A Global Glassy Layer on BaAl2B2O7

Article pubs.acs.org/crystal

A Global Glassy Layer on BaAl2B2O7 Crystals Formed during Surface Crystallization of BaO·Al2O3 ·B2O3 Glass Wolfgang Wisniewski,*,† Bernd Schröter,‡ Tilman Zscheckel,† and Christian Rüssel† †

Otto-Schott-Institut, Jena University, Fraunhoferstrasse 6, 07743 Jena, Germany Institut für Festkörperphysik, Jena University, Helmholzweg 5, 07743 Jena, Germany

‡

ABSTRACT: A glass with the composition BaO·Al2O3·B2O3 was thermally annealed at temperatures from 700 to 780 °C for 1−8 h. This resulted in the crystallization of [0001]-oriented rhombohedral BaAl2B2O7 at the surface of the glass. The crystals grow fastest perpendicular to their c-axes (parallel to the surface) and cannot circumvent barriers in the surface. The surface of samples encapsulated in alumina ceramics during annealing was covered by a thin layer of glass, indicating nucleation occurred below the surface. If the samples were not encapsulated in alumina, around 85% of the surface was glass covered. This was the result of surface contaminations such as cubic BaSO4 detected on these samples. Crystal growth occurs toward the surface until the enrichment of impurities in the glassy layer is large enough to inhibit further crystal growth. The coinciding enrichment of impurities on the bulk side of the oriented surface crystals probably initiates the secondary nucleation, leading to the second layer of oriented crystals previously described.

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INTRODUCTION Nucleation processes in glasses during surface crystallization are still not fully understood. Common descriptions of surface nucleation assume that the surface energy is smaller if a crystal is located directly at the surface.1−3 This is surely the case if the surface energy, that is, the interface energy crystal/surrounding atmosphere, is smaller than the interface energy crystal/melt. In that case, the chemical composition of the surrounding atmosphere should also play an important part. Another reason for the occurrence of surface crystallization might be that stress relaxation could occur faster near the surface.4−7 This is of special importance if the change in density is large during the crystallization process because the occurring stress is larger and hence relaxation plays a more important part. It should be noted that the occurrence of stresses during the crystallization process has been experimentally proven8 and theoretically discussed;9−12 even the formation of high pressure phases was recently described.13 If stress relaxation near the surface is decisive, the crystals near the surface should possess random orientation. In most models reported in the literature, surface crystallization of glass results in the occurrence of crystals located directly at the surface,14,15 although a model incorporating crystallization to occur a few nanometers below the physical surface has been published.1 Surface defects such as scratches or dust particles also influence nucleation significantly.16 Cordierite crystals were found to crystallize under the surface of the glass matrix,17−19 which is in full agreement with the stress relaxation model. On the other hand, the fresnoite crystals formed during the surface crystallization of a 2BaO·TiO2·2.75SiO2 glass were found to be part of the immediate surface,19,20 while the surface of glass-ceramics crystallized from a BaO·Al2O3·B2O3 glass was found to be composed of exposed crystals (15%) and crystals covered by a thin layer of glass.19,21 © 2012 American Chemical Society

Recently, some experimental work on the surface crystallization of glass using electron backscatter diffraction (EBSD) has been reported in the literature.18−21 This method is carried out in a scanning electron microscope (SEM) and enables one to identify the orientation of individual crystals. A short introduction on EBSD, inverse pole figures (IPF), and indexing parameters such as the image quality (IQ) and the confidence index (CI) has previously been given.21 It was recently shown that the method can also be used to prove the existence of a thin glass layer covering surface crystallized glass-ceramics as a thin glass layer significantly enhances the effect of EBSDpattern degradation.19 The application of EBSD to surface crystallization in glasses has shown that surface crystallized cordierite crystals are statistically oriented,18 while surface crystallized Ba2TiSi2O8 fresnoite crystals are preferably oriented with their crystallographic [101]-direction perpendicular to the surface.20 Subsequently, kinetic selection leads to a predominant [001]orientation occurring about 7 μm below the surface in the latter system. In the case of BaAl2B2O7 glass-ceramics, the crystalline layer located immediately at the surface is oriented with its crystallographic [0001]-direction perpendicular to the surface, while the crystals are preferably oriented with their [0001]direction parallel to the surface at some distance below the surface (>20 μm).21 The orientation change is continuous in the fresnoite system20 but very abrupt in BaAl2B2O7 glassceramics.21 The preferred orientation directly at the surface should have thermodynamic reasons and should occur due to the Received: December 9, 2011 Revised: January 11, 2012 Published: January 17, 2012 1586

