Applications of high energy ion beam techniques in environmental science: Investigation associated with glass and ceramic waste forms

https://doi.org/10.1016/j.elspec.2005.06.010Get rights and content

Abstract

High energy ion beam capabilities including Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA) have been very effectively used in environmental science to investigate the ion-exchange mechanisms in glass waste forms and the effects of irradiation in glass and ceramic waste forms in the past. In this study, RBS and NRA along with SIMNRA simulations were used to monitor the Na depletion and D and 18O uptake in alumina silicate glasses, respectively, after the glass coupons were exposed to aqueous solution. These results show that the formation of a reaction layer and an establishment of a region where diffusion limited ion exchange occur in these glasses during exposure to silica-saturated solutions. Different regions including reaction and diffusion regions were identified on the basis of the depth distributions of these elements. In the case of ceramics, damage accumulation was studied as a function of ion dose at different irradiation temperatures. A sigmoidal dependence of relative disorder on the ion dose was observed. The defect-dechanneling factors were calculated for two irradiated regions in SrTiO3 using the critical angles determined from the angular yield curves. The dependence of defect-dechanneling parameter on the incident energy was investigated and it was observed that the generated defects are mostly interstitial atoms and amorphous clusters. Thermal recovery experiments were performed to study the damage recovery processes up to a maximum temperature of 870 K.

Introduction

Ion beam analysis and characterization using high energy ion beams has been extensively used in many areas including thin films and interfaces, isotopic elemental uptake in materials, and defect accumulation and thermal recovery investigation of single crystal materials. In particular, Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA) have been very effectively used in environmental science to investigate the ion-exchange mechanisms in glass waste forms and the effects of irradiation in glass and ceramic waste forms in the past. In RBS, a He+ ion beam is incident on the substrate with an energy that is typically between 0.5 MeV and 2.0 MeV. In this regime, the Coulomb scattering can be treated classically (Rutherford scattering) and as a result, reasonably accurate numerical simulations of the ion scattering yield can be performed. The technique is element specific since the recoil energy of the backscattered He+ ion is mass dependent. Since the ion loses energy as it travels through the target material, an energy spectrum of the backscattered ions also yields information about the depth of the backscattering event. If the ion beam is well aligned with a major axial direction of a single crystal target, the ions channel into the crystal along the relatively open areas between the rows of atoms. Ions which backscatter from the first (surface) atom in each row give rise to the surface peak observed in the backscattered ion energy distribution curve, while the small-angle forward scattered ions form a shadow cone which extends into the solid. Subsequent atoms along the row and within the shadow cone do not lead to backscattering events, for a static model of an ideally terminated bulk lattice. By rotating the sample a few degrees with respect to the angle of incidence away from the low index direction, the channeling mode becomes a random mode. In this mode, virtually all the atoms in the sample are exposed to the ion beam and as a result the highest backscattered ion yield can be obtained. The variation of integrated yield over a small region next to the surface peak as a function of tilt or polar angle generates the angular yield curve or rocking curve, which provides information about the crystalline quality of the sample and the impurity location in the sample.

Because the Rutherford cross-section increases with the square of the atomic number, the signal from a light element in a heavy element matrix will be seen against a huge background in the RBS spectrum. For example, it is difficult to resolve oxygen RBS signal from many oxides including the glass coupons which are discussed in this paper. However, isotope-specific nuclear reactions can be effectively used to quantify the light elements including oxygen, hydrogen and their isotopes in the target material. During nuclear reaction, the projectile overcomes the Coulomb interaction and reacts with the target atom through nuclear interaction. As a result, an unstable nucleus is formed and, subsequently, the unstable nuclei reach the ground state by emitting energetic particles, such as alpha, beta and protons or gamma rays. In general, the energy of the emitted particles and gamma rays is larger than the energy of the incident beam and, as a result, the backscattered particles are well separated from the reaction products in the spectrum. The reaction products are characteristic for the elements in the target material and they can be captured using appropriate specialized detectors. If the cross-sections are known for the elements, the experimental spectra can be simulated and the quantity of the elements in the target material can be determined. On the other hand, if the cross-sections are not known, still the quantification can be performed by evaluating the experimental data against those collected from standards.

This paper briefly discusses the use of ion beam analysis, especially, RBS and NRA in investigating some of the crucial scientific problems associated with glass and ceramic waste forms in environmental science research. In the case of glass waste forms, this paper summarizes a series of ion beam measurements of the Na2O–Al2O3–SiO2 glass coupons as modified by exposure to a solution containing D218O and saturated with respect to amorphous silica. In this case, Na will be released from the glass samples while 18O and D will be incorporated into glass matrix. In the case of ceramic waste forms, to demonstrate the effectiveness of the usage of ion beams in developing the scientific understanding of radiation effects in ceramics, we will briefly describe the investigations associated with a perovskite structure, SrTiO3(1 0 0) and a pyrochlore structure, Sm2Ti2O7(1 0 0). In this case, RBS in channeling and random geometry has been effectively used to investigate the accumulation of relative disorder as a result of ion irradiation and recovery of these defects as a function of thermal annealing.

