Low Voltage Single Crystal Actuators

Transcription

1 Low Voltage Single Crystal Actuators Xiaoning Jiang *a, Paul W. Rehrig a, Jun Luo a, Wesley S. Hackenberger a Shujun Zhang b, and Thomas R. Shrout b a TRS Technologies, Inc. 282 East College Avenue, Suite J., State College, PA 1681 b Materials Research Institute, The Pennsylvania State University, University Park, PA 1682 ABSTRACT In this paper low voltage single crystal actuators were investigated using thin PMN-PT plates for applications requiring low voltage, large strain, low profile and/or actuation at cryogenic temperatures. Firstly, single crystal thickness effect on piezoelectric properties was studied by investigating the relationship between electromechanical coupling coefficient of PMN-PT crystals and the crystal thickness. It was found that electromechanical coupling coefficient (k t ) of 5 µm, 75 µm and 1 µm PMN-PT single crystal thin plates are.5,.51, and.55, respectively, which are slightly lower than that of bulk single crystal (.6). A couple of single crystal actuators were then assembled using crystal plates with thickness of 15-2 µm. These actuators were characterized by measuring strain vs. electric field at room temperature and cryogenic temperatures. A 3 mm x 3 mm x 19 mm single crystal stack actuator showed a 21 µm stroke at room temperature under 15 V, and a 1 µm stroke at 6 K under 2 V. A 5 mm x 5 mm x 12 mm single crystal actuator showed 13.5 µm stroke at room temperature under 15 V, and 6 µm stroke at 77 K under 15 V. These low voltage actuators hold promising for space precise positioning and adaptive structures and cryogenic SEM, SPM and STM applications. Keywords: single crystal piezoelectrics, piezoelectric actuator, cryogenic actuator, low voltage actuator 1. INTRODUCTION Piezoelectric actuators with low driving voltages (< 15 V) are desired for some aerospace and industrial applications. Currently, widely used low voltage piezoelectric actuators are mostly co-fired PZT actuators, where green tape thickness varies from < 1 µm to ~ 1 µm. The strain achieved is usually less than.1% because of the low piezoelectric coefficients. Single crystal piezoelectrics based on Pb(Zn 1/3 Nb 2/3 ) 1-x Ti x O 3 (PZN-PT) or Pb(Mg 1/3 Nb 2/3 ) 1-x Ti x O 3 (PMN-PT) exhibit large increases in strain over conventional piezoelectric ceramics due to the ability to orient the crystals along a preferred high strain crystallographic direction. Due to a unique ferroelectric domain configuration, the crystals piezoelectric strain remains nearly hysteresis free up to levels of ~.5 to.6% depending on the crystal composition (Figure 1) [1]. Furthermore, the crystals have been found to retain appreciable piezoactivity to temperatures as low as <2 K (Figure 2), comparing with the fact that PZT retained ~ 25% piezoelectric activity at 77 K, and < 5% piezoelectric activity at 4 K [2,3], which indicates an important advantage for many cryogenic actuation applications using single crystal piezoelectrics. However, the large strain (~.5%) can only be obtained under about 25 KV/cm driving field for most of the developed single crystal piezoelectric actuators, meaning that > 125 V is required for driving.5 mm thick single crystals, and such high voltage is not acceptable in many applications. Therefore, thin single crystal plates must be used in actuators to achieve relatively high strain under low voltages (< 15 V). In this paper low voltage single crystal actuators were investigated using thin PMN-PT plates for applications requiring low voltage, large strain, low profile and/or actuation at cryogenic temperatures. Firstly, single crystal thickness effect on piezoelectric properties was studied by investigating the relationship between electromechanical coupling coefficient of PMN-PT crystals and the crystal thickness. Single crystal actuators composed of thin plates are fabricated and characterized at both room temperature and cryogenic temperatures under driving voltage of 15 V or less. Smart Structures and Materials 26: Active Materials: Behavior and Mechanics, edited by William D. Armstrong, Proc. of SPIE Vol. 617, 617G, (26) X/6/$15 doi: / Proc. of SPIE Vol G-1

