Journal of Nuclear Materials 424 (2012) 216–219

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Journal of Nuclear Materials

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Helium release from aged palladium tritide

Zhi Zhang ⇑, Wei-Cai Yang, Yong-Jun Su, Hong-Zhi Zhu, Cheng Qin, Jin-Shui Yang, Meng Liu, Jie DuChina Academy of Engineering Physics, PO Box 919-71, Mianyang, Sichuan 621900, PR China

a r t i c l e i n f o

Article history:Received 10 March 2011Accepted 5 March 2012Available online 10 March 2012

0022-3115/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jnucmat.2012.03.005

⇑ Corresponding author.E-mail address: zhangzhi1434@163.com (Z. Zhang

a b s t r a c t

Palladium tritides at the initial tritium/palladium (T/Pd) atomic ratio of 0.65 were prepared by gaseous P–V–T method. The 3He release behavior in the aged palladium tritides for 604, 1265 and 2168 days withthe initial tritium/palladium atomic ratio of 0.65, has been studied by the methods of D–T isotopesexchange and aqua regia dissolution separately. The results of D–T exchange experiment show thatthe 3He release ratios in these three different time aged palladium tritides are 1.7%, 2.1% and 3.3%, leavingmore than 96.7% 3He kept in palladium tritides Lattice. A similar result can also been obtained by aquaregia dissolution method, and the 3He release ratios are 2.0%, 2.9% and 3.3% by comparison. The similarityreveals a strong 3He retain ability of palladium tritides. The experimental data by two methods agree wellwith each other.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Palladium plays a special role in the group of metal hydridesusually used in hydrogen isotope storage, extracting, purificationand separation due to its high resistance to oxidation and poison-ing, fast kinetic of absorption and desorption, and outstandingability to retain the 3He generated in the matrix. It has been exper-imentally and theoretically studied more for the fresh Pd–T systemand there are lots of works, but the aging Pd–T system is still lessstudied and reported. We have previously reported [1,2] aging ef-fects on thermodynamics and kinetic properties data in palladiumbeds up to 3 years old. More aging palladium tritide concerns 3Hemicroscopic behavior in palladium [3–10] and its thermodynamicscharacteristic for aging palladium tritide [11,12].

As for the 3He release behavior in aged Pd–T system, the volumnof 3He release from the solid phase is generally obtained by mea-suring gas pressure and contents in a sample holder. Coronadoet al. [5] have reported data on palladium beds up to 490 daysold, only 0.01 mol% 3He could be obtained in the gas from thePd–T beds. Thiébaut [13] have studied the 3He retention propertiesat room temperature of Pd tritides after more 13 years and showedthat Pd tritides retained more than 95% of 3He for at least 9 years ofaging. Abell et al. [14] has studied the 3He release behavior fromaged PdTx by thermal desorption experiments, and pointed that3He release from a sample with [He]/[Pd] � 0.3 required tempera-tures in excess of �327 �C, while for a sample with [He]/[Pd] � 0.02, the release required temperatures in excess of at least

ll rights reserved.

).

1000 �C. All these results revealed that palladium have a strongability to retain the 3He generated in the matrix for many years.

In this study, 3He release ratio in aged PdTx system has beenstudied using a complementary technique including D–T isotopeexchange and aqua regia dissolution with mass spectrum and gaschromatogram analyses. The specific aim of this study is to probe3He release behavior in samples with different aged time, withthe hope of further proving its strong ability to retain 3He.

2. Experimental

2.1. Preparation of Pd samples

The palladium powders used in this study consisted of 40–60mesh particles (SEM images of palladium powders are shown inFig. 1), and the purity of which is more than 99.95% (impurities:Pt�0.005%; Rh < 0.001%; Ir < 0.001%; Au < 0.001%; Ag < 0.0006%;Cu < 0.0006%; Al < 0.001%; Ni < 0.001%; Fe < 0.001% and Si <0.001%). The specific surface area of these powders, measured bythe BET method, is 0.438 m2/g.

