Although, the manufacturing processes for bulk and multilayer piezoelectric components share commonalities, they are quite different. Bulk piezoelectric components are made of one layer of piezoceramic material pressed with up to 1 MN compacting force. Multilayer piezoelectric components are made by tape casting very thin layers of piezoceramic material on which thin layers of electrode material are printed. Over 100 layers are then laminated.
Basic Manufacturing of Bulk Piezoelectric Components
A bulk piezoelectric component is defined as a product with a single layer of piezoelectric ceramic with electrically conductive metal electrodes applied to two opposing sides. These electrodes are used during poling of the material (aligning the electrical dipoles like in magnetic materials) and also used as electrical contacts for applying electric field during use. Bulk piezoelectric components are manufactured by compaction of piezoceramic powder by conventional processes like uni-axial pressing, isostatic pressing and extrusion.
The basic technology for manufacturing of bulk components is the pressing of shaped bodies using spraydried granular material. This is achieved using high-capacity presses with up to 1 MN compacting force.
The shaped bodies are either manufactured true-to-size, taking into account the sintering contraction, or with machining excesses, which are removed after sintering to achieve the required precision.
By using high-production inboard sawing equipment, it is possible to manufacture components (discs, plates, tubes, etc.) with very low thickness.
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Basic Manufacturing of Multilayer Piezoelectric Actuators
While being well adapted for sensors and applications at resonance, bulk piezoelectric components are not preferred for actuator applications. To reach a useful quasi-static displacement with bulk, a very high operating voltage, in the range of 500-1000V is required. In order to overcome this problem the multilayer technology was developed.
A multilayer component is composed of several layers of piezoelectric material, alternating with internal electrodes. Internal electrodes are successively positioned as positive and negative. All positive electrodes are connected to one external electrode on one side of the component; negative electrodes are connected on the other side of the component.
Compared to bulk piezoelectric actuators, multilayer piezoelectric actuators have the advantage of providing displacements comparable to bulk piezoelectric actuators at applied electric fields up to 100 times smaller. In addition, since the internal electrodes are embedded in the ceramic, higher electrical fields can be applied, leading to higher strain levels, and the active material is better protected against the environmental effects.
The multilayer approach enables designs with multiple functions, e.g. integration of several actuators within one component or combination of actuator and sensor.
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A flow diagram of the manufacturing processes involved with piezoceramic bulk and multilayer processing at CTS is shown below. Each process is described below the diagram.
Raw materials – both bulk and multilayer processing
High purity raw materials are used, mainly lead oxide, titanium oxide, zirconium oxide and a range of dopants.
Mixing – both bulk and multilayer processing
The raw materials are accurately weighed according to a well-documented prescription and mixed with additives and solvents.
Calcining – both bulk and multilayer processing
The mixed materials are calcinated at 900 °C to 1000 °C to remove organics in the raw material mix and also create a more homogenous material.
Milling – both bulk and multilayer processing
The calcinated materials are milled to achieve a very homogeneous suspension with the required particle size. The suspension is measured for viscosity and approved for tape casting or for bulk production.
Granulation – only for bulk processing
The powder is mixed with a binder and spray dried for further processing.
Pressing – only for bulk processing
The piezoceramic material is pressed in a specific pressing tool adapted to the shape to be produced.
Mixing – only for multilayer processing
The powder is mixed with a specific binder. The mixed materials are milled to form a homogeneous suspension (slurry).
Tape casting – only for multilayer processing
The suspension is tape casted on a carrier foil. The tape thickness is between 20-40 µm as a standard. The green ceramic tape contains binder materials, which make the tape flexible and easy to handle in the following processes.
Build-up – only for multilayer processing
The tape is cut into sheets and automatically inspected for defects
Platinum or silver/palladium electrodes with predetermined patterns are printed on the piezoceramic sheets using screen-printing process with a suitable electrode paste. A simple multilayer actuator typically consists of 2 different electrode patterns, while more complex designs can include 5 different prints.
Layers of piezoelectric ceramic sheets are automatically stacked in the right sequence to the final build-up design. All items are built in 6” x 6” blocks.
Lamination – only for multilayer processing
The piezoelectric ceramic blocks are laminated at defined pressure, temperature and for a specified duration of time. During this process, individually cut layers are firmly laminated and brought in appropriate contact with each other.
Dicing and drilling – only for multilayer processing
After lamination, the individual components are diced or milled out of the green piezoelectric ceramic blocks. Dicing needs to follow accurately the pattern of the internal electrodes. In some cases, dicing is also performed on sintered ceramic.
Binder burn out – both bulk and multilayer processing
The organic compounds in components, binder materials, additives and remaining solvents are removed by very slowly heating of the green parts to a temperature of 500-700 ºC, where the organic compounds decompose and evaporate from the ceramic.
Sintering – both bulk and multilayer processing
In order for grains to grow and diffuse to form a dense piezoelectric ceramic components need to be sintered at high temperature. Several parameters have to be optimized with this process, e.g. temperature ramp rates, holding time and the condition of atmosphere.
Machining – both bulk and multilayer processing
Most products are machined or rectified in some manner before or after termination depending on the specific product. A lapping process is commonly employed, with the grain size of the lapping medium is adapted to the desired surface roughness on the parts.
Termination – both bulk and multilayer processing
For multilayer products, external electrodes are applied to make a connection to the internal electrodes inside the component. For bulk products, the termination provides external electrodes.
Termination is performed either by screen-printing and firing of an electrode paste, typically silver, or by a sputtering process.
Poling – both bulk and multilayer processing
During the poling process, adequate DC electrical field is applied and this applied electric field orients the domains in the electric field direction and lead to a remnant polarization of the material. This alignment is achieved in the poling process, where a high electrical field, 2-3kV/mm, is applied to the external electrodes at elevated temperatures for a specific amount of time.
Final inspection – both bulk and multilayer processing
After poling the components and stacks will go through a final inspection according to customer specifications or internal procedures, which is a combination of statistical method and 100 % measurements. Various mechanical and electrical parameters may be tested – also according to customer specifications. Generally for multilayer actuators, capacitance, dielectric loss and mechanical strain level (stroke/displacement) are specified for inspection, while for bulk components the frequency response (minimum impedance, coupling) is more relevant.
Stacking – both bulk and multilayer processing
Bulk as well as individual multilayer elements can be stacked. The elements are bonded together with epoxy adhesive. Inactive piezoceramic end-plates are applied to provide electrical insulation and spread the load. The individual elements within a stack are typically connected together electrically with a bus wire.
Packaging – both bulk and multilayer processing
Components are packed properly in plastic trays, plastic bags, foam or placed on tape and then placed in a box ready for shipment. Products intended for vacuum applications undergo a specific cleaning and packaging process.