(Catio3): Effects of Surface Damage and [Ca ] in the Leachant

Home , Anatase, Brookite, Calcium titanate

Aqueous dissolution of perovskite (CaTiO3): Effects of surface damage and [Ca2+] in the leachant

Zhaoming Zhanga) and Mark G. Blackford Australian Nuclear Science & Technology Organisation, Lucas Heights, NSW 2234, Australia Gregory R. Lumpkin Australian Nuclear Science & Technology Organisation, Lucas Heights, NSW 2234, Australia; and Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, United Kingdom Katherine L. Smith and Eric R. Vance Australian Nuclear Science & Technology Organisation, Lucas Heights, NSW 2234, Australia

(Received 2 November 2004; accepted 19 May 2005)

We have characterized thermally annealed perovskite (CaTiO3) surfaces, both before and after aqueous dissolution testing, using scanning electron microscopy, cross-sectional transmission electron microscopy, x-ray photoelectron spectroscopy, and atomic force microscopy. It was shown that mechanical damage caused by polishing

was essentially removed at the CaTiO3 surface by subsequent annealing; such annealed samples were used to study the intrinsic dissolution behavior of perovskite in deionized water at RT, 90 °C, and 150 °C. Our results indicate that, although mechanical damage caused higher Ca release initially, it did not affect the long-term Ca dissolution rate. However, the removal of surface damage by annealing did lead to the subsequent

spatial ordering of the alteration product, which was identified as anatase (TiO2)by both x-ray and electron diffraction, on CaTiO3 surfaces after dissolution testing at 150 °C. The effect of Ca2+ in the leachant on the dissolution reaction of perovskite at 150 °C was also investigated, and the results suggest that under repository conditions, the release of Ca from perovskite is likely to be significantly slower if Ca2+ is present in ground water.

I. INTRODUCTION to preferential attack during room temperature (RT) dissolution tests of perovskite.13 Therefore it is highly de- Perovskite (CaTiO3) is one of the principal components of the synroc family—polyphase titanate ceramics sirable to test defect-free samples so that intrinsic disso- designed as host matrices for the immobilization of high- lution behavior can be observed. In this study, we dem- level nuclear waste (HLW).1–3 As synroc, with incorpo- onstrate that thermal annealing can remove surface rated HLW, is envisaged to be disposed in geological damage, caused by mechanical polishing, in polycrystal- repositories, it may come into contact with ground water. line CaTiO3. Dissolution testing of such samples (that are Therefore, its chemical durability in an aqueous environ- free from surface damage), as well as polished samples, ment is an important issue. was carried out to assess the effect of surface damage on the dissolution behavior. There have been many studies on the dissolution be- 2+ 2+ havior of synroc4–10 and the perovskite phase alone.11–18 The concentration of Ca in the leachant ([Ca ]) clearly controls the reactivity of perovskite with the liq- However, the samples used in previous dissolution tests 2+ of perovskite were in the form of as-cut (rough) surfaces, uid phase. It was reported that when [Ca ] is greater than5×10−4 mol/L (20 mg/L) at 150 °C, no net disso- mechanically polished discs, ion-beam-thinned speci- 18 mens, and powders. It is well known that cutting, pol- lution of CaTiO3 occurs. However, it is not clear how ishing, ion-beam thinning, or crushing in sample prepa- this value was determined, as no experimental details were provided by the authors. One of the aims of the ration can lead to surface deformation and stress. For 2+ example, polished synroc discs had a damaged surface current work is to examine the influence of [Ca ], which layer of ∼2 ␮m thickness,4 and the as-cut surfaces could be relevant to repository conditions, on the disso- exhibited damage to a depth of ∼50 ␮m.9 It was lution behavior of CaTiO3 at 150 °C. suggested that the deformation introduced by cutting led II. EXPERIMENTAL A. Sample preparation a)Address all correspondence to this author. e-mail: [email protected] Calcium titanate powder with a bulk TiO2/CaO molar DOI: 10.1557/JMR.2005.0294 ratio of 1.02 (i.e., slightly Ti-rich) was used to fabricate

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the polycrystalline samples. The following bulk impuri- coupled plasma mass spectrometry (ICP-MS) and atomic ties were indicated by the manufacturer (Ferro Corpora- emission spectroscopy (ICP-AES) to determine the el- ס ס ס tion): Al2O3 0.26 wt%, BaO 0.013 wt%, Fe2O3 emental release of Ca and Ti. Acid stripping, to remove ס ס 0.021 wt%, K2O 0.004 wt%, MgO 0.090 wt%, possible deposits on the Teflon sample holder or the ס ס ס Na2O 0.074 wt%, SiO2 0.036 wt%, SrO 0.008 reaction vessel walls, was not carried out. ס wt%, and ZrO2 0.027 wt%. Pellets were uniaxially pressed from the powder mixed with 5 wt% of paraffin as a binder. Sintering was carried out at 1415 °C in air for C. Characterization 2 h in a tube furnace after removing the added paraffin by X-ray photoelectron spectroscopy (XPS) was used to slow heating to temperatures ഛ600 °C. The density of determine the surface composition both before (polished 3 ∼ the CaTiO3 pellets was 4.00 g/cm ( 99% or annealed) and after dissolution testing. Measurements of theoretical density). Finally, the pellets (diameter were performed in ultrahigh vacuum with a Kratos ∼14 mm, thickness ∼1.2 mm) were mechanically pol- XSAM 800 system. The Mg anode of the x-ray source ished to 0.25 ␮m using diamond paste with chemically (K␣: 1253.6 eV) was operated at 225 W, and the spec- inert solvents (hydrocarbon-based lubricant and cyclo- trometer pass energy was set at 20 eV for regional scans. hexane as a rinsing agent), followed by an ultrasonic The analysis area was 4 × 6mm2, much larger than the washing in acetone. Care was taken in the polishing proc- grain size of the polycrystalline samples (1–10 ␮m). The ess to avoid unintentional chemical attack during prepa- thickness of the probed surface layer was ∼5 nm. The ration procedures. binding energies were calibrated by fixing the C 1s peak To reduce surface damage caused by mechanical pol- (due to adventitious carbon) at 285.0 eV. The surface