Three tritium-storing samples labeled as Z1, Z2 and Z3 wereprepared by placing these powders in tight stainless steel sampleholders, equipped with valves, and the weights of sample Z1, Z2and Z3 were 4.99 g, 5.00 g, 4.99 g respectively. The void volumesfor gas were 3 mL. Before tritium loading, each sample was acti-vated by three cycles absorption/desorption with deuterium at280 �C [1]. Three samples were subsequently loaded at room tem-perature with almost pure tritium (the purity of tritiumwas >98.0% and composition impurities determined by high reso-lution mass spectrometry only consisted of deuterium). Thetritium was measured by P–V–T method and the initial T/Pd was

Fig. 1. SEM images of palladium powders.

Z. Zhang et al. / Journal of Nuclear Materials 424 (2012) 216–219 217

0.65. Sample Z1 and Z2 had been aged at room temperature for604 days and 1265 days, respectively. By contrast, Z3 was replen-ished sample, which was aged 1265 days under the same condi-tions as for above two samples and, subsequently, sample Z3 wasdesorbed at 280 �C for 1 h under the vacuum condition and replen-ished with tritium to the atomic ratio T/Pd = 0.65. The total agedtimes for sample Z3 were 2168 days. In this study, the pressuremeasurements use manometers with pressure the error 0.1 KPafor 1 MPa. The temperature error is 0.1 �C for Pt100.

2.2. Theory and step of the experiment

2.2.1. Method of D–T isotope exchangeIsotope exchange is to make D2 flow through the Pd column

filled with PdTx with a constant flux at room temperature. Thefunction of D2 only serve as carrier gas. The column length and in-side diameter were 106 mm and 6 mm. This Pd column is not thesame as the thing used for the sample preparation.

The total 3He quantity (n mol) of the Pd column is composed oftwo parts. One part is origined from tritium decay in PdTx (n1 mol).And the other is from tritium decay in Pd column void (n2 mol).One part of the 3He quantity origined from tritium decay in PdTx

(n1 mol) can be retain in the Pd matrix (n3 mol) and another partrelease into the Pd column void (n4 mol).

The 3He in the void (n5 mol)of the column and held up in inter-stitial sites may be carried out. The 3He quantity retained in the Pdmatrix (n3 mol) is almost impossible to be carried out by D–T iso-tope exchange because of Pd outstanding ability of retaining the3He. So we analyze the 3He content (c%) of the gaseous mixturecarried out from the column, and obtain the total 3He quantity tocalculate the 3He release ratio (g) by P–V–T method. The followingformulae show the 3He quantity and 3He release ratio relations.

n ¼ n1 þ n2 ð1Þn1 ¼ n3 þ n4 ð2Þn5 ¼ n2 þ n4 ð3Þg ¼ n4=n1 ð4Þ

The schematic showing the general procedure of D–T isotopeexchange is presented in Fig. 2. Firstly, the source of D2 and massflowmeter should be connected to the experimental system. Then

Fig. 2. Schematic diagram of D–T isotope exchange.

opening the valves at the both ends of the Pd column to makethe D2 flow into the column and substitute tritium in the column.The 3He would be carried into the standard vacuum container bythe D–T mixture. Deuterium quantity and flux rate used for ex-change are 0.0456 mol and 25 mL/min. Finally, we get the totalquantity of 3He by analyzing the sample of gaseous mixture inthe standard container.

2.2.2. Method of aqua regia dissolutionAqua regia dissolution is employed to dissolve the samples after

D–T isotope exchange and vacuum thermal degassing at 280 �Cusing the same sample. The 3He quantity retained in the Pd matrix(n3 mol) can be obtained by analyzing the 3He content (c%) re-leased from the solution. The following formulae can calculatethe 3He release ratio (g)

g ¼ n4=n1 ¼ ðn1 � n3Þ=n1 ð5Þ

In this experiment, we plunged the aged Pd powders into thebeaker of 50 mL aqua regia placed in a sealed reactor. In order totest the dissolving time of aging palladium in aqua regia, fresh pal-ladium have been carried out in a glass beaker filled with 50 mLaqua regia for 3 times. The following reaction happened whenthe powders were dissolved. Although Aqua regia is mixed acidof HNO3 and HCl with the ratio of 1–3, it’s the ratio of 1–6 in thischemical reaction.

