Viveka Alfredsson

Influence of the block length of triblock copolymers on the formation of mesoporous silica

The effect of different block lengths of Pluronic surfactants, (EO)(x)-(PO)(y)-(EO)(x), in the formation of mesoporous silica has been investigated. The syntheses were performed in micellar solution of the surfactant under acidic conditions. The materials were characterized by SAXS, TEM and nitrogen adsorption measurements. The EO-block length of the polymers determines the mesostructure of the silica. For the hexagonal material (SBA-15) the wall thickness is largely dependent on the length of the EO-blocks, while the PO-block length has a great effect on the pore diameter. Furthermore, the PO-block length influences the templating ability, as longer PO-blocks result in more highly ordered domai s and well defined particles. The synthesis temperature also influences these parameters

Deposition of hard crystalline Al2O3 coatings by bipolar pulsed d.c. PACVD

Abstract A bipolar pulsed d.c. PACVD technique was used to deposit crystalline Al2O3 coatings onto WC/Co cemented carbide cutting inserts and onto steel substrates using a gas mixture of AlCl3, H2, O2 and Ar. Different deposition conditions were tested, substrate temperatures (Ts) between 500 and 700°C and a gas mixture ratio O/AlCl3 in the range from 2 to 20. At Ts≥600°C transparent crystalline coatings of the γ-Al2O3 phase were obtained having the best crystallinity for O/Al ratio of 4 and 6. At a substrate temperature of 700°C and an O/AlCl3 ratio of 2, α-Al2O3 or a mixture of α- and γ-Al2O3 phases was formed. The microhardness HV[0.02] of the crystalline coatings was found to be in the range 19–23 GPa. The γ-Al2O3 layers appeared in most cases transparent with smooth surfaces. The wear properties of Al2O3-coated cutting inserts produced by the PACVD method were evaluated and compared with a thermal CVD κ-Al2O3 coating in a machining test in a ball bearing steel. From the test results it could be concluded that the PACVD Al2O3 coatings showed wear properties very similar to the thermal CVD κ-Al2O3 coatings.

High-resolution transmission electron microscopy of mesoporous MCM-41 type materials

High-resolution transmission electron microscopy of MCM-41 shows for the first time clearly the hexagonal shape of the pores, and allows direct observation of the transition from a lamellar to the hexagonal phase.

TEM-tomography of FAU-zeolite crystals containing Pt-clusters.

A method for preparing ultrathin sections (- 20 nm) of inorganic solids has been developed using ultramicrotomy of resin-embedded crystal fragments. Undamaged crystals, oriented along a crystallographic direction, could be imaged with transmission electron microscopy (TEM) at a resolution better than 0.5 nm. The true internal structure of the crystals could be investigated by imaging the second in a series of at least three consecutive ultrathin sections. Such TEM-tomography proved that Pt-ion exchanged FAU zeolite crystals, after reduction and oxidation, are occupied internally and randomly of large platinum clusters mainly in the {111-twin planes. TEM-tomography could be useful in man made nanostructures like semiconductors, epitaxial thin films, hard metal coatings, ceramics, catalysts, and biomaterials.

The effect of stirring on the synthesis of intergrowths of zeolite Y polymorphs

This paper is concerned with the role of organic structure-directing agents during competitive synthesis n of zeolite intergrowths. The system chosen for study is that of the intergrowth between the cubic (FAU) and hexagonal (EMT) polymorphs of zeolite Y. These two polymorphs can be synthesised using crown ether n templates and it has been previously shown that mixtures of crown ethers produce intimate intergrowths of FAU and EMT. It was also previously suggested that concentration gradients of crown ethers at crystal n growing surfaces are responsible for an oscillation of the structure between the two phases rather than a n completely random intergrowth. In this current work we have made a detailed study of comparative syntheses, both stirred and static, in order to assess the role of diffusion of structure-directing agents to a growing crystal n surface. Using a combination of high-resolution electron microscopy, X-ray diffraction and modeling we have established that diffusion-limited transport of structure-directing agents to growing crystal surfaces is an important factor controlling the final product. We also establish the “true” threshold concentration of crown ethers to synthesise each individual phase. This work has n consequences for our general understanding of structure-directed growth of microporous crystals.

Observations on the Role of Crown Ether Templates in the Formation of Hexagonal and Cubic Polymorphs of Zeolite Y

Intergrowths of hexagonal and cubic polymorphs of zeolite Y have been synthesized using crown-ether templates and the manner of the intergrowth between the polymorphs was studied by high-resolution electron microscopy. The images reveal that both polymorphs grow via a layer mechanism and it is suggested that double-six ring units are the key unit for crystal growth. The growing mechanism accounts for surface structures, defect structures and overall particle morphology. Oscillatory growth occurs in syntheses from multiple crown ether preparations whereby the crystal makeup changes from hexagonal to cubic as surface concentrations of crown ethers fluctuate during crystallization. Finally, the structure directing role of the crown ethers is discussed.

The platinum agglomeration in the {111}-twin planes of the zeolite FAU

Abstract The preferred location of platinum clusters in the {111}-twin planes (TP) of zeolite NaY (FAU) has been observed by high-resolution transmission electron microscopy (HRTEM). The platinum was incorporated in the zeolite by ion exchange followed by oxidation and reduction. Samples from each of the three different synthesis stages were examined by HRTEM. Ion exchange provides a homogeneous platinum distribution in the zeolite, and agglomeration of platinum in the TP was only observed after the oxidation. The HRTEM observations of platinum in the TP, from two independent directions, 〈110〉 and 〈211〉, support the fact that the platinum clusters are intrazeolitic. HRTEM observations of ultrathin sections (

Fine Structures of Zeolites: Defects, Interfaces and Surface Structures An HREM Study

Abstract HREM study on fine structures of zeolites, especially defects, interfaces and surface structures is reported by taking examples of LTL, FAU and EMT. Four different boundaries in LTL are shown and an easy way to distinguish them is suggested. It is concluded from observations of atomic resolution surface-profile imaging that double-hexagonal rings act in crystal-growth process as growth-units in FAU and probably in EMT. The first observation of the effect of dealumination on FAU and EMT is reported in atomic scale. Application of HREM to determine the complicated structure, which is impossible to be solved without knowledge of the nature of heavy defects, is also shown for the case of ETS-10 in order to show the advantage of the HREM.