Crystallization of ZrTiO4 Nanocrystals in Lithium-Alumino-Silicate

Article pubs.acs.org/crystal

Crystallization of ZrTiO4 Nanocrystals in Lithium-Alumino-Silicate Glass Ceramics: Anomalous Small-Angle X‑ray Scattering Investigation Vikram Singh Raghuwanshi,*,†,‡ Christian Rüssel,§ and Armin Hoell† †

Helmholtz Zentrum Berlin für Materialien und Energie, Hahn-Meitner Platz 1, D-14109, Berlin, Germany Humboldt-Universität zu Berlin, Brook-Taylor Strasse 2, D-12489, Berlin, Germany § Otto-Schott-Institut, Friedrich-Schiller-Universität Jena, Fraunhoferstrasse 6, D-07743, Jena, Germany ‡

ABSTRACT: We report on anomalous small-angle X-ray scattering (ASAXS) investigations of the formation and structure of nanosized ZrTiO4 crystals in lithium-aluminosilicate (LAS) glass during heat treatment at 750 °C for different periods of time. For a sample annealed for 30 min, ASAXS measurements near the X-ray absorption edge of Ti and Zr reveal the formation of particles (ZrTiO4) surrounded by a shell-like region. The sample annealed for 240 min shows the formation of two different types of particles (spherical core−shell: ZrTiO4 and large spherical particles: LiAlSi2O6). Additionally, ASAXS results allow quantitative determination of the average composition and volume fraction of the nanocrystals, the surrounding region (shell), and the remaining glass matrix. Data analysis reveals the formation of an alumina enriched region around ZrTiO4. This alumina enriched layer makes the glass network rigid and hinders further growth of ZrTiO4 nanoparticles. For a prolonged annealing time (240 min), a new phase (LiAlSi2O6) is formed additionally, and the (Al/Si) ratio in the shell surrounding the ZrTiO4 crystals decreases. Moreover, the unannealed sample also shows the formation of a Ti enriched phase during cooling of the samples. The obtained quantitative information helps to understand the crystallization and growth mechanisms of ZrTiO4 nanocrystals in LAS glass ceramics.

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INTRODUCTION Crystal nucleation and growth control is a critical phenomenon in the development of any glass ceramic. Glass ceramics with low thermal expansion coefficients are widely used for various scientific, industrial, and daily applications.1−3 However, nucleation agents are used to obtain homogeneously dispersed and nanograined crystals during heat treatment of the glass ceramics.4,5 This helps to produce ultratransparent glass ceramics containing nanocrystals which make them potential candidates for applications in optical devices.6 Therefore, it is mandatory to fully understand the nucleation, growth, and stabilization mechanisms of nanoparticles in glass ceramic systems. The present interest in the formation of ZrTiO4 nanocrystals in glass ceramics motivates us to investigate their collective properties which are useful for both the fundamentals and the technological applications.7,8 Lithium-alumino-silicate (LAS) transparent glass ceramics containing ZrTiO4 and LiAlSi2O6 nanoparticles are used to manufacture high temperature resistant furnace windows, cooktop panels, and telescope mirrors due to their unique property of low thermal expansion coefficients.9,10 In these glass ceramics, the crystallization and growth of ZrTiO4 and LiAlSi2O6 crystals depend on the parent glass composition, thermal treatment, and addition of nucleation agents.11 In © 2014 American Chemical Society

earlier studies, it was reported that there is a liquid−liquid phase separation which leads to the formation of small droplets enriched in nucleation agents (Zr and Ti).12,13 Moreover, the crystallization and growth of ZrTiO4 nanocrystals upon heat treatment is accompanied by the formation of a diffusion barrier surrounding the precipitate. Using TEM enabled the observation of an alumina enriched layer at the interface of the ZrTiO4 nanocrystals. This layer prevents further growth of the ZrTiO4 nanosized crystals and suppressed Ostwald ripening.14 The alumina enriched layer also facilitates the nucleation of a secondary LiAlSi2O6 phase in the case of long time heat treatments. However, quantitative structure and composition information on nanosized crystals in the LAS glass ceramic system is still not fully revealed. Present ASAXS investigations on the studied system were performed to obtain quantitative structure and average compositions to prove the hypothesis of the crystallization and growth mechanisms of ZrTiO4 and LiAlSi2O6 nanocrystals in LAS glass ceramic systems. In this paper, the glass under study has the composition 7.6Li2O−0.16Na2O−0.13K2O−1.85MgO−0.33BaO−1.2ZnO− Received: January 23, 2014 Revised: March 27, 2014 Published: May 9, 2014 2838

dx.doi.org/10.1021/cg5001232 | Cryst. Growth Des. 2014, 14, 2838−2845

Crystal Growth & Design

Article

Figure 1. Atomic scattering amplitudes at X-ray K-absorption edges of (a) Zr (Z = 40 electrons) and (b) Ti (Z = 22 electrons) and the selected energies for the ASAXS measurements.

12.73Al2O3−72.58SiO2−2.11TiO2−0.9ZrO2−0.39As2O3 (mol %). After heat treatments at 750 °C for variable time intervals, precipitation of ZrTiO4 and LiAlSi2O6 nanocrystals in LAS glass system occurred. The main goal of this work is to estimate the crystallization kinetics and crystal growth with respect to the evaluation of the average shape, size, composition, and volume fraction of the crystals depending on the role of zirconia and titania used as nucleation agent in LAS glass system. We have performed detailed ASAXS measurements to obtain quantitatively the average structure and average composition information on the different types of nanoparticles precipitated during heat treatment of the investigated glass. The results will help to understand the role of Zr and Ti as nucleation agents in LAS glass ceramics will help to optimize different optical and thermal properties for various applications.

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Figure 1a,b shows the variation of the atomic scattering amplitude near the X-ray K-absorption edges of Zr and Ti. The quantitative information on the structure of nanoparticles from the SAXS data are obtained by fitting the scattering curves using the theoretically evaluated differential scattering cross section. The scattering intensity for a system of noninteracting particles can be described as15 I(q) =

NAD ∑i cifi (E)

Fsphere(q , r , Δη) = 3Δη

sin(qr ) − (qr ) cos(qr )

(1)

(qr )3

(4)

Similarly, the scattering amplitude of the spherical core−shell structure can be written:

where NA is the Avogadro number, D is the mass density of the phase, ci is the mole part of ith element present in that phase, and MC = Σ ciMi is the total molar mass, where Mi is the molar mass of ith element present in the phase. f i(E) is the atomic scattering amplitude and defined as f (E) = f0 + f ′(E) + if ″(E)

N (r )Vp(r )2 |F(q , r , Δη)|2 dr + cq−α + Bkg

where N(r) is the particle number distribution, Vp(r) is the volume of a particle with the radius r and F(q, r, Δη) is the scattering amplitude which contains information about shape and size of the particles. In the scattering amplitude, Δη = ηparticle − ηmatrix is the effective electron density difference between the particle and the remaining matrix as defined in eq 1 and q is the transferred momentum, which is related with the wavelength (λ) of the X-rays and the scattering angle (2θ) as |q⃗| = q = 4π sin θ/λ. Additionally, a constant background Bkg arising from resonant Raman and from fluctuation scattering is also included. The term cq−α indicates the smoothness of the particle surface. For a smooth surface the value of α = 4. The scattering amplitude for a homogeneous sphere with radius r is defined as19,20

ASAXS THEORY

MC

∞

(3)

Small angle X-ray scattering (SAXS) investigations allow determination of the spatial correlations in the scattering densities present in the sample. It allows evaluating the inhomogeneity of sizes from 1 up to 100 nm present in the system.15−17 In principle, SAXS measurements depend on the difference between the effective electron densities, termed as contrast, of the different phases present in the system. The effective electron density is defined as η(E ) =