3Pdþ 2HNO3 þ 12HCl ¼ 3H2½PdCl4� þ 2NO " þ 4H2O ð6Þ

All 3He gas in Pd was almost released into the gas except for avery small amount dissolving in aqua regia. The pressure of thegas in the container was measured by a pressure gage. After thereaction finished, we sampled the gas in the container and ana-lyzed the 3He content to calculate the total quantity of 3He.

Fig. 3. Schematic illustration of aqua regia dissolution.

218 Z. Zhang et al. / Journal of Nuclear Materials 424 (2012) 216–219

The palladium powders used for 3He measurements wereunderwent the same treatment as for the foregoing statement.Aqua regia dissolution measurements were performed using a spe-cialized system, as shown in Fig. 3. The system was composed of asealed container (two beakers of NaOH powders and aqua regiawere placed in which), pressure gages, valves, a vacuum pump, agas chromatograph, carrier gas, etc. The role of NaOH powders isto neutralize partial acids in the container void. The carrier gas ofgas chromatograph is high pure nitrogen. The aged Pd powdersare weighed by using a electronic balance system after samplingthe Z1, Z2 and Z3, and put them in the glove box. Immediately afterdividing 150 mL aqua regia into three 50 mL weighing bottles, weopened the sealed container, and placed a beaker of NaOH powdersand an empty beaker in it. The empty beaker is for filling with50 mL aqua regia. In the course of dissolution of aged Pd powders,the valve 1 was open and the valve 2 was closed. Finally, we re-corded the pressure of sealed container after Pd dissolved com-pletely, and analyzed the sample of the gas to get the contentand total quantity of 3He. When the one experiment finished, wereset NaOH and aqua regia and carry out next round of experiment.

3. Results and discussion

3.1. The quantity of 3He measured by D–T isotope exchange

The initial Pd mass of the three columns (Z1, Z2, Z3) was 4.99 g,5.00 g, 4.99 g respectively. From the knowledge of void volumes,temperatures and pressures of gas phase, the stoichiometry of thetritide, expressed as the atomic ratio of tritium over metal T/Pd,was calculated by subtracting from this amount of tritium deter-mined by the volumetric method, The obtained quantities of T2 inthe three Pd beds were almost the same. Before the D–T exchange,the quantities of 3He (n mol) in the three columns, calculated bythe radioactive decay law, are 0.0027 mol, 0.0054 mol and0.0093 mol. The He/Pd ratio of each samples is about 0.058, 0.115and 0.199. The quantity of D2 used for D–T isotope exchange exper-iments was 0.0456 mol, and the flux of D2 was controlled at 25 mL/min by a mass flowmeter. The experimental time is about 41 minand enough to carry out the 3He gas in the column voids completely.The 3He in the Pd matrix is impossible to be carried out during D–Tisotope exchange except for high temperature. The gaseous mixturewas collected by a standard container with a volume of 3 L.

After the D–T exchange, we sampled the gas in the standardcontainer when the temperature was 25 �C and the pressure was30.39 KPa, 30.39 kPa, 30.47 kPa respectively. The mixed gas ismade up of 3He, tritium and deuterium. Because the content dataof 3He is essential and tritium or deuterium are not necessary,the gaseous mixture has been analyzed and the 3He conrents fromsample Z1, Z2 and Z3, are 0.13%, 0.31% and 0.51%, respectively. Itcan be calculated that the quantities of 3He (n5 mol) brought outfrom Z1, Z2, Z3 were 4.78 � 10�5 mol, 1.14 � 10�4 mol and1.88 � 10�4 mol by formula (7). It is noticed that the 3He in thevoid of Z3 was produced in the later stage. The quantities of 3Hereleased in the first stages (�1265 days) are considered as sameas sample Z2 just because of their uniform aging conditions. Sothe total 3He quantities of sample Z3 released were3.02 � 10�4 mol.

n5 ¼PV

ð1þ vPÞRT� c% ð7Þ

Where P is container pressure, V is container volume, v is gas com-pressi-bilities, and c% is 3He conrent.

