64 research outputs found
A statistical model approximation for perovskite solid-solutions: a Raman study of lead-zirconate-titanate single crystal
Lead titanate (PbTiO3) is a classical example of a ferroelectric perovskite
oxide illustrating a displacive phase transition accompanied by a softening of
a symmetry-breaking mode. The underlying assumption justifying the soft-mode
theory is that the crystal is macroscopically sufficiently uniform so that a
meaningful free energy function can be formed. In contrast to PbTiO3,
experimental studies show that the phase transition behaviour of
lead-zirconate-titanate solid solution (PZT) is far more subtle. Most of the
studies on the PZT system have been dedicated to ceramic or powder samples, in
which case an unambiguous soft-mode study is not possible, as modes with
different symmetries appear together. Our Raman scattering study on
titanium-rich PZT single crystal shows that the phase transitions in PZT cannot
be described by a simple soft-mode theory. In strong contrast to PbTiO3,
splitting of transverse E-symmetry modes reveals that there are different
locally-ordered regions. The role of crystal defects, random distribution of Ti
and Zr at the B-cation site and Pb ions shifted away from their ideal
positions, dictates the phase transition mechanism. A statistical model
explaining the observed peak splitting and phase transformation to a complex
state with spatially varying local order in the vicinity of the morphotropic
phase boundary is given.Comment: Article contains four black-and-white figures, one colour figure and
one Table. Symmetry analysis and details of the model are given in Appendices
I and II, respectivel
Large area sheet task: Advanced dendritic web growth development
The growth of silicon dendritic web for photovoltaic applications was investigated. The application of a thermal model for calculating buckling stresses as a function of temperature profile in the web is discussed. Lid and shield concepts were evaluated to provide the data base for enhancing growth velocity. An experimental web growth machine which embodies in one unit the mechanical and electronic features developed in previous work was developed. In addition, evaluation of a melt level control system was begun, along with preliminary tests of an elongated crucible design. The economic analysis was also updated to incorporate some minor cost changes. The initial applications of the thermal model to a specific configuration gave results consistent with experimental observation in terms of the initiation of buckling vs. width for a given crystal thickness
High-pressure neutron study of the morphotropic PZT: phase transitions in a two-phase system
In piezoelectric ceramics the changes in the phase stabilities versus stress
and temperature in the vicinity of the phase boundary play a central role. The
present study was dedicated to the classical piezoelectric,
lead-zirconate-titanate (PZT) ceramic with composition
Pb(ZrTi)O at the Zr-rich side of the morphotropic phase
boundary at which both intrinsic and extrinsic contributions to
piezoelectricity are significant. The pressure-induced changes in this
two-phase (rhombohedral +monoclinic at room temperature and
above 1 GPa pressures) system were studied by high-pressure neutron
powder diffraction technique. The experiments show that applying pressure
favors the phase, whereas the phase transforms continuously to the
, which is favored at elevated temperatures due to the competing entropy
term. The phase transformation is discontinuous. The
transformation contributes to the extrinsic piezoelectricity. An important
contribution to the intrinsic piezoelectricity was revealed: a large
displacement of the cations (Zr and Ti) with respect to the oxygen anions
is induced by pressure. Above 600 K a phase transition to a cubic phase took
place. Balance between the competing terms dictates the curvature of the phase
boundary. After high-pressure experiments the amount of rhombohedral phase was
larger than initially, suggesting that on the Zr-rich side of the phase
boundary the monoclinic phase is metastable.Comment: 6 figure
A Raman Study of Morphotropic Phase Boundary in PbZr1-xTixO3 at low temperatures
Raman spectra of PbZr1-xTixO3 ceramics with titanium concentration varying
between 0.40 and 0.60 were measured at 7 K. By observing the
concentration-frequency dependence of vibrational modes, we identified the
boundaries among rhombohedral, monoclinic, and tetragonal ferroelectric phases.
The analysis of the spectra was made in the view of theory group analysis
making possible the assignment of some modes for the monoclinic phase.Comment: 5 pages, 4 figure
High pressure phases in highly piezoelectric Pb(Zr0.52Ti0.48)O3
Two novel room-temperature phase transitions are observed, via synchrotron
x-ray diffraction and Raman spectroscopy, in the Pb(Zr0.52Ti0.48)O3 alloy under
hydrostatic pressures up to 16 GPa. A monoclinic (M)-to-rhombohedral (R1) phase
transition takes place around 2-3 GPa, while this R1 phase transforms into
another rhombohedral phase, R2, at about 6-7 GPa. First-principles calculations
assign the R3m and R3c symmetry to R1 and R2, respectively, and reveal that R2
acts as a pressure-induced structural bridge between the polar R3m and a
predicted antiferrodistortive R-3c phase.Comment: REVTeX, 4 pages with 3 figures embedded. Figs 1 and 3 in colo
Towards a New Architectural Framework – The Nth Stratum Concept
Current architectures and solutions are about to reach the limits of sustainable developments. Over the years, many new requirements have emerged, and there are observations pointing to an ever-increasing diversity in applications, services, devices, types of networks at the edge and the access. Meanwhile, the infrastructures for internetworking, connectivity, and also management remain fairly the same. A new paradigm is needed that can support a continuous high pace of innovations in all the different parts and aspects of a communication system, while at the same time keeping costs of deployment and maintenance down. This new paradigm has to embrace current trends towards increased heterogeneity, but on the other hands provide support for co-existence and interoperability between alternative and various solutions all residing within a global communication system. This paper presents a new architectural framework called the Nth Stratum concept, and which takes a holistic approach to tackle these new needs and requirements on a future communication system
Competing orders in PZN-xPT and PMN-xPT relaxor ferroelectrics
Neutron and x-ray scattering studies on relaxor ferroelectric systems
Pb(ZnNb)O (PZN), Pb(MgNb)O (PMN), and
their solid solutions with PbTiO (PT) have shown that inhomogeneities and
disorder play important roles in the materials properties. Although a
long-range polar order can be established at low temperature - sometimes with
the help of an external electric field; short-range local structures called the
``polar nano-regions'' (PNR) still persist. Both the bulk structure and the PNR
have been studied in details. The coexistence and competition of long- and
short-range polar orders and how they affect the structural and dynamical
properties of relaxor materials are discussed.Comment: Article submitted for JPSJ Special Topics (Novel States of Matter
Induced by Frustration
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