ishing, a portion of the previously polished CaTiO3 concentrations of different species were determined by samples were annealed in air at 1320 °Cfor2hinatube integrating the peak areas above a linear background furnace prior to aqueous durability testing. The same with appropriate sensitivity factors (as defined in the heating/cooling rate (5 °C/min) was used for both the software package supplied by Kratos Analytical). Note sintering and annealing processes. that, because the sensitivity factors were not calibrated, the experimentally determined surface concentrations B. Dissolution tests might deviate somewhat from the actual ones. However, Static dissolution tests on single pellets were con- this is unimportant in the present context, as we are only ducted in 22 mL of deionized water (DIW) at RT, 90 °C, interested in monitoring the surface composition changes and 150 °C for up to 4 wk. Teflon reaction vessels were as a result of dissolution testing. used in dissolution tests at RT and 90 °C and Parr Teflon- Samples, both prior to and after dissolution tests, were lined digestion bombs (Parr Instrument Company) at characterized by scanning electron microscopy (SEM), 150 °C. Specimens were supported by a Teflon sample using a JEOL-6400 scanning electron microscope with holder in all dissolution tests. The majority of the dura- an energy-dispersive spectrometer (EDS). A comprehen- bility tests were conducted on annealed specimens, but sive set of standards was used for quantitative work, as-polished samples were also used for comparison. The giving a high degree of accuracy. The sample morphol- geometrical specimen surface area to leachant volume ogy and the presence of any impurity and secondary ratio, SA/V, was approximately 0.16 cm−1. Samples were phases were examined. cooled to ambient laboratory temperature by taking the The sample morphology was also investigated by reaction vessels out of the furnace at the end of each test. atomic force microscopy (AFM) in contact mode both Reacted specimens were dried in air at RT before being before and after dissolution testing. The measurements analyzed. Note that the Parr digestion bomb is a sealed were conducted in air using a Digital Instruments Dimen- system, so the sample temperature could not be directly sion 3000 scanning probe microscope fitted with a stan- measured. The heat-up time to 150 °C is therefore not dard silicon nitride cantilever. At least five different re- accurately known. This uncertainty is negligible for tests gions on the surface were measured for each sample. lasting a week or longer, but the heat-up time would be X-ray diffraction (XRD) measurements were carried a significant portion of a 2-day test time. out on pellet surfaces with a Siemens D-500 instrument 2+ To determine the effect of [Ca ] in the leachant on the using Co K␣ radiation to determine whether any impurity dissolution behavior of calcium titanate, dissolution tests phases were initially present and whether any secondary were carried out on annealed specimens at 150 °C for phases were formed after dissolution tests. 2 days in 20, 200, and 2000 mg/L of Ca2+, respectively. Transmission electron microscopy (TEM) work was

The corresponding leachant was obtained by adding the performed on ion-beam-thinned cross-sectional CaTiO3 appropriate amount of CaCl2 to DIW. The addition of samples. A JEOL 2010F field-emission gun TEM, CaCl2 did not change the pH value of DIW. equipped with a Link Si(Li) detector and an EmiSpec ES The leachates were analyzed using inductively Vision microanalysis system, was used to examine the

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damaged layer at the polished surface, as well as the with a single doublet with a spin-orbit splitting of about

annealed CaTiO3 surfaces both prior to and after dis- 3.5 and 5.7 eV, respectively. The results are in good solution tests. The composition and structure of the agreement with those reported for single crystal 20 dissolution products were determined using TEM/EDS CaTiO3. The surface chemical composition of the and selected area diffraction patterns (SADPs), respec- polished CaTiO3 surface was determined by XPS as: at.%, and 47.5 ס at.%, O 12.2 ס at.%, Ti 9.9 ס tively. Ca at.%. The carbon signal was mostly due to 30.4 ס C ubiquitous hydrocarbons present on all solid surfaces ex- III. RESULTS AND DISCUSSION posed to ambient. The O 1s peak was comprised of two 2− A. Characterization of unreacted sample surfaces components: the main O lattice peak and a small shoul- der on the high-binding-energy side indicating the pres- 1. Mechanically polished surfaces − ence of adsorbed OH /H2O species on the surface (due to The extent of the mechanical damage at the polished exposure to air). Because the absolute cation concentra- surface was examined by cross-sectional TEM. The tions were affected by the varying amount of C and O depth of the damaged layer was found to vary from al- contamination on each surface, the surface chemical most0upto∼0.3 ␮m. This variability is likely due to compositions in this paper are compared with each other large scratches from earlier coarse polishing steps not in terms of the cation-concentration ratios rather than being completely removed by subsequent fine polishing their absolute values. The Ca/Ti ratio of the polished steps.19 A typical cross-section of the polished polycrys- surface, as determined by XPS, was approximately 0.8, 13,14 talline CaTiO3 surface is shown in Fig. 1, which was in excellent agreement with previous results. recorded under two-beam illumination conditions to highlight the mechanical damage. The depth of damage varied from ∼0.02 ␮m (near the top left of the figure) to ∼0.1 ␮m (in the lower half of the figure). The bright and dark bands (roughly perpendicular to the surface) displayed in the image are bend contours. The results of SEM/EDS and XRD analyses of the

