Antonio Carlos Vieira Coelho

Surface area, crystal morphology and characterization of transition alumina powders from a new gibbsite precursor

A new procedure was used to prepare a microcrystalline powder constituted by thin euhedral hexagonal gibbsite plates, 0.2 to 0.6 µm in diameter and 32 nm thick. The powder, fired between 200 and 1000 °C, produced chi and kappa transition aluminas. Alpha-alumina is formed from 1000 °C and recrystallized up to 1500 °C. At 1000 °C, kappa- and alpha-alumina coexisted, but kappa-alumina could only be characterized by SAED. The details of the internal organization of the transition alumina pseudomorphs were clearly observable in TEM due to the great thinness of the I-gibbsite plates. The specific surface area varied from pristine I-gibbsite (24.9 m2.g-1) to chi- and kappa transition aluminas (25.4 m2.g-1) at 1000 °C to alpha-alumina (4.0 m2.g-1) at 1500 °C. The maximum value of specific surface area is 347 m2.g-1 in chi-alumina powder at 300 °C, a difference from Bayer gibbsite, in which the chi-alumina highest surface area is 370 m2.g-1 at 400 °C.

ARGILAS ESPECIAIS: ARGILAS QUIMICAMENTE MODIFICADAS - UMA REVISÃO

The simultaneous use of the specific values of some structural and chemical properties of clay minerals, such as kaolinite, montmorillonite and talc, allows the development of new properties for these materials, especially in relation to the external and internal microcrystal surfaces. These developments are very diversified for montmorillonite, due to the high specific surface area, expansible basal spacings, easy intercalation inside the 2:1 structural layers and a reversible and high cation exchance capacity. The review presents examples of chemical modifications on kaolins, montmorillonites (bentonites) and talcs.

Preparation and Characterization of Cu(II) Phthalocyanine Tetrasulfonate Intercalated and Supported on Layered Double Hydroxides

The aim of the present work was to synthesize and characterize layered doublehydroxides (LDHs), in the magnesium/aluminum form, intercalated with copper(II)phthalocyanine tetrasulfonate (CuPcTs). The metal complex was immobilized intothe LDH gallery region through the reconstitution method and this material wascharacterized by X-ray diffraction (XRD), surface area and porosity measurements,elementary analysis, thermogravimetry (TGA), vibrational (IR) and electronic(UV-visible) spectroscopies, and electronic paramagnetic resonance (EPR). Thecatalytic performance of CuPcTs intercalated and supported on the LDH wasevaluated by carrying out the hydrogen peroxide dismutation. The CuPcTs wassuccessfully intercalated into the LDH layers according to XRD data (the basalspacing of the carbonate precursors increases by approximately 15Å inthe intercalated samples). The surface area and porosity analysis suggested thatthe CuPcTs intercalated materials are not microporous solids. Samples containingthe metal complex confined between the LDH layers have an appreciable thermalstability: decomposition is not observed at least up to 400 °C. TGA experiments also show that the weight-loss curves of the CuPcTs supported samples superimpose those recorded for the CuPcTs complex and the LDH-carbonate while the curves for theintercalated materials are unique. CuPcTs intercalated or supported on LDHs is notactive in the hydrogen peroxide dismutation although the free form shows activity at pH above 8.

Structure, surface area and morphology of aluminas from thermal decomposition of Al(OH)(CH3COO)2 crystals

Crystalline aluminium hydroxiacetate was prepared by reaction between aluminium powder (ALCOA 123) and aqueous solution of acetic acid at 96 ◦ C±1 ◦ C. The white powder of Al(OH)(CH3COO)2 is constituted by agglomerates of crystalline plates, having size about 10 µm. The crystals were fired from 200 ◦ C to 1550 ◦ C, in oxidizing atmosphere and the products characterized by X-ray diffraction, scanning electron microscopy and surface area measurements by BET-nitrogen method. Transition aluminas are formed from heating at the following temperatures: gamma (300 ◦ C); delta (750 ◦ C); alpha (1050 ◦ C). The aluminas maintain the original morphology of the Al(OH)Ac2 crystal agglomerates, up to 1050 ◦ C, when sintering and coalescence of the alpha-alumina crystals start and proceed up to 1550 ◦ C. High surface area aluminas are formed in the temperature range of 700 ◦ C to 1100 ◦ C; the maximum value of 198m 2 /g is obtained at 900 ◦ C, with delta-alumina structure. The formation sequence of transition aluminas is similar to the sequence from well ordered boehmite, but with differences in the transition temperatures and in the development of high surface areas. It is suggested that the causes for these diversities between the two sequences from Al(OH) Ac2 and boehmite are due to the different particle sizes, shapes and textures of the gamma-Al2O3 which acts as precursor for the sequence gamma- to alpha-Al2O3.

