B-site Multi-element Doping Effect on Electrical Property of Bismuth Titanate Ceramics
Ferroelectrics - Physical Effects
Introduction This article represents a systematic review of the behaviour of B-site multi-element doping effect on electrical property of bismuth titanate ceramics. Bismuth titanate, Bi 4 Ti 3 O 12 (BIT) is a potential candidate for high-temperature device applications due to their high dielectric constant, Curie temperature (T c ), breakdown strength, anisotropy and, low dielectric dissipation factor, therefore attracting considerable commercial interest in applications such as high
... piezoelectric devices, memory storage and optical displays. These features make bismuth layer-structured ferroelectrics (BLSFs) attractive in the field of developing lead-free piezoelectric materials. BIT is a well-known member of the BLSFs, which can be represented by a general formula (Bi 2 O 2 ) 2+ (A m-1 B m O 3m+1 ) 2-, where A represents a mono-, di, or trivalent ion, such as La 3+ , Nd 3+ , Pr 3+ , Sm 3+ , etc., B stands for transition-metal cations like Ti 4+ , Nb 5+ , Ta 5+ , W 6+ , etc., and m is the number of BO 6 octahedra in the perovskite-like layer (m=1-5) (). The layer structure of Bi 4 Ti 3 O 12 (m=3) consists of three perovskitelike (Bi 2 Ti 2 O 10 ) 2-units with a pseudo-perovskite layer structure, sandwiched between (Bi 2 O 2 ) 2+ layers along its crystallographic c axis. In the (Bi 2 Ti 2 O 10 ) 2-units, Ti ions are enclosed by oxygen octahedra, and Bi ions occupy the spaces in the framework of octahedral (Subbarao, 1950). BIT is of interest in high-temperature piezoelectric sensors, because it remains ferroelectric up to 675 °C and offers relatively high piezoelectric property (Subbarao, 1961) . However, the high leakage current and domain pinning due to defects in BIT have appeared as obstacles for further applications. The reasons for such problems are suggested due to the instability in the oxidation state of Ti ions and the volatile property of Bi during the sintering process (Nagata, 1999) . Great efforts have been made to solve the high-leakage current, by incorporation of W, Nb or Ta dopants, as these can significantly decrease the conductivity in ). Unfortunately, the piezoelectric effect in the high Curie temperature BIT is relatively low, with coefficient values typically less than 20 pC N -1 found for both pure and modified Bi 4 Ti 3 O 12 . Thus, it is a challenge to seek a rational pathway to improve the electrical property of BIT ceramics. www.intechopen.com Ferroelectrics -Physical Effects 244 We have reported in our publications, the effects of composition and crystal lattice structure upon microstructure, dielectric, piezoelectric and electrical properties of BIT, Bi 4 Ti 3- x W x O 12+x +0.2wt%Cr 2 O 3 (BTWC), Bi 4 Ti 3-2x Nb x Ta x O 12 (BTNT) and Bi 4 Ti 3-2x Nb x Ta x-y Sb y O 12 (BTNTS) ceramics have been widely investigated (Hou et al, 2009(Hou et al, and 2011. The processing of as-synthesized BIT were optimized and the main parameters were determined, and confirmed that the piezoelectric coefficient (d 33 ) of undoped BIT is 8 pC N -1 . As for BTWC, the results have shown the systematic changes in the lattice parameters; the formation of secondary phase(s) at higher levels of W/Cr doping; and the increase in dielectric constant and loss at room temperature with increase of doping content. A higher value of d 33 , at 22 pC N -1 , was obtained for the sample with x=0.025, which may result directly from lowering conductivity. With regard to BTNT and BTNTS, the results have shown the formation of orthorhombic structure for all the samples within these family of dopants; the addition of Nb/Ta caused a remarkably suppressed grain growth while there is not much difference in grain size for BTNTS; as the doping content was increased, the Curie temperatures of BTNT decreased significantly, to as low as 630 o C while the difference is not very significant for BTNTS. The co-doping at B-site could induce the distortion of oxygen octahedral and reduce the oxygen vacancy concentration, resulting in the enhancement of d 33 . Especially, the highest values of 26 and 35pC N -1 , were obtained for Bi 4 Ti 2.98 Nb 0.01 Ta 0.01 O 12 and Bi 4 Ti 2.98 Nb 0.01 Ta 0.002 Sb 0.008 O 12 , respectively. The activation energy associated with the electrical relaxation and DC conductivity were determined from the electric modulus spectra, suggesting the movements of oxygen ions are possible for both ionic conductivity as well as the relaxation process. To ascertain the electrical conduction mechanism in the ceramics, various physical models have been proposed, suggesting the conductivity behavior of the ceramics can be explained using correlated barrier hopping model. All measurements demonstrated that BTNT ceramics are promising candidates for high temperature applications.