Scientific
rationale
Research Coordinator: Prof. Fabrizio Davì
Scientific
rationale
The research project aims to the electromechanical characterization
of ferroelectric solids with a view towards an application to an "inchworm"
motor with tunable properties. The inchworm actuator can be considered
a small linear motors that produces axial movements trough very thin footsteps.
They are able to position objects with nanometric precision on a range
of few millimeters with a practically continuous motion. Despite the low
speed (around 0.2÷1.5 mm/sec), many are the advantages that these
devices offer, particularly the elevated resolution for a single step (up
to 1 nm) that makes them suitable for many micropositioning applications.
The lack of contacts and the exploitation of the piezoelectric
effect lead to other interesting typical advantages of these actuators,
i.e.
the total absence of magnetic fields generation and radiated noise and,
in addition, the complete immunity to the noise itself. Their high response
speed is limited only by the inertia of the object being moved and the
output capability of the electronic driver. Virtually no power is expended
nor heat generated when maintaining a PZT actuator in an energized (holding)
state.
Finally, due to its extreme simplicity and energy conversion
based on a solid state dynamic, the inchworm actuator can be
operated over millions of cycles without sign of aging.
As already mentioned these devices are currently being used in the field
of the high precision micropositioning in spatial applications, life science,
industrial automation, microelectronic technologies, etc. Some application
examples are: sample positioning for electron beam litography, accurate
positioning and alignment of optical fibers for connection to a laser diode
or laser welding, electrical probes positioning for semiconductor inspection,
active alignment of imaging system, scanning confocal microscope optics,
active mirror element in space telescope, etc.
Ferroelectrics are ferroic materials which have active
domain walls, i.e. crystallographic boundaries that can be moved
by applying an external force or field. The resulting changes in the shapes
of these materials are large enough to make them useful as actuators. Moreover
they undergo two or more phase transformations, which can be used to tune
their performance.
Ferroelectric materials exhibit spontaneous electric
polarization; that is, the electric dipole moments within a domain align
in a common direction, even in the absence of an external electrical field.
A large class of ferroelectrics exhibits the perovskite structure ABO3
(A is a bivalent metal, B is a tetravalent element). On cooling from a
high temperature, this crystal structure undergoes a displacive phase transformation
and the point symmetry changes from cubic (m3m) to tetragonal (4mm). The
tetragonal state has spontaneous polarization along the [001] crystallographic
direction and we have 6 possible domain orientations which minimize the
electrostatic energy.
Domain wall motion is initiated by electric fields or
mechanical stress with associated hysteretical phenomena. The very first
studies in this field are due to Devonshire (Adv. in Physics, 1954),
Merz (Phys. Rev., 1954) and Little (Phys. Rev., 1955).
A micromechanical model of "domain switching", that is the modification which domain structure undergoes upon the application of electromechanical actions, was given in Davì (Z.A.M.P.,2001); there the ferroelectric solid was modeled as a continuum with vectorial microstructure and double-well energy. By the dissipation inequality and the balance law the evolution equations were arrived at: a characterization of both the hysteresis phenomena and the domain structure was given also. The evolution of domain wall was the subject of a further paper: Davì and Mariano (J. Mech. and Physics of Solids,2001), whereas in Davì (Math.and Mech.of Solids,2000), the effects of domain switching on the constitutive properties of piezoelectric ceramics was studied.
Further developments of this research are related to the development and electromechanical characterization of piezoelectric materials in the PZT system. Compositions near the morphotropic phase boundary are chosen. At the Curie point, PZT converts from a paraelectric state with the ideal cubic perovskite structure to a ferroelectric phase located near a morphotropic phase boundary between the tetragonal and rhombohedral states. The morphotropic phase boundary delineates two solid phases that remain in a near-equilibrium state over a very wide temperature range. Very large piezoelectric coupling between electric and mechanical variables is obtained near this phase boundary. Titanium-rich compositions in the PZT system favor a tetragonal modification, with sizable elongation along [001] and a large spontaneous polarization in the same direction. Six equivalent polar axes in the tetragonal phase correspond to the [100] direction (the unit cell edge that forms one side of a square) and directions of the cubic paraelectric state. A rhombohedral ferroelectric state is favored for zirconium-rich compositions: the distortion and polarization are along the [111] (body diagonal) directions, giving rise to eight possible domain states.
