Search

This work presents a fast and direct controlled routine for the fabrication of fully dense alumina based on the reactive spark plasma sintering (reactive-SPS) of boehmite ( γ -AlOOH) nano-powders obtained by the sol-gel technique. The evolution of the transition aluminas during sintering has been studied. Some boehmite powders were seeded with α -Al 2 O 3 particles prior to the gelation. Boehmite seeded powders exhibited a direct transition to α -Al 2 O 3 at 1070 ° C, enhancing the transformation kinetics and lowering the required temperature by more than 100 ° C. For comparison, other samples were prepared by previously annealing the seeded and unseeded boehmite powders. Thus, α -Al 2 O 3 powders were obtained and were sintered by standard-SPS. A detailed structural and mechanical characterization is presented, comparing the hardness and indentation fracture resistance for different grain sizes and porosities. Both the reactive-SPSed samples and the standard-SPSed samples showed a high hardness (18-20 GPa), whereas the reactive-SPSed samples exhibited a lower indentation fracture resistance due to a large grain size ( ∼ 10 μ m). Improvements of this procedure for obtaining smaller grain size are discussed. In summary, the presented technique brings a revolutionary fast method for the fabrication of fully dense alumina, as this process reduces the time and temperature required for alumina densification.

An analysis of the sunspot observations made by Hevelius during 1642-1645 is presented. These records are the only systematic sunspot observations just before the Maunder Minimum. We have studied different phenomena meticulously recorded by Hevelius after translating the original Latin texts. We re-evaluate the observations of sunspot groups by Hevelius during this period and obtain an average value 7% greater than that calculated from his observations given in the current group database. Furthermore, the average of the active day fraction obtained in this work from Hevelius' records previous to the Maunder Minimum is significantly greater than the solar activity level obtained from Hevelius' sunspot observations made during the Maunder Minimum (70% vs. 30%). We also present the butterfly diagram obtained from the sunspot positions recorded by Hevelius for the period 1642-1645. It can be seen that no hemispheric asymmetry exists during this interval, in contrast with the Maunder Minimum. Hevelius noted a ~3-month period that appeared to lack sunspots in early 1645 that gave the first hint of the impending Maunder Minimum. Recent studies claim that the Maunder Minimum was not a grand minimum period speculating that astronomers of that time, due to the Aristotelian ideas, did not record all sunspots that they observed, producing thus an underestimation of the solar activity level. However, we show the good quality of the sunspot records made by Hevelius indicates that his reports of sunspots were true to the observations.

A new software (Soonspot) for the determination of the heliographic coordinates and areas of sunspots from solar images is presented. This program is very user-friendly and the accuracy of its results has been checked by using solar images provided by the Debrecen Photoheliographic Data (DPD). Due to its applicability in the studies of historical solar observations, the program has been used to analyze the solar drawings carried out by Hevelius in the 17th century.

Analytical expressions are formulated that enable the lognormal distribution of nanocrystallite sizes and the mean-squared crystal lattice microstrain in both doped and undoped nanocrystalline ceramics to be routinely determined by X-ray diffractometry. Importantly, these expressions, which are also applicable to most well-annealed nanocrystalline metals/alloys, are developed under the consideration that both the experimental and instrumental X-ray diffraction peaks are modeled by split Voigt functions to account for the (hitherto widely ignored) intrinsic peak asymmetry. The general analytical expressions of this new single-peak line-broadening method are then particularized to the case of small nanocrystallites with uniform composition for which a reasonable simplification (i.e., symmetrization of the experimental peak) can be made without compromising the accuracy of the results. Next, as an example of application the method developed is applied to the characterization of a nanocrystalline ceramic powder, and the nanocrystallite sizes so-determined are validated by transmission electron microscopy. The more complete and reliable microstructural information offered by this single-peak line-broadening method could contribute towards the accurate characterization of both doped and undoped nanocrystalline ceramics, and of most well-annealed nanocrystalline metals/alloys as well, a topic of particular research interest today.