The 3He quantity origined from tritium decay in Pd column void(n2 mol) can be calculated by tritium decay law (8). The volume ofthe each column void was 3 mL.

n2 ¼ kZ

placunaVRT

dt ð8Þ

where k is tritium decay constant, placuna is T2 plateau pressure inthe void, V is column void volume.

The T2 plateau pressure in the void, decreasing with the agedtime was given by [11]:

placuna ¼ 0:0043þ 0:0027� eð�nHe=nPd=0:094Þ ðMPaÞ ð9Þ

where nHe/nPd is the atomic ratio of 3He over Pd. nHe is given by:

nHe ¼ 2nT e�kt ð10Þ

nT is the initial T2 quantity of the Pd column.According to the formula (8)–(10), the 3He quantity (n2 mol)

can be given by final formula (11).

n2 ¼ kZ

0:0043þ 0:0027� eð�2NT e�kt=nPd=0:094Þ� � V

RTdt ð11Þ

According to this formula, the 3He quantity (n2 mol) of Z1, Z2and Z3 are 1.31 � 10�6 mol, 2.86 � 10�6 mol and 2.04 � 10�6

mol, respectively. It could be calculated that the 3He release ra-tios of Z1 and Z2 are 1.7% and 2.1% according the followingformula.

g ¼ n4=n1 ¼ ðn5 � n2Þ=ðn� n2Þ ð12Þ

Sample Z3 was desorbed at 280 �C after 1265 days andreplenished with tritium to the atomic ratio T/Pd = 0.65. The to-tal aged times were 2168 days. So the 3He release ratio of Z3 in-cludes two parts. The 3He release ratio can be consider as thesame as sample Z2 in the first stage and decided by experimentin the second stage. The total 3He release ratio of Z3 is 3.3% bycalculation. The results of experiments further confirm that Pdhas a strong ability to retain the 3He generated from tritiumdecay.

3.2. The quantity of 3He measured by aqua regia solution

A small quantity of Pd powders from sample Z1, Z2 and Z3were separately put into weighing bottles and weighted by elec-tronic balance. The quality of them was 0.682 g, 0.238 g and0.795 g. It’s worthy mentioning that the weight of sample is ran-dom. The 3He content analysis will not be affected unless theweight is too little. The effective volume of the sealed containerwas 2869, 2873 and 2871 mL for three rounds of experiments.The volume of aqua regia in the container was 50 mL every time.Before the solution of aged Pd, fresh Pd of 1.5 g were dissolvedwith 50 mL aqua regia, and it took 40 min to dissolve completelythese fresh powders. Therefore, the aged Pd samples were steepedin aqua regia for over 60 min to make sure the Pd was dissolvedcompletely.

The experiment results and chromatographic analytical data areshown in Table 1. The theoretical values of 3He are5.41 � 10�4 mol/g, 1.08 � 10�3 mol/g and 1.86 � 10�3 mol/g molby subtracting n2 from n in sample Z1, Z2 and Z3.

It can be seen from Table 1 that the sealed container effectivevolumes are different each time because of glass beaker differ-ences. The pressure increases to a certain degree due to chemicalreactions between Palladium and aqua regia. The experimentalvalues of the 3He quantity in Pd were 5.30 � 10�4 mol/g,1.05 � 10�3 mol/g and 1.80 � 10�3 mol/g. The quantities of 3Heheld up in Pd were 98.0%, 97.1%, 96.7%. That is to say the ratiosof 3He release, increasing with aging time, were 2.0%, 2.9% and3.3%. The results reveal that most of 3He are still holding up inPd matrix after D–T substitution and thermal desorption at lowtemperature.

Table 1The experimental results of 3He.