2 1$ C 12 $ Field (ky/cm) Field (ky/cm) 4 2 Figure 1. Piezoelectric strain response from single PZN-PT single crystal material compared to piezoelectric and electrostrictive ceramics. The crystal strain remains nearly hysteresis free up to ~.6%. At high fields very large strains > 1% can be achieved but with an increase in hysteresis PZN-8%PT PZT-5H 1 d 33 (pc/n) d 31 (pc/n) d 33 = 393 3K 4 2 Value for PZT Ceramic at Room Temperature Temperature (K) (a) Temperature (K) (b) Figure 2. Cryogenic piezoelectric properties of Single crystal piezoelectrics. (a) d 33 vs. temperature for a crystal of PZN-8%PT (from stack actuator strain measurement). (b) d 31 vs. temperature (from resonance measurements) for a crystal of PZN-8%PT compared to Type VI (PZT-5H) piezoelectric ceramics 2. EXPERIMENTAL DESIGN AND FABRICATION PMN-PT single crystal plates with thickness of 5 µm.5 mm were prepared to investigate the thickness effect on piezoelectric properties. Basically, the dielectric constant, dielectric loss, and electromechanical coupling coefficient were measured for PMN-PT crystal plates. The thin crystal plates fabrication process flow is shown in Figure 3. After crystal orientation alignment,.3 mm thick crystal slices were cut from a crystal boule by using an ID saw or a wire saw. The crystal slices were then lapped down to the designed thickness (e.g. 1 µm, 15 µm, etc.) by using a Logitech lapper PM5. Figure 4 shows the picture of a 1 µm thick PMN-PT single crystal slice. The thin crystal slices were then coated with Cr/Au electrodes on both sides followed by final shaping to obtain thin crystal plates with designed dimensions. PMN-PT single crystal plates with dimension of 5 mm x 5 mm x.5 mm, 5 mm x 5 mm x.1 mm, 5 mm x 5 mm x.75 mm and 5 mm x 5 mm x.5 mm were prepared for the electromechanical coupling coefficient study. PMN-PT single crystal stack actuators were then assembled using thin crystal plates and a stacking process scheme shown in Figure 5. Crystal plates with dimension of 3 mm x 3 mm x.15 mm, 5 mm x 5 mm x.15 mm, and 2 mm x 6 mm x.15 mm were fabricated to be used for single crystal stack actuators. After stacking, shim tabs were soldered and two lead wires were attached. The stack was then encapsulated by a thin epoxy coating. Figure 6 shows photograph pictures of a 3 mm x 3 mm x 19 mm stack with a plate thickness of.2 mm, a 5 mm x 5 mm x 12 mm stack with a plate thickness of.15 mm and a 2 mm x.5 mm x 4 mm stack with a plate thickness of.15 mm. The actuation is along the height directions (see arrows in the pictures), e.g. 19 mm, 12 mm and 4 mm directions for the developed actuators. Proc. of SPIE Vol G-2

3 PMN-PT crystal boule Lapping Crystal orientation Electroding Slicing Shaping Figure 3. Processing flow for thin PMN-PT single crystal plates. Figure 4. Photograph picture of 1 µm thick PMN-PT crystal plates. Endcap Metal shim Tab for soldering Single crystal Epoxy Figure 5. Schematic view of single crystal stack assembly. Proc. of SPIE Vol G-3