dx.doi.org/10.1021/cg2016325 | Cryst. Growth Des. 2012, 12, 1586−1592

Crystal Growth & Design

Article

As previously reported, samples thermally annealed on an alumina substrate without a cover showed two surface morphologies: crystals covered by a glassy layer and exposed crystals without a detectable glass layer. The surface concentration of the latter was around 15%. It was suggested that this topmost, partially glass covered layer of crystals (layer 1 in ref 21) is formed by nucleation at the immediate surface followed by the crystals diving beneath the surface due to deformations occurring during crystal growth.21 As in the work presented here, ref 21 only presents results from the crystallization of polished surfaces not in contact to alumina during annealing so as to exclude heterogeneous nucleation as well as possible. To gain further insight into the crystallization behavior, surfaces were modified by artificial deformations, such as grooves introduced by a string saw (see profile a in Figure 1).

minimization of interfacial energies if a certain orientation with respect to the surface occurs.1−3 The formation of oriented layers in surface crystallized glasses was previously described to be the result of random nucleation followed by kinetic selection.22,23 Recently, however, it was shown that the initial step might also be oriented nucleation.20,21 A close to zero or even negative thermal expansion has been reported in glass-ceramics containing rhombohedral BaAl2B2O7 crystals.23,24 These glass-ceramics also show an unusually high ionic conductivity, which is accompanied by a very small activation energy of less than 0.5 eV.26 Surprisingly, the crystallization of the BaO·Al2O3·B2O3 glass is accompanied by a volume expansion, which is unusual for the crystallization of glass,25 but is for example also observed during the crystallization of cordierite from some glasses in the MgO/Al2O3/ SiO2 system.27 This Article reports on the detailed analysis of the immediate surface to allow conclusions on the growth mechanism and to explain the occurrence of glassy layer on surface crystals.

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EXPERIMENTAL SECTION

The raw materials BaCO3, Al(OH)3, and B(OH)3 were mixed and then melted at 1480 °C using an inductively heated furnace to produce a glass with the composition BaO·Al2O2·B2O3. After the melt was stirred for 2 h, it was cast on a copper block, quenched with a copper stamp, and transferred to a furnace preheated to 620 °C. The furnace was subsequently switched off, allowing the glass to cool. To eliminate any surface effects from the quenching procedure, a few millimeters of the quenched surface of the glass samples were ground away, followed by manual polishing with shrinking grain sizes down to 0.75 μm diamond suspension. Crystallization was achieved by thermal annealing of the samples at temperatures from 700 to 780 °C using a heating rate of 10 K/min. This temperature was kept for 1−8 h before turning the furnace off. Surface modifications such as grooves or scratches were introduced using a string saw or a hard metal tip. Groove profiles were obtained via laser scanning microscopy (LSM) using an Axio Imager Z1M LSM5-Pascal. Atomic force microscopy (AFM)-measurements were performed using an “Ultra Objektiv” (SIS Herzogenreuth, Germany) attached to a contrast microscope (JENAVAL, Carl Zeiss, Jena, Germany). The samples were characterized using a scanning electron microscope Jeol JSM-7001F equipped with an analyzing system EDAX Trident. EBSD-scans were captured and evaluated using a TSL Digiview 1913 EBSD-camera and the software TSL OIM Data Collection 5.31 and TSL OIM Analysis 5. To achieve a conductive surface, the samples were mounted using an Ag-paste and coated with a thin layer of carbon at about 10−3 Pa. A material file for indexing the cubic BaSO4-phase was built from the ICSD-file no. 62368. The solid samples were directly analyzed (without powdering) by X-ray diffraction (XRD, Siemens D5000) using Cu Kα-radiation in a Bragg−Brentano 2Θ/Θ setup. X-ray photoelectron spectroscopy (XPS) gives information on the chemical composition of a surface layer only a couple of nanometers thick. XPS investigations were carried out with an EA200-ESCA-system (SPECS) using nonmonochromatic Mg Kα radiation (hν = 1253.6 eV). The samples were heated to 500 °C just before the analysis to remove carbon from the surface.

