Tellurite And Fluorotellurite Glasses For Active And Passive
Tellurite And Fluorotellurite Glasses For Active And Passive Tellurite And Fluorotellurite Glasses For Active And Passive
5. Crystallisation studies; MDO 161 Fig. (5.14): DTA trace of glass MOF005 (70TeO2-10Na2O-20ZnF2 (mol. %)), showing crystallisation peaks with background removed and Gaussian deconvolution [4]. Nukui et al. [7] have shown that glasses in the binary system TeO2-ZnO devitrify into two phases: ZnTeO3 and Zn2Te3O8. For low ZnO compositions (10 mol. %), ZnTeO3 forms at a lower temperatures (≈ 380°C) than Zn2Te3O8 (≈ 480°C). As the ZnO content in the glass is increased to 30 mol. %, the Zn2Te3O8 phase field opens up, and the phase crystallises from around 440°C (i.e. from a lower temperature). The ZnTeO3 phase field remains approximately the same size, but moves to higher temperatures [7]. For the ternary glass system used in this study, TeO2-Na2O-ZnF2, the addition of fluorine and sodium to the composition lower the initial temperature of crystallisation for the Zn2Te3O8 phase further, and no ZnTeO3 was seen in the sample which crystallised at 400°C. This phase may be stable at a higher temperature, or the fluoride phase (NaZnF3) is more stable and crystallises in preference.
5. Crystallisation studies; MDO 162 5.3.1.4. Heat treated Er +3 -doped compositions Fig. (5.12) and (5.13) show XRD traces of glass MOF017 (69.86TeO2-9.98Na2O- 19.96ZnF2-0.2ErF3 mol. %) before and after a heat treatment for 1 hr. at 275°C respectively. Although the heat treated sample was opaque, no distinct crystalline phase was identified by XRD, indicating either liquid / liquid phase separation, which could result in opacity and lack of crystallites shown by fig. (5.13), or that the crystals are too small to be resolved by the diffractometer, but still contribute to scattering. As crystal size decreases, Debye diffraction rings, and hence peaks broaden [8]. If crystal size is sufficiently small (of the order of nm), peaks can become indistinguishable from the amorphous halo. The untreated glass exhibits the characteristic halo at around 30° 2θ, with a lower intensity broader halo centered at around 55° 2θ. After the heat treatment the main halos shape has developed a lower angle shoulder at around 23° 2θ and there is possibly a peak developing at around 40° 2θ. Kukkonen [9] and Beggiora [10] et al. studied the crystallisation of SiO2-Al2O3-CdF2-PbF2-ZnF2-YF3-ErF3 glass-ceramics. Erbium (III) was found to preferentially partition to PbFx nano-crystals. Crystalline peaks appeared from the amorphous halo of the untreated glass with increasing time and temperature. In the TeO2-Na2O-ZnF2-ErF3 system, Er +3 could preferentially partition to one of the phases, possibly the lower temperature fluoride phase (NaZnF3).
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5. Crystallisation studies; MDO 161<br />
Fig. (5.14): DTA trace of glass MOF005 (70TeO2-10Na2O-20ZnF2 (mol. %)), showing<br />
crystallisation peaks with background removed and Gaussian deconvolution [4].<br />
Nukui et al. [7] have shown that glasses in the binary system TeO2-ZnO devitrify into<br />
two phases: ZnTeO3 and Zn2Te3O8. <strong>For</strong> low ZnO compositions (10 mol. %), ZnTeO3<br />
forms at a lower temperatures (≈ 380°C) than Zn2Te3O8 (≈ 480°C). As the ZnO content in<br />
the glass is increased to 30 mol. %, the Zn2Te3O8 phase field opens up, and the phase<br />
crystallises from around 440°C (i.e. from a lower temperature). The ZnTeO3 phase field<br />
remains approximately the same size, but moves to higher temperatures [7]. <strong>For</strong> the<br />
ternary glass system used in this study, TeO2-Na2O-ZnF2, the addition of fluorine and<br />
sodium to the composition lower the initial temperature of crystallisation for the<br />
Zn2Te3O8 phase further, and no ZnTeO3 was seen in the sample which crystallised at<br />
400°C. This phase may be stable at a higher temperature, or the fluoride phase (NaZnF3)<br />
is more stable and crystallises in preference.
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