polished CaTiO3 sample indicated that it was essentially single phase, with minor impurity phases (∼1%) of rutile

(TiO2) and magnesium/calcium aluminum titanate [(Mg0.9Ca0.1)Al2Ti3O10] present. The polished CaTiO3 surface was also examined using XPS. Both the Ca 2p and Ti 2p XPS peaks could be fitted

FIG. 2. (a) SEM and (b) AFM images of a polished polycrystalline

CaTiO3 surface following subsequent annealing at 1320 °Cfor2h, FIG. 1. Cross-sectional TEM view of a polished polycrystalline showing terraces and steps on the surface of each grain as a result of

CaTiO3 surface, showing uneven subsurface damage. microfaceting.

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2. Annealed surfaces

After thermal annealing at 1320 °C for 2 h, the pellet

surfaces of the polycrystalline CaTiO3 were no longer flat. Figures 2(a) and 2(b) show the SEM and AFM

images of an annealed CaTiO3 surface, respectively. The terrace-and-step type of structure on the surface of each grain, as a result of microfaceting, was evident in these

images. Similar results were reported for BaTiO3 in a recent publication,21 in that the crystallographic orientations of the faceted planes were determined by TEM/ SADP as {100}, {110}, and {111} when sintering/ annealing was carried out under similar conditions as

ours. The structure of orthorhombic CaTiO3 is very similar to that of tetragonal BaTiO3: both deviate only slightly from the cubic SrTrO3 structure (which is the 22 archetypical perovskite ABX3 structure). Therefore it is likely that {100}pc, {110}pc, and {111}pc (the subscript “pc” indicates pseudocubic settings) are also the crystal-

lographic orientations of the terraces and steps in CaTiO3 (Fig. 2). Note that it is convenient to use the pseudocubic

settings when comparing the structure of CaTiO3 with that of BaTiO3 and SrTiO3; the relationship between the pseudocubic and orthorhombic cells was given by Hu et al.23 This is consistent with our TEM/SADP results. The terrace-and-step type of structure can be seen clearly in the low-magnification TEM image of an annealed

CaTiO3 surface [Fig. 3(a)]. Electron-diffraction patterns of the area suggest that the terraces and steps are likely composed of (001) and (1¯01) planes (using space group ,(Å23 5.37 ס Å, c 7.62 ס Å, b 5.44 ס Pnma with a ¯ which correspond to (101)pc and (100)pc in the pseudocubic settings. The results are consistent with the facets representing low-surface-energy planes, as the equilib- FIG. 3. (a) Low- and (b) high-resolution TEM micrographs (from rium crystal shape of SrTiO3 is composed of predomi- different regions) of a polished polycrystalline CaTiO3 surface follow- nantly {100} but also of {110} and {111} planes.24 ing subsequent annealing at 1320 °Cfor2h. There was no evidence of a damaged surface layer in the annealed sample [Fig. 3(a)]. This was confirmed by high-resolution TEM results (of a different region), detected by XPS on the annealed surface, presumably which showed lattice fringes extending all the way to the due to thermally induced segregation of trace impurities surface [Fig. 3(b)]. Clearly the mechanically damaged from the bulk (although P was not one of the bulk im- layer in the polished sample was greatly reduced (if not purities identified by the manufacturer). In addition to

removed completely), demonstrating that subsequent rutile (TiO2) and magnesium/calcium aluminum titanate thermal annealing is an effective way to remove surface [(Mg0.9Ca0.1)Al2Ti3O10] (both were identified on pol- damage caused by the mechanical polishing process. ished surfaces as well), an occasional grain of tricalcium

The amount of carbon present on the annealed surface, phosphate [Ca3(PO4)2] was also observed on some of the as determined by XPS, was much lower than that on the annealed surfaces when we used SEM/EDS. The other polished surface, indicating a “cleaner” surface. The Ca surface impurities identified by XPS (Si, Na, and Ba) 2p and Ti 2p XPS spectra were very similar to those from were possibly from a silicate glassy phase that was too the polished surface. The measured Ca/Ti ratio (0.95) thin to be observed by SEM/EDS. Such glassy phases was ∼19% higher than that of the polished surface, sug- were previously observed using TEM in synroc samples gesting that the polished surface was slightly deficient in by Cooper et al.10 This is consistent with the fact that Ca despite the fact that only nonpolar and inert solvents these impurities were no longer detected after dissolution were used during the polishing process. Small amounts reaction at RT for a week (see below), as silicates are of cation impurities (Na, Si, Al, Ba, and P) were also rapidly dissolved in water at this scale.

