Journal of Wuhan University of

Technology-Mater. Sci. Ed.

Feb. 2007

91

DOI 10.1007/s11595-005-1091-5

Relationship between Liquid Phase Content and the Orientation of PMNT Ceramics by TGG Method CAO Minghe, LI Dongliang, HAO Hua, LIU Hanxing* (State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China) Abstract: The effects of glass frit on the sintering and electric properties of PMN-PT textured ceramics were investigated. The glass frits, including PbO, Bi2O3 and ZnO, were selected since liquid phase sintering lowered the PMN-PT sintering temperature. The piezoelectric properties of PMN-PT ceramics with glass frit addition are strongly dependent on the densification. The addition of glass frits into PMN-PT matrix reduced the sintering temperature to 1 100 ℃ instead of 1 150 ℃ for samples without glass. The piezoelectric coefficients (d33) of PMN-PT textured ceramics achieved 568 pc/N with 1 wt% excess PbO. Key words:

texture; liquid phase; piezoelectric property; templated grain grouth(TGG)

1 Introduction Relaxor-based ferroelectric material such as Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) with a pervoskite structure has recently attracted much attention. Recent studies have shown that (001) oriented single crystals of PMN-PT exhibit an extraordinarily large electromechanical coupling coefficient (>90%) and piezoelectric coefficient (d 33>2 000 pc/N) coupled with an ultrahigh strain [1]. Its excellent properties are promising in medical ultrasonic imaging, sonar application. But the synthesis of single crystals is complex and the processing costs are high. The superior properties of single crystals are not observed in conventional polycrystalline ceramics because they are averaged out and clamped in polycrystalline samples in which individual grains are randomly oriented. Textured polycrystalline ceramics is possible by polycrystalline processing to produce a single-crystal like material. If the polycrystalline sample is textured, the properties become more anisotropic, and thus are improved in certain directions. An alternate means of texturing a polycrystalline ceramics is known as Templated Grain Growth (TGG) [2-5]. The TGG process has been demonstrated in a wide variety of systems, including mullite [4], Sr 0.53Ba 0.47Nb 2O 6 [5], ZnO [6] and (Received June 17,2005; Accepted: Sept.18,2006) CAO Minghe(曹明贺)：Ph D; Assoc. Prof.; E-mail: [email protected]; * Corresponding author.:LIU Hanxing(刘韩星)： Ph D; Prof.; E-mail: [email protected] Supported by the National Natural Science Foundation of China (No.50502027) and the 973 Project (No. 2002CB613303)

Sr 2Nb 2O 7[7]. In our study, single phase perovskite 0.67PMN-0.33PT powder was selected as the matrix material [8] and (001) plate-like SrTiO 3 particles as template component[9,10], PMN-PT textured ceramics was prepared by TGG method. The purpose of present work was to lower the sintering temperature and improve the piezoelectric properties of PMN-PT ceramics using a new sintering agent based upon PbO, Bi 2O 3 and ZnO. Also, the effect of glass frit on dielectric properties of PMN-PT ceramics was investigated.

2 Experimental The matrix powders and excess frit were placed in a ball mill with an ethanol/butanone tape casting solvent, respectively, and milled for 24 h, and then SrTiO3 platelets were added into the mill, along with polyvinylbutyral as a binder, the slurry was slowly milled for another 8 hours, then removed from the mill. The ratio of matrix precursor to template particles was such that the final material was 90 volume percent PMN-PT, 10 volume percent template particles. The slurry from which air bladders were removed in vacuum was tape cast using a blade of 250 μm in thickness at a shear rate of 20 cm/min. The tape was dried and cut into 1.0 cm squares and laminated in stacks of several layers to form a green body of approximately 2 mm in thickness. The green body was heated to remove the binder, then hot pressed in argon at 950 ℃ for 30 min at a pressure of 30 MPa. Then the samples were heated at 10 ℃/min to 1 050-1 150 ℃

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and annealed in an oxygen atmosphere for another 10 h in order to drive the TGG process. The sample crosssection was polished, thermally etched, and poled. Typical poling conditions involved the application of an electric field of 30 kV/cm across the sample at 80 ℃ for 15 min.

