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Morphological and Physical Analysis of the EDZF

2. Fabrication and Characterization of Zirconia–Yttria–Alumino Silicate Glass-based

2.3 Characterization of the EDZFs

2.3.1 Morphological and Physical Analysis of the EDZF

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57 It can be seen that the both the ZEr-A and ZEr-B fibres are formed as expected.

Visual inspection of the lateral surfaces of the fibre reveals a smooth polymer layer, thereby indicating that there are no bubbles or other deformities present in the polymer coating.

The surfaces of the fabricated EDZFs are also examined using an Olympus BX51 operating 200x magnification. The images obtained are shown in Figure 14.

Figure 14: View of the surface of the two EDZFs (ZEr-A, above left, and ZEr-B, above right) under 200x magnification.

It can be seen that the microscopic views of the two EDZFs are very much similar, with both EDZFs having a very distinct core, surrounded by the cladding. Both EDZFs have a core diameter of approximately 10 μm, taking the internal ring of the core. It can also be seen that the core is homogeneous and shows no observable defects at the interface between the silica cladding and the core. Therefore it can be construed that in physical terms, the fabricated fibre is well within its design parameters.

However, of greater concern is the morphology of the crystalline structures formed within the EDZFs. As ZrO2 can sustain the crystalline nature of the host matrix at high temperatures, comparable to those required for the collapsing of the substrate rod and also for the drawing of the fibre, it is thus expected that the ZrO2

will retain crystalline nature in some portions of the core glass matrix of preform as well as the fibre. The core region morphology of selected ZEr-A and ZEr-B preform samples, developed without any thermal treatment or annealing, is studied using Field-Emission Gun Scanning Electron Microscopy (FEGSEM). An analysis of the

ZEr-A ZEr-B

58 microstructures in the doping region of the preforms tested is given in Figure 15. It can be seen clearly from the figure that the ZEr-B fibre, which is richer in ZrO2 micro-crystallites than the ZEr-A fibre, has micro-crystallite structures with better defined boundaries than that of the ZEr-A fibre, as is to be expected. The presence of the ZrO2

micro-crystallites is an important factor, as this will affect the non-linear characteristics of the EDZF.

Figure 15: The microstructure of the core region of optical fibre preforms A (above left) and ZEr-B (above right)14.

The presence of the ZrO2 micro-crystallites in the ZEr-B after the drawing process is confirmed by Tunneling Electron Microscopy (TEM), as shown in Figure 16.

X-Ray Diffraction (XRD) is also carried out on samples of the fibre preform, and the XRD curve obtained is shown in Figure 17. From the curve, a small diffraction peak is detected at a 2θ value of about 30°, indicating the formation of tetragonal ZrO2 in the host matrix [60]. In bulk zirconia-silicate (ZrO2–SiO2) glass, phase-separation has been observed at temperatures below the onset of crystallization that also results in structural homogeneity. This phenomenon, known as phase separation or immiscibility, is a phenomenon that is known to exist in amorphous binary systems. However in some ZrO2–SiO2 system, immiscibility exists even in the stable liquid phase above the melting point.

14 Figure adapted from that in H. Ahmad, M. C. Paul, N. A. Awang, S. W. Harun, M. Pal and K. Thambiratnam, "Four-Wave-Mixing in Zirconia-Yttria-Aluminum Erbium," J. Europ. Opt. Soc. Rap. Public., vol. 7, pp. 12011-1 - 12011-8, 2012.

1 μm 1 μm

ZEr-A ZEr-B

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Figure 16: TEM spectroscopic analysis of ZEr-B15.

Figure 17: XRD curve obtained for ZEr-B preform, with small diffraction peak at 2θ of ~30°.

The phase diagram of ZrO2–SiO2 system, obtained from the evaluation of the Fact-Sage software16 isgiven in Figure 18. It can be seen that a stable immiscibility zone exists at a molarity ratio of 60–80 mole% in favor to SiO2 (or alternatively, 40-20%

mole% in favor of ZrO2) up to temperatures of about 2250oC. The stable immiscibility zone extent to temperatures lower than the melting point and gives a metastable

15 Figure adapted from that in Chapter 2: Nano-Engineered Glass-Based Erbium Doped Optical Fibers for Study of Multi-Channel Amplification and Four-Wave Mixing Phenomena, M. C. Paul, M. Pal, K. Thambiratnam, S. W. Harun, N. A. Awang, S. Das, S. K. Badhra, H. Ahmad and J. K. Sahu. Rare Earths: New Research (Chemistry Research and Applications: Materials Science and Technologies) (Zhaosen Liu (Editor)) Nova Science Publishers Inc. (1 July 2013, Hauppauge, New York, No. of pages: 289, ISBN-10: 162618996X) 16 www.factsage.com

60 immiscibility zone in a wide composition range where phase separation normally occurs in an amorphous state.

Figure 18: Phase-diagram of SiO2-ZrO2 system obtained using the Fact-Sage software

It is worth noticing the different forms in which the ZrO2 crystallites can take in the system, depending on the temperature. At a high temperature of 2350oC and above, the ZrO2 molecules take on a cubic structure, while at a lower temperature range of between 1170oC to 2250oC, the molecules take on a tetragonal structure instead and a mono-clinic structure when the temperature is below 1170oC. This represents a serious problem in the fabrication process - the shift from the tetragonal to monoclinic phases occurs very fast and will incur an increase of between 3 to 5% in volume. This rapid increase, encountered during the cooling process as the temperature drops quickly from more than 2400oC to less than 1000oC, results in significant cracking in the drawn fibre, especially in the core region as this is where the concentration of the ZrO2 crystallites is highest and can essentially destroy the mechanical properties of the fibre. However, this problem can be overcome by adding a minute amount of Y2O3, or even oxides such as MgO, CaO, which can suppress the changes in the crystalline structure and thus preserve the integrity of the fibre. It is also expected that the separated ZrO2 and Al2O3 phase in the EDZF will mix together when heated at high temperatures. Generally, the homogeneous ZrAlxOy amorphous mixture is thermodynamically more stable than the separated two phases, and thus the two

61 separated phases ZrO2 and Al2O3 will tend to mix into a homogeneous mixture before crystallization.

In addition to the physical structure of the fibre, the concentration levels of the various dopants in the EDZFs are analyzed through Electron Probe Micro-Analysis (EPMA). The obtained results from the EPMA are given in Table 2.

Table 2: Doping levels within core region of the preforms

It can be seen from the results of the EPMA given in Table 2 that the A and ZEr-B contain almost similar amounts of Al2O3 with dopant concentrations of between 0.24-0.25 mole%. However, the ZrO2 dopant concentration in the ZEr-B fibre is higher, at 2.21 mole% as compared to only 0.65 mole% in the ZEr-A fibre. Similarly, the Er2O3 dopant concentrations for ZEr-A and ZEr-B are 0.155 and 0.225 mole%

respectively, as is to be expected.

The next section will detail the measurement and characterization of the spectral properties of the EDZF.