∫0

Fsphere shell(q , rt , Δηs , ν , μ) = Fsphere(q , rt , Δηs) − Fsphere(q , νrt , Δηs(1 − μ))

(5)

where rt is the total radius from the center of the particle core until the outer end of the shell and r = νrt (0 < ν < 1) is the radius of the core. Δηs = ηshell − ηmatrix is the effective electron density difference between shell and matrix and μΔηs is the effective electron density difference between the core and the matrix. ASAXS investigations taking advantage of the variation of energy dependent atomic scattering amplitudes allow estimation of quantitative information on the distribution of particular element (resonant term) present in the system. The resonant

(2)

For a particular element and at an energy far from the X-ray absorption edges, the atomic scattering amplitude f(E) is equal to the number of electrons present in the respective atom. At the energies close to the X-ray absorption edges, the scattering amplitude varies significantly because of the variation in the values of f ′ and f ″ due to electronic excitation. The values of the atomic scattering amplitudes can be obtained from theoretical calculations as given by Cromer and Liberman.18 2839



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min have been selected for the ASAXS experiment. The thickness of the samples was in the range from 40 to 60 μm, which is suitable to achieve acceptable X-ray transmission. ASAXS experiments were conducted on all samples by choosing four different energies close to the X-ray K-absorption edges of Ti (4966 eV), Zr (17998 eV), and the L3 absorption edge of Ba (5247 eV) at the 7T-MPW-SAXS beamline at BESSY II synchrotron at Helmholtz-Zentrum Berlin (HZB).27 The photons scattered by the samples were collected by using a twodimensional gas filled multiwire proportional counter (MWPC, RIGAKU molmet, 200) with delay line readout. Raw scattering images were corrected for transmission, photon flux, solid angle distortion, dead-time of the detector electronics, and sensitivity of detector pixels. The final scattering curves were obtained by performing radial circular averaging on two-dimensional scattering images. A precalibrated glassy carbon standard sample of thickness 90 μm was used to scale the scattering curves to differential scattering cross sections per unit volume. The glassy carbon sample was measured with every sequence of particular energy. Moreover, each sample is measured at a particular energy three times in order to get good statistics. A silver behenate sample (first peak at q = 1.076 nm−1) was used to calibrate the q axis.28 A two-dimensional SAXS detector pattern for one of the sample annealed at 750 °C for 30 min and measured at 17600 eV is shown in Figure 2b which clearly shows an isotropic intensity distribution.

scattering term can be evaluated by solving the Stuhrmann equation given as21,22 I(q , E) = IO(q) + f ′(E)IOR (q) + (f ′(E)2 + f ″(E)2 )IR (q) (6)

where the first term IO(q) is the SAXS term (far from the absorption edge), the second term IOR(q) is the scattering cross term (contribution from SAXS and resonant scattering term), and the third term IR(q) is the resonant term which reveals the distribution of the particular element on whose X-ray absorption edge the ASAXS measurements are performed. Solving the above linear equation system for three or more Xray energies allows separation of the resonant term IR(q) from the SAXS term and the scattering cross term.23 The average composition information on multiphases present in the system can be obtained by fitting the ASAXS curves using two separate fitting routines.24−26 First, the ASAXS curves are fitted simultaneously for three or more energies to get the structural model and to obtain the relative contrast values. Second, the estimated relative contrasts are further fitted with the theoretically evaluated relative contrasts defined as η (En) − ηmatrix (En) Δη(En) = core Δη(E1) ηcore(E1) − ηmatrix (E1)

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RESULTS Figure 3 shows the SAXS curve of the unannealed sample and the two samples annealed at 750 °C for 30 and 240 min. The

(7)

The detailed information on the average composition calculation by ASAXS data analyses are given in the earlier reported literature.24,25

