomenon. In this work, we studied the absorption conditions for deuterium in titanium plates under the influence of electric discharges. We looked for the conditions under which deuterium is introduced in the titanium structure, and we “pushed” more deuterium atoms into the titanium using electric discharges. Many works have studied the thermodynamic conditions about the absorption in the hydrogen-titanium system. However, little interest has been given to the absorption conditions for the deuterium-titanium system [ 14-171,particularly with the use of electric discharges as an absorption promoter. The implantation of deuterium ions in other structures,

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such as SIC composites under glow discharges,was studied as function of the absorption and retention of deuterium [18]. The interaction of deuterium atoms with palladium under electric dischargeshas been studied earlier looking for an explanation of the excess of energy released in the process [19]. This work presents experimental data of temperature, pressure and mass of absorbed deuterium in titanium at different thermodynamic conditions. The detection of anomalous emissions of subatomic particles in the processwas monitored using CR39 plastic detectors (C,2H,806). 2. EXPERIMENTAL

SETUP

Figure 1 shows the experimental arrangement, which consists of a vacuum chamber, auxiliary and diagnostic systems.The chamber is a stainless steel cylinder, 50 cm long and 4.85 cm in radius. Inside, coinciding with the cylinder axis, is located a titanium plate of dimensions 0.03 x 5.4 x 45 cm, which is electrically isolated from the rest of the chamber by ceramic and Teflon connectors. There are two small accessesin the cylindrical wall. In one of these, a multiple connection of 4 devices is available, namely, a Pirani and a cold cathode pressure meter, a sapphire valve for the control of the deuterium supply, and four thermocouples. The Pirani detectors were connected to a TPG 100 Balzers indicator. In the accessa T connection is found, in which a quartz window and a gate valve, joined with a mechanical and a diffusive vacuum pumps, are located. In Ref. [20], a similar experimental setup for investigating the hydrogen-titanium system is described. On the titanium plate, three cromel-alumel thermocouples were placed at the ends and on the center of the plate. Another thermocouple was used to register the temperature of the chamber atmosphere. The thermocouples were connected to a DP80 Omega temperature

et al.

gauge. In Fig. 1, the asterisksindicate the positions of the thermocouples. The electric discharges were applied between the titanium plate and the body of the chamber by a voltage transformer whose maximum output is 5000 Vat with a maximum current of 30 mA. A variable direct current source with &30 V and &3000 A output was used to degasthe titanium sample by the Joule effect. The release of particles produced in the absorption/desorption process was detected by eight CR39 plastic detectors inserted with sheets of paper coated with a lithium borate layer and blocks of paraffin. The lithium borate layer increases around three hundred times the neutron detection efficiency. The paraffin blocks slow down the expected high energy of the neutrons [21]. High energy neutrons interact with the hydrogen content in the CR39 plastic detector producing proton recoils which are registered as latent damage trails of up to 80 /*m in length and 100 A in diameter. Those trails are made visible as nuclear tracks by etching the CR39 detector in a chemical solution. In this way, the density of nuclear tracks is proportional to the neutron fluence. The detectors were cut in a 10 x 7 cm rectangular geometry. The left half of each detector was covered with boron layers. The right half remained free of boron. The dimensions of the paraffin blocks were 20 x 15 x 5 cm. The arrangement was built as follows: a block of paraffin, a CR39 plastic, a boron sheet, and finally, a white sheet. Eight similar groups were constructed. The arrangement was placed perpendicularly to the chamber axis, as shown in Fig. 1. Two similar arrangements, used as control detectors, were placed in the same laboratory, but far away from the deuteration chamber to monitor the neutron tracks from the background. These tracks are produced by alpha-particles spontaneously emitted by the decay of the environmental radon radioactive gas. The plastic detectors used in these experiments have the advantage, with respect to other electronic detectors, of being transparent to the electromagnetic radiation. Active detectors were tested during the experiments, but the electromagnetic radiation originated from the electric discharges affected the reliability of the collected data. 3. RESULTS AND DISCUSSION

CL!3 s Plastic I3 ’CR39 Detectors 7-r

* - Thermocouples Ga”ge Fig. 1. Experimental

setup.

Before the absorption of deuterium in the samples,the titanium plates were degassed to reduce impurities as nitrogen, hydrogen, oxygen, water, etc. For this degassing, the samplewas heated up to 700°C in approximately 30 min at lo-’ mbar. This temperature was kept for around 10 min, and later the chamber was returned to room temperature. Finally, deuterium was added to the system. No additional heating was applied to the system during the rest of the experiments. In Figs 24, the plots show the evolution of deuterium massin the atmosphere of the chamber and the titanium temperature during the absorption phenomenon. The abscissaaxis shows the time of the experiment. The left

