Using Diffusion Bonding in Making Piezoelectric Actuators

Lyndon B. Johnson Space Center, Houston, Texas

Thursday, May 01 2003

Fabrication is simplified and performance improved.

A technique for the fabrication of piezoelectric actuators that generate acceptably large forces and deflections at relatively low applied voltages involves the stacking and diffusion bonding of multiple thin piezoelectric layers coated with film electrodes. The present technique stands in contrast to an older technique in which the layers are bonded chemically, by use of urethane or epoxy agents.

The older chemical-bonding technique entails several disadvantages, including the following:

• It is difficult to apply the bonding agents to the piezoelectric layers.
• It is difficult to position the layers accurately and without making mistakes.
• There is a problem of disposal of hazardous urethane and epoxy wastes.
• The urethane and epoxy agents are nonpiezoelectric materials. As such, they contribute to the thickness of a piezoelectric laminate without contributing to its performance; conversely, for a given total thickness, the performance of the laminate is below that of a unitary piezoelectric plate of the same thickness.

The figure depicts some aspects of the fabrication of a laminated piezoelectric actuator by the present diffusion-bonding technique. First, stock sheets of the piezoelectric material are inspected and tested. Next, the hole pattern shown in the figure is punched into the sheets. Alternatively, if the piezoelectric material is not a polymer, then the holes are punched in thermoplastic films. Then both faces of each punched piezoelectric sheet or thermoplastic film are coated with a silver-ink electrode material by use of a silk-screen printer. The electrode and hole patterns are designed for minimal complexity and minimal waste of material.

After a final electrical test, all the coated piezoelectric layers (or piezoelectric layers and coated thermoplastic films) are stacked in an alignment jig, which, in turn, is placed in a curved press for the diffusion-bonding process. In this process, the stack is pressed and heated at a specified curing temperature and pressure for a specified curing time. The pressure, temperature, and time depend on the piezoelectric material selected. At the end of the diffusion-bonding process, the resulting laminated piezoelectric actuator is tested to verify the adequacy of the mechanical output as a function of an applied DC voltage.

The principal advantages of the diffusion-bonding process over the older chemical-bonding process are the following:

• No adhesive thinner or hardening agent is needed;
• There are no waste chemicals;
• Cure can be done at a relatively low temperature;
• There is less handling of piezoelectric sheets;
• No special adhesive-handling equipment is needed;
• The thickness contributed by the thermoplastic adhesive material (if used) is minimal;
• Diffusion bonding results in high-strength bonds that impart high durability and long fatigue life.

In the case of polymeric piezoelectric layers, piezoelectric properties are improved, probably because of an increase in Young's modulus associated with annealing during diffusion bonding.

There is a significant reduction in electrical wiring: The electrode and hole patterns in the stacked layers give rise to an internal topology equivalent to that of a continuously folded length of piezoelectric material. As a result, the multiple electrical connections to the active piezoelectric layers are reduced to two terminal holes.

This work was done by Frank E. Sager of Oceaneering Space Systems for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Manufacturing category.

Title to this invention, covered by U.S. Patent No. 5,761,782 has been waived under the provisions of the National Aeronautics and Space Act {42 U.S.C. 2457 (f)}. Inquiries concerning licenses for its commercial development should be addressed to
Mr. Jeff Brown
Oceaneering Space Systems
16665 Space Center Blvd.
Houston , TX 77598
Tel. No: (281) 488-9080 Ext. 3437
Fax No: (281) 488-6485

Refer to MSC-22886, volume and number of this NASA Tech Briefs issue, and the page number.