Showing papers on "Crystallization published in 2011"

Ferroelectricity in hafnium oxide thin films

Abstract: We report that crystalline phases with ferroelectric behavior can be formed in thin films of SiO2 doped hafnium oxide. Films with a thickness of 10 nm and with less than 4 mol. % of SiO2 crystallize in a monoclinic/tetragonal phase mixture. We observed that the formation of the monoclinic phase is inhibited if crystallization occurs under mechanical encapsulation and an orthorhombic phase is obtained. This phase shows a distinct piezoelectric response, while polarization measurements exhibit a remanent polarization above 10 μC/cm2 at a coercive field of 1 MV/cm, suggesting that this phase is ferroelectric. Ferroelectric hafnium oxide is ideally suited for ferroelectric field effect transistors and capacitors due to its excellent compatibility to silicon technology.

The kinetics and mechanisms of amorphous calcium carbonate (ACC) crystallization to calcite, via vaterite.

Abstract: The kinetics and mechanisms of nanoparticulate amorphous calcium carbonate (ACC) crystallization to calcite, viavaterite, were studied at a range of environmentally relevant temperatures (7.5–25 °C) using synchrotron-based in situ time-resolved Energy Dispersive X-ray Diffraction (ED-XRD) in conjunction with high-resolution electron microscopy, ex situX-ray diffraction and infrared spectroscopy. The crystallization process occurs in two stages; firstly, the particles of ACC rapidly dehydrate and crystallize to form individual particles of vaterite; secondly, the vaterite transforms to calcitevia a dissolution and reprecipitation mechanism with the reaction rate controlled by the surface area of calcite. The second stage of the reaction is approximately 10 times slower than the first. Activation energies of calcite nucleation and crystallization are 73 ± 10 and 66 ± 2 kJ mol−1, respectively. A model to calculate the degree of calcite crystallization from ACC at environmentally relevant temperatures (7.5–40 °C) is also presented.

Petrolog3: Integrated software for modeling crystallization processes

Abstract: This paper introduces Petrolog3, software for modeling (1) fractional and equilibrium crystallization, (2) reverse fractional crystallization at variable pressure, melt oxidation state and melt H2O contents, and (3) postentrapment reequilibration of melt inclusions in olivine. Petrolog3 offers an algorithm that allows calculations with a potentially unlimited number of (1) mineral-melt equilibrium models for major and trace elements and (2) models describing melt physical parameters such as density and viscosity, melt oxidation state, and solubility of fluid components in silicate melts. The current version of the software incorporates 46 mineral-melt equilibrium models for 8 minerals; 3 models describing distribution of trace elements between minerals and melt; 4 models of melt oxidation state; 1 model for H2O solubility in silicate melts; and 4 models describing melt density and viscosity. The idea behind the program is to provide the community of igneous petrologists and geochemists with a user-friendly interface for using any combinations of available mineral-melt equilibrium models for computer simulation of the crystallization process.

Crystallization Pathways in Biomineralization

Abstract: A crystallization pathway describes the movement of ions from their source to the final product. Cells are intimately involved in biological crystallization pathways. In many pathways the cells utilize a unique strategy: They temporarily concentrate ions in intracellular membrane-bound vesicles in the form of a highly disordered solid phase. This phase is then transported to the final mineralization site, where it is destabilized and crystallizes. We present four case studies, each of which demonstrates specific aspects of biological crystallization pathways: seawater uptake by foraminifera, calcite spicule formation by sea urchin larvae, goethite formation in the teeth of limpets, and guanine crystal formation in fish skin and spider cuticles. Three representative crystallization pathways are described, and aspects of the different stages of crystallization are discussed. An in-depth understanding of these complex processes can lead to new ideas for synthetic crystallization processes of interest to mate...

