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Changes in alkali-silicate glasses induced with electron irradiation
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Journal of Non-Crystalline Solids 354 (2008) 1169–1171 www.elsevier.com/locate/jnoncrysol
Changes in alkali-silicate glasses induced with electron irradiation Ondrej Gedeon a
a,*
, Josef Zemek b, Karel Jurek
b
Department of Glass and Ceramics, Institute of Chemical Technology, Technicka 5, CZ-166 28 Prague, Czech Republic Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, CZ-182 21 Prague, Czech Republic
b
Available online 19 November 2007
Abstract Potassium-silicate, potassium–calcium-silicate and sodium–potassium-silicate glasses were irradiated with low- (1600 eV) and highenergy (50 keV) electrons. Induced changes were recorded by EPMA and XPS. Structural changes connected with transport of alkali ions are discussed. Ó 2007 Elsevier B.V. All rights reserved. PACS: 81.05.Kf; 61.80. x Keywords: Alkali silicates; Radiation; Surfaces and interfaces
1. Introduction Glass irradiated by an electron beam undergoes changes in its structure, caused mostly by ionization and ballistic interaction of high-energy particles with solid constituents [1]. Depending on the irradiating dose and intensity various effects may be introduced in the glass, among them increased disorder in the original structure accompanied with volume changes, phase separation or gas accumulation can be mentioned. Interaction of an electron beam with glass containing alkali ions is a point of interest for analysts dealing with electrons as an excitation source. As a technological material used in nuclear industry (nuclear waste deposition, insulating material in nuclear power plants) the understanding of fundamentals of the electron interaction with glass is a base for its safe use. Although the electron energies coming from the nuclear sources are significantly higher (of the order of MeV) than used in the presented study, the production of electrons of energy of orders of eV or keV are an inevitable result of the interaction of the high-energy particles with glass and therefore the low-
*
Corresponding author. Tel.: +420 220443695; fax: +420 224313200. E-mail address: [email protected] (O. Gedeon).
0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.12.125
energy electron interaction necessary accompanies each high-energy glass irradiation. One of the most interesting and wide studied phenomena is the alkali ion migration in alkali glass irradiated by electron beams. The phenomenon can be observed even for very low-energy beams due to weakly bonded alkali ions in the glass structure. These changes have been observed as the decay of alkali intensities by many authors in the electron probe microanalysis (EPMA) or in Auger electron spectroscopy (AES) [2–4]. If the macroscopic alkali migration is observed by EPMA (hereafter referred as high-energy region), a decay curve, introduced as the alkali X-ray intensity versus time, can be recorded. The measured decay curves generally display two distinct parts [5]. Instantaneously after the exposure, the recorded X-ray signal shows linear regime and after some time, denoted as the incubation period, the signal abruptly changes into an exponential-like decay. The incubation time can vary in the range from seconds to hours depending on the exposure conditions and on the type of glass. In case of the low current density of the electron source the time scale can be replaced by the dose scale as the temperature increase is negligible in this case. The alkali migration and oxygen desorption observed has been usually correlated with a mechanism proposed by
1170
O. Gedeon et al. / Journal of Non-Crystalline Solids 354 (2008) 1169–1171
Lineweaver [6]. The primary electrons produce a negatively charged layer at about the electron penetration depth. The charged layer is assumed to attract positively charged alkali ions from the surface to bulk. Upon decreasing the energy of the primary electrons, the incubation period/dose first slightly increases. The following variation of the incubation period/dose depends on glass composition but it appears that the incubation period/dose goes to infinity as the energy of primary electrons reaches some threshold value, below which no incubation period/dose is found [7]. This energy threshold is proportional to the mass of alkali ion and it also depends on the activation energy of the ion for diffusion in glass. To explain the existence of the threshold energy value for the incubation period/dose, elastic scattering assisted hopping mechanism was suggested [8] for the alkali ions releasing. On contrary, for low-energy irradiation (up to 3 keV) towards the surface migration was found. The surface relaxation processes and Gibbsian segregation may count for out-migration. A formation of neutral alkali atoms under the electron irradiation was also suggested [9]. Brow [10] has shown the formation of metallic sodium on the surface of irradiated binary sodium-silicate glass, whereas no metallic sodium was found in a CaO-containing glass. Using the top-surface sensitive ion scattering spectroscopy (ISS), Then and Pantano [11] observed alkali signal increase at the beginning of electron irradiation and then confirmed to-the-surface migration during the initial stages of electron irradiation. 