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Suppression effects of cooling rate on crystallization in ZBLAN glass
Journal of Non-Crystalline Solids 481 (2018) 306–313
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Suppression effects of cooling rate on crystallization in ZBLAN glass a,⁎
a
b
Teng-Cheong Ong , Ted Steinberg , Esa Jaatinen , John Bell a b c
T
c
Department of Energy and Process Engineering, Queensland University of Technology, 2 George St., 4000 Brisbane, Australia Department of Nanotechnology and Molecular Science, Queensland University of Technology, 2 George St., 4000 Brisbane, Australia Department of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, 2 George St., 4000 Brisbane, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords: Crystallization ZBLAN Rapid cooling Critical cooling rate Microscopy Diffraction experiment
ZBLAN glass is a heavy metal fluoride glass that shows great potential in the application of long-haul telecommunication cables. However, during processing in the fiber-drawing temperature region, the material tends to undergo heavy devitrification, resulting in a crystalline fiber that is not usable as an optical waveguide. In this study, ZBLAN glass was processed with different cooling rates to discern how the rate of cooling affects the process of crystallization. Rapidly cooled ZBLAN glass test samples were then analysed with a wide range of spectroscopy and imaging techniques including EDS, SEM, TEM and XRD. These techniques revealed there are two predominant crystal phases, one rich in zirconium and barium, the other in zirconium and sodium. Ultimately, a critical cooling rate was established to be somewhere between 900 °C/min and 4000 °C/min, with a theoretically predicted value of 1081 °C/min for a test sample volume size of 9.4 × 10− 8 m3. Cooling above this critical cooling rate yielded fully amorphous ZBLAN test samples that were completely free of nano-crystalline inclusions. Using Kramers-Kronig relations, an attenuation loss for the fully amorphous ZBLAN test samples was estimated to be 0.09 dB/Km at a wavelength of 1800 nm.
1. Introduction Through years of research, there have been many papers published exploring the material properties of ZBLAN glass and various processing techniques were attempted with the aims of achieving a material that can transmit with the theoretical minimum attenuation losses predicted for ZBLAN. As ZBLAN glass is cooled from its melt, crystallites form throughout the medium, their size and structure dependent on the rate of cooling and degree of undercooling. These crystallites act as scattering centres that degrade optical transmission through the glass, resulting in attenuation losses. This study explored various processing techniques with different cooling rates to determine the effect they have on the suppression of crystallization. Ultimately, a critical cooling rate was determined, a rate that results in the complete suppression of even nano-crystallite formation in the glass. 2. Experimental procedure ZBLAN test samples were heated and cooled with 6 different cooling rates. The samples were heated in various ways, depending on the cooling method implemented. A heating rate of 20 °C/min was applied for all test samples, and immediately after the maximum target temperature of 500 °C was reached, the test samples were cooled
⁎
immediately. For the cooling rates of 5 °C/min and 20 °C/min, a controlled heating environment was required. These test samples were processed entirely in a DSC, which could be cooled at a slow and steady rate. The majority of the other test samples were heated in a conventional furnace, and then either air cooled or quenched in different media to achieve the required cooling rates. Lastly, test samples were also processed in a device (which was named an “REPD”, Rapid Electrothermal Processing Device) that was built to rapidly heat and cool the test samples [1]. All of the samples were cooled to just above room temperature. The following test matrix in Table 1 is an outline of the heating and cooling regimes used to process the samples. All cooling rates (CR) were taken by the following relation,
CR =
Tm − Tg , Δtm – g
(1)
where Tm is the melting temperature, 450 °C, Tg is the glass transition temperature, 260 °C, and Δtm–g is the time interval for the test sample to cool between the two temperatures (in seconds). The percentage error of the cooling rate measurements was low, ranging from 0.031 to 6.3% for the first 5 thermal profiles, the 5–4000 °C/min cooling rates. For the last thermal profile, the 8000 °C/min cooling rate, the percentage error was higher at 35%.
Corresponding author. E-mail address: [email protected] (T.-C. Ong).
https://doi.org/10.1016/j.jnoncrysol.2017.11.003 Received 20 June 2017; Received in revised form 31 October 2017; Accepted 2 November 2017 Available online 01 December 2017 0022-3093/ © 2017 Elsevier B.V. All rights reserved.
Journal of Non-Crystalline Solids 481 (2018) 306–313
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Table 1 Experimental test matrix. Thermal profile no.
Heating rate (°C/ min)
Maximum temperature (°C)
Method of heating
Cooling rate (°C/ min)
Method of cooling
Analysis techniquesa
1 2 3 4
20 20 20 20
500 500 500 500
DSC heated DSC heated Furnace heated Furnace heated
5 20 100 900
XRD, XRD, XRD, XRD,
5
20
500
REPD heated
4000
DSC cooled DSC cooled Air-cooled Quenched in soapy water REPD cooled
6
20
500
Furnace heated
8000
Quenched in water
SEM, SEM, SEM, SEM,
EDS EDS TEM, EDS TEM, EDS
XRD, SEM, TEM, EDS, Ellipsometry SEM, TEM, EDS, Ellipsometry
a Analysis techniques acronyms: XRD - X-ray diffraction, SEM - Scanning Electron Microscopy, TEM - Transmission Electron Microscopy, EDS - Energy Dispersive Spectroscopy, REPD Rapid Electro-thermal Processing Device.
For the characterization of the structure and molecular composition of the test samples, XRD, EDS and TEM analyses were used. For characterizing the crystallites in the micro-sized region (and larger), EDS and XRD were used, since the peaks in the spectra and patterns they generate can be matched with various crystal phases. For crystallites in the sub-micron range, especially the nano-sized particles (observed in the high cooling rates), TEM was particularly useful, where the diffraction patterns were used to identify the nano- and micro-crystalline phases. SEM and TEM imaging techniques were used to image the crystals themselves, where backscatter imaging (SEM) and dark field/bright field imaging (TEM) made it possible to identify the different crystal phases and individual crystallites. The X-ray diffractograms were primarily used to observe the change in degree of crystal/amorphous content in each test sample. Ellipsometry was used (in conjunction with an analysis approach that incorporates Kramers-Kronig relations) for the fully amorphous test samples to determine the attenuation losses [2]. The ZBLAN material used in this experiment was provided courtesy of THORLABS, an optical equipment company based in New Jersey, United States of America.
