PZT powders are used in electrical applications such as piezoelectric ceramic ultrasound and capacitive devices.

Piezoelectric materials are ferroelectric materials with a unique range of properties. In a basic sense, if a piezoelectric material is deformed, an electric charge is generated. The opposite of this phenomenon also holds true: If an electric field is applied to a piezoelectric material, deformation occurs.

This ability to transform electrical energy into mechanical energy and vice versa makes piezoelectric materials suitable for a variety of applications, including diagnostic and therapeutic medical ultrasound devices; SONAR; automotive applications, such as knock sensors, buzzers, and back-up sensors; energy-harvesting applications; aerospace applications, such as guidance systems and vibration and noise cancellation; industrial applications, such as flow meters; piezoelectric motors and actuators; and consumer applications, such as humidifiers and BBQ grill igniters.

Soft (sensor) ceramic powders are typically used when high coupling and/or high charge sensitivity are important, such as in flow or level sensors; ultrasonic nondestructive testing/evaluation (NDT/NDE) applications; or for accurate inspections of automotive, structural, or aerospace products. Material characteristics include a high dielectric constant; high coupling; high charge sensitivity; high density with a fine grain structure; a high Curie point; and a clean, noise-free frequency response.

Hard (high power) ceramic powders are used when high power characteristics are required, including applications such as the generation of ultrasonic or high-voltage energy in ultrasonic cleaners, sonar devices, etc. Important powder characteristics include a high piezoelectric charge constant (d33); a higher mechanical quality factor that reduces mechanical loss and enables a lower operating temperature; a low dissipation factor that ensures cooler, more economical operation; high dielectric stability; and low mechanical loss under demanding conditions.

Manufacturing Process

The process of manufacturing piezoelectric lead zirconate titanate (PZT) ceramic powders consists of six distinct unit operations (see Figure 1). High-purity raw materials are evaluated, selected and sourced throughout the world. Selection criteria, in addition to purity, include material activity and limits on specific deleterious impurities.

Once each material is selected and approved for use, it is precisely weighed (according to the formulation being manufactured) and transferred to the mills. These ingredients are then wet-milled together in their proper proportions, with particle size distribution being the most critical control. Precise control over particle size distribution is necessary to ensure appropriate material activity during calcination.

The mill is then discharged and the product is dried and prepared for calcining. High-purity crucibles are necessary to guarantee that no chemical contaminants are present in the final product. The calcining operation is carried out in air at about 1000°C, where the desired PZT phase is formed.

It is important to remember that the major ingredient in PZT material is lead oxide, which is a hazardous material that has a relatively high vapor pressure at calcining temperatures. Therefore, it is possible to alter the desired composition by allowing too much lead to evaporate during the firing operations. Proprietary measures are employed to ensure that lead loss is not a factor.

After calcining, the PZT powder is returned to the mill to ensure homogeneity and to prepare the material for the addition of binder. The binder-containing slurry is then fed to a spray dryer, where water is evaporated. A successful spray drying operation requires experienced operators who can regulate the temperatures within the unit in accordance with the solids content of the slurry material, as well as the volume of slurry entering the dryer.

The purpose of spray drying PZT powder material is to provide a free-flowing product in the form of binder-containing hollow spheres of a narrow particle size distribution, which makes for an ideal dry pressable material. The morphology of the material is crucial for the consistent filling of die cavities. The uniform spheres of appropriate particle size distribution allow for air escapement throughout the compaction process, yielding lamination-free green ceramic shapes.

Figure 2 illustrates the remaining unit operations performed by the ceramic manufacturer for the conversion of the spray-dried PZT powder into dense piezoelectric ceramic shapes for ultrasonic transducer applications.

For additional information, contact:

• APC International, Ltd. at P.O. Box 180, Duck Run, Mackeyville, PA 17750; (570) 726-6961; fax (570) 726-7466; e-mail sales@americanpiezo.com; or visit www.americanpiezo.com.
• Hammond Lead Products at 10 South Grosstown Rd., Pottstown, PA 19464; (610) 327-1400 or (800) HLP-POTS (457-7687); fax (610) 327-1255 or visit www.hammondleadproducts.com.

SIDEBAR: New Partnership

APC International, Ltd. (APCI) and Hammond Lead Products (HLP), a division of The Hammond Group, Inc., have joined forces through a technology/manufacturing venture for the production of piezoelectric PZT ceramic powders. The partnership will utilize APCI's PZT materials background, technical powder manufacturing experience and propriety formulations, as well as HLP's knowledge in working with and manufacturing toxic materials. HLP will also provide a dedicated internal supply of high-purity lead oxide.

A new domestic PZT powder manufacturing plant will be located at HLP's Pottstown, Pa., facility. Currently under construction, the plant is being engineered as a fully compliant lead chemical manufacturing operation. In addition to being an ISO 9000/2000 facility, it will be designed to conform to the more stringent reduced ambient air standards for lead that will go into effect later in 2010.

Links

• APC International, Ltd.
• Hammond Lead Products