4 (a) (b) (c) Figure 6. Pictures of single crystal stack actuators with thin single crystal plates. (a) 3 mm x 3 mm x 19 mm stack actuator with 8- layer 3 mm x 3 mm x.2 mm PMN-PT single crystal thin plates. (b) 5 mm x 5 mm x 12 mm stack actuator with 7-layer 5 mm x 5 mm x.15 mm PMN-PT single crystal thin plates. (c).5 mm x 2 mm x 4 mm stack actuator with 21-layer.5 mm x 2 mm x.15 mm PMN-PT single crystal thin plates. Table 1. Measurement data for PMN-PT thin plates. Length Width Thickness Calculated (mm) (mm) (mm) fr (Hz) fa (Hz) C (nf) Loss K Note: fr: resonant frequency, fa: anti-resonant frequency, C: capacitance, K: dielectric constant. 3. EXPERIMENTAL RESULTS 3.1 Electromechanical coupling coefficient of thin crystal plates All crystal plates were poled under about 1 KV/cm at room temperature. A HP 4194A impedance analyzer was used to record resonance frequency and anti-resonance frequency, capacitance and dielectric loss. Capacitance and dielectric loss of the samples were measured at 1 KHz, and dielectric constants were then calculated using the parts dimension and the measured capacitance. Electromechanical coupling coefficients were calculated according to IEEE Standard [4]. The Proc. of SPIE Vol G-4

5 resonant frequency, anti-resonant frequency, capacitance, dielectric loss were presented in Table 1 for PMN-PT crystal plates with thickness ranged from 5 µm to 5 µm. It was found that the dielectric loss increases with the decreasing crystal thickness, while the dielectric constants decrease with the decreasing crystal thickness. The calculated electromechanical coupling coefficients of these samples vs. thickness were presented in Figure 7, and the decreased k t was found for thin plates. It is believed that the domain motion contributions to the piezoelectric and dielectric properties for thin single crystal plates are less significant comparing with thick plates or bulk single crystal, more experiments will be carried out to understand more about this phenomenon and the results will be published in another paper. Electromechanical coupling (kt) Crystal plate thickness (mm) Figure 7. Electromechanical coupling coefficients (k t ) for PMN-PT crystal plates with various thicknesses. 3.2 Stroke performance of single crystal stack actuators with thin plates The 3 mm x 3 mm x 19 mm stack actuator was fabricated with 8-layer 3 mm x 3 mm x.2 mm thin PMN-PT plates. The 5 mm x 5 mm x 12 mm stack actuator consisted of 7 layers of 5 mm x 5 mm x.15 mm plates. The.5 mm x 2 mm x 4 mm miniature stacks consisted of 21-layer.15 mm thick PMN-PT plates. The displacement of actuators was measured using LVDT system (Figure 8 ) under low driving voltages (< 15 V). Figure 9 shows the displacement vs. voltage performance of low voltage single crystal actuators prototyped at room temperature. The stroke of single crystal actuators was also measured at cryogenic temperatures and the results are shown in Figure 1. Cryogenic piezoelectric properties of PMN-PT and PZN-PT single crystals were carried out in a helium cryostat (Displex CSW-22, Advanced Research Systems, Inc., Allen Town, PA). The temperature range of this cryostat was designed to achieve 2 K-35 K without attachment. LVDT Lock-in Amplifier High V Supplier Computer Actuator Figure 8. LVDT setup for actuator strain-electric field test. Proc. of SPIE Vol G-5

