Effect of Substitution of Ca on Thermal Expansion of Cordierite (Mg2Al4Si5O18)

The effect of substitution of calcium on the anisotropic axial thermal expansion of cordierite was investigated by using a high-temperature X-ray diffraction technique. The compositions were prepared by the sol–gel route. In the Mg2-xCax-Al4Si5O18 system, single-phase cordierite can be prepared for x up to 0.5. Thermal expansion anisotropy (αa–αc) of cordierites reduces progressively by the substitution of increasing amounts of Ca for Mg.

De "de" : Estudio histórico-comparativo de los usos y la semántica de la preposición "de" en español

El presente estudio supone un intento de describir y analizar el uso de la preposición "de" sobre la base de un corpus diacrónico, con énfasis en las diferentes relaciones semánticas que establece. Partiendo de un total de más de 16.000 casos de "de" hemos establecido 48 categorías de uso, que corresponden a cuatro tipos de construcción sintáctica, a saber, el uso de "de" como complemento de nombres (CN), verbos (CV), adjetivos (CA) y, finalmente, su uso como núcleo de expresiones adverbiales independientes (CI). El estudio consta de tres partes fundamentales. En la parte I, se introduce la Lingüística Cognitiva, que constituye la base teórica esencial del trabajo. Más exactamente, se introducen conceptos como la teoría del prototipo, la teoría de las metáforas conceptuales y la gramática cognitiva, especialmente las ideas de "punto de referencia" y "relación intrínseca" (Langacker 1995, 1999). La parte II incluye el análisis de las 48 categorías. En esta parte se presentan y comentan casi 2.000 ejemplos del uso contextual de "de" extraídos del corpus diacrónico. Los resultados más importantes del análisis pueden resumirse en los siguientes puntos: El uso de "de" sigue siendo esencialmente el mismo en la actualidad que hace 800 años, en el sentido de que todas las 48 categorías se identifican en todas las épocas del corpus. El uso de "de" como complemento nominal va aumentando, al contrario de lo que ocurre con su uso como complemento verbal. En el contexto nominal son especialmente las relaciones posesivas más abstractas las que se hacen más frecuentes, mientras que en el contexto verbal las relaciones que se hacen menos frecuentes son las de separación/alejamiento, causa, agente y partitivo indefinido. Destaca la importancia del siglo XVIII como época de transición entre un primer estado de las cosas y otro posterior, en especial en relación con el carácter cada vez más abstracto de las relaciones posesivas así como con la disminución de las categorías adverbales de causa, agente y partitivo. Pese a la variación en el contexto inmediato de uso, el núcleo semántico de "de" se mantiene inalterado. La parte III toma como punto de partida los resultados del análisis de la parte II, tratando de deslindar el aporte semántico de la preposición "de" a su contexto de uso del valor de la relación en conjunto. Así, recurriendo a la metodología para determinar el significado básico y la metodología para determinar lo que constituyen significados distintos de una preposición (Tyler , Evans 2003a, 2003b), se llega a la hipótesis de que "de" posee cuatro significados básicos, a saber, 'punto de partida', 'tema/asunto', 'parte/todo' y 'posesión'. Esta hipótesis, basada en las metodologías de Tyler y Evans y en los resultados del análisis de corpus, se intenta verificar empíricamente mediante el uso de dos cuestionarios destinados a averiguar hasta qué punto las distinciones semánticas a las que se llega por vía teórica son reconocidas por los hablantes nativos de la lengua (cf. Raukko 2003). El resultado conjunto de los dos acercamientos tanto refuerza como especifica la hipótesis. Los datos que arroja el análisis de los cuestionarios parecen reforzar la idea de que el núcleo semántico de "de" es complejo, constando de los cuatro valores mencionados. Sin embargo, cada uno de estos valores básicos constituye un prototipo local, en torno al cual se construye un complejo de matices semánticos derivados del prototipo. La idea final es que los hablantes son conscientes de los cuatro postulados valores básicos, pero que también distinguen matices más detallados, como son las ideas de 'causa', 'agente', 'instrumento', 'finalidad', 'cualidad', etc. Es decir, "de" constituye un elemento polisémico complejo cuya estructura semántica puede describirse como una semejanza de familia centrada en cuatro valores básicos en torno a los cuales se encuentra una serie de matices más específicos, que también constituyen valores propios de la preposición. Creemos, además, que esta caracterización semántica es válida para todas las épocas de la historia del español, con unas pequeñas modificaciones en el peso relativo de los distintos matices, lo cual está relacionado con la observada variación diacrónica en el uso de "de".

