Preparation of nano-AlN powder by sol–gel foaming and its sintering properties

Uniformly dispersed nano-sized aluminum nitride powders were prepared by the sol–gel foaming method using aluminum nitrate as the aluminum source, sucrose as the carbon source, and ammonium chloride as the foaming agent. The effects of ammonium chloride content on the particle size and the sintering properties of aluminum nitride were investigated. The results showed that when the molar ratio of ammonium chloride to aluminum nitrate was .5, the colloidal foams were uniform, large, and fluffy, and amorphous alumina precursors with uniform particles could be prepared. Aluminum nitride powder with a particle size of 22–27 nm can be obtained by calcining these precursors in nitrogen atmosphere at 1400°C for 2 h. At the same time, aluminum nitride bulk material with a relative density of 95% can be obtained by sintering the compact samples in nitrogen atmosphere at 1700°C for 2 h.     

Synthesis and characterization of nanocrystalline zinc aluminate spinel powder by sol–gel method

Single-phase nanocrystalline zinc aluminate (ZnAl₂O₄) spinel powder has been synthesized by the sol–gel method. Zinc aluminate nanoparticles were formed at 600 °C, which is at much lower temperature than by solid state reactions. Formation of ZnAl₂O₄ and their particle size depend on the calcination temperature. Calcination temperature also affects the specific surface area and pore volume. The nanocrystalline zinc aluminate was characterized by powder X-ray diffraction, FT-IR spectroscopy, thermal gravimetric analysis, diffuse reflectance spectroscopy, surface area measurements, field emission scanning electron microscopy coupled with energy dispersive X-ray analysis and transmission electron microscopy. Catalytic reactivity of nanocrystalline zinc aluminate was tested for the reduction of 4-nitrophenol to 4-aminophenol using NaBH₄.     

Preparation of nanocrystalline SrTiO3 powder by sol–gel combustion method

In this research a sol–gel combustion route has been presented to synthesize strontium titanate (SrTiO₃:ST) nanocrystalline, using citric acid as fuel. The synthesis procedure was optimized by systematically varying the molar ratios of total metal nitrate to citric acid (MN:CA) from 1:1 to 1:3. The effect was investigated through XRD, SEM and TEM analysis. Analysis of XRD spectrum shows the complete of SrTiO₃ nanocrystalline, however, a minor phase of SrCO₃ impurity was found. Hence, an acid treatment process, with 1 mol/l HNO₃ solution and deionized water, was applied to remove the impurity. The results show that the appropriate condition to prepare the single phase nanocrystalline SrTiO₃ powders is MN:CA molar ratio of 1:3, coupled with an acid treatment process and at the lower calcination temperature of 500 °C. The particle size of powders was in nanometer ranges. The average crystallite size calculated from FWHM was about 23 nm. Morphology of powders was identified by SEM analysis. However, TEM estimated the average particle size about 7.5 nm after applying an acid treatment technique at 600 °C.     

Effects of oxide addition on structure and properties of WC–10Co cemented carbide obtained by in situ synthesized powder

Combining spray drying and in situ synthesized technology, WC–10Co cemented carbide with uniform composition was prepared by vacuum sintering. The effects of Al₂O₃ and additions of different rare-earth oxides (La₂O₃, Y₂O₃ and CeO₂) on the microstructure and mechanical properties of WC–10Co were investigated. As the Al₂O₃ content increased from .5 to 2 wt%, the hardness of the sintered sample increased, whereas the relative density and fracture toughness decreased. Compared with the addition of .5 wt% Al₂O₃, the WC–10Co alloy with .5 wt% rare-earth oxides had higher hardness. In addition, compared with the alloy without an inhibitor (.80 μm), after adding .5 wt% Al₂O₃, La₂O₃, Y₂O₃ and CeO₂, the WC grain sizes were reduced to .73, .65, .71 and .62 μm, respectively, which indicated that the addition of Al₂O₃ and rare-earth oxides could refine WC grain during sintering. Among these additives, CeO₂ had the best effect. With the addition of .5 wt% CeO₂, the hardness and the fracture toughness increased from 1299 to 1710 HV₃₀ and from 16.18 to 18.90 MPa m^1/2, respectively.     

Synthesis and characterization of nanocrystalline zirconia powder by simple sol–gel method with glucose and fructose as organic additives

The present study has devised the sol–gel method using glucose and fructose as two organic additives so as to synthesize zirconia nanoparticles. The presence of these organic additives has produced some positive effect on the phase transition from tetragonal to monoclinic and played an important role in the morphology and crystallite size of the nanoparticles. Fourier transform infrared (FT-IR) spectra have shown Zr–O–Zr bond. Crystal phase and crystallite size have been determined by X-ray Diffraction (XRD) analysis. Besides, the morphology of the samples has been investigated by field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The optical properties of the samples have been analyzed using photoluminescence (PL) spectroscopy, too. All the analyses consistently have shown fairly uniform nanoparticles with small size, containing both tetragonal and monoclinic phases with crystallite size between 10 and 30 nm.     

