Kinetic Phase Diagrams Research Articles

Synthesis of zeolites in the absence of organic structure-directing agents: factors governing crystal selection and polymorphism

Organic structure-directing agents (OSDAs) are commonly avoided in commercial zeolite synthesis because of the economic and environmental disadvantages associated with the synthesis and removal of organics occluded within zeolite micropores. Zeolite crystallization in OSDA-free media is the route by which microporous clays form in nature, and it is also the preferred method of producing zeolites in bulk for a wide range of applications. There are many synthesis parameters that influence zeolite crystallization, among which include the molar fractions of reagents (silica, alumina, and hydroxide ions), water content, temperature, synthesis aging and heating time, the selection of extraframework cations, the choice of silica and alumina sources, and the use of crystal seeds. In this review, we discuss zeolite framework types that form in OSDA-free solutions at these different synthesis conditions in an effort to highlight structure-property relationships while simultaneously emphasizing the areas where further studies are needed to optimize and/or discover new materials. Interestingly, fewer than 15% of the total reported zeolite structures have been prepared in the absence of OSDAs. For many of these structures, fundamental mechanisms governing their formation are not well understood. In addition, OSDA-free syntheses tend to be more susceptible to the formation of crystal polymorphs (or impurities) that can be generated through a series of structural transformations during the course of zeolite growth. Here we examine the driving forces for phase transitions and explore methods to control phase selection and polymorphism. In order to better facilitate comparisons among zeolite synthesis parameters, we have reinstituted the approach of using kinetic phase diagrams to identify conditions of phase stability.

Growth of a strained epitaxial film on a patterned substrate

Abstract We study the influence of the growth kinetics on the Asaro–Tiller–Grinfelʼd instability of a thin film deposited on a patterned substrate. We use a continuum model that we solve at first order in the surface slope. Both wetting interactions and elastic fields induced by the film/substrate interface introduce an explicit dependence on the film thickness. As a consequence, the translational symmetry in the growth direction is broken and the deposition flux cannot be trivially accounted for. Similarly to the evolution during annealing, the instability can skip during growth from an in-phase to an out-of-phase geometry depending on the growth duration and film thickness. We compare the evolution of the instability using different deposition fluxes. We find that the kinetic phase diagram found in the annealing case also explains the evolution during growth in usual growth conditions.

Controlling Crystal Polymorphism in Organic-Free Synthesis of Na-Zeolites

Controlling polymorphism is critical in areas such as pharmaceuticals, biomineralization, and catalysis. Notably, the formation of unwanted polymorphs is a ubiquitous problem in zeolite synthesis. In this study, we propose a new platform for controlling polymorphism in organic-free Na-zeolite synthesis that enables crystal composition and properties to be tailored without sacrificing crystal phase purity. Through systematic adjustment of multiple synthesis parameters, we identified ternary (kinetic) phase diagrams at specific compositions (i.e., Si, Al, and NaOH mole fractions) using colloidal silica and sodium aluminate. Our studies identify multiple stages of zeolite phase transformations involving the framework types FAU, LTA, EMT, GIS, SOD, ANA, CAN, and JBW. We report an initial amorphous-to-crystalline transition of core-shell particles (silica core and alumina shell) to low-density framework types and their subsequent transformation to more dense structures with increasing temperature and/or time. We show that reduced water content facilitates the formation of structures such as EMT that are challenging to synthesize in organic-free media and reduces the synthesis temperature required to achieve higher-density framework types. A hypothesis is proposed for the sequence of phase transformations that is consistent with the Ostwald rule of stages, wherein metastable structures dissolve and recrystallize into more thermodynamically stable structures. The ternary diagrams developed here are a broadly applicable platform for rational design that offers an alternative to time- and cost-intensive methods of ad hoc parameter selection without a priori knowledge of crystal phase behavior.