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EXPERIMENTAL SECTION

The glass used in our study has the composition 7.6Li2O−0.16Na2O− 0.13K2O−1.85MgO−0.33BaO−1.2ZnO−12.73Al2O3−72.58SiO2− 2.11TiO2−0.9ZrO2−0.39As2O3 (mol %) and was prepared by Schott AG, Mainz, Germany, in an electric furnace using a platinum crucible in air. This composition is similar to an industrially used glass (Robax glass ceramics) but in contrast to Robax, this glass contains arsenic. The glass samples were annealed at 750 °C (heating rate 10 K/min) for different periods of time. After annealing, the furnace was switched off and slowly cooled down to room temperature. The unannealed and the crystallized glass samples are visually transparent as shown in Figure 2a. SAXS experiments were performed to investigate the effect of heat treatment on the shape, size, volume fraction, and size distribution of the particles in the glass matrix, and the respective average compositions were determined by analyzing the ASAXS curves. One unannealed and two annealed glass samples at 750 °C for 30 and 240

Figure 3. SAXS curves measured at the energy of 4647 eV for the LAS glass samples: unannnealed and annealed samples at 750 °C for 30 and 240 min. The solid black lines show the fit for the derived structure model.

SAXS curves were measured at an energy of 4647 eV far from the X-ray absorption edges of all elements present in the studied glass. The scattering curves are scaled to differential scattering cross section per unit volume and plotted as a function of the momentum transfer vector q. The SAXS curve for the unannealed sample shows a small hump between q = 0.5 and 1 nm−1, which indicates the presence of very small particles or phase separation during cooling of the samples. The SAXS curve for 30 min annealed sample shows significantly higher intensity with respect to the unannealed sample. Moreover, it shows clearly a sharp hump at about q = 0.5 nm−1, which gives a hint of the occurrence of particles larger than those in the unannealed sample. The SAXS curve for the sample annealed for 240 min shows a broad hump between q = 0.2 and 0.6 nm−1, which is a first hint of the existence of two kinds of size distributions with large sizes and volume fractions.

Figure 2. (a) Transparent glass ceramic piece after annealing at 750 °C for 30 min. (b) 2D SAXS detector image for the sample annealed at 750 °C for 30 min and measured at 17600 eV. 2840



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with average sizes about 16 nm (in diameter) for the sample annealed for 240 min was observed. Furthermore, the ASAXS investigations were performed in order to determine quantitatively the average composition and the structure of the nanoparticles, their surrounding shell, and the remaining glass matrix. ASAXS measurements were made near the particular X-ray absorption edge of different elements (Zr, Ti, and Ba) present in the investigated glass samples. Figures 4 and 5 show the ASAXS curves measured near the

Furthermore, information on the size, shape, and size distributions of the particles in the investigated three glass samples was all evaluated by nonlinear fitting of the respective SAXS curve. The software SASfit29 was used to fit the SAXS curves. For fitting, SASfit requires predefined shape, size, size distribution, contrasts of the particles, and a constant background. On the basis of earlier investigations (TEM, XANES) on the same set of samples, the SAXS curves were fitted by assuming different structure models.12,13 The unannealed sample and the sample annealed for 30 min were fitted by using the spherical core−shell structure model (eq 5) in eq 3. The total scattering intensity is given as follows: ISAXS(q) =

∫0

∞

N (rt)|Vp(rt)Fsphere shell(q , rt , Δηs , ν , μ)|2 dr

+ cq−4 + Bkg

(8)

Similarly, the SAXS curve for the sample annealed for 240 min was fitted by assuming a sphere model (eq 4) combined with a spherical core−shell structure model (eq 5) in eq 3. The total scattering intensity can be given as ISAXS(q) =

∫0 +

∞

∫0

N (rt)|Vp(rt)Fsphere shell(q , rt , Δηs , ν , μ)|2 dr ∞

N (r )|Vp(r )Fsphere(q , r , Δη)|2 dr + cq−4

+ Bkg

(9)

It is found that a log-normal size distribution fits well for all the scattering curves. While fitting a particular SAXS curve, all the structure determining parameters, size distribution parameters, and the contrast of the particles were free variables. Figure 3 shows the assumed structure model fitted SAXS curves for all the investigated samples, and the resulting average size parameters (average diameter of core, thickness of shell) are shown in Table 1. Parameters show that the size of the particles

Figure 4. Measured ASAXS curves and their structural fits at four energies near the X-ray K absorption edge of Ti (4966 eV) for the samples annealed at 750 °C for 30 min (a) and for 240 min (b).

