Hindering the Kinetic Selection of Dendritic Ba-Fresnoite by Phase

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Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

Hindering the Kinetic Selection of Dendritic Ba-Fresnoite by Phase Separation in a Glass of the Near-Eutectic Composition Ba2TiSi2O8− 2.625SiO2 Wolfgang Wisniewski,*,† Peter Š vančaŕ ek,‡ Milan Parchovianský,† Christian Thieme,§ and Christian Rüssel§ †

Centre for Functional and Surface Functionalized Glass, Alexander Dubček University of Trenčín, 911 50 Trenčín, Slovakia Joint Glass Centre of the IIC SAS, TnU AD, and FChFT STU, Š tudentská 2, 911 50 Trenčín, Slovakia § Otto-Schott-Institut, Jena University, Fraunhoferstr. 6, 07743 Jena, Germany Crystal Growth & Design Downloaded from pubs.acs.org by STOCKHOLM UNIV on 05/07/19. For personal use only.

‡

ABSTRACT: A glass with the near-eutectic molar composition of 2BaO·TiO2·4.625SiO2 (BT2.625S) in the Ba-fresnoite− SiO2 system is produced and crystallized at 850, 880, 910, or 1100 °C. The resulting surface crystallized layer is analyzed using scanning electron microscopy (SEM) including electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). In agreement with related glass compositions, this glass shows oriented nucleation at the surface. However, a phase previously not observed in the Ba-fresnoite−SiO2 system is detected and indicated to be brookite (TiO2) by XRD, as well as EBSD results. Crystal growth into the bulk occurs via the dendritic mechanism, but when crystallizing at 1100 °C, the dendrites are severely disturbed by droplets resulting from phase separation, which also hinders kinetic selection.

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INTRODUCTION Directionally solidified eutectic ceramic oxides show challenging mechanical properties, making them advantageous candidates for numerous applications. Reported mechanical strengths for such materials frequently range from 1 GPa to 3 GPa and are, hence, much higher than those of other ceramics. Advantageous physical properties have been reported in compositions where ZrO2, Al2O3, yttrium aluminum garnet, AlYO3, or AlGdO3 crystallize.1−8 These systems usually require very high melting temperatures, which sometimes reach 2300 °C. This means that the conventional large-scale melting suitable for an industrial production using crucibles made from the usual refractories or noble metals cannot be applied. Hence, electric arc melting or laser melting must be used, making the production of eutectic ceramic oxides with advantageous compositions difficult. The microstructure of these materials is composed of alternating lamellae formed by the different ceramic oxides. These oxides have different coefficients of thermal expansion so that the entire structure is highly stressed after cooling. Together with the numerous grain boundaries, this leads to heavy crack deflection, which results in a notably increased fracture toughness.9 Phases occurring in a similarly alternating order in specific cross sections form, e.g., during the growth of dendrites in glass-ceramics,10−16 so that the mechanical effect should be similar. The compositions used for the preparation of non-glassforming eutectic ceramic oxides generally crystallize sponta© XXXX American Chemical Society

neously during cooling.. However, an eutectic composition enabling the production of a glass has been reported in the system Ba2TiSi2O8 (Ba-fresnoite) + SiO2.16 Hence, this liquid may easily be quenched to form a glass which, in turn, may undergo controlled crystallization by a subsequent thermal treatment. Crystallizing liquids may show surface or bulk nucleation, but the combination of both is also frequently observed. For glasses, the classic nucleation theory assumes a random orientation of the nuclei with respect to the surface. In the past decade, however, oriented nucleation has been proven in multiple glass compositions, as summarized in Chapter 4.3 of ref 16, using the very limited information depth of electron backscatter diffraction (EBSD).17,18 It was first detected after the surface crystallization of Ba-fresnoite19 and has been shown to occur in glasses used to crystallize Ba-, Sr-, and Gefresnoite,16 as well as in multiple other glass systems.16 The effect of increasing the concentration of added network formers to the respective glass compositions has recently been analyzed.20−22 The work concerning the crystallization of fresnoite in glasses up to 2017 has been summarized16 and the nomenclature established in this review16 will be used here. The nomenclature for glasses used to crystallize fresnoite is based on the stoichiometric mol composition 2BaO·TiO2· Received: March 28, 2019 Revised: April 26, 2019

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DOI: 10.1021/acs.cgd.9b00418 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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diffractometer while samples crystallized at 1100 °C were analyzed using a PANalytical Empyrean DY1098 X-ray powder diffractometer.