In this paper we analyze a small collection of sunspot drawings made at the Astronomical Observatory of the Spanish Navy in the period from April 1, 1884 to August 18, 1884. We calculate the area and the heliographic coordinates of the observed sunspots. From these coordinates, we obtain the solar rotation rate in this period. Finally, we compare our results with the data recorded by the Royal Greenwich Observatory.

A wide set of 3 mol% Y2O3-stabilized ZrO2 (3YSZ) powders have been synthesized by the sol–gel method without modifiers from solutions of zirconium n-propoxide and yttrium acetate prepared with different water/Zr molar ratio and zirconium concentration, and the effect of these two synthesis variables on the pressureless sinterability of the 3YSZ powders has been investigated. It was found that the sinterability increases with increasing zirconium concentration and water/Zr molar ratio, which is attributed to the smaller cluster size developed during the sol–gel synthesis. Implications for the preparation of more sinterable 3YSZ powders are discussed Fil: Mamana, Nadia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina Fil: Ortiz, Angel L.. Universidad de Extremadura. Badajoz; España Fil: Sánchez Bajo, Florentino. Universidad de Extremadura. Badajoz; España Fil: Caruso, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina

The accuracy of the Rietveld method in determining the lattice parameters of polytypes in multiphase SiC ceramics was investigated as a function of the polytype composition and of the polytype crystallite size, using as benchmarks two sets of standard X-ray diffraction (XRD) patterns obtained by computer simulation. It was found that the Rietveld method is remarkably accurate, but also that the measurement of lattice parameters is sensitive to the polytype composition and to the polytype crystallite size. In particular, the accuracy of Rietveld-calculated lattice parameters for a given polytype not only decreases with increasing number of polytypes in the SiC ceramics, but also depends on which other polytypes are present. These two findings are interpreted based on the severity of the corresponding peak overlap phenomenon in the XRD patterns. In addition, the accuracy of the Rietveld analyses also decreases with decreasing crystallite size of the polytypes, which is due to the progressive loss of definition in the location of the peak maxima and to the greater severity of the peak overlap, both resulting from the corresponding peak broadening. This identification of the confidence limits of the Rietveld-calculated lattice parameters for a wide range of microstructural features may have important implications for the future characterization of SiC ceramics by XRD.

A procedure to estimate the aerosol extinction parameters of Angstrom's formula using data acquired with photometric broadband standard filters, the Johnson–Cousins filters, is presented. The method is based on the transformation of the heterochromatic magnitudes to the equivalent mean wavelength monochromatic magnitudes by an iterative approach, correcting subsequently the effects of water-vapour extinction in the longer-wavelength filters. As an example, the procedure has been used in the determination of the extinction parameters at the location of our observatory using the filters B, V, Rc and Ic on the night of 2008 August 28.

A custom processor based on reconfigurable hardware technology is proposed in order to compute the figure of merit used to measure the quality of the fit of X-ray diffraction peaks. As the experimental X-ray profiles can present many peaks severely overlapped, it is necessary to select the best model among a large set of reasonably good solutions. Determining the best solution is computationally intensive, because this is a hard combinatorial optimization problem. The proposed processors, working in parallel, increase the performance relative to a software implementation.

A new normalized sunspot-area series has been reconstructed from the series obtained by the Royal Greenwich Observatory and other contemporary institutions for the period 1874 - 2008 and the area series compiled by De la Rue, Stewart, and Loewy from 1832 to 1868. Since the two sets of series do not overlap in time, we used as a link between them the new version of sunspot index number (Version 2) published by SILSO (Sunspot Index and Long-term Solar Observations). We also present a spectral analysis of the normalized area series in search of periodicities beyond the well-known solar cycle of 11 years and a study of the Waldmeier effect in the new version of sunspot-number and the sunspot-area series presented in this study. We conclude that while this effect is significant in the new series of sunspot number, it has a weak relationship with the sunspot-area series., Comment: 16 pages, 5 figures, to be published in Solar Physics