Table 1. Resulting Structure Parameters, Volume Fraction Evaluated by Fitting the ASAXS Curves and the Resulting Relative Contrastsa parameters

unannealed

30 min

240 min

ZrTiO4: (core) average diameter (nm) ZrTiO4: (shell) thickness (nm) LiAlSi2O6: average diameter (nm) density core (g/cm3) density shell (g/cm3) vol fraction ZrTiO4 (%) vol fraction LiAlSi2O6 (%) percent Zr in core (%) percent Li in particle (%)

0.8 ± 0.2

3.0 ± 1.0

4.5 ± 1.0

0.3 ± 0.1

0.5 ± 0.2

0.8 ± 0.2 15.8 ± 2.0

5.05 ± 0.05 3.8 ± 0.5 1.2

5.0 3.2 1.6 29.5 95 75.0

± 0.1 ± 0.5

The parameters shown in the table are for unannealed sample and the samples annealed at 750 °C for 30 and 240 min.

Figure 5. Fitted ASAXS curves measured at four energies near the Xray K-absorption edge of the Zr (17998 eV) for the samples annealed at 750 °C for 30 min (a) and for 240 min (b). For the clarity of the figure, only two energies (far and near to the absorption edge) are shown.

increases with the annealing time. The unannealed sample shows the formation of a spherical core−shell structure of small size of about 1.1 nm in diameter. The samples annealed for 30 and 240 min have nearly the same core size (about 3−5 nm in diameter) for the spherical core−shell structure. The thickness of the shell surrounding the particles also increases with the annealing time. Additionally, a large sort of spherical particles

absorption edge of Ti and Zr for the sample annealed at 750 °C for 30 and 240 min. It is observed that all the samples (including the unannealed sample shown in Figure 6) show remarkable ASAXS effects only near the X-ray K-absorption edge of Ti and Zr. This indicates that the nanoparticles precipitated during preparation (cooling and annealing) of the samples are enriched with both Ti and Zr atoms.

82 ± 6.0

± 4.0 ± 5.0

a

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would need 48 parameters. During simultaneous fitting, the size related parameters (mean r1, width s1, shell thickness t1; mean r2, width s2) are the same for each scattering curve. This reduces the number of fitting parameters to 20 variables (5 variables per curve). Second, the obtained relative experimental contrasts at both absorption edges of Ti and of Zr were further fitted simultaneously with the theoretically calculated relative contrasts. During fitting the relative experimental contrasts for the sample annealed for 30 and 240 min, it was assumed that the crystal consists of pure ZrTiO414 (core, fixed composition, and density), and the shell surrounding the crystal is enriched with Al, Si, and O and a certain combination of all other elements present in the glass. Additionally, for the sample annealed for 240 min, large spherical particles consisting of LiAlSi2O6 were assumed. The fitted relative contrast data near the Ti and Zr absorption edges for the sample annealed at 750 °C for 30 min are shown in Figure 8a,b. The respective fit Figure 6. ASAXS curves measured at four energies near the X-ray Kabsorption edge of Ti (4966 eV) for the unannealed sample.