2. Experimental Glasses were melted using high purity batch in a Pt crucible and cast on a metal plate to prevent crystallization. The plate of glass was annealed near Tg for 16 h. The glasses were transparent and contained no bubbles or cords. Their compositions were as follows: potassium–calcium-silicate glass KCS: 79.5 wt% SiO2, 8.5 wt% CaO, 12.0 wt% K2O (5.47 at.% of K), sodium–potassium-silicate
glass NKS: 5.0 mol% Na2O, 10.0 mol% K2O, 85.0 mol% SiO2; and potassium-silicate glass KS: 10 mol% K2O, 90 mol% SiO2). Glasses undergone EPMA were cut into smaller samples and afterward polished under isopropyl alcohol to prevent water corrosion of the glass surface. The samples were then coated with approximately 30 nm carbon layers. Specimens were exposed in an electron microprobe analyser; the diameter of the beam was measured at the fluorescent screen using an optical microscope and set to 50/100 lm, so that the widening of the defocused beam caused by electron scattering can be neglected compared with the beam diameter. The accelerating voltage of the electron beam was 50 kV. Glasses undergone XPS were prepared as follows. A rectangular glass sample was fixed to a sample holder. Before electron irradiation, a clean glass surface was prepared by fracturing of the glass rod under UHV condition using a standard cleaving device. The clean pristine glass surface was step-by-step irradiated by electron beam with various electron doses. The electron beam of energy of 1600 eV was scanned uniformly over the entire glass surface. The impact angle of the electron beam was perpendicular to the sample surface. The detection angle was surface normal except for NKS glass, where additional measurements were performed under 60° counted from the surface normal. More details can be found in [12,13]. 3. Results and discussion Intensity changes of potassium during continuous electron irradiation in EPMA are presented in Fig. 1. Incubation doses in KS, KCS, and NKS glasses are clearly ordered according the expected potassium mobilities in the corresponding glasses. Hence, the incubation dose is the lowest in the binary potassium-silicate KS glass, higher in the calcium-silicate glass, and the significantly highest in NKS glass. The remarkable prolongation of the incubation period/dose in the last case is caused by the so called mixed-alkali effect, which decreases potassium mobility sig-
1.1
Relative intensity
1.0 0.9 0.8 0.7 0.6 0.5 0
4000
8000
12000
16000
20000
Dose [C/m2] Fig. 1. Decay of potassium intensity with dose in NKS (long-dashed line), KS (solid line), and KCS (short-dashed line) glasses.
Relative concentration
O. Gedeon et al. / Journal of Non-Crystalline Solids 354 (2008) 1169–1171
1171
2.3
8
1.9
6
1.5
4
1.1
2
0.7 0
500
1000
1500
2000
2500
3000
0 3500
Dose [C/m2] Fig. 2. Relative potassium concentrations with respect to its concentration measured for freshly fractured glass surface. Triangles mark experimental results from KCS glass, squares from KS glass, and circles from NKS glass. Broken line represents a model fit assuming two layers on-the-bulk [10]. Open symbols represent data obtained under low-angle detection (the topmost surface layer) and their values are shown on right axis. The solid lines connected symbols serve only as guides for an eye.
nificantly. It should be mentioned that the experimentally determined incubation periods/doses are within 10% for the presented curves. However, both accuracy and reproducibility decrease with the decreasing energy of the primary electrons and with increasing of the current density (when the temperature increases). Generally, it is believed that potassium ions are driven to bulk by electric filed created by the trapped electrons. The incubation period is then a result of a period needed for the formation of the percolation paths, dense and long enough to enable the potassium ions, released from their site by the elastic scattering assistance, to migrate with significantly higher rate [14]. XPS, probing the glass surfaces up to a few nanometers yields qualitatively similar courses of the compositional changes as EPMA for KS and KCS glasses (see Fig. 2). It means that after some time the potassium intensity starts to decrease. However, the first period is now characterized by the incremental