Figure 1. Profile across a string saw groove (a) after annealing at 700 °C for 1 h and (b) after annealing at 780 °C for 1 h. The values for profile (b) are exaggerated by 50 μm. Arrows indicate the expected orientation of the crystal c-axis (not detected after annealing).

It was expected to find crystals with their c-axes oriented perpendicular to the local surface at the time of nucleation, as indicated by the arrows in Figure 1, and thus different from the strong orientation with the c-axes perpendicular to the surface generally observed in this system.21 After being annealed at 780 °C for 1 h, the introduced structures disappeared (see profile b in Figure 1), while annealing at 700 °C for 1 h led to a crystallized surface layer, as shown in Figure 2 where the

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Figure 2. SEM-micrograph of a groove (tilted by 70°) and IPF-map of a scan covering the framed area. The map has been distorted to match the distorted frame outlined on the tilted surface.

RESULTS AND DISCUSSION Analysis of Samples Encapsulated in Alumina during Annealing. All thermally annealed samples contained rhombohedral BaAl2B2O7 as a crystalline phase, which was proven by X-ray diffraction (XRD). The intensity of the XRDpeaks differed notably from those of the JCPDS patterns due to an oriented layer of crystals at the immediate sample surface.

unfiltered IPF-map of a scan covering a groove after annealing is presented. Despite the comparably low quality of the EBSDpatterns obtained from this surface, the red color indicating a preferred orientation of the c-axes perpendicular to the surface 1587



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meaning the surface is either completely covered by a thin layer of glass or entirely composed of exposed crystals because the thin glass layer reduces the quality of EBSD-patterns.21 So, if the two surface morphologies were present, different IQvalues should be observed. It was also recently shown that the degradation of EBSD patterns during scanning is more pronounced if the crystals are covered by a thin glassy layer.19 This method was applied to surfaces featured in Figure 4 under the exact conditions stated in ref 19 and showed that all samples exhibit the sensitivity toward pattern degradation also observed for crystals covered by a thin glassy layer. Hence, the homogeneous IQ-distribution from crystals in Figures 3 and 4 indicates that the entire surface is covered by this thin glassy layer. It should be noted that a glassy layer covering the whole surface was only detected on samples encapsulated in alumina during thermal annealing. By contrast, samples that were not encapsulated during annealing showed the two morphologies of exposed and glass covered crystals.19,21 Analysis of Samples Not Encapsulated in Alumina during Annealing. A more detailed examination of samples not encapsulated in alumina showed that EBSD-patterns that cannot be indexed as rhombohedral BaAl2B2O7 were obtained from very small parts of the surface (less than 0.1%), generally located within the area not covered by a glassy layer. These patterns were reliably indexed as cubic BaSO4. Because the quality of EBSD-patterns obtained from the exposed crystals was usually good, a number of EBSD-scans with a step size of 0.10 μm and a binning of 4 × 4 could be performed. Figure 5 shows an SEM-micrograph of the untreated surface of a sample not encapsulated during annealing with frames

on one side of the groove is much too dominant to be an indexing error. This conclusion is in agreement with previously published results21 and shows that the volume concentration of crystals is high enough to enable the acquisition of very faint EBSD-patterns, indicating a [0001]-orientation from this area. These crystals are clearly absent on the other side of the groove where reasonable indexing is not possible at all, indicating that the nucleation rate during the annealing process was very low (no nucleation on the left side) but crystal growth parallel to the surface was possible. The groove clearly stopped the crystals from spreading to the other side. Simply scratching the surface with a tip followed by annealing at 720 °C for 1 h led to enhanced nucleation in agreement with the literature.16 Figure 3 presents the combined

Figure 3. Combined IPF+IQ-map of an EBSD-scan performed on a surface scratched before annealing at 720 °C for 1 h.

