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Ion beam analysis of float glass surface composition
JOURNAl, OF
ELSEVIER
Journal of Non-Crystalline Solids 212 (1997) 232-242
Ion beam analysis of float glass surface composition F. Lamouroux a, N. Can b, P.D. T o w n s e n d a,* , B.W. Farmery a, D.E. Hole
a
a Department of Physics and Astronomy, Unit,ersity of Sussex, Brighton BNI 9QH, UK b Department of Physics, Science Faculty, Unirersi O, of Ege, 35100 Bornova-lzmir, Turkey
Received 23 July 1996; revised 14 November 1996
Abstract Data are reported on ion beam surface analysis methods of proton-induced X-ray emission (PIXE), and helium-induced X-ray emission (HIXE), made simultaneously with Rutherford backscattering spectrometry (RBS), which were taken with samples from across the full width of a float glass production line. Data are given for both clear and green glass. Significant variations are recorded between signals from the lower surface, (the face in contact with tin), and the upper surface, not only for the uptake of the tin, but also for the consequent changes in the near surface composition of the other main elements of the glass. In particular, the tin alters the local compositions of iron, calcium, silicon, and sodium. The changes are interpreted in terms of the furnace temperature gradients and chemical interactions. In all cases, including the green glass, iron is depleted from the non-tin face relative to the bulk composition, and the changes of surface content of the iron differs on the two faces relative to the bulk value.
1. Introduction Float glass is of considerable industrial importance [1-6] and consequently there have been numerous studies of the compositional changes in the near surface layers which are caused by the ingress of tin from the float process [7-12]. The presence of tin within the glass surface in contact with the tin bath is an inevitable consequence of the high temperature processing and contact of the two molten materials. To a lesser extent tin will also reach the upper surface of the glass by vapour transport. H o w e v e r , the chemistry at the two faces will differ, since the upper face may be under reducing conditions,
* Corresponding author: Tel.: +44-1273 678 073; fax: +441273 678 193/678 097; e-mail: [email protected].
whereas as there is no gas phase on the lower surface. The total concentration and depth of penetration will be influenced by the tin temperature and thermal gradient along the flow path, the contact time (as defined by the flow speed), the chemical composition of the glass feed material and furnace design. There are the obvious problems of non-uniform mixing of the source materials and variations in their purity when dealing with a commercial production line which handles some 600 tonnes ( ~ 5 X 105 kg) per day. However, the current study has focused on the presence of the tin, since this is generally undesirable for several reasons. Firstly it results in ' b l o o m i n g ' as the surface appearance is degraded by tin precipitation during subsequent heat treatments, for example during tempering or bending of the sheet, as used for car windscreens. The presence of the modified surface layer is also attributed to the
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F. Lamouroux et a l . / Journal of Non-Crystalline Solids 212 (1997) 232-242
poor, or variable, adhesion quality of many types of coating which are applied to the glass sheet. Overall, the tin is thought to be responsible for a wide range of technological problems. On the positive side, the presence of tin dopants in the outer glass layer provides a very low loss and low cost optical waveguide [13], although this property has not yet been commercially exploited. Assessment of the composition of the surface layer is partly confused by the fact that the tin exists in at least two charge states (Sn(II) and Sn(IV)) [14] and is associated with the accumulation of other impurities, or segregation of host constituents. Standard chemical analyses of the surface composition lack the depth resolution required for detailed study of the tin modified layers, however some of the surface specific techniques require successive sectioning of the surface [15]. It is not always recognised that the chemical, mechanical or sputtering methods used for the sectioning are not passive techniques, and in many cases can seriously perturb the original composition profiles. Details of such problems for analysis of tin in float glass will be published subsequently. Therefore, although numerous techniques have been applied, the apparent tin profiles are not identical, and in part show artifacts of the preparation techniques. For the present study this is not a problem as most measurements have been made on the original surfaces after they left the production line. In the immediate surface layer, secondary ion mass spectrometry (SIMS) data [6] suggest that there is a rapidly decreasing quantity of tin over the first 0.2 Ixm of the profile, and the quantity of tin is dependent on the sample thickness, but inconsistent interpretations occur for deeper tin penetration. For example the tin entering in the lower face is variously described as being a smoothly decreasing function ( ~ exponential) by X-ray fluorescence and HF etching [7], but electron probe micro analysis (EPMA) data [8,9] suggest that not only does the tin extend beyond 30 Ixm for thick glass (12 mm), but additionally, for all the thicknesses there is a significant buried peak at about 40% of the total diffusion range. Rather different techniques using measurements of optical waveguide refractive index profiles and cathodoluminescence (CL) [12] were interpreted as being in agreement for a profile which has a saturation property change near the upper few p~m of
233
the surface, followed by a diffusion tail further in the sample. At first sight these appear to be in conflict with the data of Verita et al. [8,9], but with the benefit of hindsight, one sees that the CL data were not monotonic, but include a small minimum at precisely the depth indicated by the EPMA data minimum for the same thickness glass. Additionally, the EPMA data indicate that not only does the total tin content rise for thicker glass samples, but also the surface concentration increases, albeit somewhat erratically with thickness. CL data obtained from the near surface region (i.e., to a depth of ~ 0.3 Ixm) are also erratic, but indicate a clear decreasing trend in signal with thickness. By contrast, RBS data (Lamouroux, unpublished [16]) give signals from the outer few txm which increase linearly with thickness. Overall it is necessary to recognise that the alternative profiling methods do not sample identical depths, and further, they may respond differently to the Sn(II) and Sn(IV) states. Thus one possible explanation for the observed decreasing CL intensities with glass thickness is that the CL responds more effectively to just one charge state, and not the total tin content in the layer. Changes in the CL signal with heat treatments or different polishing methods, may further indicate that the charge state can be modified in these treatments. Such valence state sensitivity may be of particular value in assessing the problems of chemical and physical adhesion of coating layers on the glass and so CL is a useful complement to the alternative probe methods. However, the EPMA, SIMS, RBS and CL data all are in agreement that there is at least 10 to 20 times as much tin on the lower face as the upper one; the total tin uptake increases with glass thickness (i.e., contact time with the tin bath); and the presence of tin is linked to changes in the local concentration of iron and anticorrelated with sulphate ions. Further, as might be expected for a bulk production process, there are considerable variations in the absolute quantities of the compositions between glass from different sources or positions across the float line, and these variations are in addition to the estimations of SnO 2 which vary with the analysis method. The EPMA data suggest the buried peak in the tin concentration appears to be linked to the onset of increases in Fe203 and SO 3 concentrations. In view of the difficulties of intercomparison, the
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present series of experiments were undertaken using ion beam analyses to record the concentrations of several elemental components across the full width of a single section of a float glass line, both from the lower and upper faces. All such particle counting analyses have statistical errors, which to a first approximation can be reduced by increasing the total number of counts. However, this supposes the signals from each element are in isolation and there is no ionic diffusion or surface dissociation driven by the probe beam. This assumption is probably justified for the ion beam doses used, which are limited to some 50 ~C of probe beam into a spot size of l mm 2. There will be dissociation of oxygen from a few monolayers of the surface, but since the analyses concentrate on the metal (and Si) content such changes have been ignored in the present investigation. The implantation dose is not however negligible in terms of producing measurable changes in the refractive index of the layer, and densification is known to occur, which raises the refractive index [17]. Compositional variations and ionic movement are less likely, but cannot be totally excluded. These are not problems limited to ion beam analysis, and for silicate materials they have also been seen for electron beam bombardment [ 17]. The new data for clear float glass were compared with equivalent measurements on samples taken from the same furnace for the production of green glass. The overall aims of the project were to resolve factors which could be related to the furnace design, and to obtain data which might help an understanding of the chemical interactions between the tin and other elements within the glass. Related new data from the changes in luminescence with depth, refractive index, polishing chemistry and sample thickness will be reported subsequently.
