M. Braglia

Determination of rare earth elements by ion chromatography. Separation procedure optimization

In the development of research on materials for optical fibres production, the use of pure lanthanum compounds is very important for the manufacture of glasses, since they can play an important role for signal enhancement by fluorescence emission. For this purpose, an ion exchange chromatographic method for rare earth determination has been studied and optimized. The analytical column involved was a multimode ion exchange type and the effectiveness of ligands (namely oxalic and diglycolic acids) on lanthanides elution was investigated. Elutions were performed for ligand concentrations ranging from 0 to 80 mM and pH ranges from 3.0 to 5.5 coupled with a post-column spectrophotometric detection with 4-(2-pyridylazo)resorcinol (PAR) at 520 nm. After optimization, gradient elution enabled the determination of lanthanides with in a total analysis time of 25 min. The method developed has been applied to the analysis of YbF3 (after acidic microwave digestion) for the determination of impurities due to other rare earth elements.

Glass ceramics for optical amplifiers: rheological, thermal, and optical properties

Abstract A glass ceramic system based on a glass of nominal composition (mol%) 32.4SiO 2 –8.1Al 2 O 3 –31.4CdF 2 –5.4ZnF 2 –4.3YF 3 –18.4PbF 2 (TGC) doped with 500 ppmw of PrF 3 has been prepared and studied to explore the possibility of using it for optical fibre amplifiers operating at 1.3 μm. A comparative analysis of two samples having the same nominal composition, but obtained in different conditions, showed that one sample was completely amorphous, while the other one contained crystals of the phase Pb x Cd 1− x F 2 . Crystals of the same phase were found in the amorphous sample after isochronal and isothermal treatments, obtaining a glass ceramic system transparent in the visible range. The coarsening of the crystals investigated by DSC, TEM, XRD decreased on annealing while the photoluminescence intensity increased, indicating a change in the surrounding of Pr 3+ ions during the controlled ceramming process. Finally, based on the study of the rheological properties of the glass ceramic system we suggest that the temperature interval suitable for fibre drawing is between 572°C and 614°C.

Rheology of tellurite glasses

A series of tellurite glasses containing TeO2, ZnO, and Na2O and doped with 1000 ppmw of Er2O3 has been synthesized. The series displays good thermal stability. Based on thermal analysis results, we chose to investigate the viscosity in the temperature range 310–395°C, using Al as the best material for plates in the rheological measurements. First, the viscosity vs. strain (strain sweep) at fixed temperatures and angular frequencies was investigated. Then, having fixed the strain value, the viscosity curve vs. angular frequency was determined. Finally, the viscosity–temperature curve of all the glasses prepared was determined and fitted by different equations, including the viscosity value at the glass transition temperature, Tg, measured at a heating rate of 2°C/min. The best fit was obtained using the Cohen–Grest equation. The stability of the glasses was determined by comparing ΔT values (ΔT = Tx − Tg, with Tx being the onset crystallization temperature). For a glass containing 10 mol% Na2O, ΔT reached 176°C. The crystalline phases formed on heating were always TeO2, Na6ZnO4, and Zn2Te3O8.

Core/clad compositions with high numerical aperture for single-mode fluoride fibres

Abstract ZBLYALi core glasses modified by the substitution of LiF for BaF 2 or the addition of PbF 2 and cladding glasses of ZBLYAN composition, into which HfF 4 and NaF were substituted, are described and characterised by their thermal glass parameters and refractive index changes. Glasses studied, with T g range from 533 to 573 K, yielded numerical apertures as high as 0.39, if ZBLYALiPb glasses were used as core. Since incorporation of PbF 2 in ZBLYALi glass led to a strong decrease in glass stability, a closer look at the crystallisation behaviour of ZBLYALiPb was taken. Addition of PbF 2 was found to suppress precipitation of LiBaZr 2 F 11 in favour of a (Ba, Pb)ZrF 6 solid solution with orthorhombic structure.

