Modifications Induced by Irradiation in Glasses

106 Pages·1992·13.799 MB·English

by  P. Mazzoldi (Eds.)

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MODIFICATION S INDUCE D BY
IRRADIATIO N IN GLASSE S
PROCEEDINGS OF SYMPOSIUM F ON
CHEMICAL AND PHYSICAL MODIFICATIONS
INDUCED BY IRRADIATION IN GLASSES
OF THE 1991 Å-MRS FALL CONFERENCE
STRASBOURG, FRANCE, NOVEMBER 5-7, 1991
Edited by
P.MAZZOLDI
Dipartimento di Fisica 'Galileo'
Universita degli Studi di Padova, Italy
1992
NORTH-HOLLAN D
AMSTERDAM - LONDON - NEW YORK - TOKYO
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í
FOREWOR D
The symposium on "Chemical and Physical Modification Induced by Irradiation in
Glasses" covered many aspects of the interaction of radiation with glasses,
comprising contributions from a broad scientific community.
The papers presented at the symposium treated topics in an interdisciplinary
manner.
Talks given by invited speakers covered aspects of ion implantation applications
in the field of optics (Professor P. Townsend), of mechanical properties (Professor
G.W. Arnold), of chemical modifications and compound formation (Professor J.C.
Dran and G. Battaglin).
It is worth remarking that many of the contributions arose from the pursuit of the
interdisciplinary approach required to connect technological applications to the
basic interactions of energetic ions with insulators.
The present meeting was the second organized by E.M.R.S. in the field of ion
interaction with insulators. The first was held in 1984.
The symposium represented an ideal bridge between the sixth (Weimar 1991) and
the planned seventh International Conference on Radiation Effects in Insulators
(REI '93), which will be held in Japan.
P. Mazzoldi
vi
SUPPORTIN G ORGANIZATION S AND SPONSOR S
This conference was held under the auspices of:
The Council of Europe
The Commission of the European Communities
It is our pleasure to acknowledge with gratitude the financial assistance
provided by:
Banque Populaire (France)
Centre de Recherches Nucleaires (France)
Centre National de la Recherche Scientifique (France)
Elsevier Science Publishers B.V. (The Netherlands)
Service de Documentation Touristique du Palais des Congres de
Strasbourg (France)
The Commission of the European Communities
The Council of Europe
The European Parliament
Modifications Induced by Irradiatio n in Glasses
P. Mazzoldi (editor)
© 1992 Elsevier Science Publishers B.V. All rights reserved.
Chemical modification s induced by ion implantatio n in glasses
J.-C. Dran
CSNSM-IN2P3-CNRS , Bat. 108 91405 Orsay , Franc e
Abstrac t
Chemica l effects of ion implantatio n in silicate glasse s are reviewe d with
emphasi s to thos e induce d by inert ions, namel y the radiolytic decompositio n of
oxide component s associate d with the formation of oxygen bubble s and the chang e
in the resistanc e to aqueou s corrosio n in connexio n with the long-term durability of
vitrified radioactiv e waste .
1. INTRODUCTION
Alike other material s subjecte d to ion implantation , oxide glasse s underg o
various chemica l modifications , generall y not deliberatel y designe d but rathe r
occurring as a side effect of ion-bea m processin g and to a large exten t accountin g
for change s in other materia l properties . Two broad classe s of transformation s can
be distinguished , namel y those involving the glass chemica l compositio n or its
chemica l reactivity. In the first case , the interes t lies on the final chemica l state of the
target-implan t syste m and in the secon d one , on the materia l behavio r toward s
specific reagents . It should be underline d that, althoug h compositio n and reactivity
are generall y linked, reactivity change s do not necessaril y involve significan t
compositiona l changes , but can have other causes , for exampl e bombardmen t
induce d stresse s or structura l modifications .
Compositiona l change s can in turn be subdivide d into two categorie s
(igure 1):
i. change s which are only relate d to energ y depositio n and do not depen d on the
chemica l propertie s of the incident ions, being produce d eve n by inert gas ions.
They are specific to compoun d material s like silicate glasse s and involve bond
breakin g and rearrangement . Such effects occur along the collision cascade ; they
are not specific to ion bombardment , but can be produce d by othe r types of
radiations . They belong to the field of radiation chemistry .
ii. change s which involve bond formation betwee n reactiv e ions and targe t
constituen t atoms . Such chemica l effects are encountere d when processin g any
material , including monoatomi c target s like metals . They are equivalen t to the so-
called hot atom chemistr y induce d by the recoil atom during nuclea r reactio n or
radioactiv e decay . It should be stresse d that surfac e contaminant s [most often Ï or
C] knocke d on by incident ions can also form bonds with matrix constituents .
