Ceramics: Templated Grain Growth and Piezoelectric Properties. Seongtae Kwon, Edward M. Sabolsky, Gary L. Messing, w and Susan Trolier-McKinstry.
Journal
J. Am. Ceram. Soc., 88 [2] 312–317 (2005) DOI: 10.1111/j.1551-2916.2005.00057.x
High Strain, /001S Textured 0.675Pb(Mg1/3Nb2/3)O3–0.325PbTiO3 Ceramics: Templated Grain Growth and Piezoelectric Properties Seongtae Kwon, Edward M. Sabolsky, Gary L. Messing,w and Susan Trolier-McKinstry Department of Materials Science and Engineering, Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802
plates should possess an anisometric morphology to facilitate mechanical alignment within the matrix during tape casting, extrusion, or uniaxial pressing. The single-crystal templates must have the desired crystallographic orientation and an appropriate lattice match to the crystal to be grown. To serve as a substrate for nucleation and growth of the desired phase, the template particles should be thermodynamically stable at the TGG process temperature. The matrix material must result in a fine grain size matrix since the growth process is driven by the difference in surface free energy between the template and the matrix grains during thermal processing. A dense matrix is required for TGG because porosity reduces the driving force for boundary motion.16 Li et al.17 showed that single-crystal PMN–PT can be grown on a single crystal of PMN–35PT embedded in a powder matrix of the same composition when heated at 11501C for extended times. Li et al. termed the crystal growth process seeded polycrystal conversion. The same group later showed that /111Soriented PMN–PT could be obtained by conversion from {111}SrTiO3 (ST).18,19 In both instances, the growth of the PMN–PT single crystals required excess PbO, which is known to reduce the dielectric properties of PMN–PT and causes significant processing challenges due to its volatility at high temperature. In any case, seeded conversion and templated grain growth processes for growing single crystals and textured ceramics typically use only a small volume fraction of PbO (o5 vol%) in order to avoid compositional fluctuations that are observed in flux-grown, Pb-based perovskite single crystals.20–22 The low liquid phase content dramatically increases the growth rate. Sabolsky et al.3 showed that coarse-grained, /001S-textured PMN–PT ceramics can be produced using 200–300 mm, tabular /001S BaTiO3 particles as templates. Such large template particles necessitated hot pressing in the presence of excess PbO to obtain a dense ceramic. While excellent dielectric and piezoelectric properties were demonstrated with /001S-textured ceramics,4–6 the cost of hot pressing and the use of excess PbO diminish the commercial potential of BaTiO3-templated PMN– PT. Furthermore, the large grain size of the textured ceramic (400–600 mm) significantly compromises the mechanical strength of the textured ceramics. While these difficulties could be mitigated, in principle, by the use of finer tabular BaTiO3 templates, there is currently no source for such particles. In the present work, we demonstrate that highly dense, relatively fine grain size, textured PMN–32.5PT ceramics can be obtained by TGG using {001}-ST as template particles. ST particles were selected because they can be produced in tabular form, are relatively small in size, and have a small lattice parameter mismatch with PMN–PT.23,24 In this paper, we correlate the effects of sintering time and use of excess PbO on texture development. The dielectric and piezoelectric properties of the /001S-textured PMN–PT ceramics are presented.
