The tilt casting of titanium aluminides

TiAl castings are prone to various defects including bubbles entrained during the turbulent filling of moulds. The present research has exploited the principles of the Durville tilt casting technique to develop a novel process in which the Induction Skull Melting (ISM) of TiAl alloys in a vacuum chamber has been combined with controlled tilt pouring to achieve the tranquil transfer of the metal into a hot ceramic shell mould. Practical casting equipment has been developed to evaluate the feasibility of this process in parallel with the development of novel software to simulate and optimize it. The PHYSICA CFD code was used to simulate the filling, heat transfer and solidification during tilt pouring using a number of free surface modelling techniques, including the novel Counter Diffusion Method (CDM). In view of the limited superheat, particular attention was paid to the mould design to minimize heat loss and gas entrainment caused by interaction between the counter-flowing metal and gas streams. The model has been validated against real-time X-ray movies of the tilt casting of aluminium and against TiAl blade castings. Modelling has contributed to designing a mould to promote progressive filling of the casting and has led to the use of a parabolic tilting cycle to balance the competing requirements for rapid filling to minimize the loss of superheat and slow filling minimize the turbulence-induced defects.

Synthesis and spectroscopic investigation of iron(III) complexes of N'-(thioaroyl)pyridine-2-carbohydrazides

A new series of iron(III) complexes [Fe(L(1))(HL(1))], [Fe(L(1)) Cl]; [H2L(1) = N'-(2-methoxythiobenzoyl)pyridine-2-carbohydrazide], [Fe(L(2))(acac)], [Fe(HL(2))2 Cl]; [H2L(2) = N'-(4-methoxythiobenzoyl)pyridine-2-carbohydrazide] and [Fe(L(3)) (acac)]; [H2L(3) = N'-(2-hydroxythiobenzoyl)pyridine-2-carbohydrazide] were prepared by stirring/refluxing/mixing the respective ligand with FeCl3/Fe(acac)3 in chloroform/methanol. All the compounds were characterized by elemental analyses, magnetic susceptibility, IR, UV and Mossbauer spectral data. The complexes high/low spin state and have tetrahedral/octahedral geometry.

Mechanisms and chemistry of dye adsorption on manganese oxides-modified diatomite

The investigations into structural changes which occur during adsorbent modification and the adsorption mechanisms are essential for an effective design of adsorption systems. Manganese oxides were impregnated onto diatomite to form the type known as delta-birnessite. Initial investigations established the effectiveness of manganese oxides-modified diatomite (MOMD) to remove basic and reactive dyes from aqueous solution. The adsorption capacity of MOMD for methylene blue (MB), hydrolysed reactive black (RB) and hydrolysed reactive yellow (RY) was 320, 419, and 204 mg/g, respectively. Various analytical techniques were used to characterise the structure and the mechanisms of the dye adsorption process onto MOMD such as Fourier transform infrared (FTIR), X-ray diffraction (XRD) and atomic absorption spectrometry (A.A.). A small shift to higher values of the cl-spacing of dye/MOMD was observed indicating that a small amount of the dye molecules were intercalated in the MOMD structure and other molecules were adsorbed on the external surface of MOMD. Two mechanisms of dye adsorption onto MOMD were proposed; intercalation of the dye in the octahedral layers and adsorption of the dye on the MOMD external surface. Moreover, the results demonstrated that the MOMD structure was changed upon insertion of MB and RY with an obvious decrease in the intensity of the second main peak of the MOMD X-ray pattern. (C) 2009 Elsevier Ltd. All rights reserved.

Tris(1-ethyl-3-methylimidazolium) hexabromidoeuropate(III)

The crystal structure of the title compound, (C6H11N2)(3-)[EuBr6], consists of 1-ethyl-3-methylimidazolium cations and centrosymmetric octahedral hexabromidoeuropate anions. The [EuBr6](3-) anions are located at the corners and face-centres of the monoclinic unit cell. Characteristic hydrogen-bonding interactions can be observed between the bromide anions and the acidic H atoms of the imidazolium cations.

