ABSTRACT. THE INFLUENCE OF V 2 O 5 ON THE CHARACTERISTICS OF BaBi 2 Nb 2 O 9 CERAMICS

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1 THE INFLUENCE OF V 2 O 5 ON THE CHARACTERISTICS OF BaBi 2 Nb 2 O 9 CERAMICS Cheng-Yuan Kung 1, Yin-Fang Wei 2, Ching-Chich Leu 3, Ying-Chung Chen 1, Ho-Hua Chung 4, and Chien-Chen Diao 2 Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, R.O.C. 1 Department of Electronic Engineering, Kao Yuan University, Kaohsiung, Taiwan, R.O.C. 2 Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung, Taiwan, R.O.C. 3 Department of Mechanical and Automation Engineering, Kao Yuan University, Kaohsiung, Taiwan, R.O.C. 4 ABSTRACT In this study, V 2 O 5 is used to substitute Nb 2 O 5 site of BaBi 2 Nb 2 O 9 ceramics to form BaBi 2 Nb 2-x V x O 9 compositions (0 x 0.4) and the BaBi 2 Nb 2-x V x O 9 ceramics are sintered at 950 o C ~1050 o C. The effect of V 2 O 5 content on the sintering and dielectric characteristics of BaBi 2 Nb 2-x V x O 9 (0 x 0.4) ceramics is developed. All BaBi 2 Nb 2-x V x O 9 ceramics reveal an orthorhombic structure independent on the V 2 O 5 content, and this result suggests that V 2 O 5 will completely substitute the Nb 2 O 5 site. Compared the XRD patterns of BaBi 2 Nb 2-x V x O 9 ceramics, the 2θ values are shifted to lower values and the lattice constants a and c gradually decrease with the increase of V 2 O 5 content. The dielectric characteristics are investigated in the temperature of 25~465 o C at 1MHz. It is found that the V 2 O 5 content have no apparent influence on the grain growth, and the V 2 O 5 content has large effect on the maximum dielectric constants and the Curie temperatures of BaBi 2 Nb 2-x V x O 9 ceramics. Keywords: BaBi 2 Nb 2 O 9, V 2 O 5, XRD, dielectric properties INTRODUCTION In the past year, for the developed nonvolatile random access memories (NvRAMs) materials, PbZrTiO 3 thin films degrade most of the initial amount of switching charge after 10 6 ~10 8 cycles of full polarization switching [1]. The layer structured bismuth compound ferroelectric has the general formula: A n-1 Bi 2 B n O 3n+3, where A is usually a divalent ion, such as Sr, Ba, or Pb, and B is Ti 4+, Nb 5+, or Ta 5+ [2-4]. Within the bismuth family, SrBi 2 Ta 2 O 9 and BaBi 2 Ta 2 O 9 ceramics had attracted the most attention in the past years [5-6]. Although the polarizations of SrBi 2 Ta 2 O 9 and BaBi 2 Ta 2 O 9 ceramics is less than the competing PZT-based materials, the bismuth-layer compounds are much stables to polarization fatigue free property, i.e. almost no charge loss will happen when polarization is reversed many cycles. BaBi 2 Nb 2 O 9, SrBi 2 Ta 2 O 9, and SrBi 2 Nb 2 O 9 compositions are three of the Aurivillus phases, which are a class of layer structured bismuth oxides [7]. In SrBi 2 Ta 2 O 9 and SrBi 2 Nb 2 O 9 compositions, V 2 O 5 can be used to substitute the Ta 2 O 5 and Nb 2 O 5 site to form SrBi 2 Ta 2-x V x O 9 and SrBi 2 Nb 2-x V x O 9, and their dielectric characteristics were developed before. In the SrBi 2 Ta 2 O 9,

