Microstructure Control of Lead Niobate for Piezoelectric Transducers

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The effects of La_2O_3 doping on the microstructure and piezoelectric properties of PbNb_2O_6 were examined. Extensive grain growth induced by the phase transformation form the low- to high-temperature forms was suppressed by La_2O_3 doping. Furthermore, La_2O_3 reduced the rate of normal grain growth and the phase transformation temperature. Thus, dense, fine-grained ceramics were obtained. La_2O_3 increased the frequency and dielectric constants. The elimination of pores and cracks were responsible for the increase in the frequency constant. La acts as donor ion to PbNb_2O_6 and increases the dielectric constant. Co-doping of La_2O_3 and K_2O resulted in an increase in the rate of normal grain growth. This result suggests that grain boundary segregation of La ions is responsible for the reduction of the rate of normal grain growth. These results indicates that PbNb_2O_6 doped with La_2O_3 has small mechanical quality factor and large frequency constant, and can be used as basic composition for the piezoelectric material with wideband ultrasonic transducers. The compact with the composition of Pb_<0.95>La_<0.10/3>Nb_2O_6 and sintered at 1200℃ had frequency constant of 2175 kHzmm, dielectric constant of 616, and mechanical quality factor of 147.