Longer Crystallization Time Research Articles

Modification effects of amphiphilic comb-like polysiloxane containing polyether side chains on the PVDF membranes prepared via phase inversion process

Amphiphilic comb-like polysiloxane (ACPS) containing polyether side chains was used as the modification reagent in the preparation of hydrophilic porous poly (vinylidene fluoride) (PVDF) membranes via a phase inversion process. The effects of ACPS on morphology, crystallinity, mechanical properties, reservation of ACPS in the phase inversion process, chemical structure, hydrophilicity and filterability performance of porous PVDF membranes were discussed. It was found that the addition of ACPS would result in the delayed demixing which yields “sponge-like” sublayers and longer crystallization time during the membrane formation process. It was revealed that O/F ratios of the bulk membrane were almost the same as those of the corresponding casting solutions which obviously indicated the high reservation of ACPS in the membrane formation process. The fact that the O/F ratios in the membrane surface layers were much higher than those in the bulk membrane proved the enrichment of ACPS on the surface. The filterability experiments and water contact angle testing proved the hydrophilicity of the blend membranes. Through a schematic model, the mechanism relating the membrane structure and performance was interpreted. From the observed results, it can be concluded that ACPS acts as a potential candidate material for preparing PVDF membranes with extraordinary hydrophilicity and filterability.

Synthesis and characterization of zeolite NaY from rice husk silica

Rice husk silica (RHS) in amorphous phase with 98% purity was prepared from a waste rice husk from rice milling by leaching with hydrochloric acid and calcination. The RHS was used effectively as a silica source for the synthesis of zeolite Y in sodium form (NaY). The zeolite in pure phase was obtained from a two-step synthetic route in which the starting gels were mixed, aged for 24 h at room temperature and crystallized for 24 h at 90 °C. The diameter of single crystal particles from a scanning electron microscope was approximately 1.0 μm, whereas the average particle diameter from laser diffraction particle size analyzer was approximately 10 μm because of the agglomeration of small crystals. Longer crystallization time in this route resulted in a mixed phase containing NaY and zeolite P in sodium form (NaP). In addition, a one-step synthetic route (no aging) was studied and the product was also a mixed phase zeolite.

Orientation Control in Sol–Gel‐Derived BiScO3–PbTiO3 Thin Films

BiScO3–PbTiO3 (BSPT) thin films were fabricated via a sol–gel method on Pt(111)/Ti/SiO2/Si(111) substrates. The effects of different factors on the orientation of the sol–gel‐derived BSPT thin films were investigated. The results showed that a higher lead excess concentration, longer drying time, higher pyrolysis temperature, longer pyrolysis time, higher crystallization temperature, and longer crystallization time could enhance the (100) orientation of the BSPT thin films. Based on the experimental results, a mechanism for the orientation evolution in the sol–gel‐derived BSPT thin films was proposed. The production of the (100) orientation was attributed to the (100)‐oriented PbO nanocrystals forming during the pyrolysis process due to the lattice match.

Variation of the Si/Al ratio in nanosized zeolite Beta crystals

Zeolite Beta nanocrystals were prepared from basic aluminosilicate precursor solutions upon hydrothermal treatment at 100 °C. The Si/Al ratio of the initial system was systematically changed from 25 to infinity in order to study the limits in the framework composition of BEA-type crystallites synthesized from clear basic solutions. Furthermore, the effect of the Si/Al ratio on the precursor species, ultimate crystal size, morphology and yield was investigated. The results revealed that the crystallization kinetics of nanosized Beta are dependent on the amount of Al in the precursor solutions, that is, the nucleation and growth processes are faster in Al-rich systems. The crystallization process of zeolite Beta with Si/Al ratios in the initial solutions of 14, 23 and 32 was accomplished within 72 h, whereas longer crystallization times, 140 and 264 h, were necessary to obtain crystalline products with Si/Al ratios of 42 and infinity, respectively. The intermediates and final products were investigated by complementary techniques such as XRD, HRTEM, DLS, IR, NMR spectroscopy and chemical analysis. Low temperature (77 K) CO adsorption infrared spectroscopy was used to study the Brønsted acidity of zeolite Beta samples with different Si/Al ratios. The properties of Beta nanocrystals important for the design of catalysts and selective separation materials are provided based on the results obtained from the detailed characterization.