IPF+IQ-map of a scan obtained from such a surface where numerous crystals were formed along lines presumably following the introduced scratches. Interestingly, circular crystals well separated by glassy phase (black) are located between these lines and show the same [0001]-orientation, verifying the idea of oriented nucleation. Of course, it is possible that these areas are the result of a heterogeneous nucleation due to residual dust particles from the scratching procedure.16 Alternatively, an enhanced rate of nucleation could be the result of mechanical stresses introduced during scratching. To locate the areas of larger IQ-value (see also ref 21), an annealing series (samples encapsulated in alumina) was performed to study the crystallization within a broader range of temperatures and times. Figure 4 presents the combined IPF+IQ-maps of scans performed on surfaces annealed under the different conditions Figure 5. SEM-micrograph of the untreated surface with the areas analyzed by EBSD (a) and EDX (b) framed in white. Individual EBSD-patterns were obtained at the locations 1−3.

outlining an EBSD-scan (a) and an EDX-mapping (b). The EBSD-patterns 1−3 presented in Figure 6 were obtained at the locations 1−3 in Figure 5. Table 1 presents relevant indexing parameters of the three patterns, clearly attributing patterns 1 and 2 to cubic BaSO4, while pattern 3 is attributed to rhombohedral BaAl2B2O7. The combined phase+IQ-map of a selected part of the EBSDscan in Figure 7a confirms the phase locations, while the corresponding IPF+IQ-map presented in Figure 7b provides orientation information. The BaAl2B2O7 phase shows the [0001]orientation of the surface crystallization described above. The

Figure 4. Combined IPF+IQ-maps of EBSD-scans obtained from samples annealed under the stated conditions.

given in the figure, all of which show the same [0001]orientation. Areas where the IQ-values are significantly smaller while homogeneous indexing occurs were not observed, 1588



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should always be formed if the respective components are present. The source of sulfur might either be trace impurities in the barium carbonate raw material or, however, contaminants in the furnace. As BaSO4 is not observed in samples encapsulated during annealing, the latter option seems more likely. Consequently, the nucleation of BaSO4 would occur directly at the surface and not below the surface as is the case for BaAl2B2O7. X-ray Diffraction. The XRD-patterns previously obtained from this material21−26 showed unidentified peaks of low intensity not attributable to rhombohedral BaAl2B2O7. XRD-patterns obtained from a sample encapsulated during annealing for 2 h at 780 °C are presented in Figure 8. The unidentified peaks are more intense than in the XRD-pattern of a not encapsulated

Figure 6. EBSD-patterns of cubic BaSO4 (1 and 2) and rhombohedral BaAl2B2O73 obtained at the locations 1−3 in Figure 5.

Table 1. Indexing Parameters of the Patterns 1−3 in Figure 6 Attributing Patterns 1 and 2 to Cubic BaSO4 and Pattern 3 to Rhombohedral BaAl2B2O7 pattern

phase

votes

fit [deg]

CI

1

BaAl2B2O7 BaSO4 BaAl2B2O7 BaSO4 BaAl2B2O7 BaSO4

6 118 4 39 68 4

1.26 0.44 1.51 1.03 0.69 1.42

0.008 0.950 0.017 0.283 0.533 0.025

2 3

Figure 8. XRD-patterns obtained from the surface as well as after removing 50 and 100 μm from an encapsulated sample. A comparable pattern from a not encapsulated sample is also presented as well as the theoretical patterns of BaAl2B2O7 (JCPDS no. 86-2168) and cubic BaSO4 (calculated from the ICSD file no. 62368 used for the EBSDmaterial file). Question marks indicate unidentifiable peaks.

Figure 7. (a) Phase+IQ-map (enhanced IQ-contrast) and (b) IPF +IQ-map of the EBSD-scan as well as (c) the SKα-map of the performed EDX analysis.