When T(LIESST) Meets Thermal Hysteresis – a Theoretical Approach

AbstractWe used a macroscopic master equation for a two‐level system, with simple assumptions, to calculate the kinetic behavior of a spin‐crossover system when the equilibrium temperature interplays with the T(LIESST) temperature (LIESST = light‐induced excited spin state trapping), that is, the thermal return temperature of the low‐temperature metastable state of the system. The leading role of the unstable equilibrium branch is characterized through the crossing of the cooperative energy barrier, which induces a bifurcation of the response of the system in the strongly cooperative case. The results are shown in terms of a kinetic phase diagram, which shows the eventual discontinuities that result from the bifurcation effect. The present study suggests an experimental method for locating the unstable equilibrium branch of the thermal hysteresis, which enables determination of the equilibrium temperature of the hysteretic system, and which is not accessible by usual static measurements.

Second/Additional Bistability in a CO Oxidation Reaction on Pt(111): An Extension and Compilation

The CO oxidation reaction on Pt(111) was investigated in a pressure range of reactants 10–7–10–5 mbar and a temperature range 364–643 K. In the studied dependence of reaction rate vs temperature, two loops of hysteresis were found in reaction rate measurements instead of one, as expected. The explanation for the observed phenomena leads to two regions of bistability in the kinetic phase diagram caused by two phase transitions. These two detected phase transitions were confirmed independently by variation of two external control parameters, temperature and CO partial pressure, respectively. It means that the two kinetic phase diagrams were crossed in two parameter spaces, i.e., by variation of temperature with constant partial pressure of reactants and by variation of the CO partial pressure in isothermal conditions. The presented experimental data and comparison with literature data suggest that both phase transitions were observed previously but interpreted erroneously as one phenomenon. A unifying expla...

Growth kinetics in a strained crystal film on a wavy patterned substrate

We study the combination of the Asaro–Tiller–Grinfeld morphological instability of a strained film and the organizing effect of an underlying patterned substrate. We use a continuum description of surface diffusion accounting for elasticity, surface energy and wetting interactions. We solve both the mechanical equilibrium and the growth dynamics during annealing at linear order in the film modulation amplitude. We characterize the kinetic phase diagram as a function of time, the film thickness and the ratio between the substrate and the instability wavelengths. We find that the film surface can skip from a configuration in phase with the substrate to either an out-of-phase or a non-ordered configuration, depending on the pattern wavelength and annealing time.

Relationships between molecular structure and kinetic and thermodynamic controls in lipid systems: Part III. Crystallization and phase behavior of 1-palmitoyl-2,3-stearoyl- sn-glycerol (PSS) and tristearoylglycerol (SSS) binary system

The phase behavior of 1-palmitoyl-2,3-distearoyl- sn-glycerol (PSS)/tristearoylglycerol (SSS) binary system was investigated in terms of polymorphism, crystallization and melting behavior, microstructure and solid fat content (SFC) using widely different constant cooling rates. Kinetic phase diagrams were experimentally determined from the DSC heating thermograms and analyzed using a thermodynamic model to account for non-ideality of mixing. The kinetic phase diagram presented a typical eutectic behavior with a eutectic point at the 0.5 PSS mixture with a probable precipitation line from 0.5 PSS to 1.0 PSS, regardless of the rate at which the sample was cooled. The eutectic temperature decreased only slightly with increasing cooling rate. PSS has a strong effect on the physical properties of the PSS–SSS mixtures. In fact, the overall phase behavior of the PSS–SSS binary system was determined, for a very large part, by the asymmetrical TAG. Moreover, PSS is a key driver of the high stability observed in crystal growth, polymorphism and phase development. Levels as low as 10% PSS, when cooled slowly, and 30% when cooled rapidly, were found to be sufficient to suppress the effect of thermal processing.

Particle Trapping and Banding in Rapid Colloidal Solidification

We derive an expression for the nonequilibrium segregation coefficient of colloidal particles near a moving solid-liquid interface. The resulting kinetic phase diagram has applications for the rapid solidification of clay soils, gels, and related colloidal systems. We use it to explain the formation of bandlike defects in rapidly solidified alumina suspensions.