Figure 7 shows the ASAXS curves measured near the Ba L3 absorption edge (5247 eV) for the samples annealed at 750 °C

Figure 8. Comparison of the theoretical and experimental energydependent relative contrasts for the sample annealed at 750 °C for 30 min. (a) Ti K-absorption edge; 4966 eV and (b) Zr K- absorption edge; 17998 eV.

parameters are shown in Table 1, and the evaluated compositions are given in Table 2. Moreover, in Table 2, the uncertainties in the calculated chemical compositions is about 3−4%, which were obtained by different calculations with different starting parameters for core, shell, and the remaining matrix. The unannealed sample shows a spherical core−shell structure with sizes of about 1.1 nm (in diameter), but it is difficult to make conclusions about the composition of the shell for such small particles, due to the weak signals (Figure 6). Additionally, by using the Stuhrmann method (6), the pure resonant scattering contributions from the distributions of Ti atoms in the samples annealed for different times have been separated from the total scattering SAXS curves. Figure 9 shows the resonant curves for the samples annealed for 30 and 240 min. The resonant scattering is about 1 order of magnitude smaller than the total scattering as shown in Figure 3. Both scattering curves are similar in shape. Both the resonant curves for the sample annealed for 30 and 240 min fitted with the spherical model and with same geometrical structure parameters used for the respective SAXS curves. This result indicates that the core of the small spherical core−shell structure is enriched with the Ti atoms. Figure 10a shows the comparison of the volume weighted log-normal particle size distributions evaluated from the

Figure 7. ASAXS curves measured at four energies near the X-ray L3 absorption edge of the Ba (5247 eV) for the samples annealed at 750 °C for 30 min (a) and for 240 min (b). The arrow indicates the shift of the differential scattering cross section with increasing energy. For the clarity of the figure only two energies (far and close to the absorption edge) are shown.

for 30 and 240 min. It is seen from the ASAXS curves at Ti and Zr absorption edge that the intensity of the scattering curves decreases when the energy moves toward the respective absorption edges of the element. However, in contrast to Zr and Ti ASAXS, the intensities of the scattering curves near the Ba absorption edge increase when the energy moves toward the respective absorption edge. Furthermore, quantitative average composition of nanoparticles, shells, and the remaining glass matrix was obtained by fitting the ASAXS curves and the relative contrasts by two separate fit routines as discussed in Theory section. First, the relative experimental contrasts were obtained by fitting the ASAXS curves using SASfit. The theoretical model (spherical shell + sphere) assumed for the 240 min annealed sample has 12 variable parameters for each scattering curve. This means a particular ASAXS sequence measured at four different energies 2842



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Table 2. Resulting Averaged Composition of the Core, Shell, and Matrix Evaluated by Fitting the Relative Contrasts at Ti and Zr Edge for the Sample Annealed at 750 °C for 30 and 240 min spherical core shell particles shell

matrix

samples 750 °C

core

Al2O3

SiO2

oths

ratio (Al/Si)

Al2O3

SiO2

Oths

ratio (Al/Si)

sphere particles

0 min 30 min 240 min

phase (Ti) ZrTiO4 ZrTiO4

38 (Al%) 6.3 (Al%)

49 (Si%) 71.6 (Si%)

13 22.1

1.55 0.18

12.73 12.60 11.15

72.58 74.01 77.99

14.69 13.39 10.85

0.35 0.34 0.29

LiAlSi2O6

Figure 11. Comparison of the size parameters (particle diameter) obtained by different methods; SAXS and TEM.

as a hindrance to further crystal growth. Moreover, for a long annealing time also the precipitation of another phase LiAlSi2O6 with a large size of about 16 nm is observed. SAXS data analysis shows the presence of small spherical core−shell structures (about 1.1 nm in diameter) in the unannealed sample, which reveals phase separation during casting and cooling of the samples. The samples annealed at 750 °C for 30 and 240 min show the formation of a spherical core−shell structure with nearly the same sizes (core diameter; between 3 and 5 nm) as shown in Table 1. Additionally, a large size spherical particle is observed in the sample annealed for 240 min. Figure 12 shows the schematic of the relative contrast profile (effective electron density profile) for the structure models used to fit the SAXS curves of the annealed samples. For 30 min annealing time, a steplike contrast profile is obtained for the spherical core−shell structure, where the particle (core) has high contrast with respect to the surrounding shell like region. Moreover, the additional larger sized spherical particles in the 240 min long annealed sample have lower contrast (small effective electron density difference)