Enhanced Thermal Conductivity in a Nanostructured Phase Change Composite due to Low Concentration Graphene Additives

Abstract: The liquid−solid phase change enthalpy, crystallization, and thermal conductivity of graphene/1-octadecanol (stearyl alcohol) composite, a nanostructured phase change material, was investigated as a function of graphene content. The thermal conductivity (κ) of the nanocomposite increased by nearly 2.5-fold (∼140% increase) upon ∼4% (by weight) graphene addition while the drop in the heat of fusion (i.e., storage capacity) was only ∼15.4%. The enhancement in thermal properties of 1-octadecanol obtained with the addition of graphene is markedly superior to the effect of other nanofillers such as silver nanowires and carbon nanotubes reported previously in the literature. Boosting the thermal conductivity of organic phase change materials without incurring a significant loss in the heat of fusion is one of the key issues in enabling their practical application as latent heat storage/release units for thermal management and thermal protection.

Water crystallization and its importance to freezing of foods: A review

Abstract: In this review, different aspects of water crystallization including modelling approaches, process evaluation methods and the effect of novel freezing techniques is presented. There are different methods available to explain the nucleation and growth of crystals. The characteristics of ice crystals are studied by light and electron microscopy methods for many years, and recently a number of novel methods including magnetic resonance imaging, X-ray analysis, and infrared spectroscopy are employed. Several emerging techniques are developed to improve the crystallization of water during freezing, including ultrasound assisted freezing, high pressure freezing, ice nucleating proteins, and supersession of nucleation. Understanding the mechanisms of these new techniques and their relationship to the crystallization phenomenon can be helpful for improving freezing processes.

Cubic crystals from cubic colloids

Abstract: We have studied the crystallization behavior of colloidal cubes by means of tunable depletion interactions. The colloidal system consists of novel micron-sized cubic particles prepared by silica deposition on hematite templates and various non-adsorbing watersoluble polymers as depletion agents. We have found that under certain conditions the cubes self-organize into crystals with a simple cubic symmetry, which is set by the size of the depletant. The dynamic of crystal nucleation and growth is investigated, monitoring the samples in time by optical microscopy. Furthermore, by using temperature sensitive microgel particles as depletant it is possible to fine tune depletion interactions to induce crystal melting. Assisting crystallization with an alternating electric field improves the uniformity of the cubic pattern allowing the preparation of macroscopic (almost defect-free) crystals that show visible Bragg colors.

NaSr3Be3B3O9F4: A Promising Deep‐Ultraviolet Nonlinear Optical Material Resulting from the Cooperative Alignment of the [Be3B3O12F]10− Anionic Group

Abstract: Colorless crystals of the title compound as large as 20 x 20 x 10 mm are prepared by spontaneous crystallization using B2O3 and NaF as flux media

Crystallization kinetics of poly(lactic acid)-talc composites

Abstract: The crystallization kinetics of poly(lactic acid) / talc composites were determined over a range of 0 to 15 wt% of talc. Talc was found to change the crystallization kinetics. The presence of talc increases the crystallization rate and this increase is related to talc concentration and to crystallization temperature. In order to understand the effect of talc and PLA crystallinity on mechanical properties, dynamic mechanical thermal analyses were performed on poly(lactic acid) / talc composites before and after an annealing process. It was demonstrated that the presence of crystals improves thermo- mechanical properties but in order to achieve good results at high temperatures the reinforcing effect of a filler such as talc is necessary.

Hydrothermal Etching Assisted Crystallization: A Facile Route to Functional Yolk-Shell Titanate Microspheres with Ultrathin Nanosheets-Assembled Double Shells

Abstract: We report a facile "hydrothermal etching assisted crystallization" route to synthesize Fe(3)O(4)@titanate yolk-shell microspheres with ultrathin nanosheets-assembled double-shell structure. The as-prepared microspheres possess a uniform size, tailored shell structure, good structural stability, versatile ion-exchange capability, high surface area, large magnetization, and exhibit a remarkable catalytic performance.

Na2CsBe6B5O15: an alkaline beryllium borate as a deep-UV nonlinear optical crystal.