2SiO2 (BTS) and notes only deviations from it by number, to enhance clarity. Hence, a glass with one extra mole of SiO2 is denoted as BT1S.16 While stoichiometric Ba-fresnoite (Ba2TiSi2O8) has a melting point of 1445 ± 5 °C,16 it forms a eutectic melt with SiO2 near the composition BT2.5S.16 The oriented nucleation of Ba-fresnoite has been analyzed for several glasses with a molar composition of BTS + xSiO2 (x = 0−3).16,20 Growth into the bulk generally occurs via the dendritic growth mechanism if the amount of additional SiO2 is x = 0.2 mol or higher.20 While the dendritic structures in fresnoite glassceramics range from very fine to very coarse,16 it would be interesting to know whether crystallization near the eutectic composition of BTS and SiO2 forms structures comparable to the lamellae observed in eutectic ceramics. Hence, this manuscript presents results concerning the oriented nucleation, as well as the mechanism of growth into the bulk obtained from a glass with the near-eutectic composition of BT2.625S.

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RESULTS AND DISCUSSION The DTA analysis of the powdered glass showed a glasstransition temperature of Tg = 747 °C, an onset crystallization temperature of Tx = 912 °C and a crystallization peak of Tp = 956 °C. These values are in the expected range, considering the values reported for glasses of similar composition.16 The polished samples of the BT2.625S-glass were crystallized for 2 h at 850, 880, 910, or 1100 °C. The compact surfaces of the respectively crystallized samples were analyzed by XRD and the resulting XRD patterns are presented in Figure 1, along with the theoretical patterns of Ba-fresnoite,

EXPERIMENTAL PROCEDURE

A glass with the molar composition 2BaO·TiO 2 ·4.625SiO 2 (BT2.625S) was prepared from BaCO3, TiO2, and SiO2 (quartz). The compounds were mixed and subsequently melted in a 250-mL Pt crucible at a temperature of 1550 °C using an inductive furnace. After keeping the melt at 1550 °C for 1 h, it was stirred for 1 h using a frequency of 60 min−1 and subsequently poured on a copper block. The melt was then rapidly quenched using a copper stamp to avoid crystallization. It was then transferred to a furnace preheated to 500 °C, which was subsequently switched off, allowing the glass to cool slowly. A 60 g batch of powdered glass (grain size = 250−315 μm) was analyzed by a differential thermal analyzer (DTA) in a Shimadzu DTA-50 using a heating rate of 10 K/min. Pieces of the produced glasses were polished with abrasive slurries down to diamond paste with a grain size of 0.75 μm and then crystallized in an electric furnace at 850, 880, 910, or 1100 °C for 2 h under a normal air atmosphere. In order to perform scanning electron microscopy (SEM) studies, all samples were contacted with Ag paste and coated with a thin layer of carbon at ∼10−3 Pa to avoid surface charging. Some samples were embedded in Araldite CY212 and cut perpendicular to the surface polished before crystallization in order to produce cross sections. These cross sections were manually polished with abrasive slurries down to a diamond paste of 0.75 μm grain size and a final finish of at least 30 min using colloidal silica (Logitech Syton Typ SF1 (pH 13.3, grain size = 32 nm)) was applied. The final finish applied to cross sections of samples crystallized at 1100 °C consisted of 6 h vibration polishing using a Buehler VibroMet 2 system (200 g static load) and a MasterPrep polishing suspension (50 nm sol−gel alumina). SEM analyses of samples crystallized at 850, 880, or 910 °C were performed using a Jeol Model JSM 7001F SEM system that was equipped with an EDAX Trident analyzing system containing a Digiview 3 EBSD camera. EBSD scans were performed using a voltage of 20 kV and a current of ca. 2.40 nA. The scans were captured and evaluated using the software TSL OIM Data Collection 5.31 and TSL OIM Analysis 6.2. Unreliable data points were removed in all datasets used for orientation analysis by applying a confidence index (CI) filter of 0.1. SEM analyses of samples crystallized at 1100 °C were performed using a Jeol JSM 7600F SEM equipped with a Nordlis Max EBSD camera. EBSD scans were performed using a voltage of 20 kV and a current of up to ca. 5 nA. The scans were captured using the Oxford Instruments software Aztec 3.1 and evaluated using the Channel 5 software package. X-ray diffraction (XRD) was performed using Cu Kα radiation and the θ−2θ arrangement in the 2θ range from 5° to 60°. The samples crystallized at 850, 880, or 910 °C were analyzed in a Siemens D5000