Lead magnesium niobate–lead titanate, 0.675Pb(Mg1/3Nb2/3)O3– 0.325PbTiO3 (PMN–32.5PT) ceramics were textured (grainoriented) in the /001S-crystallographic direction by the templated grain growth process. The textured PMN–32.5PT ceramics were produced by orienting {001}-SrTiO3 (ST) platelets (B10 lm in diameter and B2-lm thickness) in a submicron PMN–32.5PT matrix. The templated growth of /001S-oriented PMN–32.5PT grains on the ST platelets resulted in textured ceramics with B70% Lotgering factor and 498% theoretical density. Unlike most lead-based ceramics, excess PbO was not needed for sintering or grain growth. Based on unipolar stain-field measurements at 0.2 Hz, the textured samples displayed 40.3% strain at 50 kV/cm. Low-field d33-coefficients of 41600 pC/N (o5 kV/cm) were measured directly from unipolar measurements. The low drive field d33-piezoelectric coefficient of the highly textured samples is two times greater than polycrystalline PMN–32.5PT. I. Introduction
S
INGLE-CRYSTAL lead magnesium niobate–lead titanate, 0.675Pb(Mg1/3Nb2/3)O3–0.325PbTiO3 (PMN–32.5PT) has extremely high piezoelectric d33-coefficients (d3342000 pC/N) and longitudinal electromechanical coupling coefficients (k33)4 0.9 when measured in the /001S.1,2 The enhanced electromechanical properties of the /001S-oriented PMN–32.5PT single crystals have been attributed to domain or crystallographic engineering for rhombohedral compositions near the morphotropic phase boundary (MPB). It has been previously shown that the piezoelectric enhancement identified for this composition and orientation can also be accessed by grain-orienting PMN–32.5PT ceramics by the templated grain growth (TGG) process.3–6 TGG is a ceramic processing method for producing textured ceramics.7–15 The TGG process is dependent on the nucleation and growth of the desired crystal(s) on oriented, lattice-matched template crystal(s), resulting in an increased fraction of oriented material after thermal treatment. With continued thermal treatment, the volume fraction of oriented material will increase due to the growth of the larger, oriented grains at the expense of the finer grain matrix. It has been shown by various investigators that ferroelectric/piezoelectric ceramics, which display a high degree of texture in the polar direction, possess a large percentage of the single-crystal properties. TGG is a method for producing textured ceramics by a relatively inexpensive ceramic processing route. There are several requirements for the template particles to successfully texture Pb-based perovskite ceramics by TGG. The single-crystal tem-
H. M. Chan—contributing editor
Manuscript No. 10142. Received April 21, 2003; approved February 6, 2004. Work supported by a subcontract under Materials Systems Incorporated, Littleton, MA, under DARPA/NAVSEA Contract No. N66604-99-C-4622 and DARPA Grant No. F49620-00-1-0098. w Author to whom correspondence should be addressed. e-mail: messing@matse. psu.edu
II. Experimental Procedure Rhombohedral PMN–32.5PT was studied as the matrix composition in this work. For comparative purposes, a PMN–
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32.5PT matrix containing 3 wt% excess PbO was also prepared. The matrix powder was prepared by mixing (PbCO3)2Pb(OH)2 (Aldrich, Milwaukee, WI, mean particle size 3.7 mm), MgNb2O6 (H. C. Starck, Newton, MA, mean particle size 0.4 mm), and fumed-TiO2 (Degussa-Huls, Frankfurt-main, Germany, mean particle size 0.03 mm) in a polyethylene bottle for 10 h with highpurity ZrO2 media (3-mm diameter) in deionized water (no pH adjustment). After milling, the slurry was dried on a hot plate while stirring with a magnetic stir bar. The powder was calcined at 7001C for 1 h to form the perovskite phase.25,26 For tape casting, the calcined powder was sieved to o90 mm (170 mesh), and ball milled in toluene with an organic binder (73210, Ferro, Cleveland, OH) for 24 h. From scanning electron microscope (SEM) observations of the calcined matrix powder, the powder consisted of loosely agglomerated primary particles. The median diameter of the calcined powder measured by a light scattering method was 0.26 mm. Tabular ST particles (5–15-mm width) with an aspect ratio 45 (Fig. 1) were mixed in the slurry with a magnetic stir bar. The tabular ST particles were synthesized by a two-step molten salt process23,24 using KCl as a flux. Tapes with ST templates were cast with a blade height of 200 mm at a shear rate of 360 s1. The tapes were dried, cut, and then laminated under 1300 kg/cm2 at