Ultralow-density nanocage-based metal-oxide polymorphs

For two important metal oxides (MO, M=Mg, Zn) we predict, via accurate electronic structure calculations, that new low-density nanoporous crystalline phases may be accessible via the coalescence of nanocluster building blocks. Specifically, we consider the assembly of cagelike (MO)12 clusters exhibiting particularly high gas phase stability, leading to new polymorphs with energetic stabilities rivaling (and sometimes higher) than those of known MO polymorphs.

Spectroscopic characterization of phases formed by high-dose carbon ion implantation in silicon

High-dose carbon-ion-implanted Si samples have been analyzed by infrared spectroscopy, Raman scattering, and x-ray photoelectron spectroscopy (XPS) correlated with transmission electron microscopy. Samples were implanted at room temperature and 500°C with doses between 1017 and 1018 C+/cm2. Some of the samples were implanted at room temperature with the surface covered by a capping oxide layer. Implanting at room temperature leads to the formation of a surface carbon-rich amorphous layer, in addition to the buried implanted layer. The dependence of this layer on the capping oxide suggests this layer to be determined by carbon migration toward the surface, rather than surface contamination. Implanting at 500°C, no carbon-rich surface layer is observed and the SiC buried layer is formed by crystalline ßSiC precipitates aligned with the Si matrix. The concentration of SiC in this region as measured by XPS is higher than for the room-temperature implantation.

Análise por difração de raios x de filmes de óxidos cerâmicos compostos por IrO2/TiO2/CeO2

Independent of the sample form (powder or film), XRD analysis of Ir0,3Ti(0,7-x)Ce xO2, (nominal) mixtures, for x=0, shows the formation of a solid solution phase between IrO2 and TiO2, as well as the rutile phases of IrO2 and TiO2. The presence of the anatase phase of TiO2 is also confirmed. The introduction of 30 mol% CeO2 in the mixture reveals the presence of the CeO2 and Ce2O3 phases, besides the already mentioned ones, in the powder. In the film form, however, an amorphous phase is identified. When all of the TiO2 is substituded by CeO2, for both sample forms, the only phases found are IrO2, CeO2 and Ce2O3. This result suggests cerium oxides are not capable of forming solid solutions with either IrO2 or (Ir,Ti)O2 acting solely as a dispersant matrix for these phases. These results are consistent with the much higher electrochemically active surface area when CeO2 is introduced in the binary Ti/Ir0,3Ti0,7O2 mixture. It was possible to establish a relationship between the electrochemical stability of the supported films and their crystalline structure. The unexpected presence of TiO2 and Ti2O3 in the Ti/Ir0,3Ce0,7O2 (film sample) is attributed to oxidation of the Ti support during the calcination step.

Preparação e caracterização de pós cerâmicos de aluminatos de lítio

The preparation of gamma-LiAlO2 by coprecipitation and sol-gel synthesis was investigated. Ceramic powders obtained by coprecipitation synthesis were prepared from aqueous solutions of aluminum and lithium nitrates using sodium hydroxide as precipitant agent. By sol-gel synthesis, the ceramic powders were prepared from hydrolysis of aluminum isopropoxide. The materials obtained by two routes of synthesis were dried at 80ºC and calcined at 550, 750, 950 and 1150ºC. The characterization was done by X-ray diffraction, infrared spectroscopy, emission and absorption atomic spectrometry, helium picnometry, specific surface area (BET method) and scanning electronic microscopy. Mixtures of crystalline phases were obtained by coprecipitation synthesis: 80ºC- LiAl2(OH)7.2H2O + Al(OH)3; 550 and 750ºC- alpha-LiAlO2 + eta-Al2O3; 950 and 1150ºC- gamma-LiAlO2 + LiAl5O8. Chemical analysis showed molar ration Al/Li @ 3. Crystalline single-phases were obtained by sol-gel synthesis above 550ºC: 550ºC-alpha-LiAlO2; 750, 950 and 1150ºC-gamma-LiAlO2. These powders presented molar ration Al/Li @ 1. Thus, gamma-LiAlO2 crystalline phase was obtained at 750ºC by sol-gel synthesis while by coprecipitation synthesis, a mixture of crystalline phases was obtained. These results showed the superiority of the sol-gel synthesis for the preparation of pure gamma-LiAlO2.