Since the glass was proposed as a medium for immobilization of radioactive nuclear wastes, many studies have been conducted on glass water reaction processes for glass compositions ranging from simple binary and ternary silicate glasses to complex waste glasses with 30 or more components [1]. The general picture of the glass corrosion process in water can be summarized as follows: upon initial contact by water, alkali is extracted by ion exchange in what is thought to be a diffusion-controlled process and simultaneously, hydrolysis and dissolution of the glass network occurs. The ion-exchange rate slows in accordance with a diffusion-controlled process as a reaction layer builds up on the glass over time. The dissolution rate of the glass network slows because of the common ion effect, i.e., as the solution becomes more concentrated in glass components. As such, the difference in chemical potential between the glass and aqueous phase decreases, which decreases the dissolution rate. The dissolution rate does not go to zero because silicate glasses are thermodynamically unstable in water.

Secondary phase formation occurs in some glasses under specific environments [1], [2]. Sales and co-workers also reported the formation of complex alteration layers on nuclear waste glasses when the glass samples reacted to aqueous solution [3], [4]. Because of the dominating effect that secondary phase formation has on glass dissolution rates [1], [2] recent work on glass/water interactions has focused on understanding this process and incorporating it into models. The ion-exchange process has been largely ignored because it has been thought to be a short duration, secondary or tertiary process that had little or no bearing on long-term corrosion or radionuclide release rates.

Since a Na ion-exchange reaction can effectively increase the radionuclide release rate by a factor of over 1000 [1], the Na ion-exchange reaction is a major factor that currently limits waste loading. The discovery of the significance of ion exchange to long-term radionuclide release rates requires simulation of the coupled processes of glass dissolution, mass transport and chemical reactions in a complex disposal system [1]. This observation stresses the importance of understanding and minimizing (through the formulation of new glasses) alkali ion exchange. Ion exchange (in which an H+ or H3O+ ion exchanges for an alkali ion (M+) in the glass, thereby generating a hydrated layer on the glass surface) was, in fact, the primary process involved in traditional ideas of glass “leaching”. The overall chemical reactions describing the process are described in details elsewhere [5], [6], [7], [8], [9], [10]. Because these reactions produce characteristically different ratios of H/M in the hydrated layer, surface analytical techniques have been used to map these elemental distributions. Nuclear reaction analysis is a useful method for obtaining the elemental depth profiles of H and O because it is possible to take advantage of isotope-specific nuclear reactions, which can provide additional information on how the elements are incorporated into the hydrated layers, and thus provide clues as to specific reaction mechanisms. Although some early studies [11], [12], [13], [14], [15] showed H/Na values of ∼3, later work [16], [17], [18] showed that the H/Na ratio changes with depth and are highly variable, ranging from ∼1 to ∼3 depending on glass composition and experimental conditions. Inconsistencies in the H/Na ratio are now known to be a consequence of network repolymerization that occurs in the hydrated layers of some glasses [19], [20], [21] via a silanol condensation reaction.

Experiments run in isotopically labeled water (D218O) showed that reactive glasses incorporate significant amounts of 18O but little D in their surface layers in contrast with expectations from the primary ion-exchange reactions, which would produce values of 18O/D less than one [22]. The incorporation of 18O at the expense of D in the surface layer can be explained by a repolymerization reaction and Raman and nuclear magnetic resonance (NMR) spectroscopic analysis of the hydrolyzed layers on sodium borosilicate glasses conclusively showed that such reactions could take place [21]. Less reactive glasses show little structural change in the hydrolyzed layer, at least over the short duration of the tests, incorporate significantly less 18O, and thus have lower 18O/D ratios [22], [23]. Pederson et al. [22], [23] completed two of the few studies where the exchange kinetics was also measured in isotopically labeled solutions. These kinetics studies were important because they clearly showed an isotope effect on the exchange rate only for those reactive glasses that also exhibited network repolymerization. Pederson and co-workers concluded that network repolymerization opens the layer structure so that diffusive transport is no longer rate controlling. Ion beam studies played a major role in the earlier studies of ion exchange including the detailed description of the work related to this paper described in an earlier publication [24]. In this study, the ability of Rutherford backscattering spectrometry to quantitatively determine the Na removed from the outer part of the glass, and the ability of nuclear reaction analysis to determine similar profiles of 18O and deuterium have been effectively used.