CA\TG Sequence at the 5'end of Oligo(A)-tracts Strongly Modulates DNA Curvature

An analysis of the base pair doublet geometries in available crystal structures indicates that the often reported intrinsic curvature of DNA containing oligo-(d(A).d(T)) tracts may also depend on the nature of the flanking sequences. The presence of CA/TG doublet in particular at the 5' end of these tracts is expected to enhance their intrinsic bending property. To test this proposition, three oligonucleotides, d(GAAAAACCCCCC), d(CCCCCCAAAAAG), d(GAAAAATTTTTC), and their complementary sequences were synthesized to study the effect of various flanking sequences, at the 5' and 3' ends of the A-tracts, on the curvature of DNA in solution. An analysis of the polyacrylamide gel electrophoretic mobilities of these sequences under different conditions of salts and temperatures (below their melting points) clearly showed that the oligomer with CA/TG sequence in the center was always more retarded than the oligomer with AC/GT sequence, as well as the oligomer with AT/AT sequence. Hydroxyl radical probing of the sequences with AC/GT and CA/TG doublet junctions gives a similar cutting pattern in the A-tracts, which is quite different from that in the C-tracts, indicating that the oligo(A)-tracts have similar structures in the two oligomers. KMnO4 probing shows that the oligomer with a CA/TG doublet junction forms a kink that is responsible for its inherent curvature and unusual electrophoretic mobility. UV melting shows a reduced thermal stability of the duplex with CA/TG doublet junction, and circular dichroism (CD) studies indicate that a premelting transition occurs in the oligomer with CA/TG doublet step before global melting but not in the oligomer with AC/GT doublet step, which may correspond to thermally induced unbending of the oligomer. These observations indicate that the CA/TG doublet junction at the 5' end of the oligo(A)-tract has a crucial role in modulating the overall curvature in DNA.

Dependence of magnetism on GdFeO3 distortion in the t(2g) system ARuO(3) (A = Sr, Ca)

We have examined the stability of the ferromagnetic (FM) state in CaRuO3 and SrRuO3 as a function of the GdFeO3 distortion. Model calculations predict the dependence of the FM transition temperature (T-c) on the rotation angle theta to vary as cos(2)(2 theta) for e(g)-electron systems. However, here, we find an initial increase and then the expected decrease. Furthermore, a much faster decrease is found than predicted for e(g)-electron systems. Considering the specific case of CaRuO3, a larger deviation of the Ru-O-Ru angle from 180 degrees in CaRuO3 as compared to SrRuO3 should result in a more reduced bandwidth, thereby making the former more correlated. The absence of long-range magnetic order in the more correlated CaRuO3 is traced to the strong collapse of various exchange interaction strengths that arises primarily from the volume reduction and increased distortion of the RuO6 octahedra network that accompanies the presence of a smaller ion at the A site.

Behavior of Ca(OH)2/CaO pellet under dehydration and hydration

With construction of a thermochemical energy conversion prototype system to store solar heat, thermal dissociation of pellets of Ca(OH)2 and hydration of CaO have been investigated in some detail for its application to the system. The inorganic substance is very attractive as a material for long term heat storage, but molar density changes associated with the reaction are fairly large. Therefore, this factor has been taken into account in the kinetic equation. The importance of additives and pellet size has been discussed considering reactivity and strength of pellets. An analysis has been attempted when chemical reaction is important. The deformation of pellets was observed during hydration.