Rheological properties of sodium caprate-induced β-lactoglobulin gel and changes in secondary structure during gelation

In this study, we found that transparent gels of β-lactoglobulin (β-LG) were formed by adding different concentrations of sodium caprate to protein solutions at ambient temperature. We investigated changes in the dynamic viscoelasticity of the mixture with time at 25°C and found that more than 12% β-LG induced the formation of a viscoelastic gel with a suitable amount of sodium caprate (for example, 12% β-LG and 3.6% sodium caprate). Furthermore, we analyzed the changes in the secondary structure of proteins during the gelation step by FHR spectroscopy. Dissociation of the β-LG dimer was first observed just after mixing with sodium caprate. Furthermore, in the β-LG protein in which the original contents were predominantly β-sheets, intermolecular β-sheets attributable to aggregation increased with a decrease in the content of intramolecular β-sheets. Sodium caprate-induced gel was heated at 80°C for 30 min after the gel was formed, and a large increase in the intermolecular β-sheet bands was observed by heat treatment. These results suggest that the formation of sodium caprate-induced gels of β-LG was accompanied by less marked changes in the protein conformation than those in heat-induced gels.     

Sodium silicate activated slag-fly ash binders: Part III—Composition of soft gel and calorimetry

Sodium silicate activated, slag-fly ash binders are potential alternative binders to Portland cement. In this study, the early age properties of slag-fly ash binders namely, set time, and heats of reaction were investigated. Set time was investigated using a combination of two methods namely, the ASTM C403 penetration testing, and s-wave ultrasonic wave reflectometry (SUWR). The discrepancy in set time identified by these two methods suggested the presence of a soft gel which eventually hardened with time. The composition of this soft gel was analyzed by suspending the chemical reaction of the binder after the soft gel formed, but before it hardened. In order to analyze the composition of the soft gel, selective chemical extractions were performed on the binder. ²⁹Si Magic Angle Spinning-Nuclear Magnetic Resonance (MAS-NMR), and FTIR spectroscopy were performed on binders and extraction residues. The soft gel contained a modified calcium silicate hydrate gel (C–N–S–H where N=Na), with a short mean chain length and no observable Al incorporation. Orthosilicate units were also found to be present in relatively high proportions when compared to hardened binders at later ages.     

A rapid production of pure TMA–montmorillonite nanoclay in supercritical water: Study on powder crystallinity and adsorption capacity

Pure nano-montmorillonite (MMT) with interlayer cations of tetramethylammonium (TMA⁺) was prepared by the two different procedures in supercritical water (SCW). In single-step procedure, synthesis of a 1-month aged precursor was conducted directly in SCW medium; while, in two-step procedure, a temperature programmed-promotion approach was applied. For the later technique, first the aging of the fresh solution of reactants was carried out for 2 h at 80 °C followed by temperature increment to reach supercritical condition (T > 372 °C, P > 221 atm). The prepared powders were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The Al/Si ratio of synthesized montmorillonite was about 1.73 as determined by inductively coupled plasma optical emission spectrometry (ICP-OES). Furthermore, the effects of operational parameters, i.e. temperature, reaction time, and precursor aging were studied on purity and crystallinity of product. In spite of the high crystalline montmorillonite obtained from the two procedures, it was revealed that the two-step procedure had a significant effect on the crystallinity, surface area, and consequently, adsorption capacity of products compared to MMT obtained from a single-step method. High adsorption capacity indicates that crystalline TMA–montmorillonite can be synthesized efficiently using two-stage procedure in supercritical water. Possible mechanism of montmorillonite synthesis in supercritical water was also suggested.     

Characterization and functionalization by sol–gel route of SiC foams

Ceramic foam materials are commonly used for various applications, including catalyst supports or solar receivers. SiC foams are good candidates for the latter application as solar receivers. Its efficiency is directly related to the geometry, which can be evidenced by X-ray microtomography, and optical properties of the receiver. A promising route to add functionalities with homogenous and adhering oxide coatings onto complex SiC foams in a single step process is proposed. This oxide synthetic process is derived from the Pechini method. Foams are fully impregnated by precursor sols with a controlled viscosity making a thin and totally covering coating.     