2. Experimental The sections were taken from a slice from the standard production line of SIV (Societa Italiana Vetro) in which each section was cut from an entire width of either clear or green glass. The ion beam analyses were made on 2 × 2 cm 2 square samples taken at 47 cm intervals across the section cut from the 2.8 m wide float line. Samples also included
those obtained during a production change over cycle from green to clear manufacture, and for glasses of a variety of thickness from 2.1 to 12 ram. Alternative ion beam analyses were made using helium ion Rutherford backscattering spectrometry (RBS) and helium or proton versions of particle induced X-ray emission (PIXE). In order to differentiate them they will be referred to as PIXE for protons and HIXE when using helium ions. One advantage of HIXE is that it can be made simultaneously with RBS and is therefore on exactly the same region of the sample. The principal advantages of simultaneous analyses with respect to sequential analyses are that (a) results from both techniques correspond to identical conditions over the same region of the sample and (b) ion-induced changes in the surface structure and composition at each location are minimised by the reduced ion fluence (i.e., dose). The helium ion energy was 1.89 MeV and data were recorded for 50 ~C of incident beam. Proton beams for PIXE were at 1.5 MeV and doses were 10 ~C. Low beam currents (nanoamps or less) and close proximity to earthing regions avoided problems of charging on the glass surfaces. Note in particular that RBS, HIXE and PIXE data are unlikely to be identical, as they provide analyses over very different depth scales beneath the surface. The RBS data are analyzable from the outer ~m or so of the surface, even though the ion beam penetration is greater. By contrast, both HIXE and PIXE signals can originate from the entire ion range. These are very different for the He and H ions used as the projected ion ranges of 1.89 MeV He ions in the glass is ~ 5.6 ~m, whereas that for 1.5 MeV protons is ~ 29 ~m. Therefore for the 2.1 mm thick glass, where the tin penetration is on the scale of a few ~m at most, the HIXE beam does not probe beyond the tin doped region, this results in a different compositional view of the sample compared with the PIXE signals, since the PIXE data integrate compositional ~edistributions over a much greater depth scale, including a significant contribution from the bulk glass RBS of iioat glass is capable of giving a clear separation of the tin, iron, calcium, silicon and oxygen signals, but is less successful for the analysis of the small concentrations of the light elements such as sodium (as exemplified in Fig. 1). Quantitative measurement of the near surface concentrations were
235
F. Lamouroux et al. / Journal o f Non-Cr3,stalline Solids 212 (1997) 2 3 2 - 2 4 2
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normally possible, but for conversion into depth distributions there were some problems as the tin was present to depths of > 0.8 jxm, and so the tail of the RBS tin signal extended beneath signals from Fe and Ca. The difficulty is typified for the tin in the green glass, as shown in Fig. 1. The difference between the lower and upper faces, here called the tin and non-tin sides, is apparent. From the figure it reveals the Fe signal, superposed on that of Sn, but surprisingly the iron RBS signal is very weak on the non-tin side of the green glass, even though green colouring is achieved by intentional iron doping of the glass. The shape of the RBS edges is less steep for the Sn and Fe than one would expect, which may indicate either that there was surface charging of the glass, or that the peak concentration was beneath the surface for each of these species. Charging is less likely as the Si and O edges are not perturbed, therefore there is an indication that the Sn and Fe signals are increasing from the surface over the first ~ 0.05 txm, in contrast with the EPMA data. PIXE type signals have much greater sensitivity than those of RBS and can detect smaller quantities of impurities, however they do not offer any clear depth information. There is also an unfortunate problem in this example where we wish to resolve features from Sn and Ca ions, since for these elements there is relatively poor signal separation from the X-ray emission of Sn and Ca, and, as seen in Fig. 2, the two signals overlap• In this example tin L lines should appear at channels 346, 345, 368 and 392, which span the same energy range as for those from
Ca K shells, which should fall at 371,370 and 403. (Note an experimental shift in channel zero has not been adjusted for on the figures.) For the surface of float glass the concentrations of the two elements are not independent so one cannot merely deduce the strength of the tin signal by reference to signals from the non-tin side. In the case of the clear float glass, the calcium values at the upper face and within the glass interior were closely similar. The previously mentioned advantage of the HIXE and PIXE comparison is also seen from this figure, as the greater penetration of the protons activates much more undoped glass, therefore from the upper surface, the signal is dominated by Ca, and so the effective S n / C a ratio is changed when comparing the HIXE and PIXE data. The fact that the PIXE and HIXE signals vary by ~ 10% between the two faces is related to the changes in the cross-sections for X-ray emission which favour detection of the heavier elements (i.e., Sn rather than Ca). In earlier studies [8] the presence of sulphur was noted as being important in controlling the tin profile, but in this work the sulphur signals were inadequate for quantitative assessment, similarly Mn has been recorded [11] as a trace impurity, but here it was only detectable in one sample and so neither element will be discussed. 2400
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F. Lamouroux et al. / Journal of Non-CrT'stalline Solids 212 (1997) 232-242
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3.1. Ion beam analysis of clear and green glass The spatial and face dependent signals from the components which were analyzed are relatively complex and therefore intensity values are displayed as histograms in Fig. 3 for the sampling points across the width of the float line, from SX to DX (right to left). The variations in intensity are greater than the sampling errors and typically are reproducible within < 5% when measured on the same specimen. The results emphasise that there are some clear trends for the uptake of the tin. Nevertheless the patterns do not always vary smoothly across the width of the sets of samples and deviations from the smooth patterns occur at local regions of some samples, as shown by the data. In order to emphasise the patterns which occur, many of the figures are plotted with a suppressed zero. Fig. 3 gives a compilation of values for the RBS, HIXE and PIXE signals from sheets of clear and green float glass. Whereas the signals for tin are clearly defined for RBS in the upper figures of Fig. 3, there are only composite X-ray signals of Sn + Ca from the X-ray data for HIXE and PIXE
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Fig. 3. A compilation of RBS, HIXE and PIXE data for clear and green float glass samples across the width of the float production line. The lower face data, where the glass was in contact with the tin bath, are shown as solid bars and the upper face data as hatched areas. For the RBS of clear glass (a), the upper face signals have been increased by a factor of 18. Other data are on the same scales for each face.
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F. Lamouroux et al. / Journal of Non-C~stalline Solids 212 (1997) 232-242
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across the width. On the lower face the maximum tin concentration is in the middle of the float line, whereas on the upper face the tin is greatest near the edges• The variations in the tin concentration are > 50% on the upper face and ~ 12% on the tin face. HIXE and PIXE data for both clear and green float glass are shown in Fig. 4, and finally, Fig. 5 presents examples of HIXE and PIXE data taken from the same regions of these sets of glass samples, (but not from identical spots) which show a number of variations in the recorded Na, Si and Cr signals from the two types of glass surface.
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3.2. Measurements of compositions through the glass thickness In order to assess if the compositional variations were solely confined to the glass surface, experiments were made with samples which had been polished at an angle of 20° to the surface, so that sequential analysis could be made by lateral displacement of the same sample. For this purpose a float glass sample of 7.9 mm thickness was chamfered to give analysis at lateral spacings across the sloping face. Data are shown in Fig. 6 a - d for HIXE measurements of (Ca + Sn), Na, Fe and Si content. The figures include the values obtained from the two original faces, together with a set of 11 values taken
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from the polished sloping faces. Fig. 7 includes related PIXE data for the (Ca + Sn), Fe and Cr for the outer faces and just six equally spaced analyses between them. Both analyses reveal much stronger
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Distance (+_ 0.2 era) Fig. 6. These HIXE data were taken from 7.9 mm thick samples which have been chamfered to reveal the depth thickness distributions of several elements. The end point data of the tin and non-tin side are also shown.