We have developed a novel, analytical model for the determination of grain size distributions in nanocrystalline (nc) materials with low internal stresses by X-ray diffractometry (XRD). The model assumes explicitly that the XRD peaks are pseudo-Voigtian and that the grain size distributions are lognormal, both of which are assumptions amply supported by the experimental evidence. It was found analytically that the grain size dispersion depends on the shape of the XRD peaks only, whereas the grain size median depends on both the shape and width of the XRD peaks. In addition, the theoretical predictions resulting from the model were validated using standard XRD peaks obtained by computer simulation from first principles. Particular emphasis is given to the discussion of the validity limits of the model, and to the analysis of the influence of the characteristics of the grain size distributions on the nature of the XRD peaks. We then show how to calculate the average and apparent grain sizes from the grain size distribution determined with the model, and how this compares with the Scherrer method. Implications for the characterization of (undistorted and distorted) nc-materials are indicated, and a case study of an nc-powder of cubic ZrO 2 is presented. The application of the model itself is simple, involving only the fit of a pseudo-Voigt function to a single XRD peak followed by the use of two equations. This suggests that the model may have an important role to play in the characterization of nc-materials.

The thermal behavior of the hyperfine interaction at Zr sites on two sol-gel derived 6 mole% CaO-ZrO2 powders, obtained from the particulate and non-particulate regimes, has been studied by Perturbed Angular Correlations and complementary techniques. The aim was to get experimental support on the thermal stability of the obtained material in order to see the advantages of the different preparation regimes. The results could be interpreted in terms of the different microstructures and nanoscopic configurations exhibited by the resulting powders. After crystallization both powders showed the hyperfine nanoconfigurations of metastable tetragonal zirconia. In the sample obtained following the hydrolysis and the condensation processes via a particulate regime, the undesirable phase transformation towards the monoclinic form of zirconia is inhibited up to higher temperatures.

Results of the photometric analysis by means of the Wilson-Devinney method, combined with an empirical massradius relation for LC IV stars support the hypothesis that GX Gem is a well detached evolved system of subgiant stars. The solution is consistent with the known spectroscopic parameters obtained by Popper (1996) and the photometric data of Lacy (2002a). (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The Maunder Minimum (1645-1715 approximately) was a period of very low solar activity and a strong hemispheric asymmetry, with most of sunspots in the southern hemisphere. In this paper, two data sets of sunspot latitudes during the Maunder minimum have been recovered for the international scientific community. The first data set is constituted by latitudes of sunspots appearing in the catalogue published by Gustav Sp\"orer nearly 130 years ago. The second data set is based on the sunspot latitudes displayed in the butterfly diagram for the Maunder Minimum which was published by Ribes and Nesme-Ribes almost 20 years ago. We have calculated the asymmetry index using these data sets confirming a strong hemispherical asymmetry in this period. A machine-readable version of this catalogue with both data sets is available in the Historical Archive of Sunspot Observations (http://haso.unex.es) and in the appendix of this article., Comment: 14 pages, 1 figure, accepted for publication in "Advances in Space Research"

Two pieces of as-sintered 3mol% yttria tetragonal zirconia polycrystals were joined and the electrical response of this composite was compared with the one of its counterparts. The joined sample showed higher electrical conductivity than the assintered sample. This effect was attributed to the joining process.La respuesta eléctrica de la unión superplástica de dos muestras de 3mol% Y-TZP ha sido comparada con la de una muestra sin unir. La muestra unida presenta una conductividad eléctrica mayor que la muestra sin unir. Este aumento de conductividad se atribuye al proceso de unión.

La determinacion de las fracciones en peso de las fases mediante difraccion de rayos X es enormemente complicada en ceramicos a base de SiC debido al intenso solapamiento entre las reflexiones Bragg de los diferentes politipos. No obstante, el metodo de Rietveld constituye una herramienta poderosa para resolver este problema. En este trabajo se utiliza el metodo de Rietveld al objeto de determinar las fracciones en peso de las diferentes fases para una muestra de SiC sinterizado en fase liquida (LPS SiC). Nuestros resultados ponen tambien de manifiesto la necesidad de incorporar correcciones debidas a orientacion preferencial para efectuar un analisis cuantitativo preciso.