potassium increase. The suggested model [9] shows that the increase corresponds to the formation of the potassium rich top layer. The formation of the layer is caused by the surface relaxation in which potassium ions play an active role. The irradiation makes potassium ions easier to reach the surface as the glass structure is loosened by the electron impact. As soon as the surface relaxation is saturated the electric field assisted migration of potassium ions to bulk becomes dominant and therefore concentration decrease is observed. The results obtained from NKS glass strongly support both the role of the surface relaxation in which potassium ions take part and the formation of the potassium rich topmost layer [11]. The potassium concentration in the topmost layer (obtained for low-angle detection) increased more than six times for the small doses while the concentration in the thicker layer (obtained for the normal-angle detection) is significantly lower. The very short increase of potassium concentration
limited to the smallest doses can be explained by the interplay of sodium ions that are more mobile [10] (Fig. 2). 4. Conclusion The changes in the compositions of the electron irradiated potassium-silicate glasses are studied by means of EPMA and XPS. The potassium intensity changes are strongly correlated with the glass structure changes accompanied with the surface relaxation. Acknowledgments This work was supported by the Grant Agency of the Czech Republic through Grant No. 104/06/0202. This study was also a part of the research programme MSM 6046137302: Preparation and research of functional materials and material technologies using micro- and nanoscopic methods. References [1] W.J. Weber, R.C. Ewing, C.A. Angell, G.W. Arnold, A.N. Cormack, J.M. Delaye, D.L. Griscom, L.W. Hobbs, A. Navrotsky, D.L. Price, A.M. Stoneham, M.C. Weinberg, J. Mater. Res. 12 (8) (1997) 1946. [2] D.M. Usher, J. Phys. C: Solid State Phys. 14 (1981) 2039. [3] A. Miotelo, J. Phys. C: Solid State Phys. 19 (1986) 445. [4] P. Mazzoldi, A. Miotelo, J. Non-Cryst. Solids 95&96 (1987) 161. [5] O. Gedeon, V. Hulinsky, K. Jurek, Mikrochim. Acta (Suppl.) 132 (2000) 505. [6] J.L. Lineweaver, J. Appl. Phys. 38 (1963) 1786. [7] K. Jurek, O. Gedeon, Mikrochim. Acta 139 (2002) 67. [8] O. Gedeon, M. Lisˇka, J. Non-Cryst. Solids 286 (2001) 181. [9] R.J. Gosssink, T.P.A. Lommen, Appl. Phys. Lett. 34 (1979) 444. [10] R.K. Brow, J. Non-Cryst. Solids 175 (1994) 155. [11] A.M. Then, C.G. Pantano, J. Non-Cryst. Solids 120 (1990) 178. [12] O. Gedeon, J. Zemek, J. Non-Cryst. Solids 320 (2003) 177. [13] J. Zemek, O. Gedeon, J. Non-Cryst. Solids 337 (2004) 268. [14] O. Gedeon, K. Jurek, V. Hulı´nsky´, J. Non-Cryst. Solids 246 (1999) 1.
Journal of Non-Crystalline Solids 354 (2008) 1169–1171 www.elsevier.com/locate/jnoncrysol
Changes in alkali-silicate glasses induced with electron irradiation Ondrej Gedeon a
a,*
, Josef Zemek b, Karel Jurek
b
Department of Glass and Ceramics, Institute of Chemical Technology, Technicka 5, CZ-166 28 Prague, Czech Republic Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, CZ-182 21 Prague, Czech Republic
b
Available online 19 November 2007
Abstract Potassium-silicate, potassium–calcium-silicate and sodium–potassium-silicate glasses were irradiated with low- (1600 eV) and highenergy (50 keV) electrons. Induced changes were recorded by EPMA and XPS. Structural changes connected with transport of alkali ions are discussed. Ó 2007 Elsevier B.V. All rights reserved. PACS: 81.05.Kf; 61.80. x Keywords: Alkali silicates; Radiation; Surfaces and interfaces
1. Introduction Glass irradiated by an electron beam undergoes changes in its structure, caused mostly by ionization and ballistic interaction of high-energy particles with solid constituents [1]. Depending on the irradiating dose and intensity various effects may be introduced in the glass, among them increased disorder in the original structure accompanied with volume changes, phase separation or gas accumulation can be mentioned. Interaction of an electron beam with glass containing alkali ions is a point of interest for analysts dealing with electrons as an excitation source. As a technological material used in nuclear industry (nuclear waste deposition, insulating material in nuclear power plants) the understanding of fundamentals of the electron interaction with glass is a base for its safe use. Although the electron energies coming from the nuclear sources are significantly higher (of the order of MeV) than used in the presented study, the production of electrons of energy of orders of eV or keV are an inevitable result of the interaction of the high-energy particles with glass and therefore the low-
*
Corresponding author. Tel.: +420 220443695; fax: +420 224313200. E-mail address: [email protected] (O. Gedeon).