Hidrogenofosfato de titânio lamelar: síntese, troca iônica e intercalação

The development associated with the research field involving crystalline inorganic lamellar titanium hydrogenphosphate, Ti(HPO4).H2O, synthesized as alpha or gamma forms, is directly linked to the huge number of reactions, that occur inside the free interlamellar space. Two distinguishable well-characterized features such as ion-exchange and intercalation reactions are explored here. From the interactive point of view, the acidic OH centres distributed on the lamella can interact with cations or with basic polar molecules to exchange or to intercalate them. These chemical reactions are normally followed by an expansion of the interlamellar space, proportional to the amount intercalated, reflecting in ion radii or organic molecule size lengths used in ion-exchange or insertion processes, respectively. The effectiveness of the exchange increased when the original matrix has the proton of OH group previously ion-exchanged by an alkaline or an alkylammonium cations. Monoalkyl-, dialkyl- and heterocyclic amines are focused in this revision as clear and elucidative examples of acid-base interactive processes, that come out inside of the well-formed infinite sequence of inorganic lamellar structure.

Caracterização textural e estrutural de V2O5/TiO2 obtidos via sol-gel: comparação entre secagem convencional e supercrítica

This work describes a modified sol-gel method for the preparation of V2O5/TiO2 catalysts. The samples have been characterized by N2 adsorption at 77K, x-ray diffractometry (XRD) and Fourier Transform Infrared (FT-IR). The surface area increases with the vanadia loading from 24 m² g-1, for pure TiO2, to 87 m² g-1 for 9wt.% of V2O5. The rutile form is predominant for pure TiO2 but became enriched with anatase phase when vanadia loading is increased. No crystalline V2O5 phase was observed in the catalysts diffractograms. Two species of surface vanadium observed by FT-IR spectroscopy a monomeric vanadyl and polymeric vanadates, the vanadyl/vanadate ratio remains practically constant.

Determinação de cobre, alumínio e ferro em solos derivados do basalto através de extrações seqüenciais

Copper, aluminum and iron concentrations were determined in four geochemical fractions of three different basaltic soils from the northwest region of the Parana State, Brazil. The fractions examined were the reducible manganese dioxide and amorphous iron oxide, crystaline iron oxide, organic and residual. Metal concentrations were determined in the extracts by flame atomic absorption spectrophotometry. High Fe concentrations were extracted from the crystalline iron oxide (>20%), as well as the amorphous iron oxide (>12%). Copper was extracted from the amorphous and crystalline iron oxides in the range 5 to 12%, but low concentrations were bound to organic matter. Low concentrations of aluminum were extracted (<8%) from the amorphous and crystaline iron oxides, and organic matter. High concentrations of aluminum were found in the residual fraction.

A new experimental procedure to obtain titania powders as anatase phase by a sol-gel process

Titania powders were synthesized by a sol-gel process using titanium tetrabutoxide as precursor. The syntheses were performed in water or in solutions of dimethylformamide (dmf) or dimethylsulfoxide (dmso). It is demonstrated, by X-ray diffraction patterns of the synthesized powders, that the samples obtained in dmf or dmso solutions are crystalline (anatase phase) with some minor amount of brookite phase, whereas the sample synthesized in water is amorphous. The anatase phase can be obtained independently of any previous or further treatment of the synthesized powder, such as hydrothermal or heat treatment, providing a new, simple, quick and inexpensive route to synthesize anatase powders. From the peak broadening of the anatase (101) diffraction, the crystallite sizes were calculated as 6 nm.

Reduced ceria nanofilms from structure prediction

Experimentally, Ce2O3 films are used to study cerium oxide in its fully or partially reduced state, as present in many applications. We have explored the space of low energy Ce2O3 nanofilms using structure prediction and density functional calculations, yielding more than 30 distinct nanofilm structures. First, our results help to rationalize the roles of thermodynamics and kinetics in the preparation of reduced ceria nanofilms with different bulk crystalline structures (e.g. A-type or bixbyite) depending on the support used. Second, we predict a novel, as yet experimentally unresolved, nanofilm which has a structure that does not correspond to any previously reported bulk A2B3 phase and which has an energetic stability between that of A-type and bixbyite. To assist identification and fabrication of this new Ce2O3 nanofilm we calculate some observable properties and propose supports for its epitaxial growth.

Aplicação da calorimetria exploratória diferencial no estudo da cinética de transição alfa -> delta HMX

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) or octogen is a white crystalline substance which occurs in four polymorphous forms. It is used in a wide variety of military and industrial formulations owing to its suitable properties. Researchers have demonstrated the usefulness of this energetic material in explosive components. In the present work we apply differential scanning calorimetry (DSC) to measure the a ® d solid-solid phase transition energy of HMX. The results obtained by Kissinger's and Ozawa's methods were 487 and 495 kJ/mol, respectively.

