FAQs on Ferroperm™ Piezoceramic materials

CTS | Ferroperm is one of the few suppliers in the industry to offer a complete range of piezoceramic materials.

Here is a comparison table that summarises the traditional standard materials.

In addition to these standard materials, Ferroperm also offers a range of other specially dedicated materials - see our complete materials range 

Our customers specify more than 95% of all production in Ferroperm Piezoceramics. This means that most orders will come with a list of individual parameters and tolerances that will have to be fulfilled before the final inspection unit can approve the production, and parts can be shipped. 

In most cases, it is not necessary to specify all parameters since over-specification often leads to an unnecessarily difficult part to produce and therefore also becomes very expensive. For that reason, we have a set of standard tolerances that we use if nothing else is specified.  Link to tolerances page

Over the years, the industrial suppliers have agreed on a common terminology for their product types. Most of these have been formalised in various standardisation documents from IEC, CENELEC, IEEE etc. This pdf shows the difference between the 4 main geometric categories: Terminology.pdf

At Ferroperm Piezoceramics, the general aim for our production is to have full traceability of all production steps in any order. We are therefore always able to go back to each operation and see if systematic problems have occurred with equipment etc. This gives us a good possibility to perform corrective actions to direct or indirect problems seen in our own measurements or noted in the customer’s production or testing facilities. We therefore always welcome a dialog regarding possible problems, and we encourage our customers to send back parts for inspection and re-measurements if there are any doubts regarding their performance.

The production at Ferroperm Piezoceramics comprises all steps of processing from reaction of raw materials in form of oxides, carbonates, organometallics etc. to the delivery of finished electroded and poled piezoceramic elements, fulfilling customer drawings and specifications.

Based on 40 years of experience, it is known that the only real test of the raw materials used in the production of piezoceramics is a pilot production of real piezoelectric parts. As a consequence, Ferroperm Piezoceramics has gone a step further, performing all production in the company material-batch-specific.

This means that before any material is released for regular production, a pilot production has been completed, and all relevant parameters measured and approved. The data for a specific batch is saved in a data-base which is used by the production manager to optimize the production parameters for each new production. This system has several advantages over the more common method of using generic parameters or mean values from several earlier batches.

Examples: If two batches have different sintering shrinkage, the production manager will adjust the pressure in the uniaxial presses in order to obtain the same dimensions of a certain part from batch to batch. If a customer has very tight tolerance on for example capacitance, the production manager will select only batches with a very similar permittivity whereby a constant capacitance and thickness can be obtained over deliveries of several years.

Hard materials have a low dielectric loss and a high mechanical Qm value. This makes them suitable for applications where you want to transmit as much power as possible. The sensitivity of such materials is however not very high. Hard PZT materials in Ferroperm Piezoceramics' program are Pz24, Pz26 and Pz28.

Soft materials have higher sensitivity, higher displacement and a lower mechanical Q value. This makes them suitable for sensor applications. The loss is however higher, and they are therefore not able to transmit very high power without having problems with overheating and depolarisation. Soft PZT Materials in Ferroperm’s program are Pz21, Pz23, Pz27 and Pz29.

Data sheets for each material can be downloaded from the resources page.

More information in this document: Hard-and-soft-PZT.pdf

We use four different processes to deposit an electrode on the surface of the parts. Each of these processes has different characteristics, so the right choice of electrode will depend completely on your application. More information: Electrode-types.pdf

Ferroperm Piezoceramics offers three different options for surface finish on discs, rings and plates.

• Lapped surface Ra ≈ 1,2 µm

• Fine-grinded surface Ra ≈ 0,8 µm

• Polished surface Ra ≈ 0,3 µm

These values are defined for the ceramic material without electrodes. If a chemical or evaporated electrode is deposited on the surface, the Ra values remain in the same range. The screen-printed electrodes are however thicker and will always define a new Ra which is in the same range as a normal lapped surface. In applications with screen-printed electrodes where low Ra-values are required, Ferroperm Piezoceramics offers a polishing operation after deposition of the electrode. The Ra values will in these cases be below 0,5 µm.

The indication of polarity can be specified by the customer, but there are a few "standard" methods which are widely used within the industry.

The most common way of indicating the direction of polarisation is to apply a small ink dot on the positive side of the part. Another way, typical for small plates, is to use two parallel lines imprinted in the silver electrodes as an indicator of the positive side. If a positive electrode is connected to the side with the ink dot, the part will expand in the polarisation direction and contract in the directions perpendicular to this.

The indication of polarity on a shear plates can be specified by the customer, but there are a few "standard" methods, which are widely used within the industry.

The most common way of indicating the direction of polarisation is to remove a single, or both, corners at the face where the positive polarisation electrode was. Another method is to make a small ink-dot close to the positive edge. For shear parts it makes no difference if you turn the plate corresponding to a rotation of the polarisation vector (flip the plate over). A shear plate, however, has no sensitivity in any other direction than the polarisation direction, so turning the plate corresponding to a horizontal 90 degree turn of the polarisation vector should therefore be avoided. Normally, a shear plate is designed in rectangular shape to avoid such mistakes. The nomenclature for shear plates is always to give the dimension in the polarisation (3) direction first, then the dimension in the 2-direction and finally the direction (1) of applied/generated field (distance between electrodes).

Example: A 1 mm thick plate that is 10 mm wide, poled along a 5 mm long side and then electroded on the large faces, will thus be called: 5x10x1 mm.