Pre-evaluation method for the spectroscopic properties of YAG bulk materials by sol–gel synthetic YAG powder

We propose a convenient method for pre-evaluating the spectroscopic properties of luminescent impurity doped YAG by sol–gel synthetic powder. Three synthetic methods, the sol–gel, normal-strike, and reverse-strike methods, are examined for their ability as a predictor and the sol–gel method proved the most effective of the three in experiments on crystallinity, particle size, and spectroscopic properties. The sol–gel Nd:YAG powder agrees well with that of a single crystal or ceramic YAG regarding the luminescent spectrum, the intensity, and the lifetime, even though the powder sintered at a low temperature of 1100 °C in comparison with the YAG crystal melting point of 1970 °C. These facts suggest that the sol–gel synthetic powder is the most effective and the easiest technique for evaluating spectroscopic properties of bulk YAG materials before fabricating the final materials.     

Entrap-immobilization of invertase on composite gel fiber of cellulose acetate and zirconium alkoxide by sol–gel process

We prepared a composite gel fiber by the gel formation of cellulose acetate and zirconium tetra-n-butoxide. Gel fiber is stable in common solvents, phosphate solution, and electrolyte solution. Invertase was entrap-immobilized on the gel fiber. The immobilization was easily performed under the mild conditions. The apparent Michaelis constant (K_m) and maximum reaction velocity (V_max) were estimated from Eadie–Hofstee plot for immobilized invertase. The K_m of immobilized invertase was larger than that of native invertase, while the opposite tendency was observed for the V_max. The activity for the immobilized invertase became higher with increasing fiber diameter. It indicates that the hydrolysis of sucrose occurs in the neighborhood of the fiber surface. The thermal stability of the immobilized invertase was higher than those of its native counterpart. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2084–2088, 2001     

Available nitrogen in soils and its determination by the ‘Nmin-method’ and by electroultrafiltration (EUF)

Excess nitrogen fertilizer rates are an environmental hazard. To avoid excess rates, the level of available nitrogen in the soil must be known and considered for assessing the nitrogen fertilizer rate. In arable soils nitrate and exchangeable ammonium in the rooting depth of a crop are directly available to plant roots. These two nitrogen forms are recovered with the Nmin method and they are considered in assessing the nitrogen fertilizer rate for arable crops.Besides nitrate and ammonium recovered by the Nmin method from soil samples taken in early spring, a considerable amount of organic soil nitrogen may be mineralized during the growth period and contribute to crop nutrition. The nitrogen mineralization potential can be determined in incubation tests. The absolute quantities of mineralized nitrogen thus obtained, however, are much higher than the mineralization rates under field conditions. It is for this reason that incubation tests have not obtained a major importance for assessing nitrogen fertilizer rates.With the electro-ultrafiltration (EUF) method nitrate, ammonium, and a minor portion of organic soil nitrogen are extracted from a soil suspension. There is experimental evidence that this organic nitrogen (=Norg) is easily mineralizable. Based on field experiments with cereals and maize formulae were established by which the nitrogen fertilizer rate can be calculated. In these formulae nitrate, ammonium and organic nitrogen extracted by EUF are considered.Advantages and drawbacks of the Nmin method and EUF method are discussed.     

Preparation of carboxylate group-containing rod-like polysilsesquioxane with hexagonally stacked structure by sol–gel reaction of 2-cyanoethyltriethoxysilane

Carboxylate group-containing rod-like polysilsesquioxane (PSQ-COO^?Na⁺) with a hexagonally stacked structure was successfully prepared by the sol–gel reaction (hydrolytic polycondensation) of 2-cyanoethyltriethoxysilane (CETEOS) monomer in sodium hydroxide aqueous solution. The X-ray diffraction pattern of the cast film of PSQ-COO^?Na⁺ showed three diffraction peaks with the d-value ratio of 1 : 1/√3 : 1/2, indicating the formation of hexagonal phase. In addition, the transmission electron microscopic image of PSQ-COO^?Na⁺ showed a stripe pattern, indicating that the rod-like PSQs were stacked parallel. Self-organization of an ion pair composed of a carboxylate anion and a sodium cation, which was converted from the cyano group of CETEOS by hydrolysis under alkaline conditions, was a driving force for the formation of such regular higher-ordered structures of PSQ-COO^?Na⁺.     

Polymerization,grafting and adsorption in the presence of inorganic substrates: Thermal polymerization of styrene with untreated and γ-irradiated silica gel as a case study

The search for innovative and well-defined organic–inorganic hybrid materials demands that the interplay between polymerization reactions, chemical reactivity, and physical adsorption be fully understood. We examine the case study of the thermal polymerization of styrene in the presence of silica, and the effect of pre-irradiation of silica with γ-rays. The effect of adsorption of styrene-derived radical species on the silica support and polymerization conditions on free and grafted (unextractable) polystyrenes and on polymerization mechanism is discussed on the basis of previous literature findings and new data. Evidence is provided of silica derivatization with various species besides polystyrene (¹³C CPMAS NMR), including products from a β-scission of the adsorbed radical intermediates ensuing oxygen donation from the silica network. The polymerization mechanism can be cationic as well as radical, depending on reaction conditions, and the cationic route prevails in the grafting of polystyrene from silica, resulting in a bimodal molecular weight distribution.     

