C. Aragó

Spectral and thermal properties of quasiphase-matching second-harmonic-generation in Nd3+ :Sr0.6 Ba0.4 (NbO3)2 multiself-frequency-converter nonlinear crystals

Quasiphase-matching second-harmonic-generation experiments have been performed on the Nd3+-doped strontium barium niobate multiself-frequency-converter nonlinear crystals. Optical microscopy has been used to determine the size dispersion of cylindrical ferroelectric domains appearing in as-grown crystals. A broad distribution of domain sizes (ranging between 1.5 and 8 μm) has been obtained. This gives the possibility of broadband second-harmonic generation in the 800–1200 nm range. The dependence of second-harmonic efficiency on crystal temperature has been found to be correlated with the thermal behavior of the spontaneous polarization, so that second-harmonic efficiency is almost temperature independent in the 25–50 °C range.

Evolution of the ferroelectric transition character of partially deuterated triglycine selenate

Automatically recorded hysteresis loops and dielectric constant data on pure and partially deuterated triglycine selenate and sulphate are used to characterize the transition evolution from quasi-tricritical to clearly first order character. A quantitative assessment of the transitions displacive degree is made through the first anharmonic contribution to the effective field ( g ), obtained from hysteresis loop data. This characterization is complemented with the assessment by means of a generalized Rhodes-Wohlfarth parameter R = (NkB C /4 P 2 S 0 )1/2 involving the Curie constant C and the saturation spontaneous polarization. P S 0 as done previously by other authors.

Evaluation of Piezoelectric Properties of Bi4Ti3O12—Based Ceramics at High Temperature

Nowadays many industries as aeronautic, aerospace, etc, are claiming for new sensors and actuators that can operate in hostile environments. PZT ceramics are the common materials used in piezoelectric devices, however their low Curie temperature prevents their use at high temperatures. Bi4Ti3O12-based ceramics show a great potential for high temperature applications. Although piezoelectric parameters of BIT ceramics are well described at room temperature, its behaviour at high temperatures is almost lacking. This paper describes an experimental set-up to measure the piezoelectric properties of high temperature piezoceramics up to 700°C. The variation of piezoelectric parameters of W-doped BIT ceramics as a function of temperature indicates that their operating temperature extends up to 500°C, surpassing the expected one (½ Tc) in more than 200°C.

Deviations from Vegard's Law in the Curie Temperature of Mixed Ferroelectric Solid Solutions

Composition dependence of the Curie temperature in mixed ferroelectric-ferroelectric solid solutions with isomorphous crystalline lattices, like the perovskite solid solutions LiTaO3/LiNbO3 or PbZrO3/PbTiO3, has been analysed by means of a classical effective field approach using a two sublattice system. The observed non-linear relationship between the critical temperature and the composition, deviating from a Vegards law behavior, is shown to be explained in terms of coupling strength between the individual dipoles of the two basic sublattices. In the case of LiTa1− xNbxO3, the Curie temperature is lower than that given by Vegards law, but for PbZr1− xTixO3 is higher. This means that in the second case the coupling strength between the different individual dipoles is stronger, while in the first case the main interaction is due to the identical dipoles in each sublattice.

Quantum tunneling versus zero-point energy in double-well potential model for ferrroelectric phase transitions

The Ising model in a transverse field, which involves quantum tunneling in double potential wells, has been used for many years to describe ferroelectric phase transitions. An alternative model previously published takes into account the effect of a non-negligible zero-point energy in the double potential wells at the phase transition. A comparison of the respective Curie temperatures, spontaneous polarizations and susceptibilities shows that both models are nearly equivalent. Quantitative differences of the two methods are apparent at low temperature where the saturation polarization value with non-negligible zero-point energy is higher than the same quantity with the Ising model in a transverse field.

Investigation of the Thermal Hysteresis at the Phase Transition of Relaxor Ferroelectric Sr0.61Ba0.39(NbO3)2

Measurements of birefringence and dielectric constant in single crystals of Sr0.61Ba0.39(NbO3)2, evidence a thermal hysteresis (Δ T ≅ 5°C) related to the discontinuous first order ferroelectric transition. The experimental results are analyzed in the light of the effective field approach and compared with the parameters characteristics of a classic perovskite like BaTiO3.

Ferroelectric Direct Energy Converters in Perspective

Solar, geothermal and industrial waste heat is today available in very large quantities. Phase transitions (liquid-vapor, ferro-paramagnetic, or ferro-paraelectric) are a natural choice to look for practical materials and devices to convert directly thermal into electrical energy. In all cases the conversion efficiency is of course limited by Carnots principle (ηmax = ΔT/Tmax). Ferroelectric converters have a considerable interest whenever high voltages, accompanied by low currents, are advantageous. This works displays a more detailed discussion on the performance of ferroelectric converters using thin plates of Zr-rich PZT through the FRL-FRH phase transition. The ultimate question “Do ferroelectric converters have a future?” is also briefly discussed.

Poling effect on distribution of quenched random fields in a uniaxial relaxor ferroelectric

The frequency dependence of the dielectric permittivitys maximum has been studied for poled and unpoled doped relaxor strontium barium niobate Sr0.61Ba0.39Nb2O6:Cr3 + (SBN-61:Cr). In both cases the maximum found is broad and the frequency dispersion is strong. The present view of random fields compensation in the unpoled sample is not suitable for explaining this experimental result. We propose a new mechanism where the dispersion of quenched random electric fields, affecting the nanodomains, is minimized after poling. We test our proposal by numerical simulations on a random field Ising model. Results obtained are in agreement with the polarizations measurements presented by Granzow et al. (Phys. Rev. Lett. 92 (2004) 065701).

Analysis of the thermal hysteresis in partially deuterated tryglicine selenate

Abstract Dielectric constant data on partially deuterated triglycine selenate (TGSe)1-x, (DTGSe)x for x = 0, 0.35, 0.80 show a steady increase of thermal hysteresis ΔT(x) with deuteration, in agreement with previous data by Gesi. In this work we correlate this observed thermal hysteresis with the parameter g(x) = (γ/β)N2μ2 obtained from hysteresis loops data, in order to characterize the ferroelectric transition. For x = 0, the transition is close to a tricritical point (g ≌ 1/3) and we see that cooling very slowly through Tc under these quasitricritical conditions results in a very large residual contribution to the dielectric constant at the ferroelectric phase.

Dielectric Properties and Low Field Switching of Partially Deuterated TGS

Abstract The dependence of ferroelectric parameters (saturation polarization, transition temperature, Curie constant,…) with deuteration in mixed crystals of (DTGS) x (TGS) 1−x has been characterized by means of hysteresis loops and dielectric constant measurements. Although the transition character is known to be second order for all compositions, a slight evolution towards a tricritical point was ascertained.