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B. Characterization of reacted sample surfaces 1. Mechanically polished surfaces The main aim of this paper is to study the intrinsic

dissolution behavior of CaTiO3 in DIW by examining defect-free (annealed) samples, and to compare the results with those obtained from polished surfaces. Be- cause the dissolution behavior of mechanically polished

CaTiO3 pellets in DIW was studied extensively at temperatures below 100 °C,13,14 we only carried out dissolution testing on polished perovskite surfaces at 150 °C.

Figure 4 shows a SEM micrograph of a polished CaTiO3 surface after dissolution testing in DIW at 150 °C for 1 wk. The reacted surface was almost covered com- FIG. 5. Normalized Ca/Ti ratio (measured by XPS) with respect to

pletely by alteration products, which were identified by dissolution time for annealed polycrystalline CaTiO3 treated in deionized water at RT (squares), 90 °C (circles), and 150 °C (triangles), XRD as anatase (TiO2) crystals. This is consistent with XPS results, which showed that the surface Ca/Ti ratio respectively. decreased significantly after dissolution testing (from 0.8 to 0.02). Figure 4 also shows that the anatase layer across obtained from XPS measurements, with respect to disso- the reacted perovskite surface was inhomogeneous (pre- lution time for annealed CaTiO3 treated in DIW at RT, sumably resulting from the uneven mechanical damage 90 °C, and 150 °C, respectively. The rate of calcium loss introduced by polishing); SEM/EDS analyses indicated decreased with increasing time and decreasing tempera- that the anatase layer in area A1 was thicker than that in ture, but the difference between RT and 90 °C was small area A2. compared to that between 90 and 150 °C. At RT and 2. Annealed surfaces 90 °C, most of the (small) Ca loss occurred during the first week (168 h) of dissolution tests. At 150 °C, how- After dissolution tests at all three temperatures (RT, ever, the surface Ca/Ti ratio decreased very rapidly with 90 °C, and 150 °C), the Ti 2p XPS spectrum showed no time, and dropped to nearly zero after 1 wk. The general detectable changes, whereas the Ca 2p doublet was trend in dissolution behavior for annealed perovskite at slightly better resolved (full-width-half-maximum of the temperatures below 100 °C is similar to those reported Ca 2p peak ∼0.1 eV narrower). There was no evidence 3/2 previously for polished CaTiO pellets.13,14 However, for the formation of Ca(OH) or CaCO on the perovskite 3 2 3 the amount of reduction in the Ca/Ti ratio at the annealed surface after any dissolution tests, contrary to a previous surface, after dissolution testing at RT, was much less report17 when CaTiO was subjected to more severe hy- 3 than that at the polished surface.13,14 This is consistent drothermal treatment in DIW (300–350 °C and 500 bar). with the suggestion by Turner et al.13 that surface defor- However, a decrease in the surface Ca/Ti ratio was de- mation led to preferential attack at RT. tected by XPS after dissolution testing at all three tem- No surface alteration features were observed by SEM peratures. Figure 5 shows the normalized Ca/Ti ratio, or AFM after dissolution tests for up to 4 wk at RT and 90 °C. Previous studies reported the formation of a thin titanaceous amorphous layer on perovskite (predomi- nantly on defective crystals and at grain boundaries) after dissolution testing at RT.14 Due to the detection limit of SEM and AFM, we cannot completely rule out the possibility of a very thin amorphous layer present on an-

nealed CaTiO3 after dissolution tests at RT and 90 °C. However, if such a titanaceous amorphous layer exists, its thickness would be in the order of 1 nm, not 10 nm as observed on “defective” perovskite surfaces.14 This is based on our XPS results that the average Ca/Ti ratio (over the top 5 nm surface layer) had decreased only slightly after dissolution testing at RT and 90 °C (Fig. 5). In addition, the incorporation of Ca–OH species in the 13,14 FIG. 4. SEM image of a polished perovskite surface after dissolution amorphous layer, as suggested in previous studies, is testing in deionized water at 150 °C for 1 wk, showing underlying unlikely due to the absence of a higher binding energy perovskite (P) and secondary anatase (A1 and A2). peak in the Ca 2p XPS spectrum after dissolution testing.

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A chemical shift of ∼2 eV in the Ca 2p peaks would be expected from Ca–OH relative to the Ca–O bond.25 In contrast to the experiments at RT and 90 °C, individual crystals (as alteration products) were clearly seen on annealed perovskite surfaces after dissolution testing at 150 °C for a time as short as 2 days. Figures 6(a) and 6(b) show SEM images of a perovskite surface after dissolution tests at 150 °C for 1 wk and 4 wk, respectively.