room temperature. The sample thickness changed from 1.5 to 1 mm during lamination. Laminated samples were cut into 4 mm 5 mm rectangles and then heated at 6001C in air to remove the organic binder. The samples were encapsulated in a platinum foil and then embedded in PMN–32.5PT powder within an alumina boat. Samples were heated in flowing oxygen at 151C/min to a sintering temperature of 11501C and held for 1 to 50 h. Sintered densities were measured by the Archimedes method. Texture fraction was determined by X-ray diffraction (XRD) of samples polished to 0.05 mm using the Lotgering method. Polished samples for SEM analysis were thermally etched at 1001C below the sintering temperature for 30 min to reveal the grain structure. No grain growth occurred during thermal etching. The dielectric constant and loss (at 1 V) were measured with a multifrequency impedance meter (HP 4284A LCR meter, Hewlett-Packard Development Company, L.P. Palo Alto, CA) between 01 and 3001C over a frequency range of 1 kHz to 1 MHz. For these measurements, the major faces of the samples were polished and electroded with gold by sputtering (so that the electric field was parallel to /001S). The unipolar strain versus electric field (strain-field curves) was measured using the Sawyer–Tower circuit in conjunction with a linear variable differential transducer, driven by a lock-in amplifier (Stanford Research Systems, Sunnyvale, CA, Model SR830). The high fields were generated by a Trek 609C-6 highvoltage amplifier (Trek Equipment Company, Sausolito, CA). The dielectric polarization versus electric field (P–E hysteresis loops) was measured with the same modified Sawyer–Tower circuit. Both the strain and polarization versus field measure-
ments were completed while immersed in Galden HT-200 (Solvay Solexis, Inc., Thorofare, NJ) to prevent arcing. Prior to measurement, samples were poled in polydimethylsiloxane (Dow Corning 200 Fluid, Midland, MI) at 40 kV/cm for 15 min at room temperature.
III. Results and Discussion (1) TGG Processing Initial TGG experiments of the PMN–32.5PT matrix containing excess PbO showed that the ST templates would preferentially dissolve into the matrix when processed at a sintering temperature of 11501C. This template dissolution during the thermal processing prevented the required nucleation and growth of the oriented grains. Further experiments showed that an initial annealing step at 7501C for 1 h before subsequent sintering at a higher temperature resulted in better development of texture. Observation of the templated ceramics just after the 7501C hold showed that little templated growth had occurred. The lower temperature anneal appeared to affect the stability of the templates before the formation of PbO-based liquid phase and the growth of the templated grains. It should be noted that the melting point of PbO alone is B8401C and that this temperature is reduced in the presence of other elements. Since the ST templates are not chemically stable in liquid PbO, it is important to have oriented growth before any adverse chemical reactions occur between the templates and matrix. Since the transformation temperature of the precursor to perovskite PMN–PT is as low as B6501C,25 it can be expected that formation of perovskite PMN–PT on the (001) plane occurs before the PbO melts. In contrast, when no excess PbO was used, a similar degree of texture can be obtained without a low-temperature nucleation step. This result indicates that the low-temperature annealing step is necessary when excess PbO is present within the matrix. Figure 2 displays the XRD patterns of both a random (untemplated) and textured PMN–32.5PT (3wt% excess PbO, 3 vol% ST) ceramics sintered at 11501C for 10 h. The (110) peak is the most intense X-ray peak in random PMN–32.5PT ceramic. The two major peaks in the templated PMN–32.5PT ceramic were (100) and (200), which indicates the high degree of texture. The presence of a relatively weak (110) peak indicates that the Random (110)
Intensity
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(111) (100)
10
20
(211) (200)
30
40
50
60
70
80
Textured (100)
Intensity
(200)
(110)
10
Fig. 1. Molten salt synthesized, tabular SrTiO3 used as templates.
20
30
40 50 Degrees (2 )
60
70
80
Fig. 2. The X-ray diffraction patterns of random and textured PMN– 32.5PT ceramics.
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Fig. 3. Fracture surfaces of PMN–32.5PT samples (side view) containing 5 vol% SrTiO3 templates after heating at 11501C for 0 min (left) and 1 h (right).
