1,879 research outputs found
Design and development of a smart panel with five decentralised control units for the reduction of vibration and sound radiation
This Technical Report discusses the design and the construction of a smart panel with five decentralised direct velocity feedback control units in order to reduce the vibration of the panel dominated by well separated low frequency resonances. Each control unit consists of an accelerometer sensor and a piezoelectric patch strain actuator. The integrated accelerometer signal is fed back to the actuator via a fixed negative control gain. In this way the actuator generates a control excitation proportional and opposite to the measured transverse velocity of the panel so that it produces active damping on the panel. First the open loop frequency response function between the sensor and the actuator of a single channel has been studied and an analogue controller has been designed and tested in order to improve the stability of this control system. Following the stability of all five control units has been assessed using the generalised Nyquist criterion. Finally the performances of the smart panel have been tested with reference to the reduction of the vibrations at the error positions and with reference to the reduction of the radiated sound. Finally in appendix to this Report, a parametric study is presented on the properties of sensor-actuator FRFs measured with different types of piezoelectric patch actuators. The results of this parametric study have been used in order to choose the actuators to be used for the construction of the smart pane
Effects of coupling between octahedral tilting and polar modes on the phase diagram of PbZr1-xTixO3 (PZT)
The results are presented of anelastic and dielectric spectroscopy
measurements on large grain ceramic PZT with compositions near the two
morphotropic phase boundaries (MPBs) that the ferroelectric (FE) rhombohedral
phase has with the Zr-rich antiferroelectric and Ti-rich FE tetragonal phases.
These results are discussed together with similar data from previous series of
samples, and reveal new features of the phase diagram of PZT, mainly connected
with octahedral tilting and its coupling with the polar modes. Additional
evidence is provided of what we interpret as the onset of the tilt instability,
when is initially frustrated by lattice disorder, and the long range order is
achieved at lower temperature. Its temperature T_IT(x) prosecutes the long
range tilt instability line T_T(x) up to T_C, when T_T. It is proposed that the
difficulty of seeing the expected 1/2 modulations in diffraction
experiments is due to the large correlation volume associated with that type of
tilt fluctuations combined with strong lattice disorder. It is shown that the
lines of the tilt instabilities tend to be attracted and merge with those of
polar instabilities. Not only T_IT bends toward T_C and then merges with it,
but in our series of samples the temperature T_MPB of the dielectric and
anelastic maxima at the rhombohedral/tetragonal MPB does not cross T_T, but
deviates remaining parallel or possibly merging with T_T. These features,
together with a similar one in NBT-BT, are discussed in terms of cooperative
coupling between tilt and FE instabilities, which may trigger a common phase
transition. An analogy is found with recent simulations of the tilt and FE
transitions in multiferroic BiFeO3. An abrupt change is found in the shape of
the anelastic anomaly at T_T when x passes from 0.465 to 0.48, possibly
indicative of a rhombohedral/monoclinic boundary.Comment: 15 pages, 11 figure
Piezoelectric softening in ferroelectrics: ferroelectric versus antiferroelectric PbZrTiO
The traditional derivation of the elastic anomalies associated with
ferroelectric (FE) phase transitions in the framework of the Landau theory is
combined with the piezoelectric constitutive relations instead of being
explicitly carried out with a definite expression of the FE part of the free
energy. In this manner it is shown that the softening within the FE phase is of
electrostrictive and hence piezoelectric origin. Such a piezoelectric softening
may be canceled by the better known piezoelectric stiffening, when the
piezoelectric charges formed during the vibration are accompanied by the
depolarization field, as for example in Brillouin scattering experiments. As
experimental validation, we present new measurements on Zr-rich PZT, where the
FE phase transforms into antiferroelectric on cooling or doping with La, and a
comparison of existing measurements made on FE PZT with low frequency and
Brillouin scattering experiments
Effects of aging and annealing on the polar and antiferrodistortive components of the antiferroelectric transition in PZT
The antipolar and antiferrodistortive (AFD) components of the
antiferroelectric (AFE) transition in PbZr1-xTixO3 (x=0.054) can occur
separately and with different kinetics, depending on the sample history, and
are accompanied by elastic softening and stiffening, respectively. Together
with the softening that accompanies octahedral tilting in the fraction of phase
that is not yet transformed into AFE, they give rise to a variety of shapes of
the curves of the elastic compliance versus temperature. All such anomalies
found in samples with x=0.046 and 0.054, in addition to those already studied
at x=0.050, can be fitted consistently with a phenomenological model based on
the simple hypothesis that each of the polar and AFD transitions produces a
step in the elastic modulus, whose position in temperature and width reflect
the progress of each transition. The slowing of the kinetics of the
transformations is correlated with the formation of defect structures during
aging in the ferroelectric or AFE state, which are also responsible for a
progressive softening of the lattice with time and thermal cycling, until
annealing at high temperature recovers the initial conditions
Strong spectral evolution during the prompt emission of GRB 070616
Swift has revealed features in GRB early light curves, such as steep decays
and X-ray flares, whose properties are consistent with an internal origin
though they are far from understood. The steep X-ray decay is often explained
using the curvature effect; however a significant number of GRBs display strong
spectral evolution during this phase, and a new mechanism must be invoked to
explain this. Of particular interest are the longest duration GRBs in which the
early emission can be studied in most detail. Here we present data for GRB
070616, in which the prompt emission shows a complex multipeaked structure,
leading to one of the longest prompt emission durations ever recorded. We take
advantage of extensive coverage of such a long burst by all Swift instruments.
Combining data from Swift and Suzaku we study the evolution of the prompt
emission spectrum, following the temporal variability of the peak energy and
spectral slope.Comment: 4 pages, 2 figures (Fig 1 in colour), contributed talk, submitted to
the proceedings of Gamma Ray Bursts 2007, Santa Fe, New Mexico, November 5-9
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Processing of (BaSr)Fe12O19 for antenna miniaturization
Hexaferrites of M-type at high-frequency are investigated, owing to their higher ferro-magnetic resonances as compared to traditional magnetic materials, thus ensuring a less dispersive behavior of the material at microwaves frequencies. Here we present the experimental work concerned with the ceramic process for the production of the barium-strontium hexaferrite (BSFO). The critical issue in the production of this material is to obtain a fully dense and homogeneous microstructure. These aspects have been addressed by introducing powder grinding processes, and by varying the cold consolidation conditions of the powders. A number of samples of the material have been produced, both for morphological and microstructural characterization and a demonstrator antenna, onto which conductive patches were applied, and the radiating properties of which were tested. Barium-strontium hexaferrite as supporting material for antennas - while allowing substantial device miniaturization – displays relatively high dielectric and magnetic losses
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