Figure 9. Resonant curves evaluated at the Ti K edge by using the Stuhrmann method for the two samples annealed at 750 °C for 30 and 240 min.

structural fit for the SAXS and resonant scattering curves for all investigated samples. The size distributions determined by SAXS curves for the samples annealed for 30 and 240 min are not significantly different from each other. The size distributions determined by the resonant curves are comparable to the respective size distributions from the SAXS curves. Additionally, Figure 10b shows the volume weighted lognormal particle size distributions for the larger kind of particles present in the sample annealed for 240 min and obtained from the structural fit for the respective SAXS curve.

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DISCUSSION It has already been reported that in the investigated glass ceramics precipitation of ZrTiO4 nanocrystals occurs.13 These nanoparticles are surrounded by an Al enriched layer which acts

Figure 10. Size distributions of the particles obtained after fitting of the SAXS curves. (a) Log-normal distribution for the unannealed sample and samples annealed at 750 °C for 30 and 240 min. The figure also includes the size distribution obtained by fitting the resonant curves. (b) Log-normal distribution for the large spherical particles precipitated for the sample annealed at 750 °C for 240 min. 2843



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Figure 12. Sketches of the structure model showing the variation of the contrast. A spherical core−shell model fits to the 30 min annealed sample (top). A spherical core−shell model combined with a large size sphere model fit to the 240 min annealed sample (bottom).

for 30 min, Zr and Ti form ZrTiO4 particles surrounded by a thin shell-like region. The evaluated density of the shell is 3.8 g/cm3 and hence close to the bulk density of the Al2O3 3.9 g/ cm3, which reveals the enrichment of alumina in the surrounding shell region of the ZrTiO4 particles. The enrichment of alumina in the shell is in agreement with earlier publications.12,13 This can be explained by the presence of alumina in the phase separated structures, and during annealing it is squeezed out toward the rim of the particles. The layer surrounding the particle is very thin (about 0.5 nm), and it may not be able to stop further diffusion of Zr and Ti into the crystals for a longer annealing time. However, on further annealing nearly all Zr from the glass matrix diffuses into the crystal and completes the crystallization for ZrTiO4 particles as reported earlier by XANES investigations.12 For the 240 min annealed sample, nearly all the Zr atoms (95%) are depleted from the glass and incorporated into the ZrTiO4 particles. At this time, a new phase (LiAlSi2O6) is additionally formed, and Li, Al, Si, and O are hence depleted from the matrix for the formation of this new phase. The alumina surrounding the ZrTiO4 crystals is then incorporated into the new LAS phase. This leads to an increase of the Si concentration (including the other elements present in the glass, i.e., Na, K, Mg, and others) in the shell region surrounding ZrTiO4 in comparison to the Al atoms. This hence results in a lower Al/Si ratio of 0.18 than in the sample annealed for 30 min (Al/Si: 1.55). Moreover, the density of the shell decreases for the longer annealing time of 240 min, which can also be explained by an increase of the Si concentration and a decrease in the evaluated Al/Si ratio in the shell region with increasing annealing time (see Table 2). Moreover, the thickness of the shell like region increases with the annealing time, which should be due to a diffusion of the respective components between shell and the remaining matrix.