texture fraction is not 100%. There were no major differences in the XRD patterns between textured samples with and without excess PbO. Figure 3 shows the microstructure evolution of a templated sample with no excess PbO. Figure 3(a) shows that there is little growth on the templates when the sample initially reaches 11501C. After a 1 h hold at the same temperature, there is 30–40 mm of growth on the ST templates, see Figure 3(b). Most of the templated PMN–PT grains contain a porous region near the center. Microstructures of thermally etched surfaces of templated samples sintered at 11501C for 10 and 50 h are shown in Fig. 4. The large blocky grains are B30 mm thick and aligned along the tape casting direction. After heating for 10 h, the samples had a Lotgering factor of 65%, and a Lotgering factor of 69% after 50 h of heating. After 50 h of annealing at 11501C, the matrix grain size was still only B5 mm in diameter. As determined by energy-dispersive spectroscopy, the dark regions in the samples are relics of the original ST templates. It is noteworthy that a significant amount of the ST remains after 50 h at 11501C. Figure 5 shows texture evolution in the sample with 5 vol% ST and no excess PbO at 11501C. Considering the very long sintering time with the insignificant increase in the degree of texture after 50 h, it is apparent that the driving force for growth was depleted at some point during this process. Several possible reasons for the decay in the template growth kinetics include: (1) matrix grain growth and reduction of driving force, (2) impurity drag along grain boundaries, and (3) impingement of grown large grains. Figure 4 shows that there is only a slight increase in matrix grain size when PMN–32.5PT is heated for 10 and 50 h (2.8 and 4.2 mm, respectively as measured by the line-interception method). Also, the interfaces between the matrix grains and the large templated grains are still curved; thus, the driving force for the grain growth still exists. These observations suggest that
matrix grain growth is not a significant factor in reducing the driving force for template growth (Fig. 4(b)). The textured samples were translucent (Fig. 6), which implies clean grain boundaries and a high density. In contrast, samples with excess PbO were opaque. Thus, impurity drag does not appear to be a likely explanation for the decreased growth kinetics after extended heat treatment. Since the templates are randomly dispersed in the matrix, it is natural that the large grains impinge after a certain amount of growth. Impingement of the growth fronts results in relatively low-energy boundaries with little curvature, and thus, significantly lower driving force for growth. To determine the importance of grain impingement on the saturation of template growth, samples with 1 vol% ST template were prepared in the same manner as described above for the 5 vol% ST-containing samples. If impingement is the predominant factor for the saturated texturing, the final grown grain size will be the same as the spacing of templates (xs), which depends on the number frequency of the templates (fs) according to Eq. (1). 1=3 6 (1) xs ¼ pfs Equation (1) is derived by assuming a two-dimensional lattice of equidistant spaced particles. From Eq. (1), the grain size of the templated grains within samples containing 1 vol% ST templates is predicted to be B1.7 times larger than in samples containing 5 vol% ST. The grain size of the samples was measured on polished and thermally etched cross-sectional surfaces of the sintered samples. The cross-section of the samples is oriented perpendicular to the basal plane of the templates (i.e., the hk0-plane). When we assume that the templated grains are tetragonal bars randomly oriented on the (001) plane, the width of the templated grains can change significantly depending on the orientation of the
Fig. 4. Polished and etched surfaces of textured PMN–32.5PT samples annealed at 7401C and then sintered at 11501C for 10 h (left) and 50 h (right).
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0.8 Lotgering Factor
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
10 20 30 40 50 Total Holding Time at 1150˚C (h)
60
Fig. 5. Texture evolution of SrTiO3-templated samples heated at 11501C.
template particles in the x–y plane of the grain, and thus, this dimension is not a good indicator of grain size. Thus, the thickness of templated grains was measured to evaluate the relative growth, since the thickness of the templated grains is independent of the template in-plane orientation. The thickness was 16 mm for 5 vol% templated samples and 23 mm for 1 vol% templated samples. The difference between 1 vol% and 5 vol% templated samples is comparable to the prediction above, thus supporting the argument that impingement of growing grains is the primary reason for the saturation in texture evolution observed in Fig. 5. The above analysis is overly simplistic since the template particles are randomly distributed during processing, and thus the amount of growth before impingement between template grains varies significantly. This simplicity of the above analysis clearly underscores the need for modeling of TGG in a system with randomly distributed template particles. The microstructure of a sample textured using 3 wt% excess PbO and 3 vol% ST templates is shown in Fig. 7. The grown grains are larger and more faceted than in samples free of excess PbO and containing 5 vol% ST templates. In this sample, the PbO-based liquid phase contributes to the higher growth rate, and thus, the growth is not limited by the low-energy interfaces formed on impingement. As a result, this sample was highly textured. However, even though the degree of texture can be enhanced by the addition of excess PbO, the dielectric and piezoelectric properties of the textured samples with excess PbO are usually inferior to those of samples without excess PbO.