A resonance in a piezoceramic part can be considered as an acoustic phenomenon. If a sound wave is transmitted onto a piezoelectric part, this will create a vibration with the same frequency in the material. Read more about it here: Resonances.pdf

Based on basic knowledge on the nature of resonances, it can be seen that the smaller the determining dimension is, the higher the resonance frequency will be. The exact location of a resonance will depend on the speed of sound through the piezoelectric material for one specific mode (since this gives the wave length in the material), combined with the size of the element: More information here: Res-geometry.pdf

For a circuit diagram and a detailed answer, please read this document: Equivalent-Circuit.pdf

Soldering electrical wires to screen-printed silver electrodes makes excellent and time-stable connections. Occasionally, there can be problems with wetting the solder on the silver surface, and soldering can therefore be difficult. Please read this document for best-practice recommendations: Soldering.pdf

Ferroperm Piezoceramics is generally concentrated completely on the manufacturing of piezoceramic materials and have little expertise in the assembly of ultrasonic devices. By maintaining this strategy, we keep a status as a completely independent supplier and avoid any indirect competition with our own customers. We therefore only have very limited information on this subject:

First of all, the output from the piezoelectric material is very dependent on the electronic driving system. The electronics and ceramics should match each other very closely in terms of resonance frequency and input impedance.

Secondly, the assembly process is very critical for the vibration loss in the transducer. The gluing conditions should therefore be controlled very strictly. It is important that the glue layer is very thin and very uniform if loss is to be minimized. In some cases, the glue is deposited by screen-printing or a spray process to secure the thin and uniform thickness. It is also common to vacuum-treat the assembled part to remove trapped air in the glue joint.

Finally, the curing conditions for the glue are important for the performance of the part. If you cure the glue at a too high temperature, you will get two problems which in combination will amplify. An elevated temperature will give increased tendency of depoling the part. At the same time, increased temperature can cause mechanical stresses due to mismatched thermal expansion coefficients in the ceramic and housing materials.

The depolarisation problems in piezoceramics are, however, most often caused by a combination of electrical and mechanical stresses under operation. Since the gluing operation does not include the electrical stress, the curing temperature can often be up to 25 - 50 °C higher than that recommended maximum operation temperature. A method to check if there is a problem with depolarisation during curing is to measure if the capacitance has decreased (5-10%) in cured parts compared to fresh and unmounted parts.

From other customers, we hear that a good product for gluing is Epotek (www.epotek.com). They have a full program of conductive and insulating glues. Other brands we hear about are Ciba-Geigy and Loctite. The brand is not the most important factor as long as you choose a professional, reliable and reproducible product and fine-tune your process to work with this. 

In many cases, piezoelectric transducers are used for sending wave fronts from the piezoelectric transducer into a different media, for example body-tissue or steel, and then analysing the returning echo in order to determine the distance and nature of the various details inside this media. 
Acoustic waves (or any other form of waves) that are transmitted onto a different material unfortunately tend to reflect from its boundaries. For more details, please read this document: Matching-layers.pdf

It is difficult to say anything certain about the maximum operation voltage or power input you can use in piezoceramics since these depend on several external factors.

These factors are for example:

• Operation frequency
• Impedance matching
• Frequency fine-tuning
• Matching layers
• Backing material
• Mechanical stresses
• Mechanical load
• Operation time
• Heat dissipation
• Ambient temperature
• etc.

We do however have general guidelines based on experience from customers in several different application areas:

• Pz26
  • Max 10 W/cm 2 of radiating surface
  • Max 200 VAC/mm.
• Pz27
  • Max 2-3 W/cm 2 of radiating surface
  • Max 50-100 VAC/mm.

If the piezoceramics are used in static applications and only driven in the positive regime, it is often the break-down voltage in air that sets the practical limit. We recommend not to go above 1 kV/mm.

As you can see from the first comments, it is important to point out that these values are only general. In special cases, customers have reported much higher or lower possible values.

Ferroperm Piezoceramics is generally concentrated completely on the manufacturing of piezoceramic materials and have little expertise in the assembly of ultrasonic devices. By maintaining this strategy, we keep a status as a completely independent supplier and avoid any indirect competition with our own customers.

We therefore only have limited information on this subject, but we are of course willing to help with the general guidelines we have:

When assembling a high-power transducer, it is important to apply the right amount of pre-stress. However, if the pre-stress is too high, a depolarisation will begin to take place resulting in lower output of the transducer.

We generally recommend not to use a pre-stress higher than 25 MPa since this is the level that Pz26 under normal circumstances will withstand during high-field operation where the stress rises further in the "expansion-phase" of a cycle.

The amount of pre-stress can be monitored either by a simple torque wrench or by measuring the DC voltage over a large capacitor.

It is important to point out that the stress level can increase above the critical level if an unfavourable combination of materials is chosen in a transducer that needs to work at elevated temperatures or at some point is heat-treated (e.g. autoclaving of ultrasonic dental equipment). It is therefore important to carefully select materials whose TEC match the TEC for PZT in such a way that stresses are released, or at least kept constant, at elevated temperatures.

As a general guideline, the TEC for the thickness direction in Pz26 is 1 to 2 x10-6 K -1 in a temperature range from 0 – 100 °C

Apart from being material-dependent, the data for this parameter is unfortunately separated into several different categories. Values for our two standard PZT materials, Pz26 and Pz27 are given in this document:  TEC.pdf

In any piezoelectric material, there are four different Young’s moduli. For more details, please read the attached document: Youngs-module.pdf

Please see the attached document for the energy calculations: Energy.pdf