0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.12.125
energy electron interaction necessary accompanies each high-energy glass irradiation. One of the most interesting and wide studied phenomena is the alkali ion migration in alkali glass irradiated by electron beams. The phenomenon can be observed even for very low-energy beams due to weakly bonded alkali ions in the glass structure. These changes have been observed as the decay of alkali intensities by many authors in the electron probe microanalysis (EPMA) or in Auger electron spectroscopy (AES) [2–4]. If the macroscopic alkali migration is observed by EPMA (hereafter referred as high-energy region), a decay curve, introduced as the alkali X-ray intensity versus time, can be recorded. The measured decay curves generally display two distinct parts [5]. Instantaneously after the exposure, the recorded X-ray signal shows linear regime and after some time, denoted as the incubation period, the signal abruptly changes into an exponential-like decay. The incubation time can vary in the range from seconds to hours depending on the exposure conditions and on the type of glass. In case of the low current density of the electron source the time scale can be replaced by the dose scale as the temperature increase is negligible in this case. The alkali migration and oxygen desorption observed has been usually correlated with a mechanism proposed by
1170
O. Gedeon et al. / Journal of Non-Crystalline Solids 354 (2008) 1169–1171
Lineweaver [6]. The primary electrons produce a negatively charged layer at about the electron penetration depth. The charged layer is assumed to attract positively charged alkali ions from the surface to bulk. Upon decreasing the energy of the primary electrons, the incubation period/dose first slightly increases. The following variation of the incubation period/dose depends on glass composition but it appears that the incubation period/dose goes to infinity as the energy of primary electrons reaches some threshold value, below which no incubation period/dose is found [7]. This energy threshold is proportional to the mass of alkali ion and it also depends on the activation energy of the ion for diffusion in glass. To explain the existence of the threshold energy value for the incubation period/dose, elastic scattering assisted hopping mechanism was suggested [8] for the alkali ions releasing. On contrary, for low-energy irradiation (up to 3 keV) towards the surface migration was found. The surface relaxation processes and Gibbsian segregation may count for out-migration. A formation of neutral alkali atoms under the electron irradiation was also suggested [9]. Brow [10] has shown the formation of metallic sodium on the surface of irradiated binary sodium-silicate glass, whereas no metallic sodium was found in a CaO-containing glass. Using the top-surface sensitive ion scattering spectroscopy (ISS), Then and Pantano [11] observed alkali signal increase at the beginning of electron irradiation and then confirmed to-the-surface migration during the initial stages of electron irradiation. 2. Experimental Glasses were melted using high purity batch in a Pt crucible and cast on a metal plate to prevent crystallization. The plate of glass was annealed near Tg for 16 h. The glasses were transparent and contained no bubbles or cords. Their compositions were as follows: potassium–calcium-silicate glass KCS: 79.5 wt% SiO2, 8.5 wt% CaO, 12.0 wt% K2O (5.47 at.% of K), sodium–potassium-silicate
glass NKS: 5.0 mol% Na2O, 10.0 mol% K2O, 85.0 mol% SiO2; and potassium-silicate glass KS: 10 mol% K2O, 90 mol% SiO2). Glasses undergone EPMA were cut into smaller samples and afterward polished under isopropyl alcohol to prevent water corrosion of the glass surface. The samples were then coated with approximately 30 nm carbon layers. Specimens were exposed in an electron microprobe analyser; the diameter of the beam was measured at the fluorescent screen using an optical microscope and set to 50/100 lm, so that the widening of the defocused beam caused by electron scattering can be neglected compared with the beam diameter. The accelerating voltage of the electron beam was 50 kV. Glasses undergone XPS were prepared as follows. A rectangular glass sample was fixed to a sample holder. Before electron irradiation, a clean glass surface was prepared by fracturing of the glass rod under UHV condition using a standard cleaving device. The clean pristine glass surface was step-by-step irradiated by electron beam with various electron doses. The electron beam of energy of 1600 eV was scanned uniformly over the entire glass surface. The impact angle of the electron beam was perpendicular to the sample surface. The detection angle was surface normal except for NKS glass, where additional measurements were performed under 60° counted from the surface normal. More details can be found in [12,13]. 3. Results and discussion Intensity changes of potassium during continuous electron irradiation in EPMA are presented in Fig. 1. Incubation doses in KS, KCS, and NKS glasses are clearly ordered according the expected potassium mobilities in the corresponding glasses. Hence, the incubation dose is the lowest in the binary potassium-silicate KS glass, higher in the calcium-silicate glass, and the significantly highest in NKS glass. The remarkable prolongation of the incubation period/dose in the last case is caused by the so called mixed-alkali effect, which decreases potassium mobility sig-
1.1
Relative intensity
1.0 0.9 0.8 0.7 0.6 0.5 0
4000
8000
12000
16000
20000
Dose [C/m2] Fig. 1. Decay of potassium intensity with dose in NKS (long-dashed line), KS (solid line), and KCS (short-dashed line) glasses.
