Phase Purity and Site Selection with Lanthanum Doping in Sodium Bismuth Titanate (Na 0.5 Bi 0.5 Tio 3)-Piezoelectric Ceramic

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Phase Purity and Site Selection with Lanthanum Doping in Sodium Bismuth Titanate (Na 0.5 Bi 0.5 TiO 3)-Piezoelectric Ceramic

Humberto Foronda, Elena Aksel, and Jacob L. Jones College of Engineering University of Florida

Sodium bismuth titanate (NBT) is a lead-free piezoelectric material currently under study as a potential replacement to the commercially viable lead zirconate titanate. In order to improve the properties of the material, doping, or the small addition of ions to the material, can be used. The focus of this work is to examine the changes in the density and X-ray diffraction patterns of NBT with the addition of various lanthanum concentrations. Lanthanum was added in 0.5, 1.5, and 5 mol% to NBT using two different doping schemes. In one scheme, lanthanum was added to replace bismuth. In the other scheme, lanthanum was added to replace sodium. The addition of lanthanum with a reduction in sodium led a significant decrease in density, while density remained constant with the reduction in bismuth. Also, X-ray diffraction showed that with a doping level of 5% an extra phase is forming using both doping schemes. These findings indicate that processing lanthanum-doped NBT may be more stable with a reduction in bismuth.

Introduction

Piezoelectricity is the ability of a material to electrically The purpose of this research is to better understand the polarize in response to a mechanical stress and vice versa, structure of NBT and the changes that take place with to mechanically strain in response to an applied electric lanthanum doping. field [1]. Piezoelectrics is a rapidly developing topic in the field of materials science and engineering as it has many applications in industry. They are used in mobile phones, sonar, ultrasound, and fuel injectors of diesel engines. The most common commercially produced piezoelectric ceramic is lead zirconate titanate (PZT) [1], [2]. PZT is the most commercially viable piezoelectric ceramic because of its high piezoelectric properties and well-established means of processing [1]–[3]. However, lead (Pb), which is Figure 1: Pseudo-cubic representation of the NBT Unit Cell harmful to the human body and the environment, is released into the atmosphere during processing of PZT [4]. Experimental Sodium bismuth titanate (NBT) is one of the potential replacements for PZT and was initially developed in the Lanthanum-doped NBT was processed using two 1960s [5], [6]. different reaction schemes, each with the intention of NBT is a lead-free ceramic produced by the reaction placing lanthanum on a specified site within the NBT unit

cell. The first reaction (Scheme A) was designed for ¼Bi 2O3 + ¼Na 2CO 3+ TiO 2→ lanthanum to sit on the bismuth site of the unit cell. The Na 0.5 Bi 0.5 TiO 3 + ¼CO 2 (1) reaction scheme used is NBT has a perovskite structure [7], as shown in Figure 1. Sodium (Na) and bismuth (Bi) define the corners of the ¼(1-x) Bi 2O3 + ¼Na 2CO 3 + TiO 2+ 0.5xLa 2O3 → NBT unit cell, oxygen is located on each of faces, and Na 0.5 Bi 0.5(1-x) La xTiO 3 + ¼CO 2 (2) titanium is located in the center of the unit cell. where x is the percentage of added lanthanum (0.5%, 1.5%, Doping, the addition of a small amount of an element to or 5%). In Scheme A, bismuth is removed the perovskite structure, is often used to alter the structure stoichiometrically in order to produce a vacancy which and properties of the material [8]. This process has been lanthanum could occupy. On the contrary, Scheme B is studied to a great extent in PZT [8]-[12]; however, little designed with the intention of placing lanthanum on the work has been published on lead-free materials. Dopants, sodium site of the unit cell. The scheme is such as lanthanum (La) and iron, can be used to alter the structure and properties of NBT, such as its color, amount ¼Bi 2O3 + ¼(1-3x) Na 2CO 3 +TiO 2 + 0.5xLa 2O3 → Na 0.5(1- of oxygen vacancies, and density [8]. 3x) Bi 0.5 TiO 3 + ¼CO 2 (3)

University of Florida | Journal of Undergraduate Research | Volume 12, Issue 1 | Fall 2010 1 HUMBERTO FORONDA , ELENA AKSEL , AND JACOB L . JONES