Substitution of chromium for univalent copper in superconducting Pb2Sr2(Ca,Y)Cu3O8+delta

Following considerations of geometry and the similarity between chromate and carbonate groups in terms of size and charge, we have investigated the possibility of replacing the two-coordinate Cu-I in superconducting lead cuprates of the general formula Pb2Sr2(Ca, Y)CU3O8 by Cr. A high-resolution electron microscopy study coupled with energy dispersive X-ray analysis on small crystals of the title phases suggests that between 10 and 15% of the Cu-I can be replaced by Cr. While from the present structural study using HRTEM and Rietveld refinement of X-ray powder data we are unable to precisely obtain the oxidation state and oxygen coordination of Cr, we suggest in analogy with Cr substitution in other similar cuprates that in the title phases (CuO2)-O-I rods are partially replaced by tetrahedral CrO42- groups. Infrared spectroscopy supports the presence of CrO42- groups. The phases Pb1.75Sr2Ca0.2Y0.8O8+delta and Pb1.75Sr2Ca0.2Y0.8CCu2.85Cr0.15O8+delta are superconducting as-prepared, but the substitution of Cr for Cu-I results in a decrease of the Te as well as the superconducting volume fraction. (C) 1996 Academic Press, lnc.

Oriented films of LaNiO3 and other members of the Lan+1NinO3n+1 series, LaCuO3−δ and Pb(Zr0.52Ti0.48)O3, obtained by nebulized spray pyrolysis

The technique of nebulized spray pyrolysis has been explored to find out whether oriented films of certain important oxides can be produced on single-crystal substrates by this relatively gentle method. Starting with acetylacetonate precursors, oriented films of metallic LaNiO3 containing nearly spherical grains (30 nm) have been obtained. Films of near-stoichiometric La4Ni3O10 and La3Ni2O7 showing metallic conductivity have been obtained by this method. This is indeed gratifying since it is difficult to prepare monophasic and stoichiometric bulk samples of these materials. Films of La2NiO4 show the expected semiconducting behavior. In the La-Cu-O system, starting with acetylacetonates, we have obtained films mainly comprising semiconducting La2Cu2O5, which is generally difficult to prepare in bulk form. More interestingly, nebulized spray pyrolysis gives excellent stoichiometric films of Pb(Zr0.52Ti0.48)O-3 consisting of nearly spherical grains (30 nm) which show ferroelectric behavior. The present investigation demonstrates that nebulized spray pyrolysis provides a useful and desirable route to deposite oriented films of complex oxide materials on single-crystal substrates.

Investigation of valence states in the perovskite oxides, LaAFeNbO(6) (A = Ca, Sr)

Compounds of the type, LaAFeNbO(6) (A = Ca Sr) have been synthesized to study the electrical and magnetic properties and to examine valence degeneracy. The results show that valence degeneracy is not operative and the compounds are insulating. Magnetic susceptibility data show that part of the Fe is in Fs(2+) state, thus oxidizing part of Nb4+ to Nb5+ by an internal redox mechanism. The presence of mixed valent Fe is confirmed by Mossbauer spectra. (C) 1999 Elsevier Science B.V. All rights reserved.

Raman evidence for the superconducting gap and spin-phonon coupling in the superconductor Ca( Fe0.95Co0.05)(2)As-2

Inelastic light scattering studies on a single crystal of electron-doped Ca(Fe0.95Co0.05)(2)As-2 superconductor, covering the tetragonal-to-orthorhombic structural transition as well as the magnetic transition at T-SM similar to 140 K and the superconducting transition temperature T-c similar to 23 K, reveal evidence for superconductivity-induced phonon renormalization. In particular, the phonon mode near 260 cm(-1) shows hardening below T-c, signaling its coupling with the superconducting gap. All three Raman active phonon modes show anomalous temperature dependence between room temperature and T-c, i.e. the phonon frequency decreases with lowering temperature. Further, the frequency of one of the modes shows a sudden change in temperature dependence at TSM. Using first-principles density functional theory based calculations, we show that the low temperature phase (T-c < T < T-SM) exhibits short-ranged stripe antiferromagnetic ordering, and estimate the spin-phonon couplings that are responsible for these phonon anomalies.

Grain size effects on charge-ordering, phase segregation and related properties of rare-earth manganates, Nd(0.5)A(0.5)MnO(3) (A = Ca or Sr)

Grain size has marked effects on charge-ordering and other properties of Nd(0.5)A(0.5)MnO(3) (A=Ca or Sr). Thus, the anti-ferromagnetic (AFM) transition in Nd0.5Ca0.5MnO3 is observed distinctly only in samples sintered at 1273 K or higher. The sample with a small grain size (sintered at 1173 K) shows evidence for greater ferromagnetic (FM) interaction at low temperatures, probably due to phase segregation. The FM transition as well as the charge-ordering transition in Nd0.5Sr0.5MnO3 becomes sharper in samples sintered at 1273 K or higher. The sample sintered at 1173 K does not show the AFM-CO transition around 150 K and is FM down to low temperatures; the apparent T-c-T-co gap decreases with the increase in the grain size. The samples sintered at lower temperatures (<1673 K) show evidence for greater segregation of the AFM and FM domains. (C) 2002 Elsevier Science Ltd. All rights reserved.

