The selection of piezoelectric materials for any given application can be complex. We looked at some of the parameters that influence the performance of basic piezoelectric sensors and actuators in the lectures but did not have time to consider the complexities associated with the diverse range of devices (e.g. microphones, guitar pickups, car knock sensors, level sensors, acceleration sensors, audio sounders & buzzers, motion control actuators, power transducers for cleaning, welding, machining, sonar etc.) that exploit piezoelectrics. No single material is optimal for all applications; hence, manufacturers offer various options. CeramTec is one company that produces piezoelectric ceramics, and they offer around 20 different types of PZT, each with a unique combination of physical characteristics. This question gives you a specific application and a small selection of piezoelectric ceramics. The aim is to select the MOST suitable material for the application. You will need to do some background research to understand the application itself, the piezoelectric material's role, and which material parameters most strongly influence performance.

Application and a brief description:

Application: High-power ultrasonic cleaning baths for industrial parts

Description: Piezoelectrics are used in a wide range of ultrasonic cleaning applications ranging from small ultrasonic baths used to clean small components such as jewellery to large industrial baths that are several meters long. The large industrial baths, which you will consider in this question, run almost continuously at high powers to clean industrial parts as they come off a production line. Automated conveyors transport the components into the bath and out the other side. In this application, piezoelectric actuators are mounted to the outer surface of a liquid-filled 'bath'. Vibrations from the piezoelectric actuators operating at frequencies between 20kHz and 40kHz are transmitted into the bath, causing cavitation in a cleaning fluid. Cavitation is a process where pressure differences in a fluid cause the formation of tiny gas bubbles, which implode after a short time. The cavitation intensively cleans the contaminated surfaces of the parts, even if they have a complicated shape. To be effective, high-intensity vibrations must be generated by the piezoelectric actuators.

Material options available:

CeramTec offer around 20 different types of piezoelectric ceramics, but for this question, you MUST ONLY consider the following materials (please download the two datasheetsLinks to an external site.):

PZT5A2

Sonox P53

PZT507

PZT403

PZT406

For this question, you should assume that the materials will be orientated to actuate or sense in the 33 direction. You will notice that two of the relative permittivity parameters (the ones with a superscript 'S') have been crossed out, and you are advised to ignore these.

Questions:

To select a suitable material for the application, please answer the following questions in the space provided:

a) To generate the high-intensity ultrasonic vibrations needed for cleaning, the piezoelectric actuators are powered by a high-intensity sine wave electrical signal at frequencies between 20 and 40 kHz. In well-designed ultrasonic actuators, much of the input energy will be converted to vibrations, but some will inevitably cause 'self-heating' within the piezoelectric. Briefly explain why self-heating needs to be considered when designing actuators for the current application.

b) Identify three material-related parameters that you feel are most important to consider in relation to self-heating in the current application. For each one, briefly explain; what the parameter represents; why you selected it as important; and state if it is desirable to have a high, low or medium value. NOTE: A parameter may be considered 'important' either because it affects the heating or relates to self-heating consequences.

c) Identify two additional material parameters relating to the piezoelectric performance (not related to self-heating) that you feel are most important for the current application. For each one, briefly explain what the parameter represents; why you selected it as important; and state if it is desirable to have a high, low or medium value.

d) Out of the ceramics listed above, which would you recommend for the current application and why? In justifying your selection, you should briefly explain your selection process. NOTE: In material selection, it is often helpful to eliminate materials that are clearly unsuitable for the application at an early stage and then consider the remaining options in greater detail.

NOTE: This question is worth 35% of the coursework; hence you are expected to provide a detailed and carefully considered answer.