characterizing reliability of multilayer pzt actuators

Characterizing reliability of multilayer PZT actuators

S.A. Hooker

National Institute of Standards and Technology, 325 Broadway St., Boulder, CO, USA 80305

ABSTRACT

Many new applications are emerging for piezoelectric ceramics including adaptive structures, active-flow-control devices, and vibration and noise suppression systems. Additionally, there are opportunities to use these devices in the biomedical field for miniature pumps, ultrasonic surgical tools, micro-needle arrays, and nanorobotics. In each of these instances, actuator stability is critical, representing a significant challenge for piezoelectric ceramic materials. In particular, the properties of lead zirconate titanate (PZT) have been found to degrade, often significantly, during continuous operation due to a combination of domain pinning, relaxation of interfacial stress, and, in the worst cases, micro-crack formation. This degradation, referred to as actuator fatigue, can be even more pronounced when high voltages are used to achieve maximum displacement or more complex actuator designs are required. For example, multilayer actuators, such as co-fired stacks, are important for many emerging applications and are now being produced with very small physical dimensions, lowering power requirements. However, multilayer components may be highly susceptible to long-term fatigue due to the large number of interfaces involved in their configuration. In this work, we report a method for rapidly characterizing the reliability of multilayer PZT actuators by monitoring degradation in switching polarization over time. To verify this approach, a series of miniature (3 mm x 3 mm x 2 mm) multilayer actuators were characterized over 1 million cumulative cycles. These actuators were produced commercially from soft PZT materials, and the sintering temperature was varied to tailor the ceramic microstructure and performance characteristics. Evaluation of cyclic polarization degradation was found to be an effective method for illuminating differences among the different actuators tested, as well as serving to predict their long-term resistance to fatigue. Keywords: Reliability, fatigue, accelerated testing, actuator, multilayer actuators, PZT-5A, soft PZT

1. INTRODUCTION Piezoelectric ceramics are desirable actuator materials due to their fast response, wide operational bandwidth, high force, large power density, relatively compact size, and ease of motion control. These advantages make materials such as lead zirconate titanate (PZT) of great interest for adaptive optics, automobile fuel injectors, active flow-control devices, vibration dampers, miniature pumps, ultrasonic surgical tools, microneedle arrays, and micro- and nanorobotic systems. In each of these applications, reliability of the actuator is important, in particular stability of the induced physical displacement during repetitive use. Unfortunately, the properties of PZT can degrade during continuous operation, affecting the resulting displacement.1,2 Such degradation is often due to a combination of domain pinning, relaxation of interfacial stresses, and, in certain cases, micro-crack formation and is referred to here as actuator fatigue. Microstructure tailoring of PZT has been previously reported to optimize mechanical and electrical behavior, both of which contribute to electromechanical degradation and fatigue. For example, increases in bending strength, blocked force, free displacement, and energy density have all been reported when fired grain sizes have been held below 1 µm.3,4 However, fracture toughness has been found to increase for larger grain sizes (above 5 µm) 5,6 along with ferroelectric and piezoelectric properties.7 While the role of mechanical and electrical properties in determining fatigue resistance is complex, it is evident that variations in microstructure are likely to result in variations in fatigue behavior. At present, determining and predicting fatigue behavior in piezoelectric ceramics is not trivial and often requires testing a sample of the components under real-world operational conditions. For applications in aeronautics and spacecraft, such tests could require years of testing, even with acceleration of voltage or applied load. Here, we propose a method

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for rapidly testing actuator reliability by examining polarization degradation during continuous cycling at switching electric fields. By utilizing a series of actuators with different microstructure characteristics, it was possible to utilize this test to bring to light differences in the long-term performance of the different components. Such accelerated tests complement other methods for quality characterization, such as screening for likely defects and testing against certain initial performance metrics.

