Relaxor ferroelectrics lead magnesium niobates

Because of very high dielectric constants k > 20, 000), lead-based relaxor ferroelectrics, Pb(B, B2)02, where B is typically a low valence cation and B2 is a high valence cation, have been iavestigated for multilayer capacitor appHcations. Relaxor ferroelectrics are dielectric materials that display frequency dependent dielectric constant versus temperature behavior near the Curie transition. Dielectric properties result from the compositional disorder ia the B and B2 cation distribution and the associated dipolar and ferroelectric polarization mechanisms. Close control of the processiag conditions is requited for property optimization. Capacitor compositions are often based on lead magnesium niobate (PMN), Pb(Mg2 3Nb2 3)02, and lead ziac niobate (PZN), Pb(Zn 3Nb2 3)03. 

Relaxor-type electrostrictive materials, such as those from the lead magnesium niobate-lead titanate, Pb(Mgp 3Nb2/3)03-PbTi03 (or PMN-PT), solid solution are highly suitable for actuator applications. This relaxor ferroelectric also exhibits an induced piezoelectric effect. That is, the electromechanical coupling factor kt varies with the applied DC bias field. As the DC bias field increases, the coupling increases and saturates. Since this behavior is reproducible, these materials can be applied as ultrasonic transducers which are tunable by the bias field [12]. 

More recently, because of their high dielectric constants k > 20 000), lead-based relaxor ferroelectrics have been used as capacitor materials. These ceramics have the general chemical formula Pb(5i, 2)03) vvhere Bi is typically a low-valence cation and B2 is a high-valence cation. Compositions used in capacitor applications are frequently based on lead magnesium niobate, Pb(Mgi/3,Nb2/3)03, and lead zinc niobate, Pb(Zni/3,Nb2/3)03. Other substituents and modifiers are added so that dielectric layers of these materials can be densified at relatively low temperatures ( 900 °C). The low firing temperatures permit the use of relatively inexpensive cofired electrode materials, such as silver. Typically, tape casting is used in the preparation of the dielectric layers. 

Lead magnesium niobate, Pb(Mg 3Nb2/3)03, is a ferroelectric relaxor material which is paraelectric at room temperature. The perovskite phase is typically modifield with addition of lead titanate or barium titanate, which raises the Curie temperature to near room temperature. Very high dielectric constants can be achieved, >20,000, but at the expense of temperature stability. 

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