Table 2 List of peaks and attributed phases for X-ray diffractogram in Fig. 1. Phase ZrNaF5
β-ZrBaF6
Na Ba Zr2 F11 Na7 Zr6 F31
Symbol in Fig. 1
2θ
d-spacing (Å)
23.52 26.53 54.51 33.13 51.13 55.58 62.74 66.27
5.57 3.86 1.99 3.13 2.08 1.97 1.72 1.63
3.2. SEM results 3.2.1. Test samples cooled at 5 °C/min For this cooling rate, crystal phases of distinct morphology can be observed spreading throughout the entire test sample (only a small proportion of the sample remains in an amorphous glassy state). Fig. 4 shows an image of the crystallization pattern formed by the various crystal phases at a low magnification. Fig. 5 is a higher magnification back scatter image that shows distinct morphologies, a “feathery” type and a “bow tie/V-shaped” type [3], surrounded by a third less distinct morphology of regions of darker colouration with “black specs”. The contrast in shade indicates the difference in atomic number of each phase, heavier elements (with high atomic number) backscatter electrons more than lighter elements (with low atomic number), and therefore appear brighter in the image. Therefore “whiter” or “lighter coloured” regions in a backscatter image are composed of elements with a higher average atomic number. “Darker coloured” regions are comprised of elements that have a lower
3. Results and discussion 3.1. XRD results The primary peaks that appear in the XRD pattern of the slower cooled test samples can be seen in Fig. 1, and the phases they relate to are outlined in Table 2. Fig. 2 displays an overlay of three XRD patterns, for the following cooling rates; 5 °C/min (green), 20 °C/min (blue), 100 °C/min (red).
Fig. 1. X-ray diffractogram for 20 °C/min cooled ZBLAN test sample labelled with assigned crystal phases (shown in Table 2) as identified using the PDF-4+™ database.
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Fig. 2. Overlay of X-ray diffractograms for test samples of 3 different cooling rates, 5, 20 and 100 °C/min. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
3.2.2. Test samples cooled at 20 °C/min Fig. 6 shows a polished cross section of the ZBLAN test sample cooled at 20 °C/min. At first glance, the light coloured feathery morphology can be seen visibly, however this crystal phase is much smaller in size than the corresponding phase observed in the 5 °C/min sample. The backscatter image of Fig. 7 shows the same three morphologies of the 5 °C/min cooled test sample (as seen in Fig. 5), except each phase is smaller in size. The different phases have been highlighted in Fig. 8 to illustrate the change in size between the two cooling rates (light feathery morphology in green, dark regions in red and V-shaped in blue). Take note, the following side by side images are to equal magnification, therefore the highlighted regions show a true scale of the actual size difference.
Table 3 Indexing of diffraction pattern of Fig. 12b. Ring designation
Crystal species
d-spacing (Å)
(hkl)
1st 2nd 3rd 4th 5th
Ba Ba Ba Ba Ba
2.126 1.532 1.243 0.958 0.873
110 202 212 312 402
F2 F2 F2 F2 F2
average atomic number [4]. The light grey regions (“feather” crystal phase) of Fig. 5 contain high levels of zirconium and barium, with a deficiency in sodium, and are most likely to be a zirconium barium crystal matrix, Zr2BaF10. The atoms of this crystal phase have a much larger nucleus, therefore appear lighter in colour in the backscatter image. The regions with a bowtie/V-shaped morphology have a typical ZBLAN glass composition, which is reflected in the medium grey colouration in the backscatter image. The darkest coloured regions with black specs correlate to a zirconium sodium fluoride crystal phase, possibly NaZrF6. It is a region containing high levels of zirconium and sodium, and a deficiency in barium.
3.2.3. Summary of change in size of crystal phases (5 °C/min compared to 20 °C/min) The light feather morphology decreases in size from 500 to 30 μm, which correlates to a 90% decrease in size. Similarly, the “dark regions with black specs” crystal phase shows a 70% decrease in size and the Vshaped region shows the greatest change, a 95% decrease in size. Fig. 9 shows part of the 20 °C/min cooled test sample that has remained in an amorphous state. On the right side of the image, where
Fig. 3. X-ray diffractogram for 4000 and 8000 °C/min test sample.
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3.2.5. Test samples cooled at 900, 4000 & 8000 °C/min The SEM image of the 900 °C/min test sample in Fig. 11 displays a polished cross section of the test sample that is uniformly grey in appearance with no discernible features. This suggests the entire test sample is completely amorphous, however the 900 °C/min test samples still show evidence of nano-crystal inclusions (however, they are not visible under SEM), but are only detectable using TEM analysis. 3.3. TEM results 3.3.1. Test samples cooled at 100 °C/min Fig. 12a shows a fragment from the test sample cooled at 100 °C/ min, which contains micro-crystals. Under TEM analysis, utilizing bright field imaging, the fragments appear “grainy” and “speckled”, which indicates that at this cooling rate the test sample is still quite inhomogeneous and the ZBLAN glass matrix is scattered with a plethora of micro and nano-sized crystals. Fig. 12b displays a diffraction pattern taken from a fragment of the 100 °C/min cooled test sample. The rings of this pattern were indexed and matched to a barium fluoride crystallite (Table 3).
Fig. 4. Low magnification of polished 5 °C/min ZBLAN test sample, crystal phase with a “light feathery” morphology can be seen spreading throughout the test sample.
3.3.2. Test samples cooled at 900 °C/min The bright field image in Fig. 13a displays a fragment from the test sample cooled at 900 °C/min. The texture of this fragment has a more even consistency and is more uniform in appearance compared to the 100 °C/min test sample. However, the test sample still contains traces of nano-crystals, as is evidenced in the corresponding diffraction pattern in Fig. 13b, which shows low level crystalline diffraction. The diffraction pattern of Fig. 13b is markedly different to the one in Fig. 12b, where the rings seen in the diffraction pattern of Fig. 12b (the 100 °C/min cooled test sample) have disappeared. The spots in the diffraction pattern of Fig. 13b correlate to a ring position that have a dspacing of length 3.64 Å, 2.21 Å and 1.87 Å. Based on the matches found using PDF 4 +™ software, these d-spacings correlate with an AlF3 structure.
there are no light coloured specs, is the amorphous region. On the left side of the image, the “feathery” crystal phase is visible, however the growth of this phase abruptly stops half way across the image. 3.2.4. Test samples cooled at 100 °C/min The ZBLAN test samples cooled at a 100 °C/min were primarily amorphous, and these amorphous regions appear in the SEM images as uniform grey areas with no distinct shapes or features. Fig. 10 shows a polished cross section of the test sample, where the uniform grey area is the amorphous part of the ZBLAN glass matrix. The dark area in the bottom left corner is the epoxy resin that the test sample fragments are embedded in to be polished. Highlighted in the red square are dark grey specs, which are crystallites. An EDS analysis of the crystallites revealed that the crystallites contain high levels of aluminium and low levels of lanthanum and in particular, barium. The crystallites observed in Fig. 10 are most likely to be aluminium trifluoride crystals, AlF3 (as corroborated by TEM analysis). As can be seen by the scale ruler, these crystallites can range between 1 and 4 μm in length.