Mapping the local reaction kinetics by PEEM: CO oxidation on individual (100)-type grains of Pt foil

The locally-resolved reaction kinetics of CO oxidation on individual (100)-type grains of a polycrystalline Pt foil was monitored in situ using photoemission electron microscopy (PEEM). Reaction-induced surface morphology changes were studied by optical differential interference contrast microscopy and atomic force microscopy (AFM). Regions of high catalytic activity, low activity and bistability in a (p,T)-parameter space were determined, allowing to establish a local kinetic phase diagram for CO oxidation on (100) facets of Pt foil. PEEM observations of the reaction front propagation on Pt(100) domains reveal a high degree of propagation anisotropy both for oxygen and CO fronts on the apparently isotropic Pt(100) surface. The anisotropy vanishes for oxygen fronts at temperatures above 465K, but is maintained for CO fronts at all temperatures studied, i.e. in the range of 417 to 513K. A change in the front propagation mechanism is proposed to explain the observed effects.

EFFECTS OF AGENT'S REPULSION IN 2D FLOCKING MODELS

In nature many animal groups, such as fish schools or bird flocks, clearly display structural order and appear to move as a single coherent entity. In order to understand the complex behavior of these systems, many models have been proposed and tested so far. This paper deals with an extension of the Vicsek model, by including a second zone of repulsion, where each agent attempts to maintain a minimum distance from the others. The consideration of this zone in our study seems to play an important role during the travel of agents in the two-dimensional (2D) flocking models. Our numerical investigations show that depending on the basic ingredients such as repulsion radius (R1), effect of density of agents (ρ) and noise (η), our nonequilibrium system can undergo a kinetic phase transition from no transport to finite net transport. For different values of ρ, kinetic phase diagrams in the plane (η ,R1) are found. Implications of these findings are discussed.

Ripple rotation in epitaxial growth of MnAs(11¯00)

Rippled states formation driven by kinetic instability is evidenced in the case of MnAs(1$\overline{1}$00) hetero- and homoepitaxial growth in a narrow multistable range of growth parameters. The evolution of the surface morphology in this range, comprising slope selection and ripple rotation, maps the kinetic phase diagram recently predicted [A. Levandovsky and L. Golubovi\ifmmode \acute{c}\else \'{c}\fi{}, Phys. Rev. E 76, 041605 (2007)] for growth on rectangular symmetry surfaces, including Ehrlich-Schwoebel instability and effects related to vertical asymmetry.

Catalytic CO Oxidation on Individual (110) Domains of a Polycrystalline Pt Foil: Local Reaction Kinetics by PEEM

The reaction kinetics of catalytic CO oxidation on individual grains of a polycrystalline Pt foil has been studied simultaneously by photoemission electron microscopy (PEEM) and mass spectroscopy (MS), in the pressure range ~10−5 mbar. By processing the video-PEEM images of ongoing catalytic reaction, the kinetic transitions were tracked for individual [110]-oriented domains. The obtained local kinetic phase diagrams were contrasted to those obtained from global MS activity measurements. These data and the observation of reaction front propagation on different Pt(110) domains indicate a quasi-independent behaviour of the crystallographic domains. The observed front propagation velocities and the degree of their anisotropy on Pt foil corroborate earlier observations on Pt(110) single crystals, confirming our concept of using Pt foil to monitor and compare different surface terminations in parallel.

Dynamic and Reversible Self-Assembly of Photoelectrochemical Complexes Based on Lipid Bilayer Disks, Photosynthetic Reaction Centers, and Single-Walled Carbon Nanotubes

