| 1. |
- Hedlund, Jonas, et al.
(författare)
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Controlling the preferred orientation in silicalite-1 films synthesized by seeding
- 1999
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Ingår i: Microporous and Mesoporous Materials. - 1387-1811 .- 1873-3093. ; 28:1, s. 185-194
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Tidskriftsartikel (refereegranskat)abstract
- Single crystal silicon (100) wafers were seeded with colloidal silicalite-1 crystals and hydrothermally treated in a precursor solution to grow thin silicalite-1 films. A total of 28 experiments in eight series were investigated with SEM and XRD to evaluate the preferred orientation of the crystals constituting the films. The investigated parameters in the film formation process were seed crystal size, amount of adsorbed seed crystals and film thickness after hydrothermal treatment of the seeded substrates. In thin films, most of the crystalline material is oriented with the b-axis perpendicular to the substrate surface. In thick films, most of the crystalline material is oriented with the a-axis perpendicular to the substrate surface. The change in preferred orientation with film thickness is faster when small seeds are used. The amount of adsorbed seeds has a larger influence on the preferred orientation when large seeds are used. A mechanism explaining these trends is proposed. The choice of size and coverage of seeds can be used to control the preferred orientation of the crystals in a film of given thickness within certain limitations.
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| 2. |
- Li, Q., et al.
(författare)
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Nucleation period for TPA-silicalite-1 crystallization determined by a two-stage varying-temperature synthesis
- 1999
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Ingår i: Microporous and Mesoporous Materials. - 1387-1811 .- 1873-3093. ; 31:1, s. 141-150
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Tidskriftsartikel (refereegranskat)abstract
- A two-stage-varying-temperature synthesis procedure, which involves a rapid change in temperature at some point during the course of crystallization, was applied to the synthesis of discrete colloidal particles of TPA-silicalite-1. As the duration of the period at the initial synthesis temperature was extended, the crystal concentration and ultimate crystal size varied until they were approximately equal to those obtained for a complete synthesis at the initial temperature. At this point in the crystallization, it was concluded that the nucleation stage was completed. For syntheses performed at 60, 80 and 100°C, the duration of the nucleation period determined by this method was about 100 h, between 4 and 6 h and less than 2 h, respectively. Thus, nucleation, for this system, is a continuous process, and it was found that the rate of nucleation, which is initially high, declines, throughout the nucleation period. In all cases, nucleation occurred during an induction period when little or no crystal growth was observed, which explains why the syntheses yielded a product with a rather narrow crystal size distribution. If, for the two-stage syntheses, the temperature change was made after completion of the nucleation period, the second synthesis temperature controlled only the linear growth rate of the crystals and the final yield of silicalite-1 obtained
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| 3. |
- Mihailova, B., et al.
(författare)
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Vibrational spectroscopy study of the structure of silicalite-1 films on a gold surface
- 1999
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Ingår i: Microporous and Mesoporous Materials. - 1387-1811 .- 1873-3093. ; 32:3, s. 297-304
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Tidskriftsartikel (refereegranskat)abstract
- The structure of silicalite-1 films grown on seeded gold surfaces is investigated by modelling the observed changes in the infrared reflection absorption (IRRA) spectra of samples treated for different times in the synthesis solution. The results show that a gradual deformation of the five-membered silicon-oxygen rings occurs during the first 11 h of the hydrothermal treatment which leads to breaking of Si---O---Si linkages and to formation of linear defects along the c-axis. Interactions between the dislocations and the grain boundaries during the further growth of the film provoke the appearance of void spaces in the grain boundary interface which may cause incipient cracking in the silicalite-1 films on seeded gold surfaces. The range 1000-1300 cm-1 in the IRRA spectra is found to be appropriate for estimating the quality of silicalite-1 films grown on metal surfaces.
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| 4. |
- Schoeman, Brian J.