2 and SrBi 2 Nb 2 O 9 composition, the substitution of V 2 O 5 will help to lower the sintering temperatures and produce materials with enhanced dielectric properties that are useful in applications [8]. In this study, ceramic materials based on BaBi 2 Nb 2 O 9 composition were investigated, V 2 O 5 was used to substitute for Nb 2 O 5 to form the composition of BaBi 2 Nb 2-x V x O 9 composition, where 0 x 0.4. Bulk BaBi 2 Nb 2-x V x O 9 ceramic materials were sintered at different temperatures and their morphologies and crystal phases were examined. The relative dielectric constants were investigated as a function of temperature and sintering temperature. EXPERIMENTAL PROCEDURE The BaBi 2 Nb 2-x V x O 9 ceramic materials were prepared by the mixed oxide method. Reagent-grade raw materials of BaCO 3, Bi 2 O 3, Nb 2 O 5, and V 2 O 5 with higher than 99.5% purity were used as starting materials, mixed according to the composition BaBi 2 Nb 2-x V x O 9, where x=0, 0.1, 0.2, 0.3, and 0.4. Then, agate balls were used to mill the powders for 4 hour with deionized water. The dried and ground powders were calcined at 850 o C for 3h. After calcining, the powders were ground again, and polyvinyl alcohol (PVA) was added as binder. The calcined powders were pressed into the pellets in a steel die with the size of 1mm in thickness and 12mm in diameter. After debinding, these pellets was carried out from 950 o C to 1050 o C for 4h. The crystal structures of BaBi 2 Nb 2-x V x O 9 ceramics were investigated using X-ray diffraction patterns (XRD) for phase identification in the range 2θ=20 o ~60 o with Cu-Kα radiation. Micro-structural analyses of the sintered specimens were observed by using scanning electronic micrograph (SEM). The sintered ceramics were painted with Ag-Pd paste and sintered at 700 o C for 15min. Temperature-dependent dielectric properties were measured with an oscillate amplitude (50mV) by an HP4294A impedance/gain phase analyzer, putting the specimens in a temperature programmable testing chamber. For each sintering temperature, two samples were used to find the maximum dielectric constant and Curie temperature. RESULTS AND DISCUSSION SEM analyses (Fig. 1) revealed the changes in the morphologies of BaBi 2 Nb 2-x V x O 9 ceramics a under the sintering temperature of 1050 o C. The pores are not observed in the BaBi 2 Nb 2 O 9 ceramic (Fig.1(a)) and the sintered surfaces reveal a dense surfaces, but the grain growth is not easy observed. As the V 2 O 5 content increases, for BaBi 2 Nb 1.8 V 0.2 O 9 and BaBi 2 Nb 1.7 V 0.3 O 9 ceramics, the sintered ceramics reveal the dense surfaces and grain growth is improved (Figs.1(b) and Figs.1(c)). Too much V 2 O 5 content seems not be necessary, for example, BaBi 2 Nb 1.6 V 0.4 O 9 ceramics, even the ceramic reveals a dense surface, but the grain growth is not easy observed (Fig.1(d)). Typical X-ray diffraction (XRD) patterns of BaBi 2 Nb 2-x V x O 9 ceramics are shown in Fig.2, as a function of V 2 O 5 content and sintered at 1050 o C. Pentavalent vanadium ions (V 5+, 0.58 Å) have a smaller size compared with niobate ions (Nb 5+, 0.69Å). Although vanadium ions can have six coordination of oxygen, it is too small to form a stable layered perovskite structure. However, within the sintering temperatures of 950 o C~1050 o C, X-ray diffraction analysis on the

3 (a) (b) (c) (d) Fig.1 The micrographs of BaBi 2 Nb 2-x V x O 9 ceramics, sintered at 1050 o C and with different V 2 O 5 content. (a) x=0, (b) x=0.2, (c) x=0.3, and (d) x=0.4. x=0.4 x=0.3 Intensity x=0.2 x=0.1 x= θ value Fig.2 The X-ray patterns of BaBi 2 Nb 2-x V x O 9 ceramics, sintered at 1050 o C and with different V 2 O 5 content. BaBi 2 Nb 2-x V x O 9 ceramics indicate that only the single-phase layered perovskite structure is found, no secondary or unknown phases are detectable. These results suggest that Nb 2 O 5 will completely be substituted by V 2 O 5. Figure 3 shows the changes of the lattice constants a and c axis of BaBi 2 Nb 2-x V x O 9 ceramics as a function of V 2 O 5 content. At room temperature, the BaBi 2 Nb 2 O 9 ceramic has the orthorhombic structure. Similarly, the BaBi 2 Nb 2-x V x O 9 ceramics have the orthorhombic structure. Compared the XRD patterns shown in Fig.2, the 2θ values will shift to lower values as the V 2 O 5 content increases, and as Fig.3 shows, the lattice constants a and c gradually decrease with the

4 increase of V 2 O 5 content. The vanadium ions (V 5+, 0.58 Å) have a smaller size compared with niobate ions (Nb 5+, 0.69Å) is the reason. As the V 2 O 5 content increases, it is believed that more V 5+ ions to substitute the ions of Nb 5+ and that will lead to the decrease of the lattice constants a and c. Lattice constants (A) o c axis a axis V 2 O 5 content Fig.3 The lattice constant of BaBi 2 Nb 2-x V x O 9 ceramics. Figure 4 shows the Curie temperatures of BaBi 2 Nb 2-x V x O 9 ceramics as a function of sintering temperature and V 2 O 5 content. As Fig.4 shows, for BaBi 2 Nb 2 O 9 ceramic, the Curie temperatures change from 245 o C~265 o C as the sintering temperature increases from 950 o C~ 1050 o C. For BaBi 2 Nb 1.9 V 0.1 O 9 ceramic, the Curie temperatures are unchanged at 265 o C as the sintering temperature increases. For BaBi 2 Nb 1.8 V 0.2 O 9, BaBi 2 Nb 1.7 V 0.3 O 9, and BaBi 2 Nb 1.6 V 0.4 O 9 ceramics, the Curie temperatures change from 245 o C~225 o C as the sintering temperature increases from 950 o C~ 1050 o C. 270 Tc ( o C) 240 ε max o x=0 x=0.1 x=0.2 + x=0.3 x x= sintering temperature ( o C) Fig.4 The Curie temperatures and the maximum dielectric constants of BaBi 2 Nb 2-x V x O 9 ceramics as a function of sintering temperature and V 2 O 5 content.