3.3.3. Test samples cooled at 4000 & 8000 °C/min For the test samples cooled at a higher rate (4000 °C/min and above), the diffraction pattern displays what is considered a typical “amorphous halo” and intense band, as can be seen in Fig. 14b. Fig. 5. Backscatter image of 5 °C/min ZBLAN test sample with different crystal phases highlighted.
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possible to see how uniform the fragments actually are, that at a higher magnification there are no discernible irregularities. Fig. 15b is a HRTEM (high resolution TEM) image, which shows how highly disordered the structure of the test sample is when cooled rapidly. There are no signs of ordered clusters, which strongly corroborates the test sample's fully glassy nature as seen in the uniformity in the appearance of the other bright field images in Figs. 14a and 15a.
4. Discussion The X-ray diffractograms indicate the test samples are heavily crystalline when cooled at the cooling rate of 5 °C/min. Multiple strong distinct peaks can be seen with high intensities, which indicates a multiphase crystalline structure throughout the glass. As the cooling rate increases, these peaks diminish in intensity, but the number of peaks and their location remain consistent (see Fig. 2). This level of consistency is corroborated by the images captured by the SEM and the EDS spectra, with crystalline phases matching those identified in literature by Battezzati and Baricco [5], Carter et al. [6] and Gerard De Leede [3]. The assigned peaks primarily relate to two crystal phases. The first phase is a zirconium and barium rich phase, while the other is a zirconium and sodium rich phase. For this experiment, the peaks that were observed in the X-ray diffractograms (see Fig. 1) were identical to those observed in a paper by Battezzati and Baricco [5]. The X-ray diffractograms of the test samples processed with cooling rates faster than 900 °C/min all appear identical, with a typical “broad amorphous hump” (see Fig. 3). This would suggest that the samples are fully amorphous, however there may still be micro- and nano-crystallites present. After a certain cooling rate, XRD is no longer able to detect the presence of crystallites, this detection limit is due to the difficulty of detecting signals from a low concentration of sub-micron sized crystallites. When the SEM images from each cooling rate are put side by side, as seen in Fig. 8, it is observable how an increasing cooling rate impedes the growth of a crystal. Fig. 9 visibly illustrates the suppressive effect cooling rate has on crystallization, where the growth of the “feather” crystal phase stretches only through the left side of the SEM image for the 20 °C/min cooled test sample. This SEM image shows the growth of this crystal phase extending dendritically, akin to snowflake formation, but then the growth halts abruptly, leaving segments of the ZBLAN matrix still in an amorphous state. The TEM analysis was able to probe for crystallites in the sub-micron range, thus it was able to distinguish the test samples that were truly amorphous, absent of even nano-crystalline inclusions. The bright field images of the 900 °C/min test samples were more uniform in appearance (see Fig. 13a) when compared to the bright field images of the 100 °C/min test samples (see Fig. 12a). However, the diffraction pattern
Fig. 6. SEM image of 20 °C/min cooled ZBLAN test sample at low magnification.
Fig. 7. Backscatter image of 20 °C/min ZBLAN test sample with different crystal phases highlighted.
The bright field images in Figs. 14a and 15a also show that the test samples are completely homogenous in texture, with no specs or different shaded regions. This uniformity in appearance is indicative of a regular glass matrix that is entirely amorphous throughout. The bright field image in Fig. 15a was taken at a higher magnification, where it is
Fig. 8. Coloured regions showing various crystal phases for (left) a test sample cooled at 5 °C/min, (right) test sample cooled at 20 °C/min. Both images are to equal scale. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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of the 900 °C/min test sample (see Fig. 13b) still contains distinct spots in the intense band of the pattern. These spots are part of a ring pattern, but the lack of a full ring indicates that the concentration of crystallites in this test sample is significantly reduced. The spots are broader than the widths of the rings for the 100 °C/min diffraction pattern (see Fig. 12b), which suggests that the grain size of these crystallites must be significantly smaller. Larger grains result in narrow rings, while smaller grains result in broad rings or spots as observed in the 900 °C/min diffraction pattern of Fig. 13b. The bright field and HRTEM (high resolution TEM) images (see Figs. 14a and 15a, b) of the 4000 °C/min test sample indicate that the ZBLAN test sample processed at this cooling rate has a disordered molecular arrangement. The presence of the long range order observed from the diffraction patterns of the 100 and 900 °C/min test samples are completely absent in the diffraction pattern for the 4000 °C/min test sample (see Fig. 14b), as is evidenced by the disappearance of the spots and ring patterns, only an intense band and a “diffuse halo” typical of an amorphous material remains. Typically, amorphous phases can be interpreted as structures composed of ultrafine random nano-clustered structures with characteristic cluster size of 10–30 atoms only [7]. The 4000 °C/min ZBLAN test sample can be considered free of any crystallite inclusions, without ordered structures larger than 3 nm. The short range structure would relate to the polyhedral arrangements of the 5 heavy metal constituents and their fluoride counterparts. Raman analysis performed on this test sample yielded spectra almost identical to the spectra observed in a study by Kawamato and Sakaguchi [8], where they studied a number of different fluorozirconate glasses. They determined that the basic structure of the vitreous forms of these compounds are chains of ZrF8 dodecahedra, which would correspond to the short range order observed in the 4000 °C/min test sample. The results of these various microscopy and diffraction analysis techniques indicate a strong correlation between the impediment of the crystallization process and the cooling rate of the test samples. As the samples are cooled faster, the rate of diffusion of the molecules (which are in in a disordered arrangement in the molten state) decreases as well, therefore there is not sufficient time for the molecules to rearrange to the most energetically stable state (the crystalline state for ZBLAN). The growth of the crystal phases is strongly dependent on the diffusion rates, the rate of cooling and the availability of nucleation sites for crystallization. When the test samples were cooled slowly, with a high presence of nucleation sites (mostly heterogeneous), many small grains proliferated, were able to grow and agglomerated into large crystals. This growth mechanism is common to most highly crystal forming materials [9,10]. With an increase in the cooling rate, and a high presence of nucleation sites, instead of large crystals forming, the ZBLAN material formed a large number of small grains, resulting in micro-crystals. With a sufficiently fast cooling rate, the diffusion rate is restrictive to the point that the nuclei are unable to grow, effectively resulting in the suppression of crystallization. There have been various studies in literature examining the importance of diffusion mechanisms in the phenomenon of crystallization, most noticeably in a study by Sestak [11]. With only the sparse formation of nuclei, this creates a “glassy” matrix that can be considered fully amorphous or “nano-crystalline” (the delineation between the two however can be difficult to substantiate). Ultimately, if it were possible to “eliminate” nucleation sites entirely, a test sample could be cooled slowly and still result in a “true glass”.