Modified electrodes based on mixed bentonite vanadium(V) oxide xerogels

A novel type of composite xerogel can be obtained by the combination of polyvanadic acid and bentonite clay, displaying a characteristic green colour, associated with the presence of vanadium(IV) sites and intervalence-transfer bands in the near-infrared region. The slow evaporation of the xerogel suspensions leads to lamellar solids containing vanadium(V) oxide and silicate layers. In contrast to the vanadium-oxide films, the composite xerogels are not soluble in water, showing a good electrochemical response when applied onto electrode surfaces. Enhanced electrochemical signals are observed in the presence of electroactive species such as the hexaammineruthenium(II) complexes, reflecting their intercalation into the interlamellar space.

Indirect and direct Chapelle's methods for the determination of lime consumption in pozzolanic materials

In this work, comments are made about indirect methods and direct Chapelles method applied to the determination of the reactivity of pozzolanic materials. The Chapelles method is based on the lime-pozzolan reaction and quantifies the pozzolanic reactivity of any material intended to be applied by the cement industry. This lime consumption determination by the pozzolan through this reaction can be made with mass proportions - lime:pozzolan 1:1, as originally proposed by Chapelle or 2:1, as specified in Brazilian and French Standards. Comparative results with both proportions are presented for sugar cane bagasse ash, rice rusk ash, silica fume, fly ash, and metakaolin, commonly studied in our country. Statistical calculations showed that for some materials of similar characteristics to the researched RHA, FA e MK at issue, essays carried out with only 1g of CaO may be underestimating the amounts of CaO consumed per gram of pozzolanic material that can be obtained. Comments were made about the Brazilian and French Standards based on this method and emphasized the expression of lime consumption per mass of the amorphous phase of these materials, as determined by X ray diffraction analysis with the Rietveld refinement method.

Characterization and properties of mixed bentonite-vanadium(V) oxide xerogels

Polyvanadic acid interacts with bentonite clay in aqueous colloidal suspension, yielding a green, flocculent material. The slow evaporation of the xerogel suspensions leads to insoluble lamellar solids, exhibiting basal distances of 13.0 A, and a strong intervalence transfer band around 1150 nm. Scanning electron microscopy images show the existence of interconnected nodules containing a uniform distribution of vanadium ions and aluminum-silicates. Evidence of polyvanadate-bentonite interaction is provided by the FTIR spectra, from the appearance of a strong peak at 835 cm-1, tentatively ascribed to a V-O-Si vibration. The constitution of the green composite depends on the relative amounts of bentonite and polyvanadic acid employed in the synthesis, following the approximate composition bentonite.y(V2O5).z(H2O), where y = 1.5–4.0, and z = 5–12, and bentonite ≈ Nax(Al2-xMgx)(OH)2Si4O10. The dramatic changes in the solubility and aggregation properties corroborate the spectroscopic evidence of strong interactions between polyvanadic acid and the bentonite particles.

Solidification/stabilization of a tannery waste with blended cement and wyoming bentonite

Abstract The solidification/stabilization of a tannery waste, which has a high chromium content, was studied by thermogravimetry (TG), derivative thermogravimetry (DTG) and differential thermal analysis (DTA). Pastes prepared with water, cement, tannery waste and Wyoming bentonite were analyzed after different setting times during the first 28 days of solidification. In the present work it is clear that Wyoming bentonite combined with waste affects cement hydration, while bentonite alone does not affect it significantly. The presence of the waste results in a small acceleration of the cement hydration reactions, probably due to the presence of extra Ca and alkalinity in the waste. When waste is present in the sample during TG and DTG analysis, it is burned to carbon dioxide, which partially reacts with the calcium hydroxide formed during cement hydration. This results in an underestimate of the calcium hydroxide content of the solidified sample. The greater the degree of hydration of the waste‐containing cement sample, the greater the amount of carbon dioxide absorbed.

Microstructures of Building Materials from Huaca De La Luna, Peru

ABSTRACT The microstructures of building materials from the archaeological complex of Huaca de la Luna on the northern coast of Peru is assessed for the first time by a multiscale approach. The multiscale microstructural characterization used laboratory techniques including optical microscopy, scanning electron microscopy, X-ray microtomography, and both gas and powder pycnometry. Optical microscopy revealed the presence of organic matter and shell fragments and X-ray microtomography showed differences in the sizes and connectivity of pores. The BET (N2 gas adsorption) specific surface areas showed greater presence of clayey matrix in the mortars. No significant differences among mortars and brick fragments mineralogy were detected by X-ray diffraction, so the larger amount of fine binding matrix in the mortars suggests that they were produced blending raw material with different grain sizes. The aim of this study is to provide data that can be used for a better understanding of the Moche culture, its construction techniques, and to support conservation strategies of the archaeological heritage.

Spectral homodyne detection, resonator detection, and entanglement in the above-threshold OPO

In the spectral domain, the quantum state of the three beams emitted by an above-threshold optical parametric oscillator encompasses six modes. We use resonator detection to characterize hexapartite entanglement.