Electronic structure of ${\rm Ca}_{1-x}{\rm Sr}_x{\rm VO}_3$: A tale of two energy scales

We investigate the electronic structure of Ca1-xSrxVO3 using photoemission spectroscopy. Core level spectra establish an electronic phase separation at the surface, leading to a distinctly different surface electronic structure compared to the bulk. Analysis of the photoemission spectra of this system allowed us to separate the surface and bulk contributions. These results help us to understand properties related to two vastly differing energy scales, namely the low-energy scale of thermal excitations ( $\sim\!k_{\rm B}T$) and the high-energy scale related to Coulomb and other electronic interactions.

Dielectric Studies of laser ablated Ca doped BaTiO3 thin films

Recently, there has been growing interest in Ca modified BaTiO3 structures due to their larger electro-optic coefficients for their use in optical storage of information over conventional BaTiO3 crystals. Barium Calcium Titanate (BCT) shows promising applications in advanced laser systems, optical interconnects and optical storage devices. BaTiO3 thin films of varied Ca (3 at. % - 15 at. %) doping were deposited using pulsed laser ablation (KrF excimer laser) technique over Pt/Si substrates. The stoichiometric and the compositional analysis were carried out using EDAX and SIMS. The dielectric studies were done at the frequency regime of 40 Hz to 100 kHz at different ambient temperatures from 200 K to 600 K. The BCT thin films exhibited diffuse phase transition, which was of a typical non lead relaxor behavior and had high dielectric constant and low dielectric loss. The phase transition for the different compositions of BCT thin films was near the room temperature, showing a marked departure from the bulk phase transition. The C - V and the hysteresis behavior confirmed the ferroelectric nature below the phase transition and paraelectric at the room temperature.

Growth and Characterization Studies of ABi2ta2o9 (A = Ba, Sr and Ca) Ferroelectric Thin Films

Thin films of ferroelectric ABi2Ta2O9 bismuth-layered structure, where A = Ba, Sr and Ca, were prepared by pulsed laser deposition technique on Pt/TiO2/SiO2/Si(100) substrates. The influence of substrate temperature between 500 to 750°C, and oxygen partial pressure 100-300 mTorr, on the structural and electrical properties of the films was investigated. The films deposited above 650°C substrate temperature showed complete Aurivillius layered structure. Films annealed at 750°C for 1h in oxygen atmosphere have exhibited better electrical properties. Atomic force microscopy study of surface topography shows that the films grown at lower temperature has smaller grains and higher surface roughness. This paper discusses the pronounced influence of A-site cation substitution on the structural and ferroelectric properties with the aid of Raman spectroscopy, X-ray diffraction and electrical properties. The degradation of ferroelectric properties with Ba and Ca substitution at A-sites is attributed to the higher structural distortion caused by changing tolerance factor. A systematic proportionate variation of coercive field is attributed to electronegativity difference of A-site cations.

Properties of the Ferrimagnetic Double-Perovskites A2FeReO6 (A=Ba and Ca)

We have synthesized ceramics of A2FeReO6 double-perovskites A2FeReO6 (A=Ba, Ca). Structural characterizations indicate a cubic structure with a=8.0854(1) Å for Ba2FeReO6 and a distorted monoclinic symmetry with a=5.396(1) Å, b=5.522(1) Å, c=7.688(2) Å and β=90.4° for Ca2FeReO6. The barium compound is metallic from 5K to 385K, i.e. no metal-insulator transition has been seen up to 385K, and the calcium compound is semiconducting from 5K to 385K. Magnetization measurements show a ferrimagnetic behavior for both materials, with Tc =315 K for Ba2FeReO6 and above 385K for Ca2FeReO6. At 5K we observed, only for Ba2FeReO6, a negative magnetoresistance of 10% in a magnetic field of 5T. Electrical, magnetic and thermal properties are discussed and compared to those of the analogous compounds Sr2Fe(Mo,Re)O6 recently studied.