Structural changes induced in silica by ion irradiation observed by IR reflectance spectroscopy

Fusion Engineering and Design 98–99 (2015) 2034–2037

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Structural changes induced in silica by ion irradiation observed by IR reflectance spectroscopy Rafael Saavedra a , Piedad Martin a,∗ , David Jimenez-Rey a,b , Rafael Vila a a b

Materiales para Fusión, Laboratorio Nacional de Fusion, CIEMAT, Avda. Complutense 40, 28040 Madrid, Spain Centro Micro-Análisis Materiales (CMAM), Universidad Autónoma de Madrid (UAM), 28049 Madrid, Spain

h i g h l i g h t s • • • • •

IR reflection spectroscopy was used to study microstructural changes in silica. Electronic excitation was the predominant process of energy transfer in this work. IR reflection spectra of ion and neutron irradiated silica has been compared. New IR reflection bands related to three-four member rings appear at high ion fluence. He+ ions are the best suited to reproduce neutron microstructural changes.

a r t i c l e

i n f o

Article history: Received 26 September 2014 Accepted 28 April 2015 Available online 23 May 2015 Keywords: Ion irradiation Silica glass IR reflection Surface cracks

a b s t r a c t The structural changes produced by ion irradiation, He+ (2.5 MeV), O4+ (13.5 MeV), Si4+ (24.4 MeV), and Cu7+ (32.6 MeV), in different types of silica (KU1, KS-4V and Infrasil 301) were observed by IR reflection spectroscopy. The IR reflectance spectra were measured between 400 and 1400 cm−1 . Structural bands wavenumber of the three silica grades, irradiated with the same ion and fluence, is independent on OH or impurity content of silica. Modification in the surface structure of the irradiated face of a silica sample was studied monitoring the changes in the wavenumber of fundamental structural bands as function of the ion fluence. Samples irradiated at high ion fluence present a shift of known structural bands and new IR reflection bands around 608 cm−1 and between 920 and 990 cm−1 , corresponding to a new structure. The spectra of neutron irradiated samples at fluences 1017 and 1018 n/cm2 were also measured and compared with ion irradiated samples. © 2015 Elsevier B.V. All rights reserved.

1. Introduction It is known that energetic ions can be used to understand the effects of neutron irradiation avoiding high residual radioactivity and the problem of finding suitable test reactors. Fused silica aSiO2 , is a candidate material for fusion applications in magnetic and inertial concepts [1–3]. Many plasma diagnostics in fusion reactors will be based on analysis of optical radiation. In earlier works about microscopic and macroscopic changes in silica network induced by ion or particle radiation, Devine [4,5] suggested a “reorganisation” of the silica network with a reduction in the mean Si O Si bridging bond angle and changes in the ring distribution statics. It is known that radiation induces a macroscopic density increment of about 3%. The presence of high density

∗ Corresponding author. Tel.: +34 91 496 2581; fax: +34 91 346 6068. E-mail address: [email protected] (P. Martin). http://dx.doi.org/10.1016/j.fusengdes.2015.04.060 0920-3796/© 2015 Elsevier B.V. All rights reserved.

of self-trapped excitons in ion irradiated samples may contribute to the creation of new structures as was pointed out by Itoh et al. [6,7]. The observed frequency shifts of the IR structural bands can be interpreted as a decrease of the six member SiO-rings and an increase in three and four member rings, accompanied by a corresponding decrease of the Si O Si bond angle [8]. The shift of the silica structural bands caused by changes in fictive temperature and also several mechanical, chemical and optical properties was investigated by Tomozawa et al. [9–11]. They indicated that the Si O Si bond angle decreases with increasing fictive temperature. Modifications of structural and optical properties of silica induced by ion microbeam were studied by Nishikawa et al. [12], and they observed the formation of surface grooves. IR reflectance spectroscopy was used by Bibent et al. [13] to probe and compare the consequences of thermal quenching or ionic implantation on the structure of silica. The effect of fast neutron irradiation (from 1017 to 1021 n/cm2 ) on the structure of fused silica using IR reflection spectroscopy was studied earlier by Abdukadyrova [14] and

