Initial Synthesis Conditions Research Articles

Self-propagating high-temperature synthesis of macroporous B2+L12 Ni-Al intermetallics used in cylindrical radiant burners

The use of Ni-Al alloys proves to be highly relevant in advanced radiant burner production. The aim of this study is an in-situ fabrication of maсro-porous B2-L12 Ni-Al alloys by self-propagating high-temperature synthesis for a specified burner configuration. It has been revealed that the use of small amounts of special additives dramatically affects the synthesis of coarse-porous alloys. High-speed imaging and high-speed spectral pyrometry have demonstrated that a combustion wave consists of a set of short-lived superadiabatic drops of a melt, the temperature of which exceeds the liquidus temperature for a short time. In the formation of melt drops, up to 107 nickel and aluminum particles are involved. This makes it possible to synthesize porous alloys with Ni-Al elements of up to several millimeters in diameter, using Ni and Al powders with the size of fewer than 10 μm. The porosity structure of such alloys has been characterized in the paper. It has also been demonstrated that the structure can be controlled by changing initial synthesis conditions such as reaction mixture composition, an additive type and its amount, mixture relative density, and a preheating value. In addition, the article describes the conditions that allow obtaining alloys with Ni-Al elements in the range of 500–2500 μm in size. As for the phase composition of synthesized alloys, it has been examined by X-ray diffraction. Besides B2-L12 phases, the alloys may contain some martensite L10, which disappears if the alloys are annealed for an hour at a temperature of 1100 °C. To evaluate mechanical properties of the synthesized and annealed alloys, a compression test has been applied. The impact of the alloys phase composition on possible operation modes of Ni-Al radiant burners has been discussed in the work.

Mesoporous silicas templated by heterocyclic amino acid derivatives: Biomimetic synthesis and drug release application

The present paper reported a biomimetic synthesis of mesoporous silicas (BMSs) at room temperature by using synthesized polymers (C16-l-His, C16-l-Pro and C16-l-Trp) which derived from amino acid with ring structures as template under basic condition via co-structural-directing-agent method. The formation mechanism of BMSs and effect of initial synthesis conditions (such as surfactant structure, pH and co-solvents) on morphology and structure of BMSs were systematically studied. Synthesized BMSs were characterized by transmission electron microscope (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and nitrogen adsorption/desorption isotherms. The results showed that the surfactant structure was the dominant factor to direct the final mesostructure of BMSs, since the structure of surfactant affected the structure and size of clusters. Meanwhile the generation of BMSs required very rigorous alkaline condition which controlled the ionization degree of the surfactant and thus contributing to adequate stacking energy. Higher pH resulted in construction of channels with higher curvature. The presence of ethanol was found to facilitate the formation of BMSs with larger particle size. In application, aspirin can be loaded into BMSs with high efficiency, and the drug crystalline state of aspirin transformed from crystalline state to amorphous state during this process, which undoubtedly lead to the improvement of drug dissolution from 72.8% to 100% within 90 min. It is convincible that the biomimetic method presented here provided novel insight on precisely control of mesoporous silica and undoubtedly promoted the application of mesoporous silica materials.

