D Dierickx

Statistical extreme value modeling of particle size distributions: experimental grain size distribution type estimation and parameterization of sintered zirconia

The grain size distribution of a sintered zirconia is investigated in detail. Microstructural evidence in combination with X-ray diffraction (XRD) data reveal a bimodal microstructure consisting of a polycrystalline matrix of retained untransformed tetragonal zirconia, containing clusters of smaller transformed monoclinic zirconia grains. This necessitates a comparison between the size measurements for the untransformed zirconia and the full set of measurements made on the sintered zirconia microstructure. A statistically consistent, graphical representation of grain size density distributions is shown. The experimental frequency density distributions are compared with the Log Normal and the Gumbel model. Traditional statistical tools are used to estimate the probability of observing the experimental data set from the expected model functions. Parameter estimations for the model functions are made according to a quantile approach, an R 2 fit, a c 2 fit and a non-linear simplex optimization routine. The bimodality of the experimentally obtained grain size distribution influences the likelihood probabilities of the model functions in accordance with their respective boundary conditions. The Gumbel function is shown to provide the best correspondence to the experimental data, with a slightly higher probability for a unimodal distribution as compared with the bimodal dataset. The Log Normal function is less likely than the Gumbel function in both instances, but has a slightly higher probability for the bimodal data set. It is shown that the quantile estimation is fast and straightforward to implement and allows a full evaluation of the expectation ranges on the estimated parameters. D 2000 Elsevier Science Inc. All rights reserved.

Ceramic superconductors synthesized by sol-gel methods

A sol-gel method is described based on the use of organic complexing agents in liquid solution, such as citric acid, ethylene-diamine-tetra-acetic acid (EDTA) and others. It is shown how the procedures can be designed on a scientific basis, using pM′-pH diagrams for the respective cation and complexing agent. This technique yields powders and sintered materials with superior properties to the dry mix and calcine method. An important advantage of the method is its versatility due to the fact that stable complexes can be formed for a wide range of elements. Precise control of stoichiometry is possible, however the residual carbon level is more difficult to control.

Stress release at Y2BaCuO5 inclusions in fast melt processed YBa2Cu3O7-x observed by micro-Raman spectroscopy

We have studied the interface region between the superconducting YBa2Cu3O7−x (123) matrix and the embedded nonsuperconducting Y2BaCuO5 (211) inclusions in fast melt processed samples, by recording micro-Raman spectra along lines crossing these inclusions. The observed relative shifts of the 123 Raman modes indicate the accumulation of compressive stress in the bulk of the 123 material and a release of this stress around the 211 inclusions. These results show that for practical applications of the fast melt processed materials, beside strong pinning, proper mechanical properties around the pinning centers are important.

Effects of and platinum additions on the unidirectional solidification of

The effects of 211 and/or Pt additions on the solidification of YBaCuO according to the horizontal Bridgman method have been studied. It was found that mixing the pure 123 powder with 20 wt% very fine and regular in size 211 particles gave rise to a strong increase in the viscosity of the peritectic liquid and to a dramatic refinement of the distribution of the 211 particles finally trapped into the 123 matrix of the samples. Moreover, it was observed that the extra addition of 0.5 wt% Pt refined further this distribution, which we interpret as due to a Pt induced hindrance of the growth and the coarsening rates of the 211 particles in the peritectic liquid. The effects of 211 and Pt additions on the size of the 123 grains were also studied: it was concluded that the former increased the 123 grain size because it made the constitutional supercooling smaller, whereas the latter increased it due to heterogeneous nucleation of 123 grains on globules of a Pt compound. To quantify the various observed effects, the variations of the mean diameter of particles, their area fraction and their density were measured as a function of the distance from the first solidified end. Results are also presented for samples prepared from unreacted precursors which were simultaneously synthesized and melt textured without experiencing the peritectic melting.

Flux dynamics and critical currents of high Tc tapes

Abstract We report on detailed measurements of the temperature and field variation of the critical current density J c (T,H) and normalised relaxation rate S = d ( ln M )/ d ( ln t ) in (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x (T c ≈110 K) and (Tl,Pb,Bi)(Sr,Ba) 2 Cu 3 O x (T c ≈119 K) silver-sheeted tapes with a critical current density J c (T=5K,H=0T)≈ 10 5 A/cm 2 . For these tapes two different pinning regimes at T c (T cr ≈20 K). This anomaly is clearly observed in the wide field interval H ≤ 30kG. Comparison of J c (T) and normalised relaxation rate S demonstrates that these materials are an intermediate case between single crystals and thin films.

Melt processing and morphology of freeze dried YBa2Cu3O7-x high Tc superconductors

Abstract Although quench melt growth has been established as a suitable way to enhance the critical current density J c in bulk YBa 2 Cu 3 O 7-x , reproducibility is difficult to guarantee. One way of eliminating the quench step, is to use starting powders consisting of an (Y 2 O 3 , Ba 2 Cu 3 O x ) mixture, which is a similar phase composition as is obtained after quenching. These precursor powders can be produced by applying a freeze drying method, combined with an optimised calcination treatment. By applying a suitable melt-process, bulk YBa 2 Cu 3 O 7-x can be obtained with a range of different Y 2 BaCuO 5 morphologies and a critical current J c at 77K of ∼ 10 4 A/cm 2 in zero field and ∼4∗10 3 A/cm 2 in a field of 3T. Quench experiments have been performed in order to determine the influence of holding time and composition on green phase content and morphology of stoichiometric Y 1.2 Ba 2 Cu 3 O 7−x and YBa 2 Cu 3 O 7−x with 20 wt% Y 2 BaCuO 5 excess compositions.