To the first group belong s the generatio n of oxyge n bubble s induce d by the
decompositio n of oxide component s into lower-valenc e oxides or metals and to the
2
second , the formation of compound s such as carbide s or nitrides . Thes e two
compositiona l change s are generall y superimposed , particularl y for high ion
fluences , which can modify primary bonds .
Inorganic insulator s have bee n the most studie d material s with respec t to
ion-induce d chemica l modifications , becaus e of both the grea t wealth of observe d
effects and the broad variety of analytica l technique s which can be implemente d for
this type of materials , including optical spectrometries . Severa l review articles have
bee n publishe d [1-4], but none deals exclusivel y with glasses . The presen t article,
largely base d on a previous one [4], intends to fill this gap. Compositiona l change s
induce d by ion bombardmen t in silicate glasse s will be the scop e of the first part.
They will be restricte d to those induce d by inert ions, as compoun d formation is
discusse d by C. Battaglin in this symposium . The secon d part will be devote d to
change s in the chemica l reactivity of glasses , exemplifie d by their aqueou s
corrosion , in connexio n with the importan t issue of the long-ter m durability of
vitrified high-leve l radioactiv e waste .
Inciden t ion
Chemica l propertie s
Surfac e of surfac e
contaminan t (Chemical reactivity)
Compositiona l change s
in the cascad e
(Radiation chemistry)
Bon d formatio n with reactiv e implan t
(Hot atom chemistry)
Figure 1. The different types of chemica l transformation s induce d by ion
bombardmen t
2. COMPOSITIONA L CHANGES INDUCED BY INERT IONS
As state d above , the observe d effects are not specific to ion bombardment ,
since they are also produce d by electron s and gamm a rays [5]. Most experiment s in
this field rely on irradiation with electro n microscop e beam s and subsequen t
observatio n of microstructura l modifications in the form of gas bubble s attribute d to
molecula r oxygen . Data obtaine d with various ion beam s unequivocall y link such a
bubble formation to a radiolytic decompositio n in which the electroni c energ y loss
plays the dominan t role [6].
3
2.1. Vitreou s silic a
There is no evidenc e for a radiolytic decompositio n of pure silica unde r
electro n [7] or high-energ y ion [8] bombardment . Indee d high resolutio n transmis -
sion electro n microscop e (HRTEM) investigation s by de Natale and Howitt [7] did
not revea l the formation of gas bubbles , even after dose s two orders of magnitud e
larger than thos e necessar y to observ e such bubble s in multicomponen t glasses .
Such a high resistanc e to radiation s confirms early results by Todd et al. [9] on the
absenc e of oxygen outgassin g upon electro n irradiation of silica, in contras t to more
comple x glasses .
2.2. Multicomponen t glasse s
Numerou s investigation s clearly demonstrat e the occurrenc e of a radiolytic
decompositio n induce d by eithe r electro n or ion beam s on multicomponen t
glasses . Identification of oxygen as the sole constituen t of bubble s was establishe d
by mas s spectrometr y [9]. The conditions of bubble growth have bee n studie d in
detail by mean s of HRTEM [8, 10]; both a threshol d dose of irradiation and a strong
temperatur e dependenc e have bee n evidenced .
The compositio n dependenc e of the proces s support s a mechanis m base d
on the breakin g of oxide bonds , the diffusion of the correspondin g cation and the
formation of molecula r oxygen which aggregate s as bubble s [7]. The probability of
bond breakin g varies conversel y with the bond strengt h and thus increase s from
network forming element s to network modifiers. It is the highes t for alkalis (e.g. bond
strengt h of Na 0.87 eV, compare d to that of Si 4.62 eV). The radiolysis mechanis m
itself relies on electroni c excitation (above the band gap of abou t 9 eV for silica)
and non-radiativ e electron-hol e recombinatio n focuse d on a single atom ; the latter
is thus put into motion, since the available energ y exceed s the bond strengt h [11].
XPS experiment s provide an additiona l featur e of particula r significanc e
[12,13]. For example , bombardmen t of alkali silicate glasse s with 4 keV Ar ions
produce s a continuou s evolution of the Ï 1s peak , with a decreas e of the proportion
of non-bridgin g oxygen and a concomitan t increas e of that of bridging oxygen [13].
Since a simultaneou s surfac e desalkalinizatio n occurs , such a Si-O-Si polymeriza -
tion proces s is triggere d by the breakin g of the alkali-oxyge n bond .
3. CHANGES IN CHEMICAL REACTIVITY
The only propert y investigate d so far is the resistanc e to aqueou s corrosion .