As shown, the original CaTiO3 surface was covered by a “skin” layer composed of well-aligned individual crystals. This is confirmed by cross-sectional TEM results (Fig. 7), which showed that columns of crystals grew in

FIG. 7. Cross-sectional TEM image of an annealed perovskite grain after dissolution testing in deionized water at 150 °C for 1 wk, showing columns of well-aligned anatase crystals on top.

preferred directions with respect to the underlying perovskite surface. Comparing Figs. 6(a) and 6(b) with Fig. 4 of the polished surface, it is obvious that the removal of surface defects has led to a homogeneous alteration layer consisting of spatially ordered crystals. Therefore, annealed surfaces may well be more suitable than commonly used mechanically polished surfaces for dissolution studies, as ceramic (including vitreous) waste–form assemblages would be processed at high temperatures similar to the annealing temperatures util- ized here. The structure of the alteration products was

identified by XRD and TEM/SADP as anatase (TiO2). In addition to anatase, a second TiO2 polymorph was also observed in previous studies: brookite on ion-beam- thinned perovskite samples after dissolution tests at 15 140 °C in DIW, or TiO2(B) on naturally weathered perovskite samples.26 However, we found no evidence

for the formation of either brookite or TiO2(B) in the present study. We have also investigated the structural relationship

between CaTiO3 and anatase. Detailed results will be presented in a forthcoming paper, in which the preferred orientation between anatase crystals and underlying perovskite grains is attributed to the similarity between their anionic sublattices (despite their overall crystallographic dissimilarities). This pseudo-epitaxial relationship between anatase and perovskite also provides direct experimental evidence for “congruent dissolution” (Ca and Ti released at equal rates, followed by precipitation FIG. 6. SEM images of annealed perovskite surfaces after dissolution of anatase) as the dissolution mechanism for perovskite testing in deionized water at 150 °C for (a) 1 wk and (b) 4 wk, showing at 150 °C. alteration products covering almost the entire surface. The height of the anatase crystals corresponds to the

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thickness of the alteration layer, which increased with in the pellet after dissolution tests at 150 °C. In addition,

increasing dissolution time. Figure 8 shows an AFM im- a few hydroxyapatite [Ca10(PO4)6(OH)2] crystals (which age of a partially covered perovskite grain with indi- had the distinct shape of hexagonal rods27) were ob- vidual anatase crystals on top after dissolution testing at served sitting on top of the reacted surface, likely the 150 °C for 1 wk. From such images, the typical height of product of DIW reacting with the impurity phase of

the anatase crystals was estimated as around 50, 150, and Ca3(PO4)2 (see Sec. III. A. 2). The results of SEM/EDS 300 nm after dissolution tests at 150 °C for 2 days, 1 wk, analyses also showed that there was a small amount of Al and 4 wk, respectively. These estimates seemed to be present in the anatase “skin” layer, whereas no Al was lower than those indicated by TEM images (e.g., Fig. 7). detected in perovskite grains without the anatase over- This discrepancy is believed to be mainly caused by the layer. The combined SEM/EDS and XPS results seemed fact that AFM measured the height of isolated anatase to suggest that most of the Al detected at the annealed crystals, whereas TEM showed areas that were fully cov- surface by XPS arose from insoluble phases [such as

ered by anatase crystals. Once the adjacent anatase crys- (Mg0.9Ca0.1)Al2Ti3O10], but a small amount of Al was tals touched one another, they would be “forced” to grow also from a soluble impurity phase (likely the silicate along the height direction only, leading to more elon- glassy phase as suggested previously). Because the Al/Ti gated crystals. In addition, the TEM cross-section was ratio determined by XPS (probing depth ∼5 nm) only likely to be cut at an angle to the growth direction of increased slightly after dissolution tests at 150 °C, the anatase; therefore, the “apparent” height of the crystals, dissolved Al ions would have to be incorporated through- as displayed in Fig. 7, would be higher than their “true” out the anatase layer, not adsorbed as precipitates onto value. It should be mentioned that the shape of the ana- the anatase surface. This was confirmed by the TEM/ tase crystals was not depicted accurately by AFM im- EDS results that Al (∼1 at.%) and Ca (∼1.5 at.%) ions ages, as no attempt was made to deconvolute the influ- were incorporated throughout the anatase crystal layer. ence caused by the AFM tip (which is of comparable size To satisfy the charge neutrality condition, the anatase to the anatase crystals). layer also has to incorporate either oxygen vacancies or As mentioned earlier, the concentrations of some trace hydroxyl groups according to the mechanisms listed impurities (Na, Si, Al, and Ba) at the annealed polycrys- below7 talline CaTiO surface were much higher, according to 3 4+ 2− ⇔ 3+ ᮀ XPS analyses, than those in the bulk, as a result of ther- 2Ti +O 2Al + o mally induced segregation. After dissolution testing at all or 4+ 2− ⇔ 3+ − three temperatures, the presence of Na, Si, and Ba were Ti +O Al +OH . (1) no longer detectable by XPS, presumably as a result of the removal of a relatively soluble impurity phase. The 4+ 2− ⇔ 2+ ᮀ Ti +O Ca + o surface Al concentration decreased slightly after disso- or lution tests at RT and 90 °C but increased slightly after 3Ti4+ +3O2− ⇔ 4Ca2+ + 2OH− . (2) testing at 150 °C. The results of SEM/EDS analyses showed that the two main impurity phases [rutile (TiO ) 2 To confirm that the presence of segregated surface and (Mg Ca )Al Ti O ; see Sec. III. A. 2] remained 0.9 0.1 2 3 10 impurities did not alter the dissolution behavior of the main perovskite phase, we also carried out dissolution

testing on high-purity single-crystal CaTiO3 (100) surfaces at RT, 90 °C, and 150 °C (the single crystals were grown by ESCETE B.V., The Netherlands). The dissolution behavior of annealed single-crystal surfaces, which contained much less segregated surface impurities, was indeed similar to that of annealed polycrystalline surfaces, demonstrating that the presence of surface impurities played a negligible role in the dissolution behavior of the main perovskite phase.