(2) Dielectric and Piezoelectric Properties Figure 8 shows the dielectric constant at 1 kHz of the unpoled textured samples with 1, 3, and 5 vol% ST templates. The temperatures for the maximum dielectric constant, Kmax, are 1581, 1471, and 1201C for 1, 3, and 5 vol% ST samples, respectively. The maximum dielectric constants, Kmax, are between 23 100 and 26 000, which are significantly higher than single crystals grown from polycrystalline precursors.17 It appears that both Kmax and Tmax decrease with the addition of ST. These trends
Fig. 7. Polished surface of textured PMN–32.5PT containing 3% excess PbO and 3 vol% SrTiO3 sintered at 11501C for 10 h.
agree with Ko et al.,27 who reported that 1 mole% Sr in (1x)PMNxPT (x 5 0.3 and 0.35) results in an B101C decrease in Tmax (1 vol% of ST in PMN–32.5PT is equivalent to 1.09 mol%). The sample containing excess PbO and 3% ST has a lower Kmax than samples without excess PbO, most likely as a result of the presence of excess PbO on the grain boundaries. The dielectric loss of all the samples was B0.02 at room temperature, decreased to o0.01 at 2001C, and then increased above 2001C. Figure 9 shows a well-saturated hysteresis loop, obtained using a triangular pulse. The remanent polarization (Pr) was 24 mC/cm2 and the coercive field (Ec) was 4.8 kV/cm, which is nearly 40% higher than obtained with solid-state-grown single crystals.17 The well-saturated characteristics of the hysteresis loop correspond to a dense, textured ceramic with clean grain boundaries. The piezoelectric behavior of ST and BT6 textured samples, and a randomly oriented sample are compared in Fig. 10. Textured PMN–32.5PT with 5 vol% ST and no excess PbO showed a maximum strain level of B0.3% at an electric field of 40 kV/ cm, which is approximately twice the strain response of a random ceramic at the same field. In spite of having a higher texture fraction, the textured samples with excess PbO have strain values between random samples without excess PbO and textured samples without PbO. Again, this is likely to be due to the deleterious effect of PbO at the grain boundaries. The piezoelectric coefficient (d33) for each sample was determined from the slope of the strain versus electric field curves in the low field region between 0 and 10 kV/cm. The d33 was 580, 1310, and 990 pC/N for random samples, textured samples (0 wt% excess PbO), and textured samples (3 wt% excess PbO), respectively. The d33 of textured sample without excess PbO in the low-field region 30000 3% ST, 3% PbO
(001)
2500
Dielectric Constant
3000 (002)
2000 1500 (110)
1000 500
25000 20000 5% ST 1% ST
15000 10000 5000 0 0
0 10
30
50
70
Fig. 6. Optical view of 65% textured PMN–32.5PT containing 5 vol% SrTiO3 templates (sample is 4 mm 3 mm 0.6 mm thick).
50
100 150 200 Temperature (˚C)
250
300
Fig. 8. Temperature dependence of the dielectric constant of textured PMN–32.5PT as a function of excess PbO and SrTiO3 templates.
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between 0 and 5 kV/cm was as high as 1660 pC/N. The d33 values of textured and untextured samples are compared in Table I. The electric field in many piezoelectric applications is o10 kV/cm. In this range, textured ceramics show piezoelectric behavior equivalent to d33-coefficient 41000 pC/N. The single crystal shows an almost perfectly linear strain-field response. The amount of strain at 10 kV/cm was 0.21% for single crystal and the d33 value was 2150 pC/N. The highly textured PMN–PT ceramic displays a significant fraction of the piezoelectric response of single crystals, making them suitable replacements for single crystals in some piezoelectric applications. The piezoelectric hysteresis, the difference between the strainfield curve on the application (lower line) and release (upper line) of the electrical field, has maximum values of B0.03% and B0.02% for textured and untextured samples, respectively. It should be noted that even with the presence of a paraelectric material (ST), the properties are still significantly better than the random samples.
