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Dr John Huber
Materials World Network -
Domain Evolution in Ferroelectrics
This project is an international collaboration between University of
Texas, Austin, and University of Oxford, UK. Ferroelectric crystals
have applications as sensors, actuators and memory devices. Their
behaviour in these applications is strongly governed by defects in the
crystals such as domain walls. Understanding of these defects is at
present held back by a lack of experimental data that are carefully
matched to - and thus can directly evaluate the predictions of -
current models.
During the project, 3-dimensional mapping of domain structure using
synchrotron X-ray diffraction will be carried out. Material
configurations will be chosen to capture features such as domain needle
formation, and domain nucleation near electrodes or inclusions. This
will provide direct observations of the evolution of domain structure.
Existing phase field models, extended to 3-dimensions will then be used
to explore the observed configurations. Piezo-force microscopy and
scanning electron microscopy will be used to evaluate model predictions
at surfaces. The outcomes of the project will contribute at a
fundamental level to the understanding of domain structure evolution,
fracture, and toughening in ferroelectric crystals.
- Support
- EPSRC (UK), NSF (US)
- People
- Dan Sui, Chad Landis (University of Texas, Austin)
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Sensors and actuators: selection issues in engineering design
Choosing the best material or device to fulfil a given function is a
crucial part of engineering design. Historically, this process relied
on the experience of designers and incremental modification of previous
designs. However, in novel design problems, solutions with great
potential may be overlooked simply because they rely on a device or
material that was not in the designer’s experience. The aim is to
extend rational selection methods to the field of engineering devices.
Selection methods have been developed for making an optimal choice of
sensor, actuator, or power source given the demands of an application.
The approach involves establishing a set of common performance
characteristics shared by each member of a class of devices, and then
matching the performance characteristics of each device to the
application’s requirements. Quantitative methods such as merit indices
and formal constrained optimisation methods are employed. The
results are applicable to a very wide range of design problems.
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In the last 15 years, the field of functional materials with
applications in sensors, actuators and smart materials, has expanded
rapidly. Central to this development are piezoelectric materials which
offer solid state actuation and sensing under direct electrical
control. However, the fundamental issues of designing with and
modelling of bulk single crystals remain largely unaddressed. Although
many of the potential applications are piezoelectric in nature, the
greatest strains are achieved at high field levels, which can induce
both ferroelectric switching and phase transformations. At present,
applications are severely limited by the issue of robustness: internal
stresses give rise to cracks that grow in low-cycle fatigue. Yet there
is no reliable model for the internal stress state of large single
crystals. Similarly, a predictive understanding of the behaviour of
ferroelectric single crystals under combined electrical, mechanical and
thermal loads is needed. Such an understanding would enable the
engineering design process for existing single crystal piezoelectrics,
and would be equally applicable in the future to Lead-free
piezoelectric single crystals that have the same underlying mechanisms
of piezoelectricity and ferroelectric switching.
- Support
- EPSRC
- People
- Prashant Potnis, Nien-Ti Tsou
The aim is to develop a system for in situ observation of
microstructural evolution, using Atomic Force Microscopy. The principal
thrust of the project is the examination of ferroelectric surfaces to
address key questions about the evolution of microstructure under
mechanical and electrical loads. It is well understood that the
material behaviour is governed by processes occurring at the
microstructural length scale, such as domain wall motion and
pinning. However, relatively little has been done to model the
evolution of domain structures under load. In order to understand
this process, it is necessary to observe how the domain structure of
the materials changes under load. This is a challenging task
because conventional observation of the microstructure relies on
etching of polished surfaces. However, several methods of scanning
probe microscopy can directly identify the microstructure of
ferroelectrics. This raises the possibility of scanning a ferroelectric
surface while the material is loaded to observe the microstructural
events that control the material response.
- Support
- John Fell Fund
- People
- J.E. Huber, A.C.F. Cocks
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