R. Saavedra et al. / Fusion Engineering and Design 98–99 (2015) 2034–2037

2. Experimental procedure Optically polished samples (∼1 mm thickness) of high purity synthetic silica, KU1 (OH ∼ 820 ppm) and KS-4V (OH < 1 ppm) from Kurchatov Institute (Russian Federation), that are candidate materials for optical components in fusion devices, and Infrasil 301 (I301) from Heraeus, with higher impurity content (Al ∼ 20 ppm, OH < 8 ppm) were implanted at the Centre for Micro Analysis of Materials (CMAM) of the Universidad Autónoma of Madrid (UAM) using a 5 MV tandem accelerator. The samples were surrounded by a copper mask during irradiation, to avoid electric arcs and to define the irradiation area of 5 × 5 mm2 . The irradiations were performed at RT with different ions: He+ (2.5 MeV), O4+ (13.5 MeV), Si4+ (24.4 MeV), and Cu7+ (32.6 MeV) in a standard scattering chamber at a vacuum up to 10−5 Pa. The ion fluences were from 5 × 1012 to 1.6 × 1015 ions/cm2 . Ion masses were chosen to obtain a broad range of electronic stopping power values. Depth profiles of energy loss of ions, in both electronic and nuclear processes, were estimated from calculations using SRIM 2008 [16]. The estimated projected range of ions was 9 ␮m. The electronic stopping power at sample surface were: ∼6.2 keV/nm for Cu7+ , 3.5 keV/nm for Si4+ , 1.6 keV/nm for O4+ , and for He+ it was 0.25 keV/nm. After ion implantation the specular reflection spectra were obtained at room temperature using a single-beam FourierTransform spectrometer Nicolet 5700 with a DTGS (deuterated triglycine sulfate) detector. A 3 mm diameter beam at a fixed reflection angle of 45◦ was used. The resolution was 4 cm−1 . The spectra were analysed in the wavenumber range between 400 and 1400 cm−1 . As reference for the measurements a golden mirror was used. The spectra of the irradiated face were compared with the unirradiated face spectra.

80 70 60 Reflectance (%)

at fluences from 1018 to 1020 n/cm2 by Bates et al. [15] observing an increase in the network disorder and compaction with neutron fluence. In this work the structural modifications caused by energetic ion irradiation (He, O, Si, and Cu) at several fluences, in different types of silica (KU1, KS-4V and Infrasil 301) were observed by IR reflection spectroscopy. The change in the surface structure of the irradiated face of a silica sample was studied monitoring the wavenumber shift of fundamental structural bands with ion fluence. The reflection spectra of ion irradiated silica were compared with the spectra of neutron irradiated samples at fluences 1017 and 1018 n/cm2 . Macroscopic surface cracks were detected for O, Si and Cu irradiated samples after ion beam shutdown at low fluences.

2035

νR

νS (TO)

4+

KU1 irrad13.5 MeV O

KU1 unirradiated

50 40

5e12 ions/cm

2

1e14 ions/cm

2

5e14 ions/cm

2

1.6e15 ions/cm

νS (LO) 2

30

νB

20 10 0 400

600

800

1000

1200

1400

wavenumber (cm-1) Fig. 1. KU1 silica irradiated with O4+ (13.5 MeV) ions at different fluence from 5 × 1012 to 1.6 × 1015 ions/cm2 .

longitudinal optical mode, ∼ 783 cm−1 ␯B bending, and ∼480 cm−1 ␯R rocking mode. The evolution of the fundamental structural band ␯S (TO) ∼ 1123 cm−1 with implanted dose has been analysed for the different ions. We have observed that the amplitude of the structural ␯S (TO) band decreases when ion fluence increases and this band shifts towards a lower wavenumber value until a certain ion fluence for which no shift was detected. An example of measured spectra is shown in Fig. 1. KU1 silica was implanted with 13.5 MeV O4+ ions at different fluences from 5 × 1012 to 1.6 × 1015 ions/cm2 . The spectra are identical at fluences 5 × 1014 and 1.6 × 1015 ions/cm2 . These fluences agree those to which optical absorption saturation was reached in an earlier work [18]. The saturation was obtained for a deposited energy of ∼5 × 1024 eV/cm3 . A broadening of the bending band ␯B (∼783 cm−1 ) was also detected when ion fluence increases and this band shifts to higher wavenumber. Rocking band (∼480 cm−1 ) shifts and broadens towards lower wavenumber with increasing ion fluence. New bands around 608 cm−1 and between 920 and 990 cm−1 appear at high ion fluence. These new bands and the structural bands shift has been related to the transition of sixmember rings of SiO4 tetrahedra to planar three and four-member rings [19] and to stretching SiO− bonds [20], which were generated in the ion track. Structural bands wavenumber of the three silica grades, irradiated with the same ion and fluence, has been found to be independent on OH or impurity content of silica. The normalised spectra to maximum intensity of KU1, KS4V and I301 irradiated with 24.4 MeV Si4+ ions at the same fluence (1.6 × 1015 ions/cm2 ) is shown in Fig. 2.