Nucleation and growth of crystals in a ZBLYALiPb glass

Abstract The surface and bulk crystallisation of a ZBLYALiPb glass has been investigated. The crystalline phases grown after thermal treatments were observed by optical and scanning electron microscopy and identified by X-ray diffraction (XRD). The nucleation frequency is very low (maximum value 3×10 3 m −3 s −1 ). Growth is linear. Its rate can be described with an Arrhenius equation in the temperature range from 568 to 603 K. The apparent activation energy is 397 kJ mol−1. Values of activation energy published up to now for crystallisation in fluorozirconate glasses are collected. They range from ∼150 to ∼500 kJ mol −1 . The spread can be explained by comparison with the apparent activation energy for viscous flow which presents a huge variation in the undercooled regime above the glass transition temperature. From the temperature dependence of the activation energy for crystallisation of different fluorozirconate glasses it is concluded that the rate of crystal growth is controlled by viscous flow.

Different fluorination processes with ammonium bifluoride and their effect on fluorozirconate glasses

Object of this work is the study of each step of ZrO2 fluorination process by ammonium bifluoride, in order to identify thermal conditions for oxyfluorides formation and to avoid their incorporation into fluoride glasses. To this purpose, DSC, XRD and IR analyses have been carried out on different thermally treated mixtures of ZrO2 and NH4F·HF. IR spectroscopy on starting materials and different heated mixtures have been carried out in order to control step-by-step the formation of oxyfluorides during the heating. XRD reveals the presence of oxyfluorides species in addition to expected ammoniumzirconium-fluoride compounds. Species like ZrO0.46F3.08, ZrO0.67F2.67, …, found in mixtures after fluorination are likely to remain in the glass: DSC and IR analyses on glasses prepared by different fluorination processes show appreciable differences, in agreement with the suspect of a fluorination role on fluorozirconate glasses quality.

Crystallization behaviour of fluorozirconate glasses

Abstract Results are reported on crystallization modes in undoped ZBLAN and praseodymium doped ZBLAN glasses. The transformation usually proceeds from the surface with growth of oriented crystals. Activation energies of 154 and 216 kJ/mol were found for different crystallization stages at temperatures around 600 K.

Local structure of Pr3+ in fluorozirconate glasses

Abstract A study of the local atomic structure around praseodymium in fluorozirconate glasses is presented. Pr K-edge (at 42 keV) extended absorption fine structure anomalous X-ray scattering data and photoluminescence measurements have been performed on two series of fluorozirconate glass systems. From our study three important results have been obtained: (i) when the conventional melting method is used for the preparation of fluoride glasses, the local environment is independent of the Pr3+ doping precursor up to a concentration of 10 000 ppmw of Pr; (ii) no first shell Pr–Pr signal has been observed, even in the samples having a Pr3+ concentration of 10 000 ppmw, i.e. ten times greater than the concentration quenching limit (1000 ppmw); (iii) preliminary analysis on AXRS, at the Pr3+ K-edge, discounts the presence of Pr–F–Pr contribution in the second shell signal for the samples doped with 10 000 ppmw Pr3+.

Different processes for the preparation of fluorozirconate glasses

Abstract Different processes for the preparation of fluorozirconate glasses (ZBLAN) have been investigated by infrared spectroscopy (IR) and x-ray diffraction (XRD) on fluoride powders, during the reaction. Glasses have been prepared with 20-10-5-0 (% weight) ammonium bifluoride (NH4F·HF) or with nitrogen trifluoride (NF3) as fluorinating agent. In order to improve the quality of the glasses, small percentages (about 5%) of NH4F·HF are a good compromise between fluorination and hydration of starting products; NF3 is very effective during the fluorination step of the preparation: our electron spin resonance measurements (ESR) on these last glasses after γ-irradiation (20 Mrad), do not detect any O2 signal.

Fabrication of Er3+-doped fluoride fibres with single-mode geometry based on over-clad drilling

Abstract Fluoride preforms with single-mode geometry have been accomplished by drilling a small hole along the centre axis of an over-clad rod and inserting an Er 3+ -doped multimode preform stretched. From such preforms, single-mode fibres have been drawn, which could be used for the fabrication of optical amplifiers. Core–clad concentricity of the fibres depended on the precision of the drilling operation. Best fibres have a core non-circularity of 5.9%, a clad non-circularity of 3.3% and a core–clad concentricity of 1.7 μm. Fibre loss and bending strength were measured to be 0.48 dB/m and 0.83 GPa, respectively.