Abstract: A new deep-UV nonlinear optical crystal, Na2CsBe6B5O15, has been grown through spontaneous crystallization with a molten flux based on Na2O–Cs2O–B2O3. Na2CsBe6B5O15 contains two-dimensional alveolate beryllium borate layers [Be2BO5]∞ that are bridged via planar [BO3] groups. UV–vis diffuse reflectance analysis on a powder sample of Na2CsBe6B5O15 indicates that the short-wavelength absorption edge of Na2CsBe6B5O15 is below 200 nm. Second-harmonic generation (SHG) on the powder sample was measured with a 1064 nm laser using the Kurtz and Perry technique, which showed that Na2CsBe6B5O15 is a phase-matchable material, and its measured SHG coefficient is ∼1.17 times as large as the d36 coefficient of potassium dihydrogen phosphate. The relatively larger SHG coefficient of Na2CsBe6B5O15 originates from its shorter distance between the adjacent layers bridged via the small [BO3] groups.

Polymer Crystallization of Partially Miscible Polythiophene/Fullerene Mixtures Controls Morphology

Abstract: Mixtures of polythiophene and fullerene are intensely studied for organic photovoltaic applications. Control of nanoscale morphology of these materials is critical for device performance, but characterization and understanding of this morphology and how it arises is lacking. We use energy-filtered transmission electron microscopy (EFTEM) to obtain high-contrast images of P3HT nanocrystals in organic semiconductor mixtures. Grazing-incidence small-angle X-ray scattering correlates well with the length scales obtained from EFTEM images; we combine the two techniques to follow the morphology evolution under different material processing histories. EFTEM also measures local P3HT concentration in PCBM-rich regions, proving that these components are partially miscible. We determine the P3HT-PCBM χ parameter and Flory–Huggins phase diagram, which predicts miscibility for P3HT volume fractions above 0.42. This miscibility suppresses fullerene crystallization. The nanoscale morphology in these materials, critical ...

Nucleation of the Electroactive γ Phase and Enhancement of the Optical Transparency in Low Filler Content Poly(vinylidene)/Clay Nanocomposites

Abstract: Poly(vinylidene fluoride), PVDF, based nanocomposites with different clay structures have been processed by solvent casting and melt crystallization. Depending on the melting temperature of the polymer, the nanocomposite recrystalises in the electroactive γ or nonelectroactive α phase of the polymer. This fact is related to the thermal behavior of the clay. For montmorillonite clay, the full crystallization of the electroactive γ phase occurs for clay contents lower than 0.5 wt %, allowing the nanocomposites to maintain the mechanical properties of the polymer matrix. The electroactivity of the material has been proven by measuring the piezoelectric d33 response of the material. The obtained value of d33 is −7 pC/N, lower than in β-PVDF obtained by mechanical stretching but still among the largest coefficients obtained for polymers. Further, the optical transmittance in the visible range is strongly enhanced with respect to the transmittance of the pure polymer. Finally, it is demonstrated that the nuclea...

Real-Time Investigation of Crystallization and Phase-Segregation Dynamics in P3HT:PCBM Solar Cells During Thermal Annealing

Abstract: Crystallization and phase segregation during thermal annealing lead to the increase of power-conversion efficiency in poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk-heterojunction solar cells. An understanding of the length and time scale on which crystallization and phase segregation occur is important to improve control of the nanomorphology. Crystallization is monitored by means of grazing incidence X-ray diffraction in real time during thermal annealing. Furthermore, the change in film density is monitored by means of ellipsometry and the evolution of carrier mobilities by means of field effect transistors, both during annealing. From the combination of such measurements with those of device performance as a function of annealing time, it is concluded that the evolution of microstructure involves two important time windows: i) A first one of about 5 minutes duration wherein crystallization of the polymer correlates with a major increase of photocurrent; ii) a second window of about 30 minutes during which the aggregation of PCBM continues, accompanied by an increase in the fill factor.

In-situ monitoring of the formation of crystalline solids.