Fig. 8. The PIXE signals for iron taken for a sequence of float glass samples derived from changing concentrations of iron during conversion of the float line from dark green to light green over a period of several days. Note the significant difference in PIXE signals from the tin and upper faces of the glass. The compositions were measured from chemical analyses of bulk material. The linear fits are to the observed data and the alternative saturating exponential curve includes the zero point (see text).
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F. Lamouroux et al. / Journal of Non-Co,stalline Solids 212 (1997) 232-242
(Sn + Ca) signals on the lower face (as expected), and a significantly smaller signal from the upper face.
3.3. Effects of changing iron content during glass production An opportunity was taken to analyze the surface iron compositions relative to those in the bulk glass, from an equivalent point across the float line, over a number of days as the line was changed from dark to light green glass. PIXE analyses of the iron content from the upper and lower faces of the glass are presented in Fig. 8.
4. Discussion
4.1. Ion beam analysis of clear and green glass It is immediately apparent from the preceding results that, while it may be possible to identify trends in the data, the number of variables involved in terms of glass composition, uniformity and temperature history for different sections of the float line will inevitably result in some scatter between the analyses at different locations. Nevertheless, commencing with the RBS tin data of Fig. 3, the patterns are relatively understandable in purely physical terms, since the tin bath temperature varies across the width of the line and is a maximum in the centre, which results in greater diffusion and ion exchange in the centre, hence a central tin maximum is reasonable. At the upper face the tin arrives by vapour transport, and the flow is limited by the structure of the furnace. This has the consequences that (a) there will be much less tin contamination and, (b) less tin will be expected to reach the centre of the upper face. Additional factors involving the chemistry of the ions at each face probably contribute as well, but the chemical role is less apparent at this stage from the RBS data. As mentioned, HIXE analyses for the Sn is not ideal, because of the overlap with the Ca signal, but nevertheless Fig. 3b exhibits the same trend as from RBS in that there is more (Sn + Ca) signal from the tin face. However the combined signal shows an obvious minimum in the middle of the furnace,
which for the tin side is clearly the opposite in shape from that of the Fig. 3a. In the case of the green glass, there is a similar factor of ~ 18 in the average Sn concentration seen by RBS on the two faces, and broadly the pattern of more tin near the edges is again apparent on the upper face (except for the 141 cm datum on Fig. 3d). In percentage terms the upper values vary by ~ 50%, but by only ~ 10% on the tin side. There appear to be more fluctuations in the intensity of the tin signals in the green glass. This is most obvious for the RBS data of Fig. 3d which show no clear patterns. At the higher concentration levels, for the tin on the lower face, the average values are slightly less than for the clear material, as shown on Fig. 3a. More dramatically, Fig. 3d emphasises that there is only about 30% less tin on the upper face of the green glass, compared with the lower face, whereas Fig. 3a directly emphasises that for the clear float glass the differences were greater than a factor of 18. Further, in terms of general trends and apparent scatter in the data, the recorded variations are much larger than expected from the counting statistics. On the upper face the green glass contains considerably more tin than the equivalent face of the clear material, albeit in highly variable quantities between the samples. HIXE and PIXE data confirm that the net (Sn + Ca) signals are higher for the lower face, but in absolute terms the number of counts is more equal than would be expected if there were only additions of Sn. One therefore concludes that inward diffusion of tin is matched by a loss, or movement, of the Ca from the surface layer. There is no reason to believe that the bath temperature profile changes between the production of clear and green float glass and the lack of a central tin peak emphasises that the indiffusion of tin is a chemically, as well as a thermally, controlled process in which iron plays a pivotal role. Overall, the data suggest that the spatial variation of the uptake of tin at the surface (RBS) not only differs from, but can be the inverse of the depth integrated signal (PIXE). Indeed, since earlier reports link chemical associations between the presence of Sn and Fe, this is not unlikely. The data for HIXE and PIXE profiles for Fe were shown in Fig. 4. For the clear glass, and using the shallower probe of the HIXE, there is evidence that the iron, which is a natural contaminant of the raw
F. Lamouroux et al. / Journal of Non-Crystalline Solids 212 (1997) 232-242
material, follows the same trend as for the Sn. There is a higher concentration on the lower face, and a maximum at the 141 cm position in the middle of the sheet. The HIXE surface analysis emphasises that the iron concentrates where there is surface tin, as seen by Sn RBS, and varies by a similar amount between the centre and edge of the lower face. However, for these samples there is less Fe detected on the upper face by only a factor of 3, compared with the factor of 18 seen in RBS for the tin ratio from the two sides. The green glass measurements indicate a higher overall concentration of iron, compared with the clear material, by a factor of about three, but are confusing in that when comparing HIXE signals for Fe and (Sn + Ca), the patterns for the lower face are inverted for the clear material. While P1XE signals confirm some excess of Fe on the lower face, the similarity between the two sides (Fig. 4a, b) suggests that averaged over the larger volume detected by PIXE, the total iron content is rather similar at all points on both sides of the glass. These analyses are not inconsistent with the earlier suggestion of Verita et al. [8] in which Fe and Sn followed similar reductions in concentration over the outer 5 to 10 Ixm of their 12 mm thick glass, but deeper in the interior the Fe concentration rose markedly. An initial interpretation is that Fe moves from a uniformly distributed bulk, or interior, concentration towards the surface to associate with the Sn. At the centre of the sheet the temperature is higher and so more iron can reach the surface layers (where there is also a maximum in the Sn concentration). From Fig. 3 it appeared that additions of Sn reduced the quantity of Ca, so on proceeding to include data of Fig. 4, one may suggest that for the 2.1 mm glass the iron peaks at a greater depth than the Sn, and so displaces Ca towards the interior of the glass melt. PIXE analysis integrates signals over some 29 ~xm beneath the surface, but even on this scale there are differences between the upper and lower faces. Hence ionic migration proceeds on at least the 30 txm scale within the time the glass is being produced. The PIXE data can be interpreted as evidence that, except for the indiffusion of Sn, the compositional changes for Fe and Ca are primarily from redistribution of components from the interior of the glass. For HIXE of Fe in green glass one must recognise that the green coloration is achieved by
239
addition of Fe to the bulk material so Sn linked changes may be masked to some extent, but Fig. 4 does confirm however that, as seen by contrasting HIXE and PIXE data, there is movement of Fe towards the surface on the tin side (i.e., implying a depletion of iron from an intermediate depth beyond 5 Ixm). Analysis of a selection of other elements was given in Fig. 5. The variability of the signals is much greater than the reproducibility but two general features are that the concentrations of each element differ on the two sides of the glass, and are non-uniform across the width. It is immediately apparent that the concentrations of the trace element chromium are highly variable across the float line for these samples. The lack of any obvious trend and the presence of three anomalously high signals in the clear float glass (Fig. 5i), where there is a fourfold variation in the detected Cr, probably indicates that the Cr is a random and non-uniform dopant in the melt. Chromium PIXE data from the green glass follow a similar pattern to that of iron on the tin face. The patterns of a central minimum, or maximum, for Na and Si analyses are in line with the earlier figures and support the view of a non-uniform temperature profile for the furnace. For example the PIXE signals for the tin side of Si and Na have inverse patterns and that of the silicon appears to resemble that of the Sn, but with a lesser percentage change. Sodium seems to be in slightly higher concentrations near the sides of the float line on both the lower and upper faces, but relatively constant across the upper face. The use of a suppressed zero on these histograms perhaps overemphasises the variations, but the HIXE and PIXE signals are not identical, even when similar trends can be discerned, and more surprisingly it is the PIXE signal which shows the greater variation. To summarise, it is difficult to offer detailed models of the likely chemistry, although for the clear glass there is an obvious surface association of Sn, Fe and Si concentration profiles and a small anticorrelation with that of Na from deep within the lower glass face. The HIXE data further suggest that the distributions on the outer regions of the two faces are not identical (e.g., for Si, Fe and Na) and at some positions across the float line these surface concentrations can differ by a factor of two between the faces.
