Lead-Free Piezoelectric Freestanding Films with Sheet Geometry-Enhanced High-Field Piezoelectric Coefficients Huidong Li z and Wei-Heng Shih w Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104 Wan Y. Shih School of Biomedical Engineering, Science, and Health Systems, Drexel University, Philadelphia, Pennsylvania 19104 (Na 0.5 K 0.5 )NbO 3 -based materials are promising lead-free piezo- electrics because of their relatively high Curie temperatures and piezoelectric coefficients. Using precursor powders made by a coating method, (Na 0.5 K 0.5 ) 0.945 Li 0.055 Nb 0.96 Sb 0.04 O 3 thick films were successfully fabricated by pressureless sintering. Large high-field Àd 31 value of 1700 pm/V, 420 times larger than that of the bulk counterpart, was observed at an applied dc electric field of 6–8 kV/cm. In situ X-ray diffraction study showed that the large piezoelectric response was partially due to non-1801 domain switching under the electric field. Further in- vestigation is needed to elucidate this enhancement effect. The enhanced high-field piezoelectric property observed should en- courage the use of lead-free piezoelectrics. I. Introduction P IEZOELECTRIC films are widely used in piezoelectric micro- electromechanical systems (PMEMS). Most of the PMEMS devices are made of lead-based piezoelectric materials such as lead zirconate titanate. Because of the toxicity of lead, there have been intensive efforts around the globe to limit the use of lead. For instance, the European Union has established envi- ronmental guidelines to prohibit lead-containing materials in a majority of electronics starting from 2006. 1 Several Asian coun- tries have taken similar measures because they export electronics to European countries. Because of the reduced size and power consumption, and improved performance, PMEMS have en- joyed steadily increased applications. However, in many appli- cations especially biomedical applications the presence of lead is unacceptable. There is a need to develop lead-free PMEMS for biomedical applications. For PMEMS applications, thick films made by tape casting and sintered without a substrate are most suitable because thin films made on a substrate exhibit a much reduced piezoelectric coefficient due to pinning by the substrate and material interdiffusion with the substrate. Ease of device fabrication is another benefit of substrate-free films. So far, much of the lead-free piezoelectric research has been devoted to developing the bulk materials and substrate-based thin films; very few studies were carried out for thick films with- out a substrate 2 due to the difficulty in preparing good sub- strate-free lead-free piezoelectric films. Sodium potassium niobate, Na 0.5 K 0.5 NbO 3 (NKN)-based piezoelectrics are among the most studied lead-free piezoelectrics. 3 Numerous studies have tried to improve the piezoelectric coefficient of NKN-based piezoelectrics by solid solution and doping. 4,5 While NKN- based solid solutions offer promising piezoelectric coefficients, the difficulty of making NKN-based films without a substrate is that they require pressure-assisted sintering, which is incompat- ible with substrate-free processing. Recently, we have shown that by doping antimony in the solid solution of NKN and LiNbO 3 (LN) and with a precursor coating approach, bulk pieces of (Na 0.5 K 0.5 ) 0.945 Li 0.055 Nb 0.96 Sb 0.04 O 3 (Sb–NKN–LN) could be sintered at 11201C without pressure with a grain size 45 m that exhibited a d 33 4240 pC/N, and a dielectric constant of 1000. 6 With a processing route permitting pressure-less sinte- ring, it is then possible to make lead-free piezoelectric substrate- free films by tape casting for potential PMEMS applications. The purpose of this study is to fabricate lead-free Sb–NKN– LN films without a substrate using the precursor coating and a pressureless sintering process and evaluate their piezoelectric performance. II. Experimental Procedure The Sb–NKN–LN film made without a substrate (freestanding film hereafter) was made from the Sb–NKN–LN precursor powder prepared by coating sodium and potassium precursors on the Nb 2 O 5 and Sb 2 O 5 particles in an aqueous solution. 5 After calcination at 8501C for 2 h, the powder was ball-milled for 24 h and sieved using #45 and #100 meshes. The powder was then mixed with a proprietary dispersing resin and ball-milled in an alcohol–ketone mixture for 24 h. With the remaining resin and a phthalate-based plasticizer, the precursor powder was further ball-milled for 24 h, deaired, and cast into tapes of desired thickness. The green tapes were then placed in an alumina cru- cible sealed with packing powder that had the same composition as the tape and sintered at 11201C for 2 h. A photograph of a 40 mm thick thus sintered Sb–NKN–LN freestanding film is shown in Fig. 1(a). As observed in the figure, the film was trans- lucent with a yellowish tint. The scanning electron microscopy (SEM) (FEI/Phillips XL30, FEI, Hillsboro, OR) cross-section micrograph of the film is shown in Fig. 1(b). To measure the d 31 piezoelectric coefficient, the 40-mm-thick Sb–NKN–LN freestanding film was first coated with Pt/Pd elec- trode by sputtering, and then cut to an 11-mm-long, 1.6-mm- wide strips (the inset of Fig. 2) by a wire saw (WS22, Princeton Scientific Corp., Princeton, NJ). A microscope glass slide was glued to one end of the strip with a nonconductive epoxy. The sample was poled at 25 kV/cm and 1201C for 30 min on a hot plate, and then aged for over 24 h before the piezoelectric mea- surements. A small titanium foil (0.5 mm  0.5 mm  0.025 mm) was attached at the free end as a mirror for a Keyence LC-2450 laser displacement meter that had a 0.5 mm resolution (Keyence D. Damjanovic—contributing editor This work is financially supported in part by the National Institute of Health under Grant No. 1 R01 EB000720 and the Nanotechnology Institute (NTI) of South Eastern Pennsylvania. w Author to whom correspondence should be addressed. e-mail: shihwh@drexel.edu z Present address: TBT Group Inc., Berlin, NJ. Manuscript No. 26306. Received May 20, 2009; approved December 28, 2009. J ournal J. Am. Ceram. Soc., 93 [7] 1852–1855 (2010) DOI: 10.1111/j.1551-2916.2010.03632.x r 2010 The American Ceramic Society 1852