Abstract: The processes occurring during the early stages of the formation of crystalline solids are not well understood thus preventing the rational synthesis of new solids. The investigation of the structure-forming processes is an enormous challenge for both analytical and theoretical methods because very small particles or aggregates with different chemical composition and different sizes must be probed, both before and during nucleation. Furthermore, these precursors are present in a complex and dynamic equilibrium. This Review gives a survey of the in-situ methods available for the study of the early stages of crystallization of solids and how they can help in the synthesis of metastable polymorphs, of transient intermediates, and/or precursors displaying new or improved properties. Examples of actual research demonstrate the necessity and potentials but also the limitations of in-situ monitoring of the formation of crystalline solids.

Additive controlled crystallization

Abstract: Additives play a decisive role in crystallization processes. We survey the ongoing efforts in the realm of additive-controlled crystallization. Emphasis is placed on the commonly used and most studied types of additives and their influences on prenucleation, stabilization of amorphous precursors, nucleation, crystallization, chiral resolution, nanocrystal assembly, polymorph, and thus morphologies. We will highlight three types of nonclassical trajectories adopted by additive controlled crystallization, including oriented attachment, mesocrystals, and biomorphs.

Competition between Fullerene Aggregation and Poly(3-hexylthiophene) Crystallization upon Annealing of Bulk Heterojunction Solar Cells

Abstract: Concomitant development of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) aggregation and poly(3-hexylthiophene) (P3HT) crystallization in bulk heterojunction (BHJ) thin-film (ca. 85 nm) solar cells has been revealed using simultaneous grazing-incidence small-/wide-angle X-ray scattering (GISAXS/GIWAXS). With enhanced time and spatial resolutions (5 s/frame; minimum q ≈ 0.004 A–1), synchrotron GISAXS has captured in detail the fast growth in size of PCBM aggregates from 7 to 18 nm within 100 s of annealing at 150 °C. Simultaneously observed is the enhanced crystallization of P3HT into lamellae oriented mainly perpendicular but also parallel to the substrate. An Avrami analysis of the observed structural evolution indicates that the faster PCBM aggregation follows a diffusion-controlled growth process (confined by P3HT segmental motion), whereas the slower development of crystalline P3HT nanograins is characterized by constant nucleation rate (determined by the degree of supercooling and PCBM demixing)....

Dynamics of Crystallization and Disorder during Annealing of P3HT/PCBM Bulk Heterojunctions

Abstract: A real-time crystallographic analysis of P3HT/PCBM films during thermal annealing is reported, detailing the temporal variation of crystallization, disorder, and orientational spread during the annealing. Five P3HT/PCBM chlorobenzene solutions with different P3HT concentrations (0, 33, 50, 67, 100 wt %) were spin coated on SiO2 substrates. The thick films (100 nm) were studied during annealing (50 min at 140 °C), with in situ synchrotron grazing incidence X-ray diffraction (GI-XRD) and a sampling time <8 s. For the first time, the evolution of the crystal structure is analyzed taking into account P3HT paracrystallinity. The following were observed: a predominance of edge-on P3HT lamellae in the as-spun and annealed films; changes in concentration-dependent edge-on lamellar orientation spread along the alkyl-stacking direction and paracrystalline disorder after annealing; a permanent lamellar stretching just along the alky-stacking direction after annealing; an increase in the P3HT domain size along only the alkyl-stacking direction for the edge-on lamellae, with dynamics consistent with PCBM acting as a plasticizer for P3HT; and finally, an increase in the PCBM concentration at the sample−air interface. We show that the paracrystalline correction is important to calculate correctly the domain size as deduced from GI-XRD.

Microscopically porous, interconnected single crystal LiNi1/3Co1/3Mn1/3O2 cathode material for Lithium ion batteries

Abstract: A porous cathode material consisting of interconnected single crystal LiNi1/3Co1/3Mn1/3O2 (NCM) nanoparticles has been synthesized for lithium ion batteries. Trace nitric acid is used as pH value adjuster to form honeycomb-shaped foam, and a novel stepwise crystallization process is employed to obtain NCM particles. The modified sol–gel process followed by an optimized crystallization process results in significant improvements in chemical and physical characteristics of the NCM particles. They consist of a fully-developed single crystal NCM with uniform composition and a porous NCM architecture with a reduced degree of fusion and a large specific surface area. These structural modifications in turn significantly enhance the electrochemical properties of the NCM cathode material, in terms of improved charge/discharge capacity, cyclic stability and rate capability.

