Crystallization kinetics of yttrium aluminum garnet (Y 3 Al 5 O 12 )
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The kinetics and pathways for crystallization of solid, amorphous, yttrium aluminum garnet (YAG) were studied using isothermal differential thermal analysis, x-ray diffraction, and transmission electron microscopy. The activation energy for crystallization was 437 KJ/mol and the measured Avrami exponent was 2.74, which corresponded to three-dimensional crystal growth with a constant number of nuclei. Time–temperature–transformation (T–T–T) curves were developed from the data to predict crystallization rates as a function of temperature. The crystallization pathway for YAG in this system is compared to others reported in the literature.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURES
Yttrium aluminum garnet (Y3Al5O12; YAG) is an important ceramic material used in structural refractory applications. It is also used in optical applications such as laser materials (e.g., when doped with rare-earth cations, such as neodymium). Single-crystal YAG is one of the most creep-resistant ceramic oxides known.1,2 Its low creep, coupled with its very high melting temperature (1940 °C3) make it particularly attractive for structural applications, and it is being investigated for potential use as a structural fiber.4-11 YAG composition fibers have been successfully made using containerless methods to pull amorphous fibers from highly undercooled melts.4,12,13 This process has the advantage that fibers can be drawn from typically fragile, glass-forming, materials (such as YAG), as well as from compositions that are difficult to access due to their incongruent melting behavior (e.g., mullite).4 As formed, these fibers are amorphous and require subsequent controlled crystallization to maximize their properties at elevated temperatures. However, to crystallize amorphous YAG composition fibers with a controlled microstructure, a thorough understanding of the crystallization kinetics of quenched YAG composition glasses is necessary. Even though YAG is a well-studied engineering material, little data is available concerning its crystallization kinetics. This is in part due to the fact that YAG is primarily used in its crystalline form and also because it is a very fragile glass-forming material.14,15 The purpose of this study was to characterize the kinetics and pathways for crystallization of solid, amorphous YAG so that this information could be used in subsequent controlled crystallization experiments.
YAG powder was chemically synthesized via a complex polymerization method as described elsewhere.16–18 The precursors were yttrium nitrate hexahydrate, Y(NO3)3 ⭈ 6H2O (Alfa Aesar, Ward Hill, MA) and aluminum nitrate nonahydrate, Al(NO3)3 ⭈ 9H2O (Aldrich, Milwaukee, WI). They were dissolved in a 5 wt% solution of 78% hydrolyzed polyvinyl alcohol PVA (Gohsenol KH-17s, MW ⳱ 106,000, Nippon Gohsei, Inc., Osaka, Japan). The chemically synthesized powders were subsequently melted in a water-cooled hearth using a CO2 laser to form a pellet.19 The pellet was then levitated in an oxygen atmosphere with an aero-acoustic levitator, and melted again
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