Introduction to Piezoelectric Transducers
by Rob Carter and Richard Kensley
CHAPTER 1
Working with Piezoceramic
THIN-SHEET (SINGLE LAYER) PIEZOCERAMIC STOCK

PLEASE NOTE THAT THE PRIMARY COMPONENT OF PIEZOCERAMIC IS LEAD OXIDE (AS IN LEAD PAINT). WHEN HANDLING, USE THE PRECAUTIONS RECOMMENDED FOR HANDLING LEAD PAINT. For more, see “What You Need to Know About Working with LEAD PAINT” https://www.health.ny.gov/publications/2502.pdf or similar.

Unlike laminated bending motor stock, thin sheet piezoceramic is extremely fragile and difficult to handle. With proper care and practice it can be handled and manipulated quite easily.

Clean cutting of piezoceramic is best achieved by sending parts to diamond dicing saw or a waterjet cutting service. Some laser cutting services will cut piezoceramic as well.

With some risk to the part, rough cutting can be accomplished by placing parts on a flat surface such as a glass plate, and then lightly scribing a ruled line along its surface with a sharp razor blade until the piece cracks and separates. To retrieve the part, one edge should be lifted up by inserting the razor blade underneath and lifting until it can easily be grasped with one’s fingers (or plastic tweezers).

If the intent is to use the piece itself, then all jagged edges should be gently sanded down using 220 and/or 400 grit sandpaper. This will reduce the probability of subsequent crack propagation.

If the intent is to bond the part to make piezo-composite stock, the edges do not necessarily need to be cleaned up prior to bonding. Subsequent diamond saw or waterjet machining operations can be performed on the bonded composite.

2-LAYER GENERATOR STOCK
DURABILITY

The two-layer motor stock and generator stock are much more rugged than generally assumed. It can be handled without special care and oftentimes dropped without damage. The ceramic is nonporous and is impervious to moisture as well as chemically inert with acids and solvents. The adhesives used for lamination, the center shim, and the nickel electrodes, however, are susceptible to particular solvents and acids.

CUTTING AND SHAPING

For prototyping purposes, the generator stock can be rough cut on a band saw (having ~14 teeth/inch or more) as long as it is supported underneath by a back-up plate (plexiglass, metal, etc.). This is not recommended for dimensions less than 1/4”. Rough cutting usually produces burrs at the center shim which may make electrical contact to one of the outer electrodes. The burrs can be removed by filing or sanding the edge. Chipping will occur along the edge, but this is seldom great enough to affect performance. With some practice, the generator stock can be trimmed with scissors when one wants to remove thin slivers.

High quality cuts, necessary for long term stable performance, require the use of a high-speed diamond wheel saw or waterjet.

ACCESSING THE CENTER SHIM

A milling machine can be used to remove ceramic in order to access the center shim electrode for soldering an electrical lead. Removal of ~1 mil per pass is recommended. A handheld grinding tool (i.e. Dremel Tool) is suitable for quick center shim access. Alternatively, on rectangular parts a sharp three-corner file (¼ inch on a side or less) can be used to grind away the ceramic from one corner, exposing the brass center shim for soldering.

To make temporary contact to the center layer (in order to measure capacitance of a single layer or perhaps to repolarize it) the center shim can be contacted with a razor blade or push pin,

BONDING AND ATTACHING TO PIEZOCERAMIC

Attachments for power input or mechanical grounding are usually accomplished by bonding to the piezoceramic at its ends or middle. Holes or fasteners are put in these secondary members. Almost any adhesive bonds well to the piezoceramic nickel surface. These include epoxies, anaerobics, silicones, and cyanoacrylates. For quick and/or reversible mounting, the bending element is often clamped between two surfaces

SOLDERING & ATTACHING LEADS TO THE ELECTRODES & CENTER SHIM

Piezoceramic electrodes will be either fired silver or nickel. Silver electrodes are flat white in color while nickel electrodes are grey. Electrical connections are usually made to these electrodes by soldering, but one may also use conductive adhesive, or clips to attach wires.