Figure 1. XRD patterns acquired from the surfaces of compact samples after crystallization at the stated parameters. Theoretical patterns of Ba-fresnoite (JCPDS File No. 70-1920), stishovite (JCPDS File No. 86-2331), and brookite (ICSD File No. 31122) are presented for comparison. Peaks not attributed to Ba-fresnoite are highlighted by a question mark symbol (“?”).

stishovite, and brookite for comparison. Multiple peaks in every pattern are not attributable to Ba-fresnoite and are highlighted in the figure by a question mark symbol (“?”). All XRD patterns show enhanced (201) peaks of Bafresnoite, which is consistent with the oriented nucleation described in detail in ref 20. However, the degree of peak enhancement is very small in the pattern obtained after crystallizing at 1100 °C and the (001) peaks are barely enhanced. The latter is in contrast to comparable XRD patterns acquired from related Ba-fresnoite containing glassceramics where this peak enhancement results from the kinetic selection during growth into the bulk.16 The peaks not attributable to Ba-fresnoite appear to increase in intensity as the crystallization temperature is raised from 850 °C to 910 °C but are barely discernible after crystallization at 1100 °C. While this could indicate an increasing amount of secondary crystals with increasing temperature, it must also be remembered that related glasses show a time-shifted bulk crystallization after initial surface crystallization. Hence, the microstructure within the information volume of XRD may, for B



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example, be composed of only a surface crystallized layer or a layer of surface crystallization and a crystallized bulk beneath it. The theoretical patterns of stishovite23 and brookite24 are presented because they both have peaks at 2θ ≈ 31°, where one of the most intense peaks not attributable to Ba-fresnoite occurs. However, none of the other peaks of stishovite occur, making it unlikely that the unattributed peaks are caused by stishovite. Furthermore, stishovite is a high-pressure phase and, hence, is unlikely to form under the supplied conditions. The theoretical pattern of brookite, on the other hand, shows peaks close to the positions of all peaks highlighted by “?”, except the peak at 2θ ≈ 10°. However, other (minor intensity) peaks of brookite fail to occur in the measured XRD patterns. Hence, it may be stated that the presence of brookite is probable but not confirmed, while the origin of the peak at 2θ ≈ 10° remains unclear. As systematic artifact (i.e., from the sample holder) is unlikely as this peak occurs in patterns acquired at two completely different locations (i.e., the glass was crystallized in different furnaces and measured in different diffractometers by different operators). During the subsequent analysis of these glass-ceramics, not all EBSD patterns obtained from the surfaces of the glassceramics crystallized at 850, 880, and 910 °C were indexable as Ba-fresnoite. Since XRD indicated the possible presence of brookite, a material file for this phase was created based on ICSD File No. 31122 and supplied during the subsequent EBSD analyses. Figure 2 presents EBSD patterns 1−6: patterns

Table 1. Selected Indexing Parameters for The EBSD Patterns 1−6 Presented in Figure 2 pattern

material file

votes

fit [deg]

CI

1

Ba-fresnoite brookite

50 2

0.75 1.66

0.375 0.000

2


87 7

0.65 1.45

0.542 0.025

3


52 13

0.67 1.27

0.392 0.025

4


7 41

1.41 0.99

0.025 0.300

5


13 30

1.30 1.02

0.025 0.192

6


5 5

1.62 1.41

0.000 0.017

SEM micrographs illustrating the morphology of the immediate surfaces of the prepared glass-ceramics are presented in Figure 3. While they show large, bright areas

Figure 3. SEM micrographs acquired from the immediate surface of the respective glass-ceramics heat treated at 850, 880, 910, or 1100 °C. The 001 pole figures (PFs) of data points attributed to Bafresnoite in EBSD scans performed on these surfaces are also presented.