2. EXPERIMENTAL PROCEDURE

2.1. Multilayer Actuator Fabrication PZT powder in a soft configuration (PZT-5A-type) was obtained from a commercial vendor and attrition milled for 6 hours to reduce the particle size from approximately 600 nm to 140 nm. This material was then combined with a surfactant and modifier, dispersed in toluene, and milled at high speed for 10 minutes using a laboratory-scale shaker mill. After adding a binder and plasticizer, the mixture was ball-milled at low speed for 24 hours to ensure complete dispersion. A conventional tape casting technique8,9 was then used to fabricate the actuators. A silicone-coated plastic carrier film served as the substrate onto which the cast tape was deposited, and the continuous tape was cut into sheets to form the individual layers in the multilayer stack. A doctor blade designed for casting tape with a thickness of 0.17 mm was used throughout this work. Platinum ink was screen-printed onto several individual PZT sheets to serve as the internal electrodes within the device. The printed electrodes were then dried at 80 °C using a belt oven. Next, the multilayer configuration was prepared by stacking together the individual PZT sheets and applying temperature and pressure to form a cohesive pad. Each pad contained 36 sheets of PZT tape, including 8 cover (i.e., inactive) layers on both the top and bottom. To form the active section of the actuator across which the voltage is applied, 10 blank sheets were interleaved with 10 electroded sheets to form an interdigitated internal electrode structure similar to that shown in Figure 1. The entire pad was then tacked together at 58 °C for 30 seconds and subsequently laminated at 70 °C for 20 minutes. Separation of the individual actuator components (3 mm x 3 mm x 2 mm) was achieved using a vision-assisted dicing saw. To remove the remaining organic materials, the actuators were slowly heated to 550 °C in flowing nitrogen. A large muffle furnace was then used to sinter the components in air to temperatures ranging from 1175 to 1325°C for a soak period of 24 minutes. The parts were buried in PZT powder and fired in sealed crucibles to minimize lead loss. Relatively fast ramp rates (20 °C/min) were used for both heat-up and cool-down to minimize the total time at or near the sintering temperature. The resulting devices were terminated on two ends using a commercial silver-palladium termination paste and subsequently heated to 850 °C for 15 minutes to remove the organic constituents in the termination paste and ensure conductive contacts. Fired grain sizes were determined by evaluating representative components processed at each temperature using scanning electron microscopy (SEM).

(a) (b) (c) Figure 1. (a) Schematic of the multilayer actuator devices fabricated in this work; (b) example of the cast tape prepared

for this application; and (c) a typical SEM image of the actuator cross-section.

End Termination Internal Electrodes

Ceramic



2.2. Polarization Characterization Ferroelectric polarization for the different actuator devices was determined using a modified Sawyer-Tower circuit, with remanent polarization (Pr), saturation polarization (Psat), and coercive field (Ec)) calculated for each measurement. To determine the initial performance for each actuator, a 1 Hz sinusoidal wave served as input to the circuit, and P-E hysteresis loops were acquired through a digital oscilloscope, as shown in Figure 2a. Pr, Psat, and Ec were calculated from the loops, with the statistical average representing baseline performance. To determine the effect of continuous cyclic operation, three to five actuators from each temperature were operated for up to 1 million cycles, using an accelerated test frequency of 35 Hz. Data were acquired at regular intervals, and Pr, Psat, and Ec were determined as a function of cumulative cyclic exposure. During all measurements, the actuator devices were clamped in a Kelvin clip, providing electrical contact to the end terminations as shown in Figure 2b. Dielectric properties of the different actuators were also characterized at 1 kHz to provide baseline data.

(a) (b)

Figure 2. (a) Instrumentation used to determine the polarization degradation for the various actuator devices and (b) Kelvin clip fixture for holding the miniature actuator devices during testing.

3. RESULTS AND DISCUSSION Baseline dielectric and polarization properties for the different actuator devices are shown in Table 1. Based on these data, devices processed at 1250 °C to 1275 °C exhibited the best combination of polarization and capacitance. This range of temperatures represents a conventional sintering temperature range for these ceramic materials. SEM images of the ceramic microstructures within the active region are shown in Figure 3, illustrating the increases in grain size as a function of sintering temperature. Again, this behavior is typical for conventional sintering of PZT. Degradation in the polarization behavior of a typical multilayer actuator component processed at 1250 °C is shown in Figure 4, illustrating the rapid decrease in ferroelectric properties that occurred during continuous cycling at switching electric fields. As evidenced by the data in Figure 4, significant fatigue can be induced in these components after relatively few cycles. Based on an applied sinusoidal wave at 35 Hz, approximately 45 minutes were required in order to test each specimen to 100,000 cycles. Further testing to approximately 1 million cycles required approximately 8 hours, during which time data were acquired at 30 minute intervals. Figure 5 illustrates the changes in remanent polarization resulting from these longer cumulative cycles for the different sintering temperatures. As seen in Figure 5, the majority of degradation was found to occur during the first 250,000 cycles, with only minimal changes in polarization noted for longer testing durations. These results indicate that testing the actuator devices for a period of 2 hours (250,000 cycles) is sufficient to determine the extent of changes that will likely occur. Moreover, it should be noted that measurement of polarization degradation was sufficiently sensitive as to allow differentiation among the devices produced at the different temperatures. These results indicate that a test method based on polarization degradation can be used as an accelerated procedure for comparing among different batches of actuator devices.



Table 1. Baseline dielectric properties for the different multilayer actuator devices.

