Lead titanate glass-ceramics derived from a silicate-based melt
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Glass composition 39PbO–1BaO–25TiO2–9.8Al2O3–24.2SiO2–1B2O3 (mol%) crystallized on heat treatment at 艌600 °C to form glass-ceramics whose majority phase was PbTiO3 (PT). At 600 °C, nano-size PT crystals (∼20–50 nm) with a c/a ratio of 1.000(1) were observed, but as heat-treatment temperature increased to 1000 °C, the crystal size and c/a ratio increased to ∼1.2 m and 1.056(4), respectively. Permittivity measurements as a function of temperature revealed a broad peak at ∼400 °C associated with the nanocrystalline PT crystals, but it sharpened and increased in temperature as heat treatment temperature increased to 1000 °C. The causes of peak broadening and shift of Tc are believed to be due to either clamping of PT crystals by glass matrix, finite size effects due to their intrinsically small size, or the incorporation of dopant impurities such as Al, Si, or Ba ions in the PT phase.
I. INTRODUCTION
PbTiO3 (PT) is a typical ferroelectric material with the perovskite structure. It undergoes a first-order phase transition, Tc ∼ 490 °C, above which the structure is cubic (space group Pm3m) but below is tetragonal (space group P4mm).1–3 The large ionic displacements in PT at Tc lead to a particularly large room-temperature spontaneous polarization (>53 C/cm2), the largest in the perovskite family.1 PT exhibits large pyroelectric coefficients, low relative permittivity (∼100–200), and dielectric properties stable with time, temperature, and frequency. These properties make PT an attractive material for pyroelectric, electro-optical, and nonlinear optical applications.2–6 Undoped PT is difficult to form as a ceramic body due to its large crystal anisotropy (c/a ⳱ 1.064) and breaks into pieces on cooling below Tc. This has limited the applications of pure PT ceramics.4,7 Good-quality PT ferroelectric thin films have been fabricated with advanced thin film technology8–12 such as sol-gel and chemical vapor deposition. The glass-ceramic route offers the possibility of fabricating bulk PT by controlled crystallization of a glass, but the main advantage of glass over ceramic materials is the relatively simple methods that may be used to form complex shapes, such as casting and extrusion. Since the 1960s, there have been several studies concerning the nucleation of ferroelectric crystals such as BaTiO3,13,14
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0166 1316
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 5, May 2005 Downloaded: 15 Mar 2015
NaNbO3,15–17 KNbO3,15,16 LiNbO3,16,18 and PT19–26 in glass. The key features investigated include generic phase evolution, crystallization kinetics,20,21,27 dielectric properties,14,25,28–30 and the composition and morphology of the crystals.20,23,25,31 The properties of glass ceramic are mainly determined by the major crystalline phase but may deviate from the intrinsic properties due to the presence of surrounding glass matrix or secondary phase(s). In the case of ferroelectric glass ceramics, it is crucial to under
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