Fig. 9. 20 °C/min ZBLAN test sample, left hand side of image shows area of test sample with crystallization, right hand side is amorphous.
Fig. 10. SEM image of AlF3 crystallites scattered throughout the ZBLAN glass matrix of 100 °C/min test sample.
5. Conclusions Table 4 summarizes the cooling rates and their corresponding effect on crystal size. The greater the cooling rate is (with the same level of undercooling from the melt temperature), the more crystal growth is impeded. In the
Fig. 11. High magnification of 900 °C/min ZBLAN test sample.
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Fig. 12. a) Bright field image of a ZBLAN fragment cooled at 100 °C/min containing many crystallites. b) The ZBLAN fragment sits on top of a carbon grid of the TEM sample holder.
Fig. 13. a) Bright field image of a 900 °C/min ZBLAN test sample fragment, which appears more consistent in texture however there are still “speckles” that could potentially be nano-crystals. b) Diffraction pattern of 900 °C/min test sample, with spots belonging to an AlF3 ring pattern.
Fig. 14. a). Bright field images of 4000 °C/min cooled ZBLAN test sample. b) Diffraction pattern of 4000 °C/min test sample, showing an amorphous structure. Test samples cooled at 8000 °C/ min had identical diffraction patterns.
Fig. 15. a) Bright field image of 4000 °C/min ZBLAN test sample at higher magnification, the darker shaded region shows how uniform the ZBLAN test sample fragment is. b) HRTEM image of disordered nature of 4000 °C/min ZBLAN test sample.
order of magnitude of 101 °C/min cooling rate, the crystals grow to a length in the hundreds of micrometres. In the order of magnitude of 102 °C/min cooling rate, crystal growth is significantly impeded and only crystallites in the single digit range of micrometres form. It is only in the order of magnitude of 103, in the thousands of °C/min, that the
test samples become fully amorphous with the exclusion of even nanocrystallites. By using an approach that combines a condition as stated in a study by Lysenko et al. [12] and nucleation data from Lu et al. [13], a theoretical critical cooling rate was determined to be 1081 °C/min [2], which substantiates the empirical findings of this study. 312
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agencies in the public, commercial, or not-for-profit sectors.
Table 4 Summary of cooling rate and its effect on crystal size. Cooling rate (°C/ min)
Crystallization observed?
Crystal size
5 °C/min
Yes
20 °C/min
Yes
100 °C/min 900 °C/min 4000 °C/min 8000 °C/min
Yes Yes No No
Large crystals in excess of 100 μm Medium crystals in range of 2–30 μm Micro-crystals < 10 μm Nano-crystals Fully amorphous structure Fully amorphous structure
References [1] T.C. Ong, T. Steinberg, E. Jaatinen, J. Bell, B. Fogarty, Suppression of Crystallization in ZBLAN Glass by Rapid Heating and Cooling Processing, (2017) (Manuscript in Preparation). [2] T.C. Ong, T. Steinberg, E. Jaatinen, J. Bell, Theoretical Estimation of a Critical Cooling Rate and Attenuation Loss for Amorphous ZBLAN Glass, (2017) (Manuscript in Preparation). [3] G.D. Leede, Crystallization Behaviour of a Fluorozirconate Glass, Eindhoven University of Technology, 1989, p. 64. [4] Variable Pressure or Low Vacuum Scanning Electron Microscopy (LVSEM), Australian Microscopy & Microanalysis Research Facility, 2013, http://www. ammrf.org.au/myscope/sem/background/practical/types/lvsem.php , Accessed date: 31 October 2017. [5] L. Battezzati, M. Baricco, An experimental study of thermodynamic properties in a ZBLAN glass-forming system, Mater. Sci. Eng. A 133 (1991) 584–587. [6] S.F. Carter, P.W. France, M.W. Moore, J.M. Parker, A.G. Clare, The crystallization of a ZrF4—BaF2—LaF3—A1F3—NaF—pbF2 core glass for infrared fibers, Phys. Chem. Glasses 28 (5) (1987) 188. [7] Z. Czigany, L. Hultman, Interpretation of electron diffraction patterns from amorphous and fullerene-like carbon allotropes, Ultramicroscopy 110 (2010) 815–819. [8] Y. Kawamoto, F. Sakaguchi, Thermal properties and Raman spectra of crystalline and vitreous BaZrF6, PbZrF6 and SrZrF6, Bull. Chem. Soc. Jpn. 56 (1983) 2138–2141. [9] S. Martini, M.L. Herrera, R.W. Hartel, Effect of cooling rate on nucleation behaviour of milk fat-sunflower oil blends, J. Agric. Food Chem. 49 (2001) 3223–3229. [10] M.Z. Sanchez, V.B.F. Mathot, G.V. Poel, J.L.G. Ribelles, Effect of the cooling rate on the nucleation kinetics of poly(L-lactic acid) and its influence on morphology, Macromolecules 40 (2007) 7989–7997. [11] J. Sestak, The applicability of DTA to the study of crystallization kinetics of glasses, Phys Chem Glasses, Institute of Solid State Physics of the Czechoslovak Academy of Sciences, Prague, 1974. [12] A.B. Lysenko, I.V. Zaborulko, T.V. Kalinina, A.A. Kazantseva, Conditions of crystal nucleation processes suppression at the quenching from a liquid state, Phys. Chem. Solid State 14 (4) (2013) 886–890. [13] G. Lu, P. Hart, I. Aggarwal, Phys. Chem. Glasses 31 (1990) 205. [14] A.R. Forouhi, I. Bloomer, Optical dispersion relations for amorphous semiconductors and amorphous dielectrics, Phys. Rev. B 34 (10) (1986).