(författare)
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Analysis of the nucleation and growth of TPA-silicalite-1 at elevated temperatures with the emphasis on colloidal stability
- 1998
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Ingår i: Microporous and Mesoporous Materials. - 1387-1811 .- 1873-3093. ; 22:1-3, s. 9-22
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Tidskriftsartikel (refereegranskat)abstract
- The question as to whether silicalite-1 grows via an aggregation of smaller particles or similar sized particles has been addressed by considering the fundamentals governing colloidal stability - the quantitative theory underlying colloidal stability being given by the extended Derjaguin-Landau and Verwey-Overbeek (DLVO) theory. Application of the extended DLVO theory to discrete colloidal particles in silicalite-1 precursor sols shows that a net repulsive interactive energy exists between the negatively charged particles. The thermal energy of the colloidal particles (1/2kT at 373 K, the crystallization temperature) is not sufficient to overcome the net repulsive energy barrier. The extended DLVO theory has been applied to two growth scenarios with similar results: growth by aggregation of particles of very different sizes and growth by aggregation of similar sized particles. The significance of these conclusions is that a proposed growth mechanism of MFI type zeolite (growth by aggregation of subcolloidal particles) is deemed not to be a reasonable description of molecular sieve growth. The conclusion that the colloidal crystals are stable with respect to aggregation is supported by experimental observations. The ideas presented in this study are based upon crystallization of molecular sieves from clear solutions in the presence of quaternary ammonium cations that play a significant role in the stabilization of the colloidal crystals. Extension of the ideas presented shows that the extended DLVO theory is equally applicable to wholly inorganic heterogeneous systems.
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| 5. |
- Granqvist, Claes-Göran, et al.
(författare)
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Towards the smart window : progress in electrochromics
- 1997
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Ingår i: Journal of Non-Crystalline Solids. - 0022-3093 .- 1873-4812. ; 218, s. 273-279
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Tidskriftsartikel (refereegranskat)abstract
- Electrochromic devices have the ability to produce reversible and persistent changes of their optical properties. The phenomenon is associated with joint ion and electron transport into/out of an electrochromic thin film, in most cases being a transition metal oxide. This paper outlines the various applications of such devices in smart windows suitable for energy-conscious architecture, in variable-reflectance mirrors, and in display devices. Critical materials issues and design concepts are discussed. The paper also covers two specific research topics: computed electronic structure of crystalline WO3 incorporating ionic species, showing how reflectance modulation emerges from a first-principles calculation; and Li+ dynamics in heavily disordered Ti oxide, illustrating how diffusion constants derived from impedance spectroscopy can be reconciled with the Anderson—Stuart model.
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| 6. |
- Hofmeister, H., et al.
(författare)
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Structure of nanometersized silicon particles prepared by various gas phase processes
- 1998
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Ingår i: Journal of Non-Crystalline Solids. - : Elsevier BV. - 0022-3093 .- 1873-4812. ; 232-234, s. 182-187
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Tidskriftsartikel (refereegranskat)abstract
- We have explored various gas phase processes for the fabrication of nanometersized Si and SiOx particles and measured their structural properties (agglomeration, size, shape, crystallinity, surface roughness and internal structure) by conventional and high resolution electron microscopy. Agglomerated amorphous Si particles, 10-30 nm in size, were prepared by gas phase reactions including cluster growth processes in a low pressure silane plasma. Annealing at 900°C resulted in almost complete crystallisation of nearly spherical particles covered by an amorphous oxide shell. Inert gas arc evaporation of silicon yielded single crystalline, spherical Si particles, 4-16 nm in size, in which no defects were detected. These particles, agglomerated into chains and tangles, are covered entirely by a thin amorphous oxide layer. Thermal evaporation of solid SiO in an inert gas atmosphere produced agglomerated, nearly spherical amorphous SiOx particles, 8-24 nm in size, with considerable surface roughness. Upon annealing at 900°C, the formation of 3-6 nm sized Si crystallites in the interior of these particles was observed.
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