C. Solution analyses After each dissolution test, the leachant and the blank were analyzed for Ti and Ca using ICP-MS and ICP- FIG. 8. AFM image of an annealed perovskite grain after dissolution AES, respectively. The Ti concentration in the leachant testing in deionized water at 150 °C for 1 wk, showing individual was several ␮g/L (ppb) above that in the blank after anatase crystals on top. dissolution tests at 150 °C (comparable to those reported

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before9) but was similar to that in the blank at lower results of solution analyses are in good agreement with testing temperatures. Obviously the solubility limit of Ti those obtained by AFM. Note that because the alteration was reached at 150 °C, as evidenced by the presence of layer on the polished surface was inhomogeneous

TiO2 precipitates on the leached perovskite surfaces. The (Fig. 4), no attempt was made to either derive the thick- Ca concentration was considerably higher than that in the ness of the reacted layer or measure the height of indi- blank after dissolution tests at 90 °C for 4 wk and 150 °C vidual anatase crystals. for 2 days or longer. Because we have carried out only The total amount of Ca detected in the leachate after static dissolution testing, in which saturation effects were dissolution testing of the polished perovskite pellet at evident, no attempt was made to derive the leach rates. 150 °C for 1 wk is also listed in Table I (4.8 mg/L). It is However, qualitative information can still be obtained slightly lower than the amount of Ca release from the from comparing the Ca concentration in the fluid after annealed sample under the same dissolution conditions dissolution testing (Table I). As shown, the total elemen- (6.0 mg/L). This difference is mainly attributed to the

tal release of Ca from the annealed CaTiO3 pellets in- difference in the actual surface area of the two surfaces: creased with increasing temperature or time. Note that the annealed surface has a higher surface area as it is no the nominal testing time at 150 °C includes the unknown longer flat (Fig. 2). In other words, the surface mechani- amount of heat-up time (see Sec. II. B), leading to a cal damage does not seem to result in higher total Ca

relatively larger error (and overestimate) in dissolution release from CaTiO3 at 150 °C, although it clearly affects time for shorter tests. The above results are consistent the orientation and ordering of the alteration products with those from surface analysis (Fig. 5), which showed (anatase crystals) as shown in Fig. 4. This is consistent that the rate of Ca loss dropped markedly with increasing with previous reports that mechanical damage led to time and decreasing temperature. higher initial Ca release at RT, but the long-term release When comparing the results of dissolution testing at of Ca was unaffected.13 The difference in the amount of 90 and 150 °C, it should be borne in mind that the initial Ca release between the polished and annealed

amount of CO2 present in the leachant at 90 °C is dif- perovskite surfaces (if any) would be insignificant com- ferent from that at 150 °C. At 90 °C, the solution inside pared to the total amount of Ca released during dissolu-

the Teflon vessel equilibrates with atmospheric CO2 tion testing at 150 °C for 1 wk, as the reacted layer was quickly, but such an equilibration does not occur at estimated to be several hundreds of nanometers thick 150 °C (the Teflon-lined stainless steel vessel used in the (Table I). dissolution testing at 150 °C is a sealed system). Further- The pH values of both the leachant and the blank were more, the pH value of the leachant would also change measured after some dissolution tests (as soon as the accordingly. However, on the basis of literature data, vessels were cooled to RT). As shown in Table I, the pH 11 neither the presence of dissolved CO2 nor the variation value for the 90 °C blank is about a unit lower than that in pH16 influences the Ca dissolution rate appreciably. of the 150 °C tests. This is consistent with the 90 °C

Therefore, our comparisons should still be valid. solution containing a greater amount of dissolved CO2, Also listed in Table I are the calculated thickness of as the Teflon vessel used for the 90 °C testing is perme- the reacted layer and the measured thickness of the ana- able whereas the Teflon-lined stainless steel vessel used

tase overlayer (by AFM) from annealed CaTiO3. The at 150 °C is a sealed system. The pH change, caused by actual thickness of the reacted layer is expected to be the dissolution reaction, was monitored by the difference lower than the calculated one because the actual surface between the leachant and the blank. The largest pH area is higher than the geometric surface area. Hence the change was observed after dissolution testing at 150 °C

TABLE I. Total Ca concentration in the fluid after dissolution testing of annealed and polished CaTiO3 pellets, the derived thickness of the reacted layer and the measured thickness of the anatase layer by AFM, as well as the final pH values of the leachant and the blank.

