Lithium concentration dependence of implanted helium retention in lithium silicates

Nuclear Instruments and Methods in Physics Research B 268 (2010) 1857–1861

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Lithium concentration dependence of implanted helium retention in lithium silicates } cs a,*, E. Szilágyi a, Cs. Bogdán a, E. Kótai a, Z.E. Horváth b D.E. Szo a b

KFKI Research Institute for Particle and Nuclear Physics, H-1525 Budapest, P.O. Box 49, Hungary Research Institute for Technical Physics and Materials Science, H-1525 Budapest, P.O. Box 49, Hungary

a r t i c l e

i n f o

Article history: Available online 25 February 2010 Keywords: Helium retention Ion-implantation Proton backscattering spectrometry XRD

a b s t r a c t Helium ions of 500 keV were implanted with a fluence of 1.4  1017 ion/cm2 into various lithium silicates to investigate whether a threshold level of helium retention exists in Li-containing silicate ceramics similar to that found in SiOx in previous work. The composition and phases of the as prepared lithium silicates were determined by proton backscattering spectrometry (p-BS) and X-ray diffraction (XRD) methods with an average error of ±10%. Electrostatic charging of the samples was successfully eliminated by wrapping the samples in Al foil. The amounts of the retained helium within the samples were determined by subtracting the non-implanted spectra from the implanted ones. The experimental results show a threshold in helium retention depending on the Li concentration. Under 20 at.% all He is able to escape from the material; at around 30 at.% nearly half of the He, while over 65 at.% all implanted He is retained. With compositions expressed in SiO2 volume percentages, a trend similar to those reported of SiOx previously is found. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction It is well known that hydrogen and noble gases ion-implanted into solid matters are partly dissolved in the solid being the subject of thermal diffusion and are partly captured in various types of traps [1,2]. Experiments and first principle theories also show that in case of materials with crystalline structure helium accumulation can be associated to the traps, that can be vacancies, vacancy clusters and bubbles [3,4], as well as dislocations and grain boundaries [5]. These trapping mechanisms may result in bubble formation and gas accumulation leading to the deterioration of the material by facilitating embrittlement. Aggregation of the bubbles can also cause blistering and exfoliation [6–9]. Helium accumulation has been observed not only in crystalline materials, but also in amorphous [10] or even porous materials [11]. These microscopic mechanisms can have a major impact on the solid’s macroscopic properties such as hardness and rigidity, leading to great concern in applications where materials are inevitably or necessarily exposed to permanent and high dose ion-irradiation. For example, structural and functional materials of fusion or fission reactor vessels are constantly irradiated by a, p and n particles from the fusion or fission reactions. According to our knowledge, the only exception where no surface blistering or exfoliation was observed even at high He fluences * Corresponding author. } cs). E-mail address: [email protected] (D.E. Szo 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.02.022

is SiO2 [12]. Previously it was shown that ion-implanted helium will fully escape from silica (SiO2) even at room temperature, while silicon (Si) is able to capture all the implanted He. It was also shown that in substoichiometric oxide (SiOx) a threshold level can be observed at x = 1.3, i.e. implanted helium will escape from the oxides if x > 1.3 [13]. Helium could be detected in SiO2 only where it was insulated properly from the vacuum [14]. Lithium orthosilicate (Li4SiO4) is one of the candidates for tritium breeding material in fusion reactor blankets [15], therefore it is the subject of various investigations, mainly hydrogen retention and radiation damage experiments. Li-silicates can also be considered as a mixture of SiO2 and Li2O. In this work we investigate whether a threshold level exists for helium retention in Licontaining silicate ceramics similar to that of substoichiometric silicon oxide. The helium retention of lithium silicate ceramics was determined as a function of lithium concentration. 2. Experimental Various lithium containing silicates (Li2O)x(SiO2)1x ceramics and pure Li2O were prepared. Lithium silicates were composited from water suspension of solid LiCO3 and SiO2 via precipitation method [16], the nominal Li2O proportion of x was 0.2, 0.33, 0.5 and 0.67. Pure Li2O was synthesised from stoichiometric mixing of Li2SO4 and Ba(OH)2, that reacted and resulted in LiOH which was heated at 700 °C in a silver jar to gain Li2O. Then 10 mm diameter, 3 mm high cylinder-shaped pastilles were prepared from the