Synthesis, Structure, and Reactivity of a New Ti-Containing Microporous/Mesoporous Material

A new class of porous, mixed phase titanosilicate materials containing a microporous TS-1 phase and a mesoporous Ti-MCM-48 phase has been successfully synthesized. A novel, one-pot synthesis method was used in which the organic templates for the mesoporous and microporous phases were added sequentially to the same reaction mixture, followed by crystallization at 150 degrees C. The gemini surfactant 18-12-18 was used to form the Ti-MCM-48 mesophase; subsequent addition of tetrapropylammonium cation (TPA+) led to the formation of TS-1. The relative amounts of the two phases within the final products were controlled by optimizing the crystallization time. Crystallization times between 12 and 50 h gave materials containing both phases, with an increasing amount of microphase formed at longer crystallization times. These materials, called "Ti-MMM-2" (microporous/mesoporous materials) were characterized using powder XRD, N2 physisorption, TEM, FTIR, DR-UV/Vis spectroscopy, and 29Si MAS NMR. In the epoxidation of cyclohexene with tert-butyl hydroperoxide (TBHP), Ti-MMM-2 samples exhibited higher catalytic activity (approximately 61%) than either TS-1 (16%) or Ti-MCM-48 (42%), with a very high selectivity (97%) for formation of cyclohexene oxide.

Influence of Chain Microstructure on the Crystallization Kinetics of Metallocene-Made Isotactic Polypropylene

Highly regioregular isotactic polypropylene samples containing only rr defects of stereoregularity in a wide range of concentration have been obtained with a series of regiospecific metallocene catalysts. A study of the relationships between kinetics of melt-crystallization and the microstructure of polypropylene chains is reported. The equilibrium melting temperatures for isotactic polypropylenes of different stereoregularity have been estimated. The kinetic study has shown that the crystallization rate increases with increasing the concentration of rr defects and decreasing the molecular weight. During isothermal crystallizations from the melt, crystals of α form develop at the beginning of the crystallization, whereas the formation of crystals of γ form is observed only at longer crystallization times. According to the literature, the crystals of γ form are nucleated over the preformed crystals of α form.

Synthesis and characterization of nano-sized zeolites

Synthesis of zeolite ZSM-5 and silicalite-1 by hydrothermal crystallization of clear supersaturated homogeneous synthesis mixture has been achieved. The influence of the templates, synthesis temperature, synthesis time and SiO2/Al2O3 ration on the properties of the nano-sized zeolite has been investigated. The properties of the samples were characterized by X-ray powder diffraction, N2 adsorption/desorption, scanning electron microscopy, and transmission electron microscopy. The results show that the temperature at 170℃ was favorable to synthesize the nano-size zeolites. The use of tetrapropylammonium hydroxide as the template in the hydrothermal synthesis conditions employed was favorable to form nanozeolite than tetramethylammonium hydroxide. The synthesis time also plan an important role. 1 day was required to obtain a fully crystallized product. The longer crystallization time would produce bigger particles. A decrease in Al2O3 content would decrease the crystallite size. The nanosize ZSM-5 and silicalite with crystallite size of 150 nm have been successfully synthesized.

Synthesis of boron-beta zeolite from near-neutral fluoride-containing gels

A borosilicate beta zeolite was prepared from almost neutral fluoride aqueous gels in the presence of 1,4-diazabicyclo[2.2.2]octane and methylamine as the templates. The influence of several physical and chemical parameters was studied. In comparison with the (Si,Al) system, this (Si,B) system appears less ‘reactive’ with longer crystallization times. Moreover, the products obtained display a narrower range of compositions. Only boron-rich (Si/B=2) and fluoride-poor media (F/Si=2) led to well-crystallized samples of beta zeolite characterized by a Si/B molar ratio close to 14. The differences observed between the two systems might be related to a stronger complexation of boron by the fluoride anions. The crystallization of the boron zeolite beta occurs probably through condensation reactions after hydrolysis of fluoroborate (BF 4 −), hydroxyfluoroborate (BF 3OH − and BF 2(OH) 2 −) and fluorosilicate species. Zeolite beta was characterized by XRD, SEM, chemical analysis, thermal analysis and 11B, 19F and 13C MAS NMR spectroscopy. The latter technique shows that the as-made beta samples contain, beside the dabconium cation, a polymeric compound identified as polyethylene piperazine.

