Search

Every refinery/sugar factory has a challenge to deliver sugar within the specifications, while averting excessive wash of the crystals. Wash is needed to clean the crystals of the surface film of mother liquor containing ash and colorants, however, excessive wash dissolves good product. Dissolved sucrose sent back in the process for renewed crystallisation requires costly extra capacity, water, and energy. To avoid exceeding the highest sugar colour allowed for sugar going to the bin/silo, it is necessary to work with a safety margin to the target colour set for the daily operation. This paper will discuss how different methods for colour measurement influence the size of the safety margin. After a short overview of different methods, application of the ICUMSA "General Guidelines for Validating Indirect Technologies" will be described. These guidelines may be applied to any kind of indirect measuring technology. The focus will then be on discussing the safety margin for the ICUMSA methods for solution colour and for a real-time method measuring wet or dry crystals online, directly in the production. Further, practical examples of reduced safety margin will be given. [ABSTRACT FROM AUTHOR]

The yield surface of a material is a criterion at which macroscopic plastic deformation begins. For crystalline solids, plastic deformation occurs through the motion of dislocations, which can be captured by discrete dislocation dynamics (DDD) simulations. In this paper, we predict the yield surfaces and strain-hardening behaviors using DDD simulations and a geometric manifold learning approach. The yield surfaces in the three-dimensional space of plane stress are constructed for single-crystal copper subjected to uniaxial loading along the [ 100 ] and [ 110 ] directions, respectively. With increasing plastic deformation under [ 100 ] loading, the yield surface expands nearly uniformly in all directions, corresponding to isotropic hardening. In contrast, under [ 110 ] loading, latent hardening is observed, where the yield surface remains nearly unchanged in the orientations in the vicinity of the loading direction itself but expands in other directions, resulting in an asymmetric shape. This difference in hardening behaviors is attributed to the different dislocation multiplication behaviors on various slip systems under the two loading conditions. [ABSTRACT FROM AUTHOR]

Molecular adsorption is the first important step of many surface-mediated chemical processes, from catalysis to lubrication. This phenomenon is controlled by physical/chemical interactions, which can be accurately described by first-principles calculations. Several computational tools have been developed to study molecular adsorption based on high throughput/automatized approaches in recent years. However, these tools can sometimes be over-sophisticated for non-expert users. Here we present Xsorb, a Python-based program for identifying the accurate adsorption energy and geometry of complex molecules on crystalline (reconstructed) surfaces. The program automatically samples the potential energy surface (PES) that describes the molecule-surface interaction by generating several adsorption configurations through symmetry operations. The set of the most representative ones is automatically identified through a fast pre-optimization scheme. Finally, the PES global minimum is identified through a full structural optimization process. We show the program capabilities through an example consisting of a hydrocarbon molecule, 1-hexene, adsorbed over the (110) surface of iron and the reconstructed (001) surface of diamond. This program, despite its conceptual simplicity, is very effective in reducing the computational workload usually associated with the creation and optimization of several adsorption configurations. Program title: Xsorb CPC Library link to program files: https://doi.org/10.17632/kv97tgybx8.1 Developer's repository link: https://gitlab.com/triboteam/xsorbed/ Licensing provisions: CC by 4.0 Programming language: Python (version 3.7 and above) and Quantum ESPRESSO (for the ab initio calculations). Nature of problem: Identifying the most stable adsorption configuration of a molecule over a substrate and compute its adsorption energy. Solution method: Creating a Python-based code that generates many adsorption configurations with different molecular orientations, performs a preliminary partial geometrical optimization with density functional theory calculations of all these configurations, identifies the most relevant ones for the full geometrical optimization and computes the adsorption energy. [ABSTRACT FROM AUTHOR]

Mechanical failure such as fatigue is a process with complexities arising from the microstructures of materials and the underlying physics. Single crystals without surfaces and imperfections thus offer an ideal platform to probe the process of mechanical degradation by excluding the microstructural complexity. We explore the failure mechanisms of single-wall carbon nanotubes under constant, monotonic and cyclic tensile loads. Simulations with the full-atom details show that, under high strain, failure is dominated by bond breakage, while bond recovery and Stone–Wales transitions are promoted as strain decreases or temperature increases. By assuming the process as a single-step or multiple-step reaction, kinetic models based on the Arrhenius law are developed for the process of mechanical degradation. The models well predict the life expectation and distribution, which conform to the exponential law for single-step processes and are complicated by the memory effect resulted from the multi-step mechanisms. We find that the nanotube could conform to the damage-free, memory-less limit dominated by bond breakage at room temperature and high strain, featuring an exponential distribution of life to failure, while at elevated temperatures where the Stone–Wales transitions are activated, the memory effect becomes significant and the life distribution can be better fitted by the log-normal or Weibull model. The work offers an atomistic view of the failure process and clarifies the underlying physics of the probabilistic models used for life prediction. • Unveil failure of carbon nanotubes by atomistic simulations. • Establish multi-step kinetic models for prediction of life and mechanistic shift. • Elucidate the physics behind the probabilistic life models and the memory effects. [ABSTRACT FROM AUTHOR]