Revealing correlation of core‒rim structures,defects and stacking-faults in SiC ceramics by integrated scanning electron microscopy

Dopant distribution is key to understand sintering behavior and microstructure?property relationship of silicon carbide (SiC) ceramics. Using high-angle secondary-electron (SE) imaging in scanning electron microscope combining elemental and cathodoluminescence (CL) analysis, core?rim structures were found as common microstructural features in SiC ceramics sintered with rare-earth oxides. Characteristic emission by donor-accepter-pairs (DAP) could reveal co-solution of Al?N in α- and β-SiC phases in hot-pressed and gas-pressure sintered ceramics, with or without AlN co-dopant. Stacking-faults (SFs) were found in β-SiC grains to emit characteristic photons by local twin-structures, which confine the luminous transition by DAP into a quantum effect and suppress fully the luminescence when SF density reaches a limit by sintering with two rare-earth oxides. Systematic correlation between the development of microstructures, solution and defect luminescence, and phase transformation can lead to hierarchical microstructure?property relationship to study synergetic mechanical, thermal and opto-electrical performance of SiC ceramics as well as electronic materials.     

Carbon reactivity of binder metals in diamond–metal composites – characterization by scanning electron microscopy and X-ray diffraction

Diamond tooling is a successfully used technique in machining of very hard materials such as minerals and concrete. The type and strength of bonding between the diamond grains, that are mainly responsible for the machining process (e.g. cutting or grinding), and the metallic binder phase is directly linked to the tools quality. Therefore it is of interest to investigate the carbon reactivity of commonly used binder materials.This paper reports about the investigation of the interfacial area between diamonds and one-component metallic binder matrices. As matrix material pure chromium, cobalt, copper, iron, and nickel was used. After the sintering process the diamonds were extracted from the metallic matrix and analyzed by scanning electron microscopy and X-ray diffraction. The morphology of the diamond surface was investigated and a phase analysis was done. These experimental studies support the hypothesis that the carbon reactivity of transition metals is linked to their d-orbital electron configuration.     

Synthesis and microwave dielectric properties of pseudobrookite-type structure Mg5Nb4O15 ceramics by aqueous sol–gel technique

Pseudobrookite-type Mg₅Nb₄O₁₅ ceramics were prepared by aqueous sol–gel process and microwave dielectric properties were investigated. Highly reactive nanosized Mg₅Nb₄O₁₅ powders were successfully synthesized at 600 °C in oxygen atmosphere with particle sizes of 20–40 nm firstly and then phase evolution was detected by DTA-TG and XRD. Sintering characteristics and microwave dielectric properties of Mg₅Nb₄O₁₅ ceramics were studied at different temperatures ranging from 1200 °C to 1400 °C. With the increase of sintering temperature, density, ?_r and Q·f values increased, and then saturated at 1300 °C. Excellent microwave properties of ?_r ～11.3, Q·f ～43,300 GHz and τ_f ～?58 ppm/°C, were obtained finally. The sintering temperature of Mg₅Nb₄O₁₅ ceramics was significantly reduced by aqueous sol–gel process compared to conventional solid-state methods.     

Column adsorption of Cu(Ⅱ) by polymer-supported nano-iron oxides in the presence of sulfate: Experimental and mathematical modeling

Polymer-supported hydrous iron oxides (HFOs) are promising for heavy metals removal from aqueous systems.The ubiquitous inorganic ligands,e.g.,sulfate,are expected to exert considerable impacts on pollutants removal by these hybrid sorbents.Herein,we obtained a hybrid sorbent HFO-PS by encapsulating nanosized HFO into macroporous polystyrene (PS) resin.Both batch and column sorption experiments of Cu(Ⅱ) by HFO-PS were carried out in the presence of sulfate.Obviously,the presence of sulfate is favorable for Cu(Ⅱ) sorption onto HFO-PS.The performances of column Cu(Ⅱ) removal were fitted and predicted with Adams-Bohart,Clark,Thomas and BDST models.Thomas model is suggested best-fit to predict the breakthrough curves.Besides,a linear correlation is observed between breakthrough time and column length based on BDST model,which might be useful for predicting the breakthrough time for Cu(Ⅱ) removal by HFO-PS.