Estudio mediante rayos X de las maclas de la microclina

Se efect��a un estudio te��rico y experimental de la difraccion de rayos X por maclas de microclina. Se comparan los resultados al orientar de distintas maneras los cristales. Se llega a la conclusi��n de que el m��todo permite la diferenciaci��n de los feldespatos pot��sicos segun su modo de formaci��n.

Cation distribution and magnetization of BaFe12-2xCoxSnxO19 (x=0.9,1.28) single crystals

The distribution of Sn4+ cations within the five crystallographic sites of the magnetoplumbite (M) ���like compound BaFe12���2xCoxSnxO19 has been analyzed using single���crystal x���ray���diffraction data. The species Fe3+ and Co2+ cannot be distinguished using x rays because of their very similar atomic numbers; however, the calculation of the apparent valencies for the different sites allows an insight into the Co2+ cation segregation. The use of previous data from neutron powder diffraction allows a precise picture of the cation distribution, which indicates a pronounced site selectivity for both Sn4+ and Co2+ cations. The Sn4+ cations prefer the 4f2 sites and to a much lower extent the 12k sites, while they do not enter the octahedral 2a sites at all. Co2+ cations are distributed among tetrahedral and octahedral sites displaying a clear preference for the tetrahedral 4f1 sites. Magnetic measurements indicate that the compound still exhibits uniaxial anisotropy with the easy direction parallel to the c axis. Nevertheless, the magnetic structure shows a considerable degree of noncolinearity. A strong reduction of the magnetic anisotropy regarding that of the undoped compound is also detected.

M��todos de concentra����o de minerais 2:1 secund��rios na fra����o argila visando sua identifica����o por difratometria de raios x

Al��m dos baixos teores normalmente encontrados na fra����o argila dos solos sob clima tropical e subtropical, o tamanho reduzido e a baixa cristalinidade dos minerais 2:1 secund��rios dificultam sua identifica����o por difratometria de raios X (DRX). Este estudo objetivou avaliar m��todos qu��micos e f��sico de concentra����o de minerais 2:1 secund��rios na fra����o argila para facilitar a identifica����o por DRX, incluindo a natureza dos minerais quanto ao local de forma����o de cargas permanentes (l��mina tetraedral ou octaedral). Coletaram-se amostras de dois Cambissolos originados de argilito da Forma����o Guabirotuba na Bacia Sedimentar de Curitiba (PR): horizontes A, Bi, C1 (1,2 a 1,5 m), C2 (2,2 a 2,5 m), C3 (3,2 a 3,5 m) e C4 (4,2 a 4,5 m). Ap��s remo����o da mat��ria org��nica e dispers��o da terra fina seca ao ar, a fra����o argila foi submetida a tratamentos sequenciais com ditionito-citratobicarbonato (DCB) (amostra desferrificada - remo����o de ��xidos de Fe pedogen��ticos) e com solu����es de NaOH a quente, em diferentes concentra����es (0,5; 1,0; 1,5; 2,5; 3,5; 4,0; 4,5 e 5,0 mol L-1), para extra����o de gibbsita e caulinita, em diferentes graus. A fra����o argila desferrificada tamb��m foi submetida �� separa����o f��sica (centrifuga����o) em argila grossa (0,2 a 2 m) e fina (< 0,2 m). Foram realizados tratamentos auxiliares para identificar as esp��cies minerais 2:1 na fra����o argila: satura����o com Mg e solvata����o com etilenoglicol; satura����o com K e secagem ao ar e aquecimento a 550 ��C; e satura����o com Li (teste de Greene-Kelly). Os resultados mostraram que o m��todo cl��ssico de extra����o da caulinita, com solu����o de NaOH 5,0 mol L-1 a quente, n��o deve ser aplicado para concentra����o de minerais 2:1 secund��rios, pois tamb��m removeu grande parte desses minerais. O tratamento com DCB e com solu����o de NaOH 3,5 mol L-1 possibilitou, com maior efici��ncia, a concentra����o e identifica����o de minerais 2:1 secund��rios por DRX nas amostras dos horizontes A, Bi e C1. Nas amostras tomadas em maiores profundidades (horizontes C2, C3 e C4), devido aos maiores teores desses minerais e ao menor tamanho dos cristais (argila fina), a solu����o menos concentrada de NaOH (1,5 mol L-1) foi mais eficiente para esse prop��sito. No horizonte A, os minerais 2:1 concentraram-se na fra����o argila grossa, compat��vel com o maior grau de intemperismo desse horizonte. Identificou-se esmectita com hidroxi-Al entrecamadas nos horizontes mais superficiais (A e Bi) e esmectita nas amostras do horizonte C. A satura����o com Li permitiu a identifica����o das esmectitas dioctaedrais montmorilonita e beidelita/nontronita. As adapta����es ao m��todopadr��o (NaOH 5 mol L-1) favoreceram a concentra����o de minerais 2:1 secund��rios na fra����o argila dos solos; a concentra����o da solu����o de NaOH deve ser maior para horizontes com menor teor do mineral.