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4.2. Measurements of compositions through the glass thickness In the analyses of compositional variations with depth, the experimental difficulties of sectioning were eased by using 7.9 mm thick glass, rather than the 2.1 mm material discussed for the standard float line investigations. While it has already been suggested that the form of the near surface composition may be a function of the glass thickness (e.g., Ref. [10]), the data of Figs. 6 and 7 are unlikely to be influenced by this, and the expectations are that while the signals will differ between the two surfaces, to a first approximation, they might be uniform within the bulk material. The intensities of data from the interior mostly vary within about 3% on Fig. 6, although a slight trend appears for the sodium data that suggests this is not just a random error or inherent problems of uniform mixing within a glass whose viscosity varies steeply with temperature. Sodium data show losses at both faces, most noticeably on the non-tin side, whereas Si has a reverse trend (Fig. 6). The HIXE Fe data are erratic and at variance with those from PIXE, where the iron content appears to fall steadily across the sample. By contrast with all the other data sets, the PIXE Cr signals display a clear concentration maximum in the interior of the glass. While the act of polishing might induce a redistribution of material in the near surface layers, it seems unlikely to be the origin of the large variations seen for the Fe and Cr signals, particularly since the PIXE signals originate from up to 29 Ixm beneath the surface. Overall, the clear conclusions are that the compositions within the glass differ from those at the surfaces, and that the compositional variations are not obviously linked, and therefore may just be the result of imperfect mixing of the constituents of the glass. No clear pattern of internal compositional separations is apparent at this stage.
4.3. Effects of changing iron content during glass production Fig. 1 has already presented the RBS signals from the two faces for the sample with 0.593% Fe203 (as determined by bulk chemical analysis at SIV). From the lower face the iron signal was of approximately the intensity expected from this bulk composition.
However, an immediately obvious feature was that the iron signal from this outer txm of the glass is most apparent on the lower face, despite the fact that for this concentration within the bulk, one would have expected a comparable Fe signal on the upper face as well. It therefore appears that on the depth scale sampled by the RBS the iron is depleted from the upper surface. Indeed, this depletion was observed for all the Fe concentrations in the set of green glass samples, and Fig. 8 summarises how the PIXE analyses signals for the Fe varied with bulk changes in the iron content. Note that the PIXE signal represents a signal integrated over some 29 I~m depth (i.e., this is already much greater than the penetration range of the tin and therefore might be assumed to mirror the Fe concentration within the bulk). On the upper face there was a linear dependence between the PIXE and bulk iron signals, but of particular note is that the absolute intensities were only ~ 10% of those expected from the bulk, even when considering data from a 29 Ixm layer. It is not clear what function should be quoted for the iron dependence from the lower face, since although it is possible to describe the data by a linear dependence, as shown on Fig. 8, extrapolation of the line to the nominal zero concentration for the iron would give a significant intercept for the iron signal. Consequently it may be appropriate to consider an alternative function to describe the surface concentration of the iron at the tin side of the glass. Fig. 8 indicates that the measured data, together with the zero value, might be described by a saturating exponential function. This plot emphasises the problem and the mismatch between the bulk signal and that at the surface, but does not explain the origin of the divergence. Finally, the data of Fig. 8 for the set of samples taken during the dark green to light green glass conversion of the float line, show an iron content in the upper and lower faces which differ initially by a factor of ~ 15, increasing to ~ 50 times at the lower bulk values for iron. This changing ratio, which differs from the factor of 3 reported above for steady state green glass production, is unexpected since the upper face value is consistently below that expected in the bulk. The slow reduction in the lower face iron content may be indicative of a build up of iron in the float bath during green glass production, which leads to a non-equilibrium excess
F. Lamouroux et al. /Journal of Non-Crystalline Solids 212 (1997) 232-242
of iron in the tin side of the green float, as the line is converted to the clearer glass. The data imply that a return to totally clear float production may require an extended period of operation to purge iron from the tin bath. The chemistry for the incorporation of iron will be sensitive to the presence of other elements and their valence states, and with this in mind inspection of the data were made to separate out possible reasons for the surface dependence of the iron relative to that in the bulk glass. The combined HIXE signals of S n / C a differ for the two faces, presumably because of the uptake of tin. PIXE equivalent signals reveal a similar variation in pattern although the ratio of signals for [(0.279% F e 2 0 3 ) / ( 0 . 1 6 2 % Fe203)] are 10% for HIXE and only 5% for PIXE. This implies internal depth variations in composition. Sodium data are equally variable ~ 4% with rather more Na being included in the iron rich material. Such data do not resolve the differences between surface and bulk material for the iron and the conclusions to draw from the green glass data are that the Fe is strongly associated with the presence of the tin, but even for the green glass it is depleted near the upper surface layer, with the most surprising fact being that the concentration falls to ~ 10% that of the bulk. With such complex chemistry, and the reality of using samples from a commercial production line, one must recognise that the data can only indicate trends, rather than provide definitive models for the manner in which the indiffusion of tin alters the surface composition. Indeed, the current study has underlined that for this production process there are a number of competing processes. For example, the various surface analysis techniques used here provide information on different depth scales, and over the outer few tens of Ixm the glass compositions are varying significantly. Such information is potentially of value in float glass production, and in the related problems of understanding the variability in the quality of bonding of coatings, which are linked to the presence of surface tin. The uptake of tin is known to be temperature sensitive, as is the viscosity of the glass, hence on the lower face there is a peak in the tin doping level at the centre of the float line, and other impurities differ in their concentrations present at the surface across the width of the float line.
241
Variations in composition with depth were also noted, but these were relatively minor compared with the changes at the surfaces. The interlinked chemistry of Sn, Fe, Na and Ca is evident and expected, but the depletion of Si from the upper surface was not. Finally, in the set of varying Fe doped green glass samples there was strong evidence for surface depletion of the iron on the upper face and a non-linear dependence, relative to the bulk, for the iron in the tin face.
5. Summary Near surface compositional analysis of clear and green float glass has shown how not only the tin, but also iron and other elements, vary differently across the width of the float line. The variational trends differ from the upper and lower faces of the glass. In the case of green glass there are disparities between the surface and bulk iron contents. Analysis with depth of a thick glass sample also reveals inhomogenieties in the composition, even within the 'bulk' of the glass.
Acknowledgements We wish to thank Dr. M Capranica and Dr. C. Ergun from SIV for the glass samples and for discussions. Financial support is gratefully acknowledged from the Brite Euram contract BREU2-0357 (project BE 6111), from the British Council and the University of Ege.
References [1] W. Bettley and D.D. Ross, Glass Technol. 35 (1994) 193. [2] C.G. Pantano, V. Bojan, M. Verita, F. Geotti-Bianchini and S. Hreglish, Fundamentals of Glass Science and Technology, ESG Venice Proc. (1993). [3] A. Pilkington, Proc. R. Soc. London A314 (1969) 1. [4] W.C. Hynd, Flat Glass Manufacturing Processes, Glass: Science and Technology, ed, D.R. Uhlamm and N.J. Kreidl, Vol. 2 (Academic Press, New York, 1984) p. 83. [5] C.K. Edge, Float glass, Advances in Ceramics, ed. D.C. Boyd and J.F. MacDowell, Vol. 18 (American Ceramic Society, Westerville, OH, 1986) p. 43. [6] C.K. Edge, Am. Ceram. Soc. Bull. 71 (1992) 936.
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[7] L. Colombin, H. Charlier, A. Jelli, G. Debras and J. Verbist, J. Non-Cryst. Solids 38&39 (1980) 551. [8] M. Verita, F. Geotti-Bianchini, S. Hreglich, C.G. Pantano and V. Bojan, Bol. Soc. Esp. Ceram. Vidrio 31-C-6 (1992) 415. [9] A. Stella and M. Verita, Mikrochim. Acta 114 (1994) 475. [10] C.G. Pantano, V. Bojan, M. Verita, F. Geotti-Bianchini and S. Hreglich, Proc. 2nd Int. Conf. Eur. Soc. Glass Sci. Technol., Venice (1993). [1 I] J.F. Shackelford, S.H. Risbud, B.H. Kusko, T. Cahill, P.P. Bihuniak and M.E. Hanson, Am. Ceram. Soc. Bull. 72 (1993) 100.