Irradiation of materials with energetic ion beams can alter the physical, chemical, electrical and optical properties of the materials in the surface region. Ion irradiation and implantation have been routinely used not only to transform crystalline materials into either a fully or partially amorphous materials but also to perform low-level doping in the semiconductor industry. In this process, the near-surface region of single crystals may be transformed into either a fully or partially amorphous material with significant changes in properties. Thermal annealing of irradiated crystals can result in partial or complete recrystallization of the highly damaged or amorphous layer. Perovskite- (e.g., SrTiO3) and pyrochlore- (e.g., Sm2Ti2O7) structure materials have been recommended as alternate materials for stabilization and immobilization of high level wastes containing fission products and actinides. These materials provide the unique property of exchanging larger cations for actinides and rare earths, while exchanging Ti for smaller cations. Due to alpha decay, actinide-bearing phases will be subjected to considerable self-radiation damage. Radiation effects from alpha decay can result in amorphization, macroscopic swelling and order of magnitude increase in dissolution rates [25], [26], [27], [28], and these structural damages significantly affect the chemical and physical stability and durability of the nuclear waste forms [29], [30], [31]. As such development of fundamental scientific understanding of the effects associated with irradiation in these materials is crucial to predict the long-term performance of these materials. However, amorphization studies [26], [27] of actinide doped pyrochlores as a result of the gradual accumulation of alpha-recoil collision cascades can be time consuming, and only limited data under a few sets of experimental conditions can be obtained. Heavy and light ion irradiation can be effectively used to induce the interested damage level within a short time as a function of various parameters including irradiation dose and temperature. As such, results of amorphization, crystal swelling and dissolution rates on ion-beam irradiated materials have provided a reasonable representation of the worst-case effect of the radiation effects on chemical and physical durability over long time periods for actual actinide-containing waste forms.

The use of pyrochlores in solid oxide fuel cells and as host matrices for actinide-rich wastes is receiving increasing attention because of recent discoveries showing that the isovalent substitution of Zr for Ti in Gd2Ti2O7 or Sm2Ti2O7 results in an increase of four to five orders of magnitude in the oxygen ion conductivity at 875 K and resistance to energetic particle irradiation [32], [33]. Several experimental investigations using neutron diffraction, Raman and infrared spectroscopies, extended X-ray absorption fine structure and theoretical studies based on molecular dynamics and atomistic simulations have been carried out to understand the mechanisms responsible for the increase of oxygen ion conductivity and radiation tolerance in these materials [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45]. These studies show that the increase in the ionic conductivity in pyrochlore is most likely due to the increased oxygen vacancies at the 48f site as a result of cation and anion disordering, which are responsible for the increased ionic conductivity. The increased radiation tolerance is attributed to the ease of rearrangement and relaxation of Gd or Sm, Zr and O ions/defects within the crystal structure, which inhibits amorphization by causing the irradiation-induced defects to relax and form cation antisite defects and anion Frenkel defects. There is limited direct experimental evidence for the presence of cation antisite disorder in a highly ordered pyrochlore structure.

Recently, it has been demonstrated that a number of single-crystal perovskite oxides, such as SrTiO3 (STO) and BaTiO3 (BTO), could be successfully utilized as gate oxide materials in semiconductor technology [46]. As such, there is a renewed interest in the use of ion implantation to alter the near-surface properties of SrTiO3 because of current activities in the semiconductor industry. Many of the potential semiconductor technology applications require thermal annealing to remove the damage introduced by ion implantation. It has also been recently shown that thin single crystal SrTiO3 films can be cleaved using hydrogen implantation and subsequent annealing [47]. The ability to cleave such implanted surfaces for transfer to other substrates could have significant impact on the fabrication of electro-optical devices. Recently, Gea et al. have also shown that near-surface nanocomposites can be fabricated by irradiating SrTiO3 single crystal using energetic Au2+ ions [48].

As such, the understanding of the ion beam irradiation effects on these materials and subsequent damage recovery process are useful for many applications including waste forms which are relevant to the environmental research. In the past, we have carried out detailed investigations associated with relative damage accumulation and thermal recovery of these defects in titanate materials using various high-energy ion species. Detailed discussions of these experiments and results were discussed in previous publications [49], [50], [51], [52]. A portion of this paper is dedicated to a brief discussion that demonstrates the effective use of high-energy ion beam techniques in understanding the irradiation effects of these materials.

Section snippets

Experimental

Detailed description of experimental set up associated with the glass reaction experiments are described elsewhere [24]. During these experiments, in the case of D218O solutions, a Si-bearing solution (SPEX Si standard) was used to raise the dissolved silicon to saturation values, thereby circumventing potential problems with vaporization and isotopic exchange of oxygen in the 90 °C oven. Subsequent to silicon addition, the solutions were pH-adjusted to the desired value. Because of cost, D218O

Background hydrogen (1H)

During the melting of glass coupons, it is likely that some of the components in glass pick up water or hydrogen. In addition, when the glass coupons were cut in a water environment some water uptake might have occurred and somewhat altering the surface region. Since the data analysis in these ion beam experiments is mainly centered on the surface region, it is useful to know the hydrogen (1H) depth profile of the starting material in relation to the depth distribution of other elements

Conclusion

High energy ion beam capabilities have been successfully utilized in understanding the ion-exchange mechanisms in glass waste forms and radiation effects in ceramics that are proposed for high level radioactive waste immobilization. Rutherford backscattering spectrometry and nuclear reaction analysis were used to monitor the Na depletion and D and 18O uptake in alumina silicate glasses, respectively, after the glass coupons were exposed to aqueous solution. RBS and NRA spectra were simulated

Acknowledgements

This work is supported in part by the US Department of Energy Environmental Management Science Program, the Office of Biological and Environmental Research and the Office of Basic Energy Sciences. Work related to the accelerator facility was conducted at the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated for the U.S. Department of Energy (DOE) by Battelle Memorial Institute under Contract

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