Total Ca Calculated thickness of Measured thickness of Final pH Final pH release (mg/L) the reacted layer (nm)a the anatase layer (nm) (leachant) (blank) Annealed 90 °C, 4 weeks 0.1 7 иии 5.8 5.1 150 °C, 2 days 1.4 75 50 иии иии 150 °C, 1 week 6.0 340 150 иии иии 150 °C, 4 weeks 8.4 450 300 7.2 6.0 Polished 150 °C, 1 week 4.8 иии иии 7.3 6.3

͓C͑leachant͒ − C͑blank͔͒ × V Mass͑perovskite͒ aThickness of the reacted layer = × , SA × ␳ Mass͑Ca͒ geometric surface area ס leachant volume, SA ס Ca concentration in the blank, V ס (Ca concentration in the leachant, C(blank ס (where C(leachant .density of perovskite ס of the sample, ␳

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for 4 wk (6.0 → 7.2). These pH changes, however, are much smaller than those predicted by geochemical cal- culations28 using the EQ3/6 geochemical code package29 + → for the overall dissolution reaction: CaTiO3 +2H 2+ Ca + TiO2 +H2O. The results indicated that the elec- trical imbalance was comparable to the Ca concentrations even when the solutions were appropriately cor-

rected for the influx of CO2 in the case of the Teflon vessels at 90 °C and the limited amount of CO2 in the case of the stainless steel vessels at 150 °C. Although the solutions were not analyzed for anions, fluoride from the test vessels would have caused the pH to be much more acidic. In addition, the incorporation of OH− in the anatase layer, due to the small amount of Al and Ca ions present in the anatase crystal layer (see Sec. III. B. 2), would lower the pH as well. The results of geochemical calculations also indicated that all solutions were found FIG. 9. Normalized Ca/Ti ratio (measured by XPS) of the reacted CaTiO3 surface, after dissolution tests at 150 °C for 2 days, versus the to be saturated or over saturated with respect to anatase. [Ca2+] in the leachant. The solubility of anatase, like rutile, is not expected to be a strong function of pH (2–9) or temperature (100– 300 °C).30 To summarize, although the results of our Ca/Ti ratio of the surface treated in 2000 mg/L of [Ca2+] solution analysis are qualitatively consistent with those was 0.85, still lower than that of the original surface indicating that the presence of 2000 mg/L ,(1 ס of surface analysis, the geochemical calculations indicate (Ca/Ti a more complex solution chemistry than the simple ion of [Ca2+] in the solution did not completely inhibit the + 2+ exchange of 2H3O for Ca . It would be highly desir- dissolution reaction. able to measure the anion concentrations as well as those Excellent agreement was achieved between the XPS of cations. and SEM results. Figures 10(a)–10(d) show the SEM

2+ images of the perovskite surface, treated at 150 °C for D. Effect of [Ca ] in the leachant 2 days in DIW with 0, 20, 200, and 2000 mg/L of Ca2+, We also carried out a series of dissolution tests on respectively. As seen the perovskite surface treated in annealed polycrystalline perovskite at 150 °C for 2 days 20 mg/L of [Ca2+] was essentially identical in appearance with different Ca2+ concentrations in the leachant by to that treated in pure DIW; a uniform anatase crystal

adding appropriate amounts of CaCl2 to DIW. The high- layer covered the majority of the treated surface. When 2+ est concentration of CaCl2 used in the current study the [Ca ] was increased to 200 mg/L, only a small (∼0.55 wt%) is more than 2 orders of magnitude lower portion of the treated surface was covered by crystals than its solubility limit at 150 °C(∼66 wt%).31 Figure 9 (but the size of crystals remained similar as before),

shows the normalized Ca/Ti ratio of the reacted CaTiO3 indicating a significant slowdown in the dissolution– surface, obtained from XPS measurements, versus the precipitation reactions. When the concentration of Ca2+ [Ca2+] in the leachant. Note that all samples tested in was further increased to 2000 mg/L, no anatase crystals

CaCl2 solutions were thoroughly rinsed in DIW, to re- were visible on the reacted surface. This surface had move possible CaCl2 residues on the surface, before be- similar morphology and chemical composition to those ing dried for surface analysis. When the [Ca2+] was treated in DIW at 90 °C, suggesting that the presence of 20 mg/L, the Ca/Ti ratio of the reacted surface remained 2000 mg/L [Ca2+] in the leachant inhibits dissolution approximately the same as that of a surface treated in reactions in a similar manner as does decreased reaction pure DIW. A further increase in [Ca2+] to 200 mg/L led temperature. It was reported previously that when [Ca2+]

to a much higher surface Ca/Ti ratio, indicating a sig- is greater than 20 mg/L, no net dissolution of CaTiO3 nificant slowdown in the loss of Ca from perovskite (the occurs at 150 °C.18 However, our results clearly indicate possibility of an increase in the amount of Ti loss was that even when [Ca2+] is 2 orders of magnitude higher

ruled out by solution analyses: the addition of CaCl2 did than the reported value, there is still some net dissolution not significantly change the Ti concentration in the of CaTiO3 occurring at 150 °C. leachant, which remained in the ppb range for all tests). The average Ca2+ concentration in seawater is reported This trend continued when the [Ca2+] was increased by to be around 400 mg/L.32 However, natural brines found another 10-fold, but the difference in the Ca/Ti ratio be- deep in the earth’s crust may contain much higher [Ca2+]; tween 2000 and 200 mg/L of [Ca2+] was much smaller for example, an average Ca2+ concentration of 15,000 mg/L than that between 200 and 20 mg/L. The normalized was reported for the natural brines found deep in the

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FIG. 10. SEM images of annealed perovskite surfaces, treated at 150 °C for 2 days in (a) 0, (b) 20, (c) 200, and (d) 2000 mg/L of Ca2+, respectively. The corresponding surface Ca/Ti ratios, determined by XPS, are shown in the parentheses.