3. Results and discussion 3.1. Ion irradiated samples Normalized Reflectance

It is known that the position of the Si O stretching band (wavenumber ∼1123 cm−1 ) is directly correlated with the average Si O Si bond angle in the silica structure. At wavenumbers lower than 2000 cm−1 (wavelengths > 5 ␮m), the observation of absorption of the Si O stretching band requires very thin specimens. For bulk silica samples the observation of reflection mode of this band can be made, since only a thin surface layer of the specimen is observed [17]. We have measured the IR reflection spectra of KU1, KS-4V and I301 silica, after irradiation with He, O, Si and Cu ions at different fluences. An example of the measured spectra is shown in Fig. 1. The observed peaks are assigned to the following vibrational modes of bridging oxygen of Si O Si structure: ∼1123 cm−1 ␯S (TO) asymmetric stretching transverse optical mode, ∼1266 cm−1 ␯S (LO) asymmetric stretching

unirradiated KU1 KS-4V I301

1 0.8

νR

νS (TO)

4+

irrad 24.4MeV Si

0.6

νS (LO)

1.6 e15 ions/cm2

0.4

νB

0.2 0

400

600

800

1000

1200

1400

wavenumber (cm-1) Fig. 2. Normalised spectra at maximum intensity of KU1, KS4V and I301 irradiated with 24.4 MeV Si4+ ions at fluence 1.6 × 1015 ions/cm2 .

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Table 1 IR reflection bands wavenumber. Vibrational modes of bridging oxygen of Si O S structure. Samples (KU1, KS-4V, I301)

Asymmetr stretching ␯S (TO) (cm−1 )

Asymmetr stretching ␯S (LO) (cm−1 )

New bands stretching Si O− (cm−1 )

Bending ␯B (cm−1 )

Implanted samples new band (cm−1 )

Rocking ␯R (cm−1 )

Unirradiated

1123

1266

–

783

–

480

Neutron irrad. 1017 n/cm2 Neutron irrad. 1018 n/cm2

1123 1120

1266 1265

– –

783 783

– –

480 480

Impl. He+ 2.46 MeV 1.6 × 1015 /cm2 Impl. O4+ 13.50 MeV 1.6 × 1015 /cm2 Impl. Si4+ 24.37 MeV 1.6 × 1015 /cm2 Impl. Cu7+ 32.6 MeV 4.8 × 1014 /cm2

1120

1263

986, 945

783

618

480

1100

1250

990, 920–930

792

608

468

1100

1250

990, 920–930

792

608

468

1100

1250

990, 920–930

792

608

468

The spectrum of He+ irradiated samples is clearly different from spectra of samples irradiated with other (heavier) ions at the same fluence. Minor changes were observed in the position of bands with reference to unirradiated material; only at highest fluence new bands around 945 cm−1 and 986 cm−1 raise in He+ irradiated samples. Fig. 3 compares KU1 spectra of unirradiated and irradiated face with different ions at highest fluence (He+ , O4+ , Si4+ and Cu7+ ). The ion irradiated spectra are also compared with the spectra of neutron irradiated samples at fluences 1017 n/cm2 and 1018 n/cm2 . As can be seen in Fig. 3 the spectra of neutron irradiated samples at 1018 n/cm2 (∼10−3 dpa) is similar to spectra of He+ irradiated samples at highest fluence. Table 1 summarises the position of IR reflection bands of irradiated silica with different ions and compares with the reflection bands of neutron irradiated samples at fluences 1017 and 1018 n/cm2 . Fig. 4 shows the shift of the wavenumber position of the Si O asymmetric stretching band in irradiations with different ions, as function of the ion fluence. The frequency shift of IR absorption of ␯S (TO) band as a function of ion fluence was obtained earlier by Awazu et al. [19] with similar results. 3.2. Macroscopic cracks We have observed in an earlier work [18] that implanted samples with O4+ (13.5 MeV) and Si4+ (24.4 MeV) ions at low fluences (ion beam shutdown between ∼5 × 1012 and 1 × 1014 ions/cm2 ) show important post irradiation surface cracks in the implanted

KU1 unirradiated neutr on irrad 10e17 n/cm neutron irrad 10e18 n/cm +

70 Reflectance (%)

2

irrad He 1.6e15 ions/cm

10

irrad O

60

2

2

4+

irrad Si

4+

2

1.6e15 ions/cm

7+

irrad Cu

0

500

700

2

1.6e15 ions/cm

4.8e14 ions/cm

2

900

50 40

KU1

30 20 10 0

400

600

800 1000 1200 wavenumber (cm-1)