Sample no. Container volume (ml) Pressure (kPa) Temperature (�C) Contents of 3He in container (%) The quantity of 3He in Pd (mol/g)

Theoretical (n1) Experimental

Z1 2869 95.1 26.3 0.33 5.41 � 10�4 5.30 � 10�4

Z2 2873 94.1 26.5 0.23 1.08 � 10�3 1.05 � 10�3

Z3 2871 97.7 26.5 1.27 1.86 � 10�3 1.80 � 10�3

Z. Zhang et al. / Journal of Nuclear Materials 424 (2012) 216–219 219

3.3. Comparative analysis of the methods

The 3He release ratios of Z1, Z2, Z3 are 1.7%, 2.1%, 3.3% by D–Tisotopes exchange method and 2.0%, 2.9% and 3.3% by aqua regiadissolution method. In theory, the 3He release ratios obtained byaqua regia dissolution method will be slightly larger than usingD–T isotopes exchange method because of 3He micro- desorptionat low temperature. The experimental data by two methods agreewell with each other. Fig. 4 shows the comparable plot of relationbetween 3He release ratio and He/Pd ratio with reference values byEmig et al. [15] and Thiebaut et al. [13]. The result of this researchshows that the 3He release ratio increases with the He/Pd ratio. S.Thiebaut’s result shows that 3He releasing have three differentstages. The first one, during approximately the first 1.5 years ofs-torage (He/Pd < 0.05), is characterized by a relatively high 3He re-lease which tends to decrease. From 1.5 to 9 years (He/Pd = 0.23),the 3He release rate is stable, around 4%. Lastly, after 9 years ofaging (He/Pd > 0.23), 3He release begins to increase. J.A. Emig’s re-sult shows that the 3He release ratio keeps relatively constant inthe early stage (He/Pd < 0.33). When the He/Pd ratio is above0.37, the 3He release ratio begins to accelerate. The difference be-tween these three results attributes to various reasons. First, theinitial T/Pd ratios are not the same completely. As we can be seenfrom S. Thiebaut report that the 3He release ratios of seven samples(P1–P7) with initial stoichiometries (>0.6) are not completely thesame. Secondly, the starting material of palladium powder is notthe same entirety including Pd diameter, morphology and purity.Pd purity is a key factor which affects its thermodynamics. How-ever, the impact of Pd purity on 3He release still remains unclear.Thirdly, experimental errors also cause the difference of these re-sults. The main experimental error is due to the pressure and tem-perature measure which determined the initial (and the final)tritium and 3He content. The pressure and temperature gauge errorare estimated to be 0.2 KPa for 1 MPa and 0.1 �C for Pt100, respec-tively. This error could change the stoichiometry from 0.007 to

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.400

2

4

6

8

10

12

14

16

18

20

3 He

rele

ased

ratio

(%)

He/Pd atom ratio

Exp by aqua regia solution J.A. Emig:no replenished samples [15] S. Thi'ebaut:sample P1(T/Pd=0.64) [13]

Fig. 4. Comparable plots of relation between 3He release ratio and He/Pd ratio withreference values.

0.011, varingd from 2 to 3 mg of tritium. These factors contributingto the 3He release ratio error are less than 0.2%. The predominanterror originates in the 3He content analysis by gas chromatogram.Although the relatively error of the gas chromatogram is 1%, whichvaried 3He content ranging from 0% to 5%, and this has multiplica-tional effect on the 3He release ratio (g). The experimental errorsare signed in Fig. 4.

In conclusion, the results indicate that palladium tritides have astrong ability to retain 3He generated in their matrix. The 3He re-lease ratio increases with the He/Pd ratio but this raise is relativelyslow. And accelerated release is not reached.

4. Conclusion

The methods of D–T isotopes exchange and aqua regia dissolu-tion combined with mass spectrum and gas chromatogram analy-ses have been used to study the 3He release ratio. They are acomparatively good method to check results obtained by previousmeasurements. 3He release ratios of three samples, indicate that3He release ratio increase with aging time. The experimental databy two methods agree well with each other. The result of experi-ments and literatures indicates that palladium tritides have astrong ability to retain 3He generated in their matrix.

Acknowledgements

We would like to express our immense gratitude to Yong-JunWei, Wen-Yong Jin, Xue-Sheng Zhang, Jie Li for their continuedsupport through the years. We would like to thank Huo-Gen Huangof National Key Laboratory for Surface Physics and Chemistry forhis partial support of this work.

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