sample (see also ref 21). Removing about 50 μm from the surface of an encapsulated sample decreased the intensity of the (003)-peak of BaAl2B2O7 and the unidentifiable peaks; the latter are not observed at all 100 μm beneath the initial surface. Cubic BaSO4 is not observed in the XRD-pattern, probably because of its low volume fraction. The unknown peaks could not be attributed to any reasonable phase and only occur when an exaggerated (003)-peak of BaAl2B2O7 is observed. Hence, the unknown phase occurs either in the highly oriented surface layer 1 or just below it where crystal plates of the spherulites composing the second layer of orientation also grow parallel to the surface; that is, the c-axes are perpendicular to the surface. A correlation with annealing times, that is, a metastable phase that is transformed during annealing, is not observed. The 10% absorption length in the solid crystal phase was calculated to be 47.82 μm based on the linear absorption coefficient for Cu Kα-radiation28 and the X-ray density of rhombohedral BaAl2B2O7.29 Because of the curved surfaces of the samples after annealing, the sides of the sample were removed before grinding parallel to the surface. Surface Modifications by the Electron Beam. After an EBSD-scan was performed on a sample only partially covered by glass with a step size of 0.25 μm (binning 2 × 2, exposure 0.45 s) in the scanned parts of the SEM-micrographs in Figure 9,

cubic BaSO4 crystals in the scan are oriented with the [111]direction relatively perpendicular to the surface. With one exception, this [111]-orientation was observed in all patterns attributed to cubic BaSO4 from the prepared samples. The statistic indexing and low IQ-values of the surrounding area result from weaker patterns obtained from glass covered areas not suited for analysis with a binning of 4 × 419 as well as EBSD-pattern degradation.19 The EDX-map of sulfur presented in Figure 7c clearly shows a higher sulfur concentration in the areas appearing brighter in the SEM-micrograph in Figure 5. Most areas with high sulfur concentration could be indexed despite the parameters of the scan. The corresponding EDX-map of aluminum indicates a depletion where the higher sulfur concentrations were detected. The EDX-maps of oxygen and barium did not show significant variations and hence did not provide further information. The EDX-map was performed using an acceleration voltage of only 5 kV to minimize the information volume contributing to the analysis. Obviously trace impurities in the glass notably affect the nucleation behavior and the growth mechanism. This is shown by the formation of cubic BaSO4 crystals at the immediate surface. This compound is thermodynamically very stable and 1589



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covers a part of the surface previously irradiated by the electron beam. The crystal previously exposed to the electron beam does not show the structures observed in the scanned, glass covered area. Figure 10a shows the original AFM-micrograph, while Figure 10b shows the image after the differentiation was performed to eliminate the tilt of the surface, leading to an enhanced visualization of the structures introduced by the electron beam during scanning. A profile along the line L1 in Figure 10a covers three of the structures (three arrows) and extends into the unscanned area. In the profile, three dents of about 5 nm can be identified in the continuous slope of the surface (see arrows) with a distance of about 250 nm between them, corresponding to the step size of the performed EBSD-scan. The fourth arrow marks the boundary of the scanned area. The observed dents in the interval of the step size of the EBSD-scan are comparable to those observed after scanning cordierite crystals18 and indicate that the layer of glass covering the crystals is possibly evaporated during the exposure time during which the ESBD-pattern is obtained. The areas where the crystals are fully exposed are neither structured nor do they show significant pattern degradation under the given parameters due to their higher thermal stability (the melting point is 995 °C30) and the generally higher thermal conductivity in comparison to the amorphous phase.31 However, increasing the energy input can also lead to the destruction of the BaAl2B2O7 crystals during EBSD-analyses.19 X-ray Photoelectron Spectroscopy. The XPS-spectra presented in Figure 11 were obtained (a) from the uncrystallized

Figure 9. SEM-micrographs of crystallized surfaces structured during an EBSD-scan.

the surface is structured. While Figure 9a was obtained from an entirely glass covered area and shows continuous structures, Figure 9b contains part of the surface where crystals seem to protrude from the surface and no structuring is observed. The glass covered area surrounding the exposed crystal of the scanned part in Figure 9b, however, shows the same structure as was also observed in Figure 9a. An AFM-scan of an area containing similar structures is presented in Figure 10 where the left part of the AFM-measurement

Figure 11. XPS-data obtained from the surface of the uncrystallized glass (a) and from the surface of the glass-ceramic material after annealing (b). The values over 800 eV have been distorted to increase contrast.