The optical constants up to 1800 nm wavelength for the fully amorphous ZBLAN test samples were then characterized using a combination of refractive index matching liquids, ellipsometer data and an analysis approach (utilizing Kramers-Kronig relations) by Forouhi and Bloomer [14]. It was estimated that for the fully amorphous ZBLAN test samples (cooled at 4000 °C/min), they have an attenuation loss of 0.092 dB/Km at 1800 nm wavelength [2], which although is not at the theoretical best, is still a marked improvement on the attenuation loss of ZBLAN fibers to date. If further data could be collected further into the mid and deep IR wavelengths for this fully amorphous test sample, the attenuation loss could have reached as low as 0.05 dB/km at 2200 nm. The next step in this research would be to determine a method of drawing ZBLAN fibers with a rapid cooling rate. This fiber would then need to be tested to determine its performance capabilities for long haul fiber optic applications. Although there may be many complications in processing ZBLAN, from the conclusions based on the promising results of this study, turning ZBLAN glass into a viable material as an optical waveguide for fiber optic technology is possible. Funding This research did not receive any specific grant from funding
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Suppression effects of cooling rate on crystallization in ZBLAN glass a,⁎
a
b
Teng-Cheong Ong , Ted Steinberg , Esa Jaatinen , John Bell a b c
T
c
Department of Energy and Process Engineering, Queensland University of Technology, 2 George St., 4000 Brisbane, Australia Department of Nanotechnology and Molecular Science, Queensland University of Technology, 2 George St., 4000 Brisbane, Australia Department of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, 2 George St., 4000 Brisbane, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords: Crystallization ZBLAN Rapid cooling Critical cooling rate Microscopy Diffraction experiment
ZBLAN glass is a heavy metal fluoride glass that shows great potential in the application of long-haul telecommunication cables. However, during processing in the fiber-drawing temperature region, the material tends to undergo heavy devitrification, resulting in a crystalline fiber that is not usable as an optical waveguide. In this study, ZBLAN glass was processed with different cooling rates to discern how the rate of cooling affects the process of crystallization. Rapidly cooled ZBLAN glass test samples were then analysed with a wide range of spectroscopy and imaging techniques including EDS, SEM, TEM and XRD. These techniques revealed there are two predominant crystal phases, one rich in zirconium and barium, the other in zirconium and sodium. Ultimately, a critical cooling rate was established to be somewhere between 900 °C/min and 4000 °C/min, with a theoretically predicted value of 1081 °C/min for a test sample volume size of 9.4 × 10− 8 m3. Cooling above this critical cooling rate yielded fully amorphous ZBLAN test samples that were completely free of nano-crystalline inclusions. Using Kramers-Kronig relations, an attenuation loss for the fully amorphous ZBLAN test samples was estimated to be 0.09 dB/Km at a wavelength of 1800 nm.
1. Introduction Through years of research, there have been many papers published exploring the material properties of ZBLAN glass and various processing techniques were attempted with the aims of achieving a material that can transmit with the theoretical minimum attenuation losses predicted for ZBLAN. As ZBLAN glass is cooled from its melt, crystallites form throughout the medium, their size and structure dependent on the rate of cooling and degree of undercooling. These crystallites act as scattering centres that degrade optical transmission through the glass, resulting in attenuation losses. This study explored various processing techniques with different cooling rates to determine the effect they have on the suppression of crystallization. Ultimately, a critical cooling rate was determined, a rate that results in the complete suppression of even nano-crystallite formation in the glass. 2. Experimental procedure ZBLAN test samples were heated and cooled with 6 different cooling rates. The samples were heated in various ways, depending on the cooling method implemented. A heating rate of 20 °C/min was applied for all test samples, and immediately after the maximum target temperature of 500 °C was reached, the test samples were cooled
⁎
immediately. For the cooling rates of 5 °C/min and 20 °C/min, a controlled heating environment was required. These test samples were processed entirely in a DSC, which could be cooled at a slow and steady rate. The majority of the other test samples were heated in a conventional furnace, and then either air cooled or quenched in different media to achieve the required cooling rates. Lastly, test samples were also processed in a device (which was named an “REPD”, Rapid Electrothermal Processing Device) that was built to rapidly heat and cool the test samples [1]. All of the samples were cooled to just above room temperature. The following test matrix in Table 1 is an outline of the heating and cooling regimes used to process the samples. All cooling rates (CR) were taken by the following relation,
CR =
Tm − Tg , Δtm – g
(1)
where Tm is the melting temperature, 450 °C, Tg is the glass transition temperature, 260 °C, and Δtm–g is the time interval for the test sample to cool between the two temperatures (in seconds). The percentage error of the cooling rate measurements was low, ranging from 0.031 to 6.3% for the first 5 thermal profiles, the 5–4000 °C/min cooling rates. For the last thermal profile, the 8000 °C/min cooling rate, the percentage error was higher at 35%.
Corresponding author. E-mail address: [email protected] (T.-C. Ong).
https://doi.org/10.1016/j.jnoncrysol.2017.11.003 Received 20 June 2017; Received in revised form 31 October 2017; Accepted 2 November 2017 Available online 01 December 2017 0022-3093/ © 2017 Elsevier B.V. All rights reserved.
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Table 1 Experimental test matrix. Thermal profile no.
Heating rate (°C/ min)
Maximum temperature (°C)
Method of heating
Cooling rate (°C/ min)
Method of cooling
Analysis techniquesa
1 2 3 4
20 20 20 20
500 500 500 500
DSC heated DSC heated Furnace heated Furnace heated
5 20 100 900
XRD, XRD, XRD, XRD,
5
20
500
REPD heated
4000
DSC cooled DSC cooled Air-cooled Quenched in soapy water REPD cooled
6
20
500
Furnace heated
8000
Quenched in water
SEM, SEM, SEM, SEM,
EDS EDS TEM, EDS TEM, EDS
XRD, SEM, TEM, EDS, Ellipsometry SEM, TEM, EDS, Ellipsometry
a Analysis techniques acronyms: XRD - X-ray diffraction, SEM - Scanning Electron Microscopy, TEM - Transmission Electron Microscopy, EDS - Energy Dispersive Spectroscopy, REPD Rapid Electro-thermal Processing Device.
For the characterization of the structure and molecular composition of the test samples, XRD, EDS and TEM analyses were used. For characterizing the crystallites in the micro-sized region (and larger), EDS and XRD were used, since the peaks in the spectra and patterns they generate can be matched with various crystal phases. For crystallites in the sub-micron range, especially the nano-sized particles (observed in the high cooling rates), TEM was particularly useful, where the diffraction patterns were used to identify the nano- and micro-crystalline phases. SEM and TEM imaging techniques were used to image the crystals themselves, where backscatter imaging (SEM) and dark field/bright field imaging (TEM) made it possible to identify the different crystal phases and individual crystallites. The X-ray diffractograms were primarily used to observe the change in degree of crystal/amorphous content in each test sample. Ellipsometry was used (in conjunction with an analysis approach that incorporates Kramers-Kronig relations) for the fully amorphous test samples to determine the attenuation losses [2]. The ZBLAN material used in this experiment was provided courtesy of THORLABS, an optical equipment company based in New Jersey, United States of America.
