Chemical reactions of lead magnesium niobate titanate in the presence of a glass
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A relaxor ferroelectric of composition 0.93Pb(Mg1/3Nb2/3)O3-0.07PbTiO3 was sintered with 3 wt. % commercial sealing glass at 750 °C for 30 min to achieve ^ 9 5 % of theoretical density and a nearly pure perovskite phase. At higher glass additions (up to 20 wt. %), higher sintering temperatures (up to 800 °C), and longer sintering times (up to 4 h), the amount of perovskite (PMN type) decreases and that of pyrochlore (6PbO • MgO • 3Nb 2 O 5 or 3PbO • 2Nb 2 O 5 ) increases. On sintering at 800 °C for 4 h no perovskite phase is present in compositions with even 1% glass addition. The reaction of glass with the PMN phase was found to lead to the disappearance of the perovskite. Addition of 0.1 to 0.6 wt. % MgO to compositions containing 1 and 3 wt. % glass (and balance PMN-PT) results in essentially pure PMN perovskite phase on sintering at 700-800 °C for 30-240 min, confirming that the reaction of glass with PMN and depletion of MgO from PMN can be arrested. The sintered ceramics exhibit relaxor behavior and possess dielectric properties essentially commensurate with the phase composition.
I. INTRODUCTION Lead magnesium niobate, PbMgi/3Nb2/3O3 (PMN), is a typical relaxor ferroelectric1 in which the Mg2+ and Nb5+ ions are randomly distributed over the B sites of the ABO3 type perovskite lattice. The Curie temperature of PMN ( 15 °C) can be raised to around 25 °C by forming a solid solution (0.93PMN-0.07PT) with lead titanate, PbTiO3 (PT), which is a well-known ordered ferroelectric with a Curie temperature of 490 °C involving a first order cubic to tetragonal phase transition.2 The PMN-PT compositions exhibit larger electrostrictive strains and dielectric constants than other perovskites such as BaTiO 33 and lead zirconate titanate ceramics.4 However, the full exploitation of these materials has been hampered due to the occurrence of a pyrochlore phase with a low dielectric constant (~200) during the synthesis of PMN and the loss of PbO at the high sintering temperatures (~1300 °C) required for achieving the necessary densification. The problems in the synthesis of PMN have been reviewed by Shrout and Halliyal.5 The occurrence of the undesired pyrochlore phase can be minimized (or even eliminated) by a precursor route in which MgNb 2 O 6 of columbite structure is formed first, which then reacts with PbO to form PMN,6 or by using ultrapure starting materials,7 or by adding excess MgO (2-10%). 8 " 10 However, excess PbO and MgO lead to lowering and aging of dielectric properties of PMN.11'12
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address: Tata Research Development and Design Center, Pune, 411001, India.
1308
http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 6, Jun 1991
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The high dielectric constant of PMN-type materials makes them good candidates as dielectrics for multilayer capacitors. The current multilayer capacitor manufacturing technology requires co-firing of the stack of dielectric and electrode layers at a sufficiently low temperature to permit the use of silver-rich or base metal electrodes. It is,
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