Peculiarities of establishment of steady-state frontal polymerization of vinyl monomers

Frontal radical polymerization of vinyl monomers in non-stationary mode is investigated theoretically. It is shown that the formation and the time of establishment of steady-state polymerization heat autowaves considerably depend on the initial temperature, T_i. When T_i is less than the adiabatic heating temperature (T_a), the excess heat and relatively high conversion foster front formation before the non-stationary one. Whereas, for T_i > T_a, steady-state frontal polymerization regime is established, when the heat flow from the outer source is considerably less than the heat evolving due to the chemical reaction.     

Formation of high purity Si nanofiber from metallurgical grade Si by molten salt electrorefining

We studied the purification of metallurgical grade Si from solutions of K₂SiF₆ in fluoride melts using a molten salt electrorefining process at 700 °C. Electrorefining close to the deposition potential gave dense, coherent, and well-adherent deposits. It was shown that the deposition rate and microstructure of Si strongly depend on the process temperature. Deposited polycrystalline silicon has a well defined rod shape and crooked structure that varies with current density. The anodic dissolution rate is affected by the initial concentrations of K₂SiF₆ and the applied current density. The results of an inductively coupled plasma (ICP) analysis indicated that recovered silicon fiber deposits with purities greater than 99.98% can be obtained using the developed technique. The morphology of the electrodeposited silicon on silver substrates is discussed in the context of a cathodic reaction on the electrode surface, and a comprehensive explanation of the purification mechanism with salt removal is provided.     

Cataract halos: a driving hazard in aging populations. Implication of the Halometer DG test for assessment of intraocular light scatter

OBJECTIVE AND BACKGROUND: Cataract, regardless of etiology, results in light scatter and subjective glare. Senile cataract is emerging as a crucial factor in driving safely, particularly in night driving and adverse weather conditions. The authors examined this visual impairment using a new Halometer DG test in the eyes of older adult drivers with and without cataract. METHOD: Examined subjects consisted of n=65 older adults with cataract in one or both eyes and n=72 adult drivers who did not have a cataract in either eye. Subjects were examined for distance high contrast visual acuity (VA) and red/green disability glare (DG) with a new halo generating instrument. Subjects also completed a subjective Driving Habits Questionnaire (DHQ), designed to obtain information about driving during the past year. RESULTS: DG increased with age of the driver. VA and Halometer DG testing of better and worse eyes prognosticated impairments which significantly affect driving performance. Cataract subjects demonstrated increased Halometer DG scores and were two to four times more likely to report difficulty with driving at night and with challenging driving situations than were cataract-free drivers. CONCLUSION: DG is a specific cataract-induced functional age-related risk factor of driving difficulty, easily measured by a technician with a new Halometer DG device. APPLICATION: Optometrists and ophthalmologists should incorporate Halometer DG testing in their pre-examination vision testing rooms for patients over age 55, and also perform this test on others who complain about glare. Traffic safety engineers should incorporate automotive optical-microprocessor-aided tests for DG into cars, to alert drivers of mild functional impairments and progressive degrees of DG sensitization.     

Material‐Driven Fibronectin Assembly Promotes Maintenance of Mesenchymal Stem Cell Phenotypes

Mesenchymal stem cells (MSCs) are a research tool to investigate fundamental biology and are candidates for use in regenerative medicine. In this context, significant efforts have been devoted to develop technologies to control stem cell fate, including the use of soluble factors in media. However, material properties offer alternative approaches that avoid the use of soluble factors. Here, a material system capable of sustaining the growth of stem cells (maintaining stemness) and of promoting highly efficient differentiation upon external stimulation is described. Poly(ethyl acrylate) induces assembly of fibronectin (FN) into nanonetworks (FN fibrillogenesis) upon simple adsorption from solutions. It is shown that these FN nanonetworks allow growth of MSCs and maintenance of stemness for long periods of time (up to 30 d) using basal media in absence of soluble factors. Additionally, the system promotes enhanced levels of differentiation when defined supplemented media are used. The study reveals the critical role of the intermediate protein layer at the material interface to control MSC fate regardless of the properties of the underlying material and it introduces a new material system as a candidate to be used in MSC niche design.     

