Surface Crystal Nucleation and Growth in Poly (ε-caprolactone): Atomic Force Microscopy Combined with Fast Scanning Chip Calorimetry

By using an atomic force microscope (AFM) coupled to a fast scanning chip calorimeter (FSC), AFM-tip induced crystal nucleation/crystallization in poly (ε-caprolactone) (PCL) has been studied at low melt-supercooling, that is, at a temperature typically not assessable for melt-crystallization studies. Nanogram-sized PCL was placed on the active/heatable area of the FSC chip, melted, and then rapidly cooled to 330 K, which is 13 K below the equilibrium melting temperature. Subsequent isothermal crystallization at this temperature was initiated by a soft-tapping AFM-tip nucleation event. Crystallization starting at such surface nucleus led to formation of a single spherulite within the FSC sample, as concluded from the radial symmetry of the observed morphology. The observed growth rate in the sub-micron thin FSC sample, nucleated at its surface, was found being much higher than in the case of bulk crystallization, emphasizing a different growth mechanism. Moreover, distinct banding/ring-like structures are observed, with the band period being less than 1 µm. After crystallization, the sample was melted for gaining information about the achieved crystallinity and the temperature range of melting, both being similar compared to much slower bulk crystallization at the same temperature but for a much longer time.

Entropy-Driven Heterogeneous Crystallization of Hard-Sphere Chains under Unidimensional Confinement

We investigate, through Monte Carlo simulations, the heterogeneous crystallization of linear chains of tangent hard spheres under confinement in one dimension. Confinement is realized through flat, impenetrable, and parallel walls. A wide range of systems is studied with respect to their average chain lengths (N = 12 to 100) and packing densities (ϕ = 0.50 to 0.61). The local structure is quantified through the Characteristic Crystallographic Element (CCE) norm descriptor. Here, we split the phenomenon into the bulk crystallization, far from the walls, and the projected surface crystallization in layers adjacent to the confining surfaces. Once a critical volume fraction is met, the chains show a phase transition, starting from regions near the hard walls. The established crystal morphologies consist of alternating hexagonal close-packed or face-centered cubic layers with a stacking direction perpendicular to the confining walls. Crystal layer perfection is observed with an increasing concentration. As in the case of the unconstrained phase transition of athermal polymers at high densities, crystal nucleation and growth compete with the formation of sites of a fivefold local symmetry. While surface crystallites show perfection with a predominantly triangular character, the morphologies of square crystals or of a mixed type are also formed. The simulation results show that the rate of perfection of the surface crystallization is not significantly faster than that of the bulk crystallization.

Crystallization Behavior and Optical Properties of Glass-Ceramics Containing Nano-Diopside Crystals

Diopside is a ceramic material with excellent properties including a low dielectric constant, high thermal conductivity, low sintering temperature below 1000 °C, and high mechanical strength. It has been applied to wireless and optical communications, substrates for touch panels, lenses for UV-LED, building materials, and so on. In this study, glass-ceramics containing nano-sized diopside crystals were fabricated, and their transmittance at visible light and photoluminescence were evaluated. In particular, TiO2 was added as a nucleating agent to suppress the surface crystallization phenomenon and Mn was used as a dopant to emit red light. The glass-ceramics were prepared by heat treatment at a temperature lower than the maximum crystal growth temperature (TP) calculated from the non-isothermal analysis method using differential thermal analysis (DTA) for the formation of nano-sized crystals. For glass containing 20 wt% of TiO2, the Avrami constant was calculated to be 2.23 and the activation energy required for crystal growth to be 549 kJ/mol, reflecting typical bulk crystallization behavior. Glass-ceramics with high light transmittance up to 70% were obtained by inducing the bulk crystallization behavior, and the diopside crystal size was less than 10 nm, which was equal to or higher than that of commercialized transparent glass-ceramic products. Glass-ceramic specimens doped with Mn showed luminescence of 736∼766 nm wavelength at excitation light of 365 nm wavelength. The emission peak intensity increased with the amount of dopant added, but gradually decreased with increasing crystallinity of the diopside phase.

