I. A. Santos

Medidas de resistividade elétrica DC em sólidos: como efetuálas corretamente

This paper deals with the most common methods for determining the dc electrical resistivity in solid materials. A brief overview of the fundamental concepts related to the electrical resistivity on materials is introduced. Undoubtedly, the most common and useful procedure to determine the electrical resistivityxa0(r) is the fourpoint probe method. Some crucial mistakes regarding the experimental procedure and the appropriated correction factors are found in the literature. Thus, the correction factor for the most common sample geometries were gathered and revised in order to provide an easy way to use and apply them.

Structural and spectroscopic properties of rare-earth (Nd3+,Er3+, and Yb3+) doped transparent lead lanthanum zirconate titanate ceramics

This work presents the structural and spectroscopic characterization of transparent lead lanthanum zirconate titanate ceramics doped with Nd3+, Er3+, or Yb3+ ions. High optical quality samples presenting the perovskite structure were prepared through a mixed oxides method followed by conventional sintering or uniaxial hot pressing. Absorption and luminescence spectra were measured, and radiative emission parameters were calculated for Nd3+- and Er3+-doped samples. The results indicate the potential of these polycrystalline host–ion combinations for the construction of diode-pumped lasers in the near-infrared region.

Features of diffuse phase transition in lead barium niobate ferroelectric ceramics

Features of the diffuse phase transition in lead barium niobate (PbxBa1−xNb2O6–PBN) ferroelectric ceramics are presented and discussed. Ceramics were prepared near to the morphotropic phase boundary (x∼0.63) and the dielectric properties were investigated. The character of the ferroelectric phase transition was carefully analyzed, indicating that the diffuseness coefficient decreases as the lead concentration increases. This effect is attributed to the increase of the unit-cell polarization with the morphotropic phase boundary proximity. The polarization enhancement induces the formation and interaction between the polar microregions in PBN samples at temperatures higher than Tm. The appearance of dilatation centers, which act as strong oscillating random-field sources and are naturally created due to the peculiar A-site vacancy distribution in PBN structure, increases the maximum dielectric permittivity by the polarization mobility enhancement. On the other hand, the temperature of maximum dielectric per...

Dielectric behavior and diffuse phase transition features of rare earth doped Sr0.61Ba0.39Nb2O6 ferroelectric ceramics

The dielectric behavior and the main features of the diffuse phase transition of nondoped and rare earth (La, Nd, or Gd) doped Sr0.61Ba0.39Nb2O6 relaxor ferroelectric ceramics were carefully investigated. The dopant incorporation gives rise to one continuous decrease of the temperatures of maximum dielectric permittivity. The whole set of the effects of rare earth (RE) doping on SBN diffuse phase transition and relaxation features can be attributed to the RE site distribution. In fact, the dopant incorporates into the tungsten bronze A2 sites, giving rise to the strong random field sources that act mainly on destroying the ferroelectric long-range interactions. This A2-sites occupation also affects the dielectric permittivity frequency response, whereas the relaxation strength (ΔTrs=Tm(1u2009MHz)−Tm(100u200aHz)) increases with RE content increasing. This effect can be explained considering that the dopant roughens the free energy landscape, promoting strong changes in the volume size distribution of the interacti...

Rare earth doping effect on the elastic moduli of low silica calcium aluminosilicate glasses

Abstract Elastic moduli (E, G, K, ν) of calcium aluminosilicate glasses with m 1/2 and the thermal shock resistance is 339±102 W/m.

Hot-pressed transparent PLZT ceramics from low cost chemical processing

Lanthanum-modified lead zirconate titanate (PLZT) ceramics were obtained with high transmittance in the visible range by a combination of an inexpensive chemical processing and hot pressing. Optical, microstructural, pyroelectric, ferroelectric and dielectric properties characterized in this study attested the applicability of the employed method in the production of PLZT transparent ferroelectric ceramics. In fact, the corresponding analyzed physical parameters are in very good agreement with those obtained in samples traditionally prepared by other methods. Furthermore, due to high sample quality, a phenomenological analysis of the PLZT 10/65/35 relaxor features was performed in these ceramics.

Structure Refinement of (Sr,Ba)Nb2O6 Ceramic Powder from Neutron and X-Rays Diffraction Data

The structure of polycrystalline strontium barium niobate at room temperature was refined by the Rietveld method. Sintered ceramic samples were used to collect powder neutron and X-ray diffraction data. The ratio Sr/Ba » 64/36 was found from the initial batch composition Sr0.61Ba0.39Nb2O6, corroborating with the quantitative X-ray dispersive spectroscopy (EDS) measurements. The structure is tetragonal with cell parameters a, b = 12.4504(3) A and c = 3.9325(1) A and space group P4bm. It was not necessary to introduce any positional disorder for the oxygen atoms. Cation Nb+5 displacements not parallel to the c direction are presented, which can influence the behavior of the ferroelectric properties.

Effect of the composition and sintering process on the electrical properties in Pb(Mg1/3 Nb2/3)O3–PbTiO3 ceramics

Abstract The effects of the composition and the sintering process on the ferroelectric, dielectric and electromechanical properties in (1–x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 ceramics with 0.1⩽x⩽0.4 were investigated. It was found that the densification process was strongly dependent on the amount of PT. For the lowest concentration of PT the microstructure of the both hot-pressed and conventional sintered samples were very similar, reflecting in almost identical electrical properties. Nevertheless, it was noticed that the higher amount of PT higher was the deviation of the electrical properties between the conventional and the hot-pressed samples. The gradual decreasing of the physical properties of the conventional sintered ceramics was related to the gradual decrease of the density and inhomogeneous microstructure. The results also revealed that for the lower concentration of lead titanate a relaxor behavior is noticed with a high electrostrictive effect, which was almost hysteretic free. However, higher amount of lead titanate led to a “normal” ferroelectric behavior.

Investigation of dielectric anomalies at cryogenic temperatures in (1−x)[Pb(Mg1∕3Nb2∕3)O3]–xPbTiO3 system

Complex electrical permittivity measurements in (1−x)[Pb(Mg1∕3Nb2∕3)O3]–xPbTiO3 ceramics for 0.10⩽x⩽0.40 were performed in the frequency and temperature range from 1to100kHz and from 15to600K, respectively. Unexpected dielectric anomalies, which were frequency dependent, were found at cryogenic temperatures. It was verified that this dielectric behavior is always observed, independent of whether the composition presents a “normal” ferroelectric or a relaxor characteristic. The results were analyzed within the framework of two current models found in the literature. It was concluded that none of the proposed mechanisms uniquely satisfies the experimental results in the composition range investigated here.

EVIDENCE FOR THE STABLE EXISTENCE OF THE FE2ND PHASE IN THE FE-ND SYSTEM

We did directional solidification experiments on Fe20Nd80 alloys at several low cooling rates in order to observe the solidification sequence, displayed along the length of the sample. We also took measurements of the transition temperatures around the eutectic temperature using a Calvet type calorimeter. Our results show that the formation of the eutectic L↔(Nd)+Fe17Nd2 occurs first, and is followed by the eutectic L↔(Nd)+Fe2Nd. The calorimetric measurements give the temperature of the first eutectic to be 688u200a°C, the temperature of the peritectic formation of the phase Fe2Nd to be 682u200a°C and the temperature of its eutectoid decomposition as 659u200a°C. Thermomagnetic measurements give the Curie temperature of the Fe2Nd phase as 250u200a°C. A new phase diagram for this system is proposed.