This work investigates the effect of cation substitution on the properties of the lead-free, solid solution (Na₀.₅Bi₀.₅)TiO₃-BaTiO₃ (NBT-BT). Substitution into NBT-BT increases disorder and increases the relaxor ferroelectric properties. These relaxor ferroelectric materials show large piezoelectric strains that can be utilized in actuator applications. Ceramic samples were prepared using standard solid-state synthesis techniques using metal oxide and carbonate reagents. Materials were pressed and sintered into ceramic disks. All prepared NBT-BT materials had their structure investigated via powder x-ray diffraction. Electrical properties such as, dielectric permittivity, ferroelectricity, and piezoelectricity, were investigated and analyzed for each prepared material.
Three new group 13-substituted NBT-BT materials were prepared and analyzed to determine any periodic trends: (Na₀.₅Bi₀.₅)TiO₃-0.055BaTiO₃-Bi(M)O₃ M=Al, Ga, In. The ternary phases selected were theoretically determined to have high ferroelectric properties and were substituted in molar amounts of 0.02, 0.04 Bi(Al)O₃, 0.02, 0.04 Bi(Ga)O₃, and 0.01, 0.02 Bi(In)O₃. Powder x-ray and neutron diffraction experiments suggests that all materials had a complex, two-phase structure with a mixture of tetragonal P4bm and monoclinic Cc phases.
Most substituted NBT-BT samples displayed modest electrical properties improvements compared to the unmodified, NBT-BT. The 2% BiGaO₃ substituted sample displayed the highest room temperature and overall dielectric permittivity (Ɛᵣ = 6626 at 259 °C) with the second highest strain performance (d₃₃* = 570 pm/V). The 4% BiAlO3 substituted sample uniquely underwent a reversible ergodic relaxor to ferroelectric phase transition which enhanced the large field strain (d₃₃* = 695 pm/V). This project is described further in Chapter 3.
The ternary, solid solution of lead-free (Na₀.₅Bi₀.₅)TiO₃-BaTiO₃ (NBT-BT) and nonpolar BiGaO₃ (NBT-xBT-yBG) were systematically prepared and investigated. Samples were prepared near the morphotropic phase boundary (MPB) of NBT-BT ((1-x)NBT-xBT, x= 0.04-0.09) with 2-7% BiGaO₃ substitution. Dielectric, ferroelectric, and piezoelectric properties were analyzed for all prepared samples. A phase diagram was prepared by determining the ferroelectric to ergodic relaxor transition temperature (TF-R) via dielectric permittivity measurements on poled samples. The enhanced strain at the electric field induced relaxor to ferroelectric transition was investigated to determine which composition around the MPB displayed the largest strain performance. The highest strain achieved was near the MPB of the binary parent, 0.93NBT-0.07BT, substituted with 4% BiGaO₃(0.53 % , d33* = 883 pm/V). The largest strains for each system occurred in compositions with one to two percent BiGaO₃ past the RT ferroelectric to relaxor transition. These results reveal, similar to that of MPB properties, that the largest electric field enhanced strain occurs near the MPB of the binary NBT-BT in substituted NBT-BT. Chapter 4 contains further details on this study.