R. Pirc

Organic and inorganic relaxor ferroelectrics with giant electrocaloric effect

The electrocaloric effect (ECE) in inorganic thin film and organic relaxor ferroelectrics is investigated by directly measuring the ECE around room temperature. The results reveal that giant ECEs can be obtained in the high energy electron irradiated poly(vinylidene fluoride-trifluoroethylene) relaxor copolymer and in the La-doped Pb(ZrTi)O3 relaxor ceramic thin films, which are much larger than that from the normal ferroelectric counterparts. The large ECE observed, compared with normal ferroelectrics, is likely caused by the large number of disordered fluctuating polarization entities in relaxor ferroelectrics which can lead to extra entropy contributions and larger ECE.

Comparison of directly and indirectly measured electrocaloric effect in relaxor ferroelectric polymers

We report the directly measured electrocaloric effect (ECE) (the adiabatic temperature change ΔT) of relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer and its blend with poly(vinylidene fluoride-chlorotrifluoroethylene). The results show that the directly measured ΔT in the relaxor terpolymer is much larger than that deduced from Maxwell relation and that the relaxor terpolymer possesses a giant ECE at room temperature. The large difference between the directly measured ΔT and that deduced indicates that the Maxwell relation, which is derived for ergodic systems, is not suitable for deducing ECE in the relaxor ferroelectric polymers, which are nonergodic (polar-glass) material systems.

Vogel-Fulcher freezing in relaxor ferroelectrics

A physical mechanism for the freezing of polar nanoregions (PNRs) in relaxor ferroelectrics is presented. Assuming that the activation energy for the reorientation of a cluster of PNRs scales with the mean volume of the cluster, the characteristic relaxation time {tau} is found to diverge as the cluster volume reaches the percolation limit. Applying the mean field theory of continuum percolation, the familiar Vogel-Fulcher equation for the temperature dependence of {tau} is derived.

Influence of the critical point on the electrocaloric response of relaxor ferroelectrics

The electrocaloric effect (ECE), i.e., the conversion of electric energy into heat, is of great importance for application in new generation cooling or heating devices that would be friendlier to the environment. Here, utilizing direct measurements of the ECE change of the temperature ΔT via a high resolution calorimeter, we study the ECE as a function of the magnitude of the electric-field step E in the vicinity of the critical point in several bulk relaxor ferroelectric ceramic systems. Relatively large ΔT of ∼2 to 3 K were obtained at modest fields of 90 kV/cm, even in the case of ceramic materials. The effective responsivity ΔT/E as a function of the electric field shows a characteristic peak near the critical point, which demonstrates the importance of proximity to the critical point for the enhancement of the electrocaloric effect. Experimental results are in good agreement with the theoretical calculations based on the spherical random-bond random-field model.

Electrocaloric effect in relaxor ferroelectrics

A theoretical model for the electrocaloric effect (ECE) in relaxor ferroelectrics is presented. By solving a self-consistent relation for the ECE temperature change ΔT and minimizing numerically the mean field free energy for relaxors, the field and temperature dependence of ΔT is calculated. The corresponding harmonic Landau coefficient a=a(T), which differs from the ferroelectric case by always being positive, is derived from the spherical random bond-random field model, and the fourth-order coefficient b is treated as a phenomenological parameter, which can be either positive or negative. For b<0, a line of field-induced first-order relaxor-to-ferroelectric phase transitions exists in relaxors, which terminates at a liquid-vapor type critical point ECP,TCP. The critical behavior close to ECP,TCP is analyzed. It is shown that near the first-order phase transition a temperature or field interval or gap formally appears, where ΔT cannot be found. However, domain formation in the coexistence range should r...

Upper bounds on the electrocaloric effect in polar solids

Physical upper bounds on the electrocaloric effect (ECE) in bulk polar solids are derived using thermodynamic and statistical mechanics arguments. It is shown that the maximum ECE temperature change ΔT under saturation can be estimated from the dielectric data, such as the saturation polarization and effective Curie constant, as well as from the orientational degeneracy Ω of the elementary dipolar entities in the system and the specific heat of the material. Also obtained is a universal relation for the theoretical maximum value of ΔT, which depends only on Ω and the molar specific heat.

Negative electrocaloric effect in antiferroelectric PbZrO3

The dielectric and thermal properties of a typical antiferroelectric (AFE) material are investigated by minimising numerically the free energy as given by the Kittel model of AFEs. The phase line of second-order phase transitions in the phase diagram is shown to change to a first-order line at the tricritical point T3cp, E3cp. The static dielectric susceptibility and the electrocaloric (EC) effect are calculated as a function of temperature and the applied electric field E. It is found that in a given range of electric fields and temperatures the EC effect has negative values but generally becomes positive above the AFE ordering temperature T0. The dielectric susceptibility shows characteristic peaks at the phase transitions between the field-induced polar and the AFE antipolar phase, and diverges at the tricritical point. We present experimental results for a negative EC effect, which have been obtained by direct EC measurements in PbZrO3 ceramics, and agree qualitatively with the above model.

Dynamics of relaxor ferroelectrics

We study a dynamic model of relaxor ferroelectrics based on the spherical random-bond—random-field model and the Langevin equations of motion written in the representation of eigenstates of the random interaction matrix. The solution to these equations is obtained in the long-time limit where the system reaches an equilibrium state in the presence of random local electric fields. The complex dynamic linear and third-order nonlinear susceptibilities �1(!) and �3(!), respectively, are calculated as functions of frequency and temperature. In analogy with the static case, the dynamic model predicts a narrow frequency dependent peak in �3(T,!), which mimics a transition into a glass-like state, but a real transition never occurs in the case of non-zero random fields. A freezing transition can be described by introducing the empirical Vogel-Fulcher (VF) behavior of the relaxation time � in the equations of motion, with the VF temperature T0 playing the role of the freezing temperature Tf. The scaled third-order nonlinear susceptibility a ′ (T,!) = � ′ (!)/� ′ (3!)� ′ (!) 3 , where the bar denotes a statistical average over T0, shows a crossover from paraelectric-like to glass-like behavior in the quasistatic regime above Tf. The shape of �1(!) and �3(!)—and thus of a ′ (T,!)—depends crucially on the probability distribution of �. It is shown that for a linear distribution of VF temperatures T0, a ′ (T,!) has a peak near Tf and shows a strong frequency dispersion in the low temperature region.

High-resolution electrocaloric and heat capacity measurements in barium titanate

The electrocaloric (EC) effect in a BaTiO3 single crystal oriented along the [001] direction has been studied by direct high-resolution EC measurements in the vicinity of the field-induced critical point. It is shown that the observed behavior of the EC temperature change , as well as the heat capacity anomalies are in good agreement with the predictions of the Landau theory. We also present the electric field-temperature phase diagram for the field-induced paraelectric-to-ferroelectric phase transition line, derived from the calculated latent heat.

Tunneling model of proton glasses

AbstractThe static critical behavior of a mixed hydrogen-bonded ferro-antiferroelectric system is investigated with use of a pseudo-spin Ising model in a transverse field, which describes the tunneling of protons. Assuming an infinite-ranged random interbond coupling, then→0 replica method is applied to evaluate the averaged free energy, from which the replica-symmetric solution of the Sherrington-Kirkpatrick type is obtained. It is shown that a proton pseudo-spin glass (PG) phase exists below a freezing temperatureTf, whereTf and the order parameters for the PG and ferroelectric phase are parametrized by the tunneling frequency Ω. For Ω ≧ Ωc, where Ωc =