50 40 Polarization ( C/cm2)
30 20 10 0 -60
-40
-20
-10
0
20
40
60
-20 -30 -40 -50 Electric Field (kV/cm)
Fig. 9. Hysteresis loop of 50 h sintered PMN–32.5PT with 5 vol% SrTiO3 and no excess PbO.
0.35
Table I. Piezoelectric Coefficient of Untextured and Textured PMN–32.5PT as a Function of Sample Preparation Conditions ST (vol%)
Excess PbO
d33 (pC/N) to 10 kV/cm
d33 (pC/N) to 5 kV/cm
Sintering time (h)
0 3 0 0
580 990 1200 1310
730 1180 1500 1660
10 10 10 50
0 3 5 5
PMN–32.5PT, lead magnesium niobate–lead titanate, 0.675Pb(Mg1/3Nb2/3)O3– 0.325PbTiO3; ST, SrTiO3.
Table II summarizes the piezoelectric response of the various samples. The textured samples tested without excess PbO show higher levels of strain. The sintered density of the samples with excess PbO was lower than that of samples without excess PbO. The piezoelectric hysteresis of the textured samples without excess PbO is almost twice that of samples with excess PbO. For a comparison, the maximum piezoelectric hysteresis of the untextured samples was 0.018%. It is clear that the main portion of strain increase in textured sample is not due to strain-field hysteresis but due to grain orientation. The gap between the increasing and decreasing field traces in the strain-field measurement can be due to irreversible domain wall motion and this may limit the use of these textured ceramics in some piezoelectric applications. Rhombohedral PMN–PT single crystals, which are poled and measured in the /001S, show a nearly anhysteretic strain-field response indicating the absence of extrinsic contributions. There are several plausible reasons for the piezoelectric hysteresis in the textured samples. The splitting in the (200) peak, shown in Fig. 2, suggests the presence of tetragonal domain states in the textured samples. The presence of the tetragonal domain structure would increase hysteresis. The origin of the hysteresis can also result from porosity and internal stresses as a result of the templated grains. Internal stresses may result from the remnant ST templates embedded within the oriented grains. This would produce an internal stress field within the grains, which might, for example, locally stabilize the tetragonal PMN– PT. Any of these factors would alter the domain wall response to electrical fields and mechanical stresses.28
ST textured, no excess PbO BT textured, 1% excess PbO
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IV. Conclusions
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0.2
ST textured, 3% excess PbO
0.15
Untextured
0.1 0.05 0 0
10
20 30 40 Electric Field (kV/cm)
50
60
Fig. 10. Strain of textured, untextured, and single-crystal PMN– 32.5PT as a function of excess PbO and SrTiO3 templates.
The TGG of PMN–32.5PT ceramics using tabular ST particles as templates resulted in dense, highly (470% Lotgering factor) textured ceramics. Due to the fine size of the template particles, it was possible to obtain textured ceramics without the use of excess PbO or without extensive matrix grain growth. The textured ceramic showed piezoelectric coefficients and strain levels two times greater than random ceramics with a low-field d33 of 1660 pC/N. The TGG process is based on relatively conventional powder processing steps and constituents, which makes it an attractive low-cost alternative for fabricating high-strain materials.28 This method will be more attractive for PMN–PT when smaller BaTiO3 or other nonreactive templates become available, since it has been shown that BaTiO3 does not adversely affect the dielectric properties of PMN–PT.
Table II. Piezoelectric Properties of Textured PMN–32.5PT as a Function of Sample Preparation Conditions Strain at 10 kV/cm (%) ST (vol %)
3 5 5
Hysteresis (%)
Excess PbO
Lotgering factor
Density (g/cm3)
Sintering time (h)
Lower curve
Upper curve
Maximum
Average
3% 0 0
0.73 0.65 0.69
7.82 7.93 8.00
10 10 50
0.096 0.129 0.140
0.107 0.144 0.158
0.015 0.030 0.035
0.004 0.008 0.009
PMN–32.5PT, lead magnesium niobate–lead titanate, 0.675Pb(Mg1/3Nb2/3)O3–0.325PbTiO3; ST, SrTiO3.
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Textured PMN-PT: Processing and Piezoelectric Properties Acknowledgments
The authors would also like to thank Huseyin Yilmaz and Jeff Long for their contributions to this work. Careful editing by Kristen Brosnan is appreciated.
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