Table 2 List of peaks and attributed phases for X-ray diffractogram in Fig. 1. Phase ZrNaF5
β-ZrBaF6
Na Ba Zr2 F11 Na7 Zr6 F31
Symbol in Fig. 1
2θ
d-spacing (Å)
23.52 26.53 54.51 33.13 51.13 55.58 62.74 66.27
5.57 3.86 1.99 3.13 2.08 1.97 1.72 1.63
3.2. SEM results 3.2.1. Test samples cooled at 5 °C/min For this cooling rate, crystal phases of distinct morphology can be observed spreading throughout the entire test sample (only a small proportion of the sample remains in an amorphous glassy state). Fig. 4 shows an image of the crystallization pattern formed by the various crystal phases at a low magnification. Fig. 5 is a higher magnification back scatter image that shows distinct morphologies, a “feathery” type and a “bow tie/V-shaped” type [3], surrounded by a third less distinct morphology of regions of darker colouration with “black specs”. The contrast in shade indicates the difference in atomic number of each phase, heavier elements (with high atomic number) backscatter electrons more than lighter elements (with low atomic number), and therefore appear brighter in the image. Therefore “whiter” or “lighter coloured” regions in a backscatter image are composed of elements with a higher average atomic number. “Darker coloured” regions are comprised of elements that have a lower
3. Results and discussion 3.1. XRD results The primary peaks that appear in the XRD pattern of the slower cooled test samples can be seen in Fig. 1, and the phases they relate to are outlined in Table 2. Fig. 2 displays an overlay of three XRD patterns, for the following cooling rates; 5 °C/min (green), 20 °C/min (blue), 100 °C/min (red).
Fig. 1. X-ray diffractogram for 20 °C/min cooled ZBLAN test sample labelled with assigned crystal phases (shown in Table 2) as identified using the PDF-4+™ database.
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Fig. 2. Overlay of X-ray diffractograms for test samples of 3 different cooling rates, 5, 20 and 100 °C/min. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
3.2.2. Test samples cooled at 20 °C/min Fig. 6 shows a polished cross section of the ZBLAN test sample cooled at 20 °C/min. At first glance, the light coloured feathery morphology can be seen visibly, however this crystal phase is much smaller in size than the corresponding phase observed in the 5 °C/min sample. The backscatter image of Fig. 7 shows the same three morphologies of the 5 °C/min cooled test sample (as seen in Fig. 5), except each phase is smaller in size. The different phases have been highlighted in Fig. 8 to illustrate the change in size between the two cooling rates (light feathery morphology in green, dark regions in red and V-shaped in blue). Take note, the following side by side images are to equal magnification, therefore the highlighted regions show a true scale of the actual size difference.
Table 3 Indexing of diffraction pattern of Fig. 12b. Ring designation
Crystal species
d-spacing (Å)
(hkl)
1st 2nd 3rd 4th 5th
Ba Ba Ba Ba Ba
2.126 1.532 1.243 0.958 0.873
110 202 212 312 402
F2 F2 F2 F2 F2
average atomic number [4]. The light grey regions (“feather” crystal phase) of Fig. 5 contain high levels of zirconium and barium, with a deficiency in sodium, and are most likely to be a zirconium barium crystal matrix, Zr2BaF10. The atoms of this crystal phase have a much larger nucleus, therefore appear lighter in colour in the backscatter image. The regions with a bowtie/V-shaped morphology have a typical ZBLAN glass composition, which is reflected in the medium grey colouration in the backscatter image. The darkest coloured regions with black specs correlate to a zirconium sodium fluoride crystal phase, possibly NaZrF6. It is a region containing high levels of zirconium and sodium, and a deficiency in barium.
3.2.3. Summary of change in size of crystal phases (5 °C/min compared to 20 °C/min) The light feather morphology decreases in size from 500 to 30 μm, which correlates to a 90% decrease in size. Similarly, the “dark regions with black specs” crystal phase shows a 70% decrease in size and the Vshaped region shows the greatest change, a 95% decrease in size. Fig. 9 shows part of the 20 °C/min cooled test sample that has remained in an amorphous state. On the right side of the image, where
Fig. 3. X-ray diffractogram for 4000 and 8000 °C/min test sample.
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3.2.5. Test samples cooled at 900, 4000 & 8000 °C/min The SEM image of the 900 °C/min test sample in Fig. 11 displays a polished cross section of the test sample that is uniformly grey in appearance with no discernible features. This suggests the entire test sample is completely amorphous, however the 900 °C/min test samples still show evidence of nano-crystal inclusions (however, they are not visible under SEM), but are only detectable using TEM analysis. 3.3. TEM results 3.3.1. Test samples cooled at 100 °C/min Fig. 12a shows a fragment from the test sample cooled at 100 °C/ min, which contains micro-crystals. Under TEM analysis, utilizing bright field imaging, the fragments appear “grainy” and “speckled”, which indicates that at this cooling rate the test sample is still quite inhomogeneous and the ZBLAN glass matrix is scattered with a plethora of micro and nano-sized crystals. Fig. 12b displays a diffraction pattern taken from a fragment of the 100 °C/min cooled test sample. The rings of this pattern were indexed and matched to a barium fluoride crystallite (Table 3).
Fig. 4. Low magnification of polished 5 °C/min ZBLAN test sample, crystal phase with a “light feathery” morphology can be seen spreading throughout the test sample.
3.3.2. Test samples cooled at 900 °C/min The bright field image in Fig. 13a displays a fragment from the test sample cooled at 900 °C/min. The texture of this fragment has a more even consistency and is more uniform in appearance compared to the 100 °C/min test sample. However, the test sample still contains traces of nano-crystals, as is evidenced in the corresponding diffraction pattern in Fig. 13b, which shows low level crystalline diffraction. The diffraction pattern of Fig. 13b is markedly different to the one in Fig. 12b, where the rings seen in the diffraction pattern of Fig. 12b (the 100 °C/min cooled test sample) have disappeared. The spots in the diffraction pattern of Fig. 13b correlate to a ring position that have a dspacing of length 3.64 Å, 2.21 Å and 1.87 Å. Based on the matches found using PDF 4 +™ software, these d-spacings correlate with an AlF3 structure.
there are no light coloured specs, is the amorphous region. On the left side of the image, the “feathery” crystal phase is visible, however the growth of this phase abruptly stops half way across the image. 3.2.4. Test samples cooled at 100 °C/min The ZBLAN test samples cooled at a 100 °C/min were primarily amorphous, and these amorphous regions appear in the SEM images as uniform grey areas with no distinct shapes or features. Fig. 10 shows a polished cross section of the test sample, where the uniform grey area is the amorphous part of the ZBLAN glass matrix. The dark area in the bottom left corner is the epoxy resin that the test sample fragments are embedded in to be polished. Highlighted in the red square are dark grey specs, which are crystallites. An EDS analysis of the crystallites revealed that the crystallites contain high levels of aluminium and low levels of lanthanum and in particular, barium. The crystallites observed in Fig. 10 are most likely to be aluminium trifluoride crystals, AlF3 (as corroborated by TEM analysis). As can be seen by the scale ruler, these crystallites can range between 1 and 4 μm in length.