From bulk crystallization of inorganic nanoparticles at the air/water interface: tunable organization and intense structural colors

With minute amounts of a surfactant, a variety of nanoparticles self-assemble at the air/water interface into optically active crystalline 2D structures.

A detailed description of the devitrification mechanism of d-mannitol

The transformation of undercooled d-mannitol into “phase X” previously interpreted as a second amorphous state is actually corresponding to a surface crystallization accompanied by a very slow bulk crystallization into α form.

Kinetics of sitallization of semiconducting AsSe1.5Snx (x = 0,13, 0,20) glasses

Ранее [Школьников, 2014] исследованы структурно-химические особенноcти полупроводниковых стекол AsSe1.5Snx, склонных к ситаллизации (равномерной объемной кристаллизации). Стекла с 5 и 7.4 ат.% Sn (х = 0,13 и 0,20) синтезировали методом вакуумной плавки, как правило, из особо чистых элементных веществ при различных температурах в интервале 700−950 С с последующей закалкой ампул с расплавами в воздухе. Методами 119Sn мёссбауэровской спектроскопии, рентгенофазового анализа, измерения плотности и микротвердости закаленных образцов исследована кинетика ступенчатых превращений при изотермической cиталлизации в интервале температур 210−310 С. Анализ кинетики валовой объемной кристаллизации стекол выполнен по данным измерения плотности с использованием уравнения Колмогорова–Аврами, обобщенного на ступенчатые и неполные изотермические превращения. Установлено, что на первой ступени изотермической ситаллизации стекол в низкотемпературном интервале 210−255 С преобладают гомогенное зарождение и трехмерный рост тонкодисперсных кристаллов фазы SnSe, инициирующей на второй ступени гетерогенное зарождение и двумерный рост кристаллов основной кристаллохимическиподобной фазы As2Se3. Реконструктивная кристаллизация исследованных стекол связана с непрерывным изменением химического состава и описывается интервалом значений энергии активации. При температурах 260−310 С на первой ступени выделяется смесь фаз SnSe и SnSe2 с преобладанием фазы SnSe на начальных стадиях, а выделение основной кристаллической фазы As2Se3 сильно замедлено или не фиксируется. In the article [Shkolnikov, 2014], structural-chemical features of AsSe1.5Snx semiconducting glasses, prone to sitallization (uniform bulk crystallization), were investigated. Glasses with 5 and 7.4 at. % Sn (x = 0.13 and 0.20) were synthesized by vacuum melting, usually from extremely pure elemental substances at various temperatures in the range of 700-950 C followed by quenching ampoules with melts in air. The kinetics of stepwise transformations during bulk isothermal crystallization of AsSe1.5Snx glasses has been studied in the temperature range of 210−310 °С using 119Sn Mȍssbauer spectroscopy, x-ray phase analysis, and the density and microhardness measurements of the quenched samples. The kinetics of the gross bulk crystallization of glasses have been analyzed according to the data on density measurement using the Kolmogorov–Avrami equation, which was generalized on stepwise and incomplete isothermal transformations. It was found that the first stage of isothermal sitallization of glasses in the low-temperature range of 210–255 С is dominated by homogeneous nucleation and three-dimensional growth of finely dispersed SnSe phase crystals, which initiate heterogeneous nucleation and two-dimensional growth of crystals of the main crystallochemically similar phase of As2Se3 at the second stage. Reconstructive crystallization of the investigated glasses is associated with a continuous change in the chemical composition and is described by an interval of values of the activation energy. At the temperatures of 260–310 °C the first step separates a mixture of SnSe and SnSe2 phases with the predominance of the SnSe phase in the initial stages, and the precipitation of the basic crystalline phase of As2Se3 is strongly retarded or not fixed.