• PDF / 2,338,909 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 97 Downloads / 249 Views

DOWNLOAD

REPORT

---

B5.12.1

Ordered Structures in Ba(Cd1/3Ta2/3)O3 Microwave Ceramics: A Transmission Electron Microscopy Study J. Sun1, 4, S. J. Liu2, N. Newman2, and David. J. Smith1, 3 1 Center for Solid State Science, 2 Department of Chemical and Materials Engineering, Materials and Engineering Program, 3Department of Physics and Astronomy, Arizona State University, Tempe AZ 85287, U.S.A., 4School of Materials Science and Engineering, Shanghai Jiao-tong University, Shanghai 20003, P. R. China ABSTRACT Ordered structures of Ba(Cd1/3Ta2/3)O3 ceramics with and without boron additive were investigated systemically by electron diffraction and high resolution transmission electron microscopy. The results showed a well-ordered structure of 1:2 with hexagonal symmetry for Ba(Cd1/3Ta2/3)O3 with boron additive. No significant changes in ordered structures were observed after long-period annealing. The 1:2 ordered domain structures (average domain size ~18 nm) and high-density domain boundaries induced by ordering were observed for Ba(Cd1/3Ta2/3)O3 without boron additive sintered at relatively high temperature. The sintering process has a profound influence on the microstructure of Ba(Cd1/3Ta2/3)O3 ceramics. INTRODUCTION Ba(B'1/3 B''2/3)O3 (B' = Mg and Zn, B'' = Ta) are important dielectric ceramics for microwave communication systems because of their excellent high frequency properties [1-3]. Ba(B'1/3Ta2/3)O3 ceramics are referred to as 1:2 complex perovskites, which can be described as a hexagonal unit cell containing three close-packed BaO3 layers with B ions in an ordered arrangement in the octahedral hole (space group: P 3 m1) [4, 5]. B-site cations are ordered in 1:2 ratios with one B' layer and two B'' layers repeated along direction of the parent cubic perovskite cell. It has been reported that the degree of B-site ordering increases with extended high temperature annealing, and the B-site ordering arrangement is critical in optimizing the microwave dielectric properties for Ba(B'1/3Ta2/3)O3 ceramics [6, 7]. Ba(Cd1/3Ta2/3)O3 belongs to the family of Ba(B'1/3Ta2/3)O3 perovskite ceramics. Galasso and Pyle studied the structure of various niobates and tantalates and proposed that the B-site ordering increases as the difference in valence and size between B' and B'' atoms increased in A(B'1/3 B''2/3)O3 perovskites [4, 5]. Because the difference in ionic size between B' and B'' for Ba(Cd1/3Ta2/3)O3 is much larger than those for Ba(Zn1/3Ta2/3)O3 and Ba(Mg1/3Ta2/3)O3, the degree of ordering should be much higher for Ba(Cd1/3Ta2/3)O3 compared with Ba(Zn1/3Ta2/3)O3 and Ba(Mg1/3Ta2/3)O3 ceramics. Experimental and theoretical investigations of structure and dielectric properties of Ba(Cd1/3Ta2/3)O3 were carried out in recent years. The results show that the Ba(Cd1/3Ta2/3)O3 has a reasonably high dielectric constant and low loss tangent and is a

B5.12.2

potential material for microwave applications [8, 9]. However, the microstructure of the Ba(Cd1/3Ta2/3)O3 ceramics has not been studied, especially using transmission electron microscopy (TEM)