Investigation Of Nucleation And Initial Stage Of Gan Growth By Atomic Force Microscopy And X-Ray Diffraction

AbstractThe nucleation and initial stage of GaN growth on sapphire was investigated by atomic force microscopy, X-ray diffraction and photoluminescence. A 15 to 30 nm thick GaN buffer layer deposited at proper conditions was extremely smooth and nearly amorphous. Proper post deposition annealing resulted in the buffer crystallized. The buffer layer deposition temperature, thickness and annealing time and temperature must be coordinated. Low deposition temperature and/or insufficient annealing of the buffer results in a GaN wafer which has fine spiking surface morphology with an RMS of 3.4 nm for 1.4 μm wafer, strong yellow luminescence and wide xray rocking curve FWHM. High deposition temperature, longer crystallization time, and a low growth rate results in a wafer which exhibits strong band edge luminescence without noticeable yellow luminescence, and a narrow (002) diffraction rocking curve. However, the surface morphology exhibits well developed hexagonal feature with RMS roughness of 14.3 nm for a 570 nm thick layer. X-ray rocking curve analysis revealed buffer crystallization, domain coalescence and alignment process. The FWHM of the ω–scan of GaN (101) diffraction was 1700–2000 arc seconds for 200–1400 nm wafers which indicates that the twist of the domains is not changing much with the growth.

Computer simulation of beam crystallization under non-ideal fields and perturbations in a storage ring

In this paper, we have studied the ion beam crystallization in a storage ring, by taking into account a non-ideal field and perturbations, with the help of computer simulations. The results show that crystalline structures can still be obtained with realistic tolerances, though a longer crystallization time seems to be required. Due to close orbit distortions, the crystalline structures obtained are shifted away from the reference orbit. The maximum allowable tolerances for crystalization are also estimated.

Determination of avrami‐exponents of isotactic polypropylene by means of X‐ray methods

AbstractAvrami‐exponents of polypropylene have been measured using two methods: observing the spherulitic growth in the polarization microscope, and measuring the crystallinity as a function of crystallization time using X‐ray wide angle scattering. Both methods yield the same values, showing that the determination of the Avrami‐exponent with optical microscopy is of sufficient accuracy despite the limitations of this method imposed by the restricted crystallization volume between the glass slides. Using the X‐ray method, the Avrami‐exponent of the secondary crystallization volume between the glass slides. Using the X‐ray method, the Avrami‐exponent of the secondary crystallization occurring at longer crystallization times can be measured. It is shown that this type of crystallization takes place within the already existing spherulites, whose size distribution is not changed.

Investigations of the crystallization of Y and ZSM-3 zeolites

The crystallization of the structurally related zeolites Y and ZSM-3 was studied by kinetic experiments with a nucleation gel described in the literature for the synthesis of Y. Self crystallization of this gel resulted at the beginning in a hexagonal phase which changed to a cubic phase after longer crystallization times. The crystallization of Y-zeolite in an appropriate batch was initiated immediately by these gels. The crystallization of ZSM-3 in a Li-containing batch was supported by the initial hexagonal phase. For longer aged gels a preceding recrystallization of the nuclei took place as could be proved by adsorption measurements and studies of the product composition in dependence on the crystallization time. Experiments with different kinds of zeolites as nuclei showed that Y can be nucleated by a series of zeolites containing sixmembered rings via recrystallization processes. For ZSM-3 these processes are mostly too slow for an efficient nucleation.