In this case, sodium is removed stoichiometrically in order Results and Discussion to produce a vacancy for lanthanum to occupy. The first step in processing begins with mixing The average density measurements for the various stoichiometric amounts of powders (according to Schemes pellets are shown in Table 1. It is clear from the table that A and B above). The powders used to process NBT are as the percentage of lanthanum increases in Scheme A, the sodium carbonate (99.5% purity, Alfa Aesar), bismuth densities remain unchanged. The average densities in this oxide (99.975% purity, Alfa Aesar), and titanium dioxide case are 5.90, 5.91, and 5.74 g/cm 3, and they are all within (99.85% purity, Alfa Aesar). The dopant lanthanum oxide 5% of each other. However, in Scheme B, the density (99.99% purity, Alfa Aesar) was also added according to decreases as the percentage of lanthanum added increases. the two reaction schemes. The powders are then suspended With 0.5% lanthanum, the average density of a sintered in ethanol to form a slurry and ball milled with yttria pellet is 5.92 g/cm 3. When 5% lanthanum is added, the stabilized zirconia for approximately twenty four hours. average density drops to 4.9 g/cm 3. The theoretical density The ball milled slurry is dried and calcined at 800ºC for of undoped NBT is 5.99 g/cm 3 [8]. To calculate the two hours using heating and cooling rates of 4 and theoretical density of samples produced using the various 5°C/min, respectively, to form La-doped Na 0.5 Bi 0.5 TiO 3. doping schemes, the mass of the unit cell was calculated Afterwards, an organic polyvinyl alcohol binder is added to assuming that all of the added lanthanum went to the the powders to aid with pressing, and the mixture is ground intended site (i.e. sodium or bismuth) and that the volume to pass through a 200 micron sieve. It is then packed in a of the unit cell is constant. The percent of theoretical 10 mm diameter pressing die and pressed using a uniaxial density is found from the ratio of the measured density to press for one minute at 300 MPa. The green pellets are then the calculated theoretical density. The density for Scheme sintered in a furnace at 1100°C for up to one hour, using A remains at approximately 98% of the theoretical density, heating and cooling rates of 4 and 5°C/min, respectively, to while in Scheme B it drops from 98.2% to 79.4% with reduce porosity and consolidate the powders into a solid increased lanthanum content. body. Table 1: Density Measurements of Doped NBT Ceramics Density of the sintered ceramics was measured using the Percent of Archimedes method. The weight of the pellets in air and Lanthanum Density Site 3 Theoretical submerged in water was recorded and used to calculate the (%) (g/cm ) density with the following equation: Density (%) 0 - 5.84 97.5 W × Denisty Density = air fluid (4) 0.5 Bi 5.90 98.3 − Wair Wfluid 1.5 Bi 5.91 98.8 Where W is weight of the pellet in air, W is the air fluid 5 Bi 5.74 97.1 weight of the pellet in fluid, and Density fluid is the density of the fluid. Finally, an Inel CPS120 X-ray diffractometer, 0.5 Na 5.92 98.2 shown in Figure 2, was used to measure X-ray diffraction 1.5 Na 5.64 93.1 (XRD) patterns of the processed materials. Six minute diffraction patterns of each prepared pellet were measured 5 Na 4.91 79.4 to examine its phase purity. Figures 3 and 4 show the measured XRD patterns. Figure 3 corresponds to Scheme A, where lanthanum is intended to replace bismuth, while Figure 4 corresponds to Scheme B, where lanthanum is intended to replace sodium. The samples produced with 0.5% lanthanum in both schemes do not show the presence of any extra phases, as the peaks in the pattern correspond to the undoped NBT perovskite structure. As more of the dopant is added to the samples, extra peaks begin to appear, which indicate undesired secondary phases. This is clear when 5% dopant is used as there are several extra peaks in both figures. The extra peaks are marked with red arrows in both Figures 3 and 4 for the 5% samples. The phase purity of the samples decreases with an increase in the percentage of lanthanum.

Figure 2: Inel CPS120 X-ray diffractometer

University of Florida | Journal of Undergraduate Research | Volume 12, Issue 1 | Fall 2010 2 PHASE PURITY AND SITE SELECTION WITH LA DOPING IN NBT

Figure 3: Diffraction patterns of NBT-La produced according to scheme A, with La on Bi site

Figure 4. Diffraction patterns of NBT-La produced according to scheme B, with La on Na site

Conclusion

In this work, lanthanum-doped NBT was successfully lanthanum in both schemes. However, as the amount of produced using solid state processing. Two different doping lanthanum added increased to 5%, extra phases appeared in schemes were used (Schemes A and B) for the addition of both doping schemes. This indicates that adding 5% lanthanum. In Scheme A, the addition of lanthanum is lanthanum to NBT using either doping scheme does not compensated for with an equal reduction in bismuth, while form a stable perovskite. in Scheme B it is compensated for with a reduction in sodium. Density measurements of the prepared ceramics Acknowledgements show that when using Scheme A the density remains relatively unchanged, while in Scheme B it decreases The authors gratefully acknowledge support for this greatly with an increase in lanthanum addition. This work from the University Scholars Program at the behavior indicated that the lanthanum dopant may be more University of Florida, the Research Experience in Materials stable on the bismuth site. (REM) program in the Department of Materials Science at Also, XRD measurements of the processed ceramics the University of Florida, and the National Science show only NBT perovskite peaks at low concentrations of Foundation award DMR-0746902.

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