Effects of Niobium Doping on the Microstructure and Electrical Properties of 0.36BiScO 3 –0.64PbTiO 3 Ceramics Si Chen, Xianlin Dong, w Hong Yang, Ruihong Liang, and Chaoliang Mao Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China Recently, a new family of piezoelectric perovskite materials based on the solid solution (1x)BiScO 3 –xPbTiO 3 was devel- oped. This system was found to have a Curie temperature higher than 4501C and excellent piezoelectric properties near the MPB composition. Niobium, as a donor dopant in the piezoelectric system Pb(Zr,Ti)O 3 and other lead-based perovskite materials, has commonly been used to increase the electrical resistivity, dielectric, and piezoelectric properties. In the current work, the effect of niobium substitution in the BS–PT system has been reported. The results of niobium additions in the BS–PT system showed no large enhancement of the piezoelectric prop- erties. Niobium doping also led to lower Curie temperatures and higher dielectric loss. Further grain size effects in niobium-doped BS–PT compositions provided experimental evidence of signif- icant extrinsic contributions to the piezoelectric properties in this system. I. Introduction P IEZOELECTRIC sensors are being developed for extreme-tem- perature applications. These devices may find applications in space exploration, electric aircraft, oil and geothermal well drill- ing tools, and automotive smart brakes. Many of these appli- cations require operation temperatures higher than 3001C. 1 Traditional piezoelectric materials based upon Pb(Zr x Ti 1x )O 3 (PZT) exhibit a T c around 3601C (PZT5A), with values of the piezoelectric coefficient d 33 about 370 pC/N. However, these materials can only be used below 2001C due to the loss of polarization. 2 Recently, bismuth-based perovskite systems Bi(Me)O 3 –PbTiO 3 (Me 5 Sc 31 , In 31 , Yb 31 , etc.) were found to possess much higher Curie temperatures. 3 Specifically, the (1x)BiScO 3 –xPbTiO 3 system was found to have a Curie tem- perature around 4501C and excellent piezoelectric properties near the MPB composition, which make this kind of material a promising candidate for high-temperature applications. 3–10 PZT ceramics are almost always used with a dopant, a modi- fier, or other chemical additive to optimize their properties for specific applications. Modifications to the PZT systems are clas- sified as either donor-doped ‘‘soft’’ or acceptor-doped ‘‘hard’’ formulations. 2,11 The use of suitable donor or acceptor dopants results in either enhancement or clamping of the domain wall contributions to the dielectric and piezoelectric properties. The similarity of the piezoelectric properties and MPB behavior of the (1x)BiScO 3 –xPbTiO 3 system to PZT suggests an analo- gous approach to optimizing the properties of the (1x)BiScO 3 – xPbTiO 3 system. In the current work, we investigate the effect of donor doping on the dielectric and piezoelectric properties of 0.36BiScO 3 – 0.64PbTiO 3 . The donor dopant used in this study is Nb 51 substitution for Ti 41 on the B site, following the typical ABO 3 perovskite nomenclature. Furthermore, the influence of niobium additions and a resulting grain size effect on the dielectric and piezoelectric properties are discussed. II. Experimental Procedures (1) Sample Preparation Traditional mixed-oxide ceramic processing was used to prepare all samples in this work. Niobium-doped samples were prepared following the A-site compensated formulation, modified for preferential substitution of Nb 51 for Ti 41 . The general forma- tion used for niobium-doped 0.36BiScO 3 –0.64PbTiO 3 (BS– PNT) samples is given below. 0:36BiScO 3  0:64ðPb 1x=2 ; V Ax=2 ÞðTi 1x ; Nb x ÞO 3 (1) This batch formulation accommodated the charge difference on the B site through the formation of A-site vacancies (V A ), ignoring the effects of bismuth and lead volatility during the sintering process. In this work, niobium additions of x 5 0, 0.5, 1, 2, and 4 mol% were chosen. The starting raw materials were Bi 2 O 3 (pur- ity 99.90%, Shanghai Experimental Reagent Co. Ltd, Shanghai, China), TiO 2 (purity 99.38%, Shanghai Coking & Chemical Factory, Shanghai, China), Sc 2 O 3 (purity 99.90%, Shanghai Ex- perimental Reagent Co. Ltd), Pb 3 O 4 (purity 99.72%, Shanghai Longrun Chemical Plant, Shanghai, China), and Nb 2 O 5 (Purity 99.00%, Shanghai Experimental Reagent Co., Ltd). Raw mate- rials were mixed stoichiometrically. 0.5 wt% excess amounts of Pb 3 O 4 and Bi 2 O 3 were added to each composition to compen- sate for volatilization during sintering. Aqueous suspensions of all raw materials were ball-milled with stabilized zirconia media for 24 h and dried at 1201C. The dried powders were then cal- cined at 7501C for 6 h and ball milled again for 24 h to crush the agglomerates. After drying, about 4 wt% PVA was mixed with the powders to improve the green strength of compacts. The mixture was dried and crushed to pass through a 40-mesh sieve. The powders were pressed into disks of 16 mm diameter at 200 MPa. Following an 8001C binder burnout, pellets were then covered with powders of the same composition, and sintered in sealed crucibles at 1000, 1100, 1150, and 12001C for 90 min. (2) Sample Characterization X-ray powder diffraction was performed using an automated dif- fractometer (Model Rigaku RAX-10, Tokyo, Japan) with Cuk a1 radiation operated at room temperature to determine phase as- semblage and purity within detection limits. For electrical meas- urements, Ag paste was applied to both sides of the disks and then heated to 6501C for 30 min. The electroded specimens were poled in silicone oil at 1001C by applying a dc field of 4 kV/mm for 15 min. The piezoelectric coefficient was measured using a d 33 meter (Model ZJ-3D, Institute of Acoustics, Beijing, China). The electromechanical coupling factor (k p ) was measured by the res- onance and anti-resonance technique using an Agilent 4294A (40 Hz–110 MHz; Hewlett-Packard, Palo Alto, CA). The dielectric C. Randall—contributing editor w Author to whom correspondence should be addressed. e-mail: xldong@sunm. shcnc.ac.cn Manuscript No. 21882. Received June 6, 2006; accepted October 4, 2006. J ournal J. Am. Ceram. Soc., 90 [2] 477–482 (2007) DOI: 10.1111/j.1551-2916.2006.01443.x r 2006 The American Ceramic Society 477