Ac behaviour and phase transition in PLZT 6/80/20 ferroelectric ceramics

The Ac behaviour of PLZT 6/80/20 ferroelectric ceramics was analyzed around and above the phase transition. Two relaxation processes are identified, showing that the so-called ‘universal relaxation law’ holds for the ceramics. A critical point in the values of the Ac conductivity, around the temperature corresponding to the maximum of the dielectric losses, is observed below the transition temperature due to the relaxor behaviour. The frequency dependence of the Ac conductivity at various temperatures and the hysteresis loops show classical relaxor behaviour with a diffuse phase transition.     

Dynamical exchange corrections to the dielectric function in a two-dimensional electron gas

The effect of dynamical exchange interactions on the dielectric function of a two-dimensional electron gas is studied using a variational approach. Exchange effects are introduced via the local-field correction. The variationally obtained local-field factor is compared to the earlier perturbative result to first order in the electron-electron interaction.     

Electronic Structure and Magnetic Properties of SmCo7-xZrx

The electronic structure and magnetism of SmCo_7-xZr_x alloy are investigated using the spin-polarized MS-X. method. The results show that a few of electrons are transferred to the Sm(5d⁰) orbital due to orbital hybridization between Sm and Co atoms. The exchange interactions between 3d and 5d electrons are more important than the polarization effects of the conductive electrons, thus it is the main reason resulting in the long-range ferromagnetic order in SmCo_7-xZr_x. The Curie temperature of SmCo_7-xZr_x is generally lower than that of corresponding pure Co, which may be explained by the weaker average coupling strength between Co lattices due to some negative couplings mainly occurring of 2e site. The calculated results for the Sm₅Co₃₂Zr₂ cluster may lead to a better understanding of why SmCo_7-xZr_x is stable phase. Since the spin-up DOS peak of d electrons at E_F arises and the bonding of electrons at E_F strengthens with increasing Zr concentration, which results in the internal energy of the system decrease, the stable ferromagnetic order forms in SmCo_7-xZr_x.     

Plasmon resonances in a spherical sodium cluster and in a flat surface with a soft optical edge

Received: 27 August 1997/Revised version: 7 November 1997     

Domain-wall energy in sintered Nd15Fe77B8 permanent magnet

From measurements of the magnetic domain widthsD versus grain thicknessL, we have determined the domain wall energy in the demagnetized state for a sintered Nd₁₅Fe₇₇B₈ magnet: = 56 erg/cm². Using this wall energy and the published magnetocry-stalline constantK ₁=4.9×10⁷ erg/cm³, we have calculated the exchange constantA=4.0×10^–6 erg/cm, domain wall thickness _B=89 Å, and the critical diameter for single domain particlesD _c=0.67 m.     

The evolution of poly(lactic acid) degradability by dielectric spectroscopy measurements

Dielectric spectroscopy is a powerful method that allows the study of the dynamics of polymers in a wide frequency range. The different regimes of the dielectric spectra can be observed and the dynamics of the primary and secondary relaxations can be found.In this work, the systems investigated included industrial and purified poly(lactic acid), PLA. This is an aliphatic polyester, one of the most important biocompatible and biodegradable material that has received increasing attention in the last 10 years.Thermal analyses indicated that the glass transition temperature of the polymer was about 320 K.The β relaxation was observed between −150 °C and −30 °C, depending on the measurement frequency (1 Hz-100 kHz), and was assigned to the secondary relaxation in the glassy state. We studied the changes that are connected with the water penetration in the polymer, and directly affect that relaxation process. Water molecules confined by polymer chains and in the polymer networks play an important role in the degradation of the matrix, and then we can observe the evolution of that degradation during some weeks, in a controlled humidity environment. It is accepted that water preferentially enters in the amorphous zones, but also affects the crystalline regions. It is observed a clear evolution of the relaxation activation energy during the degradation of the polymer.The dielectric relaxation results are complemented with molar mass measurements during the degradation process with time.     

Miscibility in poly(L-lactide)- b -poly( $ \upvarepsilon$ -caprolactone) double crystalline diblock copolymers

Thermally stimulated depolarization currents, TSDC, wide-angle X-ray scattering, WAXS, differential scanning calorimetry, DSC, and polarized light optical microscopy, PLOM, have been used to examine poly(L-lactide)-b -poly( -caprolactone) diblock copolymers in a wide composition range. Both components are crystallizable and the miscibility in the amorphous phase has been determined from the behavior of the primary relaxations which are the dielectric manifestation of the glass transition, and also from the superstructural morphology revealed by PLOM and the compositional dependence of the melting points as determined by DSC. Distinct segmental mobilities in the amorphous phase which can be well resolved by TSDC are present; the mode of the slower component shifts to lower temperatures as the PCL content increases while the glass transition of neat PCL is present for all compositions. A relaxation times bimodal distribution is apparent for PCL-rich copolymers. The composition dependence of the multiple glass transitions detected in these weakly segregated copolymers are predicted by the self-concentration model for a miscible blend made of components with a large T_g contrast.     

Spin waves propagation in Fibonacci quasiperiodic metamagnet superlattices

In this work we present a fractal analysis of the spin wave modes propagating in a Fibonacci quasiperiodic metamagnetic superlattices consisting of a ferromagnetic material (layer A being Fe) and a metamagnetic material (layer B being FeBr₂). The spin wave modes are obtained in the exchange regime, using the transfer matrix technique. Our numerical results show that the power law for this system is independent of the dimensionless in-plane wavevector kxa$k_{x}a$, and that the spectrum present a multifractal characteristic, exhibiting several scale invariant windows for a fixed value of the in-plane wavevector. The results reported here can be experimentally observed by light scattering techniques.     

Study on spatial dispersion theory of excess conduction loss in normal metals surface below terahertz frequencies

We have developed a rigorous model for analyzing excess conduction loss in normal metals surface by using the spatial dispersion theory. We have used the model to account excess conduction loss and dissect the discrepancies between excess conduction loss measurements and classical theoretical predictions in normal metals surface below terahertz frequencies. Our analysis shows that the conductivity is not only frequency but also wave vector dependent. We demonstrate good quantitative agreement with the published experimental data for the room temperature excess conduction loss of normal metals surface below terahertz frequencies.     

Ti deficiency effect on the dielectric response of CaCu3Ti4O12 ceramics

Single phase ceramics CaCu₃Ti_4.0O₁₂ and CaCu₃Ti_3.9O₁₂ have been prepared using the traditional solid-state reaction method. Compared with the stoichiometric ceramics CaCu₃Ti_4.0O₁₂, Ti-deficient ceramics CaCu₃Ti_3.9O₁₂ have the larger lattice parameter, the higher force constant, and smaller dielectric constant and the lower dissipation factor, although their fundamental characters of dielectric response are similar. Their characteristic relaxation frequencies are not well fitted with the Arrhenius Law but a tentatively supposed relation. With the Cole-Cole Law, the fitted broadened factors of dissipation peaks are 0.5433 and 0.8651 for CaCu₃Ti_3.9O₁₂ and CaCu₃Ti_4.0O₁₂, respectively. All facts mentioned above imply that mutually correlated motion of Ti ions or defects may be expected to be responsible for the giant dielectric constant and high dissipation factor of CaCu₃Ti_4.0O₁₂.