Electrochemical Properties of Nanocrystalline Li4Mn5O12 for Hybrid Capacitor

The hybrid capacitor or asymmetric capacitor is the hybridization of electrochemical capacitor and rechargeable batteries. The device is developed by hybridization on the electrode level; therefore, it is based on a battery anode (charged by faradic reaction) and electrochemical capacitor electrode (charge stored in the electrochemical double layer) [1]. Sometimes, the name “lithium-ion” capacitor was introduce to described a device using a graphite negative electrode and activated carbon (AC) based positive electrode in combination with a Li+ containing electrolyte [2]. In such device, different types of materials have been proposed for faradic component. Lithium insertion compounds used in lithium-ion batteries have attracted a lot of interest due to high specific charge. They have been typically used in organic electrolyte but the aqueous solutions are not missing [3,4]. In this work, nanocrystalline Li4Mn5O12 was low-temperature synthesized by sol-gel method with citric acid as a chelating agent. Various initial synthesis conditions were studied such as: pH of solution, the ratio of citric acid to total metal ions, the precursor of lithium salts. The structure, morphology and electrochemical properties of materials are characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), cyclic voltammetry (CV) and galvanostatic cycling with potential limit (GCPL). The stoichiometric of formula was exactly determined by reduction-oxidation titration and flame atomic absorption spectrophotoscopy (FAAS). The resulting materials exhibited the particles size of 50 – 100 nm. The electrochemical results showed that Li4Mn5O12 synthesized at pH = 9 and the ratio of citric acid to total metal ions 1:1 exhibited the best electrochemical capacitive performance within potential range of 0 – 1.4 V in 1M Li2SO4. A maximum specific capacitance of 20 F.g-1 during 200 cycles was obtained for hybrid capacitor for AC/Li4Mn5O12 at high discharge rate of 5 A.g-1. Acknowledgements The authors would like to thank Office of Naval Research Global (ONRG) for the grant N62909-13-1-N235. The authors acknowledge funding from VNU-HCM under the grants HS2013-76-01.

Optical properties of serrated GaN nanowires

GaN nanowires with periodic serrated morphology have been synthesized on Si substrate by Au-catalyzed vapor-liquid-solid growth mode. The presence of Mn vapor during growth process has been found to enhance the production and quality of serrated GaN nanowires, without introducing dopants. We have performed photoluminescence and Raman spectral measurements on nanowires with different levels of serration. Temperature dependent photoluminescence revealed a broad yellow-green and red luminescence in the samples. Room temperature Raman spectra exhibits disorder-activated phonon mode at ~670 cm−1, in addition to E2(high) and A1(LO) modes of GaN. Further investigation of Raman spectra revealed the presence of tensile stress in the GaN nanowires when Mn vapor is present during the growth process. The dependence of the optical properties on the morphology of GaN nanowires shows that they can be tuned by initial synthesis conditions.

Short synthesis of ordered silicas with very large mesopores

SBA-15 silicas with cylindrical mesopores of diameter from 10 to ∼30 nm were synthesized within hours (2.5–7.5 hours) instead of one or more days required in hitherto reported syntheses of large-pore SBA-15 with (100) interplanar spacing above 13 nm. The synthesis was carried out using Pluronic P123 surfactant, 1,3,5-triisopropylbenzene swelling agent and silica hydrolysis/condensation catalyst, NH4F, under low-temperature initial synthesis conditions (12–17 °C) similar to those used in our earlier report on the synthesis of SBA-15 with a wide range of pore diameters (10–26 nm). Herein, it is shown that the low temperature step at which the ordered material forms can be shortened to one hour and the mesoporous material can be recovered at this stage. However, the attainment of a larger pore size and/or better structural ordering requires a hydrothermal treatment, which can be as short as 1–6 hours, if appropriately high temperature (150–170 °C) is selected. Additionally, improved synthesis conditions are reported for ultra-large-pore SBA-15 with (100) interplanar spacing of 25–30 nm. Standard SBA-15 synthesis without a swelling agent was also streamlined. Moreover, the synthesis of large-pore FDU-12 silica with face-centered cubic structure of spherical mesopores using Pluronic F127 surfactant and 1,3,5-trimethylbenzene swelling agent can be shortened to ∼9 hours. The streamlined synthesis procedures can be followed by surfactant removal via calcination with high ramping rate and short dwell time. The synthesis approach based on a short self-assembly step and short high-temperature hydrothermal treatment provides a convenient way to produce large-pore ordered mesoporous silicas suitable for numerous applications.

Independent Fine-Tuning of the Intrawall Porosity and Primary Mesoporosity of SBA-15

We present a study in which the intrawall porosity and primary mesoporosity of SBA-15 are independently controlled by modifying the strength of the molecular interaction that governs the formation of the material. The interactions are targeted at specific times during the process of formation, which results in selective tuning of the porosity, while other characteristics of the SBA-15 material are retained. We show that the intrawall porosity can be considerably reduced by addition of NaI, but not NaCl, and that the shape of the primary mesopores can be influenced by a decrease in temperature, while the two-dimensional hexagonal structure and the particle morphology and size remain unchanged. The timing of the “tuning event” is imperative. We show that a decrease in intrawall porosity by addition of NaI is generic to Pluronic-based mesoporous silica syntheses. This work demonstrates that the material characteristic of mesoporous silica is not necessarily restricted by the initial synthesis conditions as the material properties can be tuned by “actions” taken upon the ongoing synthesis. The triblock copolymer Pluronic P104 was used as a structure director and tetramethyl orthosilicate as a silica source. The materials have been characterized primarily with nitrogen sorption and small-angle X-ray diffraction.

A Comparative Characterization of the HPA‐MCM‐48 Type Catalysts Produced by the Direct Hydrothermal and Room Temperature Synthesis Methods

MCM‐48 type support materials synthesized by the direct hydrothermal synthesis (HTS) and room temperature synthesis (RTS) methods were incorporated with tungstophosphoric acid (TPA) in the range of 10–40 wt% by using a wet impregnation technique in methanol solutions. Resulting HPA‐MCM‐48 catalysts were characterized by the XRD, Nitrogen Physisorption, SEM, TEM, EDS, and FT‐IR methods in order to determine the effects of different initial synthesis conditions on the catalyst properties. RTS samples were found to have better crystalline structures, higher BET surface areas, and higher BJH pore volumes than HTS samples. They also had slightly higher TPA incorporation, except for the 40 wt% samples, as evidenced by the EDS results. Keggin ion structure was preserved, for both methods, even at the highest acid loading of 40 wt%. It was concluded that the simpler and more economical RTS method was more successful than the HTS method for heteropoly acid incorporation into MCM‐48 type materials.

Synthesis of monodispersed mesoporous silica spheres (MMSSs) with controlled particle size using gemini surfactant

A series of mesoporous silica spheres with tunable particle size were synthesized by the self-assembly of cationic gemini surfactant and silica source under basic condition. By varying the initial synthesis conditions, such as molar composition, temperature and addition of co-solvent, the particle sizes of the MMSSs with radially oriented pore structure were controlled in a range of 120–490nm, and the pore sizes changed from 2.2 to 3.4nm. The MMSSs have been obtained under different conditions when ethanol was used as co-solvent. With increasing the concentration of ethanol, the sphere diameter varied from 70 to 460nm. The increasing of reaction temperature from 0 to 90°C led to the average diameter of the mesoporous silica spheres changing from 90 to 410nm. Higher NaOH concentration and lower surfactant concentration have been found to facilitate the formation of MMSSs with larger particle size. MMSSs with diameter of 180–380nm were also synthesized with the addition of the co-solvents such as isopropanol, acetone, acetonitrile, dimethyl sulfoxide and methanol. The possible mechanism responsible for the shape and structure of MMSSs was discussed.

Molecular Dynamics Simulation of the Formation of Polymer Networks

AbstractThe random end linking of different amounts of trifunctional crosslinkers with 3 000 prepolymer linear chains, with length varying from 10 to 30 monomers, to form networks at different system number densities was dynamically simulated by the molecular dynamics method. Investigation of the crosslinking kinetics shows that, with a stoichiometric number of crosslinkers present, the time evolution of free ends fraction decays as a power law in time t−3/4. This scaling behavior is different from the one in a dense polymer melt. Structural analyses of the resulting networks indicate that the imperfections of the network structure strongly depend on the initial synthesis conditions including the initial system number density and the ratio of crosslinkers to precursor polymer chain ends.magnified image

Control of the cation occupancies of MnZn ferrite synthesized via reverse micelles

Variations in cation occupancy in mixed metal ferrite systems can affect their electronic and magnetic properties. It is known that different synthesis parameters can lead to various cation distributions and the ability to tune these distributions is of great interest. This study uses the extended x-ray-absorption fine structure–IR relationship to investigate the effect of various Fe2+∕Fe3+ ratios in initial synthesis conditions on cation distribution for manganese zinc ferrite (MZFO). Differences in the precipitated material before firing could lead to differences in the final material if fired under similar conditions. This work uses several different ratios of Fe3+∕Fe2+, which will affect the initial cell potential for the reaction, to synthesize nano MZFO. All samples were fired for 5h at 500°C under flowing nitrogen. Transmission electron microscopy micrographs reveal highly crystalline uniform nanoparticles of 16±2nm. The x-ray diffraction revealed single phase crystalline MZFO with an average crystallite size of around 14nm. The saturation magnetization ranged from 43to68emu∕g as measured by vibrating-sample magnetometry. The Fourier transform infrared (FTIR) analysis was used to determine the cation occupancies while changing the initial Fe3+∕Fe2+ ratios from 10∕90 to 90∕10. The FTIR spectra revealed a shift in the first absorption region in the far IR from 566.98to549.62cm−1 corresponding to the octahedral occupancies. This shift corresponds to a change in the percentage of octahedral sites occupied by manganese from roughly 25% to 12%. This change in manganese occupancy is also observed in the iron occupancies, which in turn help to explain the variation in saturation magnetization.

LiMn 2O 4 cathode materials synthesized by the cellulose–citric acid method for lithium ion batteries

A new technique was employed to synthesize spinel LiMn 2O 4 cathode materials by adding cellulose and citric acid to an aqueous solution of lithium and manganese salts. Various synthesis conditions such as the calcination temperature and the citric acid-to-metal ion molar ratio ( R) were investigated to determine the ideal conditions for preparing LiMn 2O 4 with the best electrochemical characteristics. The optimal synthesis conditions were found to be R = 1/3 and a calcination temperature of 800 °C. The initial discharge capacity of the material synthesized using the optimal conditions was 134 mAh g −1, and the discharge capacity after 40 cycles was 125 mAh g −1, at a current density of 0.15 mA cm −2 between 3.0 and 4.35 V. Details of how the initial synthesis conditions affected the capacity and cycling performance of LiMn 2O 4 are discussed.

Anatase Formation during the Synthesis of Mesoporous Titania and Its Photocatalytic Effect

Thermally stable mesoporous titania can be synthesized following different synthesis routes. In this paper, mesoporous titania was synthesized applying the evaporation-induced self-assembly (EISA) method. The mesostructured titania was treated with an NH(4)OH solution to increase the thermal stability. The influence of this postsynthesis treatment on the anatase formation was investigated using different initial synthesis conditions. Mesoporous titania was synthesized under both acidic and basic conditions, with hexadecylamine and cetyltrimethylammonium bromide as surfactants. The samples were investigated with FT-Raman to investigate when exactly anatase was formed during the synthesis. It will be shown that the anatase formation depends mainly on the initial synthesis conditions. This study shows that anatase can already be formed before calcination under certain circumstances. Furthermore, the accessibility of the anatase phase influenced the photocatalytic activity.

LiNi 0.8Co 0.2O 2 cathode materials synthesized by the maleic acid assisted sol–gel method for lithium batteries

A maleic acid assisted sol–gel method was employed to synthesize LiNi 0.8Co 0.2O 2 cathode materials, which are of interest for potential use in lithium batteries. Various synthesis conditions such as solvent, calcination time, calcination temperature, acid-to-metal ion ratio ( R), and lithium stoichiometry were studied to determine the ideal conditions for preparing LiNi 0.8Co 0.2O 2 with the best electrochemical characteristics. The optimal synthesis conditions were found to be an ethanol solvent with a calcination time of 12 h at 800°C under flowing oxygen. The first discharge capacity of the material synthesized using the above conditions was 190 mAh/g, and the discharge capacity after 10 cycles was 183 mAh/g, at a 0.1 C rate between 3.0 and 4.2 V. Details of how varying initial synthesis conditions affected capacity and cycling performance of LiNi 0.8Co 0.2O 2 are discussed.

Investigation of reaction in an Ni−Al−Cr SHS system based on high-temperature brightness pyrometry

A previously proposed model of reaction in an Ni−Al system is developed as applied to a ternary reaction system that is promising for obtaining materials for vapor-phase thermal deposition of protective coatings. We compare the features of the temperature profile of the combustion wave with data from structural investigations of the reaction products. We show that a detailed qualitative model for the reaction adequately fits the structure and phase composition of the products obtained over a wide range of concentrations of the alloying element (chromium) and initial synthesis conditions.