Helena de Souza Santos

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.

Pseudomorphic formation of aluminas from fibrillar pseudoboehmite

Abstract Bundles of microfibrils of the aluminum monohydroxide (pseudoboehmite) prepared by the thermal aging of aluminum dihydroxide monoacetate sols change into pseudomorphs of microcrystals of γ-Al 2 O 3 after firing at 400°C; after firing at 700°C, the pseudomorphs are of δ-Al 2 O 3 ; firing at 900°C gives pseudomorphs of θ-Al 2 O 3 and at 1100°C of α-Al 2 O 3 . This sequence is similar to well crystallized boehmite.

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.

The ultrastructure of the mucous granules from starfish tube feet

The ultrastructure of mucus-secreting cells of the starfish Asterina stellifera was studied by electron microscopy. Details of the internal structure of the mucous granules were clearly resolved in the electron micrographs: the granules, of ellipsoidal shape, are limited by a distinct membrane and display in their interior, at the central part, a bundle of hexagonal rods organized in a regular hexagonal array. It is suggested that these hexagonal rods are the site of the mucoprotein in the granule of this type of mucus-secreting cells.

Characterization of the aluminum hydroxide microcrystals formed in some alcohol–water solutions

Abstract The crystallization in water of non-crystalline Al(OH)3 to form somatoids of aluminum trihydroxide bayerite is altered into other aluminum hydroxides when certain water-soluble alcohols are added. The crystalline and non-crystalline phases formed under the different environments were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray elemental microanalysis by using EDS/TEM and scanning electron microscopy (SEM). Ethylene glycol leads to nordstrandite and fibrillar pseudoboehmite. Methanol produces gibbsite and fibrillar pseudoboehmite. Ethanol has different effects according to its concentration: 20% produces bayerite+nordstrandite+fibrillarpseudoboehmite, 50% yields gibbsite+bayerite+pseudoboehmite, but 94% yields only fibrillar pseudoboehmite. Glycerol leads only to fibrillar pseudoboehmite. Isopropyl alcohol produces bayerite and nordstrandite. The presence of pseudoboehmite in the several systems suggests that the monohydroxide acts as an intermediate in the crystallization of the aluminum trihydroxides during aging in those alcohol–water solutions.

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.

Ultrastructural and histochemical studies on the epithelium revestment layer in the tube feet of the starfish Asterina stellifera

The “cuticle,” which revests the starfish tube foot, has been studied by electron microscopy and the findings correlated with histochemical observations.

Thermal transformation of synthetic euhedral and fibrillar crystals of boehmite into aluminas

Abstract Fibrillar pseudoboehmite and euhedral lath-like boehmite aqueous sols were prepared from the reaction of aluminium secondary butoxide and acetic acid in order to have a solid content of 70g/1 of AlOOH. The dried solids were fired from 200 °C to 1500 °C and examined by TEM, SEM, SAED, and XRD to compare formation of aluminas and their crystal morphological changes at the same temperatures. It was observed that the sequence of transformations of fibrillar pseudoboehmite is closer to that for euhedral boehmite than to the “gelatinous boehmite” described in the literature.

Ultrastructure of a mucous gland cell found in the tube feet of the starfishAsterina stellifera

Es describida la ultraestrutura de un segundo tipo de célula glandular mucosa que es encontrada en el pié ambulacrario de la estrella del marAsterina stellifera. Los gránulos secretores de esta célula tienen en su interior una ultraestrutura en forma de pequeños puntos densos y delicadas fibrillas, mas faltando una organizatión, principalmente comparada con la organizatión del otro tipo de granulo mucoso, también encontrado en el mismo pié ambulacrario.

Caulins brasileiros: alguns aspectos da geologia e da mineralogia

The size of Brazil and the diversity of geology is reflected in varying environments in which kaolinite assemblages have been formed. The geological environments of the Brazilian kaolin may be divided into the following groups - sedimentary kaolin, kaolin derived from pegmatite, from granitic rocks, from volcanic rocks and kaolin derived from anorthosite. The sedimentary clays are mainly found in the Amazon basin and those adjacent to the Jari River are being exploited commercially for export as a paper coating clay. Amazon kaolin is characterised by high iron and titania (lattice-held) with low levels of alkali and exhibiting euhedral kaolinite crystals. The South-eastern pegmatite, when not iron stained, are extremely low in iron and titania and a mixture of kaolinite 7A/10A-halloysite occurs in all deposits. The North-eastern pegmatite produces kaolin constituted only by euhedral kaolinites with an absence of halloysite. Kaolin from granites generally has higher iron levels when compared with pegmatite and deposits constituted only by kaolinite are rare, a mixture of kaolinite/7A-halloysite being common. Both the pegmatite and granite derived kaolin are utilised as a paper filler and in general ceramics. Volcanic rocks on alteration produce a fine siliceous clay with titania levels higher than other types and are generally mixtures of kaolinite-7A. Volcanic derived clays are utilized locally in ceramics. Kaolins derived from anorthosite are similar in iron and titania levels to those from granitic kaolins. Assemblages of kaolinite and small quantities of 7A-halloysite are found. These clays are used in both whiteware ceramics and paper filler.