}cs et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 1857–1861 D.E. Szo

powders by hydraulic press and annealed at 900 °C for 4 h. The sample composition and phase analysis were performed with proton backscattering spectrometry (p-BS) and X-ray diffraction (XRD) methods. The XRD patterns were obtained with a Bruker AXS D8 Discover diffractometer using Cu Ka radiation. Both the ion implantation and backscattering experiments were carried out in a scattering chamber connected to a Van de Graaff type ion accelerator. The samples were implanted with a fluence of 1.4  1017 ion/cm2 using a 500 keV He+ ion beam. To reach better lateral homogeneity the beam was scanned over beam slits of a 2.5  2.5 mm2. The beam spot on the sample was slightly wider when the tilt angle of the sample was 37°. The beam current was about 100 nA determined by a transmission Faraday-cup [17]. The samples were measured before and after implantation with proton backscattering spectrometry (p-BS) using 20 lC dose of 2005 and 2015 keV proton beams with 15 nA at 7° and 60° tilt angles. On a few selected samples He-BS was also carried out using 2020 keV helium beam. The backscattered ions were detected using a blind detector (ORTEC type with 150 nm Al cover-layer) mounted to a scattering angle of 165° in Cornell geometry. For energy calibration of the detector a piece of Si single crystal and a Si single crystal with thin Au top layer were used. To decrease the surface contamination liquid N2 trap was used, and the vacuum was about 1  104 Pa during the measurements. To prevent electrostatic charging the samples were wrapped in Al foil leaving a 5 mm diameter hole for the beams. The spectra were evaluated with the RBX (ver. 5.23) software package [18], the same version that was used in intercomparison of ion beam analysis software sponsored by the International Atomic Energy Agency (IAEA) [19–21].

3. Results and discussion Silicate ceramics are insulators, so during the measurements they were gradually charged and discharged as the measuring proton beam hit the samples. To avoid this charging effect the samples were wrapped in aluminium foil as a commonly used technique. Wrapped and unwrapped samples were also measured by both proton and helium backscattering spectrometry. Fig. 1 shows the p-BS spectra of wrapped and unwrapped samples of the same composition accompanied by the two simulated spectra. In the spectra of unwrapped samples all the Li, O and Si edges are shifted towards higher energies. The Li, O and Si edges were shifted by 24, 12 and 6 keV, respectively. This shift corresponds to 55 keV difference in the beam energy (resulting 1950 keV) due to the charging of the samples. Moreover a huge difference was also found in the yield belonging to the Li contribution compared to the wrapped spectrum. In the cross section of the proton scattering on 7Li [22] there is a resonance peak at 2050 keV with a width of 200 keV (FWHM). The energy of the impinging protons changed at the entering edge of this resonance. The values of the cross section are 73 and 103 mbarn/srad at the energy of 1950 and 2005 keV, respectively. Therefore the yield of the measured Li is very sensitive to the charging of the samples. In order to estimate the edge shifting which is a sign of the ceramics being positively charged due to the proton beam, the following considerations are to be made. The energy of an edge belonging to a target element with mass number Mt is altered by the excess charge via the potential difference (DU) which decelerates the ions before the reaction and accelerates the reaction product of the sample: EMt ¼ ðE0  q1 DUÞK þ q2 DU, where q1 and q2 are the charge state of the beam before and after the scattering, E0 is the beam energy, and K is the kinematic factor for Mt. Subtracting the energy without the charging effect (E0K), the energy shift is DE ¼ ðq2  q1 KÞDU.

Energy [keV] 400

600

800

1000

2005 keV p-BS o o tilt=60 Θ=165

10000

1200

1400

1800

wrapped non-wr wr sim non-wr sim

Li

6.08 keV/ch E0=59.9 keV

8000

1600

O 24 keV

6000

Yield

1858

4000

12 keV

1950 keV o o tilt=60 Θ=165

2000

Si

6.08 keV/ch E0=4.9 keV

6 keV

0 50

100

150

200

250

300

Channel Fig. 1. Comparison of wrapped, non-wrapped and simulated p-BS spectra. The non-wrapped spectrum is shifted to the right compared to the wrapped one. The Li, O and Si edges are shifted towards higher energies by 24, 12 and 6 keV, respectively. The non-wrapped sample was simulated with 1950 keV, while the wrapped one with 2005 keV beam energy. The 55 keV difference is due to the charging of the non-wrapped sample so, in the simulation E0 was shifted by 55 keV accordingly.