An aqueous solution containing photosynthetic reaction centers (RCs), membrane scaffold proteins (MSPs), phospholipids, and single-walled carbon nanotubes (SWCNTs) solubilized with the surfactant sodium cholate (SC) reversibly self-assembles into a highly ordered structure upon dialysis of the latter. The resulting structure is photoelectrochemically active and consists of 4-nm-thick lipid bilayer disks (nanodisks, NDs) arranged parallel to the surface of the SWCNT with the RC housed within the bilayer such that its hole injecting site faces the nanotube surface. The structure can be assembled and disassembled autonomously with the addition or removal of surfactant. We model the kinetic and thermodynamic forces that drive the dynamics of this reversible self-assembly process. The assembly is monitored using spectrofluorimetry during dialysis and subsequent surfactant addition and used to fit a kinetic model to determine the forward and reverse rate constants of ND and ND-SWCNT formation. The calculated ND and ND-SWCNT forward rate constants are 79 mM(-1) s(-1) and 5.4 × 10(2) mM(-1) s(-1), respectively, and the reverse rate constants are negligible over the dialysis time scale. We find that the reaction is not diffusion-controlled since the ND-SWCNT reaction, which consists of entities with smaller diffusion coefficients, has a larger reaction rate constant. Using these rate parameters, we were able to develop a kinetic phase diagram for the formation of ND-SWCNT complexes, which indicates an optimal dialysis rate of approximately 8 × 10(-4) s(-1). We also fit the model to cyclic ND-SWCNT assembly and disassembly experiments and hence mimic the thermodynamic forces used in regeneration processes detailed previously. Such forces may form the basis of both synthetic and natural photoelectrochemical complexes capable of dynamic component replacement and repair.

Field emission techniques for studying surface reactions: Applying them to NO–H2 interaction with Pd tips

The adsorption of NO and its reaction with H(2) over Pd tips were investigated by means of field ion microscopy (FIM) and pulsed field desorption mass spectrometry (PFDMS) in the 10(-3)Pa pressure range and at sample temperatures between 400 and 600K. By varying the H(2) partial pressure while keeping the other control parameters constant, the NO+H(2) reaction over Pd crystallites is shown to exhibit a strong hysteresis effect. The hysteresis region narrows with increase in temperature and the H(2) pressures delimiting this hysteresis decrease as well. Abrupt transformations of the micrographs are observed by FIM from bright to dark patterns and vice versa. These transformations define the hysteresis region. The collected data allow establishing a novel kinetic phase diagram of the NO+H(2)/Pd system within the range of temperatures and pressures indicated. The observed features are correlated with a local chemical analysis by means of field pulses. NO(+) seems to be the dominating imaging species under all conditions. At high relative H(2) pressures (the "hydrogen-side"), H atoms seem to diffuse subsurface. This process is blocked at lower H(2) pressure (the "NO-side") due to NO(ad) and O(ad) accumulation on the surface. Probe-hole measurements with field pulses indicate that the Pd surface undergoes oxidation as revealed by the occurrence of PdO(2)(+) species in the mass spectra.

Growth Kinetic Processes of AlN Molecules on the Al-Polar Surface of AlN

We studied growth kinetic processes of AlN molecules on the Al-polar surface of AlN using ab initio and Monte Carlo simulations. Molecular processes were presented and analyzed during the nucleation, ripening, and coalescence stages. The results show that the nucleus number decreases as temperature rises due to the increasing diffusion of the molecules. By analyzing the growth time dependence of average cluster size, interface-limited Ostwald ripening is found to be the main ripening mechanism when the temperature is lower than 1773 K. As cluster-corner crossing diffusion is limited, the growth is fractal-like extension, and the coalescence is achieved through adhesion of clusters, leading to a generally continuous morphology with some vacancies and closure failures, which is in good agreement with our experimental results. Moreover, coverage/temperature kinetic phase diagrams under different deposition rates are presented (from 0.025 to 0.1 ML/s). Our finding suggests that a temperature higher than 1800 K is crucial for growth of an ideal atomic-scale Al-polar AlN surface.

Local Reaction Kinetics by Imaging: CO Oxidation on Polycrystalline Platinum

Local Reaction Kinetics by Imaging: CO Oxidation on Polycrystalline Platinum

The binary phase behavior of 1,3-dilauroyl-2-stearoyl- sn-glycerol and 1,2-dilauroyl-3-stearoyl- sn-glycerol