Temperature Cap (nF) Tan δ Psat (µC/cm2)

Pr (µC/cm2)

Ec (kV/cm)

1175 °C 6.48 0.023 17.65 12.82 14.06

1200 °C 6.38 0.022 20.35 15.32 14.70

1225 °C 6.67 0.021 25.79 20.95 15.73

1250 °C 6.79 0.020 30.20 24.30 16.38

1275 °C 6.65 0.019 31.99 26.17 17.19

1300 °C 6.32 0.019 31.55 26.09 18.02

1325 °C 5.17 0.017 27.13 22.40 19.97

Figure 3. SEM micrographs for the different sintering conditions tested, showing grain growth at the higher

temperatures. All images have been taken at the same magnification.

1175 °C 1200 °C 1225 °C

1250 °C 1275 °C 1300 °C



-40

-30

-20

-10

0

10

20

30

40

-40 -20 0 20 40

Electric Field, kV/cm

Pola

rizat

ion,

µC

/cm

2

baseline 10,000 cycles 100,000 cycles

Figure 4. Polarization degradation in a typical multilayer actuator component after exposure to 100,000 cycles at

switching electric fields.

0.3

0.6

0.9

1.2

0 250,000 500,000 750,000 1,000,000 1,250,000

# cycles

PR, n

orm

aliz

ed

1175°C1200°C1225°C1250°C1275°C1300°C1325°C

Figure 5. Degradation in remanent polarization as function of cumulative exposure to switching electric fields,

illustrating that the majority of degradation occurs during the initial 250,000 cycles.



4. CONCLUSIONS Ferroelectric hysteresis measurements have been successfully demonstrated as a method for accelerated fatigue testing of multilayer piezoelectric ceramic actuators. By continuously operating the devices at switching electric fields, significant degradation can be induced in these devices after relatively short interrogation times. Tests of 2 hours in duration were found to be sufficient to result in actuator fatigue, with only minimal degradation occurring over longer test exposures. This method of analysis was found to be sufficient to detect changes in fatigue behavior among components processed at different sintering temperatures. Control of grain growth through sintering optimization offers one option for tailoring fatigue resistance in PZT actuator devices. Demonstration of an accelerated test sensitive to microstructural variations provides actuator designers with the ability to assess quality control in new actuator components.

ACKNOWLEDGEMENTS

The PZT powder used in this work was EC-65 composition, produced by Edo Ceramics. Custom fabrication of the actuator components used for testing was provided by Synkera Technologies. All information on commercial suppliers is provided for reference only and does not constitute an endorsement of specific vendors. The author wishes to acknowledge Dr. Jens Mueller of the National Institute of Standards and Technology for SEM analysis of the different components. The contribution of the National Institute of Standards and Technology is not subject to copyright in the U.S.

REFERENCES

1 D.E. Dausch, “Ferroelectric Polarization Fatigue in PZT-Based RAINBOWs and Bulk Ceramics”, J.Am.Ceram.Soc., 80 [9], 2355-60 (1997). 2 J.H. Koh, et al., “Degradation and cracking behavior of 0.2(PbMg1/3Nb2/3O3)–0.8(PbZr0.475Ti0.525O3) multilayer ceramic actuators,” Sensors and Actuators A, 112 [2-3], 232-6 (2004). 3 T.R. Shrout, et al., “Recent Advances in Piezoelectric Materials,” Proceedings of SPIE, 3241, (1997). 4 C.L. Davis, D.G. Morris, and F.T. Calkins, “Characterization of Fine Grain Piezoceramic Stack Actuators,” Smart Structures and Materials 2001: Active Materials: Behavior and Mechanics, C.S. Lynch, editor, Proceedings of SPIE, 4333, (2001). 5 T. Karastamatis, et al., “The Effect of Grain Size on the R-Curve Behavior of Lead Zirconate Titanate (PZT),” SPIE Vol. 4333 (2001). 6 S.L. Dos Santos, et al., “Effect of Poling Direction on R-Curve Behavior in Lead Zirconate Titanate,” J. Am. Ceram. Soc. 83 [2] 424-6 (2000). 7 C.A. Randall, et al., “Intrinsic and Extrinsic Size Effects in Fine-Grained Morphotropic-Phase-Boundary Lead Zirconate Titanate Ceramics,” J. Am. Ceram. Soc. 81 [3] 677-88 (1998). 8 R.E. Mistler, D.J. Shanefield, and R.B. Runk, “Tape Casting of Ceramics”, pp. 414-448 in Ceramic Processing Before Firing, edited by G.Y. Onoda and L.L. Hench, New York: John Wiley and Sons, Inc., (1978). 9 J.S. Reed, Introduction to the Principles of Ceramic Processing, New York: John Wiley and Sons, Inc., (1988).



characterizing reliability of multilayer pzt actuators

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