The structure of [Co(H-tptz)Cl-3] center dot H2O (tptz=2,4,6-tri(2-pyridyl)-1,3,5-triazine) prepared by crystallization from the ionic liquid, N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide

The structure of tris-chloro[2,6-bis(2'-pyridyl)-4-(2'-pyridinium)-1,3,5-triazine]cobalt(II) monohydrate, [Co(C18H13N6)Cl-3]center dot H2O (C2/c (No. 15), a = 7.783(11), b = 22.42(3), c = 11.001(15) angstrom, beta = 90.05(2)degrees), crystallized from the open air reaction of CoCl2 and 2,4,6-tri(2-pyridyl)-1,3,5-triazine in the ionic liquid, N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide is reported. The structure consists of six coordinate cobalt in an octahedral geometry bonded to the tridentate tptz ligand and three chlorines. The non-coordinating pyridyl group in the tptz ligand is protonated (with the protonated nitrogen crystallographically disordered over two possible sites), providing overall charge neutrality for the complex.

Liquid Structure and Dynamics of Aqueous Isopropanol over gamma-Alumina

The liquid structures of thin films of aqueous solutions of 0, 7, 19, 50, and 100 mol % isopropanol above O/Al-terminated gamma-alumina surfaces have been investigated by means of classical molecular dynamics simulations. The structuring effect of the oxide oil the liquid mixtures is strong and heavily dependent on the local structure of the oxide. Two distinct re-ions are found oil the oxide Surface characterized by the degree of coordination of Al atoms. Above octahedral Al atoms, water and isopropanol molecules adsorb via the oxygen atoms to maximize the electrostatic interaction, whereas above tetrahedral Al sites the solvent molecules adsorb via hydrogen atoms with the oxygen atoms away front the surface. More mobility is found in the second layer compared with the first; however, its structure is still influenced significantly by the orientation of molecules in the first adsorbed layer. Qualitatively, the displacement of water from the surface by the adsorption of isopropanol occurs with 2.6 Water molecules lost for every alcohol molecule present based on the effective surface areas of the two species calculated from the pure simulations.

Versatile N,C,N coordination behavior of a monoanionic aryldiamine ligand in ruthenium(II) complexes: Syntheses and crystal structures of [Ru-II(C6H3(CH(2)NMe(2))(2)-2,6)X(L)] (L equals norbornadiene, X=Cl, SO3CF3; L=PPh(3), X=I) and [Ru-II{C6H3(CH(2)NMe(2))(2)-2,6}(2,2':6:2''-terpyridine)]Cl

The monoanionic ligand [C6H3(CH(2)NMe(2))(2)-2,6](-), a potentially terdentate N,C,N bonding system, has been employed to synthesize a series of new ruthenium(II) complexes [Ru{C6H3(CH(2)NMe(2))(2)-2,6}X(L)] (L = PPh(3) X = Cl (2a), I (2b); L = norbornadiene (nbd), X = Cl (4), eta(1)-OSO2CF3 (5)) and [Ru{C6H3(CH(2)NMe(2))(2)-2,6}(2,2':6',2 ''-terpyridine)]Cl (3). X-ray crystal structures of 2b and 3-5 have been determined, in which the N,C,N coordination geometry with respect to the metal center is found to differ considerably. In each complex the aryldiamine ligand is terdentate, eta(3)-N,C,N-bonded as a six electron donor system. However, depending on the other ligands in the Ru(II) coordination sphere, this ligand demonstrates considerable flexibility in adopting coordination geometries which range from meridional in 3 through pseudomeridional in 2b to pseudofacial in 4 and 5. In the structures of 4 and 5 significant distortions of the aryl ring, involving bending of the six-membered ring into a boatlike conformation, are found. The different combinations of the N,C,N ligand with sets of other ligands lead to a range of metal geometries, i.e. square pyramidal in 2b, octahedral in 3, and bicapped tetrahedral in 4 and 5.