1400

Fig. 3. KU1 neutron irradiated at fluences 1017 and 1018 n/cm2 compared with samples irradiated with different ions (He+ , O4+ Si4+ at fluence 1.6 × 1015 ions/cm2 and Cu7+ at fluence 4.8 × 1014 ions/cm2 ).

structural strech(TO) peak position (cm-1)

20

80

area. The position of the structural band ␯S (TO) of implanted samples, at increasing fluences shifts towards a lower wavenumber value till a new stable position (Fig. 4). As it has been pointed (in Section 3.1), at a determined fluence (ion beam shutdown) no shift of the new position of the structural band was observed. It is worth noting that cracks were detected at fluences just below those required to reach the new stable position of the structural band. Implanted samples at higher fluence with a new stable position of the structural band agree with samples with no appreciable macroscopic cracks and with implanted samples showing optical absorption saturation [18]. Cracks in O, Si and Cu implanted samples can be due to the creation of clusters of self-trapped excitons in the tracks which could contribute to creation of new structures [6,7]. At low fluences (ion beam shutdown), when the material is not completely covered with ion tracks, possibly a non-uniform material density is generated, cracks could be produced by the generation of local regions of different density. At higher ion fluences when the material is completely covered with ion tracks, a uniform density of material can be obtained and no appreciable macroscopic cracks were observed. Using synchrotron SAXS (small angle X-ray scattering) measurements and molecular dynamics simulations Kluth et al. [21,22] have demonstrated that swift heavy ions in a-SiO2 is characterised by density fluctuations on nanometer length scales that resemble a steady state when the material is completely covered with ion tracks. No macroscopic cracks were detected at measured fluences in He+ implanted samples. Self-trapped excitons depends on the stopping power of the ions [7] and the estimated stopping power of He+ (2.5 MeV) is ∼0.25 keV/nm, i.e., lower than the critical stopping power to produce clusters of self-trapped excitons (1 keV/nm).

1125 1120 +

1115 1110

He 2.5MeV 4+ O 13.5MeV Si 4+ 24.4MeV Cu 7+ 32.6MeV

1105 1100 1095

10 12

10 13 10 14 1015 2 Fluence (ions/cm )

10 16

Fig. 4. Shift of wavenumber position of the Si O stretching band (reflection spectra) as a function of ion fluence.

R. Saavedra et al. / Fusion Engineering and Design 98–99 (2015) 2034–2037

4. Conclusions The IR reflection spectroscopy technique allowed us to monitor the microstructural changes in irradiated silica. The evolution of the fundamental structural band ␯S (TO) with ion fluence has been analysed in three types of fused silica with different OH and impurity content and has been compared with the reflectance spectra of neutron irradiated samples at 1017 and 1018 n/cm2 fluences. The spectrum of He+ implanted samples (at fluence 1.6 × 1015 ions/cm2 ) is the closest match to the spectrum of neutron irradiated samples at 1018 n/cm2 and it is clearly different from those of implanted samples with other ions (at the same fluence). He+ ions are then the best suited to reproduce neutron microstructural changes Structural bands wavenumber of the three silica grades, irradiated with the same ion and fluence, is independent on OH or impurity content of silica. Electronic excitation was the predominant process of energy transfer in our measurements. New IR reflection bands around 608 cm−1 and between 920 and 990 cm−1 appear at high ion fluence. These new bands and the structural bands shift has been related to the transition of six-member rings of SiO4 tetrahedra to three and four member rings and to creation of non-bridging oxygen SiO− bonds. At deposited energies around 5 × 1024 eV/cm3 a new stable wavenumber value of the fundamental structural band was observed. Important surface cracking was detected for O, Si and Cu irradiated samples at low fluence, showing an excellent correlation with present fluences where maximum changes appear. This can be due to density fluctuations on nanometer length scale if the material is not completely covered by ion tracks. Acknowledgements This work has been supported by the CICYT (Comision Interministerial de Ciencia y Tecnologia, Spain) Project ENE201239787-C06-01, TechnoFusión Project of the CAM (Comunidad Autónoma Madrid) and partially by the European Communities within the European Fusion Technology Programme. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] G. Vayakis, E.R. Hodgson, V. Voitsenya, C.I. Walker, Generic diagnostic issues for a burning plasma experiment, Fusion Sci. Technol. 53 (2008) 699–750 (Chap. 12).