The binary phase behavior of purified 1,3-dilauroyl-2-stearoyl- sn-glycerol (LSL) and 1,2-dilauroyl-3-stearoyl- sn-glycerol (LLS) was investigated at a slow (0.1 °C/min) and a relatively fast (3.0 °C/min) cooling rate in terms of melting and crystallization, polymorphism, solid fat content (SFC), hardness and microstructure. Much of the behavior of the system is explained by its polymorphism and the influence of thermal processing. The α-form and the β′-form of a double chain length structure were detected in the mixtures cooled at 3.0 °C/min, whereas only the β′-form was detected in those cooled at 0.1 °C/min. X-ray diffraction data as well as thermodynamic data propose that the most stable phases are promoted by the symmetrical LSL. The measured trends in structural characteristics, thermal properties, SFC, relative hardness and microstructure delimit three groups of mixtures which imply a competition between the stabilizing effect of LSL and disordering introduced by kinetic effects: (a) LLS-rich mixtures with LSL molar fractions ( X LSL) less than 0.3, (b) mixtures with X LSL clustered around 0.5 and (c) LSL-rich mixtures with X LSL ≥ 0.7. The balance between ordering and kinetic effects determines the polymorphism of the mixtures, which in turn determines the behavior of the LSL/LLS system. The kinetic phase diagram of the LSL/LLS binary system constructed using heating differential scanning calorimetry thermograms displayed a singularity at the 0.5 LSL molar fraction which delimits two distinct behaviors: eutectic behavior in one region and monotectic behavior in the other. The molecular interactions, as depicted by a non-ideality parameter of mixing obtained from a thermodynamic model based on the Hildebrand equation, suggests an almost ideal mixing behavior and a moderate tendency to the formation of unlike-pairs in the liquid state.

A computational study of kinetic phase diagrams for CoPt alloy films during epitaxial growth

We have studied the kinetic processes of the epitaxial growth for CoPt alloy films using the master equation method. The kinetic phase diagrams of CoPt alloy films which show the phase formation conditions during the epitaxial growth are determined. From the kinetic phase diagrams, we find that the [001] ordered structure is much easy to be grown at high temperature while the [100] ordered structure is easy to be grown at low temperature although both the [001] and [100] ordering could be the equilibrium ground states. The atomic deposition, ordering and surface segregation lead to a rich variety of phases in epitaxial growth. The surface segregation is found to enhance the [001] ordering and leads to the formation of the [001] ordered phase at high temperature.

Intermediate phase evolution in YBCO thin films grown by the TFA process

The YBCO thin film growth process from TFA precursors involves a complex reaction pathwhich includes several oxide, fluoride and oxyfluoride intermediate phases, andthe final microstructure and properties of the films are strongly influenced bythe morphological and chemical evolution of these intermediate phases. In thiswork we present a study of the evolution of the intermediate phases involved inthe TFA YBCO growth process under normal pressure conditions and we showthat the oxygen partial pressure during pyrolysis of the TFA precursors is animportant parameter. The Cu phase after the TFA pyrolysis can be either CuO,Cu2O or a mixture of both as the oxygen partial pressure is modified. The kinetics evolution ofthe intermediate phases has been determined for films pyrolysed in oxygen and nitrogenatmospheres and it is concluded that non-equilibrium phase transformations influence thereaction path towards epitaxial YBCO films and its microstructure. The intermediatephase evolution in these two series of films is summarized in kinetic phase diagrams.

Influence of Lipid Vesicle Composition and Surface Charge Density on Vesicle Adsorption Events: A Kinetic Phase Diagram

Lipid vesicle adsorption on a solid surface, from a bulk liquid solution, results in different final situations on the surface depending on the vesicles' composition, properties of the solution (pH, ion types, and concentration), and surface properties. The main alternative outcomes of the adsorption event are (i) a lipid bilayer with vesicle rupture immediately upon the adsorption event or (ii) bilayer formation only at and after a critical vesicle coverage, (iii) adsorption of intact vesicles, or (iv) repulsion (no adsorption). We have simulated these different events for the case of vesicles consisting of pure neutral (zwitterionic) lipids and mixtures of neutral and positive or negative lipids, keeping the bulk conditions fixed, and have compiled the different resulting lipid structures on the surface as a function of vesicle composition and surface charge density, in a kinetic phase diagram.