3.3.3. Test samples cooled at 4000 & 8000 °C/min For the test samples cooled at a higher rate (4000 °C/min and above), the diffraction pattern displays what is considered a typical “amorphous halo” and intense band, as can be seen in Fig. 14b. Fig. 5. Backscatter image of 5 °C/min ZBLAN test sample with different crystal phases highlighted.
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possible to see how uniform the fragments actually are, that at a higher magnification there are no discernible irregularities. Fig. 15b is a HRTEM (high resolution TEM) image, which shows how highly disordered the structure of the test sample is when cooled rapidly. There are no signs of ordered clusters, which strongly corroborates the test sample's fully glassy nature as seen in the uniformity in the appearance of the other bright field images in Figs. 14a and 15a.
4. Discussion The X-ray diffractograms indicate the test samples are heavily crystalline when cooled at the cooling rate of 5 °C/min. Multiple strong distinct peaks can be seen with high intensities, which indicates a multiphase crystalline structure throughout the glass. As the cooling rate increases, these peaks diminish in intensity, but the number of peaks and their location remain consistent (see Fig. 2). This level of consistency is corroborated by the images captured by the SEM and the EDS spectra, with crystalline phases matching those identified in literature by Battezzati and Baricco [5], Carter et al. [6] and Gerard De Leede [3]. The assigned peaks primarily relate to two crystal phases. The first phase is a zirconium and barium rich phase, while the other is a zirconium and sodium rich phase. For this experiment, the peaks that were observed in the X-ray diffractograms (see Fig. 1) were identical to those observed in a paper by Battezzati and Baricco [5]. The X-ray diffractograms of the test samples processed with cooling rates faster than 900 °C/min all appear identical, with a typical “broad amorphous hump” (see Fig. 3). This would suggest that the samples are fully amorphous, however there may still be micro- and nano-crystallites present. After a certain cooling rate, XRD is no longer able to detect the presence of crystallites, this detection limit is due to the difficulty of detecting signals from a low concentration of sub-micron sized crystallites. When the SEM images from each cooling rate are put side by side, as seen in Fig. 8, it is observable how an increasing cooling rate impedes the growth of a crystal. Fig. 9 visibly illustrates the suppressive effect cooling rate has on crystallization, where the growth of the “feather” crystal phase stretches only through the left side of the SEM image for the 20 °C/min cooled test sample. This SEM image shows the growth of this crystal phase extending dendritically, akin to snowflake formation, but then the growth halts abruptly, leaving segments of the ZBLAN matrix still in an amorphous state. The TEM analysis was able to probe for crystallites in the sub-micron range, thus it was able to distinguish the test samples that were truly amorphous, absent of even nano-crystalline inclusions. The bright field images of the 900 °C/min test samples were more uniform in appearance (see Fig. 13a) when compared to the bright field images of the 100 °C/min test samples (see Fig. 12a). However, the diffraction pattern
Fig. 6. SEM image of 20 °C/min cooled ZBLAN test sample at low magnification.
Fig. 7. Backscatter image of 20 °C/min ZBLAN test sample with different crystal phases highlighted.
The bright field images in Figs. 14a and 15a also show that the test samples are completely homogenous in texture, with no specs or different shaded regions. This uniformity in appearance is indicative of a regular glass matrix that is entirely amorphous throughout. The bright field image in Fig. 15a was taken at a higher magnification, where it is
Fig. 8. Coloured regions showing various crystal phases for (left) a test sample cooled at 5 °C/min, (right) test sample cooled at 20 °C/min. Both images are to equal scale. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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of the 900 °C/min test sample (see Fig. 13b) still contains distinct spots in the intense band of the pattern. These spots are part of a ring pattern, but the lack of a full ring indicates that the concentration of crystallites in this test sample is significantly reduced. The spots are broader than the widths of the rings for the 100 °C/min diffraction pattern (see Fig. 12b), which suggests that the grain size of these crystallites must be significantly smaller. Larger grains result in narrow rings, while smaller grains result in broad rings or spots as observed in the 900 °C/min diffraction pattern of Fig. 13b. The bright field and HRTEM (high resolution TEM) images (see Figs. 14a and 15a, b) of the 4000 °C/min test sample indicate that the ZBLAN test sample processed at this cooling rate has a disordered molecular arrangement. The presence of the long range order observed from the diffraction patterns of the 100 and 900 °C/min test samples are completely absent in the diffraction pattern for the 4000 °C/min test sample (see Fig. 14b), as is evidenced by the disappearance of the spots and ring patterns, only an intense band and a “diffuse halo” typical of an amorphous material remains. Typically, amorphous phases can be interpreted as structures composed of ultrafine random nano-clustered structures with characteristic cluster size of 10–30 atoms only [7]. The 4000 °C/min ZBLAN test sample can be considered free of any crystallite inclusions, without ordered structures larger than 3 nm. The short range structure would relate to the polyhedral arrangements of the 5 heavy metal constituents and their fluoride counterparts. Raman analysis performed on this test sample yielded spectra almost identical to the spectra observed in a study by Kawamato and Sakaguchi [8], where they studied a number of different fluorozirconate glasses. They determined that the basic structure of the vitreous forms of these compounds are chains of ZrF8 dodecahedra, which would correspond to the short range order observed in the 4000 °C/min test sample. The results of these various microscopy and diffraction analysis techniques indicate a strong correlation between the impediment of the crystallization process and the cooling rate of the test samples. As the samples are cooled faster, the rate of diffusion of the molecules (which are in in a disordered arrangement in the molten state) decreases as well, therefore there is not sufficient time for the molecules to rearrange to the most energetically stable state (the crystalline state for ZBLAN). The growth of the crystal phases is strongly dependent on the diffusion rates, the rate of cooling and the availability of nucleation sites for crystallization. When the test samples were cooled slowly, with a high presence of nucleation sites (mostly heterogeneous), many small grains proliferated, were able to grow and agglomerated into large crystals. This growth mechanism is common to most highly crystal forming materials [9,10]. With an increase in the cooling rate, and a high presence of nucleation sites, instead of large crystals forming, the ZBLAN material formed a large number of small grains, resulting in micro-crystals. With a sufficiently fast cooling rate, the diffusion rate is restrictive to the point that the nuclei are unable to grow, effectively resulting in the suppression of crystallization. There have been various studies in literature examining the importance of diffusion mechanisms in the phenomenon of crystallization, most noticeably in a study by Sestak [11]. With only the sparse formation of nuclei, this creates a “glassy” matrix that can be considered fully amorphous or “nano-crystalline” (the delineation between the two however can be difficult to substantiate). Ultimately, if it were possible to “eliminate” nucleation sites entirely, a test sample could be cooled slowly and still result in a “true glass”.