[12] B. Yang, P.D. Townsend and S.A. Holgate, J. Phys. D27 (1994) 1757. [13] G. Kakarantzas, E. Glavas and P.D. Townsend, Electron. Lett. 25 (1989) 102. [14] G. Principi, A. Maddalena, A. Gupta, F. Geotti-Bianchini, S. Hreglich and M. Verita, Nucl. lnstrum. Methods B76 (1993) 215. [15] H. Bach, Fresenius' Z. Anal. Chem. 333 (1989) 373. [16] F. Lamouroux, MSc thesis, University of Sussex (1995). [17] P.D. Townsed, P.J. Chandler and L. Zhang, Optical Effects of Ion Implantation (Cambridge University, Cambridge, 1994).
ELSEVIER
Journal of Non-Crystalline Solids 212 (1997) 232-242
Ion beam analysis of float glass surface composition F. Lamouroux a, N. Can b, P.D. T o w n s e n d a,* , B.W. Farmery a, D.E. Hole
a
a Department of Physics and Astronomy, Unit,ersity of Sussex, Brighton BNI 9QH, UK b Department of Physics, Science Faculty, Unirersi O, of Ege, 35100 Bornova-lzmir, Turkey
Received 23 July 1996; revised 14 November 1996
Abstract Data are reported on ion beam surface analysis methods of proton-induced X-ray emission (PIXE), and helium-induced X-ray emission (HIXE), made simultaneously with Rutherford backscattering spectrometry (RBS), which were taken with samples from across the full width of a float glass production line. Data are given for both clear and green glass. Significant variations are recorded between signals from the lower surface, (the face in contact with tin), and the upper surface, not only for the uptake of the tin, but also for the consequent changes in the near surface composition of the other main elements of the glass. In particular, the tin alters the local compositions of iron, calcium, silicon, and sodium. The changes are interpreted in terms of the furnace temperature gradients and chemical interactions. In all cases, including the green glass, iron is depleted from the non-tin face relative to the bulk composition, and the changes of surface content of the iron differs on the two faces relative to the bulk value.
1. Introduction Float glass is of considerable industrial importance [1-6] and consequently there have been numerous studies of the compositional changes in the near surface layers which are caused by the ingress of tin from the float process [7-12]. The presence of tin within the glass surface in contact with the tin bath is an inevitable consequence of the high temperature processing and contact of the two molten materials. To a lesser extent tin will also reach the upper surface of the glass by vapour transport. H o w e v e r , the chemistry at the two faces will differ, since the upper face may be under reducing conditions,
* Corresponding author: Tel.: +44-1273 678 073; fax: +441273 678 193/678 097; e-mail: [email protected].
whereas as there is no gas phase on the lower surface. The total concentration and depth of penetration will be influenced by the tin temperature and thermal gradient along the flow path, the contact time (as defined by the flow speed), the chemical composition of the glass feed material and furnace design. There are the obvious problems of non-uniform mixing of the source materials and variations in their purity when dealing with a commercial production line which handles some 600 tonnes ( ~ 5 X 105 kg) per day. However, the current study has focused on the presence of the tin, since this is generally undesirable for several reasons. Firstly it results in ' b l o o m i n g ' as the surface appearance is degraded by tin precipitation during subsequent heat treatments, for example during tempering or bending of the sheet, as used for car windscreens. The presence of the modified surface layer is also attributed to the
0022-3093/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PH S0022-3093(97)00027-6
F. Lamouroux et a l . / Journal of Non-Crystalline Solids 212 (1997) 232-242
poor, or variable, adhesion quality of many types of coating which are applied to the glass sheet. Overall, the tin is thought to be responsible for a wide range of technological problems. On the positive side, the presence of tin dopants in the outer glass layer provides a very low loss and low cost optical waveguide [13], although this property has not yet been commercially exploited. Assessment of the composition of the surface layer is partly confused by the fact that the tin exists in at least two charge states (Sn(II) and Sn(IV)) [14] and is associated with the accumulation of other impurities, or segregation of host constituents. Standard chemical analyses of the surface composition lack the depth resolution required for detailed study of the tin modified layers, however some of the surface specific techniques require successive sectioning of the surface [15]. It is not always recognised that the chemical, mechanical or sputtering methods used for the sectioning are not passive techniques, and in many cases can seriously perturb the original composition profiles. Details of such problems for analysis of tin in float glass will be published subsequently. Therefore, although numerous techniques have been applied, the apparent tin profiles are not identical, and in part show artifacts of the preparation techniques. For the present study this is not a problem as most measurements have been made on the original surfaces after they left the production line. In the immediate surface layer, secondary ion mass spectrometry (SIMS) data [6] suggest that there is a rapidly decreasing quantity of tin over the first 0.2 Ixm of the profile, and the quantity of tin is dependent on the sample thickness, but inconsistent interpretations occur for deeper tin penetration. For example the tin entering in the lower face is variously described as being a smoothly decreasing function ( ~ exponential) by X-ray fluorescence and HF etching [7], but electron probe micro analysis (EPMA) data [8,9] suggest that not only does the tin extend beyond 30 Ixm for thick glass (12 mm), but additionally, for all the thicknesses there is a significant buried peak at about 40% of the total diffusion range. Rather different techniques using measurements of optical waveguide refractive index profiles and cathodoluminescence (CL) [12] were interpreted as being in agreement for a profile which has a saturation property change near the upper few p~m of
233
the surface, followed by a diffusion tail further in the sample. At first sight these appear to be in conflict with the data of Verita et al. [8,9], but with the benefit of hindsight, one sees that the CL data were not monotonic, but include a small minimum at precisely the depth indicated by the EPMA data minimum for the same thickness glass. Additionally, the EPMA data indicate that not only does the total tin content rise for thicker glass samples, but also the surface concentration increases, albeit somewhat erratically with thickness. CL data obtained from the near surface region (i.e., to a depth of ~ 0.3 Ixm) are also erratic, but indicate a clear decreasing trend in signal with thickness. By contrast, RBS data (Lamouroux, unpublished [16]) give signals from the outer few txm which increase linearly with thickness. Overall it is necessary to recognise that the alternative profiling methods do not sample identical depths, and further, they may respond differently to the Sn(II) and Sn(IV) states. Thus one possible explanation for the observed decreasing CL intensities with glass thickness is that the CL responds more effectively to just one charge state, and not the total tin content in the layer. Changes in the CL signal with heat treatments or different polishing methods, may further indicate that the charge state can be modified in these treatments. Such valence state sensitivity may be of particular value in assessing the problems of chemical and physical adhesion of coating layers on the glass and so CL is a useful complement to the alternative probe methods. However, the EPMA, SIMS, RBS and CL data all are in agreement that there is at least 10 to 20 times as much tin on the lower face as the upper one; the total tin uptake increases with glass thickness (i.e., contact time with the tin bath); and the presence of tin is linked to changes in the local concentration of iron and anticorrelated with sulphate ions. Further, as might be expected for a bulk production process, there are considerable variations in the absolute quantities of the compositions between glass from different sources or positions across the float line, and these variations are in addition to the estimations of SnO 2 which vary with the analysis method. The EPMA data suggest the buried peak in the tin concentration appears to be linked to the onset of increases in Fe203 and SO 3 concentrations. In view of the difficulties of intercomparison, the
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F. Lamouroux et a l . / Journal of Non-Cr3,stalline Solids 212 (1997) 232-242
present series of experiments were undertaken using ion beam analyses to record the concentrations of several elemental components across the full width of a single section of a float glass line, both from the lower and upper faces. All such particle counting analyses have statistical errors, which to a first approximation can be reduced by increasing the total number of counts. However, this supposes the signals from each element are in isolation and there is no ionic diffusion or surface dissociation driven by the probe beam. This assumption is probably justified for the ion beam doses used, which are limited to some 50 ~C of probe beam into a spot size of l mm 2. There will be dissociation of oxygen from a few monolayers of the surface, but since the analyses concentrate on the metal (and Si) content such changes have been ignored in the present investigation. The implantation dose is not however negligible in terms of producing measurable changes in the refractive index of the layer, and densification is known to occur, which raises the refractive index [17]. Compositional variations and ionic movement are less likely, but cannot be totally excluded. These are not problems limited to ion beam analysis, and for silicate materials they have also been seen for electron beam bombardment [ 17]. The new data for clear float glass were compared with equivalent measurements on samples taken from the same furnace for the production of green glass. The overall aims of the project were to resolve factors which could be related to the furnace design, and to obtain data which might help an understanding of the chemical interactions between the tin and other elements within the glass. Related new data from the changes in luminescence with depth, refractive index, polishing chemistry and sample thickness will be reported subsequently.
