Lead-Free Piezoelectric Material

Ricoh has successfully developed a lead-free piezoelectric film on a Si substrate. The film has electromechanical properties comparable to those of PZT (lead zirconate titanate).

Why a Lead-Free Piezoelectric Material?

PZT (lead zirconate titanate) has excellent piezoelectric and ferroelectric properties and is dominantly used in sensors, actuators and transducers for various products, which include car navigation systems and high-precision positioning systems. Ricoh has also used PZT for the actuator on inkjet printer heads.

Although lead is an RoHS regulated substance because of its environmental and health hazards, because there is no comparable alternative material, electronic components and devices made from PZT have so far been exempted from the regulation.

Ricoh confronted the technical challenge and has successfully developed a lead-free material, BSnT (Sn-doped BaTiO3), which shows electromechanical properties equivalent to PZT.

The lead-free material of Ricoh

1) Why it is the BSnT

PZT has outstanding piezoelectricity; it is often called a gift from the heaven. Piezoelectricity converts electrical energy into mechanical energy and the reverse. In PZT, huge piezoelectricity appears in an MPB composition. This MPB composition has two or more crystal structures. Because the opportunity for conversion increases, the amount of displacement increases. Figure 1 shows the PZT phase diagram. The crystal structures are classified according to color.

Ricoh searched for a lead-free material that achieves the MPB state. BSnT, which is a solid solution of BaTiO3 and BaSnO3, was selected. The MPB state exists at a BaSnO3 composition of approximately 10% (Figure 2).
Figure 1: PZT phase diagram Figure 2: BSnT phase diagram
Figure 1: PZT phase diagram Figure 2: BSnT phase diagram

2) Film is fabricated on Si substrate

To achieve BSnT material having good piezoelectric properties, commonly, a high temperature annealing process is needed. Until now, high quality BSnT films were only possible on ceramic substrates. Because it is difficult to integrate the films with MEMS (Micro Electro Mechanical Systems) or a drive circuit on one chip, its application is limited to several cases. Ricoh has successfully developed a high heat-resistant underlayer on Si substrate, and formed a lead-free BSnT film on it, which indicates excellent electromechanical properties equivalent to PZT. This is a world first, making it possible to broaden the application of lead-free materials in various electronic devices.

Figure 3 and Figure 4 show the surface morphology and polarization-electric (P-E) field hysteresis curves of the BSnT film fabricated on a Si substrate at 750℃ and 1000℃, respectively. It is evident that increasing the firing temperature from 750℃ to 1000℃ enlarges the grain size and improves P-E characteristics, which correlates with the electromechanical properties.
Figure 3: Surface morphology of the BSnT films fired at different temperatures.
Figure 3: Surface morphology of the BSnT films fired at different temperatures.
Figure 4: Difference in properties of BSnT material by firing temparature Figure 4: Difference in properties of BSnT material by firing temparature (fired at 1000℃)
Figure 4: Difference in properties of BSnT material by firing temparature (right: fired at 1000℃)

3) Purpose for adopting CSD method (*1)

One other unique characteristic of Ricoh’s technology is using CSD method as the precursor material. To fabricate the BSnT material from a powder requires a complicated process and expensive etching equipment for patterning. Because Ricoh adopted the CSD method as the precursor material, it is possible for us to fabricate piezoelectric devices with our original IJP (Inkjet Printing) method. Consequently, we can increase: material usage efficiency, thereby reducing manufacturing cost and environmental load during manufacture and reducing facility investment for corresponding changes in the manufacturing process; examples are the enlargement or processes suited for small lots of diverse products.

  • *1CSD method: The abbreviation of Chemical Solution Deposition method, also called the sol-gel method, is to synthesize a material by starting with gelification of the sol solution. It is a low-temperature synthesis method and can be applied to synthesize materials of various microstructures, topologies and functions.


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Application of Lead-Free Piezo Material

The lead-free piezoelectric material we developed has equivalent piezoelectric properties to PZT and can be developed on a Si substrate, which is a highly workable substrate. In the future, our lead-free piezoelectric material will open the door to application in sensors and actuators for various products in which PZT is now used.
 

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