Multilayer mirrored bubbles with spatially-chirped and elastically-tuneable optical bandgaps

We demonstrate the multifolding Origami manufacture of elastically-deformable Distributed Bragg Reflector (DBR) membranes that reversibly color-tune across the full visible spectrum without compromising their peak reflectance. Multilayer films composed of alternating transparent rubbers are fixed over a 300 mu m wide pinhole and deformed by pressure into a concave shape. Pressure-induced color tuning from the near-IR to the blue arises from both changes in thickness of the constituent layers and from tilting of the curved DBR surfaces. The layer thickness and color distribution upon deformation, the band-gap variation and the repeatability of cyclic color tuning, are mapped through micro-spectroscopy. Such spatially-dependent thinning of the film under elastic deformation produces spatial chirps in the color, and are shown to allow reconstruction of complex 3D strain distributions. (C) 2012 Optical Society of America

One-Pot, High-Yield Synthesis of Convex Au Octahedra by H2O2 - Mediated Process In Solution Phase

A simple, non-seeding and high-yield synthesis of convex gold octahedra with size of ca. 50 nm in aqueous solution is described. The octahedral nanoparticles were systematically prepared by reduction of HAuCl4 using ascorbic acid (AA) in the presence of cetyltrimethylammonium bromide (CTAB) as the stabilizing surfactant while concentrations of Au3+ were fixed. The synthesizing process is especially different to other wet synthesis of metallic nanoparticles because it is mediated by H2O2. Mechanism of the H2O2 – mediated process will be described in details. The gold octahedra were shown to be single crystals with all 8 faces belonging to {111} family. Moreover, the single crystalline particles also showed attractive optical properties towards LSPR that should find uses as labels for microscopic imaging, materials for colorimetric biosensings, or nanosensor developments.

In situ FTIR spectroscopy studies on electrochemical redox processes of high nuclearity osmium carbonyl clusters

The electrochemical redox processes of two high nuclearity osmium carbonyl clusters [(PhP)N[OsC(CO) ]·PPN (1) and Os(CO) (6) have been studied by electrochemical in situ FTIR. The five oxidation states of 1, i.e., [OsC(CO)], have been characterized. There are no significant structural changes for these species. Hence, the ability of this decanuclear cluster to act as an electron reservoir has been demonstrated. The structural rearrangement associated with the two-electron reduction of bicapped tetrahedral 6 to octahedral dianion [Os(CO)] and [Os(CO)] tetraanion has also been investigated. © 1996 American Chemical Society.

Dichloridobis(picolinohydrazide)cadmium(II)

The title compound, [CdCl2(C6H7N3O)(2)], was obtained unintentionally as a product of an attempted reaction of CdCl2 center dot 2.5H(2)O and picolinic acid hydrazide, in order to obtain a cadmium(II) complex analogous to a 15-metallacrown-5 complex of the formula [MCu5L5]X-n, with M = a central metal ion, L = picolinic acid hydrazide and X = Cl- , but with cadmium the only metal present. The coordination geometry around the Cd-II atom can be considered as distorted octahedral, with two bidentate picolinic acid hydrazide ligands, each coordinating through their carbonyl O atom and amino N atom, and two chloride anions. In the crystal structure, intermolecular N-H center dot center dot center dot Cl and N-H center dot center dot center dot N hydrogen bonds link the molecules into a two-dimensional network parallel to the ( 100) plane. The pyridine rings of adjacent networks are involved in pi-pi stacking interactions, the minimum distance between the ring centroids being 3.693 (2) angstrom.

catena-poly[mercury(II)-di-mu-bromo-mu-pyridazine-kappa N-2 : N ']

The crystal structure of [HgBr2(Pyo)](n) (Pyo = pyridazine, C4H4N2) consists of strands of octahedrally coordinated mercuric centers asymmetrically bridged by bromide and connected by the two neighboring N atoms of pyridazine molecules to complete the octahedral coordination of mercury. The Hg atoms lie on inversion centers.

Identification of MnCr2O4 nano-octahedron in catalysing pitting corrosion of austenitic stainless steels

Pitting corrosion of stainless steels, one of the classical problems in materials science and electrochemistry, is generally believed to originate from the local dissolution in MnS inclusions, which are more or less ubiquitous in stainless steels. However, the initial location where MnS dissolution preferentially occurs is known to be unpredictable, which makes pitting corrosion a major concern. In this work we show, at an atomic scale, the initial site where MnS starts to dissolve in the presence of salt water. Using in situ ex-environment transmission electron microscopy (TEM), we found a number of nano-sized octahedral MnCr2O4 crystals (with a spinel structure and a space group of Fd (3) over barm) embedded in the MnS medium, generating local MnCr2O4/MnS nano-galvanic cells. The TEM experiments combined with first-principles calculations clarified that the nano-octahedron, enclosed by eight {1 1 1} facets with metal terminations, is "malignant", and this acts as the reactive site and catalyses the dissolution of MnS. This work not only uncovers the origin of MnS dissolution in stainless steels, but also presents an atomic-scale evolution in a material's failure which may occur in a wide range of engineering alloys and biomedical instruments serving in wet environments. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Temperature and Pressure Induced Structural Changes of Cobalt (II) in a Phosphonium Based Ionic Liquid