2. Experimental The sections were taken from a slice from the standard production line of SIV (Societa Italiana Vetro) in which each section was cut from an entire width of either clear or green glass. The ion beam analyses were made on 2 × 2 cm 2 square samples taken at 47 cm intervals across the section cut from the 2.8 m wide float line. Samples also included
those obtained during a production change over cycle from green to clear manufacture, and for glasses of a variety of thickness from 2.1 to 12 ram. Alternative ion beam analyses were made using helium ion Rutherford backscattering spectrometry (RBS) and helium or proton versions of particle induced X-ray emission (PIXE). In order to differentiate them they will be referred to as PIXE for protons and HIXE when using helium ions. One advantage of HIXE is that it can be made simultaneously with RBS and is therefore on exactly the same region of the sample. The principal advantages of simultaneous analyses with respect to sequential analyses are that (a) results from both techniques correspond to identical conditions over the same region of the sample and (b) ion-induced changes in the surface structure and composition at each location are minimised by the reduced ion fluence (i.e., dose). The helium ion energy was 1.89 MeV and data were recorded for 50 ~C of incident beam. Proton beams for PIXE were at 1.5 MeV and doses were 10 ~C. Low beam currents (nanoamps or less) and close proximity to earthing regions avoided problems of charging on the glass surfaces. Note in particular that RBS, HIXE and PIXE data are unlikely to be identical, as they provide analyses over very different depth scales beneath the surface. The RBS data are analyzable from the outer ~m or so of the surface, even though the ion beam penetration is greater. By contrast, both HIXE and PIXE signals can originate from the entire ion range. These are very different for the He and H ions used as the projected ion ranges of 1.89 MeV He ions in the glass is ~ 5.6 ~m, whereas that for 1.5 MeV protons is ~ 29 ~m. Therefore for the 2.1 mm thick glass, where the tin penetration is on the scale of a few ~m at most, the HIXE beam does not probe beyond the tin doped region, this results in a different compositional view of the sample compared with the PIXE signals, since the PIXE data integrate compositional ~edistributions over a much greater depth scale, including a significant contribution from the bulk glass RBS of iioat glass is capable of giving a clear separation of the tin, iron, calcium, silicon and oxygen signals, but is less successful for the analysis of the small concentrations of the light elements such as sodium (as exemplified in Fig. 1). Quantitative measurement of the near surface concentrations were
235
F. Lamouroux et al. / Journal o f Non-Cr3,stalline Solids 212 (1997) 2 3 2 - 2 4 2
1000 0 © ¢12
O
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normally possible, but for conversion into depth distributions there were some problems as the tin was present to depths of > 0.8 jxm, and so the tail of the RBS tin signal extended beneath signals from Fe and Ca. The difficulty is typified for the tin in the green glass, as shown in Fig. 1. The difference between the lower and upper faces, here called the tin and non-tin sides, is apparent. From the figure it reveals the Fe signal, superposed on that of Sn, but surprisingly the iron RBS signal is very weak on the non-tin side of the green glass, even though green colouring is achieved by intentional iron doping of the glass. The shape of the RBS edges is less steep for the Sn and Fe than one would expect, which may indicate either that there was surface charging of the glass, or that the peak concentration was beneath the surface for each of these species. Charging is less likely as the Si and O edges are not perturbed, therefore there is an indication that the Sn and Fe signals are increasing from the surface over the first ~ 0.05 txm, in contrast with the EPMA data. PIXE type signals have much greater sensitivity than those of RBS and can detect smaller quantities of impurities, however they do not offer any clear depth information. There is also an unfortunate problem in this example where we wish to resolve features from Sn and Ca ions, since for these elements there is relatively poor signal separation from the X-ray emission of Sn and Ca, and, as seen in Fig. 2, the two signals overlap• In this example tin L lines should appear at channels 346, 345, 368 and 392, which span the same energy range as for those from
Ca K shells, which should fall at 371,370 and 403. (Note an experimental shift in channel zero has not been adjusted for on the figures.) For the surface of float glass the concentrations of the two elements are not independent so one cannot merely deduce the strength of the tin signal by reference to signals from the non-tin side. In the case of the clear float glass, the calcium values at the upper face and within the glass interior were closely similar. The previously mentioned advantage of the HIXE and PIXE comparison is also seen from this figure, as the greater penetration of the protons activates much more undoped glass, therefore from the upper surface, the signal is dominated by Ca, and so the effective S n / C a ratio is changed when comparing the HIXE and PIXE data. The fact that the PIXE and HIXE signals vary by ~ 10% between the two faces is related to the changes in the cross-sections for X-ray emission which favour detection of the heavier elements (i.e., Sn rather than Ca). In earlier studies [8] the presence of sulphur was noted as being important in controlling the tin profile, but in this work the sulphur signals were inadequate for quantitative assessment, similarly Mn has been recorded [11] as a trace impurity, but here it was only detectable in one sample and so neither element will be discussed. 2400
?.M'~.
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F. Lamouroux et al. / Journal of Non-CrT'stalline Solids 212 (1997) 232-242
236
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3.1. Ion beam analysis of clear and green glass The spatial and face dependent signals from the components which were analyzed are relatively complex and therefore intensity values are displayed as histograms in Fig. 3 for the sampling points across the width of the float line, from SX to DX (right to left). The variations in intensity are greater than the sampling errors and typically are reproducible within < 5% when measured on the same specimen. The results emphasise that there are some clear trends for the uptake of the tin. Nevertheless the patterns do not always vary smoothly across the width of the sets of samples and deviations from the smooth patterns occur at local regions of some samples, as shown by the data. In order to emphasise the patterns which occur, many of the figures are plotted with a suppressed zero. Fig. 3 gives a compilation of values for the RBS, HIXE and PIXE signals from sheets of clear and green float glass. Whereas the signals for tin are clearly defined for RBS in the upper figures of Fig. 3, there are only composite X-ray signals of Sn + Ca from the X-ray data for HIXE and PIXE
(Fe) 0
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Fig. 4. Comparison of Fe K a X-ray signals from clear and green glass samples. The tin face data are shown as solid bars.
(the lower pairs of figures). For the clear glass the RBS of the tin on the two faces differs by a factor of ~ 18 and, more interestingly has very different trends Clear
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Fig. 3. A compilation of RBS, HIXE and PIXE data for clear and green float glass samples across the width of the float production line. The lower face data, where the glass was in contact with the tin bath, are shown as solid bars and the upper face data as hatched areas. For the RBS of clear glass (a), the upper face signals have been increased by a factor of 18. Other data are on the same scales for each face.