Canadian Shield.33 Therefore, the Ca dissolution rate Anatase crystals formed readily on both polished and from the perovskite phase under “real” repository annealed perovskite surfaces after dissolution testing at conditions with elevated Ca2+ concentrations may be 150 °C. Surface damage did not result in a notable in- much lower than the rates measured in this study in DIW. crease in the total amount of Ca released from perovskite To strictly determine the effect of Ca2+ on the dissolution after 1 wk. However, the removal of surface damage by of perovskite, however, one should perform a single-pass annealing did lead to the spatial ordering of the alteration flow-through test with various fixed pH values and con- products on the perovskite surfaces. The alteration layer centrations of Ca. on the annealed surface was composed of well-aligned To confirm that the presence of [Cl−] did not affect the columns of anatase crystals grown in preferred directions dissolution reaction, an annealed perovskite surface was with respect to the underlying perovskite, in contrast to also treated at 150 °C for 1 wk in a NaCl solution with the inhomogeneous layer observed on the mechanically − similar [Cl ] (4000 mg/L) to that used in the CaCl2 so- polished surface. The height of the anatase crystals was lution with 2000 mg/L of [Ca2+]. This treated surface around 50, 150, and 300 nm on annealed perovskite sur- (after rinsing in DIW) was extensively reacted and simi- faces after dissolution tests for 2 days, 1 wk, and 4 wk, lar in appearance to those treated in pure DIW under the respectively. same experimental conditions. It was confirmed that the presence of Ca2+ in solution appears to inhibit the dissolution reaction of perovskite. IV. CONCLUSIONS Therefore, under repository conditions, the release of Ca

This is the first study to use annealed CaTiO3 surfaces from perovskite may be significantly slower than that in aqueous dissolution testing to investigate the effects of reported from laboratory tests in Ca-free aqueous media. surface damage in perovskite. Some (small) calcium loss Thermal annealing effectively removes mechanical was observed by XPS from such defect-free perovskite damage at the surface caused by the polishing process, surfaces after dissolution tests in DIW at RT and 90 °C and annealed surfaces might be more suitable for carry- for up to 4 wk. The thickness of the Ca-deficient ing out dissolution tests as they are similar to “real” layer was estimated to be in the order of 1 nm, much less surfaces of waste–form assemblages. Furthermore, this than that reported for mechanically damaged surfaces simple method to remove surface mechanical damage (∼10 nm). The results confirm that surface damage can can be extended to other oxide ceramic materials in lead to higher (initial) Ca release from perovskite. general.

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ACKNOWLEDGMENTS 11. S. Myhra, H.E. Bishop, J.C. Riviere, and M. Stephenson: Hydro- thermal dissolution of perovskite (CaTiO3). J. Mater. Sci. 22, The authors thank S. Leung, K. Short, and G. Thoro- 3217 (1987). good for their help with the SEM, AFM, and XRD meas- 12. S. Myhra, R.St.C. Smart, and P.S. Turner: The surfaces of urements, J. Cantwell and Z. Aly for carrying out the titanate minerals, ceramics and silicate glasses: Surface analytical solution analyses, Y. Zhang and P. McGlinn for valuable and electron microscope studies. Scanning Microsc. 2, 715 advice on solution chemistry, D. Mitchell for useful (1988). 13. P.S. Turner, C.F. Jones, S. Myhra, F.B. Neall, D.K. Pham, and discussions of the TEM results, D. Attard and M. Colella R.St.C. Smart: Dissolution mechanisms of oxides and titanate for preparing the TEM specimens, and G. Smith and ceramics—electron microscope and surface analytical studies, T. Roach for polishing the CaTiO3 pellets. We are in Surfaces and Interfaces of Ceramic Materials, edited by also indebted to Dr. Denis Strachan, of Pacific Northwest L.-C. Dufour, C. Monty, and G. Petot-Ervas (Kluwer Academic National Laboratory, for his critical reading of Publishers, Dordrecht, Netherlands, 1989), p. 663. our manuscript and for providing the geochemical 14. D.K. Pham, F.B. Neall, S. Myhra, R.St.C. Smart, and P.S. Turner: Dissolution mechanisms of CaTiO3—solution analysis, surface calculations. analysis and electron microscope studies—implications for synroc, in Scientific Basis for Nuclear Waste Management XII, edited by W. Lutze and R.C. Ewing (Mater. Res. Soc. Symp. Proc. 127, REFERENCES Pittsburgh, PA, 1989), p. 231. 15. T. Kastrissios, M. Stephenson, P.S. Turner, and T.J. White: 1. A.E. Ringwood: Safe Disposal of High Level Nuclear Reactor Hydrothermal dissolution of perovskite: Implications for synroc Waste: A New Strategy (Australian National University Press, formulation. J. Am. Ceram. 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