6 Displacement (um) V 1 V 5 V Displacement (um) Driving Voltage (V) (a) Driving Voltage (V) (b) Displacement (um) Driving Voltage (V) (c) Figure 9. Stroke performance for low voltage single crystal actuators. (a) displacement vs. voltage for 3 mm x 3 mm x 19 mm actuator. (b) displacement vs. voltage for 5 mm x 5 mm x 12 mm actuator. (c) displacement vs. voltage for.5 mm x 2 mm x 4 mm actuator. The active strain (considering the active length only) and the overall strain (considering the total length) are presented in Table 2 for the fabricated actuators. The active length is the total length of PMN-PT single crystal materials, e.g. active length of 3 mm x 3 mm x 19 mm actuator is 16mm (8-layer.2 mm thick crystals). The total length counts not only the active materials (e.g. PMN-PT crystal), but also non-active materials (e.g. Cr/Au, metal shim and epoxy). The active strain for the 5 mm x 5 mm x 12 mm stack is relatively low comparing with the desired strain of.2% at 1 KV/cm, which is likely caused by the claming of epoxy used in the stack assembly, which was observed in thick plate stacks, but this impact is more significant in the case of thin plate stacks. The overall strain of the developed stacks is lower than that of the active one because passive materials such as epoxy bonding layer and metal shims are counted in the overall length. Although the overall strain of the developed low voltage single crystal stack actuators is not much higher that of.1%, further decreased single crystal plate thickness could lead to higher strain. For example, if.75 mm thick plates, instead of.15 mm thick plates, were used in the stack, the strain of such actuators under 15 V would be doubled and Proc. of SPIE Vol G-6

7 higher than.2%, which is not achievable for co-fired ceramic actuators. Moreover, the developed low voltage single crystal actuators showed ~ 5% stroke at 77 K, comparing to the reported ~ 25% retained stroke for ceramic actuators. These low voltage actuators hold promising in space precise positioning and adaptive structures and cryogenic SEM, SPM and STM applications. Table 2. Strain comparison for the low voltage single crystal stack actuators. Stack Actuators Driving Field Active strain Overall Strain (KV/cm) 3mmx3mmx19mm (8-layer of %.113%.2mm thick plates) 5mmx5mmx12mm (7-layer.15 mm thick plates) 1.133%.116%.5mmx2mmx4mm (21-layer 1.19%.148%.15 mm thick plates) 3 mm PMN-PT Stack 25 Displacement (um) Temperature (K) (a) Actuation Stroke (um) 16 Measured data Expon. Fit Temperature (K) (b) Figure 1. Cryogenic stroke of single crystal actuators. (a) stroke vs. temperature for the 3mmx3mmx19mm PMN-PT crystal actuator under 2 V. (b) stroke vs. temperature for the 5mmx5mmx12mm PMN-PT single crystal actuator under driving voltage of 15 V. Proc. of SPIE Vol G-7

8 4. CONCLUSIONS Thin PMN-PT single crystal plates are required to develop low voltage single crystal piezoelectric actuators. The study shows that electromechanical coupling coefficient (k t ) and dielectric constant of PMN-PT crystal decreases with the plate thickness decreasing from.5 mm to.5 mm, while the dielectric loss increases with the decreasing thickness, which may indicate the degraded piezoelectric and dielectric properties of thin single crystal plates. More work on the thickness effect on the poling and de-poling mechanism, thin plates strain measurement, etc. could help understand more about this phenomenon. Low voltage single crystal piezoelectric actuators consisting of thin single crystal plates were fabricated and tested in this paper. Reasonably high overall strain (~.15%) was achieved under driving voltage of 15 V. The actuators also retained ~ 5% room temperature stroke at temperature of 77 K, which is very promising for various cryogenic actuation applications. Further investigation may focus on the thickness effect on the actuator performances, load capability, reliability, actuation creep, etc. ACKNOWLEDGMENTS The work was sponsored by NASA under Grant No. NNC4CA91C, NNL5AA83P, and a JPL subcontract # We gratefully acknowledge the assistance of Hua Lei at TRS Technologies for sample preparation. The authors also thank Carl Mills and Dr. William Cook at NASA LaRC, and Dr. E.H. Yang at NASA JPL for the helpful discussions. REFERENCES [1] S.E. Park and T.R. Shrout, Relaxor based ferroelectric single crystals for electromechanical actuators, Mat. Res. Innovat., 1, pp.2-25, [2] Cryogenic Actuators, [3] C. Bodefeld, personal communications, Attocube Systems AG, 25. [4] IEEE Standard on Piezoelectricity, ANSI/IEEE Standard Proc. of SPIE Vol G-8