5 The maximum dielectric constants of BaBi 2 Nb 2-x V x O 9 ceramics (ε Tc, revealed at Curie temperatures) as a function of sintering temperature and V 2 O 5 content are also shown in Fig.4. Even only 950 o C is used as the sintering temperature, the ε Tc values of BaBi 2 Nb 2-x V x O 9 ceramics about 335~409. The ε Tc values linearly increase as the sintering temperatures increase from 950 o C to 1050 o C. The decrease in pores and increase in grain growth will account for this result. In this study, BaBi 2 Nb 1.7 V 0.3 O 9 ceramics reveal a higher ε Tc values than the other BaBi 2 Nb 2-x V x O 9 ceramics do. The temperature-dependent dielectric constants of BaBi 2 Nb 2-x V x O 9 ceramics are shown in Fig.5 as a function of V 2 O 5 content and sintered at 1050 o C. For BaBi 2 Nb 2-x V x O 9 ceramics, the maximum dielectric constants first increase with the increase of V 2 O 5 content, reach a maximum in the BaBi 2 Nb 1.7 V 0.3 O 9 ceramics, then slightly decrease in the BaBi 2 Nb 1.6 V 0.4 O 9 ceramics. As Fig.5 shows, the Curie temperatures are not apparently observed, because around the Curie temperatures, the dielectric constants of BaBi 2 Nb 2-x V x O 9 ceramics reveal the broaden curves. Maximum dielectric constant o x=0 x=0.1 x=0.2 + x=0.3 x x= sintering temperature ( o C) Fig.5 The temperature-dependent dielectric constants of BaBi 2 Nb 2-x V x O 9 ceramics as a function of V 2 O 5 and sintered at 1050 o C. CONCLUSION Solid state reacted BaBi 2 Nb 2-x V x O 9 ferroelectric ceramics (x=0.1, 0.2, 0.3, and 0.4) had been investigated completely. The tunable Curie temperatures and dielectric constants are measured and observed by X-ray powder diffraction (XRD), scanning electron microscope (SEM) and the dielectric measurement in this study. 1. The maximum dielectric constants of BaBi 2 Nb 2-x V x O 9 ceramics increase with the increase of sintering temperature. 2. In the BaBi 2 Nb 2-x V x O 9 compositions (0 x 0.4), the Curie temperatures are in the ranges of 225 o C~265 o C. The Curie temperatures are not apparently observed, because around the Curie temperatures, the dielectric constants of BaBi 2 Nb 2-x V x O 9 ceramics reveal the broaden curves. 3. The maximum dielectric constants of BaBi 2 Nb 2-x V x O 9 ceramics first increase, reach a

6 maximum reach a maximum in the BaBi 2 Nb 1.7 V 0.3 O 9 ceramics, then slightly decrease in the BaBi 2 Nb 1.6 V 0.4 O 9 ceramics. 4. As the V 2 O 5 content increases, it is believed that more V 5+ ions to substitute the ions of Nb 5+ and that will lead to the decrease of the lattice constants a and c. The vanadium ions (V 5+, 0.58 Å) have a smaller size compared with niobate ions (Nb 5+, 0.69Å) is the reason. REFERENCES [1] B.Aurivillius, Ark.Kemi 1 (1949) 463. [2] M.J.Forbess, S.Seraji, Y.Wu, C.P.Nguyen and G.Z.Cao, Appl.Phys.Lett. 76 (2000) [3] X.Y.Zhang, Z.Z.Huang, H.L.W.Chan, K.W.Kwok and C.L.Choy, J.Europ.Ceram.Soc. 19(1999)985. [4] Y.Noguchi, M.Miyayama, and T.Kudo, Appl.Phys.Lett. 77 (2000) [5] AE.C.Subbarao, J.Phys.Chem.Solids 23(1962) 665. [6] R.L.Withers, J.G.Thompson, and A.D.Rae, J.Solid State Chem. 94 (1991) 404. [7] C.A.Paz de Araujo, J.E.Cuchiaro, L.D.McMillian, M.C.Scott, and J.F.Scott, Nature 374(1995) 627. [8] Y.Wu, M.J.Forbess, S.Seraji, S.J.Limmer, T.P.Chou, and G.Cao, Mater.Sci.Eng., B.86 (2001) 70.