Silver electrodes are not recommended for high electric field DC applications because silver ions are likely to migrate forming a bridge between the two electrodes, which is a resistive short circuit. Silver electrode piezoceramic is, however, often used in AC applications. Silver is applied to piezoceramic in the form of flakes suspended in a glass frit paste which is generally screened onto the ceramic and fired. The glass makes the bond between the ceramic and the silver particles. Silver is soluble in tin and a silver loaded solder should be used to prevent scavenging of silver in the electrode. Good solder joints can be made to the silver electrodes on piezoceramics with resin-core type solders with technique similar to that used for attaching components to printed circuit boards.

Nickel has good corrosion resistance and is a good choice for both AC and DC applications. It is applied to the piezoceramic either by vacuum deposition or electroless nickel process. It can usually be soldered to easily using an organic acid flux and RoHS compliant lead-free solder.

NOTE: Vacuum deposited nickel electrodes are usually very thin, making soldering somewhat tricky. Use of low wattage soldering irons with small tips and choice of the correct flux (to remove surface oxidation) makes soldering to electrode surfaces easy even under adverse conditions.

To prepare a poled for parallel operation (i.e. ‘Y’ poled) piezo bimorph for use, a wire must be attached to the center shim Generally, the center shim layer of a 2-layer piezoelectric bending elements is either .004” (.1mm) thick brass or stainless steel. Shims are soldered to in the same way as the nickel electrode.

Tools & Materials for Soldering

  • Soldering iron set – 550°- 650° F
  • Lead-free Solder
  • Supersafe #67 DSA Liquid Flux
  • Wires (preferably #30 gauge or smaller)
  • Pencil eraser and paper clip
Procedure for Soldering
  • Clean surface to be soldered with an abrasive (pencil eraser) and wipe with alcohol. This step can usually be skipped when using the proper flux.
  • Dip the tip of a paper clip into the flux and apply a small dot of Supersafe Liquid Flux to the electrode area to be soldered.
  • Apply small amount of solder to iron tip and transfer solder to the piezoceramic electrode by touching iron tip to flux dot. A good solder joint should flow rapidly (≤1 second) and look shiny. Metal shims take longer due to the increased thermal mass (~2 seconds).
  • Apply another small dot of Supersafe Liquid Flux to the solder dot on electrode.
  • Position pre-tinned wire on solder dot and apply soldering iron to the wire until the solder melts. Remove iron quickly after the solder melts and hold the wire still until the solder solidifies. A 30-gauge wire or smaller is recommended to minimize strain on the solder joint during wire handling.
  • Remove Supersafe Liquid Flux residue with clean running water. This flux residue is electrically conductive and must be removed for proper functioning of a piezo device. Any rosin residue may be removed with alcohol.
  • Wherever feasible, the wire-solder joint should be strain relieved with a drop of adhesive.

 

CHAPTER 2
Performance Testing

Generally, measurements of free deflection, blocked force, resonant frequency, and capacitance are easy to make and should be recorded for each design configuration explored. Capacitance is a good measure of the health of the element. If the capacitance of the piece decreases during operation or testing, from that of its initial value, then the element has probably depolarized, cracked, or lost electrical contact during operation.

For fundamental performance testing, a power supply and switching box with an adequate series protection resistor are suggested. Any frequency generator can be used to measure resonant frequency. The resonance patterns may be observed by sprinkling sugar on the piece and sweeping through the frequency band. At resonance, sugar will be tossed off the anti-nodes and collect at the nodes. Suitable means for measuring and deflection, such as an X-Y table and high compliance force gage, are needed. Such a system is shown in Figure 28.

Figure 2.1. System for measuring output performance (free deflection, blocked force, and resonant frequency) of a bending motor element

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