3 Results and Discussion Fig.1 shows the relationship between bulk density and sintering temperature of PMN-PT textured ceramics. For different frits, the density increased obviously from 1 050 to 1 100 ℃ but reduced slightly over 1 100 ℃. With the liquid phase content increase,

Fig 1. The relationship between bulk density and sintering temperature of PMN-PT ceramics Noting：Corresponding to specifically additive, 1 denotes 1 wt%, 2 denotes 2 wt% and 3 denotes 3 wt%.

densities of PMN-PT ceramics also have a general increasing tendency. Compared to samples without frit, the frit can decrease the sintering temperature and obtain a high density. It is considered that the samples have been fully sintered at 1 100 ℃. The piezoelectric properties of PMN-PT textured ceramics are shown in Fig.2. Compared to no excess frit in TGG process, the piezoelectric coefficients (d33) of 1 wt% excess PbO in the matrix powders increased from 480 to 568 pc/N. But in general, with the frit content increase, the piezoelectric properties of PMNPT textured ceramics presented a decline tendency, especially for B2O3. This indicates that the right kind and content of frit materials can help to improve the density and piezoelectric properties of textured ceramics. The saturation of the texturing rate can be attributed in part to the decrease in the growth rate of the single crystal layer due to the coarsening and shape change of the matrix grains. The second factor of slowing the rate of texturing is the impingement of the

templated grains, since the template particles are not ideally situated in the ceramics. Adding the glass frit can decrease sintering temperature and provide excess liquid phase. In the case of generally sintered samples, with the content of liquid phase increasing, the density of ceramics increased, and the polarization particles also increased. So the electric properties of ceramics were improved to some extent. But, if the content of frit keeps on increasing, the liquid phase is also increased, the defect in the crystal structure will also be increased. When the defect caused by excess liquid phase is far higher than the effect of structure density, the piezoelectric properties will be descended. This is also approved by dielectric properties that was measured at 1 MHz at room temperature. The relationship between dielectric properties and different frit additive is shown in Table 1. For the same composition samples, the dielectric properties became higher as the content of frit increased. This was mainly caused by the improvement of the density of the ceramics. But for the dielectric loss, with the liquid phase content increasing, the defects in crystal increased. As a result, the dielectric loss of the ceramics increased. For appropriate frit and content, the dielectric loss of ceramics should be increased slightly.

Journal of Wuhan University of

Technology-Mater. Sci. Ed.

Feb. 2007

93

Table 1 The relationship of dielectric properties and different frit additive The kind of frit Sample names Dielectric constant εr Dielectric loss tanδ/%

P-1 2 020 2.90

PbO P-2 2 180 4.60

P-3 2 480 6.50

4 Conclusions The addition of frits into the PMN-PT could reduce the sintering temperature obviously. The piezoelectric coefficients (d33) of PMN-PT ceramics can be increased from 480 to 568 pC/N with 1 wt% excess PbO as glass frit in the matrix powders. However, the piezoelectric coefficients would be reduced with ZnO or B2O3 as glass frits. The glass frits type and their content should be considered to prepare the textured ceramics with high electric properties.

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B-1 1 550 4.30

B2O 3 B-2 1 680 5.80

B-3 1 930 7.50

Z-1 1 670 3.80

ZnO Z-2 1 890 4.90

Z-3 2 260 7.20

[5] Duran C, Trolier-McKinstry S, Messing GL. Fabrication and Electrical Properties of Textured Sr0.53Ba0.47Nb2O6 Ceramics by Templated Grain Growth[J]. Journal of the American Ceramics Society,2000, 83 (9): 2 203-2 213 [6] Suvaci E, Ozer IO, Dogan A. Textured ZnO-Based Varistors via Templated Grain Growth[J]. Key Engineering Materials, 2004, 264-268: 297-300 [7] Brahmaroutu B, Messing GL, Trolier M, Kinstry S, et al. Templated Grain Growth of Textured Sr2Nb2O7[J].Ferroelectrics, 1996, 1-2 : 883-886 [8] Wen BS, Sheng WJ, Ma JH. Polar Microdomain in Relaxor Ferroelectric Ceramics and Its Electrostrictive Effect[J]. Ferroelectrics, 1997, 195 (1-4): 145-148 [9] Messing GL, Trolier-McKinstry S, Sabolsky EM, et al. Templated Grain Growth of Textured Piezoelectric Ceramics[J]. Critical Reviews in Solid State and Materials Science, 2004, 29 (2): 45-96 [10] Liu HX, Sun XQ, Zhao QL, et al. The Syntheses and Microstructures of Tabular SrTiO 3 Crystal[J]. Solid-State Electronics, 2003, 47 (12): 2 295-2 298