Search results for "CRYSTALLIZATION"

Conformation of the galactose ring adopted in solution and in crystalline form as determined by experimental and DFT 1H NMR and single-crystal X-ray …

2003

The solution-state conformations of various galactose derivatives were determined by comparison of the experimental (1)H-(1)H vicinal coupling constants to those calculated using density functional theory (DFT) at the B3LYP/cc-pVTZ//B3LYP/6-31G(d,p) level of theory. The agreement between the experimental and calculated vicinal coupling constants for 1,2:3,4-di-O-isopropylidene-alpha-d-galactopyranose was good, thereby confirming an (O)S(2) skew conformation for it and its derivatives on the basis of their similar observed couplings. Single-crystal X-ray analysis of 1,2:3,4-di-O-isopropylidene-6-O-(3,4,6-tri-O-acetyl-2-deoxy-2-N-phthalimido-beta-d-glucopyranosyl)-alpha-d-galactopyranose and …

Cell inactivation and membrane damage after long-term treatments at sub-zero temperature in the supercooled and frozen states.

2008

The survival of cells subjected to cooling at sub-zero temperature is of paramount concern in cryobiology. The susceptibility of cells to cryopreservation processes, especially freeze-thawing, stimulated considerable interest in better understanding the mechanisms leading to cell injury and inactivation. In this study, we assessed the viability of cells subjected to cold stress, through long-term supercooling experiments, versus freeze-thawing stress. The viability of Escherichia coli, Saccharomyces cerevisiae, and leukemia cells were assessed over time. Supercooled conditions were maintained for 71 days at -10 degrees C, and for 4 h at -15 degrees C, and -20 degrees C, without additives or…

Kristallisations- und schmelzverhalten von copolymeren aus 3,3-bis(chlormethyl)oxetan und ß-propiolacton

1974

Einkristalle von Copolymeren aus 3,3-Bis(chlormethyl)oxetan und s-Propiolacton wurden bezuglich Schmelzverhalten, Schmelzenthalpie, Dichte und Langperiode untersucht. Aus der Messung der Schmelzenthalpie und der Dichte berechnen sich Kristallisationsgrade von 50—60% in guter Ubereinstimmung mit einem Modell, welches den Ausschlus der aus s-Propiolacton (PL) entstandenen Grundbausteine von dem aus Sequenzen der 3,3-Bis(chlormethyl)oxetan (BCMO-)Einheiten aufgebauten Gitter verlangt. Im Vergleich zu reinem Poly-BCMO wird die Dicke der Copolymereinkristalle nur wenig durch Anderung der Kristallisationstemperatur beEinflust. Wegen des blockartigen Aufbaus der Copolymeren bestimmt die mittlere S…

High-Fluence Implantation of Erbium into Silicon-Germanium Alloys: Structural and Thermal Properties

2000

AbstractHigh-quality crystalline Si1-xGex (x=0.10 and 0.25) alloys were implanted with 70 keV Er+ ions at temperatures of 350°C and 550°C to a fluence of 1015 cm−2. In-situ Rutherford backscattering/channeling (RBS) analysis supplemented with transmission electron microscopy (TEM) showed that as-implanted alloys were in form of ternary solid solutions with a peak Er concentration of 1 at.% without any trace of Er-Si or Er-Ge precipitation.In the samples implanted at 350°C Er atoms were found to be distributed randomly in the amorphous host matrix. Post-implantation annealing at different temperatures up to 600° showed that the solid phase epitaxial regrowth of the damaged layers strongly de…

Schmelz- und kristallisationserscheinungen bei makromolekularen substanzen. VI. Ergebnisse der röntgenklein- und weitwinkeluntersuchungen an polyglyk…

1962

Rontgenographische Untersuchungen ergaben, das die Kleinwinkelperiode im Verlauf der Nachkristallisation mit zunehmendem Kristallisationsgrad abnimmt. Ferner steigt nach dem Erhitzen einer bei 140°C auskristallisierten Probe auf eine hohere Temperatur die Kleinwinkelperiode zunachst stark an und nimmt wahrend der anschliesenden Neukristallisation wieder etwas ab. Die Deutung dieser experimentellen Ergebnisse wird diskutiert, wobei der Einflus des Bausteinfaktors berucksichtigt wird. Weitwinkeluntersuchungen zeigten, das bei der isothermen Kristallisation der Kristallisationsgrad zwar zunimmt, die Ordnung in Kettenrichtung aber nicht verbessert wird; diese hangt offenbar nur von der Kristall…

Small changes in particle-size distribution dramatically delay and enhance nucleation in hard sphere colloidal suspensions.

2006

We present hard-sphere crystallization kinetics for three samples with small differences in polydispersity. We show that an increase in polydispersity of 1% is sufficient to cause dramatic changes in the crystallization kinetics: Crystallization is delayed by almost one decade in time and quantitative and qualitative changes in the crystallization scenario are observed. Surprisingly the nucleation rate density is enhanced by almost a factor of 10. We interpret these results in terms of polydispersity limited growth, where local fractionation processes lead to a delayed but faster nucleation.

Two-Step Crystallization Kinetics in Colloidal Hard-Sphere Systems

2005

The crystallization kinetics of colloidal hard spheres was studied using a special Bragg spectrometer with high sensitivity. In contrast with the classical scenario we observe a two-step nucleation process: the number of crystallites increases slowly at early times, followed by a dramatic reduction at intermediate times, prior to undergoing a rapid increase at late times. We explain these results in terms of a polydispersity limited growth of crystallites, where the crystallization at early times is governed by local fractionation processes, leading to a long delay prior to final crystallization.

Schmelz- und kristallisationserscheinungen bei makromolekularen substanzen. I. Kristallisationskinetische untersuchungen an isotaktischem polypropylen

1960

Die Messung der Bruttokristallisation hochpolymerer Substanzen erlaubt erst mit Hilfe der AVRAMIschen Theorie Ruckschlusse auf die Kristallisationskinetik (Keimbildung und Wachstumsgeschwindigkeit). Die Frage, ob die fur niedermolekulare Stoffe entwickelte AVRAMIsche Theorie wirklich auf Hochpolymere ubertragen werden kann, bedarf einer eingehenden Prufung. Es wurden daher mittels mikroskopischer Untersuchungen die Keimbildung und die Wachstumsgeschwindigkeit von isotaktischem Polypropylen in Abhangigkeit von der Kristallisationstemperatur getrennt gemessen und die so ermittelten Werte mit den aus der Bruttokristallisation nach AVRAMI berechneten Zahlen verglichen. Die ubereinstimmung ist v…

Thermal stability and crystallization kinetics of Al4Mn quasicrystals as studied by exoelectron emission and DTA methods

1995

2015

In many technical applications, but also in natural processes like ice nucleation in clouds, crystallization proceeds in the presence of stresses and flows, hence the importance to understand the crystallization mechanism in simple situations. We employ molecular dynamics simulations to study the crystallization kinetics of a nearly hard sphere liquid that is weakly sheared. We demonstrate that shear flow both enhances and suppresses the crystallization kinetics of hard spheres. The effect of shear depends on the quiescent mechanism: suppression in the activated regime and enhancement in the diffusion-limited regime for small strain rates. At higher strain rates crystallization again become…