While ion implantatio n has bee n extensivel y use d to improve the corrosio n resis -
tance of metals [14], in the cas e of glasse s and more generall y of inorganic insula-
tors, the expecte d effect is rathe r to enhanc e the corrosio n rate. In this respect ,
besid e a few early works dealing with ion bea m microlithography , ion implantatio n
has bee n mostly use d to investigat e the radiation effects associate d with á-deca y
on the chemica l durability of borosilicat e glasse s encapsulatin g high-leve l
radioactiv e waste . The quantificatio n of radiation effects is not always straightfor -
ward becaus e of the complexity of the mechanis m of aqueou s corrosio n of silicate
glasses . It is worth recalling that this phenomeno n involves severa l steps , the
importanc e of which depend s on the solution pH [15]: i) ion exchang e betwee n
mobile cations (alkalis and alkaline earths ) and H+ or H3O+; ii) permeatio n of mole-
4
cular water; iii) attac k of siloxane groups Si-O-Si by OH" and network destruction ;
iv) condensatio n of silanol groups Si-OH (repolymerization) ; v) readsoptio n of
dissolve d silica or comple x ions on the glass surface . At high pH, networ k
destructio n is predominan t and the dissolutio n is congruen t (etching) , wherea s at
acid or neutra l pH, ion exchang e and hydration are dominant , leading to a selectiv e
dissolutio n (leaching ) and the formation on the glass surfac e of a hydrate d layer
enriche d with poorly soluble transition and heav y elements . In the first case , the
weight loss or surfac e recessio n can be use d to monitor the corrosio n progress ,
while in the cas e of leachin g a convenien t monitor is the exten t of hydratio n
(thicknes s of the hydrate d layer and/o r total hydroge n content) . In the following
section s we will successivel y describ e the experiment s relevan t to both types of
corrosio n regimes .
3.1. Ion bea m effect s on etchin g
Most studie s of ion bea m effects on the etch rate of glasse s rely on experi-
ments illustrated by figure 2. Polishe d section s partially coate d with a resis t are
irradiate d through a mask , etche d with a prope r reagen t and probe d with a profilo-
mete r in order to measur e the two step heights h and h*, correspondin g to the unir-
radiate d and irradiate d zones . Typical kinetic variations are shown on figure 2 and
define the correspondin g etch rates V and V*, the enhancemen t factor K=V7V and
the etchabl e range Re, i.e. the thicknes s of the layer having an enhance d etchabi -
lity. On table 1 are reporte d literature data relevan t to etch rate enhancemen t indu-
ced by ion bombardmen t in silicate glasses . Most of the early works dea l with light
ions in the energ y range 10-100 keV/amu where electroni c energ y loss is domi-
nant. Vitreous silica is the most studie d material ; its etch rate with dilute HF is only
moderatel y increased , for exampl e by a factor of 5 unde r  implantatio n [16] and
Ions
Mask
Resist é
4
Depth
I J V
Re
v* í
Etch time
é
Figure 2. Principle of measuremen t of ion bea m enhance d etch rate.
5
Table 1
Etch rate enhancemen t induce d by ion bombardmen t in silicate glasse s
Targe t Ion energ y Fluenc e Etchan t Ê Reference s
(keV) (cm-2)
 33.165 , 012.101 · £ 5
S i 0 2
Si0 H20 60 > 12
2 Q 1 0 5
125-300
Soda-lim e H,He,P b > 1 0 12 NaCI soln. up to 50 [18]
Boro- 50-570
silicates
C, Í or Ï implantatio n [17]. The etch rate of the implante d layer has bee n found to
be constan t along the range of the ions and correlate d to the electroni c stoppin g
power [18].
In the framewor k of a systemati c study of ion bombardmen t effects on the
aqueou s dissolutio n of silicates [19], we have investigate d the dose dependenc e of
the Ê factor by implanting a grea t variety of ions on different amorphou s and crystal-
line silicates . The experimenta l set up was slightly different from that of figure 2, a s
glass polishe d section s were simply covere d with an electro n microscop e grid,
without any resis t to protec t part of the unirradiate d materia l during etching . The
measure d step height betwee n unirradiate d and irradiate d area s define s a diffe-
rential etch rate AV = V*-V = (K-1)V. When plotting AV in function of the ion fluence ,
we have observe d in many case s a sigmoida l variation, with a sudde n increas e for
a critical value Fc (figure 3). Ê strongly depend s on the solid/solutio n syste m and
100
Ion fluenc e (1012/cm2) Ion fluence
Figure 3. Dos e dependenc e of the etch rate enhancemen t for muscovit e mica (a)
and soda-lim e glass (b); 200 keV Pb ions; etchan t 4% HF solution 30°C.