Phase transformation silica

V. V. Murashov, I. M. Svishchev. Quartz family of silica polymorphs comparative simulation study of quartz, moganite, and orthorhombic silica, and their phase transformations. Phys Rev B 57 5639, 1998. 

The phase transformation behavior of Py-D3C was far simpler than that reported for tetrahydrofuran/N2- and tetrahydrofuran/Xe-D3C in reference 10. We attribute this difference in part to impurities in the samples employed, samples that contained methanol and ethylenediamine according to 13 C CPMAS NMR spectroscopy. We have observed that use of Si(OCH3)4 as a silica source or ethylenediamine as a catalyst in clathrasil synthesis introduces defects that can alter phase transition temperatures by as much as 30 °C and/or introduce new phase transformations. 

When the gel is kept in contact with the pore-filling liquid, its structure and properties keep changing as a function of time. This process is called aging. During the aging period, four processes affect the porous structure and surface area of the silica gel. These are polycondensation, syneresis, coarsening and phase transformation.6,12,13... 

Because the currently used y-alumina is not stable in all acid and basic environments used in industry [2], the development of mesoporous layers other than y-alumina deserves attention as well. Most common materials that can be used for the mesoporous layer are zirconia and ti-tania [3,4], but recently also the preparation of mesoporous hafnia is described [5], Hafnia seems to be a very interesting membrane material, because it can, unlike zirconia and titania, be fired up to 1850°C without a phase transformation of its monoclinic form. Hafnia also has a high chemical resistance toward acid and basic media. Another interesting material, currently under investigation by the group of Brinker is mesoporous silica [6,7], This material is especially interesting because a tailor made morphology and pore-size is possible. 

In the contradistinction to the titania, silica or iron oxide the zirconium hydroxide is more stable than its oxide. From this reason, an important is question, how deep may the hydration process change the surface area and dispersion of the sample. Also, how can it lead to the phase transformations. 

Liu et al. (2002) NMR Mesoporous silica, SBA-1, SBA-3 Phase transformation in synthesis + + n.a. Phase formation, drying... 

The effect of mechanical activation on mullite synthesis from the mixtures of aluminium hydroxide and silica gel was studied in [32,39]. They demonstrated that under heating of nonactivated mixtures, the components undergo phase transformations similar to those observed for individual compounds. Mullite is synthesized at 1400°C. 

Plutonium. Gardner et al. (26) have made a careful high temperature x-ray diflFraction study of the plutonium-oxygen system in the range from room temperature to 900°C. observing diffraction from oxide samples contained in silica capillaries. They review briefly previous work apropos of phase transformations (i.e., thermal and electrical measurements) and construct a phase diagram as shown in Figure 5. 

Further complications occur when some of the phases undergo a phase transformation involving a change in volume. These effects are the cause of anomalous behaviour of the expansion and of a decrease of strength. A typical example is provided by inversions of SiO which is the main phase of silica refractories. The volume changes described in Section /.I arc projected into the expansion curve of silica refractories and also into that of high-silica fireclay. (Expansion curves of various refractories are shown in Fig. 212). 

Bmno E, Pentinghaus H (1974) Substitution of cations in natural and synthetic feldspars. In MacKenzie WS, Zussmarm, J (eds) The Feldspars. Manchester Uitiv Press, Manchester, p 574-609 Buerger MJ (1951) Crystallographic aspects of phase transformations. In Smoluchowski R (ed) Phase Transformations in Solids. Wiley, New York, p 183-211 Buerger MJ (1954) The stuffed derivatives of the silica stiuctures. Am Mineral 39 600-614 Bulenda M, Schwabl F, Tauber UC (1996) Defect-induced condensation and the central peak at elastic phase transitions. Phys Rev B 54 6210-6221... 

The Fischer-Tropsch Synthesis (FTS) converts synthesis gas (a mixture of CO and H,) to hydrocarbons. Iron-based catalysts lose activity with time on stream (TOS). The rate of deactivation is dependent on the presence/absence of promoters such as potassium and/or binders such as silica [1.2]. Several possible causes of catalyst deactivation have been postulated [3] (i) Sintering, (ii) Carbon deposition, and, (iii) Phase transformations. With respect to phase transformations, there is considerable disagreement whether the active phase for the FTS is iron oxide or carbide [4,5]. In addition, certain reactor conditions, such as a high partial pressure of water, are known to cause a decline in activity [6]. 

Zhao[132] demonstrated a simple solvothermal post-treatment method to prepare ordered large-pore (>7nm) FDU-15 laid phase) via phase transformation from 2-D hexagonal mesophases (an SBA-15 prepared by the evaporation-induced self-assembly process). This synthetic strategy of solvothermal post-treatment can be simply performed in many organic solvents such as hexane at 60-100 °C, and be extended to the syntheses of other silica-based mesoporous materials. 

From a view point of reaction time, the typical preparation of mesoporous material can be divided three main steps (1) interaction between surfactant and silica (or other inorganic) species in solution and the formation of ordered mesostmcture (2) the further reaction (polymerization or condensation for silica) at a certain temperature for a time period. A possible phase transformation may occur (3) recovery of solid product by filtration, washing, and drying. The phase transformation may also occur in this step (4) removal of template from the solid product by calcination or extraction with solvent. The phase transformation is also possible even in this step. 

Once those parameters which have more influence on mesophase stability are known, the new synthetic pathways utilizing structural rearrangements to create new materials will become a reality. An investigation of phase transformation11581 under hydrothermal conditions indicated the most mildly basic conditions utilized (pH 9), which favor silica condensation, best inhibit the phase transformation, and thus produce the most kinetically... 

Careful control of the surfactant-water content and the rate of condensation of silica at high alkalinity resulted in hollow tubules 0.3 to 3 pm in diameter.[292] The wall of the tubules consisted of coaxial cylindrical pores, nanometers in size, that are characteristic of those of MCM-41. The formation of this higher-order structure may take place through a liquid-crystal-phase transformation mechanism involving an anisotropic membrane-to-tubule phase change. 

C.C. Landry, S.H. Tolbert, K.W. Gallis, A. Monnier, G.D. Stucky, F. Norby, and J.C. Hanson, Phase Transformations in Mesostructured Silica/Surfactant Composites. Mechanisms for Change and Applications to Materials Synthesis. Chem. Mater., 2001, 13, 1600-1608. 

M.C. Liu, H.S. Sheu, and S.F. Cheng, Drying Induced Phase Transformation of Mesoporous Silica. Chem. Commun., 2002, 2854-2855. 

Intuitively, one might have expected radiation to increase rather than to decrease surface area because damage implies breaking up of crystals into smaller units. There is some evidence for macroscopic effects of this sort as a result of bombardment. The formation of tridymite crystals in silica xerogel (by neutron bombardment) was aptly termed 128) a brutal destruction of the original texture the measured surface area, however, decreased in this case. A phase transformation has also been induced in beryllia by electron bombardment in an electron... 

Calcination of powders in the presence of different gases may induce solid phase transformation, which in turn affects the PZC/IEP. Hydrogen-treated and untreated zirconia were studied in [160], but no substantial shift in CIP was detected. Two titanias were heated in O2 or in H2 at 530 or 6OO C, but no substantial change in lEP or CIP was observed in one sample [161], Dehydration of titania (rutile) as a function of temperature was studied in [162]. The Og of silica was depressed by a factor of 10 by heating at 800°C for 3 hours, further heating (up to 36 hours) did not affect CTq. Rehydration of heated powders for 3-56 days brought about a gradual increase in Og [163], A few examples of different phase transformations induced in the same initial material by calcination at various temperatures are presented in Chapter 3. 

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