with respect to the spherical core−shell particles. This means the large kind of particles consist of lighter elements. ASAXS measurements near the absorption edge of Ti, Zr, and Ba give information on the distribution of respective atoms present in the particle and the remaining glass matrix. A prominent ASAXS effect for the unannealed sample near the Ti (shown in Figure 6) and Zr (not shown) absorption edge shows that the phase separated small spherical core−shell structures are enriched with Ti and Zr atoms. Earlier, TEM investigations reported by Höche et al.13 also concluded the presence of phase separated droplets enriched with alumina Al, Zr, and Ti in unannealed glass samples. Later, upon heat treatment for different periods of time inside these particles ZrTiO4 crystallizes and subsequently additional LAS nanoparticles are formed. The samples annealed for 30 and 240 min also show a remarkable ASAXS effect near the measured absorption edges (Zr, Ti, and Ba). It is seen from the ASAXS measurement near the Ti and Zr absorption edge shown in Figures 4 and 5 that the intensity of the scattering curves decreases as the measured energy moves toward the respective absorption edge. This means the effective electron density difference (particle contrast) between the particle and the matrix decreases, which reveals the particle consists mainly of Zr and Ti atoms. On the other hand, ASAXS measurements near the Ba absorption edge show an increase in the scattering intensity as the measured energy moves toward the respective absorption edge (shown in Figure 7). This means the effective electron density difference between the particle and the matrix increases, which reveals the presence of Ba atoms in the remaining glass matrix and not in the particle. Resulting parameters in Table 1 for the 30 min annealed sample show that about 82% of the total Zr atoms present in the glass are enriched in the ZrTiO4 particles. During annealing 2844



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This reduces the concentration gradient between remaining glass matrix and the shell region and results in growth of the shell. An increase in the volume fraction of the particles with the time is due to the growth of the particles. Fitting of the evaluated resonant curves shown in Figure 9 near the Ti absorption edge also reveals the formation of nearly same small size ZrTiO4 particles for annealed samples. The respective size distributions evaluated from the resonant curves are comparable with the core size of the respective SAXS curves. Moreover, the average size parameters obtained by SAXS data analyses are similar to those obtained from TEM investigations on the same set of samples as reported earlier13 (see Figure 11).

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CONCLUSION In summary, we have evaluated quantitatively the structure parameters and the composition of ZrTiO4 nanocrystals (core (crystals), shell, and matrix) in LAS glass ceramics by ASAXS investigations. Data analyses show the presence of Ti and Zr enriched spherical core−shell structures with diameters of about 1.1 nm in the unannealed samples. The sample annealed at 750 °C for 30 min shows the formation of ZrTiO4 nanoparticles (core; 3 nm in diameter) surrounded by an alumina enriched shell with the ratio of Al/Si = 1.55. Moreover, for a prolonged annealing time of 240 min, a second type of particle composed of LiAlSi2O6 with larger diameters (15.8 nm) along with the small ZrTiO4 is observed. Extended annealing time decreases the ratio of Al/Si in the shell region. These findings are in agreement with studies by TEM and XANES on this glass composition reported earlier. The growth of the particles for the 30 min annealed sample may be partially constrained by the shell-like region (Al and Si) surrounding the ZrTiO4 particles. For the sample annealed for 240 min, the depletion of Zr from the glass matrix is almost complete (95%), and hence the glass matrix does not contain enough Zr to enable further growth of the ZrTiO4 particles.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Roman Wurth for providing samples for the ASAXS measurements. VSR thanks Prof. Dr. Klaus Rademann from the Institute of Chemistry, Humboldt Universität zu Berlin for funding. Moreover, all authors thank him for valuable discussions.

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REFERENCES

(1) Grayson, M. In Encyclopedia of Glass, Ceramic, and Cement; Wiley: New York, 1985; p 548. (2) Pannhorst, W. In Low Thermal Expansion Glass Ceramics; Springer: Berlin, 1995; pp 1−12. (3) Beall, G. H. Annu. Rev. Mater. Sci. 1992, 22, 91−119. (4) Stookey, S. D. Glastech. Ber. (Fifth International Congress on Glass. Verlag der Deutschen Glastechnischen Gesellschaft, Frankfurt Am. Main) 1959, V/1−8. (5) Pinckney, L. R.; George, B. H. J. Am. Ceram. Soc. 2008, 91, 773− 779. (6) Pannhorst, W. In Zerodur-a Low Thermal Expansion Glass Ceramic for Optical Precision Applications; Springer: Berlin, Heidelberg, NY, 1995, 107−131. 2845


Crystallization of ZrTiO4 Nanocrystals in Lithium-Alumino-Silicate

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