In this paper the temperature and pressure induced paramagnetic switching of cobalt (II) complex in binary mixture of phosphonium based ionic liquid [P6,6,6,14]SCN and [Co(NCS)2], is reported. This arises from a structural change in the coordination of the cobalt (II) center from tetrahedral [Co(NCS)4]2- to octahedral [Co(NCS)6]4- when mobile thiocyanate ions are added. These properties are reflected in the abrupt change of conductivity behavior of the magnetic ionic liquid. Therefore, as demonstrated herein the reversible switching in coordination of cobalt from tetrahedral to octahedral can be easily monitored at ambient as well as elevated pressure by tracking the dc-conductivity changes.

Clinopyroxene based glasses and glass-ceramics for functional applications

As piroxenas são um vasto grupo de silicatos minerais encontrados em muitas rochas ígneas e metamórficas. Na sua forma mais simples, estes silicatos são constituídas por cadeias de SiO3 ligando grupos tetrahédricos de SiO4. A fórmula química geral das piroxenas é M2M1T2O6, onde M2 se refere a catiões geralmente em uma coordenação octaédrica distorcida (Mg2+, Fe2+, Mn2+, Li+, Ca2+, Na+), M1 refere-se a catiões numa coordenação octaédrica regular (Al3+, Fe3+, Ti4+, Cr3+, V3+, Ti3+, Zr4+, Sc3+, Zn2+, Mg2+, Fe2+, Mn2+), e T a catiões em coordenação tetrahédrica (Si4+, Al3+, Fe3+). As piroxenas com estrutura monoclínica são designadas de clinopiroxenes. A estabilidade das clinopyroxenes num espectro de composições químicas amplo, em conjugação com a possibilidade de ajustar as suas propriedades físicas e químicas e a durabilidade química, têm gerado um interesse mundial devido a suas aplicações em ciência e tecnologia de materiais. Este trabalho trata do desenvolvimento de vidros e de vitro-cerâmicos baseadas de clinopiroxenas para aplicações funcionais. O estudo teve objectivos científicos e tecnológicos; nomeadamente, adquirir conhecimentos fundamentais sobre a formação de fases cristalinas e soluções sólidas em determinados sistemas vitro-cerâmicos, e avaliar a viabilidade de aplicação dos novos materiais em diferentes áreas tecnológicas, com especial ênfase sobre a selagem em células de combustível de óxido sólido (SOFC). Com este intuito, prepararam-se vários vidros e materiais vitro-cerâmicos ao longo das juntas Enstatite (MgSiO3) - diopsídio (CaMgSi2O6) e diopsídio (CaMgSi2O6) - Ca - Tschermak (CaAlSi2O6), os quais foram caracterizados através de um vasto leque de técnicas. Todos os vidros foram preparados por fusão-arrefecimento enquanto os vitro-cerâmicos foram obtidos quer por sinterização e cristalização de fritas, quer por nucleação e cristalização de vidros monolíticos. Estudaram-se ainda os efeitos de várias substituições iónicas em composições de diopsídio contendo Al na estrutura, sinterização e no comportamento durante a cristalização de vidros e nas propriedades dos materiais vitro-cerâmicos, com relevância para a sua aplicação como selantes em SOFC. Verificou-se que Foi observado que os vidros/vitro-cerâmicos à base de enstatite não apresentavam as características necessárias para serem usados como materiais selantes em SOFC, enquanto as melhores propriedades apresentadas pelos vitro-cerâmicos à base de diopsídio qualificaram-nos para futuros estudos neste tipo de aplicações. Para além de investigar a adequação dos vitro-cerâmicos à base de clinopyroxene como selantes, esta tese tem também como objetivo estudar a influência dos agentes de nucleação na nucleação em volume dos vitro-cerâmicos resultantes á base de diopsídio, de modo a qualificá-los como potenciais materiais hopedeiros de resíduos nucleares radioactivos.