Effect of metal halide fluxes on the microstructure and luminescence of Y3Al5O12:Ce phosphors

Yellow-emitting Y_2.95Al₅O₁₂:0.05Ce (YAG:Ce) phosphor particles with high luminescence efficiency under 450 nm ultraviolet (UV) excitation were prepared by a heat treatment of submicrometer-sized oxide powder mixture in a reducing atmosphere at 1550 °C. Prior to the heat treatment, the oxide mixture was blended with 5 wt% of metal halides – BaF₂, BaCl₂, NaF, NaCl, and KF – as a flux. It was observed that YAG:Ce particles prepared with fluorine containing flux demonstrated spherical morphology, high photoluminescence properties, and diameters of 5–20 μm, whereas those prepared with chlorine were small-sized particles (≤5 μm) with relatively low intensity. The highest relative photoluminescence (PL) intensity (∼118%) was obtained for the samples prepared with 5% BaF₂. A set of rules was established for the formation of spherical YAG:Ce particles in the presence of this flux.     

Preparation and characterization of Sr4Al2O7:Eu, Eu phosphors

A new composition of red strontium aluminate phosphor (Sr₄Al₂O₇:Eu³⁺, Eu²⁺) is synthesized using a solid state reaction method in air and in a reducing atmosphere. The investigation of firing temperature indicates that a single phase of Sr₄Al₂O₇ is formed when the firing temperature is higher than 1300 °C and that a Sr₃Al₂O₆ phase is formed as the main peak below 1300 °C. The effects of firing temperature and doping concentration on luminescent properties are investigated. Sr₄Al₂O₇ phosphors exhibit the typical red luminescent properties of Eu³⁺ and Eu²⁺. A comparison photoluminescence study with Sr₃Al₂O₆ phosphor shows that Sr₄Al₂O₇ has higher emission intensity than Sr₃Al₂O₆ as a result of the higher optimum doping concentration of Sr₄Al₂O₇ phosphor.     

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.     

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.     

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.     

SHS as a new approach to synthesizing hierarchical inorganic structures

We applied SHS method (also known as combustion synthesis) to create hierarchically structured porous hollow carbon nanospheres, mesoporous carbon nanosheets, and AlN stellar and multi-storey dendrite microcrystals. Our method utilizes the heat of combustion reaction for generating liquid and gaseous reactive species that promote the nucleation and growth of hierarchical microstructures. The studies on the electrochemical characteristics of carbon hierarchical structures revealed that these materials have potential application to fabrication of functional materials for use in supercapacitors and Li–S battery electrodes.     

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.     

Controlling Al/Cu composite diffusion layer during hydrostatic extrusion by using colloidal Ag

In this study, intermetallic compound formation at the interface between aluminum and copper during hydrostatic extrusion was simulated by performing a solid state diffusion bonding experiment with various processing parameters, including bonding temperature and pressure and holding time, and by inserting an Ag colloid layer between the aluminum and copper. Regression equations were developed to predict thickness of diffusion layer and interface hardness.An intermetallic compound formed at the interface between the Al and Cu during diffusion bonding at 420 °C and 240 MPa for 60 min, and it was effectively controlled by inserting an Ag colloid. These experimental data will be useful for setting up processing parameters to prepare Al/Cu matrix composite materials by using hydrostatic extrusion.     

Thermal regimes for carbidizing wave propagation in the system titanium-halogen-containing polymer

Erevan. Translated from Fizika Goreniya i Vzryva, Vol. 28, No. 3, pp. 46–49, May–June, 1992.     

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.