X-ray photoelectron spectroscopy analysis of ion��beam��induced oxidation of GaAs and AlGaAs

The oxidation of GaAs and AlGaAs targets subjected to O2+ bombardment has been analyzed, using in situ x��ray photoelectron spectroscopy, as a function of time until steady state is reached. The oxides formed by the O2+ bombardment have been characterized in terms of composition and binding energy. A strong energy and angular dependence for the oxidation of As relative to Ga is found. Low energies as well as near normal angles of incidence favor the oxidation of As. The difference between Ga and As can be explained in terms of the formation enthalpy for the oxide and the excess supply of oxygen. In an AlGaAs target the Al is very quickly completely oxidized irrespective of the experimental conditions. The steady state composition of the altered layers show in all cases a preferential removal of As.

Gas-sensing properties of sprayed films of (CdO)x(ZnO)1-x mixed oxide

A novel NO2 sensor based on (CdO)x(ZnO)1-x mixed-oxide thin films deposited by the spray pyrolysis technique is developed. The sensor response to 3-ppm NO2 is studied in the range 50��C-350��C for three different film compositions. The device is also tested for other harmful gases, such as CO (300 ppm) and CH4 (3000 ppm). The sensor response to these reducing gases is different at different temperatures varying from the response typical for the p-type semiconductor to that typical for the n-type semiconductor. Satisfactory response to NO2 and dynamic behavior at 230��C, as well as low resistivity, are observed for the mixed-oxide film with 30% Cd. The response to interfering gas is poor at working temperature (230��C). On the basis of this study, a possible sensing mechanism is proposed.

Low-dose, simple, and fast grating-based X-ray phase-contrast imaging.

Phase sensitive X-ray imaging methods can provide substantially increased contrast over conventional absorption-based imaging and therefore new and otherwise inaccessible information. The use of gratings as optical elements in hard X-ray phase imaging overcomes some of the problems that have impaired the wider use of phase contrast in X-ray radiography and tomography. So far, to separate the phase information from other contributions detected with a grating interferometer, a phase-stepping approach has been considered, which implies the acquisition of multiple radiographic projections. Here we present an innovative, highly sensitive X-ray tomographic phase-contrast imaging approach based on grating interferometry, which extracts the phase-contrast signal without the need of phase stepping. Compared to the existing phase-stepping approach, the main advantages of this new method dubbed "reverse projection" are not only the significantly reduced delivered dose, without the degradation of the image quality, but also the much higher efficiency. The new technique sets the prerequisites for future fast and low-dose phase-contrast imaging methods, fundamental for imaging biological specimens and in vivo studies.

On the nucleolus of 2 x 2 assignment games

[spa] En este art��culo hallamos f��rmulas para el nucleolo de juegos de asignaci��n arbitrarios con dos compradores y dos vendedores. Se analizan cinco casos distintos, dependiendo de las entradas en la matriz de asignaci��n. Los resultados se extienden a los casos de juegos de asignaci��n de tipo 2 x m o m x 2.

Reliure de : M. Annaei Lucani pharsalia, sive de bello civili Caesaris et Pompeii lib. X. Ex emendatione V.C. Hug : Grotii.

[La guerre civile (latin). 1626]

Entropy change and magnetocaloric effect in Gd5(SixGe1-x)4

Isothermal magnetization curves up to 23 T have been measured in Gd5Si1.8Ge2.2. We show that the values of the entropy change at the first-order magnetostructural transition, obtained from the Clausius-Clapeyron equation and the Maxwell relation, are coincident, provided the Maxwell relation is evaluated only within the transition region and the maximum applied field is high enough to complete the transition. These values are also in agreement with the entropy change obtained from differential scanning calorimetry. We also show that a simple phenomenological model based on the temperature and field dependence of the magnetization accounts for these results.

Virtual anthropology: A comparison between the performance of conventional X-ray and MDCT in investigating the trabecular structure of long bones.

Recently, modern cross-sectional imaging techniques such as multi-detector computed tomography (MDCT) have pioneered post mortem investigations, especially in forensic medicine. Such approaches can also be used to investigate bones non-invasively for anthropological purposes. Long bones are often examined in forensic cases because they are frequently discovered and transferred to medico-legal departments for investigation. To estimate their age, the trabecular structure must be examined. This study aimed to compare the performance of MDCT with conventional X-rays to investigate the trabecular structure of long bones. Fifty-two dry bones (24 humeri and 28 femora) from anthropological collections were first examined by conventional X-ray, and then by MDCT. Trabecular structure was evaluated by seven observers (two experienced and five inexperienced in anthropology) who analyzed images obtained by radiological methods. Analyses contained the measurement of one quantitative parameter (caput diameter of humerus and femur) and staging the trabecular structure of each bone. Preciseness of each technique was indicated by describing areas of trabecular destruction and particularities of the bones, such as pathological changes. Concerning quantitative parameters, the measurements demonstrate comparable results for the MDCT and conventional X-ray techniques. In contrast, the overall inter-observer reliability of the staging was low with MDCT and conventional X-ray. Reliability increased significantly when only the results of the staging performed by the two experienced observers were compared, particularly regarding the MDCT analysis. Our results also indicate that MDCT appears to be better suited to a detailed examination of the trabecular structure. In our opinion, MDCT is an adequate tool with which to examine the trabecular structure of long bones. However, adequate methods should be developed or existing methods should be adapted to MDCT.