136 research outputs found
Some strategies for improving caloric responses with ferroelectrics
Y.L. and B.D. acknowledge the China Scholarship Council (CSC) for funding Y.L.’s stay in France and a public grant overseen by the French National Research Agency (ANR) as part of the “Investissements d’Avenir” program (Reference No. ANR-10-LABX-0035, Labex NanoSaclay).Many important breakthroughs and significant engineering developments have been achieved during the past two decades in the field of caloric materials. In this review, we address ferroelectrics emerging as ideal materials which permit both giant elastocaloric and/or electrocaloric responses near room temperature. We summarize recent strategies for improving caloric responses using geometrical optimization, maximizing the number of coexisting phases, combining positive and negative caloric responses, introducing extra degree of freedom like mechanical stress/pressure, and multicaloric effect driven by either single stimulus or multiple stimuli. This review highlights the promising perspective of ferroelectrics for developing next-generation solid-state refrigeration.Publisher PDFPeer reviewe
Direct and indirect measurements on electrocaloric effect : recent developments and perspectives
Y.L. and B.D. acknowledge the China Scholarship Council (CSC) for funding Y.L.'s stay in France and a public grant overseen by the French National Research Agency (ANR) as part of the “Investissements d'Avenir” program (Reference: ANR-10-LABX-0035, Labex NanoSaclay).It has been ten years since the discovery of the giant electrocaloric effect in ferroelectric materials showed that it is possible to employ this effect for substantial cooling applications. This last decade has been marked by increasing research interest, especially in characterizing and measuring the electrocaloric effect using both the so-called indirect and direct approaches. In this context, a comprehensive summary and careful reexamination of these approaches are very timely and of great importance to justify the assumptions used in different measurement techniques. This review is therefore dedicated to cover recent important and rapid advances from both the indirect and direct measurements and provides critical insights relevant for quantifying the electrocaloric effect. It involves electrocaloric materials from normal ferroelectrics, antiferroelectrics, and relaxors, and it fundamentally focuses on how the electrocaloric entropy changes in response to electric field in these typical electrocalorics. The article addresses recent developments, especially during the past three years, such as technical selection of proper polarization-electric field loops, negative electrocaloric effect in antiferroelectrics and relaxors, the controversial debate on the indirect method in relaxors, the important role of field dependence of specific heat, kinetic factors, and so on. Moreover, this review also is concerned with extracting reliable data by direct measurements. Four typical techniques and devices used recently, such as thermocouples, differential scanning calorimeters, specifically designed calorimeters, and scanning thermal microscopy, are briefly reviewed, while infrared cameras are emphasized. We hope that our review will not only provide a useful background to understand fundamentally the electrocaloric effect and what one really measures but also may act as a practical guide to exploit and develop electrocalorics towards the design of suitable devices.Publisher PDFPeer reviewe
Spin and lattice excitations of a BiFeO3 thin film and ceramics
We present a comprehensive study of polar and magnetic excitations in BiFeO3
ceramics and a thin film epitaxially grown on an orthorhombic (110) TbScO3
substrate. Infrared reflectivity spectroscopy was performed at temperatures
from 5 to 900 K for the ceramics and below room temperature for the thin film.
All 13 polar phonons allowed by the factor-group analysis were observed in
theceramic samples. The thin-film spectra revealed 12 phonon modes only and an
additional weak excitation, probably of spin origin. On heating towards the
ferroelectric phase transition near 1100 K, some phonons soften, leading to an
increase in the static permittivity. In the ceramics, terahertz transmission
spectra show five low-energy magnetic excitations including two which were not
previously known to be infrared active; at 5 K, their frequencies are 53 and 56
cm-1. Heating induces softening of all magnetic modes. At a temperature of 5 K,
applying an external magnetic field of up to 7 T irreversibly alters the
intensities of some of these modes. The frequencies of the observed spin
excitations provide support for the recently developed complex model of
magnetic interactions in BiFeO3 (R.S. Fishman, Phys. Rev. B 87, 224419 (2013)).
The simultaneous infrared and Raman activity of the spin excitations is
consistent with their assignment to electromagnons
Towards multicaloric effect with ferroelectrics
This work was supported in part by the National Science Foundation (Grant No. CMMI-#1361713) and by DOE Ames Laboratory on “The Caloric Materials Consortium”. L.B. acknowledges the support of ARO Grant No. W911NF-16-1-0227. B.D. acknowledges a public grant overseen by the French National Research Agency (ANR) as part of the “Investissements d’Avenir” program (Grant No. ANR-10-LABX-0035, Labex NanoSaclay) and Fonds National de la Recherche du Luxembourg (FNR) through InterMobility Project No. 16/1159210 “MULTICALOR”.Utilizing thermal changes in solid state materials strategically offers caloric-based alternatives to replace current vapor-compression technology. To make full use of multiple forms of the entropy and achieve higher efficiency for designs of cooling devices, the multicaloric effect appears as a cutting-edge concept encouraging researchers to search for multicaloric materials with outstanding caloric properties. Here we report the multicaloric effect in BaTiO3 single crystals driven simultaneously by mechanical and electric fields and described via a thermodynamic phenomenological model. It is found that the multicaloric behavior is mainly dominated by the mechanical field rather than the electric field, since the paraelectric-to-ferroelectric transition is more sensitive to mechanical field than to electric field. The use of uniaxial stress competes favorably with pressure due to its much higher caloric strength and negligible elastic thermal change. It is revealed that multicaloric response can be significantly larger than just the sum of mechanocaloric and electrocaloric effects in temperature regions far above the Curie temperature but cannot exceed this limit near the Curie temperature. Our results also show the advantage of the multicaloric effect over the mechanically-mediated electrocaloric effect or electrically-mediated mechanocaloric effect. Our findings therefore highlight the importance of ferroelectric materials to develop multicaloric cooling.PostprintPeer reviewe
Effect of high pressure on multiferroic BiFeO3
We report experimental evidence for pressure instabilities in the model
multiferroic BiFeO3 and namely reveal two structural phase transitions around 3
GPa and 10 GPa by using diffraction and far-infrared spectroscopy at a
synchrotron source. The intermediate phase from 3 to 9 GPa crystallizes in a
monoclinic space group, with octahedra tilts and small cation displacements.