glass and (b) from the crystallized surface of the samples not encapsulated during annealing. The crystallized glass-ceramic sample surprisingly showed a peak of notable intensity attributed to sodium, which is significantly weaker in line (a) attributed to the uncrystallized glass. It can thus be concluded that a significant increase of the sodium concentration in the first couple of nm beneath the surface occurred during the crystallization process. The boric acid used as reagent to melt the glass was found to contain 0.00052 ma % of sodium by atomic absorption spectroscopy (AAS). Sulfur was not detected in the XPS-spectra, probably due to the marginal part of the surface covered by BaSO4. The results of a further EDX-scan are presented in Figure 12 where the EDX-maps of the elements Na, S, and Al are presented in addition to the corresponding SEM-micrograph. The bright structures corresponding to the cubic BaSO4 in samples not encapsulated during annealing clearly show an

Figure 10. AFM-scan results: (a) original and (b) after differentiation of a surface structured by the electron beam (left) and an unscanned, unstructured surface (right). The elevation profile along line L1 is given below. Arrows mark corresponding structures in the scan and the profile. 1590



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After nucleation, the crystals at the surface grow fast perpendicular to the crystallographic c-axis due to large crystal growth velocities in these directions,21 covering the surface with a thin, highly [0001]-oriented layer (L1 in Figure 13). Crystal growth toward the surface and into the bulk is relatively slow but leads to the oriented layer 1,21 which might extend up to 50 μm into the bulk in some locations. On the other hand, the spherulitic growth beneath layer 1 also means the topmost part of layer 2 is composed of horizontal crystals with the c-axes oriented perpendicular to the surface. The lower degree of crystal orientation in this second layer21 would explain the less exaggerated (003)-peak observed in the corresponding XRDpattern in Figure 8. Hence, a clear separation of layers 1 and 2 is not possible via XRD. An unknown phase is formed near the surface and only occurs as long as the (003)-peak of BaAl2B2O7 is exaggerated; that is, the BaAl2B2O7 crystals are [0001]oriented. As the crystals grow toward the surface, occurring impurities are not incorporated into the crystal but accumulated in the moving crystallization front. This leads to an enrichment of impurities such as sodium in a small layer at the surface as shown by XPS in Figure 11. As the crystals grow toward the surface, the composition of the glassy phase increasingly deviates from stoichiometry. The crystallization of the topmost glass layer thus continues until the deviations from stoichiometry are so high that crystallization is no longer possible under the prevailing conditions, thus forming a very thin layer of residual glassy phase at the surface. It is sensitive to the electron beam and leads to the varying local pattern degradation observed during EBSD-scans of partially glass covered surfaces. It can be concluded that the formation of the glassy layer covering the crystals is due to nucleation beneath the surface and slight deviations from stoichiometry and/or the presence of trace impurities. Crystals uncovered by a thin glass layer would consequently be the result of an ideally stoichiometric composition or the incorporation of impurities in grain boundaries/residual glass phase between crystals or in the crystals themselves. The spread of the topmost layer 1 can be blocked by structures like the groove presented in Figure 2, indicating the crystals cannot grow around a barrier or change their direction of growth significantly. Hence, crystals “turning” from the oriented layer 1 into the bulk cannot trigger the formation of the secondary layer 2 (L2 in Figure 13) of oriented crystals with their fastest growing direction perpendicular to the surface.21 As the same accumulation of impurities described for the surface can be expected to occur on the bulk side of the initial surface layer, it seems plausible that chemical changes might lead to enhanced rates of bulk nucleation just below the oriented layer 1. After all, the spherulites contributing to layer 2 must practically nucleate at the same time to produce the almost straight boundary between layer 2 and the residual glass of the bulk.21 The thickness of layer 1 can be explained by the enhanced possibilities for diffusion during crystal growth into the bulk. During the growth of layer 2, impurities are likely enriched between the individual crystals of the spherulites.

