Search results for "Seed crystal"
showing 6 items of 6 documents
Modelling phase transition kinetics of chenodeoxycholic acid with the Runge–Kutta method
2009
Abstract The phase transition kinetics of two chenodeoxycholic acid polymorphic modifications— form I (stable at high temperature), form III (stable at low temperature) and the amorphous phase has been examined under various conditions of temperature and relative humidity. Form III conversion to form I was examined at high temperature conditions and was found to be non-spontaneous, requiring seed crystals for initiation. The formation kinetic model of form I was created incorporating the three-dimensional seed crystal growth, the phase transition rate proportion to the surface area of form I crystals, and the influence of the amorphous phase surface area changes with an empirical stage poin…
Application of enthalpy model for floating zone silicon crystal growth
2017
Abstract A 2D simplified crystal growth model based on the enthalpy method and coupled with a low-frequency harmonic electromagnetic model is developed to simulate the silicon crystal growth near the external triple point (ETP) and crystal melting on the open melting front of a polycrystalline feed rod in FZ crystal growth systems. Simulations of the crystal growth near the ETP show significant influence of the inhomogeneities of the EM power distribution on the crystal growth rate for a 4 in floating zone (FZ) system. The generated growth rate fluctuations are shown to be larger in the system with higher crystal pull rate. Simulations of crystal melting on the open melting front of the pol…
Dry synthesis of B-MFI, MTN- and MTW-type materials
1997
Abstract B-MFI and MTN-type materials were synthesized via the fluoride route from dry powders in the complete absence of a solution phase. Amorphous precursors obtained by drying SiO 2 · Al 2 O 3 gels at 700°C were transformed into MTN-type material in the presence of dried NH 4 F and TMACl. Amorphous precursors obtained by drying SiO 2 gels at 700°C were transformed into B-MFI in the presence of dried NH 4 F, B 2 O 3 and TPABr. Water is formed as a reaction product. However, the water vapor pressure is appreciably below the water vapor saturation pressure under the given reaction conditions. In addition, syntheses using seed crystals were carried out successfully.
Influence of synthesis conditions on the morphology of Dodecasil 1H
1993
Abstract The synthesis of Dodecasil 1H was studied in the system SiO2/1-adamantylamin/NH3/water with the objective of controlling the particle size and the morphology of the crystals. Since nucleation occurs in a very early stage of the synthesis, the heating rate to the final temperature and/or holding points during heat up have a strong influence on both parameters. Crystals smaller than 150 μm could only be obtained by the use of seed crystals, which are completely consumed in the synthesis with subsequent new nucleation. Using seed crystals, the expensive template 1-adamantylamine can be substituted with cheaper reagents such as sodium dodecyl sulfate or trimethylethylammonium bromide a…
Synthesis And Sorptive Properties Of Dodecasil 1 H With An Acessible Pore System
1994
Abstract The synthesis of Dodecasil 1H was studied in the system SiO2/1-adamantylamine/NH3/water with the objective of controlling the particle size and the morphology of the crystals. Since nucleation occurs in a very early stage of the synthesis, the heating rate to the final temperature have a strong influence on both parameters. Using seed crystals and such a heating programme, the expensive template 1-adamantylamine can be reduced to so small amounts, that a nearly template free pore system is achieved. The emptiness of the pore system was demonstrated from adsorption measurement using He and H2.
Determination of trace amounts of β tegafur in commercial α tegafur by powder X-ray diffractometric analysis.
2011
Abstract Objectives The main objective of this work was to develop a suitable analytical technique for determining trace amounts of the thermodynamically stable solid form in bulk samples of metastable form, to a sensitivity of 0.005%–1.0%. Tegafur (5-fluoro-1-(tetrahydro-2-furyl)-uracil) α and β crystalline forms were used as a model for this problem. Methods The trace content of the thermodynamically stable β polymorphic form in tegafur samples was increased by promoting phase transition from the bulk of thermodynamically metastable α form to β form, and achieving sufficient β form content for a quantitative powder X-ray diffractometry (PXRD) analysis. The phase transition was stimulated …