Síntese e caracterização de alfa-fosfato de zircônio(IV) contendo agregados de cobre metálico

The alpha-zirconium (IV) hydrogenphosphate (alpha-ZrP) has received great attention in the last years due to its properties like ion exchange, intercalation, ionic conductivity and catalytic activity. This work reports a method to produce metallic copper clusters on alpha-ZrP to be used as catalysts in petrochemical processes. It was found that the solids were non-crystalline regardless of the uptake of copper and the reduction. The specific surface area increased as a consequence of the increase of the interlayer distance to accept the copper ions between the layers. During the reduction, big clusters of copper (0,5-11µ) with different sizes and shapes were produced.

Efeito do teor metálico em catalisadores Co/Al2O3 aplicados à reação de reforma a vapor de etanol

The development of cobalt catalysts to produce hydrogen from ethanol is the goal of this investigation. Co/Al2O3 catalysts were prepared by impregnation and characterized by atomic absorption, nitrogen adsorption, X-ray diffraction, Raman spectroscopy, temperature programmed reduction and carbon analysis. The catalysts contained Co3O4 oxide and Co3+ and Co2+ species interacting with alumina. The cobalt load affects the crystal size and the crystalline structure and higher Co loads influence the reaction mechanism, changing the selectivity of the catalysts, decreasing the amount of CO produced and avoiding the formation of products catalyzed by the support. The ethanol conversion was 50-70% with 10-<1% of CO in the hydrogen.

Preparação de compostos lamelares: síntese do hidrogenofosfato de zircônio cristalino e sua intercalação com aminas. Um experimento para estudantes de graduação

The present paper describes the synthesis of crystalline zirconium hydrogen phosphate by direct precipitation and its intercalation with pyridine and n-butylamine. The simple experiment was tested in the undergraduate inorganic chemistry laboratory course for chemistry students at IQ-UNICAMP using inexpensive reagents. The materials were characterized by powder X-ray diffraction and infrared analyses in order to obtain detailed information of the solid structure changes as a result of the intercalation process. Pyridine and n-butylamine are focused in this work as clear and elucidative examples leading to acid-base interactive processes that result in the well-formed infinite sequence of inorganic lamellar structures.

Preparação e caracterização óptica de filmes de poli(estireno sulfonados) dopados com neodímio

In this work we describe the processing of poly(styrene sulphonate) films (PSS) doped with neodymium (Nd). Optical density measurements in the UV-Vis-NIR region show the typical bands observed for neodymium chloride (NdCl3) in solution. In the case of films, the intensity ratio between the peaks at 800 nm (4I9/2 -> 4F5/2 + ²H7/2) and 580 nm (4I9/2 -> 4G5/2 + ²G7/2) is equal to 0.83. Infrared spectra present an enhancement in the absorption region of aromatic rings. Site selective luminescence spectroscopy shows that the incorporation of Nd introduces a hipsochromic shift and a line shape definition in UV luminescence compared to PSS film, decreasing the interaction between aromatic groups. In addition, the film exhibits an intense radiative transition at 1061 nm (4F3/2->4I11/2), comparable to the one present in crystalline materials doped with Nd.

Morphological and Magnetic Properties of Superparamagnetic Carbon-Coated Fe Nanoparticles Produced by Arc Discharge.

Spherical carbon coated iron particles of nanometric diameter in the 5-10 nm range have been produced by arc discharge at near-atmospheric pressure conditions (using 5-8·10 4 Pa of He). The particles exhibit a crystalline dense iron core with an average diameter 7.4 ± 2.0 nm surrounded by a sealed carbon shell, shown by transmission electron microscopy (TEM), selected-area diffrac- tion (SAED), energy-dispersive X-ray analysis (STEM-EDX) and electron energy loss spectroscopy (EELS). The SAED, EDX and EELS results indicate a lack of traces of core oxidized phases showing an efficient protection role of the carbon shell. The magnetic properties of the nanoparticles have been investigated in the 5-300 K temperature range using a superconducting quantum interference device (SQUID). The results reveal a superparamagnetic behaviour with an average monodomain diameter of 7.6 nm of the nanoparticles. The zero field cooled and field cooled (ZFC-FC)magnetization curves show a blocking temperature (TB)at room temperature very suitable for biomedical applications (drug delivery, magnetic resonance imaging-MRI-, hyperthermia).