Metal-enhanced fluorescence of chlorophylls in single light-harvesting complexes

Ensemble and single-molecule spectroscopy demonstrates that both emission and absorption of peridinin-chlorophyll-protein photosynthetic antennae can be largely enhanced through plasmonic interactions. We find up to 18-fold increase of the chlorophyll fluorescence for complexes placed near a silver metal layer. This enhancement, which leaves no measurable effects on the protein structure, is observed when exciting either chlorophyll or carotenoid and is attributed predominantly to an increase of the excitation rate in the antenna. The enhancement mechanism comes from plasmon-induced amplification of electromagnetic fields inside the complex. This result is an important step toward applying plasmonic nanostructures for controlling the optical response of complex biomolecules and improving the design and functioning of artificial light-harvesting systems.     

Low-temperature combustion waves in low-energy K2TaF7–Si-additive systems

This work presents the use of a thermocouple technique for measuring temperature profiles in a condensed K₂TaF₇–Si system blended with a small amount of Teflon [(C₂F₄)_n] or potassium chlorate (KClO₃). A base experiment is described in detail to demonstrate the ability of the system to react under a low-rate self-sustaining mode at ambient temperature. The ignition temperatures, temperature–time profiles, combustion parameters, and final products are presented with respect to the additive concentration. The combustion processes begin at 340 and 450 °C for the KClO₃ and (C₂F₄)_n-containing mixtures, respectively. The maximum temperatures of both KClO₃ and (C₂F₄)_n-containing mixtures range from 470 to 960 °C and the combustion self-propagates along the sample at a speed of 0.01–0.08 cm/s. The solid combustion products produced under the optimized conditions include fine powders of tantalum, tantalum carbide, and tantalum silicides. The chemical mechanism of the combustion process and reaction parameters responsible for low-speed wave propagation are presented and discussed.     

Rare-earth hexaboride 2D nanostructures synthesis and coupling with NaAlH4 for improved hydrogen release

This study investigates two-dimensional REB₆ nanostructures formation from a reaction mixture of RE₂O₃–B₂O₃–Mg–NaCl composition (RE is Ce, La, Nd, or Eu). The reaction mixture is sealed in a metal cup and is preliminarily heated in argon to reach the flash point. A combustion wave forms on the top surface of the pellet after self-ignition at ~650 °C. This wave quickly propagates through the sample and increases the sample temperature to 800–950 °C, converting the raw mixture into the final product. Processing of the final product leads to the formation of pure hexaboride phases of RE, the particles of which are in the form of nanosheets with a thickness of 50 nm (or less) and a BET surface of 23.8–50.8 m²/g. In addition, CeB₆ nanosheets were tested as a catalyst in NaAlH₄ dehydrogenation/hydrogenation processes. The results demonstrated that the catalytic activity of CeB₆ nanosheets increases with higher portions of NaCl.     

Structural and thermal properties of boron nanoparticles synthesized from B2O3 + 3Mg + kNaCl mixture

Amorphous boron nanoparticles were synthesized by heating a B₂O₃ + 3 Mg + kNaCl (k is the number of moles of NaCl) exothermic mixture in a laboratory oven at 800 °C under argon flow. NaCl was used as inert material to decrease the maximum combustion temperature of the reaction mixture when it was self-ignited after the melting of Mg at 650 °C. The size of the boron nanoparticles extracted from the final product by acid leaching ranged between 30 and 300 nm for k values ranging from 1 to 5. Moreover, increasing the value of k from 1 to 5 resulted in an increase in the specific surface area of the nanoparticles from 40 to 74 m² g⁻¹. However, at k = 10, a decrease in the specific surface area to 47.56 m² g⁻¹ was recorded due to incomplete reduction of B₂O₃. The ignition point of boron nanoparticles in air as estimated using a thermocouple was approximately 300 °C. Digital camera recording of the combustion process of boron nanoparticles in air revealed that the burning speed of the nanoparticles increased significantly from 0.3 to 15 cm/s when k increased from 1 to 5.