The article offers information on the tourmaline pocket The Rocket Pocket, found in 2001 from Dada's Tunnel at the Pederneira mines in Brazil, along with photographs of several tourmaline crystals with rocket-like shapes found from the pocket. Topics discussed include recovery of minerals and characteristics of rocket-shaped tourmaline; the distinctive size, shape, and colors of crystals, and gem value of the crystal.

This paper describes a model for baby Skyrme crystal chunks with arbitrary potential by considering energy contributions from the bulk and surface of a crystal chunk. We focus on two potentials which yield distinct Skyrme lattices: the standard potential V=m²(1-φ³) and the easy plane potential V=12m²(φ¹)². In both models, the static energy functional is minimized over all two-dimensional period lattices, yielding the minimal-energy crystal structure(s). For the standard potential, the Skyrmions form a hexagonal crystal structure, whereas, for the easy plane potential, the minimal-energy crystal structure is a square lattice of half-charge lumps. We find that square crystal chunks are the global minima in the easy plane model for charges B>6, with 2B being a perfect square (m²=1). In contrast, we observe that hexagonal crystal chunks in the standard model become the global minima for surprisingly large charges, B>954 (m²=0.1). [ABSTRACT FROM AUTHOR]

The quasistatic approximation is a useful but questionable simplification for analyzing step instabilities during the growth/evaporation of vicinal surfaces. Using this approximation, we characterized in Part I of this work the effect on stability of different mechanisms and their interplay: elastic step-step interactions, the Schwoebel barrier, and the chemical coupling of the diffusion fields on adjacent terraces. In this second part, we present a stability analysis of the general problem without recourse to the quasistatic approximation. This analysis reveals the existence of a supplementary mechanism, which we label the "dynamics effect" as it follows from accounting for all the convective and transient terms in the governing equations. This effect can be stabilizing or destabilizing depending on the ratio of step attachment/detachment kinetics to terrace diffusion kinetics. Further, we find that this dynamics effect remains significant in the slow deposition/evaporation regime, thereby invalidating the classical postulate underlying the quasistatic approximation. Finally, revisiting experiments of crystal growth on Si(111)-7 × 7 and GaAs(001), our analysis provides an alternative explanation of the observed step bunching, one that does not require the mechanisms previously invoked in the literature. [ABSTRACT FROM AUTHOR]

Epitaxial growth on a surface vicinal to a high-symmetry crystallographic plane occurs through the propagation of atomic steps, a process called step-flow growth. In some instances, the steps tend to form close groups (or bunches), a phenomenon termed step bunching, which corresponds to an instability of the equal-spacing step propagation. Over the last fifty years, various mechanisms have been proposed to explain step bunching, the most prominent of which are the inverse Ehrlich–Schwoebel effect (i.e., the asymmetry which favors the attachment of adatoms from the upper terrace), elastically mediated interactions between steps (in heteroepitaxy), step permeability (in electromigration-controlled growth), and the chemical effect (which couples the diffusion fields on all terraces). Beyond the discussion of the influence of each of these mechanisms taken independently on the propensity to bunching, we propose a unified treatment of the effect of these mechanisms on the onset of the bunching instability, which also accounts for their interplay. This is done in the setting of the so-called quasistatic approximation, which by permitting mostly analytical treatment, offers a clear view of the influence on stability of the combined mechanisms. In particular, we find that the Ehrlich–Schwoebel effect, elastic step-interactions and the chemical effect combine in a quasi-additive fashion, whereas step permeability is neither stabilizing nor destabilizing per se but changes the relative influence of the three aforementioned mechanisms. In a companion paper, we demonstrate and discuss the importance of another mechanism, which we call the dynamics effect, that emerges when relaxing the simplifying but questionable quasistatic approximation. [ABSTRACT FROM AUTHOR]