Beating crystallization in glass-forming metals by millisecond heating and processing.

Abstract: The development of metal alloys that form glasses at modest cooling rates has stimulated broad scientific and technological interest. However, intervening crystallization of the liquid in even the most robust bulk metallic glass-formers is orders of magnitude faster than in many common polymers and silicate glass-forming liquids. Crystallization limits experimental studies of the undercooled liquid and hampers efforts to plastically process metallic glasses. We have developed a method to rapidly and uniformly heat a metallic glass at rates of 10^6 kelvin per second to temperatures spanning the undercooled liquid region. Liquid properties are subsequently measured on millisecond time scales at previously inaccessible temperatures under near-adiabatic conditions. Rapid thermoplastic forming of the undercooled liquid into complex net shapes is implemented under rheological conditions typically used in molding of plastics. By operating in the millisecond regime, we are able to “beat” the intervening crystallization and successfully process even marginal glass-forming alloys with very limited stability against crystallization that are not processable by conventional heating.

Strain-induced crystallization of natural rubber: a review of x-ray diffraction investigations

Abstract: Strain-induced crystallization of natural rubber was discovered in 1925 by the means of x-ray diffraction and has been widely investigated by this technique until today. The studies devoted to the structure of the crystalline phase of natural rubber are first reviewed. This structure is strongly anisotropic and can be related to the exceptionally good strength and fatigue properties of this material. The relationships between strain-induced crystallization of natural rubber and its mechanical response, during static or tension-retraction tests, are also reviewed and discussed; in particular, the hysteresis of the stress-strain curve is mainly explained by strain-induced crystallization. The kinetics of crystallization under both static and cyclic deformation is also discussed, as well as the influence of different factors, depending either on material composition (crosslink density, carbon black fillers) or on external parameters (temperature, strain rate…).

Crystallization Behavior of Carbon Nanotube−Polylactide Nanocomposites

Abstract: The crystallization kinetics and the resulting structure and morphology of polylactide (PLA) were investigated in the presence of carbon nanotubes (CNTs). Nanocomposite samples prepared by solution and melt mixing present homogeneous filler dispersion, as observed by scanning electron microscopy. Calorimetric characterization of the nonisothermal and isothermal crystallization behavior analyzed according to Avrami’s theory provides evidence of the significant impact of CNTs on the crystallization kinetics of the PLA matrix. The nucleating effect of the nanofillers is confirmed by Raman spectroscopy experiments. Indeed, during isothermal crystallization, the nanotube characteristic vibrations are strongly affected by the development of polymer crystalline phase. Additionally, CNTs increase the number of nucleation sites and thereby decrease the average spherolite size as observed by optical microscopy. The PLA crystal structure is not modified by the presence of CNTs, as probed by X-ray diffraction.

Surface design for controlled crystallization: the role of surface chemistry and nanoscale pores in heterogeneous nucleation.

Abstract: Current industrial practice for control of primary nucleation (nucleation from a system without pre-existing crystalline matter) during crystallization from solution involves control of supersatura...