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F. Lamouroux et al. / Journal of Non-C~stalline Solids 212 (1997) 232-242
tO0FiiCa+Sn
across the width. On the lower face the maximum tin concentration is in the middle of the float line, whereas on the upper face the tin is greatest near the edges• The variations in the tin concentration are > 50% on the upper face and ~ 12% on the tin face. HIXE and PIXE data for both clear and green float glass are shown in Fig. 4, and finally, Fig. 5 presents examples of HIXE and PIXE data taken from the same regions of these sets of glass samples, (but not from identical spots) which show a number of variations in the recorded Na, Si and Cr signals from the two types of glass surface.
-~ ~
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3.2. Measurements of compositions through the glass thickness In order to assess if the compositional variations were solely confined to the glass surface, experiments were made with samples which had been polished at an angle of 20° to the surface, so that sequential analysis could be made by lateral displacement of the same sample. For this purpose a float glass sample of 7.9 mm thickness was chamfered to give analysis at lateral spacings across the sloping face. Data are shown in Fig. 6 a - d for HIXE measurements of (Ca + Sn), Na, Fe and Si content. The figures include the values obtained from the two original faces, together with a set of 11 values taken
oI
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Fig. 7. PIXE data as for Fig. 6, in which only six sampling points were used instead of eleven.
from the polished sloping faces. Fig. 7 includes related PIXE data for the (Ca + Sn), Fe and Cr for the outer faces and just six equally spaced analyses between them. Both analyses reveal much stronger
8000
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Distance (+_ 0.2 era) Fig. 6. These HIXE data were taken from 7.9 mm thick samples which have been chamfered to reveal the depth thickness distributions of several elements. The end point data of the tin and non-tin side are also shown.
Fig. 8. The PIXE signals for iron taken for a sequence of float glass samples derived from changing concentrations of iron during conversion of the float line from dark green to light green over a period of several days. Note the significant difference in PIXE signals from the tin and upper faces of the glass. The compositions were measured from chemical analyses of bulk material. The linear fits are to the observed data and the alternative saturating exponential curve includes the zero point (see text).
238
F. Lamouroux et al. / Journal of Non-Co,stalline Solids 212 (1997) 232-242
(Sn + Ca) signals on the lower face (as expected), and a significantly smaller signal from the upper face.
3.3. Effects of changing iron content during glass production An opportunity was taken to analyze the surface iron compositions relative to those in the bulk glass, from an equivalent point across the float line, over a number of days as the line was changed from dark to light green glass. PIXE analyses of the iron content from the upper and lower faces of the glass are presented in Fig. 8.
4. Discussion
4.1. Ion beam analysis of clear and green glass It is immediately apparent from the preceding results that, while it may be possible to identify trends in the data, the number of variables involved in terms of glass composition, uniformity and temperature history for different sections of the float line will inevitably result in some scatter between the analyses at different locations. Nevertheless, commencing with the RBS tin data of Fig. 3, the patterns are relatively understandable in purely physical terms, since the tin bath temperature varies across the width of the line and is a maximum in the centre, which results in greater diffusion and ion exchange in the centre, hence a central tin maximum is reasonable. At the upper face the tin arrives by vapour transport, and the flow is limited by the structure of the furnace. This has the consequences that (a) there will be much less tin contamination and, (b) less tin will be expected to reach the centre of the upper face. Additional factors involving the chemistry of the ions at each face probably contribute as well, but the chemical role is less apparent at this stage from the RBS data. As mentioned, HIXE analyses for the Sn is not ideal, because of the overlap with the Ca signal, but nevertheless Fig. 3b exhibits the same trend as from RBS in that there is more (Sn + Ca) signal from the tin face. However the combined signal shows an obvious minimum in the middle of the furnace,
which for the tin side is clearly the opposite in shape from that of the Fig. 3a. In the case of the green glass, there is a similar factor of ~ 18 in the average Sn concentration seen by RBS on the two faces, and broadly the pattern of more tin near the edges is again apparent on the upper face (except for the 141 cm datum on Fig. 3d). In percentage terms the upper values vary by ~ 50%, but by only ~ 10% on the tin side. There appear to be more fluctuations in the intensity of the tin signals in the green glass. This is most obvious for the RBS data of Fig. 3d which show no clear patterns. At the higher concentration levels, for the tin on the lower face, the average values are slightly less than for the clear material, as shown on Fig. 3a. More dramatically, Fig. 3d emphasises that there is only about 30% less tin on the upper face of the green glass, compared with the lower face, whereas Fig. 3a directly emphasises that for the clear float glass the differences were greater than a factor of 18. Further, in terms of general trends and apparent scatter in the data, the recorded variations are much larger than expected from the counting statistics. On the upper face the green glass contains considerably more tin than the equivalent face of the clear material, albeit in highly variable quantities between the samples. HIXE and PIXE data confirm that the net (Sn + Ca) signals are higher for the lower face, but in absolute terms the number of counts is more equal than would be expected if there were only additions of Sn. One therefore concludes that inward diffusion of tin is matched by a loss, or movement, of the Ca from the surface layer. There is no reason to believe that the bath temperature profile changes between the production of clear and green float glass and the lack of a central tin peak emphasises that the indiffusion of tin is a chemically, as well as a thermally, controlled process in which iron plays a pivotal role. Overall, the data suggest that the spatial variation of the uptake of tin at the surface (RBS) not only differs from, but can be the inverse of the depth integrated signal (PIXE). Indeed, since earlier reports link chemical associations between the presence of Sn and Fe, this is not unlikely. The data for HIXE and PIXE profiles for Fe were shown in Fig. 4. For the clear glass, and using the shallower probe of the HIXE, there is evidence that the iron, which is a natural contaminant of the raw
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material, follows the same trend as for the Sn. There is a higher concentration on the lower face, and a maximum at the 141 cm position in the middle of the sheet. The HIXE surface analysis emphasises that the iron concentrates where there is surface tin, as seen by Sn RBS, and varies by a similar amount between the centre and edge of the lower face. However, for these samples there is less Fe detected on the upper face by only a factor of 3, compared with the factor of 18 seen in RBS for the tin ratio from the two sides. The green glass measurements indicate a higher overall concentration of iron, compared with the clear material, by a factor of about three, but are confusing in that when comparing HIXE signals for Fe and (Sn + Ca), the patterns for the lower face are inverted for the clear material. While P1XE signals confirm some excess of Fe on the lower face, the similarity between the two sides (Fig. 4a, b) suggests that averaged over the larger volume detected by PIXE, the total iron content is rather similar at all points on both sides of the glass. These analyses are not inconsistent with the earlier suggestion of Verita et al. [8] in which Fe and Sn followed similar reductions in concentration over the outer 5 to 10 Ixm of their 12 mm thick glass, but deeper in the interior the Fe concentration rose markedly. An initial interpretation is that Fe moves from a uniformly distributed bulk, or interior, concentration towards the surface to associate with the Sn. At the centre of the sheet the temperature is higher and so more iron can reach the surface layers (where there is also a maximum in the Sn concentration). From Fig. 3 it appeared that additions of Sn reduced the quantity of Ca, so on proceeding to include data of Fig. 4, one may suggest that for the 2.1 mm glass the iron peaks at a greater depth than the Sn, and so displaces Ca towards the interior of the glass melt. PIXE analysis integrates signals over some 29 ~xm beneath the surface, but even on this scale there are differences between the upper and lower faces. Hence ionic migration proceeds on at least the 30 txm scale within the time the glass is being produced. The PIXE data can be interpreted as evidence that, except for the indiffusion of Sn, the compositional changes for Fe and Ca are primarily from redistribution of components from the interior of the glass. For HIXE of Fe in green glass one must recognise that the green coloration is achieved by
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addition of Fe to the bulk material so Sn linked changes may be masked to some extent, but Fig. 4 does confirm however that, as seen by contrasting HIXE and PIXE data, there is movement of Fe towards the surface on the tin side (i.e., implying a depletion of iron from an intermediate depth beyond 5 Ixm). Analysis of a selection of other elements was given in Fig. 5. The variability of the signals is much greater than the reproducibility but two general features are that the concentrations of each element differ on the two sides of the glass, and are non-uniform across the width. It is immediately apparent that the concentrations of the trace element chromium are highly variable across the float line for these samples. The lack of any obvious trend and the presence of three anomalously high signals in the clear float glass (Fig. 5i), where there is a fourfold variation in the detected Cr, probably indicates that the Cr is a random and non-uniform dopant in the melt. Chromium PIXE data from the green glass follow a similar pattern to that of iron on the tin face. The patterns of a central minimum, or maximum, for Na and Si analyses are in line with the earlier figures and support the view of a non-uniform temperature profile for the furnace. For example the PIXE signals for the tin side of Si and Na have inverse patterns and that of the silicon appears to resemble that of the Sn, but with a lesser percentage change. Sodium seems to be in slightly higher concentrations near the sides of the float line on both the lower and upper faces, but relatively constant across the upper face. The use of a suppressed zero on these histograms perhaps overemphasises the variations, but the HIXE and PIXE signals are not identical, even when similar trends can be discerned, and more surprisingly it is the PIXE signal which shows the greater variation. To summarise, it is difficult to offer detailed models of the likely chemistry, although for the clear glass there is an obvious surface association of Sn, Fe and Si concentration profiles and a small anticorrelation with that of Na from deep within the lower glass face. The HIXE data further suggest that the distributions on the outer regions of the two faces are not identical (e.g., for Si, Fe and Na) and at some positions across the float line these surface concentrations can differ by a factor of two between the faces.