ABSORPTION

OF DEUTERIUM

IN TITANIUM

887

PLATES

As can be seen in Fig. 2, an additional absorption of deuterium takes place when the electric discharges are applied. With the discharges, some molecules of deuterium dissociate into individual atoms. Other molecules and atoms change from the basal state to radical forms and others transform to ions because of the colhsions with the free electrons and among them. The individual 3.1. Absorption with electric discharges atoms, in the different forms, have more freedom to travel These experiments occurred at room temperature in the electric field and acquire more energy. Thus, when (around 10°C). A small amount of deuterium, calculated the atoms of deuterium impact the surface of titanium, in 0.076 mg, was introduced and absorbed at the begin- the additional energy influences the distance that the ning of the experiment. The absorption increases when atoms penetrate into the body of the metallic structure. 1.41 g of deuterium were quickly added to the system. Without electric discharges, the atoms of deuterium penWith this additional mass,the pressureincreased. Later, a etrate a shorter distance than when the discharges are decreaseof pressurefrom 1900to 480 mbar was observed, applied. This is the reason for the additional absorption which is equivalent to an absorbed mass of 1.081 g of of deuterium. However, the mechanisms involved in the deuterium. The procedure continued, adding deuterium collision of the atoms with the structure are complex and until a total of 1.404 g of gas absorbed was obtained. are out of the scopeof this paper. In this work, we discuss This process was carried out for a period of 10 min in the global effects of these conditions. which the titanium temperature increased 1.8”C; seeFig.

ordinate axis shows the mass of deuterium in the atmosphere within the chamber. The right ordinate axis representsthe titanium temperature at the center of the plate. The results are organized in four sections: absorption at two conditions, desorption and the releaseof particles.

2.

Once the last absorption was carried out, we waited for 2 min so that the pressurewas stabilized before applying the electric discharges. During this period some desorption of deuterium was observed. The electric discharges were applied for 30 min at 2000 V. A fall of pressure from 1000 to 17 mbar, equivalent to 0.748 g of additional absorbed deuterium, was measuredduring the first 2 min. Simultaneous to this effect, an increment in the temperature of titanium, from 10.4 to 12.2”C, was registered in the next 10 min. Starting from the moment at which the electric discharges were applied, a temperature increase of 19.7‘C was observed in the titanium. The temperature remained almost constant during the rest of the discharge. Finally, the plate reached the point that, despite the continuing discharges, the pressure stabilized. This indicated that the absorption had ceased and that the titanium was saturated.

3.2. Application qf’ electric discharges qfter successive absorption of deuterium in titanium

Deuterium was injected to the system in five stages with the idea that the titanium would absorb the gas in 6 min. The initial pressure was 2.7 x lo-* mbar. The first stagewas carried out when the system was at 14°C. Gas absorption was observed, accompanied by a small increment in the temperature of the titanium. This process was repeated during the remaining stages. A maximum absorption of 1.343 g of deuterium was obtained at the end of the processeswith an increase of 5°C in the temperature of the titanium. After the last stage, a potential of around 2000 V was applied for 35 min; seeFig. 3. A desorption of 0.371 g of deuterium was observed in the first few minutes of the electric discharge. After that, there was a reversion in the process until the titanium reached the equilibrium between the absorption and the

3 30

0 25 -Q)

-

Temperature

fi 0.6 f+d ,” 0.4

&

0.2

I 0

I 5

.

I 10

Beginning of electric discharges , . , . ( 1 , I ( I 20 40 1s 25 30 35

IV 45

0

z

Time (mm) Fig. 2. Absorption

of deuterium at room temperature

Temperature Mass of deuterium

End of electric discharges

E

0.0 -0.2

A --

0.6-1

lb

1’5

io

is

3-o

3’5

40

45’-

Tie (min) Fig. 3. Absorption

of deuterium after saturating the titanium.

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desorption of deuterium. Finally, 0.425 g of deuterium was absorbed after the discharge. This was accompanied by an increase in the titanium temperature, from 18.9 to 31.6”C. The absorption promoted by the electric discharge was not significant in this case, possibly due to the saturation in the titanium before the application of the discharge. A more detailed analysis of the system behavior indicates that the temperature has four regions. In Region I the temperature increases quickly and linearly, from 14 to 19”C, at a rate of 0.833”C/min. This occurred without electric dischargesin the first 6 min of deuterium absorption, starting from room temperature. In Region II, where some desorption of deuterium takes place, the increase in temperature is slow, 0.21l”C/min. When the deuterium absorption began again with the application of electric discharges, the increase in temperature was rapid, 0.916”C/min, from 21 to 26S”C, as in Region III. Region IV shows the low absorption of deuterium and the little increment in the temperature, 0.225”C/min. 3.3. Desorption of deuterium with the application electric discharge

of an

the application of electric discharges in these conditions did not produce absorption but desorption of the gas. It should be also noted that the absorption was in the order of grams and that the desorption was in the order of milligrams. The profiles of both curves, the mass of deuterium and the titanium temperature are similar. When there is desorption in the titanium, the temperature increases, and when the mass of deuterium does not change in the chamber atmosphere, the temperature remains almost constant for around 20 min, until the desorption restarts. Figure 4 shows the behavior when a disturbance in the system takes place and when the electric discharges remain for a long time. In the first case,the dischargesare applied, turned off and applied again. The equilibrium reached with the initial discharges is modified because the atoms change their excited or ionized state to basal forms without the electric field. The electric forces in the structure are released,the ionized atoms are releasedalso and the system achieves another equilibrium. The result of these new conditions produces a small desorption in the titanium when the discharges are applied again to obtain a new equilibrium. In the second case,the desorption can be due to the increase in temperature by the constant impact of particles in the titanium. The competitive effect of the absorption promoted by the dischargesversus the desorption promoted by the increment in temperature produces an equilibrium in the flat zone of Fig. 4. After a while, the increment in temperature dominates and the desorption begins again. This region is located after the flat zone in Fig. 4.