Relative concentration
O. Gedeon et al. / Journal of Non-Crystalline Solids 354 (2008) 1169–1171
1171
2.3
8
1.9
6
1.5
4
1.1
2
0.7 0
500
1000
1500
2000
2500
3000
0 3500
Dose [C/m2] Fig. 2. Relative potassium concentrations with respect to its concentration measured for freshly fractured glass surface. Triangles mark experimental results from KCS glass, squares from KS glass, and circles from NKS glass. Broken line represents a model fit assuming two layers on-the-bulk [10]. Open symbols represent data obtained under low-angle detection (the topmost surface layer) and their values are shown on right axis. The solid lines connected symbols serve only as guides for an eye.
nificantly. It should be mentioned that the experimentally determined incubation periods/doses are within 10% for the presented curves. However, both accuracy and reproducibility decrease with the decreasing energy of the primary electrons and with increasing of the current density (when the temperature increases). Generally, it is believed that potassium ions are driven to bulk by electric filed created by the trapped electrons. The incubation period is then a result of a period needed for the formation of the percolation paths, dense and long enough to enable the potassium ions, released from their site by the elastic scattering assistance, to migrate with significantly higher rate [14]. XPS, probing the glass surfaces up to a few nanometers yields qualitatively similar courses of the compositional changes as EPMA for KS and KCS glasses (see Fig. 2). It means that after some time the potassium intensity starts to decrease. However, the first period is now characterized by the incremental potassium increase. The suggested model [9] shows that the increase corresponds to the formation of the potassium rich top layer. The formation of the layer is caused by the surface relaxation in which potassium ions play an active role. The irradiation makes potassium ions easier to reach the surface as the glass structure is loosened by the electron impact. As soon as the surface relaxation is saturated the electric field assisted migration of potassium ions to bulk becomes dominant and therefore concentration decrease is observed. The results obtained from NKS glass strongly support both the role of the surface relaxation in which potassium ions take part and the formation of the potassium rich topmost layer [11]. The potassium concentration in the topmost layer (obtained for low-angle detection) increased more than six times for the small doses while the concentration in the thicker layer (obtained for the normal-angle detection) is significantly lower. The very short increase of potassium concentration
limited to the smallest doses can be explained by the interplay of sodium ions that are more mobile [10] (Fig. 2). 4. Conclusion The changes in the compositions of the electron irradiated potassium-silicate glasses are studied by means of EPMA and XPS. The potassium intensity changes are strongly correlated with the glass structure changes accompanied with the surface relaxation. Acknowledgments This work was supported by the Grant Agency of the Czech Republic through Grant No. 104/06/0202. This study was also a part of the research programme MSM 6046137302: Preparation and research of functional materials and material technologies using micro- and nanoscopic methods. References [1] W.J. Weber, R.C. Ewing, C.A. Angell, G.W. Arnold, A.N. Cormack, J.M. Delaye, D.L. Griscom, L.W. Hobbs, A. Navrotsky, D.L. Price, A.M. Stoneham, M.C. Weinberg, J. Mater. Res. 12 (8) (1997) 1946. [2] D.M. Usher, J. Phys. C: Solid State Phys. 14 (1981) 2039. [3] A. Miotelo, J. Phys. C: Solid State Phys. 19 (1986) 445. [4] P. Mazzoldi, A. Miotelo, J. Non-Cryst. Solids 95&96 (1987) 161. [5] O. Gedeon, V. Hulinsky, K. Jurek, Mikrochim. Acta (Suppl.) 132 (2000) 505. [6] J.L. Lineweaver, J. Appl. Phys. 38 (1963) 1786. [7] K. Jurek, O. Gedeon, Mikrochim. Acta 139 (2002) 67. [8] O. Gedeon, M. Lisˇka, J. Non-Cryst. Solids 286 (2001) 181. [9] R.J. Gosssink, T.P.A. Lommen, Appl. Phys. Lett. 34 (1979) 444. [10] R.K. Brow, J. Non-Cryst. Solids 175 (1994) 155. [11] A.M. Then, C.G. Pantano, J. Non-Cryst. Solids 120 (1990) 178. [12] O. Gedeon, J. Zemek, J. Non-Cryst. Solids 320 (2003) 177. [13] J. Zemek, O. Gedeon, J. Non-Cryst. Solids 337 (2004) 268. [14] O. Gedeon, K. Jurek, V. Hulı´nsky´, J. Non-Cryst. Solids 246 (1999) 1.