The effects of a magnetic field on the crystallization of a fluorozirconate glass
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The effects of a magnetic field on the crystallization of a fluorozirconate glass Dennis S. Tuckera) and Michael R. Lapointe Marshall Space Flight Center, Marshall Space Flight Center, Alabama 35812
Zhiyong Jia Materials and Information Technology (MINT) Center, University of Alabama, Tuscaloosa, Alabama 35487 (Received 2 August 2006; accepted 16 October 2006)
An axial magnetic field of 0.1 T was applied to ZrF4–BaF2–LaF3–AlF3NaF fibers during heating to the glass crystallization temperature. Scanning electron microscopy and x-ray diffraction were used to identify crystal phases. It was shown that fibers exposed to the magnetic field did not crystallize, while fibers not exposed to the field did crystallize. A hypothesis based on magnetic work was proposed to explain the results, and was tested by measuring the magnetic susceptibilities of the glass and crystal.
Fluorozirconate glasses have been of interest to researchers for over 25 years. One class that has consistently shown the most promise is ZrF4–BaF2–LaF3– AlF3–NaF (ZBLAN) glass.1 ZBLAN fiber optics are currently used in a number of applications, such as fiber amplifiers, and lasers for cutting, drilling, and surgery, and they show promise in applications such as measurement of nuclear radiation resistant lengths and nonlinear applications.2 Intrinsic and extrinsic processes limit light propagation at low powers in ZBLAN.3 Intrinsic processes include band gap absorption, Rayleigh scatter, and multiphonon absorption. Extrinsic processes include impurities, such as rare-earth and metal ions, and crystallites formed during preform preparation and fiber drawing. The theoretical loss coefficient for ZBLAN is 0.001 dB/km at 2 m. The achievement of this lower limit is hindered by both intrinsic and extrinsic processes. Varma et al.4 stated that they felt the devitrification was due to a narrow working range and low viscosity at the drawing temperature. A low viscosity increases the likelihood of crystallization due to ionic diffusion.5 Magnetic fields have been used to affect flows in conducting fluids. Crystals grown from melts are improved by using uniform6–9 and rotating magnetic fields.10 Convective heat and mass transport in semiconductor melts
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0209 J. Mater. Res., Vol. 22, No. 6, Jun 2007
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with large electrical conductivities can be controlled by magnetic fields. The effects of direct current (dc) and alternating current (ac) fields are different. A dc field tends to produce a body force that opposes or damps the buoyant convection, while an ac field drives melt motions.11 The effects of magnetic fields on g-jitter-driven flow under microgravity have also been investigated,12 as well as the effects of a magnetic field on the crystallization of inorganic salts. The reported results vary for magnetic field effects on inorganic salt crystallization; it has been found that calcium carbonate crystalliza
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