A critical evaluation of reactive templated grain growth (RTGG) mechanisms in highly [001] textured Sr 0.61 Ba 0.39 Nb 2
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In reactive templated grain growth (RTGG), oriented template crystals are used to seed both phase formation and crystallographic orientation in textured ceramics. This mechanism differs substantially from templated grain growth (TGG), in which texture forms via grain growth mechanism. In this work, characteristics of both RTGG and TGG processes are evaluated in [001] textured Sr0.61Ba0.39Nb2O6 ceramics produced from reactive SrNb2O6 and BaNb2O6 matrix powders and acicular KSr2Nb5O15 (KSN) templates. Above 1100 °C, SrxBa1 xNb2O6 (SBN) forms by oriented nucleation and growth on KSN (the RTGG process) and by nucleation of nonoriented matrix grains. RTGG occurs without densification or coarsening until phase formation is complete (;1250 °C) and accounts for ;60% of the texture in dense SBN ceramics. A later TGG process occurs from 1250–1350 °C and is characterized by simultaneous densification, grain growth, and additional texture development.
I. INTRODUCTION
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-editor-manuscripts/ DOI: 10.1557/jmr.2011.379
subunit, which is connected one-dimensionally along the c-axis. In SBN, only 5/6 of the A sites are filled, resulting in a relatively open structure that can support substantial variation in stoichiometry. Distortion of the SBN structure at room temperature gives rise to ferroelectric switchable polarization along the c-axis, and this material undergoes a relaxor phase transition from this polar tetragonal state (4 mm) to a nonpolar tetragonal paraelectric state (4/mmm) at a temperature dependent on x.9 The ferroelectric and thermoelectric properties of SBN are maximized along the crystal c-axis, and are low or nonexistent in other directions.1,3–6 Although SBN crystals can be grown with various compositions, the high cost and limited sizes of these crystals restrict their commercial application. Randomly oriented polycrystalline SBN shows relatively poor properties as only a small fraction of grains show c-axis alignment. The properties of SBN ceramics may be greatly increased by crystallographic texturing, and the resulting ceramics offer substantial processing advantages relative to SBN crystals. Production of [001] fiber texture in SBN results in ∞m/m symmetry and allows separation of directionally dependent properties, potentially yielding crystallike performance in these materials.10 Templated grain growth (TGG) and reactive TGG (RTGG) have been used extensively to produce relatively high texture qualities in many ceramic materials.10–14 In each case, texture results in significant enhancement of directionally dependent structural or functional properties. In TGG and RTGG, aligned template particles are used to direct oriented grain growth within a dense, fine-grained
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Ó Materials Research Society 2011
Ferroelectric SrxBa1 xNb2O6 (SBN, x 5 0.25–
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