* 논 문 세 미 나 *

1. * 논 문 세 미 나 * Some effects of different additives on dielectric and piezoelectric properties of (Bi½Na½)TiO3 - BaTiO3 morphotropic-phase-boundary composition

2. Abstract • Microstructure, dielectric and piezoelectric of (Bi1/2Na1/2)TiO3 - 6BaTiO3 doped with Nb5+ , Co3+ , or La3+ were studied for a candidate as lead-free pizoelectric ceramics. • The doping with Co3+ enhanced piezoelectric coeff -icient(d33) and lower dielectric loss. • The Co3+ doped system's intermediate phase transition almost disappeared.

3. Introduction 1. • Bismuth sodium titanate, (Bi1/2Na1/2)TiO3 ➥ an attractive lead-free A-site complex-perovskite for high-Tc ferroelectric relaxor material. • The BNT-based ceramic materials are of interests as new candidates to replace the widely used lead-contented perovskite materials. ➪the free control of sintering atmosphere and lack of lead pollution during the process of preparation. • To improve its properties, several solid solutions of BNT with PbTiO3 SrTiO3 , (SraPbbCac)TiO3 , La2(TiO3)3 , CaTiO3 and BaTiO3 have been studied.

4. Introduction 2. • Among these BNT-based system, (Bi1/2Na1/2)TiO3 - BaTiO3 (BNT)is more arresting. • The aim of this paper➪to investigate the effects of single and cooperative substitution on (Bi1/2Na1/2)TiO3 - 6BaTiO3 system by studying the dielectric and piezoelectric propert -ies of (Bi1/2Na1/2)TiO3 - 6BaTiO3 doped with (1 at.%)La 3+, (1 at.%)Nb5+ or (1 at.%)Co 3+.

5. Experiment 1. • The conventional ceramic fabrication technique was used to prepare these ceramic samples. • The high purity of starting metal oxide or carbonate powders, such as Bi2O3, Na2CO3, BaCO3, TiO2, Nb2O5, Co2O3 and La2O3 were used. • The powders of these raw materials ➪ in ethanol with carnelian balls by ball-milling for 12 h➪ calcined at 1000 ℃ for 2 h ➪ were ground and ball-milled for 24 h into the fine powders and were pressed into discs, 16.4 or 12 mm in diameter and 1 mm in thickness ➪ some of these small discs as the samples were sintered at 1160-1200 ℃ for 2 h in an air atmosphere. • All samples were sintered at the temperatures leading to the maximum relative densities.

6. Experiment 2. • FESEM(field emission scanning electron microscope) ➥ The surface micrograph of sintered samples. • Atomated diffractometer ➥X-ray diffraction patterns. • Multifrequency LCR meter ➥Temperature dependence of dielectric constant and loss tangent • Piezo d33 meter ➥The piezoelectric charge coefficient

7. Results and discussion 1. • It’s not necessary to control the atmosphere during the sintering process of these ceramics ➪ the limited evaporation of the raw materials, which is not the case with conventional PZT or PZT- based ceramics. • X-ray diffraction data at room temperature ➪ that all of the samples have a structure of rhombohedral phase. • Figs 1 and 2 show temperature dependences of dielectric constant ε 33T and tan δ of all the samples, respectively. • All samples that show the curie points in Figs. 1 and 2 are shifted to higher temperatures ➪ it can be thought that the additives have influenced the crystal lattices.

8. Results and discussion 2. • For all BNBT6 samples with additives, the dielectric constant is higher than BNBT6 at room temperature and the ε 33T temperature curves of the compositions exhibit strong dielectric dispersion with the increasing temperature. ➪the diffuse phase transition * BNBT system is a typical relaxation ferroelectric with A-sit complex ions .

9. Results and discussion 3. • The pure BNBT6 and BNBT6 doped with La3+ or Nb5+➪ a sharp increase of permittivity near about 100℃➪ the tendency of the increase becomes slow and the curve begins to broaden. • "a shoulder" ➪ intermediate phase transition. • the temperature region between the shoulder and Tmax➪ an anti-ferroelectric phase .

10. Results and discussion 4. • The BNBT6 systems to which Co3+ is added, "the shoulder" almost dis- appeared. • The BNBT6-Co and BNBT6-La-Co system have the similar curves of ε-T and tg δ-T. • The loss tangent is that it almost keeps constant through a wide tempera-ture range from the room temperature to about 250℃

11. Results and discussion 5. • The SEM micrographs of all compositions' as-sintered surfaces. • The BNBT6-Co system ➪ large grain sizes. • The surface of the grains of Co-doping BNBT6 ➪ a remarkable growth of grains. ( Fig. 3(4) and (6)) • The La 3+doping samples without Co3+➪ the grain size became relatively smaller which was shown in BNBT6-La and BNBT6-La-Nb system. • BNBT6-La -Co system presents a relatively smaller grain size than BNBT6-Co system.

12. Results and discussion 6. • The comparison of BNBT6 with BNBT6-La gives a similar result➪ the sample with La 3+ doped has a smaller grain size. • The addition of La➪To restrain the grain growth of the ceramics materials • The addition of Co 3+➪ has a converse effect on grain size of the ceramics materials.

13. Results and discussion 7. • Table 1 summarizes the dielectric and piezoelectric properties of all these modifield BNBT6 compositions. • For all the modified BNBT6 compositions ➪ dielectric constant is greater than that of BNBT6 at room temperature. • Piezoelectric coefficient (d33) of BNBT6-La and BNBT6-La-Nb, in which La 3+ is added, is enhanced and the value of mechanical quality factor Qm decreases.

14. Results and discussion 8. • The piezoelectric properties of those samples doped with Co 3+ ➪ higher d33 , kt and kp and than other compositions. • BNBT6-Co ➪d33 goes up to 139 pC/N ➪ a maximal value of these samples. ➪ the remarkable growth of grain size after Co3+ -doping ➥ The sound growth of grains makes the development of domains more consummate, and thus the piezoelectricity of the ceramics is improved. • The grain size of BNBT6-Co ceramics decreases after La is added, and d33 becomes smaller as well. But BNBT6 ceramics' dielectric loss drops down to 2.3% after Co-doping. • The Nb5+ doping ➪ the kt/kp ratio of the system became bigger ➪ the anisotropy of the ceramics was enhanced.

15. Conclusions • The piezoelectric properties and the loss tangent of BNBT6 system can be improved in some extent by Co3+ doping. • The cooperative soft-doping effect of La3+ and Nb5+ could obviously enhance the value of d33 . • The intermediate phase transition of BNBT6 system existing before the Curie temperature almost disappears and grain size shows an evident when Co3+ is doped.