This assumption of DU electrostatic charging can explain the edge offsets, the difference in the Li yield contribution and the sparkling (or surface tracking of charge) which was observed on the unwrapped samples during implantation and measurements. The electrostatic charging of the ceramics was estimated to an average 55 kV by simulating the spectra in RBX software with altered beam energy (Fig. 1). This corresponds to the energy shift of the edges when comparing the wrapped and unwrapped spectra (Table 1). No enhanced energy spread can be observed on the edges, therefore it can be assumed that there are no significant changes in the charge state during the scattering reaction (q1 = q2). The spectrum of the unwrapped sample could be and was simulated using an altered beam energy of 1950 keV, and the E0 parameter of the energy calibration was also modified with this potential shift. The agreement between the simulated and measured spectra is quite satisfactory except for the Li resonance peak, which smeared out much more in the spectrum of the unwrapped sample due to the charging process. Moreover some difference can also be seen at lower energies, probably due to inaccurate Li cross section data. Wrapping the ceramics in Al foil successfully prevented them from being electrostatically charged, so without changing the experimental parameters (beam energy and energy calibration of the detector) the simulated spectra in Fig. 1 satisfactorily matched with the measured ones, except for the low energy part due to the Li cross section problem mentioned above.

Table 1 The measured and calculated energy shifts (in keV units) in the case of potential difference of 55 kV. Edge (keV)

Li O Si

Proton Measured

Calculated (q1 = q2 = 1)

24.3 12.2 6.3

23.8 15.4 7.15

}cs et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 1857–1861 D.E. Szo

Energy [keV]

Energy [keV] 600

800

1859

1000

200

1200

600

800

1000

1200

40000

2000

2015 keV p-BS o o tilt=60 , Θ=165

30000

2

He O

Yield

Yield

1800

10 He/cm non-implanted

2020 keV He-BS o o tilt=60 , Θ=165 1000

1600 17

26 at.% Li

1500

1400

Li

wrapped non-wrapped

O

400

Si

20000

1000

differencial spectrum

800 600 10000

Si

400

500

200 0 0 0

0 50

100

150

He 60

80

100

100

120

200

300

Channel

200

Channel Fig. 2. Spectra measured by 2020 keV 4He-BS on wrapped and non-wrapped samples of the same composition. An enhanced energy spread of the O and Si edges can be seen on the non-wrapped sample spectrum compared to the wrapped one. The FWHM of the edges are 5.5 channels (33 keV) both on Si and O edges of the wrapped spectrum, while these values are 8 and 8.5 channels (48 keV and 51 keV), respectively in the non-wrapped spectrum.

The charging effect is also important during helium implantation, because if the sample is able to be charged to 50 kV that would cause a 10% difference in the beam energy of 500 keV. This difference can significantly change the projected range of the helium particles. Therefore it was necessary to check if wrapping the samples in aluminium foil could prevent the samples from electrostatic charging under a helium beam. Fig. 2 shows two measured spectra taken on similar samples with and without wrapping using a 2020 keV He+ beam. The O and Si edges of the nonwrapped spectrum are somewhat shifted to the right again and spread out. The full width at half maximum (FWHM) of both edges is 33 keV in the wrapped spectrum, while in the case of unwrapped one the FWHM of the O and Si edges are 48 and 51 keV, respectively. The enhanced energy spread in the non-wrapped spectrum is due to the effect of the three possible He charge states. In principle the charge state distribution can be determined by fitting the height of step-like spectra that are shifted according to their charge states. Similar effects were already observed in the literature [23]. However, in our case the resolution is too poor to determine the charge state distribution of the scattered helium particles. Having avoided the electrostatic charging of the samples the compositions were determined by p-BS both for implanted and non-implanted ceramics. Significant amount of carbon was observed in the spectra of the pure Li2O and the highest Li-containing silicate. The origin of the carbon is that the Li2O is able to absorb the CO2 content of the air. The phases of the silicate samples

Fig. 3. Comparison of non-implanted and implanted p-BS spectra of a 29.5 Li at.% sample. The Li, O and Si edges and the implanted He hump are indicated in the figure. The inset shows the differential spectrum near the helium peak. The nonimplanted spectrum was subtracted from the implanted one and the He peak was simulated with RBX with a Gaussian helium depth distribution.

were determined by XRD method (Table 2). Most of the samples were mixtures of two or three silicate phases, e.g., lithium orthosilicate (Li4SiO4), lithium metasilicate (Li2SiO3), lithium disilicate (Li2(Si2O5)), quartz low and cristobalite alpha low (SiO2). In the low Li-containing ceramics a significant amount of silica was found. The Li2O proportion, x, was determined both with XRD and BS methods, and their results and the nominal value agreed pretty well. In order to determine the helium distribution, background spectra were recorded before the ion implantation. Fig. 3 shows the p-BS spectra of a 29.5 Li at.% sample before and after implantation. On the implanted spectrum a small broad peak can be seen which is associated with the implanted helium. The inset shows a detail of the differential spectrum, where the non-implanted spectrum was subtracted from the implanted one. On this spectrum a distinct peak can be observed between channels 75 and 110 which was identified as the implanted helium. The He peak was simulated in RBX using standard Gaussian function as the helium depth distribuðzbÞ2

