* 논 문 세 미 나 * Some effects of different additives on dielectric and piezoelectric properties of (Bi½Na½)TiO 3 - BaTiO 3 morphotropic-phase-boundary composition

Abstract  Microstructure, dielectric and piezoelectric of (Bi 1/2 Na 1/2 )TiO 3 - 6BaTiO 3 doped with Nb 5+, Co 3+, or La 3+ were studied for a candidate as lead-free pizoelectric ceramics.  The doping with Co 3+ enhanced piezoelectric coeff -icient(d33) and lower dielectric loss.  The Co 3+ doped system's intermediate phase transition almost disappeared.

Introduction 1.  Bismuth sodium titanate, (Bi 1/2 Na 1/2 )TiO 3 ➥ an attractive lead-free A-site complex-perovskite for high-T c 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 PbTiO 3 SrTiO 3, (Sr a Pb b Ca c )TiO 3, La 2 (TiO 3 ) 3, CaTiO 3 and BaTiO 3 have been studied.

Introduction 2.  Among these BNT-based system, (Bi 1/2 Na 1/2 )TiO 3 - BaTiO 3 (BNT)is more arresting.  The aim of this paper ➪ to investigate the effects of single and cooperative substitution on (Bi 1/2 Na 1/2 )TiO 3 - 6BaTiO 3 system by studying the dielectric and piezoelectric propert -ies of (Bi 1/2 Na 1/2 )TiO 3 - 6BaTiO 3 doped with (1 at.%)La 3+, (1 at.%)Nb 5+ or (1 at.%)Co 3+.

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 Bi 2 O 3, Na 2 CO 3, BaCO 3, TiO 2, Nb 2 O 5, Co 2 O 3 and La 2 O 3 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 ℃ for 2 h in an air atmosphere.  All samples were sintered at the temperatures leading to the maximum relative densities.

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 d 33 meter ➥ The piezoelectric charge coefficient

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 33 T 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.

Results and discussion 2.  For all BNBT6 samples with additives, the dielectric constant is higher than BNBT6 at room temperature and the ε 33 T 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.

Results and discussion 3.  The pure BNBT6 and BNBT6 doped with La 3+ or Nb 5+ ➪ 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 T max ➪ an anti- ferroelectric phase.

Results and discussion 4.  The BNBT6 systems to which Co 3+ 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 ℃

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 Co 3+ ➪ 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.

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 ➪ T o restrain the grain growth of the ceramics materials  The addition of Co 3+ ➪ has a converse effect on grain size of the ceramics materials.

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 Q m decreases.

Results and discussion 8.  The piezoelectric properties of those samples doped with Co 3+ ➪ higher d33, k t and k p 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 Nb 5+ doping ➪ the kt/kp ratio of the system became bigger ➪ the anisotropy of the ceramics was enhanced.

Conclusions  The piezoelectric properties and the loss tangent of BNBT6 system can be improved in some extent by Co 3+ doping.  The cooperative soft-doping effect of La 3+ and Nb 5+ 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 Co 3+ is doped.