Figure 2. EBSD patterns acquired from the immediate surface of the glass-ceramics crystallized at 850, 880, and 950 °C.

1 and 4 were obtained after crystallization at 850 °C, patterns 2 and 5 were obtained after crystallization at 880 °C, and patterns 3 and 6 were obtained after crystallization at 910 °C. As a rule of thumb, an EBSD pattern can be considered reliably indexed by the TSL software if it receives at least 30 votes, has a fit factor of 0.100. The EBSD patterns 1−6 in Figure 2 were indexed individually with the same Ba-fresnoite material file used in previous EBSD analyses, as well as the brookite material file. An interplanar angle tolerance of 2° was used, and the resulting parameters are stated in Table 1. While the patterns 1−3 are reliably indexed as Ba-fresnoite, the patterns 4 and 5 almost fulfill the rule of thumb for a reliable set of indexing parameters as brookite. Only pattern 6 completely fails to be indexed. Hence, the material file for brookite was also supplied during all subsequent analyses of the immediate surface.

similar to those observed at the surface of BT3S20 after crystallization at 850 and 880 °C, crystallization at 910 °C led to a large number of bright particles irregularly distributed across the surface. After crystallization at 1100 °C, the grains are larger and irregular surface structures are also observed. All of these surfaces show small, dark droplets pointing toward the phase separation, e.g., described to occur in the glass BT1.5S.20 Since the crystallization of this glass should ideally lead to a two-phase material containing Ba-fresnoite and SiO2, it is plausible to assume that these dark droplets contain high amounts of SiO2. These surfaces were analyzed using EBSD, and the resulting 001 pole figures (PFs) of Ba-fresnoite are shown along with the respective micrographs. The PFs of the surfaces crystallized at 850−910 °C all show the clear preference of orientations C



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where the c-axes are tilted from the surface normal by ∼60° ± 10°, which is in agreement with the 201 peak of exaggerated intensity in Figure 1. The PF obtained after crystallization at 1100 °C also shows this preference, but orientations with the caxes tilted by ca. 60°−120° occur more often than those tilted less from the surface normal, as similarly described in related glass-ceramics.20 These observations are in agreement with the oriented nucleation of Ba-fresnoite reported for the glasses BT2S and BT3S.20 The data points attributed to orthorhombic brookite in the EBSD scans featured above also showed nonrandom orientation distributions. The 100, 010, and 001 PFs of these data sets are presented in Figure 4 show that there seems to be

Figure 5. SEM micrographs featuring (a) the surface crystallized layer in a BT2.625S sample crystallized for 2 h at 880 °C and (b) the growth front in the framed area of panel (a) in greater detail.

By contrast, crystallizing this glass at 1100 °C leads to the near-surface microstructure visualized in detail in Figure 6a. It

Figure 4. 100, 010, and 001 PFs of the data points attributed to brookite in the EBSD scans performed in the immediate surfaces of the respectively crystallized glass-ceramics.

a preference for orientations with the b-axes, and after crystallization at 910 °C also with the a-axes, oriented perpendicular to the surface while the other axes rotate randomly around it (i.e., fiber textures). However, these results should be considered with caution, since brookite has not been confirmed to occur at the surfaces of these glass-ceramics. Furthermore, the number of data points contributing to these PFs is barely sufficient for texture analysis. The 181 data points attributed to brookite in the scan performed on the glassceramics crystallized at 880 °C are certainly not enough for the PF to be representative. EBSD patterns indexable as brookite could not be obtained after crystallization at 1100 °C. Please note that peaks attributable to brookite were also barely visible in the XRD pattern of samples crystallized at 1100 °C presented in Figure 1. The growth into the bulk during the crystallization at 880 °C is illustrated by Figure 5a: a clear surface layer of very delicate growth structures with a morphology resembling dendritic growth subject to kinetic selection is observed, in analogy to Ba-fresnoite in the glasses BT2S and BT3S.20 However, looking at the growth front within the framed area with a greater magnification shows that dark droplets indicating phase separation are clearly discernible in the crystallized area, as well as in the uncrystallized glass. This was also observed in the glass BT1.5S.20 Bulk nucleation is not indicated after the crystallization time of 2 h. All EBSD patterns of sufficient quality acquired from this cut plane were indexed as Bafresnoite, but generally the obtained pattern quality is too low to perform acceptable EBSD scans.

