1-3 Piezocomposite technology is well established for ultrasonic applications and we have already described its advantages in a previous article.
To resume in a nutshell, we can say that their advantages over bulk piezoelectric material are:
- an improved electromechanical efficiency
- reduced lateral resonances
- an improved sensitivity
The following electrical impedance curves demonstrates all of the advantages clearly:
1-3 Piezocomposites can also be customized to suit different applications such as sonar, biomedical imaging, non-destructive testing or borehole acoustics.
Unfortunately, this also makes them difficult to design!
There are many permutations and parameters to consider when designing such piezocomposites.
Let’s talk now about two of the most impactful aspects:
Selecting the right piezoelectric material and polymer fill materials
Selecting a suitable piezoelectric material for the PZT pillars or rods is important but care must also be taken when selecting the polymer material for the passive phase as it plays an important role in well designed piezocomposites.
For piezoelectric material, we ideally want to:
- Maximize the electromechanical coupling coefficients for efficient conversion of electrical energy to mechanical energy.
- Low acoustic impedance is desirable as we generally transmit to low impedance loads such as water and air.
- The permittivity (dielectric properties) of piezoceramics is what allows the device to be matched with the electronics it is linked to. Thus, engineers must be careful to choose a material with a permittivity which matches the drive and receive electronics.
The choice of the polymer fill depends also on the type of design we want.
For example, when designing phased array devices, we avoid crosstalk (energy coupling laterally across the device) which is unwanted.
Crosstalk adds noise to receive signals and can disrupt the directivity.
Soft materials have better characteristics for reducing crosstalk as their attenuative properties are generally higher than stiffer materials.
The drawback of soft polymer or piezoelectric materials is that their malleability tends to be high and therefore, they are not suited to high levels of hydrostatic pressure.
For high power applications, devices with softer polymer fillers can also generate heat quicker and without the proper avenues to dissipate the heat, it can lead to device failures.
Reducing the piezocomposite transducer to a “Unimodal Resonance” device
One of the main goals is to build a unimodal device, which operates like a very small piston and generates ultrasonic waves within a desired band of frequencies.
Many aspects of piezocomposites design can influence the resonance characteristics of the device, generating unwanted secondary resonances:
- Device dimensions can introduce resonances based on the width or length of the composite. To palliate against this, these dimensions should be increased enough so that the existing resonances become out of band.
- The polymer filler in the kerf can also introduce what is known as an inter-pillar resonance. Certain designs with too low a volume fraction and polymers with low acoustic velocities, the inter-pillar resonance can impinge on the operational bandwidth leading to poor surface dilation quality.
How to Optimize piezocomposite transducers?
OnScale simplifies the task to optimize such piezocomposite transducers by providing an easy access to parametric sweeps of the most important design variables (fractional volume, matching layer thickness,…).
Once your piezocomposite model is setup parametrically, hundreds of simulations can be launched on the cloud and results can be post-processed using scripts to provide exactly the metrics that you need.
For example, in the following picture issued from such an optimization analysis, you can observe that if we change the matching thickness, the main center frequency of the device is also impacted!
Check the details about this study in this article: Simulation and optimization of piezocomposite transducers with a practical example
We have also detailed guides teaching step-by-step how to simulate piezocomposites.
Take a look at them here on our support website to get started: