Effects of molten-salt processing parameters on the structural and optical properties of preformed La2Zr2O7:Eu3+ nanoparticles

Molten-salt method has been used to synthesize various functional materials, but it has not been employed to adjust the structure, particle size, and properties of preformed particles. To fill the knowledge gap, in this study, we use a molten-salt medium to reprocess preformed La₂Zr₂O₇:5%Eu³⁺ nanoparticles which were already formed by a molten-salt synthesis (MSS) method. The molten-salt processing is conducted under various conditions in terms of processing time, temperature, and medium. Specifically, within the studied molten-salt processing time (0 h–24 h) at 800°C, 3 h was found to offer the best optical output. In terms of the investigated molten-salt processing temperature (650 °C–1100 °C) for 3 h, 800 °C was identified as the most desirable annealing temperature. Regarding the explored molten-salt processing media (nitrate vs. chloride) at 3 h and 800 °C, better luminescence results were obtained for nanoparticles processed in chloride. Under each processing parameter, the change of optical properties is explained based on the balance among the crystalline size, crystal structure, crystallinity, defect, and agglomeration characteristics of the molten salt processed La₂Zr₂O₇:5%Eu³⁺ particles. We expect this study will excite other scientists to further explore molten-salt processing as an effective post-synthesis method to fine-tune the structure, particle size, and properties of preformed particles to meet the demand of functional materials.     

A Temperature Window for the Synthesis of Single-Walled Carbon Nanotubes by Catalytic Chemical Vapor Deposition of CH4 over Mo2-Fe10/MgO Catalyst

A temperature window for the synthesis of single-walled carbon nanotubes by catalytic chemical vapor deposition of CH₄ over Mo₂-Fe₁₀/MgO catalyst has been studied by Raman spectroscopy. The results showed that when the temperature is lower than 750 °C, there were few SWCNTs formed, and when the temperature is higher than 950 °C, mass amorphous carbons were formed in the SWCNTs bundles due to the self-decomposition of CH₄. The temperature window of SWCNTs efficient growth is between 800 and 950 °C, and the optimum growth temperature is about 900 °C. These results were supported by transmission electron microscope images of samples formed under different temperatures. The temperature window is important for large-scale production of SWCNTs by catalytic chemical vapor deposition method.     

Seed Layer-Assisted Chemical Bath Deposition of Cu2O Nanoparticles on ITO-Coated Glass Substrates with Tunable Morphology,Crystallinity, and Optical Properties

A seed layer-assisted chemical bath deposition method performed at low temperature has been developed to grow uniform and high-quality crystal cuprous oxide (Cu₂O) nanoparticles on transparent conductive/glass substrates. The annealing process by continuous beam (CW) of CO₂ laser was used prior to growing the Cu₂O nanoparticles. In this study, the controlled synthesis of Cu₂O films was investigated by controlling the growth temperatures at 55 °C, 60 °C, 65 °C, and 70 °C, respectively. The modified seeding substrate reflect enhanced structural properties with laser annealing temperature of 450 ℃. In addition, Cu₂O nanoparticles with flower-like stricter show a greater density containing a smaller particle with 75 nm average dimension and flower particle size was about 85 nm. Results suggest an effective synthesis route for developing high-quality Cu₂O nanoparticles for optical and electronic applications.

     

Toward a Green Chemistry Approach for the Functionalization of Melamine Foams with Silver Nanoparticles

The growing popularity of silver nanoparticles in the field of nanotechnology has created the necessity of developing new sustainable synthesis methods. This study presents a new green in situ functionalization method of melamine foams with silver nanoparticles. The synthesis pathway and the influence of the processing parameters are optimized to phase out 100% of polluting and dangerous solvents while maximizing silver transfer. A deep study of the morphological and chemical changes of the synthesized silver nanoparticles successfully demonstrated that water can be used as the only solvent for obtaining active melamine foams with potential application in multiple fields. Results showed that rising reaction temperatures from environmental to mild conditions (40 °C and 60 °C) is crucial for obtaining high functionalization yields with this green method. Following the optimum fabrication conditions using only water, highly functionalized melamine foams showed a great amount of ultrafine silver nanoparticles distributed over the porous structure.     

Radiation-curable carbon fiber prepreg composites

Radiation processing is the utilization of ionizing radiation, usually photons or electron beams, to produce useful physical and chemical changes in a material. A potential application for electron beam processing for composite manufacturing is for curing carbon fiber prepregs. These prepregs, carbon fibers or fabrics preimpregnated with liquid polymer resin, are commonly used in the aircraft industry. Their use, however, can be time consuming and labor intensive. The advantages of radiation curing over thermal or chemical curing methods include improved rate control, reduced curing times, and curing at ambient temperature. There is no need for chemical initiators. A radiation-curable prepreg has been designed to meet the mechanical and physical property specifications of a leading aircraft manufacturing company. The resin is a mixture of an expoxy diacrylate, polybutadiene diacrylate, and a multifunctional monomer. This resin was used to impregnate a plain weave carbon fabric, at a loading of 35 percent (by mass), using a solvent process. Preliminary characterization studies show that the cured polymer produced by irradiation in air to a dose of 40 kGy is amorphous with a maximum gel fraction of 85 percent. The softening point of the polymer varied from 228°C (30-kGy sample) to 237°C (50-kGy sample). The linear thermal expansion coefficient of the polymer was 1.7 × 10⁻⁴ m/m°C from 25°C to 150°C and was unaffected by varying the applied dose from 30 to 50 kGy.     

PBI‐Based Polymer Membranes for High Temperature Fuel Cells – Preparation,Characterization and Fuel Cell Demonstration

Proton exchange membrane fuel cell (PEMFC) technology based on perfluorosulfonic acid (PFSA) polymer membranes is briefly reviewed. The newest development in alternative polymer electrolytes for operation above 100 °C is summarized and discussed. As one of the successful approaches to high operational temperatures, the development and evaluation of acid doped polybenzimidazole (PBI) membranes are reviewed, covering polymer synthesis, membrane casting, acid doping, physicochemical characterization and fuel cell testing. A high temperature PEMFC system, operational at up to 200 °C based on phosphoric acid‐doped PBI membranes, is demonstrated. It requires little or no gas humidification and has a CO tolerance of up to several percent. The direct use of reformed hydrogen from a simple methanol reformer, without the need for any further CO removal, has been demonstrated. A lifetime of continuous operation, for over 5000 h at 150 °C, and shutdown‐restart thermal cycle testing for 47 cycles has been achieved. Other issues such as cooling, heat recovery, possible integration with fuel processing units, associated problems and further development are discussed.     

Carbon Nanotube Growth on Calcium Carbonate Supported Molybdenum-Transition Bimetal Catalysts

A comparison of different catalyst systems (Fe–Mo, Co–Mo or Ni–Mo nanoparticles supported on calcium carbonate) has been performed in order to optimize the carbon nanotube (CNT) growth. The influences of the reaction temperature, metal loading and carbon source on the synthesis of CNTs were investigated. Dense CNT networks have been synthesized by thermal chemical vapor deposition (CVD) of acetylene at 720 °C using the Co–Mo/CaCO₃ catalyst. The dependence of the CNT growth on the most important parameters was discussed exemplarily on the Co catalyst system. Based on the experimental observations, a phenomenological growth model for CVD synthesis of CNTs was proposed. The synergy effect of Mo and active metals was also discussed.     

A method to form self-aligned carbon-nanotube-vias using a Ta-cap layer on a Co-catalyst

A method to form self-aligned carbon nanotube (CNT) vias using a Ta-cap layer on a Co catalyst by chemical vapor deposition at 400 °C is described. The Ta-cap layer protects the Co catalyst from oxidation. This protection potentially eliminated the need for additional processing steps to control the size and activity of the Co catalyst, which are two critical parameters for CNT formation. The Ta-cap layer was observed to remain on the top of the multi-walled CNTs as a contact layer after the synthesis at 400 °C. The Ta-cap layer self-formed a continuous interface between the electrode layer and the CNT layer. Besides, it can avoid the conventional chemical mechanical polishing process. The Ta-cap layer thickness was found to be the key parameter affecting CNT growth and was optimized. O₂ plasma treatment was implemented to remove residual amorphous carbon accumulated on the top of Ta-cap layer after CNT formation. The electrical properties were also measured to verify the feasibility of this CNT-via structure.     

Microwave sintering of IR-transparent Y2O3–MgO composite ceramics

A method for preparation of dense Y₂O₃–MgO composite ceramics by the microwave sintering was developed. The initial powders were obtained by glycine-nitrate self-propagating high-temperature synthesis (SHS) with different oxidant-to-fuel ratio. Density and IR-transmission of microwave sintered Y₂O₃–MgO ceramics increase with respect to dispersity of the SHS-powders and reach its maximum values for the powder prepared in a 20% fuel excess. The sintering behavior of Y₂O₃–MgO compacts was investigated by optical dilatometry and measuring an electric conductivity upon heating. Significant microwave radiation power surges at temperatures of 900–1000 °C, caused by the decomposition of magnesium carbonate, have been found. As a result of matching the conditions for the synthesis of powders and sintering modes, a transmission of composite ceramics of 78% at a wavelength of 6 μm was achieved at a maximum processing temperature of 1500 °C.     

Synthesis of polypyrrole/polythiophene copolymers in supercritical carbon dioxide

Synthesis and processing of polymer-based materials through green friendly methods involving supercritical fluids particularly supercritical carbon dioxide (SCC) has recently received substantial technological importance because of the commercial and industrial benefits involved. In the present work, a SCC assisted green and sustainable process has been developed to synthesize polypyrrole/polythiophene copolymers (PPCs). The process of synthesis has been conducted through ferric chloride-initiated chemical oxidative polymerization in the presence of various molar proportions of pyrrole to thiophene 0:1.0, 1.0:0, 1.0:1.0 and 1.0:2.0 at temperature ranging from 50 to 90 °C, 1,200 psi over 12 h in SCC. Polymerization below 90 °C afforded PPCs in semi-solid products, whereas polymerizations conducted at 90 °C under identical conditions have afforded the end products in complete solid state. The structure and properties of PPCs have been evaluated through ultraviolet–visible absorption and Fourier transform infrared spectra, elemental analysis, atomic force microscopy, simultaneous thermogravimetric–differential thermal analysis–differential thermogravimetry and four-point probe electrical conductivity methods. With molar proportion of thiophene, time and temperature, all the polymerization reactions have been conducted to completion resulting in PPCs with enhanced polymerization yield, rheoviscosity, dispersion of polypyrrole into polythiophene matrix and thermal stability. This has contributed a simultaneous loss in the electrical conductivity of PPCs.     

Spark Plasma Sintering of Superhard B4C–ZrB2 Ceramics by Carbide Boronizing

A carbide boronizing method was first developed to produce dense boron carbide‐ zirconium diboride (“B₄C”–ZrB₂) composites from zirconium carbide (ZrC) and amorphous boron powders (B) by Spark Plasma Sintering at 1800°C–2000°C. The stoichiometry of “B₄C” could be tailored by changing initial boron content, which also has an influence on the processing. The self‐propagating high‐temperature synthesis could be ignited by 1 mol ZrC and 6 mol B at around 1240°C, whereas it was suppressed at a level of 10 mol B. B₈C–ZrB₂ ceramics sintered at 1800°C with 1 mole ZrC and 10 mole B exhibited super high hardness (40.36 GPa at 2.94 N and 33.4 GPa at 9.8 N). The primary reason for the unusual high hardness of B₈C–ZrB₂ ceramics was considered to be the formation of nano‐sized ZrB₂ grains.     

Reactivity of SiO2 fume from ferrosilicon production with Ca(OH)2 under hydrothermal conditions

Reactivity has been studied of SiO₂ fume from ferrosilicon production with Ca(OH)₂ in suspension, with stirring, under hydrothermal conditions at 190°C and 200°C from 1 to 12 hours. Reaction product after a 4-hour synthesis at 200°C is calcium silicate hydrate xonotlite C₆S₆H, with some C-S-H(I) phase. When glass fibers were added to the slurry, which was subsequently filtered, pressed, and dried, the product thus obtained was lightweight thermal insulation material, temperature-resistant up to 800°C and usable up to 1000°C. The product has excellent insulation properties with thermal conductivity 0.042–0.081 W/mK at bulk densities 150–500 kg/m³.     

Electrophoretic deposition of ZrB2 coatings in NaCl-KCl-AlF3 melt containing synthesized ZrB2 nanoparticles

A novel method for preparation of ZrB₂ coatings has been proposed by combination of molten salt synthesis of ZrB₂ nanoparticles and subsequent electrophoretic deposition of the as-synthesized ZrB₂ nanoparticles in the same molten system in this paper. Nanoscale ZrB₂ particles have been produced by borothermal reduction of ZrO₂ in NaCl-KCl-AlF₃ melt at 980°C. Then electrophoretic deposition of the as-synthesized ZrB₂ nanoparticles has been achieved in the resulting molten suspension at a reduced temperature of 900°C, yielding relatively dense ZrB₂ coatings on graphite substrates with a thickness of around 25 μm. Moreover, the effect of different cell voltages ranging from 0.8 V to 1.4 V (i.e., electric field 0.4–0.7 V/cm) on the prepared ZrB₂ coatings has been investigated. Finally, during the tests at 600°C–800°C in air, ZrB₂ coatings deposited at a cell voltage of 1.2 V have exhibited good high-temperature oxidation resistance.     

Chemically stable polyphenylene ether microcapsules prepared using a facile method for the self‐healing of polymers

Chemically stable polyphenylene ether (PPO) microcapsules (MCs) filled with epoxy resins (PPO‐EP MCs) were prepared using low‐molecular‐weight PPO with vinyl end‐groups as shell wall and epoxy resins as core material using an oil‐in‐water emulsion solvent evaporation method. This method for synthesizing MCs with PPO shell walls is simple, convenient and novel, which can avoid the influence of processing parameters on the chemical stability of the epoxy resin core material. The resulting PPO‐EP MCs exhibit good chemical stability below 255 °C mainly owing to the absence of a polymerization catalyst of the epoxy resins. The initial thermal decomposition temperature of the MCs is about 275 °C. The MCs were embedded in a 4,4′‐bismaleimidodiphenylmethane/O,O′‐diallylbisphenol A (BMI/BA) thermosetting resin system. When processed at high temperature (up to 220 °C), the microencapsulated epoxy resins could be released from the fractured MCs to matrix crack surfaces and bond the crack surfaces. An amount of 8 wt% MCs restored 91 and 112% of the original fracture toughness of the BMI/BA matrix when heated at 220 °C/2 h and 80 °C/1 h + 220 °C/2 h, respectively. The MCs only slightly decreased the thermal property of the matrix. © 2016 Society of Chemical Industry     

Removal of trace quantities of co from H2 using temperature‐swing‐adsorption

A method for the removal of trace quantities (0.2 to 10 ppm) of carbon monoxide in commercial hydrogen using a temperature‐swing‐adsorption system with 0.5 wt% Pt/Al₂O₃, adsorbent has been investigated. The adsorbent could be regenerated by treatment with air at 200°C to 250°C or by treatment with hydrogen at 270°C. The breakthrough curves have been described by a model which considers both the external‐film resistance and the intraparticle diffusion resistance. The model has been validated at the high gas velocities expected in a commercial plant.     

Hexamethylenetetramine Dinitrate (HDN): The Precursor for RDX Production by Bachmann Process

Hexamethylenetetramine dinitrate (HDN) is a rather weak explosive but is used as a precursor for the synthesis of RDX, one of the most important secondary nitramine explosives. HDN has limited application because of its hygroscopic character. This paper reports on the synthesis and characterization of HDN in high yield and purity by the reaction of hexamine with nitric acid at temperatures below 15 °C. It was characterized by FTIR and ¹H NMR spectroscopy, Scanning Electron Microscopy (SEM) and Liquid Chromatography/Mass Spectrometry (LC/MS) measurements. The thermal characteristics of HDN were determined by DSC and TG/DTA. The DSC curve of HDN shows an endothermic peak at 170.5 °C corresponding to the melting point of HDN, followed by two exothermic peaks at 174.0 °C and 200.5 °C due to the decomposition. The differences in the thermal behavior of HDN samples, which were thermally aged at 50 °C, 100 °C, and 150 °C in a nitrogen atmosphere were examined. Additionally, some quantum chemical properties of the nitration of hexamethylenetetramine were calculated.     

Metastability in the MgAl2O4–Al2O3 System

Aluminum oxide must take a spinel form (γ‐Al₂O₃) at increased temperatures in order for extensive solid solution to form between MgAl₂O₄ and α‐Al₂O₃. The solvus line between MgAl₂O₄ and Al₂O₃ has been defined at 79.6 wt% Al₂O₃ at 1500°C, 83.0 wt% Al₂O₃ at 1600°C, and 86.5 wt% Al₂O₃ at 1700°C. A metastable region has been defined at temperatures up to 1700°C which could have significant implications for material processing and properties. Additionally, initial processing could have major implications on final chemistry.     

Eggshell derived mesoporous biphasic calcium phosphate for biomedical applications using rapid thermal processing

Biphasic calcium phosphate (BCP) has received much interest for making various bone substitutes since its physicochemical properties can be easily tailored by tuning its phase composition. Due to high temperature processing, it is hard to prepare BCP with nanoscale characteristics. In the present study, we have made an attempt to optimize the heat treatment parameters for the synthesis of BCP with nanoscale characteristics from eggshell derived hydroxyapatite (HA) through rapid thermal processing (RTP). To accomplish this, eggshell derived HA was prepared by wet precipitation method and subjected to RTP at 750°C and 1150°C for 3 and 10 minutes. For comparison we have also studied conventional calcination at 750°C and 1150°C for 3 hours. XRD, FTIR, SEM, EDX, HRTEM, and BET analyses were used to understand the effect of RTP and conventional calcination on eggshell derived HA. Our results indicate that eggshell derived HA on RTP at 1150°C for 3 minutes and 10 minutes can offer nanoscale BCP with good dissolution, bioactivity, cytocompatibility, and mesoporous nature. Hence, RTP can be a potential method to prepare BCP with nanoscale features for biomedical applications.     

Novel combustion synthesis of La3+-substituted MnFe2O4

La³⁺-substituted MnFe₂O₄ compounds have been prepared by using a novel combustion synthesis method. This process was found to yield homogeneous, finely crystalline powders without intermediate decomposition and/or calcination steps. Combustion-synthesized powders were sintered at 1000°C, and structural features of thus prepared materials were characterized by XRD analysis and FT-IR spectroscopy. The dc electrical conductivity of synthesized materials has been measured as a function of temperature up to 1000°C. The materials have shown semiconducting behavior at elevated temperatures. The ac electrical conductivity of synthesized samples was found to increase with increasing applied frequency. The dielectric constant and dielectric loss tangent have also been characterized. The article is published in the original.     

Synthesis,characterization and sinterablity of 10 mol% Sm2O3-doped CeO2 nanopowders via carbonate precipitation

A carbonate coprecipitation method has been used for the facile synthesis of highly reactive 10 mol% Sm₂O₃-doped CeO₂ (20SDC) nanopowders, employing nitrates as the starting salts and ammonium hydrogen carbonate (AHC) as the precipitant. The AHC/RE³⁺ (RE = Ce + Sm) molar ratio (R) and the reaction temperature (T) affect significantly the final yield and precursor properties, including chemical composition and particle morphology. Suitable processing conditions are T = 60 °C and R = 5.0–10, under which precipitation is complete and the resultant precursors show ultrafine particle size, spherical particle shape, and good dispersion. Thus, the processed precursors are rare-earth carbonates with an approximate formula of Ce_0.8Sm_0.2(CO₃)_1.5·1.8H₂O, which directly yield oxide solid-solutions upon thermal decomposition at a low temperature of ∼440 °C. The 20SDC solid solution powders calcined at 700 °C show excellent reactivity and have been densified to ∼99% of the theoretical via pressureless sintering at a very low temperature of 1200 °C for 4 h.