Without modifying the conditions in the previous experiment, a second electric discharge was applied during 28 min to increase the desorption of deuterium from the titanium. Figure 4 shows that the desorption during the discharges was accompanied by an increase in temperature, from 17.6 to 31.3”C. This temperature increase occurred mainly during the first 10 min of electric discharge. After that, the temperature remained practically constant during the rest of the experiment. A mass 3.4. The particle emission of 0.419 mg of deuterium was desorbed from the plate. This value was obtained from the pressure increment in The detectors remained near the deuteration chamber the chamber from 5 x 10e2to 6.1 x 10-l mbar. during all the experiments. At the end, they were chemiDuring this experiment, the plate was saturated and cally developed with a solution of 10% NaOH at 70°C during 7 h. The eight CR39 detectors were cut in 1 x 1 cm pieces. Two pieces were selected from each detector forming two groups according to the position that they had before the cut, two from the left hand side with boron and two from the right hand side without it. Next, the I. 1. I ’ I ’ I. I I tracks were counted up with an optical microscope using - 32 End of electric discharge 27 fields for each piece. Figure 5 shows the track density found in the CR39 - 30 Go.4 detectors for the two groups. The plots show similar g - 28 behavior. The pieces covered with boron present more g . G tracks, with a ratio up to 2:1, with respect to the others. . 0.3- 26 The maximum and minimum values are in the same -24 1 regions for the two plots. This fact suggeststhat, at least, i . & 0.2 two groups of energies exist in the neutrons released VJ during the experiments. The plots show also another Temperature maximum located on the first 5 cm from the neutron 9 0.1-. source. To find other neutron groups with different ener- 18 gies, it might be necessaryto enhance the detection resBeginning of electric discharges olution reducing the thickness of the paraffin blocks. The o.o! , . , . , , . t t , 116 tracks found in the CR39 control detectors belong to 0 5 10 25 30 the a-particles from the radon produced in their storage before and during the experiments and from the imperFig. 4. Desorptionof deuteriumby electricdischarges.

“‘T

ABSORPTION

0

5

10

15

20

25

30

OF DEUTERIUM

35

40

45

Paraffin Thickness (cm) Fig. 5. Track density in the CR39 plastic detectors.

IN TITANIUM

PLATES

889

In this case, no more absorption of deuterium in the titanium occurred. The track density found in the CR39 detectors located near the deuteration chamber is higher than the background track density found in the CR39 control detectors. This anomaly can not be due to the emission of xparticles from the ambient radon, because these tracks are not registered by the control detectors which are transparent to electrons and electromagnetic radiation. Thus, a conclusion about this effect is that the anomalies observed in the plastic detectors are due to neutron emission because of the deuteration process. This effect suggests that the interaction of the forces in the metallic lattice affects the equilibrium in the nucleus of the deuterium atoms absorbed into the titanium structure. authors acknowledge the technical assistanceof Isaias Contreras, Satil Moran and Carlos Vgzquez from the Plasma Physics Laboratory and the logistic support of Carlos Didz and Gabriel Salinas from the Thermalfluids Laboratory at the Institute National de Investigaciones Nucleares.

Acknowledgements-The

fections of the plastics. The threshold was around 30 tracks/cm*. The bombardment of deuterium ions on titanium with high energy ion accelerators can produce the release of sub-atomic particles. However, the energy required by the bombardment is much higher than the energy required by the implantation of ions by electric discharges, like those used in this work. In this condition, the detection of sub-atomic particles released in the absorption of deuterium in the titanium structure can be considered as a consequence of other interactions different from the energy of impact. 4. CONCLUSIONS

Electric discharges can be used as a promoter of the absorption of deuterium and/or hydrogen in the structure of titanium. With this technique, the storage capacity of this metal can be used up to saturation without the need of drastically changing the thermodynamic conditions in the process. The absorption occurs easily at ambient conditions. The discharges promoted more absorption of deuterium in the titanium plates, but the increment in temperature initiates a competitive desorption process that reaches equilibrium after 10 min. After that, no more absorption or desorption occurs in the system. When the electric discharges were applied between the titanium and the chamber, there was an additional absorption of deuterium, which reached up to 20% of the initial absorption. When the electric discharges were applied again, the absorption was reversed having slow desorption in the titanium, possibly originated by the temperature increment. On the other hand, when the titanium was apparently saturated with deuterium, the discharges caused an initial desorption, followed by absorption to the same levels of saturation previous to the discharges.

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