Fig. 9. 20 °C/min ZBLAN test sample, left hand side of image shows area of test sample with crystallization, right hand side is amorphous.
Fig. 10. SEM image of AlF3 crystallites scattered throughout the ZBLAN glass matrix of 100 °C/min test sample.
5. Conclusions Table 4 summarizes the cooling rates and their corresponding effect on crystal size. The greater the cooling rate is (with the same level of undercooling from the melt temperature), the more crystal growth is impeded. In the
Fig. 11. High magnification of 900 °C/min ZBLAN test sample.
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Fig. 12. a) Bright field image of a ZBLAN fragment cooled at 100 °C/min containing many crystallites. b) The ZBLAN fragment sits on top of a carbon grid of the TEM sample holder.
Fig. 13. a) Bright field image of a 900 °C/min ZBLAN test sample fragment, which appears more consistent in texture however there are still “speckles” that could potentially be nano-crystals. b) Diffraction pattern of 900 °C/min test sample, with spots belonging to an AlF3 ring pattern.
Fig. 14. a). Bright field images of 4000 °C/min cooled ZBLAN test sample. b) Diffraction pattern of 4000 °C/min test sample, showing an amorphous structure. Test samples cooled at 8000 °C/ min had identical diffraction patterns.
Fig. 15. a) Bright field image of 4000 °C/min ZBLAN test sample at higher magnification, the darker shaded region shows how uniform the ZBLAN test sample fragment is. b) HRTEM image of disordered nature of 4000 °C/min ZBLAN test sample.
order of magnitude of 101 °C/min cooling rate, the crystals grow to a length in the hundreds of micrometres. In the order of magnitude of 102 °C/min cooling rate, crystal growth is significantly impeded and only crystallites in the single digit range of micrometres form. It is only in the order of magnitude of 103, in the thousands of °C/min, that the
test samples become fully amorphous with the exclusion of even nanocrystallites. By using an approach that combines a condition as stated in a study by Lysenko et al. [12] and nucleation data from Lu et al. [13], a theoretical critical cooling rate was determined to be 1081 °C/min [2], which substantiates the empirical findings of this study. 312
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agencies in the public, commercial, or not-for-profit sectors.
Table 4 Summary of cooling rate and its effect on crystal size. Cooling rate (°C/ min)
Crystallization observed?
Crystal size
5 °C/min
Yes
20 °C/min
Yes
100 °C/min 900 °C/min 4000 °C/min 8000 °C/min
Yes Yes No No
Large crystals in excess of 100 μm Medium crystals in range of 2–30 μm Micro-crystals < 10 μm Nano-crystals Fully amorphous structure Fully amorphous structure
References [1] T.C. Ong, T. Steinberg, E. Jaatinen, J. Bell, B. Fogarty, Suppression of Crystallization in ZBLAN Glass by Rapid Heating and Cooling Processing, (2017) (Manuscript in Preparation). [2] T.C. Ong, T. Steinberg, E. Jaatinen, J. Bell, Theoretical Estimation of a Critical Cooling Rate and Attenuation Loss for Amorphous ZBLAN Glass, (2017) (Manuscript in Preparation). [3] G.D. Leede, Crystallization Behaviour of a Fluorozirconate Glass, Eindhoven University of Technology, 1989, p. 64. [4] Variable Pressure or Low Vacuum Scanning Electron Microscopy (LVSEM), Australian Microscopy & Microanalysis Research Facility, 2013, http://www. ammrf.org.au/myscope/sem/background/practical/types/lvsem.php , Accessed date: 31 October 2017. [5] L. Battezzati, M. Baricco, An experimental study of thermodynamic properties in a ZBLAN glass-forming system, Mater. Sci. Eng. A 133 (1991) 584–587. [6] S.F. Carter, P.W. France, M.W. Moore, J.M. Parker, A.G. Clare, The crystallization of a ZrF4—BaF2—LaF3—A1F3—NaF—pbF2 core glass for infrared fibers, Phys. Chem. Glasses 28 (5) (1987) 188. [7] Z. Czigany, L. Hultman, Interpretation of electron diffraction patterns from amorphous and fullerene-like carbon allotropes, Ultramicroscopy 110 (2010) 815–819. [8] Y. Kawamoto, F. Sakaguchi, Thermal properties and Raman spectra of crystalline and vitreous BaZrF6, PbZrF6 and SrZrF6, Bull. Chem. Soc. Jpn. 56 (1983) 2138–2141. [9] S. Martini, M.L. Herrera, R.W. Hartel, Effect of cooling rate on nucleation behaviour of milk fat-sunflower oil blends, J. Agric. Food Chem. 49 (2001) 3223–3229. [10] M.Z. Sanchez, V.B.F. Mathot, G.V. Poel, J.L.G. Ribelles, Effect of the cooling rate on the nucleation kinetics of poly(L-lactic acid) and its influence on morphology, Macromolecules 40 (2007) 7989–7997. [11] J. Sestak, The applicability of DTA to the study of crystallization kinetics of glasses, Phys Chem Glasses, Institute of Solid State Physics of the Czechoslovak Academy of Sciences, Prague, 1974. [12] A.B. Lysenko, I.V. Zaborulko, T.V. Kalinina, A.A. Kazantseva, Conditions of crystal nucleation processes suppression at the quenching from a liquid state, Phys. Chem. Solid State 14 (4) (2013) 886–890. [13] G. Lu, P. Hart, I. Aggarwal, Phys. Chem. Glasses 31 (1990) 205. [14] A.R. Forouhi, I. Bloomer, Optical dispersion relations for amorphous semiconductors and amorphous dielectrics, Phys. Rev. B 34 (10) (1986).
The optical constants up to 1800 nm wavelength for the fully amorphous ZBLAN test samples were then characterized using a combination of refractive index matching liquids, ellipsometer data and an analysis approach (utilizing Kramers-Kronig relations) by Forouhi and Bloomer [14]. It was estimated that for the fully amorphous ZBLAN test samples (cooled at 4000 °C/min), they have an attenuation loss of 0.092 dB/Km at 1800 nm wavelength [2], which although is not at the theoretical best, is still a marked improvement on the attenuation loss of ZBLAN fibers to date. If further data could be collected further into the mid and deep IR wavelengths for this fully amorphous test sample, the attenuation loss could have reached as low as 0.05 dB/km at 2200 nm. The next step in this research would be to determine a method of drawing ZBLAN fibers with a rapid cooling rate. This fiber would then need to be tested to determine its performance capabilities for long haul fiber optic applications. Although there may be many complications in processing ZBLAN, from the conclusions based on the promising results of this study, turning ZBLAN glass into a viable material as an optical waveguide for fiber optic technology is possible. Funding This research did not receive any specific grant from funding
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