PHD THESIS: STRUCTURAL HEALTH MONITORING OF PIEZOELECTRIC ULTRASONIC TRANSDUCERS
USING THEIR PARASITIC MODES
key words: piezoelectric material, transducer, numerical modelling, finite element method,
structural health monitoring, electroacoustic characterization
GREMAN laboratory (UMR CNRS 7347 - University of Tours - INSA Centre Val de Loire):
GREMAN is research group on materials, microelectronics, acoustics & nanotechnology, a
laboratory of over 100 persons with 42 permanent academic staff, 16 administrative &
technical staff, 38 PhD students and 10 post-doctoral researchers. Since its creation in 2012,
this joint research laboratory of Tours University, INSA Centre Val de Loire and CNRS
focuses its activities on materials, components and devices for energy efficiency and
microelectronics through 5 key topics:
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functional oxides for energy efficiency: combinatory synthesis and nanostructuration;
magnetic and optical properties of ferroic and electronic correlation materials;
innovative materials and components for power and RF microelectronics: wide
bandgap and porous semiconductors and their applications;
piezoelectric and capacitive micro/nano systems for ultrasound transducers and
energy conversion;
methods and instrumentation for ultrasonic characterisation of complex media.
Background and motivation:
A piezoelectric transducer converts electrical energy into vibrational mechanical energy.
Here we consider the ultrasonic frequency range typically from 1 to 20 MHz. Such a
transducer is composed of 3 major parts: a piezoelectric element that converts electricity into
vibrations and/or vice-versa, a backing that damps the vibrations inside the piezoelectric
element, a matching layer that improves the transmission of ultrasonic waves both ways
between the piezoelectric element and the propagation medium.
Because the piezoelectric effect is reversible, a transducer can both generate and/or receive
an ultrasonic wave. Such transducers are used for cleaning, atomizing liquids, drilling or
milling materials, welding plastics, for medical diagnostics, therapy, and nondestructive
evaluation as well as for many other purposes. Consequently, systems incorporating either a
single piezoelectric transducer or two transducers are widely used to measure distances and
flow rates, and as fluid level sensors.
Ultrasonic transducer performance can degrade because of different factors linked to the
conditions in which they are used or stored. It has been shown that properties of a
piezoelectric element erode gradually, in a logarithmic relationship with time after fabrication.
However, in many cases, aging of the other transducer elements – including the bond layers
between them - dominates that of the piezoelectric element.
Here we propose to use parasitic vibration modes to evaluate the aging of transducer
structures of different levels of complexity (a simple piezoelectric disc or plate with a backing,
a single element ultrasonic transducer, an ultrasound array element, a fully connected array).
Objective and method:
Due to the natural anisotropy of piezoelectric materials and to the geometry of transducer
elements, several vibration modes exist in a given structure, one of which is the main mode
that is used to produce ultrasonic waves in a propagation medium, all the other modes being
considered as parasitic ones. In most applications, the main mode is the longitudinal one: the
deformations are in the same direction as the electrical field that appears between two
electrodes on each face of the piezoelectric element when a voltage is applied to them. This
mode is chosen to have a high electromechanical coupling coefficient and the transducer
design is performed in order to reduce all parasitic modes and/or to push them to frequencies
that are outside of the useful bandwidth.
For a disc shape piezoelectric element, the thickness (longitudinal) mode is often the main
one and the radial mode is then parasitic, its frequency depending on the disc’s diameter (the
harmonics of the radial mode are also parasitic). For plates operating in thickness mode,
there are several parasitic modes, the frequencies of which depend on the lateral dimensions
of the piezoelectric element.
Whatever the transducer design, parasitic modes always remain but they are rarely studied
in detail. The idea behind this work is to use them to obtain data that is linked to the
degradation of the transducer.
The transducer structures will be modeled using numerical methods, including finite
elements. Experiments will be performed to measure complex electrical impedance as a
function of frequency using a RF network/impedance analysers and mechanical
displacements using laser vibrometers/interferometers, among other characteristics. Several
transducer structures of different levels of complexity will be studied. Comparing the
experimental results with simulations based on the finite element method will allow a
validation of the model. Then structures with well-controlled degradations will be studied and
correlations between measured/simulated results and the type/size/location of defects will be
investigated.
The outcomes are procedures to monitor the transducer characteristics during its lifetime and
detect degradations before they significantly affect the performance of the system in which
the device operates.
Required profile:
The candidate should have theoretical skills in materials science and in physical acoustics
and some experience in the use of numerical modelling tools (e.g. finite element method).
Experimental capabilities are also essential and basic knowledge in the field of signal
processing would be useful. A Masters level degree that includes courses in electronics &
electrical engineering and/or applied physics and/or materials science and/or mechanical
engineering is required.
Contact persons:
Marc Lethiecq: marc.lethiecq@insa-cvl.fr, +33(0)254558424
Maxime Bavencoffe: maxime.bavencoffe@insa-cvl.fr, +33(0)254558409