18 at: at: . tion: f ðzÞ ¼ a  e 2c2 , a ¼ 0:05, b ¼ 8  1018 cm 2 , c ¼ 5  10 cm2 This peak area corresponds to the amount of retained helium of 0.7 ± 0.08  1017 at/cm2. The main results of the helium retention measurements are summarized in Fig. 4 which shows the Li concentration dependence of helium retention. The first data point (at 0 at.%) refers to SiO2 and comes from former experiment [13]. With growing concentration a growing trend of the helium retention can be observed, but there is a threshold value. Under 20 at.% Li concentration no helium can be found in the samples within measurement error. Between 20 and

Table 2 Nominal x in (Li2O)x(SiO2)1x and SiO2 V% determined by p-BS, and phases of the samples determined by XRD method. (Li2O)x(SiO2)1x

SiO2 V%

Nominal

XRD

p-BS

1 0.67 0.50 0.33 0.2

– 0.64 0.49 0.29 0.17

1 0.61 0.44 0.33 0.24

0 26.5 41.1 55.2 63.5

Li at.%

66.7 40.4 29.5 20.7 16.1

Phases (XRD) SiO2

Li2SiO3

Li2(Si2O5)

Li4SiO4

– 4.5 1.4 24 55

– 0 95.9 20 9

– 0 2.7 56 36

– 95.5 0 0 0

}cs et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 1857–1861 D.E. Szo

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4. Conclusions Retained He %

The electrostatic charging of the insulator samples due to the measuring beam was successfully eliminated by wrapping the insulator samples in Al foil. The measured p-BS spectra were simulated adequately. The compositions of the samples were determined by p-BS and XRD methods with an average error of ±10%. The amounts of the retained helium within the samples were determined by subtracting the non-implanted spectra from the implanted ones. The experimental results show a threshold in helium retention depending on the Li concentration. Under 20 at.% all the He within measurement error is able to escape from the material, at around 30 at.% nearly half, and over 65 at.% all the implanted He is retained. The compositions are expressed in SiO2 volume percentages, and a similar trend is found to that of SiOx.

100

Retention of He %

80

60

40

20

0 0

10

20

30

40

50

60

70

Acknowledgements

Li conc. [at.%] (RBS) Fig. 4. Helium retention dependence on atomic Li% in lithium silicates implanted with a fluence of 1.4  1017 4He+/cm2. The first data point (0%) refers to SiO2 with no Li, this result comes from a former experiment [13]. The solid curve is only guiding the eye. The measured points show an increasing trend with increasing Li concentration, however a threshold is clearly found.

The technical assistance of the Van de Graaff accelerator operating staff is gratefully appreciated. D.E.Sz. acknowledges the support from Apponyi Albert program ‘‘Mecenatura” Grant of the National Office for Research and Technology. References

100

Li-silicates Si Retained He (%)

80

60

40

20

0 0

20

40

60

80

100

SiO2 (V%) Fig. 5. Helium retention of lithium silicates (present work) and SiOx from Ref. [13]. The compositions are expressed in SiO2 volume percentages, for comparison. In this representation both materials show a similar trend.

30 Li at.% the yield of helium grows to half of the implanted amount. At 66 at.% of lithium all implanted helium is found in the sample so the implanted helium is fully retained. In Fig. 5 the helium retention of lithium silicates and SiOx [13] are shown. For the sake of comparison the compositions of both kinds of materials are expressed in SiO2 volume percentages. In this representation both materials show a similar trend, below 20 V% SiO2 all the implanted helium is fully retained, while above 60 V% SiO2 practically no helium remains in the materials. According to these experiments a Li concentration threshold for helium retention was observed at room temperature, therefore one could presume that similar threshold might exist at higher temperatures too. In case of tritium not only the diffusion but the chemical effects might also play an important role in the retention mechanisms. For higher tritium production higher lithium concentration is preferred in reactor blankets. In order to find the optimal lithium concentration for blanket materials of future fusion reactors further investigations are needed determining also the hydrogen retention preferably at higher temperatures.

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