The evolution of microstructure and electrical behaviour of nanocrystalline ZrO 2 - 3mol% Y 2 O 3 samples with respect to thermal cycles have been investigated. The sample, that initially consisted of 77.8%wt of tetragonal and 22.3%wt monoclinic phases, eliminates the tetragonal phase by means of a low temperature t → m phase transformation in which the content of monoclinic phase reaches 96%wt. This transformation affects significantly the electrical properties of the sample, increasing the grain boundary resistivity in successive thermal cycles, whereas the bulk contribution remains unchanged. The growth of the grain boundary resistivity is believed to be due to the presence of microcracks that accommodate the larger size of monoclinic grains.

The precise microstructural characterization of the yttria-partially stabilized zirconia (Y-PSZ) system by means of X-ray diffractometry (XRD) experiments faces a serious challenge: the discrimination between the cubic, c, and pseudocubic, t{prime}, phases. The authors show how the confusion between the two phases goes beyond a simple qualitative problem, because the relative abundances for all the present polymorphs can be substantially modified. This is particularly true for the amount of transformable tetragonal phase, which is of paramount importance for the design of zirconia-based tough ceramics. Similarly, they show that a conclusive discrimination between c and t{prime} phases is possible on the basis of XRD experiments by a careful application of the Rietveld method and by emphasizing some subtle details around the angular domain close to the {l_brace}200{r_brace} planes. The procedure is applied to samples of Y-PSZ (4.7 mol%).

A worked example of polytypism is presented, aimed at assisting undergraduates in the learning and instructors in the teaching of this topic. In particular, this crystallography concept, not necessarily obvious for beginners, is illustrated pedagogically using to that end the model case of the prolific polytypism of silicon carbide (SiC). On the basis of concepts that are easily assimilated by students (i.e. simple topological constraints) this article first presents a unified description of the polytypism phenomenon in SiC that allows one to understand without difficulty the existence of its numerous polytypic variants and how they develop. Then the various notations used to designate these different polytypes are described, and finally the crystal structures of the most common are discussed. This worked example is thus expected to contribute to motivating undergraduates in the study of a crystallography topic that often is not treated in sufficient depth in class. Ministerio de Ciencia y Tecnología MAT2010-16848

The Williams-Watts (WW) relaxation function has been widely used to describe the relaxation behaviour of many systems. In the present work the range of applicability of the WW response was extensively tested by the analysis of experiments in both the time and the frequency domains. On the other hand, the analysed experiments covered a wide time-scale for the characteristic relaxation times. Some related topics were also considered, i.e. our procedures in obtaining the associated activation energy spectra or the distribution of relaxation times. The relationship between time-domain and frequency-domain relaxation responses was also analysed. In light of our results the universality of the WW response, appears to be good at least for the time-scales and the different probes covered by our experiments.

In 1997 the Sun and the Earth passed through the equatorial plane of Jupiter and therefore through the orbital planes of its main satellites. During this period, mutual eclipses and occultations occurred and were observed. We investigate the precision of the catalogue to produce improved data for the development of dynamical models. Light curves of mutual eclipses and occultations were recorded by the observers of the international campaign PHEMU97 organized by the Institut de Mecanique Celeste, Paris, France. We made 275 observations of 148 mutual events from 42 sites. For each observation, information is given about the telescope, the receiver, the site and the observational conditions. This paper gathers together the data and gives a first estimate of the precision. The catalogue of these rare events represents a collection of improved accurate astrometric data useful for the development of dynamical models.

En este trabajo se propone un metodo polimorfico para el analisis cuantitativo de fases en la circona. Este procedimiento emplea un sistema de ecuaciones compatible y determinado que relaciona las intensidades de un conjunto de grupos de picos del espectro de difraccion con las fracciones en peso de las diferentes fases presentes en la muestra. En dicho sistema, ademas de las abundancias relativas de las fases, se utilizan como incognitas un conjunto de parametros que dan cuenta de la aportacion de las colas de los grupos de picos adyacentes al grupo de picos considerado. Este procedimiento se ha aplicado a un difractograma simulado correspondiente a una circona con 50% de fase cubica, 25% de fase tetragonal y 25% de fase monoclinica y tambien a una muestra en polvo de circona fabricada mediante tecnicas sol-gel.