When the pressure is further increased the cation displacements (and thus the
polar character) of BiFeO3 is suppressed above 10 GPa. The above 10 GPa
observed non-polar orthorhombic Pnma structure is in agreement with recent
theoretical ab-initio prediction, while the intermediate monoclinic phase was
not predicted theoretically.Comment: new version, accepted for publication in Phys. Rev.
Magnetodielectric coupling and phonon properties of compressively strained EuTiO3 thin films deposited on LSAT
Compressively strained epitaxial (001) EuTiO3 thin films of tetragonal
symmetry have been deposited on (001) (LaAlO3)_0.29-(SrAl_{1/2}Ta_{1/2}O3)_0.71
(LSAT) substrates by reactive molecular-beam epitaxy. Enhancement of the Neel
temperature by 1 K with 0.9% compressive strain was revealed. The polar phonons
ofthe films have been investigated as a function of temperature and magnetic
field by means of infrared reflectance spectroscopy. All three infrared active
phonons show strongly stiffened frequencies compared to bulk EuTiO3 in
accordance with first principles calculations. The phonon frequencies exhibit
gradual softening on cooling leading to an increase in static permittivity. A
new polar phonon with frequency near the TO1 soft mode was detected below 150
K. The new mode coupled with the TO1 mode was assigned as the optical phonon
from the Brillouin zone edge, which is activated in infrared spectra due to an
antiferrodistortive phase transition and due to simultaneous presence of polar
and/or magnetic nanoclusters. In the antiferromagnetic phase we have observed a
remarkable softening of the lowest-frequency polar phonon under an applied
magnetic field, which qualitatively agrees with first principles calculations.
This demonstrates the strong spin-phonon coupling in EuTiO3, which is
responsible for the pronounced dependence of its static permittivity on
magnetic field in the antiferromagnetic phase.Comment: Submitted to Phys. Rev.
Disorder and relaxation mode in the lattice dynamics of PbMgNbO relaxor ferroelectric
The low-energy part of vibration spectrum in PbMgNbO
relaxor ferroelectric was studied by inelastic neutron scattering. We observed
the coexistence of a resolution-limited central peak with strong quasielastic
scattering. The line-width of the quasielastic component follows a
dependence. We find that is temperature-dependent.
The relaxation time follows the Arrhenius law well. The presence of a
relaxation mode associated with quasi-elastic scattering in PMN indicates that
order-disorder behaviour plays an important r\^ole in the dynamics of diffuse
phase transitions
Multiferroic phase transition near room temperature in BiFeO3 films
In multiferroic BiFeO3 thin films grown on highly mismatched LaAlO3
substrates, we reveal the coexistence of two differently distorted polymorphs
that leads to striking features in the temperature dependence of the structural
and multiferroic properties. Notably, the highly distorted phase
quasi-concomitantly presents an abrupt structural change, transforms from a
hard to a soft ferroelectric and transitions from antiferromagnetic to
paramagnetic at 360+/-20 K. These coupled ferroic transitions just above room
temperature hold promises of giant piezoelectric, magnetoelectric and
piezomagnetic responses, with potential in many applications fields
Anomalous transverse acoustic phonon broadening in the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O_3
The intrinsic linewidth of the transverse acoustic (TA) phonon
observed in the relaxor ferroelectric compound
Pb(MgNbTiO (PMN-20%PT) begins to broaden
with decreasing temperature around 650 K, nearly 300 K above the ferroelectric
transition temperature ( K). We speculate that this anomalous
behavior is directly related to the condensation of polarized, nanometer-sized,
regions at the Burns temperature . We also observe the ``waterfall''
anomaly previously seen in pure PMN, in which the transverse optic (TO) branch
appears to drop precipitously into the TA branch at a finite momentum transfer
\AA. The waterfall feature is seen even at
temperatures above . This latter result suggests that the PNR exist as
dynamic entities above .Comment: 6 pages, 4 figure
A Universal Phase Diagram for PMN-xPT and PZN-xPT
The phase diagram of the Pb(Mg1/3Nb2/3)O3 and PbTiO3 solid solution (PMN-xPT)
indicates a rhombohedral ground state for x < 0.32. X-ray powder measurements
by Dkhil et al. show a rhombohedrally split (222) Bragg peak for PMN-10%PT at
80 K. Remarkably, neutron data taken on a single crystal of the same compound
with comparable q-resolution reveal a single resolution-limited (111) peak down
to 50 K, and thus no rhombohedral distortion. Our results suggest that the
structure of the outer layer of these relaxors differs from that of the bulk,
which is nearly cubic, as observed in PZN by Xu et al.Comment: Replaced Fig. 3 with better versio
- …