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4.2. Measurements of compositions through the glass thickness In the analyses of compositional variations with depth, the experimental difficulties of sectioning were eased by using 7.9 mm thick glass, rather than the 2.1 mm material discussed for the standard float line investigations. While it has already been suggested that the form of the near surface composition may be a function of the glass thickness (e.g., Ref. [10]), the data of Figs. 6 and 7 are unlikely to be influenced by this, and the expectations are that while the signals will differ between the two surfaces, to a first approximation, they might be uniform within the bulk material. The intensities of data from the interior mostly vary within about 3% on Fig. 6, although a slight trend appears for the sodium data that suggests this is not just a random error or inherent problems of uniform mixing within a glass whose viscosity varies steeply with temperature. Sodium data show losses at both faces, most noticeably on the non-tin side, whereas Si has a reverse trend (Fig. 6). The HIXE Fe data are erratic and at variance with those from PIXE, where the iron content appears to fall steadily across the sample. By contrast with all the other data sets, the PIXE Cr signals display a clear concentration maximum in the interior of the glass. While the act of polishing might induce a redistribution of material in the near surface layers, it seems unlikely to be the origin of the large variations seen for the Fe and Cr signals, particularly since the PIXE signals originate from up to 29 Ixm beneath the surface. Overall, the clear conclusions are that the compositions within the glass differ from those at the surfaces, and that the compositional variations are not obviously linked, and therefore may just be the result of imperfect mixing of the constituents of the glass. No clear pattern of internal compositional separations is apparent at this stage.
4.3. Effects of changing iron content during glass production Fig. 1 has already presented the RBS signals from the two faces for the sample with 0.593% Fe203 (as determined by bulk chemical analysis at SIV). From the lower face the iron signal was of approximately the intensity expected from this bulk composition.
However, an immediately obvious feature was that the iron signal from this outer txm of the glass is most apparent on the lower face, despite the fact that for this concentration within the bulk, one would have expected a comparable Fe signal on the upper face as well. It therefore appears that on the depth scale sampled by the RBS the iron is depleted from the upper surface. Indeed, this depletion was observed for all the Fe concentrations in the set of green glass samples, and Fig. 8 summarises how the PIXE analyses signals for the Fe varied with bulk changes in the iron content. Note that the PIXE signal represents a signal integrated over some 29 I~m depth (i.e., this is already much greater than the penetration range of the tin and therefore might be assumed to mirror the Fe concentration within the bulk). On the upper face there was a linear dependence between the PIXE and bulk iron signals, but of particular note is that the absolute intensities were only ~ 10% of those expected from the bulk, even when considering data from a 29 Ixm layer. It is not clear what function should be quoted for the iron dependence from the lower face, since although it is possible to describe the data by a linear dependence, as shown on Fig. 8, extrapolation of the line to the nominal zero concentration for the iron would give a significant intercept for the iron signal. Consequently it may be appropriate to consider an alternative function to describe the surface concentration of the iron at the tin side of the glass. Fig. 8 indicates that the measured data, together with the zero value, might be described by a saturating exponential function. This plot emphasises the problem and the mismatch between the bulk signal and that at the surface, but does not explain the origin of the divergence. Finally, the data of Fig. 8 for the set of samples taken during the dark green to light green glass conversion of the float line, show an iron content in the upper and lower faces which differ initially by a factor of ~ 15, increasing to ~ 50 times at the lower bulk values for iron. This changing ratio, which differs from the factor of 3 reported above for steady state green glass production, is unexpected since the upper face value is consistently below that expected in the bulk. The slow reduction in the lower face iron content may be indicative of a build up of iron in the float bath during green glass production, which leads to a non-equilibrium excess
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of iron in the tin side of the green float, as the line is converted to the clearer glass. The data imply that a return to totally clear float production may require an extended period of operation to purge iron from the tin bath. The chemistry for the incorporation of iron will be sensitive to the presence of other elements and their valence states, and with this in mind inspection of the data were made to separate out possible reasons for the surface dependence of the iron relative to that in the bulk glass. The combined HIXE signals of S n / C a differ for the two faces, presumably because of the uptake of tin. PIXE equivalent signals reveal a similar variation in pattern although the ratio of signals for [(0.279% F e 2 0 3 ) / ( 0 . 1 6 2 % Fe203)] are 10% for HIXE and only 5% for PIXE. This implies internal depth variations in composition. Sodium data are equally variable ~ 4% with rather more Na being included in the iron rich material. Such data do not resolve the differences between surface and bulk material for the iron and the conclusions to draw from the green glass data are that the Fe is strongly associated with the presence of the tin, but even for the green glass it is depleted near the upper surface layer, with the most surprising fact being that the concentration falls to ~ 10% that of the bulk. With such complex chemistry, and the reality of using samples from a commercial production line, one must recognise that the data can only indicate trends, rather than provide definitive models for the manner in which the indiffusion of tin alters the surface composition. Indeed, the current study has underlined that for this production process there are a number of competing processes. For example, the various surface analysis techniques used here provide information on different depth scales, and over the outer few tens of Ixm the glass compositions are varying significantly. Such information is potentially of value in float glass production, and in the related problems of understanding the variability in the quality of bonding of coatings, which are linked to the presence of surface tin. The uptake of tin is known to be temperature sensitive, as is the viscosity of the glass, hence on the lower face there is a peak in the tin doping level at the centre of the float line, and other impurities differ in their concentrations present at the surface across the width of the float line.
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Variations in composition with depth were also noted, but these were relatively minor compared with the changes at the surfaces. The interlinked chemistry of Sn, Fe, Na and Ca is evident and expected, but the depletion of Si from the upper surface was not. Finally, in the set of varying Fe doped green glass samples there was strong evidence for surface depletion of the iron on the upper face and a non-linear dependence, relative to the bulk, for the iron in the tin face.
5. Summary Near surface compositional analysis of clear and green float glass has shown how not only the tin, but also iron and other elements, vary differently across the width of the float line. The variational trends differ from the upper and lower faces of the glass. In the case of green glass there are disparities between the surface and bulk iron contents. Analysis with depth of a thick glass sample also reveals inhomogenieties in the composition, even within the 'bulk' of the glass.
Acknowledgements We wish to thank Dr. M Capranica and Dr. C. Ergun from SIV for the glass samples and for discussions. Financial support is gratefully acknowledged from the Brite Euram contract BREU2-0357 (project BE 6111), from the British Council and the University of Ege.
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