Lead zirconate-lead titanate ceramics PZT show extremely strong piezoelectric effects for compositions near the morphotrophic phase boundary (MPB) (Jaffe, Cook and Jaffe, Piezoelectric Ceramics, 1977) where rhombohedral and tetragonal phases coexist and are related to the presence of a maximum in the dielectric constant, a larger number of riorientable polarization directions and a maximum mechanical compliance preventing cracking during domain reorientation (Wersing et al., Silicates Industriels, 1985). The MPB of the undoped material lies near the composition with PbZrO3 53.5% mol, and is impurity-sensitive. Superior properties can be achieved through compositional modifications that can be summarized as donor and acceptor doping, isovalent substitutions and addition of low melting additives ((Jaffe, Cook and Jaffe, Piezoelectric Ceramics, 1977, Haertling, Piezoelectric and Electrooptic Ceramics, 1986). We believe that twinning is a minimizer for the elastic energy associated with spontaneous strain associated with polarization.
Processing of PZT materials is mainly based on the conventional
ceramic powder technology, the perovskitic phase being obtained by the
solid state reaction of the starting oxides. For most of the applications
high density and homogeneous microstructure are among the most important
features that must be achieved. Several limiting factors come into play,
mostly related to the high volatility of PbO at elevated temperatures,
difficulty to achieve sintered densities close to the theoretical
ones (Kingon and Clark J. Am. Ceram. Soc., 1983), nonstoichiometry
in composition as well as compositional fluctuations (Fernandez et al.,
J.
Eur. Ceram. Soc., 1998) and poor microstructure.
In the Research Institute for Ceramic Technology (IRTEC)
all the processing steps were optimized starting from the evaluation of
the raw materials,to the cold consolidation and heating profiles, to electrode
deposition and poling. Several compositions were developed from the
“soft" PZT (PbZr 0.52Ti
0.48O3doped
with niobium) (Galassi et al., NATO Science Series 3 - High Technology,
2000), to an “hard” PZT with a complex composition showing excellent electromechanical
coupling factor values while retaining high coupling factors (Galassi et
al., J. Eur. Ceram. Soc., 1999). We shall study both from a
theoretical and an experimental point of view the electromechanical properties
of PZT.
Technical
description
Inchworm Motor Systems provide true linear motion, nanometer resolution, and a range of hundreds of millimeters. Since they are not steppers, servos, or other electromagnetic motors, there's no backlash or mechanical drift, no rotational motion, gear boxes, leadscrews, or hydraulic lines. Inchworm Motors are solid-state linear positioning devices that provide a superior motion profile without loss of resolution at high speeds and no loss of force or smoothness of motion at low speeds.
The inchworm motor is a piezoelastic collar fitted
over a precision steel shaft. It is formed by three hollow cylinders in
PZT
linked through the basis. The two ends of the collar
are polarized in the radial direction such that, when the voltage is applied,
they will expand and contract to act as clamps (resembling the head and
the tail of an "inch-worm"). The central portion of the collar is polarized
in the axial direction such that, when the voltage is applied, it will
expand or contract to modulate the length of the collar (acting as the
body of the inch-worm). The signals applied to the electrodes of these
ferroelectric components are synchronized in such a way that the collar
can "inch" along the steel shaft in very small but precise steps.
Domain switching and twinning phenomena induced by strong electric fields or mechanical stress can be used to tune the "inchworm" response in real time, by varying both the direction and the intensity of the average spontaneous polarization.
Personnel
Unit 1 - Mathematical Modeling and Engineering
Design
| Fabrizio Davì (Unit and Project Coord.) | University of Ancona | Full Professor |
| Stefano Pirani | University of Ancona | Associate Professor |
| Marco Mosconi | University of Ancona | Ph. D. Student |
| Paolo Mengucci | University of Ancona | Research Assistant |
| Adriano Di Cristoforo | University of Ancona | University Technical Personnel |
| Raffaella Rizzoni | University of Ferrara | Research Assistant |
| Giovanni Lancioni | University of Roma "Tor Vergata" | Ph. D. Student |
Unit 2 - Components and Materials Development
| Carmen Galassi (Unit Coordinator) | CNR-IRTEC | CNR Researcher |
| Edoardo Roncari | CNR-IRTEC | CNR Researcher |
| Gualtiero Fabbri | CNR-IRTEC | CNR Fellowship |
| Claudio Capiani | CNR-IRTEC | CNR Technical Personnel |
Related
papers