Figure 6. (a) SEM micrograph of the surface crystallized layer featured in Figure 7 in greater detail and (b) a higher-magnification SEM view of the framed area in panel (a).

is clearly coarser and less aligned than that grown at the lower temperatures illustrated in Figure 5. Nevertheless, Figure 6b shows that even the topmost layer of crystals does not form compact grains but is instead composed of orientation domains highly permeated by residual glass (dark). While the growth structures discernible here do not show the classical dendritic morphology, they are composed of a core (elongated streaks in the cross section) surrounded by more-fragmented orientation domains. The bridges between the cores and the disturbed secondary structures share a certain degree of similarity with the final stage of interdendritc crystallization of Ba-fresnoite grown via electrochemically induced nucleation (EiN).11 D



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Figure 7a presents this near-surface area with a greater overview and superimposed by an inverse pole figure + band

Table 2. Maximum Distance and Maximum Point-to-Origin Misorientation of the Lines L1−L7 in Figure 7c, as Well as the Average Deviation Calculated from These Values profile

maximum distance [μm]

maximum p-t-o misorientation [deg]

average deviation [°/μm]

L1 L2 L3 L4 L5 L6 L7

12.75 7.91 6.43 8.17 14.83 26.72 16.99

4.39 9.95 1.49 4.43 12.13 30.80 20.74

0.34 1.26 0.23 0.54 0.82 1.15 1.22

orientation changes than those acquired further in the bulk. While this observation should not be overrated due to the steps in some of the misorientation profiles, it would support the hypothesis that these dendrites faced increasingly difficult conditions for growth as the temperature increased and the glass started to phase separate. Although acquiring EBSD patterns further in the bulk is problematic, there are some areas that allow a systematic EBSD analysis. The IPF + BC map of an EBSD scan performed on one of these is featured in Figure 8a. The area

Figure 7. (a) IPF + BC map of an EBSD scan superimposed onto an SEM micrograph featuring the surface crystallized layer in a BT2.625S sample crystallized for 2 h at 1100 °C. (b) The framed area presented in greater detail. (c) Point-to-origin (p-t-o) misorientation profiles illustrating the orientation changes along the lines L1−L7 superimposed onto the also presented IPF + BC map segment.

contrast (IPF + BC) map of an EBSD scan performed on the area. The latter shows that indexable EBSD patterns could only be obtained systematically from a near-surface layer and that the domains within this layer show an orientation development comparable to, e.g., the dendrites competing for space during kinetic selection presented in ref 19. The SEM micrograph in Figure 7b visualizes the microstructure of the framed area in greater detail. It shows that the area below the surface layer by no means only contains uncrystallized glass but is instead highly crystallized. Hence, the inability to obtain EBSD patterns of comparable quality from this area proves that, here, the crystal lattice of the orientation domains is unable to function as a diffraction lattice sufficient for EBSD pattern acquisition, although it received exactly the same polish and coating as the near-surface layer featured in Figure 7a. The most probable explanation for this observation is that the orientation domains below the near-surface layer are so small that multiple domains occur within the information volume significant to the EBSD measurement,17,18 in addition to the omnipresent residual glass. Orientation changes along the lines L1-L7 are visualized by the point-to-origin (p-t-o) misorientation profiles presented in Figure 7c. The average deviations along these lines is calculated in Table 2: these orientation domains show orientation changes of up to 30.80° over a distance of only 26.72 μm while freely grown dendrites of Ba-fresnoite have been shown to have orientation changes of

Hindering the Kinetic Selection of Dendritic Ba-Fresnoite by Phase

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