Control of Crystal Morphology in Poly(l-lactide) by Adding Nucleating Agent

Abstract: In the past few years, poly(L-lactide) (PLLA) has attracted increasing attention due to its excellent biocompatibility and processability. As an eco-friendly thermoplastic polyester, PLLA can be produced completely from renewable sources, such as corn, and degrade into carbon dioxide and water in soil or a composting environment. This sustainability makes PLLA a suitable alternative to traditional petrochemical-based polymers where their recycling still remains a challenge. PLLA has good mechanical properties at room temperature, such as high tensile strength and elastic modulus, but its article prepared by practical processing methods (e.g., injection molding) exhibits poor mechanical performance above its glass transition temperature (around 60 C) since it remains almost amorphous after processing due to its slow crystallization rate.Moreover, PLLA is brittle and its long-term behavior is poor because of a pronounced creep. The crystallization can restrict molecular mobility and then improve the long-term performance. Obviously, PLLA article with suitable crystallinity is required in many commercial applications. On the other hand, the mechanical, thermal properties, and even biodegradability of such semicrystalline polymer have been demonstrated to be strongly dependent on the crystal morphology and structure. Therefore, the realization of effective control on the crystal superstructure consisting of crystalline lamella, such as spherulites and shish-kebab structure, is of great scientific and technical significance because it might provide a route to prepare PLLA products with excellent macroscopic performance. In most cases, isotropic spherulite-like crystal morphology is observed for semicrystalline polymer including PLLA, while shish-like fibril crystals are often obtained from the sheared melt. Many researchers reported that adding nucleating agent is a useful method for controlling the crystal superstructure of polymer, such as polyolefins. For example, the exclusive formation of bundle-like superstructure for β-phase and the radiating, spherulitic superstructure for R-phase in polypropylene (PP) can be induced by adding highly active β-form and R-form nucleating agent, respectively. Moreover, the assembled structure could vary with its concentration and the thermal conditions during melting and crystallization if the nucleating agent is partially or completely dissolved in the polymer melt and recrystallize during cooling. As a result, different superstructures of PP could be induced. For instance, sorbitol-based nucleating agents, such as well-known 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS), can self-organize into nanofibils on cooling, and then lateral growth of PP lamellae occurs orthogonally to the fibrils, forming a typical shish-kebab structure. Thus, adding nucleating agent with the ability of self-organizing in polymer melt is believed to be an important method to manipulate crystal morphology as well as properties of polymer articles. Similar to DMDBS, 1,3,5-benzenetricarboxylamide derivatives, another family of nucleating agents for PP, are also found to be capable of self-organizing in PPmelt through intermolecular hydrogen bonding of the amides in the form of a three-dimensional fibrillar network. Very recently, the derivatives have been successfully used to enhance the crystallization of PLLA. However, no attention has been paid to the self-organization of the derivatives in PLLA melt and its subsequent effect on the crystal morphology of PLLA though it is very important for the macroscopic performance. Therefore, in this Communication, we use one of the above derivatives, i.e., N,N0,N00-tricyclohexyl-1,3,5-benzenetricarboxylamide (TMC-328), as a model to tailor the crystal superstructure of PLLA, and three crystal morphologies including cone-like, shish-kebab, and needle-like structures have been successfully obtained for the first time using melt crystallization. Furthermore, the evolution of crystal morphology during crystallization is investigated using in-situ POM and rheological measurement.

Fast Nucleation and Growth of ZIF‐8 Nanocrystals Monitored by Time‐Resolved In Situ Small‐Angle and Wide‐Angle X‐Ray Scattering

Abstract: Prenucleation clusters: in situ synchrotron X-ray scattering with a one-second time resolution revealed the occurrence of nano-sized clusters during the nucleation and early growth of nanocrystals of a zeolitic imidazolate framework (ZIF). The complex crystallization process exhibits similarities with crystallization processes of zeolites from solution. Hmim= 2-methylimidazole.

From Heterogeneous to Homogeneous Nucleation of Isotactic Poly(propylene) Confined to Nanoporous Alumina

Abstract: The crystallization of highly isotactic polypropylene confined in self-ordered nanoporous alumina is studied by differential scanning calorimetry. A transformation from a predominantly heterogeneous to predominantly homogeneous nucleation takes place if the pore diameter is smaller than 65 nm. Crystallization is suppressed with decreasing pore size, and the absence of nucleation below 20 nm pores indicates the critical nucleus size. The results reported here might enhance the understanding of nanocomposites containing semicrystalline polymers and reveal design criteria for polymeric nanofibers with tailored mechanical and optical properties.

Crystallization Kinetics of Linear and Long-Chain-Branched Polylactide

Abstract: In this study, the non-isothermal cold crystallization and isothermal melt crystallization of both linear and long-chain-branched (LCB) polylactide (PLA) were investigated using a differential scan...