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Piezoelectric Crystals

By: Dr. Kevin T. Zawilski, Chemfiles Volume 5 Article 13

BAE Systems, Advanced Systems
and Technology
Merrimack, NH

 

PMN–PT research performed at Stanford
University, Laboratory for Advanced
Materials, Stanford, CA

Growth of Lead Magnesium Niobate–Lead Titanate Single Crystals

Piezoelectric materials have the ability to generate a voltage in response to an applied mechanical stress or conversely change shape in response to an applied voltage. High performance piezoelectric materials have a wide range of applications including sonar arrays, ultrasonic imaging devices, and fine motion controllers. A major breakthrough in high performance piezoelectric materials was made in 1997, when the exceptional piezoelectric properties of relaxor ferroelectric single crystals were first measured.1,2 When crystals such as lead magnesium niobatelead titanate, (1–x) PbMg1/3Nb2/3O3 – (x) PbTiO3 (or PMN–PT), were measured along the <001> direction, the electromechanical coupling factor (k33) was found to be >90% with achievable strain levels of >1.5%. Previously, the best performing piezoelectric materials were PbZr(1–y)TiyO3 (PZT) ceramics with k33 ranging from 70% to 75% and achievable strain levels of 0.1%.

PMN–PT has a perovskite crystal structure as shown in Figure 1 and deforms from the high temperature cubic form to either pseudo-cubic/trigonal form at room temperature and low Ti concentration or tetragonal form at room temperature and high Ti concentration.3 The piezoelectric properties of PMN–PT peak near the morphotropic phase boundary (MPB) where the solid solution changes structure from pseudo-cubic/trigonal to tetragonal, in the range of x = 0.32–0.35.


Figure 1. Perovskite structure of PMN–PT


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Flux Growth

PMN–PT crystals were first grown by Park and Shrout1 using the flux technique, and since that time, others have successfully followed their lead.35The flux growth of this material typically involves dissolving the component oxides in a PbO or PbO/B2O3 flux. When the flux is slowly cooled, PMN–PT crystals begin to spontaneously nucleate and grow. Crystals from one to several mm in cross section have been obtained from such experiments.

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Bridgman Growth

The Bridgman method is much more conducive than the flux technique to the growth of the large, oriented crystals required for commercial devices. In the Bridgman method, a PMN–PT charge is melted in a crucible and then is unidirectionally solidified by passing the crucible through a temperature gradient. An oriented crystal is placed at the cold end of the crucible and partially melted to seed crystal growth. High-temperature furnaces and Pt crucibles are required for the growth of this material because the melting point of PMN–PT is in the range of 1285 ºC to 1320 ºC.6

Several researchers have used this technique to grow large, oriented PMN–PT crystals of good quality.69 A typical Bridgman grown crystal with a 1.8-cm diameter is shown in Figure 2. Although single crystals have been grown using this technique, compositional variations along the growth axis are a major problem that results in large portions of the crystals having non-optimal piezoelectric properties.6


Figure 2. Typical Bridgman grown PMN–PT crystal (diam. = 1.8 cm)


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Zone Leveling

The segregation behavior observed in the Bridgman growth of PMN–PT is a well-recognized consequence of thermodynamic properties of solid-solution systems, in this case the pseudo-binary system of lead magnesium niobate–lead titanate. The zone leveling technique had previously been used to ‘level’ compositional profiles in other segregating material systems and was proposed as an attractive method to eliminate the compositional variations observed in PMN–PT. Instead of melting the entire charge, as in Bridgman growth, the zone leveling method relies on melting only a small ‘zone’ of charge. By placing the initial molten zone partway into a seed and subsequently passing the zone through the entire charge, large, oriented single crystals of relatively good compositional uniformity can be grown.10

Some potentially difficult challenges remain in the production of PMN–PT for commercial devices. These include scaling laboratory processes to reproducible production scales and determining how to lower production costs (for example the expensive Pt crucibles can only be used once in the growth processes described previously). The physics and chemistry of PMN–PT are still being fully explored, making this new material very exciting and possibly opening doors to more applications.


Name Formula MW CAS MP BP Density at 25 °C Cat. No.
Boric anhydride, 99.999% B2O3 69.62 [1303-86-2] 450 °C 2065 °C 2.46 g/mL 202851-5G
202851-25G
Lead(II) oxide, 99.999% PbO 223.19 [1317-36-8] 886 °C   9.35 g/mL 203610-10G
203610-50G
Lead(II) titanate, 99+% PbTiO3 303.09 [12060-00-3]     7.52 g/mL 215805-250G
Magnesium oxide, 99.999% MgO 40.31 [1309-48-4] 2852 °C 3600 °C 3.60 g/mL 529699-10G
529699-50G
Niobium(V) oxide, 99.99% Nb2O5 265.81 [1313-96-8] 1460 °C   4.47 g/mL 203920-10G
203920-50G
Titanium(IV) oxide, rutile, 99.999% TiO2 79.87 [1317-80-2] 1800 °C   4.17 g/mL 204730-5G
204730-25G

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Materials

     

References

  1. Park, S.-E.; Shrout, T. R. J. Appl. Phys. 1997, 82, 1804.
  2. Service, R. F. Science 1997, 275, 1878.
  3. Ye, Z.-G.; Dong, M. J. Appl. Phys. 2000, 87, 2312.
  4. Dong, M.; Ye, Z.-G. J. Cryst. Growth 2000, 209, 81.
  5. Jiang, X. et al. Physica C 2001, 364-365, 678.
  6. Zawilski, K. T. et al. J. Cryst. Growth 2003, 258, 353.
  7. Lee, S.-G. et al. Appl. Phys. Lett. 1999, 74, 1030.
  8. Zawilski, K. T. et al. J. Cryst. Growth 2005, 282, 236.
  9. X. Wan, X. et al. J. Cryst. Growth 2004, 263, 251.
  10. Zawilski, K. T. et al. J. Cryst. Growth 2005, 277, 393.

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