326 research outputs found
State transition and electrocaloric effect of BaZrTiO: simulation and experiment
The electrocaloric effect (ECE) of BaZrTiO (BZT) is closely
related to the relaxor state transition of the materials. This work presents a
systematic study on the ECE and the state transition of the BZT, using a
combined canonical and microcanonical Monte Carlo simulations based a
lattice-based on a Ginzburg-Landau-type Hamiltonian. For comparison and
verification, experimental measurements have been carried on BTO and BZT
( and ) samples, including the ECE at various temperatures, domain
patterns by Piezoresponse Force Microscopy at room temperature, and the P-E
loops at various temperatures. Results show that the dependency of BZT behavior
of the Zr-concentration can be classified into three different stages. In the
composition range of , ferroelectric domains are visible,
but ECE peak drops with increasing Zr-concentration harshly. In the range of , relaxor features become prominent, and the decrease of
ECE with Zr-concentration is moderate. In the high concentration range of , the material is almost nonpolar, and there is no ECE peak visible.
Results suggest that BZT with certain low range of Zr-concentration around
can be a good candidate with relatively high ECE and
simutaneously wide temperature application range at rather low temperature
Positive and negative electrocaloric effect in BaTiO in the presence of defect dipoles
The influence of defect dipoles on the electrocaloric effect (ECE) in
acceptor doped BaTiO is studied by means of lattice-based Monte-Carlo
simulations. A Ginzburg-Landau type effective Hamiltonian is used. Oxygen
vacancy-acceptor associates are described by fixed defect dipoles with
orientation parallel or anti-parallel to the external field. By a combination
of canonical and microcanoncial simulations the ECE is directly evaluated. Our
results show that in the case of anti-parallel defect dipoles the ECE can be
positive or negative depending on the density of defect dipoles. Moreover, a
transition from a negative to positive ECE can be observed from a certain
density of anti-parallel dipoles on when the external field increases. These
transitions are due to the delicate interplay of internal and external fields,
and are explained by the domain structure evolution and related field-induced
entropy changes. The results are compared to those obtained by MD simulations
employing an {\it{ab initio}} based effective Hamiltonian, and a good
qualitative agreement is found. In addition, a novel electrocaloric cycle,
which makes use of the negative ECE and defect dipoles, is proposed to enhance
the cooling effect
Determination of optimal reversed field with maximal electrocaloric cooling by a direct entropy analysis
Application of a negative field on a positively poled ferroelectric sample
can enhance the electrocaloric cooling and appears as a promising method to
optimize the electrocaloric cycle. Experimental measurements show that the
maximal cooling does not appear at the zero-polarization point, but around the
shoulder of the P-E loop. This phenomenon cannot be explained by the theory
based on the constant total entropy assumption under adiabatic condition. In
fact, adiabatic condition does not imply constant total entropy when
irreversibility is involved. A direct entropy analysis approach based on work
loss is proposed in this work, which takes the entropy contribution of the
irreversible process into account. The optimal reversed field determined by
this approach agrees with the experimental observations. This study signifies
the importance of considering the irreversible process in the electrocaloric
cycles
Direct measurement of electrocaloric effect in lead-free Ba(SnxTi1-x)O3 ceramics
In this study, we report on investigation of the electrocaloric (EC) effect in lead-free Ba(SnxTi1-x)O3 (BSnT) ceramics with compositions in the range of 0.08 ≤ x ≤ 0.15 by the direct measurement method using a differential scanning calorimeter. The maximum EC temperature change, ΔTEC-max = 0.63 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.11 at ∼44 °C around the multiphase coexistence region. We observed that the EC effect also peaks at transitions between ferroelectric phases of different symmetries. Comparison with the results of indirect EC measurements from our previous work shows that the indirect approach provides reasonable estimations of the magnitude of the largest EC temperature changes and EC strength. However, it fails to describe correctly temperature dependences of the EC effect for the compositions showing relaxor-like behaviour (x = 0.14 and 0.15) because of their non-ergodic nature. Our study provides strong evidence supporting that looking for multiphase ferroelectric materials can be very useful to optimize EC performance
A giant electrocaloric effect of a Pb0.97La0.02(Zr0.75Sn0.18Ti0.07)O3 antiferroelectric thick film at room temperature
A 2-µm-Pb0.97La0.02(Zr0.75Sn0.18Ti0.07)O3 (PLZST) antiferroelectric (AFE) thick film with tetragonal structure was deposited on LaNiO3/Si (100) substrates via a sol-gel technique. The electrocaloric effect (ECE) of the PLZST thick film is investigated under the functions of external electric field and temperature. Giant ECEs (∆T = 53.8 oC and ∆S = 63.9 J·K-1·kg-1) are received at 5 oC, which is attributed to a field-induced AFE to ferroelectric (FE) phase transition. Moreover, a large ∆T of above 30 oC is remains at temperature range from 5 oC to 25 oC. The maximum electrocaloric coefficient (ξmax = 0.060 K·cm/kV) and refrigeration efficiency (COP = 18) of the film are also obtained at 5 oC. At room temperature, the values of ∆T, ∆S, COP and ξmax are 35.0 oC, 39.0 J·K-1·kg-1, 14 and 0.039 K·cm/kV at 900 kV/cm, respectively. The AFE thick films with giant ECEs are promising candidates for applications in cooling systems at room temperature
Enhancement of polar phases in PVDF by forming PVDF/SiC nanowire composite
Different contents of silicon carbide (SiC) nanowires were mixed with Poly(vinylidene fluoride) (PVDF) to facilitate the polar phase crystallization. It was shown that the annealing temperature and SiC content affected on the phase and crystalline structures of PVDF/SiC samples. Furthermore, the addition of SiC nanowire enhanced the transformation of non-polar α phase to polar phases and increased the relative fraction of β phase in PVDF. Due to the nucleating agent mechanism of SiC nanowires, the ion-dipole interaction between the negatively charged surface of SiC nanowires and the positive CH2 groups in PVDF facilitated the formation of polar phases in PVDF
Enhanced energy-storage performance and electrocaloric effect in compositionally graded Pb(1−3x/2)LaxZr0.85Ti0.15O3 antiferroelectric thick films
The compositionally graded multilayer Pb(1−3x/2)LaxZr0.85Ti0.15O3 (PLZT) antiferroelectric (AFE) thick films were deposited on LaNiO3/Si (100) substrates by using a sol–gel method. The effect of gradient sequence on dielectric properties, energy-storage performance, and electrocaloric effect (ECE) was investigated in detail. It is found that the compositionally graded films exhibited a significant enhancement in dielectric properties, energy-storage performance and ECE, which was, in contrast to the single-composition PLZT film, contributed by the strain and the gradient of polarization near the interfaces between the adjacent layers. A recoverable energy-storage density of 44 J/cm3 and efficiency of 71% was obtained in the up-graded PLZT AFE thick film at 1950 kV/cm. A giant reversible adiabatic temperature change of ∆T=28 °C at room temperature at 900 kV/cm was also achieved in the up-graded film. Moreover, all the thick films displayed a small leakage current density below 10−6 A/cm2 at room temperature. Thus, the compositionally graded PLZT AFE thick films with a large recoverable energy-storage density and a giant ECE could be a potential candidate for the applications in high energy-storage density capacitors and cooling devices
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