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[2] M. Decreton, T. Shikama, E.R. Hodgson, Performance of functional materials and components in a fusion reactor: the issue of radiation effects in ceramics and glass materials for diagnostics, J. Nuclear Mater. 329–333 (2004) 125–132. [3] J.F. Latkowski, A. Kubota, M.J. Caturla, S.N. Dixit, J.A. Speth, S.A. Payne, Fused silica final optics for inertial fusion energy: radiation studies and system-level analysis, Fusion Sci. Technol. 43 (2003) 540–558. [4] R.A.B. Devine, Macroscopic and microscopic effects of radiation in amorphous SiO2 , Nuclear Instrum. Methods B 91 (1994) 378–390. [5] R.A.B. Devine, Ion implantation and radiation induced structural modifications in amorphous SiO2 , J. Non-Cryst. Solids 152 (1993) 50–58. [6] N. Itoh, A. Marshall Stoneham, Excitonic model of track registration of energetic heavy ions in insulators, Nuclear Instrum. Methods Phys. Res. B146 (1998) 362. [7] N. Itoh, D.M. Duffy, S. Khakshouriand, A.M. Stoneham, Making tracks: electronic excitation roles in forming swift heavy ion tracks, J. Phys. Condens. Matter 21 (2009) 474205. [8] S. Klaumünzer, Ion tracks in quartz and vitreous silica, Nuclear Instrum. Methods B 225 (2004) 136–153. [9] S.R. Ryu, M. Tomozawa, Fictive temperature measurement of amorphous SiO2 films by IR method, J. Non-Cryst. Solids 352 (2006) 3929–3935. [10] M. Tomozawa, J.W. Hong, S.R. Ryu, Infrared (IR) investigation of the structural changes of silica glasses with fictive temperature, J. Non-Cryst. Solids 351 (2005) 1054–1060. [11] A. Agarwal, M. Tomozawa, Correlation of silica glass properties with the infrared spectra, J. Non-Cryst. Solids 209 (1997) 166–174. [12] H. Nishikawa, M. Murai, T. Nakamura, Y. Ohki, M. Oikawa, T. Sato, T. Sakai, Y. Ishii, M. Fukuda, Modification of structural and optical properties of silica glass induced by ion microbeam, Surf. Coat. Technol. 201 (2007) 8185– 8189. [13] N. Bibent, A. Faivre, G. Ferru, J.L. Bantignies, S. Peuget, Silica structural changes induced by thermal treatment or ionic implantation as probed by IR reflectance spectroscopy, J. Appl. Phys. 106 (2009), 063512-1-7. [14] I.Kh. Abdukadyrova, Effect of neutron irradiation on the structure and optical properties of fused SiO2 , Inorg. Mater. 44 (2008) 971–975. [15] J.B. Bates, R.W. Hendricks, L.B. Shaffer, Neutron irradiation effects and structure of noncrystalline SiO2 , J. Chem. Phys. 61 (1974) 4163–4176. [16] J. Ziegler, SRIM. The Stopping and Range of Ions in Matter, 2008, http://www. srim.org [17] A. Agarwal, K.M. Davis, M. Tomoazwa, A simple IR spectroscopic method for determining fictive temperature of silica glasses, J. Non-Cryst. Solids 185 (1995) 191–198. [18] P. Martin, D. Jimenez-Rey, R. Vila, F. Sanchez, R. Saavedra, Optical absorption defects created in SiO2 by Si, O and He ion irradiation, Fusion Eng. Des. 89 (2014) 1679–1683. [19] K. Awazu, S. Ishii, K. Shima, S. Roorda, J.L. Brebner, Structure of latent tracks created by swift heavy-ion bombardment of amorphous SiO2 , Phys. Rev. B 62 (2000) 3689–3698. [20] M.E. Lynch, D.C. Folz, D.E. Clark, Use of FTIR reflectance spectroscopy to monitor corrosion mechanisms on glass surfaces, J. Non-Cryst. Solids 353 (2007) 2667–2674. [21] P. Kluth, O.H. Pakarinen, F. Djurabekova, R. Giulian, M.C. Ridgway, A.P. Byrne, K. Nordlund, Nanoscale density fluctuations in swift heavy ion irradiated amorphous SiO2 , J. Appl. Phys. 110 (2011), 123520(1)-123520(5). [22] P. Kluth, C.S. Schnohr, O.H. Pakarinen, F. Djurabekova, D.J. Sprouster, R. Giulian, M.C. Ridgway, A.P. Byrne, C. Trautmann, D.J. Cookson, K. Nordlund, M. Toulemonde, Fine structure in swift heavy ion tracks in amorphous SiO2 , Phys. Rev. Lett. 101 (2008), 175503(1)-175503(4).