Figure 12. Results of a second EDX-mapping of the untreated surface. Two areas showing structures characteristic for crystals not covered by glass have been outlined in the SEM-micrograph.

increased sulfur concentration while being depleted in aluminum as also observed in the scan presented in Figure 7. The EDX-map of sodium shows a very weak contrast, but it can be discerned that the sodium concentration of surface locations corresponding to structures resembling glass covered crystals is slightly higher than in areas where the crystals are not covered by glass. This matches the results of the XPS-measurements. An effect of sodium on crystal growth itself is very unlikely due to the minuscule concentration in the sample. However, the local enrichment might contribute to the stability of the residual thin glass layer against crystallization. The Growth Model. The phases and orientations observed during the surface crystallization of the BaO·Al2O3·B2O3 glass are schematically illustrated in Figure 13. Crystal nucleation at the surface is highly oriented with the crystallographic c-axis perpendicular to the surface, indicating the surface energy of the crystal should have a minimum at this orientation. For the case of samples encapsulated in alumina during annealing, the crystals are completely covered by a thin layer of glass (L0 in Figure 13), matching the theoretical work based on cordierite.1

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CONCLUSIONS A thin layer of glass enriched in Na2O covers the BaAl2B2O7 crystals formed during surface crystallization of the corresponding glass. Hence, nucleation must occur beneath the surface. If, however, BaAl2B2O7 crystals without a covering glassy layer are observed, their formation is due to heterogeneous nucleation caused by impurities on the surface. This was illustrated by

Figure 13. Schematic illustrating the growth model of surface crystallization in the BaO·Al2O2·B2O3 glass.

The exposed crystals observed in samples not encapsulated during annealing were caused by a contamination of the surface, proven by the occurrence of cubic BaSO4 crystals. 1591



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(20) Wisniewski, W.; Nagel, M.; Völksch, G.; Rüssel, C. Cryst. Growth Des. 2010, 10, 1414−1418. (21) Wisniewski, W.; Zscheckel, T.; Völksch, G.; Rüssel, C. CrystEngComm 2010, 12, 3105−3111. (22) Rüssel, C.; Tauch, D.; Garkova, R.; Woltz, S.; Völksch, G. Phys. Chem. Glasses: Eur. J. Glass Sci. Technol., Part B 2006, 47, 397−404. (23) Tauch, D.; Rüssel, C. J. Non-Cryst. Solids 2005, 351, 2294−2298. (24) Tauch, D.; Rüssel, C. J. Non-Cryst. Solids 2005, 351, 3483−3489. (25) Rüssel, C.; Tauch, D.; Garkova, R.; Woltz, S.; Völksch, G. Phys. Chem. Glasses: Eur. J. Glass Sci. Technol., Part B 2006, 47, 397−404. (26) Tauch, D.; Keding, R.; Rüssel, C. Phys. Chem. Glasses: Eur. J. Glass Sci. Technol., Part B 2009, 50, 389−394. (27) Müller, R.; Naumann, R.; Reinsch, S. Thermochim. Acta 1996, 280&281, 119. (28) Henke, B. L.; Lee, P.; Tanaka, T. J.; Shimabukuro, R. L.; Fujikawa, B. K. At. Data Nucl. Data Tables 1982, 27, 1. (29) Tauch, D.; Rüssel, C. J. Non-Cryst. Solids 2007, 353, 2109−2114. (30) Hovhannisyan, R. M. Glass Technol. 2003, 44, 96−100. (31) Kittel, C. Phys. Rev. 1949, 75, 972−974.

means of sulfur contaminations, which led to the local crystallization of cubic BaSO4. Oriented nucleation leads to highly oriented crystals with their c-axis perpendicular to the surface. XRD-patterns indicate that an unknown phase occurs in the highly oriented, primary layer of crystallization or just below it in the topmost part of layer 2. The BaAl2B2O7 crystals in the topmost layer grow parallel to the surface, as crystal growth occurs fastest in directions perpendicular to the c-axis, but cannot circumvent barriers. Slow crystal growth will occur toward the surface until the enrichment of impurities is large enough to inhibit further crystal growth and thus a thin layer of residual glass is formed. On the bulk side of the oriented primary layer, the accumulation of impurities might initialize the secondary nucleation responsible for the second layer of oriented growth.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (0049) 03641 948501. Fax: (0049) 03641 48502. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank René de Kloe for helping with the identification of the EBSD-patterns of the BaSO4-phase and the Deutsche Forschungsgemeinschaft (DFG) in Bonn Bad Godesberg (Germany) for financial support (project no. Ru 417/14-1).

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REFERENCES

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A Global Glassy Layer on BaAl2B2O7

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